We have developed a new calorimeter array to increase our collecting area by a factor of four. The 6×6 pixel device has a total area of 144 mm2, making it one of the largest X-ray microcalorimeter arrays yet constructed. A relatively thin high-z absorber consisting of a 0.7 μm HgTe layer supported on 15 μm high-purity silicon provides good efficiency up to photon energies of 1.5 keV. The heat capacity of this composite is low enough to obtain an energy resolution of ∼6 eV FWHM on the 2 mm×2 mm pixels when operated at a base temperature of 50 mK. The infrared blocking filters have also been improved. Room temperature radiation must be attenuated by about 9 orders of magnitude between 2 μm and 2 cm to avoid having photon shot noise dominate the detectornoise. Accomplishing this while maintaining a high transmission for very soft X-rays that can penetrate only a few μg cm−2 is a problem common to all soft X-ray calorimeters that observe external targets. We are constructing monolithic silicon two-layer support meshes with a 350 μm pitch front layer on a 5 mm pitch backing layer. These are 98% open and have >95% effective transmission over a 60° field of view, while providing robust support for 38 mm diameter filters consisting of 20 nm of aluminum on 50 nm of polyimide. Five of these filters in series provide the necessary infrared attenuation. Integral deicing heaters are ion implanted in the fine mesh to remove contamination when necessary.

The electrical conductivity in doped semiconductors in the strongly localized variable range hopping regime is currently explained as phonon-assisted electron hopping. While investigating the non-Ohmic behavior of doped silicon at temperatures of 0.05-1 K, we found strong evidence for the existence of separate temperatures for the electron and phonon systems analogous to the hot-electron effect in metals. This behavior cannot easily be explained by phonon-assisted hopping and seems to favor instead a direct electron-electron interaction at low temperature. A hot-electron model makes definite predictions for the dependence of the electrical conductivity on the bias power, the frequency dependence of the resistance nonlinearities, and for an additional noise term. We have made a systematic investigation of these quantities, and find all of them in good agreement with the model predictions over a wide range of parameters.

We present a geometrical model to describe excess electrical noise in transition-edge sensors (TESs). In this model, a network of fluctuating resistors represents the complex dynamics inside a TES. The fluctuations can cause several resistors in series to become superconducting. Such events short out part of the TES and generate noise because much of the current percolates through low resistance paths. The model predicts that excess white noise increases with decreasing TES bias resistance (R/RN) and that perpendicular zebra stripes reduce noise and alpha of the TES by reducing percolation.

The square-format 32-pixel microcalorimeter array at the focal plane of the high-resolution X-ray spectrometer on the Astro-E2 X-ray Observatory is the first of a new generation of silicon-based microcalorimeters. This array has numerous advantages over its predecessor, the bilinear array that was launched on Astro-E. Foremost among its benefits are: (1) the energy resolution is improved by a factor of two at 6keV (now 6eV FWHM), (2) the thermal time constant is a factor of two faster, and (3) each pixel has a Gaussian line response. We will discuss the design changes that have led to these and other advantages.

Microcalorimeter performance is limited by non-ideal effects that were not included in the standard theory of bolometers and microcalorimeters developed 20 years ago by Mather (Appl. Opt. 21 (1982) 1125). These include the hot-electron effect, absorber decoupling, thermometer non-ohmic behavior, and all related extra noise sources. Models that include these effects have been developed and can be used to optimize the design of microcalorimeters for best performance. The design of the array for the XRS detector on the Astro-E2 satellite was completely optimized based on the required performance and on the characteristics of the materials used. The characteristic heat capacity and thermal conductivity of all the detector components have been measured and the values have been used as input to the models to design the detector geometry for best performance. Mechanical modeling has also been carried out in parallel to ensure the mechanical integrity of the microcalorimeter. We report here the analysis involved in the optimization of the detectors, and the comparison between modeled and measured performance.

Two challenges facing the use of large area (2mm×2mm) bismuth absorbers for microcalorimetry are uncertainties in the heat capacity of bismuth and the effects of lateral heat conduction and position dependence due to the absorber's large size. We have measured the heat capacity of three Bi samples to be 0.3−0.6JK−1m−3 at 100mK. These absorbers also exhibit response variations as phonons created by an X-ray event at an absorber edge will take longer to propagate to the thermometer attachment point than those at the absorber center. This effect may degrade the detector's energy resolution if the propagation time is not very short compared to the thermometer time constant. We show that the response of the largest absorber varies by ∼4% across its area.

Journal Article Microelectronic Fabrication of Transition Edge Sensors Get access K L Nelms, K L Nelms University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar S Ali, S Ali University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar A Dosaj, A Dosaj University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar D Liu, D Liu University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar D McCammon, D McCammon University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar L E Rocks, L E Rocks University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar W T Sanders, W T Sanders University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar P Timbie, P Timbie University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar J E Vaillancourt J E Vaillancourt University of Wisconsin-Madison, Department of Physics, 1150 University Ave., Madison, WI, 53706-1390 Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 9, Issue S02, 1 August 2003, Pages 122–123, https://doi.org/10.1017/S1431927603440695 Published: 06 August 2003