U.S. patent application number 11/881076 was filed with the patent office on 2009-01-29 for mems device with nanowire standoff layer.
Invention is credited to Shih-Yuan Wang, Wei Wu, Wenhua Zhang.
Application Number | 20090027763 11/881076 |
Document ID | / |
Family ID | 40223902 |
Filed Date | 2009-01-29 |
United States Patent
Application |
20090027763 |
Kind Code |
A1 |
Zhang; Wenhua ; et
al. |
January 29, 2009 |
MEMS DEVICE WITH NANOWIRE STANDOFF LAYER
Abstract
A microelectromechanical systems (MEMS) device and related
methods are described. The MEMS device comprises a first member
having a first surface and a second member having a second surface,
the first and second surfaces being separated by a gap that is
closable by a MEMS actuation force applied to at least one of the
first and second members. A standoff layer is disposed on the first
surface of the first member, the standoff layer providing standoff
between the first and second surfaces upon a closing of the gap by
the MEMS actuation force. The standoff layer comprises a plurality
of nanowires that are anchored to the first surface of the first
member and that extend outward therefrom.
Inventors: |
Zhang; Wenhua; (Sunnyvale,
CA) ; Wu; Wei; (Mountain View, CA) ; Wang;
Shih-Yuan; (Palo Alto, CA) |
Correspondence
Address: |
HEWLETT PACKARD COMPANY
P O BOX 272400, 3404 E. HARMONY ROAD, INTELLECTUAL PROPERTY ADMINISTRATION
FORT COLLINS
CO
80527-2400
US
|
Family ID: |
40223902 |
Appl. No.: |
11/881076 |
Filed: |
July 24, 2007 |
Current U.S.
Class: |
359/320 ;
257/415; 438/29; 438/52; 977/762; 977/814; 977/816; 977/818;
977/819; 977/824; 977/890; 977/952 |
Current CPC
Class: |
G02B 26/0841 20130101;
B81B 3/0054 20130101; B81B 2203/0118 20130101 |
Class at
Publication: |
359/320 ;
257/415; 438/29; 438/52; 977/762; 977/814; 977/816; 977/818;
977/819; 977/824; 977/890; 977/952 |
International
Class: |
G02F 1/29 20060101
G02F001/29; H01L 41/09 20060101 H01L041/09; H01L 41/22 20060101
H01L041/22 |
Claims
1. A MEMS device, comprising: a first member having a first
surface; a second member having a second surface, said first and
second surfaces being separated by a gap that is closable by a MEMS
actuation force applied to at least one of said first and second
members; and a standoff layer disposed on said first surface of
said first member, said standoff layer providing standoff between
said first and second surfaces upon a closing of said gap by said
MEMS actuation force; wherein said standoff layer comprises a
plurality of nanowires anchored to said first surface and extending
outward therefrom.
2. The MEMS device of claim 1, wherein each of said plurality of
nanowires comprises a material that is substantially nonconducting
at a nominal operating voltage of the MEMS device.
3. The MEMS device of claim 2, wherein said nanowire material is
selected from the group consisting of SiC and high-bandgap
dielectric materials.
4. The MEMS device of claim 1, wherein said nanowires are disposed
on said first surface according to a predetermined spatial pattern,
and wherein each of said nanowires extends outward from said first
surface at one of a plurality of predetermined angles nonparallel
to said first surface.
5. The MEMS device of claim 1, wherein said nanowires are disposed
on said first surface in a random spatial pattern.
6. The MEMS device of claim 1, wherein said nanowires extend
outward from said first surface at random angles substantially
nonparallel to said first surface.
7. The MEMS device of claim 1, said plurality of nanowires being a
first plurality of nanowires, said MEMS device further comprising a
second plurality of nanowires anchored to said second surface and
extending outward therefrom.
8. The MEMS device of claim 1, said first surface comprising a
semiconductor material, wherein said nanowires comprise a material
that can be catalytically grown from said first surface according
to a CMOS compatible process.
9. The MEMS device of claim 8, wherein said nanowire material is
selected from the group consisting of Si, Ge, SiC, Group III-V
materials, and Group II-VI materials.
