Abstract:
An electroluminescent display device has a device glass substrate and a sealing glass substrate that are attached together with a sealing resin. The device glass substrate has a pixel region provided with an organic EL element, and a horizontal driving circuit and a vertical driving circuit, both of which supply driving signals for driving the organic EL element. The sealing glass substrate is provided with a light shield layer for preventing incident light from entering the horizontal driving circuit and the vertical driving circuit. This makes edges of an organic EL panel clear, thereby improving a display contrast.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1 Field of the Invention  
           [0002]    This invention relates to an electroluminescent display device and a manufacturing method thereof, particularly to an electroluminescent display device with an improved display qualities.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, electroluminescent (hereafter, referred to as EL) display devices with an EL element have been receiving an attention as a display device replacing a CRT and an LCD.  
           [0005]    [0005]FIG. 6 is a schematic view showing a structure of a conventional organic EL panel. A pixel region and a peripheral driving circuit region are formed on a device glass substrate  1 . The pixel region includes a plurality of pixels and each of the pixels has an organic EL element, an organic EL element driving TFT (thin film transistor), TFTa, and a pixel selecting TFT (not shown). For example, the organic EL element driving TFT, TFTa, has an active layer made of a polysilicon layer. A light shield layer  3  made of Cr (chromium) is disposed below TFTa with an insulating film  2  interposed therebetween.  
           [0006]    This light shield layer  3  is placed off the position exactly below the active layer of TFTa. The reason is as follows. In the manufacturing process of this display device, the active layer of TFTa is initially made of an amorphous silicon. This amorphous silicon is then crystallized by excimer laser irradiation. At this process step, if the light shield layer  3  made of Cr is located exactly below the active layer of TFTa, thermal conductivity becomes high. This makes it difficult to control grain sizes of the active layer, thereby degrading characteristics of TFTa.  
           [0007]    A peripheral driving circuit disposed in a periphery of the pixel region includes many TFTs, TFTb, as well as related wiring (not shown). The light shield layer  3  made of Cr is not formed exactly below TFTb with the same reason as described above.  
           [0008]    The device glass substrate  1  is attached to a sealing glass substrate  5  with a sealing resin  4  made of an epoxy resin interposed therebetween.  
           [0009]    As described above, since the light shield layer  3  is not formed exactly below TFTbs in the peripheral driving circuit, light incident from the sealing glass substrate  5  penetrates to the device glass substrate  1 . Therefore, edges of the organic EL panel become blurred to an observer and a display contrast is degraded.  
         SUMMARY OF THE INVENTION  
         [0010]    The invention provides an electroluminescent display device that includes a first substrate, an electroluminescent element disposed on the first substrate, a peripheral driving circuit disposed on a peripheral portion of the first substrate and supplying a driving signal for the electroluminescent element, a second substrate attached to the first substrate, and a light shield layer covering the peripheral driving circuit.  
           [0011]    The invention also provides an electroluminescent display device that includes a first substrate, an electroluminescent element disposed on the first substrate, a peripheral driving circuit disposed on a peripheral portion of the first substrate and supplying a driving signal for the electroluminescent element, a second substrate attached to the first substrate, and means for preventing light coming from the second substrate and entering the peripheral driving circuit from passing through the first substrate.  
           [0012]    The invention further provides a manufacturing method of an electroluminescent display device. The method includes providing a first substrate having thereon an electroluminescent element and a peripheral driving circuit for supplying a driving signal for driving the electroluminescent element. The peripheral driving circuit is disposed on a peripheral portion of the first substrate. The method also includes forming a light shield layer on a second substrate, forming a photoresist layer on the light shield layer at a peripheral region of the second substrate, and removing a portion of the light shield layer by etching using the photoresist layer as a mask. The method further includes forming a pocket region on the second substrate by etching the second substrate using the photoresist layer and the remaining light shield layer as a mask, removing the photoresist layer, forming a desiccant layer in the pocket region, and attaching the second substrate to the first substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a plan view of a device glass substrate according to an embodiment of the invention.  
         [0014]    [0014]FIG. 2 is an equivalent circuit diagram of a pixel formed on a pixel region shown in FIG. 1  
         [0015]    [0015]FIGS. 3A and 3B are cross-sectional views of an organic EL panel according to the embodiment.  
         [0016]    [0016]FIG. 4 is a partial cross-sectional view of the pixel region and a peripheral driving circuit region shown in FIG. 1.  
         [0017]    FIGS.  5 A- 5 F show manufacturing steps of a sealing glass substrate of the embodiment.  
         [0018]    [0018]FIG. 6 is a cross-sectional view of a conventional organic EL panel. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Next, there will be described an embodiment of the invention with reference to FIGS.  1 - 5 F. First, a structure of a device glass substrate  200  of this embodiment will be described with reference to FIGS. 1 and 2.  
