Patent Publication Number: US-7710532-B2

Title: Reflective liquid crystal display device having cholesteric liquid crystal color filter

Description:
This application is a Divisional of application Ser. No. 10/293,434 filed Nov. 14, 2002, now U.S. Pat. No. 6,809,786 which is hereby incorporated by reference as if fully set forth herein. 
    
    
     This application claims the benefit of Korean Patent Application No. 2001-71519, filed on Nov. 16, 2001, which is hereby incorporated by reference 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 reflective LCD device having a cholesteric liquid crystal (CLC) color filter. 
     2. Discussion of the Related Art 
     Presently, LCD devices are developed as next generation display devices because of their light weight, thin profile, and low power consumption characteristics. In general, an LCD device is a non-emissive display device that displays images using a refractive index difference utilizing optical anisotropy properties of liquid crystal material that is interposed between an array (TFT) substrate and a color filter (C/F) substrate. Among the various type of LCD devices commonly used, active matrix LCD (AM-LCD) devices have been developed because of their high resolution and superiority in displaying moving images. The AM-LCD device includes a thin film transistor (TFT) per each pixel region as a switching device, a first electrode for ON/OFF, and a second electrode used for a common electrode. 
       FIG. 1  is a perspective view of an LCD device according to the related art. 
     In  FIG. 1 , first and second substrates  10  and  30  are arranged to face each other with a liquid crystal material layer  50  interposed therebetween. On an inner surface of the first substrate  10 , a color filter (C/F) layer  12  and a common electrode  16 , which functions as one electrode for applying an electric field to the liquid crystal layer  50 , are subsequently formed. The C/F layer  12  includes a color filter for transmitting only light of a specific wavelength, and a black matrix (not shown) that is disposed at a boundary of the color filter and shields light of a region in which alignment of the liquid crystal material is uncontrollable. On an inner surface of the second substrate  30 , a plurality of gate lines  32  and a plurality of data lines  34  are formed in a matrix array. A TFT “T”, which functions as a switching device, is disposed at a region where each gate line  32  and data line  34  crosses, and a pixel electrode  46  that is connected to the TFT “T” is disposed at a pixel region “P” defined by the region where the gate and data lines  32  and  34  cross. First and second polarizing plates  52  and  54 , which transmit only light parallel to a polarizing axis, are disposed on an outer surface of the first and second substrates  10  and  30 , respectively. An additional light source such as a backlight, for example, is disposed below the second polarizing plate  54 . 
     The LCD device of  FIG. 1  is a transmissive LCD device that displays images by transmitting only desired light through the first substrate using an optic/dielectric anisotropy of the liquid crystal layer after light from the backlight passes through the second substrate. 
       FIG. 2  is a schematic plan view of an LCD device according to the related art.  FIG. 2  shows gate and data pads for connection with an external circuit. 
     In  FIG. 2 , a liquid crystal panel  60  for an LCD device includes a first substrate  10 , a second substrate  30  larger than the first substrate  10 , and a liquid crystal layer  50  interposed between the first and second substrates  10  and  30 . The liquid crystal panel  60  can be divided into a display region “D,” and first and second non-display regions “N 1 ” and “N 2 ” in plan view. The first non-display region “N 1 ” is defined by the first and second substrates  10  and  30 , and the second non-display region “N 2 ” is defined by a larger portion of the second substrate  30 . Elements such as a TFT, gate and data lines, a pixel electrode, a color filter layer and a common electrode illustrated in  FIG. 1  are formed in the display region “D.” Gate and data pads  62  and  64  connected to an external circuit (not shown) are formed in the second non-display region “N 2 ” to apply a display signal to the display region “D.” Since a black matrix  66  formed in the first non-display region “N 1 ” of the first substrate  10  absorbs incident light, a boundary of the display region “D” maintains a black state. 
       FIG. 3  is a schematic cross-sectional view of an LCD device according to the related art. A boundary of a display region is mainly shown in  FIG. 3 . 
     In  FIG. 3 , a boundary of first and second substrates  10  and  30  with a liquid crystal layer  50  therebetween is sealed with a seal pattern  68 . A color filter layer  40  on an inner surface of the first substrate  10  is extended to a first non-display region “N 1 ” so that a deterioration at a boundary of a display region “D” by a step difference between the display region “D” and the first non-display region “N 1 ” can be prevented during a rubbing process for aligning the liquid crystal layer  50 . Array elements  42  such as a TFT and a pixel electrode (of  FIG. 1 ) are formed on an inner surface of the second substrate  30 . When light is emitted into the first non-display region “N 1 ,” a black matrix  66  of the first non-display region “N 1 ” absorbs the light. Accordingly, a black state is maintained in the boundary of the display region “D.” 
