Patent Publication Number: US-8981370-B2

Title: Semiconductor device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a manufacturing method thereof. 
     In addition, the present invention relates to an object, a method, a method for producing an object, a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor device, a memory device, a display device, a liquid crystal display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof. Alternatively, the present invention relates to, for example, an electronic device including the memory device, the display device, or the light-emitting device. 
     Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices. 
     2. Description of the Related Art 
     A technique by which a transistor is formed with a semiconductor film formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and an image display device. A silicon film is widely known as a semiconductor film applicable to the transistor. 
     In the silicon film used as the semiconductor film of the transistor, amorphous silicon and crystalline silicon are used in accordance with the usage. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film which can be formed using the established technique for forming a film in a large-sized area. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, a heat treatment at a high temperature or a laser beam treatment which is performed on an amorphous silicon film has been known. 
     Further, in recent years, an oxide semiconductor film has attracted attention. For example, a transistor is disclosed in which an amorphous oxide semiconductor film containing indium, gallium, and zinc and having an electron carrier density of lower than 10 18 /cm 3  is used (see Patent Document 1). 
     The oxide semiconductor film can be formed by a sputtering method and therefore can be applied to a transistor included in a large-sized display device. Moreover, a transistor including the oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. Further, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized. 
     In the case where glass is used as a substrate over which the transistor is provided, the electrical characteristics of the transistor might deteriorate due to diffusion of impurities from the glass. In order to suppress the deterioration, a film having a barrier property is provided between the substrate and the transistor. 
     [Reference] 
     [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
     SUMMARY OF THE INVENTION 
     An object is to provide a semiconductor device which has stable electrical characteristics and high reliability. Another object is to provide a semiconductor device which has stable electrical characteristics. Another object is to provide a semiconductor device which has high reliability. Another object is to provide a semiconductor device which can operate at high speed. Another object is to provide a semiconductor device with low power consumption. An object is to provide a semiconductor device with high yield. 
     According to one embodiment of the present invention, a semiconductor device includes a gate electrode over an insulating surface, a gate insulating film over the gate electrode, a semiconductor film which is over the gate insulating film and overlaps with the gate electrode, and a protective film over the semiconductor film; and the protective film includes a crystalline insulating film and an aluminum oxide film over the crystalline insulating film. 
     According to another embodiment of the present invention, a semiconductor device includes a base film, a semiconductor film over the base film, a gate insulating film over the semiconductor film, and a gate electrode which is over the gate insulating film and overlaps with the semiconductor film; and the base film includes a crystalline insulating film and an aluminum oxide film over the crystalline insulating film. 
     According to another embodiment of the present invention, a semiconductor device includes a base film, a semiconductor film over the base film, a gate insulating film over part of the semiconductor film, a gate electrode which is over the gate insulating film and overlaps with the semiconductor film, and a protective film over the semiconductor film and the gate electrode; and the protective film includes a crystalline insulating film and an aluminum oxide film over the crystalline insulating film. 
     According to another embodiment of the present invention, a semiconductor device includes a semiconductor film, a gate electrode overlapping with the semiconductor film, and a gate insulating film which is provided between the semiconductor film and the gate electrode; and the gate insulating film includes a crystalline insulating film and an aluminum oxide film over the crystalline insulating film. 
     The crystalline insulating film contains one or more kinds of Mg, Ti, V, Cr, Y, Zr, and Ta. 
     The aluminum oxide film has crystallinity. 
     The aluminum oxide film has a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3 . 
     The aluminum oxide film is provided over the crystalline insulating film, whereby the aluminum oxide film can have crystallinity and a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3 . The aluminum oxide film has a high barrier property against impurities. Accordingly, a change in electrical characteristics of a transistor, which is caused by impurities, can be suppressed. 
     Further, the aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  has high resistance to a chemical liquid, plasma, or the like and is not unintentionally etched with ease. Therefore, occurrence of shape defects of the aluminum oxide film, which is caused by unintentional etching, can be suppressed. Note that in portions where shape defects of layers that constitute a transistor occur, an etching residue occurs and another shape defect is caused by the portion. Thus, it is important to suppress occurrence of shape defects in order to provide transistor having stable electrical characteristics. Further, the aluminum oxide film has a high barrier property against impurities. Furthermore, because the aluminum oxide film has few defects, the use of the aluminum oxide film for the gate insulating film can suppress deterioration in electrical characteristics of a transistor, which is caused by defects of the gate insulating film. 
     The aluminum oxide film is provided over the crystalline insulating film, whereby the aluminum oxide film can have crystallinity and a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3 . Accordingly, a semiconductor device which has stable electrical characteristics and high reliability can be provided. A semiconductor device which has stable electrical characteristics can be provided. A semiconductor device which has high reliability can be provided. A semiconductor device which can operate at high speed can be provided. A semiconductor device with low power consumption can be provided. A semiconductor device with high yield can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 2A to 2C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 3A to 3C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 4A to 4C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 5A to 5C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 6A to 6C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 7A to 7D  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 1A to 1C . 
         FIGS. 8A to 8C  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 1A to 1C . 
         FIGS. 9A to 9D  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 2A to 2C . 
         FIGS. 10A to 10C  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 2A to 2C . 
         FIGS. 11A to 11D  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 3A to 3C . 
         FIGS. 12A to 12D  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 3A to 3C . 
         FIGS. 13A to 13D  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 4A to 4C . 
         FIGS. 14A to 14D  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 4A to 4C . 
         FIGS. 15A to 15D  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 5A to 5C . 
         FIGS. 16A to 16D  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 5A to 5C . 
         FIGS. 17A to 17D  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 6A to 6C . 
         FIGS. 18A to 18D  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 6A to 6C . 
         FIGS. 19A to 19D  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 6A to 6C . 
         FIGS. 20A to 20C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 21A to 21C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 22A to 22C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 23A to 23C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 24A to 24C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 25A to 25C  are a top view and cross-sectional views which show an example of a transistor of one embodiment of the present invention. 
         FIGS. 26A to 26C  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 20A to 20C . 
         FIGS. 27A to 27C  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 20A to 20C . 
         FIGS. 28A to 28C  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 21A to 21C . 
         FIGS. 29A to 29C  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 21A to 21C . 
         FIGS. 30A to 30C  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 22A to 22C . 
         FIGS. 31A and 31B  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 22A to 22C . 
         FIGS. 32A to 32C  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 23A to 23C . 
         FIGS. 33A and 33B  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 23A to 23C . 
         FIGS. 34A to 34C  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 24A to 24C . 
         FIGS. 35A and 35B  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 24A to 24C . 
         FIGS. 36A to 36C  are cross-sectional views illustrating an example of a method for manufacturing the transistor illustrated in  FIGS. 25A to 25C . 
         FIGS. 37A to 37C  are cross-sectional views illustrating the example of the method for manufacturing the transistor illustrated in  FIGS. 25A to 25C . 
         FIGS. 38A to 38D  are circuit diagrams and a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention, and a graph showing electrical characteristics thereof. 
         FIG. 39  is a cross-sectional view of a semiconductor device of one embodiment of the present invention. 
         FIGS. 40A to 40C  are a circuit diagram and a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention, and a graph showing electrical characteristics thereof. 
         FIG. 41  is a cross-sectional view of a semiconductor device of one embodiment of the present invention. 
         FIGS. 42A to 42C  are block diagrams illustrating a structure of a CPU of one embodiment of the present invention. 
         FIGS. 43A to 43C  are a circuit diagram of a display device including an EL element of one embodiment of the present invention, a cross-sectional view of part of a pixel of the display device, and a cross-sectional view of a light-emitting layer in the pixel. 
         FIG. 44  is a cross-sectional view of part of a pixel of a display device including an EL element of one embodiment of the present invention. 
         FIGS. 45A and 45B  are a circuit diagram of a pixel of a display device including an EL element of one embodiment of the present invention and a cross-sectional view of the pixel. 
         FIG. 46  is a cross-sectional view of a pixel of a display device including a liquid crystal element of one embodiment of the present invention. 
         FIGS. 47A to 47D  illustrate electronic devices of one embodiment of the present invention. 
         FIGS. 48A to 48C  are cross-sectional TE images of each sample, which are observed by a scanning transmission electron microscope. 
         FIGS. 49A and 49B  are cross-sectional TE images of each sample, which are observed by a scanning transmission electron microscope. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details can be modified in various ways. Therefore, the present invention should not be construed as being limited to the description in the following embodiments. In describing structures of the present invention with reference to the drawings, the same reference numerals are used in common for the same portions in different drawings. Note that the same hatch pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases. 
     Note that what is described (or part thereof) in one embodiment can be applied to, combined with, or exchanged with another content in the same embodiment and/or what is described (or part thereof) in another embodiment or other embodiments. 
     Note that in each embodiment, a content described in the embodiment is a content described with reference to a variety of diagrams or a content described with a text disclosed in this specification. 
     In addition, by combining a diagram (or part thereof) described in one embodiment with another part of the diagram, a different diagram (or part thereof) described in the same embodiment, and/or a diagram (or part thereof) described in one or a plurality of different embodiments, much more diagrams can be formed. 
     Note that the size, the thickness of layers, or regions in diagrams is sometimes exaggerated for simplicity. Therefore, embodiments of the present invention are not limited to such scales. 
     Note that drawings are schematic views of ideal examples, and the embodiments of the present invention are not limited to the shape or the value illustrated in the drawings. For example, the following can be included: variation in shape due to a manufacturing technique; variation in shape due to an error; variation in signal, voltage, or current due to noise; variation in signal, voltage, or current due to a difference in timing; or the like. 
     Note that a voltage refers, in many cases, to a potential difference between a certain potential and a reference potential (e.g., a ground potential (GND) or a source potential). Accordingly, a voltage can also be called a potential. 
     Even when the expression “to be electrically connected” is used in this specification, there is a case in which no physical connection is made and a wiring is just extended in an actual circuit. 
     Note that technical terms are in many cases used in order to describe a specific embodiment, example, or the like. However, one embodiment of the present invention should not be construed as being limited by the technical terms. 
     Note that terms which are not defined (including terms used for science and technology, such as technical terms and academic parlance) can be used as the terms which have a meaning equivalent to a general meaning that an ordinary person skilled in the art understands. It is preferred that terms defined by dictionaries or the like are construed as consistent meaning with the background of related art. 
     Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. In addition, the ordinal numbers do not denote particular names which specify the present invention. 
     The invention excluding content which is not specified in the drawings and texts in this specification can be constituted. Alternatively, when the range of a value (e.g., the maximum and minimum values) is described, the range may be freely narrowed or a value in the range may be excluded, so that the invention can be specified by a range part of which is excluded. In this manner, it is possible to specify the scope of the present invention so that a conventional technology is excluded, for example. 
     As a specific example, a diagram of a circuit including a first transistor to a fifth transistor is illustrated. In that case, it can be specified that the circuit does not include a sixth transistor in the invention. It can be specified that the circuit does not include a capacitor in the invention. Further, it can be specified that the circuit does not include a sixth transistor with a particular connection structure in the invention. It can be specified that the circuit does not include a capacitor with a particular connection structure in the invention. For example, it can be specified that a sixth transistor whose gate is connected to a gate of the third transistor is not included in the invention. For example, it can be specified that a capacitor whose first electrode is connected to the gate of the third transistor is not included in the invention. 
     As another specific example, a description of a value, “a voltage is preferably higher than or equal to 3 V and lower than or equal to 10 V” is given. In that case, for example, it can be specified that the case where the voltage is higher than or equal to −2 V and lower than or equal to 1 V is excluded from the invention. For example, it can be specified that the case where the voltage is higher than or equal to 13 V is excluded from the invention. Note that, for example, it can be specified that the voltage is higher than or equal to 5 V and lower than or equal to 8 V in the invention. For example, it can be specified that the voltage is approximately 9 V in the invention. For example, it can be specified that the voltage is higher than or equal to 3 V and lower than or equal to 10 V but is not 9 V in the invention. 
     As another specific example, a description “a voltage is preferred to be 10 V” is given. In that case, for example, it can be specified that the case where the voltage is higher than or equal to −2 V and lower than or equal to 1 V is excluded from the invention. For example, it can be specified that the case where the voltage is higher than or equal to 13 V is excluded from the invention. 
     As another specific example, a description “a film is an insulating film” is given to describe properties of a material. In that case, for example, it can be specified that the case where the insulating film is an organic insulating film is excluded from the invention. For example, it can be specified that the case where the insulating film is an inorganic insulating film is excluded from the invention. 
     As another specific example, a description of a stacked-layer structure, “a film is provided between A and B” is given. In that case, for example, it can be specified that the case where the film is a stacked film of four or more layers is excluded from the invention. For example, it can be specified that the case where a conductive film is provided between A and the film is excluded from the invention. 
     Note that various people can implement the invention described in this specification and the like. However, different people may be involved in the implementation of the invention. For example, in the case of a transmission/reception system, the following case is possible: Company A manufactures and sells transmitting devices, and Company B manufactures and sells receiving devices. As another example, in the case of a light-emitting device including a TFT and a light-emitting element, the following case is possible: Company A manufactures and sells semiconductor devices including TFTs, and Company B purchases the semiconductor devices, provides light-emitting elements for the semiconductor devices, and completes light-emitting devices. 
     In such a case, one embodiment of the invention can be constituted so that a patent infringement can be claimed against each of Company A and Company B. That is, one embodiment of the invention with which a patent infringement can be claimed against Company A or Company B is clear and can be regarded as being disclosed in this specification or the like. For example, in the case of a transmission/reception system, one embodiment of the invention can be constituted by only a transmitting device and one embodiment of the invention can be constituted by only a receiving device. Those embodiments of the invention are clear and can be regarded as being disclosed in this specification or the like. As another example, in the case of a light-emitting device including a TFT and a light-emitting element, one embodiment of the invention can be constituted by only a semiconductor device including a TFT, and one embodiment of the invention can be constituted by a light-emitting device including a TFT and a light-emitting element. Those embodiments of the invention are clear and can be regarded as being disclosed in this specification or the like. 
     [Embodiment 1] 
     In this embodiment, a transistor of one embodiment of the present invention will be described. 
       FIG. 1A  is a top view of a transistor of one embodiment of the present invention.  FIG. 1B  is a cross-sectional view taken along dashed-dotted line A 1 -A 2  in  FIG. 1A .  FIG. 1C  is a cross-sectional view taken along dashed-dotted line A 3 -A 4  in  FIG. 1A . Note that for simplicity, a gate insulating film  112  and the like are not illustrated in  FIG. 1A . 
       FIG. 1B  is a cross-sectional view of a transistor including a crystalline insulating film  132  over a substrate  100 ; an aluminum oxide film  134  over the crystalline insulating film  132 ; a gate electrode  104  over the aluminum oxide film  134 ; the gate insulating film  112  over the gate electrode  104 ; a semiconductor film  106  which is over the gate insulating film  112  and overlaps with the gate electrode  104 ; a source electrode  116   a  and a drain electrode  116   b  over the semiconductor film  106 ; a crystalline insulating film  136  over the semiconductor film  106 , and the source electrode  116   a  and the drain electrode  116   b ; and an aluminum oxide film  138  over the crystalline insulating film  136 . 
     Here, the crystalline insulating film  132  and the crystalline insulating film  136  each contain one or more kinds of Mg, Ti, V, Cr, Y, Zr, and Ta. Specifically, it is preferred to include one or more kinds of magnesium oxide, titanium oxide, vanadium oxide, chromium oxide, yttrium oxide, zirconium oxide, and tantalum oxide. For example, an insulating film containing zirconium oxide and yttrium oxide can be used. 
     The crystalline insulating film  132  and the crystalline insulating film  136  are each an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by X-ray diffraction (XRD), electron diffraction, or neutron diffraction. 
     The aluminum oxide film  134  and the aluminum oxide film  138  each have crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  134  has crystallinity even near the interface with the crystalline insulating film  132 . The aluminum oxide film  138  has crystallinity even near the interface with the crystalline insulating film  136 . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film is an aluminum oxide film having a lower density than an aluminum oxide film having crystallinity. 
     The aluminum oxide film  134  and the aluminum oxide film  138  are each a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by X-ray reflectivity (XRR) or Rutherford backscattering spectrometry (RBS). The aluminum oxide film  134  and the aluminum oxide film  138  each have a high bather property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  134  does not include a low density layer near the crystalline insulating film  132 , and the aluminum oxide film  138  does not either include a low density layer near the crystalline insulating film  136 . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     The semiconductor film  106  can be formed using a silicon film, a germanium film, a silicon germanium film, a gallium arsenide film, a silicon carbide film, or a gallium nitride film. Note that an organic semiconductor film may be used as the semiconductor film  106 . Alternatively, an oxide semiconductor film may be used as the semiconductor film  106 . 
     As the oxide semiconductor film, an In-M-Zn oxide film can be used. Here, a metal element M is an element whose bond energy with oxygen is higher than that of In and that of Zn. Alternatively, the metal element M is an element which has a function of suppressing desorption of oxygen from the In-M-Zn oxide film. Owing to the effect of the metal element m, generation of an oxygen vacancy in the oxide semiconductor film is suppressed. Note that oxygen vacancies in the oxide semiconductor film sometimes generate carriers. Therefore, the effect of the metal element M can suppress an increase in carrier density in the oxide semiconductor film and an increase in an off-state current. Moreover, a change in the electrical characteristics of the transistor, which is caused by oxygen vacancies, can be reduced, so that a highly reliable transistor can be obtained. 
     The metal element M can be, specifically, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Nb, Mo, Sn, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, or W, and is preferably Al, Ti, Ga, Y, Zr, Ce, or Hf. The metal element M can be formed using one or more elements selected from the above elements. Further, Si or Ge may be used instead of the metal element M. 
     The hydrogen concentration in the oxide semiconductor film is lower than or equal to 2×10 20  atoms/cm 3 , preferably lower than or equal to 5×10 19  atoms/cm 3 , further preferably lower than or equal to 1×10 19  atoms/cm 3 . This is because hydrogen included in the oxide semiconductor film sometimes generates unintentional carriers. The generated carriers are a factor of increasing an off-state current of the transistor and changing electrical characteristics of the transistor. Thus, with the above range of the hydrogen concentration in the oxide semiconductor film, it is possible to suppress an increase in off-state current of the transistor and a change in electric characteristics of the transistor. 
     An oxide semiconductor film may be in a non-single-crystal state, for example. The non-single-crystal state is, for example, structured by at least one of c-axis aligned crystal (CAAC), polycrystal, microcrystal, and an amorphous part. The density of defect states of an amorphous part is higher than those of microcrystal and CAAC. The density of defect states of microcrystal is higher than that of CAAC. Note that an oxide semiconductor including CAAC is referred to as a CAAC-OS (c-axis aligned crystalline oxide semiconductor). 
     For example, an oxide semiconductor film may include a CAAC-OS. In the CAAC-OS, for example, c-axes are aligned, and a-axes and/or b-axes are not macroscopically aligned. 
     For example, an oxide semiconductor film may include microcrystal. Note that an oxide semiconductor including microcrystal is referred to as a microcrystalline oxide semiconductor. A microcrystalline oxide semiconductor film includes microcrystal (also referred to as nanocrystal) with a size greater than or equal to 1 nm and less than 10 nm, for example. 
     For example, an oxide semiconductor film may include an amorphous part. Note that an oxide semiconductor including an amorphous part is referred to as an amorphous oxide semiconductor. An amorphous oxide semiconductor film, for example, has disordered atomic arrangement and no crystalline component. Alternatively, an amorphous oxide semiconductor film is, for example, absolutely amorphous and has no crystal part. 
     Note that an oxide semiconductor film may be a mixed film including any of a CAAC-OS, a microcrystalline oxide semiconductor, and an amorphous oxide semiconductor. The mixed film, for example, includes a region of an amorphous oxide semiconductor, a region of a microcrystalline oxide semiconductor, and a region of a CAAC-OS. Further, the mixed film may have a stacked structure including a region of an amorphous oxide semiconductor, a region of a microcrystalline oxide semiconductor, and a region of a CAAC-OS, for example. 
     Note that an oxide semiconductor film may be in a single-crystal state, for example. 
     An oxide semiconductor film preferably includes a plurality of crystal parts. In each of the crystal parts, a c-axis is preferably aligned in a direction parallel to a normal vector of a surface where the oxide semiconductor film is formed or a normal vector of a surface of the oxide semiconductor film. 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. An example of such an oxide semiconductor film is a CAAC-OS film. 
     Note that in most cases, a crystal part in the CAAC-OS film fits inside a cube whose one side is less than 100 nm In an image obtained with a transmission electron microscope (TEM), a boundary between crystal parts in the CAAC-OS film are not clearly detected. Further, with the TEM, a grain boundary in the CAAC-OS film is not clearly found. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is suppressed. 
     In each of the crystal parts included in the CAAC-OS film, for example, 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. Further, in each of the crystal parts, metal atoms are arranged in a triangular or hexagonal configuration when seen from the direction perpendicular to the a-b plane, 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 term “perpendicular” includes a range from 80° to 100°, preferably from 85° to 95°. In addition, a term “parallel” includes a range from −10° to 10°, preferably 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 film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. Further, when an impurity is added to the CAAC-OS film, crystallinity of the crystal part in a region to which the impurity is added is lowered 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 the film deposition is accompanied with the formation of the crystal parts or followed by the formation of the crystal parts through a crystallization treatment such as a heat treatment. Hence, the c-axes of the crystal parts are aligned in 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. 
     In a transistor using the CAAC-OS film, change in electrical characteristics due to irradiation with visible light or ultraviolet light is small. Thus, the transistor has high reliability. 
     The oxide semiconductor film has a wider band gap than a silicon film by approximately 1 eV to 2 eV. For that reason, in the transistor including the oxide semiconductor film, impact ionization is unlikely to occur and avalanche breakdown is unlikely to occur. That is, it can be said that hot-carrier degradation is unlikely to occur in the transistor. 
     In the case where the oxide semiconductor film is used as the semiconductor film  106  as described above, a channel region can be completely depleted by an electric field of the gate electrode  104  even in the case where the thickness of the semiconductor film  106  is large (e.g., greater than or equal to 15 nm and less than 100 nm) because the oxide semiconductor film generates fewer carriers. Thus, in the transistor including the oxide semiconductor film, an increase in an off-state current and a change in a threshold voltage due to a punch-through phenomenon is not caused. When the channel length is, for example, 3 μm, the off-state current can be lower than 10 −21  A or lower than 10 −24  A per micrometer of channel width at room temperature. 
     The oxygen vacancies in the oxide semiconductor film, which are a factor of generating carriers, can be evaluated by electron spin resonance (ESR). That is, an oxide semiconductor film with few oxygen vacancies can be referred to as an oxide semiconductor film which does not have a signal evaluated by ESR, which is caused by oxygen vacancies. Specifically, the spin density attributed to oxygen vacancies of the oxide semiconductor film is lower than 5×10 16  spins/cm 3 . When the oxide semiconductor film has oxygen vacancies, a signal having symmetry is found at a g value of around 1.93 in ESR. 
     Here, there is no particular limitation on the substrate  100  as long as it has heat resistance enough to withstand at least a heat treatment performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate  100 . Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon-on-insulator (SOI) substrate, or the like may be used as the substrate  100 . Still alternatively, any of these substrates further provided with a semiconductor element may be used as the substrate  100 . 
     In the case where a large glass substrate such as the 5th generation (1000 mm×1200 mm or 1300 mm×1500 mm), the 6th generation (1500 mm×1800 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2500 mm), the 9th generation (2400 mm×2800 mm), or the 10th generation (2880 mm×3130 mm) is used as the substrate  100 , minute processing is sometimes difficult due to shrinkage of the substrate  100  caused by a heat treatment or the like in a manufacturing process of a semiconductor device. Therefore, in the case where the above-described large glass substrate is used as the substrate  100 , a substrate which is unlikely to shrink through the heat treatment is preferred to be used. For example, as the substrate  100 , it is possible to use a large glass substrate in which the amount of shrinkage after a heat treatment which is performed for an hour at 400° C., preferably 450° C., more preferably 500° C. is less than or equal to 10 ppm, preferably less than or equal to 5 ppm, more preferably less than or equal to 3 ppm. 
     Further alternatively, a flexible substrate may be used as the substrate  100 . Note that as a method for forming a transistor over a flexible substrate, there is also a method in which, after a transistor is formed over a non-flexible substrate, the transistor is separated from the non-flexible substrate and transferred to the substrate  100  which is a flexible substrate. In that case, a separation layer is preferred to be provided between the non-flexible substrate and the transistor. 
     The gate electrode  104  can be formed to have a single-layer or a stacked-layer structure of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. 
     The source electrode  116   a  and the drain electrode  116   b  can be formed to have a single-layer or a stacked-layer structure of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. Note that a conductive film for forming the source electrode  116   a  and the drain electrode  116   b  may be the same or different from each other. 
     A protective insulating film  118  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     Silicon oxynitride refers to a substance that contains more oxygen than nitrogen, and silicon nitride oxide refers to a substance that contains more nitrogen than oxygen. 
     The protective insulating film  118  is preferred to contain excess oxygen. 
     In the case where the protective insulating film  118  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     An insulating film containing excess oxygen refers to an insulating film in which the amount of released oxygen which is converted into oxygen atoms is greater than or equal to 1×10 18  atoms/cm 3 , greater than or equal to 1×10 19  atoms/cm 3 , or greater than or equal to 1×10 20  atoms/cm 3  in thermal desorption spectroscopy (TDS). 
     Here, a measurement method of the amount of released oxygen using TDS analysis will be described. 
     The total amount of a released gas in TDS analysis is proportional to the integral value of intensity of ions of the released gas. Then, the integral value is compared with that of a standard sample, whereby the total amount of the released gas can be calculated. 
     For example, the amount of oxygen molecules (N O2 ) released from an insulating film can be found according to Formula 1 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 gases having a mass number of 32 which are obtained in the TDS analysis are assumed to originate from an oxygen molecule. A CH 3 OH gas, 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 also not taken into consideration because the proportion of such a molecule in the natural world is minimal. 
     
