Patent Publication Number: US-2022216243-A1

Title: Peeling method and manufacturing method of flexible device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a peeling method and a manufacturing method of a flexible device. One embodiment of the present invention relates to a display device, a display module, and an electronic device. One embodiment of the present invention relates to a display device, a display module, and an electronic device which are flexible. 
     Note that one embodiment of the present invention is not limited to the above technical field. Examples of the technical field of one embodiment of the present invention include a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, an electronic device, a lighting device, an input device (e.g., a touch sensor), an input/output device (e.g., a touch panel), a driving method thereof, and a manufacturing method thereof. 
     Note that in this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor, a semiconductor circuit, an arithmetic device, a memory device, and the like are each an embodiment of the semiconductor device. In addition, an imaging device, an electro-optical device, a power generation device (e.g., a thin film solar cell and an organic thin film solar cell), and an electronic device each may include a semiconductor device. 
     2. Description of the Related Art 
     Display devices using organic electroluminescent (EL) elements or liquid crystal elements have been known. Examples of the display device also include a light-emitting device provided with a light-emitting element such as a light-emitting diode (LED), and electronic paper performing display with an electrophoretic method or the like. 
     The organic EL element generally has a structure in which a layer containing a light-emitting organic compound is provided between a pair of electrodes. When voltage is applied to this element, light emission can be obtained from the light-emitting organic compound. With the use of such an organic EL element, thin, lightweight, high-contrast, and low-power-consumption display devices can be achieved. 
     Patent Document 1 discloses a flexible light-emitting device using an organic EL element. 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2014-197522 
     SUMMARY OF THE INVENTION 
     Flexible devices typified by flexible displays can be obtained by forming semiconductor elements, such as transistors, and other elements over flexible substrates (films). However, flexible substrates have lower heat resistance than glass substrates or the like. Thus, when transistors or the like are directly formed on flexible substrates, the electrical characteristics and reliability of the transistors cannot be improved in some cases. 
     Thus, a method described in Patent Document 1 in which a semiconductor element, a light-emitting element, or the like formed over a glass substrate over which a peeling layer is formed is peeled and transferred to a flexible substrate has been considered. In this method, the formation temperature of the semiconductor element can be increased; thus, an extremely highly reliable flexible device can be manufactured. 
     An object of one embodiment of the present invention is to provide a novel peeling method. Another object of one embodiment of the present invention is to provide a peeling method at low cost with high mass productivity. Another object of one embodiment of the present invention is to perform peeling using a large-sized substrate. 
     An object of one embodiment of the present invention is to provide a novel flexible device and a manufacturing method thereof. Another object of one embodiment of the present invention is to provide a highly reliable flexible device. Another object of one embodiment of the present invention is to manufacture a flexible device at low temperatures. Another object of one embodiment of the present invention is to provide a manufacturing method of a flexible device with a simplified manufacturing process. Another object of one embodiment of the present invention is to provide a manufacturing method of a flexible device at low cost with high mass productivity. Another object of one embodiment of the present invention is to manufacture a flexible device using a large-sized substrate. Another object of one embodiment of the present invention is to provide a device with a curved surface. Another object of one embodiment of the present invention is to provide a lightweight flexible device. Another object of one embodiment of the present invention is to provide a thin flexible device. Another object of one embodiment of the present invention is to provide a flexible device capable of being repeatedly bent. 
     Note that the descriptions of these objects do not disturb the existence of other objects. One embodiment of the present invention does not necessarily achieve all the objects. Other objects can be derived from the description of the specification, the drawings, and the claims. 
     (1) One embodiment of the present invention is a peeling method including a step of forming a resin layer having a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm over a formation substrate using a photosensitive and thermosetting material, a step of forming a transistor including an oxide semiconductor in a channel formation region over the resin layer, a step of irradiating the resin layer with light using a linear laser device, and a step of separating the transistor and the formation substrate from each other. 
     In the above embodiment (1), the resin layer is preferably formed using a solution having a viscosity of greater than or equal to 5 cP and less than 100 cP and further preferably greater than or equal to 10 cP and less than 50 cP. 
     In the above embodiment (1), the resin layer is preferably formed with a spin coater. 
     In the above embodiment (1), it is preferable that the resin layer be formed by heating the material at a first temperature and the transistor be formed at a temperature lower than or equal to the first temperature. 
     In the above embodiment (1), the resin layer is preferably irradiated with light from the formation substrate side using the linear laser device. 
     (2) Another embodiment of the present invention is a peeling method including a step of forming a first film having a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm over a formation substrate using a photosensitive and thermosetting material, a step of forming a first region and a second region which is thinner than the first region in the first film by a photolithography method, a step of forming a resin layer having the first region and the second region by heating the first film at a first temperature, a step of forming a transistor including an oxide semiconductor in a channel formation region over the resin layer, a step of forming a conductive layer to overlap with the second region of the resin layer, a step of irradiating the resin layer with light using a linear laser device, and a step of separating the transistor and the formation substrate from each other. 
     (3) Another embodiment of the present invention is a peeling method including a step of forming a first film having a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm over a formation substrate using a photosensitive and thermosetting material, a step of forming an opening in the first film by a photolithography method, a step of forming a resin layer having the opening by heating the first film at a first temperature, a step of forming a transistor including an oxide semiconductor in a channel formation region over the resin layer, a step of forming a conductive layer to overlap with the opening of the resin layer, a step of irradiating the resin layer with light using a linear laser device, and a step of separating the transistor and the formation substrate from each other. 
     In each of the above embodiments (2) and (3), the conductive layer is preferably formed using the same material and the same fabrication step as an electrode included in the transistor. 
     Another embodiment of the present invention is a method for manufacturing a flexible device including a step of exposing the conductive layer by separating the transistor and the formation substrate from each other using the peeling method of the above embodiment (2) or (3), and a step of electrically connecting the conductive layer and a circuit board to each other through the opening of the resin layer. 
     (4) Another embodiment of the present invention is a display device including a resin layer, a transistor over the resin layer, and a display element electrically connected to the transistor. The resin layer has a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm. The transistor includes an oxide semiconductor in a channel formation region. The 5% weight loss temperature of the resin layer is preferably lower than 400° C. 
     Another embodiment of the present invention is a display module including the display device of the above embodiment (4) and a circuit board. The display device includes a conductive layer. The resin layer has an opening. The conductive layer is electrically connected to the circuit board through the opening. 
     (5) Another embodiment of the present invention is a display device including a flexible substrate, a first resin layer over the substrate, a first inorganic insulating layer over the first resin layer, a second resin layer over the first inorganic insulating layer, a second inorganic insulating layer over the second resin layer, an oxide semiconductor layer over the second inorganic insulating layer, a first gate insulating layer over the oxide semiconductor layer, a first gate over the first gate insulating layer, a source and a drain each electrically connected to the oxide semiconductor layer, and a display element electrically connected to the source or the drain. The first resin layer has a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm. The 5% weight loss temperature of the first resin layer is preferably lower than 400° C. 
     In the above embodiment (5), it is preferable that a second gate be provided between the second inorganic insulating layer and the oxide semiconductor layer and a second gate insulating layer be provided between the second gate and the oxide semiconductor layer. 
     Alternatively, in the above embodiment (5), a second gate is preferably provided between the first inorganic insulating layer and the second resin layer. In that case, the second inorganic insulating layer functions as a second gate insulating layer. Furthermore, a third inorganic insulating layer is preferably provided over the second gate and the first inorganic insulating layer. 
     (6) Another embodiment of the present invention is a display device including a flexible substrate, a first resin layer over the substrate, a second resin layer over the first resin layer, an inorganic insulating layer over the second resin layer, an oxide semiconductor layer over the inorganic insulating layer, a first gate insulating layer over the oxide semiconductor layer, a first gate over the first gate insulating layer, a source and a drain each electrically connected to the oxide semiconductor layer, a second gate between the first resin layer and the second resin layer, and a display element electrically connected to the source or the drain. The inorganic insulating layer functions as a second gate insulating layer. The first resin layer has a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm. 
     Another embodiment of the present invention is a display module including the display device of the above embodiment (5) or (6) and a circuit board. The display device includes a conductive layer. The first resin layer has an opening. The conductive layer is electrically connected to the circuit board through the opening. 
     One embodiment of the present invention is a module including any of the display devices in the above embodiments. The module is provided with a connector such as a flexible printed circuit (hereinafter also referred to as an FPC) or a tape carrier package (TCP) or is mounted with an integrated circuit (IC) by a chip on glass (COG) method, a chip on film (COF) method, or the like. 
     Any of the above embodiments of the present invention may be applied to a light-emitting device or an input/output device (such as a touch panel) instead of the display device. 
     One embodiment of the present invention is an electronic device including the module with any of the above structures and at least one of an antenna, a battery, a housing, a camera, a speaker, a microphone, and an operation button. 
     One embodiment of the present invention can provide a novel peeling method. One embodiment of the present invention can provide a peeling method at low cost with high mass productivity. One embodiment of the present invention can perform peeling using a large-sized substrate. 
     One embodiment of the present invention can provide a novel flexible device and a manufacturing method thereof. One embodiment of the present invention can provide a highly reliable flexible device. One embodiment of the present invention can manufacture a flexible device at low temperatures. One embodiment of the present invention can provide a manufacturing method of a flexible device with a simplified manufacturing process. One embodiment of the present invention can provide a manufacturing method of a flexible device at low cost with high mass productivity. One embodiment of the present invention can manufacture a flexible device using a large-sized substrate. One embodiment of the present invention can provide a device with a curved surface. One embodiment of the present invention can provide a lightweight flexible device. One embodiment of the present invention can provide a thin flexible device. One embodiment of the present invention can provide a flexible device capable of being repeatedly bent. 
     Note that the descriptions of these effects do not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects. Other effects can be derived from the description of the specification, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 2A to 2D  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 3A to 3E  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 4A to 4C  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 5A and 5B  each illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 6A to 6D  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 7A to 7E  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 8A to 8C  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 9A to 9C  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 10A to 10C  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 11A to 11E  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 12A and 12B  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 13A and 13B  illustrate an example of a manufacturing method of a flexible device. 
         FIGS. 14A and 14B  illustrate an example of a flexible device. 
         FIGS. 15A to 15E  each illustrate an example of a flexible device. 
         FIGS. 16A and 16B  each illustrate an example of a flexible device. 
         FIG. 17  illustrates an example of a flexible device. 
         FIG. 18  illustrates an example of a flexible device. 
         FIGS. 19A and 19B  each illustrate an example of a flexible device. 
         FIG. 20  illustrates an example of a display module. 
         FIGS. 21A to 21F  each illustrate an example of an electronic device. 
         FIG. 22  illustrates a process member of Example 1. 
         FIGS. 23A and 23B  are photographs showing results of Example 1. 
         FIGS. 24A to 24C  each illustrate a sample of Example 2. 
         FIGS. 25A to 25C  show TDS analysis results of Example 2. 
         FIGS. 26A and 26B  each show I d -V g  characteristics of a transistor of Example 3. 
         FIGS. 27A and 27B  each show I d -V g  characteristics of a transistor of Example 3. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments will be described in detail with reference to the drawings. Note that 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, 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. Furthermore, the same hatch pattern is used for portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     The position, size, range, or the like of each structure illustrated in drawings is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like disclosed in the drawings. 
     Note that the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film”. The term “insulating film” can be changed into the term “insulating layer”. 
     Note that in this specification, a “substrate” preferably has a function of supporting at least one of a functional circuit, a functional element, a functional film, and the like. A “substrate” does not necessary have a function of supporting a functional circuit, a functional element, a functional film, and the like, and may have a function of protecting a surface of the device, or a function of sealing at least one of a functional circuit, a functional element, a functional film, and the like, for example. 
     Embodiment 1 
     In this embodiment, a peeling method and a manufacturing method of a flexible device of embodiments of the present invention will be described with reference to  FIGS. 1A to 1D ,  FIGS. 2A to 2D ,  FIGS. 3A to 3E ,  FIGS. 4A to 4C ,  FIGS. 5A and 5B ,  FIGS. 6A to 6D ,  FIGS. 7A to 7E ,  FIGS. 8A to 8C ,  FIGS. 9A to 9C ,  FIGS. 10A to 10C ,  FIGS. 11A to 11E ,  FIGS. 12A and 12B , and  FIGS. 13A and 13B . 
     One embodiment of the present invention is a peeling method which includes a step of forming a resin layer having a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm over a formation substrate using a photosensitive and thermosetting material, a step of forming a transistor including an oxide semiconductor in a channel formation region over the resin layer, a step of irradiating the resin layer with laser light using a linear laser device, and a step of separating the transistor and the formation substrate from each other. 
