Progress in developing close-packed X-ray microcalorimeter arrays of Mo/Au transition-edge sensors

X-ray microcalorimeters using transition-edge sensors (TES) show great promise for use in astronomical x-ray spectroscopy. We have obtained very high energy resolution (2.4 eV at 1.5 keV and 3.7 eV at 3.3 keV) in large, isolated TES pixels using Mo/Au proximity-effect bilayers on silicon nitride membranes. In order to be truly suitable for use behind an X-ray telescope, however, such devices need to be arrayed with a pixel size and focal-plane coverage matched to the telescope focal length and spatial resolution. This requires fitting the TES and its thermal link, a critical component of each calorimeter pixel, into a far more compact geometry than has previously been investigated. We are addressing the tradeoffs that must be made to accommodate a smaller sensor area and are exploring the range in thermal conductance that can be obtained by perforating the nitride membrane in a narrow perimeter around the sensor. We have already demonstrated that such compact pixel geometries can yield suitable thermal conductance values.