Aluminum properties beyond the thin film regime for superconducting qubit circuits*

Aluminum is the most commonly used material in superconducting qubit circuits, yet its properties are not fully characterized in the thin film regime. In particular, the penetration depth of thin film aluminum is not well documented, even though its value is of utmost importance in the design of, e. g., flux qubits and couplers. Very relevant device parameters are affected by penetration depth, particularly the kinetic inductance and the flux exclusion properties of the thin film, leading to device design uncertainties and loss, respectively.

In this work, we study the dependence of penetration depth with film thickness. Our experiment consists of measuring the resonance of LC resonators with different aluminum thickness and extracting the kinetic inductance contribution to the resonance. Separately, four-probe measurements of the normal state resistance of aluminum at 4K provide an alternative estimate of the kinetic inductance.

A proper choice of film thickness leads to penetration depths low enough to favor the removal of vortexes, which are a source of qubit decoherence and resonator noise, while keeping the rest of device properties minimally altered.