Patent Publication Number: US-11393918-B2

Title: Semiconductor device and method for manufacturing semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/292,243, filed Oct. 13, 2016, now allowed, which is a divisional of U.S. application Ser. No. 13/924,757, filed Jun. 24, 2013, now U.S. Pat. No. 9,472,682, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2012-147633 on Jun. 29, 2012, all of which are incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     One embodiment of the present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. 
     In this specification, the semiconductor device refers to all the devices that operate by utilizing semiconductor characteristics. For example, electro-optical devices, semiconductor circuits, and electronic devices are all included in the category of the semiconductor device. 
     2. Description of the Related Art 
     Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon film is widely known as the semiconductor thin film applicable to the transistor. As another film, an oxide semiconductor film has attracted attention. 
     An improvement in performance of a semiconductor device including the transistor requires an improvement in on-state characteristics and achievement of high-speed operation of a transistor including an oxide semiconductor film. As a method to improve the on-state characteristics of the transistor, the following method is known: nitrogen is injected to reduce resistance of the oxide semiconductor film and thus reduce contact resistance between the oxide semiconductor film and a source electrode layer and contact resistance between the oxide semiconductor film and a drain electrode layer. 
     For example, Patent Document 1 discloses a field-effect transistor in which an amorphous oxide semiconductor film containing zinc, indium, and oxygen is used, in which resistance of the oxide semiconductor film is reduced by adding nitrogen to the oxide semiconductor film by an ion implantation method, and in which a portion containing nitrogen serves as a source portion or a drain portion and a portion containing no nitrogen serves as a channel portion. 
     REFERENCE 
     Patent Document 
     [Patent Document 1] Japanese Published Patent Application No. 2010-93070 
     SUMMARY OF THE INVENTION 
     In a transistor including an oxide semiconductor film, unintended impurities such as hydrogen or moisture enter the oxide semiconductor film to form carriers, which causes a problem in that electric characteristics of the transistor vary. 
     Thus, an object of one embodiment of the present invention is to achieve improved on-state characteristics and high-speed operation of a transistor including an oxide semiconductor film and improve reliability of the transistor by imparting stable electric characteristics to the transistor. Alternatively, an object of one embodiment of the present invention is to improve on-state characteristics of a transistor including an oxide semiconductor film. Alternatively, an object of one embodiment of the present invention is to impart stable electric characteristics to the transistor and thus improve reliability of a transistor. Alternatively, an object of one embodiment of the present invention is to manufacture a semiconductor device without complicating the process. 
     In a top-gate transistor in which an oxide semiconductor film, a gate insulating film, a gate electrode layer, and a silicon nitride film are stacked in this order and the oxide semiconductor film includes a channel formation region, nitrogen is added to regions of part of the oxide semiconductor film and the regions become low-resistance regions by forming a silicon nitride film over and in contact with the oxide semiconductor film A source electrode layer and a drain electrode layer are in contact with the low-resistance regions of the oxide semiconductor film. A region of the oxide semiconductor film, which is not in contact with the silicon nitride film (that is, a region overlapping with the gate insulating film and the gate electrode layer) becomes the channel formation region. 
     Thus, one embodiment of the present invention is a semiconductor device which includes the following: an oxide semiconductor film including a channel formation region and a pair of low-resistance regions between which the channel formation region is sandwiched and which contains nitrogen; a stack of a gate insulating film and a gate electrode layer over the channel formation region; a silicon nitride film in contact with the low-resistance regions; and a source electrode layer and a drain electrode layer electrically connected to the respective pair of low-resistance regions. In the silicon nitride film, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 500° C. in thermal desorption spectroscopy. 
     Alternatively, one embodiment of the present invention is a semiconductor device including an oxide semiconductor film, a gate insulating film over the oxide semiconductor film, a gate electrode layer over the gate insulating film, a silicon nitride film which is over the oxide semiconductor film, the gate insulating film, and the gate electrode layer and which includes openings reaching the oxide semiconductor film, and a source electrode layer and a drain electrode layer over the silicon nitride film and in contact with the oxide semiconductor film through the openings. In the silicon nitride film, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 500° C. in thermal desorption spectroscopy. 
     The number of released hydrogen molecules in the silicon nitride film is lower than or equal to 1.5×10 20 /cm 3 , preferably lower than or equal to 1.0×10 20 /cm 3 , more preferably lower than or equal to 7.5×10 19 /cm 3  at a temperature lower than or equal to 550° C. in thermal desorption spectroscopy. Further, in thermal desorption spectroscopy, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 500° C.; thus, in thermal desorption spectroscopy, a temperature at which the number of released hydrogen molecules becomes higher than or equal to 1.0×10 20 /cm 3  is higher than or equal to 500° C., preferably higher than or equal to 550° C. 
     The channel formation region and the low-resistance regions each may be a region including a crystal having a c-axis which is substantially perpendicular to a surface of the region. Alternatively, the channel formation region may be a region including a crystal having a c-axis which is substantially perpendicular to a surface of the region and the low-resistance regions may be amorphous regions. 
     The silicon nitride film may serve as an interlayer insulating film covering the oxide semiconductor film, the gate insulating film, and the gate electrode layer. Alternatively, the silicon nitride film may serve as sidewall insulating films in contact with side surfaces of the gate insulating film and side surfaces of the gate electrode layer. 
     Further, one embodiment of the present invention is a semiconductor device in which an oxide semiconductor film includes a channel formation region overlapping with a gate electrode layer, a pair of first low-resistance regions which overlaps with sidewall insulating films and contains nitrogen and between which the channel formation region is sandwiched, and a pair of second low-resistance regions which is in contact with a source electrode layer and a drain electrode layer, respectively, and contains an impurity element and between which the channel formation region and the first low-resistance regions are sandwiched. The impurity element is phosphorus or boron, and the second low-resistance regions have lower resistance than the first low-resistance regions. 
     The oxide semiconductor film may be a region including a crystal having a c-axis which is substantially perpendicular to a surface of the region and the second low-resistance regions may be amorphous regions. 
     One embodiment of the present invention is a method for manufacturing a semiconductor device including the following steps: forming an oxide semiconductor film; forming a gate insulating film over the oxide semiconductor film; forming a gate electrode layer over the gate insulating film; etching the gate insulating film with the gate electrode layer used as a mask to expose part of the oxide semiconductor film; depositing a silicon nitride film in contact with the exposed part of the oxide semiconductor film, in the silicon nitride film, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 500° C. in thermal desorption spectroscopy, so that nitrogen is added to a region of the oxide semiconductor film which is in contact with the silicon nitride film to form a low-resistance region; and forming a source electrode layer or a drain electrode layer electrically connected to the low-resistance region. As a deposition gas for the silicon nitride film, a mixed gas containing silane and nitrogen is used. 
     One embodiment of the present invention is a method for manufacturing a semiconductor device including the following steps: forming an oxide semiconductor film; forming a gate insulating film over the oxide semiconductor film; forming a gate electrode layer over the gate insulating film; etching the gate insulating film with use of the gate electrode layer as a mask to expose part of the oxide semiconductor film; depositing a silicon nitride film in contact with the exposed part of the oxide semiconductor film, in the silicon nitride film, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 500° C. in thermal desorption spectroscopy, so that nitrogen is added to a region of the oxide semiconductor film which is in contact with the silicon nitride film to form a low-resistance region; and forming a source electrode layer or a drain electrode layer electrically connected to the low-resistance region. A mixed gas containing silane, nitrogen, and ammonia is used for deposition of the silicon nitride film and the flow ratio of ammonia to nitrogen in the deposition gas is less than or equal to 0.1. 
     One embodiment of the present invention is a method for manufacturing a semiconductor device including the following steps: forming an oxide semiconductor film; depositing an insulating film over the oxide semiconductor film; depositing a conductive film over the insulating film; processing the conductive film and the insulating film to form a gate electrode layer and a gate insulating film; depositing a silicon nitride film over the oxide semiconductor film, the gate insulating film, and the gate electrode layer, so that nitrogen is added to a region of the oxide semiconductor film which is in contact with the silicon nitride film to form a low-resistance region; etching part of the silicon nitride film to form an opening reaching the oxide semiconductor film; and forming a source electrode layer or a drain electrode layer over the oxide semiconductor film and the silicon nitride film. A deposition gas containing silane, nitrogen, and ammonia is used for deposition of the silicon nitride film and the flow ratio of ammonia to nitrogen in the deposition gas is less than or equal to 0.1. 
     Further, after the gate electrode layer is formed, an impurity element such as phosphorus or boron may be added to the oxide semiconductor film with the gate electrode layer used as a mask. 
     By etching the silicon nitride film, sidewall insulating films may be formed in contact with side surfaces of the gate electrode layer and an impurity element such as phosphorus or boron may be added to the oxide semiconductor film with the gate electrode layer and the sidewall insulating films used as a mask. 
     Further, heat treatment may be performed after the silicon nitride film is formed. 
     According to one embodiment of the present invention, on-state characteristics of a transistor including an oxide semiconductor film can be improved. Alternatively, stable electric characteristics can be imparted to a transistor including an oxide semiconductor film, so that reliability thereof can be improved. Further, a semiconductor device can be manufactured without complicating the process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are respectively a top view and a cross-sectional view illustrating a semiconductor device according to one embodiment of the present invention; 
         FIGS. 2A to 2C  are views illustrating a method for manufacturing a semiconductor device, according to one embodiment of the present invention; 
         FIG. 3  is a cross-sectional view illustrating a semiconductor device according to one embodiment of the present invention; 
         FIGS. 4A and 4B  are respectively a top view and a cross-sectional view of a semiconductor device according to one embodiment of the present invention; 
         FIGS. 5A to 5C  are views illustrating a method for manufacturing a semiconductor device, according to one embodiment of the present invention; 
         FIGS. 6A and 6B  are cross-sectional views illustrating a semiconductor device according to one embodiment of the present invention; 
         FIG. 7  is a cross-sectional view illustrating a semiconductor device according to one embodiment of the present invention; 
         FIGS. 8A and 8B  are circuit diagrams each illustrating a semiconductor device according to one embodiment of the present invention; 
         FIGS. 9A to 9C  are circuit diagrams and a perspective view illustrating a semiconductor device according to one embodiment of the present invention; 
         FIG. 10  is a cross-sectional view illustrating a semiconductor device according to one embodiment of the present invention; 
         FIGS. 11A and 11B  are circuit diagrams each illustrating a semiconductor device according to one embodiment of the present invention; 
         FIG. 12  is a block diagram of a semiconductor device according to one embodiment of the present invention; 
         FIG. 13  is a block diagram of a semiconductor device according to one embodiment of the present invention; 
         FIG. 14  is a block diagram of a semiconductor device according to one embodiment of the present invention; 
         FIGS. 15A to 15C  are views illustrating an electronic device according to one embodiment of the present invention; 
         FIG. 16  shows TDS results in Example 1; 
         FIGS. 17A and 17B  show SIMS analysis results and RBS analysis results in Example 1; and 
         FIGS. 18A and 18B  show TDS results in Example 2. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details thereof can be modified in various ways. Therefore, the present invention is not construed as being limited to description of the embodiments. 
     Further, in embodiments hereinafter described, the same parts are denoted with the same reference numerals throughout the drawings. The thickness, width, relative relation of position, and the like of elements illustrated in the drawings are exaggerated for clarification of description of the embodiments in some cases. 
     Note that the term such as “over” in this specification and the like does not necessarily mean that a component is placed “directly on” another component. For example, the expression “a gate electrode layer over an insulating film” can mean the case where there is an additional component between the insulating film and the gate electrode layer. The same applies to the term “under”. 
     In this specification and the like, the term “electrode layer” or “wiring layer” does not limit the function of components. For example, an “electrode layer” can be used as part of a “wiring layer”, and the “wiring layer” can be used as part of the “electrode layer”. In addition, the term “electrode layer” or “wiring layer” can also mean a combination of a plurality of “electrode layers” and “wiring layers”, for example. 
     Functions of a “source” and a “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification. 
     Note that in this specification and the like, the term “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. 
     Examples of an “object having any electric function” include an electrode and a wiring. 
     Embodiment 1 
     In this embodiment, a semiconductor device according to one embodiment of the present invention is described in detail with reference to drawings.  FIGS. 1A and 1B  are respectively a top view and a cross-sectional view of a transistor  420  that is a semiconductor device according to one embodiment of the present invention.  FIG. 1A  is the top view of the transistor  420 , and  FIG. 1B  is the cross-sectional view taken along dashed-dotted line A-B in  FIG. 1A . 
