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IEEE Transactions on Applied Superconductivity

A novel multilayer process for HTS SFQ circuit

H. Katsuno, S. Inoue, T. Nagano, Jiro Yoshida

Published Jun 1, 2003• 19 citations
Publisher

Abstract

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/sub c/ exceeding 90%, and an I/sub c/R/sub 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.

DOI
10.1109/tasc.2003.814050

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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/sub c/ exceeding 90%, and an I/sub c/R/sub 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.
We have developed a multilayer structure with a smooth surface and fabrication processes for HTS single-flux-quantum (SFQ) circuits. The multilayer structure with surface roughness Ra less than 2 nm, composed of a La0.2–Y0.9Ba1.9Cu3Ox ground plane and base electrode layer, and SrSnO3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La0.2–Yb0.9Ba1.9Cu3Ox counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in Ic as low as 8% for 1000 junctions. The sheet inductance values below 50 K were 0.8–1.0 pH per square. Operation of several elementary SFQ circuits including a toggle flip-flop, a confluence buffer, a set–reset flip-flop and other SFQ elements has been successfully demonstrated at 30–60 K.
We have fabricated and measured high T/sub c/ superconductor single flux quantum 1-bit flux-counting analog-to-digital converters (ADCs). The ADCs were made with a multilayer all-epitaxial process which incorporates 10 edge SNS (superconductor-normal-superconductor) or step-edge grain boundary (SEGB) Josephson junctions with a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// groundplane. The ADC consists of a quantizer connected to a Toggle flip-flop through a buffer-like stage. Direct readout of the flux state of the T flip-flop was made with a Read SQUID inductively coupled through a hole in the groundplane. The circuits were operated at 65 K and low speeds. The SNS circuits outperformed the SEGB circuits because of their higher readout voltages and tighter critical current spreads.
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Characteristics of SFQ signal propagation in oxide circuits were investigated by numerical simulation. The numerical simulation for the SFQ signal propagating in the Josephson transmission line (JTL) was performed by assuming the inductor to be a passive transmission line and by taking into account the high I/sub c/R/sub n/ value of oxide junctions. The SFQ signal was found to oscillate when the inductor length was more than 20 /spl mu/m. The origin of the oscillation and the oscillation conditions were investigated with respect to the Josephson plasma frequency. A shunt resistor connected between an inductor and the groundplane was found to be effective for suppressing the oscillation. The JTL in which shunt resistors are connected may also be used as a delay line.
The SNS (Nb/HfTi/Nb) ramp junction technology process has been improved for digital superconducting circuit operation. DC interferometers were realized on an isolated superconducting ground plane, connections being realized through windows in the insulation layers. The inductances of superconducting Nb striplines (widths w down to the sub-/spl mu/m range) were measured. For a 250 nm SiO/sub 2//Nb-oxide insulation layer, the sheet inductances of striplines were determined at 0.382 /spl plusmn/ 0.009, 0.340 /spl plusmn/ 0.005, and 0.293 /spl plusmn/ 0.008 pH (stripline widths: 1.5, 1.0, 0.54 /spl mu/m) and compared with data calculated by different inductance evaluation programs. The Josephson junctions used in the interferometers exhibit a critical current density of j/sub C/=525 kA/cm/sup 2/ and a characteristic voltage of V/sub C/=95 /spl mu/V. The design of an RSFQ converter circuit was developed on the basis of SNS ramp junction technology. For circuit applications the critical current density j/sub C/ was set to 150 kA/cm/sup 2/.
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.