Patent Publication Number: US-2023147069-A1

Title: Display device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention particularly relates to a light-emitting device, a display device, an electronic device, a lighting device, a manufacturing method thereof, a usage method thereof, an operation method thereof, or the like. In particular, the present invention relates to a light-emitting device, a display device, an electronic device, or a lighting device utilizing electroluminescence (EL), a manufacturing method thereof, a usage method thereof, an operation method thereof, or the like. 
     2. Description of the Related Art 
     Recent light-emitting devices and display devices are expected to be applied to a variety of uses and become diversified. 
     For example, light-emitting devices and display devices for mobile devices and the like are required to be thin, lightweight, capable of being provided on a curved surface, and unlikely to be broken. In addition, a light-emitting device and a display device that can be bent at any part are demanded for greater portability. 
     Light-emitting elements utilizing EL (also referred to as EL elements) have features such as ease of thinning and lightening, high-speed response to input signal, and driving with a direct-current low voltage source; therefore, application of the light-emitting elements to light-emitting devices and display devices has been suggested. 
     For example, Patent Document 1 discloses a technical idea that a thin film device layer formed on a silicon wafer, a glass substrate, or the like is transferred onto a plastic substrate having a stacked-layer structure. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2004-72050 
       
    
     SUMMARY OF THE INVENTION 
     In Patent Document 1, materials of the plastic substrate that can be used for a display device are listed; however, some of them, such as a fluorine rubber material or a silicone resin, are too soft to be favorable for transfer of a thin film device layer. Furthermore, Patent Document 1 does not disclose a material for a substrate that can be favorably used in a repeatedly bendable display device. 
     An object of one embodiment of the present invention is to provide a highly portable display device, electronic device, or lighting device. 
     Another object of one embodiment of the present invention is to provide a repeatedly bendable display device, electronic device, or lighting device. 
     Another object of one embodiment of the present invention is to provide a highly reliable display device, electronic device, or lighting device. 
     Another object of one embodiment of the present invention is to provide a display device, electronic device, or lighting device that is unlikely to be broken. 
     Another object of one embodiment of the present invention is to provide a display device, electronic device, or lighting device with low power consumption. 
     Another object of one embodiment of the present invention is to provide a novel display device, electronic device, or lighting device. 
     Note that the descriptions of these objects do not disturb the existence of other objects. Note that in one embodiment of the present invention, there is no need to achieve all the objects. Note that other objects will be apparent from the description of the specification, the drawing, the claims, and the like and other objects can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a display device that includes a first substrate, a second substrate, a third substrate, and a fourth substrate. The first substrate and the second substrate overlap with each other with a display element provided therebetween. The third substrate and the fourth substrate overlap with each other with the first substrate and the second substrate provided therebetween. The third substrate and the fourth substrate are softer than the first substrate and the second substrate. 
     One embodiment of the present invention is a display device that includes a first substrate, a second substrate, a third substrate, and a fourth substrate. The first substrate and the second substrate overlap with each other with a display element provided therebetween. The third substrate and the fourth substrate overlap with each other with the first substrate and the second substrate provided therebetween. The Young&#39;s modulus of the third substrate and the fourth substrate is smaller than the Young&#39;s modulus of the first substrate and the second substrate. 
     The Young&#39;s modulus of a material suitable for the first substrate and the second substrate is larger than or equal to 1 GPa (1×10 9  Pa) and smaller than or equal to 100 GPa (100×10 9  Pa), preferably larger than or equal to 2 GPa and smaller than or equal to 50 GPa, further preferably larger than or equal to 2 GPa and smaller than or equal to 20 GPa. 
     The Young&#39;s modulus of a material used for the third substrate and the fourth substrate is preferably smaller than or equal to one fiftieth, further preferably smaller than or equal to one hundredth, still further preferably smaller than or equal to one five hundredth of the Young&#39;s modulus of the material used for the first substrate and the second substrate. 
     In one embodiment of the present invention, a highly portable display device, electronic device, or lighting device can be provided. 
     In one embodiment of the present invention, a repeatedly bendable display device, electronic device, or lighting device can be provided. 
     In one embodiment of the present invention, a highly reliable display device, electronic device, or lighting device can be provided. 
     In one embodiment of the present invention, a display device, electronic device, or lighting device that is unlikely to be broken can be provided. 
     In one embodiment of the present invention, a display device, electronic device, or lighting device with low power consumption can be provided. 
     In one embodiment of the present invention, a novel display device, electronic device, or lighting device can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A to  1 C  illustrate one mode of a display device. 
         FIGS.  2 A to  2 C  are cross-sectional views illustrating examples of bending of a display device. 
         FIGS.  3 A and  3 B  illustrate one mode of a display device. 
         FIGS.  4 A to  4 C  are a block diagram and circuit diagrams illustrating one mode of a display device. 
         FIGS.  5 A to  5 C  illustrate one mode of a display device. 
         FIGS.  6 A to  6 D  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIGS.  7 A to  7 D  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIGS.  8 A and  8 B  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIGS.  9 A and  9 B  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIGS.  10 A and  10 B  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIGS.  11 A and  11 B  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIG.  12    is a cross-sectional view illustrating one example of a method for manufacturing a display device. 
         FIGS.  13 A to  13 D  are cross-sectional views illustrating one example of a method for manufacturing a display device. 
         FIG.  14    is a cross-sectional view illustrating one mode of a display device. 
         FIGS.  15 A and  15 B  are cross-sectional views each illustrating one mode of a display device. 
         FIGS.  16 A and  16 B  are cross-sectional views each illustrating one mode of a display device. 
         FIGS.  17 A and  17 B  are cross-sectional views each illustrating one mode of a display device. 
         FIGS.  18 A and  18 B  are cross-sectional views each illustrating one mode of a display device. 
         FIGS.  19 A and  19 B  illustrate structure examples of light-emitting elements. 
         FIGS.  20 A to  20 E  illustrate examples of electronic devices and lighting devices. 
         FIGS.  21 A and  21 B  illustrate one example of an electronic device. 
         FIGS.  22 A to  22 C  illustrate one example of an electronic device. 
         FIGS.  23 A and  23 B  are photographs for explaining Example. 
         FIGS.  24 A to  24 F  each illustrate one mode of a display device. 
         FIG.  25    illustrates one mode of a display device. 
         FIGS.  26 A to  26 H  each illustrate one mode of a display device. 
         FIGS.  27 A and  27 B  are cross-sectional views each illustrating one mode of a display device. 
         FIGS.  28 A and  28 B  are cross-sectional views each illustrating one mode of a display device. 
         FIGS.  29 A and  29 B  are cross-sectional views each illustrating one mode of a display device. 
         FIG.  30    is a cross-sectional view illustrating one mode of a lighting device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to the drawings. Note that one embodiment of the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, one embodiment of the present invention should not be construed as being limited to the description in the following embodiments. Note that in the structures of the invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description of such portions is not repeated. 
     Note that in each drawing referred to in this specification, the size of each component or the thickness of each layer might be exaggerated or a region might be omitted for clarity of the invention. Therefore, embodiments of the invention are not limited to such scales. Especially in a top view (a plan view) and a perspective view, some components might not be illustrated for easy understanding. 
     The position, size, range, and the like of each component illustrated in the drawings and the like are not accurately represented in some cases to facilitate understanding of the invention. Therefore, the disclosed invention is not necessarily limited to the position, size, range, and the like disclosed in the drawings and the like. For example, in the actual manufacturing process, a resist mask or the like might be unintentionally reduced in size by treatment such as etching, which is not illustrated in some cases for easy understanding. 
     Note that ordinal numbers such as “first” and “second” in this specification and the like are used in order to avoid confusion among components and do not denote the priority or the order such as the order of steps or the stacking order. A term without an ordinal number in this specification and the like might be provided with an ordinal number in a claim in order to avoid confusion among components. 
     In addition, in this specification and the like, the term such as an “electrode” or a “wiring” does not limit a function of a component. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa. Further, the term “electrode” or “wiring” can also mean a combination of a plurality of “electrodes” and “wirings” formed in an integrated manner. 
     Note that the term “over” or “under” in this specification and the like does not necessarily mean that a component is placed “directly above and in contact with” or “directly below and in contact with” another component. For example, the expression “electrode B over insulating layer A” does not necessarily mean that the electrode B is on and in direct contact with the insulating layer A and can mean the case where another component is provided between the insulating layer A and the electrode B. 
     Further, functions of a source and a drain might be switched depending on operation condition, e.g., when a transistor having a different polarity is employed or a direction of current flow is changed in circuit operation. Therefore, it is difficult to define which is the source (or the drain). Thus, the terms “source” and “drain” can be used to denote the drain and the source, respectively. 
     In this specification and the like, the term “electrically connected” includes the case where components are connected through an object having any electric function. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Thus, even when the expression “electrically connected” is used in this specification, there is a case in which no physical connection is made and a wiring is just extended in an actual circuit. 
     In this specification and the like, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, the term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. The term “equal” allows for a maximum error of ±5%. 
     In this specification, in the case where an etching step is performed after a photolithography process, a resist mask formed in the photolithography process is removed after the etching step, unless otherwise specified. 
     Embodiment 1 
     A structure example of a display device  100  that is one embodiment of the present invention will be described with reference to drawings.  FIG.  1 A  is a top view of the display device  100  and  FIG.  1 B  is a cross-sectional view taken along a dashed-dotted line A 1 -A 2  in  FIG.  1 A .  FIG.  1 C  is a cross-sectional view taken along a dashed-dotted line B 1 -B 2  in  FIG.  1 A . 
     The cross-sectional structure of one embodiment of the present invention is not limited to that illustrated in  FIG.  1 C . For example, any of cross-sectional structures illustrated in  FIGS.  24 A to  24 F  may also be employed. The external electrode  124  may be covered with a substrate  147  as illustrated in  FIGS.  24 B,  24 C, and  24 F , in which case a connection portion can be protected. Note that  FIGS.  24 D to  24 F  each illustrate a structure in which a semiconductor chip  910  is provided over a substrate by COG or the like. When the semiconductor chip  910  is covered with the substrate  147  as illustrated in  FIGS.  24 E and  24 F , the semiconductor chip  910  and its connection portion can be protected. 
       FIGS.  2 A to  2 C  are cross-sectional views illustrating the display device  100  in a bent state. Note that  FIGS.  2 A to  2 C  are each a cross-sectional view taken along the dashed-dotted line B 1 -B 2  in  FIG.  1 A .  FIG.  2 A  illustrates the display device  100  which is folded double.  FIG.  2 B  illustrates the display device  100  which is folded in three.  FIG.  2 C  illustrates the display device  100  which is rolled up. Note that the bending directions are not limited to those shown in  FIGS.  2 A to  2 C , and the display device  100  that is one embodiment of the present invention can be bent in any direction. 
