Abstract:
Disclosed is a liquid crystal display (LCD) with black matrixes of low reflectivity capable of reducing the reflection of back light. The black matrix of the disclosed LCD includes a photoshield layer formed on the back surface of a front substrate, and at least one internal photo-interference layer formed over the photoshield layer. The internal photo-interference layer has a refraction index different from that of the photoshield layer. The internal photo-interference layer has a double-layer structure consisting of a chromium nitride layer and a chromium oxide layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a liquid crystal display (LCD), and more particularly to an LCD with a black matrix of low reflectivity.  
           [0003]    2. Description of the Related Art  
           [0004]    LCD is a flat panel display using electro-optic properties of a liquid crystal layer interposed between two substrates. An example of a conventional LCD is illustrated in FIG. 1.  
           [0005]    [0005]FIG. 1 is a sectional view illustrating an essential part of an LCD having a conventional simple shield type black matrix.  
           [0006]    As shown in FIG. 1, the LCD having a simple shield type black matrix includes a back substrate  13  and a front substrate  15  which are arranged in parallel to each other and a liquid crystal layer  17  which is interposed between the substrates  13  and  15 . The LCD also includes a back polarizer  12  attached to the back surface of the back substrate  13 , and a front polarizer  16  attached to the front surface of the front substrate  15 .  
           [0007]    R, G, and B color filter layers  14 - 1 ,  14 - 2 , and  14 - 3  are formed on the back surface of the front substrate  15  such that they are separated from one another by black matrixes  19 - 1 ,  19 - 2 , and  19 - 3  formed on the back surface of the front substrate  15 . The black matrixes are arranged between adjacent ones of the color filter layers  14 - 1 ,  14 - 2 , and  14 - 3  and they are flush with one another.  
           [0008]    Pixel electrodes respectively corresponding to the color filter layers  14 - 1 ,  14 - 2 , and  14 - 3  and thin film transistors serving as active devices are formed on the front surface of the back substrate  13 . A back light source  11  is arranged behind the back polarizer  12 .  
           [0009]    The black matrixes  19 - 1 ,  19 - 2 , and  19 - 3  are photoshield films for shielding an external light, thereby preventing an increase in leakage current at the thin film transistors.  
           [0010]    Now, the operation of the LCD having the above mentioned configuration will be described with reference to FIG. 1.  
           [0011]    Light emitted from the back light source  11  are linearly polarized while passing through the back polarizer  12 , and then pass through the back substrate  13 . The back light emerging from the back substrate  13  reach the black matrixes  19 - 1 ,  19 - 2 , and  19 - 3  and the color filter layers  14 - 1 ,  14 - 2 , and  14 - 3 , after passing through the liquid crystal layer  17 . Assuming that the liquid crystal layer  17  has a twisted liquid crystal structure, the polarization vectors of the back light are rotated by an angle of 90°.  
           [0012]    The back light reaching each of the color filter layers  14 - 1 ,  14 - 2 , and  14 - 3  is colored, and then externally emitted through the front polarizer  16 , thereby being recognized as information.  
           [0013]    Meanwhile, the back light reaching the black matrixes,  19 - 1 ,  19 - 2  and  19 - 3 , are reflected toward the back substrate  13 , and then are applied to a channel region in an associated one of the thin film transistors, thereby resulting in a production of photocurrent noises.  
           [0014]    The conventional simple shield type black matrixes  19 - 1 ,  19 - 2 , and  19 - 3  have respectively a photoshield layer made of a chromium material. The intensity distribution, that is, luminance distribution, of the back light reflected from the simple shield type black matrix is depicted in FIGS. 2A to  2 C, respectively.  
