U.S. patent application number 13/016564 was filed with the patent office on 2011-08-04 for microelectromechanical device with restoring electrode.
Invention is credited to Mark W. Miles.
Application Number | 20110188110 13/016564 |
Document ID | / |
Family ID | 43306210 |
Filed Date | 2011-08-04 |
United States Patent
Application |
20110188110 |
Kind Code |
A1 |
Miles; Mark W. |
August 4, 2011 |
MICROELECTROMECHANICAL DEVICE WITH RESTORING ELECTRODE
Abstract
Light in the visible spectrum is modulated using an array of
modulation elements, and control circuitry connected to the array
for controlling each of the modulation elements independently, each
of the modulation elements having a surface which is caused to
exhibit a predetermined impedance characteristic to particular
frequencies of light. The amplitude of light delivered by each of
the modulation elements is controlled independently by pulse code
modulation. Each modulation element has a deformable portion held
under tensile stress, and the control circuitry controls the
deformation of the deformable portion. Each deformable element has
a deformation mechanism and an optical portion, the deformation
mechanism and the optical portion independently imparting to the
element respectively a controlled deformation characteristic and a
controlled modulation characteristic. The deformable modulation
element may be a non-metal. The elements are made by forming a
sandwich of two layers and a sacrificial layer between them, the
sacrificial layer having a thickness related to the final cavity
dimension, and using chemical (e.g., water) or a plasma based etch
process to remove the sacrificial layer.
Inventors: |
Miles; Mark W.; (San
Francisco, CA) |
Family ID: |
43306210 |
Appl. No.: |
13/016564 |
Filed: |
January 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11580967 |
Oct 13, 2006 |
7898722 |
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13016564 |
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11056571 |
Feb 11, 2005 |
7379227 |
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11580967 |
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09966843 |
Sep 28, 2001 |
6867896 |
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11056571 |
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09056975 |
Apr 8, 1998 |
6674562 |
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09966843 |
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09413222 |
Oct 5, 1999 |
7123216 |
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11580967 |
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08744253 |
Nov 5, 1996 |
5986796 |
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09413222 |
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PCT/US95/05358 |
May 1, 1995 |
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08744253 |
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Current U.S.
Class: |
359/291 ; 29/832;
359/290 |
Current CPC
Class: |
Y10T 29/4913 20150115;
G02B 6/29358 20130101; B82Y 20/00 20130101; G02B 26/0833 20130101;
G02B 26/0841 20130101; G02B 26/02 20130101; G02B 26/001 20130101;
G02B 26/085 20130101; G01J 3/26 20130101 |
Class at
Publication: |
359/291 ;
359/290; 29/832 |
International
Class: |
G02B 26/00 20060101
G02B026/00; H05K 3/30 20060101 H05K003/30 |
Claims
1. A method for resetting at least one mirror comprising: applying
a reset voltage to an encapsulation membrane in a deflectable
mirror assembly; and discontinuing application of the reset voltage
after deflection of a mirror within the mirror assembly from an
actuated position to a rest position.
2. The method of claim 1, wherein the mirror assembly comprises: an
optically transmissive substrate; an electrode located on the
substrate; at least one deflectable mirror member spaced from the
substrate by a first gap and positioned so that the electrode is
between the substrate and the at least one deflectable mirror
member; and the encapsulation membrane comprising electrically
conductive material, the encapsulation membrane spaced from the at
least one deflectable mirror member by a second gap, the at least
one deflectable mirror member being positioned between the
substrate and the encapsulation membrane.
3. The method of claim 2, wherein applying a reset voltage to the
encapsulation membrane comprises applying the reset voltage across
the encapsulation membrane and the at least one deflectable mirror
member.
4. The method of claim 1, further comprising deflecting the at
least one deflectable mirror member towards the encapsulation
membrane and to a rest position after the reset voltage is applied
to the encapsulation membrane.
5. The method of claim 4, wherein deflecting the at least one
deflectable mirror member includes overcoming electrostatic
attraction between the at least one deflectable mirror member and
the electrode.
6. The method of claim 2, wherein the electrode is positioned on
the same side of the at least one deflectable mirror member as the
substrate and spaced from the at least one deflectable mirror
member, and wherein the electrode comprises optically transparent
conductive material.
7. The method of claim 1, wherein the mirror assembly is present in
an array and applying the reset voltage comprises applying the
reset voltage to a plurality of mirror assemblies in the array.
8. A method of moving at least one mirror in a mirror assembly, the
method comprising: applying an actuation voltage across a substrate
and the at least one mirror, the at least one mirror spaced from
the substrate by a first gap; actuating the at least one mirror to
deflect towards the substrate to an actuated position; applying a
reset voltage to an encapsulation membrane comprising electrically
conductive material, the encapsulation membrane spaced from the at
least one mirror by a second gap, the at least one mirror being
positioned between the substrate and the encapsulation membrane;
and discontinuing application of the reset voltage.
9. The method of claim 8, wherein applying the reset voltage to the
encapsulation membrane comprises applying the reset voltage across
the encapsulation membrane and the at least one mirror.
10. The method of claim 8, further comprising deflecting the at
least one mirror to deflect towards the encapsulation membrane to a
rest position after the reset voltage is applied to the
encapsulation membrane.
11. The method of claim 10, wherein the mirror assembly comprises
at least one electrode located on the substrate and wherein
applying an actuation voltage comprises applying an actuation
voltage across the at least one electrode and the at least one
mirror to move the at least one mirror toward the actuated
position.
12. The method of claim 11, wherein deflecting the at least one
mirror includes overcoming electrostatic attraction between the at
least one mirror and the at least one electrode.
13. The method of claim 11, wherein the at least one electrode
comprises optically transparent conductive material.
14. The method of claim 13, wherein the optically transparent
conductive material comprises an oxide.
15. The method of claim 13, wherein the transparent conductive
material is disposed on the substrate between the substrate and the
at least one mirror.
16. The method of claim 8, wherein the mirror assembly is present
in an array and applying the reset voltage comprises applying the
reset voltage to a plurality of mirror assemblies in the array.
17. The method of claim 8, wherein the encapsulation membrane
comprises a substrate.
18. The method of claim 8, wherein the substrate is substantially
parallel to the encapsulation membrane.
19. A method of making a mirror device comprising: depositing
spacer layers and patterns on a base substrate having address
circuitry; constructing elements of the mirror device on the base
substrate, the elements including an electrode and a mirror;
constructing an optically transparent substrate in sealed overlying
relationship with the mirror after removal of the spacer layers;
and forming at least one reset electrode, the reset electrode
connected to the optically transparent substrate.
20. The method of claim 19, wherein forming the at least one reset
electrode comprises attaching an optically transparent conductive
material to at least one region of the optically transparent
substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 11/580,967, filed Oct. 13, 2006, which is a
continuation-in-part (CIP) of U.S. patent application Ser. No.
11/056,571, filed Feb. 11, 2005 (now U.S. Pat. No. 7,379,227),
which is a continuation of U.S. patent application Ser. No.
09/966,843, filed Sep. 28, 2001 (now U.S. Pat. No. 6,867,896),
which is a divisional of U.S. patent application Ser. No.
09/056,975, filed Apr. 8, 1998 (now U.S. Pat. No. 6,674,562).
[0002] U.S. patent application Ser. No. 11/580,967 is also a
continuation-in-part (CIP) of U.S. patent application Ser. No.
09/413,222, filed Oct. 5, 1999 (now U.S. Pat. No. 7,123,216), which
is a continuation-in-part (CIP) of U.S. patent application Ser. No.
08/744,253, filed Nov. 5, 1996 (now U.S. Pat. No. 5,986,796), which
is a continuation of International Application No. PCT/US95/05358,
filed on May 1, 1995.
[0003] The disclosures of all of the above-referenced prior
applications, publications, and patents are considered part of the
disclosure of this application, and are incorporated by reference
herein. Moreover, the following patents and applications are also
incorporated herein by reference: U.S. patent application Ser. No.
08/238,750, filed May 5, 1994 (now U.S. Pat. No. 5,835,255); U.S.
patent application Ser. No. 08/554,630, filed Nov. 6, 1995 (now
abandoned); and U.S. patent application Ser. No. 08/769,947, filed
Dec. 19, 1996 (now abandoned).
BACKGROUND
[0004] This invention relates to visible spectrum (including
ultra-violet and infrared) modulator arrays.
[0005] Visible spectrum modulator arrays, such as backlit LCD
computer screens, have arrays of electro-optical elements
corresponding to pixels. Each element may be electronically
controlled to alter light which is aimed to pass through the
element. By controlling all of the elements of the array, black and
white or, using appropriate elements, color images may be
displayed. Non-backlit LCD arrays have similar properties but work
on reflected light. These and other types of visible spectrum
modulator arrays have a wide variety of other uses.
SUMMARY OF THE INVENTION
[0006] In general, in one aspect, the invention features modulation
of light in the visible spectrum using an array of modulation
elements, and control circuitry connected to the array for
controlling each of the modulation elements independently, each of
the modulation elements having a surface which is caused to exhibit
a predetermined impedance characteristic to particular frequencies
of light.
[0007] Implementations of the invention may include the following
features. The surface may include antennas configured to interact
with selected frequencies of light, or the surface may be a surface
of an interference cavity. The impedance characteristic may be
reflection of particular frequencies of light, or transmission of
particular frequencies of light. Each of the modulation elements
may be an interference cavity that is deformable to alter the
cavity dimension. The interference cavity may include a pair of
cavity walls (e.g., mirrors) separated by a cavity dimension. One
of the mirrors may be a broadband mirror and the other of the
mirrors may be a narrow band mirror. Or both of the mirrors may be
narrow band mirrors, or both of the mirrors may be broad band,
non-metallic mirrors. The cavity may have a cavity dimension that
renders the cavity resonant with respect to light of the frequency
defined by the spectral characteristics of the mirrors and
intrinsic cavity spacing in an undeformed state. One of the mirrors
may be a hybrid filter. One (or both) of the walls may be a
dielectric material, a metallic material, or a composite
dielectric/metallic material. The cavity may be deformable by
virtue of a wall that is under tensile stress. The control
circuitry may be connected for analog control of the impedance to
light of each element. The analog control may be control of the
degree of deformity of the deformable wall of the cavity.
