Plasma-Surface Interactions at a “Spinning Wall” Vincent M. Donnelly Department of Chemical Engineering University of Houston Houston, Texas Students: Joydeep Guha (now at Lam Research), Rohit Khare Postdocs: Peter Kurunczi (now at Varian), Luc Stafford (now at Univ. Montreal) Visiting Professor Yi-Kang Pu (Tsinghua University, Beijing, China) Supported by the National Science Foundation, the Department of Energy, the American Chemical Society’s Petroleum Research Fund, the University of Houston, and Lam Research Corp.

Classes of Catalytic Reactions on Plasma Chamber Walls (Catalytic means walls are not consumed) Ion Neutralization and Fragmentation Knowledge / Treatment Good / Unit probability for this channel. Poor / Usually ignored or adjustable parameter Neutral Recombination and Reactions Fair to poor / A few published coefficients. Usually an adjustable parameter Poor / A handful of studies. Usually ignored or an adjustable parameter.

“SPINNING WALL” Method for Studying Plasma-Surface Interactions to pump to differentially-pumped mass spectrometer, or Auger Electron spectrometer 10-3P plasma 10-6P pres.=P high-speed motor spinning cylinder surface exposed to plasma

PLASMA REACTOR, SPINNING WALL AND MASS SPEC. Feed gases (Cl2 or Cl2/O2 in this talk) Line-of-sight gas from spinning surface = Chopper open signal – chopper closed signal differential pumping mass spectrometer (Extrel) tuning fork chopper Ionization gauge anodized Al reactor pumping differential pumping differential pumping

AUGER SPECTRA OF “SEASONED” REACTOR WALL DURING LONG EXPOSURES TO Cl2, O2, OR N2 PLASMAS, 5 mTorr, 600 W Wall is AlySixOz with ~5% Cl, O, or N in Cl2, O2 or N2 plasmas – Cl in Cl2 corresponds to ~1-2 x 1014cm-2 Cl at the surface. Si from erosion of quartz discharge tube; ~1% Ag from Ag-plated gaskets. Large amount of Cl in N2 plasmas, compared to O2 plasmas.

Cl2 MASS SPECTROMETER SIGNALS: PLASMA ON OR OFF J. Guha, V. M. Donnelly, Y-K. Pu, J. Appl. Phys. 103, 013306 (2008); Plasma-ON signals are a result of desorption of Cl2 formed by recombination of Cl on the spinning wall surface. Plasma-OFF signal is a result of desorption of physisorbed Cl2.

ATOM RECOMBINATION: Experiment Detects Delayed (L-H) Recombination, not prompt (E-R) Plasma (e.g. Cl2 plasma) Cl atoms Eley-Rideal (E-R) product (Cl2) (if occuring, not detectable) Langmuir-Hinshelwood (L-H) product (Cl2) Mass Spec. Reaction time  1/(2f)

ABSOLUTE Cl2 DESORPTION FLUXES FROM ANODIZED-Al EXPOSED TO A Cl2 PLASMA (plasma off removed) Cl(g)  Cl(ads) in plasma, followed by 2Cl(ads) Cl2 in mass spec.

TIME-RESOLVED AUGER SPECTRA OF SPINNING WALL DURING Cl2 PLASMA EXPOSURE, 5 mTorr, 600 W Eb = 1.5 keV

TIME-RESOLVED PEAK-TO-PEAK AUGER INTENSITIES Cl2 plasma, 5 mTorr, 600 W, Eb = 1.5 keV Dashed line corresponds to time-independent Cl coverage. CONCLUSION: Cl undergoing recombination accounts for <10% of the total Cl coverage.

Extracting Cl L-H Recombination Probabilities Cl2 plasma Cl atoms (L-H) Cl2 Mass Spec. When the sample is rotated much faster than the desorption rate, desorption and coverage become independent of time and achieve their average values. Therefore as f   (i.e. t  0) it is as though the sample were continuously exposed to a Cl flux of 1/3 that in the plasma, Cl. Therefore LH recombination probability,

Cl Atom LH Recombination Probabilities on Anodized Al as a Function of Cl Flux and Total Pressure Cl is small and appears to both increase and decrease with increasing Cl flux

Cl Atom LH Recombination Probabilities on Anodized Al as a Function of Cl-to-Cl2 Number Density Ratio Cl scales with Cl-to-Cl2 flux ratio. Suggests Cl2 may block sites for Cl adsorption and recombination. See J. Guha, V. M. Donnelly, Y-K. Pu, J. Appl. Phys. 103, 013306 (2008); L. Stafford, R. Khare, J. Guha, V. M. Donnelly, J-S. Poirier and J. Margot, J. Phys. D, Appl Phys. 42, 055206 (2009).

Cl Recombination on Anodized Aluminum vs. Stainless Steel Similar recombination probabilities because they are both coated with a SiOxCly layer. Stainless values actually lower, probably because the surface is smoother (electropolished).

Reported Cl Recombination Coefficients on Chlorine Plasma-Conditioned Stainless Steel

Proposed Site Blocking Mechanism for Cl Heterogeneous Recombination in Cl2 Plasmas High Cl2/Cl density Low Cl2/Cl density

“A global (volume averaged) model of a chlorine discharge” E. G. Thorsteinsson and J. T. Gudmundsson Plasma Sources Sci. Technol. 19 (2010) 015001 Points: Experiments (Malyshev and Donnelly) Lines: Their model. No adjustable parameters.

