Abstract
We have been developing a 10-kA/cm 2 advanced Nb process in order to fabricate larger scale and higher speed SFQ circuits with over 100k junctions. We have proposed a planarized multi-layer structure, which consists of a Nb/AlO x /Nb junction layer, 4 Nb wiring layers, a Nb layer for DC power, a Nb ground plane, SiO 2 insulator layers, and a Mo resistor layer. Process technologies for fabricating 1.0-μm 2 junctions with high J C of 10 kA/cm 2 and 1 σ of 1.4% and Mo resistors with R □ of 2.4 Ω have been developed. A new planarization technology called caldera, which is applicable to patterns of various sizes, has been developed. This process consists of reactive ion etching (RIE) with a reverse mask, bias sputtering, and mechanical polishing planarization (MPP). We have now successfully developed all the component technologies required for the advanced Nb process. We have implemented these technologies for the through process and fabricated a structure with six planarized Nb layers, including Nb/AlO x /Nb junctions, Mo resistors, and contacts. In this structure, we obtained excellent current–voltage characteristics for the junctions, sufficient superconducting characteristics for the contacts, and good insulation characteristics between the wiring layers.
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