Transflective liquid crystal display device and fabricating method thereof

A transflective liquid crystal display device and fabricating method thereof is disclosed in the present invention. The transflective liquid crystal display device includes first and second substrates facing into each other, each of the first and second substrates having reflective and transmissive portions, a buffer layer on the first substrate, the buffer layer having a first transmissive hole at the transmissive portion, a color filter layer on the buffer layer and the first substrate, the color filter layer of the transmissive portion being thicker than that of the reflective portion, a common electrode on the color filter layer, a pixel electrode on the second substrate, a reflecting layer over the pixel electrode, the reflecting layer having a second transmissive hole at the transmissive portion, and a liquid crystal layer between the common electrode and the reflecting layer.

This application claims the benefit of the Korean Application No. P2002-047989 filed on Aug. 14, 2003, 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 a transflective liquid crystal display device and fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for maximizing brightness and color reproducibility by improving a contrast ratio as well as preventing light leakage without reducing an aperture ratio.

2. Discussion of the Related Art

Liquid crystal display (LCD) devices are developed as next generation display devices because of their characteristics of light weight, thin profile, and low power consumption. Generally, an LCD device is a non-emissive display device that displays images using a refractive index difference having optical anisotropy properties of liquid crystal material that is interposed between a thin film transistor (TFT) array substrate and a color filter (C/F) substrate.

In the conventional LCD device, a displaying method using a backlight behind the array substrate as a light source is commonly used. However, the incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. The backlight of the conventional LCD device requires high brightness, thereby increasing power consumption by the backlight device. Thus, a relatively heavy battery is required to supply a sufficient power to the backlight of such a device, and the battery cannot be used outdoors for a long period of time.

In order to overcome the problems described above, a reflective LCD has been developed. Since the reflective LCD device uses the ambient light instead of the backlight, it becomes light weight and easy to carry. In addition, power consumption of the reflective LCD device is reduced so that the reflective LCD device can be used for a portable display device such as an electronic diary or a personal digital assistant (PDA).

However, brightness of the reflective LCD device may vary in accordance with the surrounding conditions. For example, the brightness of the indoor ambient light differs largely from that of the outdoors. Therefore, the reflective LCD device cannot be used where the ambient light is weak or does not exist. In order to overcome such problems, a transflective LCD device has been researched and developed. The transflective LCD device can be switched from a transmissive mode using transmission of light to a reflective mode using reflection of light according to the user's selection.

FIG. 1is a schematic cross-sectional view of a transflective liquid crystal display device according to a related art. As shown inFIG. 1, a liquid crystal panel40includes first and second substrates10and30facing into each other, and a liquid crystal layer20interposed therebetween. A transflective liquid crystal display (LCD) device60is composed of the liquid crystal panel40and a backlight unit50. The backlight unit50is disposed at the outside of the liquid crystal panel40and provides the liquid crystal panel40with light.

A color filter layer12for passing the light having only the specific band of wavelength is formed on the inner surface of the first substrate10. A common electrode14functioning as an electrode applying a voltage to the liquid crystal layer20is formed on the color filter layer12. An insulating layer32is formed on the inner surface of the second substrate30. A transparent pixel electrode34functioning as another electrode applying a voltage to the liquid crystal layer20is formed on the insulating layer32. A passivation layer36and a reflecting layer38that commonly have a transmissive hole35exposing a portion of the pixel electrode34are subsequently formed on the pixel electrode34. The liquid crystal panel40includes a reflective portion “r” corresponding to the reflecting layer38and a transmissive portion “t” corresponding to the transmissive hole35.

In order to maximize the light efficiency of the reflective and transmissive portions “r” and “t”, a cell gap corresponding to a thickness of the liquid crystal layer20of the reflective portion “r” is designed to be different from that of the transmissive portion “t”. This structure is referred to as a dual cell gap structure. A cell gap “d1” of the transmissive portion “t” is about twice of a cell gap “d2” of the reflective portion “r.”

A retardation “δ” of a liquid crystal layer is defined by the following equation:
δ=Δn·d,
wherein δ represents a retardation of a liquid crystal layer, Δn is a refractive index anisotropy of a liquid crystal layer, and d represents a cell gap of a liquid crystal layer. Therefore, to reduce a difference in light efficiency between the reflective and transmissive modes, the retardation of the liquid crystal layer should be kept uniform by forming a cell gap of the transmissive portion larger than that of the reflective portion.

