Electronic Design

Heat Convection Yields Integrated CMOS MEMS Accelerometer

Designers can use resistive IC-based thin-film microheating elements to manufacture relatively low-cost integrated MEMS accelerometers in a standard CMOS process. This novel method, developed by the National Institute of Standards and Technology in Gaithersburg, Md., offers cost, system robustness, and adaptability advantages over conventional MEMS accelerometers, most of which use a solid proof mass.

The new accelerometer works on the principle of heat convection. MEMS project leader Michael Gaitan of NIST's Semiconductor Electronics Division says it uses micromachined thermopiles or thermistors separated by a gap for temperature sensing (see the figure). Heated wires are placed in a gas that's encapsulated in glass and hermetically sealed to prevent any external air flow or pressure changes.

Accelerometers made in this fashion have a small linearity error of less than 0.5% under tilt conditions of ±90°, and less than 2% for accelerations of 7 g or less (1 g = 9.81 ms/s2). Good sensitivity, which is nearly a linear function of heater power, also was achieved. For operating power of about 100 mW, 115 µV/g was measured for thermopiles. A 25-µV/g sensitivity was measured for thermistors as well. Both types of devices can operate up to several hundred hertz, versus tens of hertz for other convection-type accelerometers.

A standard CMOS process provides the miniaturization inherent in high levels of integration. It allows the low-cost integration of the sensing element, drive, detection, and output circuitry on the same substrate.

A suspended polysilicon microheater is the heart of the accelerometer. The thermal difference between the heated element and the surrounding gas generates a buoyant force that causes a convective flow of the encapsulated gas. Under acceleration, the change in the convective flow causes a temperature difference between the two sides of the heated element. This difference is proportional to the acceleration.

Contact Michael Gaitan at (310) 975-2070 or at [email protected] Velkjo Milanovic, another team member, has left NIST to develop this device commercially at the Adriatic Research Institute, a nonprofit organization in Berkeley, Calif. Go to www.adriaticresearch.com for information about ARI's work.

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