Page 277 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

MECHANICAL SENSORS 257

Strain
induced
by load

Vertical
deflection

Figure 8.26 Piezoresistive measurement of the deflection of a cantilever beam

Doped silicon resistors (piezoresistors) can be made at a very low cost and have a strain
gauge factor that is much higher (~50 to 100) than that for metals (~2). However, it is
harder to control the exact resistance of the silicon piezoresistor and, more importantly,
its actual gauge factor is strongly dependent on both the doping level and the ambient
temperature. Consequently, an embedded temperature sensor is essential for a precise
measurement of the strain and hence any static displacement by this method. This problem
is not so critical in a dynamic structure where it is only necessary to measure the frequency
of oscillation; however, care is still needed because the deposition of the piezoresistor
may itself induce stress in the microstructure and cause a shift in its natural resonant
frequency!

8.4.5 Pressure Microsensors

Pressure microsensors were the first type of silicon micromachined sensors to be devel-
oped in the late 1950s and early 1960s. Consequently, the pressure microsensors represent
probably the most mature silicon micromechanical device with widespread commercial
availability today. The largest market is undoubtedly the automotive, and Table 8.9 shows
the enormous growth in the world market for automotive silicon micromachined sensors
from 1989 to 1999. The two most important silicon sensors are the pressure and microac-
celerometer (Section 8.4.6) sensors, with substantial growth expected for gyrometers
(Section 8.4.7), which will be used for navigation.

Table 8.9 Worldwide growth for automotive silicon micro-
machined sensors. From Sullivan (1993)
Year Revenue 0 Growth- Year Revenue Growth-
(MEuro) rate (%) (MEuro) rate (%)
1989 175 _ 1995 376 21
1990 283 62 1996 463 23
1991 323 14 1997 564 22
1992 321 -1 1998 679 20
1993 285 -11 1999 804.2 18
1994 312 10
a
leuro = $1.1 for September 2000