       
         
           
             
               
                 
                   
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                         N 
                         
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                         S 
                         
                           H 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                     
                     × 
                     
                       S 
                       
                         O 
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                     α 
                   
                 
               
               
                 
                   [ 
                   
                     FORMULA 
                     ⁢ 
                     
                         
                     
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                     1 
                   
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     The value N H2  is obtained by conversion of the amount of hydrogen molecules desorbed from the standard sample into densities. The integral value of ion intensity in the case where the standard sample is subjected to the TDS analysis is denoted by S H2 . Here, the reference value of the standard sample is N H2 /S H2 . The integral value of ion intensity in the case where the insulating film is subjected to the TDS analysis is denoted by S O2 . A coefficient affecting the ion intensity in the TDS analysis is denoted by α. Japanese Published Patent Application No. H6-275697 can be referred to for details of Formula 1. Note that the amount of oxygen released from the above insulating film is measured with EMD-WA1000S/W, a thermal desorption spectroscopy apparatus produced by ESCO Ltd., with the use of a silicon wafer containing a hydrogen atom at 1×10 16  atoms/cm 2  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 coefficient α includes the ionization rate of the oxygen molecules, the amount of the released oxygen atoms can be estimated through the evaluation of the amount of the released oxygen molecules. 
     Note that the amount of the released oxygen molecules is denoted by N O2 . The amount of released oxygen converted into oxygen atoms is twice the amount of the released oxygen molecules. 
     The insulating film containing excess oxygen may contain a peroxide radical. Specifically, the spin density attributed to a peroxide radical of the insulating film is higher than or equal to 5×10 17  spins/cm 3 . Note that the insulating film containing a peroxide radical has a signal having asymmetry at a g value of around 2.01 in ESR. 
     Alternatively, the insulating film containing excess oxygen may be formed using 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 RBS. 
     The gate insulating film  112  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     The gate insulating film  112  is preferred to contain excess oxygen. 
     In the case where the gate insulating film  112  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The transistor illustrated in  FIGS. 1A to 1C  is surrounded with the aluminum oxide film  134  and the aluminum oxide film  138  each having crystallinity and a high density. Thus, the transistor has a high barrier property against external impurities. Further, outward diffusion of excess oxygen contained in the transistor can be suppressed because the aluminum oxide films do not transmit oxygen. 
     Accordingly, the transistor illustrated in  FIGS. 1A to 1C  has stable electrical characteristics. 
     Note that although  FIGS. 1A to 1C  illustrate the structure including all of the crystalline insulating film  132 , the aluminum oxide film  134 , the crystalline insulating film  136 , and the aluminum oxide film  138 , one embodiment of the present invention is not limited to this structure. For example, the crystalline insulating film  132  and the aluminum oxide film  134  are not necessarily provided. Alternatively, the crystalline insulating film  136  and the aluminum oxide film  138  are not necessarily provided. 
     Next, a transistor having a structure different from that of the transistor illustrated in  FIGS. 1A to 1C  will be described with reference to  FIGS. 2A to 2C . 
       FIG. 2A  is a top view of a transistor of one embodiment of the present invention.  FIG. 2B  is a cross-sectional view taken along dashed-dotted line B 1 -B 2  in  FIG. 2A .  FIG. 2C  is a cross-sectional view taken along dashed-dotted line B 3 -B 4  in  FIG. 2A . Note that for simplicity, a gate insulating film  212  and the like are not illustrated in  FIG. 2A . 
       FIG. 2B  is a cross-sectional view of a transistor including a crystalline insulating film  232  over a substrate  200 ; an aluminum oxide film  234  over the crystalline insulating film  232 ; a gate electrode  204  over the aluminum oxide film  234 ; the gate insulating film  212  over the gate electrode  204 ; a source electrode  216   a  and a drain electrode  216   b  over the gate insulating film  212 ; a semiconductor film  206  which is over the gate insulating film  212 , and the source electrode  216   a  and the drain electrode  216   b  and overlaps with the gate electrode  204 ; a crystalline insulating film  236  over the semiconductor film  206 , and the source electrode  216   a  and the drain electrode  216   b ; and an aluminum oxide film  238  over the crystalline insulating film  236 . 
     Here, as the crystalline insulating film  232  and the crystalline insulating film  236 , a film selected from the insulating films given as examples of the crystalline insulating film  132  and the crystalline insulating film  136  can be used. 
     The crystalline insulating film  232  and the crystalline insulating film  236  are each an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  234  and the aluminum oxide film  238  each have crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  234  has crystallinity even near the interface with the crystalline insulating film  232 . The aluminum oxide film  238  has crystallinity even near the interface with the crystalline insulating film  236 . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. 
     The aluminum oxide film  234  and the aluminum oxide film  238  are each a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  234  and the aluminum oxide film  238  each have a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  234  does not include a low density layer near the crystalline insulating film  232 , and the aluminum oxide film  238  does not either include a low density layer near the crystalline insulating film  236 . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  206 . 
     The description of the substrate  100  is referred to for the substrate  200 . 
     The description of the gate electrode  104  is referred to for the gate electrode  204 . 
     The description of the source electrode  116   a  and the drain electrode  116   b  are referred to for the source electrode  216   a  and the drain electrode  216   b.    
     The description of the protective insulating film  118  is referred to for the protective insulating film  218 . 
     The protective insulating film  218  is preferred to contain excess oxygen. 
     In the case where the protective insulating film  218  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate insulating film  112  is referred to for the gate insulating film  212 . 
     The gate insulating film  212  is preferred to contain excess oxygen. 
     In the case where the gate insulating film  212  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The transistor illustrated in  FIGS. 2A to 2C  is surrounded with the aluminum oxide film  234  and the aluminum oxide film  238  each having crystallinity and a high density. Thus, the transistor has a high barrier property against external impurities. Further, outward diffusion of excess oxygen contained in the transistor can be suppressed because the aluminum oxide films do not transmit oxygen. 
     Accordingly, the transistor illustrated in  FIGS. 2A to 2C  has stable electrical characteristics. 
     Note that although  FIGS. 2A to 2C  illustrate the structure including all of the crystalline insulating film  232 , the aluminum oxide film  234 , the crystalline insulating film  236 , and the aluminum oxide film  238 , one embodiment of the present invention is not limited to this structure. For example, the crystalline insulating film  232  and the aluminum oxide film  234  are not necessarily provided. Alternatively, the crystalline insulating film  236  and the aluminum oxide film  238  are not necessarily provided. 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 1A to 1C  and  FIGS. 2A to 2C  will be described with reference to  FIGS. 3A to 3C . 
       FIG. 3A  is a top view of a transistor of one embodiment of the present invention.  FIG. 3B  is a cross-sectional view taken along dashed-dotted line C 1 -C 2  in  FIG. 3A .  FIG. 3C  is a cross-sectional view taken along dashed-dotted line C 3 -C 4  in  FIG. 3A . Note that for simplicity, a gate insulating film  312  and the like are not illustrated in  FIG. 3A . 
       FIG. 3B  is a cross-sectional view of a transistor including a crystalline insulating film  332  over a substrate  300 ; an aluminum oxide film  334  over the crystalline insulating film  332 ; a base insulating film  302  over the aluminum oxide film  334 ; a semiconductor film  306  over the base insulating film  302 ; a source electrode  316   a  and a drain electrode  316   b  over the semiconductor film  306 ; the gate insulating film  312  over the semiconductor film  306 , and the source electrode  316   a  and the drain electrode  316   b ; a gate electrode  304  which is over the gate insulating film  312  and overlaps with the semiconductor film  306 ; a crystalline insulating film  336  over the gate electrode  304 ; and an aluminum oxide film  338  over the crystalline insulating film  336 . 
     Here, as the crystalline insulating film  332  and the crystalline insulating film  336 , a film selected from the insulating films given as examples of the crystalline insulating film  132  and the crystalline insulating film  136  can be used. 
     The crystalline insulating film  332  and the crystalline insulating film  336  are each an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  334  and the aluminum oxide film  338  each have crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  334  has crystallinity even near the interface with the crystalline insulating film  332 . The aluminum oxide film  338  has crystallinity even near the interface with the crystalline insulating film  336 . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. 
     The aluminum oxide film  334  and the aluminum oxide film  338  are each a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  334  and the aluminum oxide film  338  each have a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  334  does not include a low density layer near the crystalline insulating film  332 , and the aluminum oxide film  338  does not either include a low density layer near the crystalline insulating film  336 . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  306 . 
     The description of the substrate  100  is referred to for the substrate  300 . 
     The base insulating film  302  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     The base insulating film  302  is preferred to contain excess oxygen. 
     In the case where the base insulating film  302  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate insulating film  112  is referred to for the gate insulating film  312 . 
     The gate insulating film  312  is preferred to contain excess oxygen. 
     In the case where the gate insulating film  312  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the source electrode  116   a  and the drain electrode  116   b  are referred to for the source electrode  316   a  and the drain electrode  316   b.    
     The description of the gate electrode  104  is referred to for the gate electrode  304 . 
     The transistor illustrated in  FIGS. 3A to 3C  is surrounded with the aluminum oxide film  334  and the aluminum oxide film  338  each having crystallinity and a high density. Thus, the transistor has a high barrier property against external impurities. Further, outward diffusion of excess oxygen contained in the transistor can be suppressed because the aluminum oxide films do not transmit oxygen. 
     Accordingly, the transistor illustrated in  FIGS. 3A to 3C  has stable electrical characteristics. 
     Note that although  FIGS. 3A to 3C  illustrate the structure including all of the crystalline insulating film  332 , the aluminum oxide film  334 , the crystalline insulating film  336 , and the aluminum oxide film  338 , one embodiment of the present invention is not limited to this structure. For example, the crystalline insulating film  332  and the aluminum oxide film  334  are not necessarily provided. Alternatively, the crystalline insulating film  336  and the aluminum oxide film  338  are not necessarily provided. 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 1A to 1C ,  FIGS. 2A to 2C , and  FIGS. 3A to 3C  will be described with reference to  FIGS. 4A to 4C . 
       FIG. 4A  is a top view of a transistor of one embodiment of the present invention.  FIG. 4B  is a cross-sectional view taken along dashed-dotted line D 1 -D 2  in  FIG. 4A .  FIG. 4C  is a cross-sectional view taken along dashed-dotted line D 3 -D 4  in  FIG. 4A . Note that for simplicity, a gate insulating film  412  and the like are not illustrated in  FIG. 4A . 
       FIG. 4B  is a cross-sectional view of a transistor including a crystalline insulating film  432  over a substrate  400 ; an aluminum oxide film  434  over the crystalline insulating film  432 ; a base insulating film  402  over the aluminum oxide film  434 ; a source electrode  416   a  and a drain electrode  416   b  over the base insulating film  402 ; a semiconductor film  406  over the base insulating film  402 , and the source electrode  416   a  and the drain electrode  416   b ; the gate insulating film  412  over the semiconductor film  406 ; a gate electrode  404  which is over the gate insulating film  412  and overlaps with the semiconductor film  406 ; a crystalline insulating film  436  over the gate electrode  404 ; and an aluminum oxide film  438  over the crystalline insulating film  436 . Note that the gate insulating film  412  includes a crystalline insulating film  412   a  and an aluminum oxide film  412   b  over the crystalline insulating film  412   a.    
     Here, as the crystalline insulating film  432  and the crystalline insulating film  436 , a film selected from the insulating films given as examples of the crystalline insulating film  132  and the crystalline insulating film  136  can be used. 
     The crystalline insulating film  432  and the crystalline insulating film  436  are each an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  434  and the aluminum oxide film  438  each have crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  434  has crystallinity even near the interface with the crystalline insulating film  432 . The aluminum oxide film  438  has crystallinity even near the interface with the crystalline insulating film  436 . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. 
     The aluminum oxide film  434  and the aluminum oxide film  438  are each a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  434  and the aluminum oxide film  438  each have a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  434  does not include a low density layer near the crystalline insulating film  432 , and the aluminum oxide film  438  does not either include a low density layer near the crystalline insulating film  436 . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  406 . 
     The description of the substrate  100  is referred to for the substrate  400 . 
     The description of the base insulating film  302  is referred to for the base insulating film  402 . 
     The base insulating film  402  is preferred to contain excess oxygen. 
     In the case where the base insulating film  402  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate insulating film  112  is referred to for the gate insulating film  412 . 
     The gate insulating film  412  is preferred to contain excess oxygen. 
     In the case where the gate insulating film  412  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the source electrode  116   a  and the drain electrode  116   b  are referred to for the source electrode  416   a  and the drain electrode  416   b.    
     The description of the gate electrode  104  is referred to for the gate electrode  404 . 
     The transistor illustrated in  FIGS. 4A to 4C  is surrounded with the aluminum oxide film  434  and the aluminum oxide film  438  each having crystallinity and a high density. Thus, the transistor has a high barrier property against external impurities. Further, outward diffusion of excess oxygen contained in the transistor can be suppressed because the aluminum oxide films do not transmit oxygen. 
     Accordingly, the transistor illustrated in  FIGS. 4A to 4C  has stable electrical characteristics. 
     Note that although  FIGS. 4A to 4C  illustrate the structure including all of the crystalline insulating film  432 , the aluminum oxide film  434 , the crystalline insulating film  436 , and the aluminum oxide film  438 , one embodiment of the present invention is not limited to this structure. For example, the crystalline insulating film  432  and the aluminum oxide film  434  are not necessarily provided. Alternatively, the crystalline insulating film  436  and the aluminum oxide film  438  are not necessarily provided. 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 1A to 1C ,  FIGS. 2A to 2C ,  FIGS. 3A to 3C , and  FIGS. 4A to 4C  will be described with reference to  FIGS. 5A to 5C . 
       FIG. 5A  is a top view of a transistor of one embodiment of the present invention.  FIG. 5B  is a cross-sectional view taken along dashed-dotted line E 1 -E 2  in  FIG. 5A .  FIG. 5C  is a cross-sectional view taken along dashed-dotted line E 3 -E 4  in  FIG. 5A . Note that for simplicity, a gate insulating film  512  and the like are not illustrated in  FIG. 5A . 
       FIG. 5B  is a cross-sectional view of a transistor including a crystalline insulating film  532  over a substrate  500 ; an aluminum oxide film  534  over the crystalline insulating film  532 ; a base insulating film  502  over the aluminum oxide film  534 ; a semiconductor film  506  over the base insulating film  502 ; the gate insulating film  512  over the semiconductor film  506 ; a gate electrode  504  which is over the gate insulating film  512  and overlaps with the semiconductor film  506 ; a crystalline insulating film  536  over the semiconductor film  506  and the gate electrode  504 ; and an aluminum oxide film  538  over the crystalline insulating film  536 . 
     In the cross-sectional view in  FIG. 5B , a protective insulating film  518  is provided over the aluminum oxide film  538 . Note that openings reaching the semiconductor film  506  are formed in the crystalline insulating film  536 , the aluminum oxide film  538 , and the protective insulating film  518 , and a wiring  524   a  and a wiring  524   b  provided over the protective insulating film  518  are in contact with the semiconductor film  506  through the openings. 
     Note that although the gate insulating film  512  is provided only in a region overlapping with the gate electrode  504  in  FIG. 5B , one embodiment of the present invention is not limited to this structure. For example, the gate insulating film  512  may be provided so as to cover the semiconductor film  506 . Alternatively, a sidewall insulating film may be provided in contact with a side surface of the gate electrode  504 . 
     In the case of providing the sidewall insulating film, it is preferred that, in the semiconductor film  506 , a region overlapping with the sidewall insulating film has lower resistance than a region overlapping with the gate electrode  504 . For example, in the semiconductor film  506 , a region not overlapping with the gate electrode  504  may contain an impurity that reduces the resistance of the semiconductor film  506 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  506 , the region overlapping with the sidewall insulating film has lower resistance than the region overlapping with the gate electrode  504 ; thus, the region serves as a lightly doped drain (LDD) region. With the LDD regions of the transistor, drain induced barrier lower (DIBL) and hot-carrier degradation can be suppressed. Note that in the semiconductor film  506 , the region overlapping with the sidewall insulating film may serve also as an offset region. Also with the offset region of the transistor, DIBL and hot-carrier degradation can be suppressed. 
     Here, as the crystalline insulating film  532  and the crystalline insulating film  536 , a film selected from the insulating films given as examples of the crystalline insulating film  132  and the crystalline insulating film  136  can be used. 
     The crystalline insulating film  532  and the crystalline insulating film  536  are each an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  534  and the aluminum oxide film  538  each have crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  534  has crystallinity even near the interface with the crystalline insulating film  532 . The aluminum oxide film  538  has crystallinity even near the interface with the crystalline insulating film  536 . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. 
     The aluminum oxide film  534  and the aluminum oxide film  538  are each a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  534  and the aluminum oxide film  538  each have a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  534  does not include a low density layer near the crystalline insulating film  532 , and the aluminum oxide film  538  does not either include a low density layer near the crystalline insulating film  536 . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  506 . 
     It is preferred that, in the semiconductor film  506 , the region not overlapping with the gate electrode  504  has lower resistance than a region overlapping with the gate electrode  504 . For example, in the semiconductor film  506 , the region not overlapping with the gate electrode  504  may contain an impurity that reduces the resistance of the semiconductor film  506 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  506 , the region not overlapping with the gate electrode  504  has lower resistance than the region overlapping with the gate electrode  504 ; thus, the region can serve as a source region and a drain region of the transistor. 
     The description of the substrate  100  is referred to for the substrate  500 . 
     The description of the base insulating film  302  is referred to for the base insulating film  502 . 
     The base insulating film  502  is preferred to contain excess oxygen. 
     In the case where the base insulating film  502  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate insulating film  112  is referred to for the gate insulating film  512 . 
     The gate insulating film  512  is preferred to contain excess oxygen. 
     In the case where the gate insulating film  512  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate electrode  104  is referred to for the gate electrode  504 . 
     The protective insulating film  518  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     The wiring  524   a  and the wiring  524   b  can be formed to have a single-layer or a stacked-layer structure of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. Note that the wiring  524   a  and the wiring  524   b  may be the same or different from each other. 
     In the transistor illustrated in  FIGS. 5A to 5C , a region where the gate electrode  504  overlaps with another wiring and electrode is small; therefore, parasitic capacitance is unlikely to be generated. Accordingly, the switching characteristics of the transistor can be enhanced. Moreover, the channel length of the transistor is determined by the width of the gate electrode  504 ; therefore, a miniaturized transistor having a short channel length is manufactured easily. 
     The transistor illustrated in  FIGS. 5A to 5C  is surrounded with the aluminum oxide film  534  and the aluminum oxide film  538  each having crystallinity and a high density. Thus, the transistor has a high barrier property against external impurities. Further, outward diffusion of excess oxygen contained in the transistor can be suppressed because the aluminum oxide films do not transmit oxygen. 
     Accordingly, the transistor illustrated in  FIGS. 5A to 5C  has stable electrical characteristics. 
     Note that although  FIGS. 5A to 5C  illustrate the structure including all of the crystalline insulating film  532 , the aluminum oxide film  534 , the crystalline insulating film  536 , and the aluminum oxide film  538 , one embodiment of the present invention is not limited to this structure. For example, the crystalline insulating film  532  and the aluminum oxide film  534  are not necessarily provided. Alternatively, the crystalline insulating film  536  and the aluminum oxide film  538  are not necessarily provided. 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 1A to 1C ,  FIGS. 2A to 2C ,  FIGS. 3A to 3C ,  FIGS. 4A to 4C , and  FIGS. 5A to 5C  will be described with reference to  FIGS. 6A to 6C . 
       FIG. 6A  is a top view of a transistor of one embodiment of the present invention.  FIG. 6B  is a cross-sectional view taken along dashed-dotted line F  1 -F 2  in  FIG. 6A .  FIG. 6C  is a cross-sectional view taken along dashed-dotted line F 3 -F 4  in  FIG. 6A . Note that for simplicity, a gate insulating film  612  and the like are not illustrated in  FIG. 6A . 
       FIG. 6B  is a cross-sectional view of a transistor including a crystalline insulating film  632  over a substrate  600 ; an aluminum oxide film  634  over the crystalline insulating film  632 ; a base insulating film  602  over the aluminum oxide film  634 ; a semiconductor film  606  over the base insulating film  602 ; the gate insulating film  612  over the semiconductor film  606 ; a gate electrode  604  over the gate insulating film  612 ; a sidewall insulating film  610  in contact with side surfaces of the gate electrode  604 ; a source electrode  616   a  and a drain electrode  616   b  over the semiconductor film  606  and the sidewall insulating film  610 ; a crystalline insulating film  636  over the semiconductor film  606 , and the source electrode  616   a  and the drain electrode  616   b ; an aluminum oxide film  638  over the crystalline insulating film  636 ; and an insulating film  640  over the aluminum oxide film  638 . 
     In the cross-sectional view in  FIG. 6B , a protective insulating film  618  is provided over the gate electrode  604 , the sidewall insulating film  610 , the source electrode  616   a  and the drain electrode  616   b , the crystalline insulating film  636 , the aluminum oxide film  638 , and the insulating film  640 . Note that openings reaching the source electrode  616   a  and the drain electrode  616   b  are formed in the crystalline insulating film  636 , the aluminum oxide film  638 , the insulating film  640 , and the protective insulating film  618 , and a wiring  624   a  and a wiring  624   b  provided over the protective insulating film  618  are in contact with the source electrode  616   a  and the drain electrode  616   b  through the openings. 
     Note that although part of the sidewall insulating film  610  is provided on the side surfaces of the gate insulating film  612  in  FIG. 6B , one embodiment of the present invention is not limited to this structure. For example, the sidewall insulating film  610  may be provided over the gate insulating film  612 . 
     Note that, in  FIG. 6B , the surfaces of the gate electrode  604 , the sidewall insulating film  610 , the source electrode  616   a  and the drain electrode  616   b , the crystalline insulating film  636 , the aluminum oxide film  638 , and the insulating film  640  are level with one another. 
     Here, as the crystalline insulating film  632  and the crystalline insulating film  636 , a film selected from the insulating films given as examples of the crystalline insulating film  132  and the crystalline insulating film  136  can be used. 
     The crystalline insulating film  632  and the crystalline insulating film  636  are each an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  634  and the aluminum oxide film  638  each have crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  634  has crystallinity even near the interface with the crystalline insulating film  632 . The aluminum oxide film  638  has crystallinity even near the interface with the crystalline insulating film  636 . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. 
     The aluminum oxide film  634  and the aluminum oxide film  638  are each a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  634  and the aluminum oxide film  638  each have a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  634  does not include a low density layer near the crystalline insulating film  632 , and the aluminum oxide film  638  does not either include a low density layer near the crystalline insulating film  636 . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  606 . 
     It is preferred that, in the semiconductor film  606 , a region not overlapping with the gate electrode  604  has lower resistance than a region overlapping with the gate electrode  604 . For example, in the semiconductor film  606 , the region not overlapping with the gate electrode  604  may contain an impurity that reduces the resistance of the semiconductor film  606 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  606 , the region not overlapping with the gate electrode  604  has lower resistance than the region overlapping with the gate electrode  604 ; thus, the region can serve as a source region and a drain region of the transistor. However, since the transistor illustrated in  FIG. 6B  includes the source electrode  616   a  and the drain electrode  616   b , a source region and a drain region are not necessarily provided. 
     It is preferred that, in the semiconductor film  606 , a region overlapping with the sidewall insulating film  610  has higher resistance than a region overlapping with the source electrode  616   a  and the drain electrode  616   b  and lower resistance than a region overlapping with the gate electrode  604 . For example, in the semiconductor film  606 , the region not overlapping with the gate electrode  604  may contain an impurity that reduces the resistance of the semiconductor film  606 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  606 , the region overlapping with the sidewall insulating film  610  has higher resistance than the source electrode  616   a  and the drain electrode  616   b  and lower resistance than the region overlapping with the gate electrode  604 ; thus, the region serves as an LDD region. With the LDD regions of the transistor, DIBL and hot-carrier degradation can be suppressed. Note that in the semiconductor film  606 , the region overlapping with the sidewall insulating film  610  may serve also as an offset region. Also with the offset region of the transistor, DIBL and hot-carrier degradation can be suppressed. 
     The description of the substrate  100  is referred to for the substrate  600 . 
     The description of the base insulating film  302  is referred to for the base insulating film  602 . 
     The base insulating film  602  is preferred to contain excess oxygen. 
     In the case where the base insulating film  602  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate insulating film  112  is referred to for the gate insulating film  612 . 
     The gate insulating film  612  is preferred to contain excess oxygen. 
     In the case where the gate insulating film  612  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate electrode  104  is referred to for the gate electrode  604 . 
     The sidewall insulating film  610  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     Note that it is preferred to use a crystalline insulating film and an aluminum oxide film over the crystalline insulating film for the sidewall insulating film  610 . With such a structure, shape defects of the sidewall insulating film  610  can be made unlikely to occur. 
     The source electrode  616   a  and the drain electrode  616   b  can be formed to have a single-layer or a stacked-layer structure of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. Note that the source electrode  616   a  and the drain electrode  616   b  may be the same or different from each other. 
     The description of the protective insulating film  518  is referred to for the protective insulating film  618 . 
     The description of the wiring  524   a  and the wiring  524   b  are referred to for the wiring  624   a  and the wiring  624   b.    
     In the transistor illustrated in  FIGS. 6A to 6C , a region where the gate electrode  604  overlaps with another wiring and electrode is small; therefore, parasitic capacitance is unlikely to be generated. Accordingly, the switching characteristics of the transistor can be enhanced. The source electrode  616   a  and the drain electrode  616   b  are provided, whereby parasitic resistance can be made lower than that of the transistor illustrated in  FIGS. 5A to 5C . Accordingly, an on-state current can be increased. Moreover, the channel length of the transistor is determined by the width of the gate electrode  604 ; therefore, a miniaturized transistor having a short channel length is manufactured easily. 
     The transistor illustrated in  FIGS. 6A to 6C  is surrounded with the aluminum oxide film  634  and the aluminum oxide film  638  each having crystallinity and a high density. Thus, the transistor has a high barrier property against external impurities. Further, outward diffusion of excess oxygen contained in the transistor can be suppressed because the aluminum oxide films do not transmit oxygen. 
     Accordingly, the transistor illustrated in  FIGS. 6A to 6C  has stable electrical characteristics. 
