05/11/2006 CDF Silicon Workshop at UCSB 1 Rainer Wallny Silicon Detector Workshop at UCS May 11th, 2006 Slides ruthlessly stolen from: Paula Collins, CERN Alan Honma, CERN Christian Joram, CERN Michael Moll, CERN Steve Worm, RAL Silicon Detectors – How They Work
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Rainer Wallny Silicon Detector Workshop at UCSB May 11th, 2006
Rainer Wallny Silicon Detector Workshop at UCSB May 11th, 2006 Slides ruthlessly stolen from: Paula Collins, CERN Alan Honma, CERN Christian Joram, CERN Michael Moll, CERN Steve Worm, RAL. - PowerPoint PPT Presentation
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05/11/2006 CDF Silicon Workshop at UCSB 1
Rainer Wallny
Silicon Detector Workshop at UCSBMay 11th, 2006
Slides ruthlessly stolen from: Paula Collins, CERN Alan Honma, CERN Christian Joram, CERN Michael Moll, CERN Steve Worm, RAL
Silicon Detectors – How They Work
05/11/2006 CDF Silicon Workshop at UCSB 2
o Why Silicon ?o Semiconductor Basics
– Band-gap, PN junction– Silicon strip detectors
o Some Technicalities - Wafer Production
- Wire Bondingo Radiation Damage
– Effect on Vd
– Effect on Leakage Currentso Conclusions
Outline
05/11/2006 CDF Silicon Workshop at UCSB 33
Tracking Chambers with Solid Media
o Ionization chamber medium could be gas, liquid, or solid – Some technologies (ie. bubble chambers) not applicable in collider
environments
“Solid-state detectors require high-technology devices built by specialists and appear as black boxes with unchangeable characteristics.”
-Tom Ferbel, 1987
LowModerateModerateIonization Energy
FastModerateModerateSignal Speed
ModerateModerateLowAtomic number
HighModerateLowDensity
SolidLiquidGas
o High-precision tracking advantages with solid media– Easily ionized, relatively large amount of charge– Locally high density means less charge spreading– Fast readout possible
05/11/2006 CDF Silicon Workshop at UCSB 44
o Electrical properties are good– Forms a native oxide with excellent electrical properties– Ionization energy is small enough for easy ionization, yet large enough to maintain
a low dark currento Mechanical properties are good
– Easily patterned and read out at small dimensions– Can be operated in air and at room temperature – Can assemble into complex geometries
o Availability and experience– Significant industrial experience and commercial applications– Readily available at your nearest beach
Why Silicon?
05/11/2006 CDF Silicon Workshop at UCSB 5
The Idea is Not Quite New …
Semiconductors used since 50’s for energy measurement in nuclear physics
o Precision position measurements up until 70’s
done with emulsions or bubble chambers
-> limited rates, no triggering
o Traditional gas detectors: limited to 50-100 μm
o First silicon usage for precision position
measurement: NA11 at CERN, 1981
05/11/2006 CDF Silicon Workshop at UCSB 66
fan out to readout electronics
E706 (FNAL 1987)NA11 (CERN 1981)
sensor
o 24x36 mm2 active areao 8 layers of silicono 1m2 readout electronics!
o 50x50 mm2 active areaSilicon sensor and readout electronics technology closely coupled with electronics miniaturization (transistors, ICs, ASICs …) silicon quickly took off …
Pioneering Silicon Strip Detectors
05/11/2006 CDF Silicon Workshop at UCSB 77
CDF SVX IIa half-ladder: two silicon sensors with readout electronics (SVX3b analog readout chip) mounted on first sensor
ATLAS SCT barrel module: four silicon sensors with center-tapped readout electronics (ABCD binary readout chip)
Silicon sensor and readout chip development intimately relatedBUT will concentrate on silicon only here …
Each sensor treated individually, nurtured into life in many hours of careful handling
Systematic assembly line production with decent QA systems
New Production Paradigm
P. Collins,Warwick 2006
05/11/2006 CDF Silicon Workshop at UCSB 10
Moore’s Law: Exponential growth of sensitive area and number of electronic channels with time(from Computer Science: doubling of IC integration capacity every 18 months)
‘Moore’s Law’ for Silicon Detector Systems
05/11/2006 CDF Silicon Workshop at UCSB 11
LEP Tevatron LHC
Whoops… P.Collins, ICHEP 2002
Large Silicon Detector Systems ….
