Optimization of fabrication conditions for multilayer structures with La-doped YBCO groundplane

We have examined and optimized the fabrication conditions for multilayer structures needed in high-T/sub c/ single flux quantum circuits. La/sub 0.2/Y/sub 0.9/Ba/sub 1.9/Cu/sub 3/O/sub x/ (La-YBCO) was chosen as a material for both the groundplane (GP) and base-electrode. The GP-La-YBCO was deposited at 740(degree)C by a sputtering method and the optimized 300 nm thick film exhibited a typical average roughness (Ra) of 2 nm. (La,Sr)/sub 2/AlTaO/sub 6/ (LSAT) isolation layer sputter-deposited under the same conditions as the GP-La-YBCO showed good crystallinity and a relatively flat surface (Ra~2 nm). For the deposition of La-YBCO base electrode, a slightly lower substrate temperature (720(degree)C) resulted in a smooth film surface (Ra~2 nm), while higher substrate temperatures led to appearance of precipitates and hollows on the La-YBCO and columnar morphology of the LSAT layer which was revealed by crosssectional SEM observation. Interface-engineered ramp-edge junctions fabricated on LSAT/GP-La-YBCO multilayer structures exhibited RCSJ-like I-V characteristics with typical I/sub c/R/sub n/ products of 2 mV at 4.2 K. Excellent isolation between the GP and the base electrode better than 10/sup 9/ (omega)/cm/sup 2/ was also confirmed.