Page 408 - The Mechatronics Handbook

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Micro- and Nanoaccelerometers
By the end of the 1970s it became apparent that the essentially planar processing integrated-circuit (IC)
technology could be modified to fabricate three-dimensional electromechanical structures by the micro-
machining process. Accelerometers and pressure sensors were among the first IC sensors. The first
accelerometer was developed in 1979. Since then the technology has been progressing steadily and now
an extremely diverse range of accelerometers are readily available. Most sensors use bulk micromachining
rather than surface micromachining techniques. In bulk micromachining the flexures, resonant beams,
and all other critical components of the accelerometer are made from bulk silicon in order to exploit the
full mechanical properties of silicon crystals. With proper design and film process, bulk micromachining
yields extremely stable and robust accelerometers.
The selective etching of multiple layers of deposited thin films, or surface micromachining, allows
movable microstructures to be fabricated on silicon wafers. With surface micromachining, layers of
structure material are disposed and patterned as shown in Fig. 19.25. These structures are formed by
polysilicons and sacrificial materials such as silicon dioxides. The sacrificial material acts as an interme-
diate spacer layer and is etched away to produce a freestanding structure. Surface machining technology
also allows smaller and more complex structures to be built in multiple layers on a single substrate. A
typical example of modern micromachined accelerometer is given in Fig. 19.26. Multiple accelerometers
can be mounted on a single chip, sensing accelerations in x, y, and z directions. The primary signal
conditioning is also provided in the same chip. The output from the chip is usually read in the digital form.

FIGURE 19.25 Steps of micromachining to manufac-
ture micro- and nanoaccelerometers.

FIGURE 19.26 Multiple accelerometers in a single chip.

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