U.S. patent application number 11/009506 was filed with the patent office on 2005-05-05 for semi-transmission type liquid crystal display which reflects incident light coming from outside to provide a display light source and transmits light from a light source at the back.
Invention is credited to Ikeno, Hidenori, Suzuki, Masayoshi.
Application Number | 20050094068 11/009506 |
Document ID | / |
Family ID | 19079794 |
Filed Date | 2005-05-05 |
United States Patent
Application |
20050094068 |
Kind Code |
A1 |
Ikeno, Hidenori ; et
al. |
May 5, 2005 |
Semi-transmission type liquid crystal display which reflects
incident light coming from outside to provide a display light
source and transmits light from a light source at the back
Abstract
A semi-transmission type liquid crystal display that maximizes
the luminance in reflection mode and transmission mode. The liquid
crystal display comprises a lower substrate with thin film
transistors, an opposite substrate facing the lower substrate, a
liquid crystal layer between the lower substrate and the opposite
substrate, a reflection electrode formed in a reflection area of
the lower substrate, a transparent electrode formed in a
transparent area of the lower substrate, a common electrode formed
on the opposite substrate, and a drive circuit for applying a
voltage between the reflection electrode and the transparent
electrode and the common electrode. The potential difference
between a drive voltage applied to that surface of the lower
substrate which contacts the liquid crystal layer and a drive
voltage applied to that surface of the opposite substrate which
contacts the liquid crystal layer is lower in the transparent area
than in the reflection area.
Inventors: |
Ikeno, Hidenori; (Tokyo,
JP) ; Suzuki, Masayoshi; (Tokyo, JP) |
Correspondence
Address: |
HAYES, SOLOWAY P.C.
130 W. CUSHING STREET
TUCSON
AZ
85701
US
|
Family ID: |
19079794 |
Appl. No.: |
11/009506 |
Filed: |
December 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11009506 |
Dec 10, 2004 |
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10216967 |
Aug 12, 2002 |
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6831715 |
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Current U.S.
Class: |
349/114 |
Current CPC
Class: |
G02F 1/136227 20130101;
G02F 1/133555 20130101 |
Class at
Publication: |
349/114 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 22, 2001 |
JP |
2001-251088 |
Claims
1. A liquid crystal display comprising: a lower substrate on which
interconnections and thin film transistors are formed; an opposite
substrate so arranged as to face said lower substrate; a liquid
crystal layer sandwiched between said lower substrate and said
opposite substrate; a reflection electrode formed in a reflection
area of said lower substrate; a transparent electrode formed in a
transparent area of said lower substrate; a common electrode formed
on said opposite substrate; and a drive circuit for applying a
voltage between said reflection electrode and said transparent
electrode and said common electrode, whereby a potential difference
between a drive voltage applied to that surface of said lower
substrate which contacts said liquid crystal layer and a drive
voltage applied to that surface of said opposite substrate which
contacts said liquid crystal layer is made lower in said reflection
area than in said transparent area.
2. The liquid crystal display according to claim 1, wherein said
potential difference between said drive voltage applied to that
surface of said lower substrate which contacts said liquid crystal
layer and said drive voltage applied to that surface of said
opposite substrate which contacts said liquid crystal layer is made
lower in said transparent area than in said reflection area by
capacitive division of an electrostatic capacity of said reflection
area.
3. A liquid crystal display comprising: a lower substrate on which
interconnections and thin film transistors are formed; an opposite
substrate so arranged as to face said lower substrate; a liquid
crystal layer sandwiched between said lower substrate and said
opposite substrate; a reflection electrode formed in a reflection
area of said lower substrate; a transparent electrode formed in a
transparent area of said lower substrate; a common electrode formed
on said opposite substrate; and a drive circuit for applying a
voltage between said reflection electrode and said transparent
electrode and said common electrode, wherein a potential difference
between a drive voltage applied to that surface of said lower
substrate which contacts said liquid crystal layer and a drive
voltage applied to that surface of said opposite substrate which
contacts said liquid crystal layer is made lower in said
transparent area than in said reflection area, and wherein a cell
gap which is a thickness of a liquid crystal layer in said
transparent area is substantially equal to a cell gap which is a
thickness of a liquid crystal layer in said reflection area.
4. The liquid crystal display according to claim 2, wherein a cell
gap which is a thickness of a liquid crystal layer in said
transparent area is substantially equal to a cell gap which is a
thickness of a liquid crystal layer in said reflection area.
5. A liquid crystal display comprising: a lower substrate on which
interconnections and thin film transistors are formed; an opposite
substrate so arranged as to face said lower substrate; a liquid
crystal layer sandwiched between said lower substrate and said
opposite substrate; a reflection electrode formed in a reflection
area of said lower substrate; a transparent electrode formed in a
transparent area of said lower substrate; a common electrode formed
on said opposite substrate; and a drive circuit for applying a
voltage between said reflection electrode and said transparent
electrode and said common electrode, wherein a potential difference
between a drive voltage applied to that surface of said lower
substrate which contacts said liquid crystal layer and a drive
voltage applied to that surface of said opposite substrate which
contacts said liquid crystal layer is made lower in said
transparent area than in said reflection area, and wherein an
insulating layer is deposited on said thin film transistors, said
reflection electrode and said transparent electrode are formed on
said insulating layer at predetermined regions, and said
transparent electrode is electrically connected to a source
electrode of each of said thin film transistors via a contact hole
formed in said insulating layer, and said opposite substrate is
connected to said transparent electrode via an insulating film, an
alignment film and said liquid crystal layer.
6-11. (canceled)
12. The liquid crystal display according to claim 5, wherein said
insulating film is formed of one selected from SiN, SiO.sub.2,
Ti.sub.2O.sub.3, Ta.sub.2O.sub.5, SiO, Al.sub.2O.sub.3, acryl and
arton.
13-19. (canceled)
20. The liquid crystal display according to claim 3, wherein said
potential difference between said drive voltage applied to that
surface of said lower substrate which contacts said liquid crystal
layer and said drive voltage applied to that surface of said
opposite substrate which contacts said liquid crystal layer is made
lower in said transparent area than in said reflection area by
capacitive division of an electrostatic capacity of said reflection
area.
21. The liquid crystal display according to claim 5, wherein said
potential difference between said drive voltage applied to that
surface of said lower substrate which contacts said liquid crystal
layer and said drive voltage applied to that surface of said
opposite substrate which contacts said liquid crystal layer is made
lower in said transparent area than in said reflection area by
capacitive division of an electrostatic capacity of said reflection
area.
Description
[0001] FIELD OF THE INVENTION
[0002] The present invention relates to a liquid crystal display,
and, more particularly, to a semi-transmission type liquid crystal
display which reflects incident light coming from outside to
provide a display light source and transmits light from a light
source at the back.
DESCRIPTION OF THE RELATED ART
[0003] There is a reflection type liquid crystal display (LCD)
known which has a reflector inside and reflects incident external
light by the reflector to provide a display light source, thereby
eliminating the need for a backlight as a light source and a
transmission type liquid crystal display equipped with a backlight
as a light source.
[0004] As the reflection type liquid crystal display can be
designed with lower power consumption, thinner and lighter than the
transmission type liquid crystal display, it is mainly used for a
portable terminal. This is because as light input from outside is
reflected at the reflector in the display, the light can be used as
a display light source, thus eliminating the need for a backlight.
