Patent Publication Number: US-2015076503-A1

Title: Light emitting device and manufacturing method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display device which is manufactured by using an element having a light emitting material interposed between electrodes and emitting light by applying an electric current between the electrodes (light emitting element). 
     2. Description of the Related Art 
     In recent years, a thin lightweight display using a light emitting element has been actively developed. The light emitting element is manufactured by interposing a material which emits light by applying an electric current between a pair of electrodes. Since the light emitting element itself emits light unlike in the case of liquid crystal, a light source such as back light is not required, and the element is very thin. Therefore, it is extremely advantageous to manufacture a thin lightweight display. 
     However, one background of not reaching practical use yet while having such big advantages is a problem of reliability. The light emitting element often deteriorates due to moisture (water) and has a disadvantage of being hard to obtain long-term reliability. The light emitting element which is deteriorated due to water causes a decrease in luminance or does not emit light. It is conceivable that this causes a dark spot (black spot) and shrinkage (a decrease in luminance from an edge portion of a display device) in a display device using the light emitting element. Various countermeasures are suggested to suppress such deterioration (for example, Reference 1: Japanese Patent Laid-Open No. H9-148066, and Reference 2: Japanese Patent Laid-Open No. H7-169567). 
     However, these countermeasures are insufficient and have few effects on a defect that a region where emission intensity is decreased is gradually increased with time. A major reason for such a defect is an influence of water remaining inside a light emitting device besides water entering from outside. Additional countermeasures are desired. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a structure and a manufacturing method of a light emitting device, which reduces the amount of water remaining in the light emitting device. 
     It is another object of the invention to provide a structure and a manufacturing method of a light emitting device, which suppresses the deterioration of a light emitting element due to water remaining in the light emitting device. 
     A light emitting device of the invention includes a thin film transistor, an insulating layer covering the thin film transistor, an electrode which is electrically connected to the thin film transistor through a contact hole formed on the insulating layer, and a light emitting element formed by interposing a light emitting layer between a first electrode which is electrically connected to the electrode and a second electrode. The light emitting device further includes a layer formed of a different material from that of the insulating layer only between the electrode and the first electrode and the insulating layer. 
     Another light emitting device of the invention includes a thin film transistor, an insulating layer covering the thin film transistor, an electrode which is electrically connected to the thin film transistor through a contact hole formed on the insulating layer, and a light emitting element formed by interposing a light emitting layer between a first electrode which is electrically connected to the electrode and a second electrode. The light emitting device further includes a layer formed of a different material from that of the insulating layer only between the electrode and the insulating layer. 
     Another light emitting device of the invention includes a thin film transistor, a first insulating layer covering the thin film transistor, and an electrode which is formed over the first insulating layer and is electrically connected to the thin film transistor through a contact hole formed on the first insulating layer. A wiring and a first electrode of a light emitting element are formed over a second insulating layer which is formed to cover the first insulating layer and the electrode. The first electrode of a light emitting element is electrically connected to the electrode through a contact hole formed on the second insulating layer. The light emitting element is formed by interposing a light emitting layer between the first electrode and a second electrode. The light emitting device further includes a film formed of a different material from the second insulating layer only between a top surface of the second insulating layer and the first electrode of a light emitting element and the wiring. 
     Another light emitting device of the invention includes a thin film transistor, a first insulating layer covering the thin film transistor, and an electrode which is formed over the first insulating layer and is electrically connected to the thin film transistor through a contact hole formed on the first insulating layer. A wiring and a first electrode of a light emitting element are formed over a second insulating layer which is formed to cover the first insulating layer and the electrode. The first electrode of a light emitting element is electrically connected to the wiring through a contact hole formed on the second insulating layer. The light emitting element is formed by interposing a light emitting layer between the first electrode and a second electrode. The light emitting device further includes a film formed of a different material from the second insulating layer only between a top surface of the second insulating layer and the wiring. 
     Another light emitting device of the invention includes a thin film transistor, a first insulating layer covering the thin film transistor, and an electrode which is formed over the first insulating layer and is electrically connected to the thin film transistor through a contact hole formed on the first insulating layer. A wiring and a first electrode of a light emitting element are formed over a second insulating layer which is formed to cover the first insulating layer and the electrode. The first electrode of a light emitting element is electrically connected to the wiring through a contact hole formed on the second insulating layer. The light emitting element is formed by interposing a light emitting layer between the first electrode and a second electrode. The light emitting device further includes a film formed of a different material from the second insulating layer only between a top surface of the first insulating layer and the electrode. 
     Another light emitting device of the invention includes a thin film transistor, a first insulating layer covering the thin film transistor, and an electrode which is formed over the first insulating layer and is electrically connected to the thin film transistor through a contact hole formed on the first insulating layer. A wiring and a first electrode of a light emitting element are formed over a second insulating layer which is formed to cover the first insulating layer and the electrode. The first electrode of a light emitting element is electrically connected to the wiring through a contact hole formed on the second insulating layer. The light emitting element is formed by interposing a light emitting layer between the first electrode and a second electrode. The light emitting device further includes a first film formed of a different material from the first insulating layer only between a top surface of the first insulating layer and the electrode, and a second film formed of a different material from the second insulating layer only between a top surface of the second insulating layer and the wiring and the first electrode of a light emitting element. 
     Another light emitting device of the invention includes a thin film transistor, a first insulating layer covering the thin film transistor, and an electrode which is formed over the first insulating layer and is electrically connected to the thin film transistor through a contact hole formed on the first insulating layer. A wiring and a first electrode of a light emitting element are formed over a second insulating layer which is formed to cover the first insulating layer and the electrode. The first electrode of a light emitting element is electrically connected to the wiring through a contact hole formed on the second insulating layer. The light emitting element is formed by interposing a light emitting layer between the first electrode and a second electrode. The light emitting device further includes a first film formed of a different material from the first insulating layer only between a top surface of the first insulating layer and the electrode, and a second film formed of a different material from the second insulating layer only between a top surface of the second insulating layer and the wiring. 
     According to a method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed, and the etching stopper film is removed by using the electrode and the first electrode of a light emitting element as a mask. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, the etching stopper film is removed by using the electrode as a mask, and a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface formed over a substrate, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed, the etching stopper film is removed by using the electrode and the first electrode of a light emitting element as a mask, and the substrate is heat-treated. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface formed over a substrate, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, the etching stopper film is removed by using the electrode as a mask, a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed, and the substrate is heat-treated. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface formed over a substrate, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed, the etching stopper film is removed by using the electrode and the first electrode of a light emitting element as a mask, and the substrate is heat-treated in a vacuum. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface formed over a substrate, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, the etching stopper film is removed by using the electrode as a mask, a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed, and the substrate is heat-treated in a vacuum. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface formed over a substrate, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed, and the etching stopper film is removed by using the electrode and the first electrode of a light emitting element as a mask. Then, a bank is formed to cover an edge portion of the first electrode of a light emitting element, the substrate is heat-treated in a vacuum, a light emitting film is continuously formed over the bank to the first electrode, and a second electrode is formed to cover the bank and the light emitting film. 
     According to another method for manufacturing a light emitting device of the invention, a thin film transistor is formed over an insulating surface formed over a substrate, and an insulating layer is formed over the thin film transistor. An etching stopper film is formed of a different material from the insulating layer over the insulating layer, and an electrode which penetrates the etching stopper film and the insulating layer and is electrically connected to the thin film transistor is formed. Subsequently, the etching stopper film is removed by using the electrode as a mask, and a first electrode of a light emitting element, which is electrically in contact with a part of the electrode, is formed. Then, a bank is formed to cover an edge portion of the first electrode of a light emitting element, the substrate is heat-treated in a vacuum, a light emitting film is continuously formed over the bank to the first electrode of a light emitting element, and a second electrode is formed to cover the bank and the light emitting film. 
     In a light emitting device having a structure of the invention, the amount of water remaining inside the light emitting device is reduced. Further, the deterioration of a light emitting element due to water remaining inside can be suppressed. 
     In a light emitting device manufactured by a method for manufacturing a light emitting element of the present invention, the amount of water remaining inside the light emitting device is reduced. Further, the deterioration of a light emitting element due to water remaining inside can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are cross-sectional views showing a light emitting device of the invention. 
