Patent Publication Number: US-2023141429-A1

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/983,143, filed Aug. 3, 2020, now allowed, which is a continuation of U.S. application Ser. No. 16/683,349, filed Nov. 14, 2019, now U.S. Pat. No. 10,741,695, which is a continuation of U.S. application Ser. No. 16/039,531, filed Jul. 19, 2018, now U.S. Pat. No. 10,483,404, which is a continuation of U.S. application Ser. No. 15/244,104, filed Aug. 23, 2016, now U.S. Pat. No. 10,032,926, which is a continuation of U.S. application Ser. No. 14/873,279, filed Oct. 2, 2015, now U.S. Pat. No. 9,437,747, which is a continuation of U.S. application Ser. No. 13/916,167, filed Jun. 12, 2013, now U.S. Pat. No. 9,153,699, which claims the benefit of foreign priority applications filed in Japan as Serial No. 2012-136438 on Jun. 15, 2012, and Serial No. 2012-141373 on Jun. 22, 2012, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof. 
     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 
     In recent years, semiconductor devices have been developed to be used mainly for an LSI, a CPU, or a memory. A CPU is an aggregation of semiconductor elements each provided with an electrode which is a connection terminal, which includes a semiconductor integrated circuit (including at least a transistor and a memory) separated from a semiconductor wafer. 
     A semiconductor circuit (IC chip) of an LSI, a CPU, or a memory is mounted on a circuit board, for example, a printed wiring board, to be used as one of components of a variety of electronic devices. 
     A technique for manufacturing a transistor by using an oxide semiconductor film for a channel formation region, or the like has been attracting attention. Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor film and a transistor in which InGaO 3 (ZnO) m  is used as an oxide semiconductor film. 
     Patent Document 1 discloses a three-layer structure in which a first multi-component oxide semiconductor layer is provided over a substrate, a one-component oxide semiconductor layer is stacked over the first multi-component oxide semiconductor layer, and a second multi-component oxide semiconductor layer is stacked over the one-component oxide semiconductor layer. 
     Non-Patent Document 1 discloses a transistor having a stack of oxide semiconductors. 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2011-155249 
       
    
     Non-Patent Document 
     
         
         [Non-Patent Document 1] Arokia Nathan et al., “Amorphous Oxide TFTs: Progress and issues”, SID 2012 Digest pp. 1-4. 
       
    
     SUMMARY OF THE INVENTION 
     The electrical characteristics of a transistor including an oxide semiconductor layer are varied by influence of an insulating film in contact with the oxide semiconductor layer, that is, by an interface state between the oxide semiconductor layer and the insulating film. 
     For example, in the case where an insulating film containing silicon is used as the insulating film, when the oxide semiconductor layer is deposited on the silicon oxide film by a sputtering method, silicon might enter the oxide semiconductor layer at the time of the sputtering. In the structure of Non-Patent Document 1, an oxide semiconductor functioning as a channel is in contact with silicon oxide, and thus silicon, which is a constituent atom of the silicon oxide film, might enter the channel, as an impurity. When an impurity such as silicon enters the oxide semiconductor layer, the field-effect mobility of the transistor might decrease. 
     Further, when a silicon nitride film is used as the insulating film, many carriers flow through the interface between the silicon nitride film and the oxide semiconductor layer, and thus it is difficult to obtain transistor characteristics. 
     An object of one embodiment of the present invention is to provide a structure of a transistor having high field-effect mobility. 
     Thus, in order that an oxide semiconductor layer through which carriers flow is not in contact with a gate insulating film containing silicon, a buried channel structure in which the oxide semiconductor layer through which carriers flow is separated from the gate insulating film containing silicon is employed. Further, in the case where an oxide semiconductor layer is provided over a base insulating film and a gate insulating film is provided over the oxide semiconductor layer, in order that the oxide semiconductor layer through which carriers flow is not in contact with the base insulating film containing silicon, it is preferable to employ a buried channel structure in which the oxide semiconductor layer through which carriers flow is separated from the base insulating film containing silicon. 
     Specifically, as illustrated in  FIG.  1 A , a first oxide semiconductor film  403   a , a second oxide semiconductor film  403   b , and a third oxide semiconductor film  403   c  are stacked in this order, and the second oxide semiconductor film  403   b  is made to an n-type in order that a conduction band offset (Ec) in an energy band diagram (schematic diagram) illustrated in  FIG.  1 B  is greater than or equal to 0.05 eV and preferably greater than or equal to 0.1 eV. The energy band diagram in  FIG.  1 B  is an energy band which corresponds to a portion between C and C′ in  FIG.  1 A . Note that the energy band diagram illustrated in  FIG.  1 B  is mealy an example and thus does not limit the present invention. Any structure of the energy band diagram may be employed as long as the energy level of the bottom of the conduction band in a second oxide semiconductor layer S 2  is lower than those of the bottoms of the conduction band in a first oxide semiconductor layer S 1  and a third oxide semiconductor layer S 3 . 
     As a way to make the second oxide semiconductor layer S 2  an n-type, the second oxide semiconductor layer S 2  is deposited by a sputtering method in a mixed atmosphere containing nitrogen or dinitrogen monoxide. As another way to make the second oxide semiconductor layer S 2  an n-type, deposition is performed using a sputtering target containing a very small amount of boron or phosphorus so that the second oxide semiconductor layer S 2  contains boron or phosphorus. 
     As a material of the first oxide semiconductor layer S 1 , a material which can be represented as M1 a M2 b M3 c O x  (a is a real number greater than or equal to 0 and less than or equal to 2, b is a real number greater than 0 and less than or equal to 5, c is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used. Ga, Mg, Hf, Al, Zr, Sn, or the like can be used as the constituent element M2 to function as a stabilizer for reducing the number of oxygen vacancies in an oxide semiconductor. As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may be contained. As the constituent element M1, indium or the like is used. As the constituent element M3, zinc or the like is used. 
     Typically, for the first oxide semiconductor layer S 1 , a gallium oxide film, a gallium zinc oxide film, or a material film in which the content of the constituent element M2 is higher than the content of the constituent element M1 is used. For example, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having any of atomic ratios of In:Ga:Zn=1:3:2, In:Ga:Zn=1:4:2, and In:Ga:Zn=1:5:4 is used. In forming the first oxide semiconductor layer, deposition is preferably performed by a sputtering method in a mixed atmosphere containing more oxygen than a rare gas and more preferably in an oxygen atmosphere (oxygen: 100%), and the resulting oxide semiconductor layer can also be referred to as a first I-type oxide semiconductor layer. The first I-type oxide semiconductor layer is a highly purified oxide semiconductor layer that contains impurities other than the main components of the oxide semiconductor layer as little as possible and is an I-type (intrinsic) oxide semiconductor or close thereto. In such an oxide semiconductor layer, the Fermi level (Ef) can be at the same level as the intrinsic Fermi level (Ei). 
     For the second oxide semiconductor layer S 2 , a material which can be represented as M4 d M5 e M6 f O x , (d is a real number greater than 0 and less than or equal to 5, e is a real number greater than or equal to 0 and less than or equal to 3, f is a real number greater than 0 and less than or equal to 5, and x is an arbitrary positive number) is used. Ga, Mg, Hf, Al, Zr, Sn, or the like can be used as the constituent element M5 to function as a stabilizer for reducing the number of oxygen vacancies in an oxide semiconductor. As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may be contained. As the constituent element M4, indium or the like is used. As the constituent element M6, zinc or the like is used. Typically, a material film in which the content of the constituent element M4 is higher than the content of the constituent element M5 is used. For example, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 is used. In forming the second oxide semiconductor layer, deposition is preferably performed by a sputtering method in a mixed atmosphere containing nitrogen or a mixed atmosphere containing dinitrogen monoxide, and the resulting oxide semiconductor layer is also referred to as an N-type oxide semiconductor layer. Note that the N-type oxide semiconductor layer has higher carrier density and higher conductivity a than the first I-type oxide semiconductor layer. 
     For the third oxide semiconductor layer S 3 , a material which can be represented as M7 g M8 h M9 i O x  (g is a real number greater than or equal to 0 and less than or equal to 2, h is a real number greater than 0 and less than or equal to 5, i is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used. Ga, Mg, Hf, Al, Zr, Sn, or the like can be used as the constituent element M8 to function as a stabilizer for reducing the number of oxygen vacancies in an oxide semiconductor. As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may be contained. As the constituent element M7, indium or the like is used. As the constituent element M9, zinc or the like is used. Typically, a material film in which the content of the constituent element M7 is substantially equal to the content of the constituent element M8 is used. For example, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 is used. In forming the third oxide semiconductor layer, deposition is preferably performed by a sputtering method in a mixed atmosphere containing more oxygen than a rare gas and preferably in an oxygen atmosphere (oxygen: 100%), and the resulting oxide semiconductor layer can also be referred to as a second I-type oxide semiconductor layer. 
     Note that the materials of the first and second oxide semiconductor layers S 1  and S 2  may be selected so that the energy level of the bottom of the conduction band in the second oxide semiconductor layer S 2  is lower than that of the bottom of the conduction band in the first oxide semiconductor layer S 1 , and the compositions of the above materials may be adjusted as appropriate. 
     Further, the materials of the second and third oxide semiconductor layers S 2  and S 3  may be selected so that the energy level of the bottom of the conduction band in the second oxide semiconductor layer S 2  is lower than that of the bottom of the conduction band in the third oxide semiconductor layer S 3 , and the compositions of the above materials may be adjusted as appropriate. 
     The second oxide semiconductor layer S 2 , the S 1 , and the S 3  have at least one common constituent element. 
     In the case where a transistor is formed in which the conductivity of the second oxide semiconductor layer S 2  is increased using such a three-layer structure, a distance between the second oxide semiconductor layer S 2  and a drain electrode, i.e., the thickness of the third oxide semiconductor layer S 3  becomes dominant, so that a channel length L apparently becomes smaller in the forward direction, and an on-state current can be increased. In the reverse direction, the third oxide semiconductor layer S 3  is depleted and a sufficiently small off state current can be achieved. 
     In a structure of one embodiment of the present invention disclosed in this specification, between two oxide semiconductor layers, an oxide semiconductor layer which has higher conductivity a than the two oxide semiconductor layers may be provided, and an example of the structure is a transistor  418  illustrated in  FIG.  1 A . Note that the cross-sectional view of the transistor  418  in  FIG.  1 A  corresponds to a structural view taken along a chain line A 1 -A 2  in a top view in  FIG.  1 C .  FIG.  1 D  illustrates a cross-sectional view taken along a dotted line A 2 -A 3  in  FIG.  1 C . 
     A structure of one embodiment of the present invention disclosed in this specification is a semiconductor device including a first insulating layer over an insulating surface, a first oxide semiconductor layer over the first insulating layer, a second oxide semiconductor layer over the first oxide semiconductor layer, a third oxide semiconductor layer over the second oxide semiconductor layer, and a second insulating layer over the third oxide semiconductor layer. The second oxide semiconductor layer has higher conductivity than the third oxide semiconductor layer and the first oxide semiconductor layer. 
     In the above structure, to increase the conductivity of the second oxide semiconductor layer, the second oxide semiconductor layer is made to have a higher nitrogen concentration, boron concentration, or phosphorus concentration than the third oxide semiconductor layer and the first oxide semiconductor layer. An atmosphere containing nitrogen or dinitrogen monoxide may be used for depositing the second oxide semiconductor layer, whereby the second oxide semiconductor layer having high conductivity a may be obtained. 
     In the above structure, as illustrated in  FIG.  1 A , the thickness of the first oxide semiconductor film  403   a  is set larger than that of each of the second oxide semiconductor film  403   b  and the third oxide semiconductor film  403   c , whereby influence of diffusion of silicon or the like contained in the base film is reduced. 
     In the above structure, the side surface of the second oxide semiconductor layer may be covered with the third oxide semiconductor layer. The structure in which the side surface of the second oxide semiconductor layer is not in direct contact with a source electrode layer or a drain electrode layer can lead to a reduction in leakage current. 
     In the above structure, the first insulating layer is an insulating film containing silicon, and the second insulating layer is an insulating film containing gallium. As illustrated in  FIG.  1 D , an insulating film  402  formed using an insulating film containing gallium is provided to cover and be in contact with the side surface of the second oxide semiconductor film  403   b , whereby leakage current can be reduced. 
     In the case of a top-gate transistor, a gate electrode layer is further provided over the second insulating layer in the above structure. Although the transistor  418  in  FIG.  1 A  is a top-gate transistor, one embodiment of the present invention is not particularly limited to the top-gate transistor in  FIG.  1 A . 
       FIG.  16 A  illustrates another example of a top-gate transistor. To obtain a buried channel structure, specifically, the first oxide semiconductor layer S 1 , the second oxide semiconductor layer S 2 , and the third oxide semiconductor layer S 3  are stacked in this order, and an oxide semiconductor material having high conductivity a is used for a channel formation region  103   b  illustrated in  FIG.  16 B  for the purpose of obtaining an energy difference greater than or equal to 0.05 eV, preferably greater than or equal to 0.1 eV in an energy band diagram (schematic diagram) in  FIG.  16 C . Note that the energy band diagram illustrated in  FIG.  16 C  is mealy an example and thus does not limit the present invention. Any structure of the energy band diagram may be employed as long as the energy level of the bottom of the conduction band in the second oxide semiconductor layer S 2  is lower than those of the bottoms of the conduction band in the first oxide semiconductor layer S 1  and the third oxide semiconductor layer S 3 . 
     As a way to increase the conductivity a of the second oxide semiconductor layer S 2  overlapping with a gate electrode layer  101 , the second oxide semiconductor layer S 2  is deposited by a sputtering method in an atmosphere containing nitrogen or dinitrogen monoxide. As another way to increase the conductivity a of the second oxide semiconductor layer S 2 , deposition is performed using a sputtering target containing a very small amount of boron or phosphorus so that the second oxide semiconductor layer S 2  contains boron or phosphorus. 
     The third oxide semiconductor layer S 3  is formed to overlap with the gate electrode layer  101 , and low-resistance regions  104   c  and  108   c  which do not overlap with the gate electrode layer  101  are n-type. In order that the low-resistance regions  104   c  and  108   c  not overlapping with the gate electrode layer  101  are electrically connected to a source electrode layer and a drain electrode layer, the low-resistance regions  104   c  and  108   c  are preferably regions having low resistance. Further, the low-resistance regions  104   c  and  108   c  are preferably formed in a self-aligned manner using the gate electrode layer  101  as a mask. 
     As a way to form the low-resistance regions  104   c  and  108   c  that are n-type, the gate electrode layer  101  is used as a mask, and the low-resistance regions  104   c  and  108   c  are formed by addition of nitrogen, boron, or phosphorus in a self-aligned manner by an ion implantation method. As another way to form the low-resistance regions that are n-type, a nitride insulating film (typically, a silicon nitride film  107 ) is formed in contact with the third oxide semiconductor layer S 3  or the third oxide semiconductor layer S 3  is subjected to nitrogen plasma treatment. 
     Another structure of one embodiment of the present invention disclosed in this specification is a semiconductor device including a first insulating layer over an insulating surface, a first oxide semiconductor layer over the first insulating layer, a second oxide semiconductor layer over the first oxide semiconductor layer, a third oxide semiconductor layer over the second oxide semiconductor layer, a second insulating layer on and in contact with the third oxide semiconductor layer, a gate electrode layer over the second insulating layer, and a third insulating layer on and in contact with the third oxide semiconductor layer. The second oxide semiconductor layer has a smaller thickness than the first oxide semiconductor layer and the third oxide semiconductor layer. 
