Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device (hereinafter referred to as a light-emitting device) that has an element (hereinafter referred to as a light-emitting element) where a thin film including a luminescent material is sandwiched between a pair of an anode electrode and a cathode electrode. In particular, the present invention relates to a light-emitting device whose light-emitting element includes a thin film (hereinafter referred to as a light-emitting layer) made of an electro-luminescent material (EL material). The present invention also relates to a display device that uses a substrate made of an organic resin material and, more particularly, to a display device where a pixel portion is formed on such a substrate using thin-film transistors and an EL material. 
     2. Description of the Related Art 
     Liquid crystal panels or EL materials applied to display devices may contribute to reduction in weight and thickness thereof in comparison with conventional CRTs. Therefore, attempts have been recently made to apply display devices using the liquid crystal panels or EL materials to various fields. Also, it has now become possible to connect portable telephones and personal digital assistants (PDAs) to the Internet, which leads to the dramatic increase in the amount of image information to be displayed thereon and creates increasing demand for high-definition color display devices. 
     Display devices used for such portable information terminals need to be reduced in weight and, for instance, portable telephones whose weights are below 70 g are now on the market. For the reduction in weight, almost all components, such as electronic components, housing, and batteries, of the portable information terminals are subjected to reengineering. For the further weight reduction, however, display devices need to be reduced in weight. 
     Display devices are produced using glass substrates in many cases, so that one conceivable method for weight reduction would be to reduce the thickness of the glass substrates. In this case, however, the glass substrates tend to be cracked and the shock resistance thereof is lowered. This becomes a serious hindrance to the application of display devices including such thin glass substrates to portable information terminals. To meet demand for weight reduction as well as shock resistance, the development of display devices using organic resin substrates (plastic substrates) is under consideration. 
     For instance, light-emitting devices that have light-emitting elements produced using EL materials are currently under development. Display devices whose pixel portions are formed using light-emitting elements are capable of emitting light by themselves and further do not require light sources, such as backlights, unlike liquid crystal display devices. As a result, such light-emitting elements are highly expected as an effective means for reducing weights as well as thickness of display devices. 
     The construction of a typical light-emitting element using an organic EL material is shown in  FIG. 22 . In this drawing, an insulator  2201 , an anode  2202 , a light-emitting layer  2203 , and a cathode  2204  are laminated to form a light-emitting element  2200 . 
     Before being observed by an observer  2206 , light  2205  emitted from the light-emitting layer directly passes through the anode  2202 , or is reflected by the cathode  2204  and then passes through the anode  2202 . That is, the observer  2206  observes the light  2205  that and passes through the anode  2202  to be emitted in picture elements where the light-emitting layer  2203  performs light emission. 
     A light-emitting element is composed of two electrodes: an anode that injects holes into an organic compound layer including a light-emitting layer, and a cathode that injects electrons into the organic compound layer. The light-emitting element having this construction utilizes a phenomenon where light is emitted when the holes injected from the anode are recombined with the electrons injected from the cathode within the light-emitting layer. The organic compound layer including the light-emitting layer is degraded by various factors, such as heat, light, moisture, and oxygen. To prevent this degradation, an ordinary active matrix type light-emitting device is produced by forming light-emitting elements in a pixel portion after wiring and semiconductor elements are formed therein. 
     After the formation of the light-emitting element, a first substrate, on which the light-emitting element have been formed, and a second substrate for covering the light-emitting elements are laminated and sealed (packaged) using a sealing member. This construction prevents the light-emitting elements from being exposed to the outside air. 
     It should be noted here that in this specification, all layers provided between a cathode and an anode are collectively referred to as an organic compound layer. The organic compound layer has a well-known structure where, for instance, a hole injecting layer, a light-emitting layer, an electron transporting layer, and an electron injecting layer are laminated with each other. A predetermined voltage is applied to the organic compound layer by a pair of electrodes to cause the recombination of carriers, thereby causing light emission in the light-emitting layer. 
     The light-emitting element, however, has a problem as to durability and, in particular, to oxidation resistance. The cathode that injects electrons into the organic compound layer is ordinarily made of an alkaline metal or an alkaline earth metal having a low work function. It is well known that these metals tend to react with and water, thereby having low oxidation resistance. The oxidation of the cathode means that the material of the cathode loses electrons and is coated with an oxidation layer. The reduction in the number of electrons to be injected and the oxidation coat may reduce the amount of emitted light in brightness. 
     As described above, the electrode of the light-emitting element is easily oxidized with a considerably small amount of oxygen or moisture and therefore the light-emitting element is easily degraded. Various techniques have been developed to prevent the oxidation of the light-emitting element. For instance, the light-emitting element is sealed with a metal or glass that is impermeable to oxygen and moisture. Also, the light-emitting element is produced to have a resin lamination construction or is filled with nitrogen or an inert gas. Even if the light-emitting element is seated with a metal or a resin, however, oxygen easily passes through small gaps and oxidizes the cathode and light-emitting layer. Also, moisture easily passes through the resin used to seal the light-emitting element in terms of the light-emitting element. This causes a problem in that areas (called dark spots) that do not emit light appear on a display screen and expand with the lapse of time, which makes the light-emitting element incapable of emitting light. 
     EL materials are capable of emitting blue light and thus it is possible to realize a full-color display device of a self-light emitting type with the materials. However, it is confirmed that organic light-emitting elements are degraded in various ways. This degradation prevents the actual use of the EL materials and a solution to this problem is urgently required. The dark spots are spot-shaped defects that do not emit light in the pixel portion and so degrade display quality. The dark spots are also defects that get worse over time. Even if the light-emitting element is not brought into operation, the number of the dark spots is increased by the existence of moisture. It is thought that the cause of the dark spots is the oxidation reaction of the cathode made of an alkaline metal. To prevent the occurrence of dark spots, a sealed space is filled with dryer gas or provided with a dryer agent, in which the light-emitting element is placed. 
     Also, the light-emitting element is vulnerable to heat that promotes oxidation. This means that there are many factors causing oxidation and therefore it is difficult to make actual use of light-emitting devices. In view of the problems described above, the object of the present invention is to provide a light-emitting device with a high degree of reliability and an electronic device where a high-reliability display unit is achieved using such a light-emitting device. 
     It is well known that a substrate made of an organic resin material has high permeability to moisture, in comparison with a glass substrate. For instance, the permeability to moisture of polyether imide is 36.5 g/m 2 ·24 hr, that of polyimide is 32.7 g/m 2 ·24 hr, and that of polyether terephthalate (PET) is 12.1 g/m 2 ·24 hr. 
     As is apparent from this, if a display device produced with a light-emitting element including an organic resin substrate is left standing in the air for a long time period, moisture gradually permeates and the organic light-emitting element is degraded. In addition, a sealing member used to seal a light-emitting element is also made of an organic resin material, so that it is difficult to completely prevent oxygen and moisture in the air from entering through sealed portions. 
     Also, an organic resin substrate is soft, in comparison with a metal substrate or a glass substrate, so that scratches or the like are easily made thereon. Further, the long-term exposure to the direct sunlight causes a light chemical reaction and alters the quality and color of the organic resin substrate. 
     As described above, the organic resin substrate is a highly effective means to realize a display device reduced in weight with high shock resistance, although there remain many problems that must be solved in order to ensure the reliability of the light-emitting element. In view of these problems, the object of the present invention is to provide a display device that uses a light-emitting element with a high degree of reliability. 
     Also, if the outside light (the light existing outside the light-emitting device) enters picture elements that do not emit light, the light is reflected by the back surface (the surface contacting the organic compound layer) of the cathode, so that the cathode back surface functions as a mirror and reflects the outside scenes. To solve this problem, a circular polarizing film has conventionally been applied to a light-emitting device to prevent the reflection of the outside scenes toward the observer, although this construction raises the fabrication cost because the circular polarizing film is high-priced. In view of this problem, the object of the present invention is to prevent this mirror reflection phenomenon of a light-emitting device without using a circular polarizing film. 
     SUMMARY OF THE INVENTION 
     According to the present invention, in a display device using an organic resin substrate, a hard carbon film is formed on a surface of the substrate as a protecting film that prevents from entering moisture or the like and the scratches on the surface. In particular, a DLC (Diamond like Carbon) film is used with the present invention. The DLC film has a construction where carbon atoms are bonded into a diamond bond (SP 3  bond) in terms of a short-distance order, although the film has an amorphous construction containing a graphite bond (SP 2  bond) from a macroscopic viewpoint. The DLC film contains 95 to 70 atomic % carbon and 5 to 30 atomic % hydrogen, so that the DLC film is very hard and excels in insulation. The DLC film is also characterized by low gas permeability to moisture and oxygen. Further, it is known that the hardness of the DLC film is 15 to 25 Gpa in the case of measurement using a micro-hardness meter. 
