Patent Publication Number: US-10332743-B2

Title: Method for manufacturing semiconductor device

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a semiconductor device including an oxide semiconductor. 
     BACKGROUND ART 
     In recent years, a technique by which a thin film transistor (TFT) is manufactured using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices and urgently developed particularly as switching elements in image display devices. 
     Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like. 
     Some metal oxides have semiconductor characteristics. For example, metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like, and thin film transistors in each of which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (see Patent Documents 1 to 4, Non-Patent Document 1). 
     Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO 3 (ZnO) m  (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (see Non-Patent Documents 2 to 4). 
     Further, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (see Patent Document 5, Non-Patent Documents 5 and 6). 
     REFERENCE 
     Patent Document 
     
         
         [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 
     
         
         [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 —ZnO 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 
       
    
     DISCLOSURE OF INVENTION 
     It is an object to manufacture and provide a highly reliable semiconductor device which includes a thin film transistor having stable electric characteristics. 
     In a method for manufacturing a semiconductor device which includes a thin film transistor where a semiconductor layer including a channel formation region and a semiconductor layer including source and drain regions are each formed using an oxide semiconductor layer, heat treatment which reduces impurities such as moisture (heat treatment for dehydration or dehydrogenation) is performed so as to improve the purity of the oxide semiconductor layer. In addition, not only impurities such as moisture in the oxide semiconductor layer but also those existing in a gate insulating layer are reduced, and impurities such as moisture existing in interfaces between the oxide semiconductor layer and films provided over and under and in contact with the oxide semiconductor layer are reduced. 
     In this specification, an oxide semiconductor film which is used for a semiconductor layer including a channel formation region is referred to as a first oxide semiconductor film (a first oxide semiconductor layer), and an oxide semiconductor film which is used for source and drain regions is referred to as a second oxide semiconductor film (a second oxide semiconductor layer). 
     In order to reduce impurities such as moisture, the first oxide semiconductor film and the second oxide semiconductor film are formed and then subjected to heat treatment at 200° C. or higher, preferably higher than or equal to 400° C. and lower than or equal to 600° C., under an inert gas atmosphere of nitrogen or a rare gas (such as argon or helium) or under reduced pressure, with the first oxide semiconductor film and the second oxide semiconductor film exposed. Thus, moisture which is included in the first oxide semiconductor film and the second oxide semiconductor film is reduced. After the heating, the oxide semiconductor films are slowly cooled to a range of higher than or equal to room temperature and lower than 100° C. under an inert gas atmosphere. 
     With the use of the first oxide semiconductor film and the second oxide semiconductor film in which moisture is reduced by heat treatment under an inert gas atmosphere of nitrogen or argon or under reduced pressure, electric characteristics of a thin film transistor is improved and a thin film transistor having both high productivity and high performance is realized. 
       FIG. 29  shows results of performing thermal deposition spectroscopy (TDS) measurement using a thermal desorption spectrometer on a plurality of samples on which heat treatment was performed under a nitrogen atmosphere under different heating-temperature conditions. 
     The thermal desorption spectrometer is an apparatus for detecting and identifying, using a quadrupole mass spectrometer, a gas component which is discharged or generated from a sample when the sample is heated and the temperature thereof is increased in high vacuum. With the thermal desorption spectrometer, a gas and a molecule discharged from a surface or the inside of a sample can be observed. A thermal desorption spectrometer manufactured by ESCO, Ltd. (product name: EMD-WA1000S) was used. As for the measurement condition, the rate of temperature rise was set at approximately 10° C./min and the degree of vacuum during the measurement was approximately 1×10 −7  (Pa). In addition, the SEM voltage was set to 1500 V, the dwell time was 0.2 (sec), and the number of channels to be used was 23. Note that the ionization coefficient, the fragmentation coefficient, the pass-through coefficient, and the pumping rate of H 2 O were respectively 1.0, 0.805, 1.56, and 1.0. 
       FIG. 29  is a graph showing results of TDS measurement in terms of H 2 O, where the following samples are compared: a sample (sample 1) in which an In—Ga—Zn—O-based non-single-crystal film was formed to a thickness of 50 nm over a glass substrate; a sample (sample 3) on which heat treatment was performed at 250° C. for 1 hour under a nitrogen atmosphere; a sample (sample 2) on which heat treatment was performed at 350° C. for 1 hour under a nitrogen atmosphere; a sample (sample 4) on which heat treatment was performed at 450° C. for 1 hour under a nitrogen atmosphere; and a sample (sample 5) on which heat treatment was performed at 350° C. for 10 hours under a nitrogen atmosphere. The results in  FIG. 29  indicate that the higher the temperature of the heat treatment under a nitrogen atmosphere is, the more impurities such as moisture (H 2 O) which are discharged from the In—Ga—Zn—O-based non-single-crystal film are reduced. 
     In the graph of  FIG. 29 , a first peak showing discharge of impurities such as moisture (H 2 O) can be observed in the vicinity of 200° C. to 250° C., and a second peak showing discharge of impurities such as moisture (H 2 O) can be observed at 300° C. or higher. 
     Note that when the sample on which heat treatment had been performed at 450° C. under a nitrogen atmosphere was left at room temperature even for approximately 1 week in air, discharge of moisture at 200° C. or higher was not observed. Accordingly, it is found that an In—Ga—Zn—O-based non-single-crystal film is stabilized by the heat treatment. 
     Further, TDS measurement was performed in terms of each of H, O, OH, H 2 , O 2 , N, N 2 , and Ar in addition to H 2 O. It was possible for a peak to be clearly observed in terms of each of H 2 O, H, O, and OH, but not in terms of H z , O 2 , N, N 2 , and Ar. Each sample had a structure in which an In—Ga—Zn—O-based non-single-crystal film was formed to a thickness of 50 nm over a glass substrate, and heat conditions were set as follows: at 250° C. for 1 hour under a nitrogen atmosphere; at 350° C. for 1 hour under a nitrogen atmosphere; at 350° C. for 10 hours under a nitrogen atmosphere; and at 450° C. for 1 hour under a nitrogen atmosphere. An In—Ga—Zn—O-based non-single-crystal film on which heat treatment was not performed and a glass substrate alone were each subjected to the measurement as a comparative example.  FIG. 30 ,  FIG. 31 ,  FIG. 32 , and  FIG. 33  show results of TDS measurement in terms of H, O, OH, and H 2 , respectively. Note that the oxygen concentration under a nitrogen atmosphere in the above heat conditions was 20 ppm or lower. 
     According to the above results, it is found that moisture is mainly discharged by heat treatment of an In—Ga—Zn—O-based non-single-crystal film. In other words, discharge of moisture (H 2 O) from the In—Ga—Zn—O-based non-single-crystal film is mainly caused due to heat treatment, and a substance generated by decomposition of a water molecule has an influence on values of TDS measurement in terms of H, O, and OH which are respectively shown in  FIG. 30 ,  FIG. 31 , and  FIG. 32 . Note that an In—Ga—Zn—O-based non-single-crystal film is considered to include hydrogen and OH; therefore, these are also discharged by the heat treatment. 
     In this specification, heat treatment under an inert gas atmosphere of nitrogen or a rare gas (such as argon or helium) or under reduced pressure is referred to as heat treatment for dehydration or dehydrogenation. In this specification, dehydrogenation does not refer to only elimination in the form of H 2  by the heat treatment, and dehydration or dehydrogenation also refers to elimination of H, OH, and the like for convenience. 
     After impurities (such as H 2 O, H, or OH) included in the oxide semiconductor layer are reduced by the heat treatment under an inert gas atmosphere so that the carrier concentration is increased, slow cooling is performed. After the slow cooling, for example, an oxide insulating film is formed in contact with the oxide semiconductor layer; accordingly, the carrier concentration of the oxide semiconductor layer is reduced and thus reliability is increased. 
     The resistance of the first oxide semiconductor film and the second oxide semiconductor film is reduced (the carrier concentration is increased, preferably to 1×10 18 /cm 3  or higher) by the heat treatment under a nitrogen atmosphere. Thus, the first oxide semiconductor film and the second oxide semiconductor film each of whose resistance is reduced can be formed. The first oxide semiconductor film and the second oxide semiconductor film each of whose resistance is reduced are processed through an etching step to form a first oxide semiconductor layer and a second oxide semiconductor layer, and further processed through an etching step to form a semiconductor layer and source and drain regions. 
     After that, an oxide insulating film is formed in contact with the first oxide semiconductor layer whose resistance is reduced, whereby the resistance of at least a region which is in the first oxide semiconductor layer whose resistance is reduced and in contact with the oxide insulating film is reduced (the carrier concentration is reduced, preferably to lower than 1×10 18 /cm 3 ); thus, an oxide semiconductor region whose resistance is increased can be formed. It is important to increase and reduce the carrier concentration of the first oxide semiconductor film and the second oxide semiconductor film by heating under an inert gas atmosphere (or under reduced pressure), slow cooling, formation of the oxide insulating film, and the like during a manufacturing process of a semiconductor device. In other words, the first oxide semiconductor film and the second oxide semiconductor film are subjected to heat treatment for dehydration or dehydrogenation to be oxygen-deficiency type, that is, n-type (such as n −  or n + -type) oxide semiconductor films, and then the oxide insulating film is formed so that the first oxide semiconductor layer becomes an oxygen-excess type, that is, i-type oxide semiconductor layer. Accordingly, it is possible to manufacture and provide a semiconductor device including a highly reliable thin film transistor having favorable electric characteristics. 
     Note that as the oxide insulating film formed in contact with the first oxide semiconductor layer whose resistance is reduced, an inorganic insulating film which blocks impurities such as moisture, a hydrogen ion, and OH − , specifically a silicon oxide film or a silicon nitride oxide film is used. 
     Further, after the oxide insulating film serving as a protective film is formed over the semiconductor layer and the source and drain regions, second heating may be performed. When the second heating is performed after the formation of the oxide insulating film serving as a protective film over the semiconductor layer and the source and drain regions, variation in electric characteristics of the thin film transistor can be reduced. 
     In one embodiment of the structure of the invention disclosed in this specification, a gate electrode layer is formed; a gate insulating layer is formed over the gate electrode layer; a first oxide semiconductor film is formed over the gate insulating layer; a second oxide semiconductor film is formed over the first oxide semiconductor film; the first oxide semiconductor film and the second oxide semiconductor film are heated to be dehydrated or dehydrogenated; the dehydrated or dehydrogenated first oxide semiconductor film and the dehydrated or dehydrogenated second oxide semiconductor film are selectively etched to form a first oxide semiconductor layer and a second oxide semiconductor layer; a conductive film is formed over the first oxide semiconductor layer and the second oxide semiconductor layer; the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film are selectively etched to form a semiconductor layer, a source region, a drain region, a source electrode layer, and a drain electrode layer; and an oxide insulating film which is contact with part of the semiconductor layer is formed over the gate insulating layer, the semiconductor layer, the source region, the drain region, the source electrode layer, and the drain electrode layer so that carrier concentration is reduced. 
     In another embodiment of the structure of the invention disclosed in this specification, a gate electrode layer is formed; a gate insulating layer is formed over the gate electrode layer; a first oxide semiconductor film is formed over the gate insulating layer; a second oxide semiconductor film is formed over the first oxide semiconductor film; the first oxide semiconductor film and the second oxide semiconductor film are heated under an inert gas atmosphere so that carrier concentration is increased; the first oxide semiconductor film and the second oxide semiconductor film each of whose carrier concentration is increased are selectively etched to form a first oxide semiconductor layer and a second oxide semiconductor layer; a conductive film is formed over the first oxide semiconductor layer and the second oxide semiconductor layer; the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film are selectively etched to form a semiconductor layer, a source region, a drain region, a source electrode layer, and a drain electrode layer; and an oxide insulating film which is in contact with part of the semiconductor layer is formed over the gate insulating layer, the semiconductor layer, the source region, the drain region, the source electrode layer, and the drain electrode layer so that carrier concentration is reduced. 
     In another embodiment of the structure of the invention disclosed in this specification, a gate electrode layer is formed; a gate insulating layer is formed over the gate electrode layer; a first oxide semiconductor film is formed over the gate insulating layer; a second oxide semiconductor film is formed over the first oxide semiconductor film; the first oxide semiconductor film and the second oxide semiconductor film are heated under reduced pressure so that carrier concentration is increased; the first oxide semiconductor film and the second oxide semiconductor film each of whose carrier concentration is increased are selectively etched to form a first oxide semiconductor layer and a second oxide semiconductor layer; a conductive film is formed over the first oxide semiconductor layer and the second oxide semiconductor layer; the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film are selectively etched to form a semiconductor layer, a source region, a drain region, a source electrode layer, and a drain electrode layer; and an oxide insulating film which is in contact with part of the semiconductor layer is formed over the gate insulating layer, the semiconductor layer, the source region, the drain region, the source electrode layer, and the drain electrode layer so that carrier concentration is reduced. 
