U.S. patent number RE33,651 [Application Number 07/255,918] was granted by the patent office on 1991-07-30 for variable gap device and method of manufacture.
This patent grant is currently assigned to AT&T Bell Laboratories. Invention is credited to Greg E. Blonder, Angelo A. Lamola, Robert A. Lieberman.
United States Patent |
RE33,651 |
Blonder , et al. |
July 30, 1991 |
Variable gap device and method of manufacture
Abstract
A variable gap device which comprises a suitable material with a
cavity formed therein mounted over a body. Elements are formed on
the surface of the body and on the top surface of the cavity. In
one embodiment, the elements are electrodes of a variable
capacitance device, and in another embodiment the elements are
partial or total reflectors of incident light from an optical
fiber. The surface of the cavity opposite the body is preferably
flat and of the order of microns from the body. The device is
fabricated by forming a removable layer over a temporary substrate.
An appropriate element pattern is formed over the removable layer
and the layer is etched in a configuration which will define the
cavity. The deformable material is encapsulated over this structure
and then removed from the substrate with the element remaining in
the cavity. Any removable material remaining in the cavity can then
be etched away.
Inventors: |
Blonder; Greg E. (Summit,
NJ), Lamola; Angelo A. (Sudbury, MA), Lieberman; Robert
A. (Murray Hill, NJ) |
Assignee: |
AT&T Bell Laboratories
(Murray Hill, NJ)
|
Family
ID: |
26945055 |
Appl.
No.: |
07/255,918 |
Filed: |
October 11, 1988 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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687192 |
Dec 28, 1984 |
|
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Reissue of: |
744382 |
Jun 13, 1985 |
04617608 |
Oct 14, 1986 |
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Current U.S.
Class: |
361/291; 73/705;
29/25.42 |
Current CPC
Class: |
G01L
9/0077 (20130101); G02B 6/266 (20130101); H01G
5/16 (20130101); G01L 9/0072 (20130101); G09F
9/372 (20130101); Y10T 29/435 (20150115) |
Current International
Class: |
G02B
6/26 (20060101); G01L 9/00 (20060101); H01G
5/00 (20060101); G09F 9/37 (20060101); H01G
5/16 (20060101); H01G 005/18 (); H01G 007/00 ();
G01L 009/00 () |
Field of
Search: |
;361/280,291,283,287,286
;29/25.42,588 ;350/96.14 ;338/4,47 ;73/77,705,717 ;357/60 ;381/174
;250/231P |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
J W. Berthold et al., "Fiber Optic Method for Sensing Diaphragm
Deflection", SPIE Proceedings, vol. 412, pp. 90-95, (1983). .
F. P. Milanovich, "Process, Product and Waste Stream Monitoring
with Fiber Optics", ISA International Conference Preprint, pp.
407-418, (1983)..
|
Primary Examiner: Griffin; Donald A.
Attorney, Agent or Firm: Birnbaum; L. H.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. Pat. application
Ser. No. 687,192, filed Dec. 28, 1984, assigned to the present
assignee and now abandoned.
Claims
What is claimed is:
1. A method of fabricating a variable gap device comprising the
steps of:
forming a removable layer over a substrate;
forming a first element over the removable layer;
removing portions of the removable layer which are not covered by
the element;
forming a material over the substrate so as to encapsulate the
element and the remaining portion of the removable layer
thereunder; and
separating the material from the substrate and removing the
remaining portion of the removable layer so as to form a cavity in
the material with the element adhering to the top surface of the
cavity.
2. The method according to claim 1 wherein the removable layer
comprises a photoresist material.
3. The method according to claim 1 wherein the material which
encapsulates the element comprises a rubber material.
4. The method according to claim 3 further comprising the step of
mounting the material with the cavity and element formed therein
over a substrate which includes a second element so that the cavity
is aligned over the second element.
5. The method according to claim 4 wherein the material is made to
adhere to the substrate surface by means of van der Waals
attraction.
6. The method according to claim 4 wherein the material is sealed
to the substrate surface.
7. The method according to claim 1 or 4 wherein the elements are
electrodes so as to fabricate a variable capacitance device.
8. The method according to claim 4 wherein the second element is a
partially reflecting element and the substrate is an optical
fiber.
9. The method according to claim 1 wherein the encapsulating
material is separated from the substrate by means of a parting
layer formed over the substrate prior to forming the removable
layer.
10. The method according to claim 9 wherein the parting layer
comprises a metal.
11. The method according to claim 10 wherein the encapsulating
material is separated from the substrate by immersing the structure
in a bath so that the metal layer is dissolved in the bath.