FIG. 2is a schematic cross-sectional view of a transflective liquid crystal display device having a micro reflector structure (MRS) according to another related art. InFIG. 2, first and second substrates70and90face into and are spaced apart from each other, and a liquid crystal layer65is interposed between the first and second substrates70and90. A transparent pixel electrode92is formed on the inner surface of the second substrate90, and a passivation layer96having a transmissive hole94is formed on the pixel electrode92. The transmissive hole94exposes a portion of the pixel electrode92. The passivation layer96has an uneven pattern “A” on the upper surface. A reflecting layer98formed on the passivation layer96also has the transmissive hole94and the uneven pattern “A”. A color filter layer72and a common electrode74are subsequently formed on the inner surface of the first substrate70. The reflective LCD device includes a reflective portion “rr” corresponding to the reflecting layer98and a transmissive portion “tt” corresponding to the transmissive hole94.

Since the reflecting layer98has the uneven pattern “A” on the upper surface, the incident light is diffusely reflected at the reflecting layer98along several directions. Accordingly, the efficiency of the reflected light is improved. This structure of the reflecting layer is referred to as a micro reflector structure (MRS). The passivation layer96includes a plurality of seeds96ahaving a hemispheric shape and a coating layer96bcovering the seeds96a. In the MRS, even though the efficiency of reflected light is improved, it is difficult to control a step difference between the reflective and transmissive portions “rr” and “tt” in fabricating processes. This is due to severe variations in processing conditions for the coating layer96bcovering the seeds96ain accordance with an area ratio of the transmissive portion “tt”. Moreover, in a transflective LCD device having a dual cell gap structure, light efficiency between reflective and transmissive portions is kept uniform. However, since a color filter layer has a uniform thickness at the reflective and transmissive portions, light passing through the reflective portion has a high-color reproducibility and a low-brightness as compared to light passing through the transmissive portion due to a difference between the numbers passing through color filters of the reflective and transmissive portions. Accordingly, a color difference between the reflective and transmissive portions occurs.

SUMMARY OF THE INVENTION

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

Another object of the present invention is to provide a transflective liquid crystal display device and fabricating method thereof in which light efficiency and a color characteristic are kept uniform between reflective and transmissive portions.

Another object of the present invention is to provide a transflective liquid crystal display device and fabricating method thereof having a dual thickness color filter (DCF) structure in which a color filter layer of a reflective portion has a thickness different from that of a transmissive portion.

A further object of the present invention is to provide a transflective liquid crystal display device and fabricating method thereof in which a cell gap of a reflective portion is different from that of a transmissive portion by a buffer layer.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a transflective liquid crystal display device includes first and second substrates facing into each other, each of the first and second substrates having reflective and transmissive portions, a buffer layer on the first substrate, the buffer layer having a first transmissive hole at the transmissive portion, a color filter layer on the buffer layer and the first substrate, the color filter layer of the transmissive portion being thicker than that of the reflective portion, a common electrode on the color filter layer, a pixel electrode on the second substrate, a reflecting layer over the pixel electrode, the reflecting layer having a second transmissive hole at the transmissive portion, and a liquid crystal layer between the common electrode and the reflecting layer.

In another aspect of the present invention, a fabricating method of a transflective liquid crystal display device includes forming a buffer layer on a first substrate having reflective and transmissive portions, the buffer layer having a first transmissive hole at the transmissive portion, forming a color filter layer on the buffer layer and the first substrate, the color filter layer of the transmissive portion being thicker than that of the reflective portion, forming a common electrode on the color filter layer, forming a pixel electrode on a second substrate having the reflective and transmissive portions, forming a reflecting layer over the pixel electrode, the reflecting layer having a second transmissive hole at the transmissive portion, attaching the first and second substrates such that the common electrode and the reflecting layer face into each other, and forming a liquid crystal layer between the common electrode and the reflecting layer.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to the illustrated embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 3is a schematic cross-sectional view of a transflective liquid crystal display device according to a first embodiment of the present invention.

InFIG. 3, first and second substrates110and130face into and are spaced apart from each other. A liquid crystal panel170includes the first and second substrates110and130, and a liquid crystal layer150interposed therebetween. A transflective liquid crystal display (LCD) device190is composed of the liquid crystal panel170and a backlight unit180. The backlight unit180is disposed at the outside of the liquid crystal panel170and provides the liquid crystal panel170with light.

The transflective LCD device190includes reflective and transmissive portions “R” and “T”. A buffer layer134is formed on the inner surface of the first substrate110. The buffer layer134has a first transmissive hole132at the transmissive portion “T”. Thus, the liquid crystal layer150of the transmissive portion “T” is greater than that of the reflective portion “R” (i.e., a cell gap of the reflective portion “R”). A color filter layer136is formed on the buffer layer134and the inner surface of the first substrate110. The color filter layer136of the transmissive portion “T” is thicker than that of the reflective portion “R”. Accordingly, the color filter layer136has a step difference along the boundary of the first transmissive hole132. A common electrode138is formed on the color filter layer136.

