Patent Publication Number: US-9899536-B2

Title: Method of forming semiconductor device with different energy gap oxide semiconductor stacked layers

Description:
TECHNICAL FIELD 
     The invention disclosed in this specification and the like relates to a semiconductor device and a method for manufacturing the semiconductor device. 
     In this specification and the like, a semiconductor device refers to all types of devices which can function by utilizing semiconductor characteristics; an electro-optical device, an image display device, a semiconductor circuit, and an electronic appliance are all semiconductor devices. 
     BACKGROUND ART 
     A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention. 
     For example, a technique by which a transistor is formed using zinc oxide or an In—Ga—Zn-based oxide semiconductor as an oxide semiconductor is disclosed (see Patent Documents 1 and 2). 
     Non-Patent Document discloses a transistor having a structure in which oxide semiconductors having different compositions are stacked. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2007-123861 
         [Patent Document 2] Japanese Published Patent Application No. 2007-096055 
       
    
     Non-Patent Document 
     
         
         [Non-Patent Document 1] Masashi Ono et al., “Novel High Performance IGZO-TFT with High Mobility over 40 cm 2 /Vs and High Photostability Incorporated Oxygen Diffusion”, IDW&#39;ll Late-News Paper, pp. 1689-1690. 
       
    
     DISCLOSURE OF INVENTION 
     In a transistor including an oxide semiconductor, if a trap level (also referred to as an interface state) exists at an interface between an oxide semiconductor layer and a layer in contact with the oxide semiconductor layer, the existence of the trap level causes a change in the electrical characteristics (e.g., the threshold voltage or the subthreshold swing (s value)) of the transistor. 
     For example, in a bottom-gate transistor, when a constituent element of a source electrode layer and a drain electrode layer diffuses into a back channel of an oxide semiconductor layer, the constituent element forms a trap level, so that the electrical characteristics of the transistor is changed. Further, the existence of a trap level at an interface between the oxide semiconductor layer and a gate insulating layer might cause a change in the electrical characteristics of the transistor. 
     Thus, an object of one embodiment of the present invention is to provide a highly reliable semiconductor device including an oxide semiconductor by preventing a change in its electrical characteristics. 
     One embodiment of the present invention is a bottom gate transistor including an oxide semiconductor, which has at least a stacked structure formed of a first oxide semiconductor layer which is in contact with a source electrode layer and a drain electrode layer and a second oxide semiconductor layer which is provided between the first oxide semiconductor layer and a gate insulating layer. In the above transistor, the second oxide semiconductor layer is used as a main current path (channel) of the transistor and the first oxide semiconductor layer is used as a buffer layer for preventing diffusion of a constituent element of the source and drain electrode layers, whereby a change in the electrical characteristics of the transistor can be prevented. Specifically, any of the following structures can be employed for example. 
     One embodiment of the present invention is a semiconductor device including a gate electrode layer, a gate insulating layer over the gate electrode layer, an oxide semiconductor stack overlapping with the gate electrode layer with the gate insulating layer positioned therebetween, and a source electrode layer and a drain electrode layer electrically connected to the oxide semiconductor stack. The oxide semiconductor stack includes a first oxide semiconductor layer in contact with the source electrode layer and the drain electrode layer and a second oxide semiconductor layer provided between the first oxide semiconductor layer and the gate insulating layer. The first oxide semiconductor layer contains at least indium and gallium, and the content of the indium is lower than or equal to the content of the gallium in the first oxide semiconductor layer. The second oxide semiconductor layer contains at least indium and gallium, and the content of the indium is higher than the content of the gallium in the second oxide semiconductor layer. The first oxide semiconductor layer contains a constituent element of the source electrode layer and the drain electrode layer, as an impurity. 
     Another embodiment of the present invention is a semiconductor device including a gate electrode layer, a gate insulating layer over the gate electrode layer, an oxide semiconductor stack overlapping with the gate electrode layer with the gate insulating layer positioned therebetween, and a source electrode layer and a drain electrode layer electrically connected to the oxide semiconductor stack. The oxide semiconductor stack includes a first oxide semiconductor layer in contact with the source electrode layer and the drain electrode layer, a third oxide semiconductor layer in contact with the gate insulating layer, and a second oxide semiconductor layer provided between the first oxide semiconductor layer and the third oxide semiconductor layer. The first oxide semiconductor layer and the third oxide semiconductor layer contain at least indium and gallium, and the content of the indium is lower than or equal to the content of the gallium in the first oxide semiconductor layer and the third oxide semiconductor layer. The second oxide semiconductor layer contains at least indium and gallium, and the content of the indium is higher than the content of the gallium in the second oxide semiconductor layer. The first oxide semiconductor layer contains a constituent element of the source electrode and the drain electrode layer, as an impurity. 
     Another embodiment of the present invention is a semiconductor device including a gate electrode layer, a gate insulating layer over the gate electrode layer, an oxide semiconductor stack overlapping with the gate electrode layer with the gate insulating layer positioned therebetween, and a source electrode layer and a drain electrode layer electrically connected to the oxide semiconductor stack. The oxide semiconductor stack includes a first oxide semiconductor layer in contact with the source electrode layer and the drain electrode layer, a third oxide semiconductor layer in contact with the gate insulating layer, and a second oxide semiconductor layer provided between the first oxide semiconductor layer and the third oxide semiconductor layer. The first oxide semiconductor layer and the third oxide semiconductor layer contain at least indium and gallium, the content of the indium is lower than or equal to the content of the gallium in the first oxide semiconductor layer and the third oxide semiconductor layer. The second oxide semiconductor layer contains at least indium and gallium, and the content of the indium is higher than the content of the gallium in the second oxide semiconductor layer. The first oxide semiconductor layer contains a constituent element of the source electrode and the drain electrode layer, as an impurity. The third oxide semiconductor layer contains a constituent element of the gate insulating layer, as an impurity. 
     In any one of the above semiconductor devices, the source electrode layer and the drain electrode layer preferably contain copper. 
     In any one of the above semiconductor devices, the gate insulating layer may include a silicon nitride film. 
     Effects of a structure according to one embodiment of the present invention can be described as follow. Note also that the following description is just a consideration. 
     The transistor of one embodiment of the present invention includes the first oxide semiconductor layer which is in contact with the source electrode layer and the drain electrode layer and the second oxide semiconductor layer which serves as a main current path (channel) of the transistor. Here, the first oxide semiconductor layer serves as a buffer layer for preventing a constituent element of the source and drain electrode layers from diffusing into the channel. By providing the first oxide semiconductor layer, it is possible to prevent diffusion of the constituent element into an interface between the first oxide semiconductor layer and the second oxide semiconductor layer and into the second oxide semiconductor layer. 
     Further, it is preferable that a metal oxide which is applied to the first oxide semiconductor layer have a larger energy gap (band gap) than a metal oxide which is applied to the second oxide semiconductor layer because a conduction band offset can be formed between the second oxide semiconductor layer and the first oxide semiconductor layer. When the conduction band offset exists in the oxide semiconductor stack, carriers flow through the second oxide semiconductor layer without passing through the first oxide semiconductor layer and the interface thereof; thus even if a trap level due to the diffusion of the metal element exists on the back channel side, the oxide semiconductor stack is insusceptible to the trap level. Accordingly, the electrical characteristics of the transistor can be stabilized. 
     It is preferable that the transistor of one embodiment of the present invention further include a third oxide semiconductor layer which is provided between the second oxide semiconductor layer and the gate insulating layer and is in contact with the gate insulating layer, in addition to the first oxide semiconductor layer and the second oxide semiconductor layer. The third oxide semiconductor layer contains one or a plurality of metal elements selected from the constituent elements of the second oxide semiconductor layer, and the third oxide semiconductor layer includes characteristics of the second oxide semiconductor layer. Therefore, when the third oxide semiconductor layer is provided, the interface on the gate insulating layer side of the second oxide semiconductor layer functioning as the channel can be stabilized. That is, the third oxide semiconductor layer functions as a buffer layer for preventing degradation of the interface. In particular, photodegradation (e.g., negative-bias temperature stress photodegradation) of the transistor can be reduced by prevention of capture of carriers at the interface on the gate insulating layer side of the channel, so that the transistor can have high reliability. 
     Further, it is preferable that, in a manner similar to the metal oxide applied to the first oxide semiconductor layer, a metal oxide which is applied to the third oxide semiconductor layer have a larger energy gap than the metal oxide applied to the second oxide semiconductor layer because a conduction band offset can be formed between the third oxide semiconductor layer and the second oxide semiconductor layer. Also in a general MISFET, a trap level or the like is generated at an interface between a gate insulating layer and a semiconductor and the electrical characteristics of the FET is degraded; however, when a third oxide semiconductor layer is provided, the MISFET has a structure in which carriers flow through a region which is apart from the gate insulating layer (what is called a buried channel), and therefore influence of the interface can be reduced. 
     In the case where metal oxides which are formed of the same constituent elements but have different compositions are applied to the first oxide semiconductor, the second oxide semiconductor, and the third oxide semiconductor, for example, metal oxides containing at least indium and gallium can be used as the first oxide semiconductor, the second oxide semiconductor, and the third oxide semiconductor. Here, as the ratio of the indium content to the content of the other metal element(s) becomes higher in the metal oxide, the field-effect mobility of the metal oxide is increased; as the ratio of the gallium content to the content of the other metal element(s) becomes higher in the metal oxide, the energy gap of the metal oxide is increased. Therefore, a metal oxide in which the indium content is higher than the gallium content is preferably used as the second oxide semiconductor to be a channel formation region, and metal oxides in which the indium content is lower than or equal to the gallium content are preferably used as the first oxide semiconductor and the third oxide semiconductor which function as buffer layers. 
     According to one embodiment of the present invention, a change in the electrical characteristics of a transistor including an oxide semiconductor can be prevented, so that a highly reliable semiconductor device can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1C  are a plan view and cross-sectional views illustrating one embodiment of a semiconductor device. 
         FIGS. 2A to 2D  are a plan view, cross-sectional views, and a band diagram illustrating one embodiment of a semiconductor device. 
         FIGS. 3A to 3D  illustrate an example of a method for manufacturing a semiconductor device. 
         FIGS. 4A to 4C  each illustrate one embodiment of a semiconductor device. 
         FIG. 5  illustrates one embodiment of a semiconductor device. 
         FIGS. 6A and 6B  illustrate one embodiment of a semiconductor device. 
         FIGS. 7A and 7B  illustrate one embodiment of a semiconductor device. 
         FIGS. 8A to 8C  illustrate electronic appliances. 
         FIGS. 9A to 9C  illustrate an electronic appliance. 
         FIGS. 10A and 10B  are cross-sectional views each illustrating one embodiment of a semiconductor device. 
         FIGS. 11A to 11C  illustrate one embodiment of a semiconductor device. 
         FIG. 12  shows measurement results of SSDP-SIMS in an example. 
         FIG. 13  illustrates one embodiment of a semiconductor device. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments and an example of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below and it is easily understood by those skilled in the art that the mode and details can be changed in various ways. Therefore, the invention should not be construed as being limited to the description in the following embodiments and example. 
     Note that in structures of the present invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description thereof is not repeated. Further, the same hatching pattern is applied to portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     Note that in each drawing described in this specification, the size, the film thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such scales. 
     Note that the ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention. 
     In this specification, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system. 
     Embodiment 1 
     In this embodiment, one embodiment of a semiconductor device and one embodiment of a method for manufacturing the semiconductor device are described using  FIGS. 1A to 1C ,  FIGS. 2A to 2D , and  FIGS. 3A to 3D . In this embodiment, a bottom-gate transistor including an oxide semiconductor layer is described as an example of the semiconductor device. 
       FIGS. 1A to 1C  illustrate a structure example of a transistor  300 .  FIG. 1A  is a plan view of the transistor  300 ,  FIG. 1B  is a cross-sectional view taken along dotted line X 1 -Y 1  in  FIG. 1A , and  FIG. 1C  is a cross-sectional view taken along dotted line V 1 -W 1  in  FIG. 1A . 
     The transistor  300  includes a gate electrode layer  402  which is provided over a substrate  400  having an insulating surface, a gate insulating layer  404  over the gate electrode layer  402 , an oxide semiconductor stack  408  which is on and in contact with the gate insulating layer  404  and overlaps with the gate electrode layer  402 , and a source electrode layer  410   a  and a drain electrode layer  410   b  which are electrically connected to the oxide semiconductor stack  408 . Note that an insulating layer  412  which covers the source electrode layer  410   a  and the drain electrode layer  410   b  and is in contact with the oxide semiconductor stack  408  may be included in the transistor  300  as a component. 
