Superconducting Transmon Qubits with the Niobium-buffered α-Tantalum on the Sapphire Substrates

Tantalum (Ta) is a promising material for superconducting qubits due to its stable chemical properties, formation of a single oxide, and resistance to reaction with other substances. However, achieving the desired α-Ta typically requires high-temperature substrate heating, leading to the time-consuming process and stringent requirements of the film deposition system. Here we demonstrate the deposition of high-quality α-Ta at room temperature using the niobium (Nb) film as a buffer layer. The multilayer films are characterized using X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Transmons with capacitors, transmission lines, and readout resonators fabricated with such Nb buffered Ta exhibit excellent performance. We observe an “inflection point” in the dependence of the Qi of the resonators on the input microwave photons, which can be fitted well with an expression for the superposition of the multilayer resonators. These experimental results show that the multilayer films deposited at room temperature have promising applications in superconducting devices.