Inductance Investigation of YBa2Cu3O7−δ Nano-Slit SQUIDs Fabricated With a Focused Helium Ion Beam

Focused helium ion beam material modification is ideally suited for the fabrication of YBa2Cu3O7-δ (YBCO) Josephson junctions and superconducting feature sizes down to the nanoscale. We report the fabrication and measurement of YBCO nano-slit SQUIDs and study how scaling the dimensions of the SQUID body and electrodes influences the electrical properties. Sixteen nano-slit SQUIDs with different width and length were fabricated from a single-layer 25-nm-thick YBCO film. The experimental results yield an estimation of the sheet inductance to be ~4 pH/□ and a penetration depth of ~180 nm. The temperature dependence of the inductance agree well with an impurity scattering model, indicating that the TC of the film was ~82 K, and the ratio between geometric inductance and kinetic inductance at 0 K was ~0.8. These results provide solid basis for the design of high-performance, high-TC micro-SQUID magnetometers.

I. Introduction

Focused helium ion beam (FHIB) materials modification is an emerging technique with the capability to make high-quality thin-film, high-transition temperature (high-T_C) superconducting Josephson junctions [1]. In this method, ion irradiation converts the film from a superconductor to an insulator by disordering the crystalline lattice [2]. In contrast to other techniques to fabricate high-T_C superconducting devices based on grain-boundary [3], step-edge [4], or multilayer c-axis sandwich Josephson junctions [5], no material is removed or etched, as a result, nanoscale insulating features are obtainable for both Josephson junctions [6] and superconducting electrodes for devices such as superconducting quantum interference devices (SQUIDs) [7]. Scaling down to smaller junction and SQUID loop dimensions are necessary for higher integration density of superconducting circuits and for improving performance [8]. The smaller dimensions obtainable in circuit geometry afforded by scaling yields more control over circuit parameters such as critical current I_C, resistance R and inductance L, the key variables for optimization of SQUIDs.