Patent Publication Number: US-9431545-B2

Title: Semiconductor device and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An embodiment of the present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. 
     In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices. 
     2. Description of the Related Art 
     Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (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 transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1). 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
       
    
     SUMMARY OF THE INVENTION 
     In order to achieve high-speed operation, low power consumption, high integration, or the like of a transistor, it is necessary to miniaturize a transistor. 
     One object of an embodiment of the present invention is to provide a structure of a semiconductor device which achieves high-speed response and high-speed operation by improving on-state characteristics of a miniaturized transistor (e.g., on-state current or field-effect mobility), and to provide a manufacturing method thereof, in order to achieve a high-performance semiconductor device. 
     Further, in accordance with miniaturization of the transistor, concern about a decrease in yield of a manufacturing process rises. 
     Accordingly, it is another object to provide a miniaturized transistor having high electric characteristics with high yield. 
     Further, another object of an embodiment of the present invention is to achieve high performance, high reliability, and high productivity also in the semiconductor device including the transistor. 
     In a semiconductor device including a transistor in which an oxide semiconductor film, a gate insulating film, and a gate electrode layer on side surfaces of which sidewall insulating layers are provided are stacked in this order, a source electrode layer and a drain electrode layer are provided in contact with the oxide semiconductor film and the sidewall insulating layers. In a process for manufacturing the semiconductor device, a conductive film and an interlayer insulating film are stacked to cover the oxide semiconductor film, the sidewall insulating layers, and the gate electrode layer. Then, the interlayer insulating film and the conductive film are cut (grinded or polished), so that the conductive film over the gate electrode layer is removed. Accordingly, a source electrode layer and a drain electrode layer are formed. As the cutting (grinding or polishing) method, a chemical mechanical polishing (CMP) method can be preferably used. 
     Precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film over the gate electrode layer, which is one step of the formation process of the source electrode layer and the drain electrode layer. Consequently, in a process for manufacturing the semiconductor device, a transistor having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     Further, an insulating film is preferably provided over the gate electrode layer. Part or all of the insulating film may be removed in a step for removing part of the conductive film which is to be a source electrode layer and a drain electrode layer and provided over the insulating film. 
     Low-resistance regions whose resistances are lower than that of the channel formation region and which include a dopant are formed in the oxide semiconductor film so that the channel formation region is sandwiched between the low-resistance regions, by introducing the dopant into the oxide semiconductor film in a self-aligned manner with the use of the gate electrode layer as a mask. The dopant is an impurity by which the conductivity of the oxide semiconductor film is changed. As the method for introduction the dopant, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. 
     With the oxide semiconductor film including the low-resistance regions between which the channel formation region is provided in the channel length direction, on-state characteristics (e.g., on-state current and field-effect mobility) of the transistor are increased, which enables high-speed operation and high-speed response of the transistor. 
     An embodiment of the invention disclosed in this specification is a semiconductor device including an oxide semiconductor film including a channel formation region provided over an oxide insulating film, a gate insulating film over the oxide semiconductor film, stacked layers of a gate electrode layer and an insulating film over the gate insulating film, sidewall insulating layers covering side surfaces of the gate electrode layer and side surfaces of the insulating film, a source electrode layer and a drain electrode layer in contact with the oxide semiconductor film, side surfaces of the gate insulating film, and side surfaces of the sidewall insulating layers, and an interlayer insulating film over the source electrode layer and the drain electrode layer. In the semiconductor device, the height of a top surface of the source electrode layer and the height of a top surface of drain electrode layer are lower than the height of a top surface of the insulating film, the height of a top surface of the sidewall insulating layers, and the height of a top surface of the interlayer insulating film, and higher than the height of a top surface of the gate electrode layer. Further, in the oxide semiconductor film, a region which does not overlap with the gate electrode layer and includes a region overlapping with the gate insulating film includes a dopant. 
     In the above-described structure, an embodiment of the invention disclosed in this specification is a semiconductor device in which the height of the top surface of the insulating film, the height of the top surface of the sidewall insulating layers, and the height of the top surface of the interlayer insulating film are the same. 
     In addition, in the oxide semiconductor film, a region which does not overlap with the source electrode layer or the drain electrode layer may have a higher oxygen concentration than a region which overlaps with the source electrode layer or the drain electrode layer. 
     An embodiment of the invention disclosed in this specification is a method for manufacturing a semiconductor device including the following steps of: forming an oxide insulating film; forming an oxide semiconductor film over the oxide insulating film; forming a gate insulating film over the oxide semiconductor film; stacking a gate electrode layer and an insulating film overlapping with the oxide semiconductor film over the gate insulating film; selectively introducing a dopant into the oxide semiconductor film with the use of the gate electrode layer and the insulating film as masks; forming sidewall insulating layers covering side surfaces of the gate electrode layer and side surfaces of the insulating film over the gate insulating film; forming a conductive film over the oxide semiconductor film, the gate insulating film, the gate electrode layer, the insulating film, and the sidewall insulating layers; forming an interlayer insulating film over the conductive film; and forming a source electrode layer and a drain electrode layer by removing the interlayer insulating film and the conductive film by a chemical mechanical polishing method until the insulating film over the gate electrode layer is exposed and the conductive film is divided. 
     In the above structure, an inorganic insulating film (typically an aluminum oxide film) which has high density and which is to be a protective insulating film may be provided over the insulating film, the source electrode layer, the drain electrode layer, the sidewall insulating layers, and the interlayer insulating film. 
     In the above structure, an inorganic insulating film (typically an aluminum oxide film) which has high density and which is to be a protective insulating film may be provided between the source electrode layer and the interlayer insulating film and between the drain electrode layer and the interlayer insulating film. 
     In addition to the cutting (grinding or polishing) method such as a chemical mechanical polishing method, etching (dry etching or wet etching) method, plasma treatment, or the like may be employed in combination in the step of removing the conductive film over the gate electrode layer. For example, after the removing step by a chemical mechanical polishing method, a dry etching method or plasma treatment may be performed in order to improve the planarity of a surface to be processed. 
     In the above structure, planarization treatment may be performed on a surface of the oxide insulating film over which the oxide semiconductor film is to be formed. The oxide semiconductor film having small thickness can be provided with good coverage. As the planarization treatment, a chemical mechanical polishing method, an etching method, plasma treatment, or the like or a combination of these can be employed. 
     Further, heat treatment (dehydration or dehydrogenation treatment) by which hydrogen or moisture is eliminated from the oxide semiconductor film may be performed on the oxide semiconductor film. When a crystalline oxide semiconductor film is used as the oxide semiconductor film, heat treatment for crystallization may be performed. 
     Further, oxygen may be supplied to the oxide semiconductor film. The dehydration or dehydrogenation treatment may accompany elimination of oxygen which is a main constituent material of an oxide semiconductor to lead to a reduction in oxygen. An oxygen vacancy exists in a portion where oxygen is eliminated in an oxide semiconductor film, and a donor level which leads to a change in the electric characteristics of a transistor is formed owing to the oxygen vacancy. 
     Thus, oxygen is preferably supplied to the oxide semiconductor film after being subjected to the dehydration or dehydrogenation treatment. By introduction of oxygen to the oxide semiconductor film, an oxygen vacancy in the film can be filled. 
     For example, an oxide insulating film including much (excessive) oxygen, which serves as a supply source of oxygen, may be provided to be in contact with the oxide semiconductor film, whereby oxygen can be supplied from the oxide insulating film to the oxide semiconductor film. In the above structure, heat treatment may be performed in the state where the oxide semiconductor film after being subjected to the heat treatment as the dehydration or dehydrogenation treatment and the oxide insulating film are at least partly in contact with each other to supply oxygen to the oxide semiconductor film. 
     Further, oxygen (which includes at least one of an oxygen radical, an oxygen atom, and an oxygen ion) may be introduced to the oxide semiconductor film after being subjected to the dehydration or dehydrogenation treatment to supply oxygen to the oxide semiconductor film. As the method for introduction of oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be used. 
     Further, it is preferable that the oxide semiconductor film in the transistor include a region where the oxygen content is higher than that in the stoichiometric composition of the oxide semiconductor in a crystalline state. In that case, the oxygen content is higher than that in the stoichiometric composition of the oxide semiconductor. Alternatively, the oxygen content is higher than that of the oxide semiconductor in a single crystal state. In some cases, oxygen exists between lattices of the oxide semiconductor. 
     By removing hydrogen or moisture from the oxide semiconductor to highly purify the oxide semiconductor so as not to contain impurities as much as possible, and supplying oxygen to fill an oxygen vacancy therein, the oxide semiconductor can be turned into an i-type (intrinsic) oxide semiconductor or a substantially i-type (intrinsic) oxide semiconductor. Accordingly, the Fermi level (Ef) of the oxide semiconductor can be changed to the same level as the intrinsic Fermi level (Ei). Thus, by using the oxide semiconductor film for a transistor, variation in the threshold voltage V th  of the transistor due to an oxygen vacancy and a shift of the threshold voltage Δ V th  can be reduced. 
     An embodiment of the present invention relates to a semiconductor device including a transistor or a semiconductor device including a circuit which is formed by using a transistor. For example, an embodiment of the present invention relates to a semiconductor device including a transistor in which a channel formation region is formed using an oxide semiconductor or a semiconductor device including a circuit which is formed by using such a transistor. For example, the present invention relates to an electronic device which includes, as a component, a semiconductor integrated circuit including an LSI, a CPU, a power device mounted in a power circuit, a memory, a thyristor, a converter, an image sensor, or the like, or a light-emitting display device including a light-emitting element or an electro-optical device typified by a liquid crystal display panel. 
     Accordingly, a miniaturized transistor having high electric characteristics can be provided with high yield. 
     Further, also in a semiconductor device including the transistor, high performance, high reliability, and high productivity can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are a plan view and a cross-sectional view illustrating an embodiment of a semiconductor device. 
         FIGS. 2A to 2D  are cross-sectional views illustrating an embodiment of a method for manufacturing a semiconductor device. 
         FIGS. 3A to 3D  are cross-sectional views illustrating an embodiment of a method for manufacturing a semiconductor device. 
         FIGS. 4A to 4C  are cross-sectional views each illustrating an embodiment of a semiconductor device. 
         FIGS. 5A to 5C  are a cross-sectional view, a plan view, and a circuit diagram illustrating an embodiment of a semiconductor device. 
         FIGS. 6A and 6B  are a circuit diagram and a perspective view illustrating an embodiment of a semiconductor device. 
         FIGS. 7A and 7B  are a cross-sectional view and a plan view illustrating an embodiment of a semiconductor device. 
         FIGS. 8A and 8B  are circuit diagrams each illustrating an embodiment of a semiconductor device. 
         FIG. 9  is a block diagram illustrating an embodiment of a semiconductor device. 
         FIG. 10  is a block diagram illustrating an embodiment of a semiconductor device. 
         FIG. 11  is a block diagram illustrating an embodiment of a semiconductor device. 
         FIGS. 12A to 12C  are plan views each illustrating an embodiment of a semiconductor device. 
         FIGS. 13A and 13B  are cross-sectional views each illustrating an embodiment of a semiconductor device. 
         FIGS. 14A and 14B  are a circuit diagram and a cross-sectional view illustrating an embodiment of a semiconductor device. 
         FIGS. 15A to 15F  each illustrate an electronic device. 
         FIG. 16  shows a cross-sectional STEM image of Example Transistor  1 . 
         FIG. 17  illustrates a structure of Example Transistor  2 . 
         FIG. 18  shows electric characteristics of Example Transistor  2 . 
         FIGS. 19A and 19B  are a plan view and a cross-sectional view illustrating an embodiment of a semiconductor device. 
         FIGS. 20A and 20B  are a plan view and a cross-sectional view illustrating an embodiment of a semiconductor device. 
         FIG. 21  shows a graph of normal probability distribution of an on-state current in Example Transistor  2 . 
         FIG. 22  shows a graph of normal probability distribution of a threshold voltage in Example Transistor  2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention disclosed in this specification are described in detail below with reference to drawings. Note that the invention disclosed in this specification is not limited to the description below, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways. In addition, the invention disclosed in this specification should not be construed as being limited to the description in the embodiments given below. Note that ordinal numbers such as “first” and “second” 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 any particular names to define the invention. 
     Embodiment 1 
     In this embodiment, an embodiment of a semiconductor device and a method for manufacturing the semiconductor device are described with reference to  FIGS. 1A and 1B  and the like. In this embodiment, a transistor including an oxide semiconductor film is described as an example of the semiconductor device. 
     The transistor may have a single-gate structure in which one channel formation region is formed, a double-gate structure in which two channel formation regions are formed, or a triple-gate structure in which three channel formation regions are formed. Alternatively, the transistor may have a dual-gate structure including two gate electrode layers positioned above and below a channel formation region with a gate insulating film provided therebetween. 
     A transistor  440   a  illustrated in  FIGS. 1A and 1B  is an example of a top-gate transistor.  FIG. 1A  is a plan view and  FIG. 1B  is a cross-sectional view along dashed line X-Y in  FIG. 1A . 
     As illustrated in  FIG. 1B , which is a cross-sectional view in the channel length direction, a semiconductor device including the transistor  440   a  includes, over a substrate  400  having an insulating surface over which an oxide insulating film  436  is provided, an oxide semiconductor film  403  including a channel formation region  409  and low-resistance regions  404   a  and  404   b , a source electrode layer  405   a , a drain electrode layer  405   b , a gate insulating film  402 , a gate electrode layer  401 , sidewall insulating layers  412   a  and  412   b  provided on side surfaces of the gate electrode layer  401 , an insulating film  413  over the gate electrode layer  401 , an interlayer insulating film  415  over the source electrode layer  405   a  and the drain electrode layer  405   b , and an insulating film  407  covering the transistor  440   a.    
     The interlayer insulating film  415  is provided for planarizing unevenness due to the transistor  440   a , and the height of a top surface of the interlayer insulating film  415  is substantially the same as that of each of the sidewall insulating layers  412   a  and  412   b  and the insulating film  413 . Further, the height of each of the top surface of the source electrode layer  405   a  and the drain electrode layer  405   b  is lower than the height of each of the top surface of the interlayer insulating film  415 , the sidewall insulating layers  412   a  and  412   b , and the insulating film  413 , and higher than the height of a top surface of the gate electrode layer  401 . Note that “height” here means a distance from a top surface of the substrate  400 . 
     Further, in  FIGS. 1A and 1B , the insulating film  407  is provided in contact with the interlayer insulating film  415 , the source electrode layer  405   a , the drain electrode layer  405   b , the sidewall insulating layers  412   a  and  412   b , and the insulating film  413 . 
     A dopant is introduced into the oxide semiconductor film  403  in a self-aligned manner with the use of the gate electrode layer  401  as a mask, so that the low-resistance regions  404   a  and  404   b  between which the channel formation region  409  is sandwiched, which have lower resistances than the channel formation region  409 , and which include a dopant are formed in the oxide semiconductor film  403 . The dopant is an impurity by which the conductivity of the oxide semiconductor film  403  is changed. As the method for introducing the dopant, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. 
     With the oxide semiconductor film  403  including the low-resistance regions  404   a  and  404   b  between which the channel formation region  409  is provided in the channel length direction, on-state characteristics (e.g., on-state current and field-effect mobility) of the transistor  440   a  are increased, which enables high-speed operation and high-speed response of the transistor. 
     An oxide semiconductor used for the oxide semiconductor film  403  preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. In addition, as a stabilizer for reducing the variation in electric characteristics of a transistor using the oxide, the oxide semiconductor preferably contains gallium (Ga) in addition to In and Zn. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) is preferably contained as a stabilizer. 
