In-plane switching mode liquid crystal display with alignment layers of different anchoring energies

An in-plane switching liquid crystal display device includes an inorganic alignment layer having low anchoring energy and an organic alignment layer having higher anchoring energy than that of the inorganic alignment layer. Applying the voltage between electrodes, only liquid crystal molecules in the vicinity of the inorganic alignment layer are affected by the electric field to be twisted about 90.degree. in the liquid crystal layer.

This application claims the benefit of Korean patent application No. 
1996-41513, filed Sep. 21, 1996, which is hereby incorporated by 
reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid crystal display device, and more 
particularly, to an in-plane switching mode liquid crystal display device 
having high brightness. 
2. Discussion of Related Art 
A great need has arisen recently for thin film transistor liquid crystal 
display devices (TFT LCD) having wide-angle images, for use in such 
applications as portable televisions, notebook computers, etc. However, 
the TFT LCD's in use today have a problem in that their contrast ratio 
depends on the viewing-angle. In order to overcome this problem, a number 
of approaches have been proposed, including, for example, a twisted 
nematic LCD having a mounted optical compensator, and a multi-domain LCD, 
among others. However, it is not expected that the contrast ratio and 
color shifting dependence on the viewing angle will be adequately solved 
by such LCDs. 
Another proposal discloses an in-plane switching liquid crystal display 
device. It is suggested that such a device will have a wider viewing 
angle, as disclosed, for example, in the JAPAN DISPLAY 92 P457, Japanese 
Patent Unexamined Publication No. 7-36058, Japanese Patent Unexamined 
Publication No. 7-225338, and ASIA DISPLAY 95 P707. 
FIGS. 1 and 2A-2B show a conventional in-plane switching mode liquid 
crystal display device. As shown in these figures, the alignment 
directions of the substrates 10, 11 are approximately perpendicular to the 
gate bus line 1 formed on the first substrate 10 to align the liquid 
crystal molecules of the liquid crystal layer 19. A first polarizer 25 
having a polarization axis parallel to the gate bus line 1 is attached to 
the substrate 10, and a second polarizer 26 having polarization axis 
direction parallel to the alignment direction of the alignment layers 21a, 
21b is attached to the second substrate 11. Data (or pixel) electrodes 5 
and common electrodes 6 are formed in the direction perpendicular to the 
gate bus lines 1 and the data bus lines 2. The common bus line 7 is 
parallel to the gate bus line 1. A thin film transistor, having a gate 
electrode 4 connected to the gate bus line 1 and whose source/drain 
electrodes 3 are connected to the data bus line 2 and the pixel electrode 
5, is formed at the cross region of the gate bus line 1 and the data bus 
line 2. The liquid crystal display device also includes a shielding layer 
8 such as black matrix on the second substrate 11, a color filter layer 
17, and passivation layers 16a, 16b. 
When no voltage is applied, the liquid crystal molecules are obliquely 
aligned in the direction of the data and common electrodes 5, 6 along the 
alignment direction. When voltage is applied, the liquid crystal molecules 
are rotated and aligned parallel to the gate bus line 1 because of a 
horizontal electric field parallel to the gate bus line 1. The rotation of 
the liquid crystal molecules in turn controls the transmittance through 
the liquid crystal layer 19. 
In this in-plane switching mode liquid crystal display device, since the 
liquid crystal molecules are switching parallel to the surface of the 
substrate 10, 11, the viewing-angle dependence problem is solved and 
contrast ratio is improved. 
If the liquid crystal is a negative type (N-type) liquid crystal, with 
dielectric anisotropy .DELTA..epsilon.&lt;0, the liquid crystal molecules 20 
in the liquid crystal layer 19 are aligned perpendicular to the electrodes 
5, 6 over the entire liquid crystal layer 19, when no voltage is applied, 
as shown in FIG. 2A. When voltage is applied, the liquid crystal molecules 
are rotated and aligned perpendicular to the direction of the field 
applied between the electrodes 5, 6 because of .DELTA..epsilon.&lt;0. Since 
the first and second alignment layers 21a, 21b are made of organic 
materials, such as, for example, indium tin oxide (ITO), having anchoring 
energy higher than the turning effect of the liquid crystal molecules due 
to the horizontal electric field, the only rotated liquid crystal 
molecules are those in the vicinity of the middle plane of the liquid 
crystal layer 19 and not those in the vicinity of the first and second 
substrates 10, 11. As a result, the liquid crystal molecules 29 are 
aligned as shown in FIG. 2B. 
