Patent Publication Number: US-2022216341-A1

Title: Semiconductor device and manufacturing method of the semiconductor device

Description:
TECHNICAL FIELD 
     One embodiment of the present invention relates to a transistor, a semiconductor device, and an electronic device. Another embodiment of the present invention relates to a method of manufacturing a semiconductor device. Another embodiment of the present invention relates to a semiconductor wafer and a module. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a storage device are each an embodiment of a semiconductor device. It can be sometimes said that a display device (a liquid crystal display device, a light-emitting display device, and the like), a projection device, a lighting device, an electro-optical device, a power storage device, a storage device, a semiconductor circuit, an imaging device, an electronic device, and the like include a semiconductor device. 
     Note that one embodiment of the present invention is not limited to the above technical field. One embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. Another embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. 
     BACKGROUND ART 
     A technique by which a transistor is formed using a semiconductor thin film formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). A silicon-based semiconductor material is widely known as a semiconductor thin film applicable to the transistor; in addition, an oxide semiconductor has been attracting attention as another material. 
     A CAAC (c-axis aligned crystalline) structure and an nc (nanocrystalline) structure, which are neither single crystal nor amorphous, have been found in an oxide semiconductor (see Non-Patent Document 1 and Non-Patent Document 2). 
     Non-Patent Document 1 and Non-Patent Document 2 disclose a technique for manufacturing a transistor using an oxide semiconductor having a CAAC structure. 
     REFERENCE 
     Non-Patent Document 
     
         
         [Non-Patent Document 1] S. Yamazaki et al., “SID Symposium Digest of Technical Papers”, 2012, volume 43, issue 1, pp. 183-186 
         [Non-Patent Document 2] S. Yamazaki et al., “Japanese Journal of Applied Physics”, 2014, volume 53, Number 4S, pp. 04ED18-1-04ED18-10 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     An object of one embodiment of the present invention is to provide a semiconductor device in which variation of transistor characteristics is small. Another object of one embodiment of the present invention is to provide a semiconductor device having favorable reliability. Another object of one embodiment of the present invention is to provide a semiconductor device having favorable electrical characteristics. Another object of one embodiment of the present invention is to provide a semiconductor device with a high on-state current. Another object of one embodiment of the present invention is to provide a semiconductor device that can be miniaturized or highly integrated. Another object of one embodiment of the present invention is to provide a semiconductor device with low power consumption. 
     Note that the description of these objects does not preclude the existence of other objects. One embodiment of the present invention does not have to achieve all these objects. Other objects will be apparent from the description of the specification, the drawings, the claims, and the like, and other objects can be derived from the description of the specification, the drawings, the claims, and the like. 
     Means for Solving the Problems 
     One embodiment of the present invention is a semiconductor device including a first insulator; a first oxide over the first insulator; a second oxide over the first oxide; a third oxide and a fourth oxide over the second oxide; a first conductor over the third oxide; a second conductor over the fourth oxide; a second insulator over the first conductor; a third insulator over the second conductor; a fifth oxide that is over the second oxide and is positioned between the third oxide and the fourth oxide; a sixth oxide over the fifth oxide; a fourth insulator over the sixth oxide; a third conductor over the fourth insulator; and a fifth insulator over the first insulator to the third insulator. The fifth oxide includes a region in contact with the second oxide to the fourth oxide and the first insulator. The sixth oxide includes a region in contact with the fifth oxide, the first conductor, and the second conductor. The fourth insulator includes a region in contact with the sixth oxide, the third conductor, and the fifth insulator. 
     In the above, it is preferable that the first oxide to the sixth oxide each include In, an element M (M is Al, Ga, Y, or Sn), and Zn, that the first oxide, the third oxide, the fourth oxide, and the sixth oxide each include a region having a higher concentration of the element M than the second oxide, and that the first oxide, the third oxide, the fourth oxide, and the sixth oxide each include a region having a higher concentration of the element M than the fifth oxide. 
     In the above, it is preferable that the second insulator, the third insulator, and the fifth insulator each include either one of aluminum and hafnium. 
     One embodiment of the present invention is a method for manufacturing a semiconductor device, including the steps of depositing a first insulator; depositing a first oxide film, a second oxide film, and a third oxide film in this order over the first insulator; performing first heat treatment; depositing a first conductive film, a first insulating film, and a second conductive film in this order over the third oxide film; processing the first oxide film, the second oxide film, the third oxide film, the first conductive film, the first insulating film, and the second conductive film into island shapes to form a first oxide, a second oxide, a first oxide layer, a first conductive layer, a first insulating layer, and a second conductive layer; removing the second conductive layer; depositing a fifth insulator over the first insulator, the first oxide, the second oxide, the first oxide layer, the first conductive layer, and the first insulating layer; depositing a sixth insulator over the fifth insulator; forming an opening reaching the second oxide in the first oxide layer, the first conductive layer, the first insulating layer, the fifth insulator, and the sixth insulator; through the formation of the opening, forming a third oxide and a fourth oxide from the first oxide layer, forming a first conductor and a second conductor from the first conductive layer, and forming a seventh insulator and an eighth insulator from the first insulating layer; performing second heat treatment; depositing a fourth oxide film over the sixth insulator and in the opening; depositing a fifth oxide film over the fourth oxide film; removing, by a dry etching method, the fourth oxide film and the fifth oxide film deposited on side surfaces of the fifth insulator to the eighth insulator in the opening; removing the fourth oxide film and the fifth oxide film over the sixth insulator; forming a fifth oxide and a sixth oxide by leaving the fourth oxide film and the fifth oxide film on a bottom portion of the opening; depositing a second insulating film over the sixth oxide and the sixth insulator; depositing a third conductive film over the second insulating film; and forming a ninth insulator and a third conductor in the opening by polishing the third conductive film and the second insulating film by a CMP method until the sixth insulator is reached. 
     In the above, the dry etching method is preferably performed at a pressure of higher than or equal to 66.5 Pa. 
     Effect of the Invention 
     According to one embodiment of the present invention, a semiconductor device in which variation of transistor characteristics is small can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device having favorable reliability can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device having favorable electrical characteristics can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device with a high on-state current can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device that can be miniaturized or highly integrated can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device with low power consumption can be provided. 
     Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not have to have all these effects. Note that effects other than these will be apparent from the description of the specification, the drawings, the claims, and the like and effects other than these can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top view of a semiconductor device of one embodiment of the present invention. 
         FIG. 1B  to  FIG. 1D  are cross-sectional views of the semiconductor device of one embodiment of the present invention. 
         FIG. 2A  is a table showing classifications of crystal structures of IGZO.  FIG. 2B  is a graph showing an XRD spectrum of a CAAC-IGZO film.  FIG. 2C  is an image showing a nanobeam electron diffraction pattern of the CAAC-IGZO film. 
         FIG. 3A  is a top view of a semiconductor device of one embodiment of the present invention. 
         FIG. 3B  to  FIG. 3D  are cross-sectional views of the semiconductor device of one embodiment of the present invention. 
         FIG. 4A  is a top view of a semiconductor device of one embodiment of the present invention. 
         FIG. 4B  to  FIG. 4D  are cross-sectional views of the semiconductor device of one embodiment of the present invention. 
         FIG. 5A  is a top view illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 5B  to  FIG. 5D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 6A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 6B  to  FIG. 6D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 7A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 7B  to  FIG. 7D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 8A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 8B  to  FIG. 8D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 9A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 9B  to  FIG. 9D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 10A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 10B  to  FIG. 10D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 11A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 11B  to  FIG. 11D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 12A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 12B  to  FIG. 12D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 13A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 13B  to  FIG. 13D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 14A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 14B  to  FIG. 14D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 15A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 15B  to  FIG. 15D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 16A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 16B  to  FIG. 16D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 17A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 17B  to  FIG. 17D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 18A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 18B  to  FIG. 18D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 19A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 19B  to  FIG. 19D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 20A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 20B  to  FIG. 20D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 21A  is a top view illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention.  FIG. 21B  to  FIG. 21D  are cross-sectional views illustrating the method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 22A  and  FIG. 22B  are cross-sectional views of semiconductor devices of embodiments of the present invention. 
         FIG. 23  is a cross-sectional view illustrating a structure of a storage device of one embodiment of the present invention. 
         FIG. 24  is a cross-sectional view illustrating a structure of a storage device of one embodiment of the present invention. 
         FIG. 25  is a cross-sectional view of a semiconductor device of one embodiment of the present invention. 
         FIG. 26A  and  FIG. 26B  are cross-sectional views of semiconductor devices of embodiments of the present invention. 
         FIG. 27  is a cross-sectional view of a semiconductor device of one embodiment of the present invention. 
         FIG. 28  is a cross-sectional view of a semiconductor device of one embodiment of the present invention. 
         FIG. 29  is a top view illustrating an apparatus for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIG. 30A  and  FIG. 30B  are block diagrams illustrating a structure example of a storage device of one embodiment of the present invention. 
         FIG. 31A  to  FIG. 31H  are circuit diagrams each illustrating a structure example of a storage device of one embodiment of the present invention. 
         FIG. 32  is a diagram illustrating a hierarchy of storage devices. 
         FIG. 33A  and  FIG. 33B  are schematic diagrams of semiconductor devices of embodiments of the present invention. 
         FIG. 34A  and  FIG. 34B  are diagrams illustrating examples of electronic components. 
         FIG. 35A  to  FIG. 35E  are schematic diagrams of storage devices of embodiments of the present invention. 
         FIG. 36A  to  FIG. 36H  are diagrams illustrating electronic devices of embodiments of the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments will be described with reference to the drawings. Note that the embodiments can be implemented with many different modes, and it is readily understood by those skilled in the art that modes and details thereof can be changed in various ways without departing from the spirit and scope thereof. Thus, the present invention should not be interpreted as being limited to the description of the embodiments below. 
     In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Therefore, they are not limited to the illustrated scale. Note that the drawings schematically illustrate ideal examples, and embodiments of the present invention are not limited to shapes, values, and the like shown in the drawings. For example, in the actual manufacturing process, a layer, a resist mask, or the like might be unintentionally reduced in size by treatment such as etching, which might not be reflected in the drawings for easy understanding of the invention. Furthermore, in the drawings, the same reference numerals are used in common for the same portions or portions having similar functions in different drawings, and repeated description thereof is omitted in some cases. Furthermore, the same hatch pattern is used for the portions having similar functions, and the portions are not especially denoted by reference numerals in some cases. 
     Furthermore, especially in a top view (also referred to as a “plan view”), a perspective view, or the like, the description of some components might be omitted for easy understanding of the invention. In addition, some hidden lines and the like might not be shown. 
     The ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate. In addition, the ordinal numbers in this specification and the like do not sometimes correspond to the ordinal numbers that are used to specify one embodiment of the present invention. 
     In this specification and the like, terms for describing arrangement, such as “over” and “under”, are used for convenience to describe the positional relation between components with reference to drawings. The positional relation between components is changed as appropriate in accordance with a direction in which the components are described. Thus, without limitation to terms described in this specification, the description can be changed appropriately depending on the situation. 
     When this specification and the like explicitly state that X and Y are connected, for example, the case where X and Y are electrically connected, the case where X and Y are functionally connected, and the case where X and Y are directly connected are regarded as being disclosed in this specification and the like. Accordingly, without being limited to a predetermined connection relation, for example, a connection relation shown in drawings or text, a connection relation other than one shown in drawings or text is regarded as being disclosed in the drawings or the text. Here, X and Y each denote an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). 
     In this specification and the like, a transistor is an element having at least three terminals including a gate, a drain, and a source. In addition, the transistor includes a region where a channel is formed (hereinafter also referred to as a channel formation region) between the drain (a drain terminal, a drain region, or a drain electrode) and the source (a source terminal, a source region, or a source electrode), and current can flow between the source and the drain through the channel formation region. Note that in this specification and the like, a channel formation region refers to a region through which a current mainly flows. 
     Furthermore, functions of a source and a drain might be switched when a transistor of opposite polarity is employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can sometimes be interchanged with each other in this specification and the like. 
     Note that a channel length refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is in an on state) and a gate electrode overlap with each other or a channel formation region in a top view of the transistor. Note that in one transistor, channel lengths in all regions do not necessarily have the same value. In other words, the channel length of one transistor is not fixed to one value in some cases. Thus, in this specification, the channel length is any one of the values, the maximum value, the minimum value, and the average value in a channel formation region. 
     The channel width refers to, for example, the length of a channel formation region in a direction perpendicular to a channel length direction in a region where a semiconductor (or a portion where current flows in a semiconductor when a transistor is in an on state) and a gate electrode overlap with each other, or a channel formation region in a top view of the transistor. Note that in one transistor, channel widths in all regions do not necessarily have the same value. In other words, the channel width of one transistor is not fixed to one value in some cases. Thus, in this specification, the channel width is any one of the values, the maximum value, the minimum value, and the average value in a channel formation region. 
     Note that in this specification and the like, depending on the transistor structure, a channel width in a region where a channel is actually formed (hereinafter also referred to as an “effective channel width”) is sometimes different from a channel width shown in a top view of a transistor (hereinafter also referred to as an “apparent channel width”). For example, in a transistor whose gate electrode covers a side surface of a semiconductor, the effective channel width is larger than the apparent channel width, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor whose gate electrode covers a side surface of a semiconductor, the proportion of a channel formation region formed in the side surface of the semiconductor is increased in some cases. In that case, the effective channel width is larger than the apparent channel width. 
     In such a case, the effective channel width is sometimes difficult to estimate by actual measurement. For example, estimation of an effective channel width from a design value requires assumption that the shape of a semiconductor is known. Accordingly, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure the effective channel width accurately. 
     In this specification, the simple term “channel width” refers to an apparent channel width in some cases. Alternatively, in this specification, the simple term “channel width” refers to an effective channel width in some cases. Note that values of a channel length, a channel width, an effective channel width, an apparent channel width, and the like can be determined, for example, by analyzing a cross-sectional TEM image and the like. 
     Note that impurities in a semiconductor refer to, for example, elements other than the main components of a semiconductor. For example, an element with a concentration lower than 0.1 atomic % can be regarded as an impurity. When an impurity is contained, for example, the density of defect states in a semiconductor increases and the crystallinity decreases in some cases. In the case where the semiconductor is an oxide semiconductor, examples of an impurity which changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components of the oxide semiconductor; hydrogen, lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen are given as examples. Note that water also serves as an impurity in some cases. In addition, oxygen vacancies (also referred to as V O ) are formed in an oxide semiconductor in some cases by entry of impurities, for example. 
     Note that in this specification and the like, silicon oxynitride is a material that contains more oxygen than nitrogen in its composition. Moreover, silicon nitride oxide is a material that contains more nitrogen than oxygen in its composition. 
     In this specification and the like, the term “insulator” can be replaced with an insulating film or an insulating layer. Furthermore, the term “conductor” can be replaced with a conductive film or a conductive layer. Moreover, the term “semiconductor” can be replaced with a semiconductor film or a semiconductor layer. 
     In this specification and the like, “parallel” indicates a state where two straight lines are placed at an angle greater than or equal to −10° and less than or equal to 10°. Accordingly, the case where the angle is greater than or equal to −5° and less than or equal to 5° is also included. Furthermore, “substantially parallel” indicates a state where two straight lines are placed at an angle greater than or equal to −30° and less than or equal to 30°. Moreover, “perpendicular” indicates a state where two straight lines are placed at an angle greater than or equal to 80° and less than or equal to 100°. Accordingly, the case where the angle is greater than or equal to 85° and less than or equal to 95° is also included. Furthermore, “substantially perpendicular” indicates a state where two straight lines are placed at an angle greater than or equal to 60° and less than or equal to 120°. 
     In this specification and the like, a metal oxide is an oxide of metal in a broad sense. Metal oxides are classified into an oxide insulator, an oxide conductor (including a transparent oxide conductor), an oxide semiconductor (also simply referred to as an OS), and the like. For example, in the case where a metal oxide is used in a semiconductor layer of a transistor, the metal oxide is referred to as an oxide semiconductor in some cases. That is, an OS transistor can also be called a transistor including a metal oxide or an oxide semiconductor. 
     In this specification and the like, “normally off” means that a drain current per micrometer of channel width flowing through a transistor when no potential is applied to a gate or the gate is supplied with a ground potential is 1×10 −20  A or lower at room temperature, 1×10 −18  A or lower at 85° C., or 1×10 −16  A or lower at 125° C. 
     Embodiment 1 
     In this embodiment, an example of a semiconductor device including a transistor  200  of one embodiment of the present invention and a manufacturing method thereof are described using  FIG. 1A  to  FIG. 19D . 
     &lt;Structure Example of Semiconductor Device&gt; 
     A structure of a semiconductor device including the transistor  200  is described using  FIG. 1 .  FIG. 1A  to  FIG. 1D  are a top view and cross-sectional views of a semiconductor device including the transistor  200 .  FIG. 1A  is a top view of the semiconductor device.  FIG. 1B  to  FIG. 1D  are cross-sectional views of the semiconductor device. Here,  FIG. 1B  is a cross-sectional view of a portion indicated by dashed-dotted line A 1 -A 2  in  FIG. 1A , and is a cross-sectional view in the channel length direction of the transistor  200 .  FIG. 1C  is a cross-sectional view of a portion indicated by dashed-dotted line A 3 -A 4  in  FIG. 1A , and is a cross-sectional view in the channel width direction of the transistor  200 .  FIG. 1D  is a cross-sectional view of a portion indicated by dashed-dotted line A 5 -A 6  in  FIG. 1A . Note that for clarity of the drawing, some components are not illustrated in the top view of  FIG. 1A . 
     The semiconductor device of one embodiment of the present invention includes an insulator  212  over a substrate (not shown), an insulator  214  over the insulator  212 , the transistor  200  over the insulator  214 , an insulator  280  over the transistor  200 , an insulator  282  over the insulator  280 , and an insulator  283  over the insulator  282 . The insulator  212 , the insulator  214 , the insulator  280 , the insulator  282 , and the insulator  283  function as interlayer films. A conductor  240  (a conductor  240   a  and a conductor  240   b ) that is electrically connected to a conductor  242  (a conductor  242   a  and a conductor  242   b ) included in the transistor  200  and functions as a plug is also included. Note that an insulator  241  (an insulator  241   a  and an insulator  241   b ) is provided in contact with the side surfaces of the conductor  240  functioning as a plug. A conductor  246  (a conductor  246   a  and a conductor  246   b ) that is electrically connected to the conductor  240  and functions as a wiring is provided over the insulator  283  and the conductor  240 . An insulator  286  is provided over the conductor  246  and the insulator  283 . 
     The insulator  241   a  is provided in contact with the inner wall of an opening in the insulator  280 , the insulator  282 , and the insulator  283 ; a first conductor of the conductor  240   a  is provided in contact with the side surface of the insulator  241   a ; and a second conductor of the conductor  240   a  is provided on the inner side thereof. The insulator  241   b  is provided in contact with the inner wall of an opening in the insulator  280 , the insulator  282 , and the insulator  283 ; a first conductor of the conductor  240   b  is provided in contact with the side surface of the insulator  241   b ; and a second conductor of the conductor  240   b  is provided on the inner side thereof. A groove and a slit, for example, are included in the category of the opening. A region where an opening is formed is referred to as an opening portion in some cases. The level of the top surface of the conductor  240  and the level of the top surface of the insulator  283  in a region overlapping with the conductor  246  can be substantially the same. Note that although the transistor  200  has a structure in which the first conductor of the conductor  240  and the second conductor of the conductor  240  are stacked, the present invention is not limited thereto. For example, the conductor  240  may be provided as a single layer or to have a stacked-layer structure of three or more layers. In the case where a structure body has a stacked-layer structure, layers may be distinguished by ordinal numbers corresponding to the formation order. 
     [Transistor  200 ] 
     As illustrated in  FIG. 1A  to  FIG. 1D , the transistor  200  includes an insulator  216  over the insulator  214 ; a conductor  205  (a conductor  205   a  and a conductor  205   b ) positioned to be embedded in the insulator  214  or parts of the insulator  214  and the insulator  216 ; an insulator  222  over the insulator  216  and the conductor  205 ; an insulator  224  over the insulator  222 ; an oxide  230   a  over the insulator  224 ; an oxide  230   b  over the oxide  230   a ; an oxide  243  (an oxide  243   a  and an oxide  243   b ) and an oxide  230   c  over the oxide  230   b ; the conductor  242   a  over the oxide  243   a ; an insulator  271   a  over the conductor  242   a ; the conductor  242   b  over the oxide  243   b ; an insulator  271   b  over the conductor  242   b ; an oxide  230   d  over the oxide  230   c ; an insulator  250  over the oxide  230   d ; a conductor  260  (a conductor  260   a  and a conductor  260   b ) that is positioned over the insulator  250  and overlaps with part of the oxide  230   c ; and an insulator  272  in contact with part of the insulator  224 , the side surface of the oxide  230   a , the side surface of the oxide  230   b , the side surface of the oxide  243   a , the side surface of the conductor  242   a , the side surface of the insulator  271   a , the top surface of the insulator  271   a , the side surface of the insulator  271   b , and the side surface of the conductor  242   b . In addition, the oxide  230   c  is in contact with the side surface of the oxide  243   a , the side surface of the oxide  243   b , part of the top surface of the oxide  230   b , part of the insulator  280 , and part of the top surface of the insulator  224 . The oxide  230   d  is in contact with the side surface of the conductor  242   a , the side surface of the conductor  242   b , and the top surface of the oxide  230   c . Here, as illustrated in  FIG. 1B  and  FIG. 1C , the top surface of the conductor  260  is positioned to be substantially aligned with the top surface of the insulator  250 . The insulator  282  is in contact with the top surfaces of the conductor  260 , the insulator  250 , and the insulator  280 . 
     Hereinafter, the insulator  271   a  and the insulator  271   b  are collectively referred to as an insulator  271 , in some cases. 
     An opening reaching the oxide  230   b  is provided in the insulator  280 , the insulator  272 , and the insulator  271 . The oxide  230   c , the oxide  230   d , the insulator  250 , and the conductor  260  are positioned in the opening. The insulator  250  includes a region in contact with the side surface of the conductor  260  and a region in contact with the bottom surface of the conductor  260 . The oxide  230   c  includes a region in contact with the oxide  230   b  and a region overlapping with the bottom surface of the conductor  260  with the oxide  230   d  and the insulator  250  therebetween. 
     The oxide  230  preferably includes the oxide  230   a  positioned over the insulator  224 , the oxide  230   b  positioned over the oxide  230   a , the oxide  230   c  that is positioned over the oxide  230   b  and is at least partly in contact with the oxide  230   b , and the oxide  230   d  positioned over the oxide  230   c . Including the oxide  230   a  below the oxide  230   b  makes it possible to inhibit diffusion of impurities into the oxide  230   b  from components formed below the oxide  230   a . Moreover, including the oxide  230   d  over the oxide  230   c  makes it possible to inhibit diffusion of impurities into the oxide  230   c  from components formed above the oxide  230   d.    
     Although a structure in which the oxide  230  has a four-layer stacked structure of the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d  in the transistor  200  is described, the present invention is not limited thereto. For example, the oxide  230  may be a single layer of the oxide  230   b  or has a two-layer structure of the oxide  230   a  and the oxide  230   b , a two-layer structure of the oxide  230   b  and the oxide  230   c , a three-layer structure of the oxide  230   a , the oxide  230   b , and the oxide  230   c , or a stacked-layer structure including five or more layers. Alternatively, each of the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d  may have a stacked-layer structure. 
     Here, the conductor  260  functions as a first gate (also referred to as a top gate) electrode, and the conductor  205  functions as a second gate (also referred to as a back gate) electrode. The insulator  250  functions as a first gate insulator, and the insulator  224  functions as a second gate insulator. The conductor  242   a  functions as one of a source and a drain, and the conductor  242   b  functions as the other of the source and the drain. A region of the oxide  230  that overlaps with the conductor  260  at least partly functions as a channel formation region. 
     In the transistor  200 , a metal oxide functioning as an oxide semiconductor (such a metal oxide is hereinafter also referred to as an oxide semiconductor) is preferably used for the oxide  230  (the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d ) including the channel formation region. 
     The metal oxide functioning as a semiconductor has a band gap of preferably 2 eV or higher, further preferably 2.5 eV or higher. With the use of a metal oxide having such a wide band gap, the off-state current of the transistor can be reduced. 
     For the oxide  230 , for example, a metal oxide such as an In-M-Zn oxide including indium, an element M, and zinc (the element M is one or more kinds selected from aluminum, gallium, yttrium, tin, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like) is preferably used. An In—Ga oxide, an In—Zn oxide, or an indium oxide may be used for the oxide  230 . 
     The atomic ratio of In to the element M in the metal oxide used for the oxide  230   b  or the oxide  230   c  is preferably greater than the atomic ratio of In to the metal element M in the metal oxide used for the oxide  230   a  or the oxide  230   d.    
     The oxide  230   a  is positioned under the oxide  230   b  or the oxide  230   c , whereby impurities and oxygen can be inhibited from being diffused into the oxide  230   b  or the oxide  230   c  from components formed under the oxide  230   a.    
     The oxide  230   d  is positioned over the oxide  230   b  or the oxide  230   c , whereby impurities can be inhibited from being diffused into the oxide  230   b  or the oxide  230   c  from components formed over the oxide  230   d . When the oxide  230   d  is positioned over the oxide  230   b  or the oxide  230   c , oxygen can be inhibited from diffusing upward from the oxide  230   b  or the oxide  230   c.    
     When the oxide  230   a  to the oxide  230   d  contain a common element (as the main component) besides oxygen, the density of defect states at each interface between the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d  can be low. In that case, a main carrier path is the oxide  230   b , the oxide  230   c , or the vicinity thereof, for example, the interface between the oxide  230   b  and the oxide  230   c . Since the density of defect states at the interface between the oxide  230   b  and the oxide  230   c  can be decreased, the influence of interface scattering on carrier conduction is small, and a high on-state current can be obtained. 
     The oxide  230   b  and the oxide  230   c  preferably have crystallinity. It is particularly preferable to use a CAAC-OS (c-axis aligned crystalline oxide semiconductor) for the oxide  230   b  and the oxide  230   c . The oxide  230   d  may also have crystallinity. 
     The CAAC-OS has c-axis alignment, a plurality of nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion. Note that the distortion refers to a portion where the direction of a lattice arrangement changes between a region with a regular lattice arrangement and another region with a regular lattice arrangement in a region where the plurality of nanocrystals are connected. 
     The nanocrystal is basically a hexagon but is not always a regular hexagon and is a non-regular hexagon in some cases. Furthermore, a pentagonal or heptagonal lattice arrangement, for example, is included in the distortion in some cases. Note that it is difficult to observe a clear crystal grain boundary (also referred to as grain boundary) even in the vicinity of distortion in the CAAC-OS. That is, formation of a crystal grain boundary is found to be inhibited by the distortion of a lattice arrangement. This is because the CAAC-OS can tolerate distortion owing to a low density of arrangement of oxygen atoms in the a-b plane direction, an interatomic bond length changed by substitution of a metal element, and the like. 
     A crystal structure in which a clear crystal grain boundary (grain boundary) is observed is what is called a polycrystal structure. It is highly probable that the crystal grain boundary becomes a recombination center and traps carriers and thus decreases the on-state current or field-effect mobility of a transistor. Thus, the CAAC-OS in which no clear crystal grain boundary is observed is one of crystalline oxides having a crystal structure suitable for a semiconductor layer of a transistor. Note that Zn is preferably contained to form the CAAC-OS. For example, an In—Zn oxide and an In—Ga—Zn oxide are suitable because they can inhibit generation of a crystal grain boundary as compared with an In oxide. 
     The CAAC-OS tends to have a layered crystal structure (also referred to as a layered structure) in which a layer containing indium and oxygen (hereinafter, an In layer) and a layer containing the element M, zinc, and oxygen (hereinafter, an (M,Zn) layer) are stacked. Note that indium and the element M can be replaced with each other, and when the element M in the (M,Zn) layer is replaced with indium, the layer can also be referred to as an (In,M,Zn) layer. Furthermore, when indium in the In layer is replaced with the element M, the layer can be referred to as an (In,M) layer. 
     The CAAC-OS is a metal oxide having a dense structure with high crystallinity and a low amount of impurities or defects (oxygen vacancies V O  and the like) as shown above. In particular, after the formation of a metal oxide, heat treatment is performed at a temperature at which the metal oxide does not become a polycrystal (e.g., 400° C. to 600° C.), whereby a CAAC-OS having a dense structure with higher crystallinity can be obtained. As the density of the CAAC-OS is increased in such a manner, diffusion of impurities or oxygen in the CAAC-OS can be further reduced. 
     On the other hand, a clear crystal grain boundary is difficult to observe in the CAAC-OS; thus, it can be said that a reduction in electron mobility due to the crystal grain boundary is unlikely to occur. Thus, a metal oxide including a CAAC-OS is physically stable. Therefore, the metal oxide including a CAAC-OS is resistant to heat and has high reliability. 
     In a cross-sectional view of the transistor in the channel length direction, it is preferable that a groove be provided in the oxide  230   b  and the oxide  230   c  be embedded in the groove. At this time, the oxide  230   c  is provided to cover the inner wall (the side surface and the bottom surface) of the groove. It is preferable that the thickness of the oxide  230   c  be approximately the same as the depth of the groove. 
     With such a structure, even when the opening in which the conductor  260  and the like are embedded is formed and a damaged region is formed on the surface of the oxide  230   b  at the bottom portion of the opening, the damaged region can be removed. This can inhibit poor electrical characteristics of the transistor  200  due to the damaged region. 
       FIG. 1  and the like show the structure in which the side surface of the opening in which the conductor  260  and the like are embedded is substantially perpendicular to the formation surface of the oxide  230   b  including the groove of the oxide  230   b ; this embodiment is not limited thereto. For example, the opening may have a U-shape with a bottom portion having a moderate curve. For example, the side surface of the opening may be tilted with respect to the formation surface of the oxide  230   b.    
     As shown in  FIG. 1C , a curved surface may be provided between the side surface of the oxide  230   b  and the top surface of the oxide  230   b  in a cross-sectional view of the transistor  200  in the channel width direction. That is, an end portion of the side surface and an end portion of the top surface may be curved (such a shape is hereinafter also referred to as a rounded shape). 
     The radius of curvature of the curved surface is preferably greater than 0 nm and less than the thickness of the oxide  230   b  in a region overlapping with the conductor  242 , or less than half of the length of a region that does not have the curved surface. Specifically, the radius of curvature of the curved surface is greater than 0 nm and less than or equal to 20 nm, preferably greater than or equal to 1 nm and less than or equal to 15 nm, further preferably greater than or equal to 2 nm and less than or equal to 10 nm. Such a shape can improve the coverage of the groove with the insulator  250  and the conductor  260 , which are formed in a later step. Furthermore, reduction in the length of the region that does not have the curved surface can be prevented, and decrease in the on-state current and mobility of the transistor  200  can be inhibited. Thus, a semiconductor device with favorable electrical characteristics can be provided. 
     The oxide  230  preferably has a stacked-layer structure of a plurality of oxide layers with different chemical compositions. Specifically, the atomic ratio of the element M to the metal element of the main component in the metal oxide used for the oxide  230   a  is preferably greater than the atomic ratio of the element M to the metal element of the main component in the metal oxide used for the oxide  230   b . Moreover, the atomic ratio of the element M to In in the metal oxide used for the oxide  230   a  is preferably greater than the atomic ratio of the element M to In in the metal oxide used for the oxide  230   b . Furthermore, the atomic ratio of In to the element M in the metal oxide used for the oxide  230   b  is preferably greater than the atomic ratio of In to the element M in the metal oxide used for the oxide  230   a.    
