Patent Publication Number: US-2021184041-A1

Title: Semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. 
     In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices. 
     2. Description of the Related Art 
     Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention. 
     For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1). 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a structure of a transistor including an oxide semiconductor in a channel formation region in which the threshold voltage of electric characteristics of the transistor can be positive, which is a so-called normally-off switching element, and a manufacturing method thereof. 
     Further, another object of one embodiment of the present invention is to provide a structure of a semiconductor device which achieves high-speed response and high-speed operation by improving on characteristics of a transistor (e.g., on-state current or field-effect mobility), and to provide a manufacturing method thereof, in order to achieve a high-performance semiconductor device. 
     Further, another object is to provide a semiconductor device in which reliability is high and threshold voltage does not easily shift even in a long-time use. 
     It is an object of one embodiment of the present invention to achieve at least one of the above-described objects. 
     In a transistor in which a semiconductor layer, source and drain electrode layers, a gate insulating layer, and a gate electrode layer are stacked in that order. As the semiconductor layer, an oxide semiconductor layer which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc, is used. 
     One embodiment of the present invention disclosed in this specification is a semiconductor device including an oxide semiconductor layer including a channel formation region over an oxide insulating layer, a gate insulating film over the oxide semiconductor layer, and a gate electrode layer overlapping with the oxide semiconductor layer over the gate insulating film. The oxide semiconductor layer contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc. 
     One embodiment of the present invention disclosed in this specification is a semiconductor device including an oxide semiconductor layer including a channel formation region over an oxide insulating layer, a source electrode layer and a drain electrode layer over the oxide semiconductor layer, a gate insulating film over the source electrode layer and the drain electrode layer, and a gate electrode layer overlapping with the oxide semiconductor layer over the gate insulating film. The oxide semiconductor layer contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc. 
     One embodiment of the present invention disclosed in this specification is a semiconductor device including a pair of first oxide semiconductor layers separated from each other over an oxide insulating layer, a second oxide semiconductor layer including a channel formation region on and in contact with the oxide insulating layer and the pair of first oxide semiconductor layers, a gate insulating film over the oxide insulating layer and the second oxide semiconductor layer, and a gate electrode layer overlapping with the second oxide semiconductor layer over the gate insulating film. The second oxide semiconductor layer contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc. 
     The oxide semiconductor layer or the second oxide semiconductor layer is a non-single crystal semiconductor, and may include a c-axis-aligned crystal region. 
     The oxide semiconductor layer or the second oxide semiconductor layer is a non-single crystal semiconductor, and can be formed with an oxide target having a composition ratio of indium:gallium:zinc=3:1:2. 
     In the oxide semiconductor layer or the second oxide semiconductor layer, a region which does not overlap with the gate electrode layer may include a dopant. 
     In the oxide semiconductor layer or the second oxide semiconductor layer, a region which does not overlap with the source electrode layer or the drain electrode layer may have a higher oxygen concentration than a region which overlaps with the source electrode layer or the drain electrode layer. 
     Low-resistance regions whose resistances are lower than that of the channel formation region and which include a dopant may be formed in the oxide semiconductor layer so that the channel formation region is sandwiched between the low-resistance regions, by introducing the dopant into the oxide semiconductor layer in a self-aligning manner with the use of the gate electrode layer as a mask. The dopant is an impurity by which the electrical conductivity of the oxide semiconductor layer is changed. As the method for introducing the dopant, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. 
     With an oxide semiconductor layer which includes low-resistance regions between which a channel formation region is sandwiched in the channel length direction, the transistor has excellent on-state characteristics (e.g., on-state current and field-effect mobility) and enables high-speed operation and high-speed response. 
     Further, heat treatment (dehydration or dehydrogenation treatment) by which hydrogen or moisture is released may be performed on the oxide semiconductor layer. When a crystalline oxide semiconductor layer is used as the oxide semiconductor layer, heat treatment for crystallization may be performed. 
     Through the dehydration or dehydrogenation treatment, oxygen that is a main component material of an oxide semiconductor might be detached and thus might be reduced. There is an oxygen defect in a portion where oxygen is detached in the oxide semiconductor film and a donor level which leads to variation in the electric characteristics of a transistor is formed owing to the oxygen defect. 
     Thus, oxygen is preferably supplied to the oxide semiconductor layer after being subjected to the dehydration or dehydrogenation treatment. By supplying oxygen to the stack of oxide semiconductor layers, oxygen defects in the film can be repaired. 
     For example, an oxide insulating film including much (excessive) oxygen, which serves as an oxygen supply source, may be provided so as to be in contact with the oxide semiconductor layer, whereby oxygen can be supplied to the oxide semiconductor layer from the oxide insulating film. In the above structure, heat treatment may be performed as dehydration or dehydrogenation treatment in the state where the oxide semiconductor layer and the oxide insulating film are in contact with each other at least partly to supply oxygen to the oxide semiconductor layer. 
     Further or alternatively, 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. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be used. 
     Further, it is preferable that the oxide semiconductor layer in a transistor include a region where the oxygen content is higher than that in the stoichiometric composition of the oxide semiconductor in a crystalline state. In that case, the oxygen content is higher than that in the stoichiometric composition ratio of the oxide semiconductor. Alternatively, the oxygen content is higher than that of the oxide semiconductor in a single crystal state. In some cases, oxygen exists between lattices of the oxide semiconductor. 
     By removing hydrogen or moisture from the oxide semiconductor to highly purify the oxide semiconductor so as not to contain impurities as much as possible, and supplying oxygen to repair oxygen vacancies therein, the oxide semiconductor can be turned into an i-type (intrinsic) oxide semiconductor or a substantially i-type (intrinsic) oxide semiconductor. This enables the Fermi level (E f ) of the oxide semiconductor to be at the same level as the intrinsic Fermi level (E i ). Accordingly, by using the oxide semiconductor layer for a transistor, fluctuation in the threshold voltage Vth of the transistor due to an oxygen vacancy and a shift of the threshold voltage ΔVth can be reduced. 
     One embodiment of the present invention relates to a semiconductor device including a transistor or a semiconductor device including a circuit which is formed by using a transistor. For example, one embodiment of the present invention relates to a semiconductor device including a transistor in which a channel formation region is formed using an oxide semiconductor or a semiconductor device including a circuit which is formed by using such a transistor. For example, the present invention relates to an electronic device which includes, as a component, an LSI; a CPU; a power device mounted in a power circuit; a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display panel; or a light-emitting display device including a light-emitting element. 
     According to one embodiment of the present invention, a structure of a transistor including an oxide semiconductor in a channel formation region in which the threshold voltage of electric characteristics of the transistor can be positive, which is a so-called normally-off switching element, and a manufacturing method thereof can be provided. 
     Further, according to one embodiment of the present invention, in order to achieve a semiconductor device having higher performance, a structure for improving on-state characteristics of the transistor (e.g., on-state current and field-effect mobility) and for achieving high-speed response and high-speed operation of the semiconductor device and a manufacturing method thereof can be provided. 
     Further, according to one embodiment of the present invention, a semiconductor device in which reliability is high and threshold voltage does not easily shift even in a long-time use can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  are diagrams showing one embodiment of a semiconductor device and a method for manufacturing the semiconductor device. 
         FIGS. 2A to 2C  are diagrams showing one embodiment of a semiconductor device. 
         FIGS. 3A and 3C  are diagrams showing one embodiment of a semiconductor device. 
         FIGS. 4A to 4E  are diagrams showing one embodiment of a semiconductor device and a method for manufacturing the semiconductor device. 
         FIGS. 5A to 5C  are a cross-sectional view, a plan view, and a circuit diagram showing one embodiment of a semiconductor device. 
         FIGS. 6A and 6B  are a circuit diagram and a perspective view showing one embodiment of a semiconductor device. 
         FIGS. 7A to 7C  are a plan view and cross-sectional views showing one embodiment of a semiconductor device. 
         FIGS. 8A and 8B  are circuit diagrams showing one embodiment of a semiconductor device. 
         FIG. 9  is a block diagram showing one embodiment of a semiconductor device. 
         FIG. 10  is a block diagram showing one embodiment of a semiconductor device. 
         FIG. 11  is a block diagram showing one embodiment of a semiconductor device. 
         FIG. 12  is an energy band diagram of an oxide semiconductor. 
         FIG. 13  is a graph showing results of XRD measurement of an oxide semiconductor film. 
         FIG. 14  is a graph showing evaluation results of electrical characteristics of a transistor  1 . 
         FIGS. 15A and 15B  are graphs showing evaluation results of electrical characteristics and reliability of a transistor  2 . 
         FIGS. 16A to 16C  are TEM photographs of an oxide semiconductor film. 
         FIG. 17  is a graph showing a leakage current of a transistor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the invention disclosed in this specification are described with reference to the accompanying drawings. Note that the invention disclosed in this specification is not limited to the following description, and it is easily understood by those skilled in the art that modes and details can be variously changed without departing from the spirit and the scope of the invention. Therefore, the invention disclosed in this specification is not construed as being limited to the description of the following embodiments. Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps and the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention. 
     Embodiment 1 
     In this embodiment, one embodiment of a semiconductor device and a method for manufacturing the semiconductor device are described with reference to  FIGS. 1A to 1E  and  FIGS. 3A to 3C . In this embodiment, a transistor including an oxide semiconductor film is described as an example of the semiconductor device. 
     The transistor may have a single-gate structure in which one channel formation region is formed, a double-gate structure in which two channel formation regions are formed, or a triple-gate structure in which three channel formation regions are formed. Alternatively, the transistor may have a dual-gate structure including two gate electrode layers positioned over and under a channel formation region with a gate insulating film provided therebetween. 
     A transistor  440   a  shown in  FIGS. 1A to 1E  is an example of a planar type transistor having a top-gate structure. 
     The transistor  440   a  includes, over a substrate  400  having an insulating surface over which an oxide insulating layer  436  is provided, an oxide semiconductor layer  403  including a channel formation region  409 , low-resistance regions  404   a  and  404   b , and low-resistance regions  406   a  and  406   b , a source electrode layer  405   a , a drain electrode layer  405   b , a gate insulating film  402 , and a gate electrode layer  401 . An insulating film  407  is formed over the transistor  440   a.    
     In  FIGS. 1A to 1E , the source electrode layer  405   a  and the drain electrode layer  405   b  are not overlapped with the gate electrode layer  401 , over the oxide semiconductor layer  403 ; however, the source electrode layer  405   a  and the drain electrode layer  405   b  may be partly overlapped with the gate electrode layer  401  like a transistor  440   b  shown in  FIG. 2A . 
     The oxide semiconductor layer  403  is an oxide semiconductor layer (also referred to as an IGZO layer) which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc. 
     The oxide semiconductor layer  403  can be formed by a sputtering method with an oxide target having a composition ratio of indium:gallium:zinc=3:1:2. 