The transmission type liquid crystal display has such a
characteristic as having a better visibility than the reflection
type liquid crystal display in case where ambient light is
dark.
[0005] The basic structure of the existing liquid crystal displays
comprises a liquid crystal of an TN (Twisted Nematic) type, a
single sheet polarizer type, an STN (Super Twisted Nematic) type, a
GH (Guest-Host) type, a PDLC (Polymer Dispersed Liquid Crystal)
type, a cholesteric type or the like, a switching element which
drives the liquid crystal and a reflector or backlight provided
inside or outside a liquid crystal cell. Those ordinary liquid
crystal displays employ an active matrix drive system which can
achieve high definition and high image quality using thin film
transistors (TFTs) or metal/insulating film/metal structure diodes
(MIMs) as switching elements, and are equipped with a reflector or
backlight.
[0006] As a liquid crystal display which has advantages of both the
conventional reflection type liquid crystal display and
transmission type liquid crystal display, a semi-transmission type
liquid crystal display is disclosed (see Japanese Patent No.
2955277) which, as shown in FIG. 1, has gate interconnections 2 and
source interconnections 3 so provided as to run around pixel
electrodes 1 of an active matrix substrate and intersect each other
perpendicularly, has thin film transistors 4 provided on the pixel
electrodes 1, has the gate interconnections 2 and source
interconnections 3 connected to the gate electrodes and source
electrodes of the thin film transistors 4 and has reflection areas
5 of a metal film and transparent areas 6 of ITO formed in the
pixel electrodes 1.
[0007] As the reflection areas and transparent areas are provided
in the pixel electrodes, the backlight can be turned off when the
ambient light is bright 50 that the liquid crystal display can be
used as a reflection type liquid crystal display, and thus
demonstrates lower power consumption that is the characteristic of
the reflection type liquid crystal display. When the ambient light
is dark, the backlight is turned on so that the liquid crystal
display is used as a transmission type liquid crystal display, and
thus demonstrates an improved visibility in case where ambient
light is dark, which is the characteristic of the transmission type
liquid crystal display. Hereunder, a liquid crystal display which
can be used as a reflection type liquid crystal display and as a
transmission type liquid crystal display will be called as a
semi-transmission type liquid crystal display.
[0008] According to the conventional semi-transmission type liquid
crystal display, however, incident light travels through the liquid
crystal layer back and forth in the reflection area 5 and passes
the liquid crystal layer in the transparent area 6, thus producing
a difference in light path in the liquid crystal layer. This
results in a retardation difference between both areas, which
disables the maximization of the intensity of the output light. To
solve the problem, the liquid crystal display described in Japanese
Patent No. 2955277 has an insulating layer 8 provided under an
transparent electrode 7 in the reflection area 5 and a reflector 9
arranged over or under the insulating layer 8, as illustrated in a
cross-sectional view of a liquid crystal display shown in FIG. 2,
thereby providing a difference between the thickness, dr, of the
liquid crystal layer in the reflection area 5 and the thickness,
df, of the liquid crystal layer in the transparent area 6.
[0009] FIG. 5 is a graph showing the results of computing the
intensity, Ip, of the output light in transmission mode and the
intensity, I.lambda., of the output light in reflection mode. It is
apparent that the intensities of the output light in transmission
mode and in reflection mode differ depending on the thickness of
the liquid crystal layer. The difference in light path between the
reflection area 5 and the transparent area 6 is canceled to
approximate the characteristic of the output light by setting the
ratio of the thickness dr of the liquid crystal layer in the
reflection area to the thickness dr of the liquid crystal layer in
the transparent area to about 1:2. Because the thickness of the
insulating layer 8 is about a half the thickness of the liquid
crystal layer and should be several micrometers, the number of the
fabrication processes is increased, thus impairing the
planarization of the transparent electrode 7. An alignment film
which is formed on the transparent electrode 7 in order to align
the liquid crystal molecules is affected by the planarization of
the transparent electrode 7. This brings about a problem of making
effective alignment difficult in a rubbing process.
[0010] Further, as shown in FIG. 3, a step between the reflection
area 5 and the transparent area 6 disturbs an electric line of
force 10 produced between a lover substrate 11 and an opposite
substrate 12, thus deteriorating the characteristics of the liquid
crystal display. Furthermore, as shown in FIG. 4, in a liquid
crystal layer 13 around the step portion between the reflection
area 5 and transparent area 6 on the lower substrate 11, the
relationship between the direction of alignment of the liquid
crystal molecules and the pretilt angle of the liquid crystal
molecules in the vicinity of the surface of the lower substrate 11
generates disturbance in the rotational direction of the liquid
crystal molecules (reverse tilt disclination) at the time the
liquid crystal display is operated, thus deteriorating the
characteristics of the liquid crystal display.
SUMMARY OF THE INVENTION
[0011] Accordingly, it is an object of the invention to provide a
semi-transmission type liquid crystal display which maximizes the
luminance in reflection mode as well as transmission mode, so that
the alignment of liquid crystal molecules is not disturbed even
around the boundary between the reflection area and the transparent
area.
[0012] A liquid crystal display according to the invention
comprises;
[0013] a lower substrate on which interconnections and thin film
transistors are formed;
[0014] an opposite substrate so arranged as to face the lower
substrate;
[0015] a liquid crystal layer sandwiched between the lower
substrate and the opposite substrate;
[0016] a reflection electrode formed in a reflection area of the
lower substrate;
[0017] a transparent electrode formed in a transparent area of the
lower substrate;
[0018] a common electrode formed on the opposite substrate; and
[0019] a drive circuit for applying a voltage between the
reflection electrode and the transparent electrode and the common
electrode,
[0020] whereby a potential difference between a drive voltage
applied to that surface of the lower substrate which contacts the
liquid crystal layer and a drive voltage applied to that surface of
the opposite substrate which contacts the liquid crystal layer is
made lower in the transparent area than in the reflection area.
[0021] As the drive voltage applied to the liquid crystal layer in
the transparent area is lower than the drive voltage applied to the
liquid crystal layer in the reflection area, the birefringence of
the liquid crystal layer in the reflection area becomes smaller
than the birefringence of the liquid crystal layer in the
transparent area, making it possible to ensure the optimal
birefringence in each of the reflection mode and transmission mode.
This can optimize the intensities of the output light in both
modes.
[0022] The liquid crystal display may be constructed in such a way
that the potential difference between the drive voltage applied to
that surface of the lower substrate which contacts the liquid
crystal layer and the drive voltage applied to that surface of the
opposite substrate which contacts the liquid crystal layer is made
lower in the transparent area than in the reflection area by
capacitive division of an electrostatic capacity of the reflection
area.
[0023] The capacitive division of the electrostatic capacity of the
reflection area produces a difference between the drive voltages of
the transparent area and reflection area, so that the transparent
area and reflection area can be simultaneously driven by the
different voltages using a voltage that is supplied by a single
thin film transistor. This makes it possible to prevent an increase
in the quantity of the thin film transistors and eliminate the
complexity of the drive voltage control, leading to a reduction in
the production cost of the liquid crystal display.
[0024] The liquid crystal display may be constructed in such a way
that a cell gap which is a thickness of a liquid crystal layer in
the transparent area is substantially equal to a cell gap which is
a thickness of a liquid crystal layer in the reflection area.
