1. Vacuum Beam Studies of Photoresist Etching Kinetics
SFR Workshop
November 8, 1999
David Graves, John Coburn, and Frank Greer
UC Berkeley, CA
Photoresist etching kinetics are investigated using Argon ions, Fluorine atoms, and Deuterium atoms. Etch yields and abstraction probabilities are reported.
11/18/2018

2. Schematic of the Beam Apparatus in Cross-section (Top View)
Quadrupole Mass
Spectrometer (QMS)
Atom Source F
Atom Source D
Surface Reaction Products
Analysis Section
Rotatable
Carousel
Tuning fork
chopper + +
Experimental Diagnostics
QCM
- Measures mass
change of PR
QMS
- Measures products
formed on PR
Photoresist-coated
Quartz Crystal
Microbalance (QCM)
Ion Source
Electron
Filament
Main Chamber
Deuterium used to avoid confusion with Ar2+ (m/c ratio = 20) signal in QMS
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3. Simplified Kinetic Model#
Chemisorption: s GF F * F
YCh
JAr+
Ion-Enhanced Etching:
(via Chemical
Sputtering) x x * x + F C F x
( )x *
YPh
JAr+
Physical Sputtering: + C
GF = Fluorine Flux (atoms/cm2s) JAr+= Ion Flux (ions/cm2s)
YPh = Physical Sputtering Yield (GF/JAr ) (atoms/ion) s = Sticking Coefficient of Fluorine
x = Average Stoichiometry (Fluorine atoms consumed/Carbon etched)
YCh = Chemical Sputtering Yield at Saturation (GF/JAr+ ) (atoms/ion)
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# Model formulation based on Si/SiO2 work by Sawin et al. JVST A 1991.

4. Effect of Large Hydrogen Fluxes During Etching
GD = 0
GD > 4 GF
At 1000 eV, the etch yield is not affected despite the large flux of D atoms!
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5. Why is there no effect of D flux on PR etch rate?3. 5.
QMS D
QMS F 1. D F D D F D D F D F D
QCM
QCM 2. 4. 6. F D D F D
Abstraction Kinetics Experiments
1. Expose virgin PR to F atoms Pump out deuterium from system
2. Pump out fluorine from system Expose PR to F atoms (Measure DF, Dmass)
3. Expose PR to D atoms (Measure DF, Dmass) 6. Return to step 2
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6. Abstraction Kinetics ResultsF
QMS D
QMS D D D D D F F D F
QCM
QCM
F abstracts D and replaces it on surface
DF signal declines as D is depleted from surface
gFD = 0.19
D does not abstract F to form DF
gDF ~ 0
DF QMS Signal
Plasma on
Plasma on
F and F2
D and D2
D2 only
F2 only
Time
Time
Fluorine atoms abstract deuterium readily from a deuterated PR surface
Deuterium atoms DO NOT abstract fluorine from a fluorinated PR surface
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7. Abstraction and Recombination
Fluorine Site Balance
Rate of Adsorption
Etching Rate
s GF
JAr+
YCh x
s ( GF / JAr+ )
s ( GF / JAr+ ) + YCh x
Deuterium Site Balance
Rate of Adsorption
Abstraction and Recombination
Rates
s GD qD
gFD GF qD
gDD GD qD
gFD = Abstraction prob. of D(ab) by F(g)
gDD = Recombination prob. of D(g) with D(ab) s
(s + gDD) +
gFD ( GF / GD )
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8. Deuterium recombination probability must be > 0
Deuterium recombination probability must be > 0.2 for etch yield to be unaffected.
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9. Summary of Experimental Results
D does not abstract F, but will etch PR very slowly
F readily abstracts D from the PR surface
Results consistent with gas phase kinetics:
H* CF > CF3* + HF
High activation barrier
F* CH > CH3* + HF
Low activation barrier
Abstraction Probability
Etching Probability
Deuterium Atoms
~ 0
1.5x10-4
Fluorine Atoms
0.19
Future Work
Explore D/F abstraction reactions on Si, Al2O3, and SiO2
Explore effects of D+, CFx+, and CFx on PR etch yield
11/18/2018