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. 1 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. 2 S. W. Pang
Wet Chemical Etching 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 The University of Michigan on Visiting Prof. HKU p. 3 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. 4 S. W. Pang
WET ISOTROPIC ETCHING SOLUTIONS TYPICAL ETCHANT FOR Si 1:3:8 HF:HNO 3 :CH 3 COOH HNO 3 - OXIDIZE Si; HF - ETCH SiO 2 ACETIC ACID PREVENT HNO 3 DISSOCIATION 3Si + 18HF + 4HNO 3 3H 2 SiF 6 + 8H 2 O + 4NO ETCH RATES: Si (0.5 TO 3 µm/min), SiO 2 (30 nm/min) Si 3 N 4 ETCHANT : H 3 PO 4 AT 160-180 o C Al ETCHANT : H 3 PO 4 + HNO 3 + CH 3 COOH The University of Michigan on Visiting Prof. HKU p. 5 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. 6 S. W. Pang
TYPICAL ANISOTROPIC WET ETCHANTS FOR Si ETCHANT TEMP Si RATE SiO 2 RATE Si 3 N 4 RATE (100)/(111) ( C) (µm/min) (nm/min) (nm/min) 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 EDP ETHYLENE DIAMINE PYROCATECHOL TMAH TETRAMETHYL AMMONIUM HYDROXIDE The University of Michigan on Visiting Prof. HKU p. 7 S. W. Pang WET ETCHANTS COMPARISONS KOH EDP TMAH COMMON SOLUTION, EASY DISPOSAL ORIENTATION DEPENDENT ETCH, SMOOTH SURFACE MOBIL ION CONTAMINATION SELECTIVE ETCH WITH p ++ ETCH STOP METAL ETCH MASK (e.g. Cr, Cu, Ta, ) EXCEPT Al CARCINOGENIC, CORRISIVE, REFLUX CONDENSER NEEDED NO MOBILE ION, SAFER, EASIER TO SETUP Al AS ETCH MASK WITH Si ADDED OR LOWER ph ROUGHER SURFACE (H 2 BUBBLES) IF CONCENTRATION <20% The University of Michigan on Visiting Prof. HKU p. 8 S. W. Pang
KOH ETCHING OF Si ALKALI METALS CONTAMINATION FOR INTEGRATED CIRCUITS HIGHLY SELECTIVITY ORIENTATION, SiO 2 / Si 3 N 4, DOPING ETCH STOP BORON DOPED >2X10 19 cm -3 TYPICAL MIXTURE KOH (4 g); ISOPROPANOL (100 ml); H 2 O KOH OXIDIZE Si; IPA SATURATE SOLUTION H 2 O FORM OH - Si + 2KOH + H 2 O K 2 SiO 3 + 2H 2 J. B. PRICE, PROC. SEMICONDUCTOR SILICON, ELECTROCHEM. SOC. P. 339 (1973) The University of Michigan on Visiting Prof. HKU p. 9 S. W. Pang Si ETCHED IN KOH Si (100) Si ETCHED INTERCEPT AT 54.74 o (110) Si ETCHED 80 µm DEEP INTERCEPT AT 90 o 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. 10 S. W. Pang
Wet Etchants for ICs The University of Michigan on Visiting Prof. HKU p. 11 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. 12 S. W. Pang
PLASMA GENERATION FOR DRY ETCHING 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 PARTICLE MASS (g) TEMP (K) VELOCITY (cm/s) CURRENT (A/cm 2 ) NEUTRAL 6.6x10-23 300 4x10 4 - IONS 6.6x10-23 500 5x10 4 21x10-6 ELECTRONS 9.1x10-28 23000 1x10 8 38x10-3 WHERE v = 8kT πm AND J = qnv 4 The University of Michigan on Visiting Prof. HKU p. 13 S. W. Pang Plasma Generation - I 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 The University of Michigan on Visiting Prof. HKU p. 14 S. W. Pang
Plasma Generation - II 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 The University of Michigan on Visiting Prof. HKU p. 15 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 - + O 2 e - +O +O (more reactive than O 2 ) e- + O 2 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 - + SF 6 SF - 6 e - + SF 6 SF 5- +F (dissociation attachment) Ion-Neutral Collisions - Charge transfer or further ionization. Change energy distribution of ions and neutrals in reactor Ar + + Ar Ar + + O Plasma Generation - III Ar+Ar + Ar+O + (less efficient) The University of Michigan on Visiting Prof. HKU p. 16 S. W. Pang