       FIG.  3 A  is a perspective view of the display device  100 , and  FIG.  3 B  is a cross-sectional view for specifically describing a portion taken along a dashed-dotted line X 1 -X 2  in  FIG.  3 A . Note that the cross-sectional structure may be the one illustrated in  FIG.  3 B  or the one illustrated in  FIG.  25   . 
     &lt;Configuration Example of Display Device&gt; 
     The display device  100  described in this embodiment includes a display area  131 , a driver circuit  132 , and a driver circuit  133 . The display device  100  also includes a terminal electrode  216  and a light-emitting element  125  including an electrode  115 , an EL layer  117 , and an electrode  118 . A plurality of light-emitting elements  125  are formed in the display area  131 . A transistor  232  for controlling the amount of light emitted from the light-emitting element  125  is connected to each of the light-emitting elements  125 . 
     The external electrode  124  and the terminal electrode  216  are electrically connected to each other through an anisotropic conductive connection layer  123 . In addition, the terminal electrode  216  is electrically connected to the driver circuit  132  and the driver circuit  133 . 
     The driver circuit  132  and the driver circuit  133  each include a plurality of transistors  252 . The driver circuit  132  and the driver circuit  133  each have a function of determining which of the light-emitting elements  125  in the display area  131  is supplied with a signal from the external electrode  124 . 
     The transistor  232  and the transistor  252  each include a gate electrode  206 , a gate insulating layer  207 , a semiconductor layer  208 , a source electrode  209   a,  and a drain electrode  209   b.  A wiring  219  is formed in the same layer where the source electrode  209   a  and the drain electrode  209   b  are formed. In addition, an insulating layer  210  is formed over the transistor  232  and the transistor  252 , and an insulating layer  211  is formed over the insulating layer  210 . The electrode  115  is formed over the insulating layer  211 . The electrode  115  is electrically connected to the drain electrode  209   b  through an opening formed in the insulating layer  210  and the insulating layer  211 . A partition  114  is formed over the electrode  115 , and the EL layer  117  and the electrode  118  are formed over the electrode  115  and the partition  114 . 
     In the display device  100 , a substrate  111  and a substrate  121  are attached to each other with a bonding layer  120  provided therebetween. One surface of the substrate  111  is provided with a substrate  137  with a bonding layer  138  provided therebetween. One surface of the substrate  121  is provided with the substrate  147  with a bonding layer  148  provided therebetween. 
     The other surface of the substrate  111  is provided with an insulating layer  205  with a bonding layer  112  provided therebetween. The insulating layer  205  is preferably formed as a single layer or a multilayer using silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, or the like. The insulating layer  205  can be formed by a sputtering method, a CVD method, a thermal oxidation method, a coating method, a printing method, or the like. 
     The other surface of the substrate  121  is provided with an insulating layer  145  with a bonding layer  142  provided therebetween. The other surface of the substrate  121  is provided with a light-blocking layer  264  with the insulating layer  145  provided therebetween. The other surface of the substrate  121  is also provided with a coloring layer  266  and an overcoat layer  268  with the insulating layer  145  provided therebetween. 
     Note that the insulating layer  205  functions as a base layer and can prevent or reduce diffusion of moisture and impurity elements from the substrate  111 , the bonding layer  112 , or the like to the transistor or the light-emitting element. The insulating layer  145  functions as a base layer and can prevent or reduce diffusion of moisture and impurity elements from the substrate  121 , the bonding layer  142 , or the like to the transistor or the light-emitting element. The insulating layer  145  can be formed using a material and a method similar to those of the insulating layer  205 . 
     A flexible material such as an organic resin material, or the like can be used for the substrate  111  and the substrate  121 . In the case where the display device  100  is a so-called bottom-emission display device or a dual-emission display device, a material that transmits light emitted from the EL layer  117  is used for the substrate  111 . In the case where the display device  100  is a top-emission display device or a dual-emission display device, a material that transmits light emitted from the EL layer  117  is used for the substrate  121 . 
     In a similar manner, in the case where the display device  100  is a so-called bottom-emission display device or a dual-emission display device, a material that transmits light emitted from the EL layer  117  is used for the substrate  137 . In the case where the display device  100  is a top-emission display device or a dual-emission display device, a material that transmits light emitted from the EL layer  117  is used for the substrate  147 . 
     If the mechanical strength of a material used for the substrate  111  and the substrate  121  is too low, the substrates easily become deformed at the time of manufacture of the display device  100 , which reduces yield and thus, contributes to a reduction in productivity. Yet, if the mechanical strength of the material used for the substrate  111  and the substrate  121  is too high, the display device becomes difficult to bend. An index of the mechanical strength of a material is a Young&#39;s modulus. The Young&#39;s modulus of a material suitable for the substrate  111  and the substrate  121  is larger than or equal to 1 GPa (1×10 9  Pa) and smaller than or equal to 100 GPa (100×10 9  Pa), preferably larger than or equal to 2 GPa and smaller than or equal to 50 GPa, further preferably larger than or equal to 2 GPa and smaller than or equal to 20 GPa. Note that in measurement of a Young&#39;s modulus, ISO527, JISK7161, JISK7162, JISK7127, ASTMD638, ASTMD882, or the like can be referred to. 
     The thickness of each of the substrate  111  and the substrate  121  is preferably greater than or equal to 5 μm and less than or equal to 100 μm, further preferably greater than or equal to 10 μm and less than or equal to 50 μm. One or both of the substrate  111  and the substrate  121  may be a stacked-layer substrate that includes a plurality of layers. 
     It is preferable that the substrate  111  and the substrate  121  be formed using the same material and have the same thickness. However, depending on the purpose, the substrates  111  and  121  may be formed using different materials or have different thicknesses. 
     Examples of materials that have flexibility and transmit visible light, which can be used for the substrate  111  and the substrate  121 , include a polyethylene terephthalate resin, a polyethylene naphthalate resin, a polyacrylonitrile resin, a polyimide resin, a polymethylmethacrylate resin, a polycarbonate resin, a polyethersulfone resin, a polyamide resin, a cycloolefin resin, a polystyrene resin, a polyamide imide resin, a polyvinylchloride resin, and the like. Furthermore, when a light-transmitting property is not necessary, a non-light-transmitting substrate may be used. For example, aluminum or the like may be used for the substrate  121  or the substrate  111 . 
     The thermal expansion coefficients of the substrate  111  and the substrate  121  are preferably less than or equal to 30 ppm/K, more preferably less than or equal to 10 ppm/K. In addition, on surfaces of the substrate  111  and the substrate  121 , a protective film having low water permeability may be formed in advance; examples of the protective film include a film containing nitrogen and silicon such as a silicon nitride film or a silicon nitride oxide film and a film containing nitrogen and aluminum such as an aluminum nitride film. Note that a structure in which a fibrous body is impregnated with an organic resin (also called prepreg) may be used as the substrate  111  and the substrate  121 . 
     With such substrates, a non-breakable display device can be provided. Alternatively, a lightweight display device can be provided. Alternatively, an easily bendable display device can be provided. 
     For the substrate  137 , a material softer than the substrate  111  is used. For example, a material having a smaller Young&#39;s modulus than the substrate  111  is used for the substrate  137 . For the substrate  147 , a material softer than the substrate  121  is used. For example, a material having a smaller Young&#39;s modulus than the substrate  121  is used for the substrate  147 . 
     The Young&#39;s modulus of the material used for the substrate  137  is preferably smaller than or equal to one fiftieth, further preferably smaller than or equal to one hundredth, still further preferably smaller than or equal to one five hundredth of the Young&#39;s modulus of the material used for the substrate  111 . The Young&#39;s modulus of the material used for the substrate  147  is preferably smaller than or equal to one fiftieth, further preferably smaller than or equal to one hundredth, still further preferably smaller than or equal to one five hundredth of the Young&#39;s modulus of the material used for the substrate  121 . 
     Examples of a material that can be used for the substrate  137  and the substrate  147  include a viscoelastic high molecular material such as silicone rubber or fluorine rubber. The material used for the substrate  137  and the substrate  147  preferably has a light-transmitting property. The substrate  137  and the substrate  147  may be formed using the same kind of material or different materials. 
     In the case where the substrate  137  and the substrate  147  are bonded to each other with the substrate  111  and the substrate  121  provided therebetween, the thickness of the substrate  137  is preferably equal to that of the substrate  147 . When the thickness of the substrate  137  is equal to that of the substrate  147 , the substrate  111  and the substrate  121  can be positioned close to a neutral plane of a bent portion. Accordingly, stress applied to the substrate  111  and the substrate  121  at the time of bending can be reduced. 
     A material with a small Young&#39;s modulus more easily becomes deformed than a material with a large Young&#39;s modulus does; therefore, internal stress generated by deformation is easily dispersed in the former. When a material with a Young&#39;s modulus smaller than that of the substrate  111  and the substrate  121  is used for the substrate  137  and the substrate  147 , local stress generated in the substrate  111  and the substrate  121  at the time of bending can be relaxed, whereby the substrate  111  and the substrate  121  can be prevented from being broken. The substrate  137  and the substrate  147  also function as buffers dispersing external physical pressure and impact. 
     The substrate  137  or the substrate  147  is provided on the inner side of a bent portion, whereby the radius of curvature of the substrate  111  or  121  that is positioned on the inner side of the bent portion can be prevented from being smaller than the thickness of the substrate  137  or the substrate  147 . In this manner, breakage of the substrate  111  or the substrate  121  due to bending at an excessively small radius of curvature can be prevented. 
     In one embodiment of the present invention, the display device  100  can be prevented from being broken even when the radius of curvature of the substrate  111  or  121  that is positioned on the inner side of a bent portion is 1 mm or less. 
     Note that the thickness of the substrate  137  is preferably greater than or equal to 2 times and less than or equal to 100 times that of the substrate  111 , further preferably greater than or equal to 5 times and less than or equal to 50 times that of the substrate  111 . The thickness of the substrate  147  is preferably greater than or equal to 2 times and less than or equal to 100 times that of the substrate  121 , further preferably greater than or equal to 5 times and less than or equal to 50 times that of the substrate  121 . When the substrate  137  is thicker than the substrate  111  and the substrate  147  is thicker than the substrate  121 , stress relaxation and the effect of buffers can be enhanced. 
     It is preferable that the substrate  137  and the substrate  147  be formed using the same material and have the same thickness. However, depending on the purpose, the substrates  137  and  147  may be formed using different materials or have different thicknesses. 
