Optical Technique Probes MEMS Performance

Richard Gaughan

A new excitation/detection method based on light pressure and interferometric profiling can characterize a microelectromechanical systems (MEMS) gimballed mirror array. Besides enabling accurate, noninvasive process monitoring, the all-optical technique opens the door for remotely operated optical MEMS applications.

An excitation and detection technique based on light pressure and interferometric profiling eases the characterization of microelectromechanical systems, such as this gimballed micromirror. Courtesy of Agere Systems.

Optical MEMS are miniaturized devices that are designed to manipulate light in the same fashion as large-scale optics. They offer obvious advantages in terms of mass, volume and electrical power consumption, but they also present significant design challenges because the influence of various physical forces does not scale linearly with decreasing size. As a result, new designs can be modeled, but prototype testing of the devices also must be performed.

An optical switch from Agere Systems of Murray Hill, N.J., features an array of 1-mm-diameter micromirrors that are fabricated on a silicon wafer, which then is bonded to another chip that contains the electrodes to electrostatically drive mirror rotation. Understanding the mechanical properties of the mirrors is necessary to evaluate the design suitability and to monitor the fabrication process. But when the array is assembled with the electrode chip, the mirror motion couples to the geometry of the electrostatic drive. And if mechanical shaking excites the mirrors, coupled modes involving more than one mirror are introduced.

Laser illumination has provided a solution. John E. Graebner, Stanley Pau and Peter L. Gammel of Agere directed approximately 3 mW from a 1480-nm fiber amplifier pump laser to a weak focus in an 80-µm spot on the back of a single mirror. A chopper modulated the beam at frequencies of up to 4 kHz. With the spot slightly offset from an axis of symmetry, the radiation pressure of the chopped beam excited resonant modes of the mirror.

To noninvasively monitor the resonant modes, the researchers scanned the 2-µm-diameter CW beam from a 633-nm interferometer across the 1.2 x 1.2-mm region containing a micromirror. The interferometer measured the amplitude of the surface deflection, the reflectivity at each location and the phase of the motion relative to the chopped excitation beam.

The technique excited, identified and characterized three resonant modes at 341, 530 and 2680 Hz, which featured maximum amplitudes of 26.0, 5.7 and 2.0 nm, respectively. The measurements validated the results of modeling and confirmed that the motion resulted from radiation pressure and not from localized heating effects.

The method promises remotely operated optical MEMS applications as well as improved process monitoring. "The all-optical excitation and detection allows the possibility of remote sensing in harsh environments -- such as chemical, radiation, etc. -- which may be incompatible with electronics and electrical leads," Graebner said. "It also has the advantage of interrogating sensors and sensor arrays which then do not need to carry their own power source, allowing long life and improved reliability."

In October, Agere announced the sale of aspects of its optoelectronics business, including its MEMS products, to TriQuint Semiconductor Inc. of Hillsboro, Ore., for $40 million in cash.