05/11/2006 CDF Silicon Workshop at UCSB 12
The Basics ……
05/11/2006 CDF Silicon Workshop at UCSB 13
o If the gap is large, the solid is an insulator. If there is no gap, it is a conductor. A semiconductor results when the gap is small.
o For silicon, the band gap is 1.1 eV, but it takes 3.6 eV to ionize an atom. The rest of the energy goes to phonon exitations (heat).
o In a gas, electron energy levels are discrete. In a solid, energy levels split and form a nearly-continuous band.
Semiconductor Basics – Band Gap
05/11/2006 CDF Silicon Workshop at UCSB 14
Semiconductor Basics – Principle of Operation- Basic motivation: charged particle
position measurement- Use ionization signal (dE/dx) left
behind by charged particle passageEf
E
valence band
conductance band
h
e
++
++
__
__
- In a semiconductor, ionization produces electron hole pairs- Electric fields drift electrons and holes to oppositely electrodesBUT:- In pure intrinsic (undoped) silicon, many more free charge
carriers than those produced by a charged particle. Have 4.5x108 free charge carriers; only 3.2x104 produced by MIP- Electron –hole pairs quickly re-combine …
Need to deplete free charge carriers and separate e-holes ‘quickly’!
300 m
1 cm
1 cm
05/11/2006 CDF Silicon Workshop at UCSB 15
Ef
E
VB
CBe
n-type: • In an n-type semiconductor, negative charge carriers (electrons) are obtained by adding impurities of donor ions (eg. Phosphorus (type V))• Donors introduce energy levels close to conduction band thus almost fully ionized => Fermi Level near CB
Electrons are the majority carriers.
Ef
E
VB
CB
h
p-type: • In a p-type semiconductor, positive charge carriers (holes) are obtained by adding impurities of acceptor ions (eg. Boron (type III))
• Acceptors introduce energy levels close to valence band thus ‘absorb’ electrons fromVB, creating holes => Fermi Level near VB.
Holes are the majority carriers.
Doping Silicon
05/11/2006 CDF Silicon Workshop at UCSB 16
The pn-JunctionExploit the properties of a p-n junction (diode) to collect ionization charges
+ –
+–
+
+
+
+
+
+
+
+
+
+ –
– –+
+–
+
+
–
–
–p nWhen brought together to form a junction, a gradient of electron and hole densities results in a diffuse migration of majority carriers across the junction. Migration leaves a region of net charge of opposite sign on each side, called the depletion region (depleted of charge carriers).
Electric field set up prevents further migration of carriers resulting in potential difference Vbi
Another way to look at it:
Fermi-Levels need to be adjustedso thus energy bands get distorted => potential Vbi
Ef
E
VB
CBe
Ef
E
VB
CB
hEf
E
VB
CB
p n
e.V
Funky cartoon from Brazil: http://www.agostinhorosa.com.br/artigos/transistor-6.html
05/11/2006 CDF Silicon Workshop at UCSB 17
+ –
+–
+
+
+
+
+
+
+
+ –
– –++
–
+
+
–
–
–
p n
Dopantconcentration
Space chargedensity
Carrierdensity
Electricfield
Electricpotential
o p-type and n-type doped silicon forms a region that is depleted of free charge carriers
o The depleted region contains a non-zero fixed charge and an electric field. In the depletionzone, electron – hole pairs won’t recombine but rather drift along field lines
o Artificially increasing this depleted region by applying a reversed bias voltage allow charge collection from a larger volume
pn - Junction
05/11/2006 CDF Silicon Workshop at UCSB 18
pp p
n
If we make the p-n junction at the surface of a silicon wafer with the bulk being n-type (you could also do it the opposite way), we then need to extend the depletion region throughout the n bulk to get maximum charge collection by applying a reverse bias voltage.
