Patent Publication Number: US-2006012742-A1

Title: Driving device for active matrix organic light emitting diode display and manufacturing method thereof

Description:
BACKGROUND OF THE PRESENT INVENTION  
      1. Field of Invention  
      The invention relates to an active matrix organic light emitting diode display and the manufacturing method of the same. In particular, it relates to a driving device for an organic light emitting diode display and manufacturing method of the same.  
      2. Related Art  
      As the technology of thin-film transistor liquid crystal display (TFT LCD) improves, flat screen displays become the mainstream products on the display market. The development of liquid crystal display industry increases the quality and yield of the displays, and also accelerates expectations and demands for the next generation displays. The organic light emitting diode (OLED) display has the features of light, thin, low driving voltage, self-emissive and wide viewing angles. The manufacturing process for the OLED display is also simpler than the LCD displays and the OLED easily applies to flexible displays. It is the next generation display of possibilities.  
      The driving method of the OLED display includes two types: passive and active matrix type. The passive matrix displays are used mostly in car audio displays, cellular phones, gaming consoles and PDA&#39;s. The current commercial products of the OLED display are passive matrix. The advantage of the passive matrix OLED display is no need for color filters and backlight modules due to its simple structure. The disadvantage of the passive matrix OLED is the size limitation. To develop the large size passive matrix displays has some problems such as higher energy consumption, shorter lifetime and deterioration of the OLED device. Active matrix displays provide wider viewing angles, high luminance and quick response time. They conform to the requirements of the large size and high-resolution display. Refer to  FIG. 1 , illustrating a schematic view of the thin-film transistor according to the related art. A common structure of driving device for active matrix displays includes a driving element (e.g. thin-film transistor) above a substrate  100 . An insulating layer  112  above the thin-film transistor covers a source and a drain of the thin-film transistor defined in the poly-Si layer  111  and the gate  14  is located on the insulating layer  112 . A dielectric layer  116  lays on the insulating layer  112  and electrodes  115 ,  117  pass through the layers for connecting to the drain and the source. A planarization layer  130  covers the dielectric layer  116  and the electrodes  1   15 ,  117 . A transparent conductive layer  120  installed on the planarization layer  130  connects to the electrode  1   15  through the planarization layer  130 . An organic light emitting diode (not shown) is formed on the top of the transparent conductive layer  120 ). Active matrix displays formed by the structure described above have low emissive efficiency and larger leakage current than passive matrix displays.  
      Due to the requirements for the large size and high-resolution displays, the driving device of OLED has to progress from ‘passive matrix’ to ‘active matrix’. Therefore, changing the driving device structure of the active matrix OLED display for improving the emissive efficiency and reducing leakage current has become an important subject for the next generation displays.  
     SUMMARY OF THE PRESENT INVENTION  
      In view of the foregoing, the present invention provides a new driving device for an active matrix OLED display and its manufacturing method. The new structure of the driving device is implemented by forming the pixel electrode directly on a substrate surface by means of a simple production procedure.  
      The driving device comprises a substrate, a dielectric layer, a thin-film transistor, and a transparent conductive layer. The dielectric layer is formed above the substrate and covers the source and the drain of the thin-film transistor. Source and drain electrode pass through the dielectric layer for separately connecting with the source and drain area. The transparent conductive layer gets direct contact with the substrate and is connected to the drain through the drain electrode, so the transparent conductive layer can be functioned as a pixel electrode. The driving device provides the less leakage current and higher emissive efficiency than the prior active matrix displays.  
