Post on 19-Jan-2021
The University of Michigan on – Visiting Prof. HKU p. 1 S. W. Pang
ELEC 7364 Lecture Notes Summer 2008
Etching
by STELLA W. PANG
from The University of Michigan, Ann Arbor, MI, USA
Visiting Professor at The University of Hong Kong
The University of Michigan on – Visiting Prof. HKU p. 2 S. W. Pang
Etching Requirements Flexibility to Optimize Processes Low Cost and High Throughput System With Low
Downtime Uniform Etching Better Than 5% - Minimize Etch Rate
Dependence on Feature Size, Wafer Size, Etch Depth, Aspect Ratio, Adjacent Features, Position on Wafer
High Selectivity to Mask and Layer Below Good Profile Control to Avoid Undercutting Low Device Damage With Low Ion Energy and Uniform
Plasma Low Particle Generation (<20 0.1-μm Particles/wafer) Environmental Issues to Reduce Chemical Waste
The University of Michigan on – Visiting Prof. HKU p. 3 S. W. Pang
Usually for Surface Cleaning and Complete Removal of a Layer (e.g. Photoresist, Oxide)
Advantages of Wet Etching - Low Cost, Simple System - Highly Selective to Mask and Underlying Layer - Batch Processing With Larger Number of
Wafers (>24) at a Time for High Throughput Disadvantages of Wet Etching
- Isotropic Etch With Undercut Profile - For Small and High Aspect Ratio Features,
Difficult to Get Solvents in and Out, Can Cause Non-uniform Etch
- Need to Provide Waste Treatment for Large Quantity of Solvents
Wet Chemical Etching
The University of Michigan on – Visiting Prof. HKU p. 4 S. W. Pang
WET ISOTROPIC ETCHING
SIMILAR ETCH RATES IN THE VERTICAL AND HORIZONTAL DIRECTIONS
FEATURES BECOME LARGER WITH ROUNDED PROFILE AFTER ETCHING
DIFFICULT TO CONTROL EXACT DIMENSION OR PROFILE
SURFACE ROUGHNESS DEVELOPED DUE TO PREFERENTIAL ETCHING
The University of Michigan on – Visiting Prof. HKU p. 5 S. W. Pang
WET ISOTROPIC ETCHING SOLUTIONS TYPICAL ETCHANT FOR Si
– 1:3:8 HF:HNO3:CH3COOH – HNO3 - OXIDIZE Si; HF - ETCH SiO2 – ACETIC ACID – PREVENT HNO3
DISSOCIATION – 3Si + 18HF + 4HNO3 3H2SiF6 +
8H2O + 4NO – ETCH RATES: – Si (0.5 TO 3 µm/min), SiO2
(30 nm/min) Si3N4 ETCHANT : H3PO4 AT 160-180 oC
Al ETCHANT : H3PO4 + HNO3 + CH3COOH
The University of Michigan on – Visiting Prof. HKU p. 6 S. W. Pang
ANISOTROPIC WET ETCHING
FASTER ETCH RATE IN ONE DIRECTION THAN THE OTHER
ETCH RATE DEPENDS ON CRYSTALLINE STRUCTURE
– DENSE CRYSTAL PLANES (e.g. <111> IN Si) ETCH SLOWER THAN LESS DENSE PLANES (<100> OR <110>)
– MAXIMUM ETCH DEPTH DEPENDS ON FEATURE SIZE
ETCH STOP OR SELECTIVITY BASED ON DOPING
The University of Michigan on – Visiting Prof. HKU p. 7 S. W. Pang
TYPICAL ANISOTROPIC WET ETCHANTS FOR Si
EDP – ETHYLENE DIAMINE PYROCATECHOL
TMAH – TETRAMETHYL AMMONIUM HYDROXIDE
ETCHANT TEMP
(°C)
