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Lecture 11.0

Lecture 11.0. Etching. Etching. Patterned Material Selectivity is Important!! Un-patterned. 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

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Lecture 11.0

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  1. Lecture 11.0 Etching

  2. Etching • Patterned • Material Selectivity is Important!! • Un-patterned

  3. 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 Etching y

  4. 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

  5. Wet Etch Chemistries • Layer Etchant • Photoresist H2SO4, H2O2 • SiO2HF, NH4F-HCl-NH4F • Si3N4 ?, HNO3 • Si HF

  6. Dissolution of Layer-Wet Etch • BL-Mass Transfer • A(l)+b B(s) ABb(l) • A= • Acid for metal (B) dissolution • redox reaction • Base for SiO2 (B) dissolution • Solvent for photoresist (B) dissolution

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

  8. Rate Determining Steps X

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

  10. 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

  11. 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

  12. Dry Etch • Physical Evaporation • Not typically used • Heating chip diffuses dopants out of position • Sputtering from a target • Plasma reactor with volatile reaction product

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

  14. Removal Rate • Sputtering Yield, S • S=α(E1/2-Eth1/2) • Deposition Rate  • Ion current into Target *Sputtering Yield • Fundamental Charge

  15. 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

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

  17. Dry 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

  18. Plasma • Temperature of Gas molecules, Tgas PVm/Rg • Temperature of Electrons, • Te=e2E2Mg/(6me2m2 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 Energy • kBTe > Molecular Dissociation Energy

  19. Plasma 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

  20. Gaseous (Volatile) Products • SiO(g), SiF4(v), SiCl4(v), SiBr4(v) • MFx(v),MClx(v), MBrx(v),

  21. 1st Ionization Energies • O 13.618 eV • Br 11.814 eV • Cl 12.967 eV • F 17.422 eV • H 13.598 eV • Ar 15.759 eV

  22. PreClean O2+ eO2+ + 2e O2+ e2O + 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 etch Plasma Etch Mechanism Neutrals are main reactive species!!

  23. Degree of Ionization, 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

  24. Plasma Transport Equations • Flux, J

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

  26. Etch 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 CagSurface 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

  27. Rate Determining Steps X

  28. Plasma Etch Rate of Polymers Residue Build-up

  29. Plasma Etch Rate of Polymers

  30. Clean 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

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