     The oxide semiconductor is non-single-crystal and may be either amorphous or polycrystalline. Further, the oxide semiconductor may have either an amorphous structure including a portion having crystallinity or a non-amorphous structure. 
     In an oxide semiconductor in an amorphous state, a flat surface can be obtained with relative ease, so that when a transistor is manufactured with the use of the oxide semiconductor, interface scattering can be reduced, and relatively high mobility can be obtained with relative ease. 
     In an oxide semiconductor having crystallinity, defects in the bulk can be further reduced and when a surface flatness is improved, mobility higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with the average surface roughness (R a ) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably less than or equal to 0.1 nm. 
     Note that the average surface roughness (R a ) is obtained by expanding, into three dimensions, arithmetic mean surface roughness that is defined by JIS B 0601: 2001 (ISO4287:1997) so as to be able to apply it to a curved surface. R a  can be expressed as an “average value of the absolute values of deviations from a reference surface to a designated surface” and is defined by the following formula 1. 
     
       
         
           
             
               
                 
                   
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     Here, the specific surface is a surface which is a target of roughness measurement, and is a quadrilateral region which is specified by four points represented by the coordinates (x 1 , y 1 , f(x 1 , y 1 )), (x 1 , y 2 , f(x 1 , y 2 )), (x 2 , y 1 , f(x 2 , y 1 )), and (x 2 , y 2 , f(x 2 , y 2 )). S 0  represents the area of a rectangle which is obtained by projecting the specific surface on the xy plane, and Z 0  represents the height of the reference surface (the average height of the specific surface). Ra can be measured using an atomic force microscope (AFM). 
     As the oxide semiconductor layer  403 , an oxide semiconductor layer including a crystal and having crystallinity (crystalline oxide semiconductor layer) can be used. The crystals in the crystalline oxide semiconductor layer may have crystal axes oriented in random directions or in a certain direction. 
     For example, an oxide semiconductor layer including a crystal having a c-axis which is substantially perpendicular to a surface of the oxide semiconductor film can be used as the crystalline oxide semiconductor layer. 
     The oxide semiconductor layer including a crystal having a c-axis substantially perpendicular to a surface has neither single crystal structure nor amorphous structure and is an oxide semiconductor layer including a c-axis aligned crystal (also referred to as CAAC), i.e., a CAAC-OS layer. 
     CAAC is a c-axis aligned crystal which has a triangular or hexagonal atomic arrangement when seen from the direction of an a-b plane, a surface, or an interface and in which metal atoms are arranged in a layered manner, or metal atoms and oxygen atoms are arranged in a layered manner along a c-axis, and the direction of the a-axis or the b-axis is varied in the a-b plane (or the surface or the interface), that is, which rotates around the c-axis. A thin film including CAAC is crystallized along the c-axis but alignment along the a-b planes does not necessarily appear. 
     The CAAC is, in a broad sense, non-single-crystal including a phase which has a triangular, hexagonal, regular triangular, or regular hexagonal atomic arrangement when seen from the direction perpendicular to the a-b plane and in which metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis direction. 
     A film including CAAC is not a single crystal, but this does not mean that the CAAC film is composed of only an amorphous component. Although the CAAC film includes a crystallized portion (crystalline portion), a boundary between one crystalline portion and another crystalline portion is not clear in some cases. 
     Nitrogen may be substituted for part of oxygen included in the CAAC. The c-axes of individual crystalline portions included in the CAAC film may be aligned in one direction (e.g., the direction perpendicular to a surface of a substrate over which the CAAC film is formed, a surface of the CAAC film, or an interface of the CAAC film). Alternatively, normals of the a-b planes of individual crystalline portions included in the CAAC film may be aligned in one direction (e.g., the direction perpendicular to a surface of a substrate over which the CAAC film is formed, a surface of the CAAC film, an interface of the CAAC film, or the like). 
     The crystalline oxide semiconductor layer enables a change of electric characteristics of the transistor due to irradiation with visible light or ultraviolet light to be further suppressed, leading to a highly reliable semiconductor device. 
     There are three methods for obtaining a crystalline oxide semiconductor layer having c-axis alignment. The first is a method in which an oxide semiconductor layer is deposited at a temperature higher than or equal to 200° C. and lower than or equal to 500° C. such that the c-axis is substantially perpendicular to the top surface. The second is a method in which an oxide semiconductor layer is deposited thin, and is subjected to heat treatment at a temperature(s) higher than or equal to 200° C. and lower than or equal to 700° C., so that the c-axis is substantially perpendicular to the top surface. The third is a method in which a first-layer oxide semiconductor layer is deposited thin, and is subjected to heat treatment at a temperature(s) higher than or equal to 200° C. and lower than or equal to 700° C., and a second-layer oxide semiconductor layer is deposited thereover, so that the c-axis is substantially perpendicular to the top surface. 
     The oxide semiconductor layer  403  has a thickness greater than or equal to 1 nm and less than or equal to 30 nm (preferably greater than or equal to 5 nm and less than or equal to 10 nm) and can be formed by a sputtering method, a molecular beam epitaxy (MBE) method, a CVD method, a pulse laser deposition method, an atomic layer deposition (ALD) method, or the like as appropriate. The oxide semiconductor layer  403  may be formed with a sputtering apparatus which performs deposition in the state where top surfaces of a plurality of substrates are substantially perpendicular to a top surface of a sputtering target. 
     For example, the CAAC-OS film is formed by a sputtering method with a polycrystalline oxide semiconductor sputtering target. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) may flake off from the sputtering target. In that case, the flat-plate-like sputtered particle reaches a substrate while maintaining their crystal state, whereby the CAAC-OS film can be formed. 
     For the deposition of the CAAC-OS film, the following conditions are preferably used. 
     By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used. 
     By increasing the substrate heating temperature during the deposition, migration of a sputtered particle is likely to occur after the sputtered particle reaches a substrate surface. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate, migration occurs on the substrate surface, so that a flat plane of the flat-plate-like sputtered particle is attached to the substrate. 
     Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol % or higher, preferably 100 vol %. 
     As an example of the sputtering target, an In—Ga—Zn—O compound target is described below. 
     The In—Ga—Zn—O compound target, which is polycrystalline, is made by mixing InO X  powder, GaO Y  powder, and ZnO Z  powder in a predetermined molar ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y and Z are given positive numbers. Here, the predetermined molar ratio of InO X  powder to GaO Y  powder and ZnO Z  powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target. 
       FIGS. 1A to 1E  illustrate an example of a method for manufacturing the transistor  440   a.    
     First, the oxide 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 heat resistance enough to withstand heat treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate; or the like can be used as the substrate  400 , or the substrate provided with a semiconductor element can be used as the substrate  400 . 
     The semiconductor device may be manufactured using a flexible substrate as the substrate  400 . To manufacture a flexible semiconductor device, the transistor  440   a  including the oxide semiconductor layer  403  may be directly formed over a flexible substrate; or alternatively, the transistor  440   a  including the oxide semiconductor layer  403  may be formed over a substrate, and then may be separated and transferred to a flexible substrate. Note that in order to separate the transistor  440   a  from the manufacturing substrate and transfer it to the flexible substrate, a separation layer may be provided between the manufacturing substrate and the transistor  440   a  including the oxide semiconductor film. 
     The oxide insulating layer  436  can be formed by a plasma CVD method, a sputtering method, or the like using any of silicon oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, gallium oxide, silicon nitride oxide, and aluminum nitride oxide, or a mixed material thereof. 
     The oxide insulating layer  436  may be either a single layer or a stacked layer. For example, a silicon oxide film, an In—Hf—Zn-based oxide film, and the oxide semiconductor layer  403  may be stacked in that order over the substrate  400 ; a silicon oxide film, an In—Zr—Zn-based oxide film with an atomic ratio of In:Zr:Zn=1:1:1, and the oxide semiconductor layer  403  may be stacked in that order over the substrate  400 ; or a silicon oxide film, an In—Gd—Zn-based oxide film with an atomic ratio of In:Gd:Zn=1:1:1, and the oxide semiconductor layer  403  may be stacked in that order over the substrate  400 . 
     A silicon oxide film is formed by a sputtering method as the oxide insulating layer  436  in this embodiment. 
     Further, a nitride insulating film may be provided between the oxide insulating layer  436  and the substrate  400 . The nitride insulating film can be formed using any of silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or a mixed material of any of these, by a plasma CVD method, a sputtering method, or the like. 
     Next, the oxide semiconductor layer  403  is formed over the oxide insulating layer  436 . 
     The oxide insulating layer  436 , which is in contact with the oxide semiconductor layer  403 , preferably contains oxygen which exceeds at least the stoichiometric composition ratio in the film (the bulk). For example, in the case where a silicon oxide film is used as the oxide insulating layer  436 , the composition formula is SiO 2+α  (α&gt;0). By using such a film as the oxide insulating layer  436 , oxygen can be supplied to the oxide semiconductor layer  403 , leading to favorable characteristics. By a supply of oxygen to the oxide semiconductor layer  403 , oxygen vacancies in the film can be repaired. 
     For example, when the oxide insulating layer  436  containing much (excessive) oxygen, which serves as an oxygen supply source, is provided so as to be in contact with the oxide semiconductor layer  403 , oxygen can be supplied from the oxide insulating layer  436  to the oxide semiconductor layer  403 . Heat treatment may be performed in the state where the oxide semiconductor layer  403  and the oxide insulating layer  436  are in contact with each other at least partly to supply oxygen to the oxide semiconductor layer  403 . 
     In order that hydrogen or water will be not contained in the oxide semiconductor layer  403  as much as possible in the formation step of the oxide semiconductor layer  403 , it is preferable to heat the substrate provided with the oxide insulating layer  436  in a preheating chamber in a sputtering apparatus as a pretreatment for formation of the oxide semiconductor layer  403  so that impurities such as hydrogen and moisture adsorbed to the substrate and/or the oxide insulating layer  436  are eliminated and evacuated. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. 
     Therefore, planarizing treatment may be performed on the region of the oxide insulating layer  436  which is in contact with the oxide semiconductor layer  403 . The planarization treatment may be, but not particularly limited to, polishing treatment (such as chemical mechanical polishing (CMP)), dry etching treatment, or plasma 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 top surface of the oxide insulating layer  436 . 
     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 of the oxide insulating layer  436 . 
     The oxide semiconductor layer  403  is preferably deposited under a condition such that much oxygen is contained (for example, by a sputtering method in an atmosphere where the proportion of oxygen is 100%) so as to be a film containing much oxygen (preferably having a region containing an excess of oxygen as compared to the stoichiometric composition ratio of the oxide semiconductor in a crystalline state). 
     Note that in this embodiment, a target used for forming the oxide semiconductor layer  403  by a sputtering method is, for example, an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic percentage], so that an In—Ga—Zn-based oxide film (IGZO film) is formed. 
     The relative density (the fill rate) of the metal oxide target is 90% to 100% inclusive, preferably 95% to 99.9% inclusive. By using the metal oxide target with high relative density, a dense oxide semiconductor film can be formed. 
