Patent Publication Number: US-10332996-B2

Title: Semiconductor device and manufacturing method thereof

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device using an oxide semiconductor and a manufacturing method thereof. 
     BACKGROUND ART 
     A transistor using a semiconductor film formed over an insulating surface is an essential semiconductor element for a semiconductor device. Since the manufacturing of transistors has a limitation on the allowable temperature limit of a substrate, a transistor using, as an active layer, amorphous silicon which can be formed at a relatively low temperature, polysilicon which can be obtained by crystallization using laser light or a catalytic element, or the like has been mainly used as a transistor for the semiconductor display device. 
     In recent years, a metal oxide having semiconductor characteristics which is referred to as an oxide semiconductor has attracted attention as a novel semiconductor material which has both high mobility, which is a characteristic of polysilicon, and uniform element characteristics, which is a characteristic of amorphous silicon. The metal oxide has been used for various applications; for example, indium oxide that is a well-known metal oxide has been used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors in each of which a channel formation region is formed using such a metal oxide having semiconductor characteristics have been known (Patent Documents 1 and 2). 
     REFERENCE 
     
         
         Patent Document 1: Japanese Published Patent Application No. 2007-123861 
         Patent Document 2: Japanese Published Patent Application No. 2007-096055 
       
    
     DISCLOSURE OF INVENTION 
     As for a transistor included in a semiconductor device, it is preferable to reduce the variation of the threshold voltage caused by time degradation and to reduce the off-state current. With the transistor the variation of the threshold voltage caused by time degradation of which is small, reliability of a semiconductor device can be increased. With a transistor the off-state current of which is low, power consumption of a semiconductor device can be suppressed. 
     It is an object of the present invention to provide a method for manufacturing a highly reliable semiconductor device. It is another object of the present invention to provide a method for manufacturing a semiconductor device with low power consumption. It is another object of the present invention to provide a highly reliable semiconductor device. It is another object of the present invention to provide a semiconductor device with low power consumption. 
     In a semiconductor device with high withstand voltage for controlling large current, a so-called power device, silicon has been used mainly as a semiconductor material. However, it is said that the physical characteristics of a semiconductor element using silicon reach the theoretical value limit, and a novel semiconductor material with which the characteristics can be improved has been demanded in order to realize a power device that has high withstand voltage and can control large current. As the semiconductor material that may improve the characteristics such as high withstand voltage, high conversion efficiency, or high-speed switching, for example, a compound semiconductor such as silicon carbide or gallium nitride can be given. The bandgap of silicon carbide and the bandgap of gallium nitride are 3.26 eV and 3.39 eV, respectively, which are about three times as large as that of silicon; it is known that such a compound semiconductor is advantageous for improving the withstand voltage, reducing the power loss of a semiconductor device, and the like. 
     On the other hand, the compound semiconductor such as silicon carbide or gallium nitride has a problem of high process temperature. The process temperature of silicon carbide is about 1500° C. and the process temperature of gallium nitride is about 1100° C., which does not allow film deposition on a glass substrate whose allowable temperature limit is low. Therefore, an inexpensive glass substrate cannot be used, and further, the compound semiconductor cannot be applied when the size of a substrate is increased, so that the mass productivity of semiconductor devices using the compound semiconductor such as silicon carbide or gallium nitride is low, which disturbs practical application. 
     In view of the foregoing problems, an object of one embodiment of the present invention is to provide a semiconductor device for high power application in which a novel semiconductor material having high mass productivity is used. 
     The present inventors paid their attention to the fact that impurities such as hydrogen or water existing in an oxide semiconductor film cause degradation over time, such as shifts in threshold voltage, to transistors. It has been found that the oxide semiconductor film formed by sputtering or the like includes a large amount of hydrogen or water as impurities. According to one embodiment of the present invention, in order to decrease impurities such as moisture or hydrogen in the oxide semiconductor film, after the oxide semiconductor film is formed, the exposed oxide semiconductor film is subjected to first heat treatment in a reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a rare gas, or the like, an oxygen gas atmosphere, or an ultra dry air atmosphere (in air whose moisture content is less than or equal to 20 ppm (dew point conversion, −55° C.), preferably less than or equal to 1 ppm, far preferably less than or equal to 10 ppb in the case where measurement is performed using a dew-point meter of a cavity ring-down laser spectroscopy (CRDS) system). Next, in order to further decrease impurities such as moisture or hydrogen in the oxide semiconductor film, oxygen is added to the oxide semiconductor film by an ion implantation method, an ion doping method, or the like, and after that, the exposed oxide semiconductor film is subjected to second heat treatment in a reduced-pressure atmosphere, an inert gas atmosphere of nitrogen, a rare gas, or the like, an oxygen gas atmosphere, or an ultra dry air atmosphere (in air whose moisture content is less than or equal to 20 ppm (dew point conversion, −55° C.), preferably less than or equal to 1 ppm, far preferably less than or equal to 10 ppb in the case where measurement is performed using a dew-point meter of a cavity ring-down laser spectroscopy (CRDS) system). 
     The first heat treatment decreases the impurities such as moisture or hydrogen in the oxide semiconductor film, but does not eliminate completely, so that there is room for improvement. This incompleteness of impurity elimination is considered to be caused by hydrogen or a hydroxyl group bonded to a metal that is a component of the oxide semiconductor. In accordance with the present invention, the bond between the metal as a component of the oxide semiconductor and the hydrogen or the hydroxyl group is cut and the hydrogen or the hydroxyl group is made to react with oxygen to produce water by adding oxygen in the oxide semiconductor film by an ion implantation method, an ion doping method, or the like. Then the second heat treatment is performed after the addition of oxygen, whereby impurities such as hydrogen or a hydroxyl group left can be easily eliminated as water. 
     When an oxide semiconductor which is an i-type (intrinsic) semiconductor or a substantially i-type semiconductor can be obtained by elimination of impurities such as moisture or hydrogen, deterioration of characteristics of the transistor due to the impurities, such as shifts in threshold voltage, can be prevented from being promoted and off-state current can be reduced. Specifically, impurities such as hydrogen or water contained in an oxide semiconductor are removed so that the value of the concentration of hydrogen in the oxide semiconductor measured by secondary ion mass spectroscopy (SIMS) is less than or equal to 5×10 19 /cm 3 , preferably less than or equal to 5×10 18 /cm 3 , far preferably less than or equal to 5×10 17 /cm 3 , still far preferably less than 1×10 16 /cm 3 . In addition, the carrier density of the oxide semiconductor film, which is measured by Hall effect measurement, is less than 1×10 14 /cm 3 , preferably less than 1×10 12 /cm 3 , far preferably less than or equal to 1×10 11 /cm 3  that is less than or equal to a measurement limit. In other words, the carrier density of the oxide semiconductor film is extremely close to zero. Furthermore, the band gap of the oxide semiconductor is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, far preferably greater than or equal to 3 eV. With the use of the oxide semiconductor film which is highly purified by sufficiently reducing the hydrogen concentration, the off-state current of the transistor can be reduced. 
     The above two heat treatments are preferably performed at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or lower than or equal to the strain point of a glass substrate). Note that these heat treatments are performed at a temperature not exceeding the allowable temperature limit of the substrate to be used. An effect of elimination of water or hydrogen by heat treatment has been confirmed by thermal desorption spectroscopy (TDS). 
     Heat treatment in a furnace or a rapid thermal annealing method (RTA method) is used for the heat treatment. As the RTA method, a method using a lamp light source or a method in which heat treatment is performed for a short time while a substrate is moved in a heated gas can be employed. With the use of the RTA method, it is also possible to make the time involved in heat treatment shorter than 0.1 hours. 
     For example, even in the case where the transistor using the highly purified oxide semiconductor film in the above-described manner is an element whose channel width W is 1×10 4  μm and whose channel length L is 3 μm, electrical characteristics of an off-state current of 10 −13  A or less and a subthreshold swing (S factor) of about 0.1 V/dec (the thickness of a gate insulating film is 100 nm) can be obtained. Therefore, the off-state current in the state where a source-drain voltage is 0 or less, that is, the leakage current is much smaller than that of a transistor using silicon having crystallinity. 
     Further, such a transistor using a highly-purified oxide semiconductor (purified OS) exhibits almost no temperature dependence of off-state current. This is because the conductivity type is made to be as close to an intrinsic type as possible by removing impurities which become electron donors (donors) in the oxide semiconductor to highly purify the oxide semiconductor, so that the Fermi level positions in a center of the forbidden band. This also results from the fact that the oxide semiconductor has an energy gap of 3 eV or more and includes very few thermally excited carriers. In addition, the source electrode and the drain electrode are in a degenerated state, which is also a factor for showing no temperature dependence. Transistors are mainly operated with carriers which are injected from the degenerated source electrode to the oxide semiconductor, and the above-described characteristics (no dependence of the off-state current on the temperature) can be explained by no dependence of carrier density on the temperature. 
     As the oxide semiconductor, the following can be used: an oxide of four metal elements such as an In—Sn—Ga—Zn—O-based oxide semiconductor; an oxide of three metal elements such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxide semiconductor; an oxide of two metal elements such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxide semiconductor; an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, a Zn—O-based oxide semiconductor, or the like. In this specification, for example, an In—Sn—Ga—Zn—O-based oxide semiconductor means a metal oxide including indium (In), tin (Sn), gallium (Ga), and zinc (Zn), whose stoichiometric composition ratio is not particularly limited. The above-described oxide semiconductor may include silicon. 
     Alternatively, oxide semiconductors can be represented by the chemical formula, InMO 3 (ZnO) m  (m&gt;0). Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. 
     The analysis of the concentrations of hydrogen in the oxide semiconductor film and the conductive film is described here. The concentrations of hydrogen in the oxide semiconductor film and the conductive film are measured by secondary ion mass spectrometry (SIMS). It is known that it is difficult to obtain data in the proximity of a surface of a sample or in the proximity of an interface between stacked films formed using different materials by the SIMS analysis in principle. Thus, in the case where the distribution of the hydrogen concentration of the film in a thickness direction is analyzed by SIMS, an average value in a region of the film, in which the value is not greatly changed and almost the same value can be obtained is employed as the hydrogen concentration. Further, in the case where the thickness of the film is small, such a region where almost the same value can be obtained cannot be found in some cases due to the influence of the hydrogen concentration of a film which is adjacent to the film. In that case, the maximum value or the minimum value of the hydrogen concentration in the region of the film is employed as the hydrogen concentration of the film. Further, in the case where a mountain-shaped peak having the maximum value or a valley-shaped peak having the minimum value do not exist in the region of the film, the value at the inflection point is employed as the hydrogen concentration. 
     The transistor may be a bottom-gate transistor, a top-gate transistor, or a bottom-contact transistor. A bottom-gate transistor has a gate electrode over an insulating surface; a gate insulating film over the gate electrode; an oxide semiconductor film which overlaps with the gate electrode over the gate insulating film; a source electrode and a drain electrode which are over the oxide semiconductor film; and an insulating film over the source electrode, the drain electrode, and the oxide semiconductor film. A top-gate transistor has an oxide semiconductor film over an insulating surface; a gate insulating film over the oxide semiconductor film; a gate electrode which overlaps with the oxide semiconductor film over the gate insulating film and functions as a conductive film; a drain electrode; a source electrode; and an insulating film over the source electrode, the drain electrode, and the oxide semiconductor film. A bottom-contact transistor has a gate electrode over an insulating surface; a gate insulating film over the gate electrode; a source electrode and a drain electrode which are over the gate insulating film; an oxide semiconductor film which is over the source electrode and the drain electrode and which overlaps with the gate electrode over the gate insulating film; and an insulating film over the source electrode, the drain electrode, and the oxide semiconductor film. 
     Hydrogen or water which is around the oxide semiconductor film is easily absorbed by the oxide semiconductor film not only in film deposition by sputtering or the like but also after the film deposition. Water or hydrogen easily forms a donor level and thus serves as an impurity in the oxide semiconductor itself. Therefore, according to one embodiment of the present invention, after the source electrode and the drain electrode are formed, an insulating film using an insulating material having a high barrier property may be formed so as to cover the source electrode, the drain electrode, and the oxide semiconductor film. It is preferable to use an insulating material having a high barrier property for the insulating film. For example, as the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be used. When a plurality of insulating films stacked is used, an insulating film having lower proportion of nitrogen than the insulating film having a high barrier property, such as a silicon oxide film or a silicon oxynitride film, is formed on the side close to the oxide semiconductor film. Then, an insulating film having a barrier property is formed so as to overlap with the source electrode, the drain electrode, and the oxide semiconductor film with the insulating film having lower proportion of nitrogen provided therebetween. With the insulating film having a barrier property, impurities such as moisture or hydrogen can be prevented from entering the oxide semiconductor film, the gate insulating film, or the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. 
     Between the gate electrode and the oxide semiconductor film, a gate insulating film may be formed to have a structure in which an insulating film formed using a material having a high barrier property and an insulating film having lower proportion of nitrogen, such as a silicon oxide film or a silicon oxynitride film are stacked. The insulating film such as a silicon oxide film or a silicon oxynitride film is formed between the insulating film having a barrier property and the oxide semiconductor film. With the insulating film having a barrier property, impurities in an atmosphere, such as moisture or hydrogen, or impurities included in a substrate, such as an alkali metal or a heavy metal, can be prevented from entering the oxide semiconductor film, the gate insulating film, or the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. 
     In accordance with the present invention, a method for manufacturing a semiconductor device with high reliability can be provided. A method for manufacturing a semiconductor device with low power consumption can be provided. Further, a semiconductor device with high reliability can be provided. A semiconductor device with low power consumption can be provided. 
     Further, a semiconductor element having a high withstand voltage can be manufactured at a low film deposition temperature, so that a semiconductor device for high power application with high mass productivity can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1E  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 2A to 2C  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 3A to 3C  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 4A to 4E  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 5A to 5E  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 6A to 6C  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 7A and 7B  are top views of a semiconductor device; 
         FIGS. 8A to 8C  are views illustrating a method for manufacturing a semiconductor device; 
         FIG. 9  is a top view of a semiconductor device; 
         FIGS. 10A to 10C  are views illustrating a method for manufacturing a semiconductor device; 
         FIGS. 11A and 11B  are a top view and a cross-sectional view of electronic paper, respectively; 
         FIGS. 12A and 12B  are block diagrams each of a semiconductor device; 
         FIGS. 13A and 13B  illustrate a structure of a signal line driver circuit; 
         FIGS. 14A and 14B  are circuit diagrams each illustrating a structure of a shift register; 
         FIGS. 15A and 15B  are a diagram illustrating one embodiment of a shift register and a timing chart illustrating operation thereof, respectively; 
         FIG. 16  is a cross-sectional view of a liquid crystal display device; 
         FIG. 17  is a view illustrating a structure of a liquid crystal display device module; 
         FIGS. 18A to 18C  are cross-sectional views each of a light-emitting device; 
         FIGS. 19A to 19F  are views each illustrating an electronic device using a semiconductor device; 
         FIG. 20  is a longitudinal cross-sectional view of an inverted staggered transistor using an oxide semiconductor; 
         FIG. 21  is an energy band diagram (schematic diagrams) along the section A-A′ of  FIG. 20 ; 
         FIG. 22A  illustrates the state where a positive potential (+VG) is applied to a gate electrode (GE) and  FIG. 22B  illustrates the state where a negative potential (−VG) is applied to the gate electrode (GE); and 
         FIG. 23  illustrates a relation between the vacuum level and the work function of a metal (ϕ M ), and a relation between the vacuum level and the electron affinity of an oxide semiconductor (χ). 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments and Example of the present invention will be described below with reference to the accompanying drawings. Note that the present invention is not limited to the following description, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not to be construed as being limited to the content of the embodiments included herein. 
     The present invention can be applied to manufacture of any kind of semiconductor devices including microprocessors, integrated circuits such as image processing circuits, RF tags, and semiconductor display devices. A semiconductor device means any device which can function by utilizing semiconductor characteristics, and a semiconductor display device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device. The semiconductor display devices include the following in its category: liquid crystal display devices, light-emitting devices in which a light-emitting element typified by an organic light-emitting element (OLED) is provided for each pixel, electronic papers, digital micromirror devices (DMDs), plasma display panels (PDPs), field emission displays (FEDs), and other semiconductor display devices in which a circuit element using a semiconductor film is included in a driver circuit. 
     Embodiment 1 
     A bottom-gate transistor having a channel-etched structure is taken as an example, and a structure of the transistor included in a semiconductor device according to one embodiment of the present invention and a manufacturing method thereof will be described. 
     As shown in  FIG. 1A , a gate electrode  101  is formed over a substrate  100 . 
     Although there is no particular limitation on a substrate that can be used as the substrate  100  having an insulating surface, it is necessary that the substrate has heat resistance high enough to withstand at least heat treatment to be performed later. For example, a glass substrate manufactured by a fusion method or a float method can be used. When the temperature of the heat treatment performed later is high, it is preferable to use a substrate having a strain point of 730° C. or higher as the glass substrate. For the glass substrate, a glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. In general, by containing a larger amount of barium oxide (BaO) than boron oxide, a glass substrate which is heat-resistant and more practical can be obtained. Therefore, it is preferable to use a glass substrate containing BaO and B 2 O 3  so that the amount of BaO is larger than that of B 2 O 3 . 
     Instead of the above-described glass substrate, a substrate formed of an insulator such as a ceramic substrate, a quartz substrate, or a sapphire substrate may be used. Alternatively, crystallized glass or the like may be used. Further alternatively, a metal substrate such as a substrate of a stainless steel alloy, provided with an insulating film on its surface, may be used. 
     A substrate formed from a flexible synthetic resin, such as plastic, generally tends to have a low allowable temperature limit, but can be used as the substrate  100  as long as the substrate can withstand a processing temperature in the later manufacturing process. Examples of the plastic substrate include polyester typified by polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polycarbonate (PC), polyetheretherketone (PEEK), polysulfone (PSF), polyetherimide (PEI), polyarylate (PAR), polybutylene terephthalate (PBT), polyimide, acrylonitrile-butadiene-styrene resin, polyvinyl chloride, polypropylene, polyvinyl acetate, acrylic resin, and the like. 
     An insulating film to serve as a base film may be formed between the substrate  100  and the gate electrode  101 . As the base film, for example, a single layer of a silicon oxide film, a silicon oxynitride film, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film or a stacked layer of a plurality of these films can be used. In particular, an insulating film having a high barrier property, for example, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film may be used for the base film, so that impurities in an atmosphere, such as moisture or hydrogen, or impurities included in the substrate  100 , such as an alkali metal or a heavy metal, can be prevented from entering the oxide semiconductor film, the gate insulating film, or the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. 
     In this specification, oxynitride refers to a substance which includes more oxygen than nitrogen, and nitride oxide refers to a substance which includes more nitrogen than oxygen. 
     The gate electrode  101  can be formed to have a single-layer structure or a stacked-layer structure using one or more conductive films using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, neodymium, or scandium or an alloy material which contains any of these metal materials as its main component, or a nitride which contains any of these metals. Aluminum or copper can be used as one of the metal material if aluminum or copper can withstand a temperature of heat treatment performed in a later process. Aluminum or copper is preferably combined with a refractory metal material so as to prevent a heat resistance problem and a corrosive problem. As the refractory metal material, molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium, or the like can be used. 
     For example, as a two-layer structure of the gate electrode  101 , the following structure is preferable: a two-layer structure in which a molybdenum film is stacked over an aluminum film, a two-layer structure in which a molybdenum film is stacked over a copper film, a two-layer structure in which a titanium nitride film or a tantalum nitride film is stacked over a copper film, or a two-layer structure in which a titanium nitride film and a molybdenum film are stacked. As a three-layer structure of the gate electrode  101 , the following structure is preferable: a stacked structure in which an aluminum film, an alloy film of aluminum and silicon, an alloy film of aluminum and titanium, or an alloy film of aluminum and neodymium is sandwiched by any two films selected from a tungsten film, a tungsten nitride film, a titanium nitride film, and a titanium film. 
