Patent Publication Number: US-8975634-B2

Title: Semiconductor device including oxide semiconductor film

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     An embodiment of the present invention relates to an oxide semiconductor film. In addition, an embodiment of the present invention relates to a semiconductor device. 
     It is to be noted that the semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic appliances are all semiconductor devices. 
     2. Description of the Related Art 
     In recent years, development of semiconductor devices formed using field-effect transistors including oxide semiconductor films has been advanced. The oxide semiconductor film has a function as a layer in which a channel of the field-effect transistor is formed (also referred to as a channel formation layer). 
     As the field-effect transistor, for example, there are a field-effect transistor in which an oxide semiconductor layer formed with an oxide semiconductor film containing indium (In), gallium (Ga), and zinc (Zn) is used as a channel formation layer, a field-effect transistor in which an oxide semiconductor layer formed with an oxide semiconductor film containing indium (In), tin (Sn), and zinc (Zn) is used as a channel formation layer, and the like (e.g., Patent Document 1). 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
       
    
     SUMMARY OF THE INVENTION 
     However, an oxide semiconductor film used for a field-effect transistor and the like has a problem of occurrence of oxygen deficiency. In addition, when an oxide semiconductor film having an oxygen deficiency is used as a channel formation layer of a field-effect transistor, electrical characteristics of the field-effect transistor are deteriorated; for example, unnecessary carriers are generated in a channel region and off-state current of the field-effect transistor is increased. 
     An object of an embodiment of the present invention is to suppress occurrence of oxygen deficiency in an oxide semiconductor film. An object of an embodiment of the present invention is to improve electrical characteristics of a field-effect transistor. 
     In an embodiment of the present invention, an oxide semiconductor film is formed using germanium (Ge) instead of part of or all of gallium (Ga) or tin (Sn). The bond energy between germanium (Ge) and oxygen (O) is higher than the bond energy between gallium (Ga) and oxygen (O) and the bond energy between tin (Sn) and oxygen (O). Thus, a crystal of an oxide semiconductor formed using germanium (Ge) has a low possibility of occurrence of oxygen deficiency. Accordingly, an oxide semiconductor film is formed using germanium (Ge) in order to suppress occurrence of oxygen deficiency. 
     An embodiment of the present invention is an oxide semiconductor film including a crystal represented by a general formula: A x Ge (1-x/2)(1-y) Sn (1-x/2)y Zn 1-x/2 )O 3 (ZnO) n  (A is at least one of In, Al, Ce, Nd, and Gd; x is a number greater than 0 and smaller than 2; y is a number greater than or equal to 0 and smaller than 1; and n is a number greater than or equal to 1). For example, when A is In, an oxide semiconductor film includes a crystal represented by a general formula: In x Ge (1-x/2)(1-y) Sn (1-x/2)y Zn 1-x/2 O 3 (ZnO) n  (x is a number greater than 0 and smaller than 2, y is a number greater than or equal to 0 and smaller than 1, and n is a number greater than or equal to 1). 
     Further, an embodiment of the present invention is an oxide semiconductor film including a crystal represented by a general formula: A x(1-y) Ga xy Ge 1-x/2 Zn 1-x/2 O 3 (ZnO) n  (A is at least one of In, Al, Ce, Nd, and Gd; x is a number greater than 0 and smaller than 2; y is a number greater than or equal to 0 and smaller than 1; and n is a number greater than or equal to 1). For example, when A is In, an oxide semiconductor film includes a crystal represented by a general formula: In x(1-y) Ga xy Ge 1-x/2 Zn 1-x/2 O 3 (ZnO) n  (x is a number greater than 0 and smaller than 2, y is a number greater than or equal to 0 and smaller than 1, and n is a number greater than or equal to 1). 
     In addition, an embodiment of the present invention is a semiconductor device including an oxide semiconductor layer, an insulating layer, a first conductive layer overlapping with part of the oxide semiconductor layer with the insulating layer provided therebetween, a second conductive layer electrically connected to the oxide semiconductor layer, and a third conductive layer electrically connected to the oxide semiconductor layer. Here, the oxide semiconductor layer is formed using at least part of the oxide semiconductor film and thus includes the same crystal as that of the oxide semiconductor film. In addition, the oxide semiconductor layer has a function as a channel formation layer of a field-effect transistor. The insulating layer has a function as a gate insulating layer of the field-effect transistor. The first conductive layer has a function as a gate of the field-effect transistor. The second conductive layer has a function as one of a source and a drain of the field-effect transistor. The third conductive layer has a function as the other of the source and the drain of the field-effect transistor. 
     According to an embodiment of the present invention, occurrence of oxygen deficiency in an oxide semiconductor material can be suppressed. The oxide semiconductor material is used for a channel formation layer of a field-effect transistor, so that electrical characteristics of the field-effect transistor can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic diagrams illustrating an example of a crystal structure included in an oxide semiconductor film. 
         FIGS. 2A and 2B  are schematic diagrams illustrating an example of a crystal structure included in an oxide semiconductor film. 
         FIGS. 3A and 3B  are schematic diagrams illustrating an example of a crystal structure included in an oxide semiconductor film. 
         FIGS. 4A and 4B  are schematic diagrams illustrating an example of a crystal structure included in an oxide semiconductor film. 
         FIGS. 5A and 5B  are diagrams illustrating an example of a semiconductor device. 
         FIGS. 6A and 6B  are diagrams illustrating an example of a semiconductor device. 
         FIGS. 7A to 7D  are diagrams illustrating an example of the semiconductor device. 
         FIGS. 8A to 8C  are diagrams illustrating an example of the semiconductor device. 
         FIGS. 9A and 9B  are diagrams illustrating an example of a semiconductor device. 
         FIGS. 10A and 10B  are diagrams illustrating an example of a semiconductor device. 
         FIGS. 11A to 11C  are diagrams illustrating an example of a semiconductor device. 
         FIGS. 12A and 12B  are diagrams illustrating an example of a semiconductor device. 
         FIGS. 13A and 13B  are diagrams illustrating an example of the semiconductor device. 
         FIGS. 14A and 14B  are diagrams each illustrating an example of a semiconductor device. 
         FIG. 15  is a diagram illustrating an example of a semiconductor device. 
         FIG. 16  is a diagram illustrating an example of a semiconductor device. 
         FIG. 17  is a diagram illustrating an example of a semiconductor device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of embodiments for describing the present invention will be explained below with reference to the drawings. Note that it will be readily appreciated by those skilled in the art that details of the embodiments can be modified in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be limited to the description of the following embodiments. 
     Note that the contents in different embodiments can be combined with one another as appropriate. In addition, the details of the embodiments can be replaced with each other. 
     Further, the ordinal numbers such as “first” and “second” are used to avoid confusion between components and do not limit the number of each component. 
     Embodiment 1 
     In this embodiment, an example of an oxide semiconductor film which is an embodiment of the present invention is described. 
     The oxide semiconductor film of this embodiment includes a crystal. The crystal can be represented by, for example, a general formula: In x Ge (1-x/2)(1-y) Sn (1-x/2)y Zn 1-x/2 O 3 (ZnO) n  (x is a number greater than 0 and smaller than 2, y is a number greater than or equal to 0 and smaller than 1, and n is a number greater than or equal to 1) or a general formula: In x(1-y) Ga xy Ge 1-x/2 Zn 1-x/2 O 3 (ZnO) n  (x is a number greater than 0 and smaller than 2, y is a number greater than or equal to 0 and smaller than 1, and n is a number greater than or equal to 1). Note that a value of n is not necessarily limited to an integer and may be a number greater than or equal to 1; however, when n is, for example, an integer greater than or equal to 1 and smaller than or equal to 50, a crystal can be more stabilized. As described above, the oxide semiconductor film of this embodiment includes a crystal containing germanium (Ge). Note that in the above crystal, indium (In) can be replaced with another element (e.g., at least one of Al, Ce, Nd, Gd, and the like). 
     The crystal represented by the above general formula has a layered structure including a layer containing indium (In), a layer containing germanium (Ge), and a layer containing zinc (Zn). The crystal having the above layered structure becomes a semiconductor with favorable electrical characteristics; for example, the band gap is wider than that of silicon. Note that the stacking sequence of layers of the layered structure may be changed depending on a proportion of indium (In), germanium (Ge), and zinc (Zn). In some cases, another layer containing one or more of indium (In), germanium (Ge), and zinc (Zn) is stacked in addition to the above layers. 
     Further, the oxide semiconductor film can be formed by a sputtering method over an element formation layer, for example. Here, an oxide target containing the above elements such as indium (In), germanium (Ge), and zinc (Zn) can be used as a sputtering target. 
     Further, schematic diagrams each illustrating an example of a crystal structure of the oxide semiconductor film of this embodiment are described with reference to  FIGS. 1A and 1B ,  FIGS. 2A and 2B ,  FIGS. 3A and 3B , and  FIGS. 4A and 4B . 
     A crystal illustrated in  FIGS. 1A and 1B  is a crystal of an In—Ge—Zn—O oxide semiconductor with a composition ratio of In:Ge:Zn=2:1:3. Note that  FIG. 1A  is a schematic diagram illustrating a crystal structure in a direction perpendicular to the c-axis.  FIG. 1B  is a schematic diagram illustrating a crystal structure in a direction perpendicular the a-b plane. A relatively large black sphere represents an indium (In) atom. A relatively small black sphere represents an oxygen (O) atom. A gray sphere represents a germanium (Ge) atom. A white sphere represents a zinc (Zn) atom. Note that for convenience, the size of the spheres may be different from the actual size of the atoms. Further, the coordination number of each atom is not limited to this embodiment. 
     In the crystal illustrated in  FIGS. 1A and 1B , an In layer (a layer containing indium (In))  111 , a Zn layer (a layer containing zinc (Zn))  112 , and a Ge—Zn layer (a layer containing germanium (Ge) and zinc (Zn))  113  are arranged in this order in a layered manner. That is, the crystal illustrated in  FIGS. 1A and 1B  has a structure in which a plurality of layers is stacked in the c-axis direction. Note that the stacking sequence of the Zn layer  112  and the Ge—Zn layer  113  is not particularly limited. Further, in  FIGS. 1A and 1B , a Ge—Zn layer may be included instead of the Zn layer  112 . In addition, a Zn layer may be included instead of the Ge—Zn layer  113 . 
     In the In layer  111 , each hexacoordinate indium (In) atom is bonded to six tetracoordinate oxygen (O) atoms. 
