Abstract
We are interested in the low noise heterodyne receiver used for the astronomical study in the submillimeter wavelengths range and develop a process to fabricate Nb-based superconductor-insulator-superconductor (SIS) junctions with sub-micron size for the purpose of the reduction in the coupling losses at higher radio frequency (RF) and wide bandwidth operation. We use conventional self-aligned mask trilayer process in addition with E-beam lithography to fabricate SIS junctions with well-defined size of ~0.4um×0.4um and high quality, such as low leakage, high current density of ~10kA/cm^2, and sharp nonlinearity in the I-V curve. The junction has wRnC product value of 2.4. We first attempt to use PMMA combined with polymide, which is commonly used in the IC process, and successfully fabricate sub-micron SIS with high quality. By this process, the junction size could reach ~0.1um×0.1um and even smaller. Our SIS receiver’s system is waveguide coupled with SIS junction. Under the 300GHz frequency range measurement, a 0.2um^2 area size junction with supercurrent density of 3,250A/cm^2 performs its noise temperature of about 25hv/k. To improve the SIS mixer junction qualities, especially to increase the supercurrent density with a tough junction, is important for the future studies.