Atomic Layer Deposition Josephson Junctions for Cryogenic Circuit Applications
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Atomic Layer Deposition Josephson Junctions for Cryogenic Circuit Applications C. A. Jhabvala1 · P. C. Nagler1 · T. R. Stevenson1 Received: 1 August 2019 / Accepted: 6 April 2020 © This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply 2020
Abstract Superconducting-insulating-superconducting trilayers have been produced for Josephson Junction fabrication by thermal atomic layer deposition (ALD) processes. The trilayers are composed of alternating layers of T i0.4N0.6/Al2O3/Ti0.4N0.6, deposited at 450 °C, in a thermal ALD reactor on A l2O3-coated silicon. The conformal nature of the ALD process provides excellent step coverage of superconducting and insulating films. The film thickness of a single ALD cycle, being one mono-layer, allows us to precisely control the tunnel-barrier insulator thickness by counting the number of ALD cycles during the insulator deposition step. Tunnel junctions with critical current approximately 500 A/cm2 are reported. Fabrication of Josephson Junctions and progress toward development of a single-element ALD superconducting quantum interference device are discussed. Keywords Atomic layer deposition · ALD · Josephson Junction · SQUID · Titanium nitride · Superconducting
1 ALD Trilayer Growth The conformal nature of ALD makes this technique extremely attractive for depositing and patterning multiple layers of superconductors and insulators. ALD eliminates step-coverage problems, the need for sloped-sidewall etches, and the potential for a discontinuity when the superconductor crosses over a sharp step, as is seen, for example, in the complex fabrication steps necessary for some sputter produced superconducting circuits [1]. As cryogenic detector arrays grow to larger formats, highly compact readouts will be required. ALD offers excellent step coverage and enables the complete filling, without voids, of small geometries with high aspect-ratio features. We seek to ultimately design and fabricate * C. A. Jhabvala [email protected] 1
NASA Goddard Space Flight Center, Greenbelt, MD, USA
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Journal of Low Temperature Physics
compact superconducting circuits. The need for fine line widths and reduced line spacing is becoming apparent. Using ALD-grown superconductors and insulators, we seek to shrink our line spacing to sub-micron gaps with excellent step coverage between traces. ALD is a gas-phase deposition technique. Films are grown by alternating two reactant gases, or precursors. The growth is self-limiting in nature, with one molecular layer of material grown per alternating cycle of reactants. An inert carrier gas, N2 in our deposition system, is used to continuously flow gas through the system as the precursors are added in short pulses. The deposition system used in this study is a Beneq™ Model TFS 200 multiple-wafer (25) thermal reactor. Precursors for T i0.4N0.6 are titanium tetrachloride (TiCl4) and ammonia (NH3), and for Al2O3 are tetramethylaluminum (TMA) and water (H
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