     Note that although  FIGS. 6A to 6C  illustrate the structure including all of the crystalline insulating film  632 , the aluminum oxide film  634 , the crystalline insulating film  636 , and the aluminum oxide film  638 , one embodiment of the present invention is not limited to this structure. For example, the crystalline insulating film  632  and the aluminum oxide film  634  are not necessarily provided. Alternatively, the crystalline insulating film  636  and the aluminum oxide film  638  are not necessarily provided. 
     According to this embodiment, since a gate insulating film has a high barrier property against impurities and contains fewer defects, a transistor having stable electric characteristics and high reliability can be provided. 
     This embodiment shows an example of a basic principle. Thus, part or the whole of this embodiment can be freely combined with, applied to, or replaced with part or the whole of another embodiment. 
     [Embodiment 2] 
     In this embodiment, methods for manufacturing the transistors described in Embodiment 1 will be described. 
     First, a method for manufacturing the transistor illustrated in  FIGS. 1A to 1C  will be described with reference to  FIGS. 7A to 7D  and  FIGS. 8A to 8C . Note that only cross-sectional views corresponding to  FIG. 1B  are shown for simplicity in  FIGS. 7A to 7D  and  FIGS. 8A to 8C . 
     First, the substrate  100  is prepared. As the substrate  100 , a substrate selected from the substrates given as examples of the substrate  100  can be used. 
     Next, the crystalline insulating film  132  is formed (see  FIG. 7A ). The crystalline insulating film  132  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  132  and can be formed by a sputtering method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, or a pulsed laser deposition (PLD) method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  132  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     When a microwave CVD method is employed as the CVD method, plasma damage to a surface to be formed can be made small. Since high-density plasma is used, a dense film having fewer defects can be formed even at a relatively low temperature (at approximately 325° C.). Note that the microwave CVD method is also referred to as a high-density plasma CVD method. 
     Note that a first heat treatment may be performed after the crystalline insulating film  132  is formed. The first heat treatment can be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C. The first heat treatment is performed in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, preferably 1% or more, further preferably 10% or more, or under reduced pressure. Alternatively, the first heat treatment may be performed in such a manner that a heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, preferably 1% or more, further preferably 10% or more in order to compensate desorbed oxygen. By the first heat treatment, the crystallinity of the crystalline insulating film  132  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  134  is formed (see  FIG. 7B ). The aluminum oxide film  134  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  134  is formed over the crystalline insulating film  132 , whereby the aluminum oxide film  134  having crystallinity and a high density can be formed. The aluminum oxide film  134  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element. Thus, the aluminum oxide film  134  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  134  having crystallinity and a high density is likely to be formed over the crystalline insulating film  132 . Moreover, it is preferred to form the aluminum oxide film  134  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, a conductive film to be the gate electrode  104  is formed. The conductive film to be the gate electrode  104  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  104  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  104  is processed to form the gate electrode  104  (see  FIG. 7C ). 
     Next, the gate insulating film  112  is formed (see  FIG. 7D ). The gate insulating film  112  can be formed using an insulating film selected from the insulating films given as examples of the gate insulating film  112  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a semiconductor film to be the semiconductor film  106  is formed. The semiconductor film to be the semiconductor film  106  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  106  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  106  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a second heat treatment may be performed after the oxide semiconductor film is formed. The second heat treatment can be performed under the conditions shown in the first heat treatment. By the second heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  106  is processed into an island shape to form the semiconductor film  106  (see  FIG. 8A ). 
     Note that when the semiconductor film  106  is an oxide semiconductor film, a third heat treatment may be performed after the semiconductor film  106  is formed. The third heat treatment can be performed under the conditions shown in the first heat treatment. By the third heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, a conductive film to be the source electrode  116   a  and the drain electrode  116   b  is formed. The conductive film to be the source electrode  116   a  and the drain electrode  116   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  116   a  and the drain electrode  116   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  116   a  and the drain electrode  116   b  is processed to form the source electrode  116   a  and the drain electrode  116   b  (see  FIG. 8B ). 
     Next, the protective insulating film  118  is formed. The protective insulating film  118  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  118  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     As the protective insulating film  118 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the protective insulating film  118  can contain excess oxygen. 
     Next, the crystalline insulating film  136  is formed. The crystalline insulating film  136  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  136  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a fourth heat treatment may be performed. The fourth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the fourth heat treatment, the crystallinity of the crystalline insulating film  136  can be improved and impurities such as hydrogen and water can be removed. When the semiconductor film  106  is an oxide semiconductor film and the protective insulating film  118  contains excess oxygen, defects in the semiconductor film  106  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the aluminum oxide film  138  is formed (see  FIG. 8C ). The aluminum oxide film  138  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a fifth heat treatment may be performed. The fifth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the fifth heat treatment, when the semiconductor film  106  is an oxide semiconductor film and the protective insulating film  118  contains excess oxygen, defects in the semiconductor film  106  (oxygen vacancies in the oxide semiconductor film) can be reduced. Note that the fifth heat treatment may be substituted for the fourth heat treatment. At this time, with the aluminum oxide film  138 , outward diffusion of excess oxygen can be suppressed and oxygen vacancies can be effectively reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 1A to 1C  can be manufactured. 
     When the semiconductor film  106  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the first to fifth heat treatments. Moreover, the aluminum oxide film  134  and the aluminum oxide film  138  each serve as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the first to fifth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the first to fifth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 2A to 2C  will be described with reference to  FIGS. 9A to 9D  and  FIGS. 10A to 10C . Note that only cross-sectional views corresponding to  FIG. 2B  are shown for simplicity in  FIGS. 9A to 9D  and  FIGS. 10A to 10C . 
     First, the substrate  200  is prepared. As the substrate  200 , a substrate selected from the substrates given as examples of the substrate  200  can be used. 
     Next, the crystalline insulating film  232  is formed (see  FIG. 9A ). The crystalline insulating film  232  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  232  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  232  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a sixth heat treatment may be performed after the crystalline insulating film  232  is formed. The sixth heat treatment can be performed under the conditions shown in the first heat treatment. By the sixth heat treatment, the crystallinity of the crystalline insulating film  232  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  234  is formed (see  FIG. 9B ). The aluminum oxide film  234  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  234  is formed over the crystalline insulating film  232 , whereby the aluminum oxide film  234  having crystallinity and a high density can be formed. The aluminum oxide film  234  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element. Thus, the aluminum oxide film  234  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  234  having crystallinity and a high density is likely to be formed over the crystalline insulating film  232 . Moreover, it is preferred to form the aluminum oxide film  234  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, a conductive film to be the gate electrode  204  is formed. The conductive film to be the gate electrode  204  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  204  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  204  is processed to form the gate electrode  204  (see  FIG. 9C ). 
     Next, the gate insulating film  212  is formed (see  FIG. 9D ). The gate insulating film  212  can be formed using an insulating film selected from the insulating films given as examples of the gate insulating film  212  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a conductive film to be the source electrode  216   a  and the drain electrode  216   b  is formed. The conductive film to be the source electrode  216   a  and the drain electrode  216   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  216   a  and the drain electrode  216   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  216   a  and the drain electrode  216   b  is processed to form the source electrode  216   a  and the drain electrode  216   b  (see  FIG. 10A ). 
     Next, a semiconductor film to be the semiconductor film  206  is formed. The semiconductor film to be the semiconductor film  206  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  206  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  206  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a seventh heat treatment may be performed after the oxide semiconductor film is formed. The seventh heat treatment can be performed under the conditions shown in the first heat treatment. By the seventh heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  206  is processed into an island shape to form the semiconductor film  206  (see  FIG. 10B ). 
     Note that when the semiconductor film  206  is an oxide semiconductor film, an eighth heat treatment may be performed after the semiconductor film  206  is formed. The eighth heat treatment can be performed under the conditions shown in the first heat treatment. By the eighth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at an interface between the gate insulating film  212  and the semiconductor film  206  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, the protective insulating film  218  is formed. The protective insulating film  218  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  218  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     As the protective insulating film  218 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the protective insulating film  218  can contain excess oxygen. 
     Next, the crystalline insulating film  236  is formed. The crystalline insulating film  236  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  236  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a ninth heat treatment may be performed. The ninth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the ninth heat treatment, the crystallinity of the crystalline insulating film  236  can be improved and impurities such as hydrogen and water can be removed. When the semiconductor film  206  is an oxide semiconductor film and the protective insulating film  218  contains excess oxygen, defects in the semiconductor film  206  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the aluminum oxide film  238  is formed (see  FIG. 10C ). The aluminum oxide film  238  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a tenth heat treatment may be performed. The tenth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the tenth heat treatment, when the semiconductor film  206  is an oxide semiconductor film and the protective insulating film  218  contains excess oxygen, defects in the semiconductor film  206  (oxygen vacancies in the oxide semiconductor film) can be reduced. Note that the tenth heat treatment may be substituted for the ninth heat treatment. At this time, with the aluminum oxide film  238 , outward diffusion of excess oxygen can be suppressed and oxygen vacancies can be effectively reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 2A to 2C  can be manufactured. 
     When the semiconductor film  206  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the sixth to tenth heat treatments. Moreover, the aluminum oxide film  234  and the aluminum oxide film  238  each serve as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the sixth to tenth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the sixth to tenth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 3A to 3C  will be described with reference to  FIGS. 11A to 11D  and  FIGS. 12A to 12D . Note that only cross-sectional views corresponding to  FIG. 3B  are shown for simplicity in  FIGS. 11A to 11D  and  FIGS. 12A to 12D . 
     First, the substrate  300  is prepared. As the substrate  300 , a substrate selected from the substrates given as examples of the substrate  300  can be used. 
     Next, the crystalline insulating film  332  is formed (see  FIG. 11A ). The crystalline insulating film  332  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  332  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  332  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that an eleventh heat treatment may be performed after the crystalline insulating film  332  is formed. The eleventh heat treatment can be performed under the conditions shown in the first heat treatment. By the eleventh heat treatment, the crystallinity of the crystalline insulating film  332  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  334  is formed (see  FIG. 11B ). The aluminum oxide film  334  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  334  is formed over the crystalline insulating film  332 , whereby the aluminum oxide film  334  having crystallinity and a high density can be formed. The aluminum oxide film  334  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element. Thus, the aluminum oxide film  334  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  334  having crystallinity and a high density is likely to be formed over the crystalline insulating film  332 . Moreover, it is preferred to form the aluminum oxide film  334  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, the base insulating film  302  is formed (see  FIG. 11C ). The base insulating film  302  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  302  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  302 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  302  can contain excess oxygen. 
     Next, a semiconductor film to be the semiconductor film  306  is formed. The semiconductor film to be the semiconductor film  306  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  306  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  306  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a twelfth heat treatment may be performed after the oxide semiconductor film is formed. The twelfth heat treatment can be performed under the conditions shown in the first heat treatment. By the twelfth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  306  is processed into an island shape to form the semiconductor film  306  (see  FIG. 11D ). 
     Note that when the semiconductor film  306  is an oxide semiconductor film, a thirteenth heat treatment may be performed after the semiconductor film  306  is formed. The thirteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the thirteenth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at an interface between the base insulating film  302  and the semiconductor film  306  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, a conductive film to be the source electrode  316   a  and the drain electrode  316   b  is formed. The conductive film to be the source electrode  316   a  and the drain electrode  316   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  316   a  and the drain electrode  316   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  316   a  and the drain electrode  316   b  is processed to form the source electrode  316   a  and the drain electrode  316   b  (see  FIG. 12A ). 
     Next, the gate insulating film  312  is formed (see  FIG. 12B ). The gate insulating film  312  can be formed using an insulating film selected from the insulating films given as examples of the gate insulating film  312  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a conductive film to be the gate electrode  304  is formed. The conductive film to be the gate electrode  304  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  304  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  304  is processed to form the gate electrode  304  (see  FIG. 12C ). 
     Next, the crystalline insulating film  336  is formed. The crystalline insulating film  336  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  336  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a fourteenth heat treatment may be performed. The fourteenth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the fourteenth heat treatment, the crystallinity of the crystalline insulating film  336  can be improved and impurities such as hydrogen and water can be removed. When the semiconductor film  306  is an oxide semiconductor film and the base insulating film  302  contains excess oxygen, defects in the semiconductor film  306  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the aluminum oxide film  338  is formed (see  FIG. 12D ). The aluminum oxide film  338  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a fifteenth heat treatment may be performed. The fifteenth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the fifteenth heat treatment, when the semiconductor film  306  is an oxide semiconductor film and the base insulating film  302  contains excess oxygen, defects in the semiconductor film  306  (oxygen vacancies in the oxide semiconductor film) can be reduced. Note that the fifteenth heat treatment may be substituted for the fourteenth heat treatment. At this time, with the aluminum oxide film  338 , outward diffusion of excess oxygen can be suppressed and oxygen vacancies can be effectively reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 3A to 3C  can be manufactured. 
     When the semiconductor film  306  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the eleventh to fifteenth heat treatments. Moreover, the aluminum oxide film  334  and the aluminum oxide film  338  each serve as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the eleventh to fifteenth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the eleventh to fifteenth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 4A to 4C  will be described with reference to  FIGS. 13A to 13D  and  FIGS. 14A to 14D . Note that only cross-sectional views corresponding to  FIG. 4B  are shown for simplicity in  FIGS. 13A to 13D  and  FIGS. 14A to 14D . 
     First, the substrate  400  is prepared. As the substrate  400 , a substrate selected from the substrates given as examples of the substrate  400  can be used. 
     Next, the crystalline insulating film  432  is formed (see  FIG. 13A ). The crystalline insulating film  432  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  432  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  432  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a sixteenth heat treatment may be performed after the crystalline insulating film  432  is formed. The sixteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the sixteenth heat treatment, the crystallinity of the crystalline insulating film  432  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  434  is formed (see  FIG. 13B ). The aluminum oxide film  434  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  434  is formed over the crystalline insulating film  432 , whereby the aluminum oxide film  434  having crystallinity and a high density can be formed. The aluminum oxide film  434  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element. Thus, the aluminum oxide film  434  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  434  having crystallinity and a high density is likely to be formed over the crystalline insulating film  432 . Moreover, it is preferred to form the aluminum oxide film  434  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, the base insulating film  402  is formed (see  FIG. 13C ). The base insulating film  402  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  402  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  402 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  402  can contain excess oxygen. 
     Next, a conductive film to be the source electrode  416   a  and the drain electrode  416   b  is formed. The conductive film to be the source electrode  416   a  and the drain electrode  416   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  416   a  and the drain electrode  416   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  416   a  and the drain electrode  416   b  is processed to form the source electrode  416   a  and the drain electrode  416   b  (see  FIG. 13D ). 
     Next, a semiconductor film to be the semiconductor film  406  is formed. The semiconductor film to be the semiconductor film  406  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  406  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  406  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a seventeenth heat treatment may be performed after the oxide semiconductor film is formed. The seventeenth heat treatment can be performed under the conditions shown in the first heat treatment. By the seventeenth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  406  is processed into an island shape to form the semiconductor film  406  (see  FIG. 14A ). 
     Note that when the semiconductor film  406  is an oxide semiconductor film, an eighteenth heat treatment may be performed after the semiconductor film  406  is formed. The eighteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the eighteenth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at an interface between the base insulating film  402  and the semiconductor film  406  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, the gate insulating film  412  is formed (see  FIG. 14B ). The gate insulating film  412  can be formed using an insulating film selected from the insulating films given as examples of the gate insulating film  412  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a conductive film to be the gate electrode  404  is formed. The conductive film to be the gate electrode  404  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  404  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  404  is processed to form the gate electrode  404  (see  FIG. 14C ). 
     Next, the crystalline insulating film  436  is formed. The crystalline insulating film  436  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  436  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a nineteenth heat treatment may be performed. The nineteenth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the nineteenth heat treatment, the crystallinity of the crystalline insulating film  436  can be improved and impurities such as hydrogen and water can be removed. When the semiconductor film  406  is an oxide semiconductor film and the base insulating film  402  contains excess oxygen, defects in the semiconductor film  406  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the aluminum oxide film  438  is formed (see  FIG. 14D ). The aluminum oxide film  438  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a twentieth heat treatment may be performed. The twentieth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twentieth heat treatment, when the semiconductor film  406  is an oxide semiconductor film and the base insulating film  402  contains excess oxygen, defects in the semiconductor film  406  (oxygen vacancies in the oxide semiconductor film) can be reduced. Note that the twentieth heat treatment may be substituted for the nineteenth heat treatment. At this time, with the aluminum oxide film  438 , outward diffusion of excess oxygen can be suppressed and oxygen vacancies can be effectively reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 4A to 4C  can be manufactured. 
     When the semiconductor film  406  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the sixteenth to twentieth heat treatments. Moreover, the aluminum oxide film  434  and the aluminum oxide film  438  each serve as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the sixteenth to twentieth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the sixteenth to twentieth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 5A to 5C  will be described with reference to  FIGS. 15A to 15D  and  FIGS. 16A to 16D . Note that only cross-sectional views corresponding to  FIG. 5B  are shown for simplicity in  FIGS. 15A to 15D  and  FIGS. 16A to 16D . 
     First, the substrate  500  is prepared. As the substrate  500 , a substrate selected from the substrates given as examples of the substrate  500  can be used. 
     Next, the crystalline insulating film  532  is formed (see  FIG. 15A ). The crystalline insulating film  532  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  532  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  532  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a twenty-first heat treatment may be performed after the crystalline insulating film  532  is formed. The twenty-first heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-first heat treatment, the crystallinity of the crystalline insulating film  532  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  534  is formed (see  FIG. 15B ). The aluminum oxide film  534  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  534  is formed over the crystalline insulating film  532 , whereby the aluminum oxide film  534  having crystallinity and a high density can be formed. The aluminum oxide film  534  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element. Thus, the aluminum oxide film  534  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  534  having crystallinity and a high density is likely to be formed over the crystalline insulating film  532 . Moreover, it is preferred to form the aluminum oxide film  534  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, the base insulating film  502  is formed (see  FIG. 15C ). The base insulating film  502  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  502  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  502 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  502  can contain excess oxygen. 
     Next, a semiconductor film to be the semiconductor film  506  is formed. The semiconductor film to be the semiconductor film  506  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  506  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  506  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a twenty-second heat treatment may be performed after the oxide semiconductor film is formed. The twenty-second heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-second heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  506  is processed into an island shape to form the semiconductor film  506  (see  FIG. 16A ). 
     Note that when the semiconductor film  506  is an oxide semiconductor film, a twenty-third heat treatment may be performed after the semiconductor film  506  is formed. The twenty-third heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-third heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at an interface between the base insulating film  502  and the semiconductor film  506  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, an insulating film  513  to be the gate insulating film  512  is formed (see  FIG. 16A ). The insulating film  513  can be formed using an insulating film selected from the insulating films given as examples of the gate insulating film  512  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a conductive film to be the gate electrode  504  is formed. The conductive film to be the gate electrode  504  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  504  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  504  is processed to form the gate electrode  504 . 
     Next, the gate insulating film  512  is formed by processing the insulating film  513  using a resist mask used for forming the gate electrode  504  or the gate electrode  504  as a mask (see  FIG. 16B ). 
     Next, an impurity may be added to the semiconductor film  506  using the gate electrode  504  as a mask. As the impurity, an impurity selected from the impurities that reduce the resistance of the semiconductor film  506  may be added. Note that in the case where the semiconductor film  506  is an oxide semiconductor film, as the impurity, one or more of helium, boron, nitrogen, fluorine, neon, aluminum, phosphorus, argon, arsenic, krypton, indium, tin, antimony, and xenon can be added. The impurity may be added by an ion implantation method or an ion doping method, preferably, an ion implantation method. At this time, the acceleration voltage is made higher than or equal to 5 kV and lower than or equal to 100 kV. The amount of the added impurity is made greater than or equal to 1×10 14  ions/cm 2  and less than or equal to 1×10 16  ions/cm 2 . 
     Next, a twenty-fourth heat treatment may be performed. The twenty-fourth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twenty-fourth heat treatment, a region of the semiconductor film  506 , to which an impurity is added, can be made a low-resistant region. When the semiconductor film  506  is an oxide semiconductor film and the base insulating film  502  contains excess oxygen, defects in the semiconductor film  506  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the crystalline insulating film  536  is formed. The crystalline insulating film  536  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  536  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a twenty-fifth heat treatment may be performed. The twenty-fifth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twenty-fifth heat treatment, the crystallinity of the crystalline insulating film  536  can be improved and impurities such as hydrogen and water can be removed. When the semiconductor film  506  is an oxide semiconductor film and the base insulating film  502  contains excess oxygen, defects in the semiconductor film  506  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the aluminum oxide film  538  is formed (see  FIG. 16C ). The aluminum oxide film  538  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a twenty-sixth heat treatment may be performed. The twenty-sixth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twenty-sixth heat treatment, when the semiconductor film  506  is an oxide semiconductor film and the base insulating film  502  contains excess oxygen, defects in the semiconductor film  506  (oxygen vacancies in the oxide semiconductor film) can be reduced. Note that the twenty-sixth heat treatment may be substituted for the twenty-fifth heat treatment. At this time, with the aluminum oxide film  538 , outward diffusion of excess oxygen can be suppressed and oxygen vacancies can be effectively reduced. 
     Next, the protective insulating film  518  is formed. The protective insulating film  518  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  518  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, openings reaching the semiconductor film  506  are formed by processing the crystalline insulating film  536 , the aluminum oxide film  538 , and the protective insulating film  518 . 
     Next, a conductive film to be the wiring  524   a  and the wiring  524   b  is formed. The conductive film to be the wiring  524   a  and the wiring  524   b  can be formed using a conductive film selected from the conductive films given as examples of the wiring  524   a  and the wiring  524   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the wiring  524   a  and the wiring  524   b  is processed to form the wiring  524   a  and the wiring  524   b  (see  FIG. 16D ). 
     Through the above steps, the transistor illustrated in  FIGS. 5A to 5C  can be manufactured. 
     When the semiconductor film  506  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the twenty-first to twenty-sixth heat treatments. Moreover, the aluminum oxide film  534  and the aluminum oxide film  538  each serve as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the twenty-first to twenty-sixth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the twenty-first to twenty-sixth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 6A to 6C  will be described with reference to  FIGS. 17A to 17D ,  FIGS. 18A to 18D , and  FIGS. 19A to 19D . Note that only cross-sectional views corresponding to  FIG. 6B  are shown for simplicity in  FIGS. 17A to 17D ,  FIGS. 18A to 18D , and  FIGS. 19A to 19D . 
     First, the substrate  600  is prepared. As the substrate  600 , a substrate selected from the substrates given as examples of the substrate  600  can be used. 