[0008] The predetermined impedance characteristic may include
reflection of incident electromagnetic radiation in the visible
spectrum, e.g., the proportion of incident electromagnetic
radiation of a given frequency band that is, on average, reflected
by each of the modulation elements. The modulation element may be
responsive to a particular electrical condition to occupy either a
state of higher reflectivity or a state of lower reflectivity, and
the control circuitry may generate a stream of pulses having a duty
cycle corresponding to the proportion of incident radiation that is
reflected and places the modulation element in the higher state of
reflectivity during each the pulse and in the lower state of
reflectivity in the intervals between the pulses. The
characteristic may include emission of electromagnetic radiation in
the visible spectrum. The characteristic may include the amount of
electromagnetic radiation in the visible spectrum that is emitted,
on average, by the antennas. The characteristic may be incident
electromagnetic radiation in the visible spectrum. The modulation
elements may include three sub-elements each associated with one of
three colors of the visible spectrum. The modulation element may be
responsive to a particular electrical condition to occupy either a
state of higher transmissivity or a state of lower transmissivity,
and the control circuitry may generate a stream of pulses having a
duty cycle corresponding to the proportion of incident radiation
that is transmitted and places the modulation element in the higher
state of transmissivity during each the pulse and in the lower
state of transmissivity in the intervals between the pulses. The
characteristic may include the proportion of incident
electromagnetic radiation of a given frequency band that is, on
average, transmitted by each of the modulation elements.
[0009] The visible spectrum may include ultraviolet frequencies, or
infrared frequencies.
[0010] In general, in another aspect of the invention, the control
circuitry may be connected to the array for controlling the
amplitude of light delivered by each of the modulation elements
independently by pulse code modulation.
[0011] In general, in another aspect, the invention features a
modulation element having a deformable portion held under tensile
stress, and control circuitry connected to control the deformation
of the deformable portion.
[0012] Implementations of the invention may include the following
features. The modulation element may be self-supporting, or held on
separate supports. The deformable portion may be a rectangular
membrane supported along two opposite edges by supports which are
orthogonal to the membrane. The deformable portion, under one mode
of control by the control circuitry, may be collapsed onto a wall
of the cavity. The control circuitry controls the deformable
portion by signals applied to the modulation element, and the
deformation of the control portion may be subject to hysteresis
with respect to signals applied by the control circuitry.
[0013] In general, in another aspect, the invention features
modulating light in the visible spectrum using a deformable
modulation element having a deformation mechanism and an optical
portion, the deformation mechanism and the optical portion
independently imparting to the element respectively a controlled
deformation characteristic and a controlled modulation
characteristic.
[0014] Implementations of the invention may include the following
features. The deformation mechanism may be a flexible membrane held
in tensile stress, and the optical portion may be formed on the
flexible membrane. The optical portion may be a mirror. The mirror
may have a narrow band, or a broad band, or include a hybrid
filter.
[0015] In general, in another aspect, the invention broadly
features a non-metal deformable modulation element.
[0016] In general, in another aspect, the invention features a
process for making cavity-type modulation elements by forming a
sandwich of two layers and a sacrificial layer between them, the
sacrificial layer having a thickness related to the final cavity
dimension, and using chemical (e.g., water) or a plasma based etch
process to remove the sacrificial layer.
[0017] Among the advantages of the invention are the following.
[0018] Very high-resolution, full-color images are produced using
relatively little power. The embodiment which senses the image
incident on the array has relatively low noise. Their color
response characteristics are tunable by selection of the dimensions
of the antennas. The antenna or cavity embodiments are useful in
portable, low power, full color displays, especially under high
ambient light conditions. Phase controlled reflective embodiments
are useful in passive light scanning such as optical disk readers
without moving parts. The emissive embodiments also could be used
as display devices especially in low-ambient-light conditions.
[0019] Because of the dielectric materials used in some
embodiments, the devices have the advantage of being extremely
light efficient, making them especially appropriate for high
intensity projection displays, and reducing or eliminating the need
for backlighting in low ambient light applications. In addition,
more accurate color representations are possible, as well as
designs optimized for the IR and UV. Mechanical hysteresis
precludes the need for active drivers, and this coupled with their
geometric simplicity and monolithic nature brings defect losses
down significantly. The devices are also exceptionally fast, low
power, and non-polarizing. The fact that they can be reflective
and/or transmissive enhances their flexibility.
[0020] The process for fabrication as represented in some
embodiments relies on benign chemicals, minimizing waste disposal
problems, and facilitating the fabrication of devices on a variety
of substrates (e.g., plastics or integrated circuits) using a
larger variety of materials. Devices on plastic substrates have the
potential of being extremely inexpensive. All of the manufacturing
technologies used are mature, further reducing manufacturing
costs.
[0021] Other advantages and features of the invention will become
apparent from the following description and from the claims.
DESCRIPTION
[0022] FIG. 1 is a perspective view of a display device.
[0023] FIG. 2 is a perspective schematic exploded view of a
representative portion of the screen of FIG. 1.
[0024] FIG. 3 is an enlarged top view of a tri-dipole of FIG.
2.
[0025] FIG. 4 is a schematic view of a single dipole antenna of
FIG. 3.
[0026] FIG. 5 is a schematic perspective view, broken away, of a
portion of the screen of FIG. 1.
[0027] FIG. 6 is an enlarged top view of an individual tri-bus of
FIG. 2.
[0028] FIG. 7 is an enlarged perspective view of a representative
portion of the screen of FIG. 1.
[0029] FIG. 8 is a cross-sectional view along 8-8 of FIG. 7.
[0030] FIG. 9 is a diagram of a portion of a control circuit of
FIG. 2, and a corresponding dipole antenna of FIG. 3.
[0031] FIGS. 10A, 10B, 10C are representative graphs of the input
voltage to the bias source of FIG. 9.
[0032] FIG. 11 is a diagram of portions of the control modules for
a row of pixels,
[0033] FIG. 12 is a circuit diagram of an oscillator.
[0034] FIG. 13 is a schematic diagram of a circuit module of FIG.
2, a corresponding dipole antenna of FIG. 3, and a graphical
representation of the output of a binary counter.
[0035] FIG. 14 is a circuit diagram of the pulse counter of FIG.
13.
[0036] FIGS. 15, 16, 17, 18, and 19 are top views of alternative
dipole arrangements.
[0037] FIGS. 20A through 20F are perspective views of a cavity
devices.
[0038] FIGS. 21A and 21B are side views of the cavity device.
[0039] FIGS. 22A through 22F are graphs of useful pairs of
frequency responses which can be achieved by the cavity device when
it is in one of two states.
[0040] FIGS. 22G through 22AF are a larger list of graphs of
individual frequency responses which in some combinations prove
useful in the cavity device.
[0041] FIGS. 23A and 23B are top and cutaway side views,
respectively, of a display.
[0042] FIGS. 23C and 23D are top and cutaway side views,
respectively, of another display.
[0043] FIG. 24A is a graph of an electromechanical response of the
cavity device.
[0044] FIGS. 24B and 24C are graphs of addressing and modulation
schemes for a display.
[0045] FIG. 24D is a graph of a hysteresis curve.
[0046] FIGS. 25A through 25N and FIGS. 26A through 26K are
perspective views of the device during assembly.
[0047] FIGS. 27A through 27C are side views of dielectric
mirrors.
[0048] FIG. 27D is a top view of a dielectric mirror.
[0049] FIGS. 28A, 28B are perspective and top views of a linear
tunable filter.
[0050] FIGS. 29A, 29B are perspective and top views of a deformable
mirror.
[0051] Referring to FIG. 1, device 20 includes a screen 22 for
displaying or sensing a high resolution color image (or a
succession of color images) under control of power and control
circuitry 26. The image is made up of a densely packed rectangular
array of tiny individual picture elements (pixels) each having a
specific hue and brightness corresponding to the part of the image
represented by the pixel. The pixel density of the image depends on
the fabrication process used but could be on the order of 100,000
pixels per square centimeter.
[0052] Referring to FIG. 2, each pixel is generated by one
so-called tri-dipole 30. The boundary of each tri-dipole is
T-shaped. The tri-dipoles are arranged in rows 32 in an
interlocking fashion with the "Ts" of alternating tri-dipoles
oriented in one direction and the "Ts" of intervening tri-dipoles
along the same row oriented in the opposite direction. The rows
together form a two-dimensional rectangular array of tri-dipoles
(corresponding to the array of pixels) that are arranged on a
first, external layer 34 of screen 22. The array may be called an
electrically alterable optical planar array, or a visible spectrum
modulator array.
[0053] On a second, internal layer 36 of screen 22 so-called
tri-busses 38 (shown as T-shaped blocks in FIG. 2) are arranged in
an interlocking two-dimensional array 40 corresponding to the
layout of the tri-dipoles on layer 34 above. Each tri-dipole 30 is
connected to its corresponding tri-bus 38 by a multi-conductor link
42 running from layer 34 to layer 36 in a manner described
below.
[0054] On a third, base layer 44 of screen 22 a set of circuit
modules 46 are arranged in a two-dimensional rectangular array
corresponding to the layouts of the tri-dipoles and tri-busses.
Each circuit module 46 is connected to its corresponding tri-bus 38
by a six-conductor link 48 running from layer 36 to layer 44 in a
manner described below.
[0055] Each circuit module 46 electronically controls the optical
characteristics of all of the antennas of its corresponding
tri-dipole 30 to generate the corresponding pixel of the image on
screen 22. Circuit modules 46 are connected, via conductors 50
running along layer 44, to an edge of layer 44. Wires 52 connect
the conductors 50 to control and power circuitry 26 which
coordinates all of the circuit modules 46 to 25 generate the entire
image.
[0056] Referring to FIG. 3, each tri-dipole 30 has three dipole
sections 60, 62, 64. The center points 59, 61, 63 of the three
sections are arranged at 120 degree intervals about a point 65 at
the center of tri-dipole 30. Each section 60, 62, 64 consists of a
column of dipole antennas 66, 68, 70, respectively, only ten dipole
antennas are shown in each section in FIG. 3, but the number could
be larger or smaller and would depend on, e.g., the density with
which control circuits 46 can be fabricated, the tradeoff between
bandwidth and gain implied by the spacing of the antennas, and the
resistive losses of the conductors that connect the antennas to the
control circuit 46. Only the two arms of each dipole antenna are
exposed on layer 34, as shown in FIG. 3. The dipole antennas of a
given section all have the same dimensions corresponding to a
particular resonant wavelength (color) assigned to that section.
The resonant wavelengths for the three sections 60, 62, 64 are
respectively 0.45 microns (blue), 0.53 microns (green), and 0.6
microns (red).
