Plasma cleaning

Plasma cleaning: towards high yield cavity processing

G. Wu, W-D. Moeller, C. Antoine, H. Jiang, I. Pechenezhskiy, T. Khabiboulline, J. Reid, T. Koeth, W. Muranyi, B. Tennis, E. Harms, Y. Terechkine, H. Edwards, D. Mitchell, A. Rowe, C. Boffo, C. Cooper, Fermilab

July 24, 2008 G. WU et al, TFSRF 2008, JLAB 1 Plasma cleaning

Contents

Introduction – Cavity limitation: field emission and quench Introduction to plasma cleaning General application SRF field activity ECR plasma and RF cavity Experimental plan and the issues to be addressed Potential benefit and other applications

July 24, 2008 G. WU et al, TFSRF 2008, JLAB 2 Plasma cleaning

Field emission is a continuing problem

Red represents the FE limitation

DESY cavity experience L. Lijie’s summary of DESY cavity databank, DESY, 2006

July 24, 2008 G. WU et al, TFSRF 2008, JLAB 3 Plasma cleaning

Field emission is a continuing problem

JLAB SNS cavity experience J. Ozelis, SRF 2005

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Field emission sources

Inclusions from weld-prep machining, forming Residues from chemical processing (EP,BCP) Water impurity (HPWR) Clean room particles Assembly particulates

Particulates includes: (Ni, Mn, In, Cu, C, F, Cl, Ca, Al, Si), Nb, Fe, Cr, S, etc…

Particle free is not guaranteed FE is a localized statistical problem: Success else where does not guarantee the local success Past success does not guarantee the future success Îexperienced in all Lab’s

Field emission is still a hot topic – SRF 2007 July 24, 2008 G. WU et al, TFSRF 2008, JLAB 5 Plasma cleaning

Introducing plasma

Plasma induces chemical reactions in reduced temperatures, converts some surface materials, contaminants to gaseous phase Plasma generates accelerated to bombard the surface (including loose particles)

Glow discharge, RF discharge and ECR plasma are common methods

Noble gas, N2, O2, H2, mixtures, …

Dissolve chemical compound to etching or coating

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Applications [3,4]

Semiconductor industry [1,6,9] Micro-electronics – Josephson junctions [8] Automobile industry – painting [2,10] Aircraft industry – painting [11] Medical applications – pre-cleaning for coating and sterilization Optical industry Antique preservation – surface protection Particle accelerator – beam line components [14] Microwave power – multi-MW, large-orbit, coaxial gyrotron [7]

SRF Field for Nb cavity – JLAB [13] Plasma cleaning – INFN/Lagnero, JLAB [12,18] Coupler processing – DESY (W-D. Moeller, Dennis)

July 24, 2008 G. WU et al, TFSRF 2008, JLAB 7 Plasma cleaning ECR plasma and RF cavity

Electron being accelerated clockwise by periodic electric field. External magnetic is pointing out of the paper (not shown). Color reflects energy

eB ω = m

ECR = cyclotron resonance

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ECR plasma and RF cavity

17 3 -6 1 R 4β n0= 3.2×10 /m τ=2.6×10 s Minimum field E = ( )Q P acc 0 in 2 -5 d Q0 (1+ β ) r< 0.3 mm P=1×10 torr 130 V/m for 90eV

Eacc for 150 watt RF input under different input 9-cell Cavity Eacc for 15 kilo-watt RF input under coupling for cavity Q0 different input coupling for cavity Q0

July 24, 2008 G. WU et al, TFSRF 2008, JLAB 9 Plasma cleaning

For multi-cell cavity, plasma can be stable across all the cells.

x-wave Dose rate: ~ 5x1017 /cm2/s for 10-5 torr

ωL ωR Physical process 1 µm particle (~ 1010 ) 3.9GHz 15 minutes (1% knock out ω rate) Microwave travelingω inside magnetized plasma ω 2 Chemical process can be ce 2 ωce much higher than R,L = ± + pe + 2 4 physical and does not affect surface morphology ω : function of density

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Experimental plan and the issues to be addressed Usual cleaning Cold RF test to find FE limit Plasma processing at room temperature Cold RF test to verify improvement

Gas mixtures: Ar, H2, O2, He, Kr.

Surface contamination removal? Ar implantation? Ar creates surface Gas pressure defects? Plasma density Dry-oxidation afterwards? Temperature distribution If Hydrogen, how about Q-disease? Ion energy distribution If , oxidation compound? Ion flux rate He also effective? Chemical reaction

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Coupler Qext 3x104 – 2x109

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70

Plasma heating 60

50

g 40

30

20 Temperature [deTemperature 10

0 0 1020304050 Time [min.]