[0025] As the cell gaps in the transparent area and the reflection
area are substantially identical, it is possible to eliminate an
alignment disturbance produced by the disturbance of the electric
line of force in the liquid crystal layer or an alignment
disturbance, such as the reverse tilt disclination produced by the
disturbance of the pretilt angle. This can improve the
characteristics of the liquid crystal display.
[0026] The liquid crystal display may be constructed in such a way
that an insulating layer is deposited on the thin film transistors,
the reflection electrode and the transparent electrode are formed
on the insulating layer at predetermined regions, the transparent
electrode is electrically connected to a source electrode of each
of the thin film transistors via a contact hole formed in the
insulating layer, and the opposite substrate is connected to the
transparent electrode via an insulating film.
[0027] The connection of the reflection electrode to the
transparent electrode via the insulating film allows a capacitor to
be formed by the reflection electrode and transparent electrode and
a potential difference can be produced between the transparent area
and reflection area by capacitively dividing the capacitor formed
by the liquid crystal sandwiched between the transparent electrode
and the opposite electrode into a capacitor formed by the
transparent electrode-insulating film-reflection electrode and a
capacitor formed by the reflection electrode-liquid
crystal-opposite electrode.
[0028] The liquid crystal display may be constructed in such a way
that an insulating layer is deposited on the thin film transistors,
the reflection electrode and the transparent electrode are formed
on the insulating layer at predetermined regions, the transparent
electrode is electrically connected to a source electrode of each
of the thin film transistors via a contact hole formed in the
insulating layer, the opposite substrate is electrically connected
to the transparent electrode and an insulating film is deposited on
that surface of the opposite substrate which contacts the liquid
crystal layer.
[0029] As the insulating film is deposited on the reflection
electrodes a capacitor formed by the liquid crystal sandwiched
between the reflection electrode and the opposite electrode is
capacitatively divided into a capacitor formed by the insulating
film and a capacitor formed by the liquid crystal, thereby
providing a potential difference between the transparent area and
the reflection area.
[0030] The liquid crystal display may be constructed in such a way
that an insulating layer is deposited on the thin film transistors,
the reflection electrode and the transparent electrode are formed
on the insulating layer at predetermined regions, the transparent
electrode is electrically connected to a source electrode of each
of the thin film transistors via a contact hole formed in the
insulating layer, the opposite substrate is electrically connected
to the transparent electrode and an insulating film is deposited on
that region of the opposite substrate which faces the reflection
electrode.
[0031] As the insulating film is deposited on that region of the
opposite substrate which faces the reflection electrode, a
capacitor formed by the liquid crystal sandwiched between the
reflection electrode and the opposite electrode is capacitatively
divided into a capacitor formed by the liquid crystal and a
capacitor formed by the insulating film, thereby providing a
potential difference between the transparent area and the
reflection area.
[0032] The liquid crystal display may be constructed in such a way
that an insulating layer is deposited on the thin film transistors,
the reflection electrode and the transparent electrode are formed
on the insulating layer at predetermined regions, the transparent
electrode is electrically connected to a source electrode of each
of the thin film transistors via a contact hole formed in the
insulating layer, the opposite substrate is electrically connected
to the transparent electrode and an insulating film is deposited on
the reflection electrode and that region of the opposite substrate
which faces the reflection electrode.
[0033] As the insulating film is deposited on the reflection
electrode and that region of the opposite substrate which faces the
reflection electrode, a capacitor formed by the liquid crystal
sandwiched between the reflection electrode and the opposite
electrode is capacitatively divided into a capacitor formed by the
insulating film and a capacitor formed by the liquid crystal,
thereby providing a potential difference between the transparent
area and the reflection area.
[0034] The liquid crystal display may be constructed in such a way
that an insulating layer is deposited on the thin film transistors,
the reflection electrode and the transparent electrode are formed
on the insulating layer at predetermined regions, the transparent
electrode is electrically connected to a source electrode of each
of the thin film transistors via a contact hole formed in the
insulating layer, a second source electrode is connected to the
source electrode via an insulating film, and the reflection
electrode is electrically connected to the second source electrode
via a contact hole formed in the insulating layer.
[0035] As the second source electrode is connected to the source
electrode via the insulating film, a capacitor is formed by the
reflection electrode and the transparent electrode. By
capacitatively dividing a capacitor formed by the liquid crystal
sandwiched between the transparent electrode and the opposite
electrode into a capacitor formed by the transparent
electrode-insulating film-second source electrode and a capacitor
formed by the reflection electrode-liquid crystal-opposite
electrode, a potential difference can be provided between the
transparent area and the reflection area.
[0036] The liquid crystal display may be constructed in such a way
that an insulating layer is deposited on the thin film transistors,
the transparent electrode is formed on the insulating layer, an
insulating film is deposited on the transparent electrode, the
reflection electrode is formed the insulating film, the transparent
electrode is electrically connected to a source electrode of each
of the thin film transistors via a contact hole formed in the
insulating layer, and openings are formed in the reflection
electrode and the insulating film to the transparent electrode.
[0037] As the insulating film is formed on the transparent
electrode and the reflection electrode is formed on the insulating
film, a capacitor is formed by the reflection electrode and the
transparent electrode. By capacitatively dividing a capacitor
formed by the liquid crystal sandwiched between the transparent
electrode and the opposite electrode into a capacitor formed by the
transparent electrode-insulating film-reflection electrode and a
capacitor formed by the reflection electrode-liquid
crystal-opposite electrode, a potential difference can be provided
between the transparent area and the reflection area. Because the
reflection electrode and insulating film are eliminated at the
opening, the opening serves as the transparent area.
[0038] The liquid crystal display may be constructed in such a way
that undulations are formed on the insulating layer and the
openings are formed in top peripheral regions of the undulations
and/or bottom peripheral regions thereof.
[0039] It is difficult to efficiently reflect light input from the
opposite substrate toward a viewer in the top peripheral regions
and the bottom peripheral regions of the undulations. Therefore,
the openings are formed it the top peripheral regions and the
bottom peripheral regions as transparent areas, so that efficient
liquid crystal display can be ensured in reflection mode as well as
transmission mode.
[0040] The insulating film may be formed of one selected from SiN,
SiO.sub.2, Ti.sub.2O.sub.3, Ta.sub.2O.sub.5, SiO, Al.sub.2O.sub.5,
acryl and arton.
[0041] Because SiN, SiO.sub.2, Ti.sub.2O.sub.3, Ta.sub.2O.sub.5,
SiO, Al.sub.2O.sub.5, acryl and arton can be used as the material
for the insulating film, it is possible to select the optimal
insulating film in accordance with various conditions, such as the
usage, the product quality and the material for the liquid crystal.
This increases the degree of freedom in the design stage.
[0042] The liquid crystal display may be constructed in such a way
that a first color filter is formed on the opposite substrate, a
second color filter is formed on the thin film transistors and the
reflection electrode is formed on the second color filter.