     For an oxide semiconductor layer which can be used as the semiconductor layer and the source and drain regions, for example, an oxide material having semiconductor characteristics may be used. For example, thin films expressed by InMO 3 (ZnO) m  (m&gt;0) are formed, and a thin film transistor using the thin films as a semiconductor layer and source and drain regions is manufactured. Note that M denotes one metal element or a plurality of metal elements selected from Ga, Fe, Ni, Mn, and Co. For example, M denotes Ga in some cases; meanwhile, M denotes the above metal element such as Ni or Fe in addition to Ga (Ga and Ni or Ga and Fe) in other cases. Further, the above oxide semiconductor may include Fe or Ni, another transitional metal element, or an oxide of the transitional metal as an impurity element in addition to the metal element included as M. In this specification, among the oxide semiconductors whose composition formulas are expressed by InMO 3 (ZnO) m  (m&gt;0), an oxide semiconductor which includes Ga as M is referred to as an In—Ga—Zn—O-based oxide semiconductor, and a thin film of the In—Ga—Zn—O-based oxide semiconductor is also referred to as an In—Ga—Zn—O-based non-single-crystal film. 
     As the oxide semiconductor which is applied to the oxide semiconductor layer, any of the following oxide semiconductors can be applied in addition to the above: an In—Sn—Zn—O-based oxide semiconductor; an In—Al—Zn—O-based oxide semiconductor; a Sn—Ga—Zn—O-based oxide semiconductor; an Al—Ga—Zn—O-based oxide semiconductor; a Sn—Al—Zn—O-based oxide semiconductor; an In—Zn—O-based oxide semiconductor; a Sn—Zn—O-based oxide semiconductor; an Al—Zn—O-based oxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-based oxide semiconductor; and a Zn—O-based oxide semiconductor. In addition, the above oxide semiconductor layer may include silicon oxide. Silicon oxide (SiO x  (x&gt;0)), which hinders crystallization, included in the oxide semiconductor layer can suppress crystallization of the oxide semiconductor layer in the case where heat treatment is performed after formation of the oxide semiconductor layer in the manufacturing process. Note that the oxide semiconductor layer is preferably amorphous but may be partly crystallized. 
     The oxide semiconductor is preferably an oxide semiconductor containing In, more preferably an oxide semiconductor containing In and Ga. Dehydration or dehydrogenation is effective in a process of forming an i-type (intrinsic) oxide semiconductor layer. 
     In addition, the oxide semiconductor layer used as the source and drain regions (also referred to as n +  layers or buffer layers) of the thin film transistor preferably has higher conductivity (electrical conductivity) than the oxide semiconductor layer used as a channel formation region. 
     Since a thin film transistor is easily broken due to static electricity or the like, a protective circuit for protecting a driver circuit is preferably provided over the same substrate for a gate line or a source line. The protective circuit is preferably formed using a non-linear element including an oxide semiconductor. 
     The gate insulating layer, the first oxide semiconductor film, and the second oxide semiconductor film may be successively treated (also referred to as successive treatment, an insitu process, or successive film formation) without being exposed to air. By the successive treatment without exposure to air, each interface between the stacked layers, that is, interfaces of the gate insulating layer, the first oxide semiconductor film, and the second oxide semiconductor film can be formed without being contaminated by an atmospheric component or an impurity element floating in air, such as water or hydrocarbon. Accordingly, variation in characteristics of the thin film transistor can be reduced. 
     Note that the term “successive treatment” in this specification means that during a process from a first treatment step using a PCVD method or a sputtering method to a second treatment step using a PCVD method or a sputtering method, an atmosphere in which a substrate to be processed is disposed is not contaminated by a contaminant atmosphere such as air, and is constantly controlled to be vacuum or an inert gas atmosphere (a nitrogen atmosphere or a rare gas atmosphere). By the successive treatment, treatment such as film formation can be performed while preventing moisture or the like from attaching again to the cleaned substrate to be processed. 
     Performing the process from the first treatment step to the second treatment step in the same chamber is within the scope of the successive treatment in this specification. 
     In addition, the following is also within the scope of the successive treatment in this specification: in the case of performing the process from the first treatment step to the second treatment step in different chambers, the substrate is transferred after the first treatment step to another chamber without being exposed to air and subjected to the second treatment. 
     Note that between the first treatment step and the second treatment step, a substrate transfer step, an alignment step, a slow cooling step, a step of heating or cooling the substrate to a temperature which is necessary for the second treatment step, or the like may be provided. Such a process is also within the scope of the successive treatment in this specification. 
     A step in which liquid is used, such as a cleaning step, wet etching, or resist formation, may be provided between the first treatment step and the second treatment step. This case is not within the scope of the successive treatment in this specification. 
     Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps and the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the invention. 
     Moreover, as a display device including a driver circuit, a light-emitting display device including a light-emitting element and a display device including an electrophoretic display element, which is also referred to as electronic paper, are given in addition to a liquid crystal display device. 
     In the light-emitting display device including a light-emitting element, a plurality of thin film transistors are included in a pixel portion, and in the pixel portion, there is a region where a gate electrode of a thin film transistor is connected to a source wiring or a drain wiring of another thin film transistor. In addition, in a driver circuit of the light-emitting display device including a light-emitting element, there is a region where a gate electrode of a thin film transistor is connected to a source wiring or a drain wiring of the thin film transistor. 
     In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic appliance are all semiconductor devices. 
     A thin film transistor having stable electric characteristics can be manufactured and provided. Accordingly, a semiconductor device which includes the highly reliable thin film transistor having favorable electric characteristics can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A to 1C  illustrate a method for manufacturing a semiconductor device; 
         FIGS. 2A and 2B  illustrate the method for manufacturing a semiconductor device; 
         FIGS. 3A and 3B  illustrate a semiconductor device; 
         FIGS. 4A to 4C  illustrate a method for manufacturing a semiconductor device; 
         FIGS. 5A to 5C  illustrate the method for manufacturing a semiconductor device; 
         FIGS. 6A and 6B  illustrate the method for manufacturing a semiconductor device; 
         FIG. 7  illustrates a semiconductor device; 
       FIGS.  8 A 1 ,  8 A 2 ,  8 B 1 , and  8 B 2  illustrate semiconductor devices; 
         FIG. 9  illustrates a semiconductor device; 
       FIGS.  10 A 1 ,  10 A 2 , and  10 B illustrate a semiconductor device; 
         FIGS. 11A and 11B  illustrate a semiconductor device; 
         FIG. 12  illustrates a pixel equivalent circuit of a semiconductor device; 
         FIGS. 13A to 13C  illustrate semiconductor devices; 
         FIGS. 14A and 14B  are block diagrams of semiconductor devices; 
         FIG. 15  illustrates a configuration of a signal line driver circuit; 
         FIG. 16  is a timing chart illustrating operation of a signal line driver circuit; 
         FIG. 17  is a timing chart illustrating operation of a signal line driver circuit; 
         FIG. 18  illustrates a configuration of a shift register; 
         FIG. 19  illustrates a connection configuration of the flip-flop illustrated in  FIG. 18 ; 
         FIG. 20  illustrates a semiconductor device; 
         FIG. 21  shows simulation results of oxygen density of an oxide semiconductor layer; 
         FIG. 22  is an external view illustrating an example of an electronic book reader; 
         FIGS. 23A and 23B  are external views respectively illustrating an example of a television set and an example of a digital photo frame; 
         FIGS. 24A and 24B  are external views illustrating examples of amusement machines; 
         FIGS. 25A and 25B  are external views respectively illustrating an example of a portable computer and an example of a cellular phone; 
         FIG. 26  illustrates a semiconductor device; 
         FIG. 27  illustrates a semiconductor device; 
         FIG. 28  is a cross-sectional view illustrating an electric furnace; 
         FIG. 29  is a graph showing results of TDS measurement; 
         FIG. 30  is a graph showing results of TDS measurement in terms of H; 
         FIG. 31  is a graph showing results of TDS measurement in terms of O; 
         FIG. 32  is a graph showing results of TDS measurement in terms of OH; 
         FIG. 33  is a graph showing results of TDS measurement in terms of H 2 ; and 
         FIG. 34  illustrates a structure of an oxide semiconductor layer used for simulation. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments and an example will be described in detail with reference to drawings. However, the present invention is not limited to the following description, and it is easily understood by those skilled in the art that various changes and modifications can be made in modes and details without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of the embodiments and example below. In the structures described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and repetitive description thereof is omitted. 
     Embodiment 1 
     A semiconductor device and a method for manufacturing the semiconductor device will be described with reference to  FIGS. 1A to 1C ,  FIGS. 2A and 2B , and  FIGS. 3A and 3B . 
       FIG. 3A  is a plan view of a thin film transistor  470  included in a semiconductor device, and  FIG. 3B  is a cross-sectional view taken along line C 1 -C 2  of  FIG. 3A . The thin film transistor  470  is an inverted staggered thin film transistor and includes, over a substrate  400  which is a substrate having an insulating surface, a gate electrode layer  401 , a gate insulating layer  402 , a semiconductor layer  403 , source and drain regions  404   a  and  404   b , and source and drain electrode layers  405   a  and  405   b . In addition, an oxide insulating film  407  is provided to cover the thin film transistor  470  and be in contact with the semiconductor layer  403 . 
     Heat treatment which reduces impurities such as moisture (heat treatment for dehydration or dehydrogenation) is performed at least after formation of a first oxide semiconductor film and a second oxide semiconductor film which become the semiconductor layer  403  and the source and drain regions  404   a  and  404   b , so that the resistance of the first oxide semiconductor film and the second oxide semiconductor film is reduced (the carrier concentration is increased, preferably to 1×10 18 /cm 3  or higher). Then, the oxide insulating film  407  is formed in contact with a first oxide semiconductor layer formed from the first oxide semiconductor film, whereby the resistance of the first oxide semiconductor layer is increased (the carrier concentration is reduced, preferably to lower than 1×10 18 /cm 3 , more preferably to lower than or equal to 1×10 14 /cm 3 ). Thus, the first oxide semiconductor layer whose resistance is increased can be used as a channel formation region. 
     Further, after impurities such as moisture (H 2 O) are eliminated by the heat treatment for dehydration or dehydrogenation, slow cooling is preferably performed under an inert gas atmosphere. After the heat treatment for dehydration or dehydrogenation and the slow cooling, the carrier concentration of the first oxide semiconductor layer is reduced by formation of the oxide insulating film to be in contact with the first oxide semiconductor layer, which results in that reliability of the thin film transistor  470  is improved. 
     Impurities such as moisture are reduced not only in the semiconductor layer  403  and the source and drain regions  404   a  and  404   b  but also in the gate insulating layer  402  and interfaces between the semiconductor layer  403  which is an oxide semiconductor layer and films that are provided over and under and in contact with the semiconductor layer  403 , specifically, an interface between the gate insulating layer  402  and the semiconductor layer  403  and an interface between the oxide insulating film  407  and the semiconductor layer  403 . 
     Note that the source and drain electrode layers  405   a  and  405   b  that are in contact with the semiconductor layer  403  which is an oxide semiconductor layer and the source and drain regions  404   a  and  404   b  are formed using one or more materials selected from titanium, aluminum, manganese, magnesium, zirconium, and beryllium. Further, a stack of alloy films including a combination of any of the above elements may be used. 
     For the semiconductor layer  403  including a channel formation region and the source and drain regions  404   a  and  404   b , an oxide material having semiconductor characteristics may be used. For example, an oxide semiconductor having a structure expressed by InMO 3 (ZnO) m  (m&gt;0) can be used, and in particular, an In—Ga—Zn—O-based oxide semiconductor is preferably used. Note that M denotes one metal element or a plurality of metal elements selected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), and cobalt (Co). For example, M denotes Ga in some cases; meanwhile, M denotes the above metal element such as Ni or Fe in addition to Ga (Ga and Ni or Ga and Fe) in other cases. Further, the above oxide semiconductor may include Fe or Ni, another transitional metal element, or an oxide of the transitional metal as an impurity element in addition to the metal element included as M. In this specification, among the oxide semiconductors whose composition formulas are expressed by InMO 3 (ZnO) m  (m&gt;0), an oxide semiconductor which includes at least Ga as M is referred to as an In—Ga—Zn—O-based oxide semiconductor, and a thin film of the In—Ga—Zn—O-based oxide semiconductor is also referred to as an In—Ga—Zn—O-based non-single-crystal film. 
     As the oxide semiconductor which is applied to the oxide semiconductor layer, any of the following oxide semiconductors can be applied in addition to the above: an In—Sn—Zn—O-based oxide semiconductor; an In—Al—Zn—O-based oxide semiconductor; a Sn—Ga—Zn—O-based oxide semiconductor; an Al—Ga—Zn—O-based oxide semiconductor; a Sn—Al—Zn—O-based oxide semiconductor; an In—Zn—O-based oxide semiconductor; an In—Ga—O based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor; an Al—Zn—O-based oxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-based oxide semiconductor; and a Zn—O-based oxide semiconductor. In addition, the above oxide semiconductor may include silicon oxide. 
     The source region is provided between the semiconductor layer (also referred to as a first oxide semiconductor layer) and the source electrode layer, and the drain region is provided between the semiconductor layer and the drain electrode layer. As the source and drain regions, an oxide semiconductor layer having n-type conductivity (also referred to as a second oxide semiconductor layer) can be used. 
     In addition, it is preferable that the second oxide semiconductor layer used as the source and drain regions  404   a  and  404   b  of the thin film transistor be thinner and have higher conductivity (electrical conductivity) than the first oxide semiconductor layer used as a channel formation region. 
     Further, the first oxide semiconductor layer used as the channel formation region has an amorphous structure, and the second oxide semiconductor layer used as the source and drain regions includes a crystal grain (nanocrystal) in an amorphous structure in some cases. The crystal grain (nanocrystal) in the second oxide semiconductor layer used as the source and drain regions has a diameter of 1 nm to 10 nm, typically 2 nm to 4 nm, approximately. 
