Patent Publication Number: US-9411208-B2

Title: Display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/154,534, filed Jan. 14, 2014, now allowed, which is a continuation of U.S. application Ser. No. 13/594,934, filed Aug. 27, 2012, now U.S. Pat. No. 8,637,863, which is a continuation of U.S. application Ser. No. 12/959,989, filed Dec. 3, 2010, now U.S. Pat. No. 8,269,218, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2009-276454 on Dec. 4, 2009, all of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a display device having a transistor, and the channel of the transistor includes an oxide semiconductor film. 
     BACKGROUND ART 
     Patent Document 1 discloses a thin film transistor in which a first gate electrode is formed on a substrate, a first gate insulating layer is formed so as to cover the first gate electrode, a semiconductor layer formed using an oxide semiconductor is formed over the first gate insulating layer, a second gate insulating layer is formed over the semiconductor layer, a second gate electrode is formed over the second gate insulating layer, and a drain electrode and a source electrode which are connected to the semiconductor layer are formed, wherein the thickness of the second gate electrode is equal to or larger than that of the first gate electrode is described (Claim  1 ). In Patent Document 1, the above-described thin film transistor can be used for a driving field-effect transistor of a liquid crystal display or an organic EL display (144th paragraph). 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2009-176865 
       
    
     DISCLOSURE OF INVENTION 
     In Patent Document 1, the generation of a hump of conduction property of a thin film transistor can be suppressed (26th paragraph). 
     However, the above-described structure is insufficient in order to improve characteristics of a transistor. In view of the above, it is an object of an embodiment of the present invention to provide a thin film transistor including an oxide semiconductor film, which is used for the pixel portion of a display device such as a liquid crystal display or an organic EL display and has high reliability. 
     In the following experiment, the influence of hydrogen on a transistor including an oxide semiconductor film was examined. Thus, the inventor found that characteristics of the transistor can be improved by removing hydrogen and the above problem can be solved. 
     &lt;Experiment. Hydrogen and Transistor Characteristics&gt; 
       FIGS. 1A and 1B  show schematic diagrams of experimental transistors including oxide semiconductor films (amorphous In—Ga—Zn—O films, in this experiment, also referred to as a-IGZO) and characteristics of the transistors.  FIG. 1A  shows the case where SiO x  formed by a plasma CVD method (also referred to as PECVD-SiO x ) is used for an interlayer film and  FIG. 1B  shows the case where SiO x  formed by a sputtering method (also referred to as sputtered-SiO x  or sputtering-SiO x ) is used for the interlayer film. The manufacturing method other than the above is the same in each of the transistors. When SiO x  formed by a plasma CVD method was used, normally-on transistor characteristics were obtained. In addition, the change in transistor characteristics depending on the measurement temperature was large. On the other hand, when SiO x  formed by a sputtering method was used, normally-off transistor characteristics were obtained and the change in transistor characteristics depending on the measurement temperature was small. When the hydrogen concentration of each of these two transistors was measured by secondary ion mass spectrometry, it was found that the transistor in which SiO x  formed by a plasma CVD method is used includes a large amount of hydrogen; however, the transistor in which SiO x  formed by a sputtering method is used includes a small amount of hydrogen ( FIG. 2 ). 
     Further, in order to clarify electronic characteristics of a-IGZO, analysis thereof was conducted by first principle calculation. (A) a-IGZO satisfying the stoichiometric proportion was and (B) a-IGZO to which hydrogen was added were assumed, and the electronic states were calculated. A unit cell of 84 atoms, the composition ratio of In:Ga:Zn:O=1:1:1:4, and the density of 5.9 g/cm 3  was assumed, and an amorphous structure was reproduced by the classical molecular dynamics method, and further, optimization of the structure was performed by the quantum molecular dynamic method. Then, the electronic states were calculated. 
     The calculation results are shown in  FIGS. 3A and 3B .  FIGS. 3A and 3B  show DOS (Density Of States, the electron density of states) of a-IGZO. The point where Energy shows 0 (zero) is the Fermi level. It is found that, in (A) a-IGZO satisfying the stoichiometric proportion, the Fermi level exists in the valence band; however, in (B) a-IGZO to which hydrogen is added, electrons exist also in the conduction band. That is, it is found that, in a-IGZO, hydrogen forms a donor level and supplies electrons. 
     Removing hydrogen is removing a donor in the oxide semiconductor film. The oxide semiconductor film can be a semiconductor which is made intrinsic or substantially intrinsic by removing the donor. 
     When the off-state current of the transistor (the channel length (L)=10.0 μm, the channel width (W)=1 m) whose channel has the oxide semiconductor film with reduced hydrogen concentration was measured at room temperature (25° C.), the off-state current was equal to or lower than 1×10 −12  A ( FIG. 4 ). When W is converted into 1 μm, the off-state current is equal to or lower than 1×10 −18  A. 
     The oxide semiconductor film can be a semiconductor which is made intrinsic or substantially intrinsic by minimizing the number of hydrogen atoms contained in the oxide semiconductor film. Accordingly, characteristics of the transistor can be improved and the above problem can be solved. 
     An embodiment of the present invention is a display device having a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode and a drain electrode over the oxide semiconductor film, and wherein the source electrode and the drain electrode are electrically connected to the oxide semiconductor film; a second gate insulating film over the source electrode, the drain electrode, and the oxide semiconductor film; a second gate electrode over the second gate insulating film, and wherein the second gate electrode is electrically connected to the first gate electrode; an organic resin film having flatness over the second gate insulating film; a pixel electrode over the organic resin film having flatness, and wherein the pixel electrode is electrically connected to either the source electrode or the drain electrode; a first alignment film over and in contact with the second gate electrode and the pixel electrode; a liquid crystal layer over the first alignment film; a second alignment film over the liquid crystal layer; a counter electrode over the second alignment film; and a counter substrate over the counter electrode, wherein the concentration of hydrogen atoms contained in the oxide semiconductor film and measured by secondary ion mass spectrometry is less than 1×10 16  cm −3 . 
     An embodiment of the present invention is a display device having a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode and a drain electrode over the oxide semiconductor film, and wherein the source electrode and the drain electrode are electrically connected to the oxide semiconductor film; a second gate insulating film over the source electrode, the drain electrode and the oxide semiconductor film; a second gate electrode over the second gate insulating film, and wherein the second gate electrode is electrically connected to the first gate electrode; an organic resin film having flatness over the second gate insulating film; a pixel electrode over the organic resin film having flatness, and wherein the pixel electrode is electrically connected to either the source electrode or the drain electrode; an EL layer over the pixel electrode; a counter electrode over the EL layer; a sealing material over and in contact with the second gate electrode and the counter electrode; and a counter substrate over the sealing material, wherein the concentration of hydrogen atoms contained in the oxide semiconductor film and measured by secondary ion mass spectrometry is less than 1×10 16  cm −3 . 
     An embodiment of the present invention is a display device having a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode and a drain electrode over the oxide semiconductor film, and wherein the source electrode and the drain electrode are electrically connected to the oxide semiconductor film; a second gate insulating film over the source electrode, the drain electrode and the oxide semiconductor film; a second gate electrode over the second gate insulating film, and wherein the second gate electrode is electrically connected to the first gate electrode; an organic resin film having flatness over the second gate insulating film; a pixel electrode over the organic resin film having flatness, and wherein the pixel electrode is electrically connected to either the source electrode or the drain electrode; and a filler over and in contact with the second gate electrode and the pixel electrode, wherein a spherical particle including a cavity is provided in the filler, the cavity contains a black region and a white region, and a space around the cavity is filled with a liquid, wherein the concentration of hydrogen atoms contained in the oxide semiconductor film and measured by secondary ion mass spectrometry is less than 1×10 16  cm −3 . 
