VOC Emissions Reduction from
Photolithographic Processes
Dr. Thomas W. Peterson
Chemical and Environmental Engineering,
University of Arizona
 1999 Arizona Board of Regents for The University of Arizona
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NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
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Photolithography
• Application and development of photoresist with the use of
light and heat to write a pattern of microstructures on a
wafer
• Photoresist
– Protective material used to form a resistant film on the
wafer surface
– Reacts chemically with areas exposed (not covered by a
patterned mask) to become more or less soluble with
respect to a solvent
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Basic Photo Process Steps
Incoming
Wafer
Soft Bake
Adhesion
Cool Plate
Cool Plate
Stepper
1
Interface
Post
Exposure
Bake
Cool Plate
Develop
Resist
Application
Align and
Expose
Post-Dev
Inspection
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Spin-Coat Resist Usage/Waste
~ 5 ml Dispense on
200 mm Dia Wafer
1 - 2 Microns Thick
+/- 0.01 Microns
Initial Wet Volume: ~ 5 ml
Final Wet Volume: ~ .06 ml
Approx 99% Waste
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Chemical Usage in the
Photolithography Process
• Standard photoresist chemicals
– Resin (e.g. Cresol-formaldehyde {Novolac} )
– Solvents
• Ethyl lactate (EL) (b.p 154C)
• Ethyl 3-ethoxypropionate (EEP)
• Cellosolve Acetate
– Photo-active Compounds
• Developer
• Edge Bead Remover
• Solvents
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Strategies for Reducing VOC Emissions
• Switch to Water-Based Resists
– Advantage: no VOC emissions
– Disadvantage: line width resolution, “speed”
• Optimize Use of Current Solvent-Based Resists
– Advantage: utilize current formulations, equipment
– Disadvantage: no hope for eliminating VOC emissions
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Benefits to Minimizing Resist Usage
• Resist cost (~$500/gallon)
• Environmental Impact
– Most of the Resist is “wasted”, in the sense that it is spun off the
wafer. The high “waste” is required to achieve uniformity in
thickness and proper coverage.
– Because of precise resist formulations, solvent evaporation during
spinning, and contamination concerns, it is impossible to recycle
resist that is spun from the wafer.
– Typically, Organic Solvents are used to clean the photoresist
“cups” into which the excess resist is spun. Less solvent usage
leads to less emissions of Volatile Organic Compounds (VOCs).
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Photoresist Dispense Optimization
• Objective
– Coverage of given thickness
– Coverage of given uniformity
• Control Variable(s)
– Resist properties (viscosity, solvent mass fraction,
amount dispensed)
– Spin speed
– Spin recipe
– Temperature, RH
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Three Steps to Coating Process
Surface Tension
Viscosity
Solvent Evaporation
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Spin-Coat Operating Parameters
•
•
•
•
•
•
•
•
Spin Speed (rpm)
Spin Time (sec)
Surface Temperature
Room Temperature
Room Humidity
Resist Volume
Resist Viscosity
Resist Solid Mass Fraction
Ambient T, RH
Resist Volume, Solids
Wt %, 
surface T
t,
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Properties of Photoresist
• Initial Viscosity: 5 to 100 cP
• Solvent Mass Fraction (SMF): 70-90%
• Photoresist viscosity for the experiments presented:
– 560 Resist - 17 cP
– 500 Resist - 40 cP
• Typical Viscosity Changes: 0.01 to 10,000 Poise as SMF
goes from 1.0 to 0.0
• Typical Diffusivity Changes: 10-6 to 10-12 cm2/sec as SMF
goes from 1.0 to 0.0
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Coating Process Overview
• Current Practice
– At slow speed (usually ~2000 rpm) dispense 3-5 cc resist and spin
for less than 2 seconds
– Ramp up speed (~3000 rpm) and spin for 20-30 seconds
– Apply Edge Bead Remover
– Fraction Resist “spun off”: typically 97-99%
• Ultra Casting Pre-dispense (UCP)
– At very high speed (6000 rpm) dispense 1 cc resist and spin for 1
second
– Decelerate to 2000 rpm and apply second dispense of 1 cc
– Accelerate to 3000 rpm and spin for 20-30 seconds
– Apply Edge Bead Remover
– Fraction Resist “spin off”: typically 95-97%
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How Fast Does the Solvent Evaporate?
