Self-calibrating MEMS override manufacturing variations

July 18, 2012 — Purdue University researchers have demonstrated self-calibrating micro electro mechanical systems (MEMS), which enable higher accuracy for existing and new MEMS applications.

“Each MEMS device is slightly different due to variations that occur in manufacturing,” said Jason Vaughn Clark, an assistant professor of electrical and computer engineering and mechanical engineering at Purdue University, explaining the value of self calibration. Microstructure geometry, stiffness, and mass all influence performance, and can vary MEMS device to MEMS device. Conventional MEMS test methods are impractical and expensive, with unknown accuracy and large uncertainty, Clark added.

Clark developed the self-calibration theory, then demonstrated the device alongside doctoral student Fengyuan Li, validating the thesis.

Figure. A self-calibratable MEMS. SOURCE: Purdue University Birck Nanotechnology Center image/Jason Vaughn Clark.

The self-calibrating technology makes it possible to accurately measure displacement on a scale of micrometers to less than a nanometer. “Quantities like velocity, acceleration, force, stiffness, frequency, and mass can be related to displacement,” said Clark.

The heart of the self-calibrating MEMS are two gaps of differing size, electrostatic sensors and comb drives with meshing fingers drawn toward each other when a voltage is applied, and returned to their original position when the voltage is turned off. The comb drives measure the change in capacitance while gauging the distances of the two gaps built into the device. The fine measurements reveal the difference between the device’s designed layout and the actual dimensions.

"Once you learn the difference between layout and fabrication, you have calibrated the device," Clark said. "Many MEMS designs with comb drives can be easily modified to implement our technology."

The new self-calibratable MEMS could eliminate or reduce the need for rigorous factory calibration on high-accuracy MEMS for navigation or other applications,  Clark said, estimating up to 30% of manufacturing costs relate to calibration.

The self-calibratable MEMS could lead to high-performance data storage technologies and advanced lithography to create next generation computer circuits and nanodevices. “A $15 chip that can fit on your fingertip…is able to measure MEMS displacements better than a $500,000 electron microscope,” as a result of self-calibration, Clark noted.

Researchers will use the new self-calibration approach to improve the accuracy of atomic force microscopes (AFMs), calibrating AFM displacement, stiffness, and force.

The group also will use a calibrated MEMS to measure the difference in gravity between different heights above the ground. The ability to measure gravity with such sensitivity could be used as a new tool for detecting underground petroleum deposits. "Conventional gravity meters can cost over $200,000," Clark said. "They consist of a large vacuum tube and a mirrored mass. Gravitational acceleration is determined by measuring the drop time of the mass in free fall. Since oil or mineral deposits have a different density than surrounding material, the local gravity is slightly different." Other applications abound.

The self-calibratable technology also could allow MEMS to recalibrate themselves after being exposed to harsh temperature changes or remaining dormant for long periods.

The work is based at the Birck Nanotechnology Center in Purdue’s Discovery Park. The research is funded by the National Science Foundation.

Findings are detailed in a paper to appear later this year in the IEEE Journal of Microelectromechanical Systems (JMEMS), “Self-Calibration for MEMS with Comb Drives: Measurement of Gap,” Fengyuan Li and Jason Vaughn Clark, Purdue University, Discovery Park, Birck Nanotechnology Center.

Abstract:

We present a practical method for measuring planar gaps of MEMS with comb drives by on-chip or off-chip electrical probing. We show that our method is practical, accurate, precise, and repeatable. The option of on-chip, postpackaged electrical measurement enables MEMS to be autonomously self-calibratable. We use the measurement of gap to determine the geometrical difference between layout and fabrication, which can lead to measurements of other properties such as displacement, force, stiffness, mass, etc. Our method consists of applying enough voltage to close two unequal gaps and measuring the resulting changes in capacitances. Many MEMS designs with comb drives can be easily modified to implement our technology. Our results are an order better than convention, and suggest means for further improvement.

Courtesy of Emil Venere and Jason Vaughn Clark at Purdue.

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