10. The MEMS device of claim 1, wherein each of said first and
second members including said first and second surfaces are
electrically conductive, and wherein each of said nanowires
comprises p-doped material at a first end and n-doped material at a
second end to form a diode.
11. A method of fabricating a MEMS device, comprising: forming a
first member having a first surface and a second member having a
second surface, said first and second surfaces being separated by a
gap that is closable by a MEMS actuation force applied to at least
one of said first and second members; and growing a plurality of
nanowires anchored to said first surface and growing outward
therefrom, whereby said plurality of nanowires form a standoff
layer providing standoff between said first and second surfaces
upon a closing of said gap by said MEMS actuation force.
12. The method of claim 11, further comprising forming a
predetermined pattern of metallic islands on said first surface,
wherein said growing the plurality of nanowires comprises
catalytically growing the plurality of nanowires from said first
surface at locations corresponding to said metallic islands.
13. The method of claim 12, wherein said forming said first and
second members comprises: prior to said forming the predetermined
pattern, forming said first member in a first layer; subsequent to
said forming the predetermined pattern, forming a second layer
above said first layer and said metallic islands, wherein said
second layer is a sacrificial layer; forming said second member in
a third layer above said second layer such that at least a portion
of said second layer is exposed; and prior to said catalytically
growing said plurality of nanowires, removing said second layer to
expose said metallic islands.
14. The method of claim 11, further comprising: subsequent to said
forming said first and second members including said gap
therebetween, filling said gap with a colloidal solution containing
metallic nanoparticles; and drying said colloidal solution, whereby
a first plurality of said metallic nanoparticles from said
colloidal solution remain on said first surface at a first
plurality of random locations; wherein said growing the plurality
of nanowires comprises catalytically growing said nanowires at said
first plurality of random locations.
15. The method of claim 14, said plurality of nanowires being a
first plurality of nanowires, said drying said colloidal solution
further resulting in a second plurality of metallic nanoparticles
remaining on said second surface at a second plurality of random
locations, the method further comprising catalytically growing a
second plurality of nanowires extending outward from said second
surface at said second plurality of random locations.
16. The method of claim 14, wherein said first member comprises a
uniform crystalline structure at said first surface thereof,
whereby each of said nanowires grows outward from said first
surface at one of a plurality of predetermined angles substantially
nonparallel to said first surface.
17. The method of claim 14, wherein said first member comprises a
polycrystalline structure at said first surface thereof, whereby
said nanowires grow outward from said first surface at random
angles.
18. A method comprising operating a MEMS device to cause a
mechanical closing of a gap between a first surface of a first
member of the MEMS device and a second surface of a second member
of the MEMS device, wherein at least said first surface comprises a
plurality of nanowires anchored thereto and extending outward
therefrom to form a standoff layer that provides standoff between
said first and second surfaces upon said closing of said gap.
19. The method of claim 18, wherein said nanowires are disposed on
said first surface according to a predetermined spatial pattern,
and wherein each of said nanowires extends outward from said first
surface at one of a plurality of predetermined angles substantially
nonparallel to said first surface.
20. The method of claim 19, wherein said nanowires are disposed on
said first surface in a random spatial pattern.
Description
FIELD
[0001] This patent specification relates to microelectromechanical
systems (MEMS) devices. More particularly, this patent
specification relates to standoff structures in MEMS devices.
BACKGROUND
[0002] Advances in microelectromechanical systems (MEMS) technology
continues to increase the applicability of MEMS devices to a
variety of technological endeavors. By way of example, MEMS
integrated circuit devices can be found in inkjet printers,
automobile airbag systems, automobile stability control systems,
optical switches, pressure sensors, computer and television display
systems, and optical switches. Further applications for MEMS
devices continue to be developed. As used herein, MEMS device
refers generally to a micro-scale or nano-scale apparatus having
two or more parts adapted to move relative to one another, where
the motion is based on any suitable interaction or combination of
interactions, such as mechanical, thermal, electrical, magnetic,
optical, and/or chemical interactions. MEMS devices include,
without limitation, NEMS (nanoelectromechanical systems) devices,
MOEMS (micro-opto-electromechanical systems) devices, and devices
with analogous functionalities having alternative nomenclatures as
may be currently or hereinafter used or adopted.
