Veeco 004-210-000, Instruction Manual

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Force Imaging
Force Modulation
204 MultiMode SPM Instruction Manual Rev. B
11.6 Force Modulation
11.6.1 Introduction
This section describes the operation of force modulation mode, which you can use to image local
sample stiffness or elasticity. This method is useful for imaging composite materials or soft samples
on hard substrates where you can obtain contrast between regions of different elasticity. This
section assumes knowledge of operation of Contact Mode AFM in air (see
Chapter 6 ). It is useful,
but not essential, to have experience operating in TappingMode (see Chapter 7 ).
Force modulation measures local sample elasticity by oscillating a cantilever such that the tip
indents slightly into a sample. The tip indents soft materials more easily than harder materials. The
amount of cantilever de
fl ection is inversely related to the amount of indentation. For example, a
soft sample allows the tip to indent more deeply into the surface, resulting in a very small de fl ection
of the cantilever. A hard sample allows less indentation, with the cantilever de fl ected by a larger
amount. To measure the relative elasticity of the sample the system records the amplitude of the tip
de
fl ection versus position over the sample as depicted in Figure 11.6a .
Figure 11.6a Contrast Generated in Force Modulation Mode
Force modulation requires the use of a special optional cantilever holder, shown in Figure 11.6b .
This cantilever holder uses a piezoelectric bimorph to oscillate the cantilever against a sample
surface. The force modulation cantilever holder is similar to the standard tapping cantilever holder;
however, its piezo stack is much larger, allowing larger driving amplitudes.
Figure 11.6b MultiMode Force Modulation Tipholder
Large cantilever response
on hard material
Small cantilever
response on soft material
Soft Material
Large Indentation
Hard Material
Small Indentation
Bimorph