U.S. patent application number 11/126328 was filed with the patent office on 2005-09-15 for liquid crystal display device and manufacturing method thereof.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Ariyoshi, Tomoyuki, Ashizawa, Keiichiro, Kuriyama, Hideki, Nakatani, Mitsuo, Suzuki, Masahiko, Ukisu, Masahiro, Watanabe, Kunihiko.
Application Number | 20050200786 11/126328 |
Document ID | / |
Family ID | 19139910 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050200786 |
Kind Code |
A1 |
Watanabe, Kunihiko ; et
al. |
September 15, 2005 |
Liquid crystal display device and manufacturing method thereof
Abstract
A liquid crystal display device is provided with a pair of
substrates which are arranged to face each other in an opposed
manner and a liquid crystal layer which is sandwiched between main
surfaces of a pair of substrates. On the main surface of one of the
pair of substrates which faces the liquid crystal layer, pixel
regions including switching elements, pixel electrodes which are
connected to the switching elements and a protective film which is
disposed at liquid crystal layer side of the switching elements are
formed. In such a liquid crystal device, the protective film is
formed by laminating the plurality of material layers which include
at least a first material layer and a second material layer which
is arranged closer to the liquid crystal layer side than the first
material layer. Further, the second material layer exhibits
resistivity which is lower than resistivity of the first material
layer and higher than resistivity of silicon or semiconductor
layers which constitute channels of the switching elements. Due to
such a constitution, the image retention which is generated on a
display screen of the liquid crystal display device can be
reduced.
Inventors: |
Watanabe, Kunihiko; (Chiba,
JP) ; Ukisu, Masahiro; (Mobara, JP) ;
Ariyoshi, Tomoyuki; (Mobara, JP) ; Nakatani,
Mitsuo; (Mobara, JP) ; Suzuki, Masahiko;
(Mobara, JP) ; Kuriyama, Hideki; (Mobara, JP)
; Ashizawa, Keiichiro; (Mobara, JP) |
Correspondence
Address: |
REED SMITH LLP
Suite 1400
3110 Fairview Park Drive
Falls Church
VA
22042
US
|
Assignee: |
Hitachi, Ltd.
|
Family ID: |
19139910 |
Appl. No.: |
11/126328 |
Filed: |
May 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11126328 |
May 11, 2005 |
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10274966 |
Oct 22, 2002 |
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6900853 |
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Current U.S.
Class: |
349/138 |
Current CPC
Class: |
G02F 1/133397 20210101;
G02F 1/136227 20130101; G02F 1/134363 20130101 |
Class at
Publication: |
349/138 |
International
Class: |
G02F 001/1333 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 22, 2001 |
JP |
2001-322951 |
Claims
What is claimed:
1. A liquid crystal display device, comprising: a pair of
substrates being arranged opposite to each other; and a liquid
crystal layer being interposed between main surfaces of the pair of
substrates, a plurality of pixel regions each of which includes a
switching element, a pixel electrode connected to the switching
element and at least two films, wherein the at least two films each
includes SiN.
2. A liquid crystal display device according to claim 1, wherein
one film of the at least two films is arranged closer to the liquid
crystal layer and exhibits resistivity which is lower than the
resistivity of each the other.
3. A liquid crystal display device according to claim 1, wherein
one film of the at least two films, which is arranged closer to the
liquid crystal layer and is thinner than the other of the at least
two films near the liquid crystal layer.
4. A liquid crystal display device according to claim 1, wherein
the pixel regions each includes the counter electrode, wherein the
pixel electrode and the counter electrode are disposed so that the
liquid crystal display comprises the IPS type.
5. A liquid crystal display device according to claim 4, wherein
the one film of the at least two films is disposed between the
pixel electrode and the switching.
6. A liquid crystal display device according to claim 1, wherein
the pixel regions each includes the counter electrode, wherein the
pixel electrode and the counter electrode are disposed so that the
liquid crystal display comprises the TN type.
7. A liquid crystal display device according to claim 6, wherein
the one film of the at least two films is disposed between the
pixel electrode and the switching.
8. A liquid crystal display device according to claim 1, wherein
the pixel regions each includes the counter electrode, wherein the
pixel electrode and the counter electrode are disposed so that the
liquid crystal display comprises the VA type.
9. A liquid crystal display device according to claim 8, wherein
the one film of the at least two films is disposed between the
pixel electrode and the switching.
10. A liquid crystal display device according to claim 8, wherein
the films are formed on the other substrate.
11. A liquid crystal display device, comprising: a pair of
substrates being arranged opposite to each other; and a liquid
crystal layer being interposed between main surfaces of the pair of
substrates, a plurality of pixel regions each of which includes a
switching element, a pixel electrode connected to the switching
element and a film including SiN, wherein the composition ratio of
nitrogen with respect to silicon in the closer side of the film
from the liquid crystal layer is higher than the far side.
12. A liquid crystal display device according to claim 11, wherein
the pixel regions each includes the counter electrode, wherein the
pixel electrode and the counter electrode are disposed so that the
liquid crystal display comprises the IPS type.
13. A liquid crystal display device according to claim 12, wherein
the one film of the at least two films is disposed between the
pixel electrode and the switching.
14. A liquid crystal display device according to claim 11, wherein
the pixel regions each includes the counter electrode, wherein the
pixel electrode and the counter electrode are disposed so that the
liquid crystal display comprises the TN type.
15. A liquid crystal display device according to claim 14, wherein
the one film of the at least two films is disposed between the
pixel electrode and the switching.
16. A liquid crystal display device according to claim 11, wherein
the pixel regions each includes the counter electrode, wherein the
pixel electrode and the counter electrode are disposed so that the
liquid crystal display comprises the VA type.
17. A liquid crystal display device according to claim 16, wherein
the one film of the at least two films is disposed between the
pixel electrode and the switching.
18. A liquid crystal display device according to claim 17, wherein
the films are formed on the other substrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid crystal display
device and a manufacturing method thereof, and more particularly to
the structure of a liquid crystal display panel (liquid crystal
display element) suitable for removing undesired charge remaining
on electrodes which apply an electric field to a liquid crystal
layer thereof, for promptly changing over images on a display
screen and for promptly erasing images at the time of completion of
display operation and a method for manufacturing thereof.
[0003] 2. Description of the Related Art
[0004] Liquid crystal display devices have been popularly used as
display devices for personal computers, monitors, television sets
and the like. The liquid crystal display device includes a liquid
crystal display panel which is comprised of a pair of substrates, a
liquid crystal layer sandwiched between a pair of substrates (a
layer made of liquid crystal composition sealed between a pair of
substrates), and a group of electrodes which are formed on a main
surface of at least one of these pair of substrates which faces the
liquid crystal layer in an opposed manner. The display operation of
the liquid crystal display device is performed such that an
electric field applied to the inside of the liquid crystal layer by
a group of electrodes is controlled in response to information to
be displayed so as to modulate the light transmissivity of the
liquid crystal layer. In the above-mentioned main surface of the
substrate of the liquid crystal display panel, a region where the
light transmissivity of the liquid crystal layer is modulated (a
region where the display operation is performed) is referred to as
a display screen, a display region or an effective display
region.
[0005] The liquid crystal display device is classified into two
types, that is, the active matrix type and the passive matrix type
depending on the behavior of liquid crystal molecules in the inside
of the above-mentioned liquid crystal layer during the display
operation and the electrode structure in the inside of the
above-mentioned liquid crystal display panel adapted to the
behavior. The former liquid crystal display device is characterized
in that an active element (switching element) is formed on each
pixel which constitutes the display region. As such an active
element, for example, a thin film transistor (TFT) or a thin film
diode (TFD) is used.
[0006] An example of the active matrix type liquid crystal display
device is explained in conjunction with an equivalent circuit
diagram of a liquid crystal display device using thin film
transistors shown in FIG. 15.
[0007] As shown in FIG. 15, on a display screen 50 (a region
surrounded by a broken line) of the liquid crystal display device,
a plurality of scanning signal lines 10 which extend in the x
direction and are arranged in parallel in the y direction which
intersects the x direction and a plurality of video signal lines
(also referred to as "data lines") 12 which extend in the y
direction and are arranged in parallel in the x direction are
formed. Further, on the display screen 50, a plurality of thin film
transistors TFT each of which is connected to one of a plurality of
scanning signal lines 10 and one of video signal lines 12 are
formed. A plurality of thin film transistors TFT are formed of
so-called field effect type transistors which are switched in
response to a voltage applied to gate electrodes, wherein one of a
plurality of scanning signal lines 10 is connected to each gate
electrode. One of a plurality of video signal lines 12 is connected
to a drain electrode of each thin film transistor TFT, while a
pixel electrode which applies an electric field to the liquid
crystal layer is connected to a source electrode of each thin film
transistor TFT. The pixel electrode is indicated as capacitance
C.sub.LC by being coupled with a counter electrode (also referred
to as a common electrode) which generates an electric field to be
applied to the liquid crystal layer along with the pixel electrode.
A video signal voltage Y1, Y2, Y3, . . . Yend which is generated in
response to an image to be displayed is supplied to a plurality of
thin film transistors TFT (arranged in the y direction in FIG. 15)
to which the video signal lines 12 are connected through the video
signal lines 12, while each thin film transistor TFT supplies the
above-mentioned video signal voltage X1R, X1G, X1B, . . . XendB to
the pixel electrode in accordance with the timing at which the
scanning signal voltage Y1, Y2, Y3, . . . Yend is applied to the
gate electrode through one of the scanning signal lines 10.
Accordingly, on the display screen 50 of the liquid crystal display
device, the pixels PIX each of which includes one of the plurality
of thin film transistors TFT and the capacitance C.sub.LC to which
the video signal voltage is applied through the thin film
transistor TFT are formed two-dimensionally. Here, as described
above, as viewed from the gate electrode (a channel layer in which
the movement of charge is controlled due to the gate electrode) of
the thin film transistor TFT, the electrode arranged at the video
signal line 12 side is set as the drain electrode and the electrode
at the pixel electrode (capacitance C.sub.LC) side as the source
electrode. However, the naming of these electrodes can be exchanged
based on the relative relationship between potentials of both
electrodes. In this specification, for the sake of convenience, the
electrode of the thin film transistor TFT at the video signal line
12 side is referred to as the drain electrode and the electrode of
the thin film transistor TFT at the pixel electrode side is
referred to as the source electrode.
[0008] On the other hand, to the counter electrode which
constitutes the capacitance C.sub.LC together with the pixel
electrode, the reference voltage Vcom is supplied through a
reference voltage line 11. Depending on the mode of applying
voltage to the liquid crystal layer (modulation of optical
transmissivity of the liquid crystal layer), the counter electrodes
and the reference voltage lines 11 are formed on either a substrate
(also referred to as a TFT substrate) on which the above-mentioned
scanning signal lines 10, the video signal lines 12, the thin film
transistors TFT and the pixel electrodes are formed or another
substrate which faces the TFT substrate in an opposed manner while
sandwiching a liquid crystal layer therebetween. Since the former
liquid crystal display device generates an electric field in a
liquid crystal layer along a main surface of the TFT substrate, the
liquid crystal display device is referred to as an
in-planes-switching (abbreviated as IPS) type liquid crystal
display device or a lateral electric field type liquid crystal
display device. On the other hand, since the latter liquid crystal
display device generates an electric field in a liquid crystal
layer along the thickness direction, the device is also referred to
as a vertical electric field type liquid crystal display device.
Here, in the vertical electric field type liquid crystal display
device, there may be a case in which one counter electrode
corresponds to a plurality of pixel electrodes (for example, all
pixel electrodes arranged within the above-mentioned display screen
SCR) and the above-mentioned capacitance C.sub.LC is formed for
every pixel or the counter electrode also performs a function of
the reference voltage line 11 on a main surface of another
substrate which faces the TFT substrate in an opposed manner. Such
a vertical electric field type structure is applicable to a liquid
crystal display device using twisted nematic liquid crystal which
gradually twists a long axis direction of liquid crystal molecules
in the inside of the liquid crystal layer from the TFT substrate to
the substrate which faces the TFT substrate in an opposed manner (a
so-called TN type liquid crystal display device) and a so-called
vertically aligned type (VA type) liquid crystal display device
which aligns a long axis of liquid crystal modules with respect to
the main surface of the TFT substrate with an inclination of a
given angle.
[0009] The above-mentioned scanning signal lines 10 are
respectively electrically connected to output terminals of a
driving circuit (a vertical scanning circuit or also referred to as
a gate driver) V-DRV and receive the scanning signals Y1, Y2, Y3, .
. . Yend. The above-mentioned video signal lines 12 are
respectively connected to output terminals of a driving circuit (a
video signal driving circuit or also referred to as a drain driver)
H-DRV different from the driving circuit V-DRV and receive the
video signals X1R, X1G, X1B, . . . XendB. Data on images to be
displayed on the liquid crystal display device is inputted to a
control circuit (also referred to as a timing converter) TCON from
the outside and the scanning signals and the video signals
(possibly including gray scale signals) which are suitable for
operation of the liquid crystal display device are generated.
[0010] Further, the pixel PIX shown in FIG. 15 is also provided
with another capacitance Cad besides the above-mentioned
capacitance C.sub.LC. The capacitance Cadd is also referred to as
an additional capacitance or a storage capacitance and is provided
for holding a charge supplied to the pixel electrode of each pixel
in response to the video signal until a point of time that a charge
corresponding to a next video signal is supplied to the pixel
electrode.
SUMMARY OF THE INVENTION
[0011] However, as described above, in the active matrix type
liquid crystal display device, it is also necessary to form the
active elements (switching elements) such as the field effect type
transistors or diodes on the substrate on which the pixel
electrodes are formed. Accordingly, an undulation is formed on the
main surface of the substrate due to the mounting of the active
elements.
[0012] On the other hand, to modulate the light transmissivity of
the liquid crystal layer in response to images, it is important to
satisfy a so-called initial orientation condition that the liquid
crystal molecules in the liquid crystal layer are oriented in a
given mode with respect to an uppermost surface (a surface which
faces the liquid crystal layer) of the substrate on which the pixel
electrodes are formed. To satisfy this initial orientation
condition, for example, it is necessary to apply a mechanical or
optical treatment to a main surface of the orientation film formed
on the uppermost surface of the substrate and to adjust the
direction and the inclination of the long axis direction of the
liquid crystal molecules with respect to the main surface of the
substrate on which the orientation film is formed. Accordingly, it
is ideal that the main surface of the orientation film is leveled
to an extent which is comparable with the leveling of the main
surface of the substrate. To cope with such a demand, in an actual
manufacturing, an insulation film is formed on the substrate on
which the active elements are formed such that the insulation film
covers the active elements so as to level the undulation on the
uppermost surface of the substrate (the main surface of the film
which is formed on the main surface of the substrate and comes into
contact with the liquid crystal layer) to an extent that the
above-mentioned initial orientation condition is satisfied (such
that at least stepped portions produced by active elements can be
reduced). Such an insulation film is also referred to as a
protective film, a leveling layer or a passivation layer.
[0013] However, inventors of the present invention have found
problem on a so-called "image retention" that when an operational
power supply of the liquid crystal display device is cut off, an
image which has been displayed until now on a display screen
slightly remains. The inventors also have considered that this
image retention is partially attributed to the above-mentioned
insulation film which covers the active elements.
[0014] The procedure through which the inventors have arrived at
such an idea is as follows.
