Abstract:
A display apparatus includes display pixels each having a thin film transistor and an EL element formed successively forming over a substrate. The EL element has a cathode electrode connected to the source of the thin film transistor and an anode electrode, and is driven by the thin film transistor. The EL element externally emits light from the reverse side of the substrate. For example, when the cathode electrode is formed the comblike, meshlike, or gridlike pattern on the luminous layer, the light is emitted through the slits of the cathode pattern. The display apparatus is provided that can improve the aperture ratio of a display pixel and can increase the degree of freedom in deciding the size and the drive capability of a TFT element which drives an EL element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation application of U.S. patent application Ser. No. 09/258,499, filed on Feb. 26, 1999 now U.S. Pat. No. 6,630,784, and which is herein incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a display apparatus that includes electroluminescence (hereinafter referred to as EL) elements and thin film transistors (hereinafter referred to as TFTs) which are fabricated on a substrate, and also to a method of fabricating the same. 
   2. Description of the Related Art 
   Recently, display apparatus using TFT elements and EL elements such as organic EL elements have been noted as display devices, in place of the CRTs (Cathode Ray Tubes) or LCD (Liquid Crystal Display) apparatus. 
     FIG. 1  is a cross sectional view illustrating a conventional display apparatus including organic EL elements and TFT elements.  FIG. 1  shows a laminated structure in which an organic EL element is deposited over a TFT structure. The structure is formed as follows: 
   A gate electrode  2  is formed on a transparent insulating substrate  1  such as glass or synthetic resin. An insulating film  3  is formed on the gate electrode  2 . An active layer  4  of polycrystalline silicon is formed on the insulating film  3 . A source region  4   s  and a drain region  4   d  into which impurities are implanted are formed in the active layer  4 . An interlayer insulating film  8  formed of a SiO 2  film  6  and a SiN film  7  is formed on the active layer  4 . The source region  11   s  is connected to the source electrode  10   s  via the contact hole  9  formed in the interlayer insulating film  8 . The drain region  4   d  is connected to the drain electrode  10   d  via the contact hole  9  formed in the interlayer insulating film  8 . 
   Planarization insulating film  11  is formed on the electrodes  10   s  and  10   d  and the interlayer insulating film  8 . The source electrode  10   s  is connected to an anode electrode  28  of an organic EL element formed on a TFT element via the contact hole  12  formed in the planarization insulating film  11 . 
   The organic EL element is formed by successively laminating an anode  2  formed of: a transparent electrode of ITO (Indium Tin oxide); an organic layer comprised a second hole transfer layer  27  of MTDATA (4,4′-bis(3-methylphenylphenylamino)biphenyl), a first hole transfer layer  26  of TPD (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), a luminous layer  25  of Bebq2(10-benzo[h]quinolinol-beryllium complex) containing Quinacridone derivative, and an electron transfer layer  24  of Bebq2; and a cathode electrode  23  of magnesium and indium alloy (MgIn). The organic layer is made of an organic chemical compound. Thus, an EL element is constructed the organic layer, the anode electrode  28 , and the cathode electrode  23 . 
   In the organic EL element, the holes injected from the anode electrode  28  and the electrons injected from the cathode electrode  23  are recombined inside the luminous layer  25 . Excitons are generated by exciting the organic molecules of the luminous layer  25 . The luminous layer  25  radiates light through the process of the excitons disappearing. The light is radiated out from the transparent anode electrode  28  through the transparent insulating substrate  1 . 
   However, in the conventional display apparatus structure, since the organic EL element emits light through the side of the substrate  1  on which TFT elements are formed, the TFT structure blocks the emitted light so that the display pixel aperture ratio cannot be increased. 
   Furthermore, since the TFT elements must be small-sized to the extent that the luminous light is not blocked, there are severe limitations on increasing the size of each TFT element as well as the TFT element capability. 
   SUMMARY OF THE INVENTION 
   The present invention is made to solve the above-mentioned problems involved in the conventional display apparatus. It is an object of the invention to provide a display apparatus that can improve the display pixel aperture ratio and can increase the degree of freedom in deciding the size and the drive capability of a thin film transistor which drives an EL element. 
   According to the present invention, the display apparatus comprises: a substrate; thin film transistors formed on the substrate, each of the thin film transistors having a source electrode and a drain electrode; and electroluminescence elements respectively formed over the thin film transistors, each of the electroluminescence elements having a cathode electrode, an anode electrode, and a luminous layer formed between the cathode electrode and the anode electrode; wherein each of the electroluminescence elements emits toward the reverse side of the substrate. 
   Each of the electroluminescence elements comprises the cathode electrode, the luminous layer and the anode electrode successively formed above the thin film transistor. The cathode electrodes is connected to a source or drain electrode of the corresponding thin film transistor. Moreover, each of the thin film transistors drives the corresponding electroluminescence element. 
