U.S. patent application number 10/907992 was filed with the patent office on 2006-10-26 for a non-contacting electrostatically-driven mems device.
This patent application is currently assigned to TEXAS INSTRUMENTS INCORPORATED. Invention is credited to Mark H. Strumpell.
Application Number | 20060238852 10/907992 |
Document ID | / |
Family ID | 37186561 |
Filed Date | 2006-10-26 |
United States Patent
Application |
20060238852 |
Kind Code |
A1 |
Strumpell; Mark H. |
October 26, 2006 |
A NON-CONTACTING ELECTROSTATICALLY-DRIVEN MEMS DEVICE
Abstract
An improved microelectromechanical systems (MEMS) device, which
eliminates, or at least reduces, "stiction" is described. The MEMS
device includes a central electrode and a pair of outer electrodes
formed on a substrate. The central electrode includes a plurality
of extensions defining a plurality of grooves interspersed with the
extensions. The outer electrodes include a plurality of extensions
disposed within the grooves of the central electrode.
Inventors: |
Strumpell; Mark H.; (Allen,
TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
TEXAS INSTRUMENTS
INCORPORATED
7839 Churchill Way, MS 3999
Dallas
TX
|
Family ID: |
37186561 |
Appl. No.: |
10/907992 |
Filed: |
April 22, 2005 |
Current U.S.
Class: |
359/291 |
Current CPC
Class: |
G02B 26/0841
20130101 |
Class at
Publication: |
359/291 |
International
Class: |
G02B 26/00 20060101
G02B026/00 |
Claims
1. A microelectromechanical device, comprising: a substrate; a
first electrode formed on the substrate, the first electrode having
at least one extension extending from a first portion of the first
electrode and at least one extension extending from a second
portion of the first electrode, the second portion being
substantially opposed to the first portion; a second electrode
formed on the substrate substantially adjacent to the first portion
of the first electrode, the second electrode having at least one
extension extending towards the first electrode; and a third
electrode formed on the substrate substantially adjacent to the
second portion of the first electrode, the third electrode having
at least one extension extending towards the first electrode;
wherein the at least one extension of the first portion of the
first electrode substantially overlaps the at least one extension
of the second electrode and the at least one extension of the
second portion of the first electrode substantially overlaps the at
least one extension of the third electrode.
2. A device according to claim 1, wherein the at least one
extension extending from a first portion of the first electrode is
a plurality of extensions spaced from one another to define a
plurality of grooves interspersed with the plurality of
extensions.
3. A device according to claim 2, wherein the at least one
extension extending from a second portion of the first electrode is
a plurality of extensions spaced from one another to define a
plurality of grooves interspersed with the plurality of
extensions.
4. A device according to claim 3, wherein the at least one
extension of the second electrode is a plurality of extensions
disposed within a plurality of grooves corresponding to the first
portion of the first electrode.
5. A device according to claim 4, wherein the at least one
extension of the third electrode is a plurality of extensions
disposed within a plurality of grooves corresponding to the second
portion of the first electrode.
6. A device according to claim 3, wherein at least one of the
plurality of extensions extending from each of the first and second
portions of the first electrode has a polygonal geometric
shape.
7. A device according to claim 6, wherein the polygonal geometric
shape is selected from the group consisting of a triangle, a
square, a rectangle, a parallelogram, a diamond, and a
trapezoid.
8. A device according to claim 3, wherein at least one of the
plurality of extensions extending from each of the first and second
portions of the first electrode has a plane curve geometric
shape.
9. A device according to claim 8, wherein the plane curve geometric
shape is selected from the group consisting of a circle, a
semi-circle, an ellipse, a semi-ellipse, a line, a parabola, and a
hyperbola.
10. A device according to claim 3, wherein at least one of the
plurality of extensions extending from each of the second and third
electrodes has a polygonal geometric shape.
11. A device according to claim 10, wherein the polygonal geometric
shape is selected from the group consisting of a triangle, a
square, a rectangle, a parallelogram, a diamond, and a
trapezoid.
12. A device according to claim 3, wherein at least one of the
plurality of extensions extending from each of the second and third
electrodes has a plane curve geometric shape.
13. A device according to claim 12, wherein the plane curve
geometric shape is selected from the group consisting of a circle,
a semi-circle, an ellipse, a semi-ellipse, a line, a parabola, and
a hyperbola.
14. A device according to claim 1, further comprising a yoke
operatively secured to the substrate, the yoke having a groove
formed therein.
15. A device according to claim 14, further comprising a pixel
mirror operatively secured to the yoke, the pixel mirror having a
downwardly extending post for engagement with the groove formed in
the yoke.