         FIGS. 2A to 2C  are cross-sectional views showing a light emitting device of the invention. 
         FIGS. 3A to 3D  are cross-sectional views showing a method for manufacturing a light emitting device of the invention. 
         FIGS. 4A to 4D  are cross-sectional views showing a method for manufacturing a light emitting device of the invention. 
         FIGS. 5A to 5D  are cross-sectional views showing a method for manufacturing a light emitting device of the invention. 
         FIG. 6  is a top view showing an example of a module structure of the invention. 
         FIGS. 7A to 7E  show examples of an electronic device of the invention. 
         FIGS. 8A and 8B  show examples of a structure of a light emitting device of the invention. 
         FIGS. 9A to 9F  show examples of a pixel circuit. 
         FIG. 10  shows an example of a protective circuit. 
         FIGS. 11A and 11B  are cross-sectional views showing a light emitting device of the invention. 
         FIGS. 12A and 12B  are pictures showing an upper surface of a light emitting device of the invention. 
         FIGS. 13A and 13B  are pictures showing an upper surface of a light emitting device of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment mode of the present invention will be described with reference to the attached drawings. The invention is not limited to the following description. As is easily known to a person skilled in the art, the mode and the detail of the invention can be variously changed without departing from the purpose and the scope of the invention. Thus, the invention is not interpreted while limiting to the following description of the embodiment mode. Note that a reference numeral may not be given to a portion having similar function and shape, and the description thereof may be omitted. 
     Embodiment Mode 1 
       FIGS. 1A and 1B  are cross-sectional views showing part of a light emitting device of the invention. A light emitting device of the invention includes: a base insulating layer  101  formed over a substrate  100 ; a thin film transistor  105  having a semiconductor layer  102  formed over the base insulating layer  101  as an active layer, a gate insulating layer  103 , and a gate electrode  104 ; and a light emitting element  106 . The thin film transistor  105  is covered with an insulating layer  107 . Over the insulating layer  107 , electrodes  108  and  109  of the thin film transistor  105  which electrically connect to the semiconductor layer  102  through contact holes penetrating the insulating layer  107  and the gate insulating layer  103 , and a wiring (not shown) are formed. Note that the wiring may be formed at the same time as the electrodes  108  and  109 , but may be separately formed. A first electrode  110  of the light emitting element  106  is formed to partially overlap and electrically connect to the electrode  109  of the thin film transistor  105 . An edge portion of the first electrode  110  is covered with a bank  113 . The light emitting element  106  is formed by interposing a light emitting layer  112 , which is continuously formed over the bank  113  to the first electrode  110 , between the first electrode  110  and a second electrode  111 . Light can be emitted from a portion of the light emitting layer  112 , which is directly sandwiched between the first electrode  110  and the second electrode  111 . 
     The insulating layer  107  is preferably formed of a material which has a self-planarizing property and can reduce irregularity caused by the thin film transistor  105  thereunder, in order to improve an aperture ratio. For example, a self-planarizing application film of acrylic, polyimide, siloxane (a material which is composed of a skeleton formed by the bond of silicon and oxygen and which includes either an organic group containing at least hydrogen (such as an alkyl group or aromatic hydrocarbon) or a fluoro group or both as a substituent), or the like is preferably used. 
     In  FIG. 1A , an etching stopper film  114  remains only between the electrodes  108 ,  109 , and  110  and the wiring, which are formed over the insulating layer  107 , and the insulating layer  107 . In other words, it remains only between the electrodes  108  and  109  of the thin film transistor  105 , the first electrode  110  of the light emitting element  106 , and the wiring (not shown) formed between the insulating layer  107  and the bank  113 , and a top surface of the insulating layer  107 . When the electrodes  108 ,  109 , and  110  and the wiring formed between the insulating layer  107  and the bank  113  are formed by etching, the etching stopper film  114  has a function of preventing the insulating layer  107  from being etched. In the case of the light emitting device of the invention in this embodiment mode, the etching stopper film  114  is formed to cover the insulating layer  107 ; a contact hole is formed; the electrodes  108 ,  109 , and  110  and the wiring formed between the insulating layer  107  and the bank  113  are formed thereover; and then, an exposed portion of the etching stopper film is removed. Thus, the light emitting device of the invention can be formed to have such a structure that the etching stopper film  114  remains only below the electrodes  108  and  109 , the wiring formed between the insulating layer  107  and the bank  113 , and the first electrode  110  of the light emitting element  106 . 
     The etching stopper film  114  is formed of a water impermeable material which is different from the insulating layer  107 , with or without conductivity. A film which mainly contains silicon nitride is preferably used as the etching stopper film. A material, having a sufficiently lower etching rate than those of the electrodes  108  and  109  at the time of etching to form the electrodes  108  and  109 , is used. 
     In the light emitting device of the invention having the structure, the etching stopper film  114  is not entirely formed over the insulating layer  107  at the time of performing heat treatment for removing water in the insulating layer  107 . Therefore, water is effectively removed from a portion where the etching stopper film  114  is removed, and the amount of water remaining in the insulating layer  107  can be reduced. 
     Since the etching stopper film  114  is not entirely formed over the insulating layer  107 , even when a small amount of water remaining in the insulating layer  107  seeps into the layer where the light emitting element  106  is formed, the water is not locally concentrated. Thus, an influence of water seeping from the insulating layer  107  on the light emitting element  106  can be reduced. If the etching stopper film is entirely formed over the insulating layer  107 , the water remaining in the insulating layer  107  seeps, in a concentrated manner, from a tiny pinhole in the etching stopper film or at the boundary with the electrodes  108  and  109 . Therefore, deterioration of the light emitting element located in the periphery thereof is promoted. 
     Since the deterioration of the light emitting element  106  due to water can be suppressed, improvement in display quality and reliability can be realized. 
       FIG. 1B  is different from  FIG. 1A  in the way that there is an etching stopper film  115  only below electrodes  108  and  109  of a thin film transistor  105  and a wiring (not shown) formed between an insulating layer  107  and a bank  113 , but not below a first electrode  110  of a light emitting element  106 . Only the timing of removing the etching stopper film  115  is different, but an effect and a structure of the etching stopper film are similar to that in  FIG. 1A . In other words, the steps are as follows: the insulating layer  107  and the etching stopper film  115  are formed; the electrodes  108  and  109  of the thin film transistor and the wiring are formed; and an exposed portion of the etching stopper film  115  is removed before forming the first electrode  110  of the light emitting element  106 . 
     In order to completely remove a portion of the etching stopper film  115  which is to be removed, it is effective to perform overetching to some extent. In this case, the insulating layer may be etched in some cases, depending on an etching rate of the insulating layer under removing conditions of the etching stopper film. For example, schematic diagrams of the structures corresponding to  FIGS. 1A and 1B  in the case of performing overetching are shown in  FIGS. 11A and 11B , respectively. As shown, a portion of the insulating layer  107  where the etching stopper film  115  remains is thicker than a portion of the insulating layer  107  where the etching stopper film  115  is removed. 
     Note that overetching can be applied to other structures of the invention. In that case, a portion of the insulating layer where the etching stopper remains is thicker than a portion of the insulating layer where the etching stopper film is removed. 
       FIG. 2A  is a cross-sectional view of a light emitting device of the invention, in which an electrode of a thin film transistor is formed in a different layer from a first electrode of a light emitting element. The light emitting device of the invention includes: a base insulating layer  301  formed over a substrate  300 ; a thin film transistor  315  having a semiconductor layer  302  formed over the base insulating layer  301  as an active layer, a gate insulating layer  303 , and a gate electrode  304 ; and a light emitting element  316 . The thin film transistor  315  is covered with a first insulating layer  305 . Over the first insulating layer  305 , a wiring (not shown) and electrodes  306  and  307  of the thin film transistor  315  which electrically connect to the semiconductor layer  302  through contact holes penetrating the first insulating layer  305  and the gate insulating layer  303  are formed. Note that the wiring may be formed at the same time as the electrodes  306  and  307 , or may be separately formed. The wiring (not shown), the electrodes  306  and  307  of the thin film transistor  315 , and the first insulating layer  305  are covered with a second insulating layer  309 . Over the second insulating layer  309 , a first electrode  311  of the light emitting element  316 , which electrically connects to the electrode  307  of the thin film transistor  315  through a contact hole penetrating the second insulating layer  309 , is formed. The light emitting element  316  is formed by interposing a light emitting layer  313  between the first electrode  311  and a second electrode  314  which are formed over the second insulating layer  309 . A wiring  310  may be formed over the second insulating layer  309 . When the second electrode  314  has high resistance, the wiring  310  can be used as an auxiliary wiring by forming a contact hole reaching the wiring  310  in a bank  312  and connecting the second electrode  314  to the wiring  310 . Naturally, the wiring  310  may be used for other applications as well as an auxiliary wiring. 
     In  FIG. 2A , either the first insulating layer  305  or the second insulating layer  309  is formed of a material which has a self-planarizing property and can reduce irregularity caused by the thin film transistor  315  thereunder or the like, in order to improve an aperture ratio. For example, a self-planarizing application film of acrylic, polyimide, siloxane, or the hie is used. 
     An etching stopper film  308  is formed between a top surface of the first insulating layer  305  and the electrodes  306  and  307  of the thin film transistor  315  and the wiring (not shown) formed between the first insulating layer  305  and the bank  312 . When the electrodes  306  and  307  and the wiring formed between the first insulating layer  305  and the bank  312  are formed by etching, the etching stopper film  308  has a function of preventing the first insulating layer  305  from being etched. In the case of the light emitting device of the invention in this embodiment mode, the etching stopper film is formed to cover the first insulating layer  305 ; a conductive film is formed thereover, patterned, and etched to form the electrodes  306  and  307  and the wiring; and an exposed portion of the etching stopper film is removed. Thus, the light emitting device of the invention can be formed to have such a structure that the etching stopper film  308  remains only below the electrodes  306  and  307  and the wiring formed between the first insulating layer  305  and the bank  312 . 