     An oxide semiconductor is used for the channel formation region of the transistor. With the use of an oxide semiconductor, the maximum process temperature can be lower than that of the case of using low-temperature polysilicon (LTPS). 
     In the case of using LTPS for the channel formation region of the transistor, the resin layer is required to have heat resistance because a temperature of approximately 500° C. to 550° C. is applied. The resin layer is required to have a larger thickness to relieve the damage in a laser crystallization step. When the resin layer is irradiated with laser light to peel the transistor from the formation substrate, the resin layer is required to have a larger thickness also to suppress absorption of laser light by silicon because the bandgap of silicon is as narrow as 1.1 eV. 
     In contrast, the transistor formed using an oxide semiconductor does not need heat treatment at high temperatures unlike the case of LTPS, and can be formed at a temperature lower than or equal to 350° C., or even lower than or equal to 300° C. Thus, the resin layer is not required to have high heat resistance. Accordingly, the heat resistant temperature of the resin layer can be low, and the material can be selected from a wide range. Furthermore, the transistor formed using an oxide semiconductor does not need a laser crystallization step. Furthermore, even in the case where a laser is used in a peeling step, the resin layer can be thinned because the bandgap of an oxide semiconductor is broad, which is greater than or equal to 2.0 eV and less than or equal to 3.5 eV (preferably greater than or equal to 2.5 eV and further preferably greater than or equal to 3 eV), and an oxide semiconductor absorbs less laser light than silicon. Since the resin layer is not required to have high heat resistance and can be thinned, the manufacturing cost of a device can be significantly reduced. An oxide semiconductor is preferably used, in which case the steps can be simplified as compared with the case where LTPS is used. 
     According to one embodiment of the present invention, a transistor or the like is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer. Here, the heat resistance of the resin layer can be measured by, for example, a weight loss percentage due to heat, specifically, the 5% weight loss temperature. The 5% weight loss temperature of the resin layer can be lower than or equal to 450° C., lower than or equal to 400° C., lower than 400° C., or lower than 350° C., for example. For example, a transistor is formed at a temperature lower than or equal to 350° C., or even lower than or equal to 300° C. 
     In one embodiment of the present invention, the resin layer is formed using a photosensitive material. With the photosensitive material, a resin layer with a desired shape can be easily formed. For example, a resin layer having an opening or a resin layer having two or more regions with different thicknesses can be easily formed. Accordingly, the resin layer can be prevented from hindering formation of a back gate, an external connection terminal, a through electrode, or the like. 
     According to one embodiment of the present invention, irradiation with laser light is performed with a linear laser device. A laser apparatus used in a manufacturing line for LTPS or the like can be effectively used. The linear laser device condenses laser light in a long rectangular shape (the laser light is shaped into a linear laser beam) so that the resin layer is irradiated with light. 
     A flexible device can be manufactured using a peeling method of one embodiment of the present invention. An example of a manufacturing method of a flexible device is shown with reference to  FIGS. 1A to 1D  and  FIGS. 2A to 2D . 
     First, as illustrated in  FIG. 1A , a first stack  110  and a second stack  120  are attached to each other with a bonding layer  132 . 
     The first stack  110  includes a formation substrate  111 , a resin layer  112 , an insulating layer  113 , a layer  114  including a transistor, and a display element  131 . 
     Here, the display element  131  is preferably positioned within 10 μm, further preferably 5 μm, and still further preferably 2.5 μm, from a neutral plane. 
     A region having low adhesion may be generated in the display element  131  in the case where an EL element is used for the display element  131 , for example. Stress applied to the display element  131  can be reduced by arranging the display element  131  in a position closer to the neutral plane. In addition, in a peeling step in manufacturing a display device or at the use of the display device by being bent, for example, occurrence of film separation can be suppressed. 
     The resin layer  112  is formed to a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm using a photosensitive and thermosetting material. 
     The layer  114  includes a transistor including an oxide semiconductor in a channel formation region. 
     The second stack  120  includes a formation substrate  121 , a resin layer  122 , an insulating layer  123 , and a functional layer  124 . 
     The resin layer  122  is formed to a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm using a photosensitive and thermosetting material. The functional layer  124  includes at least one of a coloring layer such as a color filter, a light-blocking layer such as a black matrix, and a sensor element such as a touch sensor. 
     Next, as illustrated in  FIG. 1B , the resin layer  112  is irradiated with laser light  160  through the formation substrate  111 . A linear laser device is preferably used for the laser light irradiation. A light source is relatively moved with respect to the formation substrate  111  to perform the laser light irradiation. Although an example in which the formation substrate  111  is separated ahead of the formation substrate  121  is shown here, one embodiment of the present invention is not limited thereto. When the formation substrate  121  is separated ahead, the resin layer  122  is irradiated with laser light through the formation substrate  121  with a linear laser device. 
     Then, as illustrated in  FIG. 1C , the formation substrate  111  and the insulating layer  113  are separated from each other.  FIG. 1C  illustrates an example in which separation occurs in the resin layer  112 . Part of the resin layer (a resin layer  112   a ) remains over the formation substrate  111 . The thickness of the resin layer  112  remaining on the insulating layer  113  side is reduced as compared with that in  FIG. 1B . Note that separation occurs at an interface between the formation substrate  111  and the resin layer  112  in some cases depending on the manufacturing conditions (a material of the resin layer  112 , the laser irradiation conditions, and the like). 
     Next, as illustrated in  FIG. 1D , the exposed resin layer  112  and a substrate  141  are attached to each other. The substrate  141  preferably has flexibility. For example, the resin layer  112  and the substrate  141  can be attached to each other with an adhesive. 
     Next, as illustrated in  FIG. 2A , the resin layer  122  is irradiated with the laser light  160  through the formation substrate  121 . A linear laser device is preferably used for the laser light irradiation. A light source is relatively moved with respect to the formation substrate  121  to perform the laser light irradiation. 
     Then, as illustrated in  FIG. 2B , the formation substrate  121  and the insulating layer  123  are separated from each other.  FIG. 2B  illustrates an example in which separation occurs in the resin layer  122 . Part of the resin layer (a resin layer  122   a ) remains over the formation substrate  121 . The thickness of the resin layer  122  remaining on the insulating layer  123  side is reduced as compared with that in  FIG. 2A . 
     Next, as illustrated in  FIG. 2C , the exposed resin layer  122  and a substrate  151  are attached to each other. The substrate  151  preferably has flexibility. 
     Through the above steps, a flexible device  100  illustrated in  FIG. 2D  can be fabricated. 
     In the peeling method and the manufacturing method of a flexible device of embodiments of the present invention, the fabrication process of the transistor can be performed at a low temperature with the use of an oxide semiconductor in the channel formation region of the transistor. Furthermore, the resin layer can have a small thickness and low heat resistance. Thus, there are advantages in that the material of the resin layer can be selected from a wide range, high mass productivity can be obtained at low cost, and peeling and fabrication of a flexible device can be performed using a large-sized substrate, for example. 
     The manufacturing method of a flexible device of one embodiment of the present invention will be more specifically described below with reference to  FIGS. 3A to 3E ,  FIGS. 4A to 4C ,  FIGS. 5A and 5B ,  FIGS. 6A to 6D ,  FIGS. 7A to 7E ,  FIGS. 8A to 8C ,  FIGS. 9A to 9C ,  FIGS. 10A to 10C ,  FIGS. 11A to 11E ,  FIGS. 12A and 12B , and  FIGS. 13A and 13B . Here, an example in which a display device including a transistor and an organic EL element (also referred to as an active matrix organic EL display device) is fabricated as the flexible device will be described. With the use of a flexible material for a substrate, the display device can be a foldable organic EL display device. 
     Note that thin films included in the display device (e.g., insulating films, semiconductor films, or conductive films) can be formed by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, and the like. As the CVD method, a plasma-enhanced chemical vapor deposition (PECVD) method or a thermal CVD method may be used. As the thermal CVD method, for example, a metal organic chemical vapor deposition (MOCVD) method may be used. 
     Alternatively, thin films included in the display device (e.g., insulating films, semiconductor films, or conductive films) can be formed by a method such as spin coating, dipping, spray coating, ink-jetting, dispensing, screen printing, or offset printing, or with a doctor knife, a slit coater, a roll coater, a curtain coater, or a knife coater. 
     When thin films included in the display device are processed, a lithography method or the like can be used for the processing. Alternatively, island-shaped thin films may be formed by a film formation method using a blocking mask. A nanoimprinting method, a sandblasting method, a lift-off method, or the like may be used for the processing of thin films. Examples of the photolithography method include a method in which a resist mask is formed over a thin film to be processed, the thin film is processed by etching or the like, and the resist mask is removed, and a method in which a photosensitive thin film is formed, exposed to light, and developed to be processed into a desired shape. 
     In the case of using light in a lithography method, as light used for exposure, for example, light with an i-line (wavelength: 365 nm), light with a g-line (wavelength: 436 nm), light with an h-line (wavelength: 405 nm), or light in which the i-line, the g-line, and the h-line are mixed can be used. Alternatively, ultraviolet light, KrF laser light, ArF laser light, or the like can be used. Exposure may be performed by liquid immersion exposure technique. As the light for the exposure, extreme ultra-violet light (EUV) or X-rays may be used. Instead of the light for the exposure, an electron beam can be used. It is preferable to use EUV, X-rays, or an electron beam because extremely minute processing can be performed. Note that in the case of performing exposure by scanning of a beam such as an electron beam, a photomask is not needed. 
     For etching of thin films, a dry etching method, a wet etching method, a sandblast method, or the like can be used. 
     Manufacturing Method Example 1 
     First, a resin layer  23  is formed over a formation substrate  14  using a photosensitive and thermosetting material ( FIG. 3A ). 
     Specifically, the resin layer  23  is formed by depositing the photosensitive and thermosetting material to a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm and performing heating. By heating, released gas components (e.g., hydrogen and/or water) in the resin layer  23  can be reduced. It is particularly preferable that the photosensitive and thermosetting material be heated at a temperature higher than or equal to the formation temperature of each layer formed over the resin layer  23 . For example, in the case where the formation temperature of the transistor is below 350° C., a film to be the resin layer  23  is preferably heated at a temperature higher than or equal to 350° C. and lower than or equal to 450° C., further preferably higher than or equal to 350° C. and lower than or equal to 400° C., still further preferably higher than or equal to 350° C. and lower than 400° C., and yet still further preferably higher than or equal to 350° C. and lower than 375° C. Thus, a gas released from the resin layer  23  in the fabrication process of the transistor can be significantly reduced. 
     Since the film to be the resin layer  23  is formed using the photosensitive material in one embodiment of the present invention, part of the film can be removed by a photolithography method. Specifically, after the material is deposited, heat treatment (also referred to as pre-baking) for removing a solvent is performed, and then light exposure is performed using a photomask. Next, development is performed, whereby an unnecessary portion can be removed. After that, heat treatment (also referred to as post-baking) is preferably performed. In the post-baking, heating is preferably performed at a temperature higher than or equal to the formation temperature of each layer formed over the resin layer  23 . 
     The resin layer  23  has flexibility. The formation substrate  14  has lower flexibility than the resin layer  23  does. Since the resin layer  23  is formed over the formation substrate  14 , the resin layer  23  can be transferred easily. 
     The resin layer  23  is preferably formed using a photosensitive polyimide resin (also referred to as a PSPI). 
     Examples of a material which can be used to form the resin layer  23  include an acrylic resin, an epoxy resin, a polyamide resin, a polyimide-amide resin, a siloxane resin, a benzocyclobutene-based resin, and a phenol resin. 
     The resin layer  23  is preferably formed with a spin coater. By a spin coating method, a thin film can be uniformly formed over a large-sized substrate. 
     The resin layer  23  is preferably formed using a solution having a viscosity of greater than or equal to 5 cP and less than 500 cP, further preferably greater than or equal to 5 cP and less than 100 cP, and still further preferably greater than or equal to 10 cP and less than or equal to 50 cP. As the viscosity of the solution is lower, application is performed more easily. As the viscosity of the solution is lower, inclusion of air bubbles can be reduced more; thus, a high-quality film can be formed. 
     The resin layer  23  preferably has a thickness greater than or equal to 0.01 μm and less than 10 μm, further preferably greater than or equal to 0.1 μm and less than or equal to 5 μm, still further preferably greater than or equal to 0.1 μm and less than or equal to 3 μm, and yet still further preferably greater than or equal to 0.5 μm and less than or equal to 1 μm. With a solution having low viscosity, the resin layer  23  having a small thickness can be easily formed. By forming the resin layer  23  thin, the display device can be fabricated at low cost. The display device can be light-weight and thin. The display device can have higher flexibility. The thickness of the resin layer  23  is not limited thereto, and may be greater than or equal to 10 μm. For example, the resin layer  23  may have a thickness greater than or equal to 10 μm and less than or equal to 200 μm. The resin layer  23  having a thickness greater than or equal to 10 μm is favorable because the rigidity of the display device can be increased. 
     The resin layer  23  can be formed by dip coating, spray coating, ink jetting, dispensing, screen printing, or offset printing, with a doctor knife, a slit coater, a roll coater, a curtain coater, or a knife coater. 
     The resin layer  23  preferably has a thermal expansion coefficient of greater than or equal to 0.1 ppm/° C. and less than or equal to 20 ppm/° C. and further preferably greater than or equal to 0.1 ppm/° C. and less than or equal to 10 ppm/° C. As the resin layer  23  has a lower thermal expansion coefficient, breakage of a transistor or the like which is caused owing to the heating can be further suppressed. 
     In the case where the resin layer  23  is positioned on the display surface side of the display device, the resin layer  23  preferably has a high visible-light transmitting property. 
     The formation substrate  14  has stiffness high enough for easy transfer and has resistance to heat applied in the fabrication process. Examples of a material that can be used for the formation substrate  14  include glass, quartz, ceramics, sapphire, a resin, a semiconductor, a metal, and an alloy. Examples of the glass include alkali-free glass, barium borosilicate glass, and aluminoborosilicate glass. 
     Next, an insulating layer  31  is formed over the resin layer  23  ( FIG. 3B ). 
     The insulating layer  31  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . The insulating layer  31  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     The insulating layer  31  can be used as a barrier layer that prevents diffusion of impurities contained in the resin layer  23  into a transistor and a display element formed later. For example, the insulating layer  31  preferably prevents moisture and the like contained in the resin layer  23  from diffusing into the transistor and the display element when the resin layer  23  is heated. Thus, the insulating layer  31  preferably has a high barrier property. 