FIGS. 3A-3B show an in-plane switching mode liquid crystal display device 
having a positive type (P-type) liquid crystal and a birefringent 
anisotropy .DELTA..epsilon.&lt;0. When no voltage is applied, as shown in 
FIG. 3A, the liquid crystal molecules 20 are aligned approximately 
parallel to the data and common electrodes 5, 6. When voltage is applied, 
as shown in FIG. 3B, the liquid crystal molecules are rotated and aligned 
parallel to the direction of the electric field. At this time, however, 
only liquid crystal molecules 20 in the vicinity of the middle of the 
liquid crystal layer 19 are rotated, excluding those molecules near the 
first and second substrates 10, 11, due to the presence of an organic 
material-coated layer 21a, 21b. 
In the conventional in-plane switching liquid crystal display device 
described above, the liquid crystal molecules are rotated only in the 
vicinity of the middle plane of the liquid crystal layer 19, so that light 
transmittance is controlled using the birefringence of the transmitting 
material. Thus, insufficient light passes through the liquid crystal layer 
causing image quality deterioration. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to an in-plane switching 
liquid crystal display that substantially obviates one or more of the 
problems due to the limitations and disadvantages of the related art. 
An object of the present invention is to provide a liquid crystal display 
device including an inorganic alignment layer having low anchoring energy 
to improve image quality. 
Additional features and advantages of the present invention will be set 
forth in the description which follows, and will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure and process particularly pointed out in the 
written description as well as in the appended claims. 
In one aspect of the present invention, there is provided a liquid crystal 
display device including: first and second substrates; a liquid crystal 
layer between the first substrate and the second substrate; a gate bus 
line and a data bus line crossing each other on the first substrate, the 
gate bus line and the data bus line defining a pixel region; a thin film 
transistor at a crossing region of the gate bus line and the data bus 
line; at least one pair of electrodes in the pixel region, the at least 
one pair of electrodes applying an electric field parallel to one of the 
first substrate and second substrate; a first alignment layer over the 
first substrate and having a first anchoring energy; and a second 
alignment layer over the second substrate and having a second anchoring 
energy different from the first anchoring energy of the first alignment 
layer. 
In another aspect of the present invention, there is provided a liquid 
crystal display device, including: first and second substrates; a liquid 
crystal layer between the first and second substrates; a plurality of the 
gate bus line and the data bus line respectively crossing each other on 
the first substrate, the gate bus line and the data bus line defining a 
plurality of pixel regions; a plurality of thin film transistors 
respectively at crossing regions of the gate bus line and the data bus 
line; at least one pair of electrodes in the pixel region over the first 
substrate; a first alignment layer over the first substrate, the first 
alignment layer having a first anchoring energy lower than a turning 
effect of the liquid crystal layer by the electric field; and a second 
alignment layer over the second substrate, the second alignment layer 
having a second anchoring energy higher than the turning effect of the 
liquid crystal layer by the electric field. 
In yet another aspect of the invention, there is provided a liquid crystal 
display device including: a first substrate having a first alignment layer 
over the first substrate; a second substrate having a second alignment 
layer over the second substrate; a liquid crystal layer between the first 
and second substrate; a gate bus line and a data bus line crossing each 
other on the first substrate, the gate bus line and the data bus line 
defining a pixel region; a thin film transistor at a crossing region of 
the gate bus line and the data bus line; at least one pair of electrodes 
in the pixel region, the at least one pair of electrodes applying an 
electric field parallel to one of the first substrate and second 
substrate; a passivation layer over the first substrate; a shielding layer 
on the second substrate; and a color filter layer on the shielding layer 
and the second substrate; wherein the first alignment layer has a first 
anchoring energy lower than a turning effect of the liquid crystal layer 
by the electric field; and wherein the second alignment layer has a second 
anchoring energy higher than the turning effect of the liquid crystal 
layer by the electric field. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE INVENTION 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
A brief description of description of the invention will first be made. 