WHAT DOES TIME DEPENDENCE OF DESORPTION TELL US? Proposed Mechanism for Cl Recombination, Cl2 Adsorption and Cl2 Desorption Plasma On Same Process? Plasma Off

Time-Dependence of Observed and Modeled Desorption plasma side AES or MS side spinning-substrate  = 0  =  2/3 tr=1/(2 f)

Predicted vs. Observed Cl2 Desorption Kinetics Why so different?: Multiple rates.  Distribution of surface sites

Time-Dependence of Observed and Modeled Desorption Cl2 adsorption – desorption (Plasma OFF) Adsorbed Cl2 formed by Cl2 adsorption. Adsorbed Cl2 also formed by Cl recombination. From our measured Cl recombination probabilities we can calculate the amount of adsorbed Cl2 due to Cl recombination. Lets assume Cl2 desorption is rate limiting. Use Cl2 desorption kinetics with plasma OFF to compute kinetics with plasma ON. NOTE: NOTHING CHANGED – just turn on plasma. Compare model to measurements. Assumed Gaussian distribution of binding energies for Cl2 adsorption and desorption, used to predict decays (lines). 10 m Anodized Al

Time-Dependence of Observed and Modeled Desorption Assumed Gaussian distribution of binding energies for Cl2 adsorption and desorption, used to predict decays (lines). Adsorbed Cl2 formed by Cl2 adsorption. Adsorbed Cl2 also formed by Cl recombination. From our measured Cl recombination probabilities we can calculate the amount of adsorbed Cl2 due to Cl recombination. Lets assume Cl2 desorption is rate limiting. Use Cl2 desorption kinetics with plasma OFF to compute kinetics with plasma ON. NOTE: NOTHING CHANGED – just turn on plasma. Compare model to measurements.

O RECOMBINATION ON ANODIZED-Al EXPOSED TO AN O2 PLASMA Similar to Cl2 plasma, but no physisorbed O2 (i.e. no increase in O2 signal vs. rpm with plasma off). P. F. Kurunczi, J. Guha, and V. M. Donnelly, J. Phys. Chem. B 109, 20989 (2005); P. F. Kurunczi, J. Guha, and V. M. Donnelly, Phys. Rev. Lett. 96, 018306 (2006); J. Guha, P. Kurunczi, L. Stafford, and V. M. Donnelly, J. Phys. Chem. C 112, 8963 (2008).

O Recombination in Oxygen Plasmas Similar kinetics, different mechanism (no O2 site blockage) Vary kd,I distrubution to get the best fit of the model to the experimental measurements. kd,i

DESORPTION MASS SPECTRA OF Cl2/O2 PLASMAS ClO and ClO2 are desorption products: O(ads) + Cl(ads)  ClO(g)  ClO(ads) 2O(ads) + Cl(ads)  ClO2(g)  ClO2(ads)  

Mixed Cl2 / O2 Plasmas: Recombination and Reactions of Cl i.e. Cl(g) + Cl(ads)  Cl2(g) Cl(g) + O(ads)  ClO(g)

Mixed Cl2 / O2 Plasmas: Why does O2 addition have little effect on Cl, yet addition of Cl2 suppresses Cl?

Proposed Site Blocking Mechanism for Cl Heterogeneous Recombination in Cl2 Plasmas Cl2 sticks and blocks sites for Cl recombination O2 does not

Effect of Trace Cu on O-Atom Recombination in an O2 Plasma Auger spectrometer O2 Plasma Cu PVD source Coat the sample with trace Cu while it is exposed to an O2 plasma – simulate contamination during via etching

O Recombination on Ti-contamined Surface in Oxygen Plasmas O recombination probability decreases by 41% after 5% Ti surface coverage.

Proposed Mechanism O Si Cu+ O Si Ti Ti4+ + Ti(g) + O Cu(g) + O Si O Dangling bond O Si Cu+ O Si Ti Ti4+ + Ti(g) + O Charge transfer (autocompensation)[2] Cu(g) + O Si O Cu+ Cu2+ O + O O ref[1] Si Si O O Si Cu2+ O Si Cu+ O2(g) [1] J. Guha et. al. J. Appl. Phys. 105, 113309 (2009) [2] J.P. Lafemina, Crit. Rev. Surface chemistry 3 (1994) 297

SUMMARY Cl Langmuir-Hinshelwood (L-H) recombination seems to be limited by Cl2 desorption. The mean binding energy for Cl2 on anodized Al is 13 kcal/mol. and the range of binding energies is ~9 to 17 kcal/mol. Cl recombination coefficient increases with Cl-to-Cl2 number density ratio. O recombination on anodized Al follows kinetics with a range of rates at distributions of sites, but the mechanism is different from Cl recombination – no O2 site blockage. Our  values have been used in a global model by Thorsteinsson and Gudmundsson. With no adjustable parameters, their model reproduces Cl densities measure by Maylshev and Donnelly in a chlorine ICP. Trace Cu surface contamination catalyzes O recombination. Small amounts of surface Ti suppresses O recombination.