     As the insulating layer  31 , an inorganic insulating film such as a silicon nitride film, a silicon oxynitride film, a silicon oxide film, a silicon nitride oxide film, an aluminum oxide film, or an aluminum nitride film can be used, for example. A hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, a neodymium oxide film, or the like may be used. A stack including two or more of the above insulating films may also be used. It is particularly preferable that a silicon nitride film be formed over the resin layer  23  and a silicon oxide film be formed over the silicon nitride film. An inorganic insulating film is preferably formed at high temperatures because the film can have higher density and a higher barrier property as the deposition temperature becomes higher. 
     In the case of using an inorganic insulating film for the insulating layer  31 , the substrate temperature during the deposition is preferably higher than or equal to room temperature (25° C.) and lower than or equal to 350° C. and further preferably higher than or equal to 100° C. and lower than or equal to 300° C. 
     In the case where the resin layer  23  has an uneven surface, the insulating layer  31  preferably covers the unevenness. The insulating layer  31  may function as a planarization layer that fills the unevenness. It is preferable to use a stack including an organic insulating material and an inorganic insulating material for the insulating layer  31 , for example. As the organic insulating material, the resin that can be used for the resin layer  23  can be used. 
     In the case of using an organic insulating film for the insulating layer  31 , a temperature applied to the resin layer  23  at the formation of the insulating layer  31  is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     Next, a transistor  40  is formed over the insulating layer  31  ( FIG. 3C ). 
     There is no particular limitation on the structure of the transistor in the display device. For example, a planar transistor, a staggered transistor, or an inverted staggered transistor may be used. A top-gate transistor or a bottom-gate transistor may be used. Gate electrodes may be provided above and below a channel. 
     Here, the case where a bottom-gate transistor including an oxide semiconductor layer  44  is formed as the transistor  40  is described. 
     In one embodiment of the present invention, an oxide semiconductor is used as a semiconductor of a transistor. A semiconductor material having a wider bandgap and a lower carrier density than silicon is preferably used because an off-state current of the transistor can be reduced. 
     The transistor  40  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . In addition, the transistor  40  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     Specifically, first, a conductive layer  41  is formed over the insulating layer  31 . The conductive layer  41  can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. 
     The substrate temperature during the deposition of the conductive film is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     The conductive layers included in the display device can each have a single-layer structure or a stacked-layer structure including 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. Alternatively, a light-transmitting conductive material such as indium oxide, indium tin oxide (ITO), indium oxide containing tungsten, indium zinc oxide containing tungsten, indium oxide containing titanium, ITO containing titanium, indium zinc oxide, zinc oxide (ZnO), ZnO to which gallium is added, or indium tin oxide containing silicon may be used. Alternatively, a semiconductor such as an oxide semiconductor or polycrystalline silicon whose resistance is lowered by containing an impurity element, for example, or silicide such as nickel silicide may be used. A film including graphene may be used as well. The film including graphene can be formed, for example, by reducing a film containing graphene oxide. A semiconductor such as an oxide semiconductor containing an impurity element may be used. Alternatively, the conductive layers may be formed using a conductive paste of silver, carbon, copper, or the like or a conductive polymer such as a polythiophene. A conductive paste is preferable because it is inexpensive. A conductive polymer is preferable because it is easily applied. 
     Next, an insulating layer  32  is formed. For the insulating layer  32 , the description of the inorganic insulating film that can be used for the insulating layer  31  can be referred to. 
     Then, the oxide semiconductor layer  44  is formed. The oxide semiconductor layer  44  can be formed in the following manner: an oxide semiconductor film is formed, a resist mask is formed, the oxide semiconductor film is etched, and the resist mask is removed. 
     The substrate temperature during the deposition of the oxide semiconductor film is preferably lower than or equal to 350° C., further preferably higher than or equal to room temperature and lower than or equal to 200° C., and still further preferably higher than or equal to room temperature and lower than or equal to 130° C. 
     The oxide semiconductor film can be formed using one or both of an inert gas and an oxygen gas. Note that there is no particular limitation on the percentage of oxygen flow rate (partial pressure of oxygen) at the time of forming the oxide semiconductor film. To fabricate a transistor having high field-effect mobility, however, the percentage of oxygen flow rate (partial pressure of oxygen) at the time of forming the oxide semiconductor film is preferably higher than or equal to 0% and lower than or equal to 30%, further preferably higher than or equal to 5% and lower than or equal to 30%, and still further preferably higher than or equal to 7% and lower than or equal to 15%. 
     As an oxide target that can be used for forming the oxide semiconductor film, an In-M-Zn-based oxide (M is Al, Ga, Y, or Sn) can be used. It is particularly preferable to use an In—Ga—Zn-based oxide. 
     The oxide semiconductor film can be formed by a sputtering method. Alternatively, a PLD method, a PECVD method, a thermal CVD method, an ALD method, or a vacuum evaporation method may be used, for example. 
     Next, a conductive layer  43   a  and a conductive layer  43   b  are formed. The conductive layers  43   a  and  43   b  can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. 
     Note that during the processing of the conductive layers  43   a  and  43   b , the oxide semiconductor layer  44  might be partly etched to be thin in a region not covered by the resist mask. 
     The substrate temperature during the deposition of the conductive film is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     In the above manner, the transistor  40  can be formed ( FIG. 3C ). In the transistor  40 , part of the conductive layer  41  functions as a gate, part of the insulating layer  32  functions as a gate insulating layer, and the conductive layers  43   a  and  43   b  function as a source and a drain. 
     Next, an insulating layer  33  that covers the transistor  40  is formed ( FIG. 3D ). The insulating layer  33  can be formed in a manner similar to that of the insulating layer  31 . 
     It is preferable to use an oxide insulating film, such as a silicon oxide film or a silicon oxynitride film, formed at a low temperature in the above range in an atmosphere containing oxygen for the insulating layer  33 . An insulating film with low oxygen diffusibility and oxygen permeability, such as a silicon nitride film, is preferably stacked over the silicon oxide film or the silicon oxynitride film. The oxide insulating film formed at low temperatures in an atmosphere containing oxygen can easily release a large amount of oxygen by heating. When a stack including such an oxide insulating film that releases oxygen and an insulating film with low oxygen diffusibility and oxygen permeability is heated, oxygen can be supplied to the oxide semiconductor layer  44 . As a result, oxygen vacancies in the oxide semiconductor layer  44  can be filled and defects at the interface between the oxide semiconductor layer  44  and the insulating layer  33  can be repaired, leading to a reduction in defect levels. Accordingly, an extremely highly reliable flexible device can be fabricated. 
     Through the above steps, the insulating layer  31 , the transistor  40 , and the insulating layer  33  can be formed over the resin layer  23  ( FIG. 3D ). 
     If the formation substrate  14  and the insulating layer  31  are separated from each other at this stage by a method described later, a flexible device including no display element can be fabricated. Forming the transistor  40  or forming a capacitor, a resistor, a wiring, and the like in addition to the transistor  40 , and separating the formation substrate  14  and the transistor  40  from each other by the method described later can provide a flexible device including a semiconductor circuit, for example. 
     Then, an insulating layer  34  is formed over the insulating layer  33  ( FIG. 3E ). The display element is formed on the insulating layer  34  in a later step; thus, the insulating layer  34  preferably functions as a planarization layer. For the insulating layer  34 , the description of the organic insulating film or the inorganic insulating film that can be used for the insulating layer  31  can be referred to. 
     The insulating layer  34  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . The insulating layer  34  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     In the case of using an organic insulating film for the insulating layer  34 , a temperature applied to the resin layer  23  at the formation of the insulating layer  34  is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     In the case of using an inorganic insulating film for the insulating layer  34 , the substrate temperature during the deposition is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to 100° C. and lower than or equal to 300° C. 
     Next, an opening that reaches the conductive layer  43   b  is formed in the insulating layer  34  and the insulating layer  33 . 
     After that, a conductive layer  61  is formed ( FIG. 4A ). Part of the conductive layer  61  functions as a pixel electrode of a display element  60 . The conductive layer  61  can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. 
     The conductive layer  61  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . The conductive layer  61  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     The substrate temperature during the deposition of the conductive film is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     Subsequently, an insulating layer  35  that covers an end portion of the conductive layer  61  is formed ( FIG. 4A ). For the insulating layer  35 , the description of the organic insulating film or the inorganic insulating film that can be used for the insulating layer  31  can be referred to. 
     The insulating layer  35  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . The insulating layer  35  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     In the case of using an organic insulating film for the insulating layer  35 , a temperature applied to the resin layer  23  at the formation of the insulating layer  35  is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     In the case of using an inorganic insulating film for the insulating layer  35 , the substrate temperature during the deposition is preferably higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to 100° C. and lower than or equal to 300° C. 
     Then, an EL layer  62  and a conductive layer  63  are formed ( FIG. 4B ). Part of the conductive layer  63  functions as a common electrode of the display element  60 . 
     The EL layer  62  can be formed by an evaporation method, a coating method, a printing method, a discharge method, or the like. In the case where the EL layer  62  is formed for each individual pixel, an evaporation method using a blocking mask such as a metal mask, an ink-jet method, or the like can be used. In the case of sharing the EL layer  62  by some pixels, an evaporation method not using a metal mask can be used. 
     Either a low molecular compound or a high molecular compound can be used for the EL layer  62 , and an inorganic compound may also be included. 
     The conductive layer  63  can be formed by an evaporation method, a sputtering method, or the like. 
     The EL layer  62  and the conductive layer  63  are each formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . The EL layer  62  and the conductive layer  63  are each preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . The conductive layer  63  is formed at a temperature lower than or equal to the allowable temperature limit of the EL layer  62 . 
     Specifically, the EL layer  62  and the conductive layer  63  are each preferably formed at a temperature higher than or equal to room temperature and lower than or equal to 350° C. and further preferably higher than or equal to room temperature and lower than or equal to 300° C. 
     In the above manner, the display element  60  can be completed ( FIG. 4B ). In the display element  60 , the conductive layer  61  part of which functions as a pixel electrode, the EL layer  62 , and the conductive layer  63  part of which functions as a common electrode are stacked. 
     Although a top-emission light-emitting element is formed as the display element  60  here, one embodiment of the present invention is not limited thereto. 
     The light-emitting element may be a top-emission, bottom-emission, or dual-emission light-emitting element. A conductive film that transmits visible light is used as the electrode through which light is extracted. A conductive film that reflects visible light is preferably used as the electrode through which light is not extracted. 
     Next, an insulating layer  74  is formed so as to cover the conductive layer  63  ( FIG. 4C ). The insulating layer  74  functions as a protective layer that suppresses diffusion of impurities such as water into the display element  60 . The display element  60  is sealed with the insulating layer  74 . 
     The insulating layer  74  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23  and lower than or equal to the allowable temperature limit of the display element  60 . The insulating layer  74  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     The insulating layer  74  preferably includes an inorganic insulating film with a high barrier property that can be used for the insulating layer  31 . A stack including an inorganic insulating film and an organic insulating film can also be used. 
     Then, a protective layer  75  is formed over the insulating layer  74  ( FIG. 5A ). The protective layer  75  can be used as a layer positioned on the outermost surface of a display device  10 . The protective layer  75  preferably has a high visible-light transmitting property. 
     The above-described organic insulating film that can be used for the insulating layer  31  is preferably used for the protective layer  75  because the surface of the display device can be prevented from being damaged or cracked. In the protective layer  75 , the organic insulating film and a hard coat layer (e.g., a silicon nitride layer) for protecting a surface from damage or the like, a layer formed of a material that can disperse pressure (e.g., an aramid resin layer), or the like may be stacked. 
       FIG. 5B  illustrates an example in which a substrate  75   a  is attached to the insulating layer  74  with a bonding layer  75   b . Examples of the substrate  75   a  include a resin and the like. The substrate  75   a  preferably has flexibility. 
     As the bonding layer  75   b , any of a variety of curable adhesives such as a reactive curable adhesive, a thermosetting adhesive, an anaerobic adhesive, and a photocurable adhesive such as an ultraviolet curable adhesive can be used. Alternatively, an adhesive sheet or the like may be used. 
     For the substrate  75   a , a polyester resin such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), a polyacrylonitrile resin, an acrylic resin, a polyimide resin, a polymethyl methacrylate resin, a polycarbonate (PC) resin, a polyethersulfone (PES) resin, a polyamide resin (e.g., nylon or aramid), a polysiloxane resin, a cycloolefin resin, a polystyrene resin, a polyamide-imide resin, a polyurethane resin, a polyvinyl chloride resin, a polyvinylidene chloride resin, a polypropylene resin, a polytetrafluoroethylene (PTFE) resin, an ABS resin, or cellulose nanofiber can be used, for example. 
     Next, the resin layer  23  is irradiated with laser light  65  through the formation substrate  14  ( FIG. 6A ). 
     For example, an excimer laser with a wavelength of 308 nm, a solid-state UV laser with a wavelength of 343 nm or 355 nm, or the like can be used. A linear laser device is preferably used for the laser light irradiation. A light source is relatively moved with respect to the formation substrate  14  to perform the laser light irradiation. 
     A solid-state laser is preferable because the solid-state laser does not use a gas and thus the running cost can be reduced to approximately ⅓ of the case of an excimer laser. 
     Then, the formation substrate  14  and the insulating layer  31  are separated from each other ( FIG. 6B ).  FIG. 6B  illustrates an example in which separation occurs in the resin layer  23 . Part of the resin layer (a resin layer  23   a ) remains over the formation substrate  14 . The thickness of the resin layer  23  remaining on the insulating layer  31  side is reduced as compared with that in  FIG. 6A . 
     The thickness of the resin layer  23   a  remaining on the formation substrate  14  side can be less than or equal to 100 nm, specifically approximately greater than or equal to 40 nm and less than or equal to 70 nm. The formation substrate  111  can be reused by removing the resin layer  23   a . For example, in the case where glass is used for the formation substrate  14  and a polyimide resin is used for the resin layer  23 , the resin layer  23   a  can be removed with fuming nitric acid. Alternatively, the resin layer  23  may be formed again over the resin layer  23   a  remaining on the formation substrate  14  using a photosensitive and thermosetting material. 