     The transistor  420  in  FIGS. 1A and 1B  includes the following: a base insulating film  436  over a substrate  400 ; an oxide semiconductor film  409  which is over the base insulating film  436  and includes a channel formation region  403 , a low-resistance region  404   a , and a low-resistance region  404   b ; a gate insulating film  402  and a gate electrode layer  401  which are over the channel formation region  403 ; a silicon nitride film  407  over the oxide semiconductor film  409 , the gate insulating film  402 , and the gate electrode layer  401 ; an interlayer insulating film  411  over the silicon nitride film  407 ; and a source electrode layer  405   a  and a drain electrode layer  405   b  which are electrically connected to the low-resistance region  404   a  and the low-resistance region  404   b , respectively, through openings provided in the silicon nitride film  407  and the interlayer insulating film  411 . 
     Since the silicon nitride film  407  is formed in contact with the oxide semiconductor film, the hydrogen concentration in the silicon nitride film  407  is preferably reduced as much as possible and the number of released hydrogen molecules in the silicon nitride film  407  is preferably reduced. For example, it is preferable that a peak of the number of released hydrogen molecules do not appear at a temperature lower than or equal to 500° C. when measurement is performed by thermal desorption spectroscopy (TDS). In the TDS, it is also preferable that at a temperature lower than or equal to 500° C., for example, the number of released hydrogen molecules be lower than or equal to 1.5×10 20 /cm 3 , preferably lower than or equal to 1.0×10 20 /cm 3 , more preferably lower than or equal to 7.5×10 19 /cm 3 . Alternatively, it is preferable that a temperature at which the number of released hydrogen molecules is higher than or equal to 1.0×10 20 /cm 3  be higher than or equal to 500° C., preferably higher than or equal to 550° C. 
     Further, it is preferable that the hydrogen concentration in the silicon nitride film be lower than or equal to 2.0×10 22  atoms/cm 3 , preferably lower than or equal to 1.5×10 22  atoms/cm 3  in secondary ion mass spectrometry (SIMS). 
     Further, when composition of the silicon nitride film is measured by Rutherford backscattering spectrometry (RBS), the hydrogen concentration is preferably lower than or equal to 15 atomic %, more preferably lower than or equal to 10 atomic %. 
     By depositing the silicon nitride film  407  over and in contact with the oxide semiconductor film  409 , nitrogen is added to a region of the oxide semiconductor film  409 , which is in contact with the silicon nitride film, whereby regions whose resistance is reduced (the low-resistance regions  404   a  and  404   b ) are formed. The low-resistance regions  404   a  and  404   b  have lower electric resistance than the channel formation region  403 . 
     Further, the oxide semiconductor film  409  is preferably a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film including a crystal having a c-axis which is substantially perpendicular to a surface of the film In this embodiment, a CAAC-OS film is used as the oxide semiconductor film  409 . 
     In this embodiment, in a deposition step of the silicon nitride film  407 , nitrogen is added to the oxide semiconductor film  409  to form the low-resistance regions  404   a  and  404   b ; thus, the low-resistance regions  404   a  and  404   b  can keep crystallinity to be referred to as a CAAC-OS film. Accordingly, in the oxide semiconductor film  409  in this embodiment, the channel formation region  403  and the low-resistance regions  404   a  and  404   b  include a crystal having a c-axis which is substantially perpendicular to a surface of the film. 
     In the transistor  420 , the source electrode layer  405   a  and the drain electrode layer  405   b  are in contact with the regions of the oxide semiconductor film  409 , which have low resistance; thus, contact resistance between the oxide semiconductor film  409  and the source electrode layer  405   a  and contact resistance between the oxide semiconductor film  409  and the drain electrode layer  405   b  are reduced. Therefore, the transistor  420  has high on-state characteristics (e.g., on-state current and field-effect mobility), and a semiconductor device capable of high-speed operation and high-speed response can be obtained. 
     Further, in the deposition step of the silicon nitride film  407  serving as an interlayer insulating film, nitrogen is added to the oxide semiconductor film  409  to form the low-resistance regions  404   a  and  404   b ; thus, the on-state characteristics of the transistor  420  can be improved without complicating the process. 
     Next, a method for manufacturing the transistor  420  is described with reference to  FIGS. 2A to 2C . 
     First, the base insulating film  436  and the base insulating film  438  are deposited over the substrate  400 . 
     There is no particular limitation on a substrate that can be used, as long as it has heat resistance high enough to withstand heat treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. 
     A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; or the like may be used as the substrate  400 . Alternatively, an SDI substrate, a semiconductor substrate over which a semiconductor element is provided, or the like can be used. 
     For the formation of the base insulating films  436  and  438 , a sputtering method, a molecular beam epitaxy (MBE) method, a chemical vapor deposition (CVD) method, a pulsed laser deposition (PLD) method, an atomic layer deposition (ALD) method, or the like can be employed as appropriate. Note that when the base insulating film  438  is formed by a sputtering method, impurity elements such as hydrogen can be reduced and hydrogen can be prevented from entering an oxide semiconductor film to be formed later. 
     An inorganic insulating film may be used as each of the base insulating films  436  and  438 . It is preferable to use, for example, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a hafnium oxide film, a gallium oxide film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film. Further, the base insulating films  436  and  438  each can be formed with a single-layer structure or a stacked-layer structure including two or more layers with the use of these compounds. The base insulating films  436  and  438  may include the same material or different materials. 
     Note that here, silicon oxynitride contains more oxygen than nitrogen and contains, for example, oxygen, nitrogen, and silicon at concentrations higher than or equal to 50 atomic % and lower than or equal to 70 atomic %, higher than or equal to 0.5 atomic % and lower than or equal to 15 atomic %, and higher than or equal to 25 atomic % and lower than or equal to 35 atomic %, respectively. Note that the above ranges are obtained in the case where measurement is performed using Rutherford backscattering spectrometry and hydrogen forward scattering (HFS). Moreover, the total percentage of the constituent elements does not exceed 100 atomic %. 
     In the transistor  420 , the base insulating film  436  preferably has a function of preventing entry of impurities such as hydrogen and moisture from the substrate  400  into the oxide semiconductor film  409  to be formed later. Accordingly, an inorganic insulating film such as a silicon nitride film, an aluminum oxide film, a silicon oxynitride film, an aluminum nitride film, or an aluminum nitride oxide film can be preferably used as the base insulating film  436 . In particular, a silicon nitride film is preferably used because it has a high shielding effect (blocking effect) against both of oxygen and impurities such as hydrogen and moisture. 
     Because the base insulating film  438  is in contact with the oxide semiconductor film  409  to be formed later, the base insulating film  438  preferably contains oxygen which exceeds at least the stoichiometric composition in the layer (the bulk). For example, in the case where a silicon oxide film is used as the base insulating film  438 , SiO 2+α  (□&gt;0) is used. 
     Further, planarization treatment may be performed on the base insulating film  438  in order to improve the planarity of the oxide semiconductor film  409  to be formed later. As the planarization treatment, for example, chemical mechanical polishing (CMP) treatment, dry etching treatment, or the like can be performed. As the etching gas, a chlorine-based gas such as a chlorine gas, a boron chloride gas, a silicon chloride gas, or a carbon tetrachloride gas, a fluorine-based gas such as a carbon tetrafluoride gas, a sulfur fluoride gas, or a nitrogen fluoride gas, or the like can be used. 
     Note that the base insulating film is not necessarily provided as long as insulation between the substrate  400  and the oxide semiconductor film  409  to be formed later can be ensured. 
     In this embodiment, a silicon nitride film is used as the base insulating film  436 , and a silicon oxide film is used as the base insulating film  438 . 
     Subsequently, an oxide semiconductor film to be the oxide semiconductor film  409  is formed over the base insulating film  438 . 
     The oxide semiconductor film preferably contains at least indium (In) or zinc (Zn). Alternatively, the oxide semiconductor film preferably contains both In and Zn. In order to reduce variation in electric characteristics of the transistor including the oxide semiconductor film, the oxide semiconductor film preferably contains one or more of stabilizers in addition to In or Zn. 
     As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), zirconium (Zr), or the like can be given. As another stabilizer, lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given. 
     As the oxide semiconductor, for example, the following can be used: a single component metal oxide such as indium oxide, tin oxide, or zinc oxide; a two-component metal oxide such as an In—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, or an In—Ga oxide; a three-component metal oxide such as an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, or an In—Lu—Zn oxide; or a four-component metal oxide such as an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, or an In—Hf—Al—Zn oxide. 
     Note that, for example, an In—Ga—Zn oxide means an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In, Ga, and Zn. The In—Ga—Zn oxide may contain a metal element other than In, Ga, and Zn. 
     Alternatively, a material represented by InMO 3 (ZnO) m  (m&gt;0 is satisfied, and m is not an integer) may be used as the oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, as the oxide semiconductor, a material represented by In 2 SnO 5 (ZnO) n  (n&gt;0 is satisfied, n is an integer) may be used. 
     For example, it is possible to use an In—Ga—Zn oxide containing In, Ga, and Zn at an atomic ratio of 1:1:1, 2:2:1, or 3:1:2, or any of oxides whose composition is in the neighborhood of the above compositions. Alternatively, an In—Sn—Zn oxide containing In, Sn, and Zn at an atomic ratio of 1:1:1, 2:1:3, or 2:1:5, or any of oxides whose composition is in the neighborhood of the above compositions may be used. 
     However, the composition is not limited to those described above, and a material having the appropriate composition may be used depending on needed semiconductor characteristics and electric characteristics (e.g., field-effect mobility, threshold voltage, and variation). In order to obtain needed semiconductor characteristics, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element and oxygen, the interatomic distance, the density, and the like be set to be appropriate. 
     For example, high carrier mobility can be obtained relatively easily in the case where the In—Sn—Zn oxide is used. However, carrier mobility can be increased by reducing the defect density in the bulk also in the case where the In—Ga—Zn oxide is used. 
     Further, the energy gap of the oxide semiconductor film used in this embodiment is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, more preferably greater than or equal to 3 eV. The off-state current of a transistor can be reduced by using an oxide semiconductor film having a wide energy gap. 
     A structure of an oxide semiconductor film is described below. 
     In this specification, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°. 
     In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system. 
     An oxide semiconductor film is classified roughly into a single-crystal oxide semiconductor film and a non-single-crystal oxide semiconductor film. The non-single-crystal oxide semiconductor film includes any of an amorphous oxide semiconductor film, a microcrystalline oxide semiconductor film, a polycrystalline oxide semiconductor film, a CAAC-OS film, and the like. 
     The amorphous oxide semiconductor film has disordered atomic arrangement and no crystalline component. A typical example thereof is an oxide semiconductor film in which no crystal part exists even in a microscopic region, and the whole of the film is amorphous. 
     The microcrystalline oxide semiconductor film includes a microcrystal (also referred to as nanocrystal) with a size greater than or equal to 1 nm and less than 10 nm, for example. Thus, the microcrystalline oxide semiconductor film has a higher degree of atomic order than the amorphous oxide semiconductor film Hence, the density of defect states of the microcrystalline oxide semiconductor film is lower than that of the amorphous oxide semiconductor film. 
     The CAAC-OS film is one of oxide semiconductor films including a plurality of crystal parts, and most of the crystal parts each fits inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. The density of defect states of the CAAC-OS film is lower than that of the microcrystalline oxide semiconductor film. The CAAC-OS film is described in detail below. 
     In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur. 
     According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts. 
     From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film. 
     A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO 4  crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film. 
     On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO 4  crystal. Here, analysis (ϕ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (ϕ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZnO 4 , six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when ϕ scan is performed with 2θ fixed at around 56°. 
     According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal. 
     Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. 
     Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies depending on regions. 
     Note that when the CAAC-OS film with an InGaZnO 4  crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ do not appear at around 36°. 
     In a transistor using the CAAC-OS film, change in electric characteristics due to irradiation with visible light or ultraviolet light is small. Thus, the transistor has high reliability. 
     Note that an oxide semiconductor film may be a stacked film including two or more films of an amorphous oxide semiconductor film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example. 
     Alternatively, the oxide semiconductor film may have a stacked-layer structure of a plurality of oxide semiconductor films. For example, the oxide semiconductor film may have a stacked-layer structure of a first oxide semiconductor film and a second oxide semiconductor film which are formed using metal oxides with different compositions. Alternatively, for example, the first oxide semiconductor film may be formed using any of two-component metal oxide, a three-component metal oxide, and a four-component metal oxide, and the second oxide semiconductor film may be formed using any of these which is different from the oxide for the first oxide semiconductor film. 
     Further, the constituent elements of the first oxide semiconductor film and the second oxide semiconductor film may be made the same and the composition of the constituent elements of the first oxide semiconductor film and the second oxide semiconductor film may be made different. For example, the first oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 1:1:1, and the second oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 3:1:2. Alternatively, the first oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 1:3:2, and the second oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 2:1:3. 