[0057] Referring to FIG. 4, each dipole antenna 80 schematically
includes two Ls 82, 84 respectively made up of bases 86, 88, and
arms 90, 92. The bases of each antenna 80 are electrically
connected to the corresponding circuit module 46. The span (X) of
arms 90, 92 is determined by the desired resonant wavelength of
dipole antenna 80; for example, for a resonant wavelength of
lambda, X would be lambda/2. Dipole antennas 66, 68, 70 have X
dimensions of 0.225 microns (lambda.sub.1/2), 0.265 microns
(lambda.sub.1/2), and 0.3 microns (lambda.sub.3/2), respectively.
The effective length (Y) of bases 86, 88 from arms 90, 92 to
circuit module 46 is also a function of the dipole antenna's
resonant wavelength; for a resonant wavelength of lambda, Y is a
multiple of lambda.
[0058] Referring to FIG. 5, each of the bases 86, 88 physically is
made up of four segments; (1) one of the conductors 96 of link 42,
(2) a portion 112 of tri-bus 38, (3) a short connecting portion 124
of tri-bus 38, and (4) one of the conductors 94 of link 48, which
together define a path (e.g., the path shown as dashed line 97)
with an effective length of Y from the arm (e.g., 92) to the
circuit module 46.
[0059] The placement of link 42 perpendicular to the surface of
layer 34 allows arms 90, 92 (formed on the surface of layer 34) to
be spaced at an actual spacing Z that is closer than lambda/2, the
minimum required effective Y dimension of bases 86, 88. Spacing Z
may be chosen based on the bandwidth/gain tradeoff, and for example
may be one quarter of the resonant wavelength for the dipole
antennas of a given section (i.e., lambda/4, or 0.1125 microns
(lambda.sub.1/4), 0.1325 microns (lambda.sub.1/4) and 0.15 microns
(lambda.sub.3/4) for antennas 66, 68, 70, respectively).
[0060] Referring to FIG. 6, each tri-bus 38 is formed of aluminum
on layer 36 and has three zigzag shaped bus pairs 100, 102, 104 for
respectively connecting dipole antennas of the corresponding
sections 60, 62, 64 of tri-dipole 30. Bus pairs 100, 102, 104 are
connected to individual dipole antennas 66, 68, 70 via conductors
of link 42 (FIG. 2) that are joined to the bus pairs at points,
e.g., 106.
[0061] Each bus pair 100, 102, 104 has two parallel buses 108, 110.
Bus 108 electrically connects together the arms of the dipole
antenna 5 of the corresponding section and, independently, the
related bus 110 electrically connects together the arms 92 of the
dipole antennas of that same section.
[0062] Points 106 delineate a series of fragments 112, 114, 116 on
each of the three bus pairs 100, 102, 104, respectively. Each
fragment forms part of one or more of the bases 86, or 88 and
therefore contributes to the effective Y dimension.
[0063] The lengths (Q) of fragments 112, 114, 116 are one-half of
the resonant wavelengths (i.e. lambda/2) of the sections 60, 62,
64, or 0.225 microns (lambda.sub.1,/2), 0.265 microns
(lambda.sub.2/2), and 0.3 microns (lambda.sub.3/2),
respectively.
[0064] The conductors of link 48 (FIG. 2) are attached to tri-bus
38 at points 118, 120, 122 at the ends of buses 108, 110. Between
points 118, 120, 122 and the first points 106 on along buses 108,
110 are fragments 124, 126, 128, which also form portions of the
bases 86, 88 and are included to adjust the effective Y dimensions
of those bases to be integer multiples of lambda/2. The lengths of
the three fragments 124, 126, 128 are 0.1125 microns, 0.1525
microns, and 0.1875 microns, respectively.
[0065] Referring to FIG. 7, each dipole antenna 80 is physically
formed (of aluminum) on an insulating semiconductor (e.g. silicon
dioxide of silicon nitride) substrate 130 (part of layer 34) by
x-ray or electron beam lithography or other technique suitable for
forming submicron-sized structures.
[0066] Tri-busses 38 (not seen in FIG. 7) are formed on the
upper-side of a second insulating semiconductor substrate 132 (part
of layer 36). Circuit modules 46 (not seen in FIG. 7) are part of a
third insulating semiconductor substrate 134 (part of layer 44) and
are connected by conductors 50 to gold contact pads 136 (only one
shown, not to scale) formed on the edge of substrate layer 134.
[0067] Referring to FIG. 8, circuit module 46 is formed in and on
substrate 134 by any one of several monolithic processes. A section
138 of the substrate 134, which has been previously coated with an
insulating semiconductor oxide layer 140, is repeatedly masked
(whereby small windows are opened in the oxide layer, exposing the
semiconductor beneath) and exposed to n and p dopants to form the
desired circuit elements (not shown in detail in FIG. 8).
[0068] The individual circuit elements are connected to each other
and to external contact pad 136 (FIG. 7) by aluminum conductors
142, 50, respectively. To form the connections, holes 144 are
opened in oxide layer 140 and a sheet of aluminum is deposited,
filling holes 144. Using a masking technique similar to the one
described above the unwanted aluminum is removed, leaving only
conductors 142, 50.
[0069] Semiconductor substrate layer 132 is deposited directly on
top of the remaining exposed oxide layer 140 and conductors 142,
50. Holes 146 (one shown) (opened using a suitable lithographic
technique) are channels for the electrical conductors 147 of links
48, which connect tri-bus 38 and circuit module 46. Tri-bus 38 is
etched from a sheet of aluminum deposited onto the surface of layer
132. The deposition process fills holes 146, thereby forming the
conductors of links 48.
[0070] Substrate layer 130 is deposited onto the surface of
substrate layer 132 and tri-bus 38. The arms of dipole antennas 80
are formed by depositing a sheet of aluminum onto the surface of
layer 130 and etching away the unwanted metal. During the
deposition process holes 148 are filled thereby forming the
conductors 149 of links 42 between the arms of dipole antenna 80
and tri-bus 38.
[0071] The conductors 149 are the uppermost parts of bases 86, 88
(FIG. 4) of dipole antennas 66, 68, 70; the lengths of conductors
149 together with the lengths of fragments 112, 114, 116 (FIG. 6),
the lengths of fragments 124, 126, 128, and the lengths of the
conductors 147 determine the effective Y dimension of bases 86,
88.
[0072] The length of the conductors 149 is determined by the
thickness of the substrate 130 through which links 42 pass.
Substrate 130 and links 42 are 0.05625 microns (i.e.
lambda.sub.1/8) thick. This thickness is achieved by controlling
the deposition rate of the semiconductor material as layer 130 is
formed.
[0073] The length of the conductors 149 is determined by the
thickness of the substrate layer 132 through which they pass. This,
layer and links 48 are therefore also 0.05625 microns 20 thick.
[0074] The Y dimensions for the dipole antennas 66, 68, 70 of
sections 60, 62, 64 therefore are as follows:
[0075] (a) For section 60, Y equals the sum of 0.05625 microns
(length of the conductor in link 42, lambda.sub.1/8)+n*0.225
microns (where 0.225 microns=lambda.sub.2/2, the length of a
fragment 112, and n=the number of fragments 112 in each base 86, 88
of the nth dipole antenna 66n)+0.1125 microns (length of fragment
124, lambda.sub.1/4)+0.05625 microns (length of the conductor in
link 48, lambda.sub.1/8), and that sum equals
(n+1)*(lambda.sub.1/2).
[0076] (b) For section 62, Y equals the sum of 0.05625 microns
(length of link 42, lambda.sub.1/8)+n*0.265 microns (where 0.265
microns=lambda.sub.2/2, the length of a fragment 114, and n=the
number of fragments 114 in each base 86, 88 of the nth dipole
antenna 68n)+0.1125 microns (length of fragment 126,
(lambda.sub.2/2)-(lambda.sub.1/4))+0.05625 microns (length of the
conductor in link 48, lambda.sub.1/8), and that sum equals
(n+1)*(lambda.sub.1/2).
[0077] (c) For section 64, Y equals the sum of 0.05625 microns
(length of conductor in link 42, lambda.sub.1/8)+n*0.3 microns
(where 0.3 microns-lambda.sub.3/2, the length of a fragment 116,
and n equals the number of fragments 116 in each base 86, 88 of the
nth dipole antenna 70n)+0.1875 microns (the length of fragment 128,
(lambda.sub.3/2)-(lambda.sub.1/4))+0.05625 microns (length of
conductor in link 48), and that sum equals
(n+1)*(lambda.sub.3/2).
[0078] Referring again to FIG. 1, in some embodiments, the
displayed image is not emitted from device 20 but is comprised of
ambient light (or light from a source, not 25 shown) selectively
reflected by the tri-dipoles 30 of screen 22.
[0079] In that case, each tri-dipole 30 receives ambient light
having a broad spectrum of wavelengths and is controlled by the
corresponding circuit module to reflect only that portion of the
ambient light manifesting the hue and brightness of the desired
corresponding pixel.
[0080] The hue generated by tri-dipole 30 depends on the relative
intensities of the light reflected by sections 60, 62, 64. The
overall brightness of that hue of light in turn depends on the
absolute intensities of the light radiation reflected by sections
60, 62, 64. Thus, both the hue and brightness of the light
generated by tri-dipole 30 can be controlled by regulating the
intensity of the light reflected by the dipole antennas in each
section of the tri-dipole; this is done by controlling the
reflectivity of each dipole antenna, i.e. the percentage of the
light of the relevant wavelength for that dipole antenna which is
reflected.
[0081] The desired percentage is attained not by regulating the
amount of light reflected at any given instant but by arranging for
the antenna to be fully reflective in each of a series of spaced
apart time slots, and otherwise non-reflective. Each dipole
antenna, in conjunction with its circuit module, has only two
possible states: either it reflects all of the light (at the
antenna's resonant frequency), or it reflects none of that light.
The intensity is regulated by controlling the percentage of total
time occupied by the time slots in which the dipole antenna
occupies the first state.
