Valente - Plasma Etching of Niobium SRF Cavities
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PLASMA ETCHING OF NIOBIUM SRF CAVITIES Anne-Marie Valente-Feliciano for Janardan Upadhyay Jefferson Lab
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http://www.surfacetreatments.it/thinfilms Plasma Etching of Niobium surfaces: Studies on samples and Single-Cell Cavities (AnneMarie Valente - 30') Speaker: AnneMarie Valente - Jefferson Lab - Newport News (VA) USA | Duration: 30 min. Abstract Plasma based surface modification provides an excellent opportunity to eliminate impurities and defects in the penetration depth region of Nb SRF cavity surfaces. It also allows a better control of the final SRF surface as final surface modifications like oxidation or nitridation can be done in the same process cycle. In the framework of a collaboration between ODU and Jefferson Lab, we are pursuing the use of environmentally friendly dry etching of SRF cavity in an Ar/Cl2 discharge. The experimental conditions in the microwave glow discharge system with a barrel-type reactor have been optimized. The viability of plasma etching as an alternative surface preparation method for bulk Nb surfaces has been demonstrated on flat samples by achieving etching rates comparable to wet processes, such as BCP or EP. The optimized experimental conditions are now being applied to the preparation of single cell cavities. The geometry of SRF cavities made of bulk Nb defines the use of asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of a driven and grounded electrode creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the plasma sheath attached to the cavity surface. A specially designed single cell cavity with sample holders is used to study these asymmetric discharges. The sample holder ports can be used for both diagnostics and sample etching purposes. The approach is to combine radially and spectrally resolved profiles of optical intensity of the discharge with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions and sequences.
Transcript of Valente - Plasma Etching of Niobium SRF Cavities
PLASMA ETCHING OF NIOBIUM SRF CAVITIES
Anne-Marie Valente-Felicianofor Janardan Upadhyay
Jefferson Lab
Acknowledgement
•Financial support by JLab and DOE
•Work done in the framework of PhD thesesJanardan Upadhyay & Marija Raskovic
• Collaboration with Prof. Leposava Vuskovic and Dr. Svetozar Popovic , Old Dominion University, Norfolk ,Virginia Dr. Larry Phillips, Jefferson Lab, Newport News , Virginia
Outline
• Motivation• Work plan• Plasma etching on Flat Samples• Single Cell Cavity Experiment• Concluding remarks
Motivation
Wet acid etching (BCP or EP) is the commonly used process in processing of SRF cavities.
Wet etching has been all but abandoned as the fabrication process in microelectronic industry, primarily due to the isotropic material removal
Plasma-assisted etch process (dry etching) is the enabling process in semiconductor industry, since it can be highly selective with respect to direction and hence indispensable in patterned removal of surface material or in removal of material from non-flat surfaces [2].
(a)
(b)
Schematic illustration of (a) isotropic action (wet)
etching, and (b) anisotropic (dry) etching
Motivation
• Low Cost• No wet chemistry• Environment and People friendly (compared to wet etching process)• Full control on the final surface
“Oxide –free” surface if kept under UHV A variety of surfaces can be intentionally created through
plasma processing Pure Nb2O5, or other cap layer superconducting NbN S-I-S Multilayer
BCP Process
Dry Process
Comparison of surface micrographs taken with KH-3000 digital microscope with magnification 10×350
M. Rašković, L. Vušković, S. Popović, L. Phillips, A. M. Valente-Feliciano, S. B. Radovanov, and L. Godet, Nucl. Instrum. Methods Phys. Res. A 569, 663 (2006
Work Plan
Phase 1: Work with flat samples, with the objective to fulfil the requirements for etching rates, surface roughness, and to demonstrate friendly and cost-reducing aspect of the plasma-assisted process.
Phase 2: Work with a single-cell cavity to establish optimum conditions for an asymmetric electronegative discharge in cavity geometry, to demonstrate the uniformity of surface treatment, and to perform the RF performance test compatible with existing standards, to establish treatment protocol, and to define the process monitoring procedure environmentally.
