VACUUM ARC DEPOSITION IN INTERIOR CAVITIES Physical and Engineering Principles and Ideas for Interior Implementations Ra

1. VACUUM ARC DEPOSITION IN INTERIOR CAVITIES Physical and Engineering Principles and Ideas for Interior Implementations Raymond L. Boxman Electrical Discharge and Plasma LaboratorySchool of Electrical EngineeringTel-Aviv University Thin Films Applied To Superconducting RF

2. Background and Objectives • Vacuum Arc Deposition • (a.k.a. cathode arc deposition, arc evaporation) • Most popular method for applying hard coatings in tool industry • …but less well known than other PVD (e.g. sputtering, e-beam evaporation) and CVD methods • Objectives of this lecture: • Review: • Physics of vacuum arc • Engineering issues in vacuum arc deposition • Suggest implementations with interior cavity Thin Films Applied To Superconducting RF

3. Outline • I. Physics of the Vacuum Arc • The Arc Discharge • Cathode Spots and Cathode Spot Plasma Jets • Observations • Theory • Macroparticles • II. Vacuum Arc Engineering • Arc Ignition • Cathode Spot Confinement and Motion • Heat Removal • Macroparticle Control • III. Suggestions for Coating Interior Cavities Thin Films Applied To Superconducting RF

4. I. Physics of the Vacuum Arc – The Arc Discharge • D.C. Discharges • Corona • High V, Low I • At sharp point • Glow Discharge • V ~ 100’s V, I ~mA’s • Cathode fall 150-550 V, depends on gas and cathode material • Arc • 10’s of volts, A-kA • Cathode spots Thin Films Applied To Superconducting RF

5. Glow ‘individual’ secondary emission of electrons by: Ions (depends on ionization energy, not kinetic energy) Excited Atoms Photons Not enough! Multiplication in avalanche near cathode Need high cathode drop (100’s of V’s) Used in sputtering to accelerate bombarding ions into ‘target’ cathode Arc Collective electron emission Current at cathode concentrated into cathode spots Combination of thermionic and field emission of electrons Can get sufficient electron current Low cathode voltage drop (10’s of V’s) High temp. in cathode spot gives high local evaporation rate – used in vacuum arc deposition Difference between Glow and Arc –cathode electron emission process Thin Films Applied To Superconducting RF

6. Cathode Spot Characteristics • Diam: m’s • Lifetime: ns’s to s’s • Extinguish, reignite at adjacent location • Apparent ‘random walk’ motion • In B field, “retrograde motion” in -JB direction Thin Films Applied To Superconducting RF

7. Cathode Spot Plasma Jets • ~Fully Ionized • Multiple ionizations common • Zav(Ti) ~2 • Ion directed kinetic energy 20-150 eV • Flow velocity ~104 m/s • ~cos distribution • Ti, Te ~few eV • Supersonic ions, thermal electrons • Ii -0.1 Iarc, Ie  1.1 Iarc Thin Films Applied To Superconducting RF

8. Cathode Spot Theory • Two Approaches: • Quasi-continuous (~steady state) • Explosive Emission • Quasi-continuous approach: • Must account simultaneously for: • Cathode heating (for e- and atom emission) • Electron emission • Atom emission • High ion energy / plasma velocity Thin Films Applied To Superconducting RF

9. Beilis Model: Cathode Spot & Cathode Plasma Jet Electron relaxation zone. Ion diffusion Cathode SHEATH Hydrodynamic Plasma Flow Acceleration Region e i e a  Kinetic flow Knudsen Layer Plasma Jet Expansion Cathode Spot Region Thin Films Applied To Superconducting RF

10. TF emission of electrons Evaporation of atoms Acceleration of electrons into vapor Collisionless sheath Collisionless Knudsen layer Electrons loose energy to vapor in relaxation zone Beilis Model Thin Films Applied To Superconducting RF

11. Back-flow of electron and ions to cathode Heats cathode spot Joule heating under cathode surface Joule heating of plasma Hydrodynamic plasma expansion Beilis Model – cont’d Thin Films Applied To Superconducting RF

12. Like in jet engine – conversion of thermaldirected kinetic energy But plasma heated all along length Continuous heating, conversion into kinetic energy, so Ti~3ev, Ei~20-150eV Beilis Model –Hydrodynamic Plasma Expansion Thin Films Applied To Superconducting RF

13. Explosive Electron Emission (Mesyats et al.) • Cathode spot is a sequence of explosion of protuberances Thin Films Applied To Superconducting RF

14. EEE (Mesyats et al.) – cont’d • Each explosion creates further protuberances, which can then explode • Idea supported by high resolution laser shadowgraphs, showing short life time and small dimensions, spike noise in ion current, etc. Thin Films Applied To Superconducting RF

15. Macroparticles Thin Films Applied To Superconducting RF

16. Macroparticles • Spray of liquid metal droplets from the cathode spot • small fraction of cathode erosion for refractory metals • large fraction of cathode erosion for low melting point cathode materials • exponentially decreasing size distribution function • most mass in the 10-20 m diam range • preferentially emitted close to cathode plane • Downward pressure from plasma jet on liquid surface Thin Films Applied To Superconducting RF

17. II. Vacuum Arc Engineering • Coating forms on any substrate intercepting part of plasma jet • In vacuum, coating composition  cathode composition • In reactive gas background, can form compounds (nitrides, oxides, carbides, etc.) Thin Films Applied To Superconducting RF

18. II. Vacuum Arc Engineering • Arc Ignition • Cathode Spot Confinement and Motion • Heat Removal • Macroparticle Control Thin Films Applied To Superconducting RF

