U.S. patent application number 09/777749 was filed with the patent office on 2001-07-19 for methods of fabricating in-plane mems thermal actuators.
Invention is credited to Cowen, Allen, Dhuler, Vijayakumar R., Hill, Edward.
Application Number | 20010008357 09/777749 |
Document ID | / |
Family ID | 23561566 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008357 |
Kind Code |
A1 |
Dhuler, Vijayakumar R. ; et
al. |
July 19, 2001 |
Methods of fabricating in-plane MEMS thermal actuators
Abstract
A MEMS thermal actuator device is provided that is capable of
providing linear displacement in a plane generally parallel to the
surface of a substrate. Additionally, the MEMS thermal actuator of
the present invention may provide for a self-contained heating
mechanism that allows for the thermal actuator to be actuated using
lower power consumption and lower operating temperatures. The MEMS
thermal actuator includes a microelectronic substrate having a
first surface and at least one anchor structure affixed to the
first surface. A composite beam extends from the anchor(s) and
overlies the first surface of the substrate. The composite beam is
adapted for thermal actuation, such that it will controllably
deflect along a predetermined path that extends substantially
parallel to the first surface of the microelectronic substrate. In
one embodiment the composite beam comprises two or layers having
materials that have correspondingly different thermal coefficients
of expansion. As such, the layers will respond differently when
thermal energy is supplied to the composite. An electrically
conductive path may extend throughout the composite beam to
effectuate thermal actuation. In one embodiment of the invention a
two layer composite beam comprises a first layer of a semiconductor
material and a second layer of a metallic material. The
semiconductor material may be selectively doped during fabrication
so as to create a self-contained heating mechanism within the
composite beam. The invention also comprises a MEMS thermal
actuator that includes two or more composite beams. The two or more
composite beams may be disposed in an array or a ganged fashion,
such that the multiple composite beams benefit from overall force
multiplication and are therefore capable of greater and more linear
displacement distances. The invention is also embodied in a method
for fabricating the MEMS thermal actuators of the present
invention.
Inventors: |
Dhuler, Vijayakumar R.;
(Raleigh, NC) ; Hill, Edward; (Chapel Hill,
NC) ; Cowen, Allen; (Cary, NC) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
23561566 |
Appl. No.: |
09/777749 |
Filed: |
February 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09777749 |
Feb 6, 2001 |
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09395068 |
Sep 13, 1999 |
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6211598 |
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Current U.S.
Class: |
310/306 |
Current CPC
Class: |
Y10S 310/06 20130101;
H01H 1/0036 20130101; F16K 2099/008 20130101; H01H 37/32 20130101;
B81B 2201/031 20130101; F16K 99/0007 20130101; F16K 99/0042
20130101; F15C 5/00 20130101; B81B 3/0024 20130101; H01H 2037/008
20130101; F16K 2099/0074 20130101; F16K 99/0001 20130101; F03G 7/06
20130101; F16K 99/0036 20130101 |
Class at
Publication: |
310/306 |
International
Class: |
H02N 010/00 |
Claims
That which is claimed:
1. A microelectromechanical (MEMS) actuator structure comprising: a
microelectronic substrate having a first surface; an anchor
structure affixed to the first surface of said microelectronic
substrate; and a composite beam extending from said anchor
structure at a proximate end and cantilevered over the first
surface of said microelectronic substrate, wherein said composite
beam is adapted for thermal actuation so as to controllably move a
distal end of said composite beam along a predetermined path that
extends substantially parallel to the first surface of said
microelectronic substrate.
2. The MEMS actuator of claim 1, wherein said composite beam
further comprises an electrically conductive path through said
composite beam, such that said composite beam is thermally actuated
in response to electrical current flowing therethrough.
3. The MEMS actuator of claim 2, wherein said anchor further
comprises at least two contacts in operable connection with the
electrically conductive path.
4. The MEMS actuator of claim 1, wherein said composite beam
further comprises at least two layers which have differing thermal
coefficients of expansion so as to respond differently to thermal
actuation.
5. The MEMS actuator of claim 4, wherein the at least two layers
are disposed in a generally perpendicular relationship with respect
to the first surface of said microelectronic substrate.
6. The MEMS actuator of claim 4, wherein the at least two layers
further comprises a first layer including a semiconductor
material.
7. The MEMS actuator of claim 1, wherein the composite beam further
includes a semiconductor material layer having a controllably doped
region.
8. The MEMS actuator of claim 6, wherein the first layer including
a semiconductor material further comprises a controllably doped
region so as to impart self-heating capabilities to said composite
beam.
9. The MEMS actuator of claim 1, wherein the composite beam further
includes a metallic material layer.
10. The MEMS actuator of claim 4, wherein the at least two layers
further comprise a metallic material layer.
11. The MEMS actuator of claim 4, wherein the at least two layers
further comprise a first layer including a semiconductor material
and a second layer including a metallic material, wherein the
second layer has a greater thermal coefficient of expansion than
the first layer, such that said composite beam deflects toward the
first layer at the distal end in response to selective thermal
actuation.
12. The MEMS actuator of claim 6, wherein the first layer includes
a semiconductor material having a controllably doped region so as
to impart self-heating capabilities to said composite beam.
13. The MEMS actuator of claim 12, wherein the controllably doped
region of said first layer is operably in contact a point removed
from the proximate end of said composite beam with said second
layer, such that a conductive resistance path is provided by said
composite beam.
