
TappingMode AFM
Basic Principle of TappingMode
104 MultiMode SPM Instruction Manual Rev. B
7.1 Basic Principle of TappingMode
Figure 7.1a represents a cantilever oscillating in free air at its resonant frequency. A piezo stack
excites the cantilever substrate vertically, causing the tip to bounce up and down. As the cantilever
bounces vertically, the reflected laser beam is deflected in a regular pattern over a photodiode array,
generating a sinusoidal electronic signal. The signal is converted to a root mean square (RMS)
amplitude value, which is displayed in volts on the topmost (TappingMode Output Signal) meter
located on the front of the MultiMode base.
Figure 7.1b represents the same cantilever at the sample surface. Although the piezo stack
continues to excite the cantilever’s substrate with the same energy, the tip is deflected in its
encounter with the surface. The reflected laser beam (return signal) reveals information about the
vertical height of the sample surface and some characteristics of the sample material itself. These
material characteristics may include elasticity (hardness), magnetic and/or electric forces present.
Figure 7.1a Tapping Cantilever in Free Air
Figure 7.1b Tapping cantilever on sample surface. Note deflection of cantilever and return signal (exaggerated).
Return signal
Cantilever
Laser beam
Return signal
(deflected)
Laser beam
Sample surface
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