     An embodiment of the present invention is a display device having a first gate electrode; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film; a source electrode and a drain electrode over the oxide semiconductor film, and wherein the source electrode and the drain electrode are electrically connected to the oxide semiconductor film; a second gate insulating film over the source electrode, the drain electrode and the oxide semiconductor film; a second gate electrode over the second gate insulating film, and wherein the second gate electrode is electrically connected to the first gate electrode; an organic resin film having flatness over the second gate insulating film; a pixel electrode over the organic resin film having flatness, and wherein the pixel electrode is electrically connected to either the source electrode or the drain electrode; and an electronic ink layer over and in contact with the second gate electrode and the pixel electrode, wherein a microcapsule in which positively charged white microparticles and negatively charged black microparticles are encapsulated is provided in the electronic ink layer, wherein the concentration of hydrogen atoms contained in the oxide semiconductor film and measured by secondary ion mass spectrometry is less than 1×10 16  cm −3 . 
     According to an embodiment of the present invention, the concentration of hydrogen atoms measured by secondary ion mass spectrometry of the oxide semiconductor film in which a channel formed is less than 1×10 16  cm −3 . Accordingly, the oxide semiconductor film is a semiconductor which is made intrinsic or substantially intrinsic. The carrier density of the oxide semiconductor film is extremely reduced. The off-state current of the transistor is extremely decreased. Further, avalanche breakdown is not easily generated. 
     Furthermore, as Patent document regarding the concentration of hydrogen atoms, Japanese Published Patent Application No. 2007-103918 is given. In Japanese Published Patent Application No. 2007-103918, a field-effect transistor having a channel layer formed of an amorphous oxide film including In or Zn, wherein the amorphous oxide film includes hydrogen atoms or deuterium atoms of equal to or greater than 10 16  cm −3  and equal to or less than 10 20  cm −3 , is described. However, hydrogen atoms are positively added, which is a technical idea of the above document. On the other hand, the number of hydrogen atoms is minimized, which is a technical idea of an embodiment of the present invention. Therefore, these technical ideas are opposite and completely different from each other. According to an embodiment of the present invention, the concentration of hydrogen atoms contained in the oxide semiconductor film is less than 1×10 16  cm −3 , whereby the oxide semiconductor film can be a semiconductor which is made intrinsic or substantially intrinsic. Moreover, the density of the carriers can be extremely reduced, and the off-state current of the transistor can be extremely decreased. 
     According to an embodiment of the present invention, the transistor which is electrically connected to the pixel electrode is not covered with the organic resin film having flatness and the pixel electrode. Hydrogen atoms of the organic resin film do not adversely affect the transistor. The potential applied to the pixel electrode does not adversely affect the transistor. 
     According to an embodiment of the present invention, the second gate insulating film is formed over the source electrode and the drain electrode. Therefore, the source electrode and the drain electrode are protected by the second gate insulating film. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are schematic diagrams of transistors and diagrams showing characteristics thereof. 
         FIG. 2  shows a hydrogen concentration profile of a transistor. 
         FIGS. 3A and 3B  show calculation results. 
         FIG. 4  shows transistor characteristics. 
         FIGS. 5A and 5B  illustrate a display device. 
         FIGS. 6A and 6B  illustrate a display device. 
         FIG. 7  is a cross sectional view of a transistor. 
         FIGS. 8A and 8B  are energy band diagrams of a transistor. 
         FIGS. 9A and 9B  are energy band diagrams of a transistor. 
         FIG. 10  is an energy band diagram of a transistor. 
         FIGS. 11A to 11C  illustrate a manufacturing method of a display device. 
         FIGS. 12A and 12B  illustrate a manufacturing method of a display device. 
         FIG. 13  illustrates a method for manufacturing a display device. 
         FIGS. 14A and 14B  each illustrate a display device. 
         FIGS. 15A and 15B  each illustrate a display device. 
         FIGS. 16A and 16B  each illustrate a display device. 
         FIG. 17  shows a C-V measurement. 
         FIGS. 18A and 18B  show a C-V measurement. 
         FIGS. 19A and 19B  each show a TEM image at a cross section. 
         FIGS. 20A and 20B  each show a TEM image at a cross section. 
         FIG. 21A  shows an enlarged view of the superficial portion of the sample A and  FIGS. 21B to 21F  each illustrate an electron diffraction pattern of a crystalline region. 
         FIGS. 22A and 22B  each illustrate a display device. 
         FIGS. 23A and 23B  show a mobile phone and a portable information terminal, respectively. 
         FIGS. 24A and 24B  illustrate a television set and a digital photo frame, respectively. 
         FIG. 25  illustrates a portable amusement machine. 
         FIG. 26  illustrates an electronic book. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described below. However, the present invention can be carried out in many different modes, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways without departing from the purpose and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiment. Note that the same reference numerals are commonly given to the same portions or portions having similar functions in different drawings, and repetitive explanation will be omitted in some cases. 
     Embodiment 1 
     This embodiment discloses a display device including a first gate electrode; a first gate insulating film formed over the first gate electrode; an oxide semiconductor film formed over the first gate insulating film; and a source electrode and a drain electrode formed over the oxide semiconductor film. The source electrode and the drain electrode are electrically connected to the oxide semiconductor film. A second gate insulating film formed over the source electrode, the drain electrode, and the oxide semiconductor film; and a second gate electrode formed over the second gate insulating film are provided. The second gate electrode is electrically connected to the first gate electrode. An organic resin film having flatness formed over the second gate insulating film and a pixel electrode formed over the organic resin film having flatness are provided. The pixel electrode is electrically connected to either the source electrode or the drain electrode, and a display medium formed over and in contact with the second gate electrode and the pixel electrode. 
     As illustrated in  FIGS. 5A and 5B , the display device includes a pixel  20 . The display device may include a plurality of pixels  20 . The pixel  20  includes a transistor  15 . The transistor  15  includes a first gate electrode  3 , a first gate insulating film  4 , an oxide semiconductor film  5 , an electrode  6 A (one of a source electrode and a drain electrode), an electrode  6 B (the other of the source electrode and the drain electrode), a second gate insulating film  7 , and a second gate electrode  8 . The pixel  20  includes an organic resin film  9  having flatness and a pixel electrode  10 .  FIG. 5A  is a top view of the pixel  20  and  FIG. 5B  illustrates a cross section A-B and a cross section C-D in  FIG. 5A . 
     The transistor  15  is formed over a substrate  1 . An insulating film  2  serving as a base film is formed over the substrate  1 . The first gate electrode  3  is formed over the insulating film  2 . The first gate insulating film  4  is formed over the first gate electrode  3  to cover the first gate electrode  3 . The oxide semiconductor film  5  is formed over the first gate insulating film  4 . The electrode  6 A and the electrode  6 B are formed over the oxide semiconductor film  5 . The second gate insulating film  7  is formed over the electrode  6 A, the electrode  6 B, and the oxide semiconductor film  5 . The second gate insulating film  7  covers the electrode  6 A, the electrode  6 B, and the oxide semiconductor film  5 . The second gate electrode  8  is formed over the second gate insulating film  7 . 
     In  FIG. 5A , the second gate electrode  8  is electrically connected to the first gate electrode  3 ; however, the second gate electrode  8  is not necessarily to be electrically connected to the first gate electrode  3  ( FIGS. 6A and 6B ). When the second gate electrode  8  is electrically connected to the first gate electrode  3 , the second gate electrode  8  has the same potential as the first gate electrode  3 . On the other hand, when the second gate electrode  8  is not electrically connected to the first gate electrode  3 , the second gate electrode  8  does not always have the same potential as the first gate electrode  3 . 
     The first gate electrode  3  is electrically connected to a scan line driver circuit (not shown). A selection signal from the scan line driver circuit is applied to the first gate electrode  3 . When the first gate electrode  3  is electrically connected to the second gate electrode  8 , the same selection signal is also applied to the second gate electrode  8 . When the first gate electrode  3  is not electrically connected to the second gate electrode  8 , the second gate electrode  8  is electrically connected to another scan line driver circuit and another selection signal from the scan line driver circuit is applied thereto. 
