Abstract:
An auxiliary capacitor for a pixel of an active matrix type liquid crystal display is provided without decreasing the aperture ratio. A transparent conductive film for a common electrode is formed under a pixel electrode constituted by a transparent conductive film with an insulation film provided therebetween. Further, the transparent conductive film for the common electrode is maintained at fixed potential, formed so as to cover a gate bus line and a source bus line, and configured such that signals on each bus line are not applied to the pixel electrode. The pixel electrode is disposed so that all edges thereof overlap the gate bus line and source bus line. As a result, each of the bus lines serves as a black matrix. Further, the pixel electrode overlaps the transparent conductive film for the common electrode to form a storage capacitor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention disclosed in this specification relates to a configuration of a pixel region of an active matrix type liquid crystal display and, more particularly, to a configuration of an auxiliary capacitor connected to a pixel electrode in parallel and a configuration of a black matrix for preventing leakage of light at boundaries between adjacent pixels. 
   2. Description of Related Art 
   Liquid crystal displays having an active matrix circuit are known. They have a configuration including a plurality of source bus lines for transmitting image data, a plurality of gate bus lines for transmitting switching signals in an intersecting relationship therewith, and a plurality of pixels provided at those intersections. Normally, transistors (specifically thin film transistors) are used as switching elements. 
   A pixel includes not only a transistor for switching but also a pixel electrode and has a structure in which the gate electrode, source, and drain of the transistor are connected to a gate bus line, a source bus line, and the pixel electrode, respectively. Distinction between the source and drain of a transistor is not fixed during the operation thereof and varies depending on signals according to a common definition of electrical circuits. In the following description, however, the terms “source” and “drain” simply refer to impurity regions provided in a transistor which are connected to a source bus line and a pixel electrode, respectively. 
   Each pixel includes one or more transistors. Specifically, two or more transistors connected in series are advantageous in that leak current can be reduced even when the transistors are not selected. The above-described definition is also applied to such a case, and no definition is given to an impurity region connected to neither a source bus line nor a pixel electrode. 
   A capacitor is formed between a pixel electrode and an electrode which is opposite to the pixel electrode across liquid crystal. A transistor as described above serves as a switching element for supplying and removing electrical charge to and from this capacitor. 
   In an actual operation, however, the capacity of the pixel electrode portion is too small to store necessary electrical charge for a sufficient period of time by itself. It is therefore necessary to provide a separate auxiliary capacitor. 
   Such an auxiliary capacitor (also referred to as “storage capacitor”) has been formed between an opaque conductive material such as a metal which is separately provided and a pixel electrode or a semiconductor layer. The gate bus line for the next row has been normally used as the opposite electrode. Although a capacitor formed using a gate bus line has been sufficient when the area of the pixel is large enough, a gate bus line has been unsuccessful in providing a sufficient capacity by itself when the area of the pixel is small. This has required a gate bus line to be expanded to accommodate the area for an electrode of an auxiliary capacitor. Such a structure results in a decrease in the aperture ratio because a pixel includes an area where light is blocked. 
   It is an object of the invention disclosed in this specification to provide a configuration wherein an auxiliary capacity is increased without reducing the aperture ratio of a pixel. It is another object of the invention to provide a configuration of a black matrix for solving the problem of leakage of light that occurs at boundaries between adjoining pixels. 
   SUMMARY OF THE INVENTION 
   The invention disclosed in this specification is characterized in that: 
   a pixel includes a pixel electrode comprising a first transparent conductive film; 
   the pixel electrode overlaps a gate bus line and a source bus line; 
   a layer of a common electrode comprising a second transparent conductive film is provided between the gate bus line and the pixel electrode and between the source bus line and the pixel electrode such that it covers the gate bus line and source bus line; and 
   the common electrode is maintained at constant potential. 
   Specifically, the common electrode is disposed such that it covers the bus lines, and the pixel electrode is disposed such that it overlaps the bus lines. Thus, the common electrode and the pixel electrode overlap each other, and this overlap provides an auxiliary capacitor. In addition, while at least one of the electrodes of a conventional auxiliary capacitor has been formed of an opaque material, both of such electrodes are formed of a transparent material according to the present invention. Thus, they will not hinder display and the aperture ratio is maintained. 
   With the above-described configuration, a black matrix can be formed by source bus lines and gate bus lines by arranging pixel electrodes in an overlapping relationship with the source bus lines and gate bus lines and by positioning boundaries between the pixel electrodes on the source bus lines and gate bus lines. In general, boundaries between pixel electrodes of a liquid crystal display are affected by the electric field of the adjacent electrodes. As a result, at such boundaries, an image can be produced differently from that to be formed by the relevant pixels or leakage of light can occur due to absence of an electric field. 
   It is therefore not appropriate to use such boundaries between pixel electrodes, and a structure is normally employed in which such areas are shaded by a black matrix. It has been conventionally necessary to configure a black matrix by a separate layer. For example, the use of bus lines as a black matrix has been proposed in Japanese unexamined patent publication (KOKAI) No. H6-216421 and etc. In practice, however, this results in unstable display because signals on bus lines affect pixel electrodes. 
