FABRICATION OF AN ALL-REFRACTORY CIRCUIT USING LIFT-OFF WITH IMAGE-REVERSAL PHOTORESIST

A four-stage shift register was fabricated using Nb/Al-Al2O3/Nb Josephson junctions, Mo resistors, Nb transmission lines, and SiO2 insulating layers. The circuit had 36 junctions (5-mu-m, 1000 A/cm2) and 61 resistors (1.2 ohms/square), with a minimum feature size of 2-mu-m. 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, non-uniformity 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 Gb/s.