Solvent Mass Fraction vs time
0.8
Solvent Mass Fraction
0.7
0.6
0.5
0.4
0.3
6000 RPM
0.2
4000 RPM
2000 RPM
0.1
0
0
5
10
15
20
25
30
Time, sec
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Solid and Solvent Mass Remaining
on Wafer
Solids Remaining on Wafer
Solvent Mass Remaining on Wafer
60
180
160
2000 rpm
140
4000 rpm
120
6000 rpm
Solvent Remaining, mg
Solids remaining, mg
50
40
30
20
2000 rpm
4000 rpm
10
100
80
60
40
6000 rpm
20
0
0
0
5
10
15
Time, sec
20
25
30
0
5
10
15
20
25
30
Time, sec
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Flow Characteristics:
Resist Dispense Nozzle
Flow Rate through Resist Nozzle
Water Flow through 2mm x 6mm Resist Nozzle
6
Flow Rate, ml/sec
5
4
3
2
Flow Rate = 0.4125 (Inches H20) + 1.4911
1
2
R = 0.9834
0
0
1
2
3
4
5
6
7
8
9
10
Pressure Drop, In H2O
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Resist Flows vs. Pressure Drop
in TRACK Pump
Resist Flows in TRACK Pump
4
560 @ 21.7C
y = 0.2659x - 0.3795
R2 = 1
3.5
500 @ 22.0C
Linear (560 @ 21.7C)
Flow Rate, gr/sec
3
Linear (500 @ 22.0C)
2.5
2
y = 0.1078x - 0.154
R2 = 1
1.5
1
0.5
0
0
2
4
6
8
10
12
14
16
Pressure Drop, PSI
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Dispense Rate vs. Temperature
Dispense Rate vs Temperature
3
2.5
g/sec
2
1.5
1
0.5
0
30 C
5 psi 560
25 C
22 C
10 psi 560
18 C
10psi 500
16 C
15psi 500
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Temperature Dependence
of Resist Viscosity
Viscosity vs Temperature
Visc = V0 exp(K/T)
1
Resist Viscosity, Poise
560
500
560
500
Resist
Resist
model
model
560: K=710.58 Deg K V0 = 0.0157 Poise
500: K=1220.8 Deg K V0 = 0.0067 Poise
0.1
3.28
3.30
3.32
3.34
3.36
3.38
3.40
3.42
3.44
3.46
1000/T (Deg K)
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Theory of Spin Coating
Conservation of solvent
 xA  xA 
 xA


( D( xA)
)0
t z z
z
Radial momentum/continuity
2  2 (h  z')
w
dz ' 
0
 [ x A ( z', t )]
z
z

0
Boundary Conditions
(
1
x
) D( x A ) A  k ( x A  x A )  0 at z = h
1  xA
z
 xA
 0 at z = 0
z
dh
 w( z  h)  k ( x A ( z  h)  x A )  0
dt
w  0 and
where
xA
mass fraction of solvent in resist
D
Diffusivity of solvent
density of resist
spin speed


h

Can show theoretically that
h  h ( , )  K 
0.25
/
1/ 2
resist thickness
viscosity of resist
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Effect of Evaporation on Film Height
During Spinning
1000 RPM, NO evap
2000 RPM
4000 RPM
8000 RPM
1000 RPM W/ evap
2000 RPM
4000 RPM
8000 RPM
2000 rpm
4000 rpm
8000 rpm
1
100
80
Solvent Mass Fraction
Film Height vs Spin Time
Effect of Solvent Evaporation
90
Film Height, um
1000 rpm
70
60
50
40
30
20
10
Solvent Mass Fraction Due to Evaporation
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0
5
10
15
Spin Time, Sec
20
25
30
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30
Spin Time, sec
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Final Film Thickness:
Single Dispense
Single Dispense Data, 500 and 560 Resists
2
1.9
hf = 10^1.38 Nu^0.4 / RPM^0.52
Final Height, um
1.8
1.7
1.6
1.5
1.4
1.3
1.2
model
data
1.1
1
0.04
0.045
0.05
0.055
0.06
0.065
0.07
0.075
0.08
0.085
Nu^0.4/RPM^0.52
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Ultra Casting Predispense
Technique
UCP
6000
6000
1cc dispense
3-5 cc dispense
4000
4000
casting rpm
casting rpm
EBR
2000
EBR
2000
time[sec]
conventional dispense
time[sec]
< 1 sec
UCP dispense
Note: All dispenses achieve complete coverage of the wafer with coating solution (Resist) ; the process
that forms the final thickness and uniformity is at the same rpm in both cases !