     The oxide semiconductor stack  408  in the transistor  300  includes an oxide semiconductor layer  408   a  which is in contact with the source electrode layer  410   a  and the drain electrode layer  410   b , and an oxide semiconductor layer  408   b  which is provided between the oxide semiconductor layer  408   b  and the gate insulating layer  404 . 
     The oxide semiconductor layer  408   b  in the oxide semiconductor stack  408  is a region serving as a channel of the transistor  300 . The oxide semiconductor layer  408   a  provided on the back channel side of the oxide semiconductor layer  408   b  functions as a buffer layer for preventing a constituent element of the source electrode layer  410   a  and the drain electrode layer  410   b  from diffusing into the oxide semiconductor layer  408   b . That is, the oxide semiconductor layer  408   a  contains the constituent element of the source electrode layer  410   a  and the drain electrode layer  410   b , as an impurity. By providing the buffer layer, formation of a trap level in the channel of the transistor  300  can be prevented; thus an increase in s-value due to the trap level can be prevented. Accordingly, variations in the electrical characteristics or degradation over time of the transistor can be prevented; thus a highly reliable semiconductor device can be provided. 
     For the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   b , oxide semiconductors which have different constituent elements or oxide semiconductors which have the same constituent elements but whose compositions are different from each other may be used. Note that it is preferable that an oxide semiconductor which has high field-effect mobility be used for the oxide semiconductor layer  408   b  functioning as the channel of the transistor  300 . 
     For example, in the case where the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   b  are formed to have the same constituent elements by using oxide semiconductors which contain at least indium and gallium, an oxide semiconductor in which the indium content is larger than the gallium content is preferably used for the oxide semiconductor layer  408   b , and an oxide semiconductor in which the indium content is lower than or equal to the gallium content is preferably used for the oxide semiconductor layer  408   a.    
     In an oxide semiconductor, the s orbital of heavy metal mainly contribute to carrier transfer, and when the percentage of indium in the oxide semiconductor is increased, overlaps of the s orbitals are likely to be increased. Accordingly, when the indium content is higher than the gallium content in the oxide semiconductor layer  408   b , it is possible that the oxide semiconductor layer  408   b  has higher field-effect mobility than an oxide in which the indium content is lower than the gallium content. 
     Further, as the ratio of the gallium content to the content of the other metal element(s) becomes higher in a metal oxide, the energy gap of the metal oxide is increased. Accordingly, when the indium content is lower than or equal to the gallium content in the oxide semiconductor layer  408   a , the oxide semiconductor layer  408   a  has a larger energy gap than the oxide semiconductor layer  408   b . Accordingly, the above structure is preferable because a conduction band offset can be formed between the oxide semiconductor layer  408   b  and the oxide semiconductor layer  408   a . Further, gallium needs large formation energy of an oxygen vacancy and thus is not likely to generate an oxygen vacancy as compared to indium. Therefore, a metal oxide in which the indium content is lower than or equal to the gallium content has stable characteristics as compared to a metal oxide in which the indium content is higher than the gallium content. Therefore, stabilization on the back channel side in the transistor  300  can be achieved. 
     For example, in the case where an In—Ga—Zn-based oxide semiconductor is used for the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   b , an In—Ga—Zn-based oxide having any of atomic ratios of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=1:3:2 (=1/6:3/6:2/6), In:Ga:Zn=2:4:3 (=2/9:4/9:3/9), and In:Ga:Zn=1:5:3 (=1/9:5/9:3/9); or a metal oxide having an atomic ratio which is close to any of the above atomic ratios can be used for the oxide semiconductor layer  408   a . As the oxide semiconductor layer  408   b , an In—Ga—Zn-based oxide having any of atomic ratios of In:Ga:Zn=3:1:2 (=3/6:1/6:2/6), In:Ga:Zn=4:2:3 (=4/9:2/9:3/9), In:Ga:Zn=5:1:3 (=5/9:1/9:3/9), In:Ga:Zn=5:3:4 (=5/12:3/12:4/12), In:Ga:Zn=6:2:4 (=6/12:2/12:4/12), or In:Ga:Zn=7:1:3 (=7/11:1/11:3/11); or a metal oxide having an atomic ratio which is close to any of the above atomic ratios can be used. 
     Note that for example, the expression “the composition of an oxide including In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in the neighborhood of the composition of an oxide containing In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b, and c satisfy the following relation: (a−A) 2 +(b−B) 2 +(c−C) 2 . For example, r may be 0.05. 
     Note that the metal oxide which is applied to the oxide semiconductor stack  408  is not limited thereto, and a metal oxide which have an appropriate composition in accordance with required electrical characteristics (field-effect mobility, a threshold voltage, variations, and the like) may be used. In order to obtain the needed electrical characteristics, the carrier concentration, the impurity concentration, the defect density, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like are preferably set to appropriate values. For example, instead of gallium, another metal element may be contained. Alternatively, a metal oxide which does not contain zinc can be used. Note that it is preferable that a metal oxide which is to be applied contain zinc because a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film can be relatively easily formed as the oxide semiconductor layer that is formed later. 
     Note that the oxide semiconductor layer can be formed by a sputtering method. Generation of particles in deposition can be prevented by using a sputtering target which contains indium. Therefore, it is preferable that the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   b  contain indium. 
     A structure of an oxide semiconductor layer is described below. 
     An oxide semiconductor layer is classified roughly into a single-crystal oxide semiconductor layer and a non-single-crystal oxide semiconductor layer. The non-single-crystal oxide semiconductor layer includes any of an amorphous oxide semiconductor layer, a microcrystalline oxide semiconductor layer, a polycrystalline oxide semiconductor layer, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, and the like. 
     The amorphous oxide semiconductor layer has disordered atomic arrangement and no crystalline component. A typical example thereof is an oxide semiconductor layer in which no crystal part exists even in a microscopic region, and the whole of the film is amorphous. 
     The microcrystalline oxide semiconductor layer includes a microcrystal (also referred to as nanocrystal) with a size greater than or equal to 1 nm and less than 10 nm, for example. Thus, the microcrystalline oxide semiconductor layer has a higher degree of atomic order than the amorphous oxide semiconductor layer. Hence, the density of defect states of the microcrystalline oxide semiconductor layer is lower than that of the amorphous oxide semiconductor layer. 
     The CAAC-OS film is one of oxide semiconductor layers including a plurality of crystal parts, and most of the crystal parts each fit inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. The density of defect states of the CAAC-OS film is lower than that of the microcrystalline oxide semiconductor layer. The CAAC-OS film is described in detail below. 
     In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film. 
     A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor layer of InGaZnO 4 , six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when φ scan is performed with 2θ fixed at around 56°. 
     According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. 
     Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies depending on regions. 
     Note that when the CAAC-OS film with an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ do not appear at around 36°. 
     In a transistor using the CAAC-OS film, change in electric characteristics due to irradiation with visible light or ultraviolet light is small. Thus, the transistor has high reliability. 
     Note that an oxide semiconductor layer may be a stacked film including two or more of an amorphous oxide semiconductor layer, a microcrystalline oxide semiconductor layer, and a CAAC-OS film, for example. 
     For example, the CAAC-OS film is formed by a sputtering method with a polycrystalline metal oxide target. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) may flake off from the sputtering target. In that case, the flat-plate-like sputtered particle reaches a substrate while maintaining their crystal state, whereby the CAAC-OS film can be formed. 
     For the deposition of the CAAC-OS film, the following conditions are preferably used. 
     By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used. 
     By increasing the substrate heating temperature during the deposition, migration of a sputtered particle is likely to occur after the sputtered particle is attached to a substrate surface. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate, migration occurs on the substrate surface, so that a flat plane of the flat-plate-like sputtered particle is attached to the substrate. 
     Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol % or higher, preferably 100 vol %. 
     As an example of the metal oxide target for sputtering, an In—Ga—Zn—O compound target is described below. 
     The In—Ga—Zn—O compound target, which is polycrystalline, is made by mixing InO X  powder, GaO Y  powder, and ZnO A  powder in a predetermined ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y, and Z are each a given positive number. Here, the predetermined molar ratio of InO X  powder to GaO Y  powder and ZnO Z  powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target. 
     Note that the oxide semiconductor layer  408   a  may have different crystallinity from the oxide semiconductor layer  408   b . Note that the CAAC-OS film is preferably applied to the oxide semiconductor layer  408   b  functioning as the channel of the transistor  300 . In the case where the oxide semiconductor layer  408   b  is formed using the CAAC-OS film, a precursor might be oriented in the oxide semiconductor layer  408   a  provided on and in contact with the oxide semiconductor layer  408   b , i.e., the precursor might have what is called orderliness, so that the oxide semiconductor layer  408   a  can be the CAAC-OS film. When the oxide semiconductor layer  408   a  provided on the back channel side is formed of an amorphous oxide semiconductor, oxygen vacancies are generated in etching treatment for forming the source electrode layer  410   a  and the drain electrode layer  410   b , and accordingly the oxide semiconductor layer  408   a  is likely to be n-type. Therefore, it is preferable to apply a crystalline oxide semiconductor to the oxide semiconductor layer  408   a.    
     Note that in this embodiment, the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   b  have the same constituent elements and form an oxide semiconductor stack. At this time, depending on materials or deposition conditions of the oxide semiconductor layers, the interfaces between the oxide semiconductor layers are unclear in some cases. Therefore, in  FIGS. 1A to 1C , the interface between the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   b  is schematically denoted by a dotted line. The same applies to drawings mentioned below. 
       FIGS. 2A to 2C  illustrate a structural example of a transistor  310 .  FIG. 2A  is a plan view of the transistor  310 ,  FIG. 2B  is a cross-sectional view taken along a chain line X 2 -Y 2  in  FIG. 2A , and  FIG. 2C  is a cross-sectional view taken along a chain line V 2 -W 2  in  FIG. 2A . In a manner similar to the transistor  300  in  FIGS. 1A to 1C , the transistor  310  in  FIGS. 2A to 2C  includes the gate electrode layer  402  provided over the substrate  400  having an insulating surface, the gate insulating layer  404  over the gate electrode layer  402 , the oxide semiconductor stack  408  which is in contact with the gate insulating layer  404  and overlaps with the gate electrode layer  402 , and the source electrode layer  410   a  and the drain electrode layer  410   b  which are electrically connected to the oxide semiconductor stack  408 . Further, the insulating layer  412  which covers the source electrode layer  410   a  and the drain electrode layer  410   b  and is in contact with the oxide semiconductor stack  408  may be included as a component of the transistor  310 . 
     The transistor  310  is different from the transistor  300  in that an oxide semiconductor layer  408   c  is provided between the oxide semiconductor layer  408   b  and the gate insulating layer  404 . That is, in the transistor  310 , the oxide semiconductor stack  408  includes a stacked structure of the oxide semiconductor layer  408   a , the oxide semiconductor layer  408   b , and the oxide semiconductor layer  408   c.    
     Note that the transistor  310  has the same structure as the transistor  300  except the oxide semiconductor layer  408   c ; thus the description of the transistor  300  can be referred to. 
     By including the oxide semiconductor layer  408   c  between the gate insulating layer  404  and the oxide semiconductor layer  408   b  in which a channel is formed, the transistor  310  has a structure in which carriers flow through a region which is apart from the gate insulating layer  404  (what is called a buried channel). Accordingly, an interface between the gate insulating layer  404  and the channel can be stabilized and formation of a trap level at the interface can be prevented. Thus, degradation, in particular, photodegradation such as negative-bias temperature stress photodegradation of the transistor is prevented, so that the transistor can have high reliability. 
     In a metal oxide containing indium and gallium, as the ratio of the gallium content to the content of the other metal element becomes higher, the energy gap of the metal oxide is increased. In addition, owing to a difference in band gap between the oxide semiconductor layer  408   c  and the oxide semiconductor layer  408   b , a conduction band offset is formed. Accordingly, it is preferable to use, as the oxide semiconductor layer  408   c , a metal oxide in which the indium content is lower than or equal to the gallium content because the channel can be effectively formed in the oxide semiconductor layer  408   b.    
     In the case where an In—Ga—Zn-based oxide semiconductor is used as the oxide semiconductor layer  408   c , an In—Ga—Zn-based oxide having any of atomic ratios of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=1:3:2 (=1/6:3/6:2/6), In:Ga:Zn=2:4:3 (=2/9:4/9:3/9), or In:Ga:Zn=1:5:3 (=1/9:5/9:3/9); or an oxide having an atomic ratio which is close to any of the above atomic ratios can be used. Note it is preferable to use a metal oxide in which the indium content is lower than or equal to the gallium content for both the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   c . Here, the composition of the oxide semiconductor layer  408   a  may be either different from or the same as the composition of the oxide semiconductor layer  408   c.    
