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Nov. 14, 2003 – If one thinks of acceleration sensors, the air bag in today’s automobile and the inertial navigation system used in modern avionics come to mind. But who would think of the various applications of accelerometers in industry, consumer markets, harsh environments or defense?

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Although accelerometers were among earliest MEMS products, the process of bringing them to market took nearly 20 years until a commercial breakthrough was achieved in the automotive and avionics markets during the last decade. With a mature technology and high-volume production at low cost, acceleration sensors are now in the best position to successfully move into newly emerging applications.

A NEXUS market report predicts that MEMS accelerometers will continue to exhibit high growth rates, representing a market evolution from $240 million in 1996 to $690 million for 2005. However, the analysis forecasts a much higher growth in the nonautomotive field (250 percent from 2000 to 2005) than in the automotive field (50 percent from 2000 to 2005).

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Four main technologies are competing to cover most applications requiring accelerometers:

• The first two technology choices, piezoelectric and piezoresistive accelerometers, generate an electrical charge (piezoelectric) or a resistance alteration (piezoresistive) in response to stress. With their low cost, they are suited for many general-purpose applications, especially for the measurements of high shock. However, as they don’t offer static load (or DC) response, they cannot be used for inertial applications.
• Micromechanical capacitive accelerometers offer true static load response and can be highly precise and sensitive, but are generally reserved for low shock applications. They work well for high-grade applications, which require mathematical integration to obtain compound velocity and position data (inertia). Considering their high cost, these have traditionally been used in aerospace, but also headed into other inertial measurement applications.
• The last available accelerometer technology is MEMS with two major variations, both measuring a deflection of a seismic mass under acceleration: In the first MEMS type, the mass structure is etched into silicon (so-called bulk micromachining), which makes it truly integrated and very small. It can be used for low frequency sensing inertial applications such as guidance and navigation. The second variation uses the deposition of layers and structures above a silicon substrate to build the seismic mass, called surface micromachining. Established applications for the second technology have principally been instrumentation and automotive with tens of millions of products mounted for example in air bag control units.

The question is: Will the race for accelerometers in the nonautomotive fields be won by the more precise aerospace accelerometer technology, which has to compete on cost, or by the lower grade technology that is used for automotive, which has to compete in terms of specifications?

Advanced inertial sensors (e.g. for aerospace) have to measure the acceleration very precisely, over a long range and in a stable way. Here, micromechanical capacitive accelerometers have traditionally been used, but bulk micromachined MEMS sensors have recently proved their ability to serve the needs. MEMS manufacturers were able to respond to the market trend, heading size and competitive operating performance (improved stability, temperature sensitivity, rectification error, high shock survival without degradation of specification and higher integration and functionality of the electronics).

Furthermore, these bulk capacitive MEMS sensors are clearly ideal for general applications in the domain of transportation, instrumentation, civilian navigation and aerospace, surface drilling, tilt measurements and military applications. Solutions are currently being implemented to provide low cost products with a minimum concession on specification to remain competitive on these markets.

Low grade MEMS sensors, historically used in the automotive industry to detect a shock (making low-cost surface micromachining the technology of choice), have the advantage to propose a product at low cost, which is a major drive today for the selection of a product. To access these new markets areas and especially the civilian inertial world, manufacturers tend to improve the specifications of their products, either by integration of an improved sensor or by implementation of a complex electronic.

MEMS accelerometers with an inertial grade are now available for these new markets at an affordable price. Considering the economic situation, low-cost products initially met with a positive response. However, there is a continuous demand to optimize accelerometers in terms of specification, size, weight and shock resistance, and bulk capacitive sensors, taking advantage of flexible scale manufacturing of the technology from small to large volume, re-enter the market with low-cost solutions.

The fight is not over, but in any case, MEMS will be the winner.

Patric Salomon runs the microsystems consultancy company 4M2C.