Page 155 - MEMS Mechanical Sensors

144 Pressure Sensors

condenser microphone is shown in Figure 6.28 [110]. This device consists of a
nitride/aluminum diaphragm and a boron-doped backplate with acoustic
holes etched through. Membrane materials successfully used include polysili-
con, both flat [111, 112] and corrugated [113, 114], nitride (as shown in Fig-
ure 6.28), and boron-doped silicon [115]. Other examples can be found in the
literature [106, 109, 116] including differential devices [117], acoustic arrays
of microphones [118], and hydrophones [119].
• Electret microphones are a form of capacitive microphone that utilizes a mate-
rial that holds a permanent charge. This avoids the need to dc bias a capacitive
device. The electret material is typically silicon dioxide, silicon nitride [107,
113], or Teflon [120]. Otherwise, the design and fabrication of these devices is
very similar to those of the capacitive microphones.
• Piezoresistive microphones consist of thin diaphragms with four piezoresistors
arranged as with standard piezoresistive pressure sensors described in Section
5.5.2 [121, 122]. These are not widely used due to their relatively low
sensitivity.
• Piezoelectric microphones utilize a thin-film piezoelectric layer deposited on
the top surface of a structure sensitive to acoustic pressures. As the structure
deforms, charge is generated. Microphones are a dynamic sensing applica-
tion and therefore well suited to piezoelectric sensing techniques. Example
membrane-based devices include bulk etched silicon nitride membranes with
thin-film ZnO and Al electrodes [123] or spin-coated P(VDF/TrFE) film [124],
a boron-doped etch stop defined diaphragm coated with a sol-gel layer of PZT
[125], and a nitride/parylene membrane with ZnO piezoelectric sensing ele-
ments [126]. This last device incorporates ZnO-coated cantilevers coated and
integrated by a 1-µm-thick parylene layer that forms the membrane. Piezoelec-
tric microphones based purely on cantilever structures have also been demon-
strated. Cantilever structures are more compliant than membranes and are
capable of larger displacements for a given acoustic pressure. The first device
of this kind used a sputtered thin-film ZnO layer on a nitride cantilever [127],
but later research on a similar structure demonstrated the improved sensitivity
of PZT films [128]. Piezoelectric microphones are capable of comparable sen-
sitivities to capacitive devices but suffer from higher noise levels.

Soundwaves

3
Si N 4 Al

+
pSi
Acoustic Air gap
holes

Figure 6.28 Typical condenser microphone. (After: [110].)