     In the Zn layer  112 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Ge—Zn layer  113 , each pentacoordinate germanium (Ge) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the case where tin (Sn) is contained in addition to indium (In), germanium (Ge), and zinc (Zn), an oxide semiconductor crystal having the above layered structure can be formed. For example, a crystal illustrated in  FIGS. 2A and 2B  is a crystal of an In—Ge—Sn—O oxide semiconductor with a composition ratio of In:Ge:Sn:Zn=4:1:1:6. Note that  FIG. 2A  is a schematic diagram illustrating a crystal structure in a direction perpendicular to the c-axis.  FIG. 2B  is a schematic diagram illustrating a crystal structure in a direction perpendicular the a-b plane. A relatively large black sphere represents an indium (In) atom. A relatively small black sphere represents an oxygen (O) atom. A relatively large gray sphere represents a tin (Sn) atom. A relatively small gray sphere represents a germanium (Ge) atom. A white sphere represents a zinc (Zn) atom. Note that for convenience, the size of the spheres may be different from the actual size of the atoms. 
     In the crystal illustrated in  FIGS. 2A and 2B , an In layer  121 , a Zn layer  122 , and a Ge—Sn—Zn layer (a layer containing germanium (Ge), tin (Sn), and zinc (Zn))  123  are arranged in this order in a layered manner. That is, the crystal illustrated in  FIGS. 2A and 2B  has a structure in which a plurality of layers is stacked in the c-axis direction. Note that the stacking sequence of the Zn layer  122  and the Ge—Sn—Zn layer  123  is not particularly limited. Further, in  FIGS. 2A and 2B , a Ge—Zn layer, a Sn—Zn layer (a layer containing tin (Sn) and zinc (Zn)), or a Ge—Sn—Zn layer may be included instead of the Zn layer  122 . Further, a Ge—Zn layer, a Sn—Zn layer, or a Zn layer may be included instead of the Ge—Sn—Zn layer  123 . 
     In the In layer  121 , each hexacoordinate indium (In) atom is bonded to six tetracoordinate oxygen (O) atoms. 
     In the Zn layer  122 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Ge—Sn—Zn layer  123 , each pentacoordinate germanium (Ge) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate tin (Sn) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the case where tin (Sn) is contained in addition to indium (In), germanium (Ge), and zinc (Zn) as illustrated in  FIGS. 2A and 2B , an oxide semiconductor crystal having the above layered structure can be formed. For example, carrier mobility can be increased by addition of tin (Sn). 
     In the case where the proportion of the crystal illustrated in  FIGS. 2A and 2B  of indium (In), germanium (Ge), tin (Sn), and zinc (Zn) is changed, an oxide semiconductor crystal having the above layered structure can be formed. For example, a crystal illustrated in  FIGS. 3A and 3B  is a crystal of an In—Ge—Sn—Zn—O oxide semiconductor with a composition ratio of In:Ge:Sn:Zn=4:1:1:10. Note that  FIG. 3A  is a schematic diagram illustrating a crystal structure in a direction perpendicular to the c-axis.  FIG. 3B  is a schematic diagram illustrating a crystal structure in a direction perpendicular to the a-b plane. A relatively large black sphere represents an indium (In) atom. A relatively small black sphere represents an oxygen (O) atom. A relatively large gray sphere represents a tin (Sn) atom. A relatively small gray sphere represents a germanium (Ge) atom. A white sphere represents a zinc (Zn) atom. Note that for convenience the size of the spheres may be different from the actual size of the atoms. 
     In the crystal illustrated in  FIGS. 3A and 3B , an In layer  131 , a Zn layer  132 , a Ge—Sn—Zn layer  133 , and a Zn layer  134  are arranged in a layered manner in this order. That is, the crystal illustrated in  FIGS. 3A and 3B  has a structure in which a plurality of layers is stacked in the c-axis direction. Note that the stacking sequence of the Zn layer  132 , the Ge—Sn—Zn layer  133 , and the Zn layer  134  is not particularly limited. Further, in  FIGS. 3A and 3B , a Ge—Zn layer, a Sn—Zn layer, or a Ge—Sn—Zn layer may be included instead of the Zn layer  132 . A Ge—Zn layer, a Sn—Zn layer, or a Zn layer may be included instead of the Ge—Sn—Zn layer  133 . A Ge—Zn layer, a Sn—Zn layer, or a Ge—Sn—Zn layer may be included instead of the Zn layer  134 . 
     In the In layer  131 , each hexacoordinate indium (In) atom is bonded to six tetracoordinate oxygen (O) atoms. 
     In the Zn layer  132 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Ge—Sn—Zn layer  133 , each pentacoordinate germanium (Ge) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate tin (Sn) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Zn layer  134 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     Even in the case where the proportion of indium (In), germanium (Ge), tin (Sn), and zinc (Zn) is changed as illustrated in  FIGS. 3A and 3B , a crystal of an oxide semiconductor can be formed. For example, carrier mobility can be increased by increasing the amount of indium (In). Further, when the amount of zinc (Zn) is increased, the oxide semiconductor film can be more easily crystallized. 
     In the case where gallium (Ga) is contained in addition to indium (In), germanium (Ge), and zinc (Zn), an oxide semiconductor crystal having the above layered structure can be formed. For example, a crystal illustrated in  FIGS. 4A and 4B  is a crystal of an In—Ga—Ge—Zn—O oxide semiconductor with a composition ratio of In:Ga:Ge:Zn=3:1:1:4. Note that  FIG. 4A  is a schematic diagram illustrating a crystal structure in a direction perpendicular to the c-axis.  FIG. 4B  is a schematic diagram illustrating a crystal structure in a direction perpendicular to the a-b plane. A relatively large black sphere represents an indium (In) atom. A relatively small black sphere represents an oxygen (O) atom. A gray sphere represents a germanium (Ge) atom. A relatively large white sphere represents a gallium (Ga) atom. A relatively small white sphere represents a zinc (Zn) atom. Note that for convenience, the size of the spheres may be different from the actual size of the atoms. 
     In the crystal illustrated in  FIGS. 4A and 4B , an In layer  141 , a Zn layer  142 , a Ge—Zn layer  143 , an In layer  144 , a Zn layer  145 , a Ga—Zn layer (a layer containing gallium (Ga) and zinc (Zn))  146 , an In layer  147 , a Zn layer  148 , and a Ga—Ge layer (a layer containing gallium (Ga) and germanium (Ge))  149  are arranged in a layered manner in this order. That is, the crystal illustrated in  FIGS. 4A and 4B  has a structure in which a plurality of layers is stacked in the c-axis direction. Note that the stacking sequence of the Zn layer  142  and the Ge—Zn layer  143  is not particularly limited. The stacking sequence of the Zn layer  145  and the Ga—Zn layer  146  is not particularly limited. The stacking sequence of the Zn layer  148  and the Ga—Zn layer  149  is not particularly limited. In  FIGS. 4A and 4B , a Ge—Zn layer, a Ga—Zn layer, or a Ga—Ge—Zn layer (a layer containing gallium (Ga), germanium (Ge), and zinc (Zn)) may be included instead of the Zn layer  142 . Further, a Ga—Ge—Zn layer, a Ge—Zn layer, or a Zn layer may be included instead of the Ge—Zn layer  143 . A Ge—Zn layer, a Ga—Zn layer, or a Ga—Ge—Zn layer may be included instead of the Zn layer  145 . A Ga—Ge—Zn layer, a Ge—Zn layer, or a Zn layer may be included instead of the Ga—Zn layer  146 . In addition, a Ge—Zn layer, a Ga—Zn layer, or a Ga—Ge—Zn layer may be included instead of the Zn layer  148 . A Ga—Ge—Zn layer, a Ge layer (a layer containing germanium (Ge)), or a Ga layer (a layer containing gallium (Ga)) may be included instead of the Ga—Ge layer  149 . 
     In the In layer  141 , each hexacoordinate indium (In) atom is bonded to six tetracoordinate oxygen (O) atoms. 
     In the Zn layer  142 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Ge—Zn layer  143 , each pentacoordinate germanium (Ge) atom is bonded to five pentacoordinate oxygen (O) atoms. Each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the In layer  144 , each hexacoordinate indium (In) atom is bonded to six tetracoordinate oxygen (O) atoms. 
     In the Zn layer  145 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Ga—Zn layer  146 , each pentacoordinate gallium (Ga) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the In layer  147 , each hexacoordinate indium (In) atom is bonded to six tetracoordinate oxygen (O) atoms. 
     In the Zn layer  148 , each pentacoordinate zinc (Zn) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the Ga—Ge layer  149 , each pentacoordinate gallium (Ga) atom is bonded to five tetracoordinate oxygen (O) atoms. Further, each pentacoordinate germanium (Ge) atom is bonded to five tetracoordinate oxygen (O) atoms. 
     In the case where gallium (Ga) is used instead of tin (Sn) as illustrated in  FIGS. 4A and 4B , an oxide semiconductor crystal having the above layered structure can be formed. 
     In addition, as the oxide semiconductor film, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film may be used. 
     CAAC-OS is a mixed phase structure of a crystal portion and an amorphous portion. Further, in the crystal included in the crystal portion, a c-axis is aligned in a direction perpendicular to a surface where the semiconductor film is formed or a surface of the semiconductor film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and layers each containing metal atoms and oxygen atoms are stacked in a layered manner. The normal vectors of the layers are aligned in the c-axis direction. Therefore, the CAAC-OS is not completely single crystal nor completely amorphous. Note that when the CAAC-OS includes a plurality of crystal portions, the directions of the a-axes and the b-axes of crystals of the plurality of crystal portions may be different from each other. Note that the crystal portion includes the crystal having any of the structures described with reference to  FIGS. 1A and 1B ,  FIGS. 2A and 2B ,  FIGS. 3A and 3B , and  FIGS. 4A and 4B . 
     Note that in most cases, the crystal portion of CAAC-OS fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (also referred to as TEM), a boundary between a crystal portion and an amorphous portion in the CAAC-OS is not always clear. Further, a grain boundary in the CAAC-OS is not found. Thus, in the CAAC-OS, a reduction in electron mobility, due to the grain boundary, is prevented. 
     Further, in the CAAC-OS film, distribution of crystal portions is not necessarily uniform in the depth direction of the film. For example, when a crystal growth occurs from a surface side of the oxide semiconductor film to form the CAAC-OS film, in some cases, the proportion of the crystal portions in the vicinity of the surface of the CAAC-OS film is high and the proportion of the amorphous portions in the vicinity of the surface where the CAAC-OS film is formed is high. 
     The c-axes of the crystal included in the crystal portion of the CAAC-OS are perpendicular to the surface where the CAAC-OS film is formed or the surface of the CAAC-OS film; thus, 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 the c-axes in the crystal portions of the CAAC-OS are substantially perpendicular to the surface where the CAAC-OS film is formed or the surface of the CAAC-OS film. 
     The above is descriptions of the examples of the crystal structure of an oxide semiconductor illustrated in  FIGS. 1A and 1B ,  FIGS. 2A and 2B ,  FIGS. 3A and 3B , and  FIGS. 4A and 4B . 
     Further, in order to examine which element tin (Sn), gallium (Ga), or germanium (Ge) has the strongest bond with oxygen (O), a value of an energy which is necessary for oxide of each of tin (Sn), gallium (Ga), and germanium (Ge) to form an oxygen-deficient state (such an energy is also referred to as deficiency formation energy E def ) and a value of the bond energy between a metal and oxygen (O) are described by using Table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Deficiency 
                   