     In the above structure, a region of the third oxide semiconductor layer which is in contact with the third insulating layer has low crystallinity and has a higher nitrogen concentration than a region of the third oxide semiconductor layer which is in contact with the second insulating layer. The region of the third oxide semiconductor layer which is in contact with the second insulating layer overlaps with a channel formation region of the second oxide semiconductor layer. The third insulating layer is a silicon nitride film, and thus the region of the third oxide semiconductor layer which is in contact with the third insulating layer can have a higher nitrogen concentration than the region of the third oxide semiconductor layer which is in contact with the second insulating layer. To further reduce resistance, phosphorus, boron, or nitrogen may be added to the region of the third oxide semiconductor layer which is in contact with the second insulating layer by an ion implantation method using the gate electrode layer as a mask. The third oxide semiconductor layer is preferably formed using a film having a crystalline structure. The region of the third oxide semiconductor layer which overlaps with the gate electrode layer has a crystalline structure and the region of the third oxide semiconductor layer to which phosphorus, boron, or nitrogen is added by an ion implantation method is a region having low crystallinity. 
     In the above structure, the third insulating layer may be provided as a sidewall provided on a side surface of the gate electrode layer. 
     In the above structure, to prevent generation of a parasitic channel, a taper angle formed by an end surface of the first oxide semiconductor layer and a surface of the first insulating layer is preferably greater than or equal to 10° and less than or equal to 60°. A taper angle formed by an end surface of the second oxide semiconductor layer and the surface of the first insulating layer is preferably greater than or equal to 10° and less than or equal to 60°. A taper angle formed by an end surface of the third oxide semiconductor layer and the surface of the first insulating layer is preferably greater than or equal to 10° and less than or equal to 60°. 
     In the above structure, the side surface of the second oxide semiconductor layer may be covered with the third oxide semiconductor layer. The structure in which the side surface of the second oxide semiconductor layer is not in direct contact with the source electrode layer or the drain electrode layer can lead to a reduction in leakage current. 
     In the case of a dual-gate transistor in which gate electrode layers are provided over and below the oxide semiconductor layers, the first gate electrode layer is provided between the insulating surface and the first insulating layer, and the second gate electrode layer is provided over the second insulating layer. 
     Each of the oxide semiconductor layers is deposited under the following conditions: a sputtering target which is polycrystalline and whose relative density (filling rate) is high is used; the sputtering target in deposition is sufficiently cooled to a room temperature; the temperature of a surface where the oxide semiconductor layer is to be deposited of a substrate where the oxide semiconductor layer is to be deposited (a deposition-target substrate) is increased to a room temperature or higher; and an atmosphere in a deposition chamber hardly contains moisture or hydrogen. 
     The higher density of the sputtering target is more preferable. When the density of the sputtering target is increased, the density of a film to be deposited can also be increased. Specifically, the relative density (filling rate) of the sputtering target is higher than or equal to 90% and lower than or equal to 100%, preferably higher than or equal to 95%, more preferably higher than or equal to 99.9%. Note that the relative density of the sputtering target refers to a ratio between the density of the sputtering target and the density of a material free of porosity having the same composition as the sputtering target. 
     The sputtering target is preferably sintered in an inert gas atmosphere (a nitrogen or rare gas atmosphere), in vacuum, or in a high-pressure atmosphere. As a sintering method, an atmospheric sintering method, a pressure sintering method, or the like is used as appropriate. A polycrystalline target obtained by any of these methods is used as a sputtering target. As the pressure sintering method, a hot pressing method, a hot isostatic pressing (HIP) method, a discharge plasma sintering method, or an impact method is preferably used. Although the maximum temperature at which sintering is performed is selected depending on the sintering temperature of the sputtering target material, it is preferably set to approximately 1000° C. to 2000° C., or more preferably, 1200° C. to 1500° C. Although the holding time of the maximum temperature is selected depending on the sputtering target material, 0.5 hours to 3 hours is preferable. 
     In the case where an In—Ga—Zn-base oxide film is deposited, a sputtering target having any of atomic ratios of In:Ga:Zn=3:1:2, In:Ga:Zn=1:1:1, and In:Ga:Zn=1:3:2 is used as the sputtering target. 
     Reduction of impurities remaining in the deposition chamber is also important for forming a dense film. The back pressure (ultimate vacuum: degree of vacuum before introducing a reaction gas) in a deposition chamber is set to be lower than or equal to 5×10 −3  Pa, preferably 6×10 −5  Pa, and pressure in deposition is set to be lower than 2 Pa, preferably lower than or equal to 0.4 Pa. When the back pressure is set to be low, impurities in the deposition chamber are reduced. 
     Reduction of impurities in a gas to be introduced in the deposition chamber, i.e., a gas to be used in deposition, is also important for forming a dense film. Further, it is important to increase the proportion of oxygen contained in a deposition gas and optimize power. By increasing the proportion of oxygen (the upper limit: 100% oxygen) contained in the deposition gas and optimizing the power, plasma damage in deposition can be alleviated. Thus, a dense film is easily formed. 
     Deposition of the oxide semiconductor film is preferably performed while a quadrupole mass analyzer (hereinafter also referred to as Q-mass) is operated continuously in order that the amount of moisture in the deposition chamber, or the like is monitored by the Q-mass before or in deposition. 
     The oxide semiconductor film is in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like. 
     When the temperature of a deposition-target substrate, which is an example of deposition conditions, is set to be higher than or equal to 200° C., a dense oxide semiconductor film including a crystal part, i.e., a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film can be obtained. 
     First, the CAAC-OS film is described. 
     The CAAC-OS film is one of oxide semiconductor films including a plurality of crystal parts, and most of each crystal part fits inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. 
     In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the TEM image of the crystal parts included in the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film. 
     A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. Here, analysis (ϕ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (ϕ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZnO 4 , six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when ϕ scan is performed with 2θ fixed at around 56°. 
     According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. 
     In this specification, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies depends on regions. 
     Note that when the CAAC-OS film with an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° is derived from the (311) plane of a ZnGa 2 O 4  crystal; such a peak indicates that a ZnGa 2 O 4  crystal is included in part of the CAAC-OS film including the InGaZnO 4  crystal. It is preferable that in the CAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ do not appear at around 36°. 
     The CAAC-OS film is an oxide semiconductor film having a low impurity concentration. The impurity is any of elements which are not the main components of the oxide semiconductor film and includes hydrogen, carbon, silicon, a transition metal element, and the like. In particular, an element (e.g., silicon) which has higher bonding strength with oxygen than a metal element included in the oxide semiconductor film causes disorder of atomic arrangement in the oxide semiconductor film because the element deprives the oxide semiconductor film of oxygen, thereby reducing crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, and the like have a large atomic radius (or molecular radius); therefore, when any of such elements is contained in the oxide semiconductor film, the element causes disorder of the atomic arrangement of the oxide semiconductor film, thereby reducing crystallinity. Note that the impurity contained in the oxide semiconductor film might become a carrier trap or a source of carriers. 
     The CAAC-OS film is an oxide semiconductor film having a low density of defect states. For example, oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein. 
     The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus has a low carrier density in some cases. Thus, in some cases, a transistor including the oxide semiconductor film in a channel formation region rarely has a negative threshold voltage (is rarely normally-on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has little variation in electrical characteristics and high reliability. A charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released. The trapped charge may behave like a fixed charge. Thus, the transistor including the oxide semiconductor film with a high impurity concentration and a high density of defect states has unstable electrical characteristics in some cases. 
     Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by deposition using a sputtering target or by performing treatment for crystallization such as heat treatment after deposition. 
     During deposition, fine sputtering particles fly from a sputtering target, and a film is formed so that the sputtering particles adhere onto the deposition-target substrate. When the temperature of the substrate is higher than or equal to 200° C., the sputtering particles are rearranged because the substrate is heated. Thus, a dense film is formed. 
     A phenomenon in the deposition is described in detail using  FIGS.  12 A to  12 C ,  FIGS.  13 A  and  13 B, and  FIGS.  14 A to  14 C . 
     When ions collide with the surface of the sputtering target, the crystal region included in the sputtering target is cleaved along an a-b plane, and sputtered particles whose top and bottom surfaces are each aligned with a layer parallel to the a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) are separated from the sputtering target. Assuming that the crystalline particle which is sputtered from a surface of a sputtering target  2002  and released has c-axis alignment and is a flat-plate-like sputtered particle  2001  as illustrated in  FIG.  12 A , a schematic model diagram in  FIG.  12 B  can be obtained. The flat-plate-like sputtered particle is preferably in a state illustrated in  FIG.  12 C , i.e., is preferably the one with an outermost surface of a (Ga or Zn)O plane. 
     When the oxygen flow rate is high and the pressure in a chamber  2003  is high during deposition, as illustrated in  FIG.  13 A , oxygen ions are attached onto the flat-plate-like sputtered particle and the sputtered particle can have a large amount of oxygen on its surface. Another flat-plate-like sputtered particle is stacked thereover before the attached oxygen is released; therefore, as illustrated in  FIG.  14 C , a large amount of oxygen can be contained in the film. The oxygen adsorbed on the surface contributes to a reduction in the number of oxygen vacancies in the oxide semiconductor film. 
     To form an oxide semiconductor film having a crystalline region with c-axis alignment, it is preferable to increase the substrate temperature at the deposition. However, when the substrate temperature is higher than 350° C., the oxygen adsorbed on the surface might be released as illustrated in  FIG.  13 B . Therefore, when the substrate temperature is set to higher than or equal to 150° C. and lower than or equal to 350° C. and preferably higher than or equal to 160° C. and lower than or equal to 230° C. and only an oxygen gas is used as a deposition gas, an oxide semiconductor film having a crystalline region with c-axis alignment, i.e., a CAAC-OS film, can be obtained. 
       FIG.  14 A  is a model of a process in the deposition, in which one flat-plate-like sputtered particle reaches a surface of a substrate  2000  to be stabilized. As illustrated in  FIG.  14 A , the flat-plate-like sputtered particle reaches the substrate surface while keeping its crystalline state, whereby formation of a CAAC-OS film is facilitated. Further, when flat-plate-like sputtered particles are stacked as illustrated in  FIG.  14 B , formation of a CAAC-OS film is facilitated. Note that the CAAC-OS film is a film which contains a large amount of oxygen and has a reduced number of oxygen vacancies as illustrated in  FIG.  14 C . 
     In the CAAC-OS film over the substrate  2000 , a series of about 2 to 20 indium atoms exist in a lateral direction to form a layer including indium atoms. Note that in some cases, the layer has a series of 20 or more indium atoms; for example, the layer may have a series of 2 to 50 indium atoms, 2 to 100 indium atoms, or 2 to 500 indium atoms in a lateral direction. 
     Layers including indium atoms overlap with each other. The number of layers is greater than or equal to 1 and less than or equal to 20, greater than or equal to 1 and less than or equal to 10, or greater than or equal to 1 and less than or equal to 4. 
     As described above, a stack of the layers including indium atoms often appears to be a cluster including several indium atoms in a lateral direction and several layers in a longitudinal direction. This is because each of the sputtering particles has a flat-plate-like shape. 
     By increasing the temperature of the deposition-target substrate, migration of sputtering particles is likely to occur on a substrate surface. With this effect, a flat-plate-like sputtered particle reaches the substrate surface, moves slightly, and then is attached to the substrate surface with a flat plane (a-b plane) of the sputtered particle facing toward the substrate surface. Therefore, an oxide semiconductor film having a crystal region which is c-axis-aligned perpendicularly to the surface of the oxide semiconductor film is easily formed. 
     Further, heat treatment at a temperature of higher than or equal to 200° C. may be performed after the deposition of the oxide semiconductor film, so that a denser film is obtained. However, in that case, oxygen vacancies might be generated when impurity elements (e.g., hydrogen and water) in the oxide semiconductor film are reduced. Therefore, before the heat treatment is performed, an insulating layer containing excess oxygen is preferably provided over or below the oxide semiconductor film, in which case oxygen vacancies in the oxide semiconductor film can be reduced by the heat treatment. 
     An oxide semiconductor film shortly after deposition is made dense; thus, a dense film which is thin and close to single crystal can be obtained. Since oxygen, hydrogen, or the like hardly diffuses within the film, a semiconductor device including a dense oxide semiconductor film can achieve improvement in reliability. 
     It is preferable that a CAAC-OS film be used for at least the second oxide semiconductor layer and the channel formation region overlapping with the gate electrode layer be formed with the CAAC-OS film. In the case where a CAAC-OS film is used for the first oxide semiconductor layer, since the first oxide semiconductor layer has the same crystal structure as the second oxide semiconductor layer, the number of levels can be small at the interface thereof, so that high field-effect mobility can be achieved. Further, it is preferable that the second oxide semiconductor layer be formed on and in contact with the first oxide semiconductor layer that is a CAAC-OS film because the second oxide semiconductor layer formed over the first oxide semiconductor layer is easily crystallized using the first oxide semiconductor layer as a crystal seed, so that the first and second oxide semiconductor layers can have the same crystal structure. 
     A transistor having high field-effect mobility can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A to  1 D  are schematic views illustrating one embodiment of the present invention. 
         FIGS.  2 A to  2 D  are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention. 
         FIGS.  3 A and  3 B  are a cross-sectional view and a top view illustrating one embodiment of the present invention. 
         FIGS.  4 A to  4 C  are cross-sectional views and a top view illustrating one embodiment of the present invention. 
         FIGS.  5 A and  5 B  are a cross-sectional view and a circuit diagram illustrating one embodiment of a semiconductor device. 
         FIGS.  6 A to  6 C  are a cross-sectional view and circuit diagrams illustrating one embodiment of a semiconductor device. 
         FIG.  7    is a circuit diagram illustrating one embodiment of a semiconductor device. 
         FIG.  8    is a perspective view illustrating one embodiment of a semiconductor device. 
         FIGS.  9 A to  9 C  are block diagrams illustrating one embodiment of a semiconductor device. 
         FIGS.  10 A to  10 C  illustrate an electronic appliance. 
         FIGS.  11 A to  11 C  illustrate electronic appliances. 
         FIG.  12 A  is a schematic view of a flat-plate-like sputtered particle,  FIG.  12 B  is a model diagram in deposition, and  FIG.  12 C  is a model diagram showing the state of the flat-plate-like sputtered particle. 
         FIG.  13 A  is a model diagram in deposition and  FIG.  13 B  is a model diagram showing the state where oxygen of a flat-plate-like sputtered particle is released. 
         FIGS.  14 A and  14 B  are model diagrams in deposition and  FIG.  14 C  is a model diagram showing the state of flat-plate-like sputtered particles. 
         FIG.  15    is a top view illustrating an example of an apparatus for manufacturing a semiconductor device. 
         FIGS.  16 A to  16 C  are a top view, a cross-sectional view, and a schematic view illustrating one embodiment of the present invention. 
         FIGS.  17 A to  17 C  are schematic views each illustrating one embodiment of the present invention. 
         FIGS.  18 A to  18 D  are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention. 
         FIGS.  19 A to  19 D  are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention. 
         FIGS.  20 A to  20 E  are top views and cross-sectional views illustrating one embodiment of the present invention. 
         FIGS.  21 A and  21 B  are cross-sectional views each illustrating one embodiment of the present invention. 
         FIGS.  22 A and  22 B  are a cross-sectional view and a circuit diagram illustrating one embodiment of a semiconductor device. 
         FIGS.  23 A to  23 C  are a cross-sectional view and circuit diagrams illustrating one embodiment of a semiconductor device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments. 
     Embodiment 1 
     In this embodiment, one embodiment of a semiconductor device and one embodiment of a method for manufacturing the semiconductor device will be described with reference to  FIGS.  2 A to  2 D . In this embodiment, an example of a method for manufacturing a transistor including an oxide semiconductor film is described. 
     First, an insulating film  433  is formed over a substrate  400  having an insulating surface and a conductive film is formed thereover by a sputtering method, an evaporation method, or the like. The conductive film is etched so that a conductive layer  491  and wiring layers  434  and  436  are formed. 
