Aluminium-oxide wires for superconducting high kinetic inductance circuits

TL;DR

This paper explores the use of aluminium-oxide thin films for superconducting circuits with high kinetic inductance, demonstrating high quality factors and tunable inductance through controlled fabrication and heat treatment.

Contribution

It introduces a optimized reactive sputtering process to produce aluminium-oxide films with high sheet resistance and kinetic inductance, and studies their microwave properties at millikelvin temperatures.

Findings

Achieved a quality factor of ~700,000 at 20 mK.

Established a correlation between sheet resistance and kinetic inductance.

Demonstrated tunability of inductance via heat treatment.

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.