     It is preferable to use a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, or hydride are removed as a sputtering gas used when the oxide semiconductor layer  403  is formed. 
     The substrate is held in a film formation chamber kept under reduced pressure. Then, a sputtering gas from which hydrogen and moisture are removed is introduced while residual moisture in the film formation chamber is removed, and the oxide semiconductor layer  403  is formed over the substrate  400  using the above target. 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. As an exhaustion unit, a turbo molecular pump to which a cold trap is added may be used. In the deposition chamber which is evacuated with an entrapment vacuum pump such as a cryopump, 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 are removed, whereby the impurity concentration in the oxide semiconductor layer  403  formed in the deposition chamber can be reduced. 
     The oxide insulating layer  436  and the oxide semiconductor layer  403  are preferably formed in succession without exposure to the air. According to successive formation of the oxide insulating layer  436  and the oxide semiconductor layer  403  without exposure to the air, impurities such as hydrogen and moisture can be prevented from being adsorbed onto a surface of the oxide insulating layer  436 . 
     The oxide semiconductor layer  403  can be formed by processing an oxide semiconductor film into an island shape by a photolithography process. 
     Further, a resist mask for forming the island-shaped oxide semiconductor layer  403  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing cost can be reduced. 
     Note that the etching of the oxide semiconductor film may be dry etching, wet etching, or both dry etching and wet etching. As an etchant used for wet etching of the oxide semiconductor film, for example, a mixed solution of phosphoric acid, acetic acid, and nitric acid, or the like can be used. In addition, ITO07N (produced by KANTO CHEMICAL CO., INC.) may also be used. 
     Further, heat treatment may be performed on the oxide semiconductor layer  403  in order to remove excess hydrogen (including water and a hydroxyl group) (to perform dehydration or dehydrogenation treatment). The temperature of the heat treatment is higher than or equal to 300° C. and lower than or equal to 700° C., or lower than the strain point of the substrate. The heat treatment can be performed under reduced pressure, a nitrogen atmosphere, or the like. For example, the substrate is put in an electric furnace which is a kind of heat treatment apparatus, and the oxide semiconductor layer  403  is subjected to the heat treatment at 450° C. for an hour in a nitrogen atmosphere. 
     Further, a heat treatment apparatus used is not limited to an electric furnace, and a device for heating a process object by heat conduction or heat radiation from a heating element such as a resistance heating element may be alternatively used. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the high-temperature gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas like argon, is used. 
     For example, as the heat treatment, GRTA may be performed as follows. The substrate is put in an inert gas heated at high temperature of 650° C. to 700° C., is heated for several minutes, and is taken out of the inert gas. 
     Note that in heat treatment, it is preferable that moisture, hydrogen, and the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is set to preferably 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, far preferably 0.1 ppm or lower). 
     In addition, after the oxide semiconductor layer  403  is heated by the heat treatment, a high-purity oxygen gas, a high-purity dinitrogen monoxide gas, or ultra dry air (the moisture amount is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, further preferably less than or equal to 10 ppb, 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. It is preferable that water, hydrogen, or the like be not contained in the oxygen gas or the dinitrogen monoxide gas. The purity of the oxygen gas or the dinitrogen monoxide gas which is introduced into the heat treatment apparatus is preferably 6N or more, far preferably 7N or more (i.e., the impurity concentration in the oxygen gas or the dinitrogen monoxide gas is preferably 1 ppm or lower, far 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  403  can be a high-purified, i-type (intrinsic) oxide semiconductor film. 
     Note that the heat treatment for dehydration or dehydrogenation can be performed in the process of manufacturing the transistor  440   a  anytime after formation of the oxide semiconductor film which has not been processed into the oxide semiconductor layer  403  and before formation of the insulating film  407 . For example, the heat treatment may be performed after formation of the oxide semiconductor film or after formation of the island-shaped oxide semiconductor layer  403 . 
     Further, the heat treatment for dehydration or dehydrogenation may be performed more than once or may be combined with another heat treatment. 
     When the heat treatment for dehydration or dehydrogenation is performed in the state where the oxide insulating layer  436  is covered with the oxide semiconductor film which has not been processed into the island-shaped oxide semiconductor layer  403 , oxygen contained in the oxide insulating layer  436  can be prevented from being released by the heat treatment, which is preferable. 
     Further or alternatively, 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. 
     Oxygen which is added to the dehydrated or dehydrogenated oxide semiconductor layer  403  to supply oxygen to the film can highly purify the oxide semiconductor layer  403  and make the film an i-type (intrinsic). Variation in electric characteristics of a transistor having a highly-purified and i-type (intrinsic) oxide semiconductor layer  403  is suppressed, and the transistor is electrically stable. 
     As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be used. 
     In the step of addition of oxygen, oxygen may be directly added to the oxide semiconductor layer  403  or added to the oxide semiconductor layer  403  through another film such as the gate insulating film  402  or the insulating film  407 . An ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like may be employed for the addition of oxygen through another film, whereas plasma treatment or the like can also be employed for the addition of oxygen directly into an exposed oxide semiconductor layer  403 . 
     As described above, the addition of oxygen into the oxide semiconductor layer  403  can be performed anytime after dehydration or dehydrogenation treatment is performed thereon. Further, oxygen may be added plural times into the dehydrated or dehydrogenated oxide semiconductor layer  403 . 
     Next, a conductive film 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 oxide semiconductor layer  403 . The conductive film is formed using a material that can withstand heat treatment in a later step. As a conductive film used for the source electrode layer and the drain electrode layer, for example, a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W and a metal nitride film containing any of the above elements as its main component (a titanium nitride film, a molybdenum nitride film, and a tungsten nitride film) can be used. A metal film having a high melting point such as Ti, Mo, W, or the like or a metal nitride film of any of these elements (a titanium nitride film, a molybdenum nitride film, and a tungsten nitride film) may be stacked on one of or both of a lower side or an upper side of a metal film of Al, Cu, or the like. Alternatively, the conductive film used for the source electrode layer and the drain electrode layer may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), indium oxide-tin oxide (In 2 O 3 —SnO 2 ), indium oxide-zinc oxide (In 2 O 3 —ZnO), or any of these metal oxide materials in which silicon oxide is contained can be used. 
     Through a photolithography process, a resist mask is formed over the conductive film, and selective etching is performed thereon, so that the source electrode layer  405   a  and the drain electrode layer  405   b  are formed, and then, the resist mask is removed. 
     Next, the gate insulating film  402  covering the oxide semiconductor layer  403 , the source electrode layer  405   a , and the drain electrode layer  405   b  is formed (see  FIG. 1A ). 
     To improve the coverage with the gate insulating film  402 , the above-described planarizing treatment may be performed also on the top surface of the oxide semiconductor layer  403 , and top surfaces of the source electrode layer  405   a  and the drain electrode layer  405   b . It is preferable that the flatness of the top surface of the oxide semiconductor layer  403  and the top surfaces of the source electrode layer  405   a  and the drain electrode layer  405   b  be good particularly when the thickness of the gate insulating film  402  is small. 
     The gate insulating film  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. Alternatively, the gate insulating film  402  may be formed with a sputtering apparatus where film formation is performed with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target. 
     The gate insulating film  402  can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. It is preferable that the gate insulating film  402  include oxygen in a portion which is in contact with the oxide semiconductor layer  403 . In particular, the gate insulating film  402  preferably contains a large amount of oxygen which exceeds at least the stoichiometric ratio in (a bulk of) the film. For example, in the case where a silicon oxide film is used as the gate insulating film  402 , the composition formula is SiO 2+α  (α&gt;0). In this embodiment, a silicon oxide film of SiO 2+α  (α&gt;0) is used as the gate insulating film  402 . By using the silicon oxide film as the gate insulating film  402 , oxygen can be supplied to the oxide semiconductor layer  403 , leading to good characteristics. Further, the gate insulating film  402  is preferably formed in consideration of the size of a transistor to be formed and the step coverage with the gate insulating film  402 . 
     When the gate insulating film  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 (HfSi x O 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. Further, the gate insulating film  402  may have either a single-layer structure or a stacked-layer structure. 
     Then, the gate electrode layer  401  is formed over the gate insulating film  402  by a plasma CVD method, a sputtering method, or the like (see  FIG. 1B ). The gate electrode layer  401  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. 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 have 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 film  402 , a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (e.g., InN or SnN) film can be used. These films each have a work function higher than or equal to 5 eV, preferably higher than or equal to 5.5 eV; thus, when these are used as the gate electrode layer, the threshold voltage of the electric characteristics of the transistor can be positive. Accordingly, a so-called normally-off switching element can be provided. 
     Next, a dopant  421  is introduced into the oxide semiconductor layer  403  with the use of the gate electrode layer  401 , the source electrode layer  405   a , and the drain electrode layer  405   b  as masks, whereby the low-resistance regions  404   a  and  404   b  are formed. 
     The dopant  421  is not added to the oxide semiconductor layer  403  in the regions under the source electrode layer  405   a  and the drain electrode layer  405   b  in some cases, or the dopant  421  is added to the oxide semiconductor layer  403  in the regions under the source electrode layer  405   a  and the drain electrode layer  405   b  such that the dopant concentration in each region is lower and the resistance in each region is higher than that of the other region in some cases, depending on the thickness of each of the source electrode layer  405   a  and the drain electrode layer  405   b  and the condition of addition of the dopant  421 . 
     In a transistor  440   c  in  FIG. 2B , a tungsten film with small thickness, for example 10 nm, is formed as the source electrode layer  405   a  and the drain electrode layer  405   b . Owing to the above-described small thickness of each of the source electrode layer  405   a  and the drain electrode layer  405   b , when a dopant is introduced into the oxide semiconductor layer  403  to form low-resistance regions, the dopant can also be introduced into the oxide semiconductor layer  403  which is below the source electrode layer  405   a  and the drain electrode layer  405   b , through the source electrode layer  405   a  and the drain electrode layer  405   b . As a result, in the transistor  440   c , the low-resistance regions  404   a  and  404   b  are formed in the oxide semiconductor layer  403  which is below the source electrode layer  405   a  and the drain electrode layer  405   b.    
     The dopant  421  is an impurity by which the electrical conductivity of the oxide semiconductor layer  403  is changed. One or more selected from the following can be used as the dopant  421 : Group 15 elements (typical examples thereof are phosphorus (P), arsenic (As), and antimony (Sb)), boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon (Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc (Zn). 
     The dopant  421  can be introduced into the oxide semiconductor layer  403  through other films (e.g., the insulating film  407 , the source electrode layer  405   a , and the drain electrode layer  405   b ) by an implantation method. As the method for introducing the dopant  421 , an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. In the case where the above method is used, it is preferable to use a single ion of the dopant  421 , a fluoride ion, or a chloride ion. 