Si RATE
(µm/min)
SiO2 RATE
(nm/min)
Si3N4 RATE
(nm/min)
(100)/(111)
RATIO
KOH 85 1.4 1.4 - 400:1
EDP 115 0.75 0.2 0.1 35:1
TMAH 95 1.3 0.2 0.02 20:1
The University of Michigan on – Visiting Prof. HKU p. 8 S. W. Pang
WET ETCHANTS COMPARISONS KOH
– COMMON SOLUTION, EASY DISPOSAL – ORIENTATION DEPENDENT ETCH, SMOOTH SURFACE – MOBIL ION CONTAMINATION
EDP – SELECTIVE ETCH WITH p++ ETCH STOP – METAL ETCH MASK (e.g. Cr, Cu, Ta, …) EXCEPT Al – CARCINOGENIC, CORRISIVE, REFLUX CONDENSER
NEEDED
TMAH – NO MOBILE ION, SAFER, EASIER TO SETUP – Al AS ETCH MASK WITH Si ADDED OR LOWER pH – ROUGHER SURFACE (H2 BUBBLES) IF CONCENTRATION
<20%
The University of Michigan on – Visiting Prof. HKU p. 9 S. W. Pang
KOH ETCHING OF Si ALKALI METALS – CONTAMINATION FOR INTEGRATED
CIRCUITS
HIGHLY SELECTIVITY – ORIENTATION, SiO2/ Si3N4, DOPING
ETCH STOP – BORON DOPED >2X1019 cm-3
TYPICAL MIXTURE
– KOH (4 g); ISOPROPANOL (100 ml); H2O
– KOH – OXIDIZE Si; IPA – SATURATE SOLUTION – H2O – FORM OH-
– Si + 2KOH + H2O K2SiO3 + 2H2
J. B. PRICE, PROC. SEMICONDUCTOR SILICON, ELECTROCHEM. SOC. P. 339 (1973)
The University of Michigan on – Visiting Prof. HKU p. 10 S. W. Pang
Si ETCHED IN KOH Si
(100) Si ETCHED INTERCEPT AT 54.74o
(110) Si ETCHED 80 µm DEEP
INTERCEPT AT 90o
W. R. RUNYAN AND K. E. BEAN, SEMICONDUCTOR INTEGRATED CURCUIT PROCESSING TECHNOLOGY, ADDISON-WESLEY, NY, 1990
The University of Michigan on – Visiting Prof. HKU p. 11 S. W. Pang
Wet Etchants for ICs
The University of Michigan on – Visiting Prof. HKU p. 12 S. W. Pang
WET VS. DRY ETCHING CHEMICAL CONSUMPTION AND DISPOSAL
– LIQUID VS. GAS
PROFILE CONTROL
– DIRECTIONAL REACTIVE SPECIES FOR VERTICAL PROFILE
– TAPERED, ROUNDED, MIRRORS, LENSES
CHEMICAL VS. PHYSICAL
– DIRECTIONALITY AND DENSITY OF NEUTRAL SPECIES VS. CHARGED PARTICLES
DAMAGE – CHARGING, ION BOMBARDMENT, CONTAMINATION
The University of Michigan on – Visiting Prof. HKU p. 13 S. W. Pang
PLASMA GENERATION FOR DRY ETCHING
PARTICLE MASS (g) TEMP (K) VELOCITY(cm/s)
CURRENT(A/cm2)
NEUTRAL 6.6x10-23 300 4x104 -
IONS 6.6x10-23 500 5x104 21x10-6
ELECTRONS 9.1x10-28 23000 1x108 38x10-3
v = 8kTπm
GAS IONIZED BY rf/MICROWAVE POWER – CONTAINS IONS (POSITIVE AND NEGATIVE), NEUTRALS,
ELECTRONS, PHOTONS – ONLY 0.1-10 % OF THE GAS IS IONIZED
REACTIVE SPECIES GENERATED BY IMPACT IONIZATION, DISSOCIATION, EXCITATION, RELAXATION, AND RECOMBINATION
WHERE AND J =qnv4
The University of Michigan on – Visiting Prof. HKU p. 14 S. W. Pang
Gases ionized by external energy (rf or microwave power) to generate ions, electrons, photons, and neutral reactive species
Still mostly gas molecules since <10% is ionized Electron impact ionization - Remove electrons from
