Patent Publication Number: US-6664564-B2

Title: Method for fabricating an organic light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of, and claims priority benefit of, U.S. application Ser. No. 09/805,418 filed on Mar. 13, 2001 U.S. Pat. No. 6,582,984. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method for fabricating a light emitting diode. More particularly, the present invention relates to a method for fabricating an organic light emitting diode. 
     2. Description of Related Art 
     The organic light emitting diode (OLED) utilizes an emissive characteristic organic film between two electrodes. When the direct currency voltage is charged to the electrodes, a hole will be injected from the anode, and an electron will be injected from the cathode. Because of the potential difference created by the applied voltage, the carrier moves and combines in the thin film, and a part of the electron-hole combines with the electron to release energy that will emit and stimulate the particle, thus forming a single stimulative particle. When the single stimulative particle releases the energy back to the substrate, there is a standard ratio of the energy that is released and radiated by emitting a photon. The above description is that of an organic light emitting diode. 
     An energy band model is usually used to describe an electric charge movable model. However, because organic material is unlike metal or a semiconductor, it will create a wide energy band. Therefore, the energy band of the organic material actually can form a continuous energy level by the electron-hole. This energy band model can easily explain the procedure after the electric charge injects from the electrode, combines between the energy gaps and emits a photon. 
     The basic substrate structure of the organic light emitting diode is glass and adds an emissive characteristic organic semiconductor between a metal cathode and a transparency indium-tin-oxide (ITO) anode. More particularly, the carrier can easily reach a balance between the injection and the transportation in the multi-layer structure of the organic semiconductor layer. The structure uses a thin film that includes an electron-hole injected by an electric charge and transmitted to the hole transport layer. The electron-hole combines with the electron and emits light on the emitting layer. The structure also includes a hole injection layer. When the voltage is low, an electron-hole still can be injected into the organic layer. 
     Accordingly, the color display can be a single color and a multi-color display. If the display is a single color display, it has one emitting layer. If it is a multi-color (red, green and blue, the tricolor) display, it can be formed by a shiftable mask. 
     FIG. 1 illustrates a conventional structure of an organic light emitting diode. Referring to FIG. 1, a glass substrate  100  has a plurality of strip-shaped anodes  110 . On top of the anodes  110  is a multi-layer structure  120 . This multi-layer structure  120  includes a hole injection layer  122 , a hole transport layer  124 , an organic emitting layer  126  and an electron transport layer  128 . On top of the multi-layer structure  120  is a plurality of strip-shaped cathodes  130 . 
     FIG.  2 A through FIG. 2D show the method for fabricating a conventional organic light emitting diode. 
     Referring to FIG. 2A, an indium-tin-oxide (ITO) layer is formed over a glass substrate  200 . Then, using a normal photolithography and etching process, the indium-tin-oxide layer is etched to form a plurality of strip-shaped anodes  210 . 
     Referring to FIG. 2B, a polymer layer  212  is formed between each anode and is used for insulation. In addition, this polymer layer  212  can be flat on the surface of the substrate. But the material of the polymer layer  212  is not very stable and easily changes its shape, so it is does not work well. The polymer layer  212  is an extra coating for the space between the anodes  210 . 
     Referring to FIG. 2C, a multi-layer structure  220  is formed on top of the anode  210 . The multi-layer structure  220  includes a hole injection layer  222 , a hole transport layer  224 , an organic emitting layer  226  and an electron transport layer  228  in sequence. However, the polymer layer  212  is not very smooth; therefore, a multi-layer structure  220  subsequently is formed on an un-smooth surface. 
     Referring to FIG. 2D, the multi-layer structure  220  is covered by a mask, and a plurality of strip-shaped metal are formed to make cathode  230 . The cathode  230  is placed perpendicular to the strip-shaped anode  210 . 
     In the conventional method for fabricating the organic light emitting diode, the thickness of the anode is greater, and the anode is located on top of the substrate and has a non-flat surface after the process. Although the long strips of the anode and the cathode are insulated by the polymer layer, a cross talk interference problem can still occur. 
     Therefore, the purpose of this invention is to avoid the non-flat surface of the cathode metal that causes cross talk interference on the organic light emitting diode, which can cause less brightness and instability. 
     Also, the conventional method uses the polymer to create a smoother surface. This method is costly and also increases the process time. Therefore, this invention does not use a polymer for insulation and flattening, and it also can save time and cost. 
     SUMMARY OF THE INVENTION 
     The invention provides a method for fabricating an organic light emitting diode. 
     As embodied and broadly described herein, the invention provides a substrate and uses a mask to etch the substrate to provide a position for the anode. A plurality of grooves are then formed in the substrate. The anode is formed on the bottom of the groove. A dot-matrix type mask is used to form the organic emitting layer between the anode and the cathode, and to form the hole injection layer, the hole transport layer and the electron transport layer inside the groove. More particularly, the total thickness of the anode, the organic emitting layer, the hole injection layer, the hole transport layer and the electron transport layer is equal to the depth of the groove. Thus, the substrate can have a smooth surface. Finally, another mask is used on the substrate and forms the cathode strips that will complete the fabrication of the organic light emitting diode. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a structure illustrating a conventional organic light emitting diode; 
     FIGS. 2A through 2D illustrate the steps for fabricating a conventional organic light emitting diode; 
     FIGS. 3A through 3E illustrate the steps of the organic light emitting diode according to one preferred embodiment of this invention. 
