All-refractory Josephson logic circuits

A process for the fabrication of Josephson integrated circuits is described which uses only refractory materials. The Josephson devices are Nb-Si-Nb tunnel junctions which are formed in the initial phase of the process. After depositing a Nb-Si-Nb `trilayer' over the entire substrate, the individual devices are isolated by the selective niobium anodization process (SNAP). Other materials used are molybdenum for the normal resistors and bias-sputtered SiO/SUB 2/ for additional insulator layers. The process uses only five photolithographic steps to produce circuits of the direct-coupled isolation type. This simplicity is achieved by using some layers for multiple purposes and by fabricating components with different functional purposes in a single step. For example, the lower electrode of the Josephson devices also functions as the ground plane and the contacts to the ground plane are actually large-area Josephson junctions formed simultaneously with the active devices. Low capacitance junctions (~0.025 pF//spl mu/m/SUP 2/) are produced with good uniformity.