Micrometer Scale Y–Ba–Cu–O SQUID Arrays Fabricated With a Focused Helium Ion Beam

We present the fabrication and testing of microarrays of superconducting quantum interference devices (SQUIDs) fabricated from the high-transition temperature superconductor YBa
₂
Cu
₃
O
_7-δ
(YBCO). The arrays consisted of 84 SQUIDs, each with loop area 25 μm
²
, connected electrically in series with a meander line structure. The entire array was contained in an area of just 100 μm × 100 μm. The Josephson junctions in the arrays were directly written into YBCO thin films with ion irradiation from a focused helium ion beam. We found that the electrical properties can be controlled by changing the diameter of the ion beam through selection of either a 20 or 5 μm beam aperture. In particular, larger aperture or beam size results in a weaker link junction with higher operating temperature and smaller critical voltage. The voltages across the dc-biased arrays were measured as a function of magnetic field. The data exhibited maximum voltage modulations of 300 μV at 65 K and 1 mV at 52 K for the 20 and 5 μm beam aperture, respectively. The magnetic field periodicity for both arrays was approximately 30 μT per flux quantum.