Transflective LCD device having dual thickness color filter

A transflective LCD device includes first and second transparent substrates spaced apart from each other and having a reflective portion and a transmissive portion, an insulator on the first transparent substrate, a passivation layer on the insulator within the reflective portion, a reflector on the passivation layer, a transparent pixel electrode disposed over the insulator covering the reflector and the passivation layer, a buffer pattern disposed on a rear surface of the second substrate, the buffer pattern having a saw-tooth shape corresponding to the reflective portion, a color filter on the rear surface of the second substrate covering the buffer pattern, the color filter having a first thickness in the transmissive portion and a second thickness in the reflective portion, a transparent common electrode on a rear surface of the color filter, and a liquid crystal layer between the transparent pixel electrode and the transparent common electrode.

The present invention claims the benefit of Korean Patent Application No. 2002-0088486, filed in Korea on Dec. 31, 2002, 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 a method of fabricating a liquid crystal display device, and more particularly, to a transflective liquid crystal display device having a dual thickness color filter and a method of fabricating the same.

2. Description of Related Art

Presently, liquid crystal display (LCD) devices having light weight, thin profiles, and low power consumption characteristics are commonly used in office automation equipment and video units. The LCD devices typically use a liquid crystal (LC) interposed between upper and lower substrates, and make use of optical anisotropy of the LC. Since molecules of the LC are thin and long, an alignment direction of the LC molecules can be controlled by application of an electric field to the LC molecules. When the alignment direction of the LC molecules is properly adjusted, the LC can be aligned such that light is refracted along the alignment direction of the LC molecules to display images.

In general, LCD devices are divided into transmissive-type LCD devices and reflective-type LCD devices according to whether the display device uses an internal or external light source. The transmissive-type LCD device includes an LCD panel and a backlight device, wherein the incident light produced by the backlight device is attenuated during transmission so that the actual transmittance is only about 7%. In addition, the transmissive-type LCD device requires a relatively high initial brightness, whereby electrical power consumption by the backlight device increases. Accordingly, a relatively heavy battery, which cannot be used for an extended period of time, is needed to supply sufficient power to the backlight device.

In order to overcome these problems, the reflective-type LCD has been developed. Since the reflective-type LCD device uses ambient light instead of the backlight device, wherein a reflective opaque material is used as a pixel electrode, the reflection-type LCD device is light and easy to carry. In addition, since the power consumption of the reflective-type LCD device is reduced, it can be used as a personal digital assistant (PDA). However, the reflective-type LCD device is easily affected by its surroundings. For example, since ambient light in an office differs largely from that of the outdoors, the reflective-type 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-type LCD device has been developed, wherein a user's may select from the transmissive mode to the reflective mode, or vise versa.

FIG. 1is a schematic cross sectional view of a transflective-type LCD device according to the related art. InFIG. 1, a transflective-type LCD device includes an upper substrate10, a lower substrate30, an interposed liquid crystal layer20therebetween, and a backlight device45disposed below the lower substrate30, wherein each of the upper and lower substrates10and30has a transparent substrate1. The upper substrate10includes a color filter12formed on a rear surface of the transparent substrate1, and an upper transparent electrode14formed on the color filter12, wherein the upper transparent electrode14serves as a common electrode. In addition, an upper polarizer16is formed on a front surface of the transparent substrate1, wherein the upper polarizer16serves as a filter for selectively transmitting portions of incident light produced by the backlight device45. Accordingly, the upper polarizer16has an optical polarizing axis along one direction such that only the portions of incident light having the same vibrating direction as the direction of the polarizing axis can pass through the upper polarizer16.

InFIG. 1, the lower substrate30includes an insulating layer33formed on the front surface of the transparent substrate1, and a lower transparent electrode32formed on the insulating layer33. In addition, a passivation layer34and a reflective electrode36are formed in series on the lower transparent electrode32, and a transmitting hole31is formed in the passivation layer34and the reflective electrode36to expose a portion of the pixel electrode32. Furthermore, a lower polarizer40is formed on the lower surface of the transparent substrate1in the lower substrate30. Thus, when an electric field is applied across the liquid crystal layer20, molecules of the liquid crystal layer20align in accordance with the electric field such that the liquid crystal layer20refracts the incident light in order to display an image.