     Reflective LCD devices without a backlight are being researched and developed. Transflective LCD devices use a backlight to provide light. However, only about 7% of the light that is emitted by the backlight passes through each cell of the LCD device. Since the backlight should emit light of a relatively high brightness, corresponding power consumption increases. Accordingly, a large capacity heavy battery is commonly used to supply sufficient power for the backlight. Moreover, use of the large capacity battery limits operating time. On the other hand, because power consumption of the reflective LCD devices greatly decreases due to use of ambient light as a light source, operating time increases. Such reflective LCD devices are used for portable information apparatuses such as electronic diaries and personal digital assistants (PDAs). In the reflective LCD devices, a pixel area, which is covered with a transparent electrode in conventional transmissive LCD devices, is covered with a reflective plate or reflective electrode having opaque reflection characteristics. 
     However, brightness of reflective LCD devices is very poor because the devices use only ambient light as a light source. The poor brightness results from operational characteristics of the reflective LCD devices in which ambient light which passes through a color filter substrate, is reflected on a reflective electrode on a second substrate, is passed through the color filter substrate again and then displays an image. Accordingly, brightness is decreased as a result of reduction of the transmittance when the ambient light passes through a color filter layer twice. Since overall thickness of the color filter layer is inversely proportional to transmittance and is directly proportional to color purity of the light, the problem of inadequate brightness of the reflective LCD devices can be remedied by forming a thin color filter layer with high transmittance and low color purity. However, there is a limit in fabricating the color filter layer below a threshold thickness due to characteristics of the resin used to form the color filter layer. 
     Accordingly, one possible solution to this problem is fabricating LCD devices using cholesteric liquid crystal (CLC) that has selective reflection and transparency characteristics. In reflective LCD devices using a CLC color filter (CCF) layer, the fabrication processes are simplified due to omission of the reflective layer, and high color purity and high contrast ratio are achieved. Moreover, since CLC has a spiral structure and spiral pitch determines a selective reflection bandwidth of the CLC, the reflection bandwidth can be controlled by a distribution of the spiral pitch at one pixel. To illustrate this in more detail, a wavelength range of visible light is from about 400 nm to about 700 nm. The wavelength of the red light region is centered at about 650 nm, the wavelength of the green light region is centered at about 550 nm, and the wavelength of the blue light region is centered at about 450 nm. The CCF layer is formed having characteristics that can selectively reflect or transmit right-handed or left-handed circularly polarized light at a bandwidth that corresponds to a pitch deviation by selecting bandwidths corresponding to the red, green, and blue light regions. In addition, the CCF layer is formed having characteristics that control conditions for right or left pitch deviations with respect to the center wavelength. Accordingly, the pitch of the liquid crystal can be artificially adjusted so that a CCF layer can selectively reflect light of an intrinsic wavelength of the color corresponding to each pixel. 
       FIG. 4  is a schematic plan view of a reflective LCD device using a CCF layer according to the related art.  FIG. 4  shows gate and data pads for connection with an external circuit. 
     In  FIG. 4 , since the reflective LCD device using a CCF layer displays images by reflecting ambient light, array elements (not shown) and a CCF layer are formed on first and second substrates  70  and  72 , respectively. Accordingly, the first substrate is larger than the second substrate  72 . As a result, even though a display region “D” is disposed as in the LCD device of  FIG. 3 , a first non-display region “N 1 ” corresponds to a boundary of the second substrate  72  and a second non-display region “N 2 ” corresponds to a larger portion of the first substrate  70 . Especially, even though a black matrix is disposed at the first non-display region adjacent to the display region in the LCD device of  FIG. 3 , an additional black matrix between adjacent pixels is omitted due to a selective reflection property of the CCF layer and a metal bar  78  of one material of a gate pad  74  and a data pad  76  is disposed at the first non-display region “N 1 ” not adjacent to the second non-display region “N 2 ” to prevent light reflection in the reflective LCD device using a CCF layer. However, an extra metal bar to prevent light reflection is not disposed at the first non-display region “N 1 ” adjacent to the second non-display region “N 2 ” where the gate pad  74  and the data pad  76  are formed. 
       FIG. 5  is a schematic cross-sectional view of a reflective LCD device using a CCF layer according to the related art. A boundary of a display region is mainly shown in  FIG. 5 . 