     In general, an oxide semiconductor layer is mostly deposited by a sputtering method. On the other hand, when the oxide semiconductor layer is formed by sputtering, in some cases, an ionized rare gas element (e.g., argon) or a particle ejected from a surface of a sputtering target flicks off a particle of a film, such as a gate insulating film, on which the oxide semiconductor layer is to be formed. The particle flicked off from the film on which the oxide semiconductor layer is to be formed might enter the oxide semiconductor layer and functions as an impurity element therein. In particular, a portion of the oxide semiconductor layer, which is in the vicinity of the surface on which the oxide semiconductor layer is formed, might have high concentration of the impurity element. Further, when the impurity element remains in the vicinity of the surface where the oxide semiconductor layer is to be formed, the resistance of the oxide semiconductor layer is increased, which causes the electrical characteristics of the transistor to be lowered. 
     However, in the transistor  310  illustrated in  FIGS. 2A to 2C , since the oxide semiconductor layer  408   c  is provided between the gate insulating layer  404  and the oxide semiconductor layer  408   b  in which the channel is formed, a constituent element of the gate insulating layer  404  can be prevented from diffusing to the channel. That is, the oxide semiconductor layer  408   c  may contain the constituent element (e.g., silicon) of the gate insulating layer  404  as an impurity. By including the oxide semiconductor layer  408   c , the transistor  310  can have more stabilized electrical characteristics; thus, a highly reliable semiconductor device can be provided. 
       FIG. 2D  is an energy band diagram in the thickness direction (D-D′) in  FIG. 2B . In this embodiment, the oxide semiconductor layers  408   a  to  408   c  are formed using In—Ga—Zn-based oxide semiconductors. Metal oxides in which the indium content is lower than or equal to the gallium content are used for the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   c , and a metal oxide in which the indium content is higher than the gallium content is used for the oxide semiconductor layer  408   b . Accordingly, the oxide semiconductor stack  408  described in this embodiment can have what is called a buried channel as shown in the energy band of  FIG. 2D . 
     An example of a method for manufacturing the transistor  310  is described below with reference to  FIGS. 3A to 3D . 
     First, the gate electrode layer  402  (including a wiring formed with the same layer) is formed over the substrate  400  having an insulating surface. 
     There is no particular limitation on the substrate that can be used as the substrate  400  having an insulating surface as long as it has heat resistance high enough to withstand heat treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like can be used. Furthermore, any of these substrates provided with a semiconductor element may be used as the substrate  400 . Further alternatively, a base insulating layer may be formed over the substrate  400 . 
     The gate electrode layer  402  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium, or an alloy material containing any of these materials as a main component. Alternatively, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used as the gate electrode layer  402 . The gate electrode layer  402  may have a single-layer structure or a stacked-layer structure. The gate electrode layer  402  may have a tapered shape with a taper angle of greater than or equal to 15° and less than or equal to 70° for example. Here, the taper angle refers to an angle formed between a side surface of a layer having a tapered shape and a bottom surface of the layer. 
     The material of the gate electrode layer  402  may be a conductive material such as indium oxide-tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium oxide-zinc oxide, or indium tin oxide to which silicon oxide is added. 
     Alternatively, as the material of the gate electrode layer  402 , an In—Ga—Zn-based oxide containing nitrogen, an In—Sn-based oxide containing nitrogen, an In—Ga-based oxide containing nitrogen, an In—Zn-based oxide containing nitrogen, an Sn-based oxide containing nitrogen, an In-based oxide containing nitrogen, or a metal nitride film (such as an indium nitride film, a zinc nitride film, a tantalum nitride film, or a tungsten nitride film) may be used. These materials have a work function of 5 eV or more. Therefore, when the gate electrode layer  402  is formed using any of these materials, the threshold voltage of the transistor can be positive, so that the transistor can be a normally-off switching transistor. 
     Next, the gate insulating layer  404  is formed so as to cover the gate electrode layer  402  (see  FIG. 3A ). As the gate insulating layer  404 , a single layer or a stack of layers including at least one of the following films formed by a plasma CVD method, a sputtering method, or the like is used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, 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, and a neodymium oxide film. 
     Note that it is preferable that a region which is included in the gate insulating layer  404  and is in contact with the oxide semiconductor stack  408  formed later be formed using an oxide insulating layer and it is further preferable that the region include an oxygen-excess region. In order to provide the oxygen-excess region in the gate insulating layer  404 , the gate insulating layer  404  may be formed in an oxygen atmosphere, for example. Alternatively, oxygen may be introduced into the deposited gate insulating layer  404  to provide the oxygen-excess region. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be used. 
     Next, an oxide semiconductor film to be the oxide semiconductor layer  408   c , an oxide semiconductor film to be the oxide semiconductor layer  408   b , and an oxide semiconductor film to be the oxide semiconductor layer  408   a  are deposited in this order over the gate insulating layer  404  and processed into an island shape by etching treatment using a photolithography method to form the oxide semiconductor stack  408  (see  FIG. 3B ). 
     The oxide semiconductor layer  408   c , the oxide semiconductor layer  408   b , and the oxide semiconductor layer  408   a  may be each an amorphous oxide semiconductor or a crystalline oxide semiconductor. However, it is preferable that the oxide semiconductor layer  408   b  functioning as the channel of the transistor  310  be a crystalline oxide semiconductor. The crystalline oxide semiconductor may be obtained by performing heat treatment on an amorphous oxide semiconductor. The heat treatment for crystallizing the amorphous oxide semiconductor is performed at a temperature higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 400° C., further preferably higher than or equal to 500° C., still further preferably higher than or equal to 550° C. Note that the heat treatment can also serve as another heat treatment in the manufacturing process. 
     The oxide semiconductor films each can be formed by a sputtering method, a molecular beam epitaxy (MBE) method, a CVD method, a pulse laser deposition method, an atomic layer deposition (ALD) method, or the like as appropriate. 
     In the formation of the oxide semiconductor films, the hydrogen concentration in the oxide semiconductor films is preferably reduced as much as possible. In order to reduce the hydrogen concentration, for example, in the case where a sputtering method is used for the deposition, a high-purity rare gas (typically, argon) from which impurities such as hydrogen, water, a hydroxyl group, or a hydride have been removed; oxygen; or a mixed gas of oxygen and the rare gas is used as appropriate as an atmosphere gas supplied to a deposition chamber of a sputtering apparatus. 
     The oxide semiconductor films are formed in such a manner that a sputtering gas from which hydrogen and moisture have been removed is introduced into the deposition chamber while moisture remaining therein is removed, whereby the hydrogen concentration in the formed oxide semiconductor films can be reduced. In order to remove the residual moisture in the deposition chamber, an entrapment vacuum pump, for example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo molecular pump provided with a cold trap. A cryopump has a high capability in removing a hydrogen molecule, a compound containing a hydrogen atom such as water (H 2 O) (preferably, also a compound containing a carbon atom), and the like; therefore, the impurity concentration in the oxide semiconductor film formed in the deposition chamber which is evacuated using a cryopump can be reduced. 
     The gate insulating layer  404  and the oxide semiconductor films are preferably formed in succession without exposure to the air. By forming the gate insulating layer  404  and the oxide semiconductor films in succession without exposure to the air, attachment of hydrogen or a hydrogen compound (e.g., adsorption water) onto surfaces of the oxide semiconductor films can be prevented, and thus mixing of an impurity can be prevented. 
     Further, in the case where the oxide semiconductor film is formed by a sputtering method, the relative density (the fill rate) of a metal oxide target which is used for forming the oxide semiconductor film is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. With the use of a metal oxide target with a high relative density, a dense oxide semiconductor film can be deposited. 
     Note that formation of the oxide semiconductor film while the substrate  400  is kept at high temperature is also effective in reducing the impurity concentration in the oxide semiconductor film. The temperature at which the substrate  400  is heated may be higher than or equal to 150° C. and lower than or equal to 450° C.; the substrate temperature is preferably higher than or equal to 200° C. and lower than or equal to 350° C. By heating the substrate at high temperature during the film formation, a crystalline oxide semiconductor film can be formed. 
     In the case where a CAAC-OS film is employed as the oxide semiconductor layer (e.g., the oxide semiconductor layer  408   b ), any of the following methods can be used for forming the CAAC-OS film, for example. One of the methods is to form an oxide semiconductor film at a deposition temperature higher than or equal to 200° C. and lower than or equal to 450° C., thereby obtaining c-axis alignment substantially perpendicular to a surface. Another method is to form a thin oxide semiconductor film and then subject the film to heat treatment at a temperature higher than or equal to 200° C. and lower than or equal to 700° C., thereby obtaining c-axis alignment substantially perpendicular to a surface. The other method is to form a first thin oxide semiconductor film, heat treatment is performed on the film at a temperature higher than or equal to 200° C. and lower than or equal to 700° C., and then form a second oxide semiconductor film, thereby obtaining c-axis alignment substantially perpendicular to a surface. 
     Oxide semiconductors used for the oxide semiconductor layers  408   a  to  408   c  contain at least indium (In). In particular, indium and zinc (Zn) are preferably contained. As a stabilizer for reducing variation in electric characteristics of a transistor using the oxide semiconductor, gallium (Ga) is preferably additionally contained. 
     Further, as described above, it is preferable that the oxide semiconductor layer  408   a  and the oxide semiconductor layer  408   c  functioning buffer layers for reducing influence of a trap level be each formed using a metal oxide in which the indium content is lower than or equal to the gallium content, and the oxide semiconductor layer  408   b  serving as a channel formation region be formed using a metal oxide in which the indium content is higher than the gallium content. 
     As a stabilizer, one or more of tin (Sn), hafnium (Hf), aluminum (Al), and zirconium (Zr) may be contained instead of gallium (Ga) or in addition to gallium (Ga). As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may be contained. 
     As the oxide semiconductor, for example, any of the following can be used: a single-component metal oxide such as indium oxide, tin oxide, or zinc oxide; a two-component metal oxide such as an In—Zn-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide. 
     For example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn. 
     Alternatively, a material represented by InMO 3 (ZnO) m  (m&gt;0 is satisfied, and m is not an integer) may be used as an oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, as the oxide semiconductor, a material expressed by a chemical formula, In 2 SnO 5 (ZnO) n  (n&gt;0, n is a natural number) may be used. 
     Further, the oxide semiconductor stack  408  is preferably subjected to heat treatment for removing excess hydrogen (including water and a hydroxyl group) contained in the films (for dehydration or dehydrogenation). The temperature of the heat treatment is higher than or equal to 300° C. and lower than or equal to 700° C., or lower than the strain point of the substrate. The heat treatment can be performed under reduced pressure, a nitrogen atmosphere, or the like. Hydrogen, which is an impurity imparting n-type conductivity, can be removed by the heat treatment. 
     Note that the heat treatment for the dehydration or dehydrogenation may be performed at any timing in the manufacturing process of the transistor as long as it is performed after the formation of the oxide semiconductor films. The heat treatment for dehydration or dehydrogenation may be performed a plurality of times, and may also serve as another heat treatment. 
     In the heat treatment, it is preferable that water, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is set to preferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, further preferably 0.1 ppm or lower). 
     In addition, after the oxide semiconductor layers (or the oxide semiconductor stack) are heated by the heat treatment, a high-purity oxygen gas, a high-purity dinitrogen gas, or ultra dry air (the moisture amount is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, further preferably less than or equal to 10 ppb, in the measurement with use of a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) may be introduced into the same furnace while the heating temperature is being maintained or being gradually decreased. It is preferable that water, hydrogen, or the like be not contained in the oxygen gas or the dinitrogen monoxide gas. The purity of the oxygen gas or the dinitrogen monoxide gas which is introduced into the heat treatment apparatus is preferably 6N or more, further preferably 7N or more (i.e., the impurity concentration in the oxygen gas or the dinitrogen monoxide gas is preferably 1 ppm or lower, further preferably 0.1 ppm or lower). The oxygen gas or the dinitrogen monoxide gas acts to supply oxygen that is a main component of the oxide semiconductor and that has been reduced by the step of removing an impurity for the dehydration or dehydrogenation, so that the oxide semiconductor layers can have high purity and be i-type (intrinsic) oxide semiconductor layers. 
     Since there is a possibility that oxygen is also released and reduced by dehydration or dehydrogenation treatment, oxygen (including at least one of an oxygen radical, an oxygen atom, and an oxygen ion) may be introduced into the oxide semiconductor layers which have been subjected to the dehydration or dehydrogenation treatment to supply oxygen to the layers. 