     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 . 
     For example, the insulating film  433  can be formed using one or more insulating films selected from the following: an oxide insulating film of silicon oxide, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, or the like; a nitride insulating film of silicon nitride, aluminum nitride, or the like; an oxynitride insulating film of silicon oxynitride, aluminum oxynitride, or the like; or a nitride oxide insulating film of silicon nitride oxide or the like. Note that “silicon nitride oxide” refers to the one that contains more nitrogen than oxygen and “silicon oxynitride” refers to the one that contains more oxygen than nitrogen. Here, for example, silicon oxynitride refers to the one that contains oxygen, nitrogen, and silicon at concentrations ranging from 50 atomic % to 70 atomic %, 0.5 atomic % to 15 atomic %, and 25 atomic % to 35 atomic %, respectively. In the case where a substrate which is provided with a semiconductor element is used, a silicon nitride film which is deposited by a plasma chemical vapor deposition (CVD) method using a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) as a supply gas is preferably used as the insulating film  433 . This silicon nitride film also functions as a barrier film and prevents hydrogen or a hydrogen compound from entering an oxide semiconductor layer to be formed later, thereby improving the reliability of the semiconductor device. A silicon nitride film deposited by a plasma CVD method using a mixed gas of silane (SiH 4 ), nitrogen (N 2 ), and ammonia (NH 3 ) as a supply gas includes fewer defects than the silicon nitride film deposited using a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) as a supply gas. When the silicon nitride film deposited using a mixed gas of silane (SiH 4 ), nitrogen (N 2 ), and ammonia (NH 3 ) is formed to a thickness greater than or equal to 300 nm and less than or equal to 400 nm, an ESD resistance can be 300 V or higher. Therefore, when a stack in which a silicon nitride film which is deposited using a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) as a supply gas is stacked over the silicon nitride film which is deposited to a thickness greater than or equal to 300 nm and less than or equal to 400 nm using a mixed gas of silane (SiH 4 ), nitrogen (N 2 ), and ammonia (NH 3 ) is used as the insulating film  433 , a barrier film having a high ESD resistance can be achieved. 
     The conductive layer  491  and the wiring layers  434  and  436  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 conductive layer  491  and the wiring layers  434  and  436 . The conductive layer  491  and the wiring layers  434  and  436  may have a single-layer structure or a stacked-layer structure. 
     The conductive layer  491  and the wiring layers  434  and  436  can also be formed using a conductive material such as indium oxide-tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium oxide-zinc oxide, or indium tin oxide to which silicon oxide is added. It is also possible that the conductive layer  491  and the wiring layers  434  and  436  have a stacked structure of the above conductive material and the above metal material. 
     In order to obtain a normally-off switching element, it is preferable that the threshold voltage of the transistor is made positive by using a material having a work function of 5 eV (electron volts) or higher, preferably 5.5 eV or higher, for a gate electrode layer. Specifically, a material which includes an In—N bond and has a specific resistivity of 1×10 −1  Ω·cm to 1×10 −4  Ω·cm, preferably 5×10 −2  Ω·cm to 1×10 −4  Ω·cm, is used for the gate electrode layer. Examples of the material are an In—Ga—Zn-based oxide film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, an In—O film containing nitrogen, and a metal nitride film (e.g., an InN film). 
     Next, an oxide insulating film is formed over the conductive layer  491  and the wiring layers  434  and  436 . The oxide insulating film has a projecting portion reflecting the shape of the conductive layer  491  on its surface. 
     The oxide insulating film 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, gallium oxide zinc, and zinc oxide, or a mixed material thereof. The oxide insulating film may have either a single-layer structure or a stacked-layer structure. 
     Then, polishing treatment (e.g., chemical mechanical polishing (CMP)) is performed, whereby an oxide insulating film  435  which is planarized is formed and top surfaces of the wiring layers  434  and  436  and a top surface of the conductive layer  491  are exposed. After the CMP treatment, cleaning is performed and heat treatment for removing moisture attached on the substrate is performed. A cross-sectional view of a structure obtained after this step corresponds to  FIG.  2 A . 
     After the planarization and the heat treatment, an insulating film  437  and a stack  403  of oxide semiconductor films are formed. A cross-sectional view of a structure obtained after this step corresponds to  FIG.  2 B . 
     Then, patterning is performed using one mask and the insulating film  437  and the stack  403  of oxide semiconductor films are selectively etched. A cross-sectional view of a structure obtained after this step corresponds to  FIG.  2 C . It is preferable that the insulating film  437  and the stack  403  of oxide semiconductor films be formed successively without being exposed to the air because interfaces of the films can be prevented from being contaminated by an impurity. 
     The insulating film  437  is formed by a plasma CVD method or a sputtering method. In the case where a plasma CVD method is used, it is particularly preferable to use a plasma CVD method in which plasma is generated utilizing electric-field energy of a microwave and a source gas for the insulating film is excited by the plasma, and the excited source gas is reacted on a surface of an object to deposit a reactant (also referred to as a microwave plasma CVD method). The insulating film formed by a plasma CVD method using a microwave is a dense film, and therefore, the insulating film  437  obtained by processing the insulating film is also a dense film. The insulating film  437  has a thickness greater than or equal to 5 nm and less than or equal to 300 nm. 
     The insulating film  437  can be formed using a single layer or a stack of layers selected from the following films: an oxide insulating film of silicon oxide, gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, or the like; an oxynitride insulating film of silicon oxynitride, aluminum oxynitride, or the like; or a nitride oxide insulating film of silicon nitride oxide or the like. 
     In this embodiment, the stack  403  of oxide semiconductor films has a three-layer structure in which the first oxide semiconductor film  403   a , the second oxide semiconductor film  403   b , and the third oxide semiconductor film  403   c  are stacked in this order, as illustrated in  FIG.  2 C . 
     For each of the oxide semiconductor films, a two-component metal oxide such as 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—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; or a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, or an In—Sn—Hf—Zn-based oxide can be used. 
     As the first oxide semiconductor film  403   a , a material film which can be represented as M1 a M2 b M3 c O x  (a is a real number greater than or equal to 0 and less than or equal to 2, b is a real number greater than 0 and less than or equal to 5, c is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used. In this embodiment, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 and has a thickness greater than or equal to 5 nm and less than or equal to 15 nm is used. Further, the first oxide semiconductor film  403   a  may have an amorphous structure but is preferably a CAAC-OS film. Note that the first oxide semiconductor film  403   a  can be referred to as a first I-type oxide semiconductor layer. 
     As the second oxide semiconductor film  403   b , a material film which can be represented as M4 d M5 e M6 f O x , (d is a real number greater than 0 and less than or equal to 5, e is a real number greater than or equal to 0 and less than or equal to 3, f is a real number greater than 0 and less than or equal to 5, and x is an arbitrary positive number) is used. In this embodiment, an In—Ga—Zn-based oxide film is deposited to a thickness greater than or equal to 5 nm and less than or equal to 30 nm by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 in a mixed atmosphere containing oxygen and nitrogen or a mixed atmosphere containing a rare gas, oxygen, and nitrogen. Further, it is preferable that the second oxide semiconductor film  403   b  be a CAAC-OS film. Note that the second oxide semiconductor film  403   b  can be referred to as an N-type oxide semiconductor layer. 
     As the third oxide semiconductor film  403   c , a material film which can be represented as M7 g M8 h M9 i O x  (g is a real number greater than or equal to 0 and less than or equal to 2, h is a real number greater than 0 and less than or equal to 5, i is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used. In this embodiment, an In—Ga—Zn-based oxide semiconductor film which is deposited by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=1:3:2 and has a thickness greater than or equal to 5 nm and less than or equal to 30 nm is used. Note that the third oxide semiconductor film  403   c  can be referred to as a second I-type oxide semiconductor layer. Further, the third oxide semiconductor film  403   c  may have an amorphous structure but is preferably a CAAC-OS film. The third oxide semiconductor film  403   c  is in contact with a source electrode layer and a drain electrode layer, whereby the threshold voltage is determined. 
     With such a stacked-layer structure, a structure in which the second oxide semiconductor film  403   b  through which carriers flow is not in contact with the insulating film containing silicon is obtained. 
     Polycrystalline targets are used as the sputtering targets for forming the first oxide semiconductor film  403   a  and the second oxide semiconductor film  403   b  so that the first oxide semiconductor film  403   a  and the second oxide semiconductor film  403   b  are CAAC-OS films. Further, when a sputtering target having a composition which easily causes crystallization is used for forming the first oxide semiconductor film  403   a , the second oxide semiconductor film  403   b  in contact with the first oxide semiconductor film  403   a  can also be crystallized. The number of levels at an interface between the first oxide semiconductor film  403   a  and the second oxide semiconductor film  403   b  is small, and thus high field-effect mobility can be achieved. It is preferable that the thicknesses and compositions are adjusted so that carriers flow only through the second oxide semiconductor film  403   b.    
     When the insulating film  437  and the first oxide semiconductor film  403   a  are deposited successively without being exposed to the air, impurity contamination of an interface between the insulating film  437  and the first oxide semiconductor film  403   a  can be prevented; when the second oxide semiconductor film  403   b  and the third oxide semiconductor film  403   c  are deposited successively without being exposed to the air, impurity contamination of an interface between the second oxide semiconductor film  403   b  and the third oxide semiconductor film  403   c  can be prevented. The third oxide semiconductor film  403   c  also functions as a protective film that protects the second oxide semiconductor film  403   b  from exposure to the air in etching or the like in a later step. 
     In the case where steps for stacking the first oxide semiconductor film  403   a , the second oxide semiconductor film  403   b , and the third oxide semiconductor film  403   c  in this order are performed successively without exposure to the air, a manufacturing apparatus a top view of which is illustrated in  FIG.  15    may be used. 
     The manufacturing apparatus illustrated in  FIG.  15    is single wafer multi-chamber equipment, which includes three sputtering devices  10   a ,  10   b , and  10   c , a substrate supply chamber  11  provided with three cassette ports  14  for holding a process substrate, load lock chambers  12   a  and  12   b , a transfer chamber  13 , a substrate heating chamber  15 , and the like. Note that a transfer robot for transferring a substrate to be treated is provided in each of the substrate supply chamber  11  and the transfer chamber  13 . The atmospheres of the sputtering devices  10   a ,  10   b , and  10   c , the transfer chamber  13 , and the substrate heating chamber  15  are preferably controlled so as to hardly contain hydrogen and moisture (i.e., as an inert atmosphere, a reduced pressure atmosphere, or a dry air atmosphere). For example, a preferable atmosphere is a dry nitrogen atmosphere in which the dew point of moisture is −40° C. or lower, preferably −50° C. or lower. An example of a procedure of the manufacturing steps with use of the manufacturing apparatus illustrated in  FIG.  15    is as follows. The process substrate is transferred from the substrate supply chamber  11  to the substrate heating chamber  15  through the load lock chamber  12   a  and the transfer chamber  13 ; moisture attached to the process substrate is removed by vacuum baking in the substrate heating chamber  15 ; the process substrate is transferred to the sputtering device  10   c  through the transfer chamber  13 ; and the first oxide semiconductor film  403   a  is deposited in the sputtering device  10   c . Then, the process substrate is transferred to the sputtering device  10   a  through the transfer chamber  13  without exposure to air, and the second oxide semiconductor film  403   b  is deposited in the sputtering device  10   a . Then, the process substrate is transferred to the sputtering device  10   b  through the transfer chamber  13 , and the third oxide semiconductor film  403   c  is deposited in the sputtering device  10   b . If needed, the process substrate is transferred to the substrate heating chamber  15  through the transfer chamber  13  without exposure to air and heat treatment is performed. As described above, with use of the manufacturing apparatus illustrated in  FIG.  15   , a manufacturing process can proceed without exposure to air. Further, with of the sputtering devices in the manufacturing apparatus in  FIG.  15   , a process performed without exposure to the air can be achieved by change of the sputtering target. As the sputtering devices in the manufacturing apparatus in  FIG.  15   , a parallel plate sputtering device, an ion beam sputtering device, a facing-target sputtering device, or the like may be used. In a facing-target type sputtering device, an object surface is separated from plasma and thus damage in deposition is small; therefore, a CAAC-OS film having high crystallinity can be formed. 
     In order that the second oxide semiconductor film  403   b  through which carriers flow is not in contact with the insulating film containing silicon, top and bottom interfaces of the second oxide semiconductor film  403   b  are protected with the first oxide semiconductor film  403   a  and third oxide semiconductor film  403   c  so that an impurity such as silicon does not enter the second oxide semiconductor film  403   b  and the interfaces thereof; accordingly, high field-effect mobility is achieved. 
     After the insulating film  437  and the stack  403  of oxide semiconductor films are formed, a conductive film is formed. This conductive film is selectively etched, so that electrode layers  445   a  and  445   b  and a conductive layer  442  are formed. A cross-sectional view of a structure obtained after this step corresponds to  FIG.  2 D . By performing etching plural times at this time, electrodes which have projecting regions in their bottom edge portions when seen in cross-section are formed. The electrode layers  445   a  and  445   b  having the projecting regions in the bottom edge portions function as a source electrode layer and a drain electrode layer of the transistor. The electrode layer  445   a  is provided on and in contact with the wiring layer  436  and the electrode layer  445   b  is provided on and in contact with the wiring layer  434 . 
     A distance between the electrode layers  445   a  and  445   b  corresponds to a channel length L of the transistor. In order that the channel length L of the transistor is less than 50 nm, for example, approximately 20 nm, it is preferable that a resist be subjected to light exposure using an electron beam and the developed mask be preferably used as an etching mask of the conductive film. At a higher acceleration voltage, an electron beam can provide a more precise pattern. The use of multiple electron beams can shorten the process time per substrate. In an electron beam writing apparatus capable of electron beam irradiation, the acceleration voltage is preferably in the range from 5 kV to 50 kV, for example. The current intensity is preferably in the range from 5×10 −12  A to 1×10 −11  A. The minimum beam size is preferably 2 nm or less. The minimum possible pattern line width is preferably 8 nm or less. Under the above conditions, a pattern with a width of, for example, 30 nm or less, preferably 20 nm or less, more preferably 8 nm or less, can be obtained. 
     The insulating film  402  is provided over the electrode layers  445   a  and  445   b  and the conductive layer  442  and also provided over the stack  403  of oxide semiconductor films. A material of the insulating film  402  can be a silicon oxide film, a gallium oxide film, a gallium oxide zinc film, a Ga 2 O 3  (Gd 2 O 3 ) film, a zinc oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. As another material of the insulating film  402 , an In—Ga—Zn-based oxide film having an insulating property can be given. The In—Ga—Zn-based oxide film having an insulating property may be formed under the following conditions: a sputtering target having an atomic ratio of In:Ga:Zn=1:3:2 is used, the substrate temperature is room temperature, and an argon gas or a mixed gas of argon and oxygen are used as sputtering gases. 
     It is preferable that the insulating film  402  include a region containing oxygen in a proportion higher than that of the stoichiometric composition (an oxygen-excess region). This is because, when an insulating layer in contact with the stack  403  of oxide semiconductor films includes an oxygen-excess region, oxygen can be supplied to the stack  403  of oxide semiconductor films, release of oxygen from the stack  403  of oxide semiconductor films can be prevented, and oxygen vacancies can be filled. In order to provide the oxygen-excess region in the insulating film  402 , the insulating film  402  is formed in an oxygen atmosphere, for example. Alternatively, oxygen may be introduced into the deposited insulating film  402  to provide the oxygen-excess region therein. Further, as illustrated in  FIG.  3 A , the insulating film  402  preferably has a stacked-layer structure of a first insulating film  402   a  and a second insulating film  402   b . The stacked-layer structure is formed in such a manner that, over an insulating film including a region containing excess oxygen (oxygen-excess region), a silicon oxide film or a silicon oxynitride film is formed on a condition where a high frequency power higher than or equal to 0.17 W/cm 2  and lower than or equal to 0.5 W/cm 2 , preferably higher than or equal to 0.26 W/cm 2  and lower than or equal to 0.35 W/cm 2 , is supplied. Specifically, the silicon oxynitride film is formed in conditions where silane (SiH 4 ) and dinitrogen monoxide (N 20 ) are supplied as source gases at 160 sccm and 4000 sccm, respectively; the pressure of a treatment chamber is adjusted to 200 Pa; and a power of 1500 W is supplied with a high-frequency power supply of 27.12 MHz. Further, the substrate temperature at which the silicon oxynitride film is formed is set to 220° C. 