19. Arc Ignition • Problem: extremely high voltage needed to break-down vacuum gap (~100 kV/cm) • Drawn-arc (most common) • Trigger electrode, mechanically operated • Connected to +voltage (e.g. anode via current limiting resistor) • Momentary contact with cathode • Arc ignited when contact broken • Current transfers to main anode • Breakdown to trigger electrode • Vacuum gap • Sliding spark • Laser ignition Thin Films Applied To Superconducting RF

20. Controlling Cathode Spot Location and Motion • Objectives: • Locate CS’s on ‘front’ surface of cathode • Maximize plasma transmission to substrates • Prevent damage to cathode structure • Methods: • Magnetic Field (retrograde and “acute angle” motion • Passive border • Strellnitski shield • Pulsed arc Thin Films Applied To Superconducting RF

21. Magnetic Control of Cathode Spots Thin Films Applied To Superconducting RF

22. Passive Border Thin Films Applied To Superconducting RF

23. Strelnitski Shield Thin Films Applied To Superconducting RF

24. Pulse Control • Basic Idea: arc duration shorter than CS travel time to edge • Short Pulse • Laser Ignition • Long Pulse - Long Cathode • Active detection of CS location – • quench arc when CS reaches edge Thin Films Applied To Superconducting RF

25. Heat Removal • Total power P = VarcIarc • Varc ~20-40 V • Iarc ~ 50-1000 A • P > 1 kW • Distribution • ~1/3 in cathode • ~2/3 in anode • Substrate: Thin Films Applied To Superconducting RF

26. Heat Removal from Cathode • Cool cathode important to • minimize MP generation • Prevent cathode damage • In best case, C.S.’s rapidly moved around to give on average a uniform heat flux on cathode surface S=P/A Thin Films Applied To Superconducting RF

27. Heat Removal from Cathode, cont’d • Then average surface Temp (far from C.S.) given by hc– contact heat transfer coefficient hw– heat transfer coefficient to water Thin Films Applied To Superconducting RF

28. Thin Films Applied To Superconducting RF

29. Substrate Temperature Control • Ts critical in determining coating properties • Measure with IR radiation detector • Ts determined by balance between heating and cooling processes • Often use heat flux from process to control Ts • Vary bias or arc current Thin Films Applied To Superconducting RF

30. Macroparticle Control • 3 Approaches • Ignore • Get good results (e.g. with tool coatings) despite (or because of?) MPs • Minimize MP Production/Transmission • Remove MPs Thin Films Applied To Superconducting RF

31. Minimize MP Production/Transmission • Choose refractory cathode material • “Poison” (i.e. nitride) cathode surface • Operate at ‘higher’ N2 background pressure • Low cathode temperature • direct cooling • lower current (lower deposition rate) • Place substrates where plasma flux max, MP flux min Thin Films Applied To Superconducting RF

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33. Macroparticle Removal • Filtered Vacuum Arc Deposition • Use magnetic field to bend plasma beam around an obstacle which blocks MP transmission Thin Films Applied To Superconducting RF

34. Thin Films Applied To Superconducting RF VENETIAN BLIND

35. Two quarter-torus filtered arcs at Tel Aviv University Thin Films Applied To Superconducting RF

36. Thin Films Applied To Superconducting RF

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38. Filtered Arc –Advantages and Disadvantages • Advantages • High quality, very smooth coatings • ‘almost’ MP free • Can achieve higher deposition rate than other ‘high quality’ techniques • Disadvantages • Usually poor plasma transmission • Material utilization efficiency low • Much slower than unfiltered arc deposition • Bulky equipment Thin Films Applied To Superconducting RF

39. Other Arc Modes • Hot Anode Vacuum Arc • Crucible anode • Hot Refractory Anode Vacuum Thin Films Applied To Superconducting RF

40. 10 mm Thin Films Applied To Superconducting RF

41. III. How can we coat the inside of: Thin Films Applied To Superconducting RF

42. Approach 1: Ignore MPs Thin Films Applied To Superconducting RF

43. Approach 1: Ignore MPs • Cavity serves as vacuum chamber and anode • Various techniques for magnetically controlling c.s. motion Thin Films Applied To Superconducting RF

44. Approach 2: Miniature Filter:Example – Welty Rect. Filter Thin Films Applied To Superconducting RF

45. Approach 2: Miniature Filter:Another Example • Progress in Use of Ultra-High Vacuum Cathodic Arcs for Deposition of Thin Film Superconducting Layers • J.Langner, M.J.Sadowski, P.Strzyzewski, R.Mirowski, J.Witkowski, S.Tazzari, L.Catani, A.Cianchi, J.Lorkiewicz, R.Russo, T.Paryjczak, J.Rogowski, J.Sekutowicz • Presentation 28 Sept at XXXIII-ISDEIV in Matsue, Japan • Showed use of a cylindrical “Venetian Blind” filter to deposit Nb inside cavity! Thin Films Applied To Superconducting RF

46. Approach III. Beilis “black-body” HRAVA deposition device Thin Films Applied To Superconducting RF

47. Interior Coatings - Considerations • Use cavity as vacuum chamber • Need complicated end seal to allow for electrical connections (main arc and trigger), cooling water, in some cases motion • Cooling can be applied directly to outside of tube • Fitting everything into cavity – difficult! • Integrity, lifetime? • Triggering – not shown Thin Films Applied To Superconducting RF

48. Summary and Conclusions • VAD uses inherent properties of cathode spot plasma jets to rapidly deposit dense, high quality coatings • Successful implementation requires “plasma engineering” to: • Confine cathode spots on desired surface • Remove process heat • Control macroparticle contamination Thin Films Applied To Superconducting RF

49. Summary and Conclusions, cont’d • Several approaches exist for efficiently and rapidly coating interior of RF cavities • But with technical difficulties Thin Films Applied To Superconducting RF