     Next, the crystalline insulating film  632  is formed (see  FIG. 17A ). The crystalline insulating film  632  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  632  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  632  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a twenty-seventh heat treatment may be performed after the crystalline insulating film  632  is formed. The twenty-seventh heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-seventh heat treatment, the crystallinity of the crystalline insulating film  632  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  634  is formed (see  FIG. 17B ). The aluminum oxide film  634  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  634  is formed over the crystalline insulating film  632 , whereby the aluminum oxide film  634  having crystallinity and a high density can be formed. The aluminum oxide film  634  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element. Thus, the aluminum oxide film  634  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  634  having crystallinity and a high density is likely to be formed over the crystalline insulating film  632 . Moreover, it is preferred to form the aluminum oxide film  634  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, the base insulating film  602  is formed (see  FIG. 17C ). The base insulating film  602  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  602  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  602 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  602  can contain excess oxygen. 
     Next, a semiconductor film to be the semiconductor film  606  is formed. The semiconductor film to be the semiconductor film  606  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  606  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  606  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a twenty-eighth heat treatment may be performed after the oxide semiconductor film is formed. The twenty-eighth heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-eighth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  606  is processed into an island shape to form the semiconductor film  606  (see  FIG. 17D ). 
     Note that when the semiconductor film  606  is an oxide semiconductor film, a twenty-ninth heat treatment may be performed after the semiconductor film  606  is formed. The twenty-ninth heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-ninth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at an interface between the base insulating film  602  and the semiconductor film  606  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, an insulating film to be the gate insulating film  612  is formed. The insulating film to be the gate insulating film  612  can be formed using an insulating film selected from the insulating films given as examples of the gate insulating film  612  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a conductive film to be the gate electrode  604  is formed. The conductive film to be the gate electrode  604  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  604  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  604  is processed to form a conductive film  605  to be the gate electrode  604 . 
     Next, the gate insulating film  612  is formed by processing the insulating film to be the gate insulating film  612  using a resist mask used for processing the conductive film  605  or the conductive film  605  as a mask (see  FIG. 18A ). 
     Next, an impurity may be added to the semiconductor film  606  using the conductive film  605  as a mask (this step is also referred to as a first impurity addition step). As the impurity, an impurity selected from the impurities that reduce the resistance of the semiconductor film  606  may be added. Note that in the case where the semiconductor film  606  is an oxide semiconductor film, as the impurity, one or more of helium, boron, nitrogen, fluorine, neon, aluminum, phosphorus, argon, arsenic, krypton, indium, tin, antimony, and xenon can be added. The impurity may be added by an ion implantation method or an ion doping method, preferably, an ion implantation method. At this time, the acceleration voltage is made higher than or equal to 5 kV and lower than or equal to 100 kV. The amount of the added impurity is made greater than or equal to 1×10 14  ions/cm 2  and less than or equal to 1×10 16  ions/cm 2 . 
     Next, an insulating film to be the sidewall insulating film  611  is formed. The insulating film to be the sidewall insulating film  611  can be formed using an insulating film selected from the insulating films given as examples of the sidewall insulating film  610  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. Next, a highly anisotropic etching treatment is performed on the insulating film to be the sidewall insulating film  611 , whereby the sidewall insulating film  611  which is in contact with side surfaces of the gate insulating film  612  and the conductive film  605  can be formed (see  FIG. 18B ). 
     Next, an impurity may be added to the semiconductor film  606  using the conductive film  605  and the sidewall insulating film  611  as masks (this step is also referred to as a second impurity addition step). The conditions of the first impurity addition step can be referred to for the second impurity addition step. Two kinds of low-resistance regions can be provided in the semiconductor film  606  by performing the first impurity addition step and the second impurity addition step. Therefore, electric-field concentration at an edge of the drain electrode is likely to be relieved and hot-carrier degradation can be effectively suppressed. Moreover, the edge of the source electrode has less influence of the electric field from the edge of the drain electrode; therefore, DIBL can be suppressed. Note that either one of the first impurity addition step and the second impurity addition step may be performed. 
     Next, a thirtieth heat treatment may be performed. The thirtieth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the thirtieth heat treatment, a region of the semiconductor film  606 , to which an impurity is added, can be made a low-resistant region. When the semiconductor film  606  is an oxide semiconductor film and the base insulating film  602  contains excess oxygen, defects in the semiconductor film  606  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, a conductive film to be the source electrode  616   a  and the drain electrode  616   b  is formed. The conductive film to be the source electrode  616   a  and the drain electrode  616   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  616   a  and the drain electrode  616   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  616   a  and the drain electrode  616   b  is processed to form a conductive film  616  (see  FIG. 18C ). 
     Next, the crystalline insulating film  637  is formed (see  FIG. 18D ). The crystalline insulating film  637  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  637  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     Next, a thirty-first heat treatment may be performed. The thirty-first heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the thirty-first heat treatment, the crystallinity of the crystalline insulating film  637  can be improved and impurities such as hydrogen and water can be removed. When the semiconductor film  606  is an oxide semiconductor film and the base insulating film  602  contains excess oxygen, defects in the semiconductor film  606  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the aluminum oxide film  639  is formed (see  FIG. 19A ). The aluminum oxide film  639  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, a thirty-second heat treatment may be performed. The thirty-second heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the thirty-second heat treatment, when the semiconductor film  606  is an oxide semiconductor film and the base insulating film  602  contains excess oxygen, defects in the semiconductor film  606  (oxygen vacancies in the oxide semiconductor film) can be reduced. Note that the thirty-second heat treatment may be substituted for the thirty-first heat treatment. At this time, with the aluminum oxide film  639 , outward diffusion of excess oxygen can be suppressed and oxygen vacancies can be effectively reduced. 
     Next, an insulating film  641  to be the insulating film  640  is formed (see  FIG. 19B ). The insulating film  641  can be formed using an insulating film selected from the insulating films given as examples of the insulating film  640  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     Next, the insulating film  641 , the aluminum oxide film  639 , the crystalline insulating film  637 , the conductive film  616 , the sidewall insulating film  611 , and the conductive film  605  are processed so that the surfaces of these films are level with one another. The processing can be performed by a dry etching treatment or a chemical mechanical polishing (CMP) treatment. By the processing, the conductive film  605  becomes the gate electrode  604 , the sidewall insulating film  611  becomes the sidewall insulating film  610 , the conductive film  616  becomes the source electrode  616   a  and the drain electrode  616   b , the crystalline insulating film  637  becomes the crystalline insulating film  636 , the aluminum oxide film  639  becomes the aluminum oxide film  638 , and the insulating film  641  becomes the insulating film  640  (see  FIG. 19C ). 
     The source electrode  616   a  and the drain electrode  616   b  are formed in this manner, whereby the distance between the gate electrode  604  and the source electrode  616   a  or the drain electrode  616   b  can be made similar to the thickness of the sidewall insulating film  610 . Thus, the length of the distance between the gate electrode  604 , and the source electrode  616   a  or the drain electrode  616   b  can be made smaller than the minimum feature size; therefore, the structure of the transistor illustrated in  FIGS. 6A to 6C  is suitable in manufacturing a miniaturized transistor. 
     Since the aluminum oxide film  638  has high resistance to a chemical, occurrence of shape defects can be suppressed. Thus, in a portion of the aluminum oxide film  638 , where shape defects occur, it is possible to suppress occurrence of an etching residue and another shape defect which is caused by the portion. Accordingly, a transistor having stable electrical characteristics can be obtained. 
     Next, the protective insulating film  618  is formed. The protective insulating film  618  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  618  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, openings exposing the source electrode  616   a  and the drain electrode  616   b  are formed by processing the crystalline insulating film  636 , the aluminum oxide film  638 , the insulating film  640 , and the protective insulating film  618 . 
     Next, a conductive film to be the wiring  624   a  and the wiring  624   b  is formed. The conductive film to be the wiring  624   a  and the wiring  624   b  can be formed using a conductive film selected from the conductive films given as examples of the wiring  624   a  and the wiring  624   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the wiring  624   a  and the wiring  624   b  is processed to form the wiring  624   a  and the wiring  624   b  (see  FIG. 18C ). 
     Through the above steps, the transistor illustrated in  FIGS. 6A to 6C  can be manufactured. 
     When the semiconductor film  606  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the twenty-seventh to thirty-second heat treatments. Moreover, the aluminum oxide film  634  and the aluminum oxide film  638  each serve as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the twenty-seventh to thirty-second heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the twenty-seventh to thirty-second heat treatments. 
     According to this embodiment, since an aluminum oxide film having a high barrier property against impurities can be formed, a transistor having stable electric characteristics and high reliability can be provided. 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     [Embodiment 3] 
     In this embodiment, a transistor of one embodiment of the present invention will be described. 
       FIG. 20A  is a top view of a transistor of one embodiment of the present invention.  FIG. 20B  is a cross-sectional view taken along dashed-dotted line G 1 -G 2  in  FIG. 20A .  FIG. 20C  is a cross-sectional view taken along dashed-dotted line G 3 -G 4  in  FIG. 20A . Note that for simplicity, a gate insulating film  2112  and the like are not illustrated in  FIG. 20A . 
       FIG. 20B  is a cross-sectional view of a transistor including a gate electrode  2104  over a substrate  2100 ; the gate insulating film  2112  over the gate electrode  2104 ; a semiconductor film  2106  which is over the gate insulating film  2112  and overlaps with the gate electrode  2104 ; a source electrode  2116   a  and a drain electrode  2116   b  over the semiconductor film  2106 ; and a protective insulating film  2118  over the semiconductor film  2106 , and the source electrode  2116   a  and the drain electrode  2116   b . Note that the gate insulating film  2112  includes a crystalline insulating film  2112   a  and an aluminum oxide film  2112   b  over the crystalline insulating film  2112   a.    
     Here, the crystalline insulating film  2112   a  contains one or more kinds of Mg, Ti, V, Cr, Y, Zr, and Ta. Specifically, it is preferred to include one or more kinds of magnesium oxide, titanium oxide, vanadium oxide, chromium oxide, yttrium oxide, zirconium oxide, and tantalum oxide. For example, an insulating film containing zirconium oxide and yttrium oxide can be used. 
     The crystalline insulating film  2112   a  is an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2112   b  has crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2112   b  has crystallinity even near the interface with the crystalline insulating film  2112   a . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film has many defects; therefore, when it is used as the gate insulating film, deterioration in electrical characteristics of the transistor might occur. Whereas, the aluminum oxide film  2112   b  has fewer defects; therefore, deterioration in electrical characteristics of the transistor, which is caused by defects in the gate insulating film  2112 , can be suppressed. 
     The aluminum oxide film  2112   b  is a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  2112   b  has a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  2112   b  does not include a low density layer near the crystalline insulating film  2112   a . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. Thus, the aluminum oxide film  2112   b  has fewer defects; therefore, the gate insulating film  2112  can be made favorable. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has high leakage current; therefore, when it is used as the gate insulating film, an off-state current of the transistor might be increased. Moreover, the aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     Further, a relative dielectric constant of the aluminum oxide film  2112   b  is greater than or equal to 7 and less than or equal to 10. Thus, a physical thickness which is twice as large as a required equivalent oxide thickness can be obtained. Accordingly, gate leakage current can be reduced in some cases. 
     The semiconductor film  2106  can be formed using a silicon film, a germanium film, a silicon germanium film, a gallium arsenide film, a silicon carbide film, or a gallium nitride film. Note that an organic semiconductor film may be used as the semiconductor film  2106 . Alternatively, an oxide semiconductor film may be used as the semiconductor film  2106 . 
     Note that the description in the above embodiments is referred to for the oxide semiconductor film used as the semiconductor film  2106 . 
     The description of the substrate  100  is referred to for the substrate  2100 . 
     The description of the gate electrode  104  is referred to for the gate electrode  2104 . 
     The description of the source electrode  116   a  and the drain electrode  116   b  are referred to for the source electrode  2116   a  and the drain electrode  2116   b.    
     The description of the protective insulating film  118  is referred to for the protective insulating film  2118 . 
     Next, a transistor having a structure different from that of the transistor illustrated in  FIGS. 20A to 20C  will be described with reference to  FIGS. 21A to 21C . 
       FIG. 21A  is a top view of a transistor of one embodiment of the present invention.  FIG. 21B  is a cross-sectional view taken along dashed-dotted line H 1 -H 2  in  FIG. 21A .  FIG. 21C  is a cross-sectional view taken along dashed-dotted line H 3 -H 4  in  FIG. 21A . Note that for simplicity, a gate insulating film  2212  and the like are not illustrated in  FIG. 21A . 
       FIG. 21B  is a cross-sectional view of a transistor including a gate electrode  2204  over a substrate  2200 ; the gate insulating film  2212  over the gate electrode  2204 ; a source electrode  2216   a  and a drain electrode  2216   b  over the gate insulating film  2212 ; a semiconductor film  2206  which is over the gate insulating film  2212 , and the source electrode  2216   a  and the drain electrode  2216   b  and overlaps with the gate electrode  2204 ; and a protective insulating film  2218  over the semiconductor film  2206 , and the source electrode  2216   a  and the drain electrode  2216   b . Note that the gate insulating film  2212  includes a crystalline insulating film  2212   a  and an aluminum oxide film  2212   b  over the crystalline insulating film  2212   a.    
     Here, the description of the crystalline insulating film  2112   a  is referred to for the crystalline insulating film  2212   a.    
     The crystalline insulating film  2212   a  is an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2212   b  has crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2212   b  has crystallinity even near the interface with the crystalline insulating film  2212   a . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film has many defects; therefore, when it is used as the gate insulating film, deterioration in electrical characteristics of the transistor might occur. Whereas, the aluminum oxide film  2212   b  has fewer defects; therefore, deterioration in electrical characteristics of the transistor, which is caused by defects in the gate insulating film  2212 , can be suppressed. 
     The aluminum oxide film  2212   b  is a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  2212   b  has a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  2212   b  does not include a low density layer near the crystalline insulating film  2212   a . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. Thus, the aluminum oxide film  2212   b  has fewer defects; therefore, the gate insulating film  2212  can be made favorable. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has high leakage current; therefore, when it is used as the gate insulating film, an off-state current of the transistor might be increased. Moreover, the aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     Further, a relative dielectric constant of the aluminum oxide film  2212   b  is greater than or equal to 7 and less than or equal to 10. Thus, a physical thickness which is twice as large as a required equivalent oxide thickness can be obtained. Accordingly, gate leakage current can be reduced in some cases. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  2206 . 
     The description of the substrate  100  is referred to for the substrate  2200 . 
     The description of the gate electrode  104  is referred to for the gate electrode  2204 . 
     The description of the source electrode  116   a  and the drain electrode  116   b  are referred to for the source electrode  2216   a  and the drain electrode  2216   b.    
     The description of the protective insulating film  118  is referred to for the protective insulating film  2218 . 
     The protective insulating film  2218  is preferred to contain excess oxygen. 
     In the case where the protective insulating film  2218  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 20A to 20C  and  FIGS. 21A to 21C  will be described with reference to  FIGS. 22A to 22C . 
       FIG. 22A  is a top view of a transistor of one embodiment of the present invention.  FIG. 22B  is a cross-sectional view taken along dashed-dotted line  11 - 12  in  FIG. 22A .  FIG. 22C  is a cross-sectional view taken along dashed-dotted line  13 - 14  in  FIG. 22A . Note that for simplicity, a gate insulating film  2312  and the like are not illustrated in  FIG. 22A . 
       FIG. 22B  is a cross-sectional view of a transistor including a base insulating film  2302  over a substrate  2300 ; a semiconductor film  2306  over the base insulating film  2302 ; a source electrode  2316   a  and a drain electrode  2316   b  over the semiconductor film  2306 ; the gate insulating film  2312  over the semiconductor film  2306 , and the source electrode  2316   a  and the drain electrode  2316   b ; and a gate electrode  2304  which is over the gate insulating film  2312  and overlaps with the semiconductor film  2306 . Note that the gate insulating film  2312  includes a crystalline insulating film  2312   a  and an aluminum oxide film  2312   b  over the crystalline insulating film  2312   a.    
     Here, the description of the crystalline insulating film  2112   a  is referred to for the crystalline insulating film  2312   a.    
     The crystalline insulating film  2312   a  is an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2312   b  has crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2312   b  has crystallinity even near the interface with the crystalline insulating film  2312   a . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film has many defects; therefore, when it is used as the gate insulating film, deterioration in electrical characteristics of the transistor might occur. Whereas, the aluminum oxide film  2312   b  has fewer defects; therefore, deterioration in electrical characteristics of the transistor, which is caused by defects in the gate insulating film  2312 , can be suppressed. 
     The aluminum oxide film  2312   b  is a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  2312   b  has a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  2312   b  does not include a low density layer near the crystalline insulating film  2312   a . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. Thus, the aluminum oxide film  2312   b  has fewer defects; therefore, the gate insulating film  2312  can be made favorable. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has high leakage current; therefore, when it is used as the gate insulating film, an off-state current of the transistor might be increased. Moreover, the aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     Further, a relative dielectric constant of the aluminum oxide film  2312   b  is greater than or equal to 7 and less than or equal to 10. Thus, a physical thickness which is twice as large as a required equivalent oxide thickness can be obtained. Accordingly, gate leakage current can be reduced in some cases. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  2306 . 
     The description of the substrate  100  is referred to for the substrate  2300 . 
     The description of the base insulating film  302  is referred to for the base insulating film  2302 . 
     The description of the source electrode  2116   a  and the drain electrode  2116   b  are referred to for the source electrode  2316   a  and the drain electrode  2316   b.    
     The description of the gate electrode  2104  is referred to for the gate electrode  2304 . 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 20A to 20C ,  FIGS. 21A to 21C , and  FIGS. 22A to 22C  will be described with reference to  FIGS. 23A to 23C . 
       FIG. 23A  is a top view of a transistor of one embodiment of the present invention.  FIG. 23B  is a cross-sectional view taken along dashed-dotted line J 1 -J 2  in  FIG. 23A .  FIG. 23C  is a cross-sectional view taken along dashed-dotted line J 3 -J 4  in  FIG. 23A . Note that for simplicity, a gate insulating film  2412  and the like are not illustrated in  FIG. 23A . 
       FIG. 23B  is a cross-sectional view of a transistor including a base insulating film  2402  over a substrate  2400 ; a source electrode  2416   a  and a drain electrode  2416   b  over the base insulating film  2402 ; a semiconductor film  2406  over the base insulating film  2402 , and the source electrode  2416   a  and the drain electrode  2416   b ; the gate insulating film  2412  over the semiconductor film  2406 ; and a gate electrode  2404  which is over the gate insulating film  2412  and overlaps with the semiconductor film  2406 . Note that the gate insulating film  2412  includes a crystalline insulating film  2412   a  and an aluminum oxide film  2412   b  over the crystalline insulating film  2412   a.    
     Here, the description of the crystalline insulating film  2112   a  is referred to for the crystalline insulating film  2412   a.    
     The crystalline insulating film  2412   a  is an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2412   b  has crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2412   b  has crystallinity even near the interface with the crystalline insulating film  2412   a . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film has many defects; therefore, when it is used as the gate insulating film, deterioration in electrical characteristics of the transistor might occur. Whereas, the aluminum oxide film  2412   b  has fewer defects; therefore, deterioration in electrical characteristics of the transistor, which is caused by defects in the gate insulating film  2412 , can be suppressed. 
     The aluminum oxide film  2412   b  is a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  2412   b  has a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  2412   b  does not include a low density layer near the crystalline insulating film  2412   a . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. Thus, the aluminum oxide film  2412   b  has fewer defects; therefore, the gate insulating film  2412  can be made favorable. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has high leakage current; therefore, when it is used as the gate insulating film, an off-state current of the transistor might be increased. Moreover, the aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     Further, a relative dielectric constant of the aluminum oxide film  2412   b  is greater than or equal to 7 and less than or equal to 10. Thus, a physical thickness which is twice as large as a required equivalent oxide thickness can be obtained. Accordingly, gate leakage current can be reduced in some cases. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  2406 . 
     The description of the substrate  100  is referred to for the substrate  2400 . 
     The description of the base insulating film  302  is referred to for the base insulating film  2402 . 
     The base insulating film  2402  is preferred to contain excess oxygen. 
     In the case where the base insulating film  2402  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the source electrode  116   a  and the drain electrode  116   b  are referred to for the source electrode  2416   a  and the drain electrode  2416   b.    
     The description of the gate electrode  104  is referred to for the gate electrode  2404 . 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 20A to 20C ,  FIGS. 21A to 21C ,  FIGS. 22A to 22C , and  FIGS. 23A to 23C  will be described with reference to  FIGS. 24A to 24C . 
       FIG. 24A  is a top view of a transistor of one embodiment of the present invention.  FIG. 24B  is a cross-sectional view taken along dashed-dotted line K 1 -K 2  in  FIG. 24A .  FIG. 24C  is a cross-sectional view taken along dashed-dotted line K 3 -K 4  in  FIG. 24A . Note that for simplicity, a gate insulating film  2512  and the like are not illustrated in  FIG. 24A . 
       FIG. 24B  is a cross-sectional view of a transistor including a base insulating film  2502  over a substrate  2500 ; a semiconductor film  2506  over the base insulating film  2502 ; the gate insulating film  2512  over the semiconductor film  2506 ; and a gate electrode  2504  which is over the gate insulating film  2512  and overlaps with the semiconductor film  2506 . Note that the gate insulating film  2512  includes a crystalline insulating film  2512   a  and an aluminum oxide film  2512   b  over the crystalline insulating film  2512   a.    
     In the cross-sectional view in  FIG. 24B , a protective insulating film  2518  is provided over the gate insulating film  2512  and the gate electrode  2504 . Note that openings reaching the semiconductor film  2506  are formed in the gate insulating film  2512  and the protective insulating film  2518 , and a wiring  2524   a  and a wiring  2524   b  provided over the protective insulating film  2518  are in contact with the semiconductor film  2506  through the openings. 
     Note that although the gate insulating film  2512  is provided so as to cover the semiconductor film  2506  in  FIG. 24B , one embodiment of the present invention is not limited to this structure. For example, the gate insulating film  2512  may be provided only in a region overlapping with the gate electrode  2504 . Alternatively, a sidewall insulating film may be provided in contact with a side surface of the gate electrode  2504 . 
     In the case of providing the sidewall insulating film, it is preferred that, in the semiconductor film  2506 , a region overlapping with the sidewall insulating film has lower resistance than a region overlapping with the gate electrode  2504 . For example, in the semiconductor film  2506 , a region not overlapping with the gate electrode  2504  may contain an impurity that reduces the resistance of the semiconductor film  2506 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  2506 , the region overlapping with the sidewall insulating film has lower resistance than the region overlapping with the gate electrode  2504 ; thus, the region serves as an LDD region. With the LDD regions of the transistor, DIBL and hot-carrier degradation can be suppressed. Note that in the semiconductor film  2506 , the region overlapping with the sidewall insulating film may serve also as an offset region. Also with the offset region of the transistor, DIBL and hot-carrier degradation can be suppressed. 
     Here, the description of the crystalline insulating film  2112   a  is referred to for the crystalline insulating film  2512   a.    
     The crystalline insulating film  2512   a  is an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2512   b  has crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2512   b  has crystallinity even near the interface with the crystalline insulating film  2512   a . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film has many defects; therefore, when it is used as the gate insulating film, deterioration in electrical characteristics of the transistor might occur. Whereas, the aluminum oxide film  2512   b  has fewer defects; therefore, deterioration in electrical characteristics of the transistor, which is caused by defects in the gate insulating film  2512 , can be suppressed. 
     The aluminum oxide film  2512   b  is a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  2512   b  has a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  2512   b  does not include a low density layer near the crystalline insulating film  2512   a . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. Thus, the aluminum oxide film  2512   b  has fewer defects; therefore, the gate insulating film  2512  can be made favorable. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has high leakage current; therefore, when it is used as the gate insulating film, an off-state current of the transistor might be increased. Moreover, the aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     Further, a relative dielectric constant of the aluminum oxide film  2512   b  is greater than or equal to 7 and less than or equal to 10. Thus, a physical thickness which is twice as large as a required equivalent oxide thickness can be obtained. Accordingly, gate leakage current can be reduced in some cases. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  2506 . 
     It is preferred that, in the semiconductor film  2506 , a region not overlapping with the gate electrode  2504  has lower resistance than a region overlapping with the gate electrode  2504 . For example, in the semiconductor film  2506 , the region not overlapping with the gate electrode  2504  may contain an impurity that reduces the resistance of the semiconductor film  2506 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  2506 , the region not overlapping with the gate electrode  2504  has lower resistance than the region overlapping with the gate electrode  2504 ; thus, the region can serve as a source region and a drain region of the transistor. 
     The description of the substrate  100  is referred to for the substrate  2500 . 
     The description of the base insulating film  302  is referred to for the base insulating film  2502 . 
     The base insulating film  2502  is preferred to contain excess oxygen. 
     In the case where the base insulating film  2502  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate electrode  104  is referred to for the gate electrode  2504 . 
     The description of the protective insulating film  518  is referred to for the protective insulating film  2518 . 
     The description of the wiring  524   a  and the wiring  524   b  are referred to for the wiring  2524   a  and the wiring  2524   b.    
     In the transistor illustrated in  FIGS. 24A to 24C , a region where the gate electrode  2504  overlaps with another wiring and electrode is small; therefore, parasitic capacitance is unlikely to be generated. Accordingly, the switching characteristics of the transistor can be enhanced. Moreover, the channel length of the transistor is determined by the width of the gate electrode  2504 ; therefore, a miniaturized transistor having a short channel length is manufactured easily. 
     Next, a transistor having a structure different from those of the transistors illustrated in  FIGS. 20A to 20C ,  FIGS. 21A to 21C ,  FIGS. 22A to 22C ,  FIGS. 23A to 23C , and  FIGS. 24A to 24C  will be described with reference to  FIGS. 25A to 25C . 
