Rapid Single-Flux-Quantum Circuits Fabricated Using $\hbox{20}\hbox{-}\hbox{kA}/\hbox{cm}^{2}$ $\hbox{Nb}/\hbox{AlO}_{x}/\hbox{Nb}$ Process

We designed and fabricated rapid single-flux-quantum (RSFQ) circuits using a new 20-kA/cm
²
process based on conventional Nb/AlO
_x
/Nb Josephson junction (JJ) technology. Circular JJs were fabricated to improve their uniformity. They had sufficiently low spread of the critical currents for large-scale RSFQ circuits. We selected the circuit design parameters carefully, including the bias voltage and McCumber-Stewart parameter, balancing the energy consumption, switching speeds, margins, and integration density. We demonstrated several logic gates and shift registers at a clock frequency of 154 GHz with the designed bias voltage of 2.5 mV, and demonstrated energy-efficient shift registers using the low-voltage RSFQ (LV-RSFQ) technique at 83 GHz.