[0003] The operation of many MEMS devices involves actuation that
closes a gap between two members such that their surfaces are
brought into actual or imminent contact with each other. By way of
example, a simple MEMS-based optical switch may comprise a
cantilevered mirror element disposed above a substrate layer, such
that a voltage applied between the mirror element and the substrate
layer causes flexing of the mirror element toward the substrate by
electrostatic attraction, whereby a light beam impinging upon the
mirror element is redirected according to the applied voltage. In
another example, large arrays of such mirror elements can be placed
on a single chip and individually driven by separate voltages to
form a DMD (deformable mirror device) for use in a computer or
television display system.
[0004] Moveable MEMS elements, such as the above-described
cantilevered mirror element, often have high surface area to volume
ratios such that surface effects can become competitive with mass,
inertia, and the various "intentional" forces in dictating the
actual movement (or lack thereof that physically occurs. So-called
stiction forces, for example, can cause a mirror element to stick
to a substrate and prevent the mirror element from moving. Various
mechanisms have been proposed to explain such adhesion, including
solid bridging, liquid bridging, "cold welding," 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. In addition to degrading device
performance or causing outright device failure, stiction issues can
also underlie increased margin requirements in device design (e.g.,
building in a higher spring coefficient for a deformable member,
increasing actuation/release voltages, etc.) which can increase
device size, cost, and power requirements, while reducing device
speed and efficiency. Other issues arise as would be apparent to
one skilled in the art in view of the present disclosure.
SUMMARY
[0005] In one embodiment, a MEMS device is provided, comprising a
first member having a first surface and a second member having a
second surface, the first and second surfaces being separated by a
gap that is closable by a MEMS actuation force applied to at least
one of the first and second members. A standoff layer is disposed
on the first surface of the first member, the standoff layer
providing standoff between the first and second surfaces upon a
closing of the gap by the MEMS actuation force. The standoff layer
comprises a plurality of nanowires that are anchored to the first
surface and that extend outward therefrom.
[0006] Also provided is a method of fabricating a MEMS device. The
method comprises forming a first member having a first surface and
a second member having a second surface. The first and second
surfaces are separated by a gap that is closable by a MEMS
actuation force applied to at least one of the first and second
members. The method further comprises growing a plurality of
nanowires anchored to the first surface and growing outward
therefrom. The plurality of nanowires forms a standoff layer that
provides standoff between the first and second surfaces upon a
closing of the gap by the MEMS actuation force.
[0007] Also provided is a method comprising operating a MEMS device
to cause a mechanical closing of a gap between a first surface of a
first member of the MEMS device and a second surface of a second
member of the MEMS device. The first surface comprises a plurality
of nanowires anchored thereto and extending outward therefrom to
form a standoff layer that provides standoff between the first and
second surfaces upon the closing of the gap.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1A-1B illustrate perspective views of a MEMS device
according to an embodiment;
[0009] FIG. 1C illustrates a side view of the MEMS device of FIGS.
1A-1B;
[0010] FIG. 2 illustrates a side view of a MEMS device according to
an embodiment;
[0011] FIG. 3 illustrates a side view of a MEMS device according to
an embodiment;
[0012] FIG. 4 illustrates a side view of a MEMS device according to
an embodiment;
[0013] FIG. 5 illustrates a side view of a MEMS device according to
an embodiment;
[0014] FIG. 6 illustrates fabricating a MEMS device according to an
embodiment; and
[0015] FIG. 7 illustrates fabricating a MEMS device according to an
embodiment.
DETAILED DESCRIPTION
[0016] FIGS. 1A-1B illustrate perspective views of a MEMS device
100 according to an embodiment, with FIG. 1A illustrating an
open-gap configuration and FIG. 1B illustrating a closed-gap
configuration. FIG. 1C illustrates a side view of the MEMS device
100 in the open-gap configuration. MEMS device 100 comprises a
lower member 102 having a first surface 104, and an upper member
106 having a second surface 108. The upper member 106 forms part of
a larger cantilever mirror element 105 that is spaced apart from
the lower member 102 by a spacer element 116 at one end. In the
open configuration, there is a gap 110 between the second surface
108 and the first surface 104. The MEMS device 100 preferably
comprises materials and structures compatible with CMOS
(complementary metal-oxide-semiconductor) fabrication technologies,
which are well known and around which large bases of installed
fabrication equipment has been built up worldwide. For higher-power
applications, the MEMS device 100 can comprise materials and
structures compatible with bipolar and/or BiCMOS (an integration of
bipolar and CMOS) fabrication technologies.