         [0020]    As shown in FIG. 1, there are disposed on the device glass substrate  200  a pixel region  300 , and a horizontal driving circuit  301  and a vertical driving circuit  302  as a peripheral driving circuit of the pixel region. The vertical driving circuit  302  supplies a gate signal Gn (a horizontal scanning signal) to each pixel of the pixel region  300 . The horizontal driving circuit  301  supplies a drain signal (a video signal Dm) to each pixel of the pixel region  300  in accordance with the horizontal scanning signal.  
         [0021]    [0021]FIG. 2 is an equivalent circuit diagram of one of the pixels placed in the pixel region  300 . A gate signal line  50  for supplying the gate signal Gn and a drain signal line  60  for supplying the drain signal, i.e. the video signal Dm intersect each other.  
         [0022]    There are disposed in the vicinity of the intersection of both signal lines an organic EL element  120 , a TFT  106  for driving the organic EL element  120  and a TFT  110  for selecting a pixel.  
         [0023]    A positive source voltage PVdd is supplied to a drain  106   d  of the organic EL element driving TFT  106 . A source  106   s  is connected to an anode  121  of the organic EL element  120 .  
         [0024]    A gate  110   g  of the pixel selecting TFT  110  is connected to the gate signal line  50 , receiving the gate signal Gn, and a drain  110   d  of the pixel selecting TFT  110  is connected to the drain signal line  60 , receiving the video signal Dm. A source  110   s  of the TFT  110  is connected to a gate  106   g  of the organic EL element driving TFT  106 . The gate signal Gn is outputted from a vertical driving circuit  302 . The video signal Dm is outputted from a horizontal driving circuit  301 .  
         [0025]    The organic EL element  120  includes the anode  121 , a cathode  122  and an emissive layer  123  disposed between the anode  121  and the cathode  122 . The cathode  122  is supplied with a negative source voltage CV.  
         [0026]    A storage capacitor  130  is connected to the gate  106   g  of the TFT  106 . That is, one of the electrodes of the storage capacitor  130  is connected to the gate  106   g , and the other electrode is connected to a storage capacitor electrode  131 . The storage capacitor  130  is provided to hold the video signal of the display pixel for one field period by keeping the charge corresponding to the video signal Dm.  
         [0027]    The operation of the EL display device having the above structure is described as follows. The pixel selecting TFT  110  turns on when the gate signal Gn becomes a high level for one horizontal period. Then, the video signal Dm is applied from the drain signal line  60  to the gate  106   g  of the organic EL element driving TFT  106  through the TFT  110 . The conductance of the TFT  106  changes in response to the video signal Dm supplied to the gate  106   g , and a driving current corresponding to the changed conductance is supplied to the organic EL element  120  through the TFT  106 . Then, the organic EL element  120  emits light.  
         [0028]    Next, there will be described a structure of an organic EL panel that includes the device glass substrate  200  and the related structures described above as well as a sealing glass substrate  100  that covers the device glass substrate  200 , with reference to FIGS. 3A and 3B. FIG. 3A is a cross-sectional view corresponding to line A-A of FIG. 1.  
         [0029]    The device glass substrate  200  and the sealing glass substrate  100  are attached together at corresponding edges with a sealing resin  101  made of a resin material that works as an adhesive, such as an epoxy resin. This prevents moisture infiltration from the outside. The device glass substrate  200  and the sealing glass substrate  100  may have the same thickness of about 0.7 mm.  
         [0030]    A plurality of pixels is disposed in a matrix in the pixel region  300  of the device glass substrate  200 . Each of the pixels includes the organic EL element  120 , the organic EL element driving TFT, TFTa, and the pixel selecting TFT, as described above. For example, TFTa has the active layer made of a polysilicon layer. A light shield layer  201  made of Cr (chromium) is disposed below TFTa with an insulating film  210  interposed therebetween. The light shield layer  201  is placed off the position exactly below the active layer of TFTa, as is the case with the display panel of FIG. 6.  
         [0031]    A concave portion (hereafter, referred to as a pocket region  102 ) is formed on an inside surface of the sealing glass substrate  100  facing the device glass substrate  200  by etching. A depth of the pocket region  102  is preferably 0.1 to 0.3 mm, and a desiccant layer  103  is formed on a bottom of the pocket region  102 . The desiccant layer  103  is formed, for example, by coating resin dissolved with powdered calcium oxide or barium oxide and an adhesive on the bottom of the pocket region  102  and hardening the resin by UV irradiation or heating. Forming of the pocket region  102  is for securing a gap between the desiccant layer  103  and the organic EL element  120  to prevent the breaking of the EL element due to contacting with the desiccant layer  103 .  
         [0032]    Furthermore, a light shield layer  104  having a laminated structure of a chromium oxide layer and a chromium layer is formed on a convex portion in the periphery of the pocket region  102 . The light shield layer  104  is disposed to cover the vertical driving circuit  302 . Although not shown in FIG. 3, the light shield layer  104  is extended to cover the horizontal driving circuit  301 .  
         [0033]    In the above-described structure, since the light shield layer  104  is disposed in the region over the vertical driving circuit  302  and the horizontal driving circuit  301 , light incident from the sealing glass substrate  100  can be shielded. Thus, edges of the organic EL panel are clear to improve a display contrast.  
