Force Imaging
Force Calibration (Contact Mode AFM)
Rev. B
MultiMode SPM Instruction Manual
195
Note:
De
fl
ection
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
de
fl
ection voltage when the cantilever is off the sample surface. You can adjust the
Setpoint
to set
the normal de
fl
ection 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 de
fl
ection of the cantilever to minimize the
contact force of the cantilever on the sample. It is possible to get a negative de
fl
ection whenever the
cantilever sticks to the surface. To engage, you must the set
Setpoint
greater than the de
fl
ection
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-de
fl
ection point and
beyond, while viewing the force curve. You can adjust the setpoint (most often made more negative)
so that it lies between the
fl
at segment of the force curve which corresponds to the zero de
fl
ection
point, and the tip of the retraction scan where the cantilever pulls off the sample surface VCSmin
(see
). The contact force is at its minimum when VCSmin is on the centerline of the
de
fl
ection-signal axis (
Setpoint
= VCSmin).
In practice, VCSmin must be a little below the centerline because VCSmin is the point where the
cantilever pulls off the surface and operation at this de
fl
ection 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 con
fi
ned 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.
