Patent Publication Number: US-9853165-B2

Title: Method for manufacturing semiconductor device

Description:
TECHNICAL FIELD 
     The present invention relates to, for example, a semiconductor, a conductor, an insulator, a transistor, or a semiconductor device. The present invention relates to, for example, a method for manufacturing a semiconductor, a conductor, an insulator, a transistor, or a semiconductor device. The present invention relates to, for example, a semiconductor, a conductor, an insulator, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, a processor, or an electronic device. The present invention relates to a method for manufacturing a semiconductor, a conductor, an insulator, a display device, a liquid crystal display device, a light-emitting device, a memory device, or an electronic device. The present invention relates to a driving method of a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a memory device, or an electronic device. 
     Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. 
     In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases. 
     BACKGROUND ART 
     A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor. 
     As silicon which is used as a semiconductor of a transistor, either amorphous silicon or polycrystalline silicon is used depending on the purpose. For example, in the case of a transistor included in a large display device, it is preferable to use amorphous silicon, which can be used to form a film on a large substrate with the established technique. In the case of a transistor included in a high-performance display device where a driver circuit and a pixel circuit are formed over the same substrate, it is preferable to use polycrystalline silicon, which can be used to form a transistor having a high field-effect mobility. As a method for forming polycrystalline silicon, high-temperature heat treatment or laser light treatment which is performed on amorphous silicon has been known. 
     In recent years, transistors including oxide semiconductors (typically, In—Ga—Zn oxide) have been actively developed. 
     Oxide semiconductors have been researched since early times. In 1988, there was a disclosure of a crystal In—Ga—Zn oxide that can be used for a semiconductor element (see Patent Document 1). In 1995, a transistor including an oxide semiconductor was invented, and its electrical characteristics were disclosed (see Patent Document 2). 
     The transistor including an oxide semiconductor has different features from a transistor including amorphous silicon or polycrystalline silicon. For example, a display device in which a transistor including an oxide semiconductor is used is known to have small power consumption. An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used in a transistor included in a large display device. Because a transistor including an oxide semiconductor has high field-effect mobility, a high-performance display device in which, for example, a driver circuit and a pixel circuit are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized. 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. S63-239117 
     [Patent Document 2] Japanese translation of PCT international application No. H11-505377 
     DISCLOSURE OF INVENTION 
     An object is to provide a minute shape. Another object is to provide a transistor with a small channel length. Another object is to provide a transistor with a small subthreshold swing value. Another object is to provide a transistor having a small short-channel effect. Another object is to provide a transistor with normally-off electrical characteristics. Another object is to provide a transistor having a low leakage current in an off state. Another object is to provide a transistor with excellent electrical characteristics. Another object is to provide a highly reliable transistor. Another object is to provide a transistor with high frequency characteristics. 
     Another object is to provide a semiconductor device including the transistor. Another object is to provide a module including the semiconductor device. Another object is to provide an electronic device including the semiconductor device or the module. Another object is to provide a novel semiconductor device. Another object is to provide a novel module. Another object is to provide a novel electronic device. 
     Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is a method for manufacturing a semiconductor device, including the steps of forming a semiconductor over a substrate; forming a first conductor over the semiconductor; forming a first insulator over the first conductor; forming a resist over the first insulator; performing light exposure and development on the resist to make a second region and a third region of the resist remain and expose part of the first insulator; applying a bias in a direction perpendicular to a top surface of the substrate and generating plasma using a gas containing carbon and halogen; depositing and etching an organic substance with the plasma; etching the first insulator using the organic substance, the second region, and the third region as masks to form a second insulator and a third insulator and expose the first conductor; etching the first conductor using the second insulator and the third insulator as masks to form a second conductor and a third conductor and expose the semiconductor; removing the organic substance, the second region, and the third region; forming a fourth insulator over an exposed part of the semiconductor; and forming a fourth conductor over the fourth insulator. In the embodiment, an etching rate of the organic substance is higher than a deposition rate of the organic substance in the exposed part of the first insulator, and the deposition rate of the organic substance is higher than the etching rate of the organic substance in a side surface of the second region. 
     In the above structure of one embodiment of the present invention, a distance between the second conductor and the third conductor is less than or equal to 80% of a distance between the second region and the third region. 
     A minute shape can be provided. A transistor with a small channel length can be provided. A transistor with a small subthreshold swing value can be provided. A transistor having a small short-channel effect can be provided. A transistor with normally-off electrical characteristics can be provided. A transistor having a low leakage current in an off state can be provided. A transistor with excellent electrical characteristics can be provided. A highly reliable transistor can be provided. A transistor with high frequency characteristics can be provided. 
     A semiconductor device including the transistor can be provided. A module including the semiconductor device can be provided. An electronic device including the semiconductor device or the module can be provided. A novel semiconductor device can be provided. A novel module can be provided. A novel electronic device can be provided. 
     Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1E  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 2A to 2E  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 3A to 3D  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 4A to 4E  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 5A to 5E  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 6A to 6E  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 7A to 7D  are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention. 
         FIGS. 8A and 8B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 9A and 9B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 10A and 10B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 11A and 11B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 12A to 12C  are cross-sectional views illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 13A to 13E  are cross-sectional views and a band diagram of a transistor of one embodiment of the present invention. 
         FIGS. 14A and 14B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 15A and 15B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 16A and 16B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 17A and 17B  are a top view and a cross-sectional view illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 18A and 18B  are a top view and cross-sectional views illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 19A to 19C  are cross-sectional views illustrating a method for manufacturing a transistor of one embodiment of the present invention. 
         FIGS. 20A and 20B  are circuit diagrams illustrating a semiconductor device of one embodiment of the present invention. 
         FIG. 21  is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIG. 22  is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIG. 23  is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 24A and 24B  are circuit diagrams illustrating a memory device of one embodiment of the present invention. 
         FIG. 25  is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIG. 26  is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIG. 27  is a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 28A and 28B  are top views each illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 29A and 29B  are block diagrams illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 30A and 30B  are cross-sectional views each illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 31A and 31B  are cross-sectional views each illustrating a semiconductor device of one embodiment of the present invention. 
       FIGS.  32 A 1  to  32 A 3  and  32 B 1  to  32 B 3  are perspective views and cross-sectional views of semiconductor devices of one embodiment of the present invention. 
         FIG. 33  is a block diagram illustrating a semiconductor device of one embodiment of the present invention. 
         FIG. 34  is a circuit diagram illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 35A to 35C  are a circuit diagram, a top view, and a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 36A and 36B  are a circuit diagram and a cross-sectional view illustrating a semiconductor device of one embodiment of the present invention. 
         FIGS. 37A to 37F  are perspective views each illustrating an electronic device of one embodiment of the present invention. 
         FIGS. 38A to 38C  are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and  FIG. 38D  is a schematic cross-sectional view of the CAAC-OS. 
         FIGS. 39A to 39D  are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS. 
         FIGS. 40A to 40C  show structural analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD. 
         FIGS. 41A and 41B  show electron diffraction patterns of a CAAC-OS. 
         FIG. 42  shows a change of crystal parts of an In—Ga—Zn oxide owing to electron irradiation. 
         FIGS. 43A and 43B  are schematic views showing deposition models of a CAAC-OS and an nc-OS. 
         FIGS. 44A to 44C  show an InGaZnO 4  crystal and a pellet. 
         FIGS. 45A to 45D  are schematic views showing a deposition model of a CAAC-OS. 
         FIGS. 46A and 46B  are STEM images. 
         FIGS. 47A and 47B  are STEM images. 
         FIGS. 48A and 48B  are a top view and a cross-sectional view of a transistor. 
         FIGS. 49A and 49B  show Id-Vg characteristics of transistors. 
         FIGS. 50A and 50B  show Id-Vg characteristics of transistors. 
         FIGS. 51A to 51C  are cross-sectional views each illustrating a transistor of one embodiment of the present invention. 
         FIGS. 52A to 52C  are cross-sectional views each illustrating a transistor of one embodiment of the present invention. 
         FIGS. 53A to 53C  are cross-sectional views each illustrating a transistor of one embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments and examples of the present invention will be described in detail with the reference to the drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. Further, the present invention is not construed as being limited to description of the embodiments and the examples. In describing structures of the present invention with reference to the drawings, common reference numerals are used for the same portions in different drawings. Note that the same hatched pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases. 
     Note that the size, the thickness of films (layers), or regions in drawings is sometimes exaggerated for simplicity. 
     In this specification, the terms “film” and “layer” can be interchanged with each other. 
     A voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a source potential or a ground potential (GND)). A voltage can be referred to as a potential and vice versa. Note that in general, a potential (a voltage) is relative and is determined depending on the amount relative to a certain potential. Therefore, a potential which is represented as a “ground potential” or the like is not always 0 V. For example, the lowest potential in a circuit may be represented as a “ground potential”. Alternatively, a substantially intermediate potential in a circuit may be represented as a “ground potential”. In these cases, a positive potential and a negative potential are set using the potential as a reference. 
     Note that the ordinal numbers such as “first” and “second” 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 correspond to the ordinal numbers which specify one embodiment of the present invention in some cases. 
     Note that a “semiconductor” has characteristics of an “insulator” in some cases when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border therebetween is not clear. Accordingly, a “semiconductor” in this specification can be called an “insulator” in some cases. Similarly, an “insulator” in this specification can be called a “semiconductor” in some cases. 
     Further, a “semiconductor” has characteristics of a “conductor” in some cases when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border therebetween is not clear. Accordingly, a “semiconductor” in this specification can be called a “conductor” in some cases. Similarly, a “conductor” in this specification can be called a “semiconductor” in some cases. 
     Note that an impurity in a semiconductor refers to, for example, elements other than the main components of the semiconductor. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. When an impurity is contained, the density of states (DOS) may be formed in a semiconductor, the carrier mobility may be decreased, or the crystallinity may be decreased. In the case where the semiconductor is an oxide semiconductor, examples of an impurity which changes characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specifically, there are hydrogen (included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, for example. In the case of an oxide semiconductor, oxygen vacancies may be formed by entry of impurities such as hydrogen. In the case where the semiconductor is silicon, examples of an impurity which changes characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements. 
     In this specification, the phrase “A has a region with a concentration B” includes, for example, “the concentration of the entire region in a region of A in the depth direction is B”, “the average concentration in a region of A in the depth direction is B”, “the median value of a concentration in a region of A in the depth direction is B”, “the maximum value of a concentration in a region of A in the depth direction is B”, “the minimum value of a concentration in a region of A in the depth direction is B”, “a convergence value of a concentration in a region of A in the depth direction is B”, and “a concentration in a region of A in which a probable value is obtained in measurement is B”. 
     In this specification, the phrase “A has a region with a size B, a length B, a thickness B, a width B, or a distance B” includes, for example, “the size, the length, the thickness, the width, or the distance of the entire region in a region of A is B”, “the average value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the median value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the maximum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the minimum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “a convergence value of the size, the length, the thickness, the width, or the distance of a region of A is B”, and “the size, the length, the thickness, the width, or the distance of a region of A in which a probable value is obtained in measurement is B”. 
     Note that the channel length refers to, for example, the 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 a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other or a region where a channel is formed in a plan view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor (or a portion where a current flows in a semiconductor when a transistor is on) and a gate electrode overlap with each other, or a region where a channel is formed. In one transistor, channel widths in all regions are not necessarily the same. In other words, the channel width of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed. 
     Note that depending on transistor structures, a channel width in a region where a channel is formed actually (hereinafter referred to as an effective channel width) is different from a channel width shown in a plan view of a transistor (hereinafter referred to as an apparent channel width) in some cases. For example, in a transistor having a three-dimensional structure, an effective channel width is greater than an apparent channel width shown in a plan view of the transistor, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a three-dimensional structure, the proportion of a channel region formed in a side surface of a semiconductor is high in some cases. In that case, an effective channel width obtained when a channel is actually formed is greater than an apparent channel width shown in the plan view. 
     In a transistor having a three-dimensional structure, an effective channel width is difficult to measure in some cases. For example, to estimate an effective channel width from a design value, it is necessary to assume that the shape of a semiconductor is known as an assumption condition. Therefore, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure an effective channel width accurately. 
     Therefore, in this specification, in a plan view of a transistor, an apparent channel width that is a length of a portion where a source and a drain face each other in a region where a semiconductor and a gate electrode overlap with each other is referred to as a surrounded channel width (SCW) in some cases. Further, in this specification, in the case where the term “channel width” is simply used, it may denote a surrounded channel width and an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may denote an effective channel width in some cases. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by obtaining and analyzing a cross-sectional TEM image and the like. 
     Note that in the case where electric field mobility, a current value per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width may be used for the calculation. In that case, the values might be different from those calculated by using an effective channel width. 
     Note that in this specification, the description “A has a shape such that an end portion extends beyond an end portion of B” may indicate, for example, the case where at least one of end portions of A is positioned on an outer side than at least one of end portions of B in a top view or a cross-sectional view. Thus, the description “A has a shape such that an end portion extends beyond an end portion of B” can be read as the description “one end portion of A is positioned on an outer side than one end portion of B in a top view,” for example. 
     In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. A term “substantially parallel” indicates that the angle formed between two straight lines is greater than or equal to −30° and less than or equal to 30°. The term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. A term “substantially perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 60° and less than or equal to 120°. 
     In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system. 
     &lt;Processing Method 1&gt; 
     A method for processing a conductor, an insulator, or a semiconductor of one embodiment of the present invention is described below. 
     First, a layer  116  and a layer  110  over the layer  116  are prepared (see  FIG. 1A ). A conductor, an insulator, or a semiconductor can be used as the layer  116 . In addition, a conductor, an insulator, or a semiconductor can be used as the layer  110 . 
     The conductor may be formed to have a single-layer structure or a stacked-layer structure including a conductor containing, for example, one or more of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     The insulator may be formed to have a single-layer structure or a stacked-layer structure including 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. For example, aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide may be used as the insulator. In particular, an oxide containing silicon is preferably used. 
     As the semiconductor, a Group 14 semiconductor such as silicon or germanium, a compound semiconductor such as silicon carbide, germanium silicide, gallium arsenide, indium phosphide, zinc selenide, cadmium sulfide, or an oxide semiconductor, an organic semiconductor, or the like may be used. An oxide semiconductor is described later. 
     Next, a bottom anti-reflective coating (BARC) is formed. Then, a resist is formed. After that, the resist is processed. To process the resist, first, the resist is exposed to light using a photomask or the like. At this time, the action of the BARC can inhibit halation. Next, a light-exposed region is removed or left using a developing solution, so that resists  122  are formed. For the light exposure of the resist, KrF excimer laser light, ArF excimer laser light, extreme ultraviolet (EUV) light, or the like may be used. Alternatively, a liquid immersion technique may be employed in which a portion between a substrate and a projection lens is filled with liquid (e.g., water) to perform light exposure. An electron beam or an ion beam may be used instead of the above-mentioned light. Note that a photomask is not necessary in the case of using an electron beam or an ion beam. 
     Next, the BARC is etched using the resists  122  as masks to form BARCs  120  (see  FIG. 1B ). Note that in some cases, an organic or inorganic substance without a function of an anti-reflection layer may be used instead of the BARCs  120 . A structure without the BARCs  120  may be employed in some cases. 
     The distance between the resists  122  is denoted by L 0 . The minimum (also referred to as the minimum feature size) of L 0  is determined by a light-exposure apparatus, a resist, or the like. 
     Next, plasma treatment is performed. The plasma treatment can be performed by a parallel plate reactive ion etching (RIE) method, an inductively coupled plasma (ICP) etching method, or the like. 
     Plasma is generated with the use of a gas containing carbon and halogen. The plasma reacts with carbon, hydrogen, and the like contained in the resists  122  and the like, whereby an organic substance is deposited over the processing surfaces (e.g., the top and side surfaces of the resists  122 , the side surfaces of the BARCs  120 , and an exposed part of the layer  110 ). The organic substance is deposited isotropically. Here, when a bias is applied in a direction perpendicular to the top surfaces of the layers  116  and  110 , deposition and etching of the organic substance occur concurrently. The etching of the organic substance is performed anisotropically because the etching rate in the direction of the bias application is high. 
     As the gas containing carbon and halogen, for example, a gas containing carbon and fluorine, such as a trifluoromethane gas, a tetrafluoromethane gas, a hexafluoroethane gas, a hexafluoropropane gas, an octafluoropropane gas, or an octafluorocyclobutane gas; a gas containing carbon and chlorine, such as a carbon tetrachloride gas; or the like can be used. Alternatively, hydrogen, a rare gas such as helium or argon, and the like may be mixed to be used. 
     The deposition and etching rates of the organic substance are determined by the composite action of various conditions. For example, increasing the proportion of carbon in the gas used for generating plasma increases the deposition rate, whereas increasing the proportion of halogen in the gas increases the etching rate. Furthermore, for example, reducing the bias reduces the etching rate, whereas increasing the bias increases the etching rate. Here, conditions that make the etching rate higher than the deposition rate in the direction of the bias application are used. Therefore, the organic substance on the top surfaces of the resists  122  and the exposed part of the layer  110  is etched almost as soon as it is deposited. Furthermore, the exposed part of the layer  110  is also etched. Note that it is possible not to etch the exposed part of the layer  110 , according to the conditions of the plasma treatment. Furthermore, the conditions of the plasma treatment can be changed in stages, e.g., in two stages or three stages. 
     Meanwhile, the etching rate of the organic substance is lower than the deposition rate thereof on the side surfaces of the resists  122  and on the side surfaces of the BARCs  120 . Accordingly, an organic substance  124  is deposited on the regions (see  FIG. 1C ). 
     Next, the layer  110  and the layer  116  are etched using the organic substance  124 , the resists  122 , and the BARCs  120  as masks to form layers  110   a  and  110   b  and layers  116   a  and  116   b  (see  FIG. 1D ). The etching of the layer  110  and the layer  116  can be performed by dry etching and/or wet etching. At this time, the organic substance  124  may be removed. Note that the layer  110   a  and the layer  110   b  may be connected to each other in the depth direction. Furthermore, the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 1E ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. 
     At this time, the layer  110   a  includes a first region, a second region, and a third region. The second region is located between the first region and the third region. The first region is a flat region. The second region and the third region each have a slope. The slope of the second region is more gentle than that of the third region. In the second region, there may be variation in the degree of the slope between the vicinity of the first region and the vicinity of the third region. For example, the vicinity of the first region may have a steep slope, and the vicinity of the third region may have a gentle slope. Such a shape of the layer  110   a  can increase step coverage with a layer to be formed over the layer  110   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  110   b . Note that the slope refers to a change in the thickness, and the slope angle may be a right angle. 
     &lt;Processing Method 2&gt; 
     It is possible to obtain a shape different from that in  FIG. 1E  by changing the conditions for the plasma treatment, as shown in  FIGS. 2A to 2E . 
     Since  FIGS. 2A and 2B  are the same as  FIGS. 1A and 1B , respectively, the description thereof is omitted. 
     Next, plasma treatment is performed. The plasma treatment causes deposition and etching of an organic substance. Furthermore, the exposed part of the layer  110  is also etched. Here, the layer  110  is etched until the layer  116  is exposed while the organic substance  124  is deposited on the side surfaces of the resists  122  and the BARCs  120 , whereby the layer  110   a  and the layer  110   b  are formed (see  FIG. 2C ). Note that the layer  110   a  and the layer  110   b  may be connected to each other in the depth direction. 
     Next, the layer  116  is etched using the organic substance  124 , the resists  122 , and the BARCs  120  as masks to form the layers  116   a  and  116   b  (see  FIG. 2D ). The etching of the layer  116  can be performed by dry etching and/or wet etching. At this time, the organic substance  124  may be removed. Note that the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 2E ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. 
