Title: Self assembled micro anti-stiction structure
Abstract: A method and apparatus are described for reducing stiction in a MEMS device having a movable element and a substrate. The method generally comprises providing the substrate with an anti-stiction member and interposing the anti-stiction member between the moveable element and the substrate. The apparatus generally comprises an anti-stiction member that is interposable between the moveable element and the substrate. Another embodiment of the invention of the invention is directed to a MEMS device, comprising: a substrate, a moveable element moveably coupled to the substrate, and an anti-stiction member that is interposable between the moveable element and the substrate. A further embodiment of the invention is directed to an optical switch having one or more moveable elements moveably coupled to a substrate, and an anti-stiction member that is interposable between at least one of the moveable elements and the substrate. The anti-stiction member may be in the form of a flexible cantilevered structure that overhangs the moveable element. Actuating the moveable element causes the anti-stiction member to flex and snap into place between the moveable element and the substrate. An additional embodiment of the invention is directed to a method of fabricating a MEMS device. The method proceeds by providing a silicon-on-insulator (SOI) substrate; defining a moveable element from a device layer of the SOI substrate; and depositing a flexible material over the device layer and the moveable element. One or more portions of the flexible material overhang the moveable element, whereby the flexible material forms one or more anti-stiction members.
Patent Number: 6,859,577 Issued on 02/22/2005 to Lin
| Inventors:
|
Lin; Chuang-Chia (San Pablo, CA)
|
| Assignee:
|
Analog Devices Inc. (Norwood, MA)
|
| Appl. No.:
|
891760 |
| Filed:
|
June 25, 2001 |
| Current U.S. Class: |
385/18; 385/25 |
| Intern'l Class: |
G02B 006//42; G02B 006//26 |
| Field of Search: |
385/15,18,23,24,25,83,88,94
|
References Cited [Referenced By]
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|
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|
| 5867302 | Feb., 1999 | Fleming | 359/291.
|
| 6025951 | Feb., 2000 | Swart et al. | 359/245.
|
| 6114044 | Sep., 2000 | Houston et al. | 428/447.
|
| 6215921 | Apr., 2001 | Lin | 385/18.
|
| 6341039 | Jan., 2002 | Flanders et al. | 359/578.
|
| 6396975 | May., 2002 | Wood et al. | 385/18.
|
| 6498870 | Dec., 2002 | Wu et al. | 385/18.
|
| 6523961 | Feb., 2003 | Ilkov et al. | 353/99.
|
| 6538798 | Mar., 2003 | Miller et al. | 359/291.
|
| 6600591 | Jul., 2003 | Anderson et al. | 359/291.
|
| 6639572 | Oct., 2003 | Little et al. | 345/55.
|
| 6668109 | Dec., 2003 | Nahum et al. | 385/18.
|
| 6674562 | Jan., 2004 | Miles | 359/291.
|
Other References
R. Legtenberg et al., "Stiction of surface micromachined structures after
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Sensors and Actuators A, 43, pp 230-238, 1994.
H. Guckel et al, "Fabrication of Micromechanical Devices from Polysilicon
Films with Smooth Surfaces", Sensors and Actuators, 20 pp 117-122 (1989).
R. Alley et al, "The Effect of Release-Etch Processing on Surface
Microstructure Stiction", Proc. IEEE Solid-State Sensor & Actuator
Workshop, Hilton Head Island, S.C., pp. 202-207 (1992).
D. Kobayashi et al., "An Integrated Lateral Tunneling Unit" Proc. IEEE
Micro Electro Mechanical Systems, Travemunde Germany, pp 214-219, (1992).
N. Takeshima et al. Proc. Int. Conf. Solid-State Sensors & Actuators
(Transducers '91), San Francisco, CA, pp. 63-66, (1991, IEEE, New York)).
C. Mastrangelo and C. Hsu, "Mechanical Stability and Adhesion of
Microstructures Under Capillary Forces--Part I: Basic Theory" Journal of
Microelectromechanical Systems, vol. 2, No. 1 (Mar. 1993) pp. 33-43.
C. Mastrangelo and C. Hsu, "Mechanical Stability and Adhesion of
Microstructures Under Capillary Forces--Part II: Experiments" Journal of
Microelectromechanical Systems, vol. 2, No. 1, (Mar. 1993) pp. 44-55.
Storment, C., Flexible, Dry-Released Process for Aluminum Electrostatic
Actuators, Sep. 1994, pp. 90-96.
Provisional Patent Application of Behrang Behin et al., "Global Mechanical
Stop for Precise Positioning of a Field of Mirrors", filed Mar. 9, 1999.
