Page 118 - MEMS Mechanical Sensors

5.6 Actuation Techniques 107

5.6.2 Piezoelectric

As we have already seen, piezoelectric devices can be used for both sensor and
actuator applications. An applied voltage across the electrodes of a piezoelectric
material will result in a deformation that is proportional to the magnitude of the
voltage (strictly electric field). The displacement across a bulk sample of PZT with
an actuation voltage of several hundred volts, for example, is only a small fraction
of a micron. When such a system is scaled down to that of a typical MEMS actuator,
a displacement of several orders of magnitude less is obtained! For this reason, some
form of mechanical amplification is needed in order to generate useful displace-
ments. Such a device can be fabricated by depositing a piezoelectric film onto a sub-
strate in the form of a cantilever beam as shown in Figure 5.16. This type of
structure is referred to as a piezoelectric unimorph. The deflection at the free end of
the beam is greater than that produced in the film itself.
Piezoelectric actuators are often used in micropumps (see Chapter 9) as a way of
deflecting a thin membrane, which in turn alters the volume within a chamber
below. Such a structure is depicted in Figure 5.17. The device comprises two silicon
wafers bonded together. The lower wafer comprises an inlet and outlet port, which
have been fabricated using bulk micromachining techniques. The upper wafer has
been etched to form the pump chamber. The shape of the ports gives rise to a prefer-
ential direction for the fluid flow, although there is a degree of flow in the reverse
direction during pumping. So the ports behave in a similar manner to valves. An
alternative structure comprises cantilever-type flaps across the ports, but these often
suffer from stiction during pumping. When a voltage is applied to the piezoelectric
material, this results in a deformation of the thin membrane and hence changes the
volume within the chamber. This is depicted in Figure 5.17(b). Typical flow rates
are in the range of nanoliters to microliters per minute, depending on the dimension
of the micropump.

5.6.3 Thermal

Thermal actuation techniques tend to consume more power than electrostatic or
piezoelectric methods, but the forces generated are also greater. One of the basic

Piezoelectric material
Pump chamber

Si
Si
(a)

Si
Si
Out In
(b)
Figure 5.17 A simple micropump with a piezoelectric actuator: (a) the situation with no applied
voltage; and (b) the effect of applying a voltage to the piezoelectric layer.