97 6
With square pixels, the shape of an object is faithfully trans-
ferred, as shown in. Fig. 6.5a, while non-square pixels in the
specimen scan result in distortion in the displayed image,
. Fig. 6.5b.
Note that for all measurements the calibration artifact must
be placed normal to the optic axis of the SEM to eliminate
image foreshortening effects (see further discussion below).
Using a Calibrated Structure in ImageJ-Fiji
The image-processing software engine ImageJ-Fiji includes a
“Set Scale” function that enables a user to transfer the image
calibration to subsequent measurements made with various
functions. As shown in. Fig. 6.6a, starting with an image
of a primary or secondary calibration artifact (i.e., where
“secondary” refers to a commercial vendor artifact that is
traceable to a primary national measurement calibration
artifact) that contains a set of defined distances, the user
can specify a vector that spans a particular pitch to establish
the calibration at that magnification setting. This calibra-
tion procedure should then be repeated to cover the range
of magnification settings to be used for subsequent measure-
ments of unknowns. Note that the calibration that has been
performed is only strictly valid for the SEM working distance
at which the calibration artifact has been imaged. When aBeam locations on specimen
and specimen pixels Beam locations in computer memory
and display pixelsl
Ll'bBeam locations on specimen
and specimen pixelsBeam locations in computer memory
and display pixelslLl. Fig. 6.5 a Careful calibration a
of the x- and y-scans produces
square pixels, and a faithful repro-
duction of shapes lying in the
scan plane perpendicular to the
optic axis. b Distortion in the dis-
play of an object caused by non-
square pixels in the image scan
6.3 · Making Dimensional Measurements With the SEM: How Big Is That Feature?