A Cryogenic Waveguide Mount for Microstrip Circuit and Material Characterization

A waveguide split-block fixture used in the characterization of thin-film superconducting planar circuitry at millimeter wavelengths is described in detail. The test fixture is realized from a pair of mode converters, which transition from rectangular-waveguide to on-chip microstrip-line signal propagation via a stepped ridge-guide impedance transformer. The observed performance of the W-band package at 4.2 K has a maximum in-band transmission ripple of 2 dB between 1.53 and 1.89 times the waveguide cutoff frequency. This metrology approach enables the characterization of superconducting microstrip test structures as a function of temperature and frequency. The limitations of the method are discussed and representative data for superconducting Nb and NbTiN thin-film microstrip resonators on single-crystal Si dielectric substrates are presented.