     The introduction of the dopant  421  may be controlled by setting the addition conditions such as the accelerated voltage and the dosage, or the thickness of the films through which the dopant passes as appropriate. In this embodiment, boron is used as the dopant  421 , whose ion is introduced by an ion implantation method. The dosage is preferably set to be greater than or equal to 1×10 13  ions/cm 2  and less than or equal to 5×10 16  ions/cm 2 . 
     The concentration of the dopant  421  in the low-resistance region is preferably greater than or equal to 5×10 18 /cm 3  and less than or equal to 1×10 22 /cm 3 . 
     The dopant  421  may be introduced while the substrate  400  is heated. 
     The introduction of the dopant  421  into the oxide semiconductor layer  403  may be performed plural times, and the number of kinds of dopant may be plural. 
     Further, heat treatment may be performed thereon after the introduction of the dopant  421 . The heat treatment is preferably performed at a temperature(s) higher than or equal to 300° C. and lower than or equal to 700° C. (further preferably higher than or equal to 300° C. and lower than or equal to 450° C.) for one hour under an oxygen atmosphere. The heat treatment may be performed under a nitrogen atmosphere, reduced pressure, or the air (ultra-dry air). 
     In the case where the oxide semiconductor layer  403  is a crystalline oxide semiconductor film, the oxide semiconductor layer  403  may be partly amorphized by the introduction of the dopant  421 . In that case, the crystallinity of the oxide semiconductor layer  403  can be recovered by performing a heat treatment thereon after the introduction of the dopant  421 . 
     Thus, the oxide semiconductor layer  403  in which the low-resistance regions  404   a  and  404   b  are formed with the channel formation region  409  sandwiched between the low-resistance regions  404   a  and  404   b.    
     Through the above-described process, the transistor  440   a  of this embodiment can be manufactured (see  FIG. 1C ). With the oxide semiconductor layer  403  which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc, the transistor  440   a  can have excellent on-state characteristics (field-effect mobility), small off-state current, and high reliability. 
     Next, the insulating film  407  is formed over the oxide semiconductor layer  403 , the source electrode layer  405   a , the drain electrode layer  405   b , the gate insulating film  402 , and the gate electrode layer  401  (see  FIG. 1D ). 
     The insulating film  407  including the metal element can be formed by a plasma-enhanced CVD method, a sputtering method, an evaporation method, or the like. As the insulating film  407 , an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or a gallium oxide film can be typically used. 
     Alternatively, as the insulating film  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 film  407  can be either a single film or a stacked film. The insulating film  407  can be a stack of a silicon oxide film and an aluminum oxide film, for example. 
     The insulating film  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 film  407 . In addition, it is preferable that the insulating film  407  include an excess of oxygen on the side closer to the oxide semiconductor layer  403  because the excess of oxygen serves as a supply source of oxygen for the oxide semiconductor layer  403 . 
     In this embodiment, a silicon oxide film with a thickness of 100 nm is formed as the insulating film  407  by a sputtering method. The silicon oxide film can be formed by a sputtering method under a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas and oxygen. 
     In order to remove residual moisture from the deposition chamber of the insulating film  407  in a manner similar to that of the deposition of the oxide semiconductor film, an entrapment vacuum pump (such as a cryopump) is preferably used. When the insulating film  407  is deposited in the deposition chamber evacuated using a cryopump, the impurity concentration of the insulating film  407  can be reduced. As an evacuation unit for removing moisture remaining in the deposition chamber of the insulating film  407 , a turbo molecular pump provided with a cold trap may be used. 
     It is preferable that a high-purity gas in which an impurity such as hydrogen, water, a hydroxyl group, or hydride is reduced be used as the sputtering gas for the formation of the insulating film  407 . 
     The aluminum oxide film which can be used as the insulating film  407  provided over the oxide semiconductor layer  403  has a high blocking effect by which both of oxygen and impurities such as hydrogen or moisture is prevented from being passed through the film. 
     Therefore, 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, 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 . 
     Further, a planarization insulating film may be formed thereover in order to reduce surface roughness due to the transistor. As the planarization insulating film, an organic material such as a polyimide-based resin, an acrylic-based 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. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. 
     Further, respective openings reaching the source electrode layer  405   a  and the drain electrode layer  405   b  are formed in the gate insulating film  402  and the insulating film  407 , and a wiring layer  465   a  and a wiring layer  465   b  electrically connected to the source electrode layer  405   a  and the drain electrode layer  405   b , respectively, are formed in the openings (see  FIG. 1E ). With the use of this wiring layers  465   a  and  465   b , the transistor is connected to another transistor, which can lead to formation of a variety of circuits. 
     Alternatively, as a transistor  440   d  in  FIG. 2C , the wirings  465   a  and  465   b  may be formed directly on the oxide semiconductor layer  403  without providing the source electrode layer  405   a  and the drain electrode layer  405   b.    
     The wiring layers  465   a  and  465   b  can be formed with a material and a method which are similar to those of the gate electrode layer  401 , the source electrode layer  405   a , and the drain electrode layer  405   b . For example, as the wiring layers  465   a  and  465   b , 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. 
     In the oxide semiconductor layer  403  which is highly purified and whose oxygen vacancies are repaired, impurities such as hydrogen and water are sufficiently removed; the hydrogen concentration in the oxide semiconductor layer  403  is less than or equal to 5×10 19 /cm 3 , preferably less than or equal to 5×10 18 /cm 3 . The hydrogen concentration in the oxide semiconductor layer  403  is measured by secondary ion mass spectrometry (SIMS). 
     The current value in the off state (off-state current value) of the transistor  440   a  using the highly purified oxide semiconductor layer  403  containing an excess of oxygen that repairs an oxygen vacancy according to this embodiment is less than or equal to 100 zA per micrometer of channel width at room temperature (1 zA (zeptoampere)=1×10 −21  A), preferably less than or equal to 50 zA/mm. 
     In this manner, a structure of a transistor including an oxide semiconductor in a channel formation region in which the threshold voltage of electric characteristics of the transistor can be positive, which is a so-called normally-off switching element, and a manufacturing method thereof can be provided. 
     Further, in order to achieve a semiconductor device having higher performance, a structure for improving on-state characteristics of the transistor (e.g., on-state current and field-effect mobility) and for achieving high-speed response and high-speed operation of the semiconductor device and a manufacturing method thereof can be provided. 
     In addition, a highly reliable semiconductor device in which a threshold voltage does not easily shift even in a long-time use can be provided. 
     Embodiment 2 
     In this embodiment, another embodiment of a semiconductor device and a method for manufacturing the semiconductor device are described with reference to  FIGS. 3A to 3C  and  FIGS. 4A to 4E . The same portion as or a portion having a function similar to those in the above embodiment can be formed in a manner similar to that described in the above embodiment, and also the steps similar to those in the above embodiment can be performed in a manner similar to that described in the above embodiment, and repetitive description is omitted. In addition, detailed description of the same portions is not repeated. 
     A transistor  450  illustrated in  FIGS. 3A to 3C  is an example of a top-gate transistor.  FIG. 3A  is a top view,  FIG. 3B  is a cross-sectional view taken along two-dot chain line X-Y in  FIG. 3A , and  FIG. 3C  is a cross-sectional view taken along two-dot chain line V-W in  FIG. 3A . 
     As shown in  FIG. 3B  which is a cross-sectional view in the channel length direction, the transistor  450  includes, over the substrate  400  having an insulating surface over which the oxide insulating layer  436  is formed, first oxide semiconductor layers  408   a  and  408   b , a second oxide semiconductor layer  403  including the channel formation region  409  and low-resistance regions  414   a  and  414   b , the source electrode layer  405   a , the drain electrode layer  405   b , the gate insulating film  402 , and the gate electrode layer  401 . The oxide semiconductor layers  408   a  and  408   b  are separated from each other on and in contact with the oxide insulating layer  436 . The oxide semiconductor layer  403  is in contact with the oxide semiconductor layers  408   a  and  408   b  and the oxide insulating layer  436 . 
       FIG. 3C  is a cross-sectional view in the channel width direction. The oxide semiconductor layer  403  has a taper angle of 20° to 50°. When the oxide semiconductor layer  403  has a perpendicular end portion, oxygen is likely to be eliminated, and thus, oxygen defects easily occur. In contrast, when the oxide semiconductor layer  403  is tapered, the occurrence of oxygen defects is suppressed and thus the occurrence of leakage current (parasitic channel) in the transistor  450  is reduced. 
     The oxide semiconductor layers  408   a  and  408   b  are provided below the oxide semiconductor layer  403  with a thickness of 3 nm to 5 nm, so that contact resistance between the oxide semiconductor layer  403  and the source and drain electrode layers  405   a  and  405   b  can be reduced. 
     The low-resistance regions  414   a  and  414   b  can be formed by introducing a dopant into the oxide semiconductor layer  403  with the use of the gate electrode layer  401  as a mask. Further or alternatively, the low-resistance regions can be formed by diffusing a metal element. When the low-resistance regions are formed by introducing a dopant and diffusing a metal element, contact resistance with the wiring layers can be further reduced. 
     Further, a sidewall insulating layer may be formed on a side surface of the gate electrode layer  401 . The transistor  450  includes thin sidewall insulating layers  412   a  and  412   b  on the side surfaces of the gate electrode layer  401 . The sidewall insulating layers  412   a  and  412   b  may be formed on the side surface of the gate electrode layer  401  in a self-aligned manner by forming an insulating film to cover the gate electrode layer  401  and then processing the insulating film by anisotropic etching by a reactive ion etching (RIE) method. There is no particular limitation on the insulating film; for example, a silicon oxide film with favorable step coverage, which is formed by reacting tetraethyl orthosilicate (TEOS), silane, or the like with oxygen, nitrous oxide, or the like, can be used. The insulating film can be formed by a thermal CVD method, a plasma CVD method, an atmospheric pressure CVD method, a bias ECRCVD method, a sputtering method, or the like. A silicon oxide film formed by a low temperature oxidation (LTO) method may also be used. 
     With the sidewall insulating layers  412   a  and  412   b , the gate electrode layer  401  and the low-resistance regions  414   a  and  414   b  can be prevented from being short-circuited. 
     When a dopant is introduced into the entire oxide semiconductor layers  408   a  and  408   b  to form the low-resistance regions, the transistor can be electrically connected with another conductive layer, below the oxide semiconductor layers  408   a  and  408   b , i.e., from the oxide insulating layer  436  side. 
     When the oxide semiconductor layer  403  is formed using an IGZO film which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc has high field-effect mobility, the thickness of the oxide semiconductor layer  403  is decreased to 3 nm to 5 nm, whereby a transistor can be prevented from being normally-on caused by a short-channel effect. 
     For the oxide semiconductor layers  408   a  and  408   b , an indium oxide, a tin oxide, a zinc oxide; a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used. 
     As the oxide semiconductor layers  408   a  and  408   b , an oxide semiconductor layer having a high conductivity can be used. 
     In this embodiment, an oxide semiconductor layer formed using an oxide target having a composition ratio of indium:gallium:zinc=1:1:1 is used as each of the oxide semiconductor layers  408   a  and  408   b.    
