Creation of Nanometre-Scale Islands, Wires and Holes on Silicon … · 8 Creation of Nanometre-Scale Islands, Wires and Holes on Silicon Surfaces for Microelectronics on the silicon

Creation of Nanometre-Scale

Islands Wires and Holes on Silicon Surfaces

for Microelectronics

PhD Thesis

V Palermo

University of Bologna

March 2003

copy All Rights Reserved

2 Creation of Nanometre-Scale Islands Wires and Holes on Silicon Surfaces for Microelectronics

VPalermo 3

Universitagrave degli studi di Bologna

Creazione di isole fili e fori nanoscopici su superfici di silicio per microelettronica

Creation of Nanometre-Scale Islands Wires and Holes on Silicon Surfaces for Microelectronics

Tesi di Dottorato di Ricerca Scienze Chimiche

XV ciclo

Presentata da Vincenzo Palermo

RelatoreProf Alberto Ripamonti

Co-Relatore Dott Derek Jones

Coordinatore di Dottorato Prof Goffredo Rosini

Marzo 2003


VPalermo 5

OMNIA IN MENSURA ET NUMERO ET PONDERE

Sapientiae Salomonis 1120


VPalermo 7

1 Introduction nanotechnology and the future of computers The motivations of nanotechnology research

In 1965 an electronic engineer named Gordon Moore one of the future founders of Intel

noted that the performance of computers and their complexity doubled every 18 months

and foresaw that computer power would continue to grow exponentially over the following

years

This prevision quite provocative for its time actually came about and gained the name of

ldquoMoorersquos Lawrdquo and continues to hold for the trends of todayrsquos computer industry Since

1965 the number of transistors present in an integrated circuit (IC) has increased from

several hundred to more than ten million and the minimum size of transistor elements has

shrunk from several millimetres to asymp130 nanometres (fig 11)

Devices of such tiny dimensions are actually fabricated using lithographic techniques

where light is passed through an optical mask to react with a photo-sensitive layer (resist)

Fig11 Evolution of the number of transistors present on commercial computers [1]


on the silicon wafer This resist is then selectively removed and used as a mask for

processing the silicon surface (fig 12 left) The maximum resolution attainable depends

upon the wavelength used and current technology is near to its intrinsic resolution limit

On the other hand there is strong scientific and economic demand for further development

in IC miniaturization to obtain more powerful and complex computers Besides every-day

life applications more powerful computers are fundamental for much scientific research

such as climate change tracking genome sequencing and fluid dynamics Increased

miniaturization is also fundamental for reducing power which has to be dissipated by the

chips which run at progressively higher frequencies Energy consumption by

microelectronic devices is already an issue and represents one of the main obstacles for

the continuing growth in wireless communication (cell phones portable computers CD

and DVD players digital cameras etc)

Thus it is expected that new production methods different from current lithographic ones

will be developed methods which allow modification of a surface well below the 100 nm

limit and even down to single atom manipulation Techniques such as Scanning

Tunnelling Microscopy and Atomic Force Microscopy are already capable of moving

single atoms (see fig 12 right) but unfortunately building a working nanodevice in this

way would take a very long time and these techniques are difficult to apply to large scale

production

Nowadays thousands of researchers are working in the nanotechnology field towards a

new generation of microelectronic devices Several possible solutions are competing for

tomorrowrsquos computer architecture and there is still no clear winner It is likely that the

final solution will be the combined use of different techniques and components (including

molecules nanowires and nanodots) as they become available together with conventional

Fig 12 Left conventional litographic process [2] Right atomic manipulation of iron atoms on copper [3]

VPalermo 9

silicon technology

Below is a brief summary of the most recent developments in nanotechnology and

nanoscience

Actual trends in nanotechnology

Perhaps the most fascinating idea for nanodevice construction is to use one single

molecule working as a complete device The first molecular diodes (ie molecules

conducting current only in one direction) were created in 1997 in 1999 a molecular fuse

and a molecular transistor were demonstrated although there was no possibility of wiring

these devices to external contacts In April 2001 James Heath and his group at UCLA

fabricated an array of overlapping crossbars and placed a small molecule of rotaxane

between each crossbar (fig 13 left) This composite molecule is made up of two

component parts the main rod-like molecular axis and a mobile ring ldquothreadedrdquo on it like a

bead on a necklace and can function as a molecular switch A working 16-bit memory

circuit was constructed using these molecules For a brief review of these works see [4] In

June 2002 a single molecule transistor was built by connecting an organic molecule to two

metal contacts the molecule contained one or two atoms of a transition metal (cobalt or

vanadium) forming the active region of the device supported by an organic backbone [5]

Fig13 Working nanodevices Left schematic representation of rotaxane molecules between crossed nanowires [4] Right SEM image of semiconductor nanowires forming a small circuit [9]


Another approach to nanodevice fabrication has become possible through the discovery of

carbon nanotubes which were observed for the first time in 1991 by a Japanese electron

microscopist studying the material deposited during arc-evaporation synthesis of fullerenes

[6] They consist of a graphite-like carbon seamless cylinder with a diameter of several

nanometers and lengths of up to a millimetre Carbon nanotubes are very stable can

behave as metals or semiconductors and can host other molecular or ionic species thus

modifying their electrical behaviour In 2001 Avouris and his group reported the first

circuit made with a single nanotube [7] A few months later Cees Dekker presented a

nanotube-based transistor able to amplify an input signal by a factor of ten and built

several logic circuits using these nanotube transistors [8]

One problem with carbon nanotubes is that it is very difficult to control their electronic

properties ie their metallic or semiconducting behaviour An alternative to carbon

nanotubes are semiconductor nanowires Silicon nanowires can be made using a laser to

vaporize the silicon together with a metal catalyst like iron or gold The vapour condenses

in nanosized drops of silicon and metal from which the wires slowly grow out as more

silicon is adsorbed In 2001 a group at Harvard University [9] created a transistor by

crossing two different nanowires After this the same group arranged four nanowires in a

noughts and crosses grid creating something like a 4-bit memory (fig 13 right) Even

metallic nanowires made of platinum and silver can be used in a crossed configuration to

store information [10]

There are some issues common to all these new technologies though First it is difficult to

imagine these methods applied to large-scale production Up to now the insertion of a

molecule between two electrodes is an occasional lucky event while nanotubes and wires

have to be positioned on the surface creating the appropriate contacts on them manually

The large-scale production of integrated circuits using these building blocks will not be

straightforward and does not seem imminent

Another issue is of an economic and not a scientific or technological nature Since 1965

the cost of IC manufacturing plants has sky-rocketed If the cost of semiconductor

production plants continues to rise exponentially in a few years such plants will cost up to

$20 billion This is a sizeable investment even for large companies such as IBM or Intel

For this reason it is likely that IC companies will resist changing to completely new

technologies closing down their existing plants As it is clear that silicon will remain the

VPalermo 11

fundamental raw material of the IC industry for the foreseeable future nanotechnology

developments for microelectronics will need to be silicon-compatible In these early days

of nanotechnology the most valid approach would seem to be the addition of molecular

functions to existing silicon technology ndash using the latter as a foundation on which to build

Fabrication of self-organised structures on silicon

The possibility of using the phenomenon of atomic or molecular self-organization to create

nanostructures on silicon has already been demonstrated The clean silicon surface shows

in some cases a high degree of order and complex surface reconstruction as will be

described later Several different ordered structures form spontaneously on this surface

such as series of monatomic steps or boundaries between reconstructed areas It has been

demonstrated that it is possible to use these structures to fabricate ordered nanodots and

nanolines on the surface [11] More recently well-defined nanometric patterns have been

obtained with selective etching of silicon using nitric oxide [12]

In this study the possibility of creating different types of nanostructures on the silicon

surface is explored Methods had to be developed which were

- Simple They must not need complex masks or lithographic steps to create the

structure but rather exploit self-organisation phenomena

- High resolution the silicon surface modifications should be on a scalelength of below

100 nm

- Fast billions of nanostructures have to form over the whole surface simultaneously to

be compatible with large-scale production requirements

- Cheap they must not require expensive equipment (such as e-beam lithography) but

exploit simple chemical andor physical treatments to produce nanostructures on the

silicon surface

In Chapter 2 the main characteristics of silicon are described Chapter 3 provides a

summary of the techniques used for this research Chapter 4 examines the chemical etching

of silicon in different liquid environments and the effects of this etching on the surface at a

nanoscopic level with the creation of nanoholes


Chapter 5 describes the growth in ultra-high vacuum (UHV) of nanoscopic voids and

islands on the silicon surface and the effect of surface oxide on this growth

Chapter 6 discusses the modification of silicon surfaces in UHV following the adsorption

of molecules and thermal heating to produce nanoislands and nanolines on silicon

The overall conclusions of our work are summarised in Chapter 7

Finally we will give some conclusions based on the results obtained and discuss possible

applications of the methods developed

Bibliography

[1] From wwwintelcom

[2] From wwwsematechorg

[3] Confinement Of Electrons In Quantum Corrals On A Metal Surface Crommie MF

Lutz CP Eigler DM Science 262 (5131) 218-220 Oct 8 1993

[4] Molecules Get Wired Service RF Science 294 (5551) 2442-2443 Dec 21 2001

[5] Coulomb Blockade And The Kondo Effect In Single-Atom Transistors Park J

Pasupathy AN Goldsmith JI Chang C Yaish Y Petta JR Rinkoski M Sethna JP

Abruna HD Mceuen PL Ralph DC Nature 417 (6890) 722-725 Jun 13 2002 Kondo

Resonance In A Single-Molecule Transistor Liang WJ Shores MP Bockrath M Long

JR Park H Nature 417 (6890) 725-729 Jun 13 2002 Nanotechnology - Electronics

And The Single Atom De Franceschi S Kouwenhoven L Nature 417 (6890) 701-702

Jun 13 2002

[6] Smallest Carbon Nanotube Ajayan PM Ijima S Nature 358 (6381) 23-23 Jul 2 1992

[7] Carbon Nanotube Inter- And Intramolecular Logic Gates Derycke V Martel R

Appenzeller J Nano Letters 1 (9) 453-456 Sep 2001

[8] Logic Circuits With Carbon Nanotube Transistors Bachtold A Hadley P Nakanishi T

Dekker C Science 294 (5545) 1317-1320 Nov 2001

VPalermo 13

[9] Logic Gates And Computation From Assembled Nanowire Building Blocks Huang Y

Duan XF Cui Y Lauhon LJ Kim Kh Lieber CM Science 294 (5545) 1313-1317

Nov 9 2001

[10] Formation And Disappearance Of A Nanoscale Silver Cluster Realized By Solid

Electrochemical Reaction Terabe K Nakayama T Hasegawa T Aono M Journal Of

Applied Physics 91 (12) 10110-10114 Jun 15 2002

[11] Fabrication And Integration Of Nanostructures On Si Surfaces Ogino T Hibino H

Homma Y Kobayashi Y Prabhakaran K Sumitomo K Omi H Accounts Of Chemical

Research 32 (5) 447-454 May 1999

[12] Ultrafine And Well-Defined Patterns On Silicon Through Reaction Selectivity

Prabhakaran K Hibino H Ogino T Advanced Materials 14 (19) 1418-1421 Oct 2

2002


VPalermo 15

2 Silicon surfaces

The name silicon (silicio in Italian) comes from the latin word silex Amorphous silicon

was first isolated by Berzelius in 1824 by reaction of potassium with silicon tetrafluoride

Thirty years later the first crystalline silicon was prepared Silicon makes up 25 of

earthrsquos crust and is the second most abundant element after oxygen Elemental silicon is

not found in nature occurring as silicon oxide (sand quartz amethyst flint etc) or

silicates (asbestos clay mica etc) Perhaps no other element and its compounds has such

a wide range of uses Silicon compounds such as sand and clay are used in the building

industry as refractory materials for high-temperature applications and for enamels and

pottery Silica is the main component of glass silicon carbide is an important abrasive and

silicones are commonly used polymers and lubricants

Here the most interesting use of silicon of course is for the production of

microelectronics devices For this application silicon of high purity (999999) and of

high crystallinity is needed Table 21 lists some of the physical characteristics of silicon

High purity polycrystalline silicon is produced by the reaction of gaseous trichlorosilane

with hydrogen in a furnace Then to prepare a single-crystal of silicon the so-called

Czochralski method is commonly used

Polycrystalline silicon is melted in a quartz furnace at 1415degC in an argon atmosphere

Then a seed of single-crystal silicon is lowered into contact with the melt and slowly

pulled out In this way the crystal grows and a crystalline cylindrical ingot several metres

long is created from the initial seed

After cooling down the ingot is sliced into thin silicon wafers The wafer surfaces are

polished using a counter-rotating lapping machine in an Al2O3 slurry until the surface is

very flat and shiny ready for the lithographic processes


Another way to obtain single crystal silicon is the Floating Zone (FZ) method in which a

silicon cylinder is slowly passed through a heating ring The area inside the ring melts and

solidifies smoothly crystallising as it comes out of the ring yielding a single silicon crystal

Microelectronic devices are built on the silicon surface which is the surface of interest

here Unfortunately silicon surfaces are normally quite dirty and uneven at the atomic

scale Atmospheric oxygen and humidity react with silicon surfaces creating a thin layer of

oxide (called ldquonative oxiderdquo) which is usually irregular and full of defects Different kinds

of contaminants also adsorb onto the surface These are usually small organic molecules

and microscopic dust particles A clean surface on exposure to the atmosphere is

completely covered with gas molecules in less than 10-9 seconds If the pressure is

reduced letrsquos say to 10-6 mbar this time increases to 1 second This is the reason why to

study a clean surface we have to work in UHV at pressures below 10-10 mbar

The atoms in the silicon crystal have a diamond-like structure each atom having 4 bonds

in a tetrahedral sp3 arrangement with bond angles of 10947 degrees At the crystal

surface some atoms will have non-bonding orbitals ldquodanglingrdquo in the vacuum ie sp3

orbitals with a lone electron which are highly reactive These orbitals are known as

dangling bonds To minimize surface energy the surface will reorganize by decreasing

the number of dangling bonds

Table 21 Physical data of silicon [1]

Atomic Weight 2809 Lattice constant (A) 543095 Crystal structure Face-centered cubic

(diamond) Melting point 1415 degC

Density (gcm3) 2328 Boiling point 2355degC Atomscm3 50E22 Minority carrier

lifetime (s) 25E-3

Dielectric Constant 119 Specific heat (Jg degC)

07

Breakdown field (Vcm)

~3E5 Thermal conductivity (Wcm degC)

15

Electron affinity x(V)

405 Vapour pressure (Pa) 1 at 1650degC 1E-6 at 900deg C

Energy gap (eV) at 300K

112 Reactivity Inert to acids Attacked by halogens and alkaline

solutions Intrinsic carrier

conc (cm-3) 145E10 Oxidation states +4 -4

Intrinsic Debye Length (microm)

24 Energy of a Si-Si bond (eV)

232

Intrinsic resistivity (Ω-cm)

23E5

VPalermo 17

Dangling bond densities and positions and thus the type of surface reconstruction will

depend upon crystal orientation as well as the temperature and kinetics of the system

Fig 21 shows a drawing of the main faces of a silicon crystal The angle α between any

(11n) face and the (100) face can be calculated from 2cos 2 += nnα The angle

between any (11n) face and the (111) face can be calculated from

)2(3)2(cos 2 ++= nnα

The chemistry and physics of the faces are very different a brief description will be given

for the most important orientations

Si (100)

On the (100) surface each atom has two Si-Si bonds connecting it to the bulk and two

dangling bonds pointing outward Surface energy is reduced by the dimerisation of the

surface atoms through overlap interaction of one dangling bond per atom forming rows of

dimers aligned along the (110) direction This is the well-known ldquo2x1rdquo reconstruction of

this silicon surface The symmetric dimers would make the silicon surface metallic but to

reduce surface stress the dimers tend to buckle and the surface is thus semiconductive It

took several years to understand that the dimers are buckled because at room temperature