     Introduction (supply) of oxygen into the dehydrated or dehydrogenated oxide semiconductor layers enables the oxide semiconductor layers to be highly purified and to be i-type (intrinsic). Variation in electric characteristics of a transistor having the highly-purified and i-type (intrinsic) oxide semiconductor is suppressed, and the transistor is electrically stable. 
     In the case where oxygen is introduced into the oxide semiconductor layers, oxygen may be directly introduced into the oxide semiconductor layers or may be introduced into the oxide semiconductor layers through an insulating layer formed in a later step. As the method for adding oxygen (containing at least one of an oxygen radical, an oxygen atom, and an oxygen ion), an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be used. A gas containing oxygen can be used for oxygen introducing treatment. As the gas containing oxygen, oxygen, dinitrogen monoxide, nitrogen dioxide, carbon dioxide, carbon monoxide, and the like can be used. Further, a rare gas may be contained in the gas containing oxygen in the oxygen introducing treatment. 
     For example, in the case where an oxygen ion is introduced into the oxide semiconductor layers by an ion implantation method, the dose can be greater than or equal to 1×10 13  ions/cm 2  and less than or equal to 5×10 16  ions/cm 2 . 
     The timing of supply of oxygen to the oxide semiconductor layers is not particularly limited to the above as long as it is after the formation of the oxide semiconductor films. Oxygen may be introduced into the oxide semiconductor layers plural times. 
     Then, a conductive film is formed over the oxide semiconductor stack  408  and processed to form the source electrode layer  410   a  and the drain electrode layer  410   b  (including a wiring formed with the same layer) (see  FIG. 3C ). 
     For the source electrode layer  410   a  and the drain electrode layer  410   b , a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, a metal nitride film containing any of the above elements as a component (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like can be used, for example. Alternatively, a film of a high-melting-point metal such as Ti, Mo, or W or a metal nitride film of any of these elements (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) may be stacked on one of or both a bottom side and a top side of a metal film of Al, Cu, or the like. Alternatively, the source electrode layer  410   a  and the drain electrode layer  410   b  may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), indium oxide-tin oxide (In 2 O 3 —SnO 2 ), indium oxide-zinc oxide (In 2 O 3 —ZnO), or any of these metal oxide materials in which silicon oxide is contained can be used. 
     For the source electrode layer  410   a  and the drain electrode layer  410   b , a metal nitride film such as an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, or an In—O film containing nitrogen can be used. These films contain the same constituent elements as the oxide semiconductor stack  408  and can therefore stabilize the interface with the oxide semiconductor stack  408 . 
     Note that it is preferable to use a conductive film containing copper for the source electrode layer  410   a  and the drain electrode layer  410   b  because wiring resistance can be reduced. In general, copper easily diffuses into a semiconductor or a silicon oxide film, and thus the operation of a semiconductor device might be unstable and yield might be significantly reduced. However, in the transistor  310 , since the oxide semiconductor layer  408   a  serving as a buffer layer for preventing the constituent element (that is copper here) of the electrode layers is provided in contact with the source electrode layer  410   a  and the drain electrode layer  410   b , influence of the trap level on the back channel side which is generated owing to diffusion of copper can be reduced, preferably can be eliminated. 
     Next, the insulating layer  412  is formed to cover the source electrode layer  410   a , the drain electrode layer  410   b , and the exposed oxide semiconductor stack  408  (see  FIG. 3D ). 
     The insulating layer  412  can be formed using a single layer or a stack of layers of one or more of the following films formed by a plasma CVD method or a sputtering method: a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, a silicon nitride oxide film, and the like. However, it is preferable that an oxide insulating layer be formed as the insulating layer  412  because the oxide insulating layer can supply oxygen to the oxide semiconductor stack  408 . 
     Heat treatment may be performed after the insulating layer  412  is formed. A temperature of the heat treatment is preferably higher than or equal to 200° C., and for example, 220° C. 
     Through the above steps, the transistor  310  of this embodiment can be formed. 
     The transistor of this embodiment includes the first oxide semiconductor layer which serves as a buffer layer for preventing diffusion of the constituent element of the source electrode layer and the drain electrode layer into the channel and the second oxide semiconductor layer which serves as the channel. With such a structure, influence of an interface state which can be formed on the back channel side of the transistor can be reduced. Further, it is preferable that the transistor described in this embodiment further include the third oxide semiconductor layer which is provided between the oxide semiconductor layer serving as the channel and the gate insulating layer and serves as a buffer layer for preventing degradation of an interface on the gate insulating layer side of the channel. When the third oxide semiconductor layer is provided in the transistor, photodegradation (e.g., negative-bias temperature stress photodegradation) of the transistor can be reduced, so that a highly reliable semiconductor device can be provided. 
     The methods and structures described in this embodiment can be combined as appropriate with any of the methods and structures described in the other embodiments. 
     Embodiment 2 
     In this embodiment, an embodiment of a semiconductor device which is different from that in Embodiment 1 will be described with reference to  FIGS. 10A and 10B . Specifically, transistors each including a gate insulating layer with a structure which is different from that of the transistor of Embodiment 1 are described. 
     A structural example of a transistor  320  is illustrated in  FIG. 10A . In a manner similar to the transistor  310  in  FIGS. 2A to 2D , the transistor  320  in  FIG. 10A  includes the gate electrode layer  402  provided over the substrate  400  having an insulating surface, the gate insulating layer  403  and  406  over the gate electrode layer  402 , the oxide semiconductor stack  408  which is in contact with the gate insulating layer  406  and overlaps with the gate electrode layer  402 , and the source electrode layer  410   a  and the drain electrode layer  410   b  which are electrically connected to the oxide semiconductor stack  408 . In the transistor  320 , the oxide semiconductor stack  408  includes the oxide semiconductor layer  408   c  in contact with the gate insulating layer  406 , the oxide semiconductor layer  408   b  on and in contact with the oxide semiconductor layer  408   c , and the oxide semiconductor layer  408   a  on and in contact with the oxide semiconductor layer  408   b  and in contact with the source electrode layer  410   a  and the drain electrode layer  410   b . Further, the insulating layer  412  which covers the source electrode layer  410   a  and the drain electrode layer  410   b  and is in contact with the oxide semiconductor stack  408  may be included as a component of the transistor  320 . 
     The transistor  320  is different from the transistor  310  in that the gate insulating layer has a structure in which a first gate insulating layer  403  which includes gate insulating layers  403   a ,  403   b , and  403   c , and a second gate insulating layer  406  are stacked from the gate electrode layer  402  side. 
     Note that the transistor  320  has the same structure as the transistor  310  except the gate insulating layer; therefore, the description of the transistor  310  can be referred to. 
     In the transistor  320 , a nitrogen-containing silicon film is applied to the first gate insulating layer  403 . A nitrogen-containing silicon film has higher relative permittivity than a silicon oxide film and needs to have a larger thickness than a silicon oxide film to obtain the same electrostatic capacity; thus, it is possible to physically increase the thickness of the gate insulating layer. Accordingly, a reduction in withstand voltage of the transistor  320  is prevented and further the withstand voltage is improved, so that electrostatic breakdown of the semiconductor device can be prevented. 
     Further, as the second gate insulating layer  406  in contact with the oxide semiconductor stack  408 , an insulating layer containing oxygen, such as a silicon oxide film, a gallium oxide film, or an aluminum oxide film, is used. It is further preferable that the second gate insulating layer  406  include a region which contains oxygen in a proportion higher than that of oxygen in the stoichiometric composition (i.e., an oxygen-excess region). This is because when the insulating layer in contact with the oxide semiconductor stack  408  includes an oxygen-excess region, oxygen can be supplied to the oxide semiconductor stack  408  and thus oxygen can be prevented from being released from the oxide semiconductor stack  408 , and oxygen vacancies in the oxide semiconductor stack  408  can be filled. In order to provide the oxygen-excess region in the second gate insulating layer  406 , the second gate insulating layer  406  is formed in an oxygen atmosphere, for example. Alternatively, oxygen may be introduced into the deposited second gate insulating layer  406  to provide the oxygen-excess region therein. 
     A silicon nitride film, a silicon nitride oxide film, and a silicon oxynitride film can be given as examples of the nitrogen-containing silicon film which is applied to the first gate insulating layer  403 . However, as the ratio of the nitrogen content to the oxygen content becomes higher, relative permittivity is increased and thus a silicon nitride film is preferably used. Further, the energy gap of silicon nitride is as small as 5.5 eV whereas the energy gap of silicon oxide is 8 eV, and the specific resistance of silicon nitride is small accordingly; therefore, use of a silicon nitride film can lead to high electrostatic discharge (ESD) resistance. Note that in this specification, a “silicon oxynitride film” refers to a film that includes more oxygen than nitrogen, and a “silicon nitride oxide film” refers to a film that includes more nitrogen than oxygen. 
     The gate insulating layer  403   a  in contact with the gate electrode layer  402  is a silicon film which contains a smaller amount of ammonia than at least the gate insulating layer  403   b . Ammonia serves as a ligand of a metal complex owing to a function of a lone electron-pair on a nitrogen atom. Therefore, for example, in the case where copper is used for the gate electrode layer  402 , when a gate insulating layer containing a large amount of ammonia is provided in contact with the gate electrode layer, the copper might diffuse into the gate insulating layer owing to reaction represented by Formula (1) described below.
 
[Formula 1]
 
Cu 2+ +4NH 3   [Cu(NH 3 ) 4 ] 2+   (1)
 
     When the gate insulating layer  403   a  containing a small amount of ammonia (a smaller amount ammonia than at least the gate insulating layer  403   b ) is provided in contact with the gate electrode layer  402  in the transistor  320 , a material (e.g., copper) of the gate electrode layer  402  can be prevented from diffusing into the first gate insulating layer  403 . In other words, the gate insulating layer  403   a  can function as a barrier film against a metal material included in the gate electrode layer  402 . By including the gate insulating layer  403   a , the transistor can have higher reliability. 
     As the gate insulating layer  403   b , a nitrogen-containing silicon film which has a larger thickness than the gate insulating layer  403   a  and in which the number of defects is reduced is used. For example, the gate insulating layer  403   b  has a thickness greater than or equal to 300 nm and less than or equal to 400 nm. Further, in the nitrogen-containing silicon film to be used as the gate insulating layer  403   b , the spin density of a signal which appears in the Nc center (the g value is 2.003) of signals measured by electron spin resonance (ESR) is preferably less than or equal to 1×10 17  spins/cm 3  and further preferably less than or equal to 5×10 16  spins/cm 3 . As described above, by providing a nitrogen-containing silicon film having a large thickness (e.g., 300 nm or more) in which the number of defects is reduced, it is possible that the gate insulating layer  403   b  has an ESD resistance of 300 V or more. 
     As the gate insulating layer  403   c , a nitrogen-containing silicon film in which the hydrogen concentration is reduced is used. The hydrogen concentration of the gate insulating layer  403   c  is lower than at least that of the gate insulating layer  403   b . For example, in the case where the gate insulating layer  403   c  is deposited by a plasma CVD method, the hydrogen concentration contained in a supply gas is reduced to lower than the hydrogen concentration of a supply gas used for depositing the gate insulating layer  403   b , whereby the hydrogen concentration of the gate insulating layer  403   c  can be lower than that of gate insulating layer  403   b . Specifically, in the case where silicon nitride films are formed as the gate insulating layer  403   b  and the gate insulating layer  403   c , the gate insulating layer  403   c  may be deposited using a supply gas containing a smaller amount of ammonia than a supply gas for depositing the gate insulating layer  403   b  or may be deposited without using ammonia. 
     When a silicon nitride film in which the hydrogen concentration is reduced is provided as the gate insulating layer  403   c , the amount of hydrogen or a hydrogen compound (e.g., water) which is mixed into the second gate insulating layer  406  and the oxide semiconductor stack  408  can be reduced. Part of hydrogen serves as a donor by being bonded to an oxide semiconductor and electrons serving as carriers are generated, which causes the threshold voltage of the transistor to be changed (shifted) in the negative direction. Accordingly, by including a silicon nitride film in which the hydrogen concentration is reduced as the gate insulating layer  403   c , the transistor can have stabilized electrical characteristics. Further, by being provided as the gate insulating layer  403   c , the silicon nitride film in which the hydrogen concentration is reduced has an effect as a barrier film for preventing impurities such as hydrogen and a hydrogen compound included in the gate insulating layer  403   b  from diffusing into the oxide semiconductor stack  408 . 
