U.S. patent application number 09/740778 was filed with the patent office on 2001-06-07 for methods of fabricating tunable capacitors.
Invention is credited to Dhuler, Vijayakumar R..
Application Number | 20010002872 09/740778 |
Document ID | / |
Family ID | 23552838 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002872 |
Kind Code |
A1 |
Dhuler, Vijayakumar R. |
June 7, 2001 |
Methods of fabricating tunable capacitors
Abstract
A tunable capacitor having low loss and a corresponding high Q
is provided. The tunable capacitor comprises first and second
substrates having first and second capacitor plates disposed,
respectively, thereon. The capacitor plates may comprise a high
temperature superconductor material. A MEMS actuator, that is
either driven by electrostatic force, heat or both, operably
contacts the second substrate so that once the actuator is engaged
it responds by displacing the second substrate, thereby varying the
capacitance between said first capacitor plate and said second
capacitor plate. As such, the capacitance can be controlled based
upon the relative spacing between the first and second capacitor
plates. The MEMS actuator may either be operably attached to the
second substrate or detached, yet supporting, the second substrate.
A method is also provided for micromachining or otherwise
fabricating a tunable capacitor having high temperature
superconductor capacitor plates and electrostatic and/or thermal
MEMS actuation such that the resulting tunable capacitor has low
signal loss and a corresponding high Q. The tunable capacitor can
therefore be used in high frequency applications, such as those
using radio frequencies.
Inventors: |
Dhuler, Vijayakumar R.;
(Raleigh, NC) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
23552838 |
Appl. No.: |
09/740778 |
Filed: |
December 19, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09740778 |
Dec 19, 2000 |
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09392987 |
Sep 9, 1999 |
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6215644 |
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Current U.S.
Class: |
361/277 |
Current CPC
Class: |
H01G 5/16 20130101 |
Class at
Publication: |
361/277 |
International
Class: |
H01G 005/00 |
Claims
That which is claimed:
1. A tunable capacitor comprising: a first substrate having a first
surface; a first capacitor plate disposed on the first surface of
said first substrate; a second substrate having a first surface; a
second capacitor plate disposed on the first surface of said second
substrate, wherein said first and second substrates are positioned
such that said first and second capacitor plates face one another
in a spaced apart relationship; and a microelectromechanical
actuator operably contacting said second substrate for displacing
said second substrate in response to electrostatic forces, thereby
varying the capacitance between said first capacitance plate and
said second capacitance plate.
2. The tunable capacitor of claim 1, wherein said first and second
capacitor plate further comprise a high temperature superconductor
material.
3. The tunable capacitor of claim 2, wherein said first and second
capacitor plates further comprise a high temperature superconductor
yttrium compound.
4. The tunable capacitor of claim 2, wherein said first and second
capacitor plates further comprise a high temperature superconductor
thallium compound.
5. The tunable capacitor of claim 1, wherein said
microelectromechanical actuator further comprises: at least one
first electrode disposed on the first surface of said first
substrate; and at least one cantilever structure at least partially
overlying said first electrode and operably contacting said second
substrate, said at least one cantilever structure comprising a
second electrode.
6. The tunable capacitor of claim 5, wherein said at least one
first electrode further comprises a high temperature superconductor
material.
7. The tunable capacitor of claim 5, wherein said second electrode
further comprises silicon.
8. The tunable capacitor of claim 5, further comprising a first
intermediary layer disposed on the first surface of said first
substrate intermediate said cantilever structure and said first
substrate.
9. The tunable capacitor of claim 8, wherein said first
intermediary layer further comprises gold.
10. The tunable capacitor of claim 5, further comprising a second
intermediary layer disposed on the first surface of said second
substrate intermediate said cantilever structure and said second
substrate, wherein said second intermediary layer operably connects
said second substrate to said cantilever structure.
11. The tunable capacitor of claim 10, wherein said first
intermediary layer further comprises gold.
12. The tunable capacitor of claim 5, wherein said cantilever
structure further comprises spring-like elements structurally
patterned in said cantilever structure that provide elasticity for
said cantilever structure.
13. The tunable capacitor of claim 5, wherein said cantilever
structure further comprises a first layer comprising the second
electrode and a second layer disposed on the first layer comprising
a biasing element.
14. The tunable capacitor of claim 13, wherein said first layer
further comprises silicon and said second layer comprises gold.
15. The tunable capacitor of claim 1, wherein said microelectronic
actuator is operably attached to said second substrate.
16. The tunable capacitor of claim 1, further comprising at least
one spring-like structure operably connected to said second
substrate, whereby said at least one spring-like structure provides
elasticity for said second substrate.
17. The tunable capacitor of claim 1, wherein said first and second
substrates further comprise low signal loss substrates.
18. The tunable capacitor of claim 17, wherein said first and
second substrates further comprise magnesium oxide (MgO).
19. A tunable capacitor comprising: a first substrate having a
first surface; a first capacitor plate disposed on the first
surface of said first substrate; a second substrate having a first
surface; a second capacitor plate disposed on the first surface of
said second substrate, wherein said first and second substrates are
positioned such that said first and second capacitor plates face
one another in a spaced apart relationship; and a
microelectromechanical actuator operably contacting said second
substrate for displacing said second substrate in response to
thermal actuation, thereby varying the capacitance between said
first capacitance plate and said second capacitance plate.
20. The tunable capacitor of claim 19, wherein said first and
second capacitor plates further comprise a high temperature super
conductor material.
21. The tunable capacitor of claim 20, wherein said first and
second capacitor plates further comprise a high temperature
superconductor yttrium compound.
22. The tunable capacitor of claim 20, wherein said first and
second capacitor plates further comprise a high temperature
superconductor thallium compound.
23. The tunable capacitor of claim 19, wherein said
microelectromechanical actuator further comprises a moveable
composite structure having at least two layers, including: a first
layer; and a second layer disposed on said first layer and operably
contacting said second substrate, wherein said first layer responds
differently to thermal actuation than said second layer.
