Lecture 11.0Lecture 11.0

Etching

EtchingEtching

Patterned– Material Selectivity is Important!!

Un-patterned

EtchingEtching

Dry Etch An-isotropic

• dy/dt:dx/dt:6

Gas Phase Reaction with volatile products

Frequent use of very reactive species in a Plasma – Si Etch– SiO2 Etch– Metal Etch

Wet Etch=Dissolution Isotropic

• dy/dt:dx/dt:1.2

– Si Etch • Strong HF

– SiO2 Etch• Strong NH4OH not NaOH (Na

ion is bad)

– Si3N4 Etch• Phosphoric Acid

– Metal Etch• Acid Solution (HNO3)

– Photoresist• Solvent• H2SO4 Solution

x

y

EtchingEtching

Wet and Dry Etch have very different chemical reactions!

Wet and Dry Etch have similar rate determining steps– Mass Transfer Limiting– Surface Reaction Limiting

Similar mathematics

Wet Etch ChemistriesWet Etch Chemistries

Layer EtchantPhotoresist H2SO4, H2O2

SiO2 HF, NH4F-HCl-NH4F

Si3N4 ?, HNO3

Si HF

Dissolution of Layer-Wet EtchDissolution of Layer-Wet Etch

BL-Mass TransferA(l)+b B(s) ABb(l)

A=– Acid for metal (B) dissolution

• redox reaction

– Base for SiO2 (B) dissolution

– Solvent for photoresist (B) dissolution

Etch ReactionsEtch ReactionsBoundary Layer Mass TransferSurface Chemical Reaction

– Like Catalytic reactionProduct diffusion away from surface

Reactant ConcentrationProfile

ProductConcentrationProfile

Rate Determining StepsRate Determining Steps

X

Global Dissolution Rate/TimeGlobal Dissolution Rate/Time

Depends on– Mass Transfer

• Diffusion Coefficient• Velocity along wafer surface• Size of wafer

– Solubility– Density of film being etched

Wet Etch ReactionWet Etch Reaction

Wafers in Carriage Placed in Etch

Solution How Long?? Boundary Layer MT

is Rate Determining– Flow over a leading

edge for MT– Derivation & Mathcad

solutionAlso a C for theConcentration profile

Local Dissolution Rate/TimeLocal Dissolution Rate/Time

Depends on– Mass Transfer

• Diffusion Coefficient• Velocity along wafer surface• Size of wafer

– Solubility– Density of film being etched– Position on the wafer

• see “photoresist dissolution” example

Dry EtchDry Etch

Physical Evaporation– Not typically used

• Heating chip diffuses dopants out of position

Sputtering from a targetPlasma reactor with volatile reaction

product

RF Plasma Sputtering for RF Plasma Sputtering for Deposition and for EtchingDeposition and for Etching

RF + DC field

Removal RateRemoval RateSputtering Yield, S

– S=α(E1/2-Eth1/2)

Deposition Rate – Ion current into Target *Sputtering Yield– Fundamental Charge

gas(x) andtarget(t) ofnumbersatomic

)(

2.53/2

4/33/23/2

i

xt

x

xt

t

Z

energybindingsurfaceU

ZZ

Z

ZZ

Z

U

PlasmaPlasma

Free Electrons accelerated by a strong electric field

Collide with gas molecules and eject e-

Collision creates more free electrons Free electrons combine with ions to form

free radicals Gas Ions/Free Radicals are very reactive

with materials at the wafer surface– Ions non-selective removal– Free Radicals

Plasma ConditionsPlasma Conditions

Reduced Pressure ~100 mtorrFlow of gases in and outDC or AC (rf) electric field

– Parallel plate electrodes– Other geometries

Dry Etch ChemistriesDry Etch Chemistries

Gas Surface Etched O2 Pre-clean

95%CF4-5% O2 Si

50%CF4-25%HBr-25%O2 Poly Si

75%Cl2-25%HBr Metal etch

CF2 layer on side walls prevents wall etching

PlasmaPlasma

Temperature of Gas molecules, Tgas PVm/Rg

Temperature of Electrons, • Te =e2E2Mg/(6me

2m2 kB)

– Accelerated by E field between collisions with gas molecules

m= momentum collision frequency=Ng vel m(v)

Te E/Ng ERgTg/Ptot >> Tgas

kBTe > Gas Ionization EnergykBTe > Molecular Dissociation Energy

Plasma Gas ChemistriesPlasma Gas Chemistries

Reactant Gases– Physical Etch = Sputtering from chip target

• Ar

– Chemical Etch• O2

• CF4

• HBr• Cl2

• CHF3

• C2F6

• Mixtures

– CF2 deposition (like a teflon polymer layer) prevents side wall etch

Gaseous (Volatile) ProductsGaseous (Volatile) Products

– SiO(g), SiF4(v), SiCl4(v), SiBr4(v)

– MFx(v), MClx(v), MBrx(v),

11stst Ionization Energies Ionization Energies

O 13.618 eVBr 11.814 eVCl 12.967 eVF 17.422 eVH 13.598 eVAr 15.759 eV

Plasma Etch MechanismPlasma Etch Mechanism

PreClean• O2+ eO2

+ + 2e

• O2+ e2O + e

• O + e O-

• O2+ + e 2O

– O + s O-s– O + Si(s) s-SiO– SiO-s SiO(g)

Metal (M) Etch• Cl2 + e 2Cl + e

• Cl2 Cl2+ + e

• Cl + s Cl-s

• x Cl-M(s) MClx(g)

– Simultaneously• e + CF4 CF3

+ +F+ 2e

• e + CF3+ CF2 + F

• CF3+ + CF2 (CF2)n+F

• Polymer on wall of etchNeutrals are main reactive species!!

Degree of Ionization, Degree of Ionization, α α

α = Ni/No= Qi N λD

– N = neutral number density • N = Ni+No

– λD = Characteristic Diffusion length (mean free path)

– Qi= ionization collision cross section

• Qi= 0.283 x 10-16(cm2) Pi(E)

– Pi(E)= ionization probability

Plasma Transport EquationsPlasma Transport Equations

Flux, J

mobilityelectronμ

mobilityionμ

e

i

electronsforEndx

dnDJ

ionsforEndx

dnDJ

neutralsfordx

dnDJ

eee

ee

iii

ii

nnn

Etch ReactionsEtch ReactionsBoundary Layer Mass TransferSurface Chemical Reaction

– Like Catalytic reactionProduct diffusion away from surface

Reactant ConcentrationProfile

ProductConcentrationProfile

Etch ReactionEtch Reaction

A(g)+bB(s) ABb(g) -(1/A) dNB/dt= -(1/A)(/MwB)dVB/dt= -(/MwB) dy/dt = - JB

– JB= b JA =b Kg(CAg-CAs) BL-MT of A– JB= b JA= b ks Cag Surface Reaction– may be catalytic– JB= b JABb = Kg(CABb-s-CABb-g)BL-MT of Abb

–

– JB= b q/Hrxn

• q = h (Ts – Tg) BL-HT• q = k dT/dy Conduction in wafer

Rate Determining StepsRate Determining Steps

X

Plasma Etch Rate of PolymersPlasma Etch Rate of PolymersResidue Build-up

Plasma Etch Rate of PolymersPlasma Etch Rate of Polymers

Clean developed Photoresist off of waferClean developed Photoresist off of wafer

Wet-chemical stripping agents (solvents)– Incomplete wetting at small scale

Supercritical CO2.-new technology

– Zero surface tension• Complete wettability• Good for small line widths