[0015] A voltage which is applied to the liquid crystal layer
during the image display operation (also referred to as liquid
crystal driving) of the liquid crystal display device is also
applied to the insulation film formed on the substrate on which the
active elements are formed (hereinafter referred to as "TFT
substrate") and a charge is stored on an upper surface and a lower
surface of the insulation film due to a dielectric constant of the
insulation film. On the TFT substrate on which the field effect
type transistors are formed as the active elements, a so-called
gate insulation film which performs an insulation between the
above-mentioned gate electrodes and a channel layer and the
above-mentioned protective film which covers the transistors are
formed. Since the gate insulation film is provided with the gate
electrodes and the scanning signal lines which come into contact
with one surface thereof and the source electrodes, drain
electrodes and video signal lines which come into contact with
other surface thereof, it is easy for the gate insulation film to
sweep out the stored charge. However, the protective film brings
only one surface thereof into contact with the source electrodes,
the drain electrodes, the video signal lines or the gate electrodes
and the scanning signal lines and brings the other surface thereof
into contact with only materials such as the orientation film and
the liquid crystal display layer which hardly allow the flow of
electricity therethrough. Accordingly, the charge stored in the
above-mentioned protective film is not discharged even when the
liquid crystal display device is turned off and remains in the
protective film for a considerable time and hence, an offset
voltage is applied to the liquid crystal layer due to this residual
charge whereby the above-mentioned image retention occurs on the
display screen. The phenomenon in which even after the power supply
to the liquid crystal display device is cut off, the image written
before cutting off the power supply is retained on the display
screen for a fixed time is also referred to as "sticking" of
image.
[0016] The inventors of the present invention also have found that
the image retention is liable to occur easily under following
situations.
[0017] (1) the dielectric film such as the gate insulation film,
the protective film or the like which is formed on the TFT
substrate is liable to relatively easily store the charge (for
example, such a film having a small film thickness and made of
material having a high dielectric constant).
[0018] (2) The driving voltage of the liquid crystal (voltage
applied to the liquid crystal layer) is relatively high.
[0019] (3) The resistivity of the liquid crystal material per se is
relatively large.
[0020] Based on such a finding, the inventors have considered that,
to solve the above-mentioned image retention, it is necessary to
set the shape and physical properties of the dielectric film such
as the gate insulation film, the protective film or the like formed
on the TFT substrate such that the charge hardly remains on the
dielectric film. For example, it is preferable to form the
dielectric film such that the film thickness of the dielectric film
is increased or the relative dielectric constant of the dielectric
film is reduced.
[0021] One solution to satisfy these requests is disclosed in
Japanese Patent 2938521, for example. This publication discloses an
invention which can suppress the luminance irregularities generated
in a liquid crystal display device wherein a silicon nitride (SiN)
film and an amorphous silicon (a-Si) film are sequentially
laminated to a thin film transistor to form a protective film and,
thereafter, a given potential is applied to the amorphous silicon
film which also constitutes a conductive material under the
irradiation of light so as to make the charge storing state in a
plane of the protective film uniform. This publication teaches that
by holding the whole area of the surface of the protective film at
a fixed potential, the above-mentioned luminance irregularities can
be suppressed even after the image display operation is performed
for a long time.
[0022] However, the inventors of the present invention have faced
following problems in applying the invention disclosed in the
above-mentioned patent publication to the above-mentioned
suppression of image retention.
[0023] First of all, in the in-plane-switching type (lateral
electric field type) liquid crystal display device, in view of
achieving the initial orientation of the liquid crystal molecules
in the liquid crystal layer, it is desirable to cover both of the
pixel electrodes and the counter electrodes which generate the
electric field for driving the liquid crystal with the protective
film. However, the electric field which is generated between the
pixel electrode and the counter electrode inevitably enters the
liquid crystal layer through the protective film and hence, when
the protective film includes a thin film made of the conductive
material as described above, the electric field is consumed in the
generation of current in the inside of the film. Accordingly, the
electric field generated between the pixel electrode and the
counter electrode substantially cannot enter the liquid crystal
layer and hence, it is not exaggerating to state that the
orientation of the liquid crystal molecules present in the liquid
crystal layer cannot be controlled (the light transmissivity of
liquid crystal layer cannot be modulated).
[0024] To confirm the advantageous effect of the invention
disclosed in the above-mentioned publication, an amorphous silicon
film which exhibits the conductive property and has a film
thickness of 20 nm is formed on a protective film of a conventional
in-plane-switching type liquid crystal display device. As a result,
it is found that such a liquid crystal display device cannot drive
the liquid crystal and it is impossible to perform even the
evaluation of the quality of images.
[0025] On the other hand, also with respect to the active matrix
type liquid crystal display device in general, the application of
the invention described in the publication to such a liquid crystal
display device gives rise to following drawbacks. For example, in a
thin film transistor which is referred to as a bottom gate type in
which a gate electrode is formed on a main surface side of a
substrate and a semiconductor film which constitutes a channel
layer is arranged on the gate electrode, the above-mentioned
amorphous silicon film faces the semiconductor film (the channel
layer of the thin film transistor) through an insulation film which
forms the protective film together with the amorphous silicon film.
Accordingly, the electric field is applied to the semiconductor
film from both of the gate electrode and the amorphous silicon film
which are held at a fixed potential so that even when the voltage
of the gate electrode is set lower than a threshold value voltage
of the thin film transistor, the semiconductor film holds a state
in which the electric field is applied thereto for some time. As a
result, there arise new drawbacks exemplified by the increase of
cut-off current of the thin film transistor and the occurrence of
image retention attributed to such an increase of cut-off current.
Further, also with respect to the active matrix type liquid crystal
display device which uses the top-gate type thin film transistors
or the thin film diodes as the switching elements in which the
semiconductor films (channel layers) are formed on the substrate
main surface side and the gate electrodes are arranged on the
semiconductor films, it has been pointed out that erroneous
operations of the switching elements are induced due to the
parasitic capacitance generated between the thin film transistor or
the thin film diode and the above-mentioned amorphous silicon film
which is held at a fixed potential.
[0026] To summarize the above-mentioned reviews, although efforts
have been made to solve the phenomenon that the charge stored in
the protective film which is the cause of the occurrence of image
retention remains in the protective film for a long time by holding
the in-plane of the protective film at a fixed potential, the
result of the effort is that although no luminance irregularities
are generated, the image retention remains as it is. Accordingly,
even with the use of the method taught in the above-mentioned
patent publication in which the conductive film is applied to the
insulating protective film, it is extremely difficult to obtain
both of the suppression of image retention and the luminance
irregularities and the applying of cut-off current to the thin film
transistors and the applying of driving electric field to the
liquid crystal layer.
[0027] There may be an idea to solve such an image retention
problem only by improving the gate insulation film. However, since
the insulation film affects the electric characteristics of the
thin film transistors which constitute the switching elements of
the liquid crystal display device, the increase of the film
thickness or the reduction of the electric capacitance which is
obtained by lowering the relative dielectric constant brings about
drawbacks to the contrary. Further, although the protective film
receives less restrictions with respect to the shape and physical
properties compared to the gate insulation film, the protective
film affects the turn-off current of the thin film transistor or
there exist restrictions on the treatment capacity of a plasma CVD
device which forms the protective film. Accordingly, it is not
realistic to adjust the film thickness and the relative dielectric
constant of the protective film only for the purpose of alleviation
of image retention in the same manner as the gate insulation
film.
[0028] Accordingly, it is one of objects of the present invention
to solve the trade-off relationship between the above-mentioned
suppression of image retention and the luminance irregularities as
well as the trade-off relationship between the electric properties
of the switching element and the efficiency of applying electric
field to the liquid crystal layer in the active matrix type liquid
crystal display device.
[0029] In view of the above-mentioned object, the present invention
provides liquid crystal display devices as exemplified as
follows.
[0030] According to one aspect of the present invention, in a
liquid crystal display device which is provided with a pair of
substrates which are arranged to face each other in an opposed
manner and a liquid crystal layer which is sandwiched between main
surfaces of a pair of substrates, on the main surface of one of the
pair of substrates which faces the liquid crystal layer, pixel
regions including switching elements and pixel electrodes which are
connected to the switching elements and a protective film which is
formed by laminating a plurality of material layers at the liquid
crystal layer side of the switching elements are formed. The
plurality of material layers include at least a first layer
(material layer) and a second layer (material layer) which is
arranged closer to the liquid crystal layer side than the first
layer (above the first layer as viewed from one main surface of the
pair of substrates). Further, the second layer exhibits resistivity
which is lower than resistivity of the first layer and higher than
resistivity of silicon. Here, silicon is exemplified a so-called
semiconductor material which forms channels of the switching
elements provided to the liquid crystal display device such as
single crystal silicon, poly-crystalline silicon, amorphous
silicon. Accordingly, the second layer included in the
above-mentioned protective film may be characterized in that the
second layer exhibits resistivity which is lower than resistivity
of the first layer included in the protective film and is higher
than resistivity of the channel layers of switching elements.
[0031] These first layer and second layer are also characterized in
that with the irradiation of light, the first layer exhibits the
resistivity of not less than 1.0.times.10.sup.13 .OMEGA.cm and the
second layer exhibits the resistivity of not more than
1.0.times.10.sup.11 .OMEGA.cm. Such photo conductive property of
the protective film can be measured by irradiating light having
illuminance of equal to or more than 500 kLx (kilo-lux) to the
first layer and the second layer or by irradiating the light to the
protective film structure from a side opposite to the main surface
of the substrate on which these layers are formed.
[0032] As described above, to describe the protective film
structure according to the present invention which sequentially
laminates the first layer and the second layer in this order on the
main surface of the substrate on which the switching elements are
formed in view of the relative dielectric constant, the second
layer has the higher relative dielectric constant than the first
layer and the second material layer assumes the relative dielectric
constant of not less than 7.5.
[0033] Further, the liquid crystal display device is characterized
in that when the protective film which is formed at the liquid
crystal layer side of the above-mentioned switching element is
formed of material: Si.sub.xN.sub.yX.sub.z (X being a general term
of other constitutional element) which contains at least silicon
and nitride, the composition ratio of nitrogen relative to silicon
(y/x) of the second layer is smaller than that of the first
material layer.
[0034] It is preferable that the first layer and the second layer
which constitute the above-mentioned protective film further
satisfy at least one of following modes.
[0035] In one mode, the resistivity of the second layer is set to a
value less than {fraction (1/100)} of the resistivity of the first
layer.
[0036] In another mode, the resistivity of the second layer is set
lower than the resistivity of the first layer.
[0037] In still another mode, a thickness of the second layer is
set to a value not more than 1/2 of a total thickness of the
protective film and the thickness of the second layer is preferably
set to a value not less than 10 nm.
[0038] Although the novel characteristics of the protective film
which covers the switching elements formed on the substrate in the
liquid crystal display device of the present invention have been
described by focusing on the cross-sectional structure, to grasp
the features of the protective film as a plan view of the substrate
at the liquid crystal layer side, they are as follows.
[0039] One feature is that in a liquid crystal display device in
which the device is provided with a pair of substrates which are
arranged to face each other in an opposed manner and a liquid
crystal layer which is sandwiched between main surfaces of a pair
of substrates, and on the main surface of one of the pair of
substrates which faces the liquid crystal layer, pixel regions
including switching elements and pixel electrodes which are
connected to the switching elements and a protective film which
comes into contact with liquid crystal layer sides of the switching
elements are formed, when light is irradiated to a surface of the
protective film which faces the liquid crystal layer, the
resistivity of the protective film is reduced to a value not more
than {fraction (1/100)} of the resistivity of the protective film
when light is not irradiated to the surface of the protective film.
Here, the light irradiation means to irradiate light having
illuminance of not less than 500 kLx, for example, to the upper
surface of the protective film.
[0040] Another feature is that in a liquid crystal display device
in which the device is provided with a pair of substrates which are
arranged to face each other in an opposed manner and a liquid
crystal layer which is sandwiched between main surfaces of a pair
of substrates, and on the main surface of one of the pair of
substrates which faces the liquid crystal layer, pixel regions
including switching elements and pixel electrodes which are
connected to the switching elements and a protective film which
comes into contact with liquid crystal layer sides of the switching
elements are formed, a surface of the protective film which faces
the liquid crystal layer in an opposed manner exhibits the dark
resistivity which falls in a range of 1.0.times.10.sup.13 .OMEGA.cm
to 1.0.times.10.sup.15 .OMEGA.cm and exhibits the resistivity of
1.0.times.10.sup.9 .OMEGA.cm to 1.0.times.10.sup.11 .OMEGA.cm when
light having illuminance of 500 kilo lux is irradiated to the
surface of the protective film.
[0041] Still another feature is that in a liquid crystal display
device in which the device is provided with a pair of substrates
which are arranged to face each other in an opposed manner and a
liquid crystal layer which is sandwiched between main surfaces of a
pair of substrates, and on the main surface of one of the pair of
substrates which faces the liquid crystal layer, pixel regions
including switching elements and pixel electrodes which are
connected to the switching elements and a protective film which
comes into contact with liquid crystal layer sides of the switching
elements are formed, a surface of the protective film which faces
the liquid crystal layer in an opposed manner exhibits the relative
dielectric constant of not less than 7.5.
[0042] Further, when the protective film is formed of material
containing at least silicon and nitrogen: Si.sub.xN.sub.yX.sub.z (X
being a general term of other constitutional element), the liquid
crystal display device is characterized in that the device is
provided with a pair of substrates which are arranged to face each
other in an opposed manner and a liquid crystal layer which is
sandwiched between main surfaces of a pair of substrates, and on
the main surface of one of the pair of substrates which faces the
liquid crystal layer, pixel regions including switching elements
and pixel electrodes which are connected to the switching elements
and a protective film which comes into contact with the channel
layers of the switching elements are formed, the composition ratio
of nitrogen relative to silicon (y/x ratio) of the surface of the
protective film which faces the liquid crystal layer assumes a
value which is larger than 0 and falls in a range below 1.0.
[0043] In a liquid crystal display device having at least one of
the above-mentioned features, it is preferable to form an
orientation film on the liquid crystal layer side of the protective
film. Further, when the liquid crystal display device is
constituted such that, as in the case of the twisted nematic (TN)
type liquid crystal display device, the counter electrodes which
apply an electric field to the liquid crystal layer along with the
pixel electrodes are formed on the other of the pair of substrates,
it is preferable to form the pixel electrodes between the
protective film and the liquid crystal layer (more preferably
between the protective film and the orientation film).
[0044] On the other hand, when the liquid crystal display device
having at least one of the above-mentioned features is constituted
such that, as in the case of the in-plane-switching type liquid
crystal display device, the pixel electrodes and the counter
electrodes are formed on one of the pair of substrates, it is
preferable to form the pixel electrodes at a side opposite to the
liquid crystal layer with respect to the protective layer (a main
surface side of one of a pair of substrates).
[0045] According to the above-mentioned liquid crystal display
device of the present invention, in a manufacturing method which
includes a first step in which a plurality of switching elements
are formed on a main surface of one of a pair of substrates and a
second step in which a protective film is formed on upper portions
of the switching elements by a chemical vapor deposition method
using a plasma gas in which a plurality of gases are introduced, at
least one of following features is reflected in the second step so
that the resistivity of the second layer which constitutes the
protective film can be set lower than the resistivity of the first
layer which constitutes the protective film.
[0046] One feature is that the first material layer having at least
the first composition is formed by vapor deposition and,
thereafter, the second material layer having at least the second
composition which is different from the first composition is formed
by vapor deposition by changing the introduction ratio of a
plurality of raw material gases.
[0047] Further, another feature is that at least the
above-mentioned first material layer is formed by vapor deposition
and, thereafter, the second material layer is formed by vapor
deposition by setting electric power supplied to the plasma lower
than electric power at the time of forming the first material layer
by vapor deposition.
[0048] In the second step, when the first gas which contains
silicon (for example, monosilane) and the second gas which contains
nitrogen (for example, ammonia) are used as the above-mentioned
plurality of raw material gases, it is preferable to set an
introduction amount ratio of the second gas relative to the first
gas to a value larger than 1.0 at the time of forming the first
material layer by vapor deposition and to a value which falls in a
range larger than 0 and not more than 1.0 at the time of forming
the second material layer by vapor deposition.
[0049] Further, in the second step, the high frequency electric
power introduced to plasma at the time of forming the second
material layer by vapor deposition is suppressed to a value lower
than high frequency electric power at the time of forming the first
material layer by vapor deposition. For example, it is preferable
to set the high frequency electric power to not more than 0.2
W/cm.sup.2.
