Transflective liquid crystal display device having a color filter and method for fabricating thereof

A color filter substrate for a liquid crystal display device and method of fabricating the color filter substrate are provided. The color filter substrate includes a base substrate having a transmissive portion and a reflective portion, the transmissive portion having a groove, a color filter layer on the base substrate, and a black matrix on the color filter layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display device.

2. Description of Related Art

Recently, liquid crystal display (LCD) devices with light, thin, and low power consumption characteristics are used in office automation equipment and video units and the like. Such LCDs typically use a liquid crystal (LC) interposed between upper and lower substrates with an optical anisotropy. Since the LC has thin and long LC molecules, the alignment direction of the LC molecules can be controlled by applying an electric field to the LC molecules. When the alignment direction of the LC molecules is properly adjusted, the LC is aligned and light is refracted along the alignment direction of the LC molecules to display images.

In general, LCD devices are divided into transmissive LCD devices and reflective LCD devices according to whether the display device uses an internal or external light source.

A conventional transmissive LCD device includes an LCD panel and a backlight device. The incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. The transmissive LCD device requires a high, initial brightness, and thus electrical power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device, and the battery can not be used for a lengthy period of time.

In order to overcome the problems described above, the reflective LCD has been developed. Since the reflective LCD device uses ambient light instead of the backlight by using a reflective opaque material as a pixel electrode, it is light and easy to carry. In addition, the power consumption of the reflective LCD device is reduced so that the reflective LCD device can be used as an electric diary or a PDA (personal digital assistant).

However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that of the outdoors. Therefore, the reflective LCD device can not be used where the ambient light is weak or does not exist. In order to overcome the problems described above, a transflective LCD device has been researched and developed. The transflective LCD device can be transferred according to the user's selection from the transmissive mode to the reflective mode, or vise versa.

FIG. 1is a schematic perspective view of a conventional transflective LCD device11.

InFIG. 1, the conventional transflective LCD device11includes upper and lower substrates15and21with an interposed liquid crystal23. The upper and lower substrates15and21are sometimes respectively referred to as a color filter substrate and an array substrate. On a surface facing the lower substrate21, the upper substrate15includes a black matrix16and a color filter layer18. The color filter layer18includes a matrix array of sub-color filters17of red (R), green (G), and blue (B) that are formed such that each color filter is bordered by the black matrix16. The upper substrate15also includes a common electrode13over the color filter layer18and over the black matrix16. On a surface facing the upper substrate15, the lower substrate21includes an array of thin film transistors (TFTs) “T” that act as switching devices. The array of TFTs is formed to correspond with the matrix of color filters. A plurality of crossing gate and data lines25and27are positioned such that a TFT is located near each crossing of the gate and data lines25and27. The lower substrate21also includes a plurality of pixel electrodes19, each in an area defined between the gate and data lines25and27. Such areas are often referred to as pixel regions “P.” Each pixel electrode19includes a transmissive portion “A” and a reflective portion “C”. The transmissive portion “A” is usually formed from a transparent conductive material having a good light transmittance, for example, indium-tin-oxide (ITO). Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion “C”.

FIG. 2is a schematic cross-sectional view of a conventional transflective LCD device such as the device11ofFIG. 1.

InFIG. 2, upper and lower substrates15and21are facing and spaced apart from each other and a liquid crystal layer23is interposed therebetween. A backlight apparatus45is disposed over the outer surface of the lower substrate21. On the inner side of the upper substrate15, a color filter layer18for passing only the light of a specific wavelength and a common electrode14functioning as one electrode for applying a voltage to the liquid crystal layer23are subsequently formed. On the inner surface of the lower substrate21, a pixel electrode32functioning as the other electrode for applying a voltage to the liquid crystal layer23, a passivation layer34having a transmissive hole31exposing a portion of the pixel electrode32, and a reflective plate36are subsequently formed. An area corresponding to the reflective plate36is a reflective portion “C” and an area corresponding to the portion of the pixel electrode32exposed by the transmissive hole31is a transmissive portion “A”.

