NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Peterson...

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingPeterson

1

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


2

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


3

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


4

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


5

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


6

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


7

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).


8

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


9

Three Steps to Coating Process

Surface Tension

Viscosity

Solvent Evaporation


10

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


11

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


12

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%


13

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


14

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


15

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


16

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


17

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


18

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


19

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


20

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


21

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


22

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


23

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


24

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


25

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


26

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


27

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


28

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


29

Acknowledgments

• Motorola MOS 12– Tom Roche– Melissa Masteller– Frank Fischer– Michelle Demumbrum

• University of Arizona– Chris Doty

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