SRF2011 - List of Authors (Stevie, F.A.)

Paper

Title

Page

Energetic Condensation Growth of Nb Thin-films

309

M. Krishnan, C. James, E.F. Valderrama
AASC, San Leandro, California, USA
A.D. Batchelor, P. Maheshwari, F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA
H.L. Phillips, C.E. Reece, J.K. Spradlin, A-M. Valente-Feliciano, X. Zhao
JLAB, Newport News, Virginia, USA
K.I. Seo
NSU, Newport News, Virginia, USA
Z.H. Sung
ASC, Tallahassee, Florida, USA

Funding: Funded by DE-FG02-08ER85162 and DE-SC0004994. The Jefferson Science Associates, LLC effort supported by DE-AC05-06OR23177, with supplemental funding from the American Recovery and Reinvestment Act.
This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma, whose 40-120eV ions enable sufficient surface mobility to ensure that the lowest energy state (crystalline structure with minimal defects) is accessible to the film. Hetero-epitaxial films of Nb were grown on a-plane sapphire and MgO crystals with good superconducting properties and crystal size (10mm × 20mm) limited only by substrate size. The substrates were heated to 700 deg C and coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. Residual resistivity ratio (RRR) values (up to a record RRR-554 on MgO and RRR-328 on a-sapphire) vary with substrate annealing and deposition temperatures. XRD spectra and pole figures reveal that RRR increases as the crystal structure of the Nb film becomes more ordered, consistent with fewer defects and hence longer electron mean free path. A transition from Nb(110) to Nb(100) orientation on the MgO(100) lattice occurs at higher temperatures. SIMS depth profiles, EBSD and SEM images complement the XRD data. Crystalline structure in Nb on amorphous borosilicate substrates has implications for future, lower-cost SRF cavities.

Slides TUIOB01 [8.959 MB]

TUPO051

High-Temperature Heat Treatment Study on a Large-Grain Nb Cavity

508

G. Ciovati, P. Dhakal, R. Myneni
JLAB, Newport News, Virginia, USA
P. Maheshwari, F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA

Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177
Improvement of the cavity performance by a high-temperature heat-treatment without subsequent chemical etching have been reported for large-grain Nb cavities treated by buffered chemical polishing, as well as for a fine-grain cavity treated by vertical electropolishing [1]. Changes in the quality factor, Q0, and maximum peak surface magnetic field achieved in a large-grain Nb single-cell cavity have been determined as a function of the heat treatment temperature, between 600 °C and 1200 °C. The highest Q0 improvement of about 30% was obtained after heat-treatment at 800 °C-1000 °C. Measurements by secondary ion mass spectrometry on large-grain samples heat-treated with the cavity showed large reduction of hydrogen concentration after heat treatment.
[1] G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, and D. Griffis, Phys. Rev. ST Accel. Beams 13, 022002 (2010)

THPO028

SIMS and TEM Analysis of Niobium Bicrystals

776

P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou
NCSU AIF, Raleigh, North Carolina, USA
G. Ciovati, R. Myneni, J.K. Spradlin
JLAB, Newport News, Virginia, USA
M. Rigsbee
Materials Science and Engineering, Raleigh, USA

The behaviour of interstitial impurities(C,O,N,H) on the Nb surface with respect to grain boundaries may affect cavity performance. Large grain Nb makes possible the selection of bicrystal samples with a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other non heat treated (control). H levels were found to be higher for the non heat treated sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both the samples. TEM results showed that the bicrystal interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both the samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both the samples. C was observed to be segregated along the grain boundary for the control sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences both across the grain boundary and between samples

THPO032

TOF-SIMS Analysis of Hydrogen in Niobium, From 160°K to 475°K

788

P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou
NCSU AIF, Raleigh, North Carolina, USA
G. Ciovati, R. Myneni
JLAB, Newport News, Virginia, USA
M. Rigsbee
Materials Science and Engineering, Raleigh, USA

Niobium (Nb) is the material of choice for superconducting radio frequency (SRF) cavities due to its high critical temperature and critical magnetic field. Interstitial impurity elements such as H directly influence the efficiency of these cavities. Quantification of H in Nb is difficult since H is extremely mobile in Nb with a very high diffusion coefficient even at room temperature. In the presented work, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was used to characterize H in Nb over a wide temperature range (160°K to 475°K) in situ to check for changes in mobility. Multiple experiments showed that as the specimen temperature is decreased below 300 °K, the H/Nb intensity changes by first increasing and then decreasing drastically at temperatures below 200°K. As specimen temperature is increased from 300°K to 450°K, the H/Nb intensity decreases. Remarkably, the H intensity with respect to Nb increases with time at 475°K (approximately 200oC). Correlation between this data and the H-Nb phase diagram appears to account for the H behaviour.

