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1SNT5039 Nano Processing (Etching Technology)
1. PLASMA ETCHING FUNDAMENTALS- PLASMA ETCHING MECHANISMS-
ETCHING PROCESS VARIABLES- ETCHING PROCESS REQUIREMENTS
2. ISSUES OF ETCHING FOR NANO-PROCESSING- PLASMA ETCHING MODELS-
CHAGE UP DAMAGE- MICROLOADING
PART III: ETCHING TECHNOLOGY
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Difference between Plasma Etching and Wet Etching
PLASMA ETCHING MECHANISMS
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PLASMA ETCHING MECHANISMS
Four Different Plasma Etching Mechanisms
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Sputtering:- Positive ions are accelerated across
the sheath with high kinetic energy.- Directional but has low
selectivity.
Chemical etching: - Active species from the gas phase
encounter the surface and react to form volatile product.
- Non-volatile reaction product would remain on the surface and
impede further etching.
- Non-directional but can have high selectivity.
PLASMA ETCHING MECHANISMS
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Ion-enhanced directional etching:- Where neutral species cause
little etching, ion bombardment can make the substrate more
reactive, the effects of radicals and ions can be synergistic to
enhance etching rate.
- Ions accelerated across the plasma sheath edge strike the
surface vertically with kinetic energy, causing directional
etching.
Inhibitor-driven ion-assisted etching:- Etching by neutral
radicals is spontaneous so ion bombardment does not cause etching
reaction.
- Ions sputter off substrate materials to form inhibitor-films
on the sidewall, resulting in anisotropic etching.
PLASMA ETCHING MECHANISMS
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PLASMA ETCHING MECHANISMS
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Plasma parameters
Plasma properties(Collision)
- RF power (source power, bias power)- Frequency of power-
Pressure- Gas combination- Gas flow rate- Temperature
- Energy of particles (Te, Ti)- Particle density (n, n*, np)-
Ionization and dissociation ratios- Ion energy (Ei)- Electron
energy distribution
Etching properties
-Etching rate-Uniformity- Selectivity-Anisotropy-Loading
(microloading)- Plasma induced damage
ETCHING PROCESS VARIABLES
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Electron-Atom CollisionsElastic scattering: e + Ar e +
ArIonization: e + Ar 2e + Ar+Excitation: e + Ar e + Ar*
Ion-Atom CollisionsElastic collision: Ar+ + Ar Ar+ + ArResonant
charge exchange: Ar+ + Ar Ar + Ar+Non-resonant charge exchange: B+
+ C B + C+
Radiative relaxation: Ar* Ar + photon (Used for OES)
Recombination: e + Ar+ Ar*** e + Ar+ + Ar Ar + Ar*** e + Ar+ +
wall Ar + wall
Q) Why are the last two reactions are dominant in the plasma,
rather than direct recombination ?
ETCHING PROCESS VARIABLES
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Pressure affects the following properties
(1) Sheath potential and ion energy bombarding the surface(2)
Electron temperature and electron energy distribution(3) Ionization
ratio and collision frequency (4) Flux of ions and radical species
to the surface(5) Surface coverage and chemical reaction rate(6)
Mass transport rate
(* Pressure range in processing plasma: 1mTorr to 10Torr)
Physical sputtering
Chemical etching
Ion-assisted etching
Pressure
Ion energy
ETCHING PROCESS VARIABLES (PRESSURE EFFECTS)
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- Electron energy and ionization rate tend to decrease with
increasing pressure, and this results in less efficiency to
regenerate the charge lost by diffusion and recombination.-
However, electron density (plasma density) does not change very
much due to the compensating effect between the larger number of
neutral reactants and lower ionization rate at higher pressure.-
That is, plasma density (1010 1011 cm-3) tends to be insensitive to
the pressure change in the low pressure processing plasmas in the
range of 1mTorr 1 Torr.- In general, the neutral gas temperature
tends to increase with pressure whereas the electron temperature
deceases. This is due to the thermalization of electrons (energy
transfer from electrons to neutrals).