     At this time, the layer  110   a  includes a first region and a second region. The first region is a flat region. The second region has a slope. In the second region, there may be variation in the degree of the slope. For example, the second region may have a shape in which the vicinity of the first region has a steep slope, and the slope gradually becomes gentle as the distance from the first region is increased. Such a shape of the layer  110   a  can increase step coverage with a layer to be formed over the layer  110   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  110   b.    
     &lt;Processing Method 3&gt; 
     It is possible to obtain a shape different from those in  FIG. 1E  and  FIG. 2E  by changing the conditions for the plasma treatment, as shown in  FIGS. 3A to 3D . 
     Since  FIGS. 3A and 3B  are the same as  FIGS. 1A and 1B , respectively, the description thereof is omitted. 
     Next, plasma treatment is performed. The plasma treatment causes deposition and etching of an organic substance. Furthermore, the exposed part of the layer  110  is also etched. Here, the layer  110  and the layer  116  are etched while the organic substance  124  is deposited on the side surfaces of the resists  122  and the BARCs  120 , whereby the layers  110   a  and  110   b  and the layers  116   a  and  116   b  are formed (see  FIG. 3C ). Note that the layer  110   a  and the layer  110   b  may be connected to each other in the depth direction. Furthermore, the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 3D ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. 
     At this time, the layer  110   a  includes a first region and a second region. The first region is a flat region. The second region has a slope. In the second region, there may be variation in the degree of the slope. For example, the second region may have a shape in which the vicinity of the first region has a steep slope, and the slope gradually becomes gentle as the distance from the first region is increased. Furthermore, the layer  116   a  includes a third region and a fourth region. The third region is a flat region. The fourth region has a slope. In the fourth region, there may be variation in the degree of the slope. For example, the fourth region may have a shape in which the vicinity of the third region has a steep slope, and the slope gradually becomes gentle as the distance from the third region is increased. Such shapes of the layer  110   a  and the layer  116   a  can increase step coverage with layers to be formed over the layer  110   a  and the layer  116   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  110   b  and the layer  116   b.    
     &lt;Processing Method 4&gt; 
     It is also possible to obtain a shape different from those in  FIG. 1E ,  FIG. 2E , and  FIG. 3D  by adding an etching step, as shown in  FIGS. 4A to 4E . 
     Since  FIGS. 4A and 4B  are the same as  FIGS. 1A and 1B , respectively, the description thereof is omitted. 
     Next, the layer  110  is etched using the resists  122  and the BARCs  120  as masks so that the layer  116  is exposed, whereby the layer  110   a  and the layer  110   b  are formed (see  FIG. 4C ). The etching of the layer  110  can be performed by dry etching and/or wet etching. Note that the layer  110   a  and the layer  110   b  may be connected to each other in the depth direction. 
     Next, plasma treatment is performed. Next, plasma treatment is performed. The plasma treatment causes deposition and etching of an organic substance. Furthermore, the exposed part of the layer  116  is also etched. Note that it is also possible not to etch the exposed part of the layer  116 , according to the conditions for the plasma treatment. 
     Meanwhile, the etching rate of the organic substance is lower than the deposition rate thereof on the side surfaces of the resists  122 , on the side surfaces of the BARCs  120 , and on the side surfaces of the layers  110   a  and  110   b . Accordingly, the organic substance  124  is deposited on the regions. 
     Next, the layer  116  is etched using the organic substance  124 , the resists  122 , and the BARCs  120  as masks to form the layers  116   a  and  116   b  (see  FIG. 4D ). The etching of the layer  116  can be performed by dry etching and/or wet etching. At this time, the organic substance  124  may be removed. Note that the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 4E ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. 
     At this time, the layer  116   a  includes a first region, a second region, and a third region. The second region is located between the first region and the third region. The first region is a flat region. The second region and the third region each have a slope. The slope of the second region is more gentle than that of the third region. In the second region, there may be variation in the degree of the slope between the vicinity of the first region and the vicinity of the third region. For example, the vicinity of the first region may have a steep slope, and the vicinity of the third region may have a gentle slope. Such a shape of the layer  116   a  can increase step coverage with a layer to be formed over the layer  116   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  116   b.    
     &lt;Processing Method 5&gt; 
     It is also possible to obtain a shape different from those in  FIG. 1E ,  FIG. 2E ,  FIG. 3D , and  FIG. 4E  by adding an etching step and changing the conditions for the plasma treatment, as illustrated in  FIGS. 5A to 5E . 
     Since  FIGS. 5A and 5B  are the same as  FIGS. 1A and 1B , respectively, the description thereof is omitted. Since  FIG. 5C  is the same as  FIG. 4C , the description thereof is omitted. 
     Next, plasma treatment is performed. The plasma treatment causes deposition and etching of an organic substance. Furthermore, the exposed part of the layer  116  is also etched, whereby the layer  116   a  and the layer  116   b  are formed. Note that the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     Meanwhile, the etching rate of the organic substance is lower than the deposition rate thereof on the side surfaces of the resists  122 , on the side surfaces of the BARCs  120 , and on the side surfaces of the layer  110   a  and  110   b . Accordingly, the organic substance  124  is deposited on the regions (see  FIG. 5D ). 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 5E ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. 
     At this time, the layer  116   a  includes a first region and a second region. The first region is a flat region. The second region has a slope. In the second region, there may be variation in the degree of the slope. For example, the second region may have a shape in which the vicinity of the first region has a steep slope, and the slope gradually becomes gentle as the distance from the first region is increased. Such a shape of the layer  116   a  can increase step coverage with a layer to be formed over the layer  116   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  116   b.    
     &lt;Processing Method 6&gt; 
     It is also possible to obtain a shape different from those in  FIG. 1E ,  FIG. 2E ,  FIG. 3D ,  FIG. 4E , and  FIG. 5E  by not providing the layer  110 , as illustrated in  FIGS. 6A to 6E . 
     First, the layer  116  is prepared (see  FIG. 6A ). 
     Next, a BARC is formed. Then, a resist is formed. After that, the resist is processed to form the resists  122 . 
     Next, the BARC is etched using the resists  122  as masks, whereby the BARCs  120  are formed (see  FIG. 6B ). Note that it is possible not to provide the BARCs  120  in some cases. 
     The distance between the resists  122  is denoted by L 0 . 
     Next, plasma treatment is performed. Next, plasma treatment is performed. The plasma treatment causes deposition and etching of an organic substance. Furthermore, the exposed part of the layer  110  is also etched. Note that it is also possible not to etch the exposed part of the layer  110 , according to the conditions for the plasma treatment. 
     Meanwhile, the etching rate of the organic substance is lower than the deposition rate thereof on the side surfaces of the resists  122  and on the side surfaces of the BARCs  120 . Accordingly, the organic substance  124  is deposited on the regions (see  FIG. 6C ). 
     Next, the layer  116  is etched using the organic substance  124 , the resists  122 , and the BARCs  120  as masks to form the layers  116   a  and  116   b  (see  FIG. 6D ). The etching of the layer  116  can be performed by dry etching and/or wet etching. At this time, the organic substance  124  may be removed. Note that the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 6E ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. A shape similar to that in  FIG. 6E  can also be obtained in such a manner that the layer  110   a  and the layer  110   b  are removed after the shape in  FIG. 4E  is obtained. 
     At this time, the layer  116   a  includes a first region, a second region, and a third region. The second region is located between the first region and the third region. The first region is a flat region. The second region and the third region each have a slope. The slope of the second region is more gentle than that of the third region. In the second region, there may be variation in the degree of the slope between the vicinity of the first region and the vicinity of the third region. For example, the vicinity of the first region may have a steep slope, and the vicinity of the third region may have a gentle slope. Such a shape of the layer  116   a  can increase step coverage with a layer to be formed over the layer  116   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  116   b.    
     &lt;Processing Method 7&gt; 
     It is also possible to obtain a shape different from those in  FIG. 1E ,  FIG. 2E ,  FIG. 3D ,  FIG. 4E ,  FIG. 5E , and  FIG. 6E  by changing the conditions for the plasma treatment and the like, as illustrated in  FIGS. 7A to 7D . 
     Since  FIGS. 7A and 7B  are the same as  FIGS. 6A and 6B , respectively, the description thereof is omitted. 
     Next, plasma treatment is performed. The plasma treatment causes deposition and etching of an organic substance. Furthermore, the exposed part of the layer  116  is also etched, whereby the layer  116   a  and the layer  116   b  are formed. Note that the layer  116   a  and the layer  116   b  may be connected to each other in the depth direction. 
     Meanwhile, the etching rate of the organic substance is lower than the deposition rate thereof on the side surfaces of the resists  122  and on the side surfaces of the BARCs  120 . Accordingly, the organic substance  124  is deposited on the regions (see  FIG. 7C ). 
     The distance between the layers  116   a  and  116   b  is denoted by L 1 . L 1  is smaller than L 0  by the thickness of the organic substance  124 . That is, a shape smaller than the minimum feature size determined by a light-exposure apparatus or a resist can be obtained. 
     Next, the organic substance  124 , the resists  122 , and the BARCs  120  are removed, whereby a hole smaller than the minimum feature size can be formed (see  FIG. 7D ). The removal of the organic substance  124 , the resists  122 , and the BARCs  120  can be performed by dry etching such as plasma ashing and/or wet etching. A shape similar to that in  FIG. 7D  can also be obtained in such a manner that the layer  110   a  and the layer  110   b  are removed after the shape in  FIG. 5E  is obtained. 
     At this time, the layer  116   a  includes a first region and a second region. The first region is a flat region. The second region has a slope. In the second region, there may be variation in the degree of the slope. For example, the second region may have a shape in which the vicinity of the first region has a steep slope, and the slope gradually becomes gentle as the distance from the first region is increased. Such a shape of the layer  116   a  can increase step coverage with a layer to be formed over the layer  116   a ; therefore, a defect in shape is less likely to occur. The same applies to the layer  116   b.    
     As described above, any of the processing methods of one embodiment of the present invention makes it possible to process a layer in a size smaller than the minimum feature size. Furthermore, any of the processing methods enables formation of a layer that is less likely to have a defect in shape. 
     &lt;Transistor 1&gt; 
     A transistor of one embodiment of the present invention is described below. 
       FIG. 8A ,  FIG. 9A ,  FIG. 10A , and  FIG. 11A  are top views illustrating a method for manufacturing a transistor.  FIG. 8B ,  FIG. 9B ,  FIG. 10B , and  FIG. 11B  are each a cross-sectional view taken along dashed dotted lines A 1 -A 2  and A 3 -A 4  shown in the corresponding top view. 
     First, a substrate  400  is prepared. 
     As the substrate  400 , an insulator substrate, a semiconductor substrate, or a conductor substrate may be used, for example. As the insulator substrate, a glass substrate, a quartz substrate, a sapphire substrate, a stabilized zirconia substrate (e.g., an yttria-stabilized zirconia substrate), or a resin substrate is used, for example. As the semiconductor substrate, a single material semiconductor substrate of silicon, germanium, or the like or a compound semiconductor substrate of silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, gallium oxide, or the like is used, for example. A semiconductor substrate in which an insulator region is provided in the above semiconductor substrate, e.g., a silicon on insulator (SOI) substrate or the like is used. As the conductor substrate, a graphite substrate, a metal substrate, an alloy substrate, a conductive resin substrate, or the like is used. A substrate including a metal nitride, a substrate including a metal oxide, or the like is used. An insulator substrate provided with a conductor or a semiconductor, a semiconductor substrate provided with a conductor or an insulator, a conductor substrate provided with a semiconductor or an insulator, or the like is used. Alternatively, any of these substrates over which an element is provided may be used. As the element provided over the substrate, a capacitor, a resistor, a switching element, a light-emitting element, a memory element, or the like is used. 
     Alternatively, a flexible substrate may be used as the substrate  400 . As a method for providing the transistor over a flexible substrate, there is a method in which the transistor is formed over a non-flexible substrate and then the transistor is separated and transferred to the substrate  400  which is a flexible substrate. In that case, a separation layer is preferably provided between the non-flexible substrate and the transistor. As the substrate  400 , a sheet, a film, or a foil containing a fiber may be used. The substrate  400  may have elasticity. The substrate  400  may have a property of returning to its original shape when bending or pulling is stopped. Alternatively, the substrate  400  may have a property of not returning to its original shape. The thickness of the substrate  400  is, for example, greater than or equal to 5 μm and less than or equal to 700 μm, preferably greater than or equal to 10 μm and less than or equal to 500 μm, or further preferably greater than or equal to 15 μm and less than or equal to 300 μm. When the substrate  400  has a small thickness, the weight of the semiconductor device can be reduced. When the substrate  400  has a small thickness, even in the case of using glass or the like, the substrate  400  may have elasticity or a property of returning to its original shape when bending or pulling is stopped. Therefore, an impact applied to the semiconductor device over the substrate  400 , which is caused by dropping or the like, can be reduced. That is, a durable semiconductor device can be provided. 
     For the substrate  400  which is a flexible substrate, metal, an alloy, resin, glass, or fiber thereof can be used, for example. The flexible substrate  400  preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate  400  is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10 −3 /K, lower than or equal to 5×10 −5 /K, or lower than or equal to 1×10 −5 /K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, and acrylic. In particular, aramid is preferably used for the flexible substrate  400  because of its low coefficient of linear expansion. 
     Next, a conductor is formed. The conductor may be formed by a sputtering method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, or the like. 
     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, the CVD method can include a metal CVD (MCVD) method and a metal organic CVD (MOCVD) method depending on a source gas. 
     In the case of a PECVD method, a high quality film can be obtained at relatively low temperature. Furthermore, a TCVD method does not use plasma and thus causes less plasma damage to an object. For example, a wiring, an electrode, an element (e.g., transistor or capacitor), or the like included in a semiconductor device might be charged up by receiving charges from plasma. In that case, accumulated charges might break the wiring, electrode, element, or the like included in the semiconductor device. Such plasma damage is not caused in the case of using a TCVD method, and thus the yield of a semiconductor device can be increased. In addition, since plasma damage does not occur in the deposition by a TCVD method, a film with few defects can be obtained. 
     An ALD method also causes less plasma damage to an object. An ALD method does not cause plasma damage during deposition, so that a film with few defects can be obtained. 
     Unlike in a deposition method in which particles ejected from a target or the like are deposited, in a CVD method and an ALD method, a film is formed by reaction at a surface of an object. Thus, a CVD method and an ALD method enable favorable step coverage almost regardless of the shape of an object. In particular, an ALD method enables excellent step coverage and excellent thickness uniformity and can be favorably used 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; thus, it is sometimes preferable to combine an ALD method with another deposition method with a high deposition rate such as a CVD method. 
     When a CVD method or an ALD method is used, composition of a film to be formed can be controlled with a flow rate ratio of the source gases. For example, by the CVD method or the ALD method, a film with a desired composition can be formed by adjusting the flow ratio of a source gas. Moreover, with a CVD method or an ALD method, by changing the flow rate ratio of the source gases while forming the film, a film whose composition is continuously changed can be formed. In the case where the film is formed while changing the flow rate ratio of the source gases, as compared to the case where the film is formed using a plurality of deposition chambers, time taken for the deposition can be reduced because time taken for transfer and pressure adjustment is omitted. Thus, semiconductor devices can be manufactured with improved productivity. 
     Next, a resist or the like is formed over the conductor and processing is performed using the resist, whereby a conductor  413  is formed. Note that the case where the resist is simply formed also includes the case where a BARC is formed below the resist. 
     The resist is removed after the object is processed by etching or the like. For the removal of the resist, plasma treatment and/or wet etching are/is used. Note that as the plasma treatment, plasma ashing is preferable. In the case where the removal of the resist or the like is not enough, the remaining resist or the like may be removed using ozone water and/or hydrofluoric acid at a concentration higher than or equal to 0.001 volume % and lower than or equal to 1 volume %, and the like. 
     Any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used for processing the conductor to form the conductor  413 . 
     The conductor to be the conductor  413  may be formed to have a single-layer structure or a stacked-layer structure using a conductor containing, for example, one or more of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     Then, an insulator  402  is formed. The insulator  402  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     The insulator  402  may be formed to have a single-layer structure or a stacked-layer structure including 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. The insulator  402  may be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     The insulator  402  may have a function of preventing diffusion of impurities from the substrate  400 . 
     Next, a semiconductor is formed. The semiconductor can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Next, heat treatment is preferably performed. By the heat treatment, the hydrogen concentration in the semiconductor can be reduced in some cases. Furthermore, oxygen vacancies in the semiconductor can be reduced in some cases. 
     Then, a resist or the like is formed over the semiconductor and processing is performed using the resist, whereby a semiconductor  406  is formed (see  FIGS. 8A and 8B ). At this time, part of the insulator  402  over which the semiconductor  406  is not provided may be etched. In this manner, the insulator  402  has a projection. When the insulator  402  has a projection, an s-channel structure, which is described later, can be easily obtained. 
     Any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used for processing the semiconductor to form the semiconductor  406 . 
     Next, a conductor  416  is formed. The conductor  416  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     The conductor  416  may be formed to have a single-layer structure or a stacked-layer structure including a conductor containing, for example, one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     Next, an insulator  410  is formed (see  FIGS. 9A and 9B ). The insulator  410  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     The insulator  410  may have a single-layer structure or a stacked-layer structure including 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. The insulator  410  may be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     Next, a resist or the like is formed over the insulator  410  and processing is performed using the resist, whereby insulators  410   a  and  410   b  and conductors  416   a  and  416   b  are formed (see  FIGS. 10A and 10B ). 
     At this time, any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used. For example, in each of  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D , the layer  116  and the layer  110  may be replaced with the conductor  416  and the insulator  410 , respectively. Here, the case where the insulator  410  and the conductor  416  are processed using a method similar to the processing method in  FIGS. 1A to 1E  is illustrated. 
     For example, when the conductor  413 , the insulator  402 , the conductor  416   a , and the conductor  416   b  serve as a gate electrode, a gate insulator, a source electrode, and a drain electrode, respectively, a bottom-gate transistor may be obtained by completing the steps up to  FIGS. 10A and 10B . 
     Next, an insulator is formed. The insulator can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Next, a conductor is formed. The conductor can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Then, a resist or the like is formed over the conductor and processing is performed using the resist, whereby a conductor  404  is formed. In addition, the insulator is processed using the resist or the conductor  404 , whereby an insulator  412  is formed (see  FIGS. 11A and 11B ). Here, the processing is performed so that the insulator  412  and the conductor  404  have the same shape when seen from the above; however, the shapes are not limited thereto. For example, the insulator  412  and the conductor  404  may be processed using different resists. For example, after the insulator  412  is formed, the conductor to be the conductor  404  may be formed; or after the conductor  404  is formed, a resist or the like may be formed over the insulator to be the insulator  412 . 
     At this time, any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used. For example, in each of  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D , the layer  116  and the layer  110  may be replaced with the insulator to be the insulator  412  and the conductor to be the conductor  404 , respectively. 
     The insulator to be the insulator  412  may be formed to have a single-layer structure or a stacked-layer structure including 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. The insulator to be the insulator  412  may be formed using, for example, aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     The conductor to be the conductor  404  may be formed to have a single-layer structure or a stacked-layer structure including a conductor containing, for example, one or more kinds of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     Next, an insulator may be formed. The insulator can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     The insulator may be formed to have a single-layer structure or a stacked-layer structure including 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. The insulator may be formed to have a single-layer structure or a stacked-layer structure including an insulator containing, for example, aluminum oxide, silicon nitride oxide, silicon nitride, gallium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     The insulator preferably has a function of a barrier layer. The insulator has, for example, a function of blocking oxygen and/or hydrogen. For example, the insulator preferably has a higher capability of blocking oxygen and/or hydrogen than any of the insulator  402  and the insulator  412 . 