     FIG. 3F is a three-dimensional diagram illustrating the organic light emitting diode corresponding to FIG.  3 A through FIG.  3 E. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the invention will be described in accompaniment with the drawings of FIG.  3 A through FIG. 3E which illustrate an organic light emitting diode. 
     Referring to FIG. 3A, a substrate  300  is provided. The substrate is made of glass or plastic, for example. A mask  302  is formed on top of the substrate  300  and marks the anode position. Then the substrate  300  is etched using the mask  302  to form the plural groove  304  on the substrate  300 . The method of etching the substrate is plasma dry etching, laser carving, dry-wet etching or engraving. The depth of the groove  304  is equal to the total thickness of the anode, the organic emitting layer, the hole injection layer, the hole transport layer and the electron transport layer. 
     The groove  304  is forming to mark a place for the anode. It can reduce the cost of the conventional method that forms a polymer for insulating between two anodes. The material of the substrate  300  is very stable, and it can replace the polymer to have an even smoother surface. 
     Referring to FIG. 3B, an anode  310  is formed on the bottom of the groove  304 . The anode is formed by depositing an indium-tin-oxide layer in the groove  304 , and then etching back the extra indium-tin-oxide that coats the top of the substrate. Or, when the substrate  300  is etched to form the grooves  304  (see FIG.  3 A), controlling the etching ratio creates a groove  304  that is narrow on the top and wide in the bottom. Then an indium-tin-oxide is formed by sputtering deposition on the bottom of the groove  304  and becomes the anode  310 . The thickness of the anode  310  is approximately 1500 Å˜2500 Å. 
     Referring to FIG. 3C, an organic emitting layer  326  is formed under the preformative cathode spots, which are not shown in the diagram. The method of forming the organic emitting layer  326  uses a dot-matrix type of a metal mask  312  to form the organic emitting layer  326  under the preformative cathode spots and over the anode  310 . The thickness of the organic emitting layer  326  is approximately 550 Å˜650 Å. If the organic light emitting diode is multi-color (including the tricolors of red, green and blue) and the organic emitting layer can be formed by using a different type of dot matrix metal mask. In addition, in the structure of an organic light emitting diode, the structure of the multi-layer  320  that allows the carriers to easily reach a balance by injection and transportation. The structure of the multi-layer  320  includes a hole injection layer  322 , a hole transport layer  324  and an electron transport layer  328 . The multi-layer structure  320  can be formed inside the groove  304 . The total thickness of the hole injection layer  322 , the hole transport layer  324  and the electron transport layer  328  is approximately 250 Å˜350 Å. 
     Referring to FIG. 3D, the material layers inside the groove  304  include the anode  310  and the multi-layer  320 , and the total thickness is equal to the depth of the groove  304 . The surface of the substrate  300  is smooth. The total thickness of the anode  310 , the organic emitting layer  326 , the hole injection layer  322 , the hole transport layer  324  and the electron transport layer  328  are equal to the depth of the groove  304 . Therefore, the surface of the substrate  300  is smoother when those materials fill in the groove. 
     Referring to FIG. 3E, another mask is defined on the substrate  300  to form a strip-shaped metal layer, which is the cathode  330  of the organic light emitting diode. The cathode is perpendicular to the groove  304 . This is the final fabricating step of the organic light emitting diode. 
     Referring to FIG. 3F, a three dimensional diagram of the organic light emitting diode according to the fabrication diagrams from FIG.  3 A through FIG. 3E is provided. The substrate includes a plurality of strips that are parallel to the groove  304 . On top of the substrate  300  are a plurality of cathode strips perpendicular to the groove  304 . On the bottom of the groove  304  is the anode  310 . On top of the anode  310  is the multi-layer structure  320  that is right under the cathode strip  330 . The multi-layer structure  320  includes the organic emitting layer  326 , the hole injection layer  322 , the hole transport layer  324  and the electron transport layer  328 . The total thickness for both the anode and the multi-layer is equal to the depth of the groove  304 . The organic light emitting diode comprises of the organic emitting layer  326  that is between the anode  310  and the cathode  330 . 
     In the conventional design, the entire anode, organic emitting layer, cathode and multi-layer structure are all located on top of the substrate surface. The thickness of the anode is very thick at approximately a few thousands Angstroms deep. The thickness of the organic emitting layer and the multi-layer structure are very thin and only a few hundred angstroms deep. Therefore in the conventional design, the polymer is formed in the space between the anodes and makes a smoother surface, but cross talk interference still occurs either between the anodes or between the anode and the cathode. Therefore, this invention provides a groove to place the anode, the organic emitting layer and the multi-layer structure. It will prevent the problem of cross talk interference. 
     In view of the foregoing description, there are three advantages to this invention: 
     First, the anode and the cathode are located at different levels, in that the anode is located on the bottom of the groove and the cathode is on top of the substrate. Therefore, they can avoid interference from their neighbor, and brightness and stability are increased. 
     Second, the organic layer must be located under the cathode. Therefore the insulation is the vacuum or the nitrogen during the later packaging process. 
     Third, the polymer is used for smoothening the surface of the substrate in the conventional method. It will increase cost and the time. Therefore, this invention utilizes the groove for insulating the anodes and smoothening the surface of the substrate. It can save fabrication time and cost. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.