     In order to make the oxide  230   c  serve as a main carrier path, the atomic ratio of indium to a metal element of a main component in the oxide  230   c  is preferably greater than the atomic ratio of indium to a metal element of a main component in the oxide  230   b . When a metal oxide having a high content of indium is used for a channel formation region, the on-state current of the transistor can be increased. Accordingly, when the atomic ratio of indium to a metal element of a main component in the oxide  230   c  is greater than the atomic ratio of indium to a metal element of a main component in the oxide  230   b , the oxide  230   c  can serve as a main carrier path. 
     The conduction band minimum of the oxide  230   c  is preferably farther from the vacuum level than the conduction band minimum of the oxide  230   a  and the oxide  230   b . In other words, the electron affinity of the oxide  230   c  is preferably larger than the electron affinity of the oxide  230   a  and the oxide  230   b . At this time, the oxide  230   c  serves as a main carrier path. 
     For the oxide  230   c , specifically, a metal oxide with In:M:Zn=4:2:3 [atomic ratio] or a composition in the neighborhood thereof, In:M:Zn=5:1:3 [atomic ratio] or a composition in the neighborhood thereof, or In:M:Zn=10:1:3 [atomic ratio] or a composition in the neighborhood thereof, indium oxide, or the like may be used. 
     A shift voltage (Vsh) measured in +GBT (Gate Bias Temperature) stress test is given as a parameter to evaluate the reliability of a transistor. The shift voltage (Vsh) is defined as gate voltage (Vg) at which, in a drain current (Id)-gate voltage (Vg) curve of a transistor, the tangent at a point where the slope of the curve is the steepest intersects the straight line of Id=1 pA. Furthermore, the amount of change in Vsh is represented as ΔVsh. 
     The ΔVsh of a transistor under a +GBT stress test shifts in the negative direction with time in some cases. In other cases, the ΔVsh shows the behavior of changing in both the negative direction and the positive direction, instead of changing in the negative direction. Note that this behavior is sometimes referred to as jagged behavior of ΔVsh in +GBT stress tests in this specification and the like. 
     When the metal oxide containing the element M not as its main component or the metal oxide with a small ratio of the element M is used for the oxide  230   c , ΔVsh can be reduced and jagged behavior of ΔVsh can be suppressed, for example, whereby the reliability of a transistor can be improved. 
     The oxide  230   b  and the oxide  230   c  are preferably an oxide having crystallinity, such as a CAAC-OS. An oxide having crystallinity, such as a CAAC-OS, has a dense structure with small amounts of impurities and defects (e.g., oxygen vacancies) and high crystallinity. This can inhibit oxygen extraction from the oxide  230   b  by the source electrode or the drain electrode. This can reduce oxygen extraction from the oxide  230   b  even when heat treatment is performed; thus, the transistor  200  is stable with respect to high temperatures in a manufacturing process (what is called thermal budget). 
     In addition, a CAAC-OS is preferably used for the oxide  230   c ; the c-axis of a crystal included in the oxide  230   c  is preferably aligned in a direction substantially perpendicular to the formation surface or top surface of the oxide  230   c . The CAAC-OS has a property of making oxygen move easily in the direction perpendicular to the c-axis. Thus, oxygen contained in the oxide  230   c  can be efficiently supplied to the oxide  230   b.    
     The oxide  230   d  preferably contains at least one of the metal elements contained in the metal oxide used for the oxide  230   c , and further preferably contains all of these metal elements. For example, it is preferable that an In-M-Zn oxide, an In—Zn oxide, or an indium oxide be used for the oxide  230   c , and an In-M-Zn oxide, a M-Zn oxide, or an oxide of the element M be used for the oxide  230   d . Accordingly, the density of defect states at the interface between the oxide  230   c  and the oxide  230   d  can be decreased. 
     The conduction band minimum of the oxide  230   d  is preferably closer to the vacuum level than the conduction band minimum of the oxide  230   c . In other words, the electron affinity of the oxide  230   d  is preferably smaller than the electron affinity of the oxide  230   c . In that case, a metal oxide that can be used for the oxide  230   a  or the oxide  230   b  is preferably used for the oxide  230   d . At this time, the oxide  230   c  serves as a main carrier path. 
     Specifically, for the oxide  230   c , a metal oxide with In:M:Zn=4:2:3 [atomic ratio] or a composition in the neighborhood thereof, In:M:Zn=5:1:3 [atomic ratio] or a composition in the neighborhood thereof, or In:M:Zn=10:1:3 [atomic ratio] or a composition in the neighborhood thereof, or an indium oxide can be used; for the oxide  230   d , a metal oxide with In:M:Zn=1:3:4 [atomic ratio] or a composition in the neighborhood thereof, M:Zn=2:1 [atomic ratio] or a composition in the neighborhood thereof, or M:Zn=2:5 [atomic ratio] or a composition in the neighborhood thereof, or an oxide of the element M can be used. 
     The oxide  230   d  is preferably a metal oxide that inhibits diffusion or passage of oxygen more readily than the oxide  230   c . Providing the oxide  230   d  between the insulator  250  and the oxide  230   c  can inhibit diffusion of oxygen contained in the insulator  280  into the insulator  250 . Thus, the oxygen can be efficiently supplied to the oxide  230   b  through the oxide  230   c.    
     When the atomic ratio of In to the metal element of the main component in the metal oxide used for the oxide  230   d  is smaller than the atomic ratio of In to the metal element of the main component in the metal oxide used for the oxide  230   c , diffusion of In to the insulator  250  side can be inhibited. Since the insulator  250  functions as a gate insulator, the transistor exhibits poor characteristics when In enters the insulator  250  and the like. Thus, the oxide  230   d  provided between the oxide  230   c  and the insulator  250  allows the semiconductor device to have high reliability. 
     Here, the conduction band minimum gradually changes at junction portions of the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d . In other words, the conduction band minimum at the junction portions of the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d  continuously changes or is continuously connected. To obtain this, the density of defect states in mixed layers formed at the interface between the oxide  230   a  and the oxide  230   b , the interface between the oxide  230   b  and the oxide  230   c , and the interface between the oxide  230   c  and the oxide  230   d  is preferably decreased. 
     Specifically, when the oxide  230   a  and the oxide  230   b , the oxide  230   b  and the oxide  230   c , and the oxide  230   c  and the oxide  230   d  contain the same element as a main component in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, in the case where the oxide  230   b  is an In-M-Zn oxide, an In-M-Zn oxide, an M-Zn oxide, an oxide of the element M, an In—Zn oxide, indium oxide, or the like may be used for the oxide  230   a , the oxide  230   c , and the oxide  230   d.    
     Specifically, for the oxide  230   a , a metal oxide with In:M:Zn=1:3:4 [atomic ratio] or a composition in the neighborhood thereof, or In:M:Zn=1:1:0.5 [atomic ratio] or a composition in the neighborhood thereof is used. For the oxide  230   b , a metal oxide with In:M:Zn=1:1:1 [atomic ratio] or a composition in the neighborhood thereof, or In:M:Zn=4:2:3 [atomic ratio] or a composition in the neighborhood thereof is used. For the oxide  230   c , a metal oxide with In:M:Zn=4:2:3 [atomic ratio] or a composition in the neighborhood thereof, In:M:Zn=5:1:3 [atomic ratio] or a composition in the neighborhood thereof, or In:M:Zn=10:1:3 [atomic ratio] or a composition in the neighborhood thereof, or indium oxide is used. Note that a composition in the neighborhood includes the range of ±30% of an intended atomic ratio. Gallium is preferably used as the element M. For the oxide  230   d , a metal oxide with In:M:Zn=1:3:4 [atomic ratio] or a composition in the neighborhood thereof, M:Zn=2:1 [atomic ratio] or a composition in the neighborhood thereof, or M:Zn=2:5 [atomic ratio] or a composition in the neighborhood thereof, or an oxide of the element M is used. 
     When the metal oxide is deposited by a sputtering method, the above atomic ratio is not limited to the atomic ratio of the deposited metal oxide and may be the atomic ratio of a sputtering target used for depositing the metal oxide. 
     When the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d  have the above structure, the density of defect states at the interface between the oxide  230   a  and the oxide  230   b , the interface between the oxide  230   b  and the oxide  230   c , and the interface between the oxide  230   c  and the oxide  230   d  can be made low. Thus, the influence of interface scattering on carrier conduction is small, and the transistor  200  can have a high on-state current and excellent frequency characteristics. 
     The insulator  212 , the insulator  214 , the insulator  271 , the insulator  272 , the insulator  282 , the insulator  283 , and the insulator  286  preferably function as barrier insulating films, each of which inhibits diffusion of impurities such as water and hydrogen from the substrate side or above the transistor  200  into the transistor  200 . Thus, for each of the insulator  212 , the insulator  214 , the insulator  271 , the insulator  272 , the insulator  282 , the insulator  283 , and the insulator  286 , an insulating material which has a function of inhibiting diffusion of impurities such as hydrogen atoms, hydrogen molecules, water molecules, nitrogen atoms, nitrogen molecules, nitrogen oxide molecules (e.g., N 2 O, NO, or NO 2 ), or copper atoms (through which the impurities are less likely to pass) is preferably used. Alternatively, it is preferable to use an insulating material which has a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like) (through which the above oxygen is less likely to pass). 
     Note that in this specification, a barrier insulating film refers to an insulating film having a barrier property. A barrier property in this specification means a function of inhibiting diffusion of a targeted substance (also referred to as having lower permeability). Alternatively, a barrier property in this specification means a function of trapping or fixing (also referred to as gettering) a targeted substance. 
     For example, it is preferable that the insulator  212 , the insulator  283 , and the insulator  286  be formed using silicon nitride or the like, and the insulator  214 , the insulator  271 , the insulator  272 , and the insulator  282  be formed using aluminum oxide or the like. Accordingly, impurities such as water and hydrogen can be inhibited from being diffused into the transistor  200  side from the substrate side through the insulator  212  and the insulator  214 . Alternatively, oxygen contained in the insulator  224  or the like can be inhibited from being diffused to the substrate side through the insulator  212  and the insulator  214 . Impurities such as water and hydrogen can be inhibited from being diffused from the insulator  280 , the conductor  246 , and the like into the oxide  230 . In this manner, the transistor  200  is preferably surrounded by the insulator  212 , the insulator  214 , the insulator  271 , the insulator  272 , the insulator  282 , and the insulator  283  having a function of inhibiting diffusion of oxygen and impurities such as water and hydrogen. 
     The resistivities of the insulator  212 , the insulator  283 , and the insulator  286  are preferably low in some cases. For example, by setting the resistivities of the insulator  212 , the insulator  283 , and the insulator  286  to approximately 1×10 13  Ωkm, the insulator  212 , the insulator  283 , and the insulator  286  can sometimes reduce charge up of the conductor  205 , the conductor  242 , the conductor  260 , or the conductor  246  in treatment using plasma or the like in the manufacturing process of a semiconductor device. The resistivities of the insulator  212 , the insulator  283 , and the insulator  286  are preferably higher than or equal to 1×10 10  Ωcm and lower than or equal to 1×10 15  Ωcm. 
     The insulator  216  and the insulator  280  preferably have a lower permittivity than the insulator  214 . When a material with a low permittivity is used for an interlayer film, parasitic capacitance generated between wirings can be reduced. For the insulator  216  and the insulator  280 , silicon oxide, silicon oxynitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, or the like is used as appropriate, for example. 
     The insulator  212 , the insulator  214 , and the insulator  216  are preferably deposited by a sputtering method. The insulator  212 , the insulator  214 , and the insulator  216  which are deposited by a sputtering method have low hydrogen concentrations in their films, which is preferable. The insulator  212 , the insulator  214 , and the insulator  216  are preferably deposited successively without being exposed to the air environment. The deposition without exposure to the air environment is preferable because impurities or moisture from the air environment can be prevented from attaching onto the insulator  212 , the insulator  214 , and the insulator  216 , so that the interface between the insulator  212  and the insulator  214 , the vicinity of the interface, the interface between the insulator  214  and the insulator  216 , and the vicinity of the interface can be kept clean. An apparatus capable of successive deposition is described later. 
     As illustrated in  FIG. 1B  and  FIG. 1C , the transistor  200  has a structure in which the oxide  230   c  and the oxide  230   d  are positioned only in the vicinity of the bottom portion of the opening provided in the insulator  280 , the insulator  272 , and the insulator  271 . 
     The above structure can inhibit generation of a current leakage path between the adjacent transistors  200  in a semiconductor device in which the plurality of transistors  200  are positioned in the channel width direction. The following shows details. 
     By applying a negative potential to the conductor  205  functioning as a second gate electrode, generation of a parasitic channel in the oxide  230   c  and the oxide  230   d  can be inhibited in some cases. In particular, when the oxide  230   c  and the oxide  230   d  in the transistor  200  have the above structure, the distance between the conductor  205  and each of the oxide  230   c  and the oxide  230   d  is relatively short. Thus, the potential of the conductor  205  can sufficiently reach the whole oxide  230   c  and oxide  230   d . Accordingly, the generation of the parasitic channel in the whole oxide  230   c  and oxide  230   d  can be inhibited. As a result, generation of a current leakage path between the adjacent transistors  200  can be inhibited. 
     Furthermore, by changing a potential applied to the conductor  205  not in conjunction with but independently of a potential applied to the conductor  260 , the threshold voltage (Vth) of the transistor  200  can be controlled. In particular, by applying a negative potential to the conductor  205 , Vth of the transistor  200  can be further increased, and the off-state current can be reduced. 
     The conductor  205  is positioned to overlap with the oxide  230  and the conductor  260 . Furthermore, the conductor  205  is preferably provided to be embedded in the insulator  214  or parts of the insulator  214  and the insulator  216 . 
     As shown in  FIG. 1A , the conductor  205  is preferably provided to be larger than a region of the oxide  230   a  and the oxide  230   b  that does not overlap with the conductor  242   a  or the conductor  242   b . As illustrated in  FIG. 1C , it is particularly preferable that the conductor  205  extend to a region outside end portions of the oxide  230   a  and the oxide  230   b  that intersect with the channel width direction. That is, the conductor  205  and the conductor  260  preferably overlap with each other with the insulators therebetween on the outer side of the side surfaces of the oxide  230   a  and the oxide  230   b  in the channel width direction. With this structure, the channel formation region of the oxide  230  can be electrically surrounded by the electric field of the conductor  260  functioning as a first gate electrode and the electric field of the conductor  205  functioning as the second gate electrode. In this specification, a transistor structure in which a channel formation region is electrically surrounded by electric fields of a first gate and a second gate is referred to as a Surrounded channel (S-channel) structure. 
     In this specification and the like, the S-channel structure refers to a transistor structure in which a channel formation region is electrically surrounded by electric fields of a pair of gate electrodes. The S-channel structure disclosed in this specification and the like is different from a Fin-type structure and a planar structure. With the S-channel structure, resistance to a short-channel effect can be enhanced, that is, a transistor in which a short-channel effect is unlikely to occur can be provided. 
     Furthermore, as shown in  FIG. 1C , the conductor  205  is extended to function as a wiring as well. However, without limitation to this structure, a structure where a conductor functioning as a wiring is provided below the conductor  205  may be employed. In addition, the conductor  205  does not necessarily have to be provided in each transistor. For example, the conductor  205  may be shared by a plurality of transistors. 
     Although the transistor  200  having a structure in which the conductor  205   a  and the conductor  205   b  are stacked as the conductor  205  is shown, the present invention is not limited thereto. For example, the conductor  205  may have a single-layer structure or a stacked-layer structure of three or more layers. In the case where a structure body has a stacked-layer structure, layers may be distinguished by ordinal numbers corresponding to the formation order. 
     Here, for the conductor  205   a , it is preferable to use a conductive material having a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, a nitrogen molecule, a nitrogen oxide molecule (N 2 O, NO, NO 2 , or the like), and a copper atom. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like). 
     When a conductive material having a function of inhibiting diffusion of oxygen is used for the conductor  205   a , the conductivity of the conductor  205   b  can be inhibited from being lowered because of oxidation. As a conductive material having a function of inhibiting diffusion of oxygen, for example, tantalum, tantalum nitride, ruthenium, or ruthenium oxide is preferably used. Thus, the conductor  205   a  is a single layer or a stack of the above conductive materials. For example, the conductor  205   a  may be a stack of tantalum, tantalum nitride, ruthenium, or ruthenium oxide and titanium or titanium nitride. Note that the conductor  205   a  may be formed by an ionization sputtering method described later. 
     Moreover, the conductor  205   b  is preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. Note that the conductor  205   b  is shown as a single layer but may have a stacked-layer structure, for example, a stack of the conductive material and titanium or titanium nitride. 
     The insulator  222  and the insulator  224  function as a gate insulator. 
     It is preferable that the insulator  222  have a function of inhibiting diffusion of hydrogen (e.g., at least one of a hydrogen atom, a hydrogen molecule, and the like). In addition, it is preferable that the insulator  222  have a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like). For example, the insulator  222  preferably has a function of further inhibiting diffusion of one or both of hydrogen and oxygen as compared to the insulator  224 . 
     For the insulator  222 , an insulator containing an oxide of one or both of aluminum and hafnium is preferably used. It is preferable that aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), or the like be used as the insulator. In the case where the insulator  222  is formed using such a material, the insulator  222  functions as a layer that inhibits release of oxygen from the oxide  230  to the substrate side and diffusion of impurities such as hydrogen from the periphery of the transistor  200  into the oxide  230 . Thus, providing the insulator  222  can inhibit diffusion of impurities such as hydrogen inside the transistor  200  and inhibit generation of oxygen vacancies in the oxide  230 . Moreover, the conductor  205  can be inhibited from reacting with oxygen contained in the insulator  224  and the oxide  230 . 
     Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxide may be added to the above insulator, for example. Alternatively, these insulators may be subjected to nitriding treatment. A stack of silicon oxide, silicon oxynitride, or silicon nitride over these insulators may be used for the insulator  222 . 
     For example, a single layer or stacked layers of an insulator containing what is called a high-k material such as aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO 3 ), or (Ba,Sr)TiO 3  (BST) may be used for the insulator  222 . With miniaturization and high integration of transistors, a problem such as leakage current might arise because of a thinner gate insulator. When a high-k material is used for the insulator functioning as a gate insulator, a gate potential during operation of the transistor can be reduced while the physical thickness of the gate insulator is maintained. 
     It is preferable that oxygen be released from the insulator  224  in contact with the oxide  230  by heating. Silicon oxide, silicon oxynitride, or the like is used as appropriate for the insulator  224 , for example. When an insulator containing oxygen is provided in contact with the oxide  230 , oxygen vacancies in the oxide  230  can be reduced and the reliability of the transistor  200  can be improved. 
     For the insulator  224 , specifically, an oxide material from which part of oxygen is released by heating, in other words, an insulating material including an excess-oxygen region is preferably used. An oxide from which oxygen is released by heating is an oxide film in which the amount of released oxygen molecules is greater than or equal to 1.0×10 18  molecules/cm 3 , preferably greater than or equal to 1.0×10 19  molecules/cm 3 , further preferably greater than or equal to 2.0×10 19  molecules/cm 3  or greater than or equal to 3.0×10 20  molecules/cm 3  in TDS (Thermal Desorption Spectroscopy) analysis. Note that the temperature of the film surface in the TDS analysis is preferably within the range of 100° C. to 700° C., or 100° C. to 400° C. 
     One or more of heat treatment, microwave treatment, and RF treatment may be performed in a state in which the insulator including an excess-oxygen region and the oxide  230  are in contact with each other. By the treatment, water or hydrogen in the oxide  230  can be removed. For example, in the oxide  230 , dehydrogenation can be performed when a reaction in which a bond of a defect where hydrogen enters an oxygen vacancy (VoH) is cut occurs, i.e., a reaction of “V O H→V O +H” occurs. Part of hydrogen generated at this time is bonded to oxygen to be H 2 O, and removed from the oxide  230  or an insulator in the vicinity of the oxide  230  in some cases. Part of hydrogen is diffused into or trapped by (also referred to as gettering) the conductor  242  in some cases. 
     For the microwave treatment, for example, an apparatus including a power supply that generates high-density plasma or an apparatus including a power supply that applies RF to the substrate side is suitably used. For example, the use of a gas containing oxygen and high-density plasma enables high-density oxygen radicals to be generated, and RF application to the substrate side allows the oxygen radicals generated by the high-density plasma to be efficiently introduced into the oxide  230  or an insulator in the vicinity of the oxide  230 . The pressure in the microwave treatment is higher than or equal to 133 Pa, preferably higher than or equal to 200 Pa, further preferably higher than or equal to 400 Pa. As a gas introduced into an apparatus for performing the microwave treatment, for example, oxygen and argon are used and the oxygen flow rate (O 2 /(O 2 +Ar)) is lower than or equal to 50%, preferably higher than or equal to 10% and lower than or equal to 30%. 
     In a manufacturing process of the transistor  200 , heat treatment is preferably performed with a surface of the oxide  230  exposed. The heat treatment is performed at higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 350° C. and lower than or equal to 400° C., for example. Note that the heat treatment is performed in a nitrogen gas or inert gas atmosphere, or an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more. For example, the heat treatment is preferably performed in an oxygen atmosphere. This can supply oxygen to the oxide  230  to reduce oxygen vacancies (V O ). The heat treatment may be performed under reduced pressure. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in a nitrogen gas or inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more in order to compensate for released oxygen. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more, and then another heat treatment is successively performed in a nitrogen gas or inert gas atmosphere. 
     Note that oxygen adding treatment performed on the oxide  230  can promote a reaction in which oxygen vacancies in the oxide  230  are repaired with supplied oxygen, i.e., a reaction of “V O +O→null”. Furthermore, hydrogen remaining in the oxide  230  reacts with supplied oxygen, so that the hydrogen can be removed as H 2 O (dehydration). This can inhibit recombination of hydrogen remaining in the oxide  230  with oxygen vacancies and formation of VoH. 
     Note that the insulator  222  and the insulator  224  may have a stacked-layer structure of two or more layers. In such cases, without limitation to a stacked-layer structure formed of the same material, a stacked-layer structure formed of different materials may be employed. 
     The oxide  243  (the oxide  243   a  and the oxide  243   b ) may be provided over the oxide  230   b.    
     The oxide  243  (the oxide  243   a  and the oxide  243   b ) preferably has a function of inhibiting passage of oxygen. The oxide  243  having a function of inhibiting passage of oxygen is preferably provided between the oxide  230   b  and the conductor  242  functioning as the source electrode and the drain electrode, in which case the electric resistance between the oxide  230   b  and the conductor  242  can be reduced. Such a structure improves the electrical characteristics of the transistor  200  and the reliability of the transistor  200 . In the case where the electric resistance between the oxide  230   b  and the conductor  242  can be sufficiently reduced, the oxide  243  is not necessarily provided. 
     A metal oxide including the element M may be used for the oxide  243 . In particular, aluminum, gallium, yttrium, or tin is preferably used as the element M. The concentration of the element M in the oxide  243  is preferably higher than that in the oxide  230   b . Furthermore, gallium oxide may be used for the oxide  243 . A metal oxide such as an In-M-Zn oxide may be used for the oxide  243 . Specifically, the atomic ratio of the element M to In in the metal oxide used for the oxide  243  is preferably greater than the atomic ratio of the element M to In in the metal oxide used for the oxide  230   b . The thickness of the oxide  243  is preferably larger than or equal to 0.5 nm and smaller than or equal to 5 nm, further preferably larger than or equal to 1 nm and smaller than or equal to 3 nm, still further preferably larger than or equal to 1 nm and smaller than or equal to 2 nm. The oxide  243  preferably has crystallinity. In the case where the oxide  243  has crystallinity, release of oxygen from the oxide  230  can be favorably inhibited. When the oxide  243  has a hexagonal crystal structure, for example, release of oxygen from the oxide  230  can sometimes be inhibited. 
     Note that an oxide film to be the oxide  230   a , an oxide film to be the oxide  230   b , and an oxide film to be the oxide  243  are preferably deposited successively without being exposed to the air environment. Depositing these films without being exposed to the air environment can prevent impurities or moisture from the air environment from attaching onto the oxide film to be the oxide  230   a , the oxide film to be the oxide  230   b , and the oxide film to be the oxide  243 ; this can keep clean the interface between the oxide film to be the oxide  230   a  and the oxide film to be the oxide  230   b , the vicinity of the interface, the interface between the oxide film to be the oxide  230   b  and the oxide film to be the oxide  243 , and the vicinity of the interface, which is preferable. An apparatus capable of successive deposition is described later. 
     The conductor  242   a  is provided over the oxide  243   a  and the conductor  242   b  is provided over the oxide  243   b . Each of the conductor  242   a  and the conductor  242   b  functions as a source electrode or a drain electrode of the transistor  200 . 
     For the conductor  242  (the conductor  242   a  and the conductor  242   b ), for example, a nitride containing tantalum, a nitride containing titanium, a nitride containing molybdenum, a nitride containing tungsten, a nitride containing tantalum and aluminum, a nitride containing titanium and aluminum, or the like is preferably used. In one embodiment of the present invention, a nitride containing tantalum is particularly preferable. As another example, ruthenium oxide, ruthenium nitride, an oxide containing strontium and ruthenium, or an oxide containing lanthanum and nickel may be used. These materials are preferable because they are conductive materials that are not easily oxidized or materials that maintain the conductivity even when absorbing oxygen. 
     When the oxide  243  is not provided, the contact between the conductor  242  and the oxide  230   b  or the oxide  230   c  may make oxygen in the oxide  230   b  or the oxide  230   c  diffuse into the conductor  242 , resulting in oxidation of the conductor  242 . It is highly possible that oxidation of the conductor  242  lowers the conductivity of the conductor  242 . Note that diffusion of oxygen in the oxide  230   b  or the oxide  230   c  into the conductor  242  can be rephrased as absorption of oxygen in the oxide  230   b  or the oxide  230   c  by the conductor  242 . 
     When oxygen in the oxide  230   b  or the oxide  230   c  is diffused into the conductor  242   a  and the conductor  242   b , a layer is sometimes formed between the conductor  242   a  and the oxide  230   b  and between the conductor  242   b  and the oxide  230   b , or between the conductor  242   a  and the oxide  230   c  and between the conductor  242   b  and the oxide  230   c . The layer contains more oxygen than the conductor  242   a  or the conductor  242   b  does, and thus the layer is assumed to have an insulating property. In this case, a three-layer structure of the conductor  242   a  or the conductor  242   b , the layer, and the oxide  230   b  or the oxide  230   c  can be regarded as a three-layer structure of a metal, an insulator, and a semiconductor and is sometimes referred to as a MIS (Metal-Insulator-Semiconductor) structure or referred to as a diode-connected structure mainly formed of the MIS structure. 
     Note that hydrogen contained in the oxide  230   b , the oxide  230   c , or the like is diffused into the conductor  242   a  or the conductor  242   b  in some cases. In particular, when a nitride containing tantalum is used for the conductor  242   a  and the conductor  242   b , hydrogen contained in the oxide  230   b , the oxide  230   c , or the like is likely to be diffused into the conductor  242   a  or the conductor  242   b , and the diffused hydrogen is bonded to nitrogen contained in the conductor  242   a  or the conductor  242   b  in some cases. That is, hydrogen contained in the oxide  230   b , the oxide  230   c , or the like is sometimes absorbed by the conductor  242   a  or the conductor  242   b  in some cases. 
     There is a curved surface between the side surface of the conductor  242  and the top surface of the conductor  242  in some cases. That is, an end portion of the side surface and an end portion of the top surface might be curved. The radius of curvature of the curved surface at an end portion of the conductor  242  is greater than or equal to 3 nm and less than or equal to 10 nm, preferably greater than or equal to 5 nm and less than or equal to 6 nm, for example. When the end portions are not angular, coverage with films in later deposition steps is improved. 
     The insulator  272  is provided to cover the side surfaces of the oxide  230   a , the oxide  230   b , the oxide  243 , the conductor  242 , and the insulator  271  and preferably functions as at least a barrier insulating film against oxygen. Thus, the insulator  272  preferably has a function of inhibiting oxygen diffusion. For example, the insulator  272  preferably has a function of further inhibiting diffusion of oxygen as compared to the insulator  280 . For example, an insulator containing an oxide of one or both of aluminum and hafnium is preferably deposited as the insulator  272 . 
     As the insulator  272  in particular, aluminum oxide or hafnium oxide is preferably deposited by a bias sputtering method in an oxygen-containing atmosphere. The bias sputtering method is a method in which sputtering is performed while RF power is applied to a substrate. The potential of the substrate applied with the RF power becomes a negative potential (bias potential) with respect to a plasma potential, and cations in plasma are accelerated by the bias potential and implanted into the substrate. The bias potential can be controlled by the amount of RF power applied to the substrate. 
     Therefore, aluminum oxide or hafnium oxide is deposited by the bias sputtering method in an oxygen-containing atmosphere, whereby oxygen can be implanted into the insulator  224 . The amount of oxygen to be implanted into the insulator  224  can be controlled by adjusting the amount of RF power applied to the substrate, so that the optimal amount of oxygen can be implanted into the insulator  224 . 
     The insulator  271  is provided in contact with the top surface of the conductor  242 , and preferably functions as at least a barrier insulating film against oxygen like the insulator  272 . Thus, the insulator  271  preferably has a function of inhibiting oxygen diffusion. For example, the insulator  271  preferably has a function of further inhibiting diffusion of oxygen as compared to the insulator  280 . For example, an insulator containing an oxide of one or both of aluminum and hafnium is preferably deposited as the insulator  271 . For another example, an insulator containing silicon nitride may be used for the insulator  271 . 
     With such an insulator  271  and the insulator  272 , the oxide  230   a , the oxide  230   b , the oxide  243 , and the conductor  242  can be separated from the insulator  280 . Thus, oxygen can be inhibited from being directly diffused from the insulator  280  into the oxide  230   a , the oxide  230   b , the oxide  243 , and the conductor  242 . Accordingly, a reduction in the carrier density in the source region and the drain region of the oxide  230  due to supply of excess oxygen to the source region and the drain region can be prevented. Furthermore, the conductor  242  can be inhibited from being excessively oxidized, so that an increase in the resistivity and a reduction in an on-state current can be inhibited. 
     The insulator  250  functions as a gate insulator. The insulator  250  is preferably in contact with the top surface of the oxide  230   d . For the insulator  250 , silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, or the like can be used. In particular, silicon oxide and silicon oxynitride are preferable because they are thermally stable. 
     Like the insulator  224 , the insulator  250  is preferably formed using an insulator from which oxygen is released by heating. When an insulator from which oxygen is released by heating is provided as the insulator  250  in contact with the top surface of the oxide  230   d , oxygen can be effectively supplied to the channel formation region of the oxide  230   b  through the oxide  230   d  and the oxide  230   c  and oxygen vacancies in the channel formation region of the oxide  230   b  can be reduced. Thus, a transistor that has stable electrical characteristics with small variation in electrical characteristics and improved reliability can be provided. Furthermore, as in the insulator  224 , the concentration of impurities such as water and hydrogen in the insulator  250  is preferably reduced. The thickness of the insulator  250  is preferably greater than or equal to 1 nm and less than or equal to 20 nm. 
