Fabrication of NbN single flux quantum (SFQ) circuit and operating properties at 10 K

We fabricated a prototype of a single flux quantum (SFQ) logic circuit for operation near 10 K by using NbN, which has a critical temperature around 16 K. This circuit is fabricated on a 2-inch silicon (Si) substrate based on the NbN/AlN/NbN tunnel junction having fluctuations in the critical current of ±5% or less from 976 Josephson junctions (JJ). The fabrication of a prototype circuit to generate, propagate, and detect SFQ pulses and the examination of its operation confirmed normal operation up to approximately 9.8 K, which is close to 10 K. In addition, at the operating temperature of 8.5 K, the operations of elementary cells such as a confluence buffer and a T flip-flop were verified and were successful in the operation of the largest scale circuit, an 8-bit shift register consisting of 105 JJ. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(10): 34–39, 2001