     The etching stopper film  308  is formed of a water impermeable material which is different from the first insulating layer  305 , with or without conductivity. A film which mainly contains silicon nitride is preferably used as the etching stopper film. A material, having a sufficiently lower etching rate than those of the electrodes  306  and  307  and the wiring formed between the first insulating layer  305  and the bank  312  at the time of etching to form the electrodes  306  and  307  and the wiring, is used. 
     An etching stopper film  317  is also formed between a top surface of the second insulating layer  309  and the first electrode  311  of the light emitting element  316  and the wiring  310 . When the first electrode  311  and the wiring  310  are formed by etching, the etching stopper film  317  has a function of preventing the second insulating layer  309  from being etched. The etching stopper film  317  is formed as in the case with the etching stopper film  308 : an etching stopper film is formed to cover the second insulating layer  309 , the first electrode  311  and the wiring  310  are formed thereover, and an exposed portion of the etching stopper film is removed. Thus, the light emitting device of the invention can be formed to have such a structure that the etching stopper film  317  remains only below the first electrode  311  and the wiring  310 . 
     The etching stopper film  317  is formed of a material different from the second insulating layer  309 , with or without conductivity. A film which mainly contains silicon nitride is preferably used as the etching stopper film. A material, having a sufficiently lower etching rate than those of the first electrode  311  and the wiring  310  at the time of etching to form the first electrode  311  and the wiring  310 , is used. 
     When either the etching stopper film  308  or the etching stopper film  317  is not required in terms of the etching rate, the etching stopper film may not be formed. 
     In the light emitting device of the invention having the structure, the etching stopper films are not entirely formed over the first insulating layer  305  and the second insulating layer  309  at the time of performing heat treatment for removing water in the insulating layers. Therefore, water is effectively removed from a portion where the etching stopper films are removed, and the amount of water remaining in the first insulating layer  305  and the second insulating layer  309  can be reduced. 
     Since the etching stopper film  317  is not entirely formed over the insulating layer, even when a small amount of water remaining in the insulating layer seeps into the layer where the light emitting element  316  is formed, the water is not locally concentrated. Thus, an influence of water seeping from the second insulating layer  309  on the light emitting element  316  can be reduced. 
     Since the deterioration of the light emitting element  316  due to water can be suppressed, improvement in display quality and reliability can be realized. 
       FIG. 2B  has a structure almost similar to  FIG. 2A , but it shows an example that a first insulating layer  350  is formed of an inorganic film of silicon oxide, silicon nitride, or the like. Since such a film often has a sufficient difference in an etching rate with electrodes  351  and  352  of a thin film transistor  353  and a wiring, an etching stopper film between the electrodes  351  and  352  and the wiring formed between a first insulating layer  350  and a second insulating layer  309  and a top surface of the first insulating layer  350  may not be formed. In that case, the first insulating layer  350  directly reflects irregularity thereunder; therefore, the second insulating layer  309  is formed of a self-planarizing material. For example, a self-planarizing application film of acrylic, polyimide, siloxane, or the like corresponds to this. Note that other structures and effects are similar to those in  FIG. 2A , so description is omitted. 
       FIG. 2C  has a structure almost similar to  FIG. 2A , but the wiring  310  in  FIG. 2A  is used not as an auxiliary wiring for reducing apparent resistance of a second electrode of a light emitting element but as a wiring  370  electrically connecting to the electrode  306  of the thin film transistor  315 . The first electrode  311  of the light emitting element is connected to the electrode  307  of the thin film transistor  315  not directly but through a wiring  371 . Note that other structures and effects are similar to those in  FIG. 2A , so description is omitted. 
     In this embodiment mode, the structure that an etching stopper is formed between an electrode of a transistor and an insulating layer thereunder, the structure that an etching stopper is formed between a wiring and an insulating layer thereunder, the structure that an etching stopper film is formed between an electrode of a light emitting element and an insulating layer thereunder, and the structure that an etching stopper is not formed between an electrode of a light emitting element and an insulating layer thereunder are disclosed, and patterns that each of the structures is applied to the structure that an electrode of a transistor and an electrode of a light emitting element are formed in the same layer, the structure that they are formed in different layers, and the like are described. However, the elements can be freely combined with each other. 
     In this embodiment mode, the base insulating layer, the insulating layer, the first insulating layer, and the second insulating layer are described as single layers; however, they may have multilayer structures of two or more layers. In addition, the etching stopper film may also be a single layer or have a multilayer structure. 
     Embodiment Mode 2 
     A method for manufacturing a light emitting element of the invention is described in this embodiment mode with reference to  FIGS. 3A to 3D  and  FIGS. 4A to 4D . 
     An insulating layer  801  is formed over a substrate  800 , and a semiconductor layer is then formed over the insulating layer  801  ( FIG. 3A ). 
     Light transmitting glass, quartz, plastic (such as polyimide, acrylic, polyethylene terephthalate, polycarbonate, polyacrylate, or polyethersulfone), or the like can be used as a material of the substrate  800 . The substrate thereof may be used after being polished by CMP or the like, if necessary. In this embodiment mode, a glass substrate is used. 
     The insulating layer  801  is formed in order to prevent an element exerting an adverse influence on characteristics of the semiconductor layer, such as an alkali metal or an alkaline earth metal contained in the substrate  800 , from diffusing into the semiconductor layer. Silicon oxide, silicon nitride, silicon oxide containing nitrogen, silicon nitride containing oxygen, or the like can be used as a material thereof, and the insulating layer is formed to be a single layer or to have a laminated structure. Note that the insulating layer  801  is not necessarily required to be formed when diffusion of an alkali metal or an alkaline earth metal need not be worried about. 
     The subsequently formed semiconductor layer is obtained by performing laser crystallization on an amorphous silicon film in this embodiment mode. An amorphous silicon film is formed to be 25 nm to 100 nm (preferably, 30 nm to 60 nm) in thickness over the insulating layer  801 . A known method such as a sputtering method, a low pressure CVD method, or a plasma CVD method can be used as a manufacturing method thereof. Subsequently, the amorphous silicon film is heat-treated at a temperature of 500° C. for one hour for dehydrogenation. 
     Then, the amorphous silicon film is crystallized with the use of a laser irradiation apparatus to form a crystalline silicon film. As to the laser crystallization in this embodiment mode, an excimer laser is used, and an emitted laser beam is processed to have a linear beam spot with an optical system. The amorphous silicon film is irradiated therewith to be a crystalline silicon film, and is used as the semiconductor layer. 
     As another method for crystallizing an amorphous silicon film, there is a crystallizing method only by heat treatment, a crystallizing method by heat treatment with the use of a catalytic element which promotes crystallization, or the like. Nickel, iron, palladium, tin, lead, cobalt, platinum, copper, gold, or the like can be used as the element which promotes crystallization. By using the element, crystallization can be performed at a lower temperature in a shorter time, compared to the case of performing crystallization only by heat treatment. Therefore, a glass substrate or the like is less damaged. In the case of performing crystallization only by heat treatment, a highly heat resistant quartz substrate or the like needs to be used as the substrate  800 . 
     Subsequently, addition of a very small amount of impurities, so-called channel doping, is performed on the semiconductor layer to control a threshold value, if necessary. An N type or P type impurity (phosphorus, boron, or the like) is added by an ion doping method to obtain a required threshold value. 
     Thereafter, the semiconductor layer is patterned to have a predetermined shape as shown in  FIG. 3A , thereby obtaining an island-shaped semiconductor layer  802 . A photoresist is applied to the semiconductor layer, exposed to light, and baked to form a resist mask having a predetermined shape over the semiconductor layer. Etching is performed using the mask. Thus, the patterning is performed. 
     A gate insulating layer  803  is formed to cover the semiconductor layer  802 . The gate insulating layer  803  is formed of an insulating layer containing silicon by a plasma CVD method or a sputtering method to be 40 nm to 150 nm in thickness. 
     A gate electrode  804  is formed over the gate insulating layer  803 . The gate electrode  804  may be formed by using an element of Ta, W, Ti, Mo, Al, Cu, Cr, or Nd, or by using an alloy material or compound material which mainly contains the element. A semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus may be used. Alternatively, an AgPdCu alloy may be used. 
     The gate electrode  804  is formed to be a single layer in this embodiment mode; however, it may be formed to have a laminated structure of two or more layers (for example, a laminated structure of a tungsten layer as a lower layer and a molybdenum layer as an upper layer). The above-mentioned material may be used, even in the case of forming a gate electrode having a laminated structure. A combination thereof may also be appropriately selected. 
     The gate electrode  804  is processed by etching using a mask of a photoresist. 
     The semiconductor layer  802  is added with a highly concentrated impurity with the use of the gate electrode  804  as a mask. According to this step, a thin film transistor including the semiconductor layer  802 , the gate insulating layer  803 , and the gate electrode  804  is formed. 
     A manufacturing step of a thin film transistor is not particularly limited, and it may be appropriately changed so that a transistor having a desired structure can be formed. 
     A top gate thin film transistor using a crystalline silicon film which is crystallized by employing laser crystallisation is used in this embodiment mode; however, a bottom gate thin film transistor using an amorphous semiconductor film can be used for a pixel portion. Silicon germanium as well as silicon can be used for an amorphous semiconductor. In the case of using silicon germanium, the concentration of germanium is preferably approximately 0.01 atomic % to 4.5 atomic %. 
     A microcrystalline semiconductor (semi-amorphous semiconductor) film, in which a crystal grain of 0.5 nm to 20 nm can be observed within an amorphous semiconductor, may be used. A microcrystalline state in which a crystal grain of 0.5 nm to 20 nm can be observed is also referred to as a microcrystal (μc). 