ETCHING PROCESS VARIABLES (PRESSURE EFFECT)
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- The ratio of neutrals to ions is higher at high pressure
(surface flux of neutrals is much larger). In high pressures,
etching is mainly performed chemically. Isotropic etching profile
and undercutting are common phenomena observed in the chemical
etching.
- The rate of chemical etching tends to be proportional to the
concentration of neutral etchant species (first order reaction
rate), whereas the rate of ion-enhanced etching tends to be
independent of the concentration of neutral etchant species (zero
order reaction rate).
- Lower pressure brings about the increase of sheath voltage and
ion bombardment energy.
ETCHING PROCESS VARIABLES (PRESSURE EFFECT)
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In the case of elementary reactions: A + B = products.
In this case, reaction rate can be expressed as:(product
formation)/(time) = k(T) nAnB (ni =xip)
Effects of wall on pressure dependenceMany cases, heterogeneous
reaction on the wall is
more efficient, since it can frequently be first
orderreaction:
A + B + wall = AB + wall.In this case, reaction rate can be
expressed as:
(product formation)/(time) = k(T) nAAw
PRESSURE EFFECT IN CHEMICAL KINETICS
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Overall reactionEffective Pressure
Dependence
F + Sisurf = SiF p
CF3 + F = CF4 p2
CF2 + F2 = CF3 + F p2
CF2 + CF2 = C2F4 p2
3CF2 = C3F6 p3
ETCHING PROCESS VARIABLES (PRESSURE)
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QAR
xQxARx
andQAR
xQxARx
seseSiO
seseSi
619
14
519
14
1021.81068.2
102.2
1087.11068.2
105
2
moleculesCFoffluxInputatomsSioffluxEtch
4
Utilization factor
Flow rate effect (low flow rate regime)
: For Si
: For SiO2
where Re = etch rate (/min), As = substrate area, Q = flow rate
(sccm).
Q) In the CF4/O2 plasma where the single 8-inch silicon wafer
has an opening ratio of 50% and its etching rate is 3000A/min with
CF4gas flow of 25sccm,1) Estimate utilization factor of CF4
molecules for Si etching (0.36)2) Estimate a maximum possible
etching rate (8300A/min)
ETCHING PROCESS VARIABLES (FLOW RATE)
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Etch rate Limited by Residence time
Overall etching rate
100% utilization line
Gas flow rate
Overall flow rate effect- When pumping speed or flow rate
exceeds to a certain point, residence
time of reactive gases becomes too short to etch the substrate.
- Short residence time can lower the utilization factor. This can
be
minimized by increasing RF power to increase the generation rate
of reactive species
- Overall etching rate will be determined by lack of reactant
gases at the low flow rate regime and by insufficient residence
time of reactant gases at the high flow rate regime.
ETCHING PROCESS VARIABLES (FLOW RATE)
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If utilization factor is high, reaction products can affect gas
composition of the etching plasma. This is often observed in the
low flow regime. The following figures show consumption of the
etching gas under the condition with high utilization factor.
Q) Explain the effect of increasing the power in both
graphs?