     At least part of the resin layer  23  can be peeled from the formation substrate  14  by applying a perpendicular tensile force to the resin layer  23 , for example. 
     Specifically, at least part of the resin layer  23  can be peeled from the formation substrate  14  by attaching a mechanism to part of the top surface of the protective layer  75  and pulling up the protective layer  75 . 
     The separation trigger is preferably formed by inserting a sharp instrument such as a knife between the formation substrate  14  and the insulating layer  31 . 
     The separation of the formation substrate  14  and the insulating layer  31  can complete the display device  10  ( FIG. 6C ). The display device  10  can remain being bent or can be bent repeatedly, for example. 
     As illustrated in  FIG. 6D , a substrate  29  may be attached to the surface exposed by the separation, with a bonding layer  28 . The substrate  29  can function as a supporting substrate of the flexible device.  FIG. 6D  illustrates an example in which the substrate  29  is attached to the resin layer  23  with the bonding layer  28 . 
     The material that can be used for the substrate  75   a  can be used for the substrate  29 . 
     Through the above steps, the display device using an oxide semiconductor for the transistor and a separate coloring method for an EL element can be fabricated. 
     Manufacturing Method Example 2 
     First, components from the resin layer  23  to the insulating layer  35  are formed in order over the formation substrate  14  in a manner similar to that in the manufacturing method example 1 ( FIG. 7A ). 
     Then, a protective layer  71  is formed as illustrated in  FIG. 7B . 
     The protective layer  71  has a function of protecting surfaces of the insulating layer  35  and the conductive layer  61  in a peeling step. The protective layer  71  can be formed using a material that can be easily removed. 
     For the protective layer  71  that can be removed, a water-soluble resin can be used, for example. A water-soluble resin is applied to an uneven surface to cover the unevenness, which facilitates the protection of the surface. A stack of a water-soluble resin and an adhesive that can be peeled by light or heat may be used for the protective layer  71  that can be removed. 
     Alternatively, for the protective layer  71  that can be removed, a base material having a property in which adhesion is strong in a normal state but weakened when irradiated with light or heated may be used. For example, a thermal peeling tape whose adhesion is weakened by heat, a UV-peeling tape whose adhesion is weakened by ultraviolet irradiation, or the like may be used. Alternatively, a weak adhesion tape with weak adhesion in a normal state or the like can be used. 
     Next, the formation substrate  14  and the insulating layer  31  are separated from each other in a manner similar to that in the manufacturing method example 1 ( FIG. 7C ).  FIG. 7C  illustrates an example in which separation occurs in the resin layer  23 . Part of the resin layer (resin layer  23   a ) remains over the formation substrate  14 . The thickness of the resin layer  23  remaining on the insulating layer  31  side is reduced as compared with that in  FIG. 7B . 
     After the formation substrate  14  and the insulating layer  31  are separated from each other, the protective layer  71  is removed ( FIG. 7C ). 
     Next, the EL layer  62  and the conductive layer  63  are formed, whereby the display element  60  is completed ( FIG. 7D ). 
     The EL layer  62  and the conductive layer  63  may be formed while the resin layer  23  (or the insulating layer  31 ) is fixed to a stage of a deposition apparatus, but are preferably formed while the resin layer  23  is fixed to a supporting substrate  73  by a tape  72  or the like and the supporting substrate  73  is placed on the stage, as illustrated in  FIG. 7D . Fixing the resin layer  23  to the supporting substrate  73  can facilitate the transfer of the resin layer  23  in an apparatus and between apparatuses. The substrate that can be used as the formation substrate  14  can be used as the supporting substrate  73 . 
     Then, a substrate  22  is attached to the display element  60  with a bonding layer  13 . Accordingly, the display element  60  can be sealed by the bonding layer  13  and the substrate  22  ( FIG. 7E ). 
     The material that can be used for the bonding layer  75   b  can be used for the bonding layer  13 . 
     The material that can be used for the substrate  75   a  can be used for the substrate  22 . 
     Note that in a manner similar to that in the manufacturing method example 1, the insulating layer  74  may be formed over the display element  60  and the display element  60  may be sealed by the insulating layer  74 . Then, the protective layer  75  may be formed over the insulating layer  74 . 
     Through the above steps, the display device  10  can be completed ( FIG. 7E ). 
     In the manufacturing method example 2, after a layer is peeled from the formation substrate  14 , the EL layer  62  and the conductive layer  63  can be formed over the layer. In the case where a region having low adhesion is generated in a stacked-layer structure of the EL layer  62  and the like, the stacked-layer structure is formed after peeling so that a decrease in the yield of peeling can be suppressed. With the use of the manufacturing method example 2, a material can be selected more freely, leading to fabrication of a highly reliable display device at lower cost. 
     Manufacturing Method Example 3 
     First, components from the resin layer  23  to the display element  60  are formed in order over the formation substrate  14  in a manner similar to that in the manufacturing method example 1 ( FIG. 8A ). 
     A resin layer  93  is formed over a formation substrate  91  using a photosensitive and thermosetting material ( FIG. 8B ). 
     The resin layer  93  has flexibility. The formation substrate  91  has lower flexibility than the resin layer  93  does. Since the resin layer  93  is formed over the formation substrate  91 , the resin layer  93  can be transferred easily. 
     For the resin layer  93 , a polyimide resin is preferably used. For the material and formation method of the resin layer  93 , the description of the resin layer  23  can be referred to. 
     The resin layer  93  preferably has a thickness greater than or equal to 0.01 μm and less than 10 μm, further preferably greater than or equal to 0.1 μm and less than or equal to 5 μm, still further preferably greater than or equal to 0.1 μm and less than or equal to 3 μm, and yet still further preferably greater than or equal to 0.5 μm and less than or equal to 1 μm. With a solution having low viscosity, the resin layer  93  having a small thickness can be easily formed. By forming the resin layer  93  thin, the display device can be fabricated at low cost. The display device can be light-weight and thin. The display device can have higher flexibility. The thickness of the resin layer  93  is not limited thereto, and may be greater than or equal to 10 μm. For example, the resin layer  93  may have a thickness greater than or equal to 10 μm and less than or equal to 200 μm. The resin layer  93  having a thickness greater than or equal to 10 μm is favorable because the rigidity of the display device can be increased. 
     In the case where the resin layer  93  is positioned on the display surface side of the display device, the resin layer  93  preferably has a high visible-light transmitting property. 
     For the formation substrate  91 , the description of the formation substrate  14  can be referred to. 
     Next, an insulating layer  95  is formed over the resin layer  93 . Then, a coloring layer  97  and a light-blocking layer  98  are formed over the insulating layer  95  ( FIG. 8B ). 
     For the insulating layer  95 , the description of the insulating layer  31  can be referred to. 
     A color filter or the like can be used as the coloring layer  97 . The coloring layer  97  is provided to overlap with the display region of the display element  60 . 
     A black matrix or the like can be used as the light-blocking layer  98 . The light-blocking layer  98  is provided to overlap with the insulating layer  35 . 
     Next, a surface of the formation substrate  14  on which the resin layer  23  and the like are formed and a surface of the formation substrate  91  on which the resin layer  93  and the like are formed are attached to each other with a bonding layer  99  ( FIG. 8C ). 
     Next, the resin layer  23  is irradiated with the laser light  65  through the formation substrate  14  ( FIG. 9A ). Here, an example in which the formation substrate  14  is separated ahead of the formation substrate  91  is shown. 
     Then, the formation substrate  14  and the insulating layer  31  are separated from each other ( FIG. 9B ).  FIG. 9B  illustrates an example in which separation occurs in the resin layer  23 . Part of the resin layer (resin layer  23   a ) remains over the formation substrate  14 . The thickness of the resin layer  23  remaining on the insulating layer  31  side is reduced as compared with that in  FIG. 9A . Then, the exposed resin layer  23  and the substrate  29  are attached to each other with the bonding layer  28  ( FIG. 9C ). 
     Next, the resin layer  93  is irradiated with the laser light  65  through the formation substrate  91  ( FIG. 10A ). 
     Then, the formation substrate  91  and the insulating layer  95  are separated from each other, and the exposed resin layer  93  and the substrate  22  are attached to each other with the bonding layer  13  ( FIG. 10B ).  FIG. 10B  illustrates an example in which separation occurs in the resin layer  93 . Part of the resin layer (a resin layer  93   a ) remains over the formation substrate  91 . The thickness of the resin layer  93  remaining on the insulating layer  95  side is reduced as compared with that in  FIG. 10A . 
     In  FIG. 10B , light emitted from the display element  60  is extracted to the outside of the display device through the coloring layer  97  and the resin layer  93 . Thus, the resin layer  93  preferably has high visible-light transmittance. In the peeling method of one embodiment of the present invention, the thickness of the resin layer  93  can be reduced. Therefore, the visible-light transmittance of the resin layer  93  can be increased. 
     As illustrated in  FIG. 10C , the resin layer  93  may be removed, and the substrate  22  may be attached to the insulating layer  95  with the bonding layer  13 . 
     Through the above steps, the display device using an oxide semiconductor for the transistor and a color filter method can be fabricated. 
     The manufacturing method example 3 is an example in which the peeling method of one embodiment of the present invention is performed twice to fabricate a flexible device. In one embodiment of the present invention, each of the functional elements and the like included in the flexible device is formed over the formation substrate; thus, even in the case where a high-resolution display device is manufactured, high alignment accuracy of the flexible substrate is not required. It is thus easy to attach the flexible substrate. 
     As described in this embodiment, in the peeling method of one embodiment of the present invention, the fabrication process of the transistor can be performed at a low temperature. Furthermore, the resin layer can have a small thickness and low heat resistance. Thus, there are advantages in that the material of the resin layer can be selected from a wide range, high mass productivity can be obtained at low cost, and peeling and fabrication of a flexible device can be performed using a large-sized substrate, for example. Warpage of the flexible device due to the thickness of the resin layer can be suppressed in some cases. 
     Manufacturing Method Example 4 
     In the peeling method of one embodiment of the present invention, the resin layer is formed using the photosensitive material; thus, the resin layer with a desired shape can be easily formed. 
     For example, by forming an opening in the resin layer and disposing a conductive layer to cover the opening, an electrode part of which is exposed (also referred to as a rear electrode or a through electrode) can be formed after a peeling step to be described later. The electrode can be used as an external connection terminal. 
     In the manufacturing method example 4, the external connection terminal is electrically connected to a circuit board such as a flexible printed circuit (FPC) through the opening formed in the resin layer. 
     First, a film  21  to be the resin layer  23  is formed over the formation substrate  14  using a photosensitive and thermosetting material ( FIG. 11A ). 
     Specifically, the photosensitive and thermosetting material is deposited to a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm. 
     Next, a solvent is removed by heat treatment and then light exposure is performed using a photomask. After that, development is performed to process the film  21  into a desired shape. 
     For example,  FIG. 11B  illustrates an example of forming an opening in the resin layer  23 . Note that  FIG. 12B  illustrates an example in which a first region and a second region which is thinner than the first region are provided in the resin layer  23 . 
     Next, the film  21  processed into a desired shape is heated, whereby the resin layer  23  is formed ( FIG. 11B ). It is particularly preferable that the film  21  be heated at a temperature higher than or equal to the formation temperature of each layer formed over the resin layer  23 . For example, in the case where the formation temperature of the transistor is below 350° C., the film  21  to be the resin layer  23  is preferably heated at a temperature higher than or equal to 350° C. and lower than or equal to 450° C., further preferably higher than or equal to 350° C. and lower than or equal to 400° C., still further preferably higher than or equal to 350° C. and lower than 400° C., and yet still further preferably higher than or equal to 350° C. and lower than 375° C. Thus, a gas released from the resin layer  23  in the fabrication process of the transistor can be significantly reduced. 
     Next, the insulating layer  31  is formed over the resin layer  23 . Then, a transistor  80  is formed over the insulating layer  31 . 
     Here, the case where a transistor including the oxide semiconductor layer  44  and two gates is formed as the transistor  80  is described. 
     The transistor  80  is formed at a temperature lower than or equal to the allowable temperature limit of the resin layer  23 . The transistor  80  is preferably formed at a temperature lower than or equal to the heating temperature in the above-described heating step of the resin layer  23 . 
     Specifically, first, a conductive layer  81  is formed over the insulating layer  31 . The conductive layer  81  can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. 
     Next, an insulating layer  82  is formed. For the insulating layer  82 , the description of the inorganic insulating film that can be used for the insulating layer  31  can be referred to. 
     Then, an oxide semiconductor layer  83  is formed. The oxide semiconductor layer  83  can be formed in the following manner: an oxide semiconductor film is formed, a resist mask is formed, the oxide semiconductor film is etched, and the resist mask is removed. For the oxide semiconductor layer  83 , the description of the material that can be used for the oxide semiconductor layer  44  can be referred to. 
     Then, an insulating layer  84  and a conductive layer  85  are formed. For the insulating layer  84 , the description of the inorganic insulating film that can be used for the insulating layer  31  can be referred to. The insulating layer  84  and the conductive layer  85  can be formed in the following manner: an insulating film to be the insulating layer  84  and a conductive film to be the conductive layer  85  are formed, a resist mask is formed, the insulating film and the conductive film are etched, and the resist mask is removed. 
     Next, the insulating layer  33  that covers the oxide semiconductor layer  83 , the insulating layer  84 , and the conductive layer  85  is formed ( FIG. 11C ). The insulating layer  33  can be formed in a manner similar to that of the insulating layer  31 . 
     Here, the insulating layer  31 , the insulating layer  82 , and the insulating layer  33  each include an opening in a region overlapping with the opening of the resin layer  23  ( FIG. 11D ). The opening in a plurality of layers may be formed at a time or may be formed in each layer. 
     Next, a conductive layer  86   a , a conductive layer  86   b , and a conductive layer  86   c  are formed ( FIG. 11E ). The conductive layers  86   a ,  86   b , and  86   c  can be formed in the following manner: a conductive film is formed, a resist mask is formed, the conductive film is etched, and the resist mask is removed. 
     In the above manner, the transistor  80  can be formed. In the transistor  80 , part of the conductive layer  81  functions as a gate, part of the insulating layer  84  functions as a gate insulating layer, part of the insulating layer  82  functions as a gate insulating layer, and part of the conductive layer  85  functions as a gate. The oxide semiconductor layer  83  includes a channel region and a low-resistance region. The channel region overlaps with the conductive layer  85  with the insulating layer  84  provided therebetween. The low-resistance region includes a region connected to the conductive layer  86   a  and a region connected to the conductive layer  86   b.    
     Next, components from the insulating layer  34  to the display element  60  are formed over the insulating layer  33 . For these steps, the manufacturing method example 1 can be referred to. 