     In this embodiment, In—Ga—Zn—O-based non-single-crystal films are used as the semiconductor layer  403  including the channel formation region and the source and drain regions (also referred to as n +  layers or buffer layers)  404   a  and  404   b.    
       FIGS. 1A to 1C  and  FIGS. 2A and 2B  are cross-sectional views illustrating a manufacturing process of the thin film transistor  470 . 
     The gate electrode layer  401  is provided over the substrate  400  which is a substrate having an insulating surface. An insulating film serving as a base film may be provided between the substrate  400  and the gate electrode layer  401 . The base film has a function of preventing diffusion of an impurity element from the substrate  400 , and can be formed to have a single-layer or stacked-layer structure using one or more films selected from a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. The gate electrode layer  401  can be formed to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as its main component. 
     For example, as a stacked-layer structure of two layers of the gate electrode layer  401 , the following structures are preferable: a two-layer structure of an aluminum layer and a molybdenum layer stacked thereover; a two-layer structure of a copper layer and a molybdenum layer stacked thereover; a two-layer structure of a copper layer and a titanium nitride layer or a tantalum nitride layer stacked thereover; and a two-layer structure of a titanium nitride layer and a molybdenum layer. As a stacked-layer structure of three layers, a stack of a tungsten layer or a tungsten nitride layer, a layer of an alloy of aluminum and silicon or a layer of an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer is preferable. 
     The gate insulating layer  402  is formed over the gate electrode layer  401 . 
     The gate insulating layer  402  can be formed using a single layer or stacked layers of any of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, and a silicon nitride oxide layer with a plasma CVD method, a sputtering method, or the like. For example, a silicon oxynitride layer may be formed with a plasma CVD method using SiH 4 , oxygen, and nitrogen as a film formation gas. In addition, it is possible to form a silicon oxide layer as the gate insulating layer  402  with a CVD method using an organosilane gas. As the organosilane gas, a compound including silicon, such as tetraethoxysilane (TEOS) (chemical formula: Si(OC 2 H 5 ) 4 ), tetramethylsilane (TMS) (chemical formula: Si(CH 3 ) 4 ), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (chemical formula: SiH(OC 2 H 5 ) 3 ), or tris(dimethylamino)silane (chemical formula: SiH(N(CH 3 ) 2 ) 3 ) can be used. 
     A first oxide semiconductor film  430  and a second oxide semiconductor film  433  are formed to be stacked over the gate insulating layer  402  (see  FIG. 1A ). The first oxide semiconductor film  430  serves as a semiconductor layer which functions as a channel formation region, and the second oxide semiconductor film  433  serves as source and drain regions. 
     Note that before the oxide semiconductor films are formed with a sputtering method, reverse sputtering in which an argon gas is introduced and plasma is generated is preferably performed to remove dust attaching to a surface of the gate insulating layer  402 . The reverse sputtering refers to a method in which, without application of voltage to a target side, an RF power source is used for application of voltage to a substrate side under an argon atmosphere to generate plasma in the vicinity of the substrate so that a surface is modified. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, N 2 O, or the like is added may be used. Further alternatively, an argon atmosphere to which Cl 2 , CF 4 , or the like is added may be used. 
     As the oxide semiconductor films, In—Ga—Zn—O-based non-single-crystal films are used. The oxide semiconductor films are formed with a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. The oxide semiconductor films can be formed with a sputtering method under a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas (typically argon) and oxygen. 
     The gate insulating layer  402 , the first oxide semiconductor film  430 , and the second oxide semiconductor film  433  may be successively formed without being exposed to air. By successive film formation without exposure to air, each interface of the stacked layers can be formed without being contaminated by an atmospheric component or an impurity element floating in air, such as water or hydrocarbon. Therefore, variation in characteristics of the thin film transistor can be reduced. 
     The first oxide semiconductor film  430  and the second oxide semiconductor film  433  are subjected to heat treatment under an atmosphere of an inert gas (such as nitrogen, helium, neon, or argon) or under reduced pressure, and then slowly cooled under an inert gas atmosphere (see  FIG. 1B ). When the heat treatment under the above atmosphere is performed on the first oxide semiconductor film  430  and the second oxide semiconductor film  433 , impurities such as hydrogen or water which are included in the first oxide semiconductor film  430  and the second oxide semiconductor film  433  can be removed. 
     Note that it is preferable that water, hydrogen, or the like be not included in nitrogen or a rare gas such as helium, neon, or argon in the heat treatment. In addition, nitrogen or a rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus preferably has a purity of 6N (99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is, the concentration of impurities is 1 ppm or lower, preferably 0.1 ppm or lower). 
     Further, in the heat treatment, a heating method using an electric furnace or an instantaneous heating method such as a gas rapid thermal anneal (GRTA) method using a heated gas or a lamp rapid thermal anneal (LRTA) method using lamp light can be used. 
     Here, as one mode of the heat treatment for the first oxide semiconductor film  430  and the second oxide semiconductor film  433 , a heating method using an electric furnace  601  is described with reference to  FIG. 28 . 
       FIG. 28  is a schematic view of the electric furnace  601 . A heater  603  is provided outside a chamber  602  and used for heating the chamber  602 . A susceptor  605  on which a substrate  604  is set is provided in the chamber  602 , and the substrate  604  is carried into or out of the chamber  602 . Further, the chamber  602  is provided with a gas supply unit  606  and an evacuation unit  607 . A gas is introduced into the chamber  602  by the gas supply unit  606 . The evacuation unit  607  evacuates the chamber  602  or reduces the pressure in the chamber  602 . Note that the temperature-rise characteristics of the electric furnace  601  are preferably set at higher than or equal to 0.1° C./min and lower than or equal to 20° C./min. In addition, the temperature-drop characteristics of the electric furnace  601  are preferably set at higher than or equal to 0.1° C./min and lower than or equal to 15° C./min. 
     The gas supply unit  606  includes a gas supply source  611 , a pressure adjusting valve  612 , a refiner  613 , a mass flow controller  614 , and a stop valve  615 . In this embodiment, the refiner  613  is preferably provided between the gas supply source  611  and the chamber  602 . When the refiner  613  is provided, impurities such as water or hydrogen in a gas which is introduced into the chamber  602  from the gas supply source  611  can be removed by the refiner  613 , so that entry of water, hydrogen, or the like into the chamber  602  can be suppressed. 
     In this embodiment, nitrogen or a rare gas is introduced into the chamber  602  from the gas supply source  611  so that the atmosphere in the chamber is a nitrogen or rare gas atmosphere, and the first oxide semiconductor film  430  and the second oxide semiconductor film  433  which are formed over the substrate  604  are heated in the chamber  602  heated to higher than or equal to 200° C. and lower than or equal to 600° C., preferably higher than or equal to 400° C. and lower than or equal to 450° C. In this manner, dehydration or dehydrogenation of the first oxide semiconductor film  430  and the second oxide semiconductor film  433  can be performed. 
     Alternatively, dehydration or dehydrogenation of the first oxide semiconductor film  430  and the second oxide semiconductor film  433  can be performed in such a manner that, with the pressure reduced by the evacuation unit, the first oxide semiconductor film  430  and the second oxide semiconductor film  433  which are formed over the substrate  604  are heated in the chamber  602  heated to higher than or equal to 200° C. and lower than or equal to 600° C., preferably higher than or equal to 400° C. and lower than or equal to 450° C. 
     Next, the heater is turned off, and the chamber  602  of the heating apparatus is slowly cooled. By the heat treatment under an inert gas atmosphere or under reduced pressure and the slow cooling, the resistance of the oxide semiconductor films is reduced (the carrier concentration is increased, preferably to 1×10 18 /cm 3  or higher). Thus, a first oxide semiconductor film  434  and a second oxide semiconductor film  435  each of whose resistance is reduced can be formed. 
     As a result, reliability of the thin film transistor to be completed later can be improved. 
     Note that when the heat treatment is performed under reduced pressure, cooling may be performed by introducing an inert gas after the heating so that the pressure returns to atmospheric pressure. 
     Alternatively, after the substrate  604  in the chamber  602  of the heating apparatus is cooled to 300° C., the substrate  604  may be transferred into an atmosphere of room temperature. This results in that cooling time for the substrate  604  can be shortened. 
     When the heating apparatus has multiple chambers, the heat treatment and the cooling treatment can be performed in different chambers. Typically, in a first chamber which is filled with nitrogen or a rare gas and heated to higher than or equal to 200° C. and lower than or equal to 600° C., preferably higher than or equal to 400° C. and lower than or equal to 450° C., the oxide semiconductor films over the substrate are heated. Next, the substrate on which the above heat treatment is performed is transferred, through a transfer chamber into which nitrogen or an inert gas is introduced, to a second chamber which is filled with nitrogen or a rare gas and whose temperature is lower than or equal to 100° C., preferably room temperature, and is subjected to cooling treatment. By the above process, throughput can be improved. 
     After the heat treatment under an inert gas atmosphere or under reduced pressure, slow cooling to higher than or equal to room temperature and lower than 100° C. is performed, the substrate provided with the first oxide semiconductor film  434  and the second oxide semiconductor film  435  is taken out of the heating apparatus, and a photolithography step is performed. 
     The first oxide semiconductor film  434  and the second oxide semiconductor film  435  after the heat treatment under an inert gas atmosphere or under reduced pressure are preferably in an amorphous state, but may be partly crystallized. 
     The first oxide semiconductor film  434  and the second oxide semiconductor film  435  are processed into a first oxide semiconductor layer  431  and a second oxide semiconductor layer  436  which are island-like oxide semiconductor layers through the photolithography step (see  FIG. 1C ). 
     A conductive film is formed over the gate insulating layer  402 , the first oxide semiconductor layer  431 , and the second oxide semiconductor layer  436 . 
     As a material for the conductive film, an element selected from Al, Cr, Ta, Ti, Mo, and W; an alloy containing any of the above elements as its component; an alloy containing a combination of any of the above elements; and the like can be given. 
     In the case where heat treatment is performed after the formation of the conductive film, it is preferable that the conductive film have heat resistance enough to withstand the heat treatment. Since Al alone has disadvantages such as low heat resistance and a tendency to be corroded, aluminum is used in combination with a heat-resistant conductive material. As the heat-resistant conductive material which is used in combination with Al, any of the following materials may be used: an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of the above elements as a component, an alloy containing a combination of any of the above elements, and a nitride containing any of the above elements as a component. 
     The first oxide semiconductor layer  431 , the second oxide semiconductor layer  436 , and the conductive film are etched through an etching step to form a first oxide semiconductor layer  432 , the source and drain regions  404   a  and  404   b , and the source and drain electrode layers  405   a  and  405   b  (see  FIG. 2A ). Note that only part of the first oxide semiconductor layer  431  is etched, so that the first oxide semiconductor layer  432  has a depression (a recessed portion). 
     A silicon oxide film is formed as the oxide insulating film  407  with a sputtering method so as to be in contact with the first oxide semiconductor layer  432 . As the oxide insulating film  407  which is formed in contact with the oxide semiconductor layer whose resistance is reduced, an inorganic insulating film which does not include impurities such as moisture, a hydrogen ion, and OH −  and blocks entry of these from the outside, specifically, a silicon oxide film or a silicon nitride oxide film is used. 
     In this embodiment, a silicon oxide film is formed to a thickness of 300 nm as the oxide insulating film  407 . The substrate temperature in the film formation may be higher than or equal to room temperature and lower than or equal to 300° C., and is set at 100° C. in this embodiment. Formation of the silicon oxide film with a sputtering method can be performed under a rare gas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas (typically argon) and oxygen. Further, either a silicon oxide target or a silicon target may be used as a target. For example, the silicon oxide film can be formed with a sputtering method using a silicon target under an atmosphere of oxygen and nitrogen. 
     When the oxide insulating film  407  is formed with a sputtering method, a PCVD method, or the like so as to be in contact with the first oxide semiconductor layer  432  whose resistance is reduced, in the first oxide semiconductor layer  432  whose resistance is reduced, the resistance of at least a region in contact with the oxide insulating film  407  is increased (the carrier concentration is reduced, preferably to lower than 1×10 18 /cm 3 , more preferably to lower than or equal to 1×10 14 /cm 3 ); thus, an oxide semiconductor region whose resistance is increased can be formed. It is important to increase and reduce the carrier concentration of the oxide semiconductor layer by heating under an inert gas atmosphere (or under reduced pressure), slow cooling, formation of the oxide insulating film, and the like in a manufacturing process of the semiconductor device. The first oxide semiconductor layer  432  serves as the semiconductor layer  403  including the oxide semiconductor region whose resistance is increased. In this manner, the thin film transistor  470  can be manufactured (see  FIG. 2B ). 
     Impurities (such as H 2 O, H, or OH) included in the first oxide semiconductor film and the second oxide semiconductor film are reduced by the above heat treatment for dehydration or dehydrogenation so that the carrier concentration is increased, and then slow cooling is performed. After the slow cooling, the first oxide semiconductor film is processed into the island-like first oxide semiconductor layer, and the oxide insulating film is formed in contact with the first oxide semiconductor layer so that the carrier concentration of the first oxide semiconductor layer is reduced. When the first oxide semiconductor layer whose carrier concentration is reduced is used as a semiconductor layer, reliability of the thin film transistor  470  can be improved. 
