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Multi-Modal Electron Microscopy Investigation on Decoherence Sources and Their Stability in Nb Based Superconducting Qubit

Jin‐Su Oh, Xiaotian Fang, Tae‐Hoon Kim, Matt Lynn, Matt J. Kramer, Mehdi Zarea, James A. Sauls, Alexander Romanenko, Sam Posen, Anna Grassellino, Cameron Kopas, Mark Field, Jayss Marshall, Hilal Cansizoglu, J. Mutus, Matthew J. Reagor, Lin Zhou

Published Jan 1, 2022• 1 citation
Publisher
DOI
10.2139/ssrn.4128730

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Niobium is commonly used for superconducting quantum systems as readout resonators, capacitors, and interconnects. The coherence time of the superconducting qubits is mainly limited by microwave dissipation attributed to two-level system defects at interfaces, such as the Nb/Si and Nb/air interface. One way to improve the Nb/air interface quality is by thermal annealing, as shown by extensive studies in 3D superconducting radio frequency (SRF) cavities. However, it is unclear how the microstructure and chemistry of the interface structures change during heat treatment. To address this knowledge gap, we comprehensively characterized Nb films deposited on Si wafers by physical vapor deposition, including (1) an Nb film from a transmon and (2) an Nb film without any patterning step, using an aberration-corrected transmission electron microscope. Both Nb films exhibit columnar growth with strong [110] textures. There is a double layer between the Nb film and Si substrate, which are amorphous niobium silicides with different Nb and Si concentrations. After in-situ heating of the heterostructure at 360°C inside the microscope, the composition of the double layers at the Nb-Si interface remains almost the same despite different thickness changes. The initial amorphous niobium oxide layer on Nb surface decomposes into face-centered cubic Nb nanograins in the amorphous Nb-O matrix upon heating.
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.
We report the first evidence of the formation of niobium hydrides within niobium films on silicon substrates in superconducting qubits fabricated at Rigetti Computing. We combine complementary techniques including room and cryogenic temperature atomic scale high-resolution and scanning transmission electron microscopy (HR-TEM and STEM), atomic force microscopy (AFM), and the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) to reveal the existence of the niobium hydride precipitates directly in the Rigetti chip areas. Electron diffraction and high-resolution transmission electron microscopy (HR-TEM) analyses are performed at room and cryogenic temperatures (~106 K) on superconducting qubit niobium film areas, and reveal the formation of three types of Nb hydride domains with different crystalline orientations and atomic structures. There is also variation in their size and morphology from small (~5 nm) irregular shape domains within the Nb grains to large (~10-100 nm) Nb grains fully converted to niobium hydride. As niobium hydrides are non-superconducting and can easily change in size and location upon different cooldowns to cryogenic temperatures, our findings highlight a new previously unknown source of decoherence in superconducting qubits, contributing to both quasiparticle and two-level system (TLS) losses, and offering a potential explanation for qubit performance changes upon cooldowns. A pathway to mitigate the formation of the Nb hydrides for superconducting qubit applications is also discussed.