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edfas.org 23 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 4 by most major SEM vendors. However, implementing an on-axis transmissiondetector canbe as straightforward as using a fluorescent screen and a digital camera (Fig. 3a). [9] Still, diffraction in an SEM presents a few challenges. For example, although it isn’t as easy as in a TEM or a STEM, changing between quasi-parallel illumination used to obtain sharp diffraction spots or rings and convergent illu- mination to obtain convergent beam electron diffraction (CBED) patterns or to emphasize Kikuchi scattering is pos- sible inanSEMby changing thebeamlimitingaperture size and the working distance (Fig. 2). To that end, beam con- vergence angles fromless than 1milliradian (i.e., ~ 0.06 de- grees) to more than 60 milliradians can be obtained in most SEMs. A recent detector development involved the adapta- tion of a conventional EBSD detector for transmission imaging. [11] Here, a phosphor centered on the optic axis is positioned below the sample (Fig. 3b). When electrons transmitted through the sample strike the phosphor, the electron scatteringdistribution (i.e., theDP) is replicated in the photon distribution emitted by the phosphor. Amirror and lens assembly is used to image the photon distribu- tion to a digital camera positioned outside the vacuum chamber. Although this detector has primarily been used for TKD applications, it can be used in a variety of ways for recording and combining DPs, including 4D STEM-in- SEM. For example, the DPs in Figs. 2 and 5 were recorded using this setup. A significant advancement of the phosphor-based detector was recently described. [12] Here, a digital micro- mirror device (DMD), a digital camera, and a photomul- tiplier tube (PMT) enable imaging and diffraction in one programmable STEM (p-STEM) detector (Fig. 3c). TheDMD comprises an array of micromirrors, each of which can be independently tilted towards the cameraor thePMT.When all mirrors are aimed towards the camera, the system is in diffraction mode and can be used for TKD or other diffraction-based analyses including 4D STEM-in-SEM. When user-specified mirrors are tilted toward the PMT, the system is in imaging mode where the photon signal reflected to the PMT is proportional to image intensity at each beam raster point. One benefit of this system is that images from any detector on the SEM can be used to choose where to collect DPs from on the sample. The user simply clicks on specific points or selects a region of interest (ROI) on the image, and DPs are automatically recorded at each position or ROI (Fig. 4). Those DPs can then be used on the fly to select specific spots for DF imaging, where the appropriate mirrors are tilted to the PMT for real-space imaging. Virtual aperture implementa- tion (i.e., tilting themirrors), can also be automatedby use of a suitable software script. The most recent development in detector technology for STEM-in-SEM is perhaps the incorporation of direct electron detection methods. [10] These small, pixelated, radiation cameras can be placed under the sample on Fig. 4 Transmission images and DPs obtainedwith the p-STEMdetector. (a) A bend center in stainless steel. (b) A Ni-Cr thin film depth profile standard. The top image shows a 010 milliradian BF image, the bottom a 50 to 100 milliradian ADF image. Inset numbers and crosses indicate positions where the diffractions patterns were obtained. (a) (b)