[0043] As color filters are formed on the opposite substrate and
the device substrate, light passes the color filter on the opposite
substrate side twice in reflection mode and light passes the color
filters on the device substrate and the opposite substrate once
each in transmission mode. This can make it possible to reduce a
change in color in both modes. It is also possible to respectively
set the hues in transmission mode and reflection mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1 is a plan view of a conventional semi-transmission
type liquid crystal display;
[0045] FIG. 2 is a cross-sectional view of the conventional
semi-transmission type liquid crystal display;
[0046] FIG. 3 is a diagram showing the disturbance of the electric
line of force in the conventional semi-transmission type liquid
crystal display;
[0047] FIG. 4 is a diagram showing the disturbance of the
rotational direction of liquid crystal molecules in the
conventional semi-transmission type liquid crystal display;
[0048] FIG. 5 is a graph showing the relationship between the
thickness of the liquid crystal layer and the intensity of output
light in reflection mode and transmission mode;
[0049] FIG. 6 is a partial cross-sectional view of a
semi-transmission type liquid crystal display according to a first
embodiment;
[0050] FIG. 7 is a cross-sectional view showing a part of a lower
substrate of the first embodiment in a simplified form;
[0051] FIG. 8 is an equivalent circuit of what is illustrated in
FIGS. 6 and 7;
[0052] FIGS. 9A through 9F are diagrams showing a fabrication
process for the lower substrate of the first embodiment;
[0053] FIG. 10 is a cross-sectional view showing a part of a lower
substrate of a second embodiment in a simplified form;
[0054] FIGS. 11A through 11E are diagrams showing a fabrication
process for the lower substrate of the second embodiment;
[0055] FIG. 12 is a cross-sectional view showing a part of a lower
substrate of a third first embodiment in a simplified form;
[0056] FIGS. 13A through 13P are diagrams showing a fabrication
process for the lower substrate of the third embodiment;
[0057] FIG. 14 is a cross-sectional view showing a part of a lower
substrate of a fourth embodiment in a simplified form; and
[0058] FIGS. 15A through 15F are diagrams showing a fabrication
process for the lower substrate of the fourth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0059] One embodiment of the invention will be described below with
reference to the accompanying drawings. The embodiment is just one
form of the invention and does not restrict the invention.
[0060] FIG. 6 is a partial cross-sectional view of a
semi-transmission type liquid crystal display according to the
first embodiment of the invention. As shown in FIG. 6, the
semi-transmission type liquid crystal display has, inside, a lower
substrate 11, an opposite substrate 12 so arranged as to face the
lower substrate 11 and a liquid crystal layer 13 sandwiched between
the lower substrate 11 and the opposite substrate 12. The
semi-transmission type liquid crystal display employs an active
matrix system which has, for example, thin film transistors (TFTs)
provided as switching elements pixel by pixel.
[0061] The lower substrate 11 has an insulative substrate 14, an
insulating protection film 15, TFTs 16, an insulating layer 17, a
reflection electrode 18 and a transparent electrode 19. The
insulating protection film 15 is deposited on the insulative
substrate 14 and the TFTs 16 are formed on the insulating
protection film 15. Each TFT 16 has a gate electrode 16a on the
insulative substrate 14, a drain electrode 16b, a semiconductor
layer 16c and a source electrode 16d, the last three electrodes
lying on the insulating protection film 15 covering the gate
electrode 16a. The planar arrangement of the drain electrode 16b
and source electrode 16d is reverse to that of the prior art shown
in FIG. 1. The reason why the illustration of the prior art differs
from that of the invention is that the names, "drain" and "source",
used in the prior art are reverse to those used in the
invention.
[0062] A contact hole 20 reaching the source electrode 16d of the
TFT 16 is bored in the insulating layer 17. The reflection
electrode 18 and transparent electrode 19 are deposited, covering
the contact hole 20 and the insulating layer 17. The transparent
electrode 19 is connected to the source electrode 16d of the TFT 16
and has a function to serve as a pixel electrode. The reflection
electrode 18 is electrically connected to the transparent electrode
19 via an insulating film 21 and has a function to serve as a
reflector and a pixel electrode.
[0063] An alignment film 22 of polyimide or the like which aligns
the liquid crystal molecules is deposited, covering the reflection
electrode 18 and transparent electrode 19. As the alignment film 22
is rubbed, the alignment direction of the liquid crystal molecules
of the liquid crystal layer 13 is determined. That surface of the
opposite substrate 12 which contacts the liquid crystal layer 13 is
also covered with an alignment film (not shown). A gate terminal
portion 23 on the insulative substrate 14 and a drain terminal
portion 24 on the insulating protection film 15 covering the gate
terminal portion 23 are formed in a terminal area provided in the
peripheral portion of the lower substrate 11.
[0064] The opposite substrate 12 has a transparent electrode 25, a
color filter 26 and an insulative substrate 27 laminated in order
from that side of the liquid crystal layer 13. Incident light input
to the opposite substrate 12 from the insulative substrate 27
travels from the opposite substrate 12 and reaches the lower
substrate 11 through the liquid crystal layer 13, and is reflected
at the reflection electrode 18 to become reflected light. The
reflected light travels through the liquid crystal layer 13 again
and comes out of the opposite substrate 12 from the transparent
electrode 25.
[0065] A backlight 28 is provided opposite side of the lower
substrate 11 to the liquid crystal layer 13. The light from the
backlight 28 reaches the liquid crystal layer 13, passing through
the insulative substrate 14, the insulating protection film 15, the
insulating layer 17 and the transparent electrode 19 and comes out
of the opposite substrate 12 from the transparent electrode 25
through the liquid crystal layer 13.
[0066] FIG. 7 is a cross-sectional view showing a part of the lower
substrate 11 of the liquid crystal display shown in the
cross-sectional view of FIG. 6. As the transparent electrode 19 is
electrically connected to the source electrode 16d of the TFT 16
via the contact hole 20, the potential supplied by the TFT 16
equals the potential of the transparent electrode 19. As the
reflection electrode 18 is connected to the transparent electrode
19 via the insulating film 21, however, the potential of the
reflection electrode 18 becomes lower than the potential of the
transparent electrode 19. At this time, a capacitor is formed by
the reflection electrode 18, the transparent electrode 19 and the
insulating film 21.
[0067] FIG. 8 is an equivalent circuit of the liquid crystal
display illustrated in FIGS. 6 and 7. Provided that the structure
of sandwiching the liquid crystal layer 13 between the lower
substrate 11 and the opposite substrate 12 is regarded as a
capacitor, let CLC1 be the combination of the transparent electrode
19 and the opposite substrate 12, let CLC2 be the combination of
the reflection electrode 18 and the opposite substrate 12, and let
C1 be the reflection electrode 18 and transparent electrode 19
connected together via the insulating film 21. Because two
capacitors, CLC2 and C1, are connected in series in the area of the
reflection electrode 18, the voltage applied by the TFT 16 is
capacitively divided so that the voltage applied to the liquid
crystal layer 13 becomes lower than the voltage applied only to the
CLC1 in the area of the transparent electrode 19.
[0068] It is known that in case where quarter-wave plates are
provided on the upper and lower substrates to produce a phase
difference of .lambda./4 in such a way that the plates pass
opposite circularly polarized lights, for example, the liquid
crystal which has initially taken the vertical alignment state goes
into a so-called normally black mode of providing black display in
both the reflection portion and transmission portion when no
voltage is applied, but if the wavelength of light used for display
is set to .lambda., the reflection type liquid crystal display
provides output light with the highest intensity when the
birefringence (retardation) of the liquid crystal layer 13 is
.lambda./4 while the transmission type liquid crystal display
provides output light with the highest intensity when the
birefringence is .lambda./2. It is also known that as the voltage
applied to the liquid crystal layer 13 is increased, the
birefringence of the liquid crystal layer 13 is increased
monotonously.
