Patent Publication Number: US-8980685-B2

Title: Method for manufacturing thin film transistor using multi-tone mask

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device including an oxide semiconductor, and a manufacturing method thereof. 
     2. Description of the Related Art 
     A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor manufactured using amorphous silicon has low field-effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor manufactured using crystalline silicon has high field-effect mobility, but a crystallization step such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate. 
     In contrast, attention has been drawn on a technique in which a thin film transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 each disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor formed into an oxide semiconductor film and is used for a switching element or the like of an image display device.
     [Patent Document 1] Japanese Published Patent Application No. 2007-123861   [Patent Document 2] Japanese Published Patent Application No. 2007-96055   

     SUMMARY OF THE INVENTION 
     A thin film transistor including an oxide semiconductor in a channel formation region has higher field-effect mobility than a thin film transistor including amorphous silicon. An oxide semiconductor film can be formed at temperatures of 300° C. or lower by a sputtering method or the like and a manufacturing process of a thin film transistor including an oxide semiconductor film is simpler than that of a thin film transistor including polycrystalline silicon. 
     There is an expectation for application of such an oxide semiconductor to liquid crystal displays, electroluminescent displays, electronic paper, and the like by forming a thin film transistor including the oxide semiconductor over a glass substrate, a plastic substrate, or the like. 
     As a method for manufacturing a thin film transistor, a method by which a stacked structure is formed by a photolithography process using a number of light-exposure masks (also referred to as photomasks) is employed. However, a photolithography process includes a number of steps and is one factor of largely affecting the manufacturing cost, yield, productivity, and the like. In particular, reducing the number of light-exposure masks whose design and manufacturing costs are high is a significant object. 
     In view of the above problems, it is an object to manufacture a semiconductor device at low cost with high productivity in such a manner that a photolithography process is simplified by reducing the number of light-exposure masks. 
     In a method for manufacturing a semiconductor device including an inverted staggered thin film transistor, an etching step is performed with the use of a mask layer formed using a multi-tone mask which is a light-exposure mask through which light is transmitted so as to have a plurality of intensities. 
     Since a mask layer formed using a multi-tone mask has a plurality of thicknesses and can be further changed in shape by performing etching, the mask layer can be used in a plurality of etching steps to provide different patterns. Therefore, a mask layer corresponding at least two kinds of different patterns can be formed using one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of corresponding photolithography processes can also be reduced, whereby simplification of a manufacturing process can be realized. 
     A process for manufacturing an inverted staggered thin film transistor includes an etching step (a first etching step) of processing a semiconductor film and a conductive film into island shapes and an etching step (a second etching step) of etching the conductive film and the semiconductor film into a source electrode layer, a drain electrode layer, and a semiconductor layer having a depression. The first etching step is performed by wet etching in which an etchant is used and the second etching step is performed by dry etching in which an etching gas is used. 
     As the etchant, a mixed solution of phosphoric acid, acetic acid, and nitric acid, or an ammonia hydrogen peroxide mixture can be used. 
     As the etching gas, a gas including chlorine (a chlorine-based gas such as Cl 2 , BCl 3 , or SiCl 4 ) is preferable. Alternatively, a gas obtained by adding oxygen or a rare gas (such as Ar) to the above gas may be used as the etching gas. 
     An oxide semiconductor used in this specification is formed into a thin film represented by InMO 3 (ZnO) m  (m&gt;0), and a thin film transistor is manufactured by using this thin film as a semiconductor layer. Note that M denotes one or more of metal elements selected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), or cobalt (Co). For example, in some cases, M denotes Ga and any of the above metal elements other than Ga, such as Ga and Ni, or Ga and Fe. The above oxide semiconductor includes, in some cases, a transition metal element such as Fe or Ni or an oxide of the transition metal as an impurity element, in addition to the metal element included as M. In this specification, this thin film is also called an In—Ga—Zn—O-based non-single-crystal film. 
     Since an In—Ga—Zn—O-based non-single-crystal film is formed by a sputtering method and then subjected to thermal treatment at 200° C. to 500° C., typically 300° C. to 400° C. for 10 minutes to 100 minutes, an amorphous structure is observed as its crystal structure in an XRD (X-ray diffraction) analysis. Moreover, as for the electrical characteristics of the thin film transistor, an on/off ratio of 10 9  or more and a mobility of 10 or more at a gate voltage of ±20 V can be achieved. 
     According to one embodiment of the present invention disclosed in this specification, a gate electrode layer is formed over a substrate having an insulating surface; a gate insulating layer, an oxide semiconductor film, and a conductive film are stacked over the gate electrode layer; a first mask layer is formed over the gate insulating layer, the oxide semiconductor film, and the conductive film; an oxide semiconductor layer and a conductive layer are formed by etching the oxide semiconductor film and the conductive film with the use of the first mask layer in a first etching step; a second mask layer is formed by ashing the first mask layer; and an oxide semiconductor layer having a depression, a source electrode layer, and a drain electrode layer are formed by etching the oxide semiconductor layer and the conductive layer with the use of the second mask layer in a second etching step, wherein the first mask layer is formed using a light-exposure mask through which light is transmitted so as to have a plurality of intensities, wherein wet etching in which an etchant is used is employed in the first etching step and dry etching in which an etching gas is used is employed in the second etching step, and wherein the oxide semiconductor layer having a depression includes a region with a smaller thickness than a region overlapping with the source electrode layer or the drain electrode layer. 
     According to another embodiment of the present invention disclosed in this specification, a gate electrode layer is formed over a substrate having an insulating surface; a gate insulating layer, a first oxide semiconductor film, a second oxide semiconductor film, and a conductive film are stacked over the gate electrode layer; a first mask layer is formed over the gate insulating layer, the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film; a first oxide semiconductor layer, a second oxide semiconductor layer, and a conductive layer are formed by etching the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film with the use of the first mask layer in a first etching step; a second mask layer is formed by ashing the first mask layer; and an oxide semiconductor layer having a depression, a source region, a drain region, a source electrode layer, and a drain electrode layer are formed by etching the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive layer with the use of the second mask layer in a second etching step, wherein the first mask layer is formed using a light-exposure mask through which light is transmitted so as to have a plurality of intensities, wherein wet etching in which an etchant is used is employed in the first etching step and dry etching in which an etching gas is used is employed in the second etching step, and wherein the oxide semiconductor layer having a depression includes a region with a smaller thickness than a region overlapping with the source region or the drain region. 
     The method for manufacturing a semiconductor device disclosed in this specification achieves at least one of the above objects. 
     Moreover, the second oxide semiconductor film used for the source region and the drain region of the thin film transistor is preferably thinner than the first oxide semiconductor film used for a channel formation region and preferably has higher conductivity (electrical conductivity) than the first oxide semiconductor film. 
     The second oxide semiconductor film has n-type conductivity and serves as the source region and the drain region. 
     Moreover, the first oxide semiconductor film has an amorphous structure and the second oxide semiconductor film includes a crystal grain (nanocrystal) in an amorphous structure in some cases. The crystal grain (nanocrystal) in the second oxide semiconductor film has a diameter of 1 nm to 10 nm, typically approximately 2 nm to 4 nm. 
     As the second oxide semiconductor film used for the source region and the drain region (n +  layer), an In—Ga—Zn—O-based non-single-crystal film can be used. 
     An insulating film may be formed so as to cover the thin film transistor and be in contact with the oxide semiconductor layer including the channel formation region. 
     Moreover, since the thin film transistor is easily destroyed by static electricity or the like, a protective circuit for protecting a driver circuit is preferably provided over the same substrate with respect to a gate wiring or a source wiring. The protective circuit is preferably formed using a non-linear element including an oxide semiconductor. 
     Note that the ordinal numbers such as “first” and “second” are used for convenience and do not define 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. 
     As a display device including a driver circuit, there are 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, in addition to a liquid crystal display device. 
     A light-emitting display device including a light-emitting element includes a pixel portion having a plurality of thin film transistors. The pixel portion includes a region where a gate electrode of one thin film transistor is connected to a source or drain wiring of another thin film transistor. A driver circuit of the light-emitting display device including a light-emitting element includes a region where a gate electrode of a thin film transistor is connected to a source or drain wiring of the thin film transistor. 
     Note that the semiconductor devices in this specification indicate all the devices which can operate by using semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic appliance are all included in the category of the semiconductor devices. 
     Further, by reducing the number of light-exposure masks, a photolithography process is simplified, whereby a reliable semiconductor device can be manufactured at low cost with high productivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  illustrate a method for manufacturing a semiconductor device. 
         FIGS. 2A and 2B  illustrate a semiconductor device. 
         FIGS. 3A to 3E  illustrate a method for manufacturing a semiconductor device. 
         FIGS. 4A and 4B  illustrate a semiconductor device. 
         FIGS. 5A to 5C  illustrate a method for manufacturing a semiconductor device. 
         FIGS. 6A to 6C  illustrate a method for manufacturing a semiconductor device. 
         FIG. 7  illustrates a method for manufacturing a semiconductor device. 
         FIG. 8  illustrates a method for manufacturing a semiconductor device. 
         FIG. 9  illustrates a method for manufacturing a semiconductor device. 
         FIG. 10  illustrates a semiconductor device. 
         FIGS. 11A ,  11 B,  11 C and  11 D illustrate semiconductor devices. 
         FIG. 12  illustrates a semiconductor device. 
         FIG. 13  illustrates a semiconductor device. 
         FIGS. 14A and 14B  are each a block diagram of a semiconductor device. 
         FIG. 15  illustrates a structure of a signal-line driver circuit. 
         FIG. 16  is a timing chart illustrating operation of the signal-line driver circuit. 
         FIG. 17  is a timing chart illustrating operation of the signal-line driver circuit. 
         FIG. 18  illustrates a structure of a shift register. 
         FIG. 19  illustrates a connection structure of a flip-flop of  FIG. 18 . 
         FIG. 20  illustrates an equivalent circuit of a pixel in a semiconductor device. 
         FIGS. 21A to 21C  each illustrate a semiconductor device. 
         FIGS. 22A ,  22 B, and  22 C illustrate semiconductor devices. 
         FIG. 23  illustrates a semiconductor device. 
         FIGS. 24A and 24B  illustrate a semiconductor device. 
         FIGS. 25A and 25B  each illustrate an example of application of electronic paper. 
         FIG. 26  is an external view illustrating an example of an electronic book. 
         FIGS. 27A and 27B  are external views illustrating examples of a television device and a digital photo frame, respectively. 
         FIGS. 28A and 28B  are external views illustrating examples of game machines. 
         FIGS. 29A and 29B  are external views illustrating examples of cellular phones. 
         FIGS. 30A to 30D  illustrate multi-tone masks. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments are described in detail with reference to the drawings. However, it is easily understood by those skilled in the art that the modes and details herein disclosed can be modified in a variety of ways without departing from the scope and the spirit of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of Embodiments given below. In the structures of the present invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and the description thereof will not be repeated. 
     (Embodiment 1) 
     A method for manufacturing a semiconductor device of this embodiment is described with reference to  FIGS. 1A to 1E  and  FIGS. 2A and 2B . 
       FIG. 2A  is a plan view of a thin film transistor  420  of a semiconductor device of this embodiment, and  FIG. 2B  is a cross-sectional view taken along C 1 -C 2  of  FIG. 2A . The thin film transistor  420  is an inverted staggered thin film transistor and includes a gate electrode layer  401 , a gate insulating layer  402 , a semiconductor layer  403 , n +  layers  404   a  and  404   b  serving as a source region and a drain region, and source and drain electrode layers  405   a  and  405   b.    
       FIGS. 1A to 1E  correspond to cross-sectional views illustrating steps of manufacturing the thin film transistor  420 . 
     In  FIG. 1A , an insulating film  407  serving as a base film is provided over a substrate  400  and the gate electrode layer  401  is provided over the insulating film  407 . The insulating film  407  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 of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. In this embodiment, as the insulating film  407 , a silicon oxide film (with a thickness of 100 nm) is used. 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 which contains any of these materials as a main component. 
     For example, as a two-layer structure of the gate electrode layer  401 , the following structures are preferable: a two-layer structure in which a molybdenum layer is stacked over an aluminum layer, a two-layer structure in which a molybdenum layer is stacked over a copper layer, a two-layer structure in which a titanium nitride layer or a tantalum nitride layer is stacked over a copper layer, and a two-layer structure in which a titanium nitride layer and a molybdenum layer are stacked. As a three-layer structure, it is preferable to stack a tungsten layer or a tungsten nitride layer, an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer. 
