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Contact AFM

Optimization of Scanning Parameters

128 Dimension 3100 Manual Rev. D

Collect Deﬂection data with low feedback gains to ensure the piezo remains at a constant position

relative to the sample. In this case, the tip and cantilever are deﬂected by the features on the sample

surface. The output ﬂuctuations in the cantilever deﬂection voltage from the top and bottom

photodiode segments are recorded as a measure of the variation in the sample surface. Deﬂection

data is not automatically calibrated in units of distance. You must measure the sensitivity using the

procedures discussed in Chapter 13.

Deﬂection data collected with high feedback gains essentially equals the derivative of the height.

This is commonly referred to as the error-signal. The error-signal provides a sensitive edge-

detection technique and can be very helpful in visualizing ﬁne details in topography that are

difﬁcult to see in regular height data. Using two channels, you must capture both height and

deﬂection data simultaneously. Deﬂection (error-signal) data alone does not yield quantitative

height information.

8.3.2 Gain Settings

The Integral, Proportional, and LookAhead gains in the Feedback Controls panel determine the

feedback on the piezo height. The feedback loop keeps the deﬂection signal constant by adjusting

the height of the piezo tube. If the gains are high, as they should be for Height data, the piezo

height changes to keep the cantilever deﬂection nearly constant. If the gains are low, as they should

be for topographical Deﬂection data, the cantilever deﬂects from its nominal position as it

encounters features in the sample.

In general, set the gain settings as follows:

1. Set the Integral and Proportional gains to 2-3 to start scanning.

2. To optimize the gains for Height data, increase the Integral gain until the piezo begins to

oscillate, then eliminate the oscillations by reducing the gain with 2-3 clicks of the left arrow

key.

3. Repeat the process for the Proportional gain.

Note: Piezo oscillations typically cause high frequency wavy lines in the Realtime

image. Piezo oscillations are more easily observed in View > Scope Mode.

4. For Deflection data, engage the microscope with the gains high, then lower them as much as

possible without losing contact with the sample once the system begins scanning.

5. Set the LookAhead gain to 0.7 initially for samples with step-like features oriented

perpendicular to the fast scan direction. Otherwise, it should be left at 0.00.

Note: The LookAhead gain includes information from the previous scan line to

determine the current gain setting.

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Dell 3100 Spezifikationen Seite 148