Abstract:
A liquid crystal display device includes a first substrate, a second substrate facing the first substrate, a liquid crystal layer made of liquid crystals injected between the first and the second substrates, and a backlight assembly arranged on an outer surface of the first substrate. The first substrate has a light guiding pattern containing a periodic structure formed from a medium whose refractive index is different from the refractive index of the first substrate. The light guiding pattern is operative to internally reflect light from the backlight assembly to a transmission region.

Description:
This application claims the benefit of priority to Korean Patent Application No. 2003-100692 filed on Dec. 30, 2003, herein incorporated by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a transflective type liquid crystal display device, and more particularly, to a transflective type liquid crystal display device and method for manufacturing the same, capable of optimizing optical efficiency. 
     2. Description of the Related Art 
     Generally, liquid crystal display (LCD) devices have advantages such as being lightweight, having a slim profile, and low power consumption, and are widely used for portable computers, office automation equipment, and audio/video apparatuses. 
     The LCD device includes two substrates and a liquid crystal layer interposed between the two substrates, and displaces liquid crystal molecules using an electric field generated upon application of a voltage. Hence, an image is displayed by manipulating the transmission of light through the liquid crystal. 
     Since the LCD device does not generate light by itself, it uses ambient light or a backlight assembly for generating light. Generally, the LCD device can be classified into two different categories: a transmission type LCD device or a reflection type LCD device. 
       FIG. 1  is a cross-sectional view schematically showing a structure of the transmission type LCD device according to the related art. In  FIG. 1 , the transmission type LCD device includes: a first substrate  102  on which a thin film transistor (TFT) functioning as a switching element is formed on each of intersection points between a plurality of gate lines and data lines; a second substrate  101  which faces the first substrate  102  and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer  103  including liquid crystals interposed between the first and the second substrates  102  and  101 ; first and the second polarizing plates  105  and  104  arranged on an outer surface of each of the first and the second substrates  102  and  101 ; and a backlight assembly  106  disposed outside the first polarizing plate  105 . 
     An optical transmission axis of the first polarizing plate  105  has an angle of 90° to that of the second polarizing plate  104 . The backlight assembly  106  generates light and provides the light toward the first substrate  102 . 
     In the related art LCD device having the foregoing construction, when the TFTs are turned on by a scanning signal applied to the plurality of gate lines and a data voltage applied to the plurality of data lines, the data voltage is applied to pixel electrodes through the turned-on TFTs. At this time, a common voltage is supplied to the common electrode of the second substrate  101 . Accordingly, the liquid crystal molecules are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block light provided from the backlight assembly  106 , so that a predetermined image is displayed. 
     However, in the transmission type LCD device of the related art, it is difficult to realize slimness and lightweight of the LCD device due to a large volume and a heavy weight of the backlight assembly  106 . Also, the power consumption of the backlight assembly  106  increases the overall power consumption of the device by a significant amount. 
     Therefore, research into reflection type LCD devices using ambient light instead of the backlight assembly  106  is actively performed. Such a reflection type LCD device is widely used as a portable display device such as an electronic organizer and a PDA (Personal Digital Assistant) thanks to low power consumption. 
       FIG. 2  is a cross-sectional view schematically showing a structure of the reflection type LCD device according to the related art. In  FIG. 2 , the reflection type LCD device includes: a first substrate  202  on which a thin film transistor (TFT) functioning as a switching element is formed on each of crossing points between a plurality of gate lines and data lines; a second substrate  201  which faces the first substrate  202  and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer  203  including liquid crystals interposed between the first and the second substrates  202  and  201 ; a first and a second polarizing plates  205  and  204  arranged on an outer surface of each of the first and the second substrates  202  and  201 ; and a reflector  206  disposed outside the first polarizing plate  205 . 
     An optical transmission axis of the first polarizing plate  205  has an angle of 90° to that of the second polarizing plate  204 . The reflector  206  reflects light provided from the outside and provides the light toward the first substrate  202 . 
     In the LCD device having the foregoing construction, when a plurality of TFTs are turned on by a scanning signal applied to a plurality of gate lines and a data signal applied to a plurality of data lines, the data signal is applied to pixel electrodes through the turned-on TFTs. At this time, a common voltage is supplied to the common electrode of the second substrate  201 . Accordingly, the liquid crystals are controlled by the electric field generated between the pixel electrodes and the common electrode to transmit or block light provided and reflected from the outside, whereby a predetermined image is displayed. 
     However, in the related art reflection type LCD device, when ambient light does not have a sufficient intensity (for example, the surrounding environment is dim), the brightness level of the display image is lowered and displayed information is not readable, which is problematic. 
     To resolve the above problems, a transflective type LCD device, which combines the reflection type LCD device and the transmission type LCD device, has been suggested. 