24. The tunable capacitor of claim 23, wherein said first layer
comprises one or more materials having a lower coefficient of
thermal expansion than one or more materials comprising said second
layer.
25. The tunable capacitor of claim 23, wherein said first layer
comprises silicon.
26. The tunable capacitor of claim 23, wherein said second layer
comprises gold.
27. The tunable capacitor of claim 23, further comprising a first
intermediary layer disposed on the first surface of said first
substrate intermediate said moveable composite structure and said
first substrate.
28. The tunable capacitor of claim 27, wherein said first
intermediary layer further comprises gold.
29. The tunable capacitor of claim 23, further comprising a second
intermediary layer disposed on the first surface of said second
substrate intermediate said moveable composite structure and said
second substrate wherein said second intermediary layer operably
connects said second substrate to said moveable composite
structure.
30. The tunable capacitor of claim 29, wherein said first
intermediary layer further comprises gold.
31. The tunable capacitor of claim 23, wherein said second layer is
operably attached to said second substrate.
32. The tunable capacitor of claim 23, wherein said moveable
composite structure further comprises spring-like elements
structurally patterned in said second layer so as to provide
elasticity for said moveable composite structure.
33. The tunable capacitor of claim 23, further comprising at least
one spring-like structure operably connected to said second
substrate, whereby said at least one spring-like structure provides
elasticity for said second substrate.
34. The tunable capacitor of claim 19, wherein said first and
second substrates further comprise a low signal loss substrate.
35. The tunable capacitor of claim 19, wherein said first and
second substrates further comprise magnesium oxide (MgO).
36. A tunable capacitor comprising: a first substrate having a
first surface; a first capacitor plate disposed on the first
surface of said first substrate; a second substrate having a first
surface; a second capacitor plate disposed on the first surface of
said second substrate, wherein said first and second substrates are
positioned such that said first and second capacitor plates face
one another in a spaced apart relationship; and a
microelectromechanical actuator operably contacting said second
substrate for displacing said second substrate in response to a
motive force selected from the group consisting of electrostatic
force and thermal actuation, thereby varying the capacitance
between said first capacitance plate and said second capacitance
plate.
37. The tunable capacitor of claim 36, wherein said first and
second capacitor plates further comprise a high temperature super
conductor material.
38. The tunable capacitor of claim 36, wherein said
microelectromechanical actuator further comprises: at least one
first electrode disposed on the first surface of said first
substrate; and at least one cantilever structure at least partially
overlying said first electrode and operably contacting said second
substrate, said at least one cantilever structure comprising at
least two layers and a second electrode.
39. The tunable capacitor of claim 38 wherein said cantilever
structure further comprises a first layer and a second layer
disposed upon said first layer, wherein said first layer responds
differently to thermal actuation than said second layer.
40. The tunable capacitor of claim 38 wherein said first layer
comprises one or more materials having a lower coefficient of
thermal expansion than one or more materials comprising said second
layer.
41. A method for making a tunable capacitor comprising: fabricating
a first capacitor plate structure comprised of a first substrate
and a first high-temperature superconductor capacitor plate
disposed on the first substrate; fabricating a micromechanical
(MEMS) actuator that is responsive to electrostatic forces;
fabricating a second capacitor plate structure comprised of a
second substrate, a second high-temperature superconductor
capacitor plate disposed on the second substrate and at least one
first actuator electrode disposed on the second substrate; and
connecting said first capacitor plate structure to said second
capacitor plate structure such that the MEMS actuator is disposed
between the first and second capacitor plate structures and in
operable contact with the first capacitor plate structure.
42. The method of claim 41, wherein the step of fabricating an
electrostatic driven MEMS actuator further comprises the substeps
of: disposing a biasing layer on the surface of a third substrate;
attaching the biasing layer to the first substrate; and etching
back the underside of the third substrate to create a second
actuator electrode layer.
43. The method of claim 41, wherein the step of fabricating an
electrostatic MEMS actuator further comprises the substeps of:
disposing a release layer on the surface of a third substrate;
disposing a biasing layer on the release layer; bonding the biasing
layer to the first substrate; etching back the underside of the
third substrate to create a second actuator electrode layer; and
releasing the biasing layer from the second actuator electrode
layer in an area proximate the first substrate.
44. The method of claim 41, wherein said connecting step further
comprises: disposing a first support layer on the surface of said
second substrate; disposing a second support, layer on the surface
of said electrostatic driven MEMS actuator; and attaching the first
support layer to the second support layer.
45. A method for making a tunable capacitor comprising: fabricating
a first capacitor plate structure comprised of a first substrate
and a first high-temperature superconductor capacitor plate
disposed on the first substrate; fabricating a
microelectromechanical (MEMS) actuator that is thermally actuated;
fabricating a second capacitor plate structure comprised of a
second substrate and a second high-temperature superconductor
capacitor plate disposed on the second substrate; and connecting
said first capacitor plate structure to said second capacitor plate
structure such that the MEM actuator is disposed between the first
and second capacitor plate structures and in operable contact with
the first capacitor plate structure.
46. The method of claim 45, wherein the step of fabricating a
thermally activated MEMS actuator further comprises the substeps
of: disposing a first layer on the surface of a third substrate,
the first layer comprising a material having a higher thermal
coefficient of expansion than said third substrate; attaching the
first layer to the first substrate; and etching back the underside
of the third substrate to create a second layer of a thermal
actuator.
47. The method of claim 45, wherein the step of fabricating a
thermally activated MEMS actuator further comprises the substeps
of: disposing a release layer on the surface of a third substrate;
disposing a first layer of a thermal acuator on the release layer;
bonding the first layer to the first substrate; etching back the
underside of the third substrate to create a second layer of the
thermal actuator; and releasing the first layer from the second
layer in an area proximate the first substrate.
