LARGE AREA EBSD SCANS WITH DISTORTION CORRECTIONS

摘要

Some types of materials require electron backscatter diffraction (EBSD) mapping from large areas to get representative results because grain sizes are in the order of several ram's and statistically significant crystal preferred orientations require several hundred or thousands of individual grain measurements. Typical examples are large silicon wafers, geological thin sections, welding, etc. EBSD maps are acquired in x- and y-directions with a certain overlap and in the end stitched together to get a large area EBSD map. The larger the individual square maps the faster the total acquisition will be as there will be less stage moves and less recombination of areas. However, and especially at low magnification, the scanned area will be distorted due to the high EBSD tilt angle, typically 70° and this scanned area will have a trapezoidal distortion if no shape correction is applied. The distortion will increase with decreasing magnification. As a consequence, stitching such areas with trapezoidal distortion leads to mismatch between neighbour areas. To correct for this distortion a theoretical or an experimental calibration method could be applied. In the present case a calibration method has been developed to correct for the distortion. A calibration specimen with square grids has been used for the calibration procedure. The calibration specimen was tilted and the SEM parameters (magnification, tilt angle, high voltage, working distance) were set to the same experimental values as for the real EBSD scan (done after the calibration procedure). The EBSD detector should be in the in-position during calibration. The calibration procedure is carried out by marking 4 points on the distorted square grid. These 4 points must be the corners on a square grid and the points should be close to the 4 corners of the tilt corrected and dynamically corrected electron image. The uncorrected and corrected images are shown on Fig. 1. They were acquired on with the grid 70° tilted at a magnification of 33. The stage moves are applied either moving only x and y in the case of a eucentric stage with a stage 70° tilted or moving x, y and z if a pre-tilted specimen holder is used. In addition of this procedure, automatically acquired, indexed and reconstructed multi-area maps will be shown as on Fig. 2. The sample is a calcite marble from Elba in Italy, it contains, in particular, some other grains of dolomite and quartz.