Wafer-Scale MgB₂ Superconducting Devices

Creators: Kim, Changsub¹; Bell, Christina^{1, 2}; Evans, Jake M.³; Greenfield, Jonathan^{1, 2}; Batson, Emma⁴; Berggren, Karl K.⁴; Lewis, Nathan S.³; Cunnane, Daniel P.¹

1. Jet Propulsion Lab
2. Arizona State University
3. California Institute of Technology
4. Massachusetts Institute of Technology

Abstract

Progress in superconducting device and detector technologies over the past decade has realized practical applications in quantum computers, detectors for far-infrared telescopes, and optical communications. Superconducting thin-film materials, however, have remained largely unchanged, with aluminum still being the material of choice for superconducting qubits and niobium compounds for high-frequency/high kinetic inductance devices. Magnesium diboride (MgB₂), known for its highest transition temperature (T_c = 39 K) among metallic superconductors, is a viable material for elevated temperature and higher frequency superconducting devices moving toward THz frequencies. However, difficulty in synthesizing wafer-scale thin films has prevented implementation of MgB₂ devices into the application base of superconducting electronics. Here, we report ultrasmooth (<0.5 nm root-mean-square roughness) and uniform MgB₂ thin (<100 nm) films over 100 mm in diameter and present prototype devices fabricated with these films demonstrating key superconducting properties including an internal quality factor over 10⁴ at 4.5 K and high tunable kinetic inductance in the order of tens of pH/sq in a 40 nm thick film. This advancement will enable development of elevated temperature, high-frequency superconducting quantum circuits, and devices.

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Acknowledgement

The research by C. Kim, C. Bell, J. Greenfield, and D. Cunnane was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). This work was primarily supported by the Nancy Grace Roman Technology Fellowship in Astrophysics and NASA Research Opportunities in Space and Earth Sciences (NNH20ZDA001N-APRA). We acknowledge the support and infrastructure provided for this work by the Microdevices Laboratory at JPL and the Kavli Nanoscience Institute at Caltech. We thank H. LeDuc, B. Bumble, and A. Beyer for advice on device fabrication, P. Day for discussions and support on RF measurements, M. Dickie for chemical vapor deposition of silicon nitride buffer layer, and A. Wertheim for discussions on sputter depositions. The XPS was carried out at the Molecular Materials Resource Center in the Beckman Institute at the California Institute of Technology and supported by the U.S. Department of Energy grant numbers DE-SC0004993 and DE-SC0022087. The STEM work was completed in MIT.nano facilities. We thank Juan Ferrera for assistance in preparing the TEM lamella. E. Batson acknowledges the National Science Foundation Graduate Research Fellowship under Grant No. 2141064 and the NSF CQN program under Grant No. EEC1941583.

Conflict of Interest

The authors declare the following competing financial interest(s): The California Institute of Technology has filed a U.S. utility patent with the title Wafer scale production of superconducting magnesium diboride thin films with high transition temperature (inventors: C.K. and D.P.C.) describing the superconducting magnesium diboride thin film and device fabrication methods described in this paper.

Kim, C.; Bell, C.; Evans, J.; Greenfield, J.; Batson, E.; Berggren, K.; Lewis, N.; Cunnane, D. Wafer-Scale MgB2 Superconducting Devices. 2023; arxiv.2305.15190, arXiv, https:// https://arxiv.org/abs/2305.15190, accessed August 21, 2024. This manuscript includes thickness dependence of DC superconducting properties (Figure and associated discussions), which is not available in the preprint versions.

Contributions

C.K. and D.P.C. conceived and designed the experimental protocol. C.K. prepared and optimized the films. C.K. performed eddy current, cryogenic DC conductivity, and AFM measurements. D.P.C. designed the resonator patterns. C.K. fabricated the devices. C.B., J.G., and D.P.C. performed cryogenic RF measurements. C.K. and D.P.C. interpreted the results. J.M.E. performed XPS depth-profiling and analyzed the data. N.S.L. secured funding for the XPS equipment and analysis. E.B. performed sampling and STEM imaging of the MgB₂ lamella. K.K.B. supervised E.B.’s work. D.P.C. provided guidance throughout the project. C.K. and D.P.C. wrote the manuscript. All the coauthors discussed the results and helped revise the manuscript.

Supplemental Material

X-ray photoelectron spectroscopy (XPS) depth profile analysis of MgB₂ thin-film samples used to create Figure 3c (PDF)

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