High Etching Rates of Niobium
in Ar/Cl2 Microwave Discharge
M. Rašković, J. Upadhyay, L. Vušković, and S. Popović
Old Dominion University, Department of Physics, Norfolk, Virginia, USA
L. Phillips and A.-M. Valente-Feliciano
Thomas Jefferson National Accelerator Facility, Newport News, USA
This work is supported by the NSF/DOE collaborative effort through the Office of High Energy
Physics, Office of Science, Department of Energy under Grant No. DE-FG02-05ER41396.
Jefferson Lab, Accelerator Division supports M. Rašković through fellowship.
Nb cavity performance is limited
by surface quality
Cavity performance figures of merit
Application
z
z
z
Quality factor
Breakdown field
Surface resistance
⇒ Cavity QED1
⇒ Accelerators
⇒ Josephson
junctions
Cavity
surface properties
z
z
z
z
z
Roughness
Impurities
Oxides
Grain boundaries
Thermal properties
Some of “recently” offered solutions:
z EP on polycrystalline material
z BCP on large grains material
z Thin films deposition on different substrates
1
H. Mabuchi and A. C. Doherty, Science 298, 1372 (2002). & S. Gleyzes, et al., Nature 446, 297 (2007).
Plasma etching process
z
Reactive species produced in gas discharge
z
Different types of discharges at low-pressure
z
Advantages
z
z
z
z
z
z
No dangerous acids and solvents
“greener” method compared to
Small amount of chemicals
currently used wet etching methods
Good process control
Control of the final oxidation phase
Reduced number of steps for the final surface preparation
Disadvantages
z
z
Some gasses are toxic and/or corrosive
Re-deposition of non-volatile compounds
Research objectives
z
achieve ETCHING RATES comparable with
etching rates during wet etching processes
z
obtain SMOOTHER SURFACE than obtained in
wet etching processes
z
lower amount of OXIDES and other IMPURITIES
in top surface layer
z
improve RF PERFORMANCE of Nb cavities
Properties of Nb and its compounds of interest
for plasma etching.2
Compound
Tb.p.
[K]
5017
Tc
[K]
ΔHo
[kJ/mol]
Vapor pressure [Pa]
298 K
350K
color
Nb
2741
NbO
2210
NbO2
1040
Nb2O5
1780
NbF5
352
512
-1813.3
1.200
331.80
Colorles
s
NbCl5
477
520
-796.3
0.048
14.06
Yellow
NbN
2300
NbC
3773
NbS2
2
Tm.p
.[K]
9.2
0
Gray
1.25
-405.8
Gray
-796.2
White
-1899.5
White
dielectric
4573
16.2
-233.7
Gray
9.8
-137.6
Gray
6.1
Black
O. Kubaschevski, Niobium: Physico-chemical properties of its compounds and alloys (International atomic energy agency,
Viena, 1968).
Mechanism of Nb plasma etching
in Ar/Cl2 MW discharge
Overall stoichiometric equation: 2Nb(s)
+ 5Cl2 (g) → 2NbCl5 (g)
ko
kl
kads
kb
kp
NbClx<5
Nb surface
Bulk Nb
kv
MW glow discharge system
for Nb samples exposure
Process monitoring
Emission spectroscopy
14000
12000
int
Ar I
Cl I
Intensity (rel. units)
10000
8000
6000
4000
2000
0
428
432
436
16000
440
444
448
452
448
452
Wavelength (nm)
Int
14000
Nb
Ar I
Intensity (rel. units)
12000
Cl I
10000
8000
6000
4000
2000
0
428
432
436
440
Wavelength (nm)
444
Etching rate
Concentration dependence
Rate equation: ER=k(Cl2)n
1000
7
800
6
600
5
ln (ER)
Etching rate (nm/min)
1200
400
4
200
3
0
2
0.0
0.5
1.0
1.5
Cl2 (%Vol)
Experimental conditions:
- Total gas flow 200 sccm
- Pressure 340 mTorr
- Input power 491 W
2.0
2.5
3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln (%Cl2)
n=1.32±0.03
Concentration of reactive species ↑ ⇒ etching rate ↑
1.0
1.5
Etching rate
Pressure dependence
4000
1600
3500
Relative intensity (a.u.)