     Next, the insulating film  402  is selectively etched to form an opening reaching the conductive layer  442 . After that, a conductive film is formed and selectively etched, whereby an electrode layer  438  which is electrically connected to the conductive layer  442  is formed and a gate electrode layer  401  is formed over the stack  403  of oxide semiconductor films with the insulating film  402  positioned therebetween. Then, an insulating film  407  functioning as a barrier film is provided to cover the gate electrode layer  401  and the electrode layer  438 . 
     As the insulating film  407 , it is preferable to use a silicon nitride film which is deposited by a plasma CVD method in which a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) is supplied. This silicon nitride film functions as a barrier film and prevents hydrogen or a hydrogen compound from entering an oxide semiconductor layer to be formed later, thereby improving the reliability of the semiconductor device. 
     The gate electrode layer  401  and the electrode layer  438  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. A semiconductor film which is doped with an impurity element such as phosphorus and is typified by a polycrystalline silicon film, or a silicide film of nickel silicide or the like can also be used as the gate electrode layer  401  and the electrode layer  438 . The gate electrode layer  401  and the electrode layer  438  each have either a single-layer structure or a stacked-layer structure. 
     In this embodiment, a tungsten film is used as the gate electrode layer  401  on and in contact with the insulating film  402 . 
     Through the above process, a transistor  415  of this embodiment can be manufactured (see  FIG.  3 A ). The transistor  415  is an example of a dual-gate transistor.  FIG.  3 A  is a cross-sectional view of the transistor  415  in the channel length direction. In the dual-gate transistor  415 , the insulating film  437  serves as a first gate insulating film and the insulating film  402  serves as a second gate insulating film. 
     The conductive layer  491  can function as a second gate electrode layer (also referred to as back gate) for controlling the electrical characteristics of the transistor  415 . For example, by setting the potential of the conductive layer  491  to GND (or a fixed potential), the threshold voltage of the transistor  415  is increased, so that the transistor  415  can be normally off. 
     Further, when the conductive layer  491  and the insulating film  437  are not provided, the top-gate transistor illustrated in  FIG.  1 A  can be manufactured. In the case where the transistor in  FIG.  1 A  is manufactured, the same process can be used except that the insulating film  437  is not provided between the conductive layer  491  and the oxide semiconductor stack and that a nitride insulating film  444  is provided below the oxide insulating film  435 ; therefore, detailed description is omitted there. Alternatively, when the layout is changed, a dual-gate transistor and a top-gate transistor both can be manufactured over the same substrate without change in the number of steps. 
     In the transistor  418  in  FIG.  1 A , the nitride insulating film  444 , the oxide insulating film  435 , the first oxide semiconductor film  403   a , the second oxide semiconductor film  403   b , the third oxide semiconductor film  403   c , the first insulating film  402   a , and the second insulating film  402   b  are stacked in this order over the substrate  400 , and the second oxide semiconductor film  403   b  is separated from the insulating film containing silicon. Further, it is preferable that a silicon nitride film be used as the nitride insulating film  444  and a silicon nitride film be used as the second insulating film  402   b  or a silicon nitride film be used as the insulating film  407 . With such a structure, moisture and hydrogen can be prevented from entering the second oxide semiconductor film  403   b  from the outside; thus the reliability of the transistor is improved. 
       FIG.  3 B  is an example of a top view of the transistor  415 .  FIG.  3 A  is a cross section taken along a chain line X-Y in  FIG.  3 B . 
     Embodiment 2 
     In this embodiment, a structural example in  FIG.  4 A  which is partly different from the structure of  FIG.  1 A  and a manufacturing method thereof are described below. 
     First, over the substrate  400 , the oxide insulating film  435  is formed. The oxide insulating film  435  can be formed by a plasma CVD method, a sputtering method, or the like, using silicon oxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, gallium oxide, gallium oxide zinc, zinc oxide, or a mixed material of any of these materials. The oxide insulating film  435  may have either a single-layer structure or a stacked-layer structure. If needed, a nitride insulating film such as a silicon nitride film may be provided between the substrate  400  and the oxide insulating film  435 . 
     Next, the first oxide semiconductor film  403   a  and the second oxide semiconductor film  403   b  are formed by patterning using the same mask, and then the third oxide semiconductor film  403   c  is formed. Since the third oxide semiconductor film  403   c  is formed using a different mask from the first and second oxide semiconductor films  403   a  and  403   b , the third oxide semiconductor film  403   c  can cover the side surface of the first oxide semiconductor film  403   a  and the side and top surfaces of the second oxide semiconductor film  403   b  as illustrated in  FIG.  4 A . Note that in this embodiment, all of the first to third oxide semiconductor layers have an amorphous structure. However, one embodiment of the present invention is not limited thereto; it is possible that all of the first to third oxide semiconductor layers are CAAC-OS films or at least one of the first to third oxide semiconductor layers has an amorphous structure. 
     Subsequently, a conductive film is formed. This conductive film is selectively etched, so that the electrode layers  445   a  and  445   b  are formed. 
     Then, the insulating film  402  is provided over the electrode layers  445   a  and  445   b , and is also provided over the third oxide semiconductor film  403   c . As illustrated in  FIG.  4 B , since the side surface of the second oxide semiconductor film  403   b  is covered with the third oxide semiconductor film  403   c , the side surface of the second oxide semiconductor film  403   b  is not in contact with the insulating film  402 . 
     Next, a conductive film is formed over the insulating film  402  and selectively etched to form the gate electrode layer  401  over the third oxide semiconductor film  403   c  with the insulating film  402  positioned therebetween. The insulating film  407  functioning as a barrier film is provided so as to cover the gate electrode layer  401 . 
     Through the above process, a transistor  416  illustrated in  FIG.  4 A  can be manufactured.  FIG.  4 C  is a top view. A cross section taken along a chain line B 1 -B 2  in  FIG.  4 C  corresponds to  FIG.  4 A  and a cross section taken along a dotted line B 2 -B 3  in  FIG.  4 C  corresponds to  FIG.  4 B . As illustrated in  FIG.  4 C , the periphery of the third oxide semiconductor film  403   c  is positioned outside the periphery of the second oxide semiconductor film  403   b.    
     This embodiment can be freely combined with Embodiment 1. Portions denoted by the same reference numerals as those of the drawings used in Embodiment 1 can be formed using the same material as those of Embodiment 1. Instead of the stack  403  of oxide semiconductor films described in Embodiment 1, a structure in which the third oxide semiconductor film  403   c  covers the side surface of the first oxide semiconductor film  403   a  and the side and top surfaces of the second oxide semiconductor film  403   b  may be employed. Since the third oxide semiconductor film  403   c  can be provided between the second oxide semiconductor film  403   b  and the electrode layer  445   a , leakage current can be reduced. 
     Embodiment 3 
     In this embodiment, an example of a semiconductor device including the transistor described in Embodiment 1 is described with reference to  FIGS.  5 A and  5 B . 
     The semiconductor device illustrated in  FIGS.  5 A and  5 B  includes transistors  740  and  750  including a first semiconductor material in a lower portion, and a transistor  610  including a second semiconductor material in an upper portion. The transistor  610  has the same structure as the transistor  415  described in Embodiment 1. The same reference numerals are used for the same parts as those in  FIGS.  3 A and  3 B .  FIG.  5 B  is a circuit diagram of the semiconductor device in  FIG.  5 A . 
     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 such as silicon can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor enables charge to be held for a long time owing to its characteristics. 
     As a substrate used in the semiconductor device, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, a silicon on insulator (SOI) substrate, or the like can be used. A channel formation region of the transistor can be formed in or over the semiconductor substrate. The semiconductor device in  FIG.  5 A  is an example in which the channel formation region is formed in the semiconductor substrate to form a lower transistor. 
     In the semiconductor device in  FIG.  5 A , a single crystal silicon substrate is used as a substrate  700 , the transistors  740  and  750  are formed using the single crystal silicon substrate, and single crystal silicon is used as the first semiconductor material. The transistor  740  is an n-channel transistor and the transistor  750  is a p-channel transistor. The transistor  740  and the transistor  750  which are electrically connected to each other form a complementary metal oxide semiconductor (CMOS) circuit  760 . 
     In this embodiment, the single crystal silicon substrate imparting p-type conductivity is used as the substrate  700 ; thus, an n-well is formed by adding an impurity element imparting n-type conductivity to a region in which the p-channel transistor  750  is to be formed. A channel formation region  753  of the transistor  750  is formed in the n-well. As the impurity element imparting n-type conductivity, phosphorus (P), arsenic (As), or the like can be used. 
     Therefore, an impurity element imparting p-type conductivity is not added to a formation region of the transistor  740  that is the n-channel transistor; however, a p-well may be formed by adding an impurity element imparting p-type conductivity. As the impurity element imparting p-type conductivity, boron (B), aluminum (Al), gallium (Ga), or the like may be used. 
     Meanwhile, when a single-crystal silicon substrate imparting n-type conductivity is used, an impurity element imparting p-type conductivity may be added to form a p-well. 
     The transistor  740  includes a channel formation region  743 , an n-type impurity region  744  functioning as a lightly doped drain (LDD) region or an extension region, an n-type impurity region  745  functioning as a source region or a drain region, a gate insulating film  742 , and a gate electrode layer  741 . The n-type impurity region  745  has a higher impurity concentration than the n-type impurity region  744 . The side surface of the gate electrode layer  741  is provided with a sidewall insulating layer  746 . With the use of the gate electrode layer  741  and the sidewall insulating layer  746  as masks, the n-type impurity region  744  and the n-type impurity region  745  which have different impurity concentrations can be formed in a self-aligned manner. 
     The transistor  750  includes the channel formation region  753 , a p-type impurity region  754  functioning as a lightly doped drain (LDD) region or an extension region, a p-type impurity region  755  functioning as a source region or a drain region, a gate insulating film  752 , and a gate electrode layer  751 . The p-type impurity region  755  has a higher impurity concentration than the p-type impurity region  754 . The side surface of the gate electrode layer  751  is provided with a sidewall insulating layer  756 . With the use of the gate electrode layer  751  and the sidewall insulating layer  756  as masks, the p-type impurity region  754  and the p-type impurity region  755  which have different impurity concentrations can be formed in a self-aligned manner. 
     In the substrate  700 , the transistor  740  and the transistor  750  are isolated from each other by an element isolation region  789 . An insulating film  788  and an insulating film  687  are stacked over the transistor  740  and the transistor  750 . A wiring layer  647  in contact with the n-type impurity region  745  through an opening in the insulating film  788  and the insulating film  687  and a wiring layer  657  in contact with the p-type impurity region  755  through an opening in the insulating film  788  and the insulating film  687  are provided over the insulating film  687 . A wiring layer  748  is provided over the insulating film  687  so as to electrically connect the transistor  740  and the transistor  750 . The wiring layer  748  is electrically connected to the n-type impurity region  745  through an opening in the insulating film  788  and the insulating film  687  and reaching the n-type impurity region  745 . Further, the wiring layer  748  is electrically connected to the p-type impurity region  755  through an opening in the insulating film  788  and the insulating film  687  and reaching the p-type impurity region  755 . 
     An insulating film  686  is provided over the insulating film  687 , the wiring layer  647 , the wiring layer  748 , and the wiring layer  657 . A wiring layer  658  is formed over the insulating film  686 . The wiring layer  658  is electrically connected to a gate wiring through an opening in the insulating films  788 ,  687 , and  686 . The gate wiring is formed over the gate insulating film  742  or the gate insulating film  752 . The gate wiring branches into the gate electrode layer  741  and the gate electrode layer  751 . 
     The semiconductor device of this embodiment is not limited to the structure in  FIG.  5 A . As the transistors  740  and  750 , a transistor containing silicide (salicide) or a transistor which does not include a sidewall insulating layer may be used. When a structure that contains silicide (salicide) is used, the resistance of the source region and the drain region can be further lowered and the operation speed of the semiconductor device is increased. Further, the semiconductor device can be operated at low voltage, so that power consumption of the semiconductor device can be reduced. 
     Next, the structures of upper elements provided over the lower transistor in the semiconductor device in  FIGS.  5 A and  5 B  are described. 
     An insulating film  684  is stacked over the insulating film  686  and the wiring layer  658 . The conductive layer  491  and a wiring layer  692  are formed over the insulating film  684 . 
     The oxide insulating film  435  is provided over the conductive layer  491  and the wiring layer  692 . The insulating film  437  is provided over the oxide insulating film  435 . The first oxide semiconductor film  403   a  is provided over the insulating film  437 . The second oxide semiconductor film  403   b  which has a different composition from the first oxide semiconductor film  403   a , and the third oxide semiconductor film  403   c  which has substantially the same composition as the first oxide semiconductor film  403   a  are provided over the first oxide semiconductor film  403   a . Further, the electrode layer  445   a  which has a projecting region in its bottom edge portion and the electrode layer  445   b  which has a projecting region in its bottom edge portion are provided over the third oxide semiconductor film  403   c . The insulating film  402  is provided on and in contact with a region of the second oxide semiconductor film  403   b  which does not overlap with the electrode layer  445   a  or the electrode layer  445   b  (the channel formation region), and the gate electrode layer  401  is provided thereover. 
     A capacitor  690  is provided over the same oxide insulating film  435  as the transistor  610  without an increase in the number of steps. In the capacitor  690 , the electrode layer  445   a  serves as one electrode, a capacitor electrode layer  693  serves as the other electrode, and the insulating film  402  provided therebetween serves as a dielectric. The capacitor electrode layer  693  is formed in the same step as the gate electrode layer  401 . 
     By setting the potential of the conductive layer  491  to GND (or a fixed potential), the conductive layer  491  serves as a back gate which controls the electrical characteristics of the transistor  610 . The conductive layer  491  has a function of preventing static electricity. In the case where the threshold voltage of the transistor  610  is not required to be controlled by the conductive layer  491  in order to make the transistor  610  be a normally-off transistor, the conductive layer  491  is not necessarily provided. In the case where the transistor  610  is used for part of a particular circuit and a problem might be caused by providing the conductive layer  491 , the conductive layer  491  is not necessarily provided in the circuit. 
     The wiring layer  692  is electrically connected to the wiring layer  658  through an opening in the insulating film  684 . In the example in this embodiment, the insulating film  684  is subjected to planarization treatment using a CMP method. 
     In the semiconductor device, the insulating film  684  is provided between the lower portion and the upper portion, and functions as a barrier film to prevent impurities such as hydrogen, which cause deterioration or a change in electrical characteristics of the transistor  610  in the upper portion, from entering the upper portion from the lower portion. Thus, a fine inorganic insulating film (e.g., an aluminum oxide film or a silicon nitride film) having a good property of blocking impurities or the like is preferably used as the insulating film  684 . The insulating film  684  can be formed by using the same material as the insulating film  433  described in Embodiment 1. 
     In the case of using the same manufacturing method as that described in Embodiment 1, the transistor  610  can be manufactured similarly to the transistor  415 . After that insulating film  407  is formed, an interlayer insulating film  485  is formed. Further, a semiconductor device having a multilayer structure in which an embedded wiring is formed in the interlayer insulating film  485  and another semiconductor element, another wiring, or the like is formed above the embedded wiring may be manufactured. 