     The electrode  6 A is electrically connected to a signal line driver circuit (not shown). An image signal from the signal line driver circuit is applied to the electrode  6 A. 
     The organic resin film  9  having flatness is formed over the second gate insulating film  7 . The organic resin film  9  is not formed over the second gate electrode  8 . The pixel electrode  10  is formed over the organic resin film  9 . A display medium is formed over the second gate electrode  8 , the second gate insulating film  7 , and the pixel electrode  10  ( FIGS. 14A, 14B, 15A, 15B, 16A, 16B, 22A and 22B ). 
     A storage capacitor is not provided for a display device in this embodiment. Since the off-state current of the transistor  15  is extremely low, the potentials which are applied to the electrode  6 B and the pixel electrode  10  are held without decreasing. Therefore, a storage capacitor is not needed. However, it is needless to say that a storage capacitor may be provided. 
     The first gate insulating film  4  and the second gate insulating film  7  may be in contact with each other in a region where neither the electrode  6 A nor the electrode  6 B is formed ( FIGS. 5B, 6B, and 7 ). When the first gate insulating film  4  and the second gate insulating film  7  are in contact with each other, the oxide semiconductor film  5  is covered with the first gate insulating film  4  and the second gate insulating film  7 , whereby impurities can be prevented from entering the oxide semiconductor film  5 . 
     Features of a display device are described below. The concentration of hydrogen atoms contained in the oxide semiconductor film  5  in which a channel is formed is less than 1×10 16  cm −3 . Therefore, the oxide semiconductor film  5  is a semiconductor which is intrinsic or substantially intrinsic. Descriptions thereof will be made later. 
     The electrode  6 A and the electrode  6 B are covered with the second gate insulating film  7 . The electrode  6 A and the electrode  6 B are protected by the second gate insulating film  7 . 
     As illustrated in  FIG. 5B  and  FIG. 6B , the organic resin film  9  and the pixel electrode  10  are not formed over the transistor  15 . Hydrogen atoms contained in the organic resin film  9  do not affect the transistor  15 . The potential which is applied to the pixel electrode  10  does not affect the transistor  15 . 
     Next, each component of the display device is described below. 
     First, the oxide semiconductor film  5  is described. A channel is formed in the oxide semiconductor film  5 . The oxide semiconductor film  5  may be formed to have a thickness of 2 nm to 200 nm. 
     (Composition) 
     As a material of the oxide semiconductor film  5 , any of the following oxide semiconductors can be used: a four-component metal oxide such as In—Sn—Ga—Zn—O; a three-component metal oxide such as In—Ga—Zn—O, In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O, Al—Ga—Zn—O, or Sn—Al—Zn—O; a two-component metal oxide such as In—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O, or In—Mg—O; In—O, Sn—O, Zn—O, or the like. Further, SiO 2  may be contained in the above oxide semiconductor. 
     As the oxide semiconductor film  5 , a thin film expressed by InMO 3  (ZnO) m  (m&gt;0) can be used. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. Among the oxide semiconductor film  5  expressed by InMO 3  (ZnO) m  (m&gt;0), an oxide semiconductor which includes Ga as M is referred to as an In—Ga—Zn—O oxide semiconductor, and a thin film of the In—Ga—Zn—O oxide semiconductor is also referred to as an In—Ga—Zn—O film. 
     (Hydrogen Concentration) 
     The hydrogen concentration of the oxide semiconductor film  5  measured by secondary ion mass spectrometry is less than 1×10 16  cm −3 , preferably less than 1×10 14  cm −3 . As described above, the oxide semiconductor  5  with reduced hydrogen concentration is a semiconductor which is made intrinsic or substantially intrinsic. Further, it is preferable that the oxide semiconductor  5  does not include an impurity element forming a donor level or an acceptor level. 
     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 secondary ion mass spectrometry analysis in principle. Thus, in the case where distribution of the hydrogen concentration of a film in thickness direction is analyzed by secondary ion mass spectrometry, an average value in a region where the film is provided, the value is not greatly changed, and almost the same intensity can be obtained is employed as the hydrogen concentration. Further, in the case where the thickness of the film is small, a region where almost the same intensity can be obtained cannot be found in some cases due to the influence of the hydrogen concentration of the film adjacent thereto. In this case, the maximum value or the minimum value of the hydrogen concentration of a region where the film is provided is employed as the hydrogen concentration of the film. Furthermore, in the case where a mountain-shaped peak having the maximum value and/or a valley-shaped peak having the minimum value do/does not exist in the region where the film is provided, the value of the inflection point is employed as the hydrogen concentration. 
     The number of hydrogen atoms contained in the oxide semiconductor film  5  is preferably zero. The concentration of hydrogen atoms is equal to or greater than 0 cm −3  and less than 1×10 16  cm −3 . However, it is difficult to detect that the number of hydrogen atoms is zero by secondary ion mass spectrometry. Therefore, it is preferable that the concentration of hydrogen atoms is at least equal to or less than the detection limit of secondary ion mass spectrometry. 
     (Carrier Density) 
     The carrier density of the oxide semiconductor film  5  having the concentration of hydrogen atoms as described above is less than 1×10 12  cm −3  at 300 K. The carrier density of the oxide semiconductor film  5  may be less than 1×10 12  cm −3  at 300 K. It is preferable that the carrier density of the oxide semiconductor film  5  is less than 1×10 10  cm −3  at 300 K. The carrier density can be estimated by the Hall measurement, or the like. The measuring method used here is described in Examples. 
     Here, the intrinsic carrier density of an oxide semiconductor is described. The intrinsic carrier density refers to the carrier density of an intrinsic semiconductor. 
     The intrinsic carrier density contained in a semiconductor, n i , can be obtained by approximating Fermi-Dirac distribution according to Fermi-Dirac statistics with the formula of Boltzmann distribution (see Formula 1). 
     
       
         
           
             
               
                 
                   
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                           N 
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                     ⁢ 
                     
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                               E 
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                   [ 
                   
                     FORMULA 
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     The intrinsic carrier density, n i , can be obtained by the approximation formula is a relational expression of density of an effective state at a conduction band, N c , density of an effective state at a valence band, N v , and a band gap, E g . According to the above Formula, the intrinsic carrier density, n i , of silicon is 1.4×10 10  cm −3  and the intrinsic carrier density, n i , of an oxide semiconductor (here, In—Ga—Zn—O film) is 1.2×10 −7  cm −3 . It is found that the intrinsic carrier density of the oxide semiconductor is extremely lower than that of silicon. 
     The carrier density of the oxide semiconductor film  5  having the concentration of hydrogen atoms as described above is extremely low and the off-state current of the transistor  15  including the oxide semiconductor film  5  is extremely low. 
     Next, structures other than the oxide semiconductor film  5  are described. 
     The substrate  1  has enough heat resistance to withstand heat treatment to be performed later. As the substrate  1 , a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like is used. Alternatively, a substrate formed of an insulator, such as a ceramic substrate, a quartz substrate, a sapphire substrate, or a crystallized glass substrate may be used as the substrate  1 . Further alternatively, a plastic film or the like formed using polyethylene terephthalate, polyimide, an acrylic resin, polycarbonate, polypropylene, polyester, polyvinyl chloride, or the like can be used as long as it has heat resistance high enough to withstand heat treatment to be performed later. 
     For the insulating film  2 , a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or the like is used. It is preferable that substances including hydrogen atoms such as hydrogen, hydroxyl, or moisture are not contained in the insulating film  2 . The insulating film  2  may be formed to have a thickness of 10 nm to 200 nm. The insulating film  2  prevents impurities contained in the substrate  1  from entering the first gate insulating film  4  and the oxide semiconductor film  5 . Note that the insulating film  2  is not necessarily to be formed if it is not necessary to consider impurities contained in the substrate  1 . 