     The DLC film is formed using a plasma CVD method, a microwave CVD method, an electron cyclotron resonance (ECR) CVD method, or a sputtering method. With any of these methods, the DLC film is formed in intimate contact without heating the organic resin substrate. The DLC film is formed under a situation where the substrate is set on a cathode. Alternatively, the DLC film is formed by applying a negative bias and utilizing ion bombardment to some extent. In the latter case, the DLC film becomes minute and hard. 
     The reaction gas used to form the DLC film is hydrocarbon gas, such as CH 4 , C 2 H 2 , and C 6 H 6 . The DLC film is formed by ionizing the reaction gas by means of glow discharge and bombarding a cathode, to which a negative self-bias is applied, with accelerated ions. In this manner, the DLC film becomes minute and flat. The DLC film may be formed without heating the substrate to a high temperature, so that the formation of the DLC film can be performed in the final manufacturing step where a display device is finished. 
     By forming the DLC film on at least one surface of the organic resin substrate, the gas barrier property is improved. Alternatively, the gas barrier property is improved by forming the DLC film on the outer surface of a sealing member used to laminate an organic resin substrate (hereinafter, an element substrate), on which TFTs and light-emitting elements are formed, with a sealing substrate for sealing the light-emitting elements. In this case, the thickness of the DLC film is in a range of 5 nm to 500 nm. Also, by forming the DLC film on a light incident surface, ultraviolet rays are blocked, the light chemical reaction of the organic resin substrate is suppressed, and the degradation of the organic resin substrate is prevented. 
     The DLC film that prevents oxygen and moisture from entering is formed to successively cover exposed portions of the sealing member and side portions of the first and second substrates that are laminated to produce the light-emitting device. The exposed portions of the sealing member and the side portions of the first and second substrates are hereinafter collectively referred to as “end surfaces”. With a conventional technique, oxygen and moisture pass through a resin provided at end portions. The construction described above, however, prevents moisture from entering through between the first and second substrates. 
     A dryer agent is provided in a space between the element substrate and the sealing substrate sealed by the sealing member, thereby suppressing the degradation of the light-emitting elements. For instance, a barium oxide can be used as the dryer agent. The dryer agent is provided at positions (for instance, on a driving circuit, on a partition wall, or within the partition wall) outside light-emitting areas. With this construction, the dryer agent absorbs gas and moisture contained in the light-emitting elements as well as oxygen and moisture passing through a sealing resin in the end portions. As a result, the degradation of the light-emitting elements is prevented. Further, by forming an organic interlayer insulating film using a black resin, the mirror reflection phenomenon (the reflection of the outside scenes) of the light-emitting device is prevented. Also, the black resin may be used in an area in which the sealing member is formed. 
     The DLC film described above is applicable to passive type display devices as well as active matrix type display devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1D  each show a position where DLC film is formed on an organic resin substrate according to the present invention; 
         FIG. 2  shows the construction of a plasma CVD apparatus used to form DLC films used in the present invention; 
         FIGS. 3A and 3B  each show the construction of the reaction chamber of the plasma CVD apparatus; 
         FIG. 4  is a cross to sectional view showing the constructions of the driving circuit and pixel portion of a display device; 
         FIGS. 5A and 5B  are respectively a top view and an equivalent circuit diagram showing the construction of the pixel portion of the display device; 
         FIG. 6  is a perspective view showing the external appearance of an EL display device of the present invention; 
         FIG. 7  shows the construction of an input terminal of the display device; 
         FIG. 8  shows the construction of the input terminal of the display device; 
         FIGS. 9A to 9C  each show an example where a dryer agent is provided in the pixel portion; 
         FIG. 10  is a cross-sectional view showing the constructions of the driving circuit and the pixel portion of the display device; 
         FIG. 11  is a system block diagram of an electronic device in which the display device is built; 
         FIGS. 12A to 12E  each show an example of the electronic device; 
         FIGS. 13A to 13D  each show an example of the electronic device; 
         FIGS. 14A and 14B  each show an embodiment mode of the present invention; 
         FIGS. 15A and 15B  each show a CVD apparatus of the present invention; 
         FIGS. 16A to 16C  each show an example of the embodiment mode of the present invention; 
         FIGS. 17A and 17B  each show an example of the embodiment mode of the present invention; 
         FIGS. 18A to 18D  each show an example of the embodiment mode of the present invention; 
         FIGS. 19A and 19B  each show an example of the embodiment mode of the present invention; 
         FIGS. 20A and 20B  each show an example of the embodiment mode of the present invention; 
         FIGS. 21A to 21C  each show an example of the electronic device that uses a light-emitting device as its display unit; 
         FIG. 22  shows an example of the conventional technique; and 
         FIGS. 23A to 23E  each show an example of the embodiment mode of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment modes and embodiments of the present invention are described in detail below with reference to the drawings. 
     Embodiment Mode 1  
     Embodiment Mode 1 is described below with reference to  FIGS. 1A to 1D  each showing a display device using a light-emitting element.  FIG. 1A  shows a state where an element substrate  101 , on which a driving circuit  108  and a pixel portion  109  are formed using TFTs (thin-film transistors) and a sealing substrate  102  are fixed using a sealing member  105 . A light-emitting element  103  is formed in the sealed space formed between the element substrate  101  and the sealing substrate  102 . A dryer agent  106  is provided on the driving circuit or in the vicinity of the sealing member  105 . It should be noted here that although not shown in this drawing, the dryer agent  106  may be contained in a partition wall  110  that is formed across the pixel portion  109  and the driving circuit  108 . 
     Each of the element substrate and sealing substrate is made of an organic resin material, such as polyimide, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), or aramid. The thickness of each of these substrates is set at around 30 to 120 μm to maintain the flexibilities of the substrates. 
     In the example shown in  FIG. 1A , DLC films  107  are formed at end portions as gas barrier layers. Note that the DLC films are not formed on an external input terminal  104 . An epoxy adhesive is used as the sealing member. To prevent from entering moisture, the DLC films  107  are formed to cover the sealing member  105  and the end portions of the element substrate  101  and the sealing substrate  102 . 
       FIG. 1B  shows a construction where a DLC film  110  is formed to cover the undersurface of the element substrate  101 , in addition to the DLC films  107  formed to cover the sealing member  105  and the end portions of the substrates  101  and  102 . Although depending on the thickness, a DLC film has low permeability to light whose wavelength is short (500 nm or less). Therefore, in this example, no DLC film is formed on the display surface (the main surface on a display side) of the sealing substrate  102 . This construction, however, completely prevents moisture from entering the element substrate  101  on which the TFTs are formed. As a result, the degradation of the TFTs and the light-emitting element does not occur. 
       FIG. 1C  shows a construction where gas barrier property is improved. In this drawing, a DLC film is formed to cover whole surfaces of the element substrate  101 , the sealing substrate  102 , and the sealing member  105 , except for the external input terminal  104 . In addition to the improvement in gas barrier property, this construction has the effect of preventing scratches or the like on the surfaces because the surfaces of the plates are protected by the DLC film. 
       FIG. 1D  shows an example where DLC films are formed on the element substrate  113  and the sealing substrate  114  beforehand. Then, other DLC films are additionally formed to cover the end portions in which the seating member for fixing these plates is formed. 
       FIG. 2  shows an example of a CVD apparatus used to form DLC films. This drawing mainly shows a vacuum chamber and other related processing means. As shown in this drawing, the vacuum chamber includes a common chamber  202  that has a transporting means for transporting a target substrate  218  to be processed, a load lock chamber  201  that inserts and removes the target substrate, and a first reaction chamber  203  and a second reaction chamber  204  that form DLC films on the target substrate. The load lock chamber  210  and the first and second reaction chambers  203  and  204  are connected to the common chamber  202  via gate valves  205  to  207 . Also, these chambers  201  to  204  are provided with exhausting means  208 ,  209 ,  211 , and  214 . 
