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edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 4 22 STEM-in-SEM is also considered a low-voltage elec- tron imaging and diffraction technique. Beam energy limits for transmission detectors range from a practical minimum of approximately 5 keV to at least 30 keV (i.e., themaximumbeamenergy inmodernSEMs). Somedetec- tors can operate at lower energy detection thresholds, but the practical low end depends on factors including detector sensitivity, exposure time, beam current, and samplemass-thickness. As with all transmission imaging, samples shouldbeas thinas possible, and this is especially important at low voltages. Benefits of lowering the beam energy include increased electron scattering probability, larger characteristic scattering angles, and little to no knock-on damage. For example, electron scattering by a sample is more likely at 30 keV than 100 keV, and the increased scattering probability can produce stronger image contrast, which can be useful for samples compris- ing low atomic number materials. Figures 2d-f show how characteristic scattering angles changewith beamenergy for an Au (001) foil. Notice how the spacing between dif- fraction spots increases as the beam energy decreases, and that numerous spots and streaks are visible around the primary reflections that aren’t commonly observed at higher energies. Because there are no lenses between the sample and the transmission detector in an SEM, the camera length (CL) is used for diffraction pattern (DP) magnification (i.e., compare Fig. 2a and Fig. 2f). CL is defined here as the distance between the sample and the effective image plane. [14] A typical CL range is approximately 2 to 25 mm depending on the size of the vacuum chamber and the sample stage. Ultimately, greater scattering probability means that more information can be gleaned froma sample provided that the sample is not damaged by the electron scatter- ing process. To that end, although the knock-on damage threshold formanymaterials is greater than themaximum SEM beam energy, [15] ionization damage may still be a concern in some instances. For example, zeolites are sus- ceptible to ionization damage at low beam energies. [16] The damage is observedmost readily as a reduction in DP spot intensity with increasing beam exposure. However, low-dosemethods canbe used tominimize beamdamage and still allow the collection of meaningful information. [17] DETECTORS Solid-state diode detectors are by far the most common transmission detector for STEM-in-SEM imaging. These devices generally enable BF, DF, and annular dark- field (ADF) imaging with concentric annular diodes. The annular diodes are commonly segmented into quadrants, and signals from the different segments can be combined to highlight material composition differences. High-angle annular dark field (HAADF) imaging for atomic number contrast (i.e., Z-contrast) is gener- ally feasible with these detectors. Provided that coherent scattering is largely absent from the HAADF signal, the image intensity, I, is approximately proportional to Z 1.8 . [18] One major benefit of the solid-state detectors is the high signal-to-noise ratio that lends itself to sharp, strong contrast images. Another benefit is the high bandwidth that can enable video recording in some instanc- es. Although they are intended for imaging, a simplemasking system can be used to glean diffraction information for applications such as texture or grain orientation mapping. [19,20] Except for TKD applications, detectors intended specifically for diffraction are not yet offered SCANNING TRANSMISSION ELECTRON MICROSCOPY (continued from page 19) Fig. 3 Schematics and images of various detectors for STEM-in-SEM. (a) An approach similar to viewingDPs by eye in a TEMwhere a camera is used instead of binoculars. (b) A modified TKD system for diffraction. (c) A modified TKD system for imaging and diffraction. The inset images showmirrors tilted to the PMT comprising a 2 x 2 matrix (bottom) and an annular aperture (top). (d) A small footprint direct electron detectormountedonamulti-stubsampleholder. Standoffs canbeaddedor removed to adjust the camera length (CL) of the 3-position sample holder. (a) (b) (c) (d)