     Further, after the oxide insulating film  407  is formed, the thin film transistor  470  may be subjected to heat treatment (preferably at higher than or equal to 150° C. and lower than 350° C.) under a nitrogen atmosphere or an air atmosphere (in air). For example, heat treatment is performed at 250° C. for 1 hour under a nitrogen atmosphere. By the heat treatment, the semiconductor layer  403  is heated while being in contact with the oxide insulating film  407 . Accordingly, variation in electric characteristics of the thin film transistor  470  can be reduced. There is no particular limitation on this heat treatment (preferably at higher than or equal to 150° C. and lower than 350° C.) as long as it is performed after the formation of the oxide insulating film  407 . When the heat treatment serves also as another step such as heat treatment in formation of a resin film or heat treatment for reducing the resistance of a transparent conductive film, the heat treatment can be performed without increase in the number of steps. 
     Embodiment 2 
     A semiconductor device and a method for manufacturing the semiconductor device will be described with reference to  FIG. 26 . The same portions as Embodiment 1 or portions having functions similar to those of Embodiment 1 can be formed in a manner similar to that of Embodiment 1, and repetitive description thereof is omitted. 
     A thin film transistor  471  illustrated in  FIG. 26  is an example in which a conductive layer  408  is provided so as to overlap with the gate electrode layer  401  and a channel region of the semiconductor layer  403  with an insulating film interposed between the conductive layer  408  and the semiconductor layer  403 . 
       FIG. 26  is a cross-sectional view of the thin film transistor  471  included in a semiconductor device. The thin film transistor  471  is a bottom-gate thin film transistor in which, over the substrate  400  which is a substrate having an insulating surface, the gate electrode layer  401 , the gate insulating layer  402 , the semiconductor layer  403 , the source and drain regions  404   a  and  404   b , the source and drain electrode layers  405   a  and  405   b , and the conductive layer  408  are provided. The conductive layer  408  is provided over the oxide insulating film  407  so as to overlap with the gate electrode layer  401 . 
     The conductive layer  408  can be formed using a material and a method similar to those of the gate electrode layer  401  and the source and drain electrode layers  405   a  and  405   b . In the case where a pixel electrode layer is provided, the conductive layer  408  may be formed using a material and a method similar to those of the pixel electrode layer. In this embodiment, a stack of a titanium film, an aluminum film, and a titanium film is used for the conductive layer  408 . 
     The conductive layer  408  may have a potential which is the same as or different from that of the gate electrode layer  401 , and can function as a second gate electrode layer. Further, the conductive layer  408  may be in a floating state. 
     When the conductive layer  408  is provided in a position overlapping with the semiconductor layer  403 , in a bias-temperature stress test (hereinafter referred to as a BT test) for examining reliability of a thin film transistor, the amount of fluctuation in threshold voltage of the thin film transistor  471  between before and after the BT test can be reduced. Especially in a—BT test in which—20 V of voltage is applied to a gate after the substrate temperature is raised to 150° C., fluctuation in the threshold voltage can be suppressed. 
     This embodiment can be implemented in appropriate combination with Embodiment 1. 
     Embodiment 3 
     A semiconductor device and a method for manufacturing the semiconductor device will be described with reference to  FIG. 27 . The same portions as Embodiment 1 or portions having functions similar to those of Embodiment 1 can be formed in a manner similar to that of Embodiment 1, and repetitive description thereof is omitted. 
     A thin film transistor  472  illustrated in  FIG. 27  is an example in which a conductive layer  409  is provided so as to overlap with the gate electrode layer  401  and a channel region of the semiconductor layer  403  with the oxide insulating film  407  and an insulating layer  410  interposed between the conductive layer  409  and the semiconductor layer  403 . 
       FIG. 27  is a cross-sectional view of the thin film transistor  472  included in a semiconductor device. The thin film transistor  472  is a bottom-gate thin film transistor in which, over the substrate  400  which is a substrate having an insulating surface, the gate electrode layer  401 , the gate insulating layer  402 , the semiconductor layer  403 , the source and drain regions  404   a  and  404   b , the source and drain electrode layers  405   a  and  405   b , and the conductive layer  409  are provided. The conductive layer  409  is provided over the oxide insulating film  407  and the insulating layer  410  so as to overlap with the gate electrode layer  401 . 
     In this embodiment, the insulating layer  410  which functions as a planarization film is stacked over the oxide insulating film  407 , and an opening which reaches the source or drain electrode layer  405   b  is formed in the oxide insulating film  407  and the insulating layer  410 . A conductive film is formed over the insulating layer  410  and in the opening which is formed in the oxide insulating film  407  and the insulating layer  410  and etched into a desired shape, so that the conductive layer  409  and a pixel electrode layer  411  are formed. In this manner, the conductive layer  409  can be formed in the step of forming the pixel electrode layer  411 , using a similar material and method. In this embodiment, indium oxide-tin oxide alloy including silicon oxide (an In—Sn—O-based oxide including silicon oxide) is used for the pixel electrode layer  411  and the conductive layer  409 . 
     Alternatively, the conductive layer  409  may be formed using a material and a method similar to those of the gate electrode layer  401  and the source and drain electrode layers  405   a  and  405   b.    
     The conductive layer  409  may have a potential which is the same as or different from that of the gate electrode layer  401 , and can function as a second gate electrode layer. Further, the conductive layer  409  may be in a floating state. 
     When the conductive layer  409  is provided in a position overlapping with the semiconductor layer  403 , in a bias-temperature stress test (hereinafter referred to as a BT test) for examining reliability of a thin film transistor, the amount of fluctuation in threshold voltage of the thin film transistor  472  between before and after the BT test can be reduced. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 4 
     A manufacturing process of a semiconductor device including a thin film transistor will be described with reference to  FIGS. 4A to 4C ,  FIGS. 5A to 5C ,  FIGS. 6A and 6B ,  FIG. 7 , and FIGS.  8 A 1 ,  8 A 2 ,  8 B 1 , and  8 B 2 . 
     In  FIG. 4A , as a substrate  100  having a light-transmitting property, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like can be used. 
     Next, a conductive layer is formed entirely over a surface of the substrate  100 , and then a first photolithography step is performed to form a resist mask. Unnecessary portions are removed by etching, so that wirings and an electrode (a gate wiring including a gate electrode layer  101 , a capacitor wiring  108 , and a first terminal  121 ) are formed. At this time, the etching is performed so that at least end portions of the gate electrode layer  101  are tapered. 
     Each of the gate wiring including the gate electrode layer  101 , the capacitor wiring  108 , and the first terminal  121  in a terminal portion is preferably formed using a heat-resistant conductive material such as an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc); an alloy containing any of these elements as its component; an alloy containing a combination of any of the above elements; or a nitride containing any of the above elements as its component. In the case where a low-resistant conductive material such as aluminum (Al) or copper (Cu) is used, the low-resistant conductive material is used in combination with the above heat-resistant conductive material because Al or Cu alone has disadvantages such as low heat resistance and a tendency to be corroded. 
     Next, a gate insulating layer  102  is formed entirely over a surface of the gate electrode layer  101  (see  FIG. 4A ). The gate insulating layer  102  is formed to a thickness of 50 nm to 250 nm with a sputtering method, a PCVD method, or the like. 
     For example, as the gate insulating layer  102 , a silicon oxide film is formed to a thickness of 100 nm with a sputtering method. Needless to say, the gate insulating layer  102  is not limited to such a silicon oxide film and may be formed to have a single-layer structure or a stacked-layer structure using another insulating film such as a silicon oxynitride film, a silicon nitride film, an aluminum oxide film, or a tantalum oxide film. 
     Next, a first oxide semiconductor film  131  (a first In—Ga—Zn—O-based non-single crystal film) is formed over the gate insulating layer  102 . The first oxide semiconductor film  131  is formed without exposure to air after plasma treatment, which is advantageous in that dust or moisture is not attached to an interface between the gate insulating layer and the semiconductor film. Here, the first oxide semiconductor film  131  is formed under an argon or oxygen atmosphere using an oxide semiconductor target having a diameter of 8 inches and including In, Ga, and Zn (In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1), with the distance between the substrate and the target set to 170 mm, under a pressure of 0.4 Pa, and with a direct current (DC) power source of 500 W. Note that a pulse direct current (DC) power source is preferable because dust can be reduced and the film thickness can be uniform. The thickness of the first oxide semiconductor film  131  is set to 5 nm to 200 nm. As the first oxide semiconductor film  131 , an In—Ga—Zn—O-based non-single-crystal film is formed to a thickness of 50 nm with a sputtering method using an In—Ga—Zn—O-based oxide semiconductor target. 
     Next, a second oxide semiconductor film  136  (a second In—Ga—Zn—O-based non-single-crystal film) is formed with a sputtering method without exposure to air (see  FIG. 4B ). Here, sputtering is performed using a target of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 under film formation conditions where the pressure is 0.4 Pa, the power is 500 W, the film formation temperature is room temperature, and an argon gas is introduced at a flow rate of 40 sccm. Although the target of In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 is used, an In—Ga—Zn—O-based non-single-crystal film including a crystal grain which has a size of 1 nm to 10 nm just after the film formation is obtained in some cases. Note that it can be said that the presence or absence of crystal grains or the density of crystal grains can be adjusted and the diameter size can be adjusted within the range of 1 nm to 10 nm by appropriate adjustment of the film formation conditions of reactive sputtering, such as the composition ratio in the target, the film formation pressure (0.1 Pa to 2.0 Pa), the power (250 W to 3000 W: 8 inches ϕ), and the temperature (room temperature to 100° C.). The second In—Ga—Zn—O-based non-single-crystal film has a thickness of 5 nm to 20 nm. Needless to say, when the film includes a crystal grain, the size thereof does not exceed the film thickness. Here, the thickness of the second In—Ga—Zn—O-based non-single-crystal film is 5 nm. 
     The first In—Ga—Zn—O-based non-single-crystal film is formed under film formation conditions different from those for the second In—Ga—Zn—O-based non-single-crystal film. For example, the first In—Ga—Zn—O-based non-single-crystal film is formed under conditions where the ratio of an oxygen gas flow rate to an argon gas flow rate is higher than the ratio of an oxygen gas flow rate to an argon gas flow rate under the film formation conditions for the second In—Ga—Zn—O-based non-single-crystal film. Specifically, the second In—Ga—Zn—O-based non-single-crystal film is formed under a rare gas (such as argon or helium) atmosphere (or an atmosphere including an oxygen gas at less than or equal to 10% and an argon gas at greater than or equal to 90%), and the first In—Ga—Zn—O-based non-single-crystal film is formed under an oxygen mixed atmosphere (an oxygen gas flow rate is higher than a rare gas flow rate). 
     A chamber used for formation of the second In—Ga—Zn—O-based non-single-crystal film may be the same as or different from the chamber in which the reverse sputtering has been performed. 
     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. 
     In addition, there are a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, and a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge. 
     Furthermore, as a film formation method by sputtering, there are also a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during film formation to form a thin compound film thereof, and a bias sputtering method in which voltage is also applied to a substrate during film formation. 
     Next, the first oxide semiconductor film  131  and the second oxide semiconductor film  136  are subjected to heat treatment for dehydration or dehydrogenation. The first oxide semiconductor film  131  and the second oxide semiconductor film  136  are subjected to heat treatment under an atmosphere of an inert gas (such as nitrogen, helium, neon, or argon) or under reduced pressure, and then slowly cooled under an inert gas atmosphere. 
     The heat treatment is preferably performed at 200° C. or higher. For example, heat treatment is performed at 450° C. for 1 hour under a nitrogen atmosphere. By this heat treatment under a nitrogen atmosphere, the resistance of the first oxide semiconductor film  131  and the second oxide semiconductor film  136  is reduced (the carrier concentration is increased, preferably to 1×10 18 /cm 3  or higher) and the conductivity thereof is increased. Thus, a first oxide semiconductor film  133  and a second oxide semiconductor film  137  each of whose resistance is reduced are formed (see  FIG. 4C ). The electrical conductivity of the first oxide semiconductor film  133  and the second oxide semiconductor film  137  is preferably higher than or equal to 1×10 −1  S/cm and lower than or equal to 1×10 2  S/cm. 
     Next, a second photolithography step is performed to form a resist mask, and the first oxide semiconductor film  133  and the second oxide semiconductor film  137  are etched. For example, unnecessary portions are removed by wet etching using a mixed solution of phosphoric acid, acetic acid, and nitric acid, so that a first oxide semiconductor layer  134  and a second oxide semiconductor layer  138  are formed. Note that etching here is not limited to wet etching and dry etching may also be performed. 
     As an etching gas for dry etching, a gas containing chlorine (a chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon chloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. 
     Alternatively, a gas containing fluorine (a fluorine-based gas such as carbon tetrafluoride (CF 4 ), sulfur fluoride (SF 6 ), nitrogen fluoride (NF 3 ), or trifluoromethane (CHF 3 )); hydrogen bromide (HBr); oxygen (O 2 ); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used. 
     As the dry etching method, a parallel plate reactive ion etching (RIE) method or an inductively coupled plasma (ICP) etching method can be used. In order to etch the films into desired shapes, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate. 
     As an etchant used for wet etching, a mixed solution of phosphoric acid, acetic acid, and nitric acid, or the like can be used. In addition, ITO-07N (produced by Kanto Chemical Co., Inc) may be used. 