12. The method according to claim 1 wherein the element is formed
over the removable layer by depositing a metal over essentially the
entire surface of the removable layer and defining the element
photolithographically.
13. The method according to claim .[.1.]. .Iadd.3 .Iaddend.wherein
the rubber is cured at room temperature.
14. The method according to claim 1 wherein the first element is a
totally reflecting element. .[.15. A variable gap device
comprising:
a first body including a first element formed on one surface
thereof;
a second body mounted to the first body, the said second body
including a cavity formed therein which has a surface opposite the
said first element;
a second element formed on said surface of the cavity opposite the
first element to form a gap between the elements, at least one of
said bodies being flexible so that the body can be deformed and the
gap between elements can be varied, while the gap in the undeformed
state is no greater than 30 .mu.m; and optical fiber mounted with
respect to the first and second elements so that light from a core
of said fiber will be incident on at least a portion of said
elements..]. .[.16. The device according to claim 15 wherein one of
the bodies comprises said optical fiber..]. .[.17. The device
according to claim 16 wherein one of the elements is a partial
reflector of light from the fiber, and the other element is a total
reflector of light from the fiber..]. .[.18. The device according
to claim 15 wherein the first body comprises an optical fiber, the
second body is the flexible material, the first element is a
partial reflector of light from the fiber, and the second element
is a total
reflector of light from the fiber..]. 19. A variable capacitance
device comprising:
a first body including a first electrode formed on one surface
thereof, said electrode comprising two interdigitated electrode
portions which are capacitively coupled together;
a second body mounted over the first body, the said second body
including a cavity formed therein which is positioned over said
first electrode and which has a surface opposite the said
electrode;
at least one of said bodies being flexible so that the body can be
deformed and the gap between the said surface of the cavity and the
electrode portions can be varied while the gap in the undeformed
state is no greater than 30 .mu.m.Iadd.; and
a second electrode formed on said surface of the cavity opposite
the first electrode so that the interdigitated portions are
capacitively coupled together through the second
electrode.Iaddend.. .[.20. The device according to claim 19 further
comprising a second electrode formed on said surface of the cavity
opposite the first electrode so that the interdigitated portions
are capacitively coupled together through the
second electrode..]. 21. A variable capacitance device
comprising:
a first body including a first electrode formed on one surface
thereof;
a second body mounted over the first body, the said second body
including a cavity formed therein which is positioned over said
first electrode and which has a surface opposite the said
electrode;
a second electrode formed on said surface of the cavity opposite
the first electrode, at least one of said bodies being flexible so
that the body can be deformed and the gap between the electrodes
can be varied while the gap in the undeformed state is no greater
than 30 .mu.m;
each of said electrodes being u-shaped and surrounding a switch
contact electrode electrically isolated therefrom.Iadd.;
means for applying an attractive electrical potential to the first
and second electrodes to narrow the gap sufficiently to cause the
switch
contact electrodes to come into electrical contact.Iaddend.. 22. A
variable capacitance device comprising:
a first body including a first electrode formed on one surface
thereof;
a second body mounted over the first body, the said second body
including a cavity formed therein which is positioned over said
first electrode and which has a surface opposite the said
electrode;
a second electrode formed on said surface of the cavity opposite
the first electrode, at least one of said bodies being flexible so
that the body can be deformed and the gap between the electrodes
can be varied while the gap in the undeformed state is no greater
than 30 .mu.m; and
optical fibers formed between the first and second bodies with
their ends positioned so that light can be coupled between the
fibers while the bodies are undeformed and light is decoupled when
one of said bodies is
deformed. 23. A variable capacitance device comprising:
a first body including a first electrode formed on one surface
thereof;
a second body mounted over the first body, the said second body
including a cavity formed therein which is positioned over said
first electrode and which has a surface opposite the said
electrode;
a second electrode formed on said surface of the cavity opposite
the first electrode, at least one of said bodies being flexible so
that the body can be deformed and the gap between the electrodes
can be varied while the gap in the undeformed state is no greater
than 30 .mu.m; and
at least three optical fibers formed through one of the bodies with
their ends positioned so that light from a first fiber will be
coupled to a second fiber when the bodies are undeformed and light
from the said first fiber will be coupled to a third fiber when one
of the bodies is deformed.
