Leveling of thin film surface by low-incident-angle ion milling for HTS SFQ circuits

We propose a new ion etching procedure to level the surfaces of lower layers in multilayer structures for HTS SFQ circuits. In order to make a flat lower layer, we have utilized a variation in the etching rate with the ion incident angle in ion etching process. The etching process under the acceleration voltage of 280 V and the ion incident angle as low as 15 deg. for 5 min improved the average surface roughness (R a) of the SrSnO3 insulating layer from 2.8 nm to 0.8 nm. This procedure was applied just before the La-YBCO base layer growth on the SrSnO3/La-YBCO groundplane bilayer, which resulted in the base layer with R a much less than 2 nm