                 Bond Energy 
               
               
                   
                 Formation 
                 Coordination 
                 per Bond 
               
               
                 Material 
                 Energy [eV] 
                 Number of O 
                 [eV] 
               
               
                   
               
             
            
               
                 Ga 2 O 3   
                 4.17 
                 3 
                 1.39 
               
               
                 SnO 2   
                 3.51 
                 3 
                 1.17 
               
               
                 GeO 2  having 
                 4.33 
                 3 
                 1.44 
               
               
                 Rutile Structure 
               
               
                 GeO 2  having 
                 3.08 
                 2 
                 1.54 
               
               
                 Quartz Structure 
               
               
                   
               
            
           
         
       
     
     The deficiency formation energy E def  is expressed by the formula below.
 
 E   def =( E ( A   M O N-1 )+ E (O))− E ( A   M O N )
 
     Here, A denotes any of tin (Sn), gallium (Ga), and germanium (Ge). Note that E(O) denotes total energy possessed by an oxygen (O) atom, E(A M O N ) denotes energy of metal oxide A M O N  having no oxygen deficiency, and E(A M O N-1 ) denotes energy of metal oxide A M O N-1  having an oxygen deficiency. M and N each denote the number of atoms. The sum of M and N is the number of atoms used for calculation. 
     The larger the value of the deficiency formation energy E def  is, the more energy is needed to form an oxygen-deficient state, which means that bond with oxygen (O) tends to be strong. 
     For the calculation of the deficiency formation energy E def , CASTEP, which is a calculation program for a density functional theory, can be used. In the calculation relating to Table 1, a plane-wave-basis pseudopotential method is used as the density functional theory, and GGA-PBE is used for a functional. The cut-off energy is 500 eV. A 2×2×3 k-point grid, a 3×3×3 k-point grid, a 3×3×2 k-point grid, and a 1×3×3 k-point grid are used for SnO 2 , GeO 2  having a rutile structure, GeO 2  having a quartz structure, and Ga 2 O 3 , respectively. In addition, a rutile structure with 48 atoms is used for SnO 2 , a β-Gallia structure with 80 atoms is used for Ga 2 O 3 , and a rutile structure with 72 atoms and a quartz structure with 72 atoms are used for GeO 2 . E(O) is a value obtained by dividing energy possessed by oxygen molecules by 2. 
     Further, a value of the above deficiency formation energy E def  is divided by the coordination number of an oxygen (O) atom, whereby a bond energy per bond between a metal atom and an oxygen (O) atom can be obtained. 
     As shown in Table 1, at least one of bonds between a germanium (Ge) atom and oxygen (O) atoms has a bond energy higher than at least one of bonds between a tin (Sn) atom and oxygen (O) atoms or a gallium (Ga) atom and oxygen (O) atoms. The larger the bond energy is, the less oxygen deficiency occurs. It is thus found that an oxide of germanium (Ge) has a stronger bond with oxygen (O) than an oxide of tin (Sn) and an oxide of gallium (Ga), and that oxygen deficiency hardly occurs in an oxide of germanium (Ge). 
     Further, values of band-gap energy (obtained by calculation) of an oxide semiconductor film including a crystal containing germanium (Ge) (an In—Ge—Zn—O oxide semiconductor crystal and an In—Ge—Sn—Zn—O oxide semiconductor crystal) and an In—Sn—Zn—O oxide semiconductor crystal (a comparative example) are described with reference to Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Band-gap 
               
               
                   
                 Material 
                 Energy [eV] 
               
               
                   
                   
               
             
            
               
                   
                 In—Ge—Zn—O 
                 0.968 
               
               
                   
                 In—Ge—Sn—Zn—O 
                 0.836 
               
               
                   
                 In—Sn—Zn—O 
                 0.759 
               
               
                   
                   
               
            
           
         
       
