Method of fabricating alignment layer for liquid crystal display device

A method of fabricating an alignment layer for a liquid crystal display device includes forming an alignment material layer on a substrate by coating an alignment material, irradiating UV rays onto the alignment material layer and pre-baking the alignment material layer; and post-baking the alignment material layer.

This application claims the benefit of Korean Patent Application No. 10-2007-0027690, filed on Mar. 21, 2007, which is hereby incorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device and more particularly to a method of fabricating an alignment layer for an LCD device.

2. Discussion of the Related Art

The related art LCD devices use an optical anisotropic property and polarization properties of liquid crystal molecules to display images. The liquid crystal molecules have orientation characteristics of arrangement resulting from their thin and long shape. Thus, an arrangement direction of the liquid crystal molecules can be controlled by applying an electrical field to them. Accordingly, when the electric field is applied to them, polarization properties of light is changed according to the arrangement of the liquid crystal molecules such that the LCD devices display images. At least one alignment layer is formed to determine an initial orientation of the liquid crystal molecules.

FIG. 1is a schematic perspective view of a related art LCD device, andFIG. 2is a schematic cross-sectional view of a related art LCD device.

Referring toFIGS. 1 and 2, an LCD device includes an array substrate B1, a color filter substrate B2and a liquid crystal layer40interposed therebetween. In the array substrate B1, a gate line12, a data line24, a thin film transistor (TFT) T and a pixel electrode28are formed on a first substrate10. The gate and data lines12and24cross each other to define the pixel region P, and the TFT T is formed at a crossing portion of the gate and data lines12and24. The pixel electrode28in each pixel region P is connected to the TFT T and receives voltages through the TFT T. The TFT T includes a gate electrode14, a gate insulating layer16, a semiconductor layer18including an active layer18aand an ohmic contact layer18b, a source electrode20and a drain electrode22. The gate electrode14is connected to the gate line12, and the semiconductor layer18on the gate insulating layer16corresponds to the gate electrode14. The source and drain electrodes20and22are disposed on the semiconductor layer18and spaced apart from each other. The source electrode20is connected to the data line24. A passivation layer26exposing a portion of the drain electrode22is formed over the TFT T, and the pixel electrode28is formed on the passivation layer26such that the pixel electrode28is connected to the portion of the drain electrode22. In addition, a first alignment layer42of polyimide is formed on an entire surface of the first substrate10having the pixel electrode28.

In the color filter substrate B2, a black matrix32, a color filter layer34and a common electrode36are formed on a second substrate30facing the first substrate10. The black matrix32is formed on the second substrate30and has a lattice shape. The black matrix32corresponds to a non-display region of the first substrate10. The non-display region of the first substrate10includes the gate line12, the data line24and the TFT T. The color filter layer34includes sub-color filters34a,34band34c, and each of the sub-color filters34a,34band34chaving one of red (R), green (G), and blue (B) colors corresponds to each pixel region P. Although not shown, a planarization layer is formed on the black matrix32and the color filter layer34. The common electrode36is formed over the black matrix32and the color filter layer34. The common electrode36generates an electric field with the pixel electrode28such that the liquid crystal layer40is driven by the electric field. In addition, a second alignment layer44is formed on the common electrode36.

The alignment layers are formed to determine an initial orientation of liquid crystal molecules of the liquid crystal layer. Orientation process, which is divided into a contact type and a non-contact type, is performed to give the alignment layers orientation properties. In the contact type orientation process, a rubbing cloth is used. There is a physical friction between the alignment layer and the rubbing cloth to form a plurality grooves on a surface of the orientation layer. Due to the grooves, the alignment layer has the orientation properties, and the liquid crystal molecules have a pre-determined orientation. On the other hand, in the non-contact type orientation process, an optical reaction is performed onto the alignment layer to give the alignment layer anisotropic properties. The liquid crystal molecules have a pre-determined orientation due to the anisotropic properties.

Unfortunately, because additional processes, such as changing the rubbing cloth, are required in the contact type orientation process, the contact type orientation process makes production costs of the LCD device increasing. Accordingly, the non-contact type orientation process is the subject of significant research and development. Particularly, when multi domains, where the alignment layer is rubbed to have different initial orientations, are required in one pixel region, the non-contact type orientation process is widely used.

In the non-contact type orientation process, the alignment layer is formed from a polyimide resin including an optical functional group. For example, the optical function group includes cyclobutane dianhydride (CBDA).

FIG. 3shows a structure of a polyimide resin used for forming an alignment layer according to the related art. Generally, polyimide is a polymeric material having an imide ring and synthesized from an aromatic anhydride and diamine. Particularly, photoreaction imide has an optical functional group, such as a cyclobutane dianhydride (CBDA) ring inFIG. 3. When ultraviolet (UV) rays is irradiated, the cyclobutane dianhydride (CBDA) ring is opened such that the photoreaction imide having the optical functional group is resolved into maleimide (MI) and a photo-oxide reactant.

FIG. 4shows structures of a polyimide resin, which has a cyclobutane dianhydride (CBDA) ring and an oxi-dianiline (ODA) group before and after UV RAY irradiating, andFIG. 5is a graph showing dichroism and absorbance of a polyimide resin.