     In this embodiment, silicon nitride films are used as the gate insulating layer  403   a , the gate insulating layer  403   b , and the gate insulating layer  403   c  which are included in the first gate insulating layer  403 ; and a silicon oxynitride film is used as the second gate insulating layer  406 . The gate insulating layers are formed successively by a plasma CVD method. Specifically, a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) is supplied and a silicon nitride film to be the gate insulating layer  403   a  is deposited; the supply gas is switched to a mixed gas of silane (SiH 4 ), nitrogen (N 2 ), and ammonia (NH 3 ) and a silicon nitride film to be the gate insulating layer  403   b  is deposited; then the supply gas is switched to a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) and a silicon nitride film to be the gate insulating layer  403   c  is deposited; and then the supply gas is switched to a mixed gas of silane (SiH 4 ) and dinitrogen monoxide (N 2 O) and a silicon oxynitride film to be the second gate insulating layer  406  is deposited. 
     The gate insulating layer  403   a  preferably has a thickness greater than or equal to 30 nm and less than or equal to 100 nm and further preferably has a thickness greater than or equal to 30 nm and less than or equal to50 nm. The gate insulating layer  403   b , which is provided as a countermeasure against electrostatic breakdown of the transistor, preferably has a thickness greater than or equal to 300 nm and less than or equal to 400 nm. The gate insulating layer  403   c , which functions as a barrier film for preventing diffusion of hydrogen into the oxide semiconductor stack  408 , preferably has a thickness greater than or equal to 25 nm and less than or equal to 150 nm. The second gate insulating layer  406  preferably has a thickness greater than or equal to 25 nm and less than or equal to 100 nm. Note that the thickness of each of the gate insulating layers is preferably adjusted so that the total of the thickness of the first gate insulating layer  403  (the total thickness of the gate insulating layer  403   a , the gate insulating layer  403   b , and the gate insulating layer  403   c ) and the thickness of the second gate insulating layer  406  is greater than or equal to 355 nm and less than or equal to 550 nm. 
     Note that as in a transistor  330  illustrated in  FIG. 10B , the gate insulating layer formed of a stack of the first gate insulating layer  403  and the second gate insulating layer  406  and the oxide semiconductor stack  408  formed of a stack of the oxide semiconductor layer  408   b  and the oxide semiconductor layer  408   a  can be used in combination. 
     The transistor of this embodiment includes the first oxide semiconductor layer which serves as a buffer layer for preventing diffusion of the constituent element of the source electrode layer and the drain electrode layer into the channel and the second oxide semiconductor layer which serves as the channel. With such a structure, influence of an interface state which can be formed on the back channel side of the transistor can be reduced. Further, it is preferable that the transistor described in this embodiment further include the third oxide semiconductor layer which is provided between the oxide semiconductor layer serving as the channel and the gate insulating layer and serves as a buffer layer for preventing degradation of an interface on the gate insulating layer side of the channel. When the third oxide semiconductor layer is provided in the transistor, photodegradation (e.g., negative-bias temperature stress photodegradation) of the transistor can be reduced, so that a highly reliable semiconductor device can be provided. 
     Further, the transistor of this embodiment has, as the gate insulating layer, a stacked structure which is formed of a first gate insulating layer including a nitrogen-containing silicon film functioning as a barrier film against a constituent element (e.g., copper) of a gate electrode layer, a nitrogen-containing silicon film having a large thickness (e.g., 300 nm) in which the number of defects is reduced, and a nitrogen-containing silicon film having a blocking property against hydrogen; and a second gate insulating layer containing oxygen. Accordingly, in the transistor of this embodiment, variations in the electrical characteristics and electrostatic breakdown are prevented. By including such a transistor, a semiconductor device can have high reliability, and the semiconductor device can be provided in a high yield. 
     The methods and structures described in this embodiment can be combined as appropriate with any of the methods and structures described in the other embodiments. 
     Embodiment 3 
     A semiconductor device having a display function (also referred to as a display device) can be manufactured using the transistor described in Embodiment 1 or 2. Further, part or all of the driver circuitry which includes the transistor can be formed over a substrate where a pixel portion is formed, whereby a system-on-panel can be formed. 
     In  FIG. 4A , a sealant  4005  is provided so as to surround a pixel portion  4002  provided over a substrate  4001 , and the pixel portion  4002  is sealed with a substrate  4006 . In  FIG. 4A , a scan line driver circuit  4004  and a signal line driver circuit  4003  which are each formed using a single crystal semiconductor film or a polycrystalline semiconductor film over an IC chip or a substrate separately prepared are mounted on the substrate  4001 , in a region that is different from the region surrounded by the sealant  4005 . Various signals and potentials which are provided to the pixel portion  4002  through the signal line driver circuit  4003  and the scan line driver circuit  4004  are supplied from flexible printed circuits (FPCs)  4018   a  and  4018   b.    
     In  FIGS. 4B and 4C , the sealant  4005  is provided to surround the pixel portion  4002  and the scan line driver circuit  4004  which are provided over the substrate  4001 . The substrate  4006  is provided over the pixel portion  4002  and the scan line driver circuit  4004 . Consequently, the pixel portion  4002  and the scan line driver circuit  4004  are sealed together with a display element by the substrate  4001 , the sealant  4005 , and the substrate  4006 . In  FIGS. 4B and 4C , the signal line driver circuit  4003  which is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over an IC chip or a substrate separately prepared is mounted on the substrate  4001 , in a region that is different from the region surrounded by the sealant  4005 . In  FIGS. 4B and 4C , various signals and potentials which are provided to the pixel portion  4002  through the signal line driver circuit  4003  and the scan line driver circuit  4004  are supplied from an FPC  4018 . 
     Although  FIGS. 4B and 4C  each illustrate an example in which the signal line driver circuit  4003  is formed separately and mounted on the substrate  4001 , one embodiment of the present invention is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted. 
     Note that a connection method of a separately formed driver circuit is not particularly limited, and a chip on glass (COG) method, a wire bonding method, a tape automated bonding (TAB) method or the like can be used.  FIG. 4A  shows an example in which the signal line driver circuit  4003  and the scan line driver circuit  4004  are mounted by a COG method.  FIG. 4B  shows an example in which the signal line driver circuit  4003  is mounted by a COG method.  FIG. 4C  shows an example in which the signal line driver circuit  4003  is mounted by a TAB method. 
     Note that the display device includes a panel in which the display element is sealed, and a module in which an IC including a controller or the like is mounted on the panel. Specifically, a display device in this specification means an image display device, a display device, or a light source (including a lighting device). Furthermore, the display device also includes the following modules in its category: a module to which a connector such as an FPC or a TCP is attached; a module having a TCP at the tip of which a printed wiring board is provided; and a module in which an integrated circuit (IC) is directly mounted on a display element by a COG method. 
     The pixel portion and the scan line driver circuit provided over the substrate include a plurality of transistors, and the transistor described in Embodiment 1 or 2 can be applied thereto. 
     As the display element provided in the display device, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. The light-emitting element includes an element with luminance which is controlled by current or voltage, in its category, and specifically includes an inorganic electroluminescent (EL) element, an organic EL element, and the like. Furthermore, a display medium whose contrast is changed by an electric effect, such as an electronic ink display device (electronic paper), can be used. 
     Embodiments of the semiconductor device are described with reference to  FIGS. 4A to 4C  and  FIG. 5 .  FIG. 5  corresponds to a cross-sectional view along a line M-N in  FIG. 4B . An example of a liquid crystal display device using a liquid crystal element as a display element is illustrated in  FIG. 5 . 
     Note that there is no limitation on the display device as long as the transistor  4010  provided in the pixel portion  4002  is electrically connected to the display element and the display element can perform display, and a variety of display elements can be used. 
     As illustrated in  FIGS. 4A to 4C  and  FIG. 5 , the semiconductor device includes a connection terminal electrode  4015  and a terminal electrode  4016 , and the connection terminal electrode  4015  and the terminal electrode  4016  are electrically connected to a terminal included in the FPC  4018  ( 4018   a  or  4018   b ) through an anisotropic conductive layer  4019 . 
     The connection terminal electrode  4015  is formed from the same conductive layer as a first electrode layer  4034 . The terminal electrode  4016  is formed from the same conductive layer as a source electrode layer and a drain electrode layer of the transistor  4010  and a transistor  4011 . 
     The pixel portion  4002  and the scan line driver circuit  4004  provided over the substrate  4001  include a plurality of transistors.  FIGS. 4A to 4C  and  FIG. 5  illustrate the transistor  4010  included in the pixel portion  4002  and the transistor  4011  included in the scan line driver circuit  4004 . In  FIG. 5 , an insulating layer  4032  is provided over the transistors  4010  and  4011 . 
     The transistor described in Embodiment 1 or 2 can be applied to the transistor  4010  and the transistor  4011 . In this embodiment, an example in which a transistor having a structure similar to that of the transistor  310  described in Embodiment 1 is used is described. The transistors  4010  and  4011  are bottom-gate transistors. 
     Each of the transistors  4010  and  4011  includes a first oxide semiconductor layer which functions as a buffer layer for preventing diffusion of a constituent element of the source and drain electrode layers in to a channel, a second oxide semiconductor layer which functions as the channel, and a third oxide semiconductor layer which is provided between a gate insulating layer and the oxide semiconductor layer functioning as the channel and functions as a buffer layer for preventing degradation of the interface on the gate insulating layer side of the channel. Accordingly, the transistors  4010  and  4011  are each a highly reliable transistor in which influence of an interface state which can be formed on the back channel side is reduced and photodegradation (e.g., negative-bias temperature stress photodegradation) is reduced. 
     Moreover, a conductive layer may be provided so as to overlap with a channel formation region of the oxide semiconductor layer of the transistor  4011  for the driver circuit. When the conductive layer is provided so as to overlap with the channel formation region of the oxide semiconductor layer, the amount of change in the threshold voltage of the transistor  4011  can be further reduced. The conductive layer may have the same potential as or a potential different from that of a gate electrode layer of the transistor  4011 , and can function as a second gate electrode layer. Alternatively, the potential of the conductive layer may be in a floating state. 
     The conductive layer also functions to block an external electric field, that is, to prevent an external electric field (particularly, to prevent static electricity) from effecting the inside (a circuit portion including a transistor). A blocking function of the conductive layer can prevent the variation in electrical characteristics of the transistor due to the effect of external electric field such as static electricity. 
     In  FIG. 5 , a liquid crystal element  4013  includes a first electrode layer  4034 , a second electrode layer  4031 , and a liquid crystal layer  4008 . Note that insulating layers  4033  and  4038  functioning as alignment films are provided so that the liquid crystal layer  4008  is positioned therebetween. The second electrode layer  4031  is provided on the substrate  4006  side, and the first electrode layer  4034  and the second electrode layer  4031  are stacked with the liquid crystal layer  4008  provided therebetween. 
     The first electrode layer  4034  and the second electrode layer  4031  can be formed using a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide, indium zinc oxide, indium tin oxide to which silicon oxide is added, or graphene. 
     Alternatively, the first electrode layer  4034  and the second electrode layer  4031  can be formed using one or more materials selected from metals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag); an alloy of any of these metals; and a nitride of any of these metals. 
     A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) can be used for the first electrode layer  4034  and the second electrode layer  4031 . As the conductive high molecule, what is called a π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof can be given. 
     A spacer  4035  is a columnar spacer obtained by selective etching of an insulating layer and is provided in order to control the thickness of the liquid crystal layer  4008  (a cell gap). Alternatively, a spherical spacer may be used. 
     In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. These liquid crystal materials may be a low molecular compound or a high molecular compound. Such a liquid crystal material (liquid crystal composition) exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     Alternatively, a liquid crystal composition exhibiting a blue phase for which an alignment film is unnecessary may be used for the liquid crystal layer  4008 . In this case, the liquid crystal layer  4008  is in contact with the first electrode layer  4034  and the second electrode layer  4031 . A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. The blue phase can be exhibited using a liquid crystal composition which is a mixture of a liquid crystal and a material. In order to increase the temperature range where the blue phase is exhibited, a liquid crystal layer may be formed by adding a polymerizable monomer, a polymerization initiator, and the like to a liquid crystal composition exhibiting a blue phase and by performing polymer stabilization treatment. The liquid crystal composition exhibiting a blue phase has a short response time, and has optical isotropy, which contributes to the exclusion of the alignment process and reduction of viewing angle dependence. In addition, since an alignment film does not need to be provided and rubbing treatment is unnecessary, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device can be reduced in the manufacturing process. Thus, productivity of the liquid crystal display device can be increased. 
     The specific resistivity of the liquid crystal material is greater than or equal to 1×10 9  Ω·cm, preferably greater than or equal to 1×10 11  Ω·cm, more preferably greater than or equal to 1×10 12  Ω·cm. Note that the specific resistance in this specification is measured at 20° C. 
