Page 151 - MEMS Mechanical Sensors

140 Pressure Sensors

Resonant pressure sensors have been successfully commercialized, and these shall be
discussed in detail below to highlight the principles involved.
The technical challenges associated with resonant pressure sensors are as
follows:

• Fabrication of mechanical resonator structure on top of pressure-sensing
structure;
• In the case of silicon resonators, the incorporation of vibration excitation and
detection mechanisms;
• The vacuum encapsulation of the resonator to negate gas-damping effects.

The earliest MEMS resonant pressure sensor was developed by Greenwood [77]
and later commercialized by Druck [78]. A cross-section of the sensor is shown in
Figure 6.25 along with a plan view of the resonator and its mode of vibration. The
butterfly-shape resonator is attached via four arms to pillars that form part of the
diaphragm. As the diaphragm deflects, the angle on the arms causes the resonator to
be placed in tension and the resonant frequency to change. The two halves of the
resonator are coupled together via a small physical link and the arms are positioned
at node points in the optimum mode of operation. The resonator and diaphragm are
fabricated using the boron etch stop technique and the resonator driven electrostati-
cally and its vibrations detected capacitively via metal electrodes on the support
chip. A vacuum is trapped around the resonator by mounting the support chip on a
glass stem and sealing the end of the stem while in a vacuum. The assembly is then
mounted by the stem, which provides some measure of isolation from packaging
stresses (see Chapter 4). The resonator has a Q-factor of 40,000, and the sensor has
a resolution of 10 ppm and total error of less than 100 ppm [79].
Another successfully commercialized device was developed by the Yokogawa
Electric Corporation (DPharp, EJA series differential pressure sensor [80]). This
consists of two resonators located on a diaphragm, the differential output of which
provides the sensor reading [81]. The resonators are driven electromagnetically by
placing the device in a magnetic field and running an alternating current through the
structure. The pressure sensor arrangement is shown in Figure 6.26. The fabrication

Pressure
Diaphragm Resonator
Glass frit
bond
Support
chip
Vacuum

Nodes
Butterfly
resonator
Resonant mode

Nodes
Figure 6.25 Druck resonant pressure sensor.