atom/molecule e- + Ar Ar+ + 2e- Neutrals Ions with ion energy - Ionization potential (minimum energy to remove
most weakly bound electrons) for Ar = 15.8 eV - Multiplication of electrons maintains plasma
and keeps the processes going Excitation - Electrons jump to a higher energy level
within an atom e- + Ar Ar* + e- Ground State Unstable Excited State - Excitation potential (lower than ionization
potential, easier to excite within same atom) for Ar = 11.56 eV
Plasma Generation - I
The University of Michigan on – Visiting Prof. HKU p. 15 S. W. Pang
Relaxation - Unstable excited state returns to ground state by emission of photons of energy equal to ΔE
Ar* Ar + hν (Photons) - Color in plasma depends on characteristics of atoms
/molecules. In visible range: 400-700 nm (violet to red of 1.7 to 3 eV)
- Optical emission spectrum – consists of excited etch and product species. Can be used to monitor reactive species in plasma and etch products. For Example: Si at 288.1 nm; F at 704 nm Photon energy – identify species Light intensity – concentration of species
Plasma Generation - II
The University of Michigan on – Visiting Prof. HKU p. 16 S. W. Pang
Recombination - Electrons and ions recombine to form neutral species, makes stable plasma with fixed number of electrons and ions. Otherwise electron and ion density will keep increasing
e- + F+ F Dissociation - Break apart molecules
e- + O2 e-+O +O (more reactive than O2) e- + O2 2e-+O++O (dissociation ionization)
Electron Attachment - Electrons join an atom to form negative ions. Mostly with halogen atoms (e.g. F, Cl, Br, …) with 1 unfilled state in outer shell
e- + SF6 SF6-
e- + SF6 SF5-+F (dissociation attachment)
Ion-Neutral Collisions - Charge transfer or further ionization. Change energy distribution of ions and neutrals in reactor
Ar+ + Ar Ar+Ar+ Ar+ + O Ar+O+ (less efficient)
Plasma Generation - III
The University of Michigan on – Visiting Prof. HKU p. 17 S. W. Pang
rf
Plasma
REACTIVE GASES (e.g. SF6, Cl2)
WAFER
-Vdc
RIE SYSTEM
A2, V2
A1, V1
D1
D2
The University of Michigan on – Visiting Prof. HKU p. 18 S. W. Pang
VOLTAGE DISTRIBUTION ACROSS ELECTRODES Vp IS POSITIVE (10-70 V)
NEGATIVE Vdc SINCE ELECTRONS ARE FASTER THAN IONS (-30 TO -500 V)
FOR HIGH ASPECT RATIO MEMS, NEED TO REDUCE |Vdc| WHILE MAINTAINING HIGH ETCH RATE
Y
V0
Vp
-Vdc
GROUNDED
rf POWERED
The University of Michigan on – Visiting Prof. HKU p. 19 S. W. Pang
TYPICAL PLASMA CHARACTERISTICS FOR RIE PLASMA CONDITIONS TYPICAL VALUESrf POWER 0.05 - 1 W/cm2
rf FREQUENCY 13.56 MHz (100 KHz-27 MHz)dc BIAS 30 - 500 VPRESSURE 10 - 200 mTorrGAS FLOW 10 - 500 sccmWAFER TEMPERATURE 300 K (-130 TO 400 oC)ELECTRON TEMPERATURE 2 - 10 eV (23000 - 115000 K)ION TEMPERATURE 0.05 eV (600 K)GAS DENSITY 101 5 cm- 3
