Krypton-sputtered tantalum films for scalable high-performance quantum devices

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.

💡 Research Summary

The paper addresses a critical bottleneck in scaling superconducting quantum processors: the incompatibility of high‑temperature tantalum (Ta) deposition with back‑end‑of‑line (BEOL) semiconductor fabrication. Conventional magnetron sputtering of Ta uses argon (Ar) as the process gas and requires substrate temperatures of 450 °C – 650 °C to stabilize the body‑centered‑cubic (BCC) α‑Ta phase, which is essential for low‑loss microwave performance. Such temperatures exceed the BEOL thermal budget (≤ 400 °C) and hinder integration into standard foundry lines.

The authors demonstrate that replacing Ar with krypton (Kr) as the sputter gas dramatically lowers the temperature needed to obtain phase‑pure α‑Ta on Si(100) wafers. With Kr, α‑Ta forms reliably at substrate temperatures as low as 200 °C, and a broad process window (200 °C – 350 °C) is compatible with BEOL constraints. In contrast, Ar‑based deposition only yields α‑Ta above ~350 °C and often produces the less‑conductive β‑Ta phase at lower temperatures.

Electrical transport measurements reveal that Kr‑deposited films have a substantially higher residual resistivity ratio (RRR) and a longer electron mean free path (ℓ) at cryogenic temperatures. The mean free path exceeds the Ginzburg‑Landau coherence length (ξ_GL), indicating that the films approach the clean‑limit superconductivity regime. Secondary‑ion mass spectrometry (SIMS) confirms lower Ar impurity incorporation in Kr films, explaining the superior conductivity.

Structural characterization (AFM, EBSD, STEM) shows that grain morphology evolves from “cell‑like” to “flower‑like” structures between 250 °C and 350 °C, with higher temperatures reducing high‑angle grain boundary density and surface roughness. Crucially, cross‑sectional STEM and EELS analyses reveal an amorphous interlayer at the Ta/Si interface whose thickness grows with deposition temperature. Low‑temperature Kr films exhibit a thin (≈1–2 nm) intermixing layer, whereas high‑temperature Ar films develop a thicker (≈5 nm) layer, likely a silicide. Since the electric field participation in microwave resonators is strongly concentrated at this interface, a thinner interlayer directly translates into lower dielectric loss.

Microwave performance is evaluated using 3 µm‑gap coplanar waveguide (CPW) resonators fabricated from the various films. At single‑photon excitation, Kr‑deposited films grown at 250 °C – 350 °C achieve internal quality factors Q_i ≈ 4 × 10⁶, and at high power (10⁵ photons) Q_i reaches ≈ 2.5 × 10⁷. These values are comparable to or surpass the best reported Ta resonators and are markedly better than resonators from high‑temperature Ar films, which suffer from increased high‑power loss and, in some cases, BOE‑induced degradation. The authors also compare with Nb‑seeded Ta films; while the seeded approach stabilizes α‑Ta at room temperature, it yields higher low‑power loss and is more sensitive to surface oxidation and aging.

Finally, the authors fabricate transmon qubits using Kr‑deposited Ta films grown at 350 °C. The qubits feature 20 µm‑gap capacitor pads etched to a depth of 500 nm to reduce surface participation. Measured energy relaxation times correspond to median quality factors Q ≈ 1.4 × 10⁷, matching the state‑of‑the‑art for Ta‑based qubits. The devices show robust performance after standard BOE cleaning and exhibit limited aging effects compared to Nb‑seeded counterparts.

In summary, krypton sputtering provides a scalable, BEOL‑compatible route to high‑quality α‑Ta films with superior electrical conductivity, reduced interfacial intermixing, and record‑low microwave loss. This process bridges the gap between laboratory‑scale quantum hardware and industrial semiconductor manufacturing, paving the way for large‑scale production of superconducting quantum processors.