+
–
h+ e-
How to Build a Silicon Detector
05/11/2006 CDF Silicon Workshop at UCSB 19
Properties of the Depletion Zone
– Depletion width is a function of the bulk resistivity , charge carrier mobility and the magnitude of reverse bias voltage Vb:
+
–Depletion zone
undepleted zone
– The bias voltage needed to completely deplete a device of thickness d is called the depletion voltage, Vd
Vb
wd
– Need a higher voltage to fully deplete a low resistivity material.– A higher voltage is needed for a p-type bulk since the carrier
mobility of holes is lower than for electrons (450 vs 1350 cm2/ V·s)
Vd = d2 /(2)
w = 2Vb
where = 1/ q N for doped material where N is the doping concentration and q is the charge of the electron and is the carrier mobility (v= E)
05/11/2006 CDF Silicon Workshop at UCSB 20
C = A / 2Vb
Properties of the Depletion Zone (cont’d)
– One normally measures the depletion behavior (finds the depletion voltage) by measuring the capacitance versus reverse bias voltage. The capacitance is simply the parallel plate capacity of the depletion zone.
capacitance vs voltage1/C2 vs voltage
Vd
05/11/2006 CDF Silicon Workshop at UCSB 2121
Leakage Current- Two main sources of (unwanted) current
flow in reversed-biased diode:– Diffusion current, charge generated in
undepleted zone adjacent to depletion zone diffuses into depletion zone (otherwise would quickly recombine)
– Generation current Jg, charge generated in depletion zone by defects/contaminants
negligible in a fully depleted device
Jg exp(-b/kT)
Exponential dependence on temperature due to thermal dependence of e-h pair creation by defects in bulk. Rate is determined by nature and concentration of defects.
05/11/2006 CDF Silicon Workshop at UCSB 22
Bias Resistor and AC Coupling
– Need to isolate strips from each other and collect/measure charge on each strip => high impedance bias connection (resistor or equivalent)
– Usually want to AC (capacitatively) couple input amplifier to avoid large DC input from leakage current.
– Both of these structures are often integrated directly on the silicon sensor. Bias resistors via deposition of doped polycrystalline silicon, and capacitors via metal readout lines over the implants but separated by an insulating dielectric layer (SiO2 , Si3N4).
+
–h+ e-
05/11/2006 CDF Silicon Workshop at UCSB 23
• Collected charge usually given for Minimum Ionizing Particle (MIP)
dE/dx)Si = 3.88 MeV/cm, for 300 m thick = 116 keVThis is mean loss, for silicon detectors use most probable loss (0.7 mean) = 81 keV
3.6eV needed to make e-h pairCollected charge 22500 e (=3.6 fC)
Mean charge
Most probable charge ≈ 0.7 x mean
The Charge Signal
05/11/2006 CDF Silicon Workshop at UCSB 24
Landau distribution has significant low energy tail which becomes even lower with noise broadening.
Noise sources:o Capacitance ENC ~ Cd
o Leakage Current ENC ~ √ Io Thermal Noise ENC ~ √( kT/R)
Landau distribution
with noise
noise distribution
One usually has low occupancy in silicon sensors most channels have no signal. Don’t want noise to produce fake hits so need to cut high above noise tail to define good hits. But if too high you lose efficiency for real signals.
Figure of Merit: Signal-to-Noise Ratio S/N.
Typical Values ~ 10-15, people get nervous below 10. Radiation Damage can degrade the S/N. Thus S/N determines detector lifetime in radiation environment.
But There Is Noizzzzzz …..