ION/ELECTRON DENSITY 101 0 cm- 3
ION FLUX 101 5 cm2/sRADICAL FLUX 101 6 cm2/sNEUTRAL FLUX 1019 cm2/sHIGH DENSITY PLASMA SYSTEMS (e.g. INDUCTIVELY COUPLED PLASMA SOURCE OR ICP) CAN BE USED TO REDUCE Vdc AND
INCREASE CONCENTRATION OF REACTIVE SPECIES
The University of Michigan on – Visiting Prof. HKU p. 20 S. W. Pang
INDUCTIVELY COUPLED PLASMA (ICP) SYSTEM
MORE FLEXIBLE – SEPARATE POWER SUPPLIES FOR SOURCE AND STAGE
HIGH ION DENSITY, LOWER |Vdc|
SUBSTRATE STAGE
TURBO/ROOTSBLOWER
SOURCEGAS RING
ALUMINACHAMBER
STAGEGAS RING
16 PERMANENTMAGNETS
WAFER CLAMPING
LOAD LOCK
ADJUSTABLESTAGE TO SOURCEDISTANCE (6-25 cm)
MASSSPECTROMETER
HEATING/COOLING
SOURCE rf POWER( 2 MHz, 0-2000 W )
4-TURN rfCOUPLING
COIL
rf POWER13.56 MHz
0-500 W
VIEWPORT
The University of Michigan on – Visiting Prof. HKU p. 21 S. W. Pang
CONTROLLABLE PARAMETERS IN DRY ETCHING
τ =pVQ
GASES - FLOW, MIXTURE
– 1 sccm = 0.0127 Torr-l/s = 2.7x1019 mol/min
PRESSURE - RESIDENCE TIME
– For 100 sccm flow (Q); V = 15 l; pressure = 10 mTorr;
τ = 0.12 s
POWER - POWER COUPLED IN; FREQUENCY; PULSING
CYCLING – SWITCHING GASES, POWER, PRESSURE
TEMPERATURE - ACTIVATION ENERGY, ADSORPTION,
DESORPTION
CHAMBER MATERIALS AND CONDITIONS
The University of Michigan on – Visiting Prof. HKU p. 22 S. W. Pang
UNCONTROLLABLE PARAMETERS IN DRY ETCHING
SAMPLE VARIATION - MATERIAL, MASK, OXIDE, RESIDUE
RESIDUAL GASES - LEAK, ADSORPTION ON WALL, GASES FROM PREVIOUS CYCLES
STABILIZATION – GAS FLOW, PRESSURE, POWER POWER LOSS - INEFFICIENT COUPLING WAFER TEMPERATURE VARIATION - POOR
THERMAL CONDUCTANCE METER OFFSET - RECALIBRATION NEEDED
PUMP SPEED VARIATION - OIL AND FILTER REPLACEMENT
The University of Michigan on – Visiting Prof. HKU p. 23 S. W. Pang
REACTIONS ON WAFER SURFACE
WAFERS ARE EXPOSED TO IONS, ELECTRONS, NEUTRALS
TRANSPORT OF REACTIVE SPECIES AND ETCH PRODUCTS
– PROCESS CONDITIONS
– GEOMETRY OF STRUCTURES
SURFACE REACTIONS
– PHYSICAL, CHEMICAL, ION ASSISTED REACTIONS
– BOTTOM SURFACE VS. SIDEWALL
– ETCHING VS. DEPOSITION
RADIATION EFFECTS
– CHARGING RELATED TO PLASMA UNIFORMITY AND HIGH DENSITY CHARGED PARTICLES
– DEFECT GENERATION DUE TO HIGH ENERGY PHOTONS
The University of Michigan on – Visiting Prof. HKU p. 24 S. W. Pang
ION ASSISTED ETCHINGPRESENCE OF IONS AND REACTIVE NEUTRALS
ETCH RATE ENHANCEMENT DUE TO IONS AND REACTIVE NEUTRALS
IS SUBSTANTIAL, NOT JUST THE TWO ADDED TOGETHER
COBRUN AND WINTERS, J. APPL. PHYS. 50, 3189 (1974)
Si ETCH RATE (nm/min)
TIME (s)XeF2 Only
XeF2 + Ar+
Ar+ Only
The University of Michigan on – Visiting Prof. HKU p. 25 S. W. Pang
EFFECTS OF GAS CHEMISTRY - 1 FORMATION OF VOLATILE ETCH PRODUCTS
Si + 4F SiF4 V.P. AT 1 Torr 144 oC
Si + 4Cl SiCl4 63 oC