[0050] The manner of operation and advantageous effects which have
been described heretofore and the detail of preferred embodiments
will become apparent from the explanation made hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is a plan view showing one of a plurality of pixels
formed on a main surface of one of a pair of substrates which
constitute an in-plane-switching type liquid crystal display device
according to the first embodiment of the present invention in an
enlarged form;
[0052] FIG. 2 is a cross-sectional view obtained by cutting a TFT
substrate along a chain line II-II' in FIG. 1;
[0053] FIG. 3 is a cross-sectional view schematically showing a
vapor deposition apparatus served for formation of a silicon
nitride film using a plasma CVD;
[0054] FIG. 4 is an explanatory view which schematically shows a
liquid crystal display panel in cross section;
[0055] FIG. 5 is an exploded perspective view of the liquid crystal
display device which incorporates a liquid crystal display panel 21
therein;
[0056] FIG. 6 is a graph showing a result of evaluation of image
retention alleviation characteristics of the in-plane switching
type liquid crystal display device of the embodiment 1 of the
present invention obtained by the relative flicker intensity
measurement at backlight luminance: 250 Cd/m.sup.2;
[0057] FIG. 7 is a graph showing a result of evaluation of image
retention alleviation characteristics of the in-plane switching
type liquid crystal display device of the embodiment 1 of the
present invention obtained by the relative flicker intensity
measurement at backlight luminance: 250 Cd/m.sup.2;
[0058] FIG. 8 is a graph showing the correlation between a growth
condition (high frequency electric power applied to plasma), dark
resistivity and photo resistivity of an amorphous silicon nitride
film 8a formed on a liquid crystal layer side of a protective film
8 according to the embodiment 1 of the present invention;
[0059] FIG. 9 is a graph showing the correlation between a growth
condition (supply ratio of raw material gases), dark resistivity
and photo resistivity of an amorphous silicon nitride film 8a
formed on a liquid crystal layer side of a protective film 8
according to the embodiment 1 of the present invention;
[0060] FIG. 10 is a view showing one example of infrared absorption
spectra of an amorphous silicon nitride film 8a (lower side) formed
on a liquid crystal layer side of a protective film 8 according to
the embodiment 1 of the present invention and of an amorphous
silicon nitride film 8b (upper side) formed on a switching element
side of the protective film 8 according to the embodiment 1 of the
present invention;
[0061] FIG. 11 is a plan view showing one of a plurality of pixels
formed on a TN type liquid crystal display device (liquid crystal
display panel) according to the second embodiment of the present
invention in an enlarged form;
[0062] FIG. 12 is a cross-sectional view obtained by cutting a TFT
substrate along a chain line XII-XII' in FIG. 11;
[0063] FIG. 13 shows one of video signal line terminals suitable
for the liquid crystal display device according to the second
embodiment of the present invention, wherein FIG. 13A is a plan
view showing a planar structure and FIG. 13B is a cross-sectional
view taken along a line B-B' in FIG. 13A;
[0064] FIG. 14 is a cross-sectional view of the pixel and the
vicinity of the pixel formed in a VA type liquid crystal display
device (liquid crystal display panel) according to the third
embodiment of the present invention; and
[0065] FIG. 15 is an equivalent circuit diagram of the liquid
crystal display device using thin film transistors.
DETAILED DESCRIPTION
[0066] Hereinafter, the specific embodiment of the present
invention is explained in conjunction with drawings relating to the
embodiment. In the drawings which are referred to the explanation
described hereinafter, parts having identical functions are given
same symbols and the repeated explanation is omitted.
First Embodiment
[0067] The liquid crystal display device of the first embodiment
according to the present invention is explained by illustrating the
structure of a TFT substrate used in the above-mentioned
in-plane-switching type liquid crystal display device and
manufacturing steps thereof.
[0068] FIG. 1 is a plan view showing one of a plurality of pixels
formed on a main surface (which faces a liquid crystal layer) of
TFT substrate which is used in the in-plane-switching type liquid
crystal display device in an enlarged form and FIG. 2 is a
cross-sectional view obtained by cutting the TFT substrate along a
chain line C-C' in FIG. 1.
[0069] FIG. 1 shows pixels which include thin film transistors TFT
each of which has a gate electrode 1a which is branched from a
scanning signal line 1 shown at a lower side of the drawing, a
semiconductor layer (channel layer) 4 which covers the gate
electrode 1a, a drain electrode 2a which is branched from a video
signal line 2 shown at a left column of the drawing, and a source
electrode 6a which is integrally formed with a pixel electrode 6,
the above-mentioned pixel electrodes 6 which extend in the y
direction and are arranged in parallel in the x direction, and
counter electrodes 3a each of which is branched in the y direction
from a reference voltage line 3 shown at the upper side of the
drawing and arranged in parallel in a spaced-apart manner from the
pixel electrode along the x direction. In such a pixel, a region
through which light transmits is defined by an opening formed in a
light shielding film (hereinafter referred to as a black matrix)
which is formed on the other substrate (also referred to as a
counter substrate or a color filter substrate) which faces the TFT
substrate in an opposed manner while sandwiching the liquid crystal
layer. In FIG. 1, the opening formed in the black matrix is a
region which is surrounded by a broken line BMO which indicates a
brim of the opening. In the cross section of the TFT substrate
shown in FIG. 2, the liquid crystal layer and the other substrate
which faces the TFT substrate in an opposed manner are omitted. On
a main surface of the TFT substrate, a plurality of pixels having
the same structure as the above-mentioned pixel shown at the center
of FIG. 1 are arranged two dimensionally. One example of the mode
or arrangement of a plurality of pixels is shown in FIG. 1 such
that eight other pixels surround one center pixel (the eight pixels
being shown partially).
[0070] First of all, the manufacturing steps of the TFT substrate
according to this embodiment are explained in conjunction with FIG.
1 and FIG. 2. Many of these manufacturing steps adopt techniques
which are adopted by a manufacturing method for manufacturing an
existing liquid crystal display substrate or a liquid crystal
display panel and hence, they are explained briefly. Further, the
characteristics manufacturing steps for manufacturing the liquid
crystal display device according to the present invention are
explained by suitably adding drawing and in conjunction with these
drawings.
[0071] Step 1: As the TFT substrate 11, a glass substrate having a
size of 370 mm.times.470 mm.times.1.1 mm is prepared. Although the
pixels shown in FIG. 1 are formed on a main surface of the glass
substrate two dimensionally, the main surface merely constitutes a
simple glass surface at a stage of step 1. With the use of such a
glass substrate as the TFT substrate 11, a liquid crystal display
panel (also referred to as a liquid crystal cell) having a display
screen of 38 cm in an orthogonal direction can be manufactured.
[0072] Step 2: After cleaning the main surface (the upper surface
in FIG. 2) of the glass substrate 11, a first conductive film made
of metal, an alloy or the like is formed on the main surface by a
sputtering method, for example. In this embodiment, a chromium (Cr)
thin film having a thickness of 200 nm is formed by a sputtering
method. A photo resist (a photosensitive organic material) is
applied onto the chromium thin film and light is irradiated to the
photo resist through a photo mask (a light shielding plate having a
plurality of slits) so as to partially photosensitize the photo
resist. Thereafter, the photosensitized portions are selectively
removed by performing a developing processing so that the chromium
thin film is exposed partially through the openings formed in the
photo resist. All of the above-mentioned processing in step 2 are
collectively referred to as a photolithography method. A plurality
of slits formed in the photo mask have shapes which correspond to
respective profiles of the above-mentioned scanning signal lines 1
(also referred to as gate bus lines), gate electrodes 1a, reference
voltage lines 3 (also referred to as counter voltage signal lines,
common bus lines in the in-line-switching type liquid crystal
display device) and counter electrodes 3a. Further, when a photo
resist which is formed at an outside (a peripheral portion) of a
portion corresponding to a display region of the TFT substrate is
exposed using the above-mentioned photo mask, it is also possible
to provide slits which correspond to terminals (not shown in the
drawing) provided to the peripheral portion of the photo mask for
connecting the TFT substrate and an external circuit and a
protective circuit (not shown in the drawing) for protecting the
thin film transistor in the inside of the display region from the
insulation breakdown derived from the static electricity or the
like. Here, depending on the photo resist and chemicals which
develops the photo resist, portions of the photo resist which are
not photosensitized are removed by developing processing, such a
difference does not obstruct the manufacturing of the TFT substrate
according to the present invention.
[0073] Step 3: At a stage that step 2 is completed, on an upper
surface of the first conductive film made of chromium, the photo
resist having shapes similar to the above-mentioned scanning signal
lines, the gate electrodes and the like remain. By treating the
upper surface of the first conductive film using chemicals such as
a nitric acid second cerium ammonium aqueous solution (an etchant
which etches chromium), for example, the first conductive film
(chromium thin film) which is not covered with the resist is
removed from the main surface of the TFT substrate 11. This
treatment is referred to as an etching method. After forming the
scanning signal lines 2, the gate electrodes 2a, the reference
voltage lines 3, the counter electrodes 3a and the like made of the
first conductive film on the TFT substrate 11, the photo resists
remaining on the upper surfaces thereof are removed using nitric
acid or the like, for example, and thereafter, these parts are
cleaned with deionized water (water filtered by ion exchange
resin).
[0074] Step 4: On an upper surface of the TFT substrate 11 on which
the pattern of the lines (scanning signal lines and the like) and
the electrodes (gate electrodes and the like) formed of the first
conductive film are formed, the insulation film 5 and the
semiconductor film 4 are laminated in this order. The insulation
film 5 also constitutes a gate insulation film in the thin film
transistor which will be explained later. In this embodiment, the
insulation film 5 is formed of an amorphous silicon nitride film
(SiN.sub.X) and the semiconductor film 4 is formed of an amorphous
silicon film (a-Si). Since the insulation film 5 and the
semiconductor film 4 are both made of materials which contain
silicon, in this embodiment these films are continuously grown on
the main surface of the TFT substrate using a chemical vapor
deposition method (also referred to as a CVD method). This
embodiment adopts a plasma enhanced CVD method in which plasma is
generated using raw material gases for the insulation film 5 and
the semiconductor film 4 and an upper surface (working surface) of
the TFT substrate 11 is made to face this plasma in an opposed
manner. An apparatus which is served for the plasma enhanced CVD
method is schematically shown in FIG. 3. Although the detail of the
apparatus will be described later, the TFT substrate is placed on a
holder HLDR as a specimen SPCM.
[0075] Immediately before the completion of the continuous growth
of the amorphous silicon nitride film and the silicon film obtained
by the plasma enhanced CVD method, an impurity element which gives
the conductive property to the semiconductor film 4 is added to the
raw material gases and, thereafter, the vapor deposition is
performed so that a region 4a of n.sup.+ type having high electron
density is formed in the vicinity of an upper surface of the
semiconductor film 4. In this embodiment, as the impurity element,
phosphorous (P) which is one of elements of Group V which having
large valence electrons compared to silicon (Si) of Group IV is
used as a dopant. This n.sup.+ type semiconductor layer 4a connects
the second metal film which is formed subsequently with the
semiconductor layer 4 at a low electric resistance (bringing the
second metal film and the semiconductor layer 4 into an ohmic
contact). In this embodiment, the thickness of portions of the
insulation film 5 and the semiconductor film 4 to which the
impurity is not intentionally introduced (indicated as the
semiconductor film 4 in FIG. 2) and the thickness of portions of
the semiconductor film to which the impurity is intentionally
introduced (indicated by the n.sup.+ type semiconductor layer 4a in
FIG. 2) are respectively set to 400 nm, 250 nm and 50 nm. In place
of forming the n.sup.+ type semiconductor layer 4a at the time of
forming the semiconductor film by vapor deposition using the CVD or
the like, for example, it is possible to adopt the ion implantation
in which ions of a dopant are implanted into the semiconductor film
after growth or a method which moves atoms of metal or alloy into
the semiconductor film by heat treatment or the like after the
second conductive film made of metal or alloy is bonded to the
semiconductor film. Further, with respect to an image obtained by a
transmission electron microscope, it is difficult to discriminate
the region in the semiconductor film to which the impurity is not
doped intentionally (also referred to as an intrinsic semiconductor
region, hereinafter referred to as an i-type region for the sake of
convenience) and a region of the semiconductor film to which the
impurity is intentionally doped (the above-mentioned n.sup.+ type
region) and hence, the semiconductor film 4 and the n.sup.+ type
semiconductor layer 4a in FIG. 2 are observed as one semiconductor
layer.
[0076] Step 5: After completing the formation of the insulation
film 5 and the semiconductor film 4, a photo resist is patterned on
the semiconductor film 4 using the previously mentioned
photolithography method. The pattern is constituted of "islands" of
photo resist which are respectively left on upper portions of the
gate electrodes 1a and upper portions of the portions where the
scanning signal line 1 and the reference voltage line 3 cross the
video signal line 2.
[0077] Step 6: Subsequently, the semiconductor film 4 having an
upper surface on which the above-mentioned islands of photo resist
are not formed is removed by a dry etching method which uses sulfur
hexafluoride (SF.sub.6) and hydrogen chloride (HCl) thus leaving
the semiconductor film 4 (including the above-mentioned n.sup.+
type region) which constitutes the channel layer of the thin film
transistor on the gate electrode 1a and the semiconductor film 4
which prevents the disconnection of the video signal line 2 on
respective portions where the scanning signal line 1 and the
reference voltage line 3 cross the video signal line 2 which will
be explained later. Thereafter, the above-mentioned islands formed
of resist are removed in accordance with step 3. Here, the
insulation film 5 remains on at least the whole area of the display
region on the main surface of the TFT substrate 11 at this
stage.
[0078] Step 7: Subsequently, the second conductive film made of
metal or alloy is formed such that the second conductive film
covers the above-mentioned semiconductor film 4 (including n.sup.+
type region 4a) and the above-mentioned insulation film 5 by a
sputtering method. In this embodiment, the second conductive film
is formed of a thin film made of chromium having a thickness of 200
nm. The photolithography method described in step 2 and the etching
method described in step 3 are applied to the second conductive
film made of chromium thus forming the video signal lines (also
referred to as drain bus lines) 2, the drain electrode 2a, the
pixel electrodes 6 and the source electrodes 6a. In a series of
these steps, as described in conjunction with step 2, terminals
(not shown in the drawing) which are connected to an external
circuit, and a pattern (not shown in the drawing) of the
above-mentioned protective circuit and the like are formed on a
periphery of a photo mask and these components are formed together
with the video signal lines 2 and the like by the etching method.
In this step, after completion of etching using an etchant for
chromium, the processing enters a next step 8 without removing the
photo resist.
[0079] Step 8: While leaving the photo resist on the drain
electrode 2a and the source electrode 6a as it is, dry etching is
applied to the above-mentioned conductive film 4 (including n.sup.+
type region 4a) using a mixed gas of sulfur hexafluoride gases and
a hydrogen chloride gas in the same manner as step 6. With respect
to this etching gas used in the etching step, the amorphous silicon
film is more liable to be etched compared to the chromium thin film
so that the semiconductor film 4 is etched using the drain
electrode 2a and the source electrode 6a as masks. Due to this dry
etching, the semiconductor film 4 including the above-mentioned
n.sup.+ type region 4a is etched with a thickness which corresponds
to 100 nm as a process designing value from the above surface. As
described in conjunction with step 4, a thickness of n.sup.+ type
region 4a is 50 nm. Accordingly, in the semiconductor film 4 on
which the second conductive film such as the drain electrodes 2a
and the source electrodes 6a and the like are not formed, the
n.sup.+ type region 4a formed along the upper surface is completely
removed. Accordingly, the n.sup.+ type region 4a which extends from
the drain electrode 2a to the source electrode 6a at the time of
completion of step 7 is separated between two electrodes and the
i-type region of the semiconductor film 4 becomes thinner during
this period. So long as the thin film transistor is concerned, the
structure is substantially completed by this step. After applying
the dry etching to the semiconductor film 4, the photo resist is
removed using chemicals in the same manner as step 3.