ETCHING PROCESS VARIABLES (FLOW RATE)
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Areas that RF frequency can influence to plasma and etching
properties
(1) Spatial distribution of species and electrical fields across
the discharge(2) Energy distribution of species (whether it is
constant or oscillating withtime)(3) Minimum voltage required to
start and operate a plasma and theenergy with which ions bombard
the surface(4) Electron energy distribution function (EEDF)
- If wt > 1, the process is too slow to respond and it
reachesa static state in equilibrium with the time average
conditions.(w is input power frequency and t is relaxation
time)
ETCHING PROCESS VARIABLES (FREQUENCY)
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ETCHING PROCESS VARIABLES (FREQUENCY)
V = 109V (V N)160MHzMomentum CollisionFrequency
experimentalVu(Vu N)10MHzElectron EnergyModulation
0.3Torr/v+, aveLitf ~ 200KHzUitf ~ 6MHz
Ion-SheathTransit
Kex = 10-10 10-9N = 1016cm-3
Kex ni N160KHz 1.6MHzCharge Exchange
Ka = 1.510-9cm3sec-1N = 1016cm-3Ka = 310-10cm3sec-1N =
1016cm-3
Ka ne N2.5MHz (0.08eV)
500KHz (3eV)
Attachment
L = 2cm(2/L2)(v/3)ne50KHz (1eV)2MHz (3ev)
FreeDiffusion
D = 250cm2sec-1L = 2cm[Cl] ~ 51015cm-3
2D[Cl] / L2100HzAtom-AtomRecombination(Heterogeneous)
Kri = 510-8cm3sec-1
n+ n- = 31010cm-3Kri n+ n-250HzIon-Ion
Recombination
Kei 10-7 cm3sec-1Kei ni ne500HzElectron-IonRecombination
ne = 31010Wp = (neoe2/moeo)1/21.55GHzPlasma Frequency
L = 2cmDa ~ 2103cm2sec-1
(2Da / L2)ne800HzAmbipolarDiffusion
Kra = 3.4710-32cm-6sec-1[Cl] ~ 51015cm-3
Kra[Cl]2N0.3HzAtom-AtomRecombination(Homogeneous)
ParametersAssumed
Rate Expression(dn/dt)
Approx. Frequency[d(2logn)/dt]-1
Process
V = 109V (V N)160MHzMomentum CollisionFrequency
experimentalVu(Vu N)10MHzElectron EnergyModulation
0.3Torr/v+, aveLitf ~ 200KHzUitf ~ 6MHz
Ion-SheathTransit
Kex = 10-10 10-9N = 1016cm-3
Kex ni N160KHz 1.6MHzCharge Exchange
Ka = 1.510-9cm3sec-1N = 1016cm-3Ka = 310-10cm3sec-1N =
1016cm-3
Ka ne N2.5MHz (0.08eV)
500KHz (3eV)
Attachment
L = 2cm(2/L2)(v/3)ne50KHz (1eV)2MHz (3ev)
FreeDiffusion
D = 250cm2sec-1L = 2cm[Cl] ~ 51015cm-3
2D[Cl] / L2100HzAtom-AtomRecombination(Heterogeneous)
Kri = 510-8cm3sec-1
n+ n- = 31010cm-3Kri n+ n-250HzIon-Ion
Recombination
Kei 10-7 cm3sec-1Kei ni ne500HzElectron-IonRecombination
ne = 31010Wp = (neoe2/moeo)1/21.55GHzPlasma Frequency
L = 2cmDa ~ 2103cm2sec-1
(2Da / L2)ne800HzAmbipolarDiffusion
Kra = 3.4710-32cm-6sec-1[Cl] ~ 51015cm-3
Kra[Cl]2N0.3HzAtom-AtomRecombination(Homogeneous)
ParametersAssumed
Rate Expression(dn/dt)
Approx. Frequency[d(2logn)/dt]-1
Process
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19SNT5039 Nano Processing (Etching Technology)
1) When excitation frequency is significantly higher than the
ion transit frequency,ions are accelerated toward the electrode
during negative half cycle, and the fastest ions with transit times
lasting over many cycles are accelerated by the sheath electric
field, E(x,t), to an energy,
Since the excitation voltage is, Vo sin wt, is applied to the
sheath,
During half cycle,
2) When excitation frequency is significantly lower than the ion
transit frequency,
n THF s dxdttxeE
nTeV
10
2/
0max),(1
tVdxtxEs
sin),(0 0
00
0max
2sin2
VtdtVeeV HF
0max VVLF
FREQUENCY EFFECT: ION TRANSIT FREQUENCY (ITF)
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20SNT5039 Nano Processing (Etching Technology)
FREQUENCY EFFECT: ELECTRON ENERGY OSCILLATION
- While electrons in a dischargelose only a small energy
duringcollisions with neutrals, gasmolecules translate energy
tosurroundings. Therefore electrontemperatures are much higherthan
gas temperature.