[0069] It is therefore possible to optimize the birefringence of
the liquid crystal layer 13 in both the transmission mode and
reflection mode by connecting the reflection electrode 18 and the
transparent electrode 19 via the insulating film 21 to provide the
equivalent circuit shown in FIG. 8, which produces a potential
difference between the reflection electrode 18 and the transparent
electrode 19. Available materials for the insulating film 21 are
SiN, SiO.sub.2, Ti.sub.2O.sub.3, T.sub.2O.sub.5, SiO,
Al.sub.2O.sub.5 and the like. Because the capacitances of the CLC1
ad CLC2 in FIG. 8 change according to the material for, and the
thickness of, the liquid crystal layer 13 and the relationship
between the applied voltage and the birefringence also varies
depending on the material for the liquid crystal layer 13, however,
it is necessary to adequately adjust the material for and the
thickness of the insulating film 21.
[0070] FIGS. 9A through 9F are explanatory diagrams showing a
fabrication process for the lower substrate in the process of
manufacturing the semi-transmission type liquid crystal display
shown in FIG. 6. As shown in FIG. 7, first, the gate electrode 16a
is formed on the insulative substrate 14, the insulating protection
film 15 is deposited on the gate electrode 16a and the drain
electrode 16b, the semiconductor layer 16 and the source electrode
16d are formed on the insulating protection film 15, thereby
forming the substrate of the TFT 16 as a switching element (See
FIG. 9A). Further, the insulating layer 17 is deposited covering
the TFT 16 and the contact hole 20 reaching the source electrode
16d is formed (see FIG. 9B). The switching element is not limited
to the TFT 16 but a substrate for other switching elements, such as
a diode, may be prepared as well.
[0071] Next, the transparent electrode 19 is formed of ITO,
covering the insulating layer 17 so that the source electrode 16d
and the transparent electrode 19 have an electrical contact with
each other via the contact hole 20 (see FIG. 9C). At the time the
transparent electrode 19 is deposited, ITO can be deposited
selectively only in the transparent area by using sputtering with
masking applied. With a mask 29 applied on the transparent
electrode 19 to expose only the boundary region with the reflection
area, the insulating film 21 is formed by anodization (see FIG.
9D).
[0072] Then, with the mask 29 applied to the transparent electrode
19 and the insulating film 21, the reflection electrode 18 or an Al
film is formed on the insulating layer 17 by vacuum deposition (see
FIG. 9E). The material for the reflection electrode 18 is not
limited to Al but other conductive materials may be used as well.
Next, the alignment film 22 of polyimide is coated on the
reflection electrode 18, the transparent electrode 19 and the
insulating film 21 and is rubbed in the direction of the intended
alignment of the liquid crystal (see FIG. 9F). As the lower
substrate 11 is fabricated in the above-described manner and is
made to face, via a frame member, the opposite substrate 12 on
which the color filter 26 and the transparent electrode 25 are
deposited, and the liquid crystal layer 13 is injected between both
substrates to thereby manufacture the liquid crystal display. As
that surface of the lower substrate 11 which contacts the liquid
crystal layer 13 can be made substantially planarized, the
alignment disturbance or the like of the liquid crystal layer 13
does not occur in the vicinity of the boundary between the
transparent area and the reflection area.
[0073] The second embodiment of the invention will be discussed
below. As in the first embodiment in FIG. 6, the lower substrate 11
has an insulative substrate 14, an insulating protection film 15
(not shown), TFTs 16, an insulating layer 17, a reflection
electrode 18 and 4 transparent electrode 19. The insulating
protection film 15 is deposited on the insulative substrate 14 and
the TFTs 16 are formed on the insulating protection film 15. Each
TFT 16 has a gate electrode 16a on the insulative substrate 14, a
drain electrode 16b, a semiconductor layer 16c and a source
electrode 16d, the last three electrodes lying on the insulating
protection film 15 covering the gate electrode 16a. A gate terminal
portion 23 on the insulative substrate 14 and a drain terminal
portion 24 on the insulating protection film 15 covering the gate
terminal portion 23 are formed in a terminal area provided in the
peripheral portion of the lower substrate 11.
[0074] FIG. 10 is a cross-sectional view showing a part of the
lower substrate of the second embodiment in a simplified form. A
contact hole 20 reaching the source electrode 16d of the TFT 16 is
bored in the insulating layer 17. The reflection electrode 18 and
transparent electrode 19 are deposited, covering the contact hole
20 and the insulating layer 17. The transparent electrode 19 is
connected to the source electrode 16d of the TFT 16 and has a
function to serve as a pixel electrode. The reflection electrode 18
is electrically connected to the transparent electrode 19 via an
insulating film 21 and has a function to serve as a reflector and a
pixel electrode. The transparent insulating film 21 of SiO2 or the
like is deposited on the reflection electrode 18. At this time, the
insulating film 21 is deposited on the entire surface of the
reflection electrode 18 in such a way as to completely cover the
reflection electrode 18. Although not illustrated, an alignment
film 22 of polyimide or the like which aligns the liquid crystal
molecules is deposited, covering the reflection electrode 18 and
transparent electrode 19. As the alignment film 22 is rubbed, the
alignment direction of the liquid crystal molecules of the liquid
crystal layer 13 is determined.
[0075] As the transparent electrode 19 is electrically connected to
the source electrode 16d of the TFT 16 via the contact hole 20, the
potential supplied by the TFT 16 equals the potential of the
transparent electrode 19. AS the reflection electrode 18 is
connected to the transparent electrode 19 directly and
electrically, the potential of the reflection electrode 18 becomes
equal to the potential of the transparent electrode 19. Because the
transparent insulating film 21 is deposited on the reflection
electrode 18, that surface of the reflection area which contacts
the liquid crystal layer 13 becomes the top surface of the
insulating film 21 so that a capacitor is formed by the top surface
of the insulating film 21 and the reflection electrode 18.
[0076] An equivalent circuit of the liquid crystal display
according to the second embodiment becomes the one shown in FIG. 8,
as per the first embodiment. Provided that the structure of
sandwiching the liquid crystal layer 13 between the lower substrate
11 and the opposite substrate 12 is regarded as a capacitor, let
CLC1 be the combination of the transparent electrode 19 and the
opposite substrate 12, let C1 be the combination of the reflection
electrode 18 and the top surface of the insulating film 21, and let
CLC2 be the combination of the top surface of the insulating film
21 and the opposite substrate 12. Because two capacitors, CLC2 and
C1, are connected in series in the area of the reflection electrode
18, the voltage applied by the TFT 16 is capacitively divided so
that the voltage applied to the liquid crystal layer 13 becomes
lower than the voltage applied only to the CLC1 in the area of the
transparent electrode 19.
[0077] It is known that with .lambda. being the wavelength of light
used for display, the reflection type liquid crystal display
provides output light with the highest intensity when the
birefringence (retardation) of the liquid crystal layer 13 is
.lambda./4 while the transmission type liquid crystal display
provides output light with the highest intensity when the
birefringence is .lambda./2. It is also known that as the voltage
applied to the liquid crystal layer 13 is increased, the
birefringence of the liquid crystal layer 13 is increased
monotonously. It is therefore possible to optimize the
birefringence of the liquid crystal layer 13 in both the
transmission mode and reflection mode by depositing the insulating
film 21 on the reflection electrode 19 so as to provide the
equivalent circuit shown an FIG. 8, which produces a potential
difference between the reflection electrode 18 and the transparent
electrode 19. Available materials for the insulating film 21 are
organic materials, such as SiN, SiO.sub.2, acryl and arton. Because
the capacitances of the CLC1 ad CLC2 in FIG. 8 change according to
the material for, and the thickness of, the liquid crystal layer 13
and the relationship between the applied voltage and the
birefringence also varies depending on the material for the liquid
crystal layer 13, however, it is necessary to adequately adjust the
material for and the thickness of the insulating film 21.
