Plasma-Surface Interactions at a Spinning Wall Vincent M. Donnelly

Department of Chemical EngineeringUniversity of HoustonHouston, Texas

Students: Joydeep Guha (now at Lam Research), Rohit KharePostdocs: 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 Societys 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 FragmentationKnowledge / TreatmentNeutral Recombination and ReactionsGood / Unit probability for this channel. Poor / Usually ignored or adjustable parameterFair to poor / A few published coefficients. Usually an adjustable parameterPoor / A handful of studies. Usually ignored or an adjustable parameter.

plasmaspinning cylinder surface exposedto plasmatopumpto differentially-pumped mass spectrometer, or

Auger Electron spectrometerhigh-speed motorSPINNING WALL Method for Studying Plasma-Surface Interactions10-3P10-6Ppres.=P

differentialpumpingdifferentialpumpingpumpingdifferentialpumpingFeed gases(Cl2 or Cl2/O2 in this talk)mass spectrometer (Extrel)tuning forkchopperPLASMA REACTOR, SPINNING WALL AND MASS SPEC.anodized Al reactorIonization gauge

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. AUGER SPECTRA OF SEASONED REACTOR WALL DURING LONG EXPOSURES TO Cl2, O2, OR N2 PLASMAS, 5 mTorr, 600 W

Cl2 MASS SPECTROMETER SIGNALS: PLASMA ON OR OFF 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.J. Guha, V. M. Donnelly, Y-K. Pu, J. Appl. Phys. 103, 013306 (2008);

Plasma (e.g. Cl2 plasma)Eley-Rideal (E-R) product (Cl2) (if occuring, not detectable)Langmuir-Hinshelwood (L-H) product (Cl2)Cl atomsATOM RECOMBINATION: Experiment Detects Delayed (L-H) Recombination, not prompt (E-R) 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 WEb = 1.5 keV

TIME-RESOLVED PEAK-TO-PEAK AUGER INTENSITIESCl2 plasma, 5 mTorr, 600 W, Eb = 1.5 keVDashed line corresponds to time-independentCl coverage.CONCLUSION: Cl undergoing recombination accounts for

Extracting Cl L-H Recombination Probabilities Cl2 plasma(L-H) Cl2Cl atomsMass Spec. When the sample is rotated much faster than the desorption rate, desorption and coverage become independent of time and achieve their average values.

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

A global (volume averaged) model of a chlorine dischargeE. G. Thorsteinsson and J. T. GudmundssonPlasma 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

Time-Dependence of Observed and Modeled Desorption

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

Time-Dependence of Observed and Modeled Desorption 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.Cl2 adsorption desorption (Plasma OFF)

Assumed Gaussian distribution of binding energies for Cl2 adsorption and desorption, used to predict decays (lines). Time-Dependence of Observed and Modeled Desorption 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).

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

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 Cli.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 PlasmasO2 does not

Effect of Trace Cu on O-Atom Recombination in an O2 Plasma

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

Proposed Mechanism Cu(g) + O SiO SiCu+ + OO SiCu2+O OO2(g)Dangling bondref[1][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.