IN MOTION
MEMS actuators fabricated for optical
and mechanical applications
D014584_9398
One of the key abilities of MEMS devices is that they
can produce mechanical motion on a very small scale.
This provides the means to create compact actuators with
motions in the range of nanometers to millimeters and with
the ability to generate forces up to the milli-newton range.
MEMS actuators are generally based on either electrostatic
or thermal operation, although piezoelectric, magnetic and
even hydraulic types also have been demonstrated. An electrostatic actuator relies on the potential difference between
a movable and a fixed surface and exerts a force on the
movable surface. These actuators are very low power
(microwatts) but require high voltage, sometimes in excess
of 100 V, to generate a sufficient electric potential. Their
response time is fast — tens to hundreds of microseconds
— but they can be limited in their maximum displacement. Such actuators have a “pull-in” voltage limit
whereby they fully deflect after moving a certain portion of
their range, making this type of actuator most suitable for
binary positioning. The well-known comb drive actuator is
of this type. Thermal actuators, on the other hand, use Joule
heating to produce thermal expansion. They have only a
slightly slower response time than electrostatic actuators
and can be operated over their entire range of motion.
One of the first MEMS projects at SwRI was an internally funded development study of several of these
micro-actuators and their use in switching applications.
Actuators were designed based on both electrostatic and
thermal methods, with a variety of force-displacement
characteristics and the ability to produce both horizontal
and vertical motion. Also developed were two novel actuators that have since been patented by SwRI — a singlematerial, in-plane, bi-directional thermal actuator and a
vertical thermal actuator based on a bi-metal design.
One switching device produced with the actuators
from this project was an optical switch. It was designed to
switch light between any of several input collimated or
parallel optical fibers to any output fiber. The switch was
based on a two-dimensional array of micromirrors
mounted on the end of electrostatic out-of-plane actuators.
The mirrors, which are smaller than the head of a pin,
were fabricated flat, but were flipped and latched into
position by a hinge mechanism on the end of the actuator
arm. The switch was scaleable to allow additional input or
output fiber channels. Such a switch can be used as a
cross-connect in fiber optic communications systems or in
optical scientific instrumentation.
Several sponsored programs for development of
unique microstructures and sensors have benefited from
this internal research. Some of the actuator designs have
been used directly in the development of the SwRI “materials lab on a chip.” The handling and testing techniques,
design procedures and measurement tools developed
under the internal research project laid the groundwork for
many subsequent MEMS projects at SwRI.
Although this MEMS device has more than 100 functioning mechanical devices, the entire chip
can rest on a fingertip. MEMS devices are incredibly small yet highly functional.
8
Technology Today • Winter 2004
stacking layers of material(s). Initially the
materials were polysilicon and metals, but
the technology has progressed to include
numerous other materials, including polymers. Typically, the layers range in thickness
from 1 micron to 1,000 microns, depending
on the fabrication process. The diameter of
a human hair is around 90 microns, fitting
easily in the realm of MEMS.
The most common processes in use
today involve photolithography, or etching a
design into photosensitive materials. In this
process, a structural material of constant
thickness is deposited onto a substrate
chip. A photoresistant material is applied
onto the structural material in a particular
pattern of interest. The structural layer is
then etched according to the photoresist
pattern, and the photoresist is removed.
This process is repeated until the desired
layers have been placed. Some of the
materials deposited throughout this process
are “sacrificial.” When the deposition and
photolithography processes are complete,
the entire chip is exposed to an etchant,