U.S. patent application number 13/441966 was filed with the patent office on 2012-08-02 for liquid crystal display and method of driving same.
This patent application is currently assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Takeshi NISHI, Shunpei YAMAZAKI.
Application Number | 20120194754 13/441966 |
Document ID | / |
Family ID | 18157379 |
Filed Date | 2012-08-02 |
United States Patent
Application |
20120194754 |
Kind Code |
A1 |
YAMAZAKI; Shunpei ; et
al. |
August 2, 2012 |
LIQUID CRYSTAL DISPLAY AND METHOD OF DRIVING SAME
Abstract
There is disclosed a lightweight and small liquid crystal
display which achieves low power consumption and in which the
optical anisotropy of the liquid crystal material is compensated
for in order to enhance the viewing angle characteristics and the
response speed of the liquid crystal material. Display electrodes
and a common electrode are formed on one of the substrates. The
orientation of the liquid crystal material is of the HAN (hybrid
alignment nematic) type. This compensates for the optical
anisotropy of the liquid crystal material and improves the response
speed.
Inventors: |
YAMAZAKI; Shunpei; (Tokyo,
JP) ; NISHI; Takeshi; (Kanagawa, JP) |
Assignee: |
SEMICONDUCTOR ENERGY LABORATORY
CO., LTD.
Kanagawa-ken
JP
|
Family ID: |
18157379 |
Appl. No.: |
13/441966 |
Filed: |
April 9, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12613725 |
Nov 6, 2009 |
8154697 |
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13441966 |
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11218436 |
Sep 6, 2005 |
7616282 |
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12613725 |
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09686653 |
Oct 10, 2000 |
6963382 |
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11218436 |
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08751365 |
Nov 18, 1996 |
6160600 |
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09686653 |
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Current U.S.
Class: |
349/33 |
Current CPC
Class: |
G02F 1/13454 20130101;
G02F 1/13306 20130101; G02F 1/13363 20130101; G02F 1/1393 20130101;
G02F 1/134363 20130101; G02F 2203/02 20130101 |
Class at
Publication: |
349/33 |
International
Class: |
G02F 1/133 20060101
G02F001/133 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 17, 1995 |
JP |
7-323677 |
Claims
1. A method of driving a liquid crystal display device, said liquid
crystal display device comprising: a first insulating substrate; a
second insulating substrate being disposed opposite to the first
insulating substrate; a first electrode being formed over the first
insulating substrate; a second electrode being formed over the
first insulating substrate; a liquid crystal material being
interposed between the first and second insulating substrates; said
method comprising the steps of: producing a parallel electric field
to the first insulating substrates, said parallel electric field
being generated between the first and second electrodes, and
driving the liquid crystal material by the parallel electric field.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/613,725, filed Nov. 6, 2009, now allowed, which is a
continuation of U.S. application Ser. No. 11/218,436, filed Sep. 6,
2005, now U.S. Pat. No. 7,616,282, which is a continuation of U.S.
application Ser. No. 09/686,653, filed Oct. 10, 2000, now U.S. Pat.
No. 6,963,382, which is a continuation of U.S. application Ser. No.
08/751,365, filed Nov. 18, 1996, now U.S. Pat. No. 6,160,600, which
claims the benefit of a foreign priority application filed in Japan
as Serial No. 07-323677 on Nov. 17, 1995, all of which are
incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an active matrix liquid
crystal display.
BACKGROUND OF THE INVENTION
[0003] Since liquid crystal displays are lighter and more compact
display devices than CRTs, they find extensive application in
computers, electronic calculators, clocks, and watches. The
principle of operation of liquid crystal displays depends on a
change in an optical property, such as interference, scattering,
diffraction, optical rotation, or absorption, of a liquid crystal
material. This change is caused by a variation in orientation of
the liquid crystal molecules or a phase transition in response to
application of an external field such as electric field or
heat.
[0004] Generally, a liquid crystal display comprises a pair
substrates with a given spacing which is maintained at 1 to tens of
micrometers. A liquid crystal material is held between these
substrates, thus forming a liquid crystal panel. At least one of
the substrates has transparency. Electrodes are formed on both or
one of the substrates. Using these electrodes, an electric field is
applied to the liquid crystal material to control the orientation
of the liquid crystal molecules in each different pixel within the
substrate plane, thus controlling the amount of light transmitted
through the liquid crystal panel. In this way, a desired image is
displayed. The liquid crystal display is constructed differently
according to which is the above-described optical properties is
utilized, i.e., depending on the mode of operation. For example,
polarizing plates are mounted on the outside of the liquid crystal
panel.
