Development of a 385 - 500 GHz SIS Mixer for ALMA Band 8

In this paper, we report on the design and experimental results of a fix-tuned Superconductor-Insulator-Superconductor (SIS) mixer for Atacama Large Millimeter/submillimeter Array (ALMA) band 8 (385-500GHz) receivers. Nb-based SIS junctions of a current density of 10kA/cm and one micrometer size (fabricated with a two-step lift-off process) are employed to accomplish the ALMA receiver specification, which requires wide frequency coverage as well as low noise temperature. Parallel-connected twin junctions (PCTJ) are designed to resonate at the band center to tune out the junction geometric capacitance. A waveguide-microstrip probe is optimized to have nearly frequency-independent impedance at the probe’s feed point, thereby making it much easier to match the low-impedance PCTJ over a wide frequency band. The SIS mixer demonstrates a minimum double-sideband receiver noise temperature of 3 times of quantum limits for an intermediate-frequency range of 4-8GHz. The mixers were measured in band 8 cartridge with a sideband separation scheme. Single-sideband receiver noise below ALMA specification was achieved over the whole band.