Force Imaging
Force Calibration (Contact Mode AFM)
Rev. D
Dimension 3100 Manual
223
Adjust Photodiode
In an analogous manner, you can use the photodiode positioner as a coarse adjustment for
Setpoint
voltage. Changing the laser beam position on the photodiode by rotating the photodiode adjustment
knobs shifts the force curve on the graph. Rotating the top photodiode adjustment knob clockwise
shifts the curve up, just as decreasing the
Setpoint
parameter shifts the curve up. Conversely,
rotating the top photodiode adjustment knob clockwise moves the curve down.
13.4.3 Advanced Techniques
Sensitivity Determination
The
Deflection Sensitivity
allows conversion from the raw photodiode signal (in Volts) to
deflection of the cantilever (in nm), and is normally set from the
Force Calibration
mode. The
sensitivity must be calibrated before accurate deflection 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 13.4d
Set the Sensitivity Parameter
5. The second click on the mouse causes the system to calculate the slope of the line segment
and enter the -1/slope as the
Sensitivity
in the panel.
6. A click of the right mouse button will remove the line segment from the screen.
Click and drag line parallel to plot
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