1. Photoresists/Coating/Lithography
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 hois a constant film is uniform
For thin films, h  -1 t-1/2

11. Evaporating Model - Heuristic Model
CN non-volatile, CV 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

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 o1/4
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. 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

19. 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

20. 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

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

22. 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

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

24. Rate Determining StepsX

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=Kgx/DAB
Kg= DAB / C
Sc=/DAB
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