     In a manner similar to that in the manufacturing method example 3, the resin layer  93 , the insulating layer  95 , the coloring layer  97 , and the light-blocking layer  98  are formed over the formation substrate  91 . 
     Next, a surface of the formation substrate  14  on which the resin layer  23  and the like are formed and a surface of the formation substrate  91  on which the resin layer  93  and the like are formed are attached to each other with the bonding layer  99 .  FIGS. 12A and 12B  are schematic cross-sectional views illustrating this state. 
       FIGS. 12A and 12B  are different from each other only in the structure of the resin layer  23 .  FIG. 12A  is an example in which the resin layer  23  has an opening and the conductive layer  86   c  is in contact with the formation substrate  14 .  FIG. 12B  is an example in which the resin layer  23  includes the first region and the second region which is thinner than the first region and the conductive layer  86   c  overlaps with the second region. 
     The following steps are described using the structure illustrated in  FIG. 12B  as an example. 
     Next, the resin layer  23  is irradiated with laser light through the formation substrate  14 . Here, an example in which the formation substrate  14  is separated ahead of the formation substrate  91  is shown. 
     Then, the formation substrate  14  and the insulating layer  31  are separated from each other ( FIG. 13A ).  FIG. 13A  illustrates an example in which separation occurs in the resin layer  23 . Part of the resin layer (resin layer  23   a ) remains over the formation substrate  14 . The thickness of the resin layer  23  remaining on the insulating layer  31  side is reduced as compared with that in  FIG. 12B . The second region (a region thinner than other regions) of the resin layer  23  is entirely positioned on the formation substrate  14  side. Therefore, the conductive layer  86   c  is exposed. 
     In the case where the resin layer  23  remains over the conductive layer  86   c , the resin layer  23  is preferably removed by ashing or the like. Alternatively, in the case where the conductive layer  86   c  can be electrically connected to an FPC without removing the resin layer  23 , for example, the resin layer  23  is not necessarily removed. 
     Then, the exposed resin layer  23  and the substrate  29  are attached to each other with the bonding layer  28 . Note that the substrate  29  and the bonding layer  28  are provided in a position that does not overlap with the conductive layer  86   c.    
     Next, the resin layer  93  is irradiated with laser light through the formation substrate  91 . Then, the formation substrate  91  and the insulating layer  95  are separated from each other. Then, the exposed resin layer  93  and the substrate  22  are attached to each other with the bonding layer  13 . 
     Then, the conductive layer  86   c  and an FPC  77  are electrically connected to each other through a connector  76  ( FIG. 13B ). 
     As the connector  76 , any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), and the like can be used. 
     In this embodiment, an example using a top-emission light-emitting element is shown. In the case where the conductive layer  86   c  is exposed from the substrate  22  side and electrically connected to the FPC  77 , the display region cannot overlap with the FPC  77  because the substrate  22  is on the display surface side, and thus a region of the display device that overlaps with the FPC  77  is limited. In contrast, in one embodiment of the present invention, the conductive layer  86   c  can be exposed from the side opposite to the display surface with the use of the photosensitive material for the resin layer  23 . Therefore, the FPC  77  can be provided to overlap with the display region and the space of an electronic device can be saved. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 2 
     In this embodiment, a flexible device of one embodiment of the present invention will be described with reference to  FIGS. 14A and 14B ,  FIGS. 15A to 15E ,  FIGS. 16A and 16B ,  FIG. 17 ,  FIG. 18 , and  FIGS. 19A and 19B . 
     In this embodiment, an example in which an active matrix organic EL display device is fabricated as the flexible device will be described. With the use of a flexible material for a substrate, the display device can be a foldable organic EL display device. Moreover, one embodiment of the present invention is not limited to the light-emitting device or the display device including an organic EL element and can be applied to a variety of devices such as a light-emitting device and a display device including another light-emitting element or a display element, a semiconductor device, and an input/output device. 
     One embodiment of the present invention is a display device including a resin layer, a transistor over the resin layer, and a display element electrically connected to the transistor. 
     An oxide semiconductor is used for a channel formation region of a transistor in one embodiment of the present invention. As described in Embodiment 1, with the use of an oxide semiconductor, the maximum manufacturing process temperature of the display device can be lowered, the manufacturing cost of the display device can be reduced, and the manufacturing process of the display device can be simplified, as compared with the case of using LTPS. 
     In one embodiment of the present invention, the resin layer has a thickness greater than or equal to 0.1 μm and less than or equal to 3 μm. By forming the resin layer thin, the display device can be fabricated at low cost. The display device can be light-weight and thin. The display device can have higher flexibility. 
     The heat resistance of the resin layer can be measured by, for example, a weight loss percentage due to heat, specifically, the 5% weight loss temperature. In one embodiment of the present invention, the 5% weight loss temperature of the resin layer can be lower than or equal to 450° C., lower than or equal to 400° C., lower than 400° C., or lower than 350° C., for example. 
     In one embodiment of the present invention, a resin layer is formed using a photosensitive material. With the photosensitive material, a resin layer with a desired shape can be easily formed. For example, a resin layer having an opening or a resin layer having two or more regions with different thicknesses can be easily formed. Accordingly, the resin layer can be prevented from hindering formation of a back gate, an external connection terminal, a through electrode, or the like. 
     Structures of a display device of one embodiment of the present invention will be specifically described below. Note that Embodiment 1 can also be referred to for a material that can be used for the display device of this embodiment and a manufacturing method of the display device. 
     Structure Example 1 
       FIG. 14A  is a top view of a display device.  FIG. 14B  is a cross-sectional view of a display portion  381  of the display device and a cross-sectional view of a connection portion for connection to the FPC  77 . 
     The display device in  FIGS. 14A and 14B  includes a pair of substrates (the substrate  22  and the substrate  29 ). The substrate  22  side corresponds to a display surface side. The display device includes the display portion  381  and a driver circuit portion  382 . The FPC  77  is attached to the display device. 
     The display device in  FIG. 14B  is a top-emission display device using a color filter method. 
     The display device in  FIG. 14B  includes the substrate  29 , the bonding layer  28 , the resin layer  23 , the insulating layer  31 , the transistor  80 , the conductive layer  86   c , the insulating layer  33 , the insulating layer  34 , the insulating layer  35 , the display element  60 , the bonding layer  99 , the coloring layer  97 , the light-blocking layer  98 , the substrate  22 , the bonding layer  13 , the resin layer  93 , and the insulating layer  95 . 
     The display device in  FIG. 14B  can be manufactured by referring to the manufacturing method example 3 and the manufacturing method example 4 in Embodiment 1. 
     The display portion  381  includes the transistor  80 . 
     The transistor  80  includes the conductive layer  81 , the insulating layer  82 , the oxide semiconductor layer  83 , the insulating layer  84 , the conductive layer  85 , and the conductive layers  86   a  and  86   b . The conductive layer  81  and the conductive layer  85  each function as a gate. The insulating layer  82  and the insulating layer  84  each function as a gate insulating layer. The conductive layer  81  overlaps with the oxide semiconductor layer  83  with the insulating layer  82  provided therebetween. The conductive layer  85  overlaps with the oxide semiconductor layer  83  with the insulating layer  84  provided therebetween. One of the conductive layers  86   a  and  86   b  is electrically connected to a source region of the oxide semiconductor layer  83  and the other is electrically connected to a drain region of the oxide semiconductor layer  83 . 
     Here, as described above, since an oxide semiconductor is used for a channel formation region of a transistor in one embodiment of the present invention, the resin layer  23  is not required to have high heat resistance and a large thickness. Therefore, the resin layer  23  can be thin. Accordingly, the display device can be fabricated at low cost. The display device can be light-weight and thin. The display device can have higher flexibility. The same applies to the resin layer  93 . 
     The resin layer  23  and the resin layer  93  each preferably have a thickness greater than or equal to 0.01 μm and less than 10 μm, further preferably greater than or equal to 0.1 μm and less than or equal to 3 μm, and still further preferably greater than or equal to 0.5 μm and less than or equal to 1 μm. 
     It is preferable that at least one of the insulating layer  33  and the insulating layer  34  be formed using a material through which impurities such as water and hydrogen are hardly diffused. Diffusion of impurities from the outside into the transistor can be effectively inhibited, leading to improved reliability of the display device. The insulating layer  34  functions as a planarization layer. 
     In this embodiment, an example in which a light emitting element is used as the display element  60  is described. The display element  60  includes the conductive layer  61 , the EL layer  62 , and the conductive layer  63 . The display element  60  emits light to the coloring layer  97  side. 
     The transistor, the capacitor, the wiring, and the like are positioned so as to overlap with a light-emitting region of the light-emitting element; accordingly, the aperture ratio of the display portion  381  can be increased. 
     One of the conductive layer  61  and the conductive layer  63  functions as an anode and the other functions as a cathode. When a voltage higher than the threshold voltage of the light-emitting element is applied between the conductive layer  61  and the conductive layer  63 , holes are injected to the EL layer  62  from the anode side and electrons are injected to the EL layer  62  from the cathode side. The injected electrons and holes are recombined in the EL layer  62  and a light-emitting substance contained in the EL layer  62  emits light. 
     The conductive layer  61  is electrically connected to a source or a drain of the transistor  80  directly or through a conductive layer. The conductive layer  61  functioning as a pixel electrode is provided for each light-emitting element. Two adjacent conductive layers  61  are electrically insulated from each other by the insulating layer  35 . 
     The EL layer  62  contains a light-emitting substance. As the light-emitting element  304 , an organic EL element including an organic compound as a light-emitting material can be favorably used. 
     The EL layer  62  includes at least one light-emitting layer. In addition to the light-emitting layer, the EL layer  62  can further include a layer containing any of a substance with a high hole-injection property, a substance with a high hole-transport property, a hole-blocking material, a substance with a high electron-transport property, a substance with a high electron-injection property, a substance with a bipolar property (a substance with a high electron- and hole-transport property), and the like. 
     Either a low molecular compound or a high molecular compound can be used for the EL layer  62 , and an inorganic compound may also be included. 
     The conductive layer  63  functioning as a common electrode is shared by a plurality of light-emitting elements. A fixed potential is supplied to the conductive layer  63 . 
     Here, the display element  60  is preferably positioned within 10 μm, further preferably 5 μm, and still further preferably 2.5 μm, from a neutral plane. 
     A region having low adhesion may be generated in the display element  60  in the case where an EL element is used for the display element  60 , for example. Stress applied to the display element  60  can be reduced by arranging the display element  60  in a position closer to the neutral plane. In addition, in a peeling step in manufacturing a display device or at the use of the display device by being bent, for example, occurrence of film separation can be suppressed. 
     The display element  60  overlaps with the coloring layer  97  with the bonding layer  99  provided therebetween. The insulating layer  35  overlaps with the light-blocking layer  98  with the bonding layer  99  provided therebetween. 
     The coloring layer  97  is a coloring layer that transmits light in a specific wavelength range. For example, a color filter for transmitting light in a red, green, blue, or yellow wavelength range can be used. Examples of materials that can be used for the coloring layer  97  include a metal material, a resin material, and a resin material containing a pigment or dye. 
     Note that one embodiment of the present invention is not limited to a color filter method, and a separate coloring method, a color conversion method, a quantum dot method, and the like may be employed. 
     The light-blocking layer  98  is provided between the adjacent coloring layers  97 . The light-blocking layer  98  blocks light emitted from an adjacent light-emitting element to inhibit color mixture between adjacent light-emitting elements. Here, the coloring layer  97  is provided such that its end portion overlaps with the light-blocking layer  98 , whereby light leakage can be reduced. For the light-blocking layer  98 , a material that blocks light emitted from the light-emitting element can be used. For example, a black matrix can be formed using a metal material or a resin material containing pigment or dye. Note that it is preferable to provide the light-blocking layer  98  in a region other than the display portion  381 , such as the driver circuit portion  382 , in which case undesired leakage of guided light or the like can be inhibited. 
     The resin layer  23  and the substrate  29  are attached to each other with the bonding layer  28 . In addition, the resin layer  93  and the substrate  22  are attached to each other with the bonding layer  13 . 
     The insulating layer  95  and the insulating layer  31  are preferably highly resistant to moisture. The display element  60 , the transistor  80 , and the like are preferably provided between a pair of insulating layers which are highly resistant to moisture, in which case impurities such as water can be prevented from entering these elements, leading an increase in the reliability of the display device. 
     Examples of the insulating film highly resistant to moisture include a film containing nitrogen and silicon (e.g., a silicon nitride film and a silicon nitride oxide film) and a film containing nitrogen and aluminum (e.g., an aluminum nitride film). Alternatively, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or the like may be used. 
     For example, the moisture vapor transmission rate of the insulating film highly resistant to moisture is lower than or equal to 1×10 −5  [g/(m 2 ·day)], preferably lower than or equal to 1×10 −6  [g/(m 2 ·day)], further preferably lower than or equal to 1×10 −7  [g/(m 2 ·day)], and still further preferably lower than or equal to 1×10 −8  [g/(m 2 ·day)]. 
     The conductive layer  86   c  can be formed using the same material and the same fabrication step as the conductive layer included in the transistor. For example, the conductive layer  86   c  can be formed using the same material and the same fabrication step as the conductive layers  86   a  and  86   b . The conductive layer  86   c  is electrically connected to an external input terminal through which a signal and a potential from the outside is transmitted to the driver circuit portion  382 . Here, an example in which the FPC  77  is provided as the external input terminal is described. The FPC  77  is electrically connected to the conductive layer  86   c  through the connector  76 . 
     As described above, the resin layer  23  can be formed using a photosensitive material. Therefore, the conductive layer  86   c  and the FPC  77  can be electrically connected to each other through the opening provided in the resin layer  23 . Such a structure allows the FPC  77  to be positioned on the side opposite to the display surface. Thus, a space for bending the FPC  77  in incorporating the display device in an electronic device can be saved, which enables the electronic device to be smaller. 
     Note that the structure of the display device of one embodiment of the present invention is not limited to the structure in  FIG. 14A .  FIGS. 15A to 15E  are each a top view of a display device including the pair of substrates (the substrate  22  and the substrate  29 ). Each display device includes one display portion  381  and one or more driver circuit portions  382 . The FPC  77  is connected to the display device. The FPC  77  is electrically connected to an external connection electrode (not illustrated) over the substrate  29 . 
