Transcript Lecture10.0 Lithography.ppt

Lecture 10.0
Photoresists/Coating/Lithography
Semiconductor Fab
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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
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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
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Additives
Photoresist
Spin Coat wafer
 Dry solvent out of film
 Expose to Light
 Develop
 Quench development
 Dissolve resist (+) or developed
resist (-)

Spin Coating
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Cylindrical Coordinates
– Navier-Stokes
– Continuity
Navier-Stokes
Spin Coating Dynamics
Continuity
1  ( rVr )
 ( Vz )

0
r
r
z
Navier  Stokes
 Vr

 Vr
 t
 Vz

 Vr
 t
Vr
Vr 
 rz
 Vz
  gr

r
z 
z
Vz
Vz 
1  ( r rz )
 Vz
  gz

r
z 
r
z
g r  r 2
 rz
Vz
 Vr
  

r
 z

, Newtonian


Newtonian Fluidnon-evaporating
 2 vr
 2    2 r
z
B.C.' s
vr  0 @ z  0
vr
 0 @ a  h( r, t )
z
 2
q   vr dz 
h (t )3
3
0
h
h
1 
 2  2 3
qr  


rh 
t
r r
3r r
B.C. h(t  0)  ho
solution
If hois a constant film is uniform
1 / 2
2
 4  2 
For thin films, h  -1 t-1/2
h(t )  ho 1 
ho t 
3


Evaporation Model - Heuristic Model
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CN non-volatile, CV volatile
e= evaporation
 q= flow rate
Spin Coater - Heuristic Model
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Flow Rate, h is thickness
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Evaporation rate due to Mass
Transfer
Spin Coating Solution
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Dimensionless Equations
Viscosity as a function of composition
Viscosity increases with loss of
solvent
Viscosity of pure
Resin is very
high
 Viscosity of
Solvent is low
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3
Viscosity(m^2/sec)
1.521 10
0.01
1 10
3
1 10
4
 ( x)
5
1 10 1 10
5
0
0
0.02
0.04
x
xo
Volume Fraction Vapor Component
Spin Coating
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Thickness  RPM-1/2 o1/4
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Observed experimentally
Results
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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
Light Source
Lithography
Light passes
thru die mask
 Light imaged on
wafer
 Stepper to new
die location
 Re-image
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Mask
Reduction
Lens
Wafer with Photoresist
Lithography
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Aspect Ratio (AR)=3.5
– AR=Thickness/Critical Dimension
• Critical Dimension=line width
• Thickness= photoresist thickness
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Lateral Resolution (R)
– R=k1 /NA
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Numerical Apparature (NA)
– NA is a design parameter of lens
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Depth of Focus (DOF)
– DOF= k2 /NA2
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
Drying solvent out of Layer
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Removal of Solvent
– Simultaneous Heat and Mass Transfer
– In Heated oven
– Some shrinkage of layer
Photoresist
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Positive
– Light induced reaction
• decomposes polymer
into Acid + monomers
– Development
• Organic Base (Tri
Methyl ammonium
hydroxide) + Water
• neutralizes Acid group
• Dissolves layer
– Salt + monomer
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Negative
– Light induced reaction
• Short polymers
crosslink to produce
an insoluble polymer
layer
– No Development
needed
– Dissolution of unreacted material
Photoresist Development
Boundary Layer Mass Transfer
 Photoresist Diffusion
 Chemical Reaction
Reactant
Concentration
 Product diffusion, etc.

Product
Concentration
Profile
Reaction Plane
Profile
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
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Depends on
– Mass Transfer
• Diffusion Coefficient
• Velocity along wafer surface
• Size of wafer
– Solubility
– Density of Photoresist Film
Local Dissolution Rate/Time
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Depends on
– Mass Transfer
• Diffusion Coefficient
• Velocity along wafer surface
• Size of wafer
– Solubility
– Density of Photoresist Film
– Position on the wafer