1. 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.

2. 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.

3. “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

4. 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

5. 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.

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

7. 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)

8. 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.

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

10. 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.

11. 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,

12. 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

13. 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, (2008); L. Stafford, R. Khare, J. Guha, V. M. Donnelly, J-S. Poirier and J. Margot, J. Phys. D, Appl Phys. 42, (2009).

14. 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).

15. Reported Cl Recombination Coefficients on Chlorine Plasma-Conditioned Stainless Steel

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

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

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

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

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

21. 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

22. 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.

23. 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, (2005); P. F. Kurunczi, J. Guha, and V. M. Donnelly, Phys. Rev. Lett. 96, (2006); J. Guha, P. Kurunczi, L. Stafford, and V. M. Donnelly, J. Phys. Chem. C 112, 8963 (2008).

24. 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

25. 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)  

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

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

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

29. 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

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

31. 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, (2009)
[2] J.P. Lafemina, Crit. Rev. Surface chemistry 3 (1994) 297

32. 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.