Imaging Modes; Feedback And Imaging Control; Amplitude Feedback In Dynamic Modes - JPK instruments nanowizard afm Handbook

2. Imaging modes

2.1 Feedback and imaging control

The detection system measures the cantilever deflection as the tip is moved over
the surface by the scanning system. It is possible to scan laterally over the surface
without changing the height of the cantilever and just measure this deflection
signal. This is known as "constant height" imaging, but is not the most common
solution. The force applied by the cantilever depends on the deflection, so higher
parts of the sample will experience higher forces in this mode.
It is much more common to use some form of feedback loop to monitor the
cantilever response, and adjust the height of the cantilever accordingly to take
account of the changes in surface height. In this case, the base of the cantilever is
moved up and down over higher and lower parts of the sample. All parts of the
sample should now experience the same force, if the system is well set up, and the
maximum force can be controlled.
A "PI" controller is often used to control the imaging, which means that
proportional-integral feedback is used. The difference between the setpoint and
actual values is used to change the height position of the cantilever. There are two
values to set how the height position is updated; a time constant for the integrator
and a value for the proportional gain. These two values control how quickly the
feedback responds to a change in sample height. The actual values need to be
optimized for different imaging conditions, depending on the sample topography
and scan speed for example.
If a value of the cantilever deflection is selected then the feedback system adjusts
the height of the cantilever to keep this deflection constant as the tip moves over
the surface. Thus the microscope images using "constant force" rather than
constant height. When the deflection of the cantilever is used as the feedback
signal, this is usually known as contact mode imaging.

2.2 Amplitude feedback in dynamic modes

There are other ways of operating the system, using dynamic modes where the
cantilever vibrates, and this oscillation of the cantilever is measured rather than the
static deflection of the tip. There are different ways to excite the oscillations - the
cantilever substrate can be shaken directly, or a magnetic field can be used to
drive the cantilever itself if it is coated with a ferromagnetic layer. In aqueous
conditions, the most common technique is to drive the cantilever acoustically
through the liquid. In all these cases, however, the measurement of the cantilever
oscillation and control systems are similar, and the cantilever is usually driven
close to resonance.
In these dynamic modes, a setpoint amplitude is chosen, and the height adjusted
to match this amplitude through the feedback system. In addition to the height and
error signal information from this constant amplitude mode, the phase between the
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JPK Instruments NanoWizard Handbook Version 2.2a 5