     This embodiment can be freely combined with Embodiment 1 or Embodiment 2. 
     Embodiment 4 
     As another example of a semiconductor device including the transistor described in Embodiment 1, a cross-sectional view of a NOR circuit, which is a logic circuit, is illustrated in  FIG.  6 A .  FIG.  6 B  is a circuit diagram of the NOR circuit in  FIG.  6 A , and  FIG.  6 C  is a circuit diagram of a NAND circuit. 
     In the NOR circuit illustrated in  FIGS.  6 A and  6 B , p-channel transistors  801  and  802  each have a structure similar to that of the transistor  750  in  FIGS.  5 A and  5 B  in that a single crystal silicon substrate is used for a channel formation region, and n-channel transistors  803  and  804  each have a structure similar to that of the transistor  610  in  FIGS.  5 A and  5 B  and that of the transistor  415  in Embodiment 1 in that an oxide semiconductor film is used for a channel formation region. 
     In the NOR circuit and the NAND circuit illustrated in  FIGS.  6 A and  6 B , a conductive layer  491  for controlling electrical characteristics of the transistors is provided to overlap with gate electrode layers with oxide semiconductor films provided therebetween in the transistors  803  and  804 . By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors  803  and  804  are increased, so that the transistors can be normally off. In the NOR circuit in this embodiment, conductive layers which are provided in the transistors  803  and  804  and can function as back gates are electrically connected to each other. However, the present invention is not limited to the structure, and the conductive layers functioning as back gates may be electrically controlled independently. 
     In the semiconductor device illustrated in  FIG.  6 A , a single crystal silicon substrate is used as a substrate  800 , the transistor  802  is formed using the single crystal silicon substrate, and the transistor  803  in which stacked oxide semiconductor films are used for a channel formation region is formed over the transistor  802 . 
     The gate electrode layer  401  of the transistor  803  is electrically connected to a wiring layer  832 . The wiring layer  832  is electrically connected to a wiring layer  835 . The gate electrode layer  401  of the transistor  803  is electrically connected to an embedded wiring, and the embedded wiring is electrically connected to an electrode layer  842 . Note that the embedded wiring includes a first barrier metal film  486 , a second barrier metal film  488 , and a low-resistance conductive layer  487  surrounded by the first barrier metal film  486  and the second barrier metal film  488 . 
     The embedded wiring is formed in such a manner that a contact hole reaching the electrode layer  842  is formed in the interlayer insulating film  485 , the first barrier metal film  486  is formed, and a copper film or a copper alloy film is formed thereover so as to form the low-resistance conductive layer  487 . Then, polishing is performed for planarization, and the second barrier metal film  488  is formed so as to protect the exposed low-resistance conductive layer  487 . 
     Each of the first barrier metal film  486  and the second barrier metal film  488  may be formed using a conductive material which suppresses diffusion of copper contained in the low-resistance conductive layer  487 . Examples of the conductive material are a tantalum nitride film, a molybdenum nitride film, and a tungsten nitride film. 
     The wiring layer  832  is provided in an opening formed in an insulating film  826  and an insulating film  830 . The wiring layer  835  is provided in an opening formed in an insulating film  833 . The electrode layer  842  is formed over the wiring layer  835 . 
     An electrode layer  825  of the transistor  802  is electrically connected the electrode layer  445   b  of the transistor  803  through wiring layers  831  and  834 . The wiring layer  831  is formed in an opening in the insulating film  830 , and the wiring layer  834  is formed in an opening in the insulating film  833 . The electrode layer  445   a  and the electrode layer  445   b  function as source and drain electrode layers of the transistor  803 . 
     The first oxide semiconductor film  403   a  is formed on and in contact with the insulating film  437 . The third oxide semiconductor film  403   c  is formed on and in contact with the second oxide semiconductor film  403   b . With the insulating film  437  and the insulating film  402 , unnecessary release of oxygen can be suppressed, and the second oxide semiconductor film  403   b  can be kept in an oxygen excess state. Thus, in the transistor  803 , oxygen vacancies in the second oxide semiconductor film  403   b  and at the interface thereof can be filled efficiently. The transistor  804  has a structure and an effect which are similar to those of the transistor  803 . 
     In the NAND circuit in  FIG.  6 C , p-channel transistors  811  and  814  each have a structure similar to that of the transistor  750  in  FIGS.  5 A and  5 B , and n-channel transistors  812  and  813  each have a structure similar to that of the transistor  610  in  FIGS.  5 A and  5 B  in that an oxide semiconductor film is used for a channel formation region. 
     In the NAND circuit illustrated in  FIG.  6 C , conductive layers controlling electrical characteristics of the transistors are provided to overlap with gate electrode layers with oxide semiconductor films provided therebetween in the transistors  812  and  813 . By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors  812  and  813  are increased, so that the transistors can be normally off. In the NAND circuit in this embodiment, the conductive layers which are provided in the transistors  812  and  813  and function as back gates are electrically connected to each other. However, the present invention is not limited to the structure, and the conductive layers functioning as back gates may be electrically controlled independently. 
     By applying a transistor including an oxide semiconductor for a channel formation region and having extremely small off-state current to the semiconductor device in this embodiment, power consumption of the semiconductor device can be sufficiently reduced. 
     A semiconductor device which is miniaturized, is highly integrated, and has stable and excellent electrical characteristics by stacking semiconductor elements including different semiconductor materials and a method for manufacturing the semiconductor device can be provided. 
     The NOR circuit and the NAND circuit including the transistors described in Embodiment 1 are described as examples in this embodiment; however, the present invention is not limited to the circuits, and an AND circuit, an OR circuit, or the like can be formed using the transistors described in Embodiment 1 or 2. For example, a semiconductor device (storage device) in which stored data can be held even when power is not supplied and which has an unlimited number of times of writing with the transistors described in Embodiment 1 or 2 can be manufactured. 
       FIG.  7    is an example of a circuit diagram of a semiconductor device. 
     In  FIG.  7   , a first wiring (a 1st line) is electrically connected to a source electrode layer of a transistor  160 . A second wiring (2nd line) is electrically connected to a drain electrode layer of the transistor  160 . Any of the transistors  750  and  802  described in this embodiment can be used as the transistor  160 . 
     A third wiring (3rd line) is electrically connected to one of a source electrode layer and a drain electrode layer of a transistor  162 , and a fourth wiring (4th line) is electrically connected to a gate electrode layer of the transistor  162 . A gate electrode layer of the transistor  160  and the other of the source electrode and the drain electrode of the transistor  162  are electrically connected to one electrode of the capacitor  164 . A fifth wiring (5th line) and the other electrode of the capacitor  164  are electrically connected to each other. 
     For the transistor  162 , any of the structures of the transistors  415 ,  416 , and  417  described in Embodiment 1 or 2 can be used. 
     The semiconductor device having the circuit configuration in  FIG.  7    utilizes a characteristic in which the potential of the gate electrode layer 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 wiring 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 wiring is supplied to the gate electrode layer of the transistor  160  and to the capacitor  164 . That is, predetermined charge is supplied to the gate electrode layer 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 a low-level charge and a high-level charge) is given. After that, the potential of the fourth wiring 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 layer of the transistor  160  is held (holding). 
     Since the off-state current of the transistor  162  is extremely small, the charge of the gate electrode layer of the transistor  160  is held for a long time. 
     Next, reading of data is described. By supplying an appropriate potential (a reading potential) to the fifth wiring while a predetermined potential (a constant potential) is supplied to the first wiring, the potential of the second wiring varies depending on the amount of charge held in the gate electrode layer of the transistor  160 . This is because in general, when the transistor  160  is an re-channel transistor, an apparent threshold voltage V th_H  in the case where the high-level charge is given to the gate electrode layer of the transistor  160  is lower than an apparent threshold voltage V th_L  in the case where the low-level charge is given to the gate electrode layer of the transistor  160 . Here, an apparent threshold voltage refers to the potential of the fifth wiring, 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 layer of the transistor  160  can be determined. For example, in the case where the 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 wiring. 
     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 regardless of the state of the gate electrode layer of the transistor  160 , that is, a potential smaller than V th_H  may be given to the fifth wiring. Alternatively, a potential at which the transistor  160  is turned on regardless of the state of the gate electrode layer, that is, a potential higher than V th_L  may be given to the fifth wiring. 
       FIG.  8    illustrates another example of one embodiment of the structure of the storage device. 
       FIG.  8    is a perspective view of a storage device. The storage device illustrated in  FIG.  8    includes a plurality of layers of memory cell arrays (memory cell arrays  3400 ( 1 ) to  3400 ( n ) (n is an integer greater than or equal to 2)) each including a plurality of memory cells as memory circuits in the upper portion, and a logic circuit  3004  in the lower portion which is necessary for operating the memory cell arrays  3400 ( 1 ) to  3400 ( n ). 
       FIG.  8    illustrates the logic circuit  3004 , the memory cell array  3400 ( 1 ), and the memory cell array  3400 ( 2 ), in which a memory cell  3170   a  and a memory cell  3170   b  are illustrated as typical examples among the plurality of memory cells included in the memory cell array  3400 ( 1 ) and the memory cell array  3400 ( 2 ). The memory cell  3170   a  and the memory cell  3170   b  can have a configuration similar to the circuit configuration described in this embodiment with reference to  FIG.  7   , for example. 
     A transistor in which a channel formation region is formed in an oxide semiconductor film is used as each transistor included in the memory cells  3170   a  and  3170   b . The structure of the transistor in which the channel formation region is formed in the oxide semiconductor film is the same as the structure described in Embodiment 1; thus, the description of the structure is omitted. 
     The logic circuit  3004  includes a transistor in which a semiconductor material other than an oxide semiconductor is used as a channel formation region. For example, the transistor can be a transistor obtained in such a manner that an element isolation insulating layer is provided on a substrate including a semiconductor material (e.g., silicon) and a region serving as the channel formation region is formed in a region surrounded by the element isolation insulating layer. Note that the transistor may be a transistor obtained in such a manner that the channel formation region is formed in a semiconductor film such as a polycrystalline silicon film formed on an insulating surface or in a silicon film of an SOI substrate. 
     The memory cell arrays  3400 ( 1 ) to  3400 ( n ) and the logic circuit  3004  are stacked with interlayer insulating layers provided therebetween, and electrical connection or the like may be established as appropriate by an electrode or a wiring penetrating the interlayer insulating layers. 
     When a transistor having a channel formation region formed using an oxide semiconductor and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed). 
     Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate; thus, the problem of deterioration of a gate insulating film does not occur. In other words, the semiconductor device according to 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. 
     As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided. 
     This embodiment can be freely combined with any of Embodiment 1, Embodiment 2, and Embodiment 3. 
     Embodiment 5 
     In this embodiment, a central processing unit (CPU) in which at least one of the transistors  415 ,  416 ,  418  described in Embodiment 1 or 2 and the transistors  120 ,  121 ,  122 ,  123 , and  130  described in Embodiment 7 or 8 is provided in part of the CPU is described as an example of a semiconductor device. 
       FIG.  9 A  is a block diagram illustrating a specific structure of a CPU. The CPU illustrated in  FIG.  9 A  includes an arithmetic logic unit (ALU)  1191 , an ALU controller  1192 , an instruction decoder  1193 , an interrupt controller  1194 , a timing controller  1195 , a register  1196 , a register controller  1197 , a bus interface (Bus I/F)  1198 , a rewritable ROM  1199 , and a ROM interface (ROM I/F)  1189  over a substrate  1190 . A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate  1190 . The ROM  1199  and the ROM interface  1189  may each be provided over a separate chip. Obviously, the CPU illustrated in  FIG.  9 A  is only an example in which the structure is simplified, and a variety of structures is applied to an actual CPU depending on the application. 
     An instruction that is input to the CPU through the bus interface  1198  is input to the instruction decoder  1193  and decoded therein, and then, input to the ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195 . 
     The ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195  conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller  1192  generates signals for controlling the operation of the ALU  1191 . While the CPU is executing a program, the interrupt controller  1194  determines an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller  1197  generates an address of the register  1196 , and reads/writes data from/to the register  1196  in accordance with the state of the CPU. 
     The timing controller  1195  generates signals for controlling operation timings of the ALU  1191 , the ALU controller  1192 , the instruction decoder  1193 , the interrupt controller  1194 , and the register controller  1197 . For example, the timing controller  1195  includes an internal clock generator for generating an internal clock signal CLK 2  based on a reference clock signal CLK 1 , and supplies the internal clock signal CLK 2  to the above circuits. 
     In the CPU illustrated in  FIG.  9 A , a memory cell is provided in the register  1196 . As the memory cell of the register  1196 , the memory cell described in Embodiment 4 can be used. 
     In the CPU illustrated in  FIG.  9 A , the register controller  1197  selects operation of holding data in the register  1196  in accordance with an instruction from the ALU  1191 . That is, the register controller  1197  selects whether data is held by a flip-flop or by a capacitor in the memory cell included in the register  1196 . When data holding by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register  1196 . When data holding by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register  1196  can be stopped. 
     The power supply can be stopped by a switching element provided between a memory cell group and a node to which a power supply potential VDD or a power supply potential VSS is supplied, as illustrated in  FIG.  9 B  or  FIG.  9 C . Circuits illustrated in  FIGS.  9 B and  9 C  are described below. 
       FIGS.  9 B and  9 C  each illustrate an example of a memory circuit in which one of the transistors  415 , and  416 , and  418  described in Embodiment 1 or 2 and the transistors  120 ,  121 ,  122 ,  123 , and  130  described in Embodiment 7 or 8 is used as a switching element for controlling supply of power supply potential to memory cells. 
     The storage device illustrated in  FIG.  9 B  includes a switching element  1141  and a memory cell group  1143  including a plurality of memory cells  1142 . Specifically, as each of the memory cells  1142 , the memory cell described in Embodiment 3 can be used. Each of the memory cells  1142  included in the memory cell group  1143  is supplied with the high-level power supply potential VDD via the switching element  1141 . Further, each of the memory cells  1142  included in the memory cell group  1143  is supplied with a potential of a signal IN and the low-level power supply potential VSS. 
     In  FIG.  9 B , any of the transistors  415 ,  416 , and  418  described in Embodiment 1 or 2 is used as the switching element  1141 , and the switching of the transistor is controlled by a signal SigA supplied to a gate electrode layer thereof. 
     Note that  FIG.  9 B  illustrates a configuration in which the switching element  1141  includes only one transistor; however, one embodiment of the present invention is not limited thereto. In the case where the switching element  1141  includes a plurality of transistors which serves as switching elements, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection. 
     Although the switching element  1141  controls the supply of the high-level power supply potential VDD to each of the memory cells  1142  included in the memory cell group  1143  in  FIG.  9 B , the switching element  1141  may control the supply of the low-level power supply potential VSS. 
       FIG.  9 C  illustrates an example of a storage device in which each of the memory cells  1142  included in the memory cell group  1143  is supplied with the low-level power supply potential VSS through the switching element  1141 . The supply of the low-level power supply potential VSS to each of the memory cells  1142  included in the memory cell group  1143  can be controlled by the switching element  1141 . 
     When a switching element is provided between a memory cell group and a node to which the power supply potential VDD or the power supply potential VSS is supplied, data can be held even in the case where an operation of a CPU is temporarily stopped and the supply of the power supply voltage is stopped; accordingly, power consumption can be reduced. Specifically, for example, while a user of a personal computer does not input data to an input device such as a keyboard, the operation of the CPU can be stopped, so that the power consumption can be reduced. 
     Although the CPU is given as an example in this embodiment, the transistor can also be applied to an LSI such as a digital signal processor (DSP), a custom LSI, or a field programmable gate array (FPGA). 
