Fabrication of vacuum-gap microstrip structuresfor impedance matching traveling-wave parametric amplifiers

In recent years, the development of microwave traveling-wave parametric amplifiers (TWPAs) has witnessed

significant progress with near quantum-limited noise performance and broad band amplification being achieved

by multiple research groups. Nevertheless, some challenges remain, offering room for further improvement.

One of these challenges is to provide enough capacitance over the length of a TWPA to compensate for the

nonlinear inductance used for amplification, without introducing extensive noise from dielectric losses. In this

work we present a novel fabrication process for vacuum-gap microstrips where the ground plane is supported

above the central conductor in a free-standing tunnel-like structure. The process is compatible with a range

of different materials and, in addition to high-capacitance transmission lines, also enables the creation of air-

bridges and compact capacitors. The performance of the fabrication is examined using distributed aluminum

and granular aluminum resonators in a cryogenic dilution refrigerator setup, already anticipating the future

application in a kinetic inductance TWPA.