       FIG. 25A  is a top view of a transistor of one embodiment of the present invention.  FIG. 25B  is a cross-sectional view taken along dashed-dotted line L 1 -L 2  in  FIG. 25A .  FIG. 25C  is a cross-sectional view taken along dashed-dotted line L 3 -L 4  in  FIG. 25A . Note that for simplicity, a gate insulating film  2612  and the like are not illustrated in  FIG. 25A . 
       FIG. 25B  is a cross-sectional view of a transistor including a base insulating film  2602  over a substrate  2600 ; a semiconductor film  2606  over the base insulating film  2602 ; the gate insulating film  2612  over the semiconductor film  2606 ; a gate electrode  2604  over the gate insulating film  2612 ; a sidewall insulating film  2610  in contact with side surfaces of the gate electrode  2604 ; and a source electrode  2616   a  and a drain electrode  2616   b  over the semiconductor film  2606  and the sidewall insulating film  2610 . Note that the gate insulating film  2612  includes a crystalline insulating film  2612   a  and an aluminum oxide film  2612   b  over the crystalline insulating film  2612   a.    
     In the cross-sectional view in  FIG. 25B , a protective insulating film  2618  is provided over the gate electrode  2604 , and the source electrode  2616   a  and the drain electrode  2616   b . Note that openings reaching the source electrode  2616   a  and the drain electrode  2616   b  are formed in the protective insulating film  2618 , and a wiring  2624   a  and a wiring  2624   b  provided over the protective insulating film  2618  are in contact with the source electrode  2616   a  and the drain electrode  2616   b , respectively, through the openings. 
     Note that although part of the sidewall insulating film  2610  is provided on the side surfaces of the gate insulating film  2612  in  FIG. 25B , one embodiment of the present invention is not limited to this structure. For example, the sidewall insulating film  2610  may be provided over the gate insulating film  2612 . 
     Here, the description of the crystalline insulating film  2112   a  is referred to for the crystalline insulating film  2612   a.    
     The crystalline insulating film  2612   a  is an insulating film having crystallinity, specifically, an insulating film whose crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2612   b  has crystallinity. Specifically, the crystallinity can be observed by XRD, electron diffraction, or neutron diffraction. 
     The aluminum oxide film  2612   b  has crystallinity even near the interface with the crystalline insulating film  2612   a . On the other hand, in the case where an aluminum oxide film is formed over a metal film or an amorphous insulating film which serves as a base, an amorphous aluminum oxide film is formed in the aluminum oxide film near the base. The amorphous aluminum oxide film has many defects; therefore, when it is used as the gate insulating film, deterioration in electrical characteristics of the transistor might occur. Whereas, the aluminum oxide film  2612   b  has fewer defects; therefore, deterioration in electrical characteristics of the transistor, which is caused by defects in the gate insulating film  2612 , can be suppressed. 
     The aluminum oxide film  2612   b  is a high-density aluminum oxide film, specifically, an aluminum oxide film having a density greater than or equal to 3.2 g/cm 3  and less than or equal to 4.1 g/cm 3  measured by XRR or RBS. The aluminum oxide film  2612   b  has a high barrier property against impurities; therefore, deterioration in electric characteristics of the transistor, which is caused by impurities, can be suppressed. 
     Note that the aluminum oxide film  2612   b  does not include a low density layer near the crystalline insulating film  2612   a . Specifically, a layer having a density less than 3.2 g/cm 3  measured by XRR is not included. Thus, the aluminum oxide film  2612   b  has fewer defects; therefore, the gate insulating film  2612  can be made favorable. On the other hand, in the case where the aluminum oxide film is formed over the metal film or the amorphous insulating film which serves as a base, the aluminum oxide film having lower density is formed in the aluminum oxide film near the base. The aluminum oxide film having low density has high leakage current; therefore, when it is used as the gate insulating film, an off-state current of the transistor might be increased. Moreover, the aluminum oxide film having low density has low resistance to a chemical liquid and might be unintentionally etched in a chemical liquid treatment at the time of manufacturing the transistor. Consequently, shape defects and malfunctions of the transistor might occur. 
     Further, a relative dielectric constant of the aluminum oxide film  2612   b  is greater than or equal to 7 and less than or equal to 10. Thus, a physical thickness which is twice as large as a required equivalent oxide thickness can be obtained. Accordingly, gate leakage current can be reduced in some cases. 
     The description of the semiconductor film  106  is referred to for the semiconductor film  2606 . 
     It is preferred that, in the semiconductor film  2606 , a region not overlapping with the gate electrode  2604  has lower resistance than a region overlapping with the gate electrode  2604 . For example, in the semiconductor film  2606 , the region not overlapping with the gate electrode  2604  may contain an impurity that reduces the resistance of the semiconductor film  2606 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  2606 , the region not overlapping with the gate electrode  2604  has lower resistance than the region overlapping with the gate electrode  2604 ; thus, the region can serve as a source region and a drain region of the transistor. However, since the transistor illustrated in  FIG. 25B  includes the source electrode  2616   a  and the drain electrode  2616   b , a source region and a drain region are not necessarily provided. 
     It is preferred that, in the semiconductor film  2606 , a region overlapping with the sidewall insulating film  2610  has higher resistance than a region overlapping with the source electrode  2616   a  and the drain electrode  2616   b  and lower resistance than a region overlapping with the gate electrode  2604 . For example, in the semiconductor film  2606 , the region not overlapping with the gate electrode  2604  may contain an impurity that reduces the resistance of the semiconductor film  2606 . Alternatively, the resistance of the region may be reduced by defects. In the semiconductor film  2606 , the region overlapping with the sidewall insulating film  2610  has higher resistance than the source electrode  2616   a  and the drain electrode  2616   b  and lower resistance than the region overlapping with the gate electrode  2604 ; thus, the region serves as an LDD region. With the LDD regions of the transistor, DIBL and hot-carrier degradation can be suppressed. Note that in the semiconductor film  2606 , the region overlapping with the sidewall insulating film  2610  may serve also as an offset region. Also with the offset region of the transistor, DIBL and hot-carrier degradation can be suppressed. 
     The description of the substrate  100  is referred to for the substrate  2600 . 
     The description of the base insulating film  302  is referred to for the base insulating film  2602 . 
     The base insulating film  2602  is preferred to contain excess oxygen. 
     In the case where the base insulating film  2602  contains excess oxygen, oxygen vacancies in the oxide semiconductor film can be reduced. 
     The description of the gate electrode  104  is referred to for the gate electrode  2604 . 
     The description of the sidewall insulating film  610  is referred to for the sidewall insulating film  2610 . 
     Note that it is preferred to use a crystalline insulating film and an aluminum oxide film over the crystalline insulating film for the sidewall insulating film  2610 . With such a structure, shape defects of the sidewall insulating film  2610  can be made unlikely to occur. 
     The description of the source electrode  616   a  and the drain electrode  616   b  are referred to for the source electrode  2616   a  and the drain electrode  2616   b.    
     The description of the protective insulating film  518  is referred to for the protective insulating film  2618 . 
     The description of the wiring  524   a  and the wiring  524   b  are referred to for the wiring  2624   a  and the wiring  2624   b.    
     In the transistor illustrated in  FIGS. 25A to 25C , a region where the gate electrode  2604  overlaps with another wiring and electrode is small; therefore, parasitic capacitance is unlikely to be generated. Accordingly, the switching characteristics of the transistor can be enhanced. The source electrode  2616   a  and the drain electrode  2616   b  are provided, whereby parasitic resistance can be made lower than that of the transistor illustrated in  FIGS. 24A to 24C . Accordingly, an on-state current can be increased. Moreover, the channel length of the transistor is determined by the width of the gate electrode  2604 ; therefore, a miniaturized transistor having a short channel length is manufactured easily. 
     According to this embodiment, since a gate insulating film has a high barrier property against impurities and contains fewer defects, a transistor having stable electric characteristics and high reliability can be provided. 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     [Embodiment 4] 
     In this embodiment, methods for manufacturing the transistors described in Embodiment 3 will be described. 
     First, a method for manufacturing the transistor illustrated in  FIGS. 20A to 20C  will be described with reference to  FIGS. 26A to 26C  and  FIGS. 27A to 27C . Note that only cross-sectional views corresponding to  FIG. 20B  are shown for simplicity in  FIGS. 26A to 26C  and  FIGS. 27A to 27C . 
     First, the substrate  2100  is prepared. As the substrate  2100 , a substrate selected from the substrates given as examples of the substrate  2100  can be used. 
     Next, a conductive film to be the gate electrode  2104  is formed. The conductive film to be the gate electrode  2104  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  2104  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     When a microwave CVD method is employed as the CVD method, plasma damage to a surface to be formed can be made small. Since high-density plasma is used, a dense film having fewer defects can be formed even at a relatively low temperature (at approximately 325° C.). Note that the microwave CVD method is also referred to as a high-density plasma CVD method. 
     Next, the conductive film to be the gate electrode  2104  is processed to form the gate electrode  2104  (see  FIG. 26A ). 
     Next, the crystalline insulating film  2112   a  is formed (see  FIG. 26B ). The crystalline insulating film  2112   a  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  2112   a  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  2112   a  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a first heat treatment may be performed after the crystalline insulating film  2112   a  is formed. The first heat treatment can be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C. The first heat treatment is performed in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, preferably 1% or more, further preferably 10% or more, or under reduced pressure. Alternatively, the first heat treatment may be performed in such a manner that a heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, preferably 1% or more, further preferably 10% or more in order to compensate desorbed oxygen. By the first heat treatment, the crystallinity of the crystalline insulating film  2112   a  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  2112   b  is formed (see  FIG. 26C ). The aluminum oxide film  2112   b  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  2112   b  is formed over the crystalline insulating film  2112   a , whereby the aluminum oxide film  2112   b  having crystallinity and a high density can be formed. The aluminum oxide film  2112   b  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element (particularly Cu). Thus, the aluminum oxide film  2112   b  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  2112   b  having crystallinity and a high density is likely to be formed over the crystalline insulating film  2112   a . Moreover, it is preferred to form the aluminum oxide film  2112   b  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     In this manner, the gate insulating film  2112  including the crystalline insulating film  2112   a  and the aluminum oxide film  2112   b  over the crystalline insulating film  2112   a  can be formed. Note that the gate insulating film  2112  is not limited to the structure including only the crystalline insulating film  2112   a  and the aluminum oxide film  2112   b . For example, another insulating film may be provided below the crystalline insulating film  2112   a  or over the aluminum oxide film  2112   b . The condition of an interface between the gate insulating film  2112  and the semiconductor film  2106  can be made favorable by providing, for example, a silicon oxide film over the aluminum oxide film  2112   b.    
     Next, a semiconductor film to be the semiconductor film  2106  is formed. The semiconductor film to be the semiconductor film  2106  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  2106  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  2106  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a second heat treatment may be performed after the oxide semiconductor film is formed. The second heat treatment can be performed under the conditions shown in the first heat treatment. By the second heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  2106  is processed into an island shape to form the semiconductor film  2106  (see  FIG. 27A ). 
     Note that when the semiconductor film  2106  is an oxide semiconductor film, a third heat treatment may be performed after the semiconductor film  2106  is formed. The third heat treatment can be performed under the conditions shown in the first heat treatment. By the third heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at the interface between the aluminum oxide film  2112   b  and the semiconductor film  2106  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, a conductive film to be the source electrode  2116   a  and the drain electrode  2116   b  is formed. The conductive film to be the source electrode  2116   a  and the drain electrode  2116   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  2116   a  and the drain electrode  2116   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  2116   a  and the drain electrode  2116   b  is processed to form the source electrode  2116   a  and the drain electrode  2116   b  (see  FIG. 27B ). 
     Next, the protective insulating film  2118  is formed (see  FIG. 27C ). The protective insulating film  2118  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  2118  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     As the protective insulating film  2118 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the protective insulating film  2118  can contain excess oxygen. 
     Next, a fourth heat treatment may be performed. The fourth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the fourth heat treatment, when the semiconductor film  2106  is an oxide semiconductor film and the protective insulating film  2118  contains excess oxygen, defects in the semiconductor film  2106  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 20A to 20C  can be manufactured. 
     When the semiconductor film  2106  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the first to fourth heat treatments. Moreover, the gate insulating film  2112  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the first to fourth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the first to fourth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 21A to 21C  will be described with reference to  FIGS. 28A to 28C  and  FIGS. 29A to 29C . Note that only cross-sectional views corresponding to  FIG. 21B  are shown for simplicity in  FIGS. 28A to 28C  and  FIGS. 29A to 29C . 
     First, the substrate  2200  is prepared. As the substrate  2200 , a substrate selected from the substrates given as examples of the substrate  2200  can be used. 
     Next, a conductive film to be the gate electrode  2204  is formed. The conductive film to be the gate electrode  2204  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  2204  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  2204  is processed to form the gate electrode  2204  (see  FIG. 28A ). 
     Next, the crystalline insulating film  2212   a  is formed (see  FIG. 28B ). The crystalline insulating film  2212   a  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  2212   a  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  2212   a  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a fifth heat treatment may be performed after the crystalline insulating film  2212   a  is formed. The fifth heat treatment can be performed under the conditions shown in the first heat treatment. By the fifth heat treatment, the crystallinity of the crystalline insulating film  2212   a  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  2212   b  is formed (see  FIG. 28C ). The aluminum oxide film  2212   b  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  2212   b  is formed over the crystalline insulating film  2212   a , whereby the aluminum oxide film  2212   b  having crystallinity and a high density can be formed. The aluminum oxide film  2212   b  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element (particularly Cu). Thus, the aluminum oxide film  2212   b  serves as a bather film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  2212   b  having crystallinity and a high density is likely to be formed over the crystalline insulating film  2212   a . Moreover, it is preferred to form the aluminum oxide film  2212   b  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     In this manner, the gate insulating film  2212  including the crystalline insulating film  2212   a  and the aluminum oxide film  2212   b  over the crystalline insulating film  2212   a  can be formed. Note that the gate insulating film  2212  is not limited to the structure including only the crystalline insulating film  2212   a  and the aluminum oxide film  2212   b . For example, another insulating film may be provided below the crystalline insulating film  2212   a  or over the aluminum oxide film  2212   b . The condition of an interface between the gate insulating film  2212  and the semiconductor film  2206  can be made favorable by providing, for example, a silicon oxide film over the aluminum oxide film  2212   b.    
     Next, a conductive film to be the source electrode  2216   a  and the drain electrode  2216   b  is formed. The conductive film to be the source electrode  2216   a  and the drain electrode  2216   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  2216   a  and the drain electrode  2216   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  2216   a  and the drain electrode  2216   b  is processed to form the source electrode  2216   a  and the drain electrode  2216   b  (see  FIG. 29A ). 
     Next, a semiconductor film to be the semiconductor film  2206  is formed. The semiconductor film to be the semiconductor film  2206  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  2206  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  2206  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a sixth heat treatment may be performed after the oxide semiconductor film is formed. The sixth heat treatment can be performed under the conditions shown in the first heat treatment. By the sixth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  2206  is processed into an island shape to form the semiconductor film  2206  (see  FIG. 29B ). 
     Note that when the semiconductor film  2206  is an oxide semiconductor film, a seventh heat treatment may be performed after the semiconductor film  2206  is formed. The seventh heat treatment can be performed under the conditions shown in the first heat treatment. By the seventh heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at the interface between the aluminum oxide film  2212   b  and the semiconductor film  2206  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, the protective insulating film  2218  is formed (see  FIG. 29C ). The protective insulating film  2218  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  2218  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     As the protective insulating film  2218 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the protective insulating film  2218  can contain excess oxygen. 
     Next, an eighth heat treatment may be performed. The eighth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the eighth heat treatment, when the semiconductor film  2206  is an oxide semiconductor film and the protective insulating film  2218  contains excess oxygen, defects in the semiconductor film  2206  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 21A to 21C  can be manufactured. 
     When the semiconductor film  2206  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the fifth to eighth heat treatments. Moreover, the gate insulating film  2212  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the fifth to eighth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the fifth to eighth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 22A to 22C  will be described with reference to  FIGS. 30A to 30C  and  FIGS. 31A and 31B . Note that only cross-sectional views corresponding to  FIG. 22B  are shown for simplicity in  FIGS. 30A to 30C  and  FIGS. 31A and 31B . 
     First, the substrate  2300  is prepared. As the substrate  2300 , a substrate selected from the substrates given as examples of the substrate  2300  can be used. 
     Next, the base insulating film  2302  is formed. The base insulating film  2302  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  2302  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  2302 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  2302  can contain excess oxygen. 
     Next, a semiconductor film to be the semiconductor film  2306  is formed. The semiconductor film to be the semiconductor film  2306  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  2306  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  2306  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a ninth heat treatment may be performed after the oxide semiconductor film is formed. The ninth heat treatment can be performed under the conditions shown in the first heat treatment. By the ninth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  2306  is processed into an island shape to form the semiconductor film  2306  (see  FIG. 30A ). 
     Note that when the semiconductor film  2306  is an oxide semiconductor film, a tenth heat treatment may be performed after the semiconductor film  2306  is formed. The tenth heat treatment can be performed under the conditions shown in the first heat treatment. By the tenth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at the interface between the base insulating film  2302  and the semiconductor film  2306  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, a conductive film to be the source electrode  2316   a  and the drain electrode  2316   b  is formed. The conductive film to be the source electrode  2316   a  and the drain electrode  2316   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  2316   a  and the drain electrode  2316   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  2316   a  and the drain electrode  2316   b  is processed to form the source electrode  2316   a  and the drain electrode  2316   b  (see  FIG. 30B ). 
     Next, the crystalline insulating film  2312   a  is formed (see  FIG. 30C ). The crystalline insulating film  2312   a  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  2312   a  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  2312   a  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that an eleventh heat treatment may be performed after the crystalline insulating film  2312   a  is formed. The eleventh heat treatment can be performed under the conditions shown in the first heat treatment. By the eleventh heat treatment, the crystallinity of the crystalline insulating film  2312   a  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  2312   b  is formed (see  FIG. 31A ). The aluminum oxide film  2312   b  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  2312   b  is formed over the crystalline insulating film  2312   a , whereby the aluminum oxide film  2312   b  having crystallinity and a high density can be formed. The aluminum oxide film  2312   b  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element (particularly Cu). Thus, the aluminum oxide film  2312   b  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  2312   b  having crystallinity and a high density is likely to be formed over the crystalline insulating film  2312   a . Moreover, it is preferred to form the aluminum oxide film  2312   b  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     In this manner, the gate insulating film  2312  including the crystalline insulating film  2312   a  and the aluminum oxide film  2312   b  over the crystalline insulating film  2312   a  can be formed. Note that the gate insulating film  2312  is not limited to the structure including only the crystalline insulating film  2312   a  and the aluminum oxide film  2312   b . For example, another insulating film may be provided below the crystalline insulating film  2312   a  or over the aluminum oxide film  2312   b . The condition of an interface between the gate insulating film  2312  and the semiconductor film  2306  can be made favorable by providing, for example, a silicon oxide film below the crystalline insulating film  2312   a.    
     Next, a conductive film to be the gate electrode  2304  is formed. The conductive film to be the gate electrode  2304  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  2304  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  2304  is processed to form the gate electrode  2304  (see  FIG. 31B ). 
     Next, a twelfth heat treatment may be performed. The twelfth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twelfth heat treatment, when the semiconductor film  2306  is an oxide semiconductor film and the base insulating film  2302  contains excess oxygen, defects in the semiconductor film  2306  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 22A to 22C  can be manufactured. 
     When the semiconductor film  2306  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the ninth to twelfth heat treatments. Moreover, the gate insulating film  2312  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the ninth to twelfth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the ninth to twelfth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 23A to 23C  will be described with reference to  FIGS. 32A to 32C  and  FIGS. 33A and 33B . Note that only cross-sectional views corresponding to  FIG. 23B  are shown for simplicity in  FIGS. 32A to 32C  and  FIGS. 33A and 33B . 
     First, the substrate  2400  is prepared. As the substrate  2400 , a substrate selected from the substrates given as examples of the substrate  2400  can be used. 
     Next, the base insulating film  2402  is formed. The base insulating film  2402  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  2402  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  2402 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  2402  can contain excess oxygen. 
     Next, a conductive film to be the source electrode  2416   a  and the drain electrode  2416   b  is formed. The conductive film to be the source electrode  2416   a  and the drain electrode  2416   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  2416   a  and the drain electrode  2416   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  2416   a  and the drain electrode  2416   b  is processed to form the source electrode  2416   a  and the drain electrode  2416   b  (see  FIG. 32A ). 
     Next, a semiconductor film to be the semiconductor film  2406  is formed. The semiconductor film to be the semiconductor film  2406  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  2406  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  2406  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a thirteenth heat treatment may be performed after the oxide semiconductor film is formed. The thirteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the thirteenth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  2406  is processed into an island shape to form the semiconductor film  2406  (see  FIG. 32B ). 
     Note that when the semiconductor film  2406  is an oxide semiconductor film, a fourteenth heat treatment may be performed after the semiconductor film  2406  is formed. The fourteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the fourteenth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at the interface between the base insulating film  2402  and the semiconductor film  2406  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, the crystalline insulating film  2412   a  is formed (see  FIG. 32C ). The crystalline insulating film  2412   a  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  2412   a  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  2412   a  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a fifteenth heat treatment may be performed after the crystalline insulating film  2412   a  is formed. The fifteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the fifteenth heat treatment, the crystallinity of the crystalline insulating film  2412   a  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  2412   b  is formed (see  FIG. 33A ). The aluminum oxide film  2412   b  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  2412   b  is formed over the crystalline insulating film  2412   a , whereby the aluminum oxide film  2412   b  having crystallinity and a high density can be formed. The aluminum oxide film  2412   b  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element (particularly Cu). Thus, the aluminum oxide film  2412   b  serves as a bather film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  2412   b  having crystallinity and a high density is likely to be formed over the crystalline insulating film  2412   a . Moreover, it is preferred to form the aluminum oxide film  2412   b  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     In this manner, the gate insulating film  2412  including the crystalline insulating film  2412   a  and the aluminum oxide film  2412   b  over the crystalline insulating film  2412   a  can be formed. Note that the gate insulating film  2412  is not limited to the structure including only the crystalline insulating film  2412   a  and the aluminum oxide film  2412   b . For example, another insulating film may be provided below the crystalline insulating film  2412   a  or over the aluminum oxide film  2412   b . The condition of an interface between the gate insulating film  2412  and the semiconductor film  2406  can be made favorable by providing, for example, a silicon oxide film below the crystalline insulating film  2412   a.    
     Next, a conductive film to be the gate electrode  2404  is formed. The conductive film to be the gate electrode  2404  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  2404  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  2404  is processed to form the gate electrode  2404  (see  FIG. 33B ). 
     Next, a sixteenth heat treatment may be performed. The sixteenth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the sixteenth heat treatment, when the semiconductor film  2406  is an oxide semiconductor film and the base insulating film  2402  contains excess oxygen, defects in the semiconductor film  2406  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Through the above steps, the transistor illustrated in  FIGS. 23A to 23C  can be manufactured. 
     When the semiconductor film  2406  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the thirteenth to sixteenth heat treatments. Moreover, the gate insulating film  2412  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the thirteenth to sixteenth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the thirteenth to sixteenth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 24A to 24C  will be described with reference to  FIGS. 34A to 34C  and  FIGS. 35A and 35B . Note that only cross-sectional views corresponding to  FIG. 24B  are shown for simplicity in  FIGS. 34A to 34C  and  FIGS. 35A and 35B . 
     First, the substrate  2500  is prepared. As the substrate  2500 , a substrate selected from the substrates given as examples of the substrate  2500  can be used. 
     Next, the base insulating film  2502  is formed. The base insulating film  2502  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  2502  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  2502 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  2502  can contain excess oxygen. 
     Next, a semiconductor film to be the semiconductor film  2506  is formed. The semiconductor film to be the semiconductor film  2506  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  2506  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  2506  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a seventeenth heat treatment may be performed after the oxide semiconductor film is formed. The seventeenth heat treatment can be performed under the conditions shown in the first heat treatment. By the seventeenth heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  2506  is processed into an island shape to form the semiconductor film  2506  (see  FIG. 34A ). 