RIE SYSTEM REACTIVE GASES (e.g. SF6, Cl2) A2, V2 D2 Plasma D1 WAFER A1, V1 -Vdc rf The University of Michigan on Visiting Prof. HKU p. 17 S. W. Pang VOLTAGE DISTRIBUTION ACROSS ELECTRODES V p IS POSITIVE (10-70 V) NEGATIVE V dc SINCE ELECTRONS ARE FASTER THAN IONS (-30 TO -500 V) FOR HIGH ASPECT RATIO MEMS, NEED TO REDUCE V dc WHILE MAINTAINING HIGH ETCH RATE Y GROUNDED Vp -Vdc 0 rf POWERED V The University of Michigan on Visiting Prof. HKU p. 18 S. W. Pang
TYPICAL PLASMA CHARACTERISTICS FOR RIE PLASMA CONDITIONS TYPICAL VALUES rf POWER 0.05-1 W/cm 2 rf FREQUENCY dc BIAS PRESSURE GAS FLOW 13.56 MHz (100 KHz-27 MHz) 30-500 V 10-200 mtorr 10-500 sccm WAFER TEMPERATURE 300 K (-130 TO 400 o C) ELECTRON TEMPERATURE 2-10 ev (23000-115000 K) ION TEMPERATURE 0.05 ev (600 K) GAS DENSITY 10 15 cm -3 ION/ELECTRON DENSITY 10 10 cm -3 ION FLUX 10 15 cm 2 /s RADICAL FLUX 10 16 cm 2 /s NEUTRAL FLUX 10 19 cm 2 /s HIGH DENSITY PLASMA SYSTEMS (e.g. INDUCTIVELY COUPLED PLASMA SOURCE OR ICP) CAN BE USED TO REDUCE V dc AND INCREASE CONCENTRATION OF REACTIVE SPECIES The University of Michigan on Visiting Prof. HKU p. 19 S. W. Pang INDUCTIVELY COUPLED PLASMA (ICP) SYSTEM MORE FLEXIBLE SEPARATE POWER SUPPLIES FOR SOURCE AND STAGE HIGH ION DENSITY, LOWER V dc SOURCE rf POWER ( 2 MHz, 0-2000 W ) VIEWPORT 4-TURN rf COUPLING COIL 16 PERMANENT MAGNETS WAFER CLAMPING MASS SPECTROMETER SOURCE GAS RING SUBSTRATE STAGE ALUMINA CHAMBER STAGE GAS RING LOAD LOCK ADJUSTABLE STAGE TO SOURCE DISTANCE (6-25 cm) HEATING /COOLING TURBO/ROOTS BLOWER rf POWER 13.56 MHz 0-500 W The University of Michigan on Visiting Prof. HKU p. 20 S. W. Pang
CONTROLLABLE PARAMETERS IN DRY ETCHING GASES - FLOW, MIXTURE 1 sccm = 0.0127 Torr-l/s = 2.7x10 19 mol/min PRESSURE - RESIDENCE TIME τ = pv Q 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. 21 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. 22 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. 23 S. W. Pang ION ASSISTED ETCHING PRESENCE OF IONS AND REACTIVE NEUTRALS Si ETCH RATE (nm/min) XeF2 + Ar + Ar + Only XeF2 Only TIME (s) 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) The University of Michigan on Visiting Prof. HKU p. 24 S. W. Pang
EFFECTS OF GAS CHEMISTRY - 1 FORMATION OF VOLATILE ETCH PRODUCTS Si + 4F SiF 4 V.P. AT 1 Torr 144 o C Si + 4Cl SiCl 4 63 o C SiO 2 + 4F + C SiF 4 + CO 2 Al + 3Cl AlCl 3 100 o C Al + 3F AlF 3 1238 o C 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. 25 S. W. Pang EFFECTS OF GAS CHEMISTRY - 2 ENHANCE REACTIVE SPECIES GENERATION O + CF X CO + F + CF X-1 ETCH RATE INCREASES DUE TO HIGH [F] AND LESS POLYMER DEPOSITION ENHANCE POLYMER FORMATION H + CF X CHF X OR HF + CF X-1 ETCH RATE DECREASES DUE TO MORE POLYMER DEPOSITION AND LESS [F] The University of Michigan on Visiting Prof. HKU p. 26 S. W. Pang
OXYGEN ADDITION IN CF 4 Si SiO2 O2 in CF4 (%) FOR SMALL O 2 %, ETCH RATE INCREASES DUE TO HIGHER [F] FOR LARGE O 2 %, ETCH RATE DECREASES DUE TO DILUTION LESS EFFECT ON SiO 2 SINCE IT HAS SELF SUPPLY OF [O] The University of Michigan on Visiting Prof. HKU p. 27 S. W. Pang SIDEWALL PASSIVATION BY POLYMER SiO2 Si DEPOSIT H2 in CF4 (%) MORE POLYMER DEPOSITION AND LESS [F] AS H 2 IS ADDED INCREASE SELECTIVITY BETWEEN SiO 2 AND Si LESS EFFECT ON SiO 2 SINCE IT HAS SELF SUPPLY OF [O] The University of Michigan on Visiting Prof. HKU p. 28 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. 29 S. W. Pang Dry Etchants for ICs The University of Michigan on Visiting Prof. HKU p. 30 S. W. Pang