L. Y. Lin, et al, Free Space Micromachined Optical Switches with
Submillisecond Switching Time for Large-Scale Optical Crossconnects, IEEE
Photonics Technology Letters, vol. 10, No. 4, Apr. 1998.
U.S. Appl. No. 09/489,264 of Robert L. Wood et al, "MEMS Optical
Cross-Connect Switch", filed Jan. 20, 2000.
U.S. Appl. No. 09/511,428 of Behrang Behin et al, "Cantilevered
Microstructure Methods and Apparatus", filed Feb. 23, 2000.
|
Primary Examiner: Prasad; Chandrika
Attorney, Agent or Firm: JDI Patent, Isenberg; Joshua D.
Claims
What is claimed is:
1. A method for reducing suction in a MEMS device having a moveable element
moveably coupled to a substrate, the method comprising:
a) providing the substrate with an anti-stiction member; and
b) interposing the anti-stiction member between the moveable element and
the substrate,
wherein step b) includes actuating the moveable element to interpose the
anti-stiction member between the moveable element and the substrate.
2. The method of claim 1 wherein step b) includes substantially immersing
the moveable element in a liquid during actuation of the moveable element.
3. A method for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the method comprising:
a) providing the substrate with an anti-stiction member; and
b) interposing the anti-stiction member between the moveable element and
the substrate,
wherein step a) includes providing an anti-stiction member that overhangs
the moveable element.
4. The method of claim 3, wherein the anti-stiction member includes one or
more flexible portions.
5. The method of claim 4, wherein the one or more flexible portions
includes at least one double-serpentine portion.
6. The method of claim 3 wherein the anti-stiction member is made of a
flexible material.
7. The method of claim 3 wherein step b) includes actuating the moveable
element whereby the moveable element engages the anti-stiction member
causing the anti-stiction member to flex.
8. The method of claim 7 wherein step b) includes flexing the anti-stiction
member sufficiently to interpose the anti-stiction member between the
moveable element and the substrate.
9. A method for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the method comprising:
a) providing the substrate with an anti-stiction member; and
b) interposing the anti-stiction member between the moveable element and
the substrate,
wherein step a) includes:
providing a silicon-on-insulator (SOI) substrate;
forming the moveable element from a device layer of the SOI substrate; and
depositing a flexible material over the device layer and the moveable
element such that the flexible material overhangs the moveable element.
10. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element, wherein the
anti-stiction member is cantilevered such that the anti-stiction member
overhangs the moveable element.
11. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element, wherein the
anti-stiction member is made from a flexible material.
12. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element, wherein the
anti-stiction member includes one or more flexible portions disposed
between a fixed end and a free end of the anti-stiction member.
13. The apparatus of claim 12 wherein the one or more flexible portions
include at least one serpentine portion.
14. The apparatus of claim 12 wherein the one or more flexible portions
include at least one double serpentine portion.
15. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element, further
comprising a standoff attached to a free end of the anti-stiction member.
16. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element,
further comprising means for electrically isolating the moveable element
from a portion of the substrate, wherein the means for electrically
isolating includes an electrically insulating standoff attached to a free
end of the anti-stiction member.
17. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element,
further comprising means for electrically isolating the moveable element
from a portion of the substrate, wherein the anti-stiction member includes
a serpentine shaped portion that is disposed between a free end and a
fixed end of the anti-stiction member.
18. An apparatus for reducing stiction in a MEMS device having a moveable
element moveably coupled to a substrate, the apparatus comprising:
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is attached to the substrate, wherein the
anti-stiction member is not attached to the moveable element,
further comprising means for electrically isolating the moveable element
from a portion of the substrate, wherein the anti-stiction member includes
one or more double-serpentine shaped portions that are disposed between a
free end and a fixed end of the anti-stiction member.
19. A MEMS device, comprising:
a substrate;
a moveable element moveably coupled to the substrate, and
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is cantilevered such that the
anti-stiction member overhangs the moveable element.
20. A MEMS device, comprising:
a substrate;
a moveable element moveably coupled to the substrate, and
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member is made from a flexible material.
21. A MEMS device, comprising:
a substrate;
a moveable element moveably coupled to the substrate, and
an anti-stiction member that is interposable between the moveable element
and the substrate,
wherein the anti-stiction member includes one or more flexible portions
disposed between a fixed end and a free end of the anti-stiction member.
22. The MEMS device of claim 21, wherein the one or more flexible portions
include a serpentine portion.
23. The MEMS device of claim 21, wherein the one or more flexible portions
include at least one double-serpentine portion.
