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

Abstract

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 $_{7-\delta }$ (YBCO). The arrays consisted of 84 SQUIDs, each with loop area 25 $\mu \text{m}^{2}$ , connected electrically in series with a meander line structure. The entire array was contained in an area of just $\text{100}\,\mu \text{m}\times \text{100}\,\mu \text{m}$ . 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 $\mu \text{m}$ 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 $\mu \text{V}$ at 65 K and 1 mV at 52 K for the 20 and 5 $\mu \text{m}$ beam aperture, respectively. The magnetic field periodicity for both arrays was approximately 30 $\mu$ T per flux quantum.