[0080] Step 9: In this step, over the above-mentioned thin film
transistors, the pixel electrodes 6 and the counter electrodes 3a,
the protective film 8 which is suitable for reducing the previously
mentioned image retention are formed. In this embodiment, in the
same manner as step 4, two kinds of thin films made of amorphous
silicon nitride (SiN.sub.X) which differ in composition from each
other are sequentially laminated to an uppermost surface (at a
stage prior to starting this step) of the TFT substrate by a plasma
enhanced CVD method thus forming the protective film 8 which is
constituted of two-layered silicon nitride films (8a, 8b from above
in FIG. 2). The two-layered silicon nitride films 8a, 8b are formed
on the whole region of a portion of the TFT substrate which
corresponds at least the display region. The formation of the
silicon nitride films by the plasma enhanced CVD method is
performed using the vapor deposition apparatus which is
schematically shown in FIG. 3.
[0081] In this vapor deposition apparatus, a holder HLDR on which a
work piece (a specimen SPCM) is placed and an electrode ELCT which
faces the holder HLDR in an opposed manner are disposed in the
inside of a housing CHMB, the electrode ELCT which is connected to
a high frequency power supply RFP is capacitively connected to the
holder HLDR which is connected to a ground potential, and plasma is
generated between the holder HLDR and the electrode ELCT. With
respect to this plasma, the ionization state of the gases supplied
to the housing CHMB is maintained by adjusting the pressure in the
inside of the housing CHMB using an exhaust device PUMP connected
to the housing CHMB through a valve GVL and by adjusting electric
force supplied to the electrode ELCT from the high frequency power
supply RFP. As gases which generate the plasma, monosilane
(SiH.sub.4), ammonia (NH.sub.3) and nitrogen (N.sub.2) are
indicated in FIG. 3. Supply amounts of these gases into the housing
CHMB are suitably adjusted by operating valves VLV1, VLV2, VLV3
while monitoring respective flow rates using flow rate meters MFC1,
MFC2, MFC3 which are provided to respective flow passages.
[0082] The above-mentioned two layered silicon nitride films 8a, 8b
are grown by changing the vapor deposition apparatus corresponding
to the composition of the film. Further, the above-mentioned two
layered silicon nitride films 8a, 8b may be grown in separate
reaction chambers corresponding to the composition of the film
using a vapor deposition apparatus provided with a plurality of
reaction chambers (housing CHMB shown in FIG. 3) and a system which
transports a work piece between these chambers and the system under
reduced pressure atmosphere. However, in the embodiment described
hereinafter, the two layered silicon nitride films 8a, 8b are
continuously formed without stopping a discharge in the inside of
the reaction chamber without changing the vapor deposition
apparatus and the reaction chambers.
[0083] Respective film forming conditions of the upper-side silicon
nitride film 8a and the lower-side silicon nitride film 8b are
described in Table 1.
1TABLE 1 upper-side lower-side silicon silicon nitride Parameter (
Unit ) nitride film 8b film 8a SiH.sub.4 - flow rate
(cm.sup.3/min.) 130 350 NH.sub.3 - flow rate (cm.sup.3/min.) 1200
100 N.sub.2 - flow rate (cm.sup.3/min.) 2500 2000 Reaction pressure
(Pa) 266 266 RF (radio frequency) power (W) 2500 350 Substrate
temperature (.degree. C.) 250 250 Film thickness (nm) 500 0, 10,
20, 30
[0084] As shown in Table 1, the inventors have prepared four kinds
of TFT substrates which differ in the thickness of the upper-side
silicon nitride film 8a and have performed the comparison of these
TFT substrates as described later. Here, one of four kinds of TFT
substrates is the substrate on which the upper-side silicon nitride
film 8a is not formed and which the inventors conventionally
produced.
[0085] The feature of this step lies in that in the process for
forming the protective film 8, the flow rate ratio of the raw
material gases, that is, the flow rate ratio between the flow rate
of monosilane gas (SiH.sub.4 flow rate) and the flow rate of
ammonia gas (NH.sub.3 flow rate) at the time of growth of the
lower-side silicon nitride film 8b is inverted at a point of time
for starting the growth of the upper-side silicone nitride film
8a.
[0086] The inventors conventionally grew the amorphous silicon
oxide film by setting a supply amount of the ammonia gas (or a
nitride gas which is capable of substituting the ammonia gas) into
the plasma larger than a supply amount of the monosilane gas (or a
silicon compound gas which is capable of substituting the
monosilane gas) and used such an amorphous silicon nitride film as
the protective film. To the contrary, when the inventors have
reviewed the growth conditions of the amorphous silicon nitride
film in reducing the previous-mentioned image retention, the
inventors have found a tendency that, immediately before the
completion of the growth of the amorphous silicon nitride film
based on the above-mentioned process conditions, the image
retention is liable to be easily resolved corresponding to the
increase of the ratio of the supply amount of monosilane gas (or
the silicon compound gas which is capable of substituting the
monosilane gas) into the plasma with respect to the supply amount
of the ammonia gas (or nitride gas which is capable of substituting
the ammonia gas). Particularly, on a display screen of a liquid
crystal display panel in which the protective film is formed by
setting the supply amount of the monosilane gas (or the silicon
compound gas which is capable of substituting the monosilane gas)
into the plasma larger than the supply amount of the ammonia gas
(or the nitride gas which is capable of substituting the ammonia
gas), the image retention at the time of completion of the
operation can be substantially ignored. Accordingly, in this
embodiment, the inventors have formed the new silicon nitride film
8a on the conventional silicon nitride film 8b under the conditions
shown in Table 1. The detail of the protective film having such a
two-layered structure is described later.
[0087] Step 10: As mentioned above, on the upper surface of the
protective film having the two-layered structure which is formed on
the portion of the TFT substrate which corresponds at least to the
display region of the liquid crystal display panel, an orientation
film 9 which is made of organic material such as polyimide is
formed. When the TFT substrate is assembled into the liquid crystal
display panel, the orientation film is brought into contact with
the liquid crystal layer. Accordingly, by providing a mechanical
treatment such as rubbing to the upper surface of the orientation
film or irradiating light having specific polarizing components to
the upper surface of the orientation film, the liquid crystal
molecules which are brought into contact with the orientation film
are oriented in the desired direction. In this embodiment, the
treatment which gives the property to orient the liquid crystal
molecules to the upper surface of the orientation film is performed
before laminating the TFT substrate and the counter substrate to
each other. Accordingly, at a point of time that the treatment of
the upper surface of the orientation film 9 is completed, the TFT
substrate having the cross-section shown in FIG. 2 is completed.
Here, when the rubbing treatment is applied to the orientation film
9, the orientation film is baked thereafter.
[0088] Subsequently, the counter substrate is laminated to the TFT
substrate which is produced in the above-mentioned manner so as to
complete an in-plane switching type liquid crystal display device.
FIG. 4 schematically depicts the cross-section of the completed
liquid crystal display panel. In the drawing, with respect to the
TFT substrate 11 having the pixel structure shown in FIG. 1 and the
cross-section shown in FIG. 2, all components including thin films
which are formed on the main surface are omitted except for the
orientation film 9. On the other hand, on a main surface of the
counter substrate 12 which faces the TFT substrate 11 in an opposed
manner, the above-mentioned black matrix BM is formed and a color
filter CF is formed in an opening (see FIG. 1) for every pixel.
Accordingly, in observing the liquid crystal display panel
macroscopically, the black matrix BM and the color filters CF are
depicted such that they are substantially arranged on the same
layer with respect to the main surface of the counter substrate 12.
The counter substrate 12 which forms the color filters on the main
surface thereof is also referred to as a color filter substrate.
Over the black matrix BM and the color filters CF, a protective
film (also referred to as an overcoat film, not shown in the
drawing) is formed, and an orientation film 9 is formed over the
protective film in the same manner as the TFT substrate. Here, in
the in-plane switching type liquid crystal display device according
to this embodiment, each pixel region (region through which light
transmits) is constituted such that the pixel electrodes and the
counter electrodes are arranged in a spaced-apart manner from each
other on the main surface of the TFT substrate 11 as shown in FIG.
1 and hence, electrodes are not formed on a main surface of the
counter substrate 12. The summary of assembling steps of this type
of liquid crystal display device is explained hereinafter.
[0089] To a periphery of at least one of the main surfaces of the
TFT substrate 11 and the counter substrate 12 which are arranged to
face each other in an opposed manner, a tacky organic material
which is referred to as a sealing material is applied.
Subsequently, as shown in FIG. 4, the TFT substrate 11 and the
counter substrate 12 are overlapped to each other, the peripheries
of both main surfaces are adhered to each other using the sealing
material and, thereafter, the sealing material is hardened by
annealing or the like. The sealing material is applied to the main
surfaces such that the sealing material substantially surrounds a
portion (portion where the pixels and the color filters are formed)
which constitutes the display region of the main surface of at
least one of the TFT substrate 11 and the counter substrate 12.
Accordingly, in a state that the TFT substrate 11 and the counter
substrate 12 are laminated to each other, with respect to the
cross-section of the liquid crystal display panel, as shown in FIG.
4, a space is defined by these main surfaces and the sealing
material 14. A liquid crystal material (or a liquid crystal
composition including a chiral agent or the like) 15 is filled into
the space through openings (sealing openings) formed in the sealing
material 14. Here, the resistivity of the liquid crystal material
16 used in this embodiment is 1.times.10.sup.13 .OMEGA.cm.
[0090] FIG. 5 is an exploded perspective view of the liquid crystal
display device in which the liquid crystal display panel 21
produced in the above-mentioned manner is assembled. A portion of
the periphery of the TFT substrate of the liquid crystal display
panel 21 is projected to the outside from the counter substrate. On
this projected portion, terminals to which driving circuits
(external circuits with respect to the liquid crystal display
panel) 22a, 22b which supply signals to the thin film transistors
and the like disposed in the inside of the liquid crystal display
panel 21 are electrically connected are formed (not shown in the
drawing).
[0091] Polarizers 23 are respectively laminated to an upper surface
and a lower surface of the liquid crystal display panel 21, a lower
surface side of the liquid crystal display panel 21 is arranged to
face a backlight unit 24 in an opposed manner, and the liquid
crystal display panel 21 and the backlight unit 24 are fixed to
each other thus substantially completing the liquid crystal display
device. Although the backlight unit 24 is classified into a side
edge type which uses a light guide plate and the like, a direct
type which arranges a plurality of linear lamps to face the lower
surface of the liquid crystal display panel 21 and the like, there
is no restriction with respect to the backlight unit which can be
adopted by this embodiment.
[0092] <<Effect with Respect to Suppression of Image
Retention>>
[0093] The result of review related to the image retention
alleviation characteristics of the in-plane switching type liquid
crystal display device according to the above-mentioned first
embodiment of the present invention is described hereinafter.
[0094] As has been described in conjunction with step 9 which
describes the manufacturing method of TFT substrate used in the
liquid crystal display device, the inventors have produced four
types of TFT substrates which differ in the thickness of the
amorphous silicon nitride film 8a which is newly provided onto the
amorphous silicon nitride film 8b which is formed as the
conventional protective film. The film thicknesses of the silicon
nitride films 8a formed on these four types of TFT substrate are
respectively 0 nm (conventional TFT substrate having no silicon
nitride film 8a), 10 nm, 20 nm and 30 nm. The performance of four
kinds of liquid crystal display devices which are produced using
respective TFT substrates is compared using two kinds of evaluation
criteria which are described hereinafter.
[0095] The comparison using either one of these criteria is based
on the fact that the difference between the reference voltage
(voltage Vcom applied to the counter electrode 3a, also referred to
as a common voltage) immediately before changing over an image
displayed on the display screen of the liquid crystal display panel
and the reference voltage immediately after changing over the image
determines time during which the image retention occurs on the
display screen. That is, the difference in the reference voltage
between before and after the changeover of the display image is
reduced (alleviated) along with a lapse of time from the changeover
of the display image. The inventors focused their attention on this
phenomenon, measured the alleviation time necessary for alleviating
the above-mentioned difference in reference voltage, and evaluated
the image retention. The reference voltage Vcom which is described
hereinafter implies a voltage which is applied to the counter
electrode 3b, the gate voltage implies a voltage applied to the
gate electrode 1a, and the drain voltage implies a voltage applied
to the drain electrode 2a.
[0096] [Evaluation A]
[0097] In the liquid crystal display panel, the counter electrode
3a at the time of performing a black display (minimizing the
optical transmissivity of the liquid crystal layer) on the display
screen is set to a ground potential. In other words, the image
display of the liquid crystal display panel is performed by a
driving method (for example, a dot inversion driving) which is
different from a so-called common inversion driving which changes
the potential of the counter electrode 3a every image display of 1
frame. When the power supply of the liquid crystal display panel in
such an operation state is turned off, due to the potential
difference between the pixel electrode 6 and the counter electrode
3a immediately before the turning off of the power supply, the
charge generated in the protective film 8 positioned between the
electrodes is stored in the protective film 8 due to the reference
voltage of the counter electrode 3a. Accordingly, as soon as the
power supply of the liquid crystal display panel is turned off, the
protective film 8 assumes a so-called charged state. The potential
of the protective film 8 is increased correspondingly and hence,
the optical transmissivity of the liquid crystal layer which faces
the protective film 8 while sandwiching the orientation film 9
between the protective film 8 and the liquid crystal layer is also
increased. Accordingly, the display screen of the liquid crystal
display device temporarily performs the white display or exhibits
the luminance close to the white display. In this specification,
"white display" implies not only the case in which the optical
transmissivity of the liquid crystal layer assumes the maximum
value within a range of gray scale voltage applied to the liquid
crystal layer but also an operation which displays color other than
white (for example, gray) when the optical transmissivity is higher
than that of the above-mentioned "black display" state.
[0098] Due to turning-off of the operation of the liquid crystal
display panel, the charge stored in the above-mentioned protective
film 8 is decreased due to leaking from the protective film 8 along
with a lapse of time from a point of time that the operation of the
liquid crystal display panel is turned off so that the charged
state of the protective film 8 is also alleviated. Accordingly, the
reference voltage from the counter electrode 3a which is connected
to the ground potential is applied to the liquid crystal layer
without being interrupted by the residual charge in the protective
film 8 so that the optical transmissivity of the liquid crystal
layer is gradually lowered whereby the display screen of the liquid
crystal display panel is also changed to the black display.
[0099] Here, the luminance of the display screen (or the pixels
arranged on the display screen) of the liquid crystal display panel
is measured and the image retention alleviation characteristics of
the above-mentioned four liquid crystal display panels are
respectively evaluated based on time necessary for halving the
maximum luminance value (also referred to as white luminance)
measured immediately after turning off the power supply of the
liquid crystal display panel. To be more specific, the light
receiving element is made to face a specific portion of the liquid
crystal display panel in an opposed manner and a so-called
stationary measurement which continuously monitors the change of
the luminance of the portion is performed. The shorter the time
from a point of time that the above-mentioned maximum luminance
value (white luminance) is measured to a point of time that the
half value is measured, it is evaluated that the protective film
formed on the liquid crystal display panel exhibits the more
favorable image retention alleviation characteristics.
[0100] The evaluation of four kinds of liquid crystal display
devices which respectively include the protective films (amorphous
silicon nitride films) 8a having film thicknesses 0 nm, 10 nm, 20
nm, 30 nm is performed by preparing two sets of liquid crystal
display panels for each kind.
[0101] First of all, the image retention alleviation times of two
sets of liquid crystal display panels in which the film thickness
of the protective film 8a is 0 nm (having no protective film 8a)
are 125 seconds and 114 seconds respectively. Then, the image
retention alleviation times of two sets of liquid crystal display
panels in which the film thickness of the protective film 8a is 10
nm are 105 seconds and 92 seconds respectively and hence, a slight
effect is confirmed.