     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, an indium oxide, a tin oxide, a zinc oxide, a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-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 (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—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, 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 can be used. 
     Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main component and there is no particular limitation on the ratio of In:Ga: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 an integer) may be used. 
     For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), In:Ga:Zn=3:1:2 (=1/2:1/6:1/3), or any of oxides whose composition is in the neighborhood of the above compositions can be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whose composition is in the neighborhood of the above compositions may be used. 
     However, without limitation to the materials given above, a material with an appropriate composition may be used depending on needed semiconductor characteristics (e.g., mobility, threshold voltage, and variation). In order to obtain the needed semiconductor characteristics, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like be set to appropriate values. 
     For example, high mobility can be obtained relatively easily in the case of using an In—Sn—Zn oxide. However, mobility can be increased by reducing the defect density in a bulk also in the case of using an In—Ga—Zn-based oxide. 
     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 including 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 ≦r 2 . For example, r may be 0.05. The same applies to other oxides. 
     The oxide semiconductor film  403  can be in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like. 
     The oxide semiconductor film is preferably a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film. 
     The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystal parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed. 
     In each of the crystal portions included in the CAAC-OS film, a c-axis is aligned in a direction perpendicular to a surface where the CAAC-OS film is formed or a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and layers each including metal atoms and oxygen atoms are overlapped with each other. Note that the direction of a normal vector of the layers is a c-axis direction. Note that, among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. 
     In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. Further, when impurities are introduced to the CAAC-OS film, the crystal part in a region to which the impurities are introduced becomes amorphous in some cases. 
     Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction perpendicular to the surface where the CAAC-OS film is formed or a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal portion is the direction perpendicular to the surface where the CAAC-OS film is formed or the surface of the CAAC-OS film. The crystal part is formed by film formation or by performing treatment for crystallization such as heat treatment after film formation. 
     With use of the CAAC-OS film in a transistor, a change in the electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Thus, the transistor has high reliability. 
     Note that part of oxygen included in the oxide semiconductor film may be substituted with nitrogen. 
     Further, in an oxide semiconductor having a crystal portion such as the CAAC-OS, defects in the bulk can be further reduced and when the surface flatness of the oxide semiconductor is improved, mobility higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (Ra) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, further preferably less than or equal to 0.1 nm. 
     Note that the average surface roughness (Ra) is obtained by expanding, into three dimensions, arithmetic mean surface roughness that is defined by JIS B 0601: 2001 (ISO4287: 1997) to be able to apply it to a curved surface. Ra can be expressed as an “average value of the absolute values of deviations from a reference surface to a designated surface” and is defined by the following formula. 
     
       
         
           
             
               
                 
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     Here, the specific surface is a surface which is a target of roughness measurement, and is a quadrilateral region which is specified by four points represented by the coordinates ((x 1 , y 1 , f(x 1 , y 1 )), (x 1 , y 2 , f(x 1 , y 2 )), (x 2 , y 1 , f(x 2 , y 1 )), and (x 2 , y 2 , f(x 2 , y 2 )). Moreover, S 0  represents the area of a rectangle which is obtained by projecting the specific surface on the xy plane, and Z 0  represents the height of the reference surface (the average height of the specific surface). Ra can be measured using an atomic force microscope (AFM). 
     The oxide semiconductor film  403  has a thickness greater than or equal to 1 nm and less than or equal to 30 nm (preferably greater than or equal to 5 nm and less than or equal to 10 nm) and 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. The oxide semiconductor film  403  may be formed with a sputtering apparatus which performs film formation in the state where top surfaces of a plurality of substrates are set substantially perpendicular to a surface of a sputtering target. 
     For example, the CAAC-OS film is formed by a sputtering method with a polycrystalline oxide semiconductor sputtering 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 formation of the CAAC-OS film, the following conditions are preferably used. 
     By reducing the amount of impurities entering the CAAC-OS film during the film formation, 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 film formation, migration of a sputtered particle is likely to occur after the sputtered particle reaches a substrate surface. Specifically, the substrate heating temperature during the film formation 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 film formation, 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 film formation. The proportion of oxygen in the deposition gas is 30 vol % or higher, preferably 100 vol %. 
     As an example of the sputtering target, 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 Z  powder in a predetermined molar 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. 
     An example of a method for manufacturing a semiconductor device including the transistor  440   a  is described with reference to  FIGS. 2A to 2D  and  FIGS. 3A to 3D . 
     First, the oxide insulating film  436  is formed over the substrate  400  having an insulating surface. 
     There is no particular limitation on a substrate that can be used as the substrate  400  having an insulating surface as long as it has heat resistance 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. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate; or the like can be used as the substrate  400 , or the substrate provided with a semiconductor element can be used as the substrate  400 . 
     The semiconductor device may be manufactured using a flexible substrate as the substrate  400 . To manufacture a flexible semiconductor device, the transistor  440   a  including the oxide semiconductor film  403  may be directly formed over a flexible substrate. Alternatively, the transistor  440   a  including the oxide semiconductor film  403  may be formed over a substrate, and then may be separated and transferred to a flexible substrate. Note that in order to separate the transistor  440   a  from the manufacturing substrate and transfer it to the flexible substrate, a separation layer is preferably provided between the manufacturing substrate and the transistor  440   a  including the oxide semiconductor film. 
     The oxide insulating film  436  can be formed by a plasma CVD method, a sputtering method, or the like using any of silicon oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, gallium oxide, or a mixed material thereof. 
     The oxide insulating film  436  may have either a single-layer structure or a stacked-layer structure. For example, a silicon oxide film, an In—Hf—Zn-based oxide film, and the oxide semiconductor film  403  may be stacked in that order over the substrate  400 ; a silicon oxide film, an In—Zr—Zn-based oxide film with an atomic ratio of In:Zr:Zn=1:1:1, and the oxide semiconductor film  403  may be stacked in that order over the substrate  400 ; or a silicon oxide film, an In—Gd—Zn-based oxide film with an atomic ratio of In:Gd:Zn=1:1:1, and the oxide semiconductor film  403  may be stacked in that order over the substrate  400 . 
     In this embodiment, a silicon oxide film formed by a sputtering method is used as the oxide insulating film  436 . 
     Further, a nitride insulating film may be provided between the oxide insulating film  436  and the substrate  400 . The nitride insulating film can be formed by a plasma CVD method, a sputtering method, or the like with the use of silicon nitride, silicon nitride oxide, aluminum nitride, or aluminum nitride oxide, or a mixed material of any of these materials. 
     Next, the oxide semiconductor film  403  is formed over the oxide insulating film  436  (see  FIG. 2A ). 
     The oxide insulating film  436  is in contact with the oxide semiconductor film  403  and therefore preferably contains a large amount of oxygen which exceeds at least the stoichiometric composition in (a bulk of) the film. For example, in the case where a silicon oxide film is used as the oxide insulating film  436 , the composition formula is SiO 2+α (α&gt;0). By using the oxide insulating film  436  described above, oxygen can be supplied to the oxide semiconductor film  403  and favorable characteristics can be obtained. By supplying oxygen to the oxide semiconductor film  403 , an oxygen vacancy in the film can be filled. 
     For example, when the oxide insulating film  436  containing much (excess) oxygen, which serves as an oxygen supply source, is provided to be in contact with the oxide semiconductor film  403 , oxygen can be supplied from the oxide insulating film  436  to the oxide semiconductor film  403 . The oxide semiconductor film  403  and the oxide insulating film  436  may be subjected to heating treatment in a state where the oxide semiconductor film  403  and the oxide insulating film  436  are at least partly in contact with each other so that oxygen is supplied to the oxide semiconductor film  403 . 
     In order that hydrogen or water is not contained in the oxide semiconductor film  403  as much as possible in the formation process of the oxide semiconductor film  403 , it is preferable to heat the substrate provided with the oxide insulating film  436  in a preheating chamber in a sputtering apparatus as a pretreatment for formation of the oxide semiconductor film  403  so that impurities such as hydrogen and moisture adsorbed to the substrate and the oxide insulating film  436  are eliminated and evacuated. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. 
     Planarizing treatment may be performed on a region of the oxide insulating film  436  which is in contact with the oxide semiconductor film  403 . As the planarizing treatment, polishing treatment (e.g., a chemical mechanical polishing method), dry-etching treatment, or plasma treatment can be used, though there is no particular limitation on the planarizing treatment. 
     As the plasma treatment, for example, a reverse sputtering in which an argon gas is introduced and plasma is produced can be performed. The reverse sputtering is a method in which voltage is applied to a substrate side with use of an RF power source in an argon atmosphere and plasma is generated in the vicinity of the substrate so that a substrate surface is modified. Instead of the argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. The reverse sputtering can remove particle substances (also referred to as particles or dust) attached to a surface of the oxide insulating film  436 . 
     As the planarizing treatment, polishing treatment, dry-etching treatment, or plasma treatment may be performed plural times and/or in combination. Further, in the case where the treatments are combined, the order of steps of such a combination is not particularly limited and may be set as appropriate in accordance with roughness of the surface of the oxide insulating film  436 . 
     Planarizing treatment may be performed on a surface of the silicon oxide film used as the oxide insulating film  436  by a chemical mechanical polishing method (the polishing conditions: a polyurethane-based polishing cloth, silica-based slurry, a slurry temperature of room temperature, a polishing pressure of 0.001 MPa, a rotation number in polishing (table/spindle) of 60 rpm/56 rpm, and a polishing time of 0.5 minutes) so that the average surface roughness (Ra) of the surface of the silicon oxide film is preferably approximately 0.15 nm. 
     The oxide semiconductor film  403  is preferably formed under a condition such that much oxygen is contained (for example, by a sputtering method in an atmosphere of 100% oxygen) to be a film containing much oxygen (preferably having a region containing an excess of oxygen as compared to the stoichiometric composition of the oxide semiconductor in a crystalline state). 
     Note that in this embodiment, as a target for forming the oxide semiconductor film  403  by a sputtering method, an oxide target containing In, Ga, and Zn in the atomic ratio of 3:1:2 is used to form an In—Ga—Zn-based oxide film (IGZO film). 
     The relative density (filling factor) of the metal oxide target 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 use of the metal oxide target with high filling factor, a dense oxide semiconductor film can be formed. 
     It is preferable that a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, or hydride are removed be used as a sputtering gas for the formation of the oxide semiconductor film  403 . 
     The substrate is held in a deposition chamber kept under reduced pressure. Then, a sputtering gas in which hydrogen and moisture are removed is introduced into the deposition chamber from which remaining moisture is being removed, so that the oxide semiconductor film  403  is formed over the substrate  400  with the use of the target. To remove moisture remaining in the deposition chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the deposition chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H 2 O), (further preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of impurities in the oxide semiconductor film  403  formed in the deposition chamber can be reduced. 
     The oxide insulating film  436  and the oxide semiconductor film  403  are preferably formed in succession without exposure to the air. According to successive formation of the oxide insulating film  436  and the oxide semiconductor film  403  without exposure to the air, impurities such as hydrogen and moisture can be prevented from being adsorbed onto the surface of the oxide insulating film  436 . 
     The oxide semiconductor film  403  can be formed by processing a film-shaped oxide semiconductor film into an island shape by a photolithography process. 
     A resist mask for forming the island-shaped oxide semiconductor film  403  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Note that the etching of the oxide semiconductor film may be dry etching, wet etching, or both dry etching and wet etching. As an etchant used for wet etching of the oxide semiconductor film, for example, a mixed solution of phosphoric acid, acetic acid, and nitric acid, or the like can be used. Alternatively, ITO-07N (produced by KANTO CHEMICAL CO., INC.) may be used. Further alternatively, the oxide semiconductor film may be etched by a dry etching method using an inductively coupled plasma (ICP) etching method. For example, an IGZO film is etched by the ICP etching method (the etching conditions: an etching gas of BCL 3  and Cl 2  (BCL 3 :Cl 2 =60 sccm:20 sccm), a power of 450 W, a bias power of 100 W, and a pressure of 1.9 Pa), so that the IGZO film is processed into an island shape. 
     Further, heat treatment may be performed on the oxide semiconductor film  403  in order to remove excess hydrogen (including water and a hydroxyl group) (to perform dehydration or dehydrogenation treatment). 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 a substrate. The heat treatment can be performed under reduced pressure, a nitrogen atmosphere, or the like. For example, the substrate is put in an electric furnace which is a kind of heat treatment apparatus, and the oxide semiconductor film  403  is subjected to heat treatment at 450° C. for one hour in a nitrogen atmosphere. 
     Further, the heat treatment apparatus is not limited to the electric furnace, and a device for heating an object by heat conduction or heat radiation from a heating element such as a resistance heating element may also be used. For example, a rapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can be used. The LRTA apparatus is an apparatus for heating an object by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. The GRTA apparatus is an apparatus for heat treatment with a high-temperature gas. As the high-temperature gas, an inert gas which does not react with an object by heat treatment, such as nitrogen or a rare gas like argon, is used. 
     For example, as the heat treatment, GRTA may be performed as follows: the substrate is put in an inert gas heated to high temperature of 650° C. to 700° C., heated for several minutes, and taken out of the inert gas. 
     In the heat treatment, it is preferable that moisture, hydrogen, and 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 preferably 6N (99.9999%) or more, further preferably 7N (99.99999%) or more (that is, the impurity concentration is preferably 1 ppm or less, further preferably 0.1 ppm or less). 
     In addition, after the oxide semiconductor film  403  is heated by the heat treatment, a high-purity oxygen gas, a high-purity dinitrogen monoxide 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 according to the measurement with a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) may be introduced into the same furnace. It is preferable that water, hydrogen, or the like be not included 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 (that is, the impurity concentration in the oxygen gas or the dinitrogen monoxide gas is preferably 1 ppm or less, further preferably 0.1 ppm or less). The oxygen gas or the dinitrogen monoxide gas acts to supply oxygen that is a main constituent material of the oxide semiconductor and that is reduced by the step for removing impurities for dehydration or dehydrogenation, so that the oxide semiconductor film  403  can be a highly-purified, i-type (intrinsic) oxide semiconductor film. 
     The heat treatment for dehydration or dehydrogenation may be performed after a film-shaped oxide semiconductor film is formed or after the island-shaped oxide semiconductor film  403  is formed. 
     The heat treatment for dehydration or dehydrogenation may be performed plural times and may also serve as another heat treatment. 
     When the heat treatment for dehydration or dehydrogenation is performed in the state where the oxide insulating film  436  is covered with the oxide semiconductor film which has not been processed into the island-shaped oxide semiconductor film  403 , oxygen contained in the oxide insulating film  436  can be prevented from being released by the heat treatment, which is preferable. 
     Further, oxygen (which includes at least one of an oxygen radical, an oxygen atom, and an oxygen ion) may be introduced to the oxide semiconductor film  403  after being subjected to the dehydration or dehydrogenation treatment to supply oxygen to the oxide semiconductor film. 
     The dehydration or dehydrogenation treatment may accompany elimination of oxygen which is a main constituent material of an oxide semiconductor to lead to a reduction in oxygen. An oxygen vacancy exists in a portion where oxygen is eliminated in an oxide semiconductor film, and a donor level which leads to a change in the electric characteristics of a transistor is formed owing to the oxygen vacancy. 
     Oxygen is introduced to the dehydrated or dehydrogenated oxide semiconductor film  403  to be supplied thereto, so that the oxide semiconductor film  403  can be highly purified and be i-type (intrinsic). Variation in electric characteristics of a transistor having a highly-purified and i-type (intrinsic) oxide semiconductor film  403  is suppressed, and the transistor is electrically stable. 
