Abstract:
Side faces of anodes have a tapered incline that becomes broader toward a lower layer. Thus, an emissive element layer is smoothly formed on the anodes making it possible to prevent field contraction of the electric field. An EL display apparatus having long life and high yield is provided by preventing the emissive element layer from rupturing between an anode and a cathode and by preventing concentration of the electric field at an upper edge of the anode facing the cathode and localized deterioration in the emissive element layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a luminescence display apparatus comprising electroluminescence elements and thin-film transistors.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, electroluminescence (referred to hereinafter as EL) display apparatuses employing EL elements as emissive elements have attracted attention as being the display apparatuses to replace CRTs and LCDs, and the research and development also have advanced on EL display apparatuses comprising thin-film transistors (referred to hereinafter as TFT) as switching elements to drive the EL elements.  
           [0005]    [0005]FIG. 1 shows an equivalent circuit of an EL display apparatus comprising a conventional EL element and TFT.  
           [0006]    [0006]FIG. 1 is an equivalent circuit of an EL display apparatus comprising a first TFT  130 , a second TFT  140 , and an organic EL element  160 , and shows the circuitry near a gate signal line Gn of row n and a drain signal line Dm of column m.  
           [0007]    The gate signal line Gn supplying a gate signal and the drain signal line Dm supplying a drain signal are perpendicular to each other, and near the intersection of both signal lines are provided the organic EL element  160  and the TFTs  130 ,  140  driving the organic EL element  160 .  
           [0008]    The first TFT  130 , which is a switching TFT, comprises gate electrodes  131  connected to the gate signal line Gn and supplied with gate signals, a drain electrode  132  connected to a data signal line (drain signal line) Dm and supplied with data signals, and a source electrode  133  connected to a gate electrode  141  of the second TFT  140 .  
           [0009]    The second TFT  140 , which is an organic EL element driver TFT, comprises the gate electrode  141  connected to the source electrode  133  of the first TFT  130 , a source electrode  142  connected to an anode  161  of the organic EL element  160 , and a drain electrode  143  connected to a driving power supply  150  that is supplied to the organic EL element  160 .  
           [0010]    Furthermore, the organic EL element  160  comprises the anode  161  connected to the source electrode  142 , a cathode  162  connected to a common electrode  164 , and an emissive element layer  163  sandwiched between the anode  161  and the cathode  162 .  
           [0011]    Furthermore, a storage capacitor  170  is provided with one electrode  171  connected between the source electrode  133  of the first TFT  130  and the gate electrode  141  of the second TFT  140  and another electrode  172  connected to a common electrode  173 .  
           [0012]    The driving method of the circuit shown in the equivalent circuit of FIG. 1 will now be described. When the gate signal from the gate signal line Gn is applied to the gate electrode  131 , the first TFT  130  turns on. As a result, the data signal from the data signal line Dm is supplied to the gate electrode  141  and the voltage of the gate electrode  141  becomes identical to the voltage of the data signal line Dm. A current proportional to the voltage value supplied to the gate electrode  141  is then supplied from the driving power supply  150  to the organic EL element  160 . As a result, the organic EL element  160  emits light at an intensity in accordance to the magnitude of the data signal.  
           [0013]    A conventional EL display apparatus will be described next with reference to FIGS. 2, 3A, and  3 B. FIG. 2 is a top view showing one pixel of the conventional EL display apparatus. In FIG. 2, a gate signal line  51  corresponds to the gate signal line Gn, a data signal line  52  corresponds to the data signal line Dm, a driving power supply  53  corresponds to the driving power supply  150 , an electrode  54  corresponds to the electrode  172  of the storage capacitor  170 , and an anode  61  corresponds to the anode  161  of the organic EL element  160 . The gate signal lines  51  are arranged in rows and the data signal lines  52  and the driving supplies  53  are arranged in columns. The storage capacitor and the emissive element layer are arranged within the area thus partitioned. The storage capacitor is formed from a semiconductor film  13  and the electrode  54 . The semiconductor film  13  is connected to the data signal line  52  via a contact C 1 , and a gate electrode  11  is arranged between a drain  13   d  and a source  13   s.    
