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NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Basic Photo Process Steps
Stepper Interface
IncomingWafer
Adhesion
Cool PlateSoft Bake
Post Exposure
Bake
Cool Plate Resist Application
Align and Expose
Cool Plate Develop Post-Dev Inspection
1
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Spin-Coat Resist Usage/Waste
1 - 2 Microns Thick+/- 0.01 Microns
~ 5 ml Dispense on 200 mm Dia Wafer
Initial Wet Volume: ~ 5 mlFinal Wet Volume: ~ .06 mlApprox 99% Waste
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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).
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Three Steps to Coating Process
Surface Tension
Viscosity
Solvent Evaporation
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
Resist Volume, Solids Wt %,
t ,
surface T
Ambient T, RH
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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%
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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How Fast Does the Solvent Evaporate?
Solvent Mass Fraction vs time
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 5 10 15 20 25 30
Time, sec
So
lven
t M
ass
Fra
ctio
n
6000 RPM
4000 RPM
2000 RPM
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Solid and Solvent Mass Remainingon Wafer
Solids Remaining on Wafer
0
10
20
30
40
50
60
0 5 10 15 20 25 30
Time, sec
Solid
s re
mai
ning
, mg
2000 rpm
4000 rpm
6000 rpm
Solvent Mass Remaining on Wafer
0
20
40
60
80
100
120
140
160
180
0 5 10 15 20 25 30
Time, sec
Solv
ent R
emai
ning
, mg
2000 rpm
4000 rpm
6000 rpm
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Flow Characteristics:Resist Dispense Nozzle
Flow Rate through Resist Nozzle
Flow Rate = 0.4125 (Inches H20) + 1.4911
R2 = 0.9834
0
1
2
3
4
5
6
0 1 2 3 4 5 6 7 8 9 10
Pressure Drop, In H2O
Flo
w R
ate,
m
l/se
cWater Flow through 2mm x 6mm Resist Nozzle
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Resist Flows vs. Pressure Dropin TRACK Pump
y = 0.2659x - 0.3795
R2 = 1
y = 0.1078x - 0.154
R2 = 1
0
0.5
1
1.5
2
2.5
3
3.5
4
0 2 4 6 8 10 12 14 16
Pressure Drop, PSI
Flo
w R
ate
, g
r/sec
560 @ 21.7C
500 @ 22.0C
Linear (560 @ 21.7C)
Linear (500 @ 22.0C)
Resist Flows in TRACK Pump
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Dispense Rate vs. Temperature
Dispense Rate vs Temperature
0
0.5
1
1.5
2
2.5
3
30 C 25 C 22 C 18 C 16 C
g/s
ec
5 psi 560 10 psi 560 10psi 500 15psi 500
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Temperature Dependence of Resist Viscosity
Viscosity vs Temperature
0.1
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)
Resis
t V
iscosit
y,
Pois
e 560 Resist500 Resist560 model500 model
Visc = V0 exp(K/T)
560: K=710.58 Deg K V0 = 0.0157 Poise
500: K=1220.8 Deg K V0 = 0.0067 Poise
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Theory of Spin Coating
x
t
x
z zD x
x
z
A AA
A ( ( ) ) 0
20
2
0
( ')
[ ( ', )]'
h z
x z tdz
w
zA
z
x
D
h
A mass fraction of solvent in resist
Diffusivity of solvent
density of resist
spin speed
resist thickness
viscosity of resist
h h K ( , ) /. / 0 25 1 2
Conservation of solvent
Radial momentum/continuity
Can show theoretically that
where
( ) ( ) ( )1
10
x
D xx
zk x x
A
AA
A A
at z = h
Boundary Conditions
w andx
zA 0 0
at z = 0
dh
dtw z h k x z h xA A ( ) ( ( ) ) 0
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Effect of Evaporation on Film Height During Spinning
Film Height vs Spin TimeEffect of Solvent Evaporation
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30
Spin Time, Sec
Fil
m H
eig
ht,
um
1000 RPM, NO evap 2000 RPM
4000 RPM 8000 RPM
1000 RPM W/ evap 2000 RPM
4000 RPM 8000 RPM
Solvent Mass Fraction Due to Evaporation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Spin Time, sec
Sol
vent
Mas
s Fr
actio
n
1000 rpm 2000 rpm
4000 rpm 8000 rpm
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Final Film Thickness:Single Dispense
Single Dispense Data, 500 and 560 Resists
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
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
Fin
al H
eig
ht,
um
model data
hf = 10 1.38 Nu 0.4 / RPM 0.52
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Ultra Casting Predispense Technique
2000
4000
6000
2000
4000
6000
time[sec]
3-5 cc dispense1cc dispense
time[sec]
EBR EBR
casting rpm casting rpm
UCP
conventional dispense 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 !
< 1 sec
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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UCP Resist Coating - 2 Layer Model
Layer 1
Layer 2
g2
wafer FORCE
g1
g2
g1
<<<
Step 1: (1cc) Overcome surface tensionUse 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
Solvent Evaporation
Viscous Flow
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Resist Thickness vs. RPM560 Resist Thickness vs. RPM
7500
9500
11500
13500
15500
17500
19500
21500
0 500 1000 1500 2000 2500 3000 3500 4000 4500
UCP
5cc
1cc
* 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
rpm
Angstrom
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Resist Thickness vs. RPMALL DATA
Film Thickness vs RPM
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
1000 2000 3000 4000 5000 6000
Spin Speed, RPM
Fil
m T
hic
knes
s, u
m
17 cP 40 cP 17 cP, UCP
40 cP, UCP 20 cStk 35 cStk50 cStk 75 cStk 125 cStk
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Final Film Thickness:Double Dispense Data
Double Dispense 500 and 560 Data
0.5
1
1.5
2
2.5
3
0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13
Nu 0.4 / RPM 0.514
Fin
al h
eig
ht,
um
model data
hf = 10 1.349 * Nu 0.4/RPM 0.514
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Final Height:Single and Double Dispense
Single and Double Dispense500 and 560 Resists
0.5
1
1.5
2
2.5
3
0.5 1 1.5 2 2.5 3
Predicted Hf
Measu
red
Hf
data,single
data,double
Model
hf = 10 1.36 NU 0.4 / RPM 0.514
r 2 = 0.9995
ALL DATA7 Resists, 4 Operating Conditions
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3 4Predicted Hf
500 and 560 Resists
Resists 20-125 cStk
r 2 = 0.96
Hf = 10 1.43 * Nu 0.48 / RPM 0.56
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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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
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson
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Acknowledgments
• Motorola MOS 12– Tom Roche– Melissa Masteller– Frank Fischer– Michelle Demumbrum
• University of Arizona– Chris Doty