     The first gate electrode  3  is formed in a single layer or a stacked layer using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which includes any of these materials as a main component. It is preferable that substances including hydrogen atoms such as hydrogen, hydroxyl, or moisture are not contained in the first gate electrode  3 . The first gate electrode  3  may be formed to have a thickness of 10 nm to 200 nm. 
     The first gate insulating film  4  is formed in a single layer or a stacked layer using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, a hafnium silicate (HfSiO x ) film, a HfSi x O y  film to which N is added, a hafnium aluminate (HfAlO x ) film to which nitrogen is added, a hafnium oxide film, and/or an yttrium oxide film. By using a high-k material such as a hafnium silicate (HfSiO x ) film, a HfSi x O y  film to which N is added, a hafnium aluminate (HfAlO x ) film to which nitrogen is added, a hafnium oxide film, or an yttrium oxide film, gate leakage can be reduced. It is preferable that substances including hydrogen atoms such as hydrogen, hydroxyl, or moisture are not contained in the first gate insulating film  4 . The first gate insulating film  4  may be formed to have a thickness of 10 nm to 500 nm. 
     A halogen element such as fluorine or chlorine may be contained in the first gate insulating film  4  at a concentration of about 5×10 18  cm −3  to 1×10 20  cm −3 . Substances including hydrogen atoms such as hydrogen, moisture, hydroxyl, or hydride which may be contained in the oxide semiconductor film  5  or the interface between the first gate insulating film  4  and the oxide semiconductor film  5  can be removed by such a halogen element. When as the first gate insulating film  4 , for example, a stacked layer of a silicon nitride film and a silicon oxide film is formed, it is preferable that the silicon oxide film contain a halogen element at the above concentration and is disposed on the side in contact with the oxide semiconductor film  5 . The silicon nitride film prevents impurities such as hydrogen, moisture, hydroxyl, or hydride (also referred to as a hydrogen compound) from entering the silicon oxide film. 
     The electrode  6 A (one of a source electrode and a drain electrode) and the electrode  6 B (the other of the source electrode and the drain electrode) are formed in a single layer or a stacked layer using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, scandium, or yttrium; an alloy material which includes any of these materials as a main component; 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. It is preferable that hydrogen, hydroxyl, or moisture is not contained in the electrode  6 A and the electrode  6 B. The electrode  6 A and the electrode  6 B may be formed to have thicknesses of 10 nm to 500 nm. 
     The second gate insulating film  7  can be formed using the same material as the first gate insulating film  4 . For example, a silicon oxide film is used. It is preferable that hydrogen, hydroxyl, or moisture is not contained in the second gate insulating film  7 . The second gate insulating film  7  may be formed to have a thickness of 10 nm to 200 nm. Note that a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or the like can be used instead of the silicon oxide film. 
     Further, the second gate insulating film  7  may include a defect. The defect can capture hydrogen atoms contained in the oxide semiconductor film  5 . Thus, the number of hydrogen atoms contained in the oxide semiconductor film  5  can be further reduced. 
     Halogen elements such as fluorine or chlorine may be contained in the second gate insulating film  7  at a concentration of about 5×10 18  cm −3  to 1×10 20  cm −3 . Impurities such as hydrogen, moisture, hydroxyl, or hydride which may be contained in the oxide semiconductor film  5  or at the interface between the second gate insulating film  7  and the oxide semiconductor film  5  can be removed by such a halogen element. 
     The second gate electrode  8  is formed as a single layer or a stacked layer using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium or an alloy material which contains any of these materials as a main component. It is preferable that substances including hydrogen atoms such as hydrogen, hydroxyl, or moisture are not contained in the second gate electrode  8 . The second gate electrode  8  may be formed to have a thickness of 10 nm to 200 nm. The second gate electrode  8  may be formed to be thicker than the first gate electrode  3 . The second gate electrode  8  may be formed using the same material as the first gate electrode  3 . 
     The organic resin film  9  is formed using an acrylic resin film having flatness, a polyimide resin film having flatness, or the like. The organic resin film  9  may be formed to have a thickness of 1.0 μm to 2.0 μm. 
     For the pixel electrode  10 , typically, a conductive material having reflectance or light-blocking properties, such as a single material formed using an element selected from aluminum, copper, titanium, tantalum, tungsten, molybdenum, chromium, neodymium, or scandium; an alloy containing any of these elements; a compound (e.g., an oxide or a nitride) containing any of these elements; or the like can be used. Alternatively, as the conductive material having a light-transmitting property, a conductive material having a light-transmitting property, such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added, can be used. A stacked structure containing any of these materials can also be used. The pixel electrode  10  may be formed to have a thickness of 50 nm to 500 nm. 
     Next, the operation of the transistor  15  having the oxide semiconductor film  5  will be described with reference to energy band diagrams. 
       FIG. 7  is a cross-sectional view of a common inverted staggered thin film transistor using an oxide semiconductor film. An oxide semiconductor film (OS) is formed over a gate electrode (GE) with a first gate insulating film (GI) interposed therebetween. A source electrode (S) and a drain electrode (D) are formed over the oxide semiconductor film and the first gate insulating film. A passivation film (PI) is formed over the source electrode and the drain electrode. In a region where neither the source electrode nor the drain electrode is not formed, the first gate insulating film and the passivation film are in contact with each other. 
       FIGS. 8A and 8B  are energy band diagrams (schematic diagrams) of a cross-section taken along line A-A′ in  FIG. 7 .  FIG. 8A  illustrates the case where the potential of a voltage applied to the source is equal to the potential of a voltage applied to the drain (VD=0 V), and  FIG. 8B  illustrates the case where a positive potential is applied to the drain (VD&gt;0) with respect to the source. 
       FIGS. 9A and 9B  are energy band diagrams (schematic diagrams) of a cross-section taken along line B-B′ in  FIG. 7 .  FIG. 9A  illustrates a state where a positive potential (+V G ) is applied to a gate electrode (GE) and an on state where carriers (electrons) flow between the source electrode and the drain electrode.  FIG. 9B  illustrates a state where a negative potential (−V G ) is applied to the gate electrode (GE) and an off state (a minority carrier does not flow). 
       FIG. 10  illustrates a relation between a work function (φ M ) of a vacuum level and a metal and electron affinity (χ) of an oxide semiconductor. 
     Because metal is degenerated, the conduction band and the Fermi level correspond to each other. On the other hand, a conventional oxide semiconductor is typically an n-type semiconductor, in which case the Fermi level (Ef) is away from the intrinsic Fermi level (Ei) located in the middle of a band gap and is located closer to the conduction band. Note that it is known that hydrogen is a donor in an oxide semiconductor and is one factor causing an oxide semiconductor to be an n-type semiconductor. 
     In contrast, the oxide semiconductor film in this embodiment is made an intrinsic (i-type) or substantially intrinsic oxide semiconductor film obtained by removal of hydrogen atoms, which is n-type impurities, from an oxide semiconductor film to increase the purity so that an impurity other than a main component of the oxide semiconductor film includes as little as possible. In other words, a feature is that a highly-purified i-type (an intrinsic semiconductor), or a semiconductor close thereto, is obtained not by adding an impurity but by removing hydrogen atoms as much as possible. In this manner, the Fermi level (Ef) can be the same level as the intrinsic Fermi level (Ei). 
     In the case where the band gap (Eg) of an oxide semiconductor is 3.15 eV, the electron affinity (χ) is said to be 4.3 eV. When the source electrode and the drain electrode are formed using titanium, for example, the work function of titanium is substantially equal to the electron affinity (χ) of the oxide semiconductor. In that case, a Schottky barrier for electrons is not formed at an interface between the metal and the oxide semiconductor. 
     In other words, in the case where the work function of metal (φ M ) and the electron affinity (χ) of the oxide semiconductor are equal to each other and the metal and the oxide semiconductor are in contact with each other, an energy band diagram (a schematic diagram) as illustrated in  FIG. 8A  is obtained. 