     Semi-amorphous silicon (also referred to as SAS) that is a semi-amorphous semiconductor can be obtained by performing glow discharge decomposition on a silicide gas. SiH 4  is used as a typical silicide gas. In addition, Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , SiF 4 , or the like can also be used as the silicide gas. The silicide gas may be diluted with hydrogen, or hydrogen and one or more rare gas elements of helium, argon, krypton, and neon, thereby making formation of the SAS easy. At this time, it is preferable to dilute the silicide gas so that a dilution ratio ranges from 10 times to 1000 times. Reaction production of a film by glow discharge decomposition may be performed with pressures in the range of 0.1 Pa to 133 Pa. High-frequency powers of 1 MHz to 120 MHz, preferably, 13 MHz to 60 MHz may be supplied to form a glow discharge. A substrate heating temperature is preferably 300° C. or less, and a recommended substrate heating temperature is in the range of 100° C. to 250° C. 
     In the thus formed SAS, a Raman spectrum is shifted to a lower frequency side than 520 cm −1 . A diffraction peak of (111) or (220) to be caused by a crystal lattice of silicon is observed in X-ray diffraction. The SAS contains hydrogen or halogen of at least 1 atomic % or more to terminate a dangling bond. It is desirable that an atmospheric constituent impurity such as oxygen, nitrogen, or carbon is 1×10 20 /cm 3  or less as an impurity element in the film; specifically, an oxygen concentration is 5×10 19 /cm 3  or less, preferably 1×10 19 /cm 3  or less. When the SAS is processed into a TFT, mobility thereof is as follows: μ=1 cm 2 /Vsec to 10 cm 2 /Vsec. The SAS may be further crystallized with a laser. 
     Subsequently, an insulating layer  805  is formed to cover the gate electrode  804  and the gate insulating layer  803 . The insulating layer  805  may be formed of acrylic, polyimide, or siloxane. In this embodiment mode, the insulating layer  805  is formed of siloxane ( FIG. 3B ). 
     An etching stopper film  806  is formed over the insulating layer  805 . The etching stopper film  806  is formed of a material different from the insulating layer  805 . Specifically, the etching stopper film is formed of a material having sufficient selectivity to the insulating layer  805  so as not to etch the insulating layer  805 , such as silicon nitride or silicon nitride containing oxygen, when subsequently formed electrodes  807  and  808  are formed by etching. Note that conductivity of a material of the etching stopper film is not necessarily required. In this embodiment mode, a silicon nitride film is formed as the etching stopper film  806  ( FIG. 3C ). 
     A contact hole penetrating the etching stopper film  806 , the insulating layer  805 , and the gate insulating layer  803  is formed to reach the semiconductor layer  802  ( FIG. 3D ). 
     The contact hole may be formed and the insulating layer on the periphery of the substrate may be removed using a resist by dry etching or wet etching. However, in some cases, the contact hole is preferably formed by etching plural times/multiple etching under different conditions, depending on a material of the insulating layer  805 , the etching stopper film  806 , and the gate insulating layer  803 . 
     When wet etching is employed or the resist is removed by using a separating solution, it is conceivable that water enters the insulating layer  805 . Therefore, heat treatment may be performed in order to remove water in the insulating layer  805  after removing the resist. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. However, water cannot be sufficiently removed at this time in some cases, since the insulating layer  805  is covered with the etching stopper film  806 . 
     A conductive layer is formed to cover the contact hole, the insulating layer  805 , and the etching stopper film  806 . The conductive layer is processed to have a desired shape, thereby forming the electrodes  807  and  808  and a wiring (not shown). They may be single layers of aluminum, copper, or the like; however, they are formed to have a laminated structure of molybdenum, aluminum, and molybdenum from a TFT side in this embodiment mode. The conductive layer may also have another laminated structure of titanium, aluminum, and titanium, or titanium, titanium nitride, aluminum, and titanium from a TFT side. 
     The conductive layer may be processed using a resist by dry etching or wet etching. The etching stopper film  806  can prevent the insulating layer  805  from being etched. As in the case of forming the contact hole, when wet etching is employed or the resist is removed by using a separating solution, it is conceivable that water enters the insulating layer  805 . Therefore, heat treatment may be performed to remove water in the insulating layer  805  after removing the resist. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. However, water cannot be sufficiently removed at this time in some cases, since the insulating layer  805  is covered with the etching stopper film  806 . 
     According to the steps, a thin film transistor  809  in a pixel portion is completed. 
     After a light transmitting conductive layer is formed to partially cover the electrode  808  of the thin film transistor  809  in the pixel portion, the light transmitting conductive layer is processed to form a first electrode  810  ( FIG. 4A ). 
     The light transmitting conductive layer may be processed using a resist by dry etching or wet etching. As in the case of forming the contact hole and the electrodes  807  and  808 , when wet etching is employed or the resist is removed by using a separating solution, it is conceivable that water enters the insulating layer  805 . Therefore, heat treatment may be performed to remove water in the insulating layer  805  after removing the resist. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. However, water cannot be sufficiently removed at this time in some cases, since the insulating layer  805  is covered with the etching stopper film  806 . 
     The first electrode  810  is electrically in contact with the electrode  808  of the thin film transistor  809 . The first electrode  810  may be formed of ITO (indium tin oxide), ITO containing silicon oxide, IZO (Indium Zinc Oxide) in which indium oxide contains zinc oxide of 2% to 20%, zinc oxide, GZO (Gallium Zinc Oxide) in which zinc oxide contains gallium, or the like. 
     An exposed portion of the etching stopper film  806  is removed by etching using the electrodes  807  and  808  of the thin film transistor  809  and the first electrode  810  of a light emitting element as a mask. The etching may be performed by either wet etching or dry etching. In this embodiment mode, dry etching is employed ( FIG. 4B ) 
     Note that the first electrode  810  and the etching stopper film  806  may be simultaneously etched, when an etching method having high selectivity between the insulating layer  805  and the etching stopper film is employed. Another method for removing the etching stopper film  806  by controlling the etching time to etch the etching stopper film  806  at the same time as the etching of the electrodes  807  and  808  is conceivable. 
     When wet etching is employed, it is conceivable that water enters the insulating layer  805 . Therefore, heat treatment may be performed to remove water in the insulating layer  805  after removing the resist. Since the insulating layer  805  is not covered with the etching stopper film  806 , water can be sufficiently removed. In the case where there is a step of exposing to water again in the following steps, heat treatment may be performed at a time later after the step. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. 
     An insulating layer made of an organic material or an inorganic material is formed to cover the etching stopper film  806  and the first electrode  810 . The insulating layer is processed to cover an edge portion of the first electrode and partially expose the first electrode, thereby forming a bank  811 . The bank  811  is preferably formed of a photosensitive organic material (acrylic, polyimide, or the like), but may be formed of a non-photosensitive organic material or an inorganic material. In this embodiment mode, photosensitive polyimide is used. An end face of the bank  811 , facing the first electrode, has curvature, and preferably has a tapered shape in which the curvature continuously changes. Note that the bank  811  may be mixed with a black material such as a pigment or carbon, and may be used as a black matrix ( FIG. 4C ). 
     The bank  811  may be processed by light-exposure and development when a photosensitive material is used, or by dry etching or wet etching using a resist when a non-photosensitive material is used. When development is performed using a photosensitive material, wet etching is employed, or the resist is removed by using a separating solution, it is conceivable that water enters the bank  811  and the insulating layer  805 . Therefore, heat treatment may be performed to remove water in the bank  811  and the insulating layer  805  after removing the resist. 
     Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition, but it is effective to perform heat treatment in a vacuum. Since the insulating layer  805  is not entirely covered with the etching stopper film  806 , water can be sufficiently removed. In this embodiment mode, heat treatment is performed in a vacuum. 
     A light emitting layer  812  is formed to cover an exposed portion of the first electrode  810  which is not covered with the bank  811 . The light emitting layer  812  may be formed by any of an evaporation method, an ink-jet method, a spin coating method, and the like ( FIG. 4D ). 
     A second electrode  813  is formed to cover the light emitting layer  812 . Thus, a light emitting element  814  including the first electrode  810 , the light emitting layer  812 , and the second electrode  813  can be manufactured. 
     Note that it is preferable to form the light emitting layer  812  and the second electrode without exposing to atmospheric air after forming the bank  811  and heat treating in a vacuum. This is because atmospheric water can be prevented from being introduced into the light emitting device. 
     A silicon oxide film containing nitrogen may be formed as a passivation film by a plasma CVD method. In the case of using a silicon oxide film containing nitrogen, a silicon oxynitride film formed from SiH 4 , N 2 O, and NH 3 , a silicon oxynitride film formed from SiH 4  and N 2 O, or a silicon oxynitride film formed from a gas in which SiH 4  and N 2 O are diluted with Ar may be formed by a plasma CVD method. 
     A silicon oxynitride hydride film formed from SiH 4 , N 2 O, and H 2  may be used as the passivation film. Naturally, the structure of the passivation film is not limited to a single layer structure. The passivation film may have a single layer structure or laminated structure of another insulating layer containing silicon. In addition, a multilayer film of a carbon nitride film and a silicon nitride film, a multilayer film of styrene polymer, a silicon nitride film, or a diamond like carbon film may be substituted for the silicon oxide film containing nitrogen. 
     Then, a display portion is sealed. In the case of using an opposing substrate for sealing, the opposing substrate is attached by using an insulating sealant so that an external connection portion is exposed. A depression may be formed on the opposing substrate, and a drying agent may be attached thereto. A space between the opposing substrate and a substrate over which an element is formed may be filled with a dry inert gas such as nitrogen, or the opposing substrate may be formed by entirely applying a sealant to the pixel portion. It is preferable to use an ultraviolet curing resin or the like as the sealant. The sealant may be mixed with a drying agent or particles for keeping a gap constant. Then, the light emitting device is completed by attaching a flexible wiring board to the external connection portion. 