[0005] To date, twisted-nematic (TN) liquid crystal displays and
supertwisted-nematic (STN) liquid crystal displays enjoy wide
acceptance. These kinds of liquid crystal displays make use of
optical properties of liquid crystal materials such as optical
rotation and interference of birefringent light. Both kinds require
polarizing plates.
[0006] Where an image is displayed by the above-described liquid
crystal display, numerous pixels must be controlled at the same
time. For this purpose, various methods have been proposed. Among
them, active matrix driving is used widely because it is a method
capable of displaying image with high information content and high
image quality. In particular, nonlinear active devices such as
diodes or transistors are arranged at pixels. The individual pixels
are electrically isolated from each other. Interference with
unwanted signals is prevented. Thus, high image quality is
accomplished. In this method, each pixel can be regarded as a
capacitor to which an electrical switch is connected. Accordingly,
the switches are turned on and off according to the need. As a
result, electric charge can be made to go into and out of the
pixels. If the switches are turned off, electric charge can be
retained in the pixels and so the pixels can retain memory.
PROBLEMS WITH THE PRIOR ART TECHNIQUES
(1) Electric Power Consumed by Liquid Crystal Display
[0007] In any kind of liquid crystal display, the liquid crystal
material itself does not emit light. In order to have good
visibility, a light source is incorporated in the equipment
(transmissive type), or light incident on the equipment from
surroundings is utilized (reflective type).
[0008] In the case of the transmissive type, if the intensity of
light emitted by the light source is increased, then a brighter
display device can be accomplished accordingly. However, this
increases the power consumption of the whole apparatus. Most of the
electric power is consumed by the light source in the transmissive
type liquid crystal display. The ratio of the electric power
consumed by the liquid crystal panel to the electric power consumed
by the light source=1:100-1:1000. Hence, lower power consumption
can be effectively accomplished by reducing the electric power
expended by the light source. However, in both TN and STN types, it
is customary to use two polarizing plates. This considerably lowers
the transmittance of the liquid crystal panel. Therefore, in order
to accomplish a bright display, it is necessary to increase the
brightness of the light source. Since it is necessary to maintain a
certain degree of brightness, a great reduction in the power
consumption cannot be expected unless a light source of extremely
high emission efficiency is employed.
[0009] On the other hand, in the case of the reflective type liquid
crystal display, any special light source is not present inside the
display device and so low power consumption and a reduction in size
are enabled. Hence, it can be said that the reflective type is an
ideal display device. However, it makes use of light coming from
the surroundings. In order to achieve a brighter display device
with a small amount of light, it is necessary to utilize the light
efficiently. Since the reflective TN and STN types use no light
sources, the brightness of the display devices is reduced
accordingly.
(2) Response Speed
[0010] As image with higher information content is displayed, it is
required that the response speed of the liquid crystal material be
increased. However, the response time of the above-described TN
liquid crystal display is tens of milliseconds. The response time
or the STN liquid crystal display is on the order of 100 ms. At
these response speeds, when the image created on the viewing screen
moves across the screen, the image appears to tail off. In this
way, the image quality is not good. In order to improve this
phenomenon, it is necessary to utilize a liquid crystal material of
higher response speed or to establish a mode of operation that
accomplishes a higher response.
(3) Viewing Angle Characteristics
[0011] In both TN and STN types, when the liquid crystal display is
viewed obliquely, a decrease in contrast, inversion of intermediate
color hues, and other undesirable phenomena are observed, because
the state of polarization of light entering obliquely changes in
the liquid crystal layer of the liquid crystal display. In view of
these problems, a method of dividing each pixel electrode into
plural parts, a method of producing plural different states of
orientation, and other methods have been proposed but no
fundamental solution to the problems is available today.
SUMMARY OF THE INVENTION
[0012] It is an object of the present invention to provide a liquid
crystal display which is lightweight, small in size, capable of
displaying image of high image quality, shows decreased electric
power consumption, and has improved response speed and improved
viewing angle characteristics.
