Micro-Machining of a Cryogenic Imaging Array of Transition Edge X-Ray Microcalorimeters

Abstract

Future X-ray astronomy missions, like NASA’s Constellation-X and ESA’s XEUS (X-ray Evolving Universe Spectroscopy Mission) require detectors with very challenging specifications. The high energy (0.2 to 10 keV) spectrometer for XEUS aims at: high energy resolution, ΔE_FWHM< 2 eV at 1 keV and ΔE_FWHM < 5 eV at 7 keV, high absorption efficiency, > 90% up to 7 keV, a counting rate > 4 kHz, and an imaging capability of typically 32×32 pixels. The type of detector considered most promising is an array of voltage biased superconducting transition edge microcalorimeters, operated at sub-Kelvin temperatures. The array will be read out using SQUID amplifiers. Based on a succesful development of single pixel microcalorimeters, SRON recently demonstrated an energy resolution ΔE_FWHM< 4.5 eV at 5.9 keV combined with an effective time constant of 150 µs, a project was started to demonstrate this performance in a small array of 5×5 pixels. From the design options for imaging arrays considered the past two years, two options survived until now. Both require a large deal of surface and bulk micro-machining. We will present here possible fabrication routes for creating a metal pixel structure on a small perforated Sinitride membrane. Key element is the use of a sacrificial layer, which can be removed from front or backside of the wafer. Critical issues in the fabrication process will be identified.