14. The MEMS actuator of claim 1, wherein said microelectronic
substrate further defines a trench generally underlying said
composite beam.
15. The MEMS actuator of claim 1, wherein said anchor further
comprises a first anchor portion and a second anchor portion which
are physically separated by an air gap.
16. The MEMS actuator of claim 15, wherein said first anchor
portion further comprises a first contact and said second anchor
portion further comprises a second contact, such that an
electrically conductive path is initiated at the first contact,
travels through said composite beam and returns via the second
contact.
17. The MEMS actuator of claim 16, wherein the first and second
contacts further define controllably doped regions within a
semiconductor material.
18. The MEMS acuator of claim 1, wherein said acuator is thermally
actuated by a heated method selected from the group consisting
internal heating, external heating and ambient temperature
variations.
19. A microelectromechanical (MEMS) actuator structure comprising:
a microelectronic substrate having a first surface; at least two
anchor structures affixed to the first surface of said
microelectronic substrate; and at least two composite beams, each
composite beam extending from at least one anchor structure at a
proximate end and overlying the first surface of said
microelectronic substrate, wherein said composite beams are adapted
for thermal actuation so as to controllably move distal ends of
said composite beams along a predetermined path that extends
substantially parallel to the first surface of said microelectronic
substrate.
20. The MEMS actuator structure of claim 19, wherein said at least
two composite beams further comprises a first and a second
composite beam disposed upon said microelectronic substrate such
that the distal ends of said first and second composite beams face
each other, and wherein the first and second composite beams are
adapted to move in unison in response to selective thermal
actuation thereof.
21. The MEMS actuator structure of claim 20, wherein said first and
second composite beams are operably connected at the distal ends by
an interconnecting element, such that upon thermal actuation of the
composite beams the interconnecting element will move in a
generally linear direction.
22. The MEMS actuator device of claim 21, wherein said
interconnecting element has a wishbone-like configuration so as to
provide flexing capabilities and impart a greater linear
displacement distance.
23. The MEMS actuator device of claim 21, wherein said
interconnecting element is controllably doped so as to permit an
electrical path between said first and second composite beams.
24. The MEMS actuator device of claim 21, wherein the first and
second composite beams are disposed proximate to a flexible beam
structure comprising at least two anchors affixed to said
microelectronic substrate and a platform operably connected between
the at least two anchors by flexible beams, and wherein the
platform is disposed proximate the distal ends of the first and
second composite beams, such that the platform will be deflected
linearly in response to thermal actuation of the first and second
composite beams.
25. The MEMS actuator of claim 19, wherein said at least two
composite beams further comprise electrically conductive paths
through each composite beam, such that said composite beams are
thermally actuated in response to electrical current flowing
therethrough.
26. The MEMS actuator of claim 25, wherein said at least two
anchors further comprises at least two contacts in operable
connection with the electrically conductive paths.
27. The MEMS actuator of claim 19, wherein said at least two
composite beams further comprise, individually, at least two layers
which have differing thermal coefficients of expansion so as to
respond differently to thermal actuation.
28. The MEMS actuator of claim 27, wherein the at least two layers
are disposed in a generally perpendicular relationship with respect
to the first surface of said microelectronic substrate.
29. The MEMS actuator of claim 27, wherein the at least two layers
further comprises a first layer including a semiconductor
material.
30. The MEMS actuator of claim 29, wherein the first layer
including a semiconductor material has a controllably doped region
so as to impart self-heating capabilities to said composite
beam.
31. The MEMS actuator of claim 27, wherein the at least two layers
further comprises a first layer including a metallic material.
32. The MEMS actuator of claim 27, wherein the at least two layers
further comprise a first layer including a semiconductor material
and a second layer including a metallic material, wherein the
second layer has a greater thermal coefficient of expansion than
the first layer, such that said composite beams deflect toward the
first layer at the distal end in response to selective thermal
actuation.
33. The MEMS actuator of claim 32, wherein the first layer
including a semiconductor material has a controllably doped region
so as to impart self-heating capabilities to said composite
beams.
34. The MEMS actuator of claim 33, wherein the controllably doped
region of said first layer is operably in contact at the distal end
of said composite beams with said second layer, such that a
conductive path of varying resistance is provided by said composite
beams.
35. The MEMS actuator of claim 19, wherein said microelectronic
substrate further defines at least two trenches generally
underlying the at least two composite beams.
36. A system for MEMS thermal actuation in an in-plane direction,
the system comprising: a voltage source; and a MEMS actuator device
operably connected to said voltage source, said MEMS actuator
device comprising a microelectronic substrate having a first
surface, at least one anchor structure affixed to the first
surface, said anchor structure having at least two electrical
contacts, a composite beam extending from said at least one anchor
structure at a proximate end and overlying the first surface, said
composite beam including an electrically conductive path, such that
the composite beam is thermally actuated by applying current from
said voltage source to the electrically conductive path via the
electrical contacts, so as to controllably move a distal end of
said composite beam along a predetermined path that extends
substantially parallel to the first surface of said microelectronic
substrate.
37. A method for fabricating a self-contained MEMS thermal actuator
having in-plane actuation capability, the method comprising the
steps of: attaching a generally planar surface of a first
semiconductor substrate to a generally planar surface of a second
semiconductor substrate; defining a portion of a MEMS thermal
actuator in the second substrate, the portion of the MEMS thermal
actuator comprising a portion of at least one anchor affixed to the
first semiconductor substrate and a first layer of a composite
beam; doping a region of the first layer of the composite beam so
as to define a conductive path in the first layer of the composite
beam; and disposing a second layer on the undoped region of the
first layer of the composite beam, the second layer comprising a
material that responds differently to thermal actuation, such that
the first and second layers of the composite beam will effectuate
actuation of said composite beam along a predetermined path that
extends substantially parallel to the generally planar surface of
said microelectronic substrate.
