Abstract:
Homogeneity and stability of electric characteristics of a thin film transistor included in a circuit are critical for the performance of a display device including said circuit. An object of the invention is to provide an oxide semiconductor film with low hydrogen content and which is used in an inverted staggered thin film transistor having well defined electric characteristics. In order to achieve the object, a gate insulating film, an oxide semiconductor layer, and a channel protective film are successively formed with a sputtering method without being exposed to air. The oxide semiconductor layer is formed so as to limit hydrogen contamination, in an atmosphere including a proportion of oxygen. In addition, layers provided over and under a channel formation region of the oxide semiconductor layer are formed using compounds of silicon, oxygen and/or nitrogen.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device which has a circuit including a thin film transistor (hereinafter also referred to as a TFT) in which a channel formation region is formed using an oxide semiconductor film and a method for manufacturing the semiconductor device. For example, the present invention relates to an electronic device having an electro-optical device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element. 
     It is to be noted that the semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic devices are all semiconductor devices. 
     2. Description of the Related Art 
     Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a light-transmitting electrode material which is needed for a liquid crystal display or the like. 
     Some metal oxides have semiconductor characteristics. Examples of metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed using such metal oxides having semiconductor characteristics have already been disclosed (Patent Documents 1 to 4, Non-Patent Document 1). 
     Further, not only single oxides but also multiple oxides are known as metal oxides. For example, InGaO 3 (ZnO) m  (m is a natural number) which is a homologous compound is known as a multiple oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4). 
     In addition, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can be used as a channel layer of a thin film transistor (Patent Document 5, and Non-Patent Documents 5 and 6). 
     REFERENCE 
     
         
         [Patent Document 1] Japanese Published Patent Application No. S60-198861 
         [Patent Document 2] Japanese Published Patent Application No. H8-264794 
         [Patent Document 3] Japanese Translation of PCT International Application No. H11-505377 
         [Patent Document 4] Japanese Published Patent Application No. 2000-150900 
         [Patent Document 5] Japanese Published Patent Application No. 2004-103957 
         [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M. Wolf, “A ferroelectric transparent thin-film transistor”,  Appl. Phys. Lett.,  17 Jun. 1996, Vol. 68, pp. 3650-3652 
         [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The Phase Relations in the In 2 O 3 —Ga 2 ZnO 4 —ZnO System at 1350° C.”,  J. Solid State Chem.,  1991, Vol. 93, pp. 298-315 
         [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura, “Syntheses and Single-Crystal Data of Homologous Compounds, In 2 O 3 (ZnO) m  (m=3, 4, and 5), InGaO 3 (ZnO) 3 , and Ga 2 O 3 (ZnO) m  (m=7, 8, 9, and 16) in the In 2 O 3 —ZnGa 2 O 4 —System”,  J. Solid State Chem.,  1995, Vol. 116, pp. 170-178 
         [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M. Isobe, “Syntheses and crystal structures of new homologous compounds, indium iron zinc oxides (InFeO 3 (ZnO) m ) (m:natural number) and related compounds”,  KOTAI BUTSURI  ( SOLID STATE PHYSICS ), 1993, Vol. 28, No. 5, pp. 317-327 
         [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and H. Hosono, “Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor”,  SCIENCE,  2003, Vol. 300, pp. 1269-1272 
         [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors”,  NATURE,  2004, Vol. 432, pp. 488-492 
       
    
     SUMMARY OF THE INVENTION 
     The field effect mobility of a thin film transistor in which a channel formation region is provided in an oxide semiconductor is higher than that of a thin film transistor using amorphous silicon. 
     Such an oxide semiconductor is expected to be used for forming a thin film transistor over a glass substrate, a plastic substrate, or the like, and to be applied to a display device such as a liquid crystal display device, an electroluminescent display device, or electronic paper. 
     In an active matrix display device, electric characteristics of a thin film transistor included in a circuit are important and performance of the display device depends on the electric characteristics. Among the electric characteristics of a thin film transistor, in particular, threshold voltage (V th ) is important. It is needless to say that high field effect mobility is favorable. When the threshold voltage value is high or is on the minus side, it is difficult to control the circuit even if the field effect mobility is high. When a thin film transistor has high threshold voltage value and a large absolute value of its threshold voltage, the TFT cannot perform switching function and may be a load when driven at low voltage. Further, when the threshold voltage value is on the minus side, current tends to flow between the source electrode and the drain electrode even if the gate voltage is 0 V, that is, the transistor tends to be normally on. 
     In the case of an n-channel thin film transistor, it is preferable that a channel is formed and drain current begins to flow after positive gate voltage is applied. A transistor in which a channel is not formed unless a high driving voltage is applied and a transistor in which a channel is formed and drain current flows even in the case of the negative voltage state are unsuitable for a thin film transistor used in a circuit. 
     It is an object to provide a structure of a thin film transistor including an oxide semiconductor film, in which gate threshold voltage at which a channel is formed is positive and as close to 0 V as possible. 
     Further, it is another object to reduce variation in electric characteristics of the thin film transistor including an oxide semiconductor film. In particular, in a liquid crystal display device where variation between elements is large, there is a risk that display unevenness due to variation in TFT characteristics is caused. 
     Further, also in a display device including a light-emitting element, in the case where there is large variation in on current (I on ) of TFTs (TFTs in a driver circuit or TFTs supplying current to light-emitting elements arranged in pixels) which are arranged so as to make constant current flow to a pixel electrode, there is a risk that variation in luminance is generated in a display screen. 
     It is another object to provide a highly reliable semiconductor device using an oxide semiconductor. 
     Each embodiment of the invention disclosed in this specification achieves at least one of the above objects. 
     In order to improve characteristics of an oxide semiconductor layer and reduce variation in the characteristics, reduction in the concentration of hydrogen in the oxide semiconductor layer is important. 
     Therefore, with the use of an oxide semiconductor in which hydrogen is thoroughly reduced, electric characteristics of a thin film transistor are improved, and the thin film transistor can have little variation in characteristics and high reliability. 
     Characteristics of a thin film transistor in which a channel formation region is provided in an oxide semiconductor layer are affected by an interface condition of the oxide semiconductor layer, that is, an interface between the oxide semiconductor layer and a gate insulating film, an interface between the oxide semiconductor layer and a protective insulating film, or an interface between the oxide semiconductor layer and an electrode. Moreover, the characteristics of a thin film transistor are largely affected by the characteristics of the oxide semiconductor layer itself. 
     In order to form such interfaces in a clean condition, a gate insulating film, an oxide semiconductor layer, and a channel protective film are successively formed without being exposed to air. Preferably, the three layers are successively formed under reduced pressure so that an oxide semiconductor layer having an excellent interface can be obtained, and leakage current of a TFT in an off state can be small and a thin film transistor having high current-driving ability can be obtained. In particular, the oxide semiconductor layer is formed with a sputtering method in an atmosphere including oxygen at a flow rate of greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%, whereby hydrogen can be prevented from being included in the oxide semiconductor layer. 
     Note that for the oxide semiconductor film, an oxide semiconductor including zinc oxide (ZnO) in an amorphous state, a polycrystalline state, or a microcrystalline state in which both amorphous and polycrystalline states exist, to which one or a plurality of impurity elements selected from the following is added can be used: Group 1 elements (for example, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs)), Group 13 elements (for example, boron (B), gallium (Ga), indium (In), and thallium (Tl)), Group 14 elements (for example, carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb)), Group 15 elements (for example, nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi)), Group 17 elements (for example, fluorine (F), chlorine (CO, bromine (Br), and iodine (I)), and the like. Alternatively, an oxide semiconductor including zinc oxide in an amorphous state, a polycrystalline state, or a microcrystalline state in which both amorphous and polycrystalline states exist, to which no impurity element is added can be used. 
     Specifically, for example, any of an oxide semiconductor such as magnesium zinc oxide (Mg x Zn (1-x) O), cadmium zinc oxide (Cd x Zn (1-x) O), or cadmium oxide (CdO); an In—Ga—Zn—O-based (a-IGZO) oxide semiconductor typified by InGaO 3 (ZnO) 5 ; an In—Sn—Zn—O-based oxide semiconductor; a Ga—Sn—Zn—O-based oxide semiconductor; an In—Zn—O-based oxide semiconductor; a Sn—Zn—O-based oxide semiconductor; an In—Sn—O-based oxide semiconductor; or a Ga—Zn—O-based oxide semiconductor can be used. Note that the In—Ga—Zn—O-based oxide semiconductor is a material having a wide energy gap (Eg); therefore, even if two gate electrodes are provided above and below an oxide semiconductor film, an increase in off current can be suppressed, which is favorable. 
     Further, as the oxide semiconductor film, an oxide semiconductor film including silicon oxide which is obtained with a sputtering method using an oxide semiconductor target including SiO x  may be used. Typically, an oxide semiconductor target including SiO 2  at greater than or equal to 0.1 weight % and less than or equal to 20 weight %, preferably at greater than or equal to 1 weight % and less than or equal to 6 weight % may be used to form a film so that the oxide semiconductor film includes SiO x  (x&gt;0) which inhibits crystallization. Thus, a thin film transistor whose gate threshold voltage at which a channel is formed is positive and as close to 0 V as possible can be realized. 
     For the formation of the oxide semiconductor layer, a vapor phase method such as a pulsed laser deposition method (a PLD method) or an electron beam deposition method can be employed, but a sputtering method in an atmosphere of only oxygen is suitable in terms of reduction in hydrogen. In general, sputtering is performed in an atmosphere including a rare gas such as Ar or Kr; however, such a rare gas element has larger mass than oxygen and thus there is a possibility that desorption of hydrogen, contained in moisture or hydrocarbon originating from gas adsorbed to an inner wall of a film formation chamber or a jig, is promoted in sputtering. 
     Sputtering is performed in an atmosphere of only oxygen, whereby desorption of a gas attached to an inner wall of the film formation chamber and a jig can be prevented. However, in order to increase deposition rate, a rare gas may be mixed with oxygen in the range where desorption of the gas from an inner wall of the film formation chamber or the like is not caused. Specifically, sputtering may be performed in an atmosphere including oxygen at a flow rate of greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%. 
     An embodiment of the invention to be disclosed in this specification is a method for manufacturing a semiconductor device, including the steps of: forming a gate electrode over a substrate having an insulating surface; stacking a first insulating film, an oxide semiconductor layer which is formed with a sputtering method in an atmosphere including oxygen at a flow rate of greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%, and a second insulating film in this order over the gate electrode without exposure to air; selectively etching the second insulating film to form a protective film in a position overlapping with the gate electrode; forming a conductive film over the oxide semiconductor layer and the protective film; and selectively etching the conductive film and the oxide semiconductor layer. 
     Another embodiment of the invention to be disclosed in this specification is a method for manufacturing a semiconductor device, including the steps of: forming a gate electrode over a substrate having an insulating surface; forming a first insulating film over the gate electrode; forming a conductive film over the first insulating film; selectively etching the conductive film to form a source electrode and a drain electrode; stacking an oxide semiconductor layer which is formed with a sputtering method in an atmosphere including oxygen at a flow rate of greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%, and a second insulating film in this order over the first insulating film, the source electrode, and the drain electrode without exposure to air; selectively etching the second insulating film and the oxide semiconductor layer to form a protective film and an island-like semiconductor layer; and forming a third insulating film which covers the protective film and the island-like semiconductor layer. 
     The present invention achieves at least one of the above objects. 
     In the above manufacturing process, silicon oxynitride films are used as the first insulating film and the second insulating film. When the oxide semiconductor layer is interposed between the silicon oxynitride films, hydrogen, moisture, or the like can be prevented from entering or diffusing into the oxide semiconductor layer. The silicon oxynitride films may be formed, for example, with a sputtering method using silicon, silicon oxide, or the like as a sputtering target in an atmosphere including oxygen and nitrogen. The silicon oxynitride films can also be formed with a so-called CVD method such as a high-density plasma CVD method. In the case of a CVD method, an appropriate mixture of silane, dinitrogen monoxide, and nitrogen may be used as a reaction gas, for example. 
     Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used as a sputtering power source, 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. 
     Further, there are a sputtering apparatus provided with a magnet system inside the chamber and used for magnetron sputtering, and a sputtering apparatus used for ECR sputtering in which plasma generated with the use of microwaves is used without using glow discharge. 
     An insulating film such as a silicon oxide film or a silicon nitride film may be used as the first insulating film or the second insulating film; however, use of the silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % can prevent hydrogen, moisture, or the like from entering or diffusing into the oxide semiconductor layer. The insulating films are preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. 
     The oxide semiconductor film is formed with a sputtering method using an oxide semiconductor target which includes at least In, Ga, and Zn, and the concentration of hydrogen included in the target needs to be as low as possible. In general, an oxide semiconductor target includes hydrogen at greater than or equal to 10 20  atoms/cm 3  and less than or equal to 10 21  atoms/cm 3  when measured by SIMS analysis; however, the hydrogen concentration is preferably 10 19  atoms/cm 3  or lower. 
     In general, a target has a structure in which a target material is bonded to a metal plate called a backing plate. For example, a target material of an oxide semiconductor is formed in such a manner that oxides including indium (In), gallium (Ga), and zinc (Zn) are mixed at the same rate (In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 [molar ratio]), and the compound is sintered at a high temperature of 800° C. or more. The sintering is performed in an inert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere), whereby hydrogen, moisture, hydrocarbon, or the like can be prevented from being included in the target material. The sintering may be performed in vacuum or in a high-pressure atmosphere, and further, may be performed with application of mechanical pressure. 
     Note that the target material may have either an amorphous structure or a crystalline structure. As described above, the target material may include SiO 2  at greater than or equal to 0.1 weight % and less than or equal to 20 weight %, preferably greater than or equal to 1 weight % and less than or equal to 6 weight %. Note that, in this specification, a target material is referred to as a target in some cases unless otherwise specified. 
     In general, a backing plate has functions of cooling a target material and being a sputtering electrode and thus is often formed using copper, which is excellent in thermal conductivity and electric conductivity. A cooling path is formed inside or on the back surface of the backing plate, and water, oil, or the like circulates through the cooling path as a coolant; thus, cooling efficiency of a target can be increased. Note that water vaporizes at 100° C.; therefore, in the case where the temperature of the target needs to be kept at 100° C. or higher, oil or the like is preferable to water. 
     It is preferable that the target material and the backing plate be bonded to each other, for example, by electron beam welding. The electron beam welding refers to a method in which electrons generated in a vacuum atmosphere are accelerated, focused, and then delivered to an object, whereby welding can be performed only on a portion which is desired to be welded without damage on the material property of portions of the object except for the welded portion. In the electron beam welding, the shape of the welded portion and the depth of welding can be controlled. Since the welding is performed in vacuum, hydrogen, moisture, hydrocarbon, or the like can be prevented from attaching to the target material. 
     The manufactured target is transported while being held in a vacuum atmosphere or in an inert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere). Accordingly, hydrogen, moisture, hydrocarbon, or the like can be prevented from attaching to the target. 
     Similarly, the target is set in a sputtering apparatus in an inert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere) without being exposed to air. Accordingly, hydrogen, moisture, hydrocarbon, or the like can be prevented from attaching to the target. 