Referring toFIGS. 4 and 5, when UV RAY is irradiated onto the polyimide having the cyclobutane dianhydride (CBDA) ring and the oxi-dianiline (ODA) group, the CBDA ring is opened such that maleimide (MI) and the photo-oxide reactant are generated. InFIG. 4, 1376 cm−1, 1240 cm−1and 1501 cm−1represent infrared absorption bands. It is possible to measure the infrared absorption band using a fourier transformation infrared (FT-IR) spectroscopy. Maleimide (MI) obtained by UV RAY irradiating has an infrared absorption band of 1397 cm−1. Moreover, as shown inFIG. 5, an infrared absorbance curve50and a dichroism curve52of maleimide (MI) having the infrared absorption band of 1397 cm−1are overlapped to each other. The infrared absorbance curve50and the dichroism curve52of maleimide (MI) are shown in (b) ofFIG. 5. It is interpreted that maleimide (MI) has directionless properties and does not affect properties of the alignment layer.

In addition, when polyimide is photodecomposited, not only a main reaction producing maleimide (MI) but also a side reaction producing undesired products is generated. The side reaction reduces a molecular weight of polyimide such that the heat-resisting property of the alignment layer of polyimide is degraded. Accordingly, the LCD device having the related art photo-oriented alignment layer, there are some problems such as after images.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a method of fabricating an alignment layer for an LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

An advantage of the present invention is to provide an improved method of fabricating an alignment layer.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, a method of fabricating an alignment layer for a liquid crystal display device includes forming an alignment material layer on a substrate by coating an alignment material; irradiating UV rays onto the alignment material layer, thereby pre-baking the alignment material layer; and post-baking the alignment material layer.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

An alignment material for a method of fabricating an alignment layer includes a polyimide resin having a cyclobutane dianhydride (CBDA) ring ofFIG. 3. A method for obtaining an alignment layer having improved properties is explained withFIGS. 6A to 6C.

FIGS. 6A to 6Cshow fabricating processes of a color filter substrate having an alignment layer according to the present invention.

First, inFIG. 6A, a black matrix102is formed on a substrate100having a plurality of pixel regions P. The black matrix102has a lattice shape corresponding to each pixel region P. Although not shown, an LCD device has a counter substrate. On the counter substrate, a gate line, a data line and a thin film transistor are formed, and the black matrix102corresponds to them. Next, red (R), green (G) and blue (B) color filter patterns102a,102band102care formed on the substrate100having the black matrix102. Each of the red (R), green (G) and blue (B) color filter patterns102a,102band102ccorresponds to each pixel region P. In more detail, a color resin is coated on an entire surface of the substrate100having the black matrix102and then patterned a color resin layer to form one of the red (R), green (G) and blue (B) color filter patterns102a,102band102c. For example, the red (R) color filter patterns102aare disposed at a first pixel line, the green (G) color filter patterns102bare disposed at a second pixel line, and the blue (B) color filter patterns102care disposed at a third pixel line. This is referred to as a stripe type. Considering color purities and electrical properties, the red (R), green (G) and blue (B) color filter patterns102a,102band102chave different thickness.

Accordingly, as shown inFIG. 6B, a planarization layer106is formed on the red (R), green (G) and blue (B) color filter patterns102a,102band102cto form a flat top surface. The planarization layer may be formed of an insulating polymer resin. Next, a common electrode108is formed on the planarization layer106. The common electrode108includes a transparent conductive material, such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO). If the color filter substrate is used for an LCD device having in-plane switching (IPS) mode, the common electrode is not required.

As shown inFIG. 6C, an alignment material layer (not shown) is formed on the common electrode108, and then an orientation process is performed onto the alignment material layer (not shown) to form an alignment layer110. By the orientation process, the alignment layer110has properties of the alignment layer110. The alignment material includes polyimide having cyclobutane dianhydride (CBDA).

When forming the alignment layer110, it is required to minimize oxidation during photodecomposition and to activate the reaction rate such that the lifetime of maleimide (MI) is minimized and the side reaction from maleimide (MI) is prevented.

To do this, in the orientation process, ultra violet (UV) rays are irradiated onto the alignment material layer (not shown). At the same time, the alignment material layer (not shown) is heated. Namely, a first condition of the orientation process is to be performed the UV RAY irradiating and heating onto the alignment material layer (not shown) at the same time. The heating process may be called as a pre-baking process. The pre-baking process is performed under a temperature with a range of about 25 centigrade to about 230 centigrade. Due to theses processes, a photo-reaction and a heat cross-linking reaction are simultaneously occurred. Accordingly, when being photodecomposition of the cyclobutane dianhydride (CBDA) group, maleimide (MI) is much produced and activated to from a network structure, and the side reaction of maleimide (MI) is prevented because of the first condition. As a result, problems of the related art, which results from the side reaction of maleimide (MI), are overcome.