     Oxygen can be introduced by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. 
     In the step of introduction of oxygen to the oxide semiconductor film  403 , oxygen may be directly introduced to the oxide semiconductor film  403  or to the oxide semiconductor film  403  through another film such as the gate insulating film  402  or the insulating film  407 . An ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be employed for the introduction of oxygen through another film, whereas plasma treatment or the like can also be employed for the direct introduction of oxygen to the exposed oxide semiconductor film  403 . 
     The introduction of oxygen to the oxide semiconductor film  403  is preferably performed after dehydration or dehydrogenation treatment is performed thereon, but the timing is not limited thereto. Further, oxygen may be introduced plural times into the dehydrated or dehydrogenated oxide semiconductor film  403 . 
     Next, a gate insulating film  442  is formed to cover the oxide semiconductor film  403  (see  FIG. 2B ). 
     To improve the coverage with the gate insulating film  442 , the above-described planarizing treatment may be performed also on the surface of the oxide semiconductor film  403 . It is preferable that the flatness of the surface of the oxide semiconductor film  403  be good particularly when the thickness of the gate insulating film  442  is small. 
     The gate insulating film  442  has a thickness greater than or equal to 1 nm and less than or equal to 20 nm and can be formed by a sputtering method, an MBE method, a CVD method, a pulse laser deposition method, an ALD method, or the like as appropriate. The gate insulating film  442  may also be formed with a sputtering apparatus which performs film formation in the state where surfaces of a plurality of substrates are substantially perpendicular to a surface of a sputtering target. 
     The gate insulating film  442  can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. It is preferable that the gate insulating film  442  include oxygen in a portion which is in contact with the oxide semiconductor film  403 . In particular, the gate insulating film  442  preferably contains an excess amount of oxygen which exceeds at least the stoichiometric composition in the film (bulk); for example, in the case where a silicon oxide film is used as the gate insulating film  442 , the composition formula is SiO 2+α (α&gt;0). In this embodiment, a silicon oxide film of SiO 2+α (α&gt;0) is used as the gate insulating film  442 . By using the silicon oxide film as the gate insulating film  442 , oxygen can be supplied to the oxide semiconductor film  403 , leading to good characteristics. Further, the gate insulating film  442  is preferably formed in consideration of the size of a transistor to be formed and the step coverage with the gate insulating film  442 . 
     When the gate insulating film  442  is formed using a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate to which nitrogen is added (HfSi x O y N Y  (x&gt;0, y&gt;0)), hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)), or lanthanum oxide, gate leakage current can be reduced. Further, the gate insulating film  442  has either a single-layer structure or a stacked-layer structure. 
     Next, stacked layers of the conductive film and the insulating film are formed over the gate insulating film  442 , and are etched, so that stacked layers of the gate electrode layer  401  and the insulating film  413  are formed (see  FIG. 2C ). 
     The gate electrode layer  401  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. A semiconductor film which is doped with an impurity element such as phosphorus and is typified by a polycrystalline silicon film, or a silicide film of nickel silicide or the like can also be used as the gate electrode layer  401 . The gate electrode layer  401  has either a single-layer structure or a stacked-layer structure. 
     The gate electrode layer  401  can also be formed using 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. It is also possible that the gate electrode layer  401  has a stacked structure of the above conductive material and the above metal material. 
     As one layer of the gate electrode layer  401  which is in contact with the gate insulating film  442 , a metal oxide film containing nitrogen, specifically, 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, an In—O film containing nitrogen, or a metal nitride (e.g., InN or SnN) film can be used. These films each have a work function higher than or equal to 5 eV (electron volts), preferably higher than or equal to 5.5 eV (electron volts); thus, when these are used as the gate electrode layer, the threshold voltage of the electrical characteristics of the transistor can be positive. Accordingly, a so-called normally-off switching element can be obtained. 
     As the insulating film  413 , typically, an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film can be used. The insulating film  413  can be formed by a plasma CVD method, a sputtering method, or the like. 
     Next, a dopant  421  is introduced into the oxide semiconductor film  403  with the use of the gate electrode layer  401  and the insulating film  413  as masks, so that the low-resistance regions  404   a  and  404   b  are formed (see  FIG. 2D ). 
     The dopant  421  is an impurity by which the conductivity of the oxide semiconductor film  403  is changed. One or more selected from the following can be used as the dopant  421 : Group 15 elements (typical examples thereof are phosphorus (P), arsenic (As), and antimony (Sb)), boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon (Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc (Zn). 
     The dopant  421  can be introduced into the oxide semiconductor film  403  through another film (e.g., the gate insulating film  442 ) by an implantation method. As the method for adding the dopant  421 , an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. In that case, it is preferable to use a single ion of the dopant  421 , a fluoride ion, or a chloride ion. 
     The introduction of the dopant  421  may be controlled by setting the introduction conditions such as the accelerated voltage and the dosage, or the thickness of the films through which the dopant passes as appropriate. In this embodiment, phosphorus is used as the dopant  421 , and boron ions are implanted by an ion implantation method. The dosage of the dopant  421  can be set to 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 concentration of the dopant  421  in the low-resistance regions is preferably greater than or equal to 5×10 18 /cm 3  and less than or equal to 1×10 22 /cm 3 . 
     The dopant  421  may be introduced while the substrate  400  is heated. 
     The introduction of the dopant  421  into the oxide semiconductor film  403  may be performed plural times, and the number of kinds of dopant may be plural. 
     Further, heat treatment may be performed thereon after the introduction of the dopant  421 . The heat treatment is preferably performed at a temperature higher than or equal to 300° C. and lower than or equal to 700° C. (further preferably higher than or equal to 300° C. and lower than or equal to 450° C.) for one hour under an oxygen atmosphere. The heat treatment may be performed under a nitrogen atmosphere, reduced pressure, or the air (ultra-dry air). 
     In this embodiment, phosphorus (P) ions are implanted into the oxide semiconductor film  403  by an ion implantation method. Note that the conditions of the phosphorus (P) ion implantation are as follows: the acceleration voltage is 30 kV and the dosage is 1.0×10 15  ions/cm 2 . 
     When the oxide semiconductor film  403  is a CAAC-OS film, the oxide semiconductor film  403  is partly amorphized by introduction of the dopant  421  in some cases. In that case, the crystallinity of the oxide semiconductor film  403  can be recovered by performing heat treatment thereon after the introduction of the dopant  421 . 
     Thus, the oxide semiconductor film  403  in which the low-resistance regions  404   a  and  404   b  between which the channel formation region  409  is sandwiched are formed. 
     Next, an insulating film is formed over the gate electrode layer  401  and the insulating film  413 , and the insulating film is etched, so that the sidewall insulating layers  412   a  and  412   b  are formed. Further, the gate insulating film  442  is etched with the use of the gate electrode layer  401  and the sidewall insulating layers  412   a  and  412   b  as masks, so that the gate insulating film  402  is formed (see  FIG. 3A ). 
     The sidewall insulating layers  412   a  and  412   b  can be formed using a material and a method similar to those of the insulating film  413 . In this embodiment, a silicon oxynitride film formed by a CVD method is used. 
     Next, a conductive film for forming a source electrode layer and a drain electrode layer (including a wiring formed of the same layer as the source electrode layer and the drain electrode layer) is formed over the oxide semiconductor film  403 , the gate insulating film  402 , the gate electrode layer  401 , the sidewall insulating layers  412   a  and  412   b , and the insulating film  413 . 
     The conductive film is formed of a material that can withstand heat treatment performed later. As the conductive film used for the source electrode layer and the drain electrode layer, for example, 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 its component (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like can be used. A metal film having a high melting point made of Ti, Mo, W, or the like or a metal nitride film made 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 of a lower side and an upper side of a metal film made of Al, Cu, or the like. Alternatively, the conductive film used for the source electrode layer and the drain electrode layer may be formed of 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. 
     A resist mask is formed over the conductive film by a photolithography process, and the conductive film is selectively etched, so that an island-shaped conductive film  445  is formed. Then, the resist mask is removed. Note that in this etching step, the conductive film  445  over the gate electrode layer  401  is not removed. 
     When a 30-nm-thick tungsten film is used as the conductive film, the tungsten film may be etched by a dry etching method to have an island shape (the etching conditions: an etching gas of CF 4 , Cl 2 , and O 2  (CF 4 :Cl 2 :O 2 =55 sccm:45 sccm:55 sccm, a power of 3000 W, a bias power of 140 W, and a pressure of 0.67 Pa). 
     An insulating film  446  to be the interlayer insulating film  415  is stacked over the island-shaped conductive film  445  (see  FIG. 3B ). 
     The insulating film  446  can be formed using a material and a method similar to those of the insulating film  413 . The insulating film  446  is formed to have a thickness which is large enough to planarize unevenness caused by the transistor  440   a . In this embodiment, a 300-nm-thick silicon oxynitride film is formed by a CVD method. 
     Next, polishing treatment is performed on the insulating film  446  and the conductive film  445  by a chemical mechanical polishing method. Part of the insulating film  446  and part of the conductive film  445  are removed so that the insulating film  413  is exposed. 
     By the polishing treatment, the insulating film  446  is processed to be the interlayer insulating film  415  and the conductive film  445  over the gate electrode layer  401  is removed; thus, the source electrode layer  405   a  and the drain electrode layer  405   b  are formed. 
     A chemical mechanical polishing method is used for removing the insulating film  446  and the conductive film  445  in this embodiment; however, another cutting (grinding or polishing) method may be used. Further, in addition to the cutting (grinding or polishing) method such as a chemical mechanical polishing method, etching (dry etching or wet etching), plasma treatment, or the like may be employed in combination for the step of removing the conductive film  445  over the gate electrode layer  401 . For example, after the removing step by a chemical mechanical polishing method, a dry etching method or plasma treatment (e.g., reverse sputtering method) may be performed in order to improve the planarity of a surface to be processed. When the cutting (grinding or polishing) method is employed in combination with an etching method, plasma treatment, or the like, the order of steps is not particularly limited and may be set as appropriate depending on the materials, the film thicknesses, and the surface roughness of the insulating film  446  and the conductive film  445 . 
     Note that in this embodiment, the source electrode layer  405   a  and the drain electrode layer  405   b  are provided in contact with side surfaces of the sidewall insulating layers  412   a  and  412   b  provided on side surfaces of the gate electrode layer  401 , and the source electrode layer  405   a  and the drain electrode layer  405   b  each of which has an upper end portion positioned a little lower than those of the sidewall insulating layers  412   a  and  412   b  cover the side surfaces of the sidewall insulating layers  412   a  and  412   b . The shapes of the source electrode layer  405   a  and the drain electrode layer  405   b  depend on the conditions of the polishing treatment for removing the conductive film  445 , and in some cases, as shown in this embodiment, the source electrode layer  405   a  and the drain electrode layer  405   b  are depressed in the film thickness direction from the surfaces of the sidewall insulating layers  412   a  and  412   b  and the insulating film  413  on which polishing treatment is performed. However, depending on the conditions of the polishing treatment, the height of each of the top ends of the source electrode layer  405   a  and the drain electrode layer  405   b  is almost equal to that of each of the top ends of the sidewall insulating layers  412   a  and  412   b  in some cases. 
     Through the above process, the transistor  440   a  of this embodiment can be manufactured (see  FIG. 3C ). 
     In the manufacturing process of the transistor  440   a , the conductive film  445  provided over the gate electrode layer  401 , the insulating film  413 , and the sidewall insulating layers  412   a  and  412   b  is removed by chemical mechanical polishing treatment, so that the conductive film  445  is divided; thus, the source electrode layer  405   a  and the drain electrode layer  405   b  are formed. 
     Further, the source electrode layer  405   a  and the drain electrode layer  405   b  are provided in contact with the exposed portion of a top surface of the oxide semiconductor film  403  and the sidewall insulating layers  412   a  and  412   b . The distance (minimum distance) between the gate electrode layer  401  and a region (contact region) in which the oxide semiconductor film  403  is in contact with the source electrode layer  405   a  or the drain electrode layer  405   b  corresponds to a width of the sidewall insulating layer  412   a  or  412   b  in the channel length direction, whereby the further miniaturization can be achieved and variation in the manufacturing process can be suppressed. 
     Accordingly, the distance between the gate electrode layer  401  and the region (contact region) in which the oxide semiconductor film  403  is in contact with the source electrode layer  405   a  or the drain electrode layer  405   b  can be made short, so that the resistance between the gate electrode layer  401  and the region (contact region) in which the oxide semiconductor film  403  is in contact with the source electrode layer  405   a  or the drain electrode layer  405   b  is reduced; thus, the on-state characteristics of the transistor  440   a  can be improved. 
     Further, precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film  445  over the gate electrode layer  401 , which is one step of the formation process of the source electrode layer  405   a  and the drain electrode layer  405   b . Consequently, in a process for manufacturing the semiconductor device, the transistor  440   a  having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     In the step for removing the conductive film  445  over the gate electrode layer  401 , which is one step of the formation process of the source electrode layer  405   a  and the drain electrode layer  405   b , part or all of the insulating film  413  may be removed.  FIG. 4C  illustrates an example of a transistor  440   c  in which all of the insulating film  413  is removed and the gate electrode layer  401  is exposed. Further, an upper part of the gate electrode layer  401  may also be removed. A structure in which the gate electrode layer  401  is removed as in the transistor  440   c  can be used for an integrated circuit in which another wiring or another semiconductor element is stacked over the transistor  440   c.    
     A highly dense inorganic insulating film (typically an aluminum oxide film) which is to be a protective insulating film may be provided over the transistor  440   a.    
     In this embodiment, the insulating film  407  is formed in contact with the insulating film  413 , the source electrode layer  405   a , the drain electrode layer  405   b , the sidewall insulating layers  412   a  and  412   b , and the interlayer insulating film  415  (see  FIG. 3D ). 
     Further, a highly-dense inorganic insulating film (typically an aluminum oxide film) which is to be a protective insulating film may be provided between the source electrode layer  405   a  and the interlayer insulating film  415  and between the drain electrode layer  405   b  and the interlayer insulating film  415 . 
       FIG. 4B  illustrates an example of a transistor  440   b  in which an insulating film  410  is provided between the source electrode layer  405   a  and the interlayer insulating film  415  and between the drain electrode layer  405   b  and the interlayer insulating film  415 . In the transistor  440   b , a top surface of the insulating film  410  is also planarized by the cutting (grinding or polishing) step which is used in the formation process of the source electrode layer  405   a  and the drain electrode layer  405   b.    
     The insulating films  407  and  410  may have either a single-layer structure or a stacked-layer structure, and preferably contain at least an aluminum oxide film. 
     The insulating films  407  and  410  can be formed by a plasma CVD method, a sputtering method, an evaporation method, or the like. 
     As the insulating films  407  and  410 , an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxynitride film, or a gallium oxide film can be typically used as well as the aluminum oxide film. Furthermore, as the insulating films  407  and  410 , a hafnium oxide film, a magnesium oxide film, a zirconium oxide film, a lanthanum oxide film, a barium oxide film, or a metal nitride film (e.g., an aluminum nitride film) can be used. 
     In this embodiment, an aluminum oxide film is formed by a sputtering method as the insulating films  407  and  410 . When the aluminum oxide film has high density (the film density is higher than or equal to 3.2 g/cm 3 , preferably higher than or equal to 3.6 g/cm 3 ), the transistors  440   a  and  440   b  can have stable electric characteristics. The film density can be measured by Rutherford backscattering spectrometry (RBS) or X-ray reflectometry (XRR). 