     After the target is set in the sputtering apparatus, dehydrogenation treatment is preferably performed to remove hydrogen which remains on a surface of or inside the target material. As the dehydrogenation treatment, a method in which the inside of the film formation chamber is heated to higher than or equal to 200° C. and lower than or equal to 600° C. under reduced pressure, a method in which introduction and exhaust of nitrogen or an inert gas are repeated while the inside of the film formation chamber is heated, and the like can be given. In this case, not water but oil or the like is preferably used as a coolant for the target. Although a certain level of effect can be obtained when introduction and exhaust of nitrogen are repeated without heating, it is more preferable to perform the treatment with the inside of the film formation chamber heated. Alternatively, oxygen, an inert gas, or both oxygen and an inert gas may be introduced into the film formation chamber, and plasma of an inert gas and/or oxygen may be generated using a high frequency or a microwave. Although a certain level of effect can be obtained when the treatment is performed without heating, it is preferable to perform the treatment with the inside of the film formation chamber heated. 
     Note that as a vacuum pump used for a vacuum apparatus such as a sputtering apparatus, for example, a cryopump may be used. The cryopump is a pump by which exhaust is performed in such a manner that a surface having extremely low temperature is provided in a vacuum chamber, and gas molecules in the vacuum chamber are condensed on or adsorbed to the surface having extremely low temperature so as to be captured. The cryopump has high capability of exhausting hydrogen or moisture. 
     In particular, the first insulating film, the oxide semiconductor layer, and the second insulating film are formed by any of the above-described heating and other methods as appropriate in an atmosphere in which the amount of hydrogen, moisture, and hydrocarbon is sufficiently reduced. 
     As a gas used in manufacturing the thin film transistor, a high-purity gas in which the concentration of hydrogen, moisture, hydrocarbon, or the like is reduced as much as possible is preferably used. Provision of a refiner between a gas supply source and each apparatus enables the further increase in gas purity. A gas whose purity is 99.9999% or higher is preferably used. Further, in order to prevent gas inclusion from an inner wall of a gas pipe, a gas pipe whose inner surface has been subjected to mirror polishing and passivation with Cr 2 O 3  or Al 2 O 3  is preferably used. As a joint or a valve of the pipe, an all-metal valve in which a resin is not used in its sealing portion is preferable. 
     In this specification, successive film formation is carried out as follows: a substrate to be processed is placed in an atmosphere which is controlled to be vacuum or an inert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere) at all times without being exposed to a contaminant atmosphere such as air during a process from a first film formation step to a second film formation step. By the successive film formation, film formation can be performed without reattachment of hydrogen, moisture, hydrocarbon, or the like to the substrate to be processed, which has been cleaned. 
     The conductive film functions as a source electrode and a drain electrode. The conductive film is formed using a single layer or a stacked layer of aluminum or an aluminum alloy to which an element to improve heat resistance or an element to prevent hillocks, such as copper, silicon, titanium, neodymium, scandium, or molybdenum, is added. Alternatively, a structure may be employed in which a layer including a high-melting-point metal such as titanium, molybdenum, or tungsten is provided on the lower side, the upper side, or both sides of a single layer or a stacked layer of aluminum or an aluminum alloy. Among all, titanium can be given as an example of a material having excellent interface characteristics with the oxide semiconductor layer. In particular, when a stack of a titanium film, an aluminum film, and a titanium film is used as the conductive film, the resistance is low. The aluminum film is interposed between the titanium films, whereby hillocks due to the aluminum film are less likely to be generated. Thus, the stack is suitable for the source electrode and the drain electrode. 
     Alternatively, between the gate electrode and the first insulating film, a silicon nitride film or a silicon oxide film may be additionally provided. In other words, the gate insulating film may have a stacked-layer structure of two or more layers. A silicon oxynitride film is preferable as the first insulating film which is the uppermost layer of the gate insulating film and in contact with the oxide semiconductor layer, while an insulating film formed under the first insulating film may be a silicon nitride film or a silicon oxide film. By provision of such a silicon nitride film or a silicon oxide film, the film serves as an etching stopper to prevent a surface of a substrate from being etched in a manufacturing process of a TFT. The silicon nitride film or the silicon oxide film can prevent mobile ions of sodium or the like from entering a semiconductor region from a glass substrate including alkali metal such as sodium, so that variation in electric characteristics of a TFT can be suppressed. 
     Thus, a structure can be realized in which gate threshold voltage at which a channel is formed is positive and as close to 0 V as possible in a thin film transistor including an oxide semiconductor film. Moreover, variation in threshold voltage can be reduced, deterioration of electric characteristics can be prevented, and shift to normally on of a TFT can be reduced, or preferably eliminated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1E  are cross-sectional views illustrating a manufacturing process of a thin film transistor which is one embodiment of the present invention; 
         FIGS. 2A to 2F  are cross-sectional views illustrating a manufacturing process of a thin film transistor which is one embodiment of the present invention; 
         FIGS. 3A to 3E  are cross-sectional views illustrating a manufacturing process of a thin film transistor which is one embodiment of the present invention; 
         FIGS. 4A to 4E  are cross-sectional views illustrating a manufacturing process of a thin film transistor which is one embodiment of the present invention; 
         FIGS. 5A and 5B  are each a block diagram of a semiconductor device; 
         FIG. 6  illustrates a configuration of a signal line driver circuit; 
         FIG. 7  is a timing chart illustrating operation of a signal line driver circuit; 
         FIG. 8  is a timing chart illustrating operation of a signal line driver circuit; 
         FIG. 9  illustrates a configuration of a shift register; 
         FIG. 10  illustrates a configuration of connections of a flip-flop; 
       FIGS.  11 A 1 ,  11 A 2 , and  11 B illustrate semiconductor devices according to one embodiment of the present invention; 
         FIG. 12  illustrates a semiconductor device according to one embodiment of the present invention; 
         FIG. 13  illustrates a semiconductor device according to one embodiment of the present invention; 
         FIG. 14  illustrates a pixel equivalent circuit of a semiconductor device according to one embodiment of the present invention; 
         FIGS. 15A to 15C  each illustrate a semiconductor device according to one embodiment of the present invention; 
         FIGS. 16A and 16B  illustrate a semiconductor device according to one embodiment of the present invention; 
         FIGS. 17A and 17B  illustrate examples of usage patterns of electronic paper; 
         FIG. 18  is an external view illustrating one embodiment of an electronic book reader; 
         FIGS. 19A and 19B  are external views respectively illustrating one embodiment of a television set and one embodiment of a digital photo frame; 
         FIGS. 20A and 20B  are external views each illustrating one embodiment of an amusement machine; 
         FIGS. 21A and 21B  are external views each illustrating one embodiment of a mobile phone handset; 
         FIGS. 22A and 22B  illustrate one embodiment of an electronic book reader; 
         FIG. 23  illustrates one embodiment of the electronic book reader; 
         FIGS. 24A to 24E  are cross-sectional views illustrating a manufacturing process of a thin film transistor which is one embodiment of the present invention; 
         FIGS. 25A to 25C  show results of Hall effect measurement of an oxide semiconductor; and 
         FIG. 26  shows a result of XRD measurement of an oxide semiconductor layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below. 
     Embodiment 1 
     In this embodiment, a thin film transistor and a manufacturing process thereof will be described with reference to  FIGS. 1A to 1E . 
     First, a gate electrode  101  is formed over a substrate  100  (see  FIG. 1A ). 
     As the substrate  100 , any of the following substrates can be used: non-alkaline glass substrates formed of barium borosilicate glass, aluminoborosilicate glass, aluminosilicate glass, and the like with a fusion method or a float method; ceramic substrates; plastic substrates having heat resistance sufficient to withstand a process temperature of this manufacturing process; and the like. Alternatively, a metal substrate such as a stainless steel alloy substrate which is provided with an insulating film over the surface may be used. As the substrate  100 , a substrate having a size of 320 mm×400 mm, 370 mm×470 mm, 550 mm×650 mm, 600 mm×720 mm, 680 mm×880 mm, 730 mm×920 mm, 1000 mm×1200 mm, 1100 mm×1250 mm, 1150 mm×1300 mm, 1500 mm×1800 mm, 1900 mm×2200 mm, 2160 mm×2460 mm, 2400 mm×2800 mm, 2850 mm×3050 mm, or the like can be used. 
     Further, a base insulating film may be formed over the substrate  100  before the gate electrode  101  is formed. The base insulating film can be formed with a single layer or stacked layers of any of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and a silicon nitride oxide film with a CVD method, a sputtering method, or the like. A small amount of a halogen element such as fluorine or chlorine may be added to the base insulating film to immobilize mobile ions of sodium or the like. The peak of the concentration of a halogen element to be included in the insulating film, when measured by secondary ion mass spectrometry (SIMS), is preferably in the range of greater than or equal to 1×10 15  cm −3  and less than or equal to 1×10 20  cm −3 . 
     The gate electrode  101  is formed by using a metal material such as titanium, molybdenum, chromium, tantalum, tungsten, or aluminum, or an alloy material thereof. The gate electrode  101  can be formed in such a manner that a conductive film is formed over the substrate  100  by a sputtering method or a vacuum evaporation method; a mask is formed over the conductive film by a photolithography technique or an inkjet method; and the conductive film is etched using the mask. Alternatively, the gate electrode  101  can be formed by discharging a conductive nanopaste of silver, gold, copper, or the like with an inkjet method and baking the conductive nanopaste. Note that, as bather metal which increases adhesion of the gate electrode  101  and prevents diffusion to the substrate or the base film, a nitride film of the above metal material may be provided between the substrate  100  and the gate electrode  101 . The gate electrode  101  may have a single-layer structure or a stacked-layer structure. For example, a structure in which a molybdenum film and an aluminum film are stacked in this order, a structure in which a molybdenum film and an alloy film of aluminum and neodymium are stacked in this order, a structure in which a titanium film and an aluminum film are stacked in this order, a structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, or the like can be formed over the substrate  100 . 
     In this embodiment, a stacked-layer film of an aluminum film and a molybdenum film is formed with a sputtering method and is selectively etched with a photolithography technique. Here, a first photomask is used. Note that, since a semiconductor film and a wiring are formed over the gate electrode  101 , the gate electrode  101  is preferably processed to have tapered end portions in order to prevent disconnection. 
     Next, a first insulating film  102  serving as a gate insulating film, a semiconductor film  103 , and a second insulating film  104  are successively formed without being exposed to air (see  FIG. 1B ). When the films are formed successively without being exposed to air, high productivity is obtained and the reliability of interfaces between the thin films is increased. In addition, each interface between the stacked layers can be formed without being contaminated by moisture, hydrocarbon, or another contaminant impurity element which is included in air; thus, hydrogen can be prevented from being taken into the semiconductor film. 
     The first insulating film  102  and the second insulating film  104  can be formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film with a CVD method, a sputtering method, or the like. Here, as the first insulating film  102  and the second insulating film  104 , silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % are formed with an RF sputtering method. By use of the silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic %, hydrogen, moisture, or the like can be prevented from entering or diffusing into the semiconductor film  103 . The insulating films are preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. 
     The first insulating film  102  can also have a stacked-layer structure of two or more layers. A silicon oxynitride film is preferable as a film which forms the uppermost layer of the first insulating film  102  and which is in contact with the oxide semiconductor layer, while an insulating film provided under the uppermost layer may be a silicon nitride film or a silicon oxide film. This film in the lower layer of the first insulating film  102  has an effect of preventing hillock generation when a material that may cause hillocks is used for the gate electrode  101 . 
     Here, an oxide semiconductor layer (an IGZO semiconductor layer) is formed as the semiconductor film  103  with a DC magnetron sputtering method. In this specification, a semiconductor layer formed using an oxide semiconductor film including In, Ga, and Zn is also referred to as an “IGZO semiconductor layer.” The IGZO semiconductor layer has high flexibility in the composition ratio of the metal elements and functions as a semiconductor layer at a wide range of mixture ratio. For example, indium oxide including zinc oxide at 10 weight %, a material in which indium oxide, gallium oxide, and zinc oxide are mixed in equimolar ratios, and oxide including In, Ga, and Zn at a ratio of 2.2:2.2:1.0 [atomic ratio] can be given. In order to reduce variation in electric characteristics of the thin film transistor, it is preferable that the IGZO semiconductor layer be in an amorphous state. 
     The semiconductor film  103  is formed in an atmosphere of only oxygen. In general, sputtering is often performed in an atmosphere including a rare gas such as Ar or Kr; however, such a rare gas element has larger mass than oxygen and thus there is a possibility that desorption of hydrogen, contained in moisture or hydrocarbon originating from gas adsorbed to an inner wall of a film formation chamber or a jig, is promoted in sputtering. However, in order to increase deposition rate, a rare gas may be mixed with oxygen in the range where desorption of a gas from an inner wall of the film formation chamber or the like is not caused. Specifically, the sputtering may be performed in an atmosphere including oxygen at a flow rate of greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%. Note that the semiconductor film  103  is preferably formed under the condition that the substrate temperature is higher than or equal to room temperature (25° C.) and lower than 200° C. 
     Next, in order to pattern the semiconductor film  103 , the second insulating film  104  is selectively etched to form an insulator  106 , and the semiconductor film  103  is selectively etched to form an IGZO semiconductor layer  105 . The etching is performed with a dry etching method using a chlorine gas. The insulator  106  functions as a channel protective film. In this stage, a surface of the gate insulating film is exposed in a region where the semiconductor film  103  is removed. Here, a second photomask is used. The mask formed over the second insulating film  104  in the patterning is removed by ashing treatment in an oxygen atmosphere. The cross-sectional structure of the substrate at this stage corresponds to the cross-sectional view of the substrate illustrated in  FIG. 1C  (see  FIG. 1C ). In order to eliminate moisture from the manufacturing process of the thin film transistor as much as possible, cleaning using water is not necessarily performed after that. 
     Next, heat treatment is preferably performed at higher than or equal to 200° C. and lower than or equal to 600° C., typically higher than or equal to 300° C. and lower than or equal to 500° C. Here, heat treatment is performed in a furnace at 350° C. for 1 hour in a nitrogen atmosphere including oxygen. This heat treatment allows atoms of the IGZO semiconductor layer  105  to be rearranged. By this heat treatment (the heat treatment may be light annealing), strain which inhibits carrier movement is released. There is no particular limitation on timing of the heat treatment as long as it is performed after the formation of the semiconductor film  103 . In this embodiment, since the IGZO semiconductor layer  105  is covered with the insulator  106 , deterioration of the IGZO semiconductor layer  105  after the heat treatment can be suppressed, which is preferable. 
     Next, part of the insulator  106  is removed, and a contact hole (opening)  107  for connecting the IGZO semiconductor layer  105  with a source electrode  108  or a drain electrode  109  which will be formed later is formed. A photolithography technique is employed to form the contact hole (opening)  107  by selective etching so that part of the IGZO semiconductor layer  105  is exposed. Here, a third photomask is used. The etching is performed with a dry etching method using a chlorine gas. The etching for forming the contact hole (opening)  107  is performed under the condition where the etching rate of the insulator  106  is sufficiently different from that of the IGZO semiconductor layer  105 . Alternatively, the contact hole (opening)  107  may be formed by selectively removing only the insulator  106  by laser irradiation. 
     The contact hole (opening)  107  is preferably formed as small as possible in order to eliminate adverse effect of hydrogen, moisture, hydrocarbon, or the like on the IGZO semiconductor layer  105  in the formation of the contact hole (opening)  107 . However, when the contact hole (opening)  107  is too small, sufficient characteristics of a completed thin film transistor cannot be obtained. Therefore, the contact hole (opening)  107  may be formed as small as possible within the range where adverse effect is not caused. 