4. A variable capacitance device comprising:
a first body including a first electrode formed on one surface
thereof;
a second body mounted over the first body, the said second body
including a cavity formed therein which is positioned over said
first electrode and which has a surface opposite the said
electrode; and
a second electrode formed on said surface of the cavity opposite
the first electrode, at least one of said bodies being flexible so
that the body can be deformed and the gap between the electrodes
can be varied while the gap in the undeformed state is no greater
than 30 .mu.m;
at least one of the bodies being transparent and at least one
electrode being at least semi-transparent so that when light is
incident through one of the bodies, the light emerging from the
device will have different wavelengths due to interference effects
depending upon whether the device
is deformed or undeformed. 25. The device according to claim 24
further
comprising an insulating layer formed over the first electrode. 26.
The device according to claim 24 wherein the device has two stable
states when the body is deformed for a particular voltage applied
across the
electrodes. 27. The device according to claim 26 wherein the second
body is flexible and the top electrode includes a plurality of
portions which are separated by stubs extending from the second
body into the cavity.
The device according to claim 26 wherein the first electrode
includes a plurality of portions and the second electrode includes
a plurality of portions which are disposed to extend in the spaces
between the first
electrode portions when one of the bodies is deformed. 29. A method
of fabricating a variable gap device comprising the steps of:
forming a removable layer over a substrate;
removing portions of the removable layer;
forming a material over the substrate so as to encapsulate the
remaining portion of the removable layer; and
separating the material from the substrate and removing the
remaining portion of the removable layer so as to form a cavity in
the material determined by the dimensions of said portion of the
removable layer.
Description
BACKGROUND OF THE INVENTION
This invention relates to variable gap devices and their method of
manufacture.
Variable gap devices are those which include a pair of elements and
a deformable material which permits a variation in the gap between
elements. In the case of variable capacitance devices, the elements
are electrodes which form a capacitor. When a bias is supplied to
the electrodes, the variable capacitance may be used for a wide
variety of functions. For example, the deformable material may take
the form of a membrane which is coupled to an electrode and
displaced in response to a pressure or audio input. The variation
in capacitance resulting from the change in electrode gap can be
detected, and the device functions as a pressure sensor or
microphone, respectively. By applying a varying bias to the
electrodes, the membrane can be caused to vibrate, and the device
functions as a loudspeaker or hearing aid. (See, for example, U.S.
Pat. application of Busch-Vishniac et al, Ser. No. 572,683, filed
Jan. 20, 1984 and assigned to Bell Telephone Laboratories now U.S.
Pat. No. 4,558,184 issued Dec. 10, 1985.) Other types of variable
capacitance devices make use of a flexible reflecting electrode for
controlling optical interference in order to provide a display
function. (See, e.g., European Patent Application Publication No.
0035299, published Sept. 9, 1981.) A still further type of device
employs a deformable waveguide between two electrodes such that
electrostatic attraction deforms the waveguide and attenuates light
propagation therethrough. A plurality of such devices forms an
optical image recorder (see, e.g., U.S. Pat. No. 4,162,118, issued
to Conwell).
While devices presently available may be adequate, the need exists
for a variable capacitance device which can operate at low
voltages. Further, precise sensor or display operations require a
highly uniform gap distance. The device should also be compact in
order to be compatible with the microminiature nature of operations
such as blood pressure sensors and the like.
In addition to achieving devices of the abovedescribed nature, it
is desirable to provide an economical method for fabricating such
devices with precise dimensions in large quantities. This requires
a technique which is capable of forming gaps of the order of
microns and extremely flat surfaces in a reproducible fashion.
Fine-line electrode dimensions approximately 20 .mu.m in width are
also desirable for many applications. Present techniques typically
require difficult alignment and etching techniques which are not
readily compatible with extremely small gap dimensions.
Consequently, it is an object of the invention to provide variable
gap devices which are small and highly accurate, yet can be
manufactured inexpensively on a large scale. It is a further object
of the invention to provide a method of manufacturing such
devices.
SUMMARY OF THE INVENTION
These and other objects are achieved in accordance with the
invention which, in one aspect, is a variable gap device comprising
a first body including a first element formed on one surface
thereof and a second body mounted to the first body. The second
body includes a cavity formed therein with a surface positioned
opposite the first element and a second element formed on this
surface to form a gap between the elements. At least one of the
bodies is flexible so that the body can be deformed and the gap
between elements can be varied. The gap in the undeformed state is
not greater than 30 .mu.m.
In accordance with a further aspect, the invention is a method of
fabricating a variable gap device. A removable layer is formed over
a substrate, and an element is formed over the removable layer.
Portions of the removable layer which are not covered by the
element are removed. A material is formed over the substrate so as
to encapsulate the element and the remaining portion of the
removable layer thereunder. The material is then separated from the
substrate, and the remaining portion of the removable layer is
removed so that a cavity is formed in the material with the element
adhering to the bottom surface of the cavity.