   The present invention solves this problem. Specifically, since a common electrode is provided between a bus line and a pixel electrode, a signal on the bus line is blocked by the common electrode and hence does not affect the pixel electrode. 
   When a top-gate type transistor (a transistor having a structure in which the gate electrode is provided on top of a semiconductor layer) is used as a switching transistor, it will be advantageous for stable operation of the transistor if light is allowed to be incident upon the substrate primarily from above, i.e., from the side of the pixel electrodes because this prevents light from entering the semiconductor layer under the gate electrode. 
   The effect of shading can be further improved for more stable operation by providing a film made of the same material as the layer of the source bus lines on top of the gate bus lines where the semiconductor layer and the gate bus lines intersect with each other. 
   As an insulator to be used between the common electrode and the pixel electrode, organic resin may be used as well as inorganic materials (e.g., silicon oxide and silicon nitride). 
   Especially, a flat insulation layer formed by means of spin coating or the like will be effective in reducing surface irregularity for improved uniformity of an electric field applied to liquid crystal molecules. 
   Materials usable for the transparent conductive film according to the invention disclosed in this specification include ITO (indium tin oxide), SnO 2 , and materials mainly composed of those materials as well. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A through 1E  are sectional views showing fabrication steps according to a first embodiment of the present invention. 
       FIGS. 2A through 2D  illustrate the configuration of wiring and the like according to the first embodiment of the invention. 
       FIGS. 3A through 3E  are sectional views showing fabrication steps according to a second embodiment of the present invention. 
       FIGS. 4A through 4D  illustrate the configuration of wiring and the like according to the second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A first embodiment of the present invention will now be described.  FIGS. 1A through 1E  and  FIGS. 2A through 2D  show a configuration of a pixel of an active matrix type liquid crystal display which employs the invention disclosed in this specification.  FIGS. 1A through 1E  are schematic sectional views showing fabrication steps according to the present embodiment, and  FIGS. 2A through 2D  show the configuration of each of a bus line, a common electrode, pixel electrodes, a semiconductor layer, and the like according to the present embodiment. The reference numbers in  FIGS. 2A through 2D  are in correspondence with those in  FIGS. 1A through 1E .  FIGS. 1A through 1E  are conceptual views and are not exactly identical to  FIGS. 2A through 2D  in configuration. 
   Further,  FIGS. 1A through 1E  and  FIGS. 2A  though  2 D show a configuration of only a substrate on which a thin film transistor is provided. In practice, there is provided another substrate opposite thereto (opposite substrate), and liquid crystal is held between the opposite substrate and the substrate shown in  FIGS. 1A through 1E  with a gap of several μm therebetween. 
   The fabrication steps will now be described with reference to  FIGS. 1A through 1E . As shown in  FIG. 1A , a semiconductor layer (active layer)  12  of a transistor is provided on a glass substrate  11  having an underlying silicon oxide film (not shown). 
   The active layer  12  is formed by a crystalline silicon film which has been crystallized by heating an amorphous silicon film or by irradiating the same with laser beams. A gate insulation film  13  is formed so as to cover the active layer  12  The gate insulation film  13  is preferably made of silicon oxide or silicon nitride and, for example, a silicon oxide film formed using a plasma CVD process may be used. A gate bus line (gate electrode)  14  made of an aluminum-titanium alloy is formed on the gate insulation film using a well known sputtering process ( FIG. 1A ). 
   The configuration of this circuit in this state is shown in  FIG. 2A . 
   Next, a well known ion doping process is performed using the gate bus line as a mask to introduce N- or P-type impurities in the active layer, thereby forming a source  15  and a drain  16 . 
   After the impurities are introduced, thermal annealing, laser annealing or the like may be performed to activate the impurities (to recrystallize the semiconductor film) if required. 
   After the above-described steps, a silicon nitride film (or a silicon oxide film)  17  is deposited by means of a plasma CVD process. It serves as a first layer insulator ( FIG. 1B ). 
   Next, contact holes are formed in the first layer insulator  17  such that they reach the source  15  and drain  16 . Then, a well known sputtering process is performed to form a multi-layer film of titanium and aluminum which is in turn etched to form a source bus line  18  and a drain electrode  19 . 
   After the above-described steps, a silicon nitride film (or a silicon oxide film)  20  is deposited by means of a plasma CVD process. It serves as a second layer insulator ( FIG. 1C ). 
   The configuration of the circuit in this state is shown in  FIG. 2B . 
   Next, a spin coating process is performed to form a first organic resin layer  21 . The organic resin layer is formed to have a flat upper surface. Then, a well known sputtering process is performed to form an ITO film which is in turn etched to form a common electrode  22  ( FIG. 1D ). 
   The configuration of the circuit in this state is shown in  FIG. 2C . The common electrode is shaded in  FIG. 2C  to show its position clearly. As apparent from  FIG. 2C , the common electrode is formed so as to cover the source bus line and gate bus line. 
