U.S. patent application number 09/918249 was filed with the patent office on 2002-01-03 for micromechanical displays and fabrication method.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Colgan, Evan G., Kosbar, Laura L., Rosenbluth, Alan E..
Application Number | 20020000959 09/918249 |
Document ID | / |
Family ID | 22611556 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000959 |
Kind Code |
A1 |
Colgan, Evan G. ; et
al. |
January 3, 2002 |
Micromechanical displays and fabrication method
Abstract
A display device, in accordance with the present invention
includes a transparent substrate and an array of pixels formed on
the substrate, each pixel comprises a transparent electrode and a
deformable member electrically actuated between a first state and a
second state, wherein in the first state a liquid including a dye
is disposed in a gap between the transparent electrode and the
deformable member and wherein in the second state the deformable
member reduces the gap between the transparent electrode and the
deformable member such that the liquid is substantially removed
between the deformable layer and the transparent electrode in the
area of contact. A plurality of switches are formed on the
substrate for supplying control signals to the array of pixels to
selectively actuate the deformable members of the pixels, wherein
each switch comprises an actuating member movable between an active
state and an inactive state, whereby in the active state any
control signal supplied to the switch passes through the switch,
and in the inactive state any control signal supplied to the switch
is prevented from passing through the switch. Fabrication methods
are also disclosed.
Inventors: |
Colgan, Evan G.; (Chestnut
Ridge, NY) ; Kosbar, Laura L.; (Mohegan Lake, NY)
; Rosenbluth, Alan E.; (Yorktown Heights, NY) |
Correspondence
Address: |
Frank Chau
F. CHAU & ASSOCIATES, LLP
1900 Hempstead Turnpike, Suite 501
East Meadow
NY
11554
US
|
Assignee: |
International Business Machines
Corporation
|
Family ID: |
22611556 |
Appl. No.: |
09/918249 |
Filed: |
July 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09918249 |
Jul 30, 2001 |
|
|
|
09168456 |
Oct 8, 1998 |
|
|
|
Current U.S.
Class: |
345/84 |
Current CPC
Class: |
G09G 2300/08 20130101;
Y10S 359/90 20130101; G09G 2300/0473 20130101; G02B 26/0841
20130101; G09G 3/346 20130101; G09G 3/3433 20130101 |
Class at
Publication: |
345/84 |
International
Class: |
G09G 003/34 |
Claims
What is claimed is:
1. A display device comprising: a transparent substrate; an array
of pixels formed on the substrate, each pixel comprises a
transparent electrode and a deformable member electrically actuated
between a first state and a second state, wherein in the first
state a liquid including a dye is disposed in a gap between the
transparent electrode and the deformable member and wherein in the
second state the deformable member reduces the gap between the
transparent electrode and the deformable member such that the
liquid is substantially removed between the deformable layer and
the transparent electrode; and a plurality of switches formed on
the substrate for supplying control signals to the array of pixels
to selectively actuate the deformable members of the pixels,
wherein each switch comprises an actuating member movable between
an active state and an inactive state, whereby in the active state
any control signal supplied to the switch passes through the
switch, and in the inactive state any control signal supplied to
the switch is prevented from passing through the switch.
2. The display device as recited in claim 1, wherein the deformable
member includes a reflective surface which reduces the gap
displacing a portion of the liquid when the deformable member is in
the second state.
3. The display device as recited in claim 2, wherein the dye is
black such that light is reflected from the deformable member, when
the deformable member is in the second state and light is absorbed
in the gap when the deformable member is in the first state.
4. The display device as recited in claim 3, wherein the dye
includes Sudan Black.
5. The display device as recited in claim 3, wherein the dye
includes Naphthol Blue Black.
6. The display device as recited in claim 1, wherein the deformable
member includes a light absorbent surface which contacts an
insulation layer over the transparent electrode in the second
state.
7. The display device as recited in claim 6, wherein the dye is
white such that light is reflected from the gap when the deformable
member is in the second state and light is absorbed on the
deformable member when the deformable member is in the first
state.
8. The display device as recited in claim 1, wherein the deformable
member is bistable having a hysteresis such that only the first and
second states are permitted.
9. The display device as recited in claim 1, wherein the switches
include microelectromechanical switches.
10. The display device as recited in claim 1, wherein the
deformable member is actuated on hinges integrally formed with the
deformable member.
11. The display device as recited in claim 1, further comprises an
active area including the liquid therein and a first seal region
for maintaining the liquid in the active area.
12. The display device as recited in claim 11, further comprises a
second seal region for maintaining an inert gas therein between the
first seal region and the second seal region such that the
plurality of switches exist in the inert gas.
13. The display device as recited in claim 11, wherein the
transparent electrode forms a data line for controlling the pixels
and the deformable member forms a gate line for controlling the
pixels such that voltage differences provided by control signals
between the gate line and the data line provide a force for
actuating the deformable member; and the plurality of switches
formed on the substrate for supplying the control signals on gate
lines and data lines to the array of pixels selectively actuate the
deformable members of the pixels.
14. The display device as recited in claim 13, further comprises a
shift register for addressing the gate lines, the shift register
including a portion of the plurality of switches and another
portion of the plurality of switches for demultiplexing the data
lines.
15. A display device comprising: a substrate; an array of pixels
formed on the substrate, each pixel comprises an electrically
actuated switch, a storage capacitor, a transparent substrate and a
deformable member electrically actuated between a plurality of
states, wherein in each of the states a liquid including a dye is
disposed in a gap between a transparent substrate and the
deformable member in an active area and wherein the gap is
adjustable according to voltages applied to the deformable member
and the storage capacitor thereby reflecting light from the active
area according to different intensities.
16. The display device as recited in claim 15, wherein the
deformable member includes a reflective surface for reflecting
light through the transparent substrate and wherein the dye is
black.
17. The display device as recited in claim 15, wherein the
deformable member includes a light absorbent surface for absorbing
light through the transparent substrate and wherein the dye is
white.
18. The display device as recited in claim 15, wherein the
deformable member is actuated on hinges integrally formed with the
deformable member.
19. The display device as recited in claim 15, further comprises an
active device area including the liquid therein and a first seal
region for maintaining the liquid in the active area.