[0082] Each dipole antenna is controlled to be reflective or not by
controlling the impedance of the dipole antenna relative to the
impedance of the medium (e.g., air) through which the light
travels. If the medium has an effective impedance of zero, then the
relationship of the reflectivity of the dipole antenna to zero (the
controlled impedance of the dipole antenna) can be derived as
follows. If we define a three-axis system x-y-z in which the x and
y axes are in the plane of the array and the z axis is the axis of
propagation of the incident and reflected waves, where z=0 is the
surface of the array, then the incident plus reflected wave for
z<0 may be represented as:
E _ _ = x ^ E _ 0 - j k z + x ^ E _ r + j k z ( 1 ) H _ _ = V
.times. E _ _ - j.omega..mu. 0 = y ^ 1 .eta. 0 [ E _ 0 - j k z - E
_ r + j k z ] ( 2 ) ##EQU00001##
where E (overbar) is the complex amplitude of the electric field of
the sum of the transmitted wave and the reflected wave; E.sub.0 is
the complex amplitude of the electric field of the transmitted 20
wave; E.sub.r is the complex amplitude of the electric field of the
reflective wave; x(hat) is the orientation of the electric field of
the wave; H is the amplitude of the magnetic field; y(hat) is the
orientation of the magnetic field; .mu.o is the permeability of
free space; .epsilon..sub.0 is the permittivity of free space;
k=.omega.sqrt [.mu..sub.0.epsilon..sub.0] is the wavenumber; and
.eta.=sqrt [.mu..sub.0/.epsilon..sub.0] is the impedance of free
space. For z>0 (i.e., within free space) only the transmitted
wave exists and is represented by
E _ _ = x ^ E _ t - j k t z ( 3 ) H _ _ = y ^ 1 .eta. t E _ t - j k
t z ( 4 ) ##EQU00002##
E(overbar) is the complex amplitude the transmitted wave at z=0,
k.sub.t=sqrt [.mu..epsilon.] is its wavenumber; .eta.=sqrt
[.mu./.epsilon.] is the impedance of the medium, i.e. z>o.
Boundary conditions (z=0) for tangential electric fields are
imposed on equations 1 and 2 and they are combined to yield,
{circumflex over (x)}[E.sub.0+E.sub.r]={circumflex over (x)}E.sub.t
(5)
In the same way, continuity for tangential magnetic fields (z=0) at
the boundary yields, [0083] Dividing equations 5 and 6 by E.sub.0,
and E.sub.0/.eta..sub.0 respectively gives the following two
equations:
[0083] y(1/.eta..sub.0)(E.sub.0-E.sub.R)=y(1/.eta..sub.t)E.sub.t
(6)
1+E.sub.r/E.sub.0=E.sub.t/E.sub.0 (7)
1-E.sub.R/E.sub.0=(.eta..sub.0/.eta..sub.t)(E.sub.t/E.sub.0)
(8)
E.sub.r/E.sub.0 is called .GAMMA. and is the complex reflection
coefficient while E.sub.t/.eta..sub.0=T is called the complex
transmission coefficient, and .eta..sub.t/.eta..sub.0=.eta..sub.n
is the normalized wave impedance. Solving for T and .GAMMA.
yields
T _ = E _ t E _ 0 = 2 1 + .eta. 0 / .eta. t = 2 .eta. n .eta. n + 1
( 9 ) .GAMMA. _ = E _ R E _ 0 = T _ - 1 = .eta. N - 1 .eta. n + 1 (
10 ) ##EQU00003##
For matched impedance values, .eta..sub.0=.eta..sub.n, the
reflection coefficient is zero, and T=1 (i.e., no reflection), and
in the case of a load at the boundary, a matched antenna, there is
complete absorption.
[0084] As .eta..sub.n approaches zero or infinity, the reflection
coefficient approaches plus or minus one, implying total
reflection.
[0085] Referring to FIG. 9, the impedance z.sub.L, of dipole
antenna 80 is controlled by a variable resistance PIN diode 160
connected across bases 86, 88. PIN line 162 to the output of a bias
high voltage or a low voltage based on a line 168 from power and
the output of bias source 164 is a high voltage, the resistance R
of PIN diode 160 (and hence the impedance (E,) of the dipole
antenna is zero causing full reflection; when the output of bias
source 164 is a low voltage 98, resistance R is set to a value such
that the resulting impedance z@ is matched to z. (the impedance of
the air surrounding the antenna), causing zero reflection.
[0086] To generate an entire image on screen 20, power and control
circuitry 26 receives a video signal (e.g. a digitized standard RGB
television signal) and uses conventional techniques to deliver
corresponding signals to modules 46 which indicate the relative
desired intensities of light reflected from all sections 60, 62, 64
of all of the tri-dipoles in the array at a given time. Circuit
modules 46 use conventional techniques to deliver an appropriate
stream of input control signal pulses to each bias source 164 on
line 168.
[0087] The pulse stream on each line 168 has a duty cycle
appropriate to achieve the proper percentages of reflectance for
the three Sections of each tri-dipole. Referring to FIGS. 10A, 10B,
and 10C, for example, pulse stream 170 has a period T and a 50%
duty cycle. For the first 50% of each period T the input to bias
source 164 is high and the Corresponding output of source 164 is a
high voltage. During this portion of the cycle dipole antenna 80
will reflect all received light having the dipole antenna's
resonant wavelength. For the second 50% of the cycle the output of
source 164 will be low and dipole antennas 80 will absorb the
received light. In FIGS. 10B, 10C, pulse streams 172, 174 represent
a 30% duty cycle and a 100% duty cycle respectively; with a 30%
duty cycle the effective intensity of the light radiation of the
dipole antennas of the section will be 30%; for a duty cycle of
100%, the effective intensity is 100%.
[0088] For example, if a particular pixel of the image is to be
brown, the relative intensities required of the three red, 25
green, and blue sections 60, 62, 64 may be, respectively, 30, 40,
and 10. The input signals to the bias sources 164, carried on lines
168, would then have duty cycles, respectively, of 30%, 40%, and
10%. An adjacent pixel which is to be a brown of the same hue but
greater brightness might require duty cycles of 45%, 60%, and
15%.
[0089] Referring to FIG. 11, to accomplish the delivery of the
pulse width modulated signals from circuitry 26 to the pixel
circuit modules 46, each circuit module 46 in the row includes
storage 180, 182 for two bits. The bit 1 storage elements 180 of
the modules 46 in the row are connected together to create one long
shift register with the pulse width modulated signals being passed
along the row data line 184 from pixel to pixel. If, for example,
the period of the modulated signals is 1 millisecond and there are
ten different intensity levels, then an entire string of bits
(representing the on or off state of the respective pixels in the
row during the succeeding 1/10 millisecond) is shifted down the row
every 1/10 millisecond. At the end of the initial 1/10 millisecond
all of the bits in elements 180 are shifted to the associated
elements 182 by a strobe Pulse on strobe line 186. The content of
each element 182 is the input to the driver 188 for the appropriate
one of the three colors of that pixel, which in turn drives the
corresponding section 60, 62, 64 of the tri-dipole. The rate at
which data is shifted along the shift registers is determined by
the number of elements on a given row, the number of rows, the
number of intensity levels, and the refresh rate of the entire
array.
[0090] In another embodiment, the light comprising the image is
emitted by tri-dipoles 30 rather than being produced by reflected
ambient light. In that case, each tri-dipole generates the light
for a single pixel with a hue and brightness governed by the
intensities of the light emitted by each of the three sections 60,
62, 64.
[0091] Each dipole antenna within a tri-dipole is caused to emit
light at the resonant wavelength of that antenna by stimulating it
using a signal whose frequency corresponds to the resonant
wavelength. Thus, the sections 60, 62, 64 will emit blue
(lambda.sub.1), green (lambda.sub.2), and red (lambda.sub.3) light
respectively when provided with signals whose frequencies equal,
respectively, lambda.sub.1, lambda.sub.2 and lambda.sub.3.
[0092] For an idealized dipole, the current I and current density
J(overbar) (r'overbar) are described by
I=j.omega.q (11)
J( r')={circumflex over (z)}Id{dot over (o)}( r') (12)
where q is the charge density; z(hat) indicates the direction of
the current (along the z-axis); .omega. is angular frequency; and d
is the distance between ideal point charges representing the
dipole. The vector potential A(overbar) in polar coordinates is
given by
A _ _ = r ^ A _ r + .theta. ^ A _ .theta. = ( r ^ cos .theta. -
.theta. ^ sin .theta. ) .mu. 0 I _ d 4 .pi. r - j k r ( 13 )
##EQU00004##
where .theta. represents the angle relative to the dipole;
.theta.(hat) is the angular orientation of the wave; .mu..sub.0 is
the permeability of free space; r is radius from the dipole; r(hat)
is radial orientation of the wave; A.sub.r is the radial component
of the vector potential; A.sub..theta. is the angular component of
the vector potential; and k is a factor which is used to represent
sinusoidally varying waves. The H field is given by
H _ _ = .PHI. 1 .mu. 0 r [ .delta. .delta..theta. ( r A _ .theta. -
.delta. .delta..theta. ( A _ r ) ] = .PHI. j k I _ d 4 .pi. r - j k
r [ 1 + 1 j k r ] sin .theta. ( 14 ) ##EQU00005##
where .phi. is elevation, with respect to the dipole. The E field
is given by, The far-field equation is given by
E _ _ = 1 j.omega..epsilon. 0 .gradient. .times. H _ _ = .mu. 0 /
.epsilon. 0 j k I _ d 4 .pi. r - j k r ( r ^ [ 1 j k r + ( 1 j k r
) 2 ] 2 cos .theta. + .theta. [ 1 + 1 j k r + ( 1 j k r ) 2 ] sin
.theta. ) ( 15 ) H _ _ = .PHI. ^ j k I _ d 4 .pi. r - j k r sin
.theta. E _ _ = .theta. ^ .mu. 0 / .epsilon. 0 j k I _ d 4 .pi. r -
j k r sin .theta. ( 16 ) ##EQU00006##
[0093] Equation (16) describes the radiation pattern away from a
dipole antenna at distances significantly greater than the
wavelength of the emitted electromagnetic wave. It is a very broad
radiation pattern providing a wide field of view at relevant
distances.
[0094] Referring to FIG. 12, the dipole antennas 66, 68, 70 of each
section 60, 62, 64 are driven by signals (e.g., sinusoidal) with
frequencies of 5.times.10.sup.14 Hz, 5.6.times.10.sup.14 Hz, and
6.6.times.10.sup.14 Hz for red, green, and blue, respectively.
These signals are supplied by three monolithic oscillators 200 (one
shown) within circuit module 46, each tuned to one of the three
required frequencies.
[0095] In circuit 200 (an a stable multivibrator), the center pair
of coupled transistors 202, 204 are the primary active elements and
will oscillate if the circuit admittance's are set appropriately.