     In  FIG. 8B , a black circle (•) represents an electron, and when a positive potential is applied to the drain, the electron is injected into the oxide semiconductor film over the barrier (h) and flows toward the drain. In that case, the height of the barrier (h) is changed depending on gate voltage and drain voltage. When positive drain voltage is applied, the height (h) of the barrier is smaller than the height of the barrier of  FIG. 8A  without application of voltage, that is, ½ of the band gap (Eg). 
     At this time, the electron moves in the bottom, which is energetically stable, on the oxide semiconductor film side at the interface between the gate insulating film and the highly purified oxide semiconductor film as illustrated in  FIG. 9A . 
     In  FIG. 9B , when a negative potential (reverse bias) is applied to the gate electrode (GE), holes which are minority carriers are substantially zero; therefore, current is substantially close to zero. 
     Further, the band gap (Eg) of an oxide semiconductor is approximately three times larger than that of silicon in the case where the band gap (Eg) of the oxide semiconductor is 3.15 eV. The oxide semiconductor having such an Eg is resistant to an avalanche breakdown. In a transistor using an oxide semiconductor film in which the number of hydrogen atoms is reduced, voltage applied to a drain electrode can be larger than that of a transistor using an oxide semiconductor film in which the number of hydrogen atoms is not reduced or a transistor using silicon. 
     As described above, by the oxide semiconductor film  5  in which the number of hydrogen atoms is reduced, the operation of the transistor  15  can be favorable. 
     Next, a method for manufacturing a display device is described. 
     The insulating film  2  serving as a base film is formed over the substrate  1  by a plasma CVD method, a sputtering method, or the like ( FIG. 11A ). Note that the insulating film  2  is preferably formed by a sputtering method in order not to include a large amount of hydrogen. 
     An example in which a silicon oxide film is formed is described. A sputtering gas containing high purity oxygen, from which hydrogen, water, hydroxyl, or hydride (also referred to as a hydrogen compound), or the like is removed, is introduced, and a silicon semiconductor target is used, whereby a silicon oxide film is formed over the substrate  1 . The temperature of the substrate  1  may be room temperature, or the substrate  1  may be heated. 
     Alternatively, a silicon oxide film is formed as follows: quartz (preferably synthesized quart) is used as the target; the substrate temperature is 108° C.; the distance between the target and the substrate is 60 mm; the pressure is 0.4 Pa; the high-frequency power is 1.5 kW; the atmosphere is oxygen and argon (flow rate ratio of oxygen to argon is 25 sccm:25 sccm=1:1); and an RF sputtering method is used. As a sputtering gas, oxygen or a mixed gas of oxygen and argon is used. 
     The insulating film  2  is preferably formed removing moisture remaining in the deposition chamber. This is for preventing hydrogen, water, hydroxyl, hydride, or the like from being contained in the insulating film  2 . 
     In order to remove residual moisture from the treatment chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo pump provided with a cold trap. In the deposition chamber in which exhaustion is performed with the use of a cryopump, hydrogen, water, hydroxyl, hydride, or the like, is exhausted. Accordingly, the concentration of impurities included in the insulating film  2  formed in the deposition chamber can be reduced. 
     It is preferable to use a high-purity gas from which an impurity such as hydrogen, water, hydroxyl, or hydride is removed to a level of parts per million or parts per billion, as a sputtering gas. 
     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. 
     Furthermore, as a deposition method using a sputtering method, there are also 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, and a bias sputtering method in which voltage is also applied to a substrate during deposition. 
     As the sputtering in this specification, the above-described sputtering device and the sputtering method can be employed as appropriate. 
     In the case where a silicon nitride film and a silicon oxide film are stacked to form the insulating film  2 , the silicon nitride film and the silicon oxide film are formed in the same deposition chamber with the use of a common silicon target. First, a sputtering gas including nitrogen is introduced, and a silicon target provided in the deposition chamber is used, whereby the silicon nitride film is formed. Then, the sputtering gas is switched to a sputtering gas including oxygen, and the same silicon target is used, whereby the silicon oxide film is formed. The silicon nitride film and the silicon oxide film can be formed in succession without being exposed to air; therefore, impurities such as hydrogen, water, hydroxyl, or hydride can be prevented from being adsorbed on the surface of the silicon nitride film. 
     A conductive film is formed over the insulating film  2 , and then etching is performed on the conductive film with the use of a resist mask formed in a photolithography process, so that the first gate electrode  3  is formed ( FIG. 11A ). Note that the conductive film is preferably formed by a sputtering method in order to prevent hydrogen, hydroxyl, or moisture from being contained in the conductive film. An end portion of the first gate electrode  3  preferably has a tapered shape because coverage with the first gate insulating film  4  stacked later can be improved. 
     The first gate insulating film  4  is formed over the first gate electrode  3 . Note that the first gate insulating film  4  is preferably formed by a sputtering method in order to prevent hydrogen, hydroxyl, or moisture from being contained in the conductive film. Therefore, as pretreatment for deposition, it is preferable that the substrate  1  provided with the first gate electrode  3  be preheated in a preheating chamber of a sputtering apparatus and impurities such as hydrogen, water, hydroxyl, or hydride adsorbed on the substrate  1  be eliminated and evacuated. The preheat temperature is equal to or greater than 100° C. and equal to or less than 400° C., preferably equal to or greater than 150° C. and equal to or less than 300° C. Note that as an evacuation means, a cryopump is preferably provided in the preheating chamber. Note that this preheating treatment can be omitted. 
     For example, in the case where a silicon oxide film is formed as the first gate insulating film  4 , 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. 
     The oxide semiconductor film is formed over the first gate insulating film  4  by a sputtering method. Note that before the oxide semiconductor film is formed by a sputtering method, powdery substances (also referred to as particles or dust) attached on a surface of the first gate insulating film  4  are 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 voltage to a target side, a high-frequency power source is used for application of voltage to a substrate side in an argon atmosphere to generate plasma in the vicinity of the substrate and modify a surface. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used. 
     A target including an oxide semiconductor is used. For example, a target of a metal oxide containing zinc oxide as a main component is used. As another example of a metal oxide target, an oxide semiconductor target containing In, Ga, and Zn (a composition ratio of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [mol %], that is, In:Ga:Zn=1:1:0.5 [atom %]) can be used. In addition, as the oxide semiconductor target containing In, Ga, and Zn, a target having a composition ratio of In:Ga:Zn=1:1:1 [atom %] or In:Ga:Zn=1:1:2 [atom %] can be used. The filling rate of the oxide semiconductor target is equal to or greater than 90% and equal to or less than to 100%, preferably equal to or greater than 95% and equal to or less than 99.9%. With the use of the oxide semiconductor target with a high filling rate, a dense oxide semiconductor film is formed. A target containing Sift at 2 wt % or more and 10 wt % or less may be used. 
     The oxide semiconductor film is deposited in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere containing a rare gas (typically argon) and oxygen. 
     It is preferable to use a high-purity gas from which an impurity such as hydrogen, water, hydroxyl, or hydride is removed to by a level of parts per million or parts per billion, as a sputtering gas. 
     The substrate  1  is held in a deposition chamber kept under reduced pressure, a sputtering gas from which an impurity such as hydrogen, water, hydroxyl, or hydride is removed is introduced while removing residual moisture in the deposition chamber, and the above target is used, whereby the oxide semiconductor film is formed. In order to remove residual moisture from the deposition chamber, an adsorption-type vacuum pump is preferably used. For example, a cryopump, an ion pump, or a titanium sublimation pump is preferably used. The evacuation unit may be a turbo pump provided with a cold trap. In a deposition chamber which is evacuated using a cryopump, for example, hydrogen atoms, compounds including hydrogen atoms such as water (H 2 O) (more preferably, compounds including carbon atoms as well), or the like are exhausted; therefore, the concentration of impurities contained in the oxide semiconductor film which is deposited in the deposition chamber can be reduced. The substrate  1  may be heated to a temperature of less than 400° C. when the oxide semiconductor film is formed. 