Poster THPO032 [0.125 MB]

THPO069

Nb Film Growth on Crystalline and Amorphous Substrates

898

E.F. Valderrama, C. James, M. Krishnan
AASC, San Leandro, California, USA
P. Maheshwari, F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA
K.I. Seo
NSU, Newport News, Virginia, USA
X. Zhao
JLAB, Newport News, Virginia, USA

Funding: Funded by DE-FG02-08ER85162 and DE-SC0004994. The Jefferson Science Associates, LLC effort supported by DE-AC05-06OR23177, with supplemental funding from the American Recovery and Reinvestment Act.
This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma on crystalline (a- and c-sapphire, MgO) and amorphous (borosilicate) substrates. The crystal substrates were heated to 700 deg C and subsequently coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. The borosilicate substrate was preheated to 700 deg C but coated at 500 deg C. XRD spectra (Bragg-Brentano) and pole figures show a change in crystal structure on c-sapphire from textured (with twin-symmetry) to hetero-epitaxial as the temperature is increased. RRR=43 was measured on c-sapphire which is lower than RRR=200 on a-sapphire and 541 on MgO. On borosilicate, the (110) and (220) planes of Nb show sharper spectra at higher temperatures with an increase to RRR=31 at 500 deg C. The growth of crystalline Nb on an amorphous substrate is driven by energetic (40-120eV) ions from the cathodic arc plasma. The significance of crystal structure on amorphous substrates has implications for future, lower-cost SRF cavities. SIMS data show the role of impurities on crystal growth and RRR.

THPO074

SRF Multilayer Structures based on NbTiN

920

A-M. Valente-Feliciano, H.L. Phillips, C.E. Reece, J.K. Spradlin
JLAB, Newport News, Virginia, USA
A.D. Batchelor, F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA
R.A. Lukaszew
The College of William and Mary, Williamsburg, USA
K.I. Seo
NSU, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
For the past three decades, bulk niobium has been the material of choice for SRF cavities applications. In the recent years, RF cavities performances have approached the theoretical limit for bulk niobium. For further improvement of RF cavity performance for future accelerator projects, an interesting alternative has been recently proposed by Alex Gurevich with the Superconductor-Insulator-Superconductor multilayer approach, using the benefit of the higher critical field Hc2 of higher-Tc superconductors without being limited with their lower Hc1. JLab is pursuing this approach with the development of multilayer structures based on NbTiN via magnetron sputtering and High Power Impulse Magnetron Sputtering (HiPIMS). Insulators such as, AlN, Al2O3 and MgO are being investigated as candidates for the insulator layers. This paper present the preliminary results on the characteristics of NbTiN and insulator layers and a first attempt of a NbTiN-based multilayer structure on bulk Nb and thick Nb films.

THPO077

Mo-Re Films for SRF Applications

930

E.F. Valderrama, C. James, M. Krishnan
AASC, San Leandro, California, USA
P. Maheshwari, F.A. Stevie
NCSU AIF, Raleigh, North Carolina, USA
K.I. Seo
NSU, Newport News, Virginia, USA
X. Zhao
JLAB, Newport News, Virginia, USA

Funding: Funded by DE-FG02-08ER85162 and DE-SC0004994. The Jefferson Science Associates, LLC effort supported by DE-AC05-06OR23177, with supplemental funding from the American Recovery and Reinvestment Act
Single (sintered composite Mo3:Re1) and dual targets of Mo/Re were used to grow superconducting films of Mo:Re, using cathodic arc plasmas. Sharp superconducting transitions (at up to 13K) were observed in ~1 μm thick films deposited on a-sapphire and MgO crystals. The measured RRR (defined as the ratio of resistivity at 300K to that at 14K) in the best films was 6, which is higher than measured by others at higher annealing temperatures. XRD (Bragg-Brentano spectra) revealed a single sharp peak of Mo-Re (611) plane, from the composite Mo3:Re1 film. SIMS measurements revealed the role of impurity concentrations on superconducting properties. For the dual-target films, stoichiometry was controlled by varying the current to each cathode. The XRD spectra in this case showed the (330) plane of Mo-Re; hero-epitaxial growth of Mo-Re depends upon the stoichiometry of the film. This dual-target approach allows other compound films (e.g. Nb3Sn, MgB2 etc.) to be grown in a single-step. For SRF cavity applications, the RRR should be increased to >100, which is our next goal.

