Fabrication of a 12-bit A/D converter using Nb/AlO/sub x//Nb Josephson junctions

For reliable fabrication of Nb/AlO/sub 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/sub x//Nb Josephson tunnel junctions with J/sub c/ as high as 5000 A/cm/sup 2/ and V/sub m/(2 mV) as large as 60 mV at 4.2 K. A ten-level process on Nb/AlO/sub x//Nb trilayers with J/sub c/ of 1500 A/cm/sup 2/ was used to fabricate a functional 12-b A/D converter. This circuit included 2.7- mu m-diameter junctions defined by anodization and 5- mu m-diameter junctions defined by reactive ion etching using Nb/sub 2/O/sub 5/ and Al as etch stops. A lift-off process with image-reversal photoresist was used to define SiO/sub 2/ 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.