[0102] Further, both of two sets of liquid crystal display panels
in which the film thickness of the protective film 8a is 20 nm and
two sets of liquid crystal display panels in which the film
thickness of the protective film 8a is 30 nm exhibit the image
retention alleviation time of approximately 0 second thus proving
the remarkable enhancement of the image retention alleviation
characteristics. Here, the image retention alleviation time of
approximately 0 second implies that the time required from a point
of time that the maximum value of luminance is measured to a point
of time that the half value of the maximum luminance is measured is
extremely short, that is, less than 1 second and hence, the
measurement is substantially impossible with the measuring method
adopted by the inventors. In any case, it is confirmed that the
formation of the protective film 8a on the protective film 8b
brings about the large improvement of the image retention
alleviation characteristics.
[0103] [Evaluation method B]
[0104] In FIGS. 1 and 2, a DC voltage 10 V is applied to the gate
electrode 1a, a voltage having a rectangular wave pattern which
fluctuates an amplitude thereof within a range of 3 to 4 V is
applied to the drain electrode 2a, and the counter electrode 3a is
connected to the ground potential. In this state, a DC voltage 1 V
is applied to the drain electrode 2a so as to fluctuate the
above-mentioned rectangular wave voltage within a range of 4 to 5
V. In this manner, the time-sequential change of luminance
(relative flicker) which is generated when the center of the
amplitude of the rectangular voltage waveform which is applied to
the drain electrode 2a is elevated is measured. As a specific
measuring method, for example, the luminance monitoring of the
liquid crystal display panel can be used in the same manner as the
above-mentioned evaluation method A. In this evaluation method, the
attenuation of the luminance of the liquid crystal display panel is
measured from a point of time that the above-mentioned DC
components are applied to the drain electrode 2a. As described
above, even when the DC components are added to the potential of
the drain electrode 2a, an extra charge is generated in the
protective film 8 and this extra charge remains in the protective
film 8 and hence, an image to be erased from the display screen
remains on the display screen as "image retention". The relative
flicker intensity reflects also the intensity which generates the
image to be erased on the display screen and hence, the evaluation
of image retention can be performed based on the relative flicker
intensity. That is, the liquid crystal display panel having the
relative flicker intensity which drops to 0 as fast as possible is
determined as a liquid crystal display panel having the favorable
image retention alleviation characteristics.
[0105] In the same manner as the above-mentioned evaluation method
A, the evaluation method B is also performed such that four kinds
of liquid crystal display devices (provided with backlight units)
which respectively include the protective films (amorphous silicon
nitride films 8a) having film thicknesses 0 nm, 10 nm, 20 nm, 30 nm
are prepared two sets for each kind. The measurement is performed
by setting the luminance of the backlight unit of the liquid
crystal display device to 25 Cd (candela)/m.sup.2 and 250
Cd/m.sup.2 respectively and the result of these two kinds of
experiments are shown in FIG. 6 (backlight luminance=25 Cd/m.sup.2)
and FIG. 7 (backlight luminance=250 Cd/m.sup.2).
[0106] From the result of the evaluation, the improvement of the
image retention alleviation characteristics of the liquid crystal
display device derived from the formation of the protective film 8a
on the protective film 8b is confirmed. Further, it is confirmed
that the image retention alleviation characteristics are remarkably
enhanced when the thickness of the protective film 8a is set to a
value equal to or more than 20 nm. Further, to compare the graph
shown in FIG. 6 with the graph shown in FIG. 7, all characteristics
curves in the graph shown in FIG. 7 measured by setting the
backlight luminance to high values exhibit favorable image
retention alleviaation characteristics than the characteristics
curves shown in the graph of FIG. 6. Based on such a tendency, the
inventors of the present invention had an understanding that the
photo conduction generated on the protective film 8, particularly
protective film (amorphous silicon nitride film) 8a which is newly
formed on the TFT substrate in accordance with the present
invention promotes the discharge of residual charge from the
protective film 8 so that the image retention alleviation with
respect to the liquid crystal display device is enhanced.
[0107] <<Features of Protective Film>>
[0108] As described above, one example of the liquid crystal
display device according to the present invention exhibits the
favorable image retention alleviation characteristics by adopting
the protective film having the novel structure. Accordingly, the
structural features and the physical properties of the protective
film are explained hereinafter from various viewpoints.
[0109] The above-mentioned novel protective film is, as explained
in step 9 of the TFT substrate manufacturing process of the liquid
crystal display device according to the first embodiment of the
present invention, is constituted of the two-layered amorphous
silicon nitride film. This two-layered silicon nitride film is
prepared as an experiment sample having a film thickness of 200 nm
using the above-mentioned vapor deposition apparatus and the
physical properties and chemical compositions of respective silicon
nitride films are arranged in Table 2.
2TABLE 2 silicon nitride silicon nitride Parameter film 8b film 8a
Si.sub.xN.sub.y- composition ratio (ratio of 1.28 0.91 y/x)
Relative dielectric constant 6.5 9.4 Dark resistivity (unit:
.OMEGA.cm) 3 .times. 10.sup.15 3 .times. 10.sup.13 Photo
resistivity (unit: .OMEGA.cm) 1 .times. 10.sup.15 1 .times.
10.sup.9
[0110] In Table 2, dark resistivity is the resistivity of the
amorphous silicon nitride film measured in a darkroom and is
measured under the same conditions as dark resistivity of selenium
photocell or an optical electronic device similar to the selenium
photocell. On the other hand, photo resistivity is the resistivity
of the silicon nitride film when the photo conduction is generated
in the inside of the silicon nitride film by irradiating light to
the amorphous silicon nitride film. The photo sensitivity is
measured by irradiating white light of 500 kLx (kilo lux) to
respective silicon nitride films. Here, 1 Lx (lux) indicates the
illuminance when the luminous flux of 1 m.sup.2 (lumen) is incident
on a surface of 1 m.sup.2 and lux and lumen have the relationship
which is expressed by an equation 1 Lx=1 Lm/m.sup.2. Further, the
luminous intensity I of a point source which emits the luminous
flux of F[Lm] at a solid angle .omega. is expressed by the
previous-mentioned unit 1 Cd (candela), wherein there exists a
relationship I[Cd]=dF/d.omega. (=F/.omega.: when .omega. is
extremely small). For reference purpose, the total luminous flux F
which is irradiated at a full solid angle 4.pi. from a uniform
point source which exhibits the uniform luminous intensity in all
directions is expressed by F=4.pi. [Lm]. In the transmission type
liquid crystal display device, light emitted from a light source
such as a backlight unit, a front light unit or the like is
incident on the main surface of the liquid crystal display panel,
while in the reflection type liquid crystal display device, light
which is incident on the main surface of the liquid crystal display
panel from the outside is reflected in the inside of the liquid
crystal display panel and is irradiated from the main surface.
Accordingly, in both cases, the protective film is subjected to a
considerable amount of light. The inventors have reviewed this fact
along with the above-mentioned finding on "effect with respect to
image retention suppression" of the liquid crystal display device
according to the present invention, and have studied the
possibility that the photo conduction generated in the novel
silicon nitride film 8a contributes to the reduction of image
retention of the liquid crystal display device due to the formation
of the novel silicon nitride film 8a on the (liquid crystal layer
side of the) silicon nitride film 8b shown in FIG. 2.
[0111] The inventors have reviewed this possibility using several
kinds of silicon nitride films 8a which are produced by changing
the growth conditions in accordance with the plasma enhanced CVD
method. First of all, in the step for forming the silicon nitride
film 8a on the silicon nitride film 8b, high frequency power (also
referred to as radio frequency power or RF power in view of a
frequency band thereof) which generates plasma in the inside of the
vapor deposition apparatus is changed thus producing several kinds
of liquid crystal display devices. The high frequency power is
applied from the electrode ELCT to plasma in the vapor deposition
apparatus shown in FIG. 3.
[0112] As a result of comparison of the image retention alleviation
characteristics of these liquid crystal display devices, a result
that the smaller the high frequency power at the time of growing
the silicon nitride film 8a, the image retention alleviation
characteristics are enhanced. Further, several kinds of test
samples of silicon nitride films 8a are prepared by changing the
high frequency power applied to the plasma in the same manner, and
the dark resistivity and the photo resistivity of these test
samples are examined and the result shown in FIG. 8 is obtained. It
is found from the result that corresponding to the decrease of the
high frequency power which is applied at the time of growth of the
silicon nitride film 8b in response to the image retention
alleviation characteristics of the liquid crystal display devices,
the resistivities of these test samples are also reduced.
[0113] Subsequently, based on the above-mentioned result, the high
frequency power is set to 350 W, the raw material gas flow rate
ratio of SiH.sub.4 and NH.sub.3 supplied to the CVD device (vapor
deposition apparatus (NH.sub.3 flow rate/SiH.sub.4 flow rate) at
the time of growth of the silicon nitride film 8a is changed thus
producing several kinds of test samples of silicon nitride films
8a. The dark resistivity and the photo resistivity of these test
samples are measured in the above-mentioned manner and the
correlation between these resistivities and the above-mentioned raw
material gas flow rate ratio is plotted thus obtaining a result
indicated in the graph shown in FIG. 9. Further, several kinds of
liquid crystal display devices are produced by forming the silicon
nitride film 8a on the silicon nitride film 8b by respectively
changing the raw material gas flow rate ratio in the same manner as
the test samples and the image retention alleviation
characteristics are examined. As the result, the smaller the raw
material gas flow rate ratio (the SiH.sub.4 flow rate being
increased with respect to the NH.sub.3 flow rate) during the growth
period of the silicon nitride film 8a, the image retention promptly
disappears from the display screen of the liquid crystal display
device which includes the silicon nitride film 8a as one of
protective films. Further, the inventors also have found an example
in which in controlling the resistivity of the silicon nitride film
8a, it is preferable to produce the silicon nitride film 8a by
setting the above-mentioned raw material gas flow rate ratio
(NH.sub.3 flow rate/SiH.sub.4 flow rate) to not more than 1.0, that
is, by setting the SiH.sub.4 flow rate not less than the NH.sub.3
flow rate.
[0114] In view of the above, the inventors have found that in
totally reviewing the relationship between the resistivities of the
test samples of the silicon nitride films 8a produced by changing
the growth conditions of the plasma enhanced CVD method and the
image retention alleviation characteristics of the liquid crystal
display devices which are respectively provided with these silicon
nitride films 8a, the relationship between the resistivities of the
silicon nitride films 8a and the resistivities of the liquid
crystal layers influences the image retention alleviation
characteristics. The resistivities of the liquid crystal layers
substantially fall within a range of
1.times.10.sup.11-1.times.10.sup.13 .OMEGA.cm although the values
may change depending on the kinds of liquid crystal display
devices. On the other hand, focusing on the photo resistivities
shown in FIG. 8 and FIG. 9, it is considered that in the inside of
the silicon nitride film 8a which is exposed to light incident from
the lighting unit of the liquid crystal display device or an
external source of the liquid crystal display device, photo
conduction is generated to some extent so that the resistivity of
the silicon nitride film 8a becomes lower than the resistivity of
the liquid crystal layer.
[0115] For example, to compare the liquid crystal display device
having the protective film consisting only of the silicon nitride
film 8b shown in Table 2 and the liquid crystal display device
having the protective film which adds the silicon nitride film 8a
to the liquid crystal layer side of the silicon nitride film 8b, it
is concluded that with respect to the former liquid crystal display
device, the charge generated on the liquid crystal layer side of
the silicon nitride film 8b is not discharged in spite of the
conductive property of the liquid crystal layer and remains on the
surface for a long time, while with respect to the latter liquid
crystal display device, such a charge is promptly discharged due to
the conductive property of the silicon nitride film 8a which is
brought into contact with the liquid crystal layer side of the
silicon nitride film 8b. Accordingly, not to mention the silicon
nitride film, it is one of the criteria that with respect to at
least two kinds of material layers which constitute the protective
film, the resistivity, that is, for example, the photo resistivity
of material layer at the liquid-crystal-layer side is set lower
than the resistivity of the material layer at the active-element
side (silicon nitride film 8b in the first embodiment). However,
when the resistivity of the material layer at the
liquid-crystal-layer side is excessively low, the drawback which is
pointed out in conjunction with the example described in the
previously-mentioned Japanese Patent 2938521 occurs. Accordingly,
the inventors proposes the following criteria with respect to the
formation of the material layer at the liquid crystal layer side
which constitutes the protective film (in other words, a layer
which is made remote from the active elements or switching elements
due to another protective film material layer).
[0116] One of the criteria is to set the resistivity of the
material layer greater than the resistivity of the semiconductor
layer. According to the review carried out by the inventors, the
amorphous silicon film described in Japanese Patent 2938521
exhibits the dark resistivity of 1.times.10.sup.10 to 10.sup.11
.OMEGA.cm and the photo conductivity of 1.times.10.sup.6 to
10.sup.7 .OMEGA.cm (with respect to white light irradiation of 500
kLx). To the contrary, according to the finding obtained through
the experiment, the above-mentioned drawback can be obviated by
setting the resistivity (photo resistivity) of the
liquid-crystal-layer-side material layer with respect to the white
light irradiation of 500 kLx to not less than 1.times.10.sup.8
.OMEGA.cm. Further, the dark resistivity of the
liquid-crystal-layer-side material layer may be set greater than
1.times.10.sup.11 .OMEGA.cm.
[0117] Another criterion focuses on the relative dielectric
constant from a viewpoint that it is preferable to make the
liquid-crystal-layer-side material layer hold the properties equal
to or close to the properties of an insulator. It is recommendable
that the relative dielectric constant of the protective film
material layer formed at the liquid crystal layer side is greater
than the relative dielectric constant of the silicon nitride film
8b (6.5) shown in Table 2 and is lower than the relative dielectric
constant of the amorphous silicon film (within a range from 10 to
13). For example, it is preferable that the relative dielectric
constant of the material layer at the liquid crystal layer side is
set to a value within a range from not less than 7 to less than 10
and the conductive property of the material layer at the liquid
crystal layer is set to a value higher than the conductive property
of the protective film material layer formed at the active element
side and lower than the conductive property of the
semiconductor.
[0118] Further, when the protective film material layer at the
liquid crystal layer side is formed of the silicon nitride film
reviewed in the above-mentioned first embodiment, in the
composition expressed by a chemical structural formula of
Si.sub.xN.sub.yX.sub.z (X being a general term of other
constitutional element), it is preferable that a condition
0<y/x.ltoreq.1.0 is satisfied. Here, even when X which is picked
up as the general term of element other than silicon and nitrogen
is not present (even z=0), the exercise of the present invention is
not obstructed. Although the composition of silicon nitride is
expressed as Si.sub.3N.sub.4 (approximately 1.3 in the
above-mentioned y/x ratio) stoichiometrically, to the contrary, the
composition of the novel protective film which is added to the
liquid crystal layer side in the liquid crystal display device
according to the present invention becomes Si-rich. It is desirable
that the silicon nitride film which constitutes the protective film
is, for enhancing the insulation property of the material layer
formed at the active element (switching element) side, formed as
the amorphous film including the material layer at the liquid
crystal layer side. One desirable example of the range of
composition of the above-mentioned Si.sub.xN.sub.yX.sub.z (X being
a general term of other constitutional element) from the viewpoint
of insulation property is 0.5<y/x.ltoreq.1.0.