     Further, a light-transmitting oxide conductive film of indium oxide, an alloy of indium oxide and tin oxide, an alloy of indium oxide and zinc oxide, zinc oxide, aluminum zinc oxide, aluminum zinc oxynitride, gallium zinc oxide, or the like may be used for the gate electrode  101 , so that the aperture ratio of a pixel portion can be increased. 
     The gate electrode  101  is formed to a thickness of 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, a conductive film for the gate electrode is formed to a thickness of 150 nm by a sputtering method using a tungsten target, and then, the conductive film is processed (patterned) into an appropriate shape by etching; in such a manner, the gate electrode  101  is formed. It is preferable that an end portion of the gate electrode be tapered because coverage with a gate insulating film formed thereover is improved. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing costs can be reduced. 
     Next, a gate insulating film  102  is formed over the gate electrode  101 . The gate insulating film  102  can be formed to have a single-layer structure or a stacked-layer structure of one or more selected from a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, a hafnium oxide film, and a tantalum oxide film by a plasma CVD method, a sputtering method, or the like. It is preferable that the gate insulating film  102  include impurities such as moisture or hydrogen as little as possible. In the case where a silicon oxide film is formed by a sputtering method, a silicon target or a quartz target is used as a target and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas. 
     An oxide semiconductor that is made to be i-type or substantially i-type (an oxide semiconductor that is highly purified) by removal of impurities is extremely sensitive to an interface state and an interface electric charge; thus, an interface between the oxide semiconductor and the gate insulating film  102  is important. Thus, a gate insulating film (GI) which is to be in contact with the highly purified oxide semiconductor needs to have high quality. 
     For example, high-density plasma CVD using microwaves (2.45 GHz) is preferable because a dense high-quality insulating film having high withstand voltage can be formed. This is because an interface state can be reduced and interface characteristics can be favorable when the highly purified oxide semiconductor and the high quality gate insulating film are in contact with each other. 
     Needless to say, other film formation methods, such as a sputtering method or a plasma CVD method, can be applied as long as a high-quality insulating film can be formed as the gate insulating film. The film quality of the gate insulating film and/or properties of an interface with an oxide semiconductor thereof may be modified by heat treatment performed after film deposition. In any case, any insulating film can be used as long as film quality as a gate insulating film is high, interface state density with an oxide semiconductor is decreased, and a favorable interface can be formed. 
     The gate insulating film  102  may have a structure in which an insulating film formed using a material having a high barrier property and an insulating film having lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film are stacked. In that case, the insulating film such as a silicon oxide film or a silicon oxynitride film is formed between the insulating film having a barrier property and the oxide semiconductor film. As the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be given, for example. With the insulating film having a barrier property, impurities in an atmosphere, such as moisture or hydrogen, or impurities included in the substrate, such as an alkali metal or a heavy metal, can be prevented from entering the oxide semiconductor film, the gate insulating film  102 , or the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. In addition, the insulating film having lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film may be formed so as to be in contact with the oxide semiconductor film, so that the insulating film formed using a material having a high barrier property can be prevented from being in contact with the oxide semiconductor film directly. 
     For example, a stacked-layer film with a thickness of 100 nm may be formed as the gate insulating film  102  as follows: a silicon nitride film (SiN y  (y&gt;0)) with a thickness greater than or equal to 50 nm and less than or equal to 200 nm is formed by a sputtering method as a first gate insulating film, and a silicon oxide film (SiO x  (x&gt;0)) with a thickness greater than or equal to 5 nm and less than or equal to 300 nm is stacked over the first gate insulating film as a second gate insulating film. The thickness of the gate insulating film  102  may be set as appropriate depending on characteristics needed for a transistor and may be about 350 nm to 400 nm. 
     In this embodiment, the gate insulating film  102  is formed to have a structure in which a silicon oxide film having a thickness of 100 nm formed by a sputtering method is stacked over a silicon nitride film having a thickness of 50 nm formed by a sputtering method. 
     In order that hydrogen, a hydroxyl group, and moisture may be contained in the gate insulating layer  102  as little as possible, it is preferable that the substrate  100  over which the gate electrode layer  101  is formed be preheated in a preheating chamber of a sputtering apparatus as pretreatment for film formation so that impurities such as hydrogen and moisture adsorbed to the substrate  100  are removed and exhausted. The temperature of the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. This preheating treatment can be omitted. 
     Next, over the gate insulating film  102 , an oxide semiconductor film  103  having a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm, far preferably greater than or equal to 3 nm and less than or equal to 20 nm is formed. The oxide semiconductor film  103  is formed by a sputtering method using an oxide semiconductor as a target. The oxide semiconductor film  103  can be formed by a sputtering method under a rare gas (for example, argon) atmosphere, an oxygen atmosphere, or an atmosphere including a rare gas (for example, argon) and oxygen. 
     Before the oxide semiconductor film  103  is formed by a sputtering method, dust attached to a surface of the gate insulating film  102  is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering refers to a method in which, without application of a voltage to a target side, an RF power source is used for application of a voltage to a substrate side in an argon atmosphere to modify a surface. Instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Further alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used. 
     The above-described oxide semiconductor can be used as the oxide semiconductor film  103 . 
     In this embodiment, as the oxide semiconductor film  103 , an In—Ga—Zn—O-based non-single-crystal film with a thickness of 30 nm, which is obtained by a sputtering method using an oxide semiconductor target including indium (In), gallium (Ga), and zinc (Zn), is used. As the target, for example, an oxide semiconductor target having a composition ratio with an atom ratio of metals, In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. Further, the oxide semiconductor film  103  can be formed by a sputtering method under a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere containing a rare gas (typically argon) and oxygen. In the case of using a sputtering method, a target containing SiO 2  at 2 wt % to 10 wt % both inclusive may be used for depositing the film. The filling rate of the oxide semiconductor target including In, Ga, and Zn is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. With the use of the oxide semiconductor target with a high filling rate, a dense oxide semiconductor film is formed. 
     The oxide semiconductor film  103  is formed over the substrate  100  in such a manner that the substrate is held in the treatment chamber maintained at reduced pressure, a sputtering gas from which hydrogen and moisture have been removed is introduced into the treatment chamber while moisture remaining therein is removed, and metal oxide is used as a target. At that time, the substrate may be heated at higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Film deposition may be performed while the substrate is heated, whereby the concentration of an impurity contained in the oxide semiconductor film deposited can be reduced. In addition, damage by sputtering can be reduced. In order to remove remaining moisture in the treatment chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. An evacuation unit may be a turbo pump provided with a cold trap. In the deposition chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H 2 O), (far preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of an impurity in the oxide semiconductor film deposited in the deposition chamber can be reduced. 
     As one example of the deposition condition, the following can be employed: the distance between the substrate and the target is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and the atmosphere is an oxygen atmosphere (the proportion of the oxygen flow rate is 100%). Note that a pulsed direct-current (DC) power source is preferable because powder substances (also referred to as particles) generated in film deposition can be reduced and the film thickness can be uniform. The oxide semiconductor film preferably has a thickness greater than or equal to 5 nm and less than or equal to 30 nm. Since appropriate thickness depends on an oxide semiconductor material used, the thickness can be determined as appropriate depending on the material. 
     Further, in order that hydrogen, a hydroxyl group, and moisture may be contained in the oxide semiconductor film  103  as little as possible, it is preferable that the substrate  100  on which the process up to and including the step of forming the gate insulating film  102  is already performed be preheated in a preheating chamber of a sputtering apparatus as pretreatment for film formation so that impurities such as hydrogen and moisture adsorbed to the substrate  100  are removed and exhausted. The temperature of the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. This preheating treatment can be omitted. Further, this preheating may be similarly performed on the substrate  100  on which the process up to and including the step of forming a source electrode  111  and a drain electrode  112  is already performed, before the formation of an insulating film  113 . 
     Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power supply, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case where an insulating film is formed, and a DC sputtering method is mainly used in the case where a metal film is formed. 
     In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber. 
     Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge can be used. 
     Further, as a deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, or a bias sputtering method in which a voltage is also applied to a substrate during deposition can be used. 
     The gate insulating film  102  and the oxide semiconductor film  103  may be formed successively without exposure to air. Successive film formation without exposure to air makes it possible to obtain each interface between stacked layers, which is not contaminated by atmospheric components or impurity elements floating in air, such as water, hydrocarbon, or the like. Accordingly, variation in characteristics of the transistor can be reduced. 
     Next, as illustrated in  FIG. 1B , the oxide semiconductor film  103  is processed (patterned) into an appropriate shape by etching or the like, whereby an island-shaped oxide semiconductor film  104  is formed over the gate insulating film  102  so as to overlap with the gate electrode  101 . 
     A resist mask for forming the island-shaped oxide semiconductor film  104  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing costs can be reduced. 
     In the case where a contact hole is formed in the gate insulating film  102 , a step of forming the contact hole can be performed at the time of formation of the island-shaped oxide semiconductor film  104 . 
     The etching for forming the island-shaped oxide semiconductor film  104  may be dry etching, wet etching, or both dry etching and wet etching. As the etching gas for the dry etching, a gas containing chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon tetrachloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. Alternatively, a gas containing fluorine (fluorine-based gas such as carbon tetrafluoride (CF 4 ), sulfur hexafluoride (SF 6 ), nitrogen trifluoride (NF 3 ), or trifluoromethane (CHF 3 )); hydrogen bromide (HBr); oxygen (O 2 ); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used. 
     As the dry etching method, a parallel plate RIE (reactive ion etching) method or an ICP (inductively coupled plasma) etching method can be used. In order to etch the film into an appropriate shape, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate. 
     As an etchant used for the wet etching, a mixed solution of phosphoric acid, acetic acid, and nitric acid, organic acid such as citric acid or oxalic acid, or the like can be used. Alternatively, ITO-07N (manufactured by Kanto Chemical Co., Inc.) may be used. The etchant after the wet etching is removed together with the etched materials by cleaning. The waste liquid including the etchant and the material etched off may be purified and the material may be reused. A material such as indium included in the oxide semiconductor film may be collected from the waste liquid after the etching and reused, whereby the resources can be efficiently used and the costs can be reduced. 
     It is preferable that reverse sputtering be performed before the formation of a conductive film in a subsequent step so that a resist residue or the like that attaches onto surfaces of the island-shaped oxide semiconductor film  104  and the gate insulating film  102  is removed. 
     Next, first heat treatment is performed on the oxide semiconductor film  104  in a reduced-pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or an ultra dry air atmosphere (in air whose moisture content is less than or equal to 20 ppm (dew point conversion, −55° C.), preferably less than or equal to 1 ppm, far preferably less than or equal to 10 ppb in the case where measurement is performed using a dew-point meter of a cavity ring-down laser spectroscopy (CRDS) system). By the first heat treatment on the oxide semiconductor film  104 , an oxide semiconductor film  105  in which moisture or hydrogen is eliminated is formed as shown in  FIG. 1C . Specifically, heat treatment may be performed at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to a strain point of a glass substrate). Since dehydration or dehydrogenation can be performed in a short time with the RTA method, the first heat treatment can be performed even at a temperature over the strain point of a glass substrate. In this embodiment, heat treatment is performed on the oxide semiconductor film  104  at a substrate temperature of 450° C. for one hour in a nitrogen atmosphere with the use of an electrical furnace that is one of heat treatment apparatuses, and then, the oxide semiconductor film is not exposed to the air and water and hydrogen are prevented from entering the oxide semiconductor film, so that the oxide semiconductor film  105  is obtained. 
     The heat treatment apparatus is not limited to an electrical furnace, and may be provided with a device for heating an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. 
     For example, as the first heat treatment, GRTA by which the substrate is moved into an inert gas heated to a high temperature as high as 650° C. to 700° C., heated for several minutes, and moved out of the inert gas heated to the high temperature may be performed. GRTA enables high-temperature heat treatment for a short period of time. 
     It is preferable that in the heat treatment, moisture, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably set to be 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, far preferably 0.1 ppm or lower). 
     Next, as shown in  FIG. 1D , oxygen is added into the oxide semiconductor film by an ion implantation method or an ion doping method. Oxygen is added into the oxide semiconductor film  105  by an ion implantation method, an ion doping method, or the like, so that an oxide semiconductor film  107  in which oxygen is added excessively is formed. By adding oxygen, a bond between a metal as a component of the oxide semiconductor and hydrogen or a bond between the metal and a hydroxyl group is cut and the hydrogen or the hydroxyl group is reacted with oxygen to produce water; this leads to easy elimination of hydrogen or a hydroxyl group that is an impurity, in the form of water by second heat treatment performed later. 
     In an ion implantation method, a source gas is made into plasma, ion species included in this plasma are extracted and mass-separated, ion species having predetermined mass are accelerated, and an object is irradiated with the accelerated ion species in the form of an ion beam. In an ion doping method, a source gas is made into plasma, ion species are extracted from this plasma by an operation of a predetermined electric field, the extracted ion species are accelerated without mass separation, and an object is irradiated with the accelerated ion species in the form of an ion beam. When the addition of oxygen is performed using an ion implantation method involving mass-separation, an impurity such as a metal element can be prevented from being added into the oxide semiconductor film. On the other hand, an ion doping method enables ion-beam irradiation to a larger area than an ion implantation method, and therefore, when the addition of oxygen is performed using an ion doping method, the takt time can be shortened. 
     In the case where an oxygen gas is used and oxygen is added by an ion implantation method, the acceleration voltage may be set in the range of 5 kV to 100 kV both inclusive and the dosage may be set in the range of 1×10 13  ions/cm 2  to 1×10 16  ions/cm 2  both inclusive. 
     Heat treatment may be performed on the substrate provided with the oxide semiconductor film  105  at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to a strain point of a glass substrate), while the addition of oxygen into the oxide semiconductor film  105  is performed by an ion implantation method. 
     Next, second heat treatment is performed. The second heat treatment can be performed in a similar condition to the first heat treatment. Specifically, heat treatment may be performed in a reduced-pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or an ultra dry air atmosphere (in air whose moisture content is less than or equal to 20 ppm (dew point conversion, −55° C.), preferably less than or equal to 1 ppm, far preferably less than or equal to 10 ppb in the case where measurement is performed using a dew-point meter of a cavity ring-down laser spectroscopy (CRDS) system). Since dehydration or dehydrogenation can be performed in a short time with the RTA method, the second heat treatment can be performed even at a temperature over the strain point of a glass substrate. In this embodiment, heat treatment is performed at a substrate temperature of 450° C. for one hour in a nitrogen atmosphere with the use of an electrical furnace that is one of heat treatment apparatuses, and then, the oxide semiconductor film is not exposed to the air and water and hydrogen are prevented from entering the oxide semiconductor film, so that an oxide semiconductor film  108  is obtained as shown in  FIG. 1E . The heat treatment may be performed plural times after the island-shaped oxide semiconductor film  108  is formed. 
     According to one embodiment of the present invention, the bond between a metal as a component of the oxide semiconductor and hydrogen or a hydroxyl group is cut and the hydrogen or the hydroxyl group is made to react with oxygen to produce water by adding oxygen in the oxide semiconductor film  105 . Thus, an impurity such as hydrogen or a hydroxyl group left in the film can be easily eliminated in the form of water by the second heat treatment after the oxygen addition. The island-shaped oxide semiconductor film  108  formed through the second heat treatment is more i-type (intrinsic) or closer to i-type than the oxide semiconductor film  105  after the first heat treatment because impurities such as moisture or hydrogen left even after the first heat treatment are removed. Impurities such as moisture or hydrogen are eliminated, and the island-shaped oxide semiconductor film becomes an i-type (intrinsic) semiconductor or a substantially i-type semiconductor; therefore, deterioration of characteristics of the transistor due to the impurities, such as shifts in threshold voltage, can be prevented from being promoted and off-state current can be reduced. 
     Further, when an oxide semiconductor containing an impurity is subjected to a gate bias-temperature stress test (BT test) for 12 hours under conditions that the temperature is 85° C. and the voltage applied to the gate is 2×10 6  V/cm, a bond between the impurity and a main component of the oxide semiconductor is cut by a high electric field (B: bias) and a high temperature (T: temperature), and a generated dangling bond induces drift of threshold voltage (V th ). However, in the above-described manner, by improving the interfacial characteristics between the gate insulating film and the oxide semiconductor film and removing impurities, particularly hydrogen, water, and the like, in the oxide semiconductor film as much as possible, a transistor which remains stable even with respect to the BT test can be obtained. 
     The heat treatment apparatus is not limited to an electrical furnace, and may be provided with a device for heating an object to be processed by heat conduction or heat radiation from a heating element such as a resistance heating element. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for heat treatment using a high-temperature gas. 
     For example, as the second heat treatment, GRTA by which the substrate is moved into an inert gas heated to a high temperature as high as 650° C. to 700° C., heated for several minutes, and moved out of the inert gas heated to the high temperature may be performed. GRTA enables high-temperature heat treatment for a short period of time. 
     It is preferable that in the heat treatment, moisture, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably set to be 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, far preferably 0.1 ppm or lower). 
     Through the above-described process, the concentration of hydrogen in the oxide semiconductor film can be reduced and the oxide semiconductor film can be highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature which is lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that mass productivity can be increased. In addition, by using the oxide semiconductor film whose hydrogen concentration is reduced and which is highly purified, it is possible to form a transistor with a high withstand voltage, less short-channel effect, and a high on/off ratio. 
     Next, as shown in  FIG. 2A , a conductive film which forms a source electrode and a drain electrode (including a wiring formed from the same layer as the source electrode or the drain electrode) is formed over the gate insulating film  102  and the oxide semiconductor film  108  and is patterned to form the source electrode  111  and the drain electrode  112 . The conductive film may be formed by a sputtering method or a vacuum evaporation method. As a material of the conductive film to be the source and drain electrodes (including the wiring formed from the same layer as the source or the drain electrode), there are an element selected from Al, Cr, Cu, Ta, Ti, Mo, or W; an alloy including any of the above elements; an alloy film containing a combination of any of these elements; and the like. A structure may be employed in which a film of a high-melting point metal such as Cr, Ta, Ti, Mo, or W and a metal film of Al, Cu, or the like are stacked. By using an Al material to which an element which prevents generation of hillocks or whisker in an Al film, such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Y is added, heat resistance can be improved. 
     The conductive film may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. 
     Alternatively, the conductive film to be the source and drain electrodes (including the wiring formed from the same layer as the source or the drain electrode) may be formed using a conductive metal oxide. As a conductive metal oxide, indium oxide (In 2 O 3 ), tin oxide (SnO 2 ), zinc oxide (ZnO), an alloy of indium oxide and tin oxide (In 2 O 3 —SnO 2 , abbreviated to ITO), an alloy of indium oxide and zinc oxide (In 2 O 3 —ZnO), or the metal oxide material to which silicon or silicon oxide is added can be used. 
     In the case where heat treatment is performed after the deposition of the conductive film, it is preferable that the conductive film have heat resistance high enough to withstand the heat treatment. 
     Then, a resist mask is formed over the conductive film, and selective etching is performed, so that the source electrode  111  and the drain electrode  112  are formed. After that, the resist mask is removed. 
