Biomedical Engineering Reference
In-Depth Information
Chapter 2
AFM instrumentation
In theory an AFM is a relatively simple instrument. However, constructing an AFM with
nanometre-scale resolution requires a considerable amount of sophisticated engineering.
The main components of an AFM are the microscope stage itself, control electronics and a
computer. The microscope stage contains the scanner (the mechanism for moving the
AFM tip relative to the sample), sample holder and a force sensor, to hold and monitor
the AFM tip. The stage usually also includes an integrated optical microscope to view the
sample and tip. Often, the stage is supported on a vibration isolation platform which
reduces noise and increases the resolution obtainable. The control electronics usually takes
the form of a large box interfaced to both the microscope stage and the computer. The
electronics are used to generate the signals used to drive the scanner and any other
motorized components in the microscope stage. They also digitize the signals coming
from the AFM so that they can be displayed and recorded by the computer. The feedback
between the signals coming out and going back into the AFM stage is handled by the
control electronics, according to parameters set via the computer. Software in the com-
puter is used by the operator to acquire and display AFM images. The user operates the
software program, and the relevant acquisition parameters are passed onto the control
electronics box. The computer usually also contains a separate program to process and
analyse the images obtained. A photograph of a typical AFM illustrating these components
is shown in Figure 2.1.
2.1 Basic concepts in AFM instrumentation
The three basic concepts that one must be familiar with in order to understand the operation
of an AFM are piezoelectric transducers (in AFM, often known as piezoelectric scanners),
force transducers (force sensors), and feedback control. Basically, the piezoelectric trans-
ducer moves the tip over the sample surface, the force transducer senses the force between
the tip and the surface, and the feedback control feeds the signal from the force transducer
back in to the piezoelectric, to maintain a fixed force between the tip and the sample.
2.1.1 Piezoelectric transducers
Piezoelectric materials are electromechanical transducers that convert electrical potential
into mechanical motion. In other applications, they may also be used in the opposite sense,
i.e. if a change is caused in the material's dimensions they will generate an electrical
potential. Piezoelectric materials are naturally occurring and may be crystalline, amorph-
ous or even polymeric, although the materials used for AFM are generally synthetic
ceramic materials. When a potential is applied across two opposite sides of the piezoelec-
tric device, it changes geometry. The magnitude of the dimensional change depends on the
 
 
 
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