Fabrication of Josephson junctions by single line etching of Nb thin films utilizing nitrogen-gas-field ion-source focused ion beam

Abstract

We focused on a focused ion beam (FIB) technology, called nitrogen gas field ion source FIB (N2 GFIS-FIB), which can etch directly at the 10 nm level or finer. We performed single line etching of Nb thin film microbridges deposited by N2 GFIS-FIB and fabricated Josephson junction (JJ) devices. The microbridge area was separated into two parts by the processing line, whose width was around 20 nm. We performed electrical characterizations of the devices at low temperature and observed typical behaviors of JJ devices, such as a superconducting current region and current jumping to the normal current region in some of the deeply etched devices. We also observed an AC resistance oscillation in some of the shallowly etched devices. The oscillation may be due to a sub-harmonic gap structure that originates from Andreev reflection. These results indicate that simple single line etching of Nb by N2 GFIS-FIB can form JJ devices.