Microsc. Microanal. 23, 1096–1106, 2017 doi:10.1017/S1431927617012697
© MICROSCOPY SOCIETY OF AMERICA 2017
Depth Resolution Dependence on Sample Thickness and Incident Energy in On-Axis Transmission Kikuchi Diffraction in Scanning Electron Microscope (SEM)
Etienne Brodu,1,2 and Emmanuel Bouzy1,2,*
1Laboratoire d’Etude des Microstructures et de Mécanique des Matériaux (LEM3), UMR CNRS 7239, Université de Lorraine, 57045 Metz, France 2Laboratory of Excellence on Design of Alloy Metals for low-MAss Structures (DAMAS), University of Lorraine, 57045 Metz, France
Abstract: Transmission Kikuchi diffraction is an emerging technique aimed at producing orientation maps of the structure of materials with a nanometric lateral resolution. This study investigates experimentally the depth resolution of the on-axis configuration, via a twinned silicon bi-crystal sample specifically designed and fabricated. The measured depth resolution varies from 30 to 65nm in the range 10–30 keV, with a close to linear dependence with incident energy and no dependence with the total sample thickness. The depth resolution is explained in terms of two mechanisms acting concomitantly: generation of Kikuchi diffraction all along the thickness of the sample, associated with continuous absorption on the way out. A model based on the electron mean free path is used to account for the dependence with incident energy of the depth resolution. In addition, based on the results in silicon, the use of the mean absorption coefficient is proposed to predict the depth resolution for any atomic number and incident energy.
Key words: Depth resolution, mean absorption coefficient, transmission Kikuchi diffraction (TKD), electron diffraction, scanning electron microscope (SEM)
INTRODUCTION
Transmission Kikuchi diffraction (TKD) in scanning electron microscope (SEM) is a recent technique derived from electron backscatter diffraction (EBSD) (Keller & Geiss, 2012; Sneddon et al., 2016). With the TKD technique, Kikuchi diffraction is produced in transmission with electron-transparent samples and patterns are typically recorded with a phosphor screen mounted on a charged-coupled device (CCD) or comple- mentary metal-oxide-semiconductor camera (a promising emerging alternative is by direct detection (Wilkinson et al., 2013; Vespucci et al., 2015, 2017)). The patterns are then indexed to produce orientation maps, from which exhaustive micro- structural information is available. The improvement in lateral resolution compared to EBSD
However, if one wants to determine the source volume of
material with which to associate the information contained in a diffraction pattern, knowing the lateral resolution is not enough, one has to determine the depth resolution as well. For this reason, the determination of the depth resolution has attracted a large interest, and was investigated in the frame of EBSD in SEM(Ren et al., 1998;Dingley, 2004; Zaefferer, 2007; Deal et al., 2008; Bhattacharyya & Eades, 2009; Wisniewski et al., 2017), for electron channeling pattern in SEM (Yamamoto, 1977) and since recently for TKD in SEM (Suzuki, 2013; Rice et al., 2014; Trimby et al., 2014). The depth resolution of EBSD was investigated experi-
enables the possibility of studying much finer microstructures, the lateral resolution of the TKD technique being below 10nm (Keller&Geiss, 2012; Trimby, 2012; Brodusch et al., 2013; Brodu et al., 2017). The lateral resolution of the TKD technique is typically investigated either via Monte Carlo simulation, which can accurately determine the lateral spread of the electrons scattered in a material (van Bremen et al., 2016; Wang et al., 2016), or experimentally via the determination of the lateral distance separating two unconvoluted diffraction patterns (Trimby, 2012; Fundenberger et al., 2016; Brodu et al., 2017). From these studies, it was easily seen that the lateral resolution is at its best at high incident energy and low sample thickness.
*Corresponding author.
emmanuel.bouzy@
univ-lorraine.fr Received July 28, 2017; accepted October 27, 2017
mentally via the use of thin amorphous layers deposited on bulk materials (Zaefferer, 2007; Wisniewski et al., 2017). Alternatively, some authors propose to evaluate the depth resolution from the angular width of the Kikuchi lines (Dingley, 2004; Zaefferer, 2007). A few values of depth resolution are available. They range from about 10nm (Dingley, 2004; Zaefferer, 2007) to several tens of nano- meters (Wisniewski et al., 2017). Then, Deal et al. (2008) showed via filtering experiments, following a suggestion by Ren et al. (1998), that the electrons contributing the most to the Kikuchi patterns are low-loss electrons. From this result, it was deduced that the source volume for Kikuchi diffraction is smaller than the whole interaction volume. Finally, two main options are investigated to improve the depth resolu- tion of EBSD: (1) energy filters can improve the depth resolution by selecting low-loss electrons with short path lengths and thus low penetration depths (Deal et al., 2008;
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