ELEC 7364 Lecture Notes Summer 2008work7364/etching.pdf · 2008. 7. 2. · ELEC 7364 Lecture Notes ... Uniform Etching Better Than 5% - Minimize Etch Rate Dependence on Feature Size,

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


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


  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


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


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


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


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


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%


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)


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


Wet Etchants for ICs


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


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


  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


  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


  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


rf

Plasma

REACTIVE GASES (e.g. SF6, Cl2)

WAFER

-Vdc

RIE SYSTEM

A2, V2

A1, V1

D1

D2


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


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


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


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


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


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


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


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


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]


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


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


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


Dry Etchants for ICs


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


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


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


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).


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


Si ETCHING USING F-BASED GASESCYCLING BETWEEN ETCHING AND PASSIVATION

SiSi

MASK MASK

REPEATED CYCLES

PREVIOUS POLYMER COATING NEW POLYMER COATING

ADDITIONALETCH DEPTH


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


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


ROUGHNESS ALONG SIDEWALLS OF DEEP TRENCHES

SCALLOPING VERTICAL STRIATIONS

CYCLE DURATION INSUFFICIENT PASSIVATION

BALANCE ETCH/PASSIVATION VARY WITH ASPECT RATIO


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


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


  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


  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


  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


  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)


  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


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

)


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)


  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


  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


  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


  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


  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+


  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)


Considerations for Dry Etching

  Deposition During Etching   Charging   Undercut due to Neutrals and Ion Scattering   Mask Erosion   Trenching   Dry Etch Induced Damage

Documents

ELEC 7364 Lecture Notes Summer 2008work7364/etching.pdf · 2008. 7. 2. · ELEC 7364 Lecture Notes ... Uniform Etching Better Than 5% - Minimize Etch Rate Dependence on Feature Size,