05/11/2006 CDF Silicon Workshop at UCSB 25
– Diffusion of charge “cloud” caused by scattering of drifting charge carriers, radius of distribution after time td:
Charge Collection and Diffusion
– Drift velocity of charge carriers v = E, so drift time,td = d/v = d/E
= 2D td , where D is the diffusion constant, D = kT/q
– Typical charge radius: ≈ 6 m
– Charge Radius determines ‘Charge Sharing’, i.e. deposition of charge on several strips.
05/11/2006 CDF Silicon Workshop at UCSB 26
pp p
n BUT: Unlike the face with the p-strips, nothing prevents
horizontal charge spread on back face. n-strips alone are not sufficient to isolate the charge because of an electron accumulation layer produced by the positively charged SiO2 layer on the surface.
Why not get a 2nd coordinate by measuring position of the (electron) charge collected on the opposite face? p+p+ p+
n n n
n-bulk
n n n
n-bulk
p+ p+
n n n
n-bulk
+ + +
SOLUTION: • Put p-strips in between the n-strips.OR• Put “field plates” (metal over oxide) over the n-strips and apply a potential to repel the electrons.
Double Sided Detectors
05/11/2006 CDF Silicon Workshop at UCSB 2727
– Voltage drop between biasing ring and edge, top edge at backplane voltage.
– Typically n-type implants put around edge of the device and a proper distance maintained between p bias ring and edge ring.
– Usually one or more “guard” rings (left floating) to assure continuous potential drop over this region.
– Defects or oxide charge build-up in this region could lead to additional leakage current contributions
– If one increases the bias voltage, eventually the field is high enough to initiate avalanche multiplication. This usually occurs around 30V/m (compared to a typical operating field of <1V/m). Local defects and inhomogeneities could result in fields approaching the breakdown point.
– .
We have treated the silicon strip device as having infinite area, but it has edges. What happens at the edges? Single guard ring structure
Guard Rings and Avalanche Breakdown
05/11/2006 CDF Silicon Workshop at UCSB 28
Some Technicalities ……
05/11/2006 CDF Silicon Workshop at UCSB 29
Using a single Si crystal seed, meltthe vertically oriented rod onto the seed using RF power and “pull” the single crystal ingot
Wafer production Slicing, lapping, etching, polishing
Mono-crystalline Ingot
Single crystal silicon
Poly silicon rod
RF Heating coil
Float Zone process
Oxygen enrichment (DOFZ) Oxidation of wafer at high temperatures
o Pull Si-crystal from a Si-melt contained in a silica crucible while rotating.
o Silica crucible is dissolving oxygen into the melt high concentration of O in CZ
o Material used by IC industry (cheap) o Recent developments (~2 years) made CZ
available in sufficiently high purity (resistivity) to allow for use as particle detector.
Czochralski Growth
Czochralski silicon
Epitaxial silicono Chemical-Vapor Deposition (CVD) of Silicono CZ silicon substrate used in-diffusion of oxygeno growth rate about 1m/mino excellent homogeneity of resistivityo up to 150 m thick layers producedo price depending on thickness of epi-layer but not
extending ~ 3 x price of FZ wafer
05/11/2006 CDF Silicon Workshop at UCSB 31
n-Si1) Start with n-doped silicon wafer, ≈ 1-10 kcm
2)
SiO2
Oxidation at 800 – 1200 0C
3) Photolithography (= mask align + photo-resist layer + developing) followed by etching to make windows in oxide
UV light
maskPhoto-resist
etch
Wafer Processing (1)
05/11/2006 CDF Silicon Workshop at UCSB 32
4)
5)
B
Doping by ion implantation (or by diffusion)
As
Annealing (healing of crystal lattice) at 600 0Cp+
n+
p+
6) Photolithography followed by Al metallizationover implanted strips and over backplane usually by evaporation.
Al
Simple DC-coupled silicon strip detector
Wafer Processing (2)
05/11/2006 CDF Silicon Workshop at UCSB 33
Bringing It All Together• Connectivity technology: some of the possibilities
– High density interconnects (HDI):industry standard and custom cables, usually flexible kapton/copper with miniature connectors.