48. The method of claim 45, wherein said connecting step further
comprises: disposing a first support layer on the surface of said
second substrate; disposing a second support layer on the surface
of said electrostatic MEMS actuator; and attaching the first
support layer to the second support layer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to tunable
capacitors and associated fabrication methods and, more
particularly, to high frequency tunable capacitors and associated
fabrication methods.
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. For example, one advantageous
MEMS device is a variable capacitor in which the interelectrode
spacing between a pair of electrodes is controllably varied in
order to selectively vary the capacitance between the electrodes.
In this regard, conventional MEMS variable capacitors include a
pair of electrodes, one of which is typically disposed upon and
fixed to the substrate and the other of which is typically carried
by a movable actuator or driver. In accordance with MEMS
technology, the movable actuator is typically formed by
micromachining the substrate such that very small and very
precisely defined actuators can be constructed.
[0003] While a variable or tunable capacitor can be utilized for
many applications, tunable filters frequently utilize variable
capacitors in order to appropriately tune the filter to allow or
reject signals having predetermined frequencies, while,
correspondingly, allowing or rejecting signals having other
frequencies. For tunable filters that are utilized for high
frequency applications, such as applications involving radio
frequency (RF) signals, the tunable filter preferably has low
signal loss and a high Q, i.e., a high quality factor.
Unfortunately, variable capacitors that include electrodes formed
of conventional metals generally do not have a sufficiently high Q
for high frequency applications. While electrodes formed of high
temperature superconductor (HTS) materials would advantageously
increase the Q of the resulting variable capacitor, the use of HTS
materials is generally not compatible with the micromachining
techniques, such as required to fabricate the actuator of a
conventional MEMS variable capacitor. For example, the chemicals,
i.e., the etchants, utilized during the micromachining of a
substrate would likely damage the superconductor materials by
altering their performance characteristics.
[0004] As such, MEMS variable capacitors that have improved
performance characteristics are desired for many applications. For
example, tunable filters having a higher Q so as to be suitable for
filtering high frequency signals are desirable, but are currently
unavailable.
SUMMARY OF THE INVENTION
[0005] A tunable capacitor is therefore provided that is
micromachined so as to be precisely defined, extremely small and
provide microelectromechanical actuation. In one embodiment the
capacitor plates are formed of a high-temperature superconductor
material. As such the tunable capacitor can be utilized for a wide
variety of high performance applications having a high Q
requirement. For example, a tunable filter using a tunable high Q
capacitor and inductor can appropriately filter high frequency
signals, such as radio frequency (rf) signals.
[0006] The tunable capacitor includes a first substrate having at
least one first capacitor plate disposed thereon. The first
capacitor plate may be formed of a high-temperature superconductor
material. Additionally, the tunable capacitor includes a second
substrate having a second capacitor plate disposed thereon. The
second capacitor plate may be formed of a high-temperature
superconductor material. The tunable capacitor also comprises a
microelectromechanical (MEMS) actuator that is operably in contact
with the second substrate so that when an electrostatic force is
applied to the actuator it responds by displacing the second
substrate, thereby varying the capacitance between the first
capacitance plate and the second capacitance plate. Generally, the
substrates may be comprised of a low signal loss material that is
compatible with the high-temperature superconductor materials
typically used to form the capacitor plates.
[0007] In one embodiment of the invention, the MEMS acuator
comprises an electrostatic actuator that includes at least one
first electrode formed on the surface of the first substrate and at
least one cantilever structure that contacts the second substrate
and provides for at least one second electrode. The electrodes can
be fabricated from a variety of materials. For example, the first
electrode may comprise a high-temperature superconductor material
and the second electrode may comprise silicon or gold. The
cantilever structure may be operably attached to the second
substrate or alternatively, the cantilever structure may support,
but be detached from, the second substrate. In the embodiment in
which the cantilever structure is operably attached to the second
substrate, spring-like structures may be patterned in the
cantilever structure to facilitate elasticity in the cantilever
structure and the second substrate. In the embodiment in which the
cantilever structure is detached from the second substrate,
spring-like structures may be attached to and connect the first and
second substrates so as to facilitate elasticity in the second
substrate.
[0008] In yet another embodiment of the invention the tunable
capacitor includes a first substrate having at least one first
capacitor plate disposed thereon. The first capacitor plate may be
formed of a high-temperature superconductor material. Additionally,
the tunable capacitor includes a second substrate having a second
capacitor plate disposed thereon. The second capacitor plate may be
formed of a high-temperature superconductor material. The tunable
capacitor also comprises a MEMS actuator that is operably in
contact with the second substrate so that when thermal actuation is
applied the actuator responds by displacing the second substrate,
thereby varying the capacitance between the first capacitor plate
and the second capacitor plate. Generally, the substrates may be
comprised of a low signal loss material that is compatible with the
high-temperature superconductor materials typically used to form
the capacitor plates.
[0009] In one embodiment of the invention, the MEMS acuator
comprises a thermal bimorph actuator that includes at least two
layers, the first layer disposed on the first substrate and the
second layer disposed on the first layer with the second layer also
being operably in contact with said second substrate. The layers
can be fabricated from a variety of materials. For example, the
first layer may comprise silicon and the second layer may comprise
gold. Characteristically, the first and second layers will comprise
materials that have different coefficients of thermal expansion so
that actuation is effected upon changing the temperature of the
thermal bimorph. The thermal bimorph structure may be operably
attached to the second substrate or alternatively, the thermal
bimorph may support, but be detached from, the second substrate. In
the embodiment in which the thermal bimorph structure is operably
attached to the second substrate, spring-like structures may be
patterned in the thermal bimorph to facilitate elasticity in the
bimorph structure and the second substrate. In the embodiment in
which the thermal bimorph structure is detached from the second
substrate, spring-like structures may be attached to and connect
the first and second substrates so as to facilitate elasticity in
the second substrate.