A cell gap “d1” at the transmissive portion “A” is about twice of a cell gap “d2” at the reflective portion “C” to reduce the light path difference. A retardation “Δn ·d” of the liquid crystal layer23is defined by a multiplication of refractive index anisotropy “Δn” with a cell gap “d” and the light efficiency of the LCD device is proportional to the retardation. Therefore, to reduce the difference of light efficiencies between the reflective and transmissive modes, the retardations of the liquid crystal layer23at two portions should be nearly equal to each other by making the cell gap of the transmissive portion larger than that of the reflective portion.

However, even though the light efficiencies of the liquid crystal layer between the reflective and transmissive modes become equal by making the cell gaps different, the light passing the color filters at different locations is different so that the brightness can be different at the front of the display device. The transmittance of the color filter resin whose absorption coefficient is high for a specific wavelength and low for the other wavelengths has the following relation considering only the absorption, i.e., the transmittance is inversely proportional to the absorption coefficient and the distance that light passes:
T=exp(−α(λ)d)
where T is transmittance, α(λ) is an absorption coefficient depending on the wavelength and d is a distance that light passes.

Since the color filter resin is a viscous material, the thickness of the color filter resin is hard to control and the color filter layer can not be made less than a specific thickness. Therefore, the color filter layers of the reflective and transmissive portions have the same thickness and the different absorption coefficient (i.e., different material) for the uniform transmittance.

However, if the color filter layers of the reflective and transmissive portions are formed of different materials, the process and the cost would be increased and the yield would be decreased.

To solve the above problems, a fabricating method of the color filter layers with the same resin is suggested. In this method, the color filter layers at the reflective and transmissive portions have the same absorption coefficient but a different thickness so that the transmittance has the same value.

FIGS. 3A and 3Bare transmittance spectrums of first and second red color filter layers for the reflective mode having a specific thickness and two times the specific thickness, respectively.

Generally, a visible light has a wavelength ranging about 400 to 700 nanometers. Red (R), green (G) and blue (B) colors roughly correspond to wavelengths of 650, 550 and 450 nanometers, respectively.

InFIG. 3A, the transmittances at wavelengths corresponding to R, G and B are about 97%, 20% and 58%, respectively. Even though the transmittance for red color is high, the transmittances for the other colors are also not negligible so that a satisfying color purity is not obtained.

InFIG. 3B, since the second red color filter layer has twice the thickness and square transmittance compared with the first red color filter layer ofFIG. 2A, the transmittances at wavelengths corresponding to R, G and B are about 94%, 4% and 34%, respectively. Although the transmittance is decreased for all colors, the decreased amount is different for the individual colors, for example, about 5%, 16% and 24% for R, G and B, respectively.

Therefore, the color purity of the second red color filter layer is improved and this result can be applied for the green and blue color filters so that the transmittance and color purity of the transflective LCD device using the same kind of color filter resin can be uniform for the reflective and transmissive portions.

A transflective LCD device using a dual thickness color filter (DCF) of the above-mentioned principle is suggested in Korean Patent Application No. 2001-9979 of the applicant.

FIG. 4is a cross-sectional view of a transflective LCD device using the DCF according to a related art.

InFIG. 4, a transparent buffer layer64is formed on the inner surface of the upper substrate15only at a reflective portion “C”, and a color filter layer62is formed on the entire upper substrate15. Therefore, the color filter layer62of a transmissive portion “A” is thicker than that of the reflective portion “C” so that the color purity of the transmissive portion “A” can be improved. The transparent buffer layer64is formed by depositing and patterning one of an insulating material group comprising acrylic resin, benzocyclobutene (BCB) and silicon nitride (SiNx). Therefore, the buffer layer64of a yellowish color is not perfectly transparent and the transmittance of the buffer layer64is lower than that of glass substrate. Moreover, since light is partially reflected at the interface between the buffer layer64and the substrate15, the transmittance at the reflective portion “C” is more decreased.

FIGS. 5A and 5Bare cross-sectional views of color filter substrates using the DCF having transparent buffer layers of first and second thicknesses, respectively, according to a related art.