     Although the insulator  250  is illustrated as a single layer in  FIG. 1B  and  FIG. 1C , a stacked-layer structure of two or more layers may be employed. In the case where the insulator  250  has a stacked-layer structure including two layers, it is preferable that a lower layer of the insulator  250  be formed using an insulator from which oxygen is released by heating and an upper layer of the insulator  250  be formed using an insulator having a function of inhibiting diffusion of oxygen. With such a structure, oxygen contained in the lower layer of the insulator  250  can be inhibited from being diffused into the conductor  260 . That is, a reduction in the amount of oxygen supplied to the oxide  230  can be inhibited. In addition, oxidation of the conductor  260  due to oxygen contained in the lower layer of the insulator  250  can be inhibited. For example, the lower layer of the insulator  250  can be formed using the above-described material that can be used for the insulator  250 , and the upper layer of the insulator  250  can be formed using a material similar to that for the insulator  222 . 
     In the case where silicon oxide, silicon oxynitride, or the like is used for the lower layer of the insulator  250 , the upper layer of the insulator  250  may be formed using an insulating material that is a high-k material having a high relative permittivity. The gate insulator having a stacked-layer structure of the lower layer of the insulator  250  and the upper layer of the insulator  250  can be thermally stable and can have a high relative permittivity. Thus, a gate potential that is applied during operation of the transistor can be reduced while the physical thickness of the gate insulator is maintained. Furthermore, the equivalent oxide thickness (EOT) of the insulator functioning as the gate insulator can be reduced. 
     Specifically, for the upper layer of the insulator  250 , a metal oxide containing one kind or two or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium, and the like, or a metal oxide that can be used for the oxide  230  can be used. In particular, an insulator containing an oxide of one or both of aluminum and hafnium is preferably used. 
     Alternatively, the lower layer of the insulator  250  may be formed using an insulator having a function of inhibiting diffusion of oxygen. With such a structure, oxygen contained in the insulator  280  can be inhibited from being diffused into the conductor  260 . That is, a reduction in the amount of oxygen supplied to the oxide  230  can be inhibited. Moreover, the lower layer of the insulator  250  is preferably formed using an insulator having a function of inhibiting passage of impurities such as water and hydrogen. With such a structure, impurities such as water and hydrogen contained in the insulator  280  can be inhibited from being diffused into the oxide  230 . For example, the lower layer of the insulator  250  can be provided using a material similar to that for the insulator  222 . 
     In the case where silicon oxide, silicon oxynitride, or the like is used for the upper layer of the insulator  250 , the lower layer of the insulator  250  may be formed using an insulating material that is a high-k material having a high relative permittivity. The gate insulator having a stacked-layer structure of the lower layer of the insulator  250  and the upper layer of the insulator  250  can be thermally stable and can have a high relative permittivity. Thus, a gate potential that is applied during operation of the transistor can be reduced while the physical thickness of the gate insulator is maintained. Furthermore, the equivalent oxide thickness (EOT) of the insulator functioning as the gate insulator can be reduced. 
     Specifically, for the lower layer of the insulator  250 , a metal oxide containing one kind or two or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium, and the like, or a metal oxide that can be used for the oxide  230  can be used. In particular, an insulator containing an oxide of one or both of aluminum and hafnium is preferably used. 
     Furthermore, a metal oxide may be provided between the insulator  250  and the conductor  260 . The metal oxide preferably inhibits diffusion of oxygen from the insulator  250  into the conductor  260 . Providing the metal oxide that inhibits diffusion of oxygen inhibits diffusion of oxygen from the insulator  250  into the conductor  260 . That is, a reduction in the amount of oxygen supplied to the oxide  230  can be inhibited. Moreover, oxidation of the conductor  260  due to oxygen in the insulator  250  can be inhibited. 
     Note that, the metal oxide preferably has a function of part of the first gate electrode. For example, a metal oxide that can be used for the oxide  230  can be used as the metal oxide. In that case, when the conductor  260   a  is deposited by a sputtering method, the metal oxide can have a reduced electric resistance value to be a conductor. Such a conductor can be referred to as an OC (Oxide Conductor) electrode. 
     With the metal oxide, the on-state current of the transistor  200  can be increased without a reduction in the influence of the electric field from the conductor  260 . Since a distance between the conductor  260  and the oxide  230  is kept by the physical thicknesses of the insulator  250  and the metal oxide, leakage current between the conductor  260  and the oxide  230  can be inhibited. Moreover, when the stacked-layer structure of the insulator  250  and the metal oxide is provided, the physical distance between the conductor  260  and the oxide  230  and the intensity of electric field applied to the oxide  230  from the conductor  260  can be easily adjusted as appropriate. 
     The conductor  260  functions as the first gate electrode of the transistor  200 . The conductor  260  preferably includes the conductor  260   a  and the conductor  260   b  positioned over the conductor  260   a . For example, the conductor  260   a  is preferably positioned to cover the bottom surface and the side surface of the conductor  260   b . Moreover, as shown in  FIG. 1B  and  FIG. 1C , the top surface of the conductor  260  is substantially level with the top surface of the insulator  250 . Although the conductor  260  has a two-layer structure of the conductor  260   a  and the conductor  260   b  in  FIG. 1B  and  FIG. 1C , the conductor  260  may have a single-layer structure or a stacked-layer structure of three or more layers. 
     For the conductor  260   a , a conductive material having a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, a nitrogen molecule, a nitrogen oxide molecule, and a copper atom is preferably used. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like). 
     In addition, when the conductor  260   a  has a function of inhibiting diffusion of oxygen, the conductivity of the conductor  260   b  can be inhibited from being lowered because of oxidation due to oxygen contained in the insulator  250 . As a conductive material having a function of inhibiting diffusion of oxygen, for example, tantalum, tantalum nitride, ruthenium, or ruthenium oxide is preferably used. 
     The conductor  260  also functions as a wiring and thus is preferably formed using a conductor having high conductivity. For example, a conductive material containing tungsten, copper, or aluminum as its main component can be used for the conductor  260   b . The conductor  260   b  may have a stacked-layer structure; for example, a stacked-layer structure of the conductive material and titanium or titanium nitride may be employed. 
     In the transistor  200 , the conductor  260  is formed in a self-aligned manner to fill the opening formed in the insulator  280  and the like. The formation of the conductor  260  in this manner allows the conductor  260  to be positioned certainly in a region between the conductor  242   a  and the conductor  242   b  without alignment. 
     As illustrated in  FIG. 1C , in the channel width direction of the transistor  200 , with reference to the bottom surface of the insulator  222 , the level of the bottom surface of the conductor  260  in a region where the conductor  260  and the oxide  230   b  do not overlap with each other is preferably lower than the level of the bottom surface of the oxide  230   b . When the conductor  260  functioning as the gate electrode covers the side surface and the top surface of the channel formation region of the oxide  230   b  with the insulator  250  and the like therebetween, the electric field of the conductor  260  is likely to act on the entire channel formation region of the oxide  230   b . Thus, the on-state current of the transistor  200  can be increased and the frequency characteristics of the transistor  200  can be improved. When the bottom surface of the insulator  222  is a reference, the difference between the level of the bottom surface of the conductor  260  in a region where the oxide  230   a  and the oxide  230   b  and the conductor  260  do not overlap with each other and the level of the bottom surface of the oxide  230   b  is greater than or equal to 0 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm, further preferably greater than or equal to 5 nm and less than or equal to 20 nm. 
     The insulator  280  is provided over the insulator  224 , the oxide  230 , the conductor  242 , the insulator  271 , and the insulator  272 . In addition, the top surface of the insulator  280  may be planarized. 
     The insulator  280  functioning as an interlayer film preferably has a low permittivity. When a material with a low permittivity is used for an interlayer film, parasitic capacitance generated between wirings can be reduced. The insulator  280  is preferably provided using a material similar to that for the insulator  216 , for example. In particular, silicon oxide and silicon oxynitride, which have thermal stability, are preferable. Materials such as silicon oxide, silicon oxynitride, and porous silicon oxide are particularly preferable because a region containing oxygen released by heating can be easily formed. 
     The concentration of impurities such as water and hydrogen in the insulator  280  is preferably reduced. Moreover, the insulator  280  preferably has a low hydrogen concentration and includes an excess-oxygen region or excess oxygen, and may be provided using a material similar to that for the insulator  216 , for example. The insulator  280  may have a stacked-layer structure of the above materials; for example, a stacked-layer structure of silicon oxide deposited by a sputtering method and silicon oxynitride deposited by a chemical vapor deposition (CVD) method to be stacked thereover can be employed. Furthermore, silicon nitride may be stacked thereover. 
     The insulator  282  or the insulator  283  preferably functions as a barrier insulating film that inhibits impurities such as water and hydrogen from diffusing into the insulator  280  from above. The insulator  282  or the insulator  283  preferably functions as a barrier insulating film that inhibits passage of oxygen. For the insulator  282  and the insulator  283 , for example, an insulator such as aluminum oxide, silicon nitride, or silicon nitride oxide may be used. The insulator  282  may be formed using aluminum oxide that has a high blocking property against oxygen and the insulator  283  may be formed using silicon nitride that has a high blocking property against hydrogen, for example. 
     For the conductor  240   a  and the conductor  240   b , a conductive material containing tungsten, copper, or aluminum as its main component is preferably used. The conductor  240   a  and the conductor  240   b  may each have a stacked-layer structure. Note that the conductor  240   a  and the conductor  240   b  each have a circular shape in the top view in  FIG. 1A ; however, the shapes of the conductors are not limited thereto. For example, in the top view, the conductor  240   a  and the conductor  240   b  may each have an almost circular shape such as an elliptical shape, a polygonal shape such as a quadrangular shape, or a polygonal shape such as a quadrangular shape with rounded corners. 
     In the case where the conductor  240  has a stacked-layer structure, a conductive material having a function of inhibiting passage of impurities such as water and hydrogen is preferably used for a conductor in contact with the insulator  283 , the insulator  282 , the insulator  280 , and the insulator  271 . For example, tantalum, tantalum nitride, titanium, titanium nitride, ruthenium, ruthenium oxide, or the like is preferably used. The conductive material having a function of inhibiting passage of impurities such as water and hydrogen may be used as a single layer or stacked layers. Moreover, impurities such as water and hydrogen contained in a layer above the insulator  283  can be inhibited from entering the oxide  230  through the conductor  240   a  and the conductor  240   b.    
     In the case where the conductor  240  has a stacked-layer structure, it is preferable to use, for the lower layer, a conductive material having a function of inhibiting passage of oxygen and impurities such as water and hydrogen. For example, tantalum, tantalum nitride, titanium, titanium nitride, ruthenium, or ruthenium oxide is preferably used. When the conductive material is used for the lower layer of the conductor  240 , entry of impurities such as water and hydrogen diffused from the insulator  280  and the like into the oxide  230  through the conductor  240  can be further reduced. Furthermore, oxygen added to the insulator  280  can be prevented from being absorbed by the conductive material of the upper layer of the conductor  240 . For the upper layer, a conductive material containing tungsten, copper, or aluminum as its main component is preferably used. 
     For the insulator  241   a  and the insulator  241   b , for example, an insulator such as silicon nitride, aluminum oxide, or silicon nitride oxide may be used. Since the insulator  241   a  and the insulator  241   b  are provided in contact with the insulator  280 , impurities such as water and hydrogen contained in the insulator  280  or the like can be inhibited from entering the oxide  230  through the conductor  240   a  and the conductor  240   b . In particular, silicon nitride is suitable because of having a high blocking property against hydrogen. Furthermore, oxygen contained in the insulator  280  can be prevented from being absorbed by the conductor  240   a  and the conductor  240   b.    
     The conductor  246  (the conductor  246   a  and the conductor  246   b ) functioning as a wiring may be provided in contact with the top surface of the conductor  240   a  and the top surface of the conductor  240   b . The conductor  246  is preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. Furthermore, the conductor may have a stacked-layer structure and may be a stack of titanium or titanium nitride and the conductive material, for example. Note that the conductor may be formed to be embedded in an opening provided in an insulator. 
     The insulator  286  is provided over the conductor  246  and the insulator  283 . Accordingly, the top surface of the conductor  246  and the side surface of the conductor  246  are in contact with the insulator  286  and the bottom surface of the conductor  246  is in contact with the insulator  283 . In other words, the conductor  246  can have a structure in which the conductor  246  is surrounded by the insulator  283  and the insulator  286 . With such a structure, the passage of oxygen from the outside can be inhibited and the oxidation of the conductor  246  can be prevented. Furthermore, impurities such as water and hydrogen can be prevented from diffusing from the conductor  246  to the outside, which is preferable. 
     &lt;Constituent Materials of Semiconductor Device&gt; 
     Constituent materials that can be used for the semiconductor device will be described below. 
     &lt;&lt;Substrate&gt;&gt; 
     As a substrate where the transistor  200  is formed, an insulator substrate, a semiconductor substrate, or a conductor substrate is used, for example. Examples of the insulator substrate include a glass substrate, a quartz substrate, a sapphire substrate, a stabilized zirconia substrate (an yttria-stabilized zirconia substrate or the like), and a resin substrate. Examples of the semiconductor substrate include a semiconductor substrate using silicon, germanium, or the like as a material and a compound semiconductor substrate including silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, or gallium oxide. Another example is a semiconductor substrate in which an insulator region is included in the semiconductor substrate, e.g., an SOI (Silicon On Insulator) substrate. Examples of the conductor substrate include a graphite substrate, a metal substrate, an alloy substrate, and a conductive resin substrate. Other examples include a substrate including a nitride of a metal and a substrate including an oxide of a metal. Other examples include an insulator substrate provided with a conductor or a semiconductor, a semiconductor substrate provided with a conductor or an insulator, and a conductor substrate provided with a semiconductor or an insulator. Alternatively, these substrates provided with elements may be used. Examples of the element provided for the substrate include a capacitor, a resistor, a switching element, a light-emitting element, and a memory element. 
     &lt;&lt;Insulator&gt;&gt; 
     Examples of an insulator include an insulating oxide, an insulating nitride, an insulating oxynitride, an insulating nitride oxide, an insulating metal oxide, an insulating metal oxynitride, and an insulating metal nitride oxide. 
     As miniaturization and high integration of transistors progress, for example, a problem such as leakage current may arise because of a thinner gate insulator. When a high-k material is used for the insulator functioning as a gate insulator, the voltage during operation of the transistor can be lowered while the physical thickness of the gate insulator is maintained. In contrast, when a material with a low relative permittivity is used for the insulator functioning as an interlayer film, parasitic capacitance generated between wirings can be reduced. Thus, a material is preferably selected depending on the function of an insulator. 
     Examples of the insulator with a high relative permittivity include gallium oxide, hafnium oxide, zirconium oxide, an oxide containing aluminum and hafnium, an oxynitride containing aluminum and hafnium, an oxide containing silicon and hafnium, an oxynitride containing silicon and hafnium, and a nitride containing silicon and hafnium. 
     Examples of the insulator with a low relative permittivity include silicon oxide, silicon oxynitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, and a resin. 
     When a transistor using a metal oxide is surrounded by an insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen, the electrical characteristics of the transistor can be stable. For the insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen, a single layer or stacked layers of an insulator containing, for example, boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum are used. Specifically, for the insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen, a metal oxide such as aluminum oxide, magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide, or a metal nitride such as aluminum nitride, silicon nitride oxide, or silicon nitride can be used. 
     The insulator functioning as the gate insulator is preferably an insulator including a region containing oxygen released by heating. For example, when a structure is employed in which silicon oxide or silicon oxynitride including a region containing oxygen released by heating is in contact with the oxide  230 , oxygen vacancies included in the oxide  230  can be filled. 
     &lt;&lt;Conductor&gt;&gt; 
     For a conductor, it is preferable to use a metal element selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium, ruthenium, iridium, strontium, lanthanum, and the like; an alloy containing any of the above metal elements; an alloy containing a combination of the above metal elements; or the like. For example, it is preferable to use tantalum nitride, titanium nitride, tungsten, a nitride containing titanium and aluminum, a nitride containing tantalum and aluminum, ruthenium oxide, ruthenium nitride, an oxide containing strontium and ruthenium, an oxide containing lanthanum and nickel, or the like. In addition, tantalum nitride, titanium nitride, a nitride containing titanium and aluminum, a nitride containing tantalum and aluminum, ruthenium oxide, ruthenium nitride, an oxide containing strontium and ruthenium, and an oxide containing lanthanum and nickel are preferable because they are oxidation-resistant conductive materials or materials that maintain their conductivity even after absorbing oxygen. A semiconductor having high electrical conductivity, typified by polycrystalline silicon containing an impurity element such as phosphorus, or silicide such as nickel silicide may be used. 
     A stack including a plurality of conductive layers formed of the above materials may be used. For example, a stacked-layer structure combining a material containing the above metal element and a conductive material containing oxygen may be employed. A stacked-layer structure combining a material containing the above metal element and a conductive material containing nitrogen may be employed. A stacked-layer structure combining a material containing the above metal element, a conductive material containing oxygen, and a conductive material containing nitrogen may be employed. 
     Note that when an oxide is used for the channel formation region of the transistor, a stacked-layer structure combining a material containing the above metal element and a conductive material containing oxygen is preferably used for the conductor functioning as the gate electrode. In that case, the conductive material containing oxygen is preferably provided on the channel formation region side. When the conductive material containing oxygen is provided on the channel formation region side, oxygen released from the conductive material is easily supplied to the channel formation region. 
     It is particularly preferable to use, for the conductor functioning as the gate electrode, a conductive material containing oxygen and a metal element contained in a metal oxide where the channel is formed. Alternatively, a conductive material containing the above metal element and nitrogen may be used. For example, a conductive material containing nitrogen, such as titanium nitride or tantalum nitride, may be used. Alternatively, indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon is added may be used. Furthermore, indium gallium zinc oxide containing nitrogen may be used. With the use of such a material, hydrogen contained in the metal oxide where the channel is formed can be trapped in some cases. Alternatively, hydrogen entering from an external insulator or the like can be trapped in some cases. 
     &lt;&lt;Metal Oxide&gt;&gt; 
     The oxide  230  is preferably formed using a metal oxide functioning as a semiconductor (an oxide semiconductor). A metal oxide that can be used for the oxide  230  of the present invention will be described below. 
     The metal oxide preferably contains at least indium or zinc. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more kinds selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, cobalt, and the like may be contained. 
     &lt;Classification of Crystal Structures&gt; 
     First, the classification of crystal structures of an oxide semiconductor will be described with reference to  FIG. 2A .  FIG. 2A  is a diagram showing the classification of crystal structures of an oxide semiconductor, typically IGZO (a metal oxide containing In, Ga, and Zn). 
     As shown in  FIG. 2A , an oxide semiconductor is roughly classified into “Amorphous”, “Crystalline”, and “Crystal”. “Amorphous” includes completely amorphous. “Crystalline” includes CAAC (c-axis-aligned crystalline), nc (nanocrystalline), and CAC (cloud-aligned composite) (excluding single crystal and poly crystal). Note that “Crystalline” excludes single crystal, poly crystal, and completely amorphous. “Crystal” includes single crystal and poly crystal. 
     Note that the structures in the thick frame in  FIG. 2A  are in an intermediate state between “Amorphous” and “Crystal”, and belong to a new crystalline phase. That is, these structures are completely different from “Amorphous”, which is energetically unstable, and “Crystal”. 
     A crystal structure of a film or a substrate can be evaluated with an X-Ray Diffraction (XRD) spectrum.  FIG. 2B  shows an XRD spectrum, which is obtained by GIXD (Grazing-Incidence XRD) measurement, of a CAAC-IGZO film classified into “Crystalline”. Note that a GIXD method is also referred to as a thin film method or a Seemann-Bohlin method. The XRD spectrum that is shown in  FIG. 2B  and obtained by GIXD measurement is hereinafter simply referred to as an XRD spectrum. The CAAC-IGZO film in  FIG. 2B  has a composition in the vicinity of In:Ga:Zn=4:2:3 [atomic ratio]. The CAAC-IGZO film in  FIG. 2B  has a thickness of 500 nm. 
     As shown in  FIG. 2B , a clear peak indicating crystallinity is detected in the XRD spectrum of the CAAC-IGZO film. Specifically, a peak indicating c-axis alignment is detected at  20  of around 31° in the XRD spectrum of the CAAC-IGZO film. As shown in  FIG. 2B , the peak at  20  of around 31° is asymmetric with respect to the axis of the angle at which the peak intensity is detected. 
     A crystal structure of a film or a substrate can also be evaluated with a diffraction pattern obtained by a nanobeam electron diffraction (NBED) method (such a pattern is also referred to as a nanobeam electron diffraction pattern).  FIG. 2C  shows a diffraction pattern of the CAAC-IGZO film.  FIG. 2C  shows a diffraction pattern obtained by the NBED method in which an electron beam is incident in the direction parallel to the substrate. The CAAC-IGZO film in  FIG. 2C  has a composition in the vicinity of In:Ga:Zn=4:2:3 [atomic ratio]. In the nanobeam electron diffraction method, electron diffraction is performed with a probe diameter of 1 nm. 
     As shown in  FIG. 2C , a plurality of spots indicating c-axis alignment are observed in the diffraction pattern of the CAAC-IGZO film. 
     &lt;&lt;Structure of Oxide Semiconductor&gt;&gt; 
     Oxide semiconductors might be classified in a manner different from that in  FIG. 2A  when classified in terms of the crystal structure. Oxide semiconductors are classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor, for example. Examples of the non-single-crystal oxide semiconductor include the above-described CAAC-OS and nc-OS. Other examples of the non-single-crystal oxide semiconductor include a polycrystalline oxide semiconductor, an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor. 
     Here, the above-described CAAC-OS, nc-OS, and a-like OS are described in detail. 
     [CAAC-OS] 
     The CAAC-OS is an oxide semiconductor that has a plurality of crystal regions each of which has c-axis alignment in a particular direction. Note that the particular direction refers to the film thickness direction of a CAAC-OS film, the normal direction of the surface where the CAAC-OS film is formed, or the normal direction of the surface of the CAAC-OS film. The crystal region refers to a region having a periodic atomic arrangement. When an atomic arrangement is regarded as a lattice arrangement, the crystal region also refers to a region with a uniform lattice arrangement. The CAAC-OS has a region where a plurality of crystal regions are connected in the a-b plane direction, and the region has distortion in some cases. Note that the distortion refers to a portion where the direction of a lattice arrangement changes between a region with a uniform lattice arrangement and another region with a uniform lattice arrangement in a region where a plurality of crystal regions are connected. That is, the CAAC-OS is an oxide semiconductor having c-axis alignment and having no clear alignment in the a-b plane direction. 
     Note that each of the plurality of crystal regions is formed of one or more fine crystals (crystals each of which has a maximum diameter of less than 10 nm). In the case where the crystal region is formed of one fine crystal, the maximum diameter of the crystal region is less than 10 nm. In the case where the crystal region is formed of a large number of fine crystals, the size of the crystal region may be approximately several tens of nanometers. 
     In the case of an In-M-Zn oxide (the element M is one or more kinds selected from aluminum, gallium, yttrium, tin, titanium, and the like), the CAAC-OS tends to have a layered crystal structure (also referred to as a layered structure) in which a layer containing indium (In) and oxygen (hereinafter, an In layer) and a layer containing the element M, zinc (Zn), and oxygen (hereinafter, an (M,Zn) layer) are stacked. Indium and the element M can be replaced with each other. Therefore, indium may be contained in the (M,Zn) layer. In addition, the element M may be contained in the In layer. Note that Zn may be contained in the In layer. Such a layered structure is observed as a lattice image in a high-resolution TEM image, for example. 
     When the CAAC-OS film is subjected to structural analysis by out-of-plane XRD measurement with an XRD apparatus using θ/2θ scanning, for example, a peak indicating c-axis alignment is detected at 2θ of 31° or around 31°. Note that the position of the peak indicating c-axis alignment (the value of 2θ) may change depending on the kind, composition, or the like of the metal element contained in the CAAC-OS. 
     For example, a plurality of bright spots are observed in the electron diffraction pattern of the CAAC-OS film. Note that one spot and another spot are observed point-symmetrically with a spot of the incident electron beam passing through a sample (also referred to as a direct spot) as the symmetric center. 
     When the crystal region is observed from the particular direction, a lattice arrangement in the crystal region is basically a hexagonal lattice arrangement; however, a unit lattice is not always a regular hexagon and is a non-regular hexagon in some cases. A pentagonal lattice arrangement, a heptagonal lattice arrangement, and the like are included in the distortion in some cases. Note that a clear crystal grain boundary (grain boundary) cannot be observed even in the vicinity of the distortion in the CAAC-OS. That is, formation of a crystal grain boundary is inhibited by the distortion of lattice arrangement. This is probably because the CAAC-OS can tolerate distortion owing to a low density of arrangement of oxygen atoms in the a-b plane direction, an interatomic bond distance changed by substitution of a metal atom, and the like. 
     A crystal structure in which a clear crystal grain boundary is observed is what is called polycrystal. It is highly probable that the crystal grain boundary becomes a recombination center and captures carriers and thus decreases the on-state current and field-effect mobility of a transistor, for example. Thus, the CAAC-OS in which no clear crystal grain boundary is observed is one of crystalline oxides having a crystal structure suitable for a semiconductor layer of a transistor. Note that Zn is preferably contained to form the CAAC-OS. For example, an In—Zn oxide and an In—Ga—Zn oxide are suitable because they can inhibit generation of a crystal grain boundary as compared with an In oxide. 
     The CAAC-OS is an oxide semiconductor with high crystallinity in which no clear crystal grain boundary is observed. Thus, in the CAAC-OS, reduction in electron mobility due to the crystal grain boundary is unlikely to occur. Moreover, since the crystallinity of an oxide semiconductor might be decreased by entry of impurities, formation of defects, or the like, the CAAC-OS can be regarded as an oxide semiconductor that has small amounts of impurities and defects (e.g., oxygen vacancies). Thus, an oxide semiconductor including the CAAC-OS is physically stable. Therefore, the oxide semiconductor including the CAAC-OS is resistant to heat and has high reliability. In addition, the CAAC-OS is stable with respect to high temperatures in the manufacturing process (what is called thermal budget). Accordingly, the use of the CAAC-OS for the OS transistor can extend the degree of freedom of the manufacturing process. 
     [nc-OS] 
     In the nc-OS, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. In other words, the nc-OS includes a fine crystal. Note that the size of the fine crystal is, for example, greater than or equal to 1 nm and less than or equal to 10 nm, particularly greater than or equal to 1 nm and less than or equal to 3 nm; thus, the fine crystal is also referred to as a nanocrystal. Furthermore, there is no regularity of crystal orientation between different nanocrystals in the nc-OS. Thus, the orientation in the whole film is not observed. Accordingly, the nc-OS cannot be distinguished from an a-like OS or an amorphous oxide semiconductor by some analysis methods. For example, when an nc-OS film is subjected to structural analysis using out-of-plane XRD measurement with an XRD apparatus using θ/2θ scanning, a peak indicating crystallinity is not detected. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS film is subjected to electron diffraction (also referred to as selected-area electron diffraction) using an electron beam with a probe diameter larger than the diameter of a nanocrystal (e.g., larger than or equal to 50 nm). Meanwhile, in some cases, a plurality of spots in a ring-like region with a direct spot as the center are observed in the obtained electron diffraction pattern when the nc-OS film is subjected to electron diffraction (also referred to as nanobeam electron diffraction) using an electron beam with a probe diameter nearly equal to or smaller than the diameter of a nanocrystal (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm). 
     [a-Like OS] 
     The a-like OS is an oxide semiconductor having a structure between those of the nc-OS and the amorphous oxide semiconductor. The a-like OS includes a void or a low-density region. That is, the a-like OS has low crystallinity as compared with the nc-OS and the CAAC-OS. Moreover, the a-like OS has higher hydrogen concentration in the film than the nc-OS and the CAAC-OS. 
     &lt;&lt;Structure of Oxide Semiconductor&gt;&gt; 
     Next, the above-described CAC-OS is described in detail. Note that the CAC-OS relates to the material composition. 
     [CAC-OS] 
     The CAC-OS refers to one composition of a material in which elements constituting a metal oxide are unevenly distributed with a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 3 nm, or a similar size, for example. Note that a state in which one or more metal elements are unevenly distributed and regions including the metal element(s) are mixed with a size greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 3 nm, or a similar size in a metal oxide is hereinafter referred to as a mosaic pattern or a patch-like pattern. 
     In addition, the CAC-OS has a composition in which materials are separated into a first region and a second region to form a mosaic pattern, and the first regions are distributed in the film (this composition is hereinafter also referred to as a cloud-like composition). That is, the CAC-OS is a composite metal oxide having a composition in which the first regions and the second regions are mixed. 
     Here, the atomic ratios of In, Ga, and Zn to the metal elements contained in the CAC-OS in an In—Ga—Zn oxide are denoted with [In], [Ga], and [Zn], respectively. For example, the first region in the CAC-OS in the In—Ga—Zn oxide has [In] higher than that in the composition of the CAC-OS film. Moreover, the second region has [Ga] higher than that in the composition of the CAC-OS film. For example, the first region has higher [In] and lower [Ga] than the second region. Moreover, the second region has higher [Ga] and lower [In] than the first region. 
     Specifically, the first region includes indium oxide, indium zinc oxide, or the like as its main component. The second region includes gallium oxide, gallium zinc oxide, or the like as its main component. That is, the first region can be referred to as a region containing In as its main component. The second region can be referred to as a region containing Ga as its main component. 
     Note that a clear boundary between the first region and the second region cannot be observed in some cases. 
     For example, energy dispersive X-ray spectroscopy (EDX) is used to obtain EDX mapping, and according to the EDX mapping, the CAC-OS in the In—Ga—Zn oxide has a structure in which the region containing In as its main component (the first region) and the region containing Ga as its main component (the second region) are unevenly distributed and mixed. 
     In the case where the CAC-OS is used for a transistor, a switching function (on/off switching function) can be given to the CAC-OS owing to the complementary action of the conductivity derived from the first region and the insulating property derived from the second region. That is, the CAC-OS has a conducting function in part of the material and has an insulating function in another part of the material; as a whole, the CAC-OS has a function of a semiconductor. Separation of the conducting function and the insulating function can maximize each function. Accordingly, when the CAC-OS is used for a transistor, high on-state current (Ion), high field-effect mobility (μ), and excellent switching operation can be achieved. 
     An oxide semiconductor has various structures with different properties. Two or more kinds among the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, the a-like OS, the CAC-OS, the nc-OS, and the CAAC-OS may be included in the oxide semiconductor of one embodiment of the present invention. 
     &lt;Transistor Including Oxide Semiconductor&gt; 
     Next, the case where the above oxide semiconductor is used for a transistor is described. 
     When the above oxide semiconductor is used for a transistor, a transistor with high field-effect mobility can be achieved. In addition, a transistor having high reliability can be achieved. 
     An oxide semiconductor with a low carrier concentration is preferably used for the transistor. For example, the carrier concentration of an oxide semiconductor is lower than or equal to 1×10 17  cm −3 , preferably lower than or equal to 1×10 15  cm −3 , further preferably lower than or equal to 1×10 13  cm −3 , still further preferably lower than or equal to 1×10 11  cm′, yet further preferably lower than 1×10 10  cm −3 , and higher than or equal to 1×10 −9  cm −3 . In order to reduce the carrier concentration of an oxide semiconductor film, the impurity concentration in the oxide semiconductor film is reduced so that the density of defect states can be reduced. In this specification and the like, a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state. Note that an oxide semiconductor having a low carrier concentration may be referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. 
     A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases. 
     Electric charge trapped by the trap states in the oxide semiconductor takes a long time to disappear and might behave like fixed electric charge. Thus, a transistor whose channel formation region is formed in an oxide semiconductor with a high density of trap states has unstable electrical characteristics in some cases. 
     Accordingly, in order to obtain stable electrical characteristics of a transistor, reducing the impurity concentration in an oxide semiconductor is effective. In order to reduce the impurity concentration in the oxide semiconductor, it is preferable that the impurity concentration in an adjacent film be also reduced. Examples of impurities include hydrogen, nitrogen, an alkali metal, an alkaline earth metal, iron, nickel, and silicon. 