           [0014]    A semiconductor film  43  is connected to the driving power supply  53  via a contact C 2 , and a gate electrode  41 , which is connected to the semiconductor film  13 , is arranged between a drain  43   d  and a source  43   s . The semiconductor film  43  is connected to the anode  61  of the organic EL element via a contact C 3 .  
           [0015]    [0015]FIG. 3A is a cross-sectional view along line A-A of FIG. 2. On a transparent substrate  10  is formed the semiconductor film  13 , on which is covered with and formed a gate insulating film  12 . On the gate insulating film  12  are formed gate electrodes  11 , which branch from the gate signal line  51 , and the storage capacitor electrode  54 , on which is covered with and formed an interlayer insulating film  15 . On the interlayer insulating film  15  is arranged the data signal line  52 , which connects to the semiconductor film  13  via the contact C 1 . On these is covered with and formed a planarization insulating film  17 .  
           [0016]    [0016]FIG. 3B is a cross-sectional view along line B-B of FIG. 1. On the substrate  10  are laminated in sequence the semiconductor film  43 , the gate insulating film  12 , the gate electrode  41 , and the interlayer insulating film  15 , and on the interlayer insulating film  15  are formed the data signal line  52  and the driving power supply  53 , on which is covered with and formed the planarization insulating film  17 . On the planarization insulating film  17  is arranged an anode  61 , which is connected to the semiconductor film  43  via the contact C 3 . On the anode  61  is arranged an emissive element layer  66 , which has a laminated structure of a first hole transport layer  62 , a second hole transport layer  63 , an emissive layer  64 , and an electron transport layer  65 . A cathode  67  is arranged so as to cover them.  
           [0017]    The anode  61  of the pattern shown in FIG. 2 is generally formed using a method in which an ITO film is first formed on the entire surface, and after forming a positive photoresist in a predetermined shape, wet etching is performed using chemicals.  
           [0018]    However, when forming the organic EL element in this manner, the emissive element layer  66  that is formed on the anode  61  is extremely thin at approximately 200 nm so that coverage at the step portion with the planarization insulating film  17  at the edge of the anode  61  deteriorates. Thus, at the points indicated by the arrows in FIG. 4, since the vertex of the anode  61  and the vertex of the cathode  67  face each other in closer proximity than at any other location, field concentration occurs here causing a problem where the emissive layer  64  positioned between layers deteriorates rapidly. As the coverage deteriorates further, the emissive element layer  66  ruptures as shown in the figure, and the cathode  67  provided on the upper layer shorts with the anode  61  to possibly cause this pixel to be defective and not display.  
         SUMMARY OF THE INVENTION  
         [0019]    It is therefore an object of the present invention to provide an EL display apparatus having long life and high yield by preventing shorts or localized deterioration of the emissive layer  64  due to the thickness of the anode.  
           [0020]    The present invention solves the aforementioned problem and is an electroluminescence display apparatus comprising an emissive element (an electroluminescence element) laminated in sequence on the substrate with the first electrode, the emissive element layer (such as hole transport layer, emissive layer, and electron transport layer), and the second electrode, with the side faces of the first electrode inclined and becoming broader toward the substrate side.  
           [0021]    The angle formed by the incline of the first electrode and the plane of the lower layer (and/or the substrate) is 10° to 45°, or further an angle of 25° to 35°. Furthermore, the side of the first electrode has a tapered shape becoming broader from the emissive element layer toward the substrate.  
           [0022]    Furthermore, the thickness of the first electrode is less than ½, or further less than ⅓ the total film thickness of the hole transport layer, the emissive layer, and the electron transport layer.  
           [0023]    As described above, the edge of the first electrode in the present invention is inclined so that the electroluminescence element that is formed thereon is formed smoothly, shorting of the first electrode and the second electrode is prevented, and an electroluminescence display apparatus having a high yield is obtained.  