     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 6 
     A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). Examples of the electronic appliances include display devices of televisions, monitors, and the like, lighting devices, desktop personal computers and laptop personal computers, word processors, image reproduction devices which reproduce still images or moving images stored in recording media such as digital versatile discs (DVDs), portable compact disc (CD) players, radio receivers, tape recorders, headphone stereos, stereos, cordless phone handsets, transceivers, portable wireless devices, mobile phones, car phones, portable game machines, calculators, portable information terminals, electronic notebooks, e-book readers, electronic translators, audio input devices, cameras such as still cameras and video cameras, electric shavers, high-frequency heating appliances such as microwave ovens, electric rice cookers, electric washing machines, electric vacuum cleaners, air-conditioning systems such as air conditioners, dishwashers, dish dryers, clothes dryers, futon dryers, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, smoke detectors, radiation counters, and medical equipment such as dialyzers. Further, the examples include industrial equipment such as guide lights, traffic lights, belt conveyors, elevators, escalators, industrial robots, and power storage systems. In addition, oil engines, moving objects driven by electric motors using power from the non-aqueous secondary batteries, and the like are also included in the category of electric appliances. Examples of the moving objects include electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats or ships, submarines, helicopters, aircrafts, rockets, artificial satellites, space probes, planetary probes, spacecrafts, and the like. Specific examples of these electronic devices are illustrated in  FIGS.  10 A to  10 C  and  FIGS.  11 A to  11 C . 
       FIGS.  10 A and  10 B  illustrate a tablet terminal that can be folded in two.  FIG.  10 A  illustrates the tablet terminal which is open (unfolded). The tablet terminal includes a housing  9630 , a display portion  9631   a , a display portion  9631   b , a switch  9034  for switching display modes, a power switch  9035 , a switch  9036  for switching to power-saving mode, a fastener  9033 , and an operation switch  9038 . 
     In such a portable device illustrated in  FIGS.  10 A and  10 B , an SRAM or a DRAM is used as a memory for temporarily storing image data. For example, the semiconductor device described in Embodiment 4 can be used as a memory. The semiconductor device described in the above embodiment employed for the memory element enables writing and reading of data to be performed at high speed, enables data to be retained for a long time, and enables power consumption to be sufficiently reduced. A CPU for performing image processing or arithmetic processing is used in the portable device illustrated in  FIGS.  10 A and  10 B . As the CPU, the CPU described in Embodiment 5 can be used, in which case the CPU described in Embodiment 5 is used, power consumption of the portable device can be reduced. 
     A touch panel region  9632   a  can be provided in a part of the display portion  9631   a , in which data can be input by touching displayed operation keys  9638 . Note that  FIG.  10 A  shows, as an example, that half of the area of the display portion  9631   a  has only a display function and the other half of the area has a touch panel function. However, the structure of the display portion  9631   a  is not limited to this, and all the area of the display portion  9631   a  may have a touch panel function. For example, all the area of the display portion  9631   a  can display keyboard buttons and serve as a touch panel while the display portion  9631   b  can be used as a display screen. 
     Like the display portion  9631   a , part of the display portion  9631   b  can be a touch panel region  9632   b . When a finger, a stylus, or the like touches the place where a button  9639  for switching to keyboard display is displayed in the touch panel, keyboard buttons can be displayed on the display portion  9631   b.    
     Touch input can be performed concurrently on the touch panel regions  9632   a  and  9632   b.    
     The switch  9034  for switching display modes allows switching between a landscape mode and a portrait mode, color display and black-and-white display, and the like. With the switch  9036  for switching to power-saving mode, the luminance of display can be optimized in accordance with the amount of external light at the time when the tablet is in use, which is detected with an optical sensor incorporated in the tablet. The tablet terminal may include another detection device such as a sensor for detecting orientation (e.g., a gyroscope or an acceleration sensor) in addition to the optical sensor. 
     Although  FIG.  10 A  shows the example where the display area of the display portion  9631   a  is the same as that of the display portion  9631   b , there is no particular limitation on the display portions  9631   a  and  9631   b . They may differ in size and/or image quality. For example, one of them may be a display panel that can display higher-definition images than the other. 
       FIG.  10 B  illustrates the tablet terminal which is closed. The tablet terminal includes the housing  9630 , a solar battery  9633 , a charge/discharge control circuit  9634 , a battery  9635 , and a DC-DC converter  9636 . As an example,  FIG.  10 B  illustrates the charge/discharge control circuit  9634  including the battery  9635  and the DC-DC converter  9636 . 
     Since the tablet terminal can be folded in two, the housing  9630  can be closed when the tablet is not in use. Thus, the display portions  9631   a  and  9631   b  can be protected, thereby providing a tablet terminal with high endurance and high reliability for long-term use. 
     The tablet terminal illustrated in  FIGS.  10 A and  10 B  can also have a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing data displayed on the display portion by touch input, a function of controlling processing by various kinds of software (programs), and the like. 
     The solar battery  9633 , which is attached on the surface of the tablet terminal, supplies electric power to a touch panel, a display portion, an image signal processor, and the like. Note that the solar battery  9633  can be provided on one or both surfaces of the housing  9630  and the battery  9635  can be charged efficiently. When a lithium ion battery is used as the battery  9635 , there is an advantage of downsizing or the like. 
     The structure and operation of the charge/discharge control circuit  9634  illustrated in  FIG.  10 B  will be described with reference to a block diagram in  FIG.  10 C .  FIG.  10 C  illustrates the solar battery  9633 , the battery  9635 , the DC-DC converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and the display portion  9631 . The battery  9635 , the DC-DC converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond to the charge/discharge control circuit  9634  illustrated in  FIG.  10 B . 
     First, an example of operation in the case where power is generated by the solar battery  9633  using external light is described. The voltage of power generated by the solar battery  9633  is raised or lowered by the DC-DC converter  9636  so that a voltage for charging the battery  9635  is obtained. When the display portion  9631  is operated with the power from the solar battery  9633 , the switch SW 1  is turned on and the voltage of the power is raised or lowered by the converter  9637  to a voltage needed for operating the display portion  9631 . In addition, when display on the display portion  9631  is not performed, the switch SW 1  is turned off and a switch SW 2  is turned on so that charge of the battery  9635  may be performed. 
     Here, the solar battery  9633  is shown as an example of a power generation means; however, there is no particular limitation on a way of charging the battery  9635 , and the battery  9635  may be charged with another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery  9635  may be charged with a non-contact power transmission module that transmits and receives power wirelessly (without contact) to charge the battery or with a combination of other charging means. 
     In a television set  8000  in  FIG.  11 A , a display portion  8002  is incorporated in a housing  8001 . The display portion  8002  displays an image and a speaker portion  8003  can output sound. 
     A semiconductor display device such as a liquid crystal display device, a light-emitting device in which a light-emitting element such as an organic EL element is provided in each pixel, an electrophoresis display device, a digital micromirror device (DMD), or a plasma display panel (PDP) can be used for the display portion  8002 . 
     The television set  8000  may be provided with a receiver, a modem, and the like. Furthermore, when the television set  8000  is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed. 
     In addition, the television set  8000  may include a CPU for performing information communication or a memory. The memory described in Embodiment 4 or the CPU described in Embodiment 5 can be used in the television set  8000 . 
     In  FIG.  11 A , an air conditioner which includes an indoor unit  8200  and an outdoor unit  8204  is an example of an electric appliance in which the CPU of Embodiment 5 is used. Specifically, the indoor unit  8200  includes a housing  8201 , an air outlet  8202 , a CPU  8203 , and the like. Although the CPU  8203  is provided in the indoor unit  8200  in  FIG.  11 A , the CPU  8203  may be provided in the outdoor unit  8204 . Alternatively, the CPU  8203  may be provided in both the indoor unit  8200  and the outdoor unit  8204 . By using the CPU described in Embodiment 5 as the CPU in the air conditioner, power consumption can be reduced. 
     In  FIG.  11 A , an electric refrigerator-freezer  8300  is an example of an electric appliance which is provided with the CPU formed using an oxide semiconductor. Specifically, the electric refrigerator-freezer  8300  includes a housing  8301 , a door for a refrigerator  8302 , a door for a freezer  8303 , a CPU  8304 , and the like. In  FIG.  11 A , the CPU  8304  is provided in the housing  8301 . When the CPU described in Embodiment 5 is used as the CPU  8304  of the electric refrigerator-freezer  8300 , power consumption of the electric refrigerator-freezer  8300  can be reduced. 
     An example of an electric vehicle which is an example of an electric appliance is described in  FIG.  11 B . An electric vehicle  9700  is equipped with a secondary battery  9701 . The output of power of the non-aqueous secondary battery  9701  is adjusted by a control circuit  9702  and the power is supplied to a driving device  9703 . The control circuit  9702  is controlled by a processing unit  9704  including a ROM, a RAM, a CPU, or the like which is not illustrated. When the CPU described in Embodiment 5 is used as the CPU in the electric vehicle  9700 , power consumption of the electric vehicle  9700  can be reduced. 
     The driving device  9703  includes a DC motor or an AC motor either alone or in combination with an internal-combustion engine. The processing unit  9704  outputs a control signal to the control circuit  9702  based on input data such as data of operation (e.g., acceleration, deceleration, or stop) by a driver or data during driving (e.g., data on an upgrade or a downgrade, or data on a load on a driving wheel) of the electric vehicle  9700 . The control circuit  9702  adjusts the electric energy supplied from the secondary battery  9701  in accordance with the control signal of the processing unit  9704  to control the output of the driving device  9703 . In the case where the AC motor is mounted, although not illustrated, an inverter which converts direct current into alternate current is also incorporated. 
     This embodiment can be implemented in combination with any of the other embodiments, as appropriate. 
     Embodiment 7 
     In this embodiment, one embodiment of a semiconductor device is described with reference to  FIGS.  16 A to  16 C . 
     Note that a cross-sectional view of a transistor  123  illustrated in  FIG.  16 B  corresponds to a structural view taken along a chain line A-B in a top view illustrated in  FIG.  16 A . 
     The transistor  123  in  FIGS.  16 A to  16 C  includes a base insulating layer  133  which is provided over a substrate  100 ; an oxide semiconductor stack  109  which is provided over the base insulating layer  133  and includes at least a channel formation region  103   b , a low-resistance region  104   c , and a low-resistance region  108   c ; a gate insulating layer  102  and a gate electrode layer  101  which are provided over the channel formation region  103   b ; a silicon nitride film  107  over the oxide semiconductor stack  109 , the gate insulating layer  102 , and the gate electrode layer  101 ; and electrode layers  105   a  and  105   b  which are electrically connected to the low-resistance region  104   c  and the low-resistance region  108   c , respectively, through openings provided in the silicon nitride film  107 . The electrode layers  105   a  and  105   b  function as a source electrode layer and a drain electrode layer. 
     In the above structure, the base insulating layer  133  is a stack in which a second base insulating layer  133   b  is provided over a first base insulating layer  133   a . A silicon nitride film is used as the first base insulating layer  133   a , and a silicon oxide film is used as the second base insulating layer  133   b . The gate insulating layer  102  is a stack in which a second gate insulating layer  102   b  is provided over a first gate insulating layer  102   a . A silicon oxide film is used as the first gate insulating layer  102   a , and a silicon nitride film is used as the second gate insulating layer  102   b . Since the oxide semiconductor stack  109  is covered with the silicon nitride film  107 , moisture and hydrogen can be prevented from entering the channel formation region  103   b  from the outside, so that the reliability of the transistor  123  is improved. 
     In the above structure, the oxide semiconductor stack  109  is formed of the following three oxide semiconductor layers: the first oxide semiconductor layer S 1  which includes a first region  104   a , a second region  103   a , and a third region  108   a ; the second oxide semiconductor layer S 2  which includes a fourth region  104   b , the channel formation region  103   b , and a fifth region  108   b ; and the third oxide semiconductor layer S 3  which includes the low-resistance region  104   c , a sixth region  103   c , and the low-resistance region  108   c . The oxide semiconductor layer S 1 , the oxide semiconductor layer S 2 , and the oxide semiconductor layer S 3  are stacked in this order. The three oxide semiconductor layers may be films having a crystalline structure or films having an amorphous structure. 
     Of the three oxide semiconductor layers, the second oxide semiconductor has the smallest thickness. The three oxide semiconductor layers each have a thickness greater than or equal to 5 nm and less than or equal to 40 nm. There is no particular limitation on a material of the second oxide semiconductor layer as long as it is an oxide semiconductor which has higher carrier density and larger conductivity a than those of the other oxide semiconductor layers. 
     For example, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 may be used as the first oxide semiconductor layer S 1 , an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 may be used as the second oxide semiconductor layer S 2 , and an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 may be used as the third oxide semiconductor layer S 3 . In the case of forming these three layers, each layer is preferably deposited by a sputtering method in a mixed atmosphere containing more oxygen than a rare gas, preferably in an oxygen atmosphere (oxygen: 100%), and all of the resulting oxide semiconductor layers can also be referred to as I-type oxide semiconductor layers. The I-type oxide semiconductor layer is a highly purified oxide semiconductor layer that contains impurities other than the main components of the oxide semiconductor layer as little as possible and is an I-type (intrinsic) semiconductor or is close thereto. In such an oxide semiconductor layer, the Fermi level (Ef) is at the same level as the intrinsic Fermi level (Ei). 
     In the case of another combination of the stacked layers, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:3:2 may be used as the first oxide semiconductor layer S 1 , an In—Ga—Zn-based oxide film which is deposited in an nitrogen atmosphere using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 may be used as the second oxide semiconductor layer S 2 , and an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 may be used as the third oxide semiconductor layer S 3 . In forming the second oxide semiconductor layer S 2 , deposition is preferably performed in an atmosphere containing more nitrogen than oxygen, more preferably in a nitrogen atmosphere (nitrogen: 100%), and the resulting second oxide semiconductor layer can also be referred to as an N + -type oxide semiconductor layer. These three layers can be expressed as “an I-type layer, an N + -type layer and an I-type layer are stacked in this order”. 
     In the case of another combination of the stacked layers, a nitrogen-containing In—Ga—Zn-based oxide film which is deposited in a mixed atmosphere of oxygen and nitrogen using a sputtering target having an atomic ratio of In:Ga:Zn=1:3:2 may be used as the first oxide semiconductor layer S 1 ; a nitrogen-containing In—Ga—Zn-based oxide film which is deposited in a nitrogen atmosphere using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 may be used as the second oxide semiconductor layer S 2 ; and a nitrogen-containing In—Ga—Zn-based oxide film which is deposited in a mixed atmosphere of oxygen and nitrogen using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 may be used as the third oxide semiconductor layer S 3 . In forming the first oxide semiconductor layer S 1  and the third oxide semiconductor layer S 3 , deposition is preferably performed by a sputtering method in a mixed atmosphere containing more oxygen than nitrogen, and the resulting first and third oxide semiconductor layers can also be referred to as N − -type oxide semiconductor layers. These three layers can be expressed as “an N − -type layer, an N + -type layer, and an N − -type layer are stacked in this order”. 
     In the case where steps for stacking the three oxide semiconductor layers in this order are performed successively without exposure to the air, a manufacturing apparatus a top view of which is illustrated in  FIG.  15    may be used. 
     As the sputtering devices in the manufacturing apparatus in  FIG.  15   , a parallel plate sputtering device, an ion beam sputtering device, a facing-target sputtering device, or the like may be used. In a facing-target type sputtering device, an object surface is separated from plasma and thus damage in deposition is small; therefore, a CAAC-OS film having high crystallinity can be formed. 
     The low-resistance region  104   c  and the low-resistance region  108   c  are provided in contact with the silicon nitride film  107  and thus have a higher nitrogen concentration and lower resistance than the sixth region  103   c . In addition, in this embodiment, the channel formation region  103   b  has higher conductivity a than the low-resistance region  104   c  and the low-resistance region  108   c.    