     Note that either an analog video signal or a digital video signal may be used for a light emitting display device of the invention having a display function. In the case of using a digital video signal, the video signal can be divided into a video signal using voltage and a video signal using current. A video signal, inputted to a pixel when a light emitting element emits light, includes a constant voltage video signal and a constant current video signal. The constant voltage video signal includes a signal in which voltage applied to a light emitting element is constant and a signal in which current applied to a light emitting element is constant. The constant current video signal includes a signal in which voltage applied to a light emitting element is constant and a signal in which current applied to a light emitting element is constant. Drive with the signal in which voltage applied to a light emitting element is constant is constant voltage drive, and that with the signal in which current applied to a light emitting element is constant is constant current drive. By constant current drive, constant current is applied to a light emitting element, regardless of a change in resistance of the light emitting element. For a light emitting display device of the invention and a driving method thereof, either a driving method using voltage of a video signal or a driving method using current of a video signal may be employed, and either constant voltage drive or constant current drive may be employed. 
     Hereinabove, a method for manufacturing a light emitting device of the invention is described. 
     Embodiment Mode 3 
     A method for manufacturing a light emitting device having the structure shown in  FIG. 2B  is described in this embodiment mode with reference to  FIGS. 5A to 5D . Description up to formation of a base insulating layer  801 , a semiconductor layer  802 , a gate insulating layer  803 , and a gate electrode  804  over a substrate  800  ( FIG. 5A ) is similar to the description of  FIG. 3A  in Embodiment. Mode 2; therefore, the description is omitted here. 
     After forming the gate electrode  804 , a first insulating layer  850  is formed. The first insulating layer  850  can be formed of an organic insulating layer of acrylic or polyimide, an inorganic insulating layer mainly containing silicon oxide or silicon nitride, siloxane, or the like. In this embodiment mode, silicon oxide is used for the first insulating layer  850 . In the case of forming the first insulating layer  850  by applying a self-planarizing material of acrylic, polyimide, siloxane, or the like, the light emitting device has such a structure as shown in  FIG. 2A . 
     A contact hole is formed to penetrate the first insulating layer  850  and the gate insulating layer  803 . Then, a conductive layer is formed to cover the contact hole and the first insulating layer  850 . The conductive layer is processed to have a desired shape, thereby forming electrodes  851  and  852  and a wiring (not shown). They may be single layers of aluminum, copper, or the like; however, they may be formed to have laminated structures. As a laminated wiring, the following laminated structure can be employed: molybdenum, aluminum, and molybdenum; titanium, aluminum, and titanium; titanium, titanium nitride, aluminum, and titanium; or the like, from a semiconductor layer side. 
     Since silicon oxide is used for the first insulating layer  850  in this embodiment mode, an etching stopper film need not be formed. However, in the case of using, for the first insulating layer  850 , a material without a sufficient difference in an etching rate at the time of etching the electrodes  851  and  852  and the wiring (not shown), an etching stopper film needs to be formed. In that case, after forming the electrodes  851  and  852  and the wiring, an exposed portion of the etching stopper film is preferably removed using the electrodes and the wiring as a mask. 
     The conductive layer may be processed and the contact hole may be formed by dry etching or wet etching using a resist. When wet etching is employed or the resist is removed by using a separating solution, the insulating layer is exposed to water. Therefore, heat treatment may be performed after removing the resist. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. However, heat treatment for dehydration is not necessarily required in some cases, when the first insulating layer  850  is highly water impermeable. 
     According to the steps, a thin film transistor  853  in a pixel portion is completed ( FIG. 5B ) 
     Subsequently, a second insulating layer  854  is formed to cover the electrodes  851  and  852 , the wiring, and the first insulating layer  850 . The second insulating layer  854  may be formed of a similar material to the above-mentioned material of the first insulating layer  850 . In this embodiment mode, the first insulating layer  850  is formed of silicon oxide which does not have a self-planarizing property and directly reflects irregularity thereunder. Therefore, the second insulating layer is formed of a self-planarizing material of acrylic, polyimide, or siloxane. In this embodiment mode, siloxane is used for the second insulating layer  854  ( FIG. 5C ). 
     An etching stopper film  855  is formed over the second insulating layer  854 . The etching stopper film  855  is formed of a water impermeable material different from the second insulating layer  854 . Specifically, the etching stopper film is formed of a material having high selectivity to a wiring  856  and a first electrode  857  of a light emitting element to be formed later, such as silicon nitride or silicon nitride containing oxygen. Note that conductivity is not necessarily required for a material of the etching stopper film. In this embodiment mode, a silicon nitride film is formed as the etching stopper film  855 . 
     After forming the etching stopper film  855 , a conductive film is formed thereover and processed to form a wiring  856 . The wiring  856  corresponds to the wiring  310  in  FIG. 2A . Since the etching stopper film  855  is formed, the wiring  856  can be formed without largely etching a lower film at the time of etching the conductive film ( FIG. 5D ). 
     A first electrode  857  of a light emitting element is formed. The first electrode  857  is electrically connected to the electrode  852  of the thin film transistor  853  through a contact hole formed in the second insulating layer  854 . After forming a light transmitting conductive layer, the first electrode  857  is formed by processing the conductive layer. The first electrode  857  may be formed of ITO (indium tin oxide), ITO containing silicon oxide, IZO (Indium Zinc Oxide) in which indium oxide contains zinc oxide of 2% to 20%, zinc oxide, GZO (Gallium Zinc Oxide) in which zinc oxide contains gallium, or the like. 
     The conductive layer may be processed by dry etching or wet etching using a resist. When wet etching is employed or the resist is removed by using a separating solution, it is conceivable that water enters the second insulating layer  854 . Therefore, heat treatment may be performed to remove water in the second insulating layer  854  after removing the resist. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. However, water cannot be sufficiently removed in some cases, since the second insulating layer  854  is covered with the etching stopper film  855 . 
     An exposed portion of the etching stopper film  855  is removed by etching using the wiring  856  and the first electrode  857  of the light emitting element as a mask. The etching may be performed by either wet etching or dry etching. In this embodiment mode, dry etching is employed. Note that the first electrode  857  and the etching stopper film  855  may be simultaneously etched, in the case of employing an etching method having higher selectivity between the first electrode  857 , and the second insulating layer  854  and the wiring  856 . When the first electrode  857  and the wiring  856  can have high selectivity to the second insulating layer  854 , the etching stopper film need not be formed. The etching stopper film may be used only at the time of forming either of them. 
     When wet etching is employed, it is conceivable that water enters the second insulating layer  854 . Therefore, heat treatment may be performed to remove water in the second insulating layer  854  after removing the resist. Since the second insulating layer  854  is not covered with the etching stopper film  855 , water can be sufficiently removed. In the case where there is a step of exposing to water again in the following steps, heat treatment may be performed at a time later after the step. Heat treatment can be performed in any condition of atmospheric air, reduced pressure, a vacuum, and a specific gas atmosphere. The treatment may be performed in a favorable condition. 
     Subsequent steps are similar to those in  FIG. 4C  and later; therefore, description is omitted. 
     Hereinabove, a method for manufacturing a light emitting device of the invention is described. Note that the method for manufacturing a light emitting device in this embodiment mode can be appropriately combined with that in Embodiment Mode 2. 
     Embodiment Mode 4 
     The appearance of a panel of a light emitting device which corresponds to one mode of the invention is described in this embodiment mode with reference to  FIG. 6 .  FIG. 6  is a top view of a panel in which a transistor and a light emitting element formed over a substrate are sealed with a sealant formed between the substrate and an opposing substrate  4006 . 
     A sealant  4005  is provided to surround a pixel portion  4002 , a signal processing circuit  4003 , a signal line driver circuit  4020 , and a scanning line driver circuit  4004  which are provided over a substrate  4001 . The opposing substrate  4006  is provided over the pixel portion  4002 , the signal processing circuit  4003 , the signal line driver circuit  4020 , and the scanning line driver circuit  4004 . Thus, the pixel portion  4002 , the signal processing circuit  4003 , the signal line driver circuit  4020 , and the scanning line driver circuit  4004  are sealed with the substrate  4001 , the sealant  4005 , and the opposing substrate  4006 , together with a filler. 
     The pixel portion  4002 , the signal processing circuit  4003 , the signal line driver circuit  4020 , and the scanning line driver circuit  4004  which are provided over the substrate  4001  have a plurality of thin film transistors. 
     A lead wiring corresponds to a wiring for supplying signals or power voltage to the pixel portion  4002 , the signal processing circuit  4003 , the signal line driver circuit  4020 , and the scanning line driver circuit  4004 . The lead wiring is connected to a connection terminal, and the connection terminal is electrically connected to a terminal included in a flexible printed circuit (FPC)  4018  through an anisotropic conductive film. 
     An ultraviolet curing resin or a thermosetting resin as well as an inert gas such as nitrogen or argon can be used as the filler. Polyvinyl chloride, acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral, or ethylene vinylene acetate can be used. 
     A display device of the invention includes, in its category, a panel provided with a pixel portion having a light emitting element and a module in which an IC is mounted on the panel. 
     Embodiment Mode 5 
     Examples of an electronic device of the invention mounted with a module, one of whose example is described in Embodiment Mode 4, can be cited as follows: a camera such as a video camera or a digital camera, a goggle type display (head mounted display), a navigation system, an audio reproducing device (a car audio component or the like), a computer, a game machine, a portable information terminal (a mobile computer, a cellular phone, a portable game machine, an electronic book, or the like), and an image reproducing device including a recording medium (specifically, a device capable of processing data in a recording medium such as a Digital Versatile Disc (DVD) and having a display that can display the image of the data), and the like. Practical examples of these electronic devices are shown in  FIGS. 7A to 7E . 
       FIG. 7A  shows a light emitting display device. A television set, a computer monitor, or the like is regarded as this. The light emitting display device includes a chassis  2001 , a display portion  2003 , a speaker portion  2004 , and the like. In the light emitting display device according to the invention, the variation in an emission spectrum depending on a viewing angle with respect to a side from which luminescence is extracted in the display portion  2003  can be reduced, and display quality is improved. A pixel portion is preferably provided with a polarizing plate or a circularly polarizing plate to enhance contrast. For example, a quarter-wave plate, a half-wave plate, and a polarizing plate may be sequentially formed over a sealing substrate. Further, an anti-reflective film may be provided over the polarizing plate. 
