Lower failure rate
Another advantage to having no moving parts, MEMSIC said, is wider latitude in operating conditions and a lower failure rate. Traditional MEMS accelerometers have to be sold in ranges, because too much shock can damage the MEMS mass used to measure displacement inside the chip, the company said. To remedy that, traditional MEMS accelerometers use damping, which limits the sensors to a range of motion.
But MEMSIC's design requires no moving mass that can be damaged. "Our failure rate is very low . . . it's in the single-digit parts per million," said Zhao.
Using hot air as a moving parta technique that MEMSIC claims to have patentedrequires a heater. The heater on MEMSIC's accelerometersthe mechanical part of its chipsets the temperature of a column of air trapped above the chip. Temperature sensors around the peripheral of the chip sense acceleration as a change in temperature, using the same principle as hot air rising, but here the hot air moves in the opposite direction of the acceleration, because it's lighter.
"Since gravity goes downward, the hotter air goes upward," said Zhao. "It is exactly the same principle when you brake your car; your body is thrown forward because it is heavier than air. But hot air is lighter than air, so it will move backward like a helium balloon."
One ingredient of MEMSIC's secret sauce is the construction of the central heater, which looks like an array of bonding pads and lines. The heater uses polysilicon resistors to generate heat, which it regulates to a temperature that MEMSIC keeps as a trade secret. Temperature sensors on the chip's periphery measure the temperature of the hot air that rushes from the center toward the edge of the chip during acceleration. Like the helium balloon that goes backward in your car when you brake, likewise, a blast of hot air will go in the opposite direction of any acceleration, enabling the measurement of acceleration and shock as a change in temperature. The temperature sensors on the chip periphery use aluminum and silicon to create thermocouples.
MEMSIC also has no-moving-parts designs in the works for pressure and gas-flow sensors, as well as some next-generation MEMS devices on the drawing board that do have a few moving parts.