Fabrication of a 12-bit A/D converter using Nb/AlOx/Nb Josephson junctions

For reliable fabrication of Nb/AlO_x/Nb Josephson integrated circuits, a combination of anodization and reactive ion etching techniques was used to make various sizes of high-quality Nb/AlO _x/Nb Josephson tunnel junctions with J_c as high as 5000 A/cm² and V_m(2 mV) as large as 60 mV at 4.2 K. A ten-level process on Nb/AlO_x/Nb trilayers with J_c of 1500 A/cm² was used to fabricate a functional 12-b A/D converter. This circuit included 2.7-μm-diameter junctions defined by anodization and 5-μm-diameter junctions defined by reactive ion etching using Nb₂O₅ and Al as etch stops. A lift-off process with image-reversal photoresist was used to define SiO₂ insulators, Mo resistors, and Au resistors. Mo resistor values were adjusted by reactive ion etching. Measurements of the completed circuit show that all circuit components had correct values to well within design tolerance