     &lt;Impurity&gt; 
     Here, the influence of each impurity in the oxide semiconductor is described. 
     When silicon or carbon, which is one of Group 14 elements, is contained in the oxide semiconductor, defect states are formed in the oxide semiconductor. Thus, the concentration of silicon or carbon in the oxide semiconductor and the concentration of silicon or carbon in the vicinity of an interface with the oxide semiconductor (the concentration obtained by secondary ion mass spectrometry (SIMS)) are each set lower than or equal to 2×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 17  atoms/cm 3 . 
     When the oxide semiconductor contains an alkali metal or an alkaline earth metal, defect states are formed and carriers are generated in some cases. Thus, a transistor using an oxide semiconductor that contains an alkali metal or an alkaline earth metal is likely to have normally-on characteristics. Thus, the concentration of an alkali metal or an alkaline earth metal in the oxide semiconductor, which is obtained by SIMS, is lower than or equal to 1×10 18  atoms/cm 3 , preferably lower than or equal to 2×10 16  atoms/cm 3 . 
     Furthermore, when the oxide semiconductor contains nitrogen, the oxide semiconductor easily becomes n-type by generation of electrons serving as carriers and an increase in carrier concentration. As a result, a transistor using an oxide semiconductor containing nitrogen as a semiconductor is likely to have normally-on characteristics. When nitrogen is contained in the oxide semiconductor, a trap state is sometimes formed. This might make the electrical characteristics of the transistor unstable. Therefore, the concentration of nitrogen in the oxide semiconductor, which is obtained by SIMS, is set lower than 5×10 19  atoms/cm 3 , preferably lower than or equal to 5×10 18  atoms/cm 3 , further preferably lower than or equal to 1×10 18  atoms/cm 3 , still further preferably lower than or equal to 5×10 17  atoms/cm 3 . 
     Hydrogen contained in the oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and thus forms an oxygen vacancy in some cases. Entry of hydrogen into the oxygen vacancy generates an electron serving as a carrier in some cases. Furthermore, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier in some cases. Thus, a transistor using an oxide semiconductor containing hydrogen is likely to have normally-on characteristics. Accordingly, hydrogen in the oxide semiconductor is preferably reduced as much as possible. Specifically, the hydrogen concentration in the oxide semiconductor, which is obtained by SIMS, is set lower than 1×10 20  atoms/cm 3 , preferably lower than 1×10 19  atoms/cm 3 , further preferably lower than 5×10 18  atoms/cm 3 , still further preferably lower than 1×10 18  atoms/cm 3 . 
     When an oxide semiconductor with sufficiently reduced impurities is used for the channel formation region of the transistor, stable electrical characteristics can be given. 
     &lt;&lt;Other Semiconductor Materials&gt;&gt; 
     A semiconductor material that can be used for the oxide  230  is not limited to the above metal oxides. A semiconductor material that has a band gap (a semiconductor material that is not a zero-gap semiconductor) can be used for the oxide  230 . For example, a single element semiconductor such as silicon, a compound semiconductor such as gallium arsenide, or a layered material functioning as a semiconductor (also referred to as an atomic layer material or a two-dimensional material) is preferably used as a semiconductor material. In particular, a layered material functioning as a semiconductor is preferably used as a semiconductor material. 
     Here, in this specification and the like, the layered material generally refers to a group of materials having a layered crystal structure. In the layered crystal structure, layers formed by covalent bonding or ionic bonding are stacked with bonding such as the Van der Waals force, which is weaker than covalent bonding or ionic bonding. The layered material has high electrical conductivity in a monolayer, that is, high two-dimensional electrical conductivity. When a material that functions as a semiconductor and has high two-dimensional electrical conductivity is used for a channel formation region, the transistor can have a high on-state current. 
     Examples of the layered material include graphene, silicene, and chalcogenide. Chalcogenide is a compound containing chalcogen. Chalcogen is a general term of elements belonging to Group 16, which includes oxygen, sulfur, selenium, tellurium, polonium, and livermorium. Examples of chalcogenide include transition metal chalcogenide and chalcogenide of Group 13 elements. 
     For the oxide  230 , a transition metal chalcogenide functioning as a semiconductor is preferably used, for example. Specific examples of the transition metal chalcogenide which can be used for the oxide  230  include molybdenum sulfide (typically MoS 2 ), molybdenum selenide (typically MoSe 2 ), molybdenum telluride (typically MoTe 2 ), tungsten sulfide (typically WS 2 ), tungsten selenide (typically WSe 2 ), tungsten telluride (typically WTe 2 ), hafnium sulfide (typically HfS 2 ), hafnium selenide (typically HfSe 2 ), zirconium sulfide (typically ZrS 2 ), and zirconium selenide (typically ZrSe 2 ). 
     &lt;Modification Example of Semiconductor Device&gt; 
     An example of the semiconductor device that is one embodiment of the present invention will be described below with reference to  FIG. 3A  to  FIG. 3D . 
       FIG. 3A  is a top view of the semiconductor device.  FIG. 3B  is a cross-sectional view corresponding to a portion indicated by dashed-dotted line A 1 -A 2  in  FIG. 3A .  FIG. 3C  is a cross-sectional view corresponding to a portion indicated by dashed-dotted line A 3 -A 4  in  FIG. 3A .  FIG. 3D  is a cross-sectional view corresponding to a portion indicated by dashed-dotted line A 5 -A 6  in  FIG. 3A . Note that for clarity of the drawing, some components are not illustrated in the top view of  FIG. 3A . 
     Note that in the semiconductor device illustrated in  FIG. 3A  to  FIG. 3D , components having the same functions as the components included in the semiconductor device described in &lt;Structure example of semiconductor device&gt; are denoted by the same reference numerals. Note that the materials described in detail in &lt;Structure example of semiconductor device&gt; can also be used as constituent materials of the semiconductor device in this section. 
     The semiconductor device shown in  FIG. 3A  to  FIG. 3D  is a modification example of the semiconductor device shown in  FIG. 1A  to  FIG. 1D . The semiconductor device in  FIG. 3A  to  FIG. 3D  is different from the semiconductor device in  FIG. 1A  to  FIG. 1D  in the shape of the insulator  283 . An insulator  274  is included, which is also a difference. 
     In the semiconductor device shown in  FIG. 3A  to  FIG. 3D , the insulator  214 , the insulator  216 , the insulator  222 , the insulator  224 , the insulator  272 , the insulator  280 , and the insulator  282  are patterned. The insulator  283  covers the insulator  214 , the insulator  216 , the insulator  222 , the insulator  224 , the insulator  272 , the insulator  280 , and the insulator  282 . That is, the insulator  283  is in contact with the top surface and the side surface of the insulator  282  and the top surface of the insulator  212 . Accordingly, the insulator  214 , the insulator  216 , the insulator  222 , the insulator  224 , the insulator  272 , the insulator  280 , and the insulator  282  in addition to the oxide  230  and the like are isolated from the outside by the insulator  283  and the insulator  212 . In other words, the transistor  200  is located in a region sealed with the insulator  283  and the insulator  212 . 
     For example, it is preferable that the insulator  214  and the insulator  282  be formed using a material having a function of trapping or fixing hydrogen, and the insulator  212  and the insulator  283  be formed using a material having a function of inhibiting the diffusion of hydrogen and oxygen. Typically, aluminum oxide can be used for the insulator  214  and the insulator  282 . Moreover, typically, silicon nitride can be used for the insulator  212  and the insulator  283 . 
     With the above structure, entry of hydrogen contained in a region outside the sealed region into the sealed region can be inhibited. 
     Although the transistor  200  having a structure in which the insulator  212  and the insulator  283  each have a single-layer structure is shown in  FIG. 3A  to  FIG. 3D , the present invention is not limited thereto. For example, each of the insulator  212  and the insulator  283  may have a stacked-layer structure of two or more layers. 
     The insulator  274  functions as an interlayer film. The permittivity of the insulator  274  is preferably lower than that of the insulator  214 . When a material with a low permittivity is used for an interlayer film, parasitic capacitance generated between wirings can be reduced. The insulator  274  can be provided using a material similar to that for the insulator  280 , for example. 
     Alternatively, an insulator with reduced hydrogen is preferably used for the insulator  274 . With the use of an insulator with reduced hydrogen, diffusion of hydrogen into the transistor  200  can be inhibited. For the insulator  274 , an insulator may be formed by a sputtering method, for example. The hydrogen concentration in the insulator formed by a sputtering method is lower than the hydrogen concentration in an insulator formed by a CVD method, which is preferable. 
     &lt;Manufacturing Method of Semiconductor Device&gt; 
     Next, a method of manufacturing the semiconductor device that is one embodiment of the present invention and is illustrated in  FIG. 3A  to  FIG. 3D  is described with reference to  FIG. 5A  to  FIG. 21D . 
     In  FIG. 5A  to  FIG. 21D , A of each drawing is a top view. Moreover, B of each drawing is a cross-sectional view corresponding to a portion indicated by dashed-dotted line A 1 -A 2  in A, and is also a cross-sectional view of the transistor  200  in the channel length direction. Furthermore, C of each drawing is a cross-sectional view corresponding to a portion indicated by dashed-dotted line A 3 -A 4  in A, and is also a cross-sectional view of the transistor  200  in the channel width direction. Furthermore, D of each drawing is a cross-sectional view of a portion indicated by dashed-dotted line A 5 -A 6  in A. Note that for simplification of the drawing, some components are not shown in the top view of A of each drawing. 
     First, a substrate (not shown) is prepared, and the insulator  212  is deposited over the substrate. The insulator  212  can be deposited by a sputtering method, a CVD method, a molecular beam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, or the like. 
     Note that CVD methods can be classified into a plasma enhanced CVD (PECVD) method using plasma, a thermal CVD (TCVD) method using heat, a photo CVD method using light, and the like. Moreover, CVD methods can be classified into a metal CVD (MCVD) method and a metal organic CVD (MOCVD) method depending on a source gas to be used. 
     A high-quality film can be obtained at a relatively low temperature by a plasma enhanced CVD method. Furthermore, a thermal CVD method is a deposition method that does not use plasma and thus enables less plasma damage to an object to be processed. For example, a wiring, an electrode, an element (a transistor, a capacitor, or the like), or the like included in a semiconductor device might be charged up by receiving electric charge from plasma. In that case, accumulated electric charge might break the wiring, the electrode, the element, or the like included in the semiconductor device. In contrast, such plasma damage does not occur in the case of a thermal CVD method, which does not use plasma, and thus the yield of the semiconductor device can be increased. In addition, a thermal CVD method does not cause plasma damage during deposition, so that a film with few defects can be obtained. 
     As an ALD method, a thermal ALD method, in which a precursor and a reactant react with each other only by a thermal energy, a PEALD (Plasma Enhanced ALD) method, in which a reactant excited by plasma is used, and the like can be used. 
     An ALD method, which enables one atomic layer to be deposited at a time using self-regulating characteristics of atoms, has advantages such as deposition of an extremely thin film, deposition on a component with a high aspect ratio, deposition of a film with a small number of defects such as pinholes, deposition with excellent coverage, and low-temperature deposition. The use of plasma in a PEALD (Plasma Enhanced ALD) method is sometimes preferable because deposition at a lower temperature is possible. Note that a precursor used in an ALD method sometimes contains impurities such as carbon. Thus, in some cases, a film provided by an ALD method contains impurities such as carbon in a larger amount than a film provided by another deposition method. Note that impurities can be quantified by X-ray photoelectron spectroscopy (XPS). 
     Unlike a deposition method in which particles ejected from a target or the like are deposited, a CVD method and an ALD method are deposition methods in which a film is formed by reaction at a surface of an object to be processed. Thus, a CVD method and an ALD method are deposition methods that enable favorable step coverage almost regardless of the shape of an object to be processed. In particular, an ALD method has excellent step coverage and excellent thickness uniformity and thus is suitable for covering a surface of an opening portion with a high aspect ratio, for example. On the other hand, an ALD method has a relatively low deposition rate, and thus is preferably used in combination with another deposition method with a high deposition rate, such as a CVD method, in some cases. 
     A CVD method and an ALD method enable control of the composition of a film to be obtained with the flow rate ratio of the source gases. For example, by a CVD method and an ALD method, a film with a certain composition can be deposited depending on the flow rate ratio of the source gases. Moreover, for example, by a CVD method and an ALD method, a film whose composition is continuously changed can be deposited by changing the flow rate ratio of the source gases during the deposition. In the case where the film is deposited while the flow rate ratio of the source gases is changed, as compared to the case where the film is deposited using a plurality of deposition chambers, the time taken for the deposition can be shortened because the time taken for transfer and pressure adjustment is omitted. Thus, the productivity of the semiconductor device can be increased in some cases. 
     In this embodiment, for the insulator  212 , silicon nitride is formed by a sputtering method. 
     When an insulator through which copper is less likely to pass, such as silicon nitride, is used for the insulator  212  in such a manner, even in the case where a metal that is likely to diffuse, such as copper, is used for a conductor in a layer (not shown) below the insulator  212 , diffusion of the metal into an upper portion through the insulator  212  can be inhibited. The use of an insulator through which impurities such as water and hydrogen are less likely to pass, such as silicon nitride, can inhibit diffusion of impurities such as water and hydrogen contained in a layer below the insulator  212 . 
     Next, the insulator  214  is deposited over the insulator  212 . The insulator  214  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, for the insulator  214 , aluminum oxide is deposited by a sputtering method. 
     Furthermore, the hydrogen concentration in the insulator  214  is preferably lower than the hydrogen concentration in the insulator  212 . When silicon nitride is deposited by a sputtering method for the insulator  212 , silicon nitride having a low hydrogen concentration can be formed. When the insulator  214  is formed using aluminum oxide, the insulator  214  can have a lower hydrogen concentration than the insulator  212 . 
     The transistor  200  is formed over the insulator  214  in a later step; it is preferable that a film adjacent to the transistor  200  have a relatively low hydrogen concentration and a film with a relatively high hydrogen concentration be positioned away from the transistor  200 . 
     Next, the insulator  216  is deposited over the insulator  214 . The insulator  216  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, for the insulator  216 , silicon oxide or silicon oxynitride is deposited by a sputtering method. 
     The insulator  212 , the insulator  214 , and the insulator  216  are preferably deposited in a reduced pressure successively without being exposed to the air. The deposition without exposure to the air is preferable because impurities or moisture from the atmospheric environment can be prevented from attaching onto the insulator  212 , the insulator  214 , and the insulator  216 , so that the interface between the insulator  212  and the insulator  214 , the vicinity of the interface, the interface between the insulator  214  and the insulator  216 , and the vicinity of the interface can be kept clean. For example, a multi-chamber deposition apparatus is used for the successive deposition. The successive deposition can reduce the manufacturing time of the semiconductor device, which is preferable. 
     Next, an opening reaching the insulator  214  is formed in the insulator  216 . Wet etching can be used for the formation of the opening; however, dry etching is preferably used for microfabrication. As the insulator  214 , it is preferable to select an insulator that functions as an etching stopper film used in forming a groove by etching the insulator  216 . For example, in the case where silicon oxide or silicon oxynitride is used for the insulator  216  in which the groove is to be formed, silicon nitride, aluminum oxide, or hafnium oxide is preferably used for the insulator  214 . 
     As a dry etching apparatus, a capacitively coupled plasma (CCP) etching apparatus including parallel plate electrodes can be used. The capacitively coupled plasma etching apparatus including the parallel plate electrodes may have a structure in which a high-frequency voltage is applied to one of the parallel plate electrodes. Alternatively, a structure may be employed in which different high-frequency voltages are applied to one of the parallel plate electrodes. Alternatively, a structure may be employed in which high-frequency voltages with the same frequency are applied to the parallel plate electrodes. Alternatively, a structure may be employed in which high-frequency voltages with different frequencies are applied to the parallel plate electrodes. Alternatively, a dry etching apparatus including a high-density plasma source can be used. As the dry etching apparatus including a high-density plasma source, an inductively coupled plasma (ICP) etching apparatus or the like can be used, for example. 
     After the formation of the opening, a conductive film to be the conductor  205   a  is deposited. The conductive film preferably includes a conductor that has a function of inhibiting passage of oxygen. For example, tantalum nitride, tungsten nitride, or titanium nitride can be used. Alternatively, a stacked-layer film of the conductor having a function of inhibiting passage of oxygen and tantalum, tungsten, titanium, molybdenum, aluminum, copper, or a molybdenum-tungsten alloy can be used. The conductive film can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     In this embodiment, the conductive film to be the conductor  205   a  has a multilayer structure. First, tantalum nitride is deposited by a sputtering method, and titanium nitride is stacked over the tantalum nitride. When such metal nitrides are used for a lower layer of the conductor  205   b , even in the case where a metal that is likely to diffuse, such as copper, is used for a conductive film to be the conductor  205   b  described below, outward diffusion of the metal from the conductor  205   a  can be inhibited. 
     Next, a conductive film to be the conductor  205   b  is deposited. The conductive film can be deposited by a plating method, a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, for the conductive film to be the conductor  205   b , a low-resistance conductive material such as copper is deposited. 
     Next, CMP treatment is performed, thereby removing part of the conductive film to be the conductor  205   a  and part of the conductive film to be the conductor  205   b  to expose the insulator  216 . As a result, the conductor  205   a  and the conductor  205   b  remain only in the opening portion. Thus, the conductor  205  whose top surface is flat can be formed (see  FIG. 5A  to  FIG. 5D ). Note that the insulator  216  is partly removed by the CMP treatment in some cases. 
     Although the conductor  205  is embedded in the opening in the insulator  216  in the above description, this embodiment is not limited to this structure. For example, the surface of the conductor  205  may be exposed in the following manner: the conductor  205  is formed over the insulator  214 , the insulator  216  is deposited over the conductor  205 , and the insulator  216  is subjected to the CMP treatment so that the insulator  216  is partly removed. 
     Next, the insulator  222  is deposited over the insulator  216  and the conductor  205 . An insulator containing an oxide of one or both of aluminum and hafnium is preferably deposited as the insulator  222 . Note that as the insulator containing an oxide of one or both of aluminum and hafnium, aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), or the like is preferably used. The insulator containing an oxide of one or both of aluminum and hafnium has a barrier property against oxygen, hydrogen, and water. When the insulator  222  has a barrier property against hydrogen and water, hydrogen and water contained in components provided around the transistor  200  are inhibited from diffusing into the transistor  200  through the insulator  222 , and generation of oxygen vacancies in the oxide  230  can be inhibited. 
     The insulator  222  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Sequentially, heat treatment is preferably performed. The heat treatment is 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., further preferably higher than or equal to 320° C. and lower than or equal to 450° C. Note that the heat treatment is performed in a nitrogen gas or inert gas atmosphere, or an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more. For example, in the case where the heat treatment is performed in a mixed atmosphere of a nitrogen gas and an oxygen gas, the proportion of the oxygen gas may be approximately 20%. The heat treatment may be performed under reduced pressure. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in a nitrogen gas or inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more in order to compensate for released oxygen. 
     The gas used in the above heat treatment is preferably highly purified. For example, the amount of moisture contained in the gas used in the above heat treatment is 1 ppb or less, preferably 0.1 ppb or less, further preferably 0.05 ppb or less. The heat treatment using a highly purified gas can prevent entry of moisture or the like into the insulator  222  and the like as much as possible. 
     In this embodiment, as the heat treatment, after the deposition of the insulator  222 , heat treatment at 400° C. for one hour is performed with a flow rate ratio of a nitrogen gas and an oxygen gas of 4 slm:1 slm. By the heat treatment, impurities such as water and hydrogen contained in the insulator  222  can be removed, for example. In the case where an oxide containing hafnium is used for the insulator  222 , the heat treatment can improve the crystallinity of the insulator  222 . The heat treatment can also be performed after the deposition of the insulator  224 , for example. 
     Next, the insulator  224  is deposited over the insulator  222 . The insulator  224  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, for the insulator  224 , silicon oxide or a silicon oxynitride film is deposited by an ALD method. The insulator  224  is preferably deposited by a deposition method using a gas in which hydrogen atoms are reduced or removed. Thus, the hydrogen concentration in the insulator  224  can be reduced. The hydrogen concentration in the insulator  224  is preferably reduced because the insulator  224  is in contact with the oxide  230   a  in a later step. 
     Here, plasma treatment containing oxygen may be performed under reduced pressure so that an excess-oxygen region can be formed in the insulator  224 . For the plasma treatment containing oxygen, an apparatus including a power source for generating high-density plasma using a microwave is preferably used, for example. Alternatively, a power source for applying an RF (Radio Frequency) to the substrate side may be included. The use of high-density plasma enables high-density oxygen radicals to be generated, and RF application to the substrate side allows the oxygen radicals generated by the high-density plasma to be efficiently introduced into the insulator  224 . Alternatively, after plasma treatment containing an inert gas is performed using this apparatus, plasma treatment containing oxygen may be performed to compensate for released oxygen. Note that impurities such as water and hydrogen contained in the insulator  224  can be removed by selecting the conditions for the plasma treatment appropriately. In that case, the heat treatment does not need to be performed. 
     Here, after aluminum oxide is deposited over the insulator  224  by a sputtering method, for example, CMP treatment may be performed until the insulator  224  is exposed. The CMP treatment can planarize and smooth the surface of the insulator  224 . When the CMP treatment is performed on the aluminum oxide positioned over the insulator  224 , it is easy to detect the endpoint of the CMP treatment. Although part of the insulator  224  is polished by the CMP treatment and the thickness of the insulator  224  is reduced in some cases, the thickness can be adjusted when the insulator  224  is deposited. Planarizing and smoothing the surface of the insulator  224  can prevent deterioration in the coverage with an oxide deposited later and a decrease in the yield of the semiconductor device in some cases. The deposition of aluminum oxide over the insulator  224  by a sputtering method is preferred because oxygen can be added to the insulator  224 . 
     Next, an oxide film  230 A and an oxide film  230 B are deposited in this order over the insulator  224  (see  FIG. 5A  to  FIG. 5D ). 
     The oxide film  230 A and the oxide film  230 B can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     For example, in the case where the oxide film  230 A and the oxide film  230 B are deposited by a sputtering method, oxygen or a mixed gas of oxygen and a rare gas is used as a sputtering gas. Increasing the proportion of oxygen contained in the sputtering gas can increase the amount of excess oxygen in the deposited oxide films. In the case where the oxide films are deposited by a sputtering method, the above In-M-Zn oxide target or the like can be used. 
     In particular, when the oxide film  230 A is deposited, part of oxygen contained in the sputtering gas is supplied to the insulator  224  in some cases. Thus, the proportion of oxygen contained in the sputtering gas is higher than or equal to 70%, preferably higher than or equal to 80%, further preferably 100%. 
     In the case where the oxide film  230 B is formed by a sputtering method and the proportion of oxygen contained in the sputtering gas for deposition is higher than 30% and lower than or equal to 100%, preferably higher than or equal to 70% and lower than or equal to 100%, an oxygen-excess oxide semiconductor is formed. In a transistor using an oxygen-excess oxide semiconductor for its channel formation region, relatively high reliability can be obtained. Note that one embodiment of the present invention is not limited thereto. In the case where the oxide film  230 B is formed by a sputtering method and the proportion of oxygen contained in the sputtering gas for deposition is higher than or equal to 1% and lower than or equal to 30%, preferably higher than or equal to 5% and lower than or equal to 20%, an oxygen-deficient oxide semiconductor is formed. In a transistor using an oxygen-deficient oxide semiconductor for its channel formation region, relatively high field-effect mobility can be obtained. Furthermore, when the deposition is performed while the substrate is being heated, the crystallinity of the oxide film can be improved. 
     In this embodiment, the oxide film  230 A is deposited by a sputtering method using an oxide target with In:Ga:Zn=1:3:4 [atomic ratio]. In addition, the oxide film  230 B is deposited by a sputtering method using an oxide target with In:Ga:Zn=4:2:4.1 [atomic ratio]. Note that each of the oxide films is preferably formed to have characteristics required for the oxide  230   a  and the oxide  230   b  by selecting the deposition conditions and the atomic ratios as appropriate. 
     Next, an oxide film  243 A is deposited over the oxide film  230 B (see  FIG. 5A  to  FIG. 5D ). The oxide film  243 A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. The atomic ratio of Ga to In in the oxide film  243 A is preferably greater than the atomic ratio of Ga to In in the oxide film  230 B. In this embodiment, the oxide film  243 A is deposited by a sputtering method using an oxide target with In:Ga:Zn=1:3:4 [atomic ratio]. 
     The oxide film  230 A, the oxide film  230 B, and the oxide film  243 A are preferably deposited successively under reduced pressure without being exposed to the air environment. Depositing these films without being exposed to the air environment can prevent impurities or moisture from the air environment from attaching onto the oxide film  230 A, the oxide film  230 B, and the oxide film  243 A; this can keep clean the interface between the oxide film  230 A and the oxide film  230 B, the vicinity of the interface, the interface between the oxide film  230 B and the oxide film  243 A, and the vicinity of the interface, which is preferable. For example, a multi-chamber deposition apparatus is used. The successive deposition can reduce the manufacturing time of the semiconductor device, which is preferable. 
     Next, heat treatment is preferably performed. The heat treatment is performed in a temperature range where the oxide film  230 A, the oxide film  230 B, and the oxide film  243 A do not become polycrystals, i.e., at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 400° C. and lower than or equal to 600° C. Note that the heat treatment is performed in a nitrogen gas or inert gas atmosphere, or an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more. For example, in the case where the heat treatment is performed in a mixed atmosphere of a nitrogen gas and an oxygen gas, the proportion of the oxygen gas may be approximately 20%. The heat treatment may be performed under reduced pressure. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in a nitrogen gas or inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more in order to compensate for released oxygen. 
     The gas used in the above heat treatment is preferably highly purified. For example, the amount of moisture contained in the gas used in the above heat treatment is 1 ppb or less, preferably 0.1 ppb or less, and further preferably 0.05 ppb or less. The heat treatment using a highly purified gas can prevent entry of moisture or the like into the oxide film  230 A, the oxide film  230 B, the oxide film  243 A, and the like as much as possible. 
     In this embodiment, the heat treatment is performed in such a manner that treatment is performed at 550° C. in a nitrogen atmosphere for one hour and then another treatment is successively performed at 550° C. in an oxygen atmosphere for one hour. By the heat treatment, impurities such as water and hydrogen in the oxide film  230 A, the oxide film  230 B, and the oxide film  243 A can be removed, for example. Furthermore, the heat treatment improves the crystallinity of the oxide film  230 B, thereby offering a dense structure with higher density. Thus, diffusion of oxygen or impurities in the oxide film  230 B can be reduced. 
     Then, a conductive film  242 A is deposited over the oxide film  243 A (see  FIG. 5A  to  FIG. 5D ). The conductive film  242 A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. Note that heat treatment may be performed before the deposition of the conductive film  242 A. This heat treatment may be performed under reduced pressure, and the conductive film  242 A may be successively formed without exposure to the air. The treatment can remove moisture and hydrogen adsorbed onto the surface of the oxide film  243 A and the like, and further can reduce the moisture concentration and the hydrogen concentration in the oxide film  230 A, the oxide film  230 B, and the oxide film  243 A. The heat treatment is preferably performed at a temperature higher than or equal to 100° C. and lower than or equal to 400° C. In this embodiment, the heat treatment is performed at 200° C. 
     Next, an insulating film  271 A is deposited over the conductive film  242 A (see  FIG. 5A  to  FIG. 5D ). The insulating film  271 A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. As the insulating film  271 A, an insulating film having a function of inhibiting passage of oxygen is preferably used. For example, as the insulating film  271 A, aluminum oxide, hafnium oxide, silicon nitride, or the like may be deposited by a sputtering method or an ALD method. 
     Next, a conductive film  248 A is deposited over the insulating film  271 A (see  FIG. 5A  to  FIG. 5D ). The conductive film  248 A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For example, a conductive film similar to the conductive film  242 A may be used as the conductive film  248 A. 
     In this embodiment, tantalum nitride, aluminum oxide, and tantalum nitride are deposited by a sputtering method for the conductive film  242 A, the insulating film  271 A, and the conductive film  248 A, respectively. 
     The conductive film  242 A, the insulating film  271 A, and the conductive film  248 A are preferably deposited under reduced pressure successively without being exposed to the air environment. Depositing these films without being exposed to the air environment can prevent impurities or moisture from the air environment from attaching onto the conductive film  242 A, the insulating film  271 A, and the conductive film  248 A; this can keep clean the interface between the conductive film  242 A and the insulating film  271 A, the vicinity of the interface, the interface between the insulating film  271 A and the conductive film  248 A, and the vicinity of the interface, which is preferable. For example, a multi-chamber deposition apparatus is used. The successive deposition can reduce the manufacturing time of the semiconductor device, which is preferable. 
     Next, the oxide film  230 A, the oxide film  230 B, the oxide film  243 A, the conductive film  242 A, the insulating film  271 A, and the conductive film  248 A are processed into island shapes by a lithography method to form the oxide  230   a , the oxide  230   b , an oxide layer  243 B, a conductive layer  242 B, an insulating layer  271 B, and a conductive layer  248  (see  FIG. 6A  to  FIG. 6D ). A dry etching method or a wet etching method can be used for the processing. Processing by a dry etching method is suitable for microfabrication. The oxide film  230 A, the oxide film  230 B, the oxide film  243 A, the conductive film  242 A, the insulating film  271 A, and the conductive film  248 A may be processed under different conditions. Note that in this step, the thickness of the insulator  224  in a region not overlapping with the oxide  230   a  is reduced in some cases. 
     Note that in the lithography method, first, a resist is exposed to light through a mask. Next, a region exposed to light is removed or left using a developing solution, so that a resist mask is formed. Then, etching treatment through the resist mask is conducted, whereby a conductor, a semiconductor, an insulator, or the like can be processed into a desired shape. The resist mask is formed through, for example, exposure of the resist to KrF excimer laser light, ArF excimer laser light, EUV (Extreme Ultraviolet) light, or the like. Alternatively, a liquid immersion technique may be employed in which a gap between a substrate and a projection lens is filled with liquid (e.g., water) in light exposure. Alternatively, an electron beam or an ion beam may be used instead of the light. Note that a mask is unnecessary in the case of using an electron beam or an ion beam. Note that the resist mask can be removed by dry etching treatment such as ashing, wet etching treatment, wet etching treatment after dry etching treatment, or dry etching treatment after wet etching treatment. 
     In addition, a hard mask formed of an insulator or a conductor may be used under the resist mask. In the case of using a hard mask, a hard mask with a desired shape can be formed in the following manner: an insulating film or a conductive film that is the material of the hard mask is formed over the conductive film  242 A, a resist mask is formed thereover, and then the hard mask material is etched. The etching of the conductive film  242 A and the like may be performed after removing the resist mask or with the resist mask remaining. In the latter case, the resist mask sometimes disappears during the etching. The hard mask may be removed by etching after the etching of the conductive film  242 A and the like. Meanwhile, the hard mask is not necessarily removed when the hard mask material does not affect later steps or can be utilized in later steps. In this embodiment, the insulating layer  271 B and the conductive layer  248  are used as hard masks. 
     Here, the insulating layer  271 B and the conductive layer  248  function as masks for the conductive layer  242 B; thus, as shown in  FIG. 6B  to  FIG. 6D , the conductive layer  242 B does not have a curved surface between the side surface and the top surface. Thus, end portions at the intersections of the side surfaces and the top surfaces of the conductor  242   a  and the conductor  242   b  shown in  FIG. 3B  to  FIG. 3D  are angular. The cross-sectional area of the conductor  242  is larger in the case where the end portion at the intersection of the side surface and the top surface of the conductor  242  is angular than in the case where the end portion is rounded. Accordingly, the resistance of the conductor  242  is reduced, so that the on-state current of the transistor  200  can be increased. 
