Force Imaging
Force Modulation
206
MultiMode SPM Instruction Manual
Rev. B
11.6.3 Operating Principle
Force modulation mode is very similar to Contact Mode AFM. The NanoScope system scans the
cantilever over the sample surface while trying to keep the cantilever de
fl
ection constant. The
de
fl
ection setpoint determines the average de
fl
ection during operation. In addition, the cantilever is
oscillated up and down by a piezoelectric bimorph in the tipholder so that the tip indents slightly
into the sample surface as it is scanned across the surface. The NanoScope system records the
amplitude of the cantilever, motion, which indicates the relative indentation of the tip into the
surface. For softer samples the tip penetrates further into the surface resulting in a smaller change
in the angle of the cantilever. A small change in angle results in a small measured amplitude which
displays as a bright area on the image. For harder samples the tip penetrates less into the surface
resulting in a larger change in angle of the cantilever. A large change in angle causes a large
measured amplitude which displays as a dark area on the image.
11.6.4 Force Modulation Procedure
This section gives instructions for operating in
Force Modulation
mode.
1. Choose the
Force Modulation
profile under
Microscope
>
Profile
.
2. Verify that the
Microscope
mode parameter in the
Other Controls
panel is set to
Tapping
and the
SPM Feedback
in the
Feedback Controls
panel is set to
TM Deflect
.
3. Load the special force modulation cantilever holder with a cantilever. The procedure for
loading a cantilever is exactly the same as for operation with the standard air cantilever
holder.
Note:
earlier in this chapter to help you choose
an appropriate cantilever.
4. Install the cantilever holder on the MultiMode SPM head.
5. Align the laser on the cantilever.
Note:
Methods for aligning the laser are discussed in
6. Adjust the
Setpoint
. Begin with a
Setpoint
of
0V
. Recall that higher setpoints cause the
cantilever to push harder on the surface during scanning.
7. Turn the photodetector adjustment knobs to center the laser spot on the laser detector. Adjust
the
Vertical De
fl
ection
to approximately -
0.5V
for FESP or TESP cantilevers, -
2V
for NP
cantilevers.
Note:
The force the cantilever applies to the surface is related to the difference
between the
Vertical De
fl
ection
and the
Setpoint
and the spring constant of the
cantilever used. If the sample is very delicate, set the
Vertical De
fl
ection
value
closer to the chosen
Setpoint
.