     The aluminum oxide film which can be used as the insulating films  407  and  410  provided over the oxide semiconductor film  403  has a high blocking effect by which both oxygen and impurities such as hydrogen or moisture are prevented from passing through the film. 
     Therefore, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of impurities such as hydrogen or moisture, which causes change, into the oxide semiconductor film  403  and release of oxygen, which is a main constituent material of the oxide semiconductor, from the oxide semiconductor film  403 . 
     The insulating films  407  and  410  are preferably formed using a method in which impurities such as water or hydrogen are prevented from entering the insulating films  407  and  410  (preferably a sputtering method or the like) as appropriate. 
     In order to remove residual moisture from the deposition chamber of the insulating films  407  and  410  in a manner similar to that of the formation of the oxide semiconductor film, an entrapment vacuum pump (such as a cryopump) is preferably used. When the insulating films  407  and  410  is formed in the deposition chamber evacuated using a cryopump, the impurity concentration of the insulating films  407  and  410  can be reduced. A turbo molecular pump provided with a cold trap may be used as an evacuation unit for removing moisture remaining in the deposition chamber used for forming the insulating films  407  and  410 . 
     It is preferable that a high-purity gas in which impurities such as hydrogen, water, a hydroxyl group, or hydride are reduced be used as the sputtering gas for the formation of the insulating films  407  and  410 . 
     Further, a planarization insulating film may be formed in order to reduce unevenness due to the transistor. As the planarization insulating film, an organic material such as a polyimide-, acrylic-, or benzocyclobutene-based resin can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. 
     Further,  FIG. 4A  illustrates an example in which openings reaching the source electrode layer  405   a  and the drain electrode layer  405   b  are formed in the interlayer insulating film  415  and the insulating film  407 , and wiring layers  435   a  and  435   b  are formed in the openings. With the use of the wiring layers  435   a  and  435   b , the transistor  440   a  is connected to another transistor or another element, which can lead to formation of a variety of circuits. 
     The wiring layers  435   a  and  435   b  can be formed using a material and a method similar to those of the gate electrode layer  401 , the source electrode layer  405   a , or the drain electrode layer  405   b . For example, 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 (e.g., a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like can be used. A metal film having a high melting point such as Ti, Mo, W, or the like or a metal nitride film of any of these elements (a titanium nitride film, a molybdenum nitride film, and a tungsten nitride film) may be stacked on one of or both of a lower side or an upper side of a metal film of Al, Cu, or the like. The wiring layers  435   a  and  435   b  may be formed using 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 example, as the wiring layers  435   a  and  435   b , a single layer of molybdenum film, a stack of a tantalum nitride film and a copper film, or a stack of a tantalum nitride film and a tungsten film can be used. 
     As described above, in a semiconductor device, the transistors  440   a ,  440   b , and  440   c  having a miniaturized structure with a small amount of variation in shapes or characteristics and high on-state characteristics can be provided with a high yield. 
     Accordingly, a miniaturized semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided. 
     Embodiment 2 
     In this embodiment, an example of a semiconductor device which includes the transistor described in this specification, which can hold stored data even when not powered, and which has an unlimited number of write cycles is described with reference to drawings. 
       FIGS. 5A and 5B  illustrate an example of a structure of a semiconductor device.  FIG. 5A  is a cross-sectional view of the semiconductor device,  FIG. 5B  is a plan view of the semiconductor device, and  FIG. 5C  is a circuit diagram of the semiconductor device. Here,  FIG. 5A  corresponds to a cross section along line C 1 -C 2  and line D 1 -D 2  in  FIG. 5B . 
     The semiconductor device illustrated in  FIGS. 5A to 5C  includes a transistor  160  including a first semiconductor material in a lower portion, and a transistor  162  including a second semiconductor material in an upper portion. The transistor  162  is an example to which a structure of the transistor  440   a  described in Embodiment 1 is applied. 
     Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, charge can be held in a transistor including an oxide semiconductor for a long time owing to its characteristics. 
     Although all the transistors are n-channel transistors here, it is needless to say that p-channel transistors can be used. It is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to the structure described here such as the transistor  162  using an oxide semiconductor used for holding data, which is described in Embodiment 1. 
     The transistor  160  in  FIG. 5A  includes a channel formation region  116  provided in a substrate  185  containing a semiconductor material (e.g., silicon), impurity regions  120  provided so that the channel formation region  116  is sandwiched therebetween, metal compound regions  124  in contact with the impurity regions  120 , a gate insulating film  108  provided over the channel formation region  116 , and a gate electrode  110  provided over the gate insulating film  108 . Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing may be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode are collectively referred to as a “source electrode,” and a drain region and a drain electrode are collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region. 
     Further, an element isolation insulating layer  106  is formed over the substrate  185  to surround the transistor  160 , and an insulating layer  128  and an insulating layer  130  are formed to cover the transistor  160 . Note that, in the transistor  160 , the sidewall insulating layers may be formed on side surfaces of the gate electrode  110  and the impurity regions  120  may include a region having a different impurity concentration. 
     The transistor  160  formed using a single crystal semiconductor substrate can operate at high speed. Thus, when the transistor is used as a reading transistor, data can be read at a high speed. Two insulating films are formed to cover the transistor  160 . As treatment prior to formation of the transistor  162  and a capacitor  164 , CMP treatment is performed on the two insulating films, whereby an insulating layer  128  and an insulating layer  130  which are planarized are formed and, at the same time, a top surface of the gate electrode  110  is exposed. 
     As each of the insulating layer  128  and the insulating layer  130 , typically, it is possible to use an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film. The insulating layer  128  and the insulating layer  130  can be formed by a plasma CVD method, a sputtering method, or the like. 
     Alternatively, an organic material such as a polyimide resin, an acrylic resin, or a benzocyclobutene-based resin can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. In the case of using an organic material, a wet process such as a spin coating method or a printing method may be used to form the insulating layer  128  and the insulating layer  130 . 
     Note that in this embodiment, a silicon nitride film is used as the insulating layer  128 , and a silicon oxide film is used as the insulating layer  130 . 
     Planarization treatment is preferably performed on the surface of the insulating layer  130  in the formation region of the oxide semiconductor film  144 . In this embodiment, the oxide semiconductor film  144  is formed over the insulating layer  130  sufficiently planarized by polishing treatment such as CMP treatment (the average surface roughness of the surface of the insulating layer  130  is preferably less than or equal to 0.15 nm). 
     The transistor  162  illustrated in  FIG. 5A  includes an oxide semiconductor in the channel formation region. Here, an oxide semiconductor film  144  included in the transistor  162  is preferably highly purified. By using a highly purified oxide semiconductor, the transistor  162  which has extremely favorable off-state current characteristics can be obtained. 
     Since the off-state current of the transistor  162  is small, stored data can be held for a long time owing to such a transistor. In other words, power consumption can be sufficiently reduced because a semiconductor device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided. 
     In the process for manufacturing the transistor  162 , a conductive film over a gate electrode  148 , an insulating film  137 , and sidewall insulating layers  136   a  and  136   b  is removed by chemical mechanical polishing treatment to form electrode layers  142   a  and  142   b  functioning as a source electrode layer and a drain electrode layer. 
     Accordingly, in the transistor  162 , the distance between the gate electrode  148  and a region (contact region) in which the oxide semiconductor film  144  is in contact with the electrode layer  142   a  or the electrode layer  142   b  which functions as a source or drain electrode layer can be made short, so that the resistance between the gate electrode  148  and the region (contact region) in which the oxide semiconductor film  144  is in contact with the electrode layer  142   a  or the electrode layer  142   b  is reduced; thus, the on-state characteristics of the transistor  162  can be improved. 
     Further, precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film over the gate electrode  148 , which is one step of the formation process of the electrode layers  142   a  and  142   b . Consequently, in a process for manufacturing the semiconductor device, a transistor having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     An interlayer insulating film  135  and an insulating film  150  each of which has a single-layer structure or a stacked-layer structure are provided over the transistor  162 . In this embodiment, an aluminum oxide film is used as the insulating film  150 . When the aluminum oxide film has high density (the film density is higher than or equal to 3.2 g/cm 3 , preferably higher than or equal to 3.6 g/cm 3 ), the transistor  162  can have stable electric characteristics. 
     In addition, a conductive layer  153  is provided in a region overlapping with the electrode layer  142   a  of the transistor  162  with the interlayer insulating film  135  and the insulating film  150  interposed therebetween, and the electrode layer  142   a , the interlayer insulating film  135 , the insulating film  150 , and the conductive layer  153  form a capacitor  164 . That is, the source electrode layer  142   a  of the transistor  162  functions as one electrode of the capacitor  164 , and the conductive layer  153  functions as the other electrode of the capacitor  164 . Note that the capacitor  164  may be omitted if a capacitor is not needed. Alternatively, the capacitor  164  may be separately provided above the transistor  162 . 
     An insulating layer  152  is provided over the transistor  162  and the capacitor  164 . In addition, a wiring  156  for connecting the transistor  162  to another transistor is provided over the insulating layer  152 . Although not illustrated in  FIG. 5A , the wiring  156  is electrically connected to the electrode layer  142   b  through an electrode formed in an opening provided in the insulating film  150 , the insulating layer  152 , a gate insulating film  146 , and the like. Here, the electrode is preferably provided to partly overlap with at least part of the oxide semiconductor film  144  of the transistor  162 . 
     In  FIGS. 5A and 5B , the transistor  160  is provided to overlap with at least part of the transistor  162 . The source region or the drain region of the transistor  160  is preferably provided to overlap with part of the oxide semiconductor film  144 . Further, the transistor  162  and the capacitor  164  are provided to overlap with at least part of the transistor  160 . For example, the conductive layer  153  of the capacitor  164  is provided to overlap with at least part of the gate electrode  110  of the transistor  160 . With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, higher integration can be achieved. 
     Note that the electrical connection between the electrode layer  142   b  and the wiring  156  may be established by contacting the electrode layer  142   b  with the wiring  156  directly or may be established through an electrode provided in an insulating layer which is between the electrode layer  142   b  and the wiring  156 . Alternatively, the electrical connection may be established through a plurality of electrodes. 
     Next, an example of a circuit configuration corresponding to  FIGS. 5A and 5B  is illustrated in  FIG. 5C . 
     In  FIG. 5C , a first line (1st Line) is electrically connected to a source electrode of the transistor  160 . A second line (2nd Line) is electrically connected to a drain electrode of the transistor  160 . A third line (a 3rd line) and one of source or drain electrodes of the transistor  162  are electrically connected to each other, and a fourth line (a 4th line) and a gate electrode of the transistor  162  are electrically connected to each other. A gate electrode of the transistor  160  and the other of the source electrode and the drain electrode of the transistor  162  are electrically connected to one electrode of a capacitor  164 , and a fifth line (a 5th line) and the other electrode of the capacitor  164  are electrically connected to each other. 
     The semiconductor device in  FIG. 5C  utilizes a characteristic in which the potential of the gate electrode of the transistor  160  can be held, and thus enables data writing, holding, and reading as follows. 
     Writing and holding of data are described. First, the potential of the fourth line is set to a potential at which the transistor  162  is turned on, so that the transistor  162  is turned on. Accordingly, the potential of the third line is supplied to the gate electrode of the transistor  160  and the capacitor  164 . That is, predetermined charge is given to the gate electrode of the transistor  160  (writing). Here, charge for supply of a potential level or charge for supply of a different potential level (hereinafter referred to as low level charge and high level charge) is given. After that, the potential of the fourth line is set to a potential at which the transistor  162  is turned off, so that the transistor  162  is turned off. Thus, the charge given to the gate electrode of the transistor  160  is held (holding). 
     Since the off-state current of the transistor  162  is extremely low, the charge of the gate electrode of the transistor  160  is held for a long time. 
     Next, reading of data is described. By supplying an appropriate potential (reading potential) to the fifth line while a predetermined potential (constant potential) is supplied to the first line, the potential of the second line varies depending on the amount of charge held in the gate electrode of the transistor  160 . This is because in general, when the transistor  160  is an n-channel transistor, an apparent threshold voltage V th   _   H  in the case where a high level charge is given to the gate electrode of the transistor  160  is lower than an apparent threshold voltage V th   _   L  in the case where a low level charge is given to the gate electrode of the transistor  160 . Here, an apparent threshold voltage refers to the potential of the fifth line, which is needed to turn on the transistor  160 . Thus, the potential of the fifth line is set to a potential V 0  which is between V th   _   H  and V th   _   L , whereby charge given to the gate electrode of the transistor  160  can be determined. For example, in the case where a high level charge is given in writing, when the potential of the fifth wiring is set to V 0  (&gt;V th   _   H ), the transistor  160  is turned on. In the case where a low level charge is given in writing, even when the potential of the fifth wiring is set to V 0  (&lt;V th   _   L ), the transistor  160  remains in an off state. Therefore, the stored data can be read by the potential of the second line. 
     Note that in the case where memory cells are arrayed to be used, only data of desired memory cells needs to be read. In the case of a memory cell in which reading is not performed, a potential at which the transistor  160  is turned off, that is, a potential smaller than V th   _   H  may be given to the fifth wiring regardless of the state of the gate electrode of the transistor  160 . Alternatively, a potential which allows the transistor  160  to be turned on regardless of a state of the gate electrode, that is, a potential higher than V th   _   L  may be applied to the fifth lines. 
       FIGS. 19A and 19B  illustrate another example of the semiconductor device.  FIG. 19A  and  FIG. 19B  are a plan view and a cross-sectional view of the semiconductor device, respectively. Here,  FIG. 19B  corresponds to the cross section along line D 3 -D 4  in  FIG. 19A . Note that in  FIG. 19A , some components of the semiconductor device illustrated in  FIG. 19B  are omitted for clarity. 
     In  FIGS. 19A and 19B , the capacitor  164  includes the gate electrode  110 , the oxide semiconductor film  144 , an insulating film  173 , and a conductive layer  174 . The conductive layer  174  is formed by the same steps as the gate electrode  148 , and a top surface of the conductive layer  174  is covered with an insulating film  176 , and side surfaces of the conductive layer  174  are covered with sidewall insulating layers  175   a  and  175   b.    
     The electrode layer  142   b  of the transistor  162  is electrically connected to the wiring  156  in an opening which is formed in the interlayer insulating film  135  and the insulating film  150  and reaches the electrode layer  142   b . Further, a conductive layer  172  is provided below and in contact with the oxide semiconductor film  144 , and electrically connects the transistor  160  to the transistor  162 . 
     As illustrated in  FIGS. 19A and 19B , the transistor  160 , the transistor  162 , and the capacitor  164  are closely stacked to overlap with each other, whereby the area occupied by the semiconductor device can be decreased; thus, the semiconductor device can be highly integrated. 
     When a transistor having a channel formation region formed using an oxide semiconductor and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed). 
     Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate; thus, the problem of deterioration of a gate insulating film does not occur. In other words, the semiconductor device according to an embodiment of the disclosed invention does not have a limit on the number of times of writing which is a problem in a conventional nonvolatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily realized. 
     As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided. 
     The structures and methods described in this embodiment can be combined as appropriate with any of the described in the other embodiments. 
     Embodiment 3 
     In this embodiment, a semiconductor device which includes the transistor described in Embodiment 1 or 2, which can hold stored data even when not powered, and which does not have a limitation on the number of write cycles, and which has a structure different from the structure described in Embodiment 2 is described with reference to  FIGS. 6A and 6B  and  FIGS. 7A and 7B . 