     
     Note that in the calculation relating to the band-gap energy shown in Table 2, band-gap energy is calculated as follows: a structure is optimized by first principles calculation using plane-wave-basis pseudopotential method based on a density functional theory, and the density of energy states of the crystal structure optimized by the first principles calculation is calculated. Here, first principle calculation software, CASTEP, is used for a calculation program. GGA-PBE is used for a functional. Ultrasoft is used for pseudopotential. The cut-off energy is 380 eV. A 4×4×1 k-point grid and a 5×5×3 k-point grid are used in the structure optimization and the calculation of density of energy states, respectively. In addition, each structure of an In—Ge—Zn—O oxide semiconductor crystal, an In—Sn—Zn—O oxide semiconductor crystal, and an In—Ge—Sn—Zn—O oxide semiconductor crystal is determined on the basis of a structure of an In—Ga—Zn—O oxide semiconductor crystal. Here, as a basic crystal structure of an In—Ga—Zn—O oxide semiconductor, a structure with 84 atoms, which is obtained by doubling the a-axis and the b-axis in the symmetry R-3 (space group number: 148) structure, is used. Further, the structure of an In—Ge—Zn—O oxide semiconductor crystal is a structure in which Ga of the In—Ga—Zn—O oxide semiconductor crystal is replaced so that the ratio of Ge:Zn becomes 1:1. Further, the structure of an In—Sn—Zn—O oxide semiconductor crystal is a structure in which Ga of the In—Ga—Zn—O oxide semiconductor crystal is replaced so that the ratio of Sn:Zn becomes 1:1. Further, the structure of an In—Ge—Sn—Zn—O oxide semiconductor crystal is a structure in which a half of Ge of the In—Ge—Zn—O oxide semiconductor crystal is replaced with Sn. Since each optimized crystal structure has band gap, each structure has the density of energy states equivalent to that of an insulator or a semiconductor. Thus, band-gap energy can be calculated from the density of energy states. Note that, in density functional theory, the band-gap energy may be estimated to be smaller, in some cases. 
     As shown in Table 2, band-gap energy of an oxide semiconductor film including a crystal containing germanium (Ge) is larger than that of an oxide semiconductor film including a crystal not containing germanium (Ge). That is, band gap of an oxide semiconductor film including a crystal containing germanium (Ge) is wider than that of an oxide semiconductor film including a crystal not containing germanium (Ge). 
     The above is the description of the example of the oxide semiconductor film in this embodiment. 
     As described by using  FIGS. 1A and 1B ,  FIGS. 2A and 2B ,  FIGS. 3A and 3B , and  FIGS. 4A and 4B , germanium (Ge) is used in the example of an oxide semiconductor film of this embodiment. Since at least one of bonds between a germanium (Ge) atom and oxygen (O) atoms has a bond energy higher than at least one bond between atoms of another element (e.g., tin (Sn) or gallium (Ga)) and an oxygen (O) atom, occurrence of oxygen deficiency can be suppressed with the use of germanium (Ge). 
     In addition, since germanium (Ge) is used in the example of an oxide semiconductor film of this embodiment, the band gap can be increased. 
     Embodiment 2 
     In this embodiment, an embodiment of a semiconductor device and a method for manufacturing the semiconductor device will be described with reference to  FIGS. 5A and 5B ,  FIGS. 6A and 6B ,  FIGS. 7A to 7D , and  FIGS. 8A to 8C . 
     The semiconductor device in this embodiment includes the oxide semiconductor film described in Embodiment 1. The semiconductor device includes an oxide semiconductor layer having a function as a channel formation layer of a field-effect transistor, an insulating layer having a function as a gate insulating layer of the field-effect transistor, a first conductive layer overlapping with part of the oxide semiconductor layer with the insulating layer provided therebetween and having a function as a gate of the field-effect transistor, a second conductive layer electrically connected to the oxide semiconductor layer and having a function as one of a source and a drain of the field-effect transistor, and a third conductive layer electrically connected to the oxide semiconductor layer and having a function as the other of the source and the drain of the field-effect transistor. 
     An example of the semiconductor device in this embodiment is illustrated in  FIGS. 5A and 5B .  FIG. 5A  is a plan view of the semiconductor device.  FIG. 5B  is a cross-sectional view taken along a line X 1 -Y 1  of  FIG. 5A . Note that in  FIG. 5A , some components of a field-effect transistor  420  (e.g., an insulating layer  407 ) are not illustrated for simplicity. 
     The field-effect transistor  420  illustrated in  FIGS. 5A and 5B  includes, over a substrate  400  having an insulating surface, a base insulating layer  436 , a source electrode layer  405   a  and a drain electrode layer  405   b , an oxide semiconductor layer  403  in contact with the source electrode layer  405   a  on one side surface in the channel length direction and in contact with the drain electrode layer  405   b  on the other side surface in the channel length direction, a gate insulating layer  402  in contact with top surfaces of the oxide semiconductor layer  403 , the source electrode layer  405   a , and the drain electrode layer  405   b , a gate electrode layer  401  provided over the oxide semiconductor layer  403  with the gate insulating layer  402  provided therebetween, a sidewall layer  412   a  in contact with one side surface of the gate electrode layer  401  in the channel length direction, and a sidewall layer  412   b  in contact with the other side surface of the gate electrode layer  401  in the channel length direction. 
     In the field-effect transistor  420 , at least part of the sidewall layer  412   a  is provided over the source electrode layer  405   a  with the gate insulating layer  402  provided therebetween. At least part of the sidewall layer  412   b  is provided over the drain electrode layer  405   b  with the gate insulating layer  402  provided therebetween. The sidewall layer  412   a  and the sidewall layer  412   b  include a conductive material and can serve as part of the gate electrode layer  401 ; thus, a region where the sidewall layer  412   a  overlaps with the source electrode layer  405   a  with the gate insulating layer  402  provided therebetween and a region where the sidewall layer  412   b  overlaps with the drain electrode layer  405   b  with the gate insulating layer  402  provided therebetween can be substantially L ov  regions. 
     In addition, the field-effect transistor  420  illustrated in  FIGS. 5A and 5B  may include, as its components, the insulating layer  407  provided over the sidewall layer  412   a , the sidewall layer  412   b , and the gate electrode layer  401 ; and a wiring layer  435   a  and a wiring layer  435   b  provided over the insulating layer  407 . The wiring layer  435   a  is electrically connected to the source electrode layer  405   a  through an opening provided in the insulating layer  407  and the gate insulating layer  402 . The wiring layer  435   b  is electrically connected to the drain electrode layer  405   b  through an opening provided in the insulating layer  407  and the gate insulating layer  402 . 
     In the case where a sidewall layer including a conductive material is not provided in the field-effect transistor  420 , an oxide semiconductor layer and a gate electrode layer each of which has a narrow line width need to be precisely aligned in order to form an L ov  region. Further, miniaturization of the field-effect transistor increases the need. However, since the field-effect transistor  420  described in this embodiment includes the sidewall layer  412   a  and the sidewall layer  412   b  including a conductive material on the side surfaces of the gate electrode layer  401  in the channel length direction, the region where the sidewall layer  412   a  overlaps with the source electrode layer  405   a  and the region where the sidewall layer  412   b  overlaps with the drain electrode layer  405   a  can substantially serve as L ov  regions. Thus, the degree of freedom of alignment in forming the gate electrode layer  401  can be increased and the field-effect transistor  420  in which a reduction in on-state current is prevented can be provided with high yield. 
     Further, the oxide semiconductor layer  403  can be formed using the oxide semiconductor film described in Embodiment 1. The oxide semiconductor layer  403  may be formed using a CAAC-OS film. 
       FIGS. 6A and 6B  are a plan view and a cross-sectional view illustrating a field-effect transistor  422  having a structure different from the structure of the filed-effect transistor  420  illustrated in  FIGS. 5A and 5B .  FIG. 6A  is a plan view of the field-effect transistor  422 .  FIG. 6B  is a cross-sectional view taken along a line X 2 -Y 2  in  FIG. 6A . Note that in  FIG. 6A , some components of the field-effect transistor  422  (e.g., the insulating layer  407 ) are not illustrated for simplicity. 
     A difference between the field-effect transistor  422  illustrated in  FIGS. 6A and 6B  and the field-effect transistor  420  illustrated in  FIGS. 5A and 5B  is the shape of the side surface of the oxide semiconductor layer  403 . In the field-effect transistor  422  illustrated in  FIGS. 6A and 6B , the side surface of the oxide semiconductor layer  403  which is in contact with the source electrode layer  405   a  or the drain electrode layer  405   b  is tapered. When the side surface of the oxide semiconductor layer  403  is tapered, coverage with a conductive film which is to be the source electrode layer  405   a  and the drain electrode layer  405   b  can be improved. 
     An example of a manufacturing process of the field-effect transistor of this embodiment is described below with reference to  FIGS. 7A to 7D  and  FIGS. 8A to 8C . Note that a manufacturing process of the field-effect transistor  422  is described below as an example. 
     First, the base insulating layer  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 at least heat resistance to withstand a subsequent heat treatment step. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, a sapphire substrate, or a plastic 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 in which a semiconductor element is provided in an upper portion can be used as the substrate  400 . 
     As the substrate  400 , a flexible substrate such as a plastic substrate may be used. 
     The base insulating layer  436  can have a single-layer or a stacked-layer structure including one or more films selected from those containing silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, hafnium oxide, gallium oxide, and a mixed material of any of these materials. Note that the base insulating layer  436  preferably has a single-layer structure or a stacked-layer structure including an oxide insulating film so that the oxide insulating film is in contact with the oxide semiconductor layer  403  to be formed later. Note that the base insulating layer  436  is not necessarily provided. 
     It is preferable that the base insulating layer  436  include a region containing oxygen in a proportion higher than that in the stoichiometric composition (hereinafter also referred to as an oxygen-excess region). This is because oxygen deficiency in the oxide semiconductor layer  403  to be formed later can be compensated owing to the oxygen-excess region. In the case where the base insulating layer  436  has a stacked-layer structure, the base insulating layer  436  preferably includes the oxygen-excess region at least in a layer in contact with the oxide semiconductor layer  403 . In order to provide the oxygen-excess region in the base insulating layer  436 , for example, the base insulating layer  436  may be formed in an oxygen atmosphere. Alternatively, the oxygen-excess region may be formed by adding oxygen (including at least one of an oxygen radical, an oxygen atom, and an oxygen ion) to the base insulating layer  436  after its formation. Oxygen can be added by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. 
     Next, an oxide semiconductor layer  413  is formed over the base insulating layer  436  (see  FIG. 7A ). The thickness of the oxide semiconductor layer  413  is 3 nm to 30 nm, preferably 5 nm to 20 nm. As the oxide semiconductor layer  413 , for example, the oxide semiconductor film described in Embodiment 1 can be used. Here, the substrate temperature in the film formation is higher than or equal to room temperature and lower than or equal to 450° C. 
     In addition, in the case where the crystallinity of the oxide semiconductor layer  413  needs to be increased, the temperature of heat treatment performed immediately after the deposition is completed is higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 400° C., further preferably higher than or equal to 500° C., still further preferably higher than or equal to 550° C. Note that the heat treatment can also serve as another heat treatment in the manufacturing process. 
     Further, the oxide semiconductor layer  413  can be formed as follows: an oxide semiconductor film is deposited by a sputtering method and then part of the oxide semiconductor film is etched. For example, the oxide semiconductor layer  413  may be deposited with a plasma sputtering apparatus. A plasma sputtering apparatus is a sputtering apparatus which performs deposition with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target. 
     For example, a sputtering target having a composition with which the oxide semiconductor layer  413  has the composition of the above oxide semiconductor film is preferably used for forming the oxide semiconductor layer  413 . For example, an In—Ge—Sn—Zn—O oxide semiconductor film can be deposited using an oxide target having a composition ratio of In:Ge:Sn:Zn=4:1:1:6. 
     In the formation of the oxide semiconductor layer  413 , the concentration of hydrogen contained in the oxide semiconductor layer  413  is preferably reduced as much as possible. In order to reduce the hydrogen concentration, for example, in the case where a sputtering method is used for the deposition, a high-purity rare gas (typically, argon) from which impurities such as hydrogen, water, a hydroxyl group, and hydride have been removed; oxygen; or a mixed gas of oxygen and the rare gas is used as appropriate as an atmosphere gas supplied to a process chamber of a sputtering apparatus. 