24. A MEMS device, comprising:
a substrate;
a moveable element moveably coupled to the substrate, and
an anti-stiction member that is interposable between the moveable element
and the substrate,
further comprising a standoff attached to a free end of the anti-stiction
member.
25. A MEMS device, comprising:
a substrate;
a moveable element moveably coupled to the substrate, and
an anti-stiction member that is interposable between the moveable element
and the substrate, further comprising means for electrically isolating the
moveable element from a portion of the substrate, wherein the means for
electrically isolating includes an electrically insulating standoff
attached to a free end of the anti-stiction member.
26. The MEMS device of claim 25, wherein the means for electrically
isolating includes an electrically insulating portion of the moveable
element.
27. A method for fabricating a MEMS device, comprising:
providing a silicon-on-insulator (SOI) substrate;
forming a moveable element from a device layer of the SOI substrate;
forming a sacrificial layer over the moveable element and a portion of the
device layer and
depositing a flexible material over the sacrificial layer, the device layer
and the moveable element such that one or more portions of the flexible
material overhang the moveable element to form an anti-stiction member,
wherein the flexible material is deposited such that the anti-stiction
member is attached to one end to a portion of the device layer,
wherein the flexible material is deposited such that the anti-stiction
member is not attached to the moveable element; and etching the
sacrificial layer to release the moveable element,
whereby the flexible material forms one or more anti-stiction members.
28. The method of claim 27, wherein the flexible material is resistant to
an etchant that is used to remove the sacrificial layer.
29. An optical switch, comprising:
a substrate;
one or more moveable elements moveably coupled to the substrate, and
an anti-stiction member that is interposable between at least one of the
moveable elements and the substrate, wherein the anti-stiction member is
cantilevered such that the anti-stiction member overhangs the moveable
element.
30. An optical switch, comprising:
a substrate;
one or more moveable elements moveably coupled to the substrate, and
an anti-stiction member that is interposable between at least one of the
moveable elements and the substrate, wherein the anti-stiction member is
made from a flexible material.
31. An optical switch, comprising:
a substrate;
one or more moveable elements moveably coupled to the substrate, and
an anti-stiction member that is interposable between at least one of the
moveable elements and the substrate, wherein the anti-stiction member
includes one or more flexible portions disposed between a fixed end and a
free end of the anti-stiction member.
32. The optical switch of claim 31, wherein the flexible portion includes a
serpentine portion.
33. The optical switch of claim 31, wherein the flexible portion includes
at least one double serpentine portion.
34. An optical switch, comprising:
a substrate;
one or more moveable elements moveably coupled to the substrate, and
an anti-stiction member that is interposable between at least one of the
moveable elements and the substrate, further comprising a standoff
attached to a free end of the anti-stiction member.
Description
FIELD OF THE INVENTION
This invention relates generally to microelectromechanical structures
(MEMS). More particularly, it relates to reducing stiction in MEMS devices
such as those used in optical switches.
BACKGROUND OF THE INVENTION
Microelectromechanical systems (MEMS) are miniature mechanical devices
manufactured using the techniques developed by the semiconductor industry
for integrated circuit fabrication. Such techniques generally involve
depositing layers of material that form the device, selectively etching
features in the layer to shape the device and removing certain layers,
known as sacrificial layers, to release the device. Such techniques have
been used, for example, to fabricate miniature electric motors as
described in U.S. Pat. No. 5,043,043.
Silicon-on-insulator (SOI) techniques have been developed for fabricating
MEMS devices. In SOI, an oxide layer is grown or deposited on a silicon
wafer. A second silicon wafer is then bonded to the oxide layer, e.g. by
fusion bonding. After bonding, the second silicon wafer is ground back and
polished such that a thin layer of silicon is left attached to the oxide
layer to form an SOI substrate. SOI substrates are particularly useful for
MEMS devices where a moveable element formed from a silicon device layer
is to be electrically insulated from an underlying support layer.
Recently, MEMS devices have been developed for optical switching. Such
systems typically include an array of mechanically actuatable mirrors that
deflect light from one optical fiber to another. Such MEMS optical
switches are described, for example in U.S. Pat. No. 5,960,132. The
mirrors are configured to translate or rotate into the path of the light
from the fiber. Mirrors that rotate into the light path generally rotate
about a substantially horizontal axis, i.e., they "flip up" from a
horizontal position into a vertical position. MEMS mirrors of this type
are usually actuated by magnetic interaction, electrostatic interaction,
thermal actuation or some combination of these.