BRIEF DESCRIPTION OF THE DRAWING
These and other features of the invention are delineated in detail
in the following description. In the drawing:
FIG. 1 is a cross-sectional view of a device in accordance with one
embodiment of the structural aspects of the invention;
FIGS. 2-6 are cross-sectional views of a device in various stages
of fabrication in accordance with one embodiment of the method
aspects of the invention;
FIGS. 7-9 are partially cut-away perspective views of devices in
accordance with further embodiments of the invention;
FIGS. 10-12 are cross-sectional views of devices according to still
further embodiments of the invention;
FIG. 13 is an illustration of voltage as a function of electrode
position for a device operated in accordance with a still further
embodiment;
FIG. 14 is a cross-sectional view of a device which can be operated
in accordance with FIG. 13;
FIGS. 15 and 16 are illustrations of pressure as a function of
electrode position for the device of FIG. 14;
FIG. 17 is an illustration of pressure as a function of electrode
position for yet another embodiment of the invention;
FIG. 18 is a cross-sectional view of a device which exhibits the
characteristics of FIG. 17; and
FIGS. 19-22 are cross-sectional views of devices in accordance with
still further embodiments of the invention.
It will be appreciated that, for purposes of illustration, these
figures are not necessarily drawn to scale.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates, in cross section, a basic form of the
invention. A substrate, 10, which in this example was glass,
included on one major surface an element, 11, which comprised
chromium. A sheet of rubber, 12, with a small inset cavity, 13, was
formed over the substrate so that the cavity overlaid the element,
11. The cavity, 13, included an element, 14, formed on the top
surface of the cavity so that it was encased on five sides by the
rubber, 12.
The rubber was chosen to have a strong van der Waals attraction so
that it would adhere to the substrate without chemically bonding
thereto. In this example, the rubber was sold by Dow Corning under
the designation Sylgard 184, but other flexible materials may be
employed as long as they are compatible with the fabrication
technique employed to be described. It is also desirable that any
such material adhere to the substrate, but some bonding medium
could be employed for this purpose if the material did not exhibit
van der Waals attraction. The material of element, 14, in this
example was aluminum and that of element, 11, was chrome, but any
reflecting or conductive material could be employed, depending upon
the function of the device.
It will be appreciated that when an external bias (not shown) is
applied to the elements, 11 and 14, the structure operates as a
variable capacitance device since the rubber material, 12, permits
element, 14, to move either as a result of an attractive potential
applied to the elements or as a result of some outside pressure
applied to the rubber material. It will also be appreciated that,
alternatively, the device of FIG. 1 can be made with a stiff
material instead of the rubber, 12, and a flexible membrane
provided in place of substrate, 10. In that embodiment, the element
11, would move while the element, 14, would be stationary during
operation. (Hereinafter, whenever it is intended to apply a bias to
elements, 11 and 14, they will be described as "electrodes".)
Typically, for a pressure sensor, the external bias would comprise
an AC source and a resistor so as to form an RC circuit with the
device of FIG. 1. The maximum voltage across the resistor will be
determined by the resonance of the circuit which in turn is a
function of the capacitance of the device. Thus, changes in
capacitance can be detected by changes in the position of maximum
voltage as known in the art. (See, for example, The Art of
Electronics, by Horowitz and Hill, Cambridge University Press,
1980.) Alternatively, a DC potential could be supplied and changes
in capacitance determined by measuring changes in the charge on the
electrodes. A distributed array of such sensors could be used as a
tactile sensor for a robot.
One of the unique features of the structure is the fact that the
height of the cavity, 13, is made so small that the gap between
electrodes, 11 and 14, permits operation of the device with an
extremely small potential. In this example, the gap between
electrodes was only approximately 1 .mu.m, and the thickness of the
electrodes was also approximately 1 .mu.m. The width of the cavity
(and electrode, 14) was approximately 1 mm, while the width of
electrode, 11, was 0.9 mm. (The electrodes and cavity were square.)
The rubber, 12, was approximately 1 mm thick. With these
dimensions, the device may be operated at an applied AC voltage of,
typically, 5-10 volts for electrostatic movement of the electrodes
and be sensitive to pressures of approximately 0.01 psi at less
than 1 volt for sensing operations where the electrodes are moved
by outside mechanical forces. In order to keep the operating
voltage low, it is generally desirable to have an electrode gap in
the undeformed state of no greater than 10 .mu.m for electrostatic
movement of the electrodes and no greater than 30 .mu.m for
mechanical movement.