InFIG. 5A, the substrate15has a transmissive portion “A” and a reflective portion “C”. A black matrix70and a transparent buffer layer64are formed in the reflective portion “C” and a color filter layer62is formed on the entire surface of the substrate15. Since the transparent buffer layer64of a first thickness has a low step52at the borderline of the transmissive portion “A” and the reflective portion “C” so that the surface of the color filter layer62can be planarized. Moreover, since the color filter layer62at the transmissive portion “A” is thicker than that at the reflective portion “C”, the color purity can be improved at the transmissive portion “A”. However, since the thickness of the transparent buffer layer64has a limit for the planarization of the color filter layer62, the thickness ratio of the color filter layer62also has a limit and the improvement of the color purity is limited.

InFIG. 5B, to have a desired thickness ratio of the color filter layer62, the transparent buffer layer64has a second thickness higher than the first thickness ofFIG. 5Aand a high step54at the borderline of the transmissive portion “A” and the reflective portion “C”. Since the color filter layer64is made of a viscous resin and formed according to the surface of the underlayer, the color filter layer64also has a step55at the top surface. Therefore, the difference “Δd” between the designed thickness d3and the fabricated thickness d4occurs and the improvement of the color purity of the transmissive portion “A” is limited.

Generally, the thickness of a conventional color filter layer for the reflective LCD device is controlled to have the average transmittance in the range of about 55 to 70%. If the thickness of the color filter layer is increased, the transmittance and the color appearance of the color filter layer are varied. For the color filter layer twice as thick as the conventional color filter, the transmittance and the color appearance are 46% and 24.9%, respectively. On the other hand, for the color filter layer 1.3 times as thick as the conventional color filter, the transmittance and the color appearance are 54.7% and 14.1%, respectively. Consequently, if the color filter layer of the transmissive portion is not formed with a desired thickness, the color property of the transmissive portion can not approach that of the reflective portion.

Furthermore, since the step of the color filter layer also degrades the planarization property of the common electrode on the color filter layer, the display quality of conventional LCDs is degraded.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a reflective liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a color filter substrate of a reflective liquid crystal display device that has a high transmittance and color purity, and a manufacturing method of the color filter substrate.

Another object of the present invention is to provide a color filter substrate of a reflective liquid crystal display device that has a high color purity, and a manufacturing method of the color filter substrate.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a color filter substrate for a liquid crystal display device according to an embodiment of the present invention includes: a substrate having a transmissive portion and a reflective portion, the transmissive portion having a groove; a black matrix on the substrate; and a color filter layer on the black matrix and on the substrate.

In another aspect, a method of fabricating a color filter substrate for a liquid crystal display device includes: forming a groove on a substrate, the substrate having a transmissive portion and a reflective portion, the transmissive portion having the groove; forming a black matrix on the substrate; and forming a color filter layer of a first color on the black matrix and the substrate.

In another aspect, a color filter substrate for a liquid crystal display device includes: a substrate having a transmissive portion and a reflective portion; a black matrix on the substrate; a plurality of buffer patterns at the reflective portion, the plurality of buffer patterns having a substantially uneven shape; and a color filter layer at the transmissive and reflective portions.

In another aspect, a method of fabricating a color filter substrate for a liquid crystal display device includes: forming a black matrix on a substrate, the substrate having a transmissive portion and a reflective portion; forming a plurality of buffer patterns at the transmissive portion, the plurality of buffer patterns having a substantially uneven shape; and forming a color filter layer at the transmissive and reflective portions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the present invention, example of which is illustrated in the accompanying drawing.

FIGS. 6A to 6Care schematic cross-sectional views of a color filter substrate illustrating a fabricating process thereof according to a first embodiment of the present invention. The color filter substrate is usable in any type of an LCD device or other display device. InFIGS. 6A to 6C, the substrate112has a transmissive portion “A” and a reflective portion “C”.

InFIG. 6A, a groove114is formed at the transmissive portion “A” of the LCD device by photolithography and etching processes, or other techniques. The depth d5from the top surface of an upper substrate112is determined considering the thickness ratio of the color filter layer between the transmissive and reflective portions “A” and “C”. Preferably, this thickness ratio may be 1:2.

InFIG. 6B, a black matrix116is formed on the substrate112by depositing and patterning a black resin or an opaque metallic material.

