Fabrication of an all-refractory circuit using lift-off with image-reversal photoresist

A four-stage shift register was fabricated using Nb/Al-Al₂ O₃/Nb Josephson junctions, Mo resistors, Nb transmission lines, and SiO₂ insulating layers. The circuit has 36 junctions (5 μm, 1000 A/cm²) and 61 resistors (1.2 Ω/square), with a minimum feature size of 2 μm. An eight-mask process was used in the fabrication. All material layers were deposited by sputtering. Patterning for all but one of the masking levels was done by lift-off using image-reversal lithography in most cases. Lift-off avoided many of the problems common to reactive ion etching (RIE), including the need for etch stops, nonuniformity in etching, and the formation of organic residue (polymer) on the wafer. RIE was used only in the patterning of the Nb counterelectrode, where a natural Al etch stop from the barrier layer is present. The simplicity associated with the lift-off of all other layers, including the trilayer, is thought to be a major factor in the successful fabrication of the circuit which operated properly up to 4.0 Gbit/s