A further unique and desirable feature is the fact that the surface
of the cavity including the element, 14, can be essentially flat so
that the capacitance or reflectivity of the device is highly
uniform across the electrodes. This is especially important when
very sensitive measurements are required, such as in monitoring
blood pressure, and when the device is used for display purposes in
the manner described subsequently.
FIGS. 2-6 illustrate, in cross section, a sequence of steps
utilized to fabricate the device of FIG. 1 in accordance with one
embodiment of the method of the invention where the elements 11 and
14 are electrodes. (In these and subsequent figures, elements
corresponding to those in FIG. 1 are similarly numbered.) While a
single device is shown, it should be appreciated that several
devices can be batch-fabricated in accordance with the method.
As shown in FIG. 2, the method employs a temporary substrate, 20,
which in this example was a silicon wafer but could be any
substrate which is flat and generally free from imperfections. A
glass sheet, for example, could also be utilized. Formed over the
substrate was a layer, 21, which is characterized herein as a
"parting layer" since it was subsequently used to separate a
portion of the device from the substrate. Several types of parting
layers may be employed. In this example, the parting layer was
magnesium which was deposited by thermal evaporation to a thickness
of approximately 3000 Angstroms so that the device can be separated
by a galvanic coupling action. It may also be desirable to choose
layers which will dissolve in a chemical or electrochemical etch to
cause separation.
If desired, the "parting layer" could comprise an entire substrate
which is dissolvable in some etchant so as not to affect the
remaining portion of the device.
After the parting layer, 21, was deposited, a removable layer, 22,
was formed thereon. In this example, the layer was a photoresist
sold by Shipley Corporation under the designation AZ1450J. However,
any standard photoresist or other material which can be etched or
dissolved in accordance with the method can be employed. The
thickness of this layer will determine the electrode gap.
Consequently, the thickness in this example was approximately 1
.mu.m.
Next, an electrode pattern was defined over the photoresist. The
pattern could be formed, for example, by depositing through a
stencil mask. In this example, however, which required fine-line
definition, the electrode was defined by first depositing a layer
of metal 23, in this case aluminum, over the entire substrate. The
thickness of the layer was approximately 1 .mu.m. The metal was
then covered by a second photoresist layer, 24, which may be
identical to the first photoresist layer. As illustrated in FIG. 3,
the electrode was then defined by standard photolithography
involving exposing the layer, 24, through a mask (not shown) and
developing the resist so that it remained over the portion, 14, of
the metal film, 23, which will comprise the electrode. The exposed
portion of layer, 23, was then etched, for example, by a phosphoric
acid based etch to define the electrode.
As illustrated in FIG. 4, all of the exposed photoresist (layers,
22 and 24) was then removed leaving the electrode, 14, over a
portion, 25, of the photoresist layer, 22. This removal was
accomplished by standard plasma etching employing an oxygen plasma,
but other techniques could be employed.
In the next step, as illustrated in FIG. 5, the rubber material,
12, was applied over the substrate so as to encapsulate the
electrode, 14, and the remaining photoresist portion, 25. In this
example, the rubber was poured over the substrate and allowed to
cure at room temperature for approximately 24 hours. Such room
temperature curing is preferred since there is essentially no
shrinkage of the rubber which would wrinkle the top surface of the
cavity and the electrode, 14. Thus, the flat surfaces ultimately
result in a uniform capacitance in the final device. However,
higher temperature cures may be appropriate for certain
applications.
The structure of FIG. 5 was then immersed in some solution which
separates the rubber, 12, along with the electrode, 14, and the
photoresist, 25, from the temporary substrate, 20, along the
parting layer, 21. As mentioned previously, separation was
accomplished by galvanic coupling. This is, when two dissimilar
metals in electrical contact are placed in a bath where electrons
are available, the metal which gives up ions will corrode while the
metal which gives up electrons remains essentially unchanged. In
this example, the magnesium layer, 21, acted as the corroding metal
and the substrate, 20, acted as the noncorroding metal when the
structure was immersed in a bath of HC1. The rubber parted from the
substrate, typically, in approximately 30 minutes.
Any remaining photoresist portion, 25, was removed subsequently
from under the electrode, 14, for example, by rinsing in acetone
followed by isopropyl alcohol. This leaves the structure shown in
FIG. 6 where the cavity, 13, with electrode, 14, on the top surface
was formed in the rubber material. If desired, the portion, 25,
could be removed at the same time as the parting of the rubber from
the substrate by including in the bath a material which also
dissolves the layer, 25.
