The performance of superconducting integrated circuits for quantum applications is fundamentally limited by material-related losses. Tantalum, as an emerging material for next-generation quantum circuits, has attracted considerable attention in recent years after demonstrating breakthrough performance in both superconducting microwave resonators and qubits. Concurrently, a growing body of work is devoted to the operation of tantalum-based circuits and related fabrication techniques. This interest is further stimulated by tantalum thin films polymorphism resulting in a variety of its crystalline structure, superconducting properties, coherence, etc. Furthermore, tantalum circuits exhibit distinctive features in cryogenic experiments, which have not been observed in aluminum- or niobium-based ones. In this review, we summarize the recent research of tantalum thin films growth and phase selection mechanisms on various substrates, key aspects of fabrication and performance of superconducting circuit, including a material first-principles theoretical study. In conclusion, we address a number of open issues, including the role of \b{eta}-phase impurities, the effect of hydrofluoric acid solutions on chain characteristics, and the anomalous behavior of α-tantalum chains at cryogenic temperatures.
Motivated by recent advancements highlighting
Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have explored the effect of cryogenic temperatures on the growth of
Ta and its integration in superconducting circuits. Cryogenic growth of
Ta using a low-temperature molecular beam epitaxy (MBE) system is found to stabilize single-phase $\ensuremath{\alpha}$-
Ta on several different substrates, which include
Al2O3(0001),
Si(001),
Si(111),
SiNx, and
GaAs(001). The substrates are actively cooled down to cryogenic temperatures and remain 20 K during the
Ta deposition. X-ray $\ensuremath{\theta}$-$2\ensuremath{\theta}$ diffraction (XRD) after warming to room temperature indicates the formation of polycrystalline $\ensuremath{\alpha}$-
Ta. The 50-nm $\ensuremath{\alpha}$-
Ta films grown on
Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (${\ensuremath{\rho}}_{300\phantom{\rule{0.1em}{0ex}}\mathrm{K}}$) of $13.4\phantom{\rule{0.1em}{0ex}}\text{\ensuremath{\mu}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}$, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature (
TC) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an rms roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher ${\ensuremath{\rho}}_{300\phantom{\rule{0.1em}{0ex}}\mathrm{K}}$, lower RRR and
Tc, and increased $\ensuremath{\beta}$-
Ta content. Coplanar waveguide resonators with a gap width of $3\phantom{\rule{0.1em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$ fabricated from cryogenically grown
Ta on
Si(111) and
Al2O3(0001) show low-power
Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline $\ensuremath{\alpha}$-
Ta films may be promising for superconducting qubit applications even though they are not fully epitaxial.
ABSTRACT Niobium (Nb) films have emerged as a crucial material in the development of superconducting qubits, which are key components in quantum computing technology. This review provides a comprehensive examination of Nb films from a materials perspective, focusing on their intrinsic properties, fabrication methods/techniques, and their influence on qubit performance, particularly through surface and interface driven loss mechanisms. We discuss the key material properties that are essential for qubit operation. Various deposition techniques for Nb thin films, such as sputtering, evaporation, molecular beam epitaxy, and atomic layer deposition, are explored, alongside their impact on film quality, uniformity, and qubit performance. Additionally, the influence of surface roughness, thin‐film thickness, and substrate materials on quantum coherence is analyzed. Challenges such as defects and material degradation in Nb films are reviewed, along with strategies to mitigate these issues. Finally, we present the latest advancements and future directions in Nb film research, including potential improvements to enhance qubit coherence and scalability for large‐scale quantum computing systems. Ultimately, a deeper understanding of surface and interface phenomena is essential for pushing the limits of qubit performance and realizing next‐generation quantum technologies.
Tantalum has been demonstrated as a promising material for superconducting qubits. However, comparatively little attention has been given to its nitrides. Tantalum nitride exhibits a range of stoichiometries, resulting in a variety of material properties, including both superconducting and non-superconducting phases. Owing to this versatility, tantalum nitrides can serve multiple purposes in superconducting qubits: as seed layers for alpha-Ta growth, as a superconducting base material and as a non-superconducting barrier in the Josephson junction. In this study, we explore the performance of superconducting TaN and Ta thin film combinations on silicon substrates in terms of internal quality factor Qi. We find that standalone TaN films exhibit Qi values of about 1.5x10^5 at 100mK in the single-photon regime. Surprisingly, a resonator made from Ta grown on a few-nanometers-thick TaN seed layer yields largely the same performance. However, adding an additional, few-nanometers-thick Ta buffer layer between the Si substrate and this TaN seed layer enhances Qi significantly up to 5.9x10^5. Supporting transmission electron microscopy measurements reveal nitrogen accumulation and structural disorder at the TaN-Si interface, while this interfacial modification is suppressed when the Ta buffer layer is introduced. The observed improvement in resonator performance is consistent with a reduction of interface-related two-level system losses and strongly supports the hypothesis that controlling the substrate-metal interface is pivotal for the performance of superconducting qubit circuitry.
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.
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.
Superconducting qubits and microwave resonators based on tantalum thin films have recently demonstrated large increases in performance. This makes Ta an at- tractive material for superconducting quantum computing applications, but, so far, direct deposition has largely relied on high substrate temperatures exceeding 400 ◦C to achieve the cubic (BCC) phase of tantalum. Here, we show that changing the sputter gas from argon to krypton promotes synthesis of very high crystallinity BCC tantalum films on silicon at temperatures as low as 200 ◦C. This provides a wide process window compatible with back-end-of-the-line fabrication standards. The microwave performance of coplanar waveguide resonators fabricated from krypton-sputtered films shows a tight performance distribution at the state of the art. Higher temperature-grown films exhibit higher losses, in correlation with the degree of tantalum/silicon intermixing. Finally, with these films, we demonstrate transmon qubits with a relatively compact, 20 μm capacitor gap, achieving a time-averaged quality factor up to 16.9 million.