
Force Imaging
Force Calibration (Contact Mode AFM)
Rev. B MultiMode SPM Instruction Manual 195
Note: Deflection Sensitivity can be expressed in terms of the photodiode voltage
versus the distance traveled by the piezo, or the photodiode voltage versus the
voltage applied to the piezo, depending on the setting of the Units parameter.
Force Minimization
Force Calibration mode allows minimization of the contact force of the cantilever on the sample
surface. The force curve clearly illustrates the relationship between the Setpoint and the cantilever
deflection voltage when the cantilever is off the sample surface. You can adjust the Setpoint to set
the normal deflection of the cantilever and, therefore, the normal force applied by the cantilever
during data collection.
You can run the microscope below the point of zero deflection of the cantilever to minimize the
contact force of the cantilever on the sample. It is possible to get a negative deflection whenever the
cantilever sticks to the surface. To engage, you must the set Setpoint greater than the deflection
voltage when the tip is not interacting with the surface. However, you can change the setpoint after
engaging.
In Force Calibration mode, you can lower the setpoint to the zero cantilever-deflection point and
beyond, while viewing the force curve. You can adjust the setpoint (most often made more negative)
so that it lies between the flat segment of the force curve which corresponds to the zero deflection
point, and the tip of the retraction scan where the cantilever pulls off the sample surface V
CSmin
(see Figure 11.4d). The contact force is at its minimum when V
CSmin
is on the centerline of the
deflection-signal axis (Setpoint = V
CSmin
).
In practice, V
CSmin
must be a little below the centerline because V
CSmin
is the point where the
cantilever pulls off the surface and operation at this deflection is unstable. Changing the Setpoint
option in the Feedback Controls panel changes the setting of the Setpoint parameter in its Image
Mode counterpart when you exit the Force Calibration Mode. After exiting, if the image looks
good, you can decrease the force further by lowering the Setpoint in small increments until the
cantilever pulls off the sample surface. Resetting the Setpoint to a value higher than the voltage
when the tip is not interacting with the surface, recaptures the cantilever. (Slowly adjust to a more
positive value until the tip is back on the surface.) Adjusting the Setpoint a few tenths of a volt
above the point where the cantilever pulled off provides a low contact force.
If a high initial contact force adversely affects the sample, engage the cantilever with a very small
scan size. Then, minimize the force while the tip is confined to a small area of the sample where it
experiences the relatively high initial engagement force. Once the force is minimized, increase the
Ramp size or offset the scan to a different area of the sample. However, keep in mind that if the
force is minimized in a smooth area of the sample, the cantilever may pull off when it translates to a
rougher part of the sample.
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