Tantalum STJ for photon counting detectors

Superconducting Tunnel Junctions (STJ´s) are currently being developed as photon detectors for a wide range of applications. These detectors are used because they are energy resolving photon counters, and they can be used from the infrared (2 μm) to X-ray wavelengths with good quantum efficiency (80%). At visible wavelengths, the spectral resolution of STJ´s can exceed one part in ten. The goal of this work is to develop STJ´s for use in low-light astronomical applications in the near infrared. This work emphasizes two points: improvement of the epitaxial tantalum absorber and the development of a new fabrication process for Ta/Al-AlOx-Al/Ta STJ´s. The main features of this process are that pixels have aligned electrodes and vias patterned through a protecting SiO₂ layer. These vias are used to contact the top electrode layer of the STJ. The AlOx tunnel barrier has symmetric Al-Ta trap layers on either side. The 150 nm thick Ta absorber is epitaxially grown on top of a thin Nb seed layer. Photon counting experiments with theses devices are presented at λ=0.78 μm. Digital filtering methods are used to compute the photon counting data in order to minimize noise.