ION ENERGY REDUCES AT HIGH PRESSURE Eion PRESSURE MORE PHYSICAL MORE CHEMICAL 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 The University of Michigan on Visiting Prof. HKU p. 31 S. W. Pang EFFECTS OF PRESSURE AND FEATURE SIZE ON UNDERCUT WIDTH MICROWAVE/rf POWER 100/100 W, 8 cm, 25 o C, 15 µm ETCH DEPTH UNDERCUT WIDTH (µm) 1.0 0.8 0.6 0.4 0.2 2 µm 10 µm SPEEDIE 0 0 5 10 15 20 25 30 3 PRESSURE (mtorr) The University of Michigan on Visiting Prof. HKU p. 32 S. W. Pang
EFFECT OF GAS FLOW RATE ON ETCH RATE OPTIMAL ETCH RATE LOW HIGH GAS FLOW RATE LOW FLOW - LIMITED BY REACTANTS HIGH FLOW - RESIDENCE TIME REDUCED, PUMPED AWAY BEFORE REACTIONS The University of Michigan on Visiting Prof. HKU p. 33 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 Cl 2 ETCH RATE DECRASES AS ASPECT RATIO BECOMES HIGHER AVERAGE ETCH RATE (nm/min) 180 170 160 150 140 R = 156 -A 130 W 120 H 110 100 0 5 10 15 20 25 30 35 TRENCH ASPECT RATIO (A=H/W) W. H. JUAN AND S. W. PANG, J. VAC. SCI. TECHNOL. 14, P. 1189 (1996). The University of Michigan on Visiting Prof. HKU p. 34 S. W. Pang
COMPARISONS OF Si DRY ETCHING USING F- AND Cl-BASED GASES SYSTEM F-BASED Cl-BASED GASES SF 6 /C 4 F 8 / O 2 Cl 2 PROCESS SPONTANEOUS ION-ASSISTED PASSIVATION POLYMER - ETCH MASK PHOTORESIST/SiO 2 SiO 2 /Ni ETCH RATE FASTER SLOWER ETCH SELECTIVITY HIGHER LOWER ASPECT RATIO >20:1 >40:1 WAFER TEMPERATURE CONTROLLED CONTROL NOT NEEDED PRESSURE >10 mtorr <1 mtorr LARGE FEATURES GOOD GOOD SMALL FEATURES POOR GOOD The University of Michigan on Visiting Prof. HKU p. 35 S. W. Pang Si ETCHING USING F-BASED GASES CYCLING BETWEEN ETCHING AND PASSIVATION PREVIOUS POLYMER COATING NEW POLYMER COATING MASK Si MASK Si ADDITIONAL ETCH DEPTH REPEATED CYCLES The University of Michigan on Visiting Prof. HKU p. 36 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. 37 S. W. Pang DEEP Si ETCHED USING PHOTORESIST MASK CYCLED BETWEEN SF 6 /O 2 FOR ETCHING AND C 4 F 8 FOR PASSIVATION PRESSURE ~35 mtorr 2 µm WIDE GAPS, 70 µm DEEP The University of Michigan on Visiting Prof. HKU p. 38 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. 39 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. 40 S. W. Pang
Sputtering Kinetics Energy transfer between incoming ions and target atoms through series of collisions Conservation of Energy 1 2 m v 2 i i = 1 2 m u 2 i i + 1 2 m u 2 t t Conservation of Momentum m i v i = m i u i + m t u t 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 The University of Michigan on Visiting Prof. HKU p. 41 S. W. Pang Sputtering Yield S = α( E i on E th ) atoms ions E ion and E th in KeV; Valid with E ion up to few KeV. Beyond that, ion implantation will dominate and there is no etching E th = threshold energy (~10-50 ev) α = 5.2 U Z t 2 / (Z 3 2 / t + Z 3 i ) ( Z i ) 2 / 3 atoms 3 / 4 Z i + Z t KeV U = Surface Binding Energy (ev/atom) Z i, Z t = Atomic number of ions and target atoms S increases with m i and E ion The University of Michigan on Visiting Prof. HKU p. 42 S. W. Pang
Etch Efficiency Example: Ar + 100 ev to etch W Z t = 74; Z i = 18; U W = 8.29 ev/atom; E th = 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(θ i ) = cos -n θ i (S(0 )) with n ~1 to 3 S(θ i ) max ~30 to 70 Maximum etch rate occurs ~45, not at normal incident The University of Michigan on Visiting Prof. HKU p. 43 S. W. Pang Angle dependent Sputter Yield 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 θ i is too large (e.g. // to surface), not sufficient energy/momentum transfer. The ions just slide // to surface at glazing angle The University of Michigan on Visiting Prof. HKU p. 44 S. W. Pang