[0017] It is to be appreciated that the relative simple MEMS device
100 is presented for clarity of description. There are many other
types of MEMS structures for which one or more of the embodiments
is applicable. Likewise, real-world MEMS structure implementations
incorporating one or more of the embodiments will invariably have
substantially more layers and elements than illustrated in FIGS.
1A-1C and the other drawings herein. For example, although in one
embodiment (i) the lower member 102 is part of a substrate layer
upon which a remainder of the MEMS structure is built and (ii)
cantilever mirror element 105 is in an uppermost surface, in other
embodiments the lower member 102 and cantilever mirror element 105
are formed in intermediate layers among tens or even hundreds of
other layers in the overall MEMS device. Also for clarity of
description, only a portion of the cantilever mirror element 105,
termed the upper member 106, is described in further detail, as
there are many different actuation mechanisms for causing movement
of the upper member 106 other than cantilever-style elements that
are within the scope of the present teachings.
[0018] It is to be appreciated that many different mechanisms can
be used to actuate the upper member 106 relative to the lower
member 102. It is to be further appreciated that as used herein,
MEMS actuation force applied to a member refers to any MEMS-based
cause of motion for that member, including direct actuation (for
example, an electrostatic force directly being exerted on the
member by application of a voltage) and indirect actuation (for
example, the member is mechanically coupled to a different
MEMS-moveable element that is being actuated).
[0019] It is to be further appreciated that, for clarity of
description, the drawings are not drawn to a uniform scale. In
particular, vertical and horizontal scales may differ from each
other and may vary from one drawing to another. In this regard,
directional terminology, such as "top," "bottom," "front," "back,"
"leading," "trailing," etc., is used with reference to the
orientation of the drawing figure(s) being described. Because
components of the embodiments can be positioned in a number of
different orientations, the directional terminology is used for
purposes of illustration and is in no way limiting.
[0020] The gap 110 is closable by a MEMS actuation force applied to
at least one of the lower member 102 and the upper member 106. In
the simplified example of FIGS. 1A-1C, a voltage "V" between the
lower member 102 and the cantilever element 105 is applied by other
circuitry in the MEMS device 100 (not shown). When the voltage is
"ON" (see FIG. 1B), the cantilever element 105 is flexed downward
toward the lower member 102 by electrostatic force, which in turn
forces the upper member 106 to move toward the lower member 102
toward a closed-gap position. For many implementations, the gap 110
will usually be about 1 micron, although the scope of the
embodiments is not so limited. The lower member 102 may comprise,
for example, crystalline Si, while the upper member 106 may
comprise a reflective metal. In other embodiments, the upper member
106 may comprise crystalline Si having a reflective layer (not
shown) coated on its upward-facing surface.
[0021] MEMS device 100 further comprises a standoff layer 112
disposed on the first surface 104 of the lower member 102, the
standoff layer 112 comprising a plurality of nanowires 114 anchored
to the first surface 104 and extending upwardly (outwardly)
therefrom. Standoff generally refers to a condition and/or
functionality whereby two surfaces that are otherwise being forced
together into contact are held back from actual contact which, in
the case of the present teachings, is provided by an outward or
repelling force provided by physical contact of the nanowires 114,
primarily at their tips, with the respective surfaces. The first
and second surfaces 102/106 would touch but for the presence of the
standoff layer 112. Advantageously, depending on their spatial
density, materials, and dimensions, the nanowires 114 will bend
somewhat as the standoff force is provided, thereby providing a
degree of springability to the standoff characteristic. Among other
advantages, stiction problems that could otherwise prevent the
release of the upper member 106 from the lower member 102 are at
least partially overcome by virtue of the standoff functionality
provided by the nanowires 114 of the standoff layer 112.
