Patent Publication Number: US-2006017871-A1

Title: Liquid crystal display

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-213268, filed on Jul. 21, 2004. The entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a liquid crystal display capable of correctly detecting an object and conducting transmissive display as well as reflective display.  
      2. Description of Related Art  
      Liquid crystal displays (LCDs) are thin, lightweight, and low in power consumption, and therefore, are widely used for cellular phones, smart-phones, PDAs (personal digital assistants), personal computers, and the like.  
      The LCD has an array substrate on which scanning lines and signal lines cross each other, a counter substrate, a liquid crystal layer held between the array substrate and the counter substrate, and a drive circuit for driving the scanning lines and the signal lines. A pixel is formed at each crossing portion of the scanning lines and the signal lines. A video signal is applied to the pixels, to change the light transmittance of the liquid crystal layer and display an image.  
      Among LCDs, those having light sources are in the mainstream because they can display images in the dark.  
      To make the LCD compact and low in cost, recent technology integrates the drive circuit into the array substrate containing pixels, each pixel having a switching element (such as a TFT (thin film transistor)) and a pixel electrode.  
      Some recent LCDs incorporate detective elements such as optical sensors to realize a scanning function.  
      The LCD having the scanning function emits light from a light source. The light is transmitted through the LCD and is reflected from a detection object such as a printed object. The intensity of the reflected light is detected by the optical sensors of the LCD and is used to provide an image of the object.  
      The optical sensors incorporated in the LCD may detect the intensity of light emitted from a light pen, to realize a light-pen input function. Instead of the optical sensors, other elements such as piezoelectric elements may be employed to realize a touch-panel function.  
      The detective elements such as the optical sensors and piezoelectric elements are formed on the array substrate so that the detective elements and TFTs serving as switching elements may be formed through common processes. This can reduce the number of manufacturing processes of the LCD. The light source is arranged behind the counter substrate of the LCD. This arrangement allows a detection object to be presented in front of the array substrate so that the detective elements may correctly detect light or pressure provided by the object.  
      To reduce power consumption, some recent LCDs conduct as a transmissive display with light emitted from a light source and also as a reflective display with external light to display, for example, the date and time. Such LCDs employ reflective electrodes for some pixel electrodes arranged on the array substrate.  
      There is a requirement for an LCD that is capable of providing the detective function and the transmissive-reflective display function.  
      Simply combining the structures of the two types of LCD by forming detective elements and reflective electrodes on an array substrate is unsatisfactory because the combination will transmit no external light through a liquid crystal layer. Then, the LCD is unable to control brightness and achieve reflective display.  
      An example of the related arts mentioned above is disclosed in Japanese Unexamined Patent Application Publication No. 2002-303863.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a liquid crystal display (LCD) capable of correctly detecting a detection object and conducting transmissive display as well as reflective display.  
      In order to accomplish the object, a first aspect of the present invention provides an LCD having an array substrate provided with a plurality of scanning lines and a plurality of signal lines that cross the scanning lines, a counter substrate, and a liquid crystal layer held between the array substrate and the counter substrate. A pixel is formed at each crossing portion of the scanning lines and the signal lines. The array substrate includes transparent pixel electrodes and detective elements. The pixel electrodes are provided for the pixels, respectively, to apply an electric field to the liquid crystal layer. The detective elements detect an object presented in front of the array substrate. The counter substrate includes a color filter to transmit light emitted from a light source that is arranged behind the counter substrate, a transparent electrode facing a first plurality of the pixel electrodes, to transmit the light transmitted through the color filter, and a reflective electrode facing a second plurality of the pixel electrodes, to reflect external light.  
      According to the first aspect, the array substrate of the LCD has the detective elements to detect an object presented in front of the array substrate. The counter substrate has the color filter that faces the first plurality of pixel electrodes and transmits light emitted from the light source, and the reflective electrode that faces the second plurality of pixel electrodes and reflects external light. The detection object is directly presented to the detective elements, so that the detective elements correctly detect the object. The light emitted from the light source and transmitted through the transparent electrode and the external light reflected from the reflective electrode are both transmitted through the liquid crystal layer, to thereby realize transmissive display as well as reflective display.  
