Fabrication of Small Biaxial High- $T_{c}$ Gradiometric SQUID

We have developed high-T
_c
superconducting quantum interference devices (SQUIDs) containing ramp-edge-type Josephson junctions and oxide multilayer structures. In this work, we propose a new design for a small biaxial gradiometric SQUID and report its successful fabrication. It is comprised of two gradiometric SQUIDs with a baseline length of 30 microns. The biaxial gradiometric SQUID contains four junctions and many crossovers and superconducting contacts between counter and base electrodes. When the counter electrode is deposited, the junctions and the superconducting contacts are simultaneously prepared. We developed a new technique to realize the distinguishing preparation of the junctions and the contacts. The ion irradiation procedure for the ramp surfaces of the base electrode prior to counter-electrode deposition is crucially important. We prepared modified layers suitable to form the junctions and the contacts through the irradiation process. The preparation was achieved by controlling the ion incident angle against the ramp surface during irradiation. In a fabricated chip, proper operations of gradiometers crossing at right angles were independently confirmed without appreciable crosstalk by passing a current through feedback coils.