     The gate insulating layer  402 , a first oxide semiconductor film  431 , a second oxide semiconductor film  432 , and a conductive film  433  are stacked in that order over the gate electrode layer  401 . 
     The gate insulating layer  402  can be formed to have a single-layer or stacked-layer structure by a plasma CVD method, a sputtering method, or the like using a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a silicon nitride oxide layer. Alternatively, the gate insulating layer  402  can be formed using a silicon oxide layer by a CVD method in which an organosilane gas is used. As the organosilane gas, a silicon-containing compound 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 (SiH(OC 2 H 5 ) 3 ), or trisdimethylaminosilane (SiH(N(CH 3 ) 2 ) 3 ) can be used. 
     Note that before the first oxide semiconductor film  431  is formed by a sputtering method, dust on a surface of the gate insulating layer  402  is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering is a method by which voltage is applied to a substrate side using an RF power source to generate plasma on the substrate side in an argon atmosphere without applying voltage to a target side, so that a surface is modified. Nitrogen, helium, or the like may be used instead of the argon atmosphere. Alternatively, oxygen, hydrogen, N 2 O, or the like may be added to the argon atmosphere. Further alternatively, Cl 2 , CF 4 , or the like may be added to the argon atmosphere. 
     A region where the second oxide semiconductor film  432  and the conductive film  433  are in contact with each other is preferably modified through plasma treatment. In this embodiment, the plasma treatment is performed on the second oxide semiconductor film  432  (in this embodiment, an In—Ga—Zn—O-based non-single-crystal film) in an argon atmosphere before the conductive film  433  is formed. 
     The plasma treatment may be performed using nitrogen, helium, or the like instead of the argon atmosphere. Alternatively, oxygen, hydrogen, N 2 O, or the like may be added to the argon atmosphere. Further alternatively, Cl 2 , CF 4 , or the like may be added to the argon atmosphere. 
     In this embodiment, an In—Ga—Zn—O-based non-single-crystal film is used as each of the first oxide semiconductor film  431  and the second oxide semiconductor film  432 . The first oxide semiconductor film  431  and the second oxide semiconductor film  432  are formed under different conditions, and the second oxide semiconductor film  432  has higher conductivity and lower resistance than the first oxide semiconductor film  431 . For example, the second oxide semiconductor film  432  is formed using an oxide semiconductor film obtained by a sputtering method in which the argon gas flow rate is set to 40 sccm. The second oxide semiconductor film  432  has n-type conductivity and has an activation energy (ΔE) of from 0.01 eV to 0.1 eV. Note that in this embodiment, the second oxide semiconductor film  432  is an In—Ga—Zn—O-based non-single-crystal film and includes at least an amorphous component. In some cases, the second oxide semiconductor film  432  has a crystal grain (nanocrystal) in an amorphous structure. The crystal grain (nanocrystal) in this second oxide semiconductor film  432  has a diameter of 1 nm to 10 nm, typically approximately 2 nm to 4 nm. 
     By the provision of the second oxide semiconductor film  432  serving as an n +  layer, in an electrical connection between the conductive film  433  formed using a metal layer and the first oxide semiconductor film  431  serving as a channel formation region, a favorable junction is obtained. This allows more thermally-stable operation than Schottky junction. In addition, willing provision of the n +  layer is effective in supplying carriers to the channel (on the source side), stably absorbing carriers from the channel (on the drain side), or preventing a resistance component from being formed at an interface with the wiring. Further, by the decrease in resistance, high mobility can be maintained even at high drain voltage. 
     The gate insulating layer  402 , the first oxide semiconductor film  431 , the second oxide semiconductor film  432 , and the conductive film  433  can be formed successively without exposure to air. By the successive formation without exposure to air, the films can be stacked without the interface therebetween contaminated by an atmospheric component or a contaminant impurity element floating in air; therefore, variation in characteristics of a thin film transistor can be decreased. 
     A mask  434  is formed over the gate insulating layer  402 , the first oxide semiconductor film  431 , the second oxide semiconductor film  432 , and the conductive film  433 . 
     In this embodiment, an example is shown in which the mask  434  is formed in such a manner that light-exposure is performed using a high-tone mask. A resist is formed in order to form the mask  434 . As the resist, a positive type resist or a negative type resist can be used. Here, a positive resist is used. 
     Next, the resist is irradiated with light with the use of a multi-tone mask  59  as a light-exposure mask, so that the resist is exposed to light. 
     Here, light exposure with the multi-tone mask  59  is described with reference to  FIGS. 30A to 30D . 
     A multi-tone mask can achieve three levels of light exposure, so that an exposed portion, a semi-exposed portion, and an unexposed portion can be formed. In other words, a multi-tone mask is a mask through which light is transmitted so as to have a plurality of intensities. One-time light exposure and development process allows a resist mask having regions with plural thicknesses (typically, two kinds of thicknesses) to be formed. Thus, the number of light-exposure masks can be reduced by using a multi-tone mask. 
     Typical examples of a multi-tone mask include a gray-tone mask  59   a  as illustrated in  FIG. 30A  and a half-tone mask  59   b  as illustrated in  FIG. 30C . 
     As illustrated in  FIG. 30A , the gray-tone mask  59   a  includes a light-transmitting substrate  63 , and a light-blocking portion  64  and a diffraction grating  65  which are formed on the light-transmitting substrate  63 . The light transmittance of the light-blocking portion  64  is 0%. The diffraction grating  65  has light-transmitting portions in a slit form, a dot form, a mesh form, or the like with intervals which are less than or equal to the resolution limit of light used for the light exposure, whereby the light transmittance can be controlled. The diffraction grating  65  can be either in a slit form, a dot form, or a mesh form with regular intervals; or in a slit form, a dot form, or a mesh form with irregular intervals. 
     As the light-transmitting substrate  63 , a light-transmitting substrate such as a quartz substrate can be used. The light-blocking portion  64  and the diffraction grating  65  can be each formed using a light-blocking material which absorbs light, such as chromium or chromium oxide. 
     When the gray-tone mask  59   a  is irradiated with light for exposure, light transmittance  66  of the light-blocking portion  64  is 0% and the light transmittance  66  of a region where the light-blocking portion  64  and the diffraction grating  65  are not provided is 100%, as illustrated in  FIG. 30B . The light transmittance  66  of the diffraction grating  65  can be controlled in the range of 10% to 70%. The light transmittance of the diffraction grating  65  can be controlled by controlling the interval and pitch of slits, dots, or meshes of the diffraction grating. 
     As illustrated in  FIG. 30C , the half-tone mask  59   b  includes the light-transmitting substrate  63 , and a semi-transmissive portion  67  and a light-blocking portion  68  which are formed on the light-transmitting substrate  63 . The semi-transmissive portion  67  can be formed using MoSiN, MoSi, MoSiO, MoSiON, CrSi, or the like. The light-blocking portion  68  can be formed using a light-blocking material which absorbs light, such as chromium or chromium oxide. 
     In the case where the half-tone mask  59   b  is irradiated with light for exposure, as illustrated in  FIG. 30D , light transmittance  69  of the light-blocking portion  68  is 0% and that of a region where the light-blocking portion  68  and the semi-transmissive portion  67  are not provided is 100%. Further, the light transmittance  69  of the semi-transmissive portion  67  can be controlled in the range of 10% to 70%. The light transmittance of the semi-transmissive portion  67  can be controlled by choosing the material of the semi-transmissive portion  67 . 
     The light exposure is performed using the multi-tone mask, and then development is performed; accordingly, the mask  434  having regions with different thicknesses can be formed as illustrated in  FIG. 1B . 
     Next, a first etching step is performed using the mask  434 ; accordingly, the first oxide semiconductor film  431 , the second oxide semiconductor film  432 , and the conductive film  433  are etched into island shapes. As a result, a first oxide semiconductor layer  435 , a second oxide semiconductor layer  436 , and a conductive layer  437  can be formed (see  FIG. 1B ). 
     In this embodiment, the first etching step is performed by wet etching in which an etchant is used. 
     As the etchant, a mixed solution of phosphoric acid, acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), or the like can be used. Alternatively, ITO07N (manufactured by Kanto Chemical Co., Inc) may be used. 
     The etching condition (etchant, etching time, temperature, or the like) is adjusted as appropriate, depending on a material used for the conductive film  433 , so that the films can be etched into desired shapes. 
     For example, in the case where an aluminum film or an aluminum alloy film is used for the conductive film  433 , wet etching using a mixed solution of phosphoric acid, acetic acid, and nitric acid can be performed. Further, in the case where a titanium film is used for the conductive film  433 , wet etching using an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant can be performed. 
     For example, in the case where an aluminum film or an aluminum alloy film is used for the conductive film  433 , the first oxide semiconductor film  431 , the second oxide semiconductor film  432 , and the conductive film  433  may be etched using a mixed solution of phosphoric acid, acetic acid, and nitric acid as the etchant of the first etching step. 
     In the first etching step, the conductive film and the oxide semiconductor films may be etched using different etchants. For example, in the case where a titanium film is used for the conductive film  433 , the conductive film  433  is etched using an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant of the first etching step, and the first oxide semiconductor film  431  and the second oxide semiconductor film  432  may be etched using a mixed solution of phosphoric acid, acetic acid, and nitric acid. 
     Through the first etching step in which the first oxide semiconductor film  431 , the second oxide semiconductor film  432 , and the conductive film  433  are wet-etched, the first oxide semiconductor film  431 , the second oxide semiconductor film  432 , and the conductive film  433  are etched isotropically. In this manner, end portions of the mask  434  are not aligned with end portions of the first oxide semiconductor layer  435 , the second oxide semiconductor layer  436 , and the conductive layer  437 , and these end portions further recede, so that these end portions have shapes with curvature. 
     Since the etching rates of the end portions of the first semiconductor layer  435 , the second oxide semiconductor layer  436 , and the conductive layer  437  are different depending on the etching conditions or oxide semiconductor materials and conductive materials, the curvatures are different and the end portions are not continuous in some cases. 
     Furthermore, the etchant after the wet etching is removed together with the etched materials by cleaning. Waste liquid of the etchant containing the removed materials may be purified to recycle the materials contained in the waste liquid. Materials such as indium contained in the oxide semiconductor layer are collected from the waste liquid after the etching and recycled, so that resources can be effectively used and cost can be reduced. 
     Next, the mask  434  is subjected to ashing. As a result, the mask is reduced in size and thickness. Through the ashing, the region of the resist mask, which has small thickness (region overlapping with part of the gate electrode layer  401 ), is removed, so that divided masks  438  can be formed (see  FIG. 1C ). 
     A second etching step is performed using the masks  438 ; accordingly, the first oxide semiconductor layer  435 , the second oxide semiconductor layer  436 , and the conductive layer  437  are etched into a semiconductor layer  403 , n +  layers  404   a  and  404   b , and source and drain electrode layers  405   a  and  405   b  (see  FIG. 1D ). Note that the semiconductor layer  403  is partly etched to become a semiconductor layer having a groove (depression) and also having an end portion which is partly etched and exposed. 
     In this embodiment, the second etching step is performed by dry etching in which an etching gas is used. 
     As the etching gas, a gas including chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon chloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. With the use of the gas including chlorine in etching, in-plane variation in etching can be reduced as compared to the case of using a gas without chlorine. 
     Alternatively, a gas including fluorine (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 RIE (reactive ion etching) method or an ICP (inductively coupled plasma) 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. 
     In this embodiment, an ICP etching method is employed and the etching condition is as follows: Cl 2  and O 2  are used, the amount of electric power applied to the coil-shaped electrode is 1500 W, the amount of electric power applied to the electrode on the substrate side is 200 W, the pressure is 1.5 Pa, and the substrate temperature is −10° C. 
     Alternatively, the ICP etching method may be performed under the following etching condition: Cl 2  (with a flow rate of 100 sccm) is used as an etching gas, the amount of electric power applied to the coil-shaped electrode is 2000 W, the amount of electric power applied to the electrode on the substrate side is 600 W, the pressure is 1.5 Pa, and the substrate temperature is −10° C. 
     In addition, in the etching process, the end of the etching (also referred to as an end point) is preferably determined by monitoring the wavelength corresponding to each atom in the oxide semiconductor films while plasma emission intensity is measured. This method makes it possible to control the etching amount of the oxide semiconductor films more precisely. 
