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arXiv:2508.01232 · APL Quantum

In situ Al2O3 passivation of epitaxial tantalum and aluminum films enables long-term stability in superconducting microwave resonators

Yi-Ting Cheng, Hsien-Wen Wan, Wei-Jie Yan, Lawrence Boyu Young, Y.H. Lin, Kuan-Hui Lai, Wan-Sin Chen, Chao-Kai Cheng, K. H. M. Chen, Tun‐Wen Pi, Yen‐Hsiang Lin, J. Kwo, Minghwei Hong

Published Feb 27, 2026• 1 citation
arXivHTMLPDF

Abstract

Long-term stability of superconducting microwave resonators is essential for scalable quantum technologies; however, surface and interface degradation continue to limit device stability. Here, we demonstrate exceptional stability in microstrip resonators fabricated from epitaxial tantalum and aluminum films, protected by in situ deposited Al2O3 under ultrahigh vacuum. These resonators initially exhibit internal quality factors (Qi) exceeding 106 and maintain high performance with minimal degradation even after fourteen months of air exposure. In contrast, devices relying on native surface oxides show substantial declines in Qi over time, indicating increased microwave losses. X-ray photoelectron spectroscopy reveals that the in situ Al2O3 effectively suppresses interfacial oxidation and preserves the chemical integrity of the underlying superconducting films, whereas native oxides permit progressive oxidation, leading to device degradation. These findings establish a robust, scalable passivation strategy that addresses a longstanding materials challenge in the development of superconducting quantum circuits.