[0119] According to the finding which the inventors have obtained
through the experiment in which the protective films were produced
using silicon nitride, the N/Si ratio in respective compositions of
the switching-element-side layer and the liquid-crystal-layer-side
layer can be controlled at the above-mentioned raw material gas
flow rate ratio (ratio of NH.sub.3 flow rate/SiH.sub.4 flow rate)
and is also regarded to be substantially equal to such a raw
material gas flow rate. Further, the compositions of the silicon
nitride films 8a, 8b which constitute the protective film 8
illustrated in FIG. 2 can be identified using an analytic method
such as the Fourier transform infrared spectroscopic method (FT-IR
method) or the Rutherford backward scattering method (RBS method)
or the like. In FIG. 10, one example of the result of analysis
(spectrum) of the silicon nitride films 8a, 8b obtained by the
FT-IR method is shown. With respect to these spectra, the wave
number (unit: wn, 1 wn=1 cm.sup.-1) of infrared rays irradiated to
the protective film is taken on the abscissas and the absorbance
(arbitrary unit) of infrared rays by the protective film is taken
on the axis of ordinates. In both of the spectrum of the silicon
nitride film 8b (SiN-8b) and the spectrum of the silicon nitride
film 8a (SiN-8a), the absorbance due to the bonding of nitrogen and
hydrogen (N--H) is observed in the vicinity of 3200 wn, the
absorbance due to the bonding of silicon and hydrogen (Si--H) is
observed in the range of 2000 to 2100 wn, and the absorbance due to
the bonding of silicon and nitrogen (Si--N) is observed in the
vicinity of 900 wn. However, compared with the spectrum of the
silicon nitride film 8b (SiN-8b), with respect to the spectrum of
the silicon nitride film 8a (SiN-8a), at least one of following
features is observed.
[0120] Feature 1: The absorbance of the silicon-nitrogen bonding in
the vicinity of 900 wn is small.
[0121] Feature 2: The ratio of absorbance h2 due to the
silicon-hydrogen bonding in the range of 2000 to 2100 wn with
respect to the absorbance h1 due to the nitrogen-hydrogen bonding
in the vicinity of 3200 wn is increased so that the peak height of
the absorbance h2 may be set equal to the absorbance h1 or exceeds
the absorbance h1 (h1<h2).
[0122] Feature 3: The peak of infrared rays absorption due to the
silicon-silicon bonding (Si--Si) is generated in the vicinity of
600 wn.
[0123] In FIG. 10, to show the spectrum of the silicon nitride film
8b (SiN-8b) and the spectrum of the silicon nitride film 8a
(SiN-8a) in parallel, a base line of the former is shifted in the
axis of ordinates from a base line of the latter. Further, the
resistivity of the illustrated silicon nitride film 8a is set lower
by at least two digits than 1.times.10.sup.15 .OMEGA.cm which is
the resistivity of the silicon nitride film 8b.
[0124] The protective film of the liquid crystal display device of
the first embodiment according to the present invention has been
discussed from three viewpoints of resistivity, relative dielectric
constant and silicon nitride film heretofore. To observe this
protective film from the liquid crystal layer side of the substrate
(TFT substrate 11 in this embodiment) on which the protective film
is formed, that is, from the uppermost surface of the thin film
structure formed on the main surface of the substrate which faces
the liquid crystal layer, the above-mentioned features are
described as follows.
[0125] First of all, from a viewpoint of the resistivity, the
above-mentioned protective film is characterized in that due to the
irradiation of light to the surface which faces the liquid crystal
layer, the resistivity can be reduced to not more than {fraction
(1/100)} of the resistivity when the light irradiation is not
performed. This feature can be confirmed, for example, by bringing
a probe for measuring resistance into contact with an upper surface
of the protective film and by irradiating light having illuminance
of 500 kLx or more to the upper surface. One example of the
resistivity of the upper surface of the protective film is in a
range of 1.times.10.sup.13 .OMEGA.cm-1.times.10.sup.15 .OMEGA.cm
when the light is not irradiated to the upper surface of the
protective film (for example, in a dark room) and is in a range of
1.times.10.sup.9 .OMEGA.cm-1.times.10.sup.11 .OMEGA.cm when the
light having the illuminance of 500 kLx or more is irradiated to
the upper surface of the protective film.
[0126] Subsequently, from a viewpoint of relative dielectric
constant, the surface of the protective film which faces the liquid
crystal layer in an opposed manner (for example, the uppermost
surface of the protective film) exhibits the relative dielectric
constant of not less than 7.5. Further, according to one preferable
example of the liquid crystal display device of the present
invention, the surface of the protective film exhibits the relative
dielectric constant of not less than 9.0.
[0127] Finally, from a viewpoint of forming the protective film
using the material containing at least silicon and nitrogen
(Si.sub.xN.sub.yX.sub.z- , X being a general term of other
constitutional element), the composition ratio of nitrogen relative
to silicon (y/x) of the surface of the protective film which faces
the liquid crystal layer in an opposed manner (for example, the
uppermost surface of the protective film) is in a range larger than
0 and not more than 1.0. In other words, the protective film has
the liquid crystal side thereof formed of material having high
silicon content based on the stoichiometric ratio of silicon
nitride: Si.sub.3N.sub.4.
[0128] Any one of the above-mentioned features of the protective
film grasped from the upper surface of the substrate is also
provided with the feature that the protective film comes into
contact with the switching elements. That is, while the protective
film exhibits the low resistivity at the liquid crystal layer side,
the protective film sufficiently suppresses short-circuiting and
leaking of charge (electrons and positive holes) at the switching
element side. The confirmation of the features brought about by the
cross-sectional structure of the protective film according to the
previously-mentioned invention in the manufacturing line of the
liquid crystal display devices substantially constitutes a
so-called destructive test of products. However, by forming the
protective film according to the present invention on the TFT
substrate, then by confirming the operation of the switching
elements covered with the protective film, and thereafter by
confirming at least one of described features from the uppermost
surface (liquid-crystal-layer-side surface) of the protective film
according to the present invention, it is possible to manage the
quality of the liquid crystal display device according to the
present invention in a manufacturing process by a non-destructive
test.
[0129] The features of the protective film which is adopted by the
liquid crystal display device of the first embodiment according to
the present invention have been discussed from various viewpoints
of resistivity, relative dielectric constant and silicon nitride
film. However, even when the protective film adopts the
three-or-more layered laminated structure which is formed by adding
other material layers to the liquid-crystal-layer-side material
layer (hereinafter referred to upper-side layer) and the
switching-element-side material layer (hereinafter referred to as
lower-side layer), the exercise of the present invention cannot be
obstructed. For example, the protective film may be constituted by
inserting a material layer which differs from both of the
upper-side layer and the lower-side layer between the upper-side
layer and the lower-side layer. Alternatively, the protective film
may be constituted by forming a material layer which differs from
the lower-side layer at the switching element side from the
lower-side layer. Further, although it is ideal that the upper-side
layer is formed as the uppermost layer of the protective film and
exhibits the resistivity sufficiently smaller than the
resistivities of the other material layers which constitute the
protective film from a viewpoint of enhancing the image retention
alleviation characteristics of the liquid crystal display device,
the advantageous effects of the present invention are not damaged
even when the protective film is constituted by forming a material
layer which differs from the upper-side layer between the
upper-side layer and the orientation film. The gist of the present
invention lies in constituting the protective film by laminating
the lower-side layer and the upper-side layer at least one of which
satisfies the above-mentioned features sequentially in this order
with respect to the main surface. Then, various features of the
upper-side layer in view of the whole protective film are
individually enumerated including the case in which the protective
film is constituted by adding other layers to the above-mentioned
upper-side layer and the above-mentioned lower-side layer.
[0130] For example, it is preferable to suppress the dark
resistivity of the upper-side layer to not more than {fraction
(1/100)} compared to the dark resistivity of other layer which
constitutes the protective film. In other words, it is preferable
to grow the upper-side layer such that the upper-side layer
exhibits the dark resistivity which is two digits smaller than the
dark resistivity of the layer among the material layers other than
the upper-side layer included in the protective film which exhibits
the lowermost dark resistivity.
[0131] In another example, it is preferable to form the layer other
than the upper-side layer and the lower-side layer which
constitutes the protective film together with the upper-side layer
such that the photo resistivity thereof becomes not more than
{fraction (1/100)} of the dark resistivity thereof.
[0132] In still another example, it is preferable that the film
thickness which is a sum of the film thickness of the upper-side
layer and the film thickness of at least one layer other than the
upper-side layer which is formed on the lower-side layer is set to
not more than one half of the film thickness of the whole
protective film. It is more preferable that the film thickness of
at least one layer which is formed at the liquid crystal layer side
(or the orientation film side) than the lower-side layer of the
protective film is set to not less than 10 nm.
[0133] In still another example, it is preferable that at least one
layer which is formed at the liquid crystal layer side (or the
orientation film side) than the lower-side layer of the protective
film exhibits the photo resistivity lower than the photo
resistivity of the liquid crystal layer. It is more preferable that
such one layer exhibits the dark resistivity lower than the dark
resistivity of the liquid crystal.
[0134] With respect to the above-mentioned protective film, the
respective material layers which constitute the protective film may
be formed by other chemical vapor deposition method besides the
above-mentioned plasma enhanced CVD method. Further, in any
chemical vapor deposition method, by laminating the material layers
which continuously differ in chemical composition while suitably
changing supply amounts of raw material gases and vapor deposition
conditions, the operation efficiency of the liquid crystal display
device is also enhanced.
[0135] In producing the protective film by the chemical vapor
deposition method using the plasma CVD device, it is preferable to
change the high frequency power (applied to plasma) with respect to
the total flow rate of the raw material gases (SiH.sub.4 gas and
NH.sub.3 gas in case of Si.sub.xN.sub.y film) supplied to a
reaction chamber (housing CHMB illustrated in FIG. 3) corresponding
to respective films to be formed. This physical quantity is defined
as a so-called "RF power per unit gas flow rate" which is obtained
by dividing the high frequency power applied to plasma with the sum
of flow rates of the raw material gases. It is preferable to set
the RF power value per unit gas flow rate at the time of
above-mentioned growth of the upper-side layer smaller than the
corresponding RF power value at the time of growth of the
lower-side layer. While the lower-side layer in which priority is
given to insulation property is formed such that the lower-side
layer exhibits the resistivity of not less than 1.times.10.sup.15
.OMEGA.cm, for example, it is recommendable that the upper-side
layer is formed such that the upper-side layer exhibits the
resistivity similar to the resistivity of the semi-insulating
semiconductor such as ZnS or GaAs. It is desirable that the
resistivity of the upper-side layer is approximately two digits or
more lower than the resistivity of the lower-side layer. In
laminating these two kinds of material layers continuously, the
vapor deposition condition control in the vicinity of an interface
between the lower-side layer and the upper-side layer determines
the properties of the whole protective film. By properly
controlling at least one of the total flow rate of the raw material
gases and the power applied to plasma, the vapor deposition
conditions in the vicinity of the interface can be surely changed
over so that it is possible to set the resistivity of the
upper-side layer in the above-mentioned desired range.
Particularly, by setting the thickness of the upper-side layer
smaller than the thickness of the lower-side layer, this
advantageous effect is enhanced.
[0136] Further, it is preferable that the protective film is formed
on at least one of the switching elements and the black matrix. Due
to such a provision, for example, the undulation which may be
generated on the main surface of the substrate due to any one of
the switching elements, the black matrix and the color filters can
be leveled by the upper-side layer. Accordingly, in forming the
orientation film or the electrode film on the protective film, the
orientation direction of the liquid crystal molecules in the liquid
crystal layer with respect to the upper surface of the orientation
film can be surely controlled. As the switching elements, thin film
diodes may be used besides the thin film transistors exemplified in
the first embodiment. Further, besides the bottom gate structure
which forms the channel layers made of semiconductor films on the
gate electrodes shown in FIG. 2, the thin film transistors may
adopt the top gate structure in which the gate electrodes are
formed on the channel layers made of semiconductor films. Further,
the semiconductor films are not limited to the above-mentioned
amorphous silicon thin films. That is, even when the semiconductor
films are formed of poly-crystalline silicon thin films or the
silicon thin films having pseudo single crystal which increases the
grain size of the poly-crystalline grain particles, the exercise of
the present invention is not obstructed.
[0137] On the other hand, by forming the above-mentioned protective
film over the pixel electrodes or over the pixel electrodes and the
counter electrodes, the above-mentioned advantageous effect becomes
more remarkable. For example, even when the above-mentioned
protective film is applied to a liquid crystal display device which
drives the liquid crystal by fringe field switching besides the
above-mentioned in-plane-switching liquid crystal display device,
it is possible to obtain the advantageous effects.
[0138] To focus on the photo conduction which is generated in at
least one layer (the above-mentioned upper-side layer) which is
formed at the liquid crystal layer side than the lower-side layer
of the protective film, by applying the protective film to a liquid
crystal display device provided with a light source device which
irradiates a liquid crystal display panel (a light source which is
referred to as a backlight unit or a front light unit), the image
retention alleviation property can be enhanced by an operation to
turn on the light source.
[0139] Some of modifications of the protective film provided to the
liquid crystal display device according to the present invention
which has been explained heretofore are introduced in the second
embodiment and the third embodiment.
Second Embodiment
[0140] As the second embodiment of the liquid crystal display
device, a twisted nematic (also referred to as TN) type liquid
crystal display device having the above-mentioned protective film
is explained mainly in conjunction with FIG. 11 and FIG. 12. The
constitution which makes the liquid 25 crystal display device of
this embodiment different from the liquid crystal display device of
the first embodiment in structure lies in that pixels each having a
switching element are formed on one of a pair of substrates which
are arranged to face each other in an opposed manner with main
surfaces thereof spaced apart from each other (a liquid crystal
layer 15 being sealed between the main surfaces of these substrates
11, 12 as shown in FIG. 4), and counter electrodes are formed on
the other substrate. However, also in this embodiment, one of the
pair of substrates on which the switching elements and the pixel
electrodes are formed is referred to as the TFT substrate for the
sake of convenience. Further, since color filters are also formed
on the other (the substrate on which the counter electrodes are
formed) of the pair of substrates, the substrate is referred to as
the color filter substrate.
[0141] FIG. 11 is a plan view showing one of a plurality of pixels
formed on a main surface (facing the liquid crystal layer) of the
TFT substrate 11 used in the liquid crystal display device
according to this embodiment, and FIG. 12 is a cross-sectional view
obtained by cutting a liquid crystal display panel (including also
the liquid crystal layer 15 and the color filter substrate 12)
along a chain line XII-XII' in FIG. 11.
[0142] FIG. 11 shows pixels which include thin film transistors TFT
each of which has a gate electrode 1a which is formed as a portion
of a scanning signal line 1 shown at a lower side of the drawing, a
semiconductor layer (channel layer) 4 which covers the gate
electrode 1a, a drain electrode 2a which is branched from a video
signal line 2 shown at a left column of the drawing, and a source
electrode 6a which is formed in a spaced-apart manner from one end
of the drain electrode 2a and faces one end of the drain electrode
2a on the semiconductor layer 4, and pixel electrodes 7 which are
connected to the source electrodes 6a. Since the pixel electrodes 7
are arranged on the protective film 8 as shown in FIG. 12, the
pixel electrode 7 is indicated by reference numeral different from
(6) used in the first embodiment. While the source electrodes 6a
are formed of a metal film such as a chromium thin film or an alloy
film such as a molybdenum(Mo)-aluminum(Al) thin film, the pixel
electrodes 7 are formed of an oxide conductive film having high
optical transmissivity as represented by indium-tin-oxide (ITO) or
indium-zinc-oxide (IZO). The source electrodes 6a may be formed
using the same material and the same process as the video signal
lines 2 and the drain electrodes 2a. Further, depending on the
resistivity which is allowed to the video signal lines 2, the
source electrodes 6a, the video signal lines 2 and the drain signal
lines 2a may be formed of the above-mentioned oxide conductive
film. Each pixel is provided with one pixel electrode 7 which is
extended over a region which is surrounded by a pair of scanning
signal lines 1 and a pair of video signal lines 2. On a main
surface of the TFT substrate 11, a plurality of these pixels are
arranged two dimensionally. One example of the mode or arrangement
of a plurality of pixels is shown in FIG. 11 such that eight other
pixels surround one center pixel (the eight pixels being shown
partially).