     Next, a metal multilayer film serving as the source electrode and the drain electrode is formed. Here, an aluminum film is stacked over a titanium film, and another titanium film is stacked over the aluminum film with a DC magnetron sputtering method. Both a titanium target and an aluminum target are set in a sputtering chamber, and the films are successively formed using shutters so as to be sequentially stacked; thus, successive stacking can be performed in one chamber. At this time, an atmosphere of only a rare gas such as Ar or Kr may be employed. This is because the IGZO semiconductor layer  105  has already been interposed between the first insulating film  102  and the second insulating film  104  and thus a channel formation region in the IGZO semiconductor layer  105  is not particularly affected by hydrogen, moisture, hydrocarbon, or the like due to gas desorption from an inner wall of the chamber. 
     Before the metal multilayer film is formed, the IGZO semiconductor layer exposed in the contact hole (opening)  107  may be etched by approximately 10 nm by reverse sputtering. The reverse sputtering is a method by which voltage is applied to a substrate side to generate plasma on the substrate side in an inert gas atmosphere or an oxygen atmosphere, without applying voltage to a target side, so that a surface is etched. By the reverse sputtering, a favorable condition of an interface between the IGZO semiconductor layer and the metal multilayer film can be realized, and the contact resistance can be reduced. 
     Note that an oxide semiconductor film serving as a buffer layer may be formed between the IGZO semiconductor layer and the metal multilayer film. For example, titanium oxide, molybdenum oxide, zinc oxide, indium oxide, tungsten oxide, magnesium oxide, calcium oxide, tin oxide, or the like can be used. Further, an Al—Zn—O-based non-single-crystal film or an Al—Zn—O-based non-single-crystal film including nitrogen, that is, an Al—Zn—O—N-based non-single-crystal film may be used. The amount of aluminum included in the Al—Zn—O-based oxide semiconductor or the Al—Zn—O—N-based oxide semiconductor is preferably greater than or equal to 1 weight % and less than or equal to 10 weight %. 
     Note that the term Al—Zn—O—N-based oxide semiconductor film is used here just for convenience in denotation and does not mean that the stoichiometric proportion of Al to Zn to O to N in the film is 1:1:1:1. The constituent ratio of these elements can be adjusted as appropriate by conditions of film formation. 
     The buffer layer may include an impurity imparting n-type or p-type conductivity. As the impurity element, indium, gallium, aluminum, zinc, tin, or the like can be used. 
     Since the buffer layer has a higher carrier concentration than the IGZO semiconductor layer and is superior in conductivity, the contact resistance can be reduced as compared to the case of directly attaching the source electrode and the drain electrode to the IGZO semiconductor layer. 
     The buffer layer can also be referred to as a drain region or a source region. 
     Next, the metal multilayer film is selectively etched to form the source electrode  108  and the drain electrode  109 . Here, a fourth photomask is used. The three-layer conductive film in which a titanium film, an aluminum film, and a titanium film are sequentially stacked can be etched with a dry etching method using a chlorine gas. Also in the case where a buffer layer is formed between the IGZO semiconductor layer and the metal multilayer film, the buffer layer can be etched at the same time as the metal multilayer film. The cross-sectional structure of the substrate at this stage corresponds to the cross-sectional view of the substrate illustrated in  FIG. 1E  (see  FIG. 1E ). 
     In this embodiment, a channel formation region is a region of the IGZO semiconductor layer  105 , where the gate electrode  101  and the IGZO semiconductor layer  105  overlap with each other. The channel formation region is a region from an end portion of the contact hole (opening)  107  for connecting the source electrode  108  with the IGZO semiconductor layer  105  to an end portion of the contact hole (opening)  107  for connecting the drain electrode  109  with the IGZO semiconductor layer  105 . L 1  in  FIG. 1D  corresponds to a channel length. 
     The channel formation region of the IGZO semiconductor layer  105  is interposed between the silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic %, whereby hydrogen, moisture, or the like can be prevented from entering or diffusing into the channel formation region. The silicon oxynitride films are preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. 
     Embodiment 2 
     In this embodiment, a thin film transistor and a manufacturing process thereof will be described with reference to  FIGS. 2A to 2F . Note that repetitive description of the same portions as Embodiment 1 or portions having functions similar to those in Embodiment 1, and steps for forming such portions will be omitted. 
     First, a gate electrode  201  is formed over a substrate  200 . Here, a first photomask is used (see  FIG. 2A ). 
     Next, a first insulating film  202  serving as a gate insulating film, a first semiconductor film  203 , and a second insulating film  204  are successively formed without being exposed to air (see  FIG. 2B ). Here, as the first insulating film  202  and the second insulating film  204 , silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % are formed with an RF sputtering method. As the first semiconductor film  203 , an oxide semiconductor layer is formed with a DC magnetron sputtering method using an oxide semiconductor target in which SiO 2  is added at greater than or equal to 0.1 weight % and less than or equal to 20 weight % to a zinc (Zn)-including oxide semiconductor (ZnO). As described in Embodiment 1, the oxide semiconductor layer is formed in an atmosphere of only oxygen; however, an atmosphere in which a rare gas is mixed, where the flow rate of oxygen is greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%, may also be employed. Note that the first semiconductor film  203  is preferably formed under the condition that the substrate temperature is higher than or equal to room temperature (25° C.) and lower than 200° C. 
     Next, the second insulating film  204  is etched so that only a portion which overlaps with the gate electrode and a portion serving as a channel formation region of the first semiconductor film  203  is left; thus, an insulator  206  is formed. The insulator  206  functions as a channel protective film. A photolithography technique is employed to form the insulator  206  by selective etching. Here, a second photomask is used. This etching for forming the insulator  206  is performed with a dry etching method under the condition where the etching rate of the second insulating film  204  is sufficiently different from that of the first semiconductor film  203  (see  FIG. 2C ). The mask formed over the second insulating film  204  in the patterning is removed by ashing treatment in an oxygen atmosphere. In order to eliminate moisture from the manufacturing process of the thin film transistor as much as possible, cleaning using water is not necessarily performed after that. 
     In addition, in the formation of the insulator  206 , a mask can be selectively formed in a position overlapping with the gate electrode in a self-alignment manner by rear-face exposure, without using a photomask. In particular, since the first semiconductor film  203  is an oxide semiconductor film and has a high light-transmitting property, it is suitable for rear-face exposure. Note that, in the case of rear-face exposure, both the first insulating film  202  and the second insulating film  204  need to include materials having sufficient light-transmitting properties. 
     Next, heat treatment is preferably performed at higher than or equal to 200° C. and lower than or equal to 600° C., typically higher than or equal to 300° C. and lower than or equal to 500° C. Here, heat treatment is performed in a furnace at 350° C. for 1 hour in a nitrogen atmosphere including oxygen. This heat treatment allows atoms of the first semiconductor film  203  to be rearranged. By this heat treatment (the heat treatment may be light annealing), strain which inhibits carrier movement is released. There is no particular limitation on timing of the heat treatment as long as it is performed after the formation of the first semiconductor film  203 . In this embodiment, since the top surface of the first semiconductor film  203  is covered with the insulator  206 , deterioration of the first semiconductor film  203  after the heat treatment can be suppressed, which is preferable. 
     Next, a second semiconductor film  212  serving as buffer layers, and a metal multilayer film  211  serving as a source electrode and a drain electrode are formed. Here, a titanium oxide film is formed as the second semiconductor film  212  with a DC magnetron sputtering method. Then, the metal multilayer film  211  is formed in such a manner that a titanium film is stacked over the second semiconductor film  212 , an aluminum film is stacked over the titanium film, and another titanium film is stacked over the aluminum film with a DC magnetron sputtering method (see  FIG. 2D ). 
     Since the second semiconductor film  212  serving as buffer layers has a higher carrier concentration than the oxide semiconductor layer and is superior in conductivity, the contact resistance can be reduced when the buffer layers are provided as compared to the case of directly attaching the source electrode and the drain electrode to the semiconductor layer. 
     After the second semiconductor film  212  serving as the buffer layers is formed, heat treatment is preferably performed at higher than or equal to 200° C. and lower than or equal to 600° C., typically higher than or equal to 300° C. and lower than or equal to 500° C. Here, heat treatment is performed in a furnace at 350° C. for 1 hour in a nitrogen atmosphere including oxygen. This heat treatment allows atoms of the second semiconductor film  212  to be rearranged. By this heat treatment (the heat treatment may be light annealing), strain which inhibits carrier movement is released. 
     Next, the metal multilayer film is selectively etched to form a source electrode  208  and a drain electrode  209 . Here, a third photomask is used. The etching is performed with a dry etching method. At this time, the etching is performed under the condition where all of the metal multilayer film  211 , the second semiconductor film  212 , and the first semiconductor film  203  can be etched, whereby the source electrode  208 , the drain electrode  209 , a source-side buffer layer  213 , a drain-side buffer layer  214 , and a semiconductor layer  205  can be formed in the same etching step. The insulator  206  functions as a channel protective film and prevents the semiconductor layer  205  in a channel formation region from being etched (see  FIG. 2E ). 
     In this embodiment, a channel formation region is a region of the semiconductor layer  205 , where the gate electrode  201 , the semiconductor layer  205 , and the insulator  206  overlap with each other. A width L 2  of the insulator  206  corresponds to a channel length. 
     As the layers provided over and under the channel formation region of the semiconductor layer  205 , silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % are formed so that the channel formation region is interposed between the silicon oxynitride films, whereby hydrogen, moisture, or the like can be prevented from entering or diffusing into the channel formation region. 
     Note that, in order to prevent entry or diffusion of hydrogen, moisture, or the like through a side surface portion of the semiconductor layer, a third insulating film  210  may be formed so as to cover the thin film transistor. The third insulating film  210  can be formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film. For example, a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % may be formed with a sputtering method. By use of the silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic %, hydrogen, moisture, hydrocarbon, or the like can be prevented from entering or diffusing into the thin film transistor. The silicon oxynitride film is preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. 
     Embodiment 3 
     In this embodiment, a thin film transistor and a manufacturing process thereof will be described with reference to  FIGS. 3A to 3E . Note that repetitive description of the same portions as Embodiment 1 or portions having functions similar to those in Embodiment 1, and steps for forming such portions will be omitted. 
     First, a gate electrode  301  is formed over a substrate  300 . Here, a first photomask is used. 
     Next, a first insulating film  302 , serving as a gate insulating film, and a metal multilayer film  311 , serving as a source electrode and a drain electrode, are formed. 
     The first insulating film  302  can be formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film with a CVD method, a sputtering method, or the like. Here, a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % is formed as the first insulating film  302  with an RF sputtering method. 
     The metal multilayer film  311  serving as the source electrode and the drain electrode is formed in such a manner that an aluminum film is stacked over a titanium film and another titanium film is stacked over the aluminum film with a DC magnetron sputtering method (see  FIG. 3A ). 
     Next, the metal multilayer film is selectively etched to form a source electrode  308  and a drain electrode  309 . Here, a second photomask is used (see  FIG. 3B ). 
     Then, a semiconductor film  303  and a second insulating film  304  are successively formed without being exposed to air (see  FIG. 3C ). As the semiconductor film  303 , an oxide semiconductor layer (a ZnO—SiO x (x&gt;0) semiconductor layer) which is formed with a DC magnetron sputtering method using a target of zinc oxide including silicon oxide at 10 weight % is used. As described in Embodiment 1, the oxide semiconductor layer is formed in an atmosphere of only oxygen; however, an atmosphere in which a rare gas is mixed, where the flow rate of oxygen is greater than or equal to 50% and less than or equal to 100%, preferably greater than or equal to 70% and less than or equal to 100%, may also be employed. Note that the semiconductor film  303  is preferably formed under the condition that the substrate temperature is higher than or equal to room temperature (25° C.) and lower than 200° C. 
     Here, results of evaluating the crystallinity of an oxide semiconductor layer (a ZnO—SiO semiconductor layer) by XRD measurement are shown. The measurement is performed on three kinds of oxide semiconductor layers (ZnO—SiO semiconductor layers) which are formed using respective targets of zinc oxide including silicon oxide at 7.5 weight %, 10 weight %, and 12.5 weight %. 
       FIG. 26  shows results of the XRD measurement. The horizontal axis represents a rotation angle (2θ) of a measurement sample and a signal detection portion with respect to an incident X-ray, and the vertical axis represents X-ray diffraction intensity.  FIG. 26  shows a measurement result  601  in the case of including silicon oxide at 7.5 weight %, a measurement result  602  in the case of including silicon oxide at 10 weight %, and a measurement result  603  in the case of including silicon oxide at 12.5 weight %. 
     According to the measurement results in  FIG. 26 , the following can be found: a peak  604  showing crystallinity is detected when 7.5 weight % of silicon oxide is included, while no peak showing crystallinity is detected and an amorphous film is formed when 10 weight % or more of silicon oxide is included. Therefore, a ZnO—SiO x  (x&gt;0) semiconductor layer which includes silicon oxide at 10 weight % or more can be kept amorphous even in heat treatment at 700° C. 
     Before the semiconductor film  303  is formed, surfaces of the first insulating film  302 , the source electrode  308 , and the drain electrode  309  may be etched by approximately 10 nm by reverse sputtering. By the reverse sputtering, hydrogen, moisture, hydrocarbon, or the like which is attached to the surfaces of the first insulating film  302 , the source electrode  308 , and the drain electrode  309  can be removed. 
     Next, in order to pattern the semiconductor film  303 , the second insulating film  304  is selectively etched to form an insulator  306 , and the semiconductor film  303  is selectively etched to form a ZnO—SiO x (x&gt;0) semiconductor layer  305 . Here, a third photomask is used. The mask formed over the second insulating film  304  in the patterning is removed by ashing treatment in an oxygen atmosphere. The insulator  306  functions as a channel protective film. The etching is performed with a dry etching method. In order to eliminate moisture from the manufacturing process of the thin film transistor as much as possible, cleaning using water is not necessarily performed after that. 
     Next, heat treatment is preferably performed at higher than or equal to 200° C. and lower than or equal to 600° C., typically higher than or equal to 300° C. and lower than or equal to 500° C. Here, heat treatment is performed in a furnace at 350° C. for 1 hour in a nitrogen atmosphere including oxygen. This heat treatment allows atoms of the semiconductor layer  305  to be rearranged. By this heat treatment (the heat treatment may be light annealing), strain which inhibits carrier movement is released. There is no particular limitation on timing of the heat treatment as long as it is performed after the formation of the semiconductor film  303 . In this embodiment, since the top surface of the semiconductor layer  305  is covered with the insulator  306 , deterioration of the semiconductor layer  305  after the heat treatment can be suppressed, which is preferable. 
     In this embodiment, a channel formation region is a region of the ZnO—SiO x  (x&gt;0) semiconductor layer  305 , which overlaps with the gate electrode  301  and is located between the source electrode  308  and the drain electrode  309 . A distance L 3  between the source electrode  308  and the drain electrode  309  corresponds to a channel length. 
     As the layers provided over and under the channel formation region of the semiconductor layer  305 , silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % are formed so that the channel formation region is interposed between the silicon oxynitride films, whereby hydrogen, moisture, or the like can be prevented from entering or diffusing into the channel formation region. The silicon oxynitride films are preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. 
     Note that, in order to prevent entry or diffusion of hydrogen, moisture, or the like through a side surface portion of the ZnO—SiO x  (x&gt;0) semiconductor layer, a third insulating film  310  may be formed so as to cover the thin film transistor. The third insulating film  310  is preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. The third insulating film  310  can be formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film. For example, a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % may be formed with an RF sputtering method. By use of the silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic %, hydrogen, moisture, or the like can be prevented from entering or diffusing into the thin film transistor. 
     Further, an oxide semiconductor film serving as a buffer layer may be formed between the ZnO—SiO x  (x&gt;0) semiconductor layer and the metal multilayer film as needed. 