DOI
10.1063/5.0301974

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Applied Physics Letters•Sep 7, 2026•0 citations
Long-term stability of superconducting microwave resonators is essential for scalable quantum technologies; however, surface and interface degradation continue to limit device stability. Here, we demonstrate exceptional stability in microstrip resonators fabricated from epitaxial tantalum and aluminum films, protected by in situ deposited Al2O3 under ultrahigh vacuum. These resonators initially exhibit internal quality factors (Qi) exceeding 106 and maintain high performance with minimal degradation even after fourteen months of air exposure. In contrast, devices relying on native surface oxides show substantial declines in Qi over time, indicating increased microwave losses. X-ray photoelectron spectroscopy reveals that the in situ Al2O3 effectively suppresses interfacial oxidation and preserves the chemical integrity of the underlying superconducting films, whereas native oxides permit progressive oxidation, leading to device degradation. These findings establish a robust, scalable passivation strategy that addresses a longstanding materials challenge in the development of superconducting quantum circuits.
Superconducting coplanar waveguide resonators are essential components in quantum processors, where their internal quality factor (Qi) constrains qubit coherence and readout fidelity. In niobium devices, microwave losses at millikelvin temperatures are strongly influenced by two-level systems (TLS) associated with the complex NbOx surface oxide. To mitigate these losses, we investigate a surface-engineering approach in which Nb films are capped in situ with a thin tantalum layer to suppress Nb2O5 formation and replace the native NbOx interface with a Ta-based oxide. We fabricate Nb/Ta bilayer and reference Nb resonators on high-resistivity silicon using identical DC sputtering and wet etching conditions, and characterize their performance at millikelvin temperatures. Fresh Ta-encapsulated devices exhibit internal quality factors up to 2.4 x 10^6 in the near-single-photon regime, with power dependence consistent with reduced TLS-related loss at the metal-air interface. A control Nb device fabricated under the same process shows comparatively lower Q_TLS, consistent with the beneficial effect of the Ta capping layer. Furthermore, ageing tests performed on Nb/Ta resonators after six months reveal a moderate reduction in Q_TLS relative to their initial values, yet the performance remains superior to newly fabricated Nb-only devices. These results suggest that thin Ta encapsulation enhances interface quality and contributes to improved temporal stability while remaining compatible with Nb-based fabrication workflows.
The recent realization of millisecond-scale coherence with tantalum-on-silicon transmon qubits showed that depositing the Al/AlOx/Al Josephson junction in a high purity, ultrahigh vacuum environment was critical for achieving lifetime-limited coherence, motivating careful examination of the aluminum surface two-level system (TLS) bath. Here, we measure the microwave absorption arising from surface TLSs in superconducting aluminum resonators, following methodology developed for tantalum resonators. We vary film and surface properties and correlate microwave measurements with materials characterization. We find that the lifetimes of superconducting aluminum resonators are primarily limited by surface losses associated with TLSs in the 2.7 nm-thick native AlOx. Treatment with 49% HF removes surface AlOx completely; however, rapid oxide regrowth limits improvements in surface loss and long term device stability. Using these measurements we estimate that TLSs in aluminum interfaces contribute around 27% of the relaxation rate of state-of-the-art tantalum-on-silicon qubits that incorporate aluminum-based Josephson junctions.
Tantalum is a promising platform for superconducting quantum circuits, yet coherence times remain limited by dielectric losses from interfacial two-level systems (TLS), exacerbated by native oxide regrowth. Here, we implement molecular surface passivation using self-assembled organic monolayers on freshly etched tantalum and silicon in coplanar waveguide resonators. Surface characterization by contact angle, XPS, FTIR and TEM confirm the formation of ordered, nanometer-thick films that suppress oxide formation. Microwave measurements in the ~5-9 GHz range reveal internal quality factors up to 1.8x10^6 in the single-photon regime at 100 mK, representing a ~140% improvement over untreated devices with native oxide. Power and temperature dependent measurements attribute this enhancement to reduced TLS-induced losses. These results demonstrate that molecular passivation effectively engineers low-loss interfaces and provides a scalable route toward high-coherence superconducting quantum devices.
Achieving high material perfection in aluminum (Al) films and their associated Al/AlOx heterostructures is essential for enhancing the coherence time in superconducting quantum circuits. We grew Al films with thicknesses ranging from 3 to 30 nanometers (nm) epitaxially on sapphire substrates using molecular beam epitaxy (MBE). An integral aspect of our work involved electron-beam (e-beam) evaporation to directly deposit aluminum oxide (Al2O3) films on the freshly grown ultrathin epitaxial Al films in an ultra-high-vacuum (UHV) environment. This in situ oxide deposition is critical for preventing the oxidation of parts of the Al films, avoiding the formation of undesired native oxides, and thereby preserving the nm-thick Al films in their pristine conditions. The thicknesses of our Al films in the study were accurately determined; for example, coherence lengths of 3.0 and 20.2 nm were measured in the nominal 3.0 and 20 nm thick Al films, respectively. These Al films were epitaxially grown on sapphire substrates, showing an orientational relationship, denoted as Al(111)⟨21¯1¯⟩∥sapphire(0001)[21¯1¯0]. The Al/sapphire interface was atomically ordered without any interfacial layers, as confirmed by in situ reflection high-energy electron diffraction (RHEED) and cross-sectional scanning transmission electron microscopy (STEM). All sample surfaces exhibited smoothness with a roughness in the range of 0.1–0.2 nm. The Al films are superconducting with critical temperatures ranging from 1.23 to around 2 K, depending on the film thickness.
The native oxide layer formed on aluminum (Al) surfaces in superconducting quantum circuits is a significant source of two-level system defects, which couple with electric fields and degrade quantum coherence. Recent research has explored etching, encapsulation, and other surface treatments as potential strategies to mitigate the formation of oxides at air interfaces in these circuits. This study demonstrates a novel approach to passivate the Al–air interface using a molecular self-assembled monolayer (SAM). Freshly prepared Al thin films were passivated with SAMs by immersing the Al-coated silicon substrates in SAM solutions. X-ray photoelectron spectroscopy (XPS) confirms the successful binding of the SAM and the absence of further aluminum oxide growth. Moreover, the passivation remains stable after aging for 15 days in ambient conditions, as evidenced by XPS and contact angle measurements. Scanning electron microscopy analyses further support the binding of the SAM to the Al surface and mitigation of oxide growth. These findings suggest that SAM-based passivation offers a promising method for reducing microwave loss and improving the performance of Al-based superconducting quantum circuits.
Native oxide formation (NbOx) at the niobium (Nb) metal–air interface is a source of microwave loss and performance variability in superconducting resonators. We introduce vapor-phase hexamethyldisilazane (HMDS) as a dry surface passivation treatment compatible with standard nanofabrication workflows. In five independent fabrication runs, HMDS-passivated resonators consistently demonstrate improved internal quality factor (Qi) and narrower performance distribution relative to paired unpassivated controls. The Qi improvement occurs across the full microwave power range, suggesting NbOx suppression as a contributing mechanism. Statistical depth profile analysis of Nb oxidation state by electron energy loss spectroscopy corroborates this mechanism, revealing smaller chemical shifts and shallower oxidation depth in passivated specimens relative to the control. Furthermore, re-measurement after 60 days of ambient storage shows that HMDS-passivated resonators retain their Qi, indicating their ambient stability. These results establish vapor-phase HMDS as a surface passivation strategy with strong potential for superconducting quantum devices.