- The diagram shows electronenergy as a function of
excitationtime for different ratios of electronenergy loss
frequency (nu) vsexcitation frequency (w).
1) When nu > w
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21SNT5039 Nano Processing (Etching Technology)
We should be able to differentiate the gas temperature from the
surfacetemperature. The gas temperature is not easily controllable
because itdepends on power input and heat transfer.
Instead, the surface temperature is more practically used,
because in theprocessing plasmas the thermal boundary layer (the
distance in which thegas temperature is maintained close to the
wall temperature due to heattransfer) is much thicker than the mean
free path.
The etching properties affected by the temperature can be
chemicalreaction rates, selectivity, surface morphology, and
degradation of photo-resist.
When the etching rates are controlled by the reaction which is a
functionof the temperature, it can be represented as Arrhenius
Equation in the formof )/exp( kTEAR ae
ETCHING PROCESS VARIABLES (TEMPERATURE)
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Q) From the below Arrhenius plotof Si and SiO2 etching
reactionsin a fluorine containing plasma,
(a) Estimate activation energyfor overall etching reactions of
Siand SiO2
(b) Find complete Arrheniusequations, and
(c) Plot the etching selectivitybetween Si and SiO2 as afunction
of the temperature )/exp( kTEAR ae
ETCHING PROCESS VARIABLES (TEMPERATURE)
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Q) Based on these results, explain why fluorine-based gases are
preferred to chlorine-based gases for Si etching?
ETCHING PROCESS VARIABLES (TEMPERATURE AND VOLATILITY)
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ETCHING PROCESS VARIABLES (WALL EFFECT)
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Plasma parameters
Plasma properties (Collision)
- RF power (source power, bias power)- Frequency of power-
Pressure- Gas combination- Gas flow rate- Temperature
- Energy of particles (Te, Ti)- Particle density (n, n*, np)-
Ionization and dissociation ratios- Ion energy (Ei)- Electron
energy distribution
Etching properties
-Etching rate-Uniformity- Selectivity-Anisotropy-Loading
(microloading)- Plasma induced damage
ETCHING PROCESS VARIABLES
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Etching process is normally characterized by etching rate,
selectivity,uniformity, and surface quality.
- Etch rate: High etch rate results in high throughput.
- Anisotropy: Most cases of IC manufacturing, vertical
andanisotropic profiles are required. Some cases, sloped profile
isnecessary to guarantee adequate step coverage during
subsequentdeposition.
- Surface quality: Undesirable combination of substrates and
gasescan result in pitting or unwanted surface residue that can
degradesmoothness of surface. This problem becomes more severe
asdevice size becomes smaller and new materials are introduced
foradvanced devices.
ETCHING PROCESS REQUIREMENTS
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Etch rate (Re)
- Overetch ratio =(Additional etch time / target etch time) x
100%
- Uniformity = [(Rmax Rmin) / (2Rave)] x 100%
- Anisotropy = 1- Rh/Rv
- Selectivity = Rf/Rs
ETCHING PROCESS REQUIREMENTS
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SELECTIVITY REQUIREMENT
Selectivity Analysis:Selectivity is the etching rate ratio
between two neighboring materials such as a) film versus mask and
b) film versus underlying layer.
Si-substrate
SiO2 thin oxide
Poly-Si gate
MaskD/2
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SELECTIVITY REQUIREMENT
-The vertical resist profile withthe angle of 90 degrees
iscommonly obtained in the ICprocesses, from the
advancedlithography machines using193nm-365nm wavelengths.
-However, it can be lowered bybaking processes beforeetching to
70-80 degrees.
- Substrate-to-mask selectivityrequirements become lessstringent
with more anisotropicetching and a vertical mask-edge profile.
