Investigation of the effect of thermal cycling on the device performance ofYBa2Cu3O7−δ DC-SQUIDs

Abstract

We investigated the effect of thermal cycling on the operational performance ofYBa2Cu3O7−δ (YBCO) direct current superconducting quantum interference devices (DC-SQUIDs) fabricated onto24° SrTiO3 (STO) bicrystal substrates. The devices under investigation consist of directly coupledDC-SQUID magnetometer configurations. Thin films having 200 nm thicknesseswere deposited by dc-magnetron sputtering and device patterns were made bya standard lithography process and chemical etching. The SQUIDs having4 µm-wide grain boundary Josephson junctions (GBJJs) were characterized by means of criticalcurrents, peak-to-peak output voltages and noise levels, depending on the thermal cycles.In order to achieve a protective layer for the junctions against the undesired effects ofthermal cycles and ambient atmosphere during the room temperature storage, the deviceswere coated with a 400 nm thick YBCO layer at room temperature. Since the second layerof amorphous YBCO is completely electrically insulating, it does not affect the operation ofthe junctions and pick-up coils of magnetometers. This two-layered configuration ensuresthe protection of the junctions from ambient atmosphere as well as from theeffect of water molecules interacting with the film structure during each thermalcycle.