Optimum deposition condition of Nb/Al multilayers for large-scale array detectors with superconducting tunnel junctions

Abstract

In order to develop fabrication processes for superconducting tunnel junction (STJ) array detectors for time-of-flight mass spectroscopy, we have focused on a roughness of film surface and a residual stress of each deposited film in a layer structure of Nb/Al/Al 2O 3/Al/Nb. The roughness of each film depending on pre-treatments of silicon substrates and deposition conditions of Al and Nb films was studied by using an atomic force microscope (AFM). The dependence of the residual stress of the Al and Nb films on Ar gas pressure was evaluated by using an interference microscope. It was found that the Al thin film has a very smooth surface with a roughness of 0.79 nm in case of deposition on silicon substrates cleaned by a wet and dry process and in contrast, the surface roughness of the Nb and Nb/Al film are the smallest of 0.4 nm and 1.0 nm, respectively, on the Si substrate cleaned by only a dry process. We found appropriate sputtering conditions at which Al and Nb films have the stress less than 10 MPa, respectively. Those low stress conditions have to be realized in order to achieve a high reproducibility of STJ characteristics at each process run. The optimum deposition conditions for Nb and Al films were obtained for the STJ fabrication.