Abstract:
There is provided a liquid crystal display device including a display screen comprised of a plurality of areas in each of which a pixel pattern is formed, wherein any two areas located adjacent to each other, among the areas, have at least two stitches therebetween.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a liquid crystal display device and a method of fabricating the same, and more particularly to an active matrix type liquid crystal display device and a method of fabricating the same.  
         [0003]     2. Description of the Related Art  
         [0004]     In a step of forming a pixel electrode and an electrode located in facing relation to the pixel electrode in a process of fabricating a liquid crystal display device, in particular, an in-plane switching type liquid crystal display device, there is usually used stepper projection alignment in which a substrate on which a thin film transistor (TFT) is to be fabricated is divided into a plurality of areas, and each of areas is exposed to light in turn by means of a stepper.  
         [0005]     In stepper projection alignment, a difference is made in a line width and registration among layers because of dispersion in a luminance of light to which divided areas of a substrate is exposed, and/or a difference in light-exposure in areas between which a stitch is located. As a result, differences in capacitance are made among electrodes formed on the TFT substrate.  
         [0006]     Hence, it is important in stepper projection alignment to reduce non-uniformity in a luminance, caused by the differences in capacitance among electrodes formed on the TFT substrate.  
         [0007]     In order to reduce such non-uniformity, Japanese Patent Application Publication No. 2000-162639 has suggested a method of fabricating a liquid crystal display device, including a step of forming a pixel electrode and an electrode located in facing relation to the pixel electrode by stepper projection alignment, in which step, the divided areas located adjacent to each other are designed to have an area overlapping each other, and patterns in the divided areas are arranged in random number sequence in the overlapping area.  
         [0008]     Thus, an average capacitance of a pixel electrode and an opposing electrode in the overlapping area is just a half of capacitances of the electrodes in the adjacent divided areas. This ensures that a luminance smoothly changes over the adjacent divided areas between which the overlapping area exists, and hence, image-display quality is enhanced.  
         [0009]     However, a problem of a difference in a line width in the adjacent divided area between which an overlapping area exists is not solved in the above-mentioned conventional method. As a result, a viewer can recognize a difference in a luminance between the adjacent areas. This means that the conventional method cannot provide sufficient quality in displaying images.  
         [0010]     Japanese Patent Application Publication No. 11-249169 has suggested a liquid crystal display device including a substrate and at least two pixels arranged on the substrate and allowing a light to pass therethrough at different transmittance.  
         [0011]     Japanese Patent Application Publication No. 2002-182242 has suggested a method of fabricating a liquid crystal display device, including the steps of dividing at least one of an electrically conductive layer and a dielectric layer to a plurality of areas, and exposing each of the divided areas to light to pattern each of the divided areas such that adjacent divided areas have a zigzag stitch therebetween.  
         [0012]     Japanese Patent Application Publication No. 2002-107758 has suggested a method of fabricating a liquid crystal display device, including the steps of dividing a substrate into a plurality of areas, and exposing each of the areas to light by means of a stepper such that two areas located adjacent to each other have an overlapping area which is exposed to light twice. Such an overlapping area is non-linearly formed in a sub-pixel area.  
         [0013]     Japanese Patent Application Publication No. 2000-29053 has suggested a method of fabricating a liquid crystal display device, including the step of repeatedly using a mask for exposing each of pixel areas to light.  
       SUMMARY OF THE INVENTION  
       [0014]     In view of the above-mentioned problems in the prior art, it is an object of the present invention to provide a liquid crystal display device in which a viewer seldom recognizes a difference in a luminance in a display screen, and hence, which has qualified characteristic in displaying images.  
         [0015]     It is also an object of the present invention to provide a method of fabricating such a liquid crystal display device.  
         [0016]     In one aspect of the present invention, there is provided a liquid crystal display device including a display screen comprised of a plurality of areas in each of which a pixel pattern is formed, wherein any two areas located adjacent to each other, among the areas, have at least two stitches therebetween.  
         [0017]     It is preferable that the stitches are located differently from one another for each of pixel rows constituting the display screen.  
         [0018]     It is preferable that the stitches are located differently from one another for each of pixel rows of color pixels constituting a unit pixel.  
         [0019]     In another aspect of the present invention, there is provided a method of fabricating a liquid crystal display device including a step of forming a pixel pattern in each of areas constituting a display screen, by stepping projection alignment, the pixel pattern being formed in each of the areas for each of pixel rows by at least twice carrying out exposure of each of the areas to light such that stitches formed in the exposure between the areas are located differently from one another for the each of pixel rows.  