     Embodiment 4 
     In this embodiment, a thin film transistor and a manufacturing process thereof will be described with reference to  FIGS. 4A to 4E . Note that repetitive description of the same portions as Embodiment 1 or portions having functions similar to those in Embodiment 1, and steps for forming such portions will be omitted. 
     First, a gate electrode  401  is formed over a substrate  400 . Here, a first photomask is used (see  FIG. 4A ). 
     Next, a first insulating film  402  serving as a gate insulating film, a first semiconductor film  403 , and a second semiconductor film  412  are successively formed without being exposed to air (see  FIG. 4B ). Here, as the first insulating film  402 , a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % is formed with an RF sputtering method. An IGZO semiconductor layer is formed as the first semiconductor film  403  with a DC magnetron sputtering method, and an Al—Zn—O—N-based oxide semiconductor film is formed as the second semiconductor film  412 . Note that the oxide semiconductor films are preferably formed under the condition that the substrate temperature is higher than or equal to room temperature (25° C.) and lower than 200° C. 
     Note that the term Al—Zn—O—N-based oxide semiconductor film is used here just for convenience in denotation and does not mean that the stoichiometric proportion of Al to Zn to O to N in the film is 1:1:1:1. The constituent ratio of these elements can be adjusted as appropriate by conditions of film formation. 
     Next, heat treatment is preferably performed at higher than or equal to 200° C. and lower than or equal to 600° C., typically higher than or equal to 300° C. and lower than or equal to 500° C. Here, heat treatment is performed in a furnace at 350° C. for 1 hour in a nitrogen atmosphere including oxygen. This heat treatment allows atoms of the IGZO semiconductor layer and the Al—Zn—O—N-based oxide semiconductor film to be rearranged. By this heat treatment (the heat treatment may be light annealing), strain which inhibits carrier movement is released. There is no particular limitation on timing of the heat treatment as long as it is performed after the formation of the first semiconductor film  403  and the second semiconductor film  412 . 
     Next, in order to pattern the first semiconductor film  403 , the second semiconductor film  412  is selectively etched, and the first semiconductor film  403  is selectively etched to form an IGZO semiconductor layer  405 . The etching is performed with a dry etching method using a chlorine gas. The second semiconductor film  412  functions as buffer layers. In this stage, a surface of the gate insulating film is exposed in a region where the first semiconductor film  403  is removed. Here, a second photomask is used. The mask formed over the second semiconductor film  412  in the patterning is removed by ashing treatment in an oxygen atmosphere. The cross-sectional structure of the substrate at this stage corresponds to the cross-sectional view of the substrate illustrated in  FIG. 4C  (see  FIG. 4C ). In order to eliminate moisture from the manufacturing process of the thin film transistor as much as possible, cleaning using water is not necessarily performed after that. 
     Since the second semiconductor film  412  serving as the buffer layers has a higher carrier concentration than the IGZO semiconductor layer and is superior in conductivity, the contact resistance can be reduced when the buffer layers are provided as compared to the case of directly attaching a source electrode and a drain electrode to the IGZO semiconductor layer. 
     Next, a metal multilayer film serving as the source electrode and the drain electrode is formed. Here, an aluminum film is stacked over a titanium film and another titanium film is stacked over the aluminum film with a DC magnetron sputtering method. Both a titanium target and an aluminum target are set in a sputtering chamber, and the films are successively formed using shutters so as to be sequentially stacked; thus, successive stacking can be performed in one chamber. 
     Next, the metal multilayer film is selectively etched to form a source electrode  408  and a drain electrode  409 . Here, a third photomask is used. The etching is performed with a dry etching method. In this case, the condition where the metal multilayer film and the second semiconductor film  412  can be etched and the etching rates thereof are sufficiently different from that of the IGZO semiconductor layer  405  is employed. Accordingly, the source electrode  408 , the drain electrode  409 , a source-side buffer layer  413 , and a drain-side buffer layer  414  can be formed in the same etching step (see  FIG. 4D ). 
     Next, in order to prevent entry or diffusion of hydrogen, moisture, or the like from the outside, a third insulating film  410  is formed so as to cover the thin film transistor. The third insulating film  410  is preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. The third insulating film  410  can be formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film. For example, a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % may be formed with an RF sputtering method. By use of the silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic %, hydrogen, moisture, or the like can be prevented from entering or diffusing into the thin film transistor. 
     Before the third insulating film  410  is formed, surfaces of the IGZO semiconductor layer  405 , the source electrode  408 , and the drain electrode  409  may be etched by approximately 10 nm by reverse sputtering. By the reverse sputtering, hydrogen, moisture, hydrocarbon, or the like which is attached to the surfaces of the source electrode  408  and the drain electrode  409  can be removed. 
     In this embodiment, a channel formation region is a region of the IGZO semiconductor layer  405 , which overlaps with the gate electrode  401  and is located between the source-side buffer layer  413  and the drain-side buffer layer  414 . A distance L 4  between the source-side buffer layer  413  and the drain-side buffer layer  414  corresponds to a channel length (see  FIG. 4E ). 
     As the layers provided over and under the channel formation region of the IGZO semiconductor layer  405 , silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % are formed so that the channel formation region is interposed between the silicon oxynitride films, whereby hydrogen, moisture, or the like can be prevented from entering or diffusing into the channel formation region. 
     Embodiment 5 
     In this embodiment, a thin film transistor and a manufacturing process thereof will be described with reference to  FIGS. 24A to 24E . Note that repetitive description of the same portions as Embodiment 1 or portions having functions similar to those in Embodiment 1, and steps for forming such portions will be omitted. 
     First, a gate electrode  701  is formed over a substrate  700 . Here, a first photomask is used (see  FIG. 24A ). 
     Next, a first insulating film  702  serving as a gate insulating film and a semiconductor film  703  are successively formed without being exposed to air (see  FIG. 24B ). Here, a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % is formed as the first insulating film  702  with an RF sputtering method, and the semiconductor film  703  is formed with a sputtering method using a target in which silicon oxide is added to an oxide including indium (In), gallium (Ga), and zinc (Zn). Note that the semiconductor film  703  is preferably formed under the condition that the substrate temperature is higher than or equal to room temperature (25° C.) and lower than 200° C. 
     On the occasion of the formation of the semiconductor film  703 , a property of an oxide semiconductor film which is used as the semiconductor film  703  was evaluated separately.  FIG. 25A  illustrates a three-dimensional view of a property-evaluation sample  510  for evaluating a property of the oxide semiconductor film. The property-evaluation sample  510  was fabricated and subjected to Hall effect measurement at room temperature. Carrier concentration and Hall mobility of the oxide semiconductor film were evaluated. The property-evaluation sample  510  was fabricated in the following manner: an insulating film  501  including silicon oxynitride was formed over a substrate  500 , an oxide semiconductor film  502  serving as an evaluation object was formed over the insulating film  501 , and electrodes  503  to  506  were formed over the oxide semiconductor film  502 . Oxide semiconductor films serving as evaluation objects were formed using three kinds of targets in which 2 weight %, 5 weight %, and 10 weight % of silicon oxide was added to respective target materials. The property-evaluation sample  510  was fabricated for each of the oxide semiconductor films, and Hall effect measurement was performed thereon at room temperature. In addition, a sample of an oxide semiconductor film formed using a target to which silicon oxide was not added was prepared as a reference, and evaluation was performed thereon in a similar manner. 
       FIG. 25B  shows the carrier concentrations of the oxide semiconductor films, which were obtained by the Hall effect measurement. In  FIG. 25B , the horizontal axis represents the additive amount of silicon oxide, and the vertical axis represents the carrier concentration. As the additive amount of silicon oxide is increased from 0 weight % to 2 weight % to 5 weight % to 10 weight %, the carrier concentration is decreased from 1.6×10 19 /cm 3  to 8.0×10 17 /cm 3  to 2.7×10 16 /cm 3  to 2.0×10 12 /cm 3 . 
       FIG. 25C  shows the Hall mobility of the oxide semiconductor films, which was obtained by the Hall effect measurement. In  FIG. 25C , the horizontal axis represents the additive amount of silicon oxide, and the vertical axis represents the Hall mobility. As the additive amount of silicon oxide is increased from 0 weight % to 2 weight % to 5 weight % to 10 weight %, the Hall mobility is decreased from 15.1 cm 2 /Vs to 8.1 cm 2 /Vs to 2.6 cm 2 /Vs to 1.8 cm 2 /Vs. 
     The results in  FIGS. 25B and 25C  show that the carrier concentration and the Hall mobility tend to be decreased as the additive amount of silicon oxide is increased while there is no large difference in the Hall mobility between the case of adding 5 weight % of silicon oxide and the case of adding 10 weight % of silicon oxide. Therefore, when silicon oxide is added to an IGZO semiconductor layer, silicon oxide may be added to a target at greater than 0 weight % and less than or equal to 10 weight %; however, the range is preferably greater than 0 weight % and less than or equal to 6 weight %. That is, the carrier concentration may be higher than or equal to 2.0×10 12 /cm 3  and lower than 1.6×10 19 /cm 3 ; however, the range is preferably higher than or equal to 2.0×10 16 /cm 3  and lower than 1.6×10 19 /cm 3 . In addition, the Hall mobility may be higher than or equal to 1.8 cm 2 /Vs and lower than 15.1 cm 2 /Vs; however, the range is preferably higher than or equal to 2.4 cm 2 /Vs and lower than 15.1 cm 2 /vs. 
     After the semiconductor film  703  is formed, heat treatment is preferably performed at higher than or equal to 200° C. and lower than or equal to 600° C., typically higher than or equal to 300° C. and lower than or equal to 500° C. Here, heat treatment is performed in a furnace at 350° C. for 1 hour in a nitrogen atmosphere including oxygen. This heat treatment allows atoms of the IGZO semiconductor layer to be rearranged. By this heat treatment (the heat treatment may be light annealing), strain which inhibits carrier movement is released. There is no particular limitation on timing of the heat treatment as long as it is performed after the formation of the semiconductor film  703 . 
     Next, in order to pattern the semiconductor film  703 , the semiconductor film  703  is selectively etched to form an IGZO semiconductor layer  705 . The etching is performed with a dry etching method using a chlorine gas. In this stage, a surface of the gate insulating film is exposed in a region where the semiconductor film  703  is removed. Here, a second photomask is used. The mask formed over the semiconductor film  703  in the patterning is removed by ashing treatment in an oxygen atmosphere. The cross-sectional structure of the substrate at this stage corresponds to the cross-sectional view of the substrate illustrated in  FIG. 24C  (see  FIG. 24C ). In order to eliminate moisture from the manufacturing process of the thin film transistor as much as possible, cleaning using water is not necessarily performed after that. 
     Next, a metal multilayer film serving as a source electrode and a drain electrode is formed. Here, an aluminum film is stacked over a titanium film and another titanium film is stacked over the aluminum film with a DC magnetron sputtering method. Both a titanium target and an aluminum target are set in a sputtering chamber, and the films are successively formed using shutters so as to be sequentially stacked; thus, successive stacking can be performed in one chamber. 
     Next, the metal multilayer film is selectively etched to form a source electrode  708  and a drain electrode  709 . Here, a third photomask is used. The etching is performed with a dry etching method. In this case, the condition where the metal multilayer film can be etched and the etching rate thereof is sufficiently different from that of the IGZO semiconductor layer  705  is employed. Accordingly, the source electrode  708  and the drain electrode  709  can be formed in the same etching step (see  FIG. 24D ). 
     Next, in order to prevent entry or diffusion of hydrogen, moisture, or the like from the outside, a third insulating film  710  is formed so as to cover the thin film transistor. The third insulating film  710  is preferably formed under the condition where hysteresis or charge buildup is not caused in the thin film transistor. The third insulating film  710  can be formed using a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or a silicon nitride oxide film. For example, a silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % may be formed with an RF sputtering method. By use of the silicon oxynitride film including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic %, hydrogen, moisture, or the like can be prevented from entering or diffusing into the thin film transistor. 
     Before the third insulating film  710  is formed, surfaces of the IGZO semiconductor layer  705 , the source electrode  708 , and the drain electrode  709  may be etched by approximately 10 nm by reverse sputtering. By the reverse sputtering, hydrogen, moisture, hydrocarbon, or the like which is attached to the surfaces of the source electrode  708  and the drain electrode  709  can be removed. 
     In this embodiment, a channel formation region is a region of the IGZO semiconductor layer  705 , which overlaps with the gate electrode  701  and is located between the source electrode  708  and the drain electrode  709 . A distance L 5  between the source electrode  708  and the drain electrode  709  corresponds to a channel length (see  FIG. 24E ). 
     As the layers provided over and under the channel formation region of the IGZO semiconductor layer  705 , silicon oxynitride films each including nitrogen at greater than or equal to 3 atomic % and less than or equal to 30 atomic % are formed so that the channel formation region is interposed between the silicon oxynitride films, whereby hydrogen, moisture, or the like can be prevented from entering or diffusing into the channel formation region. 
     In this manner, the thin film transistor including the IGZO semiconductor layer can be manufactured. 
       FIGS. 26 and 25C  indicate that addition of silicon oxide helps an oxide semiconductor layer to be amorphous and that effect of reducing variation in characteristics at the time of manufacturing a semiconductor device can be obtained. Moreover, a quantity of Ga, expensive element promoting the amorphous character of the IGZO semiconductor layer, can be reduced or cut, whereby productivity can be increased. 
     Embodiment 6 
     In this embodiment, an example of a semiconductor device, more precisely a display device in which at least part of a driver circuit and a thin film transistor provided in a pixel portion are formed over the same substrate, with be described. 
     The thin film transistor provided in the pixel portion is formed according to any of Embodiments 1 to 5. Further, the thin film transistors described in Embodiments 1 to 5 are n-channel TFTs, and thus part of a driver circuit that can include an n-channel TFT among driver circuits is formed over the same substrate as the thin film transistor of the pixel portion. 
       FIG. 5A  illustrates an example of a block diagram of an active matrix liquid crystal display device which is an embodiment of a semiconductor device. The display device illustrated in  FIG. 5A  includes, over a substrate  5300 , a pixel portion  5301  including a plurality of pixels that are each provided with a display element; a scan line driver circuit  5302  that selects a pixel; and a signal line driver circuit  5303  that controls a video signal input to the selected pixel. 
     The pixel portion  5301  is connected to the signal line driver circuit  5303  by a plurality of signal lines S 1  to Sm (not illustrated) that extend in a column direction from the signal line driver circuit  5303 , and to the scan line driver circuit  5302  by a plurality of scan lines G 1  to Gn (not illustrated) that extend in a row direction from the scan line driver circuit  5302 . The pixel portion  5301  includes a plurality of pixels (not illustrated) arranged in matrix so as to correspond to the signal lines S 1  to Sm and the scan lines G 1  to Gn. Each pixel is connected to a signal line Sj (one of the signal lines S 1  to Sm) and a scan line Gi (one of the scan lines G 1  to Gn). 
     In addition, the thin film transistors described in Embodiments 1 to 5 are n-channel TFTs, and a signal line driver circuit including n-channel TFTs is described with reference to  FIG. 6 . 
     The signal line driver circuit illustrated in  FIG. 6  includes a driver IC  5601 , switch groups  5602 _ 1  to  5602 _M, a first wiring  5611 , a second wiring  5612 , a third wiring  5613 , and wirings  5621 _ 1  to  5621 _M. Each of the switch groups  5602 _ 1  to  5602 _M includes a first thin film transistor  5603   a , a second thin film transistor  5603   b , and a third thin film transistor  5603   c.    
