Fabrication Process for Monolithic Integration of a Nitride Superconductor-Based Superconducting Qubit with a Single Flux Quantum Control Circuit

We developed a process for fabricating nitride superconductor-based superconducting qubits monolithically integrated with a single flux quantum (SFQ)-based readout and control circuits. Since we used a Josephson junction (JJ) made of an NbN/AlN/NbN tri-layer, including a critical current density ( J _c ) of 50 A/cm ² as the suitable material for superconducting qubits and SFQ readout/control circuits operating at 20 mK, the JJs for both the qubits and the SFQ circuits could simultaneously be fabricated. The device structure was also composed of three superconductive layers, comprising two superconductive layers as wiring layers and other layers as the top ground plane. Then, we used a Pd film as the resistive layer, which is required as the shunt resistors and the bias resistors for the SFQ control circuits, followed by an investigation of the properties of NbN/AlN/NbN epitaxial junctions and evaluation of the sheet inductance of the base NbN and wiring NbTiN layer. Our results showed the successful fabrication of a JJ with a sufficiently low leakage current and a J _c of around 50 A/cm ² , which was close to the target value. Furthermore, the evaluated sheet inductance exceeded 1 pH, which was several times larger than the values obtained for Nb-integrated circuits.