38. The method of claim 37, further comprising the step of defining
a trench in the first semiconductor substrate, wherein the trench
underlies the composite beam structure.
39. The method of claim 37, wherein the step of doping further
comprises doping a first sidewall of the composite beam so as to
effectuate an external surface of the composite beam having an
electrically conductive path.
40. The method of claim 37, further comprising the step of doping
areas on the at least one anchor structure so as to define
electrical contacts.
41. The method of claim 39, wherein the step of disposing a second
layer further comprises disposing the second layer on a second
sidewall opposite the first sidewall, such that the resulting
composite beam has a first and second layer disposed
perpendicularly with respect to the generally planar surface of the
first substrate.
42. A method for fabricating a MEMS thermal actuator having
in-plane actuation capability, the method comprising the steps of:
disposing a first oxide layer on a first surface of a first
semiconductor substrate; attaching a second semiconductor substrate
on the first oxide layer; disposing a second oxide layer on the
second semiconductor substrate; defining oxide structures in the
second oxide layer that characteristically outline a portion of the
MEMS actuator structure. defining a first portion of the MEMS
actuator structure in the second semiconductor substrate, said
first portion including a first sidewall of a composite beam and a
first sidewall of an anchor structure; doping selectively the
regions of the first and second semiconductor substrate not covered
by oxide. defining a second portion of the MEMS actuator structure
in the second semiconductor substrate, said second portion
including a second sidewall of said composite beam and a second
sidewall of an anchor structure; and disposing a metal layer on the
second sidewall of the composite beam, such that the metal layer is
disposed perpendicularly with respect to the first semiconductor
substrate.
43. The method of claim 42, further comprising the step of defining
a trench through the first oxide layer in to the first
semiconductor substrate, wherein the trench underlies the plan view
outline of the composite beam structure of the MEMS actuator
device.
44. The method of claim 42, further comprising the step of
releasing the second semiconductor substrate from the first oxide
layer by selectively etching the first oxide layer in those regions
underlying the composite beam.
45. The method of claim 42, wherein the step of doping further
comprises doping the first sidewall of the composite beam so as to
effectuate an external surface of the composite beam having a
self-contained heating mechanism.
46. The method of claim 42, wherein the step of doping further
comprises doping areas on the anchor structure so as to define
electrical contacts.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to microelectromechanical
actuators, and more particular to a thermal actuator having
self-contained heating capabilities and providing in-plane
actuation.
BACKGROUND OF THE INVENTION
[0002] Microelectromechanical structures (MEMS) and other
microengineered devices are presently being developed for a wide
variety of applications in view of the size, cost and reliability
advantages provided by these devices. Many different varieties of
MEMS devices have been created, including microgears, micromotors,
and other micromachined devices that are capable of motion or
applying force. These MEMS devices can be employed in a variety of
applications including hydraulic applications in which MEMS pumps
or valves are utilized and optical applications that include MEMS
light valves and shutters.
[0003] MEMS devices have relied upon various techniques to provide
the force necessary to cause the desired motion within these
microstructures. For example, cantilevers have been employed to
transmit mechanical force in order to rotate micromachined springs
and gears. In addition, some micromotors are driven by
electromagnetic fields, while other micromachined structures are
activated by piezoelectric or electrostatic forces. Recently, MEMS
devices that are actuated by the controlled thermal expansion of an
actuator or other MEMS component have been developed. For example,
U.S. patent application Ser. Nos. 08/767,192; 08/936,598, and
08/965,277 are assigned to MCNC, the assignee of the present
invention, and describe various types of thermally actuated MEMS
devices. In addition, MEMS devices have been recently developed
that include rotational connections to allow rotation with less
torsional stress and lower applied force than found with torsion
bar connections. For instance, U.S. patent application Ser. No.
08/719,711, also assigned to MCNC, describes various rotational
MEMS connections. The contents of each of these applications are
hereby incorporated by reference herein.
[0004] Thermally actuated MEMS devices that rely on thermal
expansion of the actuator have recently been developed to provide
for actuation in-plane, i.e. displacement along a plane generally
parallel to the surface of the microelectronic substrate. However,
these thermal actuators rely on external heating means to provide
the thermal energy necessary to cause expansion in the materials
comprising the actuator and the resulting actuation. These external
heaters generally require larger amounts of voltage and higher
operating temperatures to affect actuation. For examples of such
thermally actuated MEMS devices see U.S. Pat. No. 5,881,198
entitled "Microactuator for Precisely Positioning an Optical Fiber
and an Associated Method" issued Mar. 9, 1999, in the name of
inventor Haake, and U.S. Pat. No. 5,602,955 entitled "Microactuator
for Precisely Aligning an Optical Fiber and an Associated
Fabrication Method" issued Feb. 11, 1997, in the name of inventor
Haake.
[0005] As such, a need exists to provide MEMS thermal actuated
devices that are capable of generating relatively large
displacement, while operating at significantly lower temperatures
(i.e. lower power consumption) and consuming less area on the
surface of a microelectronic substrate. These attributes are
especially desirable in a MEMS thermal actuated device that
provides relatively in-plane, linear motion and affords the benefit
of having a self-contained heating mechanism.