     Through the above process, the transistor of one embodiment of the present invention can be manufactured. 
     In the transistor illustrated in  FIG. 11B , the insulator  410   a  is provided between the conductor  416   a  and the conductor  404 , and the insulator  410   b  is provided between the conductor  416   b  and the conductor  404 . Thus, parasitic capacitance due to the conductor  416   a , the conductor  416   b , or the like is small. Therefore, a semiconductor device using the transistor illustrated in  FIG. 11B  has high frequency characteristics. 
     As illustrated in  FIG. 11B , the side surface of the semiconductor  406  is in contact with the conductors  416   a  and  416   b . The semiconductor  406  can be electrically surrounded by an electric field of the conductor  404  (a structure in which a semiconductor is electrically surrounded by an electric field of a conductor is referred to as a surrounded channel (s-channel) structure). Therefore, a channel is formed in the entire semiconductor  406  (the top, bottom, and side surfaces). In the s-channel structure, a large amount of current can flow between a source and a drain of the transistor, so that a high on-state current can be achieved. 
     In the case where the transistor has the s-channel structure, a channel is formed also in the side surface of the semiconductor  406 . Therefore, as the semiconductor  406  has a larger thickness, the channel region becomes larger. In other words, the thicker the semiconductor  406  is, the larger the on-state current of the transistor is. In addition, when the semiconductor  406  is thicker, the proportion of the region with a high carrier controllability increases, leading to a smaller subthreshold swing value. For example, the semiconductor  406  has a region with a thickness greater than or equal to 10 nm, preferably greater than or equal to 20 nm, further preferably greater than or equal to 40 nm, still further preferably greater than or equal to 60 nm, yet further preferably greater than or equal to 100 nm. In addition, to prevent a decrease in the productivity of the semiconductor device, the semiconductor  406  has a region with a thickness, for example, less than or equal to 300 nm, preferably less than or equal to 200 nm, further preferably less than or equal to 150 nm. In some cases, when the channel formation region is reduced in size, electrical characteristics of the transistor with a smaller thickness of the semiconductor  406  may be improved. Therefore, the semiconductor  406  may have a thickness less than 10 nm. 
     The s-channel structure is suitable for a miniaturized transistor because a high on-state current can be achieved. A semiconductor device including the miniaturized transistor can have a high integration degree and high density. For example, the transistor includes a region having a channel length of preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm and a region having a channel width of preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, still further preferably less than or equal to 20 nm. 
     Note that the conductor  413  is not necessarily formed (see  FIG. 12A ). A shape in which the insulator  412  protrudes from the conductor  404  may be employed (see  FIG. 12B ). Furthermore, the insulator to be the insulator  412  is not necessarily processed (see  FIG. 12C ). 
     Although the case where the conductors  416   a  and  416   b  and the insulators  410   a  and  410   b  have the same shapes as those in  FIGS. 1A to 1E  is illustrated in  FIG. 11B  and the like as a typical example, one embodiment of the present invention is not limited thereto. For example, as illustrated in  FIG. 51A , a shape similar to that in  FIG. 2E  may be employed. As illustrated in  FIG. 51B , a shape similar to that in  FIG. 3D  may be employed. As illustrated in  FIG. 51C , a shape similar to that in  FIG. 4D  may be employed. As illustrated in  FIG. 52A , a shape similar to that in  FIG. 5E  may be employed. As illustrated in  FIG. 52B , a shape similar to that in  FIG. 6E  may be employed. As illustrated in  FIG. 52C , a shape similar to that in  FIG. 7D  may be employed. As illustrated in  FIG. 53A , the insulators  410   a  and  410   b  each may have a slope which becomes steeper gradually toward the edge portion. Alternatively, as illustrated in  FIG. 53B , each edge portion of the insulators  410   a  and  410   b  may have a region whose slope angle is changed stepwise. Alternatively, as illustrated in  FIG. 53C , the conductors  416   a  and  416   b  each may have a stacked-layer structure. In the stacked-layer structure, the bottom layer may protrude from the top layer, for example. These shapes can be used in combination. The shapes can be individually obtained by change in the conditions for plasma treatment, addition of an etching step, or the like. 
     &lt;Semiconductor&gt; 
     By placing a semiconductor over and under the semiconductor  406 , electrical characteristics of the transistor can be increased in some cases. The semiconductor  406  and semiconductors placed over and under the semiconductor  406  are described in detail below with reference to  FIGS. 13A to 13E . 
       FIG. 13A  is an enlarged cross-sectional view illustrating the semiconductor  406  and its vicinity of the transistor illustrated in  FIG. 11B  in the channel length direction.  FIG. 13B  is an enlarged cross-sectional view illustrating the semiconductor  406  and its vicinity of the transistor illustrated in  FIG. 11B  in the channel width direction. 
     In the transistor structure illustrated in  FIGS. 13A and 13B , a semiconductor  406   a  is placed between the insulator  402  and the semiconductor  406 . In addition, a semiconductor  406   c  is placed between the semiconductor  406  and the conductors  416   a  and  416   b  and between the semiconductor  406  and the insulator  412 . 
     Alternatively, the transistor may have a structure illustrated in  FIGS. 13C and 13D . 
       FIG. 13C  is an enlarged cross-sectional view illustrating the semiconductor  406  and its vicinity of the transistor illustrated in  FIG. 11B  in the channel length direction.  FIG. 13D  is an enlarged cross-sectional view illustrating the semiconductor  406  and its vicinity of the transistor illustrated in  FIG. 11B  in the channel width direction. 
     In the transistor structure illustrated in  FIGS. 13C and 13D , the semiconductor  406   a  is placed between the insulator  402  and the semiconductor  406 . In addition, the semiconductor  406   c  is placed between the insulator  412  and the insulator  402 , the conductors  416   a  and  416   b , the semiconductor  406   a , and the semiconductor  406 . 
     The semiconductor  406  is an oxide semiconductor containing indium, for example. The oxide semiconductor  406  can have high carrier mobility (electron mobility) by containing indium, for example. The semiconductor  406  preferably contains an element M. The element M is preferably aluminum, gallium, yttrium, tin, or the like. Other elements which can be used as the element M are boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and the like. Note that two or more of the above elements may be used in combination as the element M. The element M is an element having high bonding energy with oxygen, for example. The element M is an element whose bonding energy with oxygen is higher than that of indium. The element M is an element that can increase the energy gap of the oxide semiconductor, for example. Furthermore, the semiconductor  406  preferably contains zinc. When the oxide semiconductor contains zinc, the oxide semiconductor is easily crystallized, in some cases. 
     Note that the semiconductor  406  is not limited to the oxide semiconductor containing indium. The semiconductor  406  may be, for example, an oxide semiconductor which does not contain indium and contains zinc, an oxide semiconductor which does not contain indium and contains gallium, or an oxide semiconductor which does not contain indium and contains tin, e.g., a zinc tin oxide or a gallium tin oxide. 
     For the semiconductor  406 , an oxide with a wide energy gap may be used, for example. For example, the energy gap of the semiconductor  406  is greater than or equal to 2.5 eV and less than or equal to 4.2 eV, preferably greater than or equal to 2.8 eV and less than or equal to 3.8 eV, further preferably greater than or equal to 3 eV and less than or equal to 3.5 eV. 
     For example, the semiconductor  406   a  and the semiconductor  406   c  are oxide semiconductors including one or more elements, or two or more elements other than oxygen included in the semiconductor  406 . Since the semiconductor  406   a  and the semiconductor  406   c  each include one or more elements, or two or more elements other than oxygen included in the semiconductor  406 , a defect state is less likely to be formed at the interface between the semiconductor  406   a  and the semiconductor  406  and the interface between the semiconductor  406  and the semiconductor  406   c.    
     The semiconductor  406   a , the semiconductor  406 , and the semiconductor  406   c  preferably include at least indium. In the case of using an In—M—Zn oxide as the semiconductor  406   a , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be less than 50 atomic % and greater than 50 atomic %, respectively, further preferably less than 25 atomic % and greater than 75 atomic %, respectively. In the case of using an In—M—Zn oxide as the semiconductor  406 , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be greater than 25 atomic % and less than 75 atomic %, respectively, further preferably greater than 34 atomic % and less than 66 atomic %, respectively. In the case of using an In—M—Zn oxide as the semiconductor  406   c , when the summation of In and M is assumed to be 100 atomic %, the proportions of In and M are preferably set to be less than 50 atomic % and greater than 50 atomic %, respectively, further preferably less than 25 atomic % and greater than 75 atomic %, respectively. Note that the semiconductor  406   c  may be an oxide that is of the same type as the oxide of the semiconductor  406   a . Note that the semiconductor  406   a  and/or the semiconductor  406   c  do/does not necessarily contain indium in some cases. For example, the semiconductor  406   a  and/or the semiconductor  406   c  may be gallium oxide. Note that the atomic ratios of the elements included in the semiconductor  406   a , the semiconductor  406 , and the semiconductor  406   c  are not necessarily simple ratios of integers. 
     As the semiconductor  406 , an oxide having an electron affinity higher than those of the semiconductors  406   a  and  406   c  is used. For example, as the semiconductor  406 , an oxide having an electron affinity higher than those of the semiconductors  406   a  and  406   c  by 0.07 eV or higher and 1.3 eV or lower, preferably 0.1 eV or higher and 0.7 eV or lower, further preferably 0.15 eV or higher and 0.4 eV or lower is used. Note that the electron affinity refers to an energy difference between the vacuum level and the conduction band minimum. 
     An indium gallium oxide has small electron affinity and a high oxygen-blocking property. Therefore, the semiconductor  406   c  preferably includes an indium gallium oxide. The gallium atomic ratio [Ga/(In+Ga)] is, for example, higher than or equal to 70%, preferably higher than or equal to 80%, further preferably higher than or equal to 90%. 
     At this time, when a gate voltage is applied, a channel is formed in the semiconductor  406  having the highest electron affinity in the semiconductors  406   a ,  406 , and  406   c.    
     Here, in some cases, there is a mixed region of the semiconductor  406   a  and the semiconductor  406  between the semiconductor  406   a  and the semiconductor  406 . Furthermore, in some cases, there is a mixed region of the semiconductor  406  and the semiconductor  406   c  between the semiconductor  406  and the semiconductor  406   c . The mixed region has a low density of defect states. For that reason, the stack including the semiconductor  406   a , the semiconductor  406 , and the semiconductor  406   c  has a band structure where energy is changed continuously at each interface and in the vicinity of the interface (continuous junction) (see FIG.  13 E). Note that boundaries of the semiconductor  406   a , the semiconductor  406 , and the semiconductor  406   c  are not clear in some cases. 
     At this time, electrons move mainly in the semiconductor  406 , not in the semiconductor  406   a  and the semiconductor  406   c . As described above, when the density of defect states at the interface between the semiconductor  406   a  and the semiconductor  406  and the density of defect states at the interface between the semiconductor  406  and the semiconductor  406   c  are decreased, electron movement in the semiconductor  406  is less likely to be inhibited and the on-state current of the transistor can be increased. 
     As factors of inhibiting electron movement are decreased, the on-state current of the transistor can be increased. For example, in the case where there is no factor of inhibiting electron movement, electrons are assumed to be efficiently moved. Electron movement is inhibited, for example, in the case where physical unevenness of the channel formation region is large. 
     To increase the on-state current of the transistor, for example, root mean square (RMS) roughness with a measurement area of 1 μm×1 μm of a top surface or a bottom surface of the semiconductor  406  (a formation surface; here, the semiconductor  406   a ) is less than 1 nm, preferably less than 0.6 nm, further preferably less than 0.5 nm, still further preferably less than 0.4 nm. The average surface roughness (also referred to as Ra) with the measurement area of 1 μm×1 μm is less than 1 nm, preferably less than 0.6 nm, further preferably less than 0.5 nm, still further preferably less than 0.4 nm. The maximum difference (P−V) with the measurement area of 1 μm×1 μm is less than 10 nm, preferably less than 9 nm, further preferably less than 8 nm, still further preferably less than 7 nm. RMS roughness, Ra, and P−V can be measured using a scanning probe microscope SPA-500 manufactured by SII Nano Technology Inc. 
     Moreover, the thickness of the semiconductor  406   c  is preferably as small as possible to increase the on-state current of the transistor. For example, the semiconductor  406   c  is formed to include a region having a thickness of less than 10 nm, preferably less than or equal to 5 nm, further preferably less than or equal to 3 nm. Meanwhile, the semiconductor  406   c  has a function of blocking entry of elements other than oxygen (such as hydrogen and silicon) included in the adjacent insulator into the semiconductor  406  where a channel is formed. For this reason, it is preferable that the semiconductor  406   c  have a certain thickness. For example, the semiconductor  406   c  is formed to include a region having a thickness of greater than or equal to 0.3 nm, preferably greater than or equal to 1 nm, further preferably greater than or equal to 2 nm. The semiconductor  406   c  preferably has an oxygen blocking property to suppress outward diffusion of oxygen released from the insulator  402  and the like. 
     To improve reliability, preferably, the thickness of the semiconductor  406   a  is large and the thickness of the semiconductor  406   c  is small. For example, the semiconductor  406   a  includes a region with a thickness of, for example, greater than or equal to 10 nm, preferably greater than or equal to 20 nm, further preferably greater than or equal to 40 nm, still further preferably greater than or equal to 60 nm. When the thickness of the semiconductor  406   a  is made large, a distance from an interface between the adjacent insulator and the semiconductor  406   a  to the semiconductor  406  in which a channel is formed can be large. Since the productivity of the semiconductor device might be decreased, the semiconductor  406   a  has a region with a thickness of, for example, less than or equal to 200 nm, preferably less than or equal to 120 nm, further preferably less than or equal to 80 nm. 
     For example, a region with a silicon concentration measured by secondary ion mass spectrometry (SIMS) of higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 2×10 18  atoms/cm 3  is provided between the semiconductor  406  and the semiconductor  406   a . A region with a silicon concentration measured by SIMS of higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 2×10 18  atoms/cm 3  is provided between the semiconductor  406  and the semiconductor  406   c.    
     The semiconductor  406  includes a region with a hydrogen concentration measured by SIMS of higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 2×10 20  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 19  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 19  atoms/cm 3 , or still further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 . It is preferable to reduce the hydrogen concentration in the semiconductor  406   a  and the semiconductor  406   c  in order to reduce the hydrogen concentration in the semiconductor  406 . The semiconductor  406   a  and the semiconductor  406   c  each includes a region with a hydrogen concentration measured by SIMS of higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 2×10 20  atoms/cm 3 , preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 19  atoms/cm 3 , further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 1×10 19  atoms/cm 3 , or still further preferably higher than or equal to 1×10 16  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 . Furthermore, the semiconductor  406  includes a region with a nitrogen concentration measured by SIMS of higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 5×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 , further preferably higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 1×10 18  atoms/cm 3 , or still further preferably higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 5×10 17  atoms/cm 3 . It is preferable to reduce the nitrogen concentration in the semiconductor  406   a  and the semiconductor  406   c  in order to reduce the nitrogen concentration in the semiconductor  406 . The semiconductor  406   a  and the semiconductor  406   c  includes a region with a nitrogen concentration measured by SIMS of higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 5×10 19  atoms/cm 3 , preferably higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 5×10 18  atoms/cm 3 , further preferably higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 1×10 18  atoms/cm 3 , or still further preferably higher than or equal to 1×10 15  atoms/cm 3  and lower than or equal to 5×10 17  atoms/cm 3 . 
     The above three-layer structure is an example. For example, a two-layer structure without the semiconductor  406   a  or the semiconductor  406   c  may be employed. Alternatively, a four-layer structure in which any one of the semiconductors described as examples of the semiconductor  406   a , the semiconductor  406 , and the semiconductor  406   c  is provided under or over the semiconductor  406   a  or under or over the semiconductor  406   c  may be employed. An n-layer structure (n is an integer of 5 or more) in which one or more of the semiconductors described as examples of the semiconductor  406   a , the semiconductor  406 , and the semiconductor  406   c  is provided at two or more of the following positions: over the semiconductor  406   a , under the semiconductor  406   a , over the semiconductor  406   c , and under the semiconductor  406   c.    
     &lt;Structure of Oxide Semiconductor&gt; 
     A structure of an oxide semiconductor is described below. 
     An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, and an amorphous oxide semiconductor. 
     From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and a microcrystalline oxide semiconductor. 
     First, a CAAC-OS is described. Note that a CAAC-OS can be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC). 
     A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets). 
     In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     A CAAC-OS observed with TEM is described below.  FIG. 38A  shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be obtained with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd. 
       FIG. 38B  is an enlarged Cs-corrected high-resolution TEM image of a region (1) in  FIG. 38A .  FIG. 38B  shows that metal atoms are arranged in a layered manner in a pellet. Each metal atom layer has a configuration reflecting unevenness of a surface over which the CAAC-OS is formed (hereinafter, the surface is referred to as a formation surface) or a top surface of the CAAC-OS, and is arranged parallel to the formation surface or the top surface of the CAAC-OS. 
     As shown in  FIG. 38B , the CAAC-OS has a characteristic atomic arrangement. The characteristic atomic arrangement is denoted by an auxiliary line in  FIG. 38C .  FIGS. 38B and 38C  prove that the size of a pellet is approximately 1 nm to 3 nm, and the size of a space caused by tilt of the pellets is approximately 0.8 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). 
     Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets  5100  of a CAAC-OS over a substrate  5120  is illustrated by such a structure in which bricks or blocks are stacked (see  FIG. 38D ). The part in which the pellets are tilted as observed in  FIG. 38C  corresponds to a region  5161  shown in  FIG. 38D . 
       FIG. 39A  shows a Cs-corrected high-resolution TEM image of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface.  FIGS. 39B, 39C , and  39 D are enlarged Cs-corrected high-resolution TEM images of regions (1), (2), and (3) in  FIG. 39A , respectively.  FIGS. 39B, 39C, and 39D  indicate that metal atoms are arranged in a triangular, quadrangular, or hexagonal configuration in a pellet. However, there is no regularity of arrangement of metal atoms between different pellets. 
     Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in  FIG. 40A . This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. 
     Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak at 2θ of around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°. 
     On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is attributed to the (110) plane of the InGaZnO 4  crystal. In the case of the CAAC-OS, when analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector of the sample surface as an axis (φ axis), as shown in  FIG. 40B , a peak is not clearly observed. In contrast, in the case of a single crystal oxide semiconductor of InGaZnO 4 , when φ scan is performed with 2θ fixed at around 56°, as shown in  FIG. 40C , six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are irregularly oriented in the CAAC-OS. 
     Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO 4  crystal in a direction parallel to the sample surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in  FIG. 41A  can be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO 4  crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile,  FIG. 41B  shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in  FIG. 41B , a ring-like diffraction pattern is observed. Thus, the electron diffraction also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in  FIG. 41B  is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO 4  crystal. The second ring in  FIG. 41B  is considered to be derived from the (110) plane and the like. 
     Moreover, the CAAC-OS is an oxide semiconductor having a low density of defect states. Defects in the oxide semiconductor are, for example, a defect due to impurity and oxygen vacancies. Therefore, the CAAC-OS can be regarded as an oxide semiconductor with a low impurity concentration, or an oxide semiconductor having a small number of oxygen vacancies. 
     The impurity contained in the oxide semiconductor might serve as a carrier trap or serve as a carrier generation source. Furthermore, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein. 
     Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity. 
     An oxide semiconductor having a low density of defect states (a small number of oxygen vacancies) can have a low carrier density. Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. That is, a CAAC-OS is likely to be highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. Thus, a transistor including a CAAC-OS rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier traps. An electric charge trapped by the carrier traps in the oxide semiconductor takes a long time to be released. The trapped electric charge may behave like a fixed electric charge. Thus, the transistor which includes the oxide semiconductor having a high impurity concentration and a high density of defect states might have unstable electrical characteristics. However, a transistor including a CAAC-OS has small variation in electrical characteristics and high reliability. 