     At this time, one of the first oxide semiconductor film and the second oxide semiconductor film, which is closer to the gate electrode layer  401  to be formed later (on the channel side), preferably contains In and Ga at a proportion of In&gt;Ga. The other oxide semiconductor film, which is farther from the gate electrode layer  401  (on the back channel side), preferably contains In and Ga at a proportion of In□Ga. 
     Further, the oxide semiconductor film may have a three-layer structure of a first oxide semiconductor film, a second oxide semiconductor film, and a third oxide semiconductor film, in which the constituent elements thereof are made the same and the composition of the constituent elements of the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film is made different. For example, the first oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 1:3:2, the second oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 3:1:2, and the third oxide semiconductor film may contain In, Ga, and Zn at an atomic ratio of 1:1:1. 
     An oxide semiconductor film which contains less In than Ga and Zn at an atomic ratio, typically, the first oxide semiconductor film containing In, Ga, and Zn at an atomic ratio of 1:3:2, has a higher insulating property than an oxide semiconductor film containing more In than Ga and Zn at an atomic ratio, typically, the second oxide semiconductor film, and an oxide semiconductor film containing Ga, Zn, and In at the same atomic ratio, typically, the third oxide semiconductor film. Accordingly, the first oxide semiconductor film may be provided on a side close to the gate electrode layer  401  so as to serve as a gate insulating film. Note that the first oxide semiconductor film containing In, Ga, and Zn at an atomic ratio of 1:3:2 may have an amorphous structure. 
     Since the constituent elements of the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film are the same, the first oxide semiconductor film has fewer trap levels at the interface with the second oxide semiconductor film. Therefore, when the oxide semiconductor film has the above structure, the amount of change in the threshold voltage of the transistor due to a change over time or a BT photostress test can be reduced. 
     In an oxide semiconductor, the s orbital of heavy metal mainly contributes to carrier transfer, and when the In content in the oxide semiconductor is increased, overlap of the s orbitals is likely to be increased. Therefore, an oxide semiconductor having an atomic ratio of In&gt;Ga has higher carrier mobility than an oxide semiconductor having an atomic ratio of In≤Ga. Further, in Ga, the formation energy of an oxygen vacancy is larger and thus an oxygen vacancy is less likely to occur than in In; therefore, the oxide semiconductor having an atomic ratio of In≤Ga has more stable characteristics than the oxide semiconductor having an atomic ratio of In&gt;Ga. 
     An oxide semiconductor having an atomic ratio of In&gt;Ga is used on the channel side and an oxide semiconductor having an atomic ratio of In≤Ga is used on the back channel side, whereby the field-effect mobility and reliability of the transistor can be further improved. 
     Further, in the case where the oxide semiconductor film  409  has a stacked-layer structure, the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film may be formed using oxide semiconductor films having different crystallinity. That is, the oxide semiconductor film may be formed using any of a single crystal oxide semiconductor film, a polycrystalline oxide semiconductor film, an amorphous oxide semiconductor film, and a CAAC-OS film, as appropriate. When an amorphous oxide semiconductor film is applied to any of the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film, internal stress or external stress of the oxide semiconductor film is relieved, change in characteristics of the transistor is reduced, and reliability of the transistor can be further improved. 
     The thickness of the oxide semiconductor film is preferably greater than or equal to 1 nm and less than or equal to 100 nm, more preferably greater than or equal to 1 nm and less than or equal to 50 nm, still more preferably greater than or equal to 1 nm and less than or equal to 30 nm, further preferably greater than or equal to 3 nm and less than or equal to 20 nm. 
     In the oxide semiconductor film, the concentration of an alkali metal or an alkaline earth metal obtained by secondary ion mass spectrometry (SIMS) is preferably lower than or equal to 1×10 18  atoms/cm 3 , more preferably lower than or equal to 2×10 16  atoms/cm 3 . This is because when an alkali metal or an alkaline earth metal is bonded to an oxide semiconductor, carriers are generated in some cases, which causes an increase in off-state current of the transistor. 
     In the oxide semiconductor film, the hydrogen concentration obtained by secondary ion mass spectrometry is set to be lower than or equal to 1×10 16  atoms/cm 3 , lower than or equal to 5×10 17  atoms/cm 3 , lower than or equal to 1×10 18  atoms/cm 3 , lower than 5×10 18  atoms/cm 3 , lower than 1×10 19  atoms/cm 3 , lower than 5×10 19  atoms/cm 3 , or lower than 1×10 20  atoms/cm 3  in order of preference. 
     Hydrogen contained in the oxide semiconductor film reacts with oxygen bonded to a metal atom to produce water, and a defect is formed in a lattice from which oxygen is released (or a portion from which oxygen is released). In addition, a bond of part of hydrogen and oxygen causes generation of electrons serving as carriers. Thus, entry of the impurities containing hydrogen is reduced as much as possible in the deposition step of the oxide semiconductor film, whereby the hydrogen concentration in the oxide semiconductor film can be reduced. When hydrogen is removed as much as possible and a highly purified oxide semiconductor film is used as a channel formation region, a negative shift in the threshold voltage can be reduced. In addition, leakage current (also referred to as off-state current) per micrometer of channel width between a source and a drain of the transistor can be reduced to several yA/μm to several zA/μm. Accordingly, electric characteristics of the transistor can be improved with the use of a highly purified oxide semiconductor film. 
     The oxide semiconductor film is formed by a sputtering method, a coating method, a pulsed laser deposition method, a laser ablation method, or the like. 
     In the case where the oxide semiconductor film is formed by a sputtering method, a power supply device for generating plasma can be an RF power supply device, an AC power supply device, a DC power supply device, or the like as appropriate. 
     As a sputtering gas, a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen is preferably higher than that of a rare gas. 
     Further, a target may be selected as appropriate depending on the composition of the oxide semiconductor film to be formed. 
     When a CAAC-OS film is formed, for example, the CAAC-OS film is formed by a sputtering method using a polycrystalline oxide semiconductor sputtering target. When ions collide with the sputtering target, a crystal region included in the sputtering target might be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) might be separated from the sputtering target. In that case, the flat-plate-like sputtered particle reaches a substrate while maintaining their crystal structures, whereby the CAAC-OS film can be deposited. 
     For the deposition of the CAAC-OS film, the following conditions are preferably used. 
     By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal structure can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, and nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is lower than or equal to −80° C., preferably lower than or equal to −100° C., can be used. 
     By increasing the substrate heating temperature during the deposition, migration of a sputtered particle is likely to occur after the sputtered particle reaches a substrate surface. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate surface, migration occurs on the substrate surface, so that a flat plane of the flat-plate-like sputtered particle is attached to the substrate. 
     Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is higher than or equal to 30 vol %□ preferably 100 vol %. 
     As an example of the sputtering target, an In—Ga—Zn oxide target is described below. 
     The In—Ga—Zn oxide target, which is polycrystalline, is made by mixing InO x  powder, GaO Y  powder, and ZnO Z  powder in a predetermined molar ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y, and Z are each a given positive number. Here, the predetermined molar ratio of InO X  powder to GaO Y  powder and ZnO Z  powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target. 
     After the oxide semiconductor film is formed, heat treatment may be performed so that the oxide semiconductor film is dehydrated or dehydrogenated. The temperature of the heat treatment is typically higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 250° C. and lower than or equal to 450° C., more preferably higher than or equal to 300° C. and lower than or equal to 450° C. 
     The heat treatment is performed under an atmosphere of an inert gas including nitrogen or a rare gas such as helium, neon, argon, xenon, or krypton. Alternatively, the heat treatment may be performed under an atmosphere of an inert gas first, and then under an oxygen atmosphere. It is preferable that the above inert gas atmosphere and the above oxygen atmosphere do not contain hydrogen, water, and the like. The treatment time is 3 minutes to 24 hours. 
     Heat treatment is performed after the oxide semiconductor film is formed, so that the hydrogen concentration in the oxide semiconductor film can be lower than 1×10 20  atoms/cm 3 , lower than 5×10 19  atoms/cm 3 , lower than 1×10 19  atoms/cm 3 , lower than 5×10 18  atoms/cm 3 , lower than or equal to 1×10 18  atoms/cm 3 , lower than or equal to 5×10 17  atoms/cm 3 , or lower than or equal to 1×10 16  atoms/cm 3 . 
     Note that in the case where an oxide insulating film is used as the base insulating film  438 , when heat treatment is performed while the oxide semiconductor film is provided over the oxide insulating film, oxygen can be supplied to the oxide semiconductor film, oxygen defects in the oxide semiconductor film can be reduced, and semiconductor characteristics can be improved in some cases. Heat treatment may be performed in a state where the oxide semiconductor film and the oxide insulating film are in contact with each other at least partly to supply oxygen to the oxide semiconductor film. 
     Then, after a mask is formed over the oxide semiconductor film by a photolithography process, the oxide semiconductor film is partly etched using the mask. Accordingly, the oxide semiconductor film  409  is formed as illustrated in  FIG. 2A . After that, the mask is removed. Note that heat treatment by which oxygen is supplied from the base insulating film  438  to the oxide semiconductor film  409  may be performed before or after the oxide semiconductor film is processed into an island-like shape. Note that by performing the heat treatment before the oxide semiconductor film is processed into an island-like shape, a larger amount of oxygen can be supplied to the oxide semiconductor film  409  because a smaller amount of oxygen is released from the base insulating layer  438  to the outside. 
     With the use of a printing method for formation of the oxide semiconductor film, the island-shaped oxide semiconductor film  409  can be formed directly. 
     Next, an insulating film to be the gate insulating film  402  is formed over the oxide semiconductor film  409 . 
     The gate insulating film  402  is preferably formed using an oxide insulating film including silicon oxide, gallium oxide, aluminum oxide, silicon oxynitride, silicon nitride oxide, hafnium oxide, tantalum oxide, or the like. The use of a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi x O y  (x&gt;0, y&gt;0)), hafnium silicate to which nitrogen is added, hafnium aluminate (HfAl x O y  (x&gt;0, y&gt;0)), or lanthanum oxide enables a reduction in gate leakage current. The gate insulating film may have a single-layer structure or a stacked-layer structure. 
     The gate insulating film  402  has a thickness greater than or equal to 1 nm and less than or equal to 100 nm and can be formed by a sputtering method, an MBE method, a CVD method, a PLD method, an ALD method, or the like as appropriate. The gate insulating film may be formed with a sputtering apparatus which performs deposition with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target. 
     Like the base insulating film  438 , the gate insulating film  402  is in contact with the oxide semiconductor film. Therefore, the gate insulating film  402  preferably contains oxygen which exceeds at least the stoichiometric composition in the film (the bulk). 
     In this embodiment, a 20-nm-thick silicon oxynitride film is formed by a CVD method. 
     Next, the gate electrode layer  401  is formed over the gate insulating film  402  and the gate insulating film is etched using the gate electrode layer as a mask, so that the oxide semiconductor film is partly exposed. The structure at this stage is illustrated in  FIG. 2A . 
     The gate electrode layer  401  can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. As the gate electrode layer  401 , a semiconductor film typified by a polycrystalline silicon film to which an impurity element such as phosphorus is added, or a silicide film such as a nickel silicide film may be used. Further, the gate electrode layer  401  can also be formed using a conductive material such as indium tin oxide, indium tungsten oxide, tungsten indium zinc oxide, titanium indium oxide, titanium indium tin oxide, indium zinc oxide, or silicon indium tin oxide. It is also possible that the gate electrode layer  401  has a stacked-layer structure of the above conductive material and the above metal material. 
     As one layer of the gate electrode layer  401  which is in contact with the gate insulating film  402 , a metal oxide film containing nitrogen, specifically, an In—Ga—Zn oxide film containing nitrogen, an In—Sn oxide film containing nitrogen, an In—Ga oxide film containing nitrogen, an In—Zn oxide film containing nitrogen, a Sn oxide film containing nitrogen, an In oxide film containing nitrogen, or a metal nitride (e.g., InN or SnN) film can be used. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage of the transistor to be positive when used as the gate electrode layer. Accordingly, a so-called normally off switching element can be provided. 
     The thickness of the gate electrode layer  401  is preferably greater than or equal to 50 nm and less than or equal to 300 nm. In this embodiment, a 30-nm-thick tantalum nitride film and a 200-nm-thick tungsten film are stacked by a sputtering method. 
     After the gate electrode layer  401  is formed, the gate insulating film  402  is etched using the gate electrode layer  401  as a mask, so that the oxide semiconductor film  409  is partly exposed. 
