Force Imaging
Force Calibration (Contact Mode AFM)
194
MultiMode SPM Instruction Manual
Rev. B
11.4.3 Advanced Techniques
Sensitivity Determination
The
De
fl
ection Sensitivity
allows conversion from the raw photodiode signal (in Volts) to
de
fl
ection of the cantilever (in nm), and is normally set from the
Force Calibration
mode. The
sensitivity must be calibrated before accurate de
fl
ection data can be obtained. Sensitivity is equal to
the inverse of the slope of the force curve while the cantilever is in contact with a hard sample
surface. Complete the following steps to calculate the sensitivity:
1. Obtain a good force curve on the display monitor using a hard sample so that the sample does
not deform.
2. Position the cursor on one end of the contact portion of the curve.
3. Click on the left mouse button to fix the line segment.
4. Drag the mouse to position the “rubber band line” parallel to the contact portion of the force
curve (see
Figure 11.4c
Set the Sensitivity Parameter
5. The second click on the mouse causes the system to calculate the slope of the line segment
and enter -1/slope as the Sensitivity in the panel.
6. A click of the right mouse button will remove the line segment from the screen.
If
Sensitivity
is calibrated on a material much stiffer than the cantilever, it measures the inverse of
the value of the AFM’s optical lever sensitivity; i.e., how many volts of de
fl
ection signal are
produced by a given de
fl
ection of the cantilever tip. The sensitivity will change for different
cantilever lengths and styles (shorter cantilevers give lower
Sensitivities
).
Sensitivity
will also
change with the position of the laser on the cantilever and the quality of the laser beam re
fl
ection
from the cantilever.
Note:
It is important to calibrate the
Sensitivity
parameter on a hard substrate as
described here BEFORE using the force curve’s vertical scale for quantitative
measurements.
Click and drag line parallel to plot
