Page 110 - An Introduction to Microelectromechanical Systems Engineering

Sensors and Analysis Systems 89

The hinge structure is simple, consisting of a plate and a support arm made of a
first polysilicon layer. A staple made of a second polysilicon layer captures the plate
support arm. The staple is anchored directly to the substrate. The fabrication util-
izes the polysilicon surface micromachined process introduced in Chapter 3. The
polysilicon layers are typically 2 µm thick. The sacrificial phosphosilicate glass
(PSG) layer is 0.5 to 2.5 µm thick. Etching in hydrofluoric acid removes the PSG
layer and releases the mechanical plate from the substrate. Recent designs incorpo-
rate mechanical levers that snap into grooves defined in the plate and permanently
lock the hinge in a vertical position.
In early demonstrations, the assembly process involved manually lifting each
plate into position using sharp probes. The process has recently evolved to rely on
self assembly by designing the hinges so that they lock in place when the movable
parts are at a particular angle relative to the substrate. Random agitation while rins-
ing in water swings the structures away from the substrate; when they reach a preset
design location, they latch and lock in position. Both manual and self-assembly
tasks remain tedious and must be automated in the future before hinge assembly
gains acceptance in a mainstream manufacturing environment.

Sensors and Analysis Systems

Pressure Sensors
The first high-volume production of a pressure sensor began in 1974 at National
Semiconductor Corp. of Santa Clara, California. Pressure sensing has since grown
to a large market with an estimated 60 million silicon micromachined pressure sen-
sors manufactured in 2001. Nearly all units use bulk micromachining technology.
Manifold-absolute-pressure (MAP) [14] and disposable blood pressure [15] sensing
are the two single largest applications. The vast majority use piezoresistive sense ele-
ments to detect stress in a thin silicon diaphragm in response to a pressure load. A
few designs use capacitive methods to sense the displacement of a thin diaphragm.
The basic structure of a piezoresistive pressure sensor consists of four sense ele-
ments in a Wheatstone bridge configuration that measure stress within a thin crys-
talline silicon membrane (see Figure 4.8). The stress is a direct consequence of the
membrane deflecting in response to an applied pressure differential across the front
and back sides of the sensor. The stress is, to a first order approximation, linearly
proportional to the applied pressure differential. The membrane deflection is typi-
cally less than one micrometer. The output at full-scale applied pressure is a few mil-
livolts per volt of bridge excitation (the supply voltage to the bridge). The output
normalized to input applied pressure is known as sensitivity [(mV/V)/Pa] and is
directly related to the piezoresistive coefficients, π and π (see Chapter 2). The
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thickness and geometrical dimensions of the membrane affect the sensitivity and,
consequently, the pressure range of the sensor. Devices rated for very low pressures
(less than 10 kPa) usually incorporate complex membrane structures, such as cen-
tral bosses, to concentrate the stresses near the piezoresistive sensors and improve
both sensitivity and linearity.
A common design layout on {100} substrates positions the four diffused p-type
piezoresistors at the points of highest stress, which occur at the center edges of the