Veeco Dimension 3100, Manual

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Force Imaging
Force Calibration (Contact Mode AFM)
222 Dimension 3100 Manual Rev. D
13.4.2 Helpful Suggestions
To minimize or calculate the contact force between the tip and sample, it is important to obtain a
good force curve; however, there will be situations where you will not obtain a good force curve no
matter how much you adjust the
Ramp size , Z scan start and Setpoint . The following sections
discuss a few such situations. Suggestions on working in Force Calibration mode are included.
False Engagement
Figure 13.4c illustrates a force curve resulting from a falsely engaged tip. The photodiode receives
light reflected off the sample, causing an increase in the deflection signal until the signal equals the
setpoint and the system “engages” (even though the tip is not on the surface). Interference in the
reflected light causes the hump-shaped waveform.
The easiest way to correct a false engagement is to withdraw the tip, adjust the photodetector
positioner to make the top/bottom differential voltage more negative, then re-engage the tip. This
compensates for the slight slope in
Figure 13.4c ).
Figure 13.4c False Engagement (G Scanner)
Motor Control
Motor Control > Tip Up and Tip Down buttons provide coarse adjustment of Z center voltage.
With these buttons the SPM head moves vertically. The feedback loop causes the Z piezo to adjust
to compensate for the head movement. If you use
Tip Up , the Z piezo extends. If you use Tip
Down , the Z piezo retracts.
Note: The Step Motor function is generally used only when the scanning range of the
Z piezo is exceeded or when it is necessary to position a force measurement in
the center of the scanner range. Because the Z-axis leadscrew has some
backlash, it may be necessary to rotate the leadscrew several turns by clicking
on the
Tip Up or Tip Down buttons before obtaining movement.
Tip
Deflection
0.48 V/div
Z Position - 9.27 V/div
Setpoint
Retracting
Extending
Possible False Engage Points