     The driver IC  5601  is connected to the first wiring  5611 , the second wiring  5612 , the third wiring  5613 , and the wirings  5621 _ 1  to  5621 _M. Each of the switch groups  5602 _ 1  to  5602 _M is connected to the first wiring  5611 , the second wiring  5612 , and the third wiring  5613 , and the wirings  5621 _ 1  to  5621 _M are connected to the switch groups  5602 _ 1  to  5602 _M, respectively. Each of the wirings  5621 _ 1  to  5621 _M is connected to three signal lines (a signal line Sm−2, a signal line Sm−1, and a signal line Sm (m=3M)) via the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c . For example, the wiring  5621 _J of the J-th column (one of the wirings  5621 _ 1  to  5621 _M) is connected to a signal line Sj−2, a signal line Sj−1, and a signal line Sj (j=3J) via the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c  which are included in the switch group  5602 _J. 
     A signal is input to each of the first wiring  5611 , the second wiring  5612 , and the third wiring  5613 . 
     Note that the driver IC  5601  is preferably formed using a single crystal semiconductor. The switch groups  5602 _ 1  to  5602 _M are preferably formed over the same substrate as the pixel portion and are thus preferably not formed on the same substrate as the driver IC. Therefore, the driver IC  5601  and the switch groups  5602 _ 1  to  5602 _M are preferably connected through an FPC or the like. Alternatively, the driver IC  5601  may be formed using a single crystal semiconductor layer which is provided with a method such as bonding over the same substrate as the pixel portion. 
     Next, operation of the signal line driver circuit illustrated in  FIG. 6  is described with reference to a timing chart in  FIG. 7 . The timing chart in  FIG. 7  illustrates a case where the scan line Gi of the i-th row is selected. A selection period of the scan line Gi of the i-th row is divided into a first sub-selection period T 1 , a second sub-selection period T 2 , and a third sub-selection period T 3 . In addition, the signal line driver circuit in  FIG. 6  is operated as in  FIG. 7  even when a scan line of another row is selected. 
     Note that the timing chart in  FIG. 7  shows a case where the wiring  5621 _J of the J-th column is connected to the signal line Sj−2, the signal line Sj−1, and the signal line Sj via the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c.    
     The timing chart in  FIG. 7  shows timing at which the scan line Gi of the i-th row is selected, timing  5703   a  of on/off of the first thin film transistor  5603   a , timing  5703   b  of on/off of the second thin film transistor  5603   b , timing  5703   c  of on/off of the third thin film transistor  5603   c , and a signal  5721 _J input to the wiring  5621 _J of the J-th column. 
     In the first sub-selection period T 1 , the second sub-selection period T 2 , and the third sub-selection period T 3 , different video signals are input to the wirings  5621 _ 1  to  5621 _M. For example, a video signal input to the wiring  5621 _J in the first sub-selection period T 1  is input to the signal line Sj−2, a video signal input to the wiring  5621 _J in the second sub-selection period T 2  is input to the signal line Sj−1, and a video signal input to the wiring  5621 _J in the third sub-selection period T 3  is input to the signal line Sj. In addition, the video signals input to the wiring  5621 _J in the first sub-selection period T 1 , the second sub-selection period T 2 , and the third sub-selection period T 3  are denoted by Data_j−2, Data_j−1, and Data_j, respectively. 
     As illustrated in  FIG. 7 , in the first sub-selection period T 1 , the first thin film transistor  5603   a  is turned on, and the second thin film transistor  5603   b  and the third thin film transistor  5603   c  are turned off. At this time, Data_j−2 input to the wiring  5621 _J is input to the signal line Sj−2 via the first thin film transistor  5603   a . In the second sub-selection period T 2 , the second thin film transistor  5603   b  is turned on, and the first thin film transistor  5603   a  and the third thin film transistor  5603   c  are turned off. At this time, Data_j−1 input to the wiring  5621 _J is input to the signal line Sj−1 via the second thin film transistor  5603   b . In the third sub-selection period T 3 , the third thin film transistor  5603   c  is turned on, and the first thin film transistor  5603   a  and the second thin film transistor  5603   b  are turned off. At this time, Data_j input to the wiring  5621 _J is input to the signal line Sj via the third thin film transistor  5603   c.    
     As described above, in the signal line driver circuit in  FIG. 6 , by dividing one gate selection period into three, video signals can be input to three signal lines from one wiring  5621  in one gate selection period. Therefore, in the signal line driver circuit of  FIG. 6 , the number of connections between the substrate provided with the driver IC  5601  and the substrate provided with the pixel portion can be approximately ⅓ of the number of signal lines. The number of connections is reduced to approximately ⅓ of the number of the signal lines, so that reliability, yield, and the like of the signal line driver circuit in  FIG. 6  can be improved. 
     Note that there is no limitation on the arrangement, the number, a driving method, and the like of the thin film transistors, as long as one gate selection period is divided into a plurality of sub-selection periods and video signals are input to a plurality of signal lines from one wiring in the respective sub-selection periods as illustrated in  FIG. 6 . 
     For example, when video signals are input to three or more signal lines from one wiring in each of three or more sub-selection periods, a thin film transistor and a wiring for controlling the thin film transistor may be added. Note that, when one gate selection period is divided into four or more sub-selection periods, one sub-selection period becomes short. Therefore, one gate selection period is preferably divided into two or three sub-selection periods. 
     As another example, one selection period may be divided into a precharge period Tp, the first sub-selection period T 1 , the second sub-selection period T 2 , and the third sub-selection period T 3  as illustrated in a timing chart of  FIG. 8 . The timing chart in  FIG. 8  illustrates timing at which the scan line Gi of the i-th row is selected, timing  5803   a  of on/off of the first thin film transistor  5603   a , timing  5803   b  of on/off of the second thin film transistor  5603   b , timing  5803   c  of on/off of the third thin film transistor  5603   c , and a signal  5821 _J input to the wiring  5621 _J of the J-th column. As illustrated in  FIG. 8 , the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c  are turned on in the precharge period Tp. At this time, precharge voltage Vp input to the wiring  5621 _J is input to each of the signal line Sj−2, the signal line Sj−1, and the signal line Sj via the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c . In the first sub-selection period T 1 , the first thin film transistor  5603   a  is turned on, and the second thin film transistor  5603   b  and the third thin film transistor  5603   c  are turned off. At this time, Data_j−2 input to the wiring  5621 _J is input to the signal line Sj−2 via the first thin film transistor  5603   a . In the second sub-selection period T 2 , the second thin film transistor  5603   b  is turned on, and the first thin film transistor  5603   a  and the third thin film transistor  5603   c  are turned off. At this time, Data_j−1 input to the wiring  5621 _J is input to the signal line Sj−1 via the second thin film transistor  5603   b . In the third sub-selection period T 3 , the third thin film transistor  5603   c  is turned on, and the first thin film transistor  5603   a  and the second thin film transistor  5603   b  are turned off. At this time, Data_j input to the wiring  5621 _J is input to the signal line Sj via the third thin film transistor  5603   c.    
     As described above, in the signal line driver circuit of  FIG. 6  to which the timing chart in  FIG. 8  is applied, the video signal can be written to the pixel at high speed because the signal line can be precharged by provision of a precharge selection period before a sub-selection period. Note that portions in  FIG. 8  which are similar to those in  FIG. 7  are denoted by common reference numerals and detailed description of the same portions and portions having similar functions is omitted. 
     Further, a structure of a scan line driver circuit is described. The scan line driver circuit includes a shift register and a buffer. Additionally, the scan line driver circuit may include a level shifter in some cases. In the scan line driver circuit, when a clock signal (CLK) and a start pulse signal (SP) are input to the shift register, a selection signal is generated. The generated selection signal is buffered and amplified by the buffer, and the resulting signal is supplied to a corresponding scan line. Gate electrodes of transistors in pixels of one line are connected to the scan line. Further, since the transistors in the pixels of one line have to be turned on at the same time, a buffer which can feed a large amount of current is used. 
     An embodiment of a shift register which is used for part of a scan line driver circuit is described with reference to  FIG. 9  and  FIG. 10 . 
       FIG. 9  illustrates a circuit configuration of the shift register. The shift register illustrated in  FIG. 9  includes a plurality of flip-flops labeled flip-flops  5701 _ 1  to  5701   —   n . The shift register is operated with input of a first clock signal, a second clock signal, a start pulse signal, and a reset signal. 
     The connection relation of the shift register illustrated in  FIG. 9  is described. The flip-flop  5701 _ 1  of a first stage is connected to a first wiring  5711 , a second wiring  5712 , a fourth wiring  5714 , a fifth wiring  5715 , a seventh wiring  5717 _ 1 , and a seventh wiring  5717 _ 2 . The flip-flop  5701 _ 2  of a second stage is connected to a third wiring  5713 , the fourth wiring  5714 , the fifth wiring  5715 , the seventh wiring  5717 _ 1 , the seventh wiring  5717 _ 2 , and a seventh wiring  5717 _ 3 . 
     In a similar manner, the flip-flop  5701   —   i  (one of the flip-flops  5701 _ 1  to  5701   —   n ) of an i-th stage is connected to one of the second wiring  5712  and the third wiring  5713 , the fourth wiring  5714 , the fifth wiring  5715 , a seventh wiring  5717   —   i −1, a seventh wiring  5717   —   i , and a seventh wiring  5717   —   i+ 1. Here, when the “i” is an odd number, the flip-flop  5701   —   i  of the i-th stage is connected to the second wiring  5712 ; when the “i” is an even number, the flip-flop  5701   —   i  of the i-th stage is connected to the third wiring  5713 . 
     The flip-flop  5701   —   n  of an n-th stage is connected to one of the second wiring  5712  and the third wiring  5713 , the fourth wiring  5714 , the fifth wiring  5715 , a seventh wiring  5717   —   n− 1, a seventh wiring  5717   —   n , and a sixth wiring  5716 . 
     Note that the first wiring  5711 , the second wiring  5712 , the third wiring  5713 , and the sixth wiring  5716  may be referred to as a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. The fourth wiring  5714  and the fifth wiring  5715  may be referred to as a first power supply line and a second power supply line, respectively. 
     Next,  FIG. 10  illustrates details of the flip-flop illustrated in  FIG. 9 . A flip-flop illustrated in  FIG. 10  includes a first thin film transistor  5571 , a second thin film transistor  5572 , a third thin film transistor  5573 , a fourth thin film transistor  5574 , a fifth thin film transistor  5575 , a sixth thin film transistor  5576 , a seventh thin film transistor  5577 , and an eighth thin film transistor  5578 . Each of the first thin film transistor  5571 , the second thin film transistor  5572 , the third thin film transistor  5573 , the fourth thin film transistor  5574 , the fifth thin film transistor  5575 , the sixth thin film transistor  5576 , the seventh thin film transistor  5577 , and the eighth thin film transistor  5578  is an n-channel transistor and is turned on when the gate-source voltage (V gs ) exceeds the threshold voltage (V th ). 
     In addition, the flip-flop illustrated in  FIG. 10  includes a first wiring  5501 , a second wiring  5502 , a third wiring  5503 , a fourth wiring  5504 , a fifth wiring  5505 , and a sixth wiring  5506 . 
     Note that all the thin film transistors here are enhancement-type n-channel transistors; however, the present invention is not limited thereto. For example, the driver circuit can be operated using depletion-type n-channel transistors. 
     Next, connections of the flip-flop illustrated in  FIG. 10  are described below. 
     A first electrode (one of a source electrode and a drain electrode) of the first thin film transistor  5571  is connected to the fourth wiring  5504 . A second electrode (the other of the source electrode and the drain electrode) of the first thin film transistor  5571  is connected to the third wiring  5503 . 
     A first electrode of the second thin film transistor  5572  is connected to the sixth wiring  5506 . A second electrode of the second thin film transistor  5572  is connected to the third wiring  5503 . 
     A first electrode of the third thin film transistor  5573  is connected to the fifth wiring  5505 , a second electrode of the third thin film transistor  5573  is connected to a gate electrode of the second thin film transistor  5572 , and a gate electrode of the third thin film transistor  5573  is connected to the fifth wiring  5505 . 
     A first electrode of the fourth thin film transistor  5574  is connected to the sixth wiring  5506 , a second electrode of the fourth thin film transistor  5574  is connected to the gate electrode of the second thin film transistor  5572 , and a gate electrode of the fourth thin film transistor  5574  is connected to a gate electrode of the first thin film transistor  5571 . 
     A first electrode of the fifth thin film transistor  5575  is connected to the fifth wiring  5505 , a second electrode of the fifth thin film transistor  5575  is connected to the gate electrode of the first thin film transistor  5571 , and a gate electrode of the fifth thin film transistor  5575  is connected to the first wiring  5501 . 
     A first electrode of the sixth thin film transistor  5576  is connected to the sixth wiring  5506 , a second electrode of the sixth thin film transistor  5576  is connected to the gate electrode of the first thin film transistor  5571 , and a gate electrode of the sixth thin film transistor  5576  is connected to the gate electrode of the second thin film transistor  5572 . 
     A first electrode of the seventh thin film transistor  5577  is connected to the sixth wiring  5506 , a second electrode of the seventh thin film transistor  5577  is connected to the gate electrode of the first thin film transistor  5571 , and a gate electrode of the seventh thin film transistor  5577  is connected to the second wiring  5502 . 
     A first electrode of the eighth thin film transistor  5578  is connected to the sixth wiring  5506 , a second electrode of the eighth thin film transistor  5578  is connected to the gate electrode of the second thin film transistor  5572 , and a gate electrode of the eighth thin film transistor  5578  is connected to the first wiring  5501 . 
     Note that the points at which the gate electrode of the first thin film transistor  5571 , the gate electrode of the fourth thin film transistor  5574 , the second electrode of the fifth thin film transistor  5575 , the second electrode of the sixth thin film transistor  5576 , and the second electrode of the seventh thin film transistor  5577  are connected are each referred to as a node  5543 . The points at which the gate electrode of the second thin film transistor  5572 , the second electrode of the third thin film transistor  5573 , the second electrode of the fourth thin film transistor  5574 , the gate electrode of the sixth thin film transistor  5576 , and the second electrode of the eighth thin film transistor  5578  are connected are each referred to as a node  5544 . 
     Note that the first wiring  5501 , the second wiring  5502 , the third wiring  5503 , and the fourth wiring  5504  may be referred to as a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. The fifth wiring  5505  and the sixth wiring  5506  may be referred to as a first power supply line and a second power supply line, respectively. 
     In the flip-flop  5701   —   i  of the i-th stage, the first wiring  5501  in  FIG. 10  is connected to the seventh wiring  5717   —   i− 1 in  FIG. 9 . The second wiring  5502  in  FIG. 10  is connected to the seventh wiring  5717   —   i+ 1 in  FIG. 9 . The third wiring  5503  in  FIG. 10  is connected to the seventh wiring  5717   —   i . The sixth wiring  5506  in  FIG. 10  is connected to the fifth wiring  5715 . 
     When the “i” is an odd number, the fourth wiring  5504  in  FIG. 10  is connected to the second wiring  5712  in  FIG. 9 ; when the “i” is an even number, the fourth wiring  5504  in  FIG. 10  is connected to the third wiring  5713  in  FIG. 9 . In addition, the fifth wiring  5505  in  FIG. 10  is connected to the fourth wiring  5714  in  FIG. 9 . 
     Note that, in the flip-flop  5701 _ 1  of the first stage, the first wiring  5501  in  FIG. 10  is connected to the first wiring  5711  in  FIG. 9 . In addition, in the flip-flop  5701   —   n  of the n-th stage, the second wiring  5502  in  FIG. 10  is connected to the sixth wiring  5716  in  FIG. 9 . 
