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!

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

Motivation and Challenges Uniform plasma surface interaction on inner surface of a 3-D object with varying geometry Removal of 10-150 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

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: 900-1400 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:1605.06494 (2016). 2J. Upadhyay et al., AIP Advances 8, 085008 (2018)

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, 113301 (2015).

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

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

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

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

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

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

Thank You Supported by DOE, Grant No. DE-SC0007879

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, 065206 (2008)

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

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

Induced Field Emission for FE Suppression