[0010] According to another embodiment the tunable capacitor may be
comprised so that the MEMS acuator can serve as either or both an
electrostatic actuator and/or a thermal bimorph actuator. In this
embodiment the cantilever structure that is operably in contact
with the second substrate comprises at least two layers in which
the layers comprise materials having differing coefficients of
thermal expansion so that the cantilever structure may serve as a
thermal actuator. In addition, a first electrode is disposed on the
first substrate and a second electrode is formed within a layer of
the cantilever structure. This embodiment of the invention can be
connected to a voltage source to supply heat or an electrostatic
force to thereby activate the actuator, causing displacement of the
second substrate and resulting in variance of the capacitance
between the first capacitance plate and the second capacitance
plate.
[0011] Additionally, the present invention is embodied in a method
for making a tunable capacitor. The method comprises fabricating a
first capacitor plate construct formed of a first substrate having
a first capacitor plate disposed thereon, the first capacitor
plate, typically, comprising a high temperature superconductor
material. A MEMS actuator, either an electrostatic actuator or a
thermal bimorph actuator is then fabricated so that it is in
operable contact with the first substrate. In one embodiment the
MEMS actuator is attached to the first substrate and in another
embodiment the MEMS actuator supports, but remains detached from,
the first substrate. A second capacitor plate construct is
fabricated of a second substrate having at least one second
capacitor plate disposed thereon, the second capacitor plate,
typically, comprising a high-temperature superconductor material.
The tunable capacitor is completed by connecting the first
capacitor plate construct having the operably contacted MEMS
actuator to the second capacitor plate construct such that the MEMS
actuator is disposed between the first and second capacitor plate
constructs and is in operable contact with the first capacitor
plate construct.
[0012] According to the present invention, a tunable capacitor and
an associated fabrication method are provided which permit
micromachining techniques to be used to fabricate a tunable
capacitor actuated by electrostatic or thermal MEMS actuators. In
one embodiment the tunable capacitor plates are formed of a high
temperature, super conductor material. As such, the tunable
capacitor can be precisely defined, small in size and MEMS
actuated, while also having improved performance characteristics
relative to conventional tunable capacitors. Thus, the tunable
capacitors of the present invention can be used in a variety of
applications, including those requiring high Q. such as, filtering
signals having high frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a cross-sectional view of a tunable capacitor
having a MEMS electrostatic actuator attached to the variable
capacitor in accordance with one embodiment of the present
invention.
[0014] FIG. 2 is a plan view of the tunable capacitor of FIG. 1 in
accordance with one embodiment of the present invention.
[0015] FIG. 3 is cross-sectional view of a tunable capacitor having
a MEMS thermal bimorph actuator attached to the variable capacitor
in accordance with another embodiment of the present invention.
[0016] FIG. 4 is a cross-sectional view of a tunable capacitor
having a MEMS electrostatic actuator supporting but detached from
the variable capacitor in accordance with another embodiment of the
present invention.
[0017] FIG. 5 is a plan view of the tunable capacitor of FIG. 4 in
accordance with one embodiment of the present invention.
[0018] FIG. 6 is a cross-sectional view of a tunable capacitor
having a MEMS thermal bimorph actuator supporting but detached from
the variable capacitor in accordance with one embodiment of the
present invention.
[0019] FIGS. 7A-7E are cross-sectional views of the various
processing stages used to fabricate the tunable capacitors of the
present invention in accordance with one embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The present invention now will be described more filly
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.
[0021] Referring to FIGS. 1 and 2 shown are cross-sectional and
plan view schematics of the tunable capacitor 10 in accordance with
a present embodiment of the present invention. FIG. 1 illustrates
the variable capacitor of the tunable capacitor comprising a first
capacitor plate 12 formed on the surface of first substrate 16 and
a second capacitor plate formed on the surface of a second
substrate 18. To achieve the desired low loss and high Q required
of a tunable capacitor used in high frequency applications, such as
those that involve radio frequency (RF) signals, the first and
second capacitor plates may be formed of a high temperature super
conducting (HTS) material. By way of example, these HTS materials
include, Yttrium Barium Copper Oxide (YBCO) and Thallium compounds
(TBCCO). These HTS materials are available commercially from
Superconductor Technologies Incorporated of Santa Barbara, Calif.
In order to insure a low loss filter the first and second
substrates are, generally, formed of a low loss material. For
example, the substrates may be formed of magnesium oxide (MgO),
although other low loss materials, such as LaAlO.sub.3 or
NdCaAlO.sub.4 may also be used for the first and second
substrates.
[0022] The tunable nature of the capacitor is exhibited by altering
the spacing between the second capacitor plate and the first
capacitor plate thereby changing capacitance. In the embodiment
shown in FIG. 1 displacement of the second capacitor plate is
produced via electrostatic actuation of a cantilever actuator
structure. The electrostatic cantilever actuator 20 comprises a
first layer 22 and a second layer 24. In this embodiment the second
layer of the cantilever actuator is physically attached to the
second substrate 18. The layering construct of the cantilever
actuator shown in FIG. 1 is by way of example, it is also possible
and within the inventive concepts herein disclosed to comprise the
cantilever actuator of a single layer or more than two layers.
Additionally, the cantilever actuator may comprise one material or
various materials. Characteristically, the material(s) that make up
the cantilever actuator will provide electrical conductivity so as
to act as an electrode, provide structural support for the second
substrate and isolate the second capacitor plate 14 from materials
that pose a threat to signal loss. As electrical voltage is
supplied to the cantilever actuator it responds with displacement.
The displacement of the cantilever actuator corresponds to
deflection in the attached second substrate 18 and, likewise,
capacitance varying deflection of the second capacitor plate
14.
