Chapter 2
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Piezoelectric materials are available in a number of sizes and shapes.
Typically, the expansion coefficient for a single piezo shape is on the order
of 0.1 nm per applied volt. Thus, if the voltage used to excite the piezo
material is 2 Volts, then the material will expand approximately 0.2 nm,
or the diameter of a single hydrogen atom. Piezoelectric materials are used
for controlling the motion of the probe as it is scanned across a surface
in an AFM. Section 2.2.1 describes in greater detail how piezoelectric
materials may be configured to scan a probe in three dimensions.
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Force Transducers
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The force between a probe and a surface is measured with a force transducer. As illustrated in Figure 2-3, when the probe comes into contact with the surface, the voltage output from the transducer increases. It is important that the output of the transducer be monotonic and increase as a greater force is applied between the probe and surface. Force transducers may be constructed that measure forces as low as 10 picoNewtons between a probe and a surface. There are several types of force sensors that may be used in an AFM (presented in Section 2.2.2). |
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FIGURE 2-3 The force transducer outputs an electronic signal when the probe
interacts with forces from the surface. Left: An amplifier outputs a voltage, So.
Right: As the probe interacts with the surface, So increases monotonically. |
| Feedback Control |
Feedback control is used in AFM for maintaining a fixed relationship, or
force, between the probe and the surface. The feedback control operates
by measuring the force between the surface and probe, then controlling
a piezoelectric ceramic that establishes the relative position of the probe
and surface. Feedback control is used in many applications, Figure 2-4
illustrates the use of feedback control in an oven. Section 2.3 has a more detailed discussion of feedback control methodologies in an AFM. |
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