     In the above structure, the second region  103   a  is provided between the channel formation region  103   b  and the base insulating layer  133 , and the channel formation region  103   b  is separated from the base insulating layer  133  containing silicon. The second region  103   a  prevents the entry of silicon from the base insulating layer  133 . The sixth region  103   c  is provided between the channel formation region  103   b  and the gate insulating layer  102 , and the channel formation region  103   b  is separated from the gate insulating layer  102  containing silicon. Accordingly, the transistor  123  has a buried channel structure in which the channel formation region  103   b  through which carriers flow is separated from the insulating film containing silicon. 
       FIG.  16 C  is an energy band taken along a line C-C′ in  FIG.  16 B . As illustrated in  FIG.  16 C , the energy level of the bottom of the conduction band in the second oxide semiconductor layer S 2  are lower than those of the bottoms of the conduction band in the first oxide semiconductor layer S 1  and the third oxide semiconductor layer S 3 . 
     In the case where the second region  103   a  is provided, the second region  103   a  prevents impurities such as silicon from entering the channel formation region  103   b , thereby preventing a reduction in the field-effect mobility of the transistor. Further, when the channel formation region  103   b  is formed using an oxide semiconductor having high conductivity a, higher field-effect mobility can be achieved. Furthermore, the sixth region  103   c  provided over the channel formation region  103   b  is depleted, whereby a sufficiently low off-state current can be obtained. 
     The energy band diagram in  FIG.  16 C  is merely an example, and thus this embodiment is not limited thereto. For example, the deposition condition or the sputtering target may be changed during the deposition of the second oxide semiconductor layer S 2  to form a layer S 21  and a layer S 22 , so that an energy band diagram illustrated in  FIG.  17 A  may be obtained. The total thickness of the layer S 21  and the layer S 22  is preferably greater than or equal to 15 nm and less than or equal to 30 nm. The length of the third oxide semiconductor layer S 3  can be a substantial channel length and thus preferably has a larger thickness than the first oxide semiconductor layer S 1  and the second oxide semiconductor layer S 2 . 
     Alternatively, the deposition condition may be changed successively during the deposition of the second oxide semiconductor layer S 2  to form the layer S 21  and the layer S 22 , so that an energy band diagram illustrated in  FIG.  17 B  may be obtained. 
     Further alternatively, the deposition condition may be changed successively during the deposition of the third oxide semiconductor layer S 3  to form a layer S 31  and a layer S 32 , so that an energy band diagram illustrated in  FIG.  17 C  may be obtained. The total thickness of the layer S 31  and the layer S 32  is preferably greater than or equal to 15 nm and less than or equal to 30 nm. 
     Embodiment 8 
     In this embodiment, one embodiment of a semiconductor device and one embodiment of a method for manufacturing the semiconductor device are described with reference to  FIGS.  18 A to  18 D . In this embodiment, an example of a method for manufacturing the transistor  120  is described, in which the low-resistance regions  104   c  and  108   c  are formed by addition of an impurity element (also referred to as a dopant) so as to have lower electrical resistance than the channel formation region  103   b.    
     First, the base insulating layer  133  is formed over the substrate  100 . 
     There is no particular limitation on a substrate that can be used, as long as it has heat resistance high enough to withstand heat treatment performed later. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like or a compound semiconductor substrate of silicon germanium or the like may be used as the substrate  100 . Alternatively, an SOI substrate, a substrate over which a semiconductor element is provided, or the like can be used. Further alternatively, 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. 
     The base insulating layer  133  can be formed using a sputtering method, a molecular beam epitaxy (MBE) method, a chemical vapor deposition (CVD) method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, or the like, as appropriate. When the base insulating layer  133  is formed by a sputtering method, an impurity element such as hydrogen can be reduced. 
     The same material as the insulating film  433  described in Embodiment 1 can be used as a material of the base insulating layer  133 . 
     The base insulating layer  133 , which is in contact with a first oxide semiconductor layer which is formed later, preferably contains oxygen which exceeds at least the stoichiometric composition in the layer (the bulk). For example, in the case where a silicon oxide film is used as the base insulating layer  133 , the composition is SiO (2+α) (α&gt;0). 
     Next, an oxide semiconductor stack is formed over the base insulating layer  133 . 
     As the first oxide semiconductor layer S 1 , a material film which can be represented as M1 a M2 b M3 c O x  (a is a real number greater than or equal to 0 and less than or equal to 2, b is a real number greater than 0 and less than or equal to 5, c is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used. In this embodiment, an In—Ga—Zn-based oxide film which is deposited using a sputtering target having an atomic ratio of In:Ga:Zn=1:3:2 and has a thickness greater than or equal to 5 nm and less than or equal to 40 nm is used. Further, the first oxide semiconductor layer may have an amorphous structure but is preferably a CAAC-OS film. 
     As the second oxide semiconductor layer S 2 , a material film which can be represented as M4 d M5 e M6 f O x , (d is a real number greater than 0 and less than or equal to 5, e is a real number greater than or equal to 0 and less than or equal to 3, f is a real number greater than 0 and less than or equal to 5, and x is an arbitrary positive number) is used. In this embodiment, an In—Ga—Zn-based oxide film is deposited to a thickness greater than or equal to 5 nm and less than or equal to 40 nm by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 in an oxygen atmosphere, a mixed atmosphere containing oxygen and nitrogen, a mixed atmosphere containing a rare gas, oxygen, and nitrogen, or a nitrogen atmosphere. The second oxide semiconductor layer is preferably a CAAC-OS film. 
     As the third oxide semiconductor layer S 3 , a material film which can be represented as M7 g M8 h M9 i O x  (g is a real number greater than or equal to 0 and less than or equal to 2, h is a real number greater than 0 and less than or equal to 5, i is a real number greater than or equal to 0 and less than or equal to 5, and x is an arbitrary real number) is used. In this embodiment, an In—Ga—Zn-based oxide semiconductor film which is deposited by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 and has a thickness greater than or equal to 5 nm and less than or equal to 40 nm is used. The third oxide semiconductor layer may be amorphous but is preferably a CAAC-OS film. The third oxide semiconductor layer is in contact with a source electrode layer and a drain electrode layer, whereby the threshold voltage is determined. 
     With such a stacked-layer structure, the channel formation region that is part of the second oxide semiconductor layer and is to be formed later is not in contact with the insulating film containing silicon. 
     The three oxide semiconductor layers are formed using polycrystalline targets as sputtering targets to be CAAC-OS films. 
     Next, a mask is formed over the three oxide semiconductor layers by a photolithography step, and then part of the three oxide semiconductor layers is etched using the mask, so that a stack of oxide semiconductor layers is formed as illustrated in  FIG.  18 A . After that, the mask is removed. In this stage, to prevent generation of a parasitic channel, a taper angle formed by an end surface of the first oxide semiconductor layer and a surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60°, preferably greater than or equal to 20° and less than or equal to 40°. Further, a taper angle formed by an end surface of the second oxide semiconductor layer and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60°, preferably greater than or equal to 20° and less than or equal to 40°. Moreover, a taper angle formed by an end surface of the third oxide semiconductor layer and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60°, preferably greater than or equal to 20° and less than or equal to 40°. 
     Note that heat treatment for supplying oxygen from the base insulating layer  133  to the second oxide semiconductor layer may be performed either before or after the oxide semiconductor layers are processed into an island shape. Note that it is preferable to perform the heat treatment before the oxide semiconductor layers are processed into an island shape because the amount of oxygen released from the base insulating layer  133  to the outside is small and thus the larger amount of oxygen can be supplied to the second oxide semiconductor layer. 
     Subsequently, the gate insulating layer  102  is formed over the stack of oxide semiconductor layers. 
     As the gate insulating layer  102 , an oxide insulating layer formed of silicon oxide, gallium oxide, aluminum oxide, silicon oxynitride, silicon nitride oxide, hafnium oxide, tantalum oxide, or the like is preferably used. Alternatively, when a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate to which nitrogen is added (HfSi x O y  (x&gt;0, y&gt;0)), hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)), or lanthanum oxide is used for the gate insulating layer  102 , gate leakage current can be reduced. Further, the gate insulating layer  102  may have either a single-layer structure or a stacked-layer structure. In the case where the gate insulating layer  102  has a stacked-layer structure, a silicon nitride film can be used for the gate insulating layer  102  as long as the silicon nitride film not in contact with the third oxide semiconductor layer. 
     The thickness of the gate insulating layer  102  is greater than or equal to 1 nm and less than or equal to 100 nm, and the gate insulating layer  102  can be formed using a sputtering method, an MBE method, a CVD method, a PLD method, an ALD method, or the like as appropriate. The gate insulating layer may be formed using a sputtering apparatus which performs deposition with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target. 
     Like the base insulating layer  133 , the gate insulating layer  102  is in contact with the oxide semiconductor layer. Therefore, a large amount of oxygen, which exceeds at least the stoichiometric composition, is preferably contained in the layer (the bulk). 
     In this embodiment, a 20-nm-thick silicon oxynitride film formed by a CVD method is used as the gate insulating layer  102 . 
     Next, a conductive film is formed over the gate insulating layer  102  and a mask is formed by a photolithography step. Then, part of the conductive film is etched using the mask, whereby the gate electrode layer  101  is formed. 
     The gate electrode layer  101  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  101 . The gate electrode layer  101  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  101  has a stacked-layer structure of the above conductive material and the above metal material. 
     As one layer of the gate electrode layer  101  which is in contact with the gate insulating layer  102 , 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 of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage of the transistor to be positive when used as a gate electrode. Accordingly, what is called a normally off switching element can be provided. 
     The thickness of the gate electrode layer  101  is preferably greater than or equal to 50 nm and less than or equal to 300 nm. In this embodiment, a stack of a 30-nm-thick tantalum nitride and a 200-nm-thick tungsten is formed by a sputtering method. 
     Then, the gate insulating layer  102  is selectively removed using the gate electrode layer  101  as a mask, whereby part of the three oxide semiconductor layers is exposed. At this stage, a structure illustrated in  FIG.  18 A  can be obtained. 
     Next, an impurity element for reducing resistance is introduced into the oxide semiconductor layers using the gate electrode layer  101  as a mask, whereby low-resistance regions are formed in regions that do not overlap with the gate electrode layer. As the method for adding the impurity element, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like can be used. 
     Phosphorus, boron, nitrogen, arsenic, argon, aluminum, a molecular ion containing any of the above element, or the like can be used as the impurity element to be introduced. The dosage of such an element is preferably 1×10 13  ions/cm 2  to 5×10 16  ions/cm 2 . When phosphorus is introduced as the impurity element, the acceleration voltage is preferably 0.5 kV to 80 kV. 
     Note that the treatment for introducing the impurity element into the three oxide semiconductor layers may be performed plural times. In the case where the treatment for introducing the impurity element into the three oxide semiconductor layers is performed plural times, the impurity element may be the same in all of the plural times of treatment or may differ between the plural times of treatment. 
     Here, since the impurity element is introduced into the three oxide semiconductor layers, the resistance of each of the three oxide semiconductor layers can be reduced and part of the three oxide semiconductor layers can be amorphized, whereby nitrogen easily diffuses into the uppermost oxide semiconductor layer at the time of forming a silicon nitride film to be formed later, and the resistance of the low-resistance regions can be further reduced. 
     Although depending on conditions at the time of introducing the impurity element or the thickness of each of the three oxide semiconductor layers, the low-resistance regions are formed at least in regions of the third oxide semiconductor layer S 3  which do not overlap with the gate electrode layer. Further, the impurity element can be introduced into regions of the second oxide semiconductor layer S 2  which do not overlap with the gate electrode layer and can be introduced into regions of the first oxide semiconductor layer S 1  which do not overlap with the gate electrode layer. In this embodiment, the impurity element is introduced into the regions of the second oxide semiconductor layer S 2  which do not overlap with the gate electrode layer and the regions of the first oxide semiconductor layer S 1  which do not overlap with the gate electrode layer. At this stage, a structure illustrated in  FIG.  18 B  is obtained. 
     Next, the silicon nitride film  107  which covers the gate electrode layer  101  and is on and in contact with the third oxide semiconductor layer is formed. The silicon nitride film  107  is preferably formed using a silicon nitride film which is deposited by a plasma CVD method in which a mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) is supplied. The silicon nitride film also functions as a barrier film and prevents hydrogen or a hydrogen compound from entering an oxide semiconductor layer to be formed later, thereby improving the reliability of the semiconductor device. The silicon nitride film  107  may be formed by a sputtering method in a nitrogen atmosphere. Nitrogen is introduced into regions in the vicinity of the surface of the oxide semiconductor layer in contact with the silicon nitride film  107 , whereby the resistance of the regions is reduced. 
     Through the above process, the oxide semiconductor stack  109  that is formed of the first oxide semiconductor layer S 1  including the first region  104   a , the second region  103   a , and the third region  108   a ; the second oxide semiconductor layer S 2  including the fourth region  104   b , the channel formation region  103   b , and the fifth region  108   b ; and the third oxide semiconductor layer S 3  including the low-resistance region  104   c , the sixth region  103   c , and the low-resistance region  108   c  can be formed. 
     Since the impurity element is added, the low-resistance region  104   c  and the low-resistance region  108   c  have an amorphous structure. In addition, since nitrogen diffuses into the low-resistance region  104   c  and the low-resistance region  108   c , the low-resistance region  104   c  and the low-resistance region  108   c  contain a larger amount of nitrogen than the sixth region  103   c . Furthermore, the first region  104   a  and the third region  108   a , which are the regions of the first oxide semiconductor layer S 1  that do not overlap with the gate electrode layer, have an amorphous structure because the impurity element is added thereto. 
     Next, regions of the silicon nitride film  107  which overlap with the low-resistance region  104   c  and the low-resistance region  108   c  are partly etched, so that openings which reach the low-resistance region  104   c  and the low-resistance region  108   c  are formed. The openings are formed by selective etching using a mask or the like. Dry etching, wet etching, or both wet etching and dry etching can be used to form the openings. There is no particular limitation on the shapes of the openings as long as the openings reach the low-resistance region  104   c  and the low-resistance region  108   c.    
     Then, a conductive film is formed in and over the openings. After that, a mask is formed by a photolithography step and the conductive film is partly etched using the mask, so that the electrode layers  105   a  and  105   b  are formed (see  FIG.  18 D ). The electrode layers  105   a  and  105   b  can be formed using a material and a method similar to those used for the gate electrode layer  401 . In this embodiment, a tungsten film is used as the conductive film. 
     Through the above process, the transistor  120  can be manufactured. A top view of the transistor  120  is to the same as  FIG.  16 A  and a cross-sectional view taken along a chain line A-B in  FIG.  16 A  corresponds to  FIG.  18 D . 
     The channel formation region  103   b , the second region  103   a , and the sixth region  103   c  of the transistor  120  remain CAAC-OS films because nitrogen is not added thereto; accordingly, a highly reliable semiconductor device can be obtained. 
     The structure of the semiconductor device of this embodiment is not limited to the one illustrated in  FIG.  18 D . Alternatively, a structure of a transistor illustrated in  FIG.  19 D ,  FIG.  20 E ,  FIG.  21 A , or  FIG.  21 B  may be employed. 
     The transistor  122  illustrated in  FIG.  19 D  has a structure in which sidewall insulating layers (also referred to as sidewalls) are provided on the side surface of the gate electrode layer  101 . A method for manufacturing the transistor  122  is described below. 
     The process up to the step in  FIG.  18 A  is the same in the method; therefore, the description thereof is omitted here.  FIG.  19 A  has the same structure as  FIG.  18 A . 