       FIG. 3  is a cross-sectional view schematically showing a construction of the transflective type LCD device according to the related art. In  FIG. 3 , the transflective type LCD device includes: a first substrate  330  on which a thin film transistor (TFT) functioning as a switching element is formed on each of crossing points between a plurality of gate lines and data lines; a second substrate  310 , which faces the first substrate  330  and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer  320  including liquid crystals interposed between the first and the second substrates  330  and  310 ; a first and a second polarizing plates  331  and  311  arranged on a lower surface of the first substrate  330  and an upper surface of the second substrates  310 , respectively; and a backlight assembly  340  disposed outside the first polarizing plate  331 . 
     An optical transmission axis of the first polarizing plate  331  has an angle of 90° to that of the second polarizing plate  311 . 
     On the first substrate  330 , a pixel electrode is connected to each TFT. On the pixel electrodes, a passivation layer  322  having a transmission hole  321  exposing a portion (transmission region) of each of the pixel electrodes and a reflector  323  are sequentially formed. 
     It is assumed that a region corresponding to the reflector  323  is a reflection region ‘r’ and a region corresponding to the portion of the pixel electrode, exposed by the transmission hole  321 , is a transmission region ‘t’. The reflection region ‘r’ is the region for reflecting light provided from ambient light in a reflection mode, and the transmission region ‘t’ is the region for transmitting light provided from the backlight assembly  340  in a transmission mode. 
     At this time, to reduce the difference in the distance that the light travels between the transmission region ‘t’ and the reflection region ‘r’, the cell gap d 1  of the transmission region ‘t’ is about twice that of the cell gap d 2  of the reflection region ‘r’. 
     Generally, a phase difference δ of a liquid crystal is obtained by the following formula:
 
δ=Δ n·d  
 
     where δ is the phase difference of a liquid crystal, Δn is the refractive index of a liquid crystal, and d is the cell gap. 
     Therefore, a difference in optical efficiency is generated between the reflection mode and the transmission mode. To reduce this difference in optical efficiency, the cell gap d 1  of the transmission region ‘t’ should be greater than the cell gap d 2  of the reflection region ‘r’ such that the phase difference value of the liquid crystal layer  320  is constant. 
     However, even though the difference in optical efficiency is reduced by making the cell gap d 1  of the transmission region t different from the cell gap d 2  of the reflection region r, it is difficult to optimize the transmission region and the reflection region. Therefore, it is difficult to obtain optimized optical efficiency. For example, in the transmission mode, not all of the light provided from the backlight assembly is transmitted through the transmission region, and some of the light impinges on the reflection region and is not transmitted, whereby optical loss occurs. Also, in the reflection mode, not all the ambient light is reflected by the reflector, and some of the ambient light impinges on the backlight assembly through the transmission region, whereby optical loss occurs. 
     SUMMARY 
     By way of introduction only, a transflective type LCD device of a first embodiment includes: a first substrate having a light guiding pattern containing a medium whose refractive index is different from a refractive index of the first substrate; a second substrate facing the first substrate; a liquid crystal layer disposed between the first and the second substrates; and a backlight assembly arranged on an outer surface of the first substrate. 
     A reflector may be formed on a pixel electrode formed on the first substrate such that a reflection region and a transmission region are provided. In this case, the reflector is formed in the reflection region and is not formed in the transmission region. 
     The reflection region may have a larger width than the transmission region. The light guiding pattern may be formed at the position that corresponds to the transmission region. Also, the refractive index of the light guiding pattern may be at least greater than that of the first substrate. The light guiding pattern may be tapered towards an inside thereof. 
     According to a second embodiment, a method for manufacturing a transflective type liquid crystal display includes: forming a predetermined pattern on a lower side of the substrate adjacent to a backlight assembly; and forming, in the pattern, a light guiding pattern made of medium whose refractive index is different from the refractive index of the substrate. 
     In another embodiment, the display device contains a light provider and a substrate having a periodic light guiding pattern formed therein. The light guiding pattern and substrate have different refractive indices that are different enough such that light from the light provider entering the light guiding pattern is directed by total internal reflection towards a front surface of the display device. 