InFIG. 6C, a color filter layer118of a first color is formed over the substrate112and a portion of the black matrix116by depositing and patterning a color resin. By repeating this process for second and third colors, a color filter layer of three colors can be selectively formed on the substrate112. The step height “Δd” of the color filter layer118is within a range of about 0.1 to a few micrometers.

FIG. 7a schematic cross-sectional view of a color filter substrate according to a second embodiment of the present invention.

InFIG. 7, the color filter substrate is identical to that in the first embodiment, except that a black matrix116is formed on a color filter layer118and the dimensions of the color filter substrate may vary as needed.

In the case of a transparent plastic substrate, a groove can be formed by a molding process during a fabricating process of the substrate and the molding process is more suitable to the control of the depth than the etching process.

FIGS. 8A to 8Care schematic cross-sectional views of a color filter substrate for an LCD device illustrating a fabricating process thereof according to a third embodiment of the present invention.

InFIG. 8A, a substrate120is made of transparent plastic so that a groove114can be formed at a transmissive portion of the LCD by a molding process during a fabricating process of the substrate120. The use of a transparent plastic material allows an easy control over the profile of the groove114.

InFIG. 8B, a black matrix116is then formed selectively on the substrate112by depositing and patterning a black resin, an opaque metallic material, or the like.

InFIG. 8C, a color filter layer118of a first color is formed on the substrate120and portions of the black matrix116by depositing and patterning a color resin. By repeating this process for second and third colors, a color filter layer of three colors can be selectively formed on the substrate120. This completes the process of forming the color filter substrate according to this embodiment.

FIG. 9is a schematic cross-sectional view of a color filter substrate for an LCD device according to a fourth embodiment of the present invention.

InFIG. 9, the color filter substrate is identical to that of the third embodiment, except that a black matrix116is formed on a color filter layer118and the dimensions of the color filter substrate may vary as needed.

FIGS. 10A to 10Dare schematic cross-sectional views except that of a color filter substrate for an LCD device illustrating a fabricating process thereof according to a fifth embodiment of the present invention. In this embodiment as shownFIGS. 10A to 10D, the substrate112has a transmissive portion “A” and a reflective portion “C”.

InFIG. 10A, after depositing an opaque metallic material on the substrate112, an opaque metal pattern115is formed by selectively removing the deposited opaque metallic material in the transmissive portion “A”.

InFIG. 10B, a groove114is then formed at the transmissive portion “A” by selectively etching the substrate112in the transmissive portion “A” as the opaque metal pattern115is used as a mask for this etching process.

InFIG. 10C, a black matrix116is then formed by selectively etching the opaque metal pattern115.

InFIG. 10D, a color filter layer118of a first color is formed by depositing and patterning a color resin on the substrate112and the black matrix116. By repeating this process for second and third colors, a color filter layer of three colors can be formed selectively on the substrate112.

In the first to fifth embodiments, since the groove of the transmissive portion is formed by etching the substrate itself, a separate buffer layer is not necessary and thus, the production cost of the color filter substrate can be reduced. If the substrate of transparent plastic is used, the etching process for the groove is also unnecessary due to the molding process, which further reduces the production cost. Moreover, since the buffer layer is not used, the substrate of the reflective portion does not have an interface between the buffer layer and the substrate so that the reflection from the interface can not occur and the device performance is improved.

However, since the color filter layer has a step at its top surface, the thickness ratio of the color filter layer between the transmissive and reflective portions has a limit so that the improvement of color property can be somewhat limited. Therefore, a method is provided in the present invention for obtaining a desired thickness ratio and minimizing the step at the top surface of the color filter layer. This method employs a plurality of buffer patterns as discussed below.

FIG. 1is a schematic cross-sectional view of a color filter substrate illustrating the principle of the present invention.

InFIG. 11, a substrate112has a transmissive portion “A” and a reflective portion “C” and a plurality of buffer patterns117having a substantially uneven shape are formed at the reflective portion “C”. The shape of the buffer patterns117can be any shape. Since the plurality of buffer patterns117have a lot of fine grooves, the thickness “d6” of color filter layer of the reflective portion “C” can be reduced or minimized so that the thickness ratio (d6:d7) of the color filter layer between the transmissive and reflective portions “A” and “C” can be increased. Therefore, the color difference between the transmissive and reflective portions “A” and “C” can be further reduced.

