
Force Imaging
Force Modulation
236 Dimension 3100 Manual Rev. D
4. Install the cantilever holder on the Dimension SPM head.
5. Align the laser on the cantilever.
Note: Methods for aligning the laser are discussed in Chapter 7.
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.
8. Find the Bimorph Resonant Frequency: The cantilever is oscillated by a small
piezoelectric bimorph mounted in the cantilever holder. For Force Modulation, oscillate the
bimorph at or near its resonant frequency. The bimorph resonance frequency is usually the
largest peak in the 5-30kHz range. This ensures the cantilever moves with sufficient
amplitude to produce elasticity contrast.
Note: You need to find the bimorph’s resonant frequency only once. (Resonant
frequencies are unique to each force modulation cantilever holder.) Once you
find the resonant frequency, write it down for future use. This Drive frequency
may be used as a starting place each time you perform force modulation
imaging; however, recheck the bimorph’s resonant frequency in free air using
Cantilever Tune each time you install a new cantilever.
a. Use a FESP or TESP cantilever. NP probes have confusing low frequency resonances.
b. Select View > Sweep > Cantilever Tune or click on the Cantilever Tune icon.
c. The display monitor plots the Frequency Sweep, showing cantilever oscillation
amplitude versus frequency. To set the parameters controlling the Frequency Sweep
plot, use the parameters in the Sweep Controls panel and the commands on the top
menu bar of the display monitor.
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