Fig 21 Schematic view of the principal orientations of a silicon surface Surface atoms are white bulk atoms are black dangling bonds are gray [2]


they shift easily from one buckling direction to the other thus appearing symmetric under

STM observation Fig 22 shows an STM image of the 2x1 reconstructed surface

Even almost perfect (100) surfaces have a certain number of monoatomic steps and the

dimer rows on atomic layers are aligned at 90deg to those on adjacent layers Dimer rows are

thus perpendicular or parallel to the step When the dimers on the upper side of the step

are parallel to the step the step is called SA if they are perpendicular the step is called SB

Because of this symmetry SA step edges will be smoother compared to the more broken

fragmented edges of the SB steps

A common defect on the Si(100) surface is the presence of nickel contamination which

appear as missing dimers in STM images This type of contamination is so critical that

even if the silicon sample is only briefly brought into contact with stainless steel tools

(tweezers for example) the 2x1 reconstruction of the surface can be blocked

Silicon atoms can diffuse easily over the silicon surface as monomers and dimers

especially at elevated temperatures The anisotropy due to the 2x1 reconstruction causes a

difference in the diffusion energies of adsorbates over the surface Diffusion of these

silicon species along dimer rows for example will be much easier A list of diffusion

energies for monomers and dimers is given below [2]

Diffusion on Si(100) 2x1 Ed (eV) Monomers along dimer rows 06 Monomers across dimer rows 085 Monomer formation energy 18 Dimer along dimer rows 11 Dimer across dimer rows 15 Dimer formation energy 26 Dimer binding energy 076 Vacancies along dimer rows 17 Vacancies across dimer rows 19

SA

SB

Fig 22 STM image of a 2x1reconstructed silicon surfaceshowing the dimer rows and steps40x35 nm Nickel-induced defectsare visible as dark spots SA and SBsteps are indicated

VPalermo 19

So the diffusion energy for both monomers and dimers is nearly 40 greater if they have

to cross a dimer row This difference reduces to sim10 for vacancy diffusion

Si(113)

The (113) surface can be imaged as a sequence of alternating (100) and (111)-like

structures with two and one dangling bonds on alternate atoms respectively Interest in the

(113) surface is scientific as it has been used to study the energetics of the (100) and (111)

surfaces as well as for surface adsorption experiments

Si(100) surfaces can easily develop (113) facets

Si(111)

This surface besides being the first one imaged with STM with atomic resolution is one of

the most studied because it is the best cleavage face of silicon and because it shows one

of the most complex and elegant reconstructions in surface science

All Si-Si bonds in the silicon crystal are perpendicular to a (111) plane so this face will

have the lowest number of dangling bonds created per unit area In fact each Si atom on a

(111) surface shows a single dangling bond oriented perpendicular to the surface and

bonded to three back atoms These three bonds for each surface atom account for the great

chemical and physical stability of the Si(111) surface Surface energy is 009 eV Aring-2

compared to 015 eV Aring-2 for Si(100)

For energy minimization this surface reconstructs forming a huge 7x7 lattice cell

containing 102 atoms described by the Dimer-Adatom-Stacking fault model (DAS) For a

detailed description of cell structure see fig 23

The cell described by this model is very complex being composed of three kinds of atoms

adatoms rest atoms and corner hole atoms Furthermore a subsurface stacking fault is

present in one half of the cell making the two halves of the unit cell look different under

STM (Fig 24) It took 26 years of research to completely understand the exact structure of

the 7x7 reconstruction

Cleaving a silicon crystal along a (111) plane produces a metastable 2x1 reconstruction

the 7x7 reconstruction is easily obtained by flashing at high temperature in UHV At T gt


830degC a disordered 1x1 phase covers the surface Cooling down to 800degC leads to the

formation of the 7x7 phase If the cooling process is too rapid small 7x7 domains nucleate

and a disordered 1x1 phase is preserved between domain boundaries

Si(110)

Even though as mentioned before the (111) plane is the favoured cleaving plane of

silicon thin (100) commercial wafers will not break along this plane because the angle

between (100) and (111) is too far from 90deg (see table 22) Instead they will break along

the (110) plane because it is perpendicular to the (100) surface Each surface atom on

Si(110) has a Si-Si bond pointing downward one dangling bond pointing outward and two

Si-Si bonds parallel to the surface in a zig-zag pattern (see fig 21) Cleaved (110)

surfaces are disordered but upon annealing at high temperatures an ordered complex 16x2

Fig23 Scheme of the 7x7 DAS model [2] In each unit cell there are 9 dimers 12 adatoms and a stacking layer fault The force driving this complex reconstruction is the minimization of dangling bonddensity The DAS model shows the lowest number of dangling bonds (19) of all possiblereconstructions 12 dangling bonds are at the adatoms 6 at the rest atoms and 1 at the corner hole atom This surface is metallic

VPalermo 21

reconstruction takes place The surface appears as a series of long ridges and valleys

parallel to each other Eventually tilted facets of orientation (17 15 1) can form on this

surface The adsorption of Ge atoms on this surface leads to the formation of self-

assembled nanowires [3]

Table 22 Angles in degrees between different silicon faces [2]

Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0

Fig 24 STM image of a Si(111)surface with 7x7 reconstruction A unitcell with its adatoms is highlightedImage size 13x13 nm


Bibliography

[1] Weast RC Handbook Of Chemistry And Physics (Chemical Rubber Co Cleveland

1972)

[2] Dabrowski J Mussig H Silicon Surfaces And Formation Of Interfaces (World

Scientific Publishing Singapore 2000)

[3] The Structure Of Clean And SiGe-Covered Si(110) Surfaces Butz R Luth H Surface

Science 365 (3) 807-816 Oct 1 1996

VPalermo 23

3 STM and other surface analysis

techniques

Scanning Tunneling Microscopy

Since the invention of the optical microscope at the end of the 16th century the possibility

of examining surfaces at higher and higher magnification has fascinated mankind

Development of the technique continued and towards the end of the 19th century optical

microscopes were as good as todays standard instruments The physical limits of the

wavelength of visible light (350-800nm) had been reached

In the 1920s de Broglie showed that electrons can behave like waves and the use of these

particles for imaging with much higher resolution soon followed Atomic resolution using

this technique is only possible in the transmission mode with extremely carefully prepared

samples

In 1982 using the peculiar properties of piezoelectric materials Binnig and Rohrer brought

a metallic tip very very close to a silicon surface and scanned it across an extremly small

area (fig31) The tunneling of electrons from the tip into the sample or vice versa allowed

them to obtain a local density of electronic states (LDOS) map of that surface Although

theory (which treated the extreme point of the tip as a sphere) then excluded the possibility

of atomic resolution following a tip crash into the surface Binnig and Rohrer began to

observe the LDOS with atomic resolution For this discovery and their following work

they were awarded the Nobel Prize in Physics in 1986


The basic principle of STM is very simple A metallic tip is scanned over a surface without

making ohmic contact and a tunneling current passes between the tip and the surface An

electronic circuit keeps this current constant by raising and lowering the tip during the

scan In this way recording the tip height at each point a three-dimensional image of local

density of electronic states (LDOS) of the surface can be obtained To explain the

extremely high resolution attainable by this simple technique quantum theory is needed

According to classical physics the current will flow between sample and tip only if they

are in physical contact If there is a vacuum gap between the two the electrons will simply

remain confined for example within the surface without the possibility of passing into the

tip

In quantum physics however the electrons have a certain probability of passing

(tunneling) across the gap appearing on the other side of the gap in this way reaching the

tip It can be shown that the probability of an electron tunneling through a gap of thickness

z is

kzep 22)0( minusprop ψ h

φmk

2= (1)

where ψ(0) is the electron wavefunction at the surface-gap border m is the electron

mass=91x10-28 g and φ is the work function of the metal (ie the energy required to

remove an electron from that material For silicon it is 48 eV) The tunneling current thus

decays exponentially with z and is extremely sensitive to topographical imperfections

present on the scanned surface A rough formula giving the current as a function of z is [1]

zFS eEVI φρ 0251)( minusprop

Fig 31 Binnig and Rohrer with the first STM Image from IBM [2]

VPalermo 25

where ρs(EF) is the local density of states at the Fermi level on the given surface For

example the formula predicts that for silicon an increase in tip-surface distance of 1 Aring

will give a 95 decrease in tunneling current

This huge dependence of tunneling current upon the distance allows detection even of the

sub-nanometre changes in height given by the single atoms of which the surface is

composed and thus to resolve them in the LDOS images Of course this description of the

tunneling process is oversimplified and for a more accurate one the electronic states of

the tip of the sample and their interaction have to be taken into account Fig 32 shows a

schematic representation of the interaction between tip and sample orbitals

The exponential decay of current with distance also yields high lateral resolution If the tip

is approximated as a sphere of radius R and the current passing at the minimum tip-sample

distance is I0 then the current passing at a lateral distance x from this point will be

Rxk

eII 22

0

2minus

=

Assuming a tip radius of 100 nm the current is concentrated in an area sim15 nm wide at tip

apex

Very sharp tips with even smaller curvature radii can be produced in several ways Simple

Fig32 Schematic view of tip-sampleorbitals interaction a) no interaction b)equilibrium c) sample positive d) tippositive [1]


tungsten wires mechanically cut are capable of obtaining atomic resolution on graphite in

air but for more disordered and rougher samples sharper and more reproducible tips are

needed

STM tips are mostly made by electrochemically etching a W or Pt-Ir wire The tips we

used were prepared using methods based on the work of Fotino [3]

A tungsten wire 038 nm diameter is immersed in a KOH solution(06M) with a thicker

tungsten wire used as a counterelectrode The cathodic and anodic reactions involved in the

etching are

Cathode 6H2O + 6 e- rarr 3H2(g) + 6 OH-

Anode W(s) + 8 OH- rarr WO42- + 4H2O + 6 e-

A potential of 3V ac is applied to the tungsten and the wire is immersed in the solution

until a constant current of sim100 mA is established The etching rapidly removes metal

shaping the wire end as a sharp tip When the potential reaches 11 V the coarse tip etching

is finished The wire is then removed from the solution carefully inserted into an

insulating plastic tube leaving only the tip exposed and re-immersed in the solution with

the tip pointing upwards A more gentle etching is thus made to reduce tip radius Usually

5 minutes etching at 07 V ac is used In this configuration very small hydrogen bubbles

formed on the tip sides sliding upwards with a ldquohoningrdquo effect on the tip

This procedure yields extremely sharp and reproducible tips at the microscopic level

After the etching the tip is thoroughly rinsed in ultrapure water then dipped into

concentrated HF to remove surface oxides and hydroxides [4] The tip is dried with

nitrogen inserted into the UHV system and degassed overnight at sim150degC

The possibility of measuring sub-nanometric distances would be useless without being able

to control tip movement over such a minute scale To scan the tip over the surface a

piezoelectric scanner is used Piezos are usually made of an alloy of PbZrO3 and PbTiO3 a

material which contracts or expands when a voltage is applied to it The Omicron

instrument used in our laboratory has three such piezo scanners for xy and z tip motion

allowing one to scan the tip over the surface with sub-Aringngstrom precision (fig 33)

To isolate the instrument from ambient vibrations the whole STM stage is suspended upon

four springs which eliminate all frequencies above 1 Hz and surrounded by a crown of

VPalermo 27

copper wings and fixed magnets Parasitic currents generated by the magnets into the

copper wings contrast every movement of the stage and efficiently block stage vibrations

The STM can be used not only to explore surface topography but to measure the IV

characteristics of single atoms or molecules on the surface (Scanning Tunneling

Spectroscopy or STS) Furthermore it can be used to modify the surface with voltage

bursts digging into it or delicately moving single molecules or atoms over a surface [5] It

can be used in vacuum in air and with proper lateral isolation of the tip even in liquids

A major drawback of STM is that it works only on conducting and semiconducting

surfaces and thus cannot be used on many surfaces of biological and chemical interest

Another instrument more suitable for these and other applications is the Atomic Force

Microscope (AFM)

Atomic Force Microscopy

AFM was invented in 1986 by Binnig Quate and Gerber after calculating the possibility of

building a cantilever with a force constant of the same order of magnitude as that of a

chemical bond

In AFM a tip mounted on a microscopic cantilever (usually made of Si3N4 fabricated with

optical lithography) is brought close to a surface When the tip touches the surface the

cantilever is very slightly deflected upwards The movement is measured by observing the

Y-PIEZO

SILICON SAMPLE

Fig 33 A picture of theSTM used for theexperiments The tripodpiezo scanner is shown

Z-PIEZO

X-PIEZO

TIP


deviation of a laser beam hitting the upper face of the cantilever Fig 34 provides a

schematic view of the principle of AFM

The typical force constant of the cantilever varies from 00006 to 2 Nm the typical

resonance frequency is 3 to 120 kHz The AFM tip can apply a force on the sample of up

to 10-9 N The AFM can be used on conductive or insulating surfaces in vacuum air or

liquids Furthermore the tip can be modified to sense electrostatic potentials (electric force

microscopy) or magnetic fields (magnetic force microscopy) it can even be functionalized

with complex molecules such as proteins to interact with biological surfaces

A drawback of the AFM is that the force it exerts can damage the surface under

observation especially if the sample is soft (as in the case of cell membranes for

example) This problem can be overcome using the instrument in tapping mode (where the

tip does not move laterally during its brief contact with the surface) or in non-contact mode

in which the tip oscillates above the surface during the scan and the changes in its

frequency due to interaction with surface are monitored The shifts in the oscillating

frequency of the cantilever due to tip-sample interaction are then used for imaging the

surface In this mode interaction of the tip with the surface is minimal and soft samples

can be imaged

STM and AFM are the main techniques used for this work A brief description of other

techniques used occasionally is given below

Fig 34 Scheme of an atomic force microscope

VPalermo 29

Low Energy Electron Diffraction (LEED)

Electrons with energies in the 20-500 eV range are diffracted by a crystalline surface the

diffraction peaks are visualized on a fluorescent screen This technique probes the long

range order of the surface up to a depth of several nanometres

X-Ray Photoelectron Spectroscopy (XPS)

XPS allows both qualitative and quantitative chemical analysis of the elements present on

or near the sample surface

An X-ray source is used to photoionize the atoms on a surface and produce photoelectrons

By measuring the kinetic energy of the photoelectrons the binding energy of the electronic

levels can be calculated This energy will depend on the chemical environment of the

surface atoms

Although the soft X-rays used penetrate to a depth of ~2000Aring the sampling depth of the

technique is determined by the mean free path of the photoelectrons which allows their

escape from only the first 10-100Aring

Secondary Ion Mass Spectroscopy (SIMS)

High and low energy ions (primary ions) are used to bombard a sample and remove surface

atoms and ions The ionic fragments removed (secondary ions) are then analysed by a mass

spectrometer The surface can be consumed during the measurement and profiles obtained

giving concentrations of the materials composing the sample at different depths (depth

profiles)

A popular variant of SIMS is TOF-SIMS In this technique the secondary ion masses are

measured by a time-of-flight (TOF) measurement The secondary ions generated by the

bombarding primary ions are accelerated to a constant kinetic energy and then move

through a field-free space before they reach the detector where their intensity is measured

as a function of flight time Since ions with different masses have different velocities at a


given kinetic energy the measured flight times of the ions can easily be converted to their

masses The static nature of this latter technique allows mass spectroscopy surface analysis

with minimal damage to the surface

Bibliography

[1] Chen CJ Introduction To Scanning Tunneling Microscopy (Oxford University Press

Oxford 1993)

[2] From wwwibmcom

[3] Tip Sharpening By Normal And Reverse Electrochemical Etching Fotino M Review

Of Scientific Instruments 64 (1) 159-167 Jan 1993

[4] A Convenient Method For Removing Surface Oxides From Tungsten STM Tips

Hockett LA Creager SE Review Of Scientific Instruments 64 (1) 263-264 Jan 1993



VPalermo 31

4 Surface modification of silicon in liquid

Nano-hole creation

Liquid treatments of silicon wafers are very common in the integrated circuit (IC)

manufacturing industry They are used to clean and improve surface uniformity to create

and etch protective oxide layers and to remove photo-resist layers

Crystalline silicon with its native oxide layer is very stable and is resistant to many acids

