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Scalable architectures characterized by quantum bits (qubits) with low errorrates are essential to the development of a practical quantum computer. In thesuperconducting quantum computing implementation, understanding and minimizingmaterials losses is crucial to the improvement of qubit performance. A newmaterial that has recently received particular attention is indium, alow-temperature superconductor that can be used to bond pairs of chipscontaining standard aluminum-based qubit circuitry. In this work, wecharacterize microwave loss in indium and aluminum/indium thin films on siliconsubstrates by measuring superconducting coplanar waveguide resonators andestimating the main loss parameters at powers down to the sub-photon regime andat temperatures between 10 and 450 mK. We compare films deposited by thermalevaporation, sputtering, and molecular beam epitaxy. We study the effects ofheating in vacuum and ambient atmospheric pressure as well as the effects ofpre-deposition wafer cleaning using hydrofluoric acid. The microwavemeasurements are supported by thin film metrology including secondary-ion massspectrometry. For thermally evaporated and sputtered films, we find thattwo-level states (TLSs) are the dominating loss mechanism at low photon numberand temperature. Thermally evaporated indium is determined to have a TLS losstangent due to indium oxide of ~5x1e-05. The molecular beam epitaxial filmsshow evidence of formation of a substantial indium-silicon eutectic layer,which leads to a drastic degradation in resonator performance.

Thin film metrology and microwave loss characterization of indium and aluminum/indium superconducting planar resonators