Diodes 206, 208, 210, 212 provide coupling capacitance's between
the transistors and the inductors 214, 216 are used to tune the
operating frequency.
[0096] In a third embodiment, an image of the object is focused by
a conventional lens (not shown in FIG. 1) onto screen 22, which
then acts as an image sensor. The tri-dipoles of screen 22,
controlled by power and control circuitry 26, generate electrical
signals corresponding to pixels of the received image. The signals
are then processed by a processor which, in conventional fashion,
delivers a derived RGB video signal which can then be transmitted
or stored.
[0097] The signals generated for each tri-dipole are generated by
the corresponding circuit module 46 and represent the hue and
brightness of the light radiation received at that tri-dipole.
[0098] Each section of tri-dipole 30 can only be used to measure
light having the resonant wavelength of its respective dipole
antennas, however, because most colors can be expressed as a
combination of red, green, and blue, circuit module 46 can, by
independently measuring the intensity of the light radiation
received at each section 60, 62, 64, derive a signal which
specifies the hue and intensity of the received pixel.
[0099] Referring to FIG. 13, dipole antenna 80 will absorb incident
light radiation at its resonant wavelength when its reflection
coefficient (.GAMMA..sub.L) is zero, which occurs when its
controlled impedance (z.sub.L) matches the impedance of the medium
(z.sub.0). In those circumstances, a voltage pulse is produced
across the ends 308, 310 of dipole 80 for each incident photon. The
relative magnitude of the light radiation received by each dipole
antenna can thus be measured by counting the average number of
pulses across ends 308, 310 over a given time period.
[0100] In this embodiment, circuit module 46 includes a terminating
load resistor 315 connected across ends 308, 310. The controlled
impedance of the combination of dipole antenna 80 and resistor 315,
described by the equations set forth below, is equal to
z.sub.0.
[0101] The voltage of the pulse across resistor 315 (created by an
incident photon) is illustrated by the sine wave graph above
register 15 and is described generally by the following
equation
V(z)=V+e.sup.-jkz+.GAMMA..sub.Le.sup.jkz (17)
Because z.sub.L=z.sub.0, .GAMMA..sub.L=0, and equation 17
simplifies to
V(z)=V+e.sup.-jkz (18)
[0102] A pulse detector 318 amplifies and sharpens the resulting
pulse to a square wave form as shown, which is then used as the
clock (CLK) input 319 to a binary counter 320. The output of the
binary counter is sampled at a regular rate; collectively the
samples form a digital signal representing the intensity of
received light radiation over time. Each time counter 320 is
sampled, it is reset to zero by a pulse on control line 322,
Counter 320 thus serves as a digital integrator that indicates how
much light arrived in each one of a succession of equal length time
periods.
[0103] Referring to FIG. 14, in pulse detector 318 the pair of
transistors 322, 324 serve as a high impedance differential stage
whose output, (representing the voltage difference between points
308, 310) is delivered to an amplifier 326. Amplifier 326 serves as
a high-bandwidth gain stage and delivers a single sided output
pulse to a conditioning circuit 328 that converts slow rising
pulses to square pulses 330 for driving counter 320.
[0104] In another embodiment, the array of tri-dipoles is operated
as a phased array. The operation of phased arrays is discussed more
fully in Amitay, et al., Theory and Analysis of Phased Array
Antennas, 1972, incorporated herein by reference. By controlling
the spacing of successive tri-dipoles across the array and the
relative phases of their operation, wave cancellation or
reinforcement can be used to control the direction in three
dimensions and orientation of the radiation. Beams can thus be
generated or scanned. In the case of an array used to sense
incoming radiation, the array can be made more sensitive to
radiation received from selected directions.
[0105] Other embodiments are also possible. For example, referring
to FIG. 15, each section of tri-dipole 400 array be a single dipole
antenna 406, 407, 408. The tri-dipole antennas are then arranged
about a center Point 410 at 120 degree intervals in a radial
pattern. Bases 411, 412, as well as arms 414, 415, of the dipole
antennas, are all formed on the same surface.
[0106] Referring to FIG. 16, each section may consist of multiple
dipole antennas 406, 407, 408 connected by attaching the bases 411,
412 of each succeeding dipole antenna to the inner ends of arms
414, 415 of the preceding dipole antenna. Circuit modules 416 are
formed on the surface of layer 413.
[0107] Referring to FIG. 17, a multi-dipole 430 could have five
sections 432, 434, 436, 438, 440 composed of dipole antennas 442,
444, 446, 448, 450, respectively. The dipole antennas of the
different sections would have different resonant wavelengths. Other
multi-dipoles might have any number of sections.
[0108] The scanning of pixels could be done other than by pulse
width modulation, for example, using charge coupled devices to
shift packets of charge along the rows of pixels.
[0109] Referring to FIGS. 18, 19, other arrangements of dipole
antennas may be used in order to match the area required for the
control circuit modules.
[0110] Referring to FIG. 18, each section 470 of a tri-dipole in
the reflective mode could be formed of a number of subsections
(e.g., 472) arranged in two rows 474 and a number of columns 47.
The antennas 478 in each subsection 472 are all served by a single
PIN diode circuit 480 located at the peripheral edge of section 470
at the end of the subsection on the layer below the antenna layer.
All circuits 480 for the entire Section 470 are in turn served by a
single bias source 164 (FIG. 9). This arrangement reduces the
number of bias sources required for the entire array of
tri-dipoles. FIG. 19 shows an alternate arrangement in which there
is but one row of subsections each served by a single PIN diode
circuit at the end of the row.
[0111] In order to reduce the number of conductors 50, selected
tri-dipoles could be used to receive control signals transmitted
directly by light and to pass those control signals to the control
circuits of nearby active tri-dipoles.
[0112] The dipoles could be mono-dipoles comprised of only a single
dipole antenna, all with the same resonant wavelength.
[0113] Dipole antennas 470 could be randomly arranged on the
surface of layer 472 of screen 22.
[0114] A different color regime, e.g. cyan-magenta-yellow, could be
substituted for RGB.
[0115] Spiral, biconical, slotted, and other antenna configurations
could be substituted for the dipole configuration.
[0116] The array could be three-dimensional.
[0117] The successive tri-dipoles in the array can be oriented so
that their respective antennas are orthogonal to each other to
enable the array to interact with radiation of any arbitrary
polarization.
[0118] The PIN diodes could be replaced by other impedance
controlling elements. Such elements might include quantum well
transistors, superconducting junctions, or transistors based on
vacuum microelectronics. Further improvement could be achieved by
reducing the complexity of the third layer containing control
circuitry. The electronics required to get control signals to the
circuitry could be eliminated by the use of laser or electron beams
to provide such signals. This would have the advantage of allowing
for arrays of even higher density.
[0119] The array could be fabricated on a transparent substrate,
thus facilitating transmissive operation.
[0120] In other embodiments, the antenna arrays alone (without
control circuitry or connection buses) may be fabricated on
one-half of a microfabricated interferometric cavity. The antenna
array can be considered a frequency selective mirror whose spectral
characteristics are controlled by the dimensions of the antennas.
Such a cavity will transmit and reflect certain portions of
incident electromagnetic radiation depending on (a) the dimensions
of the cavity itself and (b) the frequency response of the mirrors.
The behavior of interferometric cavities and dielectric mirrors is
discussed more fully in Macleod, H. A., Thin Film Optical Filters,
1969, incorporated by reference.
[0121] Referring to FIG. 20a, two example adjacent elements of a
larger array of this kind include two cavities 498, 499 fabricated
on a transparent substrate 500. A layer 502, the primary
mirror/conductor, is comprised of a transparent conductive coating
upon which a dielectric or metallic mirror has been fabricated.
Insulating supports 504 hold up a second transparent conducting
membrane 506. Each array element has an antenna array 508 formed on
the membrane 506. The two structures, 506 and 508, together
comprise the secondary mirror/conductor. Conversely, the antenna
array may be fabricated as part of the primary mirror/conductor.
Secondary mirror/conductor 506/508 forms a flexible membrane,
fabricated such that it is under tensile stress and thus parallel
to the substrate, in the undriven state.
[0122] Because layers 506 and 502 are parallel, radiation which
enters any of the cavities from above or below the array can
undergo multiple reflections within the cavity, resulting in
optical interference. Depending on the dimensions of the antenna
array, as explained above, the interference will determine its
effective impedance, and thus its reflective and/or transmissive
characteristics. Changing one of the dimensions, in this case the
cavity height (i.e., the spacing between the inner walls of layers
502, 506), will alter the optical characteristics. The change in
height is achieved by applying a voltage across the two layers at
the cavity, which, due to electrostatic forces, causes layer 506 to
collapse. Cavity 498 is shown collapsed (7 volts applied), while
cavity 499 is shown uncollapsed (0 volts applied).
[0123] In another embodiment, FIG. 20b, each cavity may be formed
by a combination of dielectric or metallic mirrors on the two
layers, and without the antennas formed on either layer. In this
case the spectral characteristics of the mirror are determined by
the nature and thickness(es) of the materials comprising it.
[0124] In an alternative fabrication scheme, FIG. 20c, each cavity
is fabricated using a simpler process which precludes the need for
separately defined support pillars. Here, each secondary
mirror/conductor, 506, is formed in a U-shape with the legs
attached to the primary layer; each secondary mirror/conductor thus
is self-supporting.
[0125] In yet another scheme, FIG. 20d, the cavity has been
modified to alter its mechanical behavior. In this version, a
stiffening layer, 510, has been added to limit deformation of the
membrane while in the driven state. This assures that the two
mirrors will remain parallel as a driving voltage is gradually
increased. The resulting device can be driven in analog mode (e.g.,
cavity 511 may be driven by 5 volts to achieve partial deformation
of the cavity) so that continuous variation of its spectral
characteristics may be achieved.
[0126] FIG. 20E illustrates an additional configuration. In this
scheme a stop layer 512 has been added so that the position of the
membrane 506 in the driven state may be a fixed offset from the
wall, 502. Alternative optical, electrical, or mechanical responses
may be accommodated in this fashion. For example, the stop layer
may act as an insulator between walls 506 and 502, or its thickness
may be set to achieve a certain center frequency when the device is
driven.