       FIG. 6A  illustrates an example of a circuit configuration of a semiconductor device, and  FIG. 6B  is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in  FIG. 6A  is described, and then, the semiconductor device illustrated in  FIG. 6B  is described below. 
     In the semiconductor device illustrated in  FIG. 6A , a bit line BL is electrically connected to one of the source electrode and the drain electrode of the transistor  162 , a word line WL is electrically connected to the gate electrode of the transistor  162 , and the other of the source electrode and the drain electrode of the transistor  162  is electrically connected to a first terminal of a capacitor  254 . 
     Next, writing and holding of data in the semiconductor device (a memory cell  250 ) illustrated in  FIG. 6A  are described. 
     First, the potential of the word line WL is set to a potential at which the transistor  162  is turned on, and the transistor  162  is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor  254  (writing). After that, the potential of the word line WL is set to a potential at which the transistor  162  is turned off, so that the transistor  162  is turned off. Thus, the potential at the first terminal of the capacitor  254  is held (holding). 
     The transistor  162  including an oxide semiconductor has extremely low off-state current. For that reason, a potential of the first terminal of the capacitor  254  (or a charge accumulated in the capacitor  254 ) can be held for an extremely long period by turning off the transistor  162 . 
     Secondly, reading of data is described. When the transistor  162  is turned on, the bit line BL and the first terminal of the capacitor  254  which are in a floating state are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor  254 . As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor  254  (or the charge accumulated in the capacitor  254 ). 
     For example, the potential of the bit line BL after charge redistribution is (C B *V B0 +C*V)/(C B +C), where V is the potential of the first terminal of the capacitor  254 , C is the capacitance of the capacitor  254 , C B  is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V B0  is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell  250  is in either of two states in which the potentials of the first terminal of the capacitor  254  are V 1  and V 0  (V 1 &gt;V 0 ), the potential of the bit line BL in the case of holding the potential V 1  (=(C B *V B0 +C*V 1 )/(C B +C)) is higher than the potential of the bit line BL in the case of holding the potential V 0  (=(C B *V B0 +C*V 0 )/(C B +C)). 
     Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read. 
     As described above, the semiconductor device illustrated in  FIG. 6A  can hold charge that is accumulated in the capacitor  254  for a long time because the off-state current of the transistor  162  is extremely small. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be stored for a long time even when power is not supplied. 
     Next, the semiconductor device illustrated in  FIG. 6B  is described. 
     The semiconductor device illustrated in  FIG. 6B  includes memory cell arrays  251   a  and  251   b  including a plurality of memory cells  250  illustrated in  FIG. 6A  as memory circuits in the upper portion, and a peripheral circuit  253  in the lower portion which is necessary for operating the memory cell array  251  (the memory cell arrays  251   a  and  251   b ). Note that the peripheral circuit  253  is electrically connected to the memory cell array  251 . 
     In the structure illustrated in  FIG. 6B , the peripheral circuit  253  can be provided under the memory cell array  251  (the memory cell arrays  251   a  and  251   b ). Thus, the size of the semiconductor device can be decreased. 
     It is preferable that a semiconductor material of the transistor provided in the peripheral circuit  253  be different from that of the transistor  162 . For example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like can be used, and a single crystal semiconductor is preferably used. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed can be favorably realized by the transistor. 
     Note that  FIG. 6B  illustrates, as an example, the semiconductor device in which two memory cell arrays  251  (the memory cell array  251   a  and the memory cell array  251   b ) are stacked; however, the number of memory cell arrays to be stacked is not limited thereto. Three or more memory cell arrays may be stacked. 
     Next, a specific structure of the memory cell  250  illustrated in  FIG. 6A  is described with reference to  FIGS. 7A and 7B . 
       FIGS. 7A and 7B  illustrate an example of a structure of the memory cell  250 .  FIG. 7A  is a cross-sectional view of the memory cell  250 , and  FIG. 7B  is a plan view of the memory cell  250 . Here,  FIG. 7A  illustrates a cross section taken along line F 1 -F 2  and line G 1 -G 2  in  FIG. 7B . 
     The transistor  162  in  FIGS. 7A and 7B  can have the same structure as the transistor in Embodiment 1 or 2. 
     An insulating layer  256  having a single-layer structure or a stacked-layer structure is provided over the transistor  162  over which an insulating layer  180  is provided. In addition, a conductive layer  262  is provided in a region overlapping with the electrode layer  142   a  of the transistor  162  with the insulating layer  256  interposed therebetween, and the electrode layer  142   a , the interlayer insulating film  135 , the insulating layer  256 , and the conductive layer  262  form a capacitor  254 . That is, the electrode layer  142   a  of the transistor  162  functions as one electrode of the capacitor  254 , and the conductive layer  262  functions as the other electrode of the capacitor  254 . 
     An insulating film  258  is provided over the transistor  162  and the capacitor  254 . Further, the memory cell  250  and a wiring  260  for connecting the adjacent memory cells  250  are provided over the insulating film  258 . Although not illustrated, the wiring  260  is electrically connected to the electrode layer  142   b  of the transistor  162  through an opening provided in the insulating layer  256 , the insulating film  258 , and the like. The wiring  260  may be electrically connected to the electrode layer  142   b  through another conductive layer provided in the opening. Note that the wiring  260  corresponds to the bit line BL in the circuit diagram of  FIG. 6A . 
     In  FIGS. 7A and 7B , the electrode layer  142   b  of the transistor  162  can also function as a source electrode of a transistor included in an adjacent memory cell. With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, higher integration can be achieved. 
     When the planar layout illustrated in  FIG. 7A  is employed, the area occupied by the semiconductor device can be reduced; thus, the degree of integration can be increased. 
       FIGS. 20A and 20B  illustrate another example of the semiconductor device. 
       FIGS. 20A and 20B  are a plan view and a cross-sectional view of the semiconductor device, respectively. Here,  FIG. 20B  corresponds to a cross section taken along line F 5 -F 6  of  FIG. 20A . Note that in  FIG. 20A , some of components of the semiconductor device illustrated in  FIG. 20B  are omitted for clarity. 
     In  FIGS. 20A and 20B , the capacitor  254  includes a conductive layer  192 , an insulating film  193 , and a conductive layer  194 , and is formed in an insulating film  196 . Note that an insulating material having high dielectric constant is preferably used for the insulating film  193 . The capacitor  254  and the transistor  162  are electrically connected to each other through a conductive layer  191  provided in the opening which is formed in the interlayer insulating film  135 , the insulating film  150 , and an insulating film  195  and reaches the electrode layer  142   a  of the transistor  162 . 
     As illustrated in  FIGS. 20A and 20B , the transistor  162  and the capacitor  254  are closely stacked to overlap with each other, whereby the occupied area of the semiconductor device can be decreased; thus, the semiconductor device can be highly integrated. 
     As described above, the plurality of memory cells is formed in the upper portion with the transistors including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of the refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. 
     A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased. 
     As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Embodiment 4 
     In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to portable devices such as cellular phones, smartphones, or electronic books are described with reference to  FIGS. 8A and 8B ,  FIG. 9 ,  FIG. 10 , and  FIG. 11 . 
     In a portable device such as a cellular phone, a smartphone, or an electronic book, an SRAM or a DRAM is used to store image data temporarily. The reason why an SRAM or a DRAM is used is that a flash memory is slow in responding and is not suitable for image processing. On the other hand, an SRAM or a DRAM has the following characteristics when used for temporary storage of image data. 
     In an ordinary SRAM, as illustrated in  FIG. 8A , one memory cell includes six transistors, that is, transistors  801  to  806 , which are driven with an X decoder  807  and a Y decoder  808 . The transistor  803  and the transistor  805 , and the transistor  804  and the transistor  806  form inverters, which enables high-speed driving. However, because one memory cell includes six transistors, a large cell area is one disadvantage. Provided that the minimum feature size of a design rule is F, the area of a memory cell in an SRAM is generally 100 F 2  to 150 F 2 . Therefore, the price per bit of an SRAM is the most expensive among memory devices. 
     In a DRAM, as illustrated in  FIG. 8B , a memory cell includes a transistor  811  and a storage capacitor  812 , which are driven with an X decoder  813  and a Y decoder  814 . One cell is configured with one transistor and one capacitor and has a small area. The area of a memory cell in a DRAM is generally 10 F 2  or less. Note that the DRAM needs to be refreshed periodically and consumes electric power even when a rewriting operation is not performed. 
     On the other hand, the memory cell of the semiconductor device described in any of the above embodiments has an area of approximately 10 F 2  and does not need to be refreshed frequently. Therefore, the area of a memory cell can be decreased, and power consumption can be reduced. 
     Next,  FIG. 9  is a block diagram of a portable device. The portable device illustrated in  FIG. 9  includes an RF circuit  901 , an analog baseband circuit  902 , a digital baseband circuit  903 , a battery  904 , a power supply circuit  905 , an application processor  906 , a flash memory  910 , a display controller  911 , a memory circuit  912 , a display  913 , a touch sensor  919 , an audio circuit  917 , a keyboard  918 , and the like. The display  913  includes a display portion  914 , a source driver  915 , and a gate driver  916 . The application processor  906  includes a CPU  907 , a DSP  908 , and an interface (IF)  909 . In general, the memory circuit  912  includes an SRAM or a DRAM. By employing the semiconductor device described in any of the above embodiments for that portion, data can be written and read at high speed and can be held for a long time, and power consumption can be sufficiently reduced. 
       FIG. 10  illustrates an example of using the semiconductor device described in any of the above embodiments in a memory circuit  950  for a display. The memory circuit  950  illustrated in  FIG. 10  includes a memory  952 , a memory  953 , a switch  954 , a switch  955 , and a memory controller  951 . The memory circuit  950  is connected to a display controller  956  that reads and controls image data input through a signal line (input image data) and data stored in the memory  952  and the memory  953  (stored image data), and is also connected to a display  957  that displays an image based on a signal input from the display controller  956 . 
     First, image data (input image data A) is produced by an application processor (not illustrated). The input image data A is stored in the memory  952  through the switch  954 . Then, the image data stored in the memory  952  (stored image data A) is transmitted to the display  957  through the switch  955  and the display controller  956 , and is displayed on the display  957 . 
     When the input image data A remains unchanged, the stored image data A is read from the memory  952  through the switch  955  by the display controller  956  normally at a frequency of approximately 30 Hz to 60 Hz. 
     Next, for example, when a user performs an operation to rewrite a screen (i.e., when the input image data A is changed), the application processor produces new image data (input image data B). The input image data B is stored in the memory  953  through the switch  954 . Also during that time, the stored image data A is regularly read from the memory  952  through the switch  955 . After the completion of storing the new image data (the stored image data B) in the memory  953 , from the next frame for the display  957 , the stored image data B starts to be read, transmitted to the display  957  through the switch  955  and the display controller  956 , and is displayed on the display  957 . This reading operation continues until the next new image data is stored in the memory  952 . 
     By alternately writing and reading image data to and from the memory  952  and the memory  953  as described above, images are displayed on the display  957 . Note that the memory  952  and the memory  953  are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in any of the above embodiments for the memory  952  and the memory  953 , data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced. 
       FIG. 11  is a block diagram of an electronic book.  FIG. 11  includes a battery  1001 , a power supply circuit  1002 , a microprocessor  1003 , a flash memory  1004 , an audio circuit  1005 , a keyboard  1006 , a memory circuit  1007 , a touch panel  1008 , a display  1009 , and a display controller  1010 . 
     Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit  1007  in  FIG. 11 . The memory circuit  1007  has a function of temporarily storing the contents of a book. For example, a user may use a highlight function. In some cases, a user wants to mark a specific portion while reading an e-book reader. This marking function is called a highlight function and is used to make a difference from the other portions by changing the display color, underlining, making characters bold, changing the font of characters, or the like. The function makes it possible to store and hold data of a portion specified by a user. In order to store the data for a long time, the data may be copied to the flash memory  1004 . Also in such a case, the semiconductor device described in any of the above embodiments is used, whereby writing and reading of data can be performed at high speed, data can be stored for a long time, and power consumption can be sufficiently reduced. 
     As described above, the portable devices described in this embodiment each incorporate the semiconductor device according to any of the above embodiments. Therefore, it is possible to obtain a portable device which is capable of reading data at high speed, holding data for a long time, and reducing power consumption. 
     The configurations, methods, and the like described in this embodiment can be combined as appropriate with any of the configurations, methods, and the like described in the other embodiments. 
     Embodiment 5 
     A semiconductor device (also referred to as a display device) having a display function can be manufactured using the transistor an example of which is described in the above embodiments. Moreover, 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. 12A , a sealant  4005  is provided to surround a pixel portion  4002  provided over a first substrate  4001 , and the pixel portion  4002  is sealed with a second substrate  4006 . In  FIG. 12A , 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 a substrate separately prepared are mounted in a region that is different from the region surrounded by the sealant  4005  over the first substrate  4001 . A variety of signals and potentials are supplied to the signal line driver circuit  4003 , the scan line driver circuit  4004 , and the pixel portion  4002  from flexible printed circuits (FPCs)  4018   a  and  4018   b.    
     In  FIGS. 12B and 12C , the sealant  4005  is provided to surround the pixel portion  4002  and the scan line driver circuit  4004  which are provided over the first substrate  4001 . The second 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 first substrate  4001 , the sealant  4005 , and the second substrate  4006 . In  FIGS. 12B and 12C , the signal line driver circuit  4003  which is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant  4005  over the first substrate  4001 . In  FIGS. 12B and 12C , a variety of signals and potentials are supplied to the signal line driver circuit  4003 , the scan line driver circuit  4004 , and the pixel portion  4002  from an FPC  4018 . 
     Although  FIGS. 12B and 12C  each illustrate an example in which the signal line driver circuit  4003  is formed separately and mounted over the first substrate  4001 , embodiments of the present invention are not limited to this structure. The scan line driver circuit may be formed separately and then mounted, or only part of the signal line driver circuit or only part of the scan line driver circuit may be formed separately and then mounted. 
     The connection method of such a separately formed driver circuit is not particularly limited; for example, a chip on glass (COG) method, a wire bonding method, or a tape automated bonding (TAB) method can be used.  FIG. 12A  illustrates an example in which the signal line driver circuit  4003  and the scan line driver circuit  4004  are mounted by a COG method;  FIG. 12B  illustrates an example in which the signal line driver circuit  4003  is mounted by a COG method; and  FIG. 12C  illustrates an example in which the signal line driver circuit  4003  is mounted by a TAB method. 
     In addition, the display device includes in its category a panel (a display panel or a light-emitting panel) in which a display element is sealed, and a module in which an IC and the like including a controller are mounted on the panel. 
     The 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, a TAB tape, or a TCP is attached; a module having a TAB tape or 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 first substrate include a plurality of transistors and any of the transistors which are described in the above embodiments can be applied. 
     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 in its category an element whose luminance is controlled by a current or a voltage, and specifically includes an inorganic electroluminescent (EL) element, an organic EL element, and the like. A display medium whose contrast is changed by an electric effect, such as electronic ink, can also be used. 
     An embodiment of a semiconductor device is described with reference to  FIGS. 12A to 12C  and  FIGS. 13A and 13B .  FIGS. 13A and 13B  are cross-sectional diagrams taken along line M-N of  FIG. 12B . 
     As illustrated in  FIGS. 12A to 12C  and  FIGS. 13A and 13B , the semiconductor device includes a connection terminal electrode layer  4015  and a terminal electrode layer  4016 , and the connection terminal electrode layer  4015  and the terminal electrode layer  4016  are electrically connected to a terminal included in the FPC  4018  through an anisotropic conductive film  4019 . 
