Development of Thin Film Multilayer Structures with Smooth Surfaces for HTS SFQ Circuits

Hironori WAKANA
Seiji ADACHI
Ai KAMITANI
Kouhei NAKAYAMA
Yoshihiro ISHIMARU
Yoshinobu TARUTANI
Keiichi TANABE

Publication
IEICE TRANSACTIONS on Electronics   Vol.E88-C    No.2    pp.208-215
Publication Date: 2005/02/01
Online ISSN: 
DOI: 10.1093/ietele/e88-c.2.208
Print ISSN: 0916-8516
Type of Manuscript: Special Section INVITED PAPER (Special Section on Superconducting Electronic Devices and Their Applications)
Category: 
Keyword: 
high-temperature superconductor,  Josephson junction,  interface modified barrier,  SFQ circuit,  ramp-edge structure,  

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Summary: 
We have fabricated a multilayer structure for single flux quantum (SFQ) circuit application using a high-temperature superconductor (HTS). La0.2-Y0.9Ba1.9Cu3Ox (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 Ra less than 1.0 nm and a Tc zero value of 88 K. By using the dc-sputtered La-YBCO films, a multilayer structure of SrSnO3/La-YBCO/SrSnO3/La-YBCO on MgO substrate with Ra below 2.0 nm was obtained. Interface-modified ramp-edge junctions with La0.2-Yb0.9Ba1.9Cu3Ox (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 IcRn products of about 3 mV at 4.2 K. We also obtained a 1-σ Ic 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.