20. The display device as recited in claim 19, further comprises a
second seal region for maintaining an inert gas therein between the
first seal region and the second seal region such that the
plurality of switches exist in the inert gas.
21. The display device as recited in claim 15, further comprises a
shift register for addressing gate lines which are coupled to each
pixel.
22. The display device as recited in claim 15, wherein the switches
include microelectromechanical switches.
23. A method for fabricating a display device comprising the steps
of: patterning a black matrix layer on a transparent substrate;
depositing a first insulation layer on the patterned black matrix
layer; patterning a transparent conductor layer on the first
insulation layer; depositing a second insulation layer on the
transparent conductor layer; depositing a sacrificial layer on the
second insulation layer for forming a gap of a predetermined
distance between the second insulation layer and deformable
members; forming openings in the sacrificial layer for providing
support points for the deformable members; patterning a metal layer
to form the deformable members; and removing the sacrificial layer
to provide the gap.
24. The method as recited in claim 23, further comprising the step
of: filling the gap with a liquid including a dye such that in a
first state of the deformable member the liquid is disposed in the
gap between the transparent electrode and the deformable member and
wherein in a second state the deformable member reduces the gap
between the transparent electrode and the deformable member such
that the liquid is substantially removed between the deformable
member and the transparent electrode.
25. The method as recited in claim 23, wherein the sacrificial
layer includes copper and the step of removing the sacrificial
layer includes the step of removing the sacrificial layer by a wet
etch process.
26. The method as recited in claim 23, wherein the deformable
members include deformable mirrors.
27. A method for fabricating a deformable display device comprising
the steps of: patterning a transparent conductor layer on a
transparent substrate; forming an insulation layer over the
transparent conductor layer; patterning a conductive black matrix
layer on the insulation layer and outside the active area, the
black matrix layer forming a drain electrode for switches;
providing a source electrode and a gate electrode for switches by
patterning one of the black matrix layer and the transparent
conductor layer outside the active area; patterning a sacrificial
layer for forming features in the sacrificial layer for providing
support points for the deformable members and connections through
the sacrificial layer; and patterning a metal layer on the
sacrificial layer to form the deformable members and support points
for the deformable members, the deformable members including
deformable display members in the active area and switches outside
the active area; and removing the sacrificial layer to provide a
predetermined gap between the insulation over the transparent
conductor and the deformable display members and to provide
cantilevered conductors for the switches, the cantilevered
conductors attaching to the source electrode and including a tip
feature for contacting the drain electrode when the gate electrode
is activated.
28. The method as recited in claim 27 wherein the step of
patterning a sacrificial layer includes the steps of forming a via
hole through the sacrificial layer to the source electrode; and
forming a tip feature hole over the drain electrode such that upon
patterning the metal layer the cantilevered conductor is attached
to the source electrode and includes the tip feature for contacting
the drain electrode.
29. The method as recited in claim 27, wherein the sacrificial
layer includes a copper layer and further comprises the steps of:
forming dimples in the copper layer for forming the cantilevered
conductors for switches; and forming openings through the copper
layer to form vias through the sacrificial layer.
30. The method as recited in claim 27, wherein the deformable
display members include deformable mirrors.
31. The method as recited in claim 27, further comprising the step
of: filling the gap with a liquid including a dye such that in a
first state of the deformable display member the liquid is disposed
in the gap between the transparent electrode and the deformable
display member and wherein in a second state the deformable display
member reduces the gap between the transparent electrode and the
deformable display member such that the liquid is substantially
removed between the deformable display member and the transparent
electrode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to micro-mechanical devices
for reflective displays and, more particularly, to a reflective
display having deformable mirrors.
[0003] 2. Description of the Related Art
[0004] There are significant efforts underway to develop low power,
high resolution, "paper-like" displays using either liquid crystals
(T. Ogawa et al., "The Trends of reflective LCDs for future
electronic paper", SID '98 Digest, p. 217) or other technologies.
The liquid crystal based approaches generally suffer from low
reflectivity and poor contrast ratios. There has recently been a
number of publications on using MEM (microelectromechanical)
devices for display applications. Projection systems based on
arrays of tilting mirrors have been commercialized (See, e.g., J.
Sampsell, "An overview of the digital micromirror device (DMD) and
its application to projection displays", SID '93 Digest, p. 1012)
and projection systems proposed using a micromechanical phase
grating (See, e.g., D. Bloom "The grating light valve:
revolutionizing display technology", SPIE Vol. 3013 (1997) p.
165).
[0005] Two types of MEM based direct view displays have also been
proposed. In the first (See e.g., E. Stern, "Large-area
micromechanical flat-panel display", SID 97 Digest, p. 230), an
array of passively addressed bistable transparent beams are used to
control the release of light trapped by total internal reflection.
This device uses a back light, and due to the thick optical feed
structure (about 4 cm) is not be suitable for portable displays. A
second direct view display (See e.g., M. W. Miles, "A new
reflective FPD technology using interferrometric modulation", SID
97 Digest (1997) p. 71) includes the use of a micromachined
deformable optical cavity whose reflected color changes with
voltage. The device includes a self-supporting deformable membrane,
made of, for example aluminum, and a thin film stack, both residing
on a transparent substrate. The self-supporting deformable membrane
and the thin film stack act as mirrors for an optically resonant
cavity. When a voltage is applied, the deformable mirror collapses
and the color of the reflected light is changed. The devices are
binary and have hysteresis which allows passive addressing. The
color selection of the two states is determined by the optical
stack (which contains a conductor) and by the rest height of the
deformable mirror. The main disadvantage of such a system is that
the maximum reflectivity is limited. For a narrow color band, the
peak reflectance can be about 80%. If an 80% reflectivity is
assumed for the whole Red, Green, and Blue bands, a triad pixel
structure, and an 80% aperture ratio, the maximum white
reflectivity would be about 21%. For a paper-like display, a
reflectivity of 40% or more is necessary.
[0006] A type of display, referred to as "electroscopic displays",
have been described by T. S. Te Velde et al. which are bistable and
have an improved reflectivity compared to the interferrometric
modulation displays described above. (See Te Velde et al., "A
family of electroscopic displays", Society of Information Display
1980 technical digest, p.116-117 and the following U.S. Pat. Nos.