     Since the CAAC-OS has a low density of defect states, carriers generated by light irradiation or the like are less likely to be trapped in defect states. Therefore, in a transistor using the CAAC-OS, change in electrical characteristics due to irradiation with visible light or ultraviolet light is small. 
     &lt;Microcrystalline Oxide Semiconductor&gt; 
     Next, a microcrystalline oxide semiconductor is described. 
     A microcrystalline oxide semiconductor has a region in which a crystal part is observed and a region in which a crystal part is not clearly observed in a high-resolution TEM image. In most cases, the size of a crystal part included in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. An oxide semiconductor including a nanocrystal (nc) that is a microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as a nanocrystalline oxide semiconductor (nc-OS). In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description. 
     In the nc-OS, a microscopic region (for example, 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. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not ordered. Accordingly, the nc-OS cannot be distinguished from an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than the size of a pellet, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet (the electron diffraction is also referred to as selected-area electron diffraction). Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is applied. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots is shown in a ring-like region in some cases. 
     Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC). 
     The nc-OS is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS. 
     &lt;Amorphous Oxide Semiconductor&gt; 
     Next, an amorphous oxide semiconductor is described. 
     The amorphous oxide semiconductor is an oxide semiconductor having disordered atomic arrangement and no crystal part and exemplified by an oxide semiconductor which exists in an amorphous state as quartz. 
     In a high-resolution TEM image of the amorphous oxide semiconductor, crystal parts cannot be found. 
     When the amorphous oxide semiconductor is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak which shows a crystal plane does not appear. A halo pattern is observed when the amorphous oxide semiconductor is subjected to electron diffraction. Furthermore, a spot is not observed and only a halo pattern appears when the amorphous oxide semiconductor is subjected to nanobeam electron diffraction. 
     There are various understandings of an amorphous structure. For example, a structure whose atomic arrangement does not have ordering at all is called a completely amorphous structure. Meanwhile, a structure which has ordering until the nearest neighbor atomic distance or the second-nearest neighbor atomic distance but does not have long-range ordering is also called an amorphous structure. Therefore, the strictest definition does not permit an oxide semiconductor to be called an amorphous oxide semiconductor as long as even a negligible degree of ordering is present in an atomic arrangement. At least an oxide semiconductor having long-term ordering cannot be called an amorphous oxide semiconductor. Accordingly, because of the presence of crystal part, for example, a CAAC-OS and an nc-OS cannot be called an amorphous oxide semiconductor or a completely amorphous oxide semiconductor. 
     &lt;Amorphous-Like Oxide Semiconductor&gt; 
     Note that an oxide semiconductor may have a structure intermediate between the nc-OS and the amorphous oxide semiconductor. The oxide semiconductor having such a structure is specifically referred to as an amorphous-like oxide semiconductor (a-like OS). 
     In a high-resolution TEM image of the a-like OS, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed. 
     The a-like OS has an unstable structure because it includes a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below. 
     An a-like OS (referred to as Sample A), an nc-OS (referred to as Sample B), and a CAAC-OS (referred to as Sample C) are prepared as samples subjected to electron irradiation. Each of the samples is an In—Ga—Zn oxide. 
     First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts. 
     Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of an InGaZnO 4  crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO 4 . Each of lattice fringes corresponds to the a-b plane of the InGaZnO 4  crystal. 
       FIG. 42  shows change in the average size of crystal parts (at 22 points to 45 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe.  FIG. 42  indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose. Specifically, as shown by (1) in  FIG. 42 , a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 2.6 nm at a cumulative electron dose of 4.2×10 8  e − /nm 2 . In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10 8  e −1 /nm 2 . Specifically, as shown by (2) and (3) in  FIG. 42 , the average crystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively, regardless of the cumulative electron dose. 
     In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS. 
     The a-like OS has a lower density than the nc-OS and the CAAC-OS because it includes a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor. 
     For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO 4  with a rhombohedral crystal structure is 6.357 g/cm 3 . Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm 3  and lower than 5.9 g/cm 3 . For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm 3  and lower than 6.3 g/cm 3 . 
     Note that there is a possibility that an oxide semiconductor having a certain composition cannot exist in a single crystal structure. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density. 
     As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more films of an amorphous oxide semiconductor, an a-like OS, a microcrystalline oxide semiconductor, and a CAAC-OS, for example. 
     &lt;Deposition Model&gt; 
     Examples of deposition models of a CAAC-OS and an nc-OS are described below. 
       FIG. 43A  is a schematic view of the inside of a deposition chamber where a CAAC-OS is deposited by a sputtering method. 
     A target  5130  is attached to a backing plate. A plurality of magnets is provided to face the target  5130  with the backing plate positioned therebetween. The plurality of magnets generates a magnetic field. A sputtering method in which the disposition rate is increased by utilizing a magnetic field of magnets is referred to as a magnetron sputtering method. 
     The substrate  5120  is placed to face the target  5130 , and the distance d (also referred to as a target-substrate distance (T-S distance)) is greater than or equal to 0.01 m and less than or equal to 1 m, preferably greater than or equal to 0.02 m and less than or equal to 0.5 m. The deposition chamber is mostly filled with a deposition gas (e.g., an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol % or higher) and the pressure in the deposition chamber is controlled to be higher than or equal to 0.01 Pa and lower than or equal to 100 Pa, preferably higher than or equal to 0.1 Pa and lower than or equal to 10 Pa. Here, discharge starts by application of a voltage at a certain value or higher to the target  5130 , and plasma is observed. The magnetic field forms a high-density plasma region in the vicinity of the target  5130 . In the high-density plasma region, the deposition gas is ionized, so that an ion  5101  is generated. Examples of the ion  5101  include an oxygen cation (O + ) and an argon cation (Ar + ). 
     Here, the target  5130  has a polycrystalline structure which includes a plurality of crystal grains and in which a cleavage plane exists in at least one crystal grain.  FIG. 44A  shows a structure of an InGaZnO 4  crystal included in the target  5130  as an example. Note that  FIG. 44A  shows a structure of the case where the InGaZnO 4  crystal is observed from a direction parallel to the b-axis.  FIG. 44A  indicates that oxygen atoms in a Ga—Zn—O layer are positioned close to those in an adjacent Ga—Zn—O layer. The oxygen atoms have negative charge, whereby repulsive force is generated between the two adjacent Ga—Zn—O layers. As a result, the InGaZnO 4  crystal has a cleavage plane between the two adjacent Ga—Zn—O layers. 
     The ion  5101  generated in the high-density plasma region is accelerated toward the target  5130  side by an electric field, and then collides with the target  5130 . At this time, a pellet  5100   a  and a pellet  5100   b  which are flat-plate-like (pellet-like) sputtered particles are separated and sputtered from the cleavage plane. Note that structures of the pellet  5100   a  and the pellet  5100   b  may be distorted by an impact of collision of the ion  5101 . 
     The pellet  5100   a  is a flat-plate-like (pellet-like) sputtered particle having a triangle plane, e.g., regular triangle plane. The pellet  5100   b  is a flat-plate-like (pellet-like) sputtered particle having a hexagon plane, e.g., regular hexagon plane. Note that flat-plate-like (pellet-like) sputtered particles such as the pellet  5100   a  and the pellet  5100   b  are collectively called pellets  5100 . The shape of a flat plane of the pellet  5100  is not limited to a triangle or a hexagon. For example, the flat plane may have a shape formed by combining two or more triangles. For example, a quadrangle (e.g., rhombus) may be formed by combining two triangles (e.g., regular triangles). 
     The thickness of the pellet  5100  is determined depending on the kind of deposition gas and the like. The thicknesses of the pellets  5100  are preferably uniform; the reason for this is described later. In addition, the sputtered particle preferably has a pellet shape with a small thickness as compared to a dice shape with a large thickness. For example, the thickness of the pellet  5100  is greater than or equal to 0.4 nm and less than or equal to 1 nm, preferably greater than or equal to 0.6 nm and less than or equal to 0.8 nm. In addition, for example, the width of the pellet  5100  is greater than or equal to 1 nm and less than or equal to 3 nm, preferably greater than or equal to 1.2 nm and less than or equal to 2.5 nm. The pellet  5100  corresponds to the initial nucleus in the description of (1) in  FIG. 42 . For example, when the ion  5101  collides with the target  5130  including an In—Ga—Zn oxide, the pellet  5100  that includes three layers of a Ga—Zn—O layer, an In—O layer, and a Ga—Zn—O layer as shown in  FIG. 44B  is separated. Note that  FIG. 44C  shows the structure of the separated pellet  5100  which is observed from a direction parallel to the c-axis. The pellet  5100  has a nanometer-sized sandwich structure including two Ga—Zn—O layers (pieces of bread) and an In—O layer (filling). 
     The pellet  5100  may receive a charge when passing through the plasma, so that side surfaces thereof are negatively or positively charged. In the pellet  5100 , for example, an oxygen atom positioned on its side surface may be negatively charged. When the side surfaces are charged with the same polarity, charges repel each other, and accordingly, the pellet  5100  can maintain a flat-plate (pellet) shape. In the case where a CAAC-OS is an In—Ga—Zn oxide, there is a possibility that an oxygen atom bonded to an indium atom is negatively charged. There is another possibility that an oxygen atom bonded to an indium atom, a gallium atom, or a zinc atom is negatively charged. In addition, the pellet  5100  may grow by being bonded with an indium atom, a gallium atom, a zinc atom, an oxygen atom, or the like when passing through plasma. A difference in size between (2) and (1) in  FIG. 42  corresponds to the amount of growth in plasma. Here, in the case where the temperature of the substrate  5120  is at around room temperature, the pellet  5100  on the substrate  5120  hardly grows; thus, an nc-OS is formed (see  FIG. 43B ). An nc-OS can be deposited when the substrate  5120  has a large size because the deposition of an nc-OS can be carried out at room temperature. Note that in order that the pellet  5100  grows in plasma, it is effective to increase deposition power in sputtering. High deposition power can stabilize the structure of the pellet  5100 . 
     As shown in  FIGS. 43A and 43B , the pellet  5100  flies like a kite in plasma and flutters up to the substrate  5120 . Since the pellets  5100  are charged, when the pellet  5100  gets close to a region where another pellet  5100  has already been deposited, repulsion is generated. Here, above the substrate  5120 , a magnetic field in a direction parallel to the top surface of the substrate  5120  (also referred to as a horizontal magnetic field) is generated. A potential difference is given between the substrate  5120  and the target  5130 , and accordingly, current flows from the substrate  5120  toward the target  5130 . Thus, the pellet  5100  is given a force (Lorentz force) on the top surface of the substrate  5120  by an effect of the magnetic field and the current. This is explainable with Fleming&#39;s left-hand rule. 
     The mass of the pellet  5100  is larger than that of an atom. Therefore, to move the pellet  5100  over the top surface of the substrate  5120 , it is important to apply some force to the pellet  5100  from the outside. One kind of the force may be force which is generated by the action of a magnetic field and current. In order to apply a sufficient force to the pellet  5100  so that the pellet  5100  moves over a top surface of the substrate  5120 , it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate  5120  is 10 G or higher, preferably 20 G or higher, further preferably 30 G or higher, still further preferably 50 G or higher. Alternatively, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate  5120  is 1.5 times or higher, preferably twice or higher, further preferably 3 times or higher, still further preferably 5 times or higher as high as the magnetic field in a direction perpendicular to the top surface of the substrate  5120 . 
     At this time, the magnets and the substrate  5120  are moved or rotated relatively, whereby the direction of the horizontal magnetic field on the top surface of the substrate  5120  continues to change. Therefore, the pellet  5100  can be moved in various directions on the top surface of the substrate  5120  by receiving forces in various directions. 
     Furthermore, as shown in  FIG. 43A , when the substrate  5120  is heated, resistance between the pellet  5100  and the substrate  5120  due to friction or the like is low. As a result, the pellet  5100  glides above the top surface of the substrate  5120 . The glide of the pellet  5100  is caused in a state where its flat plane faces the substrate  5120 . Then, when the pellet  5100  reaches the side surface of another pellet  5100  that has been already deposited, the side surfaces of the pellets  5100  are bonded. At this time, the oxygen atom on the side surface of the pellet  5100  is released. With the released oxygen atom, oxygen vacancies in a CAAC-OS might be filled; thus, the CAAC-OS has a low density of defect states. Note that the temperature of the top surface of the substrate  5120  is, for example, higher than or equal to 100° C. and lower than 500° C., higher than or equal to 150° C. and lower than 450° C., or higher than or equal to 170° C. and lower than 400° C. Hence, even when the substrate  5120  has a large size, it is possible to deposit a CAAC-OS. 
     Furthermore, the pellet  5100  is heated on the substrate  5120 , whereby atoms are rearranged, and the structure distortion caused by the collision of the ion  5101  can be reduced. The pellet  5100  whose structure distortion is reduced is substantially single crystal. Even when the pellets  5100  are heated after being bonded, expansion and contraction of the pellet  5100  itself hardly occur, which is caused by turning the pellet  5100  into substantially single crystal. Thus, formation of defects such as a grain boundary due to expansion of a space between the pellets  5100  can be prevented, and accordingly, generation of crevasses can be prevented. 
     The CAAC-OS does not have a structure like a board of a single crystal oxide semiconductor but has arrangement with a group of pellets  5100  (nanocrystals) like stacked bricks or blocks. Furthermore, a grain boundary does not exist between the pellets  5100 . Therefore, even when deformation such as shrink occurs in the CAAC-OS owing to heating during deposition, heating or bending after deposition, it is possible to relieve local stress or release distortion. Therefore, this structure is suitable for a flexible semiconductor device. Note that the nc-OS has arrangement in which pellets  5100  (nanocrystals) are randomly stacked. 
     When the target  5130  is sputtered with the ion  5101 , in addition to the pellets  5100 , zinc oxide or the like may be separated. The zinc oxide is lighter than the pellet  5100  and thus reaches the top surface of the substrate  5120  before the pellet  5100 . As a result, the zinc oxide forms a zinc oxide layer  5102  with a thickness greater than or equal to 0.1 nm and less than or equal to 10 nm, greater than or equal to 0.2 nm and less than or equal to 5 nm, or greater than or equal to 0.5 nm and less than or equal to 2 nm.  FIGS. 45A to 45D  are cross-sectional schematic views. 
     As illustrated in  FIG. 45A , a pellet  5105   a  and a pellet  5105   b  are deposited over the zinc oxide layer  5102 . Here, side surfaces of the pellet  5105   a  and the pellet  5105   b  are in contact with each other. In addition, a pellet  5105   c  is deposited over the pellet  5105   b , and then glides over the pellet  5105   b . Furthermore, a plurality of particles  5103  separated from the target together with the zinc oxide is crystallized by heat from the substrate  5120  to form a region  5105   a   1  on another side surface of the of the pellet  5105   a . Note that the plurality of particles  5103  may contain oxygen, zinc, indium, gallium, or the like. 
     Then, as illustrated in  FIG. 45B , the region  5105   a   1  grows to part of the pellet  5105   a  to form a pellet  5105   a   2 . In addition, a side surface of the pellet  5105   c  is in contact with another side surface of the pellet  5105   b.    
     Next, as illustrated in  FIG. 45C , a pellet  5105   d  is deposited over the pellet  5105   a   2  and the pellet  5105   b , and then glides over the pellet  5105   a   2  and the pellet  5105   b . Furthermore, a pellet  5105   e  glides toward another side surface of the pellet  5105   c  over the zinc oxide layer  5102 . 
     Then, as illustrated in  FIG. 45D , the pellet  5105   d  is placed so that a side surface of the pellet  5105   d  is in contact with a side surface of the pellet  5105   a   2 . Furthermore, a side surface of the pellet  5105   e  is in contact with another side surface of the pellet  5105   c . A plurality of particles  5103  separated from the target  5130  together with the zinc oxide is crystallized by heat from the substrate  5120  to form a region  5105   d   1  on another side surface of the pellet  5105   d.    
     As described above, deposited pellets are placed to be in contact with each other and then growth is caused at side surfaces of the pellets, whereby a CAAC-OS is formed over the substrate  5120 . Therefore, each pellet of the CAAC-OS is larger than that of the nc-OS. A difference in size between (3) and (2) in  FIG. 42  corresponds to the amount of growth after deposition. 
     When spaces between pellets are extremely small, the pellets may form a large pellet. The large pellet has a single crystal structure. For example, the size of the pellet may be greater than or equal to 10 nm and less than or equal to 200 nm, greater than or equal to 15 nm and less than or equal to 100 nm, or greater than or equal to 20 nm and less than or equal to 50 nm, when seen from the above. In this case, in an oxide semiconductor used for a minute transistor, a channel formation region might be fit inside the large pellet. That is, the region having a single crystal structure can be used as the channel formation region. Furthermore, when the size of the pellet is increased, the region having a single crystal structure can be used as the channel formation region, the source region, and the drain region of the transistor. 
     In this manner, when the channel formation region or the like of the transistor is formed in a region having a single crystal structure, the frequency characteristics of the transistor can be increased in some cases. 
     As shown in such a model, the pellets  5100  are considered to be deposited on the substrate  5120 . Thus, a CAAC-OS can be deposited even when a formation surface does not have a crystal structure; therefore, a growth mechanism in this case is different from epitaxial growth. In addition, laser crystallization is not needed for formation of a CAAC-OS, and a uniform film can be formed even over a large-sized glass substrate or the like. For example, even when the top surface (formation surface) of the substrate  5120  has an amorphous structure (e.g., the top surface is formed of amorphous silicon oxide), a CAAC-OS can be formed. 
     In addition, it is found that in formation of the CAAC-OS, the pellets  5100  are arranged in accordance with the top surface shape of the substrate  5120  that is the formation surface even when the formation surface has unevenness. For example, in the case where the top surface of the substrate  5120  is flat at the atomic level, the pellets  5100  are arranged so that flat planes parallel to the a-b plane face downwards. In the case where the thicknesses of the pellets  5100  are uniform, a layer with a uniform thickness, flatness, and high crystallinity is formed. By stacking n layers (n is a natural number), the CAAC-OS can be obtained. 
     In the case where the top surface of the substrate  5120  has unevenness, a CAAC-OS in which n layers (n is a natural number) in each of which the pellets  5100  are arranged along the unevenness are stacked is formed. Since the substrate  5120  has unevenness, a gap is easily generated between the pellets  5100  in the CAAC-OS in some cases. Note that, even in such a case, owing to intermolecular force, the pellets  5100  are arranged so that a gap between the pellets is as small as possible even on the unevenness surface. Therefore, even when the formation surface has unevenness, a CAAC-OS with high crystallinity can be obtained. 
     Since a CAAC-OS is deposited in accordance with such a model, the sputtered particle preferably has a pellet shape with a small thickness. Note that when the sputtered particles have a dice shape with a large thickness, planes facing the substrate  5120  vary; thus, the thicknesses and orientations of the crystals cannot be uniform in some cases. 
     According to the deposition model described above, a CAAC-OS with high crystallinity can be formed even on a formation surface with an amorphous structure. 
     &lt;Transistor 2&gt; 
     Next, a method for manufacturing a transistor with a partly different shape is described.  FIG. 14A ,  FIG. 15A ,  FIG. 16A ,  FIG. 17A , and  FIG. 18A  are top views illustrating a method for manufacturing a transistor.  FIG. 14B ,  FIG. 15B ,  FIG. 16B ,  FIG. 17B , and  FIG. 18B  are each a cross-sectional view taken along dashed dotted lines F 1 -F 2  and F 3 -F 4  shown in the corresponding top view. 