[0078] FIGS. 11A through 11E are explanatory diagrams showing a
fabrication process for the lower substrate in the process of
manufacturing the semi-transmission type liquid crystal display
shown in FIG. 10. First, the gate electrode 16a is formed on the
insulative substrate 14, the insulating protection film 15 is
deposited on the gate electrode 16a and the drain electrode 16b,
the semiconductor layer 16c and the source electrode 16d are formed
on the insulating protection film 15, thereby forming the substrate
of the TFT 16 as a switching element (see FIG. 11A). Further, the
insulating layer 17 is deposited covering the TFT 16 and the
contact hole 20 reaching the source electrode 16d is formed (see
FIG. 11B). The switching element is not limited to the TFT 16 but a
substrate for other switching elements, such as a diode, may be
prepared as well.
[0079] Next, the transparent electrode 19 is formed of ITO,
covering the insulating layer 17 so that the source electrode 16d
and the transparent electrode 19 have an electrical contact with
each other via the contact hole 20 (see FIG. 11C). At the time the
transparent electrode 19 is deposited, ITO can be deposited
selectively only in the transparent area by using sputtering with
masking applied. With masking applied on the transparent electrode
19, the reflection electrode 18 is formed by sputtering or the like
(see FIG. 11D). Then, with the masking applied to the transparent
electrode 19, SiO.sub.2 is deposited on the reflection electrode 18
by CVD, thereby forming the insulating film 21 (see FIG. 1E). Next,
the alignment film 22 of polyimide is coated on the reflection
electrode 18, the transparent electrode 19 and the insulating film
21 and is rubbed in the direction of the intended alignment of the
liquid crystal (not shown). The insulating film 21 may be deposited
on that area of the opposite substrate 12 which faces the
reflection electrode 18. Alternatively, the insulating film 21 may
be deposited on both the reflection electrode 18 and that area of
the opposite substrate 12 which faces the reflection electrode
18.
[0080] The material for the reflection electrode 18 is not limited
to Al but other conductive materials may be used as well. As the
lower substrate 11 is fabricated in the above-described manner and
is made to face, via a frame member, the opposite substrate 12 on
which the color filter 26 and the transparent electrode 25 are
deposited, and the liquid crystal layer 13 is injected between both
substrates to thereby manufacture the liquid crystal display. As
that surface of the lower substrate 11 which contacts the liquid
crystal layer 13 can be made substantially planarized, the
alignment disturbance or the like of the liquid crystal layer 13
does not occur in the vicinity of the boundary between the
transparent area and the reflection area.
[0081] The third embodiment of the invention will be discussed
below. As in the first embodiment in FIG. 6, the lower substrate 11
has an insulative substrate 14, an insulating protection film 15
(not shown), TFTs 16, an insulating layer 17, a reflection
electrode 18 and a transparent electrode 19. The insulating
protection film 15 is deposited on the insulative substrate 14 and
the TFTs 16 are formed on the insulating protection film 15. Each
TFT. 16 has a gate electrode 16a on the insulative substrate 14, a
drain electrode 16b, a semiconductor layer 16c, a source electrode
16d and a second source electrode 30, the last four electrodes
lying on the insulating protection film 15 covering the gate
electrode 16a. A gate terminal portion 23 on the insulative
substrate 14 and a drain terminal portion 24 on the insulating
protection film 15 covering the gate terminal portion 23 are formed
in a terminal area provided in the peripheral portion of the lower
substrate 11.
[0082] FIG. 12 is a cross-sectional view showing a part of the
lower substrate of the second embodiment in a simplified form. The
source electrode 16d and the second source electrode 30 are
connected together via an insulating film 21 of SiO.sub.2. A
contact hole 20 reaching the source electrode 1d of the TFT 16 and
a contact hole 20 reaching the second source electrode 30 are bored
in the insulating layer 17. The reflection electrode 18 and
transparent electrode 19 are deposited, covering the contact hole
20 and the insulating layer 17. The transparent electrode 19 is
connected to the source electrode 16d or the drain electrode 16b of
the TFT 16 and has a function to serve as a pixel electrode. The
reflection electrode 18 is connected to the second source electrode
30 and has a function to serve as a reflector and a pixel
electrode. The reflection electrode 18 and the transparent
electrode 19 are not directly and electrically connected together.
Although not illustrated, an alignment film 22 of polyimide or the
like which aligns the liquid crystal molecules is deposited,
covering the reflection electrode 18 and transparent electrode 19.
As the alignment film 22 is rubbed, the alignment direction of the
liquid crystal molecules of the liquid crystal layer 13 is
determined.
[0083] As the transparent electrode 19 is electrically connected to
the source electrode 16d of the TFT 16 via the contact hole 20, the
potential supplied by the TFT 16 equals the potential of the
transparent electrode 19. As the reflection electrode 16 is
connected via the insulating film 21 to the TFT 16 and the second
source electrode 30, the potential of the reflection electrode 18
becomes lower than the potential of the transparent electrode 19.
At this time, a capacitor is formed by the reflection electrode 18,
the transparent electrode 19 and the insulating film 21.
[0084] An equivalent circuit of the liquid crystal display
according to the third embodiment becomes the one shown in FIG. 8,
as per the first and second embodiments. Provided that the
structure of sandwiching the liquid crystal layer 13 between the
lower substrate 11 and the opposite substrate 12 is regarded as a
capacitor, let CLC1 be the combination of the transparent electrode
19 and the opposite substrate 12, let CLC2 be the combination of
the reflection electrode 18 and the opposite substrate 12, and let
C1 be the reflection electrode 18 and the second source electrode
30 connected to the source electrode 16d via the insulating film
21. Because two capacitors, CLC2 and C1, are connected in series in
the area of the reflection electrode 18, the voltage applied by the
TFT 16 is capacitively divided so that the voltage applied to the
liquid crystal layer 13 becomes lower than the voltage applied only
to the CLC1 in the area of the transparent electrode 19.
[0085] It is known that with .lambda. being the wavelength of light
used for display, the reflection type liquid crystal display
provides output light with the highest intensity when the
birefringence (retardation) of the liquid crystal layer 13 is
.lambda./4 while the transmission type liquid crystal display
provides output light with the highest intensity when the is
birefringence is .lambda./2. It is also known that as the voltage
applied to the liquid crystal layer 13 is increased, the
birefringence of the liquid crystal layer 13 is increased
monotonously. It is therefore possible to optimize the
birefringence of the liquid crystal layer 13 in both the
transmission mode and reflection mode by intervening the insulating
film 21 between the source electrode 16d and the second source
electrode 30 so as to provide the equivalent circuit shown in FIG.
B, which produces a potential difference between the reflection
electrode 18 and the transparent electrode 19. Available materials
for the insulating film 21 are SiN, SiO2, Ti2O3, Ta2O5, SiO, Al2O5
and the like. Because the capacitances of the CLC1 ad CLC2 in FIG.
8 change according to the material for, and the thickness of, the
liquid crystal layer 13 and the relationship between the applied
voltage and the birefringence also varies depending on the material
for the liquid crystal layer 13, however, it is necessary to
adequately adjust the material for and the thickness of the
insulating film 21.
