Lecture 11.0 Etching

Etching Patterned –Material Selectivity is Important!! Un-patterned

Etching 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 –SiO 2 Etch –Metal Etch Wet Etch=Dissolution Isotropic dy/dt:dx/dt:1.2 –Si Etch Strong HF –SiO 2 Etch Strong NH 4 OH not NaOH (Na ion is bad) –Si 3 N 4 Etch Phosphoric Acid –Metal Etch Acid Solution (HNO 3 ) –Photoresist Solvent H 2 SO 4 Solution x y

Etching 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 Chemistries LayerEtchant Photoresist H 2 SO 4, H 2 O 2 SiO 2 HF, NH 4 F-HCl-NH 4 F Si 3 N 4 ?, HNO 3 Si HF

Dissolution of Layer-Wet Etch BL-Mass Transfer A(l)+b B(s)  AB b (l) A= –Acid for metal (B) dissolution redox reaction –Base for SiO 2 (B) dissolution –Solvent for photoresist (B) dissolution

Etch Reactions Boundary Layer Mass Transfer Surface Chemical Reaction –Like Catalytic reaction Product diffusion away from surface Reactant Concentration Profile Product Concentration Profile

Rate Determining Steps X

Global Dissolution Rate/Time Depends on –Mass Transfer Diffusion Coefficient Velocity along wafer surface Size of wafer –Solubility –Density of film being etched

Wet 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 solution Also a  C for the Concentration profile

Local 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 Etch Physical Evaporation –Not typically used Heating chip diffuses dopants out of position Sputtering from a target Plasma reactor with volatile reaction product

RF Plasma Sputtering for Deposition and for Etching RF + DC field

Removal Rate Sputtering Yield, S –S=α(E 1/2 -E th 1/2 ) Deposition Rate  –Ion current into Target *Sputtering Yield – Fundamental Charge

Plasma 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 Conditions Reduced Pressure ~100 mtorr Flow of gases in and out DC or AC (rf) electric field –Parallel plate electrodes –Other geometries

Dry Etch Chemistries GasSurface Etched O 2 Pre-clean 95%CF 4 -5% O 2 Si 50%CF 4 -25%HBr-25%O 2 Poly Si 75%Cl 2 -25%HBr Metal etch CF 2 layer on side walls prevents wall etching

Plasma Temperature of Gas molecules, T gas  PV m /R g Temperature of Electrons, T e =e 2 E 2 M g /(6m e 2 m 2 k B ) –Accelerated by E field between collisions with gas molecules – m = momentum collision frequency=N g vel  m (v) T e  E/N g  ER g T g /P tot >> T gas k B T e > Gas Ionization Energy k B T e > Molecular Dissociation Energy

Plasma Gas Chemistries Reactant Gases –Physical Etch = Sputtering from chip target Ar –Chemical Etch O 2 CF 4 HBr Cl 2 CHF 3 C 2 F 6 Mixtures –CF 2 deposition (like a teflon polymer layer) prevents side wall etch

Gaseous (Volatile) Products –SiO(g), SiF 4 (v), SiCl 4 (v), SiBr 4 (v) –MF x (v), MCl x (v), MBr x (v),

1 st Ionization Energies O eV Br eV Cl eV F eV H eV Ar eV

Plasma Etch Mechanism PreClean O 2 + e  O e O 2 + e  2O + e O + e  O - O e  2O –O + s  O-s –O + Si(s)  s-SiO –SiO-s  SiO(g) Metal (M) Etch Cl 2 + e  2Cl + e Cl 2  Cl e Cl + s  Cl-s x Cl-M(s)  MCl x (g) –Simultaneously e + CF 4  CF 3 + +F+ 2e e + CF 3 +  CF 2 + F CF CF 2  (CF 2 ) n +F Polymer on wall of etch Neutrals are main reactive species!!

Degree of Ionization, α α = N i /N o = Q i N λ D –N = neutral number density N = N i +N o –λ D = Characteristic Diffusion length (mean free path) –Q i = ionization collision cross section Q i = x (cm 2 ) P i (E) –P i (E)= ionization probability

Plasma Transport Equations Flux, J

Etch Reactions Boundary Layer Mass Transfer Surface Chemical Reaction –Like Catalytic reaction Product diffusion away from surface Reactant Concentration Profile Product Concentration Profile

Etch Reaction A(g)+bB(s)  AB b (g) -(1/A) dN B /dt= -(1/A)(  /Mw B )dV B /dt= -(  /Mw B ) dy/dt = - J B –J B = b J A =b K g (C Ag -C As )BL-MT of A –J B = b J A = b k s C ag Surface Reaction – may be catalytic –J B = b J ABb = K g (C ABb-s -C ABb-g )BL-MT of Ab b – –J B = b q/  H rxn q = h (T s – T g )BL-HT q = k dT/dyConduction in wafer

Rate Determining Steps X

Plasma Etch Rate of Polymers Residue Build-up

Plasma Etch Rate of Polymers

Clean developed Photoresist off of wafer Wet-chemical stripping agents (solvents) –Incomplete wetting at small scale Supercritical CO 2.-new technology –Zero surface tension Complete wettability Good for small line widths