SiO2 + 4F + C SiF4 + CO2
Al + 3Cl AlCl3 100 oC
Al + 3F AlF3 1238 oC
ADDITION OF INERT GASES (e.g. Ar, He)
– CHANGES ELECTRON DISTRIBUTION AND COMPOSITION OF REACTIVE SPECIES
– DILUTION; STABILIZATION; COOLING; SPUTTERING
The University of Michigan on – Visiting Prof. HKU p. 26 S. W. Pang
EFFECTS OF GAS CHEMISTRY - 2 ENHANCE REACTIVE SPECIES GENERATION
O + CFX CO + F + CFX-1
ETCH RATE INCREASES DUE TO HIGH [F] AND LESS POLYMER DEPOSITION
ENHANCE POLYMER FORMATION
H + CFX CHFX OR HF + CFX-1
ETCH RATE DECREASES DUE TO MORE POLYMER DEPOSITION AND LESS [F]
The University of Michigan on – Visiting Prof. HKU p. 27 S. W. Pang
OXYGEN ADDITION IN CF4
FOR SMALL O2%, ETCH RATE INCREASES DUE TO HIGHER [F]
FOR LARGE O2%, ETCH RATE DECREASES DUE TO DILUTION
LESS EFFECT ON SiO2 SINCE IT HAS SELF SUPPLY OF [O]
O2 in CF4 (%)
Si
SiO2
The University of Michigan on – Visiting Prof. HKU p. 28 S. W. Pang
SIDEWALL PASSIVATION BY POLYMER
MORE POLYMER DEPOSITION AND LESS [F] AS H2 IS ADDED
INCREASE SELECTIVITY BETWEEN SiO2 AND Si
LESS EFFECT ON SiO2 SINCE IT HAS SELF SUPPLY OF [O]
H2 in CF4 (%)
SiO2
DEPOSITSi
The University of Michigan on – Visiting Prof. HKU p. 29 S. W. Pang
F vs. Cl for Metal Etching
Etch Products with Lower Boiling Point are Easier to Remover with Faster Etch Rate and Perhaps More Undercut
Presence of Ions can Enhance Etch Product Removal
The University of Michigan on – Visiting Prof. HKU p. 30 S. W. Pang
Dry Etchants for ICs
The University of Michigan on – Visiting Prof. HKU p. 31 S. W. Pang
ION ENERGY REDUCES AT HIGH PRESSURE
LOWER ION ENERGY DUE TO MORE COLLISIONS
IONS AND REACTIVE NEUTRALS DO NOT NECESSARY INCREASE WITH PRESSURE DUE TO RECOMBINATION
AFFECT DISTRIBUTION OF REACTIVE SPECIES, ADSORPTION, DESORPTION
Eion
PRESSUREMORE PHYSICAL MORE CHEMICAL
The University of Michigan on – Visiting Prof. HKU p. 32 S. W. Pang
EFFECTS OF PRESSURE AND FEATURE SIZE ON UNDERCUT WIDTH
MICROWAVE/rf POWER 100/100 W, 8 cm, 25oC, 15 µm ETCH DEPTH
0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20 25 30 35
UNDE
RCUT
WID
TH (µ
m)
PRESSURE (mTorr)
2 µm 10 µm SPEEDIE
The University of Michigan on – Visiting Prof. HKU p. 33 S. W. Pang
EFFECT OF GAS FLOW RATE ON ETCH RATE
LOW FLOW - LIMITED BY REACTANTS
HIGH FLOW - RESIDENCE TIME REDUCED, PUMPED AWAY BEFORE REACTIONS
ETCH
RAT
E
GAS FLOW RATE
LOW HIGH
OPTIMAL
The University of Michigan on – Visiting Prof. HKU p. 34 S. W. Pang
Si AVERAGE ETCH RATE AS A FUNCTION OF TRENCH ASPECT RATIO
MICROWAVE/rf POWER 100/100 W, 3 mTorr, 8 cm, 20 sccm Cl2 ETCH RATE DECRASES AS ASPECT RATIO BECOMES HIGHER
100
110
120
130
140
150
160
170
180
0 5 10 15 20 25 30 35AVER
AGE
ETCH
RAT
E (n
m/m
in)
TRENCH ASPECT RATIO (A=H/W)
