Unraveling the Connection between Deposition, Microstructure, and Performance of Superconducting Quantum Circuits using Multi-modal Electron Microscopy

Abstract

Superconducting quantum circuits (SQC) are one of the contender hardware platforms for quantum computing. Nonetheless, the advent of fault-tolerant operation has been constrained by insufficient lifetime T1 [1]. The structural and chemical defects at the interfaces and surfaces of SQC, arising from the established semiconductor fabrication techniques, such as thin film deposition and lithography, negatively affect device coherence [2]. Therefore, it is essential to understand the micro and chemical structural features at the atomic scale that may act as sources of decoherence factors in both superconducting qubits and coplanar waveguide (CPW) resonators, which are key components of SQC. In this study, we performed a comprehensive investigation on microstructures, superconductivity, and internal quality factors of Nb CPW resonators deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (DCMS). Fig. 1a-c shows that niobium (Nb) resonators fabricated using HiPIMS (Fig. 1a) exhibit a smaller average grain size compared to two samples deposited by DCMS; one is deposited using power variation from low to high, and the other is deposited at high-power (Fig. 1b-c). We identified that grain size is highly related to two-level system (TLS) sources (Fig. 1d), while crystalline defects originating from discontinuous deposition are contributors to power-independent non-TLS sources (Fig. 1e). This study highlights different aspects of superconducting metal deposition engineering for enhanced performances of SQC [3]. Plan-view TEM images of Nb thin films deposited by (a)HiPIMS, (b) Low to high-power DC sputtering, and (c) High power DC sputtering. Measured (a) TLS loss and (b) power-independent loss extracted by power-dependent internal quality factor measurement.