Paper	Title	Page
TUIOB01	Energetic Condensation Growth of Nb Thin-films	309
	M. Krishnan, C. James, E.F. Valderrama AASC, San Leandro, California, USA A.D. Batchelor, P. Maheshwari, F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA H.L. Phillips, C.E. Reece, J.K. Spradlin, A-M. Valente-Feliciano, X. Zhao JLAB, Newport News, Virginia, USA K.I. Seo NSU, Newport News, Virginia, USA Z.H. Sung ASC, Tallahassee, Florida, USA
	Funding: Funded by DE-FG02-08ER85162 and DE-SC0004994. The Jefferson Science Associates, LLC effort supported by DE-AC05-06OR23177, with supplemental funding from the American Recovery and Reinvestment Act. This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma, whose 40-120eV ions enable sufficient surface mobility to ensure that the lowest energy state (crystalline structure with minimal defects) is accessible to the film. Hetero-epitaxial films of Nb were grown on a-plane sapphire and MgO crystals with good superconducting properties and crystal size (10mm × 20mm) limited only by substrate size. The substrates were heated to 700 deg C and coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. Residual resistivity ratio (RRR) values (up to a record RRR-554 on MgO and RRR-328 on a-sapphire) vary with substrate annealing and deposition temperatures. XRD spectra and pole figures reveal that RRR increases as the crystal structure of the Nb film becomes more ordered, consistent with fewer defects and hence longer electron mean free path. A transition from Nb(110) to Nb(100) orientation on the MgO(100) lattice occurs at higher temperatures. SIMS depth profiles, EBSD and SEM images complement the XRD data. Crystalline structure in Nb on amorphous borosilicate substrates has implications for future, lower-cost SRF cavities.
	Slides TUIOB01 [8.959 MB]

TUPO051	High-Temperature Heat Treatment Study on a Large-Grain Nb Cavity	508
	G. Ciovati, P. Dhakal, R. Myneni JLAB, Newport News, Virginia, USA P. Maheshwari, F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA
	Funding: This manuscript has been authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177 Improvement of the cavity performance by a high-temperature heat-treatment without subsequent chemical etching have been reported for large-grain Nb cavities treated by buffered chemical polishing, as well as for a fine-grain cavity treated by vertical electropolishing [1]. Changes in the quality factor, Q0, and maximum peak surface magnetic field achieved in a large-grain Nb single-cell cavity have been determined as a function of the heat treatment temperature, between 600 °C and 1200 °C. The highest Q0 improvement of about 30% was obtained after heat-treatment at 800 °C-1000 °C. Measurements by secondary ion mass spectrometry on large-grain samples heat-treated with the cavity showed large reduction of hydrogen concentration after heat treatment. [1] G. Ciovati, G. Myneni, F. Stevie, P. Maheshwari, and D. Griffis, Phys. Rev. ST Accel. Beams 13, 022002 (2010)

THPO028	SIMS and TEM Analysis of Niobium Bicrystals	776
	P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou NCSU AIF, Raleigh, North Carolina, USA G. Ciovati, R. Myneni, J.K. Spradlin JLAB, Newport News, Virginia, USA M. Rigsbee Materials Science and Engineering, Raleigh, USA
	The behaviour of interstitial impurities(C,O,N,H) on the Nb surface with respect to grain boundaries may affect cavity performance. Large grain Nb makes possible the selection of bicrystal samples with a well defined grain boundary. In this work, Dynamic SIMS was used to analyze two Nb bicrystal samples, one of them heat treated and the other non heat treated (control). H levels were found to be higher for the non heat treated sample and a difference in the H intensity and sputtering rate was also observed across the grain boundary for both the samples. TEM results showed that the bicrystal interface showed no discontinuity and the oxide layer was uniform across the grain boundary for both the samples. TOF-SIMS imaging was also performed to analyze the distribution of the impurities across the grain boundary in both the samples. C was observed to be segregated along the grain boundary for the control sample, while H and O showed a difference in signal intensity across the grain boundary. Crystal orientation appears to have an important role in the observed sputtering rate and impurity ion signal differences both across the grain boundary and between samples

THPO032	TOF-SIMS Analysis of Hydrogen in Niobium, From 160°K to 475°K	788
	P. Maheshwari, A.D. Batchelor, D.P. Griffis, F.A. Stevie, C. Zhou NCSU AIF, Raleigh, North Carolina, USA G. Ciovati, R. Myneni JLAB, Newport News, Virginia, USA M. Rigsbee Materials Science and Engineering, Raleigh, USA
	Niobium (Nb) is the material of choice for superconducting radio frequency (SRF) cavities due to its high critical temperature and critical magnetic field. Interstitial impurity elements such as H directly influence the efficiency of these cavities. Quantification of H in Nb is difficult since H is extremely mobile in Nb with a very high diffusion coefficient even at room temperature. In the presented work, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) was used to characterize H in Nb over a wide temperature range (160°K to 475°K) in situ to check for changes in mobility. Multiple experiments showed that as the specimen temperature is decreased below 300 °K, the H/Nb intensity changes by first increasing and then decreasing drastically at temperatures below 200°K. As specimen temperature is increased from 300°K to 450°K, the H/Nb intensity decreases. Remarkably, the H intensity with respect to Nb increases with time at 475°K (approximately 200oC). Correlation between this data and the H-Nb phase diagram appears to account for the H behaviour.
	Poster THPO032 [0.125 MB]