Etch Rate Sputtering Rate r s ( atoms cm 2 s ) = SJ ion q J ion = Ion current density (A/cm 2 ) Etch Rate r ( nm s ) = rw s ρn A W = Atomic Weight; ρ = Density; N A = Avogadro's Number Example: Ar + to etch W S = 0.1 atom/ion; J ion = 1x10-3 A/cm 2 r s ( atoms cm 2 s ) = (0.1)(10 3 ) = 6.25x10 14 1.6x10 19 r = r (6.25x10 sw 14 = ρn A atoms cm 2 s )(183.85g) (16.6 g cm 3 )(6.02x1023 mol 1 ) S = 0.115 nm/s = 6.9 nm/min (very slow) Sputtering rate is very slow and not selective The University of Michigan on Visiting Prof. HKU p. 45 S. W. Pang Chemical Reactions Increase Etch Rate Example: Spontaneous Si etching with XeF 2 Simplest case no ions needed. The presence of ions will enhance reactions Si + 2XeF 2 SiF 4 + 2Xe The University of Michigan on Visiting Prof. HKU p. 46 S. W. Pang
A. Diffusion of XeF 2 to Si Surface B. Adsorption of XeF 2 on Si 4-Step Etch Process kaf XeF2 (g) + Si kar Net adsorption rate: Si.F2 + Xe r a = k af C XeF2 C v k ar C Si.F2 C Xe (1) k af = Ae E a kt ;K A = k af k ar r a = k af (C XeF2 C v C Si.F 2 C Xe K A ) C v = Vacant Sites on Si; E a = Activation Energy The University of Michigan on Visiting Prof. HKU p. 47 S. W. Pang C. Surface reaction on Si 4-Step Etch Process - II Si.F2 + XeF2 (g) ksf ksr Net surface reaction rate: D. Desorption of etch product from Si kdf Si.SiF4 SiF4 + Si kdr Net desorption rate: Si.SiF4 + Xe r s = k sf (C XeF2 C Si.F2 C Si.SiF 4 C Xe K S )(2) r d = k df (C Si.SiF4 C SiF 4 C v K D )(3) The University of Michigan on Visiting Prof. HKU p. 48 S. W. Pang
Rate Limiting Step Total number of sites available on Si C T = C V +C Si.F2 +C Si.SiF4 (4) C T is known; C v, C Si.F2, C Si.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 r s is the rate limiting step, then k af, k df >> k sf r a k af << r s k sf The University of Michigan on Visiting Prof. HKU p. 49 S. W. Pang Relate Knowns to Unknowns Since r a is constant and k af is large, r a / k af <<1; from (1) C XeF2 C v C Si.F 2 C Xe K A 0 C Si.F2 Unknown = K AC XeF2 C v C Xe Know or can be Estimated Similarly, r d is constant and k df is large, r d / k df <<1; from (3) C Si.SiF4 C SiF 4 C v K D 0 Unknown C Si.SiF4 = C SiF 4 C v K D The University of Michigan on Visiting Prof. HKU p. 50 S. W. Pang
Find C T Relate to Etch Rate C T = C V +C Si.F2 +C Si.SiF4 C T = C V (1+ K AC XeF2 C Xe + C SiF4 K D ) In steady state, r a =r s =r d r = k sf C XeF2 C Si.F2 r = k sf C XeF2 K A C XeF2 C v C Xe r = k sf (C XeF 2 ) 2 K A C Xe C T 1+ K AC XeF2 C Xe + C SiF4 K D The University of Michigan on Visiting Prof. HKU p. 51 S. W. Pang Reaction Kinetics 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 The University of Michigan on Visiting Prof. HKU p. 52 S. W. Pang
Ion Assisted Etching Neutrals A + S k CA r=kc A where C A is the concentration of etch product due to etching of A k=k o e -Ea/kT where E a is the activation energy Ions (Faster Rate) A + + e - + S k + CA + r + =k + C + A where C + A is the concentration of etch product due to etching of A k + =k o e -(Ea Eo) /kt k + increases due to E a reduction by E o (k + > k); E o is proportional to E ion The University of Michigan on Visiting Prof. HKU p. 53 S. W. Pang Etch Rate with Ion Assisted Etching Total etch rate: r T =r+r + r T = k o C A e ( E a / kt ) (1+αe E o / kt ) whereα = C A + C A (o < α <1) α is degree of ionization Faster etch rate due to the presence of ions The University of Michigan on Visiting Prof. HKU p. 54 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 The University of Michigan on Visiting Prof. HKU p. 55 S. W. Pang