Study of long-term stability of Mo–Cu bilayer films

Based on measurements over a period of more than 650 days, we investigated the long-term stability of superconducting Mo–Cu bilayer films in the laboratory environment. The samples are based on sputtered films, which were grown for fabricating transition-edge sensor (TES) arrays. The results show an increasing trend of their superconducting transition temperature ( T c ) and transition width with time. There appears to be a turning point between 150 and 200 d, after which the rate of increasing becomes smaller. We suspect that oxidation occurred at the interface of the bilayer, through exposed edges, until the edges are fully oxidized. If proven, the slower rising trend would reflect the subsequent evolution of the bilayer films through, e.g. further oxidation of the interface, which might change T c through the proximity effect. We quantified the proximity effect with our bilayer samples of different Cu-to-Mo thickness ratios. As a function of the thickness ratio, the measured T c ...