Superconducting-normal metal interfaces produced by reactive ion etching

Abstract

We have used CHF3/O2 reactive ion etching to reliably change the superconducting transition temperature (Tc) of aluminum thin films by 10–50 mK. Microanalyses using scanning Auger and scanning electron microscopy (SEM) indicate that the etching process results in a surface layer of high fluorine and low oxygen contents. The analyses show that photolithography and etching can produce very sharp interfaces between etched (lowered Tc) and unetched (higher Tc) regions. This technique is applied to study two specific configurations. First, long strips of aluminum thin films are patterned with a periodic structure of alternating regions of high and low Tc. The resistive transition of these films shows a surprisingly long proximity effect with a length scale of more than 50 μm, nearly two orders of magnitude greater than expected from established theory. Second, an aluminum strip with a single S–N–S (high–low–high Tc ) structure possesses a resistive anomaly just above the lower Tc. This anomaly depends on the position of the measurement voltage probes relative to the interfaces and appears to have a length scale of several μm.