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Anne-Marie Valente-Feliciano<br />for JanardanUpadhyay<br />Jefferson Lab<br />
Acknowledgement<br /><ul><li>Financial support by JLab and DOE
Work done in the framework of PhD theses</li></ul>JanardanUpadhyay & MarijaRaskovic<br /><ul><li> Collaboration with  Prof...
Outline<br />Motivation<br />Work plan<br />Plasma etching on Flat Samples<br />Single Cell Cavity Experiment<br />Conclud...
Motivation<br /> Schematic illustration of <br />(a) isotropic action (wet) etching, and (b)  anisotropic (dry) etching<br...
Plasma-assisted etch process (dry etching) is the enabling process in semiconductor industry, since it can be highly selec...
Motivation<br />Comparison of surface micrographs taken with KH-3000 digital microscope with magnification 10×350<br />BCP...
A variety of surfaces can be intentionally created through plasma processing
Pure Nb2O5, or other cap layer
 superconducting NbN
S-I-S Multilayer</li></li></ul><li>Work Plan<br />In view of the relatively complex technological challenges facing the de...
Phase 2: Work with a single-cell cavity to establish optimum conditions for an asymmetric electronegative discharge in cav...
Phase 3: Work with multiple-cell cavities to demonstrate final performance of the process</li></li></ul><li>Mechanism of P...
Flat Samples - Microwave Glow Discharge System <br />
Flat Samples - Etching Rate Dependence on Discharge Parameters<br />M. Rašković, S. Popović, J. Upadhyay, and L. Vušković,...
Flat Samples - Surface Roughness <br />As received<br />BCP 1:1:2, 20 mm<br />RMS=254nm <br />Ra=210 nm<br />RMS =286nm<br...
3-step process<br />1. Pure Ar	 removes physisorbed gasses and organic residues from Nb surface without damaging surface<b...
Sample Studies - Conclusions<br /><ul><li>Etching rates of bulk Nb as high as 1.7 ± 0.2 mm/min can be achieved in a microw...
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 re...
Surface composition analyses (EDS, XPS) show that no impurities have been introduced into Nb during microwave discharge tr...
Developed 3-step process.
Emission spectroscopy result combined with measured etching rates, suggests that the Nb etching mechanism in Ar/Cl2 MW glo...
Single Cell Sample Cavity<br />Sample and holder<br />Single Cell Cavity for  Sample Etching<br />New Electrode<br />
Single Cell Cavity Process<br />Asymmetric Discharge <br />The scaling of the voltage drop in the plasma sheath with the s...
Single Cell Cavity<br />Electrode<br />Movable multiple optical fiber system attached with spectrometer used  for tomogra...
Sample cavity setup with fiber optics<br />
Tomography  of  the  Plasma  in  the  Cavity<br />Stainless Steel Tube<br />Ceramic Tube<br />Optical Fibre<br />1.4 mm di...
Next Steps<br />Choose an optimal power supply frequency between RF (50-200 MHz) and MW (2.45 GHz.)<br />Investigate the...
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Valente - Plasma Etching of Niobium SRF Cavities

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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.

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Valente - Plasma Etching of Niobium SRF Cavities