     In  FIG. 5 , a first substrate  70  including array elements  86  and a second substrate  72  including a CCF layer  82  face each other. Incident light into the CCF layer  82  displays colors by the CCF layer  82  whose pitch is adjusted according to a wavelength of each color. These colors constitute desired images by a refractive index difference at a liquid crystal layer (not shown) interposed between the first and second substrates  70  and  72 . To illustrate this in more detail, the array elements  86  are formed on an inner surface of the first substrate  70  and a light absorption layer  80  is formed on an entire inner surface of the second substrate  72 . The CCF layer  82  is formed at a display region “D” and a first non-display region “N 1 ” on the light absorption layer  80 . A common electrode  84  is formed on the light absorption layer  80  and the CCF layer  82 . The array elements  72  on the second substrate  30  of  FIG. 3  can be used as the array elements  86 . 
     When external light enters the CCF layer  82  at the first non-display region “N 1 ,” the CCF layer  82  reflects a circularly polarized light such as left-handed or right-handed circularly polarized lights and the circularly polarized light is emitted to an exterior without passing through an additional light shielding layer. Accordingly, brightness undesirably increases at a boundary of the display region and a display quality of the reflective LCD device decreases. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a reflective liquid crystal display device that substantially obviates one or more of problems due to limitations and disadvantages of the related art. 
     An advantage of the present invention is to provide a reflective liquid crystal display device of high display quality using a cholesteric liquid crystal color filter layer by reducing brightness at a boundary of a display region. 
     Another advantage of the present invention is to provide a reflective liquid crystal display device including a light shielding pattern corresponding to a cholesteric liquid crystal color filter layer at a non-display region of a lower substrate. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a reflective liquid crystal display device includes: a first substrate including a display region and a first non-display region, the first non-display region being disposed at a boundary of the display region; a second substrate facing and spaced apart from the first substrate, the second substrate including a second non-display region corresponding to a portion larger than the first substrate; a light absorption layer on an inner surface of the first substrate; a cholesteric liquid crystal color filter (CCF) layer at the display region and the first non-display region on the light absorption layer; a common electrode on the CCF layer; an array element on an inner surface of the second substrate; a light shielding pattern on the array element, the light shielding pattern corresponding to the CCF layer at the first non-display region; a retardation plate and a polarizing plate sequentially formed on the outer surface of the second substrate; and a liquid crystal layer interposed between the common electrode and the array element. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. 
       In the drawings: 
         FIG. 1  is a perspective view of an LCD device according to the related art; 
         FIG. 2  is a schematic plan view of an LCD device according to the related art; 
         FIG. 3  is a schematic cross-sectional view of an LCD device according to the related art; 
         FIG. 4  is a schematic plan view of a reflective LCD device using a CCF layer according to the related art; 
         FIG. 5  is a schematic cross-sectional view of a reflective LCD device using a CCF layer according to the related art; 
         FIG. 6  is a schematic plan view of a reflective LCD device using a CCF layer according to an embodiment of the present invention; 
         FIG. 7  is a schematic cross-sectional of a reflective LCD device according to an embodiment of the present invention; 
         FIG. 8  is a flow chart illustrating a fabricating process of a substrate including a light shielding pattern according to an embodiment of the present invention; 
         FIG. 9  is a schematic plan view of a reflective LCD device using a CCF layer according to another embodiment of the present invention; 
         FIG. 10  is a schematic plan view of a reflective LCD device using a CCF layer according to another embodiment of the present invention; and 
         FIG. 11  is a schematic cross-sectional view of a reflective LCD device using a CCF layer according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present invention, example of which is 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. 6  is a schematic plan view of a reflective LCD device using a CCF layer according to an embodiment of the present invention. A boundary of a display region is shown in  FIG. 6 . 
       FIG. 6 , a liquid crystal panel  160  includes a first substrate  110 , a second substrate  130  smaller than the first substrate  110  and a liquid crystal layer  150  interposed between the first and second substrates  110  and  130 . The liquid crystal panel  160  can be divided into a display region “D,” and first and second non-display regions “N 1 ” and “N 2 ” in plan view. The first non-display region “N 1 ” is adjacent to a boundary of the display region “D” and is defined by the first and second substrates  110  and  130 . The second non-display region “N 2 ” corresponds to a larger portion of the first substrate  110 . A gate pad  152  and a data pad  154  are disposed at the second non-display region “N 2 ” of the first substrate  110 . Moreover, a light shielding pattern  104  adjacent to the display region “D” is disposed at the first non-display region “N 1 .” The light shielding pattern  104  is made of an organic material absorbing light. The light shielding pattern  104  is formed not through an additional process but through a process of forming a black matrix (not shown) that prevents light from entering a switching element (not shown) on the first substrate  110 . 