It is easily attacked by hydrofluoric acid (HF) and alkaline solutions

The thin (~2 nm) passivating layer of native oxide (SiO2) is formed on exposure to the

atmosphere This surface layer contains many defects and contaminants so it is usually

chemically stripped and substituted with a better chemically-formed protective oxide

The most common silicon cleaning procedure is the RCA method named after the Radio

Corporation of America [1] It consists of two steps in the first one the surface is treated

with a hot alkaline solution (H2OH2O2NH4OH 411) to remove particles from the

surface following this a hot acidic solution (H2OH2O2HCl 411) is used to remove

metal contamination Other well-known cleaning methods are IMEC (a sequence of

cleaning steps in H2OO3 and dilute HF) or the Pirana etch (a hot 41 mixture of H2SO4

H2O2)


The standard RCA clean removes surface contaminants etches the native oxide and

oxidizes the silicon surface leaving a uniform layer of silicon oxide which better protects

the surface from further contamination

Etching with fluorine-based solutions

Hydrofluoric acid is one of the most common reagents used in the treatment of silicon

wafers both in the research field and in industrial processes A rapid dip in dilute HF is the

simplest way to remove the native oxide from Si(100) and leaves the surface passivated by

a layer of Si-H bonds Because of the low polarization of Si-H bonds the Si-H layer is

stable even for several days protecting the surface from contamination It has often been

assumed that this short etch does not significantly change the surface morphology of the

silicon substrate[2] even though a prolonged dip in dilute HF leads to surface roughening

[3]

Although dilute HF roughens the Si(100) surface at the atomic scale [45] immersion in

concentrated HF (49) etches the surface oxide without attacking the Si surface

uncovering in this way the buried SiSiO2 interface The final counter-intuitive result is

that dilute HF etches the silicon while concentrated HF leaves the crystalline silicon

untouched [3]

Etching Si with fluorine-containing solutions at different concentrations and pH can

produce different morphologies from rough surfaces to flat nearly ideal Si-H terminated

surfaces

Hessel et al and Higashi et al demonstrated in 1991 that very flat Si(111) surfaces can be

obtained using 40 NH4F while etching with HF always yields rough surfaces The

surface becomes smoother because the etchant rapidly attacks Si atoms at step borders

thus removing surface kinks and irregularities in a step-flow mechanism [6 7] Later on

even smoother and more perfect surfaces were obtained by removing oxygen from the

solution after it was discovered that oxygen dissolved in 40 NH4F initiates the formation

of triangular etch pits It was not possible to obtain flat surfaces by etching Si(100) with

ammonium fluoride solutions which leads to the formation of small 2x1 dimer-row

reconstructed (100) terraces together with (111) facets [8]

This difference is caused by the different hydride terminations prevailing on the (100) and

(111) faces While the ideal Si(111)-H surface is monohydride terminated the more

VPalermo 33

reactive dihydrides predominate on the Si(100)-H surface making it more vulnerable to

etching The etching reaction is thus strongly anisotropic etching (100) facets faster than

(111) thus producing (111) microfaceting on Si(100) crystals

A more uniform Si(100) surface can however be prepared by etching at low pH with an

HFHCl mixture [9] or by using very dilute HF solutions and ultrapure water with low

dissolved oxygen and carbon contents [10]

Electrochemical etching can also be used applying anodic or cathodic bias to the silicon

to obtain different morphologies [11] by varying the potential isotropic or anisotropic

etching is observed The aforementioned results show that despite the simplicity of the

reactants fluoride etching of silicon is quite a complex reaction

Fig 41 Chemical etching of silicon

HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si

Etching mechanism of silicon

HF rapidly dissolves the SiO2 passivating layer on silicon leaving the surface almost

completely hydrogenated [12] After this two different types of reactions etch the silicon

simultaneously one chemical and the other electrochemical [13] The overall etching

mechanism can be schematized in two stages (see Figure 41)

i) Si-H bonds are substituted by Si-F or Si-OH bonds creating a partial charge on the

surface silicon atom and polarizing its Si-Si backbonds

ii) these polarised backbonds are then more easily attacked by HF or H2O After

rupture of the Si-Si bond the atom is removed leaving behind new Si-H

terminations and the reaction can start again


These reactions take place although at different rates on both Si(100) and Si(111)

Stage i) is usually the rate-determining step of the reaction and the stability of the Si-H

bonds depends upon the pH the concentration of nucleophilic species in solution and an

eventual potential applied to the crystal

For pH gt5 as in the case of concentrated NH4F solutions reaction begins with attack by

water to give Si-OH (step ArarrC) The -OH group is rapidly substituted by fluorine with

polarization of the underlying Si-Si bonds These bonds are then easily attacked by water

the silicon atom being released into solution as HSiF(OH)2 The Si-OH rarr Si-F substitution

is not fundamental for the reaction and etching can proceed even for Si-OH terminated

atoms but XPS measurements showed the presence of a certain number of Si-F bonds

remaining Furthermore fluorine seems to have a catalytic effect on Si-H substitution as

indicated by the dependence of the etch rate upon the F- concentration at least for pH

values between 4 and 8

Si-F bonds can be easily removed by a water rinse In the case of strongly alkaline

solutions (pH=14) OH- groups act directly as nucleophiles and no fluorine is needed to

catalyze Si-H bond rupture

At pH lt4 almost no free F- ions are present in solution and the etch rate is very small at

pH lt2 all etching reactions are very slow and this explains the stability of Si crystals in

concentrated (50 ww) HF solutions

Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e

+H2O hellipas in fig1

Fig 42 Electrochemical etching of silicon by H2O

The electrochemical etching of silicon involves electron transfer from the surface atoms to

the valence or conduction band of the crystal (according to the type of doping of the

silicon) Several studies have been carried out at different electric potentials In the case of

p-silicon or n-silicon at anodic potentials where positive charges (h+) are available in the

crystal a silicon-centred radical can be created by capture of a hole from the bulk (fig

42) The F- ion does not participate in the reaction but is thought to contribute through

VPalermo 35

electrostatic interactions by lowering the energy of the interaction step After the

formation of the Si-OH group the reaction proceeds as shown in the scheme of fig 41

Matsumura et al [4] proposed that not only water but HF2- molecules also play a major role

in electrochemical etching of silicon leaving on the surface Si-F terminated bonds which

can be immediately attacked in an autocatalytic process (fig 43)

In the electrochemical reactions described above an external potential is applied to the

silicon crystal The chemical and electrochemical reactions in any case take place

simultaneously most of the time with the chemical path predominating at high pH Even

when no external potential is applied to the silicon partial electrochemical reactions can

take place at different ldquocathodicrdquo and ldquoanodicrdquo sites on the surface with an internal charge

exchange which ensures neutrality [11] This macroscopic silicon etching and hydrogen

bubble formation can lead to visually observable patterns on the surface when Si(100) is

immersed in ammonium fluoride even without applying a potential

Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-

Fig 43 Autocatalytic electrochemical etching of silicon by HF2-

Inhomogeneities on silicon surfaces caused by electrochemical reactions and charge

transfer have been studied extensively because they are of fundamental importance in the

formation of porous silicon

Pore formation on silicon

When Si(100) or Si(111) are etched under anodic bias in fluorine-based solutions

microscopic pores form on their surface Several different morphologies of pores have

been observed with pore diameters ranging from 10 nm to several microns with depths of

several microns [14] Pore shape is very variable too ranging from ordered straight pores

to chaotic networks of branched pores (fig 44) Porous silicon has been known since the

fifties but it was only in 1990 that interest in this material increased following the

discovery that porous silicon layers were able to emit bright red light This led to a large

amount of research and now different classes of micropores can be reproducibly created


Fig 44 Different types of Silicon micropores From ref [14]

mostly for optical and micromachining applications However there is still no unified

theory able to explain the nucleation and growth mechanism of all the different kinds of

pores

We will give a short description of some of these theories for more detail see Parkhutik et

al [15]

One model explains pore nucleation on the basis of physical processes such as hole

positive charge migration ion transport to the surface and small perturbations on the

silicon surface modelled as Fourier components The system is shown to be unstable and

some spatial frequencies that lead to pore nucleation evolve from the etching process

A second model focusses on stationary pore growth without explaining the nucleation

stage According to this model silicon dissolves preferentially at pore edges because h+

charges are attracted by the stronger electric field present at these edges

A third class of models explains pore growth as a Diffusion Limited Aggregation (DLA)

process where the random walk of h+ charge carriers through the depleted layer present at

the silicon-liquid interface controls pore shape

Finally the model by Carstensen Cristophersen and Foll [16] proposes that areas of the

surface of some characteristic size LCO are etched by synchronized ldquocurrent burstsrdquo in the

flow of h+ charges These bursts dissolve silicon through cyclic stages of surface oxidation

oxide removal and hydrogen passivation Areas where a burst has recently taken place are

less passivated and thus more likely to be etched again in this way the pore bottom

continues to dissolve while the pore walls are passivated and are thus less favourable areas

towards current bursts

VPalermo 37

EXPERIMENTAL RESULTS

In the following sections we will show some experimental results obtained from STM and

AFM measurements of fluorine-treated Si(100) surfaces In the first part the results of

mild etching using concentrated and dilute HF solutions at low pH are presented In the

second part the results of etching at high pH using ammonium fluoride are presented and

the mechanism of pore formation discussed

Etching of Si(100) in dilute and concentrated HF

Samples were cut from different areas of an 8-inch diameter p-doped silicon(100) wafer

(10 Ω-cm) supplied by MEMC Electronic Materials Each series of STM measurements

was carried out over at least six different areas on at least two identical samples Low

Electron Energy Diffraction (LEED) was used to check the surface cleanliness of the

samples before STM measurements

Table 41 summarizes the different treatments of each sample After etching with

electronic grade HF each sample underwent a final rinse in Ultra-Pure Water (UPW

resistivity gt18 MΩ-cm) Both the HF and the UPW were allowed to flow continuously

over the sample surface Some samples were not etched with HF at all but just washed with

UPW to observe the morphology of the native oxide layer (~2 nm thick) covering the

surface All of these processes were carried out under nitrogen to limit reoxidation and the

samples were then introduced from the nitrogen atmosphere directly into the vacuum

chamber and degassed overnight at sim150degC before LEED and STM measurements

STM images were obtained from each sample using the same measurement parameters

(sample bias 4 V feedback current 1 nA scan speed 800 nm s-1) These parameters


provided a satisfactory level of reproducibility for all the samples Measurements were

made over an area of 500x500 nm (image size 500x500 pixel) Slope correction was

carried out by subtracting row-wise and column-wise fitted slopes from the entire image

which gave better results than the simple subtraction of a fitted plane especially for the

rougher samples Following slope correction the rms roughness

sum minus=xy

hyxhN

22 ))((1σ

and the 2-D Fourier transform

)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π

were calculated for each image where N2 is the number of pixels composing the image

h(xy) is the surface height at each point ∆ is the distance between points h is the mean

height and u v are the spatial frequencies The radial power spectrum PS(f) of the STM image data is obtained from the angular

average of the squared Fourier transform with f 2 = u2 + v2

Fig 45 shows the STM images obtained from the various samples Sample A still covered

with its native oxide layer shows an irregular surface with RMS roughness of ~05 nm

(see Table 41) Observing the sample with LEED gave no diffraction pattern even at

relatively high incident electron energies because of the surface oxide coverage After 1

min etching in dilute HF (sample B) the morphology is similar to the original one though

Table 41 Sample treatments average RMS roughness and integrated area of power spectra

Sample Treatment RMS roughness (nm)

PS area (f lt 01 nm-1)

PS area (f gt 01 nm-1)

A Rapid dip in water 053 plusmn 013 366 032

B 1 min in HF 5 + 10 min in water 051 plusmn 008 272 034

C 30 min in HF 5 + 10 min in water 062 plusmn 008 1064 038

D 5 sec in HF 49 + 10 min in water 042 plusmn 004 183 022

VPalermo 39

Fig 45 STM images of each group of samples showing the topography of the silicon surface A) noetching original oxide surface B) after 1 min etching in dilute HF C) after 30 min etching in diluteHF D) after dipping in concentrated HF Grey scale indicates height of the surface from lower (black) to higher (white) The images are 250x250 nm ie representative portions of the images usedfor the roughness measurement and PSD analysis

some of the larger features have disappeared and the image quality is better maybe due to

improved tunnelling due to the cleaner surface The RMS roughness is comparable to that

of the original surface Clear diffraction patterns are visible using LEED though at quite

high energies (200 eV) After prolonged etching (sample C) the RMS roughness increases

to 062 nm and a long-range corrugation is visible on the surface even if the LEED pattern

is good

The samples dipped in concentrated HF (D) reveal the bare SiSiO2 interface which has a

disordered aspect and protrusions over a wide range of dimensions The quality of the

STM images of sample D is very good probably due to the cleanliness of the surface


1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min

C e tc h e d 3 0 min

D e tc h e d HF 4 9

Fig 46 Log-Log plot of the averaged power spectra of the STM images for all the samples

which gives a more stable tunnelling junction The LEED pattern is excellent showing

clear diffraction peaks at energies as low as 37 eV comparable to that obtained after high

temperature cleaning in UHV

Fig 46 shows the power spectra of the samples The high frequency and low frequency

areas of the power specturm are considered separately Table 41 shows for each sample

together with the roughness the integrated area of the power spectrum for the high and low

frequency part

We first examine the differences between the samples in the low frequency part of the

spectrum (lt01 nm-1) In this spatial range the short dip in concentrated or dilute HF

(samples BD) seems to lower the roughness of the sample removing some of the larger

features of the native oxide surface visible in Fig 45a and thus decreasing the

corresponding part of the power spectrum Sample C however etched with HF for 30 min

shows an increase in roughness peaking at 0012 nm-1 (~85 nm corresponding to the

typical dimensions of the corrugations visible in Fig 45c) On this scale sample C has a

PS density of 435 nm4 compared to 101 nm4 for sample A

Samples A B and C have the same PS in the high frequency range showing that the fine

structure of the surface is not changed by the HF etching Sample D on the other hand

shows a decrease of roughness for frequencies gt 015 nm-1 This effect could be due to the

VPalermo 41

improved surface cleanliness after etching with concentrated HF which would give a more

stable STM junction thus reducing the high frequency noise in the image

Fig 45 and the analysis of the power spectrum of each sample shows that a rapid dip in

HF removes the native oxide but does not lead to major changes in the morphology of the

Si surface its only effect being the removal of some of the larger features present on the

original surface Prolonged etching on the other hand increases the RMS roughness of the

surface

Etching of Si(100) in concentrated ammonium fluoride and nano-hole creation

Two different types of commercial p-doped Si(100) wafers (2Ω-cm and 10Ω-cm) from

MEMC were used Several different samples of 10x5 mm were immersed for 10 minutes

in 40 electronic grade NH4F solution under agitation Previous works used low

temperatures or anodic potentials applied to the silicon to avoid gaseous hydrogen

production and to obtain a uniform surface but in our experiment we worked at room

temperature to check the influence of hydrogen bubbles on surface morphology During

the etching the stirring was sufficient to provide a uniform concentration of reagents over

the whole sample surface but not to mechanically remove the hydrogen bubbles from the

silicon surface

After the etching each sample was rinsed in ultra-pure water to remove any etching

residues and observed with STM AFM and optical microscopy The AFM measurements

were made in air while for STM measurements the samples were rapidly dried with

nitrogen and inserted into the vacuum system to avoid surface reoxidation After insertion

into the vacuum surface cleanliness was checked with LEED and the surface morphology

observed by STM Parameters for STM measurements were sample bias +4 V 1 nA

current 16 Hz scan rate The images obtained were stable and reproducible over several

days Scan parameters for AFM were 20 nN force and 1 Hz scan rate

Some of the samples were cleaned with an RCA standard clean [1] before NH4F etching to

check the influence of possible surface contaminants on the final results Eight different

samples were prepared and more than sixty STM images of the samples were taken at

different points of the various samples


(110)