[0143] On the other hand, as shown in FIG. 12, over the color
filter substrate 12, the black matrix BM and the color filters CF
are formed on the main surface of the substrate and a protective
film 18 is formed such that the protective film 18 covers the black
matrix BM and the color filters CF. The protective film 18 is
formed of a silicon nitride film expressed by a chemical structural
formula Si.sub.xN.sub.y (1.0<y/x). In the same manner as the
lower-side layer 8b of the protective film 8 formed on the TFT
substrate 11 side, the undulation generated on the main surface of
the substrate is alleviated due to such a thickness. This
undulation implies steps (stepped portions) which are formed in the
substrate thickness direction due to the formation of the scanning
signal lines 1, the video signal lines 2 and the thin film
transistors on the main surface of the TFT substrate 11 as well as
due to the formation of the black matrix BM and the color filters
CF on the main surface of the color filter substrate 12. In this
embodiment, the detail of the protective film 8 formed at the TFT
substrate 11 side is explained in detail later.
[0144] A counter electrode 13 is formed on the protective film 18.
The counter electrode 13 is formed of an oxide conductive film
having high optical transmissivity (or a transparent conductive
film similar to the oxide conductive film) in the same manner as
the above-mentioned pixel electrodes 7 and has an area which is
capable of facing in an opposed manner a plurality of pixel
electrodes 7 which are formed on the TFT substrate 11 which
sandwiches liquid crystal with the counter electrode 13. That is,
different from the structure of the first embodiment which mounts
the counter electrode 3a on the TFT substrate 11 for each pixel,
each counter electrode 13 of this embodiment is formed of one oxide
conductive film or one transparent conductive film which
corresponds to at least two pixels or all pixels which constitute
the display screen when necessary. Since it is unnecessary to
arrange the counter electrode 13 of this embodiment on the main
surface of the TFT substrate 11, the counter electrode 13 is
indicated by reference numeral which is different from the
reference numeral (3a) used in the first embodiment. Although an
orientation film 9 is formed on the counter electrode 13, the
detail thereof is substantially equal to that of the orientation
film 9 formed on the TFT substrate 11 which is explained in
conjunction with the first embodiment and hence, the explanation
thereof is omitted.
[0145] Only structural component arranged at the color filter
substrate 12 side shown in FIG. 11 is a profile BMO of an opening
formed in the black matrix BM. Within the profile BMO of the
opening of the black matrix BM which is shown by a broken line, the
color filter CF is disposed as shown in FIG. 12. The TFT substrate
11 and the color filter substrate 12 are aligned such that the
projection of the profile BMO of the opening of the black matrix
onto the main surface of the TFT substrate 11 falls within the
profile of the pixel electrode 7. Further, by forming the openings
in the black matrix BM on the color filter substrate 12, the stray
entrance of light into the liquid crystal layer from the periphery
of the pixel electrode 7 can be suppressed.
[0146] On the other hand, on the main surface of the TFT substrate
11, the scanning signal lines 1, the gate electrodes 1a, the video
signal lines 2, the source electrodes 6a and the drain electrodes
2a are spaced apart from each other in the substrate thickness
direction by way of an insulation film (gate insulation film) 5 in
the same manner as the first embodiment. Between the main surface
of the substrate 11 and the insulation film 5, conductive layers 1b
each of which extends along the scanning signal line 1 (in the x
direction in FIG. 11) and conductive layers 1c each of which is
bonded to another scanning signal line 1 which is spaced apart from
the conductive layer 1b by one pixel along the video signal line 2
are formed. These conductive layers 1b, 1c are, as shown in FIG.
11, overlapped to the periphery of one pixel electrode 7 together
with the scanning signal line 1 which is bonded to the conductive
layer 1c. Between the periphery of the pixel electrode 7 and the
above-mentioned counter electrode 13, an electric field (a
so-called fringe field) which is irregular compared to an electric
field which is generated between the a region arranged inside the
periphery of the pixel electrode 7 and the counter electrode 13 (an
electric field which is suitable for controlling optical
transmissivity of the liquid crystal layer) is generated. This
fringe field causes leaking of light along the periphery of the
pixel electrode 7 even when the potential of the pixel electrode 7
is controlled to minimize the optical transmissivity of the liquid
crystal layer, for example.
[0147] To the contrary, in the so-called loop structure consisting
of the above-mentioned conducive layers 1b, 1c and scanning signal
lines 1, the conductive layers 1b, 1c are overlapped to the
periphery of the pixel electrode 7 while sandwiching the insulation
film 5 therebetween and hence, the leaking of light attributed to
the fringe field can be suppressed. In view of such an advantageous
effect, the conductive layers and the scanning signal line which
constitute the loop structure is referred to as a light shielding
film or a light shielding structure. As shown in FIG. 11, the loop
structure includes a pair of conductive layers 1c which are formed
at both sides of the pixel electrode 7 along the extension
direction of the video signal lines 2 (y direction in FIG. 11).
Further, as the scanning signal line 1 which is included in the
loop structure, the scanning signal line 1 which does not
contribute to the control of a switching element connected to the
pixel electrode 7 to which the scanning signal line 1 is
overlapped, that is, the scanning signal line 1 which contributes
to the control of the switching element to which another pixel
electrode 7 disposed close to the pixel electrode 7 along the video
signal line 2 is connected is selected.
[0148] Accordingly, the region through which light transmits in
each pixel provided to the liquid crystal display device (liquid
crystal display panel) of this embodiment is restricted by the
opening BMO of the black matrix which is overlapped along the
periphery of the main surface of the pixel electrode 7 and the
above-mentioned loop-shaped light shielding structure.
[0149] On the other hand, a semiconductor layer 4 which is formed
between the above-mentioned insulation film 5 and the video signal
line 2, the source electrode 6 and the drain electrode 2a in the
liquid crystal display device of this embodiment extends to an end
portion of the TFT substrate 11 along the video signal line 2,
while within the profile in the main surface of the TFT substrate
11, the above-mentioned video signal line 2, the source electrode
6a and the drain electrode 2a are accommodated. Such a planar shape
of the semiconductor layer 4 is attributed to etching of the
semiconductor layer 4 using the molded video signal line 2, the
source electrode 6a and the drain electrode 2a as masks. Such a
shape of the semiconductor 4 is suitable for preventing the
disconnection of the video signal lines 2 or the disconnection of
the conductive film when the conductive film (for example, the
pixel electrode 7) which is formed on the protective film 8
described later is connected to the video signal line 2, the source
electrode 6a or the drain electrode 2a through the opening formed
in the protective film.
[0150] Also in the liquid crystal display device of this
embodiment, the bottom-gate type thin film transistors are used as
switching elements and the protective film 8 is formed such that
the protective film 8 covers these components. Accordingly,
although there lies some difference between this embodiment and the
first embodiment with respect to the step 7 of the first
embodiment, the TFT substrate according to this embodiment is
manufactured by substantially following step 1 to step 8. However,
to form the pixel electrode 7 over the protective film, following
steps are added between the step 8 and the step 9 of the first
embodiment.
[0151] Step 8-1: A photo resist is formed on the protective film 8
and the protective film 8 which is positioned above a portion of
the source electrode 6a to which the pixel electrode 7 which will
be explained later is electrically connected is partially exposed
by a photolithography method and then is removed by developing.
Accordingly, over one portion of the source electrode 6a, an
opening of the photo resist is formed. Then, the protective film 8
which is exposed through the opening of the photo resist is etched
so as to form a through hole which allows a portion of the source
electrode 6a to be exposed.
[0152] Step 8-2: The photo resist formed in step 8-1 is removed
using chemicals.
[0153] Step 8-3: A transparent conductive film having a film
thickness of 150 nm which is made of indium oxide (In.sub.2O.sub.3)
and tin oxide (SnO.sub.2) is formed on the protective film 8 using
a sputtering method. The transparent conductive film is also formed
on inner walls of openings formed in the protective film in step
8-1 and is brought into contact with portions of the source
electrodes 6a at bottom portion thereof. Thereafter, a photo resist
is applied to the transparent conductive film.
[0154] Step 8-4: The photo resist formed in step 8-3 is exposed
using a photo mask having a light shielding pattern corresponding
to the arrangement of the pixel electrodes 7 in the main surface of
the substrate 11, and the photo resist other than the photo resist
where the pixel electrodes 7 are formed is removed by developing.
Subsequently, the transparent conductive film which is not covered
with the photo resist is etched so as to remove the transparent
conductive film between the pixel electrodes 7 as shown in FIG. 11
and FIG. 12. Finally, the photo resist remaining on the pixel
electrodes 7 is removed using the chemicals.
[0155] At a stage that step 8-4 is completed, a step corresponding
to step 9 in the first embodiment is started. Accordingly, in this
embodiment, the orientation film 9 is formed over the protective
film 8 and the pixel electrodes 7 which are formed over the
protective film 8.
[0156] In this embodiment, as shown in FIG. 12, the protective film
8 is formed by laminating material layers 8a, 8b, 8c in three
layers from the switching elements in the reverse order. All of
three layers are made of silicon nitride material expressed by a
chemical structural formula Si.sub.xN.sub.yX.sub.z, (X being a
general term of other constitutional element, all three layers
satisfying y/x>0). It is not exaggerating to mention that all of
three layers are made of material containing silicon (Si) and
nitrogen (Ni) as main constitutional elements.
[0157] Here, the material layer 8a corresponds to the upper-side
layer (the silicon nitride film 8a in the first embodiment) of the
above-mentioned protective film structure according to the present
invention and the material layer 8b corresponds to the lower-side
layer (the silicon nitride film 8b in the first embodiment) of the
protective film structure. The material layer 8c is served for
controlling the etching condition of the protective film 8 in the
above-mentioned step 8-1 so as to form inner walls of the openings
formed above the source electrodes 6a into proper inclined faces.
The material layer 8c is made thin compared to the material layer
8b and is a so-called Si-rich layer which exhibits the composition
ratio (y/x) of nitrogen/silicon lower than that of the material
layer 8b. To compare the material layer 8a with the material layer
8c, it is preferable to set the thickness of the material layer 8c
to a value not more than the thickness of the material layer 8a.
Further, it is preferable to set the composition ratio (y/x) of
nitrogen/silicon of the material layer 8c to a value not less than
that of the material layer 8a. However, with respect to the
relationship between the material layer 8a and the material layer
8c, even when these recommended conditions relating to the
thickness and the composition are not taken into account, the
exercise of the present invention is not obstructed.
[0158] As shown in FIG. 12, the material layer 8c is connected to
the drain electrode 2a and the source electrode 6a which are spaced
apart from each other respectively and strides over a groove
(reaching the semiconductor layer 4) which separates these
electrodes. However, it is not exaggerating to mention that the
short-circuiting of the drain electrode 2a and the source electrode
6a through the material layer 8c can be ignored. The reason is that
the resistivity of the material layer 8c is sufficiently high
compared to the resistivity of the semiconductor layer 4 (an
intrinsic semiconductor layer which constitutes a channel of a thin
film transistor) which the groove separating the drain electrode 2a
and the source electrode 6a reaches and the electric resistance of
portions of the material layer 8c which come into contact with
these electrodes 2a, 6a is sufficiently high. Accordingly, unless
the transitional metal having the high conductive property is
remarkably increased as constitutional elements of the
above-mentioned chemical structural formula Si.sub.xN.sub.yX.sub.z
other than silicon and nitrogen compared to the amounts of silicon
and nitrogen, the drain electrode 2a and the source electrode 6a
can be substantially electrically separated by the material layer
8c.
[0159] On the other hand, on the material layer 8a which exhibits
the low resistivity compared to the resistivity of the material
layer 8b, the pixel electrodes 7 which are formed of a transparent
conductive film made of indium-tin-oxide (ITO) or indium-zinc-oxide
(IZO) are formed. Accordingly, the transparent conductive film
which constitutes respective pixel electrodes 7 is separated by
etching for every pixel in the above-mentioned step 8-4 and hence,
it appears that these pixels 7 are made conductive with each other
through the material layer 8a which exhibits the remarkable photo
conduction compared to the conventional protective film such as the
material layer 8b. However, although the resistivity of the
material layer 8a is lower than the resistivity of the material
layer 8b, to re-distribute the charge distributed among respective
pixel electrodes 7 in response to the image display operation of
the liquid crystal display device, the resistivity is held at a
high value. This can be understood from the fact that the
resistivity is set to 1.7-3.0.times.10.sup.4 .OMEGA.cm in an
example in which the pixel electrodes 7 are made of
indium-tin-oxide. Accordingly, when the pixel electrodes 7 exhibit
the resistivity comparable to the resistivity of metal, that is,
1.times.10.sup.3 .OMEGA.cm, the electric short-circuiting between
the pixels 7 through the material layer 8a during the image display
operation can be ignored.
[0160] On the other hand, particularly at a point of time that the
image display operation of the liquid crystal display device is
completed, the material layer 8a exhibits a unique effect. That is,
at a point of time that the image display operation of the liquid
crystal display device is completed, applying of scanning signals
to the switching elements (thin film transistors in this
embodiment) provided to respective pixels is terminated.
Accordingly, the charge which corresponds to the video signal taken
in immediately before the completion of the image display operation
remains in the pixel electrodes 7. Although efforts have been made
to erase the image retention from the display screen of the liquid
crystal display device by removing the residual charge from the
pixel electrodes 7 from a viewpoint of driving method of the liquid
crystal display device. However, the sufficient effects have not
been obtained. To the contrary, by bonding the material layer 8a
which exhibits the resistivity lower than that of the conventional
protective film and the pixel electrode 7, it is possible to
release at least a portion of the residual charge in the inside of
the pixel electrode 7 to the material layer 8a.
[0161] The advantageous effect of this material layer 8a can be
explained as follows in view of the comparison of the material
layer 8a with the conventional protective film. In the conventional
protective film, the resistivity of the protective film is too high
to release the residual charge from the pixel electrode 7. As a
result, the residual charge of the pixel electrode 7 is discharged
from the pixel electrode 7 such that the residual charge is
gradually leaked to the video signal line 2 through the channel of
the switching element. Accordingly, even after the completion of
the image display operation of the liquid crystal display device, a
considerable amount of charge remains in the pixel electrode 7 for
a long time and hence, the image retention of a level which makes a
user of the liquid crystal display device recognize the image
retention is displayed on the display screen. To the contrary, even
assuming that the material layer 8a of this embodiment cannot
discharge the charge from the surface which faces the liquid
crystal layer of the liquid crystal display panel in an opposed
manner in a short time, the fact that the state that the charge
which is locally held in the specific pixel electrode 7 can be
solved in a short time is apparent from the above-mentioned
explanation that the weak conductive state between the pixel
electrode 7 and the material layer 8a allows the release of the
residual charge in the inside of the pixel electrode 7 to the
material layer 8a. That is, considering that the difference in the
residual charge amount among the pixel electrodes 7 of the liquid
crystal display device makes the user recognize the image retention
on the display screen, it is appreciated that the leaking of the
residual charge in the pixel electrodes 7 to the material layer 8a
in the liquid crystal display device of this embodiment narrows the
difference in the residual charge amount among the pixel electrodes
7 and hence, it is possible to obtain the advantageous effect that
the image retention on the display screen can be suppressed.
[0162] With respect to the liquid crystal display device according
to this embodiment, as an example of the structure which is
desirable for discharging the residual charge from the pixel
electrode through the above-mentioned material layer 8a, FIG. 13
shows an improved video signal line terminal. The video signal line
2 shown in FIG. 11 extends to an end portion of the TFT substrate
11 together with the semiconductor layer 4 (also including the
semiconductor layer 4a) disposed below the video signal lines 2 and
forms a terminal (the video signal line terminal) shown in FIG. 13
at a place outside the sealing material 14 (see FIG. 4). FIG. 13A
is a plan view which shows one planar structure of the terminal in
an enlarged manner and FIG. 13B is a cross-sectional view taken
along a line B-B' in FIG. 13A (however, only the TFT substrate 11
and the laminated structure on the TFT substrate 11 shown).