     Depending on the usage of the display device, it is also possible to provide only one of the substrate  137  and the substrate  147 . One or both of the substrate  137  and the substrate  147  may be a stacked-layer substrate that includes a plurality of layers. 
     In one embodiment of the present invention, a display device that is resistant to external impact and unlikely to be broken can be provided. 
     In one embodiment of the present invention, a highly reliable display device can be provided which is unlikely to be broken even when it is repeatedly bent and stretched. 
     &lt;Example of Pixel Circuit Configuration&gt; 
     Next, an example of a specific configuration of the display device  100  is described with reference to  FIGS.  4 A to  4 C .  FIG.  4 A  is a block diagram illustrating the configuration of the display device  100 . The display device  100  includes the display area  131 , the driver circuit  132 , and the driver circuit  133 . The driver circuit  132  functions as a scan line driver circuit, for example, and the driver circuit  133  functions as a signal line driver circuit, for example. 
     The display device  100  includes m scan lines  135  which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the driver circuit  132 , and n signal lines  136  which are arranged parallel or substantially parallel to each other and whose potentials are controlled by the driver circuit  133 . The display area  131  includes a plurality of pixels  134  arranged in a matrix. The driver circuit  132  and the driver circuit  133  are collectively referred to as a driver circuit portion in some cases. 
     Each of the scan lines  135  is electrically connected to the n pixels  134  in the corresponding row among the pixels  134  arranged in m rows and n columns in the display area  131 . Each of the signal lines  136  is electrically connected to the m pixels  134  in the corresponding column among the pixels  134  arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. 
       FIGS.  4 B and  4 C  illustrate circuit configurations that can be used for the pixels  134  in the display device illustrated in  FIG.  4 A . 
     [Example of Pixel Circuit for Light-Emitting Display Device] 
     The pixel  134  illustrated in  FIG.  4 B  includes a transistor  431 , a capacitor  233 , the transistor  232 , and the light-emitting element  125 . 
     One of a source electrode and a drain electrode of the transistor  431  is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a signal line DL_n). A gate electrode of the transistor  431  is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m). 
     The transistor  431  has a function of controlling whether to write a data signal to a node  435  by being turned on or off. 
     One of a pair of electrodes of the capacitor  233  is electrically connected to the node  435 , and the other is electrically connected to a node  437 . The other of the source electrode and the drain electrode of the transistor  431  is electrically connected to the node  435 . 
     The capacitor  233  functions as a storage capacitor for storing data written to the node  435 . 
     One of a source electrode and a drain electrode of the transistor  232  is electrically connected to a potential supply line VL_a, and the other is electrically connected to the node  437 . A gate electrode of the transistor  232  is electrically connected to the node  435 . 
     One of an anode and a cathode of the light-emitting element  125  is electrically connected to a potential supply line VL_b, and the other is electrically connected to the node  437 . 
     As the light-emitting element  125 , an organic electroluminescent element (also referred to as an organic EL element) or the like can be used, for example. Note that the light-emitting element  125  is not limited to organic EL elements; an inorganic EL element including an inorganic material can be used. 
     Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other. 
     In the display device including the pixel  134  in  FIG.  4 B , the pixels  134  are sequentially selected row by row by the first driver circuit  132 , whereby the transistors  431  are turned on and a data signal is written to the nodes  435 . 
     When the transistors  431  are turned off, the pixels  134  in which the data has been written to the nodes  435  are brought into a holding state. Further, the amount of current flowing between the source electrode and the drain electrode of the transistor  232  is controlled in accordance with the potential of the data written to the node  435 . The light-emitting element  125  emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed. 
     [Example of Pixel Circuit for Liquid Crystal Display Device] 
     The pixel  134  illustrated in  FIG.  4 C  includes a liquid crystal element  432 , the transistor  431 , and the capacitor  233 . 
     The potential of one of a pair of electrodes of the liquid crystal element  432  is set according to the specifications of the pixels  134  as appropriate. The alignment state of the liquid crystal element  432  depends on data written to a node  436 . A common potential may be applied to one of the pair of electrodes of the liquid crystal element  432  included in each of the plurality of pixels  134 . Further, the potential supplied to one of a pair of electrodes of the liquid crystal element  432  in the pixel  134  in one row may be different from the potential supplied to one of a pair of electrodes of the liquid crystal element  432  in the pixel  134  in another row. 
     As examples of a driving method of the display device including the liquid crystal element  432 , any of the following modes can be given: a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode, and the like. Other examples of the driving method of the display device include an electrically controlled birefringence (ECB) mode, a polymer dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Note that the present invention is not limited to these examples, and various liquid crystal elements and driving methods can be applied to the liquid crystal element and the driving method thereof. 
     The liquid crystal element  432  may be formed using a liquid crystal composition including liquid crystal exhibiting a blue phase and a chiral material. Liquid crystal exhibiting a blue phase does not need alignment treatment. In addition, the liquid crystal exhibiting a blue phase has a short response time of 1 msec or less and is optically isotropic, which makes the viewing angle dependence small. 
     Note that a display element other than the light-emitting element  125  and the liquid crystal element  432  can be used. For example, an electrophoretic element, an electronic ink, an electrowetting element, a micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator (IMOD) element, or the like can be used as the display element. 
     In the pixel  134  in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor  431  is electrically connected to a signal line DL_n, and the other is electrically connected to the node  436 . A gate electrode of the transistor  431  is electrically connected to a scan line GL_m. The transistor  431  has a function of controlling whether to write a data signal to the node  436  by being turned on or off. 
     One of a pair of electrodes of the capacitor  233  is electrically connected to a wiring to which a particular potential is supplied (hereinafter referred to as a capacitor line CL), and the other is electrically connected to the node  436 . The other of the pair of electrodes of the liquid crystal element  432  is electrically connected to the node  436 . The potential of the capacitor line CL is set in accordance with the specifications of the pixel  134  as appropriate. The capacitor  233  functions as a storage capacitor for storing data written to the node  436 . 
     For example, in the display device including the pixel  134  in  FIG.  4 C , the pixels  134  are sequentially selected row by row by the first driver circuit  132 , whereby the transistors  431  are turned on and a data signal is written to the nodes  436 . 
     When the transistors  431  are turned off, the pixels  134  in which the data signal has been written to the nodes  436  are brought into a holding state. This operation is sequentially performed row by row; thus, an image is displayed. 
     MODIFICATION EXAMPLE 
       FIGS.  5 A to  5 C  illustrate a display device  200  having a structure different from that of the display device  100 .  FIG.  5 A  is a top view of the display device  200  and  FIG.  5 B  is a cross-sectional view taken along a dashed-dotted line A 3 -A 4  in  FIG.  5 A .  FIG.  5 C  is a cross-sectional view taken along a dashed-dotted line B 3 -B 4  in  FIG.  5 A . 
     The cross-sectional structure of one embodiment of the present invention is not limited to that illustrated in  FIG.  5 C . For example, any of cross-sectional structures illustrated in  FIGS.  26 A to  26 H  may also be employed. The external electrode  124  may be covered with the substrate  147  as illustrated in  FIGS.  26 B,  26 C,  26 D,  26 G, and  26 H , in which case a connection portion can be protected. The external electrode  124  may be covered with the substrate  147  and the substrate  137  as illustrated in  FIGS.  26 D and  26 H , in which case a connection portion can be protected. When the semiconductor chip  910  is covered with the substrate  147  as illustrated in  FIGS.  26 F,  26 G, and  26 H , the semiconductor chip  910  and its connection portion can be protected. 
     The display device  200  is different from the display device  100  in that at least part of the substrate  137  and part of the substrate  147  extend beyond the edges of the substrate  111  and the substrate  121  and that the extending portion of the substrate  137  and the extending portion of the substrate  147  are connected to each other. Other components can be formed in a manner similar to that of the display device  100 . Note that the extending portions of the substrate  137  and the substrate  147  may be connected directly or connected indirectly with a bonding layer or the like provided therebetween. 
     The structure of the display device  200  can inhibit entry of impurities from the edges of the substrate  111  and the substrate  121  and thus can further improve the reliability of the display device. 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     Embodiment 2 
     In this embodiment, another example of a method for manufacturing the display device  100  will be described with reference to  FIGS.  6 A to  6 D ,  FIGS.  7 A to  7 D ,  FIGS.  8 A and  8 B ,  FIGS.  9 A and  9 B ,  FIGS.  10 A and  10 B ,  FIGS.  11 A and  11 B , and  FIG.  12   . Note that  FIGS.  6 A to  6 D ,  FIGS.  7 A to  7 D ,  FIGS.  8 A and  8 B ,  FIGS.  9 A and  9 B ,  FIGS.  10 A and  10 B ,  FIGS.  11 A and  11 B , and  FIG.  12    are each a cross-sectional view taken along a dashed-dotted line X 1 -X 2  in  FIG.  3 A . 
     &lt;Example of Method for Manufacturing Display Device&gt; 
     [Formation of Separation Layer] 
     First, a separation layer  113  is formed over an element formation substrate  101  (see  FIG.  6 A ). Note that the element formation substrate  101  may be a glass substrate, a quartz substrate, a sapphire substrate, a ceramic substrate, a metal substrate, or the like. Alternatively, the element formation substrate  101  may be a plastic substrate having heat resistance to the processing temperature in this embodiment. 
     As the glass substrate, for example, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used. Note that when the glass substrate contains a large amount of barium oxide (BaO), the glass substrate can be heat-resistant and more practical. Alternatively, crystallized glass or the like may be used. 
     The separation layer  113  can be formed using an element selected from tungsten, molybdenum, titanium, tantalum, niobium, nickel, cobalt, zirconium, ruthenium, rhodium, palladium, osmium, iridium, and silicon; an alloy material containing any of the elements; or a compound material containing any of the elements. The separation layer  113  can also be formed to have a single-layer structure or a stacked-layer structure using any of the materials. Note that the crystalline structure of the separation layer  113  may be amorphous, microcrystalline, or polycrystalline. The separation layer  113  can also be formed using a metal oxide such as aluminum oxide, gallium oxide, zinc oxide, titanium dioxide, indium oxide, indium tin oxide, indium zinc oxide, or InGaZnO (IGZO). 
     The separation layer  113  can be formed by a sputtering method, a CVD method, a coating method, a printing method, or the like. Note that the coating method includes a spin coating method, a droplet discharge method, and a dispensing method. 
     In the case where the separation layer  113  has a single-layer structure, the separation layer  113  is preferably formed using tungsten, molybdenum, or a tungsten-molybdenum alloy. Alternatively, the separation layer  113  is preferably formed using an oxide or oxynitride of tungsten, an oxide or oxynitride of molybdenum, or an oxide or oxynitride of a tungsten-molybdenum alloy. 