     Then, the silicon nitride film  407  is formed over the oxide semiconductor film  409 , the gate insulating film  402 , and the gate electrode layer  401  so as to be in contact with exposed regions of the oxide semiconductor film. By deposition of the silicon nitride film  407 , nitrogen is added to regions of the oxide semiconductor film  409 , which are in contact with the silicon nitride film  407  (also referred to as regions exposed by etching of the gate insulating film  402 ), whereby the low-resistance region  404   a  and the low-resistance region  404   b  are formed. Note that nitrogen is not added to an unexposed region of the oxide semiconductor film  409  (also referred to as a region which overlaps with the gate electrode layer  401  and the gate insulating film  402 ), whereby the channel formation region  403  is formed. 
     For the deposition of the silicon nitride film  407 , a plasma CVD method or a sputtering method can be employed. A mixed gas of silane (SiH 4 ) and nitrogen (N 2 ) or a mixed gas of silane (SiH 4 ), nitrogen (N 2 ), and ammonia (NH 3 ) can be used as a deposition gas. 
     In order to reduce the number of hydrogen molecules released from the silicon nitride film  407 , the flow rate of ammonia in the deposition gas is preferably as low as possible. For example, it is preferable that the flow ratio of ammonia to nitrogen be 0.1 or less, preferably 0.05 or less, more preferably 0.02 or less (that is, the flow ratio of nitrogen to ammonia in the deposition gas is 10 or more, preferably 20 or more, more preferably 50 or more). 
     When ammonia is used as the deposition gas, a bond between a silicon atom and a hydrogen atom in silane or a triple bond between nitrogen atoms in nitrogen is easily cut by dissociated ammonia in the deposition. Thus, decomposition of silane or nitrogen is promoted in the deposition, so that a dense silicon nitride film can be formed. The dense silicon nitride film can prevent entry of hydrogen, moisture, and the like from the outside into the oxide semiconductor film. Accordingly, in order to improve reliability of a semiconductor device, it is preferable that the flow rate of ammonia at the deposition of the silicon nitride film be set to an amount which promotes decomposition of silane and which suppresses the number of released hydrogen molecules. 
     Further, the deposition process may include a period during which the deposition gas is not supplied with keeping the substrate under nitrogen plasma conditions. When the substrate is kept under nitrogen plasma conditions, nitrogen is added to the exposed portions of the oxide semiconductor film and resistance can be further reduced in some cases. 
     The silicon nitride film  407  has a shielding effect of blocking penetration of both oxygen and impurities such as hydrogen, moisture, a hydroxyl group, and hydride (also referred to as a hydrogen compound). Accordingly, during and after the manufacturing process, the silicon nitride film functions as a protective film for preventing entry of impurities such as hydrogen and moisture, which cause a change, into the oxide semiconductor film and preventing out-diffusion of oxygen which is a main component material of the oxide semiconductor film. 
     Further, in order to add nitrogen to the oxide semiconductor film  409 , heat treatment may be performed after the silicon nitride film  407  is formed. By the heat treatment, nitrogen can be added from the silicon nitride film  407  to the oxide semiconductor film  409 . 
     Next, the interlayer insulating film  411  is formed over the silicon nitride film  407 . The interlayer insulating film  411  can be formed using a material and a method similar to those of the base insulating film  436 . Note that the interlayer insulating film  411  is not necessarily provided. 
     Next, regions of the silicon nitride film  407  and the interlayer insulating film  411 , which overlap with the low-resistance regions  404   a  and  404   b , are partly etched, so that openings reaching the low-resistance regions  404   a  and  404   b  are formed. Note that the openings are formed by selective etching with the use of a mask or the like. Dry etching, wet etching, or both wet etching and dry etching can be used to form the openings. Further, there is no particular limitation on the shapes of the openings as long as the openings reach the low-resistance regions  404   a  and  404   b.    
     Then, a conductive material is deposited in the openings, whereby the source electrode layer  405   a  and the drain electrode layer  405   b  are formed (see  FIG. 2C ). The source electrode layer  405   a  and the drain electrode layer  405   b  can be formed using a material and a method similar to those of the gate electrode layer  401 . 
     Through the above-described steps, the transistor  420  can be manufactured. 
     In the transistor described in this embodiment, by deposition of the silicon nitride film over the oxide semiconductor film, nitrogen is added to the oxide semiconductor film; thus, the low-resistance regions can be formed in the oxide semiconductor film  409 . Further, since the oxide semiconductor film is electrically connected to the source electrode layer and the drain electrode layer in the low-resistance regions, contact resistance between the oxide semiconductor film and the source electrode layer and contact resistance between the oxide semiconductor film and the drain electrode layer are reduced. Thus, a semiconductor device with excellent on-state characteristics and favorable electric characteristics can be provided. 
     Further, in the deposition step of the silicon nitride film  407  serving as an interlayer insulating film, nitrogen is added to the oxide semiconductor film  409  to form the low-resistance regions  404   a  and  404   b ; thus, the on-state characteristics of the transistor  420  can be improved without complicating the process. 
     Further, the silicon nitride film  407  has a reduced hydrogen concentration and a small number of released hydrogen molecules. Thus, impurities such as hydrogen and moisture can be prevented from entering the oxide semiconductor film  409  from the silicon nitride film  407 . In addition, since the silicon nitride film has a property of blocking entry of hydrogen and moisture from the outside, entry of impurities such as hydrogen and moisture from the outside can be prevented. Accordingly, a semiconductor device which has stable electric characteristics and high reliability can be provided. 
     The low-resistance regions  404   a  and  404   b  are formed by addition of nitrogen in the deposition step of the silicon nitride film  407 , and a step of adding an impurity element which reduces resistance of the oxide semiconductor film may be performed in combination. By addition of an impurity element, resistance of the low-resistance regions  404   a  and  404   b  can be further reduced. 
     However, when an impurity element is added to the oxide semiconductor film  409  after its deposition, the crystallinity of the added region is decreased in some cases. For example, in the case where the low-resistance regions  404   a  and  404   b  are formed in the oxide semiconductor film  409  which is a CAAC-OS film by performing a step of adding nitrogen in the deposition and a step of adding an impurity element, the low-resistance regions  404   a  and  404   b  might have an amorphous structure (or a state with many amorphous structures). In this case, in the oxide semiconductor film  409 , the channel formation region  403  might be a CAAC-OS film and the low-resistance regions  404   a  and  404   b  might be amorphous films (or films having many amorphous structures). 
     An ion implantation method, an ion doping method, or the like can be used as a method for adding an impurity element. 
     Phosphorus, boron, nitrogen, arsenic, argon, aluminum, a molecular ion containing any of these elements, or the like can be used as the impurity element to be added. The dosage of such an element is preferably 1×10 13  ions/cm 2  to 5×10 16  ions/cm 2 . When phosphorus is added as the impurity element, the acceleration voltage is preferably 0.5 kV to 80 kV. 
     The treatment of adding the impurity element to the oxide semiconductor film  409  may be performed plural times. In the case where the treatment of adding the impurity element to the oxide semiconductor film  409  is performed plural times, the kind of the impurity element may be the same in the plural treatments or different in every treatment. 
     The impurity element may be added immediately after the oxide semiconductor film to be the oxide semiconductor film  409  is deposited or immediately after the oxide semiconductor film is processed into the island-shaped oxide semiconductor film  409 . Alternatively, the impurity element may be added after the insulating film to be the gate insulating film  402  is deposited or in a state where the gate insulating film  402  is formed and the oxide semiconductor film is partly exposed. Further alternatively, the impurity element may be added after the silicon nitride film  407  is deposited. In the case where the impurity element is added in a state where the gate insulating film  402  or the silicon nitride film  407  covers the oxide semiconductor film  409 , the oxide semiconductor film  409  is less damaged because the impurity element is added through the gate insulating film  402  or the silicon nitride film  407 . 
     Note that an oxide semiconductor film having an amorphous structure is likely to absorb an impurity such as hydrogen, which serves as a donor, from an oxide semiconductor film having crystallinity such as a CAAC-OS film; thus, when the low-resistance regions between which the channel formation region  403  is sandwiched have an amorphous structure, the low resistance regions absorb the impurity (perform gettering) from the channel formation region  403  and the transistor  420  can have favorable electric characteristics in some cases. 
     Although not illustrated, an insulating film may be further provided over the transistor  420 . As the insulating film, a single layer or a stack of one or more inorganic insulating films, typical examples of which are a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a hafnium oxide film, a gallium oxide film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, and an aluminum nitride oxide film, can be used. 
     A heating step may be additionally performed after the insulating film is formed. For example, a heating step may be performed at a temperature higher than or equal to 100° C. and lower than or equal to 200° C. in the air for longer than or equal to 1 hour and shorter than or equal to 30 hours. This heating step may be performed at a fixed heating temperature. Alternatively, the following change in the heating temperature may be conducted plural times repeatedly: the heating temperature is increased from room temperature to a temperature higher than or equal to 100° C. and lower than or equal to 200° C. and then decreased to room temperature. 
     In addition, a planarization insulating film may be formed in order to reduce surface unevenness due to the transistor  420 . As the planarization insulating film, an organic material such as a polyimide resin, an acrylic resin, or a benzocyclobutene-based resin can be used. Other than such organic materials, it is also possible to use a low dielectric constant material (low-k material) or the like. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed from these materials. 
     The transistor described in this embodiment is not limited to the above structure. For example, as in the transistor  430  illustrated in  FIG. 3 , the source electrode layer  405   a  and the drain electrode layer  405   b  which are in contact with the oxide semiconductor film  409  may be formed before the silicon nitride film  407  is formed. 
     When the source electrode layer  405   a  and the drain electrode layer  405   b  are formed before the silicon nitride film  407  is formed, nitrogen is not added to the regions of the oxide semiconductor film  409 , which overlap with the source electrode layer  405   a  and the drain electrode layer  405   b , in the deposition of the silicon nitride film  407 ; thus, the resistance of the regions is not reduced, so that a region  403   a  and a region  403   b  which have a resistance similar to that of the channel formation region  403  are formed. 
     Note that in the transistor  430  in  FIG. 3 , occurrence of parasitic channel is reduced because resistance of end portions of the oxide semiconductor film  409 , which are in contact with the source electrode layer  405   a  and the drain electrode layer  405   b , is not reduced. The low-resistance region  404   a  and the low-resistance region  404   b  are formed between the channel formation region  403  and the source electrode layer  405   a  and between the channel formation region  403  and the drain electrode layer  405   b , which enables a reduction in resistance between the channel formation region  403  and the source electrode layer  405   a  and resistance between the channel formation region  403  and the drain electrode layer  405   b ; thus, the transistor  430  can have favorable electric characteristics. 
     Note that in order to put emphasis on the on-state characteristics of the transistor  430  and reduce resistance of the source electrode layer  405   a , the drain electrode layer  405   b , the region  403   a , and the region  403   b , an impurity element may be added to the oxide semiconductor film  409  before the source electrode layer  405   a  and the drain electrode layer  405   b  are formed. By addition of the impurity element, resistance of the region  403   a  and the region  403   b  which overlap with the source electrode layer  405   a  and the drain electrode layer  405   b , respectively, can have lower resistance. In this case, since the impurity element is also added to the low-resistance regions  404   a  and  404   b , the low-resistance regions  404   a  and  404   b  have much lower resistance. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 2 
     In this embodiment, transistors according to one embodiment of the present invention which are different from those of Embodiment 1 are described. Note that, in this embodiment, portions that are similar to the portions in Embodiment 1 are denoted by the same reference numerals, and detailed description thereof is omitted. 
       FIGS. 4A and 4B  illustrate a transistor  520  according to one embodiment of the present invention.  FIG. 4A  is a top view of the transistor  520  and  FIG. 4B  is a cross-sectional view taken along dashed-dotted line C-D in  FIG. 4A . 
     The transistor  520  includes the following: the base insulating film  436  and the base insulating film  438  over the substrate  400 ; an oxide semiconductor film  509  over the base insulating film  438 ; the gate insulating film  402  over the oxide semiconductor film  509 ; the gate electrode layer  401  over the gate insulating film  402 ; a sidewall insulating film  413   a  and a sidewall insulating film  413   b  which are in contact with side surfaces of the gate insulating film  402  and side surfaces of the gate electrode layer  401 ; the interlayer insulating film  411  and an interlayer insulating film  414  which are over the oxide semiconductor film  509 , the gate electrode layer  401 , and the sidewall insulating films  413   a  and  413   b ; and the source electrode layer  405   a  and the drain electrode layer  405   b  which are in contact with the oxide semiconductor film  509  in openings provided in the interlayer insulating films  411  and  414 . 
