Plasma processes as advanced methods for cavity cleaning

1. Plasma processes as advanced methods for cavity cleaning N. Patron, R. Baracco, L. Phillips, M. Rea, C. Roncolato, D. Tonini and V. Palmieri … pushing the limits of RFS Legnaro 2006

2. ETCHING a main process CLEANING a post processs • Hydrocarbons • Removal of ~ 100 μm • Water • Reduce surface roughness • Reduce surface contamination • Oxygen, Nitrogen and other adsorbed gases • Remove damaged layers

3. Sputtering • PLASMA • Reactive ion etching • DRY ETCHING • Ion beam cleaning • ION GUN • Reactive ion beam etching • Chemical etching • WET ETCHING • Electropolishing • Electromachining

4. Let’s analyze one by one the different DRY ETCHINGtechniques

5. Sputtering • PLASMA • Reactive ion etching • DRY ETCHING • Ion beam cleaning • ION GUN • Reactive ion beam etching One example from our experience:

6. CUORECryogenic Underground Observatory for Rare Events • Cu frame used in CUORE experiment for the detencion of a dobble  decadiment • We have been given the task to find a way to eliminate ppb contamination of 232 Th from the Cu surface

8. Dry etching methods are very clean

9. Smooth surface • Thin grain boundaries But Physical Methods treatment can become an enemy…..

10. A deeper etching • Coarsening of grain boudaries • Rough surface Cleaner surface, but higher demagnetization factor

11. Sputtering Plasma Etching • For cleaning it might good • It isn’t a fast routine method (vacuum systems, flanges to be mounted…) • Whenever applying dry etching a fundamental comprehension of the role of Grain boundaries and grain Demagnetization factor is necessary.

12. Sputtering • PLASMA • Reactive ion etching • DRY ETCHING • Ion beam cleaning • ION GUN • Reactive ion beam etching

13. Reactive gasses are injected in the plasma • Mostly developed for Nb-based Josephson junctions switching devices. • Gas mixture more frequently used are: CF4/O2(a,b), CCl3F(c), SF6/O2(d); I2, XeF2(e). a) M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983 b) J. N. Sasserath and John Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990 c) J. W. Noè, Nucl. Inst. and Meth. 212 (1083) 73 d) B. J. Curtis and H. Mantle, J.Vac.Sci. Technol. A, Vol. 11, No. 5, Sep/Oct 1993 e) X. L. Fu, P. G. Li, A. Z. Jin, H. Y. Zhang, H. F. Yang, W. H. Tang, J.Vac.Sci. Technol. B, Vol. 23, No. 2, Mar/Apr 2005

14. From Literature RF reactive ion etching device • Parallel plate RF powered etcher operating at 13.56 MHz • Using CF4 and O2 as the reactive gas mixture M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983

15. From Literature Etching rates are functions of O2 percentage M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983 J. N. Sasserath, J. Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990

16. From Literature • Niobium etching rate = 30 μm/h Jay N. Sasserath and John Vivalda, J.Vac.Sci. Technol. A, Vol. 8, No. 6, Nov/Dec 1990 • Niobium etching rate = 2,4 μm/h M. Chen and R. H. Wang, J.Vac.Sci. Technol. A, Vol. 1, No. 2, Apr/June 1983

17. CCl3F-vapour rf discharge processing • Eliminate secondary electron emission problems of multipactoring from lead-plated copper quarter-wave resonators. • Flurine ions and radicals are very agressive, Noè suggests that CF4 should work too. J. W. Noè, Nucl. Inst. and Meth. 212 (1083) 73

18. LNL ACTUAL RESULTS • Niobium DC diode sputtering with CF4 • Pressure of 410-2 mbar • Sample voltage: - 1250 V Etching rate: 12,7 μm/h

19. Sputtering • PLASMA • Reactive ion etching • DRY ETCHING • Ion beam cleaning • ION GUN • Reactive ion beam etching

20. Two main type of sources

21. Kaufman sources Broad-beam sourcewith an extracting grid in wich a cathodic filament sustains a magnetical confined plasma

22. Gridless sources Best confinament condition for λ<<w

23. MAGNETRON SOURCE Positive ions are accelerated from the ionization region toward the cathode’s surface by Vdc GRIDLESS SOURCE It works just like a magnetron source where the anode is above ground potential and the cathode has a hole from where ions can exit and form the ion beam Gridless source

24. We used a gridless source • It is more simple and it’s easier to be modified if eventually we want to reduce its dimension to use it inside of a cavity • It needs only one power supply