     When the etching is performed using a chlorine-based gas (Cl 2 ) to which an oxygen gas (O 2 ) is added (preferably, the content of oxygen in the etching gas is set to be 15 vol % or more), in the case of using a silicon oxynitride film as the gate insulating layer  402 , the selectivity ratio of the In—Ga—Zn—O-based non-single-crystal film used for the first oxide semiconductor layer  435  and the second oxide semiconductor layer  436  with respect to the gate insulating layer  402  can be increased. Therefore, the first oxide semiconductor film  431  and the second oxide semiconductor film  432  can be etched more than the gate insulating layer  402 , and the damage on the gate insulating layer  402  can be sufficiently decreased. 
     In a manner similar to the above, through the second etching step in which the first oxide semiconductor layer  435 , the second oxide semiconductor layer  436 , and the conductive layer  437  are dry-etched, the first oxide semiconductor layer  435 , the second oxide semiconductor layer  436 , and the conductive layer  437  are etched anisotropically. In this manner, the end portions of the masks  438  are aligned with end portions and the depression of the semiconductor layer  403  and end portions of the n +  layers  404   a  and  404   b  and the source and drain electrode layers  405   a  and  405   b , and these end portions become continuous. 
     In addition, since the etching rates of the end portions of the semiconductor layer  403 , the n +  layers  404   a  and  404   b , and the source and drain electrode layers  405   a  and  405   b  are different depending on the etching conditions or oxide semiconductor materials and conductive materials, the tapered angles are different and the end portions are not continuous in some cases. 
     After that, the masks  438  are removed. 
     The material of the source and drain electrode layers  405   a  and  405   b  preferably has a higher etching rate than that of the semiconductor layer  403 . This is because, in the case of etching the source and drain electrode layers  405   a  and  405   b  and the semiconductor layer  403  in one time by etching, decreasing the etching rate of the semiconductor layer  403  so as to be lower than that of the source and drain electrode layers  405   a  and  405   b  can suppress the excessive etching of the semiconductor layer  403 . As a result, the removal of the semiconductor layer  403  can be suppressed. 
     After that, thermal treatment at 200° C. to 600° C., typically 300° C. to 500° C. is preferably performed. Here, thermal treatment is performed at 350° C. for an hour in a nitrogen atmosphere. Through this thermal treatment, rearrangement at the atomic level of the In—Ga—Zn—O-based oxide semiconductor used for the semiconductor layer  403  and the n +  layers  404   a  and  404   b  occurs. This thermal treatment (also including photo-annealing or the like) is important in that the distortion that interrupts carrier transport in the semiconductor layer  403  and the n +  layers  404   a  and  404   b  can be released. Note that there is no particular limitation on when to perform the thermal treatment, as long as it is performed after the first oxide semiconductor film  431  and the second oxide semiconductor film  432  are formed. 
     In addition, oxygen radical treatment may be performed on the exposed depression of the semiconductor layer  403 . By the oxygen radical treatment, the thin film transistor in which the channel formation region is formed using the semiconductor layer  403  can serve as a normally-off transistor. Moreover, by the radical treatment, the damage of the semiconductor layer  403  due to the etching can be repaired. The radical treatment is preferably performed in an atmosphere of O 2  or N 2 O, or an atmosphere of N 2 , He, Ar, or the like which includes oxygen. Alternatively, an atmosphere obtained by adding Cl 2  or CF 4  to the above atmosphere may be used. Note that the radical treatment is preferably performed with no bias voltage applied to the substrate  100  side. 
     Through the above steps, the inverted staggered thin film transistor  420  illustrated in  FIG. 1E  can be completed. 
     With the use of the resist mask having regions with a plurality of (typically two kinds of) thicknesses, which is formed using the multi-tone mask, as in this embodiment, the number of resist masks can be reduced; therefore, the process can be simplified and cost reduction can be achieved. Accordingly, a reliable semiconductor device can be manufactured at low cost with high productivity. 
     (Embodiment 2) 
     Here, an example of a semiconductor device including a thin film transistor with a structure where the source and drain electrode layers are in contact with the semiconductor layer in Embodiment 1 is described with reference to  FIGS. 3A to 3E  and  FIGS. 4A and 4B . 
       FIG. 4A  is a plan view of a thin film transistor  460  in a semiconductor device of this embodiment, and  FIG. 4B  is a cross-sectional view taken along D 1 -D 2  of  FIG. 4A . The thin film transistor  460  is an inverted staggered thin film transistor and includes a gate electrode layer  451 , a gate insulating layer  452 , a semiconductor layer  453 , and source and drain electrode layers  455   a  and  455   b.    
       FIGS. 3A to 3E  are cross-sectional views illustrating steps of manufacturing the thin film transistor  460 . 
     In  FIG. 3A , an insulating film  457  serving as a base film is provided over a substrate  450  and the gate electrode layer  451  is provided over the insulating film  457 . In this embodiment, a silicon oxide film (with a thickness of 100 nm) is used as the insulating film  457 . The gate insulating layer  452 , an oxide semiconductor film  481 , and a conductive film  483  are stacked in that order over the gate electrode layer  451 . 
     A region where the oxide semiconductor film  481  and the conductive film  483  are in contact with each other is preferably modified by plasma treatment. In this embodiment, plasma treatment is performed on the oxide semiconductor film  481  (an In—Ga—Zn—O-based non-single-crystal film in this embodiment) in an argon atmosphere before the conductive film  483  is formed. 
     The plasma treatment may be performed using nitrogen, helium, or the like instead of the argon atmosphere. Alternatively, an argon atmosphere to which oxygen, hydrogen, 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. 
     The gate insulating layer  452 , the oxide semiconductor film  481 , and the conductive film  483  can be formed successively without exposure to air. By the successive formation without exposure to air, the films can be stacked without the interface therebetween contaminated by an atmospheric component or a contaminant impurity element floating in air; therefore, variation in characteristics of a thin film transistor can be decreased. 
     A mask  484  is formed over the gate insulating layer  452 , the oxide semiconductor film  481 , and the conductive film  483 . 
     In this embodiment, an example is described in which light-exposure is performed using a multi-tone (high-tone) mask in order to form the mask  484 . The mask  484  can be formed in a manner similar to that of the mask  434  of Embodiment 1. 
     The light exposure is performed using the multi-tone mask through which light is transmitted so as to have a plurality of intensities, and then development is performed, whereby the mask  484  having regions with different thicknesses can be formed as illustrated in  FIG. 3B . By using a multi-tone mask, the number of light-exposure masks can be reduced. 
     Next, a first etching step is performed using the mask  484 ; accordingly, the oxide semiconductor film  481  and the conductive film  483  are etched into island shapes. As a result, an oxide semiconductor layer  485  and a conductive layer  487  can be formed (see  FIG. 3B ). 
     In this embodiment, the first etching step is performed by wet etching in which an etchant is used. 
     As the etchant, a mixed solution of phosphoric acid, acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), or the like can be used. Alternatively, ITO07N (manufactured by Kanto Chemical Co., Inc) may be used. 
     The etching condition (etchant, etching time, temperature, or the like) is adjusted as appropriate, depending on a material used for the conductive film  483 , so that the films can be etched into desired shapes. 
     For example, in the case where an aluminum film or an aluminum alloy film is used for the conductive film  483 , wet etching using a mixed solution of phosphoric acid, acetic acid, and nitric acid can be performed. Further, in the case where a titanium film is used for the conductive film  483 , wet etching using an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2) can be performed. 
     For example, in the case where an aluminum film or an aluminum alloy film is used for the conductive film  483 , an oxide semiconductor film  481 , a conductive film  483 , an oxide semiconductor layer  485 , and a conductive layer  487  may be etched using a mixed solution of phosphoric acid, acetic acid, and nitric acid as the etchant of the first etching step. 
     In the first etching step, the conductive film and the oxide semiconductor films may be etched using different etchants. 
     For example, in the case where a titanium film is used for the conductive film  483 , the conductive film  483  is etched using an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant of the first etching step, and the oxide semiconductor film  481  may be etched using a mixed solution of phosphoric acid, acetic acid, and nitric acid. 
     Through the first etching step in which the oxide semiconductor film  481  and the conductive film  483  are wet-etched, the oxide semiconductor film  481  and the conductive film  483  are etched isotropically. In this manner, end portions of the mask  484  are not aligned with end portions of the oxide semiconductor layer  485  and the conductive layer  487 , and these end portions further recede, so that these portions have shapes with curvature. 
     Since the etching rates of the end portions of the oxide semiconductor layer  485  and the conductive layer  487  are different depending on the etching conditions or oxide semiconductor materials and conductive materials, the curvatures are different and the end portions are not continuous in some cases. 
     Furthermore, the etchant after the wet etching is removed together with the etched materials by cleaning. Waste liquid of the etchant containing the removed materials may be purified to recycle the materials contained in the waste liquid. Materials such as indium contained in the oxide semiconductor layer are collected from the waste liquid after the etching and recycled, so that resources can be effectively used and cost can be reduced. 
     Next, the mask  484  is subjected to ashing. As a result, the mask is reduced in size and thickness. Through the ashing, a region of the resist mask, which has small thickness (region overlapping with part of the gate electrode layer  451 ), is removed, so that divided masks  488  can be formed (see  FIG. 3C ). 
     A second etching step is performed using the masks  488 ; accordingly, the oxide semiconductor layer  485  and the conductive layer  487  are etched into a semiconductor layer  453  and source and drain electrode layers  455   a  and  455   b  (see  FIG. 3D ). Note that the semiconductor layer  453  is partly etched to become a semiconductor layer having a groove (depression) and also having an end portion which is partly etched and exposed. 
     In this embodiment, the second etching step is performed by dry etching in which an etching gas is used. 
     As the etching gas, a gas including chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon chloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. By the use of the gas including chlorine in etching, in-plane variation in etching can be reduced as compared to the case of using a gas without chlorine. 
     Alternatively, a gas including fluorine (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 RIE (reactive ion etching) method or an ICP (inductively coupled plasma) 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. 
     In this embodiment, an ICP etching method is employed and the etching condition is as follows: Cl 2  and O 2  are used, the amount of electric power applied to the coil-shaped electrode is 1500 W, the amount of electric power applied to the electrode on the substrate side is 200 W, the pressure is 1.5 Pa, and the substrate temperature is −10° C. 
     When the etching is performed using a chlorine-based gas (Cl 2 ) to which an oxygen gas (O 2 ) is added (preferably, the content of oxygen in the etching gas is set to be 15 vol % or more), in the case of using a silicon oxynitride film as the gate insulating layer  452 , the selectivity ratio of the In—Ga—Zn—O-based non-single-crystal film used for the oxide semiconductor layer  485  with respect to the gate insulating layer  452  can be increased. Therefore, the oxide semiconductor film  481  can be etched more than the gate insulating layer  452 . 
     Through the second etching step in which the oxide semiconductor layer  485  and the conductive layer  487  are dry-etched, the oxide semiconductor layer  485  and the conductive layer  487  are etched anisotropically. In this manner, the end portions of the masks  488  are aligned with end portions and the depression of the semiconductor layer  453  and end portions of the source and drain electrode layers  455   a  and  455   b , and these end portions become continuous. 
     Since the etching rates of the end portions of the semiconductor layer  453  and the source and drain electrode layers  455   a  and  455   b  are different depending on the etching conditions or oxide semiconductor materials and conductive materials, the tapered angles are different and the end portions are not continuous in some cases. 
     After that, the masks  488  are removed. 
     Through the above steps, the inverted staggered thin film transistor  460  illustrated in  FIG. 3E  can be completed. 
     With the use of the resist mask having a plurality of (typically two kinds of) thicknesses, which is formed using a multi-tone mask, as in this embodiment, the number of resist masks can be reduced; therefore, the process can be simplified and cost reduction can be achieved. Accordingly, a reliable semiconductor device can be manufactured at low cost with high productivity. 
     (Embodiment 3) 
     In this embodiment, a process for manufacturing a display device including a thin film transistor is described with reference to  FIGS. 5A to 5C ,  FIGS. 6A to 6C ,  FIG. 7 ,  FIG. 8 ,  FIG. 9 ,  FIG. 10 ,  FIGS. 11A ,  11 B,  11 C, and  11 D, and  FIG. 12 . 
     As for a substrate  100  having a light-transmitting property illustrated in  FIG. 5A , a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like which is typified by #7059 glass, #1737 glass, or the like manufactured by Corning, Inc. can be used. 