     The oxide semiconductor layer  413  is formed in such a manner that a sputtering gas from which hydrogen and moisture are removed is introduced into a deposition chamber while moisture remaining in the deposition chamber is removed, whereby the concentration of hydrogen in the oxide semiconductor layer  413  can be reduced. In order 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. The evacuation unit may be a turbo molecular pump provided with a cold trap. In the deposition chamber which is evacuated with a cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H 2 O), (more preferably, also a compound containing a carbon atom), and the like have high evacuation capability; therefore, the concentration of an impurity contained in the oxide semiconductor layer  413  deposited in the deposition chamber can be reduced. 
     In order to reduce the impurity concentration in the oxide semiconductor layer  413 , it is also effective to deposit the oxide semiconductor layer  413  while the substrate  400  is kept at high temperature. The substrate  400  is heated at higher than or equal to 150° C. and lower than or equal to 450° C. 
     It is preferable to use a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, and hydride are removed as a sputtering gas used when the oxide semiconductor layer  413  is formed. 
     Before the oxide semiconductor layer  413  is deposited, planarization treatment may be performed on the surface where the oxide semiconductor layer  413  is deposited. As the planarization treatment, polishing treatment (e.g., a chemical mechanical polishing method), dry-etching treatment, or plasma treatment can be used, although there is no particular limitation on the planarization treatment. 
     As plasma treatment, reverse sputtering in which an argon gas is introduced and plasma is generated 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. Note that instead of an 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 the surface where the oxide semiconductor layer  413  is deposited. 
     As the planarization treatment, polishing treatment, dry etching treatment, or plasma treatment may be performed plural times, or these treatments may be performed in combination. In the case where the treatments are combined, the order of steps is not particularly limited and may be set as appropriate depending on the roughness of the surface where the oxide semiconductor layer  413  is deposited. 
     Further, the oxide semiconductor layer  413  is preferably subjected to heat treatment for removing excess hydrogen (including water and a hydroxyl group) contained in the oxide semiconductor layer  413  (dehydration or dehydrogenation). The temperature of the heat treatment is higher than or equal to 300° C. and lower than or equal to 700° C., or lower than the strain point of the substrate. The heat treatment can be performed under reduced pressure, a nitrogen atmosphere, or the like. 
     Hydrogen, which is an n-type impurity, can be removed from the oxide semiconductor by the heat treatment. For example, the hydrogen concentration in the oxide semiconductor layer  413  after the dehydration or dehydrogenation treatment can be 5×10 19 /cm 3  or lower, preferably 5×10 18 /cm 3  or lower. 
     Note that the heat treatment for the dehydration or dehydrogenation may be performed at any timing in the process of manufacturing the field-effect transistor  422  as long as it is performed after the deposition of the oxide semiconductor layer  413 . In the case where an aluminum oxide film is formed as the gate insulating layer  402  or the insulating layer  407 , the heat treatment is preferably performed before the aluminum oxide film is formed. The heat treatment for the dehydration or dehydrogenation may be performed plural times, and may double as another heat treatment. 
     Note that in the case where a base insulating layer containing oxygen is provided as the base insulating layer  436 , the heat treatment for the dehydration or dehydrogenation is preferably performed before the oxide semiconductor layer  413  is processed into an island shape, in which case release of oxygen contained in the base insulating layer  436  by the heat treatment can be prevented. 
     Note that in the heat treatment, it is preferable that water, hydrogen, and the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or a rare gas such as helium, neon, or argon introduced into the heat treatment apparatus is preferably 6N (99.9999%) or more, more preferably 7N (99.99999%) or more (i.e., the impurity concentration is preferably 1 ppm or lower, more preferably 0.1 ppm or lower). 
     In addition, after the oxide semiconductor layer  413  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 20 ppm or lower (−55° C. by conversion into a dew point), preferably 1 ppm or lower, more preferably 10 ppb or lower, in the measurement with the use of a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) may be introduced into the same furnace while the heating temperature is being maintained or being gradually decreased. It is preferable that water, hydrogen, or the like be not contained in the oxygen gas or the dinitrogen monoxide gas. The purity of the oxygen gas or the dinitrogen monoxide gas which is introduced into the heat treatment apparatus is preferably 6N or higher, further preferably 7N or higher (i.e., the impurity concentration in the oxygen gas or the dinitrogen monoxide gas is preferably 1 ppm or lower, further preferably 0.1 ppm or lower). The oxygen gas or the dinitrogen monoxide gas acts to supply oxygen that is a main component of the oxide semiconductor and that is reduced by the step for removing an impurity for the dehydration or dehydrogenation, so that the oxide semiconductor layer  413  can be a highly purified, i-type (intrinsic) oxide semiconductor film. 
     Further, oxygen (which includes at least one of an oxygen radical, an oxygen atom, and an oxygen ion) may be added to the oxide semiconductor layer after being subjected to the dehydration or dehydrogenation treatment to supply oxygen to the oxide semiconductor layer. 
     In the transistor described in this embodiment, oxygen is added to the dehydrated or dehydrogenated oxide semiconductor layer to be supplied thereto, so that the oxide semiconductor layer can be highly purified and be i-type (intrinsic). Variation in electrical characteristics of a field-effect transistor having a highly-purified and i-type (intrinsic) oxide semiconductor layer is suppressed, and the field-effect transistor is electrically stable. 
     In the step of addition of oxygen to the oxide semiconductor layer, oxygen may be directly added to the oxide semiconductor layer or may be added to the oxide semiconductor layer  413  through another film such as the gate insulating layer  402  or the insulating layer  407  to be formed later. An ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like may be employed for the addition of oxygen through another film, whereas a plasma treatment or the like can also be employed in addition to the above methods for the direct addition of oxygen to the exposed oxide semiconductor layer  413 . 
     Oxygen can be introduced into the oxide semiconductor layer anytime after dehydration or dehydrogenation treatment is performed thereon. Further, oxygen may be added plural times to the oxide semiconductor layer after the dehydration or dehydrogenation treatment is performed. 
     Next, the oxide semiconductor layer  413  is processed into an island-shaped oxide semiconductor layer  403  by a photolithography step. As a mask used for the processing into the island-shaped oxide semiconductor layer  403  here, a mask which is formed by a photolithography method or the like and has a finer pattern formed by a slimming process is preferably used. 
     As the slimming process, an ashing process in which oxygen in a radical state (an oxygen radical) or the like is used can be employed, for example. However, the slimming process is not limited to the ashing process as long as the mask formed by a photolithography method or the like can be processed into a finer pattern. Note that the channel length (L) of a field-effect transistor is determined by the mask formed by the slimming process. Therefore, a process with high controllability is preferably employed as the slimming process. 
     As a result of the slimming process, the line width of the mask formed by a photolithography method or the like can be reduced to a length shorter than or equal to the resolution limit of a light exposure apparatus, preferably less than or equal to half of the resolution limit of a light exposure apparatus, more preferably less than or equal to one third of the resolution limit of the light exposure apparatus. For example, the line width can become more than or equal to 30 nm and less than or equal to 2000 nm, preferably more than or equal to 50 nm and less than or equal to 350 nm. This enables further miniaturization of the field-effect transistor. 
     Next, a conductive film  415  which is to be a source electrode layer and a drain electrode layer (including a wiring formed in the same layer as the source electrode layer and the drain electrode layer) is formed over the island-shaped oxide semiconductor layer  403  (see  FIG. 7B ). 
     The conductive film  415  is formed using a material that can withstand heat treatment in a later step. 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 (a titanium nitride film, a molybdenum nitride film, a tungsten nitride film), or the like can be used. A film of a high melting point metal 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 and an upper side of a metal film of Al, Cu, or the like. Alternatively, the conductive film  415  may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), indium oxide-tin oxide (In 2 O 3 —SnO 2 ; abbreviated to ITO), indium oxide-zinc oxide (In 2 O 3 —ZnO), or any of these metal oxide materials to which silicon oxide is added can be used. 
     Next, polishing (cutting or grinding) treatment is performed on the conductive film  415  to remove part of the conductive film  415  so that the oxide semiconductor layer  403  is exposed. By the polishing treatment, a region of the conductive film  415  overlapping with the oxide semiconductor layer  403  is removed and a conductive film  415   a  having an opening in the region is thus formed (see  FIG. 7C ). As the polishing (cutting or grinding) method, chemical mechanical polishing (also referred to as CMP) treatment is preferably used. In this embodiment, the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  is removed by CMP treatment. 
     Note that the CMP treatment may be performed only once or plural times. When the CMP treatment is performed plural times, it is preferable that first polishing be performed with a high polishing rate followed by final polishing with a low polishing rate. By performing polishing steps with different polishing rates in combination, the flatness of the surfaces of the source electrode layer  405   a , the drain electrode layer  405   b , and the oxide semiconductor layer  403  can be further increased. 
     Although CMP treatment is used for removing the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  in this embodiment, another polishing (cutting or grinding) treatment may be used. Alternatively, polishing treatment such as CMP treatment may be combined with etching (dry etching or wet etching) treatment, plasma treatment, or the like. For example, after the CMP treatment, dry etching treatment or plasma treatment (e.g., reverse sputtering) may be performed in order to improve the flatness of a surface to be processed. When the polishing treatment is combined with etching treatment, 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 conductive film  415 . 
     The upper end portion of the conductive film  415   a  is substantially aligned with the upper end portion of the oxide semiconductor layer  403 . Note that the shape of the conductive film  415   a  (or the source electrode layer  405   a  and the drain electrode layer  405   b  which are formed by processing the conductive film  415   a ) differs depending on the conditions of polishing treatment for removing the conductive film  415 . For example, in some cases, a thickness of the conductive film  415   a  may be less than a thickness of the oxide semiconductor layer  403 . 
     Next, the conductive film  415   a  is processed by a photolithography step to form the source electrode layer  405   a  and the drain electrode layer  405   b  (including a wiring formed using the same layer as the source electrode layer  405   a  and the drain electrode layer  405   b ) (see  FIG. 7D ). 
     Note that in this embodiment, a method is described in which the conductive film  415  is deposited, the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  is removed by polishing treatment, and then, part of the conductive film is etched to form the source electrode layer  405   a  and the drain electrode layer  405   b ; however, a method is not limited thereto. The source electrode layer  405   a  and the drain electrode layer  405   b  may be formed by such a method that the conductive film  415  is deposited and processed by selective etching and then the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  is removed by polishing treatment. Note that in the case where etching treatment is performed before polishing treatment, there is a need to prevent the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  from being removed by the etching treatment. 
     In the example of a method for manufacturing a field-effect transistor described in this embodiment, etching treatment using a resist mask is not used in the step of removing the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  to form the source electrode layer  405   a  and the drain electrode layer  405   b . Thus, even when the width of the source electrode layer  405   a  and the drain electrode layer  405   b  in the channel length direction is miniaturized, a processing can be precisely performed. Consequently, in a process for manufacturing the semiconductor device, the field-effect transistor  420  having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield. 