     The sidewall insulating films  413   a  and  413   b  are formed using a silicon nitride film deposited using conditions similar to those of the silicon nitride film  407  described in Embodiment 1. 
     The oxide semiconductor film  509  includes the channel formation region  403  overlapping with the gate electrode layer  401 , a first low-resistance region  406   a  and a first low-resistance region  406   b  between which the channel formation region  403  is sandwiched and which overlap with the sidewall insulating film  413   a  and the sidewall insulating film  413   b , respectively, and a second low-resistance region  408   a  and a second low-resistance region  408   b  between which the first low-resistance regions  406   a  and  406   b  are sandwiched and which are electrically connected to the source electrode layer  405   a  and the drain electrode layer  405   b , respectively. 
     The first low-resistance regions  406   a  and  406   b  and the second low-resistance regions  408   a  and  408   b  each have low resistance by addition of nitrogen caused by the deposition of a silicon nitride film to be the sidewall insulating films  413   a  and  413   b.    
     Moreover, since an impurity element which reduces conductivity is added to the second low-resistance regions  408   a  and  408   b  with the gate electrode layer  401  and the sidewall insulating films  413   a  and  413   b  used as a mask, resistance of the second low-resistance regions  408   a  and  408   b  is lower than that of the first low-resistance regions  406   a  and  406   b.    
     Thus, the first low-resistance regions  406   a  and  406   b  serve as lightly doped drain (LDD) regions. By providing the LDD region, off-state current can be reduced or the withstand voltage of the transistor can be increased (reliability can be improved). 
     Since an impurity element  421  is not added to the first low-resistance regions  406   a  and  406   b , the first low-resistance regions  406   a  and  406   b  each are a film having a crystalline state (or including more crystal structures than amorphous structures) such as a crystalline semiconductor film or a CAAC-OS film. 
     By the addition of the impurity element, the ratio of amorphous components with respect to crystal components in the second low-resistance regions  408   a  and  408   b  is higher than those in the channel formation region  403  and the first low-resistance regions  406   a  and  406   b  in some cases. Since an oxide semiconductor film having an amorphous structure might be likely to absorb impurities such as hydrogen, which serve as donors, from an oxide semiconductor film having crystallinity such as a CAAC-OS film, the second low-resistance regions  408   a  and  408   b  absorb impurities (also can be expressed as “perform gettering”) from the channel formation region  403  and the first low-resistance regions  406   a  and  406   b ; thus, the transistor  520  can have favorable electric characteristics. Further, the channel formation region  403  is not in contact with a region having a high concentration of an impurity element because the first low-resistance regions  406   a  and  406   b  are provided between the channel formation region  403  and the second low-resistance regions  408   a  and  408   b  which absorb an impurity element to have a high concentration of the impurity element; thus, reliability of the transistor  520  can be improved. 
     Next, a method for manufacturing the transistor  520  is described with reference to  FIGS. 5A to 5C . Note that detailed description of portions that are similar to those in Embodiment 1 is omitted. 
     A semiconductor device illustrated in  FIG. 5A  corresponds to the semiconductor device illustrated in  FIG. 2B . Therefore, for a method for manufacturing the semiconductor device illustrated in  FIG. 5A , the description in Embodiment 1 can be referred to. Since the resistance of the oxide semiconductor film  409  is reduced by the addition of nitrogen to a region in contact with the silicon nitride film  407  in the deposition of the silicon nitride film  407 , the oxide semiconductor film  409  includes, at this stage, the low-resistance regions  404   a  and  404   b  and the channel formation region  403  which is sandwiched between the low-resistance regions  404   a  and  404   b.    
     Next, anisotropic etching is performed on the silicon nitride film  407 , whereby part of the oxide semiconductor film  409  is exposed. The anisotropic etching is performed until the top surface of the gate electrode layer  401  is exposed. At the stage at which the gate electrode layer  401  is exposed by the anisotropic etching, the silicon nitride film remains on side portions of the gate electrode layer  401  and the gate insulating film  402 . Thus, by stopping etching at this stage, the sidewall insulating films  413   a  and  413   b  can be formed in a self-aligned manner without using a mask. By forming the sidewall insulating films  413   a  and  413   b  without using a mask, the number of masks and manufacturing cost can be reduced. 
     Next, the impurity element  421  is added to the oxide semiconductor film  409  using the gate electrode layer  401  and the sidewall insulating films  413   a  and  413   b  as a mask. An ion implantation method, an ion doping method, or the like can be used as a method for adding the impurity element  421 . 
     Phosphorus, boron, nitrogen, arsenic, argon, aluminum, a molecular ion containing any of these elements, or the like can be used as the impurity element to be added. The dosage of such an element is preferably 1×10 13  ions/cm 2  to 5×10 16  ions/cm 2 . When phosphorus is added as the impurity element, the acceleration voltage is preferably 0.5 kV to 80 kV. 
     The treatment for adding the impurity element to the oxide semiconductor film  409  may be performed plural times. In the case where the treatment for adding the impurity element to the oxide semiconductor film  409  is performed plural times, the kind of the impurity element may be the same in the plural treatments or different in every treatment. 
     By adding the impurity element  421 , resistance of exposed regions of the low-resistance regions  404   a  and  404   b  (regions which do not overlap with any of the gate electrode layer  401  and the sidewall insulating films  413   a  and  413   b ) is reduced. Thus, the oxide semiconductor film  509  includes the channel formation region  403 , the first low-resistance regions  406   a  and  406   b  whose resistance is reduced by the addition of nitrogen in the deposition of the silicon nitride film, and the second low-resistance regions  408   a  and  408   b  whose resistance is reduced by the addition of nitrogen in the deposition of the silicon nitride film and treatment of adding the impurity element  421  (see  FIG. 5B ). 
     The first low-resistance regions  406   a  and  406   b  and the second low-resistance regions  408   a  and  408   b  are regions whose resistance is reduced by the addition of nitrogen in the deposition of the silicon nitride film  407 . In addition, the second low-resistance regions  408   a  and  408   b  are regions which are made to have lower resistance than the first low-resistance regions  406   a  and  406   b  by the addition of the impurity element  421 . 
     Note that in the addition of the impurity element  421 , acceleration voltage of the impurity element may be adjusted so that the impurity element is added to regions (the first low-resistance regions  406   a  and  406   b ) of the oxide semiconductor film  509 , which overlap with the sidewall insulating films  413   a  and  413   b , through the sidewall insulating films  413   a  and  413   b . The impurity element is also added to the first low-resistance regions  406   a  and  406   b , whereby resistance of the first low-resistance regions  406   a  and  406   b  can be further reduced. In the case where the impurity element is also added to the first low-resistance regions  406   a  and  406   b , more impurity elements may be added to the second low-resistance regions  408   a  and  408   b  than to the first low-resistance regions  406   a  and  406   b.    
     Next, the interlayer insulating films  411  and  414  are formed over the oxide semiconductor film  509 , the sidewall insulating films  413   a  and  413   b , and the gate electrode layer  401 . As each of the interlayer insulating films  411  and  414 , a single layer or a stack of one or more inorganic insulating films such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a hafnium oxide film, a gallium oxide film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, and an aluminum nitride oxide film can be used. The interlayer insulating films  411  and  414  may be different films or the same film. Here, the interlayer insulating film  411  is a silicon oxide film containing added oxygen in excess of the stoichiometric composition to supply oxygen to the oxide semiconductor film  509  and the interlayer insulating film  414  is a silicon nitride film which has a property of blocking entry of impurities such as hydrogen and water from the outside into the oxide semiconductor film. 
     Then, openings reaching the second low-resistance regions  408   a  and  408   b  are formed in the interlayer insulating films  411  and  414  and the source electrode layer  405   a  and the drain electrode layer  405   b  are formed in the openings using a conductive material (see  FIG. 5C ). 
     The source electrode layer  405   a  and the drain electrode layer  405   b  are in contact with the oxide semiconductor film  509  in the second low-resistance regions  408   a  and  408   b ; thus, contact resistance between the oxide semiconductor film  509  and the source electrode layer  405   a  and contact resistance between the oxide semiconductor film  509  and the drain electrode layer  405   b  can be reduced. 
     Further, since the first low-resistance regions  406   a  and  406   b  serve as LDD regions, off-state current can be reduced or the withstand voltage of the transistor can be increased (reliability can be improved). 
     Through the above-described steps, the transistor  520  can be manufactured. 
     In the transistor described in this embodiment, nitrogen is diffused into the regions of the oxide semiconductor film, which are in contact with the silicon nitride film, by formation of the silicon nitride film over the oxide semiconductor film, the gate insulating film, and the gate electrode layer, whereby low-resistance regions are formed. In the low-resistance regions, the oxide semiconductor film is electrically connected to the source electrode layer and the drain electrode layer; thus, contact resistance between the source electrode layer and the oxide semiconductor film and contact resistance between the drain electrode layer and the oxide semiconductor film can be reduced. 
     Further, the silicon nitride film has a reduced hydrogen concentration and a small number of released hydrogen molecules. Thus, impurities such as hydrogen and moisture, which cause degradation of the oxide semiconductor film, can be prevented from entering the oxide semiconductor film from the silicon nitride film. In addition, since the silicon nitride film has a property of blocking entry of hydrogen and moisture from the outside, entry of impurities such as hydrogen and moisture from the outside can be prevented. Accordingly, a semiconductor device which has stable electric characteristics and high reliability can be provided. 
     Moreover, in the semiconductor device described in this embodiment, LDD regions can be formed in a self-aligned manner by the addition of the impurity element with the gate electrode layer and the sidewall insulating films used as a mask. By providing the LDD region, off-state current of the transistor can be reduced or the withstand voltage of the transistor can be increased (reliability can be improved). 
     The transistor described in this embodiment is not limited to the above structure. For example, as in a transistor  530  illustrated in  FIG. 6A , the oxide semiconductor film  509  may be in contact with the source electrode layer  405   a  and the drain electrode layer  405   b  without the openings in the interlayer insulating film  411 . An insulating film  412  may be formed over the gate electrode layer  401 . 
     A method for manufacturing the transistor  530  is as follows. First, an oxide semiconductor film is formed, the gate insulating film  402  and the gate electrode layer  401  are formed over the oxide semiconductor film  509 , nitrogen is added to the oxide semiconductor film  509  by depositing the silicon nitride film in order to reduce resistance, and anisotropic etching is performed on the silicon nitride film, whereby the sidewall insulating films  413   a  and  413   b  are formed. After that, the source electrode layer  405   a  and the drain electrode layer  405   b  are formed, and the impurity element is added to the oxide semiconductor film  509  with the gate electrode layer  401 , the sidewall insulating films  413   a  and  413   b , the source electrode layer  405   a , and the drain electrode layer  405   b  used as masks. 
     Thus, the oxide semiconductor film  509  of the transistor  530  includes the following: the channel formation region  403  overlapping with the gate electrode layer  401 ; the first low-resistance regions  406   a  and  406   b  between which the channel formation region  403  is sandwiched and which overlap with the sidewall insulating films  413   a  and  413   b ; the second low-resistance regions  408   a  and  408   b  between which the first low-resistance regions  406   a  and  406   b  are sandwiched and which have lower resistance than the first low-resistance regions  406   a  and  406   b  by treatment of adding the impurity element; and a third low-resistance region  410   a  and a third low-resistance region  410   b  between which the second low-resistance regions  408   a  and  408   b  are sandwiched and which are in contact with the source electrode layer  405   a  and the drain electrode layer  405   b.    
     In the transistor  530 , occurrence of parasitic channel is reduced because resistance of end portions of the oxide semiconductor film  509 , which are electrically connected to the source electrode layer  405   a  and the drain electrode layer  405   b , is not reduced. When emphasis is put on the on-state characteristics of the transistor  530 , an impurity element may be added before the source electrode layer  405   a  and the drain electrode layer  405   b  are formed, so that the third low-resistance regions  410   a  and  410   b  which overlap with the source electrode layer  405   a  and the drain electrode layer  405   b  also include an impurity element. 
     Further, a semiconductor device of another mode of this embodiment may have, as in a transistor  540  illustrated in  FIG. 6B , a structure in which the insulating film  412  is formed over the gate electrode layer  401  and the source electrode layer  405   a  and the drain electrode layer  405   b  are formed so as to be in contact with side surfaces of the sidewall insulating films  413   a  and  413   b  and to cover the oxide semiconductor film  409 . 