[0022] Nanowires 114 preferably comprise material that can be
catalytically grown outward from the first surface 104. The
particular selection of nanowire material will be influenced by the
nature of the material of the lower member 102 at the first surface
104. In one embodiment, the lower member 102 comprises crystalline
or polycrystalline Si at the first surface 104 (or amorphous Si
having sufficient short-term crystalline signature for nanowire
growth), and the nanowires 114 also comprise Si. More generally,
the materials for lower member 102 at the first surface 104 and the
nanowires 114 can be selected from a variety of nanowire
growth-amenable combinations, as would be identifiable by a person
skilled in the art, selected from a group comprising Group IV
materials (e.g., Si, Ge, SiC), Group III-V materials (e.g., GaN,
GaP, InP, InAs, AlN), and Group II-VI materials (e.g., ZnO, CdS).
For clarity of description and not by way of limitation, a
particularly useful implementation in which silicon is used as both
the substrate and nanowire material is presented further
herein.
[0023] In one embodiment, the nanowires 114 have lengths between
about 5 percent and 50 percent of the gap distance 110 which, for
the case of a 1 .mu.m gap, are lengths between about 50 nm and 500
nm. In other embodiments, the nanowires 114 can be shorter than 50
nm or longer than 500 nm, and the lengths of the nanowires can vary
spatially within one MEMS cell or spatially across different MEMS
cells. The length of the nanowires 114 can even approach 100
percent of the gap distance without departing from the scope of the
present teachings. In one embodiment, the nanowires 114 have
diameters in the range of 10 nm-200 nm, although the scope of the
present teachings is not so limited. A large range of nanowire
population densities are within the scope of the present teachings,
ranging from thousands (or fewer) of nanowires per square
centimeter to millions (or more) of nanowires per square
centimeter.
[0024] In one embodiment, the nanowires 114 are anchored to the
surface 104 by virtue of having been catalytically grown therefrom
and being structurally integral therewith. In distinction to
sprinkled coatings of unanchored nanowires on the approaching
surfaces, many of which would simply take on an orientation
parallel to the approaching surfaces, anchoring of the nanowires
facilitates springability of the standoff functionality. The
anchoring of the nanowires 114 also inhibits them from loosely
floating about the MEMS device 100, which would be especially
problematic if they were electrically conducting, in which case
various electrical components/connections of the MEMS device could
become shorted by the loosely floating nanowires.
[0025] For one embodiment, the nanowires 114 comprise a material
that is substantially nonconducting at a nominal operating voltage
of the MEMS device, such that they effectively act as insulator
materials. SiC or other high-bandgap dielectric materials will
often be suitable for many practical implementations.
Advantageously, in the event of breakage from their anchored
position and movement about the MEMS device, various electrical
components/connections of the MEMS device would not become shorted
by the insulative nanowires. However, the scope of the present
teachings is not so limited, and in other embodiments the nanowires
114 can comprise conductive materials.
[0026] For the embodiment of FIGS. 1A-1C, the nanowires 114 are
regularly spaced on the first surface 104 according to a
predetermined pattern, which can be achieved by patterning the
first surface 104 with islands of catalyst metal such as titanium
or gold prior to growing the nanowires, as described further infra.
For the embodiment of FIGS. 1A-1C, the nanowires 114 extend upward
(outward) from the first surface 104 at a vertical (normal) angle.
For Si nanowires growing out of crystalline Si, this can be
achieved by ensuring that the lower member 102 consists of
<111> Si at the first surface 104. In other embodiments, the
nanowires 114 may extend upward at a uniform off-vertical angle, as
may be achieved, for example, using other crystalline orientations,
various surface-tilting strategies during fabrication, or other
methods known in the art.
[0027] FIG. 2 illustrates a side view of a MEMS device 200
according to an embodiment, comprising a lower member 202 having a
first surface 204 separated by a gap 210 from a second surface 208
on an upper member 206, and further comprising a standoff layer 212
formed by a plurality of nanowires 214 anchored to the lower member
202 at the first surface 204. As illustrated in FIG. 2, each of
said nanowires 214 extends outward from the first surface 204 at
one of a small number of predetermined angles nonparallel to the
first surface 204. The regular angular orientation of nanowires can
be beneficial over purely vertical nanowires for reasons including
more predictable springability functionality. As would be known to
a person skilled in the art, the angular growth of the nanowires
can be achieved by using a crystalline lower member 202 with
judicious selection of crystal directions at the surface 204. By
way of example, for Si nanowires growing out of crystalline Si,
each nanowire will grow at one of three predetermined possible
orientations if the lower member 202 comprises <100> Si at
the first surface 204.