SUMMARY OF THE INVENTION
[0006] A MEMS thermal actuator device is therefore provided that is
capable of providing linear displacement in a plane generally
parallel to the surface of a substrate. Additionally, the MEMS
thermal actuator of the present invention may provide for a
self-contained heating mechanism that allows for the thermal
actuator to be actuated using lower power consumption and lower
operating temperatures.
[0007] The MEMS thermal actuator includes a microelectronic
substrate having a first surface and an anchor structure affixed to
the first surface. A composite beam extends from the anchor and
overlies the first surface of the substrate. The composite beam is
adapted for thermal actuation, such that it will controllably
deflect along a predetermined path that extends substantially
parallel to the first surface of the microelectronic substrate. In
one embodiment the composite beam comprises two or more layers
having materials that have correspondingly different thermal
coefficients of expansion. As such, the layers will respond
differently when thermal energy is supplied to the composite. An
electrically conductive path may extend throughout the composite
beam to effectuate thermal actuation.
[0008] In one embodiment of the invention a two layer composite
beam comprises a first layer of a semiconductor material and a
second layer of a metallic material. The semiconductor material may
be selectively doped during fabrication so as to create a
self-contained heating mechanism within the composite beam. The
doped semiconductor region may afford a path of least resistance
for electrical current. The doping process may also enhance the
fabrication of contacts on the surface of the anchor. Additionally,
the composite beam, which is characterized by a high aspect ratio
in the z plane direction, may be constructed so that the first and
second layers lie in planes that are generally perpendicular to the
first surface of the microelectronic substrate. The vertical layer
of the composite beam provides for deflection of the beam along a
predetermined path that extends generally parallel to the surface
of the microelectronic substrate.
[0009] In another embodiment of the invention, a MEMS thermal
actuator includes two or more composite beams. The two or more
composite beams may be disposed in an array or a ganged fashion,
such that the multiple composite beams benefit from overall force
multiplication. In one such embodiment, two composite beams are
disposed on the surface of a microelectronic substrate such that
the ends of the beam farthest from the anchor structure face one
another. An interconnecting element is operably connected to the
free ends of the composite beam. The interconnecting element is
configured so as to impart flexibility when the two composite beams
are actuated in unison. The flexible nature of the interconnecting
element allows for the overall MEMS thermal actuator device to
impart a greater distance of linear deflection.
[0010] In yet another multi composite beam embodiment, two
composite beams are disposed on the surface of a microelectronic
substrate such that the ends of the beam farthest from the anchor
structure face one another and the beams are proximate a flexible
beam structure. The flexible beam structure comprises a platform
disposed between two or more anchors affixed to the substrate. One
or more flexible beams operably connect the platform and the
anchors. The platform is adjacent to the free ends of the composite
beams such that thermal actuation of the composite beam causes the
beams to operably contact the platform and deflect it in a
generally linear fashion. The flexible beam structure that houses
the platform compensates for variances that may occur in the
thermal actuation of the composite beams.
[0011] The invention is also embodied in a method for fabricating
the MEMS thermal actuators of the present invention. The method
comprises affixing a first microelectronic substrate to a second
microelectronic substrate. After the second substrate has been
thinned to a predetermined thickness, a first portion of the MEMS
thermal actuator construct is then defined in the second
microelectronic substrate, including the first layer of a composite
beam and a portion of the anchor structure. A doping process may be
undertaken to define a path of least resistance in the first layer
of the composite beam. A second layer is disposed on the first
layer, the second layer and first layer comprising different
materials that respond differently to thermal actuation. The
variance in thermal coefficients of expansion causing the first and
second layers of the composite beam to actuate the composite beam
along a predetermined path that extends substantially parallel to
the surface of the microelectronic substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a plan view of a self-contained, in-plane, MEMS
thermal actuator in accordance with an embodiment of the present
invention.
[0013] FIG. 1A is a perspective view of a self-contained, in-plane,
MEMS thermal actuator in accordance with an embodiment of the
present invention.
[0014] FIG. 2 is a cross-sectional view of the self-contained,
in-plane, MEMS thermal actuator shown in FIG. 1, in accordance with
an embodiment of the present invention.
[0015] FIG. 3 is a plan view of a dual self-contained, in-plane
MEMS thermal actuator including an interconnecting element used to
facilitate greater linear displacement, in accordance with another
embodiment of the present invention.
[0016] FIG. 4 is a plan view of a dual self-contained, in plane
MEMS thermal actuator including a flexible beam construct used to
facilitate greater linear displacement, in accordance with another
embodiment of the present invention.
[0017] FIGS. 5A-5G illustrate various stages in the fabrication
process of the self-contained, in-plane MEMS thermal actuator, in
accordance with a method for making embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
[0019] The following description details MEMS thermal actuated
devices that are capable of generating in-plane, linear motion and
provide for a self-contained heating mechanism to effectuate
thermal expansion of the materials comprising the actuator. The
resulting MEMS thermal actuated devices are thereby capable of
operating at significantly lower temperature and power while
providing relatively large displacements. In addition, methods for
fabricating such devices are provided herein.