     Here, the oxide  230   a , the oxide  230   b , the oxide layer  243 B, the conductive layer  242 B, the insulating layer  271 B, and the conductive layer  248  are formed to at least partly overlap with the conductor  205 . It is preferable that the side surfaces of the oxide  230   a , the oxide  230   b , the oxide layer  243 B, the conductive layer  242 B, the insulating layer  271 B, and the conductive layer  248  be substantially perpendicular to the top surface of the insulator  222 . When the side surfaces of the oxide  230   a , the oxide  230   b , the oxide layer  243 B, the conductive layer  242 B, the insulating layer  271 B, and the conductive layer  248  are substantially perpendicular to the top surface of the insulator  222 , a plurality of transistors  200  can be provided in a smaller area and at a higher density. Alternatively, a structure may be employed in which an angle formed by the side surfaces of the oxide  230   a , the oxide  230   b , the oxide layer  243 B, the conductive layer  242 B, the insulating layer  271 B, and the conductive layer  248  and the top surface of the insulator  222  is a small angle. In that case, the angle formed by the side surfaces of the oxide  230   a , the oxide  230   b , the oxide layer  243 B, the conductive layer  242 B, the insulating layer  271 B, and the conductive layer  248  and the top surface of the insulator  222  is preferably greater than or equal to 60° and less than 70°. With such a shape, in later steps, the coverage with the insulator  272  and the like can be improved, so that defects such as a void can be reduced. 
     Then, the conductive layer  248  is removed. The conductive layer  248  is removed by a dry etching method (see  FIG. 7A  to  FIG. 7D ). 
     Next, the insulator  272  is formed over the insulator  224 , the oxide  230   a , the oxide  230   b , the oxide layer  243 B, the conductive layer  242 B, and the insulating layer  271 B (see  FIG. 8A  to  FIG. 8D ). The insulator  272  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, for the insulator  272 , aluminum oxide is deposited by a sputtering method. In particular, the insulator  272  is preferably deposited by a bias sputtering method. In the bias sputtering method, the amount of oxygen implanted into the insulator  224  serving as a base of the insulator  272  can be controlled with the amount of RF power applied to the substrate. For example, as the RF power, a bias with a power density of 0.31 W/cm 2  or more, preferably 0.62 W/cm 2  or more, further preferably 1.86 W/cm 2  or more is applied to the substrate. In other words, an appropriate amount of oxygen for the transistor characteristics can be implanted by changing the amount of RF power used for the formation of the insulator  272 . Moreover, an appropriate amount of oxygen for improving the reliability of the transistor can be implanted. The frequency of the RF is preferably 10 MHz or more; typically 13.56 MHz. The higher the RF frequency is, the less damage the substrate gets. 
     As described above, the insulator  272  has a function of implanting oxygen to the film serving as a base, but the insulator  272  itself has a function of inhibiting passage of oxygen. Accordingly, when the insulator  280  is formed over the insulator  272  and oxygen is diffused from the insulator  280  in a later step, the oxygen can be prevented from being directly diffused from the insulator  280  into the oxide  230   a , the oxide  230   b , the oxide layer  243 B, and the conductive layer  242 B. 
     Next, an insulating film to be the insulator  280  is deposited over the insulator  224  and the insulator  272 . The insulating film can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For example, as the insulating film, a silicon oxide film is deposited by a sputtering method, and a silicon oxide film is deposited thereover by a PEALD method or a thermal ALD method. The insulating film is preferably deposited by a deposition method using a gas in which hydrogen atoms are reduced or removed. Thus, the hydrogen concentration in the insulator  280  can be reduced. Note that heat treatment may be performed before the insulating film is deposited. The heat treatment may be performed under reduced pressure, and the insulating films may be successively deposited without exposure to the air. The treatment can remove moisture and hydrogen adsorbed onto the surfaces of the insulator  224 , the insulator  272 , and the like and further can reduce the moisture concentration and the hydrogen concentration in the oxide  230   a , the oxide  230   b , the oxide layer  243 B, and the insulator  224 . For the heat treatment, the above heat treatment conditions can be used. 
     Next, the insulating film is subjected to CMP treatment, so that the insulator  280  having a flat top surface is formed (see  FIG. 9A  to  FIG. 9D ). Note that in a manner similar to that of the insulator  224 , aluminum oxide may be deposited over the insulator  280  by a sputtering method, for example, and the aluminum oxide may be subjected to CMP until the insulator  280  is reached. 
     Here, microwave treatment may be performed. The microwave treatment is preferably performed in an atmosphere containing oxygen under reduced pressure. By performing the microwave treatment, an electric field by a microwave can be supplied to the insulator  280 , the oxide  230   b , the oxide  230   a , and the like to divide VoH in the oxide  230   b  and the oxide  230   a  into an oxygen vacancy (V O ) and hydrogen (H). Part of hydrogen divided at this time is bonded to oxygen contained in the insulator  280  and is removed as water molecules in some cases. Some hydrogen is gettered by the conductive layer  242 B through the insulator  272  and the insulating layer  271 B in some cases. 
     After the microwave treatment, heat treatment may be performed with the reduced pressure being maintained. Such treatment enables hydrogen in the insulator  280 , the oxide  230   b , and the oxide  230   a  to be removed efficiently. Note that the temperature of the heat treatment is preferably higher than or equal to 300° C. and lower than or equal to 500° C. 
     Performing the microwave treatment improves the film quality of the insulator  280 , thereby inhibiting diffusion of hydrogen, water, impurities, and the like. Accordingly, hydrogen, water, impurities, and the like can be inhibited from diffusing into the oxide  230  through the insulator  280  in a later step after the formation of the insulator  280 , heat treatment, or the like. 
     Then, part of the insulator  280 , part of the insulator  272 , part of the insulating layer  271 B, part of the conductive layer  242 B, part of the oxide layer  243 B, and part of the oxide  230   b  are processed to form an opening reaching the oxide  230   b . The opening is preferably formed to overlap with the conductor  205 . The insulator  271   a , the insulator  271   b , the conductor  242   a , the conductor  242   b , the oxide  243   a , and the oxide  243   b  are formed through the formation of the opening (see  FIG. 10A  to  FIG. 10D ). 
     An upper portion of the oxide  230   b  is removed when the opening is formed. When part of the oxide  230   b  is removed, a groove portion is formed in the oxide  230   b . The groove portion may be formed in the same step as the formation of the opening or in a step different from the formation of the opening in accordance with the depth of the groove portion. 
     The part of the insulator  280 , the part of the insulator  272 , the part of the insulating layer  271 B, the part of the conductive layer  242 B, the part of the oxide layer  243 B, and the part of the oxide  230   b  can be processed by a dry etching method or a wet etching method. Processing by a dry etching method is suitable for microfabrication. The processing may be performed under different conditions. For example, the part of the insulator  280  may be processed by a dry etching method, the part of the insulator  272  and the part of the insulating layer  271 B may be processed by a wet etching method, and the part of the oxide layer  243 B, the part of the conductive layer  242 B, and the part of the oxide  230   b  may be processed by a dry etching method. Processing of the part of the oxide layer  243 B and the part of the conductive layer  242 B and processing of the part of the oxide  230   b  may be performed under different conditions. 
     When the oxide  230   b  is partly removed to form a groove by a dry etching method, a strong bias power is preferably applied. A bias power density is, for example, more than or equal to 0.02 W/cm 2 , preferably more than or equal to 0.03 W/cm 2 , further preferably more than or equal to 0.06 W/cm 2 . The dry etching treatment time may be set as appropriate depending on the depth of the groove portion. 
     Here, it is preferable to remove impurities that are attached onto the surfaces of the oxide  230   a , the oxide  230   b , and the like or diffused into the oxide  230   a , the oxide  230   b , and the like. It is also preferable to remove a damaged region that is formed on the surface of the oxide  230   b  by the above dry etching. The impurities come from components contained in the insulator  280 , part of the insulator  272 , part of the insulating layer  271 B, and the conductive layer  242 B; components contained in a member of an apparatus used to form the opening; and components contained in a gas or a liquid used for etching, for instance. Examples of the impurities include aluminum, silicon, tantalum, fluorine, and chlorine. 
     In particular, impurities such as aluminum and silicon block the oxide  230   b  and the oxide  230   c , which is formed in a later step, from becoming a CAAC-OS. It is thus preferable to reduce or remove impurity elements such as aluminum and silicon, which block the oxide from becoming a CAAC-OS. For example, the concentration of aluminum atoms at the interface between the oxide  230   b  and the oxide  230   c  and in the vicinity thereof is lower than or equal to 5.0 atomic %, preferably lower than or equal to 2.0 atomic %, further preferably lower than or equal to 1.5 atomic %, still further preferably lower than or equal to 1.0 atomic %, and yet further preferably lower than 0.3 atomic %. 
     Note that in a metal oxide, a region that is hindered from becoming a CAAC-OS by impurities such as aluminum and silicon and becomes an amorphous-like oxide semiconductor (a-like OS) is referred to as a non-CAAC region in some cases. In the non-CAAC region, the density of the crystal structure is reduced to increase VoH; thus, the transistor is likely to be normally on. Hence, the non-CAAC regions in the oxide  230   b  and the oxide  230   c  are preferably reduced or removed. 
     In contrast, the oxide  230   b  and the oxide  230   c  preferably have CAAC structures. In particular, the CAAC structure preferably reaches a lower edge portion of a drain in the oxide  230   b  and the oxide  230   c . Here, in the transistor  200 , the conductor  242   a  or the conductor  242   b , and its vicinity function as a drain. In other words, one or both of the oxide  230   b  and the oxide  230   c  in the vicinity of the lower edge portion of the conductor  242   a  (conductor  242   b ) preferably have a CAAC structure. In this manner, the damaged region of the oxide  230   b  is removed and the CAAC structure is formed in the edge portion of the drain, which significantly affects the drain withstand voltage, so that variation of the electrical characteristics of the transistor  200  can be further suppressed. The reliability of the transistor  200  can be improved. 
     In order to remove the above impurities and the like, cleaning treatment is performed. Examples of the cleaning method include wet cleaning using a cleaning solution, plasma treatment using plasma, and cleaning by heat treatment, and any of these cleanings may be performed in appropriate combination. The cleaning treatment sometimes makes the groove portion deeper. 
     As the wet cleaning, cleaning treatment may be performed using an aqueous solution in which ammonia water, oxalic acid, phosphoric acid, hydrofluoric acid, or the like is diluted with carbonated water or pure water; pure water; carbonated water; or the like. Alternatively, ultrasonic cleaning using such an aqueous solution, pure water, or carbonated water may be performed. Further alternatively, such cleaning methods may be performed in combination as appropriate. 
     Note that in this specification and the like, in some cases, an aqueous solution in which pure water is added to hydrofluoric acid for dilution is referred to as diluted hydrofluoric acid, and an aqueous solution in which pure water is added to ammonia water for dilution is referred to as diluted ammonia water. The concentration, temperature, and the like of the aqueous solution may be adjusted as appropriate in accordance with an impurity to be removed, the structure of a semiconductor device to be cleaned, or the like. The concentration of ammonia in the diluted ammonia water is higher than or equal to 0.01% and lower than or equal to 5%, preferably higher than or equal to 0.1% and lower than or equal to 0.5%. The concentration of hydrogen fluoride in the diluted hydrofluoric acid is higher than or equal to 0.01 ppm and lower than or equal to 100 ppm, preferably higher than or equal to 0.1 ppm and lower than or equal to 10 ppm. 
     A frequency greater than or equal to 200 kHz, preferably greater than or equal to 900 kHz is preferably used for the ultrasonic cleaning. Damage to the oxide  230   b  and the like can be reduced with this frequency. 
     The cleaning treatment may be performed a plurality of times, and the cleaning solution may be changed in every cleaning treatment. For example, the first cleaning treatment may use diluted hydrofluoric acid or diluted ammonia water and the second cleaning treatment may use pure water or carbonated water. 
     As the cleaning treatment in this embodiment, wet cleaning using diluted hydrofluoric acid is performed, and then, wet cleaning using pure water or carbonated water is performed. The cleaning treatment can remove impurities that are attached onto the surfaces of the oxide  230   a , the oxide  230   b , and the like or diffused into the oxide  230   a , the oxide  230   b , and the like. The crystallinity of the oxide  230   c  formed over the oxide  230   b  can be increased. 
     By the processing such as dry etching or the cleaning treatment, the thickness of the insulator  224  in a region that overlaps with the opening and does not overlap with the oxide  230   b  and the insulator  272  might become smaller than the thickness of the insulator  224  in a region that overlaps with the oxide  230   b.    
     After the etching or the cleaning treatment, heat treatment may be performed. The heat treatment is performed at a temperature higher than or equal to 100° C. and lower than or equal to 450° C., preferably higher than or equal to 350° C. and lower than or equal to 400° C. Note that the heat treatment is performed in a nitrogen gas or inert gas atmosphere, or an atmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or 10% or more. For example, the heat treatment is preferably performed in an oxygen atmosphere. Accordingly, oxygen can be supplied to the oxide  230   a  and the oxide  230   b  to reduce the amount of oxygen vacancies V O . This heat treatment can improve the crystallinity of the oxide  230   b  and the crystallinity of the oxide  230   c  which is formed in the groove portion of the oxide  230   b . The heat treatment may be performed under reduced pressure. Alternatively, the heat treatment may be performed in an oxygen atmosphere, and then another heat treatment may be successively performed in a nitrogen atmosphere without exposure to the air. 
     Next, an oxide film  230 C is deposited (see  FIG. 11A  to  FIG. 11D ). Heat treatment may be performed before the oxide film  230 C is deposited. It is preferable that the heat treatment be performed under reduced pressure and the oxide film  230 C be successively deposited without exposure to the air. The heat treatment is preferably performed in an atmosphere containing oxygen. Such treatment can remove moisture and hydrogen adsorbed onto the surface of the oxide  230   b  or the like and can reduce the moisture concentration and the hydrogen concentration in the oxide  230   a  and the oxide  230   b . The heat treatment is preferably performed at a temperature higher than or equal to 100° C. and lower than or equal to 400° C. In this embodiment, the heat treatment is performed at 200° C. 
     The oxide film  230 C can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. The oxide film  230 C may be formed by a deposition method similar to that for the oxide film  230 A or the oxide film  230 B depending on characteristics required for the oxide film  230 C. In this embodiment, the oxide film  230 C is deposited by a sputtering method using an oxide target with In:Ga:Zn=4:2:3 [atomic ratio], an oxide target with In:Ga:Zn=5:1:3 [atomic ratio], an oxide target with In:Ga:Zn=10:1:3 [atomic ratio], or an indium oxide target. 
     Part of oxygen contained in the sputtering gas is sometimes supplied to the oxide  230   a  and the oxide  230   b  during the deposition of the oxide film  230 C. Alternatively, during the deposition of the oxide film  230 C, part of oxygen contained in the sputtering gas is supplied to the insulator  280  in some cases. Therefore, the proportion of oxygen in the sputtering gas for the oxide film  230 C is preferably 70% or higher, further preferably 80% or higher, still further preferably 100%. By depositing the oxide film  230 C in an atmosphere containing much oxygen, the oxide film  230 C is likely to be CAAC-OS. 
     The oxide film  230 C is preferably deposited while the substrate is being heated. In that case, the substrate temperature is set to higher than or equal to 200° C., so that oxygen vacancies in the oxide film  230 C and the oxide  230   b  can be reduced. The deposition is performed while the substrate is heated, whereby the crystallinity of the oxide film  230 C and the oxide  230   b  can be improved. 
     Next, an oxide film  230 D is deposited (see  FIG. 11A  to  FIG. 11D ). The oxide film  230 D is preferably successively deposited after the deposition of the oxide film  230 C without being exposed to the air. 
     The oxide film  230 D can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. The oxide film  230 D is deposited by a deposition method similar to that for the oxide film  230 A or the oxide film  230 B depending on with characteristics required for the oxide film  230 D. In this embodiment, the oxide film  230 D is deposited by a sputtering method using an oxide target with In:Ga:Zn=1:3:4 [atomic ratio]. 
     Part of oxygen contained in the sputtering gas is sometimes supplied to the oxide film  230 C during the deposition of the oxide film  230 D. Alternatively, during the deposition of the oxide film  230 D, part of oxygen contained in the sputtering gas is supplied to the insulator  280  in some cases. Therefore, the proportion of oxygen contained in the sputtering gas for the oxide film  230 D is higher than or equal to 70%, preferably higher than or equal to 80%, further preferably 100%. 
     Next, the oxide film  230 C and the oxide film  230 D on the side surface of the insulator  280 , the side surface of the insulator  272 , and the side surface of the insulator  271  in the opening are removed by a dry etching method. At the same time, the oxide film  230 C and the oxide film  230 D over the insulator  280  are also removed. The oxide film  230 C and the oxide film  230 D are left on the bottom portion of the opening, whereby the oxide  230   c  and the oxide  230   d  are formed (see  FIG. 12A  to  FIG. 12D ). 
     The above processing is possible when dry etching conditions are appropriately set. For example, a pressure at the dry etching is set high. Specifically, the pressure at the dry etching is preferably higher than or equal to 66.5 Pa, further preferably higher than or equal to 133 Pa. With such a pressure, reactive ions, which are etching species, move in random directions due to a short mean free path of the reactive ions in plasma, and thus the reactive ions are less likely to reach the bottom portion of the opening in some cases. Accordingly, the oxide film  230 C and the oxide film  230 D on the bottom portion of the opening can be inhibited from being removed to be left. 
     Here, a method for forming the oxide  230   c  and the oxide  230   d , which is different from the above method, is described below. The oxide film  230 C and the oxide film  230 D are each deposited by an ionization sputtering method. An ionization sputtering method is a deposition method in which sputtering particles are ionized by applying a high-frequency power or the like to the sputtering particles. With the use of the ionization sputtering method, the films are deposited mainly on the bottom portion of the opening and over the insulator  280 , whereby the thicknesses of the films on the bottom portion of the opening can be made larger than the thicknesses of the films on the side surface of the opening. 
     Next, the oxide film  230 C and the oxide film  230 D on the bottom portion of the opening are left with the use of the above-described dry etching method, so that the oxide  230   c  and the oxide  230   d  are formed. A semiconductor device including the transistor  200  manufactured in this manner is illustrated in  FIG. 4A  to  FIG. 4D . 
     Next, an insulating film  250 A is deposited (see  FIG. 13A  to  FIG. 13D ). Heat treatment may be performed before the deposition of the insulating film  250 A. It is preferable that the heat treatment be performed under reduced pressure and the insulating film  250 A be successively deposited without exposure to the air. The heat treatment is preferably performed in an atmosphere containing oxygen. Such treatment can remove moisture and hydrogen adsorbed onto the surface of the oxide  230   d  or the like and can reduce the moisture concentration and the hydrogen concentration in the oxide  230   a , the oxide  230   b , the oxide  230   c , and the oxide  230   d . The heat treatment is preferably performed at a temperature higher than or equal to 100° C. and lower than or equal to 400° C. 
     The insulating film  250 A can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. The insulating film  250 A is preferably deposited by a deposition method using a gas in which hydrogen atoms are reduced or removed. This can reduce the hydrogen concentration in the insulating film  250 A. The hydrogen concentration in the insulating film  250 A is preferably reduced because the insulating film  250 A becomes the insulator  250  that is in contact with the oxide  230   d  in a later step. 
     In the case where the insulator  250  has a stacked-layer structure including two layers, it is preferable that a lower layer of the insulator  250  be formed using an insulator from which oxygen is released by heating and an upper layer of the insulator  250  be formed using an insulator having a function of inhibiting diffusion of oxygen. With such a structure, oxygen contained in the lower layer of the insulator  250  can be inhibited from being diffused into the conductor  260  to be formed in a later step. That is, reduction in the amount of oxygen supplied to the oxide  230  can be inhibited. In addition, oxidation of the conductor  260  due to oxygen contained in the lower layer of the insulator  250  can be inhibited. For example, the lower layer of the insulator  250  can be formed using the above-described material that can be used for the insulator  250 , and the upper layer of the insulator  250  can be formed using a material similar to that for the insulator  222 . 
     In the case where silicon oxide, silicon oxynitride, or the like is used for the lower layer of the insulator  250 , the upper layer of the insulator  250  may be formed using an insulating material that is a high-k material having a high relative permittivity. The gate insulator having a stacked-layer structure of the lower layer of the insulator  250  and the upper layer of the insulator  250  can be thermally stable and can have a high relative permittivity. Thus, a gate potential that is applied during operation of the transistor can be reduced while the physical thickness of the gate insulator is maintained. Furthermore, the equivalent oxide thickness (EOT) of the insulator functioning as the gate insulator can be reduced. 
     Alternatively, an insulator having a function of inhibiting oxygen diffusion may be formed as the insulator serving as the lower layer of the insulator  250  and an insulator from which oxygen is released by heating may be formed as the upper layer of the insulator  250 . With such a structure, oxygen contained in the insulator  280  can be inhibited from being diffused into the conductor  260  to be formed in a later step. Moreover, an insulator having a function of inhibiting passage of impurities such as water and hydrogen is preferably formed for the lower layer of the insulator  250 . With such a structure, diffusion of impurities such as water and hydrogen contained in the insulator  280  into the oxide  230  can be inhibited. 
     An insulator containing an oxide of one or both of aluminum and hafnium is preferably deposited as the insulator having a barrier property against oxygen, hydrogen, and water. Note that as the insulator containing an oxide of one or both of aluminum and hafnium, aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), or the like is preferably used. The above insulator is preferably deposited by a thermal ALD method using an inorganic precursor. An insulator with reduced hydrogen can be deposited by this method. 
     Here, after the insulating film  250 A is deposited, microwave treatment may be performed in an atmosphere containing oxygen under reduced pressure. By performing the microwave treatment, an electric field from microwaves is supplied to the insulating film  250 A, the oxide film  230 D, the oxide film  230 C, the oxide  230   b , and the oxide  230   a , so that VoH in the oxide film  230 D, the oxide film  230 C, the oxide  230   b , and the oxide  230   a  can be divided into V O  and hydrogen. Part of hydrogen divided at this time is bonded to oxygen and is removed as H 2 O from the insulating film  250 A, the oxide film  230 D, the oxide film  230 C, the oxide  230   b , and the oxide  230   a  in some cases. Part of hydrogen is gettered by the conductor  242  (the conductor  242   a  and the conductor  242   b ) in some cases. Performing the microwave treatment in such a manner can reduce the hydrogen concentration in the insulating film  250 A, the oxide film  230 D, the oxide film  230 C, the oxide  230   b , and the oxide  230   a . Furthermore, oxygen is supplied to V O  that can exist after VoH in the oxide  230   a , the oxide  230   b , the oxide film  230 C, and the oxide film  230 D is divided into V O  and hydrogen, so that V O  can be repaired or filled. 
     After the microwave treatment, heat treatment may be performed with the reduced pressure being maintained. Such treatment can remove hydrogen in the insulating film  250 A, the oxide film  230 D, the oxide film  230 C, the oxide  230   b , and the oxide  230   a  efficiently. Part of hydrogen is gettered by the conductor  242  (the conductor  242   a  and the conductor  242   b ) in some cases. Alternatively, it is possible to repeat the step of performing microwave treatment and the step of performing heat treatment with the reduced pressure being maintained after the microwave treatment. Repetitions of the heat treatment can remove hydrogen in the insulating film  250 A, the oxide film  230 D, the oxide film  230 C, the oxide  230   b , and the oxide  230   a  more efficiently. Note that the temperature of the heat treatment is preferably higher than or equal to 300° C. and lower than or equal to 500° C. 
     Furthermore, the microwave treatment improves the film quality of the insulating film  250 A, thereby inhibiting diffusion of hydrogen, water, impurities, and the like. Accordingly, hydrogen, water, impurities, and the like can be inhibited from being diffused into the oxide  230   b , the oxide  230   a , and the like through the insulator  250  in a later step such as deposition of a conductive film to be the conductor  260  or a later treatment such as heat treatment. 
     Next, a conductive film  260 A and a conductive film  260 B are deposited in this order (see  FIG. 14A  to  FIG. 14D ). The conductive film  260 A and the conductive film  260 B can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, the conductive film  260 A is deposited by an ALD method, and the conductive film  260 B is successively deposited by a CVD method without exposure to the air. 
     Then, the oxide film  230 C, the oxide film  230 D, the insulating film  250 A, the conductive film  260 A, and the conductive film  260 B are polished by CMP treatment until the insulator  280  is exposed, whereby the oxide  230   c , the oxide  230   d , the insulator  250 , and the conductor  260  (the conductor  260   a  and the conductor  260   b ) are formed (see  FIG. 15A  to  FIG. 15D ). Accordingly, the oxide  230   c  is positioned to cover the inner walls (the side wall and the bottom surface) of the opening reaching the oxide  230   b  and the groove portion of the oxide  230   b . The oxide  230   d  is positioned to cover the inner walls of the opening and the groove portion with the oxide  230   c  therebetween. The insulator  250  is positioned to cover the inner walls of the opening and the groove portion with the oxide  230   d  therebetween. The conductor  260  is positioned to fill the opening and the groove portion with the oxide  230   c , the oxide  230   d , and the insulator  250  therebetween. 
     Then, heat treatment may be performed under conditions similar to those of the above heat treatment. In this embodiment, treatment is performed at 400° C. in a nitrogen atmosphere for one hour. The heat treatment can reduce the moisture concentration and the hydrogen concentration in the insulator  250  and the insulator  280 . After the heat treatment, the insulator  282  may be successively deposited without exposure to the air. 
     Next, the insulator  282  is formed over the oxide  230   d , the oxide  230   c , the insulator  250 , the conductor  260 , and the insulator  280  (see  FIG. 16A  to  FIG. 16D ). The insulator  282  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. Aluminum oxide is preferably deposited for the insulator  282  by a sputtering method, for example. The insulator  282  is deposited by a sputtering method in an oxygen-containing atmosphere, whereby oxygen can be added to the insulator  280  during the deposition. Thus, excess oxygen can be contained in the insulator  280 . At this time, the insulator  282  is preferably deposited while the substrate is being heated. It is preferable to form the insulator  282  in contact with the top surface of the conductor  260  because oxygen contained in the insulator  280  can be inhibited from being absorbed by the conductor  260  in later heat treatment. 
     Next, part of the insulator  282 , part of the insulator  280 , part of the insulator  272 , part of the insulator  224 , part of the insulator  222 , part of the insulator  216 , and part of the insulator  214  are processed to form an opening reaching the insulator  212  (see  FIG. 17A  to  FIG. 17D ). The opening is formed to surround the transistor  200  in some cases. In other cases, the opening is formed to surround the plurality of the transistors  200 . Accordingly, part of the side surface of the insulator  282 , part of the side surface of the insulator  280 , part of the side surface of the insulator  272 , part of the side surface of the insulator  224 , part of the side surface of the insulator  222 , part of the side surface of the insulator  216 , and part of the side surface of the insulator  214  are exposed in the opening. 
     The part of the insulator  282 , the part of the insulator  280 , the part of the insulator  272 , the part of the insulator  224 , the part of the insulator  222 , the part of the insulator  216 , and the part of the insulator  214  can be processed by a dry etching method or a wet etching method. Processing by a dry etching method is suitable for microfabrication. The processing may be performed under different conditions. 
     Then, the insulator  283  is formed to cover the insulator  282 , the insulator  280 , the insulator  224 , the insulator  222 , the insulator  216 , and the insulator  214  (see  FIG. 18A  to  FIG. 18D ). The insulator  283  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In this embodiment, silicon nitride is deposited by a sputtering method. As shown in  FIG. 18B  to  FIG. 18D , the insulator  283  is in contact with the insulator  212  at the bottom surface of the opening. That is, the top surface and the side surface of the transistor  200  are surrounded by the insulator  283  and the bottom surface of the transistor  200  is surrounded by the insulator  212 . Surrounding the transistor  200  by the insulator  283  and the insulator  212  having high barrier properties can prevent entry of moisture and hydrogen from the outside. 
     Then, an insulating film to be the insulator  274  is deposited over the insulator  283 . The insulating film to be the insulator  274  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For example, silicon nitride is preferably deposited by a CVD method. The insulating film to be the insulator  274  is preferably deposited by a deposition method using the gas in which hydrogen atoms are reduced or removed. Thus, the hydrogen concentration in the insulating film to be the insulator  274  can be reduced. 
     Next, the insulating film to be the insulator  274  is subjected to CMP treatment, whereby the insulator  274  having a flat top surface is formed (see  FIG. 19A  to  FIG. 19D ). 
     Subsequently, openings reaching the conductor  242  are formed in the insulator  271 , the insulator  272 , the insulator  280 , the insulator  282 , and the insulator  283  (see  FIG. 20A  to  FIG. 20D ). The openings are formed by a lithography method. Note that the openings in the top view in  FIG. 20A  each have a circular shape; however, the shapes of the openings are not limited thereto. For example, the openings in the top view may each have an almost circular shape such as an elliptical shape, a polygonal shape such as a quadrangular shape, or a polygonal shape such as a quadrangular shape with rounded corners. 
     Subsequently, an insulating film to be the insulator  241  is deposited and the insulating film is subjected to anisotropic etching, so that the insulator  241  is formed (see  FIG. 20A  to  FIG. 20D ). The insulating film to be the insulator  241  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. The insulating film to be the insulator  241  preferably has a function of inhibiting passage of oxygen. For example, aluminum oxide is preferably deposited by an ALD method. Alternatively, silicon nitride is preferably deposited by a PEALD method. Silicon nitride is preferable because it has a high blocking property against hydrogen. 
     As an anisotropic etching for the insulating film to be the insulator  241 , a dry etching method may be performed, for example. When the insulator  241  is provided on the side surfaces of the openings, passage of oxygen from the outside can be inhibited and oxidation of the conductor  240   a  and the conductor  240   b  to be formed next can be prevented. Furthermore, impurities such as water and hydrogen can be prevented from diffusing from the conductor  240   a  and the conductor  240   b  to the outside. 
     Next, a conductive film to be the conductor  240  is deposited. The conductive film to be the conductor  240  desirably has a stacked-layer structure which includes a conductor having a function of inhibiting passage of impurities such as water and hydrogen. In the case where the conductive film to be the conductor  240  has a stacked-layer structure, tantalum nitride, titanium nitride, or the like can be used for a film in the lower layer. Moreover, tungsten, molybdenum, copper, or the like can be used for a film in the upper layer, for example. The conductive film to be the conductor  240  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     It is particularly preferable to deposit the lower layer of the conductive film to be the conductor  240  by an ionization sputtering method. Deposition by an ion sputtering method enables uniform deposition of a film on the bottom portion and the side surface of an opening. 
     Uniform deposition of the lower layer of the conductive film to be the conductor  240  is preferable because the lower layer of the conductive film to be the conductor  240  favorably functions as a seed layer of the upper layer of the conductive film to be the conductor  240 . 
     Then, part of the conductive film to be the conductor  240   a  and the conductor  240   b  is removed by CMP treatment to expose the top surfaces of the insulator  283  and the insulator  274 . As a result, the conductive film remains only in the openings, so that the conductor  240   a  and the conductor  240   b  having flat top surfaces can be formed (see  FIG. 20A  to  FIG. 20D ). Note that the top surface of the insulator  283  and the top surface of the insulator  274  are partly removed by the CMP treatment in some cases. 
