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_c) of the bilayers for the different device types through variation of the bilayer thicknesses. The latest results on these "hybrid" arrays will be presented.

Keywords: Arrays; X-ray spectroscopy; low temperature detectors; microcalorimeters; transition-edge sensors (TES).

Fig. 1

We have successfully yielded a hybrid array with two distinct pixel types. The SPA (red circle) in the center region is a 10×10 array of 50 μm pitch pixels with a measured T_c ~ 164 ± 3 mK under low ~ 50 μA bias. The SPA rests on a single 500×500 μm SiN membrane. The surrounding LPA has 250 μm pitch pixels each on their own SiN membrane with a measured T_c ~ 72 ± 2 mK.

Fig. 2

Close up of the inner SPA and some surrounding LPA pixels without the cantilevered absorbers. The SPA TES is 35 μm square and the LPA TES is 140 μm square. Select pixels have been wired up with Nb leads.

Fig. 3

SEM image at 45 degree inclination demonstrating our ability to successfully yield a hybrid array with two distinct absorber pixel types. An inner 47 μm square array of 4.0 μm thick all Au absorbers is surrounded by an outer 247 μm square array of 1.6 μm Au + 3.9 μm thick Bi absorbers pixels.

Fig. 4

We have successfully yielded a hybrid array with two distinct pixel types preserving a 3 μm gap between pixels for an 88.3% fill factor in the inner array of small pixels and 97.6% fill factor in the outer array of large pixels.

Fig. 5

The graph on top compares two step height measurements using a stylus profilometer of electroplated Au films under different bath conditions along a line crossing the 500×500 μm square SPA as seen in fig 3. The dips in the blue profile caused by the stylus crossing the dot stems were avoided by better placement of the stylus for the red profile. By carefully adjusting the bath conditions we were able to eliminate the “rabbit-ear” film growth and yield a uniformly thick Au layer over the area of the SPA. The bottom cartoon illustrates how E-field crowding can result in excessive film growth along the side wall of the resist mold and a rabbit-ear film thickness profile.

Fig. 6

Cartoon of the backside heat sinking concept for a hybrid microcalorimeter array of two pixel types (not to scale). We deposit Cu in rotation at a shallow angle (φ₁) to cover the sidewall of the SPA membrane, and then at a larger angle (φ₂) to coat the entire SPA membrane as well as the Si muntin support structure. Then we wire bond both the backside Cu and front side Au to a nearby thermal heat sink.

Fig. 7

Optical microscope image of a cross section demonstrating successful 3 μm Cu deposition on the 500×500 μm nitride membrane after deep etching through the backside of a 300 μm silicon wafer. This shows that we have the ability to thermally heat sink the entire inner small pixel array in our hybrid array design concept.