[0127] FIG. 20F shows an encapsulated version of the device of the
cavity. Encapsulation membrane 514 is fabricated in the same
fashion and using similar materials as the original cavity, 502 and
506 by use of 4 `the described processes. In this case, the process
is used to build structures on top of an array of cavities which
have already been fabricated. Encapsulation membrane 514 is a
continuous structure designed to be rigid and inflexible. The
function of this encapsulation is multifold. First it acts as a
hermetic seal so that the entire array can be purged with an inert
gas and maintained at an appropriate pressure. Second, the
electrical and optical properties may also be useful in the overall
operation of the array. Using electrically conducting materials, a
voltage may be applied to encapsulation membrane 514, and the
resulting electrostatic forces between membranes 514 and 506 can
alter the hysteresis of the underlying cavity in a useful fashion.
The collapse and release thresholds can be modified beyond what is
dictated by the structure of the cavity itself. The electrostatic
forces may also aid in releasing the membrane 502 from the
collapsed state should it become stuck during the normal course of
operation. Using the appropriate optical materials, the
encapsulation membrane 514 may also be incorporated into the
overall optical design of the cavity, providing another element
with which to achieve the desired optical responses. Finally, the
exposed side of the encapsulation membrane 514 can provide a
surface upon which drive circuitry may be fabricated or
mounted.
[0128] Referring to FIGS. 21A and 21B, the modulation effect on
incident radiation is shown. The binary modulation mode is shown in
FIG. 21A. In the undriven state (shown on the left) incident light
512 (the delta lambda represents a range of incident frequencies,
e.g., the range of visible light) contains a spectral component
which is at the resonant frequency of the device in the undriven
state. Consequently this component (delta lamba n) is transmitted,
516, and the remaining components (at nonresonant frequencies,
delta lambda minus delta lamba n) are reflected, 514. This
operation is in the nature of the operation of a fabry-perot
interference cavity.
[0129] When the device is driven and the geometry altered to
collapse (right side of figure), the resonant frequency of the
device also changes. With the correct cavity dimensions, all of the
incident light (delta lamba) is reflected.
[0130] FIG. 21A shows a binary mode of operation while FIG. 21B
shows an analog mode, where a continuously variable voltage may be
used to cause a continuously variable degree of translation of
secondary mirror/conductor 506. This provides a mechanism for
continuous frequency selection within an operational range because
the resonant frequency of the cavity can be varied continuously. In
the left side of FIG. 21A, the transmitted wavelengths are delta
lambda n zero, while in the right hand side they are delta lambda n
one.
[0131] The equations which explain the performance of the cavity
are set forth at the bottom of FIG. 21B. Equation 1 defines the
transmission T through a fabry-perot cavity. Ta, Tb, Ra, Rb are the
transmittances and reflectances of the primary (a) and secondary
(b) mirrors. Phi a and Phi b are the phase changes upon reflectance
at the primary and secondary mirrors, respectively. Delta is the
phase thickness. Equation 2 defines the phase thickness in terms of
the cavity spacing ds, the index of refraction of the spacer ns,
and the angle of incidence, theta s. Equation 3 shows that the
transmission T becomes the transmission of the second mirror when
the transmission of the first mirror approaches 0.
[0132] There are a number of particular frequency response pairs
which would be useful in applying the invention to displays. FIGS.
22A through 22F illustrate some of the possibilities and relate to
the equations of FIG. 21B. These are idealized plots which
illustrate transmission and reflectivity (T/R) of the cavity for
wavelengths in the visible range in driven and undriven states for
each of the driven and undriven response modes. The different modes
are achieved by using different combinations of mirrors and cavity
spacings. They are idealized in the sense that only approximations
to these responses can be achieved in actual designs, with
differences ranging from extraneous sidelobes to higher losses.
Differences can be tolerated as long as the overall spectral
response results in the perception of the desired color.
[0133] The spectral characteristics of the mirrors used can be
referred to as broad-band and narrow-band. The mirrors are
optimized for the visible range with a broad band mirror operating
across the entire visible range (i.e., reflecting over a minimum
range of 400 nm to 700 nm). Such a mirror is denoted in the stack
formula 1.671 0.775(ERS) 0.833M (ERS) 1.671 where ERS denotes an
equal ripple filter. The ERS filter has layers which are a quarter
wavelength thick, and their refractive indices are n0=1.671,
n1=1.986, n2=1.663, n=2.122, n4=1.562, n5=2.240, n6=1.495,
n7=2.316, n8=1.460, n9=2.350, n10=1.450. A narrow-band filter
optimized for the color green would reflect only over the range of
500 nm to 570 nm, and transmit everywhere else. Such a filter is
described by the stack formula 1 C1 2C2 (3A/2 3B1 3A/2)2 (3A/2 3B
3A/2)6 (3A/2 3B1 3A/2)2 1.52 where the refractive indices are
nA=0.156, nC2=nB1=1.93, and nB=2.34.
[0134] The cavity spacing (i.e., cavity height) in both driven and
undriven states can be set to a predetermined value by the film
thicknesses used in its construction. These two values determine
whether a cavity is resonant or non-resonant. For a resonant
cavity, the spacing is determined such that the undriven state
coincides with the resonant peak of the narrower of the two
mirrors. When a device is non-resonant, it must be driven in order
for the device to become resonant.
[0135] For example, if the undriven cavity spacing were 535 nm
then, because this coincides with the center of the previously
defined narrow-band mirror, there would be a transmission peak at
this frequency. Peak spacing for this cavity is 267 nm so the other
peaks, which would occur in the standard Fabry-Perot fall outside
of range of the narrow band mirror. This would be considered a
resonant cavity because the peak occurs in the undriven state.
Driving the cavity so that the spacing were 480 nm would result in
no transmission peak because all of the cavity peaks are outside
the range of the narrow-band mirror. For all practical purposes the
narrow-band mirror does not exist at this frequency and therefore
the transmission peak disappears.
[0136] FIG. 22A shows a T/R plot of a cavity having broad band
mirrors on both layers of the cavity. When undriven, this results
in transmission/reflection peaks which occur at wavelengths which
are approximately integral multiples of the cavity spacing. (the
notation m delta lambda n in FIG. 21A denotes the fact that there
may be a series of peaks.) This is classic fabry-perot behavior. In
the driven state (shown to the right in FIG. 22A), the cavity
resonance is shifted out of the visible range causing the device to
act like a broadband mirror.
[0137] FIG. 22B shows a T/R plot for a cavity having one broad band
and one narrow band mirror. This device has a resonant cavity,
causing a transmission peak at the center of the narrow-band
mirror's passband when the device is in the undriven state. Driving
the device (right hand side of FIG. 22B) shifts the cavity
resonance away from that of the narrow band mirror, and the device
acts like a broadband mirror.
[0138] In FIG. 22C, the cavity is like that of FIG. 22B, except the
cavity is non-resonant which results in broadband mirror cavity
behavior in the undriven state. When driven, the cavity spacing
shifts into resonance, causing a transmission peak centered on the
narrow-band mirror.
[0139] FIG. 22D shows the performance of a resonant cavity with two
narrow-band mirrors. When undriven, there is a transmission peak
centered on the mirrors' response. Since the mirrors are
narrow-band, the overall cavity response is that of a broad-band
transmitter. Driving the device out of resonance (i.e. active)
results in a reflective peak at the narrow-band center
frequency.
[0140] Like FIG. 22D, the cavity of FIG. 22E has two narrow band
mirrors, but it is a non-resonant cavity. Consequently its behavior
is opposite that of the cavity portrayed in FIG. 22D.
[0141] Thus, when one of the mirrors is narrow banded, mirror a for
example, the transmission approaches zero for frequencies outside
its range. This is essentially the transmission of mirror b. When
both of the mirrors are narrow banded, the transmission becomes a
maximum outside the frequency range. In either case, the spurious
peaks typical of a fabry-perot are avoided. The result is a device
which can be described as a single mode resonant cavity.
[0142] When both of the mirrors are narrow banded, then fabry-perot
type behavior occurs only when the cavity spacing is correct.
Making the mirrors narrow enough allows only a single peak to
occur. Then it is unnecessary to be concerned about spurious peaks
that might occur within the visible range.
[0143] FIG. 22F is for a cavity with a simpler design involving
only a metallic mirror on one wall and a hybrid filter on the other
wall. The hybrid filter is a combination of a narrow bandpass
filter (outer surface) and an induced absorber (inner surface). The
induced absorber is a simple structure which can consist of one or
more dielectric or dielectric and metallic films. The function of
the absorber is to cause incident light of a specified frequency
range to be absorbed by a reflective surface (i.e. mirror). One
such design can be achieved with a single film of refractive index
n=1.65 and a thickness of 75.8 nm. The induced absorber only
functions when it is in contact with the mirror, otherwise it is
inconsequential.
[0144] In the undriven state, the hybrid filter (a green centered
narrow bandpass/induced absorber) reflects everything but the green
light, which is unaffected by the induced absorber and subsequently
reflected by the metallic mirror. Thus the overall cavity response
is like that of a broad-band mirror. When in the driven state, the
hybrid filter comes into contact with the metallic mirror. The
absorber couples the green light into the mirror, and the result is
an absorption peak at that wavelength.
[0145] Each of the driven and undriven states shown in FIGS. 22A
through 22F can be considered optical responses which in some
combinations represent modulator designs which can be used to build
a display. FIGS. 22G through 22AF illustrate additional idealized
optical responses useful in fabricating a full color or monochrome
display. Specifically, FIGS. 22G through 22N portray broadband
responses covering the entire visible range. These would be useful
in creating a black and white display or a color display if they
were used in conjunction with an external color filter mechanism.
The responses shown in FIGS. 22O through 22T act on 1/3 of the
visible spectrum. FIGS. 22U through 22AF are representative of
responses which act on 2/3 of the visible spectrum. For example,
FIGS. 22AC and 22AD illustrate a response where the crossover has
blue on one side and green/red on the other while FIGS. 22W and 22X
have red on one side and green/blue on the other. The responses
illustrated in FIGS. 22G through 22AF could be combined in pairs to
produce two state modulators, some of which are shown in FIGS. 22B
through 22F.
[0146] Referring to FIG. 23A, a red 3.times.3 pixel (i.e., 9
cavities) subtractive mode display array based on the cavity device
using the N-N (narrow band-narrow band) configuration of FIG. 22D
is shown. The cavity pixels are formed at the intersections of
primary mirror/conductors 602 and secondary mirror/conductors 604.
The display is fabricated on substrate 608 and driven via contact
pads 606 connected to each conductor/mirror 604.