[0020] Referring to FIGS. 1, 1A and 2, shown, respectively, are a
top plan view, a perspective view and a cross-sectional end view of
a self-contained, in-plane, MEMS thermal actuator 10 in accordance
with one embodiment of the present invention. The thermal actuator
comprises a composite beam 12 that is affixed to the
microelectronic substrate 14. The composite beam is affixed, at
proximal end 16, to the substrate via an anchor, shown in FIG. 1 as
first anchor portion 18 and second anchor portion 20. From the
proximal end, the composite beam extends outward, overlying the
microelectronic substrate, and concluding in a distal end 22
disposed furthest from the anchor(s). As such, the composite beam
overlies and is suspended above the microelectronic substrate in a
cantilever-like configuration. Optionally, the thermal actuator of
the present invention may define a trench 24 disposed in the
surface 26 of the microelectronic substrate 14 that provides for
additional thermal isolation between the composite beam and the
microelectronic substrate. For example, the cross-sectional view of
FIG. 2 depicts the suspended composite beam disposed above a trench
defined in the t microelectronic substrate.
[0021] The composite beam will comprise at least two materials that
characteristically have different thermal coefficients of
expansion. As depicted in FIG. 1, the composite beam includes a
first layer 28 and a second layer 30. It is also possible and
within the inventive concepts herein disclosed to construct the
composite beam with more than two layers. As shown in FIG. 2, the
first and second layers are disposed vertically in relationship to
the surface of the microelectronic substrate. The vertical
relationship of the layers is required to affect actuation in an
in-plane direction, parallel with the generally planar surface of
the microelectronic substrate, as shown by arrow 31. The first and
second layers will, typically, be thin layers of about 1 micron to
about 10 microns so as to facilitate flexibility and movement in
the overall composite beam. Since the layers have different thermal
coefficients of expansion, the layers will respond differently to
thermal actuation resulting in deflection of the composite
beam.
[0022] In one embodiment of the invention the first layer 28 may
comprise a semiconductor material, such as silicon, and the second
layer 30 may comprise a metallic material, such as gold or nickel.
In this embodiment the second layer has a characteristically higher
coefficient of thermal expansion than the first layer. Since the
layer having the higher coefficient of expansion will expand more
readily as temperature increases, the distal end of the composite
beam will bend toward the layer having the lower coefficient of
thermal expansion when thermal energy is supplied to the composite
beam. In the embodiment described, in which the second layer 30 has
the higher coefficient of thermal expansion, the layering
relationship will effect movement of the beam to the right, toward
the first layer 28, when reviewed in FIG. 2. It will be readily
apparent to those having skill in the art, that the layering may be
reversed, so that the material having the higher coefficient of
thermal expansion is on the opposite side in the depicted
embodiment and, thus, the movement of the beam will be to the left
as viewed in FIG. 2. Altering various composite beam and thermal
actuator characteristics can vary the amount of deflection observed
in the composite beam. Included among these characteristics are the
choice of materials used for the layers, the thickness of the
layers, and the amount of heat supplied to the composite beam, and
structural discontinuities in the layers.
[0023] The first and second anchor portions 18 and 20 serve to
affix the composite beam 12 to the underlying substrate. The shape
of the overall anchor and/or the anchor portions is shown by way of
example only. The general shape and location of the anchors at the
proximal end 16 will be dictated by many design factors, including
but not limited to, the amount of composite beam deflection
desired, the coefficients of thermal expansion of the composite
beam materials, the desired rigidity of the overall thermal
actuator, etc. First and second contacts 32 and 34 are disposed on
respective first and second anchor portions 18 and 20. The contacts
are interconnected with a source of electrical energy (not shown in
FIG. 1) and serve as connection points for an electrical current
that runs through the composite beam. The heat generated by the
electrical current in the beam acts as the means for actuating the
composite beam. In the embodiment shown the two anchor portions are
separated by a gap 36. The gap serves as an insulator providing
electrical insulation between the first and second contacts.
[0024] As mentioned above, the microelectronic substrate 14 may
define a trench 24 in the region underlying the composite beam.
Typically, the trench will be slightly longer than the length of
the composite beam and the maximum deflection distance of the beam
will dictate the width of the trench. The trench provides heating
efficiency advantages. For example, the gap created by the trench
between the composite beam and the microelectronic substrate
provides thermal isolation and, thus, less heat loss is experienced
between the composite beam and the substrate. Additionally, the
trench simplifies the fabrication process used to deposit the
metallic second layer 30 upon the first layer 28. In contrast with
conventional horizontal type layering constructs, the layers of the
composite beam are stacked vertically. To effectuate vertical
layering the metallic second layer 30 is deposited at an angle to
assure uniform coverage of the vertical sidewall 38 of the first
layer (shown in FIG. 2). Without a trench in place, there is a
likelihood that depositing the metallic second layer will lead to
electrical shorting of the underlying microelectronic substrate and
any metal elements defined on the substrate. Alternately, the MEMS
thermal actuator of the present invention may be constructed
without a trench defined in the microelectronic substrate. In such
embodiments a release layer, typically an oxide layer, is used to
release the composite beam from the surface of the substrate during
fabrication. The releasing process results in a small gap,
typically less that 1 micron, existing between the microelectronic
substrate and the composite beam.
[0025] The composite beam is adapted for thermal actuation through
direct, self-heating techniques using electric current. As
previously discussed prior art in-plane, thermal actuators have
used indirect heating techniques and have, thus, required large
currents and high operating temperatures. As such, the high
temperature, large current nature of prior art thermal actuators
makes them less efficient than the thermal actuator of the present
invention. In order to permit direct heating, each layer of the
composite beam defines an electrically conductive path that runs in
a continuous loop through the composite beam and between a first
and second contacts 32 and 34 disposed upon the anchor portions 18
and 20. In this fashion, an electrical circuit is formed by passing
current from one contact and through one of the layers of the
composite beam to the distal end of the composite beam prior to
returning to the other contact via the other layer. Preferably, the
electrically conductive path is disposed so as to substantially
surround at least one of the layers comprising the composite beam.