     In another embodiment, the display device contains a light supplier, a reflector to reflect light from a light source external to the device towards the front surface, and means for redirecting light from the light supplier through total internal reflection towards the front surface. 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention 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 embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1  is a cross-sectional view schematically showing a structure of a transmission type LCD device of the related art; 
         FIG. 2  is a cross-sectional view schematically showing a structure of a reflection type LCD device of the related art; 
         FIG. 3  is a cross-sectional view schematically showing a structure of a transflective type LCD device of the related art; 
         FIG. 4  is a cross-sectional view schematically showing a structure of a transflective type LCD device according to the first embodiment of the present invention; 
         FIG. 5  is a drawing showing a status in which light progresses by a light guiding pattern of the present invention in transmission mode; 
         FIGS. 6A ,  6 B,  6 C and  6 D show the condition under which the total internal reflection occurs generally; 
         FIG. 7  is a drawing showing a total internal reflection path of light by a light guiding pattern of the present invention; 
         FIGS. 8A ,  8 B and  8 C are sectional views illustrating a method for forming a light guiding pattern on a first substrate of a transflective type LCD device; and 
         FIG. 9  illustrates a light guiding pattern formed in a rectangular pattern. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 4  is a cross-sectional view schematically showing a structure of a transflective type LCD device according to a first embodiment of the present invention. In  FIG. 4 , the transflective type LCD device includes: a first substrate  430  on which a thin film transistor (TFT) functioning as a switching element is formed on each of crossing points between a plurality of gate lines and data lines and a pixel electrode  437  is formed; a second substrate  410  facing the first substrate  430  and on which a black matrix (BM) layer, a color filter layer, and a common electrode are formed; a liquid crystal layer  420  including liquid crystals interposed between the first and the second substrates  430  and  410 ; first and second polarizing plates  431  and  411  arranged on a lower surface of the first substrate  430  and an upper surface of the second substrates  410 , respectively; and a backlight assembly  440  disposed on an outer surface of the first polarizing plate  431 . 
     An optical transmission axis of the first polarizing plate  431  has an angle of 90° to that of the second polarizing plate  411 . 
     The transflective type LCD device further includes a collimator  433  disposed between the first polarizing plate  431  and the backlight assembly  440 . The collimator  433  modulates an incident angle of light provided from the backlight assembly  440  such that parallel light is incident into the first substrate  430 . 
     Though not shown in  FIG. 4 , each thin film transistor is connected to a gate line and a data line, and each pixel electrode is connected to the drain electrode of the TFT. Accordingly, the pixel region may include the TFT and the pixel electrode. 
     The pixel region can be divided into a reflection region ‘r’ and a transmission region ‘t’. Namely, a transmission hole  421  exposing a portion of the pixel electrode  437 , and a passivation layer  422  and a reflector  423  thereon are alternately arranged on the pixel electrode  437 . The region corresponding to the transmission hole  421  exposed by the pixel electrode  437  is the transmission region ‘t’ and the region corresponding to the reflector  423  is the reflection region ‘r’. The reflection region ‘r’ is the region that reflects light provided from ambient light in the reflection mode and the transmission region ‘t’ is the region that transmits light provided from the backlight assembly  440  in the transmission mode. 
     To reduce the difference between distances traveled by light through the transmission region t and the reflection region r, the cell gap d 1  of the transmission region t is about twice that of the cell gap d 2  of the reflection region r. 
     In the embodiment shown, the ratio of the width of the reflection region r to that of the transmission region t is 3:2. Namely, by making the width of the reflection region r greater than the width of the transmission region t, more ambient light can be reflected in the reflection mode, whereby the brightness can be increased. Therefore, optical loss is reduced and optical efficiency is improved, compared to the related art. 
     However, if the width of the reflection region r is greater than that of the transmission region, the width of the transmission region t is relatively small, so that the amount of light provided from the backlight assembly  440  through the transmission region t is reduced. The light guiding pattern  432  helps to mitigate this problem. 
       FIG. 5  shows how light is affected by the light guiding pattern in the transmission mode. As shown in  FIG. 5 , in the transmission mode, light generated from the backlight assembly  440  is modulated into parallel light by the collimator  433  and provided to the first substrate  430  by way of the first polarizing plate  431 . 
     A light guiding pattern  432  capable of guiding light is formed on the first substrate  430 . The light guiding pattern  432  transmits incident light to be provided without any optical loss, to the transmission region t, through total internal reflection of the incident light. The light guiding pattern  432  is formed at the position that corresponds to the transmission region, which permits light that has traveled through using total internal reflection by the light guiding pattern  432  can be directly provided to the corresponding transmission region t. 
     Namely, light provided to the light guiding pattern  432  of the first substrate  430  is subject to total internal reflection inside the light guiding pattern  432  and is provided to the transmission region t. Therefore, since light generated from the backlight assembly  440  is provided to the transmission region t without any optical loss, the brightness is increased and the optical efficiency can be improved. 
       FIG. 6  is a schematic view showing a condition under which the total internal reflection occurs generally. As shown in  FIG. 6A , the relation between light transmitted and provided to and from media having different refractive indexes n i  and n t , is given by the following formula:
 sin θ i   =n   t   /n   i  sin θ t    
     Here, θ i  represents an incident angle, θ t  represents a transmission angle, n i  represents a refractive index of a medium through which light is provided, and n t  represents a refractive index of a medium to which light is transmitted. 