In addition, a second condition of the orientation process is to be performed under nitrogen rather than under oxygen to minimize oxidation.FIG. 7is a graph showing absorbance of carbonyl groups in an alignment layer. The numeral number120ashows a first absorbance graph of carbonyl groups in the alignment layer being oriented under nitrogen, and the numeral number120bshows a second absorbance graph of carbonyl groups in the alignment layer being oriented under oxygen. As shown inFIG. 7, the second absorbance graph120bhas a width greater than the first absorbance graph120a. The oxidation is much activated in the orientation process under oxygen than under nitrogen. The oxidation causes a bad affection to orientation properties. Accordingly, the orientation process is preferred to perform under nitrogen rather than oxygen.

A third condition of the orientation process is that the irradiated UV RAY has an energy density with a range of about 0.05 J/cm2to about 3 J/cm2. This condition is understood byFIG. 8showing a graph of relative intensity versus time. As shown inFIG. 8, when a temperature of a sample is one of 170 centigrade, 200 centigrade and 230 centigrade and UV energy density is about 1 J/cm2, appearance and disappearance of maleimide (MI) has a equilibrium state. Namely, when the UV energy density has about 1 J/cm2, the production rate of maleimide (NI) has a maximum value. In other energy densities, the production rate is increased such that properties of the alignment layer may be degraded. Accordingly, an energy density is required to be controlled. For example, an energy density has a range of about 0.05 J/cm2to 3 J/cm2.

A fourth condition of the orientation process is to perform a post-baking process after UV RAY irradiating and heating. If the heating process explained in the first condition is called as a first baking process, the post-baking process may be called as a second baking process. The reason why the second baking process is required is explained withFIG. 9.FIG. 9is a graph showing difference of retardation values depending on UV density before and after post-baking (PB). As shown inFIG. 9, the alignment layer after the post-baking (PB) process has the retardation value higher than the alignment layer before the post-baking. The higher retardation value the alignment layer has, the higher anisotropic properties the alignment layer has. Accordingly, when the post-baking process is performed, the alignment layer has improved properties.

Moreover, a temperature property depends on the post-baking process. It is explained withFIG. 10showing a glass transition temperature of an alignment layer depending on UV density before and after the post-baking (PB) process. As shown inFIG. 10, when UV RAY is irradiated, the alignment layer after the post-baking (PB) process has a glass transition temperature higher than the alignment layer before the post-baking process (PB). It is because the maleimide (MI) has a network structure by the post-baking (PB) process. A temperature of the post-baking process may be a range of about 25 centigrade to about 230 centigrade.

The alignment layer has much improved properties with a cleaning process before the above-mentioned post-baking process. It is because side-reaction products are removed by the cleaning process. In the cleaning process, a cleaning solution may include isopropyl alcohol. The alignment layer is cleaned by the cleaning solution, and then the alignment layer is further cleaned by deionized (DI) water. After cleaning process, the alignment layer is dried.

Black image produced by an LCD device having the alignment layer, which is photo-oriented under the first, second and third conditions, has brightness as shown in Table 1. The brightness of the black image is measured on points 1 to 6 in the alignment layer. Four samples are obtained by a photo-orientation process under different UV energy densities, and fifth sample is a related art polyimide alignment layer rubbed by rubbing clothes is represented by “R/B PI”.

In the table 1, a black image of the LCD device having the photo-oriented alignment layer according to the present invention has brightness of about 1.1 to about 1.2, while a black image of the LCD device having the related art alignment layer has brightness of about 1.6. Since the black image in the LCD device according to the present invention has a relatively low brightness with compared to the black image in the related art LCD device, contrast ratio is improved in the present invention. The relatively high brightness of the black image in the related art LCD device is resulted from scratches by the rubbing clothes in the rubbing process. In the present invention, since the alignment layer is photo-oriented, there is no problem such as the scratches.

As explained above, the alignment material layer including includes a polyimide resin having a cyclobutane dianhydride (CBDA) ring is coated onto the substrate. Under nitrogen, UV rays having an energy density of about 0.05 J/cm2 to about 3 J/cm2 onto the alignment layer, and the pre-baking process is performed onto the alignment layer at the same time. A temperature of the pre-baking process has a range of about 25 centigrade to about 230 centigrade. Next, the substrate having the alignment material layer is cleaned. Finally, the post-baking process is performed under a temperature with a range of about 25 centigrade to about 230 centigrade. Consequently, the alignment layer is fabricated. In this case, the temperature of the pre-baking process and the post-baking process is determined by considering temperature properties of the planarization layer in the color filter substrate. When the planarization layer has excellent temperature properties, the temperature of the pre-baking process and the post-baking process may be increased.

In the present invention, the alignment layer is photo-oriented by UV RAY irradiation. At the same time the pre-baking process is performed. Moreover, since the photo-orientation process is performed under nitrogen, a side reaction is minimized. Accordingly, the alignment layer has improved properties.

In addition, since UV RAY irradiation has a density with a range of about 0.05 J/cm2to about 3 J/cm2, a production rate of maleimide (MI) is maximized.

Furthermore, a retardation value is increased due to the post-baking process. In addition, since a cleaning process is performed after the pre-baking process and before the post-baking, side reaction products are removed such that properties of the alignment layer are further improved.