         [0020]     It is preferable that the pixel rows are comprised of a pixel row of first color pixels, a pixel row of second color pixels, and a pixel row of third color pixels, the first, second and third color pixels defining a unit pixel.  
         [0021]     It is preferable that a reticle having a certain pixel pattern is commonly used for carrying out exposure of each of the areas to light for each of the pixel rows, and a relative position between the reticle and a substrate is made different in exposure of the each of the areas to light for each of the pixel rows.  
         [0022]     In accordance with the above-mentioned present invention, it is possible to reduce a difference in a brightness between adjacent divided areas between which a stitch exists which stitch is inevitably caused in stepper projection alignment. Thus, a liquid crystal display fabricated in accordance with the present invention would have enhanced quality in displaying images.  
         [0023]     The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a perspective view of a liquid crystal display device in accordance with a first embodiment of the present invention.  
         [0025]      FIG. 2  is a plan view of a TFT array substrate in the liquid crystal display device in accordance with the first embodiment.  
         [0026]      FIG. 3A  is a cross-sectional view taken along the line III-III in  FIG. 2 , showing one of steps of a process of fabrication of the TFT array substrate in the liquid crystal display device in accordance with the first embodiment.  
         [0027]      FIG. 3B  is a cross-sectional view taken along the line III-III in  FIG. 2 , showing one of steps of a process of fabrication of the TFT array substrate in the liquid crystal display device in accordance with the first embodiment.  
         [0028]      FIG. 3C  is a cross-sectional view taken along the line III-III in  FIG. 2 , showing one of steps of a process of fabrication of the TFT array substrate in the liquid crystal display device in accordance with the first embodiment.  
         [0029]      FIGS. 4A, 4B  and  4 C are plan views of pixel rows to explain stepper projection alignment in the first embodiment.  
         [0030]      FIG. 5  is a plan view of a reticle used in stepper projection alignment in the first embodiment.  
         [0031]      FIG. 6  is a graph showing exposure of a substrate to light in the conventional stepper projection alignment.  
         [0032]      FIG. 7  is a graph showing exposure of a substrate to light in the stepper projection alignment in the first embodiment.  
         [0033]      FIG. 8  is a graph showing a luminance in a unit pixel in the liquid crystal display device in accordance with the first embodiment.  
         [0034]      FIG. 9A  is a cross-sectional view of a liquid crystal display device in accordance with the second embodiment, showing one of steps of a process of fabrication of the TFT array substrate in the second embodiment.  
         [0035]      FIG. 9B  is a cross-sectional view of a liquid crystal display device in accordance with the second embodiment, showing one of steps of a process of fabrication of the TFT array substrate in the second embodiment.  
         [0036]      FIG. 9C  is a cross-sectional view of a liquid crystal display device in accordance with the second embodiment, showing one of steps of a process of fabrication of the TFT array substrate in the second embodiment.  
         [0037]      FIG. 9D  is a cross-sectional view of a liquid crystal display device in accordance with the second embodiment, showing one of steps of a process of fabrication of the TFT array substrate in the second embodiment.  
         [0038]      FIGS. 10A, 10B ,  10 C and  10 D are plan views of pixel rows to explain stepper projection alignment in the second embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]     Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings.  
       FIRST EMBODIMENT  
       [0040]      FIG. 1  is a perspective view of a liquid crystal display device in accordance with the first embodiment of the present invention.  
         [0041]     The liquid crystal display device in accordance with the first embodiment is of an in-plane switching (IPS) type.  
         [0042]     As illustrated in  FIG. 1 , the liquid crystal display device is comprised of a TFT array substrate  101  on which thin film transistors (TFTs) are fabricated in array, and a color filter substrate  102  located in facing relation to the TFT array substrate  101  with a predetermined gap therebetween.  
         [0043]      FIG. 2  is a plan view of the TFT array substrate  101  in the first embodiment.  
         [0044]     On the TFT array substrate  101  are arranged lines of red (R) pixel rows  201 , lines of green (G) pixel rows  202 , and lines of blue (B) pixel rows  203 . A red pixel, a green pixel and a blue pixel located adjacent to one another cooperate to one another to define a unit pixel.  
         [0045]      FIG. 3C  is a cross-sectional view of the TFT array substrate  101 , taken along the line III-III in  FIG. 2 .  
