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

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 11/18/2018

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+ 0) (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) 11/18/2018 # Model formulation based on Si/SiO2 work by Sawin et al. JVST A 1991.

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! 11/18/2018

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 4. Pump out deuterium from system 2. Pump out fluorine from system 5. Expose PR to F atoms (Measure DF, Dmass) 3. Expose PR to D atoms (Measure DF, Dmass) 6. Return to step 2 11/18/2018

Abstraction Kinetics Results F 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 11/18/2018

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 ) 11/18/2018

Deuterium recombination probability must be > 0 Deuterium recombination probability must be > 0.2 for etch yield to be unaffected. 11/18/2018

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 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