BIAS MAGNETRON SPUTTERING FOR NIOBIUM THIN FILMS

1. BIAS MAGNETRON SPUTTERING FOR NIOBIUM THIN FILMS A.Frigo, G.Lanza,A.Minarello H.Padamsee, V.Palmieri Università degli Studi di Padova Istituto Nazionale di Fisica Nucleare Cornell University The International Workshop on THIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY

2. Bias Magnetron Sputtering for Niobium thin films • Advantages and disadvanteges of the bias tecnique • Preliminary results of a mixed bias-magnetron sputtering configuration for coating Niobium on copper 1.5 GHz cavities • First applications of a large area cavity shaped cathode in the bias diode sputtering configuration.

3. Bias Sputtering The positive bias applyed to the grid between target and substrates promotes IONIC BOMBARDMENT OF THE GROWING FILM - Target + Biased Grid Substrate

4. Diode Bias Sputtering IONIC BOMBARDMENT OF THE GROWING FILM

5. Diode Bias Sputtering Impurities re-sputtering during the film growth

6. Diode Bias Sputtering Impurities are preferentially removed relative to the atoms of the main film. fraction of impurities trapped into the film i = impurities sticking coefficient Ni = atoms impurities arriving on the film β= function of the bias current due to impurities ions R= sputtering rate L.I.Maissel, P.M.Schaible; J.Appl.Phys. 36, 237 (1965)

7. Advantages • Densification of the crystal structure • Higher sputtering rate • Lattice rearrangement • Films quality improvement

8. Advantages • Increasing of the coating hardness • Similar defect annealing as does an elevated substrate temperature (E.Kay,G.Heim;J.Appl.Phys 49 (9) 4862 (1978)) • Electrons bombardment reduction • Adhesion improvement

9. Disadvantages • Noble gas atoms embedding • Lattice defects • Thickness reduction • Biased grid shadowing • Still hydrogen removal is low

10. Bias Sputtering 500Å 1000Å 5000Å Ta Resistivity (microhom-cm) High Resistivity Cathode Ta Resistivity (microhom-cm) Low Resistivity Cathode Substrate Bias (Volts) Substrate Bias (Volts) High bias voltage reduce differences between films sputtered from different cathodes and of different thickness. (Tantalum films studies-L.I.Maissel,P.M.Schaible,J.Appl.Phys. 36,237 (1965) )

11. The Niobium case Ta Resistivity (microhom-cm) Temperature coefficient of resistance (x10-3) Negative Bias Potential (Volts) Electrical resistivity and temperature coefficient of resistance of niobium films deposited on negatively biased substrates as a function of bias potential. ( J.Sosniak,J.Appl.Phys. 39,4157 (1968) )

12. The Niobium case Ic R Ib Film Deposition Rate Å/min Current (milliamperes) Negative Bias Potential (Volts) Deposition rate increases with increasing negative bias. (J.Sosniak,J.Appl.Phys. 39,4157 (1968) )

13. How could we apply that to cavities?

14. Standard CERN coating configurations Magnet Cylindrical Magnetron Cavity Niobium cathode

15. Standard CERN coating configurations Cooling air Ceramic insulator Niobium cathode - 450 V Stainless steel vacuum chamber with cavity shaped sample holders Moving magnet Niobium sputtered atoms Glow discharge Argon entrance To the vacuum pumps

16. INFN-LNL coating configuration

17. Biased Grid +100 V INFN-LNL coating configuration Cathode - 250 V Magnet Grounded Cavity

18. Second Improvement Combination of the CERN coating configuration and the bias sputtering technique made from INFN-LNL - S N S Magnets S N N Target + Biased Grid Substrate

19. Biased Magnetron Sputtering:the construction

20. Water out Biased Magnetron Sputtering:the construction Improvement of the cooling system Water in

21. Biased Magnetron Sputtering:parameters

22. BIAS CERN type Biased Magnetron Sputtering: RRR results The grid still doesn’t affect much the equator part

23. BIAS CERN type Biased Magnetron Sputtering: thickness Sputtering rate obtained from thickness measurement

24. Biased Magnetron Sputtering: Tc results BIAS All samples with RRR>8 show a Tc higher than 9,3 K

25. Biased Magnetron Sputtering: lattice results BIAS Film show a lattice parameter lower than the Nb bulk They are grown with compressive stress

26. INFN-LNL coating configuration II

27. - Target + Biased Grid INFN-LNL coating configuration II The grid is behind the cathode Substrate

28. INFN-LNL coating configuration II The grid is behind the cathode • Advantages: • Anode-cathode distance reduction • Higher cathodic area • No shadowing due to the grid

29. A B Substrate Cathode BIAS INFN-LNL coating configuration II Plasma is conductive The bias grid can be placed behind the cathode

30. Bias Sputtering Bias CERN Low ratio cathode/substrate area Low sputtering rate (1 micron /day)

31. Cavity Shaped Cathode High ratio cathode/substrate area

32. Cavity Shaped Cathode

33. Cavity Shaped Cathode Biased stainless steel tube Cathode -300 V Grounded Cavity Insulator

34. Cavity Shaped Cathode Vc = -300 V i = 5 A p = 6x10-2mbar

35. Summary • Mixed Bias Magnetron Sputtering • preliminary results (RRR, Tc, lattice) • studies with different bias and parameters • studies with shaped grid • test the cavity • Large Area Cavity Shaped Cathode • construction and first run • improvement of the structure stability • characterization of the films • test the cavity

36. to be continued… Thanks

37. Cavity Shaped Cathode

38. INFN-LNL coating configuration Biased stainless steel tube Cathode -300 V Grounded Cavity Insulator

39. Cavity Shaped Cathode V=250 V i=8 A p=1x10-2mbar 10 cm 60 G