The liquid crystal display device of the present invention includes first 
and second substrates, a plurality of gate bus lines and data bus lines 
crossing each other over the first substrate, a plurality of thin film 
transistors at the crossing region of the gate bus lines and the data bus 
lines, a plurality of data and common electrodes parallel to the data bus 
lines to apply the electric field, a passivation layer over the total area 
of the first substrate, a first alignment layer including an inorganic 
layer on the passivation layer, a shielding layer preventing a current 
leakage through the gate bus line, the data bus line, and the thin film 
transistor, a color filter layer on the shielding layer and the second 
substrate, and a second alignment layer including an organic layer on the 
color filter layer. 
The inorganic alignment layer has an anchoring energy lower than that of 
the organic alignment layer. When no voltage is applied between the pixel 
electrodes and the common electrodes, the liquid crystal molecules are 
aligned perpendicular to the data and common electrodes for an N-type 
liquid crystal. When voltage is applied, only molecules in the vicinity of 
the first substrate are rotated and aligned parallel to the electrodes to 
be twisted from the first substrate to the second substrate because the 
anchoring energy of the first organic alignment layer is higher than the 
turning effect of the liquid crystal molecules. 
FIGS. 4A--4B is a sectional view of the in-plane switching liquid crystal 
device according to the present invention. Although only one pixel is 
represented in this figure, a typical display would include a plurality of 
pixels. 
As shown in FIG. 4A, the common electrodes 106 are formed on the first 
substrate 110, and the gate insulating layer 115 is deposited thereon. The 
data bus line 102 and the pixel electrodes 105 are formed on the gate 
insulating layer 115, and the first passivation layer 116a is deposited 
thereon. Although not shown in this figure, the gate bus line crossing the 
data bus line 102 is formed between the first substrate 110 and the gate 
insulating layer 115, and the thin film transistor is positioned at the 
crossing region of the data bus line 102 and the gate bus line. The gate 
electrode and the source/drain electrode of the TFT are connected to the 
gate bus line and the data bus line 102, respectively. The common 
electrodes 106 are connected to the common bus line (not shown in this 
figure) and are extended in the parallel direction of the gate bus line. 
The data bus line 102, the pixel electrodes 105, and the common electrodes 
106 are arranged at regular intervals. 
The gate bus line, the common bus line, and the common electrodes are 
formed by etching a sputtered metal thin film, including AlTa film. The 
gate insulating layer 115, (including, e.g., SiO.sub.x and SiN.sub.x), and 
the semiconductor layer (including amorphous silicon) of the thin film 
transistor are formed by a plasma chemical vapor deposition process and a 
photo-etching process. At that time, the gate insulating layer 115 and the 
semiconductor layer are successively deposited by a single process. 
The data bus line 102, the source/drain electrodes of the thin film 
transistor, and the pixel electrode 105 are formed by sputtering and 
patterning a metal layer, such as a Cr layer. 
The first passivation layer 116a is deposited over the entire area of the 
first substrate 110 by a plasma chemical vapor deposition process. On the 
first passivation layer 116a, an inorganic material (for example, 
EXP-OA002, produced by NISSAN CHEMICAL) is coated to achieve a thickness 
of about 900 .ANG. and then baked at a temperature of about 210.degree. C. 
to form the first alignment layer 121a. This inorganic alignment layer 
121a has a low anchoring energy. 
A shielding layer 108 is formed over the second substrate 111 to prevent a 
current leakage through the gate bus line, the data bus line 102, the 
common bus line, and the thin film transistor. This shielding layer 108 is 
formed by depositing and patterning a metal such as Cr/CrO.sub.x. The 
color filter layer 117 is deposited on the shielding layer 108 and the 
substrate 111, and the second passivation layer 116b is deposited thereon. 