4,178,077, 4,519,676, 4,729,636.) The article entitled, "A family
of electroscopic displays".(hereinafter Velde), describes an
electroscopic fluid display where a plate or grid which is
reflective and is movable is sealed with a glass plate and filled
with a nonconducting black or other colored solvent. If the
penetration depth of the incident light in the solvent is much
smaller than the thickness of the cell, than when the white grid is
located near the bottom plate, the grid will not be visible and the
cell will appear black. However, when the white grid is attracted
to the front side, the white grid will be visible and the cell will
appear white.
[0007] Two possible configurations are described in Velde, a
springy capacitor and a triode. For the springy capacitor, the grid
is mechanically fastened to the bottom plate via conductive springs
and when a large enough voltage is applied, the springs are
stretched and the grid rushes to the upper electrode. This
arrangement requires careful cell gap control since the threshold
voltage is a function of the cell gap.
[0008] In the triode configuration, the springs are made very weak
so that mechanical forces can be neglected and electrodes on the
top and bottom plates are used to electrostatically control the
position of the reflective plate. In U.S. Pat. No. 4,178,077, a
triode configuration is described where electrostatic forces are
used to control a movable electrode in an opaque liquid. A
fabrication process is also described which uses an underetching
process where apertures in a second layer provide access for the
etchant to the first layer. This requires a timed etch to leave
portions of the first layer in place to support the second layer.
In U.S. Pat. No. 4,519,676, a triode configuration is again
described, but with the resilient elements below the display part
to increase the aperture ratio. A more complicated fabrication
process is described which again uses timed underetching. In U.S.
Pat. No. 4,729,636, engaging points are formed between the movable
electrode and its engaging surface to improve the response time by
allowing liquid flow in and out during closure and release. The
triode configuration is complicated and requires electrical
contacts for addressing to be formed on both top and bottom plates.
Both the triode and springy capacitor (when fastened to the bottom
plate) require precise cell gap control since the threshold voltage
depends on the cell gap. The fabrication processes described
require the etching step to be stopped by a certain time or the
first layer will be fully removed and the second layer will no
longer be attached to the substrate.
[0009] For a high information content display, such as one for an
8.5 inch by 11 inch sized display with 150 dot per inch resolution,
it is advantageous to integrate some of the addressing electronics
on the display itself to reduce cost and improve yield. For the
display size described above, approximately 1,275 gate line and
about 1,650 data line connections and driver chip outputs are
needed. If the display technology used can also provide switches,
the row selection circuits (i.e., shift register) and data driver
demultiplexing circuits may be made with the display and greatly
reduce the number of connections and drivers. (See "Silicon light
valve array chip for high resolution reflective liquid crystal
projection displays", by J. L. Sanford et al., IBM J. Res.
Develop., Vol. 42 No. 3/4, May/June 1998, pp.347-358, incorporated
herein by reference.)
[0010] Therefore, a need exists for a portable display having high
reflectivity and a high contrast ratio. A further need exists for a
display which permits switches to be fabricated at the same time as
the display device. A still further need exists for a method for
fabricating the display device and switches in an efficient and
economical manner.
SUMMARY OF THE INVENTION
[0011] A display device, in accordance with the present invention
includes a transparent substrate and an array of pixels formed on
the substrate, each pixel including a transparent electrode and a
deformable member electrically actuated between a first state and a
second state, wherein in the first state a liquid including a dye
is disposed in a gap between the transparent electrode and the
deformable member and wherein in the second state the deformable
member contacts the transparent electrode to define an area of
contact thereby closing the gap such that the liquid is
substantially removed between the deformable layer and the
transparent electrode in the area of contact. A plurality of
switches is formed on the substrate for supplying control signals
to the array of pixels to selectively actuate the deformable
members of the pixels, wherein each switch comprises an actuating
member movable between an active state and an inactive state,
whereby in the active state any control signal supplied to the
switch passes through the switch, and in the inactive state any
control signal supplied to the switch is prevented from passing
through the switch.
[0012] In alternate embodiments, the deformable member may include
a reflective surface which contacts an insulation layer over the
transparent electrode in the second state. The dye may be black
such that light is reflected from the area of contact when the
deformable member is in the second state and light is absorbed in
the gap when the deformable member is in the first state. The dye
may include Sudan Black or Naphthol Blue Black. The deformable
member may include a light absorbent surface which contacts the
transparent electrode in the second state. The dye may be white
such that light is reflected from the gap when the deformable
member is in the second state and light is absorbed in the area of
contact when the deformable member is in the first state. The
deformable member is bistable having a hysteresis such that only
the first and second states are permitted. Alternatively, the
deformable member may be adjustable between a plurality of states
thereby adjusting the gap to provide reflected light on a grey
scale (i.e., various intensities). The switches preferably include
microelectromechanical switches and are formed simultaneously with
the display elements.
[0013] In other embodiments, the deformable member is preferably
actuated on hinges integrally formed with the deformable member.
The device may include an active area which may include a first
seal region for maintaining the liquid in the active area. The
display device may further include a second seal region for
maintaining an inert gas therein between the first seal region and
the second seal region such that the plurality of switches exist in
the inert gas. The transparent electrode may form a data line for
controlling the pixels and the deformable member may form a gate
line for controlling the pixels such that voltage differences
provided by control signals between the gate line and the data line
provide a force for actuating the deformable member. The plurality
of switches may be formed on the substrate for supplying the
control signals on gate lines and data lines to the array of pixels
to selectively actuate the deformable members of the pixels. A
shift register may be included using a portion of the plurality of
switches, the shift register is employed for addressing the gate
lines. Switches may also be employed for demultiplexing the data
lines to reduce the number of data driver chips and electrical
connections needed.
[0014] Another display device, in accordance with the invention
includes a substrate and an array of pixels formed on the
substrate, each pixel including a transparent substrate and a
deformable member electrically actuated between a plurality of
states. In each of the states, a liquid including a dye is disposed
in a gap between the transparent substrate and the deformable
member in an active area and wherein the gap is adjustable
according to voltages applied to and stored by each pixel thereby
reflecting light from the active area according to a grey scale
(i.e., by varying the intensity of the reflected light).
