Depth resolution in on-axis TKD 1097
Bhattacharyya & Eades, 2009) and (2) the use of a low inci- dent energy also reduces the penetration depth and improves the depth resolution (Ren et al., 1998). If the existence of a depth resolution, or selectivity, was
obvious for EBSD because electrons penetrate a bulk mate- rial only for a given depth anyway, the depth resolution in transmission with TKD was less obvious because electron– matter interactions occur across the whole thickness. Suzuki (2013) produced a first solid proof: an orientation map produced by TKD on a lamella matching the EBSD map obtained from the back face of the same lamella. More evi- dence of selectivity can be found in Trimby et al. (2014), where it is highlighted that many grains displayed in orien- tation maps are much smaller than the thickness of the lamella. Then, Rice et al. (2014) showed with amorphous layers that the scattering of prime importance for the for- mation of Kikuchi patterns occurs near the back surface. Although the depth resolution of the TKD technique was observed, it has not yet been quantified, and the mechanisms involved are not clearly identified. Therefore, this study
intends to quantify the depth resolution of the on-axis con- figuration of the TKD technique for the first time on cubic silicon, determine its dependence with incident energy and sample thickness, and propose a physical model. Because the depth resolution probably depends on the beam-sample- detector geometry, the depth resolution of the classical TKD configuration (i.e., with a detector vertical like for EBSD) might be different (and better), as outlined by Trimby et al. (2014).
EXPERIMENTAL
On-Axis TKD Set-Up This study was performed with a field emission gun SEM Zeiss Supra 40 (Zeiss, Oberkochen, Germany) equipped with an on-axis TKD system that consists of a Bruker e-Flash1000 (Bruker) camera mounted on a Bruker OPTIMUSTM (Bruker, Billerica, MA, USA) detector head. The specificity of this detector head is that the scintillator is set horizontally instead of vertically like in the conventional TKD system
(Fig. 1). This on-axis detector was first designed, built and tested at LEM3 (Fundenberger et al., 2016) and is now commercialized by Bruker under the name OPTIMUSTM. With this on-axis configuration, the sample is set perpendi- cular to the electron beam, with the horizontal scintillator set underneath it in order to collect the electrons transmitted through the sample. The on-axis TKD in SEM is thus similar to the orientation mapping from Kikuchi diffraction devel- oped for TEM (Fundenberger et al., 2003; Morawiec et al., 2014). The diffraction patterns formed on the scintillator are then captured by the CCD camera, which remains vertical in the camera block, via a mirror inclined at 45°. The main advantage of this configuration is that the scintillator is positioned in the direction of maximum signal intensity, enabling a very high acquisition speed (Yuan et al., 2017), while the lateral resolution is expected to be at least as good as conventional TKD (Brodu et al., 2017). With on-axis TKD, the distance between the detector and the sample is adjustable, typically in the range 5–30mm, which allows adjustment of the range of solid angle observed in reciprocal space. This distance is adjusted by tilting the full camera block. The consequence is that the scintillator does not remain exactly perpendicular to the electron beam, although it remains in the range 90°±2° degrees for a detector-sample distance in the range 5–30mm. The acquisition parameters are listed in the caption for each figure showing diffraction patterns.
Sample Design and Preparation
A focused ion beam (FIB) lamella was specifically designed and fabricated for the determination of the depth resolution in on-axis TKD. This lamella is composed of two twinned cubic silicon crystals, whose twin boundary makes a low angle relative to the surface. The point with such a lamella is that the in-depth position of the twin boundary inside the lamella is known for all locations, because the twin boundary is a straight plane. For the preparation of the lamella, a Si wafer presenting multiple twins was selected. Then, an area of the wafer containing a roughly perpendicular twin boundary relative to the surface was identified. The perpen- dicularity was simply evaluated from the twin boundary
Figure 1. On-axis transmission Kikuchi diffraction (TKD) configuration (left) and comparison with conventional TKD (middle) and electron backscatter diffraction (EBSD) (right) in scanning electron microscope (SEM). Note that the chamber holds two EBSD cameras simultaneously, each on its own port, one for EBSD and conventional TKD, the other one being dedicated to on-axis TKD.
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