     The first reaction chamber  203  is provided with a gas introducing means  212  and a discharge causing means  213 . Similarly, the second reaction chamber  204  is provided with a gas introducing means  215  and a discharge causing means  216 . These gas introducing means introduce above-described hydrocarbon gas or Ar, H 2 , and the like into the chambers. Each discharge causing means is composed of a cathode and an anode, which are arranged in respective reaction chambers, and a high-frequency (1 to 120 MHz) power source. DLC films are formed by setting the target substrate on the cathode side in the reaction chamber. Therefore, if DLC films are to be formed on both of the element substrate and the sealing substrate, as shown in  FIG. 1C , the posture of the target substrate need to be changed (for instance, the target substrate is required to be turned around). 
       FIGS. 3A and 3B  each show a state where a DLC film is formed on one surface of the target substrate in the first reaction chamber  203  and another DLC film is formed on the other surface of the target substrate in the second reaction chamber  204 . 
     In  FIG. 3A , a reaction chamber  301  is connected to a gas introducing means  302  and includes a cathode  305 , to which a high-frequency power source  304  is connected, and an anode  306  having a shower plate  309  for supplying gas to the reaction chamber. The reaction chamber  301  is also connected to an exhausting means  303 . A target substrate  308  is placed on the cathode  305 . Pressure pins  307  are used to transport the target substrate. With this construction, a DLC film is formed on one-surface and end portions of the target substrate in the reaction chamber. Also, if the cathode has a stepped cross section, as shown in  FIG. 3A , it becomes possible to have the formed DLC film also cover undersurface areas in the vicinity of the end portions of the target substrate. Needless to say, the DLC film covering the undersurface areas is thinner than that covering other areas. 
       FIG. 3B  shows a example of construction of a reaction chamber where a DLC film is formed on a surface opposing to that processed in  FIG. 3A  (the undersurface of the target substrate). A reaction chamber  310  is connected to a gas introducing means  312  and includes a cathode  315 , to which a high-frequency power source  314  is connected, and an anode  316  having a shower plate  320  for supplying gas to the reaction chamber  310 . The reaction chamber  310  is also connected to an exhausting means  313 . A target substrate  318  is required to be set at the cathode  315 , so that the reaction chamber  310  is further provided with a holder  319  and a mechanism  311  for moving the holder up or down. The target substrate  318  is first held by pressure pins  317  and then is set at the cathode  315  by the holder  319  that is elevated by the mechanism  311 . In this manner, a DLC film is formed on the surface opposing to that processed in  FIG. 3A  (the undersurface of the target substrate). 
     As described above, with the plasma CVD apparatus shown in  FIGS. 2 ,  3 A, and  3 B, it becomes possible to realize the display devices shown in  FIGS. 1A to 1D  where DLC films are formed as gas barrier layers. Needless to say,  FIGS. 2 ,  3 A, and  3 B each show an example construction of the CVD apparatus, so that the display devices shown in  FIGS. 1A to 1D  may be produced with a film forming apparatus having another construction. For instance, DLC films may be formed with a CVD apparatus that utilizes a microwave or electron cyclotron resonance. 
     The DLC films used as gas barrier layers more effectively prevent moisture and oxygen from entering a sealed space and thus enhances the stability of a light-emitting element. For instance, this construction reduces the number of dark spots resulting from the oxidation of a cathode. 
     Embodiment Mode 2 
       FIGS. 14A and 14B  each show an example where a pixel portion and a driving circuit are formed on a substrate having an insulating surface (such as a glass substrate, a ceramic substrate, a crystallized glass substrate, a metal substrate, or a plastic substrate). 
     In these drawings, reference numeral  1401  represents a gate-side driving circuit; numeral  1402 , a source-side (data-side) driving circuit; and numeral  1403 , a pixel portion. Signals transmitted to the gate-side driving circuit  1401  and the source-side driving circuit  1402  are supplied from an FPC (flexible print circuit)  1405  via input wiring  1404 . 
     A sealing substrate  1406  is used to seal light-emitting elements. The light-emitting elements emit light toward the sealing substrate  1406 , so that the sealing substrate  1406  is required to have transparency. Numeral  1407  represents a sealing resin used to seal the sealing substrate  1406  and the element substrate  1400 . A cross-sectional view taken along the line A–A′ in  FIG. 14A  is shown in  FIG. 14B . In this drawing, the sealing substrate  1406  is also covered with a DLC film to prevent the penetration of oxygen. 
     After an insulating film  1411  is formed on the element substrate  1400 , a light-emitting element  1412  composed of a cathode  1413 , an organic compound layer (including a light-emitting layer)  1414 , and an anode  1415  is formed on the insulating film  1411 . A protecting layer  1417  is further formed on the cathode  1413  to protect the light-emitting element  1412  that is easily oxidized by oxygen and moisture. It is preferable that the insulating film is transparent or translucent to visible radiation. 
     The cathode  1413  and the anode  1415  are also transparent or translucent to visible radiation. Here, transparency to visible radiation means that the permeability to visible radiation is around 80 to 100% and translucency to visible radiation means that the permeability to visible radiation is around 50 to 80%. The anode  1415  and the cathode  14113  must be respectively made of a conductive oxide film with a work function of 4.5 to 5.5 and a conductive film with a work function of 2.0 to 3.5 (typically a metal film including an element belonging to Group 1 or 2 of the periodic table). In many cases, however, the metal coat is not transparent to visible radiation, so that it is preferable that the construction shown in  FIGS. 14A and 14B  is used. The cathode  1413  that is translucent to visible radiation is formed by laminating a thin metal film with a thickness of 5 to 70 nm (preferably, 10 to 30 nm) and a conductive oxide film (ITO, for instance). Note that the organic compound layer (including the light-emitting layer)  1414  may adopt a well-known structure and the organic compound layer may be used alone or laminated with a carrier (electrons or holes) injecting layer, a carrier transporting layer, or a carrier blocking layer. 
     To prevent the degradation of the light-emitting element due to oxygen and moisture, DLC films are formed at the end portions of the display device and a dryer agent is further provided between the first substrate  1400  and the second substrate  1406 . Note that the dryer agent is provided by forming a barium oxide (BaO 2 ) layer on the second substrate using an EB vapor deposition method or by sealing the dryer agent in a powder state between the substrates. Alternatively, the dryer agent may be provided to function as a spacer by mixing the dryer agent with a resin and providing the mixture on partition walls or at positions (such as on the driving circuit or wiring that connects the driving circuit to picture elements) outside light-emitting areas. Further, the dryer agent may be mixed with a resin that is the material of the partition walls. The dryer agent may be provided with any of the methods described above. Note that in this embodiment mode, powder of barium oxide is provided as the dryer agent in a space  1409  between a sealing resin  1407  and a resin  1408 , as shown in  FIG. 14B . 
     With the construction shown in  FIGS. 14A and 14B , emitted light passes through the cathode and is directly observed by an observer. Most of the outside light is absorbed by an organic interlayer insulating film  1419  made of a black resin, so that the amount of the outside light reflected toward an observer is reduced to a level where no problem arises. As a result, the reflected light does not reach the observer and the outside scenes are not reflected by the surface facing the observer. 
     The following is a description of the method of forming DLC films at end portions of the light-emitting device produced by laminating the element substrate  1400  and the sealing substrate  1406 , with reference to  FIGS. 15A and 15B . A light-emitting device  1501  is held by a holding means  1502   a  in a reaction chamber  1500 . The reaction chamber  1500  is provided with an introducing opening  1508  and an exhausting opening  1509  that respectively introduces and exhausts gas used to form DLC films. Also, means (RF electrodes)  1503  for causing plasma are provided in the reaction chamber  1500 . The holding means  1502   a  is fixed to the reaction chamber and the light-emitting device  1501  on the holding means  1502   a  is pressed against the holding means  1502   a  by the movable holding means  1502   b.    
     The electrodes  1503  are connected to (high-frequency) power sources  1505  and matching circuits  1504 . Typical RF power sources are used as the power sources  1505 . The electrodes  1503  are connected to the RF power sources  1505  that apply voltages to the electrodes  1503 . A phase adjuster  1510  is provided to adjust the phases of the RF power sources  1505 . With this construction, the electrodes are supplied with power, whose phases differ from each other by 180°, from the RF power sources.  FIG. 15A  shows a state where one pair of electrodes is provided in the reaction chamber, however, a plurality of pairs of electrodes or cylindrical electrodes may be used. 
     To form DLC films in end portions of the light-emitting device  1501 , surfaces in the end portions need to be subjected to ion bombardment. Therefore, the holding means  1502   a  is connected to a power source  1507 . To generate a self-bias, a capacitor  1511  is arranged between the power source  1507  and the holding means  1502   a . The holding means  1502   a  is provided as a means for applying a bias to the substrate. Also, the holding means  1502   b  is provided to prevent the DLC films from being formed on the entire surface of the light-emitting device  1501 . That is, the holding means  1502  functions as a mask that covers a light-emitting area and the external input terminal (FPC) to thereby prevent the DCL films from forming thereon. Note that the layer forming conditions are appropriately set by an operator of the film forming apparatus. 