[0086] FIGS. 13A through 13F are explanatory diagrams showing a
fabrication process for the lower substrate in the process of
manufacturing the semi-transmission type liquid crystal display
shown in FIG. 12. First, the gate electrode 16a is formed on the
insulative substrate 14, the insulating protection film 1S is
deposited on the gate electrode 16a and the drain electrode 16b,
the semiconductor layer 16c and the source electrode 16d are formed
on the insulating protection film 15, thereby forming the substrate
of the TFT 16 as a switching element (see FIG. 13A). The switching
element is not limited to the TFT 16 but a substrate for other
switching elements, such as a diode, may be prepared as well. With
a mask 29 applied on the insulating protection film 15 and TFT 16
to expose only the end portion of the source electrode 16d, the
insulating film 21 is formed by anodization (see FIG. 13B).
[0087] Next, the second source electrode 30 is patterned on the
insulating protection film 15 at a predetermined position in such a
way as to be connected to the source electrode 16d via the
insulating film 21 (see FIG. 13C). Further, the insulating layer 17
is deposited covering the TFT 16 and the contact hole 20 reaching
the source electrode 16d and the contact hole 20 reaching the
second source electrode 30 are formed in the insulating layer 17
(see FIG. 13D). Next, the transparent electrode 19 of ITO is
deposited over the insulating layer 17 so as to electrically
contact the source electrode 16d via the contact hole 20. At the
time the transparent electrode 19 is deposited, ITO can be
deposited selectively only in the transparent area by using
sputtering with masking applied (see FIG, 13B).
[0088] Thereafter, the reflection electrode 18 of Al is formed on
and over the insulating layer 17 so as to electrically contact the
second source electrode 30 via the contact hole 20. At the time the
reflection electrode 18 is deposited, the Al film can be formed
selectively only in the reflection area by vacuum deposition with
masking applied (see FIG. 13F). The material for the reflection
electrode 18 is not limited to Al, but other conductive materials
can be used as well. Next, the alignment film 22 of polyimide is
coated on the reflection electrode 18, the transparent electrode 19
and the insulating film 21 and is rubbed in the direction of the
intended alignment of the liquid crystal (not shown).
[0089] AR the lower substrate 11 is fabricated in the
above-described manner and is made to face, via a frame mer, the
opposite substrate 12 on which the color filter 26 and the
transparent electrode 25 are deposited, and the liquid crystal
layer 13 is injected between both substrates to thereby manufacture
the liquid crystal display. As that surface of the lower substrate
11 which contacts the liquid crystal layer 13 can be made
substantially planarized, the alignment disturbance or the like of
the liquid crystal layer 13 does not occur in the vicinity of the
boundary between the transparent area and the reflection area.
[0090] A further embodiment of the invention will be discussed
below. As in the first embodiment in FIG. 6, the lower substrate 11
has an insulative substrate 14, an insulating protection film 15
(not shown), TFTs 16, an insulating layer 17, a reflection
electrode 18 and a transparent electrode 19. The insulating
protection film 15 is deposited on the insulative substrate 14 and
the TFTs 16 are formed on the insulating protection film 15. Each
TFT 16 has a gate electrode 16a on the insulative substrate 14, a
drain electrode 16b, a semiconductor layer 16c and a source
electrode 16d, the last three electrodes lying on the insulating
protection film 15 covering the gate electrode 16a. A gate terminal
portion 23 on the insulative substrate 14 and a drain terminal
portion 24 on the insulating protection film 15 covering the gate
terminal portion 23 are formed in a terminal area provided in the
peripheral portion of the lower substrate 11.
[0091] FIG. 14 is a cross-sectional view showing a part of the
lower substrate of the fourth embodiment in a simplified form. A
contact hole 20 reaching the source electrode 16d of the TFT 16 is
bored in the insulating layer 17. The transparent electrode 19, the
insulating film 21 and the reflection electrode 18 are deposited,
covering the contact hole 20 and the insulating layer 17. The
transparent electrode 19 is connected to the source electrode 16d
of the TFT 16 and has a function to serve as a pixel electrode. The
transparent insulating film 21 of SiO2 or the like is deposited
between the transparent electrode 19 and the reflection electrode
18. The reflection electrode 18 is electrically connected to the
transparent electrode 19 via an insulating film 21 and has a
function to serve as a reflector and a pixel electrode.
[0092] The insulating layer 17 has an undulating surface and the
transparent electrode 19 and the reflection electrode 18 deposited
on the insulating layer 17 also have undulating surfaces. The
reflection electrode 18 and the insulating film 21 are removed in
the top region and bottom region of the undulating surface of the
reflection electrode 18, and opening 31 are formed in such a way
that the transparent electrode 19 contacts the liquid crystal layer
13.
[0093] Although not illustrated, an alignment film 22 of polyimide
or the like which aligns the liquid crystal molecules is deposited,
covering the reflection electrode 18 and transparent electrode 19.
As the alignment film 22 is rubbed, the alignment direction of the
liquid crystal molecules of the liquid crystal layer 13 is
determined. As the transparent electrode 19 is electrically
connected to the source electrode 16d of the TFT 16 via the contact
hole 20, the potential supplied by the TFT 16 equals the potential
of the transparent electrode 19. As the reflection electrode 18 is
connected to the transparent electrode 19 via the insulating film
21, however, the potential of the reflection electrode 18 becomes
lower than the potential of the transparent electrode 19. At this
time, a capacitor is formed by the reflection electrode 18, the
transparent electrode 19 and the insulating film 21.
[0094] An equivalent circuit of the liquid crystal display
according to the fourth embodiment becomes the one shown in FIG. 8,
as per the first to third embodiments. Provided that the structure
of sandwiching the liquid crystal layer 13 between the lower
substrate 11 and the opposite substrate 12 is regarded as a
capacitor, let CLC1 let the combination of the transparent
electrode 19 and the opposite substrate 12 at the associated
opening 31, let CLC2 be the combination of the reflection electrode
18 and the opposite substrate 12, and let C1 be the reflection
electrode 18 and transparent electrode 19 connected together via
the insulating film 21. Because two capacitors, CLC2 and C1, are
connected in series in the area of the reflection electrode 18, the
voltage applied by the TFT 16 is capacitively divided so that the
voltage applied to the liquid crystal layer 13 becomes lower than
the voltage applied only to the CLC1 in the area of the transparent
electrode 19.
[0095] It is known that with .lambda. being the wavelength of light
used for display, the reflection type liquid crystal display
provides output light with the highest intensity when the
birefringence (retardation) of the liquid crystal layer 13 it
.lambda./4 while the transmission type liquid crystal display
provides output light with the highest intensity when the
birefringence is .lambda./2. It is also known that as the voltage
applied to the liquid crystal layer 13 is increased, the
birefringence of the liquid crystal layer 13 is increased
monotonously. It is therefore possible to optimize the
birefringence of the liquid crystal layer 13 in both the
transmission mode and reflection mode by depositing the insulating
film 21 on the reflection electrode 18 so as to provide the
equivalent circuit shown in FIG. 9, which produces a potential
difference between the reflection electrode 18 and the transparent
electrode 19. Available materials for the insulating film 21 are
organic materials, such as SiN, SiO.sub.2, acryl and arton. Because
the capacitances of the CLC1 ad CLC2 in FIG. 8 change according to
the material for, and the thickness of, the liquid crystal layer 13
and the relationship between the applied voltage and the
birefringence also varies depending on the material for the liquid
crystal layer 13, however, it is necessary to adequately adjust the
material for and the thickness of the insulating film 21.
