Reduced Critical Current Spread in Planar MgB₂ Josephson Junction Array Made by Focused Helium Ion Beam

We have fabricated series arrays of closely spaced planar Josephson junctions on MgB2 films using a 30 keV focused helium ion beam. Uniformity of junction parameters within the arrays is sufficient for achieving phase-lock into an applied microwave signal, and flat giant Shapiro steps are observed. The spread in critical current of a 60-Josephson junction array is estimated to be less than 3.5%, significantly better than reported in MgB2 junctions fabricated by other techniques. These results demonstrate the potential of the focused He + ion beam irradiation technique in MgB2 Josephson multi-junction circuit applications such as quantum voltage standards.