Anne-Marie Valente-Feliciano Jefferson Lab

Acknowledgement Financial support by JLab and DOE Work done in the framework of PhD theses Janardan 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 Nb 2 O 5, 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) + 5Cl 2 (g) 2NbCl 5 (g) No oxidizing agent needed

Flat Samples - Microwave Glow Discharge System

Flat Samples - Etching Rate Dependence on Discharge Parameters M. Rašković, S. Popović, J. Upadhyay, and L. Vušković, L. Phillips, A. M. Valente-Feliciano, “High etching rates of bulk Nb in Ar/Cl 2 microwave discharge,” J. Vac. Sci. Technol. A. 27 (2), 301 (2009)

Flat Samples - Surface Roughness BCP 1:1:2, 20  m BCP + PE AFM scans (50 μm  50 μm) RMS =286nm Ra=215nm RMS =215nm Ra=169 nm As received RMS=254nm Ra=210 nm 120  m 3-steps PE,120 mm RMS=234nm Ra=174 nm

3-step process 1. Pure Ar removes physisorbed gasses and organic residues from Nb surface without damaging surface Time 30 min Total flow 150 sccm Input power density 2.08 W/cm 3 Pressure 500 mTorr Etching rate  nm/min 2. 3 Vol% Cl 2 in Ar removes surface necessary for cavity production Time 120 min Total flow sccm Input power density 2.08 W/cm 3 Pressure 550 mTorr Etching rate 1  m/min removes ~120  m of surface in 2 h Vol% Cl 2 in Ar removes surface under conditions more favorable for surface smoothening Time 240 min Total flow sccm Input power density 1.4 W/cm 3 Pressure 1250 mTorr Etching rate 0.5  m/min

Sample Studies - Conclusions  Etching rates of bulk Nb as high as 1.7 ± 0.2  m/min can be achieved in a microwave glow  Discharge using Cl 2 as the reactive gas.  Nb etching rate depends on Cl 2 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/Cl 2 MW glow discharge is more a chemical etching than a physical sputtering process.

Single Cell Cavity Setup Bell-Jar System Schematic Diagram Plasma in The Cavity

Single Cell Sample Cavity Single Cell Cavity for Sample Etching Sample and holder New Electrode

Single Cell Cavity Process Optical Intensity Asymmetric Discharge The scaling of the voltage drop in the plasma sheath with the surface area of the electrode : 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

Next Steps Choose an optimal power supply frequency between RF ( 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 as at least 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 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  m.

Optical Emission Spectroscopy Intensity of Cl 2 continua around 308 nm as function of input power density in Ar/Cl 2 discharge. Intensity of Cl 2 continua around 257 nm.

Flat Samples Etching Rate Dependence on Discharge Parameters M. Rašković, S. Popović, J. Upadhyay, and L. Vušković, L. Phillips, A. M. Valente-Feliciano, “High etching rates of bulk Nb in Ar/Cl 2 microwave discharge,” submitted to J. Vac. Sci. Technol. A.

 The RF Ar-Cl 2 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 + ≈ n e ).  For pressures 1 and 2 mtorr the Cl 2 gas flow rate was 20sccm and for pressures 5 and 20 mtorr the Cl 2 gas flow rate was 100 sccm. Electronegative plasma

Radial distribution of spectral irradiance of Ar [(2P* ).4p - 3s2.3p5.(2P* ).6s] spectral line at nm. Fixed voltage 600V. Driven electrode is on the right side, and surface to be treated (cavity wall) on the left side. Electron temperature distribution in the discharge reflects also the sheath formation near the wall and the driven electrode.