doi:10.1017/S1551929515000048 32 www.microscopy-today.com • 2015 March Beam Broadening in Transmission EBSD Katherine P. Rice, 1 * Robert R. Keller, 2 and Mark P. Stoykovich 3 1 Cameca Instruments, 5500 Nobel Dr. Madison, WI 53711 2 Applied Chemicals and Materials Division, NIST, 325 Broadway, Boulder CO 80305 3 Department of Chemical and Biological Engineering, University of Colorado, Boulder CO 80309 *katherine.rice@ametek.com Introduction Transmission electron backscatter diffraction (t-EBSD) [1] also known as transmission electron forward scatter diffraction (t-EFSD) [2] or transmission Kikuchi diffraction in the SEM (TKD-SEM) [3], can provide significant improve- ments in spatial resolution in many cases over conventional EBSD performed in reflection. Over the last few years, truly remarkable results have been demonstrated, including phase identification of nanoparticles as small as 5 nm [2], effective orientation mapping resolutions of 2 nm to 3 nm for thin metal films [3], and high-contrast patterns from films as thin as 5 nm in plan-view [4]. Perhaps the most exciting thing about this technique, however, is that it requires no special equipment beyond the conventional EBSD setup. The sample simply requires mounting such that the beam can pass through and the diffraction pattern can be collected by the camera, as shown in Figure 1 [4]. Some users mount the sample horizontally, and some tilt it at a shallow angle from horizontal, mainly to prevent shadowing effects on the camera. Te increase in spatial resolution achieved with this tech- nique has important implications outside the area of nanoma- terials and even can provide more information about the microstructure of traditionally prepared thin specimens or geological samples. In fact, most samples that are well suited for transmission electron microscopy (TEM) generally provide good results for transmission EBSD, including nanoparticles deposited from solution, electrolytically thinned bulk materials, and focused ion beam (FIB)-prepared lamellae. Materials and Methods One of the primary factors to consider while discussing spatial resolution in the transmission confguration is where the detectable difraction occurs within the specimen thickness; this afects the achievable lateral resolution and determines which regions of the specimen give rise to difraction patterns. We have demonstrated through use of bilayer samples that the detectable difraction occurs within the bottom few nanometers of the crystalline region of a specimen [4], that is, the surface that is closest to the EBSD camera. Figure 2 (reproduced in part from [4]), shows a crystalline Au flm deposited on two diferent thicknesses of amorphous Si 3 N 4 (a- Si 3 N 4 ). When the sample is mounted such that the beam passes through the Au frst, then exits the a-Si 3 N 4, patterns can be seen for the 20 nm a- Si 3 N 4 layer, but not for the 50 nm a- Si 3 N 4 flm. When the sample is reversed such that the beam enters the a- Si 3 N 4 layer frst and then exits the crystalline Au, patterns can be seen for both thicknesses of the amorphous layer. Difraction is occurring everywhere throughout the sample, but electrons that difract above the bottom surface generally do not maintain coherence because they have to travel through more material where they have the potential to re-difract or incoherently scatter. Tis experiment therefore suggests the patterns that are detected likely come from the bottom few nanometers of the sample. Armed with this information, we can guide our sample preparation techniques such that the region of interest is face-down toward the detector. Another practical note that should also be considered is that FIB-prepared samples can have a signifcant amount of ion-beam-induced damage (essentially an amorphous layer) at the surface of interest, and thus a low-kV cleanup may be necessary to maintain crystallinity in that region. Because the incident beam must traverse nearly the full thickness before undergoing detectable Kikuchi scattering, transmission EBSD is unlike conventional EBSD, where the detectable difraction occurs near the top surface [5]. As the beam travels within the sample, electron energies and trajectories change as a result of both elastic and inelastic scattering. Tese efects lead to broadening of the incident beam and can be estimated to a frst approximation by Monte Carlo scattering simulations [6]. Beam broadening has for some time also been a consideration in TEM, but the order- of-magnitude higher energies used in TEM imaging, and the specimen thicknesses typically used, make it less of a concern compared to STEM-in-SEM techniques. Results Figure 3 shows an example of the electron trajectories as they scatter throughout a copper flm. Te exit trajectories of the transmitted electrons are color-coded for the energy they have at each point, which leads to estimates about efective Figure 1: Experimental setup showing sample position for transmission EBSD. Downloaded from https://www.cambridge.org/core. IP address: 54.161.69.107, on 18 Jul 2020 at 01:41:39, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929515000048