Page 152 - MEMS Mechanical Sensors

6.5 MEMS Technology Pressure Sensors 141

Figure 6.26 Yokogawa differential resonant pressure sensor.

process associated with this device is particularly impressive. The beams are vacuum
encapsulated at wafer level using a series of epitaxial depositions, selective
etches, and finally annealing in nitrogen, which drives the trapped gases left by the
sealing process through the cavity walls or into the silicon. This leaves a final cavity
pressure of below 1 mTorr, and the resonator possesses a Q-factor of more than
50,000 [82].
Other similar devices have been fabricated using a variety of techniques includ-
ing silicon fusion bonding [83], surface-micromachined resonators on bulk etches
diaphragms [84], and more recently using SOI wafer technology [85] and entirely
surfaced-micromachined sensors [86]. Surface micromachining offers the opportu-
nity for using comb-drive structures to excite and detect lateral resonances, but the
polycrystalline materials used to fabricate the resonator are inferior to single crystal
silicon. An alternative coupling mechanism to using the resonator as a strain gauge
on the top surface of a diaphragm is to use a hollow structure open to the measu-
rand. Changes in the applied pressure alter the shape of the resonator and hence the
frequency shifts [87]. This approach means the media is in contact with the resona-
tor and this introduces a cross-sensitivity to media density changes in which will
shift resonant frequency in a manner indistinguishable from the pressure measu-
rand. This device has also been used to demonstrate burst operation of the resona-
tor, which involves exciting and detecting the vibrations at separate intervals [88].
Another pressure coupling mechanism has been demonstrated by Andrews et al.
[89], where the measured pressure surrounds the resonator. Squeezed film damping
effects, which vary with the pressure around the resonator, alter the resonant
frequency. This device is designed as a vacuum sensor for use between 1 Pa and
atmosphere.
Quartz is an attractive material for resonant applications given its piezoelectric
properties and single crystal material properties. The piezoelectric nature of quartz
simplifies the excitation and detection of resonant modes, and quartz is routinely
used in high-stability time-based applications. The main drawback associated with
quartz is the limited choice of micromachining options compared with silicon and