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edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 4 18 A BRIEF OVERVIEW OF SCANNING TRANSMISSION ELECTRON MICROSCOPY IN A SCANNING ELECTRON MICROSCOPE Jason Holm National Institute of Standards and Technology jason.holm@nist.gov EDFAAO (2021) 4:18-26 1537-0755/$19.00 ©ASM International ® INTRODUCTION Scanning electron microscopes (SEMs) and the transmission electron detectors used therein are widely available. Both are generally easy to use, making the col- lectionof imaging anddiffraction techniques referred toas scanning transmission electronmicroscopy in a scanning electronmicroscope (STEM-in-SEM)moreaccessible today than ever before. These techniques are well suited to a host of applications including, for example, nanoparticle metrology, [1] imaging beam sensitive materials, [2] grain texture studies, [3] and defect analyses. [4] In this article, some of the pros and cons of STEM-in- SEMand recent advancements in detector technology are described. To illustrate imaginganddiffractioncapabilities of STEM-in-SEM, a few applications are shown including 4 dimensional (4D) STEM-in-SEM, a recently developed method that leverages both real and diffraction space to obtain useful information. STEM-IN-SEM Scanning electronmicroscopes (SEMs) are ubiquitous in laboratories. The wide array of signals and the numer- ous detectors available to collect those signals for imaging and analytical studies make SEMs practically indispens- able. Many SEMs are also equipped with a focused ion beam (FIB) column that enables electron transparent samples to be extracted from specific locations in bulk samples. Additionally, the number of SEMs equipped with detectors enabling transmission electron imaging and diffraction is increasing rapidly, and for good reason. Combined, the SEM and its various detectors, a FIB, and a transmission detector provide a powerful combination of instruments for sample preparation, imaging, diffrac- tion, and analysis with practical length scales spanning nanometers to several centimeters. Moreover, these capabilities can be implemented without removing the sample from the SEM vacuum chamber. Although scanning transmission imaging was an early sample visualization mode in SEMs, [5] it was rapidly overshadowed by advancements in transmission elec- tron microscopy (TEM) and eventually higher energy (i.e., > 100 keV) scanning transmissionelectronmicroscopy (STEM). Within the last couple decades, however, STEM- in-SEM (referred to as such to differentiate it from higher energy STEM platforms) has seen an increasingly rapid revival starting approximately with the works by Morandi and Merli, who studied contrast in semiconductor multi- layer structures. [6] Shortly thereafter, transmissionKikuchi diffraction (TKD) methods in an SEM were described. [7,8] This technique is commonly used for phase and grain orientation mapping in crystalline samples. The order of magnitude improvement in spatial resolution reported in transmissionmode compared to conventional backscatter detection mode (i.e., ~ 2 nm compared to a couple tens of nanometers) made this technique especially appeal- ing for electron transparent samples. Advancements in solid-state detector technology for transmission imaging were happening simultaneously, and today every SEM vendor offers on-axis solid-state detectors for transmis- sion imaging. Some SEMs also havemultiple transmission detectors (Fig. 1). STEM-in-SEM is a logical combination of conventional SEM and higher energy STEM technology, and it makes sense to leverage developments made in other platforms to the SEM. For example, electron scattering physics is effectively the same at SEM energies as it is at STEM ener- gies provided the sample is sufficiently thin. Therefore, the extensive theory developed to understand transmission