[0163] With respect to this video signal line terminal, an opening
which reaches the video signal line 2 from the uppermost surface of
the protective film 8 prepared in the above-mentioned step 8 is
formed together with the opening which reaches the source electrode
6a from the uppermost surface of the protective film 8 in the
above-mentioned steps 8-1 and 8-2. Subsequently, the transparent
conductive film 7a which extends to the uppermost surface of the
protective film 8 from a bottom portion of the opening is formed
together with the above-mentioned pixel electrode 7 in the
above-mentioned steps 8-3 and 8-4. Accordingly, the transparent
conductive film 7a which is formed in the opening or the through
hole shown in FIG. 13A and FIG. 13B constitutes the terminal which
receives video signals to be supplied to the video signal line 2.
To this terminal, the output terminal of the video signal driving
circuit H-DRV shown in FIG. 15 is electrically connected directly
or through a flexible printed circuit board.
[0164] Compared to a case in which the transparent conductive film
7a is not formed in the opening shown in FIG. 13A and FIG. 13B and
an output from the video signal driving circuit H-DRV is connected
to the video signal line 2 exposed through the opening, in this
terminal structure which forms the transparent conductive film 7a
in the opening, it is possible to increase the area of the electric
connection with the electrode of the semiconductor device which
constitutes the video signal driving circuit or the terminal of the
line which transmits the output signal from the semiconductor
device, and it is also possible to prevent the corrosion of the
conductive layer of the video signal line 2 in the atmosphere of
the liquid crystal display panel.
[0165] In this embodiment, since the transparent conductive film 7a
also comes into contact with the material layer 8a which
constitutes the protective film 8, the residual charge of the pixel
electrode 7 which leaks to the material layer 8a in the
above-mentioned manner can be released to the external circuit of
the liquid crystal display panel (video signal driving circuit in
the case shown in FIG. 13) through the terminal. Even when another
material layer having resistivity higher than the resistivity of
the material layer 8a is formed on the material layer 8a, the
material layer 8a and the transparent conductive film 7a are
brought into contact with each other in the inner wall of the
opening (the inclined surface shown in FIG. 13A) and hence, the
above-mentioned advantageous effect is not damaged. However, when
this another material layer exhibits the insulation property
similar to the insulation property of the material layer 8b, it is
preferable to make the thickness of another material layer smaller
than the thickness of the material layer 8a so as to generate a
tunneling current between the material layer 8a and the pixel
electrode 7. When such a terminal structure is provided as a
terminal which applies the reference potential or the ground
potential to the TFT substrate or a desired portion (for example,
the common electrode 13 provided to the color filter substrate 12)
of the liquid crystal display panel through the TFT substrate 11,
the advantageous effect is further enhanced. Here, in view of the
above-mentioned explanation that the image display operation is not
damaged even when the pixel electrode 7 is brought into contact
with the material layer 8a, it is apparent that such a terminal
structure does not obstruct the supply of signals from the video
signal driving circuit H-DRV to the video signal line 2. Further,
by forming an opening which reaches the scanning signal line 1 from
the uppermost surface of the protective film 8 through the
insulation film 5 and then by providing a scanning signal line
terminal formed of the transparent conductive film which extends to
the uppermost surface of the protective film 8 from the bottom
portion of the opening, it is possible to obtain the same
advantageous effects as the above-mentioned video signal line
terminal.
[0166] The terminal structure shown in FIG. 13A and FIG. 13B is
applicable to the in-plane-switching type liquid crystal display
device which is explained in the first embodiment. In this case, it
is preferable to perform the above-mentioned steps 8-1 to 8-4 after
the above-mentioned step 9. Further, in step for forming the
opening in the protective film 8 using the photolithography, it is
preferable to apply the photo resist to the upper surface of the
protective film 8 while covering the substantially entire area of
the main surface of the TFT substrate 11. However, it is possible
to limit the exposure and developing of the photo resist to a
peripheral portion of the TFT substrate 11.
[0167] Here, the transparent conductive film described in this
specification indicates, for the sake of convenience, a conductive
film which has the optical transmissivity sufficient to propagate
light irradiated from the liquid crystal layer to the substrate
made of material having high optical transmissivity such as glass
or plastic. Here, the transparent conductive film does not exclude
the conductive film which has property to absorb light incident on
the conductive film. Further, in this embodiment, although the
substrate which faces the TFT substrate in an opposed manner is
referred to as the color filter substrate, the exercise of the
present invention is not obstructed even when the color filters are
formed on the pixel electrodes 7 using a technique such as
electrodeposition. In such an embodiment, the color filter
substrate is replaced with the term "the counter substrate which
faces the TFT substrate in an opposed manner". Further, even when
the thin film transistors which are used as the switching elements
in this embodiment are replaced with diodes adopting the MIM
(Metal-Insulator-Metal) type laminating structure, the exercise of
the present invention is not obstructed. To include such a case, it
is possible to replace the above-mentioned term "TFT substrate
(substrate provided with the switching elements)" with the term
"the first substrate" and to replace the above-mentioned term "the
substrate which faces the TFT substrate in an opposed manner (the
color filter substrate in this embodiment)" with the term "the
second substrate".
Third Embodiment
[0168] As the third embodiment of the liquid crystal display device
of the present invention, a case in which the above-mentioned
protective film is adopted by the vertically aligned type (also
referred to as VA type) liquid crystal display device is explained
in conjunction with FIG. 14 which is a cross-sectional view.
[0169] Since the detail of the VA type liquid crystal display
device is explained in Japanese Laid-open Patent Publication
122065/2000, for example, the explanation relating to the
orientation mode of liquid crystal molecules and the behavior of
the liquid crystal molecules in response to an electric field which
features the VA type liquid crystal display device is omitted. Only
the pixel structure which features the VA type liquid crystal
display device is explained hereinafter.
[0170] In the VA type liquid crystal display device, switching
elements and pixel electrodes 7 which are connected to the
switching elements are formed on a main surface of the first
substrate 11 and counter electrodes 13 which form electric fields
in a liquid crystal layer 15 together with the pixel electrodes 7
are formed on a main surface of the second substrate 12 (facing the
first substrate 11 in an opposed manner while sandwiching the
liquid crystal layer 15 therebetween). Such a structure is
substantially in common with the structure of the TN type liquid
crystal display device of the above-mentioned second embodiment.
However, the VA type liquid crystal display device is characterized
in that at least one of the pixel electrode 7 and the counter
electrode 13 is constituted of a plurality of conductive layers
which are spaced apart from each other within the pixel (portion
which faces the color filter CF in FIG. 14) or at least two kinds
of inclined faces are formed on the main surface at the
liquid-crystal-layer 15 side. Such an electrode structure generates
at least two kinds of electric fields which differ in the electric
field applying direction with respect to the liquid crystal layer
15 in one pixel. As one of the features of the VA type liquid
crystal display device, it is pointed out that at least one
electrodes out of the pixel electrodes and the counter electrodes
include portions where one electrodes do not face the other
electrodes within the main surface of the substrate in the inside
of the pixels (for example, openings formed in a black matrix or
regions defined by profiles of the color filter layers).
[0171] In this embodiment, as shown in FIG. 14, each pixel
electrode 7 and each counter electrode 13 are respectively divided
into portions within the pixel. Respective divided portions of the
pixel electrode 7 and the counter electrode 13 are made conductive
to each other at the periphery of the pixel in the same manner as
the pixel electrode 6 and the counter electrode 3a of the
in-plane-switching type liquid crystal display device shown in FIG.
1. Further, two kinds of inclined faces which differ in the
inclination with respect to the main surface of the so-called first
substrate 11, that is, the first inclined face directed in the
right upward direction and the second inclined face directed in the
left upward direction are formed on each portion of the pixel
electrode 7. On the other hand, two kinds of inclined faces which
differ in the inclination with respect to the main surface of the
so-called second substrate 12, that is, the first inclined face
directed in the left downward direction and the second inclined
face directed in the right downward direction are formed on each
portion of the counter electrode 13. The respective portions of the
pixel electrode 7 and the respective portions of the counter
electrode 13 are arranged such that the first inclined face of the
former faces the first inclined face of the latter while
sandwiching the liquid crystal layer therebetween and the second
inclined face of the former faces the second inclined face of the
latter while sandwiching the liquid crystal layer therebetween. Due
to such a constitution, an electric field E1 is generated between
the first inclined face of the pixel electrode 7 and the first
inclined face of the counter electrode 13 due to applying of a
signal voltage to the pixel electrode 7, while an electric field E2
is generated between the second inclined face of the pixel
electrode 7 and the second inclined face of the counter electrode
13 due to applying of a signal voltage to the pixel electrode 7.
However, the behavior (displacement of orientation direction) of
liquid crystal molecules due to the electric field E1 and the
behavior of liquid crystal molecules due to the electric field E2
appear to be different from each other at the side (upper side in
FIG. 14) from which the display image on the liquid crystal display
panel is observed. That is, with respect to the video signal
supplied to one pixel, the different orientation states of liquid
crystal molecules are present in the pixel. Here, it has been
reported that by removing at least one or both of the orientation
films 9 on the first substrate side and the second substrate side
shown in FIG. 4, the liquid crystal molecules can be oriented as
mentioned above.
[0172] As a problem attributed to the image display by the liquid
crystal display device, the reduction of contrast corresponding to
the increase of an angle with respect to the normal direction of
the display screen (a so-called viewing angle) is named. This is
attributed to a phenomenon that the intensity of light irradiated
from a certain pixel through the liquid crystal layer is deviated
from a desired value due to the increase of the viewing angle.
[0173] However, with respect to the pixel of this embodiment, even
when the intensity of light which the liquid crystal molecules
oriented by the electric field E1 propagates is deviated
corresponding to the viewing angle, the deviation is compensated by
the intensity of light which the liquid crystal molecules
orientated by the electric field E2 propagates so as to prevent the
reduction of contrast. Alternatively, with respect to the pixel of
this embodiment, the reverse compensation may be performed so as to
prevent the reduction of contrast.
[0174] The TFT substrate of this embodiment adopts the thin film
transistors having the top gate structure. The feature of the top
gate structure lies in the structure that the gate electrode 1a is
arranged on the semiconductor layer 4 which constitutes the channel
of the thin film transistor with respect to the main surface of the
first substrate 11. Compared to the structure of the thin film
transistors illustrated in the first embodiment and the second
embodiment, the arrangement of the gate electrode 1a, the source
electrode 6a and the drain electrode 2a is reversed while
sandwiching the insulation film 5 between them. The top gate
structure adopted by this embodiment is suitable for a case in
which the semiconductor layer 4 has to be produced at the time of
starting the processing step of the first substrate different from
other wiring layer. For example, it is preferable to apply
annealing to the amorphous semiconductor layer 4 by laser
irradiation so as to turn the semiconductor layer 4 into a state
close to a poly-crystalline state or or a single crystal state. In
the liquid crystal display device of this embodiment, there exists
no problem in changing the thin film transistors into the bottom
gate structure as shown in the first embodiment and the second
embodiment.
[0175] To form the above-mentioned inclined faces on the upper
surface of the pixel electrode 7, holding capacitance electrodes 1d
are formed onto the insulation film 5 together with the gate
electrodes 1a and inclinations are provided to the upper surface of
the insulation film 5 by properly selecting etchants. The
protective film 8 is formed on the insulation film 5 such that the
protective film 8 covers the gate electrodes 1a and the holding
capacitance electrodes 1d. Openings which reach the upper surfaces
of the source electrodes 6a of the thin film transistors are formed
in the protective film 8. The pixel electrodes 7 are formed such
that the transparent conductive film extends to an upper surface of
the protective film 8 from the source electrodes 6 disposed at
bottom portions of the openings. The material layers 8a, 8b which
constitute the protective film 8 of this embodiment are
respectively formed substantially in the same manner as the
material layers 8a, 8b of the second embodiment and, at the same
time, the pixel electrodes 7 are formed substantially in the same
manner as the pixel electrodes of the second embodiment. Further,
the protective film 8 of this embodiment can also obtain the
advantageous effects similar to those described in the second
embodiment. Still further, by forming the scanning signal line
terminals of this embodiment as shown in FIG. 13A and FIG. 13B, the
above-mentioned advantageous effects become remarkable.
[0176] On the other hand, also on the second substrate of this
embodiment, the protective film 18 which is formed by laminating
material layers 18a, 18b having compositions equal to those of the
material layers 8a, 8b formed on the first substrate is formed. On
an upper surface (a lower layer in FIG. 14) of the material layer
18a which exhibits the lower resistivity compared with the material
layer 18b, the counter electrodes 13 made of a transparent
conductive film similar to the above-mentioned pixel electrodes 7
are formed. In forming the inclined faces on upper surfaces (lower
surfaces in FIG. 14) of the counter electrodes 13, taking into an
account the fact that light is propagated from the liquid crystal
layer 15 to the second substrate 12 side, the formation of the
metal films or alloy films having a triangular shape such as the
above-mentioned holding capacitance electrodes 1d as background
films is obviated. In place of the formation of such metal films or
alloy films, the counter electrodes 13 are patterned respectively
by etching the transparent conductive film and, thereafter, the
inclined faces are formed by further etching the transparent
conductive film after changing the etching conditions.
[0177] Also when the counter electrodes 13 are formed in a
spaced-apart manner from each other on the second substrate 12 as
in the case of this embodiment, it is preferable to adopt the
protective film 18 (also referred to as an overcoat film) according
to the present invention. Particularly, in the common inversion
driving in which the potential of the counter electrodes of the
liquid crystal display device is changed every frame or every time
a given number of scanning signal lines 2 are operated, it is
possible that the residual charge amount differ between the counter
electrodes 13 also at the second substrate 12 side. Accordingly, in
erasing the image retention, it is important to discharge the
residual charge from each counter electrode 13 and to eliminate the
potential difference which is generated between the counter
electrodes 13. In this manner, the formation of the protective film
18 according to the present invention onto the second substrate
which faces the first substrate on which the switching elements are
formed in an opposed manner brings about the image retention
reduction effect when each counter electrode 13 is divided in
response to a group of pixels even in the TN type liquid crystal
display device described in the second embodiment.
[0178] Although the liquid crystal display devices which adopt the
protective film structure of the present invention have been
explained heretofore in conjunction with the first to third
embodiments, the scope to which the protective film structure is
applicable is not limited to the disclosure of the embodiments.
Further, the novel protective film which exhibits the low
resistivity and is formed at a position close to the liquid crystal
layer side is not always referred to as the silicon nitride layer
or the material layer as mentioned above in response to the mode
for carrying out the present invention. That is, the novel
protective film may be referred to other terms. For example, in the
step in which the above-mentioned material layer 8b and the
material layer 8a are laminated to the main surface of the
substrate in this order, the former may be also referred to as the
first protective film layer (8b) and the latter may be also
referred to as the second protective film layer (8a).
[0179] The liquid crystal display devices of the present invention
which have been described heretofore bring about following
advantageous effects with respect to the liquid crystal display
panel as a single body as well as the whole liquid crystal display
module product in which the liquid crystal display panel is
incorporated.
[0180] First of all, since the characteristics to alleviate the
image retention which is generated on the display screen of the
liquid crystal display device is enhanced, it is possible to
provide the liquid crystal display device and the liquid crystal
display module which exhibit the excellent image display
quality.
[0181] Secondly, the above-mentioned liquid crystal display device
(liquid crystal display panel) which exhibits the excellent image
display quality can be manufactured by only controlling the film
forming conditions using the existing vapor deposition apparatus
without introducing new manufacturing device. Accordingly, the mass
production process conditions can be easily determined. Further,
the manufacturing yield rate of the liquid crystal display device
and the liquid crystal display module which incorporates the liquid
crystal display device therein can be maintained at a high
level.
[0182] Thirdly, it becomes no more necessary to make the driving
method of the liquid crystal display device complicated for erasing
the image retention and hence, the driving circuits mounted in the
liquid crystal display panel can be simplified and
miniaturized.
[0183] Fourthly, irrespective of the configuration of the
backlight, the side light (edge light) and the front light, in the
liquid crystal display panel which incorporates the light source
therein, the photo conduction (photo conductive phenomenon) is
generated on the surface of (or in the vicinity of) the
above-mentioned protective film according to the present invention
due to light irradiated from the light source. Accordingly, it is
possible to instantly erase the image retention from the display
screen.
* * * * *