     Ultraviolet, a KrF laser beam, or an ArF laser beam is used for light exposure for forming the resist mask in a photolithography step. The channel length L of a transistor to be formed later is determined by a pitch between a lower end of the source electrode and a lower end of the drain electrode that are adjacent to each other over the oxide semiconductor film  108 . Note that when light exposure is performed in the case where the channel length L is shorter than 25 nm, extreme ultraviolet with extremely short wavelengths of several nanometers to several tens of nanometers is used for light exposure for forming the resist mask in the photolithography step. Light exposure with extreme ultraviolet leads to a high resolution and a large depth of focus. Thus, the channel length L of the transistor can be greater than or equal to 10 nm and less than or equal to 1000 nm and the operation speed of a circuit can be increased and furthermore the value of off-state current is extremely small, so that low power consumption can be achieved. 
     Each material and etching conditions are adjusted as appropriate so that the oxide semiconductor film  108  is not removed in etching of the conductive film as much as possible. 
     In this embodiment, a titanium film is used as the conductive film, a solution (an ammonia hydrogen peroxide mixture) containing ammonia and hydrogen peroxide is used, and the conductive film is wet-etched, whereby the source electrode  111  and the drain electrode  112  are formed. As the ammonia hydrogen peroxide mixture, specifically, a solution in which oxygenated water of 31 wt %, ammonia water of 28 wt %, and water are mixed at a volume ratio of 5:2:2 is used. Alternatively, dry etching may be performed on the conductive film  105  with the use of a gas containing chlorine (Cl 2 ), boron chloride (BCl 3 ), or the like. 
     In the patterning for forming the source electrode  111  and the drain electrode  112 , an exposed portion of the island-shaped oxide semiconductor film  108  may be partly etched so that a groove (a recessed portion) is formed in the island-shaped oxide semiconductor film  108 . The resist mask used for forming the source electrode  111  and the drain electrode  112  may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing costs can be reduced. 
     In order to reduce the number of photomasks and steps in a photolithography step, etching may be performed with the use of a resist mask formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. Such a resist mask formed with the use of a multi-tone mask has a plurality of thicknesses and further can be changed in shape by etching; therefore, the resist mask can be used in a plurality of etching steps for processing into different patterns. Therefore, a resist mask corresponding to at least two kinds of different patterns can be formed by one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of corresponding photolithography steps can be also reduced, whereby simplification of a process can be realized. 
     Next, plasma treatment is performed thereon, using a gas such as N 2 O, N 2 , or Ar. By the plasma treatment, adsorbed water or the like which attaches to an exposed surface of the oxide semiconductor film is removed. Plasma treatment may be performed using a mixture gas of oxygen and argon as well. 
     After the plasma treatment, as shown in  FIG. 2B , an insulating film  113  is formed to cover the source electrode  111 , the drain electrode  112 , and the oxide semiconductor film  108 . The insulating film  113  preferably includes no impurities such as moisture or hydrogen as much as possible, and may be formed using a single-layer insulating film or a plurality of insulating films stacked. When hydrogen is contained in the insulating film  113 , entry of hydrogen to the oxide semiconductor film or extraction of oxygen contained in the oxide semiconductor film by hydrogen is caused; thus, a backchannel portion of the oxide semiconductor film might have low resistance (n-type conductivity) and a parasitic channel might be formed. Therefore, it is preferable that a formation method in which hydrogen is not used is employed in order to form the insulating film  113  containing as little hydrogen as possible. A material having a high barrier property is preferably used for the insulating film  113 . For example, as the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be used. When a plurality of insulating films stacked is used, an insulating film having lower proportion of nitrogen than the insulating film having a high barrier property, such as a silicon oxide film or a silicon oxynitride film, is formed on the side close to the oxide semiconductor film  108 . Then, the insulating film having a barrier property is formed so as to overlap with the source electrode  111 , the drain electrode  112 , and the oxide semiconductor film  108  with the insulating film having lower proportion of nitrogen provided therebetween. With the insulating film having a barrier property, impurities such as moisture or hydrogen can be prevented from entering the oxide semiconductor film  108 , the gate insulating film  102 , or the interface between the oxide semiconductor film  108  and another insulating film and the vicinity thereof. In addition, by forming the insulating film having lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film so as to be in contact with the oxide semiconductor film  108 , the insulating film formed using a material having a high barrier property can be prevented from being in contact with the oxide semiconductor film  108  directly. 
     In this embodiment, the insulating film  113  has a structure in which a silicon nitride film having a thickness of 100 nm formed by a sputtering method is stacked over a silicon oxide film having a thickness of 200 nm formed by a sputtering method. The substrate temperature in the film formation may be higher than or equal to room temperature and lower than or equal to 300° C. and is 100° C. in this embodiment. 
     After the insulating film  113  is formed, heat treatment may be performed. The heat treatment is performed in a nitrogen atmosphere or a rare gas atmosphere (argon, helium, or the like) at a temperature, preferably, higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C. In this embodiment, heat treatment for 1 hour at 250° C. in a nitrogen atmosphere is performed. Alternatively, an RTA treatment for a short time at a high temperature may be performed before the formation of the source electrode  111  and the drain electrode  112  in a similar manner to the heat treatment performed on the oxide semiconductor film. By the heat treatment performed on the state where the insulating film  113  containing oxygen is in contact with an exposed region of the oxide semiconductor film  108 , provided between the source electrode  111  and the drain electrode  112 , oxygen is supplied to the oxide semiconductor film  108 , whereby the region of the oxide semiconductor film  108 , which is in contact with the insulating film  113  can be selectively made an oxygen-excess state. Consequently, a structure which satisfies the stoichiometric composition ratio can be obtained, and a channel formation region which overlaps with the gate electrode  101  becomes i-type, which leads to improvement of the electrical characteristics of the transistor and suppression of variation of the electrical characteristics. The timing of this heat treatment is not particularly limited as long as it is after the formation of the insulating film  113 , and can be performed without increasing the number of manufacturing steps by doubling as another step such as a heat treatment for a formation of a resin film or a heat treatment for reduction of the resistance of a transparent conductive film. 
     Through the above-described process, a transistor  114  is formed. 
     Note that a cross-sectional view taken along dashed line A 1 -A 2  in  FIG. 1C  corresponds to  FIG. 1A .  FIG. 2C  is a top view of the transistor  114  shown in  FIG. 2B . A cross-sectional view along dashed line A 1 -A 2  in  FIG. 2C  corresponds to  FIG. 2B . 
     The transistor  114  includes the gate electrode  101  formed over the substrate  100  having an insulating surface, the gate insulating film  102  over the gate electrode  101 , the oxide semiconductor film  108  which overlaps with the gate electrode  101  over the gate insulating film  102 , and a pair of the source electrode  111  and the drain electrode  112  formed over the oxide semiconductor film  108 . The transistor  114  may include the insulating film  113  provided over the oxide semiconductor film  108 . The transistor  114  shown in  FIG. 2C  has a channel-etched structure in which part of the oxide semiconductor film  108  is etched between the source electrode  111  and the drain electrode  112 . 
     Although the transistor  114  is described as a single-gate transistor in Embodiment 1, a multi-gate transistor including a plurality of channel formation regions can be formed when needed. 
     Next, a conductive film may be formed over the insulating film  113  and may be patterned so that a back gate electrode  115  is formed so as to overlap with the oxide semiconductor film  108  as shown in  FIG. 3A . The back gate electrode  115  can be formed using a material and a structure which are similar to those of the gate electrode  101  or the source electrode  111  or the drain electrode  112 . 
     The thickness of the back gate electrode  115  is set to 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, a conductive film in which a titanium film, an aluminum film, and a titanium film are stacked is formed. Then, a resist mask is formed by a photolithography method, and an unnecessary portion is removed by etching so that the conductive film is processed (patterned) into an appropriate shape; thus, the back gate electrode  115  is formed. 
     Next, as shown in  FIG. 3B , an insulating film  116  is formed so as to cover the back gate electrode  115 . The insulating film  116  is preferably formed using a material with a high barrier property that can prevent moisture, hydrogen, oxygen, and the like in an atmosphere from affecting the characteristics of the transistor  114 . For example, the insulating film having a high barrier property can be formed to have a single-layer structure or a stacked-layer structure of a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like by a plasma CVD method, a sputtering method, or the like. In order to obtain an effect of a barrier property, the insulating film  116  is preferably formed to a thickness of 15 nm to 400 nm, for example. 
     In this embodiment, an insulating film with a thickness of 300 nm is formed by a plasma CVD method. The insulating film is formed under the following conditions: the flow rate of a silane gas is 4 sccm; the flow rate of dinitrogen monoxide (N 2 O) is 800 sccm; and the substrate temperature is 400° C. 
       FIG. 3B  is a cross-sectional view taken along dashed line A 1 -A 2  in  FIG. 3C . A top view of the semiconductor device shown in  FIG. 3B  is  FIG. 3C .  FIG. 3B  is a cross-sectional view along dashed line A 1 -A 2  in  FIG. 3C . 
     Although the back gate electrode  115  covers the oxide semiconductor film  108  entirely in  FIG. 3B , one embodiment of the present invention is not limited to this structure. The back gate electrode  115  overlaps with at least part of the channel formation region included in the oxide semiconductor film  108 . 
     The back gate electrode  115  may be electrically insulated to be in a floating state, or may be in a state where the back gate electrode  115  is supplied with a potential. In the latter case, to the back gate electrode  115 , a potential which is the same level as the gate electrode  101  may be applied, or a fixed potential such as ground may be applied. The level of the potential supplied to the back gate electrode  115  is controlled, whereby the threshold voltage of the transistor  114  can be controlled. 
     How characteristics of the transistor are influenced by high purification of the oxide semiconductor film by removal of impurities such as hydrogen, water, or the like contained in the oxide semiconductor film as in this embodiment is described below. 
       FIG. 20  is a cross-sectional view of a transistor including an oxide semiconductor. An oxide semiconductor film (OS) is provided over a gate electrode (GE) with a gate insulating film (GI) provided therebetween, and a source electrode (S) and a drain electrode (D) are provided thereover, and an insulating film is provided over the source electrode (S) and the drain electrode (D). 
       FIG. 21  is an energy band diagram (schematic diagrams) along section A-A′ in  FIG. 20 . In  FIG. 20 , a black circle (●) represents an electron, a white circle (◯) represents a hole, and each has electric charges (+q and −q). With a positive voltage (V D &gt;0) applied to the drain electrode (D), the dashed line shows the case where no voltage is applied to the gate electrode (V G =0) and the solid line shows the case where a positive voltage is applied to the gate electrode (V G &gt;0). In the case where no voltage is applied to the gate electrode, carriers (electrons) are not injected to the oxide semiconductor side from an electrode because of high potential barrier, so that a current does not flow, which means an off state. On the other hand, when a positive voltage is applied to the gate electrode, potential barrier is lowered, and thus a current flows, which means an on state. 
       FIGS. 22A and 22B  are energy band diagrams (schematic diagrams) along section B-B′ in  FIG. 20 .  FIG. 22A  illustrates the state where a positive voltage (VG&gt;0) is applied to the gate electrode (GE) and an on state where carriers (electrons) flow between the source electrode and the drain electrode.  FIG. 22B  illustrates the state where a negative voltage (VG&lt;0) is applied to the gate electrode (GE) and an off state (a minority carrier does not flow). 
       FIG. 23  illustrates the relation between the vacuum level, the work function of metal (φ M ), and the electron affinity of an oxide semiconductor (χ). 
     Because electrons in metal are degenerated at room temperature, the Fermi level is located in a conduction band. On the other hand, in general, a conventional oxide semiconductor is an n-type semiconductor, and the Fermi level (Ef) thereof is located near the conduction band (Ec) away from an intrinsic Fermi level (Ei) which is located in the center of the band gap. It is known that part of hydrogen in the oxide semiconductor becomes a donor and one of factors that make the oxide semiconductor an n-type semiconductor. Further, an oxygen defect is known as one of factors that make the conductivity type an n-type. 
     According to one embodiment of the present invention, hydrogen that is an n-type impurity is removed from an oxide semiconductor to highly purify the oxide semiconductor so that impurities that are not main components of the oxide semiconductor are not contained as much as possible and to remove an oxygen defect, whereby an i-type (intrinsic) or substantially intrinsic oxide semiconductor is obtained. That is, an oxide semiconductor is made to be an oxide semiconductor which is an i-type (intrinsic) semiconductor or is a substantially i-type (intrinsic) semiconductor not by adding an impurity but by removing impurities such as hydrogen or water or an oxygen defect as much as possible to have high purity. With the above-described structure, the Fermi level (Ef) can be substantially close to the same level as the intrinsic Fermi level (Ei), as indicated by arrows. 
     The band gap (Eg) of an oxide semiconductor is said to be 3.15 eV, and the electron affinity (χ) is said to be 4.3 eV. The work function of titanium (Ti) included in the source electrode and the drain electrode is substantially equal to the electron affinity (χ) of the oxide semiconductor. In that case, a Schottky barrier to electrons is not formed at an interface between the metal and the oxide semiconductor. 
     That is to say, in the case where the work function of metal (φ M ) is equal to the electron affinity of an oxide semiconductor (χ), the energy band diagram (schematic diagram) as shown in  FIG. 21  is obtained when the metal and the oxide semiconductor are in contact with each other. 
     In  FIG. 21 , a black circle (●) indicates an electron, and when a positive potential is applied to the drain electrode, the electron which crosses over a barrier (h) is injected in the oxide semiconductor, and flows to the drain electrode. In that case, the height of the barrier (h) changes depending on the gate voltage and the drain voltage; in the case where a positive drain voltage is applied, the height of the barrier (h) is smaller than the height of the barrier in  FIG. 21  where no voltage is applied, i.e., ½ of the band gap (E g ). 
     In that case, as shown in  FIG. 22A , the electron moves along the lowest part of the oxide semiconductor, which is energetically stable, at an interface between the gate insulating film and the highly-purified oxide semiconductor. 
     In  FIG. 22B , when a negative voltage is applied to the gate electrode (GE), holes which are minority carriers are substantially zero; therefore, current is substantially close to zero. 
     For example, even when the transistor has a channel width (W) of 1×10 4  μm and a channel length (L) of 3 μm, an off-state current of 10 13  A or less and a subthreshold swing (S value) of 0.1 V/dec. (the thickness of the gate insulating film: 100 nm) can be obtained. 
     In this manner, the oxide semiconductor film is highly purified so that impurities such as water or hydrogen except a main component of the oxide semiconductor are not contained as much as possible, whereby favorable operation of the transistor can be obtained. 
     Although the manufacturing method in which the oxide semiconductor film  103  is processed into a predetermined shape to form the island-shaped oxide semiconductor film  104 , and after that, the first heat treatment, the addition of oxygen, and the second heat treatment are performed is described in Embodiment 1, the present invention is not limited to this structure. The first heat treatment, the addition of oxygen, and the second heat treatment may be performed on the oxide semiconductor film  103  before being processed into the island-shaped oxide semiconductor film  104 , and after that, the oxide semiconductor film may be processed into a predetermined shape to form the island-shaped oxide semiconductor film  104 . Alternatively, the first heat treatment may be performed on the oxide semiconductor film  103 , and after that, the oxide semiconductor film may be processed into a predetermined shape to form the island-shaped oxide semiconductor film, and then, the addition of oxygen and the second heat treatment may be performed on the island-shaped oxide semiconductor film. Further alternatively, the first heat treatment and the addition of oxygen may be performed on the oxide semiconductor film  103 , and after that, the oxide semiconductor film may be processed into a predetermined shape to form the island-shaped oxide semiconductor film, and then, the second heat treatment may be performed on the island-shaped oxide semiconductor film. 
     A manufacturing method for the case where the first heat treatment, the addition of oxygen, and the second heat treatment are performed on the oxide semiconductor film  103 , and then, the oxide semiconductor film is processed into an island-shaped oxide semiconductor film is described using  FIGS. 4A to 4E . 
     First, in accordance with the above-described manufacturing process, a gate electrode  101  is formed over a substrate  100 , and a gate insulating film  102  and an oxide semiconductor film  103  are stacked in this order over the gate electrode  101 , whereby the state shown in  FIG. 1A , that is, a state shown in  FIG. 4A  is formed. Next, as shown in  FIG. 4B , first heat treatment is performed on the oxide semiconductor film  103 , so that an oxide semiconductor film  150  from which moisture and hydrogen are eliminated is formed. The conditions of the first heat treatment are already described and description thereof is omitted here. 
     Next, as shown in  FIG. 4C , oxygen is added into the oxide semiconductor film  150  by an ion implantation method, an ion doping method, or the like, so that an oxide semiconductor film  151  in which oxygen is added excessively is formed. A specific method of adding oxygen is already described, and description thereof is omitted here. By adding oxygen, a bond between a metal as a component of the oxide semiconductor and hydrogen or a bond between the metal and a hydroxyl group is cut and the hydrogen or the hydroxyl group is reacted with oxygen to produce water; this leads to easy elimination of hydrogen or a hydroxyl group that is an impurity, in the form of water by second heat treatment performed later. Heat treatment may be performed on the substrate provided with the oxide semiconductor film  150  at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to a strain point of a glass substrate) while the addition of oxygen into the oxide semiconductor film  150  is performed by an ion implantation method. 
     Next, second heat treatment is performed. The second heat treatment can be performed in a similar condition to the first heat treatment, so that an oxide semiconductor film  152  is obtained as shown in  FIG. 4D . The bond between a metal as a component of the oxide semiconductor and hydrogen or a hydroxyl group is cut and the hydrogen or the hydroxyl group is made to react with oxygen to produce water by adding oxygen in the oxide semiconductor film  105 . Thus, an impurity such as hydrogen or a hydroxyl group left in the film can be easily eliminated in the form of water by the second heat treatment after the oxygen addition. The island-shaped oxide semiconductor film  152  formed through the second heat treatment is more i-type (intrinsic) or closer to i-type than the oxide semiconductor film  150  after the first heat treatment because impurities such as moisture or hydrogen left even after the first heat treatment are removed. Impurities such as moisture or hydrogen are eliminated, and the island-shaped oxide semiconductor film becomes an i-type (intrinsic) semiconductor or a substantially i-type semiconductor; therefore, deterioration of characteristics of the transistor due to the impurities, such as shifts in threshold voltage, can be prevented from being promoted and off-state current can be reduced. 
     Next, as illustrated in  FIG. 4E , the oxide semiconductor film  152  is processed (patterned) into an appropriate shape by etching or the like, whereby an island-shaped oxide semiconductor film  153  is formed over the gate insulating film  102  so as to overlap with the gate electrode  101 . A specific etching method is already described, and description thereof is omitted here. 
     It is preferable that reverse sputtering be performed before the formation of a source electrode and a drain electrode in a subsequent step so that a resist residue or the like that attaches onto surfaces of the island-shaped oxide semiconductor film  153  and the gate insulating film  102  is removed. 
     Through the above-described process, the concentration of hydrogen in the oxide semiconductor film can be reduced and the oxide semiconductor film can be highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature which is lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that mass productivity can be increased. In addition, by using the oxide semiconductor film whose hydrogen concentration is reduced and which is highly purified, it is possible to form a transistor with a high withstand voltage, less short-channel effect, and a high on/off ratio. 
     Embodiment 2 
     In Embodiment 2, a structure and a manufacturing method of a transistor for a power device capable of higher voltage or higher current control will be described. Embodiment 1 can be applied to the same portions as Embodiment 1 or portions having functions similar to those of Embodiment 1, and therefore, description thereof is not omitted. 
     As shown in  FIG. 5A , an insulating film  201  which serves as a base film is formed over a substrate  200 , and then, a first electrode  202  is formed. 
     The description on the substrate  100  described in Embodiment 1 can be referred to for a substrate to be used as the substrate  200 . The description on the base film described in Embodiment 1 can be referred to for a material, the structure, and the thickness of the insulating film  201 . 
