Patent Publication Number: US-9905516-B2

Title: Semiconductor device and method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a semiconductor device and a manufacturing method thereof. 
     Note that a semiconductor device refers to a device which can function by utilizing semiconductor characteristics; an electro-optical device, a semiconductor circuit, and an electronic appliance are all included in the category of the semiconductor device. 
     2. Description of the Related Art 
     In the case of forming a thin film transistor (hereinafter also simply referred to as a transistor) which is used for a semiconductor device, an insulating layer formed using a silicon oxide film, a silicon nitride film, or the like is provided over the thin film transistor so as to prevent corrosion of a wiring portion and change in electrical characteristics of the thin film transistor which are caused by permeation of external moisture (for example, see Patent Document 1). 
     Note that “moisture” in this specification is used as a generic term covering water, water vapor, water molecules, hydrogen molecules, and hydrogen atoms. Note that the expression “moisture permeability is low,” for example, is construed that “permeability to one or more of water, water vapor, water molecules, hydrogen molecules, and hydrogen atoms is low.” 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2003-59939 
       
    
     SUMMARY OF THE INVENTION 
     As one of methods for achieving an enhanced effect of preventing moisture permeation into a thin film transistor, there is a method in which a crystalline metal oxide layer having low moisture permeability is placed in contact with an insulating layer. 
     In a crystalline metal oxide layer, however, interfaces between crystal grains (also referred to as grain boundaries), at which atomic arrangement in crystal lattices is irregular or less continuous, have comparatively wide grid intervals as compared to crystal grain portions. For this reason, while the crystal grain portion has extremely low moisture permeability, the crystal grain boundary portion relatively easily transmits moisture as compared to the crystal grain portion. 
     In view of the above problem, an object of one embodiment of the present invention disclosed in this specification is to provide a semiconductor device in which degradation of electrical characteristics due to moisture is suppressed. Another object is to provide a method for manufacturing the semiconductor device. 
     The following structure is employed: a metal oxide layer is placed in contact with an insulating layer covering a thin film transistor, and the metal oxide layer includes a first metal oxide layer having an amorphous structure and a second metal oxide layer having a polycrystalline (also referred to as polycrystal) structure. 
     In the first metal oxide layer having an amorphous structure, while moisture transmits more easily than in a metal oxide layer in a crystalline state, moisture permeation through crystal grain boundary portions does not occur because there are no crystal grain boundaries; moreover, moisture is readily trapped between lattices. 
     Further, in the second metal oxide layer having a polycrystalline structure, while moisture permeation occurs in crystal grain boundary portions a little more easily than in crystal parts, the crystal parts have dense structures and extremely low moisture permeability. 
     Thus, when the metal oxide layer includes the first metal oxide layer having an amorphous structure and the second metal oxide layer having a polycrystalline structure, the second metal oxide layer can prevent moisture permeation and the first metal oxide layer can prevent permeation of moisture which enters through the crystal grain boundary portions of the second metal oxide layer (or can take in the moisture). In addition, formation of the metal oxide layer in contact with the insulating layer can further prevent moisture permeation. Therefore, moisture permeation into the thin film transistor can be effectively prevented. 
     Therefore, one embodiment of the present invention is a semiconductor device which includes a transistor, an insulating layer over the transistor, and a metal oxide layer in contact with the insulating layer. The transistor includes a semiconductor layer, a gate insulating layer, a gate electrode, and a pair of electrodes functioning as a source electrode and a drain electrode. The metal oxide layer at least includes a first metal oxide layer having an amorphous structure and a second metal oxide layer having a polycrystalline structure. 
     With the structure of one embodiment of the present invention, the insulating layer and the metal oxide layer including the first metal oxide layer and the second metal oxide layer can effectively prevent moisture permeation into a thin film transistor. 
     Note that the insulating layer over the transistor, the first metal oxide layer, and the second metal oxide layer preferably have either of the following structures. 
     In the first structure, the insulating layer is placed over the transistor, the first metal oxide layer is placed over the insulating layer, and the second metal oxide layer is placed over the first metal oxide layer. In this structure, permeation of external moisture is blocked by the second metal oxide layer first. Then, the moisture entering through portions which transmit moisture relatively easily such as the crystal grain boundary portions of the second metal oxide layer is blocked by the first metal oxide layer or trapped in the first metal oxide layer. Moreover, permeation of a minute amount of moisture through the first metal oxide layer is blocked by the insulating layer. Thus, moisture outside the second metal oxide layer can be effectively prevented from reaching the transistor. 
     In the second structure, the first metal oxide layer is placed over the transistor, the second metal oxide layer is placed over the first metal oxide layer, and the insulating layer is placed over the second metal oxide layer. With this structure, even if there are not only permeation of external moisture but also moisture taken in the insulating layer at the time of formation of the insulating layer, or moisture adsorbed at an interface between the second metal oxide layer and the insulating layer, since the first metal oxide layer and the second metal oxide layer are placed between the transistor and the insulating layer, the moisture taken in the insulating layer or the moisture adsorbed at the interface between the second metal oxide layer and the insulating layer can be effectively prevented from reaching the transistor. 
     In addition, when the semiconductor layer is a film including an oxide semiconductor material, the transistor can have high mobility. 
     Another embodiment of the present invention is a method for manufacturing a semiconductor device, which includes the following steps: a transistor including a semiconductor layer, a gate insulating layer, a gate electrode, and a pair of electrodes functioning as a source electrode and a drain electrode is formed; an insulating layer is formed over the transistor; a first metal oxide layer having an amorphous structure is formed over the insulating layer; and a second metal oxide layer having a polycrystalline structure is formed over the first metal oxide layer. 
     When the manufacturing method of one embodiment of the present invention is employed, a semiconductor device which is highly effective in preventing moisture permeation into a transistor can be manufactured. 
     A further embodiment of the present invention is a method for manufacturing a semiconductor device, which includes the following steps: a transistor including a semiconductor layer, a gate insulating layer, a gate electrode, and a pair of electrodes functioning as a source electrode and a drain electrode is formed; a first metal oxide layer having an amorphous structure is formed over the transistor; a second metal oxide layer having a polycrystalline structure is formed over the first metal oxide layer; and an insulating layer is formed over the second metal oxide layer. 
     When the manufacturing method of one embodiment of the present invention is employed, a semiconductor device can be provided which is highly effective in preventing not only permeation of external moisture but also permeation of the moisture taken in the insulating layer and the moisture adsorbed at (or taken in) the interface between the second metal oxide layer and the insulating layer. 
     Note that the first metal oxide layer and the second metal oxide layer are preferably formed successively in the same apparatus, in which case entry of moisture into the first metal oxide layer and the second metal oxide layer and attachment of moisture to an interface between the first metal oxide layer and the second metal oxide layer can be suppressed. 
     Further, when the first metal oxide layer and the second metal oxide layer are formed using the same target, an increase in takt time due to transfer of a substrate from a deposition chamber to another can be suppressed. In addition, only one kind of target is needed for the formation of the metal oxide layer, so that an increase in manufacturing cost can be suppressed. 
     Moreover, when a film including an oxide semiconductor material is formed as the semiconductor layer, the transistor can have high mobility. 
     When the metal oxide layer includes the first metal oxide layer having an amorphous structure and the second metal oxide layer having a polycrystalline structure, the second metal oxide layer can prevent moisture permeation and the first metal oxide layer can prevent permeation of moisture which enters through the crystal grain boundary portions of the second metal oxide layer (or can take in the moisture). In addition, formation of the metal oxide layer in contact with the insulating layer can further prevent moisture permeation. Thus, a semiconductor device in which degradation of electrical characteristics due to moisture is suppressed can be provided. Further, a method for manufacturing the semiconductor device can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a structure of a semiconductor device according to one embodiment of the present invention. 
         FIGS. 2A to 2D  illustrate a method for manufacturing a semiconductor device, according to one embodiment of the present invention. 
         FIGS. 3A to 3D  illustrate a method for manufacturing a semiconductor device, according to one embodiment of the present invention. 
         FIGS. 4A and 4B  illustrate a method for manufacturing a semiconductor device, according to one embodiment of the present invention. 
         FIGS. 5A and 5B  illustrate a structure of a semiconductor device according to one embodiment of the present invention. 
         FIGS. 6A to 6C  illustrate structures of a semiconductor device according to one embodiment of the present invention. 
         FIGS. 7A to 7C  illustrate examples of an electronic appliance using a semiconductor device according to one embodiment of the present invention. 
         FIG. 8  shows a state of a cross section of a semiconductor device according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. In addition, the present invention is not construed as being limited to description of the embodiments below. 
     Further, in embodiments hereinafter described, the same parts are denoted with the same reference numerals throughout the drawings. The thickness, width, relative relation of position, and the like of elements such as a layer and a region illustrated in the drawings are exaggerated for clarification of description of the embodiments in some cases. 
     Note that in this specification and the like, ordinal numbers such as “first” and “second” are used in order to avoid confusion among components and do not limit the components numerically. 
     In addition, in this specification and the like, the term “electrode” or “wiring” does not limit a function of a component. For example, an “electrode” is sometimes used as part of a “wiring,” and vice versa. In addition, the term “electrode” or “wiring” can also mean a combination of a plurality of “electrodes” or “wirings,” for example. 
