Preserving Nb Superconducvity in Thin Film Flexible Structures

Abstract

Similar to observations from other groups, we have observed degradation of the superconducting properties of Nb thin films that have been subjected to subsequent high temperature fabrication processes, such as curing of a passivating polyimide layer at 350°C. This degradation may be caused by interaction with material that diffuses into the Nb during the subsequent processes, and is the subject of ongoing research. To counteract these degradation effects, we have explored multiple materials as barrier layers to attempt to isolate and protect the Nb. The effectiveness of the barrier layer depends on the use of an appropriate layer stack that minimizes degradation in the superconducting thin film, is compatible with subsequent fabrication steps, and is stable and repeatable. We have investigated multiple material stack-ups to protect Nb-based superconducting thin film in flexible structures. We show that curing polymers above a certain temperature on top of a Nb layer can adversely affect the superconducting properties including critical transition temperature (Tc) and critical current (Ic). DC electrical characterization of patterned film test structures were carried out using a closed-cycle cryostat to determine Tc and Ic for the samples. Details of the fabrication processes, experimental procedures and performance results will be presented. Results of these experiments are expected to provide insight into possible materials stack-ups for packaging and interconnect structures for future cryogenic electronics systems.