– Soldering still standard for surface mount components, packaged chips and some cables. Conductive adhesives are often a viable low temperature alternative, especially for delicate substrates.
– Wire bonding: the standard method for connecting sensors to each other and to the front-end chips. Usually employed for all connections of the front-end chips and bare die ASICs. A “mature” technology (has been around for about 40 years).
4 x 640 wire bonds~200 wire bonds Total ~2700 wire bondsOPAL (LEP) module
05/11/2006 CDF Silicon Workshop at UCSB 34
Wire Bonding• Uses ultrasonic power to vibrate needle-
like tool on top of wire. Friction welds wire to metallized substrate underneath.
• Can easily handle 80m pitch in a single row and 40m in two staggered rows (typical FE chip input pitch is 44m).
• Generally use 25m diameter aluminium wire and bond to aluminium pads (chips) or gold pads (hybrid substrates).
• Heavily used in industry (PC processors) but not with such thin wire or small pitch.
Microscope view of wire bonds connecting sensor to fan-out circuit
Electron micrograph of bond “foot”
05/11/2006 CDF Silicon Workshop at UCSB 35
Radiation Damage …
05/11/2006 CDF Silicon Workshop at UCSB 36
Radiation Damage in Silicon SensorsTwo general types of radiation damage to the detector materials:
Bulk (Crystal) damage due to Non Ionizing Energy Loss (NIEL) - displacement damage, built up of crystal defects –
I. Change of effective doping concentration (higher depletion voltage, under- depletion)
II. Increase of leakage current (increase of shot noise, thermal runaway)
III. Increase of charge carrier trapping (loss of charge)
Surface damage due to Ionizing Energy Loss (IEL) - accumulation of positive in the oxide (SiO2) and the Si/SiO2 interface – affects: interstrip capacitance (noise factor), breakdown behavior, …
Impact on detector performance and Charge Collection Efficiency (depending on detector type and geometry and readout electronics!)
Sensors can fail from radiation damage by virtue of…– Noise too high to effectively operate – Depletion voltage too high to deplete– Loss of inter-strip isolation (charge spreading)
Signal/Noise Ratio is the quantity to watch !
05/11/2006 CDF Silicon Workshop at UCSB 37
Run I Experience: SVX’ Signal-to-Noise
Radiation Damage limits the ultimate lifetime of the Detector-Need S/N >8 to perform online b-tagging with SVT-Need S/N >5 for offline b-tagging
05/11/2006 CDF Silicon Workshop at UCSB 38
Surface Damageo Surface damage generation over time:
– Ionizing radiation creates electron/hole pairs in the SiO2
– Many recombine, electrons migrate quickly away– Holes slowly migrate to Si/SiO2 interface.
Hole mobility is much lower than for electrons (20 cm2/Vs vs. 2x105 cm2/Vs)
– Some holes ‘stick’ in the boundary layero Surface damage results in
– Increased interface trapped charge (see picture)– Increase in fixed oxide charges– Surface generation centers
Metal (Al)Oxide (SiO2)
Semiconductor (Si)
-- --
--
- --
++
+++
+
+
++
+
--- -
--++ +
++
++ -
+
+
++ +
+
+
+ +
+ + + +
+
After electrontransport:
After transportof the holes:
– Electron accumulation under the oxide interface can alter the depletion voltage (depends on oxide quality and sensor geometry)
– In silicon strip sensors, surface damage effects (oxide charge) saturate at a few hundred kRad
Interface (SiOx)
05/11/2006 CDF Silicon Workshop at UCSB 39
Bulk Damage
O
P
Vacancy
Disordered region
Di-vacancy
Interstitial
C
Vacancy/OxygenCenter
CarbonInterstitial
CC
Phosphorous dopant
Carbon-CarbonPair
C O
Carbon-Oxygen pair
o Bulk damage is mainly from hadrons displacing primary lattice atoms (for E > 25 eV)– Results in silicon interstitial, vacancy,
and typically a large disordered region– 1 MeV neutron transfers 60-70 keV to
recoiling silicon atom, which in turn displaces ~1000 additional atoms
o Defects can recombine or migrate through the lattice to form more complex and stable defects– Annealing can be beneficial, but… – Defects can be stable or unstable
o Displacement damage is directly related to the non-ionizing energy loss (NIEL) of the interaction– Varies by incident particle type and
energy– Normalize fluence to 1 MeV n-equivalent
05/11/2006 CDF Silicon Workshop at UCSB 40
Microscopic defects
I
I
V
V
Distribution of vacancies created by a 50 keV Si-ion
in silicon (typical recoil energy for 1 MeV neutrons):
Schematic[Van Lint 1980]
Simulation[M.Huhtinen 2001]
Defects can be electrically active (levels in the band gap) - capture and release electrons and holes from conduction and valence band
can be charged - can be generation/recombination centers - can be trapping centers
Vacancy + Interstitial
“point defects”, mobile in silicon,can react with impurities (O,C,..)