     Functions of a “source” and a “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification and the like. 
     In this specification and the like, average surface roughness (R a ) is obtained by three-dimensional expansion of arithmetic mean surface roughness (R a ) which is defined by JIS B 0601:2001 (ISO 4287:1997) so that R a  can be applied to a curved surface, and is an average value of the absolute values of deviations from a reference surface to a specific surface. R a  is defined by Formula 1 below. 
     
       
         
           
             
               
                 
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     Here, the specific surface is a surface that is a target of roughness measurement, and is a quadrilateral region specified by four points represented by the coordinates (x 1 , y 1 , f(x 1 , y 1 )), (x 1 , y 2 , f(x 1 , y 2 )), (x 2 , y 1 , f(x 2 , y 1 )), and (x 2 , y 2 , f(x 2 , y 2 )). Moreover, S 0  represents the area of a rectangle which is obtained by projecting the specific surface on the xy plane, and Z 0  represents the height of the reference surface (the average height of the specific surface). R a  can be measured using an atomic force microscope (AFM). 
     Embodiment 1 
     In this embodiment, a semiconductor device including a transistor, according to one embodiment of the present invention and a method for manufacturing the semiconductor device will be described with reference to  FIGS. 1A and 1B ,  FIGS. 2A to 2D ,  FIGS. 3A to 3D ,  FIGS. 4A and 4B , and  FIGS. 5A and 5B . 
     (Structure of Semiconductor Device of this Embodiment) 
       FIGS. 1A and 1B  are a top view and a cross-sectional view of a semiconductor device which includes a coplanar transistor having a top-gate top-contact structure.  FIG. 1A  is a top view of the transistor.  FIG. 1B  is a cross-sectional view along dashed-dotted line A-B in  FIG. 1A . Note that in  FIG. 1A , some components are not shown for easy understanding of a structure of the transistor. 
     As illustrated in  FIG. 1B , a transistor  150  includes a substrate  100 , a base insulating layer  102  provided over the substrate  100 , a semiconductor layer  106  provided over the base insulating layer  102  and having a high-resistance region  106   a  and low-resistance regions  106   b , a gate insulating layer  108  provided over the semiconductor layer  106 , a gate electrode  110  provided to overlap with the semiconductor layer  106  with the gate insulating layer  108  provided therebetween, sidewall insulating layers  112  provided in contact with side surfaces of the gate electrode  110 , and a pair of electrodes  114  provided in contact with the semiconductor layer  106 . Further, over the transistor  150 , there are an insulating layer  116  provided to cover the gate electrode  110 , the sidewall insulating layers  112 , and the pair of electrodes  114 ; a metal oxide layer  118  provided over the insulating layer  116  and including a first metal oxide layer  118   a  and a second metal oxide layer  118   b ; and wirings  120  which are electrically connected to the pair of electrodes  114  through opening portions provided in the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b.    
     What is characteristic in the structure of this embodiment is that the insulating layer  116  and the metal oxide layer  118  are placed over the semiconductor layer  106  where a channel region of the transistor  150  is formed, and that the metal oxide layer  118  has a stacked-layer structure of the first metal oxide layer  118   a  and the second metal oxide layer  118   b.    
     The insulating layer  116  is formed using an insulating material having low relative permittivity so as to avoid generation of parasitic capacitance between conductive layers (the wirings  120  in this embodiment) which are formed above the insulating layer  116  and conductive layers (the gate electrode  110  and the pair of electrodes  114  in this embodiment) which are formed below the insulating layer  116  as much as possible. Note that when formed using an inorganic material, the insulating layer can be provided with a high barrier property against external moisture permeating the semiconductor layer  106 . 
     For the first metal oxide layer  118   a , a metal oxide layer having an amorphous structure is used. In the metal oxide layer having an amorphous structure, there are no crystal grain boundaries, and grid intervals are wide as compared to those in a metal oxide layer in a crystalline state; thus, the metal oxide layer having an amorphous structure easily traps moisture between the lattices. 
     For the second metal oxide layer  118   b , a metal oxide layer having a polycrystalline structure is used. In the metal oxide layer having a polycrystalline structure, crystal parts other than crystal grain boundary portions have dense structures and extremely low moisture permeability. 
     Therefore, when the semiconductor device has the above-described structure, permeation of external moisture into the transistor  150  can be effectively prevented, allowing suppression of change in electrical characteristics of the semiconductor device of this embodiment which is caused by permeation of external moisture. 
     Note that in the transistor illustrated in  FIGS. 1A and 1B , the low-resistance regions  106   b  (the regions in the semiconductor layer  106  which are connected to the pair of electrodes  114  serving as a source electrode and a drain electrode, and which can also be called a source region and a drain region) and the high-resistance region  106   a  (the region in the semiconductor layer  106  which includes a region where a channel portion is formed, and which can also be called a channel formation region) of the semiconductor layer  106  can be formed in a self-aligned manner using the gate electrode  110  as a mask. Thus, a minute transistor can be obtained. Moreover, cost can be low and a yield can be high as compared to those in the case where photolithography or the like is employed to form the high-resistance region  106   a  and the low-resistance region  106   b . In addition, because the low-resistance region  106   b  and the gate electrode  110  hardly overlap, parasitic capacitance in an overlapped portion is hardly (or not at all) generated, whereby the transistor can operate at high speed. 
     In the transistor illustrated in  FIGS. 1A and 1B , when the transistor is in an on state, current flows from one of the pair of electrodes  114  to the high-resistance region  106   a  through the low-resistance regions  106   b . With the low-resistance regions  106   b , deterioration such as hot carrier degradation can be suppressed even in a minute transistor with a short channel length, which can make the transistor highly reliable. 
     Note that in the transistor illustrated in  FIGS. 1A and 1B , regions of the semiconductor layer  106  which overlap with the sidewall insulating layers  112  are included in the low-resistance regions  106   b ; however, the present invention is not limited thereto. For example, the regions of the semiconductor layer  106  which overlap with the sidewall insulating layers  112  may be included in the high-resistance region  106   a . The above-described deterioration such as hot carrier degradation can be further reduced in the case of employing such a structure. In the case where the above-described structure is employed, the low-resistance regions  106   b  and the high-resistance region  106   a  can be formed in the semiconductor layer  106  in a self-aligned manner using the gate electrode  110  and the sidewall insulating layers  112  as masks. 
     (Manufacturing Method of Semiconductor Device of this Embodiment) 
     A method for manufacturing the transistor illustrated in  FIGS. 1A and 1B  will be described with reference to  FIGS. 2A to 2D ,  FIGS. 3A to 3D , and  FIGS. 4A and 4B . 
     First, the base insulating layer  102  is formed over the substrate  100  (see  FIG. 2A ). 
     For the substrate  100 , a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used. In the mass production, a mother glass with the following size is preferably used for the substrate  100 : the 8th generation (2160 mm×2460 mm); the 9th generation (2400 mm×2800 mm, or 2450 mm×3050 mm); the 10th generation (2950 mm×3400 mm); or the like. A mother glass considerably shrinks when the treatment temperature is high and the treatment time is long. Thus, in the case where mass production is performed with the use of the mother glass, the heating temperature in the manufacturing process is preferably 700° C. or lower, further preferably 450° C. or lower, still further preferably 350° C. or lower. 
     The base insulating layer  102  is formed by a PECVD method or a sputtering method to have a thickness greater than or equal to 50 nm and less than or equal to 600 nm with the use of one of a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, and a silicon nitride oxide film or a stacked-layer film including any of these films. Formation of the base insulating layer  102  can suppress permeation of impurities from the substrate  100  side into the semiconductor layer  106  which is formed in a later step. Note that the base insulating layer  102  is not necessarily provided; for example, in the case where the impurity content in the substrate  100  is sufficiently low, the base insulating layer  102  may be omitted. 
     Note that in an “oxynitride” in this specification such as silicon oxynitride, the oxygen content is higher than the nitrogen content. Further, in a “nitride oxide” such as silicon nitride oxide, the nitrogen content is higher than the oxygen content. 
     Note that in the case where a layer including an oxide semiconductor material is used as the semiconductor layer  106  which is formed in the later step, electric charge can be generated due to oxygen vacancies in the semiconductor layer  106 . In general, some of oxygen vacancies in an oxide semiconductor layer serve as a donor and causes release of an electron which is a carrier. As a result, the threshold voltage of the transistor shifts in the negative direction. Thus, for the base insulating layer  102 , an insulating film from which oxygen is released by heat treatment is preferably used. In this manner, oxygen vacancies in the semiconductor layer  106 , which cause a negative shift in the threshold voltage, can be reduced. For easier crystal growth in the semiconductor layer  106  which is formed in the later step, it is preferable that the base insulating layer  102  be sufficiently flat. 
     The above expression “release oxygen by heat treatment” means that the released amount of oxygen which is converted into oxygen atoms is estimated to be 1.0×10 18  atoms/cm 3  or higher, preferably 3.0×10 20  atoms/cm 3  or higher in thermal desorption spectroscopy (TDS) analysis. 
     Here, a method in which the amount of released oxygen is measured by conversion into oxygen atoms using TDS analysis will be described. 