     Note that when the semiconductor film  2506  is an oxide semiconductor film, an eighteenth heat treatment may be performed after the semiconductor film  2506  is formed. The eighteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the eighteenth heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at the interface between the base insulating film  2502  and the semiconductor film  2506  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, the crystalline insulating film  2512   a  is formed (see  FIG. 34B ). The crystalline insulating film  2512   a  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  2512   a  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  2512   a  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a ninteenth heat treatment may be performed after the crystalline insulating film  2512   a  is formed. The ninteenth heat treatment can be performed under the conditions shown in the first heat treatment. By the ninteenth heat treatment, the crystallinity of the crystalline insulating film  2512   a  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  2512   b  is formed (see  FIG. 34C ). The aluminum oxide film  2512   b  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  2512   b  is formed over the crystalline insulating film  2512   a , whereby the aluminum oxide film  2512   b  having crystallinity and a high density can be formed. The aluminum oxide film  2512   b  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element (particularly Cu). Thus, the aluminum oxide film  2512   b  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  2512   b  having crystallinity and a high density is likely to be formed over the crystalline insulating film  2512   a . Moreover, it is preferred to form the aluminum oxide film  2512   b  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     In this manner, the gate insulating film  2512  including the crystalline insulating film  2512   a  and the aluminum oxide film  2512   b  over the crystalline insulating film  2512   a  can be formed. Note that the gate insulating film  2512  is not limited to the structure including only the crystalline insulating film  2512   a  and the aluminum oxide film  2512   b . For example, another insulating film may be provided below the crystalline insulating film  2512   a  or over the aluminum oxide film  2512   b . The condition of an interface between the gate insulating film  2512  and the semiconductor film  2506  can be made favorable by providing, for example, a silicon oxide film below the crystalline insulating film  2512   a.    
     Next, a conductive film to be the gate electrode  2504  is formed. The conductive film to be the gate electrode  2504  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  2504  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  2504  is processed to form the gate electrode  2504  (see  FIG. 35A ). 
     Next, an impurity may be added to the semiconductor film  2506  using the gate electrode  2504  as a mask. As the impurity, an impurity selected from the impurities that reduce the resistance of the semiconductor film  2506  may be added. Note that in the case where the semiconductor film  2506  is an oxide semiconductor film, as the impurity, one or more of helium, boron, nitrogen, fluorine, neon, aluminum, phosphorus, argon, arsenic, krypton, indium, tin, antimony, and xenon can be added. The impurity may be added by an ion implantation method or an ion doping method, preferably, an ion implantation method. At this time, the acceleration voltage is made higher than or equal to 5 kV and lower than or equal to 100 kV. The amount of the added impurity is made greater than or equal to 1×10 14  ions/cm 2  and less than or equal to 1×10 16  ions/cm 2 . 
     Next, a twentieth heat treatment may be performed. The twentieth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twentieth heat treatment, a region of the semiconductor film  2506 , to which an impurity is added, can be made a low-resistant region. When the semiconductor film  2506  is an oxide semiconductor film and the base insulating film  2502  contains excess oxygen, defects in the semiconductor film  2506  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, the protective insulating film  2518  is formed. The protective insulating film  2518  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  2518  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the protective insulating film  2518  is processed to form openings exposing the semiconductor film  2506 . 
     Next, a conductive film to be the wiring  2524   a  and the wiring  2524   b  is formed. The conductive film to be the wiring  2524   a  and the wiring  2524   b  can be formed using a conductive film selected from the conductive films given as examples of the wiring  2524   a  and the wiring  2524   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the wiring  2524   a  and the wiring  2524   b  is processed to form the wiring  2524   a  and the wiring  2524   b  (see  FIG. 35B ). 
     Through the above steps, the transistor illustrated in  FIGS. 24A to 24C  can be manufactured. 
     When the semiconductor film  2506  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the seventeenth to twentieth heat treatments. Moreover, the gate insulating film  2512  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the seventeenth to twentieth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the seventeenth to twentieth heat treatments. 
     Next, a method for manufacturing the transistor illustrated in  FIGS. 25A to 25C  will be described with reference to  FIGS. 36A to 36C  and  FIGS. 37A to 37C . Note that only cross-sectional views corresponding to  FIG. 25B  are shown for simplicity in  FIGS. 36A to 36C  and  FIGS. 37A to 37C . 
     First, the substrate  2600  is prepared. As the substrate  2600 , a substrate selected from the substrates given as examples of the substrate  2600  can be used. 
     Next, the base insulating film  2602  is formed. The base insulating film  2602  can be formed using an insulating film selected from the insulating films given as examples of the base insulating film  2602  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. 
     As the base insulating film  2602 , for example, a silicon oxide film is preferred to be formed by an RF sputtering method under the following conditions: a quartz (preferably synthetic quartz) target is used; the substrate heating temperature is higher than or equal to 30° C. and lower than or equal to 450° C. (preferably higher than or equal to 70° C. and lower than or equal to 200° C.); the distance between the substrate and the target (the T-S distance) is greater than or equal to 20 mm and less than or equal to 400 mm (preferably greater than or equal to 40 mm and less than or equal to 200 mm); the pressure is greater than or equal to 0.1 Pa and less than or equal to 4 Pa (preferably greater than or equal to 0.2 Pa and less than or equal to 1.2 Pa); the high-frequency power source is greater than or equal to 0.5 kW and less than or equal to 12 kW (preferably greater than or equal to 1 kW and less than or equal to 5 kW); and the ratio of O 2 /(O 2 +Ar) in the deposition gas is greater than 20% and less than or equal to 100% (preferably greater than or equal to 50% and less than or equal to 100%). Note that a silicon target may be used as the target instead of the quartz (preferably synthetic quartz) target. Note that an oxygen gas or a mixed gas of oxygen and argon is used as a deposition gas. With such a method, the base insulating film  2602  can contain excess oxygen. 
     Next, a semiconductor film to be the semiconductor film  2606  is formed. The semiconductor film to be the semiconductor film  2606  can be formed using a semiconductor film selected from the semiconductor films given as examples of the semiconductor film  2606  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. The semiconductor film to be the semiconductor film  2606  is preferred to be formed using an oxide semiconductor film by a sputtering method. Note that it is preferred to employ a sputtering method because an oxide semiconductor film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the oxide semiconductor film while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an oxide semiconductor film having crystallinity and a high density is likely to be formed. 
     Note that a twenty-first heat treatment may be performed after the oxide semiconductor film is formed. The twenty-first heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-first heat treatment, the crystallinity of the oxide semiconductor film can be improved and further impurities such as hydrogen and water can be removed from the oxide semiconductor film. 
     Next, the semiconductor film to be the semiconductor film  2606  is processed into an island shape to form the semiconductor film  2606  (see  FIG. 36A ). 
     Note that when the semiconductor film  2606  is an oxide semiconductor film, a twenty-second heat treatment may be performed after the semiconductor film  2606  is formed. The twenty-second heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-second heat treatment, which is performed with the side surfaces of the oxide semiconductor film exposed, impurities such as hydrogen and water are likely to be removed from the side surfaces of the oxide semiconductor film and thus the impurities can be effectively removed. Moreover, impurities such as hydrogen and water that exist at the interface between the base insulating film  2602  and the semiconductor film  2606  can also be removed. Note that when the oxide semiconductor film is a CAAC-OS film, impurities diffuse easily along a layer of crystal; thus, impurities such as hydrogen and water are further likely to be removed form the side surfaces. 
     Next, the crystalline insulating film  2613   a  is formed. The crystalline insulating film  2613   a  can be formed using an insulating film selected from the insulating films given as examples of the crystalline insulating film  2612   a  and can be formed by a sputtering method, a CVD method, a MBE method, an ALD method, or a PLD method. Note that it is preferred to employ a sputtering method because an insulating film having crystallinity and a high density is likely to be formed. Moreover, it is preferred to form the crystalline insulating film  2613   a  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Note that a twenty-third heat treatment may be performed after the crystalline insulating film  2613   a  is formed. The twenty-third heat treatment can be performed under the conditions shown in the first heat treatment. By the twenty-third heat treatment, the crystallinity of the crystalline insulating film  2613   a  can be improved and impurities such as hydrogen and water can be removed. 
     Next, the aluminum oxide film  2613   b  is formed (see  FIG. 36B ). The aluminum oxide film  2613   b  can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     The aluminum oxide film  2613   b  is formed over the crystalline insulating film  2613   a , whereby the aluminum oxide film  2613   b  having crystallinity and a high density can be formed. The aluminum oxide film  2613   b  having crystallinity and a high density is unlikely to transmit hydrogen, water, oxygen, and a metal element (particularly Cu). Thus, the aluminum oxide film  2613   b  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. 
     Note that it is preferred to employ a sputtering method because the aluminum oxide film  2613   b  having crystallinity and a high density is likely to be formed over the crystalline insulating film  2613   a . Moreover, it is preferred to form the aluminum oxide film  2613   b  while the substrate is heated at a temperature higher than or equal to 100° C. and lower than or equal to 450° C. because an insulating film having crystallinity and a high density is likely to be formed. 
     Next, a conductive film to be the gate electrode  2604  is formed. The conductive film to be the gate electrode  2604  can be formed using a conductive film selected from the conductive films given as examples of the gate electrode  2604  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the gate electrode  2604  is processed to form the gate electrode  2604 . 
     Next, the crystalline insulating film  2612   a  and the aluminum oxide film  2612   b  are formed by processing the crystalline insulating film  2613   a  and the aluminum oxide film  2613   b  using a resist mask used for forming the gate electrode  2604  or the gate electrode  2604  as a mask (see  FIG. 36C ). 
     In this manner, the gate insulating film  2612  including the crystalline insulating film  2612   a  and the aluminum oxide film  2612   b  over the crystalline insulating film  2612   a  can be formed. Note that the gate insulating film  2612  is not limited to the structure including only the crystalline insulating film  2612   a  and the aluminum oxide film  2612   b . For example, another insulating film may be provided below the crystalline insulating film  2612   a  or over the aluminum oxide film  2612   b . The condition of an interface between the gate insulating film  2612  and the semiconductor film  2606  can be made favorable by providing, for example, a silicon oxide film below the crystalline insulating film  2612   a.    
     Next, an impurity may be added to the semiconductor film  2606  using the gate electrode  2604  as a mask (this step is also referred to as a first impurity addition step). As the impurity, an impurity selected from the impurities that reduce the resistance of the semiconductor film  2606  may be added. Note that in the case where the semiconductor film  2606  is an oxide semiconductor film, as the impurity, one or more of helium, boron, nitrogen, fluorine, neon, aluminum, phosphorus, argon, arsenic, krypton, indium, tin, antimony, and xenon can be added. The impurity may be added by an ion implantation method or an ion doping method, preferably, an ion implantation method. At this time, the acceleration voltage is made higher than or equal to 5 kV and lower than or equal to 100 kV. The amount of the added impurity is made greater than or equal to 1×10 14  ions/cm 2  and less than or equal to 1×10 16  ions/cm 2 . 
     Next, an insulating film to be the sidewall insulating film  2610  is formed. The insulating film to be the sidewall insulating film  2610  can be formed using an insulating film selected from the insulating films given as examples of the sidewall insulating film  2610  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. Next, a highly anisotropic etching treatment is performed on the insulating film to be the sidewall insulating film  2610 , whereby the sidewall insulating film  2610  which are in contact with side surfaces of the gate insulating film  2612  and the gate electrode  2604  can be formed (see  FIG. 37A ). 
     Next, an impurity may be added to the semiconductor film  2606  using the gate electrode  2604  and the sidewall insulating film  2610  as masks (this step is also referred to as a second impurity addition step). The conditions of the first impurity addition step can be referred to for the second impurity addition step. Two kinds of low-resistance regions can be provided in the semiconductor film  2606  by performing the first impurity addition step and the second impurity addition step. Therefore, electric-field concentration at an edge of the drain electrode is likely to be relieved and hot-carrier degradation can be effectively suppressed. Moreover, the edge of the source electrode has less influence of the electric field from the edge of the drain electrode; therefore, DIBL can be suppressed. Note that either one of the first impurity addition step and the second impurity addition step may be performed. 
     Next, a twenty-fourth heat treatment may be performed. The twenty-fourth heat treatment may be performed under conditions selected from conditions similar to those of the first heat treatment. By the twenty-fourth heat treatment, a region of the semiconductor film  2606 , to which an impurity is added, can be made a low-resistant region. When the semiconductor film  2606  is an oxide semiconductor film and the base insulating film  2602  contains excess oxygen, defects in the semiconductor film  2606  (oxygen vacancies in the oxide semiconductor film) can be reduced. 
     Next, a conductive film to be the source electrode  2616   a  and the drain electrode  2616   b  is formed. The conductive film to be the source electrode  2616   a  and the drain electrode  2616   b  can be formed using a conductive film selected from the conductive films given as examples of the source electrode  2616   a  and the drain electrode  2616   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the source electrode  2616   a  and the drain electrode  2616   b  is processed to form the source electrode  2616   a  and the drain electrode  2616   b  (see  FIG. 37B ). 
     Next, the protective insulating film  2618  is formed. The protective insulating film  2618  can be formed using an insulating film selected from the insulating films given as examples of the protective insulating film  2618  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the protective insulating film  2618  is processed to form openings exposing the source electrode  2616   a  and the drain electrode  2616   b.    
     Next, a conductive film to be the wiring  2624   a  and the wiring  2624   b  is formed. The conductive film to be the wiring  2624   a  and the wiring  2624   b  can be formed using a conductive film selected from the conductive films given as examples of the wiring  2624   a  and the wiring  2624   b  and can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. 
     Next, the conductive film to be the wiring  2624   a  and the wiring  2624   b  is processed to form the wiring  2624   a  and the wiring  2624   b  (see  FIG. 37C ). 
     Through the above steps, the transistor illustrated in  FIGS. 25A to 25C  can be manufactured. 
     When the semiconductor film  2606  is an oxide semiconductor film, a transistor having stable electrical characteristics and high reliability can be provided by performing the twenty-first to twenty-fourth heat treatments. Moreover, the gate insulating film  2612  serves as a barrier film against impurities which cause deterioration in electrical characteristics of the transistor. Thus, even in the case where diffusion of the impurities occurs, the twenty-first to twenty-fourth heat treatments can prevent deterioration in electrical characteristics from being caused. However, one embodiment of the present invention is not limited to performing all of the twenty-first to twenty-fourth heat treatments. 
     According to this embodiment, since a gate insulating film has a high barrier property against impurities and contains fewer defects, a transistor having stable electric characteristics and high reliability can be provided. 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     [Embodiment 5] 
     In this embodiment, a semiconductor device including the transistor described in any of the above embodiments, in which an oxide semiconductor film is used as a semiconductor film, will be described. 
     The off-state current of the transistor described in any of the above embodiments can be significantly reduced when an oxide semiconductor film is used as the semiconductor film. That is, the transistor has electrical characteristics in which leakage of charge through the transistor is unlikely to occur. 
     A semiconductor device including the transistor having such electrical characteristics and having a memory element which is functionally superior to a known memory element will be described below. 
     First, the semiconductor device is specifically described with reference to  FIGS. 38A to 38D . Note that  FIG. 38A  is a circuit diagram illustrating a memory cell array in the semiconductor device.  FIG. 38B  is a circuit diagram of the memory cell.  FIG. 38C  shows an example of a cross-sectional structure corresponding to the memory cell in  FIG. 38B .  FIG. 38D  is a graph showing electrical characteristics of the memory cell in  FIG. 38B . 
     The memory cell array in  FIG. 38A  includes a plurality of memory cells  556 , a plurality of bit lines  553 , a plurality of word lines  554 , a plurality of capacitor lines  555 , and a plurality of sense amplifiers  558 . 
     Note that the bit lines  553  and the word lines  554  are arranged in grid patterns, and each memory cell  556  is provided at an intersection of the bit line  553  and the word line  554 . The bit line  553  is connected to the sense amplifier  558 , and the sense amplifier  558  has a function of reading a potential of the bit line  553  as data. 
     It is seen from  FIG. 38B  that the memory cell  556  includes a transistor  551  and a capacitor  552 . A gate of the transistor  551  is electrically connected to the word line  554 . A source of the transistor  551  is electrically connected to the bit line  553 . A drain of the transistor  551  is electrically connected to one terminal of the capacitor  552 . The other terminal of the capacitor  552  is electrically connected to the capacitor line  555 . 
       FIG. 38C  shows an example of a cross-sectional structure of the memory cell.  FIG. 38C  is a cross-sectional view of the semiconductor device including the transistor  551 ; the wiring  524   a  and the wiring  524   b  connected to the transistor  551 ; an insulating film  520  provided over the transistor  551 , and the wiring  524   a  and the wiring  524   b ; and a capacitor  552  provided over the insulating film  520 . 
     Note that in  FIG. 38C , the transistor illustrated in  FIGS. 5A to 5C  is used as the transistor  551 . Therefore, for components of the transistor  551 , which are not particularly mentioned below, the description in the above embodiments is referred to. The case where an oxide semiconductor film is used as the semiconductor film  506  of the transistor  551  is described below. 
     Note that in  FIG. 39 , the transistor illustrated in  FIGS. 24A to 24C  is used as a transistor  551 . Therefore, for components of the transistor  551 , which are not particularly mentioned below, the description in the above embodiments can be referred to. The case where an oxide semiconductor film is used as the semiconductor film  2506  of the transistor  551  is described below. However, the transistor that can be applied to the transistor  551  is not limited to the transistor illustrated in  FIGS. 5A to 5C  and the transistor illustrated in  FIGS. 24A to 24C . 
     The insulating film  520  can be provided using methods and an insulating film which are similar to those of the protective insulating film  518 . Alternatively, a resin film of a polyimide resin, an acrylic resin, an epoxy resin, a silicone resin, or the like may be used as the insulating film  520 . 
     The capacitor  552  includes an electrode  526  in contact with the wiring  524   b , an electrode  528  overlapping with the electrode  526 , and an insulating film  522  provided between the electrode  526  and the electrode  528 . 
     The electrode  526  can be formed to have a single-layer or a stacked-layer structure of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. 
     The electrode  528  can be formed to have a single-layer or a stacked-layer structure of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. 
     The insulating film  522  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     Note that although  FIG. 38C  shows an example where the transistor  551  and the capacitor  552  are provided in different layers, one embodiment of the present invention is not limited to this structure. For example, the transistor  551  and the capacitor  552  may be provided in the same layer. With such a structure, over the memory cell, a memory cell having a similar structure can be stacked. By stacking a plurality of memory cells, the plurality of memory cells can be integrated in the area of one memory cell. Accordingly, the degree of integration of the semiconductor device can be increased. 
     Here, the wiring  524   a  in  FIG. 38C  is electrically connected to the bit line  553  in  FIG. 38B . The gate electrode  504  in  FIG. 38C  is electrically connected to the word line  554  in  FIG. 38B . Further, the electrode  528  in  FIG. 38C  is electrically connected to the capacitor line  555  in  FIG. 38B . 
     As shown in  FIG. 38D , voltage held in the capacitor  552  is gradually decreased over time due to leakage through the transistor  551 . A voltage originally charged from V 0  to V 1  is decreased with time to VA that is a limit for reading out data  1 . This period is called a holding period T_ 1 . In the case of a two-level memory cell, refresh operation needs to be performed within the holding period T_ 1 . 
     For example, in the case where the off-state current of the transistor  551  is not small enough, a voltage held in the capacitor  552  over time changes significantly; therefore, the holding period T_ 1  becomes short. Thus, refresh operation needs to be performed frequently. When the frequency of refresh operation is increased, power consumption is increased. 
     In this embodiment, the off-state current of the transistor  551  is extremely small; therefore, the holding period T_ 1  can be made extremely long. That is, frequency of refresh operation can be reduced, which results in reduction in power consumption. For example, when a memory cell includes the transistor  551  having an off-state current of 1×10 −21  A to 1×10 −25  A, data can be held for several days to several decades without supply of power. 
     As described above, according to one embodiment of the present invention, a semiconductor device with high degree of integration and low power consumption can be obtained. 
     Next, a semiconductor device having a structure different from that of the semiconductor device in  FIGS. 38A to 38D  is described with reference to  FIGS. 40A to 40C . Note that  FIG. 40A  is a circuit diagram of a memory cell and wirings included in the semiconductor device.  FIG. 40B  is a graph showing electrical characteristics of the memory cell in  FIG. 40A .  FIG. 40C  shows an example of a cross-sectional structure corresponding to the memory cell in  FIG. 40A . 
     It is seen from  FIG. 40A  that the memory cell includes a transistor  671 , a transistor  672 , and a capacitor  673 . Here, a gate of the transistor  671  is electrically connected to a word line  676 . A source of the transistor  671  is electrically connected to a source line  674 . A drain of the transistor  671  is electrically connected to a gate of the transistor  672  and one terminal of the capacitor  673 , and this connection portion is referred to as a node  679 . A source of the transistor  672  is electrically connected to a source line  675 . A drain of the transistor  672  is electrically connected to a drain line  677 . The other terminal of the capacitor  673  is electrically connected to a capacitor line  678 . 
     The semiconductor device illustrated in  FIGS. 40A to 40C  utilizes variation in the apparent threshold voltage of the transistor  672 , which depends on the potential of the node  679 . For example,  FIG. 40B  shows a relation between a voltage V CL  of the capacitor line  678  and a drain current I d     —     2  flowing through the transistor  672 . 
     The potential of the node  679  can be controlled through the transistor  671 . For example, the potential of the source line  674  is set to a power supply potential VDD. In this case, when the potential of the word line  676  is set to be higher than or equal to a potential obtained by adding the power supply potential VDD to the threshold voltage Vth of the transistor  671 , the potential of the node  679  can be HIGH. Further, when the potential of the word line  676  is set to be lower than or equal to the threshold voltage Vth of the transistor  671 , the potential of the node  679  can be LOW. 
     Thus, the electrical characteristics of the transistor  672  is either a V CL -I d     —     2  curve denoted as LOW or a V CL -I d     —     2  curve denoted as HIGH. That is, when the potential of the node  679  is LOW, I d     —     2  is small at a V CL  of 0 V; accordingly, data  0  is stored. Further, when the potential of the node  679  is HIGH, I d     —     2  is large at a V CL  of 0 V; accordingly, data  1  is stored. In this manner, data can be stored. 
       FIG. 40C  shows an example of a cross-sectional structure of the memory cell.  FIG. 40C  is a cross-sectional view of the semiconductor device including the transistor  672 ; an insulating film  668  provided over the transistor  672 ; the transistor  671  provided over the insulating film  668 ; the wiring  624   a  and the wiring  624   b  connected to the transistor  671 ; an insulating film  620  provided over the transistor  671 , and the wiring  624   a  and the wiring  624   b ; and a capacitor  673  provided over the insulating film  620 . 
     The description of the protective insulating film  118  is referred to for the insulating film  620 . Alternatively, a resin film of a polyimide resin, an acrylic resin, an epoxy resin, a silicone resin, or the like may be used as the insulating film  620 . 
     Note that in  FIG. 40C , the transistor illustrated in  FIGS. 6A to 6C  is used as the transistor  671 . Therefore, for components of the transistor  671 , which are not particularly mentioned below, the description in the above embodiments is referred to. The case where an oxide semiconductor film is used as the semiconductor film  606  of the transistor  671  is described below. 
     Note that in  FIG. 41 , the transistor illustrated in  FIGS. 25A to 25C  is used as a transistor  671 . Therefore, for components of the transistor  671 , which are not particularly mentioned below, the description in the above embodiments can be referred to. The case where an oxide semiconductor film is used as the semiconductor film  2606  of the transistor  671  is described below. However, the transistor that can be applied to the transistor  671  is not limited to the transistor illustrated in  FIGS. 6A to 6C  and the transistor illustrated in  FIGS. 25A to 25C . 
     In this embodiment, the case where a transistor including crystalline silicon is used as the transistor  672  will be described. Note that any of the transistors described in the above embodiments may be used for the transistor  672 . 
     The transistor including crystalline silicon has an advantage over the transistor including an oxide semiconductor film in that on-state characteristics can be easily improved. Thus, it can be said that the transistor including crystalline silicon is suitable for the transistor  672  for which excellent on-state characteristics are required. 
     Here, the transistor  672  includes a base insulating film  652  provided over a substrate  650 ; a crystalline silicon film  656  provided over the base insulating film  652 ; a gate insulating film  662  provided over the crystalline silicon film  656 ; a gate electrode  654  which is over the gate insulating film  662  and provided so as to overlap with the crystalline silicon film  656 ; and a sidewall insulating film  660  in contact with a side surface of the gate electrode  654 . 
     The description of the substrate  100  is referred to for the substrate  650 . 
     The description of the base insulating film  302  is referred to for the base insulating film  652 . 
     A silicon film such as a single crystal silicon film or a polycrystalline silicon film can be used as the crystalline silicon film  656 . 
     Note that the crystalline silicon film is used for the transistor  672  in this embodiment; however, in the case where the substrate  650  is a semiconductor substrate such as a silicon wafer, the transistor  672  may include a channel region, and a source region and a drain region in the semiconductor substrate. 
     The gate insulating film  662  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     The description of the gate electrode  104  is referred to for the gate electrode  654 . 
     The sidewall insulating film  660  can be formed to have a single-layer or a stacked-layer structure of an insulating film containing one or more of the following: aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. 
     The description of the protective insulating film  118  is referred to for the insulating film  668 . Alternatively, a resin film of a polyimide resin, an acrylic resin, an epoxy resin, a silicone resin, or the like may be used as the insulating film  668 . 
     The insulating film  668  and the base insulating film  602  include an opening reaching the gate electrode  654  of the transistor  672 . The drain electrode  616   b  of the transistor  671  is in contact with the gate electrode  654  of the transistor  672  through the opening. 
     The capacitor  673  includes an electrode  626  in contact with the wiring  624   b , an electrode  628  overlapping with the electrode  626 , and an insulating film  622  provided between the electrode  626  and the electrode  628 . 
     The description of the electrode  526  is referred to for the electrode  626 . 
     The description of the electrode  528  is referred to for the electrode  628 . 
     Here, the wiring  624   a  in  FIG. 40C  is electrically connected to the source line  674  in  FIG. 40A . The gate electrode  604  in  FIG. 40C  is electrically connected to the word line  676  in  FIG. 40A . Further, the electrode  628  in  FIG. 40C  is electrically connected to the capacitor line  678  in  FIG. 40A . 
     Note that although  FIG. 40C  shows an example where the transistor  671  and the capacitor  673  are provided in different layers, one embodiment of the present invention is not limited to this structure. For example, the transistor  671  and the capacitor  673  may be provided in the same layer. With such a structure, over the memory cell, a memory cell having a similar structure can be stacked. By stacking a plurality of memory cells, the plurality of memory cells can be integrated in the area of one memory cell. Accordingly, the degree of integration of the semiconductor device can be increased. 
     Here, when any of the transistors including an oxide semiconductor film described in the above embodiments is used as the transistor  671 , leakage of charge held in the node  679 , which occurs through the transistor  671 , can be suppressed because the off-state current of the transistor  671  is extremely small. Therefore, data can be held for a long period. Further, high voltage is not needed in data writing; therefore, power consumption can be made small and operation speed can be made high compared to a flash memory. 
     As described above, according to one embodiment of the present invention, a semiconductor device with high degree of integration and low power consumption can be obtained. 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     [Embodiment 6] 
     A central processing unit (CPU) can be formed with the use of any of the transistors described in the above embodiments or any of the semiconductor devices described in the above embodiment for at least part of the CPU. 