     The thickness of each of the oxide semiconductor layers  408   a  and  408   b  is preferably 20 nm to 50 nm. 
     An example of a method for manufacturing the transistor  450  is described with reference to  FIGS. 4A to 4E . 
     First, the oxide insulating layer  436  is formed over the substrate  400  having an insulating surface and the oxide semiconductor film  444  is formed over the oxide insulating layer  436  (see  FIG. 4A ). In this embodiment, the oxide semiconductor film  444  is formed using an oxide target having a composition ratio of indium:gallium:zinc=1:1:1, by a sputtering method. 
     Then, the oxide semiconductor film  444  is processed into an island shape by a photolithography process to form the pair of oxide semiconductor layers  408   a  and  408   b  which are separated from each other. The oxide semiconductor layer  403  is formed in contact with the oxide semiconductor layers  408   a  and  408   b  and the oxide insulating layer  436  (see  FIG. 4B ). The oxide semiconductor layer  403  is formed using an oxide target having a composition ratio of indium:gallium:zinc=3:1:2 by a sputtering method. The oxide semiconductor layer  403  is preferably tapered, and in this embodiment, has a taper angle of 30°. 
     Next, over the oxide semiconductor layer  403 , the gate insulating film  402 , the gate electrode layer  401 , the sidewall insulating layers  412   a  and  412   b  which cover side surfaces of the gate electrode layer  401  are formed (see  FIG. 4C ). The gate insulating film  402  can be formed by forming an insulating film over the oxide semiconductor layer  403  and etching the insulating film with the use of the gate electrode layer  401  and the sidewall insulating layers  412   a  and  412   b  as masks. Note that part of the oxide semiconductor layer  403  is exposed. 
     Next, a film  417  including a metal element is formed over the oxide semiconductor layer  403 , the gate insulating film  402 , and the gate electrode layer  401  to be in contact with the part of the oxide semiconductor layer  403 . 
     As the film  417  including the metal element, a metal film, a metal oxide film, a metal nitride film, and the like are used. 
     As the metal element included in the film including the metal element, one or more selected from aluminum (Al), titanium (Ti), molybdenum (Mo), tungsten (W), hafnium (Hf), tantalum (Ta), lanthanum (La), barium (Ba), magnesium (Mg), zirconium (Zr), and nickel (Ni) can be used. As the film including the metal element, a metal film, a metal oxide film, or a metal nitride film including at least one of the above-described metal elements (such a metal nitride film is, for example, a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) can be used. Further, a dopant such as phosphorus (P) or boron (B) may be included in the film including the metal element. In this embodiment, the film  417  including the metal element has electrical conductivity. 
     The film  417  including the metal element can be formed by a plasma-enhanced CVD method, a sputtering method, an evaporation method, or the like. The thickness of the film  417  including the metal element may be greater than or equal to 5 nm and less than or equal to 30 nm. 
     In this embodiment, a 10-nm-thick aluminum film is formed by a sputtering method as the film  417  including the metal element. 
     Next, the dopant  421  is selectively introduced into the oxide semiconductor layer  403  through the film  417  including the metal element with the gate insulating film  402 , the gate electrode layer  401 , and the sidewall insulating layers  412   a  and  412   b  as a mask, so that low-resistance regions are formed (see  FIG. 4D ). 
     The dopant  421  is an impurity by which the electrical conductivity of the oxide semiconductor layer  403  is changed. One or more selected from the following can be used as the dopant  421 : Group 15 elements (typical examples thereof are phosphorus (P), arsenic (As), and antimony (Sb)), boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon (Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc (Zn). 
     The dopant may be included in the film  417  including the metal element. 
     The dopant  421  is introduced into the oxide semiconductor layer  403  through the film  417  including the metal element by an implantation method. As the method for introducing the dopant  421 , an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. In the case where the above method is used, it is preferable to use a single ion of the dopant  421  or a hydride ion, a fluoride ion, or a chloride ion. 
     The introduction of the dopant  421  may be controlled by setting the addition conditions such as the accelerated voltage and the dosage, or the thickness of the film  417  including the metal element as appropriate. The dose can be greater than or equal to 1×10 13  ions/cm 2  and less than or equal to 5×10 16  ions/cm 2 . For example, for introduction of an boron ion by an ion implantation method using boron, the accelerated voltage and the dosage may be set to 15 kV and 1×10 15  ions/cm 2 , respectively. 
     The concentration of the dopant  421  in the low-resistance region is preferably greater than or equal to 5×10 18 /cm 3  and less than or equal to 1×10 22 /cm 3 . 
     The substrate  400  may be heated in introducing the dopant. 
     The introduction of the dopant  421  into the oxide semiconductor layer  403  may be performed more than once, and the number of kinds of dopant may be plural. 
     Further, heat treatment may be performed thereon after the introduction of the dopant  421 . The heat treatment is preferably performed at a temperature(s) higher than or equal to 300° C. and lower than or equal to 700° C. (further preferably higher than or equal to 300° C. and lower than or equal to 450° C.) for one hour in an oxygen atmosphere. The heat treatment may be performed in a nitrogen atmosphere, reduced pressure, or the air (ultra-dry air). 
     Next, heat treatment is performed in the state where the film  417  including the metal element and the oxide semiconductor layer  403  are partly in contact with each other. The heat treatment is preferably performed in an oxygen atmosphere. The heat treatment can also be performed under reduced pressure or a nitrogen atmosphere. The heating temperature may be set to be higher than or equal to 100° C. and lower than or equal to 700° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. 
     For example, the substrate is put in an electric furnace which is one of heat treatment apparatuses, and heat treatment is performed on the film  417  including the metal element and the oxide semiconductor layer  403  at 200° C. for one hour in an oxygen atmosphere. 
     Further, a heat treatment apparatus used is not limited to an electric furnace, and a device for heating a process object by heat conduction or heat radiation from a heating element such as a resistance heating element may be alternatively used. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. As the high-temperature gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas like argon, is used. 
     For example, as the heat treatment, GRTA may be performed as follows. The substrate is put in an inert gas heated at high temperature of 650° C. to 700° C., is heated for several minutes, and is taken out of the inert gas. 
     The heat treatment may be performed in an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or lower, preferably 1 ppm or lower, more preferably 10 ppb or lower), or a rare gas (argon, helium, or the like). Note that it is preferable that water, hydrogen, or the like be not contained in the atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas. It is also preferable that the purity of nitrogen, oxygen, or the rare gas which is introduced into a heat treatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). 
     By the heat treatment, the metal element is introduced into the oxide semiconductor layer  403  from the film  417  including the metal element, so that low-resistance regions  414   a  and  414   b  are formed. In this manner, in the oxide semiconductor layer  403 , the low-resistance regions  414   a  and  414   b  including the dopant and the metal element are formed between which a channel formation region  409  is provided. 
     In this embodiment, boron is used as the dopant and aluminum is used as the metal element, and therefore the low-resistance regions  414   a  and  414   b  contain boron and aluminum. 
     Then, the film  417  including the metal element is removed by etching. The film  417  including the metal element is removed by wet etching, in this embodiment. 
     Through the above process, the transistor  450  of this embodiment can be manufactured. With the oxide semiconductor layer  403  including the low-resistance regions  414   a  and  414   b  with the channel formation region  409  provided therebetween in the channel length direction, on-state characteristics (e.g., on-state current and field-effect mobility) of the transistor  450  are increased, which enables high-speed operation and high-speed response of the transistor  450 . 
     The low-resistance regions  414   a  and  414   b  each can be functioned as a source region or a drain region. With the low-resistance regions  414   a  and  414   b , the electrical field applied to the channel formation region  409  formed between the low-resistance regions  414   a  and  414   b  can be relaxed. Further, electrical connection between the oxide semiconductor layer  403  and each of the source electrode layer  405   a  and the drain electrode layer  405   b  in the low-resistance regions  414   a  and  414   b , respectively, can reduce the contact resistance between the oxide semiconductor layer  403  and each of the source electrode layer  405   a  and the drain electrode layer  405   b.    
     Further, a planarization insulating film may be formed thereover in order to reduce surface roughness due to the transistor. As the planarization insulating film, an organic material such as a polyimide-based resin, an acrylic-based 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. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. 
     In this embodiment, a planarization insulating film  415  is formed over the transistor  450 . Further, openings reaching the oxide semiconductor layer  403  are formed in the planarization insulating film  415 , and the source electrode layer  405   a  and the drain electrode layer  405   b  are formed so as to be electrically connected to the oxide semiconductor layer  403  through the openings (see  FIG. 4E ). 
     In this manner, a structure of a transistor including an oxide semiconductor in a channel formation region in which the threshold voltage of electric characteristics of the transistor can be positive, which is a so-called normally-off switching element, and a manufacturing method thereof can be provided. 
     Further, in order to achieve a semiconductor device having higher performance, a structure for improving on-state characteristics of the transistor (e.g., on-state current and field-effect mobility) and for achieving high-speed response and high-speed operation of the semiconductor device and a manufacturing method thereof can be provided. 
     In addition, a semiconductor device in which reliability is high and threshold voltage does not easily shift even in a long-time use can be provided. 
     This embodiment can be implemented combining with the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, an example of a semiconductor device which includes the transistor described in Embodiment 1 or 2, which can hold stored data even when not powered, and which does not have a limitation on the number of write cycles, will be described with reference to drawings. Note that a transistor  162  included in the semiconductor device in this embodiment is the transistor described in Embodiment 1 or 2. Any of the transistors described in Embodiment 1 or 2 can be used as the transistor  162 . 
     Since the off-state current of the transistor  162  is small, stored data can be held for a long time owing to such a transistor. In other words, power consumption can be sufficiently reduced because a semiconductor storage device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided. 
       FIGS. 5A and 5B  illustrate an example of a structure of a semiconductor device.  FIG. 5A  is a cross-sectional view of the semiconductor device,  FIG. 5B  is a plan view of the semiconductor device, and  FIG. 5C  is a circuit diagram of the semiconductor device. Here,  FIG. 5A  corresponds to a cross section along line C1-C2 and line D1-D2 in  FIG. 5B . 
     The semiconductor device illustrated in  FIGS. 5A and 5B  includes a transistor  160  including a first semiconductor material in a lower portion, and a transistor  162  including a second semiconductor material in an upper portion. The transistor  162  can have the same structure as that described in Embodiment 1 or 2. 
     Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold charge for a long time owing to its characteristics. 
     Although all the transistors are n-channel transistors here, p-channel transistors can be used. The technical nature of the disclosed invention is to use an oxide semiconductor in the transistor  162  so that data can be held. Therefore, it is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to the structure described here. 