When the mirror is in the horizontal position, it rests against a substrate
that forms a base. Often, the mirror is subject to electromechanical
forces, sometimes referred to as "stiction" that cause the mirror to stick
to the substrate and prevent the mirror from rotating. In addition,
stiction forces can also prevent the mirror from being properly released
from the substrate during manufacture. The mechanism by which stiction
occurs can be divided into two stages: (a) mechanical collapse of the
released portion of the microstructure to contact or move very close to
the substrate and (b) adhesion of the released portion of the
microstructure to the substrate. The microstructure's mechanical collapse
can be initiated by high surface tension forces resulting from etchant
rinse liquid trapped in the capillary-like spaces between the
microstructure and the substrate, or by residual electric charges on the
microstructure and/or the substrate. Several mechanisms have been proposed
to explain the adhesion of the microstructure to the substrate, including
solid bridging, liquid bridging, Van der Waals forces, and hydrogen
bonding. Often the stuck part can be separated with increased force, but
sometimes a permanent bond is formed after the initial contact.
A number of techniques have been developed to avoid stiction. One technique
is to reduce the real contact area between the released portion of the
microstructure and the underlying substrate either through nanoscale
roughness intrinsic to one or both surfaces or through the formation of
microscale standoffs in the form of bumps or "dimples" on the
microstructure. However, such standoffs are difficult to fabricate,
particularly when they are to be fabricated from the device layer of an
SOI substrate. Consequently the stand-offs add an additional level of
complexity to the fabrication of the MEMS device. The additional
complexity increases the cost and reduces the yield of usable MEMS
devices. Another group of stiction-inhibition techniques eliminates the
source of surface tension between the released portion of microstructure
and the substrate and prevents the microstructure's initial collapse by
eliminating the gas-liquid interface. A third alternative procedure
utilizes a self-assembled monolayer to reduce the surface energy. Often, a
combination of two or more of these methods is required to eliminate the
problem of stiction. All of these techniques add to the complexity and
cost of the MEMS device.
Thus, there is a need in the art, for a simple, low-cost way of reducing
stiction in MEMS devices.
SUMMARY OF THE INVENTION
The disadvantages associated with the prior art are overcome by embodiments
of the present the present invention directed to a method and apparatus
for reducing stiction in a MEMS device having a movable element and a
substrate. The method generally comprises providing the substrate with an
anti-stiction member and interposing the anti-stiction member between the
moveable element and the substrate. The apparatus generally comprises an
anti-stiction member that is interposable between the moveable element and
the substrate. Another embodiment of the invention of the invention is
directed to a MEMS device, comprising: a substrate, a moveable element
moveably coupled to the substrate, and an anti-stiction member that is
interposable between the moveable element and the substrate. A further
embodiment of the invention is directed to an optical switch. The optical
switch generally includes a substrate, one or more moveable elements
moveably coupled to the substrate, and an anti-stiction member that is
interposable between at least one of the moveable elements and the
substrate. The anti-stiction member may be in the form of a flexible
cantilevered structure that overhangs the moveable element. Actuating the
moveable element causes the anti-stiction member to flex and snap into
place between the moveable element and the substrate.
An additional embodiment of the invention is directed to a method of
fabricating a MEMS device. The method proceeds by providing a
silicon-on-insulator (SOI) substrate; defining a moveable element from a
device layer of the SOI substrate; and depositing a flexible material over
the device layer and the moveable element. One or more portions of the
flexible material overhang the moveable element, whereby the flexible
material forms one or more anti-stiction members.
BRIEF DESCRIPTION OF THE DRAWINGS
The teachings of the present invention can be readily understood by
considering the following detailed description in conjunction with the
accompanying drawings, in which:
FIGS. 1A depict an isometric diagram illustrating an apparatus for reducing
stiction in a MEMS device according to an embodiment of the present
invention;
FIGS. 1B-1D depict schematic diagrams illustrating alternative
configurations for anti-stiction members for use with an apparatus of the
type depicted in FIG. 1A;
FIGS. 1E-1G depict a series of isometric diagram illustrating a method for
reducing stiction in a MEMS device according to an embodiment of the
present invention;
FIGS. 1H depicts an isometric diagram illustrating an alternative version
of an apparatus for reducing stiction in a MEMS device according to an
embodiment of the present invention;
FIGS. 2A-2C depict cross-section schematic diagrams illustrating a MEMS
device according to an embodiment of the invention;
FIGS. 3A-3E depict a series of cross-sectional schematic diagrams
illustrating the fabrication of a MEMS device according to an embodiment
of the present invention;
FIGS. 4A-4B depict alternative versions of MEMS devices according to an
embodiment of the present invention;
FIG. 5 depicts an isometric schematic diagram illustrating an optical
switch according to an embodiment of the present invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
This invention proposes a different method to reduce the contact area by
building a novel structure that self-assembled. The approach of the
present invention may complement other methods to more effectively
eliminate stiction. This method is particularly suitable for (but is not
limited to) devices built on SOI wafers. Although the following detailed
description contains many specific details for the purposes of
illustration, anyone of ordinary skill in the art will appreciate that
many variations and alterations to the following details are within the
scope of the invention. Accordingly, the examples of embodiments of the
invention described below are set forth without any loss of generality to,
and without imposing limitations upon, the claimed invention.
