Operating instructions

AFM THEORY
72
In dynamic operating modes, the cantilever is excited using a piezo ele-
ment. This piezo is oscillated with a fixed amplitude at an operating fre-
quency close to the free resonance frequency of the cantilever. The repulsive
force acting on the tip will increase the resonance frequency of the canti-
lever. This will cause the vibration amplitude of the cantilever to decrease.
The vibration of the cantilever is also detected using the laser beam deflec-
tion system.
The measured laser beam deflection or cantilever vibration amplitude can
now be used as an input for a feedback loop that keeps the tip-sample inter-
action constant by changing the tip height. The output of this feedback
loop thus corresponds to the local sample height.
An image of the surface is made by recording the sample height as the tip is
scanned over the sample surface in the x and y direction. The direction of
the x- and y-axes of the scanner is shown in the figure Scanner coordinate
system. The scanner axes may not be the same as the measurement axes,
when the measurement is rotated, or when the X or Y measurement plane
is changed. Therefore, the image x- and y-axes are denoted by an asterisk to
avoid confusion (i.e. X*, Y*).
The sample structure image is now obtained by recording the output of the
height control loop as a function of the tip position.
Cantilever: 228 µm long micro-fabricated silicon cantilever with integrated tip