           [0015]    [0015]FIG. 2A illustrates the luminance distribution of the back light reflected by the black matrix  19 - 1  after passing through the R color filter layer  14 - 1 . FIG. 2B illustrates the luminance distribution of the back light reflected by the black matrix  19 - 2  after passing through the G color filter layer  14 - 2 . FIG. 2C illustrates the luminance distribution of the back light reflected by the black matrix  19 - 3  after passing through the B color filter layer  14 - 3 . Referring to FIGS. 2A to  2 C, it can be found that the back light reflected by the black matrix  19 - 1  after passing through the R color filter layer  14 - 1  exhibits a luminance of 83.9 cd/m 2 , the back light reflected by the black matrix  19 - 2  after passing through the G color filter layer  14 - 2  exhibits a luminance of 90.4 cd/m 2 , the back light reflected by the black matrix  19 - 3  after passing through the B color filter layer  14 - 3  exhibits a luminance of 90.6 cd/m 2 . In this case, the luminance of the back light emitted from the back light source  11  is 1,300 cd/m 2 .  
           [0016]    Meanwhile, the luminance distribution of back light beams of a visible range passing through the back light source  11  and the back polarizer  12  in the LCD including the above mentioned conventional simple shield type black matrix is depicted in FIG. 3. In FIG. 3, the solid line is indicative of the luminance of the back light emitted from the back light source  11  whereas the dotted line is indicative of the luminance of the back light emerging from the back polarizer  12 .  
           [0017]    Referring to FIG. 3, it can be found that the back light emerging from the back polarizer  12  has a luminance of 559 cd/m 2 . It can also be found that the peak luminance wavelengths of the back light coincide with respective wavelength of red, green, and blue light.  
           [0018]    In this case, the back polarizer  12  has a transmittance of 43% and a reflectivity of 47% ((83.9+90.4+90.6)/559×100).  
           [0019]    In the conventional LCD exhibiting such a reflectivity, the off current of each thin film transistor increases from several pA to several ten pA for every frame, and the voltage applied to the liquid crystal layer  17  decreases gradually. As a result, there is a voltage difference between positive and negative voltages, thereby causing the display screen to flicker. The generation of such flicker increases as the luminance of back light source increases according to the gradual increase in the size of a display screen.  
           [0020]    On the other hand, when an external light, such as a light emitted from a fluorescent lamp, is reflected by the black matrix, a degradation occurs in contrast and visual recognizability. In order to preventing such a degradation in visual recognizability, an LCD including an external light anti-reflection type black matrix has been proposed.  
           [0021]    [0021]FIGS. 4 and 5 are sectional views respectively illustrating the conventional external light anti-reflection type black matrix.  
           [0022]    The external light anti-reflection type black matrix illustrated in FIG. 4 has a double-layer structure consisting of a chromium oxide layer  21  and a chromium layer  22  sequentially formed over a front substrate  20 . The chromium layer  22  serves as a photoshield film whereas the chromium oxide layer  21  serves as a photo-interference layer.  
           [0023]    The external light anti-reflection type black matrix illustrated in FIG. 5 has a triple-layer structure consisting of a chromium nitride (CrN y ) layer  31 , a chromium oxide layer  32  and a chromium layer  33  sequentially formed over a front substrate  30 . The chromium layer  33  serves as a photoshield film whereas the chromium nitride layer  31  and chromium oxide layer  32  serve as photo-interference layers.  
           [0024]    [0024]FIG. 6 is a graph depicting the reflectivity of the external light anti-reflection type black matrix in FIG. 5. The abscissa of the graph is indicative of the wavelength of the external light whereas the ordinate is indicative of the reflectivity or reflection rate. Referring to FIG. 6, it can be found that the black matrix exhibits a reflectivity of 5 to 7% at the wavelength of the external light ranging from 380 nm to 780 nm. Thus, this black matrix exhibits a reduced reflectivity of less than 10% and provides an improvement in a visual recognizability.  
           [0025]    In LCDs including the above mentioned external light anti-reflection type black matrix of FIG. 4 or  5 , however, back light is reflected in a range of 40 to 50% by the chromium layer of the black matrix serving as a photoshield layer.  
