Page 150 - MEMS Mechanical Sensors

6.5 MEMS Technology Pressure Sensors 139

(UTI)). However, in order to reduce the effects of parasitic capacitance and achieve
higher performance devices, the pressure sensor should ideally be integrated with
electronics. This has been achieved by combining a bulk-etched device similar to that
shown in Figure 6.21 with basic CMOS circuitry [65, 66], but the more common
solution is to employ surface micromachining. Standard sacrificial surface
micromachining processes have been combined with CMOS capacitance measure-
ment circuitry in a number of devices [67–69]. A common theme with these sensors
is the use of an array of sensing diaphragms to increase the measured capacitance sig-
nal. In some instances, diaphragms with different pressure sensitivities have been
incorporated onto the same die in order to broaden the range of operation [70, 71].
A common application of capacitive pressure sensor arrays with integrated electron-
ics is intravascular blood pressure measurement [72] and intracranial pressure [73].
This last device was coated in a silicon elastomer, NUSIL, for reasons of biocompati-
bility. A discussion of biocompatible coatings is included in Chapter 4.
Similar devices to the surface-micromachined pressure sensors have also been
realized using SOI wafers [74]. These devices use the buried oxide as the sacrificial
layer, and the hole to allow the undercutting etch is located at the center of the dia-
phragm. The hole is sealed afterwards by silicon nitride deposition, which results in
a ring shaped diaphragm as described previously. The buried oxide also isolates the
diaphragm from the surrounding silicon, thereby reducing parasitic capacitances. A
cross-section of the device is shown in Figure 6.24.
Another more recent development is the integration of planar coils on the
capacitive pressure sensor chip. The capacitor and coil form a resonant LC circuit
the frequency of which varies with applied pressure. By integrating the coil on the
sensor chip itself, it can also be used to inductively couple power into the sensor chip
from an external coil. After energizing the sensor circuit, the external coil is used as
an antenna to detect the resonant frequency. This approach is attractive for wireless
sensing and can be used in applications where wire links are not suitable (e.g., harsh
environments). Several devices have been reported in the literature from different
research groups including two integrated devices using electroplated coils [75, 76]
and a prototype microsystem on a ceramic substrate with a printed gold coil [64].

6.5.4 Resonant Pressure Sensors

Resonant pressure sensors typically use a resonating mechanical structure as a strain
gauge to sense the deflection of the pressure-sensitive diaphragm. Resonant sensing
has been discussed in Chapter 5. The resonant approach is more technically chal-
lenging for a number of reasons discussed below, but it does offer performance
specifications beyond that achievable with piezoresistive and capacitive techniques.

Nitride seal
Diaphragm
Metal contact

Sacrifical oxide

Figure 6.24 Cross-section through SOI capacitive pressure sensor.