16. A device according to claim 1, wherein the first electrode
comprises a plurality of electrodes.
17. A microelectromechanical device, comprising: a substrate; a
first electrode formed on the substrate, the first electrode having
a plurality of spaced apart extensions extending from opposing
sides of the first electrode, the spaced apart extensions defining
a plurality of grooves interspersed with the extensions; a pair of
additional electrodes formed on the substrate adjacent to the
opposing sides of the first electrode, the additional electrodes
having a plurality of spaced apart extensions disposed within the
grooves defined at opposing sides of the first electrode; wherein
the at least one extension of the first portion of the first
electrode substantially overlaps the at least one extension of the
second electrode and the at least one extension of the second
portion of the first electrode substantially overlaps the at least
one extension of the third electrode.
18. A device according to claim 17, further comprising a yoke
operatively secured to the substrate.
19. A device according to claim 18, further comprising a pixel
mirror operatively secured to the yoke.
20. A method for reducing stiction associated with operation of
microelectromechanical devices, comprising: forming a
microelectromechanical device to include a first electrode and a
pair of electrodes flanking the first electrode, the first
electrode interfacing with the pair of electrodes via a plurality
extensions disposed within a plurality of grooves defined in the
pair of electrodes, wherein the at least one extension of the first
portion of the first electrode substantially overlaps the at least
one extension of the second electrode and the at least one
extension of the second portion of the first electrode
substantially overlaps the at least one extension of the third
electrode; whereby the surface area defined at the interface
between the first electrode and the pair of electrodes generates an
electrostatic force large enough to overcome surface adhesion
forces associated with operation of the microelectromechanical
device.
Description
BACKGROUND
[0001] Microelectromechanical systems (MEMS) devices are small
structures typically fabricated on a semiconductor wafer using
techniques such as optical lithography, doping, metal sputtering,
oxide deposition, and plasma etching, which have been developed for
the fabrication of integrated circuits. Digital micromirror devices
(DMDs), sometimes referred to as deformable micromirror devices,
are a type of MEMS device used in projection displays by
controlling light through reflection. Other types of MEMS devices
include accelerometers, pressure and flow sensors, and gears and
motors.
[0002] A conventional DMD 100 is illustrated in FIG. 1. As shown,
the DMD 100 is constructed of three metal layers: a top layer 102,
a middle layer 104, and a bottom layer 106. The three metal layers
are situated over an integrated circuit (not shown), which provides
electrical commands and signals. The top layer 102 includes a pixel
mirror 108 that resides over the middle layer 104 supported via a
mirror support post 110. The middle layer 104, in turn, resides
over the bottom layer 106 supported by four hinge support posts
112. The mirror support post 110 of the top layer 102 is attached
to a yoke 114. As the yoke 114 rotates on its torsion hinges 118,
it drives the mirror support post 110 to rotate and tilt
accordingly. Consequently, as the mirror support post 110 rotates
and tilts, it dictates the angle, direction, and magnitude that the
pixel mirror 108 will rotate and tilt. The yoke 114, in essence,
controls the pixel mirror 108 by this relay effect.
[0003] One problem associated with a conventional MEMS device, such
as the DMD 100, is "stiction", which occurs when the yoke 114
rotates on the torsion hinges 118 and the yoke landing tips 116
come in physical contact with landing sites 120 located within the
underlying bottom layer 106. In some cases, when surface adhesion
forces are high enough, the yoke landing tips 116 may stick to the
landing sites 120 in the underlying bottom layer 106, and thereby
adversely affect the response time of the pixel mirror 108 and the
overall device performance. In other cases, the landing tips 116
may adhere to the landing sites 120 and remain stuck if an applied
mechanical restoring force is not strong enough to overcome the
existing surface adhesion forces. The pixel mirror 108 will then be
considered permanently defective because it will remain fixated at
only one angle.
[0004] Stiction has heretofore been addressed by applying
lubrication or passivation layers to the yoke landing tips 116 and
the landing sites 120 in the hopes of making these metal surfaces
slippery enough to minimize sticking. In addition, reset
electronics 122 have been employed to pump additional electrical
energy into the yoke 114 in order to help it break free from the
constraining surface adhesion forces between the yoke landing tips
116 and the landing sites 120. These techniques require extra
fabrication processes and additional cost.
