1. SRF Cavity Etching Using an RF Ar/Cl2 Plasma
Jeremy Peshl,a J. Upadhyay,b M. Nikolic,c R. McNeill,a S. Popovic,a
A. Palczewski,d A.-M. Valente-Feliciano,d and L. Vuskovica
aDepartment of Physics, Center for Accelerator Sciences
Old Dominion University, Norfolk, VA
bLos Alamos National Laboratory, Los Alamos, NM
cUniversity of San Francisco, San Francisco, CA
dThomas Jefferson National Accelerator Facility, Newport News, VA
Not using, developing.
Disposing is not "easy"
Slide 6: Modular system
Slide 8: Difference in order of magnitude!

2. Outline Motivations and Challenges Current Successes
First Plasma Etched Cavity
Etching Parameters
Foundations of a Discharge Model
Emission and Laser Absorption Spectroscopy
Current Results for Ar and Ar/Cl2 Plasmas
Etching Parameters and Surface Roughness
Concluding Remarks

3. Motivation and Challenges
Uniform plasma surface interaction on inner surface of a 3-D object with varying geometry
Removal of microns of material from large surface area
Efficient transfer of the removed material outside of the system
Characterization and modeling of the coaxial electronegative plasma
Establish correlation between etching parameters and discharge parameters
Increasing the etching rate and reducing the surface roughness
Motivation
Low cost in setup and operation
No wet chemistry (no HF)
Environment and people friendly
(compared to wet etching process)
Full control on the final surface
Removal of bulk material
Surface roughness control
Possible Applications
Doping
Thin Films

4. First RF Test of Plasma Etched SRF Cavity1
8/27/2014: BCP, 6000 C 10-hr bake, degreased, and high water rinsed
2/2/2015: 24-hr plasma etch, high water rinsed, ultrasonic cleaning
2/12/2015: C 14 bake
11/03/2015: Chemical aqua regia solution, phosphoric acid rinse (No HF!)
Schematic of coaxial cylindrical electrodes2
Before plasma etching
After plasma etching
1J. Upadhyay et al, arXiv: (2016).
2J. Upadhyay et al., AIP Advances 8, (2018)

5. Nb Etching Rate vs. Parameters
Pressure
RF Power
Surface Temperature
Gas Species and DC Bias
DC Bias
No Added Bias
Added Bias
J. Upadhyay et al., J. Appl. Phys. 117, (2015).

6. Connecting Etching and Discharge
Etching Parameters
RF Power, DC Bias, Pressure, Gas Composition
Discharge Parameters
Tg, N1s, Te, EEDF, Ne
Discharge Diagnostics
Laser Absorption Spectroscopy
Doppler Broadening
Emission Spectroscopy
Line Ratios of Spectral Intensities
Modular Experimental Setup
End-on View
Top View

7. Discharge Diagnostics
Optical Emission Spectroscopy
Laser Absorption Spectroscopy
Tg and N1s
Discharge Parameters
Line Ratio Techniques
N1s
Kinematic Model
Te, EEDF
Argon
Line Ratios
696/826: 1/5
840/738: 6/3
738/706: 3/2
706/840: 2/6
852/794: 7/4 1 2 3 4 6 5 7
Optical Emission Spectroscopy
Laser Absorption Spectroscopy

8. Density Comparisons Ar
Pressure
RF Power
Ar/Cl2
Pressure
RF Power

9. Density Comparisons Ar Ar/Cl2 Large array of relationships
DC Bias
Large array of relationships
Emission vs. Laser Spectroscopy
DC Bias uniquely significant
No data in literature
Ar/Cl2
DC Bias

10. Surface Roughness Measurements
Surface Temperature and DC Bias
Initial Results
Start RMS roughness ~ 23 nm
End RMS roughness ~ 264 nm (well under 1 micron)
Work in progress

11. Concluding Remarks Plasma etching parameters determined
DC Bias, Pressure, RF Power, Surface Temperature, Gas Composition
Relationships between etching and discharge parameters are being established
Foundation for a discharge model
Decreases need for trial and error experiments
Define future experiments and the final etch process
The correlation between etching parameters and surface roughness in progress

