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Applied Physics Letters

Physical patterning of high-Q superconducting niobium resonators via ion beam etching

Miguel Manzo-Perez, Moeid Jamalzadeh, Man Nguyen, Christopher Nadeau, Alexander Madden, Iliya Shiravand, Kim Kisslinger, Xiao Tong, Kasra Sardashti, Michael Senatore, Matthew LaHaye, Davood Shahrjerdi

Published Sep 1, 2025• 4 citations
Publisher

Abstract

The development of superconducting quantum circuits increasingly involves the exploration of chemically distinct materials and complex multilayered structures. Accelerating this trend may benefit from low-damage, materials-agnostic patterning techniques that are compatible with a broad range of materials. Here, we investigate the utility of low-energy ion beam etching (IBE), a physical patterning technique, as an alternative to reactive ion etching for fabricating low-loss superconducting resonators. We use niobium (Nb) resonators as a test platform, leveraging their well-characterized performance metrics for benchmarking. To address IBE-induced surface redeposition, we introduce an in situ aluminum capping layer combined with targeted post-fabrication chemical treatment. This strategy yields resonators with internal quality factors as high as 6 × 105 in the single-photon regime at 50 mK. These results establish low-energy IBE as a promising patterning technique for superconducting devices, with the potential to accelerate development across chemically diverse and multilayered material platforms.

