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MEMS Micro-Mirrors

MEMS micro-mirrors: if you look carefully at the silicon die, you can see four MEMS devices on each of them.

An Engineering Design Project that began in the fall of 2008, recently culminated in the delivery of a small plastic box now displayed on a shelf in Dr. Eddie Law's office. In the box are tiny squares of silicon, which look like chips, but which are in fact MEMS devices.

MEMS devices are mechanical devices with moving parts, constructed out of silicon using surface micro-machining techniques, that are scaled down to a microscopic scale— one cannot fully appreciate the complexity of a MEMS device with the naked eye. Anyone who has used a Wii controller, an iPod touch, or an iPhone has held a MEMS accelerometer in their hand. Dr. Law's MEMS devices, however, are gold-covered micro-mirrors which can be raised and lowered to either reflect a laser beam (when the mirror is raised) or to let it pass when the mirror is lowered.

This MEMS design uses a linear motor to raise and lower the micro-mirror. Using an oscillating input signal, the linear motor is driven with inductive force.

This MEMS design uses a comb drive to raise and lower the micro-mirror. Using capacitive force, one of the two combs is pulled inwards, which then applies force on the torsion bar, which raises the mirror.

STEP-BY-STEP: [1] Power is applied (red), [2] comb moves to the right (blue), [3] force is applied on the beam (green) and [4] mirror rises up (gold).

STEP-BY-STEP: [1] Power is applied (red), [2] comb moves to the right (blue), [3] force is applied on torsion bar (green) and [4] mirror rises up (gold).

A year ago, Dr. Law began supervising a team of three undergraduate students: John Clayton, Terence Fisher and Jan Anthony Mendoza, who designed the MEMS devices. The objective of the project was to design a two-dimensional array of MEMS mirrors that could be raised and lowered using electrostatic forces. Two different designs were tried: one used capacitive force to raise the mirror while the second design used inductive force. The students who undertook the project had to understand the basics of mechanical system design, the concepts of electrostatic theory, the fundamentals of MEMS technology including the various fabrication processes and they had learn to use MEMSPro software to design the mirrors. (The Electrical and Computer Engineering Department was awarded two MEMSPro licenses worth $12,000 by CMC Microsystems.)

In May of this year, the student's designs were uploaded to the CMC fabrication site and the silicon devices were returned a month ago. As for the future direction of his MEMS research, Dr. Law says, "My next goal is to design a 3D MEMS-based optical switch. We need to control and synchronize the multiple mirror operations with incoming laser beams. The work that was done so far was only mirror design; this is far from my final goal."

The accompanying images of the MEMS micro-mirrors were photographed with a scanning electron microscope (SEM) belonging to the Department of Mechanical and Industrial Engineering at Ryerson.