     First, a substrate  500  is prepared. For the substrate  500 , the description of the substrate  400  is referred to. 
     Next, an insulator is formed. The insulator can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Next, a resist or the like is formed over the insulator and processing is performed using the resist, whereby an insulator  503  is formed. 
     Any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used for processing the insulator to form the insulator  503 . 
     Next, a conductor is formed. The conductor can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Next, etching is performed from the top surface of the conductor toward the bottom surface thereof so that the etched surface is parallel to the bottom surface of the substrate  500 ; thus, a conductor  513  can be formed (embedded) in a groove of the insulator  503  (see  FIGS. 14A and 14B ). When the conductor  513  is formed in this way, the top surface of the conductor  513  can be positioned at substantially the same level as the top surface of the insulator  503 . Therefore, a defect in shape in a later step can be inhibited. 
     For the insulator  503 , the description of the insulator  402  is referred to. For the conductor  513 , the description of the conductor  413  is referred to. 
     Next, an insulator  502  is deposited. The insulator  502  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For the insulator  502 , the description of the insulator  402  is referred to. 
     Next, a semiconductor  536  is formed. The semiconductor  536  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For the semiconductor  536 , the description of the semiconductor  406  is referred to. 
     Next, heat treatment is preferably performed. 
     Next, a conductor  546  is formed. The conductor  546  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For the conductor  546 , the description of the conductor  416  is referred to. 
     Next, an insulator  540  is formed (see  FIGS. 15A and 15B ). The insulator  540  can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. For the insulator  540 , description of an insulator  510  is referred to. 
     Then, a resist or the like is formed over the insulator  540  and processing is performed using the resist, whereby the insulator  510 , a conductor  516 , and a semiconductor  506  are formed (see  FIGS. 16A and 16B ). At this time, the conductor  516  and/or the semiconductor  506  may be formed in such a manner that processing is performed using the insulator  510  after the resist is removed. Furthermore, part of the insulator  502  over which the semiconductor  506  is not provided may be etched. Thus, the insulator  502  has a projection. 
     At this time, any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used. For example, in each of  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D , the layer  116  and the layer  110  may be replaced with the conductor  516  and the insulator  510 , respectively. 
     Next, a resist or the like is formed over the insulator  510  and processing is performed using the resist, whereby insulators  510   a  and  510   b  and conductors  516   a  and  516   b  are formed (see  FIGS. 17A and 17B ). Here, the case where the insulator  510  and the conductor  516  are processed using a method similar to the processing method in  FIGS. 4A to 4E  is illustrated. Note that the conductors  516   a  and  516   b  and the insulators  510   a  and  510   b  may be processed to have any of the shapes in  FIGS. 11A and 11B ,  FIGS. 51A to 51C ,  FIGS. 52A to 52C ,  FIGS. 53A to 53C , and the like. 
     Next, an insulator is formed. The insulator can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Next, a conductor is formed. The conductor can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     Then, a resist or the like is formed over the conductor and processing is performed using the resist, whereby a conductor  504  is formed. In addition, the insulator is processed using the resist or the conductor  504 , whereby an insulator  512  is formed (see  FIGS. 18A and 18B ). Here, the processing is performed so that the insulator  512  and the conductor  504  have the same shape when seen from the above; however, the shapes are not limited thereto. For example, the insulator  512  and the conductor  504  may be processed using different resists. For example, after the insulator  512  is formed, the conductor to be the conductor  404  may be formed; or after the conductor  504  is formed, a resist or the like may be formed over the insulator to be the insulator  512 . 
     At this time, any of the processing methods illustrated in  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D  may be used. For example, in each of  FIGS. 1A to 1E ,  FIGS. 2A to 2E ,  FIGS. 3A to 3D ,  FIGS. 4A to 4E ,  FIGS. 5A to 5E ,  FIGS. 6A to 6E , and  FIGS. 7A to 7D , the layer  116  and the layer  110  may be replaced with the insulator to be the insulator  512  and the conductor to be the conductor  504 , respectively. 
     For the insulator  512 , the description of the insulator  412  is referred to. For the conductor  504 , the description of the conductor  404  is referred to. 
     Next, an insulator may be formed. The insulator can be formed by a sputtering method, a CVD method, an MBE method, a PLD method, an ALD method, or the like. 
     The insulator may be formed to have a single-layer structure or a stacked-layer structure including 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. The insulator may be formed to have a single-layer structure or a stacked-layer structure including an insulator containing, for example, aluminum oxide, silicon nitride oxide, silicon nitride, gallium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     The insulator preferably has a function of a barrier layer. The insulator has, for example, a function of blocking oxygen and/or hydrogen. For example, the insulator preferably has a higher capability of blocking oxygen and/or hydrogen than any of the insulator  502  and the insulator  512 . 
     Through the above process, the transistor of one embodiment of the present invention can be manufactured. 
     In the transistor illustrated in  FIG. 18B , the insulator  510   a  is provided between the conductor  516   a  and the conductor  504 , and the insulator  510   b  is provided between the conductor  516   b  and the conductor  504 . Thus, parasitic capacitance due to the conductor  516   a , the conductor  516   b , or the like is small. Therefore, a semiconductor device using the transistor illustrated in  FIG. 18B  has high frequency characteristics. When the length of part of the conductor  516   a  which protrudes beyond the insulator  510   a  is greater than or equal to 70% and less than or equal to 130%, preferably greater than or equal to 80% and less than or equal to 120%, further preferably greater than or equal to 90% and less than or equal to 110% of the thickness of the insulator  512 , parasitic capacitance and on-state resistance can be reduced. The same applies to the conductor  516   b.    
     As illustrated in  FIG. 18B , the transistor has an s-channel structure. The electric field from the conductor  504  is less likely to be inhibited by the conductor  516   a , the conductor  516   b , and the like at the side surface of the semiconductor  506 . 
     Note that the conductor  513  is not necessarily formed (see  FIG. 19A ). In addition, the insulator  512  may protrude from the conductor  504  (see  FIG. 19B ). Furthermore, the insulator to be the insulator  512  is not necessarily processed (see  FIG. 19C ). 
     &lt;Circuit&gt; 
     An example of a circuit of a semiconductor device including a transistor or the like of one embodiment of the present invention is described below. 
     &lt;CMOS Inverter&gt; 
     A circuit diagram in  FIG. 20A  shows a configuration of a so-called CMOS inverter in which a p-channel transistor  2200  and an n-channel transistor  2100  are connected to each other in series and in which gates of them are connected to each other. 
     &lt;Structure 1 of Semiconductor Device&gt; 
       FIG. 21  is a cross-sectional view of the semiconductor device of  FIG. 20A . The semiconductor device shown in  FIG. 21  includes the transistor  2200  and the transistor  2100 . The transistor  2100  is placed above the transistor  2200 . Although an example where the transistor shown in  FIGS. 18A and 18B  is used as the transistor  2100  is shown, a semiconductor device of one embodiment of the present invention is not limited thereto. For example, any of the transistors illustrated in  FIGS. 11A and 11B ,  FIGS. 12A to 12C ,  FIGS. 19A to 19C ,  FIGS. 51A to 51C ,  FIGS. 52A to 52C , and  FIGS. 53A to 53C  can be used as the transistor  2100 . Therefore, the description regarding the above-mentioned transistors is referred to for the transistor  2100  as appropriate. 
     The transistor  2200  shown in  FIG. 21  is a transistor using a semiconductor substrate  450 . The transistor  2200  includes a region  472   a  in the semiconductor substrate  450 , a region  472   b  in the semiconductor substrate  450 , an insulator  462 , and a conductor  454 . 
     In the transistor  2200 , the regions  472   a  and  472   b  have functions of a source region and a drain region. The insulator  462  has a function of a gate insulator. The conductor  454  has a function of a gate electrode. Thus, the resistance of a channel formation region can be controlled by a potential applied to the conductor  454 . In other words, conduction or non-conduction between the region  472   a  and the region  472   b  can be controlled by the potential applied to the conductor  454 . 
     For the semiconductor substrate  450 , a single-material semiconductor substrate of silicon, germanium, or the like or a compound semiconductor substrate of silicon carbide, silicon germanium, gallium arsenide, indium phosphide, zinc oxide, gallium oxide, or the like may be used, for example. A single crystal silicon substrate is preferably used as the semiconductor substrate  450 . 
     For the semiconductor substrate  450 , a semiconductor substrate including impurities imparting n-type conductivity is used. However, a semiconductor substrate including impurities imparting p-type conductivity may be used as the semiconductor substrate  450 . In that case, a well including impurities imparting the n-type conductivity may be provided in a region where the transistor  2200  is formed. Alternatively, the semiconductor substrate  450  may be an i-type semiconductor substrate. 
     A top surface of the semiconductor substrate  450  preferably has a ( 110 ) plane. Thus, on-state characteristics of the transistor  2200  can be improved. 
     The regions  472   a  and  472   b  are regions including impurities imparting the p-type conductivity. Accordingly, the transistor  2200  has a structure of a p-channel transistor. 
     Note that the transistor  2200  is apart from an adjacent transistor by a region  460  and the like. The region  460  is an insulating region. 
     The semiconductor device shown in  FIG. 21  includes an insulator  464 , an insulator  466 , an insulator  468 , a conductor  480   a , a conductor  480   b , a conductor  480   c , a conductor  478   a , a conductor  478   b , a conductor  478   c , a conductor  476   a , a conductor  476   b , a conductor  474   a , a conductor  474   b , a conductor  474   c , a conductor  496   a , a conductor  496   b , a conductor  496   c , a conductor  496   d , a conductor  498   a , a conductor  498   b , a conductor  498   c , an insulator  490 , an insulator  492 , and an insulator  494 . 
     The insulator  464  is placed over the transistor  2200 . The insulator  466  is placed over the insulator  464 . The insulator  468  is placed over the insulator  466 . The insulator  490  is placed over the insulator  468 . The transistor  2100  is placed over the insulator  490 . The insulator  492  is placed over the transistor  2100 . The insulator  494  is placed over the insulator  492 . 
     The insulator  464  includes an opening reaching the region  472   a , an opening reaching the region  472   b , and an opening reaching the conductor  454 . In the openings, the conductor  480   a , the conductor  480   b , and the conductor  480   c  are embedded. 
     The insulator  466  includes an opening reaching the conductor  480   a , an opening reaching the conductor  480   b , and an opening reaching the conductor  480   c . In the openings, the conductor  478   a , the conductor  478   b , and the conductor  478   c  are embedded. 
     The insulator  468  includes an opening reaching the conductor  478   b  and an opening reaching the conductor  478   c . In the openings, the conductor  476   a  and the conductor  476   b  are embedded. 
     The insulator  490  includes an opening overlapping a channel formation region of the transistor  2100 , an opening reaching the conductor  476   a , and an opening reaching the conductor  476   b . In the openings, the conductor  474   a , the conductor  474   b , and the conductor  474   c  are embedded. 
     The conductor  474   a  may have a function of a gate electrode of the transistor  2100 . The electrical characteristics of the transistor  2100 , such as the threshold voltage, may be controlled by application of a predetermined potential to the conductor  474   a , for example. The conductor  474   a  may be electrically connected to the conductor  404  having a function of the gate electrode of the transistor  2100 , for example. In that case, on-state current of the transistor  2100  can be increased. Furthermore, a punch-through phenomenon can be suppressed; thus, the electrical characteristics of the transistor  2100  in a saturation region can be stable. 
     The insulator  492  includes an opening reaching the conductor  474   b  through the conductor  416   b  that is one of a source electrode and a drain electrode of the transistor  2100 , an opening reaching the conductor  416   a  that is the other of the source electrode and the drain electrode of the transistor  2100 , an opening reaching the conductor  404  that is the gate electrode of the transistor  2100 , and an opening reaching the conductor  474   c . In the openings, the conductor  496   a , the conductor  496   b , the conductor  496   c , and the conductor  496   d  are embedded. Note that in some cases, the openings are provided through any of components of the transistor  2100  or the like. 
     The insulator  494  includes an opening reaching the conductor  496   a , an opening reaching the conductor  496   b  and the conductor  496   d , and an opening reaching the conductor  496   c . In the openings, the conductor  498   a , the conductor  498   b , and the conductor  498   c  are embedded. 
     The insulators  464 ,  466 ,  468 ,  490 ,  492 , and  494  may each be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulator containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. 
     The insulator  401  may be formed using, for example, aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, or tantalum oxide. 
     The insulator that has a function of blocking oxygen and impurities such as hydrogen is preferably included in at least one of the insulators  464 ,  466 ,  468 ,  490 ,  492 , and  494 . When an insulator that has a function of blocking oxygen and impurities such as hydrogen is placed near the transistor  2100 , the electrical characteristics of the transistor  2100  can be stable. 
     An insulator with a function of blocking oxygen and impurities such as hydrogen may be formed to have a single-layer structure or a stacked-layer structure including 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. 
     Each of the conductor  480   a , the conductor  480   b , the conductor  480   c , the conductor  478   a , the conductor  478   b , the conductor  478   c , the conductor  476   a , the conductor  476   b , the conductor  474   a , the conductor  474   b , the conductor  474   c , the conductor  496   a , the conductor  496   b , the conductor  496   c , the conductor  496   d , the conductor  498   a , the conductor  498   b , and the conductor  498   c  may be formed to have, for example, a single-layer structure or a stacked-layer structure including a conductor containing one or more kinds selected from boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound containing the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     Note that a semiconductor device in  FIG. 22  is the same as the semiconductor device in  FIG. 21  except the structure of the transistor  2200 . Therefore, the description of the semiconductor device in  FIG. 21  is referred to for the semiconductor device in  FIG. 22 . In the semiconductor device in  FIG. 22 , the transistor  2200  is a FIN-type transistor. The effective channel width is increased in the FIN-type transistor  2200 , whereby the on-state characteristics of the transistor  2200  can be improved. In addition, since contribution of the electric field of the gate electrode can be increased, the off-state characteristics of the transistor  2200  can be improved. 
     Note that a semiconductor device in  FIG. 23  is the same as the semiconductor device in  FIG. 21  except the structure of the transistor  2200 . Therefore, the description of the semiconductor device in  FIG. 21  is referred to for the semiconductor device in  FIG. 23 . Specifically, in the semiconductor device in  FIG. 23 , the transistor  2200  is formed using an SOI substrate. In the structure in  FIG. 23 , a region  456  is apart from the semiconductor substrate  450  with an insulator  452  provided therebetween. Since the SOI substrate is used, a punch-through phenomenon and the like can be suppressed; thus, the off-state characteristics of the transistor  2200  can be improved. Note that the insulator  452  can be formed by turning part of the semiconductor substrate  450  into an insulator. For example, silicon oxide can be used as the insulator  452 . 
     In each of the semiconductor devices shown in  FIG. 21 ,  FIG. 22 , and  FIG. 23 , a p-channel transistor is formed utilizing a semiconductor substrate, and an n-channel transistor is formed above that; therefore, an occupation area of the element can be reduced. That is, the integration degree of the semiconductor device can be improved. In addition, the manufacturing process can be simplified compared to the case where an n-channel transistor and a p-channel transistor are formed utilizing the same semiconductor substrate; therefore, the productivity of the semiconductor device can be increased. Moreover, the yield of the semiconductor device can be improved. For the p-channel transistor, some complicated steps such as formation of lightly doped drain (LDD) regions, formation of a shallow trench structure, or distortion design can be omitted in some cases. Therefore, the productivity and yield of the semiconductor device can be increased in some cases, compared to a semiconductor device where an n-channel transistor is formed utilizing the semiconductor substrate. 
     &lt;CMOS Analog Switch&gt; 
     A circuit diagram in  FIG. 20B  shows a configuration in which sources of the transistors  2100  and  2200  are connected to each other and drains of the transistors  2100  and  2200  are connected to each other. With such a configuration, the transistors can function as a so-called CMOS analog switch. 
     &lt;Memory Device 1&gt; 
     An example of a semiconductor device (memory device) which includes the transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles is shown in  FIGS. 24A and 24B . 
     The semiconductor device illustrated in  FIG. 24A  includes a transistor  3200  using a first semiconductor, a transistor  3300  using a second semiconductor, and a capacitor  3400 . Note that any of the above-described transistors can be used as the transistor  3300 . 
     Note that the transistor  3300  is preferably a transistor with a low off-state current. For example, a transistor using an oxide semiconductor can be used as the transistor  3300 . Since the off-state current of the transistor  3300  is low, stored data can be retained for a long period at a predetermined node of the semiconductor device. In other words, power consumption of the semiconductor device can be reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. 
     In  FIG. 24A , a first wiring  3001  is electrically connected to a source of the transistor  3200 . A second wiring  3002  is electrically connected to a drain of the transistor  3200 . A third wiring  3003  is electrically connected to one of the source and the drain of the transistor  3300 . A fourth wiring  3004  is electrically connected to the gate of the transistor  3300 . The gate of the transistor  3200  and the other of the source and the drain of the transistor  3300  are electrically connected to the one electrode of the capacitor  3400 . A fifth wiring  3005  is electrically connected to the other electrode of the capacitor  3400 . 
     The semiconductor device in  FIG. 24A  has a feature that the potential of the gate of the transistor  3200  can be retained, and thus enables writing, retaining, and reading of data as follows. 
     Writing and retaining of data are described. First, the potential of the fourth wiring  3004  is set to a potential at which the transistor  3300  is on, so that the transistor  3300  is turned on. Accordingly, the potential of the third wiring  3003  is supplied to a node FG where the gate of the transistor  3200  and the one electrode of the capacitor  3400  are electrically connected to each other. That is, a predetermined charge is supplied to the gate of the transistor  3200  (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the fourth wiring  3004  is set to a potential at which the transistor  3300  is off, so that the transistor  3300  is turned off. Thus, the charge is held at the node FG (retaining). 
     Since the off-state current of the transistor  3300  is low, the charge of the node FG is retained for a long time. 
     Next, reading of data is described. An appropriate potential (a reading potential) is supplied to the fifth wiring  3005  while a predetermined potential (a constant potential) is supplied to the first wiring  3001 , whereby the potential of the second wiring  3002  varies depending on the amount of charge retained in the node FG. This is because in the case of using an n-channel transistor as the transistor  3200 , an apparent threshold voltage V th   _   H  at the time when the high-level charge is given to the gate of the transistor  3200  is lower than an apparent threshold voltage V th   _   L  at the time when the low-level charge is given to the gate of the transistor  3200 . Here, an apparent threshold voltage refers to the potential of the fifth wiring  3005  which is needed to make the transistor  3200  be in “on state.” Thus, the potential of the fifth wiring  3005  is set to a potential V 0  which is between V th   _   H  and V th   _   L , whereby charge supplied to the node FG can be determined. For example, in the case where the high-level charge is supplied to the node FG in writing and the potential of the fifth wiring  3005  is V 0  (&gt;V th   _   H ), the transistor  3200  is brought into “on state.” In the case where the low-level charge is supplied to the node FG in writing, even when the potential of the fifth wiring  3005  is V 0  (&lt;V th   _   L ), the transistor  3200  still remains in “off state.” Thus, the data retained in the node FG can be read by determining the potential of the second wiring  3002 . 
     Note that in the case where memory cells are arrayed, it is necessary that data of a desired memory cell be read in read operation. In the case where data of the other memory cells is not read, the fifth wiring  3005  may be supplied with a potential at which the transistor  3200  is in “off state” regardless of the charge supplied to the node FG, that is, a potential lower than V th   _   H . Alternatively, the fifth wiring  3005  may be supplied with a potential at which the transistor  3200  is brought into “on state” regardless of the charge supplied to the node FG, that is, a potential higher than V th   _   L . 