12. Thank You
Supported by DOE, Grant No. DE-SC

13. Photon Escape Factor Method
The rate of photons for a transition i->j incident on a detector
Ratio of two transitions from the same radiating upper level
The photon escape factor6
𝑘 𝑖𝑗 is the reabsorption coefficient
Nonlinear least square method
𝝋 𝒊𝒋 =𝒄 𝜸 𝒊𝒋 ( 𝒏 𝒋 ) 𝑨 𝒊𝒋 𝒏 𝒊
𝝋 𝒊𝒋 𝝋 𝒊𝒌 = 𝜸 𝒊𝒋 ( 𝒏 𝒋 ) 𝑨 𝒊𝒋 𝜸 𝒊𝒌 ( 𝒏 𝒌 )𝑨 𝒊𝒌
𝜸 𝒊𝒋 ≈ 𝟏 𝟏+𝝉 ,
(𝝉= 𝒌 𝒊𝒋 𝒍<𝟏𝟎𝟎)
𝒌 𝒊𝒋 = 𝝀 𝒊𝒋 𝟑 𝟖 𝝅 𝟑/𝟐 𝒈 𝒊 𝒈 𝒋 𝑨 𝒊𝒋 𝒏 𝒋 𝑴 𝟐 𝒌 𝑩 𝑻 𝒈
𝒎=𝟏 𝟓 𝝋 𝟏 𝑨 𝟐 𝝋 𝟐 𝑨 𝟏 𝒎 − 𝜸 𝟏 𝜸 𝟐 𝒎 𝟐
6 R Mewe, Z. Naturf. A 25, (1970)
M.Schulze et al., J. Phys. D : Appl. Phys. 41, (2008)

14. TDLAS
𝑻 𝒈 = 𝟕.𝟖 ∗ 𝟏𝟎 𝟏𝟑 𝜟 𝝂 𝑭𝑾𝑯𝑴 𝝂 𝟎 𝟐
𝒌 𝒊𝒋 = 𝝀 𝒊𝒋 𝟐 𝟖 𝝅 𝟑 𝟐 𝒈 𝒊 𝒈 𝒋 𝑨 𝒊𝒋 𝒏 𝒋 𝝀 𝒊𝒋 𝑴 𝟐 𝒌 𝑩 𝑻 𝒈
𝑰 𝝂 𝑰 𝟎 (𝝂) = 𝒆 −𝒌 𝝂 𝒍
𝒏 𝒋 = 𝒌 𝒊𝒋 𝒌 𝟎 𝜟 𝝂 𝑭𝑾𝑯𝑴
𝒌 𝟎
𝜟 𝝂 𝑭𝑾𝑯𝑴
𝐥𝐧 𝑰 𝝂 𝑰 𝟎 𝝂

15. Collisional Radiative Model
𝒏 𝟐𝒑𝒙 𝒊=𝟐 𝟓 𝜸 𝟐𝒑𝒙,𝟏𝒔𝒊 𝑨 𝟐𝒑𝒙,𝟏𝒔𝒊 = 𝒏 𝒆 𝒏 𝒈 𝒌 𝒈,𝟐𝒑𝒙 + 𝒏 𝒆 𝒊=𝟐 𝟓 𝒏 𝟏𝒔𝒊 𝒌 𝟏𝒔𝒊,𝟐𝒑𝒙
𝝓 𝝀𝟏 𝝓 𝝀𝟐 = 𝜞 𝝀𝟏 𝒆𝒇𝒇 𝒏 𝒈 𝒌 𝒈,𝟐𝒑𝒙 + 𝒊=𝟐 𝟓 𝒏 𝟏𝒔𝒊 𝒌 𝟏𝒔𝒊,𝟐𝒑𝒙 𝜞 𝝀𝟐 𝒆𝒇𝒇 𝒏 𝒈 𝒌 𝒈,𝟐𝒑𝒚 + 𝒊=𝟐 𝟓 𝒏 𝟏𝒔𝒊 𝒌 𝟏𝒔𝒊,𝟐𝒑𝒚
Electron Energy Distribution Function:
𝑭 𝜺 = 𝑪 𝟏 𝒙 𝑻 𝒆 − 𝟑 𝟐 𝜺 𝒆 − 𝑪 𝟐 𝒙 𝜺 𝑻 𝒆 𝟐
𝒙: Defines the distribution
Transmission Coefficient:
𝒌= 𝟐 𝒎 𝒆 𝟎 ∞ 𝝈 𝜺 𝑭 𝜺 𝜺 𝒅𝜺
𝝈 𝜺 : Theoretically calculated and experimentally measured values

16. Induced Field Emission for FE Suppression