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Acta Materialia

Exploring the relationship between deposition method, microstructure, and performance of Nb/Si-based superconducting coplanar waveguide resonators

Jin‐Su Oh, Cameron Kopas, Jayss Marshall, Xiaotian Fang, Kamal R. Joshi, Amlan Datta, Sunil Ghimire, Joong‐Mok Park, Richard H. J. Kim, Daniel Setiawan, Ella Lachman, J. Mutus, Akshay A. Murthy, Anna Grassellino, Alexander Romanenko, John Francis Zasadzinski, Jigang Wang, R. Prozorov, Kameshwar K. Yadavalli, Matthew Joseph Kramer, Lin Zhou

Published Jul 4, 2024• 17 citations
Publisher

Abstract

Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit’s structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. In this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electron microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at GB, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC.

DOI
10.1016/j.actamat.2024.120153

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Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit’s structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. In this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electron microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at GB, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC.
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.
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.
Understanding the correlation between material properties and microwave losses in superconducting films is a crucial subject for developing low-loss materials for quantum circuits. We focus on vanadium (V) as a novel material for superconducting quantum devices and discuss loss in V films in relation to their structural properties. Using a sputtering method, we grow four V-film structures on (001)-oriented Si wafers, employing Nb and Ta as the buffer and capping layer materials, respectively: Nb/V/Ta, Nb/V, V/Ta, and V. X-ray diffraction and atomic force microscopy reveal that the V films grown on the Nb buffer layers have higher uniformity of lattice orientation and smaller grain size than that directly grown on the Si wafer. Coplanar waveguide resonators are fabricated from the four V-film structures, and averaged photon number (⟨nph⟩\langle n_{\rm ph} \rangle⟨nph​⟩) dependences of internal quality factor (QintQ_{\rm int}Qint​) are obtained by performing microwave measurements. By analyzing the obtained QintQ_{\rm int}Qint​ vs ⟨nph⟩\langle n_{\rm ph} \rangle⟨nph​⟩, it is found that loss at the V surface is dominated by ⟨nph⟩\langle n_{\rm ph} \rangle⟨nph​⟩-independent non-two-level-system (non-TLS) losses, which can be mitigated by introducing the Ta capping layer. Furthermore, the V films on the Nb buffer layers exhibit lower QintQ_{\rm int}Qint​ in the ⟨nph⟩\langle n_{\rm ph} \rangle⟨nph​⟩ range from 100^{0}0 to 106^{6}6 and higher non-TLS loss than that directly grown on Si wafers, even though the former has higher lattice-orientation uniformity than the latter. Origins of these trends might be relevant to V oxides, of which presence at surfaces and grain boundaries in bulk regions in the V resonators is suggested by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, and/or V hydrides.
Improving qubit lifetime is essential for fault-tolerant quantum computing. It is widely acknowledged that intricate processing techniques introduce defects at the interfaces and surfaces of superconducting quantum circuits, which can create sources of decoherence. Therefore, a precise understanding of how a material's structure contributes to decoherence is critical for enhancing the performance of superconducting qubits. This talk highlights our recent advancements in understanding surface oxides in resonators and ultrathin aluminum oxide barriers in Josephson junctions (JJs) using transmission electron microscopy and spectroscopy techniques [1-4]. The Superconducting Quantum Materials & Systems Center (SQMS) demonstrated that encapsulating niobium surfaces with a ∼10nm tantalum layer results in 2-5x longer T1​ than bare niobium [2]. Our collaborative investigation shows that this improvement is attributed to tantalum's less lossy surface oxide, which contains fewer sub-oxides than niobium oxide. Moreover, the amorphous Ta2O5 shows a bonding nature closer to crystalline form than amorphous Nb2O5, potentially reducing hydrogen diffusion toward the oxide/metal interface [1, 2]. Moreover, we will briefly discuss our preliminary study of the thin film structure under cryogenic conditions down to 20K, conducted using a liquid helium TEM holder in a Thermo Cs-corrected Titan. We also explored the effect of aluminum deposition rates on aluminum oxide microstructure and qubit lifetime. Although increasing the deposition rate from 0.5 Å/s to 5 Å/s significantly improved oxide layer roughness and thickness variation, the qubits’ coherence remained unchanged. This is likely due to Cooper pairs only tunneling through the thinnest parts of the barrier, making them less affected by the grain boundaries in the thin aluminum films. Additionally, we uncovered a stress-induced grain boundary sliding mechanism, which contributes to short-circuit failures in JJs [4]. Our discovery provides vital insight into oxide microstructure and superconducting qubits’ functionality [5].
The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over one million at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and demonstrate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.
The performance of superconducting microwave circuits is strongly influenced by the material properties of the superconducting film and substrate. While progress has been made in understanding the importance of surface preparation and the effect of surface oxides, the complex effect of superconductor film structure on microwave losses is not yet fully understood. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975K. The lowest-loss resonators that we measure have quality factors of over 106 at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasiparticle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Our results stress the connection between the crystal structure of superconducting films and the loss mechanisms suffered by the resonators and indicate that even a moderate change in temperature during thin film deposition can significantly affect the resulting quality factors.