Micromachining of SrTiO₃ steps for high-T_c step edge junction dc SQUIDs

In this paper, we establish a model of the micromachined SrTiO₃ substrate steps for high-T_c direct-current (dc) superconducting quantum interference devices (SQUIDs). The step angle is determined by the local ion milling rate and re-deposition rate, which is caused by back sputtering of ion milling. At dynamic balance, the maximum possible step angle is predicted to be 75° with an ion beam incidence angle of 45°, which agrees well with the measured value of 71°. In order to obtain a better step sidewall profile, we consider the influences of Nb metal mask micromachining. To avoid a rounded angle at the step upper corner, the minimum thickness of the Nb mask should be 440 nm when the desired step height is 300 nm. At optimized process conditions, steps with sharp, steep angles, and flawless profiles have been fabricated. Nine of the twelve dc SQUIDs thus obtained exhibited resistively shunted junction current-voltage behavior and magnetic field modulation at 77 K.