Instruction manual

Force Imaging

Force Calibration (Contact Mode AFM)

192 MultiMode SPM Instruction Manual Rev. B

3. In the Channel 1 panel select Deflection as the Data Type.

4. Maximize the Data Scale parameter.

5. Adjust the Ramp size parameter to about 1µm.

6. If the tip does not reach the sample surface (for example, see Figure 11.4a, between points 2

and 3), slowly increase the Z scan start value.

7. As the Z scan start increases, the traces on the force curve move to the right.

8. Adjust the Setpoint parameter.

Note: Adjusting the Setpoint value moves traces up and down on the graph.

Decreasing the value moves the curve up while increasing the value moves the

curve down. (Remember that the green horizontal centerline of the graph is

equal to the Setpoint if the Data Center = 0, so that changing the Setpoint

actually shifts the graph up and down.) Typically, changing the Setpoint value

to -2 volts shows the desired features of the force curve. If it does not, decrease

the Setpoint value further.

Note: Sometimes the entire ramp will occur in the contact portion of the curve (see

Figure 11.4a, between points 3 and 5). If this occurs, increase the Ramp size

and/or decrease the Z scan start.

11.4.2 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 11.4b 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 11.4b).