The Investigation of MEMS-Fabricated Actuators for Use in Optical and Mechanical Applications, 14-9158

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Principal Investigators
Joseph N. Mitchell
Heather S. Hanson

Inclusive Dates: 09/15/99 - 03/31/01

Background - The greatest promise of microelectromechanical systems (MEMS) lies in their ability to produce mechanical motion on a very 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 their technical challenges. 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-dependant, 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 where a small, timed relay would be desirable, such as military and computer industries. For instance, 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. Again, MEMS actuators can provide such a solution.

Approach - The project had three goals: 1) to produce and evaluate a number of micromachined actuators, 2) to develop a MEMS-based optical switch and timer relay switch from these actuators, and 3) to allow SwRI staff to become proficient in the design and testing of MEMS devices. Actuators that operate via both electrostatic and thermal methods were evaluated in a range of force, speed, and size characteristics. 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 insulating microplate that separates two electrical contacts. An actuator that uses small discrete steps to produce smooth linear motion causes the micro-plate to slide away, allowing the contacts to complete a circuit.

Three iterations of mask designs were fabricated by a vendor using the MUMPS process. All other aspects of the project were performed using Institute facilities. Modeling and layout of the devices was performed using software tools tailored to MEMS design. A MEMS release and test facility were established in the Applied Physics Division that continues to serve other MEMS projects.

Accomplishments - A wide variety of MEMS actuators were produced and tested. Four were evaluated extensively, including the Residual Stress Cantilever (RSC) and Vertical Thermal Actuator (VTA), which operate perpendicular to the plane of the substrate, and the Scratch-Drive Actuator (SDA) and Horizontal Thermal Actuator (HTA), which move parallel to the substrate plane. The RSC and SDA are electrostatic devices that function by snapping down and releasing a beam or plate via application of a voltage between the device and substrate. The RSC is an electrostatically actuated cantilever fixed at one end with an initial upwardly curved shape due to residual stress between layers. The cantilever is quickly pulled flat to the substrate when a voltage is applied between the RSC and substrate. The SDA consists of an array of electrostatically actuated plates mounted freely on rails, incorporating a bushing on the front, which causes the array to step forward in small increments. This design not only produces smooth linear motion but also very high forces for a MEMS actuator. 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 as one arm is heated more than the other, whereas the VTA utilizes a coefficient mismatch between two materials to produce vertical flexing. A novel bidirectional HTA has also been developed. Typical parameters of these actuators are shown in the table below.

The RSCs were selected as actuators for the optical switch. Micromirrors were designed on the ends of the RSCs that are configured to be flipped up and automatically lock into position. A 2x2 switch was designed and tested using external fiber optic collimating elements that transmit a laser beam beneath a mirror until that mirror is actuated and the beam reflected. The switch exhibited optical losses of less than two decibels and a switching time of ten milliseconds. The timer relay switch used the SDAs due to their high force characteristics. An array of 100 SDAs attached to a 1- by 1-millimeter micro-plate was used to separate the fine-wire electrical contacts. The switch was successfully operated and handled currents up to 1 amp.

Typical parameters as determined for the MEMS actuators
 

  Actuator Size 
(one element)
Operating Voltage Power Consump-
tion
Displace-
ment
Force (for array) Speed   

RSC

1000 x 
100 mm
110 V DC < 1 mW 250 mm Not determined 1 msec

SDA

120 x 
60 mm
±150 V AC < 1 mW Unlimited 1000 mN 20 mm/sec

VTA

1000 x 
200 mm
1.5 V 300 mW 200 mm Not determined 2 msec

HTA 200 x 
20 mm
12 V 75 mW 15 mm 40 mN Not 
determined
 					

SDA-actuated microplate for electrical relay switching shown partially extended off the chip to the left. Actuation of the SDA array pulls the plate back onto the chip, allowing the electrical leads to make contact.
 

A 2x2 optical switch using RSC actuators and fold-up mirrors. When actuated, the mirror is pulled down to the substrate, reflecting the optical beam as shown.

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