Biomedical Engineering Reference
In-Depth Information
Fig. 2.3. Scheme of force transducer operation. The function of the transducer is to measure the
force between the AFM probe tip and the sample surface.
directions across the sample surface. Section 2.2.1 describes in greater detail how piezo-
electric materials are configured to scan a probe in three dimensions.
2.1.2 Force transducers
The force between an AFM 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 increases 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. Typically, the force transducer in an AFM is a cantilever with
integrated tip (the probe), and an optical lever; however, there are several types of force
sensors that may be used in an AFM (these are described in Section 2.2.2).
2.1.3 Feedback control
The reason an AFM is more sensitive than a stylus profiler that simply drags a tip over the
sample surface, is that feedback control is used to maintain a set force between the
probe and the sample. As illustrated in Figure 2.4, the control electronics take the signal
from the force transducers, and use it to drive the piezoelectrics so as to maintain the
probe-sample distance, and thus the interaction force at a set level. Thus, if the probe
registers an increase in force (for instance, while scanning the tip encounters a particle on
the surface), the feedback control causes the piezoelectrics to move the probe away from
the surface. Conversely, if the force transducer registers a decrease in force, the probe is
moved towards the surface. Section 2.3.2 has a more detailed discussion of feedback
control methodologies in AFM.
2.1.4 AFM block diagram and requirements
In general terms the design of an AFM is as shown in Figure 2.5. The force transducer
measures the force between the probe and surface; the feedback controller keeps the force
constant by controlling the expansion of the
z
piezoelectric transducer. Maintaining the
tip-sample force at a set value effectively also maintains the tip-sample distance fixed.
Then, the
x-y
piezoelectric elements are used to scan the probe across the surface in a
raster-like pattern. The amount the
z
piezoelectric moves up and down to maintain the