     &lt;Structure 2 of Semiconductor Device&gt; 
       FIG. 25  is a cross-sectional view of the semiconductor device of  FIG. 24A . The semiconductor device shown in  FIG. 25  includes the transistor  3200 , the transistor  3300 , and the capacitor  3400 . The transistor  3300  and the capacitor  3400  are placed above the transistor  3200 . Note that for the transistor  3300 , the description of the above transistor  2100  is referred to. Furthermore, for the transistor  3200 , the description of the transistor  2200  in  FIG. 21  is referred to. Note that although the transistor  2200  is illustrated as a p-channel transistor in  FIG. 21 , the transistor  3200  may be an n-channel transistor. 
     The transistor  3200  illustrated in  FIG. 25  is a transistor using a semiconductor substrate  450 . The transistor  3200  includes a region  472   a  in the semiconductor substrate  450 , a region  472   b  in the semiconductor substrate  450 , an insulator  462 , and a conductor  454 . 
     The semiconductor device illustrated in  FIG. 25  includes insulators  464 ,  466 , and  468 , conductors  480   a ,  480   b ,  480   c ,  478   a ,  478   b ,  478   c ,  476   a ,  476   b ,  474   a ,  474   b ,  474   c ,  496   a ,  496   b ,  496   c ,  496   d ,  498   a ,  498   b ,  498   c , and  498   d , and insulators  490 ,  492 , and  494 . 
     The insulator  464  is provided over the transistor  3200 . The insulator  466  is provided over the insulator  464 . The insulator  468  is provided over the insulator  466 . The insulator  490  is provided over the insulator  468 . The transistor  3300  is provided over the insulator  490 . The insulator  492  is provided over the transistor  3300 . The insulator  494  is provided over the insulator  492 . 
     The insulator  464  has an opening reaching the region  472   a , an opening reaching the region  472   b , and an opening reaching the conductor  454 . In the openings, the conductor  480   a , the conductor  480   b , and the conductor  480   c  are embedded. 
     The insulator  466  includes an opening reaching the conductor  480   a , an opening reaching the conductor  480   b , and an opening reaching the conductor  480   c . In the openings, the conductor  478   a , the conductor  478   b , and the conductor  478   c  are embedded. 
     The insulator  468  includes an opening reaching the conductor  478   b  and an opening reaching the conductor  478   c . In the openings, the conductor  476   a  and the conductor  476   b  are embedded. 
     The insulator  490  includes an opening overlapping the channel formation region of the transistor  3300 , an opening reaching the conductor  476   a , and an opening reaching the conductor  476   b . In the openings, the conductors  474   a , the conductor  474   b , and the conductor  474   c  are embedded. 
     The conductor  474   a  may have a function as a bottom gate electrode of the transistor  3300 . Alternatively, for example, electric characteristics such as the threshold voltage of the transistor  3300  may be controlled by application of a predetermined potential to the conductor  474   a . Further alternatively, for example, the conductor  474   a  and the conductor  404  that is the top gate electrode of the transistor  3300  may be electrically connected to each other. Thus, the on-state current of the transistor  3300  can be increased. A punch-through phenomenon can be suppressed; thus, stable electric characteristics in the saturation region of the transistor  3300  can be obtained. 
     The insulator  492  includes an opening reaching the conductor  474   b  through the conductor  416   b  that is one of a source electrode and a drain electrode of the transistor  3300 , an opening reaching the conductor  414  that is the other of the source electrode and the drain electrode of the transistor  3300 , an opening reaching the conductor  404  that is the gate electrode of the transistor  3300 , and an opening reaching the conductor  474   c  through the conductor  416   a  that is the other of the source electrode and the drain electrode of the transistor  3300 . In the openings, the conductor  496   a , the conductor  496   b , the conductor  496   c , and the conductor  496   d  are embedded. Note that in some cases, a component of the transistor  3300  or the like is through other components. 
     The insulator  494  includes an opening reaching the conductor  496   a , an opening reaching the conductors  496   b , an opening reaching the conductor  496   c , and an opening reaching the conductor  496   d . In the openings, the conductors  498   a    498   b ,  498   c , and  498   d  are embedded. 
     At least one of the insulators  464 ,  466 ,  468 ,  490 ,  492 , and  494  preferably has a function of blocking oxygen and impurities such as hydrogen. When an insulator that has a function of blocking oxygen and impurities such as hydrogen is placed near the transistor  3300 , the electrical characteristics of the transistor  3300  can be stable. 
     The conductor  498   d  may be formed to have a single-layer structure or a stacked-layer structure including a conductor containing, for example, one or more kinds selected from boron, nitrogen, oxygen, fluorine, silicon, phosphorus, aluminum, titanium, chromium, manganese, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium, molybdenum, ruthenium, silver, indium, tin, tantalum, and tungsten. An alloy or a compound of the above element may be used, for example, and a conductor containing aluminum, a conductor containing copper and titanium, a conductor containing copper and manganese, a conductor containing indium, tin, and oxygen, a conductor containing titanium and nitrogen, or the like may be used. 
     The source or drain of the transistor  3200  is electrically connected to the conductor  416   b  that is one of a source electrode and a drain electrode of the transistor  3300  through the conductor  480   b , the conductor  478   b , the conductor  476   a , the conductor  474   b , and the conductor  496   c . The conductor  454  that is the gate electrode of the transistor  3200  is electrically connected to the conductor  416   a  that is the other of the source electrode and the drain electrode of the transistor  3300  through the conductor  480   c , the conductor  478   c , the conductor  476   b , the conductor  474   c , and the conductor  496   d.    
     The capacitor  3400  includes an electrode electrically connected to the other of the source electrode and the drain electrode of the transistor  3300 , the conductor  414 , and an insulator  411 . Because the insulator  411  can be formed by the same step as a gate insulator of the transistor  3300 , productivity can be increased. When a layer formed by the same step as a gate electrode of the transistor  3300  is used as the conductor  414 , productivity can be increased. 
     For the structures of other components, the description of  FIG. 21  and the like can be referred to as appropriate. 
     A semiconductor device in  FIG. 26  is the same as the semiconductor device in  FIG. 25  except the structure of the transistor  3200 . Therefore, the description of the semiconductor device in  FIG. 25  is referred to for the semiconductor device in  FIG. 26 . Specifically, in the semiconductor device in  FIG. 26 , the transistor  3200  is a FIN-type transistor. For the FIN-type transistor  3200 , the description of the transistor  2200  in  FIG. 22  is referred to. Note that although the transistor  2200  is illustrated as a p-channel transistor in  FIG. 22 , the transistor  3200  may be an n-channel transistor. 
     A semiconductor device in  FIG. 27  is the same as the semiconductor device in  FIG. 25  except a structure of the transistor  3200 . Therefore, the description of the semiconductor device in  FIG. 25  is referred to for the semiconductor device in  FIG. 27 . Specifically, in the semiconductor device in  FIG. 27 , the transistor  3200  is provided in the semiconductor substrate  450  that is an SOI substrate. For the transistor  3200 , which is provided in the semiconductor substrate  450  that is an SOI substrate, the description of the transistor  2200  in  FIG. 23  is referred to. Note that although the transistor  2200  is illustrated as a p-channel transistor in  FIG. 23 , the transistor  3200  may be an n-channel transistor. 
     &lt;Memory Device 2&gt; 
     The semiconductor device in  FIG. 24B  is different from the semiconductor device in  FIG. 24A  in that the transistor  3200  is not provided. Also in this case, data can be written and retained in a manner similar to that of the semiconductor device in  FIG. 24A . 
     Reading of data in the semiconductor device in  FIG. 24B  is described. When the transistor  3300  is brought into on state, the third wiring  3003  which is in a floating state and the capacitor  3400  are brought into conduction, and the charge is redistributed between the third wiring  3003  and the capacitor  3400 . As a result, the potential of the third wiring  3003  is changed. The amount of change in the potential of the third wiring  3003  varies depending on the potential of the one electrode of the capacitor  3400  (or the charge accumulated in the capacitor  3400 ). 
     For example, the potential of the third wiring  3003  after the charge redistribution is (C B ×V B0 +C×V)/(C B +C), where V is the potential of the one electrode of the capacitor  3400 , C is the capacitance of the capacitor  3400 , C B  is the capacitance component of the third wiring  3003 , and V B0  is the potential of the third wiring  3003  before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the one electrode of the capacitor  3400  is V 1  and V 0  (V 1 &gt;V 0 ), the potential of the third wiring  3003  in the case of retaining the potential V 1  (=(C B ×V B0 +C×V 1 )/(C B +C)) is higher than the potential of the third wiring  3003  in the case of retaining the potential V 0  (=(C B ×V B0 +C×V 0 )/(C B +C)). 
     Then, by comparing the potential of the third wiring  3003  with a predetermined potential, data can be read. 
     In this case, a transistor including the first semiconductor may be used for a driver circuit for driving a memory cell, and a transistor including the second semiconductor may be stacked over the driver circuit as the transistor  3300 . 
     When including a transistor using an oxide semiconductor and having a low off-state current, the semiconductor device described above can retain stored data for a long time. In other words, power consumption of the semiconductor device can be reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be retained for a long time even when power is not supplied (note that a potential is preferably fixed). 
     In the semiconductor device, high voltage is not needed for writing data and deterioration of elements is less likely to occur. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of an insulator is not caused. That is, the semiconductor device of one embodiment of the present invention does not have a limit on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the on/off state of the transistor, whereby high-speed operation can be achieved. 
     &lt;Imaging Device&gt; 
     An imaging device of one embodiment of the present invention is described below. 
       FIG. 28A  is a plan view illustrating an example of an imaging device  200  of one embodiment of the present invention. The imaging device  200  includes a pixel portion  210  and peripheral circuits for driving the pixel portion  210  (a peripheral circuit  260 , a peripheral circuit  270 , a peripheral circuit  280 , and a peripheral circuit  290 ). The pixel portion  210  includes a plurality of pixels  211  arranged in a matrix with p rows and q columns (p and q are each a natural number greater than or equal to 2). The peripheral circuit  260 , the peripheral circuit  270 , the peripheral circuit  280 , and the peripheral circuit  290  are each connected to a plurality of pixels  211 , and a signal for driving the plurality of pixels  211  is supplied. In this specification and the like, in some cases, “a peripheral circuit” or “a driver circuit” indicate all of the peripheral circuits  260 ,  270 ,  280 , and  290 . For example, the peripheral circuit  260  can be regarded as part of the peripheral circuit. 
     The imaging device  200  preferably includes a light source  291 . The light source  291  can emit detection light P 1 . 
     The peripheral circuit includes at least one of a logic circuit, a switch, a buffer, an amplifier circuit, and a converter circuit. The peripheral circuit may be provided over a substrate where the pixel portion  210  is formed. Part or the whole of the peripheral circuit may be mounted using a semiconductor device such as an IC. Note that as the peripheral circuit, one or more of the peripheral circuits  260 ,  270 ,  280 , and  290  may be omitted. 
     As illustrated in  FIG. 28B , the pixels  211  may be provided to be inclined in the pixel portion  210  included in the imaging device  200 . When the pixels  211  are obliquely arranged, the distance between pixels (pitch) can be shortened in the row direction and the column direction. Accordingly, the quality of an image taken with the imaging device  200  can be improved. 
     &lt;Configuration Example 1 of Pixel&gt; 
     The pixel  211  included in the imaging device  200  is formed with a plurality of subpixels  212 , and each subpixel  212  is combined with a filter which transmits light with a specific wavelength band (color filter), whereby data for achieving color image display can be obtained. 
       FIG. 29A  is a plan view showing an example of the pixel  211  with which a color image is obtained. The pixel  211  illustrated in  FIG. 29A  includes a subpixel  212  provided with a color filter transmitting light with a red (R) wavelength band (also referred to as a subpixel  212 R), a subpixel  212  provided with a color filter transmitting light with a green (G) wavelength band (also referred to as a subpixel  212 G), and a subpixel  212  provided with a color filter transmitting light with a blue (B) wavelength band (also referred to as a subpixel  212 B). The subpixel  212  can function as a photosensor. 
     The subpixel  212  (the subpixel  212 R, the subpixel  212 G, and the subpixel  212 B) is electrically connected to a wiring  231 , a wiring  247 , a wiring  248 , a wiring  249 , and a wiring  250 . In addition, the subpixel  212 R, the subpixel  212 G, and the subpixel  212 B are connected to respective wirings  253  which are independent from one another. In this specification and the like, for example, the wiring  248  and the wiring  249  that are connected to the pixel  211  in the n-th row are referred to as a wiring  248 [n] and a wiring  249 [n]. For example, the wiring  253  connected to the pixel  211  in the m-th column is referred to as a wiring  253 [m]. Note that in  FIG. 29A , the wirings  253  connected to the subpixel  212 R, the subpixel  212 G, and the subpixel  212 B in the pixel  211  in the m-th column are referred to as a wiring  253 [m]R, a wiring  253 [m]G, and a wiring  253  [m]B. The subpixels  212  are electrically connected to the peripheral circuit through the above wirings. 
     The imaging device  200  has a structure in which the subpixel  212  is electrically connected to the subpixel  212  in an adjacent pixel  211  which is provided with a color filter transmitting light with the same wavelength band as the subpixel  212 , via a switch.  FIG. 29B  shows a connection example of the subpixels  212 : the subpixel  212  in the pixel  211  arranged in an n-th (n is an integer greater than or equal to 1 and less than or equal to p) row and an m-th (m is an integer greater than or equal to 1 and less than or equal to q) column and the subpixel  212  in the adjacent pixel  211  arranged in an (n+1)-th row and the m-th column. In  FIG. 29B , the subpixel  212 R arranged in the n-th row and the m-th column and the subpixel  212 R arranged in the (n+1)-th row and the m-th column are connected to each other via a switch  201 . The subpixel  212 G arranged in the n-th row and the m-th column and the subpixel  212 G arranged in the (n+1)-th row and the m-th column are connected to each other via a switch  202 . The subpixel  212 B arranged in the n-th row and the m-th column and the subpixel  212 B arranged in the (n+1)-th row and the m-th column are connected to each other via a switch  203 . 
     The color filter used in the subpixel  212  is not limited to red (R), green (G), and blue (B) color filters, and color filters that transmit light of cyan (C), yellow (Y), and magenta (M) may be used. By provision of the subpixels  212  that sense light with three different wavelength bands in one pixel  211 , a full-color image can be obtained. 
     The pixel  211  including the subpixel  212  provided with a color filter transmitting yellow (Y) light may be provided, in addition to the subpixels  212  provided with the color filters transmitting red (R), green (G), and blue (B) light. The pixel  211  including the subpixel  212  provided with a color filter transmitting blue (B) light may be provided, in addition to the subpixels  212  provided with the color filters transmitting cyan (C), yellow (Y), and magenta (M) light. When the subpixels  212  sensing light with four different wavelength bands are provided in one pixel  211 , the reproducibility of colors of an obtained image can be increased. 
     For example, in  FIG. 29A , in regard to the subpixel  212  sensing a red wavelength band, the subpixel  212  sensing a green wavelength band, and the subpixel  212  sensing a blue wavelength band, the pixel number ratio (or the light receiving area ratio) thereof is not necessarily 1:1:1. For example, the Bayer arrangement in which the pixel number ratio (the light receiving area ratio) is set at red:green:blue=1:2:1 may be employed. Alternatively, the pixel number ratio (the light receiving area ratio) of red and green to blue may be 1:6:1. 
     Although the number of subpixels  212  provided in the pixel  211  may be one, two or more subpixels are preferably provided. For example, when two or more subpixels  212  sensing the same wavelength band are provided, the redundancy is increased, and the reliability of the imaging device  200  can be increased. 
     When an infrared (IR) filter that transmits infrared light and absorbs or reflects visible light is used as the filter, the imaging device  200  that senses infrared light can be achieved. 
     Furthermore, when a neutral density (ND) filter (dark filter) is used, output saturation which occurs when a large amount of light enters a photoelectric conversion element (light-receiving element) can be prevented. With a combination of ND filters with different dimming capabilities, the dynamic range of the imaging device can be increased. 
     Besides the above-described filter, the pixel  211  may be provided with a lens. An arrangement example of the pixel  211 , a filter  254 , and a lens  255  is described with cross-sectional views in  FIGS. 30A and 30B . With the lens  255 , the photoelectric conversion element can receive incident light efficiently. Specifically, as illustrated in  FIG. 30A , light  256  enters a photoelectric conversion element  220  through the lens  255 , the filter  254  (a filter  254 R, a filter  254 G, and a filter  254 B), a pixel circuit  230 , and the like which are provided in the pixel  211 . 
     As indicated by a region surrounded with dashed-dotted lines; however, part of the light  256  indicated by arrows might be blocked by some wirings  257 . Thus, a preferable structure is that the lens  255  and the filter  254  are provided on the photoelectric conversion element  220  side, so that the photoelectric conversion element  220  can efficiently receive the light  256  as illustrated in  FIG. 30B . When the light  256  enters the photoelectric conversion element  220  from the photoelectric conversion element  220  side, the imaging device  200  with high sensitivity can be provided. 
     As the photoelectric conversion element  220  illustrated in  FIGS. 30A and 30B , a photoelectric conversion element in which a p-n junction or a p-i-n junction is formed may be used. 
     The photoelectric conversion element  220  may be formed using a substance that has a function of absorbing a radiation and generating electric charges. Examples of the substance that has a function of absorbing a radiation and generating electric charges include selenium, lead iodide, mercury iodine, gallium arsenide, cadmium telluride, and cadmium zinc alloy. 
     For example, when selenium is used for the photoelectric conversion element  220 , the photoelectric conversion element  220  can have a light absorption coefficient in a wide wavelength range, such as visible light, ultraviolet light, infrared light, X-rays, and gamma rays. 
     One pixel  211  included in the imaging device  200  may include the subpixel  212  with a first filter in addition to the subpixel  212  illustrated in  FIGS. 29A and 29B . 
     &lt;Configuration Example 2 of Pixel&gt; 
     An example of a pixel including a transistor using silicon and a transistor using an oxide semiconductor is described below. 
       FIGS. 31A and 31B  are each a cross-sectional view of an element included in an imaging device. The imaging device illustrated in  FIG. 31A  includes a transistor  351  including silicon over a silicon substrate  300 , transistors  352  and  353  which include an oxide semiconductor and are stacked over the transistor  351 , and a photodiode  360  provided in a silicon substrate  300 . The transistors and the photodiode  360  are electrically connected to various plugs  370  and wirings  371 . In addition, an anode  361  of the photodiode  360  is electrically connected to the plug  370  through a low-resistance region  363 . 
     The imaging device includes a layer  310  including the transistor  351  provided on the silicon substrate  300  and the photodiode  360  provided in the silicon substrate  300 , a layer  320  which is in contact with the layer  310  and includes the wirings  371 , a layer  330  which is in contact with the layer  320  and includes the transistors  352  and  353 , and a layer  340  which is in contact with the layer  330  and includes a wiring  372  and a wiring  373 . 
     In the example of cross-sectional view in  FIG. 31A , a light-receiving surface of the photodiode  360  is provided on the side opposite to a surface of the silicon substrate  300  where the transistor  351  is formed. With this structure, a light path can be secured without an influence of the transistors and the wirings. Thus, a pixel with a high aperture ratio can be formed. Note that the light-receiving surface of the photodiode  360  can be the same as the surface where the transistor  351  is formed. 
     In the case where a pixel is formed with use of transistors using an oxide semiconductor, the layer  310  may include the transistor using an oxide semiconductor. Alternatively, the layer  310  may be omitted, and the pixel may include only transistors using an oxide semiconductor. 
     In the case where a pixel is formed with use of transistors using silicon, the layer  330  may be omitted. An example of a cross-sectional view in which the layer  330  is not provided is shown in  FIG. 31B . In the case where the layer  330  is not provided, the wiring  372  of the layer  340  can be omitted. 