     The first electrode  202  is formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, tungsten, or yttrium; an alloy containing any of these metal elements as a component; an alloy containing these metal elements in combination; or the like. Alternatively, one or more material elements selected from manganese, magnesium, zirconium, beryllium, and thorium can be used. In addition, the first electrode  202  can have a single-layer structure or a stacked-layer structure having two or more layers. For example, a single-layer structure of an aluminum film containing silicon; a two-layer structure of an aluminum film and a titanium film stacked thereover; a two-layer structure of a tungsten film and a titanium film stacked thereover; a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in that order; and the like can be given. Alternatively, a film, an alloy film, or a nitride film which contains aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium may be used. 
     Alternatively, a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added, can be used as the first electrode  202 . A stacked-layer structure of the above-described light-transmitting conductive material and the above-described metal element may be employed. 
     The first electrode  202  can be formed in such a manner that a conductive film is formed over the substrate  200  by a sputtering method, a CVD method, or a vacuum evaporation method, a resist mask is formed over the conductive film in a photolithography step, and the conductive film is etched using the resist mask. Alternatively, the first electrode  202  can be formed by a printing method or an inkjet method without using a photolithography step, so that the number of steps can be reduced. Note that end portions of the first electrode  202  preferably have a tapered shape, so that the coverage with a gate insulating film formed later improves. When the angle between the end portion of the first electrode  202  and the insulating film  201  is greater than or equal to 30° and less than or equal to 60°, preferably greater than or equal to 40° and less than or equal to 50°, the coverage with the gate insulating film formed later can be improved. 
     In this embodiment, as the conductive film serving as the first electrode  202 , a 50-nm-thick titanium film is formed by a sputtering method, a 100-nm-thick aluminum film is formed, and a 50-nm-thick titanium film is formed. Next, the conductive film is etched with the use of a resist mask formed by a photolithography step, so that the first electrode  202  is formed. Instead of the resist mask formed by the photolithography step, a resist mask may be formed using an ink-jet method, so that the number of steps can be reduced. 
     Next, an island-shaped oxide semiconductor film  203  is formed over the first electrode  202 . The oxide semiconductor film  203  can be formed by a sputtering method, a coating method, a printing method, or the like. In this embodiment, the island-shaped oxide semiconductor film  203  is formed as follows: an oxide semiconductor film is formed over the first electrode  202  by a sputtering method and is processed into an appropriate shape by etching or the like. The oxide semiconductor film can be formed by a sputtering method under a rare gas (for example, argon) atmosphere, an oxygen atmosphere, or an atmosphere including a rare gas (for example, argon) and oxygen. 
     The etching for forming the island-shaped oxide semiconductor film  203  may be performed according to the description on the etching for forming the island-shaped oxide semiconductor film described in Embodiment 1. The angle formed between the first electrode  202  and an end portion of the island-shaped oxide semiconductor film  203  formed by the etching is set to greater than or equal to 30° and less than or equal to 60°, preferably greater than or equal to 40° and less than or equal to 50°, which is preferable because the coverage with the gate insulating film formed later can be improved. 
     Note that before the oxide semiconductor film is formed by a sputtering method, reverse sputtering in which an argon gas is introduced and plasma is generated is preferably performed to remove dust attached to the top surface of the first electrode  202 . The reverse sputtering refers to a method in which, without application of a voltage to a target side, an RF power source is used for voltage application to a substrate side in an argon atmosphere to modify a surface. Instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Further alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used. 
     The above-described oxide semiconductor can be used for the oxide semiconductor film  203 . 
     In this embodiment, as the oxide semiconductor film  203 , an In—Ga—Zn—O-based non-single-crystal film with a thickness of 30 nm, which is obtained by a sputtering method using an oxide semiconductor target including indium (In), gallium (Ga), and zinc (Zn) is used. As the target, for example, an oxide semiconductor target having a composition ratio with an atom ratio of metals, In:Ga:Zn=1:1:0.5, In:Ga:Zn=1:1:1, or In:Ga:Zn=1:1:2 can be used. Further, the oxide semiconductor film can be formed by a sputtering method under a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere containing a rare gas (typically argon) and oxygen. In the case of using a sputtering method, a target containing SiO 2  at 2 wt % to 10 wt % both inclusive may be used for depositing the film. The filling rate of the oxide semiconductor target including In, Ga, and Zn is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. With the use of the oxide semiconductor target with a high filling rate, a dense oxide semiconductor film is formed. 
     The oxide semiconductor film  203  is formed over the substrate  200  in such a manner that the substrate is held in the treatment chamber maintained at reduced pressure, a sputtering gas from which hydrogen and moisture have been removed is introduced into the treatment chamber while moisture remaining therein is removed, and metal oxide is used as a target. At that time, the substrate may be heated at higher than or equal to 100° C. and lower than or equal to 600° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Film deposition may be performed while the substrate is heated, whereby the concentration of an impurity contained in the oxide semiconductor film deposited can be reduced. In addition, damage by sputtering can be reduced. In order to remove remaining moisture in the treatment chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. An evacuation unit may be a turbo pump provided with a cold trap. In the deposition chamber which is evacuated with the cryopump, for example, a hydrogen atom, a compound containing a hydrogen atom, such as water (H 2 O), (far preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of an impurity in the oxide semiconductor film deposited in the deposition chamber can be reduced. 
     As one example of the deposition condition, the following can be employed: the substrate temperature is room temperature, the distance between the substrate and the target is 110 mm, the pressure is 0.4 Pa, the direct-current (DC) power is 0.5 kW, and the atmosphere is an atmosphere of oxygen and argon (the oxygen flow rate 15 sccm: the argon flow rate 30 sccm). Note that a pulsed direct-current (DC) power source is preferable because powder substances which are referred to as particles generated in film deposition can be reduced and the film thickness can be uniform. The oxide semiconductor film has a thickness greater than or equal to 1 μm, preferably greater than or equal to 3 μm, far preferably 10 μm. Since appropriate thickness depends on an oxide semiconductor material used, the thickness can be determined as appropriate depending on the material. 
     Further, in order that hydrogen, a hydroxyl group, and moisture may be contained in the oxide semiconductor film  203  as little as possible, it is preferable that the substrate  200  on which the process up to and including the step of forming the first electrode  202  is already performed be preheated in a preheating chamber of a sputtering apparatus as pretreatment for film formation so that impurities such as hydrogen and moisture adsorbed to the substrate  200  are removed and exhausted. The temperature of the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. This preheating treatment can be omitted. Further, this preheating may be similarly performed on the substrate  200  on which the process up to and including the step of forming a gate electrode is already performed, before the formation of a gate insulating film. 
     Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power supply, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. An RF sputtering method is mainly used in the case where an insulating film is formed, and a DC sputtering method is mainly used in the case where a metal film is formed. 
     In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber. 
     Alternatively, a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, or a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge can be used. 
     Further, as a deposition method using a sputtering method, a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, or a bias sputtering method in which a voltage is also applied to a substrate during deposition can be used. 
     Next, first heat treatment is performed on the oxide semiconductor film  203  in a reduced-pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or an ultra dry air atmosphere (in air whose moisture content is less than or equal to 20 ppm (dew point conversion, −55° C.), preferably less than or equal to 1 ppm, far preferably less than or equal to 10 ppb in the case where measurement is performed using a dew-point meter of a cavity ring-down laser spectroscopy (CRDS) system). By the first heat treatment on the oxide semiconductor film  203 , an oxide semiconductor film  205  in which moisture or hydrogen is eliminated is formed as shown in  FIG. 5B . Specifically, heat treatment may be performed at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to a strain point of a glass substrate). Since dehydration or dehydrogenation can be performed in a short time with the RTA method, the first heat treatment can be performed even at a temperature over the strain point of a glass substrate. In this embodiment, heat treatment is performed on the oxide semiconductor film  203  at a substrate temperature of 450° C. for one hour in a nitrogen atmosphere with the use of an electrical furnace that is one of heat treatment apparatuses, and then, the oxide semiconductor film is not exposed to the air and water and hydrogen are prevented from entering the oxide semiconductor film, so that the oxide semiconductor film  205  is obtained. 
     Detailed description on the heat treatment apparatus used for the first heat treatment is already made in Embodiment 1 and therefore omitted here. 
     It is preferable that in the heat treatment, moisture, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably set to be 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, far preferably 0.1 ppm or lower). 
     Next, as shown in  FIG. 5C , oxygen is added into the oxide semiconductor film  205  by an ion implantation method or an ion doping method. Oxygen is added into the oxide semiconductor film  205  by an ion implantation method, an ion doping method, or the like, so that an oxide semiconductor film  207  in which oxygen is added excessively is formed. By adding oxygen, a bond between a metal as a component of the oxide semiconductor and hydrogen or a bond between the metal and a hydroxyl group is cut and the hydrogen or the hydroxyl group is reacted with oxygen to produce water; this leads to easy elimination of hydrogen or a hydroxyl group that is an impurity, in the form of water by second heat treatment performed later. 
     In the case where an oxygen gas is used and oxygen is added by an ion implantation method, the acceleration voltage may be set in the range of 5 kV to 100 kV both inclusive and the dosage may be set in the range of 1×10 13  ions/cm 2  to 1×10 16  ions/cm 2  both inclusive. 
     Heat treatment may be performed on the substrate provided with the oxide semiconductor film  205  at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to a strain point of a glass substrate) while the addition of oxygen into the oxide semiconductor film  205  is performed by an ion implantation method. 
     Next, second heat treatment is performed. The second heat treatment can be performed in a similar condition to the first heat treatment. Specifically, heat treatment may be performed in a reduced-pressure atmosphere, an inert gas atmosphere such as a nitrogen atmosphere or a rare gas atmosphere, an oxygen gas atmosphere, or an ultra dry air atmosphere (in air whose moisture content is less than or equal to 20 ppm (dew point conversion, −55° C.), preferably less than or equal to 1 ppm, far preferably less than or equal to 10 ppb in the case where measurement is performed using a dew-point meter of a cavity ring-down laser spectroscopy (CRDS) system) at a temperature higher than or equal to 300° C. and lower than or equal to 850° C. (or a temperature lower than or equal to a strain point of a glass substrate). Since dehydration or dehydrogenation can be performed in a short time with the RTA method, the second heat treatment can be performed even at a temperature over the strain point of a glass substrate. In this embodiment, heat treatment is performed at a substrate temperature of 450° C. for one hour in a nitrogen atmosphere with the use of an electrical furnace that is one of heat treatment apparatuses, and then, the oxide semiconductor film is not exposed to the air and water and hydrogen are prevented from entering the oxide semiconductor film, so that an oxide semiconductor film  208  is obtained. The heat treatment may be performed plural times after the island-shaped oxide semiconductor film  208  is formed. 
     According to one embodiment of the present invention, the bond between a metal as a component of the oxide semiconductor and hydrogen or a hydroxyl group is cut and the hydrogen or the hydroxyl group is made to react with oxygen to produce water by adding oxygen in the oxide semiconductor film  205 . Thus, an impurity such as hydrogen or a hydroxyl group left in the film can be easily eliminated in the form of water by the second heat treatment after the oxygen addition. The island-shaped oxide semiconductor film  208  formed through the second heat treatment is more i-type (intrinsic) or closer to i-type than the oxide semiconductor film  205  after the first heat treatment because impurities such as moisture or hydrogen left even after the first heat treatment are removed. Impurities such as moisture or hydrogen are eliminated, and the island-shaped oxide semiconductor film becomes an i-type (intrinsic) semiconductor or a substantially i-type semiconductor; therefore, deterioration of characteristics of the transistor due to the impurities, such as shifts in threshold voltage, can be prevented from being promoted and off-state current can be reduced. 
     Further, when an oxide semiconductor containing an impurity is subjected to a gate bias-temperature stress test (BT test) for 12 hours under conditions that the temperature is 85° C. and the voltage applied to the gate is 2×10 6  V/cm, a bond between the impurity and a main component of the oxide semiconductor is cut by a high electric field (B: bias) and a high temperature (T: temperature), and a generated dangling bond induces drift of threshold voltage (V th ). However, in the above-described manner, by improving the interfacial characteristics between the gate insulating film and the oxide semiconductor film and removing impurities, particularly hydrogen, water, and the like, in the oxide semiconductor film as much as possible, a transistor which remains stable even with respect to the BT test can be obtained. 
     Detailed description on the heat treatment apparatus used for the second heat treatment is already made in Embodiment 1 and therefore omitted here. 
     It is preferable that in the heat treatment, moisture, hydrogen, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably set to be 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, far preferably 0.1 ppm or lower). 
     Through the above-described process, the concentration of hydrogen in the oxide semiconductor film can be reduced and the oxide semiconductor film can be highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature which is lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that mass productivity can be increased. In addition, by using the oxide semiconductor film whose hydrogen concentration is reduced and which is highly purified, it is possible to form a transistor with a high withstand voltage, less short-channel effect, and a high on/off ratio. 
     Next, as shown in  FIG. 5E , a second electrode  211  is formed over the oxide semiconductor film  208 . The similar mode of the first electrode  202  can be applied to a material and the structure of a conductive film used as the second electrode  211 . A manufacturing method of the second electrode  211  can be carried out in a similar manner to that of the first electrode  202 . 
     In this embodiment, a resist mask is formed over the conductive film which serves as the second electrode  211  in a photolithography step, and the conductive film is etched using the resist mask, so that the second electrode  211  is formed. Here, as the conductive film that serves as the second electrode  211 , a 50-nm-thick titanium film, a 100-nm-thick aluminum film, and a 50-nm-thick titanium film are stacked in this order. The angle formed between the oxide semiconductor film  208  and an end portion of the second electrode  211  may be greater than or equal to 30° and less than or equal to 60°, preferably greater than or equal to 40° and less than or equal to 50°, which is preferable because the coverage with a gate insulating film to be formed later can be improved. Then, the second electrode  211  is formed away from the first electrode  202  so as not to be in contact with the first electrode  202 . 
     One of the first electrode  202  and the second electrode  211  functions as a source electrode of a transistor and the other functions as a drain electrode thereof. 
     Heat treatment may be performed after the formation of the second electrode  211 . The temperature of the heat treatment is higher than or equal to 400° C. and lower than or equal to 850° C., preferably higher than or equal to 400° C. and lower than a strain point of the substrate. In this embodiment, the substrate is introduced into an electric furnace that is one of heat treatment apparatuses, and heat treatment is performed on the oxide semiconductor film  208  in an inert gas atmosphere, such as a nitrogen atmosphere or a rare gas atmosphere, at 450° C. for one hour, and then the oxide semiconductor film is not exposed to air so that hydrogen, water, a hydroxyl group, hydride, or the like can be prevented from entering the oxide semiconductor film, whereby hydrogen concentration is further reduced, and the oxide semiconductor film is highly purified, so that an i-type or a substantially i-type oxide semiconductor film can be obtained. 
     It is preferable that in the heat treatment, hydrogen, water, a hydroxyl group, a hydride, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into the heat treatment apparatus is preferably set to be 6N (99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is, the impurity concentration is preferably 1 ppm or lower, far preferably 0.1 ppm or lower). 
       FIG. 7A  is a top view of the first electrode  202 , the oxide semiconductor film  208 , and the second electrode  211  shown in  FIG. 5E . A cross-sectional view along dashed line B 1 -B 2  in  FIG. 7A  corresponds to  FIG. 5E . 
     Next, as shown in  FIG. 6A , a gate insulating film  212  is formed to cover the first electrode  202 , the oxide semiconductor film  208 , and the second electrode  211 . A gate electrode  213  is formed over the gate insulating film  212 . The gate insulating film  212  can be formed to have a single-layer structure or a stacked-layer structure of one or more selected from a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, an aluminum nitride oxide film, a hafnium oxide film, and a tantalum oxide film by a plasma CVD method, a sputtering method, or the like. 
     When the gate insulating film  212  is formed using a high-k material such as hafnium silicate (HfSiO x ), HfSi x O y  to which N is added, hafnium aluminate (HfAlO x ) to which N is added, hafnium oxide, or yttrium oxide, gate leakage can be reduced. Further, a stacked-layer structure can be used in which a high-k material and one or more selected from a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, and an aluminum oxide film are stacked. The thickness of the gate insulating film  212  may be greater than or equal to 50 nm and less than or equal to 500 nm. Leakage current can be reduced by increasing the thickness of the gate insulating film  212 . 
     It is preferable that the gate insulating film  212  include impurities such as moisture or hydrogen as little as possible. In the case where a silicon oxide film is formed by a sputtering method, a silicon target or a quartz target is used as a target and oxygen or a mixed gas of oxygen and argon is used as a sputtering gas. 
     An oxide semiconductor that is made to be i-type or substantially i-type (an oxide semiconductor that is highly purified) by removal of impurities is extremely sensitive to an interface state and an interface electric charge; thus, an interface between the oxide semiconductor and the gate insulating film  212  is important. Thus, a gate insulating film (GI) which is to be in contact with the highly purified oxide semiconductor needs to have high quality. 
     For example, high-density plasma CVD using microwaves (2.45 GHz) is preferable because a dense high-quality insulating film having high withstand voltage can be formed. This is because an interface state can be reduced and interface characteristics can be favorable when the highly purified oxide semiconductor and the high quality gate insulating film are in contact with each other. 
     Needless to say, other film formation methods, such as a sputtering method or a plasma CVD method, can be applied as long as a high-quality insulating film can be formed as the gate insulating film  212 . The film quality of the gate insulating film  212  and/or properties of an interface with an oxide semiconductor thereof may be modified by heat treatment performed after film deposition. In any case, any insulating film can be used as long as film quality as a gate insulating film is high, interface state density with an oxide semiconductor is decreased, and a favorable interface can be formed. 
     The gate insulating film  212  may have a structure in which an insulating film formed using a material having a high barrier property and an insulating film having lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film are stacked. In that case, the insulating film such as a silicon oxide film or a silicon oxynitride film is formed between the insulating film having a barrier property and the oxide semiconductor film. As the insulating film having a high barrier property, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like can be given, for example. With the insulating film having a barrier property, impurities in an atmosphere, such as moisture or hydrogen, or impurities included in the substrate, such as an alkali metal or a heavy metal, can be prevented from entering the oxide semiconductor film, the gate insulating film  212 , or the interface between the oxide semiconductor film and another insulating film and the vicinity thereof. In addition, the insulating film having lower proportion of nitrogen such as a silicon oxide film or a silicon oxynitride film may be formed so as to be in contact with the oxide semiconductor film, so that the insulating film formed using a material having a high barrier property can be prevented from being in contact with the oxide semiconductor film directly. 
     For example, a stacked-layer film with a thickness of 100 nm may be formed as the gate insulating film  212  as follows: a silicon oxide film (SiO X  (X&gt;0)) with a thickness greater than or equal to 5 nm and less than or equal to 300 nm is formed as a first gate insulating film, and a silicon nitride film (SiN Y  (Y&gt;0)) with a thickness greater than or equal to 50 nm and less than or equal to 200 nm is stacked by a sputtering method over the first gate insulating film as a second gate insulating film. In this embodiment, the silicon oxide film having a thickness of 100 nm is formed by an RF sputtering method under the following conditions: the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; and the atmosphere contains oxygen and argon (oxygen flow rate of 25 sccm: argon flow rate of 25 sccm=1:1). 
     Further, in order that hydrogen, a hydroxyl group, and moisture may be contained in the gate insulating film  212  as little as possible, it is preferable that the substrate  200  provided for the first electrode  202 , the oxide semiconductor film  208 , and the second electrode  211  be preheated in a preheating chamber of a sputtering apparatus as pretreatment for film formation so that impurities such as hydrogen and moisture adsorbed to the substrate  200  are removed and exhausted. The temperature of the preheating is higher than or equal to 100° C. and lower than or equal to 400° C., preferably higher than or equal to 150° C. and lower than or equal to 300° C. As an exhaustion unit provided in the preheating chamber, a cryopump is preferable. This preheating treatment can be omitted. 