(110)

Fig47 abc) STMtopographic images of differentetching morphologies Eachimage is 500x500 nm Z-ranges are 10 10 and 18 nmrespectively d) STM image of a bridgecreated by etching of the lowerlayers of silicon (black arrow)Image is 250x250x6 nm

After ~2 min of immersion in the solution hydrogen bubbles become visible on the sample

surface The production is slow and the bubbles are quite stable on the sample without

detaching Thus some areas of the surface are masked from the liquid etching action

STM observations (fig 47) show that at the nanometer scale the surface is unevenly

covered with holes of radii ranging from 10 to 200 nm with depths of 2-4 nm These holes

have a wide range of different shapes and distributions In most cases the surface was

covered with a uniform distribution of round-shaped holes (fig47a) indicating isotropic

etching The dimension and the density of the holes changed greatly from sample to

sample and even over the surface of a single sample In some cases the etching was

anisotropic yielding nearly square holes and layered structures as shown in fig 47b

Square holes have been previously observed in cases where the etching speed in the (110)

direction is significantly smaller than in the (100) direction [17]

Over these areas (fig 47b and especially 47c) it is clearly visible how once the surface

had been attacked the reaction continued to preferentially remove atoms at step

irregularities (kink atoms) straightening step edges Eventually the exposed underlying

silicon was also attacked and further holes created inside the previously etched larger

ones It was not possible to detect monatomic steps on this kind of surface The smallest

step height observed was ~15 nm corresponding to several atomic layers In the image

shown in Figure 47a the etching was not strong and created only anisotropic holes on the

surface In fig 47b and c the stronger etch proceeded laterally for several tens of

VPalermo 43

nanometres leaving straight steps several tens of nanometers long and roughly rectangular

holes as expected given the structure of the (100) crystal face In some cases a

significative underetch is observed and the formation of suspended bridges and tunnels

can be deduced from the STM images (fig 47d)

The formation of branched pores and suspended structures has been attributed during pore

formation to diffusion limited aggregation effects where the h+ charge carriers necessary

for silicon etching have a higher probability of reacting at pore bottoms than reaching the

upper part of the silicon surface In the case of very deep pores quantum wire effects have

been invoked to explain the pore growth mechanism [15] In our case though the pores

formed were very shallow the underetch depth being only a few nanometres on pores of

sim100 nm width Thus more than diffusion effects the main contribution to the

underetching process must come from anisotropic etching and some kind of autocatalytic

reaction path analogous to the one described by Matsumura et al [4] with some areas of

the silicon surface hydrogenated and thus less vulnerable to etching

Pre-treatment with RCA cleaning has no effect on the final morphology and this seems to

exclude pore nucleation being caused by presence of metallic or organic surface

contaminants

The morphology and the distribution density of the pits was quite uniform over

microscopic areas of the sample but changes were observed over the millimetre scale This

suggests that etching intensity is influenced by some large-scale parameter

Large-area measurements made with AFM or with an optical microscope (fig 48)

showed that the inhomogeneity of surface etching can be correlated with the masking

action of the bubbles While the fluoride dissolved the silicon hydrogen bubbles formed

by the reaction covered some areas of the surface thus blocking the etching over that area

generating macroscopic steps at the bubble-liquid border As the reaction proceeded more

hydrogen accumulated and the bubble diameter increased producing in this way a circular

pattern of steps The increase in bubble diameter was not continuous with time otherwise a

uniform surface slope gradient would have been obtained The formation of this circular

ldquoetching staircaserdquo indicates that the bubble growth was stepwise the bubble accumulating

more and more hydrogen without enlarging across the surface until it relaxed increasing

its diameter stepwise and covering more silicon The circular structures in fig 48a are not

co-axial and their asymmetry could derive from physical processes due to stirring or

irregularities on the surface


a b Fig 48 a) optical micrograph of etching patterns on Si(100) created by NH4F 12x09 mm b) AFM image of the circles border xy range is 40x24 microm z-range is 30 nm

The step structure was not destroyed by the etching even after the bubble detached from

the surface but on the contrary the etching process seemed to be influenced by the

presence of the step

Observing in detail a series of steps (fig 48b) a quite deep trench is visible at the base of

each step A close-up image of a step and the corresponding line profile of the trench is

shown in fig 49 The trench is sim5 nm deep with respect to the lower surface compared to

a step height of 22 nm

A similar structure has been recently obtained with electrochemical etching of p-type

Si(100) in 4 HF [18] in which a ldquocurrent burstrdquo etching model previously described

was assumed for silicon dissolution In that experiment the trench was created at the

border of silicon nitride masks and began to grow after a nucleation stage Preferential

trench etching was along the (110) direction and the trench growth was explained as an

effect of mechanical stress induced by the nitride mask and of electric field enhanced

dissolution which depended upon an external applied potential

While it is clear that in our system the gas bubbles have a masking effect similar to a

classical solid nitride mask it is unlikely that hydrogen present on the surface can induce a

significant stress in the silicon lattice as in the case of a nitride mask Furthermore no

external field was applied to drive preferential etching at the trench site

It has been proposed [19] that the cathodic and anodic part of the etching reaction

(hydrogen production and silicon oxidative etching respectively) take place at different

points on the surface with a net charge transfer between the different areas In this case

the highest reaction rates will correspond to the silicon area surrounding the bubble border

where a high number of positive charges will be available for the reaction Furthermore a

sharp trench extending into the silicon crystal will be a preferential electrostatic attractor

VPalermo 45

Fig 49 AFM image of the etched surface showing a step created on the surface by bubblemasking A stronger etching action is visible on the right side of the step as well as theprotected area on the upper side of the step (indicated by the arrows) Image is 10x10 micromz-range is 30 nm The profile on the right is taken from the central area of the image

for the h+ charge carriers coming from other ldquocathodic areasrdquo of the sample either from

other regions on the surface or from the back of the silicon chip [18]

In the areas where the hydrogen bubble had detached and the surface was exposed to the

etching the reaction was not uniform in the neighbourhood of the steps It is possible to

observe (fig 48b and 49) an area on the upper side of the step where less or even no

etching at all seems to have taken place as if the step was able to protect the surface from

etching While etching on the lower side with trench formation can be attributed to the

presence of the bubble the surface on the upper step side can be etched only after bubble

detachment so no masking effect can account for this result However a further

preferential attraction of h+ charge carriers from the already formed trench can be

hypothised electrochemically shielding the surrounding area from further etching If this is

true the shielding effect would be very strong with a relatively shallow 5 nm-deep trench

protecting an area of sim1 microm parallel to the step

To summarise the etching of Si(100) in NH4F is a complex process in which different

reaction paths both chemical and electrochemical co-exist Hydrogen bubbles formed by

the reaction act as a mask on the surface and create etching paths and inhomogeneous

etching of the surface Different kinds of pores are observed on the surface and in some

cases the anisotropy of the process is so strong as to give square-shaped holes and

underetching


The diffusion of h+ charge carriers in the crystal is one of the main rate-determining steps

of the reaction and leads to the formation of a deep trench immediately outside the bubble

perimeter These trenches act as charge collectors and reduce the etching of the upper step

surface in the proximity of the steps

Bibliography

[1] The Evolution Of Silicon-Wafer Cleaning Technology Kern W Journal Of The

Electrochemical Society 137 (6) 1887-1892 Jun 1990

[2] Spectroscopic Ellipsometry Studies Of HF Treated Si (100) Surfaces Yao H Woollam

Ja Alterovitz SA Applied Physics Letters 62 (25) 3324-3326 Jun 21 1993 Influence

Of HF-H2O2 Treatment On Si(100) And Si(111) Surfaces Graf D Bauermayer S

Schnegg A Journal Of Applied Physics 74 (3) 1679-1683 Aug 1 1993 Kinetics Of

Oxidation On Hydrogen-Terminated Si(100) And (111) Surfaces Stored In Air Miura

T Niwano M Shoji D Miyamoto N Journal Of Applied Physics 79 (8) 4373-4380

Part 1 Apr 15 1996

[3] Hydrogen On Si - Ubiquitous Surface Termination After Wet-Chemical Processing

Pietsch GJ Applied Physics A-Materials Science amp Processing 60 (4) 347-363 Apr

1995 Structure Of The Stepped SiSiO2 Interface After Thermal-Oxidation -

Investigations With Scanning Tunneling Microscopy And Spot-Profile Analysis Of

Low-Energy Electron-Diffraction Pietsch GJ Kohler U Jusko O Henzler M Hahn

PO Applied Physics Letters 60 (11) 1321-1323 Mar 16 1992

[4] Enhanced Etching Rate Of Silicon In Fluoride Containing Solutions At pH 64

Matsumura M Fukidome H Journal Of The Electrochemical Society 143 (8) 2683-

2686 Aug 1996

[5] A Study Comparing Measurements Of Roughness Of Silicon And SiO2 Surfaces And

Interfaces Using Scanning Probe Microscopy And Neutron Reflectivity Crossley A

Sofield CJ Goff JP Lake ACI Hutchings MT Menelle A Journal Of Non-Crystalline

Solids 187 221-226 Jul 1995

VPalermo 47

[6] Step-Flow Mechanism Versus Pit Corrosion - Scanning-Tunneling Microscopy

Observations On Wet Etching Of Si(111) By Hf Solutions Hessel HE Feltz A Reiter

M Memmert U Behm RJ Chemical Physics Letters 186 (2-3) 275-280 Nov 8 1991

[7] Comparison Of Si(111) Surfaces Prepared Using Aqueous-Solutions Of NH4F Versus

HF Higashi GS Becker RS Chabal YJ Becker AJ Applied Physics Letters 58 (15)

1656-1658 Apr 15 1991

[8] Wet Chemical Etching Of Si(100) Surfaces In Concentrated NH4F Solution -

Formation Of (2x1)H Reconstructed Si(100) Terraces Versus (111) Faceting Neuwald

U Hessel HE Feltz A Memmert U Behm RJ Surface Science 296 (1) L8-L14 Oct

10 1993

[9] Ideal Hydrogen Termination Of Si(001) Surface By Wet-Chemical Preparation Morita

Y Tokumoto H Applied Physics Letters 67 (18) 2654-2656 Oct 30 1995

[10] Atomic Structures Of Hydrogen-Terminated Si(001) Surfaces After Wet Cleaning

By Scanning Tunneling Microscopy Endo K Arima K Kataoka T Oshikane Y Inoue

H Mori Y Applied Physics Letters 73 (13) 1853-1855 Sep 28 1998

[11] On The Potential-Dependent Etching Of Si(111) In Aqueous NH4F Solution

Houbertz R Memmert U Behm RJ Surface Science 396 (1-3) 198-211 Jan 20 1998

[12] Etching Process Of SiO2 By HF Molecules Hoshino T Nishioka Y Journal Of

Chemical Physics 111 (5) 2109-2114 Aug 1 1999

[13] Etching Mechanism And Atomic-Structure Of H-Si(111) Surfaces Prepared In

NH4F Allongue P Kieling V Gerischer H Electrochimica Acta 40 (10) 1353-1360

Jul 1995

[14] Pore Formation Mechanisms For The Si-HF System Carstensen J Christophersen

M Foll H Materials Science And Engineering B-Solid State Materials For Advanced

Technology 69 23-28 Sp Iss Si Jan 19 2000

[15] Porous Silicon - Mechanisms Of Growth And Applications Parkhutik V Solid-

State Electronics 43 (6) 1121-1141 Jun 1999

[16] Formation And Application Of Porous Silicon Foll H Christophersen M

Carstensen J Hasse G Materials Science amp Engineering R-Reports 39 (4) 93-141 Nov

1 2002

[17] Pore Morphology And The Mechanism Of Pore Formation In N-Type Silicon

Searson PC Macaulay JM Ross FM Journal Of Applied Physics 72 (1) 253-258 Jul 1

1992


[18] Deep Electrochemical Trench Etching With Organic Hydrofluoric Electrolytes

Christophersen M Merz P Quenzer J Carstensen J Foll H Sensors And Actuators A-

Physical 88 (3) 241-246 Mar 5 2001

[19] Morphology Of Anodically Etched Si(111) Surfaces - A Structural Comparison Of

NH4F Versus HF Etching Houbertz R Memmert U Behm RJ Journal Of Vacuum

Science amp Technology B 12 (6) 3145-3148 Nov-Dec 1994

VPalermo 49

5 Surface modification of silicon in vacuum void creation and oxide desorption

The main reason for the huge success of silicon in the microelectronics industry is not due

to its superior properties as a semiconductor Other materials for example germanium

have better qualities such as higher mobility of charge carriers and lower noise levels

which would allow the construction of faster and higher performance devices

The widespread use of silicon however is mainly due to the outstanding characteristics of

its oxide Silicon dioxide (SiO2) is a very good electrical insulator easy to form

chemically and thermally stable and is compatible with lithographic and metal deposition

processes Germanium oxide on the contrary is too reactive to be used

Even the use of Si(100) substrates for nearly all microelectronic devices is dictated by

oxide quality The (111) face of silicon crystal can be easily cleaved and flattened and

almost atomically perfect surfaces can be obtained with simple chemical procedures (as

described above) But the density of interfacial defects is highest for Si(111)-SiO2

interfaces and lowest for Si(100)-SiO2 ones so microchips will continue to be fabricated

on Si(100) wafers

SiO2 (silica) is present in 95 of the earthrsquos minerals in different allotropic forms such as

quartz tridymite and cristobalite In the bulk each silicon atom is bonded to four oxygens

in a Si-O-Si tri-dimensional network Si-O bonds are 016 nm long and form an angle

ranging from 120deg to 150deg

Three typical intrinsic defects are present in SiO2 The so-called Ersquo centres are oxygen

vacancies with a hole localised on a silicon atom with only three Si-O bonds


O3Si +SiO3 Whereas the PR (peroxy radical) defects are holes trapped by a charged

peroxy moiety with a O3Si-O-O+ SiO3 structure The NBOHC (non-bridging oxygen hole

centres) derive from water or hydrogen contamination and are schematized as O3Si- O- H-

O-SiO3

The atomic structure of the Si-SiO2 interface varies enormously Local domains resembling

the tridymite and the cristobalite structure of silica are present but it seems that only 10

of the interface is ordered [1] Far from the interface the SiO2 bulk is completely

disordered The passage from bulk Si to stoichiometric SiO2 passes through a non-

stoichiometric SiOx layer 07 nm thick

When a clean silicon surface is exposed to atmospheric oxygen a thin sim2 nm thick layer

of native oxide forms spontaneously which is usually removed and substituted with

thicker better quality oxide layers before further processing

Silicon is usually oxidised by thermal annealing at temperatures between 800deg and 1100deg

in an atmosphere of pure O2 with some water eventually added to increase oxidation speed

Thermal oxides made in pure oxygen (dry oxides) grow more slowly than oxides produced

in an oxygen-water atmosphere (wet oxides) but are usually of better quality

According to the Deal-Groove formula the time t needed to grow an oxide of thickness X

is given by [1]

1

212minus

minusminus

+=

ABXBXt α

where the constant B and BA decrease exponentially with temperature as

minus

kTEexp

with activation energies for dry oxidation of EB =123 and EBA =20 eV respectively

EB is related to the diffusion of oxygen in silicon while the value of EBA is interpreted as

the energy required to break a Si-Si bond The exponent α is 1 for wet oxidation and 0 for

oxidation at high temperatures and low oxygen pressures It has intermediate values for dry

oxidation This formula does not work well for low values of X at the initial stages of

oxidation and usually empirical corrections are used

An interesting characteristic of silicon is that at high temperature and in vacuum oxygen

can actually etch the silicon crystal giving gaseous products and the oxidized layer present

on the crystal becomes unstable (fig 51) [2]

VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)