25. Source IG1: parameters The cathode is grounded The anode is at +2kV Gas process is Argon

26. LNL ACTUAL RESULTS • ION BEAM ETCHING • Energy: 2 KeV • Pressure of 410-2 mbar • Substrate to source:170 mm • REACTIVE ION ETCHING • Diode sputterind with CF4 • Pressure of 410-2 mbar Ar CF4 2,3 μm/h 12,7 μm/h

27. A possible cavity application Gas flux Plasma region Rotational extracting grid

28. Sputtering • PLASMA • Reactive ion etching • DRY ETCHING • Ion beam cleaning • ION GUN • Reactive ion beam etching

29. Atmospheric-pressure Plasma

30. CORONA • DC • RF resonance • AP plasma • RF • AP Plasma Jet • MICROWAVE • MW plasma torch

31. Why could ATM plasma be useful…? • To clean surfaces from carbon contamination or adsorbed gases. • To etch surfaces using plasma activated chemicals, without any need of a vacuum system. • To add an efficient cleaning step to the cavities surface treatments • To substitute some dungerous steps of Nb cavity chemistry

32. An example of a surface treatment

33. CORONA • DC • RF resonance • AP plasma • RF • AP Plasma Jet • MICROWAVE • MW plasma torch

34. DC corona plasma • Corona discharges accur only if the electric field is sharply NONUNIFORM, typically where the size “r” of one electrode is much lower than the distance. It’ may be seen as luminous glow around the more curved electrode. The electric field’s minimun value for the ignition is around 30 kV/cm. High field gradient Low field gradient Corona Discharge Electrodes

35. A non-self-sustaining current of 10-14 A can be detected. It is due to ions produced by cosmic rays. The corona is ignited. A luminous layer around the electrode where the E field is the highest can be seen. A self sustaining discharge makes the current jumpto ~10-6 A. Massive production of O3 DC Corona discharge Vapplied << Vcorona Vapplied > Vcorona Coronas are operated at currents/voltages below the onset of arcing

36. The Corona Mechanism • The extablisment of a corona begins with an external ionization event generating a primary electron and it is followed by an electron avalanche. • The second avalanches are due to energetic photons : NEGATIVE CORONA POSITIVE CORONA

37. Positive Corona • It appears more uniform than the corresponding negative corona thanks to the homogeneous source of secondary avalanche electrons (photoionization). • The electrons are concentrated close to the surface of the curved conductor, in a region of high-potential gradient and therefore the electrons have a higher energy than in negative corona. • Produce O3

38. Negative Corona • It appears a little larger as electrons are allowed to drift out of the ionizing region, and so the plasma continues some distance beyond it. • The electron density is much greater than in the corresponding positive corona but they are of a predominantly lower energy, being in a region of lower potential-gradien. • The lower energy of the electrons will mean that eventual reactions which require a higher electron energy may take place at a lower rate. • Produce a larger amount of O3

39. Why could corona plasma be useful? • UV/O3 treatments has been proved to be capable of producing clean surfaces in less than 1 minute(f). • Ozone production could be easily used to clean the cavities surfaces from carbon contaminants. f) J. R. Vig, J.Vac.Sci. Technol. A, Vol. 3, No. 3, May/Jun 1985

40. The early stage of our studies 1,5 GHz seamless Cu Cavity • Negative Corona inside a 1,5 GHz cavity • Discharge voltage 30kV • Strong production of O3

41. Positive Corona inside a 1,5 GHz cavity 1,5 GHz seamless Cu Cavity • Discharge voltage 25kV • Production of O3

42. To have a more uniform corona plasma it is necessary to have the same electrode distance along all the lenght of the cavity. • It is important to verify if the 2-6 eV electron and ion energy could be used for surface chemical etching or cleaning using reactive gases.

43. Corona ignited at the edges Cavity Cavity shaped catode Catode’s edges facing the cavity Attempts for understanding and studies

44. Cathode cavity shaped Negative corona inside the cavity

45. CORONA • DC • RF resonance • AP plasma • RF • AP Plasma Jet • MICROWAVE • MW plasma torch

46. RF Resonance plasma • Our purpose was to ignite an atmosferic resonance plasma inside a cavity. • Relate the mode exctitation to the shape of the plasma inside the cavity in order to control and eventually direct the plasma more or less close to the internal surface of the cavity. • Study the surface modification due to the plasma physical or chemical action.

47. Excitation mode TM010 Electric field Module of Magnetic field Lateral view Module of Electric field Base view Magnetic field

48. 6 GHz cavity Cavity TM010 plasma at a power of 50 W

49. 1,5 GHz cavity antenna upper iris Plasma at a power of 150 W lower iris

50. Pill-box cavity for the excitation mode TE111 RF power supply frequency range