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The figure shows process uniformityas a function of the
selectivity foretching poly-silicon on the underlying400A gate
oxide. The cross-hatcheddamage region indicates theunacceptable
region in which morethan 100A oxide is removed. Withimproved
uniformity, processselectivity requirement can berelaxed. The
numbers in the mapshows the amounts of oxide removed.The results
show that, the higher thepoly-silicon to oxide selectivity, theless
the gate oxide removed. Also, ifthe process uniformity is poor,
higherselectivity is necessary to ensure theremaining oxide thicker
than theminimum requirement.
SELECTIVITY REQUIREMENT (TRADE-OFF WITH UNIFORMITY)
Si-substrate
SiO2 thin oxide
Poly-Si gate
Si-substrate
SiO2 thin oxide
Poly-Si gate
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(a) In practical etching applications, there always exist
non-uniformities in the filmthickness and in the etching rate
across the wafer. We can define the uniformity offilm thickness ,
the uniformity of film thickness , and etch rate of the film Rf.
Whenthe etching is carried out, we can define the time to take for
all the films to be cleared,tc. Considering other factors to affect
device properties, the total etching time, ttot,usually include
some over-etching,
(b) Since the etching selectivity is required to prevent the
under-layer from beingattacked (For example, think of the
conventional gate structure where the thin gateoxide is between the
poly-silicon gate conductor and the Si substrate.), the time totake
to expose the substrate first, tmin, needs to be considered
(c) If the maximum allowable consumption of thin underlayer, ys
max, was alreadydetermined, it can be expressed as a function of
its etching rate Rs,ave. In this way,minimum selectivity
requirement of film to substrate, Sf/s,min, is determined
SELECTIVITY ANALYSIS DURING ETCHING
Selectivity is the etching rate ratio between two neighboring
materials such as a) substrate versus mask and b) film versus
underlying layer
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32SNT5039 Nano Processing (Etching Technology)
- Due to non-uniformity in thicknesses and etch rates across the
wafer
)1()1)(1(
)1(,
,
avef
avefctot R
ytt
)1()1(
,
,min
avef
avef
Ry
t
)1()1(
)1()1)(1()(
,
,min,max,
avef
avefstotavess R
yRttRy
)1()1(
)1()1)(1(/
max,
,,,min,/
s
avefavesavefsf y
yRRS
ETCHING PROCESS REQUIREMENTS (SELECTIVITY)
Si-substrate
SiO2 thin oxide
Poly-Si gate
Si-substrate
SiO2 thin oxide
Poly-Si gate
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Q1) Using the given equation, estimate minimum etching
selectivity for poly-silicon to gate oxide in the following gate
structure and processing condition, and also estimate the etch rate
of gate oxide.
a) average thickness of poly-silicon gate is 2000Ab) thickness
uniformity of poly-silicon gate is 5%c) average etch rate of
poly-silicon is 4000A/mind) etch rate uniformity of poly-silicon
gate is 7%e) over-etching toward the gate oxide is 100%, and f)
allowable oxide consumption is 50A
(Ans: Selectivity is 55 and etch rate is 73A/min)Q2) Compare the
results of Q1 with the results of in the following cases.
a) average thickness of poly-silicon gate changes to 1000Ab)
average etch rate of poly-silicon changes to 3000A/minc)
over-etching toward the gate oxide changes to 50%d) allowable oxide
consumption changes to 30A
)1()1(
)1()1)(1(/
max,
,,,min,/
s
avefavesavefsf y
yRRS
ETCHING PROCESS REQUIREMENTS (SELECTIVITY)
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Metalelectrode
Si wafer
Selectivity(Channel/SiO2) SiO2
Channel
Selectivity (Metal/Channel)
Photoresist
Selectivity(PR/Metal)
SELECTIVITY REQUIREMENTS IN GATE STACK ETCHING
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Where etching rate is controlled by the transport of etchant
gases. ie, the amount of available gases on the wafer surface, the
etching rate decreases as the open area on the wafer surface
increases. This is known as Loading Effect.
