Instruction manual

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 deflection constant. The
deflection setpoint determines the average deflection 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: Refer to the section on Tip Selection 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 Chapter 5.
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 Deflection 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 Deflection and the Setpoint and the spring constant of the
cantilever used. If the sample is very delicate, set the Vertical Deflection value
closer to the chosen Setpoint.