       FIG. 7  is a schematic cross-sectional of a reflective LCD device according to an embodiment of the present invention. A boundary of a display region is mainly shown in  FIG. 7 . 
     In  FIG. 7 , a first substrate  110  and a second substrate  130  face and are spaced apart from each other. A light absorption layer  132  is formed on an inner surface of the second substrate  130  and a CCF layer  134  is formed on the light absorption layer  132  at a display region “D” and a first non-display region “N 1 .” A common electrode  136  is formed on the CCF layer  134  and the light absorption layer  132 . Array elements  102  are formed on an inner surface of the first substrate  110  and a light shielding pattern  104  is formed on the array elements  102  at the first non-display region “N 1 ” adjacent to the display region “D.” A protection layer  106  is formed on the light shielding pattern  104  and the array elements  102 . The protection layer  106 , which covers, the array elements  102 , is different from a passivation layer for a TFT in that the protection layer  106  functions as a buffer layer between the light shielding pattern  104  and a liquid crystal layer  150 . A retardation plate  108  and a polarizing plate  109  are sequentially formed on an outer surface of the first substrate  110 . The polarizing plate  109  is a linear polarizing plate transmitting only light parallel to a polarization axis of the polarizing plate  109  and the retardation plate  108  is a quarter wave plate (QWP) whose slow axis makes one angle of +45° and −45° with respect to the polarization axis. The slow axis is defined by a direction parallel to the direction along which the refractive index of the QWP is larger than along any other direction. The QWP transforms linearly polarized light into circularly polarized light by a retardation value of λ/4, and vice versa. When the slow axis makes an angle of +45° with respect to the polarization axis, the QWP transforms linearly polarized light into left-handed circularly polarized light. On the other hand, when the slow axis makes an angle of −45° with respect to the polarization axis, the QWP transforms linearly polarized light into right-handed circularly polarized light. Furthermore, a seal pattern  152  is formed at the first non-display region “N 1 ” to attach the first and second substrates  110  and  130 . 
     Light passing through each cell at the boundary of the display region “D” of the reflective LCD device using the CCF layer is illustrated in detail with an assumption that the CCF layer  134  selectively reflects only a right-handed circularly polarized light and the retardation plate  108  has a slow axis making an angle of −45° with respect to the polarization axis. External light is transformed into linearly polarized light for passing through the polarizing plate  109  and the linearly polarized light is transformed into right-handed circularly polarized light for passing through the retardation plate  108 . Since the right-handed circularly polarized light is absorbed into the light shielding pattern of a material absorbing light, a black state is obtained. Accordingly, a white phenomenon at the boundary of the display region can be prevented. 
       FIG. 8  is a flow chart illustrating a fabricating process of a substrate including a light shielding pattern according to an embodiment of the present invention. 
     At step ST 1 , after a first metallic material is deposited on a substrate, a gate line including a gate electrode and a gate pad is formed through a photolithography including exposure, development and etch processes. 
     At step ST 2 , after silicon nitride (SiNx), amorphous silicon (a-Si) and impurity-doped amorphous silicon (n+ a-Si) are sequentially deposited on an entire surface of the substrate, the a-Si and the n+ a-Si are etched through a photolithography to form an active layer and an ohmic contact layer, respectively. The SiNx functions as a gate insulating layer. The active layer and the ohmic contact layer constitute a semiconductor layer. 
     At step ST 3 , after a second metallic material is deposited on an entire surface of the substrate, a data line including source and drain electrodes spaced apart from each other is formed through a photolithography. In this step, a channel is formed through exposing the active layer between the source and drain electrodes. Thus, a thin film transistor (TFT) including the gate electrode, the semiconductor layer, and the source and drain electrodes is completed. 
     At step ST 4 , a passivation layer is formed on the TFT, the gate line and the data line through depositing or coating a material for the passivation layer. The passivation layer has a drain contact hole exposing the drain electrode through a photolithography. 
     At step ST 5 , a black matrix and a light shielding pattern are formed on the passivation layer. The black matrix and the light shielding pattern correspond to the TFT and a first non-display region adjacent to a display region, respectively. 
     At step ST 6 , after an organic insulating material is coated on the black matrix and the light shielding pattern, a protection layer having a contact hole corresponding to the drain contact hole is formed through a photolithography. 
     At step ST 7 , after a transparent conductive material is deposited on the protection layer, a pixel electrode is formed through a photolithography. The pixel electrode is connected to the drain electrode through the drain contact hole. Moreover, an orientation film is formed on the pixel electrode to align a liquid crystal layer. 