     Furthermore, the etchant after the wet etching is removed together with the etched material by cleaning. The waste liquid of including the etchant and the material etched off may be purified and the material may be reused. When a material such as indium included in the oxide semiconductor layer is collected from the waste liquid after the etching and reused, the resources can be efficiently used and the cost can be reduced. 
     In order to obtain a desired shape by etching, the etching conditions (such as an etchant, etching time, and temperature) are adjusted as appropriate in accordance with the material. 
     Next, a conductive film  132  is formed using a metal material over the first oxide semiconductor layer  134  and the second oxide semiconductor layer  138  with a sputtering method or a vacuum evaporation method (see  FIG. 5B ). 
     As a material of the conductive film  132 , an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any of these elements as a component, an alloy containing a combination of any of these elements, and the like can be given. 
     In the case where heat treatment is performed after the formation of the conductive film  132 , the conductive film preferably has heat resistance enough to withstand the heat treatment. 
     Next, a third photolithography step is performed to form a resist mask, and unnecessary portions are removed by etching, so that source and drain electrode layers  105   a  and  105   b , a first oxide semiconductor layer  135 , source and drain regions  104   a  and  104   b , and a second terminal  122  are formed (see  FIG. 5C ). Wet etching or dry etching is employed as an etching method at this time. For example, when an aluminum film or an aluminum-alloy film is used as the conductive film  132 , wet etching using a mixed solution of phosphoric acid, acetic acid, and nitric acid can be performed. Alternatively, by wet etching using an ammonia peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), the conductive film  132  may be etched to form the source and drain electrode layers  105   a  and  105   b . In the etching step, part of an exposed region of the first oxide semiconductor layer  134  is etched, whereby the first oxide semiconductor layer  135  is formed. Accordingly, the first oxide semiconductor layer  135  has a region whose thickness is small between the source and drain electrode layers  105   a  and  105   b . In  FIG. 5C , etching to form the source and drain electrode layers  105   a  and  105   b , the first oxide semiconductor layer  135 , and the source and drain regions  104   a  and  104   b  is performed by dry etching at one time; therefore, end portions of the source and drain electrode layers  105   a  and  105   b , the first oxide semiconductor layer  135 , and the source and drain regions  104   a  and  104   b  are aligned with one another, and a continuous structure is formed. 
     In the third photolithography step, the second terminal  122  which is formed using the same material as the source and drain electrode layers  105   a  and  105   b  is left in the terminal portion. Note that the second terminal  122  is electrically connected to a source wiring (a source wiring including the source and drain electrode layers  105   a  and  105   b ). 
     Further, by using a resist mask which is formed using a multi-tone mask and has regions with plural thicknesses (typically, two different thicknesses), the number of resist masks can be reduced, resulting in simplified process and lower costs. 
     Next, the resist mask is removed, and a protective insulating layer  107  is formed to cover the gate insulating layer  102 , the first oxide semiconductor layer  135 , the source and drain regions  104   a  and  104   b , and the source and drain electrode layers  105   a  and  105   b . A silicon oxynitride film formed with a PCVD method is used as the protective insulating layer  107 . The silicon oxynitride film serving as the protective insulating layer  107  is provided in contact with an exposed region of the first oxide semiconductor layer  135  between the source and drain electrode layers  105   a  and  105   b , whereby the resistance of the region of the first oxide semiconductor layer  135 , which is in contact with the protective insulating layer  107 , is increased (the carrier concentration is reduced, preferably to lower than 1×10 18 /cm 3 , more preferably to lower than or equal to 1×10 14 /cm 3 ). Thus, a semiconductor layer  103  including a channel formation region whose resistance is increased can be formed. 
     Through the above process, a thin film transistor  170  can be manufactured. 
     Next, a fourth photolithography step is performed to form a resist mask, and the protective insulating layer  107  and the gate insulating layer  102  are etched so that a contact hole  125  reaching the source or drain electrode layer  105   b  is formed. In addition, a contact hole  127  reaching the second terminal  122  and a contact hole  126  reaching the first terminal  121  are also formed in the same etching step.  FIG. 6A  is a cross-sectional view at this stage. 
     Next, the resist mask is removed, and then a transparent conductive film is formed. The transparent conductive film is formed using indium oxide (In 2 O 3 ), indium oxide-tin oxide alloy (In 2 O 3 —SnO 2 , abbreviated to ITO), or the like with a sputtering method, a vacuum evaporation method, or the like. Such a material is etched with a hydrochloric acid-based solution. However, since a residue is easily generated particularly in etching ITO, indium oxide-zinc oxide alloy (In 2 O 3 —ZnO) may be used to improve etching processability. Alternatively, indium oxide-tin oxide alloy including silicon oxide (In—Sn—O-based oxide including silicon oxide) may be used. 
     In addition, when a reflective electrode layer is used as a pixel electrode layer, it can be formed using one or more kinds of materials selected from a metal such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag); an alloy thereof; and a nitride thereof. 
     Next, a fifth photolithography step is performed to form a resist mask, and unnecessary portions are removed by etching, so that a pixel electrode layer  110  is formed. 
     In the fifth photolithography step, a storage capacitor is formed with the capacitor wiring  108  and the pixel electrode layer  110 , in which the gate insulating layer  102  and the protective insulating layer  107  in a capacitor portion are used as a dielectric. Note that  FIG. 7  is a plan view at this stage 
     In addition, in the fifth photolithography step, the first terminal  121  and the second terminal  122  are covered with the resist mask, and transparent conductive films  128  and  129  are left in the terminal portion. The transparent conductive films  128  and  129  serve as electrodes or wirings connected to an FPC. The transparent conductive film  128  formed over the first terminal  121  is a connection terminal electrode which functions as an input terminal of the gate wiring. The transparent conductive film  129  formed over the second terminal  122  is a connection terminal electrode which functions as an input terminal of the source wiring. 
     Next, the resist mask is removed.  FIG. 6B  is a cross-sectional view at this stage. 
     Heat treatment may be performed after the formation of the protective insulating layer  107  or the formation of the pixel electrode layer  110 . The heat treatment may be performed at higher than or equal to 150° C. and lower than 350° C. under an air atmosphere or a nitrogen atmosphere. In the heat treatment, the semiconductor layer  103  is heated while being in contact with the protective insulating layer  107 ; accordingly, the resistance of the semiconductor layer  103  is further increased, and thus improvement and less variation in electric characteristics of the transistor can be achieved. As for the heat treatment (preferably at higher than or equal to 150° C. and lower than 350° C.), there is no particular limitation as long as it is performed after the formation of the protective insulating layer  107 . When the heat treatment also serves as another step such as heat treatment in formation of a resin film or heat treatment for reducing the resistance of a transparent conductive film, the heat treatment can be performed without increase in the number of steps. 
     Further, FIGS.  8 A 1  and  8 A 2  are a cross-sectional view of a gate wiring terminal portion at this stage and a plan view thereof, respectively. FIG.  8 A 1  is a cross-sectional view taken along line E 1 -E 2  of FIG.  8 A 2 . In FIG.  8 A 1 , a transparent conductive film  155  formed over a protective insulating film  154  is a connection terminal electrode which functions as an input terminal. Furthermore, in the terminal portion of FIG.  8 A 1 , a first terminal  151  formed using the same material as the gate wiring and a connection electrode layer  153  formed using the same material as the source wiring overlap with each other with a gate insulating layer  152  interposed therebetween, and are electrically connected to each other through the transparent conductive film  155 . Note that a portion where the transparent conductive film  128  and the first terminal  121  are in contact with each other in  FIG. 6B  corresponds to a portion where the transparent conductive film  155  and the first terminal  151  are in contact with each other in FIG.  8 A 1 . 
     FIGS.  8 B 1  and  8 B 2  are respectively a cross-sectional view and a plan view of a source wiring terminal portion which is different from that illustrated in  FIG. 6B . FIG.  8 B 1  is a cross-sectional view taken along line F 1 -F 2  of FIG.  8 B 2 . In FIG.  8 B 1 , the transparent conductive film  155  formed over the protective insulating film  154  is a connection terminal electrode which functions as an input terminal. Furthermore, in the terminal portion of FIG.  8 B 1 , an electrode layer  156  formed using the same material as the gate wiring is located below and overlaps with a second terminal  150  which is electrically connected to the source wiring with the gate insulating layer  152  interposed therebetween. The electrode layer  156  is not electrically connected to the second terminal  150 , and a capacitor for preventing noise or static electricity can be formed if the potential of the electrode layer  156  is set to a potential different from that of the second terminal  150 , such as floating, GND, or 0 V. The second terminal  150  is electrically connected to the transparent conductive film  155  with the protective insulating film  154  interposed therebetween. 
     A plurality of gate wirings, source wirings, and capacitor wirings are provided depending on the pixel density. Also in the terminal portion, the first terminal at the same potential as the gate wiring, the second terminal at the same potential as the source wiring, the third terminal at the same potential as the capacitor wiring, and the like are each arranged in plurality. The number of each of the terminals may be any number, and the number of the terminals may be determined by a practitioner as appropriate. 
     Through these five photolithography steps, the storage capacitor and a pixel thin film transistor portion including the thin film transistor  170  which is a bottom-gate thin film transistor having a staggered structure can be completed using the five photomasks. By disposing the thin film transistor and the storage capacitor in each pixel of a pixel portion in which pixels are arranged in matrix, one of substrates for manufacturing an active matrix display device can be obtained. In this specification, such a substrate is referred to as an active matrix substrate for convenience. 
     In the case of manufacturing an active matrix liquid crystal display device, an active matrix substrate and a counter substrate provided with a counter electrode are fixed to each other with a liquid crystal layer interposed therebetween. Note that a common electrode which is electrically connected to the counter electrode on the counter substrate is provided over the active matrix substrate, and a fourth terminal which is electrically connected to the common electrode is provided in the terminal portion. The fourth terminal is a terminal for setting the common electrode at a fixed potential such as GND or 0 V. 
     Alternatively, a storage capacitor may be formed with a pixel electrode which overlaps with a gate wiring of an adjacent pixel with a protective insulating film and a gate insulating layer interposed therebetween, without provision of the capacitor wiring. 
     In an active matrix liquid crystal display device, pixel electrodes arranged in matrix are driven so that a display pattern is formed on a screen. Specifically, voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, so that a liquid crystal layer provided between the pixel electrode and the counter electrode is optically modulated and this optical modulation is recognized as a display pattern by an observer. 
     In displaying moving images, a liquid crystal display device has a problem in that a long response time of liquid crystal molecules themselves causes afterimages or blurring of moving images. As a technique for improving moving image characteristics of a liquid crystal display device, there is a driving technique called black insertion by which a black image is displayed on the whole screen every other frame period. 
     Alternatively, a driving technique called double-frame rate driving may be employed in which a vertical synchronizing frequency is 1.5 times or more, preferably, 2 times or more as high as a usual vertical synchronizing frequency to improve the moving-image characteristics. 
     Further, as a technique for improving moving image characteristics of a liquid crystal display device, there is another driving technique in which a surface light source including a plurality of LED (light-emitting diode) light sources or a plurality of EL light sources is used as a backlight, and each light source included in the surface light source is independently driven in a pulsed manner in one frame period. As the surface light source, three or more kinds of LEDs may be used and an LED emitting white light may be used. Since a plurality of LEDs can be controlled independently, the light emission timing of LEDs can be synchronized with the timing at which a liquid crystal layer is optically modulated. According to this driving technique, LEDs can be partly turned off; therefore, an effect of reducing power consumption can be obtained particularly in the case of displaying an image having a large part on which black is displayed. 
     By combining these driving techniques, the display characteristics of a liquid crystal display device, such as moving-image characteristics, can be improved as compared to those of conventional liquid crystal display devices. 
     The n-channel transistor disclosed in this specification includes an oxide semiconductor film used for a channel formation region and has favorable dynamic characteristics; thus, it can be combined with any of these driving techniques. 
     In manufacturing a light-emitting display device, one electrode (also referred to as a cathode) of an organic light-emitting element is set to a low power supply potential such as GND or 0 V; thus, a terminal portion is provided with a fourth terminal for setting the cathode to a low power supply potential such as GND or 0 V. Further, in manufacturing a light-emitting display device, a power supply line is provided in addition to a source wiring and a gate wiring. Accordingly, the terminal portion is provided with a fifth terminal which is electrically connected to the power supply line. 
     In manufacturing a light-emitting display device, a partition wall including an organic resin layer is provided between organic light-emitting elements in some cases. In that case, heat treatment performed on the organic resin layer can also serve as the heat treatment which increases the resistance of the semiconductor layer  103  so that improvement and less variation in electric characteristics of the transistor are achieved. 
     The use of an oxide semiconductor for a thin film transistor leads to reduction in manufacturing cost. In particular, by the heat treatment for dehydration or dehydrogenation, impurities such as moisture are reduced and the purity of the oxide semiconductor film is increased. Therefore, a semiconductor device including a highly reliable thin film transistor having favorable electric characteristics can be manufactured without using a special sputtering apparatus in which dew point in a film formation chamber is lowered or an ultrapure oxide semiconductor target. 
     Since the semiconductor layer in the channel formation region is a region whose resistance is increased, electric characteristics of the thin film transistor are stabilized, and increase in off current or the like can be prevented. Accordingly, a semiconductor device including the highly reliable thin film transistor having favorable electric characteristics can be provided. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 5 
     An example will be described below, in which at least part of a driver circuit and a thin film transistor arranged in a pixel portion are formed over the same substrate in a display device which is one example of a semiconductor device. 
