Page 148 - MEMS Mechanical Sensors

6.5 MEMS Technology Pressure Sensors 137

temperature compensation, a second beam with piezoresistor is positioned along-
side but not coupled to the diaphragm. The diaphragm is a polysilicon layer that
coats the entire chip surface (except bond pads), thereby physically, electrically, and
thermally isolating the strain gauges and beams from the pressurized media.

6.5.3 Capacitive Pressure Sensors
Capacitive pressure sensors are typically based upon a parallel plate arrangement
whereby one electrode is fixed and the other flexible. As the flexible electrode
deflects under applied pressure, the gap between electrodes decreases and the
capacitance increases. The principles of capacitive sensing have been described in
Chapter 5. Capacitive pressure sensors were first developed in the late 1970s and
early 1980s [18, 48]. An early device, shown in Figure 6.21, consists of an ani-
sotropically etched silicon diaphragm with the fixed electrode being provided by a
metallized Pyrex 7740 glass die [49]. The glass and silicon die were joined using
anodic bonding at die level. This device demonstrated the main attractions of
capacitive sensing, these being high sensitivity to pressure, low power consumption,
and low temperature cross-sensitivity. The combination of materials and bonding
mechanisms demonstrated remain a common choice for capacitive sensors [50, 51].
All silicon devices fabricated by silicon fusion bonding [52, 53] and glass frit bond-
ing [54] have also been reported along with many surface-micromachined devices,
which are discussed below. An example of an all-silicon fusion bonded device is a
vacuum sensor developed by NASA [55]. This sensor uses a circular diaphragm and
–1
demonstrates a sensitivity of ∼1 pF mbar . Quartz has also been used to realize
micromachined capacitive sensors [56]. This technology uses fused quartz compo-
nents laser-welded together, and the fixed electrode is another diaphragm that is
free to deflect but does not experience any pressure (see Figure 6.22). This means it
is free to deflect under acceleration and will therefore move in the same manner as

Figure 6.21 Early silicon/Pyrex capacitive pressure sensor.

Pressure

Acceleration
Figure 6.22 Acceleration compensated quartz capacitive pressure sensor.