[0013] The above object is achieved in accordance with the
teachings of the invention by a liquid crystal display comprising:
a first insulating substrate having transparency; a second
insulating substrate disposed opposite to said first insulating
substrate and reflecting light; a liquid crystal material
sandwiched between said first and second substrates; first pixel
electrodes formed on said first insulating substrate; first
conducting lines for applying electrical signals to said first
pixel electrodes, said first conducting lines being formed on said
first insulating substrate; second pixel electrodes formed on said
first insulating substrate and electrically insulated from said
first pixel electrodes and from said first conducting lines; and
second conducting lines for applying electrical signals to said
second pixel electrodes, said second conducting lines being formed
on said first insulating substrate.
[0014] The above object is also achieved by a liquid crystal
display comprising: a first insulating substrate having
transparency; a second insulating substrate disposed opposite to
said first insulating substrate and reflecting light; a liquid
crystal material sandwiched between said first and second
substrates; first pixel electrodes formed on said first insulating
substrate; first conducting lines for applying electrical signals
to said first pixel electrodes, said first conducting lines being
formed on said first insulating substrate; second pixel electrodes
formed on said first insulating substrate and electrically
insulated from said first pixel electrodes and from said first
conducting lines; and second conducting lines for applying
electrical signals to said second pixel electrodes, said second
conducting lines being formed on said first insulating substrate.
The liquid crystal material is oriented in a first way near the
first insulating, substrate and in a second way near the second
insulating substrate. The first way is different from the second
way.
[0015] The above object is also achieved by a liquid crystal
display comprising: a first insulating substrate having
transparency; a second insulating substrate disposed opposite to
said first insulating substrate and reflecting light; a liquid
crystal material sandwiched between said first and second
substrates; first pixel electrodes formed on said first insulating
substrate; first conducting lines for applying electrical signals
to said first pixel electrodes, said conducting lines being formed
on said first insulating substrate; second pixel electrodes formed
on said first insulating substrate and electrically insulated from
said pixel electrodes and from said first conducting lines; and
second conducting lines for applying electrical signals to said
second pixel electrodes, said second conducting lines being formed
on said first insulating substrate. The liquid crystal material is
oriented parallel or substantially parallel to the first insulating
substrate near the first insulating substrate and oriented
vertically or substantially vertically to the second insulating
substrate near the second insulating substrate.
[0016] The above object is also achieved by a liquid crystal
display comprising: a first insulating substrate having
transparency; a second insulating substrate disposed opposite to
said first insulating substrate and reflecting light; a liquid
crystal material, sandwiched between said first and second
substrates; first pixel electrodes formed on said first insulating
substrate; first conducting lines for applying electrical signals
to said first pixel electrodes, said conducting lines being formed
on said first insulating substrate; second pixel electrodes formed
on said first insulating substrate and electrically insulated from
said first pixel electrodes and from said first conducting lines;
second conducting lines for applying electrical signals to said
second pixel electrodes, said second conducting lines being formed
on said first insulating substrate; and nonlinear devices connected
with said first or second pixel electrodes.
[0017] Furthermore, the present invention provides a method driving
a liquid crystal display comprising a first insulating substrate
having transparency, a second insulating substrate disposed
opposite to said first insulating substrate and reflecting light, a
liquid crystal material sandwiched between said first and second
substrates, first pixel electrodes formed on said first insulating
substrate, first conducting lines for applying electrical signals
to said first pixel electrodes, said first conducting lines being
formed on said first insulating substrate, second pixel electrodes
formed on said first insulating substrate and electrically
insulated from said first pixel electrodes and from said first
conducting lines, and second conducting lines for applying
electrical signals to said second pixel electrodes, said second
conducting lines being formed on said first insulating substrate.
This method consists of producing an electric field which has a
component parallel to the substrates and drives on the liquid
crystal material.
[0018] The structure of a liquid crystal display according to the
present invention is shown in FIG. 1, where display electrodes 110
and a common electrode 111 are formed on a first substrate 101. A
reflecting layer 103 is formed on a second substrate 102. The
liquid crystal display further includes a biaxial film 104, a
polarizing plate 105, and liquid crystal molecules 106. The optical
axis of the polarizing plate is indicated by the arrows 107. Liquid
crystal molecules close to the first substrate are oriented in one
direction indicated by the arrows 108. Where the novel liquid
crystal display is operated as an active matrix display, nonlinear
devices 109 (only one is shown) are formed. The liquid crystal
display further includes scanning (gate) lines 112 (only one is
shown) and signal (source) lines 113 (only one is shown).
