Straightforward planarization method for multilayered SFQ device fabrication

We developed a method of planarization that can be used to fabricate large-scale single-flux-quantum (SFQ) circuits with more than 100-k junctions. Most conventional planarization methods have problems with being able to obtain sufficient planarity in devices with Nb wiring having various pattern sizes and area densities ( pattern dependence problem). We eliminate this pattern dependence problem directly by removing the convex areas of SiO 2 insulator layer covering Nb wiring layer using the Nb wiring pattern array information. The practical process involves the combination of three steps to form the SiO 2 insulator layer, i.e., (1) bias-sputtering, (2) etching with a reversal mask of the underneath wiring pattern, and (3) chemical mechanical polishing. The two- to six-level wiring structures we fabricated, consisting of 300-nm-thick Nb and SiO 2 layers, had excellent layer flatness, independent of the wiring characteristics (width, length, and density). The electrical characteristics also remained at satisfactory levels, i.e., the leakage current between the Nb layers was sufficiently low. Two hundred to four thousand chains of stepwise and stacked contacts yielded a sufficiently large critical current, typically more than 10 mA at 4.2 K, which is two orders of magnitude larger than the critical current of Josephson junctions.