[0028] FIG. 3 illustrates a side view of a MEMS device 300
according to an embodiment, comprising a lower member 302 that
includes a polycrystalline layer 303 that can be relatively thin
(e.g., 100 nm), the lower member 302 having a first surface 304 at
the top of the polycrystalline layer 303. The first surface 304 is
separated by a gap 310 from a second surface 308 on an upper member
306, and the MEMS device 300 further comprises a standoff layer 312
formed by a plurality of nanowires 314 anchored to the lower member
302 at the first surface 304. As illustrated in FIG. 3, the
nanowires 314 are disposed across the first surface 304 in a random
spatial pattern and extend outward from the first surface 304 at
random angles substantially nonparallel to the first surface 304.
The spatial randomness of the nanowires 314 can be advantageous
over regular spacing for reasons including ease of fabrication.
[0029] The angular randomness of the nanowires 314 is facilitated
by the polycrystalline layer 303 (such as polycrystalline Si),
which provides short-term crystal signatures that are usually
sufficient for nanowire growth. In another embodiment, the layer
303 can instead comprise certain kinds of amorphous Si having
sufficient short-term crystalline signatures for nanowire growth.
The angular randomness of the nanowires 314 can be advantageous
over specified, predetermined angles for reasons including one or
more of (i) a "flatter" springability response, (ii) an ability to
statistically tailoring a springability response, (iii) avoidance
of stiction problems that could be associated with short-term or
long-term regularities or uniformities in the interactions between
the nanowire tips and the approaching surface during MEMS
actuation, or (iv) for other reasons.
[0030] FIG. 4 illustrates a side view of a MEMS device 400
according to an embodiment, comprising a lower member 402 that
includes a polycrystalline layer 403, the lower member 402 having a
first surface 404 at the top of the polycrystalline layer 403. The
first surface 404 is separated by a gap 410 from a second surface
408 on an upper member 406. The MEMS device 400 further comprises a
first standoff layer 412 formed by a first plurality of nanowires
414 anchored to first surface 404 at random locations and extending
upward therefrom at random angles.
[0031] For further increasing standoff functionality according to
an embodiment, the MEMS device 400 further comprises a second
standoff layer 416 formed by a second plurality of nanowires 418
anchored to the upper member 406 at the second surface 408 and
extending downward therefrom. Ranges of diameter, length, and
density for the second plurality of nanowires 418 may be similar,
to or different than, those for the first plurality of nanowires
414 without departing from the scope of the present teachings.
Fabrication of the second plurality of nanowires 418 can be formed
in a same fabrication step using a same colloidal suspension of
metallic catalytic nanoparticles as described infra with respect to
FIG. 7. For the particular example of FIG. 4, the upper member
comprises <111> Si at the second surface 408 such that the
nanowires 418, although being randomly placed, extend downward at a
vertical (normal) angle from the second surface 408.
[0032] FIG. 5 illustrates a side view of a MEMS device 500
according to an embodiment, comprising a lower member 502 having a
first surface 504 separated by a gap 510 from a second surface 508
on an upper member 506, and further comprising a standoff layer 512
formed by a plurality of nanowires 514 anchored to the lower member
502 at the first surface 504. According to an embodiment, for each
nanowire 514, a first type of dopant (p-type, e.g., boron) is added
to the gasses during a first phase of the catalytic growth process
to create a p-type semiconductor portion 522, whereas a second type
of dopant (n-type, e.g., phosphorus) is added to the gasses during
a second phase to create an n-type semiconductor portion 520.
Accordingly, each nanowire 514 can also be functional as a
semiconductor heterojunction diode, which can be used for various
advantageous purposes.
[0033] By way of example, in one of many possible scenarios, the
first and second members 502/506 may be conductive members across
which a voltage (having a positive polarity at the second member
506) is applied to result in an electrostatic attraction. If that
voltage is increased beyond a certain tolerable level, the
attraction force may be too great and various components (for
example, a cantilever element to which the upper member 506 may be
connected) may be irreparably damaged. To avoid this event, the
nanowire/diode 514 (along with a similar population of
nanowire/diodes) can be configured as a zener diode, as illustrated
by a zener diode symbol in FIG. 5. The nanowire/zener diode 514 can
be designed to enter avalanche breakdown mode when the voltage gets
too high, thereby protecting the device from damage.