It is possible and within the inventive concepts herein disclosed
to alter the configuration of the electrically conductive path to
form a circuit loop that will effectuate heat in the composite
beam. The electrically conductive path has a predetermined
electrical resistance so as to permit thermal actuation of the
composite beam when electrical energy is supplied thereto.
[0026] The present invention uses controlled doping to facilitate
self-heating and to customize the resistance characteristics of the
non-metallic materials used in the thermal acuator construct. In
this fashion the doped region of the composite beam acts as the
heater, self-contained within the composite beam structure.
Materials such as silicon are highly resistant and, thus, the
doping of such materials aids in constructing a highly conductive
path for the passage of electrical current. Highly doping silicon
can be achieved with materials such as phosphorus or boron. The use
of doping techniques is well known by those of ordinary skill in
the art. It should also be noted that the embodiments herein
described are not limited to internal heating to effectuate
actuation. The MEMS thermal actuators shown here will operate with
external heating and in certain embodiments, depending on the
composition of the substrate and the materials used to form the
actuator, ambient temperature actuation is possible.
[0027] In the embodiment shown in FIGS. 1 and 2, in which the first
layer 28 comprises a semiconductor material, such as silicon, and
the second layer 30 comprises a metal, such as gold, the external
surface 40 of the first layer has been controllably doped. The
second layer is electrically connected at junction 41 to the doped
conductive surface of the first layer at the tip 42 of the distal
end of the composite beam. The tip portion of the distal end may be
fabricated so that it is either an extension of the second layer
(as shown in FIG. 1) or it may comprise a continuation of the doped
conductive region of the first layer leading into the second layer.
Both alternative configurations will allow for the second layer to
be electrically connected with the doped conductive surface of the
first layer. Since the doped regions define a path of least
electrical resistance, the electrical current will mostly follow
the path defined by the doped portions of the first layer and the
metallic second layer, with the undoped portion 28 being an
electrical insulator. Accordingly, the conductive path is provided
between the contacts 32 and 34 by the doped portion of the first
layer and the metallic second layer. As shown, the contacts have
also been doped to increase electrical conductivity. It is also
possible to devise contacts that would not require doping. By way
of example, when a source of electrical energy is applied between
the contacts electrical current would flow from the first contact
32 along the external surface 40 of the first layer (i.e. the doped
region of the first layer), through junction 41 into the
interconnected second layer 30 and return back through the second
layer to the second contact 34. In an alternate embodiment the
electrical current could flow in the opposite direction, emanating
from the second contact, flowing through the composite beam and
completing the path at the first contact.
[0028] Referring to FIG. 3, in another embodiment of the present
invention the thermal actuator includes dual composite beam
actuators 102 and 104. A first and second composite beam 106 and
108 are disposed proximately such that the distal ends 110 of the
respective beams face each other. The composite beams are adapted
to move in unison in response to the selective application of
thermal actuation. To assure uniform movement between the composite
beams the beams will generally be comprised of identical layers;
similar in material, quantity of layers, layer thickness and doping
characteristics. The dual composite beams may generally be
perceived as mirror images of one another.
[0029] As shown in FIG. 3, first and second composite beams are
adapted to deflect in the direction of the arrow 112 when thermal
energy is applied to the beams. When a single composite beam
thermal actuator is used initial deflection of the beam is
generally linear, however as the beam continues to deflect the
pattern of deflection takes on an arcuate path. In this regard, as
thermal energy is applied to the single composite beam actuator the
beam has a limited range of relatively linear displacement before
the displacement becomes increasingly more rotary or angular. In
the FIG. 3 embodiment affixing an interconnecting element 114 to
the distal ends of each composite beam increases linear
displacement. The interconnecting element may be formed during the
fabrication process that defines one of the layers of the composite
beam. Thus, the interconnecting element may comprise silicon, gold,
nickel or a similar suitable material. In a silicon embodiment the
interconnecting element may be doped or undoped. If the
interconnecting element is doped, the doping will typically occur
simultaneous with the doping of the contacts 32 and 34 and the
external surface 40 of the first layer. Additionally, if the
interconnecting element is doped an actuating electrical conductive
path may exist between the contact 32 of the first thermal actuator
102, the interconnecting element and the contact 32 of the second
thermal actuator 104. This electrical configuration would make the
second pair of contacts 34 optional. Alternately, the electrical
path may exist between the second pair of contacts 34 and the doped
interconnecting element thus, eliminating the need for the first
set of contacts 32. Preferably, the interconnecting element is
shaped and sized so as to impart flexibility. The flexible nature
of the interconnecting element increases the linear displacement
distance. In the FIG. 3 illustration, the interconnecting element
has a preferred wishbone-like configuration. As the composite beams
begin to deflect upward, the legs 116 of the wishbone-like
configuration flex outward and result in an overall linear
displacement of the interconnecting element in the direction of the
arrow.