     To form DLC films at end portions of the light-emitting device produced by laminating an element substrate and a sealing substrate, the holding means  1502   a  is divided into two masking portions: a masking portion (hereinafter, a light-emitting area mask) that covers the light-emitting area, and a masking portion (hereinafter, an external input terminal mask) that covers the external input terminal. These masking portions are partially connected to each other. It is preferable that the width of the connection between the light-emitting area mask and the external input terminal mask is set at 5 mm or less (see  FIG. 15B ). It is also preferable that the relation between the width of the connection and the height of the holding means  1502   b  satisfies a condition “Height/Width≧around 2” (see  FIG. 15B ). 
     Aside from the holding means composed of the light-emitting area mask and the external input terminal mask, an ordinary masking tape may be used in the CVD apparatus to cover the external input terminal to thereby prevent the formation of a DLC film thereon. To prevent the degradation of the light-emitting element due to oxygen and moisture, DLC films need to be formed in four end portions of the light-emitting device  1501 . To effectively and evenly form the DLC films, a member  1506  supporting the holding means  1502   a  may be given a rotating function. 
     The holding means  1502   a  doubles as an electrode that applies a negative self-bias to the light-emitting device  1501 . The power source  1507  applies a negative self-bias to the electrode  1502 . Minute DLC films are formed in the end surfaces of the light-emitting device  1501  using a source gas accelerated by the negative self-bias voltage. Note that the source gas is an unsaturated hydrocarbon gas (such as methane, ethane, propane, or butane), an aromatic gas (such as benzene or toluene), or a halogenated hydrocarbon where at least one hydrocarbon molecular is replaced by a halogen element, such as F, Cl, or Br. 
     In the manner described above, DLC films  1510  with a thickness of 5 to 100 nm (preferably, 10 to 30 nm) are formed to coat the end portions of the light-emitting device.  FIG. 23  shows a state where DLC films are formed on a light-emitting device using the film forming apparatus of the present invention. DLC films are directly formed on the side surfaces and edge portions of the surfaces of a substrate in this embodiment mode. However, to bring the DLC films into intimate contact, nitride films (such as silicon nitride films or silicon oxynitride films) may be formed as base films before the DLC films are formed. In this case, the thickness of the nitride films is set at 2 to 20 nm. 
     Embodiment 1 
     The present invention is applicable to various types of display devices so long as the display devices use light-emitting elements.  FIG. 4  shows an example of display device to which the present invention is applied. The display device in this drawing is an active matrix type display device produced using TFTs. TFTs are classified into amorphous silicon TFTs and polysilicon TFTs, depending on what materials are used to produce semiconductor films that form channel formation regions. The present invention is applicable to both types of TFTs. 
     It is impossible to produce an organic resin substrate, which is resistant to heat processing at 450° C. or higher, using a commercially available material. A laser anneal technique, however, makes it possible to produce polysilicon TFTs only by heating the substrate to 300° C. or below. Also, in many cases, hydrogenation processing is required to be performed during the production of polysilicon TFTs. A plasma-aided hydrogenation processing makes it possible to produce polysilicon TFTs only by heating the substrate to around 200° C. 
     In  FIG. 4 , an N-channel type TFT  452  and a P-channel type TFT  453  are formed in a driving circuit portion  450 , and a switching TFT  454  and a current control TFT  455  are formed in a pixel portion  451 . These TFTs are formed using various components, such as island-like semiconductor layers  403  to  406 , a gate insulating film  407 , and gate electrodes  408  to  411 . 
     A substrate  401  is made of an organic resin material (such as polyimide, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), or aramid) to have a thickness of 30 to 120 μm (typically, 75 μm). A blocking layer  402  is made of silicon oxynitride (SiO x N y ) or a silicon nitride film to have a thickness of 50 to 200 nm, thereby preventing the precipitation of oligomer or the like from the substrate  401 . An interlayer insulating film includes an inorganic insulating film  418  made of silicon nitride or silicon oxynitride and an organic insulating film  419  made of acrylic or polyimide. 
     The driving circuit potion  450  includes a gate-signal-side driving circuit and a data-signal-side driving circuit having different circuit constructions, although the circuit constructions are not described here. The N-channel type TFT  452  and the P-channel type TFT  453  are connected to wirings  412  and  413  and are used to form a shift resister, a latch circuit, and a buffer circuit. 
     In the pixel portion  451 , data wiring  414  is connected to the source of the switching TFT  454  and drain-side wiring  415  is connected to the gate electrode  411  of the current control TFT  455 . Also, the source of the current control TFT  455  is connected to power source wiring  417  so as to connect a drain-side electrode  416  with the anode of the light-emitting element.  FIGS. 5A and 5B  each show a top view of the pixel portion constructed in this manner. For ease of explanation, the same reference numerals as in  FIG. 4  are used in  FIGS. 5A and 5B . Also, a cross-sectional view taken along the line A–A′ in  FIG. 5A  is shown in  FIG. 4 . 
     As shown in  FIG. 4 , partition walls  420  and  421  are formed using an organic resin, such as acrylic or polyimide, or preferably a photosensitive organic resin to cover the wiring. The light-emitting element  456  is composed of an anode  422  made of ITO (indium tin oxide), an organic compound layer  423  including a luminescent material, and a cathode  424  made of MgAg, LiF, or the like. The partition walls  420  and  421  are provided to cover the end portion of the anode  422 , thereby preventing shorts between the cathode and the anode. 
     It does not matter whether the organic compound layer is made of a low molecular material or a high molecular material. A vapor deposition method is used in the case of the low molecular material, while a spin coat method, a printing method, or an ink jet method is used in the case of the high molecular material. 
     A well-known high molecular material is a π-conjugated polymer material. The typical examples thereof are crystalline semiconductor film p-phenylene vinylene (PPV) derivatives, poly vinyl carbazole (PVK) derivatives, and polyfluorene derivatives. The organic compound layer made of such a material may be used alone or laminated with other layers to form a laminated structure, although a higher luminous efficiency is obtained in the latter case. Generally, the laminated structure is formed by stacking an anode, a hole injecting layer, a hole transporting layer, a light-emitting layer, and an electron transporting layer in this order. However, the laminated structure may be formed by stacking an anode, a hole transporting layer, a light-emitting layer, an electron transporting layer or a hole injecting layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, and an electron injecting layer in this order. The present invention can be made with any of well-known laminated constructions. Also, the organic compound layer may be doped with a fluorescent coloring agent. 
     Typical materials are, for instance, disclosed in U.S. Pat. No. 4,356,429, U.S. Pat. No. 4,539,507, U.S. Pat. No. 4,720,432, U.S. Pat. No. 4,769,292, U.S. Pat. No. 4,885,211, U.S. Pat. No. 4,950,950, U.S. Pat. No. 5,059,861, U.S. Pat. No. 5,047,687, U.S. Pat. No. 5,073,446, U.S. Pat. No. 5,059,862, U.S. Pat. No. 5,061,617, U.S. Pat. No. 5,151,629, U.S. Pat. No. 5,294,869, U.S. Pat. No. 5,294,870, Japanese Patent Application Laid-open No. Hei 10-189525, Japanese Patent Application Laid-open No. Hei 8-241048, and Japanese Patent Application Laid-open No. Hei 8-78159. 
     It should be noted here that there are four major methods of displaying color images. With the first method, three types of light-emitting elements each corresponding to one of R (red), G (green), and B (blue) are formed. With the second method, a light-emitting element that emits white light is combined with a color filter. With the third method, a light-emitting element that emits blue or cyan light is combined with a fluorescent member (a fluorescent color changing layer: CCM). With the fourth method, light-emitting elements each corresponding to one of R (red), G (green), and B (blue) are stacked using a transparent electrode as a cathode (an opposing electrode). 
     In more detail, an organic compound layer that emits red light is made of cyanopolyphenylene, an organic compound layer that emits green light is made of polyphenylenevinylene, and an organic compound layer that emits blue light is made of polyphenylenevinylene or polyalkylphenylene. Each organic compound layer is 30 to 150 nm in thickness. 