     The connection terminal electrode layer  4015  is formed using the same conductive film as a first electrode layer  4030 , and the terminal electrode layer  4016  is formed using the same conductive film as source and drain electrode layers of transistors  4010  and  4011 . 
     Each of the pixel portion  4002  and the scan line driver circuit  4004  which are provided over the first substrate  4001  includes a plurality of transistors. In  FIGS. 12A to 12C  and  FIGS. 13A and 13B , the transistor  4010  included in the pixel portion  4002  and the transistor  4011  included in the scan line driver circuit  4004  are illustrated as an example. An interlayer insulating film  4020  and an insulating film  4024  are provided over the transistors  4010  and  4011  in  FIG. 13A , and an insulating film  4021  is further provided in  FIG. 13A . An insulating film  4023  is an insulating film serving as a base film. A light-blocking film  4050  is provided to overlap with the transistors  4010  and  4011 . 
     Any of the transistors described in the above embodiments can be applied to the transistors  4010  and  4011 . In this embodiment, an example in which a transistor having a structure similar to that of the transistor  440   a  described in Embodiment 1 is used is described. 
     In the manufacturing process of the transistors  4010  and  4011 , a conductive film provided over a gate electrode layer, an insulating film, and sidewall insulating layers is removed by chemical mechanical polishing treatment, so that the conductive film is divided; thus, a source electrode layer and a drain electrode layer are formed. 
     Accordingly, the distance between the gate electrode layer and a region (contact region) in which an oxide semiconductor film is in contact with the source electrode layer or the drain electrode layer can be made short, so that the resistance between the gate electrode layer and the region (contact region) in which the oxide semiconductor film is in contact with the source electrode layer or the drain electrode layer is reduced; thus, the on-state characteristics of the transistors  4010  and  4011  can be improved. 
     Precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film over the gate electrode layer, which is one step of the formation process of the source electrode layer and the drain electrode layer. Consequently, in a process for manufacturing the semiconductor device, the transistors  4010  and  4011  each having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     Accordingly, a highly reliable semiconductor device can be provided as the semiconductor device of this embodiment illustrated in  FIGS. 12A to 12C  and  FIGS. 13A and 13B . 
     Further, a conductive layer may be further provided to overlap with a channel formation region of the oxide semiconductor film in the transistor  4011  for the driver circuit. By providing the conductive layer to overlap with the channel formation region in the oxide semiconductor film, the amount of change in the threshold voltage of the transistor  4011  by a bias-temperature stress test (BT test) 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. The potential of the conductive layer may be GND, 0V, or in a floating state. 
     The conductive layer also has a function of blocking 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. 
     The transistor  4010  included in the pixel portion  4002  is electrically connected to a display element to constitute a part of a display panel. There is no particular limitation on the kind of the display element as long as display can be performed, and various kinds of display elements can be used. 
     An example of a liquid crystal display device using a liquid crystal element as a display element is illustrated in  FIG. 13A . In  FIG. 13A , a liquid crystal element  4013  which is a display element includes a first electrode layer  4030 , a second electrode layer  4031 , and a liquid crystal layer  4008 . Insulating films  4032  and  4033  serving as orientation films are provided so that the liquid crystal layer  4008  is provided therebetween. The second electrode layer  4031  is provided on the second substrate  4006  side, and the first electrode layer  4030  and the second electrode layer  4031  are stacked with the liquid crystal layer  4008  provided therebetween. 
     A spacer  4035  is a columnar spacer obtained by selective etching of an insulating film and is provided in order to control the thickness of the liquid crystal layer  4008  (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 low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. 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 not used may be used for the liquid crystal layer  4008 . In that case, the liquid crystal layer  4008  is in contact with the first electrode layer  4030  and the second electrode layer  4031 . The blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of a 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 chiral agent. 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, so that the alignment process is not requisite and the viewing angle dependence is small. In addition, since an alignment film does not need to be provided and thus rubbing treatment is not requisite, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. Thus, productivity of the liquid crystal display device can be improved. A transistor using an oxide semiconductor film has a possibility that the electric characteristics of the transistor may fluctuate significantly by the influence of static electricity and deviate from the designed range. Therefore, it is more effective to use a liquid crystal composition exhibiting a blue phase for the liquid crystal display device including the transistor using an oxide semiconductor film. 
     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, further preferably greater than or equal to 1×10 12  Ω·cm. The specific resistivity in this specification is measured at 20° C. 
     The magnitude of a storage capacitor in the liquid crystal display device is set considering the leakage current of the transistor in the pixel portion or the like so that charge can be held for a predetermined period. The magnitude of the storage capacitor may be set considering the off-state current of the transistor or the like. 
     In the transistor using an oxide semiconductor film used in this embodiment, the current in an off state (off-state current) can be suppressed to be small. Accordingly, an electric signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. The frequency of refresh operation can be accordingly reduced, which leads to an effect of suppressing power consumption. 
     Further, the transistor using an oxide semiconductor film used in this embodiment can exhibit relatively high field-effect mobility and thus can operate at high speed. For example, with such a transistor which can operate at high speed 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. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced. In addition, the transistor which can operate at high speed can be used also in the pixel portion, whereby 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 anti-ferroelectric 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 may be used. Some examples are given as the vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or an advanced super view (ASV) mode 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 (light-blocking layer), an optical member (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 provided by 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. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display. 
     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 the 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 the organic EL element, by application of voltage to the 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 the ground state from the excited state, thereby emitting light. This light-emitting element is called a current-excitation light-emitting element after such a mechanism. 
     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. The 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. The 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. An organic EL element is used as the light-emitting element for description here. 
     To extract light emitted from the light-emitting element, it is necessary that at least one of the pair of electrodes has a light-transmitting property. A transistor and the light-emitting element are formed over a substrate. The light-emitting element can have a top emission structure in which light emission is extracted through a surface opposite to the substrate; a bottom emission structure in which light emission is extracted through a 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; a light-emitting element having any of these emission structures can be used. 
     An example of a light-emitting device (light-emitting panel) in which a light-emitting element is used as a display element is illustrated in  FIG. 13B . A light-emitting element  4513  which is the display element is electrically connected to the transistor  4010  provided in the pixel portion  4002 . A structure of the light-emitting element  4513  is not limited to the shown stacked-layer structure, which is the stacked-layer structure including the first electrode layer  4030 , an electroluminescent layer  4511 , and the second electrode layer  4031 . The structure of the light-emitting element  4513  can be changed as appropriate depending on a direction in which light is extracted from the light-emitting element  4513 , or the like. 
     A partition wall  4510  is formed using an organic insulating material or an inorganic insulating material. It is preferable that the partition wall  4510  be formed using a photosensitive resin material and have an opening over the first electrode layer  4030  so that a sidewall of the opening is formed as a tilted surface with continuous curvature. 
     The electroluminescent layer  4511  has either a single layer or a plurality of layers stacked. 
     A protective film may be formed over the second electrode layer  4031  and the partition wall  4510  in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element  4513 . As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed. In addition, in a space which is formed with the first substrate  4001 , the second substrate  4006 , and the sealant  4005 , a filler  4514  is provided for sealing. It is preferable that a panel be packaged (sealed) with a protective film (such as a laminate film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the panel is not exposed to the outside air, in this manner. 
     As the filler  4514 , an ultraviolet curable resin or a thermosetting resin can be used as well as an inert gas such as nitrogen or argon. For example, polyvinyl chloride (PVC), an acrylic resin, a polyimide resin, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. For example, nitrogen is used as the filler. 
     In addition, as 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/or depressions on the surface to reduce the glare can be performed. 
     Further, electronic paper in which electronic ink is driven can be provided as the display device. The electronic paper is also called electrophoretic display device (electrophoretic display) and is advantageous in that it exhibits the same level of readability as plain paper, it has lower power consumption than other display devices, and it can be made thin and lightweight. 
     Although the electrophoretic display device can have various modes, the electrophoretic display device contains a plurality of microcapsules dispersed in a solvent or a solute, each microcapsule containing 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. The first particles and the second particles each contain a pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless). 
     Thus, an electrophoretic display device is a display device that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a high-electric field region. 
     A solution in which the above microcapsules are dispersed in a solvent is referred to as electronic ink. This electronic ink can be printed on a surface of glass, plastic, cloth, paper, or the like. Further, by using a color filter or particles that have a pigment, color display can also be achieved. 
     The first particles and the second particles in the microcapsules may each be formed of a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material, or formed of a composite material of any of these. 
     As the electronic paper, a display device using a twisting ball display system can be used. The twisting ball display system refers to a method in which spherical particles each colored in black and white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed. 
     In  FIGS. 12A to 12C  and  FIGS. 13A and 13B , a flexible substrate as well as a glass substrate can be used as any of the first substrate  4001  and the second substrate  4006 . For example, a plastic substrate having a light-transmitting property or the like can be used. As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. In the case where the light-transmitting property is not requisite, a metal substrate (metal film) of aluminum, stainless steel, or the like may be used. For example, a sheet with a structure in which an aluminum foil is interposed between PVF films or polyester films can be used. 
     An oxide insulating film can be used for the interlayer insulating film  4020  and the insulating film  4024 . The interlayer insulating film  4020  and the insulating film  4024  can be formed by a plasma CVD method, a sputtering method, or the like using any of silicon oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, and gallium oxide, or a mixed material thereof. Further, a nitride insulating film may be stacked over the oxide insulating film, and the nitride insulating film can be formed using any of silicon nitride, silicon nitride oxide, aluminum nitride, and aluminum nitride oxide, or a mixed material thereof. 
     In this embodiment, an aluminum oxide film is used as the insulating film  4024 . The insulating film  4024  can be formed by a sputtering method or a plasma CVD method. 
     The aluminum oxide film which is provided as the insulating film  4024  over the oxide semiconductor film has a high blocking effect by which both oxygen and impurities such as hydrogen or moisture are prevented from passing through the film. 
     Therefore, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of impurities such as hydrogen or moisture, which causes change, into the oxide semiconductor film and release of oxygen, which is a main constituent material of the oxide semiconductor, from the oxide semiconductor film. 
     The insulating film  4021  serving as a planarization insulating film can be formed using an organic material having heat resistance, such as an acrylic resin, a polyimide resin, a benzocyclobutene-based resin, a polyamide resin, or an epoxy resin. Other than such organic materials, it is also possible to use a low-dielectric constant material (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. The insulating film may be formed by stacking a plurality of insulating films formed of these materials. 
     There is no particular limitation on the method of forming the insulating film  4021 , and the following method or tool (equipment) can be used depending on the material: a sputtering method, an SOG 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), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. 
     The display device displays an image by transmitting light from the light source or the display element. Therefore, the substrate and the thin films such as the insulating film and the conductive film provided for the pixel portion where light is transmitted have light-transmitting properties with respect to light in the visible light wavelength range. 
     The first electrode layer and the second electrode layer (also called pixel electrode layer, common electrode layer, counter electrode layer, or the like) for applying voltage to the display element may have light-transmitting properties or light-reflecting properties, which depends on the direction in which light is extracted, the position where the electrode layer is provided, the pattern structure of the electrode layer, and the like. 
     The first electrode layer  4030  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. 
     The first electrode layer  4030  and the second electrode layer  4031  can be formed using one or plural kinds selected from a metal 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), or silver (Ag); an alloy thereof; and a nitride thereof. 
     A conductive composition containing a conductive high molecule (also called conductive polymer) can be used for the first electrode layer  4030  and the second electrode layer  4031 . As the conductive high molecule, a so-called 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. 
     Since the transistor is likely to be broken owing to static electricity or the like, a protection circuit for protecting the driver circuit is preferably provided. The protection circuit is preferably formed using a nonlinear element. 
     Any of the transistors described in any of the above embodiments as described above enables semiconductor devices having a variety of functions to be provided. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Embodiment 6 
     Any of the transistors described in Embodiment 1 enables a semiconductor device having an image sensor function of reading data on an object to be manufactured. 
       FIG. 14A  illustrates an example of a semiconductor device having an image sensor function.  FIG. 14A  is an equivalent circuit diagram of a photosensor, and  FIG. 14B  is a cross-sectional diagram of part of the photosensor. 
     One electrode of a photodiode  602  is electrically connected to a photodiode reset signal line  658 , and the other electrode of the photodiode  602  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 photosensor reference signal line  672 , and the other of the source and the drain of the transistor  640  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 photosensor output signal line  671 . 
     Note that in circuit diagrams in this specification, a transistor including an oxide semiconductor film is denoted by a symbol “OS” so that it can be identified as a transistor including an oxide semiconductor film. In  FIG. 14A , the transistor  640  and the transistor  656  are transistors each using an oxide semiconductor film, to which any of the transistors described in any of the above embodiments can be applied. Described in this embodiment is an example in which a transistor having a structure similar to that of the transistor  440   a  described in Embodiment 1 is used. 
       FIG. 14B  is a cross-sectional diagram of the photodiode  602  and the transistor  640  in the photosensor. The photodiode  602  functioning as a sensor and the transistor  640  are provided over a substrate  601  (TFT substrate) having an insulating surface. A substrate  613  is provided over the photodiode  602  and the transistor  640  with the use of an adhesive layer  608 . 
     An interlayer insulating film  632 , an insulating film  633 , and an interlayer insulating film  634  are provided over the transistor  640  provided over the insulating film  631 . The photodiode  602  is provided over the insulating film  633 . In the photodiode  602 , a first semiconductor film  606   a , a second semiconductor film  606   b , and a third semiconductor film  606   c  are sequentially stacked from the insulating film  633  side, between an electrode layer  642  formed over the interlayer insulating film  634  and electrode layers  641   a  and  641   b  which are formed over the insulating film  633 . 
     Note that a light-blocking film  650  is provided in a region which overlaps with the transistor  640 . 
     The electrode layer  641   b  is electrically connected to a conductive layer  643  formed over the interlayer insulating film  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. The first semiconductor film  606   a  is formed by a plasma CVD method with the use of a semiconductor source gas containing an impurity element belonging to Group 13 (e.g., boron (B)). As the semiconductor source 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 by a diffusion method or an ion implantation method. Heating or the like may be performed after the impurity element is introduced by an ion implantation method or the like to diffuse the impurity element. In that case, as a method of forming the amorphous silicon film, an LPCVD method, a 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 with the use of a semiconductor source gas. As the semiconductor source 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. The third semiconductor film  606   c  is formed by a plasma CVD method with the use of a semiconductor source gas containing an impurity element belonging to Group 15 (e.g., phosphorus (P)). As the semiconductor source 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 by a diffusion method or an ion implantation method. Heating or the like may be performed after the impurity element is introduced by an ion implantation method or the like to diffuse the impurity element. In that case, as the 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). 
     Considering Gibbs free energy, the microcrystalline semiconductor is in a metastable state that is intermediate between an amorphous state and a single crystal state. That is, the microcrystalline semiconductor is a semiconductor having a third state which is stable in terms of free energy and has a short range order and lattice distortion. Columnar-like or needle-like crystals grow in a normal direction with respect to a substrate surface. The Raman spectrum of microcrystalline silicon, which is a typical example of a microcrystalline semiconductor, is located in lower wave numbers than 520 cm −1 , which represents a peak of the Raman spectrum of single crystal silicon. That is, the peak of the Raman spectrum of the microcrystalline silicon exists between 520 cm −1  which represents single crystal silicon and 480 cm −1  which represents amorphous silicon. In addition, microcrystalline silicon contains hydrogen or halogen of at least 1 at. % in order to terminate a dangling bond. Moreover, microcrystalline silicon contains a rare gas element such as helium, argon, krypton, or neon to further promote lattice distortion, so that the stability is increased and thus a favorable microcrystalline semiconductor film can be obtained. 