An insulating layer112is formed on the inner surface of the second substrate130, and a reflecting layer120is formed on the insulating layer112. A passivation layer116of a transparent insulating material is formed on the reflecting layer120including the insulating layer112. A transparent pixel electrode114is formed on the passivation layer116. The reflecting layer120has a second transmissive hole118at the transmissive portion “T” exposing a portion of the insulating layer112. The reflecting layer120and the second transmissive hole118correspond to the reflective and transmissive portions “R” and “T”, respectively, in a liquid crystal display region170.

The color filter layer136can be divided into first, second, and third regions “I”, “II”, and “III”: the first region “I” is a first flat portion on the buffer layer134, the second region “II” is a second flat portion on the inner surface of the first substrate110, and the third region “III” is a first inclined portion between the first and second regions. The first and third regions “I” and “III” correspond to the reflecting layer120, and the second region “II” corresponds to the second transmissive hole118. A black matrix (not shown) may be disposed at a portion corresponding to the border of the color filter layer136.

A defect in a rubbing process for an alignment layer (not shown) easily occurs at the third region “III” due to the step difference, and the defect causes a “disclination” in the third region “III”. The “disclination” causes light leakage when a black image is displayed, and a contrast ratio is reduced much more in the transmissive portion “T” than in the reflective portion “R”. When a black matrix is disposed to shield the third region “III” in order to prevent the reduction of the contrast ratio, an aperture ratio is severely reduced. Accordingly, in order to prevent the light leakage without reduction of the aperture ratio, the third portion “III” is disposed to correspond to the reflecting layer120of the first substrate110so that the light from the backlight unit180cannot penetrate the third region “III”, when forming the buffer layer134and the color filter layer136.

The buffer layer134may be formed of one of a transparent organic material and a transparent inorganic material. When the color filter layer136is formed by coating color resin, the color filter layer136is formed to be thicker in the transmissive portion “T” than in the reflective portion “R”. That is, a first thickness “d7” of the color filter layer136of the transmissive portion “T” is greater than a second thickness “d8” of the color filter layer136of the reflective portion “R” because of the first transmissive hole132. However, since the thickness of the buffer layer134is greater than a difference between the first and second thicknesses “d7” and “d8”, a first cell gap “d5” defined by a thickness of the liquid crystal layer150of the transmissive portion “T” is greater than a second cell gap “d6” defined by a thickness of the liquid crystal layer150of the reflective portion “R”.

A difference between the first and second cell gaps “d5” and “d6”, and color reproducibility of the reflective and transmissive portions “R” and “T” can be controlled by a thickness of the buffer layer134, a type of the color resin and a coating condition of the color resin. For example, a step difference of the color filter layer136between the reflective and transmissive portions “R” and “T” of about 2.0 μm to about 2.5 μm can be obtained by forming the buffer layer134of a thickness of about 2.0 μm to about 5.0 μm. Therefore, the transflective LCD device190has the following advantages. A color difference between the reflective and transmissive portions “R” and “T” is reduced by forming the color filter layer136of the transmissive portion “T” thicker than that of the reflective layer “R”. Also, light efficiency is kept uniform between the reflective and transmissive portions “R” and “T” by forming the first cell gap “d5” of the transmissive portion “T” greater than the second cell gap “d6” of the reflective portion “R”. Finally, a contrast ratio of the transmissive portion “T” is improved by disposing the inclined portion “III” of the color filter layer136to correspond to the reflecting layer120. Also, the color filter layer of the reflective portion and the color filter layer of the transmissive portion has a relative thickness ratio of about 1:1.5 to 1:2.5.

Although not shown inFIG. 3, the pixel electrode114and the reflecting layer120are formed at each sub-pixel, which is a unit for displaying images. A voltage is applied to the pixel electrode114through a switching element (not shown). The reflecting layer120functions either as an electrode to which a voltage is applied or as a reflection plate without applying a voltage. Also, the reflecting layer120may be formed over the pixel electrode114.

FIG. 4is a schematic plane view of the transflective liquid crystal display device according to the first embodiment of the present invention.

InFIG. 4, a transflective LCD device includes a transmissive portion “T” and a reflective portion “R” surrounding the transmissive portion “T”. The reflective portion “R” is divided into a first flat portion “I” and an inclined portion “III” of a first substrate (not shown). The transmissive portion “T” is a second flat portion “II”. Since the reflective portion “R” includes the inclined portion “III”, a reduction in a contrast ratio due to light leakage at the inclined portion “III” is prevented.

FIG. 5is a schematic cross-sectional view of a transflective liquid crystal display device according to a second embodiment of the present invention.