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30 Plasma flexibility 12

25 10

20 8 w W 15 6

10 4 Cavity power [ Normalized cavityNormalized po 5 2

0 0 10 15 20 25 30 35 9.4 6 6.5 7 7.5 8 8.5 9 9.5 9.6 Input power [W] Coil current [A]

9.4 9.3

9.2 9.2

9 9.1 w 8.8 9 8.6 8.9 8.4 8.8 Normalized cavity po cavity Normalized 8.2 Normalized cavity power power cavity Normalized

8 8.7

7.8 8.6 0.00E+00 5.00E-05 1.00E-04 1.50E-04 2.00E-04 2.50E-04 3.00E-04 3.50E-04 3.865 3.87 3.875 3.88 3.885 3.89 3.895 3.9 3.905 3.91 Pressure [torr] Freq [GHz] Coupler Qext ~1000 July 24, 2008 G. WU et al, TFSRF 2008, JLAB 15 Plasma cleaning ECR plasma results

Targeted sulfur contamination S: 115.21 °C (M), 444.6 °C (B)

Far way from iris location in beam pipe

Before plasma treating After plasma treating Ar plasma 20 minutes, Air plasma 30 minutes, highest temperature 48°C

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Elements (K) Red S Green C, O, S Near iris location in beam pipe Orange Nb

Before plasma treating After plasma treating Ar plasma 20 minutes, Air plasma 30 minutes, highest temperature 48°C

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ECR plasma results

Close to iris location in beam pipe

Before plasma treating After plasma treating Ar plasma 20 minutes, Air plasma 30 minutes, highest temperature 48°C

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Oxygen content

A. Wu, JLAB

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Potential benefit and other applications

Reduce field emission, increase cavity production yield through in-situ processing, improve cryomodule performance Potential new design for cryomodule recovery (Build-in magnetic coil)

CF4+O2, Cl2 in plasma – etching of niobium [15, 16]

NbCl5 – coating of niobium

Sn/SnClx vapor – Nb3Sn formation

B2H6+Mg – similar to Penn State HPCVD [17]

Cavity RF test is being prepared.

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1.Kim, H.-w. and R. Reif, In-situ low-temperature (600[deg]C) wafer surface cleaning by electron cyclotron resonance hydrogen plasma for silicon homoepitaxial growth. Thin Solid Films, 1996. 289(1-2): p. 192-198. 2.Steffen, H., et al., Process control of RF plasma assisted surface cleaning. Thin Solid Films, 1996. 283(1-2): p. 158-164. 3.Kruger, P., R. Knes, and J. Friedrich, Surface cleaning by plasma-enhanced desorption of contaminants (PEDC). Surface and Coatings Technology, 1999. 112(1-3): p. 240-244. 4.Kegel, B. and H. Schmid, Low-pressure plasma cleaning of metallic surfaces on industrial scale. Surface and Coatings Technology, 1999. 112(1-3): p. 63-66. 5.Li, H., et al., An in situ XPS study of oxygen plasma cleaning of aluminum surfaces. Surface and Coatings Technology, 1997. 92(3): p. 171-177. 6.Choi, K., et al., Removal efficiency of organic contaminants on Si wafer by dry cleaning using UV/O3 and ECR plasma. Applied Surface Science, 2003. 206(1-4): p. 355-364. 7.William E. Cohen, R.M.G., Reginald L. Jaynes, Christopher W. Peters, and Y.Y.L. Mike R. Lopez, Scott A. Anderson, and Mary L. Brake, Thomas A. Spencer, Radio-frequency plasma cleaning for mitigation of high-power microwave-pulse shortening in a coaxial gyrotron. APPLIED PHYSICS LETTERS, 2000. 77(23): p. 3. 8.T. S. Kuan, S.I.R., and R. E. Drake Journal of Applied Physics, 1982. 53(11): p. 7. 9.Th. Schäpers, R.P.M., G. Crecelius, H. Hardtdegen, and H. Lüth Preparation of transparent Nb/two-dimensional electron gas contacts by using electron cyclotron resonance plasma cleaning. Journal of Applied Physics, 2000. 88(7): p. 3. 10 D.F. O'Kane, K. L. Mittal, Plasma cleaning of metal surfaces. Journal of Vacuum Science and Technology, 1974. 11(3): p. 3. 11.Petasch, W., et al., Low-pressure plasma cleaning: a process for precision cleaning applications. Surface and Coatings Technology, 1997. 97(1-3): p. 176-181. 12.N. Patron, R.B., L. Phillips, M. Rea, C. Roncolato, D. Tonini, and V. Palmieri. Plasma cleaning of cavities. in Thin Films and new ideas for pushing the limits of RF superconductivity. 2006. Padua, Italy. 13. M. Raskovic, SRF 2007 14. H.F. Dylla, J. Vac. Sci. Technol.. A6 (1988) 1276. 15. M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983 16. Jay N. Sasserath and John Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990 17. Xi, X.X., In Situ Growth of MgB2 Thin Films by Hybrid Physical-Chemical Vapor Deposition. IEEE Transactions on Applied Supconductivity, 2003. 13(2): p. 5 18. N. Patron et al, SRF 2007

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Acknowledgement

H. Padamsee, A. Romanov Cornell P. Kneisel, L. Phillips, G. Bialas, R. Rimmer, H. Wang, B. Manus, G. Slack JLAB B. Tennis, R Schuessler, R. Padilla, D. Burk Femilab

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