[0030] Additionally, the interconnecting element 114 may serve to
simplify the overall heating arrangement of the dual composite beam
thermal actuator. The interconnecting element may serve as a bridge
that allows the electrical current to flow from one composite beam
to the next. In the embodiment shown in FIG. 3 the interconnecting
element uses the doped region of the first layer, typically
silicon, as the transfer path between the first composite beam and
the second composite beam. In such a configuration the electrical
bridging capabilities of the interconnecting element may eliminate
the need to supply electrical current individually to both
composite beams. In an embodiment in which the interconnect is used
as an electrical link it is more desirable to have the interconnect
element comprise a metal or doped silicon so that electrical
resistance can be minimized.
[0031] Another multi beam thermal actuator in accordance with yet
another embodiment of the present invention is shown in FIG. 4.
This embodiment provides for a flexible beam 130 construct disposed
proximate the distal ends 110 of the dual composite beams 106 and
108. In much the same fashion as the dual composite beam thermal
actuator shown in FIG. 3, the composite beams shown in the FIG. 4
embodiment will generally be perceived as mirror images of one
another. The dual composite beams are in operable contact with a
platform 132. The platform is disposed so that it is generally
midway between the distal end of the first composite beam and the
distal end of the second composite beam. The platform is operably
connected to at least two anchors that are affixed to the
microelectronic substrate, shown here as first and second anchors
134 and 136. In the embodiment shown in FIG. 4 the anchors are
operably connected to the platform via first flexible beam 138 and
second flexible beam 140. A series of springs 142 are located along
the beams and serve to provide elasticity to the overall flexible
beam structure. The fabrication of the platform, the anchors, the
flexible beams and the springs can be part of the same patterning
and etch processes used to form the first layer of the thermal
actuator construct or they may entail separate processing steps. As
such, the platform, the anchors, the flexible beams and the springs
may comprise silicon, gold, nickel or any other suitable
material.
[0032] In operation the dual thermal actuators are activated by
thermal energy and provide for the generally simultaneous
deflection of the distal ends of the composite beams. Upon
actuation the distal ends of the composite beams contact the
platform and provide the force necessary to move the platform in a
linear direction (the path of the platform is shown as arrow 144).
The beams and the springs allow for the platform to move in the
linear direction and allow for the platform to relax into a
non-actuated position upon deactivation of the thermal actuators.
This embodiment of a dual beam actuator is advantageous because the
spring-like beam structure compensates for any unequal movement of
the respective composite beams and accordingly provides enhanced
linear displacement characteristics as compared to the
aforementioned described single composite beam embodiment.
[0033] Numerous other multi beam thermal actuator embodiments are
also feasible and within the inventive concepts herein disclosed.
For example, the composite beams may be arranged radially, with the
distal ends directed toward a control hub to effectuate rotational
movement. The hub may have levers extending from it that provide
added actuation force. For a detailed discussion of rotary type
MEMS structures see U.S. patent application Ser. No. 09/275,058
entitled "Microelectromechanical Rotary Structures" filed on May
23, 1999, in the name of inventors Hill et al. and assigned to
MCNC, the assignee of the present invention. That application is
herein incorporated by reference as if set forth fully herein.
Additionally, the composite beams may be ganged together and/or
configured in an array to benefit from force multiplication,
thereby, effectively increasing the ability to move objects a
greater distance and/or move larger objects.
[0034] FIGS. 5A-5G illustrate cross-sectional views of various
fabrication stages in accordance with a method of making the
thermal actuator of the present invention. Referring to FIG. 5A, a
first microelectronic substrate 200 having a first oxide layer 202
is formed on the substrate and a trench 204 is defined through the
first oxide layer and into the first microelectronic substrate. The
first microelectronic substrate may comprise silicon although other
suitable substrate material may also be used. The first oxide layer
is typically disposed on the substrate by a conventional thermal
oxidation process in which the substrate is exposed to an oxygen
environment at elevated temperature and the oxide then grows on the
substrate. In the embodiment in which the substrate is silicon, the
first oxide layer may comprise silicon dioxide (SiO.sub.2). The
thickness of the first oxide layer will typically be about 2000
angstroms to about 8000 angstroms. The first oxide layer serves as
a dielectric insulating layer and provides a means for subsequent
defining and etching of the trench. Standard photolithography
techniques may be used to pattern the region within the first oxide
layer that will define the trench. A conventional wet etch process
may then be used to develop the trench through the first oxide
layer and into the substrate. The resulting trench typically has a
depth within the substrate of about 10 microns to about 20 microns.
The trench will also typically have a length slightly longer than
the desired predetermined length of the composite beam and a width
consistent with the maximum deflection of the beam, generally about
100 microns.
[0035] FIG. 5B illustrates the first microelectronic substrate
having a trench defined therein after a second microelectronic
substrate 206 has been attached and polished back. The second
microelectronic substrate will subsequently form the first layer of
the composite beam and a portion of the beam anchor. In a preferred
embodiment the second substrate will comprise silicon. A standard
fusion bonding process is used to affix the second microelectronic
substrate to the first microelectronic substrate at the oxide layer
interface. After the second microelectronic substrate is bonded it
is polished back to the desired predetermined thickness. The
thickness of the second microelectronic substrate will be
consistent with the desired thickness or height of the resulting
composite beam. Typically, the second microelectronic substrate
will be polished back to a thickness of about 25 microns to about
50 microns.