     The thin film transistor to be disposed in the pixel portion is formed in accordance with any of Embodiments 1 to 4. Further, the thin film transistor described in any of Embodiments 1 to 4 is an n-channel TFT, 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. 14A  illustrates an example of a block diagram of an active matrix liquid crystal display device which is an example of a semiconductor device. The display device illustrated in  FIG. 14A  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 each 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) which 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 transistor described in any of Embodiments 1 to 4 is an n-channel TFT, and a signal line driver circuit including the n-channel TFT is described with reference to  FIG. 15 . 
     The signal line driver circuit illustrated in  FIG. 15  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 , 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 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−1, a signal line Sj, and a signal line Sj+1 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 inputted 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 over a single crystal substrate. Further, the switch groups  5602 _ 1  to  5602 _M are preferably formed over the same substrate as the pixel portion. Therefore, the driver IC  5601  and the switch groups  5602 _ 1  to  5602 _M are preferably connected through an FPC or the like. 
     Next, operation of the signal line driver circuit illustrated in  FIG. 15  is described with reference to a timing chart in  FIG. 16 . The timing chart in  FIG. 16  illustrates the 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. 15  operates similarly to that in  FIG. 16  even when a scan line of another row is selected. 
     Note that the timing chart in  FIG. 16  illustrates the case where the wiring  5621 _J of the J-th column is connected to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 via the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c.    
     Note that the timing chart in  FIG. 16  illustrates timing at which the scan line Gi of the i-th row is selected, timing  5703   a  at which the first thin film transistor  5603   a  is turned on/off, timing  5703   b  at which the second thin film transistor  5603   b  is turned on/off, timing  5703   c  at which the third thin film transistor  5603   c  is turned on/off, and a signal  5721 _J inputted 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 inputted to the wirings  5621 _ 1  to  5621 _M. For example, a video signal inputted to the wiring  5621 _J in the first sub-selection period T 1  is inputted to the signal line Sj−1, a video signal inputted to the wiring  5621 _J in the second sub-selection period T 2  is inputted to the signal line Sj, and a video signal inputted to the wiring  5621 _J in the third sub-selection period T 3  is inputted to the signal line Sj+1. In addition, the video signals inputted 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−1, Data_j, and Data_j+1. 
     As illustrated in  FIG. 16 , 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−1 inputted to the wiring  5621 _J is inputted to the signal line Sj−1 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 inputted to the wiring  5621 _J is inputted to the signal line Sj 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+1 inputted to the wiring  5621 _J is inputted to the signal line Sj+1 via the third thin film transistor  5603   c.    
     As described above, in the signal line driver circuit in  FIG. 15 , by dividing one gate selection period into three, video signals can be inputted to three signal lines from one wiring  5621  in one gate selection period. Therefore, in the signal line driver circuit in  FIG. 15 , 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. 15  can be improved. 
     Note that there is no particular 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 inputted to a plurality of signal lines from one wiring in the respective sub-selection periods as illustrated in  FIG. 15 . 
     For example, when video signals are inputted to three or more signal lines from one wiring in each of three or more sub-selection periods, it is only necessary to add a thin film transistor and a wiring which is configured to control the thin film transistor. Note that when one gate selection period is divided into four or more sub-selection periods, one sub-selection period becomes shorter. Therefore, one gate selection period is preferably divided into two or three sub-selection periods. 
     As another example, one gate 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 in  FIG. 17 . Further, the timing chart in  FIG. 17  illustrates the timing at which the scan line Gi of the i-th row is selected, timing  5803   a  at which the first thin film transistor  5603   a  is turned on/off, timing  5803   b  at which the second thin film transistor  5603   b  is turned on/off, timing  5803   c  at which the third thin film transistor  5603   c  is turned on/off, and a signal  5821 _J inputted to the wiring  5621 _J of the J-th column. As illustrated in  FIG. 17 , 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 V p  inputted to the wiring  5621 _J is inputted to each of the signal line Sj−1, the signal line Sj, and the signal line Sj+1 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−1 inputted to the wiring  5621 _J is inputted to the signal line Sj−1 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 inputted to the wiring  5621 _J is inputted to the signal line Sj 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+1 inputted to the wiring  5621 _J is inputted to the signal line Sj+1 via the third thin film transistor  5603   c.    
     As described above, in the signal line driver circuit in  FIG. 15  to which the timing chart in  FIG. 17  is applied, the video signal can be written to the pixel at high speed because the signal line can be precharged by providing a precharge period before a sub-selection period. Note that portions of  FIG. 17  which are similar to those of  FIG. 16  are denoted by common reference numerals and detailed description of the same portions and portions having similar functions is omitted. 
     Further, a configuration of a scan line driver circuit is described. The scan line driver circuit includes a shift register. Additionally, the scan line driver circuit may include a level shifter or a buffer in some cases. In the scan line driver circuit, when a clock signal (CLK) and a start pulse signal (SP) are inputted 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. Since the transistors in the pixels of one line have to be turned on all at once, a buffer which can supply large current is used. 
     One mode of a shift register used for part of the scan line driver circuit will be described with reference to  FIG. 18  and  FIG. 19 . 
       FIG. 18  illustrates a circuit configuration of the shift register. The shift register illustrated in  FIG. 18  includes a plurality of flip-flops: 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 relationship of the shift register in  FIG. 18  will be described. In the i-th stage flip-flop  5701 _ i  (one of the flip-flops  5701 _ 1  to  5701 _ n ) in the shift register in  FIG. 18 , a first wiring  5501  illustrated in  FIG. 19  is connected to a seventh wiring  5717 _ i −1, a second wiring  5502  illustrated in  FIG. 19  is connected to a seventh wiring  5717 _ i +1, a third wiring  5503  illustrated in  FIG. 19  is connected to a seventh wiring  5717 _ i , and a sixth wiring  5506  illustrated in  FIG. 19  is connected to a fifth wiring  5715 . 
     Further, a fourth wiring  5504  illustrated in  FIG. 19  is connected to a second wiring  5712  in flip-flops of odd-numbered stages, and is connected to a third wiring  5713  in flip-flops of even-numbered stages. A fifth wiring  5505  illustrated in  FIG. 19  is connected to a fourth wiring  5714 . 
     Note that the first wiring  5501  of the first stage flip-flop  5701 _ 1  which is illustrated in  FIG. 19  is connected to a first wiring  5711 . Moreover, the second wiring  5502  of the n-th stage flip-flop  5701 _ n  which is illustrated in  FIG. 19  is connected to 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. 19  illustrates details of the flip-flop shown in  FIG. 18 . A flip-flop illustrated in  FIG. 19  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 ). 
     Next, a connection configuration of the flip-flop illustrated in  FIG. 19  will be 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 , and a second electrode of the third thin film transistor  5573  is connected to a gate electrode of the second thin film transistor  5572 . 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 , and a second electrode of the fourth thin film transistor  5574  is connected to the gate electrode of the second thin film transistor  5572 . 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 , and a second electrode of the fifth thin film transistor  5575  is connected to the gate electrode of the first thin film transistor  5571 . 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 , and a second electrode of the sixth thin film transistor  5576  is connected to the gate electrode of the first thin film transistor  5571 . 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 , and a second electrode of the seventh thin film transistor  5577  is connected to the gate electrode of the first thin film transistor  5571 . 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 , and a second electrode of the eighth thin film transistor  5578  is connected to the gate electrode of the second thin film transistor  5572 . A gate electrode of the eighth thin film transistor  5578  is connected to the first wiring  5501 . 
     Note that the point 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 is referred to as a node  5543 . Further, the point 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 is 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 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 4. The n-channel TFTs described in any of Embodiments 1 to 4 have a high mobility, and thus a driving frequency of the driver circuits can be increased. Further, since parasitic capacitance is reduced in the n-channel TFT described in any of Embodiments 1 to 4, high frequency characteristics (referred to as f characteristics) is obtained. For example, a scan line driver circuit using the n-channel TFT described in any of Embodiments 1 to 4 can operate at high speed, and thus a frame frequency can be increased and insertion of black images can be realized, for example. 
     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, for example, higher frame frequency can be realized. When a plurality of scan line driver circuits are provided, a scan line driver circuit which is configured to drive scan lines of even-numbered rows is provided on one side and a scan line driver circuit which is configured to drive scan lines of odd-numbered rows is provided on the opposite side; thus, 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 example 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. 14B  illustrates an example of a block diagram of an active matrix light-emitting display device. 
     The light-emitting display device illustrated in  FIG. 14B  includes, over a substrate  5400 , a pixel portion  5401  including 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 each pixel; and a signal line driver circuit  5403  that controls a video signal input to the selected pixel. 
     When the video signal inputted to a pixel of the light-emitting display device illustrated in  FIG. 14B  is a digital signal, a pixel emits light or does not emit light by switching a transistor on/off. Thus, grayscale can be displayed using an area grayscale method or a time grayscale method. An area grayscale method refers to a driving method in which one pixel is divided into a plurality of sub-pixels and the respective sub-pixels are driven independently based on video signals so that grayscale is displayed. Further, a time grayscale method refers to a driving method in which a period during which a pixel emits light is controlled so that grayscale is displayed. 
     Since the response speed of a light-emitting element is higher than that of a liquid crystal element or the like, the light-emitting element is more suitable for a time grayscale method than the liquid crystal element. Specifically, in the case of displaying with a time grayscale method, one frame period is divided into a plurality of sub-frame periods. Then, in accordance with video signals, the light-emitting element in the pixel is brought into a light-emitting state or a non-light-emitting state in each sub-frame period. By dividing one frame period into a plurality of sub-frame periods, the total length of time in which a pixel actually emits light in one frame period can be controlled by video signals so that grayscale can be displayed. 
     Note that in the example of the light-emitting display device illustrated in  FIG. 14B , when two switching TFTs are arranged in one pixel, the first scan line driver circuit  5402  generates a signal which is inputted to a first scan line serving as a gate wiring of one of the two switching TFTs, and the second scan line driver circuit  5404  generates a signal which is inputted to a second scan line serving as a gate wiring of the other of the two switching TFTs. However, one scan line driver circuit may generate both the signal which is inputted to the first scan line and the signal which is inputted to the second scan line. In addition, for example, there is a possibility that a plurality of scan lines used to control the operation of the switching element are provided in each pixel, depending on the number of the switching TFTs included in one pixel. In this case, one scan line driver circuit may generate all signals that are inputted to the plurality of scan lines, or a plurality of scan line driver circuits may generate signals that are inputted to the plurality of scan lines. 
     Also in the light-emitting display device, part of a driver circuit that can include an n-channel TFT 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 4. 
     Through the above process, a highly reliable display device as a semiconductor device can be manufactured. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 6 
     When a thin film transistor is manufactured and used for a pixel portion and further for a driver circuit, a semiconductor device having a display function (also referred to as a display device) can be manufactured. Furthermore, when part or whole of a driver circuit using a thin film transistor is formed over the same substrate as a pixel portion, 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. Furthermore, a display medium whose contrast is changed by an electric effect, such as 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 also relates to an element substrate which corresponds to one mode before the display element is completed in a manufacturing process of the display device, and the element substrate is provided with a unit which is configured to supply current to the display element in each of a plurality of pixels. Specifically, the element substrate may be in a state where only a pixel electrode of the display element is formed, a state after a conductive film to be 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). Furthermore, the display device also includes the following modules in its category: a module to which a connector such as a flexible printed circuit (FPC), a tape automated bonding (TAB) tape, or a tape carrier package (TCP) is attached; a module having a TAB tape or a TCP at the tip of which a printed wiring board is provided; and a module in which an integrated circuit (IC) is directly mounted on a substrate provided with a display element with a chip on glass (COG) method. 
     The appearance and cross section of a liquid crystal display panel which is one embodiment of a semiconductor device will be described with reference to FIGS.  10 A 1 ,  10 A 2 , and  10 B. FIGS.  10 A 1  and  10 A 2  are each a plan view of a panel in which a liquid crystal element  4013  and highly reliable thin film transistors  4010  and  4011  each including an oxide semiconductor layer, which are like the thin film transistor described in Embodiment 4 and are formed over a first substrate  4001 , are sealed between the first substrate  4001  and a second substrate  4006  with a sealant  4005 .  FIG. 10B  is a cross-sectional view taken along line M-N of FIGS.  10 A 1  and  10 A 2 . 
     The sealant  4005  is provided to surround a pixel portion  4002  and a scan line driver circuit  4004  that 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 different from the region surrounded by the sealant  4005  over the first substrate  4001 . 
     Note that there is no particular limitation on the connection method of a driver circuit which is separately formed, and a COG method, a wire bonding method, a TAB method, or the like can be used. FIG.  10 A 1  illustrates an example of mounting the signal line driver circuit  4003  with a COG method, and FIG.  10 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  each include a plurality of thin film transistors, and  FIG. 10B  illustrates, as an example, 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 . Insulating layers  4020  and  4021  are provided over the thin film transistors  4010  and  4011 . 
     As the thin film transistors  4010  and  4011 , a highly reliable thin film transistor including an oxide semiconductor layer like the thin film transistor described in Embodiment 4 can be employed. Alternatively, the thin film transistor described in any of Embodiments 1 to 3 may be employed. In this embodiment, the thin film transistors  4010  and  4011  are n-channel thin film transistors. 
     A pixel electrode layer  4030  included in 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 with 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, each of which functions as an alignment film. The liquid crystal layer  4008  is sandwiched between the pixel electrode layer  4030  and the counter electrode layer  4031  with the insulating layers  4032  and  4033  interposed therebetween. 