ETCHING PROCESS REQUIREMENTS (LOADING)
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1. PLASMA ETCHING FUNDAMENTALS- PLASMA ETCHING MECHANISMS-
ETCHING PROCESS VARIABLES- ETCHING PROCESS REQUIREMENTS
2. ISSUES OF ETCHING FOR NANO-PROCESSING- PLASMA ETCHING MODELS-
CHAGE UP DAMAGE- MICROLOADING
ETCHING TECHNOLOGY
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A. Ionization: e + Cl, Cl2 Cl+, Cl2+ + 2e
B. Dissociation: e + Cl2 2Cl + 2e
C. Adsorption: Cl, Cl2 Sisurf-nCl
D. Product formation: Si-nCl SiClx (adsorbed)
E. Product desorption: SiClx (adsorbed) SiClx (gas)
PLASMA ETCHING MODELS
Various steps of ion-assisted etching of silicon in chlorine
plasmas are illustrated as follows:
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Si + CF4 = SiF4 + C
SiO2 + CF4 = SiF4 + CO2
H + F = HF
C + O2 = CO or CO2
Q) Is CF4 reactive ?
PLASMA ETCHING MODELS (F/C)
- F for etching and C for polymerization
F/C ratio = ?
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39SNT5039 Nano Processing (Etching Technology)
(1) Increasing the F/C ratio increases Si etching rates and
decreasing the F/C ratio lowers them. The etching rates of SiO2,
Si3N4, Ti, and W are affected similarly by the F/C ratio.
(2) F/C ratio of CF4 gas is four. As the etching of Si is
carried out and the etch products SiF4 are generated, and F/C ratio
decreases.
(3) Addition of H2 to CF4 forms HF, thereby the F/C ratio and
the etching rate are reduced. This effect is also observed for CHF3
and C3F8 where the F/C ratio is lower than 4.
(4) Addition of O2 can increase the F/C ratio, because oxygen
tends to consume more carbon than fluorine by forming CO or
CO2.
PLASMA ETCHING MODELS (F/C)
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PLASMA ETCHING MODELS (F/C)
The high selectivity of etching ratesof SiO2 versus Si in CF4+H2
plasmascan be explained by the F/C model
- Although the F/C ratio is less than 4due to the formation of
HF, SiO2 cancompensate for the decreased F/Cratio since it
generates oxygen
- Etching rates of SiO2 do notdecrease significantly with
theaddition of H2
- Etching rate of Si is decreasedsignificantly as the F/C ratio
islowered with the addition of H2
Si-substrate
SiO2
Mask
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41SNT5039 Nano Processing (Etching Technology)
PLASMA ETCHING MODELS (F/C)
- The F/C ratio can also be used to differentiate the process
regimes between etching and polymerization.
- In reactive ion etching, the self-bias voltage enhances ion
bombardment at the surface and thereby promotes the removal of
nonvolatile etch products.
- This enlarges the possible etching process regime towards
lower F/C ratios.
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PLASMA ETCHING MODELS (ETCHANT-UNSATURATE MODEL)
(1) Halocarbons and their mixtures with oxidants are widely used
for plasma etching applications. Unsaturated halocarbon radicals
generated from the gas usually become sidewall inhibitors for
anisotropic etching.
(2) In the halocarbon plasmas, balance between unsaturated
species and etchant/oxidant atoms is required. Etchant-unsaturate
model can be a guide in predicting the effects of the plasma
composition.
(3) Unsaturated fluoro- and chloro-carbon polymers that are
generated in the plasmas can be saturated during reactions with
atoms and reactive molecules. The most reactive species are
preferentially removed by the saturation reactions.
- Continued
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43SNT5039 Nano Processing (Etching Technology)
(4) Relative reactivity of atoms and molecules in the saturation
reactions are F~O > Cl > Br and F2 > Cl2 > Br2. This
order is used to predict the predominant etching species and
reaction products for given gas compositions.
