Tantalum superconducting tunnel junctions for infrared photon counting

Superconducting Tunnel Junctions (STJ) are currently being developed as photon detectors for a wide range of applications. Interest comes from their ability to cumulate photon counting with chromaticity (i.e. energy resolution) from the Near InfraRed (NIR) to the X-rays wavelengths and excellent quantum efficiency up to 80%. Resolving power can reach more than 10 in the visible. Our main goal is to use STJ for astronomical observation at low light level in the near infrared. This paper relates mainly to two points: The improvement of the Tantalum absorber epitaxy and the development of a new version of the fabrication process for making Ta/Al–AlOx–Al/Ta photon counting STJ. The main features of this process are that pixels have aligned electrodes and vias patterned through a protecting SiO2 layer. This vias are then used to contact the top electrode layer. We use a double thin aluminum trapping layer on top of a 150 nm thick Ta absorber grown epitaxially.