Advantageously, the nanowire/zener diode 514 would otherwise not
disturb the operation of the device, acting simply as a
reverse-biased diode for voltages below the avalanche breakdown
threshold.
[0034] FIG. 6 illustrates fabricating a MEMS device according to an
embodiment in which only random patterns of standoff nanowires are
needed. At step 652, the basic MEMS structure is formed using known
methods, the MEMS structure comprising a lower member 602 having a
first surface 604 separated by a gap 610 from a second surface 608
of an upper member 606. At step 654, subsequent to the forming of
the first and second members 602/606 including the gap 610, the gap
610 is filled with a colloidal solution containing metallic
nanoparticles 620. For nanowires, the metallic nanoparticles 620
can comprise titanium or gold. The metallic nanoparticles 620
should be sized according to the desired diameter of the nanowires.
At step 656, the colloidal solution is dried, whereupon some of the
metallic nanoparticles 620 remain on the first surface 604 at
random locations, and whereupon others of the metallic
nanoparticles 620 remain on the second surface 608 at random
locations. At step 658, nanowires 614 are catalytically grown from
the first surface 604 of the lower member 602 (and residue metal is
removed, etc.).
[0035] In the particular example of FIG. 6, the upper member 606
comprises a metallic hinge material at the second surface 608 and,
therefore, no nanowires are grown therefrom at step 658. In an
alternative embodiment in which upper member 606 comprises a
crystalline material at the second surface 608, nanowires can also
grow therefrom in a downward direction at step 658. In the
particular example of FIG. 6, <111> Si is used for the lower
member 602 at the first surface 604, and therefore the nanowires
614 grow vertically therefrom. In accordance with principles
described supra, random nanowire orientations can be achieved by
having polycrystalline Si at the first surface 604, while
predetermined angular orientations can be achieved by having
<100> Si at the first surface 604.
[0036] FIG. 7 illustrates fabricating a MEMS device according to an
embodiment in which standoff nanowires according to a predetermined
spatial pattern are desired. At step 752, the lower member 702 is
formed and islands 720 of metallic catalytic growth material such
as titanium are formed thereon according to a predetermined
pattern. The patterning can be achieved using any of a variety of
known methods based on nanoimprint lithography, photolithography,
electron beam lithography, etc. In many cases the metallic islands
will extend about 5-10 nm above a first surface 704 of the lower
member 702. At step 754, a sacrificial layer 724 is formed atop the
first surface 704 as well as atop the metallic islands 720, and an
upper layer 726 comprising the intended material for the upper
member is formed. At step 757, the upper member 706 is patterned
from the upper layer 726 in a manner that exposes at least a
portion of the sacrificial layer 724. At step 758 an isotropic etch
is performed to remove the sacrificial layer 724 while keeping the
upper member 706, lower member 702, and metallic islands 720
intact. At step 760, nanowires 714 are catalytically grown from the
first surface 704 of the lower member 702 (and residue metal is
removed, etc.) in a manner similar to that described supra with
respect to step 658 of FIG. 6.
[0037] It is to be appreciated that practical fabrication processes
for a MEMS device according to one or more of the embodiments will
typically involve many prior preparation steps, sub-steps, and
post-processing steps with respect to the steps of the
above-described fabrication methods. However, one skilled in the
art of MEMS fabrication, CMOS processing, and related arts would be
readily able to fabricate a MEMS device according to one or more of
the embodiments without undue experimentation in view of the
present disclosure.
[0038] In one or more embodiments, the standoff nanowires can be
considered as forming a grass-like, forest-like, artificial
turf-like, or felt-like structure that facilitates release of
MEMS-actuated surfaces that might otherwise stick when the gap
between them closes. Although the specific mechanisms at work may
be different, the effect can analogized to picking up a playing
card from a felt table versus picking it up from a smooth glass
table, or picking up a smooth plastic placard from an artificial
turf surface versus picking it up from a smooth linoleum
surface.