O, O2 + CxF2x COF2, CO, CO2 + F, F2O, O2 + CxF2x-yCly COF2,
COFCl, CO, CO2 + F, F2, Cl, Cl2
(5) When atoms can react with the substrate to form volatile
products, etching can occur. When excessive unsaturates adsorb on
the surface, films are formed via polymerization. The unsaturates
also form sidewall films, resulting in anisotropic etching with the
help of ion bombardment.
(6) Addition of oxidants to the plasmas changes the
concentration of halogen atoms and unsaturates. As the oxidants
consume unsaturates, the relative concentration of less reactive
halogen atoms increases.
PLASMA ETCHING MODELS (ETCHANT-UNSATURATE MODEL)
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O, O2 + CxF2x COF2, CO, CO2 + F, F2
O, O2 + CxF2x-yCly COF2, COFCl, CO, CO2 + F, F2, Cl, Cl2
PLASMA ETCHING MODELS (ETCHANT-UNSATURATE MODEL)
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GAS ADDITIVES IN ETCHING
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Charge-up Model of Poly-Si GateNOTCH FORMATION: PROFILE CONTROL
OF POLY-SI ETCHING
-Sidewall facing the open area is irradiated by electrons, and
therefore the potential of the edge line is lower than that of the
other inner lines in the normal array structure. However, the
recombination between ions and electrons occurs at the inner
sidewall of the edge line pattern. This induces the notching at the
edge lines.
-Extent of notching is dependant on the electron flux to the
side wall facing the open area, electron temperature and ion
current density towards the wafer surface.
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47SNT5039 Nano Processing (Etching Technology)
Devices can be damaged by plasma-generated charges trapped in
thin gate oxides, and can show leaky behavior of thin gate oxides
and notching in the poly-silicon gates. Also this causes
non-uniform breakdown of gate oxides and non-uniform conductivity
of gates.
Electron ShadingPlasma (or topography) non-uniformity
PLASMA INDUCED DAMAGE
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Positive ions, accelerated by the electric field of the plasma
sheath region,bombard the wafer surface perpendicularly. Electrons,
repelled by theelectric field of the plasma sheath region, reach
the surface with largeangular distribution.
Electrons are easily trapped at the sidewall of small patterns
(usually< 0.2mm) causing excess positive ions at the bottom of
high aspect ratiopatterns. Hence, notching occurs at the bottom of
poly-Si gate structures.
PLASMA INDUCED DAMAGE (CHARGE-UP)
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A. Oxygen radicals generated in oxygen plasmas can attack
organic photo-resist materials to break polymer chains and form
volatile products CO, CO2, and H2O.
B. Stripping rates of photo-resist increase with the increase of
the oxygen concentration and temperature. Processing temperature is
above the glass transition temperature, Tg.
C. Exposure of device structures to the plasma during
photo-resist stripping can cause electrical damage and charging to
the devices. Downstream plasma systems are widely used to avoid
these problems.
PHOTO-RESIST STRIPPING (ASHING)
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50SNT5039 Nano Processing (Etching Technology)
MICROSCOPIC UNIFORMITY IN ETCHING
MICROLOADING
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51SNT5039 Nano Processing (Etching Technology)
MICROSCOPIC UNIFORMITY IN ETCHING
- It is differently named asRIE-lag,micro-loading,aspect-ratio
dependent etching (ARDE), andpattern dependent etching.
- Defined as a phenomenon where the etching rates in thesmall
patterns are either lower or higher than the etchingrates in the
open area, it can be understood as aphenomenon where the etching
rates change as etching iscarried out and thereby the aspect ratio
of structuresbecomes higher.
MICROLOADING
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52SNT5039 Nano Processing (Etching Technology)
According to the equation, the relative deposition and etching
rates are alsodependent on the flux ratio of ions over
neutrals.