H
W
R = 156 -A
W. H. JUAN AND S. W. PANG, J. VAC. SCI. TECHNOL. 14, P. 1189 (1996).
The University of Michigan on – Visiting Prof. HKU p. 35 S. W. Pang
COMPARISONS OF Si DRY ETCHING USING F- AND Cl-BASED GASES
SYSTEM F-BASED Cl-BASEDGASES SF6/C4F8/ O2 Cl2
PROCESS SPONTANEOUS ION-ASSISTEDPASSIVATION POLYMER -ETCH MASK PHOTORESIST/SiO2 SiO2/NiETCH RATE FASTER SLOWER
ETCHSELECTIVITY HIGHER LOWER
ASPECT RATIO >20:1 >40:1WAFER
TEMPERATURE CONTROLLED CONTROL NOTNEEDED
PRESSURE >10 mTorr <1 mTorrLARGE FEATURES GOOD GOODSMALL FEATURES POOR GOOD
The University of Michigan on – Visiting Prof. HKU p. 36 S. W. Pang
Si ETCHING USING F-BASED GASESCYCLING BETWEEN ETCHING AND PASSIVATION
SiSi
MASK MASK
REPEATED CYCLES
PREVIOUS POLYMER COATING NEW POLYMER COATING
ADDITIONALETCH DEPTH
The University of Michigan on – Visiting Prof. HKU p. 37 S. W. Pang
ADVANTAGES AND DISADVANTAGES OF ETCHING USING F-BASED GASES AND PASSIVATION
ADVANTAGES
– FAST ETCH RATE
– HIGH SELECTIVITY
– FLEXIBLE PROFILE CONTROL
DISADVANTAGES
– SURFACE ROUGHNESS
– SENSITIVE PROCESS THAT REQUIRES PRECISE BALANCE BETWEEN ETCHING AND PASSIVATION
– ETCH RATE AND PROFILE VARY WITH ETCH DEPTH AND FEATURE SIZE
– FREQUENT SWITCHING OF INSTRUMENTS
The University of Michigan on – Visiting Prof. HKU p. 38 S. W. Pang
DEEP Si ETCHED USING PHOTORESIST MASK CYCLED BETWEEN SF6/O2 FOR ETCHING AND C4F8 FOR PASSIVATION
PRESSURE ~35 mTorr 2 µm WIDE GAPS, 70 µm DEEP
The University of Michigan on – Visiting Prof. HKU p. 39 S. W. Pang
ROUGHNESS ALONG SIDEWALLS OF DEEP TRENCHES
SCALLOPING VERTICAL STRIATIONS
CYCLE DURATION INSUFFICIENT PASSIVATION
BALANCE ETCH/PASSIVATION VARY WITH ASPECT RATIO
The University of Michigan on – Visiting Prof. HKU p. 40 S. W. Pang
Sputtering or Ion Beam Etching
Physical bombardment by ions only, no chemical reaction, simplest dry etching
1. Sputtered target atoms - etching 2. Reflected ions, mostly neutralized 3. Ejected secondary electrons 4. Ion implantation with ions staying inside target 5. Displacement of target materials - Radiation damage
creates vacancies, interstitials, traps, amorphous layer, stoichiometry changes; Could induce substantial Device Damage
The University of Michigan on – Visiting Prof. HKU p. 41 S. W. Pang
Sputtering Kinetics
Energy transfer between incoming ions and target atoms through series of collisions
Conservation of Energy
Conservation of Momentum
Sputtering Yield (S) - Number of target atoms ejected per incident ion. S depends on ion energy, atomic number of incoming ion and target atoms, surface binding energy of target, and angle of ion incidence. High S will provide high etch rate