[0039] The use of a nanowire population as a MEMS standoff
mechanism according to one or more of the embodiments can be
advantageous over a simple surface "roughening" at least because a
variety of different nanowire lengths, nanowire patch locations,
and springabilities are offered and can be custom matched for a
particular application, whereas the options for simple roughening
are more limited. With regard to springability, it is believed
unlikely that springability can be provided by simple roughened
surfaces or, even if so, the amount of springability would be
relatively crudely adjustable and not as readily tuned (e.g., by
choice of nanowire length, material, orientation etc.) as nanowire
standoff layers according to one or more of the embodiments.
[0040] The use of a nanowire population as a MEMS standoff
mechanism according to one or more of the embodiments can be
advantageous over the use of one or more large stand-off
bumps/posts at least because a more distributed quality of standoff
support can be provided (e.g., causing fewer surface strains in the
contacting member(s)), along with the above-mentioned
customizability and springability advantages. Embodiments in which
the standoff nanowires comprise substantially nonconducting
materials, such as SiC and high-bandgap dielectric materials, would
be advantageous over embodiments based on carbon nanotubes because
the latter are highly electrically conductive and, if they broke
off from their locations, undesired electrical shorts in various
portions of the MEMS device could occur, whereas nonconducting
materials would not cause such shorts if they broke off.
[0041] Silicon is the material most commonly used to create the
integrated circuits used in most consumer electronics in the modern
world. Economies of scale, ready availability of inexpensive
high-quality silicon materials, and the ability to incorporate
electronic functionality also make silicon attractive for a wide
variety of MEMS applications. Technologies built up around the use
silicon materials include CMOS (complementary
metal-oxide-semiconductor), bipolar, and BiCMOS (integration of
bipolar and CMOS technologies), and associated silicon-based
micromachining processes. Advantageously, fabricating nanowire
standoff layers according to one or more of the embodiments is
highly compatible with CMOS, bipolar, and/or BiCMOS processing such
that the same set of reactor equipment can be used for fabricating
both the nanowire standoff layers and the various surrounding MEMS
structures. This can be advantageous over embodiments based on
carbon nanotubes and having fabrication based on materials (such as
iron or nickel) and/or process chemicals (such as acetylene-ammonia
mixtures) that are generally incompatible with CMOS, bipolar,
and/or BiCMOS processing.
[0042] Fabrication of MEMS devices according to one or more of the
embodiments can be achieved using known fabrication methods
including, but not limited to: deposition methods such as chemical
vapor deposition (CVD), metal-organic CVD (MOCVD), plasma enhanced
CVD (PECVD), chemical solution deposition (CSD), sol-gel based CSD,
metal-organic decomposition (MOD), Langmuir-Blodgett (LB)
techniques, thermal evaporation/molecular beam epitaxy (MBE),
sputtering (DC, magnetron, RF), and pulsed laser deposition (PLD);
lithographic methods such as optical lithography, extreme
ultraviolet (EUV) lithography, x-ray lithography, electron beam
lithography, focused ion beam (FIB) lithography, and nanoimprint
lithography; removal methods such as wet etching (isotropic,
anisotropic), dry etching, reactive ion etching (RIE), ion beam
etching (IBE), reactive IBE (RIBE), chemical-assisted IBE (CAIBE),
and chemical-mechanical polishing (CMP); modifying methods such as
radiative treatment, thermal annealing, ion beam treatment, and
mechanical modification; and assembly methods such as wafer
bonding, surface mount, and other wiring and bonding methods.
[0043] Whereas many alterations and modifications of the
embodiments will no doubt become apparent to a person of ordinary
skill in the art after having read the foregoing description, it is
to be understood that the particular embodiments shown and
described by way of illustration are in no way intended to be
considered limiting. By way of example, within the scope of the
embodiments is a MEMS device having a large population of cells
with closably-gapped members and nanowire standoff layers similar
to at least one of the above-described embodiments, and being
further designed such that the nanowires of different cells are
patterned differently (e.g., different sections of the surfaces
populated, different nanowire orientation strategies, etc.) or the
nanowires of different cells are formed from different materials
(e.g., conducting, insulating, diode-forming, etc.) for achieving
different mechanical and/or electrical device goals. Thus,
reference to the details of the described embodiments is not
intended to limit their scope.
* * * * *