      According to a second aspect of the present invention, the reflective electrode of the LCD of the first aspect has irregularities.  
      According to the second aspect, the reflective electrode of the LCD has irregularities to scatter light reflected from the reflective electrode, to increase the view angle.  
      According to a third aspect of the present invention, the reflective electrode of the LCD of any one of the first and second aspects is arranged on the liquid crystal layer side of the color filter.  
      According to the third aspect, the reflective electrode of the LCD is arranged on the liquid crystal layer side of the color filter, so that no external light is transmitted through the color filter under the reflective electrode, to thereby realize monochromatic reflective display.  
      In this way, the LCD according to the present invention includes the array substrate having the detective elements to detect an object presented in front of the array substrate. The counter electrode of the LCD includes the color filter to transmit light emitted from the light source, the transparent electrode that faces the first plurality of pixel electrodes and transmits the light transmitted through the color filter, and the reflective electrode that faces the second plurality of pixel electrodes and reflects external light. The detection object is directly presented to the detective elements, so that the detective elements may correctly detect the object. The light emitted from the light source and transmitted through the transparent electrode and the external light reflected from the reflective electrode are both transmitted through the liquid crystal layer, to thereby realize transmissive display as well as reflective display. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view partly showing an LCD  1  according to an embodiment of the present invention;  
       FIGS. 2A  to  2 J are sectional views showing, in particular, an optical sensor  1112  in a series of processes for forming the optical sensor  1112 , an n-channel TFT  100 , and a p-channel TFT  200  on an array substrate  11 ;  
       FIGS. 3A  to  3 J are sectional views showing, in particular, the n-channel TFT  100  in the series of forming processes;  
       FIGS. 4A  to  4 J are sectional views showing, in particular, the p-channel TFT  200  in the series of forming processes; and  
       FIG. 5  is a sectional view similar to  FIG. 1 , showing a detective operation of the LCD  1 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENT  
      A liquid crystal display (LCD) according to an embodiment of the present invention will be explained.  
       FIG. 1  is a sectional view partly showing the LCD  1  according to an embodiment of the present invention.  
      The LCD  1  has an array substrate  11  on which a plurality of scanning lines (not shown) and a plurality of signal lines (not shown) are formed. The scanning lines and the signal lines cross each other. The array substrate  11  faces a counter substrate  13 , and a liquid crystal layer  12  is held between the array substrate  11  and the counter substrate  13 .  
      In the LCD  1 , red (R), green (G), and blue (B) pixels are regularly arranged at crossing portions of the scanning lines and the signal lines, respectively. An image displayed with the pixels is viewed in front of the array substrate  11 . Behind the counter substrate  13 , there is a surface light source  14 .  
      The number of the signal lines is, for example, 240 for each of the R, G, and B colors. The number of the scanning lines is, for example, 320. Then, the number of pixels formed at the crossing portions of the signal and scanning lines will be about 230,000 to realize a QVGA (quarter video graphics array) display. All pixels may be arranged in a display area having a 2.2-inch diagonal size. With this size, a pixel pitch in a horizontal scanning direction is about 50 μm, a pixel pitch in a vertical scanning direction is about 150 μm, and a distance (cell gap) between the array substrate  11  and the counter substrate  13  is about 5 μm.  
      The LCD  1  has a transmissive display area A that allows light from the light source  14  to be transmitted through the liquid crystal layer  12  and a reflective display area B that reflects external light. The transmissive display area A may involve a majority of the pixels of the whole display area, and the reflective display area B involves the remaining pixels.  
      The array substrate  11  has a transparent glass substrate  111  of, for example, 0.7 mm thick. Each pixel region of the glass substrate  111  includes a switching element  1111  connected to a signal line (not shown) and a scanning line (not shown). On the liquid crystal layer  12  side of the glass substrate  111 , there is a transparent resin layer  112  through which each switching element  1111  is connected to a transparent pixel electrode  113  made of, for example, ITO (indium tin oxide). The signal lines are connected to a signal line driver (not shown), and the scanning lines are connected to a scanning line driver (not shown).  
