Rapid Single-Flux-Quantum Circuits Fabricated Using <inline-formula> <tex-math notation="TeX">$\hbox{20}\hbox{-}\hbox{kA}/\hbox{cm}^{2}$</tex-math></inline-formula> <inline-formula> <tex-math notation="TeX">$\hbox{Nb}/\hbox{AlO}_{x}/\hbox{Nb}$</tex-math></inline-formula> Process

Abstract

We designed and fabricated rapid single-fluxquantum (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.Index Terms-Energy efficient, low power, low-voltage, rapid single-flux-quantum logic, shift register, superconducting integrated circuits.

Fabrication Process for Superconducting Digital Circuits

“…The I c variation for the JJs with areas exceeding 1.0 μm 2 remained less than 2%. If we apply the criterion that 1σ must be less than 2% for large-scale digital circuits, A = 0.5 μm 2 is the lowest limit for obtaining such circuits [28], [29].…”