Phase 3: Work with multiple-cell cavities to demonstrate final performance of the process
In view of the relatively complex technological challenges facing the development of plasma-assisted surface treatment, we have adopted a 3-phase approach:
Mechanism of Plasma Etching
Nb surface
1. Generation of reactive species
2. Diffusion of reactive species
3. Adsorption on surface 4. Reactions
5. Desorption of products
Bulk Nb
2Nb (s) + 5Cl2 (g) 2NbCl5 (g) No oxidizing agent needed
Flat Samples - Microwave Glow Discharge System
Flat Samples - Etching Rate Dependence on Discharge Parameters
0
200
400
600
800
1000
1200
1400
0.5 1 1.5 2 2.5 3
Power density (W/cm3)
ER
(n
m/m
in)
/ R
MS
(n
m)
Etching rate
RMS
0
200
400
600
800
1000
1200
0 0.5 1 1.5 2 2.5 3 3.5
% Vol Cl2
ER
(n
m/m
in)
/ R
MS
(n
m)
Etching rate
RMS
0 200 400 600 800 1000 1200 14000
200
400
600
800
1000
1200
1400
1600
Pressure (mTorr)
Etc
hing
rat
e (n
m/m
in)
0
100
200
300
400
500
600
700
800
900
1000
RM
S (nm
)
M. Rašković, S. Popović, J. Upadhyay, and L. Vušković, L. Phillips, A. M. Valente-Feliciano, “High etching rates of bulk Nb in Ar/Cl2 microwave discharge,” J. Vac. Sci. Technol. A. 27 (2), 301 (2009)
Flat Samples - Surface Roughness
BCP 1:1:2, 20 mm
BCP + PE
AFM scans (50 μm 50 μm)
RMS =286nmRa=215nm
RMS =215nmRa=169 nm
As receivedRMS=254nm Ra=210 nm
120 mm
3-steps PE ,120 mmRMS=234nmRa=174 nm
3-step process1. Pure Ar removes physisorbed gasses and organic residues from Nb surface without damaging surface
Time 30 min Total flow 150 sccmInput power density 2.08 W/cm3 Pressure 500 mTorr
Etching rate 0nm/min2. 3 Vol% Cl2 in Ar removes surface necessary for cavity production
Time 120 min Total flow 198.6 sccmInput power density 2.08 W/cm3 Pressure 550 mTorr
Etching rate 1 mm/minremoves ~120 mm of surface in 2 h3. 1.5 Vol% Cl2 in Ar removes surface under conditions more favorable for surface smootheningTime 240 min Total flow 348.6 sccmInput power density 1.4 W/cm3 Pressure 1250 mTorr
Etching rate 0.5 mm/min
Sample Studies - ConclusionsEtching rates of bulk Nb as high as 1.7 ± 0.2 mm/min can be
achieved in a microwave glow
Discharge using Cl2 as the reactive gas.
Nb etching rate depends on Cl2 reactive gas concentration and discharge parameters: input power density and pressure in reaction chamber.
Surface composition analyses (EDS, XPS) show that no impurities have been introduced into Nb during microwave discharge treatment.
Developed 3-step process.
Emission spectroscopy result combined with measured etching rates, suggests that the Nb etching mechanism in Ar/Cl2 MW glow discharge is more a chemical etching than a physical sputtering process.
Single Cell Cavity SetupBell-Jar System
Schematic Diagram
Plasma in The Cavity
Single Cell Sample CavitySingle Cell Cavity for Sample
Etching Sample and holder
New Electrode
Single Cell Cavity Process
-0.02 -0.01 0.00 0.01 0.02 0.03 0.040.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
PM
T s
ign
al (
a.u
)Time (ms)
Sheath b Sheath a
Optical Intensity
Asymmetric Discharge The scaling of the voltage drop in the plasma sheath with the surface area of the electrode :
52
a b
b a
V A
V A
Time waveforms of spectral line intensity at both sheaths in the
discharge
Single Cell Cavity
Spectrometer
Electrode
Movable multiple optical fiber system attached with spectrometer used for tomographic study of plasma inside the cavity.
Sample cavity setup with fiber optics
Movable multiple optical fiber system combined with electrical probe attached to
the spectrometer will be used for tomographic study of plasma inside the cavity.
Tomography of the Plasma in the Cavity
Ceramic Tube
Stainless Steel Tube
Optical Fibre
1.4 mm diameter
1.6 mm diameter
1.0 mm length of langmuir
probe
Next Steps
Choose an optimal power supply frequency between RF (50-200 MHz) and MW (2.45 GHz.)
Investigate the plasma etching behavior on samples placed on actual geometry of the single cell cavity (plasma distribution, roughness uniformity.. .)
Plasma etching of single cell cavities and RF performance measurements.
Conclusion•The RF performance is the single feature that remains to be
compared to the “wet” process, since all other characteristics of the “dry etching ” technology, such as etching rates, surface roughness, low cost, and non-HF feature, have been demonstrated comparable to the currently used technologies.
•Final surface modifications (final oxidation, nitridation…) can be done in the same process cycle with the plasma etching process.
•The geometry of the inner surface of the cavity implies that the plasma discharge has to be asymmetric.
•In order for the asymmetric discharge to be effective, the lower sheath voltage at the treated surface (large area, undriven electrode) has to be at least equal or higher to the plasma floating potential at every point of the surface.
