Econo-MEMS has no moving parts

PORTLAND, Ore. — Ever since microelectromechanical system (MEMS) accelerometer chips became standard equipment on consumer devices such as the iPhone and Wii, their prices have been plummeting.

But volume manufacturing is just one reason accelerometers could eventually become $1 devices. Another is that the most expensive manufacturing-cost components of MEMS—the moving parts—are being eliminated, according to MEMSIC Inc., which has developed a no-moving-parts design (Andover, Mass.).

Eliminating the moving mechanical parts means that instead of using an expensive, custom MEMS manufacturing process, MEMSIC can run its accelerometers on any standard CMOS line.

"MEMSIC can manufacture its sensors at a lower cost," said Marlene Bourne, principal analyst at Bourne Research (Scottsdale, Ariz.). "I believe that may be partly why there have been such strong price reductions in the cost of accelerometers—coupled, of course, with accelerometers' moving into the consumer market, where the volumes are higher."

Actually, there is one "moving part" in MEMSIC's design: heated air. By monitoring the moving air currents trapped in a cavity above a standard CMOS chip, MEMSIC claims to be uniquely positioned to leverage low-cost CMOS wafer processing.

By contrast, other MEMS chip makers must use special fabrication techniques to craft their moving parts separately, then wire bond that die to a CMOS application-specific integrated circuit (ASIC) in the same package.

"Ours is the only company that can use a standard CMOS line to build MEMS, because we do not build a three-dimensional structure hanging above the silicon; for that, you need a custom process, which is very expensive," said Yang Zhao, MEMSIC's founder and inventor of its no-moving-parts design. "Instead of building a mass to sense acceleration, we use a convection flow of hot air."

All other MEMS accelerometers have to sculpt a moving bar (or other shape) out of silicon, then monitor its movement with piezoelectric or capacitive sensors. Instead, MEMSIC senses the flow of hot air above its chip. MEMSIC processes its wafers at a conventional CMOS foundry, then finishes the assembly at its own plant in China, which dices the wafers and packages each chip with an air cavity above each die.

MEMSIC said its ability to fabricate its wafers in any foundry and add its proprietary step during assembly at its own plant offers better scaling than is possible for MEMS chips that use custom processes to sculpt their moving elements.

"TSMC doesn't have to do anything special to make our MEMS wafers. There are no extra costs, our quality is high, our yields are high, the costs are low—it's just standard CMOS," said Zhao, who was an engineer at Analog Devices Inc. in the 1990s before starting MEMSIC, which is now publicly traded.