     In addition, the signal line driver circuit and the scan line driver circuit can be formed using only the n-channel TFTs described in any of Embodiments 1 to 5. The n-channel TFT described in any of Embodiments 1 to 5 has high mobility, and thus a driving frequency of a driver circuit can be increased; therefore, the n-channel TFT described in any of Embodiments 1 to 5 has high frequency characteristics (referred to as f characteristics). For example, since a scan line driver circuit using the n-channel TFT described in any of Embodiments 1 to 5 can be operated at high speed, a frame frequency can be increased; thus, insertion of black images can be realized. 
     In addition, when the channel width of the transistor in the scan line driver circuit is increased or a plurality of scan line driver circuits are provided, a higher frame frequency can be realized. When a plurality of scan line driver circuits are provided, a scan line driver circuit for driving scan lines of even-numbered rows is provided, for example, on one side and a scan line driver circuit for driving scan lines of odd-numbered rows is provided on the opposite side; thus, an increase in frame frequency can be realized. Furthermore, the use of the plurality of scan line driver circuits for output of signals to the same scan line is advantageous in increasing the size of a display device. 
     Further, when an active matrix light-emitting display device which is an embodiment of a semiconductor device is manufactured, a plurality of thin film transistors are arranged in at least one pixel, and thus a plurality of scan line driver circuits are preferably arranged.  FIG. 5B  is a block diagram illustrating an example of an active matrix light-emitting display device. 
     The light-emitting display device illustrated in  FIG. 5B  includes, over a substrate  5400 , a pixel portion  5401  having a plurality of pixels each provided with a display element, a first scan line driver circuit  5402  and a second scan line driver circuit  5404  that select a pixel, and a signal line driver circuit  5403  that controls input of a video signal to the selected pixel. 
     When the video signal input to a pixel of the light-emitting display device illustrated in  FIG. 5B  is a digital signal, a pixel emits or does not emit light by switching on/off of a transistor. Thus, grayscale can be displayed using an area ratio grayscale method or a time ratio grayscale method. An area ratio grayscale method refers to a driving method by which one pixel is divided into a plurality of subpixels and the subpixels are driven independently based on video signals so that grayscale is displayed. A time ratio grayscale method refers to a driving method by which a period during which a pixel emits light is controlled so that grayscale is displayed. 
     Since the response speed of light-emitting elements is higher than that of liquid crystal elements or the like, the light-emitting elements are more suitable for a time ratio grayscale method than liquid-crystal display elements. Specifically, in the case of displaying with a time gray scale method, one frame period is divided into a plurality of subframe periods. Then, in accordance with video signals, the light-emitting element in the pixel is set in a light-emitting state or in a non-light-emitting state during each subframe period. By dividing one frame period into a plurality of subframe periods, the total length of time, in which pixels actually emit light in one frame period, can be controlled with video signals so that gray scales are displayed. 
     In the example of the light-emitting display device illustrated in  FIG. 5B , in the case where two switching TFTs are arranged in one pixel, the first scan line driver circuit  5402  generates a signal which is input to a first scan line serving as a gate wiring of one of the switching TFTs, and the second scan line driver circuit  5404  generates a signal which is input to a second scan line serving as a gate wiring of the other of the switching TFTs; however, one scan line driver circuit may generate both the signal which is input to the first scan line and the signal which is input to the second scan line. In addition, for example, there is a possibility that a plurality of scan lines used for controlling the operation of the switching elements be provided in each pixel depending on the number of switching TFTs included in one pixel. In that case, one scan line driver circuit may generate all signals that are input to the plurality of scan lines, or a plurality of scan line driver circuits may generate signals that are input to the plurality of scan lines. 
     In addition, also in the light-emitting display device, part of the driver circuit that can include n-channel TFTs among driver circuits can be formed over the same substrate as the thin film transistors of the pixel portion. Alternatively, the signal line driver circuit and the scan line driver circuit can be formed using only the n-channel TFTs described in any of Embodiments 1 to 5. 
     Moreover, the above-described driver circuit can be used for electronic paper that drives electronic ink using an element electrically connected to a switching element, without being limited to applications to a liquid crystal display device or a light-emitting display device. The electronic paper is also referred to as an electrophoretic display device (electrophoretic display) and has advantages in that it has a level of readability comparable to that of plain paper, it has lower power consumption than other display devices, and it can be made thin and lightweight. 
     Embodiment 7 
     The thin film transistor described in any of Embodiments 1 to 5 is manufactured, and a semiconductor device having a display function (also referred to as a display device) can be manufactured using the thin film transistor in a pixel portion and further in a driver circuit. Further, part or whole of a driver circuit can be formed over the same substrate as a pixel portion, using the thin film transistor described in any of Embodiments 1 to 5, whereby a system-on-panel can be obtained. 
     The display device includes a display element. As the display element, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. Light-emitting elements include, in its category, an element whose luminance is controlled by current or voltage, and specifically include an inorganic electroluminescent (EL) element, an organic EL element, and the like. Further, a display medium whose contrast is changed by an electric effect, such as an electronic ink, can be used. 
     In addition, the display device includes a panel in which the display element is sealed, and a module in which an IC or the like including a controller is mounted on the panel. An embodiment of the present invention relates to one embodiment of an element substrate before the display element is completed in a manufacturing process of the display device, and the element substrate is provided with a means for supplying current to the display element in each of a plurality of pixels. Specifically, the element substrate may be in a state provided with only a pixel electrode of the display element, a state after a conductive film serving as a pixel electrode is formed and before the conductive film is etched to form the pixel electrode, or any of other states. 
     Note that a display device in this specification means an image display device, a display device, or a light source (including a lighting device). Further, the display device includes any of the following modules in its category: a module to which a connector such as a flexible printed circuit (FPC), tape automated bonding (TAB) tape, or a tape carrier package (TCP) is attached; a module having TAB tape or a TCP which is provided with a printed wiring board at the end thereof; and a module having an integrated circuit (IC) which is directly mounted on a display element by a chip on glass (COG) method. 
     In this embodiment, the appearance and a cross section of a liquid crystal display panel which is one embodiment of a semiconductor device will be described with reference to FIGS.  11 A 1 ,  11 A 2 , and  11 B. FIGS.  11 A 1  and  11 A 2  are top views of a panel in which highly reliable thin film transistors  4010  and  4011  each including an oxide semiconductor layer, which are described in any of Embodiments 1 to 5, and a liquid crystal element  4013  formed over a first substrate  4001  are sealed with a sealant  4005  between the first substrate  4001  and a second substrate  4006 .  FIG. 11B  is a cross-sectional view taken along line M-N of FIGS.  11 A 1  and  11 A 2 . 
     The sealant  4005  is provided so as to surround a pixel portion  4002  and a scan line driver circuit  4004  which are provided over the first substrate  4001 . The second substrate  4006  is provided over the pixel portion  4002  and the scan line driver circuit  4004 . Therefore, the pixel portion  4002  and the scan line driver circuit  4004  are sealed together with a liquid crystal layer  4008 , by the first substrate  4001 , the sealant  4005 , and the second substrate  4006 . A signal line driver circuit  4003  that is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant  4005  over the first substrate  4001 . 
     Note that the connection method of a driver circuit which is separately formed is not particularly limited, and a COG method, a wire bonding method, a TAB method, or the like can be used. FIG.  11 A 1  illustrates an example of mounting the signal line driver circuit  4003  with a COG method, and FIG.  11 A 2  illustrates an example of mounting the signal line driver circuit  4003  with a TAB method. 
     The pixel portion  4002  and the scan line driver circuit  4004  provided over the first substrate  4001  include a plurality of thin film transistors.  FIG. 11B  illustrates the thin film transistor  4010  included in the pixel portion  4002  and the thin film transistor  4011  included in the scan line driver circuit  4004 . Over the thin film transistors  4010  and  4011 , insulating layers  4020  and  4021  are provided. 
     The highly reliable thin film transistor including an oxide semiconductor layer, which is described in any of Embodiments 1 to 5, can be applied to each of the thin film transistors  4010  and  4011 . In this embodiment, the thin film transistors  4010  and  4011  are n-channel thin film transistors. 
     A pixel electrode layer  4030  of the liquid crystal element  4013  is electrically connected to the thin film transistor  4010 . A counter electrode layer  4031  of the liquid crystal element  4013  is formed on the second substrate  4006 . A portion where the pixel electrode layer  4030 , the counter electrode layer  4031 , and the liquid crystal layer  4008  overlap one another corresponds to the liquid crystal element  4013 . Note that the pixel electrode layer  4030  and the counter electrode layer  4031  are provided with an insulating layer  4032  and an insulating layer  4033 , respectively, which each function as an alignment film. Further, the liquid crystal layer  4008  is interposed between the pixel electrode layer  4030  and the counter electrode layer  4031  with the insulating layers  4032  and  4033  each interposed between the liquid crystal layer  4008  and their respective pixel electrode layer. 
     Note that the first substrate  4001  and the second substrate  4006  can be formed using glass, metal (typically, stainless steel), ceramic, or plastic. As a plastic substrate, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. In addition, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used. 
     Reference numeral  4035  denotes a columnar spacer obtained by selectively etching an insulating film and is provided to control the distance (a cell gap) between the pixel electrode layer  4030  and the counter electrode layer  4031 . Note that a spherical spacer may also be used. The counter electrode layer  4031  is electrically connected to a common potential line provided over the same substrate as the thin film transistor  4010 . With the use of the common connection portion, the counter electrode layer  4031  can be electrically connected to the common potential line through conductive particles provided between the pair of substrates. Note that the conductive particles are included in the sealant  4005 . 
     Alternatively, a 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 appears just before a cholesteric phase changes into an isotropic phase while temperature of a cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow range of temperature, the liquid crystal layer  4008  is formed using a liquid crystal composition including a chiral agent at 5 weight % or more so that the temperature range is widen. The liquid crystal composition including a liquid crystal exhibiting a blue phase and a chiral agent has a short response time of 10 μs to 100 μs, has optical isotropy, which makes the alignment process unneeded, and has small viewing angle dependence. 
     Although an example of a transmissive liquid crystal display device is described in this embodiment, the present invention can also be applied to a reflective liquid crystal display device and a transflective liquid crystal display device. 
     As an example of the present embodiment, a liquid crystal display device in which a polarizing plate is provided on the outer side of the substrate (on the viewer side) and a coloring layer and the electrode layer used for the display element are provided on the inner side of the substrate in that order; however, the polarizing plate may be provided on the inner side of the substrate. The stacked-layer structure of the polarizing plate and the coloring layer is not limited to this embodiment and may be set as appropriate depending on materials of the polarizing plate and the coloring layer or conditions of manufacturing steps. Further, a light-blocking film functioning as a black matrix may be provided. 
     In this embodiment, in order to reduce surface unevenness of the thin film transistor and to improve reliability of the thin film transistor, the thin film transistor obtained in any of Embodiments 1 to 5 is covered with the insulating layers (the insulating layer  4020  and the insulating layer  4021 ) functioning as a protective film and a planarizing insulating film. Note that the protective film is provided to prevent entry of contaminant impurities such as an organic substance, metal, or moisture present in air and is preferably a dense film. The protective film may be formed with a single layer or a stacked layer of any of 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, and an aluminum nitride oxide film with a sputtering method. Although an example in which the protective film is formed with a sputtering method is described in this embodiment, the present invention is not limited to this example, and the protective film may be formed with a variety of methods. 
     In this embodiment, the insulating layer  4020  having a stacked-layer structure is formed as a protective film. Here, as a first layer of the insulating layer  4020 , a silicon oxide film is formed with a sputtering method. The use of a silicon oxide film as a protective film has an effect of preventing hillocks of an aluminum film which is used as a source electrode layer and a drain electrode layer. 
     As a second layer of the protective film, an insulating layer is formed. In this embodiment, as the second layer of the insulating layer  4020 , a silicon nitride film is formed with a sputtering method. The use of the silicon nitride film as the protective film can prevent mobile ions of sodium or the like from entering a semiconductor region so that variation in electric characteristics of the TFT can be suppressed. 
     After the protective film is formed, the oxide semiconductor layer may be annealed (at 300° C. to 400° C.). 
     The insulating layer  4021  is formed as the planarizing insulating film. As the insulating layer  4021 , an organic material having heat resistance such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that the insulating layer  4021  may be formed by stacking a plurality of insulating films formed using these materials. 
     Note that a siloxane-based resin is a resin formed using a siloxane-based material as a starting material and having the bond of Si—O—Si. The siloxane-based resin may include an organic group (such as an alkyl group or an aryl group) or a fluoro group as a substituent. The organic group may include a fluoro group. 
     A formation method of the insulating layer  4021  is not particularly limited, and the following method can be employed depending on the material: a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, a droplet discharge method (such as an inkjet method, screen printing, or offset printing), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. In the case of forming the insulating layer  4021  using a material solution, annealing (300° C. to 400° C.) of the oxide semiconductor layer may be performed at the same time as a baking step. The baking step of the insulating layer  4021  also serves as the annealing of the oxide semiconductor layer, whereby a semiconductor device can be manufactured efficiently. 
     The pixel electrode layer  4030  and the counter electrode layer  4031  can be formed using a light-transmitting conductive material such as indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, indium tin oxide to which silicon oxide is added, or the like. 
     A conductive composition including a conductive macromolecule (also referred to as a conductive polymer) can be used for the pixel electrode layer  4030  and the counter electrode layer  4031 . The pixel electrode formed using the conductive composition preferably has a sheet resistance of less than or equal to 10000 ohms per square and transmittance of greater than or equal to 70% at a wavelength of 550 nm. Further, the resistivity of the conductive polymer included in the conductive composition is preferably less than or equal to 0.1 Ω·cm. 
     As the conductive polymer, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more kinds of them, and the like can be given. 
     Further, a variety of signals and potentials are supplied to the signal line driver circuit  4003  which is formed separately, the scan line driver circuit  4004 , or the pixel portion  4002  from an FPC  4018 . 
     In this embodiment, a connection terminal electrode  4015  is formed from the same conductive film as the pixel electrode layer  4030  included in the liquid crystal element  4013 , and a terminal electrode  4016  is formed from the same conductive film as the source and drain electrode layers of the thin film transistors  4010  and  4011 . 
     The connection terminal electrode  4015  is electrically connected to a terminal included in the FPC  4018  through an anisotropic conductive film  4019 . 
     FIGS.  11 A 1 ,  11 A 2 , and  11 B illustrate an example in which the signal line driver circuit  4003  is formed separately and mounted on the first substrate  4001 ; however, this embodiment is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted. 
       FIG. 12  illustrates an example in which a liquid crystal display module is formed as a semiconductor device by using a TFT substrate  2600  manufactured by application of the TFT described in any of Embodiments 1 to 5. 
       FIG. 12  illustrates an example of a liquid crystal display module, in which the TFT substrate  2600  and a counter substrate  2601  are fixed to each other with a sealant  2602 , and a pixel portion  2603  including a TFT or the like, a display element  2604  including a liquid crystal layer, and a coloring layer  2605  are provided between the substrates to form a display region. The coloring layer  2605  is necessary to perform color display. In the case of the RGB system, respective coloring layers corresponding to colors of red, green, and blue are provided for respective pixels. Polarizing plates  2606  and  2607  and a diffusion plate  2613  are provided outside the TFT substrate  2600  and the counter substrate  2601 . A light source includes a cold cathode tube  2610  and a reflective plate  2611 , and a circuit board  2612  is connected to a wiring circuit portion  2608  of the TFT substrate  2600  through a flexible wiring board  2609  and includes an external circuit such as a control circuit or a power source circuit. The polarizing plate and the liquid crystal layer may be stacked with a retardation plate interposed therebetween. 