     Next, a conductive layer is formed entirely over a surface of the substrate  100 , and then a first photolithography process is performed to form a resist mask. Then, an unnecessary portion is removed by etching, so that wirings and electrodes (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 an end portion of the gate electrode layer  101  is tapered.  FIG. 5A  is a cross-sectional view illustrating this state. Note that  FIG. 7  corresponds to a top view of this state. 
     Each of the gate wiring including the gate electrode layer  101 , the capacitor wiring  108 , and the first terminal  121  at 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), or scandium (Sc); an alloy including any of these elements; an alloy film including any of these elements in combination; or a nitride including any of these elements. In the case of using a low-resistant conductive material such as aluminum (Al) or copper (Cu), the low-resistant conductive material is used in combination with the above heat-resistant conductive material because Al alone has problems of low heat resistance, tendency to corrode, and the like. 
     Next, a gate insulating layer  102  is formed entirely over the gate electrode layer  101 . The gate insulating layer  102  is formed to a thickness of 50 nm to 250 nm by a sputtering method or the like. 
     For example, a silicon oxide film is formed to a thickness of 100 nm as the gate insulating layer  102  by a sputtering method. Needless to say, the gate insulating layer  102  is not limited to such a silicon oxide film and another insulating film such as a silicon oxynitride film, a silicon nitride film, an aluminum oxide film, or a tantalum oxide film may be formed to have a single-layer or stacked-layer structure. 
     Note that reverse sputtering in which an argon gas is introduced and plasma is generated is preferably performed before the formation of the oxide semiconductor film, in order to remove dust on the surface of the gate insulating layer. Nitrogen, helium, or the like may be used instead of the argon atmosphere. Alternatively, oxygen, hydrogen, N 2 O, or the like may be added to the argon atmosphere. Further alternatively, Cl 2 , CF 4 , or the like may be added to the argon atmosphere. 
     Next, a first oxide semiconductor film  109  (a first In—Ga—Zn—O-based non-single-crystal film in this embodiment) is formed over the gate insulating layer  102 . It is effective to deposit the first In—Ga—Zn—O-based non-single-crystal film without exposure to air after the plasma treatment because dust and moisture are not attached to the interface between the gate insulating layer and the semiconductor film. Here, the first In—Ga—Zn—O-based non-single-crystal film is formed under the following condition: the target is an oxide semiconductor target including In, Ga, and Zn (In2O3:Ga2O3:ZnO=1:1:1) with a diameter of 8 inches, the distance between the substrate and the target is 170 mm, the pressure is 0.4 Pa, the direct current (DC) power supply is 0.5 kW, and the atmosphere is argon or oxygen. A pulse direct current (DC) power supply is preferable because dust can be reduced and film thickness becomes uniform. The thickness of the first In—Ga—Zn—O-based non-single-crystal film is set in the range of 5 nm to 200 nm. In this embodiment, the thickness of the first In—Ga—Zn—O-based non-single-crystal film is 100 nm. 
     Next, a second oxide semiconductor film  111  (a second In—Ga—Zn—O-based non-single-crystal film in this embodiment) is formed without exposure to air by a sputtering method. Here, sputtering deposition is performed under the following condition: the target is In2O3:Ga2O3:ZnO=1:1:1, the pressure is 0.4 Pa, the amount of electric power is 500 W, the deposition temperature is room temperature, and the argon gas flow rate is 40 sccm. Although the target of In2O3:Ga2O3:ZnO=1:1:1 is used intentionally, 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 deposition is obtained in some cases. By adjusting the target composition ratio, the deposition pressure (0.1 Pa to 2.0 Pa), the amount of electric power (250 W to 3000 W: 8 inches φ), the temperature (room temperature to 100° C.), the deposition condition of reactive sputtering, and the like as appropriate, the presence or absence of the crystal grains and the density of the crystal grains can be controlled and the diameter of the crystal grain can be adjusted within the range of 1 nm to 10 nm. The thickness of the second In—Ga—Zn—O-based non-single-crystal film is 5 nm to 20 nm. Needless to say, in the case where the film includes the crystal grain, the size of the crystal grain does not exceed the film thickness. In this embodiment, 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 and the second In—Ga—Zn—O-based non-single-crystal film are formed under different conditions from each other. For example, the flow ratio of an oxygen gas to an argon gas under the deposition condition of the first In—Ga—Zn—O-based non-single-crystal film is higher than that under the deposition condition of 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 in a rare gas (such as argon or helium) atmosphere (or an atmosphere including an oxygen gas for 10% or less and an argon gas for 90% or more), and the first In—Ga—Zn—O-based non-single-crystal film is formed in an oxygen atmosphere (or a flow rate of an oxygen gas is equal to or more than a flow rate of an argon gas). 
     The second In—Ga—Zn—O-based non-single-crystal film may be formed in the chamber where reverse sputtering has been performed previously, or in a different chamber from the chamber where reverse sputtering has been performed previously. 
     As the sputtering method, there are an RF sputtering method in which a high-frequency power source is used as a sputtering power source, a DC sputtering method, and a pulse DC sputtering method by which bias is applied in a pulsed manner. The RF sputtering method is used mainly in the case of forming an insulating film, and the DC sputtering method is used mainly in the case of forming a metal film. 
     Moreover, there is 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 a chamber and used for a magnetron sputtering method, and a sputtering apparatus used for an ECR sputtering method in which plasma generated with use of microwaves is used without using glow discharge. 
     In addition, as a deposition method by a sputtering method, there are also a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin film of a compound thereof, and a bias sputtering method in which voltage is also applied to a substrate during deposition. 
     Next, a conductive film  132  is formed using a metal material over the first oxide semiconductor film  109  and the second oxide semiconductor film  111  by a sputtering method or a vacuum evaporation method.  FIG. 5B  is a cross-sectional view illustrating this state. 
     As the material of the conductive film  132 , there are an element selected from Al, Cr, Ta, Ti, Mo, or W, an alloy including the these elements, an alloy film including any of the above elements in combination, and the like. In the case of performing thermal treatment at 200° C. to 600° C., the conductive film  132  is preferably formed so as to resist such thermal treatment. In the case of using Al, Al is used in combination with a heat-resistant conductive material because Al alone has problems of low heat resistance, tendency to corrode, and the like. As the heat-resistant conductive material used in combination with Al, an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), or scandium (Sc); an alloy including any of these elements; an alloy film including any of these elements in combination; or a nitride including any of these elements is used. 
     Here, the conductive film  132  is a titanium film of a single-layer structure. Alternatively, the conductive film  132  may have a two-layer structure; for example, a titanium film is stacked over an aluminum film. Further alternatively, the conductive film  132  may have a three-layer structure; for example, a Ti film is formed, an aluminum film including Nd (Al—Nd film) is stacked over the Ti film, and a Ti film is further formed thereover. The conductive film  132  may be an aluminum film including silicon of a single-layer structure. 
     Next, a second photolithography process is performed to form a mask  133 , which is a resist mask. In this embodiment, an example is described in which light exposure is performed using a multi-tone (high-tone) mask for forming the mask  133 . The mask  133  can be formed in a manner similar to that of the mask  434  of Embodiment 1. 
     The light exposure is performed using the multi-tone mask through which light is transmitted so as to have a plurality of intensities, and then development is performed, whereby the mask  133  including regions with different thicknesses can be formed as illustrated in  FIG. 5C . Accordingly, with the use of a multi-tone mask, the number of light-exposure masks can be reduced. 
     Next, a first etching step is performed using the mask  133 ; accordingly, the first oxide semiconductor film  109 , which is the first In—Ga—Zn—O-based non-single-crystal film, the second oxide semiconductor film  111 , which is the second In—Ga—Zn—O-based non-single-crystal film, and the conductive film  132  are etched into island shapes. Accordingly, a first oxide semiconductor layer  134 , a second oxide semiconductor layer  135 , and a conductive layer  136  can be formed (see  FIG. 5C ).  FIG. 8  is a top view illustrating this state. 
     In this embodiment, the first etching step is performed by wet etching in which an etchant is used. 
     As the etchant, a mixed solution of phosphoric acid, acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), or the like can be used. 
     The etching condition (etchant, etching time, temperature, or the like) is adjusted as appropriate, depending on a material used for the conductive film  132 , so that the films can be etched into desired shapes. 
     For example, in the case where an aluminum film or an aluminum alloy film is used for the conductive film  132 , wet etching using a mixed solution of phosphoric acid, acetic acid, and nitric acid can be performed. Further, in the case where a titanium film is used for the conductive film  132 , wet etching using an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2) can be performed. 
     For example, in the case where an aluminum film or an aluminum alloy film is used for the conductive film  132 , the first oxide semiconductor film  109 , the second oxide semiconductor film  111 , the conductive film  132 , the first oxide semiconductor layer  134 , the second oxide semiconductor layer  135 , and the conductive layer  136  may be etched using a mixed solution of phosphoric acid, acetic acid, and nitric acid as the etchant of the first etching step. 
     In the first etching step, the conductive film and the oxide semiconductor films may be etched using different etchants. 
     For example, in the case where a titanium film is used for the conductive film  132 , the conductive film  132  is etched using an ammonia hydrogen peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2) as the etchant of the first etching step, and the first oxide semiconductor film  109  and the second oxide semiconductor film  111  may be etched using a mixed solution of phosphoric acid, acetic acid, and nitric acid. 
     Through the first etching step in which the first oxide semiconductor film  109 , the second oxide semiconductor film  111 , and the conductive film  132  are wet-etched, the first oxide semiconductor film  109 , the second oxide semiconductor film  111 , and the conductive film  132  are etched isotropically. In this manner, end portions of the mask  133  are not aligned with end portions of the first oxide semiconductor layer  134 , the second oxide semiconductor layer  135 , and the conductive layer  136 , and these end portions, and these end portions further recede, so that these end portions have shapes with curvature. 
     Furthermore, the etchant after the wet etching is removed together with the etched materials by cleaning. Waste liquid of the etchant containing the removed materials may be purified to recycle the materials contained in the waste liquid. Materials such as indium contained in the oxide semiconductor layer are collected from the waste liquid after the etching and recycled, so that resources can be effectively used and cost can be reduced. 
     Next, the mask  133  is subjected to ashing. As a result, the mask is reduced in size and thickness. Through the ashing, a region of the resist mask, which has small thickness (region overlapping with part of the gate electrode layer  101 ), is removed, so that divided masks  131  can be formed (see  FIG. 6A ). 
     A second etching step is performed using the masks  131 ; accordingly, the first oxide semiconductor layer  134 , the second oxide semiconductor layer  135 , and the conductive layer  136  are etched into a semiconductor layer  103 , n +  layers  104   a  and  104   b  serving as a source region and a drain region, and source and drain electrode layers  105   a  and  105   b . Note that the semiconductor layer  103  is partly etched to become a semiconductor layer having a groove (depression) and also having an end portion which is partly etched and exposed. 
     In this embodiment, the second etching step is performed by dry etching in which an etching gas is used. 
     As the etching gas, a gas including chlorine (chlorine-based gas such as chlorine (Cl 2 ), boron chloride (BCl 3 ), silicon chloride (SiCl 4 ), or carbon tetrachloride (CCl 4 )) is preferably used. By the use of the gas including chlorine in etching, in-plane variation in etching can be reduced as compared with the case of using a gas without chlorine. 
     Alternatively, a gas including fluorine (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 RIE (reactive ion etching) method or an ICP (inductively coupled plasma) 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. 
     In this embodiment, an ICP etching method is employed and the etching condition is as follows: Cl 2  and O 2  are used, the amount of electric power applied to the coil-shaped electrode is 1500 W, the amount of electric power applied to the electrode on the substrate side is 200 W, the pressure is 1.5 Pa, and the substrate temperature is −10° C. 
     When the etching is performed using a chlorine-based gas (Cl 2 ) to which an oxygen gas (O 2 ) is added (preferably, the content of oxygen in the etching gas is set to be 15 vol % or more), in the case of using a silicon oxynitride film as the gate insulating layer  102 , the selectivity ratio of the In—Ga—Zn—O-based non-single-crystal film used for the first oxide semiconductor layer  134  and the second oxide semiconductor layer  135  with respect to the gate insulating layer  102  can be increased. Therefore, the oxide semiconductor film  481  can be etched more than the gate insulating layer  102 . 
     By the second etching step in which the first oxide semiconductor layer  134 , the second oxide semiconductor layer  135 , and the conductive layer  136  are dry-etched, the first oxide semiconductor layer  134 , the second oxide semiconductor layer  135 , and the conductive layer  136  are etched anisotropically. In this manner, the end portions of the masks  131  are aligned with the depression of the semiconductor layer  103  and end portions of the n +  layers  104   a  and  104   b , and the source and drain electrode layers  105   a  and  105   b , and these end portions become continuous. 