     An example of the deposition condition is as follows: the temperature of the substrate  1  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 containing oxygen and argon (the flow ratio of oxygen to argon is 15 sccm:30 sccm). Note that a pulse direct current (DC) power is preferable because particles can be reduced and the film thickness can be uniform. The preferable thickness of the oxide semiconductor film is equal to or greater than 2 nm and equal to or less than 200 nm. Note that an appropriate thickness differs depending on an oxide semiconductor material, and the thickness may be set as appropriate depending on the material. 
     Then, the oxide semiconductor film is etched with a resist mask that is formed by a photolithography process, so that the oxide semiconductor film  5  is formed ( FIG. 11A ). Note that the etching of the oxide semiconductor film may be dry etching, wet etching, or both dry etching and wet etching. 
     As the etching gas for dry etching, a gas containing chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron trichloride (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. 
     As an etchant used for wet etching, a solution obtained by mixing phosphoric acid, acetic acid, and nitric acid, an ammonia peroxide mixture (hydrogen peroxide water at 31 wt %:ammonia water at 28 wt %:water=5:2:2), or the like can be used. In addition, ITO07N (manufactured by KANTO CHEMICAL CO., INC.) may also be used. 
     Heat treatment (also referred to as a first heat treatment) may be performed on the oxide semiconductor film  5 . The temperature of the heat treatment is equal to or greater than 400° C. and equal to or less than 750° C., preferably equal to or greater than 400° C. and less than the strain point of the substrate  1 . The heat treatment time may be 0.5 hours to 5 hours. By this heat treatment, hydrogen atoms contained in the oxide semiconductor film  5  can be removed. For example, in an electric furnace, heat treatment is performed on the oxide semiconductor film  5  in a nitrogen atmosphere at 450° C. for one hour or at 650° C. for one hour, and then, the oxide semiconductor film is not exposed to the air, whereby hydrogen atoms can be prevented from entering the oxide semiconductor film  5 . Note that the oxide semiconductor film  5  is crystallized in some cases through the first heat treatment. TEM analysis in the case where heat treatment is performed at 650° C. for one hour is shown in Example 2. 
     The apparatus for the heat treatment is not limited to the electric furnace and may be provided with a device for heating the oxide semiconductor film  5 , using 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 the oxide semiconductor film  5  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. As the gas, an inert gas which does not react with the oxide semiconductor film  5  by heat treatment, such as nitrogen or a rare gas such as argon is used. 
     GRTA heats the oxide semiconductor film  5  for 2 minutes to 5 minutes in an inert gas atmosphere at a temperature as high as 650° C. to 700° C. With GRTA, high-temperature heat treatment for a short period of time can be achieved. 
     Note that it is preferable that in the heat treatment, hydrogen, water, hydroxyl, or hydride, or the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. It is preferable that the purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). 
     As described above, depending on the conditions of the heat treatment or the material of the oxide semiconductor film, the oxide semiconductor film may be crystallized to be a microcrystalline film or a polycrystalline film. For example, the oxide semiconductor film may crystallize to become a microcrystalline oxide semiconductor film having a degree of crystallization of 90% or more, or 80% or more. Further, depending on the conditions of the first heat treatment or the material of the oxide semiconductor film, the oxide semiconductor film may become an amorphous oxide semiconductor film which does not contain a crystalline component. Furthermore, the oxide semiconductor film may become an oxide semiconductor film in which a microcrystalline portion (with a grain diameter equal to or greater than 1 nm and equal to or less than or 20 nm (typically equal to or greater than 2 nm and equal to or less than 4 nm)) is mixed in an amorphous oxide semiconductor. 
     Moreover, after the above heat treatment is performed, a second heat treatment may be performed in atmosphere of oxygen which does not contain hydrogen, water, hydroxyl, hydride, or the like, or in an atmosphere of nitrogen and oxygen which does not contain hydrogen, water, hydroxyl, hydride, or the like. Since oxygen in the oxide semiconductor film  5  is removed in some cases through the above first heat treatment, oxygen can be introduced in the oxide semiconductor film  5  through the second heat treatment. 
     Note that it is preferable to perform reverse sputtering before formation of a conductive film in the following step so that a resist residue and the like attached to surfaces of the oxide semiconductor film  5  and the first gate insulating film  4  can be removed. 
     The conductive film is formed by a sputtering method over the oxide semiconductor film  5  and the first gate insulating film  4 . The above pretreatment is preferably performed before the film formation so that hydrogen, hydroxyl, and moisture are contained as little as possible in the conductive film. The conductive film is etched with a resist mask that is formed by a photolithography step, so that the electrode  6 A (one of a source electrode and a drain electrode) and the electrode  6 B (the other of the source electrode and the drain electrode) are formed ( FIG. 11B ). 
     Next, plasma treatment using a gas such as N 2 O, N 2 , or Ar is performed to remove water or the like adsorbed on a surface of the oxide semiconductor film  5  which is exposed. Alternatively, plasma treatment is performed using a mixture gas of oxygen and argon. 
     After the plasma treatment is performed, the second gate insulating film  7  is formed over the oxide semiconductor film  5 , the electrode  6 A, and the electrode  6 B, without exposure to the air ( FIG. 11C ). In a region where the oxide semiconductor film  5  is not in contact with any one of the electrode  6 A and the electrode  6 B, the oxide semiconductor film  5  is in contact with the second gate insulating film  7 . Further, the second gate insulating film  7  covers the electrode  6 A and the electrode  6 B. 
     As the second gate insulating film  7 , a silicon oxide film having a defect, for example, is formed as follows: a sputtering gas containing high-purity oxygen in which hydrogen and moisture are removed is introduced while heating the substrate  1  at room temperature or at a temperature of less than 100° C., and a silicon semiconductor target is used. 
     For example, a silicon oxide film is formed by a pulsed DC sputtering method with use of a boron-doped silicon target which has a purity of 6N (a resistivity of 0.01 Ωcm, in which the distance between the substrate and the target is 89 mm, the pressure is 0.4 Pa, the direct current (DC) power is 6 kW, and the atmosphere is an oxygen atmosphere (the proportion of oxygen flow is 100%). Note that instead of a silicon target, quartz (preferably, synthetic quartz) can be used as a target used when the silicon oxide film is formed. As a sputtering gas, oxygen or a mixed gas of oxygen and argon is used. 
     In this case, it is preferable that the second gate insulating film  7  be formed while residual moisture in the treatment chamber is removed. This is for preventing hydrogen, hydroxyl, or moisture from being contained in the oxide semiconductor film  5  and the second gate insulating film  7 . In order to remove residual moisture from the treatment chamber, an adsorption-type vacuum pump is preferably used. 
     Heat treatment may be performed at 100° C. to 400° C. while the second gate insulating film  7  and the oxide semiconductor film  5  are in contact with each other. The second gate insulating film  7  has a lot of defects. By this heat treatment, an impurity such as hydrogen, moisture, hydroxyl, or hydride included in the oxide semiconductor film  5  can be diffused into the second gate insulating film  7  and the number of hydrogen atoms contained in the oxide semiconductor film  5  can be further reduced. 
     A conductive film is formed over the second gate insulating film  7 . The conductive film is etched with a resist mask that is formed by a photolithography step, so that the second gate electrode  8  is formed ( FIG. 11C ). Note that the conductive film is preferably formed by a sputtering method in order to prevent hydrogen, hydroxyl, or moisture from being contained in the conductive film. In the case where the second gate electrode  8  is electrically connected to the first gate electrode  3 , a contact hole is formed in the second gate insulating film  7  and the first gate insulating film  4  before the conductive film is formed. The contact hole reaches the first gate electrode  3 . The conductive film is formed after the contact hole is formed. The second gate electrode  8  is formed of the conductive film. 
     An organic resin film is formed over the second gate insulating film  7  as a planarizing film. The organic resin film is formed by a spin-coating method or the like. The organic resin film is selectively etched to form the organic resin film  9  ( FIG. 12A ). 