InFIG. 1, an area corresponding to the reflective plate36is a reflective portion “r” and an area corresponding to the portion of the pixel electrode32exposed by the transmissive hole31is a transmissive portion “t”. In addition, a first cell gap “d1” at the transmissive portion “t” is about twice that of a second cell gap “d2” at the reflective portion “r,” thereby reducing a light path difference. A retardation “δ” of the liquid crystal layer20is defined as a product of refractive index anisotropy “Δn” with a cell gap “d” (i.e., δ=Δn·d ), wherein a light efficiency of the LCD device is proportional to the retardation “δ.” Accordingly, in order to reduce the difference of light efficiencies between the reflective and transmissive modes, the retardations of the liquid crystal layer20at two portions should be nearly equal to each other by making the first cell gap “d1” of the transmissive portion “t” larger than that of the reflective portion “r.”

However, although the light efficiencies of the liquid crystal layer20between the reflective and transmissive modes become equal by making the cell gaps different, the light passing through the color filters at different locations is different, wherein 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 other wavelengths has the following relationship 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 of the light, α(λ) is an absorption coefficient depending on the wavelength of the light, and d is a distance that the light passes.

Since the color filter resin is a viscous material, the thickness of the color filter resin is hard to control and can not be fabricated at less than a specific thickness. Therefore, the color filter layers of the reflective and transmissive portions have the same thickness and different absorption coefficients (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 time and production costs would increase, thereby decreasing yield of the display device.

To solve the above problems, a fabricating method of color filter layers using the same resin has been suggested. During the fabricating method, the color filter layers at the reflective and transmissive portions have the same absorption coefficient, but have different thicknesses so that the transmittance has the same value.

FIG. 2Ais a transmittance spectrum measured during a reflective mode of a first red color filter layer having a certain thickness according to the related art, andFIG. 2Bis a transmittance spectrum measured during a reflective mode of a second red color filter layer having twice the certain thickness according to the related art. In general, visible light has a wavelength with a range of about 400 to about 700 nanometers, wherein red, green, and blue colors roughly correspond to wavelengths of 650, 550, and 450 nanometers, respectively.

InFIG. 2A, the transmittances at wavelengths corresponding to the red, green, and blue colored light are about 97%, 20% and 58%, respectively. Although the transmittance for the red colored light is high, the transmittances for the other colors are not negligible such that color purity is not obtained.

InFIG. 2B, since the second red color filter layer has twice the thickness and a square transmittance compared with the first red color filter layer ofFIG. 2A, the transmittances at wavelengths corresponding to the red, green, and blue colored light 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 the red, green, and blue colored lights, respectively. Therefore, the color purity of the second red color filter layer is improved and results can be applied for the green and blue color filters so that the transmittance and color purity of the transflective-type LCD device using the same kind of color filter resin can be uniform for the reflective and transmissive portions. An example of a transflective-type LCD device having a dual thickness color filter (DCF) using the above-detailed principles may be found in Korean Patent Application No. 2000-9979.

FIG. 3is a cross sectional view of a transflective-type LCD device having a dual thickness color filter layer according to the related art. InFIG. 3, a transparent buffer layer64is formed on an inner surface of an upper substrate15only at a reflective portion “rr,” and a color filter layer62is formed along an entire upper substrate15. Accordingly, a color filter layer62of a transmissive portion “tt” is thicker than that of the reflective portion “rr” so that the color purity of the transmissive portion “tt” 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).

FIG. 4Ais a cross sectional view of a dual thickness color filter substrate having a transparent buffer layer of a first thicknesses according to the related art, andFIG. 4Bis a cross-sectional view of a dual thickness color filter substrate having a transparent buffer layer of a second thickness according to the related art. InFIG. 4A, a substrate15has a transmissive portion “tt” and a reflective portion “rr.” In addition, a black matrix70and a transparent buffer layer64are formed in the reflective portion “rr,” and a color filter layer62is formed along an entire surface of the substrate15. Since the transparent buffer layer64of a first thickness has a low step at a borderline of the transmissive portion “tt” and the reflective portion “rr,” a surface of the color filter layer62can be planarized. Moreover, since the color filter layer62at the transmissive portion “tt” is thicker than that at the reflective portion “rr”, the color purity can be improved at the transmissive portion “tt”. However, since the thickness of the transparent buffer layer64is limited for the planarization of the color filter layer62, the thickness ratio of the color filter layer62is limited and improvement of the color purity is limited.