[0019] The first insulating substrate 101 is made from a material
which has transparency and shows some degree of resistance to an
external force. For example, glass, quartz, or other inorganic
material is used as the material of the first substrate 101. Where
TFTs are fabricated on the substrate, non-alkali glass or quartz is
used. Where a decrease in the weight of the liquid crystal panel is
required, a film showing only a small amount of birefringence such
as PES (polyethylene sulfate) can be employed.
[0020] The material of the second substrate 102 exhibits some
degree of resistance to an external force and reflects light. For
instance, a thin film 103 of Al, Cr, or other material, may be
formed on the surface of the first substrate. It is also possible
to use a substrate of Si.
[0021] In accordance with the present invention, in order to
operate the liquid crystal material, an electric field having a
component parallel to the substrates is produced inside the liquid
crystal cell.
[0022] This electric field will hereinafter be referred to as the
lateral field. The orientation of the liquid crystal molecules is
controlled by the strength of the lateral field. For this purpose,
two kinds of electrodes which are electrically insulated from each
other are formed on the first substrate. That is, they are the
display electrodes 110 and the common electrode 111. The two kinds
of electrodes are made from a conductive material such as Al or Cr.
If the electrodes are made from a material having transparency such
as ITO, then the aperture ratio of the pixels can be enhanced. The
fundamental structure of the electrodes is shown in. FIG. 2. The
display electrodes 110 and common electrode 111 have alternately
protruding portions and alternately nested in each other with a
given spacing. If necessary, a reflector formed on the second
substrate may be used as the electrodes.
[0023] The liquid crystal display according to the invention can
use an optically compensated birefringence (OCB) mode to improve
the viewing angle. In the OCB mode, the refractive index is
effectively uniform in three dimensions. A fundamental structure
for establishing the OCB mode comprises two polarizing plates
together with a bend cell and a biaxial film inserted between them.
This compensates for the refractive index in three dimensions. The
liquid crystal molecules in the bend cell are oriented in the
manner described now. The major axis of the liquid crystal
molecules in the liquid crystal panel are parallel to the
substrates near the substrates. The major axis rotate from one
substrate to the other through 180.degree. vertical to the
substrates.
[0024] However, if the above-described structure is used as it is,
then it follows that two polarizing plates are used. This makes it
impossible to realize a bright liquid crystal display. Accordingly,
a reflective liquid crystal display according to the invention
needs only one polarizing plate. Specifically, in the OCB mode, the
liquid crystal molecules are grouped in two classes about the
midway plane between the two substrates. These two groups of liquid
crystal molecules have a mirror-image relation to each other. If
this midway plane is used as a turning point and the device is
designed as a reflection type, then the liquid crystal molecules
are oriented in exactly the same way as liquid crystal molecules in
a HAN-mode (HAN: hybrid alignment nematic) liquid crystal display.
In the HAN mode, liquid crystal molecules on the side of one
substrate are oriented vertically, while liquid crystal molecules
on the side of the other substrate are oriented horizontally. The
orientation of the liquid crystal molecules 106 when no electric
field is applied is shown in FIG. 1. A nematic material having
positive or negative dielectric anisotropy is used as the liquid
crystal material. To achieve the orientation, the substrate surface
over which vertical orientation is assumed is processed with
monobasic chromium complex, or a silane-coupling agent is applied
in a manner not illustrated (not shown). Polyimide or the like is
applied to the substrate surface over which horizontal orientation
is assumed (not shown). The polyimide layer is rubbed in a
well-known manner.
[0025] The liquid crystal material may be activated by a
multiplexing method or active matrix method. In the multiplex
method, only two kinds of electrodes, i.e., display electrodes and
common electrode, are required to be formed on the first substrate.
However, in the active matrix method, a switching device consisting
of a nonlinear device 109 such as a thin-film transistor (TFT) or
diode must be added to each pixel. The active layer of each TFT can
be made from amorphous silicon or polysilicon (polycrystalline
silicon). The nonlinear devices 109 are connected with the display
electrodes 110.
