Aluminium-oxide wires for superconducting high kinetic inductance circuits

Authors :: Rotzinger, H.
Skacel, S. T.
Pfirrmann, M.
Voss, J. N.
Münzberg, J.
Probst, S.
Bushev, P.
Weides, M. P.
Ustinov, A. V.
Mooij, J. E.
Publication Year :: 2014
Abstract: We investigate thin films of conducting aluminium-oxide, also known as granular aluminium, as a material for superconducting high quality, high kinetic inductance circuits. The films are deposited by an optimised reactive DC magnetron sputter process and characterised using microwave measurement techniques at milli-Kelvin temperatures. We show that, by precise control of the reactive sputter conditions, a high room temperature sheet resistance and therefore high kinetic inductance at low temperatures can be obtained. For a coplanar waveguide resonator with 1.5\,k$\Omega$ sheet resistance and a kinetic inductance fraction close to unity, we measure a quality factor in the order of 700\,000 at 20\,mK. Furthermore, we observe a sheet resistance reduction by gentle heat treatment in air. This behaviour is exploited to study the kinetic inductance change using the microwave response of a coplanar wave guide resonator. We find the correlation between the kinetic inductance and the sheet resistance to be in good agreement with theoretical expectations.<br />Comment: 16 pages, 7 figures