     In the case where the separation layer  113  has a stacked-layer structure including, for example, a layer containing tungsten and a layer containing an oxide of tungsten, the layer containing an oxide of tungsten may be formed as follows: the layer containing tungsten is formed first and then an oxide insulating layer is formed in contact therewith, so that the layer containing an oxide of tungsten is formed at the interface between the layer containing tungsten and the oxide insulating layer. Alternatively, the layer containing an oxide of tungsten may be formed by performing thermal oxidation treatment, oxygen plasma treatment, treatment with a highly oxidizing solution such as ozone water, or the like on the surface of the layer containing tungsten. 
     In this embodiment, a glass substrate is used as the element formation substrate  101 . The separation layer  113  is formed of tungsten over the element formation substrate  101  by a sputtering method. 
     [Formation of Insulating Layer] 
     Next, the insulating layer  205  is formed as a base layer over the separation layer  113  (see  FIG.  6 A ). The insulating layer  205  is preferably formed as a single layer or a multilayer using silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, or the like. The insulating layer  205  may have, for example, a two-layer structure of silicon oxide and silicon nitride or a five-layer structure in which materials selected from the above are combined. The insulating layer  205  can be formed by a sputtering method, a CVD method, a thermal oxidation method, a coating method, a printing method, or the like. 
     The thickness of the insulating layer  205  may be greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 50 nm and less than or equal to 400 nm. 
     The insulating layer  205  can prevent or reduce diffusion of impurity elements from the element formation substrate  101 , the separation layer  113 , or the like. Even after the element formation substrate  101  is replaced by the substrate  111 , the insulating layer  205  can prevent or reduce diffusion of impurity elements into the light-emitting element  125  from the substrate  111 , the bonding layer  112 , or the like. In this embodiment, the insulating layer  205  is formed by stacking a 200-nm-thick silicon oxynitride film and a 50-nm-thick silicon nitride oxide film by a plasma CVD method. 
     [Formation of Gate Electrode] 
     Next, the gate electrode  206  is formed over the insulating layer  205  (see  FIG.  6 A ). The gate electrode  206  can be formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing any of these metal elements in combination; or the like. Further, one or more metal elements selected from manganese and zirconium may be used. The gate electrode  206  may have a single-layer structure or a stacked structure of two or more layers. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a two-layer structure in which a copper film is stacked over a titanium film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used. 
     The gate electrode  206  can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element. 
     First, a conductive film to be the gate electrode  206  is stacked over the insulating layer  205  by a sputtering method, a CVD method, an evaporation method, or the like, and a resist mask is formed over the conductive film by a photolithography process. Next, part of the conductive film to be the gate electrode  206  is etched with the use of the resist mask to form the gate electrode  206 . At the same time, a wiring and another electrode can be formed. 
     The conductive film may be etched by a dry etching method, a wet etching method, or both a dry etching method and a wet etching method. Note that in the case where the conductive film is etched by a dry etching method, ashing treatment may be performed before the resist mask is removed, whereby the resist mask can be easily removed using a stripper. 
     Note that the gate electrode  206  may be formed by an electrolytic plating method, a printing method, an inkjet method, or the like instead of the above formation method. 
     The thickness of the gate electrode  206  is greater than or equal to 5 nm and less than or equal to 500 nm, preferably greater than or equal to 10 nm and less than or equal to 300 nm, more preferably greater than or equal to 10 nm and less than or equal to 200 nm. 
     The gate electrode  206  may be formed using a light-blocking conductive material, whereby external light can be prevented from reaching the semiconductor layer  208  from the gate electrode  206  side. As a result, a variation in electrical characteristics of the transistor due to light irradiation can be suppressed. 
     [Formation of Gate Insulating Layer] 
     Next, the gate insulating layer  207  is formed (see  FIG.  6 A ). The gate insulating layer  207  can be formed to have a single-layer structure or a stacked-layer structure using, for example, any of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, a mixture of aluminum oxide and silicon oxide, hafnium oxide, gallium oxide, Ga—Zn-based metal oxide, and the like. 
     The gate insulating layer  207  may be formed using a high-k material such as hafnium silicate (HfSiO x ), hafnium silicate to which nitrogen is added (HfSi x O y N z ), hafnium aluminate to which nitrogen is added (HfAl x O y N z ), hafnium oxide, or yttrium oxide, so that gate leakage current of the transistor can be reduced. For example, a stacked layer of silicon oxynitride and hafnium oxide may be used. 
     The thickness of the gate insulating layer  207  is preferably greater than or equal to 5 nm and less than or equal to 400 nm, further preferably greater than or equal to 10 nm and less than or equal to 300 nm, still further preferably greater than or equal to 50 nm and less than or equal to 250 nm. 
     The gate insulating layer  207  can be formed by a sputtering method, a CVD method, an evaporation method, or the like. 
     In the case where a silicon oxide film, a silicon oxynitride film, or a silicon nitride oxide film is formed as the gate insulating layer  207 , a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, nitrogen dioxide, and the like can be given as examples. 
     The gate insulating layer  207  can have a stacked-layer structure in which a nitride insulating layer and an oxide insulating layer are stacked in this order from the gate electrode  206  side. When the nitride insulating layer is provided on the gate electrode  206  side, hydrogen, nitrogen, an alkali metal, an alkaline earth metal, or the like can be prevented from moving from the gate electrode  206  side to the semiconductor layer  208 . Note that nitrogen, an alkali metal, an alkaline earth metal, or the like generally serves as an impurity element of a semiconductor. In addition, hydrogen serves as an impurity element of an oxide semiconductor. Thus, an “impurity” in this specification and the like includes hydrogen, nitrogen, an alkali metal, an alkaline earth metal, or the like. 
     In the case where an oxide semiconductor is used for the semiconductor layer  208 , the density of defect states at the interface between the gate insulating layer  207  and the semiconductor layer  208  can be reduced by providing the oxide insulating layer on the semiconductor layer  208  side. Consequently, a transistor whose electrical characteristics are hardly degraded can be obtained. Note that in the case where an oxide semiconductor is used for the semiconductor layer  208 , an oxide insulating layer containing oxygen in a proportion higher than that in the stoichiometric composition is preferably formed as the oxide insulating layer. This is because the density of defect states at the interface between the gate insulating layer  207  and the semiconductor layer  208  can be further reduced. 
     In the case where the gate insulating layer  207  is a stacked layer of a nitride insulating layer and an oxide insulating layer as described above, it is preferable that the nitride insulating layer be thicker than the oxide insulating layer. 
     The nitride insulating layer has a dielectric constant higher than that of the oxide insulating layer; therefore, an electric field generated from the gate electrode  206  can be efficiently transmitted to the semiconductor layer  208  even when the gate insulating layer  207  has a large thickness. When the gate insulating layer  207  has a large total thickness, the withstand voltage of the gate insulating layer  207  can be increased. Accordingly, the reliability of the semiconductor device can be improved. 
     The gate insulating layer  207  can have a stacked-layer structure in which a first nitride insulating layer with few defects, a second nitride insulating layer with a high blocking property against hydrogen, and an oxide insulating layer are stacked in that order from the gate electrode  206  side. When the first nitride insulating layer with few defects is used in the gate insulating layer  207 , the withstand voltage of the gate insulating layer  207  can be improved. Particularly when an oxide semiconductor is used for the semiconductor layer  208 , the use of the second nitride insulating layer with a high blocking property against hydrogen in the gate insulating layer  207  makes it possible to prevent hydrogen contained in the gate electrode  206  and the first nitride insulating layer from moving to the semiconductor layer  208 . 
     An example of a method for forming the first and second nitride insulating layers is described below. First, a silicon nitride film with few defects is formed as the first nitride insulating layer by a plasma CVD method in which a mixed gas of silane, nitrogen, and ammonia is used as a source gas. Next, a silicon nitride film in which the hydrogen concentration is low and hydrogen can be blocked is formed as the second nitride insulating layer by switching the source gas to a mixed gas of silane and nitrogen. By such a formation method, the gate insulating layer  207  in which nitride insulating layers with few defects and a blocking property against hydrogen are stacked can be formed. 
     The gate insulating layer  207  can have a stacked-layer structure in which a third nitride insulating layer with a high blocking property against an impurity, the first nitride insulating layer with few defects, the second nitride insulating layer with a high blocking property against hydrogen, and the oxide insulating layer are stacked in that order from the gate electrode  206  side. When the third nitride insulating layer with a high blocking property against an impurity is provided in the gate insulating layer  207 , hydrogen, nitrogen, alkali metal, alkaline earth metal, or the like, can be prevented from moving from the gate electrode  206  to the semiconductor layer  208 . 
     An example of a method for forming the first to third nitride insulating layers is described below. First, a silicon nitride film with a high blocking property against an impurity is formed as the third nitride insulating layer by a plasma CVD method in which a mixed gas of silane, nitrogen, and ammonia is used as a source gas. Next, a silicon nitride film with few defects is formed as the first nitride insulating layer by increasing the flow rate of ammonia. Then, a silicon nitride film in which the hydrogen concentration is low and hydrogen can be blocked is formed as the second nitride insulating layer by switching the source gas to a mixed gas of silane and nitrogen. By such a formation method, the gate insulating layer  207  in which nitride insulating layers with few defects and a blocking properly against an impurity are stacked can be formed. 
     Moreover, in the case of forming a. gallium oxide film as the gate insulating layer  207 , a metal organic chemical vapor deposition (MOCVD) method can be employed. 
     Note that the threshold voltage of a transistor can be changed by stacking the semiconductor layer  208  in which a channel of the transistor is formed and an insulating layer containing hafnium oxide with an oxide insulating layer provided therebetween and injecting electrons into the insulating layer containing hafnium oxide. 
     [Formation of Semiconductor Layer] 
     The semiconductor layer  208  can be formed using an amorphous semiconductor, a microcrystalline semiconductor, a polycrystalline semiconductor, or the like. For example, amorphous silicon or microcrystalline germanium can be used. Alternatively, a compound semiconductor such as silicon carbide, gallium arsenide, an oxide semiconductor, or a nitride semiconductor, an organic semiconductor, or the like can be used. 
     In the case of using an organic semiconductor for the semiconductor layer  208 , a low molecular organic material having an aromatic ring, a π-electron conjugated conductive polymer, or the like can be used. For example, rubrene, tetracene, pentacene, polylenediimide, tetracyanoquinodimethane, polythiophene, polyacetylene, or polyparaphenylene vinylene can be used. 
