Page 280 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

260 MICROSENSORS

100 Urn
NMOS device

50 \im
500 urn 'Pressure inlet\ n+ Bottom capacitor plate

760 urn

-2.45
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Pressure (PSI)

Figure 8.28 Polysilicon capacitive pressure sensor: (a) cross section with integrated electronics,
(b) voltage response from a 100 um square diaphragm of thickness 1 um. From Kung and Lee
(1992)

Figure 8.29 A vertical resonant capacitive pressure sensor based on the torsional oscillation of a
strained bulk-micromachined structure. From Greenwood (1988)

pressure sensor proved to have excellent resolution (a few centimeters in air) and stability
(parts per million (ppm) per year) through the running of the resonator in a partial vacuum.
Accordingly, it is possible to achieve a high mechanical Q factor, here about 18 000 at a
pressure of approximately 1 Pa, and hence achieve very high pressure sensitivities.