High-quality planar high- T c Josephson junctions

Reproducible high- T c Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge (1 to 5 μ m wide) is firstly designed by ion irradiating a c -axis-oriented YBa 2 Cu 3 O 7 − δ film through a gold mask such as the nonprotected part becomes insulating. A lower T c part is then defined within the bridge by irradiating with a much lower fluence through a narrow slit (20 nm) opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. This process can be used to produce complex Josephson circuits.