         [0034]    In this embodiment, the forming of the light shield layer  104  is not restricted to the top surface of the convex portion in the periphery of the pocket region  102 , but can be in any position as long as the position is over the vertical driving circuit  302  and the horizontal driving circuit  301 . For example, the light shield layer  104  may be formed on an outside surface of the device glass substrate  200  instead of on the surface of the sealing glass substrate  100 . Specifically, the light shield layer  104  may be placed on the surface of an insulating layer that covers the TFTs, TFTa, TFTb, formed on the device glass substrate  200 , as shown in FIG. 3B. In addition, the pocket region  102  may not be formed in the sealing glass substrate  100 .  
         [0035]    [0035]FIG. 4 is a partial cross-sectional view of the pixel region  300  and the peripheral driving circuit region, for example, the region of the vertical driving circuit  302 . The organic EL element  120  and TFTa are shown in the pixel region  300 , and TFTb is shown in the peripheral driving circuit region. In the pixel region, TFTa is formed over an insulating substrate  202  made of a silica glass or a non-alkali glass with an insulating film  210  interposed therebetween. TFTa includes an active layer  21   1  formed by poly-crystallizing an amorphous silicon film by laser irradiation, a gate insulating film  212 , and a gate electrode  213  made of a metal having a high melting point such as Cr or Mo (molybdenum). The active layer  211  is provided with a channel, and a source  21  Is and a drain  211   d  on each side of the channel.  
         [0036]    The light shield layer  201  is formed on an insulating substrate  202  below TFTa.  
         [0037]    The light shield layer  201  is formed in a region except the region exactly below the active layer  211 .  
         [0038]    An interlayer insulating film  214  laminated with an SiO 2  film, an SiNx film and an SiO 2  film sequentially is formed on the whole surfaces of the gate insulating film  212  and the active layer  211 . There is disposed a driving source line  215  (a drain electrode) connected to a driving source PVdd by filling a contact hole provided correspondingly to a drain  211   d  with a metal such as Al. Furthermore, a first planarization insulating film  216  for planarizing the surface, which is made of, for example, an organic resin is formed on the whole surface. A contact hole is formed in a position corresponding to a source  211   s  in the planarization insulating film  216 . There is formed on the first planarization insulating film  216  a transparent electrode made of ITO (Indium Tin Oxide) and contacting to a source electrode  217  through the contact hole, i.e., an anode layer  218  of the organic EL element  120 . The anode layer  218  is formed in each of the pixels, being isolated as an island.  
         [0039]    A second planarization insulating film  219  is formed in a periphery of the anode layer  218 . The second planarization insulating film  219  on the anode layer  218  is removed. The organic EL element is formed by laminating the anode layer  218 , a hole transport layer  220 , an emissive layer  221 , an electron transport layer  222  and a cathode layer  223  in this order.  
         [0040]    A TFT, TFTb, is formed in the peripheral driving circuit region. A structure of TFTb is the same as that of TFTa in the pixel region except that the light shield layer  201  is not provided in its lower layer. The light shield layer  104  is provided on the sealing glass substrate  100  as described above.  
         [0041]    Next, a manufacturing method of the sealing glass substrate  100  provided with the light shield layer  104  will be described with reference to FIGS.  5 A- 5 F.  
         [0042]    First, as shown in FIG. 5A, chromium oxide and chromium layers  104   a  is formed on the sealing glass substrate  100  by sputtering. Then, as shown in FIG. 5B, a photoresist layer  105  is formed on the chromium oxide and chromium layers  104   a.  The photoresist layer  105  is formed in a region corresponding to the peripheral driving circuit.  
         [0043]    Next, as shown in FIG. 5C, the chromium oxide and chromium layers  104   a  is removed by etching using the photoresist layer  105  as a mask. The light shield layer  104  remains under the photoresist layer  105 .  
         [0044]    As shown in FIG. 5D, the surface of the sealing glass substrate  100  is removed by etching with hydrofluoric acid (HF) by using the photoresist layer  105  and the light shield layer  104  as a mask. The etching continues, for example, for two hours and the sealing glass substrate  100  is etched by approximately 0.3 mm. Here, the light shield layer  104  functions as a mask for etching with the photoresist layer  105 . Thus, the pocket region  102  is formed.  
         [0045]    Then, as shown in FIG. 5E, the photoresist layer  105  is removed. The light shield layer  104  is not removed and remains as it is. As shown in FIG. 5F, the desiccant layer  103  is formed on the bottom of the pocket region  102 . The sealing glass substrate  100  thus produced is attached to the device glass substrate  200  with the sealing resin  101 . Thus, the organic EL panel shown in FIGS. 3A and 3B is completed.  
         [0046]    In this manufacturing method, the chromium oxide and chromium layers are used for forming the pocket region  102  and as the light shield layer  104  in the completed display device. Therefore, an additional step for forming the light shield layer is not required when the display panel has the pocket region.