       FIG. 7B  shows a cellular phone, which includes a main body  2101 , a chassis  2102 , a display portion  2103 , an audio input portion  2104 , an audio output portion  2105 , an operation key  2106 , an antenna  2108 , and the like. In the cellular phone according to the invention, the deterioration of a light emitting element in the display portion  2103  is suppressed, thereby improving reliability. 
       FIG. 7C  shows a computer, which includes a main body  2201 , a chassis  2202 , a display portion  2203 , a keyboard  2204 , an external connection port  2205 , a pointing mouse  2206 , and the like. In the computer according to the invention, the variation in an emission spectrum depending on a viewing angle with respect to a side from which luminescence is extracted in the display portion  2203  can be reduced, and display quality is improved. Although a laptop computer is shown in  FIG. 7C  as an example, the invention can be applied to a desktop computer in which a hard disk and a display portion are integrated, and the like. 
       FIG. 7D  shows a mobile computer, which includes a main body  2301 , a display portion  2302 , a switch  2303 , operation keys  2304 , an infrared port  2305 , and the like. In the mobile computer according to the invention, the deterioration of a light emitting element in the display portion  2302  is suppressed, thereby improving reliability. 
       FIG. 7E  shows a portable gaming machine, which includes a chassis  2401 , a display portion  2402 , a speaker portion  2403 , operation keys  2404 , a recording medium insertion portion  2405 , and the like. In the portable gaming machine according to the invention, the deterioration of a light emitting element in the display portion  2402  is suppressed, thereby improving reliability. 
     As described above, the applicable range of the invention is so wide that the invention can be applied to electronic devices of various fields. 
     Embodiment Mode 6 
     A structure of a light emitting layer is described in detail in this embodiment mode. 
     The light emitting layer may be made of a charge injection transport material and a light emitting material including an organic compound or inorganic compound. The light emitting layer includes one or plural kinds of layers of a low molecular weight organic compound, an intermediate molecular weight organic compound (referring to an organic compound which does not have sublimation property and has the number of molecules of 20 or less or a molecular chain length of 10 μm or less), and a high molecular weight organic compound. The light emitting layer may be combined with an electron injection transport or hole injection transport inorganic compound. 
     As a highly electron transporting material among charge injection transport materials, a metal complex that has a quinoline skeleton or a benzoquinoline skeleton such as tris(8-quinolinolato)aluminum [Alq 3 ], tris(5-methyl-8-quinolinolato)aluminum [Almq 3 ], bis(10-hydroxybenzo[h]-quinolinato)beryllium [BeBq 2 ], Or bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum [BAlq], or the like can be used. As a highly hole transporting material, an aromatic amine compound (that is, a compound having a benzene ring-nitrogen bond) such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl [α-NPD], 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl [TPD], 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine [TDATA], or 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]triphenylamine [MTDATA] can be used. 
     As a highly electron injecting material among charge injection transport materials, a compound of an alkali metal or an alkaline earth metal such as lithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride (CaF 2 ) can be used. In addition, the highly electron injecting material may be a mixture of a highly electron transporting material such as Alq 3  and an alkaline earth metal such as magnesium (Mg). 
     As a highly hole injecting material among charge injection transport materials, metal oxide such as molybdenum oxide (MoOx), vanadium oxide (VOx), ruthenium oxide (RuOx), tungsten oxide (WOx), or manganese oxide (MnOx) can be used. In addition, a phthalocyanine compound such as phthalocyanine [H 2 Pc] or copper phthalocyanine (CuPC) can be used. 
     The light emitting layer may have a structure for performing color display by providing each pixel with light emitting layers having different emission wavelength bands. Typically, a light emitting layer corresponding to each color of R (red), G (green), and B (blue) is formed. In this case, color purity can be increased and a pixel portion can be prevented from having a mirror surface (glare) by providing a light emitting side of a pixel with a filter (colored layer) which transmits light of an emission wavelength band. Providing a filter (colored layer) can omit a circularly polarizing plate or the like which is conventionally used to prevent a pixel portion from having a mirror surface (glare) and can eliminate the loss of light that light is reduced to half due to the use of a polarizing plate. Further, a change in hue, which occurs when a pixel portion (display screen) is obliquely seen, can be reduced. 
     A light emitting material includes various materials. As a low molecular weight organic light emitting material, 4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran [DCJT], 4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyl julolidine-9-yl)ethenyl]-4H-pyran [DCJTB], perifianthene, 2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl julolidine-9-yl)ethenyl]benzene, N,N′-dimethyl quinacridon [DMQd], coumarin 6, coumarin 545T, tris(8-quinolinolate)aluminum [Alq 3 ], 9,9′-bianthryl, 9,10-diphenylanthracene [DPA], 9,10-bis(2-naphthyl)anthracene [DNA], or the like can be used. In addition, another material can also be used. 
     A high molecular weight organic light emitting material is physically stronger than a low molecular weight material and is superior in durability of the element. In addition, a high molecular weight material can be used for application; therefore, the element is relatively easily manufactured. A structure of a light emitting element using a high molecular weight organic light emitting material is basically the same as that of a light emitting element using a low molecular weight organic light emitting material: a cathode, an organic light emitting layer, and an anode from the side of a semiconductor layer. However, it is difficult to form such a laminated structure as in the case of using a low molecular weight organic light emitting material, when a light emitting layer using a high molecular weight organic light emitting material is formed. A two-layer structure is employed in many cases. Specifically, the light emitting element using a high molecular weight organic light emitting material has a structure of a cathode, a light emitting layer, a hole transport layer, and an anode from the side of a semiconductor layer. 
     The emission color is determined by the material of the light emitting layer. Therefore, a light emitting element that emits desired light can be formed by selecting an appropriate material of the light emitting layer. As a high molecular weight light emitting material that can be used to form the light emitting layer, a polyparaphenylene-vinylene-based material, a polyparaphenylene-based material, a polythiophen-based material, or a polyfluorene-based material can be used. 
     As a polyparaphenylene-vinylene-based material, a derivative of poly(paraphenylene vinylene) [PPV], for example, poly(2,5-dialkoxy-1,4-phenylene vinylene) [RO-PPV], poly(2-(2′-ethyl-hexoxy)-5-metoxy-1,4-phenylene vinylene) [MEH-PPV], poly(2-(dialkoxyphenyl)-1,4-phenylene vinylene) [ROPh-PPV], or the like can be used. As a polyparaphenylene-based material, a derivative of polyparaphenylene [PPP], for example, poly(2,5-dialkoxy-1,4-phenylene) [RO-PPP], poly(2,5-dihexoxy-1,4-phenylene), or the like can be used. As a polythiophene-based material, a derivative of polythiophene [PT], for example, poly(3-alkylthiophene) [PAT], poly(3-hexylthiophene) [PHT], poly(3-cyclohexylthiophene) [PCHT], poly(3-cyclohexyl-4-methylthiophene) [PCHMT], poly(3,4-dicyclohexylthiophene) [PDCHT], poly[3-(4-octylphenyl)-thiophene] [POPT], poly[3-(4-octylphenyl)-2,2 bithiophene] [PTOPT], or the like can be used. As the polyfluorene-based material, a derivative of polyfluorene [PF], for example, poly(9,9-dialkylfluorene) [PDAF], poly(9,9-dioctylfluorene) [PDOF], or the like can be used. 
     Note that a hole injection property from an anode can be enhanced by interposing a high molecular weight organic light emitting material having a hole transporting property between an anode and a high molecular weight organic light emitting material. This hole transporting material is generally dissolved into water together with an acceptor material, and the solution is applied by a spin coating method or the like. Since the hole transporting material is insoluble in an organic solvent, a laminate with the above-described organic light emitting material can be formed. A mixture of PEDOT and camphor sulfonic acid (CSA) that serves as an acceptor material, a mixture of polyaniline [PANT] and polystyrene sulfonic acid [PSS] that serves as an acceptor material, and the like can be used as the hole transporting high molecular weight organic light emitting material. 
     In addition, the light emitting layer can be formed to emit monochrome or white light. In the case of using a white light emitting material, a filter (colored layer) which transmits light having a specific wavelength is provided on a light emitting side of a pixel, thereby performing color display. 
     In order to form a light emitting layer which emits white light, for example, Alq 3 , Alq 3  partially doped with Nile red that is a red light emitting pigment, p-EtTAZ, and TPD (aromatic diamine) are sequentially laminated by an evaporation method to obtain white light. When the light emitting layer is formed by an application method using spin coating, the layer after application is preferably baked by vacuum heating. For example, an aqueous solution of poly(ethylene dioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) may be entirely applied and baked to form a layer that functions as a hole injection layer. Then, a polyvinyl carbazole (PVK) solution doped with a light emitting center pigment (such as 1,1,4,4-tetraphenyl-1,3-butadiene (TPB), 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM1), Nile red, or coumarin 6) may be entirely applied and baked to form a layer that functions as a light emitting layer. 
     The light emitting layer may be formed to be a single layer. For example, a 1,3,4-oxadiazole derivative (PBD) having an electron transporting property may be dispersed in polyvinyl carbazole (PVK) having a hole transporting property. Another method to obtain white light emission is to disperse PBD of 30 wt % and to disperse four kinds of pigments (TPB, coumarin 6, DCM1, and Nile red) in appropriate amounts. In addition to the light emitting elements described here that provide white light emission, a light emitting element that provides red light emission, green light emission, or blue light emission can be manufactured by appropriately selecting materials of the light emitting layer. 
     Further, a triplet excitation light emitting material including a metal complex or the like as well as a singlet excitation light emitting material may be used for the light emitting layer. For example, among pixels emitting red, green, and blue light, a pixel emitting red light whose luminance is reduced by half in a relatively short time is made of a triplet excitation light emitting material and the rest are made of a singlet excitation light emitting material. A triplet excitation light emitting material has a characteristic that the material has a good luminous efficiency and consumes less power to obtain the same luminance. When a triplet excitation light emitting material is used for a red pixel, only small amount of current needs to be applied to a light emitting element. Thus, reliability can be improved. A pixel emitting red light and a pixel emitting green light may be formed of a triplet excitation light emitting material and a pixel emitting blue light may be formed of a singlet excitation light emitting material to achieve low power consumption. Low power consumption can be further achieved by forming a light emitting element which emits green light that has high visibility with a triplet excitation light emitting material. 
