Deposition and Characterization of Superconducting Tantalum/Aluminum Bilayers

Reducing sources of loss in the materials that make up superconducting qubits is critical to improving their lifetime and performance.  Tantalum has been identified as a promising low loss base metal material for qubits, but to achieve high performance tantalum thin films must be deposited such that they are primarily α phase.  This may be accomplished on silicon and sapphire substrates either by deposition at elevated temperature or by deposition at lower temperatures on an appropriate seed layer.  In this work we examine the deposition of tantalum on aluminum seed layers to allow for deposition of α-tantalum films on Si substrates at temperatures less than 100°C.  The structure and properties of those tantalum/aluminum bilayers are examined as tantalum film and aluminum seed thickness is systematically varied.  Changes in superconducting critical temperature are correlated to the relative tantalum and aluminum layer thicknesses and the structure of the metal films, and are compared to a theoretical model of the proximitized bilayer system.

This material is based upon work supported under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. government or the U.S. Air Force.