     In the display device illustrated in  FIG. 15A , the driver circuit portion  382  is provided on one side. The display device in  FIG. 15A  is different from the display device in  FIG. 14A  in that the FPC  77  is attached to the display surface side. 
     In the display device illustrated in  FIG. 15B , the driver circuit portion  382  is provided on one side. In the display device in  FIG. 14A , the driver circuit portion  382  is provided along a short side of the display portion  381 , whereas in the display device in  FIG. 15B , the driver circuit portion  382  is provided along a long side of the display portion  381 . 
     In each of the display devices illustrated in  FIGS. 15C and 15D , the driver circuit portion  382  is provided on two sides. In  FIG. 15C , the driver circuit portions  382  are provided along the opposite sides. In  FIG. 15D , the driver circuit portions  382  are provided along a short side and a long side of the display device. 
       FIG. 15E  is an example of the display device in which the top surface shape of the display portion  381  is circular. The display portion  381  does not necessarily have a polygonal top surface shape and may have any of a variety of top surface shapes such as circular and elliptical shapes. 
     The display device does not necessarily have a polygonal top surface shape and may have any of a variety of top surface shapes such as circular and elliptical shapes. The display device in  FIG. 15E  has a top surface shape including both a curved portion and a linear portion. 
     Structure Example 2 
       FIGS. 16A and 16B ,  FIG. 17 , and  FIG. 18  are cross-sectional views each illustrating a display portion  381  of a display device, which is different from the display portion  381  in  FIG. 14B . In the structure example below, components similar to those in the above structure example 1 will not be described in some cases. The cross-sectional view in  FIG. 17  also illustrates a connection portion for connection to the FPC  77 . 
     The display devices in  FIGS. 16A and 16B ,  FIG. 17 , and  FIG. 18  are each a top-emission display device using a color filter method. 
     The display devices in  FIGS. 16A and 16B  each include the substrate  29 , the bonding layer  28 , the resin layer  23 , an insulating layer  24 , a resin layer  25 , the insulating layer  31 , the transistor  80 , the insulating layer  33 , the insulating layer  34 , the insulating layer  35 , the display element  60 , the bonding layer  99 , the coloring layer  97 , the light-blocking layer  98 , the substrate  22 , the bonding layer  13 , the resin layer  93 , and the insulating layer  95 . 
     The display devices in  FIGS. 16A and 16B  are each a display device in which the insulating layer  24  and the resin layer  25  are added to the components of  FIG. 14B . 
     In  FIG. 16B , the conductive layer  81  functioning as a gate of the transistor  80  is positioned between the insulating layer  24  and the resin layer  25 . 
     The resin layer  25  is formed using a photosensitive material. With the photosensitive material, the resin layer  25  with a desired shape can be easily formed. Therefore, the conductive layer  81  can be electrically connected to another conductive layer through an opening, which is formed in the resin layer  25 . 
     Since an oxide semiconductor is used for a channel formation region of the transistor, the resin layer  25  is not required to have high heat resistance and a large thickness. Therefore, the resin layer  25  can be thin. Accordingly, an electric field of the conductive layer  81  can be effectively applied to the oxide semiconductor layer  83 . An electric field for inducing a channel can be effectively applied to the oxide semiconductor layer  83  by the conductive layer  81  and the conductive layer  85 ; thus, the current drive capability of the transistor  80  can be improved and high on-state current characteristics can be obtained. 
     The resin layer  25  preferably has a planarizing function because steps due to the conductive layer  81  are planarized and thus film formation in a later step can be easily performed. 
     In some cases, adhesion can be improved by providing the insulating layer  24  between the resin layer  23  and the resin layer  25  as compared to the case where the resin layer  23  and the resin layer  25  are provided in contact with each other. 
     The display device in  FIG. 17  includes the substrate  29 , the bonding layer  28 , the resin layer  23 , the resin layer  25 , the transistor  80 , a conductive layer  81   a , the conductive layer  86   c , the insulating layer  33 , the insulating layer  34 , the insulating layer  35 , the display element  60 , the bonding layer  99 , the coloring layer  97 , the light-blocking layer  98 , the substrate  22 , the bonding layer  13 , the resin layer  93 , and the insulating layer  95 . 
     The display device in  FIG. 17  is a display device in which the insulating layer  31  is removed from and the resin layer  25  is added to the components of  FIG. 14B . 
     In  FIG. 17 , the conductive layer  81  functioning as a gate of the transistor  80  is positioned between the resin layer  23  and the resin layer  25 . 
     In the manufacturing process of the display device, part of the resin layer  23  might be removed in processing a conductive film to be the conductive layer  81 . In the case where the resin layer  23  is not a uniform film or in the case where a large opening is formed in the resin layer  23 , the yield of peeling of the formation substrate might be decreased. The display device in  FIG. 17  includes the resin layer  25  which is in contact with the resin layer  23  and the conductive layer  81 . Therefore, the formation substrate can be peeled using the resin layer  25  in a portion from which the resin layer  23  is removed. Thus, the manufacturing yield of the display device can be increased. 
     The conductive layer  81   a  and the conductive layer  86   c  can be formed using the same material and the same fabrication step as the conductive layer included in the transistor. For example, the conductive layer  81   a  can be formed using the same material and the same fabrication step as the conductive layer  81 . The FPC  77  is electrically connected to the conductive layer  86   c  through the conductive layer  81   a  and the connector  76 . 
     The display device in  FIG. 18  includes the substrate  29 , the bonding layer  28 , the resin layer  23 , the insulating layer  24 , the resin layer  25 , an insulating layer  26 , the transistor  80 , the insulating layer  33 , the insulating layer  34 , the insulating layer  35 , the display element  60 , the bonding layer  99 , the coloring layer  97 , the light-blocking layer  98 , the substrate  22 , the bonding layer  13 , the resin layer  93 , and the insulating layer  95 . 
     The display device in  FIG. 18  is a display device in which the insulating layer  26  is added to the components of  FIG. 16B . 
     In  FIG. 18 , the conductive layer  81  is surrounded by the insulating layer  24  and the insulating layer  26 . For example, in the case where copper or the like is used for the conductive layer  81 , the conductive layer  81  is preferably surrounded by the insulating layer  24  and the insulating layer  26  to prevent oxidation. A silicon nitride film is favorable for the insulating layer  24  and the insulating layer  26 , for example. 
     Structure Example 3 
       FIGS. 19A and 19B  are each a cross-sectional view illustrating a display portion  381  of a display device. The cross-sectional view in  FIG. 19B  also illustrates the driver circuit portion  382  and a connection portion for connection to the FPC  77 . 
     The display device in  FIG. 19A  is a bottom-emission, top-emission, or dual emission display device using a separate coloring method. The display device in  FIG. 19B  is a bottom-emission display device using a color filter method. 
     The display device in  FIG. 19A  includes the substrate  29 , the bonding layer  28 , the resin layer  23 , the insulating layer  31 , the transistor  40 , the insulating layer  33 , the insulating layer  34 , the insulating layer  35 , the display element  60 , the insulating layer  74 , and the protective layer  75 . 
     The display device in  FIG. 19A  can be manufactured by referring to the manufacturing method example 1 and the manufacturing method example 2 in Embodiment 1. 
     The transistor  40  includes the conductive layer  41 , the insulating layer  32 , the oxide semiconductor layer  44 , and the conductive layers  43   a  and  43   b . The conductive layer  41  functions as a gate. The insulating layer  32  functions as a gate insulating layer. The conductive layer  41  overlaps with the oxide semiconductor layer  44  with the insulating layer  32  provided therebetween. The conductive layers  43   a  and  43   b  are connected to the oxide semiconductor layer  44 . One of the conductive layers  43   a  and  43   b  functions as a source, and the other functions as a drain. 
     The insulating layer  74  functions as a protective layer that suppresses diffusion of impurities such as water into the display element  60 . The display element  60  is sealed with the insulating layer  74 . 
     The protective layer  75  can be used as a layer positioned on the outermost surface of the display device. The protective layer  75  preferably has a high visible-light transmitting property. A material that can be used for the resin layer  23  can be used for the protective layer  75 . 
     The display device in  FIG. 19B  includes the substrate  29 , the bonding layer  28 , the resin layer  23 , the insulating layer  31 , the transistor  40 , a transistor  50 , the conductive layer  86   c , a conductive layer  78 , the insulating layer  33 , the insulating layer  34 , the insulating layer  35 , the display element  60 , the bonding layer  75   b , the substrate  75   a , and the coloring layer  97 . 
       FIG. 19B  is an example including the transistor  40  and the transistor  50  which are each a transistor in which a conductive layer  45  functioning as a gate is added to the components of the transistor  40  in  FIG. 19A . 
     The display element  60  emits light to the coloring layer  97  side. 
     The conductive layer  78  can be formed using the same material and the same fabrication step as the conductive layer  61 . The FPC  77  is electrically connected to the conductive layer  86   c  through the conductive layer  78  and the connector  76 . 
     As illustrated in  FIG. 19B , it is not necessary to electrically connect the conductive layer  86   c  to the FPC  77  through an opening in the resin layer  23 . The conductive layer  86   c  is electrically connected to the FPC  77  through an opening in the insulating layer  33 , the insulating layer  34 , and the insulating layer  35 . 
     As described in this embodiment, in the flexible device of one embodiment of the present invention, the fabrication process of the transistor can be performed at a low temperature because an oxide semiconductor is used for the transistor. Furthermore, the resin layer can have a small thickness and low heat resistance. Therefore, the display device can be light-weight and thin. The display device can have higher flexibility. Warpage of the flexible device due to the thickness of the resin layer can be suppressed in some cases. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, a display module and electronic devices of one embodiment of the present invention will be described with reference to  FIG. 20  and  FIGS. 21A to 21F . 
     In a display module  8000  in  FIG. 20 , a touch panel  8004  connected to an FPC  8003 , a display panel  8006  connected to an FPC  8005 , a frame  8009 , a printed circuit board  8010 , and a battery  8011  are provided between an upper cover  8001  and a lower cover  8002 . 
     The display device of one embodiment of the present invention can be used for the display panel  8006 , for example. 
     The shapes and sizes of the upper cover  8001  and the lower cover  8002  can be changed as appropriate in accordance with the sizes of the touch panel  8004  and the display panel  8006 . 
     The touch panel  8004  can be a resistive touch panel or a capacitive touch panel and may be formed to overlap with the display panel  8006 . Instead of providing the touch panel  8004 , the display panel  8006  can have a touch panel function. 
     The frame  8009  protects the display panel  8006  and also functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed circuit board  8010 . The frame  8009  may function as a radiator plate. 
     The printed circuit board  8010  has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the battery  8011  provided separately may be used. The battery  8011  can be omitted in the case of using a commercial power source. 
     The display module  8000  can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet. 
     Highly reliable electronic devices with curved surfaces can be fabricated by one embodiment of the present invention. In addition, flexible and highly reliable electronic devices can be fabricated by one embodiment of the present invention. 
     Examples of electronic devices include a television set, a desktop or notebook personal computer, a monitor of a computer or the like, a digital camera, a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, an audio reproducing device, and a large game machine such as a pachinko machine. 
     The electronic device of one embodiment of the present invention 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. 
     Furthermore, the electronic device of one embodiment of the present invention may include a secondary battery. Preferably, the secondary battery is capable of being charged by contactless power transmission. 
     Examples of the secondary battery include a lithium-ion secondary battery such as a lithium polymer battery (lithium ion polymer battery) using a gel electrolyte, a nickel-hydride battery, a nickel-cadmium battery, an organic radical battery, a lead-acid battery, an air secondary battery, a nickel-zinc battery, and a silver-zinc battery. 
     The electronic device of one embodiment of the present invention may include an antenna. When a signal is received by the antenna, an image, data, or the like can be displayed on a display portion. When the electronic device includes the antenna and a secondary battery, the antenna may be used for contactless power transmission. 
     The electronic device of one embodiment of the present invention may include a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays). 
     The electronic device of one embodiment of the present invention can have a variety of functions, for example, a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on a display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of executing a variety of software (programs), a wireless communication function, and a function of reading a program or data stored in a recording medium. 
     Furthermore, the electronic device including a plurality of display portions can have a function of displaying image information mainly on one display portion while displaying text information mainly on another display portion, a function of displaying a three-dimensional image by displaying images where parallax is considered on a plurality of display portions, or the like. Furthermore, the electronic device including an image receiving portion can have a function of taking a still image or a moving image, a function of automatically or manually correcting a photographed image, a function of storing a photographed image in a recording medium (an external recording medium or a recording medium incorporated in the electronic device), a function of displaying a photographed image on a display portion, or the like. Note that the functions of the electronic device of one embodiment of the present invention are not limited thereto, and the electronic devices can have a variety of functions. 
       FIGS. 21A to 21C  illustrate examples of an electronic device including a display portion  7000  with a curved surface. The display surface of the display portion  7000  is bent, and images can be displayed on the bent display surface. The display portion  7000  may have flexibility. 
     The display portion  7000  can be formed using the display device of one embodiment of the present invention. One embodiment of the present invention makes it possible to provide a highly reliable electronic device having a curved display portion. 
       FIG. 21A  illustrates an example of a mobile phone. A mobile phone  7110  illustrated in  FIG. 21A  includes a housing  7101 , the display portion  7000 , operation buttons  7103 , an external connection port  7104 , a speaker  7105 , a microphone  7106 , a camera  7107 , and the like. 
     The mobile phone  7110  includes a touch sensor in the display portion  7000 . Operations such as making a call and inputting text can be performed by touch on the display portion  7000  with a finger, a stylus, or the like. 
     With the operation buttons  7103 , power can be on or off. In addition, types of images displayed on the display portion  7000  can be switched; for example, switching from a mail creation screen to a main menu screen can be performed. 
     When a detection device such as a gyroscope sensor or an acceleration sensor is provided inside the mobile phone, the direction of display on the screen of the display portion  7000  can be automatically changed by determining the orientation of the mobile phone (whether the mobile phone is placed horizontally or vertically). Furthermore, the direction of display on the screen can be changed by touch on the display portion  7000 , operation with the operation button  7103 , sound input using the microphone  7106 , or the like. 