     After the gate insulating film  212  is formed, heat treatment may be performed. The heat treatment is performed in an inert gas atmosphere (nitrogen, helium, neon, argon, or the like) at a temperature, preferably, higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C. In this embodiment, heat treatment for 1 hour at 250° C. in a nitrogen atmosphere is performed. By the heat treatment performed on the state where silicon oxide contained in the gate insulating film  212  is in contact with the oxide semiconductor film  208 , oxygen is supplied from the silicon oxide even when an oxygen defect is generated in the second heat treatment, whereby the number of oxygen defects which form donors can be reduced, a structure which satisfies the stoichiometric composition ratio can be obtained, and the oxide semiconductor film  208  is made to be an i-type or substantially i-type. The timing of this heat treatment is not particularly limited as long as it is after the formation of the gate insulating film  212 , and may be performed after another step, for example, after any one of the gate electrode  213 , an insulating film  214 , and wirings  215 ,  216 , and  217  is formed. This heat treatment can be performed without increasing the number of manufacturing steps by doubling as another step such as a heat treatment for reduction of the resistance of a transparent conductive film. 
     The gate electrode  213  can be formed to have a single-layer structure or a stacked-layer structure using one or more conductive films using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, neodymium, or scandium or an alloy material which contains any of these metal materials as its main component, or a nitride which contains any of these metals. Aluminum or copper can be used as one of the metal material if aluminum or copper can withstand a temperature of heat treatment performed in a later process. Aluminum or copper is preferably combined with a refractory metal material so as to prevent a heat resistance problem and a corrosive problem. As the refractory metal material, molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium, or the like can be used. 
     For example, as a two-layer structure of the gate electrode  213 , the following structure is preferable: a two-layer structure in which a molybdenum film is stacked over an aluminum film, a two-layer structure in which a molybdenum film is stacked over a copper film, a two-layer structure in which a titanium nitride film or a tantalum nitride film is stacked over a copper film, or a two-layer structure in which a titanium nitride film and a molybdenum film are stacked. As a three-layer structure of the gate electrode  213 , the following structure is preferable: a stacked structure in which an aluminum film, an alloy film of aluminum and silicon, an alloy film of aluminum and titanium, or an alloy film of aluminum and neodymium is sandwiched by any two films selected from a tungsten film, a tungsten nitride film, a titanium nitride film, and a titanium film. 
     Further, a light-transmitting oxide conductive film of indium oxide, an alloy of indium oxide and tin oxide, an alloy of indium oxide and zinc oxide, zinc oxide, aluminum zinc oxide, aluminum zinc oxynitride, gallium zinc oxide, or the like may be used for the gate electrode  213 , so that the aperture ratio of a pixel portion can be increased. 
     The gate electrode  213  is formed to a thickness of 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, a conductive film for the gate electrode is formed to a thickness of 150 nm by a sputtering method using a tungsten target, and then, the conductive film is processed (patterned) into an appropriate shape by etching; in such a manner, the gate electrode  213  is formed. The gate electrode  213  is formed at least so as to overlap with the end portion of the oxide semiconductor film  208  with the gate insulating film  212  provided therebetween. In the end portion of the oxide semiconductor film  208 , a channel formation region is formed in a portion  218  which overlaps with the gate electrode  213  with the gate insulating film  212  provided therebetween. It is preferable that an end portion of the gate electrode  213  be tapered because coverage with the insulating film  214  formed thereover is improved. Note that a resist mask may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, manufacturing costs can be reduced. 
     Next, as shown in  FIG. 6B , the insulating film  214  is formed to cover the first electrode  202 , the oxide semiconductor film  208 , the second electrode  211 , the gate insulating film  212 , and the gate electrode  213 , and then, contact holes  221 ,  222 , and  223  are formed. The insulating film  214  preferably includes no impurities such as moisture or hydrogen as much as possible, and may be formed using a single-layer insulating film or a plurality of insulating films stacked. The insulating film  214  is formed, for example, using an oxide insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or an aluminum oxynitride film; or a nitride insulating film such as a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film. Alternatively, an oxide insulating film and a nitride insulating film can be stacked. An insulating film having a high barrier property, for example, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, or an aluminum nitride oxide film may be used for the insulating film  214 , so that impurities such as moisture or hydrogen can be prevented from entering the oxide semiconductor film  208 , the gate insulating film  212 , or the interface between the oxide semiconductor film  208  and another insulating film and the vicinity thereof. 
     In this embodiment, the insulating film  214  has a structure in which a silicon nitride film having a thickness of 100 nm formed by a sputtering method is stacked over a silicon oxide film having a thickness of 200 nm formed by a sputtering method. Note that when the insulating film  214  is formed by a sputtering method, the substrate  200  may be heated to a temperature of 100° C. to 400° C., a high-purity sputtering gas which contains nitrogen, from which hydrogen, water, a hydroxyl group, hydride, or the like is removed may be introduced, and an insulating film may be formed using a silicon target. Also in that case, the insulating film is preferably formed while hydrogen, water, a hydroxyl group, hydride, or the like remaining in the treatment chamber is removed. 
     After the insulating film  214  is formed, heat treatment may be performed. The heat treatment is performed in an inert gas atmosphere (nitrogen, helium, neon, argon, or the like) at a temperature, preferably, higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C. 
     The contact holes  221 ,  222 , and  223  are formed as follows: a resist mask is formed by a photolithography step and parts of the gate insulating film  212  and the insulating film  214  are selectively etched using the resist mask. A part of the gate electrode  213  is exposed in the contact hole  221 ; a part of the second electrode  211  is exposed in the contact hole  222 ; and a part of the second electrode  213  is exposed in the contact hole  222 . At the time of the formation of these contact holes, a contact hole so as to expose the first electrode  202  may be formed in a region of the first electrode  202 , which is not covered with the gate electrode  213 . 
     Next, as shown in  FIG. 6C , a conductive film is formed over the insulating film  214  to cover the contact holes  221 ,  222 , and  223  and is processed into a desired shape by etching or the like, so that the wirings  215 ,  216 , and  217  are formed. A resist mask used for the etching may be formed by an inkjet method. No photomask is used when a resist mask is formed by an ink-jet method; therefore, manufacturing costs can be reduced. 
     The wiring  215  is connected to the gate electrode  213  through the contact hole  221 . The wiring  216  is connected to the second electrode  211  through the contact hole  222 . The wiring  217  is connected to the gate electrode  213  through the contact hole  223 . At the time of the formation of these wirings, a wiring which is connected to the first electrode  202  through a contact hole may be formed. 
     The wirings  215 ,  216 , and  217  can be formed using a conductive film having a similar structure and a similar material to the first electrode  202  by a similar manufacturing method to the first electrode  202 . 
     Through the above-described process, a transistor  220  is formed. 
       FIG. 7B  is a top view of the transistor  220  shown in  FIG. 6C . A cross-sectional view along dashed line B 1 -B 2  in  FIG. 7B  corresponds to  FIG. 6C . In  FIG. 7B , a wiring  230  is a wiring formed at the same time as the wirings  215 ,  216 , and  217  and is connected to the first electrode  202  through a contact hole  231 . 
     In this manner, the concentration of hydrogen in the oxide semiconductor film can be reduced and the oxide semiconductor film can be highly purified. Thus, the oxide semiconductor film can be stabilized. In addition, heat treatment at a temperature which is lower than or equal to the glass transition temperature makes it possible to form an oxide semiconductor film with a wide band gap in which carrier density is extremely low. Therefore, a transistor can be manufactured using a large-sized substrate, so that mass productivity can be increased. In addition, by using the oxide semiconductor film whose hydrogen concentration is reduced and which is highly purified, it is possible to form a transistor with a high withstand voltage, less short-channel effect, and a high on/off ratio. 
     Although all the part of the oxide semiconductor film  208 , which is formed in a region which is different from the second electrode  211  is covered with the gate electrode  213  in this embodiment, the present invention is not limited to this structure as long as at least a part of the part of the oxide semiconductor film  208 , which is formed in a region which is different from the second electrode  211  is covered with the gate electrode  213 . Further, the electrode of the first electrode  202  or the second electrode  211 , which functions as a drain electrode, may be connected to the gate electrode  213 . When the electrode which functions as a drain electrode is connected to the gate electrode  213 , the transistor can function as a diode. 
     The terms of the “source electrode” and the “drain electrode” included in the transistor interchange with each other depending on the polarity of the transistor or difference between the levels of potentials applied to the respective electrodes. In general, in an n-channel transistor, an electrode to which a lower potential is applied is called a source electrode, and an electrode to which a higher potential is applied is called a drain electrode. Further, in a p-channel transistor, an electrode to which a lower potential is applied is called a drain electrode, and an electrode to which a higher potential is applied is called a source electrode. In this specification and the like, for convenience, connection relation of the transistor is described assuming that the source electrode and the drain electrode are fixed; however, actually, the terms of the source electrode and the drain electrode interchange with each other depending on relation between the above potentials. 
     Note that “connection” in this specification and the like refers to electrical connection and corresponds to the state in which current or voltage can be conducted. 
     Here, a drain withstand voltage of the transistor described in this embodiment is described below. 
     When the electric field in the semiconductor reaches a certain threshold value, impact ionization occurs, and carriers accelerated by the high electric field impact crystal lattices in a depletion layer, thereby generating pairs of electrons and holes. As the electric field becomes higher, the pairs of electrons and holes generated by the impact ionization are further accelerated by the electric field, and the impact ionization is repeated, which causes an avalanche breakdown in which current is increased exponentially. The impact ionization occurs because carriers (electrons and holes) have kinetic energy that is larger than or equal to the band gap of the semiconductor. Therefore, the electric field which causes the impact ionization is increased as the band gap is increased. 
     Since the band gap of an oxide semiconductor is 3.15 eV, which is larger than the band gap of silicon, i.e., 1.74 eV, the avalanche breakdown is unlikely to occur. Therefore, a transistor using the oxide semiconductor has a high drain withstand voltage, and an exponential sudden increase of on-state current is unlikely to occur even when a high electric field is applied. 
     Next, hot-carrier degradation of a transistor using an oxide semiconductor is described. 
     The hot-carrier degradation means deterioration of transistor characteristics, e.g., variation in the threshold voltage or a gate leakage current, which is caused as follows: electrons that are accelerated to be rapid are injected into a gate insulating film in the vicinity of a drain in a channel and become fixed electric charge or form trap levels at the interface between the gate insulating film and the oxide semiconductor. The factors of the hot-carrier degradation are, for example, channel-hot-electron injection (CHE injection) and drain-avalanche-hot-carrier injection (DAHC injection). 
     Since the band gap of silicon is narrow, electrons are likely to be generated like an avalanche owing to an avalanche breakdown, and the number of electrons that are accelerated to be so rapid as to go over a barrier to the gate insulating film is increased. However, the oxide semiconductor described in this embodiment has a wide band gap; therefore, the avalanche breakdown is unlikely to occur and resistance to the hot-carrier degradation is higher than that of silicon. Note that although the band gap of silicon carbide which is one of materials having high withstand voltage and that of an oxide semiconductor are substantially equal to each other, electrons are less likely to be accelerated, hot-carrier degradation is less likely to be caused than in the case of silicon carbide, and drain withstand voltage is high in the oxide semiconductor because the mobility of the oxide semiconductor is two orders of magnitude smaller than that of silicon carbide. 
     From the above, the transistor using an oxide semiconductor has high drain withstand voltage; specifically, such a transistor can have a drain withstand voltage greater than or equal to 100 V, preferably greater than or equal to 500 V, far preferably greater than or equal to 1 kV. 
     Comparison between a transistor using silicon carbide, which is a typical example of a transistor, and a transistor using an oxide semiconductor is described below. Here, 4H—SiC is used as the silicon carbide. 
     An oxide semiconductor and 4H—SiC are common in some points. One example thereof is intrinsic carrier density. According to the Fermi-Dirac distribution, the intrinsic carrier density of the oxide semiconductor is estimated to about 10 −7  cm −3 , which is extremely low like the carrier density of 4H—SiC, i.e., 6.7×10 −11  cm −3 . 
     In addition, the energy band gap of the oxide semiconductor is 3.0 eV to 3.5 eV and that of 4H—SiC is 3.26 eV, which means that the oxide semiconductor and the silicon carbide are both wide-gap semiconductors. 
     However, the manufacturing temperatures of a transistor using an oxide semiconductor and a transistor using silicon carbide are largely different from each other. Heat treatment at 1500° C. to 2000° C. is generally needed in the case of using silicon carbide. In contrast, in the case of using an oxide semiconductor, manufacturing can be performed by heat treatment at 300° C. to 500° C. (lower than or equal to the glass transition temperature, about 700° C. at maximum), which allows manufacturing of a transistor over a large-sized substrate. In addition, throughput can be increased. 
     Further, a manufacturing process of the transistor using silicon carbide, which uses a PN junction, involves a step of doping with an impurity that can be a donor or an acceptor (e.g., phosphorus or boron); therefore, the number of manufacturing steps is increased. On the other hand, the transistor using an oxide semiconductor is not needed to be provided with a PN junction; therefore, the number of manufacturing steps can be decreased and the throughput can be improved, and further, a large-sized substrate can be used. 
     Note that considerable researches have been done on properties of oxide semiconductors such as density of state (DOS) in the band gap; however, the research does not include the idea of sufficiently reducing the DOS itself. In this embodiment, a highly purified oxide semiconductor is manufactured by removing water or hydrogen which might induce the DOS from the oxide semiconductor. This idea is based on the idea that the DOS itself is reduced sufficiently; thus, excellent industrial products can be manufactured. 
     Further, it is also possible to form a more highly purified (i-type) oxide semiconductor by supplying oxygen to a dangling bond of metal which is generated by an oxygen defect and reducing the DOS due to the oxygen defect. For example, an oxide film containing an excessive amount of oxygen is formed in close contact with a channel formation region and oxygen is supplied from the oxide film, whereby the DOS due to an oxygen defect can be reduced. 
     It is said that a defect of the oxide semiconductor is caused by a shallow level of 0.1 eV to 0.2 eV below the conduction band due to an excessive amount of hydrogen, a deep level due to lack of oxygen, or the like. The technical idea that hydrogen is reduced as much as possible and oxygen is sufficiently supplied in order to eliminate such a defect would be right. 
     An oxide semiconductor is generally considered as an n-type semiconductor; however, in this embodiment, an i-type oxide semiconductor is realized by removing an impurity, particularly water or hydrogen. In this point, different from the case of silicon which is made to be an i-type silicon by adding an impurity, one embodiment of the present invention includes a novel technical idea. 
     By making the oxide semiconductor an i-type semiconductor, favorable temperature characteristics of the transistor can be obtained; specifically, in terms of the current vs. voltage characteristics of the transistor, on-state current, off-state current, field-effect mobility, an S value, and a threshold voltage are hardly fluctuated at temperatures ranging from −25° C. to 150° C. That is, the current vs. voltage characteristics are hardly degraded by the temperature. 
     In the transistor using an oxide semiconductor which is described in this embodiment, mobility is about two orders of magnitude smaller than that of a transistor using silicon carbide; however, a current value and device characteristics of the transistor can be improved by increasing the drain voltage and the channel width (W). 
     A technical idea of this embodiment is that an impurity is not added to an oxide semiconductor and the oxide semiconductor itself is highly purified by removing an impurity such as water or hydrogen which exists therein. In other words, the oxide semiconductor is highly purified by removing water or hydrogen which forms a donor level, reducing oxygen deficiency, and sufficiently supplying oxygen that is a main component of the oxide semiconductor. 
     In an oxide semiconductor as deposition, a hydrogen concentration on the order of 10 20  cm −3  is measured by secondary ion mass spectrometry (SIMS). In accordance with the present invention is to highly purify an oxide semiconductor to obtain an electrically i-type (intrinsic) oxide semiconductor by removing an impurity such as water or hydrogen which forms a donor level and further by adding oxygen (one of components of the oxide semiconductor) which decreases at the same time as the removal of water or hydrogen, to the oxide semiconductor. 
     In this embodiment, it is preferable that the amount of water and hydrogen in the oxide semiconductor and the number of carriers in the oxide semiconductor be small as much as possible. In other words, the carrier density is preferably less than 1×10 14  cm −3 , far preferably less than 1×10 12  cm −3 , still far preferably less than 1×10 11  cm −3  that is less than the measurement limit. Further, in the technical idea of this embodiment, an ideal carrier density is 0 or as close to 0 as possible. By reducing the number of and preferably removing carriers in the oxide semiconductor, the oxide semiconductor is made to function as a path through which carriers pass in the transistor. As a result, the oxide semiconductor is made to be an i-type (intrinsic) oxide semiconductor which is highly purified and includes an extremely small number of or no carriers, whereby the off-state current can be extremely small when the transistor is off, which is the technical idea of this embodiment. 
     In addition, when the oxide semiconductor functions as a path and the oxide semiconductor itself is an i-type (intrinsic) oxide semiconductor which is highly purified so as to include an extremely small number of or no carriers, carriers are supplied from a source and drain electrodes. 
     The transistor having the structure described in Embodiment 2 can less occupy a substrate surface than a horizontal transistor in which a channel is formed substantially in parallel with a substrate as described in Embodiment 1. As a result of this, it is possible to miniaturize the transistor. 
     As described above, the oxide semiconductor film is highly purified so that an impurity that is not a main component of the oxide semiconductor film, typically hydrogen, water, a hydroxy group, or hydride, is contained as little as possible, whereby good operation of the transistor can be obtained. In particular, withstand voltage can be higher, a short channel effect can be reduced, and a high on-off ratio can be realized. 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiment. 
     Embodiment 3 
     In Embodiment 3, a structure and a method for manufacturing a semiconductor device will be described using a bottom-gate transistor having a channel protective structure as an example. Embodiment 1 can be applied to the same portions as Embodiment 1 or portions having functions similar to those of Embodiment 1, and therefore, description thereof is not omitted. 
     The state shown in  FIG. 1E  described in Embodiment 1 is obtained through the process up to and including the second heat treatment in a similar manner. Next, as shown in  FIG. 8A , a channel protective film  130  is formed over the oxide semiconductor film  108  so as to overlap with a region of the oxide semiconductor film  108 , which overlaps with the gate electrode  101 , that is, so as to overlap with a channel formation region. The channel protective film  130  can prevent the portion of the oxide semiconductor film  108 , which serves as the channel formation region, from being damaged in a later step (for example, reduction in thickness due to plasma or an etchant in etching). Accordingly, reliability of the transistor can be improved. 
     The channel protective film  130  can be formed using an inorganic material including oxygen (such as silicon oxide, silicon oxynitride, or silicon nitride oxide). The channel protective film  130  can be formed by a vapor deposition method such as a plasma enhanced CVD method or a thermal CVD method, or a sputtering method. After the deposition of the channel protective film  130 , the shape thereof is processed by etching. In this embodiment, the channel protective film  130  is formed in such a manner that a silicon oxide film is formed by a sputtering method and processed by etching using a mask formed by photolithography. 