     The amount of released gas in TDS analysis is proportional to the integral value of a spectrum. Therefore, the amount of released gas can be calculated from the ratio between the integral value of a measured spectrum and the reference value of a standard sample. The reference value of a standard sample refers to the ratio of the density of a predetermined atom contained in a sample to the integral value of a spectrum. 
     For example, the amount of the released oxygen molecules (N O2 ) from an insulating film can be obtained according to Formula 2 below with the TDS analysis results of a silicon wafer containing hydrogen at a predetermined density which is the standard sample and the TDS analysis results of the insulating film. Here, all spectra having a mass number of 32 which are obtained by the TDS analysis are assumed to originate from an oxygen molecule. CH 3 OH, which is given as a gas having a mass number of 32, is not taken into consideration on the assumption that it is unlikely to be present. Further, an oxygen molecule including an oxygen atom having a mass number of 17 or 18 which is an isotope of an oxygen atom is not taken into consideration either because the proportion of such a molecule in the natural world is minimal 
     
       
         
           
             
               
                 
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     N H2  is the value obtained by conversion of the amount of hydrogen molecules released from the standard sample into density. S H2  is the integral value of a spectrum when the standard sample is subjected to TDS analysis. Here, the reference value of the standard sample is set to N H2 /S H2 . S O2  is the integral value of a spectrum when the insulating film is subjected to TDS analysis. α is a coefficient affecting the intensity of the spectrum in the TDS analysis. Refer to Japanese Published Patent Application No. H6-275697 for details of Formula 2. Note that the amount of released oxygen from the above insulating film is measured with a thermal desorption spectroscopy apparatus produced by ESCO Ltd., EMD-WA1000S/W, using a silicon wafer containing hydrogen atoms at 1×10 16  atoms/cm 3  as the standard sample. 
     Further, in the TDS analysis, oxygen is partly detected as an oxygen atom. The ratio between oxygen molecules and oxygen atoms can be calculated from the ionization rate of the oxygen molecules. Note that since the above a includes the ionization rate of the oxygen molecules, the amount of the released oxygen atoms can also be estimated through the evaluation of the amount of the released oxygen molecules. 
     Note that N O2  is the amount of the released oxygen molecules. The amount of released oxygen when converted into oxygen atoms is twice the amount of the released oxygen molecules. 
     In the above structure, the layer from which oxygen is released by heat treatment may be oxygen-excess silicon oxide (SiO X  (X&gt;2)). In the oxygen-excess silicon oxide (SiO X  (X&gt;2)), the number of oxygen atoms per unit volume is more than twice the number of silicon atoms per unit volume. The number of silicon atoms and the number of oxygen atoms per unit volume are measured by Rutherford backscattering spectrometry. 
     Then, the semiconductor layer  106  is formed by a sputtering method, an evaporation method, a PECVD method, a PLD method, an atomic layer deposition (ALD) method, a molecular beam epitaxy (MBE) method, or the like (see  FIG. 2B ). For the semiconductor layer  106 , a silicon film such as an amorphous silicon film, a polycrystalline silicon film, a microcrystalline (also referred to as microcrystal or semi-amorphous silicon) silicon film, or a single crystal silicon film can be used. Alternatively, a compound semiconductor film such as a gallium arsenide film or a silicon germanium film can be used. Further alternatively, a film including an oxide semiconductor material (hereinafter referred to as an oxide semiconductor layer) may be used. In recent years, a transistor in which an oxide semiconductor material is used for a semiconductor layer has been actively researched and developed. In view of this circumstance, the structure in which an oxide semiconductor layer is used as the semiconductor layer  106  is described below. It is needless to say that the semiconductor layer  106  is not limited to an oxide semiconductor layer. 
     The semiconductor layer  106  is preferably formed such that impurities which are not main components of the semiconductor layer  106  and serve as carrier donors are contained as little as possible. Therefore, a material (e.g., a target used in a sputtering apparatus) and a film formation atmosphere for formation of the semiconductor layer  106  are preferably a highly purified material and a highly purified atmosphere which contain impurities serving as carrier donors as little as possible. 
     When the semiconductor layer  106  contains an alkali metal or an alkaline earth metal, the alkali metal or the alkaline earth metal and an oxide semiconductor are bonded to each other, so that carriers are generated in some cases, which causes an increase in the off-state current of a transistor. Accordingly, the concentration of an alkali metal or an alkaline earth metal in the semiconductor layer  106  is 1×10 18  atoms/cm 3  or lower, preferably 2×10 16  atoms/cm 3  or lower. 
     Further, when the semiconductor layer  106  contains a large amount of hydrogen, the hydrogen and an oxide semiconductor are bonded to each other, so that part of the hydrogen serves as a donor and causes generation of an electron which is a carrier. As a result, the threshold voltage of the transistor shifts in the negative direction. Accordingly, the hydrogen concentration in the semiconductor layer  106  is preferably lower than 5×10 18  atoms/cm 3 , more preferably 1×10 18  atoms/cm 3  or lower, still more preferably 5×10 17  atoms/cm 3  or lower, further more preferably 1×10 16  atoms/cm 3  or lower. Note that the concentration of hydrogen in the semiconductor layer  106  is measured by secondary ion mass spectrometry (SIMS). 
     In an oxide semiconductor where defect levels in an energy gap due to oxygen vacancies are reduced by sufficiently reducing the concentration of hydrogen and supplying a sufficient amount of oxygen, the carrier density can be lower than 1×10 12 /cm 3 , preferably lower than 1×10 11 /cm 3 , further preferably lower than 1.45×10 10 /cm 3 . For example, the off-state current (per unit channel width (1 μm) here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10 −21  A) or less, preferably 10 zA or less. In this manner, an oxide semiconductor in which impurities such as hydrogen are sufficiently removed and a sufficient amount of oxygen is supplied is used for the semiconductor layer  106 , whereby a transistor with extremely favorable off-state current characteristics can be obtained. 
     Note that off-state current refers to drain current which flows in a transistor in an off state. An off state of a transistor refers to a state where gate voltage is lower than threshold voltage in an n-channel transistor. Alternatively, an off state of a transistor refers to a state where gate voltage is higher than threshold voltage in a p-channel transistor. Drain current refers to current between a source and a drain of a transistor. Further, gate voltage refers to a potential difference between a source potential and a gate potential when the source potential is used as a reference potential. 
     In some cases, off-state current refers to drain current which flows in a normally-off transistor when gate voltage is 0 V. A normally-off transistor refers to an n-channel transistor whose threshold voltage is higher than 0 V. Alternatively, a normally-off transistor refers to a p-channel transistor whose threshold voltage is lower than 0 V. 
     The concentration of hydrogen in the semiconductor layer  106 , which is measured by secondary ion mass spectrometry (SIMS), is lower than 5×10 19  cm −3 , preferably 5×10 18  cm −3  or lower, more preferably 1×10 18  cm −3  or lower, still more preferably 5×10 17  cm −3  or lower. Further, as for alkali metal concentration, which is measured by SIMS, the concentration of sodium is 5×10 16  cm −3  or lower, preferably 1×10 16  cm −3  or lower, more preferably 1×10 15  cm −3  or lower. The concentration of lithium is 5×10 15  cm −3  or lower, preferably 1×10 15  cm −3  or lower. The concentration of potassium is 5×10 15  cm −3  or lower, preferably 1×10 15  cm −3  or lower. 
     The semiconductor layer  106  can be single crystal, polycrystalline (also referred to as polycrystal), or amorphous, for example. 
     The semiconductor layer  106  is preferably a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film. 
     The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystal parts and amorphous parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between the amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed. 
     Note that the transistor in which the CAAC-OS film is used as a semiconductor layer can have higher mobility than a transistor in which an oxide semiconductor film in an amorphous state is used as a semiconductor layer by improvement in surface flatness. In order to improve the surface flatness, the semiconductor layer is preferably formed over a flat surface. Specifically, the semiconductor layer is preferably formed over a surface with the average surface roughness (R a ) of 1 nm or lower, preferably 0.3 nm or lower, more preferably 0.1 nm or lower. Note that the average surface roughness (R a ) is preferably close to 0. In this embodiment, a surface of the base insulating layer  102  or that of the semiconductor layer  106  may be made to have the above flatness. 
     In each of the crystal parts included in the CAAC-OS film, a c-axis is aligned in a 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, triangular or hexagonal atomic arrangement when seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°. 
     In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor layer, the proportion of crystal parts in the vicinity of the surface of the CAAC-OS film is higher than that in the vicinity of the surface where the CAAC-OS film is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous 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 the 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 film formation or by performing treatment for crystallization such as heat treatment after film formation. 
     In the transistor where the CAAC-OS film is used as the semiconductor layer  106  (at least the high-resistance region  106   a ), change in electrical characteristics due to irradiation with visible light or ultraviolet light can be reduced. Thus, the transistor has high reliability. 
     Note that part of oxygen included in the oxide semiconductor film may be substituted with nitrogen. 
     The semiconductor layer  106  can be formed by a sputtering method, an MBE method, a pulsed laser deposition method, an ALD method, or the like as appropriate. In this embodiment, the semiconductor layer  106  can be formed using a sputtering apparatus which performs film formation with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target. 
