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6.6 Microphones 143

sensing applications since they can be remotely operated by rf electromagnetic
waves. This approach is being explored commercially for type pressure-
sensing applications.
• MOS transistors can also utilize the piezoresistive effect to sense strain and
therefore pressure [100]. The piezoresistive effect alters channel carrier mobil-
ity and therefore the characteristics of the transistor [101].
• Inductive coupling has also been used on a MEMS pressure sensing by
micromachining two planar coils, one fixed beneath a diaphragm and the
other located on top of the diaphragm. An ac current is applied through the
primary coil on the diaphragm, and the induced current in the second coil var-
ies with applied pressure [102].
• Force balance is an established sensing principle whereby and actuating force
is applied to maintain the sensor structure in position during the application
of the measurand. Electrostatic actuation has been applied to diaphragm
structures for pressure-sensing applications. The actuating voltage required
provides a measure of the applied pressure [103, 104]. This approach compli-
cates the fabrication of the diaphragm since an actuation electrode is required
in addition to the diaphragm deflection sensing mechanism. However, this
approach can improve dynamic range and linearity [105].

6.6 Microphones

Microphones are a particular type of MEMS pressure sensor designed to trans-
duce acoustic signals into electrical output. MEMS technology is an attractive
approach for mass-producing miniature devices in, for example, hearing aid appli-
cations. Microphone diaphragms, or membranes, should be highly sensitive,
exhibit suitable dynamic behavior, and be packaged so as to remain insensitive to
static pressures [106]. Different membrane designs have been simulated and fabri-
cated including corrugated [107] and even one based upon the ear of the parasitic
fly Ormia ochracea [108]. This approach was adopted in order to mimic the direc-
tionality achieved by the fly’s ear. Immunity to static pressures is typically achieved
by ensuring both sides of the diaphragm are open to atmosphere, but only one
side is subject to the incoming acoustic pressure waves. The microphone die
is typically packaged within a chamber designed to tune the response of the
diaphragm.
The distinction between types of microphone is typically based upon the sensing
technology used to detect the membrane displacements. These can be summarized
as follows.

• Capacitive microphones (also known as condenser microphones) are the most
widely used form of MEMS device. They have demonstrated the highest
achievable levels of sensitivity and very low noise levels [109]. These devices
consist of a parallel plate-based capacitive pressure sensor with a flexible
membrane positioned in close proximity to a fixed electrode. This fixed elec-
trode, or backplate, is normally perforated with acoustic holes to minimize
damping and ensure suitable dynamic characteristics. A schematic of a typical