9 K operation of RSFQ logic cells fabricated by NbN integrated circuit technology

We report on the fabrication and the operation of all-NbN rapid single flux quantum (RSFQ) circuits. The NbN integrated circuit consists of NbN/AlN/NbN tunnel junctions, Mo resistors, sputtered SiO₂ insulating layers, and 400-nm-thick NbN ground plane and wiring layer. The circuits were fabricated with the minimum junction size of 2 μm×2 μm and the alignment margin of +/-0.5 μm. We designed RSFQ logic cells under critical current density of 2.5 kA/cm ², sheet resistance of 2 Ω and sheet inductance of 1.4 pH. We experimentally investigated dc bias margin of the fabricated RSFQ cells at 9 K. We observed relatively large dc bias margin of more than +/-25% for a pulse splitter, a confluence buffer and an RS flip-flop, while the dc bias margin of a T flip-flop was less than +/-5%. We succeeded to demonstrate the operation of 16-bit concurrent flow shift register at 9 K