     As a method for manufacturing the transistor  540 , after the sidewall insulating films  413   a  and  413   b  are formed by a method similar to that of the transistor  520 , a conductive film to be the source electrode layer  405   a  and the drain electrode layer  405   b  and an insulating film to be the interlayer insulating film  411  are formed over the oxide semiconductor film  509 , the gate electrode layer  401 , and the sidewall insulating films  413   a  and  413   b . Regions of the conductive film and the insulating film, which overlap with the gate electrode layer  401  are removed and thus, the conductive film is divided, so that the source electrode layer  405   a  and the drain electrode layer  405   b  can be formed in a self-aligned manner. 
     The conductive film and the insulating film can be removed by chemical mechanical polishing treatment, anisotropic etching, or the like. 
     With the insulating film  412  over the gate electrode layer  401 , insulation between the gate electrode layer  401  and the source electrode layer  405   a  and insulation between the gate electrode layer  401  and the drain electrode layer  405   b  can be ensured. 
     This embodiment can be combined with any of the other embodiments as appropriate. 
     Embodiment 3 
     In this embodiment, an example of a semiconductor device including any of the transistors described in Embodiments 1 and 2 is described with reference to  FIG. 7  and  FIGS. 8A and 8B . 
       FIG. 7  is a cross-sectional view of part of a semiconductor device described in this embodiment. The cross-sectional view illustrated in  FIG. 7  is an example, and the structure of the semiconductor device described in this embodiment is not limited thereto. 
     The semiconductor device illustrated in  FIG. 7  includes a transistor  160  including a first semiconductor material in a lower portion, and a transistor  162  including a second semiconductor material in an upper portion. The transistor  162  can have a structure similar to that of any of the transistors described in Embodiments 1 and 2. Here, as the transistor  162 , the transistor described in Embodiment 1 is used. 
     Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor enables charge to be held for a long time owing to its characteristics and thus allows a reduction in power consumption. 
     The transistor  160  in  FIG. 7  includes the following: a base insulating film  102  over a substrate  100  including a semiconductor material (e.g., silicon); a semiconductor layer  116  over the base insulating film  102 ; a gate insulating film  108  over the semiconductor layer  116 ; a gate electrode layer  110  over the gate insulating film  108 ; a sidewall insulating film  120   a  and a sidewall insulating film  120   b  which are over the gate insulating film  108  and in contact with side surfaces of the gate electrode layer  110 ; a stack of an insulating film  125 , an insulating film  131 , and an insulating film  133  which covers the semiconductor layer  116 , the gate insulating film  108 , the gate electrode layer  110 , and the sidewall insulating films  120   a  and  120   b ; and a source electrode layer  124   a  and a drain electrode layer  124   b  which are in contact with the semiconductor layer  116  through openings provided in the insulating film  125 , the insulating film  131 , and the insulating film  133 . 
     In order to achieve high integration, the transistor  160  preferably has a structure in which the sidewall insulating films  120   a  and  120   b  are not provided. On the other hand, in the case where emphasis is put on characteristics of the transistor  160 , the sidewall insulating films  120   a  and  120   b  are preferably provided as illustrated in  FIG. 7 . 
     An insulating film  135  is provided over the transistor  160 . An insulating film  137  is provided over the insulating film  135 , and a conductive layer  141   a  and a conductive layer  141   b  are provided in the insulating film  137 . The conductive layer  141   a  is electrically connected to the drain electrode layer  124   b  of the transistor  160  through an opening provided in the insulating film  135 . 
     An insulating film  151 , an insulating film  153 , and an insulating film  155  are provided over the insulating film  137 . The insulating films  151  and  153  each are preferably have a blocking property so that an impurity caused by the transistor  160  is not added to the transistor  162 . For example, the insulating film  151  may contain silicon oxide and the insulating film  153  may contain silicon nitride. The insulating film  155  improves planarity of an oxide semiconductor film  144  to be formed later and supplies oxygen to the oxide semiconductor film. Thus, as the insulating film  155 , it is preferable to use a silicon oxide film or the like which contains oxygen in excess of the stoichiometric composition and whose planarity is improved by CMP treatment or the like. 
     The transistor  162  is formed over the insulating film  155 . The transistor  162  is the transistor including an oxide semiconductor described in Embodiment 1. Resistance of the oxide semiconductor film  144  included in the transistor  162  is reduced by treatment of adding an impurity element with a gate electrode layer  148  used as a mask and deposition of a silicon nitride film  143  over the oxide semiconductor film  144 . Consequently, the oxide semiconductor film  144  includes a low-resistance region  144   a , a low-resistance region  144   b , a low-resistance region  144   c , a low-resistance region  144   d , and a channel formation region  144   e . The treatment of adding an impurity element is performed between the formation of the gate electrode layer  148  and the deposition of the silicon nitride film  143 . 
     The impurity element is added to the low-resistance regions  144   a  to  144   d  with the gate electrode layer  148  used as a mask. Accordingly, concentration of the impurity element in each of the low-resistance regions  144   a  to  144   d  is higher than that in the channel formation region  144   e . Since carrier density in the oxide semiconductor film  144  is increased by increase in the concentration of the impurity element, favorable ohmic contact can be obtained between a wiring layer and the oxide semiconductor film, so that on-state current and field-effect mobility of the transistor are improved. 
     In addition, resistance of the low-resistance regions  144   c  and  144   d  is reduced because nitrogen is added to the low-resistance regions  144   c  and  144   d  by deposition of the silicon nitride film  143  over the oxide semiconductor film  144 . Accordingly, the low-resistance regions  144   c  and  144   d  have lower resistance than the low-resistance regions  144   a  and  144   b.    
     In the transistor  162 , nitrogen is added to the oxide semiconductor film by deposition of the silicon nitride film in contact with the oxide semiconductor film, so that the low-resistance regions  144   c  and  144   d  can be formed in the oxide semiconductor film  144 . Consequently, a transistor having excellent on-state characteristics and favorable electric characteristics can be obtained, which enables high-speed response and high-speed operation of a semiconductor device. 
     Further, the silicon nitride film has a reduced hydrogen concentration and has a property of blocking entry of hydrogen and moisture from the outside. Thus, entry of impurities such as water and hydrogen, which causes degradation of the oxide semiconductor film, can be prevented; accordingly, a semiconductor device having stable electric characteristics and high reliability can be provided. 
     The transistor  162  includes the following: the above-described oxide semiconductor film  144 ; a gate insulating film  146  over the oxide semiconductor film  144 ; the gate electrode layer  148  over the gate insulating film  146 ; a source electrode layer  142   a  and a drain electrode layer  142   b  which are in contact with the low-resistance region  144   a  and the low-resistance region  144   b  of the oxide semiconductor film  144 , respectively; and the silicon nitride film  143  covering the oxide semiconductor film  144 , the source electrode layer  142   a , the drain electrode layer  142   b , the gate insulating film  146 , and the gate electrode layer  148 . The source electrode layer  142   a  is electrically connected to the conductive layer  141   a  through an opening provided in the insulating film  151 , the insulating film  153 , and the insulating film  155 . 
     A stack of an insulating film  147  and an insulating film  149  is provided over the transistor  162 . As each of the insulating films  147  and  149 , an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a hafnium oxide film, a gallium oxide film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film can be used. The insulating films  147  and  149  may be the same film or different films. 
     Over the insulating films  147  and  149 , a wiring layer  145  electrically connected to the source electrode layer  142   a  through an opening provided in the insulating films  147  and  149  is provided. An insulating film  157  is provided over the wiring layer  145 , and a wiring layer  159  electrically connected to the wiring layer  145  through an opening provided in the insulating film  157  is provided over the insulating film  157 . 
     As examples of the semiconductor device including the transistor described in this specification, a NOR circuit and a NAND circuit which are logic circuits are illustrated in  FIGS. 8A and 8B .  FIG. 8A  shows the NOR circuit, and  FIG. 8B  shows the NAND circuit. 
     In the NOR circuit and the NAND circuit illustrated in  FIGS. 8A and 8B , a transistor  201 , a transistor  202 , a transistor  211 , and a transistor  214  each are a p-channel transistor and have a structure similar to that of the transistor  160  in  FIG. 7 . A transistor  203 , a transistor  204 , a transistor  212 , and a transistor  213  each are an n-channel transistor and have a structure similar to that of the transistor  162  in  FIG. 7  or that of any of the transistors described in Embodiments 1 and 2. 
     In the semiconductor device described in this embodiment, the transistor in which an oxide semiconductor is used for the channel formation region and which has extremely small off-state current is employed as the transistor  162 ; therefore, power consumption can be sufficiently reduced. Further, with any of the transistors described in Embodiments 1 and 2, contact resistance between the oxide semiconductor film and the source electrode layer and contact resistance between the oxide semiconductor film and the drain electrode layer are reduced; thus, excellent on-state characteristics are obtained, which enables high-speed response and high-speed operation of the semiconductor device. 
     Further, with a stack of semiconductor elements using different semiconductor materials, a miniaturized and highly integrated semiconductor device with stable electric characteristics and a method for manufacturing the semiconductor device can be provided. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the other structures, methods, and the like described in the other embodiments 
     Embodiment 4 
     In this embodiment, an example of a semiconductor device (memory device) which includes the transistor described in this specification, which can hold stored data even when not powered, and which has an unlimited number of write cycles is described with reference to drawings. 
       FIG. 9A  is a circuit diagram illustrating the semiconductor device of this embodiment. 
     A transistor  260  illustrated in  FIG. 9A  can have a structure similar to that of the transistor  160  illustrated in  FIG. 7  and easily operates at high speed. Further, a transistor  262  can have a structure similar to that of the transistor  162  illustrated in  FIG. 7  or that of any of the transistors described in Embodiments 1 and 2 and enables charge to be held for a long time owing to its characteristics. 
     Although all the transistors are n-channel transistors here, p-channel transistors can be used as the transistors used for the semiconductor device described in this embodiment. 
     In  FIG. 9A , a first wiring (1st Line) is electrically connected to a source electrode layer of the transistor  260 , and a second wiring (2nd Line) is electrically connected to a drain electrode layer of the transistor  260 . A third wiring (3rd Line) is electrically connected to one of the source electrode layer and the drain electrode layer of the transistor  262 , and a fourth wiring (4th Line) is electrically connected to a gate electrode layer of the transistor  262 . A gate electrode layer of the transistor  260  and the other of the source electrode layer and the drain electrode layer of the transistor  262  are electrically connected to one electrode of the capacitor  164 . A fifth wiring (5th Line) is electrically connected to the other electrode of the capacitor  164 . 
     The semiconductor device illustrated in  FIG. 9A  utilizes a characteristic in which the potential of the gate electrode layer of the transistor  260  can be held, and thus enables data writing, holding, and reading as follows. 
     Writing and holding of data will be described. First, the potential of the fourth wiring is set to a potential at which the transistor  262  is turned on, so that the transistor  262  is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode layer of the transistor  260  and to the capacitor  164 . That is, predetermined charge is supplied to the gate electrode layer of the transistor  260  (writing). Here, charge for supplying either of two different potential levels (hereinafter referred to as charge corresponding to a low level and charge corresponding to a high level) is given. After that, the potential of the fourth wiring is set to a potential at which the transistor  262  is turned off, so that the transistor  262  is turned off. Thus, the charge given to the gate electrode layer of the transistor  260  is held (holding). 
     Since the off-state current of the transistor  262  is extremely low, the charge of the gate electrode layer of the transistor  260  is held for a long time. 
     Next, reading of data will be described. By supplying an appropriate potential (a reading potential) to the fifth wiring while supplying a predetermined potential (a constant potential) to the first wiring, the potential of the second wiring varies depending on the amount of charge held at the gate electrode layer of the transistor  260 . This is because in general, when the transistor  260  is an n-channel transistor, an apparent threshold voltage V th_H  in the case where the charge corresponding to the high level is given to the gate electrode layer of the transistor  260  is lower than an apparent threshold voltage V th_L  in the case where the charge corresponding to the low level is given to the gate electrode layer of the transistor  260 . Here, an apparent threshold voltage refers to the potential of the fifth line, which is needed to turn on the transistor  260 . Thus, the potential of the fifth wiring is set to a potential V 0  which is between V th_H  and V th_L , whereby charge given to the gate electrode layer of the transistor  260  can be determined. For example, in the case where the charge corresponding to the high level is given in writing, when the potential of the fifth wiring is set to V 0  (&gt;V th_H ), the transistor  260  is turned on. In the case where the charge corresponding to the low level is given in writing, even when the potential of the fifth wiring is set to V 0  (&lt;V th_L ), the transistor  260  remains in an off state. Therefore, the stored data can be read by the potential of the second line. 
     Note that in the case where memory cells are arrayed to be used, only data of desired memory cells needs to be read. In the case where such reading is not performed, a potential at which the transistor  260  is turned off regardless of the state of the gate electrode layer of the transistor  260 , that is, a potential smaller than V th_H  may be given to the fifth wiring. Alternatively, a potential at which the transistor  260  is turned on regardless of the state of the gate electrode layer, that is, a potential higher than V th_L  may be given to the fifth wiring. 