     Organic EL materials that can be used as a light-emitting layer are given above, although the present invention is not limited to them. Any of available combinations of materials of a light-emitting layer, a charge transporting layer, and a charge injecting layer may be freely selected. The organic compound layer in this embodiment has a construction where a light-emitting element is combined with a hole injecting layer made of PEDOT (polythiophene) or PAni (polyaniline). 
     The cathode  424  placed on the organic compound layer  423  is made of a material including magnesium (Mg), lithium (Li), or calcium (Ca) each having a low work function. It is preferable that an MgAg electrode (Mg:Ag=10:1) is used as the cathode  424 . An MGAgAl electrode, LiAl electrode, and LiFAl electrode may also be used as the cathode  424 . 
     It is preferred to successively form the organic compound layer  423  and the cathode  424  without leaving them in the air. This is because the condition of the interface between the cathode  424  and the organic compound layer  423  greatly effects the luminous efficiency of the light-emitting element. Note that in this specification, a light-emitting element means a light-emitting element composed of an anode (pixel electrode), an organic compound layer, and a cathode. 
     One laminated structure including the organic compound layer  423  and the cathode  424  is required to be formed for each picture element, but the organic compound layer  423  is extremely vulnerable to moisture. Therefore, an ordinary photolithograph technique cannot be used to form the laminated structure. Also, the cathode  424  made of an alkaline metal is easily oxidized. As a result, it is preferred to selectively form the lamination member with a vapor phase method, such as a vacuum deposition method, a sputtering method, or a plasma CVD method, using a physical mask, such as a metal mask. Note that it is possible to selectively form the organic compound layer with another method, such as an ink-jet method or a screen printing method, although it is currently impossible to successively form cathodes with these methods. As a result, it is preferable to use the vapor phase method. 
     Also, a protecting electrode for protecting the cathode  424  from the outside moisture and the like may be stacked on the cathode  424 . It is preferable that the protecting electrode is made of a low resistant material including aluminum (Al), copper (Cu), or silver (Ag). Alternatively, the protecting electrode may be a transparent electrode. In this case, light is emitted in the direction of the arrow shown in  FIG. 4  (the light emission in this direction is hereinafter referred to as a “top surface emission”, for ease of explanation). In this case, by mixing a black pigment into the organic resin interlayer insulating film  419 , no polarizing plate is required to form a black screen during a non-light-emission period. This protecting electrode is also expected to achieve a heat dissipation effect that lowers the temperature of the organic compound layer. It is also effective to successively form the organic compound layer  423 , the cathode  424 , and the protecting electrode without leaving them in the air. 
     In  FIG. 4 , the switching TFT  454  has a multi-gate construction and the current control TFT  455  is provided with an LDD overlapping the gate electrode. A TFT produced using a polysilicon operates at high speed and therefore degradation, such as hot carrier injection, tends to occur for the TFT. Therefore, TFTs are formed to have different constructions according to their functions and are provided in a pixel portion (in the case of  FIG. 2 , the switching TFT whose OFF current is sufficiently reduced is combined with the current control TFT that is resistant to hot carrier injection), which is highly effective in producing a display device that achieves high reliability and superior image display (high operation performance). 
       FIG. 6  shows the external appearance of such a display device. The direction in which an image is displayed depends on the construction of the light-emitting element, although light is emitted upward to display image in this drawing. In  FIG. 6 , an element substrate  601 , on which driving circuit portions  604  and  605  and a pixel portion  603  have been formed using TFTs, and a sealing substrate  602  are laminated using a sealing member  610 . One end of the element substrate  601  is provided with an input terminal  608  via which an FPC is connected to the display device. The input terminal  608  includes a plurality of terminals that receive an image data signal, various timing signals, and electricity from an external circuit. Here, the interval between the terminals is set at 500 μm. The input terminal  608  is connected to the driving circuit portion via wiring  609 . Here, an IC ship  607  on which a CPU and a memory have been formed may be mounted on the element substrate  601  using a COG (Chip on Glass) method or the like, as necessary. 
     A DLC film  611  is formed in end portions to prevent moisture and oxygen from entering through sealed portions and being degraded in light-emitting elements. In the case where the element substrate  601  and the sealing substrate  602  are made of an organic resin material, the DLC film may be formed to coat the entire surface of the display device, except for an input terminal, as described by referring to  FIG. 1C . In this case, the input terminal is covered with a masking tape or a shadow mask prior to the formation of the DLC film. 
     As shown in  FIG. 7 , the input terminal is formed by stacking an ITO  706  formed as an anode on wiring  705  made of titanium (Ti) and aluminum (Al). Incidentally,  FIG. 8  is a cross-sectional view of the input terminal taken along the line C–C′. An element substrate  701  and a cover substrate  702  are laminated using a sealing member  703  and a DLC film  704  is formed to cover the sealing member  703  and the end portions of the element substrate  701  and the cover substrate  702 . In the driving circuit portion, an organic compound layer  707  and a cathode  708  are formed on a partition wall  709  and a contact region  710  is formed to establish the contact between the cathode  708  and the wiring, as shown in  FIG. 7 . 
     By forming DLC films on a display device that uses an organic resin substrate, the degradation of light-emitting elements is prevented and the stability of the display device is ensured for the long term. The display device using the organic resin substrate is in particular suitable as a display device for a portable device. If the portable device is used outdoors, however, it is required to increase the reliability of the display device in consideration of the exposure to the direct sunlight, wind, and rain. The DLC films also satisfy this requirement for increasing the reliability of the display device. 
     Embodiment 2 
     In this embodiment, the degradation of a light-emitting element is prevented using a means for sealing a dryer agent, such as barium oxide, in gaps of a light-emitting device or a space in which the light-emitting element is sealed. In  FIGS. 1A to 1D , a dryer agent is provided on a driving circuit or in areas in which a sealing member has been formed. In the present embodiment, a dryer agent is provided in a different manner, as shown in  FIGS. 9A to 9C . As can be seen from these drawings, a dryer agent is arranged in partition walls that are provided to separate adjacent picture elements in a pixel portion.  FIGS. 9A to 9C  are each a cross-sectional view taken alone the line B–B′ in  FIG. 5 . For ease of explanation, the same reference numerals as in  FIGS. 4 ,  5 A, and  5 B are used in  FIGS. 9A to 9C . 
       FIG. 9A  shows an example where a dryer agent  480  is dispersed in the partition wall  421 . The partition wall  421  is made of a thermosetting or photosensitive organic resin material. The dryer agent is dispersed in the organic resin material prior to the polymerization of the organic resin material, and then the organic resin material including the dryer agent is applied as it is to form the partition wall  421 . 
       FIG. 9B  shows an example where a dryer agent  481  is formed on an organic resin insulating film  419 . In this case, the dryer agent is formed at a predetermined position to have a predetermined pattern using a vacuum deposition method or a printing method. Then, the partition wall  421  is formed on the dryer agent  481 . 
       FIG. 9C  shows an example where a dryer agent  482  is formed on the partition wall  421 . In this case, the dryer agent  482  is formed using a vacuum deposition method or a printing method, similarly to the case shown in  FIG. 9B . 
       FIGS. 9A to 9C  show examples of the formation of the dryer agent, and these examples may be combined with each other as appropriate. Also, the constructions of the present embodiment may be combined with the construction shown in  FIG. 1 . If the stated formations of the dryer agent are applied to the display device of Embodiment 1, a display device with high reliability is realized by the dryer agent combined with the gas barrier property of the DLC films. 
     Embodiment 3 
       FIG. 10  shows an example of a display device that uses an inverted stagger type TFT. A substrate  501  and a light-emitting element  556  used in this embodiment are the same as those of Embodiment 1 and therefore are not described here. 
     The inverted stagger type TFT is formed by stacking the substrate  501 , gate electrodes  508  to  511 , gate insulating films  507 , and semiconductor films  503  to  506  in this order. In  FIG. 10 , an N-channel type TFT  552  and a P-channel type TFT  553  are formed in a driving circuit portion  550 . Also, a switching TFT  554 , a current control TFT  555 , and a light-emitting element  556  are formed in a pixel portion  551 . An interlayer insulating film is composed of an inorganic insulating film  518  made of silicon nitride or silicon oxynitride and an organic resin film  519  made of acrylic or polyimide. 
     The driving circuit portion  550  includes a gate-signal-side driving circuit and a data-signal-side driving circuit having different circuit constructions, although the circuit constructions are not described here. The N-channel type TFT  552  and the P-channel type TFT  553  are connected to wiring  512  and  513  and form a shift resister, a latch circuit, and a buffer circuit. 