The electrode, 11, (see FIG. 1) was formed on the permanent
substrate, 10, by chrome thermal evaporation followed by standard
photolithography. The cavity from the structure of FIG. 6 was then
aligned with the substrate electrode and the rubber made to contact
the substrate surface. Since the rubber adheres well to most
surfaces by van der Waals attraction, only slight pressure is
needed to cause the rubber to adhere to the substrate. One of the
advantages of the rubber is that it will adjust itself to conform
to imperfections or to ride over electrode interconnections on the
substrate surface. If desired, the rubber material may be sealed to
the substrate by a nubmer of techniques. In this example, the
sealing was accomplished by including a wire (not shown) on the
substrate around the periphery of the cavity and sending current
through the wire to resistively heat the rubber.
The device of FIG. 1 can be utilized, for example, as a pressure
sensor by providing leads to the top and bottom electrodes, 14 and
11, during the electrode definition step (FIG. 3) and applying an
external AC or DC bias. Any change in capacitance as a result of
moving electrode, 14, due to pressure can be detected as previously
described. As shown in FIG. 7, which shows the rubber, 12,
partially cut away, the lead, 30, to the bottom electrode could be
formed on the substrate, 10, and the lead, 31, to the top electrode
can be formed along the top surface of the cavity. (The cavity is
therefore extended in the area under the lead, 31, and would have a
reduced width corresponding to that of the lead in the extended
area, less any undercutting of layer, 25, which occurred during the
plasma etching.) The leads may be formed orthogonally if desired.
The same device could be used as a pressure switch which is turned
on only when the two electrodes, 11 and 14, make contact.
An array of devices such as shown in FIG. 7 formed in the same
rubber material, 12, can be utilized as a touch screen as long as
the rubber and electrodes are transparent or they obscure only a
small fraction of the viewing area.
As an alternative to providing leads on the top surface of a
cavity, the structure shown in FIG. 8 may be utilized. Here, the
bottom electrode is replaced by two interdigitated electrodes, 32
and 33, each with a lead, 34 and 35, formed on the substrate. Each
lead electrically connects its corresponding electrode to a bias
(not shown). The two electrodes, 32 and 33, are therefore
capacitively coupled through the top electrode and can sense the
position of the top electrode without any leads applied thereto.
Further, the device may be operated even if cracks develop in the
top electrode since the AC capacitance would not be affected.
It will also be appreciated that the devices shown in FIGS. 7 and 8
can be operated as speakers or microphones with the appropriate
potential applied to the electrode.
A further variation in electrode shape results in the high
impedance switch illustrated in FIG. 9. Both top and bottom
electrodes comprise a U-shaped portion, 40 and 42, and an
electrically isolated switch contact portion, 41 and 43. (Leads
coupled to these electrodes are not shown in this figure for the
sake of simplicity.) An attractive potential applied between
U-shaped portions, 40 and 42, pulls down the top electrode allowing
the switch contacts, 41 and 43, which are electrically isolated
from the U-shaped portions, to come in contact and close whatever
circuit is coupled to these contacts. Thus, an electrostatic relay
with high electrical isolation and requiring low power can be
produced.
As illustrated in FIG. 10, the variable capacitance device may also
be used as a low frequency optical switch. Here, two optical
fibers, 50 and 51, are provided in alignment grooves in the
substrate, 10, with the air cavity, 13, between the fiber ends. In
this example, a well, 52, is also provided in the substrate. An
optical matching fluid could be provided in the area of the cavity
and well to improve coupling. In its undeformed state, light would
be coupled between the fibers. When a sufficiently attractive
voltage is supplied to the electrodes, 11 and 14, the deformed
rubber material, which is opaque, would be interposed between the
fibers to block the coupling therebetween.
A different form of optical switch is shown in FIG. 11. An input
fiber, 53, and two output fibers, 54 and 55, are provided through
the substrate, 10, so that their ends extend into the well, 52. The
well, 52, and a portion of the cavity, 13, are provided with an
optical matching material, 56 and 57, respectively. The index of
refraction of the optical matching material is chosen so that, in
the device's undeformed state, the light from input fiber, 53, will
be totally internally reflected at the surface of the matching
material, 56, so as to be incident on optical fiber, 54. When a
sufficiently attractive voltage is applied between electrodes, 11
and 14, the space between the optical matching materials, 56 and
57, will be eliminated so that the light will no longer be
reflected at the surface of material, 56, but will continue to the
electrode, 14. The light will be reflected by this electrode and be
incident on output fiber, 55, thereby switching the light output
between the output fibers.
The device may also be used as a display, a basic form of which is
illustrated in FIG. 12. In addition to the components previously
described, this device may include a layer of insulating material,
60, formed over the bottom electrode. In this particular example,
the layer was silicon dioxide having a thickness of approximately
500 Angstroms. The top electrode, 14, was reflecting and had a
thickness of approximately 1 .mu.m. The bottom electrode, 11, was
made from a semi-transparent material, in this case chrome, and was
approximately 60 Angstroms thick. The substrate, 10, was
transparent and in this example was glass.