   Since light is emitted from the reverse side of the substrate, the thin film transistor formed on the substrate side does not block the light, so that the aperture ratio can be increased. 
   It is not required to miniaturize the thin film transistor to the extent that the light is not shielded. Hence, the thin film transistor can be designed with high freedom of size. Thin film transistors with high performance can be formed without constraints in size. 
   According to the present invention, the electroluminescence element is constructed by successively forming a cathode electrode, a luminous layer, and an anode electrode over the thin film transistor. 
   The anode electrode is made of a metal material and can cover over only a part of the display pixel area within a unit display pixel area. 
   The above-mentioned planar structure can externally emit light from the reverse side of the substrate, that is, the anode side. Moreover, the anode electrode in, for example, a comblike, meshlike or gridlike form can externally emit a sufficient amount of light. 
   The anode electrode of the electroluminescence element can be formed by a vapor evaporation process. 
   Furthermore, according to the present invention, the display apparatus is fabricated through the steps of forming the thin film transistors on a substrate; forming an insulating film to cover the thin film transistor; forming contact hole at predetermined position of the insulating film, and then forming the cathode electrode of each of the electroluminescence elements to respectively make contact with a source electrode or a drain electrode of the thin film transistor via the holes; forming said luminous layer over the cathode electrode; and forming an anode electrode over the luminous layer using an opaque metal material through a vapor evaporation method.
 
The anode electrode is preferably formed to partially occupy a unit display pixel region.
 
   The electroluminescence element comprises an organic electroluminescence element using an organic material for the luminous layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and drawings, in which: 
       FIG. 1  is a cross sectional view schematically illustrating one display pixel structure in a conventional color display apparatus; 
       FIG. 2  is a cross sectional view schematically illustrating one display pixel structure in a fabrication step according to an embodiment of the present invention; 
       FIGS. 3A and 3B  are cross sectional views each schematically illustrating the anode electrode for a display pixel according to an embodiment of the present invention; and 
       FIGS. 4A ,  4 B,  4 C,  4 D and  4 E are cross sectional views each schematically illustrating a fabrication step according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Next, an embodiment of a display apparatus according to the present invention will be described below with reference to the attached drawings. 
     FIG. 2  is a cross sectional view illustrating one pixel of a display apparatus having organic EL elements and TFT elements, according to the present applicant. 
   The display apparatus shown in  FIG. 2  differs from that shown in  FIG. 1  in that the TFT element and the organic element are reversely formed on the substrate  1 . 
   In each display pixel, an TFT element and an organic EL element are laminated over an insulating substrate which is made of glass synthetic resin, or a conductive substrate or a semiconductor substrate on which an insulating film such as SiO 2  film or SiN film is formed. The substrate  1  may be a transparent or opaque substrate. 
   The TFT structure formed on the substrate  1  is the same as that of the conventional TFT structure, and so repeated explanation will be omitted here. The source electrode  10   s  is connected to the cathode electrode  13  of an organic EL element formed over the TFT element via the contact hole formed in the planarization insulating film  11 . The source electrode  10   s  supplies the drain signal from the TFT element to the organic EL element via the drain signal line. 
   The organic EL element is formed by successively laminating a cathode electrode  13 , an electron transfer layer  14 , a luminous layer  15 , first and second hole transfer layers  16  and  17 , and an anode electrode  18 . The cathode electrode  13  comprises a magnesium and indium (MgIn) alloy or aluminum and lithium (AlLi) alloy and is connected to the source electrode  10 S of the TFT element. The electron transfer layer  14  comprises of Bebq2. The luminous layer  15  comprises of Bebq2(10-benzo[h]quinolinol-beryllium complex) containing Quinacridone derivative. The first hole transfer layer  16  comprises TPD: triphenylamine dimer (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine). the second hole transfer layer  17  comprises MTDATA (4,4′-bis(3-methylphenylphenylamino)biphenyl). The anode electrode  18  comprises a metal electrode such as Pt, Rh or Pd and is shaped in comblike, meshlike or gridlike pattern (FIGS.  3 A and  3 B)). 
   In the luminous layer of each organic EL element, each of the electron transfer layer, the luminous layer, the first hole transfer layer and the second hole transfer layer comprises an organic chemical compound. Each organic layer is sandwiched between the anode electrode and the cathode electrode. The hole transfer layer may be a single layer. 
   The holes injected from the anode electrode  18  and the electrons injected from the cathode electrode  13  are recombined together inside the luminous layer  15 . Organic molecules comprising the luminous layer  15  are excited so that excitons are produced. The luminous layer  15  emits light through the process of the excitons disappearing. The emitted light is radiated out from the anode electrode  18  (to the upper vertical orientation in  FIG. 2 ) 
   The light emitted from the luminous layer travels toward both the cathode electrode and the anode electrode but is reflected back from the metal cathode electrode without externally penetrating it. As a result, the light is radiated out through the slits of the anode electrode in a comblike, meshlike or gridlike pattern. 