[0026] A more advanced structure has a peripheral driver circuit
composed of thin-film transistors which are integrated on a
substrate. In this structure, pixel regions are integrated with the
peripheral driver circuit region on the substrate. Therefore, it is
easier to exploit the liquid crystal panel.
[0027] To improve the display characteristics, a black matrix 328
(FIG. 3(F)) is formed on those portions (i.e., conductive
interconnects, nonlinear device portions, and peripheral driver
circuit) which are not associated with image display. The black
matrix 328 is made of a metal such as Cr or made of a transparent
substance in which a black substance is dispersed to prevent
contrast deterioration due to irregular reflection inside the
liquid crystal display. An inorganic material such as glass or
quartz or an organic material such as resins can be used as the
transparent substance. Because of ease of fabrication, resinous
materials are preferably used. Acrylic-based materials can be used
as the resinous material. Carbon black or a pigment can be used as
the black substance.
[0028] The method of dispersing the black material in the resinous
material can be appropriately selected according to the used black
substance. The materials can be stirred by a stirrer, by ball
milling, or with three rolls. Where the black material is
dispersed, if a small amount of surfactant or other
dispersion-assisting agent is added, the dispersibility of the
black material can be enhanced. In order to stabilize the
dispersion and to make the black matrix layer thinner, the diameter
of the particles of the black material is preferably less than 1
.mu.m.
[0029] The black material can be formed on the TFT substrate by the
same method as used to form a resist pattern by a conventional
photolithographic process. First, an organic solution in which the
black substance is dispersed is applied to the TFT substrate by
spin-coating or print-coating techniques. The layer is patterned by
a well-known photolithographic method. The layer is subsequently
postbaked at about 200.degree. C.
[0030] In the HAN mode described above, color control is possible
by controlling the voltage applied to the liquid crystal material.
Therefore, color filters which have been used in the prior art
liquid crystal display are dispensed with.
[0031] The layers on the substrates are oriented in this way. The
substrates are so placed that their surfaces having the oriented
layers, TFTs, and transparent electrode are located opposite to
each other. A liquid crystal material is held between the opposite
substrates. Spacers are dispersed over the substrates to maintain
the substrate spacing uniform. The used spacers have a diameter of
1 to 10 .mu.m. The two substrates are bonded together with an epoxy
adhesive or the like. The adhesive pattern is formed outside the
substrates so that the pixel regions and peripheral driver circuit
region are located inside.
[0032] In the novel liquid crystal display, display electrodes and
a common electrode are formed on one substrate. An electric field
parallel to the substrates drives the liquid crystal molecules. The
liquid crystal molecules are made to assume the HAN orientation.
This compensates for the optical anisotropy of the liquid crystal
material and improves the viewing angle characteristics.
Furthermore, the HAN orientation gives higher response speed than
conventional TN and STN orientations. Where a reflector is formed
on the other substrate, a reflective liquid crystal display can be
built. Since it is not necessary to incorporate a light source in
the display device, lower power consumption can be
accomplished.
[0033] Other objects and features of the invention will appear in
the course of the description thereof, which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a schematic exploded perspective view of a liquid
crystal display according to the present invention;
[0035] FIG. 2 is a fragmentary top view of an electrode structure
included in the liquid crystal display shown in FIG. 1;
[0036] FIGS. 3(A)-3(F) are cross-sectional views of pixel TFTs and
a peripheral driver circuit included in a liquid crystal display
according to the invention; and
[0037] FIG. 4 is a graph illustrating the transmission-voltage
characteristics of Embodiment 3 of liquid crystal display according
to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
[0038] The present embodiment is a liquid crystal display making
use of a multiplexing drive method. First, a glass substrate 101
was prepared as a first insulating substrate. An ITO film having a
thickness of 1200 .ANG. was formed and patterned into display
electrodes. Then; an insulating film of SiN was grown on the ITO
film to a thickness of 1000 .ANG.. Another ITO film was formed on
this insulating film to a thickness of 1200 .ANG. and patterned to
form a common electrode. The structure of the display electrodes
and common electrode is shown in FIG. 2. The display electrodes,
indicated by 110, took a comb-like form. The display electrodes 110
had horizontally extending portions and vertically extending
portions. The vertically extending portions had a width, indicated
by 203, of 10 .mu.m. Also, the horizontally extending portions had
a width, indicated by 204, of 10 .mu.m. The common electrode 111
had horizontally extending portions and vertically extending
portions. The vertically extending portions had a width, indicated
by 205, of 10 .mu.m. Also, the horizontally extending portions had
a width, indicated by 206, of 10 .mu.m. The length, 207, of the
portions which are alternately nested in each other was 60 .mu.m.
