These high quality calibration specimens provide a means of testing scanning
electron microscopes, scanning transmission electron microscopes (in the
secondary electron mode), and electron probe microanalysis systems
particularly for
magnification accuracy above 20X, for tilt specifications on the stage, for
image distortion and for the measurement of the depth of field. However
they are also usable for tests of stage stability, centering of stub
rotation, checks for symptoms of electrical and mechanical interference, etc.
Two metal (resin-backed) grating replicas, one of a 19.7 and the other of a
2160 lines/mm original are mounted on sample mounts and gold coated. Each
replica contains lines in two orthogonal directions with line frequencies
guaranteed to be within 1% of the stated figure on delivery. The specimens
are flat with a usable area of over 60 square mm, and were originally
developed by SIRA EM Techniques Group.
For more detailed information, the reader is referred to an early research
note: "A New Magnification Test Specimen for SEMs" by I. M. Watt and N. A.
Wright, Metron (UK) 3, No 6, 153-156 (1971).
Using the Specimen
1. Magnification Calibration
The 19.7 lines/mm sample (Figure 1) is suitable from about 20X to 1600X magnification
on the instrument "CRT"; the finer specimen ruled to 2160 lines/mm
(Figure 2) can be used
from 1000X and up. It is essential that the user first ensures that the
image distortion levels are within acceptable tolerances at the
magnification levels of interest before embarking on the magnification
calibration. In addition, on some instruments it is necessary to work the
magnification calibration routine in well defined and reproducible
operational conditions (e.g. working distances, KV). Further it is better
to work with zero tilt in the direction used for calibration. For example,
Figures 1 and 2 were recorded with X-tilt
on the stage but zero Y tilt. The calibration was performed
using only the vertical lines which are not disturbed by the X-tilt.
Magnification calibration routines should include checks on recorded images
since the recording video display unit (CRT) may give a different
magnification than the visual CRT.
The 19.7 lines/mm sample,
use from about 20-1600X
The 2160 lines/mm sample,
use from about 1000X
and upwards
2. Calibration of the stage tilt controls
At normal incidence of the electron probe on the specimen (zero X- and Y-
tilt), the unit cells in the pattern are square. They become rectangular as
the specimen plane is inclined to the electron-optical axis.
Figure 3 shows this effect for the case of the X-tilt, that is, tilt around
the Y-axis. The ratio of short to long dimensions of the rectangle is the
cosine of the actual tilt angle provided that the tilt axis is parallel to
one set of the ruling. The coarse (19.71 lines/mm) sample is recommended
for tilt-angle calibration.
Demonstration of the effect of X-tilt around the Y- axis
3. Observation of image distortion in the microscope
The microscopist might want to measure and/or correct the degree of image
distortion at various working distances, magnifications, and operational KV
levels, normal incidence Figure 4
is recommended. It is also important to remember
that distortions arise all along the imaging chain to the recording CRT.
Hence it should not be forgotten that recorded images are important at this
point and should never be over looked.
Measure and correct the degree of distortion at different working
distances, magnifications, and KV settings.
4. Measurement of depth of field
Use a moderate (30 - 40o) X-tilt to demonstrate
or check the depth of field by assessing the Z-depth for good focus. The
depth of field is higher when the final aperture is smaller, when the
working distance is longer, and when the SEM is operated in the lower
magnification ranges.
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