BIAS MAGNETRON SPUTTERING FOR

NIOBIUM THIN FILMS

A.Frigo, G.Lanza,A.Minarello

H.Padamsee, V.PalmieriUniversità degli Studi di Padova

Istituto Nazionale di Fisica NucleareCornell University

The International Workshop onTHIN FILMS AND NEW IDEAS FOR PUSHING THE LIMITS OF RF SUPERCONDUCTIVITY

• 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.

Bias Magnetron Sputtering for Niobium thin films

The positive bias applyed to the grid between target and substrates promotes IONIC BOMBARDMENT OF THE GROWING FILM

Bias Sputtering

-

+

Target

Substrate

Biased Grid

IONIC BOMBARDMENT OF THE GROWING FILM

Diode Bias Sputtering

Impurities re-sputtering during the film growth

Diode Bias Sputtering

Impurities are preferentially removed relative to the atoms of the main film.

RN

Nf

ii

iii

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 rateL.I.Maissel, P.M.Schaible; J.Appl.Phys. 36, 237 (1965)

Diode Bias Sputtering

Densification of the crystal structure

Higher sputtering rate

Lattice rearrangement

Films quality improvement

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

Advantages

Noble gas atoms embedding

Lattice defects

Thickness reduction

Biased grid shadowing

Still hydrogen removal is low

Disadvantages

Bias Sputtering

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) )

Ta R

esis

tivit

y (

mic

roh

om

-cm

)

Substrate Bias (Volts)

High Resistivity CathodeLow Resistivity Cathode

Ta R

esis

tivit

y (

mic

roh

om

-cm

)

500Å

1000Å

5000Å

Substrate Bias (Volts)

The Niobium case

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) )

Ta R

esis

tivit

y (

mic

roh

om

-cm

)

Tem

pera

ture

coeffi

cie

nt

of

resis

tan

ce

(x1

0-3)

Negative Bias Potential (Volts)

The Niobium case

Deposition rate increases with increasing negative bias. (J.Sosniak,J.Appl.Phys. 39,4157 (1968) )

Film

Dep

osit

ion

Rate

Å

/min

Negative Bias Potential (Volts)

Cu

rren

t (m

illiam

pere

s)

Ic

Ib

R

How could we apply that to cavities?

Niobiu

m cathode

Cavity

Magnet

Standard CERN coating configurations

Cylindrical

Magnetron

Cooling air

Niobium cathode - 450 V

To the vacuum pumps

Stainless steel vacuum chamber with cavity shaped sample holders

Moving magnet

Argon entrance

Ceramic insulator

Glow discharge Niobium sputtered atoms

Standard CERN coating configurations

INFN-LNL coating configuration

Grounded Cavity

Magnet

Cathode - 250 V

Biased Grid

+100 V

INFN-LNL coating configuration

Second Improvement

Combination of the CERN coating configuration and the bias sputtering technique made from INFN-LNL

-

+Target

Substrate

Biased Grid

MagnetsNS

NSN

S

Biased Magnetron Sputtering:the construction

Biased Magnetron Sputtering:the construction

Improvement of the cooling system

Water in

Water out

Biased Magnetron Sputtering:parameters

CERN typeBIAS

INFN-LNL

Cathode Current (A) 3 7

Cathode Power (kW) 1.38 1.86

Bias Voltage (V) 0 100

Pressure (mbar) 2x10-3 3x10-3

Time (min) 15 20

Biased Magnetron Sputtering: RRR results

The grid still doesn’t affect much the equator part

0

5

10

15

20

25

30

35

40

45

50

55

1 2 3 4 5 6

Position

RR

R

Run B16

Run B17

Run17B PPMS

Run24

Run 26

1 6

43

52

BIAS

CERN type

Sputtering rate obtained from thickness measurement

Biased Magnetron Sputtering: thickness

0,1

3

0,0

9 0,1

0

0,1

2

0,1

2

0,0

8 0,1

0

0,1

30,1

4

0,1

4

0,1

5

0,1

9

0,1

5

0,1

3

0,1

3

0,1

6

0,0

0,1

0,1

0,2

0,2

0,3

1 2 3 4 5 6

Posizione nella cella

Sp

utt

erin

g R

ate

(mic

rom

/min

)

Run 16B Run 17B

Run 24 Run 26

1 6

43

52

BIAS CERN type

All samples with RRR>8 show a Tc higher than 9,3 K

Biased Magnetron Sputtering: Tc results

BIAS

Film show a lattice parameter lower than the Nb bulk

They are grown with compressive stress

Biased Magnetron Sputtering: lattice results

BIAS

INFN-LNL coating configuration II

-Target

Substrate

+Biased Grid

INFN-LNL coating configuration II

The grid is behind the cathode

INFN-LNL coating configuration II

Advantages:

• Anode-cathode distance reduction

• Higher cathodic area

• No shadowing due to the grid

The grid is behind the cathode

Plasma is conductive

The bias grid can be placed

behind the cathode

Substrate

Cathode

BIAS

A B

INFN-LNL coating configuration II

Bias CERN

Low ratio cathode/substrate

area

Low sputtering rate (1 micron /day)

Bias Sputtering

High ratio cathode/substrate

area

Cavity Shaped Cathode

Cavity Shaped Cathode

Cavity Shaped Cathode

Grounded Cavity

Insulator

Cathode -300 V

Biased stainless steel tube

Cavity Shaped Cathode

Vc = -300 V

i = 5 A

p = 6x10-2mbar

Summary

• Mixed Bias Magnetron Sputtering√ preliminary results (RRR, Tc, lattice)o studies with different bias and

parameterso studies with shaped grido test the cavity

• Large Area Cavity Shaped Cathode √ construction and first runo improvement of the structure stabilityo characterization of the filmso test the cavity

to be continued…

Thanks

Cavity Shaped Cathode

Grounded Cavity

INFN-LNL coating configuration

Insulator

Cathode -300 V

Biased stainless steel tube

Cavity Shaped Cathode

10 cm

V=250 V

i=8 A

p=1x10-2mbar

60 G