Abstract:
A method for manufacturing organic light emitting diode with improved electrical leakage employs a Chemical-Mechanical Polishing (CMP) process to flatten the surface of an Indium Tin Oxide (ITO) layer thereby to reduce leakage current occurred to an organic light emitting diode (OLED) element and increase efficiency and service life thereof.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for manufacturing organic light emitting diode (OLED) with improved electrical leakage and particularly a method that employs Chemical-Mechanical Polishing (CMP) technology to flatten the surface of an ITO (Indium Tin Oxide) layer to reduce electrical leakage occurred to the OLED.  
         BACKGROUND OF THE INVENTION  
         [0002]    Organic light emitting diodes (OLEDs) that are known today can be classified, based on the driving type, in simple matrix driving type and active matrix driving type. The commercialized OLEDs nowadays mostly are simple matrix driving type. This is mainly due to its simpler structure. It also does not use TFT (Thin-Film Transistor) or color filter, thus the cost is lower than LCD. However it has disadvantages such as not suitable for applications that require high resolution, consumes too much electric power in large dimension application, lower service life, accelerating degradation of the display elements, etc.  
           [0003]    Among the factors that cause the phenomena of too much power consumption, shorter service life and degradation of the display elements that occur to the OLED, one major factor is that the surface of the transparent ITO anode used in the OLED is rough and uneven. There are general two approaches to overcome this problem:  
           [0004]    1. Making a flat and more uniform ITO film by using methods other than the conventional RF sputter techniques, such as the HDAP technique developed by SHI Co.  
           [0005]    2. Performing mechanical polishing on the rough film surface formed on the ITO glass to improve the flatness and evenness.  
           [0006]    The first approach mentioned above requires expensive equipment for forming films. The second approach often generates scratches due to polishing problems and results in damages of the ITO surface and dropping of efficiency and production yield.  
         SUMMARY OF THE INVENTION  
         [0007]    Therefore the primary object of the invention is to resolve the aforesaid disadvantages. The invention employs Chemical-Mechanical Polishing (CMP) to flatten the surface of ITO layer to prevent the ITO layer in the OLED from generating point discharge thereby to reduce electrical leakage of the OLED.  
           [0008]    In order to achieve the foregoing object, the invention employs CMP technique to flatten the ITO layer. The invention includes a polishing table for polishing the surface of the ITO layer and a holder for gripping the ITO layer. The holder grips the back side of the ITO layer and the front side of the ITO layer is pressed on a polishing pad located on the polishing table so that CMP process may be executed. During the CMP process, the polishing table and the holder rotate in one direction, and a reagent is dispensed continuously through a delivery pipe to the polishing table. The CMP technique uses the chemical reaction generated by the reagent and the mechanical polishing occurred to the ITO layer on the polishing table to flatten and smooth the rough and uneven surface of the ITO layer.  
           [0009]    The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic view of the structure of a conventional OLED.  
         [0011]    [0011]FIG. 2 is a schematic view of the structure of an OLED of the invention.  
         [0012]    [0012]FIGS. 3A and 3B are a simplified top view and a side view of the equipment of CMP process of the invention.  
         [0013]    [0013]FIG. 4 is a photo taken by an AFM showing the surface of an ITO layer before being polished by means of the invention.  
         [0014]    [0014]FIG. 5 is a photo taken by an AFM showing the surface of an ITO layer after being polished by means of the invention.  
         [0015]    [0015]FIG. 6 is a chart showing graphs of electrical leakage before and after the surface of the ITO layer being polished. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring to FIG. 1, a conventional OLED  1  includes at least one ITO layer  11 , a HIL (Hole Injection layer)  12  located above the ITO layer  11 , an organic layer  13  located above the HIL layer  12 , an EIL (Electron Injection Layer) layer  14  above the organic layer  13 , and a metal layer  15  located above the EIL layer  14 . The OLED element serve as a carrier to be injected into the organic layer for coupling therewith to transfer energy and generate light. The injection effectiveness of the carrier greatly affects the bonding interface between the surface of the ITO layer  11  and the organic layer  13 . The thickness of the organic layer  13  is about 1000 A-1500 A. The HIL layer  12  adjacent to the ITO layer  11  has a thickness about 150 A. As the thickness of the HIL layer  12  is very small and the surface of the ITO layer is rough and uneven, the jutting peak  2  on the surface of the ITO layer  11  tends to generate point discharge and leakage current. As a result, greater power consumption and cross-talk are prone to occur to the OLED element  1 .  