[0023] In a present embodiment of the invention the first layer 22
of the cantilever actuator may act as the second electrode of the
electrostatic actuator. It is also possible to configure the
cantilever actuator so that the second layer acts as the second
electrode of the electrostatic actuator. The first layer may be
formed of a conductive material, such as silicon, although other
suitable conductive materials may also be used to form the first
layer. The second layer 24 of the actuator construct may comprise
gold, although other suitable materials may be used to form the
second layer. In this embodiment the second layer acts as a biasing
element, provides a point of attachment to the second substrate 18
and structurally supports the second substrate. The second layer is
connected to the second substrate through the intermediary support
structure 26. The support structure forms the second intermediary
layer of the overall tunable capacitor device. Additionally, the
second layer, in the electrostatically actuated embodiment,
provides for an electrical connection between an external voltage
source (not shown in FIG. 1) and the first layer 22 (i.e. the
second electrode). Additionally, the material(s) chosen to comprise
the second layer should not provide a means for signal loss for the
second capacitor plate 14. The second capacitor plate being,
typically, formed of a HTS material is susceptible to signal loss
if it lies in close proximity to loss-inducing materials. As shown
in the plan view of FIG. 2 the second layer 24 of this embodiment
may have spring-like structures 30 patterned during fabrication to
provide for the requisite elasticity in the deflection of the
second substrate.
[0024] The electrostatic actuation of the cantilever actuator is
realized through at least one actuator electrode 28 formed on the
first substrate 16. When different voltages are applied to the
actuator electrode and the cantilever actuator, the electrostatic
force results in the electrostatic cantilever actuator 20 being
either attracted or repelled by the actuator electrode 28 resulting
in the cantilever actuator being pulled down toward or pushed away
from the actuator electrode, respectively. This actuation allows
for the second substrate 18 and the attached second capacitor plate
14 to be deflected, thereby, varying the capacitance between the
first and second capacitor plates. As shown in FIG. 2 the actuator
electrode may be patterned on the first substrate as individual
actuator electrodes 28. Alternatively, the actuator electrode may
be one continuous ring shaped electrode formed beneath the pattern
of cantilever actuators. The number and arrangement of individual
actuator electrodes may vary in accordance with the number and
arrangement of cantilever actuators. In a present embodiment, the
actuator electrodes are formed of the same material as the first
capacitor plate, typically, HTS material. While other conductive
materials may be used to form the actuator electrodes it may be
desirable to pattern and form the actuator electrodes during the
same processing steps used to form the first capacitor plate 12 of
the tunable capacitor.
[0025] By electrically connecting the first layer 22 (i.e. the
second electrode) and the actuator electrode 28 (i.e. the first
electrode,) and the first and second capacitor plates 12, 14 to
respective electrical leads in a manner known by those of ordinary
skill in the art, the variable capacitor can be used in a tunable
filter. By varying the spacing between the capacitor plates the
filtering characteristics can be controllably modified to either
allow or reject signals having a predetermined range of
frequencies, while correspondingly rejecting or allowing signals
having frequencies outside the predetermined range. Since the
capacitor plates are, typically, constructed of an HTS material,
the tunable capacitor is particularly advantageous for filtering
signals having high frequencies, such as signals having radio
frequencies.
[0026] The plan view of FIG. 2 shows, by way of example, an
acceptable configuration of the cantilever actuators 20 leading
from the tunable capacitor frame 32 to the underside surface (not
shown in FIG. 2) of second substrate 18. The quantity and
configuration of the cantilever actuators will be dependant upon
many factors, including but not limited to, the desired robustness
of the overall structure, the elasticity of the spring-like
structures and the desired degree of capacitance variance in the
tunable capacitor. FIG. 2 also illustrates an example of the top
view relationship between the first layer 22 and second layer 24 of
the cantilever actuators 20.
[0027] FIG. 3 is a cross-sectional view of a tunable capacitor that
incorporates the use of a thermal bimorph 40 as the actuator in
accordance with another embodiment of the present invention. The
variable capacitor of the tunable capacitor comprises a first
capacitor plate 12 and a second capacitor plate 14 formed,
respectively, on the surfaces of first substrate 16 and second
substrate 18. In the same manner as the electrostatically actuated
embodiment discussed above in FIGS. 1 and 2, the first and second
capacitor plates may be formed of a high temperature super
conducting (HTS) material. In order to insure low signal loss the
first and second substrates, typically, comprise a low loss
material, such as MgO or LaAlO.sub.3.
[0028] Actuation in the FIG. 3 embodiment is accomplished by a
thermal bimorph 40. The thermal bimorph structure includes two or
more layers of materials having different thermal coefficients of
expansion that respond differently to thermal actuation. Shown in
FIG. 3 are first layer 42 and second layer 44 that comprise the
thermal bimorph structure. In this embodiment when electrical
current supplied by an external source (not shown in FIG. 3) is
passed through the second layer and/or the first layer, the overall
thermal bimorph structure becomes heated and responds by bending in
the direction of the material having the lower coefficient of
thermal expansion. In the embodiment shown in FIG. 3 the second
layer 44 will comprise a higher thermal expansion material, such as
gold, nickel or another metallic material. The first layer 42 will
comprise a lower thermal expansion material, such as silicon or
another suitable semiconductor material. This layering structure
will cause the second layer to expand more readily upon application
of heat. The expansion will cause the second layer to bend
downwards toward the first layer. In effect the overall thermal
bimorph structure will be displaced in a downward direction. This
actuation allows for the second substrate 18 and the attached
second capacitor plate 14 to be pulled down closer to the first
capacitor plate 12; thereby, effectively varying the capacitance
between the second and first capacitor plates.
[0029] In embodiments that use HTS materials to form the capacitor
plates the tuning of the capacitor is accomplished by initially
cooling the tunable capacitor to the superconducting temperature,
for a YBCO HTS material this temperature has been determined to be
77 degrees Kelvin. At the superconducting temperature the HTS
materials take on superconducting characteristics. For thermal
bimorph structures, such as the one described above, the second
layer that has the higher coefficient of expansion will contract
more readily causing the overall bimorph actuator to move in the
direction of the second layer. This actuation will result in the
second substrate and the corresponding second capacitor plate
moving away from the first substrate and the first capacitor plate.