     An opening reaching the electrode  6 B is formed in the organic resin film  9  and the second gate insulating film  7 . The pixel electrode  10  is formed in the opening ( FIG. 12B ). The pixel electrode  10  is electrically connected to the electrode  6 B. 
     A display medium is provided over the pixel electrode  10 , the second gate electrode  8 , and the second gate insulating film  7  ( FIGS. 14A, 15A, 16A, and 22A ). 
     An insulating film  21  serving as a passivation film may be formed to have a thickness of 10 nm to 200 nm over the second gate electrode  8  and the second gate insulating film  7  ( FIG. 13 ). A silicon oxide film, a silicon nitride film, or the like can be used for the insulating film  21 . As the insulating film  21 , a silicon oxide film, a silicon nitride film, or the like, is formed as follows: a sputtering gas containing high-purity oxygen in which hydrogen and moisture are removed is introduced while heating the substrate  1  at room temperature or at a temperature of less than 100° C., and a silicon semiconductor target is used. 
     The organic resin film  9  is formed over the insulating film  21  as a planarizing film. The opening reaching the electrode  6 B is formed in the organic resin film  9 , the insulating film  21 , and the second gate insulating film  7 . The pixel electrode  10  is formed in the opening ( FIG. 13 ). A display medium is provided over the pixel electrode  10  and the insulating film  21  ( FIGS. 14B, 15B, 16B, and 22B ). 
     Hereinafter, as a display medium, liquid crystal, EL, and electronic paper are described. Note that the display medium is not limited to liquid crystal, EL, and electronic paper. 
     &lt;Liquid Crystal&gt; 
       FIG. 14A  illustrates a case where the display medium is liquid crystal. A first alignment film  22  is formed over the pixel electrode  10 . The first alignment film  22  is also formed over and in contact with the second gate electrode  8  and the second gate insulating film  7 . A liquid crystal layer  23  is formed over the first alignment film  22 . A second alignment film  24  is formed over the liquid crystal layer  23 . The liquid crystal layer  23  is provided between the first alignment film  22  and the second alignment film  24 . An electrode  25  serving as a counter electrode is formed over the second alignment film  24  and a substrate  26  serving as a counter substrate is formed over the electrode  25 . The first alignment film  22  and the second alignment film  24  are not necessarily formed as long as the liquid crystal layer  23  is aligned. A spacer may be provided in order to keep a cell gap. 
     As for a mode which is used in the liquid crystal display, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be employed. Alternatively, a blue phase mode may be used. 
     The first alignment film  22 , the second alignment film  24 , the electrode  25 , and the substrate  26  are formed using known materials. 
     The first alignment film  22 , the liquid crystal layer  23 , the second alignment film  24 , and the electrode  25  are formed by known methods. 
     As described above, the insulating film  21  serving as a passivation film may be formed ( FIG. 14B ). The first alignment film  22  is formed over and in contact with the insulating film  21  and the pixel electrode  10 . 
     &lt;EL&gt; 
       FIG. 15A  illustrates a case where the display medium is EL. An EL layer  31  is formed over the pixel electrode  10 . An electrode  32  serving as a counter electrode is formed over the EL layer  31 . The EL layer  31  and the electrode  32  need not to be formed over the second gate electrode  8  and the second gate insulating film  7 . A sealing material  33  is formed over and in contact with the second gate electrode  8 , the second gate insulating film  7 , and the electrode  32 . A substrate  34  serving as a sealing substrate is formed over the sealing material  33 . 
     EL layers are classified into an organic EL layer and an inorganic EL layer. Further, inorganic EL layers are classified into a dispersion type inorganic EL and a thin-film type inorganic EL. 
     In the case of an organic EL layer, the EL layer  31  includes, for example, a hole-injection layer, a hole-transport layer, a light-emitting layer, an electron-transport layer, and an electron-injection layer. In the light-emitting layer, a dopant material is added to a host material. As for a dopant material, a phosphorescent light-emitting material or a fluorescent light-emitting material is used. 
     In the case of a dispersion type inorganic EL, the EL layer  31  includes a light-emitting layer in which particles of the light-emitting material are dispersed in a binder. In the case of a thin-film type inorganic EL, the EL layer  31  has a structure in which a light-emitting layer is provided between dielectric layers. 
     As for the sealing material  33 , an ultraviolet curable resin or a thermosetting resin can be used, as well as an inert gas such as nitrogen or argon. For example, PVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate) can be used. 
     The EL layer  31 , the electrode  32 , the sealing material  33 , and the substrate  34  are formed using known materials. In addition, the EL layer  31 , the electrode  32 , and the sealing material  33  are formed by known methods. 
     As described above, the insulating film  21  serving as a passivation film may be formed ( FIG. 15B ). The sealing material  33  is formed over and in contact with the insulating film  21  and the electrode  32 . 
     &lt;Electronic Paper&gt; 
       FIG. 16A  illustrates a case where the display medium is electronic paper. In  FIG. 16A , a twisting ball display system is used. The twist ball display system refers to a method in which spherical particles each colored in black and white which are arranged between the pixel electrode  10  and the electrode  44  are used for a display element, and a potential difference is generated between the pixel electrode  10  and the electrode  44  to control the orientation of the spherical particles, so that display is performed. 
     A filler  43  such as a resin is formed over and in contact with the pixel electrode  10 , the second gate electrode  8 , and the second gate insulating film  7 . In the filler  43 , each of spherical particles including a cavity  42  is provided. The cavity  42  includes a black region  41  and a white region  40 . The electrode  44  serving as a counter electrode is formed over the filler  43 . A substrate  45  serving as a counter substrate is provided over the electrode  44 . 
     Instead of the twist ball display system, an electrophoretic display system can be used ( FIG. 22A ). An electronic ink layer  51  is formed over and in contact with the pixel electrode  10 , the second gate electrode  8 , and the second gate insulating film  7 . In the electronic ink layer  51 , a microcapsule  52  having a diameter of about 10 μm to 200 μm in which positively charged white microparticles and negatively charged black microparticles are encapsulated, is provided. An electrode  53  serving as a counter electrode is formed over the electronic ink layer  51  and a substrate  54  serving as a counter substrate is provided over the electrode  53 . 
     The filler  43 , the cavity  42 , the electrode  44 , the substrate  45 , the electronic ink layer  51 , the microcapsule  52 , the electrode  53  and the substrate  54  are formed using known materials. In addition, the filler  43 , the cavity  42 , the electrode  44 , the electronic ink layer  51 , the microcapsule  52 , and the electrode  53  are formed by known methods. 
     As described above, the insulating film  21  serving as a passivation film may be formed ( FIG. 16B  and  FIG. 22B ). The filler  43  or the electronic ink layer  51  is formed over and in contact with the insulating film  21  and the pixel electrode  10 . 
     A display device according to the present invention can be applied to a variety of electronic devices (including an amusement machine). Examples of electronic devices are a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a digital camera, a digital video camera, a digital photo frame, a mobile phone (also referred to as a portable telephone or a mobile phone device), a portable game console, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like. 
       FIG. 23A  illustrates an example of a mobile phone. A mobile phone  1100  is provided with a display portion  1102  incorporated in a housing  1101 , operation buttons  1103 , an external connection port  1104 , a speaker  1105 , a microphone  1106 , and the like. The above display device is provided for the display portion  1102 . 
       FIG. 23B  illustrates an example of a portable information terminal. The portable information terminal includes a housing  2800  and a housing  2801 . The housing  2800  includes a display panel  2802 , a speaker  2803 , a microphone  2804 , a pointing device  2806 , a camera lens  2807 , an external connection terminal  2808 , and the like. The housing  2801  includes a keyboard  2810 , an external memory slot  2811 , and the like. Further, an antenna is incorporated in the housing  2801 . The above display device is provided for the display panel  2802 . 
       FIG. 24A  illustrates an example of a television set. In the television set  9600 , a display portion  9603  is incorporated in a housing  9601 . The display portion  9603  can display images. Here, the housing  9601  is supported by a stand  9605 . 
