Instruction manual

TappingMode AFM
Beyond Basics with Resonating Techniques
118 MultiMode SPM Instruction Manual Rev. B
7.5.3 Optimization of Scanning Parameters
Careful selection of the scan parameters is important to the successful application of the MultiMode
in TappingMode. In general, the effects of the various scan parameters are the same for the
TappingMode as they are for contact AFM mode. The user is encouraged to review Section 5.1.
which discusses parameter optimization for contact AFM.
This section focuses on parameters specific to TappingMode. As with other operating modes, the
optimal parameter selection depends greatly on the sample. All of the parameters in the Realtime
control panel are discussed in the Command Reference Manual. The user is encouraged to review
all parameter descriptions. This manual analyzes the effects of the most important parameters.
Data Type (Channel Panels)
Data type is the first parameter to set because the settings of other parameters depend on it. The
Data type parameter in the Channel 1, Channel 2 and Channel 3 panels selects the type of data
that is collected by the system. Height data corresponds to the change in piezo height needed to
keep the vibrational amplitude of the cantilever constant. Amplitude data describes the change in
the amplitude directly. The Sensitivity parameter in the Force Calibration control panel must be
determined, as described in the discussion of the Force Calibration mode, before Amplitude data is
accurate.
The scan parameters required to collect good Height data are different than the optimal parameters
for Amplitude data. To collect Height data while tracking the sample surface with minimal change
in the tip’s vibrational amplitude, the feedback gains must be high. The position of the piezo during
the scan reflects the height of the sample.
TappingMode microscopy should not be conducted with low feedback gain values, as this will
cause damage to both the tip and sample. The maximum amplitude of the cantilever oscillation is
not sufficient to track tall features.
Amplitude 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. With the dual screen mode it is possible to capture both Height and Amplitude
data simultaneously.
Gain Settings
The Integral, Proportional, and Look ahead gains on the Feedback Controls panel must be high
enough to force the feedback system to track the sample surface. When scanning in TappingMode,
the Integral and Proportional gains must be set to lower values than are used in the contact mode.
Oscillation usually occurs with Integral gains of 1-2. The Proportional gain can usually be set a
factor of 2-10 times higher than the Integral gain. To optimize the gains, 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 Real-time image. The LookAhead gain is typically
set at 0.0.