     Note that the first substrate  4001  and the second substrate  4006  can be made of glass, metal (typically, stainless steel), ceramic, or plastic. As plastic, a fiberglass-reinforced plastic (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. Alternatively, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used. 
     A columnar spacer denoted by reference numeral  4035  is obtained by selective etching of an insulating film and is provided in order 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 contained in the sealant  4005 . 
     Alternatively, liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of the liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase is only generated within a narrow range of temperatures, a liquid crystal composition containing a chiral agent at 5 wt % or more is used for the liquid crystal layer  4008  in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral agent has a high response speed of 1 msec or less, has optical isotropy, which makes the alignment process unneeded, and has a small viewing angle dependence. 
     An embodiment of the present invention can also be applied to a reflective liquid crystal display device or a semi-transmissive liquid crystal display device, in addition to a transmissive liquid crystal display device. 
     An example of the liquid crystal display device is described in which a polarizing plate is provided on the outer surface of the substrate (on the viewer side) and a coloring layer and an electrode layer used for a display element are provided on the inner surface of the substrate in this order; however, the polarizing plate may be provided on the inner surface of the substrate. The stacked-layer structure of the polarizing plate and the coloring layer is not limited to that described in this embodiment and may be set as appropriate in a manner that depends on materials of the polarizing plate and the coloring layer or conditions of manufacturing steps. Furthermore, a light-blocking film functioning as a black matrix may be provided. 
     In the thin film transistors  4010  and  4011 , the insulating layer  4020  is formed as a protective insulating film so as to be in contact with a semiconductor layer including a channel formation region. The insulating layer  4020  may be formed using a material and a method similar to those of the oxide insulating film  407  described in Embodiment 1. In addition, in order to reduce the surface roughness of the thin film transistors, the thin film transistors are covered with the insulating layer  4021  functioning as a planarizing insulating film. 
     In this embodiment, the insulating layer  4020  having a stacked-layer structure is formed as the protective film. As a first layer of the insulating layer  4020 , a silicon oxide film is formed with a sputtering method. The use of the silicon oxide film as the protective film has the effect of preventing a hillock of an aluminum film used for the source and drain electrode layers. 
     An insulating layer is formed as a second layer of the protective film. As a 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 such as sodium ions from entering a semiconductor region, thereby suppressing variations in electric characteristics of the TFTs. 
     After the protective film is formed, heat treatment (at 300° C. or less) may be performed under a nitrogen atmosphere or an air atmosphere. 
     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 any of these materials. 
     Note that a siloxane-based resin is a resin formed from a siloxane-based material as a starting material and having a Si—O—Si bond. The siloxane-based resin may include an organic group (e.g., an alkyl group or an aryl group) or a fluoro group as a substituent. The organic group may include a fluoro group. 
     There is no particular limitation on the method for forming the insulating layer  4021 , and any of the following can be used depending on a material thereof: a method such as a sputtering method, an SOG method, spin coating, dipping, spray coating, or a droplet discharging method (e.g., an ink-jet method, screen printing, or offset printing); a tool such as doctor knife, roll coater, curtain coater, or knife coater; or the like. The baking step of the insulating layer  4021  also serves as the annealing step of the 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 containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added. 
     A conductive composition containing 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 10000 ohms per square or less and a transmittance of 70% or more at a wavelength of 550 nm. Furthermore, the resistivity of the conductive macromolecule contained in the conductive composition is preferably 0.1 Ω·cm or less. 
     As the conductive macromolecule, a so-called  7   c -electron conjugated conductive polymer can be used. For example, it is possible to use polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, or a copolymer of two or more kinds of them. 
     In addition, a variety of signals and potentials are supplied from an FPC  4018  to the signal line driver circuit  4003  that is formed separately, and the scan line driver circuit  4004  or the pixel portion  4002 . 
     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 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 . 
     Note that FIGS.  10 A 1 ,  10 A 2 , and  10 B illustrate an example in which the signal line driver circuit  4003  is formed separately and mounted on the first substrate  4001 ; however, the present invention 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. 20  illustrates an example of a liquid crystal display module which is formed as a semiconductor device by using a TFT substrate manufactured in accordance with the manufacturing method disclosed in this specification. 
       FIG. 20  illustrates an example of a liquid crystal display module, in which a substrate  2600  and a counter substrate  2601  are bonded 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 substrate  2600  and the counter substrate  2601 . A light source includes a cold cathode tube  2610  and a reflective plate  2611 . A circuit board  2612  is connected to a wiring circuit portion  2608  of the 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. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 7 
     An example of electronic paper will be described as a semiconductor device. 
     The semiconductor device may be used for electronic paper. The electronic paper is also referred to as an electrophoretic display device (an electrophoretic display) and is advantageous in that it has the same level of readability as plain paper, it has lower power consumption than other display devices, and it can be made thin and lightweight. 
     Electrophoretic displays can have various modes. An electrophoretic display contains a plurality of microcapsules dispersed in a solvent or a solute, and each microcapsule contains first particles which are positively charged and second particles which are negatively charged. By application of an electric field to the microcapsules, the particles in the microcapsules move in opposite directions to each other and only the color of the particles gathering on one side is displayed. Note that the first particles and/or the second particles each contain pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless). 
     An electrophoretic display is thus a display that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a high-electric field region. An electrophoretic display does not need to use a polarizing plate which is required in 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 pigment, color display can also be achieved. 
     In addition, when a plurality of the 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 thus display can be performed by application of an electric field to the microcapsules. For example, the active matrix substrate obtained using the thin film transistor described in any of Embodiments 1 to 4 can be used. 
     Note that the first particles and the second particles in the microcapsules may each 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 formed using a composite material of any of these. 
       FIG. 9  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 in a manner similar to that of the thin film transistor described in Embodiment 1 and is a highly reliable thin film transistor including an oxide semiconductor layer. The thin film transistor described in any of Embodiments 2 to 4 can also be used as the thin film transistor  581  of this embodiment. 
     The electronic paper in  FIG. 9  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 each 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  sealed between a substrate  580  and a substrate  596  is a thin film transistor having a bottom-gate structure and is covered with an insulating film  583  that is in contact with the semiconductor layer. A source or drain electrode layer of the thin film transistor  581  is in contact with a first electrode layer  587  through an opening formed in the insulating film  583  and 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 , spherical particles  589  each having a black region  590   a , a white region  590   b , and a cavity  594  which is filled with liquid around the black region  590   a  and the white region  590   b  are provided. A space around the spherical particles  589  is filled with a filler  595  such as a resin (see  FIG. 9 ). 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  580  as the thin film transistor  581 . With the use of a common connection portion, the second electrode layer  588  can be electrically connected to the common potential line through conductive particles provided between the substrate  580  and the substrate  596 . 
     Instead of the twisting ball, an electrophoretic element can also 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 from each other, so that white or black can be displayed. A display element using this principle is an electrophoretic display element and is generally called electronic paper. 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 displayed once can be maintained. Accordingly, a displayed image can be stored even if a semiconductor device having a display function (which may be referred to simply as a display device or a semiconductor device provided with a display device) is distanced from an electric wave source. 
     Through above process, highly reliable electronic paper as a semiconductor device can be manufactured. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 8 
     An example of a light-emitting display device will be described as a semiconductor device. As a display element included in the display device, a light-emitting element utilizing electroluminescence is described in this embodiment. 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 containing a light-emitting organic compound, and current flows. Then, the carriers (electrons and holes) recombine, so that 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. 
     The 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 which 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 in this embodiment using an organic EL element as a light-emitting element. 
       FIG. 12  illustrates an example of a pixel configuration to which digital time grayscale driving can be applied as an example of a semiconductor device. 
     The configuration and operation of a pixel to which digital time grayscale driving can be applied will be described. An example is described in this embodiment in which one pixel includes two n-channel transistors using an oxide semiconductor layer in a channel formation region. 
     A pixel  6400  includes a switching transistor  6401 , a driving transistor  6402 , a light-emitting element  6404 , and a capacitor  6403 . In the switching transistor  6401 , a gate thereof is connected to a scan line  6406 , a first electrode thereof (one of source and drain electrodes) is connected to a signal line  6405 , and a second electrode thereof (the other of the source and drain electrodes) is connected to a gate of the driving transistor  6402 . In the driving transistor  6402 , the gate thereof is connected to a power supply line  6407  through the capacitor  6403 , a first electrode thereof is connected to the power supply line  6407 , and a second electrode thereof 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. 
     Note that the second electrode (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 reference to the high power supply potential that is supplied to the power supply line  6407 . As the low power supply potential, GND, 0 V, or the like may be employed, for example. The difference between the high power supply potential and the low power supply potential is applied to the light-emitting element  6404  so that a current flows through the light-emitting element  6404 , whereby the light-emitting element  6404  emits light. Thus, each potential is set so that the difference between the high power supply potential and the low power supply potential is greater than or equal to forward threshold voltage of the light-emitting element  6404 . 
     When the gate capacitance of the driving transistor  6402  is used as a substitute for the capacitor  6403 , 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 using a voltage-input voltage driving method, a video signal is inputted 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  operates in a linear region, and thus 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 the following is applied to the signal line  6405 : power supply line voltage+V th  of the driving transistor  6402 . 
     In the case of performing analog grayscale driving instead of digital time grayscale driving, the same pixel configuration as  FIG. 12  can be employed by inputting signals in a different way. 
     In the case of performing analog grayscale driving, voltage higher than or equal to the following is applied to the gate of the driving transistor  6402 : forward voltage of the light-emitting element  6404 +V th  of the driving transistor  6402 . The forward voltage of the light-emitting element  6404  refers to voltage to obtain a desired luminance, and includes at least forward threshold voltage. By input of a video signal which enables the driving transistor  6402  to operate in a saturation region, it is possible to feed current to the light-emitting element  6404 . In order that the driving transistor  6402  can operate in the saturation region, the potential of the power supply line  6407  is set higher than a 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 configuration is not limited to that illustrated in  FIG. 12 . For example, the pixel illustrated in  FIG. 12  may further include a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like. 
     Next, structures of the light-emitting element will be described with reference to  FIGS. 13A to 13C . A cross-sectional structure of a pixel will be described by taking an n-channel driving TFT as an example. Driving TFTs  7001 ,  7011 , and  7021  used for semiconductor devices illustrated in  FIGS. 13A, 13B, and 13C , respectively, can be manufactured in a manner similar to that of the thin film transistor described in Embodiment 1 and are highly reliable thin film transistors each including an oxide semiconductor layer. Alternatively, the thin film transistors described in any of Embodiment 2 to 4 can be employed as the driving TFTs  7001 ,  7011 , and  7021 . 
     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 is extracted through the surface opposite to the substrate, a bottom emission structure in which light is extracted through the surface on the substrate side, or a dual emission structure in which light is extracted through the surface opposite to the substrate and the surface on the substrate side. The pixel configuration can be applied to a light-emitting element having any of these emission structures. 
     A light-emitting element having a top emission structure will be described with reference to  FIG. 13A . 
       FIG. 13A  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. 13A , 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 as a single layer or a plurality of layers stacked. When the light-emitting layer  7004  is formed as 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 . Note that not all of these layers need to be provided. The anode  7005  may be formed using a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing 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. 13A , 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 will be described with reference to  FIG. 13B .  FIG. 13B  is a cross-sectional view of a pixel in the case where the driving TFT  7011  is an n-channel TFT and light is emitted from a light-emitting element  7012  to a cathode  7013  side. In  FIG. 13B , the cathode  7013  of the light-emitting element  7012  is formed over a light-transmitting conductive film  7017  which 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 . Note that a light-blocking film  7016  for reflecting or blocking light may be formed so as to cover the anode  7015  when the anode  7015  has a light-transmitting property. As in the case of  FIG. 13A , the cathode  7013  can be formed using a variety of conductive materials as long as they have a low work function. Note that 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 . As in the case of  FIG. 13A , 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. 13A . As the light-blocking film  7016 , a metal which reflects light can be used for example; however, the light-blocking film  7016  is not limited to a metal film. For example, a resin to which black pigment is 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. 13B , 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 will be described with reference to  FIG. 13C . In  FIG. 13C , 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. 13A , the cathode  7023  can be formed using a variety of conductive materials as long as they have a low work function. Note that the cathode  7023  is formed to have a thickness that can transmit light. For example, an aluminum film with a thickness of 20 nm can be used as the cathode  7023 . As in  FIG. 13A , 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. 13A . 
     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. 13C , light is emitted from the light-emitting element  7022  to both the anode  7025  side and the cathode  7023  side as indicated by arrows. 
     Although an organic EL element is described in this embodiment as a light-emitting element, an inorganic EL element can also be provided as a light-emitting element. 
     Note that although 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, a structure may be employed in which a TFT for current control is connected between the driving TFT and the light-emitting element. 
     Note that the structure of the semiconductor device described in this embodiment is not limited to those illustrated in  FIGS. 13A to 13C  and can be modified in various ways based on the spirit of techniques disclosed in this specification. 
     Next, the appearance and cross section of a light-emitting display panel (also referred to as a light-emitting panel), which is one embodiment of the semiconductor device, will be described with reference to  FIGS. 11A and 11B .  FIG. 11A  is a plan view of a panel in which a thin film transistor and a light-emitting element formed over a first substrate are sealed between the first substrate and a second substrate with a sealant.  FIG. 11B  is a cross-sectional view taken along line H-I of  FIG. 11A . 