     Note that the silicon substrate  300  may be an SOI substrate. Furthermore, the silicon substrate  300  can be replaced with a substrate made of germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, or an organic semiconductor. 
     Here, an insulator  380  is provided between the layer  310  including the transistor  351  and the photodiode  360  and the layer  330  including the transistors  352  and  353 . However, there is no limitation on the position of the insulator  380 . 
     Hydrogen in an insulator provided in the vicinity of a channel formation region of the transistor  351  terminates dangling bonds of silicon; accordingly, the reliability of the transistor  351  can be improved. In contrast, hydrogen in the insulator provided in the vicinity of the transistor  352 , the transistor  353 , and the like becomes one of factors generating a carrier in the oxide semiconductor. Thus, the hydrogen may cause a reduction of the reliability of the transistor  352 , the transistor  353 , and the like. Therefore, in the case where the transistor using an oxide semiconductor is provided over the transistor using a silicon-based semiconductor, it is preferable that the insulator  380  having a function of blocking hydrogen be provided between the transistors. When the hydrogen is confined below the insulator  380 , the reliability of the transistor  351  can be improved. In addition, the hydrogen can be prevented from being diffused from a part below the insulator  380  to a part above the insulator  380 ; thus, the reliability of the transistor  352 , the transistor  353 , and the like can be increased. 
     As the insulator  380 , an insulator having a function of blocking oxygen or hydrogen is used, for example. 
     In the cross-sectional view in  FIG. 31A , the photodiode  360  in the layer  310  and the transistor in the layer  330  can be formed so as to overlap each other. Thus, the degree of integration of pixels can be increased. In other words, the resolution of the imaging device can be increased. 
     As illustrated in FIG.  32 A 1  and FIG.  32 B 1 , part or the whole of the imaging device can be bent. FIG.  32 A 1  illustrates a state in which the imaging device is bent in the direction of a dashed-dotted line X 1 -X 2 . FIG.  32 A 2  is a cross-sectional view illustrating a portion indicated by the dashed-dotted line X 1 -X 2  in FIG.  32 A 1 . FIG.  32 A 3  is a cross-sectional view illustrating a portion indicated by a dashed-dotted line Y 1 -Y 2  in FIG.  32 A 1 . 
     FIG.  32 B 1  illustrates a state where the imaging device is bent in the direction of a dashed-dotted chain X 3 -X 4  and the direction of a dashed-dotted line Y 3 -Y 4 . FIG.  32 B 2  is a cross-sectional view illustrating a portion indicated by the dashed-dotted line X 3 -X 4  in FIG.  32 B 1 . FIG.  32 B 3  is a cross-sectional view illustrating a portion indicated by the dashed-dotted line Y 3 -Y 4  in FIG.  32 B 1 . 
     The bent imaging device enables the curvature of field and astigmatism to be reduced. Thus, the optical design of lens and the like, which is used in combination of the imaging device, can be facilitated. For example, the number of lens used for aberration correction can be reduced; accordingly, a reduction of size or weight of electronic devices using the imaging device, and the like, can be achieved. In addition, the quality of a captured image can be improved. 
     &lt;CPU&gt; 
     A CPU including a semiconductor device such as any of the above-described transistors or the above-described memory device is described below. 
       FIG. 33  is a block diagram illustrating a configuration example of a CPU including any of the above-described transistors as a component. 
     The CPU illustrated in  FIG. 33  includes, over a substrate  1190 , an arithmetic logic unit (ALU)  1191 , an ALU controller  1192 , an instruction decoder  1193 , an interrupt controller  1194 , a timing controller  1195 , a register  1196 , a register controller  1197 , a bus interface  1198 , a rewritable ROM  1199 , and a ROM interface  1189 . A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate  1190 . The ROM  1199  and the ROM interface  1189  may be provided over a separate chip. Needless to say, the CPU in  FIG. 27  is just an example in which the configuration has been simplified, and an actual CPU may have a variety of configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU illustrated in  FIG. 33  or an arithmetic circuit is considered as one core; a plurality of such cores are included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be 8, 16, 32, or 64, for example. 
     An instruction that is input to the CPU through the bus interface  1198  is input to the instruction decoder  1193  and decoded therein, and then, input to the ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195 . 
     The ALU controller  1192 , the interrupt controller  1194 , the register controller  1197 , and the timing controller  1195  conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller  1192  generates signals for controlling the operation of the ALU  1191 . While the CPU is executing a program, the interrupt controller  1194  judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller  1197  generates an address of the register  1196 , and reads/writes data from/to the register  1196  in accordance with the state of the CPU. 
     The timing controller  1195  generates signals for controlling operation timings of the ALU  1191 , the ALU controller  1192 , the instruction decoder  1193 , the interrupt controller  1194 , and the register controller  1197 . For example, the timing controller  1195  includes an internal clock generator for generating an internal clock signal based on a reference clock signal, and supplies the internal clock signal to the above circuits. 
     In the CPU illustrated in  FIG. 33 , a memory cell is provided in the register  1196 . For the memory cell of the register  1196 , any of the above-described transistors, the above-described memory device, or the like can be used. 
     In the CPU illustrated in  FIG. 33 , the register controller  1197  selects operation of retaining data in the register  1196  in accordance with an instruction from the ALU  1191 . That is, the register controller  1197  selects whether data is retained by a flip-flop or by a capacitor in the memory cell included in the register  1196 . When data retention by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register  1196 . When data retention by the capacitor is selected, the data is rewritten in the capacitor, and supply of a power supply voltage to the memory cell in the register  1196  can be stopped. 
       FIG. 34  is an example of a circuit diagram of a memory element  1200  that can be used as the register  1196 . The memory element  1200  includes a circuit  1201  in which stored data is volatile when power supply is stopped, a circuit  1202  in which stored data is nonvolatile even when power supply is stopped, a switch  1203 , a switch  1204 , a logic element  1206 , a capacitor  1207 , and a circuit  1220  having a selecting function. The circuit  1202  includes a capacitor  1208 , a transistor  1209 , and a transistor  1210 . Note that the memory element  1200  may further include another element such as a diode, a resistor, or an inductor, as needed. 
     Here, the above-described memory device can be used as the circuit  1202 . When supply of a power supply voltage to the memory element  1200  is stopped, GND (0 V) or a potential at which the transistor  1209  in the circuit  1202  is turned off continues to be input to a gate of the transistor  1209 . For example, the gate of the transistor  1209  is grounded through a load such as a resistor. 
     Shown here is an example in which the switch  1203  is a transistor  1213  having one conductivity type (e.g., an n-channel transistor) and the switch  1204  is a transistor  1214  having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor). A first terminal of the switch  1203  corresponds to one of a source and a drain of the transistor  1213 , a second terminal of the switch  1203  corresponds to the other of the source and the drain of the transistor  1213 , and conduction or non-conduction between the first terminal and the second terminal of the switch  1203  (i.e., the on/off state of the transistor  1213 ) is selected by a control signal RD input to a gate of the transistor  1213 . A first terminal of the switch  1204  corresponds to one of a source and a drain of the transistor  1214 , a second terminal of the switch  1204  corresponds to the other of the source and the drain of the transistor  1214 , and conduction or non-conduction between the first terminal and the second terminal of the switch  1204  (i.e., the on/off state of the transistor  1214 ) is selected by the control signal RD input to a gate of the transistor  1214 . 
     One of a source and a drain of the transistor  1209  is electrically connected to one of a pair of electrodes of the capacitor  1208  and a gate of the transistor  1210 . Here, the connection portion is referred to as a node M 2 . One of a source and a drain of the transistor  1210  is electrically connected to a line which can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch  1203  (the one of the source and the drain of the transistor  1213 ). The second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) is electrically connected to the first terminal of the switch  1204  (the one of the source and the drain of the transistor  1214 ). The second terminal of the switch  1204  (the other of the source and the drain of the transistor  1214 ) is electrically connected to a line which can supply a power supply potential VDD. The second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ), the first terminal of the switch  1204  (the one of the source and the drain of the transistor  1214 ), an input terminal of the logic element  1206 , and one of a pair of electrodes of the capacitor  1207  are electrically connected to each other. Here, the connection portion is referred to as a node M 1 . The other of the pair of electrodes of the capacitor  1207  can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor  1207  can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor  1207  is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor  1208  can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor  1208  can be supplied with the low power supply potential (e.g., GND) or the high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor  1208  is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). 
     The capacitor  1207  and the capacitor  1208  are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized. 
     A control signal WE is input to the gate of the transistor  1209 . As for each of the switch  1203  and the switch  1204 , a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD which is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state. 
     A signal corresponding to data retained in the circuit  1201  is input to the other of the source and the drain of the transistor  1209 .  FIG. 34  illustrates an example in which a signal output from the circuit  1201  is input to the other of the source and the drain of the transistor  1209 . The logic value of a signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) is inverted by the logic element  1206 , and the inverted signal is input to the circuit  1201  through the circuit  1220 . 
     In the example of  FIG. 34 , a signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) is input to the circuit  1201  through the logic element  1206  and the circuit  1220 ; however, one embodiment of the present invention is not limited thereto. The signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) may be input to the circuit  1201  without its logic value being inverted. For example, in the case where the circuit  1201  includes a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is retained, the signal output from the second terminal of the switch  1203  (the other of the source and the drain of the transistor  1213 ) can be input to the node. 
     In  FIG. 34 , the transistors included in the memory element  1200  except for the transistor  1209  can each be a transistor in which a channel is formed in a film formed using a semiconductor other than an oxide semiconductor or in the substrate  1190 . For example, the transistor can be a transistor whose channel is formed in a silicon film or a silicon substrate. Alternatively, all the transistors in the memory element  1200  may be a transistor in which a channel is formed in an oxide semiconductor. Further alternatively, in the memory element  1200 , a transistor in which a channel is formed in an oxide semiconductor may be included besides the transistor  1209 , and a transistor in which a channel is formed in a film formed using a semiconductor other than an oxide semiconductor or in the substrate  1190  can be used for the rest of the transistors. 
     As the circuit  1201  in  FIG. 34 , for example, a flip-flop circuit can be used. As the logic element  1206 , for example, an inverter or a clocked inverter can be used. 
     In a period during which the memory element  1200  is not supplied with the power supply voltage, the semiconductor device of one embodiment of the present invention can retain data stored in the circuit  1201  by the capacitor  1208  which is provided in the circuit  1202 . 
     The off-state current of a transistor in which a channel is formed in an oxide semiconductor is extremely low. For example, the off-state current of a transistor in which a channel is formed in an oxide semiconductor is significantly lower than that of a transistor in which a channel is formed in silicon having crystallinity. Thus, when the transistor is used as the transistor  1209 , a signal held in the capacitor  1208  is retained for a long time also in a period during which the power supply voltage is not supplied to the memory element  1200 . The memory element  1200  can accordingly retain the stored content (data) also in a period during which the supply of the power supply voltage is stopped. 
     Since the above-described memory element performs pre-charge operation with the switch  1203  and the switch  1204 , the time required for the circuit  1201  to retain original data again after the supply of the power supply voltage is restarted can be shortened. 
     In the circuit  1202 , a signal retained by the capacitor  1208  is input to the gate of the transistor  1210 . Therefore, after supply of the power supply voltage to the memory element  1200  is restarted, the signal retained by the capacitor  1208  can be converted into the one corresponding to the state (the on state or the off state) of the transistor  1210  to be read from the circuit  1202 . Consequently, an original signal can be accurately read even when a potential corresponding to the signal retained by the capacitor  1208  varies to some degree. 
     By applying the above-described memory element  1200  to a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Furthermore, shortly after the supply of the power supply voltage is restarted, the memory device can be returned to the same state as that before the power supply is stopped. Therefore, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor, resulting in lower power consumption. 
     Although the memory element  1200  is used in a CPU, the memory element  1200  can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and a radio frequency (RF) device. 
     &lt;Display Device&gt; 
     A display device of one embodiment of the present invention is described below with reference to  FIGS. 35A to 35C  and  FIGS. 36A and 36B . 
     Examples of a display element provided in the display device include a liquid crystal element (also referred to as a liquid crystal display element) and a light-emitting element (also referred to as a light-emitting display element). The light-emitting element includes, in its category, an element whose luminance is controlled by a current or voltage, and specifically includes, in its category, an inorganic electroluminescent (EL) element, an organic EL element, and the like. A display device including an EL element (EL display device) and a display device including a liquid crystal element (liquid crystal display device) are described below as examples of the display device. 
     Note that the display device described below includes in its category a panel in which a display element is sealed and a module in which an IC such as a controller is mounted on the panel. 
     The display device described below refers to an image display device or a light source (including a lighting device). The display device includes any of the following modules: a module provided with a connector such as an FPC or TCP; a module in which a printed wiring board is provided at the end of TCP; and a module in which an integrated circuit (IC) is mounted directly on a display element by a COG method. 
       FIGS. 35A to 35C  illustrate an example of an EL display device of one embodiment of the present invention.  FIG. 35A  is a circuit diagram of a pixel in an EL display device.  FIG. 35B  is a plan view showing the whole of the EL display device. 
       FIG. 35A  illustrates an example of a circuit diagram of a pixel used in an EL display device. 
     Note that in this specification and the like, it might be possible for those skilled in the art to constitute one embodiment of the invention even when portions to which all the terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected are not specified. In other words, one embodiment of the invention can be clear even when connection portions are not specified. Further, in the case where a connection portion is disclosed in this specification and the like, it can be determined that one embodiment of the invention in which a connection portion is not specified is disclosed in this specification and the like, in some cases. Particularly in the case where the number of portions to which a terminal is connected might be more than one, it is not necessary to specify the portions to which the terminal is connected. Therefore, it might be possible to constitute one embodiment of the invention by specifying only portions to which some of terminals of an active element (e.g., a transistor or a diode), a passive element (e.g., a capacitor or a resistor), or the like are connected. 
     Note that in this specification and the like, it might be possible for those skilled in the art to specify the invention when at least the connection portion of a circuit is specified. Alternatively, it might be possible for those skilled in the art to specify the invention when at least a function of a circuit is specified. In other words, when a function of a circuit is specified, one embodiment of the present invention can be clear. Further, it can be determined that one embodiment of the present invention whose function is specified is disclosed in this specification and the like. Therefore, when a connection portion of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a function is not specified, and one embodiment of the invention can be constituted. Alternatively, when a function of a circuit is specified, the circuit is disclosed as one embodiment of the invention even when a connection portion is not specified, and one embodiment of the invention can be constituted. 
     The EL display device illustrated in  FIG. 35A  includes a switching element  743 , a transistor  741 , a capacitor  742 , and a light-emitting element  719 . 
     Note that  FIG. 35A  and the like each illustrate an example of a circuit structure; therefore, a transistor can be provided additionally. In contrast, for each node in  FIG. 35A  and the like, it is possible not to provide an additional transistor, switch, passive element, or the like. 
     A gate of the transistor  741  is electrically connected to one terminal of the switching element  743  and one electrode of the capacitor  742 . A source of the transistor  741  is electrically connected to the other electrode of the capacitor  742  and one electrode of the light-emitting element  719 . A drain of the transistor  741  is supplied with a power supply potential VDD. The other terminal of the switching element  743  is electrically connected to a signal line  744 . A constant potential is supplied to the other electrode of the light-emitting element  719 . The constant potential is a ground potential GND or a potential lower than the ground potential GND. 
     It is preferable to use a transistor as the switching element  743 . When the transistor is used as the switching element, the area of a pixel can be reduced, so that the EL display device can have high resolution. As the switching element  743 , a transistor formed through the same step as the transistor  741  can be used, so that EL display devices can be manufactured with high productivity. Note that as the transistor  741  and/or the switching element  743 , any of the above-described transistors can be used, for example. 
       FIG. 35B  is a plan view of the EL display device. The EL display device includes a substrate  700 , a substrate  750 , a sealant  734 , a driver circuit  735 , a driver circuit  736 , a pixel  737 , and an FPC  732 . The sealant  734  is provided between the substrate  700  and the substrate  750  so as to surround the pixel  737 , the driver circuit  735 , and the driver circuit  736 . Note that the driver circuit  735  and/or the driver circuit  736  may be provided outside the sealant  734 . 
       FIG. 35C  is a cross-sectional view of the EL display device taken along part of dashed-dotted line M-N in  FIG. 35B . 
       FIG. 35C  illustrates a structure of the transistor  741  including a conductor  704   a  over the substrate  700 ; an insulator  712   a  over the conductor  704   a ; an insulator  712   b  over the insulator  712   a ; a semiconductor  706  that is over the insulator  712   b  and overlaps the conductor  704   a ; a conductor  716   a  and a conductor  716   b  in contact with the semiconductor  706 ; an insulator  710   a  over the conductor  716   a ; an insulator  710   b  over the conductor  716   b ; an insulator  718   a  over the semiconductor  706 , the conductor  716   a , the conductor  716   b , the insulator  710   a , and the insulator  710   b ; an insulator  718   b  over the insulator  718   a ; an insulator  718   c  over the insulator  718   b ; and a conductor  714   a  that is over the insulator  718   c  and overlaps the semiconductor  706 . Note that the structure of the transistor  741  is just an example; the transistor  741  may have a structure different from that illustrated in  FIG. 35C . 
     Thus, in the transistor  741  illustrated in  FIG. 35C , the conductor  704   a  serves as a gate electrode, the insulator  712   a  and the insulator  712   b  serve as a gate insulator, the conductor  716   a  serves as a source electrode, the conductor  716   b  serves as a drain electrode, the insulator  718   a , the insulator  718   b , and the insulator  718   c  serve as a gate insulator, and the conductor  714   a  serves as a gate electrode. Note that in some cases, electrical characteristics of the semiconductor  706  change if light enters the semiconductor  706 . To prevent this, it is preferable that one or more of the conductor  704   a , the conductor  716   a , the conductor  716   b , and the conductor  714   a  have a light-blocking property. 
     Note that the interface between the insulator  718   a  and the insulator  718   b  is indicated by a broken line. This means that the boundary between them is not clear in some cases. For example, in the case where the insulator  718   a  and the insulator  718   b  are formed using insulators of the same kind, the insulator  718   a  and the insulator  718   b  are not distinguished from each other in some cases depending on an observation method. 
       FIG. 35C  illustrates a structure of the capacitor  742  including a conductor  704   b  over the substrate; the insulator  712   a  over the conductor  704   b ; the insulator  712   b  over the insulator  712   a ; the conductor  716   a  that is over the insulator  712   b  and overlaps the conductor  704   b ; the insulator  718   a  over the conductor  716   a ; the insulator  718   b  over the insulator  718   a ; the insulator  718   c  over the insulator  718   b ; and a conductor  714   b  that is over the insulator  718   c  and overlaps the conductor  716   a . In this structure, part of the insulator  718   a  and part of the insulator  718   b  are removed in a region where the conductor  716   a  and the conductor  714   b  overlap each other. 
     In the capacitor  742 , each of the conductor  704   b  and the conductor  714   b  serves as one electrode, and the conductor  716   a  serves as the other electrode. 
     Thus, the capacitor  742  can be formed using a film of the transistor  741 . The conductor  704   a  and the conductor  704   b  are preferably conductors of the same kind, in which case the conductor  704   a  and the conductor  704   b  can be formed through the same step. Furthermore, the conductor  714   a  and the conductor  714   b  are preferably conductors of the same kind, in which case the conductor  714   a  and the conductor  714   b  can be formed through the same step. 