      Each pixel region in the transparent display area A of the array substrate  11  has an optical sensor  1112  that is connected to a detective circuit (not shown).  
      The counter substrate  13  has a glass substrate  131 . The whole display area of the glass substrate  131  on the liquid crystal layer  12  side is covered with a color filter  132 .  
      The color filter  132  is provided with a lattice-like light shield film made of resin. The color filter  132  of a specific color is arranged in each pixel region encircled by the signal lines and the scanning lines.  
      The whole display area of the color filter  132  of the counter substrate  13  is covered with a transparent resin layer  133 . In the transparent display area A, the transparent resin layer  133  is flat, and in the reflective display area B, it is irregular.  
      The whole display area of the transparent resin layer  133  is covered with a transparent electrode  134  made of, for example, ITO. Corresponding to the shape of the transparent resin layer  133 , the electrode  134  has irregularities in the reflective display area B.  
      The reflective display area B on the transparent electrode  134  is covered with an opaque reflective electrode  135  made of, for example, aluminum, to reflect external light. Corresponding to the irregular shape of the electrode  134 , the reflective electrode  135  has irregularities.  
      The transparent display area A on the transparent electrode  134  serves as a transparent electrode  134   a  that is flat.  
      The transparent electrode  134   a  and reflective electrode  135  are covered with an orientation film that has been rubbed to provide the liquid crystal layer  12  with a pre-tilt of, for example, 6° in a predetermined direction.  
      On the light source  14  side, the counter substrate  13  has a polarizing plate  13 A. On the front side of the array substrate  11 , there is a polarizing plate  11 A.  
       FIGS. 2A  to  4 J show a series of processes for forming, on the array substrate  11 , the optical sensors  1112 , n-channel TFTs  100  serving as the switching elements  1111 , and p-channel TFTs  200  that form a drive circuit. These processes may involve a polysilicon process. Among  FIGS. 2A  to  4 J,  FIGS. 2A  to  2 J show a part of the array substrate  11  where the optical sensor  1112  is formed,  FIGS. 3A  to  3 J show a part of the array substrate  11  where the n-channel TFT  100  is formed, and  FIGS. 4A  to  4 J show a part of the array substrate  11  where the p-channel TFT  200  is formed.  
      Now, the series of processes for forming the optical sensors  1112 , n-channel TFTs  100 , and p-channel TFTs  200  on the array substrate  11  will be explained.  
      In  FIGS. 2A, 3A , and  4 A, an undercoat layer is formed on a glass substrate  111  from SiNx (silicon nitride) or SiOx (silicon oxide) by CVD (chemical vapor deposition). The undercoat layer prevents impurities such as phosphor and boron from diffusing to the glass substrate  111 . Over the undercoat layer, amorphous silicon is deposited to about 50 angstroms by PECVD (plasma enhanced chemical vapor deposition) or spattering, to form an amorphous silicon film.  
      In  FIGS. 2B, 3B , and  4 B, a laser beam is emitted to the amorphous silicon film, to crystallize the amorphous silicon film into a polysilicon film.  
      In  FIGS. 2C, 3C , and  4 C, low-concentration boron ions are implanted into the whole surface of the polysilicon film. A mask is formed on the polysilicon film, and the polysilicon film is exposed and etched to form a p −  layer.  
      In  FIGS. 2D, 3D , and  4 D, an SiOx film serving as a first insulating layer is formed by, for example, PECVD.  
      In  FIGS. 2E, 3E , and  4 E, a resist mask is formed, and high-concentration phosphor ions are implanted into an n-type electrode region  11121  of each optical sensor  1112  and a source region  101  and drain region  102  of each n-channel TFT  100 , to form an n +  layer.  
      In  FIGS. 2F, 3F , and  4 F, the resist mask is removed, and a first metal layer is formed over the first insulating layer from an Mo (molybdenum)-Ta (tantalum) alloy, an Mo—W (tungsten) alloy, or the like.  
