Advances in High-T_c Single Flux Quantum Device Technologies

Abstract

We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-T_c single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating rampedge Josephson junctions with I_cR_n products above 1mV at 40K and small I_c spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the J_c spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200GHz at 40K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-Δ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50GHz.