Vapor-phase HMDS passivation improves performance and ambient stability of niobium superconducting resonators
Moeid Jamalzadeh, Miguel Manzo-Perez, Man Nguyen, Christopher Nadeau, Sooyeon Hwang, Jiwon Park, Iliya Shiravand, Kim Kisslinger, Xiao Tong, Matthew LaHaye, Zhujun Huang, Davood Shahrjerdi
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.
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.
Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over a timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms a niobium nitride layer at the metal–air interface, which prevents such oxidation. X-ray photoelectron spectroscopy confirms the doping of nitrogen more than 5 nm into the surface and a suppressed oxygen presence. This passivation remains stable after aging for 15 days in an ambient environment. Cryogenic microwave characterization shows an average filling-factor-adjusted two-level-system loss tangent FδTLS of (2.9±0.5)·10−7 for resonators with a 3 μm center strip and (1.0±0.3)·10−7 for a 20 μm center strip, exceeding the performance of unpassivated samples by a factor of four.
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.
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.
Abstract One main limiting factor towards achieving high coherence times in superconducting circuits is two-level system (TLS) losses. Mitigating such losses requires controlling the formation of native oxides at the metal-air interface. Here, we report the growth of alkyl-phosphonate self-assembled monolayers (SAMs) on Nb thin films following oxide removal. The impact of passivation was evaluated via the performance of coplanar waveguide resonators at 10 mK, in terms of quality factor and resonant frequency, over six days of air exposure. Un-passivated resonators exhibited an ~80% increase in loss at single-photon power levels, whereas SAM-passivated resonators maintained excellent temporal stability, attributed to suppressed oxide regrowth. By employing a two-component TLS model, we discern distinct prominent loss channels for each resonator type and quantified the characteristic TLS loss of the SAMs to be ~5×10 -7 . We anticipate our passivation methodology to offer a promising route toward industrial-scale qubit fabrication, particularly where long-term device stability is critical.
Resonators and qubits with a niobium (Nb) base metal layer achieve some of the highest coherence times in superconducting quantum devices. The performance of such devices is often limited by loss associated with two-level systems, which are found primarily at material surfaces and interfaces. The metal-air (MA) interface is a major contributor to device loss. In this work, we develop a fast-sealing microwave cavity that enables devices to be placed under vacuum within five minutes of oxide removal, thereby significantly reducing the MA interface loss compared to common device processing and packaging approaches. Using coplanar stripline resonators, we demonstrate that devices sealed in such a cavity exhibit internal quality factors exceeding one million at single-photon power. After re-exposure to air, the devices show downward resonance frequency shifts and quality factor degradations, quantitatively consistent with a model of Nb oxide regrowth. The fast-sealing microwave cavity provides a practical and consistent method to mitigate MA interface loss and sustain high coherence in Nb devices, and establishes a controlled platform for studying metal oxide regrowth kinetics and dielectric properties, the understanding of which is critical to achieving high coherence in superconducting quantum devices.
Superconducting coplanar waveguide (CPW) microwave resonators in quantum circuits are the best components for reading and changing the state of artificial atoms because of their excellent coupling to quantum systems. This coupling forms the basis of the developing circuit quantum electrodynamic architecture. In quantum processors, oscillators are used to store and transmit quantum information using microwave-frequency wave packets. However, the presence of amorphous thin-film defects is deleterious and can result in an irrevocable loss of coherent information with uncontrolled degrees of freedom. Although there has been extensive research into techniques to reduce the coherent loss of such devices, the precise structure of amorphous dielectric layers on surfaces and interfaces and their associated loss mechanism are being actively studied. In particular, planar superconducting resonators are very sensitive to defects on their surfaces, such as two-level systems in oxidized metals and nonequilibrium quasiparticles, making these devices suitable probes for the different loss mechanisms. In this work, we present the design, fabrication, and characterization of Nb CPW resonators with different surface treatments with self-assembled monolayers (SAMs), which mitigate the growth of oxides in superconducting circuits. We demonstrate SAM-passivated resonators having internal quality factors of greater than 10 6 at a single-photon excitation power (measured at 100 mK), which were probed using scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy to demonstrate the efficiency of our surface treatment. Finally, we compared the improvements in the experimental quality factors to those obtained by numerical simulation.