[0015] In alternate embodiments, the deformable member may include
a reflective surface for reflecting light through the transparent
substrate and wherein the dye is black. The deformable member may
include a light absorbent surface for absorbing light through the
transparent substrate and wherein the dye is white. The deformable
member is preferably actuated on hinges integrally formed with the
deformable member. An active device area may be included, and a
first seal region may also be included for maintaining the liquid
in the active area. The display device may include a second seal
region for maintaining an inert gas therein between the first seal
region and the second seal region such that the plurality of
switches exist in the inert gas. A shift register may be included,
and a portion of a plurality of switches may be used to construct
the shift register. The shift register is preferably for addressing
gate lines which are used to activate switches in each pixel to
connect the data lines to storage capacitors in each pixel.
Switches may also be employed for demultiplexing the data lines to
reduce the number of data driver chips and electrical connections
needed. Data lines are independent of gate lines.
[0016] A method for fabricating a display device includes providing
a top plate, patterning a transparent electrode in an active region
on the top plate, forming an insulating layer on the transparent
electrode, patterning a low reflectivity conductive material to
form a source electrode, a gate electrode and a drain electrode on
the insulating layer outside the active area, patterning a
sacrificial layer, patterning a metal layer to form deformable
members having a gap between the metal layer and the transparent
electrode in the active area and switches outside the active area
such that upon activating the gate electrode an electrical
connection is made between the source electrode and the drain
electrode, removing the sacrificial layer and filling the gap with
a liquid including a dye such that in a first state of the
deformable member the liquid is disposed in the gap between the
transparent electrode and the deformable member and wherein in a
second state the deformable member contacts the insulating layer
over the transparent electrode to define an area of contact thereby
closing the gap such that the liquid is substantially removed
between the deformable layer and the insulating layer over the
transparent electrode in the area of contact.
[0017] In alternate methods, the step of patterning the sacrificial
layer preferably includes the steps of forming a via hole through
the sacrificial layer to the source electrode and forming a tip
feature hole over the drain electrode such that upon patterning the
metal layer a cantilevered conductor is attached to the source
electrode and includes a tip feature for contacting the drain
electrode. The step of forming the source electrode, the gate
electrode, the drain electrode and a black matrix concurrently from
a low reflectivity conductive material may also be included.
[0018] Another method for fabricating a display device includes the
steps of patterning a black matrix layer on a transparent
substrate, depositing a first insulation layer on the patterned
black matrix layer, patterning a transparent conductor layer on the
first insulation layer, depositing a second insulation layer on the
transparent conductor layer, depositing a sacrificial layer on the
second insulation layer for forming a gap of a predetermined
distance between the second insulation layer over the transparent
conductor layer and deformable members, forming openings in the
sacrificial layer for providing support points for deformable
members, patterning a metal layer to form deformable members and
removing the sacrificial layer to provide the gap.
[0019] In alternate methods, the step of filling the gap with a
liquid including a dye such that in a first state of the deformable
member the liquid is disposed in the gap between the transparent
electrode and the deformable member and wherein in a second state
the deformable member reduces the gap between the second insulation
layer over the transparent electrode and the deformable member such
that the liquid is substantially removed between the deformable
member and the second insulation layer over the transparent
electrode is preferably included. The sacrificial layer may include
copper and the step of removing the sacrificial layer may include
the step of removing the sacrificial layer by a wet etch process.
The deformable members include deformable mirrors.
[0020] Yet another method for fabricating a deformable display
device includes the steps of patterning a transparent conductor
layer in an active area of a transparent substrate, forming an
insulation layer over the transparent conductor layer, patterning a
conductive black matrix layer on the insulation layer outside the
active area, the black matrix layer used for forming a drain
electrode for switches, providing a source electrode and a gate
electrode for switches by patterning one of the black matrix layer
and the transparent conductor layer outside the active area,
patterning a sacrificial layer for forming features in the
sacrificial layer for providing support points for the deformable
member's connections through the sacrificial layer (including tip
features for the switches) and patterning a metal layer on the
sacrificial layer to form the deformable members and support points
for the deformable members, the deformable members including
deformable display members in the active area and switches outside
the active area removing the sacrificial layer to provide a
predetermined gap between the insulation layer over the transparent
conductor and the deformable display members and to provide
cantilevered conductors for the switches, the cantilevered
conductors attaching to the source electrode and including a tip
feature for contacting the drain electrode when the gate electrode
is activated.
[0021] In other methods, the step of patterning a sacrificial layer
may include the steps of forming a via hole through the sacrificial
layer to the source electrode and forming a tip feature hole over
the drain electrode such that upon patterning the metal layer the
cantilevered conductor is attached to the source electrode and
includes the tip feature for contacting the drain electrode. The
sacrificial layer may include a conductive top portion and a lower
insulating portion and may further include the steps of forming
dimples in the top portion and in a portion of the bottom portion
for forming the cantilevered conductors for switches and forming
openings through the top and bottom portions to form vias through
the sacrificial layer. The conductive top portion may include
copper and the lower insulating portion may include polyimide, the
method may further include the steps of removing the top portion
with a wet etching process and removing the lower portion by a
plasma etching process. The deformable display members preferably
include deformable mirrors. The method may also include the step of
filling the gap with a liquid including a dye such that in a first
state of the deformable display member the liquid is disposed in
the gap between the transparent electrode and the deformable
display member and wherein in a second state the deformable display
member reduces the gap between the insulation over the transparent
electrode and the deformable display member such that the liquid is
substantially removed between the deformable display member and the
transparent electrode.
[0022] These and other objects, features and advantages of the
present invention will become apparent from the following detailed
description of illustrative embodiments thereof, which is to be
read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0023] The invention will be described in detail in the following
description of preferred embodiments with reference to the
following figures wherein:
[0024] FIG. 1 is a cross-sectional view of an embodiment in
accordance with the present invention with a deformable mirror in a
relaxed state;
[0025] FIG. 2 is a top plan view of the embodiment of FIG. 1 in
accordance with the present invention with a deformable mirror in a
relaxed state;
[0026] FIG. 3 is a cross-sectional view of the embodiment of FIG. 1
in accordance with the present invention with a deformable mirror
in a collapsed state;
[0027] FIG. 4 is a top plan view of the embodiment of FIG. 1 in
accordance with the present invention with the deformable mirror in
the collapsed state.
