Effects of the step structure on the yield, operating temperature, and the noise in step-edge Josephson junction rf-SQUID magnetometers and gradiometers

Abstract

Step-edge Josephson junction rf-SQUID magnetometers and gradiometers were made using YBCO films on LaAlO3(1 0 0) and SrTiO3(1 0 0) substrates. Designs with 150×150 μm2 loop and 3.6 mm diameter washer area for the magnetometers (230 pH), and 1.5 mm baseline and 1.5 mm diameter washer areas with a loop of 75×75 μm2 for the Gradiometers (490 pH) were used. Effects of the step structure on the yield, optimal operating temperature range, and the 1/f noise of the devices were investigated. The step structure was controlled using different ion beam etching (IBE) processes. The devices on LaAlO3 showed high sensitivity to the IBE parameters and the step structure while this was much less for the SrTiO3 substrate samples. This is mainly due to a considerable re-deposition of the substrate material on the step during the IBE process, in particular for LaAlO3, resulting in very low yield and high 1/f noise devices. The film structure at the step was also found to be essentially dependent on the step structure strongly affecting the 1/f noise of the devices. Using an optimized “combinational IBE” process, surface modified sharp steps were prepared resulting in high yield of low 1/f noise devices when combined with high quality YBCO film. A typical 1/f noise corner frequency of less than about 10 Hz with a white noise level of about 20 μΦo/Hz at liquid nitrogen temperature was obtained for these devices. The devices have shown stability over many thermal cycles and the time (over half a year since their fabrication) while kept at the room temperature environment. The operating temperature range of the devices was found to be controllable by the step depth and the film thickness for the steps.