InFIG. 4B, in order to have a desired thickness ratio of the color filter layer62, the transparent buffer layer64has a second thickness higher than the first thickness ofFIG. 4A, and a high step at the borderline of the transmissive portion “tt” and the reflective portion “rr”. Since the color filter layer62is made of a viscous resin and is formed according to a surface of an underlayer, the color filter layer62also has a step at a top surface. Therefore, the difference “Δd” between the designed thickness d3and the fabricated thickness d4occurs, and improvement of the color purity of the transmissive portion “tt” is limited.

Accordingly, it is very difficult to form the transparent buffer layer having a color filter thickness to be the desired thickness in the transmissive portion, whereby the color difference occurs between the transmissive portion and the reflective portion of the DCF structure. If the color filter in the transmissive portion does not have a desired thickness to obtain the desired color purity, color reproduction of the transmissive portion will not increase as much as that of the reflective portion.

Moreover, when the transparent buffer layer has the high step at the borderline of the transmissive portion and the reflective portion, the color filter thereon has an uneven surface so that planarization of the common electrode formed on the color filter is degraded. Specifically, the uneven surface of the color filter causes the common electrode to have a rough surface, thereby deteriorating image display quality of the LCD device.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a transflective liquid crystal display device and method of fabricating the same 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 transflective liquid crystal display device to produce uniform color purity and uniform color reproduction.

Another object of the present invention is to provide a method of fabricating a color filter substrate of a transflective liquid crystal display device to produce uniform color purity and uniform color reproduction.

Another object of the present invention is to provide a transflective liquid crystal display device having an easily controllable buffer layer pattern in order to control a color filter thickness in a transmissive portion to optimized image color.

Another object of the present invention is to provide a method of fabricating a transflective liquid crystal display device having an easily controllable buffer layer pattern in order to control a color filter thickness in a transmissive portion to optimized image color.

Another object of the present invention is to provide a transflective liquid crystal display device having high transmittance and color purity.

Another object of the present invention is to provide a method of fabricating a transflective liquid crystal display device having high transmittance and color purity.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a transflective LCD device includes first and second transparent substrates spaced apart from each other and having a reflective portion and a transmissive portion, an insulator on the first transparent substrate, a passivation layer on the insulator within the reflective portion, a reflector on the passivation layer, a transparent pixel electrode disposed over the insulator covering the reflector and the passivation layer, a buffer pattern disposed on a rear surface of the second substrate, the buffer pattern having a saw-tooth shape corresponding to the reflective portion, a color filter on the rear surface of the second substrate covering the buffer pattern, the color filter having a first thickness in the transmissive portion and a second thickness in the reflective portion, a transparent common electrode on a rear surface of the color filter, and a liquid crystal layer between the transparent pixel electrode and the transparent common electrode.

In another aspect, a transflective LCD device includes first and second transparent substrates spaced apart from each other and having a reflective portion and a transmissive portion, an insulator on the first transparent substrate, a passivation layer on the insulator within the reflective portion, a reflector on the passivation layer, a transparent pixel electrode disposed over the insulator covering the reflector and the passivation layer, a buffer pattern disposed on a rear surface of the second substrate, the buffer pattern having a striped shape and a plurality of holes therein corresponding to the reflective portion, a color filter on the rear surface of the second substrate covering the buffer pattern, the color filter having a first thickness in the transmissive portion and a second thickness in the reflective portion, a transparent common electrode on a rear surface of the color filter, and a liquid crystal layer between the transparent pixel electrode and the transparent common electrode.

In another aspect, a method of fabricating a transflective LCD device includes providing first and second transparent substrates spaced apart from each other and having a reflective portion and a transmissive portion, forming an insulator on the first transparent substrate, forming a passivation layer on the insulator within the reflective portion, forming a reflector on the passivation layer, forming a transparent pixel electrode over the insulator to cover the reflector and the passivation layer, forming a buffer pattern on a rear surface of the second substrate, the buffer pattern having a saw-tooth shape corresponding to the reflective portion, forming a color filter on the rear surface of the second substrate to cover the buffer pattern, the color filter having a first thickness in the transmissive portion and a second thickness in the reflective portion, forming a transparent common electrode on a rear surface of the color filter, and forming a liquid crystal layer between the transparent pixel electrode and the transparent common electrode.