SiO2 + Si rarr 2SiO(g) Oxide decomposition

Si+O2 rarr SiO2(s) oxide formation Fig51 phase diagram of the

oxygen-silicon system

Silicon oxidation apart from the initial nucleation stages at the monolayer level proceeds

uniformly over the whole surface with a planar reaction front moving from the surface

into the bulk

If heated under low oxygen partial pressure (vacuum or inert atmosphere) SiO2 is known to

decompose following the reaction

SiO2 + Si rarr 2SiOuarr (1)

The reaction begins with nucleation at defect points on the SiSiO2 interface and proceeds

in a spatially inhomogeneous manner with the formation of large voids on the oxide

surface [3]

Several studies have been made on the dynamics of void growth both on thick [4] and thin

[5] layers of SiO2 The process has been used to decorate otherwise unobservable defects at

the SiSiO2 interface [6] or to grow nanoislands of silicon on the void surface [7] It has

been suggested that the defects acting as nucleation centres could be metallic contaminants

present on the native surface which aggregate and catalyze SiO2 decomposition [8]

After oxide desorption the silicon surface is very rough In particular on Si(100) square

islands are observed several nanometres high which act as lsquopinning sitesrsquo on the motion of

monatomic steps on the Si surface It has been proposed that these islands can be composed

of silicon carbide coming from the organic contaminants present on the surface [9] or of

oxide clusters during partial reoxidation [10]

In the following section some experimental results obtained studying voids and nanoisland

growth are presented


Nanoisland growth on silicon

We used two different samples the first rougher than the second (i) p-type Czochralski Si

(100) with a RMS roughness of 05 nm shown in fig52a and (ii) p-type epitaxial Si(100)

with a RMS roughness of 019 nm shown in fig 52b The roughness was measured with

STM on the lsquoas receivedrsquo samples on different areas of 500x500 nm Every sample was

covered by a layer of native oxide ~2 nm thick

Some of the samples (AC) were introduced into the UHV system without any cleaning

others (BD) were dipped in concentrated HF (49) to remove the surface oxide without

etching the silicon [11]

After degassing the samples were heated resistively increasing the temperature slowly to

keep the pressure within the 10-10 mbar range during heating Each sample was held at

900˚C for 30 min For some of the samples the heating was stopped at lower temperatures

to obtain incomplete oxide desorption and surface roughening thus enabling observation

of the different steps of the process Table 51 summarizes the different treatments for each

group of samples

Fig52 Original surfaces of Czochralski (a) and epitaxial (b) silicon Every image is 500x500 nm

VPalermo 53

Table 51 Summary of different treatements

Silicon type Oxide removed

Heating Islands density (microm-2)

Av Island Volume (nm3)

A Czochralski No 30rsquo at 900˚C 24x103 353

B Czochralski Yes ldquo 20x104 71

C Epitaxial No ldquo 28x102 2450

D Epitaxial Yes ldquo 23x104 63

Fig53 ab) Surface roughening on group A samples after heating 30 min at 800deg and900deg respectively in presence of an oxide layer Image size 500x500 nm cd) Surface roughening on group B samples after heating 30 min at 700deg and 900degrespectively after removing the oxide layer with HF Image size 250x250 nm

Fig 52a is the native oxide surface of sample A quite disordered and irregular This

surface was stable when heated to 700˚C while at higher temperatures the oxide began to

desorb Fig 53a was taken after heating the sample at 800˚C for 30 min


The oxide began to desorb in correspondence with defect points creating oxide-free

surface voids which enlarge radially uncovering the silicon surface Some surface silicon

atoms are removed through reaction (1) Mobile silicon atoms moving randomly over the

surface aggregate forming a nanocrystal at the initial defect point surrounded by a stable

flat oxide-free surface with some steps The initial SiSiO2 interface as observed after

oxide etching with HF [12] is similar to the one shown in fig 52a and thus the surface

observed in the void area is a completely new one coming from surface reorganization

The reaction continues leading to a coalescence of the voids the final result of the process

is shown in fig 53b with protruding islands aligned along the main crystal axes and flat

areas with some steps LEED measurements confirmed that this surface is crystalline

silicon Atomic resolution was obtained on the flat areas of the surface showing the

typical 2x1 reconstruction of Si(100) Figures 53c and 53d show the evolution of the

roughening on sample B after chemically removing the native oxide layer before

introducing the sample into UHV As mentioned above the initial morphology is similar to

that shown in fig 52a though the oxide has been removed as confirmed by LEED

measurements Fig 53c shows the surface after heating at 700˚ for 30 min Without the

oxide layer the surface began to reorganize at lower temperatures and the small irregular

grains of the original surface begin to grow and become more rectangular After heating to

900˚C (fig 53d) the surface is flat with small rectangular islands The shape of the islands

is similar to that shown in fig 53b but the lateral island dimension is one order of

magnitude smaller

To assess the importance of the initial surface on the process the same treatments were

repeated starting from flat epitaxial silicon (samples CD) The initial epitaxial surface is

smooth with periodic steps due to a small miscut angle (fig52b) On heating the epitaxial

silicon without etching (sample C) relatively large islands are obtained similar to the ones

shown in fig 53b where the ldquostep-pinningrdquo effect of the islands is evident The average

dimension of sample C islands was sim2500 nm3 while only 353 nm3 for sample A Island

density decreased from 24000 to 280 microm-2 This result is maybe due to a lower

concentration of defects in the oxide layer of the epitaxial silicon which allows the

expanding void to grow much larger before coalescence with neighbouring voids the

nanostructures within them thus reaching greater dimensions To verify this hypothesis we

applied the same treatment to clean not-so clean and purposely contaminated samples To

check that the defects inducing void nucleation and nanoisland growth derive from carbon

VPalermo 55

contamination we used electronic-grade methanol to contaminate the silicon surface

Figure 54 shows the results of this comparison

State-of-the-art cleaned samples obtained with multi-step RCA cleaning [13] showed the

development of very few voids the oxide desorption leaving large smooth areas of oxide-

and nanostructure-free silicon as shown in Fig 54a where the presence of residual oxide

can be seen Samples cleaned with a simpler one-stage Pirana cleaning solution showed a

higher number of islands per unit area (Fig 54b) and samples purposely contaminated

with methanol (Fig 54c) had the highest density of island nucleation of all three samples

When the native oxide had been chemically removed from the epitaxial silicon before

heating (sample D) the island size and density is similar to the standard as-received

silicon case (sample B) Thus starting from a flat substrate the process of surface

roughening and island creation was the same even when the oxide layer had been

previously removed

From these results some important indications can be drawn The first is that oxide

desorption temperature depends upon the contamination levels of the oxide layers (fig 54)

with desorption at lower temperatures for contaminated surfaces

Second once the oxide layer is removed the silicon atoms become mobile on the surface

at relatively low temperatures (~700˚C) Their initial configuration is disordered and has a

high surface energy so they crystallize in the small islands shown in fig53d reducing

their surface area and lowering the energy of the system When the oxide is present the

surface cannot reorganize at 700˚C and is stable At 800˚C the oxide begins to desorb in

correspondence with defect points on the surface As the void area enlarges a small

cba Fig 54 The effect of contamination on nano-island production Gradient-filtered STM images of (a)sample cleaned two times with RCA 400x400 nm (b) Sample cleaned with a Pirana solution 500x500nm (c) sample contaminated with MeOH before insertion into the UHV system 500x500 nm Annealingtime is 40 min at 800degC for all the samples


crystallite grows at the centre of the voids Further oxide desorption uncovers larger areas

freeing more silicon atoms which feed the initial island making it bigger

Fig 55 shows the scaled size distribution of the islands for each group of samples Every

distribution has been obtained using several images of different samples for each group

The size distribution of all the samples follows an exponential decay and the decay seems

the same for all the samples though the average volume of the islands differs by more than

one order of magnitude (see Table 51 for details)

The point defects which catalyze void nucleation can be metallic contaminants present on

the initial oxide surface [8] or organic contaminants which at high temperatures can

form SiC nanocrystals [14] We used Scanning Tunneling Spectroscopy (STS) to look for

differences between the islands and the surrounding flat silicon surface but no difference

was found Furthermore the total island volume per surface unit is very high (more than

104 nm3microm2) and it seems unlikely that such a huge volume could consist of surface

contaminants Islands of pure silicon on silicon have been grown without evidence of

surface contamination[14]

To summarise the formation of nano-sized islands on silicon through surface diffusion

was studied At high temperatures the oxide layer covering the surface decomposes non

uniformly and circular voids of clean silicon are created The presence of the oxide layer

blocks surface silicon atomic motion and surface reorganization except within the voids

causing the growth of islands more than 10 nm high and 30 nm wide on the silicon surface

According to STM and LEED measurements we can say that the islands are mostly

Fig 55 Size distribution (scaled) for the samples described in table 1 The line is a reference for the eye

VPalermo 57

composed of silicon with traces of other substances such as carbon Final island densities

and dimensions depend upon the initial purity of the oxide layer

On the other hand if the oxide is removed chemically before heating the island growth

process is not so localised and takes place simultaneously over the whole surface yielding

smaller and more numerous islands


Bibliography

[1] J Dabrowski H Mussig Silicon Surfaces And Formation Of Interfaces (World


[2] Oxidation And Roughening Of Silicon During Annealing In A Rapid Thermal

Processing Chamber Mohadjeri B Baklanov Mr Kondoh E Maex K Journal Of

Applied Physics 83 (7) 3614-3619 Apr 1 1998

[3] High-Temperature SiO2 Decomposition At The Sio2Si Interface Tromp R Rubloff

GW Balk P Legoues FK Physical Review Letters 55 2332-2335 Nov 1985

[4] Defect Microchemistry At The SiO2Si Interface Rubloff GW Hofmann K Liher M

Young DR Physical Review Letters 582379-2382 Jun 1987 Defect Formation In

Thermal SiO2 By High Temperature Annealing Hofmann K Rubloff GW Mccorkle

RA Applied Physics Letters 49 1525-1527 Dec 1986 Kinetics Of High-Temperature

Thermal Decomposition Of SiO2 On Si(100) Liher M Lewis JE Rubloff GW Journal

Of Vacuum Science amp Technology A-Vacuum Surfaces And Films 5 1559-1562 Aug

1987

[5] Thermal-Decomposition Of Very Thin Oxide Layers On Si(111) Kobayashi Y Sugii

K Journal Of Vacuum Science amp Technology A-Vacuum Surfaces And Films 10 (4)

2308-2313 Part 3 Jul-Aug 1992 Scanning Tunneling Microscope Study On Mid-

Desorption Stages Of Native Oxides On Si(111) Kobayashi Y Sugii K Journal Of

Vacuum Science amp Technology B 9 (2) 748-751 Part 2 Mar-Apr 1991 Controlled

Growth Of SiO2 Tunnel Barrier And Crystalline Si Quantum Wells For Si Resonant

Tunneling Diodes Wei Y Wallace RM Seabaugh AC Journal Of Applied Physics 81

(9) 6415-6424 May 1 1997

[6] Defect Formation In SiO2Si(100) By Metal Diffusion And Reaction Liher M

Dallaporta H Lewis Je Appl Phys Lett 53 589-591 Aug 1988 SiO2 Film

Decomposition Reaction Initiated By Carbon Impurities Located At A Si- SiO2

Interface Raider Si Herd Sr Walkup Re Applied Physics Letters 59 (19) 2424-2426

Nov 4 1991

[7] Nanometer-Scale Si Selective Epitaxial Growth On Si(001) Surfaces Using The

Thermal Decomposition Of Ultrathin Oxide Films Fujita K Watanabe H Ichikawa M

Applied Physics Letters 70 (21) 2807-2809 May 26 1997 Pyramidal Si Nanocrystals

VPalermo 59

With A Quasiequilibrium Shape Selectively Grown On Si(001) Windows In Ultrathin

SiO2 Films Shibata M Nitta Y Fujita K Ichikawa M Physical Review B 61 (11)

7499-7504 Mar 15 2000

[8] Stacking-Fault-Induced Defect Creation In SiO2 On Si(100) Liher M Bronner Gb

Lewis Je Appl Phys Lett 52 1982-1985 May 1988

[9] Scanning Tunneling Microscopy Study Of Si(001) And Si(110) Surface Structures

Resulting From Different Thermal Cleaning Treatments Dijkkamp D Van Loenen Ej

Hoeven Aj Dieleman J Journal Of Vacuum Science amp Technology A-Vacuum

Surfaces And Films 8 218-221 Jan 1990 AFM And XPS Characterization Of The

Si(111) Surface After Thermal-Treatment Lamontagne B Guay D Roy D Sporken R

Caudano R Applied Surface Science 90 (4) 481-487 Dec 1995

[10] Scanning-Tunneling-Microscopy Study Of Oxide Nucleation And Oxidation-

Induced Roughening At Elevated-Temperatures On The Si(001)-(2x1) Surface Seiple

JV Pelz JP Physical Review Letters 73 (7) 999-1002 Aug 15 1994 Evolution Of

Atomic-Scale Roughening On Si(001)-(2x1) Surfaces Resulting From High-

Temperature Oxidation Seiple JV Pelz JP Journal Of Vacuum Science amp Technology

A-Vacuum Surfaces And Films 13 (3) 772-776 Part 1 May-Jun 1995



1995

[12] Morphological Changes Of The Si [100] Surface After Treatment With

Concentrated And Diluted HF Palermo V Jones D Materials Science In

Semiconductor Processing 4 (5) 437-441 Oct 2001



[14] Nanoscale Roughening Of Si(001) By Oxide Desorption In Ultrahigh Vacuum

Gray SM Johansson MKJ Johansson LSO Journal Of Vacuum Science amp Technology

B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61

6 Adsorption and diffusion of molecules on silicon creation of nano-islands and nano-lines

When the silicon surface is not protected by a native oxide layer or by a hydrogenated

passivating layer it is very reactive The surface chemistry of both Si(100) and Si(111) has

been extensively studied starting from clean surfaces prepared in UHV because of its great

relevance to the microelectronics industry and its technological interest The diffusion and

reaction of molecules and atoms on silicon is an interesting scientific problem on its own

apart from technological issues because silicon reconstruction yields a very complex and

anisotropic surface

We give here a brief summary of the adsorption behaviour of several elements on Si

surfaces The summary is not comprehensive and only the most interesting characteristics

for each substance are given

Table 61 Summary of adsorption behaviour of atoms and molecules on silicon surfaces [12]

Hydrogen

Molecular H2 shows low reactivity towards silicon while atomic hydrogen easily forms

Si-H bonds and can even break Si-Si bonds

Alkali metals

Alkali metals diffuse rapidly into SiO2 and can damage silicon-based transistors

Transition

metals

All transition metals apart from gold and silver react with Si forming metal silicides


Nickel A very common contaminant of silicon Nickel from even slight contact with stainless

steel tweezers can contaminate silicon samples forming its silicide and is very difficult

to remove even at high temperatures because it segregates on the silicon surface

Palladium Forms silicides especially Pd2Si and can be used to make contacts possessing a low

Schottky barrier

Titanium Widely used to fabricate contacts for silicon-based microdevices TiSi2 is one of the

more stable and highly conductive silicides

Tungsten The silicide is more stable than TiSi2 on polysilicon and is thus used for polysilicon

gate contacts

Platinum Silicide is used for bipolar transistors PtSi contacts on n-silicon give Schottky diodes

Cobalt Cobalt silicide gives better quality contacts than TiSi2 at the nanometrelevel but is less

used at the moment because it consumes too much silicon during its growth

Iron Silicides have been studied for potential optoelectronic applications because FeSi2 has a

directly accessible bandgap and is optically active

Group 13

Group 13 metals are used in IC technology to dope silicon (giving p-type doping) and

to make III-V type semiconductors When evaporated on Si(100) or Si(111) they react

strongly with the surface giving surface reconstruction and kink formation at steps

Boron Widely used for doping Usually deposited with decomposition of boron hydrides

Sticking coefficient of simple hydrides is very low so decaboranes (B10H14) are used for

deposition

Aluminum Aluminum is widely used for microelectronic contacts but in some cases it reacts with

silicon diffusing into the crystal and with SiO2 stealing oxygen atoms to form Al2O3