     A sealant  4505  is provided 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 packaging (sealing) be thus performed with a protective film (such as a bonding film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that at least the pixel portion is not exposed to the outside air. 
     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  formed 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. 11B . 
     As the thin film transistors  4509  and  4510 , the highly reliable thin film transistor including an oxide semiconductor layer, which is described in Embodiment 3, can be employed. Alternatively, the thin film transistor described in any of Embodiments 1, 2, and 4 can be employed. 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  that is a pixel electrode included in the light-emitting element  4511  is electrically connected to a source or drain electrode layer of the thin film transistor  4510 . Note that a structure of the light-emitting element  4511  is not limited to the stacked-layer structure described in this embodiment, which includes the first electrode layer  4517 , an electroluminescent layer  4512 , and a second electrode layer  4513 . The structure of the light-emitting element  4511  can be changed as appropriate in a manner that depends on the direction in which light is extracted from the light-emitting element  4511 , for example. 
     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 to have an opening over the first electrode layer  4517  so that a sidewall of the opening is formed as an inclined surface with continuous curvature. 
     The electroluminescent layer  4512  may be formed as 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 oxygen, hydrogen, moisture, carbon dioxide, or the like from entering 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. 
     A variety of signals and potentials are supplied from FPCs  4518   a  and  4518   b  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 . 
     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 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 of the FPC  4518   a  through an anisotropic conductive film  4519 . 
     The second substrate  4506  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. Nitrogen may be used for the filler, for example. 
     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. Furthermore, 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 mounted as driver circuits formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared. Alternatively, 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. The present invention is not limited to the structure illustrated in  FIGS. 11A and 11B . 
     Through the above process, a highly reliable light-emitting display panel (light-emitting panel) as a semiconductor device can be manufactured. 
     This embodiment can be implemented in appropriate combination with any of the structures described in the other embodiments. 
     Embodiment 9 
     A semiconductor device disclosed in this specification can be applied to electronic paper. Electronic paper can be used for electronic appliances of a variety of fields as long as they can display data. For example, electronic paper can be applied to an electronic book reader (electronic book), a poster, an advertisement in a vehicle such as a train, or displays of various cards such as a credit card. An example of the electronic appliances is illustrated in  FIG. 22 . 
       FIG. 22  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 operate like a paper book. 
     A display portion  2705  and a display portion  2707  are incorporated in the housing  2701  and the housing  2703 , respectively. The display portion  2705  and the display portion  2707  may display one image or different images. In the case where the display portion  2705  and the display portion  2707  display different images, for example, text can be displayed on a display portion on the right side (the display portion  2705  in  FIG. 22 ) and pictures can be displayed on a display portion on the left side (the display portion  2707  in  FIG. 22 ). 
       FIG. 22  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, and the like may be provided on the same surface as the display portion of the housing. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, a terminal that can be connected to various cables such as an AC adapter and a USB cable, or the like), a recording medium insertion portion, or the like may be provided on the back surface or the side surface of the housing. Moreover, the electronic book reader  2700  may have a function of an electronic dictionary. 
     Further, the electronic book reader  2700  may send and receive information wirelessly. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server. 
     Embodiment 10 
     A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including amusement machines). Examples of electronic appliances include television sets (also referred to as televisions or television receivers), monitors of computers or the like, cameras such as digital cameras or digital video cameras, digital photo frames, cellular phones (also referred to as mobile phones or mobile phone sets), portable game consoles, portable information terminals, audio reproducing devices, large-sized game machines such as pachinko machines, and the like. 
       FIG. 23A  illustrates an example of a television set  9600 . In the television set  9600 , a display portion  9603  is incorporated in a housing  9601 . Images can be displayed on the display portion  9603 . Here, the housing  9601  is supported by a stand  9605 . 
     The television set  9600  can be operated with an operation switch of the housing  9601  or a separate remote controller  9610 . Channels and volume can be controlled with an operation key  9609  of the remote controller  9610  so that an image displayed on the display portion  9603  can be controlled. Furthermore, the remote controller  9610  may be provided with a display portion  9607  which displays data outputted 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. Furthermore, 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, between receivers, or the like) data communication can be performed. 
       FIG. 23B  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 . Various images can be displayed on the display portion  9703 . For example, the display portion  9703  can display image data taken with a digital camera or the like to function as a normal photo frame. 
     Note that the digital photo frame  9700  is provided with an operation portion, an external connection portion (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 they may be provided on the same surface as the display portion  9703 , it is preferable to provide them on the side surface or the back surface because the design thereof is improved. For example, a memory in which image data taken with a digital camera is stored is inserted in the recording medium insertion portion of the digital photo frame  9700 , whereby the image data can be displayed on the display portion  9703 . 
     The digital photo frame  9700  may send and receive information wirelessly. Through wireless communication, desired image data can be downloaded to be displayed. 
       FIG. 24A  illustrates a portable amusement machine including two housings, a housing  9881  and a housing  9891 . The housings  9881  and  9891  are connected with a connection portion  9893  so as to be opened and closed. A display portion  9882  and a display portion  9883  are incorporated in the housing  9881  and the housing  9891 , respectively. In addition, the portable amusement machine illustrated in  FIG. 24A  includes a speaker portion  9884 , a recording medium insertion portion  9886 , an LED lamp  9890 , input units (an operation key  9885 , a connection terminal  9887 , a sensor  9888  (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), and a microphone  9889 ), and the like. It is needless to say that the structure of the portable amusement machine is not limited to the above and other structures provided with at least a semiconductor device disclosed in this specification may be employed. The portable amusement machine may include other accessory equipment as appropriate. The portable amusement machine illustrated in  FIG. 24A  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 amusement machine by wireless communication. The portable amusement machine illustrated in  FIG. 24A  can have various functions without limitation to the above. 
       FIG. 24B  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  includes an operation unit such as a start lever or a stop switch, a coin slot, a speaker, and the like. It is needless to say that the structure of the slot machine  9900  is not limited to the above and other structures provided with at least a semiconductor device disclosed in this specification may be employed. The slot machine  9900  may include other accessory equipment as appropriate. 
       FIG. 25A  is a perspective view illustrating an example of a portable computer. 
     In the portable computer in  FIG. 25A , a top housing  9301  having a display portion  9303  and a bottom housing  9302  having a keyboard  9304  can overlap with each other by closing a hinge unit which connects the top housing  9301  and the bottom housing  9302 . The portable computer in  FIG. 25A  is convenient for carrying, and in the case of using the keyboard for input, the hinge unit is opened so that the user can input looking at the display portion  9303 . 
     The bottom housing  9302  includes a pointing device  9306  with which input can be performed, in addition to the keyboard  9304 . Further, when the display portion  9303  is a touch input panel, input can be performed by touching part of the display portion. The bottom housing  9302  includes an arithmetic function portion such as a CPU or hard disk. In addition, the bottom housing  9302  includes another device, for example, an external connection port  9305  into which a communication cable conformable to communication standards of a USB is inserted. 
     The top housing  9301  further includes a display portion  9307  which can be stored in the top housing  9301  by being slid therein. Thus, a large display screen can be realized. In addition, the user can adjust the orientation of a screen of the storable display portion  9307 . When the storable display portion  9307  is a touch input panel, input can be performed by touching part of the storable display portion. 
     The display portion  9303  or the storable display portion  9307  is formed using an image display device of a liquid crystal display panel, a light-emitting display panel such as an organic light-emitting element or an inorganic light-emitting element, or the like. 
     In addition, the portable computer in  FIG. 25A , which can be provided with a receiver and the like, can receive a television broadcast to display an image on the display portion. While the hinge unit which connects the top housing  9301  and the bottom housing  9302  is kept closed, the whole screen of the display portion  9307  is exposed by sliding the display portion  9307  out and the angle of the screen is adjusted; thus, the user can watch a television broadcast. In this case, the hinge unit is not opened and display is not performed on the display portion  9303 . In addition, start up of only a circuit which displays the television broadcast is performed. Therefore, power consumption can be minimized, which is advantageous for the portable computer whose battery capacity is limited. 
       FIG. 25B  is a perspective view illustrating an example of a cellular phone that the user can wear on the wrist like a wristwatch. 
     This cellular phone includes a main body which includes a battery and a communication device having at least a telephone function; a band portion  9204  which enables the main body to be wore on the wrist; an adjusting portion  9205  which adjusts the band portion  9204  to fit the wrist; a display portion  9201 ; a speaker  9207 ; and a microphone  9208 . 
     In addition, the main body includes an operation switch  9203 . The operation switch  9203  serves, for example, as a switch for starting a program for the Internet when the switch is pushed, in addition to serving as a switch for turning on a power source, a switch for shifting a display, a switch for instructing to start taking images, or the like, and can be used so as to correspond to each function. 
     Input to this cellular phone is operated by touching the display portion  9201  with a finger, an input pen, or the like, by operating the operation switch  9203 , or by inputting voice into the microphone  9208 . Note that displayed buttons  9202  which are displayed on the display portion  9201  are illustrated in  FIG. 25B . Input can be performed by touching the displayed buttons  9202  with a finger or the like. 
     Further, the main body includes a camera portion  9206  including an image pick-up unit having a function of converting an image of an object, which is formed through a camera lens, to an electronic image signal. Note that the camera portion is not necessarily provided. 
     The cellular phone illustrated in  FIG. 25B , which can be provided with a receiver of a television broadcast and the like, can display an image on the display portion  9201  by receiving a television broadcast. In addition, the cellular phone illustrated in  FIG. 25B  may be provided with a storage device and the like such as a memory, and thus can record a television broadcast in the memory. The cellular phone illustrated in  FIG. 25B  may have a function of collecting location information, such as the GPS. 
     The display portion  9201  is formed using an image display device of a liquid crystal display panel, a light-emitting display panel such as an organic light-emitting element or an inorganic light-emitting element, or the like. The cellular phone illustrated in  FIG. 25B  is compact and lightweight and thus has limited battery capacity. Therefore, a panel which can be driven with low power consumption is preferably used as a display device for the display portion  9201 . 
     Note that  FIG. 25B  illustrates the electronic appliance which is worn on the wrist; however, this embodiment is not limited thereto as long as a portable shape is employed. 
     EXAMPLE 1 
     In this example, in an oxide semiconductor layer including a region having high oxygen density and a region having low oxygen density, the simulation result thereof in change of the oxygen density before and after heat treatment will be described with reference to  FIG. 34  and  FIG. 21 . As software for the simulation, Materials Explorer 5.0 manufactured by Fujitsu Limited was used. 
       FIG. 34  illustrates a model of an oxide semiconductor layer which was used for the simulation. Here, a structure in which a layer  703  having low oxygen density and a layer  705  having high oxygen density are stacked was employed for an oxide semiconductor layer  701 . 
     For the layer  703  having low oxygen density, an amorphous structure was employed in which the numbers of In atoms, Ga atoms, and Zn atoms were each 15 and the number of 0 atoms was 54. 
     In addition, for the layer  705  having high oxygen density, an amorphous structure was employed in which the numbers of In atoms, Ga atoms, and Zn atoms were each 15 and the number of 0 atoms was 66. 
     Moreover, the density of the oxide semiconductor layer  701  was set at 5.9 g/cm 3 . 
     Next, the classical molecular dynamics (MD) simulation was performed on the oxide semiconductor layer  701  under conditions of NVT ensemble and a temperature of 250° C. The time step was set at 0.2 fs, and the total simulation time was set at 200 ps. In addition, Born-Mayer-Huggins potential was used for the potentials of metal-oxygen bonding and oxygen-oxygen bonding. Moreover, movement of atoms at an upper end portion and a lower end portion of the oxide semiconductor layer  701  was fixed. 
     Next, the simulation result is shown in  FIG. 21 . In z-axis coordinates, the range of 0 nm to 1.15 nm indicates the layer  703  having low oxygen density, and the range of 1.15 nm to 2.3 nm indicates the layer  705  having high oxygen density. The distribution of oxygen densities before the MD simulation is indicated by a solid line  707 , and the distribution of oxygen densities after the MD simulation is indicated by a dashed line  709 . 
     The solid line  707  shows that the oxide semiconductor layer  701  has higher oxygen densities in a region on a side of the layer  705  having high oxygen density from the interface between the layer  703  having low oxygen density and the layer  705  having high oxygen density. On the other hand, in the dashed line  709 , it is found that the oxygen density is uniform in the layer  703  having low oxygen density and the layer  705  having high oxygen density. 
     As described above, when there is non-uniformity in the distribution of the oxygen density as in the stack of the layer  703  having low oxygen density and the layer  705  having high oxygen density, it is found that the oxygen diffuses from where the oxygen density is higher to where the oxygen density is lower by heat treatment and thus the oxygen density becomes uniform. 
     That is, as described in Embodiment 1, since the oxygen density at the interface between the first oxide semiconductor layer  432  and the oxide insulating film  407  is increased by formation of the oxide insulating film  407  over the first oxide semiconductor layer  432 , the oxygen diffuses to the first oxide semiconductor layer  432  where the oxygen density is lower and thus the first oxide semiconductor layer  432  has higher resistance. As described above, reliability of a thin film transistor can be improved. 
     This application is based on Japanese Patent Application serial no. 2009-156411 filed with Japan Patent Office on Jun. 30, 2009, the entire contents of which are hereby incorporated by reference.