     In the pixel portion  551 , data wiring  514  is connected to the source side of the switching TFT  554  and drain-side wiring  515  is connected to a gate electrode  511  of the current control TFT  555 . Also, the source of the current control TFT  555  is connected to a power supplying wiring  517  so as to connect a drain-side electrode  516  to an anode of the light-emitting element. 
     Partition walls  520  and  521  are formed using an organic resin, such as acrylic or polyimide, or preferably a photosensitive organic resin to cover the wiring. The light-emitting element  556  is composed of an anode  522  made of ITO (indium tin oxide), an organic compound layer  523  produced using an organic EL material, and a cathode  524  made of MgAg, LiF, or the like. The partition walls  520  and  521  are provided to cover the end portion of the anode  522 , thereby preventing shorts between the cathode and the anode. 
     Components other than the TFTs, such as the pixel portion, of the display device have the same constructions as in Embodiment 1. It is advantageous to use the inverted stagger type TFT produced using polysilicon because the manufacturing line for amorphous silicon TFTs (usually formed as inverted stagger type TFTs) can be used as it is. Needless to say, a laser anneal technique using an eximer laser makes it possible to produce polysilicon TFTs at a processing temperature of 300° C. or below. 
     Embodiment 4 
     In this embodiment, an example of construction of an electronic device using the display device of Embodiment 1 is described with reference to  FIG. 11 . A display device  900  in  FIG. 11  includes a pixel portion  921 , which is composed of picture elements  920  formed by TFTs on a substrate, and a data-signal-side driving circuit  915  and a gate-signal-side driving circuit  914  that are used to drive the pixel portion. In the example shown in  FIG. 11 , the data-signal-side driving circuit  915  uses a digital driving method and includes a shift register  916 , latch circuits  917  and  918 , and a buffer circuit  919 . Also, the gate-signal-side driving circuit  914  includes various components, such as a shift register and a buffer (not shown). 
     In the case of VGA, the pixel portion  921  includes 640 picture elements wide by 480 picture elements high. Also, as described by referring to  FIGS. 4. 5A , and  5 B, a switching TFT and a current control TFT are arranged for each picture element. The light-emitting element operates as follows. When gate wiring is selected, the gate of the switching TFT opens, data signal on source wiring is accumulated in a capacitor, and the gate of the current control TFT opens. That is, data signal inputted from the source wiring causes the flow of current into the current control TFT and the light-emitting element emits light. 
     The system block diagram shown in  FIG. 11  relates to the application of the display device of Embodiment 1 to a portable information terminal, such as a PDA. The display device of Embodiment 1 includes a pixel portion  921 , a gate-signal-side driving circuit  914 , and a data-signal-side driving circuit  915 . 
     An external circuit connected to the display device includes a power circuit  901  composed of a stabilized power source and a high-speed and high-precision operational amplifier, an external interface port  902  provided with a USB terminal or the like, a CPU  903 , an input means composed of a pen input tablet  910  and detection circuit  911 , a clock signal oscillator  912 , and a control circuit  913 . 
     The CPU  903  includes an image signal processing circuit  904  and a tablet interface  905  for receiving signals from the pen input tablet  910 , and is connected to a VRAM  906 , a DRAM  907 , a flash memory  908 , and a memory card  909 . Information processed in the CPU  903  is sent as an image signal (data signal) from the image signal processing circuit  904  to the control circuit  913 . The control circuit  913  has a function of converting the image signal and a clock signal to signals which can be used corresponding to the data-signal-side driving circuit  915  and the gate-signal-side driving circuit  914 , respectively. 
     In more detail, the control circuit  913  has a function of dividing the image signal into a plurality of pieces of data corresponding to respective picture elements. The control circuit  913  also has a function of converting a horizontal synchronizing signal and a vertical synchronizing signal inputted from the outside into two signals: a start signal used by the driving circuit, and a timing control signal required to convert the current generated by an internal power circuit into an alternating current. 
     It is desired that a portable information terminal, such as a PDA, can be used outdoors (in a train, for instance) for a long time using a rechargeable battery as a power supply (that is, without connecting the terminal to an AC outlet). Such an electronic device is also required to be easily portable and thus the weight and size thereof need to be reduced. The battery occupying the majority of weight of the electronic device increases in weight in accordance with the increase in battery capacity. Accordingly, various measures based on software techniques need to be used to reduce the power consumption of the electronic device. For instance, the time period in which a backlight is turned on is controlled or a standby mode is used. 
     In the case of the electronic device of the present embodiment, if no input signal is inputted from the pen input tablet  910  into the tablet interface  905  of the CPU  903  for a predetermined time period, the electronic device is placed in a standby mode and the components enclosed with dotted lines in  FIG. 11  stop their operations in synchronization with each other. Also, the display device reduces the strength of light emitted by the light-emitting element or stops the image displaying operation. Alternatively, memories corresponding to respective picture elements may be used to change the electronic device into a still image displaying mode. With these measures, the power consumption of the electronic device is reduced. 
     Also, a still image may be displayed by stopping the operations of the image signal processing circuit  904  of the CPU  903  and the VRAM  906  to reduce the power consumption. In  FIG. 11 , the components that continue to operate even in the still image displaying mode are indicated using dotted lines. Also, as shown in  FIG. 6 , the control circuit  913  may be mounted on the element substrate using an IC chip with a COG method, or integrally formed in the display device. 
     The display device using the organic resin substrate of the present invention contributes to the weight reduction of an electronic device. If a display device whose size is five inches or the like is used for an electronic device, the weight of the electronic device becomes around 60 g with a glass substrate. However, with a display device using the organic resin substrate of the present invention, the weight of the electronic device is reduced to 10 g or less. Further, because DLC films coat the surface of the display device, the surface increases in hardness and becomes resistant to scratches or the like. As a result, the beautiful condition of the display screen is continued. As described above, the present invention achieves a superior effect for an electronic device, such as a portable information terminal. 
     Embodiment 5 
     In this embodiment, a method of forming a cathode of a light-emitting element is described with reference to  FIGS. 16A to 16C . In these drawings, an insulating film  1601 , an anode  1602  formed as a first electrode, an organic compound layer  1603 , a cathode  1604  formed as a second electrode, and a DLC film  1605  are stacked in this order. 
     First, the description is given of  FIG. 16A  below. In this drawing, a silicon oxide film is used as the insulating film  1601 , a conductive oxide film (thickness=120 nm) formed by adding gallium oxide to zinc oxide is used as the anode  1602 , and a lamination film composed of copper-phthalocyanine (a hole injecting layer) with a thickness of 20 nm and Alq 3  (quinolilato-aluminum complex: light-emitting layer) with a thickness of 50 nm is used as the organic compound layer  1603 . The cathode  1604  has a laminated construction where a transparent electrode  1604   b  is stacked on a translucent electrode  1604   a  formed using an ultra-thin metal film. For instance, the translucent electrode  1604   a  is formed using an MgAg film with a thickness of 20 nm (alloy film formed by evaporating magnesium and silver) and the transparent electrode  1604   b  is formed using a conductive oxide film (thickness=200 nm) formed by adding gallium oxide to zinc oxide. A protecting film  1605  is formed using a DLC film. 
     Also, in  FIG. 16B , the insulating film  1601 , the anode  1602 , the organic compound layer  1603 , and an electron injecting layer  1606  that is a LiF film are stacked in this order. The cathode  1604  that is a conductive oxide film (thickness=200 nm) formed by adding gallium oxide to zinc oxide and a protecting film  1605  formed using a DLC film are stacked on the electron injecting layer  1606 . 
     In  FIG. 16C , the insulating film  1601 , the anode  1602 , and the organic compound layer  1603  are stacked in this order. Then an LiF film  1606  is stacked on the organic compound layer  1603  as an electron injecting layer, and the cathode  1604  is stacked on the film  1606 . The cathode  1604  is composed of the translucent electrode  1604   a  that is an MgAg film with a thickness of 50 nm or less (preferably, 20 nm) (alloy film formed by evaporating magnesium and silver) and the transparent electrode  1604   b  that is a conductive oxide film (thickness=200 nm) formed by adding gallium oxide to zinc oxide. The protecting film  1605  that is formed using a DLC film is stacked on the cathode  1604 . 
     After a light-emitting element is formed to have any one of the constructions described above, the light-emitting element is sealed and DLC films are formed at end portions with any one of the aforementioned methods. In this manner, the degradation due to oxygen and moisture is prevented. 