      The present invention also provides a method of manufacturing the driving device for the active matrix OLED display which can simplify process. The method of manufacturing the driving device includes the steps of: providing a substrate; forming a thin-film transistor above the substrate; providing a dielectric layer to cover the source and the drain of the thin-film transistor; forming a contact area that exposes the substrate and holes connecting the source and the drain by executing a photolithography process; filling holes with a conductive layer and form the source electrode and the drain electrode; and forming a transparent conductive layer that directly contact with the substrate through the contact area and is connected to the drain through the drain electrode. Because of the characteristics of the structure, the present invention provides easier manufacturing steps than the prior art of active matrix displays. The present invention uses photolithography process to form separately the contact area, the source and the drain connection holes in the driving device by several steps or one step. Thus, it decreases the amount of masks and the steps of the photolithography process. The present invention can be complete by the current production equipments and there is no need to acquire new equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will become more fully understood from the detailed description given hereinafter that for illustration only, and thus are not limited thereby, and wherein:  
       FIG. 1 , illustrates a schematic view of the thin-film transistor according to related art;  
       FIG. 2  is a schematic view of a first embodiment of the present invention;  
       FIG. 3A  to  3 H are schematic views illustrating a manufacturing process according to the first embodiment of the present invention; and  
       FIG. 4  is a schematic view of the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
      The present invention provides a driving device and its manufacturing method for an active matrix OLED display, wherein a the pixel electrode is directly formed on the surface of a substrate so as to reduce leakage current and to increase emissive efficiency.  
      Referring to  FIG. 2 , a schematic view of a first embodiment according to the present invention is illustrated. An insulated substrate is made up of a substrate  10  and a buffering layer  11 . A poly-Si layer  13  is deposited on the surface of the buffering layer  11 . A drain, a source, and a channel of a thin-film transistor are defined in the poly-Si layer  13 . An insulating layer  12  covers the buffering layer  11  and the poly-Si layer  13 . A gate  14  is isolated by the insulating layer  12  and located on the top of the channel in the thin-film transistor. A dielectric layer  16  covers the surface of the gate  14 . The dielectric layer  16  and the insulating layer  12  are each provided with connection holes going through to the source and the drain. The connection holes are filled with conductive materials and form the source electrode  17  and the drain electrode  15 . A transparent conductive layer  20  contacts the substrate  10  directly, and is connected to the drain through the drain electrode  15 . A planarization layer  30  covers the dielectric layer  16  and the source and the drain electrode  15 .  
      The OLED element can be formed on the top surface of the transparent conductive layer  20 . An indium tin oxide layer can be used as the transparent conductive layer  20 . In addition, a pixel electrode made up of the transparent conductive layer contacts with the substrate directly, the planarization layer lays over the edges of the transparent conductive layer to reduce the roughness of the transparent conductive layer. The leakage current between the pixel electrode and the other electrodes is thus reduced.  
       FIG. 3A  to  4 H are schematic views illustrating a process according to the first embodiment of the present invention. First of all, the substrate  10  that is covered by the buffering layer  11  on its surface ( FIG. 3A ) is provided and from the poly-Si layer  13  is formed on the top of the buffering layer  11  ( FIG. 3B ), which has the source, the drain, and the channel the insulating layer  12  is formed to cover the source and the drain of the thin-film transistor ( FIG. 3C ) and then the gate  14  ( FIG. 3D ) is formed, which is isolated by the insulating layer  12  and located on the top of the channel area of the thin-film transistor. Then, the dielectric layer  16  is formed to cover the gate  14  ( FIG. 3E ), a contact area that exposes the substrate  10  and the connection holes for the source and the drain is formed by executing a photolithography process ( FIG. 3F ). The connection holes is filled with metal ( FIG. 3G ) to form the source electrode  17  and the drain electrode  15 . The transparent conductive layer  20  is formed directly contact with the substrate  10  through the contact area ( FIG. 3H ) and is connected to the drain through the drain electrode  15 . Finally, cover a planarization layer  30  is formed to lay over the dielectric layer  16  and the source and the drain electrode  15  ( FIG. 2 ).  
      The transparent conductive layer can be provided either above or below the extended part of the drain electrode, referring to  FIG. 4 , a schematic view of a second embodiment according to the present invention is illustrated. The transparent conductive layer is provided below an extended part of the drain electrode.  
      The present invention is thus described. However, it will be obvious that this invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications would be obvious to one skilled in the art and are intended to be included within the scope of the following claims.