V
I
point defects and clusters of defects
o particle SiS EK>25 eV
EK > 5 keV
Damage to the silicon crystal: Displacement of lattice atoms
Impact on detector properties can be calculated if all defect parameters are known:Impact on detector properties can be calculated if all defect parameters are known:n,pn,p : cross sections : cross sections E : ionization energy NE : ionization energy Ntt : concentration : concentration
Trapping (e and h) CCE
shallow defects do not contribute at room
temperature due to fast detrapping
charged defects
Neff , Vdepe.g. donors in upper
and acceptors in lower half of band
gap
generation leakage current
Levels close to midgap
most effective
enhanced generation leakage current space charge
Inter-center chargeInter-center charge transfer model transfer model
(inside clusters only)(inside clusters only)
Impact of Defects on Detector properties
05/11/2006 CDF Silicon Workshop at UCSB 42
Change of Depletion Voltage Vdep (Neff) …. with particle fluence:
before inversion
after inversion
n+ p+ n+
• “Type inversion”: Neff changes from positive to negative (Space Charge Sign Inversion)
10-1 100 101 102 103
eq [ 1012 cm-2 ]
1
510
50100
5001000
5000
Ude
p [V
] (d
= 3
00m
)
10-1
100
101
102
103
| Nef
f | [
1011
cm
-3 ]
600 V
1014cm-2
type inversion
n-type "p-type"
[M.Moll: Data: R. Wunstorf, PhD thesis 1992, Uni Hamburg]
• Short term: “Beneficial annealing” • Long term: “Reverse annealing” - time constant depends on temperature: ~ 500 years (-10°C) ~ 500 days ( 20°C) ~ 21 hours ( 60°C) - Consequence: Detectors must be cooled even when the experiment is not running!
…. with time (annealing):
NC
NC0
gC eq
NYNA
1 10 100 1000 10000annealing time at 60oC [min]
0
2
4
6
8
10
N
eff [
1011
cm-3
]
[M.Moll, PhD thesis 1999, Uni Hamburg]
p+
Radiation Damage: Effect on Neff
05/11/2006 CDF Silicon Workshop at UCSB 43
Depletion Voltage: Death of SVX Layer 0
Integrated Luminosity (fb–1)
Dep
letio
n V
olta
ge (V
)Central Prediction+1σ Prediction–1σ Prediction
100
200
300
00 2 4 6 8
Data & Extrapolation
SVXII L0 lifetime prediction based on Hamburg Model (M.Moll)
- Will SVXII L0 survive Run II ? -> Antonio’s Talk
[M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund][M.Moll; Data: O.Krasel, PhD thesis 2004, Uni Dortmund]
where defectsheeff
N,
1
Radiation Damage – Trapping
05/11/2006 CDF Silicon Workshop at UCSB 47
o Two basic mechanisms reduce collectable charge:– trapping of electrons and holes (depending on drift and shaping time !)– under-depletion (depending on detector design and geometry !)
