edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 4 4 EDFAAO (2023) 4:4-11 1537-0755/$19.00 ©ASM International® SUPERCONDUCTING X-RAY SENSORS FOR TOMOGRAPHY OF MICROELECTRONICS Joseph W. Fowler1,2, Zachary H. Levine3, Paul Szypryt1,2, and Daniel S. Swetz1 1Quantum Electromagnetics Division, National Institute of Standards and Technology Boulder, Colorado 2Department of Physics, University of Colorado, Boulder 3Quantum Measurement Division, National Institute of Standards and Technology Gaithersburg, Maryland joe.fowler@nist.gov INTRODUCTION Integrated circuits (ICs) are highly complex manufactured devices. They contain billions of 3D structures covering a wide range of size scales and are composed of multiple metallic, semiconducting, and dielectric materials. Individual transistor gates and their wiring can be as small as a few nanometers, while a complete die can exceed a centimeter across. To determine the internal structure and composition of an IC after it is manufactured is an important and extremely challenging problem. Many kinds of users could benefit from tomographic analysis of microelectronic devices. Manufacturers with an eye on process control and improvement could use 3D imaging, especially for new processes still in the development or research stage. Imaging could also help researchers to connect functional failures of ICs to the physical defects that caused them. ICs are often designed and manufactured by different organizations in different countries, which raises questions of hardware security assurance. A tomographic imaging system could help to detect disabled features, hardware trojans, counterfeit designs, or other deliberate design changes introduced just before the fabrication stage. Several technical challenges hinder tomographic x-ray imaging at the sub-micrometer length scales required to analyze an IC. An intense x-ray source must be confined to a spot not much larger than the desired resolution, while its position relative to an IC sample must be both measured and controlled with similar precision. Tomographic imaging at fine spatial resolutions also demands the detection of a very large number of x-ray photons, a requirement that grows rapidly with improving resolution. Specifically, a 3D measurement of x-ray attenuation through an optically thin sample of fixed total volume requires detection of total photons scaling as the inverse fourth power of the voxels’ linear dimensions. Two powers come from the increased number of narrower voxels across any fixed cross-section, and two more arise from the reduced attenuation signal in each, thinner voxel. At the same time, making x-ray sources smaller tends to decrease the photon production rate by at least the square of the resolution scale unless the intensity can be made to grow as the source shrinks. In short, even modest improvements in Fig. 1 Design of the Tomcat instrument, shown from the outside, left, and inside the vacuum chamber, right. Right-hand figure reprinted from Ref. 3 with permission.
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