      In  FIGS. 2G, 3G , and  4 G, the semifinished product is patterned to open a p-type electrode region  11122  of each optical sensor  1112  and a source region  201  and drain region  202  of each p-channel TFT  200 . Then, high-concentration boron ions are implanted into the semifinished product.  
      At this time, the first metal layer serves as a mask to form a p +  layer in the p-type electrode region  11122 , source region  201 , and drain region  202 . For the p-channel TFT  200 , the first metal layer patterned at this time forms a gate electrode  200 G.  
      In  FIGS. 2H, 3H , and  4 H, the first metal layer is patterned to open a light receiving part  1112 J of each optical sensor  1112  and an n −  region  103  and n −  region  104  of each n-channel TFT  100 . For the n-channel TFT  100 , the first metal layer patterned at this time forms a gate electrode  100 G. For the optical sensor  1112 , the first metal layer patterned at this time forms a gate electrode  1112 G.  
      A resist mask is formed over the optical sensors  1112 , and low-concentration phosphor ions are implanted.  
      With the first metal layer and resist serving as a mask, an n −  layer is formed in the n −  region  103  and n −  region  104  of each n-channel TFT  100 .  
      The light receiving part  1112 J of the optical sensor  1112  is made of a p −  layer, and therefore, the optical sensor  1112  is a PIN-type optical sensor.  
      The resist mask is removed. To activate the implanted ions, the semifinished product is annealed at about 500° C. and is exposed to hydrogen plasma for hydrogenation.  
      In  FIGS. 2I, 3I , and  4 I, a second insulating layer is formed over the first insulating layer from SiOx by, for example, CVD.  
      In  FIGS. 2J, 3J ,  4 J, contact holes are formed to expose the n-type electrode region  11121  and p-type electrode region  11122  of each optical sensor  1112 , the source region  101  and drain region  102  of each n-channel TFT  100 , and the source region  201  and drain region  202  of each p-channel TFT  200 . The exposed parts are covered with a second metal layer, which is patterned to form the p-type electrode  1112 P, n-type electrode  1112 N, and light shield band  1112 S of each optical sensor  1112 , the source electrode  100 S and drain electrode  100 D of each n-channel TFT  100 , and the source electrode  200 S and drain electrode  200 D of each p-channel TFT  200 .  
      Returning to  FIG. 1 , a display operation of the LCD  1  will be explained.  
      In the LCD  1 , the scanning lines are sequentially driven and switching elements  1111  corresponding to, for example, red (R) pixels to be written with a selected scanning line are driven to be conductive. Then, a video signal supplied to the signal lines is applied to the pixel electrodes  113  of the selected red pixels. At the same time, a predetermined signal is supplied to each of the reflective electrode  135  and transparent electrode  134   a.  This results in applying an electric field to the liquid crystal layer  12  between the pixel electrodes  113  and the reflective electrode  135  and transparent electrode  134   a.  The strength of the electric field is dependent on the amplitude of the video signal and influences the light transmittance of the liquid crystal layer  12 .  
      Part of the light emitted from the light source  14  to the transmissive display area A is sequentially transmitted through the polarizing plate  13 A, glass substrate  131 , color filter  132 , transparent resin layer  133 , transparent electrode  134   a,  orientation film (not shown), liquid crystal layer  12 , orientation film (not shown), pixel electrodes  113 , transparent resin layer  112 , glass substrate  111 , and polarizing plate  11 A and is emitted to the outside.  
      External light made incident to the reflective display area B is sequentially transmitted through the polarizing plate  11 A, glass substrate  111 , transparent resin layer  112 , pixel electrodes  113 , orientation film (not shown), liquid crystal layer  12 , and orientation film (not shown) and reaches the reflective electrode  135 . The light is reflected from the reflective electrode  135 , is sequentially transmitted through the orientation film (not shown), liquid crystal layer  12 , orientation film (not shown), pixel electrodes  113 , transparent resin layer  112 , glass substrate  111 , and polarizing plate  11 A, and is emitted outside. The reflective electrode  135  has irregularities, and therefore, the reflected light from the reflective electrode  135  scatters.  
