Design and development of a 224-pixel TES X-ray microcalorimeter system for microanalysis with STEM

Abstract Studies of astromaterials, including sample return missions such as HAYABUSA2 and OSIRIS-REX, provide valuable insights into the formation and evolution of the solar system. Astromaterials consist of both organic and inorganic materials containing various types and amounts of elements. To analyze such astromaterials on a sub-micrometer scale, one of the most useful tools is energy-dispersive X-ray spectroscopy (EDS) in conjunction with scanning transmission electron microscope (STEM). We developed an analysis method for determining the concentration of astromaterials using a transition edge sensor (TES) X-ray microcalorimeter. The conventional semiconductor-based EDS system is sometimes insufficient to resolve emission lines at closely adjacent energies. The TES X-ray microcalorimeter is a promising solution to overcome this problem. We developed a 64-pixel TES X-ray microcalorimeter array with two different-thickness absorbers in the same device for the broadband X-ray, which had an energy resolution of approximately 7 eV (FWHM) at an energy band from B Kα (183 eV) to Cu Kα (8 keV) [1]. However, the counting rate was only approximately 1000 count/(s∙array), which is not enough for astromaterial analysis, as a longer observation time will lead to sample damage. The counting rate was limited by the number of pixels and effective area.A simple solution to increase the counting rate is to use many pixels and a large absorber. An X-ray event from a sample was focused by poly-capillary X-ray optics to increase the solid angle of the specimen in the STEM-EDS system. We calculated the photon map on the detector surface using the Monte Carlo simulation of poly-capillary X-ray optics [2]. Based on the simulations, we arranged the pixels within a diameter of 4.8 mm, with 224 pixels packed tightly. By fabricating a cantilevered gold absorber to cover the dead space of the electrodes, we achieved a 15-fold increase in the effective area. For large absorbers,the residual resistivity ratio (RRR) is an indicator of thermal diffusion and limits the TES performance. We introduced an electroplating environment of gold and obtained an RRR of approximately 23. A large absorber was fabricated using this electroplating environment in conjunction with a double photoresist layer process. In a test chip, we confirmed the performance of one pixel, which had a critical temperature of 160 mK and an energy resolution of approximately 10 eV. In this paper, we provide more details of the Monte Carlo simulation, fabrication process of the overhang absorber, and device performance.