       FIG. 21B  illustrates an example of a portable information terminal. A portable information terminal  7210  illustrated in  FIG. 21B  includes a housing  7201  and the display portion  7000 . The portable information terminal  7210  may also include an operation button, an external connection port, a speaker, a microphone, an antenna, a camera, a battery, or the like. The display portion  7000  is provided with a touch sensor. The operation of the portable information terminal can be performed by touching the display portion  7000  with a finger, a stylus, or the like. 
     The portable information terminal illustrated in this embodiment functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminals each can be used as a smartphone. The portable information terminal illustrated in this embodiment is capable of executing, for example, a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game. 
     The portable information terminal  7210  can display characters, image information, and the like on its plurality of surfaces. For example, three operation buttons  7202  can be displayed on one surface, and information  7203  indicated by a rectangle can be displayed on another surface.  FIG. 21B  illustrates an example in which the operation buttons  7202  are displayed at the top of the portable information terminal  7210  and the information  7203  is displayed on the side of the portable information terminal  7210 . Note that the operation buttons  7202  may be displayed on the side of the portable information terminal  7210  and the information  7203  may be displayed at the top of the portable information terminal  7210 , for example. 
     Information may be displayed on three or more surfaces of the portable information terminal  7210 . 
     Examples of the information  7203  include notification from a social networking service (SNS), display indicating reception of an e-mail or an incoming call, the title of an e-mail or the like, the sender of an e-mail or the like, the date, the time, remaining battery, and the reception strength of an antenna. Alternatively, the operation button, an icon, or the like may be displayed instead of the information  7203 . 
       FIG. 21C  illustrates an example of a television set. In a television set  7300 , the display portion  7000  is incorporated into a housing  7301 . Here, the housing  7301  is supported by a stand  7303 . 
     The television set  7300  illustrated in  FIG. 21C  can be operated with an operation switch of the housing  7301  or a separate remote controller  7311 . The display portion  7000  may include a touch sensor, and can be operated by touch on the display portion  7000  with a finger or the like. The remote controller  7311  may be provided with a display portion for displaying data output from the remote controller  7311 . With operation keys or a touch panel of the remote controller  7311 , channels and volume can be controlled and images displayed on the display portion  7000  can be controlled. 
     Note that the television set  7300  is provided with a receiver, a modem, and the like. A general television broadcast can be received with the receiver. When the television set is connected to a communication network with or without wires via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers) data communication can be performed. 
       FIGS. 21D to 21F  each illustrate an example of a portable information terminal including a flexible and bendable display portion  7001 . 
     The display portion  7001  is fabricated using the display device of one embodiment of the present invention. For example, a display device that can be bent with a radius of curvature of greater than or equal to 0.01 mm and less than or equal to 150 mm can be used. The display portion  7001  may include a touch sensor so that the portable information terminal can be operated by touch on the display portion  7001  with a finger or the like. One embodiment of the present invention makes it possible to provide a highly reliable electronic device including a display portion having flexibility. 
       FIG. 21D  illustrates an example of a wrist-watch-type portable information terminal. A portable information terminal  7800  includes a band  7801 , the display portion  7001 , an input/output terminal  7802 , operation buttons  7803 , and the like. The band  7801  functions as a housing. A flexible battery  7805  can be included in the portable information terminal  7800 . The battery  7805  may be arranged to overlap with the display portion  7001 , or the band  7801  and the like, for example. 
     The band  7801 , the display portion  7001 , and the battery  7805  have flexibility. Thus, the portable information terminal  7800  can be easily curved to have a desired shape. 
     With the operation buttons  7803 , a variety of functions such as time setting, turning on or off of the power, turning on or off of wireless communication, setting and cancellation of silent mode, and setting and cancellation of power saving mode can be performed. For example, the functions of the operation buttons  7803  can be set freely by the operating system incorporated in the portable information terminal  7800 . 
     By touch on an icon  7804  displayed on the display portion  7001  with a finger or the like, an application can be started. 
     The portable information terminal  7800  can employ near field communication conformable to a communication standard. For example, mutual communication between the portable information terminal and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. 
     The portable information terminal  7800  may include the input/output terminal  7802 . In the case where the input/output terminal  7802  is included in the portable information terminal  7800 , data can be directly transmitted to and received from another information terminal via a connector. Charging through the input/output terminal  7802  is also possible. Note that charging of the portable information terminal described as an example in this embodiment can be performed by contactless power transmission without using the input/output terminal. 
       FIGS. 21E and 21F  illustrate an example of a foldable portable information terminal.  FIG. 21E  illustrates a portable information terminal  7650  that is folded so that the display portion  7001  is on the inside.  FIG. 21F  illustrates the portable information terminal  7650  that is folded so that the display portion  7001  is on the outside. The portable information terminal  7650  includes the display portion  7001  and a non-display portion  7651 . When the portable information terminal  7650  is not used, the portable information terminal  7650  is folded so that the display portion  7001  is on the inside, whereby the display portion  7001  can be prevented from being contaminated or damaged. Note that although  FIGS. 21E and 21F  illustrate an example of the portable information terminal  7650  that is folded in two, the portable information terminal  7650  may be folded in three, four, or more. The portable information terminal  7650  may include an operation button, an external connection port, a speaker, a microphone, an antenna, a camera, a battery, or the like. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Example 1 
     In this example, results of peeling a formation substrate from a process member by a peeling method of one embodiment of the present invention will be described. 
       FIG. 22  is a cross-sectional schematic view of a process member  500 . Materials used for the process member  500  are as follows. Note that an oxide semiconductor layer used as a semiconductor layer of a transistor and an insulating layer used as a gate insulating layer are formed over a resin layer on the assumption that the transistor is separated from a formation substrate in the process member  500 . 
     As the formation substrate  111 , an approximately 0.7-mm-thick glass substrate was used. As the resin layer  112 , an approximately 1.55-μm-thick polyimide resin film was used. As an insulating layer  116 , a stack of an approximately 200-nm-thick silicon oxynitride film  116   a , an approximately 400-nm-thick silicon nitride film  116   b , and an approximately 50-nm-thick silicon oxynitride film  116   c  was used. As an oxide semiconductor layer  114   a , an approximately 40-nm-thick In-Ga-Zn oxide film was used. As an insulating layer  115 , an approximately 150-nm-thick silicon oxynitride film was used. As the bonding layer  132 , an approximately 5-μm-thick thermosetting epoxy resin film was used. As a flexible substrate  155 , an approximately 23-μm-thick film was used. As a protective film  156 , an approximately 100-μm-thick film was used. 
     Note that a polyimide resin film was formed using a photosensitive and thermosetting material having a viscosity of approximately 30 cP. 
     Next, a step of peeling the formation substrate  111  from the process member  500  will be described. 
     The process member  500  was irradiated with laser light from the formation substrate  111  side. In a top view, the entire surface of the process member  500  was irradiated with the laser light. Note that a shielding mask was provided in the peripheral portion of the process member  500  in irradiation. 
     As a laser emitting laser light, a XeCl excimer laser with a wavelength of 308 nm was used. The energy, repetition rate, and scan rate of the oscillator were set to 980 mJ, 60 Hz, and 11.7 mm/second, respectively. The cross section of the laser light is shaped into a linear form with a size of 0.6 mm×300 mm by adjusting an optical system. An attenuator was used for the optical system. Irradiation energy attenuation by the attenuator was 10%. 
     After the laser light irradiation, the inner side than the peripheral portion of the process member  500  was cut by a cutter from the protective film  156  side, so that the formation substrate  111  was peeled from the process member  500 . 
       FIG. 23A  is a photograph showing the appearances of the process member  500  from which the formation substrate  111  was peeled and the peeled formation substrate  111 .  FIG. 23B  is a STEM photograph of a cross section around the surface of the peeled formation substrate  111 . According to  FIG. 23B , it is found that the approximately 50-nm-thick resin layer  112  remains over the peeled formation substrate  111 . 
     According to  FIGS. 23A and 23B , it is found that the formation substrate can be peeled at an interface which is around the boundary between the formation substrate and the resin layer by the peeling method of one embodiment of the present invention. 
     Example 2 
     In this example, results of thermal desorption spectroscopy (TDS) analysis of samples fabricated in order to examine release of water from a polyimide resin layer used for one embodiment of the present invention will be described. 
     The samples used in the TDS analysis are nine samples, Samples A1, A2, A3, B1, B2, B3, C1, C2, and C3, the fabrication methods of which will be described below. 
     First, in all of the nine samples, an approximately 1.55-μm-thick polyimide resin layer  812  was formed over a glass substrate  811 . Heat treatment in a nitrogen atmosphere for one hour was performed to form the polyimide resin layer  812 . The temperature of the heat treatment was different in each sample as follows. That is, the heat treatment was performed at 350° C. in Samples A1, B1, and C1, 400° C. in Samples A2, B2, and C2, and 450° C. in Samples A3, B3, and C3. 
     Next, in each of Samples B1 to B3 and C1 to C3, an approximately 200-nm-thick silicon oxynitride film  813   a  was formed over the polyimide resin layer  812 . Then, in each of Samples C1 to C3, an approximately 400-nm-thick silicon nitride film  813   b  and an approximately 50-nm-thick silicon oxynitride film  813   c  were formed in this order over the silicon oxynitride film  813   a.    
     In this manner, the nine samples were fabricated.  FIGS. 24A to 24C  are cross-sectional schematic views of the samples. 
       FIGS. 25A to 25C  show the results of TDS analyses performed on the above nine samples.  FIG. 25A ,  FIG. 25B , and  FIG. 25C  are the results of TDS analyses performed on Samples A1 to A3, Samples B1 to B3, and Samples C1 to C3, respectively. Note that in each of the TDS analyses, the amount of a released gas with a mass-to-charge ratio m/z=18, which corresponds to a water molecule, was measured. In each of  FIGS. 25A to 25C , the horizontal axis represents substrate heating temperature [° C.] and the vertical axis represents intensity proportional to the amount of the released gas with m/z=18. 
     According to  FIG. 25A , for example, in the case where the formation temperature of a transistor including an oxide semiconductor was below 350° C. when the polyimide resin layer  812  was heated at a temperature higher than or equal to 400° C. in formation, it is found that release of water can be suppressed. Furthermore, according to  FIGS. 25B and 25C , in a structure in which an inorganic film was provided over the polyimide resin layer  812 , when the polyimide resin layer  812  is heated at a temperature higher than or equal to 400° C. in formation, it is found that release of moisture below 400° C. can be suppressed. 
     As described above, according to the results of this example, in the case where the transistor including an oxide semiconductor was formed over the polyimide resin layer, when the polyimide resin layer was heated at a temperature higher than or equal to 400° C. in formation, it is found that release of moisture from the polyimide resin layer can be suppressed. The above suggests that variation in electrical characteristics of the transistor due to release of water from the polyimide resin layer can be suppressed. 
     Example 3 
     In this example, results of comparing electrical characteristics of a transistor before and after peeling from a formation substrate will be described. Note that the transistor is formed over the formation substrate with a resin layer provided therebetween. The peeling method of the formation substrate is similar to that described in Example 1. 
     The structure of the transistor fabricated in this example is similar to the transistor  80  in  FIG. 12A  and the like. 
     As the formation substrate  14 , an approximately 0.7-mm-thick glass substrate was used. As the resin layer  23 , an approximately 1.55-μm-thick polyimide resin film was used. As the insulating layer  31 , an approximately 200-nm-thick silicon oxynitride film was used. As the conductive layer  81  functioning as a back gate electrode of the transistor, an approximately 100-nm-thick titanium film was used. As the insulating layer  82 , a stack of an approximately 400-nm-thick silicon nitride film and an approximately 50-nm-thick silicon oxynitride film was used. As the oxide semiconductor layer  83 , an approximately 40-nm-thick In-Ga-Zn oxide semiconductor film formed using an oxide target with an atomic ratio where In:Ga:Zn=4:2:3 was used. As the insulating layer  84 , an approximately 150-nm-thick silicon oxynitride film was used. As the conductive layer  85  functioning as a gate electrode of the transistor, an approximately 100-nm-thick In-Ga-Zn oxide film formed using an oxide target with an atomic ratio where In:Ga:Zn=4:2:3 was used. As the insulating layer  33 , a stack of an approximately 100-nm-thick silicon nitride film and an approximately 300-nm-thick silicon oxynitride film was used. As the conductive layers  86   a  and  86   b , a stack of an approximately 10-nm-thick titanium film and an approximately 100-nm-thick copper film was used. As the insulating layer  34 , an approximately 1.5-μm-thick acrylic resin film was used. 
     The I d -V g  characteristics (drain current-gate voltage characteristics) of the above transistor were measured. The measurement conditions were different as follows before and after peeling of the formation substrate. Before the peeling, the I d -V g  characteristics were measured in such a manner that the drain voltage was set to 0.1 V or 20 V and the back gate voltage and the gate voltage were swept from ˜8 V to 8 V in increments of 0.25 V. After the peeling, the I d -V g  characteristics were measured in such a manner that the drain voltage was set to 0.1 V or 10 V and the back gate voltage and the gate voltage were swept from ˜8 V to 8 V in increments of 0.25 V. 
     Measurement results of I d -V g  characteristics are shown in  FIGS. 26A and 26B  and  FIGS. 27A and 27B .  FIGS. 26A and 26B  show the measurement results of transistors before and after peeling each having a channel length L of 3 μm and a channel width W of 50 μm.  FIGS. 27A and 27B  show the measurement results of transistors before and after peeling each having a channel length L of 6 μm and a channel width W of 50 μm. In  FIGS. 26A and 26B  and  FIGS. 27A and 27B , the horizontal axis represents gate voltage V g  [V], the left vertical axis represents drain current I d  [A], and the right vertical axis represents field-effect mobility μ FE  [cm 2 /V s ] In addition, in  FIGS. 26A and 26B  and  FIGS. 27A and 27B , a thick solid line indicates I d -V g  characteristics at a drain voltage of 10 V or 20 V, a thick dashed-dotted line indicates I d -V g  characteristics at a drain voltage of 0.1 V, and a thin dashed line indicates field-effect mobility μ FE  at a drain voltage of 10 V or 20 V. The transistors measured before and after peeling are different transistors formed over the same resin layer  23 . 
     As shown in  FIGS. 26A and 26B  and  FIGS. 27A and 27B , it is found that there is little difference between the electrical characteristics of the transistors formed over the resin layer before and after peeling of the formation substrate from the resin layer. 
     According to this example, the above suggests that the use of the peeling method of one embodiment of the present invention allows the formation substrate to be peeled from the resin layer without influencing the electrical characteristics of the transistors provided over the resin layer. 
     Example 4 
     Items shown in Table 1 in the peeling method of one embodiment of the present invention will be described below in detail. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Processing on 
                 Not performed 
               