FIG. 1A depicts an example of an apparatus 99 for reducing stiction
according to an embodiment of the present invention. The apparatus 99
generally includes a MEMS device 100 having a moveable element 106
moveably coupled to a substrate 101. In the example depicted in FIG. 1A
the MEMS device 100 is formed from a silicon-on-insulator (SOI) substrate
101. The SOI substrate 101 includes an insulator layer 102 disposed
between a support layer 103 and a device layer 104. The apparatus 99
includes one or more an anti-stiction members 110 that are interposable
between the moveable element 106 and the support layer 103. The moveable
element 106 may be formed from a portion of the device layer 104. The
moveable element 106 may include a light-deflecting component 107 so that
the apparatus 100 may operate as part of a MEMS optical switch. By way of
example, the light-deflecting component 107 may be a simple plane
reflecting (or partially reflecting) surface, curved reflecting (or
partially reflecting) surface, prismatic reflector, refractive element,
prism, lens, diffractive element, e.g. grating or fresnel lens, a dichroic
coated surface for wavelength specific and bandpass selectivity, a
waveguide or some combination of these.
A hinge 108 moveably attaches the moveable element 106 to the rest of the
device layer 104. The hinge 108 is attached to the device layer and the
moveable element. The hinge 108 may be made of a flexible material that
flexes when a force or torque is exerted on the moveable element 106. In
the embodiment shown, the hinge 108 allows the moveable element 106 to
rotate with respect to the substrate 101. The hinge 108 may provide a
torque that counters rotation of the movable element 106 with respect to
the plane of the substrate 101. The hinge may be any suitable structure
such as one or more torsion hinges, cantilever flexures, serpentine
flexures, or pin-and-staple hinges combined with one or more springs. The
hinge 108 may also be a flexible member that allows vertical movement of
the movable element with respect to the plane of the substrate.
The anti-stiction members significantly decrease the area of contact
between the moveable element 106 and the substrate 101. Many designs are
possible for the anti-stiction members 110. In the example depicted in
FIG. 1A, the anti-stiction members 110 are in the form of cantilevered
bars that are attached to the device layer 102 but not to the moveable
element 106. The anti-stiction members 110 substantially overhang the
moveable element 106. The anti-stiction members 110 may be made from a
flexible material such as polysilicon or metals commonly used in the
semiconductor industry, e.g., Nickel, Tungsten, and the like.
Alternatively, the anti-stiction members 110 may be made from a suitable
polymer material. Furthermore, if the moveable element 106 is formed using
a lithography and etch process, it is often desirable that the
anti-stiction members 110 are made from a material that is resistant to
the final release etch process that forms the moveable element 106. For
example, polysilicon is resistant to hydrofluoric acid (HF). Although
bar-shaped anti-stiction members are depicted in FIG. 1A, the invention is
not limited to this particular configuration. Anti-stiction members having
other shapes, such as serpentine, U-shaped, or L-shaped may also be used.
As used herein, the term flexible means that the anti-stiction members 110
have at least one portion that is capable of flexing. Although,
flexibility may often be imparted by choice of material, the shape of the
anti-stiction member may also impart some degree of flexibility.
By way of example, and without loss of generality, FIGS. 1B-1D depict
possible alternative shapes for the anti-stiction member 110. In FIG. 1B
an anti-stiction member 110B has a serpentine portion 113B disposed
between an anchor 111B and a stand-off 112B. The serpentine portion may
impart flexibility to the anti-stiction member 110B. The anti-stiction
member 110B may be attached to a substrate at the anchor 111B. The
stand-off 112B at a free end of the anti-stiction member 110B reduces the
contact area between the anti-stiction member and the underside of a MEMS
device.
