EBSD is a microanalysis technique used in the SEM in order to obtain crystal-lographic information from crystalline materials. It can be used to deduce the crystal structure, identify phases, reveal texture, etc. When high energy elec-trons strike a specimen surface, back scattered elecelec-trons are generated which undergo Bragg diffraction. The intersection of the diffraction cone and a phos-phor screen placed near the specimen produces a pattern of Kikuchi lines called
as electron backscatter patterns (EBSP). This is used as the basis for identi-fication of the orientation of the crystal and the phase. The EBSD detector consists of a phosphor screen that can be moved inside the SEM chamber to-wards the sample which is place at a steep angle (70°) to the normal plane of the beam (Figure 4.1). Typically, a voltage of 20 kV is used under high current conditions in order to generate a high number of backscatter electrons. The lateral resolution of this technique is limited by the backscatter resolution of the microscope due to the larger interaction volume. Unlike other SEM tech-niques the horizontal and vertical resolutions are not the same for EBSD due to the high tilt angle.
Figure 4.1: Schematic of EBSD setup inside the SEM
EBSD was used for multiple purposes in this work. One was in the case of flat Cr2O3 cantilevers, where the orientations the the grains on the surface was measured so that it could be later compared with the images of the fracture surface and deduce the mode of fracture as transgranular or intergranular.
Focused ion beam (FIB) milling was used to prepare the surface for EBSD.
Low milling currents of 10-100 pA were used to mill away the rough surface of the grains at a gracing incidence angle to minimise ion implantation. For EBSD, a defect free surface is important, otherwise Kikuchi patterns will not be clear. The sample is mounted on an SEM stage using a 70° pre-tilted holder.
This can be done using stage tilt as well, but depending on the bulkiness of the sample and location of the detector, it can be difficult. It is recommended to mount the sample on aluminium stub using silver paste or using clamps.
The use of carbon tape can lead to mechanical drifts, especially due to the high tilt angle. The sample should be such that the surface where the data is recorded has to face the EBSD detector. The detector is usually retractable and can be brought in close to the sample after aligning the stage so that the sample faces the detector (see Figure 4.1). The maximum insertion distance
is recommended, but it depends on the geometry of the sample. The working distance was maintained close to 10 mm, but depends on the model of the detector and its position in the SEM chamber. The voltage is maintained at 20 kV and a high current condition is used to generate more electrons.
The size of the map and the pixel size (step size) is decided based on the feature being analysed. Since the oxide grains were a mixture of sub-micron to few micron sized grains, a fine step size of 20-50 nm was used. Once the area of interest is chosen and the image parameters are adjusted, the stage is not moved again and the sample rests in the chamber for a while to stabilise.
This is followed by mapping and the time required for mapping depends on the exposure time for the EBSD camera, step size and the area being mapped. The phase being analysed need to be known since the software does a comparison of the projected Kickuchi patterns to its database to identify the orientation.
The results only consist of orientations and pixel positions. The patterns need to be saved for each pixel if a reanalysis needs to be done. The obtained raw data has to be processed to obtain meaningful results.
Processing of EBSD data can be done using software associated with the model of the detector being used. In our case, an Oxford detector was used, so their Channel5 software was used for most analyses. Different forms of maps can be obtained from the raw data. One map that is of very good use is the band contrast map, which is a map showing the Kikuchi band contrasts obtained for each pixel. The advantage of this is that even if orientations in certain pixels are not identified, they still have a band contrast value and the resulting map reveals the microstructure of the sample being analysed, even though the exact orientations are unknown, as in the case of the analysed Cr2O3cantilevers. An inverse pole figure (IPF) map was also used, which shows the sample directions (e.g. growth direction) in terms of crystallographic directions of the material.
This gives an idea on the crystallographic texture of the material. In addition to these, Euler maps can also be obtained which gives the absolute orientations of grains with respect to a reference coordinate system.
EBSD was also useful for identification of the exact orientation of single crystal Cr2O3 samples used in Paper III and IV. In this case mechanical prepara-tion of the surface was used. The purchased wafers were already polished.
Therefore, only the final polishing step, that the oxide polishing to remove the surface damage layer was done. The procedure of mechanical preparation by grinding and polishing is the same as the one described in the previous section.
For the oxide layers, obtained from oxidation exposures, transmission Kikuchi diffraction (TKD) was performed which provides a higher resolution compared to EBSD.