     Embodiment 6 
     In this embodiment, the cathode of a light-emitting element is formed with a method differing from that of Embodiment 1. Here, the method of forming a cathode is below described with reference to  FIGS. 17A and 17B . In  FIG. 17A , a cathode  1702  made of an alkaline metal (Li or Mg, for instance) with a low work function is formed on an insulating film  1701 . Then, an organic compound layer  1703 , an anode  1704 , and a protecting layer  1705  (a DLC film) are formed on the cathode  1702 . 
     In  FIG. 17B , a transparent electrode  1702   a  that is a transparent conductive film ITO and a translucent electrode  1702   b  that is an ultra-thin (thickness=50 nm or less) metal film (Al—Li alloy film or MgAg alloy film, for instance) are stacked in this order on the insulating film  1701  to form the cathode  1702 . Then, the organic compound layer  1703 , the anode  1704 , and the protecting film  1705  that is a DLC film are formed on the cathode  1702 . 
     Embodiment 7 
     In this embodiment, the organic compound layer is described in more detail. Accordingly, it is possible to combine the present embodiment with any construction of the embodiment modes and Embodiments 1 to 6. Note that in this embodiment, an anode  1801  that is a first electrode is formed using a conductive oxide film. Also, a cathode that is a second electrode is formed using a conductive film to have any of constructions described with reference to  FIGS. 18A to 18D . 
       FIG. 18A  shows a construction where an anode  1801 , a hole injecting layer  1802 , a hole transporting layer  1803 , a light-emitting layer  1804 , an electron transporting layer  1805 , an electron injecting layer  1806 , and a cathode  1807  are formed and stacked in this order.  FIG. 18B  shows a construction where the anode  1801 , the hole injecting layer  1802 , the light-emitting layer  1804 , the electron transporting layer  1805 , the electron injecting layer  1806 , and the cathode  1807  are formed and stacked in this order.  FIG. 18C  shows a construction where the anode  1801 , the hole injecting layer  1802 , the light-emitting layer  1804 , the electron injecting layer  1806 , and the cathode  1807  are formed and stacked in this order.  FIG. 18D  shows a construction where the anode  1801 , the hole injecting layer  1802 , the hole transporting layer  1803 , the light-emitting layer  1804 , and the cathode  1807  are formed and stacked in this order. 
     These are just a few examples of the construction of the organic compound layer and therefore there are various different constructions that can be used for the present invention. It is possible to use the stated constructions of the organic compound layer in combination with Embodiments 1 to 6. 
     Embodiment 8 
     In this embodiment, in addition to DLC films formed at end portions of a light-emitting device, a dryer agent is provided in a light-emitting element to prevent the degradation due to oxygen and moisture. This construction is described with reference to  FIGS. 19A and 19B . Reference numeral  1901  represents a glass substrate that is a first substrate, and a base insulating film  1902  is formed on the first substrate  1901 . An amorphous silicon layer is formed on the base insulating film  1902  and is crystallized using a well-known technique to produce a crystalline silicon film, then the crystalline silicon film is processed to have an island-like pattern, thereby forming an active layer  1904  of each TFT. 
     A gate insulating film (not shown), gate electrodes  1905 , interlayer insulating films  1906 , and pixel electrodes (first electrodes)  1907  made of an alkaline metal or an alkaline earth metal with a low work function are formed on the active layer. An organic compound layer  1908  is formed on the pixel electrodes  1907 , and an anode (second electrode)  1909  is formed on the organic compound layer  1908  using a conductive oxide film (ITO film, in this embodiment) made of a compound of an indium oxide and a tin oxide. 
     A partition wall  1910  is formed under the organic compound layer to cover each TFT. Here, if the partition wall is made of a material produced by mixing a dryer agent with a resin, moisture existing under the protecting layer  1911  is absorbed by the partition wall and the degradation of the light-emitting element is prevented. 
       FIG. 19B  shows another example where a resin (hereinafter, a dryer agent)  1912  mixed with a dryer agent is provided on the protective layer  1911  in the area of a driving circuit. This dryer agent  1912  also functions as a spacer. Note that the arrangement positions of the dryer agent  1912  may be freely determined so long as the agent is not arranged on input wiring or in areas in which pixel electrodes emit light. Also, the dryer agent  1912  may be provided by combining the stated arrangement methods. Further, the present embodiment may be combined with any of the constructions described in Embodiments 1 to 7. 
     Embodiment 9 
     In this embodiment, a first substrate (such as a glass substrate)  2001  is laminated with a third substrate (a film-like substrate, such as a plastic film or an ultra-thin stainless substrate)  2004  on which a light-emitting element is to be formed. After the formation of the light-emitting element, the third substrate  2004  is laminated with a second substrate  2003 . Then the glass substrate  2001  is peeled off using a laser or an agent and a film-like substrate is instead laminated. This processing is described in detail below with reference to  FIGS. 20A and 20B . 
     After the light-emitting element formed on the third substrate  2004  is sealed with the second substrate  2003 , a laser light is applied onto the undersurface of the glass substrate  2001  to evaporate a bonding layer  2002  (such as polyimide, polyamide, polyimideamide, an urethane resin, a photo-curing resin, a thermosetting resin, a polychlorinated vinyl resin, an epoxy resin, an acrylic adhesive, and a gum adhesive). In this manner, the glass substrate  2001  is peeled off. In this embodiment, a linear beam is formed using the second harmonic (wavelength=532 nm) of a YAG laser and is irradiated onto the bonding layer  2002  through the glass substrate  2001 . As a result, the bonding layer  2002  is evaporated and the glass substrate  2001  is peeled off. 
     After this, a plastic film substrate or a thin metal substrate is laminated instead of the peeled glass substrate. This realizes a flexible light-emitting device whose weight and thickness are both reduced. Note that it does not matter whether the bonding layer  2002  is laminated with the first substrate  2001  and then the third substrate  2004  or is laminated with the third substrate  2004  and then the first substrate  2001 . The present embodiment may be combined with any of the embodiment modes and Embodiments 1 to 8. 
     Embodiment 10 
     In this embodiment, an organic compound layer is produced by combining an organic compound (hereinafter, a singlet compound) that emits light by a singlet exciton (singlet) and an organic compound (hereinafter, a triplet compound) that emits light by a triplet exciton (triplet). Here, the singlet compound means a compound that emits light only via a singlet excited state and the triplet compound means a compound that emits light via a triplet excited state. 
     Typical organic compounds that can be used as the triplet compound are described in the following theses.
     (1) T. Tsutsui, C. Adachi, S. Saito, Photochemical Processes in Organized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p.437   (2) M. A. Baldo, D. F. O&#39;Brien. Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Rorrest, Nature 395 (1998) p.151
 
*This thesis discloses an organic compound expressed by the following formula.
   (3) M. A. Baldo, S. Lamansky. P. E. Burrows, M. E. Thompson, S. R. Forrest. Appl. Phys. Lett., 75 (1999) p.4   (4) T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nalamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502   

     In addition to the luminescent materials described in the above theses, it is thought that luminescent materials (in more detail, metal complexes and organic compounds) expressed by the following molecular formulas may also be used. 
                                
(“Et” indicates ethyl group. “M” indicates an element belonging to VIII–X groups in periodic table.)
 
                                
(“M” indicates an element belonging to VIII–X groups in periodic table.)
 
     In the above molecular formulas, M is an element belonging to Group 8, 9, or 10 of the periodic table. In the above theses, platinum and iridium are used. However, the inventors of the present invention consider that it is preferable to use nickel, cobalt, or palladium because these materials are inexpensive compared with platinum and iridium and therefore suitable for reducing the cost of fabricating light-emitting devices. It is thought that nickel is in particular preferable because nickel complexes are easy to form and thus the productivity is increased. 
     The triplet compound has a higher luminous efficiency than the singlet compound and it is possible to reduce an operating voltage (voltage required to have a light-emitting element emit light) without reducing the amount of emitted light and brightness. This embodiment is made using this feature. 
     If a low-molecular organic compound is used as a light-emitting layer, the life span of a light-emitting layer that emits red light is shorter than those of light-emitting layers that emit other colored lights under present circumstances. This is because the luminous efficiency of the red-light-emitting layer is lower than those of the other-colored-light-emitting layers and the operating voltage thereof is required to be increased to obtain the same brightness as those of the other-colored-light-emitting layers. This promotes the degradation of the red-light-emitting layer. 