           [0024]    Furthermore, since concentration of the electric field at the edge of the first electrode is prevented, the electroluminescence element is prevented from locally deteriorating, and a luminescence display apparatus having a long life is obtained.  
           [0025]    Since the angle formed by the incline of the first electrode with the plane of the lower layer (and/or the substrate) is 10° to 45°, or further an angle of 25° to 35°, this ensures the emissive element layer can be formed without loss of reproducibility of the shape of the first electrode.  
           [0026]    Furthermore, since the thickness of the first electrode is less than ½, or further less than ⅓, the film thickness of the emissive element layer, this ensures the emissive element layer can be formed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is an equivalent circuit diagram of an EL display apparatus.  
         [0028]    [0028]FIG. 2 is a cross-sectional view of the EL display apparatus of the present invention.  
         [0029]    [0029]FIGS. 3A and 3B are cross-sectional views of the EL display apparatus of the present invention.  
         [0030]    [0030]FIG. 4 is a cross-sectional view illustrating a problem of a conventional EL display apparatus.  
         [0031]    [0031]FIG. 5 is a top view of an active-matrix EL display apparatus of the present invention.  
         [0032]    [0032]FIG. 6 is a cross-sectional view of the active-matrix EL display apparatus of the present invention.  
         [0033]    [0033]FIGS. 7A and 7B are enlarged cross-sectional views showing the edge of the first electrode of the present invention.  
         [0034]    [0034]FIGS. 8A, 8B, and  8 C are cross-sectional views showing a formation method of the first electrode of the present invention.  
         [0035]    [0035]FIG. 9 is a top view and a cross-sectional view of a simple-matrix EL display apparatus of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    A first embodiment of the present invention will be described hereinafter. The first embodiment is an example applying the present invention to an active-matrix organic EL display apparatus. One display pixel of the EL display apparatus of the first embodiment is shown in FIG. 5 and a cross-sectional view along line A-A in FIG. 5 is shown in FIG. 6.  
         [0037]    A driver circuit for each pixel is identical to the circuit shown in FIG. 1, and the difference with the prior art shown in FIGS. 2, 3A, and  3 B is the cross-sectional configuration of an anode  1 , or first electrode.  
         [0038]    The gate signal line  51 , the data signal line  52 , the driving power supply  53 , the electrode  54 , and the anode  1  respectively correspond to the gate signal line Gn, the data signal line Dm, the driving power supply  150 , the electrode  172  of the storage capacitor  170 , and the anode  161  of the organic EL element  160 . The gate signal lines  51  are arranged in rows and the data signal lines  52  and the driving supplies  53  are arranged in columns. A capacitor and an emissive layer are arranged within the area that is partitioned by the signal lines and power supply lines. The storage capacitor is formed from the semiconductor film  13  and the electrode  54 . The semiconductor film  13  is connected to the data signal line  52  via the contact C 1 , and the gate electrode  11  is arranged between the drain  13   d  and the source  13   s.    
         [0039]    The semiconductor film  43  is connected to the driving power supply  53  via the contact C 2 , and the gate electrode  41 , which is connected to the semiconductor film  13 , is arranged between the drain  43   d  and the source  43   s . The semiconductor film  43  is connected to the anode  1  of the organic EL element via the contact C 3 .  
         [0040]    As shown in FIG. 6, the organic EL display apparatus is formed by laminating in sequence a TFT and an organic EL element on the substrate  10 , such as a substrate formed from glass or synthetic resin, a conductive substrate, or a semi-conductive substrate. However, when a conductive substrate or a semi-conductive substrate is used for the substrate  10 , an insulating film of SiO 2  or SiN is formed on the substrate  10 , upon which the TFT and organic EL element are formed.  
         [0041]    In the present embodiment, a first TFT  30  and a second TFT  40  are both so-called top-gate TFTs provided with a gate electrode at the top of the active layer, and a case is given where a semiconductor film formed from poly-silicon is used for the active layer. Furthermore, the case is given where the TFT has the gate electrode  11  with a double-gate structure.  
