Page 100 - MEMS Mechanical Sensors

5.2 Piezoelectricity 89

metal foil strain gauges. The adhesives can contribute to a phenomenon called
creep, whereby the gauge can effectively slip and therefore produce false readings as
the adhesive softens with increasing temperature or over long periods of time.
Thick-film resistors, often used in hybrid circuits, have also been shown to be
piezoresistive. Their gauge factor is around 10, and therefore, they offer a sensitivity
between that of a semiconductor and foil strain gauge. The TCR is around 100 parts
per million (ppm) per degree Celsius and matching between adjacent resistors is
often less that 10 ppm/°C, making them well suited for use as active elements in
Wheatstone bridge circuits, which reduce the overall temperature sensitivity.
An associated effect that has been observed in semiconductors is the so-called
piezojunction effect, whereby a shift in the I-V characteristic of a p-n junction is
observed as a result of an applied stress. Although this is an interesting physical
effect, it has found little use in commercial micromachined devices.

5.2 Piezoelectricity

Certain classes of crystal exhibit the property of producing an electric charge when
subjected to an applied mechanical force (direct effect). They also deform in
response to an externally applied electric field (inverse effect). This is an unusual
effect as the material can act as both a sensor and actuator. It was first discovered in
quartz by Jacques and Pierre Curie in 1880. The physical origin of piezoelectricity
arises because of charge asymmetry within the crystal structure. Such crystals are
often termed noncentrosymmetric, and because of the lack of symmetry, they have
anisotropic characteristics. Owing to its symmetric, cubic crystal structure, silicon is
not, therefore, piezoelectric. Some crystals such as quartz and Rochelle salt are
naturally occurring piezoelectrics, while others like the ceramic materials barium
titanate, lead zirconate titanate (PZT), and the polymer material polyvinylidene
fluoride (PVDF) are ferroelectric. Ferroelectric materials are those that exhibit
spontaneous polarization upon the application of an applied electric field. In other
words, ferroelectrics must be poled (polarized) in order to make them exhibit
piezoelectric behavior. They are analogous to ferromagnetic materials in many
respects. Figure 5.3 shows how an applied force gives rise to an electric charge (and
hence voltage) across the faces of a slab of piezoelectric material.

3 (direction of polarization)
Piezoelectric material
2

Applied force
1

V Metal electrodes

Figure 5.3 An illustration of the piezoelectric effect. The applied force results in the generation of
a voltage across the electrodes.