Once this initial displacement occurs, the capacitor is then tuned
by heating the thermal bimorph. The heating operation will displace
the second capacitor plate closer to the first capacitor plate.
Typically, the temperature is increased above the superconducting
temperature by about 100 degrees Kelvin. For electrostatic
actuators the same initial cooling step is undertaken to activate
the HTS material, although no subsequent heating is required as the
capacitance is varied by applying an electrostatic force to the
actuator. Depending on the materials used to form the cantilever
actuator of the electrostatic embodiment, the cantilever actuator
may respond to the initial cooling by affecting movement in the
second substrate.
[0030] It should be noted that the thermal actuated embodiment of
the present invention does not require the use of actuator
electrodes 28 (shown in FIG. 1). Actuator electrodes are only
required in applications where an electrostatic field is
implemented to achieve actuation. Since the thermal
bimorph-actuated, tunable capacitor does not rely on an
electrostatic field there is no need to pattern actuator electrodes
on the surface of the first substrate. However, it is possible, and
within the inventive concepts herein disclosed to create a tunable
capacitor per the present invention that may be actuated either
thermally and/or electrostatically. In this embodiment the filter
may be tuned by providing electrostatic acuation, thermal actuation
or a combination of both thermal and electrostatic actuation. In
the dual electrostatic/thermal actuation embodiment the actuator
electrode(s) 28 will be required to be formed on the surface of the
first substrate. The actuator electrodes may be formed of a
suitable HTS material. While other conductive materials may be used
to form the actuator electrodes it may be desirable to pattern and
form the actuator electrodes during the same processing steps used
to form the first capacitor plate 12 of the tunable capacitor.
Additionally, in the dual electrostatic/thermal actuation
embodiment the actuator cantilevers will be required to be at least
two layers of differing materials with the layers
characteristically having contrasting thermal coefficients of
expansion (i.e. a bimorph).
[0031] Referring to FIGS. 4 and 5 shown are cross-sectional and
plan view schematics of the tunable capacitor 10 in accordance with
another present embodiment of the present invention. In this
embodiment of the invention the second substrate 18 is physically
attached to the frame 32 by the spring-like structures 50 (shown in
FIG. 5). The actuator member in this configuration, either an
electrostatic cantilever actuator or a thermal bimorph, is not
physically attached to the second substrate or the support
structure 26. The actuator member in this embodiment serves the
isolated purpose of providing the force necessary to deflect the
second capacitor plate 14. This embodiment provides for a less
constrained actuator that is free to provide greater movement and
thereby impart more deflection to the second capacitor plate.
Greater deflection in the capacitor plate provides for a wider
range of capacitance in the tunable capacitor.
[0032] FIG. 4 depicts the electrostatic cantilever version of the
tunable capacitor 10. Similar to the embodiments previously
discussed, FIG. 4 shows the variable capacitor of the tunable
capacitor comprising a first capacitor plate 12 and a second
capacitor plate 14 formed, respectively, on the surfaces of first
substrate 16 and second substrate 18. The first and second
capacitor plates may be formed of a high temperature super
conducting (HTS) material. The first and second substrates are,
typically, formed of a low loss material in order to increase the Q
of the resulting variable capacitor. For example, the substrate may
be formed of magnesium oxide (MgO), although other low loss
materials, such as LaAlO.sub.3 or NdCaAlO.sub.4 may also be used
for the first and second substrates.
[0033] The electrostatic cantilever actuator 20 comprises a first
layer 22 and a second layer 24 with the first layer structurally
supporting the second substrate 18, but not being physically
attached to the second substrate or the support structure 26. The
support structure is attached to a support platform 52 that is
formed during the second layer processing stage. A releasing
process allows for the support platform to rest atop the first
layer of the cantilever actuator without being physically attached
to the first layer. The support platform has spring-like structures
50 (shown in FIG. 5) physically connecting the support platform and
the second substrate to the frame 32. The layering construct of the
cantilever actuator shown in FIG. 4 is by way of example; it is
also possible and within the inventive concepts herein disclosed to
comprise the cantilever actuator of a single layer or more than two
layers. Additionally, the cantilever actuator may comprise one
material or various materials. Characteristically, the materials
that make up the cantilever actuator will provide electrical
conductivity so as to act as an electrode and isolate the second
capacitor plate 14 from materials that pose a threat to signal
loss.
[0034] In a present embodiment of the invention the first layer 22
of the cantilever actuator may act as the second electrode of the
electrostatic actuator. The first layer may be formed of a
conductive material, such as silicon, although other suitable
conductive materials may also be used to form the first layer. The
second layer 24 of the actuator construct may comprise gold,
although other suitable conductive materials may be used to form
the second layer. In this embodiment voltage is supplied to the
second layer of the cantilever actuator by an external voltage
source (not shown in FIG. 4), the voltage is transferred to the
first layer 22 and an electrostatic field is created between the
first layer and an actuator electrode 28. This electrostatic force
draws the cantilever actuator downward toward the stationary
actuator electrode. This downward actuation causes the second
substrate 18 and the second capacitor plate 14, which are
structurally supported by, but not physically attached to, the
cantilever actuator, to deflect downward. This deflection varies
the capacitance between the second and first capacitance
plates.
[0035] The electrostatic actuation of the cantilever actuator is
realized through at least one actuator electrode 28 formed on the
first substrate 16. As shown in FIG. 5 the actuator electrode may
be patterned on the first substrate as individual actuator
electrodes. Alternatively, the actuator electrode may be one
continuous ring shaped electrode formed beneath the pattern of
cantilever actuators. The number and arrangement of individual
electrodes may vary in accordance with the number and arrangement
of cantilever actuators. In a present embodiment, the actuator
electrodes may be formed of HTS material. While other conductive
materials may be used to form the actuator electrodes it may be
desirable to pattern and form the actuator electrodes during the
same processing steps used to form the first capacitor plate 12 of
the tunable capacitor.