o Example: ATLAS microstrip detectors + fast electronics (25ns)
o n-in-n versus p-in-n - same material, ~ same fluence- over-depletion needed
0 100 200 300 400 500 600bias [volts]
0.20
0.40
0.60
0.80
1.00
CC
E (a
rb. u
nits
)n-in-n (7.1014 23 GeV p/cm2)
n-in-n
p-in-n (6.1014 23 GeV p/cm2)
p-in-n
Laser (1064nm) measurements
[M.Moll: Data: P.Allport et al. NIMA 513 (2003) 84]
o p-in-n : oxygenated versus standard FZ- beta source- 20% charge loss after 5x1014 p/cm2 (23 GeV)
0 1 2 3 4 5p [1014 cm-2]
0
20
40
60
80
100
Q/Q
0 [%
]
oxygenatedstandard
max collected charge (overdepletion)
collected at depletion voltage
M.Moll [Data: P.Allport et all, NIMA 501 (2003) 146]
Decrease of Charge Collection Efficiency
05/11/2006 CDF Silicon Workshop at UCSB 48
Oxygenation Benefits
Michael Moll, IWORID Glasgow 2004• oxygenation increases radiation hardness
• sometimes, standard FZ exhibits similar radiation hardness - reasons unclear
• Concentrate R&D on CZ and EPI silicon
05/11/2006 CDF Silicon Workshop at UCSB 49
Summary• Silicon strip detectors built on simple pn junction principle have become a ‘mature’ technology over 25 years.
• Provide reliable tracking in high density/high rate environment
• Widespread use thanks to cost drop and advances in microelectronic industry
• Silicon Radiation hardness to a few 1015 p/cm2 - radiation hardness frontier > 1016 p/cm2 (SLHC inner pixel layer) - CZ, EPI, new materials/structures?
• Silicon People are fun to work with, outgoing and (usually) in a good mood (eh …)
•
Have fun in California - You deserve it!
05/11/2006 CDF Silicon Workshop at UCSB 50
Backup
05/11/2006 CDF Silicon Workshop at UCSB 51
Property Diamond GaN 4H SiC Si Eg [eV] 5.5 3.39 3.26 1.12 Ebreakdown [V/cm] 107 4·106 2.2·106 3·105 e [cm2/Vs] 1800 1000 800 1450 h [cm2/Vs] 1200 30 115 450 vsat [cm/s] 2.2·107 - 2·107 0.8·107 Z 6 31/7 14/6 14 r 5.7 9.6 9.7 11.9 e-h energy [eV] 13 8.9 7.6-8.4 3.6 Density [g/cm3] 3.515 6.15 3.22 2.33 Displacem. [eV] 43 15 25 13-20
o Wide bandgap (3.3eV) lower leakage current
than silicon
o Signal:Diamond 36 e/mSiC 51 e/mSi 89 e/m
more charge than diamond
o Higher displacement threshold than silicon
radiation harder than silicon (?)
R&D on diamond detectors:RD42 – Collaboration
http://cern.ch/rd42/
Recent review: P.J.Sellin and J.Vaitkus on behalf of RD50 “New materials for radiation hard semiconductor detectors”, submitted to NIMA
Sensor Materials: Diamond, SiC and GaN
05/11/2006 CDF Silicon Workshop at UCSB 52
Microscopic defects
I
I
V
V
Distribution of vacancies created by a 50 keV Si-ion
in silicon (typical recoil energy for 1 MeV neutrons):
Schematic[Van Lint 1980]
Simulation[M.Huhtinen 2001]
Defects can be electrically active (levels in the band gap) - capture and release electrons and holes from conduction and valence band
can be charged - can be generation/recombination centers - can be trapping centers
Vacancy + Interstitial
“point defects”, mobile in silicon,can react with impurities (O,C,..)