In another aspect, a method of fabricating a transflective LCD device includes providing first and second transparent substrates spaced apart from each other and having a reflective portion and a transmissive portion, forming an insulator on the first transparent substrate, forming a passivation layer on the insulator within the reflective portion, forming a reflector on the passivation layer, forming a transparent pixel electrode over the insulator to cover the reflector and the passivation layer, forming a buffer pattern on a rear surface of the second substrate, the buffer pattern having a striped shape and a plurality of holes therein corresponding to the reflective portion, forming a color filter on the rear surface of the second substrate to cover the buffer pattern, the color filter having a first thickness in the transmissive portion and a second thickness in the reflective portion, forming a transparent common electrode on a rear surface of the color filter, and forming a liquid crystal layer between the transparent pixel electrode and the transparent common electrode.

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 drawings.

FIG. 5is a schematic plan view of an exemplary color filter substrate having a shaped buffer layer pattern according to the present invention. InFIG. 5, transmissive regions200and reflective regions202may be alternately disposed along up-and-down directions in each of red (R), green (G), and blue (B) color regions. In addition, buffer patterns210may be disposed within the reflective regions202, wherein each of the buffer patterns210may have a saw-tooth shape along sides of the buffer patterns210. The saw-tooth shape may control flow of a viscous color photoresist when forming the R, G, and B color filters. Specifically, the saw-tooth shape may provide the color photoresist to be formed in the reflective regions202having a desired thickness, although the buffer patterns may have a high step.

FIG. 6is a schematic plan view of another exemplary color filter substrate having a shaped buffer layer pattern according to the present invention. InFIG. 6, transmissive regions250and reflective regions252may be alternately disposed along up-and down directions in each of red (R), green (G), and blue (B) color regions. In addition, buffer patterns260may include a saw-tooth shape. However, each of the buffer patterns260may include a plurality of holes270to capture viscous color photoresist when forming the R, G, and B color filters. For example, the holes270and the saw-tooth shape of the buffer pattern260may not allow the viscous color photoresist from flowing down from the buffer patterns260, thereby controlling thicknesses of the R, G, and B color filters between the transmissive regions250and the reflective regions252.

FIG. 7is a schematic plan view of another color filter substrate having a shaped buffer layer pattern according to the present invention. InFIG. 7, a buffer pattern290located in a reflective region287may not have a saw-tooth shape, but only a plurality of holes292therein. As described with reference toFIG. 6, the plurality of holes292may capture viscous color photoresist and function to prevent the viscous color photoresist from flowing down from a top of the buffer pattern287.

InFIGS. 5,6, and7, the buffer patterns210,260, and290may have a striped shape, and a maximum width of each of the buffer patterns210,260, and290may be about 14 micrometers. Accordingly, each of the buffer patterns210,260, and290may have a color filter thickness in the transmissive portion to be as large as that in the reflective portion. However, if the maximum width of the buffer pattern is larger than about 50 micrometers, it not be possible for the color filter in the transmissive portion to have a thickness of about 1.3 times larger than that in the reflective portion. Thus, the maximum width of each of the buffer patterns210,260, and290may be less than about 50 micrometers in order to obtain a desired color filter thickness in the transmissive portion to be at least about 1.3 times larger than that in the reflective portion.

FIG. 8is a cross sectional view of an exemplary transflective-type LCD device incorporating the color filter substrate ofFIG. 5according to the present invention. InFIG. 8, a transflective LCD device may include an upper substrate110, a lower substrate130, a liquid crystal layer120interposed therebetween, and a backlight device (not shown) disposed below the lower substrate130, wherein each of the upper and lower substrates110and130may include a transparent substrate118. In addition, the transflective LCD may be divided into a reflective portion R and a transmissive portion T depending on whether a reflector140is used.

On the transparent substrate118of the lower substrate130, an insulator132may be formed, and a passivation layer138and the reflector140may be formed in series on the insulator132, especially in the reflective portion R. Since the passivation layer138may have an opening136corresponding to the transmissive portion T, the transflective LCD may have different cell gaps between the reflective portion R and the transmissive portion T. In addition, a transparent pixel electrode134may be formed over the transparent substrate118to cover the reflector140, wherein the transparent pixel electrode134may also be formed on the insulator132within the opening136of the transmissive portion T. Since the reflector140may be disposed in the reflective portion R, light incident from an exterior may be re-reflected toward the exterior. In the transmissive portion T, light generated from the backlight device (not shown) may pass through the transparent pixel electrode134.