[0036] The plan view of FIG. 5 shows, by way of example, an
acceptable configuration of the cantilever actuators 20 attached to
the tunable capacitor frame 32 and leading to the underside surface
(not shown in FIG. 5) of the support platform 52. Additionally,
FIG. 5 shows, by way of example, an acceptable configuration for
the spring-like structures 50 attached to the tunable capacitor
frame and the support platform. The spring-like structures serve to
provide elasticity and additional structural support to the second
substrate 18. The physical design of the springs is shown by way of
example, other spring designs can be used without departing from
the inventive concepts herein disclosed. The quantity and
configuration of the cantilever actuators and the spring-like
structures will be dependant upon many factors, including but not
limited to, the desired robustness of the overall structure, the
elasticity of the spring-like structures and the desired degree of
capacitance variance in the tunable capacitor. FIG. 5 also
illustrates the top view relationship between the second substrate
18 and the underlying support platform 52.
[0037] FIG. 6 is a cross-sectional view of a tunable capacitor that
incorporates the use of thermal actuation in accordance with one
embodiment of the present invention. The variable capacitor of the
tunable capacitor comprises a first capacitor plate 12 and a second
capacitor plate 14 formed, respectively, on the surfaces of first
substrate 16 and second substrate 18. In the same manner as the
electrostatically actuated embodiment discussed above in FIGS. 4
and 5, the first and second capacitor plates may be formed of a
high temperature super conducting (HTS) material. In order to
insure a low loss filter the first and second substrates,
generally, comprise a low loss material, such as MgO or
LaAlO.sub.3.
[0038] Actuation in the FIG. 6 embodiment is accomplished by a
thermal bimorph 40. The thermal bimorph structure includes two or
more layers of materials having different thermal coefficients of
expansion that respond differently to thermal actuation. Shown in
FIG. 6 are first layer 42 and second layer 44 that comprise the
thermal bimorph structure. The second layer will comprise a higher
thermal expansion material, such as gold, nickel or another
metallic material. The first layer will comprise a lower thermal
expansion material, such as silicon or another suitable
semiconductor material. As previously discussed in embodiments
using HTS capacitor plates, this structural relationship will cause
the second layer to contract more readily during the initial super
cooling process. This will cause the overall bimorph actuator to
move in the direction of the second layer. This actuation will
result in the second substrate and the corresponding second
capacitor plate moving away from the first substrate and the first
capacitor plate. Once this initial displacement occurs, the
capacitor is then tuned by heating the thermal bimorph. The heating
operation will displace the second capacitor plate closer to the
first capacitor plate. This actuation allows for the second
substrate 18 and the second capacitor plate 14 to be deflected down
closer to the first capacitor plate 12; thereby, effectively
varying the capacitance between the second and first capacitor
plates.
[0039] As previously discussed it is possible, and within the
inventive concepts herein disclosed to create a tunable capacitor
per the actuator support mechanism of FIGS. 4-6 that may be
actuated either thermally and/or electrostatically. In the dual
electrostatic/thermal actuation embodiment the actuator
electrode(s) 28 will be required to be formed on the surface of the
first substrate 16. Additionally, in the dual electrostatic/thermal
actuation embodiment the actuator cantilevers 20 will be required
to be at least two layers of differing materials with the layers
characteristically having contrasting thermal coefficients of
expansion (i.e. a bimorph).
[0040] As shown in FIGS. 1-6, the first substrate has one first
capacitor plate. It is also possible and within the inventive
concepts herein disclosed to dispose two first capacitor plates on
the surface of the first substrate. In the instance where two
capacitor plates are disposed on the first substrate, the second
capacitor plate disposed on the second substrate is used to vary
the capacitance between the two first capacitor plates. In this
sense, the second capacitor plate serves as a bridge to control the
field between the two first capacitor plates on the first
substrate.
[0041] Characteristically HTS materials are generally not
compatible with the silicon micromachining used to fabricate MEMS
actuators. Therefore, in accordance with a method for fabrication
of the tunable capacitors of the present invention, the filter and
the MEMS actuators are fabricated separately and then assembled
into a tunable capacitor structure.
[0042] FIGS. 7A-7E illustrate various processing steps in the
method for fabricating the tunable capacitors in accordance with an
embodiment of the present invention. It will be understood by those
having ordinary skill in the art that when a layer or element is
described herein as being "on" another layer or element, it may be
formed directly on the layer, at the top, bottom or side surface
area, or one or more intervening layers may be provided between the
layers. Referring to FIG. 7A shown is a cross-sectional view of two
silicon wafers used in the fabrication of the tunable capacitor.
The first silicon wafer 70 (not shown in FIGS. 1-6) is used as a
cap for the overall tunable capacitor and serves to facilitate the
mounting of the second substrate 18 (not shown in FIG. 7A). The use
of silicon wafers is shown by way of example, other semiconductor
substrate materials may be used to form the cap structure. A cavity
72 and opening 74 are etched in the first silicon wafer in
accordance with the configuration and dimensions of the overall
tunable capacitor and the second substrate. Preferably, a
conventional wet etch procedure is used to form the cavity and a
dry etch process is used to form the opening in the first silicon
wafer.
[0043] The second silicon wafer 76 has a gold layer 78 deposited on
the topside surface of the wafer. The structures that comprise the
gold layer are deposited using conventional mask, photoresist and
electroplating processes or a standard evaporation process. These
processes are well known by those having ordinary skill in the art.
The gold layer is preferably about 5 to about 6 microns in
thickness, although other gold layer thicknesses may be suitable.
The second silicon wafer is used to form the first layer 22 or 42
of the cantilever actuator 20 and/or thermal bimorph 40, as well
as, a portion of the frame 32 of the tunable capacitor. As
discussed previously, the use of silicon as a layering material in
the cantilever actuator and the thermal bimorph is by way of
example only, other similar materials may be used to create layers
of the cantilever actuator and the thermal bimorph.