      By controlling the light transmittance of the liquid crystal layer  12 , the LCD  1  can control the intensity of light emitted from the liquid crystal layer  12 , i.e., the brightness of the pixels to display characters and images.  
      Scattering the reflected light from the reflective electrode  135  with the irregularities of the reflective electrode  135  can widen a view angle.  
      The reflective electrode  135  is arranged on the liquid crystal layer  12  side of the color filter  132 , so that external light is not transmitted through the color filter  132 . As a result, the reflective display area B can display a monochromatic image.  
      With reference to  FIG. 5 , which is a sectional view similar to  FIG. 1 , a detective operation of the LCD  1  will be explained.  
      An image signal that has been controlled to equalize the light transmittance of the liquid crystal layer  12  in the transmissive display area A is supplied to the signal lines. Thereafter, an object P such as a printed object to be detected is positioned in front of the transmissive display area A of the array substrate  11 .  
      Light emitted from the light source  14  is transmitted through the polarizing plate  13 A, glass substrate  131 , and color filter  132 . Part of the light transmitted through the color filter  132  and reflective display area B is transmitted through the transparent resin layer  133  and is reflected from the reflective electrode  135 .  
      The remaining part of the light transmitted through the color filter  132  is sequentially transmitted through the transparent resin layer  133 , transparent electrode  134   a,  orientation film (not shown), pixel electrodes  113 , transparent resin layer  112 , glass substrate  111 , and polarizing plate  11 A and is transmitted outside. Then, this light is reflected from the detection object P and enters the array substrate  11 . The intensity of the entered light is converted by each optical sensor  1112  into an electronic signal, which is detected by the detective circuit and is used to form an image of the object P.  
      As mentioned above, the array substrate  11  of the LCD  1  has the optical sensors  1112  serving as detective elements to detect light from a detection object that is presented in front of the array substrate  11 . The counter substrate  13  has the color filter  132  to transmit light emitted from the light source  14 , the transparent electrode  134   a  that faces some pixel electrodes  113  and passes the light transmitted through the color filter  132 , and the reflective electrode  135  that faces the remaining pixel electrodes  113  and reflects external light. With this arrangement, light from the detection object is directly given to the optical sensors  1112  serving as detective elements and is properly used to provide a correct image of the object. The light emitted from the light source  14  and transmitted through the transparent electrode  134   a  and the external light reflected from the reflective electrode  135  are both transmitted through the liquid crystal layer  12 , to realize transmissive display and reflective display.  
      The reflective electrode  135  has irregularities to scatter light reflected from the reflective electrode  135 , thereby widening the view angle.  
      The reflective electrode  135  is arranged on the liquid crystal layer  12  side of the color filter  132  to prevent external light from entering the color filter  132 . Namely, the reflective electrode  135  realizes monochromatic reflective display.  
      In the LCD  1  according to this embodiment, the optical sensors  1112  are provided for the pixels in the transmissive display area A, respectively. Instead, the optical sensors  1112  may be provided for the pixels in the reflective display area B, respectively. Alternatively, the optical sensors  1112  may be provided for the pixels in the transmissive display area A and reflective display area B, respectively. It is also possible to regularly arrange the optical sensors  1112  in a predetermined detective area in the array substrate  11 . The optical sensors  1112  are not limited to the PIN-type sensors. Any other type, for example, PN-type optical sensors are acceptable for the present invention.  
      In the LCD  1  according to the present invention, the optical sensors  1112  may detect the intensity of light emitted from a light pen to allow pen input. Instead of the optical sensors  1112 , piezoelectric elements may be employed as pressure detecting elements to realize a touch panel function. The piezoelectric elements provided for the array substrate  11  can correctly detect objective pressure.  
      When detecting light from a light pen or pressure from an object, the optical sensors  1112  or piezoelectric elements may be arranged outside the display area of the array substrate  11 .