     The thickness of the semiconductor layer  106  can be greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 40 nm, more preferably greater than or equal to 5 nm and less than or equal to 20 nm. In particular, in the case of the transistor whose channel length is 30 nm or less, when the semiconductor layer  106  has a thickness of around 5 nm, a short channel effect can be suppressed and stable electrical characteristics can be obtained. 
     To form a CAAC-OS film as the semiconductor layer  106 , the film formation is performed by a sputtering method with a polycrystalline oxide semiconductor sputtering target, for example. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) may flake off from the sputtering target. In that case, the flat-plate-like sputtered particle reaches a substrate while maintaining their crystal state, whereby the CAAC-OS film can be formed. 
     For the deposition of the CAAC-OS film, the following conditions are preferably used. 
     By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used. 
     By increasing the substrate heating temperature during the deposition, migration of a sputtered particle is likely to occur after the sputtered particle reaches a substrate surface. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate, migration occurs on the substrate surface, so that a flat plane of the flat-plate-like sputtered particle is attached to the substrate. 
     Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol. % or higher, preferably 100 vol. %. 
     As an example of the sputtering target, an In—Ga—Zn—O compound target is described below. 
     The In—Ga—Zn—O compound target, which is polycrystalline, is made by mixing InO X  powder, GaO Y  powder, and ZnO Z  powder in a predetermined molar ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y and Z are given positive numbers. Here, the predetermined molar ratio of InO X  powder to GaO Y  powder and ZnO Z  powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target. 
     Note that before the deposition of the semiconductor layer  106 , it is preferable that a deposition chamber be heated and evacuated to remove impurities such as moisture in the deposition chamber. It is particularly important to remove the impurities adsorbed on an inner wall of the deposition chamber. Here, heat treatment may be performed at a temperature higher than or equal to 100° C. and lower than or equal to 450° C., for example. The deposition chamber is preferably evacuated with a rough vacuum pump such as a dry pump and a high vacuum pump such as a sputter ion pump, a turbo molecular pump, or a cryopump in appropriate combination. The turbo molecular pump has an outstanding capability of evacuating large-sized molecules, whereas it has a low capability of evacuating moisture or hydrogen. Further, it is effective to use a combination of the turbo molecular pump and a cryopump having a high capability of evacuating moisture or a sputter ion pump having a high capability of evacuating hydrogen. At this time, when impurities are removed while an inert gas is introduced, the rate of elimination of moisture or the like, which is difficult to eliminate only by evacuation, can be further increased. By removing impurities in the deposition chamber by such treatment before the deposition of the oxide semiconductor, entry of hydrogen, moisture, a hydroxyl group, hydride, and the like into the semiconductor layer  106  can be suppressed. 
     An oxide semiconductor used for forming the oxide semiconductor layer  106  preferably contains at least indium (In) or zinc (Zn). In particular, the oxide semiconductor preferably contains In and Zn. In addition, as a stabilizer for reducing the variation in electrical characteristics of a transistor using the oxide, gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer. Titanium (Ti) is preferably contained as a stabilizer. Zirconium (Zr) is preferably contained as a stabilizer. 
     As another stabilizer, the oxide semiconductor may contain 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), and lutetium (Lu). 
     The semiconductor layer  106  is preferably formed using a material whose band gap is 2.5 eV or more, preferably 2.8 eV or more, further preferably 3.0 eV or more in order to reduce the off-state current of the transistor. 
     The off-resistance of the transistor (resistance between source and drain when the transistor is in an off-state) is inversely proportional to the concentration of carriers thermally excited in the semiconductor layer where a channel is formed. Since the band gap of silicon is 1.1 eV even in a state where there is no carrier caused by a donor or an acceptor (i.e., even in the case of an intrinsic semiconductor), the concentration of thermally excited carriers at room temperature (300 K) is approximately 1×10 11  cm −3 . 
     On the other hand, in the case of a semiconductor whose band gap is 3.2 eV (an oxide semiconductor, here), the concentration of thermally excited carriers is approximately 1×10 −7  cm −3 . When the electron mobility is the same, the resistivity is inversely proportional to the carrier concentration; therefore, the resistivity of the semiconductor whose band gap is 3.2 eV is 18 orders of magnitude higher than that of silicon. 
     Such a transistor in which an oxide semiconductor having a wide band gap is used for the semiconductor layer  106  (OS transistor) can realize an extremely small off-state current. 
     For the semiconductor layer  106 , for example, any of the following can be used: a single-component metal oxide such as indium oxide, tin oxide, or zinc oxide; a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide. 
     Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main component and there is no particular limitation on the ratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn. 
     Alternatively, for the semiconductor layer  106 , an oxide represented by InMO 3 (ZnO) m  (m&gt;0 is satisfied, and m is not an integer) may be used. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, as the oxide semiconductor, a material represented by In 2 SnO 5 (ZnO) n  (n&gt;0, and n is an integer) may be used. 
     For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or any of In—Ga—Zn-based oxides whose composition is in the neighborhood of the above compositions can be used for the semiconductor layer  106 . Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of In—Sn—Zn-based oxides whose composition is in the neighborhood of the above compositions may be used. 
     However, without limitation to the materials given above, a material with an appropriate composition may be used depending on needed semiconductor characteristics (e.g., mobility, threshold voltage, and variation). In order to obtain the needed semiconductor characteristics, it is preferable that the carrier concentration, the impurity concentration, the defect density, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like be set to appropriate values. 
     For example, high mobility can be obtained relatively easily in the case of using an In—Sn—Zn-based oxide. However, the mobility can be increased by reducing the defect density in the bulk also in the case of using the In—Ga—Zn-based oxide. 
     Note that for example, the expression “the composition of an oxide including In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in the neighborhood of the composition of an oxide including In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b, and c satisfy the following relation: (a−A) 2 +(b−B) 2 +(c−C) 2 ≦r 2 , and r may be 0.05, for example. The same applies to other oxides. 
     In the case of depositing an In—Ga—Zn-based oxide for the semiconductor layer  106  by a sputtering method, it is preferable to use an In—Ga—Zn—O target having the following atomic ratio: the atomic ratio of In:Ga:Zn is 1:1:1, 4:2:3, 3:1:2, 1:1:2, 1:3:2, 2:1:3, or 3:1:4. When the semiconductor layer  106  is formed using an In—Ga—Zn—O target having the aforementioned atomic ratio, a polycrystalline film or a CAAC-OS film is easily formed. 
     In the case of depositing an In—Sn—Zn-based oxide for the semiconductor layer  106  by a sputtering method, it is preferable to use an In—Sn—Zn—O target having the following atomic ratio: the atomic ratio of In:Sn:Zn is 1:1:1, 2:1:3, 1:2:2, or 20:45:35. When the semiconductor layer  106  is formed using an In—Sn—Zn—O target having the aforementioned atomic ratio, a polycrystalline film or a CAAC-OS film is easily formed. 
     In the case of depositing an In—Zn-based oxide for the semiconductor layer  106  by a sputtering method, it is preferable to use an In—Zn—O target having an atomic ratio of In:Zn=50:1 to 1:2 (In 2 O 3 :ZnO=25:1 to 1:4 in a molar ratio), preferably, In:Zn=20:1 to 1:1 (In 2 O 3 :ZnO=10:1 to 1:2 in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 (In 2 O 3 :ZnO=15:2 to 3:4 in a molar ratio). When the semiconductor layer  106  is formed using an In—Zn—O target having the aforementioned atomic ratio, a polycrystalline film or a CAAC-OS film is easily formed. 
     Note that although in the above description, the atomic ratio of the metal elements in the target used for the formation of the semiconductor layer  106  is expressed with integers, the atomic ratio of the metal elements in the target may vary a little from the above-mentioned values when the atomic ratio of the metal elements included in the semiconductor layer  106  is set to integers. For example, in the case where the semiconductor layer  106  in which In:Ga:Zn=1:1:1 is formed, a target having a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1+α:1+β:1+γ [molar ratio] can be used. Although the values of α, β, and γ vary depending on the film formation conditions, the values are in a range of approximately greater than or equal to −0.5 and less than or equal to 0.5. 
     The semiconductor layer  106  is preferably formed under conditions such that much oxygen is contained during film formation (for example, by a sputtering method in an atmosphere where the proportion of oxygen is 100%) so as to be a layer containing much oxygen (preferably including a region containing oxygen in excess of the stoichiometric composition in the semiconductor layer). 
     The deposition atmosphere may be a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas and oxygen. Here, in the case where a volume ratio of oxygen is made to be higher than that of the rare gas at the time of the deposition, much oxygen is taken in the semiconductor layer  106 , so that oxygen vacancies in the semiconductor layer  106  can be reduced. Further, in order to prevent moisture from entering the semiconductor layer  106 , it is preferable to use an atmosphere of a high-purity gas from which moisture is sufficiently removed. 
     Note that when a CAAC-OS film is used for the semiconductor layer  106 , the CAAC-OS film may have a structure in which a plurality of oxide semiconductor films are stacked. For example, the semiconductor layer  106  may be a stack of a first oxide semiconductor film, a second oxide semiconductor film, and a third oxide semiconductor film which have different compositions. For example, the following structure can be employed: the first oxide semiconductor film and the third oxide semiconductor film are formed using three-component metal oxides and the second oxide semiconductor film is formed using a two-component metal oxide; or the first oxide semiconductor film and the third oxide semiconductor film are formed using two-component metal oxides and the second oxide semiconductor film is formed using a three-component metal oxide. 
