edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 4 28 VOLTAGE CONTRAST WITHIN ELECTRON MICROSCOPY: FROM A CURIOUS EFFECT TO DEBUGGING MODERN ICs James Vickers, Blake Freeman, and Neel Leslie Thermo Fisher Scientific, Fremont, California jim.vickers@thermofisher.com EDFAAO (2023) 4:28-34 1537-0755/$19.00 ©ASM International® INTRODUCTION It has become commonplace to image nanometerscale features of modern integrated circuits (ICs) using charged-particle microscopes. Transmission electron microscopes (TEMs), which have sub-nanometer imaging resolution, routinely inspect devices extracted from modern ICs, and can resolve, for instance, the lattice of silicon atoms that form transistor channels. Meanwhile, scanning electron microscopes (SEMs) and focused-ion beam (FIB) microscopes routinely image IC features with low-nanometer resolution. Nanoscale features on ICs that have become too small to be imaged using optical techniques remain well resolved when using chargedparticle microscopes. Improved imaging resolution is reason enough to use charged-particle microscopes for characterizing and debugging modern ICs, but the interaction of charged particles with IC substrates has enabled many adjacent applications for IC-specific characterization and debug. In this paper, discussion will be limited to a set of applications that rely on voltage vontrast (VC) measurements in SEM systems, showing how VC measurements can probe electrical activity running at speeds as high as 2 GHz on modern active ICs. SECONDARY ELECTRON EMISSION There are many reasons why VC shows up in chargedparticle imaging, but fundamentally all VC begins with the creation of secondary electrons. In the field of VC-related IC debug and characterization, a SEM directs a focused beam of electrons at a device-under-test (DUT). This beam of primary electrons is raster scanned over the DUT and generates secondary electrons that are then emitted by the DUT. Capturing these secondary electrons forms the video values we “see” when looking at SEM images. Figure 1 shows a typical SEM image taken of a commercially available sub-10-nm FinFET device. Secondary electron generation results from the interaction of an energetic beam of particles striking a solid material such as the DUT. As the energetic primary electron beam strikes the DUT, some electrons scatter, elastically or inelastically, from shell electrons or atomic cores. Electrons that elastically scatter can emerge back from the DUT with energy comparable to their arrival energy, and these are known as backscattered electrons. Electrons may also scatter inelastically from shell electrons, losing energy on each scattering event, and creating secondary electrons within the DUT. Some of these secondary electrons may escape the DUT material, where they can be captured by a secondary electron detector within the SEM to form secondary-electron SEM images. By convention, secondary electrons are those electrons emitted with energy below 50 eV, whereas backscattered electrons form the remainder and have energies that can approach Fig. 1 Typical SEM image showing a commercially available sub-10nm FinFET device that has been deprocessed from the backside down to the active layer. Vertical features are the individual fins of the transistors.
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