The spacing 208 between the display and common electrodes was 5
.mu.m. A film of Cr was formed on a second insulating substrate 102
to a thickness of 1200 .ANG.. Reflecting, function was imparted to
the second insulating substrate.
[0039] In the present embodiment, the liquid crystal molecules were
made to assume HAN orientation. For this purpose, an orientation
film (not shown) was formed on each of the first substrate 101 and
second substrate 102. A layer of polyimide was formed on the first
substrate 101 by well-known spin-coating or DIP techniques. In
order to orient the liquid crystal molecules parallel to the
substrates, the polyimide film on the first substrate 101 was
rubbed in a direction indicated by the arrows 108 in portions
corresponding to the teeth of the display and common electrodes. A
silane-coupling agent was applied to the second substrate 102. As a
result, the liquid crystal molecules on the surface of the second
substrate were oriented vertically.
[0040] The first substrate 101 and the second substrate 102 formed
in this way were stacked on top of each other, thus forming a
liquid crystal panel. Spherical spacers (not shown) having a
diameter of 3 .mu.m were sandwiched between the two substrates to
make the substrate spacing uniform over the whole panel plane. An
epoxy adhesive was used to bond together the substrates and to seal
the panel. The patterned sealing adhesive was applied to a region
(not shown) surrounding the pixel regions and the peripheral driver
circuit region. The substrates were cut into a desired shape. A
liquid crystal material having positive dielectric anisotropy was
then injected between the substrates. Nematic liquid crystal
ZLI-2293 (.DELTA..di-elect cons.=+10 at 1 kHz and at 20.degree. C.)
was used as the liquid crystal material.
[0041] Then, a biaxial film 104 and a polarizing plate 105 were
successively stuck on the first substrate. The direction 107 of the
optical axis of the polarizing plate made an angle of 45.degree. to
the rubbing direction.
[0042] This liquid crystal display was operated at a voltage of 3
V. An image display could be provided at a contrast of 100, a
response speed of 2 ms, and a wide viewing angle.
Embodiment 2
[0043] A method of fabricating a substrate for use in a liquid
crystal display utilizing the active matrix circuit of the present
embodiment is next described.
[0044] A process sequence for obtaining a monolithic active matrix
circuit of the present embodiment is described below by referring
to FIGS. 3(A)-3(F). This process sequence makes use of a
low-temperature polysilicon process. The left sides of FIGS.
3(A)-3(F) show steps for manufacturing TFTs forming a driver
circuit. The right sides show steps for manufacturing TFTs forming
the active matrix circuit.
[0045] First, a glass substrate 301 was prepared as the first
insulating substrate. A silicon oxide film was formed as a buffer
oxide film 302 on the glass substrate 301 to a thickness of 1000 to
3000 .ANG. by sputtering techniques or plasma CVD in an oxygen
ambient.
[0046] Then, an amorphous silicon film was formed to a thickness of
300 to 1500 .ANG., preferably 500 to 1000 .ANG., by plasma CVD or
LPCVD. Thermal annealing was carried out above 500.degree. C.,
preferably at a temperature of 500-600.degree. C., to crystallize
the silicon film or to enhance the crystallinity. Thereafter,
photo-annealing might be effected, using a laser or the like, to
enhance the crystallinity further. During crystallization utilizing
the thermal annealing, catalytic element such as nickel for
promoting the crystallization of silicon may be added, as described
in detail in Japanese Patent Laid-Open Nos. Hei 6-244103 and Hei
6-244104.
[0047] Subsequently, the silicon film was etched to form islands of
active layers 303, 304 forming TFTs of the driver circuit and
islands of an active layer 305 forming TFTs of the matrix circuit
(pixel TFTs). The active layer 303 was used to form P-channel TFTs,
while the active layer 304 was used to form N-channel TFTs. Then, a
gate insulating film 306 of silicon oxide having a thickness of 500
to 2000 .ANG. was formed by sputtering in an oxygen ambient.
