edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 4 10 developments in pixelated, hybrid photon-counting x-ray cameras enable an interesting possibility. We will replace the TES detectors with a commercial detector of this type. This camera is in some ways the opposite of the cryogenic microcalorimeter camera: it gives up the energy resolution of the TESs in favor of large collecting area. It should increase the overall photon-counting rate by a factor of 1000 for signal photons, though the exact penalty that comes with the higher background rate remains difficult to assess through modeling. We expect to find that the optimal approach would blend the virtues of these two contrasting detection techniques. The best instrument might combine them; it might employ both a large-area camera for fast imaging and a camera based on TESs with excellent energy resolution that could exclude the more diffuse bremsstrahlung emission, allowing accurate measurement of the smallest features in a sample. Another reason to use the energyresolving TES camera along with a large-area camera is to enable pan-chromatic imaging. This possibility would mean using the energy spectrum of transmitted x-rays to determine the elemental composition of wiring and other features inside an IC sample. The TES spectrometer would disentangle elements by their characteristic absorption edges, identifying multiple materials and determining their distinctive distributions across an IC. Peering inside the complex 3D structures that make up a modern IC with x-ray tomography is not easy. It requires exquisite control over positioning, intense and tiny x-ray sources, and efficient detection of transmitted photons over large areas. Making the measurements in a laboratory rather than a synchrotron only amplifies the challenges. Superconducting detectors can play an important role with their ability to identify fluorescence emission and to distinguish multiple materials in a sample according to their x-ray transmission. We expect the continued refinement of sources, detectors, and positioning equipment to bring practical tomographic imaging of nanoscale structures to the laboratory in the very near future. ACKNOWLEDGMENTS Tomcat is the result of many years of effort by numerous scientists and engineers who designed, operated, and redesigned many aspects of the Tomcat tool. We are grateful to all the members of the Tomcat team at NIST Boulder Labs and Sandia National Laboratory, i.e., the authors of Ref. 19, as well as an earlier generation of collaborators at BAE Systems.[3] We thank Tom Gurrieri for the generous contribution of a sample IC and detailed information about its design. We also thank Kelsey Morgan for helpful discussions, Nate Ortiz for instrument drawings, and Dan Schmidt for the instrument photographs. The information, data, or work presented herein was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via agreements D2019-1908080004, D2019-1906200003, D2021-2106170004, and FA8702-15-D-0001. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, or the U.S. Government. REFERENCES 1. Z.H. 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