Local Suppression of Superconductivity in Niobium Films by Focused Silicon Ion Implantation

Abstract

Magnetic flux trapped in a superconducting circuit shifts the critical currents of nearby Josephson junctions and disturbs the operation of the circuit. The usual remedy is a moat etched through the ground plane to hold the flux away from the logic. We report direct write silicon ion implantation with a focused ion beam as a way to make moats without etching the film, by locally suppressing superconductivity in niobium (Nb). Single implanted lines were written across Nb electrodes in films of three thicknesses over two orders of magnitude in ion fluence, and the transition temperature, resistance, and critical current density of each line were measured. The transition temperature of the line falls with fluence while the surrounding Nb is unchanged, and the critical current density falls by about two orders of magnitude before the line becomes resistive at 4.2 K. The product of the critical current and the resistance of the lines follows a single power law in reduced temperature. This gives an estimate of the critical current of a line at any temperature from its transition temperature and resistance. A time dependent Ginzburg-Landau simulation indicates that an implanted region can capture and hold a vortex. From these results we suggest a starting ion energy and fluence for implanted moats in a 100 nm Nb ground plane at 4.2 K and estimate the equivalent broad beam fluence.