THPO069	Nb Film Growth on Crystalline and Amorphous Substrates	898
	E.F. Valderrama, C. James, M. Krishnan AASC, San Leandro, California, USA P. Maheshwari, F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA K.I. Seo NSU, Newport News, Virginia, USA X. Zhao JLAB, Newport News, Virginia, USA
	Funding: Funded by DE-FG02-08ER85162 and DE-SC0004994. The Jefferson Science Associates, LLC effort supported by DE-AC05-06OR23177, with supplemental funding from the American Recovery and Reinvestment Act. This paper describes Energetic Condensation Growth of Nb films using a cathodic arc plasma on crystalline (a- and c-sapphire, MgO) and amorphous (borosilicate) substrates. The crystal substrates were heated to 700 deg C and subsequently coated at 300, 500 and 700 deg C. Film thickness varied from ~0.25μm up to >3μm. The borosilicate substrate was preheated to 700 deg C but coated at 500 deg C. XRD spectra (Bragg-Brentano) and pole figures show a change in crystal structure on c-sapphire from textured (with twin-symmetry) to hetero-epitaxial as the temperature is increased. RRR=43 was measured on c-sapphire which is lower than RRR=200 on a-sapphire and 541 on MgO. On borosilicate, the (110) and (220) planes of Nb show sharper spectra at higher temperatures with an increase to RRR=31 at 500 deg C. The growth of crystalline Nb on an amorphous substrate is driven by energetic (40-120eV) ions from the cathodic arc plasma. The significance of crystal structure on amorphous substrates has implications for future, lower-cost SRF cavities. SIMS data show the role of impurities on crystal growth and RRR.

THPO074	SRF Multilayer Structures based on NbTiN	920
	A-M. Valente-Feliciano, H.L. Phillips, C.E. Reece, J.K. Spradlin JLAB, Newport News, Virginia, USA A.D. Batchelor, F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA R.A. Lukaszew The College of William and Mary, Williamsburg, USA K.I. Seo NSU, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. For the past three decades, bulk niobium has been the material of choice for SRF cavities applications. In the recent years, RF cavities performances have approached the theoretical limit for bulk niobium. For further improvement of RF cavity performance for future accelerator projects, an interesting alternative has been recently proposed by Alex Gurevich with the Superconductor-Insulator-Superconductor multilayer approach, using the benefit of the higher critical field Hc2 of higher-Tc superconductors without being limited with their lower Hc1. JLab is pursuing this approach with the development of multilayer structures based on NbTiN via magnetron sputtering and High Power Impulse Magnetron Sputtering (HiPIMS). Insulators such as, AlN, Al2O3 and MgO are being investigated as candidates for the insulator layers. This paper present the preliminary results on the characteristics of NbTiN and insulator layers and a first attempt of a NbTiN-based multilayer structure on bulk Nb and thick Nb films.

THPO077	Mo-Re Films for SRF Applications	930
	E.F. Valderrama, C. James, M. Krishnan AASC, San Leandro, California, USA P. Maheshwari, F.A. Stevie NCSU AIF, Raleigh, North Carolina, USA K.I. Seo NSU, Newport News, Virginia, USA X. Zhao JLAB, Newport News, Virginia, USA
	Funding: Funded by DE-FG02-08ER85162 and DE-SC0004994. The Jefferson Science Associates, LLC effort supported by DE-AC05-06OR23177, with supplemental funding from the American Recovery and Reinvestment Act Single (sintered composite Mo3:Re1) and dual targets of Mo/Re were used to grow superconducting films of Mo:Re, using cathodic arc plasmas. Sharp superconducting transitions (at up to 13K) were observed in ~1 μm thick films deposited on a-sapphire and MgO crystals. The measured RRR (defined as the ratio of resistivity at 300K to that at 14K) in the best films was 6, which is higher than measured by others at higher annealing temperatures. XRD (Bragg-Brentano spectra) revealed a single sharp peak of Mo-Re (611) plane, from the composite Mo3:Re1 film. SIMS measurements revealed the role of impurity concentrations on superconducting properties. For the dual-target films, stoichiometry was controlled by varying the current to each cathode. The XRD spectra in this case showed the (330) plane of Mo-Re; hero-epitaxial growth of Mo-Re depends upon the stoichiometry of the film. This dual-target approach allows other compound films (e.g. Nb3Sn, MgB2 etc.) to be grown in a single-step. For SRF cavity applications, the RRR should be increased to >100, which is our next goal.