     Next, thermal treatment at 200° C. to 600° C., typically 300° C. to 500° C., is preferably performed. Here, thermal treatment is performed at 350° C. for an hour in a nitrogen atmosphere in a furnace. Through this thermal treatment, rearrangement at the atomic level of the In—Ga—Zn—O-based non-single-crystal film occurs. This thermal treatment (including photo-annealing) is important in that the distortion that interrupts carrier transport can be released. Note that there is no particular limitation on when to perform the thermal treatment, as long as it is performed after the second In—Ga—Zn—O-based non-single-crystal film is formed. For example, the thermal treatment may be performed after a pixel electrode is formed. 
     Further, oxygen radical treatment may be performed on the exposed channel formation region of the semiconductor layer  103 . By the oxygen radical treatment, the thin film transistor can serve as a normally-off transistor. Moreover, by the radical treatment, the damage of the semiconductor layer  103  due to the etching can be repaired. The radical treatment is preferably performed in an atmosphere of O 2  or N 2 O, or an atmosphere of N 2 , He, Ar, or the like which includes oxygen. Alternatively, an atmosphere obtained by adding Cl 2  or CF 4  to the above atmosphere may be used. Note that the radical treatment is preferably performed with no bias applied. 
     Through the above process, a thin film transistor  170 , the channel formation region of which is formed using the semiconductor layer  103  can be completed.  FIG. 6A  is a cross-sectional view illustrating this state. Note that  FIG. 9  corresponds to the top view of this state. 
     The second etching step is performed so that a terminal layer  124  formed of the same material as the semiconductor layer  103 , a terminal  123  formed of the same material as the n +  layers  104   a  and  104   b , and a second terminal  122  formed of the same material as the electrode and drain electrode layers  105   a  and  105   b  are left in a 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 ). 
     With the use of the resist mask having a plurality of (typically two kinds of) thicknesses, which is formed using a multi-tone mask, the number of resist masks can be reduced; therefore, the process can be simplified and cost reduction can be achieved. 
     Next, the masks  131  are removed, and a protective insulating layer  107  is formed so as to cover the thin film transistor  170 . The protective insulating layer  107  can be formed using a silicon nitride film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, a tantalum oxide film, or the like which is obtained by a sputtering method or the like. 
     Next, a third photolithography process is performed to form a resist mask. The gate insulating layer  102  and the protective insulating layer  107  are etched to form a contact hole  125  that reaches the drain electrode layer  105   b . Moreover, by this etching, a contact hole  127  that reaches the second terminal  122  and a contact hole  126  that reaches the first terminal  121  are also formed.  FIG. 6B  is a cross-sectional view illustrating this state. 
     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 as ITO), or the like by a sputtering method, a vacuum evaporation method, or the like. Films of these materials are etched using a solution including hydrochloric acid. However, since etching of ITO particularly tends to leave residue, indium oxide-zinc oxide alloy (In 2 O 3 —ZnO) may be used in order to improve etching processability. 
     Next, a fourth photolithography process is performed to form a resist mask. An unnecessary portion is removed by etching, whereby a pixel electrode layer  110  is formed. 
     Moreover, by this fourth photolithography process, the capacitor wiring  108  and the pixel electrode layer  110  together form a storage capacitor by using the gate insulating layer  102  and the protective insulating layer  107  in a capacitor portion as a dielectric. 
     Furthermore, in the fourth photolithography process, the first terminal and the second terminal are covered with the resist mask. Accordingly, transparent conductive films  128  and  129  formed in the terminal portions are left. The transparent conductive films  128  and  129  each serve as an electrode or a wiring used for connection with an FPC. The transparent conductive film  128  formed over the first terminal  121  is used for a terminal electrode used for connection which serves as an input terminal of the gate wiring. The transparent conductive film  129  formed over the second terminal  122  is used for a terminal electrode used for connection which serves as an input terminal of the source wiring. 
     Next, the resist mask is removed.  FIG. 6C  is a cross-sectional view illustrating this state. Note that  FIG. 10  corresponds to a top view of this state. 
       FIGS. 11A and 11B , respectively, are a cross-sectional view and a top view and illustrate the gate wiring terminal portion in this state.  FIG. 11A  corresponds to a cross-sectional view taken along E 1 -E 2  of  FIG. 11B . In  FIG. 11A , a transparent conductive film  155  formed over a protective insulating film  154  is used for a terminal electrode for connection which serves as an input terminal. In the terminal portion of  FIG. 11A , a first terminal  151  formed of the same material as the gate wiring overlaps with a connection electrode layer  153  formed of the same material as the source wiring with a gate insulating layer  152 , a semiconductor layer  157  and an n +  layer  158  interposed therebetween, and the first terminal  151  and the connection electrode layer  153  are brought into conduction via the transparent conductive film  155 . Note that the portion where the transparent conductive film  128  is in contact with the first terminal  121  in  FIG. 6C  corresponds to a portion where the transparent conductive film  155  is in contact with the first terminal  151  in  FIG. 11A . 
       FIGS. 11C and 11D , respectively, are a cross-sectional view and a top view and illustrate a source wiring terminal portion which is different from the source wiring terminal portion of  FIG. 6C . Moreover,  FIG. 11C  corresponds to a cross-sectional view taken along F 1 -F 2  of  FIG. 11D . In  FIG. 11C , the transparent conductive film  155  formed over the protective insulating film  154  is used for a terminal electrode for connection which serves as an input terminal. In the terminal portion of  FIG. 11C , an electrode layer  156  formed of the same material as the gate wiring is disposed under a second terminal  150  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 as a countermeasure against noise or static electricity can be formed by setting the potential of the electrode layer  156  so as to be different from that of the second terminal  150 , for example, floating, GND, 0 V, or the like. The second terminal  150  is electrically connected to the transparent conductive film  155  via the protective insulating film  154 . 
     A plurality of gate wirings, source wirings, and capacitor wirings are provided in accordance with the pixel density. In the terminal portion, a plurality of terminals is arranged: the first terminal having the same potential as the gate wiring, the second terminal having the same potential as the source wiring, a third terminal having the same potential as the capacitor wiring, and the like. The numbers of the respective terminals may be determined as appropriate by a practitioner. 
     Through the four photolithography processes performed in this manner, the storage capacitor and the pixel thin film transistor portion including the thin film transistor  170  which is a bottom-gate n-channel thin film transistor can be completed by using four photomasks. Then, they are arranged in matrix corresponding to pixels, so that a pixel portion is formed; thus, one substrate for use in manufacturing an active matrix display device is obtained. In this specification, such a substrate is called to as an active matrix substrate for convenience. 
     In the case of manufacturing an active matrix liquid crystal display device, a liquid crystal layer is provided between an active matrix substrate and a counter substrate which is provided with a counter electrode and then the active matrix substrate and the counter substrate are fixed to each other. Note that a common electrode which is electrically connected to the counter electrode of 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 used for setting the potential of the common electrode to be fixed, for example GND, 0 V, or the like. 
     The pixel structure is not limited to the pixel structure illustrated in  FIG. 10 .  FIG. 12  illustrates an example of a top view which is different from  FIG. 10 . In the example illustrated in  FIG. 12 , the capacitor wiring is not provided and a storage capacitor is formed in such a manner that the pixel electrode overlaps with the gate wiring of the adjacent pixel with the protective insulating film and the gate insulating layer interposed therebetween; in this case, the capacitor wiring and the third terminal connected to the capacitor wiring can be eliminated. Note that in  FIG. 12 , the same portion as in  FIG. 10  is denoted with the same reference numeral. 
     In an active matrix liquid crystal display device, a display pattern is formed on a screen by driving the pixel electrodes arranged in matrix. Specifically, the liquid crystal layer provided between the pixel electrode and the counter electrode is optically modulated by applying voltage between the selected pixel electrode and the counter electrode corresponding to the selected pixel electrode, and this optical modulation is recognized as a display pattern by an observer. 
     In the display of motion pictures by a liquid crystal display device, the response speed of a liquid crystal molecule is slow. Therefore, there are problems of afterimages or blur of motion pictures. In order to improve the characteristics of a liquid crystal display device regarding motion pictures, there is a driving technique by which black display on the entire screen is performed every one frame, which is so-called black insertion. 
     Moreover, there is a driving technique by which the normal vertical cycle is increased 1.5 times or more times (preferably 2 times or more times) in order to improve the characteristics regarding motion pictures, which is so-called double frame rate driving. 
     Moreover, in order to improve the characteristics of a liquid crystal display device regarding motion pictures, there is a driving technique by which a plane light source is formed using a plurality of LEDs (light-emitting diodes), a plurality of EL light sources, or the like and each light source of the plane light source is used independently to perform intermittent lighting driving within one frame period. As the plane light source, three or more kinds of LEDs may be used or an LED that emits white light may be used. Since the plurality of LEDs can be controlled independently, the time when the LEDs emit light can be synchronized in accordance with the time when the optical modulation of the liquid crystal layer is switched. By this driving technique, the LEDs can be partly turned off; therefore, particularly in the case of displaying a picture including a black display region in most of a screen, reduction in power consumption is achieved. 
     By the use of any of these driving techniques in combination, the display characteristics of a liquid crystal display device, such as the characteristic in displaying motion pictures can be improved as compared to those of conventional liquid crystal display devices. 
     In the n-channel transistor obtained in this embodiment, the channel formation region is formed using the In—Ga—Zn—O-based non-single-crystal film and this transistor has favorable dynamic characteristics. Therefore, these driving techniques can be used in combination. 
     In the case of manufacturing a light-emitting display device, the potential of one electrode of an organic light-emitting element (also called a cathode) is set to a low power supply potential, for example to GND, 0 V, or the like. Therefore, a terminal portion is provided with a fourth terminal for setting the potential of the cathode to a low power supply potential, for example to GND, 0 V, or the like. Moreover, in the case of manufacturing a light-emitting display device, a power supply line is provided in addition to the source wiring and the gate wiring. Therefore, the terminal portion is provided with a fifth terminal electrically connected to the power supply line. 
     As described in this embodiment, the use of the oxide semiconductor for the thin film transistor leads to reduction in manufacturing cost. 
     As in this embodiment, with the use of the resist mask having a plurality of (typically two kinds of) thicknesses, which is formed using a multi-tone mask, the number of resist masks can be reduced; therefore, the process can be simplified and cost reduction can be achieved. Accordingly, a reliable semiconductor device can be manufactured at low cost with high productivity. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments, as appropriate. 
     (Embodiment 4) 
     In this embodiment, an example of manufacturing at least part of a driver circuit and a thin film transistor of a pixel portion over one substrate in a display device which is an example of a semiconductor device will be described below. 
     The thin film transistor in the pixel portion is formed in accordance with any of Embodiments 1 to 3. The thin film transistor described in any of Embodiments 1 to 3 is an n-channel TFT; therefore, part of a driver circuit which can be formed using an n-channel TFT is formed over the same substrate as the thin film transistor of the pixel portion. 
       FIG. 14A  shows 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 shown in  FIG. 14A  includes, over a substrate  5300 , a pixel portion  5301  having a plurality of pixels that is each provided with a display element; a scanning-line driver circuit  5302  that selects each pixel; and a signal-line driver circuit  5303  that controls a video signal input to a selected pixel. 
     The pixel portion  5301  is connected to the signal-line driver circuit  5303  with a plurality of signal lines S 1  to Sm (not shown) extending in a column direction from the signal-line driver circuit  5303  and is connected to the scanning-line driver circuit  5302  with a plurality of scanning lines G 1  to Gn (not shown) extending in a row direction from the scanning-line driver circuit  5302 . The pixel portion  5301  includes a plurality of pixels (not shown) arranged in matrix corresponding to the signal lines S 1  to Sm and the scanning lines G 1  to Gn. In addition, each of the pixels is connected to a signal line Sj (any one of the signal lines S 1  to Sm) and a scanning line Gi (any one of the scanning lines G 1  to Gn). 
     The thin film transistor described in any of Embodiments 1 to 3 is an n-channel TFT, and a signal-line driver circuit including an n-channel TFT is described with reference to  FIG. 15 . 
     The signal-line driver circuit shown 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 one of the wirings  5621 _ 1  to  5621 _M corresponding 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 through the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c . For example, a wiring  5621 _J of the J-th column (any 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 through the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c  of a switch group  5602 _J. 
