Investigation for Low-Rate Fenceless Al Etching Applied in Fabrication of Superconducting Circuits

Ion beam etching (IBE) is introduced in the fabrication process developed from Nb03 in SIMIT, for the etching of Al-AlOx layer. However, we found that IBE creates fences at the edges of etched profiles because of redeposition of the Al. To solve this problem, we systematically investigated the yield of fenceless patterns and the junction quality as a function of ion beam incidence angle. The yield increases as the incidence angle decreases, 0° causing the least fence formation. The yield dependence was explained by a fence formation model developed from a two-dimensional redeposition algorithm. Given that the un-etched foots due to the shadow effect become visible as angle decreases, the optimal condition is finally set to be 400 eV with incidence angle of 10°. Nb/Al-AlOx/Nb junctions fabricated by Al-AlOx layer etching at incidence angle of 10° showed good junction quality with a low subgap leakage current. We could also obtain a high yield of shunted and unshunted Nb/Al-AlOx/Nb junctions with the minimum size of 1.4 μm in diameter on 4-inch Si wafers. These results prove the feasibility to achieve fabrication without anodization process and patterning of Al to improve the reliability.