[0028] FIGS. 5 and 6 are a top plan view and cross-sectional view,
respectively, showing a black matrix layer and an insulating layer
deposited on a top plate in accordance with the present
invention;
[0029] FIGS. 7 is a top plan view showing a transparent electrode
layer and an additional insulating layer deposited on the
insulating layer of FIG. 6 in accordance with the present
invention;
[0030] FIG. 8 is a cross-sectional view of a section taken along
section line 8-8 of FIG. 7.
[0031] FIGS. 9 and 10 are a top plan view and cross-sectional view,
respectively, showing a sacrificial layer deposited and patterned
on the device of FIG. 8 in accordance with the present
invention;
[0032] FIGS. 11 and 12 are a top plan view and cross-sectional
view, respectively, showing the sacrificial layer of FIGS. 9 and 10
removed and a metal layer used to form deformable mirrors and
switches deposited in accordance with the present invention;
[0033] FIG. 13 is a top plan view of four pixels in accordance with
the present invention;
[0034] FIG. 14 is a plot of mirror gap versus drive voltage in
accordance with the present invention;
[0035] FIGS. 15 and 16 are a top plan view and cross-sectional
view, respectively, showing a conductor deposited in accordance
with the present invention;
[0036] FIG. 17 is a cross-sectional view showing a sacrificial
layer deposited and patterned on the device of FIG. 16 in
accordance with the present invention;
[0037] FIGS. 18 and 19 are a top plan view and cross-sectional
view, respectively, showing the sacrificial layer of FIG. 17
removed and a metal layer used to form deformable mirrors and
switches deposited in accordance with the present invention;
[0038] FIG. 20 is a schematic diagram of an assembled display in
accordance with the present invention;
[0039] FIG. 21 is a partial cross-sectional view taken along
section line 21-21 of FIG. 20 in accordance with the present
invention;
[0040] FIG. 22 is a schematic diagram showing a shift register and
data multiplexing circuits constructed from MEM switches in
accordance with the present invention;
[0041] FIG. 23 is a top plan view of a display pixel having a black
matrix material deposited and patterned on a transparent substrate
and covered by a blanket insulation layer in accordance with the
present invention;
[0042] FIG. 24 is a top plan view of the display pixel of FIG. 23
having a gate metal deposited and patterned thereon in accordance
with the present invention;
[0043] FIG. 25 is a top plan view of the display pixel of FIG. 24
having a sacrificial layer deposited and patterned thereon, the
sacrificial layer shows dimples and hole for features to be formed
in later processing steps in accordance with the present
invention;
[0044] FIG. 26 is a cross-sectional view of the display pixel of
FIG. 25 taken at section line 26-26 of FIG. 25 in accordance the
present invention;
[0045] FIG. 27 is a top plan view of the display pixel of FIG. 25
having a data metal/deformable member metal deposited and patterned
thereon in accordance with the present invention;
[0046] FIG. 28 is a cross-sectional view of the display pixel of
FIG. 27 taken at section line 28-28 of FIG. 27 in accordance the
present invention;
[0047] FIG. 29 is a top plan view of the display pixel of FIG. 27
showing a switch and a deformable mirror formed after the
sacrificial layer etching in accordance with the present
invention;
[0048] FIG. 30 is a cross-sectional view of the display pixel of
FIG. 29 taken at section line 30-30 of FIG. 29 in accordance the
present invention; and
[0049] FIG. 31 is a cross-sectional view of the display pixel of
FIG. 29 taken at section line 31-31 of FIG. 29 in accordance the
present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0050] The present invention includes an electrically actuated self
supporting deformable mirror which is operated in a liquid which
includes a dye to form a reflective "paperlike-like" display. The
deformable mirror is made by surface micro-machining methods and
has hysteresis so that passive addressing can be used. The
deformable mirrors are formed on a top substrate of the display so
that a gap is advantageously determined by the thickness of a
sacrificial layer.
[0051] The invention also includes the operation of the
micro-machined deformable mirrors in the fluid containing the dye.
One advantage of using the dye is that a black and white display
with high reflectivity and good contrast ratio is formed where the
dye provides the black state when the deformable mirrors are in a
relaxed state. When the mirrors are collapsed against a transparent
substrate (displacing the black dye), a high reflectivity metal
such as Al or Ag provides a bright white state. A gap between the
deformable mirror and top glass of about 2-3 microns exists to
permit flexure of the mirror.
[0052] Assuming a reflectivity of about 90% and an aperture ratio
of about 80%, a brightness of about 72% is achievable.
[0053] Further, the fabrication process includes few masking steps
which permit reduced manufacturing costs. With an additional
masking step, the process is compatible with microelectromechanical
(MEM) switches. The following describe MEM switches and are
incorporated herein by reference: P. M. Zavracky, S. Majumder and
N. E. McGruer, "Micromechanical Switches Fabricated using Ni
surface micromachining", Journal of Microelectromechanical systems,
Vol. 6 No. 1 (1997) p.3; U.S. Pat. No. 4,674,180 to P. M. Zavracky
et al.; and U.S. Pat. No. 5,638,946 P. M. Zavracky. The MEM
switches described in the above documents may be used for some of
the addressing circuits such as a shift register for addressing
gate lines and data demultiplexing to reduce the number of required
contacts in these devices. This is desirable for an economically
feasible high information content display where individual
electrical contacts for each gate and data line to the associated
driver chips increase total display costs.
[0054] Referring to the figures in which like numerals represent
the same or similar elements and initially to FIGS. 1 and 2, a
display element 18 is shown in a relaxed state in accordance with
the present invention. In a preferred embodiment a gate line 12 is
provided using deformable mirrors 33 which may be electrically
interconnected to form gate lines 12. Further, a combination of
voltages may be applied to data lines 14, which may formed by a
continuous stripe of a transparent conductive electrode 30, such as
an indium-tin oxide electrode. Deformable mirror 33 may include at
least two positions. One position is a relaxed position (FIG. 1)
and another is a collapsed position (FIG. 3). In one embodiment,
the relaxed position corresponds to an applied voltage less than a
voltage (threshold voltage) needed to pull deformable mirror 33
down, i.e. to collapse it. With deformable mirror 33 in the relaxed
position, a gap 35 between mirror, 33, and an insulator layer 32 is
occupied by a black dye 34.