     This microcrystalline semiconductor film can be formed by a radio-frequency plasma CVD method with a frequency of greater than or equal to several tens of megahertz and less than or equal to several hundreds of megahertz, or a microwave plasma-enhanced CVD apparatus with a frequency of greater than or equal to 1 GHz. As a typical example, the microcrystalline semiconductor can be formed using a compound containing silicon such as SiH 4 , Si 2 H 6 , SiH 2 Cl 2 , SiHCl 3 , SiCl 4 , or SiF 4 , which is diluted with hydrogen. The microcrystalline semiconductor film can also be formed with dilution with one or plural kinds of rare gas elements selected from helium, argon, krypton, and neon in addition to the compound containing silicon (e.g., silicon hydride) and hydrogen. In those cases, the flow ratio of hydrogen to the compound containing silicon (e.g., silicon hydride) is 5:1 to 200:1, preferably 50:1 to 150:1, further preferably 100:1. Further, a carbide gas such as CH 4  or C 2 H 6 , a germanium gas such as GeH 4  or GeF 4 , F 2 , or the like may be mixed into the gas containing silicon. 
     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 film. A surface on the n-type semiconductor film side can alternatively be used as the light-receiving plane. 
     With the use of an insulating material, the insulating film  631 , the interlayer insulating film  632 , and the insulating film  633  can be formed, depending on the material, by a method or a tool (equipment) such as a sputtering method, a plasma CVD method, an SOG 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), a doctor knife, a roll coater, a curtain coater, or a knife coater. 
     In this embodiment, an aluminum oxide film is used as the insulating film  633 . The insulating film  633  can be formed by a sputtering method or a plasma CVD method. 
     The aluminum oxide film which is provided as the insulating film  633  over the oxide semiconductor film has a high blocking effect by which both oxygen and impurities such as hydrogen or moisture are prevented from passing through the film. 
     Therefore, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of impurities such as hydrogen or moisture, which causes change, into the oxide semiconductor film and release of oxygen, which is a main constituent material of the oxide semiconductor, from the oxide semiconductor film. 
     In this embodiment, in the manufacturing process of the transistor  640 , a conductive film provided over a gate electrode layer, an insulating film, and sidewall insulating layers are removed by chemical mechanical polishing treatment, so that the conductive film is divided; thus, a source electrode layer and a drain electrode layer are formed. 
     Accordingly, the distance between the gate electrode layer and a region (contact region) in which the oxide semiconductor film is in contact with the source electrode layer or the drain electrode layer can be made short, so that the resistance between the gate electrode layer and the region (contact region) in which the oxide semiconductor film is in contact with the source electrode layer or the drain electrode layer is reduced; thus, the on-state characteristics of the transistor  640  can be improved. 
     Precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film over the gate electrode layer, which is one step of the formation process of the source electrode layer and the drain electrode layer. Consequently, in a process for manufacturing the semiconductor device, the transistor  640  having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     An inorganic insulating material is used for the insulating film  631 , the interlayer insulating film  632 , and the insulating film  633 . The insulating film  631 , the interlayer insulating film  632 , and the insulating film  633  can have a single-layer structure or a stacked-layer structure including any of oxide insulating films such as a silicon oxide layer, a silicon oxynitride layer, an aluminum oxide layer, and an aluminum oxynitride layer; and nitride insulating films such as a silicon nitride layer, a silicon nitride oxide layer, an aluminum nitride layer, and an aluminum nitride oxide layer. 
     To reduce surface roughness, an insulating film functioning as a planarization insulating film is preferably used as the interlayer insulating film  634 . For the interlayer insulating film  634 , for example, an organic insulating material having heat resistance, such as a polyimide resin, an acrylic resin, a benzocyclobutene-based resin, polyamide, or an epoxy resin, 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 (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. 
     With detection of light  622  that enters the photodiode  602 , data on an object to be detected can be read. A light source such as a backlight can be used at the time of reading data on the object. 
     As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Embodiment 7 
     In this embodiment, electronic devices according to an embodiment of the present invention are described. Specifically, electronic devices which include a display panel or a light-emitting panel including the transistor described in the above embodiments are described with reference to  FIGS. 15A to 15F . 
     Examples of an electronic device to which a semiconductor device is applied include a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game machine, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like. Specific examples of these electronic devices are illustrated in  FIGS. 15A to 15F . 
       FIG. 15A  illustrates an example of a television set. In a television set  7100 , a display portion  7103  is incorporated in a housing  7101 . Images can be displayed on the display portion  7103 , and a display panel can be used for the display portion  7103 . In this structure, the housing  7101  is supported by a stand  7105 . 
     The television set  7100  can be operated by an operation switch of the housing  7101  or a separate remote controller  7110 . With operation keys  7109  of the remote controller  7110 , channels and volume can be controlled and images displayed on the display portion  7103  can be controlled. Further, the remote controller  7110  may be provided with a display portion  7107  for displaying data output from the remote controller  7110 . 
     The television set  7100  is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, the television set can be connected to a communication network with or without wires via the modem, whereby one-way (from sender to receiver) or two-way (between sender and receiver or between receivers) data communication can be performed. 
       FIG. 15B  illustrates a computer, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connecting port  7205 , a pointing device  7206 , and the like. This computer is formed using the display panel for the display portion  7203 . 
       FIG. 15C  illustrates a portable game machine having two housings, a housing  7301  and a housing  7302 , which are connected with a joint portion  7303  so that the portable game machine can be opened or folded. A display portion  7304  is incorporated in the housing  7301  and a display portion  7305  is incorporated in the housing  7302 . In addition, the portable game machine illustrated in  FIG. 15C  includes a speaker portion  7306 , a recording medium insertion portion  7307 , an LED lamp  7308 , an input means (an operation key  7309 , a connection terminal  7310 , a sensor  7311  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, tilt angle, vibration, smell, or infrared rays), or a microphone  7312 ), and the like. It is needless to say that the structure of the portable game machine is not limited to the above as long as the display panel is used for at least either the display portion  7304  or the display portion  7305 , or both, and can include other accessories arbitrarily. The portable game machine illustrated in  FIG. 15C  has a function of reading a program or data stored in a recording medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. The portable game machine illustrated in  FIG. 15C  can have a variety of functions without limitation to the above. 
       FIG. 15D  illustrates an example of a mobile phone. A cellular phone  7400  is provided with a display portion  7402  incorporated in a housing  7401 , operation buttons  7403 , an external connection port  7404 , a speaker  7405 , a microphone  7406 , and the like. The mobile phone  7400  is manufactured using the display panel for the display portion  7402 . 
     When the display portion  7402  of the cellular phone  7400  illustrated in  FIG. 15D  is touched with a finger or the like, data can be input into the cellular phone  7400 . Further, operations such as making a call and creating e-mail can be performed by touch on the display portion  7402  with a finger or the like. 
     There are mainly three screen modes of the display portion  7402 . The first mode is a display mode mainly for displaying images. The second mode is an input mode mainly for inputting data such as text. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are combined. 
     For example, in the case of making a call or composing an e-mail, a text input mode mainly for inputting text is selected for the display portion  7402  so that text displayed on a screen can be input. In that case, it is preferable to display a keyboard or number buttons on almost all the area of the screen of the display portion  7402 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the cellular phone  7400  (whether the cellular phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are switched by touching the display portion  7402  or operating the operation buttons  7403  of the housing  7401 . Alternatively, the screen modes can be switched depending on kinds of images displayed on the display portion  7402 . For example, when a signal of an image displayed on the display portion is a signal of moving image data, the screen mode is switched to the display mode. When the signal is a signal of text data, the screen mode is switched to the input mode. 
     Moreover, in the input mode, when input by touching the display portion  7402  is not performed within a specified period while a signal detected by an optical sensor in the display portion  7402  is detected, the screen mode may be controlled to be switched from the input mode to the display mode. 
     The display portion  7402  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion  7402  with the palm or the finger, whereby personal authentication can be performed. Further, by providing a backlight or a sensing light source which emits a near-infrared light in the display portion, an image of a finger vein, a palm vein, or the like can be taken. 
       FIG. 15E  illustrates an example of a flat shaped computer. A flat shaped computer  7450  includes a housing  7451 L and a housing  7451 R which are connected to each other by a hinge  7454 . Further, in addition to an operation button  7453 , a left side speaker  7455 L, and a right side speaker  7455 R, an external connection port  7456  which is not illustrated is further provided on the side surface of the computer  7450 . When the hinge  7454  is folded so that a display portion  7452 L provided in the housing  7451 L and a display portion  7452 R provided in the housing  7451 R face each other, the display portions can be protected by the housings. 
     Each of the display portions  7452 L and  7452 R is a component which can display images and to which information can be input by touch with a finger or the like. For example, an icon indicating an installed program is selected by touching with a finger, so that a program is started up. Further, when the distance between fingers touching the two portions of the displayed image is changed, the image can be enlarged or shrank. When a finger touching one portion of the displayed image is moved, the image can be moved. When an image of a keyboard is displayed and a displayed letter or symbol is selected by touching with a finger, information can be input. 
     Further, a gyroscope, an acceleration sensor, a global positioning system (GPS) receiver, a fingerprint sensor, and a video camera can be provided for the computer  7450 . When a detection device including a sensor which detects gradients such as a gyroscope or an acceleration sensor is provided, the orientation of an image can be automatically switched by determining the orientation of the computer  7450  (whether the mobile phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     Further, the computer  7450  can be connected to a network. The computer  7450  can display the information which is on the internet, and can be used as a remote-control terminal for another device which is connected to a network. 
       FIG. 15F  illustrates an example of a lighting device. In a lighting device  7500 , light-emitting panels  7503   a  to  7503   d  of an embodiment of the present invention are incorporated in a housing  7501  as light sources. The lighting device  7500  can be attached to a ceiling, a wall, or the like. 
     The light-emitting device according to an embodiment of the present invention includes a light-emitting panel in a thin film form. Thus, when the light-emitting device is attached to a base with a curved surface, a semiconductor device with a curved surface can be obtained. In addition, when the light-emitting panel is located in a housing with a curved surface, an electronic device or a lighting device with a curved surface can be obtained. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Example 1 
     In this example, a transistor described in Embodiment 1 was formed, and a cross-section of the transistor was observed. 
     As the transistor, Example Transistor  1  which has a similar structure to the transistor  440   a  illustrated in  FIGS. 1A and 1B  was formed. A method for manufacturing Example Transistor  1  is described below. 
     As an insulating layer  11 , a 100-nm-thick silicon oxide film was formed over a silicon substrate  10  by a sputtering method (the formation conditions: an atmosphere of oxygen (oxygen=50 sccm), a pressure of 0.4 Pa, a power (power supply output) of 5.0 kW, a distance between the silicon substrate and a target of 60 mm, and a substrate temperature of 100° C.). 
     As an oxide semiconductor film  12 , a 20-nm-thick IGZO film was formed over the silicon oxide film by a sputtering method with the use of an oxide target containing In, Ga, and Zn in the atomic ratio of 3:1:2. The formation conditions were as follows: the atmosphere was an atmosphere of argon and oxygen (argon:oxygen=30 sccm:15 sccm), the pressure was 0.4 Pa, the power was 0.5 kW, and the substrate temperature was 200° C. 
     Next, as a gate insulating film, a 20-nm-thick silicon oxynitride film was formed over the IGZO film by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=1 sccm:800 sccm), a pressure of 40 Pa, a power of the RF power supply (power supply output) of 150 W, a power supply frequency of 60 MHz, and a substrate temperature of 400° C.). 
     A 100-nm-thick tungsten film was formed over the gate insulating film by a sputtering method (the formation conditions: an atmosphere of argon (argon=100 sccm), a pressure of 0.2 Pa, and a power of 1 kW) and a 200-nm-thick silicon oxynitride film was stacked over the tungsten film by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=27 sccm:1000 sccm), a pressure of 133.3 Pa, a power of the RF power supply of 60 W, a power supply frequency of 13.56 MHz, and a substrate temperature of 325° C.). 
     The silicon oxynitride film was etched by a dry etching method to form an insulating film  15  (the etching conditions: an etching gas of CF 3 , He, and CH 4  (CF 3 :He:CH 4 =22.5 sccm:127.5 sccm:5 sccm), a power of the ICP power supply of 475 W, a bias power of 300 W, and a pressure of 3.5 Pa). 
     Next, the tungsten film was etched by a dry etching method to form a gate electrode layer  14  (the etching conditions: an etching gas of CF 4 , Cl 2 , and O 2  (CF 4 :Cl 2 :O 2 =25 sccm:25 sccm:10 sccm), a power of the ICP power supply of 500 W, a bias power of 100 W, a pressure of 1.0 Pa, and a substrate temperature of 70° C.). 
     As an insulating film, a 70-nm-thick silicon oxynitride film was formed over the gate electrode layer  14  and the insulating film  15  by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=1 sccm:800 sccm), a pressure of 40 Pa, a power of the RF power (power supply output) of 150 W, a power supply frequency of 60 MHz, and a substrate temperature of 400° C.). Further, the silicon oxynitride film was etched by a dry etching method (the etching conditions: an etching gas of CHF 3  and He (CHF 3 :He=56 sccm:144 sccm), a power of the ICP power supply of 25 W, a bias power of 425 W, a pressure of 7.5 Pa, and a substrate temperature of 70° C.). Accordingly, sidewall insulating layers  16   a  and  16   b  were formed. The gate insulating film was etched with the use of the gate electrode layer  14  and the sidewall insulating layers  16   a  and  16   b  as masks, so that a gate insulating film  13  was formed. 
     A 30-nm-thick tungsten film was formed over the oxide semiconductor film  12 , the gate insulating film  13 , the sidewall insulating layers  16   a  and  16   b , and the insulating film  15  by a sputtering method (the formation conditions: an atmosphere of argon (argon=80 sccm), a pressure of 0.8 Pa, a power of 1 kW, and a substrate temperature of 230° C.). 
     A 500-nm-thick silicon oxynitride film was formed over the tungsten film by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=27 sccm:1000 sccm), a pressure of 133.3 Pa, a power of the RF power supply of 60 W, a power supply frequency of 13.56 MHz, and a substrate temperature of 325° C.). 
     Next, the silicon oxynitride film and the tungsten film were polished by a chemical mechanical polishing method (the polishing conditions: a hard polyurethane-based polishing cloth, alkaline silica-based slurry, a slurry temperature of room temperature, a polishing pressure of 0.08 MPa, a rotation number in polishing (table/spindle) of 50 rpm/50 rpm, and a polishing time of 2 minutes), so that the silicon oxynitride film and the tungsten film which was over the gate electrode layer  14  were removed to expose the insulating film  15 . 
     By the polishing treatment, the silicon oxynitride film was processed to form an insulating film  18 , and the tungsten film was divided to form a source electrode layer  17   a  and a drain electrode layer  17   b.    
     Through the process, Example Transistor  1  was formed. 
     An end face of Example Transistor  1  was cut, and a cross section of Example Transistor  1  was observed using a scanning transmission electron microscopy (STEM). In this example, as the STEM, an Ultra-thin Film Evaluation System HD-2300 manufactured by Hitachi High-Technologies Corporation was used.  FIG. 16  illustrates a cross-sectional STEM image of Example Transistor  1 . 