A serpentine shape such as that depicted in FIG. 1B may have an undesirable
tendency to twist. To overcome this an anti-stiction member 110C may have
double-serpentine hinge portion 113C located between a fixed end 111C and
a free end 112C, as shown in FIG. 1C. The double-serpentine hinge portion
113C may be formed by making a hole in a widened portion of the
anti-stiction member 110C. The double-serpentine hinge 113C is less
susceptible to undesired twisting that the serpentine portion 113B
depicted in FIG. 1B. Additional flexibility may be imparted by using two
double-serpentine hinges 113D as shown in FIG. 1D. The double-serpentine
hinges 113D are disposed between a fixed end 111D and a free end 112D of
an anti-stiction member 110D.
The operation of the anti-stiction bars is best understood by reference to
FIGS. 1E-1G, which depict an example of a method of reducing stiction in a
MEMS device according to an embodiment of the invention. The method begins
at FIG. 1E by providing the substrate 101 with one or more anti-stiction
members 110. The anti-stiction members 110 are then interposed between the
moveable element 106 and the substrate 101 as illustrated in FIGS. 1F-1G.
By way of example, the anti-stiction members 110 may be interposed between
the moveable element 106 and the substrate 101 as follows. First, the
moveable element 106 is actuated such that it engages the anti-stiction
members 110, thereby causing them to flex. Any suitable mechanism may be
used to actuate the moveable element 106. For example, a magnetic force or
an electrostatic force may actuate the moveable element 106. The actuating
force may cause the moveable element to rotate as shown in FIG. 1F. The
more the moveable element 106 rotates, the more the anti-stiction members
110 flex. At some point the moveable element 106 will move so far that the
anti-stiction members 110 flex past the moveable element 106 and snap into
place between the moveable element 106 and the substrate 101. More
specifically, the anti-stiction members 110 flex into position between the
moveable element 106 and the support layer 103 as shown in FIG. 1G. In
this position, the anti-stiction members 110 support the moveable element
106 and inhibit direct contact between the moveable element 106 and the
underlying portion of the substrate 101, e.g. either the support layer 103
or the oxide layer 102. Although the anti-stiction members 110 may bias
the moveable element 106 in a position that is slightly out of the plane
and/or out of parallel with respect to the device layer 104 this is not a
serious drawback. In MEMS applications, this position may correspond to an
"OFF" state where the alignment of the moveable element is not critical.
The out-of-parallel orientation may be corrected by using many pairs of
anti-stiction members 110 to bias the moveable element 106 in a position
that is substantially parallel to the device layer 104.
In a particular version of the method, the moveable element 106 may be
actuated while it is immersed in a liquid. The surface tension forces that
tend to cause stiction between the moveable element 106 and the substrate
101 may be eliminated when both are immersed in a liquid. Such actuation
may be motivated, e.g., by a magnetic field provided by a magnet located
outside the liquid. Post release stiction problems may be avoided by
actuating the moveable element 106 in liquid and interposing the
anti-stiction members 110 between the movable element 106 and the
substrate 101 before removing the moveable element 106 and substrate 101
from the liquid. Such a procedure is useful, for example, after a wet
etching process that releases the moveable element 106.
It is often desirable to electrically isolate the moveable element 106 from
the substrate 101. The moveable anti-stiction member 110 must not create
an undesirable short circuit between the moveable element 106 and the
substrate 101. For example, if the moveable element 106 is to be
electrostatically clamped to the substrate 101 a short circuit between
them will undesirably cause a current to flow. The moveable element 106
may be electrically isolated, e.g., by an insulating material disposed
between the anti-stiction member 110 and the device layer 104.
Alternatively, a portion of the oxide layer 102 may electrically isolate
the moveable element 106 from the support layer 103.
An alternative scheme for electrically insulating a moveable element from
anti-stiction members is depicted in FIG. 1H, which shows an apparatus 150
that has features in common with the apparatus 100 of FIG. 1A. In the
apparatus 150 a moveable element 156 is formed from a device layer 154 of
a substrate 151, which may also include an insulating layer 152 and a
support layer 153. A hinge 158 moveably connects the moveable element to
the device layer 154. Anti-stiction members 160 are interposeable between
the moveable element 156 and the rest of the substrate 151. The moveable
element 156 includes insulating portions 157 that contact anti-stiction
members 160. The insulating portions 160 electrically isolate the
anti-stiction members 160 from an electrically conductive portion of the
moveable element 156 thereby electrically isolating the anti-stiction
members 160 from the device layer 154. The insulating portions 157 may be
formed by etching out sections of the moveable element 156 and filling in
the etched out sections with insulating material. Similar insulating
portions may be used to isolate the hinge 158 from the device layer 154.
The present invention also includes embodiments directed to MEMS devices.