     The television set  9600  can be operated with an operation switch of the housing  9601  or a separate remote controller  9610 . Channels and volume can be controlled with an operation key  9609  of the remote controller  9610  so that an image displayed on the display portion  9603  can be controlled. Furthermore, the remote controller  9610  may be provided with a display portion  9607  for displaying data output from the remote controller  9610 . The above display device is provided for the display portion  9603  and the display portion  9607 . 
       FIG. 24B  illustrates an example of a digital photo frame. For example, in the digital photo frame  9700 , a display portion  9703  is incorporated in a housing  9701 . The display portion  9703  can display a variety of images. For example, the display portion  9703  can display data of an image taken with a digital camera or the like and function as a normal photo frame. The above display device is provided for the display portion  9703 . 
       FIG. 25  illustrates a portable game console including a housing  9881  and a housing  9891  which are jointed with a connector  9893  so as to be opened and closed. A display portion  9882  and a display portion  9883  are incorporated in the housing  9881  and the housing  9891 , respectively. The above display device is provided for the display portion  9883 . 
       FIG. 26  illustrates an example of an electronic book. For example, the electronic book  2700  includes a housing  2701  and a housing  2703 . The housing  2701  and the housing  2703  are combined with a hinge  2711  so that the electronic book  2700  can be opened and closed with the hinge  2711  as an axis. With such a structure, the electronic book  2700  can operate like a paper book. 
     A display portion  2705  and a display portion  2707  are incorporated in the housing  2701  and the housing  2703 , respectively. The display portion  2705  and the display portion  2707  may display one image or different images. In the case where the display portion  2705  and the display portion  2707  display different images, for example, text can be displayed on a display portion on the right side (the display portion  2705  in  FIG. 26 ) and graphics can be displayed on a display portion on the left side (the display portion  2707  in  FIG. 26 ). 
       FIG. 26  illustrates an example in which the housing  2701  is provided with an operation portion and the like. For example, the housing  2701  is provided with a power switch  2721 , an operation key  2723 , a speaker  2725 , and the like. With the operation key  2723 , pages can be turned. Note that a keyboard, a pointing device, or the like can also be provided on the same face as the display portion of the housing. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, a terminal that can be connected to various cables such as an AC adapter and a USB cable, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Moreover, the electronic book  2700  may have a function of an electronic dictionary. 
     The above display device is provided for the display portion  2705  and the display portion  2707 . 
     Example 1 
     In this example, a method for calculating the carrier density of an oxide semiconductor film is described with reference to  FIGS. 17, 18A and 18B . 
     First, the structure of a sample used in a C-V (Capacitance-Voltage) measurement is described with reference to  FIG. 17 . 
     A titanium film  503  with a thickness of 300 nm was formed by a sputtering method over a glass substrate  501  and a titanium nitride film  505  with a thickness of 100 nm was formed by a sputtering method thereover. 
     As an oxide semiconductor film  507  in which the number of hydrogen atoms were reduced, an In—Ga—Zn—O film with a thickness of 2000 nm was formed by a sputtering method over the titanium nitride film  505 . The deposition condition at this time was as follows: as the sputtering gas, Ar at a flow rate of 30 sccm and oxygen at a flow rate of 15 sccm were used, the distance between the target and the substrate was 60 mm, the direct current (DC) power was 0.5 kW, and the deposition atmosphere temperature was room temperature. 
     Next, a silicon oxynitride film  509  with a thickness of 300 nm was formed by a CVD method and a silver film  511  with a thickness of 300 nm was formed thereover. 
     Next, C-V measurement results of the sample at 300 K are shown in  FIG. 18A . According to the measurement results shown in  FIG. 18A , a curve of C −2  relative to the voltage is shown in  FIG. 18B . Here, the carrier density can be obtained by substituting the gradient of a curve of C −2  in a weak inversion state of the sample into Formula 2. In addition, in  FIG. 18B , a curve of C −2  is indicated by a solid line and the gradient of C −2  in a weak inversion state is indicated by a broken line. The gradient of the curve was −1.96×10 18  F −2  V −1 . 
     
       
         
           
             
               
                 
                   
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     Note that e is the amount of charge per electron, ∈ is relative permittivity, ∈ 0  is permittivity of vacuum, and N d  is carrier density. 
     According to Formula 2, the carrier density of the oxide semiconductor in this example was 6×10 10  cm −3 . Accordingly, it is found that the carrier density of the oxide semiconductor in this example is extremely low. 
     Example 2 
     In this example, with regard to an oxide semiconductor film in which the number of hydrogen atoms are reduced by heat treatment, the results of TEM analysis are described. 
     First, a method for manufacturing a sample is described. 
     An oxide semiconductor film was formed by a sputtering method over a substrate  601 . 
     Here, as the substrate  601 , EAGLE XG substrate (manufactured by Corning Incorporated) was used. As the oxide semiconductor film, an In—Ga—Zn—O film  603  was deposited with the use of an oxide semiconductor target of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1. The sample is referred to as sample B which is a comparative example. The number of hydrogen atoms is not reduced in the sample B. 
     Next, heat treatment was performed on the In—Ga—Zn—O film  603  at 650° C. for one hour in a nitrogen gas atmosphere with the use of an electric furnace, whereby the number of hydrogen atoms was reduced. The In—Ga—Zn—O film on which heat treatment was performed is referred to as an oxide semiconductor film  605 . The sample is referred to as sample A. 
     A cross section of each sample was observed with a high-resolution transmission electron microscope (TEM: “H9000-NAR” manufactured by Hitachi, Ltd.) at an acceleration voltage of 300 kV to examine a crystalline state.  FIGS. 19A and 19B  show cross-sectional photographs of the sample A and  FIGS. 20A and 20B  show cross-sectional photographs of the sample B. Note that  FIG. 19A  and  FIG. 20A  are low magnification photographs (two million-fold magnification) and  FIG. 19B  and  FIG. 20B  are high magnification photographs (four million-fold magnification). 
     A continuous lattice image was observed in a superficial portion in a cross section of the sample A on which heat treatment was performed in an electric furnace at 650° C. for one hour, as shown in  FIGS. 19A and 19B . In particular, in the high magnification photograph of  FIG. 19B , a clear lattice image is observed in a region surrounded by a white frame, and the existence of crystals whose crystal axes are aligned is indicated. Accordingly, it is found that the superficial portion of the In—Ga—Zn—O-based non-single-crystal film is crystallized by performing heat treatment in an electric furnace at 650° C. for one hour and a crystalline region is provided. Note that a clear continuous lattice image was not observed in a region except for the superficial portion and a state where microcrystalline particles exist here and there in an amorphous region was found. The particle size of the microcrystals was equal to or greater than 2 nm and equal to or less than or 4 nm. 
     On the other hand, a clear lattice image was not observed in any region in a thickness direction in the cross-sectional photographs of  FIGS. 20A and 20B  (the sample B) and it was found that the sample B is amorphous. 
     Next,  FIG. 21A  shows an enlarged photograph of the superficial portion of the sample A on which heat treatment was performed in an electric furnace at 650° C. for one hour and  FIG. 21B  shows an electron diffraction pattern of a crystalline region. Directional arrows  1  to  5  indicating directions where lattice images are aligned are illustrated in the enlarged photograph of the superficial portion ( FIG. 21A ), and needle crystals are grown in a direction perpendicular to a surface of the film. The electron diffraction patterns shown in  FIGS. 21B, 21C, 21D, 21E, and 21F  were observed at positions indicated by the arrows  1 ,  2 ,  3 ,  4 , and  5 , respectively, and a c-axis orientation is found. 
     From the results of analysis, it is found that the superficial portion of the sample on which heat treatment is performed in an electric furnace at 650° C. for one hour has a crystal region. 
     This application is based on Japanese Patent Application serial no. 2009-276454 filed with Japan Patent Office on Dec. 4, 2009, the entire contents of which are hereby incorporated by reference.