     The size of storage capacitor formed in the liquid crystal display device is set considering the leakage current of the transistor provided in the pixel portion or the like so that charge can be held for a predetermined period. The size of the storage capacitor may be set considering the off-state current of a transistor or the like. By using a transistor including an oxide semiconductor layer, which is disclosed in this specification, it is enough to provide a storage capacitor having a capacitance that is ⅓ or less, preferably ⅕ or less of liquid crystal capacitance of each pixel. 
     In the transistor including an oxide semiconductor layer, which is disclosed in this specification, the current in an off state (off-state current) can be made small. Accordingly, an electric signal such as image data can be held for a longer period and a writing interval can be set longer. Accordingly, the frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption. 
     The transistor including an oxide semiconductor layer, which is disclosed in this specification, can have relatively high field-effect mobility; thus, the driver circuit can operate at high speed. For example, when such a transistor is used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. In addition, by using such a transistor in a pixel portion, a high-quality image can be provided. 
     For the liquid crystal display device, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used. 
     A normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode is preferable. Some examples are given as the vertical alignment mode. For example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an Advanced Super View (ASV) mode, and the like can be used. Furthermore, this embodiment can be applied to a VA liquid crystal display device. The VA liquid crystal display device has a kind of form in which alignment of liquid crystal molecules of a liquid crystal display panel is controlled. In the VA liquid crystal display device, liquid crystal molecules are aligned in a vertical direction with respect to a panel surface when no voltage is applied. Moreover, it is possible to use a method called domain multiplication or multi-domain design, in which a pixel is divided into some regions (subpixels) and molecules are aligned in different directions in their respective regions. 
     In the display device, a black matrix (a light-blocking layer), an optical member (an optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, and the like are provided as appropriate. For example, circular polarization may be obtained by using a polarizing substrate and a retardation substrate. In addition, a backlight, a side light, or the like may be used as a light source. 
     As a display method in the pixel portion, a progressive method, an interlace method or the like can be employed. Further, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, R, G, B, and W (W corresponds to white); R, G, B, and one or more of yellow, cyan, magenta, and the like; or the like can be used. Further, the sizes of display regions may be different between respective dots of color elements. Note that one embodiment of the disclosed invention is not limited to the application to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display. 
       FIGS. 11A to 11C  illustrate examples in which a common connection portion (pad portion) for electrically connection with the second electrode layer  4031  provided on the substrate  4006  in the display device in  FIG. 5  is formed over the substrate  4001 . 
     The common connection portion is provided in a position that overlaps with a sealant for bonding the substrate  4001  and the substrate  4006 , and is electrically connected to the second electrode layer  4031  via conductive particles contained in the sealant. Alternatively, the common connection portion is provided in a position that does not overlap with the sealant (except for the pixel portion) and a paste containing conductive particles is provided separately from the sealant so as to overlap with the common connection portion, whereby the common connection portion is electrically connected to the second electrode layer  4031 . 
       FIG. 11A  is a cross-sectional view of the common connection portion and corresponds to a cross section along G 1 -G 2  in the top view in  FIG. 11B . 
     A common potential line  491  is provided over a gate insulating layer  4020 , and is formed using a material and a process similar to those of the source and drain electrodes of the transistors  4010  and  4011  illustrated in  FIG. 5 . 
     Further, the common potential line  491  is covered with the insulating layer  4032 , and the insulating layer  4032  includes a plurality of opening portions overlapping with the common potential line  491 . The opening portions are formed in the same process as a contact hole for connecting one of the source and drain electrodes of the transistor  4010  and the first electrode layer  4034 . 
     A common electrode  492  is provided over the insulating layer  4032  and formed using a material and a process similar to those of the connection terminal electrode  4015  and the first electrode layer  4034  of the pixel portion. 
     In this manner, the common connection portion can be formed by utilizing the process for manufacturing the switching element of the pixel portion  4002 . 
     Note that the common electrode  492  is an electrode in contact with the conductive particles contained in the sealant, and is electrically connected to the second electrode layer  4031  of the substrate  4006 . 
     As illustrated in  FIG. 11C , the common potential line  491  may be formed using a material and a process similar to those of the gate electrodes of the transistors  4010  and  4011 . 
     In the common connection portion illustrated in  FIG. 11C , the common potential line  491  is provided below the gate insulating layer  4020  and the insulating layer  4032 , and the gate insulating layer  4020  and the insulating layer  4032  have a plurality of opening portions overlapping with the common potential line  491 . The opening portions are formed in such a manner that the insulating layer  4032  is etched in the same process as the contact hole for connecting one of the source and drain electrode layers of the transistor  4010  and the first electrode layer  4034 , and then the gate insulating layer  4020  is selectively etched. 
     Alternatively, as the display element included in the display device, a light-emitting element utilizing electroluminescence can be used. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element. 
     In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element. In this embodiment, an organic EL element is used as a light-emitting element. 
     The inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an example of an organic EL element as a light-emitting element is described here. 
     In order to extract light emitted from the light-emitting element, at least one of the pair of electrodes has a light-transmitting property. The light-emitting element can have a top emission structure in which light emission is extracted through the surface opposite to the substrate; a bottom emission structure in which light emission is extracted through the surface on the substrate side; or a dual emission structure in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side, and a light-emitting element having any of these emission structures can be used. 
       FIGS. 6A and 6B  and  FIG. 13  illustrate an example of a light-emitting device including the light-emitting element as a display element. 
       FIG. 6A  is a plan view of the light-emitting device, and  FIG. 6B  is a cross-sectional view taken along dashed-dotted lines S 1 -T 1 , S 2 -T 2 , and S 3 -T 3  in  FIG. 6A .  FIG. 13  is a cross-sectional view taken along a dashed-dotted S 4 -T 4  in  FIG. 6A . Note that, an electroluminescent layer  542  and a second electrode layer  543  are not illustrated in the plan view in  FIG. 6A . 
     The light-emitting device illustrated in  FIGS. 6A and 6B  includes, over a substrate  500 , a transistor  510 , a capacitor  520 , and a wiring layer intersection  530 . The transistor  510  is electrically connected to a light-emitting element  540 . Note that  FIGS. 6A and 6B  illustrate a bottom-emission light-emitting device in which light from the light-emitting element  540  is extracted through the substrate  500 . 
     The transistor described in Embodiment 1 or 2 can be applied to the transistor  510 . In this embodiment, an example in which a transistor having a structure similar to that of the transistor  330  described in Embodiment 2 is used is described. The transistor  510  is a bottom-gate transistor. 
     The transistor  510  includes gate electrode layers  511   a  and  511   b ; a gate insulating layer  502  including gate insulating layers  502   a ,  502   b , and  502   c ; an oxide semiconductor layer  512  including oxide semiconductor layers  512   b  and  512   a ; and conductive layers  513   a  and  513   b  serving as a source electrode layer and a drain electrode layer. Further, an insulating layer  525  is formed over the transistor  510 . 
     The capacitor  520  includes conductive layers  521   a  and  521   b , the gate insulating layer  502 , an oxide semiconductor stack  522  including oxide semiconductor layers  522   b  and  522   a , and a conductive layer  523 . The gate insulating layer  502  and the oxide semiconductor stack  522  are sandwiched between the conductive layer  523  and the conductive layers  521   a  and  521   b , whereby the capacitor is formed. 
     The wiring layer intersection  530  is an intersection of a conductive layer  533  and the gate electrode layers  511   a  and  511   b . The conductive layer  533  and the gate electrode layers  511   a  and  511   b  intersect with each other with the gate insulating layer  502  provided therebetween. 
     In this embodiment, a 30-nm-thick titanium film is used as each of the gate electrode layer  511   a  and the conductive layer  521   a , and a 200-nm-thick copper thin film is used as each of the gate electrode layer  511   b  and the conductive layer  521   b . Thus, the gate electrode layer has a stacked-layer structure of the titanium film and the copper thin film. 
     The transistor  510  includes the oxide semiconductor layer  512   b  functioning as a channel, and the oxide semiconductor layer  512   a  functioning as a buffer layer for preventing diffusion of a constituent element of the conductive layers  513   a  and  513   b  into the channel. Accordingly, the transistor  510  is a highly reliable transistor in which influence of an interface state which can be formed on the back channel side is reduced. 
     Further, in the transistor  510 , a nitrogen-containing silicon film which functions as a barrier film against copper and whose ammonia content is reduced is included as the gate insulating layer  502   c , a nitrogen-containing silicon film having a large thickness (e.g., 300 nm) in which the number of defects is reduced is included as the gate insulating layer  502   a , and a nitrogen-containing silicon film in which the hydrogen concentration is reduced is included as the gate insulating layer  502   b . With such a structure, the transistor  510  can have favorable electrical characteristics and electrostatic breakdown of the transistor  510  can be prevented. Accordingly, a highly reliable semiconductor device can be provided in a high yield. 
     An interlayer insulating layer  504  is formed over the transistor  510 , the capacitor  520 , and the wiring layer intersection  530 . Over the interlayer insulating layer  504 , a color filter layer  505  is provided in a region overlapping with the light-emitting element  540 . An insulating layer  506  functioning as a planarization insulating layer is provided over the interlayer insulating layer  504  and the color filter layer  505 . 
     The light-emitting element  540  having a stacked-layer structure in which a first electrode layer  541 , the electroluminescent layer  542 , and the second electrode layer  543  are stacked in this order is provided over the insulating layer  506 . The first electrode layer  541  and the conductive layer  513   a  are in contact with each other in an opening formed in the insulating layer  506  and the interlayer insulating layer  504 , which reaches the conductive layer  513   a ; thus the light-emitting element  540  and the transistor  510  are electrically connected to each other. Note that a partition  507  is provided so as to cover part of the first electrode layer  541  and the opening. 
     Further, a 1500-nm-thick photosensitive acrylic film and a 1500-nm-thick photosensitive polyimide film can be used as the insulating layer  506  and the partition  507 , respectively. 
     As the color filter layer  505 , for example, a chromatic light-transmitting resin can be used. As the chromatic color light-transmitting resin, a photosensitive organic resin or a non-photosensitive organic resin can be used. The photosensitive organic resin is preferably used, in which case the number of resist masks can be reduced, which results in the simplification of the process. 
     Chromatic colors are all colors except achromatic colors such as black, gray, and white. The color filter layer is formed using a material which transmits only light of the chromatic colors. As chromatic color, red, green, blue, or the like can be used. Alternatively, cyan, magenta, yellow, or the like may also be used. “Transmitting only light of a chromatic color” means that light passing through the color filter layer has a peak at a wavelength of the light of the chromatic color. The thickness of the color filter layer may be controlled as appropriate in consideration of the relationship between the concentration of the coloring material to be included and the transmittance of light. For example, the color filter layer  505  may have a thickness greater than or equal to 1500 nm and less than or equal to 2000 nm. 
     The partition  507  can be formed using an organic insulating material or an inorganic insulating material. It is particularly preferable that the partition  507  be formed using a photosensitive resin material to have an opening over the first electrode layer  541 . A sidewall of the opening is preferably formed as a tilted surface with continuous curvature. 
     The electroluminescent layer  542  may be formed using either a single layer or a stack of a plurality of layers. 
     A protective film may be formed over the second electrode layer  543  and the partition  507  in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element  540 . As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed. 
     Further, the light-emitting element  540  may be covered with a layer containing an organic compound deposited by an evaporation method so that oxygen, hydrogen, moisture, carbon dioxide, or the like do not enter the light-emitting element  540 . 
     In addition, if needed, an optical film, such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter, may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed. 
     Further, an electronic paper in which electronic ink is driven can be provided as the display device. The electronic paper is also called an electrophoretic display device (electrophoretic display) and has advantages in that it has the same level of readability as regular paper, it has less power consumption than other display devices, and it can be set to have a thin and light form. 
     An electrophoretic display device can have various modes. An electrophoretic display device includes a plurality of microcapsules dispersed in a solvent, and each microcapsule contains first particles which are positively charged and second particles which are negatively charged. By applying an electric field to the microcapsules, the particles in the microcapsules move in opposite directions to each other and only the color of the particles gathering on one side is displayed. Note that the first particles and the second particles each contain pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless). 
     A dispersion of the above microcapsules in a solvent is referred to as electronic ink. Furthermore, by the use of a color filter or particles that have a pigment, color display is also possible. 
     The insulating layer  506  functioning as a planarization insulating layer can be formed using an organic material having heat resistance, such as an acrylic resin, polyimide, a benzocyclobutene-based resin, polyamide, or an epoxy resin. Other than such organic materials, it is also possible to use a low-dielectric constant material (low-k material) such as a siloxane-based resin, phosphosilicate glass (PSG), or borophosphosilicate glass (BPSG). Note that the insulating layer  506  may be formed by stacking a plurality of insulating layers formed using any of these materials. 