An example of such a MEMS device 200 is depicted in the cross-sections
shown in FIGS. 2A-2C. The MEMS device 200 generally includes a moveable
element 206, a substrate 201 and one or more an anti-stiction members 210
that are interposable between the moveable element 206 and the substrate
201. A hinge 208 moveably attaches the moveable element 206 to the rest of
the device layer 204. The hinge 208 is attached to the device layer and
the moveable element 206. The hinge 208 may be made of a flexible material
that flexes when a torque is exerted on the moveable element 206. In the
example depicted in FIGS. 2A-2C the MEMS device 200 is formed from a
silicon on insulator (SOI) substrate 201 having an insulator layer 202
disposed between a support layer 203 and a device layer 204. The moveable
element 206 is formed from a portion of the device layer 204. The moveable
element 206 may include a light-deflecting component 207 of any of the
types described above with respect to FIGS. 1A-1D. A magnetic material 209
such as nickel may be deposited on the moveable element 206 for magnetic
actuation. The moveable element 206 may optionally include one or more
standoffs 213 formed on an underside of the moveable element.
The anti-stiction member 210 significantly decreases the area of contact
between the moveable element 206 and the underlying portion of the
substrate 201, e.g. insulating layer 204 and/or support layer 203. In the
example depicted in FIGS. 2A-2C, the anti-stiction member 210 is in the
form of a cantilevered bar that is attached to the device layer 204 but
not to the moveable element 206. The anti-stiction member 210
substantially overhangs the moveable element 206. The overlap between the
anti-stiction member 210 and the moveable element is preferably smaller
than the overlap between the anti-stiction member and the device layer
204. The anti-stiction member may include a standoff 212 that minimizes
the contact area between the anti-stiction member and the moveable element
206. The standoff 212 may be made from an insulating material to help
electrically isolate the moveable element 206 from the substrate 201. The
anti-stiction members 210 may be made from a flexible material and may
have any suitable shape as described above. Although bar-shaped
anti-stiction members are depicted in FIGS. 2A-2C, the invention is not
limited to this particular configuration.
The operation of the anti-stiction members 210, as illustrated in FIGS.
2B-2C, proceeds substantially as described above with respect to FIGS.
1B-1D. Specifically, the anti-stiction members 210 may be interposed
between the moveable element 206 and the substrate 201 by actuating the
moveable element 206 such that it engages the anti-stiction members 210,
thereby causing them to flex as shown in FIG. 2B. For example, a magnetic
field B may exert a force on the magnetic material 209 to actuate the
moveable element 206. At some point the moveable element 206 will move so
far that the anti-stiction members 210 flex past the moveable element 206
and snap into place between the moveable element 206 and the substrate
201. In this position, the anti-stiction members 210 inhibit direct
contact between the moveable element 206 and the underlying portion of the
substrate 201, e.g. the oxide layer 202.
There are many ways of making a MEMS apparatus or device with anti-stiction
members for reducing stiction as described above. FIGS. 3A-3E depict a
series of cross-sections that illustrate an example of a method of
fabricating of a MEMS device according to another embodiment of the
invention. The method begins as shown in FIG. 3A with an SOI substrate 301
having an oxide layer 302 disposed between a support layer 303 and a
device layer 304. One or more trenches 305 are etched in the device layer
to define a moveable element 306 from the device layer 304 as shown in
FIG. 3B. A light-deflecting component (not shown) may be formed on the
moveable element either before or after forming the trenches 305. The
trenches 305 are formed all the way through the device layer 304 to the
oxide layer 302. Next a sacrificial layer 307 is formed over the device
layer 304 as shown in FIG. 3C. The sacrificial layer may be, e.g., an
oxide layer such as SiO.sub.2. The sacrificial layer 307 is patterned with
vias 309A, 309B, and 309C.
One or more patterns of flexible material are then deposited over the
sacrificial layer 307 and into the vias 309A, 309B, and 309C as shown in
FIG. 3D. By way of example, the flexible material may be polysilicon
deposited by low pressure chemical vapor deposition (LPCVD).
Alternatively, the flexible material may be a metal, such as Nickel or
Tungsten that may be deposited by evaporation, sputtering, plating and the
like. The flexible material provides a hinge 308 and an anti-stiction
member 310. The anti-stiction member 310 substantially overhangs the
moveable element 306 but is not attached to it. Via 309A provides a point
of attachment between the anti-stiction member 310 and the device layer
304. Via 309B provides a point of attachment between the hinge 308 and the
moveable element 306. Via 309C provides a point of attachment between the
hinge 308 and the device layer 304. The anti-stiction member 310 and the
hinge 308 may be formed from the same flexible material and they may be
formed at the same time. Alternatively, the hinge 308 and the
anti-stiction member 310 may be formed of different materials at different
times. A standoff 312 may be formed at a free end 311 of the anti-stiction
member 310, e.g. by patterned deposition of an insulating material.
