Characterization of Al-based Airbridges for Superconducting Microwave Devices*

The realization of a co-planar waveguide (CPW) for quantum circuits requires only a single wiring layer. However, the usability of the structure is limited by parasitic slotline modes which can modify couplings and act as a loss channel in complex superconducting integrated circuits. Free-standing metallic crossovers known as airbridges [1,2] commonly provide electrical connectivity between ground planes to suppress the parasitic modes. Here, we demonstrate our latest development of Al airbridges in conjunction with Nb CPWs. Our fabrication method relies on three separate optical lithography steps with Al deposition combined with the CPW structures using re-flowed photoresist as a scaffold. We have observed that our airbridges are superconducting and mechanically robust, also in response to sonication. Characterizing the added microwave loss is work in progress.
[1] Z. Chen et al., Applied Physics Letters 104, 052602 (2014).
[2] Y. Kwon et al., IEEE microwave and wireless components letters 11, 59 (2001).