         SUMMARY OF THE INVENTION  
         [0026]    Therefore, an object of the present invention is to provide an LCD with black matrixes of low reflectivity capable of reducing the reflection of back light in order to solve the above mentioned problems involved in the related art.  
           [0027]    The present invention provides an LCD including a front substrate on which color filters and black matrixes are formed such that the black matrixes are respectively arranged between adjacent ones of the color filters. In order to prevent back light beams passing through a liquid crystal layer from being reflected toward the liquid crystal layer, thereby achieving a reduction in flicker, the black matrix comprises a photoshield layer formed on a back surface of the front substrate, and at least one internal photo-interference layer formed over the photoshield layer. The internal photo-interference layer has a refraction index different from that of the photoshield layer.  
           [0028]    The black matrix may further comprise at least one external photo-interference layer formed between the front substrate and the photoshield layer. The external photo-interference layer has a refraction index different from that of the photoshield layer.  
           [0029]    The above objects, and other features and advantages of the present invention will become more apparent after a reading of the following detailed description taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a sectional view illustrating an essential part of an LCD having a conventional simple shield type black matrix;  
         [0031]    [0031]FIGS. 2A to  2 C are graphs respectively depicting the luminance distributions of back light reflected by each of the black matrixes in the LCD of FIG. 1, and more particularly, FIG. 2A illustrates the luminance distribution of the back light reflected by the black matrix  19 - 1  after passing through the R color filter layer  14 - 1 , FIG. 2B illustrates the luminance of the back light reflected by the black matrix  19 - 2  after passing through the G color filter layer  14 - 2 , and FIG. 2C illustrates the luminance of the back light reflected by the black matrix  19 - 3  after passing through the B color filter layer  14 - 3 ;  
         [0032]    [0032]FIG. 3 is a graph depicting the luminance distributions of back light of a visible range passing through a back light source and a back polarizer in the LCD of FIG. 1;  
         [0033]    [0033]FIGS. 4 and 5 are sectional views respectively illustrating an external light anti-reflection type black matrix;  
         [0034]    [0034]FIG. 6 is a graph depicting the reflectivity of the external light anti-reflection type black matrix in FIG. 5;  
         [0035]    [0035]FIG. 7 is a sectional view illustrating black matrixes of low reflectivity according to one embodiment of the present invention;  
         [0036]    [0036]FIG. 8 is a schematic view illustrating the principle of the reflection of back light being reduced in FIG. 7;  
         [0037]    [0037]FIGS. 9A to  9 C are graphs respectively depicting the luminance distributions of back light reflected by each of black matrixes in FIG. 7, and more particularly, FIG. 7A illustrates the luminance distribution of the back light reflected by the black matrix  19 - 1 ′ after passing through the R color filter layer, FIG. 7B illustrates the luminance of the back light reflected by the black matrix  19 - 2 ′ after passing through the G color filter layer, and FIG. 7C illustrates the luminance of the back light reflected by the black matrix  19 - 3 ′ after passing through the B color filter layer;  
         [0038]    [0038]FIG. 10 is a graph depicting a variation in reflectivity depending on the thickness of a chromium oxide layer in the LCD of FIG. 7;  
         [0039]    [0039]FIG. 11 is a graph depicting a variation in off current in each thin film transistor included in the LCD of FIG. 7;  
         [0040]    [0040]FIG. 12 is a sectional view illustrating the black matrixes of low reflectivity according to another embodiment of the present invention; and  
         [0041]    [0041]FIG. 13 is a graph depicting the relation between the thickness and reflectivity of a chromium oxide layer depending on the thickness of a chromium nitride layer in the LCD of FIG. 12.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    Now, preferred embodiments of the present invention will be described in detail, with reference to the annexed drawings. For the convenience of description, elements having the same functions as those of the above mentioned conventional LCDs are denoted by the same reference numerals and names as those of the conventional LCDs.  
         [0043]    [0043]FIG. 7 is a sectional view illustrating the black matrixes of low reflectivity according to one embodiment of the present invention.  
