Modeling the Effect of Fabrication Process on Grain Boundary Formation in Nb/Al-AlO$_x$/Nb Josephson Junction Circuit

By modeling the propagation of a seed layer with various crystal orientations, this study explores the influence of process variations on grain formation with the help of a physics-based process simulator. Grain boundaries allow easy diffusion of foreign atoms through the lattice, which causes Al to move inside the Nb bottom electrode layer and more interestingly, O to penetrate through the Al layer during oxidation and create a barrier with non-uniform thickness. In addition to thickness variations, the grain structure exhibited by Nb and Al can cause significant suppression of supercurrent at the boundaries depending on the degree of lattice mismatch, impurity deposition, etc. This work details the process simulation of grain boundary formation and aims to provide geometrical models that may be used in the simulation of device performance to account for process-induced variations.