Instead, plasma CVD could be used to form the gate insulating film,
in which case nitrous oxide (nitrogen monoxide) (N.sub.2O) or
mixture of oxygen (O.sub.2) and monosilane (SiH.sub.4) was
advantageously used as a gaseous raw material.
[0048] Then, an aluminum film having a thickness of 2000 to 6000
.ANG. was formed by sputtering techniques over the whole surface of
the substrate. Silicon, scandium, palladium, or other element may
be added to the aluminum to prevent generation of hillocks during
thermal processes conducted later. The aluminum film was etched
into gate electrodes 307, 308, and 309 (FIG. 3(A)).
[0049] Using these gate electrodes 307, 308, and 309 as a mask,
phosphorus ions were introduced into every island of the active
layers 303-305 by a self-aligned ion implantation process. At this
time, phosphine (PH.sub.3) was used as a dopant gas. The dose was
1.times.10.sup.12 to 5.times.10.sup.13 atoms/cm.sup.2. As a result,
weak N-type regions 310, 311, and 312 were formed (FIG. 3(B)).
[0050] Then, a photoresist mask 313 was formed over the active
layer 303 forming the P-channel TFTs. Another photoresist mask 314
was formed parallel to the gate electrode 309 and over those
portions of the active layer 305 forming the pixel TFTs which
terminated in locations spaced 3 .mu.m from the end of the gate
electrode 309.
[0051] Again, phosphorus ions were introduced by an ion
implantation process, using phosphine as a dopant gas. The dose was
1.times.10.sup.14 to 5.times.10.sup.15 atoms/cm.sup.2. As a result,
strong N-type regions (source and drain) 315, 316 were created.
During this implantation step, no phosphorus ions were introduced
into that region 317 of the weak N-type region 312 of the pixel TFT
active layer 305 which was coated with the mask 314. Hence, this
region 317 remained lightly doped N-type (FIG. 3(C)).
[0052] Then, the active layers 304 and 305 for the N-channel TFTs
were coated with a photoresist mask 318. Boron ions were introduced
into the islands 303 by ion implantation technology, using diborane
(B.sub.2H.sub.6) as a dopant gas. The dose was 5.times.10.sup.14 to
8.times.10.sup.15 ions/cm.sup.2. Since the dose of boron was in
excess of the dose of phosphorus (FIG. 5(C)), the previously formed
weak N-type region 310 was converted into a strong P-type region
319.
[0053] As a result of these ion implantation steps, strong N-type
regions (source/drain) 315, 316, strong P-type region
(source/drain) 319, and weak N-type region (lightly doped region)
317 were formed. In the present embodiment, the width x of the
lightly doped region 317 was about 3 .mu.m (FIG. 3(D)).
[0054] Thereafter, a thermal annealing step was carried out at 450
to 850.degree. C. for 0.5 to 3 hours to heal the damage caused by
the ion implantation, to activate the dopants and to recover the
crystallinity of the silicon. Then, a silicon oxide film was formed
as an interlayer insulator 320 to a thickness of 3000 to 6000 .ANG.
by plasma CVD over the whole surface. This interlayer insulator can
be a silicon nitride film or a multilayer film consisting of
silicon oxide and silicon nitride. The interlayer insulator film
320 was etched by wet etching techniques or dry etching techniques
to form contact holes in the source/drain regions.
[0055] Subsequently, an aluminum film or a multilayer film of
titanium and aluminum was formed to a thickness of 2000 to 6000
.ANG. by sputtering techniques. This film was etched to form
electrodes and conductive interconnects 321, 322, 323 of the
peripheral circuit and electrodes and conductive interconnects 324,
325 of the pixel TFTs. This was followed by formation of a
passivation film 326 having a thickness of 1000 to 3000 .ANG.. The
passivation film was made of silicon nitride. The passivation film
was etched to create an interlayer insulator film. An ITO film 327
for forming display electrodes was formed to a thickness of 1200
.ANG. on the interlayer insulator film and patterned in exactly the
same way as the display electrodes of Embodiment 1.
