Low-Loss Superconducting Resonators Fabricated from Tantalum Films Grown at Room Temperature

TL;DR

This study demonstrates that room-temperature grown tantalum films with niobium seed layers can achieve superconducting resonator quality factors comparable to high-temperature films, enabling better integration with temperature-sensitive components.

Contribution

It introduces a room-temperature growth method for tantalum films that maintains high resonator quality, facilitating integration into complex superconducting circuits.

Findings

Room-temperature tantalum films have high quality factors.

Despite more grain boundaries, these films perform comparably to high-temperature grown films.

This challenges previous beliefs about material properties and microwave loss.

Abstract

The use of $\alpha$-tantalum in superconducting circuits has enabled a considerable improvement of the coherence time of transmon qubits. The standard approach to grow $\alpha$-tantalum thin films on silicon involves heating the substrate, which takes several hours per deposition and prevents the integration of this material with wafers containing temperature-sensitive components. We report a detailed experimental study of an alternative growth method of $\alpha$-tantalum on silicon, which is achieved at room temperature through the use of a niobium seed layer. Despite a substantially higher density of oxygen-rich grain boundaries in the films sputtered at room temperature, resonators made from these films are found to have state-of-the-art quality factors, comparable to resonators fabricated from tantalum grown at high temperature. This finding challenges previous assumptions about correlations between material properties and microwave loss of superconducting thin films, and opens a new avenue for the integration of tantalum into fabrication flows with limited thermal budget.