1. Lecture 10.0 Photoresists/Coating/Lithography

2. Semiconductor Fab Land$0.05 Billion Building$0.15 Billion Tools & Equipment $1 Billion Air/Gas Handling Sys$0.2 Billion Chemical/Electrical Sys$0.1 Billion Total$1.5 Billion 10 year Amortization~$1 Million/day

4. 80nm Line width with =193 nm Lithography

5. Photoresist - Sales $1.2 billion/yr. in 2001 Resins –phenol-formaldehyde, I-line Solvents Photosensitive compounds –Polymethylmethacrylate or poly acrylic acid = 638 nm RED LIGHT –diazonaphthoquinone Hg lamp, = 365 nm, I-line –o-nitrobenzyl esters – acid generators Deep UV, = 248 nm, KrF laser –Cycloolefin-maleic anhydride copolymer –Poly hydroxystyrene =193 nm gives lines 100 nm = 157 nm F laser Additives

6. Photoresist Spin Coat wafer Dry solvent out of film Expose to Light Develop Quench development Dissolve resist (+) or developed resist (-)

7. Spin Coating Cylindrical Coordinates –Navier-Stokes –Continuity

8. Navier-Stokes

9. Spin Coating Dynamics

10. Newtonian Fluid- non-evaporating If h o is a constant film is uniform For thin films, h   -1 t -1/2

11. Evaporation Model - Heuristic Model C N non-volatile, C V volatile e= evaporation q= flow rate

12. Spin Coater - Heuristic Model Flow Rate, h is thickness Evaporation rate due to Mass Transfer

13. Spin Coating Solution Dimensionless Equations Viscosity as a function of composition

14. Viscosity increases with loss of solvent Viscosity of pure Resin is very high Viscosity of Solvent is low

15. Spin Coating Thickness  RPM -1/2 o 1/4 Observed experimentally

16. Results Effect of Mass Transfer –  = dimensionless Mass transfer Coefficient –Increase MT  Increase in Film Thickness –MT increases viscosity and slows flow leading to thicker film Dimensionless Film Thickness

17. Dissolve edge of photoresist So that no sticking of wafer to surfaces takes place So that no dust or debris attaches to wafers Wafer with Photoresist

18. Lithography Light passes thru die mask Light imaged on wafer Stepper to new die location Re-image Wafer with Photoresist Mask Light Source Reduction Lens

19. Lithography Aspect Ratio (AR)=3.5 –AR=Thickness/Critical Dimension Critical Dimension=line width Thickness= photoresist thickness Lateral Resolution (R) –R=k 1 /NA Numerical Apparature (NA) –NA is a design parameter of lens Depth of Focus (DOF) –DOF= k 2 /NA 2

20. Lithography - Photoreaction –Photo Reaction Kinetics dC(x,t)/dt = k o exp(-E A /RT) C(x,t) I(x, ) –Beer’s Law I(x, )/I o =exp(-  ( ) C(x,t) x)  ( ) = extinction coefficient –Solution? dC(x,t)/dt = k o exp(-E A /RT) C(x,t) I o exp(-  ( ) C(x,t) x) –C=C o at t=0, 0<x<L

21. Drying solvent out of Layer Removal of Solvent –Simultaneous Heat and Mass Transfer –In Heated oven –Some shrinkage of layer

22. Photoresist Positive –Light induced reaction decomposes polymer into Acid + monomers –Development Organic Base (Tri Methyl ammonium hydroxide) + Water neutralizes Acid group Dissolves layer –Salt + monomer Negative –Light induced reaction Short polymers crosslink to produce an insoluble polymer layer –No Development needed –Dissolution of un- reacted material

23. Photoresist Development Boundary Layer Mass Transfer Photoresist Diffusion Chemical Reaction Product diffusion, etc. Reaction Plane Reactant Concentration Profile Product Concentration Profile

24. Rate Determining Steps X

25. Dissolution of Uncrosslinked Photoresist Wafers in Carriage Placed in Solvent 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

26. Mass transfer correlation - flow over leading edge Sh=K g x/D AB K g = D AB /  C Sc= /D AB Re=V  x/

27. Global Dissolution Rate/Time Depends on –Mass Transfer Diffusion Coefficient Velocity along wafer surface Size of wafer –Solubility –Density of Photoresist Film

28. Local Dissolution Rate/Time Depends on –Mass Transfer Diffusion Coefficient Velocity along wafer surface Size of wafer –Solubility –Density of Photoresist Film –Position on the wafer