     The transistor  160  in  FIG. 5A  includes a channel formation region  116  provided over a substrate  185  including a semiconductor material (e.g., silicon), impurity regions  120  with the channel formation region  116  provided therebetween, metal compound regions  124  in contact with the impurity regions  120 , a gate insulating layer  108  provided over the channel formation region  116 , and a gate electrode  110  provided over the gate insulating layer  108 . Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing may be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode are collectively referred to as a “source electrode,” and a drain region and a drain electrode are collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region. 
     An element isolation insulating layer  106  is provided over the substrate  185  to surround the transistor  160 . An insulating layer  130  is provided to cover the transistor  160 . Note that in order to realize high integration, it is preferable that, as in  FIG. 5A , the transistor  160  does not have a sidewall insulating layer. On the other hand, when the characteristics of the transistor  160  have priority, the sidewall insulating layer may be formed on a side surface of the gate electrode  110  and the impurity regions  120  may include a region having a different impurity concentration. 
     The transistor  162  shown in  FIG. 5A  includes an oxide semiconductor in the channel formation region. Here, an oxide semiconductor layer  144  included in the transistor  162  is preferably highly purified. By using a highly purified oxide semiconductor, the transistor  162  can have extremely favorable off-state current characteristics. 
     An insulating layer  150  having a single-layer structure or a stacked-layer structure is provided over the transistor  162 . In addition, a conductive layer  148   b  is provided in a region overlapping with the electrode layer  142   a  of the transistor  162  with the insulating layer  150  provided therebetween, and the electrode layer  142   a , the insulating layer  150 , and the conductive layer  148   b  form a capacitor  164 . That is, the electrode layer  142   a  of the transistor  162  functions as one electrode of the capacitor  164 , and the conductive layer  148   b  functions as the other electrode of the capacitor  164 . Note that the capacitor  164  may be omitted if a capacitor is not needed. Alternatively, the capacitor  164  may be separately provided above the transistor  162 . 
     The insulating layer  152  is provided over the transistor  162  and the capacitor  164 . Further, a wiring  156  for connecting the transistor  162  to another transistor is provided over the insulating layer  152 . Although not illustrated in  FIG. 5A , the wiring  156  is electrically connected to the electrode layer  142   b  through an electrode formed in an opening provided in the insulating layer  150 , the insulating layer  152 , the gate insulting film  146 , and the like. Here, the electrode is preferably provided so as to partly overlap with at least the oxide semiconductor layer  144  of the transistor  162 . 
     In  FIGS. 5A and 5B , the transistor  160  is provided so as to overlap with at least part of the transistor  162 . The source region or the drain region of the transistor  160  is preferably provided so as to overlap with part of the oxide semiconductor layer  144 . Further, the transistor  162  and the capacitor  164  are provided so as to overlap with at least part of the transistor  160 . For example, the conductive layer  148   b  of the capacitor  164  is provided so as to overlap with at least part of the gate electrode  128  of the transistor  160 . With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, higher integration can be achieved. 
     Note that the electrical connection between the electrode layer  142   b  and the wiring  156  may be established by direct contact of the electrode layer  142   b  and the wiring  156  with each other or through an electrode provided in an insulating layer lying therebetween. Alternatively, the electrical connection may be established through a plurality of electrodes. 
     Next, an example of a circuit configuration corresponding to  FIGS. 5A and 5B  is illustrated in  FIG. 5C . 
     In  FIG. 5C , a first wiring (1st Line) is electrically connected to a source electrode of the transistor  160 . A second wiring (2nd Line) is electrically connected to a drain electrode of the transistor  160 . A third wiring (a 3rd line) and one of source or drain electrodes of the transistor  162  are electrically connected to each other, and a fourth wiring (a 4th line) and a gate electrode of the transistor  162  are electrically connected to each other. A gate electrode of the transistor  160  and one of the source electrode and the drain electrode of the transistor  162  are electrically connected to one electrode of the capacitor  164 . A fifth line (a 5th Line, also referred to as a word line) and the other electrode of the capacitor  164  are electrically connected to each other. 
     The semiconductor device in  FIG. 5C  utilizes a characteristic in which the potential of the gate electrode of the transistor  160  can be held, and thus enables data writing, holding, and reading as follows. 
     Writing and holding of data are described. First, the potential of the fourth line is set to a potential at which the transistor  162  is turned on, so that the transistor  162  is turned on. Accordingly, the potential of the third line is supplied to the gate electrode of the transistor  160  and the capacitor  164 . That is, predetermined charge is given to the gate electrode of the transistor  160  (writing). Here, charge for supply of a potential level or charge for supply of a different potential level (hereinafter referred to as Low level charge and High level charge) is given. After that, the potential of the fourth line is set to a potential at which the transistor  162  is turned off, so that the transistor  162  is turned off. Thus, the charge given to the gate electrode of the transistor  160  is held (storing). 
     Since the off-state current of the transistor  162  is extremely low, the charge of the gate electrode of the transistor  160  is held for a long time. 
     Next, reading of data is described. By supplying an appropriate potential (reading potential) to the fifth line while a predetermined potential (constant potential) is supplied to the first line, the potential of the second line varies depending on the amount of charge held in the gate electrode of the transistor  160 . This is because in general, when the transistor  160  is an n-channel transistor, an apparent threshold voltage V th_H  in the case where a high-level charge is given to the gate electrode of the transistor  160  is lower than an apparent threshold voltage V th_L  in the case where a low-level charge is given to the gate electrode of the transistor  160 . Here, an apparent threshold voltage refers to the potential of the fifth line, which is needed to turn on the transistor  160 . Thus, the potential of the fifth wiring is set to a potential V 0  which is between V th_H  and V th_L  whereby charge given to the gate electrode of the transistor  160  can be determined. For example, in the case where a high-level charge is given in writing, when the potential of the fifth wiring is set to V 0  (&gt;V th_H ), the transistor  160  is turned on. In the case where a low level charge is given in writing, even when the potential of the fifth wiring is set to V 0  (&lt;V th_L ), the transistor  160  remains in an off state. Therefore, the stored data can be read by the potential of the second line. 
     Note that in the case where memory cells are arrayed to be used, only data of desired memory cells needs to be read. In the case where such reading is not performed, a potential at which the transistor  160  is turned off, that is, a potential smaller than V th_H  may be given to the fifth wiring regardless of the state of the gate electrode of the transistor  160 . Alternatively, a potential which allows the transistor  160  to be turned on regardless of a state of the gate electrode, that is, a potential higher than V th_L  may be applied to the fifth lines. 
     When a transistor having a channel formation region formed using an oxide semiconductor and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed). 
     Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate, 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 one 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 the on state and the off state of the transistor, whereby high-speed operation can be easily realized. 
     Since the transistor  162  includes an oxide semiconductor layer which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc, the transistor  162  can have a positive threshold voltage. With the transistor employed, a high-quality semiconductor device can be provided. Further, the semiconductor device in this embodiment includes a transistor in which the threshold voltage does not easily shift even in a long-period of use; thus, the semiconductor device can have high reliability. 
     The methods and structures described in this embodiment can be combined as appropriate with any of the methods and structures described in the other embodiments. 
     Embodiment 4 
     In this embodiment, a semiconductor device which includes the transistor described in Embodiment 1 or 2, which can hold stored data even when not powered, and which does not have a limitation on the number of write cycles, and which has a structure different from the structure described in Embodiment 3 is described with reference to  FIGS. 6A and 6B  and  FIGS. 7A to 7C . Note that the transistor  162  included in the semiconductor device in this embodiment is the transistor described in Embodiment 1 or 2. Any of the transistors described in Embodiment 1 or 2 can be used as the transistor  162 . 
       FIG. 6A  illustrates an example of a circuit configuration of a semiconductor device, and  FIG. 6B  is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in  FIG. 6A  is described, and then, the semiconductor device illustrated in  FIG. 6B  is described. 
     In the semiconductor device illustrated in  FIG. 6A , a bit line BL is electrically connected to the source electrode or the drain electrode of the transistor  162 , a word line WL is electrically connected to the gate electrode of the transistor  162 , and the source electrode or the drain electrode of the transistor  162  is electrically connected to a first terminal of a capacitor  254 . 
     The transistor  162  including an oxide semiconductor has extremely low off-state current. For that reason, a potential of the first terminal of the capacitor  254  (or a charge accumulated in the capacitor  254 ) can be held for an extremely long period by turning off the transistor  162 . 
     Next, writing and holding of data in the semiconductor device (a memory cell  250 ) illustrated in  FIG. 6A  are described. 
     First, the potential of the word line WL is set to a potential at which the transistor  162  is turned on, so that the transistor  162  is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor  254  (writing). After that, the potential of the word line WL is set to a potential at which the transistor  162  is turned off, so that the transistor  162  is turned off. Thus, the charge at the first terminal of the capacitor  254  is held (holding). 
     Because the off-state current of the transistor  162  is extremely small, the potential of the first terminal of the capacitor  254  (or the charge accumulated in the capacitor) can be held for a long time. 
     Secondly, reading of data is described. When the transistor  162  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 , G B  is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V B0  is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell  250  is in either of two states in which the potentials of the first terminal of the capacitor  254  are V 1  and V 0  (V 1 &gt;V 0 ), the potential of the bit line BL in the case of holding the potential V 1  (=(C B *V B0 +C*V 1 )/(C B +C)) is higher than the potential of the bit line BL in the case of holding the potential V 0  (=(C B *V B0 +C*V 0 )/(C B +C)). 
     Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read. 
     As described above, the semiconductor device illustrated in  FIG. 6A  can hold charge that is accumulated in the capacitor  254  for a long time because the off-state current of the transistor  162  is extremely low. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be stored for a long time even when power is not supplied. 
     Next, the semiconductor device illustrated in  FIG. 6B  is described. 
     The semiconductor device illustrated in  FIG. 6B  includes a memory cell array  251  (memory cell arrays  251   a  and  251   b ) including a plurality of memory cells  250  illustrated in  FIG. 6A  as memory circuits in the upper portion, and a peripheral circuit  253  in the lower portion which is necessary for operating the memory cell array  251  (the memory cell arrays  251   a  and  251   b ). Note that the peripheral circuit  253  is electrically connected to the memory cell array  251 . 
     In the structure illustrated in  FIG. 6B , the peripheral circuit  253  can be provided under the memory cell array  251  (the memory cell arrays  251   a  and  251   b ). Thus, the size of the semiconductor device can be decreased. 
     It is preferable that a semiconductor material of the transistor provided in the peripheral circuit  253  be different from that of the transistor  162 . For example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like can be used, and a single crystal semiconductor is preferably used. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed can be favorably realized by the transistor. 
     Note that  FIG. 6B  illustrates, as an example, the semiconductor device in which two memory cell arrays (the memory cell array  251   a  and the memory cell array  251   b ) are stacked; however, the number of memory cell arrays to be stacked is not limited thereto. Three or more memory cell arrays may be stacked. 
     Next, a specific structure of the memory cell  250  illustrated in  FIG. 6A  is described with reference to  FIGS. 7A to 7C . 