[0036] Referring to FIG. 5C shown is an oxide structure 208 formed
on the second microelectronic substrate 206. The oxide structure(s)
generally overlie the area that will comprise the composite beam
and a portion of the anchors. The oxide structure(s) result from a
second oxide layer (not shown in FIG. 5C) being disposed on the
second substrate. The similar oxidation process to the one
previously used for disposing the first oxide layer on the first
substrate is typically used to dispose the second oxide layer. In
the embodiment in which the second microelectronic substrate is
silicon, the second oxide layer may comprise silicon dioxide
(SiO.sub.2). The thickness of the second oxide layer will typically
be about 2000 angstroms to about 8000 angstroms. Standard
photolithography techniques may be used to pattern the requisite
oxide structure. A conventional wet etch process may then be used
to develop the oxide structure(s). The resulting oxide structure(s)
provide dielectric separation, preventing subsequent doping in
areas underlying the oxide structure(s) and electrical isolation
between subsequent conductive regions (i.e., doped regions and
metallic regions).
[0037] Further processing results in the structure shown in FIG. 5D
in which a portion of the second microelectronic substrate has been
etched away to expose one side of the thermal actuator. Standard
photolithography techniques may be used to pattern a side of the
overall thermal actuator, including a portion of the anchor
structure and the first sidewall 210 of the first layer of the
composite beam into the second substrate. A deep silicon reactive
ion dry etch process may then be used to etch away a portion of the
second substrate and expose a sidewall of the first layer of the
composite beam and a portion of the anchor structure. A dry etch
process is preferred at this stage to create the high aspect ratio
of the composite beam (about 25-50 microns in depth relative to an
about 5-7 microns width).
[0038] Referring to FIG. 5E, shown is the thermal actuator
construct after exposed silicon surfaces have been subjected to a
conventional diffusion doping process. The doping process provides
for a continuous conductive path along the periphery of the
composite beam and defines the contacts on the anchor. In a typical
doping process phosphorus may be used as the doping impurity,
although other materials may be used to create a highly doped
region in the second silicon substrate. The doping process will
create doped regions in all those area not protected by an oxide.
As shown in FIG. 5E the resulting doped regions may include the
first sidewall 210 of the first layer of the composite beam, the
surface of the trench 204, the exposed portion of the remainder of
the second microelectronic substrate 206 and the regions defining
the contacts on the anchor (for the sake of not confusing the
reader, the anchor structure and, thus, the contacts are not shown
in the FIGS. 5A-5G illustrations). The doping of the surface of the
trench and exposed portion of the remainder of the second
microelectronic substrate is incidental. The doped region will
typically have a depth into the substrate of about 2000 angstroms
to about 8000 angstroms.
[0039] FIG. 5F depicts the thermal actuator after an additional
etch process has revealed the definition of the second side of the
thermal actuator. After the completion of this etch process all
that remains of the second substrate are those structures of the
thermal actuator; the first layer and a portion of the anchor
structure, that comprise the material of the second substrate. FIG.
5F illustrates the definition of the second sidewall 212 of the
first layer after completion of the additional etch process.
Standard photolithography techniques may be used to pattern into
the substrate the remaining side of the overall thermal actuator,
including a portion of the anchor structure and the second sidewall
of the first layer of the composite beam. A conventional reactive
ion dry etch process may then be used to etch away the remainder of
the second substrate and expose the second sidewall of the first
layer of the composite beam and the remainder of the anchor
structure.
[0040] The thermal actuator of the present invention is shown in
its completed form in FIG. 5G. A second layer 214, typically a
metallic layer is disposed on the second sidewall 212 of the first
layer. The second layer may comprise gold, nickel or another
suitable material that has a coefficient of thermal expansion that
differs from the coefficient of thermal expansion of the material
chosen to comprise the first layer. In the embodiment in which the
second layer comprises gold a conventional evaporation process is
used to dispose the layer, typically the layer is about 2 to about
3 microns thick. In order to properly dispose the second layer on
the vertical sidewall of the first layer and to assure proper
uniform thickness of the second layer the overall thermal actuator
construct may be placed at an angle during the evaporation process.
The evaporation process will result in the second layer being
disposed on surfaces not requiring such. In order to remove
unnecessary second layer coverage, a standard photolithography
process is used to pattern the areas requiring the second layer and
a conventional wet etch process is used to define the areas
requiring second layer coverage. In most instances, the etch
process will result in second layer coverage extending above the
second sidewall 212 and partially covering the surface of the
second oxide layer 208. Additionally, the second layer may remain
at the tip of the distal end of the composite beam (as shown in
FIG. 1). Alternately, the tip of the distal end of the composite
beam may comprise a doped region of the first layer. In the
embodiment shown in FIG. 5G the first and second oxide layers 202
and 208 remain on the completed thermal actuator. It is also
possible and within the inventive concepts herein disclosed to
remove the oxide layer after the second layer has been disposed.
The plan and cross-sectional views of FIGS. 1 and 2 illustrate an
embodiment in which the oxide layers have been removed. In most
instances leaving the oxide layers intact may be desirable as it
eliminates the need for further processing and may prevent possible
electrical shorting.
[0041] Accordingly, the fabrication method of this aspect of the
present invention provides an efficient and repeatable technique
for creating a vertical layered MEMS structure having a doped
region that provides for a self-contained heating mechanism. The
resulting MEMS thermal actuated devices are capable of generating
large displacement forces across an in-plane, generally linear
direction. These devices benefit from the capability of being able
to operate at significantly lower temperature and power while
consuming less surface area on the substrate.
[0042] Many modifications and other embodiments of the invention
will come to mind to one skilled in the art to which this invention
pertains having the benefit of the teachings presented in the
foregoing descriptions and the associated drawings. Therefore, it
is to be understood that the invention is not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
* * * * *