Surface Roughness
Figure 13. Comparison of surface micrographs taken with KH-3000 digital microscope with magnification 10×350: (a) an untreated sample; (b) BCP sample; (c) plasma-etched
(a)
(a) (b)
Surface micrographs obtained with scanning electron microscope: (a) sample treated with BCP technique – magnification 500x, (b) plasma treated sample –
magnification 1500x. Black lines indicate distance of 10 mm.
Optical Emission Spectroscopy
300 302 304 306 308 310 3120
5000
10000
15000
20000
25000
Inte
nsity
(a.
u.)
(nm)
0.73 W/cm3
1.30 W/cm3
1.51 W/cm3
2.73 W/cm3
248 250 252 254 256 258 2605.0x103
1.0x104
1.5x104
2.0x104
2.5x104
3.0x104
Inte
nsity
(a.
u.)
(nm)
Ar + Cl2
Ar + Cl2 + Nb
Intensity of Cl2 continua around 308 nm as function of input power density in Ar/Cl2 discharge.
Intensity of Cl2 continua around 257 nm .
470 475 480 485 4900.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
inte
nsity
(nm)
Ar Ar + Cl
2
Ar + Cl2 + Nb
Flat SamplesEtching Rate Dependence on Discharge Parameters
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
200
400
600
800
1000
1200
Etc
hing
ra
te (
nm
/min
)
Cl2 (%Vol)
0 200 400 600 800 1000 1200 1400200
400
600
800
1000
1200
1400
1600
Pressure (mTorr)
Etc
hing
rat
e (n
m/m
in)
0.5 1.0 1.5 2.0 2.5 3.0
600
700
800
900
1000
1100
1200
E
tchi
ng r
ate
(nm
/min
)
Power density (W/cm3)
M. Rašković, S. Popović, J. Upadhyay, and L. Vušković, L. Phillips, A. M. Valente-Feliciano, “High etching rates of bulk Nb in Ar/Cl2 microwave discharge,” submitted to J. Vac. Sci. Technol. A.
The RF Ar-Cl2 discharge contains large number density of negative ions at low powers (capacitive mode) [5].
In this case plasma exists as an electronegative core (n+ ≈ n-) and electropositive halo (n+ ≈ ne).
For pressures 1 and 2 mtorr the Cl2 gas flow rate was 20sccm and for pressures 5 and 20 mtorr the Cl2 gas flow rate was 100 sccm.
Surface Roughness
Figure 13. Comparison of surface micrographs taken with KH-3000 digital microscope with magnification 10×350: (a) an untreated sample; (b) BCP sample; (c) plasma-etched
(a)
(a) (b)
Figure 15. Surface micrographs obtained with scanning electron microscope: (a) sample treated with BCP technique – magnification 500x, (b) plasma treated sample – magnification 1500x. Black lines indicate distance of 10
mm.
Single Cell Cavity-Data from Flat SampleSome Spectroscopic Results
300 302 304 306 308 310 3120
5000
10000
15000
20000
25000
Inte
nsity
(a.
u.)
(nm)
0.73 W/cm3
1.30 W/cm3
1.51 W/cm3
2.73 W/cm3
Intensity of Cl2 continua around 308 nm as function of input power density in Ar/Cl2 discharge.
5000
6000
7000
8000
9000
10000
0 1 2 3 4 5 6 7 8 9 10
Time (min)
Excit
ati
on
Tem
pera
ture
(K
)
Ar set B
P = 1.3 W/cm3, Ar: 97%,, 3% Cl
Excitation temperature jumps about 3 min after the discharge inception.
0
200
400
600
800
1000
1200
1400
1600
0 1 2 3 4 5 6 7 8 9 10
Time (min)
Lin
e i
nte
nsit
y (
rel.
un
its) Cl I
Nb I
Nb I lines become prominent only after about three minutes from start.
P = 1.3 W/cm3, Ar: 97%,, 3% Cl
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3
Power density (W/cm3)
Excit
ati
on
Tem
pera
ture
(K
)
A
B
C
P = 1 Torr, Ar: 97% , Cl2: 3%
Excitation temperature in the absence of Nb sample is practically constant in the applied power density range.
process Material removed Roughness (rms)
Non polished 0 micron 254 nm
BCP 20 micron 286 nm
Plasma etched 120 micron 231 nm
sample code
min
g nm/min
Material removed (in micron)
RMS (10x10) nm
RMS (20x20) nm
2 11 15 0.0453 696 10.44 6070 172
2 12 30 0.093 714 21.42 35 6846 85
2 13 45 0.1311 671 30.19 49 9037 105
2 14 60 0.2058 790 47.4 69 27070 128
2 15 75 0.2583 793 59.47 67 8841 125
•Financial support by JLab and DOE
•Work done in collaboration with
S. Popovic and L. Vuskovic
Old Dominion University, Norfolk , Virginia
A.– M. Valente –Feliciano and L. Phillips
JLab, Newport News , Virginia
Acknowledgement
