All-refractory Josephson logic circuits

A method is developed for the fabrication of Josephson integrated circuits which uses only refractory materials. Nb-Si-Nb tunnel junctions which are formed in the initial phase of the method are used as the Josephson devices. A Nb-Si-Nb trilayer is deposited over the entire substrate and the individual devices are then isolated by using the selective niobium anodization process. Molybdenum is utilized for the normal resistors and bias-sputtered SiO2 is employed for additional insulator layers. Only five photolithographic steps are required to produce circuits of the direct-coupled isolation type, since some layers are used for multiple purposes and components with different functional purposes are fabricated in a single step. Low capacitance junctions (approximately 0.025 pF/sq micron) have been produced with good uniformity using this method. The performance of strings of JAWS gates is analyzed.