     In the case of using an oxide semiconductor for the semiconductor layer  208 , a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous oxide semiconductor, or the like can be used. 
     Note that an oxide semiconductor has an energy gap as wide as 3.0 eV or more and high visible-light transmissivity. In a transistor obtained by processing an oxide semiconductor under appropriate conditions, the off-state current at ambient temperature (e.g., 25° C.) can be less than or equal to 100 zA (1×10 −19  A), less than or equal to 10 zA (1×10 −20  A), and further less than or equal to 1 zA (1×10 −21  A). Therefore, a display device with low power consumption can be provided. 
     In the case where an oxide semiconductor is used for the semiconductor layer  208 , an insulating layer containing oxygen is preferably used as an insulating layer that is in contact with the semiconductor layer  208 . 
     The thickness of the semiconductor layer  208  is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, more preferably greater than or equal to 3 nm and less than or equal to 50 nm. In this embodiment, as the semiconductor layer  208 , an oxide semiconductor film with a thickness of 30 nm is formed by a sputtering method. 
     Next, a resist mask is formed over the oxide semiconductor film, and part of the oxide semiconductor film is selectively etched using the resist mask to form the semiconductor layer  208 . The resist mask can be formed by a photolithography method, a printing method, an inkjet method, or the like as appropriate. Formation of the resist mask by an inkjet method needs no photomask; thus, fabrication cost can be reduced. 
     Note that the etching of the oxide semiconductor film may be performed by either one or both of a dry etching method and a wet etching method. After the etching of the oxide semiconductor film, the resist mask is removed (see  FIG.  6 B ). 
     [Formation of Source Electrode, Drain Electrode, and the Like] 
     Next, the source electrode  209   a,  the drain electrode  209   b,  the wiring  219 , and the terminal electrode  216  are formed (see  FIG.  6 C ). First, a conductive film is formed over the gate insulating layer  207  and the semiconductor layer  208 . 
     The conductive film can have a single-layer structure or a stacked-layer structure containing any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten or an alloy containing any of these metals as its main component. For example, the following structures can be given: a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order, and a three-layer structure in which a tungsten film, a copper film, and a tungsten film are stacked in this order. 
     Note that a conductive material containing oxygen such as indium tin oxide, zinc oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added, or a conductive material containing nitrogen such as titanium nitride or tantalum nitride may be used. It is also possible to use a stacked-layer structure formed using a material containing the above metal element and conductive material containing oxygen. It is also possible to use a stacked-layer structure formed using a material containing the above metal element and a conductive material containing nitrogen. It is also possible to use a stacked-layer structure formed using a material containing the above metal element, a conductive material containing oxygen, and a conductive material containing nitrogen. 
     The thickness of the conductive film is greater than or equal to 5 nm and less than or equal to 500 nm, preferably greater than or equal to 10 nm and less than or equal to 300 nm, more preferably greater than or equal to 10 nm and less than or equal to 200 nm. In this embodiment, a 300-nm-thick tungsten film is formed as the conductive film. 
     Then, part of the conductive film is selectively etched using a resist mask to form the source electrode  209   a,  the drain electrode  209   b,  the wiring  219 , and the terminal electrode  216  (including other electrodes and wirings formed using the same film). The resist mask can be formed by a photolithography method, a printing method, an inkjet method, or the like as appropriate. Formation of the resist mask by an inkjet method needs no photomask; thus, fabrication cost can be reduced. 
     The conductive film may be etched by a dry etching method, a wet etching method, or both a dry etching method and a wet etching method. Note that an exposed portion of the semiconductor layer  208  is removed by the etching step in some cases. After the etching of the conductive film, the resist mask is removed. 
     [Formation of Insulating Layer] 
     Next, the insulating layer  210  is formed over the source electrode  209   a,  the drain electrode  209   b,  the wiring  219 , and the terminal electrode  216  (see  FIG.  6 D ). The insulating layer  210  can be formed using a material and a method similar to those of the insulating layer  205 . 
     In the case where an oxide semiconductor is used for the semiconductor layer  208 , an insulating layer containing oxygen is preferably used for at least part of the insulating layer  210  that is in contact with the semiconductor layer  208 . For example, in the case where the insulating layer  210  is a stack of a plurality of layers, at least a layer that is in contact with the semiconductor layer  208  is preferably formed using silicon oxide. 
     [Formation of Opening] 
     Next, part of the insulating layer  210  is selectively etched using a resist mask to form an opening  128  (see  FIG.  6 D ). At the same time, another opening that is not illustrated is also formed. The resist mask can be formed by a photolithography method, a printing method, an inkjet method, or the like as appropriate. Formation of the resist mask by an inkjet method needs no photomask; thus, fabrication cost can be reduced. 
     The insulating layer  210  may be etched by a dry etching method, a wet etching method, or both a dry etching method and a wet etching method. 
     The drain electrode  209   b  and the terminal electrode  216  are partly exposed by the formation of the opening  128 . The resist mask is removed after the formation of the opening  128 . 
     [Formation of Planarization Film] 
     Next, the insulating layer  211  is formed over the insulating layer  210  (see  FIG.  7 A ). The insulating layer  211  can be formed using a material and a method similar to those of the insulating layer  205 . 
     Planarization treatment may be performed on the insulating layer  211  to reduce unevenness of a surface on which the light-emitting element  125  is formed. The planarization treatment may be, but not particularly limited to, polishing treatment (e.g., chemical mechanical polishing (CMP)) or dry etching treatment. 
     Forming the insulating layer  211  using an insulating material with a planarization function can make polishing treatment unnecessary. As the insulating material with a planarization function, for example, an organic material such as a polyimide resin or an acrylic resin can be used. Besides such organic materials, a low-dielectric constant material (a low-k material) or the like can be used. Note that the insulating layer  211  may be formed by stacking a plurality of insulating layers formed of any of these materials. 
     Part of the insulating layer  211  that overlaps with the opening  128  is removed to form an opening  129 . At the same time, another opening that is not illustrated is also formed. In addition, the insulating layer  211  in a region to which the external electrode  124  is connected later is removed. Note that the opening  129  or the like can be formed in such a manner that a resist mask is formed by a photolithography process over the insulating layer  211  and a region of the insulating layer  211  that is not covered with the resist mask is etched. A surface of the drain electrode  209 h is exposed by the formation of the opening  129 . 
     When the insulating layer  211  is formed using a photosensitive material, the opening  129  can be formed without the resist mask. In this embodiment, a photosensitive polyimide resin is used to form the insulating layer  211  and the opening  129 . 
     [Formation of Anode] 
     Next, the electrode  115  is formed over the insulating layer  211  (see  FIG.  7 B ). The electrode  115  is preferably formed using a conductive material that efficiently reflects light emitted from the EL layer  117  formed later. Note that the electrode  115  may have a stacked-layer structure of a plurality of layers without limitation to a single-layer structure. For example, in the case where the electrode  115  is used as an anode, a layer in contact with the EL layer  117  may be a light-transmitting layer, such as an indium tin oxide layer, having a work function higher than that of the EL layer  117 , and a layer having high reflectance (e.g., aluminum, an alloy containing aluminum, or silver) may be provided in contact with the layer. 
     Note that although the display device having a top-emission structure is described as an example in this embodiment, a display device having a bottom-emission structure or a dual-emission structure may be used. 
     In the case where the display device  100  has a bottom-emission structure or a dual-emission structure, the electrode  115  is preferably formed using a light-transmitting conductive material. 
     The electrode  115  can be formed in such a manner that a conductive film to be the electrode  115  is formed over the insulating layer  211 , a resist mask is formed over the conductive film, and a region of the conductive film that is not covered with the resist mask is etched. The conductive film can be etched by a dry etching method, a wet etching method, or both a dry etching method and a wet etching method. The resist mask can be formed by a photolithography method, a printing method, an inkjet method, or the like as appropriate, Formation of the resist mask by an inkjet method needs no photomask; thus, fabrication cost can be reduced. The resist mask is removed after the formation of the electrode  115 . 
     [Formation of Partition] 
     Next, the partition  114  is formed (see  FIG.  7 C ). The partition  114  is provided in order to prevent an unintended electrical short-circuit between light-emitting elements  125  in adjacent pixels and unintended light emission from the light-emitting element  125 . In the case of using a metal mask for formation of the EL layer  117  described later, the partition  114  has a function of preventing the contact of the metal mask with the electrode  115 . The partition  114  can be formed of an organic resin material such as an epoxy resin, an acrylic resin, or an imide resin or an inorganic material such as silicon oxide. The partition  114  is preferably formed so that its sidewall has a tapered shape or a tilted surface with a continuous curvature. The sidewall of the partition  114  having the above-described shape enables favorable coverage with the EL layer  117  and the electrode  118  formed later. 
     [Formation of EL Layer] 
     A structure of the EL layer  117  is described in Embodiment 6. 
     [Formation of Cathode] 
     The electrode  118  is used as a cathode in this embodiment, and thus is preferably formed using a material that has a low work function and can inject electrons into the EL layer  117  described later. As well as a single-layer of a metal having a low work function, a stack in which a metal material such as aluminum, a conductive oxide material such as indium tin oxide, or a semiconductor material is formed over a several-nanometer-thick buffer layer formed of an alkali metal or an alkaline earth metal having a low work function may be used as the electrode  118 . 
     In the case where light emitted from the EL layer  117  is extracted through the electrode  118 , the electrode  118  preferably has a property of transmitting visible light. The light-emitting element  125  includes the electrode  115 , the EL layer  117 , and the electrode  118  (see  FIG.  7 D ). 
     [Formation of Counter Element Formation Substrate] 
     An element formation substrate  141  provided with the light-blocking layer  264 , the coloring layer  266 , the overcoat layer  268 , the insulating layer  145 , and a separation layer  143  is formed over the element formation substrate  101  with the bonding layer  120  therebetween (see  FIG.  8 A ). Note that the element formation substrate  141  is formed to face the element formation substrate  101  and may thus be called a “counter element formation substrate”. A structure of the element formation substrate  141  (counter element formation substrate) is described later. 
     The element formation substrate  141  is fixed over the element formation substrate  101  by the bonding layer  120 . A light curable adhesive, a reactive curable adhesive, a thermosetting adhesive, or an anaerobic adhesive can be used as the bonding layer  120 . For example, an epoxy resin, an acrylic resin, or an imide resin can be used. In a top-emission structure, a drying agent (e.g., zeolite) having a size less than or equal to the wavelength of light or a filler (e.g., titanium oxide or zirconium) with a high refractive index is preferably mixed into the bonding layer  120 , in which case the efficiency of extracting light emitted from the EL layer  117  can be improved. 