       FIGS. 7A to 7C  illustrate a structure example of the memory cell  250 .  FIG. 7A  is a plan view of the memory cell  250 .  FIG. 7B  is a cross-sectional view taken along line A-B in  FIG. 7A . 
     The transistor  162  in  FIGS. 7A and 7B  can have the same structure as the transistor in Embodiment 1 or 2. 
     As illustrated in  FIG. 7B , the transistor  162  is formed over an electrode  502  and an electrode  504 . The electrode  502  serves as a bit line BL in  FIG. 6A  and is in contact with the low-resistance region of the transistor  162 . The electrode  504  serves as one electrode of the capacitor  254  in  FIG. 6A  and is in contact with the low-resistance region of the transistor  162 . Over the transistor  162 , the electrode  506  provided in a region overlapping with the electrode  504  serves as the other electrode of the capacitor  254 . 
     As illustrated in  FIG. 7A , the other electrode  506  of the capacitor  254  is electrically connected to a capacitor line  508 . A gate electrode  148   a  over the oxide semiconductor layer  144  with the gate insulating film  146  provided therebetween is electrically connected to a word line  509 . 
       FIG. 7C  is a cross-sectional view in a connection portion between the memory cell array  251  and the peripheral circuit. The peripheral circuit can include, for example, an n-channel transistor  510  and a p-channel transistor  512 . The n-channel transistor  510  and the p-channel transistor  512  are preferably formed using a semiconductor material other than an oxide semiconductor (e.g., silicon). With such a material, the transistor included in the peripheral circuit can operate at high speed. 
     When the planar layout in  FIG. 7A  is employed, the area occupied by the semiconductor device can be reduced; thus, the degree of integration can be increased. 
     As described above, the plurality of memory cells formed in multiple layers in the upper portion each include a transistor including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor which contains at least four kinds of elements of indium, gallium, zinc, and oxygen, and has a composition ratio (atomic percentage) of indium as twice or more as a composition ratio of gallium and a composition ratio of zinc, is low, stored data can be held for a long time owing to the transistor. In other words, the frequency of refresh operation can be significantly lowered, which leads to a sufficient reduction in power consumption. Further, as illustrated in  FIG. 7B , the capacitor  254  is formed by stacking the electrode  504 , the oxide semiconductor layer  144 , the gate insulating film  146 , and the electrode  506 . Since the relative permittivity of the oxide semiconductor layer with the above-described composition is extremely high (a relative permittivity of 66), the area required for the capacitor  254  can be reduced when the oxide semiconductor layer is used as a dielectric film. 
     A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased. 
     This embodiment can be implemented in appropriate combination with the structures described in the other embodiments. 
     Embodiment 5 
     In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to portable devices such as cellular phones, smartphones, or electronic books will be described with reference to  FIGS. 8A and 8B , 
       FIG. 9 ,  FIG. 10 , and  FIG. 11 . 
     In portable electronic 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. 8A , one memory cell includes six transistors, that is, transistors  801  to  806 , which are driven with an X decoder  807  and a Y decoder  808 . The transistors  803  and  805  and the transistors  804  and  806  each serve as an inverter, and high-speed driving can be performed therewith. However, an SRAM has a disadvantage of large cell area because one memory cell includes six transistors. 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. 
     In a DRAM, as illustrated in  FIG. 8B , a memory cell includes a transistor  811  and a storage capacitor  812 , which are driven with an X decoder  813  and a Y decoder  814 . One cell includes one 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 in the case of a DRAM, a refresh operation is always necessary and power is consumed even when a rewriting operation is not performed. 
     However, the area of the memory cell of the semiconductor device described the above embodiments is about 10 F 2  and frequent refreshing is not needed. Therefore, the area of the memory cell is reduced, and the power consumption can be reduced. 
     Next,  FIG. 9  is a block diagram of a portable device. The portable device illustrated in  FIG. 9  includes an RF circuit  901 , an analog baseband circuit  902 , a digital baseband circuit  903 , a battery  904 , a power supply circuit  905 , an application processor  906 , a flash memory  910 , a display controller  911 , a memory circuit  912 , a display  913 , a touch sensor  919 , an audio circuit  917 , a keyboard  918 , and the like. The display  913  includes a display portion  914 , a source driver  915 , and a gate driver  916 . The 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  909  (IF  909 ). In general, the memory circuit  912  includes an SRAM or a DRAM; by employing the semiconductor device described in any of the above embodiments for the memory circuit  912 , 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. 10  illustrates an example of using the semiconductor device described in any of the above embodiments in a memory circuit  950  for a display. The memory circuit  950  illustrated in  FIG. 10  includes a memory  952 , a memory  953 , a switch  954 , a switch  955 , and a memory controller  951 . Further, in the memory circuit  950 , a signal line (input image data), a display controller  956  which reads and controls data held in the memories  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 held in the memory  952  though the switch  954 . The image data (stored image data A) held in the memory  952  is transmitted and displayed to the display  957  through the switch  955  and the display controller  956 . 
     In the case where the input image data A is not changed, the stored image data A is read from the display controller  956  through the memory  952  and the switch  955  with a frequency of 30 Hz to 60 Hz in general. 
     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 held in the memory  953  through the switch  954 . The stored image data A is read periodically from the memory  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  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 is continued until another new image data is held in the memory  952 . 
     By alternately writing and reading image data to and from the memory  952  and the memory  953  as described above, images are displayed on the display  957 . Note that the memory  952  and the memory  953  are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in any of the above embodiments for the memory  952  and the memory  953 , data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced. 
       FIG. 11  is a block diagram of an electronic book.  FIG. 11  includes a battery  1001 , a power supply circuit  1002 , a microprocessor  1003 , a flash memory  1004 , an audio circuit  1005 , a keyboard  1006 , a memory circuit  1007 , a touch panel  1008 , a display  1009 , and a display controller  1010 . 
     Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit  1007  in  FIG. 11 . The memory circuit  1007  has a function of temporarily storing the contents of a book. For example, users use a highlight function in some cases. When the user reads an e-book, the user will put a mark on a specific part in some cases. Such a marking function is called a highlighting function, by which characters are changed in color or type, underlined, or bold-faced, for example, so that a specific part is made to look distinct from the other part. In the function, information about the part specified by the user is stored and retained. In the case where the information is stored for a long time, the information may be copied to the flash memory  1004 . Even in such a case, by employing the semiconductor device described in any of the above embodiments, 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. 
     As described above, the semiconductor device in any of the above embodiments is mounted on each of the portable devices described in this embodiment. Therefore, a portable electric 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. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the other structures, methods, and the like described in the other embodiments. 
     Example 1 
     In this example, an oxide semiconductor film (IGZO film) containing indium, gallium, and zinc was formed, ionization potentials of the oxide semiconductor film were measured, and an energy band diagram was generated according to the measurement results. In this specification, the level of the ionization potential corresponds to the sum of the band gap (energy gap) and the electron affinity, and the value of the band gap is a value obtained by measuring a single material film by spectroscopic ellipsometry. In addition, composition analysis was performed on the oxide semiconductor film. 
     First, the values of the band gap measured by spectroscopic ellipsometry are shown. 
     As a sample oxide semiconductor film, a 100-nm-thick IGZO film was formed over a quartz substrate by a sputtering method. For the deposition conditions, the substrate temperature was 300° C., and an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio] was used. 
     The band gaps of the samples were about 2.8 eV to 2.9 eV: when a sample was deposited in an argon and oxygen atmosphere (argon:oxygen=30 sccm:15 sccm) and heat treatment after the deposition was not performed, a band gap was 2.83 eV; when a sample was deposited in an argon and oxygen atmosphere (argon:oxygen=30 sccm:15 sccm) and heat treatment was performed at 450° C. (in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour) after the deposition, a band gap was 2.90 eV; when a sample was deposited in an argon and oxygen atmosphere (argon:oxygen=30 sccm:15 sccm) and heat treatment was performed at 650° C. (in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour) after the deposition, a band gap was 2.94 eV; when a sample was deposited in an oxygen atmosphere (the proportion of oxygen in the atmosphere is 100%) and heat treatment was not performed after the deposition, a band gap was 2.82 eV; when a sample was deposited in an oxygen atmosphere (the proportion of oxygen in the atmosphere is 100%) and heat treatment was performed at 450° C. (in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour) after the deposition, a band gap was 2.89 eV; when a sample was deposited in an oxygen atmosphere (the proportion of oxygen in the atmosphere is 100%) and heat treatment was performed at 650° C. (in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour) after the deposition, a band gap was 2.94 eV. 
     In addition, an IGZO film was deposited to a thickness of 15 nm by irradiating ultraviolet light from a surface side over a single-crystal silicon substrate at a substrate temperature of 300° C. in an oxygen atmosphere (the proportion of oxygen in the atmosphere is 100%) with the use of an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio] and the ionization potential was measured by ultraviolet photoelectron spectroscopy (UPS) while the surface of the IGZO film is irradiated by ultraviolet light. Note that the ionization potential corresponds to an energy difference between a vacuum level and a valence band. 
     The energy of the conduction band was obtained by subtracting the band gap measured by spectroscopic ellipsometry from the value of the ionization potential, and the band structure of the IGZO film which was deposited with the use of an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio] was formed. Note that the band gap of the IGZO film was 2.8 eV.  FIG. 12  shows the band gap. 
     Next, the composition of the IGZO film which was deposited to a thickness of 15 nm over a single-crystal silicon substrate by sputtering at a substrate temperature of 300° C. in an oxygen atmosphere (the proportion of oxygen in the atmosphere is 100%) with the use of an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio], was evaluated by being quantified by X-ray photoelectron spectroscopy (XPS) analysis. 
     The IGZO film contained a 23.7 atomic % of indium (In), a 7.5 atomic % of gallium (Ga), a 9 atomic % of zinc (Zn), and a 59.7 atomic % of oxygen (O). 
     Further, X-ray diffraction (XRD) measurement was performed on the IGZO film deposited with the use of an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]. 
     As a sample, a 100-nm-thick IGZO film was formed over a quartz substrate by a sputtering method. Deposition conditions were as follows: a substrate temperature of room temperature, 200° C., 300° C., or 400° C., an atmosphere of argon and oxygen (argon:oxygen=30 sccm:15 sccm), and an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]. 
       FIG. 13  shows measurement results of the out-of-plane XRD spectra of the IGZO films. In  FIG. 13 , the vertical axis indicates the X-ray diffraction intensity (arbitrary unit) and the horizontal axis indicates the rotation angle  20  (degree). Note that the XRD spectra were measured with an X-ray diffractometer D8 ADVANCE manufactured by Bruker AXS K.K. 
     In the IGZO film deposited at a room temperature, a peak indicating crystallinity was not observed in the XRD spectrum as shown in  FIG. 13 , that is, it was confirmed that the IGZO film is an amorphous oxide semiconductor film. In addition, in each of the IGZO films deposited at 200° C., 300° C., or 400° C., a peak attributed to crystallinity was observed at around 31° (=20) in the XRD spectrum as shown in  FIG. 13 , that is, it was confirmed that the IGZO films are crystalline oxide semiconductor films. 
