Development of Thin Film Multilayer Structures with Smooth Surfaces for HTS SFQ Circuits(Digital Applications, <Special Section>Superconducting Electronic Devices and Their Applications)

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We have fabricated a multilayer structure for single flux quantum (SFQ) circuit application using a high-temperature superconductor (HTS). La_<0.2>-Y_<0.9>Ba_<1.9>Cu_3O_x (La-YBCO) base electrode layers were prepared by a dc or rf magnetron sputtering method. The reproducibility of film quality for dc-sputtered La-YBCO films was better than that for rf-sputtered films, and the dc sputtered films exhibited the average surface roughness R_a less than 1.0 nm and a T_c zero value of 88 K. By using the dc-sputtered La-YBCO films, a multilayer structure of SrSnO_3/La-YBCO/SrSnO_3/La-YBCO on MgO substrate with R_a below 2.0 nm was obtained. Interface-modified ramp-edge junctions with La_<0.2>-Yb_<0.9>Ba_<1.9>Cu_3O_x (La-YbBCO) counter electrodes have been fabricated by using this multilayer structure with dc-sputtered films. The fabricated junctions exhibited RSJ-type I-V characteristics with I_cR_n products of about 3 mV at 4.2 K. We also obtained a 1-σ I_c spread of 8% for a 1000-junction series-array. The sheet inductance values at 4.2 K for the base and counter electrodes on La-YBCO ground planes were 0.8 pH and 0.7 pH per square, respectively. Operation of several types of elementary SFQ circuits has been successfully demonstrated by using this multilayer structure.
社団法人電子情報通信学会の論文
2005-02-01