         [0044]    The black matrixes  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ shown in FIG. 7 respectively correspond to the black matrixes  19 - 1 ,  19 - 2 , and  19 - 3  of FIG. 1. The black matrix comprises a chromium oxide layer  41  formed over the back surface of a front substrate  40  corresponding to the front substrate  15  of FIG. 1, a chromium layer  42  formed over the chromium oxide layer  41 , and a chromium oxide layer  43  formed over the chromium layer  42 . The chromium oxide layer  41  serves as an external photo-interference layer. The chromium layer  42  serves as a photoshield layer whereas the chromium oxide layer  43  serves as an internal photo-interference layer.  
         [0045]    The principle of the reflection of back light being reduced by the black matrix portions  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ each having the above mentioned layer structure will be described hereinafter, with reference to FIG. 8 along with FIG. 1.  
         [0046]    Light beams emitted from the back light source  11  reach the chromium oxide layer  41  and the color filter layers  14 - 1 ,  14 - 2 , and  14 - 3 , respectively, after passing through the back polarizer  12 , the back substrate  13 , and the liquid crystal layer  17 . The back light beam reaching each of the color filter layers  14 - 1 ,  14 - 2 , and  14 - 3  is colored, and then externally emitted through the front polarizer  16 , thereby being recognized as information. In this case, the luminance of the back light emitted from the back light source  11  is 1,300 cd/m 2 . The back light exhibits a luminance of 559 cd/m 2  after passing though the back polarizer  12 . Accordingly, the transmittance of the back polarizer  12  is 43%.  
         [0047]    Meanwhile, the back light beam reaching the chromium oxide layer  43  is partially reflected at the interface between the liquid crystal layer  17  and chromium oxide layer  43  (reflection angle of θ 1 ) while the unreflected back light partially passing through the chromium oxide layer  43  (refraction angle of θ 2 ). A large fraction of the back light beam passing through the chromium oxide layer  43  is reflected at the interface between the chromium oxide layer  43  and chromium layer  42  while the unreflected back light passing through the chromium layer  42  with a refraction angle of θ 3 .  
         [0048]    An interference occurs between the back light reflected at the interface between the liquid crystal layer  17  and chromium oxide layer  43  and the back light passing through the liquid crystal layer  17  after being reflected at the interface between the chromium layer  42  and chromium oxide layer  43 . This interference will now be described. For the convenience of description, the back light reflected again at the interface between the liquid crystal layer  17  and chromium oxide layer  43  toward the chromium layer  42  is disregarded.  
         [0049]    Assuming that λ represents the wavelength of the back light, the external complex Fresnel reflection coefficient r for S and P waves is expressed by the following expression:  
           r =( r   12   +r   23  exp(2 jβ ))/(1+ f   12   r   23  exp(2 jβ ))  
         [0050]    where, β corresponds to “(2π n 2 hcosθ 2 )/λ” (where, h represents the thickness of the chromium oxide layer  43 , n 2  represents the refraction index of the chromium oxide layer  43 , θ 2  represents the refraction angle at the interface between the liquid crystal layer  17  and the chromium oxide layer  43 ), r 12  represents the Fresnel reflection coefficient at the interface between the liquid crystal layer  17  and chromium oxide layer  43 , and r 23  represents the Fresnel reflection coefficient at the interface between the chromium oxide layer  43  and chromium layer  42 .  
         [0051]    Using the Law of Snell, the reflection angle θ 2  and the reflection angle θ 3  can be expressed by a function of the reflection angle θ 1  at the interface between the liquid crystal layer  17  and chromium oxide layer  43 , as follows:  
         cos θ 2 =(1−sin 2  θ 1   n   1   2   /n   2   t ) ½    
         cos θ 3 =(1−sin 2  θ 1   n   1   2   /n   3   2 ) ½    
         [0052]    where, n 1  represents the refraction index of the liquid crystal layer  17 , n 2  represents the refraction index of the chromium oxide layer  43 , and n 3  represents the refraction index of the chromium layer  42 .  