[0044] The gold layer 78 is used to form the second layer 24 or 44
of the cantilever actuator 20 and/or thermal bimorph 40.
Additionally, in the embodiment having the cantilever actuator
and/or thermal/bimorph detached from the second substrate (as shown
in FIGS. 4-6) the gold layer is also used to form the substrate
platform 52 and the spring-like structures 50. In the application
having the detached cantilever actuator and/or thermal bimorph a
release layer (not shown in FIG. 7A) may be disposed between the
second silicon wafer and the gold layer. After the second silicon
wafer is polished back and masking and etching are performed to
define the silicon structure, (see FIG. 7C), a time dependent wet
etch process is used to release the substrate platform from the
underlying silicon layer. The release process serves to detach the
cantilever actuator and/or thermal bimorph from the second
substrate 18.
[0045] Prior to placing the second substrate in the opening, the
support structure 26 and the second capacitor plate 14 are formed
on the underside surface of the second substrate 18. The support
structure, which is formed around the periphery of the underside
surface of the second substrate preferably, comprises gold. Gold is
preferred because the support structure is, typically, in close
proximity to capacitor plate formed of HTS and gold has low signal
loss characteristics. Other low signal loss materials could also be
used to fabricate the support structure. The support structure is
typically disposed on the second substrate by a conventional
masking and evaporation process. The support structure forms the
second intermediary layer of the completed tunable capacitor
device. The second capacitor plate, which may comprise a HTS
material, is disposed on the underside surface of the second
substance. In instances where both the support structure and the
buffer layer comprise gold a single masking and evaporation process
may be used to dispose the gold layer. In embodiments having the
capacitor plate formed of HTS it is then patterned and disposed on
the substrate using standard Metal Organic Chemical Vapor
Deposition (MOCVD) techniques or Pulsed Laser Deposition (PLD)
techniques. The use of MOCVD and PLD techniques is well known by
those of ordinary skill in the art. The resulting capacitor plate
is preferably about 3 to about 4 microns in thickness, although
other capacitor plate thicknesses may be suitable. Once the support
structures and capacitor plates have been formed on the substrate
it may then be diced into the individual circuits that are
subsequently placed in the opening 74 of the first silicon wafer
70.
[0046] FIG. 7B shows a cross-sectional view of silicon wafers 70
and 76 after they have been bonded together. Additionally, the
second substrate 18 has been placed into the opening 74 and bonded
to the gold layer 78. A low temperature eutectic bond is typically
used to bond the two silicon wafers together. In the application
where the second layer 24 or 44 is formed of gold and the support
structure 26 is formed of gold, a conventional gold to gold
(Au--Au) eutectic bond procedure may be used to bond the second
substrate to the second layer.
[0047] Referring to FIG. 7C shown is a cross-sectional view of the
tunable capacitor construct after the second silicon wafer 76 has
been polished back to the desired thickness and the first layer of
the cantilever actuator and/or thermal bimorph has been patterned
and fabricated in the second silicon wafer. The desired thickness
will be dependent upon the configuration of the first layer of the
cantilever actuator and/or thermal bimorph. Typically, the silicon
wafer may be polished back to about 5 to about 6 microns, although
any other suitable silicon thickness may be desirable. After the
silicon has been polished back the first layer or actuator/bimorph
beams are patterned and a conventional dry etch process is
undertaken to fabricate the desired mechanical structure.
[0048] FIG. 7D shows a cross-sectional view of the second substrate
18 and the first substrate 16 prior to the two constructs being
bonded together to form the tunable capacitor of the present
invention. A first bonding pad 80 is disposed on the underside
surface of the silicon layer 76 and a second bonding pad 82 has
been electroplated or evaporated around the periphery of the first
substrate 16. The first and second bonding pads, preferably,
comprise gold and serve as the attachment point for bonding the
second substrate of the tunable capacitor to the first substrate of
the tunable capacitor. In the completed tunable capacitor construct
(shown in FIG. 7E) the bonding pads 80, 82 form the first
intermediate layer 84 of the overall tunable capacitor device.
[0049] The first substrate 16 is similar in fabrication to the
second substrate 18. The first substrate has second bonding pad 82
formed around the periphery of the topside surface of the first
substrate. The second bonding pad, preferably, comprises gold. The
first capacitor plate 12, which may comprise a HTS material, is
disposed on the topside surface of the first substrate. The
capacitor plate and, in the electrostatic embodiment, the actuator
electrodes 28 are then patterned and formed on the substrate using
standard Metal Organic Chemical Vapor Deposition (MOCVD) techniques
or Pulsed Laser Deposition (PLD) techniques. In embodiments of the
present invention using only a thermal bimorph for actuation the
actuator electrodes are not warranted. The resulting capacitor
plate and actuator electrodes are preferably about 3 to about 4
microns in thickness, although other thickness may be suitable.
[0050] Referring to FIG. 7E shown is a cross-sectional view of the
completed tunable capacitor. The first bonding pad 80 of the second
substrate construct is bonded to the second bonding pad 82 of the
first substrate construct. In applications where the first and
second bonding pad are formed of gold, a standard gold-to-gold
eutectic bonding procedure may be used to form the attachment that
completes the fabrication of the tunable capacitor.
[0051] Accordingly, the fabrication method of this aspect of the
present invention provides an efficient and repeatable technique
for introducing the use of high temperature superconductor
materials as capacitors in the field of micromachined tunable
capacitors. As such, the resulting tunable capacitor can be
precisely defined, small in size and MEMS actuated, while also
having improved performance characteristics relative to
conventional tunable capacitors. Thus, the tunable capacitors of
the present invention can be used in a variety of applications,
including those requiring high Q, such as, filtering signals having
high frequencies.
[0052] 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.
* * * * *