         [0046]     As illustrated in  FIG. 3C , the TFT array substrate  101  is comprised of an electrically insulating transparent substrate  301 , a gate electrode  302  formed on the substrate  301 , an electrode  307  facing a pixel (hereinbelow, referred to as “pixel-facing electrode”) and formed on the substrate  301 , an electrically insulating inorganic film  308  formed on the substrate  301 , covering the gate electrode  302  and the pixel-facing electrode  307  therewith, an amorphous silicon layer  303  formed on the electrically insulating inorganic film  308  above the gate electrode  302 , a drain electrode  304  formed on the electrically insulating inorganic film  308  and partially covering the amorphous silicon layer  303  therewith, a source electrode  305  formed on the electrically insulating inorganic film  308  and partially covering the amorphous silicon layer  303  therewith, a pixel electrode  306  formed on the electrically insulating inorganic film  308 , and a passivation film  309  covering the amorphous silicon layer  303 , the drain electrode  304 , the source electrode  305  and the pixel electrode  306  therewith.  
         [0047]     The gate electrode  302 , the amorphous silicon layer  303 , the drain electrode  304  and the source electrode  305  constitute a thin film transistor (TFT).  
         [0048]     Since the liquid crystal display device in accordance with the first embodiment is of an in-plane switching (IPS) type, the drain electrode  304  and the pixel electrode  306  are formed in a common layer, the gate electrode  302  and the pixel-facing electrode  307  are formed in a common layer, and the pixel electrode  306  and the pixel-facing electrode  307  are formed in different layers.  
         [0049]      FIGS. 3A  to  3 C are cross-sectional views taken along the line III-III in  FIG. 2 , each showing one of steps of a process of fabrication of the TFT array substrate  101 . Hereinbelow, a process of fabricating the TFT array substrate  101  is explained with reference to  FIGS. 3A  to  3 C.  
         [0050]     First, as illustrated in  FIG. 3A , the gate electrode  302  and the pixel-facing electrode  307  are formed on the substrate  301 .  
         [0051]     The substrate  301  is composed of transparent material which is not deformed and denaturalized in annealing. For instance, the substrate  301  is composed of glass, quartz or plastic. In the first embodiment, the substrate  301  is composed of glass.  
         [0052]     The gate electrode  302  and the pixel-facing electrode  307  are comprised of a metal film such as a chromium (Cr) or aluminum (Al) film, and formed by sputtering, for instance.  
         [0053]     Then, as illustrated in  FIG. 3B , the electrically insulating inorganic film  308  is formed on the substrate  301  such that the gate electrode  302  and the pixel-facing electrode  307  are entirely covered with the electrically insulating inorganic film  308 .  
         [0054]     Then, an amorphous silicon film is formed on the electrically insulating inorganic film  308  by chemical vapor deposition (CVD), for instance. Then, the amorphous silicon film is patterned by photolithography and etching into the amorphous silicon layer  303 , as illustrated in  FIG. 3B .  
         [0055]     Then, the drain electrode  304  and the source electrode  305  are formed on the electrically insulating inorganic film  308  such that they partially cover the amorphous silicon layer  303 . Concurrently with the formation of the drain electrode  304  and the source electrode  305 , the pixel electrode  306  is formed on the electrically insulating inorganic film  308 . That is, are formed in a common layer.  
         [0056]     The drain electrode  304 , the source electrode  305  and the pixel electrode  306  are comprised of a metal film such as a chromium (Cr) or aluminum (Al) film, and formed by sputtering, for instance.  
         [0057]     Then, as illustrated in  FIG. 3C , the passivation film  309  comprised of a silicon oxide film or a silicon nitride film is formed entirely over the electrically insulating inorganic film  308 , the drain electrode  304 , the amorphous silicon laser  303 , the source electrode  305  and the pixel electrode  306  by sputtering or CVD.  
         [0058]     Thus, the TFT array substrate  101  is completed.  
         [0059]     In each of the steps in the above-mentioned method, the gate electrode  302 , the pixel-facing electrode  307 , the amorphous silicon film  303 , the drain electrode  304 , the source electrode  305  and the pixel electrode  306  are all patterned by photolithography. The step of exposing them to light, which is a most important step in patterning them, is carried out as stepper projection alignment which is usually used in fabrication of middle- and large-sized panels of five-inches or more size.  
         [0060]     In particular, in the step of exposing a metal layer to light for fabrication of the pixel electrode  306  and the pixel-facing electrode  307 , stepper projection alignment is carried out for each of color pixels by shifting a stitch.  
         [0061]     Specifically, as illustrated in  FIG. 2 , stepper projection alignment is carried out at a first stitch position  204  for the red pixel rows  201 , at a second stitch position  205  for the green pixel rows  202 , and at a third stitch position  206  for the blue pixel rows  203 .  