         [0017]    The invention aims at preventing the foregoing phenomena from occurring to the ITO layer  11  that might result in damage of the OLED element  1  or cross-talk. The invention also enables the EIL layer  14  to function properly. The surface roughness of the ITO layer  11  in the OLED must be less than 10 A. The flattened surface of the ITO layer  11  can reduce the probability of forming crystallization centers. Therefore the amorphous organic layer  13  above the ITO layer  11  is more stable and has a longer service life. However, the surface roughness of ITO layer  11  now being used on STN-LCD and TFT-LCD is about 30 A-50 A. This condition is not applicable to OLED under the strict demand imposed to the OLED element  1 .  
         [0018]    The OLED element  1  has a strict demand for surface roughness on the ITO layer  11  being used. The average value must be lower than 1.5 nm, and a high degree of uniformity is also required for the roughness. In order to achieve flattened and smooth surface on the ITO layer  11 , the invention employs CMP technique. The CMP technique is widely used in the semiconductor IC manufacturing processes. However it is rarely used in flat display panel production. Applicant discovers that after the surface of the ITO layer  11  of OLED element  1  is flattened and polished by employing the CMP technique, leakage current occurred to the element decreases and efficiency is improved, and service life also increases.  
         [0019]    The CMP process mentioned above is one of the flatten processes known in the art that is quite easy to understand. It adopts a principle substantially like a common “grinding knife” practice. In the CMP process, a suitable reagent is used to aid grinding operation to polish the uneven surface contour of the ITO layer  11 . Once the parameters of the process are properly controlled, the surface formed by CMP can reach the flatness of 94% or higher.  
         [0020]    Refer to FIGS. 3A and 3B for a simplified apparatus for CMP flattening process. It basically includes a polishing table  3  for polishing the ITO layer  11  and a holder  4  for gripping the ITO layer  11 . The holder  4  grips the back side of the ITO layer  11  and the front side of the ITO layer  11  is pressed on a polishing pad  5  located on the polishing table  3  so that the CMP process may be executed. During the CMP process, the polishing table  3  and the holder  4  rotate in one direction, and a slurry  6  is dispensed continuously through a delivery pipe  7  to the polishing table  3 . The CMP technique uses the chemical reaction generated by the reagent and the mechanical polishing occurred to the ITO layer  11  on the polishing table  3  to flatten and smooth the rough and uneven surface of the ITO layer  11  (as shown in FIG. 2).  
         [0021]    Refer to FIGS. 4 and 5 for comparisons of the ITO layer surface before and after being polished by means of the invention. As shown in the photos, before the ITO layer is polished, the surface roughness Ra is about 2 nm, and Rz is 11.6 nm. After having been polished by the CMP process, the Ra is 0.87 nm and Rz is 4.67 nm. The surface roughness of the ITO layer  11  is greatly improved after polishing.  
         [0022]    Refer to FIG. 6 for comparisons of leakage current before and after the polishing of the ITO layer surface. The OLED element  1  made by including the ITO layer  11  without prior polishing was detected with leakage current of 5.2 mA/cm2. The OLED element  1  made by including the ITO layer  11  that has been processed by the CMP technique was detected with leakage current as low as 0-0.5 mA/cm2.  
         [0023]    The results of adopting the invention may further be seen from the following table:  
                                                                                     15   30   45   60       CMP processes   No CMP   seconds   seconds   seconds   seconds                                Leakage current   5.2   0.15   0.12   0.1   0.12       (mA/cm2)                  
 
         [0024]    Based on discussions set forth above, it is obvious that after the ITO layer  1  was polished by CMP technique, leakage current occurred to the OLED element  1  is greatly reduced. And efficiency and service life of the OLED element  1  increased. Thus the invention can provide a great benefit to the industry.