Fabrication of X-Ray Microcalorimeter Focal Planes Composed of Two Distinct Pixel Types

Abstract

We are developing superconducting transition-edge sensor (TES) microcalorimeter focal planes for versatility in meeting specifications of X-ray imaging spectrometers including high count-rate, high energy resolution, and large field-of-view. In particular, a focal plane composed of two sub-arrays: one of fine-pitch, high count-rate devices and the other of slower, larger pixels with similar energy resolution, offers promise for the next generation of astrophysics instruments, such as the X-ray Integral Field Unit (X-IFU) instrument on the European Space Agency's Athena mission. We have based the sub-arrays of our current design on successful pixel designs that have been demonstrated separately. Pixels with an all gold X-ray absorber on 50 and 75 micron scales where the Mo/Au TES sits atop a thick metal heatsinking layer have shown high resolution and can accommodate high count-rates. The demonstrated larger pixels use a silicon nitride membrane for thermal isolation, thinner Au and an added bismuth layer in a 250 micron square absorber. To tune the parameters of each sub-array requires merging the fabrication processes of the two detector types. We present the fabrication process for dual production of different X-ray absorbers on the same substrate, thick Au on the small pixels and thinner Au with a Bi capping layer on the larger pixels to tune their heat capacities. The process requires multiple electroplating and etching steps, but the absorbers are defined in a single ion milling step. We demonstrate methods for integrating heatsinking of the two types of pixel into the same focal plane consistent with the requirements for each sub-array, including the limiting of thermal crosstalk. We also discuss fabrication process modifications for tuning the intrinsic transition temperature (T) of the bilayers for the different device types through variation of the bilayer thicknesses. The latest results on these "hybrid" arrays will be presented.

Toward Large Field-of-View High-Resolution X-ray Imaging Spectrometers: Microwave Multiplexed Readout of 28 TES Microcalorimeters

“…Each TES sits on a SiN membrane to control the conductance to the thermal bath and has a 240 × 240 µm 2 overhanging Au/Bi X-ray absorber. Reference [11] presents fabrication details of the GSFC TES arrays. For the first array, the TESs are 140 × 140 µm 2 squares and the absorber-contact stem area is 'T'-shaped.…”

Update on X-Ray Microcalorimeter Arrays Based on Thermal MKIDs (TKIDs)

“…Originally, we tried using niobium and hafnium, respectively, but hafnium had too low a transition temperature, reacted with the niobium when the two were in contact, and both were incompatible with the piranha cleans we used to remove post-etch residues. The XR-9 fabrication process for the absorbers and the membranes is modeled on that developed by the group at NASA Goddard for their transition edge sensor based x-ray microcalorimeter arrays, [11]. Our version of this process involves five frontside mask steps and one backside mask step.…”

Development of a Ti/Au TES Microcalorimeter Array as a Backup Sensor for the Athena/X-IFU Instrument

“…The grid that consists of 240×240 μm 2 absorber regions with a gap of 10 μm is defined on top of the seed layer by using a positive photoresist mold. We use "Techni gold 25E RTU" electrolyte for Au electroplating [10]. The temperature of the solution is kept at 56 ℃ using a hot plate and agitation of 300 rpm is applied with a magnetic stirrer.…”