Lecture 11.0
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Transcript of Lecture 11.0
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
EtchingEtchingWet 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/TimeDepends 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
)(2.5
3/2
4/33/23/2
i
xt
x
xt
t
ZenergybindingsurfaceU
ZZZ
ZZZ
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/(6me2m
2 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
mobilityelectronμmobilityionμ
e
i
electronsforEndxdnDJ
ionsforEndxdnDJ
neutralsfordxdnDJ
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
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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