Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

Abstract

We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged $T_1$ energy relaxation times of up to $270\mu s$, corresponding to Q = 5 million, and a highest observed value of $501\mu s$. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.

13 pages, 11 figures, 2 tables

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• Fast ZZ-Free Entangling Gates for Superconducting Qubits Assisted by a Driven Resonator
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• Scalable quantum simulator with an extended gate set in giant atoms
• Microwave-activated high-fidelity three-qubit gate scheme for fixed-frequency superconducting qubits
• In situ Al${}_2$O${}_3$ passivation of epitaxial tantalum and aluminum films enables long-term stability in superconducting microwave resonators
• Spectroscopy and Coherent Control of Two-Level System Defect Ensembles Using a Broadband 3D Waveguide
• Surface Optimization of Superconducting Aluminum Resonators for Robust Quantum Device Fabrication
• Optimizing CMOS-compatible, superconducting titanium nitride resonators: Deposition conditions and structuring processes