Once the anti-stiction member 310 and hinge 308 have been formed, the
moveable element 306 may be released by etching away the sacrificial layer
307 as show in FIG. 3E. Such an etch process may be an isotropic etch in
HF The process that etches the sacrificial layer 307 may also remove a
portion of the oxide layer 302. The moveable element 306 remains attached
to the device layer 304 by the hinge 308. The anti-stiction member 310 is
attached to the device layer 304 but not the moveable element 306. The
free end 311 of the anti-stiction member overhangs the moveable element
306 and may be interposed between the moveable element 306 and the support
layer 303 in a manner similar to that shown and described above with
respect to FIGS. 1B-1D and 2A-2C.
The MEMS devices described above may be varied in many ways without
departing from the scope of the invention. For example, anti-stiction
members may be employed in beam steering MEMS elements. FIG. 4A depicts an
isometric schematic diagram of such a MEMS device 400. The device 400
generally comprises a substrate 401 having, e.g., an insulator layer 402
disposed between a support layer 403 and a device layer 404. A moveable
element 406 is formed from the device layer 404 and is attached to the
rest of the device layer 404 by torsion hinges 408A, 408B. The moveable
element 406 may include a light-deflecting element 407. The moveable
element 406 may rotate about an axis through the torsion hinges 408A,
408B, e.g. under the influence of an actuating force, e.g., an
electrostatic or magnetic force. Alternatively, the moveable element may
move by translation, e.g., in a direction substantially perpendicular to
the plane of the device layer 404. Anti-stiction members 410A, 410B may be
interposed between the moveable element 406 and the support layer 403 as
described above. Specifically, the moveable element 406 may rotate in one
direction to interpose anti-stiction member 410A and then in an opposite
direction to interpose anti-stiction member 410B. The anti-stiction
members 410A, 410B may also provide mechanical biases to the moveable
element 406.
Although, moveable elements that rotate are described herein, the present
invention is in no way limited to in rotating devices. An example of a
MEMS device 450 that uses anti-stiction members with a translating
moveable element is depicted in FIG. 4B. The device 450 generally
comprises a substrate 451 and a moveable element 456. Flexible
anti-stiction members 458 are interposable between the moveable element
456 and the substrate 451. In the device 450, the moveable element 456 is
configured to translate in direction substantially perpendicular to the
substrate 451 as shown by the double-ended arrow. By way of example, the
moveable element is retained between the substrate 451 and a cap 455. The
moveable element may move under the influence of a pneumatic force, e.g.
provided by gas that enters the space between the substrate and the cap
through a passage 453. Alternatively, the moveable element 456 may move
under the influence of an electrostatic or magnetic force. The
anti-stiction members 460 may be interposed between the substrate 451 and
the movable element 456 by exerting an actuating force on the moveable
element 456 causing it to move away from the substrate. Once the moveable
element moves far enough, the anti-stiction members 460 flex past the
moveable element 456 and into position between the moveable element 456
and the substrate 451.
The present invention also includes embodiments directed to systems that
incorporate two or more MEMS apparatus, e.g. arranged in an array. An
example of such an array is an optical switch 500 depicted in FIG. 5. The
switch 500 generally comprises a substrate 501 having an array of moveable
elements 502. Each moveable element is associated with one or more
anti-stiction members 504. The anti-stiction members 504 are interposable
between the associated moveable element 502 and the substrate 501. Each
moveable element includes a light-deflecting component 503, e.g. of any of
the types described above. By way of example, and without loss of
generality, the light deflecting component 503 one each moveable element
502 may be a mirror. The light deflecting components 503 on the moveable
elements 502 selectively couple optical signals 505 between one or more
input fibers 506 and one or more output fibers 508.
While the above includes a complete description of the preferred embodiment
of the present invention, it is possible to use various alternatives,
modifications and equivalents. It should be understood that, though
specific example applications are shown that relate to optical
communications, the present invention may be applied to reduce stiction
effects in a plurality of applications utilizing a moveable element. Such
applications may include, but not be limited to, relays, mixers, pumps,
accelerometers, RFMEMS, bioMEMS etc. Therefore, the scope of the present
invention should be determined not with reference to the above description
but should, instead, be determined with reference to the appended claims,
along with their full scope of equivalents. The appended claims are not to
be interpreted as including means-plus-function limitations, unless such a
limitation is explicitly recited in a given claim using the phrase "means
for."
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