     Note that 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 on 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  is preferably connected to the switch groups  5602 _ 1  to  5602 _M through an FPC or the like. 
     Next, operation of the signal-line driver circuit shown in  FIG. 15  is described with reference to a timing chart of  FIG. 16 .  FIG. 16  shows the timing chart where the scanning line Gi in the i-th row is selected. Further, a selection period of the scanning line Gi in 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 . Furthermore, the signal-line driver circuit of  FIG. 15  operates similarly to that in  FIG. 16  even when a scanning line of another row is selected. 
     Note that the timing chart of  FIG. 16  shows the case where the wiring  5621 _J in the J-th column is connected to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 through the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c.    
     The timing chart of  FIG. 16  shows timing when the scanning line Gi in the i-th row is selected, timing  5703   a  of on/off of the first thin film transistor  5603   a , timing  5703   b  of on/off of the second thin film transistor  5603   b , timing  5703   c  of on/off of the third thin film transistor  5603   c , and a signal  5721 _J inputted to the wiring  5621 _J in 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. 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, respectively. 
     As shown 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 through 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 through 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 through the third thin film transistor  5603   c.    
     As described above, in the signal-line driver circuit of  FIG. 15 , one gate selection period is divided into three; thus, video signals can be inputted to three signal lines through one wiring  5621  in one gate selection period. Therefore, in the signal-line driver circuit of  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 reduced to approximately one third of the number of signal lines. When the number of connections is reduced to approximately one third of the number of signal lines, the reliability, yield, and the like of the signal-line driver circuit of  FIG. 15  can be improved. 
     Note that there is no particular limitation on the arrangement, number, driving method, and the like of the thin film transistor 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 each of the plurality of sub-selection periods as shown 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, a thin film transistor and a wiring for controlling the thin film transistor may be added. Note that when one gate selection period is divided into four or more sub-selection periods, one sub-selection period becomes shorter. Therefore, one gate selection period is preferably divided into two or three sub-selection periods. 
     As another example, as shown in a timing chart of  FIG. 17 , one selection period may be divided into a pre-charge period Tp, the first sub-selection period T 1 , the second sub-selection period T 2 , and the third sub-selection period T 3 . Further, the timing chart of  FIG. 17  shows timing when the scanning line Gi in the i-th row is selected, timing  5803   a  of on/off of the first thin film transistor  5603   a , timing  5803   b  of on/off of the second thin film transistor  5603   b , timing  5803   c  of on/off of the third thin film transistor  5603   c , and a signal  5821 _J inputted to the wiring  5621 _J in the J-th column. As shown 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 pre-charge period Tp. At this time, a pre-charge voltage Vp inputted to the wiring  5621 _J is inputted to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 through the first thin film transistor  5603   a , the second thin film transistor  5603   b , and the third thin film transistor  5603   c , respectively. 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 through 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 through 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 through the third thin film transistor  5603   c.    
     As described above, in the signal-line driver circuit of  FIG. 15 , to which the timing chart of  FIG. 17  is applied, a signal line can be pre-charged by providing a pre-charge selection period before sub-selection periods. Thus, a video signal can be written to a pixel at high speed. Note that portions in  FIG. 17  similar to those in  FIG. 16  are denoted by the same reference numerals, and detailed description of the same portions and portions having similar functions is omitted. 
     In addition, a configuration of the scanning-line driver circuit is described. The scanning-line driver circuit includes a shift register and a buffer. Moreover, a level shifter may be included in some cases. In the scanning-line driver circuit, when a clock signal (CLK) and a start pulse signal (SP) are inputted to the shift register, a selection signal is produced. The generated selection signal is buffered and amplified by the buffer, and the resulting signal is supplied to a corresponding scanning line. Gate electrodes of transistors in pixels corresponding to one line are connected to the scanning line. Further, since the transistors in the pixels of one line have to be turned on at the same time, a buffer which can feed a large amount of current. 
     One mode of the shift register used for part of the scanning-line driver circuit is described with reference to  FIG. 18  and  FIG. 19 . 
       FIG. 18  shows a circuit configuration of the shift register. The shift register shown in  FIG. 18  includes a plurality of flip-flops, flip-flops  5701 _ 1  to  5701   —   n . Further, the shift register operates by inputting a first clock signal, a second clock signal, a start pulse signal, and a reset signal. 
     Connection relationships of the shift register in  FIG. 18  are described. In a flip-flop  5701   —   i  (any one of the flip-flops  5701 _ 1  to  5701   —   n ) in an i-th stage of the shift register in  FIG. 18 , a first wiring  5501  shown in  FIG. 19  is connected to a seventh wiring  5717   —   i− 1; a second wiring  5502  shown in  FIG. 19  is connected to a seventh wiring  5717   —   i+ 1; a third wiring  5503  shown in  FIG. 19  is connected to a seventh wiring  5717   —   i ; and a sixth wiring  5506  shown in  FIG. 19  is connected to a fifth wiring  5715 . 
     Further, a fourth wiring  5504  shown in  FIG. 19  is connected to a second wiring  5712  in a flip-flop in an odd-numbered stage, and is connected to a third wiring  5713  in a flip-flop of an even-numbered stage. A fifth wiring  5505  shown in  FIG. 19  is connected to a fourth wiring  5714 . 
     Note that the first wiring  5501  shown in  FIG. 19  of the flip-flop  5701 _ 1  of a first stage is connected to a first wiring  5711 , and the second wiring  5502  shown in  FIG. 19  of the flip-flop  5701   —   n  of an n-th stage is connected to a sixth wiring  5716 . 
     The first wiring  5711 , the second wiring  5712 , the third wiring  5713 , and the sixth wiring  5716  may be called a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. Further, the fourth wiring  5714  and the fifth wiring  5715  may be called a first power supply line and a second power supply line, respectively. 
     Next,  FIG. 19  shows details of the flip-flop shown in  FIG. 18 . The flip-flop shown 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 . Note that 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  are n-channel transistors, and are brought into an on state when a voltage between a gate and a source (V gs ) exceeds a threshold voltage (V th ). 
     Next, a connection structure of the flip-flop shown in  FIG. 19  is 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 , and a second electrode (the other of the source electrode and the drain electrode) of the first thin film transistor  5571  is connected to the third wiring  5503 . 
     A first electrode of the second thin film transistor  5572  is connected to the sixth wiring  5506 . A second electrode of the second thin film transistor  5572  is connected to the third wiring  5503 . 
     A first electrode of the third thin film transistor  5573  is connected to the fifth wiring  5505 . A second electrode of the third thin film transistor  5573  is connected to a gate electrode of the second thin film transistor  5572 . A gate electrode of the third thin film transistor  5573  is connected to the fifth wiring  5505 . 
     A first electrode of the fourth thin film transistor  5574  is connected to the sixth wiring  5506 . A second electrode of the fourth thin film transistor  5574  is connected to the gate electrode of the second thin film transistor  5572 . A gate electrode of the fourth thin film transistor  5574  is connected to a gate electrode of the first thin film transistor  5571 . 
     A first electrode of the fifth thin film transistor  5575  is connected to the fifth wiring  5505 . A second electrode of the fifth thin film transistor  5575  is connected to the gate electrode of the first thin film transistor  5571 . A gate electrode of the fifth thin film transistor  5575  is connected to the first wiring  5501 . 
     A first electrode of the sixth thin film transistor  5576  is connected to the sixth wiring  5506 . A second electrode of the sixth thin film transistor  5576  is connected to the gate electrode of the first thin film transistor  5571 . A gate electrode of the sixth thin film transistor  5576  is connected to the gate electrode of the second thin film transistor  5572 . 
     A first electrode of the seventh thin film transistor  5577  is connected to the sixth wiring  5506 . A second electrode of the seventh thin film transistor  5577  is connected to the gate electrode of the first thin film transistor  5571 . A gate electrode of the seventh thin film transistor  5577  is connected to the second wiring  5502 . A first electrode of the eighth thin film transistor  5578  is connected to the sixth wiring  5506 . A second electrode of the eighth thin film transistor  5578  is connected to the gate electrode of the second thin film transistor  5572 . 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 . 
     The first wiring  5501 , the second wiring  5502 , the third wiring  5503 , and the fourth wiring  5504  may be called 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 called a first power supply line and a second power supply line, respectively. 
     Alternatively, the signal-line driver circuit and the scanning-line driver circuit can be manufactured using only the n-channel TFTs described in Embodiment 1. Since the n-channel TFTs described in Embodiment 1 have high mobility, the driving frequency of the driver circuits can be increased. In addition, parasitic capacitance of the n-channel TFTs described in Embodiment 1 is reduced because of source and drain regions are formed using an In—Ga—Zn—O-based non-single-crystal film; therefore, the frequency characteristics (which are called f characteristics) of the n-channel TFTs are high. For example, the scanning-line driver circuit including the n-channel TFTs described in Embodiment 1 can operate at high speed; therefore, it is possible to increase the frame frequency or to achieve insertion of a black screen, for example. 
     In addition, when the channel width of the transistor in the scanning-line driver circuit is increased or a plurality of scanning-line driver circuits is provided, for example, much higher frame frequency can be realized. When a plurality of scanning-line driver circuits are provided, a scanning-line driver circuit for driving even-numbered scanning lines is provided on one side and a scanning-line driver circuit for driving odd-numbered scan lines is provided on the opposite side; thus, increase in frame frequency can be realized. In addition, when a signal is outputted to the same scanning line from the plurality of scanning-line driver circuits, it is advantageous for increase in size of a display device. 
     In the case of manufacturing an active matrix light-emitting display device which is an example of a semiconductor device, a plurality of scanning-line driver circuits is preferably arranged because a plurality of thin film transistors is arranged in at least one pixel. An example of a block diagram of an active matrix light-emitting display device is shown in  FIG. 14B . 
     The light-emitting display device shown in  FIG. 14B  includes, over a substrate  5400 , a pixel portion  5401  having a plurality of pixels each provided with a display element; a first scanning-line driver circuit  5402  and a second scanning-line driver circuit  5404  that select each pixel; and a signal-line driver circuit  5403  that controls a video signal input to a selected pixel. 
     In the case of inputting a digital video signal to the pixel of the light-emitting display device shown in  FIG. 14B , the pixel is put in a light-emitting state or a non-light-emitting state by switching on/off of a transistor. Thus, grayscale can be displayed using an area-ratio grayscale method or a time-ratio grayscale method. An area-ratio grayscale method refers to a driving method by which one pixel is divided into a plurality of sub-pixels and the respective sub-pixels are driven separately based on video signals so that grayscale is displayed. Further, a time-ratio grayscale method refers to a driving method by which a period during which a pixel is in a light-emitting state is controlled so that grayscale is displayed. 
     Since the response time of light-emitting elements is shorter than that of liquid crystal elements or the like, the light-emitting elements are suitable for a time-ratio grayscale method. Specifically, in the case of displaying with a time gray scale 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 put in a light-emitting state or a non-light-emitting state in each sub-frame period. By dividing a frame into a plurality of sub-frames, the total length of time, in which pixels actually emit light in one frame period, can be controlled with video signals to display gray scales. 
     Note that in the light-emitting display device shown in  FIG. 14B , in the case where one pixel includes two switching TFTs, a signal which is inputted to a first scanning line serving as a gate wiring of one of the switching TFTs is generated from the first scanning-line driver circuit  5402  and a signal which is inputted to a second scanning line serving as a gate wiring of the other of the switching TFTs is generated from the second scanning-line driver circuit  5404 . However, the signal which is inputted to the first scanning line and the signal which is inputted to the second scanning line may be generated together from one scanning-line driver circuit. In addition, for example, there is a possibility that a plurality of the scanning lines used for controlling the operation of the switching element be provided in each pixel depending on the number of switching TFTs included in one pixel. In this case, the signals which are inputted to the plurality of scanning lines may be generated all from one scanning-line driver circuit or may be generated from a plurality of scanning-line driver circuits. 
     Even in the light-emitting display device, part of the driver circuit which can be formed using the n-channel TFTs can be provided over the same substrate as the thin film transistors of the pixel portion. Moreover, the signal-line driver circuit and the scanning-line driver circuit can be manufactured using only the n-channel TFTs described in any of Embodiments 1 to 3. 
     The aforementioned driver circuits may be used for not only a liquid crystal display device or a light-emitting display device but also electronic paper in which electronic ink is driven by utilizing an element electrically connected to a switching element. The electronic paper is also called an electrophoretic display device (electrophoretic display) and has advantages in that it has the same level of readability as regular paper, it has less power consumption than other display devices, and it can be set to have a thin and light form. 