     In the example of a method for manufacturing a field-effect transistor described in this embodiment, the region of the conductive film  415  overlapping with the oxide semiconductor layer  403  is removed, whereby the side surface of the oxide semiconductor layer  403  in the channel length direction can be in contact with the source electrode layer  405   a  or the drain electrode layer  405   b . Since the oxide semiconductor layer  403  has a thickness as small as 3 nm to 30 nm, preferably 5 nm to 20 nm, contact area between the oxide semiconductor layer  403  and the source electrode layer  405   a  or the drain electrode layer  405   b  which is in contact with the side surface of the oxide semiconductor layer  403  can be reduced and contact resistance at the contact interface can be increased. Accordingly, even when the channel length (L) of the field-effect transistor  422  is shortened, electric field between the source electrode layer  405   a  and the drain electrode layer  405   b  can be relieved, and a short-channel effect such as shift of the threshold voltage can be controlled. 
     Next, the gate insulating layer  402  is formed over the oxide semiconductor layer  403 , the source electrode layer  405   a , and the drain electrode layer  405   b.    
     The gate insulating layer  402  can be formed to have a thickness greater than or equal to 1 nm and less than or equal to 20 nm by a sputtering method, an MBE method, a CVD method, a pulse laser deposition method, an ALD method, or the like as appropriate. In addition, the gate insulating layer  402  may be formed by depositing an insulating film with a plasma sputtering apparatus. 
     As the thickness of the gate insulating layer  402  becomes larger, a short channel effect is enhanced more and the threshold voltage tends to shift more to the negative side. However, in the method for manufacturing the semiconductor device in this embodiment, the top surfaces of the source electrode layer  405   a , the drain electrode layer  405   b , and the oxide semiconductor layer  403  are planarized by polishing treatment; thus, coverage with the gate insulating layer  402  with a small thickness can be improved. 
     As a material for the gate insulating layer  402 , for example, silicon oxide, gallium oxide, aluminum oxide, silicon nitride, silicon oxynitride, aluminum oxynitride, silicon nitride oxide, or the like can be used. It is preferable that the gate insulating layer  402  contain oxygen in a portion which is in contact with the oxide semiconductor layer  403 . In particular, it is preferable that the oxygen content of the gate insulating layer  402  in (a bulk of) the film be in excess of that in the stoichiometric composition. For example, in the case where a silicon oxide film is used as the gate insulating layer  402 , the composition formula thereof is preferably 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 layer  402 . By using the silicon oxide film as the gate insulating layer  402 , oxygen can be supplied to the oxide semiconductor layer  403 , leading to favorable characteristics. Further, the gate insulating layer  402  is preferably formed in consideration of the size of a field-effect transistor to be formed and the step coverage with the gate insulating layer  402 . 
     When the gate insulating layer  402  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 (HfSiO x N y  (x&gt;0, y&gt;0)) to which nitrogen is added, hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)), or lanthanum oxide, gate leakage current can be reduced. The gate insulating layer  402  may be formed with either a single-layer structure or a stacked-layer structure. 
     Next, the gate electrode layer  401  is formed over the island-shaped oxide semiconductor layer  403  with the gate insulating layer  402  provided therebetween (see  FIG. 8A ). The gate electrode layer  401  can be formed by a plasma CVD method, a sputtering method, or the like. The gate electrode layer  401  can be formed using a metal film containing an element selected from molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, and scandium; a metal nitride film containing any of the above elements as its component (e.g., a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film); or the like. Alternatively, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used as the gate electrode layer  401 . The gate electrode layer  401  may be formed with 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 tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible 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 layer  402 , a metal oxide containing nitrogen, specifically, an IGZO 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 volt), preferably higher than or equal to 5.5 eV (electron volt); thus, when these are used as the gate electrode layer, the threshold voltage of the electrical characteristics of a field-effect transistor can be positive. Accordingly, a so-called normally-off switching element can be obtained. 
     Note that the gate electrode layer  401  can be formed by processing a conductive film (not illustrated) provided over the gate insulating layer  402  using a mask. As the mask used for the processing here, a mask which is formed by a photolithography method or the like and has a finer pattern formed by a slimming process is preferably used. 
     Then, a film containing a conductive material is deposited over the gate electrode layer  401  and the gate insulating layer  402  and is partly etched to form the sidewall layer  412   a  and the sidewall layer  412   b  (see  FIG. 8B ). 
     The sidewall layer  412   a  and the sidewall layer  412   b  have conductivity and can be formed, for example, by processing a metal film of tungsten, titanium, or the like, a silicon film containing an impurity element such as phosphorus or boron, or the like. Alternatively, the sidewall layer  412   a  and the sidewall layer  412   b  having conductivity can be formed as follows: a polycrystalline silicon film is deposited over the gate electrode layer  401  and the gate insulating layer  402 , the polycrystalline silicon film is etched to form a sidewall layer which is in contact with the gate electrode layer  401 , an impurity element such as phosphorus or boron is introduced into the sidewall layer by doping, and heat treatment for activation is performed. 
     Next, the insulating layer  407  is formed over the gate insulating layer  402 , the gate electrode layer  401 , the sidewall layer  412   a , and the sidewall layer  412   b.    
     The insulating layer  407  can be formed, for example, by a plasma CVD method, a sputtering method, an evaporation method, or the like. As the insulating layer  407 , 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. 
     Furthermore, as the insulating layer  407 , an aluminum oxide film, 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. 
     The insulating layer  407  can be formed with either a single-layer structure or a stacked-layer structure, for example, a stack of a silicon oxide film and an aluminum oxide film can be used. The aluminum oxide film can be preferably used because it has a high shielding effect (blocking effect), which is impermeable to either or both oxygen and impurities such as hydrogen and moisture, and, in and after the manufacturing process, the aluminum oxide film functions as a protective film for preventing entry of an impurity such as hydrogen or moisture, which causes a change in characteristics, into the oxide semiconductor layer  403  and release of oxygen, which is a main constituent material of the oxide semiconductor, from the oxide semiconductor layer  403 . 
     The insulating layer  407  is preferably formed by a method such as a sputtering method, in which an impurity such as water or hydrogen does not enter the insulating layer  407 . 
     In order to remove residual moisture from the deposition chamber of the insulating layer  407  in a manner similar to that of the deposition of the oxide semiconductor layer  403 , an entrapment vacuum pump (such as a cryopump) is preferably used. When the insulating layer  407  is deposited in the deposition chamber evacuated using a cryopump, the impurity concentration of the insulating layer  407  can be reduced. In addition, as an evacuation unit for removing moisture remaining in the film formation chamber of the insulating layer  407 , a turbo molecular pump provided with a cold trap may be used. 
     In this embodiment, as the insulating layer  407 , a stacked-layer structure in which an aluminum oxide film and a silicon oxide film are stacked in this order from the side which is in contact with the gate electrode layer  401  is employed. Note that the aluminum oxide film has a high density (film density higher than or equal to 3.2 g/cm 3 , preferably higher than or equal to 3.6 g/cm 3 ), whereby the field-effect transistor  420  can have stable electrical characteristics. The film density can be measured by Rutherford backscattering spectrometry (also referred to as RBS) or X-ray reflectometer (also referred to as XRR). 
     Next, openings reaching the source electrode layer  405   a  and the drain electrode layer  405   b  are formed in the insulating layer  407  and the gate insulating layer  402 , and the wiring layer  435   a  and the wiring layer  435   b  are formed in the openings (see  FIG. 8C ). In the semiconductor device of this embodiment, for example, with the use of the wiring layer  435   a  and the wiring layer  435   b , the field-effect transistor is connected to another field-effect transistor or another element, which can lead to formation of a variety of circuits. 
     For example, the wiring layer  435   a  and the wiring layer  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 (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film), or the like can be used. A film of a high melting point metal 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, 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 of Al, Cu, or the like. The wiring layer  435   a  and the wiring layer  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 (ITO), 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 layer  435   a  and the wiring layer  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. 
     Through the above-described process, the field-effect transistor  422  of this embodiment can be formed. 
     Note that when the length of the island-shaped oxide semiconductor layer  403  in the channel length direction is larger than the length of the gate electrode layer  401  in the channel length direction, the degree of freedom of alignment for forming the gate electrode layer  401  can be increased. In that case, in order to decrease the channel length of the field-effect transistor, an impurity region may be provided in the oxide semiconductor layer  403 . 
     For example, a field-effect transistor  424  illustrated in  FIGS. 9A and 9B  and a field-effect transistor  426  illustrated in  FIGS. 10A and 10B  are examples which are formed as follows: the gate electrode layer  401  is formed and then an impurity is added to the oxide semiconductor layer  403  using the gate electrode layer  401  as a mask, so that an impurity region  403   a  and an impurity region  403   b  are formed in a self-aligned manner. 
     The field-effect transistor  424  has a structure similar to that of the field-effect transistor  420 ; the field-effect transistor  424  is different from the field-effect transistor  420  in that the oxide semiconductor layer  403  included in the field-effect transistor  424  has a pair of impurity regions (the impurity region  403   a  and the impurity region  403   b ) containing a dopant and a channel formation region  403   c  sandwiched between the pair of impurity regions. The field-effect transistor  426  illustrated in  FIGS. 10A and 10B  has a structure similar to that of the field-effect transistor  422 ; the field-effect transistor  426  is different from the field-effect transistor  422  in that the oxide semiconductor layer  403  included in the field-effect transistor  426  has a pair of impurity regions (the impurity region  403   a  and the impurity region  403   b ) containing a dopant and the channel formation region  403   c  sandwiched between the pair of impurity regions. Note that  FIG. 9A  is a plan view.  FIG. 9B  is a cross-sectional view taken along a line X 3 -Y 3  of  FIG. 9A .  FIG. 10A  is a plan view.  FIG. 10B  is a cross-sectional view taken along a line X 4 -Y 4  of  FIG. 10A . 
     The dopant is an impurity which changes the conductivity of the oxide semiconductor layer  403 . As the method for adding 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 layer including the pair of impurity regions between which the channel formation region  403   c  is provided in the channel length direction, on-state characteristics (e.g., on-state current and field-effect mobility) of the field-effect transistors  424  and  426  are increased, which enables high-speed operation and quick response. 
     As described above with reference to  FIGS. 5A and 5B ,  FIGS. 6A and 6B ,  FIGS. 7A to 7D ,  FIGS. 8A to 8C ,  FIGS. 9A and 9B , and  FIGS. 10A and 10B , an example of the semiconductor device in this embodiment includes a conductive layer (e.g., a gate electrode layer) having a function as a gate of a field-effect transistor, an insulating layer (e.g., a gate insulating layer) having a function as a gate insulating layer of the field-effect transistor, a semiconductor layer (e.g., a channel formation layer) having a function as a channel formation layer of the field-effect transistor, a conductive layer (e.g., a source electrode layer) having a function as one of a source and a drain of the field-effect transistor, and a conductive layer (e.g., a drain electrode layer) having a function as the other of the source and the drain of the field-effect transistor. The semiconductor layer is formed using the oxide semiconductor film in Embodiment 1. 
     In an example of the semiconductor device in this embodiment, the oxide semiconductor film described in Embodiment 1 is used as the semiconductor layer having a function as a channel formation layer of the field-effect transistor. The oxide semiconductor film described in Embodiment 1 has less oxygen deficiency and thus has a low possibility of occurrence of unnecessary carriers. Thus, off-state current of the field-effect transistor can be lowered and electrical characteristics of the field-effect transistor can be improved. In addition, since the oxide semiconductor film described in Embodiment 1 contains germanium (Ge), the oxide semiconductor film has a wide band gap. Thus, withstand voltage of the field-effect transistor can be improved, for example; accordingly, electrical characteristics of the field-effect transistor can be improved. 