         [0062]     With reference to  FIGS. 4A  to  4 C, stepper projection alignment is explained hereinbelow. In  FIGS. 4A  to  4 C, an upper half indicates a location of a reticle, and a lower half indicates a pixel pattern of the TFT array substrate  101  in association with each of patterns of the reticle.  
         [0063]     In the conventional stepper projection alignment, the TFT array substrate  101  is exposed to light with one stitch  151 , as illustrated in  FIG. 4A .  
         [0064]     In contrast, in the first embodiment, stepper projection alignment is carried out three times in an area. Specifically, stepper projection alignment is carried out at the stitch  151  for exposure of red pixels, then, at two stitches  152   a  and  152   b  (see  FIG. 4B ) for exposure of green pixels, and finally, at two stitches  153   a  and  153   b  (see  FIG. 4C ) for exposure of blue pixels. Thus, the stitches are located different from one another for each of red, green and blue color pixels, as illustrated in  FIGS. 4A, 4B  and  4 C.  
         [0065]     In each of the exposure steps for each of red, green and blue color pixels, a reticle is commonly used by shifting in each of red, green and blue color pixels.  FIG. 5  is a plan view illustrating an example of the reticle. The reticle is designed to have a common pattern  501  for each of red, green and blue color pixels.  
         [0066]     Hence, with respect to a unit pixel comprised of red, green and blue pixels, the stitches are located different from one another for each of red, green and blue color pixels by shifting an exposure area in the first embodiment, unlike the conventional stepper projection alignment in which the TFT array substrate is divided into unit pixel areas. Hence, the first embodiment provides an advantage that a difference in a capacitance and an aperture ratio can be reduced which difference is caused by a difference in a line width and/or registration of a layer between adjacent areas between which a stitch exists.  
         [0067]     The above-mentioned advantage of the first embodiment is further explained hereinbelow with reference to  FIG. 6  to  8 .  
         [0068]      FIG. 6  shows a relation between exposure and a location of a TFT array substrate in the conventional stepper projection alignment.  
         [0069]     As is obvious in view of  FIG. 6 , a stitch remains in the same position for each of red, green and blue pixels when a substrate is exposed to light, and resultingly, exposure remarkably increases at the stitch  601 .  
         [0070]      FIG. 7  shows a relation between exposure and a location of the TFT array substrate  101  in the first embodiment.  
         [0071]     In accordance with the first embodiment, a first stitch  701  for exposure of the TFT array substrate  101  to light for red pixels, a second stitch  702  for exposure of the TFT array substrate  101  to light for green pixels, and a third stitch  703  for exposure of the TFT array substrate  101  to light for blue pixels are located differently from one another, as illustrated in  FIG. 7 .  
         [0072]     Though exposure remarkably varies at each of the first to third stitches  701 ,  702  and  703 , a degree in which exposure varies in each of red, green and blue pixels in the first; embodiment is equal to the same in the conventional stepper projection alignment.  
         [0073]     By fabricating the pixel electrode  306  and the pixel-facing electrode  307  by stepper projection alignment in accordance with the exposure profiles illustrated in  FIGS. 6 and 7 , a luminance in a panel would have a profile in accordance with the exposure profile.  
         [0074]     In color pixels in which exposure smoothly varied across a stitch, a luminance would not vary discontinuously, and hence, quality in displaying images is not deteriorated. Accordingly, attention may be paid to variance in a luminance in color pixels exposure for which varies discontinuously across a stitch.  
         [0075]     Hence, a luminance profile in a panel in a unit pixel is such a profile as illustrated in  FIG. 8 .  
         [0076]     As is obvious in view of  FIG. 8 , in accordance with the first embodiment, a luminance varies at a stitch only in any one of red, green and blue pixels with respect to a unit pixel comprised of red, green and blue pixels. Hence, comparing to the conventional stepper projection alignment in which a luminance varies for all of red, green and blue pixels in stepper projection alignment, a luminance difference across a stitch is about one-third (⅓). That is, the first embodiment can reduce a luminance difference between adjacent divided areas to a greater degree than the conventional method.  
       SECOND EMBODIMENT  
       [0077]      FIG. 9D  is a cross-sectional view of a TFT array substrate in a liquid crystal display device in accordance with the second embodiment.  