     For the liquid crystal display module, 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 used. 
     Through the above process, a highly reliable liquid crystal display panel as a semiconductor device can be manufactured. 
     Note that the structure described in this embodiment can be combined with any of the structures described in other embodiments as appropriate. 
     Embodiment 8 
     In this embodiment, an example of electronic paper will be described as an embodiment of a semiconductor device to which the thin film transistor described in any of Embodiments 1 to 5 is applied. 
       FIG. 13  illustrates active matrix electronic paper as an example of a semiconductor device. A thin film transistor  581  used for the semiconductor device can be manufactured by application of the thin film transistor described in any of Embodiments 1 to 5. 
     The electronic paper in  FIG. 13  is an example of a display device using a twisting ball display system. The twisting ball display system refers to a method in which spherical particles colored in black and white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed. 
     The thin film transistor  581  which is formed over a substrate  596  is a thin film transistor with a bottom gate structure, and a source or drain electrode layer thereof is in contact with a first electrode layer  587  through an opening formed in an insulating layer  585 , whereby the thin film transistor  581  is electrically connected to the first electrode layer  587 . Between the first electrode layer  587  and a second electrode layer  588  which is formed on a substrate  597 , spherical particles  589  each having a black region  590   a , a white region  590   b , and a cavity  594  around the regions which is filled with liquid are provided. A space around the spherical particles  589  is filled with a filler  595  such as a resin (see  FIG. 13 ). In this embodiment, the first electrode layer  587  corresponds to a pixel electrode and the second electrode layer  588  corresponds to a common electrode. The second electrode layer  588  is electrically connected to a common potential line provided over the same substrate as the thin film transistor  581 . With the use of the common connection portion described in any one of Embodiments 1 to 5, the second electrode layer  588  is electrically connected to the common potential line through conductive particles provided between the pair of substrates. 
     Further, instead of the twisting ball, an electrophoretic element can be used. A microcapsule having a diameter of approximately 10 μm to 200 μm in which transparent liquid, positively charged white microparticles, and negatively charged black microparticles are encapsulated, is used. In the microcapsule which is provided between the first electrode layer and the second electrode layer, when an electric field is applied by the first electrode layer and the second electrode layer, the white microparticles and the black microparticles move to opposite sides, so that white or black can be displayed. A display element using this principle is an electrophoretic display element and is called electronic paper in general. The electrophoretic display element has higher reflectance than a liquid crystal display element, and thus, an auxiliary light is unnecessary, power consumption is low, and a display portion can be recognized even in a dim place. In addition, even when power is not supplied to the display portion, an image which has been once displayed can be maintained. Accordingly, an image display can be maintained even when a semiconductor device having a display function (which may be simply referred to as a display device or a semiconductor device provided with a display device) is distanced from a wireless electric power source. 
     An electrophoretic display element is a display element that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a region with high electric field. An electrophoretic display element does not need to have a polarizing plate, which is necessary in a liquid crystal display device, and the thickness and weight of a device including the electrophoretic display element can be reduced as compared to those of a liquid crystal display device. 
     A solution in which the above microcapsules are dispersed in a solvent is referred to as electronic ink. This electronic ink can be printed on a surface of glass, plastic, cloth, paper, or the like. Furthermore, by using a color filter or particles that have a pigment, color display can also be achieved. 
     In addition, if a plurality of the above microcapsules are arranged as appropriate over an active matrix substrate so as to be interposed between two electrodes, an active matrix display device can be completed, and display can be performed by application of an electric field to the microcapsules. For example, the active matrix substrate obtained with the thin film transistor described in any of Embodiments 1 to 5 can be used. 
     Note that the microparticles in the microcapsules may be formed using a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material, or may be formed using a composite material of any of these. 
     Note that the structure described in this embodiment can be combined with any of the structures described in other embodiments as appropriate. 
     Embodiment 9 
     In this embodiment, an example of a light-emitting display device will be described as an embodiment of a semiconductor device to which the thin film transistor described in any of Embodiments 1 to 5 is applied. As a display element included in a display device, a light-emitting element utilizing electroluminescence is described here. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element. 
     In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer including a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element. 
     Inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. A dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that description is made here using an organic EL element as a light-emitting element. 
       FIG. 14  illustrates an example of a pixel structure to which digital time grayscale driving can be applied, as an example of a semiconductor device to which an embodiment of the present invention is applied. 
     A structure and operation of a pixel to which digital time grayscale driving can be applied are described. Here, one pixel includes two n-channel transistors each of which includes an oxide semiconductor layer in a channel formation region and is described in any of Embodiments 1 to 5. 
     A pixel  6400  includes a switching transistor  6401 , a driving transistor  6402 , a light-emitting element  6404 , and a capacitor  6403 . A gate of the switching transistor  6401  is connected to a scan line  6406 , a first electrode (one of a source electrode and a drain electrode) of the switching transistor  6401  is connected to a signal line  6405 , and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor  6401  is connected to a gate of the driving transistor  6402 . The gate of the driving transistor  6402  is connected to a power supply line  6407  through the capacitor  6403 , a first electrode of the driving transistor  6402  is connected to the power supply line  6407 , and a second electrode of the driving transistor  6402  is connected to a first electrode (pixel electrode) of the light-emitting element  6404 . A second electrode of the light-emitting element  6404  corresponds to a common electrode  6408 . The common electrode  6408  is electrically connected to a common potential line provided over the same substrate. 
     The second electrode (the common electrode  6408 ) of the light-emitting element  6404  is set to a low power supply potential. Note that the low power supply potential is a potential satisfying the low power supply potential &lt;a high power supply potential with the high power supply potential set to the power supply line  6407  as a reference. As the low power supply potential, GND, 0 V, or the like may be employed, for example. A potential difference between the high power supply potential and the low power supply potential is applied to the light-emitting element  6404  and current is supplied to the light-emitting element  6404 , so that the light-emitting element  6404  emits light. Here, in order to make the light-emitting element  6404  emit light, each potential is set so that the potential difference between the high power supply potential and the low power supply potential is higher than or equal to a forward threshold voltage of the light emitting element  6404 . 
     Note that gate capacitance of the driving transistor  6402  may be used as a substitute for the capacitor  6403 , so that the capacitor  6403  can be omitted. The gate capacitance of the driving transistor  6402  may be formed between the channel region and the gate electrode. 
     In the case of a voltage-input voltage driving method, a video signal is input to the gate of the driving transistor  6402  so that the driving transistor  6402  is in either of two states of being sufficiently turned on and turned off. That is, the driving transistor  6402  is operated in a linear region. Since the driving transistor  6402  is operated in a linear region, a voltage higher than the voltage of the power supply line  6407  is applied to the gate of the driving transistor  6402 . Note that voltage higher than or equal to (voltage of the power supply line+V th  of the driving transistor  6402 ) is applied to the signal line  6405 . 
     In the case of performing analog grayscale driving instead of digital time grayscale driving, the same pixel structure as  FIG. 14  can be used by changing signal input. 
     In the case of performing analog grayscale driving, voltage higher than or equal to the sum of the forward voltage of the light-emitting element  6404  and the V th  of the driving transistor  6402  is applied to the gate of the driving transistor  6402 . The forward voltage of the light-emitting element  6404  indicates voltage at which a desired luminance is obtained, and includes at least forward threshold voltage. The video signal by which the driving transistor  6402  is operated in a saturation region is input, so that current can be supplied to the light-emitting element  6404 . In order for the driving transistor  6402  to be operated in a saturation region, the potential of the power supply line  6407  is set higher than the gate potential of the driving transistor  6402 . When an analog video signal is used, it is possible to feed current to the light-emitting element  6404  in accordance with the video signal and perform analog grayscale driving. 
     Note that the pixel structure illustrated in  FIG. 14  is not limited thereto. For example, a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like may be added to the pixel illustrated in  FIG. 14 . 
     Next, structures of the light-emitting element are described with reference to  FIGS. 15A to 15C . A cross-sectional structure of a pixel is described by taking an n-channel driving TFT as an example. Driving TFTs  7001 ,  7011 , and  7021  used for semiconductor devices illustrated in  FIGS. 15A to 15C  can be formed in a manner similar to that of the thin film transistor described in any of Embodiments 1 to 5 and are highly reliable thin film transistors each including an oxide semiconductor layer typified by an In—Ga—Zn—O-based non-single-crystal film. 
     In order to extract light emitted from the light-emitting element, at least one of the anode and the cathode is required to transmit light. A thin film transistor and a light-emitting element are formed over a substrate. A light-emitting element can have a top emission structure, in which light emission is extracted through the surface opposite to the substrate; a bottom emission structure, in which light emission is extracted through the surface on the substrate side; or a dual emission structure, in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side. A pixel structure of an embodiment of the present invention can be applied to a light-emitting element having any of these emission structures. 
     A light-emitting element having a top emission structure is described with reference to  FIG. 15A . 
       FIG. 15A  is a cross-sectional view of a pixel in the case where the driving TFT  7001  is an n-channel TFT and light is emitted from a light-emitting element  7002  to an anode  7005  side. In  FIG. 15A , a cathode  7003  of the light-emitting element  7002  is electrically connected to the driving TFT  7001 , and a light-emitting layer  7004  and the anode  7005  are stacked in this order over the cathode  7003 . The cathode  7003  can be formed using a variety of conductive materials as long as they have a low work function and reflect light. For example, Ca, Al, MgAg, AlLi, or the like is preferably used. The light-emitting layer  7004  may be formed using a single layer or a plurality of layers stacked. When the light-emitting layer  7004  is formed using a plurality of layers, the light-emitting layer  7004  is formed by stacking an electron-injection layer, an electron-transport layer, a light-emitting layer, a hole-transport layer, and a hole-injection layer in this order over the cathode  7003 . It is not necessary to form all of these layers. The anode  7005  is formed using a light-transmitting conductive film such as a film of indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added. 
     The light-emitting element  7002  corresponds to a region where the light-emitting layer  7004  is sandwiched between the cathode  7003  and the anode  7005 . In the case of the pixel illustrated in  FIG. 15A , light is emitted from the light-emitting element  7002  to the anode  7005  side as indicated by an arrow. 
     Next, a light-emitting element having a bottom emission structure is described with reference to  FIG. 15B .  FIG. 15B  is a cross-sectional view of a pixel in the case where the driving TFT  7011  is an n-channel transistor and light is emitted from a light-emitting element  7012  to a cathode  7013  side. In  FIG. 15B , the cathode  7013  of the light-emitting element  7012  is formed over a light-transmitting conductive film  7017  that is electrically connected to the driving TFT  7011 , and a light-emitting layer  7014  and an anode  7015  are stacked in this order over the cathode  7013 . A light-blocking film  7016  for reflecting or blocking light may be formed to cover the anode  7015  when the anode  7015  has a light-transmitting property. For the cathode  7013 , a variety of materials can be used as in the case of  FIG. 15A  as long as they are conductive materials having a low work function. The cathode  7013  is formed to have a thickness that can transmit light (preferably, approximately 5 nm to 30 nm). For example, an aluminum film with a thickness of 20 nm can be used as the cathode  7013 . In a manner similar to the case of  FIG. 15A , the light-emitting layer  7014  may be formed using either a single layer or a plurality of layers stacked. The anode  7015  is not required to transmit light, but can be formed using a light-transmitting conductive material as in the case of  FIG. 15A . As the light-blocking film  7016 , for example, metal or the like that reflects light can be used; however, it is not limited to a metal film. For example, a resin or the like to which black pigments are added can also be used. 
     The light-emitting element  7012  corresponds to a region where the light-emitting layer  7014  is sandwiched between the cathode  7013  and the anode  7015 . In the case of the pixel illustrated in  FIG. 15B , light is emitted from the light-emitting element  7012  to the cathode  7013  side as indicated by an arrow. 
     Next, a light-emitting element having a dual emission structure is described with reference to  FIG. 15C . In  FIG. 15C , a cathode  7023  of a light-emitting element  7022  is formed over a light-transmitting conductive film  7027  which is electrically connected to the driving TFT  7021 , and a light-emitting layer  7024  and an anode  7025  are stacked in this order over the cathode  7023 . As in the case of  FIG. 15A , the cathode  7023  can be formed using a variety of conductive materials as long as they have a low work function. The cathode  7023  is formed to have a thickness so that it can transmit light. For example, a film of Al having a thickness of 20 nm can be used as the cathode  7023 . As in  FIG. 15A , the light-emitting layer  7024  may be formed using either a single layer or a plurality of layers stacked. The anode  7025  can be formed using a light-transmitting conductive material as in the case of  FIG. 15A . 
     The light-emitting element  7022  corresponds to a region where the cathode  7023 , the light-emitting layer  7024 , and the anode  7025  overlap with one another. In the case of the pixel illustrated in  FIG. 15C , light is emitted from the light-emitting element  7022  to both the anode  7025  side and the cathode  7023  side as indicated by arrows. 
     Note that, although an organic EL element is described here as a light-emitting element, an inorganic EL element can also be provided as a light-emitting element. 
     In this embodiment, the example is described in which a thin film transistor (a driving TFT) which controls the driving of a light-emitting element is electrically connected to the light-emitting element; however, a structure may be employed in which a TFT for current control is connected between the driving TFT and the light-emitting element. 
     A semiconductor device described in this embodiment is not limited to the structures illustrated in  FIGS. 15A to 15C  and can be modified in various ways based on the spirit of techniques according to the present invention. 
     Next, the appearance and a cross section of a light-emitting display panel (also referred to as a light-emitting panel), which is one embodiment of a semiconductor device to which the thin film transistor described in any of Embodiments 1 to 5 is applied, are described with reference to  FIGS. 16A and 16B .  FIG. 16A  is a top view of a panel in which a thin film transistor and a light-emitting element formed over a first substrate are sealed with a sealant between the first substrate and a second substrate.  FIG. 16B  is a cross-sectional view taken along line H-I of  FIG. 16A . 
     A sealant  4505  is provided so as to surround a pixel portion  4502 , signal line driver circuits  4503   a  and  4503   b , and scan line driver circuits  4504   a  and  4504   b  which are provided over a first substrate  4501 . In addition, a second substrate  4506  is provided over the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b , and the scan line driver circuits  4504   a  and  4504   b . Accordingly, the pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b , and the scan line driver circuits  4504   a  and  4504   b  are sealed together with a filler  4507 , by the first substrate  4501 , the sealant  4505 , and the second substrate  4506 . It is preferable that a panel be packaged (sealed) with a protective film (such as a laminate film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the panel is not exposed to the outside air as described above. 
     The pixel portion  4502 , the signal line driver circuits  4503   a  and  4503   b , and the scan line driver circuits  4504   a  and  4504   b  provided over the first substrate  4501  each include a plurality of thin film transistors, and a thin film transistor  4510  included in the pixel portion  4502  and a thin film transistor  4509  included in the signal line driver circuit  4503   a  are illustrated as an example in  FIG. 16B . 
     The highly reliable thin film transistor including an oxide semiconductor layer typified by an In—Ga—Zn—O-based non-single-crystal film, which is described in any of Embodiments 1 to 5, can be employed as the thin film transistors  4509  and  4510 . In this embodiment, the thin film transistors  4509  and  4510  are n-channel thin film transistors. 