     After the channel protective film  130  is formed, heat treatment may be performed. The heat treatment is performed in an inert gas atmosphere (nitrogen, helium, neon, argon, or the like) at a temperature, preferably, higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C. In this embodiment, heat treatment for 1 hour at 250° C. in a nitrogen atmosphere is performed. By the heat treatment performed on the state where the portion of the oxide semiconductor film  108  which forms the channel formation region is in contact with the channel protective film  130  that is an insulating film containing oxygen, oxygen is supplied to the oxide semiconductor film  108 , whereby the region of the oxide semiconductor film  108 , which is in contact with the channel protective film  130  can be selectively made an oxygen-excess state. Consequently, even when an oxygen defect occurs due to the second heat treatment in a region of the oxide semiconductor film  108 , which is in contact with at least the channel protective film  130 , the number of oxygen defects which become donors is reduced, a structure which satisfies the stoichiometric composition ratio can be obtained, and a channel formation region which overlaps with the gate electrode  101  becomes i-type or substantially i-type, which leads to improvement of the electrical characteristics of the transistor and suppression of variation of the electrical characteristics. The timing of this heat treatment is not particularly limited as long as it is after the formation of the channel protective film  130 , and can be performed without increasing the number of manufacturing steps by doubling as another step such as a heat treatment for a formation of a resin film or a heat treatment for reduction of the resistance of a transparent conductive film. 
     Next, as shown in  FIG. 8B , a conductive film which forms a source electrode and a drain electrode (including a wiring formed from the same layer as the source electrode or the drain electrode) is formed over the oxide semiconductor film  108  and is processed into an appropriate shape by etching or the like to form a source electrode  131  and a drain electrode  132 . The description on the source electrode  111  and the drain electrode  112  described in Embodiment 1 can be referred to for a material, the structure, and the thickness of each of the source electrode  131  and the drain electrode  132 . 
     Next, plasma treatment is performed thereon, using a gas such as N 2 O, N 2 , or Ar. By the plasma treatment, adsorbed water or the like which attaches to an exposed surface of the oxide semiconductor film is removed. Plasma treatment may be performed using a mixture gas of oxygen and argon as well. 
     After the plasma treatment, as shown in  FIG. 8C , an insulating film  133  is formed to cover the source electrode  131 , the drain electrode  132 , the channel protective film  130 , and the oxide semiconductor film  108 . The description on the insulating film  113  described in Embodiment 1 can be referred to for a material, the thickness, the structure, and the manufacturing method of the insulating film  133 . 
     After the insulating film  133  is formed, heat treatment may be performed. The heat treatment is performed in an inert gas atmosphere (nitrogen, helium, neon, argon, or the like) at a temperature, preferably, higher than or equal to 200° C. and lower than or equal to 400° C., for example, at a temperature higher than or equal to 250° C. and lower than or equal to 350° C. In this embodiment, heat treatment for 1 hour at 250° C. in a nitrogen atmosphere is performed. 
     Through the above-described process, a transistor  140  is formed. 
       FIG. 9  is a top view of the transistor  140  shown in  FIG. 8C . A cross-sectional view along dashed line C 1 -C 2  in  FIG. 9  corresponds to  FIG. 8C . 
     The transistor  140  formed according to the above-described manufacturing method includes the gate electrode  101 , the gate insulating film  102  over the gate electrode  101 , the oxide semiconductor film  108  over the gate insulating film  102 , the channel protective film  130  over the oxide semiconductor film  108 , and the source electrode  131  and the drain electrode  132  over the oxide semiconductor film  108 . The transistor  140  may further include the insulating film  133  over the oxide semiconductor film  108 , the source electrode  131  and the drain electrode  132 , and the channel protective film  130 . 
     Although the transistor  140  is described as a single-gate transistor, a multi-gate transistor including a plurality of channel formation regions can be formed when needed. 
     Next, a conductive film may be formed over the insulating film  133  and may be patterned so that a back gate electrode  145  is formed so as to overlap with the oxide semiconductor film  108  as shown in  FIG. 10A . The back gate electrode  145  can be formed using a material and a structure which are similar to those of the gate electrode  101  or the source electrode  131  or the drain electrode  132 . 
     The thickness of the back gate electrode  145  is set to 10 nm to 400 nm, preferably 100 nm to 200 nm. In this embodiment, a conductive film in which a titanium film, an aluminum film, and a titanium film are stacked is formed. Then, a resist mask is formed by a photolithography method, and an unnecessary portion is removed by etching so that the conductive film is processed (patterned) into an appropriate shape; thus, the back gate electrode  145  is formed. 
     Next, as shown in  FIG. 10B , an insulating film  146  is formed so as to cover the back gate electrode  145 . The insulating film  146  is preferably formed using a material with a high barrier property that can prevent moisture, hydrogen, oxygen, and the like in an atmosphere from affecting the characteristics of the transistor  140 . For example, the insulating film having a high barrier property can be formed to have a single-layer structure or a stacked-layer structure of a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like by a plasma CVD method, a sputtering method, or the like. In order to obtain an effect of a barrier property, the insulating film  146  is preferably formed to a thickness of 15 nm to 400 nm, for example. 
     In this embodiment, an insulating film with a thickness of 300 nm is formed by a plasma CVD method. The insulating film is formed under the following conditions: the flow rate of a silane gas is 4 sccm; the flow rate of dinitrogen monoxide (N 2 O) is 800 sccm; and the substrate temperature is 400° C. 
     A top view of the semiconductor device shown in  FIG. 10B  is  FIG. 10C .  FIG. 10B  is a cross-sectional view along dashed line C 1 -C 2  in  FIG. 10C . 
     Although the back gate electrode  145  covers the oxide semiconductor film  108  entirely in  FIG. 10B , one embodiment of the present invention is not limited to this structure. The back gate electrode  145  overlaps with at least part of the channel formation region included in the oxide semiconductor film  108 . 
     The back gate electrode  145  may be electrically insulated to be in a floating state, or may be in a state where the back gate electrode  145  is supplied with a potential. In the latter case, to the back gate electrode  145 , a potential which is the same level as the gate electrode  101  may be applied, or a fixed potential such as ground may be applied. The level of the potential supplied to the back gate electrode  145  is controlled, whereby the threshold voltage of the transistor  140  can be controlled. 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiments. 
     Embodiment 4 
     In Embodiment 4, a structure of a semiconductor display device which is referred to as electronic paper or digital paper that is one of the semiconductor display devices formed using a manufacturing method of the present invention will be described. 
     A display element which can control grayscale by voltage application and has a memory property is used for electronic paper. Specifically, in the display element used for electronic paper, the following display element can be used: a non-aqueous electrophoretic display element; a display element using a PDLC (polymer dispersed liquid crystal) method, in which liquid crystal droplets are dispersed in a high polymer material between two electrodes; a display element which includes chiral nematic liquid crystal or cholesteric liquid crystal between two electrodes; a display element which includes charged fine particles between two electrodes and employs a particle-moving method by which the charged fine particles are moved through fine particles by using an electric field; or the like. Non-aqueous electrophoretic display elements include in its category a display element in which a dispersion liquid, in which charged fine particles are dispersed, is sandwiched between two electrodes; a display element in which a dispersion liquid in which charged fine particles are dispersed is included over two electrodes between which an insulating film is interposed; a display element in which twisting balls having hemispheres which are different colors which charge differently are dispersed in a solvent between two electrodes; a display element which includes microcapsules, in which a plurality of charged fine particles are dispersed in a solution, between two electrodes; and the like. 
       FIG. 11A  is a top view of a pixel portion  700 , a signal line driver circuit  701 , and a scan line driver circuit  702  of electronic paper. 
     The pixel portion  700  includes a plurality of pixels  703 . A plurality of signal lines  707  is led into the pixel portion  700  from the signal line driver circuit  701 . A plurality of scan lines  708  is led into the pixel portion  700  from the scan line driver circuit  702 . 
     Each pixel  703  includes a transistor  704 , a display element  705 , and a storage capacitor  706 . A gate electrode of the transistor  704  is connected to one of the scan lines  708 . One of a source electrode and a drain electrode of the transistor  704  is connected to one of the signal lines  707 , and the other of the source electrode and the drain electrode of the transistor  704  is connected to a pixel electrode of the display element  705 . 
     In  FIG. 11A , the storage capacitor  706  is connected in parallel to the display element  705  so that voltage applied between the pixel electrode and a counter electrode of the display element  705  may be stored; however, in the case where the memory property of the display element  705  is high enough to maintain display, the storage capacitor  706  is not necessarily provided. 
     Although a structure of an active matrix pixel portion in which one transistor which serves as a switching element is provided in each pixel is illustrated in  FIG. 11A , electronic paper of one embodiment of the present invention is not limited to this structure. A plurality of transistors may be provided in each pixel. In addition to the transistor, another element such as a capacitor, a resistor, or a coil may be provided. 
     A cross-sectional view of the display element  705  provided in each pixel  703  is illustrated in  FIG. 11B  taking electrophoretic electronic paper having microcapsules as an example. 
     The display element  705  includes a pixel electrode  710 , a counter electrode  711 , and a microcapsule  712  to which voltage is applied by the pixel electrode  710  and the counter electrode  711 . A source electrode or a drain electrode  713  of the transistor  704  is connected to the pixel electrode  710 . 
     In the microcapsule  712 , a positively charged white pigment such as titanium oxide and a negatively charged black pigment such as carbon black are encapsulated together with a dispersion medium such as oil. Voltage is applied between the pixel electrode and the counter electrode in accordance with the voltage of a video signal applied to the pixel electrode  710 , and the black pigment and the white pigment are drawn to a positive electrode side and a negative electrode side, respectively, whereby grayscales can be displayed. 
     In  FIG. 11B , the microcapsule  712  is fixed by a light-transmitting resin  714  between the pixel electrode  710  and the counter electrode  711 . However, one embodiment of the present invention is not limited to this structure, and a space formed by the microcapsule  712 , the pixel electrode  710 , and the counter electrode  711  may be filled with a gas such as air, an inert gas, or the like. In that case, the microcapsule  712  is preferably fixed to one of or both the pixel electrode  710  and the counter electrode  711  by an adhesive agent or the like. 
     The number of the microcapsules  712  included in the display element  705  is not necessarily plural as illustrated in  FIG. 11B . One display element  705  may have a plurality of the microcapsules  712 , or a plurality of the display elements  705  may have one microcapsule  712 . For example, two display elements  705  share one microcapsule  712 , and positive voltage and negative voltage are applied to the pixel electrode  710  included in one of the display elements  705  and the pixel electrode  710  included in the other display element  705 , respectively. In that case, in the microcapsule  712  in a region overlapping with the pixel electrode  710  to which positive voltage is applied, the black pigment is drawn to the pixel electrode  710  side and the white pigment is drawn to the counter electrode  711  side. On the other hand, in the microcapsule  712  in a region overlapping with the pixel electrode  710  to which negative voltage is applied, the white pigment is drawn to the pixel electrode  710  side and the black pigment is drawn to the counter electrode  711  side. 
     Next, a specific driving method of electronic paper is described using the above-described electronic paper of the electrophoretic system as an example. 
     The operation of the electronic paper can be described in accordance with the following periods: an initialization period, a writing period, and a holding period. 
     First, the grayscale levels of each of the pixels in a pixel portion are set to be equal in the initialization period before a display image is switched in order to initialize display elements. Initialization of the display elements prevents a residual image from remaining. Specifically, in an electrophoretic system, displayed grayscale level is adjusted by the microcapsule  712  included in the display element  705  such that the display of each pixel is white or black. 
     In this embodiment, an operation for initialization in the case where an initialization video signal for displaying black is input to a pixel and then an initialization video signal for displaying white is input to the pixel is described. For example, in the case where image display is performed with respect to the counter electrode  711  side in the electronic paper of an electrophoretic system, voltage is applied to the display element  705  such that the black pigment in the microcapsule  712  moves to the counter electrode  711  side and the white pigment in the microcapsule  712  moves to the pixel electrode  710  side. Then, voltage is applied to the display element  705  such that the white pigment in the microcapsule  712  moves to the counter electrode  711  side and the black pigment in the microcapsule  712  moves to the pixel electrode  710  side. 
     Further, in the case where the initialization video signal is input to the pixel only once, the white pigment and the black pigment in the microcapsule  712  do not finish moving completely depending on the grayscale level displayed before the initialization period; thus, it is possible that a difference between grayscale levels for display of pixels occurs even after the initialization period ends. Therefore, it is preferable that negative voltage −Vp with respect to common voltage Vcom be applied to the pixel electrode  710  a plurality of times so that black is displayed and positive voltage Vp with respect to the common voltage Vcom be applied to the pixel electrode  710  a plurality of times so that white is displayed. 
     In the case where grayscale levels for display before the initialization period differ depending on display elements of each of the pixels, the minimum number of times for inputting an initialization video signal also varies. Accordingly, the number of times for inputting an initialization video signal may be changed between pixels in accordance with a grayscale level for display before the initialization period. In that case, the common voltage Vcom is preferably input to a pixel to which the initialization video signal is not necessarily input. 
     In order for the voltage Vp or the voltage −Vp which is an initialization video signal to be applied to the pixel electrode  710  plural times, the following operation sequence is performed plural times: the initialization video signal is input to a pixel of a line including the scan line in a period during which a pulse of a selection signal is supplied to each scan line. The voltage Vp or the voltage −Vp which is an initialization video signal is applied to the pixel electrode  710  plural times, whereby movement of the white pigment and the black pigment in the microcapsule  712  converges in order to prevent a difference of grayscale levels between pixels from occurring. Thus, initialization of a pixel in the pixel portion can be performed. 
     Note that in each pixel in the initialization period, black may be displayed after white is displayed instead of that white is displayed after black is displayed. Alternatively, in each pixel in the initialization period, black may be displayed after white is displayed; and further, after that white may be displayed. 
     Further, as for all of the pixels in the pixel portion, timing of starting the initialization period is not necessarily the same. For example, timing of starting the initialization period may be different for every pixel, or every group of pixels belonging to the same line, or the like. 
     Next, in the writing period, a video signal having image data is input to the pixel. 
     In the case where an image is displayed on the entire pixel portion, in one frame period, a selection signal in which a pulse of voltage is shifted is sequentially input to all of the scan lines. Then, in one line period in which a pulse appears in a selection signal, a video signal having image data is input to all of the signal lines. 
     The white pigment and the black pigment in the microcapsule  712  are moved to the pixel electrode  710  side and the counter electrode  711  side in accordance with the voltage of the video signal applied to the pixel electrode  710 , so that the display element  705  displays a grayscale. 
     Also in the writing period, the voltage of a video signal is preferably applied to the pixel electrode  710  plural times as in the initialization period. Accordingly, the following operation sequence is performed plural times: the video signal is input to a pixel of a line including the scan line in a period during which a pulse of a selection signal is supplied to each scan line. 
     Next, in the holding period, the common voltage Vcom is input to all of the pixels through signal lines, and after that, a selection signal is not input to a scan line or a video signal is not input to a signal line. Accordingly, the positions of the white pigment and the black pigment in the microcapsule  712  included in the display element  705  are maintained unless positive or negative voltage is applied between the pixel electrode  710  and the common electrode  711 , so that the grayscale level displayed on the display element  705  is held. Therefore, an image written in the writing period is maintained even in the holding period. 
     Note that voltage needed for changing the grayscale level of the display element used for electronic paper tends to be higher than that of a liquid crystal element used for a liquid crystal display device or that of a light-emitting element such as an organic light-emitting element used for a light-emitting device. Therefore, the potential difference between the source electrode and the drain electrode of the transistor  704  of a pixel, which serves as a switching element in a writing period is large; as a result, off-state current is increased, and disturbance of display is likely to occur due to fluctuation of the potential of the pixel electrode  710 . However, in one embodiment of the present invention, an oxide semiconductor film whose impurity concentration is extremely low is used for an active layer of the transistor  704 . Therefore, the off-state current of the transistor  704  in the state where voltage between the gate electrode and the source electrode is substantially zero, that is, leakage current is considerably low. Therefore, even when a potential difference between the source electrode and the drain electrode of the transistor  704  increases in the writing period, off-state current can be suppressed, and generation of disturbance of display due to the change in the potential of the pixel electrode  710  can be prevented. In addition, the potential difference between the source electrode and the drain electrode of the transistor  704  used as a switching element in the pixel increases in the writing period, so that the transistor  704  easily deteriorates. However, in one embodiment of the present invention, variation in threshold voltage of the transistor  704  due to degradation over time can be reduced, so that reliability of the electronic paper can be enhanced. 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiments. 
     Embodiment 5 
       FIG. 12A  illustrate an example of a block diagram of an active matrix semiconductor display device. Over a substrate  5300  in the display device, a pixel portion  5301 , a first scan line driver circuit  5302 , a second scan line driver circuit  5303 , and a signal line driver circuit  5304  are provided. In the pixel portion  5301 , a plurality of signal lines which is extended from the signal line driver circuit  5304  is provided and a plurality of scan lines which is extended from the first scan line driver circuit  5302  and the second scan line driver circuit  5303  is provided. Pixels which include display elements are provided in a matrix form in respective regions where the scan lines and the signal lines intersect with each other. Further, the substrate  5300  in the display device is connected to a timing control circuit  5305  (also referred to as a controller or a controller IC) through a connection point such as a flexible printed circuit (FPC). 
     In  FIG. 12A , the first scan line driver circuit  5302 , the second scan line driver circuit  5303 , and the signal line driver circuit  5304  are formed over one substrate  5300  provided with the pixel portion  5301 . Therefore, since the number of components provided outside, such as a driver circuit is reduced, it is possible not only to downsize the display device but also to reduce cost owing to the decrease in the number of assembly steps and inspection steps. In addition, if the driver circuit is provided outside the substrate  5300 , a wiring would need to be extended and the number of wiring connections would be increased; therefore, the driver circuit is provided over the substrate  5300 , so that the number of connections of the wirings can be reduced. Therefore, the decrease in yield due to defective connection of the driver circuit and the pixel portion can be prevented, and the decrease in reliability due to low mechanical strength at a connection point can be prevented. 
     Note that as an example, the timing control circuit  5305  supplies a first scan line driver circuit start signal (GSP 1 ) and a scan line driver circuit clock signal (GCK 1 ) to the first scan line driver circuit  5302 . The timing control circuit  5305  supplies, for example, a second scan line driver circuit start signal (GSP 2 ) (also referred to as a start pulse) and a scan line driver circuit clock signal (GCK 2 ) to the second scan line driver circuit  5303 . Moreover, the timing control circuit  5305  supplies a signal line driver circuit start signal (SSP), a signal line driver circuit clock signal (SCK), video signal data (DATA, also simply referred to as a video signal), and a latch signal (LAT) to the signal line driver circuit  5304 . Note that one of the first scan line driver circuit  5302  and the second scan line driver circuit  5303  can be omitted. 
       FIG. 12B  illustrates a structure in which circuits with low driving frequency (for example, the first scan line driver circuit  5302  and the second scan line driver circuit  5303 ) are formed over one substrate  5300  provided with the pixel portion  5301  and the signal line driver circuit  5304  is formed over another substrate which is different from the substrate provided with the pixel portion  5301 . It is possible to form a circuit with low driving frequency such as an analog switch used for a sampling circuit in the signal line driver circuit  5304  partly over one substrate  5300  provided with the pixel portion  5301 . Thus, a system-on-panel is partly employed, so that advantages of the system-on-panel such as the above-described prevention of decrease in yield due to the defective connection or decrease in mechanical strength at a connection point, and reduction in cost due to the decrease in the number of assembly steps and inspection steps can be obtained more or less. Further, as compared with the system-on-panel in which the pixel portion  5301 , the first scan line driver circuit  5302 , the second scan line driver circuit  5303 , and the signal line driver circuit  5304  are formed over one substrate, it is possible to increase the performance of a circuit with high driving frequency, and it is possible to form a pixel portion having a large area, which is difficult to realize in the case of using a single crystal semiconductor. 
     Next, a structure of a signal line driver circuit using an n-channel transistor is described below. 