     Next, a conductive film to be the conductor  246  is deposited. The conductive film to be the conductor  246  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Then, the conductive film to be the conductor  246  is processed by a lithography method, thereby forming the conductor  246   a  in contact with the top surface of the conductor  240   a  and the conductor  246   b  in contact with the top surface of the conductor  240   b  (see  FIG. 21A  to  FIG. 21D ). At this time, part of the insulator  283  in a region where the conductor  246   a  and the conductor  246   b  do not overlap with the insulator  283  is sometimes removed, which is not shown. 
     Next, the insulator  286  is deposited over the conductor  246  and the insulator  283  (see  FIG. 3A  to  FIG. 3D ). The insulator  286  can be deposited by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. In addition, the insulator  286  may have a multilayer structure. For example, silicon nitride may be deposited by a sputtering method and silicon nitride may be deposited by a CVD method over the silicon nitride. 
     Through the above process, the semiconductor device including the transistor  200  shown in  FIG. 3A  to  FIG. 3D  can be manufactured. As shown in  FIG. 5A  to  FIG. 21D , the transistor  200  can be manufactured with the use of the method for manufacturing the semiconductor device described in this embodiment. When the semiconductor device including the transistor  200  shown in  FIG. 1A  to  FIG. 1D  is manufactured, the semiconductor device can be manufactured without the processes shown in  FIG. 17A  to  FIG. 19D . 
     &lt;Application Example of Semiconductor Device&gt; 
     Examples of a semiconductor device including the transistor  200  of one embodiment of the present invention which is different from the semiconductor device described in the above &lt;Structure example of semiconductor device&gt; and the above &lt;Modification example of semiconductor device&gt; will be described below with reference to  FIG. 22A  and  FIG. 22B . Note that in the semiconductor devices illustrated in  FIG. 22A  and  FIG. 22B , structures having the same functions as the structures in the semiconductor device described in &lt;Modification example of semiconductor device&gt; (see  FIG. 3A  to  FIG. 3D ) are denoted by the same reference numerals. Note that also in this section, the materials described in detail in &lt;Structure example of semiconductor device&gt; and &lt;Modification example of semiconductor device&gt; can be used as the materials for the transistor  200 . 
       FIG. 22A  and  FIG. 22B  each show a structure in which a plurality of transistors  200 _ 1  to  200 _ n  are sealed with the insulator  283  and the insulator  212 . Note that although the transistor  200 _ 1  to the transistor  200 _ n  appear to be arranged in the channel length direction in  FIG. 22A  and  FIG. 22B , the present invention is not limited thereto. The transistor  200 _ 1  to the transistor  200 _ n  may be arranged in the channel width direction or may be arranged in a matrix. Depending on the design, the transistors may be arranged without regularity. 
     As shown in  FIG. 22A , a portion where the insulator  283  is in contact with the insulator  212  (hereinafter, sometimes referred to as a sealing portion  265 ) is formed outside the plurality of transistors  200 _ 1  to  200 _ n . The sealing portion  265  is formed to surround the plurality of transistors  200 _ 1  to  200 _ n . Such a structure enables the plurality of transistors  200 _ 1  to  200 _ n  to be surrounded by the insulator  283  and the insulator  212 . Thus, a plurality of transistor groups surrounded by the sealing portion  265  are provided over a substrate. 
     A dicing line (sometimes referred to as a scribe line, a dividing line, or a cutting line) may be provided to overlap with the sealing portion  265 . The above substrate is divided at the dicing line, so that the transistor group surrounded by the sealing portion  265  is taken out as one chip. 
     Although the plurality of transistors  200 _ 1  to  200 _ n  are surrounded by one sealing portion  265  in the example shown in  FIG. 22A , the present invention is not limited thereto. As shown in  FIG. 22B , the plurality of transistors  200 _ 1  to  200 _ n  may be surrounded by a plurality of sealing portions. In  FIG. 22B , the plurality of transistors  200 _ 1  to  200 _ n  are surrounded by a sealing portion  265   a  and are further surrounded by an outer sealing portion  265   b.    
     When the plurality of transistors  200 _ 1  to  200 _ n  are surrounded by the plurality of sealing portions in this manner, a portion where the insulator  283  is in contact with the insulator  212  increases, which further can improve adhesion between the insulator  283  and the insulator  212 . As a result, the plurality of transistors  200 _ 1  to  200 _ n  can be more reliably sealed. 
     In that case, a dicing line may be provided to overlap with the sealing portion  265   a  or the sealing portion  265   b , or may be provided between the sealing portion  265   a  and the sealing portion  265   b.    
     One embodiment of the present invention can provide a semiconductor device in which variation of transistor characteristics is small. Another embodiment of the present invention can provide a semiconductor device with favorable reliability. Another embodiment of the present invention can provide a semiconductor device having favorable electrical characteristics. Another embodiment of the present invention can provide a semiconductor device with a high on-state current. Another embodiment of the present invention can provide a semiconductor device that can be miniaturized or highly integrated. Another embodiment of the present invention can provide a semiconductor device with low power consumption. 
     The structure, method, and the like described above in this embodiment can be used in an appropriate combination with the structures, the methods, and the like described in the other embodiments and the like. 
     Embodiment 2 
     In this embodiment, one embodiment of a semiconductor device is described with reference to  FIG. 23  to  FIG. 27 . 
     [Storage Device  1 ] 
       FIG. 23  shows an example of a semiconductor device (a storage device) of one embodiment of the present invention. The semiconductor device of one embodiment of the present invention includes the transistor  200 , a transistor  300 , and a capacitor  100 . The transistor  200  is provided above the transistor  300 , and the capacitor  100  is provided above the transistor  300  and the transistor  200 . The transistor  200  described in the above embodiment can be used as the transistor  200 . 
     The transistor  200  is a transistor in which a channel is formed in a semiconductor layer containing an oxide semiconductor. The off-state current of the transistor  200  is low; thus, by using the transistor  200  in a storage device, stored data can be retained for a long time. In other words, such a storage device does not require refresh operation or has extremely low frequency of the refresh operation, which leads to a sufficient reduction in power consumption of the storage device. 
     In the semiconductor device shown in  FIG. 23 , a wiring  1001  is electrically connected to a source of the transistor  300 , and a wiring  1002  is electrically connected to a drain of the transistor  300 . In addition, a wiring  1003  is electrically connected to one of the source and the drain of the transistor  200 , a wiring  1004  is electrically connected to the first gate of the transistor  200 , and a wiring  1006  is electrically connected to the second gate of the transistor  200 . A gate of the transistor  300  and the other of the source and the drain of the transistor  200  are electrically connected to one electrode of the capacitor  100 , and a wiring  1005  is electrically connected to the other electrode of the capacitor  100 . 
     The storage devices shown in  FIG. 23  can form a memory cell array when arranged in a matrix. 
     &lt;Transistor  300 &gt; 
     The transistor  300  is provided on a substrate  311  and includes a conductor  316  functioning as a gate, an insulator  315  functioning as a gate insulator, a semiconductor region  313  formed of part of the substrate  311 , and a low-resistance region  314   a  and a low-resistance region  314   b  functioning as a source region and a drain region. The transistor  300  may be a p-channel transistor or an n-channel transistor. 
     Here, in the transistor  300  shown in  FIG. 23 , the semiconductor region  313  (part of the substrate  311 ) in which a channel is formed has a protruding shape. In addition, the conductor  316  is provided to cover the side surface and the top surface of the semiconductor region  313  with the insulator  315  therebetween. Note that a material adjusting the work function may be used for the conductor  316 . Such a transistor  300  is also referred to as a FIN-type transistor because it utilizes a protruding portion of a semiconductor substrate. Note that an insulator functioning as a mask for forming the protruding portion may be included in contact with an upper portion of the protruding portion. Furthermore, although the case where the protruding portion is formed by processing part of the semiconductor substrate is described here, a semiconductor film having a protruding shape may be formed by processing an SOI substrate. 
     Note that the transistor  300  shown in  FIG. 23  is an example and the structure is not limited thereto; an appropriate transistor is used in accordance with a circuit structure or a driving method. 
     &lt;Capacitor  100 &gt; 
     The capacitor  100  is provided above the transistor  200 . The capacitor  100  includes a conductor  110  functioning as a first electrode, a conductor  120  functioning as a second electrode, and an insulator  130  functioning as a dielectric. Here, for the insulator  130 , the insulator that can be used for the insulator  286  described in the above embodiment is preferably used. 
     For example, a conductor  112  and the conductor  110  over the conductor  240  can be formed at the same time. Note that the conductor  112  functions as a plug or a wiring that is electrically connected to the capacitor  100 , the transistor  200 , or the transistor  300 . 
     Although the conductor  112  and the conductor  110  having a single-layer structure are shown in  FIG. 23 , the structure is not limited thereto; a stacked-layer structure of two or more layers may be employed. For example, between a conductor having a barrier property and a conductor having high conductivity, a conductor that is highly adhesive to the conductor having a barrier property and the conductor having high conductivity may be formed. 
     For the insulator  130 , for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitride oxide, hafnium nitride, or the like is used, and a stacked layer or a single layer can be provided. 
     For example, for the insulator  130 , a stacked-layer structure using a material with high dielectric strength such as silicon oxynitride and a high permittivity (high-k) material is preferably used. In the capacitor  100  having such a structure, a sufficient capacitance can be ensured owing to the high permittivity (high-k) insulator, and the dielectric strength can be increased owing to the insulator with high dielectric strength, so that the electrostatic breakdown of the capacitor  100  can be inhibited. 
     As the insulator of a high permittivity (high-k) material (a material having a high relative permittivity), gallium oxide, hafnium oxide, zirconium oxide, an oxide containing aluminum and hafnium, an oxynitride containing aluminum and hafnium, an oxide containing silicon and hafnium, an oxynitride containing silicon and hafnium, a nitride containing silicon and hafnium, or the like can be given. 
     As the material having a high dielectric strength (a material having a low relative permittivity), silicon oxide, silicon oxynitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, a resin, or the like can be given. 
     &lt;Wiring Layer&gt; 
     Wiring layers provided with an interlayer film, a wiring, a plug, and the like may be provided between the components. In addition, a plurality of wiring layers can be provided in accordance with design. Here, a plurality of conductors functioning as plugs or wirings are collectively denoted by the same reference numeral in some cases. Furthermore, in this specification and the like, a wiring and a plug electrically connected to the wiring may be a single component. That is, there are cases where part of a conductor functions as a wiring and another part of the conductor functions as a plug. 
     For example, an insulator  320 , an insulator  322 , an insulator  324 , and an insulator  326  are sequentially stacked over the transistor  300  as interlayer films. A conductor  328 , a conductor  330 , and the like that are electrically connected to the capacitor  100  or the transistor  200  are embedded in the insulator  320 , the insulator  322 , the insulator  324 , and the insulator  326 . Note that the conductor  328  and the conductor  330  function as a plug or a wiring. 
     The insulators functioning as interlayer films may also function as planarization films that cover uneven shapes therebelow. For example, the top surface of the insulator  322  may be planarized by planarization treatment using a chemical mechanical polishing (CMP) method or the like to increase planarity. 
     A wiring layer may be provided over the insulator  326  and the conductor  330 . For example, in  FIG. 23 , an insulator  350 , an insulator  352 , and an insulator  354  are stacked sequentially. Furthermore, a conductor  356  is formed in the insulator  350 , the insulator  352 , and the insulator  354 . The conductor  356  functions as a plug or a wiring. 
     Similarly, a conductor  218 , a conductor (the conductor  205 ) included in the transistor  200 , and the like are embedded in an insulator  210 , the insulator  212 , the insulator  214 , and the insulator  216 . Note that the conductor  218  functions as a plug or a wiring that is electrically connected to the capacitor  100  or the transistor  300 . In addition, an insulator  150  is provided over the conductor  120  and the insulator  130 . 
     Here, like the insulator  241  described in the above embodiment, an insulator  217  is provided in contact with the side surface of the conductor  218  functioning as a plug. The insulator  217  is provided in contact with the inner wall of an opening formed in the insulator  210 , the insulator  212 , the insulator  214 , and the insulator  216 . That is, the insulator  217  is provided between the conductor  218  and the insulator  210 , the insulator  212 , the insulator  214 , and the insulator  216 . Note that the conductor  205  and the conductor  218  can be formed in parallel; thus, the insulator  217  is sometimes formed in contact with the side surface of the conductor  205 . 
     For the insulator  217 , an insulator such as silicon nitride, aluminum oxide, or silicon nitride oxide may be used. Since the insulator  217  is provided in contact with the insulator  210 , the insulator  212 , the insulator  214 , the insulator  216 , and the insulator  222 , the entry of impurities such as water and hydrogen into the oxide  230  through the conductor  218  from the insulator  210 , the insulator  216 , or the like can be inhibited. In particular, silicon nitride is suitable because of having a high blocking property against hydrogen. Moreover, oxygen contained in the insulator  210  or the insulator  216  can be prevented from being absorbed by the conductor  218 . 
     The insulator  217  can be formed in a manner similar to that of the insulator  241 . For example, silicon nitride is deposited by a PEALD method and an opening reaching the conductor  356  is formed by anisotropic etching. 
     As an insulator that can be used for an interlayer film, an insulating oxide, an insulating nitride, an insulating oxynitride, an insulating nitride oxide, an insulating metal oxide, an insulating metal oxynitride, an insulating metal nitride oxide, or the like is given. 
     For example, when a material having a low relative permittivity is used for the insulator functioning as an interlayer film, parasitic capacitance generated between wirings can be reduced. Thus, a material is preferably selected depending on the function of an insulator. 
     For example, the insulator  150 , the insulator  280 , the insulator  210 , the insulator  352 , the insulator  354 , and the like preferably include an insulator having a low relative permittivity. For example, the insulator preferably includes silicon nitride oxide, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, a resin, or the like. Alternatively, the insulator preferably has a stacked-layer structure of a resin and silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, or porous silicon oxide. When silicon oxide or silicon oxynitride, which is thermally stable, is combined with a resin, the stacked-layer structure can have thermal stability and a low relative permittivity. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon and aramid), polyimide, polycarbonate, and acrylic. 
     When a transistor using an oxide semiconductor is surrounded by an insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen, the electrical characteristics of the transistor can be stable. Thus, the insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen can be used for the insulator  214 , the insulator  212 , the insulator  350 , and the like. 
     As the insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen, a single layer or stacked layers of an insulator containing, for example, boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum are used. Specifically, as the insulator having a function of inhibiting passage of oxygen and impurities such as hydrogen, a metal oxide such as aluminum oxide, magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide; silicon nitride oxide; silicon nitride; or the like can be used. 
     As the conductor that can be used for a wiring or a plug, a material containing one or more kinds of metal elements selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium, ruthenium, and the like can be used. A semiconductor having a high electrical conductivity, typified by polycrystalline silicon containing an impurity element such as phosphorus, or silicide such as nickel silicide may be used. 
     For example, for the conductor  328 , the conductor  330 , the conductor  356 , the conductor  218 , the conductor  240 , the conductor  112 , and the like, a single-layer structure or a stacked-layer structure using a conductive material such as a metal material, an alloy material, a metal nitride material, or a metal oxide material that is formed using the above materials can be used. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is preferable to use tungsten. Alternatively, it is preferable to form the plugs and the wirings with a low-resistance conductive material such as aluminum or copper. The use of a low-resistance conductive material can reduce wiring resistance. 
     &lt;Wiring or Plug in Layer Provided with Oxide Semiconductor&gt; 
     In the case where an oxide semiconductor is used in the transistor  200 , an insulator including an excess-oxygen region is provided in the vicinity of the oxide semiconductor in some cases. In that case, an insulator having a barrier property is preferably provided between the insulator including the excess-oxygen region and a conductor provided in the insulator including the excess-oxygen region. 
     For example, the insulator  241  is preferably provided between the conductor  240  and the insulator  224  and the insulator  280  that include excess oxygen in  FIG. 23 . Since the insulator  241  is provided in contact with the insulator  222 , the insulator  282 , and the insulator  283 , the insulator  224  and the transistor  200  can be sealed with the insulators having a barrier property. 
     That is, the insulator  241  can inhibit excess oxygen contained in the insulator  224  and the insulator  280  from being absorbed by the conductor  240 . In addition, diffusion of hydrogen, which is an impurity, into the transistor  200  through the conductor  240  can be inhibited when the insulator  241  is provided. 
     The insulator  241  is preferably formed using an insulating material having a function of inhibiting diffusion of impurities such as water and hydrogen and oxygen. For example, silicon nitride, silicon nitride oxide, aluminum oxide, hafnium oxide, or the like is preferably used. In particular, silicon nitride is preferably used because silicon nitride has a high blocking property against hydrogen. Other than that, a metal oxide such as magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, or tantalum oxide can be used, for example. 
     As in the above embodiment, the transistor  200  is preferably sealed with the insulator  212 , the insulator  214 , the insulator  282 , and the insulator  283 . Such a structure can inhibit entry of hydrogen contained in the insulator  274 , the insulator  150 , or the like into the insulator  280  or the like. 
     Here, the conductor  240  penetrates the insulator  283  and the insulator  282 , and the conductor  218  penetrates the insulator  214 , the insulator  212 , and the insulator  210 ; however, as described above, the insulator  241  is provided in contact with the conductor  240 , and the insulator  217  is provided in contact with the conductor  218 . This can reduce the amount of hydrogen entering the inside of the insulator  212 , the insulator  214 , the insulator  282 , and the insulator  283  through the conductor  240  and the conductor  218 . In this manner, the transistor  200  is sealed more surely with the insulator  212 , the insulator  214 , the insulator  282 , the insulator  283 , the insulator  241 , and the insulator  217 , so that impurities such as hydrogen contained in the insulator  274  or the like can be inhibited from entering from the outside. 
     As described in the above embodiment, the insulator  216 , the insulator  224 , the insulator  280 , the insulator  250 , and the insulator  274  are preferably formed by the deposition method using a gas in which hydrogen atoms are reduced or removed. Thus, the hydrogen concentrations in the insulator  216 , the insulator  224 , the insulator  280 , the insulator  250 , and the insulator  274  can be lowered. 
     In this manner, the hydrogen concentration in silicon-based insulating films in the vicinity of the transistor  200  can be reduced; thus, the hydrogen concentration in the oxide  230  can be reduced. 
     &lt;Dicing Line&gt; 
     A dicing line (sometimes referred to as a scribe line, a dividing line, or a cutting line) which is provided when a large-sized substrate is divided into semiconductor elements so that a plurality of semiconductor devices are each formed in a chip form is described below. Examples of a dividing method include the case where a groove (a dicing line) for dividing the semiconductor elements is formed on the substrate, and then the substrate is cut along the dicing line to divide (split) it into a plurality of semiconductor devices. 
     Here, for example, as shown in  FIG. 23 , a region in which the insulator  283  and the insulator  212  are in contact with each other is preferably designed to overlap with the dicing line. That is, an opening is provided in the insulator  282 , the insulator  280 , the insulator  272 , the insulator  224 , the insulator  222 , the insulator  216 , and the insulator  214  in the vicinity of a region to be the dicing line that is provided on an outer edge of the memory cell including the plurality of transistors  200 . 
     That is, in the opening provided in the insulator  282 , the insulator  280 , the insulator  272 , the insulator  224 , the insulator  222 , the insulator  216 , and the insulator  214 , the insulator  212  is in contact with the insulator  283 . When the insulator  212  and the insulator  283  are formed using the same material and the same method, the adhesion therebetween can be increased. For example, silicon nitride is preferably used. 
     With such a structure, the transistors  200  can be surrounded by the insulator  212 , the insulator  214 , the insulator  282 , and the insulator  283 . Since at least one of the insulator  212 , the insulator  214 , the insulator  282 , and the insulator  283  has a function of inhibiting diffusion of oxygen, hydrogen, and water, even when the substrate is divided into circuit regions each of which is provided with the semiconductor elements described in this embodiment to be processed into a plurality of chips, entry and diffusion of impurities such as hydrogen and water from the direction of the side surface of the divided substrate to the transistor  200  can be inhibited. 
     With the structure, excess oxygen in the insulator  280  and the insulator  224  can be prevented from diffusing to the outside. Accordingly, excess oxygen in the insulator  280  and the insulator  224  is efficiently supplied to the oxide where the channel is formed in the transistor  200 . The oxygen can reduce oxygen vacancies in the oxide where the channel is formed in the transistor  200 . Thus, the oxide where the channel is formed in the transistor  200  can be an oxide semiconductor with a low density of defect states and stable characteristics. That is, the transistor  200  can have a small variation in the electrical characteristics and higher reliability. 
     Note that although the capacitor  100  of the storage device shown in  FIG. 23  has a planar shape, the storage device described in this embodiment is not limited thereto. For example, the capacitor  100  may have a cylindrical shape as shown in  FIG. 24 . Note that the structure below and including the insulator  150  of a storage device shown in  FIG. 24  is similar to that of the semiconductor device shown in  FIG. 23 . 
     The capacitor  100  shown in  FIG. 24  includes the insulator  150  over the insulator  130 , an insulator  142  over the insulator  150 , a conductor  115  positioned in an opening formed in the insulator  150  and the insulator  142 , an insulator  145  over the conductor  115  and the insulator  142 , a conductor  125  over the insulator  145 , and an insulator  152  over the conductor  125  and the insulator  145 . Here, at least parts of the conductor  115 , the insulator  145 , and the conductor  125  are positioned in the opening formed in the insulator  150  and the insulator  142 . 
     The conductor  115  functions as a lower electrode of the capacitor  100 , the conductor  125  functions as an upper electrode of the capacitor  100 , and the insulator  145  functions as a dielectric of the capacitor  100 . The capacitor  100  has a structure in which the upper electrode and the lower electrode face each other with the dielectric positioned therebetween on the side surface as well as the bottom surface of the opening in the insulator  150  and the insulator  142 ; thus, the capacitance per unit area can be increased. Thus, the deeper the opening is, the larger the capacitance of the capacitor  100  can be. Increasing the capacitance per unit area of the capacitor  100  in this manner can promote miniaturization or higher integration of the semiconductor device. 
     An insulator that can be used for the insulator  280  can be used for the insulator  152 . The insulator  142  preferably functions as an etching stopper at the time of forming the opening in the insulator  150  and is formed using an insulator that can be used for the insulator  214 . 
     The shape of the opening formed in the insulator  150  and the insulator  142  when seen from above may be a quadrangular shape, a polygonal shape other than a quadrangular shape, a polygonal shape with rounded corners, or a circular shape including an elliptical shape. Here, the area where the opening and the transistor  200  overlap with each other is preferably large in the top view. Such a structure can reduce the area occupied by the semiconductor device including the capacitor  100  and the transistor  200 . 
     The conductor  115  is positioned in contact with the opening formed in the insulator  142  and the insulator  150 . The top surface of the conductor  115  is preferably substantially level with the top surface of the insulator  142 . Furthermore, the bottom surface of the conductor  115  is in contact with the conductor  110  through an opening in the insulator  130 . The conductor  115  is preferably deposited by an ALD method, a CVD method, or the like; for example, a conductor that can be used for the conductor  205  is used. 
     The insulator  145  is positioned to cover the conductor  115  and the insulator  142 . The insulator  145  is preferably deposited by an ALD method or a CVD method, for example. The insulator  145  can be provided to have stacked layers or a single layer using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, zirconium oxide, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitride oxide, or hafnium nitride. As the insulator  145 , an insulating film in which zirconium oxide, aluminum oxide, and zirconium oxide are stacked in this order can be used, for example. 
     For the insulator  145 , a material with s high dielectric strength, such as silicon oxynitride, or a high permittivity (high-k) material is preferably used. Alternatively, a stacked-layer structure using a material with a high dielectric strength and a high permittivity (high-k) material may be employed. 
     As an insulator of a high permittivity (high-k) material (a material having a high relative permittivity), gallium oxide, hafnium oxide, zirconium oxide, an oxide containing aluminum and hafnium, an oxynitride containing aluminum and hafnium, an oxide containing silicon and hafnium, an oxynitride containing silicon and hafnium, a nitride containing silicon and hafnium, and the like can be given. The use of such a high-k material can ensure sufficient capacitance of the capacitor  100  even when the insulator  145  has a large thickness. When the insulator  145  has a large thickness, leakage current generated between the conductor  115  and the conductor  125  can be inhibited. 
     Examples of a material with a high dielectric strength include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, and a resin. For example, it is possible to use an insulating film in which silicon nitride (SiN x ) deposited by an ALD method, silicon oxide (SiO x ) deposited by a PEALD method, and silicon nitride (SiN x ) deposited by an ALD method are stacked in this order. The use of such an insulator with a high dielectric strength can increase the dielectric strength and inhibit electrostatic breakdown of the capacitor  100 . 
     The conductor  125  is positioned to fill the opening formed in the insulator  142  and the insulator  150 . The conductor  125  is electrically connected to the wiring  1005  through a conductor  140  and a conductor  153 . The conductor  125  is preferably deposited by an ALD method, a CVD method, or the like and is formed using a conductor that can be used for the conductor  205 , for example. 
     The conductor  153  is provided over an insulator  154  and is covered with an insulator  156 . The conductor  153  is formed using a conductor that can be used for the conductor  112 , and the insulator  156  is formed using an insulator that can be used for the insulator  152 . Here, the conductor  153  is in contact with the top surface of the conductor  140  and functions as a terminal of the capacitor  100 , the transistor  200 , or the transistor  300 . 
     [Storage Device  2 ] 
       FIG. 25  shows an example of a semiconductor device (a storage device) of one embodiment of the present invention. 
     &lt;Structure Example of Memory Device&gt; 
       FIG. 25  is a cross-sectional view of a semiconductor device  290  including a memory device. The semiconductor device  290  in  FIG. 25  includes a capacitor device  292  besides the transistor  200  shown in  FIG. 1A  to  FIG. 1D .  FIG. 25  corresponds to a cross-sectional view in the channel length direction of the transistor  200 . 
     The capacitor device  292  includes the conductor  242   b , the insulator  271   b  over the conductor  242   b , the insulator  272  provided in contact with the top surface of the insulator  271   b , the side surface of the insulator  271   b , and the side surface of the conductor  242   b , and a conductor  294  covering the insulator  272 . In other words, the capacitor device  292  forms a MIM (Metal-Insulator-Metal) capacitor. Note that one of a pair of electrodes included in the capacitor device  292 , i.e., the conductor  242   b , can also serve as the source electrode of the transistor. The dielectric layer of the capacitor device  292  can also serve as a protective layer provided in the transistor, i.e., the insulator  271  and the insulator  272 . Thus, the manufacturing process of the capacitor device  292  can also serve as part of the manufacturing process of the transistor; therefore, the productivity of the semiconductor device can be improved. Furthermore, the one of the pair of electrodes included in the capacitor device  292 , that is, the conductor  242   b , also serves as the source electrode of the transistor; therefore, the area in which the transistor and the capacitor device are positioned can be reduced. 
     Note that the conductor  294  can be formed using, for example, a material that can be used for the conductor  242 . 
     &lt;Modification Example of Memory Device&gt; 
     Examples of a semiconductor device of one embodiment of the present invention including the transistor  200  and the capacitor device  292 , which are different from the one described above in &lt;Structure example of memory device&gt;, will be described below with reference to  FIG. 26A ,  FIG. 26B ,  FIG. 27 , and  FIG. 28 . Note that in the semiconductor devices shown in  FIG. 26A ,  FIG. 26B ,  FIG. 27 , and  FIG. 28 , structures having the same function as those included in the semiconductor device described in the above embodiment and &lt;Structure example of memory device&gt; (see  FIG. 25 ) are denoted by the same reference numerals. Note that the materials described in detail in the above embodiment and &lt;Structure example of memory device&gt; can be used as constituent materials of the transistor  200  and the capacitor device  292  in this section. 
     Modification Example 1 of Memory Device 
     An example of a semiconductor device  600  of one embodiment of the present invention including a transistor  200   a , a transistor  200   b , a capacitor device  292   a , and a capacitor device  292   b  is described with reference to  FIG. 26A . 
       FIG. 26A  is a cross-sectional view of the semiconductor device  600  including the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b  in the channel length direction. The semiconductor device  600  includes the conductor  242   b , the insulator  271   b  provided over the conductor  242   b , the insulator  272  provided in contact with the top surface of the insulator  271   b , the side surface of the insulator  271   b , and the side surface of the conductor  242   b , and the conductor  294  provided to cover the insulator  272  are provided. The capacitor device  292   a  includes the conductor  242   a , the insulator  271   a  provided over the conductor  242   a , the insulator  272  provided in contact with the top surface of the insulator  271   a , the side surface of the insulator  271   a , and the side surface of the conductor  242   a , and a conductor  294   a  provided to cover the insulator  272 . The capacitor device  292   b  includes the conductor  242   b , the insulator  271   b  provided over the conductor  242   b , the insulator  272  provided in contact with the top surface of the insulator  271   b , the side surface of the insulator  271   b , and the side surface of the conductor  242   b , and a conductor  294   b  provided to cover the insulator  272 . 
     The semiconductor device  600  has a line-symmetric structure with respect to dashed-dotted line A 3 -A 4  as shown in  FIG. 26A . A conductor  242   c  serves as one of a source electrode and a drain electrode of the transistor  200   a  and one of a source electrode and a drain electrode of the transistor  200   b . Note that an insulator  271   c  is provided over the conductor  242   c . The conductor  240  functioning as a plug connects the conductor  246  functioning as a wiring to the transistor  200   a  and the transistor  200   b . Accordingly, when the connection of the two transistors, the two capacitor devices, the wiring, and the plug have the above-described structure, a semiconductor device that can be miniaturized or highly integrated can be provided. 
     The structure examples of the semiconductor device in  FIG. 1A  to  FIG. 1D  and  FIG. 25  can be referred to for the structures and the effects of the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b.    
     Modification Example 2 of Memory Device 
     In the above description, the semiconductor device including the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b  is given as a structure example; however, the semiconductor device of this embodiment is not limited thereto. For example, as shown in  FIG. 26B , a structure in which the semiconductor device  600  and a semiconductor device having a structure similar to that of the semiconductor device  600  are connected through a capacitor portion may be employed. In this specification, the semiconductor device including the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b  is referred to as a cell. For the structures of the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b , the above description of the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b  can be referred to. 
       FIG. 26B  is a cross-sectional view in which the semiconductor device  600  including the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b , and a cell having a structure similar to that of the semiconductor device  600  are connected through a capacitor portion. 
     As shown in  FIG. 26B , the conductor  294   b  functioning as one electrode of the capacitor device  292   b  included in the semiconductor device  600  also serves as one electrode of a capacitor device included in a semiconductor device  601  having a structure similar to that of the semiconductor device  600 . Although not shown, the conductor  294   a  functioning as one electrode of the capacitor device  292   a  included in the semiconductor device  600  also serves as one electrode of a capacitor device included in a semiconductor device on the left side of the semiconductor device  600 , that is, a semiconductor device adjacent to the semiconductor device  600  in the A 1  direction in  FIG. 26B . The cell on the right side of the semiconductor device  601 , that is, the cell in the A 2  direction in  FIG. 26B , has a similar structure. That is, a cell array (also referred to as a memory device layer) can be formed. With this structure of the cell array, the space between the adjacent cells can be reduced; thus, the projected area of the cell array can be reduced and high integration can be achieved. When the cells shown in  FIG. 26B  are arranged in a matrix, a matrix-shape cell array can be formed. 
     When the transistor  200   a , the transistor  200   b , the capacitor device  292   a , and the capacitor device  292   b  are formed to have the structures described in this embodiment as described above, the area of the cell can be reduced and the semiconductor device including a cell array can be miniaturized or highly integrated. 
