Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 55

sensors are transducers that convert one form of energy, such as mechani-
cal force, to another form of energy, usually an electrical signal. There are
several basic physical principals by which sensors function, including resis-
tive, magnetic, photoconductive piezoresistive, piezoelectric, thermocouples
and thermopiles, diodes, and capacitive. All of these sensing principles have
been successfully demonstrated in MEMS sensor devices. There are far too
many variations of MEMS sensors to review even a fraction of them reported
in the literature, and therefore we will limit ourselves to reviewing only a
few of the transduction principles used in the implementation of microsen-
sors. Readers are referred to [9, 11, 12, 37, 38] for more information.
A piezoresistive material is one in which the resistance is influenced by
applied mechanical strains [9, 11, 12, 37, 38]. This phenomenon is most promi-
nent in semiconductors where the strain induces changes in the electronic
band structure of the material, thereby making the carrier scattering rates
dependent on direction of transport. This effect can be used to make a variety
of sensors by placing the piezoresistive elements at a position where the strain
is maximized. A useful quality factor of piezoresistive materials is the gauge
factor (GF), which is given by the normalized change in resistance divided
by the strain. A higher gauge factor implies a more sensitive piezoresistor-
to-mechanical deformation. Silicon as a material can have a very high gauge
factor, approaching 200 in some special configurations. In comparison, metal
resistors typically have gauge factors of around 2. Piezoresistors are used
primarily as strain measurement sensors where the resistor (strain-sensing
element) is placed on a compliant surface or structure such as illustrated
in Figure 3.21 for pressure sensors and accelerometers. Note that microma-
chining allows the substrate to be selectively removed, which significantly
reduces the stiffness in the sensing region of the device. This allows the stain
to be maximized in a localized region of the sensor where the piezoresistors
are located, thereby also maximizing the sensitivity of the sensor device.
Typically, the piezoresistors are placed into a Wheatstone bridge circuit con-
figuration, although other configurations can be used as well. The fabrication
of two types of pressure sensors using the piezoresistive effect to transduce
the mechanical strain into an electrical signal is described below.

Diffused Resistors Diffused Resistor

Proof Mass

Pressure Sensor Accelerometer

FIGuRE 3.21
Two types of MEMS silicon sensors made using the piezoresistive effect in semiconductors.
Piezoresistors are formed by diffusing a suitable type and concentration of dopants into the
substrate material. The substrate is partially removed to reduce the mechanical stiffness where
the piezoresistors are located. This has the effect of increasing the strain under loading at these
locations and, more importantly, of increasing the sensitivity of the sensors.