Image Calibration
So that the beam scan is calibrated accurately to the stage coordinate system, the user should utilize the “@” button to calibrate the beam scan based on an acquired image. The image calibration is performed by selecting 2 points on a diagonal in the image for which the stage coordinates are known (by centering the beam on those two stage positions).
A couple of points to consider when performing the image calibration:
· Scan rotation: not only does the scan rotation need to be set exactly (within 0.2 degrees or so) but the scan rotation need to be set to the stage orthogonal when the light optics are properly focused because the working distance changes the scan rotation.
· Scan speed: the scan speed affects the image shift significantly, especially in the X direction- the scan rotation (so long as it’s a horizontal reference) is not very affected. But the use of a slow scan speed is very important for accurate image calibration.
· Electron beam focus: the scan rotation rotates dramatically when adjusting the focus. Be sure you are focused properly before setting the scan rotation.
· Prior to any image calibration efforts it is desirable to check the beam orthogonality of the instrument on a calibration standard.