[0055] Black dye 34 has a high optical extinction coefficient, is a
good electrical insulator, has a high dielectric constant to lower
the threshold voltage for the mirrors and has a low viscosity.
Black dye 34 may include a single or combination of disazo dyes
dissolved in a solvent such as EGME (2-methoxyethanol), acetone,
toluene, etc. A preferred dye may be Sudan Black B (also known as
flat black BB or Solvent Black 3). Another possible disazo dye is
Naphthol Blue Black (also known as Acid Black 1 or Amido Black
10B).
[0056] Using Sudan Black B, with a gap thickness of about 3 um, the
reflected light in the dark state would be about 1.6%. For Naphthol
Blue Black, with a gap thickness of about 2 um, the reflected light
in the dark state would be about 1.2%. If the reflectivity of the
mirror is 80%, the contrast ratio of the active area of the pixels
would range from about 50 to about 67. Note that the display
operation does not require that absolutely all of the dye be
removed from between deformable mirror 33 and insulator layer 32
when the mirror is in the collapsed state. Using Sudan Black B, if
a gap of, for example, 100 nm remains in the collapsed state, the
reflected light would be about 87% of the brightness of reflected
light if the gap was completely empty of dye. This would degrade
the brightness and contrast ratio slightly but could be acceptable
for many applications.
[0057] As shown in FIG. 2, black dye 34 absorbs light which is
transmitted through a top glass 10, a transparent insulator 39 and
transparent electrode 30. Any remaining light is reflected by
deformable mirror 33 back through black dye 34. This results in
little or no light being reflected back to the viewer through the
top plate 10. Black dye 34 may be disposed in a reservoir 22
between deformable mirror 33 and a substrate 20. A black matrix
layer 31 is also included to absorb light in regions surrounding
transparent electrode 30.
[0058] Referring to FIGS. 3 and 4, element 18 is shown in a
collapsed state where the combination of the voltage applied to
gate line 12 and to data line 14 is above the threshold voltage
necessary to collapse mirror 33. When mirror 33 is collapsed, a
mirror portion 27 moves toward insulator 32 on integrally formed
hinges 26, black dye 34 is displaced from gap 35 and any light
transmitted through top plate 10, transparent electrode 30, and
insulator layers 32 and 39 is reflected by mirror 33. As shown in
FIG. 4, a pixel (mirror 33) appears bright when viewed through top
plate 10. If gaps 35 and reservoirs 22 do not provide adequate area
for rapid transport of black dye 34, i.e. dye displacement,
additional openings can be made in the deformable mirrors. The
displaced dye moves into reservoir 22 between deformable mirror 33
and substrate 20. Reservoir 22 provides a region for which dye is
displaced into as individual deformable mirrors are collapsed or
from which dye is provided as individual deformable mirrors return
to the relaxed position.
[0059] Referring to FIGS. 5-12, the processing steps for
fabricating the deformable mirrors of the present invention wherein
MEM switches are not included is shown. Referring to FIGS. 5 and 6,
a black matrix layer 31 is deposited on top plate 10. Top plate 10
may include a glass such as silicon based glass. Black matrix layer
31 preferably includes a low reflectivity material, preferably
formed of chromium oxide (Cr.sub.xO.sub.y) and/or chromium (Cr), to
reduce the reflection of light from the areas outside of an active
pixel region. Black matrix layer 31 may be conductive. Black matrix
layer 31 may be patterned by standard lithography and wet etching
techniques and overcoated with a conformally deposited transparent
insulator layer 39 such as silicon oxide (SiO.sub.2) or Silicon
Nitride (SiN.sub.x). As shown in FIGS. 7 and 8, transparent
electrode 30 is formed by depositing a transparent conductive
layer, preferably indium-tin oxide (ITO), or another transparent
conductor, to form data lines 14. Transparent electrode 30 is
patterned by lithography and wet or dry etching and overcoated with
an insulator 32 which is also transparent.
[0060] As shown in FIGS. 9 and 10, the deposition of a sacrificial
layer 36 with a thickness equal to the desired thickness of gap 35
(FIG. 1) is performed. Sacrificial layer 36 may include a material
such as sputtered copper (Cu), or Cu plated into a conducting seed
layer. Sacrificial layer 36 may be textured on its surface which
contacts mirror 33 to provide diffuse reflectivity for the mirror
when formed in the subsequent steps. The thickness of sacrificial
layer 36 determines the distance of the gap, 35, between deformable
mirror 33 and insulator layer 32 on top plate 10 when the mirror is
in the relaxed position (See FIG. 1). Sacrificial layer 36 is
patterned as shown in FIG. 9 by coating the device with a
photoresist, exposing and developing the resist, and using a
suitable wet etch. A mixture of phosphoric acid, acetic acid,
nitric acid and water in the ratios of about 80%/5%/5%/10%,
respectively, for example, may be used to perform the wet etch.
Insulator layer 32 adjacent to sacrificial layer 36 is preferably
patterned at the same time as sacrificial layer 36 using wet or dry
etching after which the photoresist is removed. In this example,
transparent electrode 30 is used as an etch stop so that only
insulator 32 is patterned, and not insulator layer 39. Also, this
ensures deformable mirror 33 is not electrically connected to black
matrix layer 31 as will be apparent from FIG. 12.
[0061] Referring to FIGS. 11 and 12, a photoresist layer is
deposited and patterned over sacrificial layer 36. Deformable
mirrors 33 are formed by plating in exposed conductor pattern areas
created by the photoresist to form a metal layer 37. The
photoresist is spun on and patterned to define the mask for plating
mirrors 33, hinges 26, and other areas where the metal used for the
deformable mirrors is desired. The patterned photoresist is baked
at about 150.degree. C. to improve its chemical resistance during
the plating step(s). Preferably, a preclean is performed in about
10% aqueous hydrochloric acid prior to plating. The metal layer may
include nickel (Ni) deposited to the desired thickness by
electroplating from a commercial electroplating solution containing
Ni. One alternative is to plate an initial layer of silver (Ag)
prior to the Ni plating. Other metals are contemplated for the
metal layer, for example aluminum (Al). After the metal layer is
plated, the photoresist layer is stripped and a selective wet etch
is used to remove sacrificial layer 36 but not etch the metal
layer. It is to be understood that the present invention does not
employ timed etching processes. Overetching will not damage the
structure or render it nonfunctional as described above in the
prior art.
