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Lecture 10.0

Lecture 10.0. Photoresists/Coating/Lithography. 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.

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Lecture 10.0

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  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

  3. 80nm Line width with =193 nm Lithography

  4. 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

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

  6. Spin Coating • Cylindrical Coordinates • Navier-Stokes • Continuity

  7. Navier-Stokes

  8. Spin Coating Dynamics

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

  10. Evaporation Model - Heuristic Model • CN non-volatile, CV volatile • e= evaporation • q= flow rate

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

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

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

  14. Spin Coating • Thickness  RPM-1/2 o1/4 • Observed experimentally

  15. 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

  16. 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

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

  18. Lithography • Aspect Ratio (AR)=3.5 • AR=Thickness/Critical Dimension • Critical Dimension=line width • Thickness= photoresist thickness • Lateral Resolution (R) • R=k1/NA • Numerical Apparature (NA) • NA is a design parameter of lens • Depth of Focus (DOF) • DOF= k2/NA2

  19. Lithography - Photoreaction • Photo Reaction Kinetics • dC(x,t)/dt = koexp(-EA/RT) C(x,t) I(x,) • Beer’s Law • I(x, )/Io=exp(- () C(x,t) x) • () = extinction coefficient • Solution? • dC(x,t)/dt = koexp(-EA/RT) C(x,t) Io exp(- () C(x,t) x) • C=Co at t=0, 0<x<L

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

  21. 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 Photoresist

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

  23. Rate Determining Steps X

  24. 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

  25. Mass transfer correlation - flow over leading edge • Sh=Kgx/DAB • Kg= DAB / C • Sc=/DAB • Re=V x/

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

  27. 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

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