Page 117 - An Introduction to Microelectromechanical Systems Engineering

96 MEM Structures and Systems in Industrial and Automotive Applications

because it can be deposited under low tensile stress, and it retains its structural integ-
rity in most anisotropic etch solutions. The thin-film heaters and sense elements are
deposited next by sputtering a thin metal layer (e.g., platinum or nickel) or by the
chemical vapor deposition of a heavily doped layer of polysilicon. The thin metal
film or polysilicon are then patterned using standard lithography followed by an
appropriate etch step. An insulating passivation layer, preferably made of silicon
nitride, encapsulates and protects the heating and sense elements. Both silicon
nitride layers must then be lithographically patterned in the shape of the two sus-
pended membranes and consequently etched to expose the silicon regions outside of
the membrane outline. The final step involves the etching of the silicon in potassium
hydroxide or a similar anisotropic etch solution to form the deep cavity. The etch
first proceeds in the open silicon regions, and then it progresses underneath the sili-
con nitride thin film, removing all the silicon and resulting in the suspended silicon
nitride membranes. The reason the etch proceeds underneath the silicon nitride layer
is because its orientation is in the <100> direction. The etch stops on the {111} crys-
tallographic planes along the periphery of the open silicon areas.

Acceleration Sensors
The first demonstration of a micromachined accelerometer took place in 1979 at
Stanford University [17], but it took nearly 15 years before such devices became
accepted mainstream products for large-volume applications (see Table 4.3). The
overall market for silicon microaccelerometers has been steadily increasing, reach-
ing an estimated $319 million in 2000 [18] and driven primarily by the need for
crash sensing in airbag deployment systems. The increase in unit volume has been
accompanied by a steady decrease in pricing for automotive applications from an
estimated $10 per unit in the early 1990s to less than $2 per unit in 2002. Clearly,
low-volume pricing for custom designs remains well above quoted figures for the
high-volume automotive markets.

Table 4.3 Some Applications for Micromachined Accelerometers
Measurement Application
Acceleration Front and side airbag crash sensing
Electrically controlled car suspension
Safety belt pretensioning
Vehicle and traction control systems
Inertial measurement, object positioning, and navigation
Human activity for pacemaker control
Vibration Engine management
Condition-based maintenance of engines and machinery
Security devices
Shock and impact monitoring
Monitoring of seismic activity
Angles of inclination Inclinometers and tilt sensing
Vehicle stability and roll
Headlight leveling
Computer peripherals (e.g., joystick, head mounted displays)
Handwriting recognition (e.g., SmartQuill from British Telecom plc)
Bridges, ramps, and construction