         [0042]    The first TFT  30 , which is a switching TFT, will be described first.  
         [0043]    As shown in FIG. 6, on the insulating substrate  10 , which is formed from quartz glass, non-alkaline glass, or the like, are formed in sequence the semiconductor film  43  and the gate insulating film  12 . The semiconductor film  43  is the active layer of the second TFT, and has the source  43   s , the drain  43   d , and the channel  43   c . On the gate insulating film  12  is formed the gate electrode  41 , which is formed from a refractory metal such as chromium (Cr), molybdenum (Mo), or the like, on which is covered with and formed the interlayer insulating film  15 , which is formed by laminating in sequence a SiO 2  film, a SiN film, and a SiO 2  film. Thereon is formed the data signal line  52  and the driving power supply  53 .  
         [0044]    The TFT has a so-called Lightly Doped Drain (LDD) structure. Namely, ion doping is performed using the gate electrode  41  on a channel  13   c  as a mask. Furthermore, the gate electrode  41  and an area up to a fixed distance from both sides of the gate electrode  41  are covered with resist, and ion doping is performed again to provide a low concentration area on both sides of the gate electrode  41  and beyond these areas the source  43   s  and the drain  43   d  of a high concentration area.  
         [0045]    Furthermore, the planarization insulating film  17 , which is formed from an organic resin or the like, is formed on the entire surface so as to planarize the surface. A contact hole is then formed at a position corresponding to the source  43   s  in the planarization insulating film  17 , and a transparent first electrode formed from ITO and contacting the source  43   s  via the contact C 3 , namely, the anode  1  of the organic EL element, is formed on the planarization insulating film  17 .  
         [0046]    The emissive element layer  66  adopts a common structure and is formed by laminating in sequence the anode  1  formed from a transparent electrode, such as ITO, the first hole transport layer  62  formed from MTDATA (4,4′,4″-tris (3-methylphenylphenylamino)triphenylamine), the second hole transport layer  63  formed from TPD (N,N′-diphenyl-N,N′-di (3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), the emissive layer  64  formed from Bebq 2  (bis(10-hydroxybenzo[h]quinolinato) beryllium) including an inductor Quinacridon, the electron transport layer  65  formed from Bebq 2 , and the cathode  67  formed from a magnesium-indium alloy or a magnesium-silver alloy or a lithium fluoride-aluminum lamination.  
         [0047]    Furthermore, in the organic EL element, holes injected from the anode and electrons injected from the cathode recombine within the emissive layer, and the organic molecules included in the emissive layer are excited to yield exitons. Light is released from the emissive layer in the process where the exitons undergo radiation deactivation, and this light is released to the outside from the transparent anode via the transparent insulating substrate to make the light emission visible.  
         [0048]    Arranging display pixels configured in this manner in a matrix on the substrate  10  forms an organic EL display apparatus capable of displaying a desired overall image by controlling each pixel.  
         [0049]    The anode  1  of the present embodiment has edges forming tapered inclines as shown in FIG. 6. Due to these inclines, the emissive element layer  66  is smoothly formed from the anode  1  on the planarization insulating layer  17 , thereby preventing the coverage from deteriorating and the anode  1  and the cathode  67  from shorting. Furthermore, since the inclines become broader on the substrate side, there are no sharp edges on the top edge of the anode  1  facing the cathode  67 , making field concentration less likely to occur. Therefore, the emissive layer  64  emits light uniformly on the entire surface and partial deterioration does not occur rapidly.  
         [0050]    It is preferable for the angle θ formed from the incline of the anode  1  shown in FIGS. 7A and 7B with the plane of the planarization insulating film  17  to be small so as to prevent rupture or field concentration. However, if the angle θ is too small, the edge of the anode  1  becomes extremely thin so that a problem arises where reproducibility of the shape decreases. Therefore, the angle formed by the plane of the bottom layer or the substrate  10  with the inclined side faces of the anode  1  is set from 10° to 45°, and preferably around 30°. Furthermore, it is preferable for the top edge of the anode  1  to have a smooth curve as shown in FIG. 7B.  
