Microwave Packaging for Voltage Standard Applications

Improved packages for Josephson Voltage Standard (JVS) circuits have increased operating margins, reliability, and longevity of JVS systems. By using the ldquoflip-chip-on-flexrdquo technique, reliable chip and cryoprobe mounting have been demonstrated. The microwave structures on these packages have been improved such that more power can be delivered to the JVS chip over a wider frequency range: DC to over 30 GHz. Detailed finite-element simulations were performed to optimize the chip-to-flex launches as well as the on-flex transmission lines. It was found that coplanar waveguide transmission line designs had improved insertion and return losses compared to those of the microstrip transmission line designs, in large part due to the large discontinuities associated with through-substrate vias for microstrip ground connections. The improved coplanar-waveguide package/probe yielded insertion loss dominated by the ~ 0.25 dB/GHz cable loss and VSWR better than 2:1 for the entire 0-30 GHz band. Substantially larger JVS system operating margins were measured using the coplanar-waveguide package; for a 5120 junction array a quantized voltage step greater than 1 mA bias current range is shown for a 10-30 GHz band.