     A metal complex used as a dopant is an example of a triplet excitation light emitting material, and a metal complex having platinum that is a third transition series element as a metal center, a metal complex having iridium as a metal center, and the like are known. A triplet excitation light emitting material is not limited to the above compounds. A compound having the above described structure and an element belonging to any of Groups 8 to 10 of the periodic table as a metal center can also be used. 
     The above described materials for forming the light emitting layer are just examples. A light emitting element can be formed by appropriately laminating functional layers such as a hole injection transport layer, a hole transport layer, an electron injection transport layer, an electron transport layer, a light emitting layer, an electron blocking layer, and a hole blocking layer. Further, a mixed layer or a mixed junction may be formed by combining these layers. The layer structure of the light emitting layer can be varied. Instead of providing a specific electron injection region or light emitting region, modification such as providing an electrode for the purpose or providing a dispersed light emitting material is acceptable as long as it does not deviate from the scope of the invention. 
     A light emitting element formed with the above described material emits light by being biased in a forward direction. A pixel of a display device formed with a light emitting element can be driven by a simple matrix mode or an active matrix mode. In either mode, each pixel emits light by applying a forward bias thereto in specific timing; however, the pixel is in a non-light-emitting state for a certain period. Reliability of a light emitting element can be improved by applying a reverse bias at this non-light-emitting time. In a light emitting element, there is a deterioration mode in which emission intensity is decreased under specific driving conditions or a deterioration mode in which a non-light-emitting region is enlarged in the pixel and luminance is apparently decreased. However, progression of deterioration can be slowed down by alternating driving of applying a forward bias and a reverse bias. Thus, the reliability of a light emitting device can be improved. 
     Embodiment Mode 7 
     An example of a light emitting device using the invention is described in this embodiment mode with reference to  FIGS. 8A and 8B . In this embodiment mode, a thin film transistor  809  having an LDD structure is connected to a light emitting element  814  through an electrode  808  of the thin film transistor. 
       FIG. 8A  shows a structure in which a first electrode  810  is formed of a light transmitting conductive film and light emitted from a light emitting layer  812  is extracted to the side of a substrate  800 . Note that reference numeral  815  denotes an opposing substrate and is fixed to the substrate  800  with the use of a sealant or the like after the light emitting element  814  is formed. A space between the opposing substrate  815  and the element is filled with a light transmitting resin  816  or the like, and sealing is performed. Accordingly, the deterioration of the light emitting element  814  due to moisture can be further suppressed. The resin  816  is preferably hygroscopic. When a highly light transmitting drying agent is dispersed in the resin  816 , an influence of the moisture can be further reduced. Therefore, it is a more preferable mode. 
       FIG. 8B  shows a structure in which both a first electrode  810  and a second electrode  813  are formed of a light transmitting conductive film and light can be emitted to both sides of a substrate  800  and an opposing substrate  815 . In this structure, a screen can be prevented from being transparent by providing a polarizing plate  817  outside the substrate  800  and the opposing substrate  815 , and visibility is increased. A protective film  818  is preferably provided outside the polarizing plate  817 . 
     Embodiment Mode 8 
     A pixel circuit, a protective circuit, and operation thereof are described in this embodiment mode. 
     In a pixel shown in  FIG. 9A , a signal line  1410  and power supply lines  1411  and  1412  are arranged in a column direction and a scanning line  1414  is arranged in a row direction. In addition, the pixel includes a switching TFT  1401 , a driving TFT  1403 , a current control TFT  1404 , a capacitor element  1402 , and a light emitting element  1405 . 
     A pixel shown in  FIG. 9C  is different in the way that a gate electrode of a TFT  1403  is connected to a power supply line  1412  arranged in a row direction, but other than that, the pixel has a similar structure to that of the pixel shown in  FIG. 9A . In other words, equivalent circuit diagrams of both of the pixels shown in  FIGS. 9A and 9C  are the same. However, each power supply line is formed using a conductive layer in a different layer when the power supply line  1412  is arranged in a column direction ( FIG. 9A ) and when the power supply line  1412  is arranged in a row direction ( FIG. 9C ). Here, a wiring connected to the gate electrode of the driving TFT  1403  is focused and the figures are separately shown in  FIGS. 9A and 9C  to show that the wirings are formed in different layers. 
     In the pixels shown in  FIG. 9A  and  FIG. 9C , the TFTs  1403  and  1404  are connected in series. A channel length L(1403) and a channel width W(1403) of the TFT  1403  and a channel length L(1404) and a channel width W(1404) of the TFT  1404  are preferably set to satisfy L(1403)/W(1403): L(1404)/W(1404)=5 to 6000:1. 
     Note that the TFT  1403  operates in a saturation region and has a role of controlling the amount of electric current flowing through the light emitting element  1405 , and the TFT  1404  operates in a linear region and has a role of controlling supply of electric current to the light emitting element  1405 . It is preferable, from the viewpoint of the manufacturing steps, that the TFTs have the same conductivity. In this embodiment mode, the TFTs are formed to be n-channel TFTs. Further, the TFT  1403  may be a depletion mode TFT as well as an enhancement mode TFT. In the present invention having the above structure, the TFT  1404  operates in a linear region, so that slight variation in gate-source voltage (Vgs) of the TFT  1404  does not affect the amount of electric current of the light emitting element  1405 . In other words, the amount of electric current of the light emitting element  1405  is determined by the TFT  1403  which operates in a saturation region. According to the above-described structure, luminance variation of the light emitting element, which is caused by the variation in characteristics of the TFT, can be improved, and a display device with improved image quality can be provided. 
     In pixels shown in  FIGS. 9A to 9D , the TFT  1401  controls the input of a video signal to the pixel. When the TFT  1401  is turned on, the video signal is inputted to the pixel. Then, voltage of the video signal is stored in the capacitor element  1402 .  FIGS. 9A and 9C  each show a structure in which the capacitor element  1402  is provided; however, the invention is not limited thereto. When a gate capacitor or the like can be used as the capacitor that can hold a video signal, the capacitor element  1402  may not be provided. 
     The pixel shown in  FIG. 9B  has the same structure as that of the pixel shown in  FIG. 9A  except that a TFT  1406  and a scanning line  1415  are added. In the same manner, the pixel shown in  FIG. 9D  has the same structure as that of the pixel shown in  FIG. 9C  except that a TFT  1406  and a scanning line  1415  are added. 
     In the TFT  1406 , “on” or “off” is controlled by the scanning line  1415  that is newly arranged. When the TFT  1406  is turned on, an electric charge held in the capacitor element  1402  is discharged, and the TFT  1404  is then turned off. In other words, it is possible to make a state in which current is forced not to flow through the light emitting element  1405  by arranging the TFT  1406 . Therefore, the TFT  1406  can be referred to as an erasing TFT. Accordingly, in the structures in  FIGS. 9B and 9D , a lighting period can be started simultaneously with or immediately after the start of a writing period without waiting for the writing of signals in all pixels. Consequently, a duty ratio can be improved. 
     In a pixel shown in  FIG. 9E , a signal line  1410  and a power supply line  1411  are arranged in a column direction, and a scanning line  1414  is arranged in a row direction. In addition, the pixel includes a switching TFT  1401 , a driving TFT  1403 , a capacitor element  1402 , and a light emitting element  1405 . A pixel shown in  FIG. 9F  has the same structure as that of the pixel shown in  FIG. 9E  except that a TFT  1406  and a scanning line  1415  are added. A duty ratio can be increased by arranging the TFT  1406  also in the structure of  FIG. 9F . 
     As described above, various pixel circuits can be adopted. It is preferable to make a semiconductor film of a driving TFT large specifically in the case of forming a thin film transistor with an amorphous semiconductor film or the like. Therefore, the pixel circuit is preferably a top emission type, which emits light from an electroluminescent layer to the side of a sealing substrate. 
     Such an active matrix light emitting device is considered to be advantageous to low voltage driving when a pixel density is increased, since each pixel is provided with TFTs. 
     In this embodiment mode, an active matrix light emitting device in which each pixel is provided with TFTs is described. However, a passive matrix light emitting device in which every column is provided with TFTs can be formed. In the passive matrix light emitting device, TFTs are not provided for each pixel; therefore, a high aperture ratio can be obtained. In the case of a light emitting device which emits light to the both sides of an electroluminescent layer, transmittance can be increased by using the passive matrix display device. 
     The case of providing a scanning line and a signal line with a diode as a protective circuit is described with reference to an equivalent circuit shown in  FIG. 9E . 
     In  FIG. 10 , a pixel portion  1500  is provided with TFTs  1401  and  1403 , a capacitor element  1402 , and a light emitting element  1405 . A signal line  1410  is provided with diodes  1561  and  1562 . The diodes  1561  and  1562  are manufactured according to the above embodiment mode as in the case of the TFT  1401  or  1403  and include a gate electrode, a semiconductor layer, a source electrode, a drain electrode, and the like. The diodes  1561  and  1562  operate by connecting the gate electrode to the drain electrode or the source electrode. 
     Common potential lines  1554  and  1555  connected to the diodes are formed in the same layer as the gate electrode. Therefore, a contact hole needs to be formed in a gate insulating layer to connect the gate electrode to the source electrode or the drain electrode of the diode. 
     A diode provided for a scanning line  1414  has a similar structure. 
     Thus, a protective diode provided for an input stage can be simultaneously formed according to the invention. Note that a position where the protective diode is formed is not limited thereto, and the protective diode can be provided between a driver circuit and a pixel. 
     Embodiment 1 
     A comparison of the number of regions where emission intensity was decreased was made, between a light emitting device to which the invention was applied (Example 1) and a light emitting device to which the invention was not applied as a comparative example (Comparative Example 1), after using the light emitting devices for a certain period of time under given conditions. Note that a structure of the light emitting device, Example 1, corresponds to the structure shown in  FIG. 1A . A structure of the light emitting device, Comparative Example 1, corresponds to a structure in which the etching stopper film  114  is entirely formed over the insulating layer  107  in  FIG. 1A . The result of comparison is shown in Table 1. Note that the etching stopper film was formed of a material which mainly contains silicon nitride in this embodiment. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 number of 
               
