Force Imaging
Force Modulation
Rev. D
Dimension 3100 Manual
241
13.6.5 Notes About Artifacts
It is possible to see artifacts in force modulation images that are not due to differences in elasticity.
Some artifacts to look for are outlined below:
Aliasing
Under some conditions, unwanted oscillations appear in the data due to aliasing of the
Drive
Frequency
with the image pixel rate. This problem can be eliminated by changing the
Drive
Frequency
by small increments. Use the arrow keys to change the
Drive Frequency
up or down
very slightly until the oscillation disappears. On some materials, shifting the
Drive Frequency
slightly (1-3Hz) below or above a resonant peak value may improve image contrast. Operators are
encouraged to experiment.
Edge Effects
Sometimes force modulation images show changes in amplitude at the edge of topographic features
like steps or bumps. These artifacts typically look like the derivative of the sample topography.
To see if a feature is an edge effect, try reversing the
Scan direction
from
Trace
to
Retrace,
for
example. If the contrast reverses or the amplitude change now appears on the other
side of the
topographic feature, then the amplitude change is likely due to the topographic edge, not
differences in elasticity. To minimize edge effects, reduce
Drive amplitude, Setpoint
or
Scan
speed.
Set the
Integral gain
and
Proportional gain
as high as possible without causing unwanted
oscillations.
Frictional Effects
Because the cantilever is held at an angle to the sample surface, the cantilever tip will slide laterally
(“skate”) as the tip pushes into the sample (see
). The amplitude of cantilever motion is
affected by differences in friction between two different materials. For this reason, force
modulation images may contain information about differences in local frictional forces. To reduce
the influence of frictional effects, use a smaller
Setpoint
or
Drive amplitude
. Or, if you are using
“Negative LiftMode” (see
), use a more positive
Lift
Height
.
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