Group 14

Carbon Carbon can form a composite semiconductor with silicon (silicon carbide) widely used

to fabricate integrated circuits for use at elevated temperatures or in the presence of

ionising radiation When carbon is deposited on silicon usually polycrystalline films

with properties between diamond and graphite are obtained

CO adsorbs molecularly on silicon without breakage of the C=O bond Other

unsaturated hydrocarbons do not react with silicon Even very reactive strained

molecules like cyclopropane have low sticking coefficient Ethylene acetylene and

propylene adsorb molecularly on Si(100) each molecule sitting on a silicon dimer

interacting with the dangling bonds of the surface Acetylene also adsorbs molecularly

on Si(111) on the 7x7 reconstructed surface preferential adsorption on centre atoms

with respect to corner atoms is observed Benzene and other aromatic ring compounds

adsorb molecularly on the surface They can form σ or π-type bonds with the silicon

laying parallel or tilted respect to the surface according to the bond type

Silicon Silicon atoms can be deposited on the surface with molecular beam epitaxy or

VPalermo 63

decomposition of silanes and chlorosilanes Silanes with single Si-Si bonds have a high

sticking probability and dissociate upon adsorption giving trihydrides and

monohydrides

Chlorosilanes have high sticking coefficients too and decompose on the surface but

they can etch the surface through the reaction SiCl4 + Si rarr 2SiCl2

Germanium Germanium has a lattice constant similar to silicon (Ge lc is 4 larger than Si) so that

various alloys of Si1-xGex can be formed The growth of Germanium on silicon is quite

peculiar with several monolayers adsorbing uniformly on the surface followed by

island formation (Stranski-Krastanov growth)

Group 15

Nitrogen Exposure to ammonia and nitrogen gas at high temperatures leads to the formation of

silicon nitride layers (Si3N4) NH3 dissociates on the surface and reacts with the

dangling bonds giving Si-NH2 and Si-H termination Silicon nitride is stable on the

surface up to 1100degC above this temperature it desorbs as Si2N

Phosphorous Phosphine (PH3) adsorbs dissociatively as Si-PH2 and Si-H and behaviour is similar to

that of ammonia

Arsenic A monolayer of As is often deposited on silicon as a substrate for GaAs growth Arsenic

forms dimers on Si(100) creating dimer rows which are parallel (perpendicular) to the

underlying silicon dimer rows when it is deposited at high (low) temperature On

Si(111) Arsenic breaks the 7x7 reconstruction giving a 1x1 pattern This 1x1 As

monolayer acts as a passivating layer and can resist further As adsorption oxygen and

air

Antimony and

Bismuth

Due to their large covalent radii Sb and Bi form only short dimer rows on silicon and

only at high temperatures

Group 16

Oxygen

Oxygen can oxidize or etch silicon according to the reactions

Si(s) + O2 rarr SiO2 (s)

Si(s) + frac12O2 rarr SiO(g)uarr

Silicon oxidation has already been described in the previous chapter Molecular oxygen

adsorption on silicon is mostly dissociative O atoms break the Si-Si bonds forming an

Si-O-Si bridge or a peroxy bridge Si-O-O-Si Hydrided silicon surfaces do not have

surface dangling bonds and are thus more protected from oxygen attack Etching is

favoured at high temperatures and low oxygen pressures [3] At T gt700degC silicon oxide

on silicon decomposes as SiO(g)

Water

H2O adsorbs dissociatively on silicon easily on Si (100) and with more difficulty on

Si(111) The molecule decomposes giving Si-OH and Si-H on adjacent sites


Group 17

Fluorine

Fluorine adsorbs and reacts easily on silicon dissociating on the surface and forming

Si-F bonds It can then penetrate the surface and attack polarised Si-Si backbonds The

surface is etched with production of SiF4 and SiF2

Chlorine

Chlorine reacts aggressively with silicon etching the surface Cl2 and F2 are often used

commercially to etch silicon usually in a plasma Atomic Cl and Cl2 adsorb on Si(100)

up to saturation The most stable configuration seems to be a Cl atom bonded to each Si

atom of a surface dimer in a symmetric structure but metastable configurations with

two Cl atoms bonded to a buckled dimer and even a central Cl atom bridged across a

dimer have been observed

On Si(111) silicon mono- di- and tri-chlorides are formed In particular heating at

Tgt400degC a uniform Si-Cl monolayer is obtained and the 7x7 reconstruction changes

into a bulk like 1x1 lattice The surface transition induced by Cl adsorption is fully

reversible The 7x7 reconstruction can be restored on desorbing the chlorine by heating

at Tgt1100degC The 1x1 domains nucleate at the lower terrace side of steps the 7x7 at the

upper step edge

Bromine

Like hydrogen bromine maintains the 2x1 structure of Si(100) forming Si-Br bonds

with the dangling bonds of the silicon dimer rows At high exposure some etching of

silicon by formation of volatile SiBr3 species has been observed

Surface diffusion on silicon

In the previous chapter we observed nano-island growth on silicon surfaces with native

oxide and surfaces contaminated with organic impurities While this phenomenon has been

observed in several experiments the formation and growth mechanisms of nanoislands has

still not been satisfactorily described theoretically Various substances have been

hypothesised as nanoisland nucleating agents such as organic or metallic contaminants

present on the surface or oxide clusters (see previous chapter) The island growth process

has been attributed by various workers to lsquosome sort of kinetic instabilityrsquo [4] to the

pinning of step flow by SiC clusters [5] or to a mesoscopic atom flux from areas with low

step density to areas of higher step density [6]

One mechanism proposed to explain surface roughening and island growth is the Ehrlich-

Schwoebel effect ie the presence of an energetic barrier that reflects atoms approaching a

VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below

Table 62 Treatment island density and island volume for each molecule tested

Dose angmuir)

Heating time at 800degC (min)

Island density (microm-2)

Ave Volume (nm3)

Equivalent layer thickness (nm)

280 10 no islands - -

90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -

180 10 no islands - - - 10 no islands - -

step [7] Recent experiments however indicate that the Schwoebel effect is

licon [48]

f our experiments was to study the reaction of simple molecules with the silicon

d to check the ability of these molecules to nucleate nanoisland growth

a better understanding of the process we chose simple molecules starting from

which are more likely to be present as traces in UHV chambers where

s nanoisland growth is often observed after sample heating Using STM we

ved at the atomic level the possible interactions of these molecules with the

e then heated the surface and checked the density and dimensions of the

s produced

ntal procedures and results

i(111) wafers p-doped 0015 Ω cm Gross contamination and particles were

rom the sample surface with a standard Pirana etch (15 min dip in a H2S4H2O2

e at T=80degC) After this the samples were thoroughly washed in ultra-pure

introduced into the UHV system After degassing for 5 hours at 600degC the

ting stage was allowed to cool down and a sequence of rapid flashes to 1100deg-

moved the native oxide layer and cleaned the surface Pressure during flashes is

1x10-9 mbar


Surface flatness and cleanliness were checked with STM and the contaminant molecules

introduced into the UHV system through a leak valve Doses of several Langmuirs were

used for the experiments (1 Langmuir=10-6 torr sec-1) Pressure was kept constant at 10-7

mbar during adsorption The STM tip was retracted during gas deposition to avoid any tip

shielding effect After the adsorption process the leak valve was closed the base pressure

in the UHV chamber returning to below 10-10 mbar and the presence of adsorbed

molecules checked on the surface with STM Finally the sample was heated at 800degC for

several minutes to allow nanoisland growth Several different areas were examined on each

sample

Table 62 summarizes the dose of exposure and the heating time for each molecule Other

experiments were carried out using smaller dosing times but only the significant results

are reported here

As expected oxygen and water react with the surface oxidising it The adsorption process

can be easily followed with the STM by lowering the tip from time to time and taking an

image No effect of the STM tip is observed over the scanned areas Upon molecule

adsorption the order of the crystalline surface rapidly degenerates and after several

minutes the surface looks completely covered by irregular atomic-sized protrusions

corresponding to Si-OH or Si-O-Si species On heating to 800degC the contaminating species

were easily desorbed through SiO formation the surface became clean and the crystal

surface periodicity re-established No residues or nanoislands were observed

Amongst the molecules tested was methanol CH3OH reacts readily with the surface and

covers it with a disorderd layer already after sim 30 Langmuirs exposure (fig 61)

Fig 61 Si(111) surface during methanol adsorption at 0 1 and 5 minutes respectively (corresponding to 0 6 and 30 L exposure) The dark irregular lines are borders between different 7x7 domains Image size 60x60 nm

VPalermo 67

Synchroton radiation photoemission spectroscopy experiments by Carbone et al [9]

showed that methanol reacts with the rest atoms of the 7x7 silicon cell already after 1

Langmuir exposure but that longer exposures are needed for methanol to react with the

remaining silicon atoms Methanol adsorbs dissociatively forming SiOCH3 and Si-CHx

species on the surface

After methanol adsorption the sample is heated to 800degC and observed again with STM

As in the case of water and oxygen the heating restores the crystalline surface and the 7x7

pattern returns visible But this time quite a dense array of nanoislands is observed on the

surface (fig 62) The islands have an average diameter of 20 nm and a height of sim2 nm

At temperatures above 400degC the Si-OCH3 and Si-CHx species decompose oxygen and

hydrogen are desorbed and according to [9] carbon atoms remain as SiC dispersed

uniformly on the surface Our STM measurements indicate that the carbide does not

randomly cover the surface but that C atoms are concentrated at the nanoislands Rough

calculations based on island volume suggest that the islands must be composed of a SixC1-x

alloy with x varying between 05 and 1

Following these results we expected carbon monoxide to behave in a similar manner

adsorbing onto the surface and with nanoisland growth However no adsorption was

observed with STM and the surface looked perfectly clean even after a dose of more than

100 L of carbon monoxide Some species did adsorb onto the surface though because

after 10 minutes subsequent heating nanoisland growth was observed Island density in

this case was only 18 of the density obtained with methanol and island dimensions are

Fig62 Nanoislands on Si(111) created after 30 L adsorption of methanol and 10 min heating at 800degC Image size 200x200 nm


much bigger (see table 62 and fig 63) To check whether the islands had been nucleated

by some other contaminant present in the vacuum chamber we heated a blank sample

without introducing any molecule No island growth was observed Exposing the surface to

other simple molecules including carbon dioxide and methane also gave no nanoisland

formation

We can estimate the amount of carbon deposited on the surface during gas dosing The

total flux of molecules hitting the surface is obtained from the Hertz-Knudsen formula

TkmPF

π2=

Fig 63 STM images of Si(111) after exposure to methanol (left) and carbonmonoxide (right) contaminating molecules and subsequent nanoisland growthImage size 400x400 nm The size distribution of nanoislands is shown below eachimage

VPalermo 69

Fig64 Left Si(111) after 180 L contamination of CO and 16 hours heating at 800degC Right Si(111)without CO contamination after 16 hours heating at 800degC Image size 250x250 nm Vertical scale is thesame for both images z-ranges are 35 and 5 nm respectively

where P is the pressure in Pascal k=138x10-23 J K-1 is the Boltzmann constant T is the

temperature and m is the molecular weight (sim32 for methanol and sim28 for CO) in

kilograms

Using this formula we obtain a flux of 027 molecules nm-2 sec-1 for methanol and 029

molecules nm-2 sec-1 for CO After ten minutes exposure at 10-7 mbar pressure more than

150 molecules will have hit each square nanometer of the surface Surface density of

Si(111) 7x7 is sim16 atomsnm2 so each surface atom will be hit by several molecules which

could react or be adsorbed even assuming a sticking coefficient much lower than unity

To explain nanoisland growth we hypothesize that CO molecules adsorb molecularly and

-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm

10 min40 min16 h16 h- no CO

Fig65 STM profiles of nanoislandsgrown for different annealing times The lower curve correspond to the surface heated at 16 hours without CO contamination


Table 63 Island density and volume for the CO-contaminated surface at different heating times For each sample the thickness of a uniform layer having the same volume per unit area of the islands is calculated

Molecule Dose

(Langmuir) Heating time at

800degC (min) Island density

(microm-2) Ave Volume

(nm3) Equivalent layer thickness (nm)

CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186

clean surface - 16 h rough - -

very weakly on Si(111) moving rapidly on the surface Because of their rapid diffusion

the STM is not able to image the CO molecules on the silicon [10]

Increasing the temperature part of the CO molecules will surely desorb but some of them

will dissociatively adsorb onto the surface The oxygen will then desorb as SiO while

carbon will remain fixed forming very stable Si-C bonds

CO decomposition will be favored at surface defects surface steps or in the proximity of

already formed SiC clusters An increased reactivity of the CO molecule on a surface in

correspondence with phase boundaries has already been observed on Pt surfaces [10]

In this case a reduced number of nucleation centres will react with the CO molecules

yielding large and fewer islands with respect to the case of methanol which does not have

the possibility of travelling long distances over the surface

Other factors are likely to influence the process of island nucleation after CO adsorption

the co-adsorption of other molecules which can slow down CO and favour its

decomposition cannot be ruled out as well as the formation of new defects at higher

temperatures Measurements with a variable temperature STM or with some other surface

analysis technique are planned in the future to confirm the proposed mechanism

When the surface was further annealed the nanoislands grew in size After 16 hours

heating very large nanoislands with diameters of sim35 nm and heights of sim20 nm became

visible (fig64 left) As a comparison when the same surface was heated without

nucleation centres deriving from contamination an irregular surface was obtained (fig64

right)

While the nanoisland density seemed to reach an asymptotic limit of 500 islandsmicrom2

(table 63) island size continued to grow with time (fig65) even when no further carbon

was supplied to the surface This indicates that even though the initial nucleating core of

the nanoisland is likely to be an SixC1-x alloy further growth is due to silicon atoms

diffusing from the crystal and being adsorbed by the growing island The final volume

VPalermo 71

occupied by the islands corresponds roughly to a 18 nm thick overall layer of removed

silicon

It is noteworthy that the clean sample after 16 hours annealing even though smoother than

the CO-contaminated one looked much more disordered with hardly any flat area visible

on nanometres scale In the presence of nanoislands however further annealing increases

island size but keeps part of the surface quite flat and ordered with flat areas visible

between the islands Thus the nanoislands can be imagined to act as ldquoimpurity sinksrdquo for

further contaminants approaching the surface yielding a greater but more ordered surface

roughening with respect to the uncontaminated silicon surface

Circular arrays of nanoislands

We used the voids described in the previous chapter formed during oxide layer

decomposition as nanoscopic masks to test the validity of the hypothesis outlined above

The small circular areas uncovered during oxide decomposition can act as ldquoskating rinksrdquo

where contaminants and diffusing silicon atoms can move freely over the surface while

the oxide layer all around and higher than the void area as well as being chemically

bonded to the surface silicon atoms will hinder surface diffusion Furthermore the oxide

passivating layer will at least in some cases prevent contaminating molecules from

adsorbing and decomposing on the surface In this way selective nanoisland growth inside

void areas can take place

The experimental procedure consisted of the following steps

1) An oxide-passivated sample was heated for 10 minutes at 800degC Oxide decomposition

began in correspondence with surface defects and spread laterally creating naked areas

of silicon

2) While the void enlarges the same defect that initiated oxide decomposition behaved as

an attractor for moving silicon atoms and nucleated the growth of a nanoisland at the

void centre

3) The void surface was exposed to contaminating molecules

4) The sample was then heated again at 800degC During the second heating the void

continued to enlarge and new nanoislands were created The freshly uncovered silicon

surface provided mobile silicon atoms to feed nanoisland growth


5) When the heating continued for long enough all the oxide was desorbed and a clean

silicon surface covered by circular groups of nanoislands was obtained

Figure 66 shows the different stages of the process

We thus heated an oxidized sample at 800degC for 10 minutes in UHV to create multiple

voids in the oxide layer After this we exposed the sample to the contaminating molecules

Then we further annealed the surface at the same temperature to promote island growth