       FIG. 9B  illustrates an example of one embodiment of a structure of a memory device which is different from that in  FIG. 9A .  FIG. 9B  illustrates an example of a circuit configuration of a semiconductor device, and  FIG. 9C  is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in  FIG. 9B  will be described, and then, the semiconductor device illustrated in  FIG. 9C  will be described. 
     In the semiconductor device illustrated in  FIG. 9B , a bit line BL is electrically connected to the source electrode layer or the drain electrode layer of the transistor  262 , a word line WL is electrically connected to the gate electrode layer of the transistor  262 , and the source electrode layer or the drain electrode layer of the transistor  262  is electrically connected to a first terminal of a capacitor  254 . 
     Here, the transistor  262  including an oxide semiconductor has extremely low off-state current. For that reason, a potential of the first terminal of the capacitor  254  (or a charge accumulated in the capacitor  254 ) can be held for an extremely long time by turning off the transistor  262 . 
     Next, writing and holding of data in the semiconductor device (a memory cell  250 ) illustrated in  FIG. 9B  will be described. 
     First, the potential of the word line WL is set to a potential at which the transistor  262  is turned on, and the transistor  262  is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor  254  (writing). After that, the potential of the word line WL is set to a potential at which the transistor  262  is turned off, so that the transistor  262  is turned off. Thus, the potential of the first terminal of the capacitor  254  is held (holding). 
     Because the off-state current of the transistor  262  is extremely small, the potential of the first terminal of the capacitor  254  (or a charge accumulated in the capacitor) can be held for an extremely long period. 
     Next, reading of data will be described. When the transistor  262  is turned on, the bit line BL which is in a floating state and the capacitor  254  are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor  254 . As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor  254  (or the charge accumulated in the capacitor  254 ). 
     For example, the potential of the bit line BL after charge redistribution is represented by (C B ·V B0 +C·V)/(C B ·C), where V is the potential of the first terminal of the capacitor  254 , C is the capacitance of the capacitor  254 , C B  is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V B0  is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell  250  is in either of two states in which the potentials of the first terminal of the capacitor  254  are V 1  and V 0  (V 1 &gt;V 0 ), the potential of the bit line BL in the case of holding the potential V 1  (=(C B ·V B0 +C·V 1 )/(C B +C)) is higher than the potential of the bit line BL in the case of holding the potential V 0  (=(C B ·V B0 +C·V 0 )/(C B +C)). 
     Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read. 
     As described above, the semiconductor device illustrated in  FIG. 9B  can hold charge that is accumulated in the capacitor  254  for a long time because the amount of the off-state current of the transistor  262  is extremely small. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long time even when power is not supplied. 
     Next, the semiconductor device illustrated in  FIG. 9C  will be described. 
     The semiconductor device illustrated in  FIG. 9C  includes memory cell arrays  251   a  and  251   b  including a plurality of memory cells  250  illustrated in  FIG. 9B  as memory circuits in an upper portion, and a peripheral circuit  253  in a lower portion which is necessary for operating a memory cell array  251  (the memory cell arrays  251   a  and  251   b ). Note that the peripheral circuit  253  is electrically connected to the memory cell array  251 . 
     In the structure illustrated in  FIG. 9C , the peripheral circuit  253  can be provided directly under the memory cell array  251  (the memory cell arrays  251   a  and  251   b ). Thus, the size of the semiconductor device can be reduced. 
     It is preferable that a semiconductor material of the transistor provided in the peripheral circuit  253  be different from that of the transistor  262 . For example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like can be used, and a single crystal semiconductor is preferably used. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, a variety of circuits (e.g., a logic circuit or a driver circuit) which need to operate at high speed can be favorably realized by the transistor. 
     Note that  FIG. 9C  illustrates, as an example, the semiconductor device in which two memory cell arrays  251  (the memory cell arrays  251   a  and  251   b ) are stacked; however, the number of memory cell arrays to be stacked is not limited thereto. Three or more memory cell arrays may be stacked. 
       FIG. 10  is an example of a cross-sectional view of the semiconductor device illustrated in  FIG. 9C . In the cross-sectional view illustrated in  FIG. 10 , the transistor  260  which is included in the peripheral circuit  253  and the transistor  262  and the capacitor  254  which are included in each of the memory cell arrays  251   a  and  251   b  are illustrated. 
     The transistors  260  and  262  can have structures similar to those of the transistors  160  and  162  illustrated in  FIG. 7 , respectively; thus, detailed description thereof is omitted. 
     The capacitor  254  included in each of the memory cell arrays  251   a  and  251   b  includes the source electrode layer  142   a  of the transistor  262 , the silicon nitride film  143 , the insulating film  147 , the insulating film  149 , and a conductive layer  242 . In other words, the source electrode layer  142   a  of the transistor  262  functions as one electrode of the capacitor  254 , the silicon nitride film  143 , the insulating film  147 , and the insulating film  149  function as a dielectric of the capacitor  254 , and the conductive layer  242  functions as the other electrode of the capacitor  254 . 
     Wiring layers for electrically connecting the peripheral circuit  253  and the memory cell array  251   a  are provided between the peripheral circuit  253  and the memory cell array  251   a . A wiring layer  258  electrically connects a drain electrode layer of the transistor  260  included in the peripheral circuit  253  and the conductive layer  256 , and a conductive layer  257  formed in the same step as the conductive layer  256  is electrically connected to a source electrode layer of the transistor  262  included in the memory cell array  251   a  through a wiring layer  259 . 
     Similarly, wiring layers for electrically connecting the memory cell arrays  251   a  and  251   b  are provided between the memory cell arrays  251   a  and  251   b . Although detailed description is omitted, the memory cell arrays  251   a  and  251   b  are electrically connected to each other through a wiring layer  271 , a wiring layer  272 , a conductive layer  273 , a conductive layer  274 , a conductive layer  275 , a conductive layer  276 , and a conductive layer  277 . 
     When a transistor including an oxide semiconductor in a channel formation region is used as the transistor  262 , stored data can be held for a long time. In other words, power consumption can be sufficiently reduced because a semiconductor device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided. 
     As the transistor  262 , any of the transistors described in Embodiments 1 and 2 can be used. In the transistor  262 , nitrogen is added to an oxide semiconductor film by depositing a silicon nitride film over and in contact with the oxide semiconductor film, so that low-resistance regions can be formed in the oxide semiconductor film  144 . Thus, a transistor with excellent on-state characteristics and favorable electric characteristics can be provided, which enables high-speed response and high-speed operation of a semiconductor device. 
     Embodiment 5 
     In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to electronic devices such as a mobile phone, a smartphone, or an e-book reader will be described with reference to  FIGS. 11A and 11B ,  FIG. 12 ,  FIG. 13 , and  FIG. 14 . 
     In electronic devices such as a mobile phone, a smart phone, and an e-book reader, an SRAM or a DRAM is used so as to store image data temporarily. This is because response speed of a flash memory is low and thus a flash memory is not suitable for image processing. On the other hand, an SRAM or a DRAM has the following characteristics when used for temporary storage of image data. 
     In a normal SRAM, as illustrated in  FIG. 11A , one memory cell includes six transistors, which are a transistor  801 , a transistor  802 , a transistor  803 , a transistor  804 , a transistor  805 , and a transistor  806 , and they are driven by an X decoder  807  and a Y decoder  808 . A pair of the transistors  803  and  805  and a pair of the transistors  804  and  806  each serve as an inverter, and high-speed driving can be performed therewith. However, an SRAM has a disadvantage of large cell area because one memory cell includes six transistors. Provided that the minimum feature size of a design rule is F, the area of a memory cell in an SRAM is generally 100 F 2  to 150 F 2 . Therefore, the price per bit of an SRAM is the highest among a variety of memory devices. 
     On the other hand, as illustrated in  FIG. 11B , a memory cell in a DRAM includes a transistor  811  and a storage capacitor  812 , and is driven by an X decoder  813  and a Y decoder  814 . One cell includes one transistor and one capacitor and has a small area. The area of a memory cell in a DRAM is generally less than or equal to 10 F 2 . Note that the DRAM needs to be refreshed periodically and consumes electric power even when a rewriting operation is not performed. 
     However, the area of the memory cell of the semiconductor device described in any of the above embodiments is about 10 F 2  and frequent refreshing is not needed. Therefore, the area of the memory cell can be reduced, which results in a reduction in power consumption. Further, with the semiconductor device described in any of the above embodiments, a semiconductor device which is hardly affected by entry of hydrogen, water, and the like from the outside and has high reliability can be provided. 
       FIG. 12  is a block diagram of an electronic device. An electronic device illustrated in  FIG. 12  includes an RF circuit  901 , an analog baseband circuit  902 , a digital baseband circuit  903 , a battery  904 , a power supply circuit  905 , an application processor  906 , a flash memory  910 , a display controller  911 , a memory circuit  912 , a display  913 , a touch sensor  919 , an audio circuit  917 , a keyboard  918 , and the like. The display  913  includes a display portion  914 , a source driver  915 , and a gate driver  916 . The application processor  906  includes a CPU  907 , a DSP  908 , and an interface (IF)  909 . In general, the memory circuit  912  includes an SRAM or a DRAM; by employing any of the semiconductor devices described in the above embodiments for the memory circuit  912 , writing and reading of data can be performed at high speed, data can be held for a long time, and power consumption can be sufficiently reduced. 
       FIG. 13  illustrates an example in which any of the semiconductor devices described in the above embodiments is used for a memory circuit  950  in a display. The memory circuit  950  illustrated in  FIG. 13  includes a memory  952 , a memory  953 , a switch  954 , a switch  955 , and a memory controller  951 . Further, the memory circuit is connected to a display controller  956  which reads and controls image data input through a signal line (input image data) and data stored in the memories  952  and  953  (stored image data), and is also connected to a display  957  which displays an image based on a signal input from the display controller  956 . 
     First, image data (input image data A) is formed by an application processor (not illustrated). The input image data A is stored in the memory  952  though the switch  954 . The image data (stored image data A) stored in the memory  952  is transmitted to the display  957  through the switch  955  and the display controller  956 , and is displayed on the display  957 . 
     In the case where the input image data A is not changed, the stored image data A is read from the memory  952  through the switch  955  by the display controller  956  normally at a frequency of approximately 30 Hz to 60 Hz. 
     Next, for example, when a user performs an operation to rewrite a screen (i.e., when the input image data A is changed), the application processor produces new image data (input image data B). The input image data B is stored in the memory  953  through the switch  954 . Also during this time, the stored image data A is regularly read from the memory  952  through the switch  955 . After the completion of storing the new image data (stored image data B) in the memory  953 , from the next frame for the display  957 , the stored image data B starts to be read, is transmitted to the display  957  through the switch  955  and the display controller  956 , and is displayed on the display  957 . This reading operation continues until another new image data is stored in the memory  952 . 
     By alternately writing and reading image data to and from the memory  952  and the memory  953  as described above, images are displayed on the display  957 . Note that the memory  952  and the memory  953  are not necessarily separate memories and a single memory may be divided and used. By employing any of the semiconductor devices described in the above embodiments for the memory  952  and the memory  953 , data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced. Further, a semiconductor device which is hardly affected by entry of water, moisture, and the like from the outside and has high reliability can be provided. 
       FIG. 14  is a block diagram of an electronic book. The electronic book in  FIG. 14  includes a battery  1001 , a power supply circuit  1002 , a microprocessor  1003 , a flash memory  1004 , an audio circuit  1005 , a keyboard  1006 , a memory circuit  1007 , a touch panel  1008 , a display  1009 , and a display controller  1010 . 
     Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit  1007  in  FIG. 14 . The memory circuit  1007  has a function of temporarily storing the contents of a book. For example, when a user uses a highlight function, the memory circuit  1007  stores and holds data of a portion specified by the user. Note that the highlight function is used to make a difference between a specific portion and the other portions while reading an e-book, by marking the specific portion, e.g., by changing the display color, underlining, making characters bold, changing the font of characters, or the like. In order to store the data for a short time, the data may be stored in the memory circuit  1007 . In order to store the data for a long time, the data stored in the memory circuit  1007  may be copied to the flash memory  1004 . Also in such a case, by employing the semiconductor devices described in any of the above embodiments, data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced. Further, a semiconductor device which is hardly affected by entry of water, moisture, and the like from the outside and which has high reliability can be provided. 
