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Chinese Physics B

Low-loss, high-coherence airbridge interconnects fabricated by single-step lithography

Ji-Bang Fu, Bo Ren, Jian-Dong Ouyang, Cong Li, Ke-Cheng-Qi Zhu, Yong-gang Che, Xiang Fu, Shi-Chuan 诗川 Xue 薛, Zhao-Hua Yang, Mingtang Deng, Junjie Wu

Published Mar 12, 2026• 0 citations
Publisher

Abstract

Abstract Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process that enables the fabrication of nanoscale airbridges with just one step of electron-beam lithography. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve burr-free, suspended metallic bridges with sub-micron features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T 1 , while enabling a 2.5-fold enhancement of the dephasing time T 2 * . This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.

DOI
10.1088/1674-1056/ae50e2

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Abstract Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process that enables the fabrication of nanoscale airbridges with just one step of electron-beam lithography. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve burr-free, suspended metallic bridges with sub-micron features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T 1 , while enabling a 2.5-fold enhancement of the dephasing time T 2 * . This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.
Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization and introduce process-related defects. We report a simplified process for fabricating nanoscale airbridges using only a single electron-beam lithography step. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve smooth, suspended metallic bridges with sub-200-nm features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T1T_1T1​, while enabling a 2.5-fold enhancement of the dephasing time T2∗T_2^*T2∗​. This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.
Superconducting circuit devices require electrical interconnects between different circuit elements on the chip, for which conventional device architectures use a combination of two structural elements: \textit{airbridges} to connect non-adjacent elements in the base layer, and \textit{bandages} to connect the electrodes forming the Josephson junctions to the base layer. Bandages introduce unwanted parasitic material interfaces and increase the manufacturing complexity. Here, we overcome the limitations imposed by \emph{bandages} by establishing \textit{all} electrical interconnects with airbridges of varying size fabricated in a single step. The airbridges show a high yield and mechanical stability over a wide range of sizes from 0.5 μm0.5\,μ\mathrm{m}0.5μm to 4 μm4\,μ\mathrm{m}4μm in width and from 5 μm5\,μ\mathrm{m}5μm to 40 μm40\,μ\mathrm{m}40μm in length, and show low loss when integrated in coplanar waveguide resonators and transmon qubits. Measured relaxation times up to more than 250 μs250\,μ\mathrm{s}250μs in standard transmon geometries show that the process achieves high coherence while substantially easing and accelerating device fabrication.
Grounding is crucial in coplanar superconducting circuit architectures, since chip complexity and scalability increase, asymmetries and discontinuities in the ground plane arise. These irregularities excite parasitic slotline modes, which can be coupled to qubits or other resonant modes and degrade circuit performance. Wirebonds are one of the most frequently used crossovers for grounding—also in Quanten Nano Zentrum Tirol (QNZT) — however, due to their size, they introduce large inductive losses and are not practical for large-scale chips. The small structure of airbridges causes lower inductive losses and is compatible with scalable superconducting devices because the density of airbridges can be increased simply by modifying the fabrication design. In this project, airbridges are fabricated for the first time in the QNZT using a single step grayscale electron beam lithography technique, inspired by the work of Janzen et al. [1]. To optimize the grayscale electron beam lithography process, special techniques are employed, such as delayed development [2] and 3D proximity effect correction (PEC) [3]. In addition, pure Methyl Isobutyl Ketone (MIBK) is used as the developer. The lower contrast characteristic of the positive resist is achieved by pure MIBK and delayed development, so that this technique increases the precision of the resist height after development. A high temperature reflow process is avoided, which can alter the characteristics of Josephson junctions. In addition, PEC is employed to increase the precision of the lithographic process, which is essential in low acceleration voltage Electron Beam Lithography(EBL). Eventually, we achieved a smooth arcshaped airbridge mask with over 99% yield for airbridges ranging from 45 µm to 65 µm and over 80% yield for 85 µm airbridges. These structures also demonstrated durability against dicing, ultrasonication and N2 blow-drying. Finally, two chips with CPW resonators grounded by airbridges are characterized at cryogenic temperatures.
Quantum hardware based on circuit quantum electrodynamics makes extensive use of airbridges to suppress unwanted modes of wave propagation in coplanar-waveguide transmission lines. Airbridges also provide an interconnect enabling transmission lines to cross. Traditional airbridge fabrication produces a curved profile by reflowing resist at elevated temperature prior to metallization. The elevated temperature can affect the coupling energy and even yield of pre-fabricated Josephson elements of superconducting qubits, tunable couplers, and resonators. We employ grayscale lithography to enable reflow and thereby reduce the peak temperature of our airbridge fabrication process from 200 to 150 °C and link this change to a substantial increase in the physical yield of transmon qubits with Josephson elements realized using Al-contacted InAs nanowires.
Airbridges are extensively employed in superconducting quantum circuits to suppress parasitic slotline modes in coplanar waveguide and minimize crosstalk between control lines. Here, we introduce a fabrication technique for airbridges, leveraging niobium as the bridge layer and aluminum as the sacrificial layer to preclude the introduction of lossy dielectrics or residues upon release. Additionally, we utilize a triangular evaporation method to significantly bolster the structural integrity of the airbridges. Our experimental evaluation, focused on resonators equipped with these airbridges, reveals that the resultant additional loss per bridge is minimal, quantified at (5.0±2.8)×10−9 in the single-photon regime and (6.3±0.9)×10−9 at high drive powers. This advancement underscores the potential of niobium airbridges in facilitating the development of large-scale and high-performance superconducting quantum circuits.
Josephson junctions are the principal circuit element in numerous superconducting quantum information devices and can be readily integrated into large-scale electronics. However, device integration at the wafer scale necessarily depends on having a reliable, high-fidelity, and high-yield fabrication method for creating Josephson junctions. When creating Al/AlO x based superconducting qubits, the standard Josephson junction fabrication method relies on a sub-micron suspended resist bridge, known as a Dolan bridge, which tends to be particularly fragile and can often times fracture during the resist development process, ultimately resulting in device failure. In this work, we demonstrate a unique Josephson junction lithography mask design that incorporates stress-relief channels. Our simulation results show that the addition of stress-relief channels reduces the lateral stress in the Dolan bridge by more than 70% for all the bridge geometries investigated. In practice, our novel mask design significantly increased the survivability of the bridge during device processing, resulting in 100% yield for over 100 Josephson junctions fabricated.