Fabrication and test of 9K NbN SFQ circuits

Kazumasa Makise

Shuichi Nagasawa, T. Satoh, K. Hinode +7 more

Kazumasa Makise, Hirotaka Terai, Shigehito Miki +2 more

We have developed an all-NbN integrated circuit technology for future integration of single flux quantum (SFQ) circuit with superconducting single-photon detector array. The all-NbN integrated circuits consist of epitaxial NbN/AlN/NbN tunnel junctions fabricated on single-crystal MgO substrates, Mo shunt resistors and 500-nm-thick NbN ground plane on the top. Critical current density and sheet resistance of the Mo resistor were set at 2.5 kA/cm2and 1.2 Ω, respectively. Current uniformity of the NbN/AlN/NbN tunnel junctions was measured to be σ = 3% for 3 μm × 3 μm junction size. The NbN-based SFQ logic cells were designed for and tested at 4.2 K. We actually measured circuit inductances of the fabricated SFQ cells. Circuit simulation revealed that the measured circuit inductances in the fabricated SFQ cells should allow operating margins of more than +/-20%.

Hirotaka Terai, Zhen Wang

Abstract We fabricated a prototype of a single flux quantum (SFQ) logic circuit for operation near 10 K by using NbN, which has a critical temperature around 16 K. This circuit is fabricated on a 2‐inch silicon (Si) substrate based on the NbN/AlN/NbN tunnel junction having fluctuations in the critical current of ±5% or less from 976 Josephson junctions (JJ). The fabrication of a prototype circuit to generate, propagate, and detect SFQ pulses and the examination of its operation confirmed normal operation up to approximately 9.8 K, which is close to 10 K. In addition, at the operating temperature of 8.5 K, the operations of elementary cells such as a confluence buffer and a T flip‐flop were verified and were successful in the operation of the largest scale circuit, an 8‐bit shift register consisting of 105 JJ. © 2001 Scripta Technica, Electron Comm Jpn Pt 2, 84(10): 34–39, 2001

Shigeyuki Miyajima, Taro Yamashita, Masamitsu Tanaka +3 more

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 (Jc) of 50 A/cm2as 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 aJcof around 50 A/cm2, 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.

Kazunori Miyahara, Shuichi Nagasawa, H. Hasegawa +3 more