               
                 formation substrate 
                   
               
            
           
           
               
               
               
            
               
                 Material of resin 
                 Photosensitive material including polyimide 
                 Nonphotosensitive material 
               
               
                 layer 
                 resin precursor 
                 including soluble polyimide 
               
               
                   
                   
                 resin 
               
               
                 Heat condition of 
                 Baking in N 2  atmosphere at 350° C. 
                 Baking in N 2  atmosphere at 
               
               
                 resin layer 
                   
                 350° C. 
               
               
                 Ease of removing 
                 ○ 
                 ○ 
               
               
                 resin layer before 
                   
                   
               
               
                 curing 
                   
                   
               
               
                 Processability of 
                 ○ 
                 Δ 
               
               
                 resin layer 
                 Removal by light exposure 
                 Removal by etching 
               
               
                 Marker recognition 
                 Δ 
                 ○ 
               
            
           
           
               
               
               
               
               
            
               
                 Removal of resin 
                 Performed 
                 Not performed 
                 Performed 
                 Not performed 
               
               
                 layer after peeling 
                   
                   
                   
                   
               
               
                 Method for 
                 (a) Exposing 
                 (b) Exposing through 
                 Refer to 
                 Refer to (b) 
               
               
                 forming contact 
                 through electrode 
                 electrode after peeling 
                 (a) 
                 (opening is 
               
               
                 with through 
                 by removing resin 
                 step by forming opening 
                   
                 formed with 
               
               
                 electrode 
                 layer by ashing 
                 in resin layer in formation 
                   
                 resist mask) 
               
               
                   
                 after peeling 
                 and forming through 
                   
                   
               
               
                   
                   
                 electorde (forming 
                   
                   
               
               
                   
                   
                 opening by light 
                   
                   
               
               
                   
                   
                 exposure) 
                   
                   
               
               
                 Finishing coloring 
                 ⊚ 
                 x 
                 ⊚ 
                 ⊚ 
               
               
                   
                 Non-colored 
                 Colored 
                 Non-colored 
                 Non-colored 
               
               
                 Peelability 
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ⊚ 
               
               
                   
               
            
           
         
       
     
     As described in Example 1 and the like, particular processing such as plasma treatment on the formation substrate is not necessary. As the material of the resin layer, a photosensitive material including a polyimide resin precursor is favorable. Alternatively, as the material of the resin layer, a nonphotosensitive material including a soluble polyimide resin is favorable. As the heat condition to form the resin layer, baking in an N 2  atmosphere at 350° C. is favorable. 
     Here, when the material of the resin layer is applied to the formation substrate, the material might be non-uniformly applied to part of the peripheral portion of the substrate or the like. It is preferable to remove such an unnecessary portion easily before curing the resin layer. For example, the unnecessary portion can be removed with an organic solvent such as thinner. Depending on the material of the resin layer, turbidity, gelation, or solidification might occur by reaction with thinner. The material of the resin layer used in Example 1 and the like is dissolved in an organic solvent such as thinner; therefore, the unnecessary portion can be easily removed before curing the resin layer. 
     The photosensitive material is preferably used because the resin layer can be easily processed. The resin layer can be processed by applying the material and then performing light exposure and development. The fabrication process can be shortened because a resist mask is not needed. 
     In the case of using the nonphotosensitive material, a resist mask is formed as follows: the material is applied and cured by heating, a resist is applied to the resin layer, and then light exposure and development are performed. After that, the resin layer can be processed by dry etching. 
     It is preferable that a fabrication apparatus easily read an alignment marker in a step of forming a layer to be peeled (e.g., a transistor or a display element) over the resin layer and a step of attaching a substrate to the resin layer. The nonphotosensitive material has in some cases a higher visible-light transmitting property than the photosensitive material. The resin layer having a high visible-light transmitting property is preferable because a fabrication apparatus can easily recognize the marker and the degree of freedom of the layout is increased as compared with a colored resin layer. 
     In the case of removing the resin layer after peeling, a through electrode can be exposed. The resin layer is preferably removed by ashing. In the case of using ashing, dry etching, or the like, the shape of an opening of the resin layer is close to a perpendicular shape. Since the resin layer is removed, the finishing device is not affected by the color of the resin layer. 
     In the case of not removing the resin layer after peeling, the through electrode is preferably exposed by peeling. When the resin layer is formed, an opening is formed in the resin layer to form the through electrode in the opening. In the case of using the photosensitive material, an opening can be formed in the resin layer by light-exposure technique. At this time, the opening has a tapered shape. In the case of using the nonphotosensitive material, an opening can be formed in the resin layer with a resist mask. At this time, the shape of the opening is close to a perpendicular shape. Then, the resin layer and the through electrode are exposed by peeling. Note that a material having low adhesion to the formation substrate is preferably used for the through electrode. In addition, a contact area with the through electrode and the formation substrate is preferably as small as possible. Since the resin layer is not removed, the finishing device is affected by the color of the resin layer. In the case of using a colored resin, the resin layer is preferably not provided in an unnecessary portion to suppress a decrease in light extraction efficiency. The resin layer having a high visible-light transmitting property is preferably used because light extraction efficiency is less likely to decrease even when the resin layer is left. 
     Both materials are comparably preferable in terms of ease of peeling. 
     This application is based on Japanese Patent Application serial no. 2016-077668 filed with Japan Patent Office on Apr. 7, 2016 and Japanese Patent Application serial no. 2016-077667 filed with Japan Patent Office on Apr. 7, 2016, the entire contents of which are hereby incorporated by reference.