       FIG. 16  is the cross-sectional STEM image of Example Transistor  1  in the channel length direction, and it can be confirmed that that the source electrode layer  17   a  and the drain electrode layer  17   b  are separated by the polishing treatment. The source electrode layer  17   a  and the drain electrode layer  17   b  are provided in contact with side surfaces of the sidewall insulating layers  16   a  and  16   b  provided on side surfaces of the gate electrode layer  14 , and in this example, the source electrode layer  17   a  and the drain electrode layer  17   b  each of which has an upper end portion positioned a little lower than those of the sidewall insulating layers  16   a  and  16   b  cover the side surfaces of the sidewall insulating layers  16   a  and  16   b . The shapes of the source electrode layer  17   a  and the drain electrode layer  17   b  depend on the conditions of the polishing treatment for dividing the conductive film, and in some cases, the source electrode layer  17   a  and the drain electrode layer  17   b  are depressed in the film thickness direction from the surfaces of the sidewall insulating layers  16   a  and  16   b  and the insulating film  15  which are polished was performed as shown in this example. 
     Note that in  FIG. 16 , the width of the lower base of the gate electrode layer  14  which is trapezoid is approximately 382 nm and the width of the upper base thereof is approximately 364 nm, the width of the sidewall insulating layers  16   a  and  16   b  in the channel length direction is approximately 51.6 nm, the thickness of the insulating film  15  which is provided over the gate electrode layer  14  is approximately 44.1 nm, the thickness of the insulating film  18 , which is from a portion where the source electrode layer  17   a  or the drain electrode layer  17   b  is in contact with the oxide semiconductor film  12  to the surface of the insulating film  18 , is approximately 139.8 nm. 
     In Example Transistor  1 , a conductive film provided over the gate electrode layer  14 , the insulating film  15 , and the sidewall insulating layers  16   a  and  16   b  was removed by chemical mechanical polishing treatment, so that the conductive film was divided; thus, the source electrode layer  17   a  and the drain electrode layer  17   b  are formed. 
     Accordingly, the distance between the gate electrode layer  14  and a region (contact region) in which the oxide semiconductor film  12  is in contact with the source electrode layer  17   a  or the drain electrode layer  17   b  can be made short, so that the resistance between the gate electrode layer  14  and the region (contact region) in which the oxide semiconductor film  12  is in contact with the source electrode layer  17   a  or the drain electrode layer  17   b  is reduced; thus, the on-state characteristics of the transistor can be improved. 
     Precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film over the gate electrode layer  14 , which was one step of the formation process of the source electrode layer  17   a  and the drain electrode layer  17   b . Consequently, in a process for manufacturing the semiconductor device, a transistor having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     As described in this example, a miniaturized transistor having high electric characteristics can be provided with high yield. In addition, a semiconductor device including the transistor can have high performance, high reliability, and high productivity. 
     Example 2 
     In this example, a transistor according to one embodiment of the semiconductor device disclosed in this specification was formed and the electric characteristics were evaluated. 
     As the transistor, Example Transistor  2  which has a similar structure to a transistor  340  illustrated in  FIG. 17  was formed. A method for manufacturing Example Transistor  2  is described below. 
     As an insulating film  336 , a 300-nm-thick silicon oxide film was formed over a silicon substrate  300  by a sputtering method (the formation conditions: an atmosphere of oxygen (oxygen=50 sccm), a pressure of 0.4 Pa, a power (power supply output) of 1.5 kW, a distance between the silicon substrate and a target of 60 mm, and a substrate temperature of 100° C.). 
     Polishing treatment was performed on a surface of the insulating film  336  by a chemical mechanical polishing method (the polishing pressure was 0.08 MPa and the polishing time was 0.5 minutes). 
     As an oxide semiconductor film, a 10-nm-thick IGZO film was formed over the insulating film  336  subjected to the polishing treatment, by a sputtering method with the use of an oxide target containing In, Ga, and Zn in the atomic ratio of 3:1:2. The formation conditions were as follows: the atmosphere was an atmosphere of argon and oxygen (argon:oxygen=30 sccm:15 sccm), the pressure was 0.4 Pa, the power was 0.5 kW, and the substrate temperature was 200° C. 
     Next, the oxide semiconductor film was etched by a dry etching method to form an island-shaped oxide semiconductor film  303  (the etching conditions: an etching gas of BCL 3  and Cl 2  (BCL 3 :Cl 2 =60 sccm:20 sccm), a power of the ICP power supply of 450 W, a bias power of 100 W, and a pressure of 1.9 Pa). 
     Next, as the gate insulating film, a 20 nm-thick silicon oxynitride film was formed over the island-shaped oxide semiconductor film  303  by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=1 sccm:800 sccm), a pressure of 40 Pa, a power of the RF power supply (power supply output) of 150 W, a power supply frequency of 60 MHz, and a substrate temperature of 400° C.). 
     A stack of a 30-nm-thick tantalum nitride film and a 135-nm-thick tungsten film were formed over the gate insulating film by a sputtering method. The formation conditions of the tantalum nitride film were as follows: the atmosphere was an atmosphere of argon and nitrogen (argon:nitrogen=50 sccm:10 sccm), the pressure was 0.6 Pa, and the power was 1 kW. The formation conditions of the tungsten film were as follows: the atmosphere was an atmosphere of argon (argon=100 sccm), the pressure was 2.0 Pa, and the power was 4 kW. 
     Next, a 200-nm-thick silicon oxynitride film was stacked over the tungsten film by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=27 sccm:1000 sccm), a pressure of 133.3 Pa, a power of the RF power supply of 60 W, a power supply frequency of 13.56 MHz, and a substrate temperature of 325° C.). 
     The silicon oxynitride film was etched by a dry etching method to form an insulating film  313  (the etching conditions: an etching gas of CHF 3 , He, and CH 4  (CHF 3 :He:CH 4 =22.5 sccm:127.5 sccm:5 sccm), a power of the ICP power supply of 475 W, a bias power of 300 W, and a pressure of 3.5 Pa). 
     The tantalum nitride film and the tungsten film were etched by dry etching methods, so that a gate electrode layer  301  was formed. The first etching conditions were as follows: the etching gas was CF 4 , Cl 2 , and O 2  (CF 4 :Cl 2 :O 2 =25 sccm:25 sccm:10 sccm), the power of the ICP power supply was 500 W, the bias power was 100 W, and the pressure was 1.0 Pa. The second etching conditions were as follows: the etching gas was Cl 2  (Cl 2 =100 sccm), the power was 2 kW, the bias power was 50 W, and the pressure was 1.0 Pa. The third etching conditions were as follows: the etching gas was Cl 2  (Cl 2 =100 sccm), the power was 1 kW, the bias power was 25 W, and the pressure was 2.0 Pa). 
     Phosphorus (P) ions were implanted into the oxide semiconductor film  303  by an ion implantation method with the use of gate electrode layer  301  as a mask, whereby low-resistance regions  304   a  and  304   b  are formed. Note that the conditions of the phosphorus (P) ion implantation were as follows: the acceleration voltage was 25 kV and the dosage was 1.0×10 15  ions/cm 2 . 
     As an insulating film, a 90-nm-thick silicon oxynitride film was formed over the gate electrode layer  301  and the insulating film  313  by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=1 sccm:800 sccm), a pressure of 40 Pa, a power of the RF power supply (power supply output) of 150 W, a power supply frequency of 60 MHz, and a substrate temperature of 400° C.). Then, the silicon oxynitride film was etched by a dry etching method to form sidewall insulating layers  312   a  and  312   b . The gate insulating film was etched with the use of the gate electrode layer  301  and the sidewall insulating layers  312   a  and  312   b  as masks, so that a gate insulating film  302  was formed. The etching conditions used for forming the sidewall insulating layers  312   a  and  312   b  and the gate insulating film  302  were as follows: the etching gas was CHF 3  and He (CHF 3 :He=30 sccm:120 sccm), the power of the power supply was 3 kW, the bias power was 200 W, the pressure was 2.0 Pa, and the substrate temperature was −10° C.) 
     A 30-nm-thick tungsten film was formed over the oxide semiconductor film  303 , the gate electrode layer  301 , the sidewall insulating layers  312   a  and  312   b , and the insulating film  313  by a sputtering method (the formation conditions: an atmosphere of argon (argon=80 sccm), a pressure of 0.8 Pa, a power of 1 kW, and a substrate temperature of 230° C.). 
     Next, a tungsten film was etched by a dry etching method (the etching conditions: an etching gas of CF 4 , Cl 2 , and O 2  (CF 4 :Cl 2 :O 2 =55 sccm:45 sccm:55 sccm), a power of 3 kW, a bias power of 140 W, and a pressure of 0.67 Pa), so that an island-shaped tungsten film was formed. 
     Next, as an insulating film, a 70-nm-thick aluminum oxide film was formed over the oxide semiconductor film  303 , the gate electrode layer  301 , the sidewall insulating layers  312   a  and  312   b , the insulating film  313 , and the tungsten film by a sputtering method. The formation conditions were as follows: the atmosphere was an atmosphere of argon and oxygen (argon:oxygen=25 sccm:25 sccm), the pressure was 0.4 Pa, the power was 2.5 kW, a distance between the silicon substrate and a target was 60 mm, and the substrate temperature was 250° C. 
     A 430-nm-thick silicon oxynitride film was formed over the aluminum oxide film by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=27 sccm:1000 sccm), a pressure of 133.3 Pa, a power of the RF power supply of 60 W, a power supply frequency of 13.56 MHz, and a substrate temperature of 325° C.). 
     Next, the silicon oxynitride film, the aluminum oxide film, and the tungsten film were polished by a chemical mechanical polishing method (the polishing conditions: a hard polyurethane-based polishing cloth, alkaline silica-based slurry, a slurry temperature of room temperature, a polishing pressure of 0.08 MPa, and a rotation number in polishing (table/spindle) of 51 rpm/50 rpm), so that the silicon oxynitride film, the aluminum oxide film, and the tungsten film each of which was over the gate electrode layer  301  were removed to expose the insulating film  313 . 
     By the polishing treatment, the silicon oxynitride film was processed to form an insulating film  315 , the aluminum oxide film was processed to form an insulating film  310 , and the tungsten film was divided to form a source electrode layer  305   a  and a drain electrode layer  305   b.    
     As an insulating film  307 , a 400-nm-thick silicon oxynitride film was formed over the gate electrode layer  301 , the insulating film  313 , the source electrode layer  305   a , the drain electrode layer  305   b , the insulating film  310 , and the insulating film  315  by a CVD method (the formation conditions: an atmosphere of SiH 4  and N 2 O (SiH 4 :N 2 O=27 sccm:1000 sccm), a pressure of 133.3 Pa, a power of the RF power supply of 60 W, a power supply frequency of 13.56 MHz, and a substrate temperature of 325° C.). 
     Openings reaching the source electrode layer  305   a  and the drain electrode layer  305   b  were formed in the insulating film  307 , the insulating film  315 , and the insulating film  310  (the first etching conditions: an etching gas of CHF 3  and He (CHF 3 :He=7.5 sccm:142.5 sccm), a power of the ICP power supply of 475 W, a bias power of 300 W, a pressure of 5.5 Pa, and a time of 192 seconds; and the second etching conditions: an etching gas of CHF 3  and He (CHF 3 :He=7.5 sccm:142.5 sccm), a power of the ICP power supply of 475 W, a bias power of 150 W, a pressure of 5.5 Pa, and a time of 78 seconds). 
     In the openings, a 50-nm-thick titanium film (the formation conditions: an atmosphere of argon (argon=20 sccm), a pressure of 0.1 Pa, and a power of 12 kW), a 100-nm-thick aluminum film (the formation conditions: an atmosphere of argon (argon=50 sccm), a pressure of 0.4 Pa, and a power of 1 kW), and a 50-nm-thick titanium film (the formation conditions: an atmosphere of argon (argon=20 sccm), a pressure of 0.1 Pa, and a power of 12 kW) were stacked by a sputtering method. 
     The stack of the titanium film, the aluminum film, and the titanium film was etched (the etching conditions: an etching gas of BCL 3  and Cl 2  (BCL 3 :Cl 2 =60 sccm:20 sccm), a power of the ICP power supply of 450 W, a bias power of 100 W, and a pressure of 1.9 Pa), so that wiring layers  335   a  and  335   b  were formed. 
     Through the above steps, a transistor  340  was formed as Example Transistor  2 . 
     Note that in Example Transistor  2 , the channel width (W) was 10 μm, the distance between the gate electrode layer  301  and an opening in which the oxide semiconductor film  303  was in contact with the source electrode layer  305   a  or the drain electrode layer  305   b  was 0.07 μm. 
     The electric characteristics of Example Transistor  2  were evaluated. 
       FIG. 18  shows gate voltage (V g )-drain current (I d ) characteristics when the drain voltage (V d ) was 1 V, and gate voltage (V g )-drain current (I d ) characteristics when the drain voltage (V d ) was 0.1 V. Note that  FIG. 18  shows the electric characteristics of Example Transistor  2  in which a case where the channel length (L) was 0.35 μm and the measurement range was a gate voltage from −4 to +4. 
     As shown in  FIG. 18 , Example Transistor  2  shows electric characteristics as a switching element, and when the drain voltage (V d ) is 1 V and the gate voltage (V g ) is 2.7 V, the on-state current of Example Transistor  2  is 45.1 μA, the threshold voltage (V th ) is −0.27 V, the subthreshold swing (S value) is 73.8 mV/dec., and when the drain voltage (V d ) is 0.1 V, the field effect mobility is 3.0 cm 2 /Vs. Note that the on-state current, the threshold voltage (V th ), the subthreshold swing (S value), and the field effect mobility are each a median value of the measured data. 
     In-plane variation was evaluated in the case where the electric characteristics of Example Transistor  2  were measured at 100 points. 
       FIG. 21  shows a normal probability graph of the on-state current when the drain voltage is 1 V and the gate voltage (V g ) is 2.7 V. When the variation of the on-state current is denoted by 3σ, in a case of the channel length of 0.35 μm, 3σ is 16.5 μA and the percentage of 3σ divided by a median value is 36.3%, in a case of the channel length of 0.55 μm, 3σ is 11.8 μA and the percentage of 3σ divided by a median value is 36.0%, and in a case of the channel length of 1 μm, 3σ is 6.4 μA and the percentage of 3σ divided by a median value is 30.0%. 
       FIG. 22  shows a normal probability graph of the threshold voltage when the drain voltage is 1 V. When the variation of the threshold voltage is denoted by 3σ, in a case of the channel length of 0.35 μm, 3σ is 0.22 V, in a case of the channel length of 0.55 μm, 3σ is 0.26 V, and in a case of the channel length of 1 μm, 3σ is 0.12V. 
     From the results shown in  FIG. 21  and  FIG. 22 , the slopes in the graphs are large in both the on-state current and the threshold voltage, and the variation is small. 
     Accordingly, the transistor of this example has sufficient electronic characteristics as a switching element and less variation in electronic characteristics even though the transistor has a miniaturized structure in which the channel length is 0.35 μm. 
     As described in this example, a miniaturized transistor having high electric characteristics can be provided with high yield. In addition, a semiconductor device including the transistor can have high performance, high reliability, and high productivity. 
     This application is based on Japanese Patent Application serial No. 2011-208229 filed with Japan Patent Office on Sep. 23, 2011 and Japanese Patent Application serial No. 2011-225519 filed with Japan Patent Office on Oct. 13, 2011, the entire contents of which are hereby incorporated by reference.