For device structures of the high aspect ratio, the relative
flux ratio betweenions and neutrals is high due to the
directionality of the ions. Thus, theetching dominates the polymer
deposition but the selectivity is low.
In the open area, relative flux ratio of ions over neutrals is
lower comparedto that of the high aspect ratio structures. Here the
deposition dominates theetching, but the selectivity can be
high.
nniii
iiie JSVJEV
JEVR0/1
MICROSCOPIC NON-UNIFORMITY IN ETCHING (MICROLOADING)
Consider a situation where significantpolymeric species are
generated in the plasmaand utilized for anisotropic, selective
etching.For example, consider the etching of SiO2 withSi as an
underlayer in the CHF3 plasmas wherethe etching selectivity is
determined by polymerdeposition and etching of these materials.
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53SNT5039 Nano Processing (Etching Technology)
Ion-Neutral Synergy Effects In Etching
Etching rate from the point of ions
Etching rate from the point of neutrals
Combining both,
nne JSVR )1(0
iiie JEVR
nniii
iiie JSVJEV
JEVR
0/1
where Vi is the volume removed per unit bombardment energy
(cm3/eV)for the saturated surface, is the surface coverage of the
chemicallyassisting neutral species, Ei is the average ion energy
(eV), and Ji is theion flux (cm-2 sec-1) to the surface. Vn is the
volume removed per reactingneutral (cm3), S0 is the sticking
coefficient on the bare surface, and Jn isthe neutral flux (cm-2
sec-1) to the surface.
MICROSCOPIC NON-UNIFORMITY IN ETCHING (MICROLOADING)
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54SNT5039 Nano Processing (Etching Technology)
The etch rate becomes larger when there are appreciable
contributions fromboth ions and neutrals than when there is only
one major contribution eitherfrom neutrals or ions. When the
surface is saturated with neutrals and the etching rate
becomescontrolled by ions: . Also, when the surface is depleted
with neutralsand the etching rate becomes controlled by neutrals: .
At constant ion energy flux, the etching rate increases in
proportion to theneutral flux and then saturates with the further
increase. This is also thesame for the increase of the ion energy
flux.
nniii
iiie JSVJEV
JEVR0/1
Etch
rate
s
Ion energy flux or neutral flux
Ion-neutral synergistic effects can beunderstood by the
equation. That is, theetching rate becomes very small when
theneutral flux is negligible. Also, the etchingrate becomes very
small when the ion energyflux is negligible.
iiie JEVR nne JSVR 0
MICROSCOPIC NON-UNIFORMITY IN ETCHING (MICROLOADING)
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55SNT5039 Nano Processing (Etching Technology)
Mask
Large contact
Small contact
Inverse RIE-lag
SiO2
Mask
Large contact
Small contact
RIE-lag
SiO2
Flux ratio for ions over neutrals
Etch rates of SiO2
Higher A/R structuresLower pressure
MICROSCOPIC NON-UNIFORMITY IN ETCHING (MICROLOADING)
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56SNT5039 Nano Processing (Etching Technology)
For a given pressure and gas composition, the optimum flux ratio
of ions over neutrals can be obtained for a certain aspect
ratio.
(a) At low pressures, the flux ratio of ions over neutrals is so
large that the etching process becomes dominated by neutral flux at
the high aspect ratio structures but not at open areas where
appreciable neutral flux is available. Thus the etching rates at
the open areas can be higher than those at the high aspect ratio
structures.
(b) At high pressures, the flux ratio of ions over neutrals
becomes low and the etching process becomes faster at the high
aspect ratio structures because enough neutral flux is available.
But at open structures, the etching rates become slower due to
polymer deposition that is resulted from the increased neutral
flux. Thus the etching rates at the open structures become lower
than those at the high aspect ratio structures.
Q) Considering (a) and (b), plot qualitatively etch rates as a
function of aspect ratio for cases of low pressure and high
pressure.
MICROSCOPIC NON-UNIFORMITY IN ETCHING (MICROLOADING)