The grain orientation is calculated from the Kikuchi pattern projected on the phosphor screen. Therefore, the orientation of the coordinate system which is fixed to the screen has to be exactly known with respect to the reference coordinate system fixed to the specimen, in particular the x direction in the specimen surface has to be perfectly aligned parallel with the phosphor screen and the specimen tilt must be known. The correct position of the specimen surface needs to be checked before any other measurement can be carried out.
The angle of specimen tilt and its rotation around the optical axis (direction of the non-deflected primary beam, z axis) is usually calibrated by using a <100> silicon crystal as a standard reference specimen [D. Dingley, 1984]. For a tilt angle of 70°30' out of the horizontal to the primary beam direction, the <114> pole falls on the pattern center in a direction normal to the optic axis, and for a tilt angle of 35°16' the <112> pole direction is normal to the primary beam. Hence the actual specimen tilt can readily be obtained from the spacing between the pattern center and the <114> or the <112> pole, respectively.
Caution: The surface of silicon wafers is usually inclined by several 0.1° to [001]. Check for and eliminate this inaccuracy by rotating the standard specimen about is normal.
Provision has to be made that the x direction in the specimen surface is perfectly parallel with the x direction, the lines, of the scanning field. Otherwise a rectangular scanning field is heavily distorted, due to the steep specimen tilt, to a trapezoid. The y loci on the specimen deviate from the supposed positions of a horizontal line by a factor of about (sin e * tan 70°30’) whereby e is the angle of deviation. Hence a static system calibration as well as the focus may not be correct. In this case the orientation data are not correct as well, unless the specimen is rotated by -e around the optical axis of the SEM. Modern SEM provide a scan rotation unit to align the scanning directions interactively.
If the focus is corrected dynamically across large scanned areas at low magnifications, the scanning field is usually rotating as well with increasing field strength of the probe forming lens.
Dynamic calibration can correct the orientation data for small misalignments e of the scanning field with respect to the x axis of the specimen.