       FIG. 42A  is a block diagram illustrating a specific structure of the CPU. The CPU illustrated in  FIG. 42A  includes an arithmetic logic unit (ALU)  1191 , an ALU controller  1192 , an instruction decoder  1193 , an interrupt controller  1194 , a timing controller  1195 , a register  1196 , a register controller  1197 , a bus interface (Bus I/F)  1198 , a rewritable ROM  1199 , and an ROM interface (ROM I/F)  1189  over a substrate  1190 . A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate  1190 . The ROM  1199  and the ROM interface  1189  may be provided over a separate chip. Obviously, the CPU illustrated in  FIG. 42A  is just an example in which a configuration is simplified, and an actual CPU may have various configurations depending on the application. 
     An instruction that is input to the CPU through the bus interface  1198  is input to the instruction decoder  1193  and decoded therein, and then, input to the ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195 . 
     The ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195  conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller  1192  generates signals for controlling the operation of the ALU  1191 . While the CPU is executing a program, the interrupt controller  1194  judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller  1197  generates an address of the register  1196 , and reads/writes data from/to the register  1196  in accordance with the state of the CPU. 
     The timing controller  1195  generates signals for controlling operation timings of the ALU  1191 , the ALU controller  1192 , the instruction decoder  1193 , the interrupt controller  1194 , and the register controller  1197 . For example, the timing controller  1195  includes an internal clock generator for generating an internal clock signal CLK 2  based on a reference clock signal CLK 1 , and supplies the clock signal CLK 2  to the above circuits. 
     In the CPU illustrated in  FIG. 42A , a memory element is provided in the register  1196 . As the register  1196 , any of the semiconductor devices described in the above embodiment can be used. 
     In the CPU illustrated in  FIG. 42A , the register controller  1197  selects operation of holding data in the register  1196  in accordance with an instruction from the ALU  1191 . That is, the register controller  1197  selects whether data is retained by a flip flop or a capacitor in the memory element included in the register  1196 . When data is held by the flip flop, a power supply voltage is supplied to the memory element in the register  1196 . When data is held by the capacitor, the data in the capacitor is rewritten, and supply of the power supply voltage to the memory element in the register  1196  can be stopped. 
     The power supply can be stopped by providing a switching element between a memory element group and a node to which a power supply potential VDD or a power supply potential VSS is supplied, as illustrated in  FIG. 42B  or  42 C. Circuits illustrated in  FIGS. 42B and 42C  are described below. 
       FIGS. 42B and 42C  each show an example of a structure including any of the transistors described in the above embodiments as a switching element for controlling supply of a power supply potential to a memory element. 
     The memory device illustrated in  FIG. 42B  includes a switching element  1141  and a memory element group  1143  including a plurality of memory elements  1142 . Specifically, as each of the memory elements  1142 , any of the semiconductor devices described in the above embodiment can be used. Each of the memory elements  1142  included in the memory element group  1143  is supplied with the high-level power supply potential VDD through the switching element  1141 . Further, each of the memory elements  1142  included in the memory element group  1143  is supplied with a potential of a signal IN and a potential of the low-level power supply potential VSS. 
     In  FIG. 42B , as the switching element  1141 , any of the transistors described in the above embodiments is used. With the use of an oxide semiconductor film as the semiconductor film of the transistor, a transistor whose off-state current is extremely small can be obtained. The switching of the transistor is controlled by a signal SigA input to the gate thereof. 
     Note that although  FIG. 42B  shows the structure in which the switching element  1141  includes only one transistor; however, the switching element  1141  is not limited to one transistor and may include a plurality of transistors. In the case where the switching element  1141  includes a plurality of transistors which serves as switching elements, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection. 
       FIG. 42C  shows an example of a memory device in which each of the memory elements  1142  included in the memory element group  1143  is supplied with the low-level power supply potential VSS through the switching element  1141 . The supply of the low-level power supply potential VSS to each of the memory elements  1142  included in the memory element group  1143  can be controlled by the switching element  1141 . 
     When a switching element is provided between a memory element group and a node to which the power supply potential VDD or the power supply potential VSS is supplied, data can be held even in the case where an operation of a CPU is temporarily stopped and the supply of the power supply voltage is stopped; accordingly, power consumption can be reduced. For example, while a user of a personal computer does not input data to an input device such as a keyboard, the operation of the CPU can be stopped, so that the power consumption can be reduced. 
     Although the CPU is given as an example here, the transistor and the semiconductor device can also be applied to an LSI such as a digital signal processor (DSP), a custom LSI, or a field programmable gate array (FPGA). 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     [Embodiment 7] 
     In this embodiment, a display device to which any of the transistors described in the above embodiments is applied will be described. 
     As a display element provided in the display device, a liquid crystal element (also referred to as a liquid crystal display element), a light-emitting element (also referred to as a light-emitting display element), or the like can be used. The light-emitting element includes, in its category, an element whose luminance is controlled by a current or a voltage, and specifically an inorganic electroluminescent (EL) element, an organic EL element, and the like. Furthermore, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used as the display element. In this embodiment, a display device including an EL element and a display device including a liquid crystal element will be described as examples of the display device. 
     Note that the display device in this embodiment includes in its category a panel in which a display element is sealed, and a module in which an IC such as a controller is mounted on the panel. 
     Additionally, the display device in this embodiment refers to an image display device, a display device, or a light source (including a lighting device). The display device includes any of the following modules in its category: a module provided with a connector such as an FPC or TCP; a module in which a printed wiring board is provided at the end of TCP; and a module in which an integrated circuit (IC) is mounted directly on a display element by a COG method. 
       FIG. 43A  is an example of a circuit diagram of the display device including an EL element. 
     The display device in  FIG. 43A  includes a switching element  743 , a transistor  741 , a capacitor  742 , and a light-emitting element  719 . 
     A gate of the transistor  741  is electrically connected to one terminal of the switching element  743  and one terminal of the capacitor  742 . A source of the transistor  741  is electrically connected to one terminal of the light-emitting element  719 . A drain of the transistor  741  is electrically connected to the other terminal of the capacitor  742  and is supplied with a power supply potential VDD. The other terminal of the switching element  743  is electrically connected to a signal line  744 . The other terminal of the light-emitting element  719  is supplied with a fixed potential. Note that the fixed potential is a ground potential GND or lower. 
     Note that, as the transistor  741 , any of the transistors described in the above embodiments is used. The transistor has stable electrical characteristics and high reliability. Therefore, a display device having stable display quality can be obtained. 
     As the switching element  743 , it is preferred to use a transistor. With a transistor, the area of a pixel can be reduced, so that a display device having a high resolution can be obtained. Moreover, as the switching element  743 , any of the transistors described in the above embodiments may be used. With the use of any of the transistors described in the above embodiments as the switching element  743 , the switching element  743  can be formed in the same process as the transistor  741 ; thus, the productivity of the display device can be improved. 
       FIG. 43B  illustrates part of a cross section of a pixel including the transistor  741 , the capacitor  742 , and the light-emitting element  719 . 
     Note that  FIG. 43B  shows an example where the transistor  741  and the capacitor  742  are provided in the same plane. With such a structure, the capacitor  742  can be formed in the same layer and using the same conductive film as a gate electrode, a gate insulating film, and a source electrode (drain electrode), which are included in the transistor  741 . When the transistor  741  and the capacitor  742  are provided in the same plane in this manner, the number of manufacturing steps of the display device can be reduced; thus, the productivity can be increased. 
     In  FIG. 43B , the transistor illustrated in  FIGS. 1A to 1C  is used as the transistor  741 . Therefore, for components of the transistor  741 , which are not particularly mentioned below, the description in the above embodiments is referred to. 
     In  FIG. 44 , the transistor illustrated in  FIGS. 20A to 20C  is used as a transistor  741 . Therefore, for components of the transistor  741 , which are not particularly mentioned below, the description in the above embodiments is referred to. However, the transistor that can be applied to the transistor  741  is not limited to the transistor illustrated in  FIGS. 1A to 1C  and the transistor illustrated in  FIGS. 20A to 20C . 
     An insulating film  720  is provided over the transistor  741  and the capacitor  742 . 
     Here, an opening reaching the source electrode  116   a  of the transistor  741  is provided in the insulating film  720  and the protective insulating film  118 . 
     An electrode  781  is provided over the insulating film  720 . The electrode  781  is in contact with the source electrode  116   a  of the transistor  741  through an opening provided in the protective insulating film  118 , the crystalline insulating film  136 , the aluminum oxide film  138 , and the insulating film  720 . 
     A partition  784  having an opening reaching the electrode  781  is provided over the electrode  781 . 
     A light-emitting layer  782  in contact with the electrode  781  through the opening provided in the partition  784  is provided over the partition  784 . 
     An electrode  783  is provided over the light-emitting layer  782 . 
     A region where the electrode  781 , the light-emitting layer  782 , and the electrode  783  overlap with one another serves as the light-emitting element  719 . 
     Note that description of the protective insulating film  118  is referred to for the insulating film  720 . Alternatively, a resin film of a polyimide resin, an acrylic resin, an epoxy resin, a silicone resin, or the like may be used as the insulating film  720 . 
     The light-emitting layer  782  is not limited to a single layer, and may be a stack of plural kinds of light-emitting materials. For example, a structure illustrated in  FIG. 43C  may be employed.  FIG. 43C  illustrates a structure in which an intermediate layer  785   a , a light-emitting layer  786   a , an intermediate layer  785   b , a light-emitting layer  786   b , an intermediate layer  785   c , a light-emitting layer  786   c , and an intermediate layer  785   d  are stacked in this order. In that case, when materials emitting light of appropriate colors are used for the light-emitting layer  786   a , the light-emitting layer  786   b , and the light-emitting layer  786   c , the light-emitting element  719  with a high color rending property or higher emission efficiency can be formed. 
     Plural kinds of light-emitting materials may be stacked to obtain white light. Although not illustrated in  FIG. 43B , white light may be extracted through coloring layers. 
     Although the structure in which three light-emitting layers and four intermediate layers are provided is shown here, the number of light-emitting layers and the number of intermediate layers can be changed as appropriate without being limited to the above. For example, the light-emitting layer  782  can be formed with only the intermediate layer  785   a , the light-emitting layer  786   a , the intermediate layer  785   b , the light-emitting layer  786   b , and the intermediate layer  785   c . Alternatively, the light-emitting layer  782  may be formed with the intermediate layer  785   a , the light-emitting layer  786   a , the intermediate layer  785   b , the light-emitting layer  786   b , the light-emitting layer  786   c , and the intermediate layer  785   d ; the intermediate layer  785   c  may be omitted. 
     Moreover, the intermediate layer can be formed using a stacked-layer structure of a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, or the like. Note that not all of these layers need to be provided as the intermediate layer. Any of these layers may be selected as appropriate to form the intermediate layer. Note that layers having similar functions may be provided. Further, an electron-relay layer or the like may be added as appropriate as the intermediate layer, in addition to a carrier generation layer. 
     The electrode  781  can be formed using a conductive film having a transmitting property with respect to visible light. “Having a transmitting property with respect to visible light” means that the average transmittance of light in a visible light region (e.g., a wavelength range from 400 nm to 800 nm) is higher than or equal to 70%, particularly higher than or equal to 80%. 
     As the electrode  781 , for example, an oxide film such as an In—Zn—W oxide film, an In—Sn oxide film, an In—Zn oxide film, an In oxide film, a Zn oxide film, or a Sn oxide film can be used. The above oxide film may contain a minute amount of Al, Ga, Sb, F, or the like. Further, a metal thin film having a thickness enough to transmit light (a metal thin film having a thickness of approximately 5 nm to 30 nm is preferred) can also be used. For example, an Ag film, an Mg film, or an Ag—Mg alloy film having a thickness of 5 nm may be used. 
     The electrode  781  is preferred to be a film which efficiently reflects visible light. For example, a film containing lithium, aluminum, titanium, magnesium, lanthanum, silver, silicon, or nickel can be used as the electrode  781 . 
     The electrode  783  can be formed using any of the films for the electrode  781 . Note that when the electrode  781  has a transmitting property with respect to visible light, it is preferred that the electrode  783  efficiently reflects visible light. When the electrode  781  efficiently reflects visible light, it is preferred that the electrode  783  has a transmitting property with respect to visible light. 
     Positions of the electrode  781  and the electrode  783  are not limited to the structure illustrated in  FIG. 43B , and the electrode  781  and the electrode  783  may be replaced with each other. It is preferred to use a conductive film having a high work function for the electrode which serves as an anode and a conductive film having a low work function for the electrode which serves as a cathode. Note that in the case where a carrier generation layer is provided in contact with the anode, a variety of conductive films can be used for the anode regardless of their work functions. 
     Note that description of the protective insulating film  118  is referred to for the partition  784 . Alternatively, a resin film of a polyimide resin, an acrylic resin, an epoxy resin, a silicone resin, or the like may be used as the partition  784 . 
     The transistor  741  connected to the light-emitting element  719  has stable electrical characteristics and high reliability. Therefore, a display device having stable display quality can be obtained. 
     Next, the display device including a liquid crystal element is described. 
       FIG. 45A  is a circuit diagram showing a structure example of the pixel of the display device including a liquid crystal element. A pixel  750  illustrated in  FIG. 45A  includes a transistor  751 , a capacitor  752 , and an element in which liquid crystal is filled between a pair of electrodes (hereinafter also referred to as a liquid crystal element)  753 . 
     One of a source and a drain of the transistor  751  is electrically connected to a signal line  755 , and a gate of the transistor  751  is electrically connected to a scan line  754 . 
     One of electrodes of the capacitor  752  is electrically connected to the other of the source and the drain of the transistor  751 , and the other of the electrodes of the capacitor  752  is electrically connected to a wiring for supplying a common potential. 
     One of electrodes of the liquid crystal element  753  is electrically connected to the other of the source and the drain of the transistor  751 , and the other of the electrodes of the liquid crystal element  753  is electrically connected to a wiring for supplying a common potential. Note that the common potential supplied to the wiring electrically connected to the other of the electrodes of the liquid crystal element  753  may be different from the common potential supplied to the wiring electrically connected to the other of the electrodes of the capacitor  752 . 
       FIG. 45B  illustrates part of a cross section of the pixel  750 . 
     Note that  FIG. 45B  shows an example where the transistor  751  and the capacitor  752  are provided in the same plane. With such a structure, the capacitor  752  can be formed in the same layer and using the same conductive film as a gate electrode, a gate insulating film, and a source electrode (drain electrode), which are included in the transistor  751 . When the transistor  751  and the capacitor  752  are provided in the same plane in this manner, the number of manufacturing steps of the display device can be reduced; thus, the productivity can be increased. 
     Note that, as the transistor  751 , any of the transistors described in the above embodiments can be used. In  FIG. 45B , the transistor illustrated in  FIGS. 1A to 1C  is used as the transistor  751 . Therefore, for components of the transistor  751 , which are not particularly mentioned below, the description in the above embodiments is referred to. 
     In  FIG. 46 , the transistor illustrated in  FIGS. 20A to 20C  is used as a transistor  751 . Therefore, for components of the transistor  751 , which are not particularly mentioned below, the description in the above embodiments is referred to. However, the transistor that can be applied to the transistor  751  is not limited to the transistor illustrated in  FIGS. 1A to 1C  and the transistor illustrated in  FIGS. 20A to 20C . 
     Note that in the case where an oxide semiconductor film is used as the semiconductor film  106  of the transistor  751 , the off-state current of the transistor  751  can be made extremely small. Thus, the charge held in the capacitor  752  is unlikely to be leaked and a voltage applied to the liquid crystal element  753  can be retained for a long time. Accordingly, when a motion image with less movement or a still image is displayed, a voltage for operating the transistor  751  is not needed by turning off the transistor  751 , whereby a display device with low power consumption can be obtained. 
     An insulating film  721  is provided over the transistor  751  and the capacitor  752 . 
     Here, an opening reaching the source electrode  116   b  of the transistor  751  is provided in the protective insulating film  118 , the crystalline insulating film  136 , the aluminum oxide film  138 , and the insulating film  721 . 
     An electrode  791  is provided over the insulating film  721 . The electrode  791  is in contact with the drain electrode  116   b  of the transistor  751  through the opening provided in the insulating film  721 , the aluminum oxide film  138 , the crystalline insulating film  136 , and the protective insulating film  118 . 
     An insulating film  792  serving as an alignment film is provided over the electrode  791 . 
     A liquid crystal layer  793  is provided over the insulating film  792 . 
     An insulating film  794  serving as an alignment film is provided over the liquid crystal layer  793 . 
     A spacer  795  is provided over the insulating film  794 . 
     An electrode  796  is provided over the spacer  795  and the insulating film  794 . 
     A substrate  797  is provided over the electrode  796 . 
     Note that description of the protective insulating film  118  is referred to for the insulating film  721 . Alternatively, a resin film of a polyimide resin, an acrylic resin, an epoxy resin, a silicone resin, or the like may be used as the insulating film  721 . 
     For the liquid crystal layer  793 , a thermotropic liquid crystal, a low-molecular liquid crystal, a polymer liquid crystal, a polymer-dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions. 
     Note that as the liquid crystal layer  793 , a liquid crystal exhibiting a blue phase may be used. In that case, the insulating films  792  and  794  serving as an alignment film are not necessarily provided. 
     The electrode  791  can be formed using a conductive film having a transmitting property with respect to visible light. 
     As the electrode  791 , for example, an oxide film such as an In—Zn—W oxide film, an In—Sn oxide film, an In—Zn oxide film, an In oxide film, a Zn oxide film, or a Sn oxide film can be used. The above oxide film may contain a minute amount of Al, Ga, Sb, F, or the like. Further, a metal thin film having a thickness enough to transmit light (a metal thin film having a thickness of approximately 5 nm to 30 nm is preferred) can also be used. 
     Alternatively, the electrode  791  is preferred to be a film which efficiently reflects visible light. For example, a film containing aluminum, titanium, chromium, copper, molybdenum, silver, tantalum, or tungsten can be used as the electrode  791 . 
     The electrode  796  can be formed using any of the films for the electrode  791 . Note that when the electrode  791  has a transmitting property with respect to visible light, it is in some cases preferred that the electrode  796  efficiently reflects visible light. When the electrode  791  efficiently reflects visible light, it is in some cases preferred that the electrode  796  has a transmitting property with respect to visible light. 
     Positions of the electrode  791  and the electrode  796  are not limited to the structure illustrated in  FIG. 45B , and the electrode  791  and the electrode  796  may be replaced with each other. 
     Each of the insulating films  792  and  794  can be formed using an organic compound insulating film or an inorganic compound insulating film. 
     The spacer  795  can be formed using an organic compound insulating film or an inorganic compound insulating film. Note that the spacer  795  can have a variety of shapes such as a columnar shape and a spherical shape. 
     A region where the electrode  791 , the insulating film  792 , the liquid crystal layer  793 , the insulating film  794 , and the electrode  796  overlap with one another serves as the liquid crystal element  753 . 
     For the substrate  797 , a glass substrate, a resin substrate, a metal substrate, or the like can be used. The substrate  797  may have flexibility. 
     The transistor  751  connected to the liquid crystal element  753  has stable electrical characteristics and high reliability. Therefore, a display device having stable display quality can be obtained. Further, a display device with low power consumption can be provided with the use of an oxide semiconductor film as the semiconductor film  106  of the transistor  751 . 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     [Embodiment 8] 
     In this embodiment, examples of an electronic device including any of the semiconductor devices described in the above embodiment will be described. 
       FIG. 47A  illustrates a portable information terminal The portable information terminal illustrated in  FIG. 47A  includes a housing  9300 , a button  9301 , a microphone  9302 , a display portion  9303 , a speaker  9304 , and a camera  9305 , and has a function as a mobile phone. One embodiment of the present invention can be applied to an arithmetic unit, a wireless circuit, or a memory circuit in a main body. Alternatively, one embodiment of the present invention can be applied to the display portion  9303 . 
       FIG. 47B  illustrates a display. The display illustrated in  FIG. 47B  includes a housing  9310  and a display portion  9311 . One embodiment of the present invention can be applied to an arithmetic unit, a wireless circuit, or a memory circuit in a main body. Alternatively, one embodiment of the present invention can be applied to the display portion  9311 . 
       FIG. 47C  illustrates a digital still camera. The digital still camera illustrated in  FIG. 47C  includes a housing  9320 , a button  9321 , a microphone  9322 , and a display portion  9323 . One embodiment of the present invention can be applied to an arithmetic unit, a wireless circuit, or a memory circuit in a main body. Alternatively, one embodiment of the present invention can be applied to the display portion  9323 . 
       FIG. 47D  illustrates a foldable portable information terminal. The foldable portable information terminal illustrated in  FIG. 47D  includes a housing  9630 , a display portion  9631   a , a display portion  9631   b , a hinge  9633 , and an operation switch  9638 . One embodiment of the present invention can be applied to an arithmetic unit, a wireless circuit, or a memory circuit in a main body. Alternatively, one embodiment of the present invention can be applied to the display portion  9631   a  and the display portion  9631   b.    
     Part or whole of the display portion  9631   a  and/or the display portion  9631   b  can function as a touch panel. By touching an operation key displayed on the touch panel, a user can input data, for example. 
     With the use of a semiconductor device according to one embodiment of the present invention, a highly reliable and high performance electronic device with low power consumption can be provided. 
     This embodiment is obtained by performing change, addition, modification, removal, application, superordinate conceptualization, or subordinate conceptualization on part or the whole of another embodiment. Thus, part or whole of this embodiment can be freely combined with, applied to, or replaced with part or all of another embodiment. 
     Note that in this specification and the like, in a diagram or a text described in one embodiment, it is possible to take out part of the diagram or the text and constitute one embodiment of the invention. Thus, in the case where a diagram or a text related to a certain portion is described, the context taken out from part of the diagram or the text is also disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. Consequently, for example, in a diagram or a text including one or more of active elements (e.g., transistors and diodes), wirings, passive elements (e.g., capacitors and resistors), conductive layers, insulating layers, semiconductor layers, organic materials, inorganic materials, components, devices, operation methods, manufacturing methods, and/or the like, it is possible to take out part of the diagram or the text and constitute one embodiment of the invention. For example, from a circuit diagram in which N circuit elements (e.g., transistors or capacitors; N is an integer) are provided, it is possible to constitute one embodiment of the invention by taking out M circuit elements (e.g., transistors or capacitors; M is an integer, where M&lt;N). As another example, it is possible to constitute one embodiment of the invention by taking out M layers (M is an integer, where M&lt;N) from a cross-sectional view in which N layers (N is an integer) are provided. As another example, it is possible to constitute one embodiment of the invention by taking out M elements (M is an integer, where M&lt;N) from a flow chart in which N elements (N is an integer) are provided. 
     Note that in the case where at least one specific example is described in a diagram or a text described in one embodiment in this specification and the like, it will be readily appreciated by those skilled in the art that a broader concept of the specific example can be derived. Therefore, in the diagram or the text described in one embodiment, in the case where at least one specific example is described, a broader concept of the specific example is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. 
     Note that in this specification and the like, a content described in at least a diagram (which may be part of the diagram) is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. Therefore, when a certain content is described in a diagram, the content is disclosed as one embodiment of the invention even when the content is not described with a text, and one embodiment of the invention can be constituted. In a similar manner, part of a diagram, which is taken out from the diagram, is disclosed as one embodiment of the invention, and one embodiment of the invention can be constituted. 
     EXAMPLE 1 
     In this example, an aluminum oxide film was formed over a crystalline insulating film, scanning transmission electron microscope (STEM) images were observed, and XRR was performed. As a comparative example, an aluminum oxide film was formed over an amorphous insulating film and similar evaluation was performed. 
     A method for manufacturing samples will be described below. 
     First, a silicon oxide film was formed over a glass substrate. The silicon oxide film was formed by a sputtering method to a thickness of 300 nm under the following conditions: a circle synthesized quartz target with a diameter of 12 inches was used; an argon gas with a flow rate of 25 sccm and an oxygen gas with a flow rate of 25 sccm were used; the power was 5 kW (13.56 MHz); the pressure was 0.4 Pa; the distance between the target and the substrate was 60 mm; and the substrate heating temperature was 100° C. 
     Next, a crystalline insulating film was formed. Here, in a sample 1, a zirconium oxide film to which yttrium oxide is added as a stabilizing material (such a zirconium oxide film is also referred to as yttria-stabilized zirconia film or a YSZ film) was formed. In a sample 2, a titanium oxide film was formed. Note that in a comparative sample, a crystalline insulating film was not formed. 
     Here, the YSZ film was formed by a sputtering method to a thickness of 100 nm under the following conditions: a circle YSZ target with a diameter of 6 inches (ZrO 2 :Y 2 O 3 =92:8 [molar ratio]) was used; an argon gas with a flow rate of 20 sccm and an oxygen gas with a flow rate of 20 sccm were used; the power was 250 W (13.56 MHz); the pressure was 0.4 Pa; the distance between the target and the substrate was 135 mm; and substrate heating was not performed. 
     The titanium oxide film was formed by a sputtering method to a thickness of 100 nm under the following conditions: a circle titanium target with a diameter of 6 inches was used; an argon gas with a flow rate of 40 sccm was used; the power was 400 W (13.56 MHz); the pressure was 0.4 Pa; the distance between the target and the substrate was 150 mm; and substrate heating was not performed. 
     Next, in each sample, an aluminum oxide film was formed. The aluminum oxide film was formed by a sputtering method to a thickness of 100 nm under the following conditions: a circle aluminum oxide target with a diameter of 12 inches was used; an argon gas with a flow rate of 25 sccm and an oxygen gas with a flow rate of 25 sccm were used; the power was 2.5 kW (13.56 MHz); the pressure was 0.4 Pa; the distance between the target and the substrate was 60 mm; and the substrate heating temperature was 250° C. 
     Cross-sectional STEM images of the sample 1, the sample 2, and the comparative sample which were formed in the above-described manner were observed. For the observation of the cross-sectional STEM images, an Ultra-thin Film Evaluation System HD-2300 manufactured by Hitachi High-Technologies Corporation was used. Note that the magnification was 200000 times.  FIG. 48A  shows a phase contrast image (also referred to as transmitted electron (TE) image) of the sample 1,  FIG. 48B  shows a TE image of the sample 2, and  FIG. 48C  shows a TE image of the comparative sample. 
     It was found from  FIG. 48A  that in the sample 1 the YSZ film had crystallinity, and the aluminum oxide film entirely had crystallinity. It was found from  FIG. 48B  that in the sample 2 the titanium oxide film had crystallinity, and the aluminum oxide film entirely had crystallinity. It was found from  FIG. 48C  that in the comparative sample not only the silicon oxide film but also a region of the aluminum oxide film near the interface with the silicon oxide film was amorphous. 
     Next, the sample 1, the sample 2, and the comparative sample were subjected to XRR. For the XRR, X-ray diffractometer ATX-G manufactured by Rigaku Corporation was used. As a result of the XRR, it was found that the sample 1, the sample 2, and the comparative Sample had a stacked-layer structure as in Table 1, a stacked-layer structure as in Table 2, and a stacked-layer structure as in Table 3, respectively. Note that the respective crystal states of these layers were evaluated from the TE images shown in  FIGS. 48A to 48C  and are shown in the tables below. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Thickness 
                 Density 
                   