[0147] A full nine-pixel display comprises three replications of
the array of FIG. 23A arranged on top of one another and fabricated
on separate substrates or color planes 610, 612, 614, as shown in
FIG. 23B. Each of the individual color planes interacts only with
and reflects one color (e.g., red, green, or blue), while passing
all other colors. This is done by selecting the mirror spectral
characteristic and cavity spacing in each color plane
appropriately. The color planes are physically aligned and
electrically driven through the contact pads to produce full color
images. The image would be viewed from below in FIG. 23B.
[0148] Referring to FIGS. 23C and 23D, a single layer composite
approach is shown. Such a device would be more complicated to
fabricate (though the mirror designs are simpler) but may suffer
from inferior resolution. In this case, all three colors reside on
the same array 616. Devices using either the B-B, B-N, N-N, B-H or
the N-H (B=broad band, N=narrow band, H=hybrid filter)
configuration are used for this display.
[0149] Either the three plane, dual plane, or the single plane
approach may be used in either transmissive and reflective modes.
Pixel size and overall display size can be altered to make the
displays useful in many different display and spatial light
modulator applications. These include direct view and projection
displays, optical computing, holographic memory, and any other
situation where a one or two dimensional modulator of light can be
used.
[0150] Because these structures depend on electrostatic attraction,
which varies in an exponential fashion with cavity spacing, while
the mechanical restoring force of the membrane varies linearly with
cavity spacing, they exhibit hysteresis. As seen in FIG. 24A, the
straight line labelled membrane tension shows that the restoring
force on the membrane varies inversely linearly with distance
(i.e., cavity spacing). That is, the smaller the spacing, the
stronger the mechanical force tending to restore the original, at
rest spacing. On the other hand, electrical attraction between the
two layers of the cavity (the curved line) varies exponentially
with smaller spacing, that is, as the two layers get closer there
is an exponential increase in the attractive force between them.
This causes a hysteresis effect as follows. With a low driving
voltage applied, the secondary mirror/conductor experiences a
displacement towards the substrate until the force of restoration
balances the electrical attraction. However if the voltage is
raised past a point known as a collapse threshold, the force of
restoration is completely overcome and the membrane is pressed
tightly against the substrate. The voltage can then be lowered
again to some degree without affecting the position of the
membrane. Only if the voltage is then lowered significantly or
eliminated will the membrane be released. Because the membrane is
under tensile stress, it will then pull itself away from the
substrate when the voltage is released. This hysteresis can be used
to achieve matrix addressing of a two-dimensional array, as
explained with reference to FIG. 24B.
[0151] The display can be addressed and brightness controlled using
control pulse sequences in the driving voltage. Shown is a timing
diagram for a 3.times.3 pixel array analogous to that shown in FIG.
23A. During operation, a continuous series of -5 volts scanning
pulses is applied to the rows (rows 1-3) of the pixel array in a
sequential fashion, as seen in the charts labelled "Row". The
pulses appear at the same frequency on each of the rows but the
pulses for different rows are staggered. These pulses are
insufficient in their own right to cause the membrane to collapse.
The columns (cols. 1-3) of the pixel array (see charts labelled
"Col) are maintained at a bias voltage of 5 volts so that the
nominal voltage across each unactivated pixel is 5 volts. At times
when the scan pulses are delivered to that pixel, the nominal row
and column potentials are 5 and -5 volts respectively, resulting in
a cavity voltage of 10 volts. With a 10 volts potential applied to
the row and a -5 volts potential to the column, the total voltage
across the cavity becomes 15 volts which is sufficient to drive the
secondary mirror/conductor into the collapsed state, where it will
remain until the end of the scan when all of the column voltages
are pulsed to zero. The three charts at the bottom of FIG. 24B show
the on and off states of the three pixels identified there by row
and column numbers.
[0152] The intensity or brightness of a pixel may be varied by
changing the fraction of the scan during which the pixel is
activated. The scan cycle begins at 198 and ends at 199. The
frequency of the scan pulses is such that six pulses of a given row
fall within the scan cycle, providing an opportunity to activate a
pixel at any one of six times during each cycle. Once the pixel is
activated it stays on until the end of the cycle. Therefore six
different intensities are possible for each pixel. For the scan
shown, pixel C1R1 is at full brightness, pixel C2R2 is at 4/6
brightness, and pixel C3R2 is at 1/6 brightness. All pixels are
cleared at the end of the scan and the cycle begins again. Since
these structures can be driven at frequencies as high as 50 kHz,
this method of brightness control is practical. Assuming six
brightness levels, there would be a possibility of more than 8
thousand row scans per second.
[0153] Another way of achieving brightness control would utilize
differential driving voltages and a more efficient addressing
scheme. Shown in FIG. 24C is an addressing scheme based on voltages
derived from the hysteresis curve illustrated in FIG. 24D. Like the
previous scheme a series of scanning pulses is applied sequentially
to the rows. When the column is sitting at the bias voltage of
[0154] C.sub.b=V.sub.1, the total row/column voltage when the
scanning pulse is applied is
C.sub.b-R.sub.s=V.sub.1+1/2(V.sub.2-V.sub.1). The diagram of FIG.
24D shows that this is insufficient to move a pixel from the on
state to the off state or from the off state to the on state. When
the column voltage is set to [0155] C.sub.1=V.sub.2 and a scanning
pulse is applied, the resulting difference is
C.sub.1-R.sub.s=V.sub.2+1/2(V.sub.2-V.sub.1). This is sufficient to
turn a pixel on, however the voltage difference is not sufficient
to turn pixels with a scan voltage of R.sub.b=0 on. Conversely, the
application of C.sub.0=-V.sub.1 volts to the column during a scan
pulse brings the total difference to
V.sub.0=C.sub.0-R.sub.s=-V.sub.1+1/2(V.sub.2-V.sub.1) which is
sufficient to turn a pixel off. In this fashion, pixels may be
driven arbitrarily into one state or another, precluding the need
for a reset pulse as is required in the previous approach.
[0156] Addressing can be accomplished by breaking the maximum on
time (i.e., maximum brightness) into eight different segments. Each
segment represents a bit ranging from most significant to least
significant in an 8 bit word. The least significant bit is the
smallest in length, equal to 1/256 of the maximum time, while the
next most significant is twice that length. Each subsequent bit is
doubled in length until the most significant bit. In this fashion,
a single row only has to be addressed a total of eight times (for
eight bits of gray scale) to create arbitrary values of brightness
for a given pixel, thus significantly reducing the required number
of row scans.
[0157] Two processes for fabricating the arrays will be discussed;
others may also be possible.
[0158] Referring to FIG. 25A, substrate 700 is first cleaned using
standard procedures. The substrate may be of many different
materials including silicon, plastic, mylar, or quartz. The primary
requirement is that the material be able to support an optically
smooth, though not necessarily flat, finish. A preferred material
would likely be glass, which would permit both transmissive and
reflective operation in the visible range.
[0159] The substrate is then coated with the primary
conductor/mirror layer(s) 702. This can be achieved using a
physical vapor deposition (PVD) method such as sputtering or e-beam
evaporation. Other possible methods include chemical vapor
deposition and molecular beam epitaxy. The dimensions and nature of
the layer(s) depend on the specific configuration desired. Detailed
examples are discussed below.
[0160] Referring to FIG. 25B, a photoresist 704 has been patterned
on the primary conductor/mirror. The photoresist may be of a
positive or negative type. The standard procedure for this step
involves spinning of the resist, softbaking at 90 C, exposing
through an appropriate mask, developing to produce the pattern, and
hardbaking at 130 C.
[0161] Referring to FIG. 25C, the photoresist pattern is defined in
the primary conductor/mirror by an etching process. This step can
be achieved either by wet chemical means or by plasma or reactive
ion etching (RIE). The choice of etching technique depends on the
nature of the conductor/mirror. In the case of an aluminum
conductor/mirror, chlorine gas may be used to perform the etch,
with a standard chamber power of 100 watts producing an etch rate
of 100 angstroms/min. Some mirror materials may resist RIE and in
such cases a technique such as ion milling may be used. All RIE
steps are performed at a pressure of 30 mtorr unless otherwise
noted. All plasma etch steps are performed at a pressure of 100
mtorr unless otherwise noted. The photoresist is removed using
standard solvents.
[0162] Alternatively, the conductor/mirror may be defined using the
well-known procedure called lift-off. This procedure is used to
define a layer in a subsequent step and is described below.
[0163] Referring to FIG. 25B, support rail material 706, has been
deposited using one of the methods mentioned previously. This
material should be an insulator, for example silicon dioxide or
silicon nitride. The material should be deposited uniformly, and at
a thickness equal to thickness of the spacer layer, which will be
deposited later. This thickness should in general be at least a
multiple of the wavelength of light of interest. A thickness of 0.5
microns would place such a device near the middle of the visible
spectrum.
[0164] Referring to FIG. 25E, photoresist layer 708 is spun on and
hardbaked. Since this layer will not be photolithographically
defined, other polymeric materials may be used instead. The only
requirement is that they dissolve in solvents such as acetone or
methanol, and be able to withstand a vacuum. This is the first step
in defining a lift-off stencil.
[0165] Referring to FIG. 25F, template layer 710 has been deposited
using one of the methods of PVD. The layer is of silicon dioxide
though other materials are possible. Ideally the material should be
etched using a process which does not affect the underlying resist.
Buffered Oxide Etch (BOE) which consists of Hydrofluoric acid
diluted 7:1 with water can perform this step in 15 seconds. The
layer need only be a thousand angstroms thick.
[0166] In FIG. 25G, photoresist layer 712 has been spun-on and
patterned in a manner already discussed.
[0167] Referring to FIG. 25H, using a combination of BOE and RIE,
the pattern of resist layer 711 has been transferred to layers 710
and 708. In the first step, the BOE is used to etch through the
silicon dioxide layer 710. An oxygen plasma is used to etch through
resist layer 708, and to remove resist layer 711. Plasma etching
differs from RIE in that it tends to be less anisotropic, yielding
profiles that are not purely vertical.
[0168] Referring to FIG. 25I, using an oxygen plasma, resist layer
708 has been slightly underetched in order to facilitate removal of
the lift-off stencil. RIE using a carbon tetrafluoride chemistry
(CF4/02 6:4) is then applied to etching through layer 706.
[0169] Referring to FIG. 25J, spacer layer 712 is deposited using
PVD techniques. This material can be any number of different
compounds which can be deposited using this technique. There are
two key requirements for such a material. The first is that the
material be soluble in water or chemically removed by a liquid
etchant other than solvents such as acetone or methanol which will
be required to remove the lift-off stencil. An example of such an
etchant would be water and appropriate materials include lithium
fluoride, aluminum fluoride, and sodium chloride. The second is
that it be deposited with extreme uniformity and thickness
control.