     [Separation of Element Formation Substrate from Insulating Layer] 
     Next, the element formation substrate  101  attached to the insulating layer  205  with the separation layer  113  therebetween is separated from the insulating layer  205  (see  FIG.  8 B ). As a separation method, mechanical force (a separation process with a human hand or a gripper, a separation process by rotation of a roller, ultrasonic waves, or the like) may be used. For example, a cut is made in the separation layer  113  with a sharp edged tool, by laser light irradiation, or the like and water is injected into the cut. Alternatively, the cut is sprayed with a mist of water. A portion between the separation layer  113  and the insulating layer  205  absorbs water through capillarity action, so that the element formation substrate  101  can be separated easily from the insulating layer  205 . 
     [Bonding of Substrate] 
     Next, the substrate  111  is attached to the insulating layer  205  with the bonding layer  112  therebetween (see  FIGS.  9 A and  9 B ). The bonding layer  112  can be formed using a material similar to that of the bonding layer  120 . In this embodiment, a 20-μm-thick aramid (polyamide resin) with a Young&#39;s modulus of 10 GPa is used for the substrate  111 . 
     [Separation of Counter Element Formation Substrate from Insulating Layer] 
     Next, the element formation substrate  141  overlapping with the insulating layer  145  with the separation layer  143  therebetween is separated from the insulating layer  145  (see  FIG.  10 A ). The element formation substrate  141  can be separated in a manner similar to that of the above-described separation method of the element formation substrate  101 . 
     [Bonding of Substrate] 
     Next, the substrate  121  is attached to the insulating layer  145  with the bonding layer  142  therebetween (see  FIG.  10 B ). The bonding layer  142  can be formed using a material similar to that of the bonding layer  120 . The substrate  121  can be formed using a material similar to that of the substrate  111 . 
     [Formation of Opening] 
     Next, the substrate  121 , the bonding layer  142 , the insulating layer  145 , the coloring layer  266 , the overcoat layer  268 , and the bonding layer  120  in a region overlapping with the terminal electrode  216  and the opening  128  are removed to form the opening  122  (see  FIG.  11 A ). A surface of the terminal electrode  216  is partly exposed by the formation of the opening  122 . 
     [Formation of External Electrode] 
     Next, the anisotropic conductive connection layer  123  is formed in and on the opening  122 , and the external electrode  124  for inputting electric power or a signal to the display device  100  is formed over the anisotropic conductive connection layer  123  (see  FIG.  11 B ). The terminal electrode  216  is electrically connected to the external electrode  124  through the anisotropic conductive connection layer  123 . For example, a flexible printed circuit (FPC) can be used as the external electrode  124 . 
     The anisotropic conductive connection layer  123  can be formed using any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), and the like. 
     The anisotropic conductive connection layer  123  is formed by curing a paste-form or sheet-form material that is obtained by mixing conductive particles to a thermosetting resin or a thermosetting, light curable resin. The anisotropic conductive connection layer  123  exhibits alp anisotropic conductive property by light irradiation or thermocompression bonding. As the conductive particles used for the anisotropic conductive connection layer  123 , for example, particles of a spherical organic resin coated with a thin-film metal such as Au, Ni, or Co can be used. 
     [Bonding of Substrates] 
     Then, the substrate  137  is bonded to the substrate  111  with the bonding layer  138  provided therebetween. The substrate  147  is bonded to the substrate  121  with the bonding layer  148  provided therebetween (see  FIG.  12   ). The bonding layers  138  and  148  can be formed using a material similar to that of the bonding layer  120 . In this embodiment, for the substrate  137  and the substrate  147 , silicone rubber that has a light-transmitting property with respect to visible light, a thickness of 200 μm, and a Young&#39;s modulus of 0.03 GPa is used. 
     [Components Formed Over Counter Element Formation Substrate] 
     Next, components, such as the light-blocking layer  264 , formed over the element formation substrate  141  are described with reference to  FIGS.  13 A to  13 D . 
     First, the element formation substrate  141  is prepared. The element formation substrate  141  can be formed using a material similar to that of the element formation substrate  101 . Then, the separation layer  143  and the insulating layer  145  are formed over the element formation substrate  141  (see  FIG.  13 A ). The separation layer  143  can be formed using a material and a method similar to those of the separation layer  113 . The insulating layer  145  can be formed using a material and a method similar to those of the insulating layer  205 . 
     Next, the light-blocking layer  264  is formed over the insulating layer  145  (see  FIG.  13 B ). After that, the coloring layer  266  is formed (see  FIG.  13 C ). 
     The light-blocking layer  264  and the coloring layer  266  each are formed in a desired position with any of various materials by a printing method, an inkjet method, a photolithography method, or the like. 
     Next, the overcoat layer  268  is formed over the light-blocking layer  264  and the coloring layer  266  (see  FIG.  13 D ). 
     For the overcoat layer  268 , an organic insulating layer of an acrylic resin, an epoxy resin, polyimide, or the like can be used. With the overcoat layer  268 , for example, an impurity or the like contained in the coloring layer  266  can be prevented from diffusing into the light-emitting element  125  side. Note that the overcoat layer  268  is not necessarily formed. 
     A light-transmitting conductive film may be formed as the overcoat layer  268 . The light-transmitting conductive film is formed as the overcoat layer  268 , so that the light  235  emitted from the light-emitting element  125  can be transmitted through the overcoat layer  268  and layers overlapping with the overcoat layer  268 , while ionized impurities can be prevented from passing through the overcoat layer  268 . 
     The light-transmitting conductive film can be formed using, for example, indium oxide, indium tin oxide (ITO), indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added. Graphene or a metal film that is thin enough to have a light-transmitting property can also be used. 
     Through the above-described steps, the components such as the light-blocking layer  264  can be formed over the element formation substrate  141 . 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     Embodiment 3 
     A display device  150  having a bottom-emission structure can be fabricated by modification of the structure of the display device  100  having a top-emission structure. 
     &lt;Display Device Having Bottom-Emission Structure&gt; 
       FIG.  14    illustrates an example of a cross-sectional structure of the display device  150  having a bottom-emission structure. Note that  FIG.  14    is a cross-sectional view of a portion similar to the portion denoted by the dashed-dotted line X 1 -X 2   FIG.  3 A  that is a perspective view of the display device  100 . The display device  150  having a bottom-emission structure differs from the display device  100  in the position where the light-blocking layer  264 , the coloring layer  266 , and the overcoat layer  268  are formed. Specifically, in the display device  150 , the light-blocking layer  264 , the coloring layer  266 , and the overcoat layer  268  are formed over the substrate  111 . 
     In the display device  150 , the substrate  121  on which the insulating layer  145  is directly formed can be attached to the substrate  111  with the bonding layer  120  therebetween. In other words, the insulating layer  145  does not need to be transferred from the element formation substrate  141 ; thus, the element formation substrate  141 , the separation layer  143 , and the bonding layer  142  are not needed. This can improve the productivity, yield, and the like of the display device. Note that other components of the display device  150  can be formed as in the case of the display device  100 . 
     In the display device  150  having a bottom-emission structure, the electrode  115  is formed using a light-transmitting conductive material, and the electrode  118  is formed using a conductive material that efficiently reflects light emitted from the EL layer  117 . 
     In the display device  150 , the light  235  emitted from the EL layer  17  can be extracted from the substrate  111  side through the coloring layer  266 . 
     &lt;Back Gate Electrode&gt; 
     Note that the display device  150  is an example of a display device in which a transistor  272  is used as a transistor included in the driver circuit  133 . Although the transistor  272  can be formed in a manner similar to that of the transistor  252 , the transistor  272  differs from the transistor  252  in that an electrode  263  is formed over the insulating layer  210  in a region overlapping with the semiconductor layer  208 . The electrode  263  can be formed using a material and a method similar to those of the gate electrode  206 . 
     The electrode  263  can also serve as a gate electrode. In the case where one of the gate electrode  206  and the electrode  263  is simply referred to as a “gate electrode”, the other may be referred to as a “back gate electrode”. One of the gate electrode  206  and the electrode  263  may be referred to as a “first gate electrode”, and the other may be referred to as a “second gate electrode”. 
     In general, the back gate electrode is formed using a conductive film and positioned so that the channel formation region of the semiconductor layer is positioned between the gate electrode and the back gate electrode. Thus, the back gate electrode can function in a manner similar to that of the gate electrode. The potential of the back gate electrode may be the same as that of the gate electrode or may be a GND potential or a predetermined potential. By changing a potential of the back gate electrode, the threshold voltage of the transistor can be changed. 
     Furthermore, the gate electrode and the hack gate electrode are formed using conductive films and thus each have a function of preventing an electric field generated outside the transistor from influencing the semiconductor layer in which the channel is formed (in particular, a function of blocking static electricity). 
     In the case where light is incident on the back gate electrode side, when the back gate electrode is formed using a light-blocking conductive film, light can be prevented from entering the semiconductor layer from the back gate electrode side. Thus, photodegradation of the semiconductor layer can be prevented and deterioration in electrical characteristics of the transistor, such as a shift of the threshold voltage, can be prevented. 
     By providing the gate electrode  206  and the electrode  263  with the semiconductor layer  208  therebetween and setting the potentials of the gate electrode  206  and the electrode  263  to be equal, a region of the semiconductor layer  208  through which carriers flow is enlarged in the film thickness direction; thus, the number of transferred carriers is increased. As a result, the on-state current and the field-effect mobility of the transistor are increased. 
     The gate electrode  206  and the electrode  263  each have a function of blocking an external electric field; thus, charges in a layer under the gate electrode  206  and in a layer over the electrode  263  do not affect the semiconductor layer  208 . Thus, there is little change in the threshold voltage in a stress test (e.g., a negative gate bias temperature (−GBT) stress test in which a negative voltage is applied to a gate or a +GBT stress test in which a positive voltage is applied to a gate). In addition, changes in the rising voltages of on-state current at different drain voltages can be suppressed. 
     The BT stress test is one kind of accelerated test and can evaluate, in a short time, change in characteristics (i.e., a change over time) of transistors, which is caused by long-term use. In particular, the amount of change in threshold voltage of the transistor in the BT stress test is an important indicator when examining the reliability of the transistor. As the amount of change in the threshold voltage in the BT stress test is small, the transistor has higher reliability. 
     By providing the gate electrode  206  and the electrode  263  and setting the potentials of the gate electrode  206  and the electrode  263  to be the same, the amount of change in the threshold voltage is reduced. Accordingly, variation in electrical characteristics among a plurality of transistors is also reduced. 
