NbN-Based Overdamped Josephson Junctions for Quantum Voltage Standards

Hirotake YAMAMORI  Takahiro YAMADA  Hitoshi SASAKI  Satoshi KOHJIRO  

Publication
IEICE TRANSACTIONS on Electronics   Vol.E95-C   No.3   pp.329-336
Publication Date: 2012/03/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E95.C.329
Print ISSN: 0916-8516
Type of Manuscript: INVITED PAPER (Special Section on Josephson Junctions – Past 50 years and Future –)
Category: 
Keyword: 
NbN,  TiN,  SNS,  voltage standard,  cryocooler,  

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Summary: 
524,288 NbN-based Josephson junctions were integrated to produce a programmable Josephson voltage standard (PJVS) on a die of 15 mm 15 mm, and the PJVS circuit was cooled to 10 K using a cryocooler and operated with a current margin of about 1.0 mA. Although an output voltage of 10 V was required for a voltage standard, the circuit was designed to generate the maximum output voltage of 17 V because it was difficult to avoid a reduction of the output voltage due to defects. Although a perfect chip without any defect was rarely fabricated, the high voltage chip that generated at least 10 V was fabricated with the fabrication yield of larger than 30%. The fabrication yield was also improved by optimizing the film growth conditions to reduce the film stress and the number of particles. Applications for a secondary voltage standard and an ac Josephson voltage standard are also described.