SIS Mixer for 385 – 500 GHz with On-Chip LO injection

We present the design and first experimental results of a 385-500 GHz fixed-tuned double sideband (DSB) receiver based on a superconductor-insulator-superconductor (SIS) junction mixer with on-chip LO injection circuitry. At high frequency, branch waveguide couplers are difficult to manufacture with required accuracy as the branches (slots) become extremely narrow. In order to solve this problem, we propose a coupler integrated onto the mixer chip and fabricated together with the SIS junction and the tuning circuitry. The on-chip LO directional coupler is made of superconducting lines with slot lines in the ground plane. Thus, the coupler is integrated into conventional SIS junction fabrication steps, benefiting from the processing accuracy better than 0.5 μm by using optical lithography only. Furthermore, the mixer design includes a novel component, an ellipse termination for the idle LO port, made of thin-film resistive material. This termination gives very broadband performance using a compact area. Moreover, it is very tolerant to the sheet resistivity of the film, geometry and does not require any physical grounding [1]. The mixer is to be used at the Atacama Pathfinder EXperiment (APEX) telescope in Chile [2].