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 are striving for low-loss airbridges that are robust against sonication and etches during further processing. At present, more than 90% of our airbridges survive the sonication.
[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).