Vacuum Beam Studies of the Plasma Etching Kinetics of Photoresist

Vacuum Beam Studies of the Plasma Etching Kinetics of Photoresist

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. 02/03/20 2 Schematic of the Beam Apparatus in Cross-section Quadrupole Mass (Top View) Spectrometer (QMS) Atom Source F Atom Source D Analysis Section Surface Reaction Products Rotatable Carousel Experimental Diagnostics 1. QCM - Measures mass change of PR 2. QMS - Measures products formed on PR Tuning fork chopper

+ Ion Source Electron Filament Photoresist-coated Quartz Crystal Microbalance (QCM) Main Chamber Deuterium used to avoid confusion with Ar2+ (m/c ratio = 20) signal in QMS 02/03/20 3 Chemisorption: Simplified Kinetic Model# F( g ) * F Ion-Enhanced Etching: (via Chemical Sputtering) Physical Sputtering: s F F( ab ) F * YCh JAr+ C( s ) x F( ab) CFx ( g ) x * + C( s )

F C F * x ( )x YPh JAr+ + F = Fluorine Flux (atoms/cm2s) C( g ) C JAr+= Ion Flux (ions/cm2s) YPh = Physical Sputtering Yield (F/JAr+ 0) (atoms/ion) s = Sticking Coefficient of Fluorine x = Average Stoichiometry (Fluorine atoms consumed/Carbon etched) YCh = Chemical Sputtering Yield at Saturation (F/JAr+ ) (atoms/ion) 02/03/20 # Model formulation based on Si/SiO2 work by Sawin et al. JVST A 1991. 4 Effect of Large Hydrogen Fluxes During Etching Photoresist Etch Yield (Olin i-line Resist) Etch Yield (Carbon Atoms Removed/Ion) 35 30 1000 eV 1000eV D = 0 D > 4 F 25 20

15 10 5 0 0 50 100 150 200 Fluorine Atom/Argon Ion Flux Ratio 250 At 1000 eV, the etch yield is not affected despite the large flux of D atoms! 02/03/20 300 5 Why is there no effect of D flux on PR etch rate? 1. D F 3. D QMS D D F D D F 5.

F QMS D F D QCM QCM 2. D F 4. D 6. D F Abstraction Kinetics Experiments 1. Expose virgin PR to F atoms 4. Pump out deuterium from system 2. Pump out fluorine from system 5. Expose PR to F atoms (Measure DF, mass) 3. Expose PR to D atoms (Measure DF, mass) 6. Return to step 2 02/03/20 6 Abstraction Kinetics Results F QMS DF QMS Signal D D D

F D D F F2 only QCM D does not abstract F to form DF DF ~ 0 FD = 0.19 F and F2 D F QCM F abstracts D and replaces it on surface DF signal declines as D is depleted from surface Plasma on D QMS Plasma on D2 only D and D2 Time Time Fluorine atoms abstract deuterium readily from a deuterated PR surface Deuterium atoms DO NOT abstract fluorine from a fluorinated PR surface 02/03/20

7 Fluorine Site Balance Rate of Adsorption Etching Rate d F s F (1 F D ) JAr+ YCh x F 0 dt s ( F / JAr+ ) (1 D ) F s ( F / JAr+ ) + YCh x * Deuterium Site Balance Rate of Adsorption Abstraction and Recombination Rates d D s D (1 F ) FD F D DD D D D dt = Abstraction prob. ( 1 F ) of D by F s D = Recombination (s + DD) + FD ( F / D ) prob. of D with D * FD (ab) (g) DD

(g) 02/03/20 (ab) 8 Model Predictions of Effect of D Atom Flux 35 PR Etch Yield w/ Ar+/F/D Etch Yield (Carbon Atoms Removed/Ion) 30 Etch Yield with D Recomb. Prob. = 0 Etch Yield with D Recomb. Prob. = 0.2 25 D 4 F Etch Yield with D Recomb. Prob. = 1 20 15 10 5 0 0 50 100 150 200 Fluorine Atom/Argon Ion Flux Ratio

250 300 Deuterium recombination probability must be > 0.2 for etch yield to be unaffected. 02/03/20 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* + CF4 -----> CF3* + HF High activation barrier F* + CH4 -----> CH3* + HF Low activation barrier Abstraction Probability Etching Probability Deuterium Atoms ~0 1.5x10-4 Fluorine Atoms 0.19 ~0 Future Work Explore D/F abstraction reactions on Si, Al2O3, and SiO2 Explore effects of D+, CFx+, and CFx on PR etch yield 02/03/20

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