[0062] If copper (Cu) is used for sacrificial layer 36 and Ni for
the metal layer, sacrificial layer 36 may be etched in a mixture of
approximately 40 parts water, 1 part hydrogen peroxide, and 8 parts
ammonium hydroxide without damage to the Ni, for example. If a Ag
layer has not already been added and higher reflectivity mirror is
desired, the Ni metal layer can be electroplated with Ag after
sacrificial layer 36 is removed. The pattern in which the metal
layer is plated produces deformable mirror 33 with hinges 26 on at
least two sides.
[0063] Referring to FIG. 13, deformable mirrors 33 for each pixel
are electrically interconnected to the adjoining deformable mirrors
33 in the direction of arrow "A" through hinges 26 to form
individual gate lines 12. Deformable mirrors 33 are electrically
isolated from transparent electrodes 30 (FIG. 12). Segments of
transparent electrodes 30 in each pixel are electrically
interconnected to the adjoining segments of transparent electrodes
30 in the vertical direction to form the individual data lines 14.
Thus, an array of pixels 50 is formed on top plate 10. Array of
pixels 50 is electrically connected by gate lines 12 and data lines
14 to which driver chips can be attached at the edges (at the end
of the array). Data lines 14 and mirror metal gate lines 12 are
extended beyond the active display area and past a glue seal region
at the periphery of the array to electrically connect with bond
pads in a tab area (formed either of ITO, gate metal, black matrix
material, or a combination of these conductive layers) where driver
chips can subsequently be attached by the use of anisotropic
conductive film (ACF) or other techniques.
[0064] Referring to FIG. 14, a display device having array of
pixels 50 is addressed according to the present invention by
sequentially selecting each of the gate lines and using the data
lines to address each 6f the pixels on the selected gate line. The
needed addressing voltages can best be understood with reference to
FIG. 14 which shows a schematic of mirror gap 35 versus the data to
gate voltage. In a preferred embodiment, the threshold voltage for
collapse of the mirror "Vm(cp)" is about 19V and the threshold
voltage for release "Vm(rl)" of a collapsed mirror is about 1 V.
The actual magnitude of the threshold voltage for collapsing the
deformable mirror and the mirror gap value is illustratively shown
in FIG. 14. The selected gate line is held at "Vg(on)" and the gate
lines not being addressed are held at "Vg(hold)".The data voltages
are "Vd(on)" for a white pixel (collapse position of deformable
mirror) and "Vd(off)" for a black pixel (relaxed position of
deformable mirror). The combination of the Vg(on) and Vg(hold) and
the Vd(on) and Vd(off) are selected so that:
Vg(on)+Vd(on)>Vm(cp)
Vg(on)+Vd(off)<Vm(cp)
Vm(rl)<Vg(hold)+Vd(on or off)<Vm(cp)
[0065] Appropriate drive voltage values for the above case may be,
for example: Vg(on)=15V, Vg(hold)=5V, Vd(on)=10V, and Vd(off)=0V.
Prior to selecting a line and writing the data to it, it is
necessary to release any collapsed mirrors. This may be
accomplished by applying "Vg(clear)" to the next line to be
addressed just prior to selecting it where:
Vg(clear)+Vd(on or off)<Vm(rl)
[0066] or, alternatively, when selecting a line, prior to
application of Vg(on), Vg(clear) can be applied while setting the
data voltages to Vd(clear) such that:
Vg(clear)+Vd(clear)<Vm(rl)
[0067] Since display elements 18 are bistable (i.e. have hysteresis
as shown in FIG. 14), advantageously, there is no degradation of
contrast ratio as the number of lines is increased as is found for
passive matrix liquid crystal displays. In other embodiments,
mirror gap is varied proportionally with the data-gate voltage to
provide a non-hysteresis mode wherein light may be reflected
according to a grey scale (varying intensities of light), i.e.,
proportionally with the gap.
[0068] Referring now to FIGS. 15-19, the processing steps to
fabricate an alternate embodiment of the present invention are
described. Microelectromechanical (MEM) switches are provided
outside an array of pixels to reduce the number of driver chips and
electrical contacts needed. FIGS. 15-19 show the fabrication of an
MEM switch only. The processing for the pixels is the same as shown
and described above except that transparent electrode 30 is
patterned first, an insulator 32 is deposited, and when black
matrix layer 31 is patterned, black matrix layer 31 is segmented
and used to provide redundancy for the gate lines. This change
ensures that there is no insulator over black matrix layer 31 so
that black matrix layer 31 can be used for a drain contact pad.
Also, a lithography step is used to define a tip feature region of
the MEM switches. Contacts to the data lines are formed using the
mirror metal. The MEM switches are fabricated concurrently with the
deformable mirrors in accordance with the present invention.
[0069] Referring to FIGS. 15 and 16, the fabrication of an MEM
switch 100 begins by patterning a conductive layer 131 to form a
source 104, gate 106, and drain 108 of the switch on an insulator
132 and a top plate 110. Source 104, gate 106 and drain 108 may be
patterned from different layers. Drain 108 is preferably patterned
from the black matrix but source 104 and gate 106 may be patterned
either from the ITO or the conductive black matrix layer. If ITO is
used for source 104, the insulator layer over source 104 is removed
during processing so that deformable mirror 133 makes electrical
contact with source 104. The black matrix material or transparent
electrode material for source 104, gate 106, and drain 108 is
preferably deposited concurrently with the similar materials
included for the active areas, i.e., for processing the deformable
mirrors.
[0070] As shown in FIG. 17, a sacrificial layer 136 is deposited.
Two patterning steps are preferably used to first open a switch tip
feature 102 preferably to a depth of slightly greater than about
2/3 of sacrificial layer 136 thickness, and to second open
sacrificial layer 36 fully down to black matrix layer 131 to form a
source contact hole 103. The depth of switch tip feature 102 is
adequate to ensure that switch 100 operates in a non-hysteresis
mode.