In operation, light from a source, 61, such as ordinary ambient
light, will be incident on the substrate and be paritally reflected
at all interfaces where there is a change in index of refraction
and fully reflected by electrode, 14. In the undeformed state, the
optical interference between the reflected rays will be relatively
weak due to the wide air cavity, 13. Reflected light which emerges
from the substrate will therefore have a grayish color. When a
sufficiently attractive voltage, in this example approximately 15
volts, is applied to electrodes, 11 and 14, the electrode, 14, will
be brought into contact with insulating layer, 60. This eliminates
the air cavity and causes a strong interference effect among light
reflected by electrode, 11, layer 60, and by electrode, 14. A
bright, intense color is therefore produced by the reflected light
at a wavelength dependent upon the thickness of the bottom
electrode and the SiO.sub.2 layer. In this example, the color of
the display was purple.
Thus, each air cavity and electrode pair formed in material, 12,
can comprise a segment of a numeric or alpha numeric display. If
desired, a filter could be provided to filter out the gray color
when the segment is in its undeformed (off) state. The top
electrode could also be made semi-transparent and a light source
provided above that electrode to produce a back-lit display
operating by transmissive rather than reflective interference.
Further, an interdigitated bottom electrode, such as that shown in
FIG. 8, could be employed. A high impedance switch, for example the
type shown in FIG. 9, can also be incorporated into each display
segment if it is desirable to have the segment remain on after the
voltage is removed. That is, the high impedance when the switch is
opened causes the capacitor to remain charged for a period of time
after the bias to the segment is removed, thus providing a latching
phenomenon.
It is also possible to design a display which, at a particular
voltage, has two stable modes, thereby eliminating the requirement
of a separate switch for each segment if latching is desired. A
display can therefore be made to operate according to the
hysteresis plot of FIG. 13. Here, X.sub.o is the displacement of
the electrodes in the undeformed state. A bias of V=V.sub.hold is
applied to each display segment to deform the device to produce a
gap of X.sub.A. For a desired segment to be turned "on", a voltage
of V>V' would be applied to cause an electrode separation of
X<X.sub.C. When this additional voltage is removed, the position
of the electrodes is returned to X=X.sub.C rather than X.sub.A,
thus establishing a bistable mode. In order to turn all segments
"off", the voltage is set to zero and all the electrode gaps return
to X=X.sub.o.
One example of a structure which can be operated in such a manner
is illustrated in FIG. 14. In this example, which illustrates a
single display segment, the top electrode is broken into portions
14a-14d and interspersed between the portions are stubs, 61-63,
which are formed from the rubber material, 12. Initially, the
displacement of the rubber, 12, as a function of pressure will be
linear according to Hooke's Law. Once the stubs make contact with
the lower electrode, 11, the pressure versus displacement curve
will experience a sharp change in slope due to the resistance
provided by the stubs against further displacement. FIG. 15
illustrates the resulting pressure versus displacement curve, 70,
along with the capacitance curve, 71, which indicates how much
pressure is needed to keep the electrodes from coming together when
an attractive potential of V' is applied thereto. As shown, these
curves intersect at point A' and C' which are the two stable states
for the particular voltage applied to the electrodes. As shown in
FIG. 16, when the voltage is lowered to V", the stable states
become A" and C". Thus, operation in accordance with the hysteresis
loop of FIG. 13, previously described, can be achieved.
An alternative way of achieving bistable modes is to vary the shape
of the capacitance curve rather than the mechanical curve. In FIG.
17, for example, the curve, 73, is the normal linear Hooke's Law
displacement curve. Curve, 72, is the capacitance curve which
represents the case of interdigitated electrodes being pulled into
a parallel plane. Points A and C are stable for such a
configuration.
One embodiment which is capable of achieving such pressure versus
position curves is illustrated in FIG. 18. Here, again the top
electrode is divided into portions, 14a-14d. However, the bottom
electrode is also divided into portions, 11a-11d, which, optimally,
are disposed on pedestals, 75-78, fabricated from the substrate,
10. The bottom electrode portions are displaced from the top
electrode portions so that the top portions move in the area
between the bottom portions when the device is deformed. The device
therefore takes advantage of the fringing fields between top and
bottom electrode portions to produce the irregular capacitance
curve of FIG. 17.