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UCP Resist Coating - 2 Layer Model
Solvent Evaporation
Layer 2
Viscous Flow
g2
wafer
Layer 1
g1
FORCE
g2 <<< g1
Step 1: (1cc)
Overcome surface tension
Use radial force (ultra casting rpm, 5000-6000 rpm)
Create interface
Step 2: (1cc)
Dispense at or below casting rpm,
Minimal resist needed because very little resistance
from surface tension
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Resist Thickness vs. RPM
560 Resist Thickness vs. RPM
Angstrom
21500
19500
17500
15500
UCP
13500
5cc
1cc
11500
9500
7500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
rpm
* Once surface is wetted, only a small additional amount is needed to built thickness.
* Key concept: initial wetting at ultra casting rpm, second dispense at or below casting
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Resist Thickness vs. RPM
ALL DATA
Film Thickness vs RPM
5
17 cP
40 cP, UCP
50 cStk
4.5
Film Thickness, um
4
40 cP
20 cStk
75 cStk
17 cP, UCP
35 cStk
125 cStk
3.5
3
2.5
2
1.5
1
0.5
0
1000
2000
3000
4000
5000
6000
Spin Speed, RPM
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Final Film Thickness:
Double Dispense Data
Double Dispense 500 and 560 Data
3
hf = 10^1.349 * Nu^0.4/RPM^0.514
Final height, um
2.5
2
1.5
model
1
0.5
0.04
0.05
0.06
0.07
0.08
data
0.09
0.1
0.11
0.12
0.13
Nu^0.4 / RPM^0.514
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Final Height:
Single and Double Dispense
Single and Double Dispense
500 and 560 Resists
3
4
data,single
Measured Hf
500 and 560 Resists
Resists 20-125 cStk
3.5
data,double
2.5
ALL DATA
7 Resists, 4 Operating Conditions
3
Model
2.5
2
2
1.5
1.5
r^2 = 0.9995
r^2 = 0.96
1
1
0.5
hf = 10^1.36 ^ NU^0.4 / RPM^0.514
Hf = 10^1.43 * Nu^0.48 / RPM ^ 0.56
0
0.5
0.5
1
1.5
2
Predicted Hf
2.5
3
0
1
2
3
Predicted Hf
4
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Conclusions
• Resist usage:
– ~5ml for single dispense
– ~2ml for UCP method
• Resist thickness and uniformity: identical for both methods
• Comparison to theoretical models: identical for both methods; slightly
stronger viscosity dependence than predicted theoretically
• Resist and solvent usage reduction yields substantial reduction in VOC
emissions
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Acknowledgments
• Motorola MOS 12
– Tom Roche
– Melissa Masteller
– Frank Fischer
– Michelle Demumbrum
• University of Arizona
– Chris Doty
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