     Furthermore, the cell array may have a stacked-layer structure instead of a single-layer structure.  FIG. 27  shows a cross-sectional view of n layers of cell arrays  610  that are stacked. When a plurality of cell arrays (a cell array  610 _ 1  to a cell array  610 _ n ) are stacked as shown in  FIG. 27 , cells can be integrally positioned without increasing the area occupied by the cell arrays. In other words, a 3D cell array can be formed. 
     Modification Example 3 of Memory Device 
       FIG. 28  shows an example in which a memory unit  470  includes a transistor layer  413  including a transistor  200 T and a memory device layer  415  of four layers (a memory device layer  415 _ 1  to a memory device layer  415 _ 4 ). 
     The memory device layer  415 _ 1  to the memory device layer  415 _ 4  each include a plurality of memory devices  420 . 
     The memory device  420  is electrically connected to the memory device  420  included in a different memory device layer  415  and the transistor  200 T included in the transistor layer  413  through a conductor  424  and the conductor  205 . 
     The memory unit  470  is sealed with the insulator  212 , the insulator  214 , the insulator  282 , and the insulator  283  (such a structure is referred to as a sealing structure below for convenience). The insulator  274  is provided in the periphery of the insulator  283 . A conductor  440  is provided in the insulator  274 , the insulator  283 , and the insulator  212 , and is electrically connected to an element layer  411 . 
     The insulator  280  is provided in the sealing structure. The insulator  280  has a function of releasing oxygen by heating. Alternatively, the insulator  280  includes an excess-oxygen region. 
     The insulator  212  and the insulator  283  are suitably formed using a material having a high blocking property against hydrogen. The insulator  214  and the insulator  282  are suitably formed using a material having a function of capturing hydrogen or fixing hydrogen. 
     Examples of the material having a high blocking property against hydrogen include silicon nitride and silicon nitride oxide. Examples of the material having a function of trapping or fixing hydrogen include aluminum oxide, hafnium oxide, and an oxide containing aluminum and hafnium (hafnium aluminate). 
     For the crystal structure of materials used for the insulator  212 , the insulator  214 , the insulator  282 , and the insulator  283 , an amorphous or crystalline structure may be employed, although the crystal structure is not limited thereto. For example, it is preferable to use an amorphous aluminum oxide film for the material having a function of trapping or fixing hydrogen. Amorphous aluminum oxide may trap or fix hydrogen more than aluminum oxide with high crystallinity. 
     Here, as the model of excess oxygen in the insulator  280  with respect to diffusion of hydrogen from an oxide semiconductor in contact with the insulator  280 , the following model can be given. 
     Hydrogen in the oxide semiconductor diffuses to other structure bodies through the insulator  280  in contact with the oxide semiconductor. The hydrogen in the oxide semiconductor react with the excess oxygen in the insulator  280 , which yields the OH bonding to diffuse in the insulator  280 . The hydrogen atom having the OH bonding reacts with the oxygen atom bonded to an atom (such as a metal atom) in the insulator  282  in reaching a material which has a function of capturing or fixing hydrogen (typically the insulator  282 ), and is trapped or fixed in the insulator  282 . The oxygen atom which had the OH bonding of the excess oxygen may remain as an excess oxygen in the insulator  280 . That is, it is highly probable that the excess oxygen in the insulator  280  serves as a bridge in the diffusion of the hydrogen. 
     A manufacturing process of the semiconductor device is one of important factors for the model. 
     For example, the insulator  280  containing excess oxygen is formed over the oxide semiconductor, and then the insulator  282  is formed. After that, heat treatment is preferably performed. Specifically, the heat treatment is performed at 350° C. or higher, preferably 400° C. or higher under an atmosphere containing oxygen, an atmosphere containing nitrogen, or a mixed atmosphere of oxygen and nitrogen. The heat treatment is performed for one hour or more, preferably four hours or more, further preferably eight hours or more. 
     The heat treatment enables diffusion of hydrogen from the oxide semiconductor to the outside through the insulator  280  and the insulator  282 . That is, the absolute amount of hydrogen in and near the oxide semiconductor can be reduced. 
     The insulator  283  is formed after the heat treatment. The insulator  283  is formed using a material having a high blocking property against hydrogen; thus, entry of hydrogen diffusing to the outside or external hydrogen into the inside, specifically, the oxide semiconductor or the insulator  280  side can be inhibited. 
     An example where the heat treatment is performed after the insulator  282  is formed is shown; however, one embodiment of the present invention is not limited thereto. For example, the heat treatment may be performed after the transistor layer  413  is formed or after the memory device layer  415 _ 1  to the memory device layer  415 _ 3  are formed. When hydrogen is diffused to the outside by the heat treatment, hydrogen is diffused to above the transistor layer  413  or in a lateral direction. Similarly, in the case where heat treatment is performed after the memory device layer  415 _ 1  to the memory device layer  415 _ 3  are formed, hydrogen is diffused into an upper area or in the lateral direction. 
     Through the above manufacturing process, the insulator  212  and the insulator  283  are bonded, whereby the sealing structure is formed. 
     With the above-described structure and the above-described manufacturing process, a semiconductor device using an oxide semiconductor with reduced hydrogen concentration can be provided. Accordingly, a semiconductor device with high reliability can be provided. According to another embodiment of the present invention, a semiconductor device with favorable electrical characteristics can be provided. 
     The structures, methods, and the like described in this embodiment can be used in an appropriate combination with the structures, configurations, methods, and the like described in the other embodiments and the like. 
     Embodiment 3 
     In this embodiment, an apparatus that can be used to manufacture a semiconductor device of one embodiment of the present invention is described with reference to  FIG. 29 . 
     In manufacture of the semiconductor device of one embodiment of the present invention, it is preferable to use what is called a multi-chamber apparatus including a plurality of treatment chambers enabling successive deposition of different kinds of films. In each treatment chamber, deposition treatment such as sputtering, CVD, or ALD can be performed. For example, in the case where one treatment chamber is a sputtering chamber, the sputtering chamber can be connected to a gas supply device, a gas refining device connected to the gas supply device, a vacuum pump, a target, or the like. The sputtering chamber may have a structure in which an ionization sputtering method can be performed. Furthermore, the sputtering chamber may have a structure in which a bias sputtering method can be performed. 
     In each treatment chamber, a substrate cleaning treatment, a plasma treatment, a reverse sputtering treatment, an etching treatment, an ashing treatment, a heat treatment, or the like may be performed. Different treatments are performed between treatment chambers as appropriate, whereby an insulator, a conductor, and a semiconductor film can be formed without exposure to the air. 
     A typical example of the semiconductor film used in one embodiment of the present invention includes an oxide semiconductor film. In particular, the oxide semiconductor film having a low impurity concentration and a low density of defect states (the amount of oxygen vacancies is small) enables a transistor with excellent electrical characteristics to be manufactured. Here, the state in which the impurity concentration is low and the density of defect states is low is referred to as highly purified intrinsic or substantially highly purified intrinsic. 
     A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus the carrier density can be reduced. Thus, a transistor in which a channel formation region is formed in the oxide semiconductor film rarely has electrical characteristics in which the threshold voltage is negative (also referred to as normally on). Moreover, a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly may have a low density of trap states. Furthermore, a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width of 1×10 6  μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10 −13  A, at a voltage between a source electrode and a drain electrode (drain voltage) ranging from 1 V to 10 V. 
     Note that impurities in the oxide semiconductor film are typically water, hydrogen, and the like. In this specification and the like, reducing or removing water and hydrogen from the oxide semiconductor film may be referred to as dehydration or dehydrogenation. Moreover, adding oxygen to the oxide semiconductor film may be referred to as oxygen addition and a state in which oxygen in excess of the stoichiometric composition is contained due to the oxygen addition may be referred to as an oxygen-excess state. 
     Here, as an oxide semiconductor, an insulator or a conductor positioned under the oxide semiconductor, and an insulator or a conductor positioned over the oxide semiconductor, different kinds of films are successively deposited without being exposed to the air, whereby a substantially highly purified intrinsic oxide semiconductor film whose impurity (hydrogen and water, in particular) concentration is reduced can be deposited. 
     First, a structure example of the apparatus that can be used to manufacture a semiconductor device of one embodiment of the present invention will be described in detail with reference to  FIG. 29 . With the use of the apparatus shown in  FIG. 29 , it is possible to successively deposit a semiconductor film, an insulator or a conductor positioned under the semiconductor film, and an insulator or a conductor positioned over the semiconductor film. Thus, impurities (hydrogen and water, in particular) can be prevented from entering the semiconductor film. 
       FIG. 29  schematically shows a top view of a single wafer multi-chamber apparatus  4000 . 
     The apparatus  4000  includes an atmosphere-side substrate supply chamber  4010 , an atmosphere-side substrate transfer chamber  4012  that transfers a substrate from the atmosphere-side substrate supply chamber  4010 , a load lock chamber  4020   a  that loads a substrate and switches the pressure in the chamber from an atmospheric pressure to a reduced pressure or from a reduced pressure to an atmospheric pressure, an unload lock chamber  4020   b  that unloads a substrate and switches the pressure in the chamber from a reduced pressure to an atmospheric pressure or from an atmospheric pressure to a reduced pressure, a transfer chamber  4029  and a transfer chamber  4039  that transfer a substrate in a vacuum, a transport chamber  4030   a  and a transport chamber  4030   b  that connect the transfer chamber  4029  and the transfer chamber  4039 , and a treatment chamber  4024   a , a treatment chamber  4024   b , a treatment chamber  4034   a , a treatment chamber  4034   b , a treatment chamber  4034   c , a treatment chamber  4034   d , and a treatment chamber  4034   e  that perform deposition or heating. 
     Note that different treatments can be performed in a plurality of treatment chambers in parallel. Thus, a stacked-layer structure of different kinds of films can be easily fabricated. The number of parallel treatments that can be conducted at maximum is equated to the number of treatment chambers. For example, the apparatus  4000  shown in  FIG. 29  is an apparatus that includes seven treatment chambers. Therefore, seven deposition treatments can be successively performed without exposure to the air using one apparatus (which is referred to as “in-situ” in this specification). 
     On the other hand, the number of stacked layers that can be fabricated in a stacked-layer structure without exposure to the air is not necessarily the same as the number of treatment chambers. For example, in the case where a stacked-layer structure to be required includes a plurality of layers formed using the same material, the layers can be provided in one treatment chamber; thus, a stacked-layer structure can have stacked layers the number of which is larger than the number of provided treatment chambers. 
     The atmosphere-side substrate supply chamber  4010  includes a cassette port  4014  that holds a substrate and an alignment port  4016  that aligns a substrate. Note that a plurality of cassette ports  4014  may be provided (for example, in  FIG. 29 , three cassette ports are provided). 
     The atmosphere-side substrate transfer chamber  4012  is connected to the load lock chamber  4020   a  and the unload lock chamber  4020   b . The transfer chamber  4029  is connected to the load lock chamber  4020   a , the unload lock chamber  4020   b , the transport chamber  4030   a , the transport chamber  4030   b , the treatment chamber  4024   a , and the treatment chamber  4024   b . The transport chamber  4030   a  and the transport chamber  4030   b  are connected to the transfer chamber  4029  and the transfer chamber  4039 . The transfer chamber  4039  is connected to the transport chamber  4030   a , the transport chamber  4030   b , the treatment chamber  4034   a , the treatment chamber  4034   b , the treatment chamber  4034   c , the treatment chamber  4034   d , and the treatment chamber  4034   e.    
     Note that a gate valve  4028  or a gate valve  4038  is provided for a connecting portion of each chamber so that each of the chambers except for the atmosphere-side substrate supply chamber  4010  and the atmosphere-side substrate transfer chamber  4012  can be independently kept under vacuum. The atmosphere-side substrate transfer chamber  4012  includes a transfer robot  4018 . The transfer chamber  4029  includes a transfer robot  4026 , and the transfer chamber  4039  includes a transfer robot  4036 . Each of the transfer robot  4018 , the transfer robot  4026 , and the transfer robot  4036  includes a plurality of movable portions and an arm for holding a substrate and can transfer a substrate to each chamber. 
     Note that the numbers of transfer chambers, treatment chambers, load lock chambers, unload lock chambers, and transport chambers are not limited to the above, and the numbers thereof can be set as appropriate depending on the space for placement or the process conditions. 
     In particular, in the case where a plurality of transfer chambers are provided, two or more transport chambers are preferably provided between one transfer chamber and another transfer chamber. For example, in the case where the transfer chamber  4029  and the transfer chamber  4039  are provided as shown in  FIG. 29 , the transport chamber  4030   a  and the transport chamber  4030   b  are preferably provided in parallel between the transfer chamber  4029  and the transfer chamber  4039 . 
     When the transport chamber  4030   a  and the transport chamber  4030   b  are provided in parallel, for example, a step of carrying a substrate to the transport chamber  4030   a  by the transfer robot  4026  and a step of carrying a substrate to the transport chamber  4030   b  by the transfer robot  4036  can be concurrently performed. Furthermore, a step of carrying out a substrate from the transport chamber  4030   b  by the transfer robot  4026  and a step of carrying out a substrate from the transport chamber  4030   a  by the transfer robot  4036  can be concurrently performed. That is, when a plurality of transfer robots are driven concurrently, the production efficiency is improved. 
     Although an example in which one transfer chamber includes one transfer robot and is connected to a plurality of treatment chambers is shown in  FIG. 29 , the present invention is not limited to this structure. One transfer chamber may be provided with a plurality of transfer robots. 
     In addition, one or both of the transfer chamber  4029  and the transfer chamber  4039  are connected to a vacuum pump and a cryopump through valves. Accordingly, the transfer chamber  4029  and the transfer chamber  4039  can be evacuated with the use of the vacuum pump from the atmospheric pressure to low or medium vacuum (approximately several hundred pascals to 0.1 Pa) and then, by switching the valve, be evacuated with the use of the cryopump from the medium vacuum to high or ultra-high vacuum (approximately 0.1 Pa to 1×10 −7  Pa). 
     Alternatively, two or more cryopumps may be connected in parallel to one transfer chamber, for example. With a plurality of cryopumps, even when one of the cryopumps is in regeneration, exhaust can be performed using the other of the cryopumps. Note that the above regeneration refers to treatment for discharging molecules (or atoms) entrapped in the cryopump. When molecules (or atoms) are entrapped too much in a cryopump, the exhaust capability is lowered; therefore, it is preferable to perform regeneration regularly. 
     The treatment chamber  4024   a , the treatment chamber  4024   b , the treatment chamber  4034   a , the treatment chamber  4034   b , the treatment chamber  4034   c , the treatment chamber  4034   d , and the treatment chamber  4034   e  can perform different treatments in parallel. In other words, the treatment chambers can perform, on the substrates provided, different treatments out of a deposition treatment by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like, a heat treatment, and a plasma treatment. In the treatment chamber, a deposition treatment may be performed after a heat treatment or a plasma treatment. 
     In the apparatus  4000 , it is possible to transfer a substrate without exposure of the substrate to the air between treatments since a plurality of treatment chambers are provided; therefore, adsorption of impurities on the substrate can be inhibited. The treatment chambers can perform a deposition treatment for different kinds of films, a heat treatment, or a plasma treatment, which makes it possible to freely determine the order of deposition, a heat treatment, and the like. 
     Note that each treatment chamber may be connected to the vacuum pump through a valve. As the vacuum pump, a dry pump, a mechanical booster pump, or the like can be used, for example. 
     Furthermore, each treatment chamber may be connected to a power source capable of generating plasma. As the power source, a DC power source, an AC power source, a high-frequency (RF, microwave, or the like) power source, or the like may be provided. A pulsed generator may be connected to the DC power source. 
     Moreover, the treatment chamber may be connected to a gas refining device through a gas supply device. Note that the number of provided gas supply devices and the number of provided gas refining devices may be equated to the number of kinds of gases. 
     For example, in the case where the deposition treatment is performed by sputtering in the treatment chamber, the treatment chamber may include a target, a backing plate connected to the target, a cathode positioned to face the target with the backing plate therebetween, a deposition shield, a substrate stage, or the like. For example, the substrate stage may include a substrate holding mechanism which holds the substrate, a rear heater which heats the substrate from the back surface, or the like. 
     Note that the substrate stage is held substantially perpendicularly to a floor during deposition and is held substantially parallel to the floor when the substrate is delivered. Here, when the substrate stage is held substantially perpendicularly to the floor, the probability that dust or a particle which might be mixed into the film during deposition is attached to the substrate can be lowered as compared to the case where the substrate stage is held parallel to the floor. However, there is a possibility that the substrate falls when the substrate stage is held perpendicularly (90°) to the floor; therefore, the angle of the substrate stage to the floor is preferably greater than or equal to 80° and less than 90°. 
     Note that the structure of the substrate stage is not limited to the above structure. A structure in which the substrate stage is substantially parallel to the floor may be employed, for example. In the case of such a structure, a target may be provided below the substrate stage, and a substrate may be provided between the target and the substrate stage. The substrate stage may be provided with a jig for fixing a substrate so as not to fall or a mechanism for fixing a substrate. 
     Furthermore, when a deposition shield is provided for the treatment chamber, particles sputtered from the target can be inhibited from being deposited on a region where deposition is not needed. Moreover, the deposition shield is preferably processed to prevent accumulated sputtered particles from being separated. For example, blasting treatment which increases surface roughness or projections and depressions may be provided on the surfaces of the deposition shield. 
     The backing plate has a function of holding the target, and the cathode has a function of applying voltage (for example, negative voltage) to the target. 
     Note that the target can be formed using a conductor, an insulator, or a semiconductor. For example, when the target is an oxide semiconductor such as a metal oxide, an oxide semiconductor film can be deposited in the treatment chamber. When the target is a metal oxide, an oxynitride semiconductor film can be formed by using a nitrogen gas as the deposition gas. 
     Each treatment chamber may be connected to a gas supply device through a gas heating mechanism. The gas heating mechanism is connected to a gas refining device through the gas supply apparatus. As the gas introduced to the treatment chamber, a gas whose dew point is −80° C. or lower, preferably −100° C. or lower, further preferably −120° C. or lower can be used; for example, an oxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) can be used. With the gas heating mechanism, a gas which is introduced to the treatment chamber can be heated to a temperature higher than or equal to 40° C. and lower than or equal to 400° C. Note that the number of provided gas heating mechanisms, the number of provided gas supply devices, and the number of provided gas refining devices may be equated to the number of kinds of gases. 
     Each treatment chamber may be connected to a turbo molecular pump and a vacuum pump through valves. Each treatment chamber may be provided with a cryotrap. 
     The cryotrap is a mechanism which can adsorb a molecule (or an atom) having a relatively high melting point, such as water. The turbo molecular pump has excellent productivity because it stably exhausts a large-sized molecule (or atom) and needs low frequency of maintenance, whereas it has a low capability in removing hydrogen and water. Thus, a cryotrap can be used to improve the evacuation capability of water or the like. The temperature of a freezer of the cryotrap is set lower than or equal to 100 K, preferably lower than or equal to 80 K. Furthermore, in the case where the cryotrap has a plurality of freezers, it is preferable to set the freezers at different temperatures because efficient exhaust is possible. For example, the temperature of a first-stage freezer is set lower than or equal to 100 K and the temperature of a second-stage freezer is set lower than or equal to 20 K. 
     Note that the exhaust method for the treatment chamber is not limited thereto, and a system similar to that in the exhaust method described for the transfer chamber connected thereto (the exhaust method using the cryopump and the vacuum pump) may be employed. Note that the exhaust method for the transfer chamber may have a system similar to that of the treatment chamber (the exhaust method using the turbo molecular pump and the vacuum pump). 
     In particular, the vacuum pump and the cryotrap may be combined as the exhaust method for the treatment chamber where an oxide semiconductor film is deposited. The exhaust method for the treatment chamber where an oxide semiconductor film is deposited preferably has at least a function of adsorbing water molecules. 
     In the treatment chamber where the oxide semiconductor film is deposited, the partial pressure of hydrogen molecules is preferably lower than or equal to 1×10 −2  Pa, and the partial pressure of water molecules is preferably lower than or equal to 1×10 −4  Pa. In the treatment chamber where the oxide semiconductor film is deposited in a standby state, the pressure is lower than or equal to 8.0×10 −5  Pa, preferably lower than or equal to 5.0×10 −5  Pa, further preferably lower than or equal to 1.0×10 −5  Pa. The above values of the partial pressure of hydrogen molecules and the partial pressure of water molecules are the values of both of the standby state and in the deposition state (a plasma discharge state) of the treatment chamber in which sputtering is performed. 
     Note that a total pressure and a partial pressure in the treatment chamber can be measured using a mass analyzer. For example, Qulee CGM-051, a quadrupole mass analyzer (also referred to as Q-mass) produced by ULVAC, Inc. can be used. 
     When the partial pressure of hydrogen molecules, the partial pressure of water molecules, and the pressure in the standby state in the treatment chamber are set to the values in above ranges, the impurity concentration in an oxide semiconductor film to be formed can be reduced. 
     In particular, each treatment chamber is used for deposition treatment by sputtering, so that part of the structure of the transistor  200  described in the above embodiment can be manufactured with a stacked-layer structure by successive in-situ deposition. 
     In the method for manufacturing the transistor  200 , the insulator  212 , the insulator  214 , and the insulator  216  are successively deposited using the apparatus  4000 . The oxide film  230 A, the oxide film  230 B, and the oxide film  243 A are successively deposited using the apparatus  4000 . The conductive film  242 A, the insulating film  271 A, and the conductive film  248 A are successively deposited using the apparatus  4000 . 
     That is, the insulator  212 , the insulator  214 , and the insulator  216  can be successively deposited without being exposed to the air. The oxide film  230 A, the oxide film  230 B, and the oxide film  243 A can be successively deposited without being exposed to the air. The conductive film  242 A, the insulating film  271 A, and the conductive film  248 A can be successively deposited without being exposed to the air. 
     With the above structure, a stacked film from which impurities (typically, water, hydrogen, and the like) are thoroughly removed can be formed. Furthermore, each interface of the stacked films is not exposed to the air; thus, the impurity concentration is reduced. 
     Alternatively, in the case where heat treatment is performed in a treatment chamber, for example, the treatment chamber may have a plurality of heating stages that can hold a substrate. Note that the heating stage may have several stages. By increasing the number of the heating stages, a plurality of substrates can be subjected to heat treatment at the same time, which leads to an increase in productivity. 
     A heating mechanism which can be used for the treatment chamber may be a heating mechanism which performs heating with a resistance heater or the like, for example. Alternatively, a heating mechanism that uses heat conduction or heat radiation from a medium such as a heated gas for heating may be used. For example, RTA (Rapid Thermal Anneal) apparatus such as a GRTA (Gas Rapid Thermal Anneal) apparatus or an LRTA (Lamp Rapid Thermal Anneal) apparatus can be used. In the LRTA, an object is heated through radiation of light (electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. In GRTA, heat treatment is performed using a high-temperature gas. An inert gas is used as the gas. 
     The load lock chamber  4020   a  may include a substrate delivery stage, a rear heater for heating a substrate from the back surface, or the like. When the pressure in the load lock chamber  4020   a  is increased from a reduced pressure state to an atmospheric pressure and becomes an atmospheric pressure, the substrate delivery stage receives a substrate from the transfer robot  4018  provided in the atmosphere-side substrate transfer chamber  4012 . After that, the load lock chamber  4020   a  is evacuated into a vacuum to make a reduced pressure state, and then, the transfer robot  4026  provided in the transfer chamber  4029  receives the substrate from the substrate delivery stage. 
     Furthermore, the load lock chamber  4020   a  is connected to a vacuum pump and a cryopump through valves. Note that the unload lock chamber  4020   b  can have a structure similar to that of the load lock chamber  4020   a.    
     Since the atmosphere-side substrate transfer chamber  4012  includes the transfer robot  4018 , delivery and receipt of a substrate between the cassette port  4014  and the load lock chamber  4020   a  can be performed using the transfer robot  4018 . Furthermore, a mechanism for inhibiting entry of dust or a particle, such as an HEPA filter (High Efficiency Particulate Air Filter), may be provided above the atmosphere-side substrate transfer chamber  4012  and the atmosphere-side substrate supply chamber  4010 . The cassette port  4014  can hold a plurality of substrates. 
     Entry of impurities into a semiconductor film can be suitably inhibited when an insulating film, a semiconductor film, and a conductive film are successively deposited without exposure to the air with the use of the apparatus  4000  described above. 
     As described above, a stacked-layer structure including a semiconductor film can be formed through successive film deposition with the use of the apparatus of one embodiment of the present invention. Therefore, impurities such as hydrogen and water that might enter a semiconductor film can be inhibited and a semiconductor film with a low density of defect states can be formed. 
     The structure, method, and the like described above in this embodiment can be used in an appropriate combination with the structures, the methods, and the like described in the other embodiments and the like. 
     Embodiment 4 
     In this embodiment, a storage device of one embodiment of the present invention including a transistor in which an oxide is used as a semiconductor (hereinafter referred to as an OS transistor in some cases) and a capacitor (hereinafter referred to as an OS memory device in some cases), is described with reference to  FIG. 30A ,  FIG. 30B , and  FIG. 31A  to  FIG. 31H . The OS memory device is a storage device including at least a capacitor and the OS transistor that controls the charging and discharging of the capacitor. Since the OS transistor has an extremely low off-state current, the OS memory device has excellent retention characteristics and thus can function as a nonvolatile memory. 
     &lt;Structure Example of Storage Device&gt; 
       FIG. 30A  shows a structure example of the OS memory device. A storage device  1400  includes a peripheral circuit  1411  and a memory cell array  1470 . The peripheral circuit  1411  includes a row circuit  1420 , a column circuit  1430 , an output circuit  1440 , and a control logic circuit  1460 . 
     The column circuit  1430  includes, for example, a column decoder, a precharge circuit, a sense amplifier, a write circuit, and the like. The precharge circuit has a function of precharging wirings. The sense amplifier has a function of amplifying a data signal read from a memory cell. Note that the wirings are connected to the memory cell included in the memory cell array  1470 , and are described later in detail. The amplified data signal is output as a data signal RDATA to the outside of the storage device  1400  through the output circuit  1440 . The row circuit  1420  includes, for example, a row decoder and a word line driver circuit, and can select a row to be accessed. 
     As power supply voltages from the outside, a low power supply voltage (VS S), a high power supply voltage (VDD) for the peripheral circuit  1411 , and a high power supply voltage (VIL) for the memory cell array  1470  are supplied to the storage device  1400 . Control signals (CE, WE, and RE), an address signal ADDR, and a data signal WDATA are also input to the storage device  1400  from the outside. The address signal ADDR is input to the row decoder and the column decoder, and the data signal WDATA is input to the write circuit. 
     The control logic circuit  1460  processes the control signals (CE, WE, and RE) input from the outside, and generates control signals for the row decoder and the column decoder. The control signal CE is a chip enable signal, the control signal WE is a write enable signal, and the control signal RE is a read enable signal. Signals processed by the control logic circuit  1460  are not limited thereto, and other control signals are input as necessary. 
     The memory cell array  1470  includes a plurality of memory cells MC arranged in a matrix and a plurality of wirings. Note that the number of the wirings that connect the memory cell array  1470  to the row circuit  1420  depends on the structure of the memory cell MC, the number of the memory cells MC in a column, and the like. The number of the wirings that connect the memory cell array  1470  to the column circuit  1430  depends on the structure of the memory cell MC, the number of the memory cells MC in a row, and the like. 
     Note that  FIG. 30A  shows an example in which the peripheral circuit  1411  and the memory cell array  1470  are formed on the same plane; however, this embodiment is not limited thereto. For example, as shown in  FIG. 30B , the memory cell array  1470  may be provided to overlap with part of the peripheral circuit  1411 . For example, the sense amplifier may be provided below the memory cell array  1470  so that they overlap with each other. 
       FIG. 31A  to  FIG. 31H  show structure examples of a memory cell that can be applied to the memory cell MC. 
     [DOSRAM] 
       FIG. 31A  to  FIG. 31C  show circuit structure examples of DRAM memory cells. In this specification and the like, a DRAM using a memory cell including one OS transistor and one capacitor is referred to as a DOSRAM (Dynamic Oxide Semiconductor Random Access Memory) in some cases. A memory cell  1471  shown in  FIG. 31A  includes a transistor M 1  and a capacitor CA. Note that the transistor M 1  includes a gate (also referred to as a top gate in some cases) and a back gate. 
     A first terminal of the transistor M 1  is connected to a first terminal of the capacitor CA, a second terminal of the transistor M 1  is connected to a wiring BIL, the gate of the transistor M 1  is connected to a wiring WOL, and the back gate of the transistor M 1  is connected to a wiring BGL. A second terminal of the capacitor CA is connected to a wiring CAL. 
     The wiring BIL functions as a bit line, and the wiring WOL functions as a word line. The wiring CAL functions as a wiring for applying a predetermined potential to the second terminal of the capacitor CA. In the time of data writing and data reading, a low-level potential is preferably applied to the wiring CAL. The wiring BGL functions as a wiring for applying a potential to the back gate of the transistor M 1 . By applying a given potential to the wiring BGL, the threshold voltage of the transistor M 1  can be increased or decreased. 
     Here, a memory cell  1471  shown in  FIG. 31A  corresponds to the storage device shown in  FIG. 25 . That is, the transistor M 1  and the capacitor CA correspond to the transistor  200  and the capacitor device  292 , respectively. 
     The memory cell MC is not limited to the memory cell  1471 , and the circuit structure can be changed. For example, as in a memory cell  1472  shown in  FIG. 31B , the back gate of the transistor M 1  may be connected not to the wiring BGL but to the wiring WOL in the memory cell MC. Alternatively, for example, the transistor M 1  may be a single-gate transistor, that is, a transistor without a back gate in the memory cell MC as in a memory cell  1473  shown in  FIG. 31C . 
     In the case where the semiconductor device described in the above embodiment is used in the memory cell  1471  or the like, the transistor  200  can be used as the transistor M 1 , and the capacitor  100  can be used as the capacitor CA. When an OS transistor is used as the transistor M 1 , the leakage current of the transistor M 1  can be extremely low. That is, with the use of the transistor M 1 , written data can be retained for a long period of time, and thus the frequency of the refresh operation for the memory cell can be decreased. In addition, refresh operation for the memory cell can be omitted. In addition, since the transistor M 1  has an extremely low leakage current, multi-level data or analog data can be retained in the memory cell  1471 , the memory cell  1472 , and the memory cell  1473 . 
     In addition, in the DOSRAM, when the sense amplifier is provided below the memory cell array  1470  to overlap with the memory cell array  1470  as described above, the bit line can be shortened. This reduces bit line capacity, which reduces the storage capacity of the memory cell. 
     [NOSRAM] 
       FIG. 31D  to  FIG. 31G  show circuit structure examples of gain-cell memory cells each including two transistors and one capacitor. A memory cell  1474  shown in  FIG. 31D  includes a transistor M 2 , a transistor M 3 , and a capacitor CB. Note that the transistor M 2  includes a top gate (simply referred to as a gate in some cases) and a back gate. In this specification and the like, a storage device including a gain-cell memory cell using an OS transistor as the transistor M 2  is referred to as a NOSRAM (Nonvolatile Oxide Semiconductor RAM) in some cases. 
     A first terminal of the transistor M 2  is connected to a first terminal of the capacitor CB, a second terminal of the transistor M 2  is connected to a wiring WBL, the gate of the transistor M 2  is connected to the wiring WOL, and the back gate of the transistor M 2  is connected to the wiring BGL. A second terminal of the capacitor CB is connected to the wiring CAL. A first terminal of the transistor M 3  is connected to a wiring RBL, a second terminal of the transistor M 3  is connected to a wiring SL, and a gate of the transistor M 3  is connected to the first terminal of the capacitor CB. 