     Note that a back gate electrode may be provided in the transistor  232  formed in the display area  131 . 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     Embodiment 4 
     A display device  160  in which the coloring layer  266 , the light-blocking layer  264 , the overcoat layer  268 , and the like are not provided can be manufactured by modification of the structure of the display device  100  having a top-emission structure. 
       FIG.  15 A  illustrates an example of a cross-sectional structure of the display device  160 . Note that  FIGS.  15 A and  15 B  are cross-sectional views of a portion similar to the portion denoted by the dashed-dotted line X 1 -X 2  in  FIG.  3 A  that is a perspective view of the display device  100 . In the display device  160 , color display can be performed by using an EL layer  117 A, an EL layer  117 B, an EL layer  117 C (not shown), and the like instead of the light-blocking layer  264 , the coloring layer  266 , and the overcoat layer  268 . The EL layer  117 A, the EL layer  117 B, and the like can emit light of the respective colors such as red, blue, and green. For example, the EL layer  117 A emits light  235 A of a red wavelength, the EL layer  117 B emits light  235 B of a blue wavelength, and the EL layer  117 C emits light  235 C (not shown) of a green wavelength. 
     Since the coloring layer  266  is not provided, a reduction in luminance caused when the light  235 A, light  235 B, and light  235 C are transmitted through the coloring layer  266  can be prevented. The thicknesses of the EL layer  117 A, EL layer  117 B, and EL layer  117 C are adjusted in accordance with the wavelengths of the light  235 A, light  235 B, and light  235 C, whereby a higher color purity can be achieved. 
     Note that in a manner similar to that of the display device  160 , a display device  170  in which the coloring layer  266 , the light-blocking layer  264 , the overcoat layer  268 , and the like are not provided can also be manufactured by modification of the structure of the display device  150  having a bottom emission structure.  FIG.  15 B  illustrates an example of a cross-sectional structure of the display device  170 . 
     Note that as illustrated in  FIGS.  27 A and  27 B , an optical film  911 , examples of which include a polarizing plate, a retardation plate, and a quarter-wave plate, may be additionally provided. The optical film  911  is bonded with the use of a bonding layer  148 A or a bonding layer  138 A. This structure can reduce reflection at a screen surface. Moreover, the optical film  911  can be protected. 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     Embodiment 5 
     In the display device  100 , a substrate provided with a touch sensor may be provided over the substrate  147  as illustrated in  FIG.  16 A . The touch sensor is formed using a conductive layer  991 , a conductive layer  993 , and the like. In addition, an insulating layer  992  is formed between the conductive layers. 
     As the conductive layer  991  and/or the conductive layer  993 , a transparent conductive film of indium tin oxide, indium zinc oxide, or the like is preferably used. Note that a layer containing a low-resistance material may be used for part or the whole of the conductive layer  991  and/or the conductive layer  993  in order to reduce resistance. For example, the conductive layer  991  and/or the conductive layer  993  can be formed as a single layer or a stack using any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten and an alloy containing any of these metals as a main component. Alternatively, a metal nanowire may be used as the conductive layer  991  and/or the conductive layer  993 . Silver or the like is preferably used as a metal for the metal nanowire, in which case the resistance value can be reduced and the sensitivity of the sensor can be improved. 
     The insulating layer  992  is preferably formed as a single layer or a multilayer using silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, or the like. The insulating layer  992  can be formed by a sputtering method, a CVD method, a thermal oxidation method, a coating method, a printing method, or the like. 
     Although the touch sensor is provided over the substrate  994 , one embodiment of the present invention is not limited thereto. The touch sensor may be provided under the substrate  994  (i.e., between the substrate  121  and the substrate  994 ). 
     The substrate provided with the touch sensor may be positioned under the substrate  137  in the display device  150 .  FIG.  16 B  illustrates an example of this case. 
     The touch sensor may be positioned over the substrate  121  with the bonding layer  148 A provided therebetween as illustrated in  FIG.  28 A , or may be positioned under the substrate  111  with the bonding layer  138 A provided therebetween as illustrated in  FIG.  28 B . 
     Note that the substrate  994  may have a function of an optical film. That is, the substrate  994  may have a function of a polarizing plate, a retardation plate, or the like. 
     In the display device  100 , the substrate  121  may be provided with a touch sensor.  FIG.  17 A  illustrates an example in which the substrate  121  is provided with a touch sensor and the substrate  147  is formed over the touch sensor and the bonding layer  142 . 
     In the display device  150 , the substrate  111  may be provided with a touch sensor.  FIG.  17 B  illustrates an example in which the substrate  111  is provided with a touch sensor and the substrate  137  is formed under the touch sensor and the bonding layer  138 . 
     In the display device  160 , the substrate  121  may be provided with a touch sensor.  FIG.  18 A  illustrates an example in which the substrate  121  is provided with a touch sensor and the substrate  147  is formed over the touch sensor and the bonding layer  142 . 
     In the display device  170 , the substrate  111  may be provided with a touch sensor.  FIG.  18 B  illustrates an example in which the substrate  111  is provided with a touch sensor and the substrate  137  is formed under the touch sensor and the bonding layer  138 . 
     Note that in  FIGS.  18 A and  18 B , the optical film  911  may be provided.  FIGS.  29 A and  29 B  illustrate an example of this case. The optical film  911  is bonded with the use of a bonding layer  142 A or the bonding layer  138 A. 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     Embodiment 6 
     In this embodiment, structure examples of a light-emitting element that can be applied to the light-emitting element  125  are described. Note that an EL layer  320  described in this embodiment corresponds to the EL layer  117  described in the above embodiment. 
     &lt;Structure of Light-Emitting Element&gt; 
     In a light-emitting element  330  illustrated in  FIG.  9 A , the EL layer  320  is interposed between a pair of electrodes (an electrode  318  and an electrode  322 ). Note that the electrode  318  is used as an anode and the electrode  322  is used as a cathode as an example in the following description of this embodiment. 
     The EL layer  320  includes at least a light-emitting layer and may have a stacked-layer structure including a functional layer other than the light-emitting layer. As the functional layer other than the light-emitting layer, a layer containing a substance having a high hole-injection property, a substance having a high hole-transport property, a substance having a high electron-transport property, a substance having a high electron-injection property, a bipolar substance (a substance having high electron- and hole-transport properties), or the like can be used. Specifically, functional layers such as a hole-injection layer, a hole-transport layer, an electron-transport layer, and an electron-injection layer can be used in combination as appropriate. 
     The light-emitting element  330  illustrated in  FIG.  19 A  emits light when current flows because of a potential difference generated between the electrode  318  and the electrode  322  and holes and electrons are recombined in the EL layer  320 . That is, the light-emitting region is formed in the EL layer  320 . 
     In the present invention, light emitted from the light-emitting element  330  is extracted to the outside from the electrode  318  side or the electrode  322  side. Therefore, one of the electrode  318  and the electrode  322  is formed of a light-transmitting substance. 
     Note that a plurality of EL layers  320  may be stacked between the electrode  318  and the electrode  322  as in a light-emitting element  331  illustrated in  FIG.  19 B . In the case where n (n is a natural number of 2 or more) layers are stacked, a charge generation layer  320   a  is preferably provided between an m-th EL layer  320  and an (m+1)-th EL layer  320 . Note that m is a natural number greater than or equal to 1 and less than n. 
     The charge generation layer  320   a  can be formed using a composite material of an organic compound and a metal oxide, a metal oxide, a composite material of an organic compound and an alkali metal, an alkaline earth metal, or a compound thereof; alternatively, these materials can be combined as appropriate. Examples of the composite material of an organic compound and a metal oxide include composite materials of an organic compound and a metal oxide such as vanadium oxide, molybdenum oxide, and tungsten oxide. As the organic compound, a variety of compounds can be used; for example, low molecular compounds such as an aromatic amine compound, a carbazole derivative, and aromatic hydrocarbon and oligomers, dendrimers, and polymers of these low molecular compounds. As the organic compound, it is preferable to use the organic compound which has a hole-transport property and has a hole mobility of 10 −6  cm 2 /Vs or higher. However, substances other than the substances given above may also be used as long as the substances have hole-transport properties higher than electron-transport properties. These materials used for the charge generation layer  320   a  have excellent carrier-injection properties and carrier-transport properties; thus, the light-emitting element  330  can be driven with low current and with low voltage. 
     Note that the charge generation layer  320   a  may be formed with a combination of a composite material of an organic compound and a metal oxide with another material. For example, a layer containing a composite material of the organic compound and the metal oxide may be combined with a layer containing a compound of a substance selected from substances with an electron-donating property and a compound with a high electron-transport property. Moreover, a layer containing a composite material of the organic compound and the metal oxide may be combined with a transparent conductive film. 
     The light-emitting element  331  having such a structure is unlikely to suffer the problem of energy transfer, quenching, or the like and has an expanded choice of materials, and thus can easily have both high emission efficiency and a long lifetime. Moreover, it is easy to obtain phosphorescence from one light-emitting layer and fluorescence from the other light-emitting layer. 
     The charge generation layer  320   a  has a function of injecting holes to one of the EL layers  320  that is in contact with the charge generation layer  320   a  and a function of injecting electrons to the other EL layer  320  that is in contact with the charge generation layer  320   a,  when voltage is applied between the electrode  318  and the electrode  322 . 
     The light-emitting element  331  illustrated in  FIG.  19 B  can provide a variety of emission colors by changing the type of the light-emitting substance used for the EL layer  320 . In addition, a plurality of light-emitting substances emitting light of different colors may be used as the light-emitting substances, whereby light emission having a broad spectrum or white light emission can be obtained. 
     In the case of obtaining white light emission using the light-emitting element  331  illustrated in  FIG.  19 B , as for the combination of a plurality of EL layers, a structure for emitting white light including red light, green light, and blue light may be used; for example, the structure may include a light-emitting layer containing a blue fluorescent substance as a light-emitting substance and a light-emitting layer containing red and green phosphorescent substances as light-emitting substances. Alternatively, a structure including a light-emitting layer emitting red light, a light-emitting layer emitting green light, and a light-emitting layer emitting blue light may be employed. Further alternatively, with a structure including light-emitting layers emitting light of complementary colors, white light emission can be obtained. In a stacked-layer element including two light-emitting layers in which light emitted from one of the light-emitting layers and light emitted from the other light-emitting layer have complementary colors to each other, the combinations of colors are as follows: blue and yellow, blue-green and red, and the like. 
     Note that in the structure of the above-described stacked-layer element, by providing the charge generation layer between the stacked light-emitting layers, the element can have a long lifetime in a high-luminance region while keeping the current density low. In addition, the voltage drop due to the resistance of the electrode material can be reduced, whereby uniform light emission in a large area is possible. 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     Embodiment 7 
     In this embodiment, examples of an electronic device and a lighting device including the display device of one embodiment of the present invention are described with reference to drawings. 