SUMMARY
[0005] The present disclosure relates to a microelectromechanical
system (MEMS) device, and more particularly, to an
electrostatically-driven digital micromirror device (DMD) that
prevents or at least reduces stiction. A central electrode includes
interspersed extensions initially formed on a substrate. Two outer
electrodes with interspersed extensions are subsequently formed on
the substrate such that the two outer electrodes flank the central
electrode. The extensions of the central and outer electrodes are
interdigitated whereby a low bias voltage applied to the outer
electrodes generates an electrostatic force upon the central
electrode enabling a pixel mirror that is formed on top of the
central electrode to freely move, rotate, and tilt.
BRIEF DESCRIPTION
[0006] FIG. 1 is an exploded view of a prior-art digital
micromirror device (DMD); and
[0007] FIG. 2 is an exploded view of a DMD according to the present
disclosure.
DETAILED DESCRIPTION
[0008] Referring to the conventional digital micromirror device
(DMD) of FIG. 1, the pixel mirror 108 tilts and rotates according
to the tilt and rotation of the yoke. In practice, the pixel mirror
108 also rotates and tilts due to the electrostatic forces
generated by the electric fields between the pixel mirror 108 and
the mirror address electrodes 113, as well as the fields generated
between the yoke 114 and the yoke address electrodes 121.
Electrical signals are fed and carried through metal contact holes
from the underlying integrated circuit (not shown).
[0009] Reference is now made to FIG. 2, which illustrates a digital
micromirror device (DMD) 200 according to the present disclosure.
The DMD 200 includes a top layer 202, a middle layer 204, and a
bottom layer 206. As illustrated in the figure, the top layer 202
includes a pixel mirror 208 connected to a downwardly extending
mirror support post 210. The mirror support post 210 is adapted for
engagement with a corresponding post-receiving hole 211 formed in
the middle layer 204 as will be further described. In some
embodiments, the pixel mirror 208 has a thickness of about 2,000 to
5,000 .ANG. and is constructed of aluminum using known methods and
techniques. Preferably, the thickness of the pixel mirror 208 of
the presently disclosed embodiment has a thickness of about 3,300
.ANG.. In addition to aluminum, other materials such as silicon
oxide, silicon nitride, polysilicon, and phosphosilicate glass
(PSG) may also be used in constructing the pixel mirror 208. In
some embodiments, the mirror support post 210 has a thickness of
about 500 to 1,000 .ANG. and is constructed of an aluminum alloy
using known methods and techniques. The mirror support post 210 may
also be formed of aluminum, titanium, and silicon metal alloys.
Preferably, the thickness of the mirror support post 210 of the
presently disclosed embodiment has a thickness of about 700
.ANG..
[0010] The middle layer 204, disposed beneath the top layer 202,
includes a yoke 212 supported by a plurality of yoke support posts
214. The yoke support posts 214 may be formed according to the same
or similar materials and methods as the mirror support posts 210.
Furthermore, the yoke support posts 214 may also have the same or
similar thickness as that of the mirror support post 210. The
middle layer 204 also includes a post-receiving hole 211, which may
be formed using known materials and methods.
[0011] The bottom layer 206, situated below the middle layer 204,
includes a yoke address electrode 216 and mirror address electrodes
220. The bottom layer 206 further includes contact pads 224, which
are provided for receiving the yoke support posts 214. Still
further, the bottom layer 206 includes a pair of metal contact
openings 217 separated by the yoke address electrode 216. Of
course, other metal contact opening arrangements are contemplated,
such as additional metal contact openings and alternatively
configured metal contact openings. Electrical signals and
connections from an integrated circuit (not shown) positioned
beneath the bottom layer 206 may be sent through the pair of metal
contact openings 217 into either the yoke address electrode 216 or
the mirror address electrodes 220. The integrated circuit may be a
static random access memory (SRAM) cell or an integrated
complementary metal oxide semiconductor (CMOS) device. In other
embodiments, the integrated circuit may be a multi-chip module
(MCM) where many devices are assembled together by stacking one on
top of another into a single module for faster electronic devices
with added functionalities.
[0012] The yoke address electrode 216 generally resides in a middle
portion of the bottom layer 206 and is flanked by two outer mirror
address electrodes 220. The yoke address electrode 216 includes a
plurality of interspersed extensions 218, thereby defining a
plurality of interspersed grooves 221. In one embodiment, the
pluralities of interspersed extensions 218 are situated at opposing
lateral sides of the yoke address electrode 216. Disposed within
the plurality of grooves 221 are a plurality of corresponding
interspersed extensions 222 of the laterally disposed mirror
address electrodes 220. Accordingly, the extensions 218, 222 are
substantially interdigitated to form a comb-like structure. In some
embodiments, the yoke address electrode 216 and the two mirror
address electrodes 220 have a thickness of about 500 to about 3,000
.ANG.. Preferably, the thickness of the yoke address electrode 216
and the two mirror address electrodes 220 within the presently
disclosed embodiment is about 1,500 .ANG.. Additionally, the
interspersed extensions 218, 222 may have a corresponding width and
length of about 20 .mu.m and a thickness of about 500 to about
3,000 .ANG.. Preferably, the thickness of the interspersed
extensions 218, 222 within the presently disclosed embodiment is
about 1,500 .ANG.. Still further, the spacing between the
interspersed extensions 218, 222 can vary from about 5 to 10 .mu.m.