       FIG. 9  is a schematic plan view of a reflective LCD device using a CCF layer according to another embodiment of the present invention. Illustrations for the same portion as the reflective LCD device of  FIG. 6  will be omitted. 
     In  FIG. 9 , a plurality of light shielding patterns  202  are formed at a first non-display region “N 1 ” adjacent to a second non-display region “N 2 .” The plurality of light shielding patterns  202  are made of the same material as a gate pad  152  and a data pad  154 . Here, the plurality of light shielding patterns  202  adjacent to a display region “D” prevent a white phenomenon of a boundary of the display region “D.” If the plurality of light shielding patterns  202  cover a gate line and a data line (not shown), a parasitic capacitor degrading a display quality can be generated between the plurality of light shielding patterns  202  and the gate and data lines. Accordingly, the plurality of light shielding patterns  202  are formed at spaces between the adjacent gate lines and between the adjacent data lines. However, if the plurality of light shielding patterns  202  are formed exactly at the spaces between the gate lines and between the data lines, a light leakage can occur in the case of misalignment. Therefore, the plurality of light shielding patterns  202  overlap the gate and data lines within a range of about 1 μm to about 3 μm to maintain a black state at the boundary of the display region “D.” Moreover, the plurality of light shielding patterns  202  are made of the same material as the data line at the space between the gate lines, and made of the same material as the gate line at the space between the data lines to prevent an electric short between the plurality of light shielding patterns  202  and the gate and data lines without an additional process. A metal bar  204  of one material of the gate pad  152  and the data pad  154  is disposed at the first non-display region “N 1 ” not adjacent to the second non-display region “N 2 ” to prevent light reflection in the reflective LCD device using a CCF layer. 
     As in the reflective LCD device of  FIG. 6 , under the assumption that the CCF layer (not shown) selectively reflects only a right-handed circularly polarized light and the retardation plate (not shown) has a slow axis making an angle of −45° with respect to a polarization axis, external light is transformed into linearly polarized light for passing through the polarizing plate and the linearly polarized light is transformed into right-handed circularly polarized light for passing through the retardation plate. Since the right-handed circularly polarized light is absorbed into the light shielding pattern of the same material as the gate and data lines, a black state is obtained. Accordingly, a white phenomenon at the boundary of the display region can be prevented. 
       FIG. 10  is a schematic plan view of a reflective LCD device using a CCF layer according to another embodiment of the present invention. Illustration for the same portion as the reflective LCD device of  FIG. 9  will be omitted. 
     In  FIG. 10 , a light shielding pattern  302  of a transparent conductive material is formed at a first non-display region “N 1 ” adjacent to a display region “D.” The light shielding pattern  302  is simultaneously formed with a pixel electrode using the same material as the pixel electrode. Moreover, the light shielding pattern  302  may be electrically isolated from the pixel electrode. Instead, the light shielding pattern  302  may be connected to a common electrode (not shown) of a second substrate (not shown) through a silver (Ag) dot point for an electric connection between first and second substrates. Accordingly, a same signal is applied to the light shielding pattern  302  and the common electrode. 
       FIG. 11  is a schematic cross-sectional view of a reflective LCD device using a CCF layer according to another embodiment of the present invention. Illustration for the same portion as the reflective LCD device of  FIG. 7  will be omitted. 
     In  FIG. 11 , since a same signal is applied to a light shielding pattern  302  at a first non-display region “N 1 ” adjacent to a display region “D” and a common electrode  136  over a second substrate  330 , a liquid crystal layer  150  between the light shielding pattern  302  and the common electrode  136  functions as a π-cell to have a retardation of λ/2. 
     As in the reflective LCD device of  FIG. 7 , under the assumption that a CCF layer  134  selectively reflects only a right-handed circularly polarized light and the retardation plate  108  has a slow axis making an angle of −45° with respect to a polarization axis of a polarizing plate  109 , external light is transformed into linearly polarized light for passing through the polarizing plate  109  and the linearly polarized light is transformed into right-handed circularly polarized light for passing through the retardation plate  108 . Since the right-handed circularly polarized light is transformed into a left-handed circularly polarized light for passing through the liquid crystal layer  150  having a retardation of λ/2, the left-handed circularly polarized light is absorbed into a light absorption layer  132  under the CCF layer  134  at the first non-display region “N 1 .” Accordingly, a black state is obtained at a boundary of the display region “D” and a white phenomenon at the boundary of the display region “D” is effectively prevented. 
     Consequently, since a light shielding pattern is included in a reflective LCD device using a CCF layer according to the present invention, a black state is kept at a boundary of a display region and a display quality is improved. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in 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.