Instruction manual

Contact AFM Mode
Optimization of Scanning Parameters
100 MultiMode SPM Instruction Manual Rev. B
the height of the sample. Deflection data should be collected with low feedback gains so the piezo
remains at a constant position relative to the sample. In this case, the tip and cantilever will be
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. To collect accurate topographical data, the Data type parameter should be set
to Height in most instances.
Deflection data collected with high feedback gains is essentially the derivative of the height. This is
commonly referred to as the error-signal mode. The error-signal mode provides a sensitive edge-
detection technique. Using dual screen mode, it is possible to capture both Height and Deflection
data simultaneously.
6.5.2 Gain settings
The Integral, Proportional, and LookAhead gains in the Feedback Controls panel control the
feedback on the piezo height. The feedback loop tries to keep 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 will change to keep the cantilever deflection nearly constant. If the gains are low, as
they should be for topographical Deflection data, the cantilever will deflect from its nominal
position as features in the sample are encountered. In general, the Integral and Proportional gain
can be set to 2–3 to start scanning. 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 two or
three clicks of the left-arrow key. Repeat the process for the Proportional gain. Piezo oscillations
typically cause high frequency wavy lines in the Realtime image. For deflection data, engage the
microscope with the gains high, then lower them as much as possible once the system is scanning.
The LookAhead gain includes information from the previous scan line to determine the current
gain setting. It should only be used on samples with step-like features which are oriented
perpendicular to the fast scan direction. Otherwise, it should be left at 0.
6.5.3 Scan size and Scan rate
In general, the Scan rate must be decreased as the Scan size is increased. Scan rates of 1.5–2.5Hz
should be used for large scans on samples with tall features. High scan rates help reduce drift, but
they can only be used on very flat samples with small scan sizes.