[0071] Referring to FIGS. 18 and 19, a metal is deposited as
described above to form an actuating member 133. A voltage applied
between gate 106 and source 104 (to which actuating member 133 is
cantilevered from) of the MEM switch which exceeds the threshold
voltage actuates switch 100. Switch 100 closure shorts tip feature
101 of actuating member 133 to drain 108, thereby electrically
connecting source 104 and drain 108. When the applied voltage is
reduced below the threshold voltage, switch 100 opens up and source
104 and drain 108 are again electrically isolated. Further details
on these processing steps and the operation of MEM shunts can be
found in Zavracky et al.
[0072] An assembled display is shown schematically in FIGS. 20 and
21 for the embodiment with integrated MEM switches. A top plate 10
has deformable mirrors in a display or active area 141. Top plate
10 includes MEM switches 100 along one or more edges to form a
shift register to address the gate lines. Also included are
additional MEM switches to demultiplex data signals on the data
lines. Top plate 10 is attached to substrate 20 using a glue seal
region formed from materials such as epoxy. In the case of
integrated MEM switches, two separate regions may be formed between
top plate 10 and bottom plate 20. One region includes a dye glue
seal 142 which includes deformable mirrors 133 to be used in
conjunction with dye and another region including the MEM switches
includes dry nitrogen or another inert gas and is sealed by a gas
glue seal 143. This is advantageous as the switches operate faster
in gas than liquid due to the lower viscosity of gas. External to
region 143 is a tab region 144 where external drivers may be
attached to metal bond pads or other connectors.
[0073] Referring to FIG. 22, one embodiment of a display 150 in
accordance with the present invention is shown. Display 150
includes an array of pixels 154 including deformable mirrors 133.
Integrated MEM switches 152 form a shift register 156 to address
gate lines 12. Integrated MEM switches 152 may also form circuits
157 to demultiplex data signals on data lines 14. This reduces the
number of electrical contacts needed. Deformable mirrors 133 and
switches 152 are formed concurrently during device fabrication. The
display devices and switches used are both fabricated with the same
process steps and are both electrostatically actuated with a
mechanical restoring force, but the switches are constrained by the
tip feature to operate in a non-hysteresis mode whereas the display
elements are not constrained and hence are bistable.
[0074] Referring to FIGS. 23-31, in an alternative embodiment of a
reflective display, an active matrix is used and the deformable
member may be adjusted to a number of positions to vary the
intensity of the reflected light. A top plan view and their
respective cross-sections of a single pixel are shown in FIGS.
23-31. The processing steps and structure are similar to those
described above. In FIG. 23, a conductive black matrix (BM) 202 is
deposited and patterned on a transparent substrate 200. Transparent
substrate 200 includes a thin insulating layer 203 thereon (FIG.
26). Black matrix 202 is covered with a transparent insulator layer
204. In FIG. 24, a gate metal layer 206 is deposited and patterned
by conventional methods such as plating Ni on a seed layer in
patterned resist and subsequently removing the resist and seed
layer to form portion of a storage capacitor 207 and gate lines
209. In FIGS. 25 and 26, a sacrificial layer 208 is deposited and
patterned. Sacrificial layer 208 preferably includes two layers. A
bottom portion 221 may include 2/3 or more of the total thickness
of sacrificial layer 208 which is preferably polyimide (a
transparent polymer), and a top portion 223 may include 1/3 or less
of the total thickness of sacrificial layer 208 which is preferably
copper. The tip regions 210 are formed by patterning the copper
layer and part of the polyimide layer so that the tip depth is
about 2/3 of or greater than the total gap thickness. Via holes 212
are patterned through both the copper, polyimide layer and
insulation stopping at the gate metal or BM layers.
[0075] In FIGS. 27 and 28, conductive material for data lines 214
and deformable beams or hinges 216 are patterned by plating in
patterned resist, using a metal 215 such as Ni. Than sacrificial
layer 208 is removed by selective wet etching (top copper portion)
and the polyimide layer (bottom portion)is selectively removed by
plasma etching except in a region under a deformable mirror 218
where the polyimide remains to form a spacer 220 as shown in FIGS.
29 and 30. The plasma etching of polyimide can be directional,
depending on the process conditions used, and proceeds laterally
under the deformable metal features at a controlled rate. This
allows the polyimide to be removed from under narrow features such
as switches 222, shown in FIG. 31, or bending beams (hinges) 216
but not from under large features such as deformable mirror 218. As
an alternative, sacrificial layer 208 may include a bottom 1/3 or
less of copper and a top 2/3 or more of polyimide. In this case,
polyimide spacers 220 is attached to the bottom of deformable
mirror 218 and the polyimide is etched first and the copper second
where the tip features is only patterned in the polyimide
layer.
[0076] Deformable mirror 218 is constrained by the polyimide to a
non-hysteresis mode where the gap is controlled by the voltage
stored on storage capacitor 207. Storage capacitor 207 is formed
between the gate metal and the black matrix. Electrical contacts to
the black matrix are formed outside the array region using the via
pattern and the same metal as the data lines. As is usual for an
active matrix device, the voltage on the storage capacitor is
transferred from the data line when the gate line is selected and
switch 222 is closed connecting the data line to the storage
capacitor. When the gate line is not selected, the switch is open
and the voltage is maintained by the storage capacitor. The voltage
difference between the storage capacitor and the previous
(non-selected) gate line, to which bending beam 216 and deformable
mirror 218 are connected, controls the deflection of the bending
beam and hence the displacement of the deformable mirror. The gap
(gap 35 as shown in FIG. 1) between deformable mirror and the
polyimide spacer determines the thickness of dye which incident
light traverses before being reflected from the mirror and hence
the intensity of the reflected light.
[0077] Although described in terms of black dye and a reflective
mirror, the present invention is applicable to other types of
deformable mirror displays, for example, white dye and a black
(non-reflective) mirror. Also, a sub-frame time modulation could be
implemented to provide grey scale.
[0078] Having described preferred embodiments of a micromechanical
displays and method for fabrication of same (which are intended to
be illustrative and not limiting), it is noted that modifications
and variations can be made by persons skilled in the art in light
of the above teachings. It is therefore to be understood that
changes may be made in the particular embodiments of the invention
disclosed which are within the scope and spirit of the invention as
outlined by the appended claims. Having thus described the
invention with the details and particularity required by the patent
laws, what is claimed and desired protected by Letters Patent is
set forth in the appended claims.
* * * * *