The first time we observed nanoisland growth in the void area was by chance after heating

for a second time samples which had been stored for 2 weeks in vacuum Even at pressures

as low as 10-11 mbar some molecules will adsorb onto the surface slowly creating defects

and thus potential nanoisland nucleation centres It is possible to observe with STM that

the silicon crystal surface looks very clean just after a high temperature flash but even

after only a few days randomly adsorbed species will be observable on the surface

Unfortunately it is not possible to identify these species simply from in situ STM

measurements

Mass spectrometry measurements show that the residual gas contaminants in vacuum are

usually He Ar H2 CH4 CO CO2 and N2 [11] Some of these molecules such as

hydrogen will stick to the silicon surface at room temperature but will simply desorb

during annealing without nucleating nanoislands Others like CH4 or CO2 will not react

with the surface to generate nanoislands as demonstrated previously The best candidate

for contamination of samples stored in UHV seems thus carbon monoxide has the ability

321

4 5

Fig 66 Selective nanoisland growth within oxide voids See text for details

VPalermo 73

as shown before to nucleate nanoislands although co-adsorption mechanisms can not be

ruled out Fig 67 (left) shows the circular groups of nano-islands obtained after heating

void-covered samples stored for 2 weeks in UHV Small islands are observed in the former

void area The void itself has enlarged slightly uncovering a clean oxide-free area of the

silicon surface Apart from the central island created during the first period of heating the

surrounding ones have dimensions decreasing from the void border to the inner area of the

void

This confirms that most of the material needed for island growth comes from the silicon

atoms diffusing from the freshly uncovered areas around the original void The outer

nucleating centres are nearest to the silicon atom source and thus generate larger islands

For a faster process we directly exposed the void to a significant concentration of CO

Fig67 (centre) shows the islands grown after a 180 Langmuir exposure and subsequent

heating The islands are preferentially located around the original void perimeter the rest

of the original oxide-free surface remaining untouched This suggests that although the

whole surface is exposed to CO the gas only interacts chemically with the surface at the

SiSiO2 interface around the oxide-free void perimeter The SiSiO2 interface thus provides

preferential nucleation sites for nanoisland growth in the successive thermal annealing

step This recalls a similar phenomenon observed in the catalytic oxidation of CO on the

Pt(111) surface where CO molecules reacted with adsorbed O species only along the

perimeters of oxygen islands [10]

On repeating the CO exposure and thermal annealing steps a second circle of nano-islands

Fig67 Left circular area of nanoislands obtained from a sample stored 2 weeks in UHV 500x500 nmCentre nanoisland circle obtained after exposing the voids to 180 L of CO and heating for 10 min at800degC 400x400 nm Right two concentric circles obtained with further CO adsorption followed byheating 650x650 nm


was formed around the newly-expanded surface void perimeter (fig67 right) This

process can in principle be repeated again to obtain a series of concentric groups of

nanoislands the only limit depending on the minimal distance between neighbouring

expanding voids We have thus observed that contaminants adsorbed at low pressure over

long periods of time generate random island growth over the whole void area while CO

molecules adsorbed at relatively high pressures over short periods yield selective

nanoisland growth at the former void border Several theories can explain the different

results CH4 and CO2 as mentioned above did not nucleate islands when when the silicon

surface was exposed to them for short periods However the lower surface mobility of

these contaminants and the greater time available for reaction with the surface could be

the cause of nucleation of SiC clusters over the whole oxide-free void area and not only at

its border The coadsorption of water another UHV residual gas with CO on the naked

silicon areas could perhaps allow a surface reaction similar to that between CO and the

SiSiO2 interface during CO exposure Another explanation for the formation of these

random nucleation sites within the voids could be the coadsorption of residual carbon-

containing species present in the UHV system with hydrogen the major residual gas in

stainless steel UHV systems

The possibility of decorating silicon oxide borders with nanoislands using the high

mobility and selective decomposition of CO molecules suggests interesting applications

for nanofabrication processes that will be discussed below

Silicon nanowire creation on Si(100)

We performed most of the experiments described above on silicon (111) 7x7

reconstructed because this surface is easily prepared in UHV and above all because it is

isotropic and has no preferential directions for atom diffusion The 2x1 reconstruction of

silicon (100) even though it has a much simpler unit cell than Si(111) 7x7 shows a more

complicated morphology with lots of monoatomic and biatomic steps and dimer rows

parallel to each other The orientation of the dimer rows changes by 90deg on alternate

atomic layers Surface diffusion in this case is much more complicated because atoms will

experience the effects of the surface anisotropy and diffuse preferentially along or across

the dimer rows The diffusion energy of a silicon atom as example is 06 eV along a

dimer row and 085 eV across different dimer rows This while being a complication

VPalermo 75

offers interesting possibilities for the creation of ordered structures The monoatomic steps

and the dimer rows can for example act as templates for the formation of elongated

structures of composition similar to the islands described previously

On heating a Si(100) surface oxide decomposition and void growth takes place as

described for Si(111) In some cases the void shape reflects the surface symmetry

especially for very thin oxide layers and the voids often have a slightly squared shape

Nanoisland growth is also influenced by the substrate symmetry and the islands look

rectangular as irregular clusters surrounded by concentric patterns of monoatomic steps

The whole structure reminds vaguely a ldquozigguratrdquo a kind of stepped pyramid found in

Mesopotamia (fig68 inset)

Apart from nanoisland growth in some cases we observed spontaneous formation of

nanowire-like SiC structures on the Si(100) surface The wires were generated after

exposing a Si(100) surface in vacuum to traces of CO followed by sample annealing at

800degC for 15 minutes The wires exhibit lengths ranging from 10 to 100 nm and average

widths of ca 5 nm Although being randomly positioned on the surface they are perfectly

aligned along the crystal axes of the substrate (Fig68)

Several descending monoatomic steps can be observed around each nano-line and nano-

island Very often one of the line extremities coincided with an island

Nanoline

Nanoisland

Fig 68 A Si(100) surface covered by nanoislands and nanowires The monoatomic steps present on the surface are visible The two insets show a typical nanoisland and a nanowire obtained on this surface The typical ldquodimer-row ldquostripes are visible along the sides of the nanowire


Couples of parallel lines starting from the same nano-island were also observed At high

resolution (fig 69) the lines showed a lamellar periodic structure with a periodicity of

around 4 nm and frequent defects Scanning Tunneling Spectroscopy measurements

showed no significant difference between the line and the surrounding silicon surface

The proposed formation mechanism for this kind of structure involves two stages in the

first one organic contaminating molecules adsorb and diffuse on the surface from the

vacuum environment stopping preferentially at surface singularities including steps and

defects

Several substances such as atomic carbon carbon monoxide ethylene and fullerene

decompose when adsorbed on silicon surfaces at temperatures above 500degC and create

localised defects on the surface inducing strain deformations and a metastable surface

reconstruction [12]

If these molecules remain attached to the steps they can diffuse rapidly along step border

until they meet a line and decompose In this way long lines of SiC defects parallel to the

surface steps can be produced In the second phase the clusters of silicon carbide can

-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon

Current- Voltage behaviour measured with STS Fourier analysis of wire periodicity

asymp 4 Aring

High resolution image of a line

Fig 69 Top high resolution image of a nanowire showing the lamellar structure Fourier analysis (bottom left) shows that the lamellae periodicity is sim 04 nm bottom right the IV characteristics measured with the STM tip on the line and the silicon surface

VPalermo 77

25-600degC

Final linear structure

600-800degC

Contaminant Silicon

Fig 610 Schematic representation of nanowire formation mechanism

locally inhibit the spontaneous surface diffusion of silicon atoms acting as templates for

the reorganisation of surface steps (fig 610)

The final morphology shows disordered lines and islands each surrounded by a complex

pattern of silicon monatomic layers

At 800degC silicon atoms are highly mobile on the surface Due to atom diffusion the

surface reorganizes with a step-flow mechanism The presence of a fixed line of

contaminants blocks atom diffusion and step flow generating the monoatomic steps

around each wire (fig 611) SiC clusters act as lsquopinning sitesrsquo on silicon atom surface

diffusion and can be purposely used to direct the localised growth of nano-islands [13]

Although the possibility of using methanol and carbon monoxide as nano-island precursors

has been demonstrated the exact nature of the substances nucleating nano-wire growth is

still unknown

Further experiments will be needed to confirm these hypotheses as well as to identify the

contaminating species nucleating nanowire growth Carbon monoxide is a main candidate

for nanowire growth but the real nucleation process is likely to be a complex one


Step flowblocked

Fig 611 Representation of step flow blocked by the presence of a nanowire 105x105nm

Bibliography

[1] Surface-Chemistry Of Silicon Waltenburg HN Yates JT Chemical Reviews 95 (5)

1589-1673 Jul-Aug 1995






[4] Growth Of Si On The Si(111) Surface Lanczycki Cj Kotlyar R Fu E Yang Yn

Williams Ed Das Sarma S Physical Review B 57 (20) 13132-13148 May 15 1998



VPalermo 79


Surfaces And Films 8 218-221 Jan 1990

[6] Nanoscale roughening of Si(001) by oxide desorption in ultrahigh vacuum Gray SM

Johansson MKJ Johansson LSO Journal Of Vacuum Science amp Technology B 14 (2)

1043-1047 Mar-Apr 1996

[7] Step Motion On Crystal Surfaces Schwoebel Rl Journal Of Applied Physics 40 614-

618 Feb 1969

[8] Scanning Tunneling Microscopy Investigation At High Temperatures Of Islands And

Holes On Si(111)7x7 In Real Time Evidence For Diffusion-Limited Decay

Hildebrandt S Kraus A Kulla R Wilhelmi G Hanbucken M Neddermeyer H Surface

Science 486 (1-2) 24-32 Jul 1 2001

[9] Methanol Adsorption On Si(111)-(7x7) Investigated By Core-Line Photoemission And

Mass Spectrometry Of Photodesorbed Ions Carbone M Piancastelli Mn Zanoni R

Comtet G Dujardin G Hellner L Surface Science 370 (1) L179-L184 Jan 1 1997

[10] Atomic And Macroscopic Reaction Rates Of A Surface-Catalyzed Reaction

Wintterlin J Volkening S Janssens Tvw Zambelli T Ertl G Science 278 (5345)

1931-1934 Dec 12 1997

[11] Redhead PA Hobson JP Kornelsen EV The Physical Basis Of Ultrahigh

Vacuum Chapter 12 (Chapman amp Hall London 1968)

[12] The Si(001) C(4 X 4) Surface Reconstruction A Comprehensive Experimental

Study Norenberg H Briggs Gad Surface Science 430 (1-3) 154-164 Jun 21 1999

[13] Production Of Nanostructures Of Silicon On Silicon By Atomic Self-Organization

Observed By Scanning Tunneling Microscopy Jones D Palermo V Applied Physics

Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81

7 Conclusions and proposals for future work It is very difficult to foresee the future of nanoscience and nanotechnology even over the

next few years exciting discoveries are continuously made opening up new possibilities

and progress in this sector is now one of the fastest and most varied So it is impossible to

predict whether computers will in the future be made with nanowires nanotubes single

electron devices or some kind of quantum-based transistor What does seem clear

however is that silicon devices can still further extend their levels of miniaturization to

remain competitive for at least the next ten years [1] During this period new emerging

technologies will reach scientific maturity and arrive at the production lines

Even when new technology prototypes do become available for full-scale production the

astronomical cost of changing from silicon to new technologies will be a major concern

Moreover a vast amount of extremely detailed knowledge is already available on all

aspects of silicon technology Thus the possibility of integrating innovative

nanotechnologies with standard CMOS silicon technology already seems an attractive

prospect [2]

For this and other reasons we have focussed our research efforts on surface phenomena on

silicon wafers which could be potentially useful for the development of silicon-compatible

nano-devices

Some important conclusions can be drawn from the work described in this thesis

bull Silicon surfaces of different crystal faces were studied at atomic resolution in ultra-

high-vacuum following transformations induced on the surface by chemical (etching)

and physico-chemical (organic contamination heating) treatments


bull The presence of surface contaminants induces at high temperature the growth of

nanometre-sized islands and lines on silicon

bull Final island densities and dimensions strongly depend upon the chemical nature of the

contaminant molecules and the treatments used Molecules interacting only weakly

with silicon will desorb upon heating whereas molecules which decompose easily on

Si will give widespread random nano-island nucleation Between these two extreme

cases complex behaviours of diffusion and subsequent decomposition are possible

bull Nanoscopic naked silicon voids were produced in oxide layers through carefully-

controlled thermal annealing It was found that void density and size depends upon

oxide purity surface contamination levels and annealing conditions

bull The growth of nano-islands and nano-voids was exploited simultaneously and the

voids used as nanoscopic masks to control the positions of growing islands Complex

silicon-on-silicon nano-structures were obtained with multi-step growth processes

bull Finally the use of macroscopic treatments and simple physical processes to produce

large quantities of nanometre-sized structures on silicon surfaces was demonstrated

These processes and the structures produced are of potential interest to the

microelectronics industry

We showed in previous chapters how it is possible to modify silicon surfaces using

chemical and physical methods The use of self-organization for the production of Si-on-Si

20 nm

Fig 71 Left SEM image of field emitter arrays made by conventional lithography [3] Center 3-D STM image of a self-organized void with a central nano-island Right Lateral schematic view of a field emitter device

VPalermo 83

nano-structures has the advantage with respect to other emerging techniques of being

silicon compatible and thus is a potential candidate for its implementation in the

production of new devices Furthermore this technique can create billions of strucures on a

wafer in only a few minutes being thus applicable to large-scale production

Even when ordered structures are obtained these methods still show a poor reproducibility

and control Chemically modified surfaces look very disordered at high magnification

with trenches and holes etched into the surface and progress has recently been made in

producing very small structures electrochemically [4] Oxide decomposition with

nanoisland growth in UHV can also yield ordered structures The void holes with

nanoislands located at their centres although quite variable in dimensions are more similar

to engineered devices than to a spontaneous random surface-roughening process In fact

there is a surprising similarity between the nanovoid-nanoisland structures and well-known

commercially available devices called field emitter arrays (FEA) These are small tips used

as electron guns in various kinds of displays and other devices Both the commercial

devices and our self-organized structures consist of a protruding tip at the centre of

microscopic holes on an insulating layer covering a semiconductor surface (see fig71)

Of course the order and size uniformity of commercial FEAs is much better than that

obtained with our process but it is encouraging to see that a self-organized structure

obtained with simple heating has a very similar surface topography and composition of

commercial working devices obtained after many expensive stages of optical lithography

deposition and etching

Nano-island density and size can to some extent be controlled and different

contaminants can be adsorbed onto the surface to modify the growth process In the future

with a better understanding of diffusion dynamics and growth mechanisms a multi-stage

fabrication process can be envisaged where purposely-designed molecules are adsorbed

onto silicon surfaces to diffuse organize in ordered arrays and favour surface

reorganization at the nanometre scale The same molecules could be used to deliver

dopants to obtain for example highly conducting nanostructures on otherwise undoped

highly resistive silicon The selective formation of nanostructures at the SiSiO2 border

can be used to scale down mesoscopic patterns to the nanometric level A silicon oxide

layer can be easily patterned on the surface with normal optical lithography and then

nanostructures can be built following the pattern edge In this way the production of very

small nanowire based devices may be possible (fig 72)


D

Gate

S

D

S

SiO2

SiO2

2 Heating nanowire creation

and oxide removal

3 Source-drain deposition by conventional lithography

1 Creation of a silicon-oxide pattern by conventional lithography Gas adorption and selective decomposition at the border

4 Insulating layer (oxide) and gate (metal) deposition

Fig 72 Schematic drawing of the construction of a self-organized nanowire-based transistor