DOI
10.1063/5.0278956

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•Sep 26, 2024•0 citations
The development of superconducting quantum circuits increasingly involves the exploration of chemically distinct materials and complex multilayered structures. Accelerating this trend may benefit from low-damage, materials-agnostic patterning techniques that are compatible with a broad range of materials. Here, we investigate the utility of low-energy ion beam etching (IBE), a physical patterning technique, as an alternative to reactive ion etching for fabricating low-loss superconducting resonators. We use niobium (Nb) resonators as a test platform, leveraging their well-characterized performance metrics for benchmarking. To address IBE-induced surface redeposition, we introduce an in situ aluminum capping layer combined with targeted post-fabrication chemical treatment. This strategy yields resonators with internal quality factors as high as 6 × 105 in the single-photon regime at 50 mK. These results establish low-energy IBE as a promising patterning technique for superconducting devices, with the potential to accelerate development across chemically diverse and multilayered material platforms.
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.
Aluminum (Al) remains the central material for superconducting qubits, and considerable effort has been devoted to optimizing its deposition and patterning for quantum devices. However, post-processing strategies focused on oxide removal of niobium (Nb) and tantalum (Ta) -based resonators using buffered oxide etch (BOE), which can not be used for Al. This challenge becomes particularly relevant for industry-scale fabrication with multi-chip bonding, where delays between sample preparation and cooldown require surface treatments that preserve low dielectric loss during extended exposure to ambient conditions. In this work, we investigate surface modification approaches for Al resonators subjected to a 24-hour delay prior to cryogenic measurement. Passivation using self-limiting oxygen and fluorine chemistries was evaluated utilizing different plasma processes. Remote oxygen plasma treatment reduced dielectric losses, in contrast to direct oxygen plasma. A fluorine-based plasma process was developed that passivated the Al surface for subsequent BOE treatment. However, the fluorine content in the surface resulted in higher loss, identifying fluorine as an unsuitable passivation material for Al resonators. Above all, selective oxide removal using HF (hydrogen fluoride) vapor and phosphoric acid yielded median dielectric losses as low as δ~LP=5.7×10−7\tildeδ_\mathrm{LP} = 5.7 \times 10^{-7}δ~LP​=5.7×10−7 (QLP≈1.7 MQ_\mathrm{LP} \approx 1.7\,\mathrm{M}QLP​≈1.7M) with δ~TLS=3.6×10−7\tildeδ_\mathrm{TLS} = 3.6 \times 10^{-7}δ~TLS​=3.6×10−7 (QTLS≈2.8 MQ_\mathrm{TLS} \approx 2.8\,\mathrm{M}QTLS​≈2.8M) in the single photon regime. Selective oxide removal provides a promising pathway for robust Al-based qubit fabrication, as it preserves low dielectric losses for a 24-hour delay before cooldown.
Material losses in superconducting circuits fundamentally limit qubit coherence times and resonator quality factors. Most research efforts focus on mitigating losses at circuit interfaces, including metal--substrate, substrate--air, and metal--air interfaces. However, the correlation between TLS and non-TLS losses with the intrinsic properties of the superconducting metal and the dielectric edge smoothness is not well studied. In this work, we link the aluminum film grain size to non-TLS losses and the dielectric trimming profile and roughness to TLS loss; both loss mechanisms are subsequently mitigated. To reduce metal-related losses, we engineer the aluminum microstructure by heating during deposition, increasing grain size and reducing grain boundary density. Beyond mitigating metal losses, we introduce a two-step etching technique, Tropic etching, to suppress dielectric TLS loss by producing an ultra-smooth silicon surface with minimal defects and redeposition. These results lay out the fabrication pathway for aluminum resonators with lower loss, demonstrating two-orders-of-magnitude improvement in quality factor from 6×1046\times10^{4}6×104 to 2.3×1062.3\times10^{6}2.3×106. Since aluminum is the basis for most high-coherence Josephson junctions and dielectric edges are inherent to all common device geometries, the improvements in aluminum microstructure and edge profiling, presented here, can enhance the performance of superconducting quantum devices.
Abstract : Over the last decade, quantum information experiments with trapped ions have demonstrated essential steps towards quantum computing and quantum simulation. Large fields are required to achieve strong coupling to the ions via dipolar interactions, and so we fabricated transmission line microresonators - capable of producing large fields in a standing wave at resonance - for eventual integration into 2D ion trap structures. The resonators were superconducting to minimize loss and maximize quality factor. We fabricated the resonators as two dimensional coplanar waveguides in niobium on R-plane sapphire using optical lithography. Resist was patterned on the niobium using optical lithography, developed then reactive-ion etched to transfer the pattern into the niobium. The resonators were cooled and tested in a cryogenic probe station and characterized with a network analyzer. Additionally, the resonator geometry was reproduced in commercial microwave simulation software. Results from our fabricated resonators showed first-resonance quality factors of 1.2 x 104 at 3.23 GHz at device temperatures of 3-4 K.
For quantum computing with superconducting qubits, fabricating transmon qubits with increased coherence time is crucial, especially in view of multi-qubit quantum processors. The fabrication process, geometries and materials used to fabricate a transmon qubit are similar to the fabrication of CPW resonators. However, due to shorter fabrication time and straightforward measurement scheme of the latter, finding a way to increase the internal quality factor Qi in a CPW resonator is a quick way to gain insight into methods to increase the coherence time of a transmon qubit. The main goal of this research is to characterize the Qi at cryogenic temperature and single photon level of CPW resonators that are fabricated with novel surface treatments and etching techniques. The highest Qi is observed for resonators etched using Deep Reactive Ion Etching and HDMS surface treatment, which reached ∼106 at single photon level. This is comparable to the highest Qi reported in literature so far.
Niobium and aluminum have been widely used to realize superconducting resonators and qubits. However, the native oxide at the metal air interface limits the devices results. This layer is highly disordered, and it induces losses limiting the performances of the resonators or qubits. Recently, tantalum led to great improvement in qubit relaxation time. This improvement has been attributed to a thinner and less disordered oxide layer compared to Nb and Al. This study is aimed to limit the formation of the oxide layer, by the introduction of novel superconducting materials such as rhenium or by encapsulating the Nb surface with other superconducting materials such as Ta, Al, Re or other non-superconducting materials such as Au. To better understand the results, we are realizing these studies on coplanar waveguide resonators where the relation between surface losses and quality factor is clear. We are also analyzing different fabrication techniques to completely characterize our devices, linking material properties to the performances. In this work we are also varying the participation ratio of the devices in order to extract with higher fidelity and precision the losses introduced by the involved interfaces.