     There is no particular limitation on the method of forming the insulating layer  506 ; the following method can be used depending on the material: a sputtering method, spin coating, dipping, spray coating, a droplet discharge method (an inkjet method), screen printing, offset printing, or the like. 
     Materials similar to those of the first electrode layer  4034  and the second electrode layer  4031  illustrated in  FIG. 5  can be used for the first electrode layer  541  and the second electrode layer  543 . 
     In this embodiment, since the light-emitting device illustrated in  FIGS. 6A and 6B  has a bottom-emission structure, the first electrode layer  541  has a light-transmitting property and the second electrode layer  543  has a light-reflecting property. Accordingly, in the case of using a metal film as the first electrode layer  541 , the film is preferably thin enough to keep the light-transmitting property; on the other hand, in the case of using a light-transmitting conductive film as the second electrode layer  543 , a conductive layer having a light-reflecting property is preferably stacked thereon. 
     A protective circuit for protecting the driver circuit may be provided. The protection circuit is preferably formed using a nonlinear element. 
     By using the transistor described in Embodiment 1 or 2 as described above, the semiconductor device can have a variety of functions. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the other structures, methods, and the like described in the other embodiments. 
     Embodiment 4 
     A semiconductor device having an image sensor function of reading data on an object can be manufactured using the transistors described in Embodiment 1 or 2. 
     An example of a semiconductor device having an image sensor function is illustrated in  FIG. 7A .  FIG. 7A  illustrates an equivalent circuit of a photo sensor, and  FIG. 7B  is a cross-sectional view illustrating part of the photo sensor. 
     In a photodiode  602 , one electrode is electrically connected to a photodiode reset signal line  658 , and the other electrode is electrically connected to a gate of a transistor  640 . One of a source and a drain of the transistor  640  is electrically connected to a photo sensor reference signal line  672 , and the other of the source and the drain thereof is electrically connected to one of a source and a drain of a transistor  656 . A gate of the transistor  656  is electrically connected to a gate signal line  659 , and the other of the source and the drain thereof is electrically connected to a photo sensor output signal line  671 . 
     Note that in circuit diagrams in this specification, a transistor using an oxide semiconductor layer is denoted by a symbol “OS” so that it can be identified as a transistor including an oxide semiconductor layer. In  FIG. 7A , the transistor  640  and the transistor  656  are each a transistor using an oxide semiconductor layer, to which the transistor described in Embodiment 1 or 2 can be applied. This embodiment shows an example of using a transistor having a structure similar to that of the transistor  320  described in Embodiment 1. The transistor  640  is a bottom-gate transistor. 
       FIG. 7B  is a cross-sectional view of the photodiode  602  and the transistor  640  in the photosensor. The transistor  640  and the photodiode  602  functioning as a sensor are provided over a substrate  601  (an element substrate) having an insulating surface. A substrate  613  is provided over the photodiode  602  and the transistor  640  with an adhesive layer  608  interposed therebetween. 
     An insulating layer  632 , an interlayer insulating layer  633 , and an interlayer insulating layer  634  are provided over the transistor  640 . The photodiode  602  includes an electrode layer  641   b  formed over the interlayer insulating layer  633 , semiconductor films (a first semiconductor film  606   a , a second semiconductor film  606   b , and a third semiconductor film  606   c  stacked over the electrode layer  641   b  in this order), an electrode layer  642  which is provided over the interlayer insulating layer  634  and electrically connected to the electrode layer  641   b  through the first to third semiconductor films, and an electrode layer  641   a  which is provided in the same layer as the electrode layer  641   b  and electrically connected to the electrode layer  642 . 
     The electrode layer  641   b  is electrically connected to a conductive layer  643  formed over the interlayer insulating layer  634 , and the electrode layer  642  is electrically connected to a conductive layer  645  through the electrode layer  641   a . The conductive layer  645  is electrically connected to a gate electrode layer of the transistor  640 , and the photodiode  602  is electrically connected to the transistor  640 . 
     Here, a pin photodiode in which a semiconductor film having p-type conductivity as the first semiconductor film  606   a , a high-resistance semiconductor film (i-type semiconductor film) as the second semiconductor film  606   b , and a semiconductor film having n-type conductivity as the third semiconductor film  606   c  are stacked is illustrated as an example. 
     The first semiconductor film  606   a  is a p-type semiconductor film and can be formed using an amorphous silicon film containing an impurity element imparting p-type conductivity type. The first semiconductor film  606   a  is formed by a plasma CVD method using a semiconductor source gas containing an impurity element belonging to Group 13 (e.g., boron (B)). As the semiconductor material gas, silane (SiH 4 ) may be used. Alternatively, Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , SiF 4 , or the like may be used. Further alternatively, an amorphous silicon film which does not contain an impurity element may be formed, and then, an impurity element may be introduced to the amorphous silicon film using a diffusion method or an ion injecting method. Heating or the like may be conducted after introducing the impurity element by an ion injecting method or the like in order to diffuse the impurity element. In this case, as a method of forming the amorphous silicon film, an LPCVD method, a chemical vapor deposition method, a sputtering method, or the like may be used. The first semiconductor film  606   a  is preferably formed to have a thickness greater than or equal to 10 nm and less than or equal to 50 nm. 
     The second semiconductor film  606   b  is an i-type semiconductor film (intrinsic semiconductor film) and is formed using an amorphous silicon film. As for formation of the second semiconductor film  606   b , an amorphous silicon film is formed by a plasma CVD method using a semiconductor source gas. As the semiconductor material gas, silane (SiH 4 ) may be used. Alternatively, Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , SiF 4 , or the like may be used. The second semiconductor film  606   b  may be formed by an LPCVD method, a vapor deposition method, a sputtering method, or the like. The second semiconductor film  606   b  is preferably formed to have a thickness greater than or equal to 200 nm and less than or equal to 1000 nm. 
     The third semiconductor film  606   c  is an n-type semiconductor film and is formed using an amorphous silicon film containing an impurity element imparting n-type conductivity type. The third semiconductor film  606   c  is formed by a plasma CVD method using a semiconductor source gas containing an impurity element belonging to Group 15 (e.g., phosphorus (P)). As the semiconductor material gas, silane (SiH 4 ) may be used. Alternatively, Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , SiF 4 , or the like may be used. Further alternatively, an amorphous silicon film which does not contain an impurity element may be formed, and then, an impurity element may be introduced to the amorphous silicon film with use of a diffusion method or an ion injecting method. Heating or the like may be conducted after introducing the impurity element by an ion injecting method or the like in order to diffuse the impurity element. In this case, as a method of forming the amorphous silicon film, an LPCVD method, a chemical vapor deposition method, a sputtering method, or the like may be used. The third semiconductor film  606   c  is preferably formed to have a thickness greater than or equal to 20 nm and less than or equal to 200 nm. 
     The first semiconductor film  606   a , the second semiconductor film  606   b , and the third semiconductor film  606   c  are not necessarily formed using an amorphous semiconductor, and may be formed using a polycrystalline semiconductor or a microcrystalline semiconductor (semi-amorphous semiconductor: SAS). 
     The mobility of holes generated by the photoelectric effect is lower than the mobility of electrons. Therefore, a PIN photodiode has better characteristics when a surface on the p-type semiconductor film side is used as a light-receiving plane. Here, an example in which light received by the photodiode  602  from a surface of the substrate  601 , over which the pin photodiode is formed, is converted into electric signals is described. Further, light from the semiconductor film having the conductivity type opposite to that of the semiconductor film on the light-receiving plane is disturbance light; therefore, the electrode layer is preferably formed using a light-blocking conductive layer. Note that the n-type semiconductor film side may alternatively be a light-receiving plane. 
     The transistor  640  includes a first oxide semiconductor layer which functions as a buffer layer for preventing diffusion of a constituent element of the source and drain electrode layers into a channel, a second oxide semiconductor layer which functions as the channel, and a third oxide semiconductor layer which is provided between a gate insulating layer  631  and the oxide semiconductor layer functioning as the channel and functions as a buffer layer for preventing degradation of the interface on the gate insulating layer  631  side of the channel. Accordingly, the transistor  640  is a highly reliable transistor in which influence of an interface state which can be formed on the back channel side is reduced and photodegradation (e.g., negative-bias temperature stress photodegradation) is reduced. 
     The insulating layer  632 , the interlayer insulating layer  633 , and the interlayer insulating layer  634  can be formed using an insulating material by a sputtering method, a plasma CVD method, spin coating, dipping, spray coating, a droplet discharge method (such as an inkjet method), a printing method (such as screen printing or offset printing), or the like depending on the material. 
     For reduction of surface roughness, an insulating layer functioning as a planarization insulating layer is preferably used as each of the interlayer insulating layers  633  and  634 . For the interlayer insulating layers  633  and  634 , for example, an organic insulating material having heat resistance, such as polyimide, an acrylic resin, a benzocyclobutene-based resin, polyamide, or epoxy, can be used. Other than such organic insulating materials, it is possible to use a single layer or stacked layers of a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. 
     With detection of light that enters the photodiode  602 , data on an object can be read. Note that a light source such as a backlight can be used at the time of reading information on an object. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the other structures, methods, and the like described in the other embodiments. 
     Embodiment 5 
     A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). Examples of electronic appliances include a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, cameras such as a digital camera and a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, an audio reproducing device, a game machine (e.g., a pachinko machine or a slot machine), a game console, and the like. Specific examples of these electronic appliances are illustrated in  FIGS. 8A to 8C . 
       FIG. 8A  illustrates a table  9000  having a display portion. In the table  9000 , a display portion  9003  is incorporated in a housing  9001  and an image can be displayed on the display portion  9003 . Note that the housing  9001  is supported by four leg portions  9002 . Further, a power cord  9005  for supplying power is provided for the housing  9001 . 
     The semiconductor device described in any of the above embodiments can be used in the display portion  9003 , so that the electronic appliance can have high reliability. 
     The display portion  9003  has a touch-input function. When a user touches displayed buttons  9004  which are displayed on the display portion  9003  of the table  9000  with his/her finger or the like, the user can carry out operation of the screen and input of information. Further, when the table may be made to communicate with home appliances or control the home appliances, the table  9000  may function as a control device which controls the home appliances by operation on the screen. For example, with use of the semiconductor device having an image sensor described in Embodiment 3, the display portion  9003  can function as a touch panel. 
     Further, the screen of the display portion  9003  can be placed perpendicular to a floor with a hinge provided for the housing  9001 ; thus, the table  9000  can also be used as a television device. When a television device having a large screen is set in a small room, an open space is reduced; however, when a display portion is incorporated in a table, a space in the room can be efficiently used. 
       FIG. 8B  illustrates a television set  9100 . In the television set  9100 , a display portion  9103  is incorporated in a housing  9101  and an image can be displayed on the display portion  9103 . Note that the housing  9101  is supported by a stand  9105  here. 
     The television set  9100  can be operated with an operation switch of the housing  9101  or a separate remote controller  9110 . Channels and volume can be controlled with an operation key  9109  of the remote controller  9110  so that an image displayed on the display portion  9103  can be controlled. Furthermore, the remote controller  9110  may be provided with a display portion  9107  for displaying data output from the remote controller  9110 . 
     The television set  9100  illustrated in  FIG. 8B  is provided with a receiver, a modem, and the like. With the use of the receiver, the television set  9100  can receive general TV broadcasts. Moreover, when the television set  9100  is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed. 
     The semiconductor device described in any of the above embodiments can be used in the display portions  9103  and  9107 , so that the television set and the remote controller can have high reliability. 
       FIG. 8C  illustrates a computer which includes a main body  9201 , a housing  9202 , a display portion  9203 , a keyboard  9204 , an external connection port  9205 , a pointing device  9206 , and the like. 
     The semiconductor device described in any of the above embodiments can be used in the display portion  9203 , so that the computer can have high reliability. 
       FIGS. 9A and 9B  illustrate a tablet terminal that can be folded. In  FIG. 9A , the tablet terminal is opened, and includes a housing  9630 , a display portion  9631   a , a display portion  9631   b , a switch  9034  for switching display modes, a power switch  9035 , a switch  9036  for switching to power-saving mode, a clip  9033 , and an operation switch  9038 . 
     The semiconductor device described in any of the above embodiments can be used in the display portion  9631   a  and the display portion  9631   b , so that the tablet terminal can have high reliability. 
