Force Imaging
Force Modulation
210
MultiMode SPM Instruction Manual
Rev. B
14. Adjust the
Integral Gain
,
Proportional Gain
,
Setpoint,
and
Scan Speed
to obtain a good
topography (
Height
) image. For force modulation operation, set the
Integral Gain
and
Proportional Gain
to values of
1
-
10
and set the
Setpoint
as low as possible using the cursor
keys (or by typing in new
Setpoint
values) until the cantilever pulls off the surface and the Z-
center voltage jumps to -
220V
.
15. Record the
Setpoint
value where the cantilever pulls off the surface (the “pull-off value”).
16. Increase the
Setpoint
until the cantilever touches the surface, and an image appears. Then
decrease the setpoint to a value just above the value where the pull-off occurred.
Note:
If you adjust the
Setpoint
very close to the pull-off value, imaging perform
with the smallest and least damaging force. However, in this condition imaging
may be more unstable, as the cantilever may pull off the surface unexpectedly.
The setpoint value affects the force modulation contrast. Examine the contrast
of the force modulation image.
17. Optimize
Drive amplitude
for Force Modulation imaging. The amount of contrast and the
quality of both
Height
and
Amplitude
images depends on the
Drive amplitude
. In general,
increasing the
Drive amplitude
provides greater contrast in the force modulation image.
Note:
It is possible to set the
Drive amplitude
too high. If this occurs, the
Drive
amplitude
no longer increases the contrast in the amplitude image. Instead, the
overall contrast in the force modulation image remains roughly constant, but
you will observe more artifacts in the image. For example, if the drive
amplitude is too high, the force modulation image becomes contaminated by
“edge effects” or “friction effects.” Operate at the lowest drive amplitude that
gives suf
fi
cient contrast to examine the sample. Low drive amplitudes also help
extend the life of the cantilever tip and reduce sample damage.
Procedure for Optimizing Drive Amplitude
1. Start with a small
Drive amplitude
value of
50mV
.
2. Increase the
Drive amplitude
with the right arrow keys or type in new values. The contrast
of the
Amplitude
(force modulation) image increases.
3. Continue increasing the
Drive amplitude
until sufficient contrast appears in the
Amplitude
image. If the
Drive amplitude
increases to the point where contrast is no longer improving,
reduce the
Drive amplitude
slightly.
Note:
Force modulation contrast also depends on the
Setpoint
. In general, if using a
larger
Setpoint
(larger tracking force) use a smaller
Drive Amplitude
to obtain
good force modulation images without artifacts. If it is dif
fi
cult to obtain a clear
force modulation image that is free of artifacts, try reducing or increasing the
Setpoint
and then optimizing the
Drive Amplitude
.
4. Readjust gains (if necessary).