     As examples of the electronic devices including a flexible display device, the following can be given: television devices (also called televisions or television receivers), monitors of computers or the like, digital cameras, digital video cameras, digital photo frames, mobile phones (also called cellular phones or mobile phone devices), portable game machines, portable information terminals, audio reproducing devices, large game machines such as pachinko machines, and the like. 
     In addition, a lighting device or a display device can be incorporated along a curved inside/outside wall surface of a house or a building or a curved interior/exterior surface of a car. 
       FIG.  20 A  illustrates an example of a mobile phone. A mobile phone  7400  is provided with a display portion  7402  incorporated in a housing  7401 , an operation button  7403 , an external connection port  7404 , a speaker  7405 , a microphone  7406 , and the like. Note that the mobile phone  7400  is manufactured using the display device in the display portion  7402 . 
     When the display portion  7402  of the mobile phone  7400  illustrated in  FIG.  20 A  is touched with a finger or the like, data can be input to the mobile phone  7400 . Further, operations such as making a call and inputting a character can be performed by touch on the display portion  7402  with a finger or the like. 
     The power can be turned on or off with the operation button  7403 . In addition, types of images displayed on the display portion  7402  can be switched; for example, switching images from a mail creation screen to a main menu screen is performed with the operation button  7403 . 
     Here, the display portion  7402  includes the display device of one embodiment of the present invention. Thus, the mobile phone can have a curved display portion and high reliability. 
       FIG.  20 B  illustrates an example of a wristband-type display device. A portable display device  7100  includes a housing  7101 , a display portion  7102 , an operation button  7103 , and a sending and receiving device  7104 . 
     The portable display device  7100  can receive a video signal with the sending and receiving device  7104  and can display the received video on the display portion  7102 . In addition, with the sending and receiving device  7104 , the portable display device  7100  can send an audio signal to another receiving device. 
     With the operation button  7103 , power ON/OFF, switching displayed videos, adjusting volume, and the like can be performed. 
     Here, the display portion  7102  includes the display device of one embodiment of the present invention. Thus, the portable display device can have a curved display portion and high reliability. 
       FIGS.  20 C to  20 F , illustrate examples of lighting devices. Lighting devices  7200 ,  7210 , and  7220  each include a stage  7201  provided with an operation switch  7203  and a light-emitting portion supported by the stage  7201 . 
     The lighting device  7200  illustrated in  FIG.  20 C  includes a light-emitting portion  7202  with a wave-shaped light-emitting surface and thus is a good-design lighting device. 
     A light-emitting portion  7212  included in the lighting device  7210  illustrated in  FIG.  20 D  has two convex-curved light-emitting portions symmetrically placed. Thus, light radiates from the lighting device  7210 . 
     The lighting device  7220  illustrated in  FIG.  20 E  includes a concave-curved light-emitting portion  7222 . This is suitable for illuminating a specific range because light emitted from the light-emitting portion  7222  is collected to the front of the lighting device  7220 . 
     The light-emitting portion included in each of the lighting devices  7200 ,  7210 , and  7220  is flexible; thus, the light-emitting portion may be fixed on a plastic member, a movable frame, or the like so that an emission surface of the light-emitting portion can be bent freely depending on the intended use. 
     The light-emitting portions included in the lighting devices  7200 ,  7210 , and  7220  each include the display device of one embodiment of the present invention. Thus, the lighting devices whose display portions can be curved into any shape and which has high reliability can be provided. 
       FIG.  30    shows a cross-sectional view of a lighting device. 
       FIG.  21 A  illustrates an example of a portable display device. A display device  7300  includes a housing  7301 , a display portion  7302 , operation buttons  7303 , a display portion pull  7304 , and a control portion  7305 . 
     The display device  7300  includes a rolled flexible display portion  7302  in the cylindrical housing  7301 . 
     The display device  7300  can receive a video signal with the control portion  7305  and can display the received video on the display portion  7302 . In addition, a battery is included in the control portion  7305 . Moreover, a connector may be included in the control portion  7305  so that a video signal or power can be supplied directly. 
     With the operation buttons  7303 , power ON/OFF, switching of displayed videos, and the like can be performed. 
       FIG.  21 B  illustrates a state in which the display portion  7302  is pulled out with the display portion pull  7304 . Videos can be displayed on the display portion  7302  in this state. Further, the operation buttons  7303  on the surface of the housing  7301  allow one-handed operation. 
     Note that a reinforcement frame may be provided for an edge portion of the display portion  7302  in order to prevent the display portion  7302  from being curved when pulled out. 
     Note that in addition to this structure, a speaker may be provided for the housing so that sound is output with an audio signal received together with a video signal. 
     The display portion  7302  includes the display device of one embodiment of the present invention. Thus, the display portion  7302  is a flexible, highly reliable display device, which makes the display device  7300  lightweight and highly reliable. 
       FIGS.  22 A and  22 B  illustrate a double foldable tablet terminal  9600  as an example.  FIG.  22 A  illustrates the tablet terminal  9600  which is unfolded. The tablet terminal  9600  includes a housing  9630 , a display portion  9631 , a display mode switch  9626 , a power switch  9627 , a power-saving mode switch  9625 , a clasp  9629 , and an operation switch  9628 . 
     The housing  9630  includes a housing  9630   a  and a housing  9630   b,  which are connected with a hinge portion  9639 . The hinge portion  9639  makes the housing  9630  double foldable. 
     The display portion  9631  is provided on the housing  9630   a,  the housing  9630   b,  and the hinge portion  9639 . By the use of the display device that is disclosed in the present specification and the like, the tablet terminal in which the display portion  9631  can be bent and which has high reliability can be provided. 
     Part of the display portion  9631  can be a touchscreen region  9632  and data can be input when a displayed operation key  9638  is touched. A structure can be employed in which half of the display portion  9631  has only a display function and the other half has a touchscreen function. The whole display portion  9631  may have a touchscreen function. For example, keyboard buttons may be displayed on the entire region of the display portion  9631  so that the display portion  9631  can be used as a data input terminal. 
     The display mode switch  9626  can switch the display between portrait mode, landscape mode, and the like, and between monochrome display and color display, for example. The power-saving switch  9625  can control display luminance in accordance with the amount of external light in use of the tablet terminal detected by an optical sensor incorporated in the tablet terminal. Another detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, may be incorporated in the tablet terminal, in addition to the optical sensor. 
       FIG.  22 B  illustrates the tablet terminal  9600  which is folded. The tablet terminal  9600  includes the housing  9630 , a solar cell  9633 , and a charge and discharge control circuit  9634 . As an example,  FIG.  22 B  illustrates the charge and discharge control circuit  9634  including the battery  9635  and the DC-to-DC converter  9636 . 
     By including the display device that is disclosed in the present specification and the like, the display portion  9631  is foldable. Since the tablet terminal  9600  is double foldable, the housing  9630  can be closed when the tablet terminal is not in use, for example; thus, the tablet terminal is highly portable. Moreover, since the display portion  9631  can be protected when the housing  9630  is closed, the tablet terminal can have high durability and high reliability for long-term use. 
     The tablet terminal illustrated in  FIGS.  22 A and  22 B  can have other functions such as a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, and a function of controlling processing by various kinds of software (programs). 
     The solar cell  9633  provided on a surface of the tablet terminal can supply power to the touchscreen, the display portion, a video signal processing portion, or the like. Note that the solar cell  9633  is preferably provided on both surfaces of the housing  9630 , in which case the battery  9635  can be charged efficiently. When a lithium ion battery is used as the battery  9635 , there is an advantage of downsizing or the like. 
     The structure and operation of the charge and discharge control circuit  9634  illustrated in  FIG.  22 B  is described with reference to a block diagram of  FIG.  22 C .  FIG.  22 C  illustrates the solar cell  9633 , the battery  9635 , the DC-to-DC converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and the display portion  9631 . The battery  9635 , the DC-to-DC converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond. to the charge and discharge control circuit  9634  illustrated in  FIG.  22 B . 
     First, description is made on an example of the operation in the case where power is generated by the solar cell  9633  with the use of external light. The voltage of the power generated by the solar cell is raised or lowered by the DC-to-DC converter  9636  so as to be voltage for charging the battery  9635 . Then, when power from the solar cell  9633  is used for the operation of the display portion  9631 , the switch SW 1  is turned on and the voltage of the power is raised or lowered by the converter  9637  so as to be voltage needed for the display portion  9631 . When images are not displayed on the display portion  9631 , the switch SW 1  is turned off and the switch SW 2  is turned on so that the battery  9635  is charged. 
     Although the solar cell  9633  is described as an example of a power generation unit, the power generation unit is not particularly limited, and the battery  9635  may be charged by another power generation unit such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery  9635  may be charged by a non-contact power transmission module which is capable of charging by transmitting and receiving power by wireless (without contact), or another charge unit used in combination. 
     It is needless to say that one embodiment of the present invention is not limited to the above-described electronic devices and lighting devices as long as the display device of one embodiment of the present invention is included. 
     This embodiment can be implemented in an appropriate combination with any of the structures described in other embodiments. 
     EXAMPLE 
     A display device  500  was formed by the method described in Embodiment 2. As a display element, a light-emitting element including an organic EL material was used. Note that for the substrate  111  and the substrate  121 , a 20-μm-thick aramid with a Young&#39;s modulus of approximately 10 GPa was used. For the substrate  137  and the substrate  147 , 200-μm-thick silicone rubber with a Young&#39;s modulus of approximately 0.03 GPa was used. 
     Then, the entire display region of the display device  500  was made to emit light and the display device  500  was folded double such that the display region on the left of the bent portion and that on the right of the bent portion overlapped with each other  FIG.  23 A  is a photograph showing the display device  500  folded double in an emission state. 
     Then, the display device  500  folded double was unfolded, and a display state of the display device  500  was observed.  FIG.  23 B  is a photograph showing the display device  500  that was unfolded. Even after the display device  500  was folded and unfolded, the entire display region thereof emitted light and the display state remained to be excellent. 
     Note that a display device which was not provided with the substrate  137  and the substrate  147  was fabricated as a comparative sample by the method described in Embodiment 2, and folded double. In this display device, the substrate  111  and the substrate  121  were cracked, and light emission was not maintained in the entire display region. 
     This application is based on Japanese Patent Application serial no. 2013-169542 filed with Japan Patent Office on Aug. 19, 2013, the entire contents of which are hereby incorporated by reference.