         [0053]    The reflectivity R of the liquid crystal layer  17  is expressed by the following expression:  
         R=|r| 2 ≡R(θ 1 , h, n 1 , n 1 , n 1 )  
         [0054]    In accordance with this expression, the reflectivity R is a function of the refraction index n 3  of the chromium layer  43 , the refraction index n 2  of the chromium layer  42 , and the thickness of the chromium oxide layer  43 , assuming that reflection angle θ 1  is zero. For example, a destructive interference occurs at an optical path difference of m(an integer) times 2hn 2 /λ, thereby resulting in a reduction in the reflected amount of light. At an optical path difference of (m +½) times 2hn 2 /λ, a construction interference occurs, thereby resulting in an increase in the reflected amount of light. Accordingly, the reflection of the back light can be reduced by appropriately adjusting the thicknesses and refraction index of the chromium oxide layer  43  and the reflection index of the chromium layer  42 .  
         [0055]    Respective luminance distributions of back light reflected by the black matrix  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ in FIG. 7 are depicted in FIGS. 9A to  9 C.  
         [0056]    [0056]FIG. 9A illustrates the luminance distribution of the back light reflected by the black matrix  19 - 1 ′ after passing through the R color filter layer, FIG. 9B illustrates the luminance distribution of the back light reflected by the black matrix  19 - 2 ′ after passing through the G color filter layer, and FIG. 9C illustrates the luminance distribution of the back light reflected by the black matrix  19 - 3 ′ after passing through the B color filter layer.  
         [0057]    Referring to FIGS. 9A to  9 C, it can be found that the back light reflected by the black matrix  19 - 1 ′ after passing though the R color filter layer  14 - 1  exhibits a luminance of 8.7 cd/m 2 , the back light reflected by the black matrix  19 - 2 ′ after passing though the G color filter layer  14 - 2  exhibits a luminance of 9.7 cd/m 2 , and the back light reflected by the black matrix  19 - 3 ′ after passing though the B color filter layer  14 - 3  exhibits a luminance of 10.5 cd/m 2 . Since the luminance sum of the back light reflected by the black matrix  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ is 28.9 cd/m 2 , the black matrix exhibits a reflectivity of 5.2% (28.9/559×100). As compared to the LCD having the conventional simple shield type black matrix, the LCD having black matrix in FIG. 7 exhibits a reduced reflectivity of 10.3% (8.7/839×100) in the case of the black matrix  19 - 1 ′, 10.7% (8.7/90.4×100) in the case of the black matrix  19 - 2 ′, and 10.9% (28.9/265×100) in the case of the black matrix  19 - 3 ′. It can also be found that the sum of back light reflected by the black matrix  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ is reduced to 10.9% (28.9/265×100).  
         [0058]    [0058]FIG. 10 is a graph depicting a variation in reflectivity depending on the thickness of the chromium oxide layer  43  in the LCD of FIG. 7. In the graph of FIG. 10, the abscissa is indicative of the thickness of the chromium oxide layer  43  whereas the ordinate is indicative of the reflectivity or reflection rate. The depicted reflectivity is based on the wavelength of 589 nm. Referring to FIG. 10, it can be found that a minimum reflectivity of about 0.05 is obtained when the chromium oxide layer  43  has a thickness ranging from about 150 Å to 1,000 Å.  
         [0059]    [0059]FIG. 11 is a graph depicting a variation in off current in each thin film transistor included in the LCD of FIG. 7. In the graph of FIG. 11, the abscissa is indicative of the luminance sum of back light respectively reflected by the black matrixes  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ whereas the ordinate is indicative of the off current in the thin film transistor. Referring to FIG. 11, it can be found that the off current A of the thin film transistor included in the LCD having the black matrixes  19 - 1 ′,  19 - 2 ′, and  19 - 3 ′ in FIG. 7 is considerably reduced, as compared to the off current B of the thin film transistor in the conventional LCD of FIG. 1.  