Preferably, the spacing between the interspersed extensions 218,
222 within the presently disclosed embodiment is about 7.5
.mu.m.
[0013] Although the interspersed extensions 218, 222 are depicted
as being square in shape, they can take on a variety of polygonal
shapes and sizes. For example, the interspersed extensions 218, 222
may be in the shape of a rectangle, a triangle, a parallelogram, a
diamond, a trapezoid or any other suitable shape. In addition, the
interspersed extensions 218, 222 may also take on plane-curve
shapes such as circles, semi-circles, ellipses, semi-ellipses,
lines, parabolas, or hyperbolas. Furthermore, the interspersed
extensions 218, 222 may be uniformly spaced or non-uniformly spaced
and uniform in shape and size or non-uniform in shape and size.
Uniform and non-uniform combinations of shapes and sizes are also
contemplated.
[0014] One benefit of the DMD 200 is realized through the amount of
electrostatic force that can be generated between the extensions
218, 222. In particular, an electrostatic force F acting upon a
charged object Q.sub.1 as a result of the presence of another
charged object Q.sub.2 can be calculated by Coulomb's law
(F=k.times.Q.sub.1.times.Q.sub.2/d.sup.2), where k is a constant
and d is the distance between the objects. The magnitude of a
charged object Q can be calculated by multiplying the surface
density .sigma. with the surface area of the charged object A
(Q=.sigma.A). Accordingly, the electrostatic force F scales
proportionally with the surface area of the charged object A
(F.alpha.A). The DMD 200 has a larger surface area when compared
with conventional DMDs, such as DMD 100 of FIG. 1. More
specifically, the interspersed extensions 218, 222 increase the
surface area of the electrodes of the DMD 200, thereby facilitating
the generation of a greater electrostatic force than that of a
conventional DMD 100.
[0015] In practice, an electrostatic field is generated by pulsing
the mirror address electrodes 220. The generated electric field in
turn generates an electrostatic force that causes the pixel mirror
208 to tilt or rotate. Unlike a conventional DMD 100, wherein the
pixel mirror 108 can experience stiction during tilting or
rotation, the DMD 200 can generate much greater electrostatic
forces thereby eliminating or at least reducing the chance that the
pixel mirror 208 will stick to underlying layers of the DMD 200. In
addition, the increased electrostatic force eliminates the need for
reset electronics.
[0016] It will be appreciated by those of ordinary skill in the art
that the invention can be embodied in other specific forms without
departing from the spirit or essential character thereof. For
example, the DMD 200 may be manufactured by surface micromachining,
where the structures are built up in layers of thin film on the
surface of a silicon wafer or any other suitable substrate. Another
technique of manufacturing a DMD is bulk micromachining. In
addition, the presently disclosed embodiments may also be applied
to MEMS devices for useful applications in the study and
understanding of biological proteins and gene functions. The
presently disclosed embodiments are therefore considered in all
respects to be illustrative and not restrictive. The scope of the
invention is indicated by the appended claims rather than the
foregoing description, and all changes that come within the meaning
and ranges of equivalents thereof are intended to be embraced
therein.
[0017] Additionally, the section headings herein are provided for
consistency with the suggestions under 37 C.F.R. .sctn. 1.77 or
otherwise to provide organizational cues. These headings shall not
limit or characterize the invention(s) set out in any claims that
may issue from this disclosure. A description of a technology in
the "Background" is not to be construed as an admission that
technology is prior art to any embodiment(s) in this disclosure.
Neither is the "Summary" to be considered as a characterization of
the embodiment(s) set forth in the claims found herein.
Furthermore, any reference in this disclosure to "embodiment" in
the singular should not be used to argue that there is only a
single point of novelty claimed in this disclosure. Multiple
embodiments may be set forth according to the limitations of the
multiple claims associated with this disclosure, and the claims
accordingly define the embodiment(s), and their equivalents, that
are protected thereby. In all instances, the scope of the claims
shall be considered on their own merits in light of the
specification, but should not be constrained by the headings set
forth herein.
* * * * *