     The capacitor  742  illustrated in  FIG. 35C  has a large capacitance per area occupied by the capacitor. Therefore, the EL display device illustrated in  FIG. 35C  has high display quality. Note that although the capacitor  742  illustrated in  FIG. 35C  has the structure in which the part of the insulator  718   a  and the part of the insulator  718   b  are removed to reduce the thickness of the region where the conductor  716   a  and the conductor  714   b  overlap with each other, the structure of the capacitor according to one embodiment of the present invention is not limited to the structure. For example, a structure in which a part of the insulator  718   c  is removed to reduce the thickness of the region where the conductor  716   a  and the conductor  714   b  overlap with each other may be used. 
     An insulator  720  is provided over the transistor  741  and the capacitor  742 . Here, the insulator  720  may have an opening portion reaching the conductor  716   a  that serves as the source electrode of the transistor  741 . A conductor  781  is provided over the insulator  720 . The conductor  781  may be electrically connected to the transistor  741  through the opening portion in the insulator  720 . 
     A partition wall  784  having an opening portion reaching the conductor  781  is provided over the conductor  781 . A light-emitting layer  782  in contact with the conductor  781  through the opening portion provided in the partition wall  784  is provided over the partition wall  784 . A conductor  783  is provided over the light-emitting layer  782 . A region where the conductor  781 , the light-emitting layer  782 , and the conductor  783  overlap with one another serves as the light-emitting element  719 . 
     So far, examples of the EL display device are described. Next, an example of a liquid crystal display device is described. 
       FIG. 36A  is a circuit diagram illustrating a configuration example of a pixel of a liquid crystal display device. A pixel shown in  FIGS. 36A and 36B  includes a transistor  751 , a capacitor  752 , and an element (liquid crystal element)  753  in which a space between a pair of electrodes is filled with a liquid crystal. 
     One of a source and a drain of the transistor  751  is electrically connected to a signal line  755 , and a gate of the transistor  751  is electrically connected to a scan line  754 . 
     One electrode of the capacitor  752  is electrically connected to the other of the source and the drain of the transistor  751 , and the other electrode of the capacitor  752  is electrically connected to a wiring for supplying a common potential. 
     One electrode of the liquid crystal element  753  is electrically connected to the other of the source and the drain of the transistor  751 , and the other electrode of the liquid crystal element  753  is electrically connected to a wiring to which a common potential is supplied. The common potential supplied to the wiring electrically connected to the other electrode of the capacitor  752  may be different from that supplied to the other electrode of the liquid crystal element  753 . 
     Note that the description of the liquid crystal display device is made on the assumption that the plan view of the liquid crystal display device is similar to that of the EL display device.  FIG. 36B  is a cross-sectional view of the liquid crystal display device taken along dashed-dotted line M-N in  FIG. 35B . In  FIG. 36B , the FPC  732  is connected to the wiring  733   a  via the terminal  731 . Note that the wiring  733   a  may be formed using the same kind of conductor as the conductor of the transistor  751  or using the same kind of semiconductor as the semiconductor of the transistor  751 . 
     For the transistor  751 , the description of the transistor  741  is referred to. For the capacitor  752 , the description of the capacitor  742  is referred to. Note that the structure of the capacitor  752  in  FIG. 36B  corresponds to, but is not limited to, the structure of the capacitor  742  in  FIG. 35C . 
     Note that in the case where an oxide semiconductor is used as the semiconductor of the transistor  751 , the off-state current of the transistor  751  can be extremely small. Therefore, an electric charge held in the capacitor  752  is unlikely to leak, so that the voltage applied to the liquid crystal element  753  can be maintained for a long time. Accordingly, the transistor  751  can be kept off during a period in which moving images with few motions or a still image are/is displayed, whereby power for the operation of the transistor  751  can be saved in that period; accordingly a liquid crystal display device with low power consumption can be provided. Furthermore, the area occupied by the capacitor  752  can be reduced; thus, a liquid crystal display device with a high aperture ratio or a high-resolution liquid crystal display device can be provided. 
     An insulator  721  is provided over the transistor  751  and the capacitor  752 . The insulator  721  has an opening portion reaching the transistor  751 . A conductor  791  is provided over the insulator  721 . The conductor  791  is electrically connected to the transistor  751  through the opening portion in the insulator  721 . 
     An insulator  792  serving as an alignment film is provided over the conductor  791 . A liquid crystal layer  793  is provided over the insulator  792 . An insulator  794  serving as an alignment film is provided over the liquid crystal layer  793 . A spacer  795  is provided over the insulator  794 . A conductor  796  is provided over the spacer  795  and the insulator  794 . A substrate  797  is provided over the conductor  796 . 
     Owing to the above-described structure, a display device including a capacitor occupying a small area, a display device with high display quality, or a high-resolution display device can be provided. 
     For example, in this specification and the like, a display element, a display device which is a device including a display element, a light-emitting element, and a light-emitting device which is a device including a light-emitting element can employ various modes or can include various elements. For example, the display element, the display device, the light-emitting element, or the light-emitting device includes at least one of an EL element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), a light-emitting diode (LED) for white, red, green, blue, or the like, a transistor (a transistor that emits light depending on current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, and a display element including a carbon nanotube. Display media whose contrast, luminance, reflectivity, transmittance, or the like is changed by electrical or magnetic effect may be included. 
     Note that examples of display devices having EL elements include an EL display. Examples of a display device including an electron emitter include a field emission display (FED), an SED-type flat panel display (SED: surface-conduction electron-emitter display), and the like. Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). Examples of a display device having electronic ink or an electrophoretic element include electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes. Thus, the power consumption can be further reduced. 
     Note that in the case of using an LED, graphene or graphite may be provided under an electrode or a nitride semiconductor of the LED. Graphene or graphite may be a multilayer film in which a plurality of layers are stacked. As described above, provision of graphene or graphite enables easy formation of a nitride semiconductor thereover, such as an n-type GaN semiconductor including crystals. Furthermore, a p-type GaN semiconductor including crystals or the like can be provided thereover, and thus the LED can be formed. Note that an MN layer may be provided between the n-type GaN semiconductor including crystals and graphene or graphite. The GaN semiconductors included in the LED may be formed by MOCVD. Note that when the graphene is provided, the GaN semiconductors included in the LED can also be formed by a sputtering method. 
     &lt;Electronic Device&gt; 
     The semiconductor device of one embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can be equipped with the semiconductor device of one embodiment of the present invention are mobile phones, game machines including portable game consoles, portable data terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines.  FIGS. 37A to 37F  illustrate specific examples of these electronic devices. 
       FIG. 37A  illustrates a portable game console including a housing  901 , a housing  902 , a display portion  903 , a display portion  904 , a microphone  905 , a speaker  906 , an operation key  907 , a stylus  908 , and the like. Although the portable game console in  FIG. 37A  has the two display portions  903  and  904 , the number of display portions included in a portable game console is not limited to this. 
       FIG. 37B  illustrates a portable data terminal including a first housing  911 , a second housing  912 , a first display portion  913 , a second display portion  914 , a joint  915 , an operation key  916 , and the like. The first display portion  913  is provided in the first housing  911 , and the second display portion  914  is provided in the second housing  912 . The first housing  911  and the second housing  912  are connected to each other with the joint  915 , and the angle between the first housing  911  and the second housing  912  can be changed with the joint  915 . An image on the first display portion  913  may be switched in accordance with the angle at the joint  915  between the first housing  911  and the second housing  912 . A display device with a position input function may be used as at least one of the first display portion  913  and the second display portion  914 . Note that the position input function can be added by providing a touch panel in a display device. Alternatively, the position input function can be added by providing a photoelectric conversion element called a photosensor in a pixel portion of a display device. 
       FIG. 37C  illustrates a laptop personal computer, which includes a housing  921 , a display portion  922 , a keyboard  923 , a pointing device  924 , and the like. 
       FIG. 37D  illustrates an electric refrigerator-freezer, which includes a housing  931 , a door for a refrigerator  932 , a door for a freezer  933 , and the like. 
       FIG. 37E  illustrates a video camera, which includes a first housing  941 , a first housing  942 , a display portion  943 , operation keys  944 , a lens  945 , a joint  946 , and the like. The operation keys  944  and the lens  945  are provided for the first housing  941 , and the display portion  943  is provided for the second housing  942 . The first housing  941  and the second housing  942  are connected to each other with the joint  946 , and the angle between the first housing  941  and the second housing  942  can be changed with the joint  946 . Images displayed on the display portion  943  may be switched in accordance with the angle at the joint  946  between the first housing  941  and the second housing  942 . 
       FIG. 37F  illustrates a car including a car body  951 , wheels  952 , a dashboard  953 , lights  954 , and the like. 
     Example 1 
     In this example, an example in which the processing method of one embodiment of the present invention is used is described. 
     First, a 126.6-mm-square silicon substrate was prepared. Next, a 400-nm-thick silicon oxide was formed by a thermal oxidation method. Then, a 40-nm-thick In—Ga—Zn oxide was formed by a sputtering method. After that, a 50-nm-thick tungsten film was formed by a sputtering method. 
     Subsequently, a 170-nm-thick BARC was formed. Next, a resist was formed. Then, a resist was exposed to light using a photomask. After that, the resist was developed to form a groove with a size of 300 nm. 
     Subsequently, plasma was generated using a trifluoromethane gas at a flow rate of 50 sccm and a helium gas at a flow rate of 100 sccm, and plasma treatment was performed. Note that the pressure was 5.5 Pa, the substrate temperature was 70° C., the ICP power was 475 W, the bias power was 300 W, and the process time was 80 seconds. At that time, an organic substance was attached to the side surfaces of the resists and the BARCs in a manner similar to that illustrated in  FIG. 6C . 
     Next, plasma was generated using a chlorine gas at a flow rate of 45 sccm, a carbon tetrafluoride gas at a flow rate of 55 sccm, and an oxygen gas at a flow rate of 55 sccm, and plasma treatment was performed to etch the tungsten film. Note that the pressure was 0.67 Pa, the ICP power was 3000 W, the bias power was 110 W, and the process time was 15 seconds (the conditions are referred to as Condition A for etching tungsten). At this time, the tungsten film was etched in a manner similar to that illustrated in  FIG. 6D . 
     Next, the organic substance, the resists, and the BARCs were removed by plasma ashing and wet etching, so that a shape as illustrated in  FIG. 6E  was obtained. The results are shown in  FIGS. 46A and 46B . Note that  FIG. 46A  shows a cross-sectional scanning transmission electron microscope (STEM) image of a center part of the silicon substrate, and  FIG. 46B  is a cross-sectional STEM image of a corner part of the silicon substrate. 
     Both in  FIGS. 46A and 46B , the distance between the tungsten films was 177 nm. That is, a groove smaller than the groove in the resist was able to be formed in the tungsten film. 
     In addition, similar evaluation was also performed in the case where the conditions for etching tungsten film were changed. Specifically, plasma was generated using a carbon tetrafluoride gas at a flow rate of 60 sccm and an oxygen gas at a flow rate of 40 sccm, and plasma treatment was performed to etch the tungsten film. Note that the pressure was 2 Pa, the ICP power was 1000 W, the bias power was 25 W, and the process time was 35 seconds (the conditions are referred to as Condition B for etching tungsten) 
     Next, the organic substance, the resists, and the BARCs were removed by plasma ashing and wet etching. The results are shown in  FIGS. 47A and 47B . Note that  FIG. 47A  shows a cross-sectional STEM image of a center part of the silicon substrate, and  FIG. 47B  is a cross-sectional STEM image of a corner part of the silicon substrate. 
     Both in  FIGS. 47A and 47B , the distance between the tungsten films was 158 nm. Also under this condition, a groove smaller than the groove in the resist was able to be formed in the tungsten film. 
     Example 2 
     In this example, a transistor with a reduced channel length was fabricated with the use of the tungsten films described in Example 1 as the source and drain electrodes of the transistor. 
       FIGS. 48A and 48B  illustrate the structure of the fabricated transistor.  FIG. 48A  is a top view of the transistor, and  FIG. 48B  is a cross-sectional view thereof taken along dashed dotted lines G 1 -G 2  and G 3 -G 4  in  FIG. 48A . Since the structure of the transistor in  FIGS. 48A and 48B  is similar to that of the transistor illustrated in  FIG. 12A , the same reference numerals are used. Furthermore, in a manner similar to that illustrated in  FIGS. 13C and 13D , the semiconductors  406   a  and  406   c  were provided above and below the semiconductor  406 . 
     A 126.6-mm-square silicon substrate was used as the substrate  400 . As the insulator  402 , a stack of a 100-nm-thick silicon oxide and a 300-nm-thick silicon oxynitride was used. As the semiconductor  406   a , a 20-nm-thick In—Ga—Zn oxide formed using an In—Ga—Zn oxide target with an atomic ratio of In:Ga:Zn=1:3:4 was used. As the semiconductor  406 , a 20-nm-thick In—Ga—Zn oxide formed using an In—Ga—Zn oxide target with an atomic ratio of In:Ga:Zn=1:1:1 was used. As the semiconductor  406   c , a 5-nm-thick In—Ga—Zn oxide formed using an In—Ga—Zn oxide target with an atomic ratio of In:Ga:Zn=1:3:4 was used. As each of the conductors  416   a  and  416   b,  50-nm-thick tungsten was used. As the insulator  412 , a 10-nm-thick silicon oxynitride was used. As the conductor  404 , 30-nm-thick tantalum nitride and 135-nm-thick tungsten were used. 
     Id-Vg characteristics of 25 transistors fabricated using Condition A for etching tungsten, at a drain voltage Vd of 0.1 V or 1.8 V, were measured. Note that, Id means a drain current and Vg means gate voltage.  FIG. 49A  shows Id-Vg characteristics of transistors with a channel length of 180 nm and a channel width of 500 nm.  FIG. 49B  shows Id-Vg characteristics of transistors with a channel length of 180 nm and a channel width of 800 nm. 
     Id-Vg characteristics of 25 transistors fabricated using Condition B for etching tungsten, at a drain voltage Vd of 0.1 V or 1.8 V, were measured.  FIG. 50A  shows Id-Vg characteristics of transistors with a channel length of 180 nm and a channel width of 500 nm.  FIG. 50B  shows Id-Vg characteristics of transistors with a channel length of 180 nm and a channel width of 800 nm. 
     According to  FIGS. 49A and 49B  and  FIGS. 50A and 50B , the transistors fabricated in this example have small variation and favorable electrical characteristics. In particular, the transistors fabricated using Condition A for etching tungsten had small variation. That is, the shapes on the 126.6-mm-square silicon substrate are uniform. 
     EXPLANATION OF REFERENCE 
       110 : layer;  110   a : layer;  110   b : layer;  116 : layer;  116   a : layer;  116   b : layer;  120 : BARC;  122 : resist;  124 : organic substance;  200 : imaging device;  201 : switch;  202 : switch;  203 : switch;  210 : pixel portion;  211 : pixel;  212 : subpixel;  212 B: subpixel;  212 G: subpixel;  212 R: subpixel;  220 : photoelectric conversion element;  230 : pixel circuit;  231 : wiring;  247 : wiring;  248 : wiring;  249 : wiring;  250 : wiring;  253 : wiring;  254 : filter;  254 B: filter;  254 G: filter;  254 R: filter;  255 : lens;  256 : light;  257 : wiring;  260 : peripheral circuit;  270 : peripheral circuit;  280 : peripheral circuit;  290 : peripheral circuit;  291 : light source;  300 : silicon substrate;  310 : layer;  320 : layer;  330 : layer;  340 : layer;  351 : transistor;  352 : transistor;  353 : transistor;  360 : photodiode;  361 : anode;  363 : low-resistance region;  370 : plug;  371 : wiring;  372 : wiring;  373 : wiring;  380 : insulator;  400 : substrate;  401 : insulator;  402 : insulator;  404 : conductor;  406 : semiconductor;  406   a : semiconductor;  406   c : semiconductor;  410 : insulator;  410   a : insulator;  410   b : insulator;  411 : insulator;  412 : insulator;  413 : conductor;  414 : conductor;  416 : conductor;  416   a : conductor;  416   b : conductor;  450 : semiconductor substrate;  452 : insulator;  454 : conductor;  456 : region;  460 : region;  462 : insulator;  464 : insulator;  466 : insulator;  468 : insulator;  472   a : region;  472   b : region;  474   a : conductor;  474   b : conductor;  474   c : conductor;  476   a : conductor;  476   b : conductor;  478   a : conductor;  478   b : conductor;  478   c : conductor;  480   a : conductor;  480   b : conductor;  480   c : conductor;  490 : insulator;  492 : insulator;  494 : insulator;  496   a : conductor;  496   b : conductor;  496   c : conductor;  496   d : conductor;  498   a : conductor;  498   b : conductor;  498   c : conductor;  498   d : conductor;  500 : substrate;  502 : insulator;  503 : insulator;  504 : conductor;  506 : semiconductor;  510 : insulator;  510   a : insulator;  510   b : insulator;  512 : insulator;  513 : conductor;  516 : conductor;  516   a : conductor;  516   b : conductor;  536 : semiconductor;  540 : insulator;  546 : conductor;  700 : substrate;  704   a : conductor;  704   b : conductor;  706 : semiconductor;  710   a : insulator;  710   b : insulator;  712   a : insulator;  712   b : insulator;  714   a : conductor;  714   b : conductor;  716   a : conductor;  716   b : conductor;  718   a : insulator;  718   b : insulator;  718   c : insulator;  719 : light-emitting element;  720 : insulator;  721 : insulator;  731 : terminal;  732 : FPC;  733   a : wiring;  734 : sealant;  735 : driver circuit;  736 : driver circuit;  737 : pixel;  741 : transistor;  742 : capacitor;  743 : switching element;  744 : signal line;  750 : substrate;  751 : transistor;  752 : capacitor;  753 : liquid crystal element;  754 : scan line;  755 : signal line;  781 : conductor;  782 : light-emitting layer;  783 : conductor;  784 : partition wall;  791 : conductor;  792 : insulator;  793 : liquid crystal layer;  794 : insulator;  795 : spacer;  796 : conductor;  797 : substrate;  901 : housing;  902 : housing;  903 : display portion;  904 : display portion;  905 : microphone;  906 : speaker;  907 : operation key;  908 : stylus;  911 : housing;  912 : housing;  913 : display portion;  914 : display portion;  915 : joint;  916 : operation key;  921 : housing;  922 : display portion;  923 : keyboard;  924 : pointing device;  931 : housing;  932 : refrigerator door;  933 : freezer door;  941 : housing;  942 : housing;  943 : display portion;  944 : operation key;  945 : lens;  946 : joint;  951 : car body;  952 : wheel;  953 : dashboard;  954 : light;  1189 : ROM interface;  1190 : substrate;  1191 : ALU;  1192 : ALU controller;  1193 : instruction decoder;  1194 : interrupt controller;  1195 : timing controller;  1196 : register;  1197 : register controller;  1198 : bus interface;  1199 : ROM;  1200 : memory element;  1201 : circuit;  1202 : circuit;  1203 : switch;  1204 : switch;  1206 : logic element;  1207 : capacitor;  1208 : capacitor;  1209 : transistor;  1210 : transistor;  1213 : transistor;  1214 : transistor;  1220 : circuit;  2100 : transistor;  2200 : transistor;  3001 : wiring;  3002 : wiring;  3003 : wiring;  3004 : wiring;  3005 : wiring;  3200 : transistor;  3300 : transistor;  3400 : capacitor;  5100 : pellet;  5100   a : pellet;  5100   b : pellet;  5101 : ion;  5102 : zinc oxide layer;  5103 : particle;  5105   a : pellet;  5105   a   1 : region;  5105   a   2 : pellet;  5105   b : pellet;  5105   c : pellet;  5105   d : pellet;  5105   d   1 : region;  5105   e : pellet;  5120 : substrate;  5130 : target;  5161 : region. 
     This application is based on Japanese Patent Application serial no. 2014-191690 filed with Japan Patent Office on Sep. 19, 2014, the entire contents of which are hereby incorporated by reference.