     As revealed by the above formula, if the refractive index n i  of the medium through which light is provided is greater than the refractive index n t  of the medium to which light is transmitted, the transmission angle θ t  is greater than the incident angle θ i . 
     As shown in  FIG. 6B , as the incident angle θ i  increases the transmission angle θ t  also increases. Accordingly, the transmitted light approaches the boundary between the two media and the amount of transmitted light is greater than in the amount of reflected light. 
     Eventually, as shown in  FIG. 6C , when the transmission angle θ t  becomes 90°, the incident light is neither transmitted nor reflected. The incident angle θ i  when the transmission angle θ t  becomes 90°, is called a critical angle θ c . As shown in  FIG. 6D , light provided at an angle greater than the critical angle θ c  is completely reflected by total internal reflection. 
       FIG. 7  is a drawing showing a total internal reflection path of light by the light guiding pattern. As shown in  FIG. 7 , to meet the total internal reflection condition, n 1  is greater than n 2  (n 1 &gt;n 2 ). Here, n 1  represents the refractive index of the light guiding pattern  432  and n 2  represents the refractive index of the first substrate  430 . 
     Also, the incident angle θ is greater than the critical angle (θ c =arcsin (n 2 /n 1 )). Therefore, light that satisfies the above two conditions is not transmitted but completely reflected by total internal reflection. Here, θ represents an incident angle of the light guiding pattern  432  and θ c  represents the critical angle. At this time, it should be noted that the reflective angle equals the incident angle. 
     Therefore, the refractive index n 1  of the light guiding pattern  432  is at least greater than the refractive index n 2  of the first substrate  430 . Generally, since the refractive index n 2  of the first substrate  430  is about 1.5, the refractive index n 1  of the light guiding pattern  432  is at least greater than 1.5. 
     Also, to get the incident light to be provided in the direction of the first substrate  430  by total internal reflection, the light guiding pattern  432  is tapered from a lower part thereof to an upper part. By tapering the width of the upper part compared to that of the lower part, incident light is repeatedly reflected inside the light guiding pattern  432  by total internal reflection and provided to the first substrate  430 . 
     Therefore, the transflective type LCD device of the present invention can improve the optical efficiency through increase in optical transmittance by forming a light guiding pattern  432  for guiding light in the direction of the first substrate  430  and providing the light from the backlight assembly  440  completely to the transmission region t of the first substrate  430  without any optical loss. 
     Also, the transflective type LCD device having the foregoing construction improves reflection efficiency by making the reflection region r having a larger width than that of the transmission region t so that a greater amount of incident ambient light is reflected upon the reflection mode, and improves optical transmittance by forming the light guiding pattern  432  so that light provided from the backlight assembly  440  is completely guided to the transmission region t by total internal reflection. As described above, the transflective type LCD device of the present invention can maximize the optical efficiency in both the reflection mode and the transmission mode. 
     In the meantime,  FIGS. 8A through 8C  are drawings explaining a manufacturing process for forming the light guiding pattern on the first substrate of the transflective type LCD device. 
     As shown in  FIG. 8A , a V-shaped pattern is formed on one side of the first substrate  430  by etching. For example, if the first substrate  430  is etched using photolithography, a positive or negative type photoresist is coated on the first substrate  430  so that a photoresist layer  435  is formed. 
     Subsequently, as shown in  FIG. 8B , an exposure mask (not shown) is positioned above the photoresist layer  435  and a specific portion of the photoresist layer  435  is exposed to exposure light of a particular wavelength. Thereafter, the exposed photoresist layer  435  is developed so that a predetermined pattern is formed. Etching is then performed using the patterned photoresist layer  435  as a mask. 
     More specifically, etchant partially passes through the patterned photoresist layer  435  and reacts with the first substrate  430 . Subsequently, by removing the patterned photoresist layer  435 , a V-shaped pattern is formed on the first substrate  430 . The V-shaped pattern is tapered such that the surface of the pattern has a larger width than the end of the V-shaped pattern inside the first substrate  430 . 
     As shown in  FIG. 8C , the light guiding pattern  432  made of a medium having the refractive index different from the first substrate  430  is formed on the V-shaped pattern. The light guiding pattern  432  has a refractive index greater than the refractive index of the first substrate  430  so that total internal reflection may occur. 
     Alternatively, as shown in  FIG. 9 , the light guiding pattern  532  may be formed on a first substrate  530  in rectangular shape. 
     As is apparent from the foregoing, the optical transmittance is increased by forming a light guiding pattern on a first substrate so that light provided from a backlight assembly is guided in the transmission mode. Also, the optical reflectance is increased by increasing the width of the reflection region formed on the first substrate so that more ambient light is reflected in the reflection mode. Thus, the optical efficiency is increased by increasing the optical reflectance and transmittance in the transflective type LCD device having a reflection mode and a transmission mode. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.