A novel multilayer process for HTS SFQ circuit

We have developed a novel multilayer process for HTS-SFQ circuits adopting a NBCO groundplane and tin-oxide isolation layers, and fabricated an HTS-SFQ ring oscillator circuit including 21 Josephson junctions. The junctions on the buried groundplane exhibited excellent Josephson characteristics with a magnetic modulation of I_c exceeding 90%, and an I_cR_n product of 0.75 mV at 30 K. The sheet inductance of the wiring layer at 30 K was evaluated to be 1.1 pH for the counter-electrode layer, and 1.3 pH for the base-electrode layer. We confirmed the correct operation of the 10-stage ring oscillator at 20-30 K. The maximum dc output voltage of the ring oscillator was 0.06 mV at 30 K and 0.12 mV at 20 K, indicating the signal delay per stage of 3.4 ps and 1.8 ps, respectively.