Influence of beam energy, flux, and composition on junction parameters in ion beam tunnel barrier processing

An ion beam process was used to fabricate planar and edge-defined niobium-lead alloy Josephson junctions. Critical current density was found to be determined primarily by the dose of oxygen ions delivered during tunnel barrier growth. Comparison of spatial distributions of critical currents with ion flux profiles indicated that both preclean and oxidation were important in determining the magnitude of the on-wafer spatial variations. Uniformity of angle of incidence of the ion beam was very important in fabrication of edge junctions. The first ion beam experiments involving intentional additions of carbon-containing ions to the beam, to facilitate controlled growth of a carbide interfacial layer which is important in high quality devices, are also reported.