     Further, the constituent elements of the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film may be the same and the compositions of the constituent elements of the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film may be different. For example, the first oxide semiconductor film and the third oxide semiconductor film may have an atomic ratio of In:Ga:Zn=1:1:1, and the second oxide semiconductor film may have an atomic ratio of In:Ga:Zn=3:1:2. Alternatively, the first oxide semiconductor film and the third oxide semiconductor film may have an atomic ratio of In:Ga:Zn=1:3:2, and the second oxide semiconductor film may have an atomic ratio of In:Ga:Zn=3:1:2. 
     At this time, the second oxide semiconductor film preferably contains more In than Ga. Further, the first oxide semiconductor film and the third oxide semiconductor film preferably contain In and Ga at a proportion of In≦Ga. 
     In an oxide semiconductor, the s orbital of heavy metal mainly contributes to carrier transfer, and when the In content in the oxide semiconductor is increased, overlap of the s orbital is likely to be increased. Therefore, an oxide having a composition of In&gt;Ga has higher mobility than an oxide having a composition of In≦Ga. Further, in Ga, the formation energy of oxygen vacancies is larger and thus oxygen vacancies are less likely to occur, than in In; therefore, the oxide having a composition of In≦Ga has more stable characteristics than the oxide having a composition of In&gt;Ga. 
     Note that when the film (the base insulating layer  102 , an insulating layer  107 , or the like in the drawings) which is in contact with and is different from the oxide semiconductor film is formed, an impurity might be diffused into the oxide semiconductor film from the film formed to be in contact with the oxide semiconductor film. For example, when silicon, carbon, or the like contained in the base insulating layer  102  or the insulating layer  107  is diffused into the oxide semiconductor film, electrical characteristics of the transistor may be adversely affected. 
     However, the adverse effect on the electrical characteristics of the transistor (e.g., a reduction in mobility) which is attributed to impurity diffusion can be reduced when a stacked-layer structure of the oxide semiconductor films is provided as described above and the oxide semiconductor film (i.e., the oxide semiconductor film having a composition of In≦Ga; the first oxide semiconductor film and the third oxide semiconductor film in this embodiment) which has fewer oxygen vacancies and more stable characteristics than the oxide semiconductor film that has high mobility (i.e., the oxide semiconductor film having a composition of In&gt;Ga; the second oxide semiconductor film in this embodiment) is provided in contact with the oxide semiconductor film that has high mobility, whereby the oxide semiconductor film that has high mobility is provided apart from the film (the base insulating layer  102 , the insulating layer  107 , or the like in the drawings) which is in contact with the stacked-layer structure of the oxide semiconductor films. In this manner, the mobility and reliability of the transistor can be improved. 
     Further, after the semiconductor layer  106  is formed, heat treatment for removal of moisture taken in the semiconductor layer  106  (dehydration or dehydrogenation) may be performed. The heat treatment can further remove moisture in the semiconductor layer  106 . The heat treatment is performed under an inert gas atmosphere at a temperature higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C., or lower than a strain point of the substrate. Note that the inert gas atmosphere is preferably an atmosphere which contains nitrogen or a rare gas (such as helium, neon or argon) as its main component and does not contain moisture, hydrogen or the like. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus can be set to 6N (99.9999%) or more, preferably 7N (99.99999%) or more (i.e., the impurity concentration is 1 ppm or less, preferably 0.1 ppm or less). 
     For example, after the substrate is introduced into an electric furnace including a resistance heater or the like, the heat treatment can be performed at 450° C. under a nitrogen atmosphere for one hour. 
     The heat treatment apparatus is not limited to the electric furnace and may be an apparatus for heating an object to be processed by thermal conduction or thermal radiation from a medium such as a heated gas. For example, a rapid thermal annealing (RTA) apparatus such as a lamp rapid thermal annealing (LRTA) apparatus or a gas rapid thermal annealing (GRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon is used. Note that in the case where a GRTA apparatus is used as the heat treatment apparatus, the substrate may be heated in an inert gas heated to high temperature of 650° C. to 700° C. because the heat treatment time is short. 
     The above heat treatment can be referred to as dehydration treatment, dehydrogenation treatment, or the like because of its advantageous effect of removing moisture. The timing of performing the heat treatment is not limited to the timing just after the formation of the semiconductor layer  106 ; for example, the heat treatment can be performed at another timing such as the timing after the semiconductor layer  106  is processed into an island shape. Such dehydration treatment or dehydrogenation treatment may be conducted once or plural times. 
     Further, after the semiconductor layer  106  is heated by the heat treatment, heat treatment for adding oxygen to the semiconductor layer  106  (also referred to as supply of oxygen) may be performed in the same furnace. The heat treatment may be performed at a temperature higher than or equal to 200° C. and lower than the strain point of the substrate in a heat treatment apparatus to which a high-purity oxygen gas, a high-purity N 2 O gas, or ultra dry air (with a moisture content of 20 ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppm or less, further preferably 10 ppb or less, in the case where measurement is performed with the use of a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) is introduced. The heat treatment is performed preferably at a temperature higher than or equal to 250° C. and lower than or equal to 450° C. It is preferable that moisture, hydrogen, and the like be not contained in these gases in particular. The purity of the oxygen gas or the N 2  gas that is introduced into the same furnace is preferably 6N or more, more preferably 7N or more (i.e., the impurity concentration is preferably 1 ppm or less, more preferably 0.1 ppm or less). Through this step, oxygen vacancies in the semiconductor layer  106  generated by dehydration or dehydrogenation treatment can be compensated. 
     Note that the above heat treatment has an advantageous effect for compensating oxygen vacancies generated in the semiconductor layer  106  by dehydration treatment or dehydrogenation treatment; thus, the heat treatment can also be referred to as oxygen supplying treatment or the like. As in the case of the dehydration treatment or dehydrogenation treatment, this heat treatment can also be performed at another timing such as the timing after the semiconductor layer  106  is processed into an island shape. In addition, this heat treatment may be conducted once or plural times. 
     By performing the above heat treatment on the semiconductor layer  106 , the impurity levels in the film can be significantly reduced. Accordingly, the field-effect mobility of the transistor can be increased so as to be close to ideal field-effect mobility. 
     Next, the semiconductor layer  106  is processed into an island shape by a photolithography process or the like (see  FIG. 2C ). 
     Then, the insulating layer  107  and a conductive layer are formed in this order over the semiconductor layer  106  by a sputtering method, an evaporation method, a PECVD method, a PLD method, an ALD method, an MBE method, or the like. The conductive layer is processed by a photolithography process or the like, so that the gate electrode  110  is formed (see  FIG. 2D ). 
     For the insulating layer  107 , a material and a method similar to those for the base insulating layer  102  can be used. Note that as in the case of the base insulating layer  102 , an insulating layer from which oxygen is released by heat treatment (preferably an insulating layer which includes a region containing oxygen in excess of the stoichiometric composition in the insulating layer) is preferably used as the insulating layer  107 . Thus, oxygen can be prevented from moving from the semiconductor layer  106  to the insulating layer  107 , and oxygen can be supplied from the insulating layer  107  to the semiconductor layer  106 . 
     The gate electrode  110  may be formed by a sputtering method or the like to have a single-layer structure or a stacked-layer structure using one or more of aluminum, titanium, chromium, cobalt, nickel, copper, molybdenum, silver, tantalum, and tungsten, a nitride of any of these elements, an oxide of any of these elements, and an alloy of any of these elements. Alternatively, an oxide or an oxynitride which contains at least In and Zn may be used. For example, an In—Ga—Zn—O—N-based material can be used. 
     Then, by an ion doping method or an ion implantation method, impurity ions  130  which have a function of reducing the resistance value of the semiconductor layer  106  are added to the semiconductor layer  106  through the insulating layer  107 . At this time, the gate electrode  110  serves as a mask, so that the semiconductor layer  106  including the high-resistance region  106   a  and the low-resistance regions  106   b  is formed in a self-aligned manner (see  FIG. 3A ). Note that because an ion implantation method uses a mass separator with which only necessary ions (the impurity ions  130  in this embodiment) are extracted, only the impurity ions  130  can be selectively added to an object (the semiconductor layer  106  in this embodiment) by an ion implantation method. An ion implantation method is thus preferably employed, in which case entry of impurities (e.g., hydrogen) into the semiconductor layer  106  is reduced as compared to the case where the ions are added by an ion doping method. Note that the use of an ion doping method is not excluded. 
     Note that by addition of the impurity ions  130  through the insulating layer  107 , damage (e.g., generation of lattice defects in the semiconductor layer  106 ) caused at the time of addition of the impurity ions  130  into the semiconductor layer  106  can be reduced. 
     Next, an insulating layer is formed with the use of a material and a method similar to those for the base insulating layer  102  and then etched, so that the sidewall insulating layers  112  are formed (see  FIG. 3B ). The sidewall insulating layers  112  can be formed in a self-aligned manner by performing highly anisotropic etching on the insulating layer. For example, dry etching is preferably employed. As an etching gas used for dry etching, for example, a gas including fluorine such as trifluoromethane, octafluorocyclobutane, or tetrafluoromethane can be used. A rare gas or hydrogen may be added to the etching gas. As the dry etching, a reactive ion etching (RIE) method in which high-frequency voltage is applied to a substrate, is preferably used. 
