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

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.
Resputtering from the substrate occurs by the noble-gas ions which are accelerated by the bias voltage towards the film surface.

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. IONIC BOMBARDMENT OF THE GROWING FILM
Diode Bias Sputtering
IONIC BOMBARDMENT OF THE GROWING FILM
Ions are noble gas positive atoms and a few percentage of cathode niobium atoms (usually and insignificant percentage)

5. Impurities re-sputtering during the film growth
Diode Bias Sputtering
Impurities re-sputtering
during the film growth
Desorption and breakup by argon bombardment of the surface adsorbed species
Whether or not this will occur depends on the relative strenghts of the metal-to-impurity and the metal-to-metal bonds.

6. L.I.Maissel, P.M.Schaible; J.Appl.Phys. 36, 237 (1965)
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
β tiene conto che la corrente di bias è formata da ioni di gas nobile e ioni di impurezze, lo sticking coefficient delle impurezze è considerato unitario. Inoltre tiene conto della densità di corrente di bias e dell carica elettronica degli ioni.
L.I.Maissel, P.M.Schaible; J.Appl.Phys. 36, 237 (1965)

7. Densification of the crystal structure Higher sputtering rate
Advantages
Densification of the crystal structure
Higher sputtering rate
Lattice rearrangement
Films quality improvement
Reduction of the distinct columnar microstructure

8. Increasing of the coating hardness
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
the Ar embedded would limit the dislocations movement in the grains thus increasing hardness
large number of lattice defects and sub-grains

9. Noble gas atoms embedding Lattice defects Thickness reduction
Disadvantages
Noble gas atoms embedding
Lattice defects
Thickness reduction
Biased grid shadowing
Still hydrogen removal is low
The sticking coefficient of hydrogen is higher that that of niobium at energies of the gas ions good not to damage the film

10. Bias Sputtering 500Å 1000Å Ta Resistivity (microhom-cm)
Substrate Bias (Volts)
High Resistivity Cathode
Low Resistivity Cathode
Ta Resistivity (microhom-cm)
500Å
1000Å
5000Å
Substrate Bias (Volts)
The high resistivity of the niobium film is mostly due to their lower density. In addition noble gas atoms bombarding the film at low energies become trapped and make a contribution to the resistivity. At higher biases the adsorbed ions are preferentially sputtered off so the fraction of bias current due to the impurity ions drops. After the peak the annealing process counteract the damaging influence. At higher energy the damage behaviour increases again (radiation damage). The same results also form impurities coming from the sputtering gas
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 Negative Bias Potential (Volts)
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
Film Deposition Rate Å/min
Negative Bias Potential (Volts)
Current (milliamperes) Ic Ib R
The increase in the current is mostly due to an increase in the ionization rate of the argon, and hence, of the plasma density
LA VERA EFFICACIA DEL BIAS E’ L’AUMENTO DEL RATE DI DEPOSIZIONE
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
Ora un breve cenno sui metodi utilizzati fino ad ora per ottenere cavità di Nb/Cu.
Ai laboratori del CERN le cavità superconduttive si ottengono tramite deposizione di niobio via magnetron sputtering.
Un catodo di Nb è collocato coassialmente alla cavità, mentre il magnete che vi scorre all’interno fornisce il confinamento magnetico necessario all’accensione del plasma e quindi alla deposizione.
La foto si riferisce allo scarica collocata in posizione equatoriale durante la depos della cella.
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
Glow discharge
Niobium sputtered atoms
Resputterig of the growing film occurs to a small degree by energetic noble-gas neutrals which are reflected from the cathode in the direction of the the substrates.
Argon
entrance
To the vacuum pumps

16. INFN-LNL coating configuration

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

18. Second Improvement
Combination of the CERN coating configuration and the bias sputtering technique made from INFN-LNL - S N S
Magnets N S N
Target +
Biased Grid
The magnetic film plays the role of confining high-energy electrons, enhancing the ionization and increasing the sputtring rate even at low pressure. The low pressure improves the efficiency of the deposition and the quality of the deposited film.
Substrate

19. Biased Magnetron Sputtering:the construction
Diverse griglie sono state progettate e costruite…
La prima in acciaio a maglie quadre… fusione
la seconda in titanio a romboedriche..
la terza in fili di acciaio.
Le foto rappresentano le griglie costruite e montate sul catodo per mezzo di un sistema di isolamento appositamente progettato. Si notino le molle

20. Biased Magnetron Sputtering:the construction
Improvement of the cooling system
Water in
Water out
Diverse griglie sono state progettate e costruite…
La prima in acciaio a maglie quadre… fusione
la seconda in titanio a romboedriche..
la terza in fili di acciaio.
Le foto rappresentano le griglie costruite e montate sul catodo per mezzo di un sistema di isolamento appositamente progettato. Si notino le molle

21. Biased Magnetron Sputtering:parameters
CERN type
BIAS
INFN-LNL
Cathode Current (A) 3 7
Cathode Power (kW)
1.38
1.86
Bias Voltage (V)
100
Pressure (mbar)
2x10-3
3x10-3
Time (min) 15 20

22. Biased Magnetron Sputtering: RRR results
CERN type
The higher part is better than the lower part, that should be investigated better. In addition the grid could be shaped in the cell part.
The grid still doesn’t affect much the equator part

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

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

25. Biased Magnetron Sputtering: lattice results
Film show a lattice parameter lower than the Nb bulk
Descrivo grafici, la linea nera rappresenta il parametro reticolare del Nb bulk.
Campioni analizzati sono due conf bias
They are grown with compressive stress

26. INFN-LNL coating configuration II

27. INFN-LNL coating configuration II
The grid is behind the cathode -
Target +
Biased Grid
Resputtering from the substrate occurs by the noble-gas ions which are accelerated by the bias voltage towards the film surface.
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
Resputtering from the substrate occurs by the noble-gas ions which are accelerated by the bias voltage towards the film surface.

29. INFN-LNL coating configuration II
Substrate
Cathode
BIAS A B
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)
3 spicchi rotanti

31. High ratio cathode/substrate area
Cavity Shaped Cathode
High ratio cathode/substrate area

32. Cavity Shaped Cathode

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

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

35. Mixed Bias Magnetron Sputtering
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

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