hanna skliarova porosity of nb magnetron sputtered thin films and dependence on sputtering...
DESCRIPTION
Pinholes (or through film porosity) in Nb thin film deposited on the inner walls of SRF cavities are harmful for cavity performance because they may expose inferior copper that has much higher resistance than niobium at 4.2 K. Aluminated quartz substrates allowed us to make visible the pore sites for inspection and counting by production visible corrosion products. We showed the correlation between the amount of pinholes in niobium thin film prepared by magnetron sputtering and the deposition parameters, such as sputtering gas pressure, substrate temperature, applied bias, placing of the sample in UBM sputtering mode. Thus low temperature of the substrate and high sputtering gas pressure promoted growth of a voided film (that corresponds to SZM approach) with high amount of pinholes. Heating of the substrate during deposition has resulted in moderate decrease of the pinhole amount, while negative bias applied to the substrate showed stronger decrease of the pinhole amount thanks to additional bombardment of the substrate by Ar+ serving to remove weakly bounded particles during deposition.TRANSCRIPT
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of Nb magnetron sputtered
thin films and dependence on sputtering parameters
Anna Skliarova, O. Azzolini, V. Palmieri
Porosity
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Pinholes in Nb film in SRF cavities
Resistance
to RF at
4.2K
RCu≈105RNb
Pinholes
expose
underlying
Cu
Cavity
surface
resistance
increase
Q-factor
EAcc
are lower
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• To correlate Nb film porosity vs
sputtering parameters
• To find approaches to less porous Nb
films
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Aim of the research:
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4S. Amorosi, C. Benvenuti, P. Chiggiato, M. Malabaila, Vacuum, V. 60, 1–2, 2001, 275-278
Method proposed by Amorosi et al.
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Our method of porosity evaluation
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Al is providing fast reaction with
visible product revealing the pore sites
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Our method of porosity evaluation
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Sputtering of Al onto polished quartz allowed
minimizing the influence of the substrate defects
and focus on sputtering parameters
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Our method of porosity evaluation
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1 2 3 4 5
Acid test evaluation: SCALE
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Standard conditions
- conditions to be considered standard if other
information is not provided below
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Standard conditions:
2” Planar UBM II type
STANDARD SAMPLE POSITION
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Standard conditions:
SAMPLE HOLDER
6 cm
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Standard conditions:
Ar as sputtering gas
Nb target RRR 300
Pbase ~10-6 mbar
IDC = 0.5 A
No heating/cooling substrate
Film thickness ~1.5 µm
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Magnetron position: acid test
MAGNETRON
MAGNETRON
SAMPLE HOLDER
Standard
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Pressure influence: acid porosity test
310-2
mbar310-3
mbar
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Pressure influence: acid porosity test
310-2
mbar310-3
mbar
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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44
33
3
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Temperature influence: acid test
Floating (~250°C)
400°C
500°C
0°C
-100°C
-50°C
300°C
Standard
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44
33
3
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Temperature influence: acid test
Floating (~250°C)
400°C
500°C
0°C
-100°C
-50°C
300°C
Standard
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Optical profilometry
Scan area: 200 μm × 200 μm
Result: % of area covered by
holes deeper than 0.5 μm
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4 0.5 %
-100 °C
4 0.4 %
0 °C
4 1 %
Floating
3 0.04 %
500 °C
- 0 %
800 °C
Temperature influence: SEM
Acid test (1÷5)
Optical profilometry
(%)
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4 0.5 %
-100 °C
4 0.4 %
0 °C
4 1 %
Floating
3 0.04 %
500 °C
- 0 %
800 °C
Temperature influence: FIB SEM
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Influence of film thickness: acid test
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28 µm
6 µm
1.5 µm
4 µm
10 µmStandard
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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UBM II type: sample position
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UBM II type: sample position
ON AXIS OUT OF AXIS
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UBM II type: sample position
ON AXIS OUT OF AXIS
1 5Acid porosity test :
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1
ON AXIS
5
OUT OF AXIS
UBM II type: position influence
Acid test (1÷5)
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1
ON AXIS
5
OUT OF AXIS
UBM II type: position influence
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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Investigated parameters:
• Magnetron position1
• Pressure2
• Temperature3
• Film thickness4
• Sample position 5
• DC-Biased MS6
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DC bias -80 V -50 V -80 V -80 V -150 V
Arpressure,
mbar310-2 510-3 510-2 310-3 310-3
Porosity acid test 3 2 1 1 2
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DC-biased MS
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DC bias -80 V -50 V -80 V -80 V -150 V
Arpressure,
mbar310-2 510-3 510-2 310-3 310-3
Porosity acid test 3 2 1 1 2
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DC-biased MS
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DC bias -80 V -50 V -80 V -80 V -150 V
Arpressure,
mbar310-2 510-3 510-2 310-3 310-3
Porosity acid test 3 2 1 1 2
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DC-biased MS
Amount of holes deeper 0.5 μm is 0.1 %
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DC-biased MS
SEM HR SEM
FIB SEM
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Winners!
132
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Winners!
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Biased MS
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Winners!
132
Biased MSHigh T MS
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Winners!
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Biased MSHigh T MS
Thick film
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• Increase film thickness1
• Substrate above magnetron2
• Proper substrate preparation3
• Avoiding UBM far out of axis4
• Clean room + vacuum cleaner5
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Methods to decrease porosity:
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Research team:
LNL INFN:
H.Skliarova, O. Azzolini, V. Palmieri
LIME Roma-Tre:
M. Renzelli, D. De Felicis, E. Bemporad