Contact AFM Mode
Optimization of Scanning Parameters
100
MultiMode SPM Instruction Manual
Rev. B
the height of the sample. De
fl
ection 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
de
fl
ected by the features on the sample surface. The output
fl
uctuations in the cantilever de
fl
ection
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.
De
fl
ection 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
De
fl
ection
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 de
fl
ection 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 de
fl
ection nearly constant. If the gains are low, as
they should be for topographical
De
fl
ection
data, the cantilever will de
fl
ect 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 de
fl
ection 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
fl
at samples with small scan sizes.
