The current status of plasma etching as a complete cleaning and surface preparation process for niobium will be discussed. A comparison with other methods will be given.

1. PLASMA ETCHING OF
NIOBIUM CAVITIES

2. THE PLASMA ETCHING PROCESS
A. Advantages
- Low cost
- Good control over the final surface state- no accidental
oxides and other residue from wet chemistry
- The process does not introduce a significant quantity of
hydrogen as compared to an EP process with hydrogen
generation.
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3. THE PLASMA ETCHING PROCESS
B. The Process
-The cavity RF surface is etched using an Argon/Chlorine
plasma (97%Ar,3% Cl) driven either at 2.45 GHz. or 13.56
MHz.
-It is done in two stages:
a. bulk removal of the “damage layer” (~100
microns)
b. finer removal to smooth surface, - current
finish achieved is comparable to EP [1.].
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4. Plasma etching of Nb surfaces
Plasma etching of Nb surfaces (small samples)
Cylindrical plasma etching for Nb cavities- transition from planar to cylindrical geometry
Coaxial capacitively coupled RF plasmaon large area curved Nb
samples
Study of the effect of discharge asymmetry by correlation of the
inner electrode diameter with the self-bias voltage and the
etching rate.
Determine the dependence of etching rate on the driven
electrode diameter, RF power, gas pressure, and Cl
concentration (at 422 K).
Small Nb samples etching in a microwave plasma at 2.45 GHz inside a quartz tube
Etching rates up to 1.7 μm/min for 97% Ar and 3% Cl gas mixture.
Surface roughness of plasma etched sample was equal or lower than the chemically etched samples.
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EtchingRate[nm/min]
Diameter of the Driven Electrode [cm]
Etching Rate
Self-bias Voltage
15% Cl2
P = 100 W
T = 422 K
p = 150 mTorr
Self-biasVoltage[100V]
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EtchingRate[nm/min]
Pressure [mTorr]
15 % Cl2
P = 150 W
T = 422 K
D = 5.08 cm
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EtchingRate[nm/min]
RF Power [W]
15% Cl2
T = 422 K
p = 150 mTorr
D = 5.08 cm
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EtchingRate[nm/min]
Chlorine Concentration [%]
P = 150 W
T = 422 K
p = 150 mTorr
D = 5.08 cm
Robust etching of a cylindrical Nb sample of diameter comparable to cavity wall dimensions
Next step : Plasma etching of a single cell Nb cavity
Development of Cl/Ar plasma etching of Nb surfaces as an alternative to wet etching for SRF cavity treatment

5. OTHER NEW PROCESSES
A. Niobium cavity electro polishing using choline chloride as an
electrolyte [2.]
- Very successful with small samples
- No RF test results to date
- Single cell tests are currently being prepared [Hui Tian,JLAB].
B. Bipolar pulsed EP using dilute sulfuric acid as an electrolyte[3]
- RF tests completed at Fermilab with excellent results [fig.1]
- These are both wet processes, whereas, plasma etching is a dry
process. This difference is only relevant with respect to the
introduction of hydrogen, which is virtually absent with plasma
etching.
C. A typical surface preparation process is given in figure 2.
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6. A TYPICAL SURFACE PREPARATION PROCESS - figure
2.
• BCP or BP
• Bulk BCP or EP
• Degas 800 C., 3 hours, nitrogen doping
• Light EP, 10 microns
• Ultrasonic rinse
• HPR open cavity
• HPR with blank flanges
• Dry overnight
• Clean assembly

7. HOW DO THESE PROCESSES COMPARE?
-Assuming standard industrial practices were used, the
process costs would be comparable. The tradeoff between
these costs and RF performance cannot be made without
substantial RF testing for each process.
-In evaluating process cost, it is not clear that a cavity that
has not been subject to BCP or EP would benefit from an
800C. bake.
-- Would nitrogen doping be required for a cavity free of
hydrogen a having the same mean free path as a nitrogen
doped cavity?
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8. • How much of the Q enhancement from
nitrogen doping is due to hydride
suppression and how much from
something else? And what is the
something else?
• Is there a tradeoff between nitrogen
doping and quench field limit ? Thermal
conductivity?,,,,, Hsh ?