  1. 1. Anne-Marie Valente-Feliciano<br />for JanardanUpadhyay<br />Jefferson Lab<br />
  2. 2. Acknowledgement<br /><ul><li>Financial support by JLab and DOE
  3. 3. Work done in the framework of PhD theses</li></ul>JanardanUpadhyay & MarijaRaskovic<br /><ul><li> Collaboration with Prof. LeposavaVuskovic and Dr. SvetozarPopovic , Old Dominion University, Norfolk ,Virginia </li></ul>Dr. Larry Phillips, Jefferson Lab, Newport News , Virginia<br />
  4. 4. Outline<br />Motivation<br />Work plan<br />Plasma etching on Flat Samples<br />Single Cell Cavity Experiment<br />Concluding remarks<br />
  5. 5. Motivation<br /> Schematic illustration of <br />(a) isotropic action (wet) etching, and (b) anisotropic (dry) etching<br /><ul><li>Wet acid etching (BCP or EP) is the commonly used process in processing of SRF cavities. </li></ul>(a)<br /><ul><li>Wet etching has been all but abandoned as the fabrication process in microelectronic industry, primarily due to the isotropic material removal
  6. 6. 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].</li></ul>(b)<br />
  7. 7. Motivation<br />Comparison of surface micrographs taken with KH-3000 digital microscope with magnification 10×350<br />BCP Process<br />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 <br />Dry Process<br />Low Cost<br />No wet chemistry<br />Environment and People friendly (compared to wet etching process)<br />Full control on the final surface <br /><ul><li>“Oxide –free” surface if kept under UHV
  8. 8. A variety of surfaces can be intentionally created through plasma processing
  9. 9. Pure Nb2O5, or other cap layer
  10. 10. superconducting NbN
  11. 11. S-I-S Multilayer</li></li></ul><li>Work Plan<br />In view of the relatively complex technological challenges facing the development of plasma-assisted surface treatment, we have adopted a 3-phase approach:<br /><ul><li>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.
  12. 12. 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.
  13. 13. Phase 3: Work with multiple-cell cavities to demonstrate final performance of the process</li></li></ul><li>Mechanism of Plasma Etching<br />2Nb (s) + 5Cl2 (g) 2NbCl5 (g) No oxidizing agent needed<br />1. Generation of reactive species<br />2. Diffusion of reactive species<br />5. Desorption of products<br />4. Reactions <br />3. Adsorption on surface <br />Nb surface<br />Bulk Nb<br />
  14. 14. Flat Samples - Microwave Glow Discharge System <br />
  15. 15. Flat Samples - Etching Rate Dependence on Discharge Parameters<br />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)<br />
  16. 16. Flat Samples - Surface Roughness <br />As received<br />BCP 1:1:2, 20 mm<br />RMS=254nm <br />Ra=210 nm<br />RMS =286nm<br />Ra=215nm<br />BCP + PE<br />RMS =215nm<br />Ra=169 nm <br />3-steps PE ,120 mm<br />RMS=234nm<br />Ra=174 nm <br />120 mm<br />AFM scans (50 μm 50 μm) <br />
  17. 17. 3-step process<br />1. Pure Ar removes physisorbed gasses and organic residues from Nb surface without damaging surface<br />Time 30 min Total flow 150 sccm<br /> Input power density 2.08 W/cm3 Pressure 500 mTorr<br />Etching rate 0nm/min<br />2. 3 Vol% Cl2 in Ar removes surface necessary for cavity production<br />Time 120 min Total flow 198.6 sccm<br />Input power density 2.08 W/cm3 Pressure 550 mTorr<br />Etching rate 1 mm/min<br />removes ~120 mm of surface in 2 h<br />3. 1.5 Vol% Cl2 in Ar removes surface under conditions more favorable for surface smoothening<br />Time 240 min Total flow 348.6 sccm<br />Input power density 1.4 W/cm3 Pressure 1250 mTorr<br />Etching rate 0.5 mm/min<br />
  18. 18. Sample Studies - Conclusions<br /><ul><li>Etching rates of bulk Nb as high as 1.7 ± 0.2 mm/min can be achieved in a microwave glow
  19. 19. Discharge using Cl2 as the reactive gas.
  20. 20. Nb etching rate depends on Cl2 reactive gas concentration and discharge parameters: input power density and pressure in reaction chamber.
  21. 21. Surface composition analyses (EDS, XPS) show that no impurities have been introduced into Nb during microwave discharge treatment.
  22. 22. Developed 3-step process.
  23. 23. 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.</li></li></ul><li>Single Cell Cavity Setup<br />Bell-Jar System <br />Plasma in The Cavity<br />Schematic Diagram<br />
  24. 24. Single Cell Sample Cavity<br />Sample and holder<br />Single Cell Cavity for Sample Etching<br />New Electrode<br />
  25. 25. Single Cell Cavity Process<br />Asymmetric Discharge <br />The scaling of the voltage drop in the plasma sheath with the surface area of the electrode : <br />Optical Intensity <br />Time waveforms of spectral line intensity at both sheaths in the discharge<br />
  26. 26. Single Cell Cavity<br />Electrode<br />Movable multiple optical fiber system attached with spectrometer used for tomographic study of plasma inside the cavity. <br />Spectrometer<br />
  27. 27. Sample cavity setup with fiber optics<br />
  28. 28.
  29. 29. Tomography of the Plasma in the Cavity<br />Stainless Steel Tube<br />Ceramic Tube<br />Optical Fibre<br />1.4 mm diameter<br />1.6 mm diameter<br />1.0 mm length of langmuir probe<br />Movable multiple optical fiber system combined with electrical probe attached to the spectrometer will be used for tomographic study of plasma inside the cavity.<br />
  30. 30. Next Steps<br />Choose an optimal power supply frequency between RF (50-200 MHz) and MW (2.45 GHz.)<br />Investigate the plasma etching behavior on samples placed on actual geometry of the single cell cavity (plasma distribution, roughness uniformity.. .)<br />Plasma etching of single cell cavities and RF performance measurements.<br />
  31. 31. Conclusion<br />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.<br />Final surface modifications (final oxidation, nitridation…) can be done in the same process cycle with the plasma etching process.<br />The geometry of the inner surface of the cavity implies that the plasma discharge has to be asymmetric. <br />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. <br />
  32. 32. Surface Roughness<br />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<br /> (a)<br />(a) (b)<br />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.<br />
  33. 33. Optical Emission Spectroscopy<br />Intensity of Cl2 continua around 257 nm .<br />Intensity of Cl2 continua around 308 nm as function of input power density in Ar/Cl2 discharge.<br />
  34. 34. Flat Samples<br />Etching Rate Dependence on Discharge Parameters<br />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.<br />
  35. 35. Electronegative plasma<br /><ul><li>The RF Ar-Cl2 discharge contains large number density of negative ions at low powers (capacitive mode) [5].
  36. 36. In this case plasma exists as an electronegative core (n+ ≈ n-) and electropositive halo (n+ ≈ ne).
  37. 37. 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.</li></li></ul><li>Surface Roughness<br />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<br /> (a)<br />(a) (b)<br />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.<br />
  38. 38. Single Cell Cavity-Data from Flat SampleSomeSpectroscopic Results<br />P = 1.3 W/cm3, Ar: 97%,, 3% Cl<br />Excitation temperature jumps about 3 min after the discharge inception.<br />Intensity of Cl2 continua around 308 nm as function of input power density in Ar/Cl2 discharge.<br />P = 1.3 W/cm3, Ar: 97%,, 3% Cl<br />P = 1 Torr, Ar: 97% , Cl2: 3%<br />Excitation temperature in the absence of Nb sample is practically constant in the applied power density range.<br />Nb I lines become prominent only after about three minutes from start. <br />
  39. 39.
  40. 40.
  41. 41. Acknowledgement<br /><ul><li>Financial support by JLab and DOE
  42. 42. Work done in collaboration with </li></ul>S. Popovic and L. Vuskovic <br />Old Dominion University, Norfolk , Virginia <br />A.– M. Valente –Feliciano and L. Phillips <br />JLab, Newport News , Virginia<br />

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