         [0051]    A method for forming the anode  1  into an incline will be described next. As described above, although the etching of the ITO film employed the conventional wet etch method, the angle θ of the incline becomes substantially 90°. In the present embodiment, a positive photoresist is formed on the ITO film, which has been formed on the entire surface, and dry etching is performed using a chlorine-based gas, such as Cl 2  or HCl, to form an incline on the ITO edge. FIGS. 8A to  8 C are cross-sectional views showing the formation method of the anode  1 . First, as shown in FIG. 8A, an ITO film  21  is formed on the entire surface of the planarization insulating film  17 . Next, a positive photoresist  22  is formed at a predetermined area. When this is exposed to a chlorine-based gas, such as chlorine gas or hydrogen chloride gas, the ITO film  21  and the photoresist  22  are etched isotropically. With dry etching using chlorine-based gas, the selectivity is low between the ITO film  21  and the photoresist  22  so that the photoresist  22  is etched simultaneously with the ITO film  21 . However, since etching is faster for the ITO film  21 , the etching proceeds as shown in FIG. 8B even though the selectivity is low. The etching continues and completes as shown in FIG. 8C. In the present embodiment, the angle θ of the incline becomes approximately 30°. In this manner, the isotropic etching is performed using an etching gas having low selectivity between the ITO film and the resist so that the anode  1  is formed with sloping edges.  
         [0052]    The film thickness of the anode  1  is described next. The film thickness of the anode  1  is thinly formed compared to the total film thickness of the emissive element layer  66 . When the film thickness of the anode  1  is thin, a step developing between it and the planarization insulating film  17  is reduced so that a rupture of the emissive element layer  66  can be prevented. Since the color of the display changes depending on the thickness of the anode  1 , an arbitrary thickness cannot necessarily be set. The film thickness of the anode  1  is set to ½ the total thickness of the emissive element layer  66  or less if possible, and preferably to ⅓ or less. However, if the anode  1  is formed too thin, the reproducibility of the shape decreases due to chipping of part of the anode  1  and so forth. In the present embodiment, the anode  61  has a thickness of approximately 85 nm, the emissive element layer  66  has a total thickness of approximately 200 nm, and the cathode  67  has a thickness of approximately 200 nm.  
         [0053]    The present invention is also applicable to a simple-matrix EL display apparatus. FIG. 9 shows a top view and a cross-sectional view along line A-A of the simple-matrix EL display apparatus representing a second embodiment of the present invention.  
         [0054]    Arranged on a transparent substrate  70  are an anode  71 , which is a first electrode extending longitudinally, and a cathode  72 , which is a second electrode extending transversely and crossing the first electrode  71 . In the emissive element layer  66 , the emissive layer  64  is formed at each intersection of the anode  71  and the cathode  72 .  
         [0055]    Although the TFT was illustrated in the aforementioned embodiments as having the top-gate structure in which the gate electrode is located on the active layer, it may have a bottom-gate structure instead. Furthermore, although a semiconductor film was used for the active layer in the aforementioned embodiments, a micro-crystalline silicon film or amorphous silicon may be used instead.  
         [0056]    In this embodiment also, the edge of the anode  71  inclines and becomes broader toward the substrate so that the emissive element layer  66  smoothly covers the anode  71 , thereby preventing shorts between the anode  71  and the cathode  72 .  
         [0057]    Furthermore, although an organic EL display apparatus was described in the aforementioned embodiments, the present invention is not limited thereto, and may be also applicable to an inorganic EL display apparatus having a emissive layer formed from inorganic materials, while yielding a similar effect.  
         [0058]    Furthermore, although the first electrode was described in the present specification as an anode, the first electrode is arranged between the substrate and the EL element (EL layer) and is the electrode covered by the EL layer so that in some cases it may be a cathode.  
         [0059]    While there has been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.