20 ${\hbox {kA/cm}}^{2}$ Process Development for Superconducting Integrated Circuits With 80 GHz Clock Frequency

Results of the development of an advanced fabrication process for superconductor integrated circuits (ICs) with 20 kA/cm² Nb/AlO_x/Nb Josephson junctions is presented. The process has 4 niobium superconducting layers, one MoN_x resistor layer with 4.0 Ohm per square sheet resistance for the junction shunting and circuit biasing, and employs circular Josephson junctions with the minimum diameter of 1 mum; total 11 photolithography levels. The goal of this process development is the demonstration of the feasibility of 80 GHz clock speeds in superconducting ICs for digital signal processing (DSP) and high performance computing. Basic components of rapid single flux quantum (RSFQ) logic such as DC/SFQ, SFQ/DC converters, Josephson transmission lines (JTLs), and simple digital circuits such as T-flip-flops and 4-bit digital counters have been fabricated and tested. The T-flip-flops were shown to operate up to 400 GHz with the widest margin of operation of plusmn13% at 325 GHz. Digital testing results on the 4-bit counters as well as the junctions, resistors, and other process parameters are also presented. Prospects for yet higher speeds and very large scale integration are discussed.