Controlled electromigration of thin Pb films started from the normal and from the superconducting state

Abstract

We have investigated the electromigration of thin Pb films with a weak link, performing the main sample fabrication steps in ultra-high vacuum and avoiding organic chemicals. In the normal state, upon a current flow, joule heating leads to an increased temperature and electromigration proceeds due to thermal activation. Cyclic controlled electromigration at 300 K and at 77 K can be well-described by a model used previously where the feed resistance and the power dissipated in the contact remain constant during the thinning process with the new addition of a parallel resistance. This arises because the Pb film is not laterally confined. Controlled electromigration relies on current-heating a sample. This seems contradictory to superconductivity, because the superconducting current does not dissipate energy in the sample. Here we discuss how edge effects can be used to heat the sample first over the superconducting transition temperature and subsequently heating can be used to thin the sample. The superconducting transition of the thin film occurs at well-defined voltages, i.e. well-defined heating currents in the feeds. The same conductance steps during the superconducting transition are found in current–voltage measurements as well as during electromigration cycles. Due to a weak link in the thin film, the current density and the temperature distribution is not uniform in the sample, and therefore several conductance steps are found.