Characterization of Nb films for superconducting qubits using phase boundary measurements

TL;DR

This study demonstrates that high-resolution measurements of the Nb film's superconducting phase boundary, combined with x-ray diffraction, can rapidly assess film quality and grain structure relevant for superconducting qubits.

Contribution

It introduces a novel, rapid characterization method for Nb films using phase boundary measurements correlated with structural analysis.

Findings

Phase boundary measurements reveal film quality variations.

Structural differences in Nb films are detectable via superconducting properties.

Method enables quick assessment of Nb film suitability for qubit fabrication.

Abstract

Continued advances in superconducting qubit performance require more detailed understandings of the many sources of decoherence. Within these devices, two-level systems arise due to defects, interfaces, and grain boundaries, and are thought to be a major source of qubit decoherence at millikelvin temperatures. In addition to Al, Nb is a commonly used metalization layer for superconducting qubits. Consequently, a significant effort is required to develop and qualify processes that mitigate defects in Nb films. As the fabrication of complete superconducting qubits and their characterization at millikelvin temperatures is a time and resource intensive process, it is desirable to have measurement tools that can rapidly characterize the properties of films and evaluate different treatments. Here we show that measurements of the variation of the superconducting critical temperature $T_c$ with an applied external magnetic field $H$ (of the phase boundary $T_c - H$) performed with very high resolution show features that are directly correlated with the structure of the Nb films. In combination with x-ray diffraction measurements, we show that one can even distinguish variations quality and crystal orientation of the grains in a Nb film by small but reproducible changes in the measured superconducting phase boundary.