         [0060]    Although the internal photo-interference layer of the black matrix according to the above-described embodiment has a single-layer structure, it may have a double-layer structure, as shown in FIG. 12.  
         [0061]    [0061]FIG. 12 is a sectional view illustrating black matrixes of low reflectivity according to another embodiment of the present invention.  
         [0062]    The black matrixes  19 - 1 ″,  19 - 2 ″, and  19 - 3 ″ shown in FIG. 12 respectively correspond to the black matrixes  19 - 1 ,  19 - 2 , and  19 - 3  of FIG. 1. The black matrix comprises a chromium nitride layer  51  formed over the back surface of a front substrate  50  corresponding to the front substrate  15  of FIG. 1, a chromium oxide layer  52  formed over the chromium nitride layer  51 , a chromium layer  53  formed over the chromium oxide layer  52 , a chromium oxide layer  54  formed over the chromium layer  53 , and a chromium nitride layer  55  formed over the chromium oxide layer  54 . The chromium nitride layer  51  and chromium oxide layer  52  serve as an external photo-interference layer. The chromium layer  53  serves as a photoshield layer whereas the chromium oxide layer  54  and chromium nitride layer  55  serve as an internal photo-interference layer.  
         [0063]    The principle of the reflection of back light being reduced by the black matrixes having the internal photo-interference layer with a double-layer structure is identical to that of the black matrix having the internal photo-interference layer with a single-layer structure illustrated in FIG. 7. That is, the reflection of back light by the black matrixes can be reduced by adjusting the thickness and refraction index of the chromium nitride layer  55 , the thickness and refraction index of the chromium oxide layer  54 , and the thickness and refraction index of the chromium layer  53  such that both the back light reflected at the interface between the chromium nitride layer  55  and chromium oxide layer  54  and the back light reflected at the interface between chromium oxide layer  54  and the chromium layer  53  destructively interfere with the back light reflected at the interface between the liquid crystal layer  17  and chromium nitride layer FIG. 13 is a graph depicting the relation between the thickness and reflectivity of the chromium oxide layer depending on the thickness of the chromium nitride layer in the LCD of FIG. 12. In the graph of FIG. 13, the abscissa is indicative of the thickness of the chromium oxide layer  54  whereas the ordinate is indicative of the reflectivity. Respective relations between thickness and reflectivity depicted in FIG. 13 are based on the wavelength of 589 nm and the conditions in which: the chromium nitride layer  55  has respective thicknesses of 200 Å, 250 Å, 300 Å, 350 Å, 400 Å, 450 Å, and 500 Å; the chromium nitride layer  55  has a refraction index of 2.8; the chromium oxide layer  54  has a thickness of 0 to 1,000 Å; the chromium oxide layer  54  has a refraction index of 3.5; and the chromium layer  53  has a refraction index of 2.0.  
         [0064]    Referring to the graph of FIG. 13, it can be found that a minimum reflectivity of about 0.03 is obtained when the chromium oxide layer  54  has a thickness of about 50 Å or about 800 Å.  
         [0065]    The previously described versions of the present invention have many advantages, including the following advantages.  
         [0066]    In accordance with the present invention, it is possible to reduce the reflection of the back light, already passing though the liquid crystal layer, again into the liquid crystal layer by adding an internal photo-interference layer to the black matrix having the conventional simple shield type structure or the conventional external light anti-reflection type structure. Accordingly, it is possible to achieve a reduction in flicker and an improvement in visual recognizability.  
         [0067]    Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible.  
         [0068]    For example, the external photo-interference layer may have a double-layer structure while the internal photo-interference layer may have a single-layer structure, and vice versa. In this case, the double-layer structure may consists of a chromium oxide layer, and a chromium nitride layer whereas the single-layer structure may consists of a chromium oxide layer.  
         [0069]    Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.