BIAS MAGNETRON SPUTTERING FOR NIOBIUM THIN FILMS A.Frigo, G.Lanza,A.Minarello H.Padamsee, V.Palmieri...
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Transcript of BIAS MAGNETRON SPUTTERING FOR NIOBIUM THIN FILMS A.Frigo, G.Lanza,A.Minarello H.Padamsee, V.Palmieri...
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