     In this embodiment, however, a triplet compound having a high luminous efficiency is used as the red-light-emitting layer, so that the operating voltage thereof does not be required to be increased to obtain the same brightness as those of the light-emitting layers that emit green and blue lights. Accordingly, a situation is avoided where the degradation of the red-light-emitting-element is extremely accelerated. As a result, it becomes possible to display color images without causing problems, such as color deviations. The reduced operating voltage is also preferable because it becomes unnecessary for transistors to have high withstand voltages. 
     It should be noted here that the triplet compound is used as the light-emitting layer that emits red light in this embodiment, although the triplet compound may also be used as the light-emitting layer that emits green light or the light-emitting layer that emits blue light. 
     In the case of RGB color display, three types of light-emitting elements that respectively emit red light, green light, and blue light need to be provided in a pixel portion. In this case, it is possible to use the triplet compound for the light-emitting element that emits red light and use the singlet compound for other light-emitting elements. 
     By selectively using the triplet compound and the singlet compound in this manner, it becomes possible to have each light-emitting element operate at the same operating voltage (10V or less, preferably 3 to 10V). Accordingly, all power sources for the light-emitting device can have the same voltage (3V or 5V), which allows circuit design to be carried out without difficulty. Note that the construction described in this embodiment may be combined with any of the constructions of Embodiments 1 to 6. 
     Embodiment 11 
     A light-emitting device formed by implementing the present invention can be incorporated to various electric-equipment, and a pixel portion is used as an image display portion. Given as such electronic equipment of the present invention are cellular phones, PDAs, electronic books, video cameras, notebook computers, and image play back devices with the recording medium, for example, DVD (digital versatile disc), digital cameras, and the like. Specific examples of those are shown in  FIGS. 12A to 13D . 
       FIG. 12A  shows a cellular phone, which is composed of a display panel  9001 , an operation panel  9002 , and a connecting portion  9003 . The display panel  9001  is provided with a display device  9004 , an audio output portion  9005 , an antenna  9009 , etc. The operation panel  9002  is provided with operation keys  9006 , a power supply switch  9002 , an audio input portion  9008 , etc. The present invention is applicable to the display device  9004 . 
       FIG. 12B  also shows a cellular phone, which is composed of a main body or a housing  9101 , a display device  9102 , an audio output portion  9103 , an audio input portion  9104 , and an antenna  9105 . The display device  9102  can be provided with a touch sensor so as to operate buttons on the display. By using the organic resin substrate of the present invention, the substrate can be bent after the completion of the display device. Therefore, while such characteristics are used, the housing with 3 dimensional curing surfaces, which is designed based on the human engineering can be employed by the display device without difficulty. 
       FIG. 12C  shows a mobile computer, or a portable information terminal, which is composed of a main body  9201 , a camera portion  9202 , an image receiving portion  9203 , operation switches  9204 , and a display device  9205 . The present invention can be applied to the display device  9205 . In such electronic devices, the display device of 3 to 5 inches is employed, however, by employing the display device of the present invention, the reduction of the weight in the portable information terminal can be attained. 
       FIG. 12D  shows a portable book, which is composed of a main body  9301 , display devices  9303 , and a recording medium  9304 , an operation switch  9305 , and an antenna  9306 , and which displays the data recorded in MD or DVD and the data received by the antenna. The present invention can be applied to the display devices  9302 . In the portable book, the display device of the 4 to 12 inches is employed. However, by employing the display device of the present invention, the reduction of the weight and thickness in the portable book can be attained. 
       FIG. 12E  shows a video camera, which is composed of a main body  9401 , a display device  9402 , an audio input portion  9403 , operation switches  9404 , a battery  9405 , and the like. The present invention can be applied to the display device  9402 . 
       FIG. 13A  shows a personal computer, which is composed of a main body  9601 , an image input portion  9602 , a display device  9603 , and a key board  9604 . The present invention can be applied to the display device  9601 . 
       FIG. 13B  shows a player employing a recording medium with programs recorded thereon (hereinafter referred to as recording medium), which is composed of a main body  9701 , a display device  9702 , a speaker portion  9703 , a recording medium  9704 , and an operation switch  9705 . The device employs DVD (digital versatile disc), CD, etc. as the recording medium so that music can be listened, movies can be seen and games and internet can be done. The present invention can be applied to the display device  9702 . 
       FIG. 13C  shows a digital camera, which is composed of a main body  9801 , a display device  9802 , an eyepiece portion  9803 , an operation switch  9804 , and an image receiving portion (not shown). The present invention can be applied to the display device  9802 . 
       FIG. 13D  also shows a digital camera, which is composed of a main body  9901 , a display device  9902 , an image receiving portion  9903 , an operation switch  9904 , a battery  9905 , etc. The present invention can be applied to the display device  9902 . By using the organic resin substrate of the present invention, the substrate can be bent after the completion of the display device. Therefore, while such characteristics are used, the housing with 3 dimensional curing surfaces, which is designed based on the human engineering can be employed by the display device without difficulty. 
     The display device of the present invention is employed in the cellular phones in  FIGS. 12A and 12B , the mobile computer or the portable information terminal in  FIG. 12C , the portable book in  FIG. 12D , and the personal computer in  FIG. 13A . The display device can reduce the power consumption of the above device by displaying white letters on the black display in a standby mode. 
     In the operation of the cellular phones shown in  FIGS. 12A and 12B , luminance is lowered when the operation keys are used, and the luminance is raised after usage of the operation switch, whereby the low power consumption can be realized. Further, the luminance of the display device is raised at the receipt of a call, and the luminance is lowered during a call, whereby the low power consumption can be realized. Besides, in the case where the cellular phone is continuously used, the cellular phone is provided with a function of turning off a display by time control without resetting, whereby the low power consumption can be realized. Note that the above operations may be conducted by manual control. 
       FIGS. 21A and 21B  show cellular phones. Reference numeral  2701  denotes a display panel, and reference numeral  2702  denotes an operation panel. The display panel  2701  and the operation panel  2702  are connected in the connection portion  2703 . The cellular phone has a display portion  2704 , an audio output portion  2705 , operation keys  2706 , a power supply switch  2707 , and an audio input portion  2708 . The present invention can be applied to the display portion  2704 .  FIGS. 21A and 21B  show the lengthwise cellular phone and the widthwise cellular phone, respectively. 
       FIG. 21C  shows a car audio system, which is composed of a main body  2801 , a display portion  2802 , and operation switches  2803  and  2804 . The light-emitting device of the present invention can be applied to the display portion  2802 . In this embodiment the car audio system for being mounted in a car is shown. However, it can be applied to the standstill car audio. The display portion  2804  can reduce the power consumption by displaying white letters in the black display. 
     Further, it is effective to incorporate an optical sensor and to provide a function of modulating emission luminance in accordance with brightness in a usage environment by providing means for detecting the brightness in the usage environment. A user can recognize image or character information without problems if brightness of 100 to 150 in contrast ratio in comparison with the brightness of the usage environment is secured. That is, it is possible that the luminance of an image is raised in the bright usage environment to make the image easy to see while the luminance of an image is suppressed in the dark usage environment to thereby suppress the power consumption. 
     Although it is not shown here, the present invention can be applied to the display device which is employed in a navigation system, a refrigerator, a washing machine, a micro-wave oven, a telephone, a fax machine, etc. As described above, the applicable range of the present invention is so wide that the present invention can be applied to various products. 
     According to the present invention described above, in a display device that uses an organic resin substrate, DLC films are formed on the outer surfaces of a sealing member and an outer surface or end portions of the organic resin substrate. This construction improves the gas barrier property of the display device and prevents the degradation of light-emitting elements. Also, if a DLC film is formed on a light incident surface, ultraviolet rays are blocked, the light chemical reaction of the organic resin substrate is suppressed, and the degradation of the organic resin substrate is prevented. 
     Such a display device realizes an electronic device whose weight is reduced and shock resistance is improved. Also, the surface on which a DLC film has been formed is hardened, so that the surface of an organic resin substrate becomes resistant to flaws. As a result, a high-quality display screen is achieved and remains clear for a long time. 
     By forming a DLC film to cover end portions of substrates, from entering oxygen and moisture through between the substrates is prevented. This achieves the prolonged life spans of light-emitting elements and a light-emitting device. Also, by providing a DLC film to cover the entire surface except for an area in which light emission is performed, it becomes unnecessary to strictly control the formation of the DLC film. Further, by forming an interlayer insulating film using a black resin, the reflection of light by the first substrate is prevented. As a result, a problem in that outside scenes, such as the face of an observer, is reflected by a light-emitting device is solved without using an expensive circular polarizing film.

Technology Category: h