[0056] Then, a silicon nitride film 329 was formed as a passivation
film to a thickness of 1000 to 3000 .ANG. by plasma CVD. An ITO
film (not shown) having a thickness of 1200 .ANG. was formed as a
common electrode on the passivation film. This film was patterned
into the same stripes as the common electrode of Embodiment 1 (FIG.
3(E)).
[0057] A black matrix 328 was formed on the laminate. In this
embodiment, the black matrix 328 was at the highest level. The ITO
layer and the black matrix may be interchanged in level. As the
black matrix material, carbon black particles having an average
diameter of 1000 .ANG. were dispersed in an acrylic-based resinous
material. The obtained solution was applied by spin-coating or
printing techniques. Then, a prebake operation was performed at
100.degree. C. for 2 minutes. The laminate was patterned by a
well-known photolithography method. At this time, the intensity of
the used ultraviolet irradiation was greater (more than 20
mW/cm.sup.2) than the intensity used in normal, patterning
processes. Alternatively, after the formation of the black matrix,
an oxygen-shielding film was made from PVA (polyvinyl alcohol) or
the like. A developer solution obtained by dissolving 2.36% by
weight of TMAH in water was used for development. Consequently, a
black matrix having a thickness of 1 .mu.m could be formed over all
of the peripheral driver circuit, the pixel TFTs, and gate and
source lines. The aperture ratio of the pixel regions was 60%.
[0058] In the present embodiment, the liquid crystal molecules were
made to assume HAN orientation. For this purpose, an orientation
film was formed on each of the aforementioned first and second
substrates. A polyimide film was formed on the first substrate by
well-known spin-coating or DIP techniques. To orient the liquid
crystal molecules parallel to the substrates, the polyimide film on
the TFT substrate was rubbed in a direction parallel to those
portions which corresponded to the teeth of the display and common
electrodes. The silane-coupling agent was applied to the second
substrate. As a result, the liquid crystal molecules on the surface
of the color filter substrate were oriented vertically.
[0059] The TFT substrate and counter substrate fabricated in this
way were stacked on top of each other, thus forming a liquid
crystal panel. The spherical spacers having a diameter of 3 .mu.m
were interposed between the two substrates to make the substrate
spacing uniform over the whole panel plane. An epoxy adhesive was
used to bond together the substrates and to seal the panel. The
patterned sealing adhesive surrounded the pixel regions and the
peripheral driver circuit region. The substrates were cut into a
desired shape. A liquid crystal material was then injected between
the substrates. Nematic liquid crystal ZLI-2293 was used as the
liquid crystal material.
[0060] Then, a biaxial film 104 and a polarizing plate 105 were
successively stuck on the first substrate. The direction 107 of the
optical axis of the polarizing plate made an angle of 45.degree. to
the rubbing direction.
[0061] This liquid crystal display was operated at a voltage of 3
V. An image display could be provided at a contrast of 100, a
response speed of 2 ms, and a wide viewing angle.
Embodiment 3
[0062] In the present embodiment, a color display was provided,
using the liquid crystal display of Embodiment 2. As the pixel
voltage applied across the liquid crystal display was, varied, the
intensity of transmitted light having a wavelength of 554.6 nm was
varied. This is illustrated in FIG. 4. As can be seen from the
graph of FIG. 4, the transmission varied continuously with varying
the voltage. No clear threshold value existed. Variations in color
hues were observed. When no voltage was applied, the device
exhibited a color of yellow-green. When a voltage of 0.5 V was
applied, it exhibited a color of green. When a voltage of 0.9 V was
applied, it exhibited a color of blue. When a voltage of 1.2 V was
applied, it exhibited a color of red.
[0063] A color display was provided, by making use of this
phenomenon and by controlling the pixel voltage applied across the
present embodiment of liquid crystal display. A multicolor display
could be provided at an operating voltage of 3 V with a wide
viewing angle.
[0064] The configuration of the present invention permits
fabrication of a bright display device which needs no backlighting
arrangement. Furthermore, the novel display device requires only
one polarizing plate, unlike the prior art liquid crystal display.
Consequently, a power consumption reduction can be accomplished.
Furthermore, the operating voltage decrease makes it possible to
use batteries as a power supply. This facilitates application to
various portable electrical appliances.
* * * * *