Fabrication of high-temperature superconductor single-flux-quantum circuits using a multilayer structure with a smooth surface

Abstract

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 R a less than 2 nm, composed of a La 0.2 -Y 0.9 Ba 1.9 Cu 3 O x ground plane and base electrode layer, and SrSnO 3 insulating layers, was deposited by off-axis magnetron sputtering. We have fabricated interface-engineered junctions based on the multilayer structure using an La 0.2 -Yb 0.9 Ba 1.9 Cu 3 O x counter-electrode layer prepared by pulsed laser deposition. The fabricated junctions exhibited excellent Josephson characteristics with a 1-σ spread in I c 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.

Preparation of multilayer films for integrated high-Tc SQUIDs with ramp-edge Josephson junctions

“…For an insulating layer, we chose SrSnO 3 (SSO) which has been used for insulating layers in our high-T c SFQ devices for recent several years [6,7,26]. We have never experienced any troubles induced by poor insulation of the SSO layer.…”

“…To achieve further improvement in the devices, an advanced fabrication technology to construct more complicated structures is needed [5]. Recently, we have developed the fabrication technology for high-T c single-flux-quantum (SFQ) digital circuits containing ramp-edge JJs and multilayer structures [6,7]. We believe that our technology can be applicable to realize the advanced SQUIDs.…”

Influence of Circuit Pattern Layout on Characteristics of Interface-Modified Ramp-Edge Junctions

“…Soutome et al [7] reported that the multilayer with a (YBCO) ground plane exhibited a sheet inductance of 1.0 pH per square for the counter-electrode layer, and the junctions exhibited a 1-sigma spread in of 7.9% for a 100-junction series-array and an product of 1.1 mV at 4.2 K. Katsuno et al [6] prepared the multilayer structures with a (NBCO) ground plane and demonstrated the operation of a ring oscillator at 20-30 K. We have successfully developed a fabrication process for a multilayer structure 7containing three superconducting layers with a smooth surface [8]. We obtained sheet inductance values of 1.0 pH per square or less for the base and counter electrode layers below 50 K, which are small enough for operation of SFQ circuits [9]. The interface-modified ramp-edge junctions fabricated on a superconducting ground plane exhibited a 1-sigma spread in of approximately 8% for a 1000-junction series-array [10] and an product higher than 1 mV at 40 K [5].…”

Characterization of La-doped YBCO superconducting films deposited by DC magnetron sputtering at various off-axis geometries

“…7) To establish the reproducible process, preparation technique of high-quality films with multi-layer structures has been required. In particular, the lowest layer must have excellent crystallinity and a flat surface, because the layer is the foundation of the multi-layer structures.…”