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The Japan Society of Applied Physics

Fabrication of magnetic Josephson junctions on Nb 4-layer superconducting integrated circuits

Daiki Hasegawa, Yuto Takeshita, Kyosuke Sano, Masamitsu Tanaka, Taro Yamashita, Akira Fujimaki

Published Jul 10, 2019• 0 citations
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Fabrication of ferromagnetic Josephson junctions toward quantum computing applications

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The Japan Society of Applied Physics•Jul 21, 2017•0 citations
We investigate a Magnetic Josephson Junction (MJJ) - a superconducting device with ferromagnetic barrier for a scalable high-density cryogenic memory compatible with energy-efficient single flux quantum (SFQ) circuits. The superconductor-insulator-superconductor-ferromagnet-superconductor (SIS'FS) MJJs are analyzed both experimentally and theoretically. We found that the properties of SIS'FS junctions fall into two distinct classes based on the thickness of S'layer. We fabricate Nb-Al/AlOx-Nb-PdFe-Nb SIS'FS MJJs using a co-processing approach with a combination of HYPRES and ISSP fabrication processes. The resultant SIS'FS structure with thin superconducting S'-layer is substantially affected by the ferromagnetic layer as a whole. We fabricate these type of junctions to reach the device compatibility with conventional SIS junctions used for superconducting SFQ electronics to ensure a seamless integration of MJJ-based circuits and SIS JJ-based ultra-fast digital SFQ circuits. We report experimental results for MJJs, demonstrating their applicability for superconducting memory and digital circuits. These MJJs exhibitIcRnproduct only ~ 30% lower than that of conventional SIS junctions co-produced in the same fabrication. Analytical calculations for these SIS'FS structures are in a good agreement with the experiment. We discuss application of MJJ devices for memory and programmable logic circuits.
We successfully fabricated magnetic Josephson junctions integrated on a Nb four-layer superconducting structure for demonstrating half flux quantum (HFQ) circuits. In the developed process, we adopted a 0-0-π SQUID instead of the 0-π SQUID as the basic element of the HFQ circuits. In this work, we characterized the Nb/PdNi/Nb magnetic Josephson junctions on the Nb four-layer structures fabricated by two different processes. We measured the I-V characteristics of junctions in liquid helium at 4.2 K. The results showed that the fabricated junctions had properties comparable to conventional ones on Si wafers.
We have developed a fabrication technology for superconducting integrated circuits with Nb-based Josephson junctions. The standard fabrication process with 10 mask levels includes 3 Nb superconducting layers and a Mo resistor layer. The influence of deposition parameters on film stress, electrical properties, and surface roughness were studied systematically. High quality Nb, Al, Mo, and SiO2films were successfully deposited for the subsequent fabrication of circuits. The circuit fabrication started with the fabrication of Mo resistors with a target sheet resistanceRshof 2 Ω, followed by the deposition of Nb/Al-AlOx/Nb trilayer Josephson-junction. The target critical current densityJcwas set at 6 kA/cm2. Small-scale circuits such as our standard library cells have been successfully fabricated and tested, confirming the capability of our fabrication technology for superconducting integrated circuits.