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ACS Applied Electronic Materials

Improved Interface of Niobium Superconducting Resonator with Ruthenium as a Capping Layer

Senthil Kumar Karuppannan, Ding Huang, Naga Manikanta Kommanaboina, Anil Kamma, Guangxu Yan, D. V. Maheswar Repaka, Yiyu Zhang, Kuan Eng Johnson Goh, Seng Kai Wong, Nelson Lim Chee Beng, Sherry Lee Koon Yap, Manas Mukherjee

Published Oct 4, 2024• 17 citations
Publisher

Abstract

The current performance of superconducting circuit-based quantum processors is limited by the poor understanding of interface physics, including native surface oxide formation on the superconducting metal, which causes two-level system (TLS) loss. Niobium (Nb), a superconducting metal with a high energy gap, is an ideal choice for superconducting processors, but unfortunately, it is marred by TLS. Several methods have been proposed to minimize surface oxide on the Nb film, and considerable improvement in TLS loss has been demonstrated. These methods include surface passivation through metal capping, self-assembly of organic molecules, and post-cleaning processes. Among these, metal capping is a suitable choice despite forming a 3–5 nm thick oxide, as self-assembly and post-treatment do not protect the Nb film surface during further fabrication. Here, we have proposed ruthenium (Ru) as a capping layer, forming a self-limiting oxidation with a 0.6 nm oxide thickness and predominantly producing fewer oxide compositions while being chemically resistant for further wafer fabrication processes, thus fulfilling all the criteria of an ideal capping layer. Our investigation suggests that Nb/Ru resonators have great potential as versatile and promising tools for advancing superconducting quantum technologies and integrating quantum interconnects into qubits with minimized TLS loss.

DOI
10.1021/acsaelm.4c01268

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The current performance of superconducting circuit-based quantum processors is limited by the poor understanding of interface physics, including native surface oxide formation on the superconducting metal, which causes two-level system (TLS) loss. Niobium (Nb), a superconducting metal with a high energy gap, is an ideal choice for superconducting processors, but unfortunately, it is marred by TLS. Several methods have been proposed to minimize surface oxide on the Nb film, and considerable improvement in TLS loss has been demonstrated. These methods include surface passivation through metal capping, self-assembly of organic molecules, and post-cleaning processes. Among these, metal capping is a suitable choice despite forming a 3–5 nm thick oxide, as self-assembly and post-treatment do not protect the Nb film surface during further fabrication. Here, we have proposed ruthenium (Ru) as a capping layer, forming a self-limiting oxidation with a 0.6 nm oxide thickness and predominantly producing fewer oxide compositions while being chemically resistant for further wafer fabrication processes, thus fulfilling all the criteria of an ideal capping layer. Our investigation suggests that Nb/Ru resonators have great potential as versatile and promising tools for advancing superconducting quantum technologies and integrating quantum interconnects into qubits with minimized TLS loss.
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.
Abstract The performance of Nb superconducting quantum devices is predominantly limited by dielectric loss at the metal–air interface, where Nb2O5 is considered the main loss source. Here, we suppress the formation of native oxides by in-situ deposition of a TiN capping layer on the Nb film. With TiN capping layers, no Nb2O5 forms on the surface of the Nb film. The quality factor Q i of the Nb resonator increases from 5.6 × 105 to 7.9 × 105 at low input power and from 6.8 × 106 to 1.1 × 107 at high input power. Furthermore, the TiN capping layer also shows good aging resistance in Nb resonator devices, with no significant performance fluctuations after one month of aging. These findings highlight the effectiveness of TiN capping layers in enhancing the performance and longevity of Nb superconducting quantum devices.
Scalable quantum computing currently requires a large array of qubit integration, but present two-dimensional interconnects face challenges such as wiring congestion, electromagnetic interference, and limited cryogenic space. To overcome this challenge, implementing three-dimensional (3D) vertical architectures becomes crucial. Niobium (Nb), due to its excellent superconducting characteristics and strong fabrication process compatibility, stands out as a prime material choice. The main challenge in Nb-Nb bonding is the presence of an oxide layer at the interface, even after post-bonding annealing across various bonding methods. The native Nb oxide forms rapidly in air, creating a resistive barrier to supercurrent flow and introducing two-level system losses that degrade qubit coherence while increasing the overall thermal budget. These issues show the need for effective surface engineering to suppress oxidation during bonding. This study introduces an ultrathin gold (Au) capping layer as a passivation strategy to prevent oxygen incorporation at the Nb surface. This approach enables low-temperature Nb-Nb thermocompression bonding at 350 °C under a reduced bonding pressure of 0.495 MPa. Detailed microstructural and interfacial analyses confirm that Au passivation effectively suppresses oxide formation and hence enhances bonding uniformity and strength with keeping the superconductivity, establishing a robust route toward low-temperature, low-pressure Nb-Nb bonding for scalable 3D superconducting quantum computing architectures.
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.
Abstract Superconducting circuit-based quantum processors are leading platforms for quantum computing. In these circuits, microwave photons are stored as qubits in ultra-low-loss planar resonators and non-linear inductors formed by Josephson junctions. Resonators are typically made from high-energy-gap superconductors like Nb or Ta, while junctions are made of Al. Resonators occupy much of the circuit, making defect-free fabrication and understanding microwave energy dissipation crucial. Losses arise from noise, two-level systems (TLS), quasi-particles, and impurities. TLS losses dominate at operating temperatures below the critical temperature of the metal, whereas photon loss due to quasi-particles, often stemming from grain boundaries and pinholes in the metal film, becomes more pronounced at higher photon numbers or temperatures approaching the metal's critical temperature. To mitigate these, substrate cleaning, surface control, and non-superconducting film capping prevent oxide formation and reduce impurities. High-frequency drives, coupled with impurities at grain boundaries, lead to nonuniform quality factors among resonators. By controlling oxygen plasma exposure to minimize surface roughness and pinhole depth, we observed an area-dependent quality factor, which we attribute to changes in surface resistivity. This approach minimized variations in quality factors across resonators, improving uniformity in Nb-based devices and more consistent qubit readout performance.
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.