Etching rate (nm/min)
1400
1200
1000
800
600
400
200
Cl I 469 nm
3000
2500
Cl II 482 nm
2000
1500
1000
Ar I 470 nm
Ar II 480 nm
Ar II 302 nm
500
0
0
200
400
600
800 1000 1200 1400
Pressure (mTorr)
Cl2 306 nm
0
200
400
600
800
1000 1200 1400 1600
Pressure (mTorr)
Experimental conditions:
- 3 vol% Cl2 in gas mixture
- Input power 491 W
Pressure ↑ ⇒ concentration of reactive species↑ ⇒ etching rate↑
Etching rate
Power dependence
2500
Cl I 469 nm
Relative intensity (a.u)
2000
1200
Etching rate (nm/min)
1100
1000
Ar I 470 nm
1500
1000
500
Cl II 482 nm
Ar II 302 nm
Ar II 480 nm
900
0
800
0.0
Cl2 306 nm
0.5
2.0
2.5
3.0
390
380
600
370
1.0
1.5
2.0
2.5
3
Power density (W/cm )
Experimental conditions:
- Total gas flow 196 sccm
- Pressure 340 mTorr
- 3 vol% Cl2 in gas mixture
3.0
360
T (oC)
0.5
1.5
Power density (W/cm3)
400
700
1.0
100% Ar plasma
350
340
330
320
3%Cl2 in Ar plasma
310
300
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Power density (W/cm3)
Input power density ↑ ⇒ plasma density and T ↑ ⇒ etching rate↑
Impurities content
X-ray photoelectron spectroscopy
Survey spectra of Nb sample surface
180000
O 1s
180000
160000
Nb 3d
Nb 3s
100000
O KLL
counts/s
120000
120000
counts/s
after exposure to Ar/Cl2 dischagre
140000
Nb 3p
140000
before exposure to Ar/Cl2 discharge
160000
C 1s
80000
C 1s
100000
80000
60000
60000
40000
40000
Nb 4s
20000
Cl 2s would be here
20000
0
0
1000
800
600
400
200
0
300
290
280
B.E. (eV)
High resolution spectra of Nb 3d line
250
1.0
Normalized intensity
1.2
Normailzed Intensity
260
High resolution spectra of O 1s line
1.4
1.0
0.8
0.6
0.8
0.6
0.4
0.2
0.4
0.2
0.0
220
270
B.E. (eV)
0.0
540
215
210
205
B. E. (eV)
200
195
538
536
534
532
B.E. (eV)
530
528
526
Surface roughness & impurities content
Scanning electron microscopy with EDX
Surface before exposure
Surface after exposure to Ar/Cl2 discharge
™ Exposure to Ar/Cl2 discharge.
™ Energy Dispersive X-ray (EDX) surface composition analysis.
Single cell system
Cage
Bell jar
Magnetron
Power line
Reactive volume
3 dm3
Single cell (Nb)
Gas line
Adsorbent
Gas in
Power in
To vacuum pumps
Conclusion
z
z
Etching rates as high as 1.5 μm/min were achieved using:
•
small concentration of reactive gas (3 vol% Cl2)
•
moderate power density (~2 W/cm3)
•
moderate pressure (800 mTorr).
Etching rate of 1.7 ± 0.2 μm/min was achieved under following experimental
conditions:
ƒ
ƒ
ƒ
ƒ
3% Cl2 in Ar.
Input power density 2.5 W/cm3.
Pressure in reaction chamber 1200 mTorr.
Two step process includes pre-etching exposure to pure Ar gas designed to remove adsorbents from Nb
sample surface prior to exposure to reactive gas.
z
No impurities were introduced in the Nb surface.
z
Multi-step process was performed to improve surface smoothness. Results will be
available next week.
z
Single-cell experimental set-up ready for exposure to plasma etching.
ƒ
Subsequently RF properties measurement
(accelerating field dependence of cavity quality factor)
ACKNOWLEDGEMENT:
•
A. Wilkinson and O. Trofimova
Surface Characterization Lab, ARC William & Mary
•
Surface science Lab, SRF Institute, Jlab
•
C. Reece, H. Tian & G. Ciovati
Thank you for your attention.
Sheath in plasmas and surface roughness
d sheath ≈ 10
1. Difference in incoming ions energy
2. Stronger ion current
6
Te
ne
Where:
Te electron temperature
ne plasma density
dsheath >> roughness of surface ⇒ smoothening
dsheath ~ roughness of surface ⇒ sheath conforms to surface shape ⇒ no smoothening
Influence of N2addition to reactive gas
mixture on Nb etching rate
900
1.2
800
1.0
0.9
700
0.8
0.7
600
0.6
0.5
500
0.4
0.3
400
0.2
0.1
300
0.0
0
10
20
30
N2 (%vol)
40
50
ER/ER0%N2
Etching rate (nm/min)
1.1