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Advanced Lithography in Semiconductor Manufacturing

Explore the intricate process of lithography in semiconductor fabrication, including photoresist coating and development. Learn about photoresist materials, lithography tools, and chemical reactions in this billion-dollar industry. Understand the dynamics and complexities of spin coating and dissolution processes, as well as photoreaction kinetics and mass transfer correlations. Enhance your knowledge of lithography equipment, resist film dissolution, and global/local dissolution rates in this essential lecture.

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Advanced Lithography in Semiconductor Manufacturing

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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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