[0096] FIGS. 15A through 15F are explanatory diagrams showing a
fabrication process for the lower substrate in the process of
manufacturing the semi-transmission type liquid crystal display
shown in FIG. 14. First, the gate electrode 16a is formed on the
insulative substrate 14, the insulating protection film 15 is
deposited on the gate electrode 16a and the drain electrode 16b,
the semiconductor layer 16c and the source electrode 16d are formed
on the insulating protection film 15, thereby forming the substrate
of the TFT 16 as a switching element (see FIG. 15A). The switching
element is not limited to the TFT 16 but a substrate for other
switching elements, such as a diode, may be prepared as well.
[0097] Further, the insulating layer 17 is deposited over the TFT
16. To form an undulating surface on the insulating layer 17, the
insulating layer 17 is formed flat after which with the insulating
layer 17 masked, steps are formed on the insulating layer 17 using
a photoresist. Thereafter, annealing is performed to make the
corner portions of the steps of the insulating layer 17 round, so
that the insulating layer 17 formed has gentle undulations on the
surface. The contact hole 20 reaching the source electrode 16d is
formed in the insulating layer 17 (see FIG. 15B). Next, the
transparent electrode 19 is formed of ITO over the insulating layer
17 by sputtering, so that the source electrode 16d and the
transparent electrode 19 electrically contact each other via the
contact hole 20 (see FIG. 15C). Further, the insulating film 21 of
SiO.sub.2 is deposited on the transparent electrode 19 by CVD (see
FIG. 15D). Then, the reflection electrode 19 which is an Al film is
formed on the insulating film 21 by vacuum deposition (see FIG.
15E).
[0098] Based on the mask that has been used to form the undulations
in the process in FIG. 15B, the top regions and bottom regions of
the undulating surface of the reflection electrode 18 are
specified, and with a mask having holes at the positions
corresponding to the top regions and bottom regions, the reflection
electrode 18 and the transparent electrode 19 at the top regions
and bottom regions are removed using etching and a photoresist,
thereby forming the openings 31. In each opening 31, the
transparent electrode 19 is exposed (see FIG. 15F).
[0099] The material for the reflection electrode 18 is not limited
to Al but other conductive materials may be used as well. As the
lower substrate 11 is fabricated in the above-described manner and
is made to face, via a frame member, the opposite substrate 12 on
which the color filter 26 and the transparent electrode 25 are
deposited, and the liquid crystal layer 13 is injected between both
substrates to thereby manufacture the liquid crystal display. As
that surface of the lower substrate 11 which contacts the liquid
crystal layer 13 can be made substantially planarized, the
alignment disturbance or the like of the liquid crystal layer 13
does not occur in the vicinity of the boundary between the
transparent area and the reflection area.
[0100] As the openings 31 are formed in the reflection electrode
18, light is allowed to pass through the liquid crystal layer 13 to
ensure liquid crystal display by irradiating the light from the
opposite side of the lower substrate 11 to the liquid crystal layer
13 by means of a backlight or the like in transmission mode, so
that display can be ensure even under a dark environment. The
regions where the openings 31 are formed cannot reflect input light
from the opposite substrate 12 toward a viewer, so that the
luminance is not significantly reduced even in the reflection mode
that reflects the input light from the opposite substrate 12 at the
reflection electrode 18 for liquid crystal display.
[0101] A description will now be given of a liquid crystal display
in which the insulating layer 17 deposited on the TFTs 16 on the
lower substrate 11 is replaced with the color filter 26 as a
different embodiment. The partial cross-sectional view of the
semi-transmission type liquid crystal display and the process of
forming the reflection electrode in the fabrication process of the
semi-transmission type liquid crystal display are the same as those
of the first to fourth embodiments. The difference lies only in the
replacement of the insulating layer 17 with the color filter
26.
[0102] In the display of the reflection mode, input light from the
opposite substrate 12 passes the color filter 26 provided on the
opposite substrate 12 twice until it becomes output light, In the
display of the transmission mode, the light from the backlight 29
passes the color filter 26 provided on the lower substrate 11 and
the color filter 26 provided on the opposite substrate 12 until it
becomes output light. In reflection and transmission modes, the
light passes a color filter twice, making it possible to ensure the
same color display in both modes. It is also possible to determine
the balance of colors displayed independently between the
transmission mode and reflection mode.
[0103] According to the invention, as the drive voltage applied to
the liquid crystal layer in the transparent area is lower than the
drive voltage applied to the liquid crystal layer in the reflection
area, the birefringence of the liquid crystal layer in the
reflection area becomes smaller than the birefringence of the
liquid crystal layer in the transparent area, making it possible to
ensure the optimal birefringence in each of the reflection mode and
transmission mode. This can optimize the intensities of the output
light in both modes. The capacitive division produces a difference
between the drive voltages of the transparent area and reflection
area, so that the transparent area and reflection area can be
simultaneously driven by the voltage that is supplied by a single
thin film transistor. This makes it possible to prevent an increase
in the quantity of the thin film transistors and eliminate the
complexity of the drive voltage control, leading to a reduction in
the production cost of the liquid crystal display. As the cell gaps
in the transparent area and the reflection area are substantially
identical, it is possible to eliminate an alignment disturbance
produced by the disturbance of the electric line of force in the
liquid crystal layer or an alignment disturbance, such as the
reverse tilt disclination produced by the disturbance of the
pretilt angle. This can improve the characteristics of the liquid
crystal display.
[0104] As the reflection electrode is connected to the transparent
electrode via the insulating film, the insulating film is deposited
on the reflection electrode, the insulating film is deposited on
that area of the opposite substrate which faces the reflection
electrode, the insulating film is deposited on the reflection
electrode and that area of the opposite substrate which faces the
reflection electrode, the second source electrode is connected to
the source electrode via the insulating film, the insulating film
is deposited on the transparent electrode and the reflection
electrode is formed on the insulating film, a capacitor is formed
by the reflection electrode and the transparent electrode and a
potential difference can be provided between the transparent area
and the reflection area by capacitive division. Because the
reflection electrode and insulating film are eliminated at the
opening, the opening serves as the transparent area.
[0105] It is difficult to efficiently reflect light input from the
opposite substrate toward a viewer in the top peripheral regions
and the bottom peripheral regions of the undulations. Therefore,
the openings are formed in the top peripheral regions and the
bottom peripheral regions as transparent areas, so that efficient
liquid crystal display can be ensured in reflection mode as well as
transmission mode.
[0106] Because SiN, SiO.sub.2, Ti.sub.2O.sub.3, Ta.sub.2O.sub.5,
SiO, Al.sub.2O.sub.5, acryl and arton can be used as the material
for the insulating film, it is possible to select the optimal
insulating film in accordance with various conditions, such as the
usage, the product quality and the material for the liquid crystal.
This increases the degree of freedom in the design stage.
[0107] As the color filters are formed on the opposite substrate
and the device substrate, light passes the color filter on the
opposite substrate side twice in reflection mode and light passes
the color filters on the device substrate and the opposite
substrate once each in transmission mode. This can make it possible
to reduce a change in color in both modes. It is also possible to
respectively set the hues in transmission mode and reflection
mode.
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