     After the sidewall insulating layers  112  are formed, the insulating layer  107  is processed using the gate electrode  110  and the sidewall insulating layers  112  as masks, so that the gate insulating layer  108  is formed (see  FIG. 3C ). 
     Note that although the impurity ions  130  are added to the semiconductor layer  106  using the gate electrode  110  as a mask just after the formation of the gate electrode  110  in this embodiment, the impurity ions  130  may be added to the semiconductor layer  106  after the formation of the sidewall insulating layers  112 , with the use of the gate electrode  110  and the sidewall insulating layers  112  as masks. In this manner, regions of the semiconductor layer  106  which overlap with the sidewall insulating layers  112  can be included in the high-resistance region  106   a.    
     Then, over the semiconductor layer  106 , the gate electrode  110 , and the sidewall insulating layers  112 , a conductive film is formed with a material and a method similar to those for the gate electrode  110 , and is processed by a photolithography process or the like, so that the pair of electrodes  114  which function as the source electrode and the drain electrode are formed. Thus, the transistor  150  is formed (see  FIG. 3D ). 
     Next, the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b  are formed in this order over the gate electrode  110 , the sidewall insulating layers  112 , and the pair of electrodes  114  (see  FIG. 4A ). Note that the specific method for forming the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b  will be described in detail in Example 1. 
     As the insulating layer  116 , one inorganic material film or stacked inorganic material films which are selected from a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and a silicon nitride oxide film can be formed by a PECVD method or a sputtering method. 
     As in the case of the formation of the semiconductor layer  106 , when one inorganic material film or stacked inorganic material films which are selected from the foregoing are deposited as the insulating layer  116 , it is preferable that a deposition chamber be heated and evacuated before the deposition and that the deposition be performed in an atmosphere including a high-purity gas from which impurities such as moisture are sufficiently removed so that entry of moisture into the insulating layer  116  is suppressed as much as possible. 
     Alternatively, the insulating layer  116  may be formed in such a manner that a material having an insulating property is applied by a spin coating method, a printing method, a dispensing method, an ink-jet method, or the like, and cure treatment (e.g., heat treatment or light irradiation treatment) is performed depending on the applied material. As the material having an insulating property, an organic resin such as an acrylic resin, a polyimide resin, a polyamide resin, a polyamide-imide resin, or an epoxy resin, or an organic-inorganic hybrid material of organic polysiloxane and the above-mentioned inorganic material, can be used. 
     The insulating layer  116  is preferably formed using an insulating material having low relative permittivity so as to avoid generation of parasitic capacitance between the conductive layers (the wirings  120  in this embodiment) which are formed above the insulating layer  116  and the conductive layers (the gate electrode  110  and the pair of electrodes  114  in this embodiment) which are formed below the insulating layer  116  as much as possible. The insulating layer  116  preferably has a high barrier property against external moisture permeating the transistor  150 . In view of the above, the structure in which one inorganic material film or stacked inorganic material films are used for the insulating layer  116  is favorably selected from the aforementioned structures. Note that the above organic materials are not excluded. When such an organic material is used for the insulating layer  116 , it is preferable that a material with a low hygroscopic property such as a polyimide resin be used and a desiccant such as silica gel or porous alumina be mixed in the organic material. 
     Note that when parasitic capacitance and a barrier property which are described above, contact characteristics of the wirings  120  which are formed in a later step, and productivity are taken into account, the insulating layer  116  is preferably formed to a thickness of greater than or equal to 200 nm and less than or equal to 2000 nm in the case where an inorganic material film is used, or is preferably formed to a thickness of greater than or equal to 200 nm and less than or equal to 5000 nm in the case where an organic material film is used. 
     The first metal oxide layer  118   a  and the second metal oxide layer  118   b  can be formed by a sputtering method, an MBE method, a pulsed laser deposition method, an ALD method, or the like. 
     For the first metal oxide layer  118   a , a film which is made of any one of aluminum oxide, gallium oxide, titanium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, and molybdenum oxide and has an amorphous structure can be used. Note that the expression “having an amorphous structure” means including a region in which a clear lattice image or a clear lattice fringe is not confirmed in observation of a cross section of the first metal oxide layer  118   a  with a transmission electron microscope (TEM); it is preferable to use a film in which the region accounts for 60% or more, preferably 80% or more of the whole as the first metal oxide layer  118   a . Note that the expression “accounting for 60% or more of the whole” can be applied to the case where an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts in the first metal oxide layer  118   a  are observed and the region where a clear lattice image or a clear lattice fringe is not confirmed accounts for 60% or more in the observed parts. 
     The expression “having an amorphous structure” can also be applied to the case where film densities of an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts in the cross section of the first metal oxide layer  118   a  are measured by X-ray reflectometry (XRR) and the film densities in all the observed parts are in a range of greater than or equal to 2.9 g/cm 3  and less than or equal to 3.3 g/cm 3 . 
     The expression “having an amorphous structure” can also be applied to the case where electron diffraction images of an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts in the cross section of the first metal oxide layer  118   a  are observed with a transmission electron microscope and a broad and blurred pattern (also referred to as a halo pattern, a halo ring, or a halo) is observed in all the observed parts. 
     The first metal oxide layer  118   a  “has an amorphous structure” when at least one of the above interpretations is applied to the first metal oxide layer  118   a , which means that it is not necessary that all the above interpretations should be applied to the first metal oxide layer  118   a.    
     For the second metal oxide layer  118   b , a film which is made of any one of aluminum oxide, gallium oxide, titanium oxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide, and molybdenum oxide and has a polycrystalline structure can be used. Note that the expression “having a polycrystalline structure” means including a region in which a clear lattice image or a clear lattice fringe is confirmed in observation of a cross section of the second metal oxide layer  118   b  with a transmission electron microscope (TEM); it is preferable to use a film in which the region accounts for 60% or more, preferably 80% or more of the whole as the second metal oxide layer  118   b . Note that the expression “accounting for 60% or more of the whole” can be applied to the case where an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts in the second metal oxide layer  118   b  are observed and the region where a clear lattice image or a clear lattice fringe is confirmed accounts for 60% or more in the observed parts. 
     The expression “having a polycrystalline structure” can also be applied to the case where film densities of an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts in the cross section of the second metal oxide layer  118   b  are measured by XRR and the film densities in all the observed parts are in a range of greater than or equal to 3.7 g/cm 3  and less than or equal to 4.1 g/cm 3 . 
     The expression “having a polycrystalline structure” can also be applied to the case where electron diffraction images of an arbitrary number (one or more, preferably three or more, further preferably five or more) of parts in the cross section of the second metal oxide layer  118   b  are observed with a transmission electron microscope and a Debye-Scherrer pattern (also referred to as a Debye-Scherrer ring) is observed in all the observed parts. 
     The second metal oxide layer  118   b  “has a polycrystalline structure” when at least one of the above interpretations is applied to the second metal oxide layer  118   b , which means that it is not necessary that all the above interpretations should be applied to the second metal oxide layer  118   b.    
     Note that it is preferable that the first metal oxide layer  118   a  and the second metal oxide layer  118   b  be successively formed in the same apparatus in which high vacuum is maintained. In this manner, at the time of formation of the first metal oxide layer  118   a  and the second metal oxide layer  118   b , entry of moisture into the films and attachment of moisture to the interface can be suppressed, whereby deterioration of the transistor due to moisture permeation can be further suppressed. 
     In the case where the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b  are successively formed in the same apparatus with high vacuum maintained, at the time of formation of the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b , entry of moisture into the films and attachment of moisture to the interfaces can be suppressed, whereby deterioration of the transistor due to moisture permeation can be further suppressed. 
     Further, in the case where the first metal oxide layer  118   a  and the second metal oxide layer  118   b  are successively formed in the same apparatus with high vacuum maintained, the first metal oxide layer  118   a  and the second metal oxide layer  118   b  can be formed using the same target, in which case an increase in takt time due to transfer of a substrate from a deposition chamber to another can be suppressed. In addition, only one kind of target is needed for the formation of the metal oxide layer, so that an increase in manufacturing cost can be suppressed. 
     As already described above, the insulating layer  116  is formed using an insulating material having low relative permittivity so as to avoid generation of parasitic capacitance between the conductive layers (the wirings  120  in this embodiment) which are formed above the insulating layer  116  and the conductive layers (the gate electrode  110  and the pair of electrodes  114  in this embodiment) which are formed below the insulating layer  116  as much as possible. Therefore, when the second metal oxide layer  118   b  having a polycrystalline structure is in contact with the insulating layer  116 , for example, because there is a large difference in relative permittivity between the second metal oxide layer  118   b  and the insulating layer  116 , polarization charges from both the sides are not compensated at an interface in terms of dielectric constant (i.e., interface between the insulating layer  116  and the second metal oxide layer  118   b ) in some cases so that induced charges are generated to adversely affect on the channel region formed in the semiconductor layer  106 . Thus, in some cases, the electrical characteristics of the transistor are adversely affected (for example, Vth (also referred to as threshold voltage) is fluctuated). 