   An organic EL display apparatus is constructed by arranging the thus-configured display pixels in a matrix form. 
   Here, the form of an anode electrode as well as the anode electrode fabricating method will be described below. 
   In the display apparatus of the invention having a laminated structure where an EL element is placed on a TFT element, the anode electrode is formed on the luminous element layer. 
   If the anode electrode  18  is formed on the luminous element layer by the ITO (Indium Tin Oxide) sputtering process in the prior art, the luminous element layer previously formed will be damaged. 
   According to the present invention, the anode electrode  18  is formed by vapor evaporating an opaque metal. This approach allows the anode electrode  18  to be formed over the luminous element layer with no occurrence of damage. 
   However, if the anode electrode  18  is formed of a metal material over the entire surface of a luminous element layer, the metal material blocks the emitted light, so that the emitted light cannot be radiated outward. This means that the display apparatus does function normally. 
   In order to deal with such problems, the anode electrode  18  is formed in a comblike (FIG.  3 A), meshlike or gridlike pattern (FIG.  3 B), so that light is emitted from the reverse side of the substrate, that is, through the slits in the anode electrode  18  (in the arrow direction of FIGS.  3 A and  3 B). The gap between comb teeth or the aperture size of the mesh is selected to a brightness required as a display apparatus. 
   Next, the display apparatus fabricating method will be described below.  FIGS. 4A  to  4 E are cross sectional views illustrating a process flow in manufacturing a display apparatus according to the present invention. 
   In the step  1 , as shown in  FIG. 4A , a gate electrode  2  of a refractory metal (a high-melting point metal) such as chromium (Cr) or molybdenum (Mo) is formed on the substrate  1  of which at least the surface is insulative. 
   An insulating film  3  and a p-Si active layer  4  are formed all over the surface of the substrate to cover the gate electrode  2 . A stopper  5  of SiO 2  film is formed on the p-Si film  4 . 
   With the stopper  5  acting as a mask, P-type or N-type ions are doped into the p-Si film  4  to form the source region  4   s  and the drain region  4   d . The region masked by the stopper  5  and not doped with ions is defined between the source region  4   s  and the drain region  4   d  and will act as a channel. An interlayer insulating film  8  formed of a SiO 2  film  6  and a SiN film  7  is formed on the channel. A first contact hole  9  penetrating the interlayer insulating film  8  is formed at the position corresponding to the source region  4   s  while a first contact hole  9  penetrating the interlayer insulating film  8  is formed at the position corresponding to the drain region  4   d . A source electrode  10   s  is formed to connect to the source region  10   s  via the first contact hole  9  while a drain electrode  10   d  is formed to connect to the drain region  10   d  via the first contact hole  9 . 
   Thus, a TFT (poly-silicon thin film transistor, hereinafter referred to as “p-SiTFT”) which has a p-Si active layer and drives an organic EL element is fabricated. The material for the active layer is not limited to p-Si, but may be amorphous silicon or fine crystalline silicon. 
   Next, the step of forming an organic EL element on the TFT element will be described below. 
   In the step  2 , as shown in  FIG. 4B , a planarization insulating film  11  comprises on the insulating film  8  and the electrodes  10   s  and  10   d  of the p-Si TFT element. The planarization insulating film  11  comprises a silicon oxide film, a silicon nitride film, a silicon nitride oxide film, a silicate glass film, a SOG (Spin On Glass) film, or a synthetic resin film (ex. polyimide resin film, organic silica film or acrylic resin film). A contact hole  12  is formed in the planarization insulating film  11 . 
   In the step  3 , as shown in  FIG. 4C , the cathode electrode  13  of either magnesium and indium alloy (MgIn) or aluminum and lithium (AlLi) alloy of an organic EL element is formed on the planarization insulating film  11 . The cathode electrode  13  is connected to the source electrode  10   s  via the contact hole  12  formed in the planarization insulating film  11 . 
   In the step  4  shown in  FIG. 4D , an electron transfer layer  14  comprises Bebq2, a luminous layer  15  comprised Bebq2(10-benzo[h]quinolinol-beryllium complex) containing quinacridone derivative, a first hole transfer layer  16  comprised TPD(4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), and a second hole transf r layer  17  comprised MTDATA (4,4′-bis(3-methylphenylphenylamino)biphenyl) are successively over the cathode electrode  13 . 
   In the step  5  shown in  FIG. 4E , a metal film of, for example, Pt, Rh or Rd is vapor evaporated on the second hole transfer layer  17  to form the anode electrode  18 . 
   The anode electrode  18  may be formed by vacuum evaporating metal in an ion state using the ion cluster method.