Fabrication of a 12-bit A/D converter using Nb/AlO(x)/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 Jc as high as 5000 A/sq cm and Vm(2 mV) as large as 60 mV at 4.2 K. A ten-level process on Nb/AlOx/Nb trilayers with Jc of 1500 A/cm2 was used to fabricate a functional 12-b A/D converter. This circuit included 2.7-microns-diameter junctions defined by anodization and 5-microns-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.