     Moreover, reference numeral  4511  denotes a light-emitting element. A first electrode layer  4517  which is a pixel electrode included in the light-emitting element  4511  is electrically connected to a source electrode layer or a drain electrode layer of the thin film transistor  4510 . Note that, although the light-emitting element  4511  has a stacked-layer structure composed of the first electrode layer  4517 , an electroluminescent layer  4512 , and the second electrode layer  4513 , the structure of the light-emitting element  4511  is not limited to the structure described in this embodiment. The structure of the light-emitting element  4511  can be changed as appropriate depending on the direction in which light is extracted from the light-emitting element  4511 , or the like. 
     A partition wall  4520  is formed using an organic resin film, an inorganic insulating film, or organic polysiloxane. It is particularly preferable that the partition wall  4520  be formed using a photosensitive material and an opening portion be formed over the first electrode layer  4517  so that a sidewall of the opening portion is formed as an inclined surface with continuous curvature. 
     The electroluminescent layer  4512  may be formed with a single layer or a plurality of layers stacked. 
     A protective film may be formed over the second electrode layer  4513  and the partition wall  4520  in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element  4511 . As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed. 
     In addition, a variety of signals and potentials are supplied to the signal line driver circuits  4503   a  and  4503   b , the scan line driver circuits  4504   a  and  4504   b , or the pixel portion  4502  from FPCs  4518   a  and  4518   b.    
     In this embodiment, a connection terminal electrode  4515  is formed from the same conductive film as the first electrode layer  4517  included in the light-emitting element  4511 , and a terminal electrode  4516  is formed from the same conductive film as the source and drain electrode layers included in the thin film transistors  4509  and  4510 . 
     The connection terminal electrode  4515  is electrically connected to a terminal included in the FPC  4518   a  through an anisotropic conductive film  4519 . 
     The second substrate located in the direction in which light is extracted from the light-emitting element  4511  needs to have a light-transmitting property. In that case, a light-transmitting material such as a glass plate, a plastic plate, a polyester film, or an acrylic film is used. 
     As the filler  4507 , an ultraviolet curable resin or a thermosetting resin can be used, in addition to an inert gas such as nitrogen or argon. For example, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. In this embodiment, nitrogen is used as the filler. 
     In addition, if needed, an optical film such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so that the glare is reduced can be performed. 
     The signal line driver circuits  4503   a  and  4503   b  and the scan line driver circuits  4504   a  and  4504   b  may be provided as driver circuits formed using a single crystal semiconductor film or polycrystalline semiconductor film over a substrate separately prepared. In addition, only the signal line driver circuits or part thereof, or only the scan line driver circuits or part thereof may be separately formed and mounted. This embodiment is not limited to the structure illustrated in  FIGS. 16A and 16B . 
     Through the above process, a highly reliable light-emitting display device (display panel) as a semiconductor device can be manufactured. 
     Note that the structure described in this embodiment can be combined with any of the structures described in other embodiments as appropriate. 
     Embodiment 10 
     A semiconductor device to which the thin film transistor described in any of Embodiments 1 to 5 is applied can be used as electronic paper. Electronic paper can be used for electronic devices of a variety of fields as long as they can display data. For example, electronic paper can be applied to an electronic book (e-book) reader, a poster, an advertisement in a vehicle such as a train, displays of various cards such as a credit card, and the like. Examples of the electronic devices are illustrated in  FIGS. 17A and 17B  and  FIG. 18 . 
       FIG. 17A  illustrates a poster  2631  formed using electronic paper. In the case where an advertising medium is printed paper, replacement of the advertisement is performed by manpower; however, by using electronic paper, the advertising display can be changed in a short time. Further, an image can be stably displayed without being distorted. Note that the poster may be configured to transmit and receive data wirelessly. 
       FIG. 17B  illustrates an advertisement  2632  in a vehicle such as a train. In the case where an advertising medium is printed paper, replacement of the advertisement is performed by manpower; however, by using electronic paper, the advertising display can be changed in a short time without a lot of manpower. Further, an image can be stably displayed without being distorted. Note that the advertisement in a vehicle may be configured to transmit and receive data wirelessly. 
       FIG. 18  illustrates an example of an electronic book reader  2700 . For example, the electronic book reader  2700  includes two housings, 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 reader  2700  can be opened and closed with the hinge  2711  as an axis. With such a structure, the electronic book reader  2700  can be operated 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 be configured to display one image or different images. In the case where the display portion  2705  and the display portion  2707  display different images, for example, a display portion on the right side (the display portion  2705  in  FIG. 18 ) can display text and a display portion on the left side (the display portion  2707  in  FIG. 18 ) can display graphics. 
       FIG. 18  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 may be provided on the surface of the housing, on which the display portion is provided. Further, 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, or the like may be provided on the back surface or the side surface of the housing. Further, the electronic book reader  2700  may have a function of an electronic dictionary. 
     The electronic book reader  2700  may be configured to transmit and receive data wirelessly. The structure can be employed in which desired book data or the like is purchased and downloaded from an electronic book server wirelessly. 
     Note that the structure described in this embodiment can be combined with any of the structures described in other embodiments as appropriate. 
     Embodiment 11 
     A semiconductor device using the thin film transistor described in any of Embodiments 1 to 5 can be applied to a variety of electronic devices (including an amusement machine). Examples of electronic devices include a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone 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. 19A  illustrates an example of a television set  9600 . In the television set  9600 , a display portion  9603  is incorporated in a housing  9601 . The display portion  9603  can display an image. Further, 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. Further, the remote controller  9610  may be provided with a display portion  9607  for displaying data output from the remote controller  9610 . 
     Note that the television set  9600  is provided with a receiver, a modem, and the like. With the receiver, a general television broadcast can be received. Further, when the television set  9600  is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers) data communication can be performed. 
       FIG. 19B  illustrates an example of a digital photo frame  9700 . For example, in the digital photo frame  9700 , a display portion  9703  is incorporated in a housing  9701 . The display portion  9703  can display various 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. 
     Note that the digital photo frame  9700  is provided with an operation portion, an external connection terminal (a USB terminal, a terminal that can be connected to various cables such as a USB cable, or the like), a recording medium insertion portion, and the like. Although these components may be provided on the surface on which the display portion is provided, it is preferable to provide them on the side surface or the back surface for the design of the digital photo frame  9700 . For example, a memory storing data of an image taken with a digital camera is inserted in the recording medium insertion portion of the digital photo frame, whereby the image data can be transferred and then displayed on the display portion  9703 . 
     The digital photo frame  9700  may be configured to transmit and receive data wirelessly. The structure may be employed in which desired image data is transferred wirelessly to be displayed. 
       FIG. 20A  illustrates a portable game machine and includes two housings, a housing  9881  and a housing  9891 , which are connected with a joint portion  9893  so that the portable game machine can be opened or folded. A display portion  9882  is incorporated in the housing  9881 , and a display portion  9883  is incorporated in the housing  9891 . In addition, the portable game machine illustrated in  FIG. 20A  is provided with a speaker portion  9884 , a recording medium insertion portion  9886 , an LED lamp  9890 , input means (an operation key  9885 , a connection terminal  9887 , a sensor  9888  (having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotation number, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radial ray, flow rate, humidity, gradient, vibration, odor, or infrared ray), and a microphone  9889 ), and the like. Needless to say, the structure of the portable game machine is not limited to that described above. The portable game machine may have a structure in which additional accessory equipment is provided as appropriate as long as at least a semiconductor device according to an embodiment of the present invention is provided. The portable game machine illustrated in  FIG. 20A  has a function of reading a program or data stored in a recording medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. Note that a function of the portable game machine illustrated in  FIG. 20A  is not limited to those described above, and the portable game machine can have a variety of functions. 
       FIG. 20B  illustrates an example of a slot machine  9900  which is a large-sized amusement machine. In the slot machine  9900 , a display portion  9903  is incorporated in a housing  9901 . In addition, the slot machine  9900  is provided with operation means such as a start lever and a stop switch, a coin slot, a speaker, and the like. Needless to say, the structure of the slot machine  9900  is not limited to that described above. The slot machine may have a structure in which additional accessory equipment is provided as appropriate as long as at least an embodiment of the present invention is provided. 
       FIG. 21A  illustrates an example of a mobile phone handset  1000 . The mobile phone handset  1000  is provided with a display portion  1002  incorporated in a housing  1001 , operation buttons  1003 , an external connection port  1004 , a speaker  1005 , a microphone  1006 , and the like. 
     When the display portion  1002  of the mobile phone handset  1000  illustrated in  FIG. 21A  is touched with a finger or the like, data can be input into the mobile phone handset  1000 . Further, operation such as making calls and composing mails can be performed by touching the display portion  1002  with a finger or the like. 
     There are mainly three screen modes of the display portion  1002 . The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting data such as text. The third mode is a mode which is a combination of the two modes of the display mode and the input mode. 
     For example, in the case of making a call or composing a mail, a text input mode mainly for inputting text is selected for the display portion  1002  so that text displayed on a screen can be input. In that case, it is preferable to display a keyboard or number buttons on almost all area of the screen of the display portion  1002 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the mobile phone handset  1000 , display on the screen of the display portion  1002  can be automatically changed by determining the orientation of the mobile phone handset  1000  (whether the mobile phone handset  1000  is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are switched by touching the display portion  1002  or using the operation buttons  1003  of the housing  1001 . Alternatively, the screen modes may be switched depending on the kind of the image displayed on the display portion  1002 . For example, when a signal of an image displayed on the display portion is the one of moving image data, the screen mode is switched to the display mode. When the signal is the one of text data, the screen mode is switched to the input mode. 
     Further, in the input mode, when input by touching the display portion  1002  is not performed for a certain period while a signal detected by the optical sensor in the display portion  1002  is detected, the screen mode may be controlled so as to be switched from the input mode to the display mode. 
     The display portion  1002  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken when the display portion  1002  is touched with the palm or the finger, whereby personal identification can be performed. Further, by providing a backlight or a sensing light source which emits a near-infrared light in the display portion, an image of a finger vein, a palm vein, or the like can be taken. 
       FIG. 21B  illustrates another example of a mobile phone handset. The mobile phone handset in  FIG. 21B  has a display device  9410  in a housing  9411 , which includes a display portion  9412  and an operation button  9413 , and a communication device  9400  in a housing  9401 , which includes operation buttons  9402 , an external input terminal  9403 , a microphone  9404 , a speaker  9405 , and a light-emitting portion  9406  that emits light when a phone call is received. The display device  9410  which has a display function can be detached from or attached to the communication device  9400  which has a phone function in two directions indicated by arrows. Thus, the display device  9410  and the communication device  9400  can be attached to each other along their short sides or long sides. In addition, when only the display function is needed, the display device  9410  can be detached from the communication device  9400  and used alone. Images or input data can be transmitted or received by wireless or wired communication between the communication device  9400  and the display device  9410 , each of which has a rechargeable battery. 
     Note that the structure described in this embodiment can be combined with any of the structures described in other embodiments as appropriate. 
     Embodiment 12 
     In this embodiment, an example of an electronic book reader will be described as one embodiment of a semiconductor device to which the thin film transistor described in any of Embodiments 1 to 5 is applied. In this embodiment, an example in which a double-sided third display panel  4313  is provided between a first display panel  4311  and a second display panel  4312  will be described with reference to  FIGS. 22A and 22B  and  FIG. 23 .  FIG. 22A  illustrates an opened electronic book reader, and  FIG. 22B  illustrates a closed electronic book reader.  FIG. 23  is a cross-sectional view of the electronic book reader in a lateral direction. 
     The electronic book reader illustrated in  FIGS. 22A and 22B  includes the first display panel  4311  having a first display portion  4301 , the second display panel  4312  having an operation portion  4304  and a second display portion  4307 , the third display panel  4313  having a third display portion  4302  and a fourth display portion  4310 , and a binding portion  4308  provided in one end portion of the first display panel  4311 , the second display panel  4312 , and the third display panel  4313 . The third display panel  4313  is inserted between the first display panel  4311  and the second display panel  4312 . The electronic book reader of  FIGS. 22A and 22B  includes four display screens of the first display portion  4301 , the second display portion  4307 , the third display portion  4302 , and the fourth display portion  4310 . 
     The first display panel  4311 , the second display panel  4312 , and the third display panel  4313  are flexible and easy to bend. When the first display panel  4311  and the second display panel  4312  are formed using plastic substrates and the third display panel  4313  is formed using a thin film, the electronic book reader can be made thin. In other words, as illustrated in the lateral cross-sectional view of the electronic book reader in  FIG. 23  as an example, the electronic book reader can have a structure in which the third display panel  4313  is more flexible than the first display panel  4311  and the second display panel  4312 . Thus, the display panels provided on the outer sides of the third display panel  4313  are made harder, whereby the electronic book reader can be handled just as a paper book. In addition, damage to the third display panel  4313  can be suppressed. 
     The third display panel  4313  is a double-sided display panel having the third display portion  4302  and the fourth display portion  4310 . The third display panel  4313  may be formed using a dual-emission type display panel or one-side emission type display panels which are bonded to each other. Alternatively, two liquid crystal display panels between which a backlight (preferably a thin-shape EL light-emitting panel) is interposed may be used. 
     The electronic book reader illustrated in  FIGS. 22A and 22B  includes a scan line driver circuit (not illustrated) which controls display of the first display portion  4301 , scan line driver circuits  4322   a  and  4322   b  which control display of the second display portion  4307 , a scan line driver circuit (not illustrated) which controls display of the third display portion  4302  and/or the fourth display portion  4310 , and a signal line driver circuit  4323  which controls display of the first display portion  4301 , the second display portion  4307 , the third display portion  4302 , and/or the fourth display portion  4310 . Note that the scan line driver circuit which controls display of the first display portion  4301  is provided in the first display panel  4311 , the scan line driver circuits  4322   a  and  4322   b  are provided in the second display panel  4312 , and the signal line driver circuit  4323  is provided inside the binding portion  4308 . 
     In the electronic book reader illustrated in  FIGS. 22A and 22B , the second display panel  4312  includes the operation portion  4304  which can have various functions such as a switch for turning on a power source, and a switch for shifting the display. 
     Input operation to the electronic book reader illustrated in  FIGS. 22A and 22B  is performed by touching the first display portion  4301  or the second display portion  4307  with a finger, an input pen, or the like or by operating the operation portion  4304 . Note that a displayed button  4309  which is displayed on the second display portion  4307  is illustrated in  FIG. 22A . Input can be performed by touching the displayed button  4309  with a finger or the like. 
     The electronic book reader illustrated in  FIGS. 22A and 22B , in which the third display panel  4313  is inserted, is useful when used in the following manner: for example, texts are read on the first display portion  4301  and the fourth display portion  4310 , and illustrations are referred to on the second display portion  4307  and the third display portion  4302 . In this case, the third display panel  4313  cannot perform display on both the third display portion  4302  and the fourth display portion  4310  at the same time; therefore, when a page is turned, display on the third display portion  4302  is switched to display on the fourth display portion  4310 . 
     When one finishes reading the first display portion  4301  and the third display portion  4302  and starts turning the next page, the third display panel  4313 , the fourth display portion  4310  and the second display portion  4307  start performing display for the next page at a certain angle. When one finishes reading the fourth display portion  4310  and the second display portion  4307  and starts turning the third display panel  4313 , the first display portion  4301  and the third display portion  4302  start displaying the next page at a certain angle. Thus, the switching of displays can be invisible, and visual incongruity or the like can be suppressed. 
     Note that the structure described in this embodiment can be combined with any of the structures described in other embodiments as appropriate. 
     This application is based on Japanese Patent Application serial no. 2009-131616 filed with Japan Patent Office on May 29, 2009, the entire contents of which are hereby incorporated by reference.