[0170] The former allows resulting structures to be underetched
without damage by using water as the final etchant. Water is an
extremely benign solvent, and makes possible the incorporation of
many different mirror, conductor, and structural materials in the
final device.
[0171] The latter insures that subsequent layers conform to the
variations of the substrate and therefore primary conductor/mirror.
This parallelism is essential for the optical behavior of the
resonant cavity devices.
[0172] The spacer may also be composed of a polymeric material such
as hardbaked photoresist or polyimide. To achieve the required
thickness uniformity, a technique other than spinning must be used
to deposit the polymer. Two such techniques include extrusion and
capillary coating. The consequence of using such a spacer is that
all subsequent process steps must be low temperature in nature to
prevent outgassing and shrinkage of this layer. In this case, the
spacer is ultimately removed using an oxygen plasma.
[0173] Alternatively there exist some organic materials which can
be deposited using physical vapor deposition via sublimation or
evaporation in a vacuum, diamine is one example of such a material.
In this case the required uniformity could be obtained and the
spacer layer again removed using an oxygen based plasma. The use of
different plasma chemistries would also permit etching of inorganic
spacers as well. Chlorinated plasmas for example could be used to
remove an aluminum spacer.
[0174] The stencil is subsequently removed using an ultrasonic
acetone bath and methanol rinse or other polymer dissolving
solvent. This also removes or lifts off excess deposited spacer
material and is what constitutes the final step of the lift-off
process. An oxygen plasma may also be used to accomplish this step
though the material which has been lifted off must be removed. This
can be done using a high pressure gas jet.
[0175] Alternatively, by using negative photoresist and an
oppositely polarized mask, a natural overhang may be produced via
overexposure. The same may be accomplished with positive
photoresist using a technique known as image-reversal. This would
preclude the need to put down a sacrificial photoresist layer and a
subsequent SiO2 layer.
[0176] Referring to FIG. 25K, secondary conductor/mirror layer(s)
and support membrane (714) are deposited. The nature of the
conductor/mirror is dependent on the application. The support
membrane must have a tensile residual stress. Tensile stress is
required for two reasons. First so that the resulting membranes
will be flat and therefore parallel to the substrate in the
quiescent state. Secondly, such structures have a tendency to stick
when the membranes come in contact with the substrate. Sufficient
tensile stress is required pull the membrane away when the drive
voltage is reduced.
[0177] The membrane must have the appropriate optical
characteristics as well. For visible light this would mean
transparency in the visible region. Silicon nitride is one
candidate for this role for it can be deposited using plasma
enhanced chemical vapor deposition (PECVD) with controlled stress.
Other candidates include titanium dioxide, magnesium fluoride and
calcium fluoride, all of which can be deposited using e-beam
evaporation with a resulting tensile stress.
[0178] In FIG. 25L, photoresist layer 716 has been spun-on and
patterned in a manner discussed above.
[0179] In FIG. 25M, using RIE or ion milling, layer(s) 714 have
been etched according to the pattern of resist layer 716.
[0180] Referring to FIG. 25N, the final etch is accomplished by
placing the device in water for a period of time. The water is
agitated, and when the devices are fully etched they are dried.
[0181] One variation on this step involves the use of t-butyl
alcohol to displace the water when the etch is finished. The
devices are then placed in a chamber at approximately 32 degrees
centigrade to cause the alcohol to freeze. After this step the
devices are placed into a vacuum chamber where the air is then
evacuated. This causes the alcohol to sublime and can reduce
membrane sticking during the drying phase.
[0182] Another alternative process has initial steps of assembly
shown in FIGS. 26A through 26C, analogous to those shown in FIGS.
25A through 25C.
[0183] Thereafter, in FIG. 26D, photoresist or polymer layer 806 is
spun on and a stencil layer, 208, of silicon dioxide is deposited
using PVD. This layer must be thicker than the spacer layer to be
deposited.
[0184] In FIG. 26E, resist layer 810 has been spun-on and patterned
using standard procedures.
[0185] In FIG. 26F, this step uses BOE and an oxygen plasma etch to
define a lift-off stencil.
[0186] In FIG. 26G, the spacer material is chosen and deposited as
described in FIG. 25J.
[0187] In FIG. 26H, the stencil is subsequently removed using an
ultrasonic acetone bath and methanol rinse.
[0188] The step shown in FIG. 26I is analogous to that shown in
FIG. 25K.
[0189] In FIG. 26J, photoresist layer 814 has been spun-on and
patterned.
[0190] In FIG. 26K, using RIE or ion milling, layer(s) 812 have
been etched according to the pattern of resist layer 214.
[0191] The final etch is accomplished in a manner described
above.
[0192] It should be noted that a greater variety of spacers and
etchants could be accommodated if the mirrors were protected by an
encapsulating material. One candidate would be silicon nitride
deposited by PECVD though others are possible. However this would
have the negative effect of complicating the process and the
structure, while having a potentially negative impact on the
optical performance of the device.
[0193] All of the materials used for the mirrors must be deposited
in such a way that their stress can be controlled. Ideally, they
are deposited as "stress balanced" which means that the overall
stress of the film or film stack is zero. Since the ultimate goal
is that the support membrane conductor/mirror combination have an
overall tensile stress, conductor/mirror films with compressive
stress may be accommodated by having a high stress support
membrane. The technique of ion assisted deposition (IAD) precludes
the need for such accommodation. Using this technique, the residual
stress of almost any film of interest may be controlled by
bombarding the substrate with an independently controlled stream of
neutral ions during the deposition process. This make possible the
total mechanical decoupling of the support material from the
optical material. As a result, emphasis can be placed on depositing
an optically neutral support membrane with ideal mechanical
characteristics. In the same manner, a "mechanically neutral" (i.e.
stressless) conductor/mirror can be deposited with ideal optical
characteristic.
[0194] Other techniques including reactive sputtering, ion-plating,
and PECVD may also be used to obtain stress control but are not as
flexible or accommodating as IAD. Use of IAD or one of these
techniques would allow for the total elimination of the support
membrane. In this case the mechanical properties of the
conductor/mirror could be tailored, without compromising the
optical properties, so that the support membrane would become
superfluous.
[0195] Referring to FIG. 27A, the simplest conductor/mirror
configuration for an individual cavity is formed from a layer 900
that is either the substrate for the primary conductor/mirror, or
the support membrane if this is the secondary. Layer 902 is a
metallic film with a thickness on the order of several hundred
angstroms. The film can be of aluminum, silver, or any number of
metals, based on the spectral, and resistive properties as well as
the ease with which the metal can be etched.
[0196] The use of a metal as both mirror and conductor simplifies
fabrication. Unfortunately the spectral characteristics of the
metallic layer cannot be tailored, and the performance limits
devices to very specific kinds of behavior. Layer 904 is an
insulator and/or reflection enhancement film. This can be formed by
oxidizing the metal, if aluminum is being used, in an oxygen plasma
thus forming a thin layer of aluminum oxide. Alternatively,
insulating layers or reflection enhancement layers may be deposited
in a manner discussed before. Metallic mirrors must be somewhat
transmissive and therefore no more than several hundred angstroms
thick. Insulator films can have thicknesses from one hundred to
several thousand angstroms. Their thickness is determined by the
kind of voltages expected in driving the devices.
[0197] Referring to FIG. 27B, a more elaborate configuration is
shown. This is a compound conductor/mirror, with layer 900 as the
substrate or the support membrane. The conductor 906 is either a
transparent film such as indium tin oxide (ITO), or a very thin
metallic layer such as gold. Either can be deposited using suitable
film deposition methods. Thicknesses for the ITO should be in the
range of several thousand angstroms, and metallic conductors less
than 100 angstroms. 908 is a multilayer dielectric stack comprising
the mirror. Such a mirror consists of alternating dielectric films
with differing indexes of refraction deposited by a suitable PVD
process. By choosing films with appropriate thicknesses and
indexes, mirrors with tailorable spectral characteristics can be
fabricated as is well known in the art. In general, the thickness
of the individual layers is one quarter the wavelength of the light
of interest.
[0198] Alternatively, these mirrors may be deposited using a
technique known as codeposition. In this case, PVD is used to
deposit two materials with different refractive indices
simultaneously. Using computer control the refractive index of the
resulting film can be varied continuously between those of either
film. This deposition technique makes possible mirrors with
virtually any spectral characteristic.
[0199] The ability to design the characteristics of this mirror
allow for devices with a greater variety of modes of operation.
Unfortunately, because the conductive layer is not perfectly
transparent, additional losses are incurred as a result.
[0200] Referring to FIGS. 27C and 27D, a dielectric mirror 908 is
deposited directly on substrate 900. Metallic conductor 902 and
insulator 904 are deposited and patterned such that they form a
border around the periphery of the mirror. Using this
configuration, it is possible to provide drive voltages to the
devices without compromising throughput since the conductor can be
placed outside the active area of the device. FIG. 27D shows a
planar view of this mirror configuration.
[0201] Response times of this device may suffer as a result of
decreased conductor area.
[0202] Referring to FIG. 28a, a linear tunable filter is shown
which has been fabricated using the process sequence defined above.
The major difference is the nature of the mask used to define the
self-supporting membrane, which is comprised of support 1006 and
1008. The substrate, 1000, is transparent in the frequency region
of interest, and electrodes 1004 are used to drive the device.
Dielectric mirror 1002 are defined separately to produce a
configuration like that of FIGS. 27c, 27d. Three filters are shown
though many more can be fabricated. Each filter 1010, 1012, and
1014 is driven independently so that individual frequencies may be
separated from an incident light beam. Such a device can find use
in spectroscopic analysis, as a demultiplexor in a wavelength
division multiplexed fiber optic communication system, a color
printer, or any other application where independent frequency
selection is required. FIG. 28b is a top view of the structure.
[0203] Referring to FIGS. 29a and 29b, a device known as a
deformable mirror includes a self-supporting membrane 1102
fabricated on a substrate 1100. When a potential is applied between
actuator electrodes 1104 and conducting mirror 1106, the surface of
the mirror can be deformed in a controllable manner. Such a device
can be used as a component in an adaptive optics system, or in any
situation where control of an incident wavefront is required.
[0204] Other embodiments are within the scope of the following
claims.
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