At SRON,we have been working on the development of X-ray Transition Edge Sensor
(TES)devices to optimize the performance of a TES array that serves as a backup sensor
to the X-ray Integral Field Unit (X-IFU)onboard Athena. Athena is an ESA-lead X-ray
observatory,to be launched in 2028,with the aim of high-resolution spectroscopy enabling
the exploration of new horizons of science themes on the hot and energetic universe. The
main sensor of the X-IFU is a large array of TES calorimeters designed to offer 2.5 eV
spectral resolution with a 5¡Éimage pixel,corresponding to a pixel size of 250 ¦Ìm on
the side.

An X-ray TES microcalorimeter consists of three components: an X-ray absorber made of
high-quantum efficiency material(s),a sensitive thermometer that can measure a
temperature rise caused by the absorbed X-ray energy,and a thermal link to a
low-temperature bath. Especially the design and fabrication technologies of the X-ray
absorber play important roles in achieving the requirements for the X-IFU. We have
improved electroplating techniques for few microns thick bismuth and gold layers,which
can be used as X-ray stopping and heat diffusion layer in the absorber,respectively.

I will focus on the design and fabrication details of our X-ray TES devices in my talk.
Some preliminary characterization results with the devices of the first generation will
be also presented.