For EBSD measurements surfaces are usually investigated at high tilt angles, typically 70 degrees. However, at low magnifications (< x200) an ideally rectangular scanned ares degenereates into a trapezium so that remarkable distortions appear. Problems mainly arise when regular structures are mapped, or many mapped are stitched together in order to image huge areas in the scale of several centimeters. Is there any SEM which does not only correct the tilt (by stretching the image into one direction) and the the focus position (dynamic focus) but also the trapezoid distortion?
I'm not really sure what you mean by 'trapezoidal distortion' if it's something other than tilt angle correction and dynamic focus.
Here at Dartmouth College in New Hampshire, USA we've been using an older FEI XL-30 FEG SEM with EBSD attachments and software by HKL and Cambridge Software. The software, Channel 5, can make the tilt angle correction you're describing. But, the XL-30 operating software itself can also make the correction too. The version of Channel 5 we're running is a little old, but I'm sure the newest version is even more capable.
We use our system for metallurgy and also for studying polycrystalline ice. With ice, the grain size is quite large and we frequently work with magnifications less than 100x without significant distortion issues.
I hope this helps!
The trapezoidal distortion is the result of the deflection of the beam during scanning, Only in case that the sample would be scanned by beam directions which would be parallel for each scan position, the applied tilt correction would be perfect. This usually works in first approximation for higher magnifications. For lower magnifications - as you mentioned below x100 (actually it is not related to he magnification but to the beam deflection!) - you will get a map distortion so that for stitched maps the single sub-maps don't match anymore. Therefore, commonlly submaps are scanned with magnifications higher than x300 or bigger what increases the number of submaps dramatically.
For a more detailed explanation please see:
Image distortions in SEM and their influences on EBSD measurements. Ultramicroscopy (2007), 107(2-3),172-183
https://www.researchgate.net/publication/6780070_Image_distortions_in_SEM_and_their_influences_on_EBSD_measurements
Article Image distortions in SEM and their influences on EBSD measurements
Scanning deflection is made by a two stages system of deflection coils that are called upper and lower. In a perfect world they should be orthogonaly fixed in the column but, of course, they are not. There is a special circuit to correct that mechanical deviation which can be ajustable by service engineer. Do you think this can be the origin of what you call trapezoidal distorsion ? Of course this defect is more sensitive with large deflection of very low magnification.
As far as I know, the tilt correction only adapts the changing step width perpendicular to the tilt axis. The amount of correction is simply too big since a) in that case each position must be corrected separately. This is PERHAPS possible (although I doubt for imaging) but the market for such a huge and also quite powerful work is too small. However, the positioning for EBSD maps is actually o problem. But also her: if nobody complains no change :-).
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