   Further, a spin coating process is performed to form a second organic resin layer  23 . Then, a well known sputtering process is performed to form an ITO film which is in turn etched to form pixel electrodes  24   a  and  24   b . The pixel electrode  24   b  is a pixel electrode for the transistor as described above, and the pixel electrode  24   a  is a pixel electrode adjacent thereto. Capacitors  25   a  and  25   b  are respectively formed at regions where the pixel electrodes  24   a  and  24   b  overlap the common electrode  22  ( FIG. 1E ). 
   The configuration of the circuit in this state is shown in  FIG. 2D . In  FIG. 2D , the pixel electrodes and the regions where the pixel electrodes overlap the common electrode (regions where the capacitors are located) are shaded to show their positions clearly. As apparent from  FIG. 2D , the pixel electrodes are formed so as to overlap the source bus line and gate bus line. As a result, the boundaries of the pixel electrodes are all located on the bus lines which consequently serve as a black matrix ( FIG. 2D ). 
   A second embodiment of the present invention will now be described.  FIGS. 3A through 3E  and  FIGS. 4A through 4D  show a configuration of a pixel of an active matrix type liquid crystal display which employs the invention disclosed in this specification.  FIGS. 3A through 3E  are schematic sectional views showing fabrication steps according to the present embodiment, and  FIGS. 4A through 4D  show the configuration of each of a bus line, a common electrode, pixel electrodes, a semiconductor layer, and the like according to the present embodiment. The reference numbers in  FIGS. 4A through 4D  are in correspondence with those in  FIGS. 3A through 3E .  FIGS. 3A through 3E  are conceptual views and are not exactly identical to  FIGS. 4A through 4D  in configuration. 
   As shown in  FIG. 3A , a semiconductor layer (active layer)  32  of a transistor is provided on a glass substrate  31  having an underlying silicon oxide film (not shown). A gate insulation film  33  is formed so as to cover the active layer  32 . A gate bus line (gate electrode)  34  made of an aluminum-titanium alloy is formed on the gate insulation film ( FIG. 3A ). 
   The configuration of this circuit in this state is shown in  FIG. 4A . Unlike the first embodiment, the gate bus line of the present embodiment is configured to be reduced in width at the region of the gate electrode of the transistor ( FIG. 4A ). 
   Next, N- or P-type impurities are introduced to form a source  35  and a drain  36 . After the above-described steps, a first layer insulator  37  which is a silicon nitride film (or a silicon oxide film) is deposited ( FIG. 3B ). 
   Next, contact holes are formed in the first layer insulator  37  such that they reach the source  35  and drain  36 . Then, a source bus line  38 , a drain electrode  39 , and a protective film  40  are formed. After the above-described steps, a second layer insulator  41  which is a silicon nitride film (or a silicon oxide film) is deposited ( FIG. 3C ). 
   The configuration of the circuit in this state is shown in  FIG. 4B . The protective film  40  is insulated from the source bus line  38 , the drain electrode  39 , and other wiring and electrodes to be at floating potential. Such a protective film  40  is effective in blocking light incident upon the transistor from above ( FIG. 4B ). 
   Next, a common electrode  42  is formed by an ITO film. Further, an organic resin layer  43  is formed ( FIG. 3D ). 
   The configuration of the circuit in this state is shown in  FIG. 4C . The common electrode is shaded in  FIG. 4C  to show its position clearly. As apparent from  FIG. 4C , the common electrode is formed so as to cover the source bus line and gate bus line. Strictly speaking, it is not essential to cover the protective film  40  with the common electrode. This is because there is a bare possibility that the protective film has some influence on the pixel electrodes as it is at floating potential. In the present embodiment, however, the protective film  40  is also covered by the common electrode  42  as illustrated ( FIG. 4C ). 
   Then, pixel electrodes  44   a  and  44   b  are formed by ITO films. The pixel electrode  44   b  is a pixel electrode for the transistor as described above, and the pixel electrode  44   a  is a pixel electrode adjacent thereto. Capacitors  45   a  and  45   b  are respectively formed at regions where the pixel electrodes  44   a  and  44   b  overlap the common electrode  42  ( FIG. 3E ). 
   The configuration of the circuit in this state is shown in  FIG. 4D . In  FIG. 4D , the pixel electrodes and the regions where the pixel electrodes overlap the common electrode (regions where the capacitors are located) are shaded to show their positions clearly. As apparent from  FIG. 4D , the pixel electrodes are formed so as to overlap the source bus line and gate bus line. As a result, the boundaries of the pixel electrodes are all located on the bus lines which consequently serve as a black matrix ( FIG. 4D ). 
   By forming an electrode opposite to a pixel electrode that constitutes an auxiliary capacitor using a transparent conductive film, a great auxiliary capacitor can be formed without decreasing the aperture ratio. In addition, a source bus line and a gate bus line can be used as a black matrix. More particularly, the present invention is effective especially in improving an aperture ratio when the pixel is small and, especially, with design rules kept unchanged. As described above, the present invention has advantages from an industrial point of view. 
   It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.