Fabrication of micron size Nb/Al-Al₂O₃/Nb junctions with a trilevel resist liftoff process

A trilevel resist, consisting of polyimide planarization, SiO₂ barrier, and photoresist, was used to pattern junction areas in Nb/Al-Al₂O₃/Nb trilayer films. After reactive ion etching to define the junction areas, the perimeter of the junction was revealed, and excellent liftoff structures were defined with an oxygen plasma shrink of the exposed polyimide sidewalls. A subsequently deposited insulation layer seals the sides and the top surface along the perimeter of the Nb counter electrode button. High-quality superconductor-insulator-superconductor (SIS) junctions with diameters as small as 1.2 μm and V_m as large as 1500 mV at 2.0 K have been fabricated. An SIS receiver using these junctions with integrated tuning elements has a DSB noise temperature of 58 K at 230 GHz. This is believed to be the lowest receiver noise temperature ever reported at this frequency