     However, when the first metal oxide layer  118   a  having an amorphous structure is placed between the insulating layer  116  and the second metal oxide layer  118   b  having a polycrystalline structure as in this embodiment, the first metal oxide layer  118   a  has a function of filling the gap between dielectric constants of the insulating layer  116  and the second metal oxide layer  118   b  having a polycrystalline structure, and generation of induced charges can be suppressed; thus, a highly reliable transistor with less fluctuation in electrical characteristics such as the threshold voltage can be obtained. 
     Next, opening portions reaching the pair of electrodes  114  are formed in the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b  by a photolithography process or the like; then, a conductive layer is formed over the second metal oxide layer  118   b  and processed by a photolithography process or the like, so that the wirings  120  are formed (see  FIG. 4B ). Note that the conductive layer to be the wirings  120  can be formed with the use of a material and a method similar to those for the gate electrode  110 . 
     Note that although not shown, a structure may be employed in which a protective film is provided to cover the second metal oxide layer  118   b  and the wirings  120 . With this structure, a minute amount of leakage current generated by surface conduction of the second metal oxide layer  118   b  can be reduced and thus the off-state current of the transistor can be reduced. 
     The above is the method for manufacturing the semiconductor device of this embodiment. 
     In the semiconductor device manufactured by the above method, moisture permeation into the transistor  150  can be effectively suppressed owing to the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b ; therefore, a semiconductor device with less fluctuation in electrical characteristics can be provided. 
     Embodiment 2 
     In this embodiment, examples of a structure and a manufacturing method of a semiconductor device whose structure is different from that of the semiconductor device described in Embodiment 1 will be described with reference to  FIGS. 5A and 5B . Note that the same portions as those in Embodiment 1 and portions having functions similar to those in Embodiment 1 and the same steps as those in Embodiment 1 and steps similar to those in Embodiment 1 can be conducted as in Embodiment 1, and repeated description is omitted. In addition, detailed description of the same portions is not repeated. 
     (Structure and Manufacturing Method of Semiconductor Device of this Embodiment) 
       FIGS. 5A and 5B  are a top view and a cross-sectional view of a semiconductor device which includes a coplanar transistor having a top-gate top-contact structure.  FIG. 5A  is a top view of the transistor.  FIG. 5B  is a cross-sectional view along dashed-dotted line C-D in  FIG. 5A . Note that in  FIG. 5A , some components are not shown for easy understanding of a structure of the transistor. 
     The semiconductor device including the transistor  150  in  FIGS. 5A and 5B  is different from the semiconductor device which is described in Embodiment 1 with reference to  FIGS. 1A and 1B  in that the metal oxide layer  118  is formed below the insulating layer  116 . 
     As described in Embodiment 1, it is preferable that the insulating layer  116  be formed such that impurities such as moisture are not taken in the film as much as possible. However, moisture or the like may be accidentally taken in the film at the time of the film formation or the like. Although a semiconductor device having the structure described in Embodiment 1 has a high barrier property against permeation of external moisture, the first metal oxide layer  118   a  and the second metal oxide layer  118   b  can hardly suppress moisture permeation in the case where moisture is taken in the insulating layer  116  as described above, in which case the suppressing effect on fluctuation in the electrical characteristics is reduced. 
     However, when the semiconductor device has the structure illustrated in  FIG. 5B , moisture permeation into the transistor  150  can be effectively suppressed by the first metal oxide layer  118   a  and the second metal oxide layer  118   b  even if impurities such as moisture are taken in the insulating layer  116 . 
     As in Embodiment 1, the semiconductor device illustrated in  FIGS. 5A and 5B  can be manufactured in the following manner: the base insulating layer  102 , the semiconductor layer  106 , the gate insulating layer  108 , the gate electrode  110 , the sidewall insulating layers  112 , and the pair of electrodes  114  are formed to complete the transistor  150 ; the first metal oxide layer  118   a , the second metal oxide layer  118   b , and the insulating layer  116  are formed in this order; and the wirings  120  connected to the pair of electrodes  114  are formed. Note that Embodiment 1 can be referred to for materials and formation methods used for the components. 
     Note that although the second metal oxide layer  118   b  is placed over the first metal oxide layer  118   a , and the insulating layer  116  is placed over the second metal oxide layer  118   b  in  FIG. 5B , the positions of the first metal oxide layer  118   a  and the second metal oxide layer  118   b  may be reversed. 
     The above is the structure and the manufacturing method of the semiconductor device of this embodiment. 
     In the semiconductor device manufactured by the above method, moisture permeation into the transistor  150  can be effectively suppressed even when moisture is taken in the insulating layer  116  at the time of formation of the insulating layer  116 ; therefore, a semiconductor device with less fluctuation in electrical characteristics can be provided. 
     Embodiment 3 
     In this embodiment, examples of a structure and a manufacturing method of a semiconductor device whose structure is different from that of the semiconductor device described in Embodiment 1 will be described with reference to  FIGS. 6A to 6C . Note that the same portions as those in the above embodiment and portions having functions similar to those in the above embodiment and the same steps as those in the above embodiment and steps similar to those in the above embodiment can be conducted as in the above embodiment, and repeated description is omitted. In addition, detailed description of the same portions is not repeated. 
     Semiconductor devices described in this embodiment with reference to  FIGS. 6A to 6C  are examples of a semiconductor device including a bottom-gate transistor.  FIGS. 6A to 6C  are cross-sectional views of a transistor  651 , a transistor  652 , and a transistor  653  in a channel length direction. 
     As a mode of the semiconductor device, a semiconductor device including the transistor  651  with a bottom-gate and top-contact structure is illustrated in  FIG. 6A . The transistor  651  can also be referred to as an inverted staggered transistor. 
     As illustrated in  FIG. 6A , the transistor  651  includes the substrate  100 , the base insulating layer  102  over the substrate  100 , the gate electrode  110  over the base insulating layer  102 , the gate insulating layer  108  over the gate electrode  110  and the base insulating layer  102 , the semiconductor layer  106  over the gate insulating layer  108 , and the pair of electrodes  114  over the semiconductor layer  106 . Further, over the transistor  651 , there are the insulating layer  116  provided to cover the semiconductor layer  106  and the pair of electrodes  114 ; the first metal oxide layer  118   a  provided over the insulating layer  116 ; the second metal oxide layer  118   b  provided over the first metal oxide layer  118   a ; and the wirings  120  which are electrically connected to the pair of electrodes  114  through opening portions provided in the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b.    
     The above semiconductor device is different from that in the above embodiment in the formation order of the components (for example, the gate electrode  110  is formed before the gate insulating layer  108  is formed); however, materials and formation methods for the components are basically the same as those for the components in Embodiment 1 and thus the semiconductor device can be manufactured referring to the manufacturing method described in the above embodiment. 
     As another mode of the semiconductor device, a semiconductor device including the bottom-gate and top-contact transistor  652  with a channel-protective (channel-stop) structure is illustrated in  FIG. 6B . The transistor  652  can also be referred to as an inverted staggered transistor. 
     As illustrated in  FIG. 6B , the transistor  652  includes the substrate  100 , the base insulating layer  102  over the substrate  100 , the gate electrode  110  over the base insulating layer  102 , the gate insulating layer  108  over the gate electrode  110  and the base insulating layer  102 , the semiconductor layer  106  over the gate insulating layer  108 , an insulating layer  602  over the semiconductor layer  106 , and the pair of electrodes  114  over the semiconductor layer  106  and the insulating layer  602 . Further, over the transistor  652 , there are the insulating layer  116  provided to cover the semiconductor layer  106 , the insulating layer  602 , and the pair of electrodes  114 ; the first metal oxide layer  118   a  provided over the insulating layer  116 ; the second metal oxide layer  118   b  provided over the first metal oxide layer  118   a ; and the wirings  120  which are electrically connected to the pair of electrodes  114  through opening portions provided in the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b.    
     The semiconductor device is different from the semiconductor device in  FIG. 6A  in that the insulating layer  602  is provided over the semiconductor layer  106 . When the pair of electrodes  114  are formed by etching process (e.g., dry etching process or wet etching process), the insulating layer  602  can prevent a layer below the pair of electrodes  114  from being also etched (also referred to as overetching). The above structure is preferable especially when the semiconductor layer  106  is thin. The reason for this is that electrical characteristics of the transistor  652  can considerably fluctuate when the semiconductor layer  106  having an extremely small thickness (specifically 100 nm or less, more specifically 40 nm or less, further specifically 20 nm or less) is reduced a little in thickness by etching. Note that materials and formation methods for the components are basically the same as those for the components in Embodiment 1 and thus the semiconductor device can be manufactured referring to the manufacturing method described in the above embodiment. 
     As a further mode of the semiconductor device, a semiconductor device including the transistor  653  with a bottom-gate and bottom-contact structure is illustrated in  FIG. 6C . The transistor  653  can also be referred to as an inverted staggered transistor. 
     As illustrated in  FIG. 6C , the transistor  653  includes the substrate  100 , the base insulating layer  102  over the substrate  100 , the gate electrode  110  over the base insulating layer  102 , the gate insulating layer  108  over the gate electrode  110  and the base insulating layer  102 , the pair of electrodes  114  over the gate insulating layer  108 , and the semiconductor layer  106  over the gate insulating layer  108  and the pair of electrodes  114 . Further, over the transistor  653 , there are the insulating layer  116  provided to cover the semiconductor layer  106  and the pair of electrodes  114 ; the first metal oxide layer  118   a  provided over the insulating layer  116 ; the second metal oxide layer  118   b  provided over the first metal oxide layer  118   a ; and the wirings  120  which are electrically connected to the pair of electrodes  114  through opening portions provided in the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b.    
