Page 141 - MEMS Mechanical Sensors

130 Pressure Sensors

maximize the sensitivity of the gauge. Metal gauges can be incorporated onto the
diaphragm face by bonding foil gauges or by depositing and patterning insulator
and metal materials using thin-film techniques such as sputtering or CVD [5].
Another resistive approach is the use of screen printed thick-film strain gauge resis-
tors. These can be printed on the top surface of a metal diaphragm, previously
coated with a printed dielectric layer, and offer improved sensitivity compared with
bonded strain gauges. Maximum resistive strain gauge sensitivity can be achieved by
bonding a silicon strain gauge to the metal diaphragm. This approach utilizes the
piezoresistive nature of silicon, which increases the output of the strain gauge for a
given deflection. The relative merits of these resistive methods and their associated
gauge factors are discussed in Chapter 5.
Other transduction techniques include capacitance, inductance, reluctance, and
piezoelectric. The capacitive approach uses the diaphragm as one electrode of a
parallel capacitor structure. Diaphragm displacement causes a change in capaci-
tance between it and a fixed electrode. Inductance can be used to monitor the
displacement of the diaphragm by mechanically linking it to the core of a linear vari-
able differential transformer (LVDT). This consists of a symmetrical arrangement of
a primary coil and two secondary coils. Movement of the magnetic core causes the
mutual inductance of each secondary coil to vary relative to the primary. Variable
reluctance transducers remove the mechanical link to the core and use the perme-
ability of the diaphragm material itself to alter the inductance within two coils posi-
tioned on either side of the diaphragm. The coils are typically wired in an inductive
half bridge, and a change in inductance alters the impedance of each coil unbalanc-
ing the bridge. Unbalances result in the ac drive signal being coupled across to the
output, and the physical arrangement is suitable for differential pressure-sensing
applications. Piezoelectric pressure sensors utilize a piezoelectric sensing element
mechanically linked to the diaphragm. Movements in the diaphragm induce a strain
in the piezoelectric and hence a charge is generated. These sensors are only suitable
for measuring dynamic pressures and are not suitable for static applications because
piezoelectric materials only respond to changing strains.

6.5 MEMS Technology Pressure Sensors

Research into solid-state pressure sensors began as far back as the 1960s [6–8]. Since
then there have been many developments both in micromachining and sensing tech-
niques, which have enabled MEMS pressure sensors to mature into a commercially
successful solution for many sensing applications. The mechanical sensor element is
typically (but not exclusively) a micromachined diaphragm. This section commences
with a brief analysis of rectangular silicon diaphragms. The different sensing princi-
ples employed to date will be introduced and illustrated with both commercially
available and research based devices. Finally, the state of the art in micromachined
pressure sensor technology will be discussed.

6.5.1 Micromachined Silicon Diaphragms
MEMS pressure sensors typically employ a diaphragm as the sensor element. This is
because of its compatibility with a range of bulk and surface silicon micromachining