     Part of the display portion  9631   a  can be a touch panel region  9632   a , and data can be input by touching operation keys  9638  that are displayed. Note that  FIG. 9A  shows, as an example, that half of the area of the display portion  9631   a  has only a display function and the other half of the area has a touch panel function. However, the structure of the display portion  9631   a  is not limited to this, and all the area of the display portion  9631   a  may have a touch panel function. For example, all the area of the display portion  9631   a  can display keyboard buttons and serve as a touch panel while the display portion  9631   b  can be used as a display screen. 
     Like the display portion  9631   a , part of the display portion  9631   b  can be a touch panel region  9632   b . When a finger, a stylus, or the like touches the place where a button  9639  for switching to keyboard display is displayed in the touch panel, keyboard buttons can be displayed on the display portion  9631   b.    
     Touch input can be performed concurrently on the touch panel regions  9632   a  and  9632   b.    
     The switch  9034  for switching display modes allows switching between a landscape mode and a portrait mode, color display and black-and-white display, and the like. With the switch  9036  for switching to power-saving mode, the luminance of display can be optimized in accordance with the amount of external light at the time when the tablet terminal is in use, which is detected with an optical sensor incorporated in the tablet terminal. The tablet terminal may include another detection device such as a sensor for detecting orientation (e.g., a gyroscope or an acceleration sensor) in addition to the optical sensor. 
     Although the display portion  9631   a  and the display portion  9631   b  have the same display area in  FIG. 9A , an embodiment of the present invention is not limited to this example. The display portion  9631   a  and the display portion  9631   b  may have different areas or different display quality. For example, one of them may be a display panel that can display higher-definition images than the other. 
       FIG. 9B  illustrates the tablet terminal folded, which includes the housing  9630 , a solar battery  9633 , and a charge and discharge control circuit  9634 . Note that  FIG. 9B  shows an example in which the charge and discharge control circuit  9634  includes the battery  9635  and the DCDC converter  9636 . 
     Since the tablet can be folded in two, the housing  9630  can be closed when the tablet is not in use. Thus, the display portions  9631   a  and  9631   b  can be protected, thereby providing a tablet with high endurance and high reliability for long-term use. 
     The tablet terminal illustrated in  FIGS. 9A and 9B  can have other functions such as a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, and a function of controlling processing by various kinds of software (programs). 
     The solar battery  9633 , which is attached on the surface of the tablet terminal, supplies electric power to a touch panel, a display portion, an image signal processor, and the like. Note that the solar battery  9633  can be provided on one or both surfaces of the housing  9630  and the battery  9635  can be charged efficiently. When a lithium ion battery is used as the battery  9635 , there is an advantage of downsizing or the like. 
     The structure and operation of the charge and discharge control circuit  9634  illustrated in  FIG. 9B  are described with reference to a block diagram of  FIG. 9C .  FIG. 9C  illustrates the solar battery  9633 , the battery  9635 , the DCDC converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and the display portion  9631 . The battery  9635 , the DCDC converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond to the charge and discharge control circuit  9634  in  FIG. 9B . 
     First, an example of operation in the case where power is generated by the solar battery  9633  using external light is described. The voltage of power generated by the solar battery  9633  is raised or lowered by the DCDC converter  9636  so that a voltage for charging the battery  9635  is obtained. When the display portion  9631  is operated with the power from the solar battery  9633 , the switch SW 1  is turned on and the voltage of the power is raised or lowered by the converter  9637  to a voltage needed for operating the display portion  9631 . In addition, when display on the display portion  9631  is not performed, the switch SW 1  is turned off and a switch SW 2  is turned on so that charge of the battery  9635  may be performed. 
     Here, the solar battery  9633  is shown as an example of a power generation means; however, there is no particular limitation on a way of charging the battery  9635 , and the battery  9635  may be charged with another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery  9635  may be charged with a non-contact power transmission module that transmits and receives power wirelessly (without contact) to charge the battery or with a combination of other charging means. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments. 
     EXAMPLE 
     In this example, measurement results of the concentrations of a constituent element of a source electrode layer or a drain electrode layer which is contained in stacked oxide semiconductor layers in a transistor are described. Specifically, an electrode layer including a copper film was formed for a source electrode layer and a drain electrode layer and the copper concentration in each oxide semiconductor layer was measured. For the measurement of the copper concentration, substrate side depth profile secondary ion mass spectrometry (SSDP-SIMS) was used. 
     A method for forming a transistor used in the measurement in this example is described below. In this example, a transistor having a structure similar to that of the transistor  300  in Embodiment 1 was formed. The same reference numerals as those of the transistor  300  are used in the following description. 
     First, a 100-nm-thick tungsten film was formed as the gate electrode layer  402  over the substrate  400 . 
     Next, a 50-nm-thick silicon nitride film and a 200-nm-thick silicon oxynitride film were formed successively by a plasma CVD method as the gate insulating layer  404  covering the gate electrode layer  402 . 
     The silicon nitride film was deposited in a condition where the pressure in a treatment chamber of a plasma CVD apparatus was set to 60 Pa, a power of 150 W was supplied at a high frequency power source of 27.12 MHz, the substrate temperature was set to 350° C., and a mixed gas of silane and nitrogen (SiH 4 :N 2 =50 sccm:5000 sccm) was supplied. Note that the plasma CVD apparatus is a parallel plate plasma CVD apparatus having an electrode area of 6000 cm 2 . The silicon oxynitride film was deposited in the same treatment chamber in a condition where the pressure was set o 40 Pa, the high-frequency power source and the substrate temperature were maintained, and a mixed gas of silane and dinitrogen monoxide (SiH 4 :N 2 O=20 sccm:3000 sccm) was supplied. 
     Next, the oxide semiconductor layer  408   b  with a thickness of 10 nm was formed over the gate insulating layer  404  by a sputtering method using a metal oxide target having an atomic ratio of In:Ga:Zn=1:1:1. The deposition conditions are as follows: the atmosphere contains oxygen at 50%, the pressure was 0.6 Pa, the electric power was 5 kW, and the substrate temperature was 170° C. 
     After that, the oxide semiconductor layer  408   a  with a thickness of 30 nm was formed over the oxide semiconductor layer  408   b  by a sputtering method using a metal oxide target having an atomic ratio of In:Ga:Zn=3:1:2. The deposition conditions are as follows: the atmosphere contains oxygen at 50%, the pressure was 0.6 Pa, the electric power was 5 kW, and the substrate temperature was 170° C. 
     Processing was performed so that the island-shaped oxide semiconductor stack  408  was obtained. Then, the oxide semiconductor stack  408  was subjected to heat treatment in a nitrogen atmosphere at 450° C. for one hour and then subjected to heat treatment in an atmosphere containing nitrogen and oxygen at 450° C. for one hour. 
     Next, the source electrode layer  410   a  and the drain electrode layer  410   b  which were in contact with the oxide semiconductor stack  408  were formed. 
     In this example, the source electrode layer  410   a  and the drain electrode layer  410   b  were formed in such a manner that a 35-nm-thick titanium film and a 200-nm-thick copper film were stacked over the gate insulating layer  404  and the oxide semiconductor stack  408  and part of the titanium film and part of the copper film were selectively etched. 
     After that, a 400-nm-thick silicon oxynitride film was deposited by a plasma CVD method as the insulating layer  412  covering the oxide semiconductor stack  408 , the source electrode layer  410   a , and the drain electrode layer  410   b.    
     The silicon oxynitride film was deposited in a condition where the pressure of a treatment chamber of a plasma CVD apparatus was controlled to 200 Pa, a power of 1500 W was supplied at a high frequency power source of 27.12 MHz, the substrate temperature was set to 220° C., and a mixed gas of silane and dinitrogen monoxide (SiH 4 :N 2 O=160 sccm:4000 sccm) was supplied. 
     Heat treatment was performed in an atmosphere containing nitrogen and oxygen at 300° C. for one hour, and then a 1.5-μm-thick acrylic resin film was formed as a planarization film. After that, heat treatment was performed in a nitrogen atmosphere at 250° C. for one hour. 
     Through the above process, the transistor of this example was formed. 
     The copper concentration of the oxide semiconductor stack  408  included in the obtained transistor was measured by SSDP-SIMS. The measurement results by SSDP-SIMS are shown in  FIG. 12 . Note that the measurement was performed on a region between the source electrode layer  410   a  and the drain electrode layer  410   b  (a region where a channel is formed). 
       FIG. 12  shows that diffusion of copper which is a constituent element of the source electrode layer  410   a  and the drain electrode layer  410   b  into the oxide semiconductor stack  408  was observed but copper was not diffused beyond the oxide semiconductor layer  408   a . Thus, it was confirmed that the copper does not reach the oxide semiconductor layer  408   b , which functions as a current path (channel) of the transistor. 
     The above results show that, in the oxide semiconductor layer  408 , the oxide semiconductor layer  408   a  can function as a buffer layer for preventing diffusion of the constituent element of the source electrode layer  410   a  and the drain electrode layer  410   b  by being provided on the back channel side of the oxide semiconductor layer  408   b  serving as a current path. Accordingly, it is suggested that variations in the electrical characteristics of the transistor including the oxide semiconductor stack  408  can be prevented. 
     REFERENCE NUMERALS 
       300 : transistor;  310 : transistor;  320 : transistor;  330 : transistor;  400 : substrate;  402 : gate electrode layer;  403 : gate insulating layer;  403   a : gate insulating layer;  403   b : gate insulating layer;  403   c : gate insulating layer;  404 : gate insulating layer;  406 : gate insulating layer;  408 : oxide semiconductor stack;  408   a : oxide semiconductor layer;  408   b : oxide semiconductor layer;  408   c : oxide semiconductor layer;  410   a : source electrode layer;  410   b : drain electrode layer;  412 : insulating layer;  491 : common potential line;  492 : common electrode;  500 : substrate;  502 : gate insulating layer;  502   a : gate insulating layer;  502   b : gate insulating layer;  502   c : gate insulating layer;  504 : interlayer insulating layer;  505 : color filter layer;  506 : insulating layer;  507 : partition;  510 : transistor;  511   a : gate electrode layer;  511   b : gate electrode layer;  512 : oxide semiconductor stack;  512   a : oxide semiconductor layer;  512   b : oxide semiconductor layer;  513   a : conductive layer;  513   b : conductive layer;  520 : capacitor;  521   a : conductive layer;  521   b : conductive layer;  522 : oxide semiconductor stack;  522   a : oxide semiconductor layer;  522   b : oxide semiconductor layer;  523 : conductive layer;  525 : insulating layer;  530 : wiring layer intersection;  533 : conductive layer;  540 : light-emitting element;  541 : electrode layer;  542 : electroluminescent layer;  543 : electrode layer;  601 : substrate;  602 : photodiode;  606   a : semiconductor film;  606   b : semiconductor film;  606   c : semiconductor film;  608 : adhesive layer;  613 : substrate;  631 : gate insulating layer;  632 : insulating layer;  633 : interlayer insulating layer;  634 : interlayer insulating layer;  640 : transistor;  641   a : electrode layer;  641   b : electrode layer;  642 : electrode layer;  643 : conductive layer;  645 : conductive layer;  656 : transistor;  658 : photodiode reset signal line;  659 : gate signal line;  671 : photo sensor output signal line;  672 : photo sensor reference signal line;  4001 : substrate;  4002 : pixel portion;  4003 : signal line driver circuit;  4004 : scan line driver circuit;  4005 : sealant;  4006 : substrate;  4008 : liquid crystal layer;  4010 : transistor;  4011 : transistor;  4013 : liquid crystal element;  4015 : connection terminal electrode;  4016 : terminal electrode;  4018 : FPC;  4019 : anisotropic conductive layer;  4020 : gate insulating layer;  4031 : electrode layer;  4032 : insulating layer;  4033 : insulating layer;  4034 : electrode layer;  4035 : spacer;  4038 : insulating layer;  9000 : table;  9001 : housing;  9002 : leg portion;  9003 : display portion;  9004 : displayed button;  9005 : power cord;  9033 : clip;  9034 : switch;  9035 : switch;  9036 : switch;  9038 : operation switch;  9100 : television set;  9101 : housing;  9103 : display portion;  9105 : stand;  9107 : display portion;  9109 : operation key;  9110 : remote controller;  9201 : main body;  9202 : housing;  9203 : display portion;  9204 : keyboard;  9205 : external connection port;  9206 : pointing device;  9630 : housing;  9631 : display portion;  9631   a : display portion;  9631   b : display portion;  9632   a : region;  9632   b : region;  9633 : solar battery;  9634 : charge and discharge control circuit;  9635 : battery;  9636 : DCDC converter;  9637 : converter;  9638 : operation key;  9639 : button. 
     This application is based on Japanese Patent Application serial no. 2012-125432 filed with Japan Patent Office on May 31, 2012, the entire contents of which are hereby incorporated by reference.