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arXiv:2206.09489 · Applied Physics Letters

Aluminum air bridges for superconducting quantum devices realized using a single-step electron-beam lithography process

N. Janzen, Michal Kononenko, S. Ren, Adrian Lupaşcu

Published Aug 29, 2022• 14 citations
arXivHTMLPDF

Abstract

In superconducting quantum devices, air bridges enable increased circuit complexity and density, and mitigate the risk of microwave loss arising from mode mixing. We implement aluminum air bridges using a simple process based on single-step electron-beam gradient exposure. The resulting bridges have sizes ranging from 20 µm to 100 µm, with a yield exceeding 99% for lengths up to 36 µm. When used to connect ground planes in coplanar waveguide resonators, the induced loss contributed to the system is negligible, corresponding to a loss per bridge less than 1.0×10−8. The bridge process is compatible with Josephson junctions and allows for the simultaneous creation of low loss bandages between superconducting layers.

DOI
10.1063/5.0103165

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In superconducting quantum devices, air bridges enable increased circuit complexity and density, and mitigate the risk of microwave loss arising from mode mixing. We implement aluminum air bridges using a simple process based on single-step electron-beam gradient exposure. The resulting bridges have sizes ranging from 20 µm to 100 µm, with a yield exceeding 99% for lengths up to 36 µm. When used to connect ground planes in coplanar waveguide resonators, the induced loss contributed to the system is negligible, corresponding to a loss per bridge less than 1.0×10−8. The bridge process is compatible with Josephson junctions and allows for the simultaneous creation of low loss bandages between superconducting layers.
Proper grounding between different ground planes in coplanar superconducting qubit circuits is important to avoid spurious resonances which increase decoherence. Here, the authors present a possible solution to suppress such undesired modes using superconducting aluminum air-bridges which have been fabricated on top of aluminum coplanar waveguide transmission lines. 3D electromagnetic simulations were done to guide the design of the air-bridges such that the input reflection (S11) of the bridges was kept at a minimum level. A fabrication method based on optical lithography techniques was developed and it resulted in air-bridges with a height of approximately 10 μm and lengths of up to 500 μm. The method can be generalized to arbitrary length air-bridge with heights even exceeding 15 μm.
Extraneous high frequency chip modes parasitic to superconducting quantum circuits can result in decoherence when these modes are excited. To suppress these modes, superconducting air bridges (AB) are commonly used to electrically connect ground planes together when interrupted by transmission lines. Here, we demonstrate the use of two-photon photolithography to build a supporting 3D resist structure in conjunction with a liftoff process to create AB. The resulting aluminum AB have a superconducting transition temperature of Tc=1.08 K and exhibit good mechanical strength up to lengths of 100 μm. A measurable amount of microwave loss is observed when 35 AB were placed over a high-Q Ta quarter-wave coplanar waveguide resonator.
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.
Scaling up superconducting quantum processors necessitates the use of a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting coplanar waveguide resonators incorporating several niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding Q int = 8.2 × 10 6 . Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium, compared with conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration in transmon qubits by achieving median qubit lifetimes of T 1 = 51.6 μ s .
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.
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.