Surface diffusion and decomposition of molecules can be sensitive to even smaller surface

features The nanoscopic lines formed on Si(100) are very regular and neat if their

nucleation and growth mechanism could be controlled ordered networks of nanowires

could be made on silicon

VPalermo 85

The techniques developed here are still experimental ones the careful control and

reproducibility of each step is still an issue and further studies will be needed to clearly

understand the dynamics of formation of these structures Nonetheless the results obtained

so far open up the possibilities of producing novel devices such as for example nano-

transistors (see scheme in fig73 where the mechanism described in fig 72 is applied for

the fabrication of a device array) The possibility of using the border of a lithographic

pattern to draw nanolines can in principle allow the creation of nanodevices using normal

IC manufacturing techniques

The results obtained and their description in this thesis are not the first examples of the use

of self-organization for the production of ordered structures and will certainly not be the

last In our opinion the most important outcome of these findings is that it is possible to

use surface diffusion to overcome the inherent limits of lithographic techniques in

microdevice production processes and to force properly selected molecules to react with

the surface in a spatially differentiated manner at particular sites thereby creating ordered

series of nanostructures The role of the surface dynamics of the substrate atoms

themselves is extremely important in this process

Outstanding results have been and continue to be obtained in nanoscience and

nanotechnology research promising the emergence of new production technologies Sooner

or later those emerging technologies will have to face the issues of cost and compatibility

with the enormous investment made in existing technologies and processing facilities The

commercial production of nanostructures based on surface diffusion processes could in

principle be cheap simple and compatible with existing technology Moreover the

processes of diffusion in bulk silicon is well-known in the microelectronics industry which

has both the know-how and the equipment to better understand and exploit the surface

diffusion processes on silicon surfaces studied here

Extremely important and innovative results are often obtained by exploiting very simple

ideas and in our opinion physical processes such as surface diffusion used to create

complex nanostructures on silicon surfaces is a prime example



2 Heating nanowire creation and

oxide removal



Fig 73 Schematic macroscopic outline of the construction of a self-organized nanowire-based transistor

VPalermo 87

Bibliography

[1] P Gargini Intel Technology Coordinator ldquoFrom Microelectronics To

Nanotechnologiesrdquo Invited Campus Colloquia Lecture CNR Research Area Bologna

February 6th 2003

[2] Toward A Hybrid Micro-Nanoelectronics Cerofolini Gf Ferla G Journal Of

Nanoparticle Research 4 (3) 185-191 Jun 2002

[3] Recent Progress In Field Emitter Array Development For High Performance

Applications Temple D Materials Science amp Engineering R-Reports 24 (5) 185-239

Jan 25 1999

[4] Silicon Dioxide Micropillars For Sieving Fabricated By Macroporous Silicon-Based

Micromachining Izuo S Ohji H French Pj Tsutsumi K Kimata M Sensors And

Materials 14 (5) 239-251 2002 Electrochemical Etching In HF Solution For Silicon

Micromachining Barillaro G Nannini A Piotto M Sensors And Actuators A-Physical

102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements

This work would not have been possible without the help and guidance of Dr Derek Jones

he introduced me to the use of STM and other techniques found the funding for this PhD

and helped me with the thousands of small and big problems I had to face during my

research I am grateful to Prof Alberto Ripamonti too for being the supervisor of this

thesis Thanks are also due to Dr Giancarlo Seconi director of ISOF-CNR where most of

this work was carried out Financial support from the Italian National Research Council

(CNR) is also gratefully acknowledged

My passion for surface science began when Fabio Biscarini showed me that it was possible

to really see the atoms and for this I will always thank him

During these years I had the luck to collaborate with many good scientists among those I

would like to cite Enrichetta Susi Massimo Cocchi and Anna Mazzone from CNR

Claudio Zannoni Anna Cavallini Daniela Cavalcoli and Antonio Castaldini from the

University of Bologna Sergio Pizzini and Maurizio Acciarri from the University of Milan

One of the most stimulating periods of my PhD was the one I spent at the Steacie Institute

for Molecular Sciences in Ottawa working with Robert Wolkow Dan Wayner Greg

Lopinski and Peter Kruse I thank them for discussing together many exciting scientific

ideas and trying to put some of them into practice

Special thanks go to my colleagues Paolo Samorigrave and Stefano De Cesari for long and

useful discussions on Science Life and their interactions often with the help of a good

pint

Finally I would like to thank Vassilia Gaetano Simone Claudia Silvia Letizia

Alessandro Angela and Sebastien even though not directly involved in this work they

have been over these three years a continuous source of happiness and support

Bologna March 2003


SCIENTIFIC PUBLICATIONS bull Lateral diffusion of titanium disilicide as a way to contacting

hybrid Si-organic nanostructures Palermo V Buchanan M Bezinger A Wolkow RA APPLIED PHYSICS LETTERS 2002 v81 p 3636 (this article has been selected for publication on the VIRTUAL JOURNAL OF NANOSCALE SCIENCE amp TECHNOLOGY 2002 v6 issue 20) bull Surface modifications in Si after Rapid Thermal Annealing Castaldini A Cavalcoli D Cavallini A Jones D Palermo V Susi E JOURNAL OF THE ELECTROCHEMICAL SOCIETY 2002 v 149 pG633 bull Production of nanostructures of silicon on silicon by atomic self-

organisation observed by scanning tunnelling microscopy Jones D Palermo V APPLIED PHYSICS LETTERS 2002 v 80 p 673 (this article has been selected for publication on the VIRTUAL JOURNAL OF NANOSCALE SCIENCE amp TECHNOLOGY 2002 v5 issue 5) bull Nucleation of nanostructures from surface defects on silicon Palermo V Jones D SOLID STATE PHENOMENA 2002 v 82-84 p 687 bull Self-Organised Growth of Silicon Structures on Si(100) During

Oxide Desorption Jones D Palermo MATERIALS SCIENCE AND ENGINEERING B 2002 v88 (2-3) p220 bull Morphological changes of Si[100] surface after treatment with

concentrated and dilute HF Palermo V Jones D MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING 2001 v 4 p 437 bull Electrical and structural properties of processed silicon surfaces Susi E Cavallini A Castaldini A Cavalcoli D Jones D Palermo V ldquoRECENT RESEARCH DEVELOPMENTS IN VACUUM SCIENCE amp TECHNOLOGYrdquo 2001 v 3 p189 bull Numerical Solutions of the Stochastic Equations of Crystal

Growth Mazzone A M Palermo V INTERNATIONAL JOURNAL OF MODERN PHYSICS C 2000 v 11 Part 1 p195-204 bull Advances in silicon surface characterisation using light beam

injection techniques

VPalermo 91

Acciarri M Pizzini S Simone G Jones D Palermo V MATERIALS SCIENCE AND ENGINEERING B 2000 V73 (1-3) p 235 - 239 bull Abrupt orientational changes for liquid crystals adsorbed on a

graphite surface Palermo V Biscarini F Zannoni C PHYSICAL REVIEW -SERIES E- 1998 V 57 NUMBER 3A p R2519-R2522 Oral presentations bull The use of oxide desorption and surface diffusion for the

creation of silicon on silicon nanostructures Palermo V Jones D 1ST NATIONAL WORKSHOP ON CURRENT TRENDS IN NANOTECHNOLOGIES Catania (Italy) Feb 2002 bull Self-Organised Growth of Silicon Structures on Si(100) During

Oxide Desorption Jones D Palermo V E-MRS Spring Meeting Strasbourg (France) June 2001 bull STM study of surface transformations on silicon during UHV

annealing Palermo V Jones D 4TH MULTINATIONAL CONGRESS ON ELECTRON MICROSCOPY Veszprem (Hungary) Sept 1999 bull Advances in silicon surface characterisation using light beam

injection technique Acciarri M Pizzini S Simone G Jones D Palermo V E_MRS SPRING MEETING June 1999 Posters bull Spontaneous nano-wire growth on silicon Palermo V Jones D SMARTON workshop Leuven (Belgium) October 2002 bull Ordered circles of nano-islands on silicon from CO adsorption Palermo V Jones D TRENDS IN NANOTECHNOLOGY 2002 Santiago de Compostela (Spain) September 2002 bull Production of nanostructures of silicon on silicon by atomic self-

organisation Palermo V Jones D EUROMAT Rimini (Italy) June 2001 bull Etching holes and anisotropic corrosion on silicon [100] Palermo V Jones D Susi E Asoli B SILICON WORKSHOP Genova (Italy) February 2001


bull Morphological and electrical characteristics of damaged silicon surfaces

Susi E Castaldini A Cavalcoli D Cavallini A Jones D Palermo V SILICON WORKSHOP Genova (Italy) February 2001 bull Effect of HF etching on the roughness of a silicon surface Palermo V Jones D NATIONAL CONFERENCE ON PHYSICS OF MATTER Genova (Italy) June 2000 bull Effect of Native Oxide Desorption upon the Surface Morphology of Si[100] by STM and LEED Palermo V Jones D SILICON WORKSHOP Genova (Italy) February 2000 bull Characterisation of silicon surfaces for microelectronics through STM measurements Palermo V Jones D 4TH MULTINATIONAL CONGRESS ON ELECTRON MICROSCOPY Veszprem (Hungary) Sept 1999

VPalermo 93

Contact Vincenzo Palermo ISOF- Institute for Organic Synthesis and Photoreactivity Via Gobetti 101 40129 Bologna ITALY Tel +39-051-6398336 Fax +39-051-6398349 Mail palermoisofcnrit


Index

1 Introduction nanotechnology and the future of computers

2 Silicon surfaces

3 STM and other surface analysis techniques

4 Surface modification of silicon in liquid Nano-hole creation


6 Adsorption and diffusion of molecules on silicon creation of nano-islands and

nano-lines

7 Conclusions and proposals for future work


VPalermo 3





XV ciclo





Marzo 2003


VPalermo 5




VPalermo 7





years




























production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


VPalermo 3





XV ciclo





Marzo 2003


VPalermo 5




VPalermo 7





years




























production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines



VPalermo 5




VPalermo 7





years




























production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


VPalermo 5




VPalermo 7





years




























production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines



VPalermo 7





years




























production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


VPalermo 7





years




























production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines





















production







VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


VPalermo 9

silicon technology


nanoscience

















































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines



































VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


VPalermo 11



















100 nm





silicon surface













Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines










Bibliography












Jun 13 2002






VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


VPalermo 13



Nov 9 2001






Research 32 (5) 447-454 May 1999



2002


VPalermo 15

2 Silicon surfaces















































lifetime (s) 25E-3


07



15









232


23E5

VPalermo 17






)2(3)2(cos 2 ++= nnα



Si (100)




























SA

SB


VPalermo 19



Si(113)






Si(111)
























Si(110)









VPalermo 21






Orientation

113

110

111

100

100

2524

9000

5474

0

111

2950

3526

0

110

6476

0

113

0



Bibliography


1972)




Science 365 (3) 807-816 Oct 1 1996

VPalermo 23


techniques










samples



















tip




z is


φmk

2= (1)








VPalermo 25













Rxk

eII 22

0

2minus

=


apex






needed





etching are























VPalermo 27











Microscope (AFM)




chemical bond




Y-PIEZO

SILICON SAMPLE


Z-PIEZO

X-PIEZO

TIP


















can be imaged




VPalermo 29











surface atoms









profiles)










Bibliography


Oxford 1993)

[2] From wwwibmcom







VPalermo 31


Nano-hole creation















H2O2)













[3]





untouched [3]



surfaces












VPalermo 33












HO+H

H HO

H

H

H

FSi

Si

SiSi

+H2O

-OH-

+F-

H2O H H

Si H

F OH

H Si

Si Si

-H2

+H2O

H

H

H

FSi

Si

SiSi

H

H

H

OHSi

Si

SiSi

H

H

H

H Si

Si

Si Si































Si Si

Si Si

H

H

H

H Si

Si

Si Si

H

H

H Si

Si

SiSi

H

H

H

OH-H+ -e

(F-)

-H+

-e









VPalermo 35














Si Si

Si Si

H

H

H

F Si

Si

Si SiH

F F-H+ -2e

+HF2-

H

HF

SiSi

SiSi + F

F F

FH

H

H

F -H+ -2e

+HF2-


















pores


al [15]


















VPalermo 37





























sum minus=xy

hyxhN

22 ))((1σ


)(22

2

)(4

)( vyuxi

xyeyxhvuF +sum∆

= π

π


















VPalermo 39







is good





1 E -0 2

1 E -0 1

1 E + 0 0

1 E + 0 1

1 E + 0 2

1 E + 0 3

0 0 0 0 0 1 0 1 0 1 0 0f (1 nm )

nm^4

A a s re c e iv e d

B e tc h e d 1 min


D e tc h e d HF 4 9








frequency part












VPalermo 41







surface










silicon surface














(110)

(110)






















VPalermo 43

















contaminants











































VPalermo 45




















underetching






Bibliography









Part 1 Apr 15 1996









2686 Aug 1996




Solids 187 221-226 Jul 1995

VPalermo 47






1656-1658 Apr 15 1991




10 1993












Jul 1995








1 2002



1992




Physical 88 (3) 241-246 Mar 5 2001




VPalermo 49















on Si(100) wafers











O-SiO3














is given by [1]

1

212minus

minusminus

+=

ABXBXt α


minus

kTEexp










VPalermo 51

1E-10

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

0607080911112

1000T (1K)

P (T

orr)






into the bulk






surface [3]




























group of samples


VPalermo 53


































magnitude smaller













VPalermo 55













previously removed


































VPalermo 57







Bibliography














1987








(9) 6415-6424 May 1 1997





Nov 4 1991




VPalermo 59



7499-7504 Mar 15 2000

















1995








B 14 (2) 1043-1047 Mar-Apr 1996


VPalermo 61








anisotropic surface





Hydrogen



Alkali metals


Transition

metals







Schottky barrier




gate contacts






Group 13






deposition



Group 14















VPalermo 63



monohydrides







Group 15






that of ammonia






air

Antimony and

Bismuth



Group 16

Oxygen










Water




Group 17

Fluorine




Chlorine












upper step edge

Bromine
















VPalermo 65

Molecule (L

O2

H2O MeOH

CO CO2 CH4

clean surface

downward

weak on si

The aim o

surface an

To obtain

the ones

spontaneou

first obser

surface W

nanoisland

Experime

We used S

removed f

41 mixtur

water and

whole hea

1200degC re

kept below


Dose angmuir)



Ave Volume (nm3)



90 10 no islands - - 30 10 2000 19 004

180 10 360 516 019 30 60 no islands - -



licon [48]








s produced








1x10-9 mbar










sample



are reported here












VPalermo 67




























formation



TkmPF

π2=


VPalermo 69




kilograms







-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120

nm

nm





Molecule Dose





CO 180 10 362 517 019 CO 180 40 550 1072 059 CO 180 16 h 505 3675 186






















right)






VPalermo 71


silicon





















of silicon



void centre



















measurements







321

4 5


VPalermo 73







void
















































VPalermo 75



















Nanoline

Nanoisland










defects




reconstruction [12]




-005

0

005

01

015

02

025

03

035

04

-3 -2 -1 0 1 2 3

V

dId

V lt

dId

Vgt

linesilicon


asymp 4 Aring



VPalermo 77

25-600degC


600-800degC

Contaminant Silicon













still unknown





Step flowblocked


Bibliography


1589-1673 Jul-Aug 1995










VPalermo 79





1043-1047 Mar-Apr 1996


618 Feb 1969




Science 486 (1-2) 24-32 Jul 1 2001






1931-1934 Dec 12 1997







Letters 80 (4) 673-675 Jan 28 2002


VPalermo 81














prospect [2]



nano-devices

























20 nm


VPalermo 83



































D

Gate

S

D

S

SiO2

SiO2


and oxide removal









VPalermo 85
































oxide removal




VPalermo 87

Bibliography



February 6th 2003





Jan 25 1999





102 (1-2) 195-201 Dec 1 2002


VPalermo 89

Acknowledgements




















pint




Bologna March 2003









VPalermo 91











VPalermo 93



Index


2 Silicon surfaces





nano-lines


Creation of Nanometre-Scale Islands, Wires and Holes on Silicon … · 8 Creation of Nanometre-Scale Islands, Wires and Holes on Silicon Surfaces for Microelectronics on the silicon

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