     There is a variety of modes of electrophoretic displays. The electrophoretic display is a device in which a plurality of microcapsules each including first particles having positive charge and second particles having negative charge are dispersed in a solvent or a solute, and an electrical field is applied to the microcapsules so that the particles in the microcapsules move in opposite directions from each other, and only a color of the particles gathered on one side is displayed. Note that the first particles or the second particles include a colorant, and does not move when there is no electric field. In addition, a color of the first particles is different from a color of the second particles (the particles may also be colorless). 
     Thus, the electrophoretic display utilizes a so-called dielectrophoretic effect in which a substance with high dielectric constant moves to a region with high electric field. The electrophoretic display does not require a polarizing plate and a counter substrate, which are necessary for a liquid crystal display device, so that the thickness and weight thereof are about half. 
     That which the microcapsules are dispersed in a solvent is called electronic ink, and this electronic ink can be printed on a surface of glass, plastic, fabric, paper, or the like. Color display is also possible with the use of a color filter or particles including a coloring matter. 
     In addition, an active matrix display device can be completed by providing, as appropriate, a plurality of the microcapsules over an active matrix substrate so as to be interposed between two electrodes, and can perform display by application of electric field to the microcapsules. For example, the active matrix substrate obtained using the thin film transistor of any of Embodiments 1 to 3 can be used. 
     Note that the first particles and the second particles in the microcapsule may be formed from one of 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 a composite material thereof. 
     Through the above process, a highly reliable light-emitting display device as a semiconductor device can be manufactured. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments, as appropriate. 
     (Embodiment 5) 
     A thin film transistor can be manufactured, and the thin film transistor can be used for a pixel portion and further for a driver circuit, so that a semiconductor device having a display function (also referred to as a display device) can be manufactured. Moreover, a thin film transistor can be used for part of a driver circuit or an entire driver circuit formed over the same substrate as a pixel portion, so that a system-on-panel can be formed. 
     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. A light-emitting element includes, in its scope, an element whose luminance is controlled by current or voltage, and specifically includes an inorganic electroluminescent (EL) element, an organic EL element, and the like. Further, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used. 
     In addition, the display device includes a panel in which a display element is sealed, and a module in which an IC and the like including a controller are mounted on the panel. Regarding one mode of an element substrate before the display element is completed in a process for manufacturing the display device, the element substrate is provided with a unit which can supply current to the display element in each of a plurality of pixels. Specifically, the element substrate may be in a state provided with only a pixel electrode of the display element, a state after a conductive film to be a pixel electrode is formed and before the conductive film is etched to form the pixel electrode, or any other states. 
     A display device in this specification refers to an image display device, a display device, or a light source (including a lighting device). Further, the display device includes any of the following modules in its category: a module including a connector such as a flexible printed circuit (FPC), a tape automated bonding (TAB) tape, or a tape carrier package (TCP); a module having a TAB tape or a TCP which is provided with a printed wiring board at the end thereof; and a module having an integrated circuit (IC) which is directly mounted on a display element by a chip-on-glass (COG) method. 
     The appearance and a cross section of a liquid crystal display panel which is one mode of a semiconductor device will be described in this embodiment with reference to  FIGS. 22A and 22B , and  22 C.  FIGS. 22A and 22B  are top views of a panel in each of which highly reliable thin film transistors  4010  and  4011  that include semiconductor layers of the In—Ga—Zn—O-based non-single-crystal films described in Embodiment 1 and a liquid crystal element  4013 , which are formed over a first substrate  4001 , are sealed with a sealant  4005  between the first substrate  4001  and a second substrate  4006 .  FIG. 22C  corresponds to a cross-sectional view of  FIGS. 22A and 22B  taken along M-N. 
     The sealant  4005  is provided so as to surround a pixel portion  4002  and a scanning-line driver circuit  4004  which are provided over the first substrate  4001 . The second substrate  4006  is provided over the pixel portion  4002  and the scanning-line driver circuit  4004 . Thus, the pixel portion  4002  and the scanning-line driver circuit  4004  as well as a liquid crystal layer  4008  are sealed with the sealant  4005  between the first substrate  4001  and the second substrate  4006 . A signal-line driver circuit  4003  which is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate which is prepared separately is mounted in a region that is different from the region surrounded by the sealant  4005  over the first substrate  4001 . 
     Note that there is no particular limitation on a connection method of the driver circuit which is separately formed, and a COG method, a wire bonding method, a TAB method, or the like can be used.  FIG. 22A  shows an example in which the signal-line driver circuit  4003  is mounted by a COG method, and  FIG. 22B  shows an example in which the signal-line driver circuit  4003  is mounted by a TAB method. 
     Each of the pixel portion  4002  and the scanning-line driver circuit  4004  which are provided over the first substrate  4001  includes a plurality of thin film transistors.  FIG. 22C  shows the thin film transistor  4010  included in the pixel portion  4002  and the thin film transistor  4011  included in the scanning-line driver circuit  4004 . Insulating layers  4020  and  4021  are provided over the thin film transistors  4010  and  4011 . 
     As each of the thin film transistors  4010  and  4011 , the highly reliable thin film transistor shown in Embodiment 3 including of the In—Ga—Zn—O-based non-single-crystal film as the semiconductor layer can be used. Alternatively, the thin film transistor described in Embodiment 1 or 2 may be applied. In this embodiment, the thin film transistors  4010  and  4011  are each an n-channel thin film transistor. 
     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 each other 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  serving as orientation films, respectively, and the liquid crystal layer  4008  is interposed between the insulating layers  4032  and  4033 . 
     Note that the first substrate  4001  and the second substrate  4006  can be formed from glass, metal (typically, stainless steel), ceramic, or plastic. As plastic, a fiberglass-reinforced plastics (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  4035  which is formed by etching an insulating film selectively is provided to control a distance (a cell gap) between the pixel electrode layer  4030  and the counter electrode layer  4031 . Alternatively, a spherical spacer may be used. In addition, the counter electrode layer  4031  is electrically connected to a common potential line provided over the same substrate as the thin film transistor  4010 . The counter electrode layer  4031  and the common potential line are electrically connected to each other through conductive particles which are arranged between the pair of substrates using a common connection portion. Note that the conductive particles are contained in the sealant  4005 . 
     Alternatively, a blue phase liquid crystal without an orientation film may be used. A blue phase is a type of liquid crystal phase which appears just before a cholesteric liquid crystal changes into an isotropic phase when the temperature of the cholesteric liquid crystal is increased. A blue phase appears only within narrow temperature range; therefore, the liquid crystal layer  4008  is formed using a liquid crystal composition in which a chiral agent of 5 wt. % or more is mixed in order to expand the temperature range. The liquid crystal composition including a blue phase liquid crystal and a chiral agent has a short response time of 10 μs to 100 μs and is optically isotropic; therefore, orientation treatment is not necessary and viewing angle dependence is small. 
     Note that this embodiment describes an example of a transmissive liquid crystal display device; however, the present invention can be applied to a reflective liquid crystal display device or a semi-transmissive liquid crystal display device. 
     Although a liquid crystal display device of this embodiment has a polarizer provided outer than the substrate (the viewer side) and a color layer and an electrode layer of a display element provided inner than the substrate, which are arranged in that order, the polarizer may be inner than the substrate. The stacked structure of the polarizer and the color layer is not limited to that shown in this embodiment and may be set as appropriate in accordance with the materials of the polarizer and the color layer and the condition of the manufacturing process. Further, a light-blocking film serving as a black matrix may be provided. 
     In this embodiment, in order to reduce the unevenness of the surface of the thin film transistors and to improve the reliability of the thin film transistors, the thin film transistors which are obtained in Embodiment 3 are covered with protective films or insulating layers (the insulating layers  4020  and  4021 ) serving as planarizing insulating films. Note that the protective film is provided to prevent entry of a contaminant impurity such as an organic substance, a metal substance, or moisture floating in the atmosphere, and therefore a dense film is preferable. The protective film may be formed using a single layer or a stack of layers of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, or an aluminum nitride oxide film. Although the protective film is formed by a sputtering method in this embodiment, the method is not particularly limited and may be selected from a variety of methods. 
     Here, the insulating layer  4020  is formed to have a stacked structure as the protective film. Here, a silicon oxide film is formed by a sputtering method as a first layer of the insulating layer  4020 . The use of a silicon oxide film for the protective film provides an advantageous effect of preventing hillock of an aluminum film used for a source electrode layer and a drain electrode layer. 
     Moreover, an insulating layer is formed as a second layer of the protective film. Here, a silicon nitride film is formed by a sputtering method as a second layer of the insulating layer  4020 . When a silicon nitride film is used for the protective film, it is possible to prevent movable ions such as sodium from entering a semiconductor region to vary the electrical characteristics of the TFT. 
     Further, after the protective film is formed, the semiconductor layer may be annealed (at 300° C. to 400° C.). 
     Further, the insulating layer  4021  is formed as the planarizing insulating film. The insulating layer  4021  can be formed from an organic material having heat resistance, such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy. As an alternative to such organic materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. Note that the insulating layer  4021  may be formed by stacking a plurality of insulating films formed of these materials. 
     Note that a siloxane-based resin is a resin formed from a siloxane-based material as a starting material and having the bond of Si—O—Si. The siloxane-based resin may include as a substituent an organic group (for example, an alkyl group or an aryl group) or a fluoro group. Alternatively, the organic group may include a fluoro group. 
     The method for the formation of the insulating layer  4021  is not particularly limited and any of the following methods can be used depending on the material of the insulating layer  4021 : a sputtering method, an SOG method, spin coating, dip coating, spray coating, a droplet discharge method (e.g., an inkjet method, screen printing, or offset printing), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. In the case of forming the insulating layer  4021  with the use of a material solution, annealing (300° C. to 400° C.) may be performed on the semiconductor layer at the same time as a baking step. When the baking of the insulating layer  4021  and the annealing of the semiconductor layer are performed at the same time, a semiconductor device can be manufactured efficiently. 
     The pixel electrode layer  4030  and the counter electrode layer  4031  can be formed from 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 including a conductive macromolecule (also referred to as a conductive polymer) can be used for the pixel electrode layer  4030  and the counter electrode layer  4031 . The pixel electrode formed using the conductive composition has preferably a sheet resistance of 10000 ohm/square or less and a transmittance of 70% or more at a wavelength of 550 nm. Further, the resistivity of the conductive high molecule included in the conductive composition is preferably 0.1 Ω·cm or less. 
     As the conductive macromolecule, a so-called π-electron conjugated conductive macromolecule can be used. As examples thereof, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more kinds of them, and the like can be given. 
     Further, a variety of signals and potentials are supplied from an FPC  4018  to the signal-line driver circuit  4003  which is formed separately, the scanning-line driver circuit  4004 , and the pixel portion  4002 . 
     In this embodiment, a connecting terminal electrode  4015  is formed using the same conductive film as the pixel electrode layer  4030  included in the liquid crystal element  4013 . A terminal electrode  4016  is formed using the same conductive film as the source and drain electrode layers included in the thin film transistors  4010  and  4011 . Note that the connecting terminal electrode  4015  and the terminal electrode  4016  are formed over an n +  layer  4025  and a semiconductor layer  4026 . 
     The connecting terminal electrode  4015  is electrically connected to a terminal of the FPC  4018  through an anisotropic conductive film  4019 . 
     Although  FIGS. 22A and 22B , and  22 C show an example in which the signal-line driver circuit  4003  is formed separately and mounted on the first substrate  4001 , this embodiment is not limited to this structure. The scanning-line driver circuit may be separately formed and then mounted, or only part of the signal-line driver circuit or part of the scanning-line driver circuit may be separately formed and then mounted. 
       FIG. 23  shows an example in which a liquid crystal display module is formed as a semiconductor device using a TFT substrate  2600  which is manufactured according to the manufacturing method disclosed in this specification. 