V
I
point defects and clusters of defects
o particle SiS EK>25 eV
EK > 5 keV
Damage to the silicon crystal: Displacement of lattice atoms
80 nm
05/11/2006 CDF Silicon Workshop at UCSB 53
Radiation Damage in Silicon
o Two general types of radiation damage– “Bulk” damage due to physical impact within the crystal– “Surface” damage in the oxide or Si/SiO2 interface
o Cumulative effects– Increased leakage current (increased Shot noise)– Silicon bulk type inversion (n-type to p-type)– Increased depletion voltage– Increased capacitance
o Sensors can fail from radiation damage by virtue of…– Noise too high to effectively operate – Depletion voltage too high to deplete– Loss of inter-strip isolation (charge spreading)
o Ratio of signal/noise is the important quantity to watch
Close proximity to the interaction region means the sensors are subject tohigh doses of radiation
05/11/2006 CDF Silicon Workshop at UCSB 54
Depletion voltage is often parameterized in three parts (Hamburg model): Neff(T,t,) = NA + NC + NYo Short term annealing (NA)
NA = eq iga,iexp(-ka,i(T)t)– Reduces NY (beneficial)– Time constant is a few days at 20 C
o Stable component (Nc)Nc = Nc0(1-exp(-ceq))+gceq– Does not anneal (does not depend on time
or temperature)– Partial donor removal (exponential or
limited exponential)– Creation of acceptor sites (linear)
o Long term reverse annealing (NY)NY = NY,∞[1-1/(1+ NY,∞kY(T)t)], NY,∞= gYeq – Strong temperature dependence– 1 year at T=20 C is the same as <1 day at
T=60 C or ~100 years at T= -7 C (ATLAS)– Can be significant long term; must cool Si
Figure 9: Dependence of Neff on the accumulated 1 MeV neutron equivalent fluence for standard and oxygen enriched FZ silicon irradiated with reactor
Fig.13: Annealing behaviour of the radiation induced change in the effective doping concentration Neff at 60C.
05/11/2006 CDF Silicon Workshop at UCSB 55
Bulk Damage – Leakage Currento Defects created by bulk damage provide intermediate states within
the band gap– intermediate states act as ‘stepping stones’ of thermal generation of
electron/hole pairs– Some of these states anneal away; the bulk current reduces with time
(and temperature) after irradiation o Annealing function (t)
– Parameterized by the sum of several exponentials iexp(-t/i)– Full annealing (for the example below) reached after ~1 year at 20ºC– At low temperatures, annealing effectively stops
– Depletion voltage is often parameterized in three parts:
• Short term annealing (Na)• A stable component (Nc)• Long term reverse annealing (NY)
P N
P N
Conc
ent ra
tion
(log
scal
e)Co
ncen
t ratio
n(lo
g sc
ale)
Small difference: small depletion V
Large area: large current
Large difference: Large depletion V
Small area: small current
Before Irradiation:
After Irradiation:
05/11/2006 CDF Silicon Workshop at UCSB 57
Bulk Damage – Leakage Resultso Measured values of (t)
– Typically one quotes measured values of (t) after complete annealing at T=20ºC: ∞ = (t=∞)
– Some typical ‘world averages’ for ∞ are • 2.2 x 10-17 A/cm3 for protons, pions• 2.9 x 10-17 A/cm3 for neutrons
– Recent results show (t=80min,T=60ºC) = 4.0 x 10-17 A/cm3 for all types of silicon, levels of impurities, and incident particle types (NIM A426 (1999)86).
parameterisation for standard silicon parameterisation for standard silicon
Fig. 7.: Fluence dependence of leakage current for detectors produced by various process technologies from different silicon materials. The current was measured after a heat treatment for
80 min at 60C [14].
Fig.8: Current related damage rate as function of cumulated annealing time at 60C. Comparison between data obtained for
oxygen diffused silicon and parameterisation given in Ref. [14].
05/11/2006 CDF Silicon Workshop at UCSB 58
Rainer Wallny
Silicon Detector Workshop at UCSBMay 11th, 2006
Slides ruthlessly stolen from: Paula Collins, CERN Alan Honma, CERN Christian Joram, CERN Michael Moll, CERN Steve Worm, RAL