     A signal line driver circuit illustrated in  FIG. 13A  includes a shift register  5601  and a sampling circuit  5602 . The sampling circuit  5602  includes a plurality of switching circuits  5602 _ 1  to  5602 _N (N is a natural number). The switching circuits  5602 _ 1  to  5602 _N each include a plurality of n-channel transistors  5603 _ 1  to  5603 _ k  (k is a natural number). 
     A connection relation in the signal line driver circuit is described taking the switching circuit  5602 _ 1  as an example. Note that one of a source electrode and a drain electrode of a transistor is referred to as a first terminal, and the other is referred to as a second terminal below. 
     First terminals of the transistors  5603 _ 1  to  5603 _ k  are connected to respective wirings  5604 _ 1  to  5604 _ k . A video signal is input to each of the wirings  5604 _ 1  to  5604 _ k . Second terminals of the transistors  5603 _ 1  to  5603 _ k  are connected to respective signal lines S 1  to Sk. Gate electrodes of the transistors  5603 _ 1  to  5603 _ k  are connected to the shift register  5601 . 
     The shift register  5601  has the function of sequentially selecting the switching circuits  5602 _ 1  to  5602 _N by sequentially outputting timing signals having high level (H-level) voltage to wirings  5605 _ 1  to  5605 _N. 
     The switching circuit  5602 _ 1  has a function of controlling respective conduction states between the wirings  5604 _ 1  to  5604 _ k  and the signal lines S 1  to Sk (conduction between the first terminal and the second terminal), namely a function of controlling whether or not to supply respective potentials of the wirings  5604 _ 1  to  5604 _ k  to the signal lines S 1  to Sk with the use of switching of the transistors  5603 _ 1  to  5603 _ k.    
     Next, the operation of the signal line driver circuit in  FIG. 13A  is described with reference to a timing chart in  FIG. 13B .  FIG. 13B  illustrates the timing chart of the timing signals Sout_ 1  to Sout_N which are input to the respective wirings  5605 _ 1  to  5605 _N and video signals Vdata_ 1  to Vdata_k which are input to the respective wirings  5604 _ 1  to  5604 _ k  from the shift register  5601 , as an example. 
     Note that one operation period of the signal line driver circuit corresponds to one line period in a display device. Illustrated in  FIG. 13B  is the case where one line period is divided into periods T 1  to TN. The periods T 1  to TN are each periods for writing a video signal into one pixel in a selected row. 
     In the periods T 1  to TN, the shift register  5601  sequentially outputs H-level timing signals to the wirings  5605 _ 1  to  5605 _N. For example, in the period T 1 , the shift register  5601  outputs an H level signal to the wiring  5605 _ 1 . Accordingly, the transistors  5603 _ 1  to  5603 _ k  included in the switching circuit  5602 _ 1  are turned on, so that the wirings  5604 _ 1  to  5604 _ k  and the signal lines S 1  to Sk are electrically connected to each other. In that case, Data(S 1 ) to Data(Sk) are input to the respective wirings  5604 _ 1  to  5604 _ k . The Data(S 1 ) to Data(Sk) are input to respective pixels in the first to kth columns in the selected row through the respective transistors  5603 _ 1  to  5603 _ k . Thus, in the periods T 1  to TN, video signals are sequentially written to the pixels in the selected row by k columns. 
     By writing video signals to the pixels by a plurality of columns, the number of video signals or the number of wirings can be reduced. Thus, the number of connections with an external circuit such as a controller can be reduced. By writing video signals to the pixels per plural columns, writing time can be extended and lack of writing of video signals can be prevented. 
     Next, one embodiment of a shift register which is used for a signal line driver circuit or a scan line driver circuit is described with reference to  FIGS. 14A and 14B  and  FIGS. 15A and 15B . 
     The shift register includes first to N-th pulse output circuits  10 _ 1  to  10 _N (N is a natural number of 3 or more) (see  FIG. 14A ). A first clock signal CK 1 , a second clock signal CK 2 , a third clock signal CK 3 , and a fourth clock signal CK 4  are supplied from a first wiring  11 , a second wiring  12 , a third wiring  13 , and a fourth wiring  14 , respectively, to the first to N-th pulse output circuits  10 _ 1  to  10 _N. A start pulse SP 1  (a first start pulse) from a fifth wiring  15  is input to the first pulse output circuit  10 _ 1 . Further, a signal from the pulse output circuit  10 _( n− 1) of the previous stage (referred to as a previous stage signal OUT(n−1)) (n is a natural number of 2 or more) is input to the n-th pulse output circuit  10 _ n  (n is a natural number of 2 or more and N or less) in a second or subsequent stage. A signal from the third pulse output circuit  10 _ 3  which is two stages after the first pulse output circuit  10 _ 1  is input to the first pulse output circuit  10 _ 1 . In a similar way, a signal from the (n+2)-th pulse output circuit  10 _( n+ 2) which is two stages after the n-th pulse output circuit  10 _ n  (referred to as the subsequent stage signal OUT(n+2)) is input to the n-th pulse output circuit  10 _ n  in the second stage or subsequent stage. Thus, the pulse output circuits of the respective stages output first output signals (OUT( 1 )(SR) to OUT(N)(SR)) to be input to the pulse output circuits of their respective subsequent stages and/or to the pulse output circuits of their respective stages before their respective previous stages, and second output signals (OUT( 1 ) to OUT(N)) to be input to another circuit or the like. Note that as illustrated in  FIG. 14A , a subsequent stage signal OUT(n+2) is not input to the last two stages of the shift register; therefore, as an example, a second start pulse SP 2  and a third start pulse SP 3  may be input thereto. 
     Note that the clock signal (CK) is a signal which alternates between an H level and an L level (low level voltage) at regular intervals. The first to fourth clock signals (CK 1 ) to (CK 4 ) are delayed by a ¼ period sequentially. In this embodiment, by using the first to fourth clock signals (CK 1 ) to (CK 4 ), control or the like of driving of a pulse output circuit is performed. 
     A first input terminal  21 , a second input terminal  22 , and a third input terminal  23  are electrically connected to any of the first to fourth wirings  11  to  14 . For example, in  FIG. 14A , the first input terminal  21  of the first pulse output circuit  10 _ 1  is electrically connected to the first wiring  11 , the second input terminal  22  of the first pulse output circuit  10 _ 1  is electrically connected to the second wiring  12 , and the third input terminal  23  of the first pulse output circuit  10 _ 1  is electrically connected to the third wiring  13 . In addition, the first input terminal  21  of the second pulse output circuit  10 _ 2  is electrically connected to the second wiring  12 , the second input terminal  22  of the second pulse output circuit  10 _ 2  is electrically connected to the third wiring  13 , and the third input terminal  23  of the second pulse output circuit  10 _ 2  is electrically connected to the fourth wiring  14 . 
     Each of the first to N-th pulse output circuits  10 _ 1  to  10 _N includes the first input terminal  21 , the second input terminal  22 , the third input terminal  23 , a fourth input terminal  24 , a fifth input terminal  25 , a first output terminal  26 , and a second output terminal  27  (see  FIG. 14B ). In the first pulse output circuit  10 _ 1 , the first clock signal CK 1  is input to the first input terminal  21 ; the second clock signal CK 2  is input to the second input terminal  22 ; the third clock signal CK 3  is input to the third input terminal  23 ; the start pulse is input to the fourth input terminal  24 ; the subsequent stage signal OUT( 3 ) is input to the fifth input terminal  25 ; the first output signal OUT( 1 )(SR) is output from the first output terminal  26 ; and the second output signal OUT( 1 ) is output from the second output terminal  27 . 
     Next, an example of a specific circuit structure of a pulse output circuit will be described with reference to  FIG. 15A . 
     Pulse output circuits each include 1st to 13th transistors  31  to  43  (see  FIG. 15A ). Signals or power supply potentials are supplied to the 1st to 13th transistors  31  to  43  from a power supply line  51  to which a first high power supply potential VDD is supplied, a power supply line  52  to which a second high power supply potential VCC is supplied, and a power supply line  53  to which a low power supply potential VSS is supplied, in addition to the above-described first to fifth input terminals  21  to  25 , the first output terminal  26 , and the second output terminal  27 . The relation of the power supply potentials of the power supply lines in  FIG. 15A  is as follows: the first power supply potential VDD is higher than or equal to the second power supply potential VCC, and the second power supply potential VCC is higher than the third power supply potential VSS. The first to fourth clock signals (CK 1 ) to (CK 4 ) are signals which become H-level signals and L-level signals repeatedly at regular intervals. The potential is VDD when the clock signal is at the H level, and the potential is VSS when the clock signal is at the L level. By making the potential VDD of the power supply line  51  higher than the potential VCC of the power supply line  52 , a potential applied to a gate electrode of a transistor can be lowered, shift in the threshold voltage of the transistor can be reduced, and deterioration of the transistor can be suppressed without an adverse effect on the operation of the transistor. 
     In  FIG. 15A , a first terminal of the 1st transistor  31  is electrically connected to the power supply line  51 , a second terminal of the 1st transistor  31  is electrically connected to a first terminal of the 9th transistor  39 , and a gate electrode of the 1st transistor  31  is electrically connected to the fourth input terminal  24 . A first terminal of the 2nd transistor  32  is electrically connected to the power supply line  53 , a second terminal of the 2nd transistor  32  is electrically connected to the first terminal of the 9th transistor  39 , and a gate electrode of the 2nd transistor  32  is electrically connected to a gate electrode of the 4th transistor  34 . A first terminal of the 3rd transistor  33  is electrically connected to the first input terminal  21 , and a second terminal of the 3rd transistor  33  is electrically connected to the first output terminal  26 . A first terminal of the 4th transistor  34  is electrically connected to the power supply line  53 , and a second terminal of the 4th transistor  34  is electrically connected to the first output terminal  26 . A first terminal of the 5th transistor  35  is electrically connected to the power supply line  53 , a second terminal of the 5th transistor  35  is electrically connected to the gate electrode of the 2nd transistor  32  and the gate electrode of the 4th transistor  34 , and a gate electrode of the 5th transistor  35  is electrically connected to the fourth input terminal  24 . A first terminal of the 6th transistor  36  is electrically connected to the power supply line  52 , a second terminal of the 6th transistor  36  is electrically connected to the gate electrode of the 2nd transistor  32  and the gate electrode of the 4th transistor  34 , and a gate electrode of the 6th transistor  36  is electrically connected to the fifth input terminal  25 . A first terminal of the 7th transistor  37  is electrically connected to the power supply line  52 , a second terminal of the 7th transistor  37  is electrically connected to a second terminal of the 8th transistor  38 , and a gate electrode of the 7th transistor  37  is electrically connected to the third input terminal  23 . A first terminal of the 8th transistor  38  is electrically connected to the gate electrode of the 2nd transistor  32  and the gate electrode of the 4th transistor  34 , and a gate electrode of the 8th transistor  38  is electrically connected to the second input terminal  22 . The first terminal of the 9th transistor  39  is electrically connected to the second terminal of the 1st transistor  31  and the second terminal of the 2nd transistor  32 , a second terminal of the 9th transistor  39  is electrically connected to a gate electrode of the 3rd transistor  33  and a gate electrode of the 10th transistor  40 , and a gate electrode of the 9th transistor  39  is electrically connected to the power supply line  52 . A first terminal of the 10th transistor  40  is electrically connected to the first input terminal  21 , a second terminal of the 10th transistor  40  is electrically connected to the second output terminal  27 , and the gate electrode of the 10th transistor  40  is electrically connected to the second terminal of the 9th transistor  39 . A first terminal of the 11th transistor  41  is electrically connected to the power supply line  53 , a second terminal of the 11th transistor  41  is electrically connected to the second output terminal  27 , and a gate electrode of the 11th transistor  41  is electrically connected to the gate electrode of the 2nd transistor  32  and the gate electrode of the 4th transistor  34 . A first terminal of the 12th transistor  42  is electrically connected to the power supply line  53 , a second terminal of the 12th transistor  42  is electrically connected to the second output terminal  27 , and a gate electrode of the 12th transistor  42  is electrically connected to the gate electrode of the 7th transistor  37 . A first terminal of the 13th transistor  43  is electrically connected to the power supply line  53 , a second terminal of the 13th transistor  43  is electrically connected to the first output terminal  26 , and a gate electrode of the 13th transistor  43  is electrically connected to the gate electrode of the 7th transistor  37 . 
     In  FIG. 15A , a connection point of the gate electrode of the 3rd transistor  33 , the gate electrode of the 10th transistor  40 , and the second terminal of the 9th transistor  39  is referred to as a node A. A connection point where the gate electrode of the 2nd transistor  32 , the gate electrode of the 4th transistor  34 , the second terminal of the 5th transistor  35 , the second terminal of the 6th transistor  36 , the first terminal of the 8th transistor  38 , and the gate electrode of the 11th transistor  41  are connected is referred to as a node B (see  FIG. 15A ). 
       FIG. 15B  is a timing chart of the shift register including a plurality of pulse output circuits illustrated in  FIG. 15A . 
     The provision of the 9th transistor  39  in which the second power supply potential VCC is applied to the gate electrode as illustrated in  FIG. 15A  has the following advantages before and after bootstrap operation. 
     Without the 9th transistor  39  in which the second high power supply potential VCC is applied to the gate electrode, as the potential of the node A is raised by the bootstrap operation, the potential of the source electrode which is the second terminal of the 1st transistor  31  rises to a value higher than the first power supply potential VDD. Then, the first terminal of the 1st transistor  31 , namely the power supply line  51  side thereof, becomes to serve as the source electrode. Consequently, in the 1st transistor  31 , high bias voltage is applied and thus significant stress is applied between the gate electrode and the source electrode and between the gate electrode and the drain electrode, which might cause deterioration of the transistor. By providing of the 9th transistor  39  in which the second power supply potential VCC is applied to the gate electrode, an increase in the potential of the second terminal of the 1st transistor  31  can be prevented, though the potential of the node A is raised by the bootstrap operation. In other words, the provision of the 9th transistor  39  can lower the level of negative bias voltage applied between the gate electrode and the source electrode of the 1st transistor  31 . Accordingly, with a circuit structure in this embodiment, negative bias voltage applied between the gate electrode and the source electrode of the 1st transistor  31  can be lowered, so that deterioration of the 1st transistor  31 , which is due to stress, can be suppressed. 
     The 9th transistor  39  is provided so as to be connected between the second terminal of the 1st transistor  31  and the gate electrode of the 3rd transistor  33  through the first terminal and the second terminal thereof. Note that when the shift register including a plurality of pulse output circuits in this embodiment is included in a signal line driver circuit having a larger number of stages than a scan line driver circuit, the 9th transistor  39  may be omitted, which is advantageous in that the number of transistors can be reduced. 
     Note that an oxide semiconductor is used for active layers of the 1st to 13th transistors  31  to  43 ; thus, the off-state current of the transistors can be reduced, the on-state current and field effect mobility can be increased, and the degree of deterioration of the transistors can be reduced; thus, a malfunction in a circuit can be reduced. Further, the degree of deterioration of the transistor formed using an oxide semiconductor by application of a high potential to a gate electrode is smaller than that of a transistor formed using amorphous silicon. Therefore, even when the first power supply potential VDD is supplied to a power supply line to which the second power supply potential VCC is supplied, similar operation can be performed, and the number of power supply lines which are provided for a circuit can be reduced, so that the circuit can be miniaturized. 
     A similar function to the above is obtained even when the connection relation is changed so that a clock signal that is supplied to the gate electrode of the 7th transistor  37  from the third input terminal  23  and a clock signal that is supplied to the gate electrode of the 8th transistor  38  from the second input terminal  22  may be supplied from the second input terminal  22  and the third input terminal  23 , respectively. Note that in the shift register shown in  FIG. 15A , if the state where the 7th transistor  37  and the 8th transistor  38  are both on is changed through the state where the 7th transistor  37  is off and the 8th transistor  38  is on to the state where the 7th transistor  37  is off and the 8th transistor  38  is off, potential reduction at the node B, which is caused by potential reduction of the second input terminal  22  and the third input terminal  23 , is caused twice due to the potential reduction of the gate electrode of the 7th transistor  37  and the potential reduction of the gate electrode of the 8th transistor  38 . On the other hand, in the case where states of the 7th transistor  37  and the 8th transistor  38  in the shift register illustrated in  FIG. 15A  are changed in such a manner that both the 7th transistor  37  and the 8th transistor  38  are on, then the 7th transistor  37  is on and the 8th transistor  38  is off, and then the 7th transistor  37  and the 8th transistor  38  are off, the potential reduction at the node B, which is caused by the potential reduction of the second input terminal  22  and the third input terminal  23 , is caused only once by the potential reduction of the gate electrode of the 8th transistor  38 . Consequently, a connection relation in which the clock signal CK 3  is supplied from the third input terminal  23  to the gate electrode of the 7th transistor  37  and the clock signal CK 2  is supplied from the second input terminal  22  to the gate electrode of the 8th transistor  38  is preferable. This is because the number of times of the change in the potential of the node B can be reduced, and the noise can be decreased. 
     In this way, in a period during which the potentials of the first output terminal  26  and the second output terminal  27  are held at the L level, the H level signal is regularly supplied to the node B, whereby malfunction of a pulse output circuit can be suppressed. 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiments. 
     Embodiment 6 
     In a liquid crystal display device according to one embodiment of the present invention, a highly reliable transistor with low off-state current is used; therefore, high visibility and high reliability are obtained. In Embodiment 6, a structure of the liquid crystal display device according to one embodiment of the present invention will be described. 
       FIG. 16  is a cross-sectional view of a pixel of a liquid crystal display device according to one embodiment of the present invention, as an example. A transistor  1401  illustrated in  FIG. 16  has a gate electrode  1402  formed over an insulating surface, a gate insulating film  1403  over the gate electrode  1402 , an oxide semiconductor film  1404  which is over the gate insulating film  1403  and which overlaps with the gate electrode  1402 , and a pair of conductive films  1406   a  and  1406   b  which function as a source electrode and a drain electrode and which are stacked over the oxide semiconductor film  1404 . Further, the transistor  1401  may include an insulating film  1407  formed over the oxide semiconductor film  1404  as a component. The insulating film  1407  is formed so as to cover the gate electrode  1402 , the gate insulating film  1403 , the oxide semiconductor film  1404 , and the conductive films  1406   a  and  1406   b.    
     An insulating film  1408  is formed over the insulating film  1407 . Part of the insulating film  1407  and the insulating film  1408  is provided with an opening, and a pixel electrode  1410  is formed so as to be in contact with the conductive film  1406   b  in the opening. 
     Further, a spacer  1417  for controlling a cell gap of a liquid crystal element is formed over the insulating film  1408 . The spacer  1417  can be formed by etching an insulating film into an appropriate shape. Alternatively, the cell gap may be controlled by dispersing a filler over the insulating film  1408 . 
     An alignment film  1411  is formed over the pixel electrode  1410 . Further, a counter electrode  1413  is provided in a position opposed to the pixel electrode  1410 , and an alignment film  1414  is formed on the side of the counter electrode  1413  which is close to the pixel electrode  1410 . The alignment film  1411  and the alignment film  1414  can be formed using an organic resin such as polyimide or polyvinyl alcohol. Alignment treatment such as rubbing is performed on their surfaces in order to align liquid crystal molecules in certain direction. Rubbing can be performed by rolling a roller wrapped with cloth of nylon or the like while applying pressure on the alignment film so that the surface of the alignment film is rubbed in certain direction. Note that it is also possible to form the alignment films  1411  and  1414  that have alignment characteristics by using an inorganic material such as silicon oxide by an evaporation method, without alignment process. 