     End planes of the IGZO films were cut out, and cross sections of the IGZO films thereof were observed with a high resolution transmission electron microscope (TEM) (“H9000-NAR” manufactured by Hitachi High-Technologies Corporation) at an acceleration voltage of 300 kV. 
     As a sample, a 100-nm-thick IGZO film was formed over a quartz substrate by a sputtering method. Deposition conditions were as follows: a substrate temperature of 300° C., an atmosphere of argon and oxygen (argon:oxygen=30 sccm:15 sccm), and an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]. 
       FIG. 16A  is a cross-sectional TEM image of an IGZO film which is not subjected to heat treatment after deposition.  FIG. 16B  is a cross-sectional TEM image of an IGZO film which is subjected to heat treatment at 450° C. (in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour) after deposition.  FIG. 16C  is a cross-sectional TEM image of an IGZO film which is subjected to heat treatment at 650° C. (in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour) after deposition. 
     As shown in  FIGS. 16A to 16C , crystals having a c-axis substantially perpendicular to a surface (CAAC) are confirmed in these IGZO films. 
     As described above, it was confirmed that a non-single crystal IGZO film is obtained with the use of an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]. 
     Example 2 
     In this example, a transistor including an IGZO film formed with an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio] was manufactured and electrical characteristics and reliability of the transistor were evaluated. 
     As the transistor, a transistor  1  having the structure of the transistor  440   a  in  FIGS. 1A to 1E  and a transistor  2  having the structure of the transistor  440   b  in  FIG. 2A  were manufactured. A method for manufacturing the transistor  1  and the transistor  2  is described below. 
     As an insulating layer, a 300-nm-thick silicon oxide film was deposited over a glass substrate by a sputtering method (deposition conditions: an oxygen atmosphere, a pressure of 0.4 Pa, a power of 1.5 kW, a distance between the glass substrate and a target of 60 mm, and a substrate temperature of 100° C.). 
     The surface of the silicon oxide film was polished and then, a 20-nm-thick IGZO film was deposited as an oxide semiconductor film, by a sputtering method with the use of an oxide target having a composition ratio of In:Ga:Zn=3:1:2 [atomic ratio]. Deposition conditions were as follows: an atmosphere of argon and oxygen (argon:oxygen=30 sccm:15 sccm), a pressure of 0.4 Pa, a power of 1.5 kW, a distance between the glass substrate and the target of 60 mm, and a substrate temperature of 200° C. 
     Then, heat treatment was performed at 450° C. in a nitrogen atmosphere for one hour and then in an oxygen atmosphere for one hour. The IGZO film was processed into an island shape by an inductively coupled plasma etching (etching conditions: an etching gas of BCl 3 :Cl 2 =60 sccm:20 sccm, a power of 450 W, a bias power of 100 W, and a pressure of 1.9 Pa). 
     A 50-nm-thick tungsten film was deposited by sputtering (deposition conditions: an argon atmosphere, a pressure of 0.8 Pa, and a power of 1 kW) and was etched (etching conditions: an etching gas of CF 4 :Cl 2 :O 2 =25 sccm:25 sccm:10 sccm, a power of 500 W, a bias power of 150 W, and a pressure of 1.0 Pa) to form a source electrode layer and a drain electrode layer. 
     Next, a 30-nm-thick silicon oxynitride film was deposited as a gate insulating film by a CVD method. 
     A stack of a 15-nm-thick tantalum nitride film (deposition conditions: an atmosphere of argon and nitrogen (Ar:N 2 =50 sccm:10 sccm), a pressure of 0.6 Pa, and a power of 1 kW) and a 135-nm-thick tungsten film (deposition conditions: an argon atmosphere, a pressure of 2.0 Pa, and a power of 4 kW) was formed by a sputtering, and was etched (first etching conditions: an etching gas of Cl 2 :SF 6 :O 2 =33 sccm:33 sccm:10 sccm, a power of 2000 W, a bias power of 50 W, and a pressure of 0.67 Pa; and second etching conditions: Cl 2 =100 sccm, a power of 2000 W, a bias power of 50 W, and a pressure of 0.67 Pa), so that a gate electrode layer was formed. 
     Ion implantation of phosphorus (P) was performed on the IGZO film of the transistor  1  with the use of the gate electrode layer, the source electrode layer, and the drain electrode layer as masks. Note that the conditions of the phosphorus (P) ion implantation were as follows: an acceleration voltage of 40 kV and a dosage of 1.0×10 15  ions/cm 2 . 
     As an insulating film, an aluminum oxide film was deposited over the gate electrode layer, by sputtering (deposition conditions: an atmosphere of argon and oxygen (argon:oxygen=25 sccm:25 sccm), a pressure of 0.4 Pa, a power of 2.5 kW, a distance between the glass substrate and the target of 60 mm, and a substrate temperature of 250° C.). Then, a 300-nm-thick silicon oxynitride film was stacked over the aluminum oxide film by a CVD method. 
     Next, an opening reaching the IGZO film was formed in the gate insulating film and the insulating film, and a stack of a 50-nm-thick titanium film (deposition conditions: an argon atmosphere (Ar=20 sccm), a pressure of 0.1 Pa, a power of 12 kW), a 100-nm-thick aluminum film (deposition conditions: an argon atmosphere (Ar=50 sccm), a pressure of 0.4 Pa, a power of 1 kW), and a 50-nm-thick titanium film (deposition conditions: an argon atmosphere (Ar=20 sccm), a pressure of 0.1 Pa, a power of 12 kW) was formed in the opening and was etched (etching conditions: an etching gas of BCl 3 :Cl 2 =60 sccm:20 sccm, a power of 450 W, a bias power of 100 W, and a pressure of 1.9 Pa), so that a wiring layer was formed. 
     Through the above process, the transistor  1  and the transistor  2  were manufactured. Note that in the transistor  1 , the channel length (L) was 3.2 μm, the channel width (W) was 10.1 μm, and the width in the channel length direction (also referred to as Loff) of a region which does not overlap with the source electrode layer, the drain electrode layer, and the gate electrode layer over the oxide semiconductor film, was 0.15 μm. In the transistor  2 , the channel length (L) was 2.9 μm, the channel width (W) was 10.1 μm, and the width in the channel length direction (also referred to as Lov) of a region where the source electrode layer or the drain electrode layer overlaps with the gate electrode layer over the oxide semiconductor film, was 1.15 μm. 
     Electrical characteristics of the transistors  1  and  2  and reliability of the transistor  1  were evaluated.  FIG. 14  shows gate voltage (V g )-drain current (I d ) characteristics of the transistor  2  when drain voltages (V d ) are 3 V and 0.1 V and field-effect mobility when the drain voltage (V d ) is 0.1V.  FIGS. 15A and 15B  show gate voltage (V g )-drain current (I d ) characteristics of the transistor  1  when drain voltages (V d ) are 3 V and 0.1 V and field-effect mobility when the drain voltage (V d ) is 0.1V. 
     As shown in  FIG. 14  and  FIGS. 15A and 15B , the field-effect mobility was approximately 20 cm 2 /Vs, particularly the field-effect mobility of the transistor  2  was over 20 cm 2 /Vs, which shows that the transistors  1  and  2  have excellent ON characteristics. 
     One of methods for examining reliability of transistors is a bias-temperature stress test (hereinafter, referred to as a gate bias temperature (GBT) test). The GBT test is one kind of accelerated test and a change in characteristics, caused by long-term use, of transistors can be evaluated in a short time. In particular, the amount of shift in threshold voltage of the transistor between before and after a GBT test is an important indicator for examining reliability. The smaller the shift in the threshold voltage between before and after a GBT test is, the higher the reliability of the transistor is. 
     The temperature of a substrate over which a transistor is formed is set at a fixed temperature. A source and a drain of the transistor are set at the same potential, and a gate is supplied with a potential different from those of the source and the drain for a certain period. The temperature of the substrate may be determined depending on the purpose of the test. Further, the potential applied to the gate is higher than the potential of the source and the drain (the potential of the source and the drain is the same) in a “+GBT test” while the potential applied to the gate is lower than the potential of the source and the drain (the potential of the source and the drain is the same) in a “−GBT test.” 
     Strength of the GBT test may be determined based on the temperature of a substrate and electric field intensity and time period of application of the electric field to a gate insulating layer. The electric field intensity in the gate insulating layer is determined as the value of a potential difference between a gate, and a source and a drain divided by the value of the thickness of the gate insulating layer. 
     In this example, the GBT test was performed on the transistor  1 . First, as a +GBT test, V g  I d  characteristics of the transistor  1  were measured at a substrate temperature of 40° C. and a drain voltage V d  of 3 V. Then, the substrate temperature was set to 150° C. and V d  was set to 0.1 V. After that, a gate voltage V g  of 6 V was applied so that the intensity of an electric field applied to the gate insulating film was 2 MV/cm, and the condition was kept for one hour in an air. Next, V g  was set to 0 V. Then, V g  I d  characteristics of the transistor  1  were measured at a substrate temperature of 40° C. and V d  of 10 V.  FIG. 15A  shows results of the +GBT test. 
     Similarly, V g -I d  characteristics of the transistor  1  were measured at a substrate temperature of 40° C. and V d  of 10 V. Then, the substrate temperature was set to 150° C. and V d  was set to 0.1 V. After that, V g  of −6 V was applied so that the intensity of an electric field applied to the gate insulating film was −2 MV/cm, and the condition was kept for one hour in an air atmosphere. Next, V g  was set to 0 V. Then, V g -I d  characteristics of the transistor  1  were measured at a substrate temperature of 40° C. and V d  of 10 V.  FIG. 15B  shows results of the −GBT test. 
     Note that in  FIGS. 15A and 15B , a thick line represents results before a GBT test and a thin line represents results after the GBT test. 
     As shown in  FIGS. 15A and 15B , the transistor  1  shows substantially no change in the threshold voltage through the +GBT test and the −GBT test. Thus, it was confirmed that the amount of changes in the threshold voltage through the +GBT test and the −GBT test is small and reliability is high in the transistor in this example. 
     Further, a transistor in which a channel length (L) was 0.8 μm, a channel width (W) was 1000 μm, and an Loff was 0.3 μm was manufactured by a manufacturing process similar to that of the transistor  1 , and the off-state leakage current (off-state current) of the transistor was measured. The measurement was performed at 125° C. and 85° C. The measurement results are shown in  FIG. 17 . 
     As shown in  FIG. 17 , the off-leakage current when the transistor in this example was operated at 85° C. for 41.5 hours was 0.5 zA/μm, which is extremely low. 
     As described above, it was confirmed that the transistor in this example has an extremely low off-state current and has high reliability. 
     This application is based on Japanese Patent Application serial No. 2011-161383 filed with Japan Patent Office on Jul. 22, 2011, the entire contents of which are hereby incorporated by reference.