Thermal design issues and performance of microcalorimeter arrays at sub-Kelvin temperatures

We have produced 5×5 pixel arrays of microcalorimeters using bulk micromachining. Analysis of our data provides the thermal conductivity parameters of Si_xN_y 1 μm thick membranes at 100 mK. Moreover we find that the thermal transport at 100 mK in Si beams, with dimensions 1.25 mm × 0.35mm × 35μm (length × height × width) is dominated by ballistic phonons with a mean free path of 110 μm. These thermal parameters can be used for modelling future 32 × 32 pixel arrays. In addition we operated three pixels in a 5 × 5 array of microcalorimeters and find that the pixel to pixel reproducibility is very good. When used as an X-ray microcalorimeter individual pixels have a thermal decay time of 200 μs is and their energy resolution is between 6 and 7 eV for 5.89 keV X-ray photons.