     The semiconductor device is different from the semiconductor device in  FIG. 6A  in that the pair of electrodes  114  are formed before the semiconductor layer  106  is formed; however, materials and formation methods for the components are basically the same as those for the components in Embodiment 1 and thus the semiconductor device can be manufactured referring to the manufacturing method described in the above embodiment. 
     Note that although the first metal oxide layer  118   a  and the second metal oxide layer  118   b  are placed over the insulating layer  116  in  FIGS. 6A to 6C , a structure may be employed in which the first metal oxide layer  118   a  and the second metal oxide layer  118   b  are placed below the insulating layer  116  as in Embodiment 2. For a structure and a manufacturing method in such a case, this embodiment and Embodiment 2 can be referred to. 
     The above is the structures and the manufacturing methods of the semiconductor devices of this embodiment. 
     Embodiment 4 
     In this embodiment, the case where the semiconductor device described in the above embodiment is applied to an electronic appliance will be described with reference to  FIGS. 7A to 7C . In this embodiment, the case where the above-described semiconductor device is applied to an electronic appliance such as a computer, a portable information terminal (including a mobile phone, a portable game machine, an audio reproducing device, and the like), electronic paper, a television set (also referred to as television or television receiver), or a digital video camera is described. 
       FIG. 7A  illustrates a portable information terminal, which includes a housing  701 , a housing  702 , a first display portion  703   a , a second display portion  703   b , and the like. A variety of electronic components (e.g., CPU, MPU, or a memory element) are incorporated inside the housing  701  and the housing  702 . Further, electronic circuits (e.g., a driver circuit or a selection circuit) necessary for displaying an image are mounted on the first display portion  703   a  and the second display portion  703   b . In these electronic components and electronic circuits, the semiconductor devices according to any of the above embodiments in which moisture permeation into the transistors is suppressed are provided. Thus, in the portable information terminal, degradation of performance (e.g., in the case of a memory element, storing of false data which is caused by fluctuation in threshold voltage of a semiconductor device in the memory element; in the case of a driving device, output of false driving signals which is caused by fluctuation in threshold voltage of a semiconductor device in the driving device) is suppressed for a long period. Note that the semiconductor device according to any of the above embodiments may be provided in at least one of the housing  701  and the housing  702 . 
     At least one of the first display portion  703   a  and the second display portion  703   b  has a touch input function, and for example, as illustrated in a left part of  FIG. 7A , which of “touch input” and “keyboard input” is performed can be selected by selection buttons  704  displayed on the first display portion  703   a . Because the selection buttons with a variety of sizes can be displayed, the portable information terminal can be easily used by people of any generation. In the case where “keyboard input” is selected, for example, a keyboard  705  is displayed on the first display portion  703   a  as illustrated in a right part of  FIG. 7A . With the keyboard  705 , letters can be input quickly by keyboard input as in the case of using a conventional information terminal, for example. 
     Further, the housing  701  and the housing  702  of the portable information terminal in  FIG. 7A  can be separated as illustrated in the right part of  FIG. 7A . This structure enables very convenient operations; for example, screen data can be controlled from the housing  701  while the screen data is shared by a plurality of people with the housing  702  hung on a wall. Note that in the case where the device is not in use, the housing  701  and the housing  702  are preferably made to overlap such that the first display portion  703   a  faces the second display portion  703   b . In this manner, the first display portion  703   a  and the second display portion  703   b  can be protected from an external shock. 
     The portable information terminal illustrated in  FIG. 7A  can have a function of displaying various kinds of information (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 function of handling or editing the information displayed on the display portion, a function of controlling processing by various kinds of software (programs), and the like. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. 
     The portable information terminal illustrated in  FIG. 7A  may transmit and receive data wirelessly. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server. 
     Further, the housings  701  and  702  illustrated in  FIG. 7A  may be equipped with an antenna, a microphone function, or a wireless communication function to be used as a mobile phone. 
       FIG. 7B  shows a television device including a housing  721 , a display portion  722 , a stand  723 , a remote controller  724 , and the like. Also in the television device, a variety of electronic components and electronic circuits are incorporated in (or mounted on) the housing  721  and the display portion  722  as in the aforementioned portable information terminal. In these electronic components and electronic circuits, the semiconductor devices according to any of the above embodiments in which moisture permeation into the transistors is suppressed are provided, whereby in the television device, degradation of performance is suppressed for a long period. 
       FIG. 7C  illustrates a digital camera including a housing  731 , an operation switch  732 , a battery  733 , and the like. Further, the digital camera is provided with a display portion (not illustrated but positioned on the back surface of the main body) and the like on its back surface. The semiconductor devices according to any of the above embodiments are also provided in the housing  731  or the display portion of the digital camera. Thus, in the digital camera, degradation of performance is suppressed for a long period. 
     As described above, the semiconductor device according to any of the above embodiments is mounted on each of the electronic appliances described in this embodiment. Accordingly, power consumption can be reduced in the electronic appliances. 
     Example 1 
     In this example, a specific example of the method for forming the insulating layer  116 , the first metal oxide layer  118   a , and the second metal oxide layer  118   b  in Embodiment 1 will be described. Results of some kinds of analysis of the layers will also be described. 
     For the insulating layer  116 , a silicon oxide film was formed with a sputtering apparatus. Formation conditions of the insulating layer  116 , which was formed to a thickness of 400 nm, were as follows: silicon was used as a target, substrate temperature was room temperature, O 2  was 100% in a deposition atmosphere, the flow rate of the O 2  gas was 300 sccm, pressure inside a chamber was 0.7 Pa, a power source was an AC power source, and a power of 6 kW was applied. 
     Then, the metal oxide layer  118  was formed successively without exposure to the air in the apparatus that was used for the insulating layer  116 . Thus, adsorption of moisture at the interface between the insulating layer  116  and the metal oxide layer  118  can be suppressed. 
     For the metal oxide layer  118 , an aluminum oxide film was formed to a thickness of 50 nm under the following conditions: pure aluminum was used as a target, substrate temperature was 150° C., O 2  was 100% in a deposition atmosphere, the flow rate of the O 2  gas was 300 sccm, pressure inside the chamber was 0.7 Pa, a power source was an AC power source, and a power of 30 kW was applied. 
       FIG. 8  shows an image of a cross section of the metal oxide layer  118  formed over the insulating layer  116  observed with a scanning transmission electron microscope (STEM). As shown in  FIG. 8 , the metal oxide layer  118  has a two-layer structure in which the first metal oxide layer  118   a  and the second metal oxide layer  118   b  are successively stacked. This structure was formed in the following manner: in an early stage of the formation of the metal oxide layer  118 , elements of the layer below the metal oxide layer  118  (here, the silicon oxide film) were sputtered out by main elements (here, aluminum and oxygen) of the metal oxide layer  118 , which was formed with application of high power, and were mixed into the metal oxide layer  118  (this is referred to as a mixing effect) to hinder crystallization in the metal oxide layer  118 , whereby the first metal oxide layer  118   a  having an amorphous structure was formed; then, as the film formation proceeded, the mixing effect was reduced, so that the second metal oxide layer  118   b  having a polycrystalline structure was formed. 
     Note that in the formation of the aluminum oxide film, by adjusting the atmosphere and pressure in the chamber, and power to apply, thicknesses of the layers (the first metal oxide layer  118   a  and the second metal oxide layer  118   b ) in the above-mentioned two-layer structure can be deliberately adjusted. 
     Note that although this example describes successive stacking of the first metal oxide layer  118   a  and the second metal oxide layer  118   b  with the use of aluminum oxide, it is needless to say that also in the case of using a different kind of metal oxide, a structure in which layers are successively stacked may be formed by adjusting parameters in the film formation as described above. 
     To examine film qualities of the first metal oxide layer  118   a  and the second metal oxide layer  118   b , film densities at three points in each of the layers were measured by XRR. As a result, it was found that the film densities in the first metal oxide layer  118   a  were 3.04 g/cm 3 , 3.02 g/cm 3 , and 3.02 g/cm 3 , and the film densities in the second metal oxide layer  118   b  were 3.80 g/cm 3 , 3.78 g/cm 3 , and 3.76 g/cm 3 . The measurement results indicate that the first metal oxide layer  118   a  is aluminum oxide having an amorphous structure and the second metal oxide layer  118   b  is aluminum oxide having a polycrystalline structure. 
     This example describes the case where aluminum oxide was used for the first metal oxide layer  118   a  and the second metal oxide layer  118   b ; however, also in the case where a different kind of metal oxide is used, the state of a layer (e.g., whether an amorphous state or a polycrystalline state) can be examined by measuring film density by XRR and comparing the measurement results with values in literature or the like. 
     This example can be implemented in combination with any of the other embodiments as appropriate. 
     This application is based on Japanese Patent Application serial no. 2011-208468 filed with Japan Patent Office on Sep. 26, 2011, the entire contents of which are hereby incorporated by reference.