     The wiring WBL functions as a write bit line, the wiring RBL functions as a read bit line, and the wiring WOL functions as a word line. The wiring CAL functions as a wiring for applying a predetermined potential to the second terminal of the capacitor CB. During data writing, data retention, and data reading, a low-level potential is preferably applied to the wiring CAL. The wiring BGL functions as a wiring for applying a potential to the back gate of the transistor M 2 . By applying a given potential to the wiring BGL, the threshold voltage of the transistor M 2  can be increased or decreased. 
     Here, the memory cell  1474  shown in  FIG. 31D  corresponds to the storage device shown in  FIG. 23 . That is, the transistor M 2 , the capacitor CB, the transistor M 3 , the wiring WBL, the wiring WOL, the wiring BGL, the wiring CAL, the wiring RBL, and the wiring SL correspond to the transistor  200 , the capacitor  100 , the transistor  300 , the wiring  1003 , the wiring  1004 , the wiring  1006 , the wiring  1005 , the wiring  1002 , and the wiring  1001 , respectively. 
     In addition, the memory cell MC is not limited to the memory cell  1474 , and the circuit structure can be changed as appropriate. For example, as in a memory cell  1475  shown in  FIG. 31E , the back gate of the transistor M 2  may be connected not to the wiring BGL but to the wiring WOL in the memory cell MC. Alternatively, for example, the transistor M 2  may be a single-gate transistor, that is, a transistor without a back gate in the memory cell MC as in a memory cell  1476  shown in  FIG. 31F . For example, the memory cell MC may have a structure in which the wiring WBL and the wiring RBL are combined into one wiring BIL as in a memory cell  1477  shown in  FIG. 31G . 
     In the case where the semiconductor device described in the above embodiment is used in the memory cell  1474  or the like, the transistor  200  can be used as the transistor M 2 , the transistor  300  can be used as the transistor M 3 , and the capacitor  100  can be used as the capacitor CB. When an OS transistor is used as the transistor M 2 , the leakage current of the transistor M 2  can be extremely low. Consequently, with the use of the transistor M 2 , written data can be retained for a long period of time, and thus the frequency of the refresh operation for the memory cell can be decreased. In addition, refresh operation for the memory cell can be omitted. In addition, since the transistor M 2  has an extremely low leakage current, multi-level data or analog data can be retained in the memory cell  1474 . The same applies to the memory cell  1475  to the memory cell  1477 . 
     Note that the transistor M 3  may be a transistor containing silicon in a channel formation region (hereinafter referred to as a Si transistor in some cases). The conductivity type of the Si transistor may be either an n-channel type or a p-channel type. A Si transistor has higher field-effect mobility than an OS transistor in some cases. Therefore, a Si transistor may be used as the transistor M 3  functioning as a read transistor. Furthermore, the use of a Si transistor as the transistor M 3  enables the transistor M 2  to be stacked over the transistor M 3 , in which case the area occupied by the memory cell can be reduced and high integration of the storage device can be achieved. 
     Alternatively, the transistor M 3  may be an OS transistor. When OS transistors are used as the transistor M 2  and the transistor M 3 , the circuit of the memory cell array  1470  can be formed using only n-channel transistors. 
       FIG. 31H  shows an example of a gain-cell memory cell including three transistors and one capacitor. A memory cell  1478  shown in  FIG. 31H  includes a transistor M 4  to a transistor M 6  and a capacitor CC. The capacitor CC is provided as appropriate. The memory cell  1478  is electrically connected to the wiring BIL, a wiring RWL, a wiring WWL, the wiring BGL, and a wiring GNDL. The wiring GNDL is a wiring for supplying a low-level potential. Note that the memory cell  1478  may be electrically connected to the wiring RBL and the wiring WBL instead of the wiring BIL. 
     The transistor M 4  is an OS transistor including a back gate, and the back gate is electrically connected to the wiring BGL. Note that the back gate and a gate of the transistor M 4  may be electrically connected to each other. Alternatively, the transistor M 4  does not necessarily include the back gate. 
     Note that each of the transistor M 5  and the transistor M 6  may be an n-channel Si transistor or a p-channel Si transistor. Alternatively, the transistor M 4  to the transistor M 6  may be OS transistors, in which case the circuit of the memory cell array  1470  can be formed using only n-channel transistors. 
     In the case where the semiconductor device described in the above embodiment is used in the memory cell  1478 , the transistor  200  can be used as the transistor M 4 , the transistor  300  can be used as the transistor M 5  and the transistor M 6 , and the capacitor  100  can be used as the capacitor CC. When an OS transistor is used as the transistor M 4 , the leakage current of the transistor M 4  can be extremely low. 
     Note that the structures of the peripheral circuit  1411 , the memory cell array  1470 , and the like described in this embodiment are not limited to the above. The arrangement and functions of these circuits and the wirings, circuit components, and the like connected to the circuits can be changed, removed, or added as needed. 
     In general, a variety of storage devices (memories) are used in semiconductor devices such as a computer in accordance with the intended use.  FIG. 32  shows a hierarchy diagram showing various storage devices with different levels. The storage devices at the upper levels of the diagram require high access speeds, and the storage devices at the lower levels require large memory capacity and high record density. In  FIG. 32 , sequentially from the top level, a memory combined as a register in an arithmetic processing device such as a CPU, an SRAM (Static Random Access Memory), a DRAM (Dynamic Random Access Memory), and a 3D NAND memory are shown. 
     A memory combined as a register in an arithmetic processing device such as a CPU is used for temporary storage of arithmetic operation results, for example, and thus is very frequently accessed by the arithmetic processing device. Accordingly, rapid operation is more important than the memory capacity of the memory. The register also has a function of retaining setting information of the arithmetic processing device, for example. 
     An SRAM is used for a cache, for example. The cache has a function of retaining a copy of part of data retained in a main memory. By copying data which is frequently used and holding the copy of the data in the cache, the access speed to the data can be increased. 
     A DRAM is used for the main memory, for example. The main memory has a function of retaining a program or data which are read from a storage. The record density of a DRAM is approximately 0.1 to 0.3 Gbit/mm 2 . 
     A 3D NAND memory is used for a storage, for example. The storage has a function of retaining data that needs to be retained for a long time and programs used in an arithmetic processing device, for example. Therefore, the storage needs to have a high memory capacity and a high record density rather than operating speed. The record density of a storage device used for a storage is approximately 0.6 to 6.0 Gbit/mm 2 . 
     The storage device of one embodiment of the present invention operates fast and can retain data for a long time. The storage device of one embodiment of the present invention can be favorably used as a storage device in a boundary region  901  including both the level in which a cache is placed and the level in which s main memory is placed. Alternatively, the storage device of one embodiment of the present invention can be favorably used as a storage device in a boundary region  902  including both the level in which a main memory is placed and the level in which a storage is placed. 
     The structure described in this embodiment can be used in appropriate combination with the structures described in the other embodiments and the like. 
     Embodiment 5 
     In this embodiment, an example of a chip  1200  on which the semiconductor device of the present invention is mounted is described with reference to  FIG. 33A  and  FIG. 33B . A plurality of circuits (systems) are mounted on the chip  1200 . A technique for integrating a plurality of circuits (systems) into one chip is referred to as system on chip (SoC) in some cases. 
     As shown in  FIG. 33A , the chip  1200  includes a CPU  1211 , a GPU  1212 , one or a plurality of analog arithmetic units  1213 , one or a plurality of memory controllers  1214 , one or a plurality of interfaces  1215 , one or a plurality of network circuits  1216 , and the like. 
     A bump (not shown) is provided on the chip  1200 , and as shown in  FIG. 33B , the chip  1200  is connected to a first surface of a printed circuit board (PCB)  1201 . In addition, a plurality of bumps  1202  are provided on a rear side of the first surface of the PCB  1201 , and the PCB  1201  is connected to a motherboard  1203 . 
     Storage devices such as DRAMs  1221  and a flash memory  1222  may be provided over the motherboard  1203 . For example, the DOSRAM described in the above embodiment can be used as the DRAM  1221 . In addition, for example, the NOSRAM described in the above embodiment can be used as the flash memory  1222 . 
     The CPU  1211  preferably includes a plurality of CPU cores. In addition, the GPU  1212  preferably includes a plurality of GPU cores. Furthermore, the CPU  1211  and the GPU  1212  may each include a memory for temporarily storing data. Alternatively, a common memory for the CPU  1211  and the GPU  1212  may be provided in the chip  1200 . The NOSRAM or the DOSRAM described above can be used as the memory. Moreover, the GPU  1212  is suitable for parallel computation of a number of data and thus can be used for image processing or product-sum operation. When an image processing circuit or a product-sum operation circuit using an oxide semiconductor of the present invention is provided in the GPU  1212 , image processing and product-sum operation can be performed with low power consumption. 
     In addition, since the CPU  1211  and the GPU  1212  are provided on the same chip, a wiring between the CPU  1211  and the GPU  1212  can be shortened, and the data transfer from the CPU  1211  to the GPU  1212 , the data transfer between the memories included in the CPU  1211  and the GPU  1212 , and the transfer of arithmetic operation results from the GPU  1212  to the CPU  1211  after the arithmetic operation in the GPU  1212  can be performed at high speed. 
     The analog arithmetic unit  1213  includes one or both of an A/D (analog/digital) converter circuit and a D/A (digital/analog) converter circuit. Furthermore, the product-sum operation circuit may be provided in the analog arithmetic unit  1213 . 
     The memory controller  1214  includes a circuit functioning as a controller of the DRAM  1221  and a circuit functioning as an interface of the flash memory  1222 . 
     The interface  1215  includes an interface circuit for an external connection device such as a display device, a speaker, a microphone, a camera, or a controller. Examples of the controller include a mouse, a keyboard, and a game controller. As such an interface, a USB (Universal Serial Bus), an HDMI (registered trademark) (High-Definition Multimedia Interface), or the like can be used. 
     The network circuit  1216  includes a network circuit such as a LAN (Local Area Network). The network circuit  1216  may further include a circuit for network security. 
     The circuits (systems) can be formed in the chip  1200  through the same manufacturing process. Therefore, even when the number of circuits needed for the chip  1200  increases, there is no need to increase the number of steps in the manufacturing process; thus, the chip  1200  can be manufactured at low cost. 
     The motherboard  1203  provided with the PCB  1201  on which the chip  1200  including the GPU  1212  is mounted, the DRAMs  1221 , and the flash memory  1222  can be referred to as a GPU module  1204 . 
     The GPU module  1204  includes the chip  1200  using SoC technology, and thus can have a small size. In addition, the GPU module  1204  is excellent in image processing, and thus is suitably used in a portable electronic device such as a smartphone, a tablet terminal, a laptop PC, or a portable (mobile) game machine. Furthermore, the product-sum operation circuit using the GPU  1212  can perform a method such as a deep neural network (DNN), a convolutional neural network (CNN), a recurrent neural network (RNN), an autoencoder, a deep Boltzmann machine (DBM), or a deep belief network (DBN); hence, the chip  1200  can be used as an AI chip or the GPU module  1204  can be used as an AI system module. 
     The structure described in this embodiment can be used in an appropriate combination with the structures described in the other embodiments and the like. 
     Embodiment 6 
     In this embodiment, examples of electronic components and electronic devices in which the storage device or the like described in the above embodiment is incorporated will be described. 
     &lt;Electronic Component&gt; 
     First,  FIG. 34A  and  FIG. 34B  show examples of an electronic component including a storage device  720 . 
       FIG. 34A  is a perspective view of an electronic component  700  and a substrate (circuit board  704 ) on which the electronic component  700  is mounted. The electronic component  700  in  FIG. 34A  includes the storage device  720  in a mold  711 .  FIG. 34A  omits part of the electronic component to show the inside of the electronic component  700 . The electronic component  700  includes a land  712  outside the mold  711 . The land  712  is electrically connected to an electrode pad  713 , and the electrode pad  713  is electrically connected to the storage device  720  via a wire  714 . The electronic component  700  is mounted on a printed circuit board  702 , for example. A plurality of such electronic components are combined and electrically connected to each other on the printed circuit board  702 , which forms the circuit board  704 . 
     The storage device  720  includes a driver circuit layer  721  and a storage circuit layer  722 . 
       FIG. 34B  is a perspective view of an electronic component  730 . The electronic component  730  is an example of a SiP (System in package) or an MCM (Multi Chip Module). In the electronic component  730 , an interposer  731  is provided over a package substrate  732  (printed circuit board) and a semiconductor device  735  and a plurality of storage devices  720  are provided over the interposer  731 . 
     The electronic component  730  using the storage device  720  as a high bandwidth memory (HBM) is illustrated as an example. An integrated circuit (a semiconductor device) such as a CPU, a GPU, or an FPGA can be used as the semiconductor device  735 . 
     As the package substrate  732 , a ceramic substrate, a plastic substrate, a glass epoxy substrate, or the like can be used. As the interposer  731 , a silicon interposer, a resin interposer, or the like can be used. 
     The interposer  731  includes a plurality of wirings and has a function of electrically connecting a plurality of integrated circuits with different terminal pitches. The plurality of wirings have a single-layer structure or a layered structure. The interposer  731  has a function of electrically connecting an integrated circuit provided on the interposer  731  to an electrode provided on the package substrate  732 . Accordingly, the interposer is sometimes referred to as a “redistribution substrate” or an “intermediate substrate”. A through electrode may be provided in the interposer  731  to be used for electrically connecting the integrated circuit and the package substrate  732 . In the case of using a silicon interposer, a through-silicon via (TSV) can also be used as the through electrode. 
     A silicon interposer is preferably used as the interposer  731 . The silicon interposer can be manufactured at lower cost than an integrated circuit because the silicon interposer is not necessarily provided with an active element. Moreover, since wirings of the silicon interposer can be formed through a semiconductor process, the formation of minute wirings, which is difficult for a resin interposer, is easily achieved. 
     An HBM needs to be connected to many wirings to achieve a wide memory bandwidth. Therefore, an interposer on which an HBM is mounted requires minute and densely formed wirings. For this reason, a silicon interposer is preferably used as the interposer on which an HBM is mounted. 
     In an SiP, an MCM, or the like using a silicon interposer, a decrease in reliability due to a difference in expansion coefficient between an integrated circuit and the interposer is less likely to occur. Furthermore, a surface of a silicon interposer has high planarity, and a poor connection between the silicon interposer and an integrated circuit provided thereon is less likely to occur. It is particularly preferable to use a silicon interposer for a 2.5D package (2.5D mounting) in which a plurality of integrated circuits are arranged side by side on the interposer. 
     A heat sink (radiator plate) may be provided to overlap with the electronic component  730 . In the case of providing a heat sink, the heights of integrated circuits provided on the interposer  731  are preferably equal to each other. In the electronic component  730  of this embodiment, the heights of the storage device  720  and the semiconductor device  735  are preferably equal to each other, for example. 
     An electrode  733  may be provided on the bottom portion of the package substrate  732  to mount the electronic component  730  on another substrate.  FIG. 34B  shows an example in which the electrode  733  is formed of a solder ball. Solder balls are provided in a matrix on the bottom portion of the package substrate  732 , whereby a BGA (Ball Grid Array) can be achieved. Alternatively, the electrode  733  may be formed of a conductive pin. When conductive pins are provided in a matrix on the bottom portion of the package substrate  732 , a PGA (Pin Grid Array) can be achieved. 
     The electronic component  730  can be mounted on another substrate by various mounting methods not limited to BGA and PGA. For example, a mounting method such as SPGA (Staggered Pin Grid Array), LGA (Land Grid Array), QFP (Quad Flat Package), QFJ (Quad Flat J-leaded package), or QFN (Quad Flat Non-leaded package) can be employed. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments and the like, as appropriate. 
     Embodiment 7 
     In this embodiment, application examples of the storage device using the semiconductor device described in the above embodiment are described. The semiconductor device described in the above embodiment can be applied to, for example, storage devices of a variety of electronic devices (e.g., information terminals, computers, smartphones, e-book readers, digital cameras (including video cameras), video recording/reproducing devices, and navigation systems). Here, the computers refer not only to tablet computers, notebook computers, and desktop computers, but also to large computers such as server systems. Alternatively, the semiconductor device described in the above embodiment is applied to a variety of removable storage devices such as memory cards (e.g., SD cards), USB memories, and SSDs (solid state drives).  FIG. 35A  to  FIG. 35E  schematically show some structure examples of removable storage devices. The semiconductor device described in the above embodiment is processed into a packaged memory chip and used in a variety of storage devices and removable memories, for example. 
       FIG. 35A  is a schematic view of a USB memory. A USB memory  1100  includes a housing  1101 , a cap  1102 , a USB connector  1103 , and a substrate  1104 . The substrate  1104  is held in the housing  1101 . The substrate  1104  is provided with a memory chip  1105  and a controller chip  1106 , for example. The semiconductor device described in the above embodiment can be incorporated in the memory chip  1105  or the like. 
       FIG. 35B  is a schematic external view of an SD card, and  FIG. 35C  is a schematic view of the internal structure of the SD card. An SD card  1110  includes a housing  1111 , a connector  1112 , and a substrate  1113 . The substrate  1113  is held in the housing  1111 . The substrate  1113  is provided with a memory chip  1114  and a controller chip  1115 , for example. When the memory chip  1114  is also provided on the back side of the substrate  1113 , the capacity of the SD card  1110  can be increased. In addition, a wireless chip with a radio communication function may be provided on the substrate  1113 . With this, data can be read from and written in the memory chip  1114  by radio communication between a host device and the SD card  1110 . The semiconductor device described in the above embodiment can be incorporated in the memory chip  1114  or the like. 
       FIG. 35D  is a schematic external view of an SSD, and  FIG. 35E  is a schematic view of the internal structure of the SSD. An SSD  1150  includes a housing  1151 , a connector  1152 , and a substrate  1153 . The substrate  1153  is held in the housing  1151 . The substrate  1153  is provided with a memory chip  1154 , a memory chip  1155 , and a controller chip  1156 , for example. The memory chip  1155  is a work memory of the controller chip  1156 , and a DOSRAM chip can be used, for example. When the memory chip  1154  is also provided on the back side of the substrate  1153 , the capacity of the SSD  1150  can be increased. The semiconductor device described in the above embodiment can be incorporated in the memory chip  1154  or the like. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments and the like, as appropriate. 
     Embodiment 8 
     The semiconductor device of one embodiment of the present invention can be used as a processor such as a CPU and a GPU or a chip.  FIG. 36A  to  FIG. 36H  show specific examples of electronic devices including a chip or a processor such as a CPU or a GPU of one embodiment of the present invention. 
     &lt;Electronic Device and System&gt; 
     The GPU or the chip of one embodiment of the present invention can be mounted on a variety of electronic devices. Examples of electronic devices include a digital camera, a digital video camera, a digital photo frame, an e-book reader, a mobile phone, a portable game machine, a portable information terminal, and an audio reproducing device in addition to electronic devices provided with a relatively large screen, such as a television device, a monitor for a desktop or notebook information terminal or the like, digital signage, and a large game machine like a pachinko machine. In addition, when the GPU or the chip of one embodiment of the present invention is provided in the electronic device, the electronic device can include artificial intelligence. 
     The electronic device of one embodiment of the present invention may include an antenna. When a signal is received by the antenna, the electronic device can display a video, data, or the like on a display portion. When the electronic device includes the antenna and a secondary battery, the antenna may be used for contactless power transmission. 
     The electronic device of one embodiment of the present invention may include a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, a chemical substance, sound, time, hardness, an electric field, current, voltage, power, radioactive rays, flow rate, humidity, a gradient, oscillation, odor, or infrared rays). 
     The electronic device of one embodiment of the present invention can have a variety of functions. For example, the electronic device can have a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of executing a variety of software (programs), a wireless communication function, and a function of reading out a program or data stored in a recording medium.  FIG. 36A  to  FIG. 36H  show examples of electronic devices. 
     [Information Terminal] 
       FIG. 36A  shows a mobile phone (smartphone), which is a type of information terminal. An information terminal  5100  includes a housing  5101  and a display portion  5102 . As input interfaces, a touch panel is provided in the display portion  5102  and a button is provided in the housing  5101 . 
     When the chip of one embodiment of the present invention is applied to the information terminal  5100 , the information terminal  5100  can execute an application utilizing artificial intelligence. Examples of the application utilizing artificial intelligence include an application for recognizing a conversation and displaying the content of the conversation on the display portion  5102 ; an application for recognizing letters, figures, and the like input to the touch panel of the display portion  5102  by a user and displaying them on the display portion  5102 ; and an application for performing biometric authentication using fingerprints, voice prints, or the like. 
       FIG. 36B  shows a notebook information terminal  5200 . The notebook information terminal  5200  includes a main body  5201  of the information terminal, a display portion  5202 , and a keyboard  5203 . 
     Like the information terminal  5100  described above, when the chip of one embodiment of the present invention is applied to the notebook information terminal  5200 , the notebook information terminal  5200  can execute an application utilizing artificial intelligence. Examples of the application utilizing artificial intelligence include design-support software, text correction software, and software for automatic menu generation. Furthermore, with the use of the notebook information terminal  5200 , novel artificial intelligence can be developed. 
     Note that although  FIG. 36A  and  FIG. 36B  show a smartphone and a notebook information terminal, respectively, as examples of the electronic device in the above description, an information terminal other than a smartphone and a notebook information terminal can be used. Examples of information terminals other than a smartphone and a notebook information terminal include a PDA (Personal Digital Assistant), a desktop information terminal, and a workstation. 
     [Game Machines] 
       FIG. 36C  shows a portable game machine  5300  as an example of a game machine. The portable game machine  5300  includes a housing  5301 , a housing  5302 , a housing  5303 , a display portion  5304 , a connection portion  5305 , an operation key  5306 , and the like. The housing  5302  and the housing  5303  can be detached from the housing  5301 . When the connection portion  5305  provided in the housing  5301  is attached to another housing (not shown), an image to be output to the display portion  5304  can be output to another video device (not shown). In that case, the housing  5302  and the housing  5303  can each function as an operating unit. Thus, a plurality of players can play a game at the same time. The chip described in the above embodiment can be incorporated into the chip provided on a substrate in the housing  5301 , the housing  5302  and the housing  5303 . 
       FIG. 36D  shows a stationary game machine  5400  as an example of a game machine. A controller  5402  is wired or connected wirelessly to the stationary game machine  5400 . 
     Using the GPU or the chip of one embodiment of the present invention in a game machine such as the portable game machine  5300  and the stationary game machine  5400  achieves a low-power-consumption game machine. Moreover, heat generation from a circuit can be reduced owing to low power consumption; thus, the influence of heat generation on the circuit, a peripheral circuit, and a module can be reduced. 
     Furthermore, when the GPU or the chip of one embodiment of the present invention is applied to the portable game machine  5300 , the portable game machine  5300  including artificial intelligence can be achieved. 
     In general, the progress of a game, the actions and words of game characters, and expressions of an event and the like occurring in the game are determined by the program in the game; however, the use of artificial intelligence in the portable game machine  5300  enables expressions not limited by the game program. For example, it becomes possible to change expressions such as questions posed by the player, the progress of the game, time, and actions and words of game characters. 
     In addition, when a game requiring a plurality of players is played on the portable game machine  5300 , the artificial intelligence can create a virtual game player; thus, the game can be played alone with the game player created by the artificial intelligence as an opponent. 
     Although the portable game machine and the stationary game machine are shown as examples of game machines in  FIG. 36C  and  FIG. 36D , the game machine using the GPU or the chip of one embodiment of the present invention is not limited thereto. Examples of the game machine to which the GPU or the chip of one embodiment of the present invention is applied include an arcade game machine installed in entertainment facilities (a game center, an amusement park, and the like), and a throwing machine for batting practice installed in sports facilities. 
     [Large Computer] 
     The GPU or the chip of one embodiment of the present invention can be used in a large computer. 
       FIG. 36E  shows a supercomputer  5500  as an example of a large computer.  FIG. 36F  shows a rack-mount computer  5502  included in the supercomputer  5500 . 
     The supercomputer  5500  includes a rack  5501  and a plurality of rack-mount computers  5502 . The plurality of computers  5502  are stored in the rack  5501 . The computer  5502  includes a plurality of substrates  5504  on which the GPU or the chip shown in the above embodiment can be mounted. 
     The supercomputer  5500  is a large computer mainly used for scientific computation. In scientific computation, an enormous amount of arithmetic operation needs to be processed at a high speed; hence, power consumption is large and chips generate a large amount of heat. Using the GPU or the chip of one embodiment of the present invention in the supercomputer  5500  achieves a low-power-consumption supercomputer. Moreover, heat generation from a circuit can be reduced owing to low power consumption; thus, the influence of heat generation on the circuit, a peripheral circuit, and a module can be reduced. 
     Although a supercomputer is shown as an example of a large computer in  FIG. 36E  and  FIG. 36F , a large computer using the GPU or the chip of one embodiment of the present invention is not limited thereto. Other examples of large computers in which the GPU or the chip of one embodiment of the present invention is usable include a computer that provides service (a server) and a large general-purpose computer (a mainframe). 
     [Moving Vehicle] 
     The GPU or the chip of one embodiment of the present invention can be applied to an automobile, which is a moving vehicle, and the periphery of a driver&#39;s seat in the automobile. 
       FIG. 36G  shows an area around a windshield inside an automobile, which is an example of a moving vehicle.  FIG. 36G  shows a display panel  5701 , a display panel  5702 , and a display panel  5703  that are attached to a dashboard and a display panel  5704  that is attached to a pillar. 
     The display panel  5701  to the display panel  5703  can provide a variety of kinds of information by displaying a speedometer, a tachometer, mileage, a fuel gauge, a gear state, air-condition setting, and the like. In addition, the content, layout, or the like of the display on the display panels can be changed as appropriate to suit the user&#39;s preference, so that the design quality can be increased. The display panel  5701  to the display panel  5703  can also be used as lighting devices. 
     The display panel  5704  can compensate for view obstructed by the pillar (a blind spot) by showing an image taken by an imaging device (not shown) provided for the automobile. That is, displaying an image taken by the imaging device provided outside the automobile leads to compensation for the blind spot and an increase in safety. In addition, displaying an image to compensate for a portion that cannot be seen makes it possible for the driver to confirm the safety more naturally and comfortably. The display panel  5704  can also be used as a lighting device. 
     Since the GPU or the chip of one embodiment of the present invention can be applied to a component of artificial intelligence, the chip can be used for an automatic driving system of the automobile, for example. The chip can also be used for a system for navigation, risk prediction, or the like. A structure may be employed in which the display panel  5701  to the display panel  5704  display navigation information, risk prediction information, or the like. 
     Note that although an automobile is described above as an example of a moving vehicle, the moving vehicle is not limited to an automobile. Examples of the moving vehicle include a train, a monorail train, a ship, and a flying vehicle (a helicopter, an unmanned aircraft (a drone), an airplane, and a rocket), and these moving vehicles can each include a system utilizing artificial intelligence when the chip of one embodiment of the present invention is applied to each of these moving vehicles. 
     [Household Appliance] 
       FIG. 36H  shows an electric refrigerator-freezer  5800  as an example of a household appliance. The electric refrigerator-freezer  5800  includes a housing  5801 , a refrigerator door  5802 , a freezer door  5803 , and the like. 
     When the chip of one embodiment of the present invention is applied to the electric refrigerator-freezer  5800 , the electric refrigerator-freezer  5800  including artificial intelligence can be achieved. Utilizing the artificial intelligence enables the electric refrigerator-freezer  5800  to have a function of automatically making a menu based on foods stored in the electric refrigerator-freezer  5800 , expiration dates of the foods, or the like, a function of automatically adjusting temperature to be appropriate for the foods stored in the electric refrigerator-freezer  5800 , and the like. 
     Although the electric refrigerator-freezer is described in this example as a household appliance, examples of other household appliances include a vacuum cleaner, a microwave oven, an electric oven, a rice cooker, a water heater, an IH cooker, a water server, a heating-cooling combination appliance such as an air conditioner, a washing machine, a drying machine, and an audio visual appliance. 
     The electronic devices, the functions of the electronic devices, the application examples of artificial intelligence, their effects, and the like described in this embodiment can be combined as appropriate with the description of another electronic device. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments and the like, as appropriate. 
     REFERENCE NUMERALS 
     
         
         M 1 : transistor, M 2 : transistor, M 3 : transistor, M 4 : transistor, M 5 : transistor, M 6 : transistor,  100 : capacitor,  110 : conductor,  112 : conductor,  115 : conductor,  120 : conductor,  125 : conductor,  130 : insulator,  140 : conductor,  142 : insulator,  145 : insulator,  150 : insulator,  152 : insulator,  153 : conductor,  154 : insulator,  156 : insulator,  200 : transistor,  200 _ n : transistor,  200 _ 1 : transistor,  200   a : transistor,  200   b : transistor,  200 T: transistor,  205 : conductor,  205   a : conductor,  205   b : conductor,  210 : insulator,  212 : insulator,  214 : insulator,  216 : insulator,  217 : insulator,  218 : conductor,  222 : insulator,  224 : insulator,  230 : oxide,  230   a : oxide,  230 A: oxide film,  230   b : oxide,  230 B: oxide film,  230   c : oxide,  230 C: oxide film,  230   d : oxide,  230 D: oxide film,  240 : conductor,  240   a : conductor,  240   b : conductor,  241 : insulator,  241   a : insulator,  241   b : insulator,  242 : conductor,  242   a : conductor,  242 A: conductive film,  242   b : conductor,  242 B: conductive layer,  242   c : conductor,  243 : oxide,  243   a : oxide,  243 A: oxide film,  243   b : oxide,  243 B: oxide layer,  246 : conductor,  246   a : conductor,  246   b : conductor,  248 : conductive layer,  248 A: conductive film,  250 : insulator,  250 A: insulating film,  260 : conductor,  260   a : conductor,  260 A: conductive film,  260   b : conductor,  260 B: conductive film,  265 : sealing portion,  265   a : sealing portion,  265   b : sealing portion,  271 : insulator,  271   a : insulator,  271 A: insulating film,  271   b : insulator,  271 B: insulating layer,  271   c : insulator,  272 : insulator,  274 : insulator,  280 : insulator,  282 : insulator,  283 : insulator,  286 : insulator,  290 : semiconductor device,  292 : capacitor device,  292   a : capacitor device,  292   b : capacitor device,  294 : conductor,  294   a : conductor,  294   b : conductor,  300 : transistor,  311 : substrate,  313 : semiconductor region,  314   a : low-resistance region,  314   b : low-resistance region,  315 : insulator,  316 : conductor,  320 : insulator,  322 : insulator,  324 : insulator,  326 : insulator,  328 : conductor,  330 : conductor,  350 : insulator,  352 : insulator,  354 : insulator,  356 : conductor,  411 : element layer,  413 : transistor layer,  415 : memory device layer,  415 _ 1 : memory device layer,  415 _ 3 : memory device layer,  415 _ 4 : memory device layer,  420 : memory device,  424 : conductor,  440 : conductor,  470 : memory unit,  600 : semiconductor device,  601 : semiconductor device,  610 : cell array,  610 _ n : cell array,  610 _ 1 : cell array,  700 : electronic component,  702 : printed circuit board,  704 : circuit board,  711 : mold,  712 : land,  713 : electrode pad,  714 : wire,  720 : storage device,  721 : driver circuit layer,  722 : storage circuit layer,  730 : electronic component,  731 : interposer,  732 : package substrate,  733 : electrode,  735 : semiconductor device,  901 : boundary region,  902 : boundary region,  1001 : wiring,  1002 : wiring,  1003 : wiring,  1004 : wiring,  1005 : wiring,  1006 : wiring.