FIGS. 12A to 12Fare schematic top plan views of a plurality of buffer patterns usable in a color filter substrate according to several embodiments of the present invention. InFIGS. 12A to 12F, a hatched region means an etched region, i.e., a concave region and a white region means a buffer pattern, i.e., a convex region.

As shown inFIGS. 12A and 12B, a buffer pattern having a plurality of circular concave holes124, and a plurality of circular convex buffer patterns125are provided.

InFIGS. 12C and 12D, a buffer pattern having a plurality of concave holes126and convex buffer patterns127have a rectangular shape.

InFIGS. 12E and 12F, a plurality of buffer patterns128and129are formed along the direction of columns and rows, respectively.

FIGS. 13A to 13Care schematic cross-sectional views of a color filter substrate illustrating a fabricating process thereof according to a sixth embodiment of the present invention.

InFIG. 13A, a black matrix116is formed on a substrate112. The black matrix116has a structure of a single layer of chromium (Cr) or a double layer of chromium (Cr) and chromium oxide (CrOx).

InFIG. 13B, a plurality of buffer patterns117covering the black matrix116are formed only at the reflective portion “C” by depositing and etching a transparent material such as benzocyclobutene (BCB), acrylic resin, or silicon nitride (SiNx). The plurality of buffer patterns117have a substantially uneven shape, e.g., fine grooves142between projections.

InFIG. 13C, a color filter layer118is formed on the plurality of buffer patterns117. Even though the color filter layer118is not planarized, a reduced step155is produced due to the fine grooves142between the plurality of buffer patterns. Here, the height “d8” of the bottom of the grooves142equals the height “d9” of the surface of the substrate112at the transmissive portion “A”. By repeating this process for three color filters, a full color filter layer of three colors can be formed.

FIG. 14is a schematic cross-sectional view of a color filter substrate according to a seventh embodiment of the present invention.

InFIG. 14, the color filter substrate is identical to that shown inFIG. 13C, except that the height “d10” from the bottom surface of grooves144to the bottom surface of the substrate112is larger than the height “d11” of the substrate112at the transmissive portion “A”. In this structure, since the plurality of buffer patterns117can be lowered, a surface step156of the color filter layer118can be further reduced.

FIGS. 15A to 15Dare schematic cross-sectional views of a color filter substrate illustrating a fabricating process thereof according to an eighth embodiment of the present invention.

As shown inFIG. 15A, in this embodiment of the color filter substrate, a substrate200has a transmissive portion “A” and a reflective portion “C”. An opaque metal pattern204is formed selectively on the substrate200only at the reflective portion “C”.

InFIG. 15B, a plurality of buffer patterns206are formed by selectively etching the substrate200while the opaque metal pattern204is used as a mask. In this etching process, the substrate200under the opaque metal pattern204is not etched to become a convex portion of the plurality of buffer patterns206. Also, a portion of the substrate200in the transmissive portion “A” is removed to provide a groove250.

InFIG. 15C, a black matrix208is then formed by further etching the opaque metal pattern204.

InFIG. 15D, a color filter layer210is formed over the substrate200. Even though the color filter layer210has a surface step255between the transmissive and reflective portions “A” and “C”, the surface step255is reduced due to the plurality of buffer patterns206.

FIG. 16is a cross-sectional view of a color filter substrate according to a ninth embodiment of the present invention.

As shown inFIG. 16, the color filter substrate is identical to that shown inFIG. 15D, except that the height “d12” from the bottom surface of the plurality of buffer patterns206to the bottom surface of the substrate200is bigger than the height “d13” of the substrate200at the transmissive portion “A”. The ninth embodiment can be acquired by adding an etching process for the transmissive portion “A” to the process of the eighth embodiment shown inFIGS. 15A to 15D. In the ninth embodiment structure, since the plurality of buffer patterns206can be lowered, a surface step256of the color filter layer210is further reduced so that the color purity of the display device can be further improved.

The plurality of buffer patterns according to the embodiments of the present invention can have a uniform pitch in the range of about 14 to 45 micrometers.

The present invention is not limited to LCD devices, but is applicable to other types of display devices and apparatuses.