€
12mivi
2 =12miui
2 +12mtut
2
€
mivi = miui +mtut
The University of Michigan on – Visiting Prof. HKU p. 42 S. W. Pang
Eion and Eth in KeV; Valid with Eion up to few KeV. Beyond that, ion implantation will dominate and there is no etching
Eth = threshold energy (~10-50 eV)
U = Surface Binding Energy (eV/atom) Zi, Zt = Atomic number of ions and target
atoms S increases with mi and Eion
Sputtering Yield
€
S = α( Eion − Eth )atomsions
€
α =5.2U
Zt(Zt
2 / 3 + Zi2 / 3 )3 / 4
( ZiZi + Zt
)2 / 3 atomsKeV
The University of Michigan on – Visiting Prof. HKU p. 43 S. W. Pang
Example: Ar+ 100 eV to etch W Zt = 74; Zi = 18; UW = 8.29 eV/atom; Eth = 33 eV S = 0.19 atom/ion Similar to experimental result: S ~0.1 atom/ion
S also depends on angle of incident ions S(θi) = cos-nθi (S(0°)) with n ~1 to 3 S(θi)max ~30° to 70° Maximum etch rate occurs ~45°, not at normal incident
Etch Efficiency
The University of Michigan on – Visiting Prof. HKU p. 44 S. W. Pang
Ejection of target atoms in forward direction is easier with less directional change of momentum
Off normal incidence confine action to surface rather than deep in substrate
When θi is too large (e.g. // to surface), not sufficient energy/momentum transfer. The ions just slide // to surface at glazing angle
Angle dependent Sputter Yield
The University of Michigan on – Visiting Prof. HKU p. 45 S. W. Pang
Sputtering Rate
Jion = Ion current density (A/cm2)
Etch Rate
W = Atomic Weight; ρ = Density; NA = Avogadro's Number
Example: Ar+ to etch W S = 0.1 atom/ion; Jion = 1x10-3 A/cm2
S = 0.115 nm/s = 6.9 nm/min (very slow) Sputtering rate is very slow and not selective
Etch Rate
€
rs(atomscm2 − s
) =SJionq
€
r ( nms) =
rsWρNA
€
rs(atomscm2 − s
) =(0.1)(10−3 )1.6x10−19
= 6.25x1014
€
r =rsWρNA
=(6.25x1014 atoms
cm2 − s)(183.85g)
(16.6 gcm3 )(6.02x10
23mol−1)
The University of Michigan on – Visiting Prof. HKU p. 46 S. W. Pang
Example: Spontaneous Si etching with XeF2
Simplest case – no ions needed. The presence of ions will enhance reactions
Si + 2XeF2 SiF4 + 2Xe
Chemical Reactions Increase Etch Rate
The University of Michigan on – Visiting Prof. HKU p. 47 S. W. Pang
A. Diffusion of XeF2 to Si Surface
B. Adsorption of XeF2 on Si
Net adsorption rate:
Cv = Vacant Sites on Si; Ea = Activation Energy
4-Step Etch Process
XeF2 (g) + Si Si.F2 + Xekaf
kar
€
ra = kafCXeF2Cv − karCSi .F2
CXe(1)
kaf = Ae−EakT ;KA =
kafkar
ra = kaf (CXeF2Cv −
CSi .F2CXe
KA
)
The University of Michigan on – Visiting Prof. HKU p. 48 S. W. Pang
C. Surface reaction on Si
Net surface reaction rate:
D. Desorption of etch product from Si
Net desorption rate:
4-Step Etch Process - II
Si.F2 + XeF2 (g) Si.SiF4 + Xeksr
ksf
€
rs = ksf (CXeF2CSi .F2
−CSi .SiF4
CXe
KS
)(2)
Si.SiF4 SiF4 + Sikdr
kdf
€
rd = kdf (CSi .SiF4−CSiF4
Cv
KD
)(3)
The University of Michigan on – Visiting Prof. HKU p. 49 S. W. Pang
Total number of sites available on Si
CT is known; Cv, CSi.F2, CSi.SiF4 are unknown
Need to find out which one is the rate limiting step - All reactions have to wait for the rate limiting step to finish before they can
proceed
For example, if rs is the rate limiting step, then kaf, kdf >> ksf
Rate Limiting Step
€
CT = CV +CSi .F2+CSi .SiF4
(4)
€
rakaf
<<rsksf
The University of Michigan on – Visiting Prof. HKU p. 50 S. W. Pang
Since ra is constant and kaf is large, ra/ kaf <<1; from (1)
Unknown Know or can be Estimated
Similarly, rd is constant and kdf is large, rd/ kdf <<1; from (3)
Unknown
Relate Knowns to Unknowns
€
CXeF2Cv −
CSi .F2CXe
KA
≈ 0
CSi .F2=KACXeF2
Cv
CXe
€
CSi .SiF4−CSiF4
Cv
KD
≈ 0
CSi .SiF4=CSiF4
Cv
KD
The University of Michigan on – Visiting Prof. HKU p. 51 S. W. Pang
Find CT
In steady state, ra=rs=rd
Relate to Etch Rate
€
CT = CV +CSi .F2+CSi .SiF4
CT = CV (1+KACXeF2
CXe
+CSiF4
KD
)
€
r = ksfCXeF2CSi .F2
r = ksfCXeF2
KACXeF2Cv
CXe
r =ksf (CXeF2
)2KA
CXe
CT
1+KACXeF2
CXe
+CSiF4
KD
The University of Michigan on – Visiting Prof. HKU p. 52 S. W. Pang
Make an assumption of the rate limiting step Solve for rate in terms of etch products, etch species,
rate constants, and surface coverage Check and see if rate dependence agrees with results.
If not, a different rate limiting step has to be used Ion Assisted Reactions - Reaction rates are enhanced
when ions are present besides neutrals
Reaction Kinetics
The University of Michigan on – Visiting Prof. HKU p. 53 S. W. Pang
Neutrals
r=kCA where CA is the concentration of etch product due to etching of A k=koe-Ea/kT where Ea is the activation energy
Ions (Faster Rate)
r+=k+CA+ where CA
+ is the concentration of etch product due to etching of A k+=koe-(Ea–Eo) /kT
k+ increases due to Ea reduction by Eo (k+ > k); Eo is proportional to Eion
Ion Assisted Etching
A + S CAk
A+ + e- + S CA+k+
The University of Michigan on – Visiting Prof. HKU p. 54 S. W. Pang
Total etch rate: rT=r+r+
α is degree of ionization
Faster etch rate due to the presence of ions
Etch Rate with Ion Assisted Etching
€
rT = koCAe(−Ea / kT )(1+αeEo / kT )
whereα =CA
+
CA
(o < α <1)
The University of Michigan on – Visiting Prof. HKU p. 55 S. W. Pang
Considerations for Dry Etching
Deposition During Etching Charging Undercut due to Neutrals and Ion Scattering Mask Erosion Trenching Dry Etch Induced Damage