On the second passivation layer 116b, a polyimide (such as RN 7492, which 
is an organic alignment material produced by NISSAN CHEMICAL), is 
deposited onto the substrate to a thickness of about 900 .ANG.. The 
deposition takes place at a temperature of 210.degree. C. to form the 
second alignment layer 121b. The first and second alignment layers 121a, 
121b are twice rubbed by a rubbing cloth having a turning speed of about 
500 rpm, the moving speed of 10 mm/sec, and a pressing amount of 0.4 mm to 
form the alignment direction on the surface of the alignment layers 121a, 
121b parallel to each other. 
The N-type liquid crystal, having .DELTA..epsilon.&lt;0, is inserted between 
the first and second substrates 110, 111. 
When no voltage is applied between the pixel electrodes 105 and the common 
electrodes 106 in the liquid crystal display device of the present 
invention, the liquid crystal molecules 120 are aligned along the 
alignment directions of the first and second alignment layers 121a, 121b, 
as shown in FIG. 4A. That is, all of the liquid crystal molecules 120 in 
the liquid crystal layer 119 are aligned perpendicular to the electrodes 
105, 106. When a voltage is applied, an electric field is generated 
between the data (or pixel) electrode 105 and the common electrode 106. 
This electric field is strongest at the surface of the first substrate 110 
and weakest at the surface of the second substrate 111. In other words, 
the field in the liquid crystal layer 119 becomes weaker from the first 
substrate 110 to the second substrate 111. This means that the liquid 
crystal molecules are most subject to restraint by the electric field in 
the vicinity of the first substrate 110. 
Further, the inorganic alignment layer 121a over the first substrate 110 
has an anchoring energy lower than the turning effect of the liquid 
crystal molecules 120 caused by the electric field, so that the liquid 
crystal molecules 120 in the vicinity of the first substrate 110 are 
rotated and then aligned parallel to the electrodes 105, 106. Since the 
organic alignment layer 121b over the second substrate 111 has an 
anchoring energy higher than the turning effect of the liquid crystal 
molecules 120, the liquid crystal molecules 120 in the vicinity of the 
second substrate 111 are not subject to restraint by the electric field. 
As a result, the liquid crystal molecules 120 are aligned perpendicular to 
the electrodes 105, 106 along the alignment direction of the alignment 
layer 121b. Accordingly, the liquid crystal molecules 120 are twisted 
about 90.degree. from the first substrate 110 to the second substrate 111. 
The liquid crystal display device of the present invention has a greater 
brightness than a conventional liquid crystal device in which only the 
liquid crystal molecules in the vicinity of the it middle plane of the 
liquid crystal layer rotate. 
FIGS. 5A-5B show a P-type liquid crystal-injected device. This structure is 
similar to the structure in FIGS. 4A-4B except for the type of liquid 
crystal. When no voltage is applied between the data (or pixel) electrodes 
105 and the common electrodes 106, as shown in FIG. 5A, the liquid crystal 
molecules 120 in the liquid crystal layer 119 are aligned parallel to the 
electrodes 105, 106. When voltage is applied, as shown in FIG. 5B, only 
those molecules 120 in the vicinity of the first substrate 110 are rotated 
and aligned perpendicular of the electrodes 105, 106, so that the liquid 
crystal molecules 120 are twisted about 90.degree. from the first 
substrate 110 to the second substrate 111. 
In the present invention, since the first alignment layer is an inorganic 
layer having anchoring energy lower than that of the second alignment 
layer, only liquid crystal molecules in the vicinity of the first 
substrate are affected by the electric field, and are twisted from the 
first substrate to the second substrate. Thus, the amount of light passing 
through the liquid crystal layer increases and resultant brightness 
improves. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof. Thus, it is intended that the 
present invention cover the modifications and variations of this invention 
provided they come within the scope of the appended claims and their 
equivalents.