     Furthermore, a liquid crystal  1415  is provided in a region which is surrounded by a sealant  1416  between the pixel electrode  1410  and the counter electrode  1413 . Injection of the liquid crystal  1415  may be performed by a dispenser method (dripping method) or a dipping method (pumping method). A filler may be mixed in the sealant  1416 . 
     The liquid crystal element formed using the pixel electrode  1410 , the counter electrode  1413 , and the liquid crystal  1415  may overlap with a color filter through which light in a particular wavelength region can pass. The color filter may be formed on a substrate (counter substrate)  1420  provided with the counter electrode  1413 . The color filter can be selectively formed by photolithography after application of an organic resin such as an acrylic-based resin in which a pigment is dispersed on the substrate  1420 . Alternatively, the color filter can be selectively formed by etching after application of a polyimide-based resin in which a pigment is dispersed on the substrate  1420 . Further alternatively, the color filter can be selectively formed by a droplet discharge method such as an ink jet method. 
     A light-blocking film which can block light may be formed between the pixels so that disclination due to variations between the pixels in the alignment of the liquid crystal  1415  is prevented from seeing. The light-blocking film can be formed using an organic resin containing a black pigment such as a carbon black or a low-valent titanium oxide. Alternatively, the light-blocking film may be formed using a film using chromium. 
     The pixel electrode  1410  and the counter electrode  1413  can be formed using a transparent conductive material such as indium tin oxide including silicon oxide (ITSO), indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO), for example. Although an example of manufacturing a transmissive liquid crystal element by using a light-transmitting conductive film for the pixel electrode  1410  and the counter electrode  1413  is described in Embodiment 6, one embodiment of the present invention is not limited to this structure. The liquid crystal display device according to one embodiment of the present invention may be a transreflective liquid crystal display device or a reflective liquid crystal display device. 
     Although a liquid crystal display device of a TN (twisted nematic) mode is described in Embodiment 6, the transistor of the present invention can be used for other liquid crystal display devices of a VA (vertical alignment) mode, an OCB (optically compensated birefringence) mode, an IPS (in-plane-switching) mode, and the like. 
     Alternatively, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase as temperature of cholesteric liquid crystal is increased. Since the blue phase is generated within an only narrow range of temperature, a liquid crystal composition containing a chiral agent at 5 wt % or more so as to improve the temperature range is used for the liquid crystal  1415 . The liquid crystal composition including liquid crystal exhibiting a blue phase and a chiral agent has a short response time of greater than or equal to 10 μsec and less than or equal to 100 μsec and is optically isotropic; therefore, alignment treatment is not necessary and viewing angle dependence is small. 
       FIG. 17  is an example of a perspective view showing a structure of a liquid crystal display device of the present invention. The liquid crystal display device illustrated in  FIG. 17  is provided with a liquid crystal panel  1601  in which a liquid crystal element is formed between a pair of substrates; a first diffusion plate  1602 ; a prism sheet  1603 ; a second diffusion plate  1604 ; a light guide plate  1605 ; a reflection plate  1606 ; a light source  1607 ; and a circuit board  1608 . 
     The liquid crystal panel  1601 , the first diffusion plate  1602 , the prism sheet  1603 , the second diffusion plate  1604 , the light guide plate  1605 , and the reflection plate  1606  are sequentially stacked. The light source  1607  is provided at an end portion of the light guide plate  1605 . The liquid crystal panel  1601  is uniformly irradiated with light from the light source  1607  which is diffused inside the light guide plate  1605 , due to the first diffusion plate  1602 , the prism sheet  1603 , and the second diffusion plate  1604 . 
     Although the first diffusion plate  1602  and the second diffusion plate  1604  are used in this embodiment, the number of diffusion plates is not limited thereto. The number of diffusion plates may be one, or may be three or more. It is acceptable as long as the diffusion plate is provided between the light guide plate  1605  and the liquid crystal panel  1601 . Therefore, a diffusion plate may be provided only on the side closer to the liquid crystal panel  1601  than the prism sheet  1603 , or may be provided only on the side closer to the light guide plate  1605  than the prism sheet  1603 . 
     Further, the cross section of the prism sheet  1603  is not limited to a sawtooth-shape illustrated in  FIG. 17 . The prism sheet  1603  may have any shape as long as light from the light guide plate  1605  can be concentrated on the liquid crystal panel  1601  side. 
     The circuit board  1608  is provided with a circuit which generates various kinds of signals input to the liquid crystal panel  1601 , a circuit which processes the signals, or the like. In  FIG. 17 , the circuit board  1608  and the liquid crystal panel  1601  are connected to each other via a flexible printed circuit (FPC)  1609 . Note that the circuit may be connected to the liquid crystal panel  1601  by using a chip on glass (COG) method, or part of the circuit may be connected to the FPC  1609  by using a chip on film (COF) method. 
       FIG. 17  illustrates an example in which the circuit board  1608  is provided with a control circuit which controls driving of the light source  1607  and the control circuit and the light source  1607  are connected to each other via an FPC  1610 . Note that the above-described control circuit may be formed over the liquid crystal panel  1601 . In that case, the liquid crystal panel  1601  and the light source  1607  are connected to each other via an FPC or the like. 
     Although  FIG. 17  illustrates an edge-light type light source in which the light source  1607  is provided at an end portion of the liquid crystal panel  1601 , a liquid crystal display device of the present invention may be a direct-below type in which the light source  1607  is provided directly below the liquid crystal panel  1601 . 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiments. 
     Embodiment 7 
     In Embodiment 7, a structure of a light-emitting device using the transistor according to one embodiment of the present invention for a pixel will be described. In this embodiment, a cross-sectional structure of a pixel when a transistor for driving a light-emitting element is an n-channel transistor is described with reference to  FIG. 18A to 18C . Although the case where a first electrode is a cathode and a second electrode is an anode is described in  FIGS. 18A to 18C , the first electrode may be an anode and the second electrode may be a cathode. 
       FIG. 18A  is a cross-sectional view of a pixel in the case where an n-channel transistor is employed as a transistor  6031 , and light emitted from a light-emitting element  6033  is extracted from a first electrode  6034  side. The transistor  6031  is covered with an insulating film  6037 , and a partition  6038  having an opening is formed over the insulating film  6037 . In the opening of the partition  6038 , the first electrode  6034  is partly exposed, and the first electrode  6034 , an electroluminescent layer  6035 , and a second electrode  6036  are sequentially stacked in the opening. 
     The first electrode  6034  is formed using a material or to a thickness such that light transmits therethrough, and can be formed using a material having a low work function of a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like. Specifically, an alkali metal such as Li or Cs, an alkaline earth metal such as Mg, Ca, or Sr, an alloy containing such metals (e.g., Mg:Ag, Al:Li, or Mg:In), a compound of such materials (e.g., calcium fluoride or calcium nitride), or a rare-earth metal such as Yb or Er can be used. Further, in the case where an electron injection layer is provided, another conductive layer such as an aluminum layer may be used. Then, the first electrode  6034  is formed to a thickness such that light transmits therethrough (preferably, about 5 nm to 30 nm). Furthermore, the sheet resistance of the first electrode  6034  may be suppressed by formation of a light-transmitting conductive layer of a light-transmitting oxide conductive material so as to be in contact with and over or under the above-described conductive layer with a thickness such that light transmits therethrough. Alternatively, the first electrode  6034  may be formed using only a conductive layer of another light-transmitting oxide conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO). Furthermore, a mixture in which zinc oxide (ZnO) is mixed at 2% to 20% in ITSO or in indium oxide including silicon oxide may be used. In the case of using the light-transmitting oxide conductive material, it is preferable to provide an electron injection layer in the electroluminescent layer  6035 . 
     The second electrode  6036  is formed using a material and to a thickness such that light is reflected or blocked, and can be formed using a material suitable for being used as an anode. For example, a single-layer film including one or more of titanium nitride, zirconium nitride, titanium, tungsten, nickel, platinum, chromium, silver, aluminum, and the like, a stacked layer of a titanium nitride film containing titanium nitride as a main component and a film containing aluminum as a main component, a three-layer structure of a titanium nitride film, a film containing aluminum as a main component, and a titanium nitride film, or the like can be used for the second electrode  6036 . 
     The electroluminescent layer  6035  is formed using a single layer or a plurality of layers. When the electroluminescent layer  6035  is formed with a plurality of layers, these layers can be classified into a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer, and the like in view of the carrier transport property. In the case where the electroluminescent layer  6035  includes at least one of a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer in addition to a light-emitting layer, the electron injection layer, the electron transport layer, the light-emitting layer, the hole transport layer, and the hole injection layer are sequentially stacked over the first electrode  6034 . Note that the boundary between layers which are adjacent to each other is not necessarily clear, and there may be the case where the boundary is unclear since materials for forming the layers are mixed. Each layer may be formed with an organic material or an inorganic material. As the organic material, any of a high molecular weight material, a medium molecular weight material, and a low molecular weight material may be used. Note that the medium molecular weight material corresponds to a low polymer in which the number of repetitions of a structural unit (the degree of polymerization) is about 2 to 20. A distinction between a hole injection layer and a hole transport layer is not always distinct, and they are the same in the sense that a hole transport property (hole mobility) is a particularly important characteristic. A layer being in contact with the anode is referred to as a hole injection layer and a layer being in contact with the hole injection layer is referred to as a hole transport layer for convenience. The same is also true for the electron transport layer and the electron injection layer; a layer being in contact with the cathode is referred to as an electron injection layer and a layer being in contact with the electron injection layer is referred to as an electron transport layer. In some cases, the light-emitting layer also functions as the electron transport layer, and it is therefore referred to as a light-emitting electron transport layer, too. 
     In the case of the pixel illustrated in  FIG. 18A , light emitted from the light-emitting element  6033  can be extracted from the first electrode  6034  side as shown by a hollow arrow. 
     Next, a cross-sectional view of a pixel in the case where an n-channel transistor is used as a transistor  6041 , and light emitted from a light-emitting element  6043  is extracted from a second electrode  6046  side is illustrated in  FIG. 18B . The transistor  6041  is covered with an insulating film  6047 , and a partition  6048  having an opening is formed over the insulating film  6047 . In the opening of the partition  6048 , a first electrode  6044  is partly exposed, and the first electrode  6044 , an electroluminescent layer  6045 , and the second electrode  6046  are sequentially stacked in the opening. 
     The first electrode  6044  is formed using a material and to a thickness such that light is reflected or blocked, and can be formed using a material having a low work function of a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like. Specifically, an alkali metal such as Li or Cs, an alkaline earth metal such as Mg, Ca, or Sr, an alloy containing such metals (e.g., Mg:Ag, Al:Li, or Mg:In), a compound of such materials (e.g., calcium fluoride or calcium nitride), or a rare-earth metal such as Yb or Er can be used. Further, in the case where an electron injection layer is provided, another conductive film such as an aluminum film may be used. 
     The second electrode  6046  is formed using a material or to a thickness such that light transmits therethrough, and formed using a material suitable for being used as an anode. For example, another light-transmitting oxide conductive material such as indium tin oxide (ITO), zinc oxide (ZnO), indium zinc oxide (IZO), or gallium-doped zinc oxide (GZO) can be used for the second electrode  6046 . Further, a mixture in which zinc oxide (ZnO) is mixed at 2% to 20% in ITSO or indium oxide including silicon oxide may be used for the second electrode  6046 . Furthermore, other than the above-described light-transmitting oxide conductive material, a single-layer film including one or more of titanium nitride, zirconium nitride, titanium, tungsten, nickel, platinum, chromium, silver, aluminum, and the like, a stacked layer of a film containing titanium nitride as a main component and a film containing aluminum as a main component, a three-layer structure of a titanium nitride film, a film containing aluminum as a main component, and a titanium nitride film, or the like can be used for the second electrode  6046 . However, in the case of using a material other than the light-transmitting oxide conductive material, the second electrode  6046  is formed to have a thickness such that light transmits therethrough (preferably, about 5 nm to 30 nm). 
     The electroluminescent layer  6045  can be formed in a manner similar to the electroluminescent layer  6035  of  FIG. 18A . 
     In the case of the pixel illustrated in  FIG. 18B , light emitted from the light-emitting element  6043  can be extracted from the second electrode  6046  side as shown by a hollow arrow. 
     Next, a cross-sectional view of a pixel in the case where an n-channel transistor is used as a transistor  6051 , and light emitted from a light-emitting element  6053  is extracted from a first electrode  6054  side and a second electrode  6056  side is illustrated in  FIG. 18C . The transistor  6051  is covered with an insulating film  6057 , and a partition  6058  having an opening is formed over the insulating film  6057 . In the opening of the partition  6058 , the first electrode  6054  is partly exposed, and the first electrode  6054 , an electroluminescent layer  6055 , and the second electrode  6056  are sequentially stacked in the opening. 
     The first electrode  6054  can be formed in a manner similar to that of the first electrode  6034  in  FIG. 18A . The second electrode  6056  can be formed in a manner similar to that of the second electrode  6046  of  FIG. 18B . The electroluminescent layer  6055  can be formed in a manner similar to that of the electroluminescent layer  6035  of  FIG. 18A . 
     In the case of the pixel illustrated in  FIG. 18C , light emitted from the light-emitting element  6053  can be extracted from both sides of a first electrode  6054  side and a second electrode  6056  side as shown by hollow arrows. 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiments. 
     Example 1 
     A semiconductor device according to one embodiment of the present invention can be used so that a highly reliable electronic device and an electronic device with low power consumption can be provided. In addition, a semiconductor display device according to one embodiment of the present invention can be used so that a highly reliable electronic device, an electronic device with high visibility, and an electronic device with low power consumption can be provided. In particular, in the case where a portable electronic device which has difficulty in continuously receiving power, a semiconductor device or a semiconductor display device with low power consumption according to one embodiment of the present invention is added to the component of the device, whereby an advantage in increasing the continuous duty period can be obtained. Further, by use of a transistor with low off-state current, redundant circuit design which is needed to compensate high off-state current is unnecessary; therefore, the density of an integrated circuit used for the semiconductor device can be increased, and a higher performance semiconductor device can be formed. 
     Moreover, with the semiconductor device of the present invention, the heat treatment temperature in the manufacturing process can be suppressed; therefore, a highly reliable thin film transistor with excellent characteristics can be formed even when the thin film transistor is formed over a substrate formed using a flexible synthetic resin of which heat resistance is lower than that of glass, such as plastic. Accordingly, with the use of the manufacturing method according to one embodiment of the present invention, a highly reliable, lightweight, and flexible semiconductor device can be provided. Examples of a plastic substrate include polyester typified by polyethylene terephthalate (PET), polyethersulfone (PES), polyethylene naphthalate (PEN), polycarbonate (PC), polyetheretherketone (PEEK), polysulfone (PSF), polyetherimide (PEI), polyarylate (PAR), polybutylene terephthalate (PBT), polyimide, an acrylonitrile-butadiene-styrene resin, polyvinyl chloride, polypropylene, polyvinyl acetate, an acrylic resin, and the like. 
     The semiconductor device according to one embodiment of the present invention can be used for display devices, laptops, or image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other than the above, as an electronic device which can be provided with the semiconductor device according to one embodiment of the present invention, mobile phones, portable game machines, portable information terminals, e-book readers, video cameras, digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), vending machines, and the like can be given.  FIGS. 19A to 19F  illustrate specific examples of these electronic devices. 
       FIG. 19A  illustrates an e-book reader including a housing  7001 , a display portion  7002 , and the like. The semiconductor display device according to one embodiment of the present invention can be used for the display portion  7002 , so that a highly reliable e-book reader, an e-book reader capable of displaying an image with high visibility, and an e-book reader with low power consumption can be provided. The semiconductor device according to one embodiment of the present invention can be used for an integrated circuit for controlling driving of the e-book reader, so that a highly reliable e-book reader, an e-book reader with low power consumption, and a higher performance e-book reader can be provided. When a flexible substrate is used, a semiconductor device and a semiconductor display device can have flexibility, whereby a user-friendly e-book reader which is flexible and lightweight can be provided. 
       FIG. 19B  illustrates a display device including a housing  7011 , a display portion  7012 , a supporting base  7013 , and the like. The semiconductor display device according to one embodiment of the present invention can be used for the display portion  7012 , so that a highly reliable display device, a display device capable of displaying an image with high visibility, and a display device with low power consumption can be provided. The semiconductor device according to one embodiment of the present invention can be used for an integrated circuit for controlling driving of the display device, so that a highly reliable display device, a display device with low power consumption, and a higher performance display device can be provided. The display device includes in its category any kind of display device for displaying information, such as display devices for personal computers, for receiving television broadcast, and for displaying advertisement. 
       FIG. 19C  illustrates a display device including a housing  7021 , a display portion  7022 , and the like. The semiconductor display device according to one embodiment of the present invention can be used for the display portion  7022 , so that a highly reliable display device, a display device capable of displaying an image with high visibility, and a display device with low power consumption can be provided. The semiconductor device according to one embodiment of the present invention can be used for an integrated circuit for controlling driving of the display device, so that a highly reliable display device, a display device with low power consumption, and a higher performance display device can be provided. When a flexible substrate is used, a semiconductor device and a semiconductor display device can have flexibility, whereby a user-friendly display device which is flexible and lightweight can be provided. Accordingly, as illustrated in  FIG. 19C , a display device can be used while being fixed to fabric or the like, and an application range of the display device is dramatically widened. 
       FIG. 19D  illustrates a portable game machine including a housing  7031 , a housing  7032 , a display portion  7033 , a display portion  7034 , a microphone  7035 , speakers  7036 , an operation key  7037 , a stylus  7038 , and the like. The semiconductor display device according to one embodiment of the present invention can be used for the display portion  7033  and the display portion  7034 , so that a highly reliable portable game machine, a portable game machine capable of displaying an image with high visibility, and a portable game machine with low power consumption can be provided. The semiconductor device according to one embodiment of the present invention can be used for an integrated circuit for controlling driving of the portable game machine, so that a highly reliable portable game machine, a portable game machine with low power consumption, and a higher performance portable game machine can be provided. Although the portable game machine illustrated in  FIG. 19D  includes two display portions  7033  and  7034 , the number of display portions included in the portable game machine is not limited to two. 
       FIG. 19E  illustrates a mobile phone including a housing  7041 , a display portion  7042 , an audio input portion  7043 , an audio output portion  7044 , operation keys  7045 , a light-receiving portion  7046 , and the like. Light received in the light-receiving portion  7046  is converted into electrical signals, whereby an outside image can be downloaded. The semiconductor display device according to one embodiment of the present invention can be used for the display portion  7042 , so that a highly reliable mobile phone, a mobile phone capable of displaying an image with high visibility, and a mobile phone with low power consumption can be provided. The semiconductor device according to one embodiment of the present invention can be used for an integrated circuit for controlling driving of the mobile phone, so that a highly reliable mobile phone, a mobile phone with low power consumption, and a higher performance mobile phone can be provided. 
       FIG. 19F  illustrates a portable information terminal including a housing  7051 , a display portion  7052 , operation keys  7053 , and the like. A modem may be incorporated in the housing  7051  of the portable information terminal illustrated in  FIG. 19F . The semiconductor display device according to one embodiment of the present invention can be used for the display portion  7052 , so that a highly reliable portable information terminal, a portable information terminal capable of displaying an image with high visibility, and a portable information terminal with low power consumption can be provided. The semiconductor device according to one embodiment of the present invention can be used for an integrated circuit for controlling driving of the portable information terminal, so that a highly reliable portable information terminal, a portable information terminal with low power consumption, and a higher performance portable information terminal can be provided. 
     This embodiment can be implemented by being combined as appropriate with the above-described embodiments. 
     This application is based on Japanese Patent Application serial No. 2009-277086 filed with Japan Patent Office on Dec. 4, 2009, the entire contents of which are hereby incorporated by reference.