     Embodiment 3 
     In this embodiment, an example of a semiconductor device which includes the field-effect transistor described in this specification, which can hold stored data even when not powered, and which does not have a limitation on the number of write cycles, will be described. 
       FIGS. 11A to 11C  illustrate an example of a structure of a semiconductor device.  FIG. 11A  is a cross-sectional diagram of the semiconductor device.  FIG. 11B  is a plan view of the semiconductor device.  FIG. 11C  is a circuit diagram of the semiconductor device. Note that  FIG. 11A  corresponds to a cross-section taken along line C 1 -C 2  and line D 1 -D 2  of  FIG. 11B . 
     The semiconductor device illustrated in  FIGS. 11A and 11B  includes a field-effect transistor  560  including a first semiconductor material in a lower portion and a field-effect transistor  562  including a second semiconductor material (in this embodiment, the oxide semiconductor film described in Embodiment 1) in an upper portion. Note that  FIGS. 11A to 11C  illustrate an example in which the structure of the field-effect transistor  420  described in Embodiment 2 is applied to that of the field-effect transistor  562 . 
     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 field-effect 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 field-effect transistor including an oxide semiconductor for a long time owing to its characteristics. 
     Although all the field-effect transistors are n-channel field-effect transistors, it is needless to say that p-channel field-effect transistors can be used. The specific structure of the semiconductor device is not necessarily limited to those described here such as the material used for the semiconductor device and the structure of the semiconductor device (e.g., the use of the field-effect transistor described in Embodiment 2, which is formed using an oxide semiconductor, as the field-effect transistor  562  for holding information). 
     The field-effect transistor  560  in  FIG. 11A  includes a channel formation region  516  provided over a semiconductor substrate  500  including a semiconductor material (e.g., silicon), impurity regions  520  with the channel formation region  516  provided therebetween, intermetallic compound regions  524  which are in contact with the impurity regions  520 , a gate insulating layer  508  provided over the channel formation region  516 , and a gate electrode layer  510  provided over the gate insulating layer  508 . Note that a transistor whose source electrode layer and drain electrode layer are not illustrated in a drawing may also be referred to as a field-effect transistor for the sake of convenience. Further, in such a case, in description of a connection of a field-effect transistor, a source region may be collectively referred to as a source electrode layer, and a drain region may be collectively referred to as a drain electrode layer. That is, in this specification, the term “source electrode layer” may include a source region. 
     An element isolation insulating layer  506  is provided over the substrate  500  to surround the field-effect transistor  560 . An insulating layer  528  and an insulating layer  530  are provided to overlap the field-effect transistor  560 . Note that, in the field-effect transistor  560 , sidewall insulating layers may be formed on side surfaces of the gate electrode layer  510  and the impurity regions  520  may include a region having a different impurity concentration. 
     The field-effect transistor  560  formed using a single crystal semiconductor substrate can operate at high speed. With the use of that field-effect transistor as a field-effect transistor for reading, data can be read at high speed. In this embodiment, two insulating films are formed to cover the field-effect transistor  560 . Note that a single insulating film or a stack of three of more insulating films may be formed. As treatment prior to formation of the field-effect transistor  560  and a capacitor  564 , CMP treatment is performed on the insulating films formed over the field-effect transistor  560 , whereby an insulating layer  528  and an insulating layer  530  which are planarized are formed and, at the same time, an upper surface of the gate electrode layer  510  is exposed. 
     As the insulating layers  528  and  530 , 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 layers  528  and  530  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, the insulating layer  528  and the insulating layer  530  may be formed by a wet method such as a spin coating method or a printing method. 
     Note that in this embodiment, a silicon nitride film is used as the insulating layer  528 , and a silicon oxide layer is used as the insulating layer  530 . 
     Planarization treatment is preferably performed on the surface of the insulating layer  530  in the formation region of the oxide semiconductor layer  544 . In this embodiment, the oxide semiconductor layer  544  is formed over the insulating layer  530  sufficiently planarized by polishing treatment (e.g., CMP treatment) (the average surface roughness of the surface of the insulating layer  530  is preferably less than or equal to 0.15 nm). 
     The field-effect transistor  562  illustrated in  FIG. 11A  is a field-effect transistor in which the oxide semiconductor film described in Embodiment 1 is used as the oxide semiconductor layer having a function as a channel formation layer. Here, the oxide semiconductor layer  544  included in the field-effect transistor  562  is preferably highly purified. By using a highly purified oxide semiconductor, the field-effect transistor  562  which has extremely favorable off-state current characteristics can be obtained. 
     Since the off-state current of the field-effect transistor  562  is small, stored data can be held for a long time owing to such a field-effect 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. 
     The field-effect transistor  562  includes the oxide semiconductor layer  544  whose side surfaces in the channel length direction are in contact with an electrode layer  542   a  and an electrode layer  542   b . Thus, resistances of regions where the oxide semiconductor layer  544  is in contact with the electrode layers  542   a  and  542   b  can be increased, whereby an electric field between the source and the drain can be relieved. Accordingly, a short-channel effect in accordance with miniaturization of a size of the field-effect transistor can be suppressed. 
     In addition, the field-effect transistor  562  includes conductive sidewall layers  537   a  and  537   b  on side surfaces of a gate electrode layer  548  in the channel length direction. Thus, the conductive sidewall layer  537   a  and the conductive sidewall layer  537   b  overlap with the electrode layer  542   a  and the electrode layer  542   b , respectively, with the gate insulating layer  546  provided therebetween. That is, the field-effect transistor substantially includes L ov  regions. Accordingly, a decrease in on-state current of the field-effect transistor  562  can be suppressed. 
     An interlayer insulating film  535  and an insulating layer  550  which have a single-layer structure or a stacked-layer structure are provided over the field-effect transistor  562 . In this embodiment, an aluminum oxide film is used as the insulating layer  550 . The density of the aluminum oxide film is made to be high (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 ), whereby stable electrical characteristics can be given to the field-effect transistor  562 . 
     Further, a conductive layer  553  is provided to overlap with the electrode layer  542   a  of the field-effect transistor  562  with the gate insulating layer  546  provided therebetween. The capacitor  564  is formed of the electrode layer  542   a , the gate insulating layer  546 , and the conductive layer  553 . That is, the electrode layer  542   a  of the field-effect transistor  562  functions as one electrode of the capacitor  564 , and the conductive layer  553  functions as the other electrode of the capacitor  564 . Note that in the case where a capacitor is not needed, the capacitor  564  may be omitted. Alternatively, the capacitor  564  may be separately provided above the field-effect transistor  562 . 
     In this embodiment, the conductive layer  553  can be formed in the same manufacturing step as that of the gate electrode layer  548  of the field-effect transistor  562 . Note that in the step of forming the sidewall layer  537   a  and the sidewall layer  537   b  on the side surfaces of the gate electrode layer  548 , sidewall layers may be formed on side surfaces of the conductive layer. 
     In addition, a wiring  556  for connecting the field-effect transistor  562  to another field-effect transistor is provided over the insulating layer  550 . The wiring  556  is electrically connected to the electrode layer  542   b  through an electrode layer  536  formed in an opening formed in the insulating layer  550 , the interlayer insulating film  535 , the gate insulating layer  546 , and the like. 
     In  FIGS. 11A and 11B , the field-effect transistor  560  is provided so as to overlap with at least part of the field-effect transistor  562 . The source region or the drain region of the field-effect transistor  560  is preferably provided so as to overlap with part of the oxide semiconductor layer  544 . In addition, the field-effect transistor  562  and the capacitor  564  are provided so as to overlap with the field-effect transistor  560  at least partly. For example, the conductive layer  553  of the capacitor  564  is provided so as to overlap with the gate electrode layer  510  of the field-effect transistor  560  at least partly. 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  542   b  and the wiring  556  may be established by direct contact of the electrode layer  542   b  and the wiring  556  without providing the electrode layer  536 . Alternatively, the electrical connection may be established through a plurality of electrode layers. 
     Next, an example of a circuit configuration corresponding to  FIGS. 11A and 11B  is illustrated in  FIG. 11C . 
     In  FIG. 11C , a first wiring (a 1st Line) is electrically connected to the source electrode layer of the field-effect transistor  560 , and a second wiring (a 2nd Line) is electrically connected to a drain electrode layer of the field-effect transistor  560 . A third wiring (3rd Line) is electrically connected to one of the source electrode layer and the drain electrode layer of the field-effect transistor  562 , and a fourth wiring (4th Line) is electrically connected to a gate electrode layer of the field-effect transistor  562 . A gate electrode of the field-effect transistor  560  and one of the source electrode layer and the drain electrode layer of the field-effect transistor  562  are electrically connected to one electrode of the capacitor  564 . A fifth wiring (5th Line) and the other electrode of the capacitor  564  are electrically connected to each other. 
     The semiconductor device in  FIG. 11C  can write, hold, and read data as described below, utilizing a characteristic in which the potential of the gate electrode layer of the field-effect transistor  560  can be held. 
     Writing and holding of data are described. First, the potential of the fourth wiring is set to a potential at which the field-effect transistor  562  is turned on, whereby the field-effect transistor  562  is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode layer of the field-effect transistor  560  and the capacitor  564 . In other words, a predetermined charge is supplied to the gate electrode layer of the field-effect transistor  560  (i.e., writing of data). Here, one of electric charges for supply of two different potentials (hereinafter referred to as a low-level electric charge and a high-level electric charge) is given. After that, the potential of the fourth wiring is set to a potential at which the field-effect transistor  562  is turned off, so that the field-effect transistor  562  is turned off. Thus, the potential given to the gate electrode layer of the field-effect transistor  560  is held (i.e., holding of data). 
     Since the off-state current of the field-effect transistor  562  is extremely low, the charge of the gate electrode layer of the field-effect transistor  560  is held for a long time. 
     Secondly, reading of data is described. By supplying an appropriate potential (reading potential) to the fifth wiring with a predetermined potential (constant potential) supplied to the first wiring, the potential of the second wiring varies depending on the amount of charge held in the gate electrode of the field-effect transistor  560 . This is because in general, when the field-effect transistor  560  is an n-channel transistor, an apparent threshold voltage V th     —     H  in the case where the high-level potential is given to the gate electrode layer of the field-effect transistor  560  is lower than an apparent threshold voltage V th     —     L  in the case where the low-level charge is given to the gate electrode layer of the field-effect transistor  560 . Here, the apparent threshold voltage refers to the potential of the fifth wiring, which is needed to turn on the field-effect transistor  560 . Thus, the potential of the fifth wiring is set to potential V 0  that is intermediate between V th     —     H  and V th     —     L , whereby charge supplied to the gate electrode layer of the field-effect transistor  560  can be determined. For example, in the case where the high-level electric charge is given in writing, when the potential of the fifth wiring is set to V 0  (&gt;V th     —     H ), the field-effect transistor  560  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 field-effect transistor  560  remains in an off state. Therefore, the stored data can be read by the potential of the second wiring. 
     Note that in the case where memory cells are arrayed, only data of desired memory cells needs to be read. In the case where such reading is not performed, a potential at which the field-effect transistor  560  is turned off regardless of the state of the gate electrode layer of the field-effect transistor  560 , that is, a potential smaller than V th     —     H  may be given to the fifth wiring. Alternatively, a potential at which the field-effect transistor  560  is turned on regardless of the state of the gate electrode layer, that is, a potential higher than V th     —     L  may be given to the fifth wiring. 