               
                   
                 regions where emission intensity is decreased 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Comparative Example 1 
                 0 
               
               
                 Example 1 
                 9 
               
               
                   
               
               
                 *Temperature: 85° C., after 100 hours 
               
            
           
         
       
     
     Table 1 shows the increased number of regions where emission intensity was decreased, which was visually counted by examining a picture taken after performing a storage test at the temperature of 85° C. for 100 hours. The number of large-area regions across a plurality of pixels, where emission intensity was decreased was counted. As shown in Table 1, a region where emission intensity was decreased did not newly appear in the light emitting device having the structure of the invention, even after 100 hours at 85° C. On the other hand, in Comparative Example 1, many regions where emission intensity was decreased newly appeared after the same period of time under the same conditions. 
     Embodiment 2 
     A comparison of the number of regions where emission intensity was decreased was made, between a light emitting device to which the invention was applied (Example 2) and light emitting devices to which the invention was not applied as comparative examples (Comparative Examples 2 to 5), after using the light emitting devices for a certain period of time under given conditions. Note that a structure of the light emitting device, Example 2, corresponds to the structure shown in  FIG. 1B . The structure was manufactured by controlling etching time to etch at the same time as etching of the electrodes  108  and  109  so that the etching stopper film  115  was removed. 
     Structures of the light emitting devices, Comparative Examples 2 to 5, correspond to a structure in which the etching stopper film  115  is entirely formed over the insulating layer  107  in  FIG. 1B . A remaining film thickness means a thickness of the etching stopper film  115  except between the electrodes  108  and  109  and the insulating layer  107 . Conditions were set by changing a thickness of the etching stopper film  115  at the time of formation. The result of comparison is shown in Table 2. Note that the etching stopper film was formed of a material which mainly contains silicon nitride in this embodiment. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 remaining 
                 number 
               
               
                   
                 film thickness 
                 of pixels where emission 
               
               
                   
                 (nm) 
                 intensity is decreased 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Example 2 
                 0 
                 0 
               
               
                 Comparative Example 2 
                 30 
                 84 
               
               
                 Comparative Example 3 
                 60 
                 104 
               
               
                 Comparative Example 4 
                 60 
                 62 
               
               
                 Comparative Example 5 
                 90 
                 42 
               
               
                   
               
               
                 *Temperature: 85° C., after 80 hours 
               
            
           
         
       
     
     Table 2 shows the increased number of pixels where emission intensity was decreased, which was visually counted after performing a storage test at a temperature of 85° C. for 80 hours. Table 2 shows the number counted pixel by pixel, differently from Table 1. As shown in Table 2, a pixel where emission intensity was decreased did not newly appear in the light emitting device having a structure of the invention, even after 80 hours at 85° C. On the other hand, in Comparative Examples 2 to 5, many pixels where emission intensity was decreased newly appeared after the same period of time under the same conditions, although there were differences in the number. 
       FIGS. 12A and 12B  are pictures of the light emitting device having a structure corresponding to  FIG. 1B , similarly to Example 2, which are taken with all pixels turned on. A brightly displaying portion corresponds to a pixel. Note that a light emitting material which emits green light is used for the light emitting device shown in  FIGS. 12A and 12B .  FIGS. 12A and 12B  show four corners, side portions between the corners, and the center of a pixel portion in the light emitting device, but do not show an entire pixel portion. 
       FIG. 12A  is a picture taken before performing a storage test at 85° C. Display is favorably performed without a pixel where emission intensity is decreased.  FIG. 12B  is a picture taken with all pixels turned on after storage at 85° C. for 1024 hours. The favorable display is maintained without generating a pixel where light intensity is decreased, even after 1024 hours at 85° C. 
       FIGS. 13A and 13B  are pictures of a light emitting device, Comparative Example 6, in which the etching stopper film  115  is entirely formed over the insulating layer  107  in  FIG. 1B , which are taken with all pixels turned on. Similarly to Example 2, a brightly displaying portion corresponds to a pixel, and a light-emitting material which emits green light is used.  FIGS. 13A and 13B  show four corners, side portions between the corners, and the center of a pixel portion in the light emitting device, but do not show an entire pixel portion. 
       FIG. 13A  is a picture taken before performing a storage test at 85° C. Regions where emission intensity is decreased exist in parts. However, initial failure and deterioration cannot be distinguished from each other at this time.  FIG. 13B  is a picture taken with all pixels turned on after storage at 85° C. for 1024 hours. It is found that the number and the area of regions where pixel emission intensity is decreased are drastically increased in the light emitting device, Comparative example 6, having the above described structure, after 1024 hours at 85° C., compared with the state before performing the storage test. In particular, the area of the region where emission intensity is decreased, which existed across a plurality of pixels, is significantly enlarged compared with the state before performing the storage test. In the structure of  FIG. 13A  or  13 B, the etching stopper film is entirely formed over the insulating layer. Therefore, water in the insulating layer cannot be sufficiently removed during the step of forming the light emitting device. Further, the water remaining in the insulating layer seeps, in a concentrated manner, from a tiny pinhole in the etching stopper film or at the boundary with an electrode or the like. Therefore, the pixel located in the periphery of the portion where water seeps is deteriorated. This is a conceivable reason for generating the region where emission intensity is decreased and enlarging the region. 
     It is found that the deterioration of a light emitting element can be effectively suppressed by using the structure of the invention as described above. 
     This application is based on Japanese Patent Application serial No. 2004-091710 filed in Japan Patent Office on Mar. 26, 2004, the entire contents of which are hereby incorporated by reference.