Fabrication of X-Ray Microcalorimeter Focal Planes Composed of Two Distinct Pixel Types
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Date
2016-12-01
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Citation of Original Publication
E. J. Wassell et al., "Fabrication of X-Ray Microcalorimeter Focal Planes Composed of Two Distinct Pixel Types," in IEEE Transactions on Applied Superconductivity, vol. 27, no. 4, pp. 1-5, June 2017, Art no. 2300205, doi: 10.1109/TASC.2016.2633783.
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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Public Domain Mark 1.0
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Abstract
We develop superconducting transition-edge sensor
(TES) microcalorimeter focal planes for versatility in meeting
the 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 subarrays: 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 Instrument on the European Space Agency’s ATHENA mission. We
have based the subarrays 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 µm pitch, 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-µm2 absorber. To tune the parameters of each subarray 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 the heatsinking of the two types of
pixel into the same focal plane consistent with the requirements for
each subarray, including the limiting of thermal crosstalk. We also
discuss fabrication process modifications for tuning the intrinsic
transition temperature (Tc) of the bilayers for the different device
types through variation of the bilayer thicknesses. The latest results
on these “hybrid” arrays will be presented.