       FIGS. 15A to 15C  illustrate a specific example of an electronic device.  FIGS. 15A and 15B  illustrate a foldable tablet terminal. The tablet terminal is opened in  FIG. 15A . The tablet terminal includes a housing  9630 , a display portion  9631   a , a display portion  9631   b , a display mode switch  9034 , a power switch  9035 , a power saver switch  9036 , a clasp  9033 , and an operation switch  9038 . 
     The semiconductor device described in Embodiment 1 or 2 can be used for the display portion  9631   a  and the display portion  9631   b , so that the tablet terminal can have high reliability. In addition, the memory device described in the above embodiment may be applied to the semiconductor device of this embodiment. 
     Part of the display portion  9631   a  can be a touch panel region  9632   a  and data can be input when a displayed operation key  9638  is touched. Although a structure in which a half region in the display portion  9631   a  has only a display function and the other half region also has a touch panel function is shown as an example, the display portion  9631   a  is not limited to the structure. For example, the display portion  9631   a  can display keyboard buttons in the whole region to be a touch panel, and the display portion  9631   b  can be used as a display screen. 
     As in the display portion  9631   a , part of the display portion  9631   b  can be a touch panel region  9632   b . When a keyboard display switching button  9639  displayed on the touch panel is touched with a finger, a stylus, or the like, a keyboard can be displayed on the display portion  9631   b.    
     Touch input can be performed in the touch panel region  9632   a  and the touch panel region  9632   b  at the same time. 
     The display mode switch  9034  can switch the display between portrait mode, landscape mode, and the like, and between monochrome display and color display, for example. The power saver switch  9036  can control display luminance in accordance with the amount of external light in use of the tablet terminal detected by an optical sensor incorporated in the tablet terminal. In addition to the optical sensor, another detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, may be incorporated in the tablet terminal. 
     Although the display portion  9631   a  and the display portion  9631   b  have the same display area in  FIG. 15A , one embodiment of the present invention is not limited to this structure. The display portion  9631   a  and the display portion  9631   b  may have different areas or different display quality. For example, one of them may be a display panel that can display higher-definition images than the other. 
     The tablet terminal is closed in  FIG. 15B . The tablet terminal includes the housing  9630 , a solar cell  9633 , a charge and discharge control circuit  9634 , a battery  9635 , and a DCDC converter  9636 . In  FIG. 15B , a structure including the battery  9635  and the DCDC converter  9636  is illustrated as an example of the charge and discharge control circuit  9634 . 
     Since the tablet terminal is foldable, the housing  9630  can be closed when the tablet terminal is not used. As a result, the display portion  9631   a  and the display portion  9631   b  can be protected; thus, a tablet terminal which has excellent durability and excellent reliability in terms of long-term use can be provided. 
     In addition, the tablet terminal illustrated in  FIGS. 15A and 15B  can have a function of displaying a variety of kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, a function of controlling processing by a variety of kinds of software (programs), and the like. 
     The solar cell  9633  provided on a surface of the tablet terminal can supply power to the touch panel, the display portion, a video signal processing portion, or the like. Note that the solar cell  9633  can be provided on one or both surfaces of the housing  9630  and the battery  9635  can be charged efficiently. The use of a lithium ion battery as the battery  9635  is advantageous in downsizing or the like. 
     The structure and the operation of the charge and discharge control circuit  9634  illustrated in  FIG. 15B  will be described with reference to a block diagram in  FIG. 15C . The solar cell  9633 , the battery  9635 , the DCDC converter  9636 , a converter  9637 , switches SW 1  to SW 3 , and a display portion  9631  are illustrated in  FIG. 15C , and the battery  9635 , the DCDC converter  9636 , the converter  9637 , and the switches SW 1  to SW 3  correspond to the charge and discharge control circuit  9634  illustrated in  FIG. 15B . 
     First, an example of the operation in the case where power is generated by the solar cell  9633  using external light is described. The voltage of power generated by the solar cell is stepped up or down by the DCDC converter  9636  so that the power has a voltage for charging the battery  9635 . Then, when the power from the solar cell  9633  is used for the operation of the display portion  9631 , the switch SW 1  is turned on and the voltage of the power is stepped up or down by the converter  9637  so as to be a voltage needed for the display portion  9631 . In addition, when display on the display portion  9631  is not performed, the switch SW 1  is turned off and the switch SW 2  is turned on so that the battery  9635  may be charged. 
     Here, the solar cell  9633  is shown as an example of a power generation means; however, there is no particular limitation on a way of charging the battery  9635 , and the battery  9635  may be charged with another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery  9635  may be charged with a non-contact power transmission module which is capable of charging by transmitting and receiving power wirelessly (without contact), or another charging means may be used in combination. 
     As described above, the semiconductor device in any of the above embodiments is mounted on each of the electronic devices described in this embodiment. Thus, it is possible to obtain an electronic device which is capable of reading data at high speed, storing data for a long time, and reducing power consumption. Further, a semiconductor device which is hardly affected by entry of water, moisture, and the like from the outside and which has high reliability can be provided. 
     The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the other structures, methods, and the like described in the other embodiments 
     Example 1 
     In this example, evaluation results of silicon nitride films applicable to a semiconductor device according to one embodiment of the present invention are described. Specifically, results obtained by TDS, SIMS, and RBS are shown. 
     Methods for manufacturing samples evaluated in this example are described. 
     As a sample A1, a 300-nm-thick silicon nitride film was deposited over a silicon wafer. The silicon nitride film was deposited as follows. A silicon wafer was placed in a treatment chamber of a plasma CVD apparatus, the pressure in the treatment chamber was controlled so as to be 100 Pa, and a power of 2000 W was supplied from a 27.12 MHz high-frequency power source. Note that the plasma CVD apparatus is a parallel plate plasma CVD apparatus having an electrode area of 6000 cm 2 , and the power density is 3.2×10 −1  W/cm 2 . The substrate temperature was set to 350° C. As the supply gas, a mixed gas of silane and nitrogen (SiH 4  flow rate: 200 sccm, N 2  flow rate: 5000 sccm) was used. 
     As a sample A2, a 300-nm-thick silicon nitride film was deposited over a silicon wafer. The silicon nitride film was deposited as follows. A silicon wafer was placed in a treatment chamber of a plasma CVD apparatus, the pressure in the treatment chamber was controlled so as to be 100 Pa, and a power of 2000 W (a power density of 3.2×10 −1  W/cm 2 ) was supplied from a 27.12 MHz high-frequency power source. The substrate temperature was set to 350° C. As the supply gas, a mixed gas of silane, nitrogen, and ammonia (SiH 4  flow rate: 200 sccm, N 2  flow rate: 2000 sccm, NH 3  flow rate: 100 sccm) was used. 
     As a reference sample, a 300-nm-thick silicon nitride film was deposited over a silicon wafer. The silicon nitride film was deposited as follows. A silicon wafer was placed in a treatment chamber of a plasma CVD apparatus, the pressure in the treatment chamber was controlled so as to be 100 Pa, and a power of 2000 W (a power density of 3.2×10 −1  W/cm 2 ) was supplied from a 27.12 MHz high-frequency power source. The substrate temperature was set to 350° C. As the supply gas, a mixed gas of silane, nitrogen, and ammonia (SiH 4  flow rate: 200 sccm, N 2  flow rate: 2000 sccm, NH 3  flow rate: 2000 sccm) was used. 
     The sample A1 and the sample A2 each are a silicon nitride film deposited under a condition that the flow ratio of ammonia to nitrogen in the supply gas is less than or equal to 0.1, which is a condition suitable for a silicon nitride film used for a semiconductor device according to one embodiment of the present invention. On the other hand, the reference sample is a silicon nitride film deposited under a condition which is not suitable for the silicon nitride film used for a semiconductor device according to one embodiment of the present invention. 
     The samples A1 and A2 and the reference sample were subjected to TDS measurement.  FIG. 16  shows the number of released hydrogen molecules of each of the samples with respect to substrate temperature. In  FIG. 16 , the horizontal axis indicates substrate temperature and the vertical axis indicates TDS intensity corresponding to the number of released hydrogen molecules. 
     A peak of a curve showing TDS results is a peak which is attributed to release of atoms or molecules contained in an analyzed sample (the samples A1 and A2 and the reference sample in this example) to the outside. The total amount of the atoms or molecules released to the outside is proportional to the integral value of each curve. Thus, the total amounts of the atoms or molecules contained in the silicon nitride films can be compared based on the height of the peak intensity. In this example, the number of released hydrogen molecules in the range from 50° C. to 570° C. was obtained as a quantitative value. 
     It is found from  FIG. 16  that hydrogen molecules in the reference sample start to be released by heat treatment at a substrate temperature around 350° C. and a peak of the number of released hydrogen molecules in the reference sample appears at around 500° C. On the other hand, in each of the samples A1 and A2, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 500° C. 
     The quantitative values of the number of released hydrogen molecules in the sample A1, the sample A2, and the reference sample were 7.35×10 19 /cm 3 , 9.00×10 19 /cm 3 , and 7.75×10 20 /cm 3 , respectively. 
     Next, SIMS measurement was performed on the samples A1 and A2 and the reference sample to measure the hydrogen concentrations in the silicon nitride films  FIG. 17A  shows SIMS results. 
     The hydrogen concentrations in the sample A1, the sample A2, and the reference sample were 1.25×10 22  atoms/cm 3 , 1.67×10 22  atoms/cm 3 , and 2.48×10 22  atoms/cm 3 , respectively. Thus, it is confirmed that the hydrogen concentration in each of the samples A1 and A2 is lower than that in the reference sample. 
     Next, the compositions of the samples A1 and A2 and the reference sample were measured using RBS.  FIG. 17B  shows quantitative values obtained by RBS analysis. 
     The vertical axis of  FIG. 17B  indicates values of the proportions (atomic %) of silicon (Si), nitrogen (N), and hydrogen (H) contained in each sample. The following was found from the measurement. In the sample A1, silicon, nitrogen, and hydrogen are contained at 39.5 atomic %, 50.6 atomic %, and 9.9 atomic %, respectively. In the sample A2, silicon, nitrogen, and hydrogen are contained at 36.2 atomic %, 50.3 atomic %, and 13.5 atomic %, respectively. In the reference sample, silicon, nitrogen, and hydrogen are contained at 31.6 atomic %, 47.6 atomic %, and 20.8 atomic %, respectively. Thus, it is confirmed that the proportion of hydrogen of each of the samples A1 and A2 is lower than that in the proportion of the reference sample. 
     The film formed in this example has a small number of released hydrogen molecules and low hydrogen concentration. Therefore, the film is suitable for a film applied to a semiconductor device according to one embodiment of the present invention. 
     Example 2 
     In this example, a property of blocking hydrogen of a silicon nitride film used for a semiconductor device according to one embodiment of the present invention is evaluated. As the evaluation method, TDS was used. 
     First, a silicon nitride film B1 was deposited to a thickness of 275 nm over a silicon wafer. The silicon nitride film B1 was deposited using the same conditions as the reference sample described in Example 1. 
       FIG. 18A  shows TDS results of the silicon nitride film B1. In  FIG. 18A , the horizontal axis indicates substrate temperature and the vertical axis indicates the number of released hydrogen molecules with respect to the substrate temperature. Hydrogen molecules in the silicon nitride film B1 start to be released at around 350° C. by heating during TDS and a peak of the number of released hydrogen molecules in the silicon nitride film B1 appears at around 500° C. The quantitative value of the number of released hydrogen molecules in the TDS shown in  FIG. 18A  is 7.75×10 20 /cm 3 . 
     Next, a silicon nitride film B2 applicable to a semiconductor device according to one embodiment of the present invention was deposited to a thickness of 50 nm as described in Example 1 to be stacked over the silicon nitride film B1, whereby a sample C was manufactured. The silicon nitride film B2 was formed using the same conditions as the silicon nitride film used for the sample A1 described in Example 1. 
       FIG. 18B  shows the number of released hydrogen molecules with respect to substrate temperature in the sample C. 
     As shown in  FIG. 18B , in the sample C, a peak of the number of released hydrogen molecules does not appear at a temperature lower than or equal to 550° C. Further, the quantitative value of the number of released hydrogen molecules of the sample C is 1.10×10 20 /cm 3 . Thus, it is confirmed that release of hydrogen molecules, which is observed in the case of the silicon nitride film B1 alone, can be suppressed by stacking the silicon nitride film B2 used for a semiconductor device according to one embodiment of the present invention over the silicon nitride film B1. 
     As a result, it is confirmed that the silicon nitride film applicable to a semiconductor device according to one embodiment of the present invention has a property of blocking hydrogen. 
     This application is based on Japanese Patent Application serial no. 2012-147633 filed with Japan Patent Office on Jun. 29, 2012, the entire contents of which are hereby incorporated by reference.