Photoresists/Coating/Lithography 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

80nm Line width with =193 nm Lithography

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

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

Spin Coating Cylindrical Coordinates Navier-Stokes Continuity

Navier-Stokes

Spin Coating Dynamics

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

Evaporating Model - Heuristic Model CN non-volatile, CV volatile e= evaporation q= flow rate

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

Spin Coating Solution Dimensionless Equations

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

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

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

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

Wafer with Photoresist 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

Lithography Aspect Ratio (AR)=3.5 Lateral Resolution (R) AR=Thickness/Critical Dimension Critical Dimesion=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

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

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

Photoresist Positive Negative Light induced reaction Development 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 Development Boundary Layer Mass Transfer Photoresist Diffusion Chemical Reaction Product diffusion, etc. Reactant Concentration Profile Product Concentration Profile Reaction Plane

Rate Determining Steps X

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

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

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

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