InFIG. 8, a color filter layer112having different thicknesses may be formed on a rear surface of the transparent substrate118of the upper substrate110. Then, a transparent common electrode114may be formed on a rear surface of the color filter layer112, wherein the transparent common electrode114and the transparent pixel electrode134may supply an electric field to the liquid crystal layer120. In the upper substrate110, a buffer pattern116is disposed between the color filter112and the transparent substrate118, especially within the reflective portion R that corresponds to the reflector140of the lower substrate130. Accordingly, the buffer pattern116may include the saw-tooth shape, as shown inFIG. 5, and may be formed of a transparent material or by etching the transparent substrate118. As described with reference toFIG. 5, the buffer pattern116may capture the viscous color photoresist (i.e., the color filter112) so that it can control the thicknesses of the color filter112in the transmissive portion T and in the reflective portion R. Moreover, the thickness of the color filter112in the transmissive portion T may be at least about 1.3 times larger than that in the reflective portion R. The color filter thicknesses in the transmissive and reflective portions T and R may be controlled by a total number of the saw-teeth formed in the buffer pattern116.

FIG. 9is a cross sectional view of another exemplary transflective-type LCD device that incorporates the color filter substrate of one ofFIGS. 6 and 7according to the present invention. InFIG. 9, a transflective LCD device may be divided into a reflective portion R and a transmissive portion T depending on whether a reflector320is used. Unlike the transflective LCD device ofFIG. 8, the transflective LCD device ofFIG. 9may include the reflective portion R within a center portion of a pixel region. Alternatively, the transflective LCD device may have the transmissive portion T within the center portion of the pixel region and the reflective portion R may be disposed along a peripheral portion of the pixel region, similar to the transflective LCD device ofFIG. 8.

InFIG. 9, an insulator layer335may be formed on a lower transparent substrate340, and a passivation layer325and the reflector320may be formed in series on the insulator335, especially in the reflective portion R. Since there is no passivation layer325in the transmissive portion T, different cell gaps may be obtained between the transmissive portion T and the reflective portion R. In addition, a transparent pixel electrode330may be formed over the lower transparent substrate340to cover the reflector320and the underlying passivation layer325. The transparent pixel electrode330also may be disposed on both sides of the reflector320in the transmissive portion T. Accordingly, light incident from the exterior may be re-reflected toward the exterior in the reflective portion R, and light generated from the backlight device (not shown) may pass through the transparent pixel electrode330in the transmissive portion T.

InFIG. 9, a color filter layer310having different thicknesses may be formed on a rear surface of an upper transparent substrate305. Then, a transparent common electrode315may be formed on a rear surface of the color filter layer310. Accordingly, the transparent common electrode315and the transparent pixel electrode330may supply an electric field to the liquid crystal layer. Furthermore, a buffer pattern345may be disposed between the color filter310and the upper transparent substrate310, especially within the reflective portion R corresponding to the reflector320. Thus, exemplary transflective-type LCD device ofFIG. 9may include the buffer pattern345having a plurality of holes therein, as shown inFIGS. 6 and 7. In addition, the buffer pattern345may have a saw-tooth shape, as shown inFIG. 6, or may have a stripe shape, as shown inFIG. 7. The buffer pattern345may be formed of a transparent material, or may be formed by etching the upper transparent substrate305. As described with reference toFIGS. 6 and 7, the buffer pattern345may capture the viscous color photoresist (i.e., the color filter310) to control the thicknesses of the color filter310both in the transmissive portion T and in the reflective portion R. Moreover, the thickness of the color filter310in the transmissive portion T may be at least about 2 times larger than that in the reflective portion R. The color filter thicknesses in the transmissive and reflective portions T and R may be controlled by the number of holes and sizes of the holes. Furthermore, if the buffer pattern345has the saw-tooth shape, as shown inFIG. 6, the color filter thickness may also be controlled by the number of saw-teeth formed in the buffer pattern345.

According to the present invention, the buffer pattern disposed in the color filter substrate may have a saw-tooth shape and/or a plurality of holes. Thus, the buffer pattern may control flowing of the viscous color resin, and the saw-tooth shape and/or the plurality of holes may capture the viscous color resin when forming the color filter. Eventually, the color filter in the transmissive portion may have a color purity and a chromaticity almost double as much as the color filter in the reflective portion.

It will be apparent to those skilled in the art that various modifications and variation can be made in the transflective lcd device having dual thickness color filter of the present invention without departing from the spirit or scope of the invention. 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.