Investigation of Microelectromechanical Systems-Fabricated Actuators for Use in Optical and Mechanical Applications, 15-9158

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Principal Investigators
Joseph N. Mitchell
Heather S. Hanson

Inclusive Dates: 09/15/1999 - Current

Background - The greatest promise of microelectromechanical systems (MEMS) lies in the ability to produce mechanical motion on a small scale. Such devices are typically low power and fast, taking advantage of such microscale phenomenon as strong electrostatic forces and rapid thermal responses. Although MEMS-based sensors have been widely deployed, few MEMS-based actuators have achieved more than laboratory-level development due to the technical challenges they present. The market for such devices is growing rapidly, especially for optical and electronic applications.

The explosive growth of data traffic, such as the Internet, has produced a pressing need for large-capacity optical networks. Optical switches are now in high demand in the telecommunications industry for their ability to reconfigure an optical network for traffic management or circuit protection without having to resort to low-bandwidth, protocol-dependent, opto-electronic conversions. To be widely deployed, such switches must be small, low cost, batch fabricated, and have a high port count. A MEMS-based optical switch is well suited to addressing these requirements.

The traditional method of creating a timed electronic switch requires a timer circuit or discrete mechanical components. Such devices tend to be large or unable to handle high currents. There are a number of applications in which a small, timed relay would be desirable, such as military and computer industries. For example, the device could be incorporated into a computer, so that after a series of incorrect password entries, the computer locks out all communications and cannot be overcome without physical access.

Approach - SwRI is developing a number of different MEMS actuators that can be used for optical switching or as a timed relay switch. The optical switch is based on an NxN array of cantilever actuators that move micromirrors perpendicular to the substrate to redirect an optical beam to any output. The optical beams are transmitted through free space with low loss via collimating optical fibers. The relay switch is based on an actuated plate that is moved off-chip to separate two electrical contacts. To produce a smooth linear motion (for timing), actuators that move in small, discrete steps are used. Progress in development of these devices follows an incremental approach. At each design iteration, the team performs modeling and layout of the fabrication mask design using two software packages tailored to MEMS design. Fabrication is performed at an external foundry. Parts release, device test, and evaluation are performed using Institute facilities.

Accomplishments - In the first design iteration, a variety of basic MEMS actuators were produced to establish an understanding of their operational characteristics. The team developed methods for part release through wet chemical etching, electrical and mechanical testing with a microscope probe station, optical analysis, and microassembly. Suitable actuators included two that operate perpendicular to the chip substrate [a residual stress cantilever (RSC) and a vertical thermal actuator (VTA)] and two that operate in the plane of the substrate [a scratch-drive actuator (SDA) and a horizontal thermal actuator (HTA)]. The RSC and SDA function by snapping down and releasing a beam or plate via application of a voltage between the device and substrate. The RSC is fixed at one end and has an initial upwardly flexed position due to residual stress between layers, whereas the SDA is mounted freely on rails and incorporates a bushing on the front that causes it to step forward in small increments, producing smooth linear motion. The VTA and HTA operate by current-generated, resistive heating along two arms, causing a thermal expansion. In the HTA, the arms have different widths, causing a lateral flexing, whereas the VTA uses a coefficient mismatch between two materials to produce vertical flexing.

In the second iteration, the team further developed these four actuators to determine optimal physical parameters and incorporated additional components to produce the final devices. The RSC and VTA were fitted with micromirrors that uses a locking hinge design, which allows assembly in a single step and prevents disassembly from shock forces. The HTAs were integrated into an indexing actuator that causes a sliding plate to be pushed forward or backward via a rack-and-pinion-type design. SDAs were also attached to large sliding plates that could be moved up to one millimeter off-chip. Test results show that the RSC operates at a switching speed of one millisecond at approximately 90 volts, whereas the VTA switches in about three milliseconds using three volts, both for 250-micrometer-deflection. The VTA can also be used as a variable attenuator, beamsplitter, or scanner, since its vertical position can be precisely controlled. The SDAs have generated very high forces for a MEMS device of 400 micronewtons for a four-plate array. They operate at 80 volts with velocities of four micrometers per second and have a travel distance limited only by the track length. The HTAs function at 15 volts and can produce forces of 20 micronewtons with a displacement of  up to 12 micrometers.

Schematic of MEMS-based crossbar optical switch concept. Using a microactuator to move a mirror, collimated light from any input optical fiber can be switched to any output.
Scanning electron microscope (SEM) photograph of residual stress cantilevers. Application of a voltage to the cantilever arm causes it to snap down to the substrate. These actuators are the basis of the optical switch.
SEM photograph of a vertical thermal actuator with integrated micromirror. Application of a current to the actuator arm produces vertical motion of the mirror, which can either reflect an optical beam or allow it to be transmitted.
SEM photo of a Scratch-Drive actuated plate. The plate can move off-chip to the right.

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