     After the same state as  FIG.  19 A  is obtained, the silicon nitride film  107  is formed. By forming the silicon nitride film  107 , the low-resistance region  104   c  and the low-resistance region  108   c  are formed. A state at this stage corresponds to  FIG.  19 B . 
     Next, the silicon nitride film  107  is partly etched, so that sidewall insulating layers  113   a  and  113   b  are formed. 
     Then, the impurity element for reducing resistance is introduced into the oxide semiconductor layer using the gate electrode layer  101  and the sidewall insulating layers  113   a  and  113   b  as masks. A state at this stage corresponds to  FIG.  19 C . 
     Note that a seventh region  106   a  which overlaps with the sidewall insulating layer  113   a  and an eighth region  106   b  which overlaps with the sidewall insulating layer  113   b  are formed in the third oxide semiconductor layer. The seventh region  106   a  and the eighth region  106   b  contain a larger amount of nitrogen than the sixth region  103   c . Further, since phosphorus or boron is added to the low-resistance region  104   c  and the low-resistance region  108   c  after the formation of the sidewall insulating layers  113   a  and  113   b , the low-resistance region  104   c  and the low-resistance region  108   c  have lower resistance than the seventh region  106   a  and the eighth region  106   b  to which phosphorus or boron is not added. 
     Next, a conductive film is formed and then a mask is formed by a photolithography step. After that, the conductive film is partly etched using the mask, whereby the electrode layers  105   a  and  105   b  are formed. Areas where the electrode layers  105   a  and  105   b  are in contact with the oxide semiconductor layer are large; therefore, the resistance can be reduced, so that the operation speed of the semiconductor device can be increased. 
     Then, to reduce surface roughness due to the transistor  122 , an interlayer insulating layer  111  serving as a planarization film is provided. The interlayer insulating layer  111  can be formed using an organic material such as a polyimide resin or an acrylic resin. Other than such an organic material, it is also possible to use a low dielectric constant material (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. 
     Through the above process, the transistor  122  illustrated in  FIG.  19 D  can be manufactured. 
     A top view of the transistor  122  is illustrated in  FIG.  20 A . A cross section taken along a chain line C-D in  FIG.  20 A  corresponds to  FIG.  19 D . In addition, a cross section taken along a chain line E-F in  FIG.  20 A  is illustrated in  FIG.  20 B . 
     As illustrated in  FIG.  20 B , an end surface of the second region  103   a  is covered with the gate insulating layer  102 . A taper angle formed by the end surface of the second region  103   a  and a surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60° and preferably greater than or equal to 20° and less than or equal to 40°. Similarly, an end surface of the channel formation region  103   b  is covered with the gate insulating layer  102 . A taper angle formed by the end surface of the channel formation region  103   b  and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60° and preferably greater than or equal to 20° and less than or equal to 40°. Similarly, an end surface of the sixth region  103   c  is covered with the gate insulating layer  102 . A taper angle formed by the end surface of the sixth region  103   c  and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60° and preferably greater than or equal to 20° and less than or equal to 40°. In this manner, the use of the structure in which at least the end surface of the channel formation region  103   b  is covered with the gate insulating layer  102  and the taper angle formed by the end surface of each oxide semiconductor layer and the surface of the base insulating layer  133  is greater than or equal to 20° and less than or equal to 40° can prevent generation of a parasitic channel. 
     To reduce leakage, a structure of a transistor illustrated in  FIG.  20 E  may be employed. A transistor  124  illustrated in  FIG.  20 E  is a structural example partly different from the structure in  FIG.  19 D . The transistor  124  in  FIG.  20 E  has a structure in which the side surface of the first oxide semiconductor layer and the side and top surfaces of the second oxide semiconductor layer are covered with the third oxide semiconductor layer. The third oxide semiconductor layer has a larger planar area than the second oxide semiconductor layer and the first oxide semiconductor layer.  FIG.  20 C  is a top view of the transistor  124 , in which the peripheries of the second and first oxide semiconductor layers are represented by a chain line inside the periphery of the third oxide semiconductor layer. A cross section taken along a chain line G-H and a cross section taken along a dotted line K-J in  FIG.  20 C  correspond to  FIG.  20 E  and  FIG.  20 D , respectively. 
     The transistor  124  in  FIG.  20 D  is formed in such a manner that the base insulating layer  133  is formed over the substrate  100 , the first oxide semiconductor layer and the second oxide semiconductor layer are formed by patterning using one mask, and then the third oxide semiconductor layer is formed. By forming the first and second oxide semiconductor layers using a different mask from the third oxide semiconductor layer, the structure in which the side surface of the first oxide semiconductor layer and the side and top surfaces of the second oxide semiconductor layer are covered with the third oxide semiconductor layer can be obtained as illustrated in  FIG.  20 D . Such a structure enables a reduction in leakage current which is generated between the electrode layer  105   a  and the electrode layer  105   b.    
     Also in the transistor  124 , a taper angle formed by an end surface of the second region  103   a  and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60° and preferably greater than or equal to 20° and less than or equal to 40°. Similarly, a taper angle formed by an end surface of the channel formation region  103   b  and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60° and preferably greater than or equal to 20° and less than or equal to 40°. Further, a taper angle formed by an end surface of the sixth region  103   c  and the surface of the base insulating layer  133  is greater than or equal to 10° and less than or equal to 60° and preferably greater than or equal to 20° and less than or equal to 40°. 
     The transistor  121  illustrated in  FIG.  21 A  is a structural example partly different from the structure in  FIG.  18 D . The transistor  121  in  FIG.  21 A  has a structure in which the side surface of the first oxide semiconductor layer and the side and top surfaces of the second oxide semiconductor layer are covered with the third oxide semiconductor layer. 
     The transistor  121  in  FIG.  21 A  is formed in such a manner that the base insulating layer  133  is formed over the substrate  100 , the first oxide semiconductor layer and the second oxide semiconductor layer are formed by patterning using one mask, and then the third oxide semiconductor layer is formed. By forming the first and second oxide semiconductor layers using a different mask from the third oxide semiconductor layer, the structure in which the side surface of the first oxide semiconductor layer and the side and top surfaces of the second oxide semiconductor layer are covered with the third oxide semiconductor layer can be obtained as illustrated in  FIG.  21 A . 
     Then, the gate insulating layer  102  is formed, and the gate electrode layer  101  is formed. After an impurity element is added, the silicon nitride film  107  is formed. Subsequently, openings are formed in the silicon nitride film and a conductive film is formed. After the conductive film is formed in and over the openings, a mask is formed by a photolithography step. Then, the conductive film is partly etched using the mask, so that the electrode layers  105   a  and  105   b  functioning as a source electrode layer and a drain electrode layer are formed. 
     Through the above process, the transistor  121  illustrated in  FIG.  21 A  can be manufactured. 
     A transistor  130  illustrated in  FIG.  21 B  is formed in such a manner that after the three oxide semiconductor layers are formed, the electrode layers  105   a  and  105   b  are formed; the gate electrode layer  101  is formed; and then the impurity element is added using the electrode layers  105   a  and  105   b  and the gate electrode layer  101  as masks. The transistor  130  has substantially the same structure as the transistor in  FIG.  18 D  except for formation order of the electrode layers  105   a  and  105   b  and the gate electrode layer  101 . 
     A method for manufacturing the transistor  130  is described below. 
     First, the base insulating layer  133  is formed over the substrate  100 . Next, the first oxide semiconductor layer S 1 , the second oxide semiconductor layer S 2 , and the third oxide semiconductor layer S 3  are formed in this order. 
     As the first oxide semiconductor layer S 1 , an In—Ga—Zn-based oxide film which is deposited in an oxygen atmosphere (oxygen: 100%) by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=1:1:1 is used. 
     As the second oxide semiconductor layer S 2 , an In—Ga—Zn-based oxide film which is formed in an oxygen atmosphere (oxygen: 100%) by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=3:1:2 is used. 
     As the third oxide semiconductor layer S 3 , an In—Ga—Zn-based oxide film which is formed in an oxygen atmosphere (oxygen: 100%) by a sputtering method using a sputtering target having an atomic ratio of In:Ga:Zn=1:3:2 is used. 
     Next, a mask is formed by a photolithography step, and then the three oxide semiconductor layers are partly etched using the mask, whereby a stack of oxide semiconductor layers is formed. 
     Then, a conductive film covering the stack of oxide semiconductor layers is formed. After that, a mask is formed by a photolithography step and the conductive film is partly etched using the mask, so that the electrode layers  105   a  and  105   b  are formed. 
     Subsequently, an insulating film is formed to cover the electrode layers  105   a  and  105   b , and then a conductive film is formed. After a mask is formed by a photolithography step, the conductive film is partly etched using the mask, whereby the gate electrode layer  101  is formed. Then, regions of the insulating film which do not overlap with the gate electrode layer  101  are etched using the same mask, so that the gate insulating layer  102  is formed. 
     Next, the impurity element for reducing resistance is introduced into at least the third oxide semiconductor layer using the gate electrode layer  101  and the electrode layers  105   a  and  105   b  as masks, whereby the low-resistance regions  104   c  and  108   c  are formed in regions which do not overlap with the gate electrode layer and the electrode layers  105   a  and  105   b.    
     Then, the silicon nitride film  107  which covers the gate electrode layer  101  and is on and in contact with the low-resistance regions  104   c  and  108   c  is formed. 
     Through the above process, the transistor  130  which includes the oxide semiconductor stack  109  that is formed of the first oxide semiconductor layer S 1  including the first region  104   a , the second region  103   a , and the third region  108   a ; the second oxide semiconductor layer S 2  including the fourth region  104   b , the channel formation region  103   b , and the fifth region  108   b ; and the third oxide semiconductor layer S 3  including the low-resistance region  104   c , the sixth region  103   c , and the low-resistance region  108   c  can be formed. 
     Note that in the case where a conductive film which can function as a back gate is provided below the oxide semiconductor stack  109  of the transistor  130 , the conductive film may be provided between the substrate  100  and the base insulating layer  133 . In such a case, the base insulating layer  133  is preferably subjected to planarization treatment by chemical mechanical polishing (CMP). 
     This embodiment can be freely combined with any of other embodiments. 
     Embodiment 9 
     As another example of a semiconductor device including the transistor described in Embodiment 7, a cross-sectional view of a NOR circuit, which is a logic circuit, is illustrated in  FIG.  23 A .  FIG.  23 B  is a circuit diagram of the NOR circuit in  FIG.  23 A , and  FIG.  23 C  is a circuit diagram of a NAND circuit. 
     In the NOR circuit illustrated in  FIGS.  23 A and  23 B , the p-channel transistors  801  and  802  each have a structure similar to that of the transistor  750  in  FIGS.  22 A and  22 B  in that a single crystal silicon substrate is used for a channel formation region, and the n-channel transistors  803  and  804  each have a structure similar to that of the transistor  610  in  FIGS.  22 A and  22 B  and that of the transistor  130  in Embodiment 7 in that an oxide semiconductor film is used for a channel formation region. 
     In the NOR circuit and the NAND circuit illustrated in  FIGS.  23 A and  23 B , a conductive layer  191  for controlling electrical characteristics of the transistors is provided to overlap with gate electrode layers with oxide semiconductor films provided therebetween in the transistors  803  and  804 . By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors  803  and  804  are increased, so that the transistors can be normally off. In the NOR circuit in this embodiment, conductive layers which are provided in the transistors  803  and  804  and can function as back gates are electrically connected to each other. However, the present invention is not limited to the structure, and the conductive layers functioning as back gates may be electrically controlled independently. 
     In the semiconductor device illustrated in  FIG.  23 A , a single crystal silicon substrate is used as the substrate  800 , the transistor  802  is formed using the single crystal silicon substrate, and the transistor  803  in which stacked oxide semiconductor films are used for a channel formation region is formed over the transistor  802 . 
     The gate electrode layer  101  of the transistor  803  is electrically connected to the wiring layer  832 . The wiring layer  832  is electrically connected to the wiring layer  835 . The gate electrode layer  101  of the transistor  803  is electrically connected to an embedded wiring, and the embedded wiring is electrically connected to the electrode layer  842 . Note that the embedded wiring includes a first barrier metal film  186 , a second barrier metal film  188 , and a low-resistance conductive layer  187  surrounded by the first barrier metal film  186  and the second barrier metal film  188 . 
     The embedded wiring is formed in such a manner that a contact hole reaching the electrode layer  842  is formed in an interlayer insulating film  185 , the first barrier metal film  186  is formed, and a copper film or a copper alloy film is formed thereover so as to form the low-resistance conductive layer  187 . Then, polishing is performed for planarization, and the second barrier metal film  188  is formed so as to protect the exposed low-resistance conductive layer  187 . 
     Each of the first barrier metal film  186  and the second barrier metal film  188  may be formed using a conductive material which suppresses diffusion of copper contained in the low-resistance conductive layer  187 . Examples of the conductive material are a tantalum nitride film, a molybdenum nitride film, and a tungsten nitride film. 
     The wiring layer  832  is provided in an opening formed in an insulating film  826  and an insulating film  830 . The wiring layer  835  is provided in an opening formed in an insulating film  833 . The electrode layer  842  is formed over the wiring layer  835 . 
     The electrode layer  825  of the transistor  802  is electrically connected the electrode layer  105   b  of the transistor  803  through the wiring layers  831  and  834 . The wiring layer  831  is formed in an opening in the insulating film  830 , and the wiring layer  834  is formed in an opening in the insulating film  833 . The electrode layer  105   a  and the electrode layer  105   b  function as source and drain electrode layers of the transistor  803 . 
     Three oxide semiconductor layers are formed on and in contact with the insulating film  137 . With the insulating film  137  and the gate insulating layer  102 , unnecessary release of oxygen can be suppressed, and the channel formation region  103   b  can be kept in an oxygen excess state. Thus, in the transistor  803 , oxygen vacancies in the channel formation region and at the interface thereof can be filled efficiently. The transistor  804  has a structure and an effect which are similar to those of the transistor  803 . 
     In the NAND circuit in  FIG.  23 C , the p-channel transistors  811  and  814  each have a structure similar to that of the transistor  750  in  FIGS.  22 A and  22 B , and the n-channel transistors  812  and  813  each have a structure similar to that of the transistor  610  in  FIGS.  22 A and  22 B  in that an oxide semiconductor film is used for a channel formation region. 
     In the NAND circuit illustrated in  FIG.  23 C , conductive layers controlling electrical characteristics of the transistors are provided to overlap with gate electrode layers with oxide semiconductor films provided therebetween in the transistors  812  and  813 . By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors  812  and  813  are increased, so that the transistors can be normally off. In the NAND circuit in this embodiment, the conductive layers which are provided in the transistors  812  and  813  and function as back gates are electrically connected to each other. However, the present invention is not limited to the structure, and the conductive layers functioning as back gates may be electrically controlled independently. 
     By applying a transistor including an oxide semiconductor for a channel formation region and having extremely small off-state current to the semiconductor device in this embodiment, power consumption of the semiconductor device can be sufficiently reduced. 
     A semiconductor device which is miniaturized, is highly integrated, and has stable and excellent electrical characteristics by stacking semiconductor elements including different semiconductor materials and a method for manufacturing the semiconductor device can be provided. 
     The NOR circuit and the NAND circuit including the transistors described in Embodiment 7 are described as examples in this embodiment; however, the present invention is not limited to the circuits, and an AND circuit, an OR circuit, or the like can be formed using the transistors described in Embodiment 7 or 8. For example, a semiconductor device (storage device) in which stored data can be held even when power is not supplied and which has an unlimited number of times of writing with the transistors described in Embodiment 7 or 8 can be manufactured. 
     This application is based on Japanese Patent Application serial no. 2012-136438 filed with Japan Patent Office on Jun. 15, 2012 and Japanese Patent Application serial no. 2012-141373 filed with Japan Patent Office on Jun. 22, 2012, the entire contents of which are hereby incorporated by reference.