     In the semiconductor device described in this embodiment, the field-effect transistor in which an oxide semiconductor layer described in Embodiment 1 is used as an oxide semiconductor layer having a function as a channel formation layer and in which off-state current is extremely low is applied, whereby stored data can be held for an extremely long time. 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). 
     The semiconductor device in this embodiment does not need high voltage for writing data and has 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, and thus a problem such as deterioration of a gate insulating layer does not occur at all. In other words, the semiconductor device according to an embodiment of the present 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 ON and OFF of the field-effect transistor, whereby high-speed operation can be easily realized. 
     As described above, a miniaturized and highly-integrated semiconductor device having high electrical characteristics and a method for manufacturing the semiconductor device can be provided. 
     Embodiment 4 
     In this embodiment, a semiconductor device which includes the field-effect transistor described in Embodiment 2, which can hold stored data even when not powered, which has an unlimited number of write cycles, and which has a structure different from the structure described in Embodiment 3 is described with reference to  FIGS. 12A and 12B  and  FIGS. 13A and 13B . 
       FIG. 12A  is an example illustrating a circuit configuration of a semiconductor device.  FIG. 12B  is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in  FIG. 12A  is described, and then, the semiconductor device illustrated in  FIG. 12B  is described. 
     In the semiconductor device illustrated in  FIG. 12A , a bit line BL is electrically connected to the source electrode layer or the drain electrode layer of the field-effect transistor  562 , a word line WL is electrically connected to the gate electrode layer of the field-effect transistor  562 , and the source electrode layer or the drain electrode layer of the field-effect transistor  562  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. 12A  are described. 
     First, the potential of the word line WL is set to a potential at which the field-effect transistor  562  is turned on, so that the field-effect transistor  562  is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor  254  (i.e., writing of data). After that, the potential of the word line WL is set to a potential at which the field-effect transistor  562  is turned off, so that the field-effect transistor  562  is turned off. Thus, the charge at the first terminal of the capacitor  254  is held (i.e., holding of data). 
     The field-effect transistor  562  in which the oxide semiconductor film described in Embodiment 1 is used as the oxide semiconductor layer having a function as a channel formation layer has a characteristic of 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 field-effect transistor  562 . 
     Secondly, reading of data is described. When the field-effect transistor  562  is turned on, the bit line BL which is in a floating state and the capacitor  254  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. 12A  can hold charge that is accumulated in the capacitor  254  for a long time because the off-state current of the field-effect transistor  562  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 retained for a long time even when power is not supplied. 
     Next, the semiconductor device illustrated in  FIG. 12B  is described. 
     The semiconductor device illustrated in  FIG. 12B  includes a memory cell array  251  (memory cell arrays  251   a  and  251   b ) including a plurality of memory cells  250  illustrated in  FIG. 12A  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. 12B , the peripheral circuit  253  can be provided under the memory cell array  251  (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 field-effect transistor provided in the peripheral circuit  253  be different from that of the field-effect transistor  562 . 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 field-effect 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 need to operate at high speed can be favorably realized with the use of the field-effect transistor. 
     Note that  FIG. 12B  illustrates, as an example, the semiconductor device in which two memory cell arrays  251  (the memory cell arrays  251   a  and  251   b ) are stacked; however, the number of memory cells to be stacked is not limited thereto. Three or more memory cells may be stacked. 
     Next, a specific structure of the memory cell  250  illustrated in  FIG. 12A  is described with reference to  FIGS. 13A and 13B . 
       FIGS. 13A and 13B  illustrate an example of a structure of the memory cell  250 .  FIG. 13A  illustrates a cross-sectional view of the memory cell  250 .  FIG. 13B  is a plan view of the memory cell  250 . Here,  FIG. 13A  illustrates a cross section taken along line F 1 -F 2  and line G 1 -G 2  in  FIG. 13B . 
     The field-effect transistor  562  in  FIGS. 13A and 13B  can have the same structure as the field-effect transistor in Embodiment 2. 
     In addition, a conductive layer  262  is provided in a region overlapping with the electrode layer  542   a  of the field-effect transistor  562  with the gate insulating layer  546  provided therebetween, and the electrode layer  542   a , the gate insulating layer  546 , and the conductive layer  262  form the capacitor  254 . That is, the electrode layer  542   a  of the field-effect transistor  562  functions as one electrode of the capacitor  254 , and the conductive layer  262  functions as the other electrode of the capacitor  254 . 
     The interlayer insulating film  535  and the insulating layer  256  are formed with a single-layer structure or a stacked-layer structure over the field-effect transistor  562  and the capacitor  254 . Further, a wiring  260  for connecting the memory cell  250  to an adjacent memory cell  250  is provided over the insulating layer  256 . The wiring  260  is electrically connected to the electrode layer  542   b  of the field-effect transistor  562  through an opening formed in the insulating layer  256 , the interlayer insulating film  535 , the gate insulating layer  546 , and the like. Note that the wiring  260  and the electrode layer  542   b  may be directly connected to each other. Note that the wiring  260  corresponds to the bit line BL in the circuit diagram of  FIG. 12A . 
     In  FIGS. 13A and 13B , the electrode layer  542   b  of the field-effect transistor  562  can also function as a source of a field-effect 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. 
     Adoption of the planar layout illustrated in  FIG. 13A  enables the area occupied by the semiconductor device to be reduced, whereby the degree of integration can be increased. 
     As describe above, the plurality of memory cells is formed in multiple layers with the field-effect transistor in which the oxide semiconductor film described in Embodiment 1 is used as the oxide semiconductor layer having a function as a channel formation layer. The field-effect transistor in which the oxide semiconductor film described in Embodiment 1 is used as the oxide semiconductor layer having a function as a channel formation layer has a low off-state current. Thus, with such a field-effect transistor, stored data can be held for a long time. That is, the frequency of 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 field-effect transistor including a material other than an oxide semiconductor (in other words, a field-effect transistor capable of operating at sufficiently high speed) and a memory circuit including the field-effect transistor including an oxide semiconductor film described in Embodiment 1 as an oxide semiconductor layer having a function as a channel formation layer (in a broader sense, a field-effect 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 electrical characteristics and a method for manufacturing the semiconductor device can be provided. 
     Embodiment 5 
     In this embodiment, examples of application of the semiconductor device described in Embodiments 3 and 4 to a portable device such as a mobile phone, a smartphone, or an e-book reader are described with reference to  FIGS. 14A and 14B ,  FIG. 15 ,  FIG. 16 , and  FIG. 17 . 
     In portable devices such as a mobile phone, a smart phone, and an e-book reader, an SRAM or a DRAM is used so as to store image data temporarily. This is because response speed of a flash memory is low and thus a flash memory 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. 14A , one memory cell includes six field-effect transistors, that is, field-effect transistors  801  to  806 , which are driven with an X decoder  807  and a Y decoder  808 . The field-effect transistors  803  and  805 , and the field-effect transistors  804  and  806  form inverters, which enables high-speed driving. However, since one memory cell consists of six field-effect transistors, there is a drawback of large cell area. 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, a price per bit of an SRAM is the most expensive among a variety of memory devices. 
     On the other hand, as shown in  FIG. 14B , a memory cell in a DRAM includes a field-effect transistor  811  and a storage capacitor  812 , and is driven by an X decoder  813  and a Y decoder  814 . One cell includes one field-effect transistor and one capacitor and thus the area of a memory cell is small. The area of a memory cell of a DRAM is generally less than or equal to 10 F 2 . 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 the above embodiments has an area of approximately 10 F 2  and does not need to be refreshed frequently. Therefore, the area of the memory cell is reduced, and the power consumption can be reduced. 
       FIG. 15  is a block diagram of a portable device. The portable device illustrated in  FIG. 15  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 memory  910  such as a flash memory, a display controller  911 , a memory  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  912  includes an SRAM or a DRAM; by employing the semiconductor device described in Embodiment 3 or 4 for the memory  912 , it is possible to provide a portable device in which writing and reading of data can be performed at high speed, data can be held for a long time, and power consumption can be sufficiently reduced. 
       FIG. 16  shows an example in which the semiconductor device described in the above embodiments is used for a memory  950  in a display. The memory  950  illustrated in  FIG. 16  includes a memory circuit  952 , a memory circuit  953 , a switch  954 , a switch  955 , and a memory controller  951 . Further, in the memory  950 , image data input from a signal line (input image data), a display controller  956  which reads and controls data held in the memory circuits  952  and  953 , and a display  957  which displays data by a signal from the display controller  956  are connected. 
     First, image data (input image data A) is formed by an application processor (not shown). The input image data A is stored in the memory circuit  952  through the switch  954 . Then, the image data stored in the memory circuit  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 circuit  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 data displayed on the screen is rewritten by a user (that is, in the case where the input image data A is changed), new image data (input image data B) is formed by the application processor. The input image data B is stored in the memory circuit  953  through the switch  954 . The stored image data A is read periodically from the memory circuit  952  through the switch  955  even during that time. After the completion of storing the new image data (the stored image data B) in the memory circuit  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 displayed on the display  957 . This reading operation continues until the next new image data is stored in the memory circuit  952 . 
     By alternately writing and reading image data to and from the memory circuits  952  and  953  as described above, images are displayed on the display  957 . Note that the memory circuit  952  and the memory circuit  953  are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in Embodiment 3 or 4 for the memory circuits  952  and  953 , data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced. 
       FIG. 17  is a block diagram of an e-book reader. The e-book reader in  FIG. 17  includes a battery  1001 , a power supply circuit  1002 , a microprocessor  1003 , a memory  1004  such as a flash memory, an audio circuit  1005 , a keyboard  1006 , a memory  1007 , a touch panel  1008 , a display  1009 , and a display controller  1010 . 
     Here, the semiconductor device described in Embodiment 3 or 4 can be used for the memory  1007  in  FIG. 17 . The memory  1007  has a function of temporarily holding the contents of a book. For example, users use a highlight function in some cases. In some cases, users put a mark on a letter while reading on an e-book reader. This marking refers to a highlight function, and users can make difference from other places by, for example, changing the color of a letter displayed, underlining a word, making a letter bold, or changing the font type of a letter. That is, there is a function of storing and holding information of a place specified by users. In the case where the information is stored for a long time, the information may be copied to the memory  1004 . Even in such a case, by employing the semiconductor device described in Embodiment 3 or 4, writing and reading of data can be performed at high speed, stored data can be held 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 Embodiment 3 or 4. Therefore, a portable device in which writing and reading of data are performed at high speed, data is held for a long time, and power consumption is sufficiently reduced, can be obtained. 
     This application is based on Japanese Patent Application serial No. 2011-223228 filed with Japan Patent Office on Oct. 7, 2011, the entire contents of which are hereby incorporated by reference.