Photoresists/Coating/Lithography

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Lecture 10.0

• Photoresists/Coating/Lithography

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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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80nm Line width with ?193 nm Lithography
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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

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Photoresist

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

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Spin Coating

• Cylindrical Coordinates
• Navier-Stokes
• Continuity

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Navier-Stokes
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Spin Coating Dynamics
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Newtonian Fluid-non-evaporating
If hois a constant film is uniform For thin
films, h ? ?-1 t-1/2
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Evaporation Model - Heuristic Model

• CN non-volatile, CV volatile
• e evaporation
• q flow rate

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Spin Coater - Heuristic Model

• Flow Rate, h is thickness
• Evaporation rate due to Mass Transfer

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Spin Coating Solution

• Dimensionless Equations

Viscosity as a function of composition
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Viscosity increases with loss of solvent

• Viscosity of pure Resin is very high
• Viscosity of Solvent is low

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Spin Coating

• Thickness ? RPM-1/2 ?o1/4
• Observed experimentally

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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
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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
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Lithography
Light Source

• Light passes thru die mask
• Light imaged on wafer
• Stepper to new die location
• Re-image

Mask
Reduction Lens
Wafer with Photoresist
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Lithography

• Aspect Ratio (AR)3.5
• ARThickness/Critical Dimension
• Critical Dimensionline width
• Thickness photoresist thickness
• Lateral Resolution (R)
• Rk1 ?/NA
• Numerical Apparature (NA)
• NA is a design parameter of lens
• Depth of Focus (DOF)
• DOF k2 ?/NA2

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Lithography - Photoreaction

• Photo Reaction Kinetics
• dC(x,t)/dt koexp(-EA/RT) C(x,t) I(x,?)
• Beers Law
• I(x, ?)/Ioexp(- ?(?) C(x,t) x)
• ?(?) extinction coefficient
• Solution?
• dC(x,t)/dt koexp(-EA/RT) C(x,t) Io exp(- ?(?)
  C(x,t) x)
• CCo at t0, 0ltxltL

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Drying solvent out of Layer

• Removal of Solvent
• Simultaneous Heat and Mass Transfer
• In Heated oven
• Some shrinkage of layer

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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

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Photoresist Development

• Boundary Layer Mass Transfer
• Photoresist Diffusion
• Chemical Reaction
• Product diffusion, etc.

Reactant Concentration Profile
Product Concentration Profile

Reaction Plane
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Rate Determining Steps
X
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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
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Mass transfer correlation - flow over leading
edge

• ShKgx/DAB
• Kg DAB / ?C
• Sc?/DAB
• ReV? x/?

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Global Dissolution Rate/Time

• Depends on
• Mass Transfer
• Diffusion Coefficient
• Velocity along wafer surface
• Size of wafer
• Solubility
• Density of Photoresist Film

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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