InFIG. 5, first and second substrates210and230face into and are spaced apart from each other. A liquid crystal panel270includes the first and second substrates210and230, and a liquid crystal layer250interposed therebetween. A transflective liquid crystal display (LCD) device290is composed of the liquid crystal panel270and a backlight unit280. The backlight unit280is disposed at the outside of the liquid crystal panel270and provides the liquid crystal panel270with light.

The transflective LCD device290includes reflective and transmissive portions “R” and “T”. A buffer layer234is formed on the inner surface of the first substrate210. The buffer layer234has a first transmissive hole232corresponding to the transmissive portion “T”. Thus, the liquid crystal layer250of the transmissive portion is thicker than that of the reflective portion “R”, (i.e., a cell gap of the reflective portion “R”). A color filter layer236is formed on the buffer layer234and the inner surface of the first substrate210. The color filter layer236of the transmissive portion “T” is thicker than that of the reflective portion “R”. Accordingly, the color filter layer236has a step difference along the boundary of the first transmissive hole232. A common electrode238is formed on the color filter layer236.

A first insulating layer212is formed on the inner surface of the second substrate230. A passivation layer216including first and second sub-passivation layers216aand216bis formed on the first insulating layer212. A reflecting layer220is formed on the passivation layer216. A second insulating layer219is formed on the reflecting layer220. A transparent pixel electrode214is then formed on the reflecting layer220.

The first sub-passivation layer216ais a seed for an uneven surface of the passivation layer216. The second sub-passivation layer216bcovers the entire surface of the second substrate230including the first sub-passivation layer216a. Accordingly, the passivation layer216is divided into a flat region “IV” and an uneven region “V”. A reflecting layer220is formed on the passivation layer216. The reflecting layer220has a second transmissive hole218at the transmissive portion “T” exposing a portion of the passivation layer216of the flat region “IV”. The reflecting layer220and the second transmissive hole218correspond to the reflective and transmissive portions “R” and “T”, respectively.

In forming the uneven region “V” of the upper surface of the passivation layer216to improve the efficiency of the reflected light, an additional step structure of the passivation layer216between the reflective and transmissive portions “R” and “T” is not used. Accordingly, a processing condition for the first and second sub-passivation layers216aand216bis simplified, and characteristics of the passivation layer216in the uneven region “V” are improved.

In another method of forming an uneven surface of the passivation layer216, the uneven surface of the passivation layer can be obtained through a photolithographic process without a seed. After the passivation layer216of a photosensitive material is formed thereon, the passivation layer216is selectively exposed with a light source. Also,FIG. 6is a schematic cross-sectional view of a transflective liquid crystal display device according to a third embodiment of the present invention in which the reflecting layer220may be formed over the pixel electrode214.

TABLE 1 is experimental data showing various thickness of a color filter layer and a buffer layer, and a step of a color filter layer between reflective and transmissive portions.

In TABLE 1, one sub-pixel of a 10.4″ super video graphics adapter (SVGA) transflective LCD device is measured when an area ratio of each of the reflective and transmissive portions “R” and “T” (shown inFIG. 5) is about 6:4. A color reproducibility of each of the reflective and transmissive portions “R” and “T” (shown inFIG. 5) is about 15% or 20% using a color coordinate of the national television system committee (NTSC). When the buffer layer234(shown inFIG. 5) has a thickness of about 3 μm, a thickness ratio of the color filter layer236(shown inFIG. 5) between the reflective and transmissive portions “T” and “R” (shown inFIG. 5) is about 1:2. A step difference of the color filter layer236(shown inFIG. 5) between the reflective and transmissive portions “T” and “R” (shown inFIG. 5) is about 2.2 μm. Accordingly, a uniform color property of the color filter layer236(shown inFIG. 5) in the reflective and transmissive portions “T” and “R” (shown inFIG. 5) and a uniform light efficiency in the reflective and transmissive portions “T” and “R” (shown inFIG. 5) are simultaneously obtained by forming the buffer layer234(shown inFIG. 5) thicker than a cell gap of the reflective portion “R” (shown in FIG.5).

In the liquid crystal display device in the present invention can be operated under one of an ECB mode and a VA mode.

A transflective liquid crystal display device in the present invention has the following advantages. Since a color filter layer is easily formed to have desired thickness in reflective and transmissive portions, a color reproducibility is kept uniform in the reflective and transmissive portions. Also, since an additional step structure of an array substrate for a dual cell gap is not required, a short circuit between the electrodes can be prevented, and a micro reflector structure (MRS) improving reflection efficiency can easily be formed. Furthermore, since an inclined portion of the color filter layer is disposed to correspond to a reflecting layer of the array substrate, light leakage is prevented and a contrast ratio of a transmissive portion is improved without reducing an aperture ratio.