While the previous examples have all involved operation of the
inventive device by applying an external bias to elements, 11 and
14, it is also possible to utilize the device with a purely optical
address and readout. As illustrated, for example, in FIG. 19, an
optical fiber sensor can be fabricated since the inventive device
can be made small enough to fit on the end of a fiber, 80. The
fiber protective coating, 90, was stripped from the end portion of
the fiber and replaced with hypodermic tubing, 91. Element 11,
which was a layer of chrome having a thickness of approximately 70
Angstroms in this example, was deposited on the end of the fiber
and tubing by thermal evaporation. The remainder of the device was
fabricated as previously described, with element, 14, encased in
rubber, 12. Element, 14, was, again, aluminum with a thickness of
approximately 1 .mu.m, but had an area approximately corresponding
to that of the fiber core, 81, which in this example was a 50 .mu.m
diameter circle. The dimensions of the rubber, 12, were 0.2 mm
thick and 0.5 mm in diameter. The portion including the element,
14, and rubber, 12, was then mounted on the fiber end so that
element, 14, was in alignment with the fiber core, 81. Additional
hypodermic tubing, 92 and 93, was provided around the rubber, 12,
and original tubing, 91, for added strength. The remainder of the
volume defined by tubing, 93, not occupied by rubber, 12, was
filled with an appropriate filler material, 94, such as
silicone.
The air gap, 13, between elements, 11 and 14, in the undeformed
state was, again, approximately 1 .mu.m. This gap will vary due to
changes in outside pressure. The structure of FIG. 19 can therefore
be utilized, for example, as a blood pressure sensor by sending
light from a source (not shown) through the fiber core, 81, so that
the light is partially reflected by element, 11, and totally
reflected by element, 14. The reflected light will form an
interference pattern returning through the fiber core. Since the
interference pattern will be dependent upon the gap distance,
pressure can be monitored by detecting changes in light intensity
for one or more wavelengths of the incident light. (For a more
detailed discussion of the readout of a fiber optic sensor, see
U.S. Pat. application of S. R. Forrest, R. A. Lieberman, and R. L.
Panock, Ser. No. 744,465, filed on June 13, 1985 and assigned to
the same assignee.)
The same device depicted in FIG. 19 can also be used as a
temperature sensor utilizing the same basic optical addressing and
readout scheme. This is because the rubber material, 12, can be
made to expand as temperature increases to cause bowing of the
reflector, 14, thereby varying the air gap and the interference
pattern of the reflected light.
A variation of the optical sensor of FIG. 19 is illustrated in FIG.
20. Here, the gap, 13, between reflecting elements, 11 and 14, is
formed by etching a portion of the core, 81, of the fiber, 80, to
form a well, 82, in the surface. The partially reflected element,
11, is then formed by depositing metal over the surface of the
fiber including the well (which will also form portions, 83 and 84,
on the fiber surface). A suitable flexible material, 12, such as
rubber, glass, or silicon, with the fully reflecting element, 14,
formed on one surface, is then mounted on the end of the fiber.
Appropriate tubing the filler material (not shown) could also be
provided. The gap distance is determined by the depth of the well,
82, and thickness of the portions, 83 and 84. Alternatively, the
reflecting layer, 11, can be deposited selectively in the well by
means of a suitable masking layer such as photoresist.
As a further embodiment, illustrated in FIG. 21, a photoresist
layer, 85, may be utilized to form the gap between reflecting
elements, 11 and 14. This can be the same photoresist which is used
to selectively deposit layer, 11, over the fiber core.
Alternatively, the layer, 11, could be formed over the entire fiber
surface, while the photoresist pillar, 85, is formed over layer,
14, and material, 12, and then the piece part subsequently mounted
on the layer, 11.
While FIGS. 20 and 21 show possible variations in the pressure
sensor, other types of temperature sensors can be fabricated by
simply mounting a thick reflecting bridge, 86 of FIG. 22, over a
layer, 11, on the fiber surface, including a mound of photoresist
(not shown). The photoresist can be dissolved to leave the air gap,
13. The bridge will expand relative to the fiber as temperature is
increased, thereby increasing the air gap spacing and changing the
reflected interference pattern. The reflecting bridge is typically
a metal or plastic which has a higher temperature coefficient than
the fiber. Similarly, the reflecting bridge, 86, can be composed of
two different layers of material with different thermal expansion
coefficients. If only a portion of the bridge is attached to only a
portion of layer 11, the bridge can bend depending upon the
temperature, and the gap will again change as a function of
temperature.
Various additional modifications of the invention will become
apparent to those skilled in the art. All such variations which
basically relay on the teachings through which the invention has
advanced the art are properly considered within the spirit and
scope of the invention.
* * * * *