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

Abstract

In superconducting quantum devices, air bridges enable increased circuit complexity and density as well as 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 reduction of the quality factor exceeding 1.0 × 10 8 per bridge. The bridge process is compatible with Josephson junctions and allows for the simultaneous creation of low loss bandages between superconducting layers. Superconducting circuits have emerged as one of the primary candidates for the implementation of quantum computing 1,2 . One of the primary advantages of superconducting circuits is the flexibility in design and scalability resulting from their realization as microfabricated circuits on a chip. As the complexity and scale of superconducting circuits increases, microfabrication methods that enable increased density and quantum coherence are essential. Relevant approaches for scalability include multi-chip fabrication 3-6 and three dimensional wiring integration 7,8 .Air bridges are one of the most relevant components of high-complexity superconducting circuits. They enhance functionality by enabling increased circuit density and by enhancing coherence in distributed resonators and qubits by prevention of mode mixing 9,10 . Several methods have been developed to date to implement air bridge crossovers for superconducting circuits. These processes are carried out using techniques including using a photo resist scaffold followed by etching of the excess aluminum to separate the bridge 10,11 , employing resist stacks that use different resists to define both a scaffold as well as a resist window above the scaffold with an undercut for liftoff 12,13 , and forming a dielectric scaffold that is etched after Al deposition 14 . In this paper, we present a method to fabricate aluminum air bridges that uses a simple single-step fabrication process that relies on gradient exposure electron-beam lithography (EBL). This process produces bridges with lengths ranging from 20 µm to 100 µm, with a yield exceeding 99 % for bridges of lengths up to 36 µm. We characterize coplanar waveguide resonators with integrated air bridges and find that negligible microwave loss is introduced when these bridges are used to connect ground planes. The process is compatible with fabrication of Josephson junctions for superconducting qubits and it has the additional desirable feature a)