Experimental realization of a 3D integrated RSFQ T-flip-flop using stacktrons

Superconducting circuits based upon Rapid Single Flux Quantum (RSFQ) Logic are an interesting approach for future digital electronics with clock frequencies in the range of 100 GHz. Our intention is to increase the integration density of such circuits. Therefore we have successfully fabricated and tested an RSFQ-circuit based on a new kind of stacked three terminal device, henceforth called a "stacktron". A stacktron consists of two vertically arranged all-niobium tunnel junctions, each separately shunted by a thin film resistor, and an access to the intermediate niobium layer. An RSFQ frequency divider (T-flip-flop) including two stacktrons has been tested up to 20 GHz. As a shunt resistor we used Pd/Au, so that /spl beta//sub c/<1 and the IV-curve is nonhysteretic. The junctions showed a current density of approximately 250 A/cm/sup 2/. The I/sub c/R/sub n/-product was 100 /spl mu/V. These first steps toward a three dimensional (3D) RSFQ architecture will be discussed in the framework of highly integrated and complex RSFQ-circuits.