                   
               
               
                 Sample 1 
                 [nm] 
                 [g/cm 3 ] 
                 Roughness [nm] 
                 Crtstal state 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 aluminum oxide 
                 4.7 
                 3.64 
                 2.12 
                 crystalline 
               
               
                 aluminum oxide 
                 101.3 
                 3.59 
                 1.30 
                 crystalline 
               
               
                 aluminum oxide 
                 7.4 
                 3.49 
                 4.50 
                 crystalline 
               
               
                 YSZ 
                 80.7 
                 5.99 
                 1.99 
                 crystalline 
               
               
                 silicon oxide 
                 300.0 
                 1.97 
                 1.52 
                 amorphous 
               
               
                 glass 
                 — 
                 2.51 
                 4.31 
                 amorphous 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Thickness 
                 Density 
                   
                   
               
               
                 Sample 2 
                 [nm] 
                 [g/cm 3 ] 
                 Roughness [nm] 
                 Crtstal state 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 aluminum oxide 
                 3.1 
                 3.86 
                 2.47 
                 crystalline 
               
               
                 aluminum oxide 
                 108.8 
                 3.58 
                 3.09 
                 crystalline 
               
               
                 aluminum oxide 
                 5.5 
                 3.92 
                 2.12 
                 crystalline 
               
               
                 titanium oxide 
                 98.5 
                 3.80 
                 4.80 
                 crystalline 
               
               
                 silicon oxide 
                 300.0 
                 2.16 
                 2.11 
                 amorphous 
               
               
                 glass 
                 — 
                 2.51 
                 4.21 
                 amorphous 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Comparative 
                 Thickness 
                 Density 
                   
                   
               
               
                 Sample 
                 [nm] 
                 [g/cm 3 ] 
                 Roughness [nm] 
                 Crtstal state 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 aluminum oxide 
                 3.6 
                 3.76 
                 1.91 
                 crystalline 
               
               
                 aluminum oxide 
                 102.8 
                 3.61 
                 2.17 
                 crystalline 
               
               
                 aluminum oxide 
                 13.2 
                 2.70 
                 4.82 
                 amorphous 
               
               
                 silicon oxide 
                 300.0 
                 1.91 
                 1.34 
                 amorphous 
               
               
                 glass 
                 — 
                 2.51 
                 1.08 
                 amorphous 
               
               
                   
               
            
           
         
       
     
     It was found from Tables 1 to 3 that the aluminum oxide film in each sample included two interface layers and a layer provided therebetween. Between the two interface layers, attention is focused on the density and the thickness of the interface layer on a base film (the YSZ film, the titanium oxide film, or the silicon oxide film) side (such an interface layer is also referred to as a lower-side layer). It was found that although there were no great difference between the density of the lower-side layer and the densities of the other layers in the sample 1 and the sample 2, the lower-side layer had a lower density than the other layers in the comparative sample. Moreover, in the comparative sample, the thickness of the lower-side layer having a low density was 13.2 nm. 
     As described above, it was found from the TE images and stacked-layer structures obtained by XRR of these samples that in the sample 1 and the sample 2, the aluminum oxide film is entirely crystallized and the lower-side layer had substantially the same density as the other layers. It was found that in the comparative sample, the region of the aluminum oxide film near the interface with the silicon oxide film was amorphous and the lower-side layer had a lower density than the other layers. Note that the amorphous region of the aluminum oxide film in the comparative sample is considered to be a lower-side layer having a low density. 
     According to this example, it is found that a silicon oxide film provided over a crystalline insulating film is entirely crystallized and has a high density. 
     EXAMPLE 2 
     In this example, a TE image of a sample (an example sample) having an aluminum oxide film where shape defects had suppressed was observed. Further, as a comparative example, a TE image of a sample (a comparative example sample) having an aluminum oxide film where shape defects had occurred was observed. 
     A method for manufacturing the samples will be described below. 
     First, a silicon oxynitride film was formed over a silicon wafer which is a substrate. The silicon oxynitride film was formed by a CVD method to a thickness of 400 nm under the following conditions: a SiH 4  gas with a flow rate of 27 sccm and a N 2 O gas with a flow rate of 1000 sccm were used; the power was 60 W (13.56 MHz); the pressure was 13.3 Pa; and the substrate heating temperature was 325° C. 
     Next, an oxide semiconductor film was formed. The oxide semiconductor film was formed by a sputtering method to a thickness of 20 nm under the following conditions: a circle In—Ga—Zn—O compound target with a diameter of 12 inches (In:Ga:Zn=3:1:2 [molar ratio]) was used; an argon gas with a flow rate of 30 sccm and an oxygen gas with a flow rate of 15 sccm were used; the power was 500 W (DC); the pressure was 0.4 Pa; the distance between the target and the substrate was 60 mm; and the substrate heating temperature was 200° C. 
     Next, a silicon oxynitride film was formed. The silicon oxynitride film was formed by a CVD method to a thickness of 20 nm under the following conditions: a SiH 4  gas with a flow rate of 1 sccm and a N 2 O gas with a flow rate of 800 sccm were used; the power was 150 W (60 MHz); the pressure was 40 Pa; and the substrate heating temperature was 400° C. 
     Next, a tantalum nitride film was formed. The tantalum nitride film was formed by a sputtering method to a thickness of 30 nm under the following conditions: a circle tantalum target with a diameter of 12 inches was used; an argon gas with a flow rate of 40 sccm and a nitrogen gas with a flow rate of 10 sccm were used; the power was 1 kW (DC); the pressure was 0.6 Pa; the distance between the target and the substrate was 60 mm; and substrate heating was not performed. 
     Next, a tungsten film was formed. The tungsten film was formed by a sputtering method to a thickness of 200 nm under the following conditions: a circle tungsten target with a diameter of 12 inches was used; an argon gas with a flow rate of 110 sccm was used; the power was 4 kW (DC); the pressure was 2 Pa; the distance between the target and the substrate was 60 mm; and the substrate heating temperature was 200° C. 
     Next, the 200-nm-thick tungsten film and the 30-nm-thick tantalum nitride film were processed to form a gate electrode. 
     Next, a silicon oxynitride film was formed. The silicon oxynitride film was formed by a CVD method to a thickness of 90 nm under the following conditions: a SiH 4  gas with a flow rate of 1 sccm and a N 2 O gas with a flow rate of 800 sccm were used; the power was 150 W (60 MHz); the pressure was 40 Pa; and the substrate heating temperature was 400° C. 
     Next, a highly anisotropic etching treatment was performed, whereby a sidewall insulating film was formed. Note that dry etching was employed as the highly anisotropic etching treatment. At this time, the 20-nm-thick silicon oxynitride film was simultaneously etched to form a gate insulating film. 
     Next, a tungsten film was formed. The tungsten film was formed by a sputtering method to a thickness of 30 nm under the following conditions: a circle tungsten target with a diameter of 12 inches was used; an argon gas with a flow rate of 90 sccm was used; the power was 1 kW (DC); the pressure was 0.8 Pa; the distance between the target and the substrate was 60 mm; and the substrate heating temperature was 200° C. 
     Next, the 30-nm-thick tungsten film was processed. 
     Next, in the example sample, a crystalline insulating film was formed. The crystalline insulating film was formed by a sputtering method to a thickness of 10 nm under the following conditions: a circle YSZ target with a diameter of 6 inches (ZrO 2 :Y 2 O 3 =92:8 [molar ratio]) was used; an argon gas with a flow rate of 20 sccm and an oxygen gas with a flow rate of 20 sccm were used; the power was 250 W (RF); the pressure was 0.4 Pa; the distance between the target and the substrate was 135 mm; and substrate heating was not performed. Note that in the comparative example sample, a crystalline insulating film was not formed. 
     Next, in both the example sample and the comparative example sample, aluminum oxide films were formed. The aluminum oxide films were each formed by a sputtering method to a thickness of 70 nm under the following conditions: a circle aluminum oxide target with a diameter of 12 inches was used; an argon gas with a flow rate of 25 sccm and an oxygen gas with a flow rate of 25 sccm were used; the power was 2.5 kW (RF); the pressure was 0.4 Pa; the distance between the target and the substrate was 60 mm; and the substrate heating temperature was 250° C. 
     Next, a silicon oxynitride film was formed. The silicon oxynitride film was formed by a CVD method to a thickness of 460 nm under the following conditions: a SiH 4  gas with a flow rate of 5 sccm and a N 2 O gas with a flow rate of 1000 sccm were used; the power was 35 W (13.56 MHz); the pressure was 133.3 Pa; and the substrate heating temperature was 325° C. 
     Next, a CMP treatment was performed. The CMP treatment was performed until the tungsten film serving as the gate electrode is partly exposed. 
     Cross-sectional STEM images of the example sample and the comparative example sample which were formed in the above-described manner were observed. For the observation of the cross-sectional STEM images, an Ultra-thin Film Evaluation System HD-2300 manufactured by Hitachi High-Technologies Corporation was used. Note that the magnification was 150000 times.  FIG. 49A  shows a TE image of the example sample, and  FIG. 49B  shows a TE image of the comparative example sample. 
     In the example sample, occurrence of shape defects of the aluminum oxide film was not observed from  FIG. 49A  (see a dashed-line circle in the figure). In the comparative example sample, it was found from  FIG. 49B  that a region of the aluminum oxide film near the tungsten film was etched and shape defects occur (see a dashed-line circle in the figure). 
     A reason why shape defects did not occur in the example sample is considered that the crystalline insulating film was provided and the aluminum oxide film had an increased density and is crystallized. 
     According to this example, it is found that shape defects which are caused by an aluminum oxide film can be suppressed with the use of an aluminum oxide film having crystallinity and a high density. 
     This application is based on Japanese Patent Application serial No. 2012-051261 filed with the Japan Patent Office on Mar. 8, 2012 and Japanese Patent Application serial No. 2012-051263 filed with the Japan Patent Office on Mar. 8, 2012, the entire contents of which are hereby incorporated by reference.