Articulated MEMS electrostatic rotary actuator

Details Articulated MEMS electrostatic rotary actuator Articulated MEMS electrostatic rotary actuator

2002-12-04

2004-07-06

Ben, Loha (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S291000, C359S295000, C359S198100, C310S090000, C310S309000, C216S002000

Reexamination Certificate

active

06760144

ABSTRACT:

MICROFICHE APPENDIX
Not Applicable
FIELD OF THE INVENTION
The present invention relates generally to optical communication systems and more particularly to electro-mechanical systems (MEMS) devices and methods of fabricating the same.
BACKGROUND OF THE INVENTION
Optical communication systems are increasingly being used to communicate data, voice, multimedia and/or other communications. Optical communication systems may employ optical fibers and/or free space optical communication paths. It will be understood by those having skill in the art that optical communication systems may use optical radiation in the visible, ultraviolet, infrared and/or other portions of the electromagnetic radiation spectrum.
Reflectors, such as mirrors, are widely used in optical communications systems. For example, optical cross-connect switches often include an array of reflectors to reflect optical energy from any switch input to any switch output. Similarly, many add-drop optical switches, wavelength blockers, and/or dynamic gain equalizers also use an array of reflectors such as mirrors to couple various optical paths.
It has been proposed to fabricate arrays of reflectors using micro-electro-mechanical system (MEMS) technology. As is well known to those having skill in the art, MEMS devices are potentially low cost devices, due to the use of microelectronic fabrication techniques. New functionality also may be provided, because MEMS devices can be much smaller than conventional electro-mechanical devices.
Many of the fabrication processes for MEMS, called micromachining, are borrowed from the integrated circuit industry, where semiconductor devices are fabricated using a sequence of patterning, deposition, and etch steps (e.g., on silicon). Silicon micromachining has been utilized since the early 1960s. At its early stage, bulk silicon micromachining was employed in the majority of the research efforts by etching away the bulk of the silicon wafer. Bulk micromachining was first practiced using anisotropic wet chemical etches, such as potassium hydroxide, which preferentially etch faster in certain crystallographic planes of single-crystal silicon. In the early 1980s, surface micromachining using sacrificial etching gave rise to new types of microsensors and microactuators. Typically, surface micromachining has used a deposited layer of polysilicon as the structural micro-mechanical material, which is deposited over a sacrificial layer onto a substrate, which is typically silicon, such that when the sacrificial layer is removed, the polysilicon remains free standing. Recent advancements in reactive ion etching (RIE) technology have made practical, and in many ways preferential, the use of dry plasma etching to define micro-mechanical structures. Reactive ion etching techniques are independent of crystal orientation, and can create devices exceeding the functionality of surface micro-machined devices. The use of single-crystal materials, particularly silicon, can be beneficial for mechanical applications because of the lack of defects and grain boundaries, maintaining excellent structural properties even as the size of the device shrinks. Fabrication techniques involving the bonding of two separate single-crystal wafers also have been proposed, wherein one wafer serves as the substrate and another wafer forms the structural micro-mechanical material/layer.
For many reflective MEMs applications, it is desirable that the reflectors are electrostatically actuated. Electrostatic actuation provides effective analog positioning and tuning. Furthermore, electrostatic actuators are relatively easy to fabricate and provide high operational speeds due to their relatively small mass. This is in contrast to other actuators, such as piezoelectric actuators, which are typically much heavier. Electrostatic actuation of a structure is typically accomplished by applying a voltage between an electrode on the structure and an electrode separated from the structure. The resulting attractive electrostatic force between the electrodes enables actuation of the structure toward the separated electrode. This applied electrostatic force is opposed by a characteristic mechanical restoring force that is a function of the structure's geometric and materials properties. However, the electrostatic force is a nonlinear function of distance. As the structure moves toward the separated electrode, such that the electrodes' separation distance decreases, the electrostatic force between the electrodes typically increases superlinearly. In contrast, the mechanical restoring force of the structure typically is a linear function of distance. Accordingly, not all positions between the electrodes are stable. In particular, when the air-gap between electrodes reaches a minimum spacing characteristic of the structure, the structure position is unstable and causes uncontrollable travel of the structure through the remaining distance to the separated electrode. This instability condition is generally known as “pull-in”, and can result in stiction (i.e., where the reflector is stuck to the electrode) and/or actuator deformation. This pull-in phenomenon typically reduces the actuation range of electrostatic MEMS devices.
An example of a reflector array, which can be fabricated from the above processes, is shown in FIG.
1
. The micro-mechanical structure
10
includes an array of single axis mirrors
12
disposed about a common rotation axis
14
, which is parallel to the array axis. This type of structure is commonly referred to as a piano MEMS array. Each mirror
12
a
is suspended above a substrate
20
with a torsional hinge
16
a
, which ideally is coaxial with the rotation axis
14
, such that the mirror
12
a
is able to pivot about the rotation axis
14
. Each end of the torsional hinge
16
a
is connected to a mechanical anchor
18
a.
Referring to
FIGS. 2
a
and
2
b
, the pivotal movement of each mirror
12
a
in
FIG. 1
is electrostatically actuated by first
22
and second
24
lower electrodes deposited on the substrate
20
, under the mirror
12
a
. In particular, a voltage applied between an upper electrode (i.e., the micro-mirror) and a first underlying electrode
22
will tilt the mirror in a first direction, while a voltage applied between the upper electrode and a second underlying electrode
24
will tilt the mirror in a second opposite direction, as shown by the dotted lines in
FIG. 2
b.
Unfortunately, the single-axis design illustrated in
FIGS. 1 and 2
a,b
is associated with a number of disadvantages. In general, these disadvantages are related to the fact that the micro-mirrors (i.e., the optical components) are part of the actuators (i.e., the electromechanical components), and thus the optimization of each component is compromised. For example, in terms of optimizing the optical design it is often desired to have a micro-mirror size that is large enough to facilitate alignment and reduce insertion loss. In practice, the electromechanical requirements can limit the size of the optical components, since it is faster and requires less energy to move a lighter object. Similarly, using the piano MEMS shown in
FIGS. 2
a
and
2
b
as an example, a longer arm provides a stronger electromechanical design due to increased leverage and/or torque and thus less actuation energy, whereas a shorter arm provides a stronger optical design, since it provides greater rotation angles and/or reduces the effect of pull-in. Unfortunately, these contrasting requirements can introduce challenges in the MEMS design.
U.S. Pat. No. 6,480,320 to Nasiri discloses a micro-electromechanical mirror and mirror array that addressed some of these concerns. However, in this micro-electromechanical device, the mirror is supported on a post above the actuation layer (i.e., there are two structural micro-mechanical layers, the upper mirror layer and the lower actuator layer). Accordingly, the device proposed by Nasiri is relatively complex to fabricate. Moreover, it is expected that the excess mass will lower the resonant frequency.
It is

Affiliated with

Also associated with

Rating

Articulated MEMS electrostatic rotary actuator does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Articulated MEMS electrostatic rotary actuator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Articulated MEMS electrostatic rotary actuator will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3230241