Abstract 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.
Abstract 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.
Abstract 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 a time-consuming process and stringent requirements for the film deposition system. Here we demonstrate the deposition of high-quality α -Ta at room temperature using a niobium (Nb) film as a buffer layer. The buffered 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 observed an “inflection point” in the dependence of the Q i of the resonators on the input microwave photons, which can be fitted well with an expression for the superposition of the buffered resonators. These experimental results show that the buffered films deposited at room temperature have promising applications in superconducting devices.
Abstract We use a magnetron sputtering system to grow a tantalum film on a sapphire substrate with a niobium buffer layer at room temperature. The tantalum film is in α-phase, which is preferred for superconducting quantum devices. We fabricate a superconducting heavy fluxonium qubit with this niobium-buffered tantalum film, which demonstrates good performance. Forbidden transitions and sideband transition phenomena between specific energy levels are observed. Due to the specific design parameters, the fluxonium described herein exhibits high anharmonicity while maintaining a long decoherence time in the higher energy states, making it well-suited for use as a qudit and enabling high-dimensional quantum computation. With further optimization of the film deposition process, superconducting qubits other than fluxoniums with higher performance are expected to be obtained using these multilayer films grown at room temperature.
Tantalum films incorporated into superconducting circuits have exhibited low surface losses, resulting in long-lived qubit states. The remaining loss pathways originate in microscopic defects that manifest as two level systems (TLSs) at low temperatures. These defects limit performance, so careful attention to tantalum film structures is critical for optimal use in quantum devices. In this work, we investigate the growth of tantalum using magnetron sputtering on sapphire, Si, and photoresist substrates. In the case of sapphire, we present procedures for the growth of fully-oriented films with α-Ta [1 1 1]//Al2O3 [0 0 0 1] and α-Ta [1 −1 0]//Al2O3 [1 0 −1 0] orientational relationships and having residual resistivity ratio (RRR) ∼ 60 for 220 nm thick films. On Si, we find a complex grain texturing with Ta [1 1 0] normal to the substrate and RRR ∼ 30. We further demonstrate airbridge fabrication using Nb to nucleate α-Ta on photoresist surfaces. For the films on sapphire, resonators show TLS-limited quality factors of 1.3 ± 0.3 × 106 at 10 mK (for a waveguide gap and conductor width of 3 and 6 μm, respectively). Structural characterization using scanning electron microscopy, x-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy reveal the dependence of residual impurities and screw dislocation density on processing conditions. The results provide practical insights into the fabrication of advanced superconducting devices including qubit arrays and guide future works on crystallographically deterministic qubit fabrication.
Abstract Tantalum (Ta) films with a body-centered cubic α-phase play a crucial role in high-performance Ta-based superconducting quantum processors. The growth of pure α-Ta at low temperatures has been a long-term goal. Existing experience with Ta growth on silicon substrates has led to the belief that α-Ta can only be stabilized at high temperatures. In this study, we systematically analyzed the growth mechanism of Ta films on sapphire substrates using first-principles calculations, revealing a narrow growth energy window for pure α-Ta at room temperature (RT). This finding indicates the feasibility of obtaining pure α-Ta at significantly lower temperatures. Experimentally, we successfully achieved pure α-Ta at RT via magnetron sputtering by meticulously adjusting the growth parameters. The resulting superconducting resonators based on the developed RT α-Ta exhibit an internal quality factor ( Q i ) of approximately 2.1 × 10 6 at single-photon levels, comparable to those fabricated at high temperatures. Additionally, fixed-frequency transmon qubits fabricated using the RT α-Ta film demonstrate a coherence time exceeding 280 µs, underscoring the film’s high quality. The RT α-Ta without a seed layer simplifies the fabrication process, offering a more efficient and flexible approach for producing Ta-based superconducting quantum processors.
Superconducting qubits based on tantalum (Ta) thin films have demonstrated the highest-performing microwave resonators and qubits. This makes Ta an attractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding \SI{400}{\celsius} to achieve the body-centered cubic phase, BCC (\textalpha-Ta). This leads to compatibility issues for scalable fabrication leveraging standard semiconductor fabrication lines. Here, we show that changing the sputter gas from argon (Ar) to krypton (Kr) promotes BCC Ta synthesis on silicon (Si) at temperatures as low as \SI{200}{\celsius}, providing a wide process window compatible with back-end-of-the-line fabrication standards. Furthermore, we find these films to have substantially higher electronic conductivity, consistent with clean-limit superconductivity. We validated the microwave performance through coplanar waveguide resonator measurements, finding that films deposited at \SI{250}{\celsius} and \SI{350}{\celsius} exhibit a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of Ta/Si intermixing revealed by cross-sectional transmission electron microscopy. Finally, with these films, we demonstrate transmon qubits with a relatively compact, \SI{20}{\micro\meter} capacitor gap, achieving a median quality factor up to 14 million.
Abstract The use of α -tantalum in superconducting circuits has enabled a considerable improvement in the coherence time of transmon qubits. The standard approach to grow α -tantalum thin films on silicon involves heating the substrate, which takes several hours per deposition and prevents the integration of this material with wafers containing temperature-sensitive components. We report a detailed experimental study of an alternative growth method of α -tantalum on silicon, which is achieved at room temperature through the use of a niobium seed layer. Despite a substantially higher density of oxygen-rich grain boundaries in the films sputtered at room temperature, resonators made from these films are found to have state-of-the-art quality factors, comparable to resonators fabricated from tantalum grown at high temperature. This finding challenges previous assumptions about correlations between material properties and microwave loss of superconducting thin films, and opens a new avenue for the integration of tantalum into fabrication flows with limited thermal budget.