         [0078]     As illustrated in  FIG. 9D , the TFT array substrate  101 A is comprised of an electrically insulating transparent substrate  901 , a gate electrode  902  formed on the substrate  901 , an electrically insulating inorganic film  903  formed on the substrate  301 , covering the gate electrode  902  therewith, an amorphous silicon layer  904  formed on the electrically insulating inorganic film  903  above the gate electrode  902 , a drain electrode  905  formed on the electrically insulating inorganic film  903  and partially covering the amorphous silicon layer  904  therewith, a source electrode  906  formed on the electrically insulating inorganic film  903  and partially covering the amorphous silicon layer  904  therewith, a passivation film  907  covering the amorphous silicon layer  904 , the drain electrode  905 , the source electrode  906  and the electrically insulating inorganic film  903  therewith, a pixel electrode  910  formed on the passivation film  907  and making electrical contact with the source electrode  906  through a contact hole, and an electrode  911  facing a pixel (hereinbelow, referred to as “pixel-facing electrode”), formed on the passivation film  907  and making electrical contact with the gate electrode through a contact hole.  
         [0079]     In the TFT array substrate  101 A in the second embodiment, the pixel electrode  910  and the pixel-facing electrode  911  are concurrently formed on the passivation film  907 . The present invention may be applied to the step of concurrently forming the pixel electrode  910  and the pixel-facing electrode  911 .  
         [0080]      FIGS. 9A  to  9 D are cross-sectional of the TFT array substrate  101 A, each showing one of steps of a process of fabrication of the TFT array substrate  101 A. Hereinbelow, a process of fabricating the TFT array substrate  10 A is explained with reference to  FIGS. 9A  to  9 D.  
         [0081]     First, as illustrated in  FIG. 9A , the gate electrode  902  is formed on the substrate  901 .  
         [0082]     Then, as illustrated in  FIG. 9B , the electrically insulating inorganic film  903  is formed on the substrate  901  such that the gate electrode  902  is entirely covered with the electrically insulating inorganic film  903 .  
         [0083]     Then, the amorphous silicon layer  904  is formed on the electrically insulating inorganic film  903  above the gate electrode  902 , as illustrated in  FIG. 9B .  
         [0084]     Then, as illustrated in  FIG. 9C , the drain electrode  905  and the source electrode  906  are formed on the electrically insulating inorganic film  903  such that they partially cover the amorphous silicon film  904  therewith.  
         [0085]     Then, the passivation film  907  is formed entirely over the substrate  901 , as illustrated in  FIG. 9C .  
         [0086]     Then, contact holes  908 A and  908 B are formed throughout the passivation film  907  in alignment with the source electrode  906  and the gate electrode  902 , as illustrated in  FIG. 9C .  
         [0087]     Then, the pixel electrode  910  is formed on the passivation film  907  and making electrical contact with the source electrode  906  through the contact hole  908 A, and concurrently, the pixel-facing electrode  911  is formed on the passivation film  907  and making electrical contact with the gate electrode  902  through the contact hole  908 B, as illustrated in  FIG. 9D .  
         [0088]     Thus, the TFT array substrate  101 A in the second embodiment is completed.  
         [0089]     In the first and second embodiments, a step of exposing the substrate to light is carried out three times for each of red, green and blue pixels in a divided area. However, it should be noted that a step of exposing the substrate to light may be carried out four or more times.  
         [0090]     With reference to  FIGS. 10A  to  10 D, a process of carrying out stepper projection alignment four times is explained hereinbelow. In  FIGS. 10A  to  10 D, an upper half indicates a location of a reticle, and a lower half indicates a pixel pattern of the TFT array substrate  101 A in association with each of patterns of the reticle.  
         [0091]     It is assumed that four pixel rows A, B, C and D are arranged on the TFT array substrate  101 A.  
         [0092]     First, stepper projection alignment is carried out at a stitch  161  for exposure of the pixel rows A, as illustrated in  FIG. 10A .  
         [0093]     Second, stepper projection alignment is carried out at a stitch  162  for exposure of the pixel rows B, as illustrated in  FIG. 10B .  
         [0094]     Third, stepper projection alignment is carried out at a stitch  163  for exposure of the pixel rows C, as illustrated in  FIG. 10C .  
         [0095]     Finally, stepper projection alignment is carried out at a stitch  164  for exposure of the pixel rows D, as illustrated in  FIG. 10D .  
         [0096]     Thus, the four stitches  161  to  164  are located different from one another for each of the pixel rows A to D, as illustrated in  FIGS. 10A  to  10 D.  
         [0097]     In each of the exposure steps for each of pixel rows A to D, a reticle such as one illustrated in  FIG. 5  is commonly used by shifting in each of pixel rows A to D.  FIG. 5  is a plan view illustrating an example of the reticle.  
         [0098]     While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.  
         [0099]     The entire disclosure of Japanese Patent Application No. 2002-335264 filed on Nov. 19, 2002 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.