       FIG. 23  shows an example of a liquid crystal display module, in which the TFT substrate  2600  and a counter substrate  2601  are fixed to each other with a sealant  2602 , and a pixel portion  2603  including a TFT and the like, a display element  2604  including a liquid crystal layer, and a color layer  2605  are provided between the substrates to form a display region. The color layer  2605  is necessary to perform color display. In the case of the RGB system, respective colored layers corresponding to colors of red, green, and blue are provided for respective pixels. Polarizing plates  2606  and  2607  and a diffuser plate  2613  are provided outside the TFT substrate  2600  and the counter substrate  2601 . A light source includes a cold cathode tube  2610  and a reflective plate  2611 . A circuit board  2612  is connected to a wiring circuit portion  2608  of the TFT substrate  2600  through a flexible wiring board  2609  and includes an external circuit such as a control circuit and 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 TN (Twisted Nematic) mode, an IPS (In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, an MVA (Multi-domain Vertical Alignment) mode, a PVA (Patterned Vertical Alignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, an OCB (Optically Compensated Birefringence) mode, an FLC (Ferroelectric Liquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) 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 combination with any of the structures described in the other embodiments, as appropriate. 
     (Embodiment 6) 
     In this embodiment, an example of electronic paper is shown as a semiconductor device. 
       FIG. 13  shows active matrix electronic paper as an example of a semiconductor device. A thin film transistor  581  used for a semiconductor device, which can be manufactured in a manner similar to that of the thin film transistor described in Embodiment 3, is a highly reliable thin film transistor including an In—Ga—Zn—O-based non-single-crystal film as a semiconductor layer. Alternatively, the thin film transistor described in Embodiment 1 or 2 can be employed as the thin film transistor  581  described in this embodiment. 
     The electronic paper in  FIG. 13  is an example of a display device in which a twisting ball display system is employed. 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  formed over a substrate  580  has a bottom-gate structure in which the source and drain electrode layer is electrically connected to a first electrode layer  587  through an opening formed in an insulating layer  583 , an insulating layer  584  and an insulating layer  585 . Between the first electrode layer  587  and a second electrode layer  588 , spherical particles  589  are provided. Each spherical particle  589  includes a black region  590   a  and a white region  590   b , and a cavity  594  filled with liquid around the black region  590   a  and the white region  590   b . The circumference of the spherical particle  589  is filled with filler  595  such as a resin (see  FIG. 13 ). In this embodiment, the first electrode layer  587  corresponds to a pixel electrode, and the second electrode layer  588  corresponds to a common electrode. The second electrode layer  588  provided with a substrate  596  is electrically connected to a common potential line provided over the same substrate  580  as the thin film transistor  581 . The second electrode layer  588  and the common potential line are electrically connected through conductive particles arranged between a pair of substrates using the common connection portion. 
     Further, instead of the twisting ball, an electrophoretic element can be used. A microcapsule having a diameter of approximately 10 μm to 200 μm, which is filled with transparent liquid, positively-charged white microparticles, and negatively-charged black microparticles, 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 to each other, so that white or black can be displayed. A display element using this principle is an electrophoretic display element, and is called electronic paper in general. The electrophoretic display element has higher reflectance than a liquid crystal display element, and thus an assistant light is unnecessary. Moreover, power consumption is low and a display portion can be recognized even in a dusky place. Furthermore, an image which is displayed once can be retained even when power is not supplied to the display portion. Accordingly, a displayed image can be stored even though a semiconductor device having a display function (which is also simply referred to as a display device or a semiconductor device provided with a display device) is distanced from an electric wave source. 
     Through the above process, highly reliable electronic paper as a semiconductor device can be manufactured. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments, as appropriate. 
     (Embodiment 7) 
     This embodiment describes an example of a light-emitting display device as a semiconductor device. As an example of a display element of the display device, here, a light-emitting element utilizing electroluminescence is used. 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 thus current flows. Then, those carriers (electrons and holes) are recombined, and thus the light-emitting organic compound is excited. When the light-emitting organic compound returns to a ground state from the excited state, light is emitted. Owing to such a mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element. 
     Inorganic EL elements are classified according to their element structures into a dispersion type inorganic EL element and a thin-film type inorganic EL element. A dispersion type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film type 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 using an organic EL element as a light-emitting element. 
       FIG. 20  shows an example of a pixel structure to which digital time grayscale driving can be applied, as an example of a semiconductor device. 
     A structure and operation of a pixel to which digital time grayscale driving can be applied are described. In this example, one pixel includes two n-channel transistors in each of which a channel formation region includes an oxide semiconductor layer (In—Ga—Zn—O-based non-single-crystal film). 
     A pixel  6400  includes a switching transistor  6401 , a driver transistor  6402 , a light-emitting element  6404 , and a capacitor  6403 . A gate of the switching transistor  6401  is connected to a scanning line  6406 , a first electrode (one of a source electrode and a drain electrode) of the switching transistor  6401  is connected to a signal line  6405 , and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor  6401  is connected to a gate of the driver transistor  6402 . The gate of the driver transistor  6402  is connected to a power supply line  6407  through the capacitor  6403 , a first electrode of the driver transistor  6402  is connected to the power supply line  6407 , and a second electrode of the driver transistor  6402  is connected to a first electrode (pixel electrode) of the light-emitting element  6404 . A second electrode of the light-emitting element  6404  corresponds to a common electrode  6408 . The common electrode  6408  is electrically connected to a common potential line formed over one substrate. 
     The second electrode (common electrode  6408 ) of the light-emitting element  6404  is set to a low power supply potential. The low power supply potential is a potential satisfying the low power supply potential&lt;a high power supply potential when the high power supply potential set to the power supply line  6407  is a reference. As the low power supply potential, for example, GND, 0 V, or the like may be employed. A potential difference between the high power supply potential and the low power supply potential is applied to the light-emitting element  6404  and current is supplied to the light-emitting element  6404 , so that the light-emitting element  6404  emits light. Here, in order to make the light-emitting element  6404  emit light, each potential is set so that the potential difference between the high power supply potential and the low power supply potential is greater than or equal to forward threshold voltage of the light-emitting element  6404 . 
     Note that gate capacitance of the driver transistor  6402  may be used as a substitute for the capacitor  6403 , so that the capacitor  6403  can be omitted. The gate capacitance of the driver transistor  6402  may be formed between the channel region and the gate electrode. 
     In the case of a voltage-input voltage driving method, a video signal is inputted to the gate of the driver transistor  6402  so that the driver transistor  6402  is in either of two states of being sufficiently turned on and turned off. That is, the driver transistor  6402  operates in a linear region. In order for the driver transistor  6402  to operate in a linear region, a voltage higher than the voltage of the power supply line  6407  is applied to the gate of the driver transistor  6402 . Note that a voltage higher than or equal to (voltage of the power supply line+Vth of the driver transistor  6402 ) is applied to the signal line  6405 . 
     In the case of performing analog grayscale driving instead of digital time grayscale driving, the same pixel structure as that in  FIG. 20  can be used by changing signal input. 
     In the case of performing analog grayscale driving, a voltage higher than or equal to (forward voltage of the light-emitting element  6404 +Vth of the driver transistor  6402 ) is applied to the gate of the driver transistor  6402 . The forward voltage of the light-emitting element  6404  indicates a voltage at which a desired luminance is obtained, and includes at least forward threshold voltage. The video signal by which the driver transistor  6402  operates in a saturation region is inputted, so that current can be supplied to the light-emitting element  6404 . In order for the driver transistor  6402  to operate in a saturation region, the potential of the power supply line  6407  is set higher than the gate potential of the driver transistor  6402 . When an analog video signal is used, it is possible to supply current to the light-emitting element  6404  in accordance with the video signal and perform analog grayscale driving. 
     Note that the pixel structure shown in  FIG. 20  is not limited thereto. For example, a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like may be added to the pixel shown in  FIG. 20 . 
     Next, structures of a light-emitting element are described with reference to  FIGS. 21A to 21C . A cross-sectional structure of a pixel is described here by taking an n-channel driver TFT as an example. TFTs  7001 ,  7011 , and  7021  serving as driver TFTs used for a semiconductor device, which are illustrated in  FIGS. 21A ,  21 B, and  21 C, can be manufactured in a manner similar to that of the thin film transistor described in Embodiment 3. The TFTs  7001 ,  7011 , and  7021  are highly reliable thin film transistors each including an In—Ga—Zn—O-based non-single-crystal film as a semiconductor layer. Alternatively, the thin film transistors described in Embodiment 1 or 2 can be employed as the TFTs  7001 ,  7011 , and  7021 . 
     In order to extract light emitted from the light-emitting element, at least one of an anode and a cathode is required to be transparent. A thin film transistor and a light-emitting element are formed over a substrate. A light-emitting element can have a top-emission structure in which light emission is extracted through the surface opposite to the substrate; a bottom-emission structure in which light emission is extracted through the surface on the substrate side; or a dual-emission structure in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side. The pixel structure can be applied to a light-emitting element having any of these emission structures. 
     A light-emitting element having a top-emission structure is described with reference to  FIG. 21A . 
       FIG. 21A  is a cross-sectional view of a pixel in the case where the TFT  7001  serving as a driver TFT is an n-channel TFT and light generated in a light-emitting element  7002  is emitted to pass through an anode  7005 . In  FIG. 21A , a cathode  7003  (pixel electrode) of the light-emitting element  7002  is electrically connected to the TFT  7001  serving as a driver TFT, and a light-emitting layer  7004  and the anode  7005  (common electrode) are stacked in this order over the cathode  7003 . The cathode  7003  can be formed using any of a variety of conductive materials as long as it has a low work function and reflects light. For example, Ca, Al, CaF, MgAg, AlLi, or the like is preferably used. The light-emitting layer  7004  may be formed using a single layer or by stacking a plurality of layers. When the light-emitting layer  7004  is formed by stacking a plurality of layers, the light-emitting layer  7004  is formed by stacking an electron-injecting layer, an electron-transporting layer, a light-emitting layer, a hole-transporting layer, and a hole-injecting layer in this order over the cathode  7003 . It is not necessary to form all of these layers. The anode  7005  is formed using a light-transmitting conductive material, for example, a light-transmitting conductive film such as a film of indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added may be used. 
     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. 21A , light is emitted from the light-emitting element  7002  to the anode  7005  side as indicated by an arrow. 
     Next, a light-emitting element having a bottom-emission structure is described with reference to  FIG. 21B .  FIG. 21B  is a cross-sectional view of a pixel in the case where the driver TFT  7011  is an n-channel TFT, and light generated in a light-emitting element  7012  is emitted to a cathode  7013  side. In  FIG. 21B , the cathode  7013  of the light-emitting element  7012  is formed over a light-transmitting conductive film  7017  which is electrically connected to the driver TFT  7011 , and a light-emitting layer  7014  and an anode  7015  are stacked in this order over the cathode  7013 . When the anode  7015  has a light-transmitting property, a light-blocking film  7016  for reflecting or blocking light may be formed so as to cover the anode  7015 . As in the case of  FIG. 21A , the cathode  7013  can be formed using any of a variety of conductive materials as long as it has 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. 21A , the light-emitting layer  7014  may be formed using a single layer or by stacking a plurality of layers. As in the case of  FIG. 21A , the anode  7015  is not required to transmit light, but can be formed using a light-transmitting conductive material. For the light-blocking film  7016 , for example, metal or the like that reflects light can be used; however, the light-blocking film  7016  is not limited to a metal film. For example, a resin or the like to which black pigment is added can 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. 21B , light is emitted from the light-emitting element  7012  to the cathode  7013  side as indicated by an arrow. 
     Next, a light-emitting element having a dual-emission structure is described with reference to  FIG. 21C . In  FIG. 21C , a cathode  7023  of a light-emitting element  7022  is formed over a light-transmitting conductive film  7027  which is electrically connected to the driver 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. 21A , the cathode  7023  can be formed using any of a variety of conductive materials as long as it has a low work function. Note that the cathode  7023  is formed to have a thickness that can transmit light. For example, an Al film with a thickness of 20 nm can be used as the cathode  7023 . As in the case of  FIG. 21A , the light-emitting layer  7024  may be formed using a single layer or by stacking a plurality of layers. As in the case of  FIG. 21A , the anode  7025  can be formed using a light-transmitting conductive material. 
     The light-emitting element  7022  corresponds to a region where the cathode  7023 , the light-emitting layer  7024 , and the anode  7025  overlap with each other. In the pixel illustrated in  FIG. 21C , 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 here as a light-emitting element, an inorganic EL element can be alternatively provided as a light-emitting element. 
     Note that this embodiment describes the example in which a thin film transistor (a driver TFT) which controls the driving of a light-emitting element is electrically connected to the light-emitting element; however, a structure may be employed in which a current control TFT is connected between the driver TFT and the light-emitting element. 
     The semiconductor device described in this embodiment is not limited to the structures illustrated in  FIGS. 21A to 21C , 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 corresponds to one embodiment of a semiconductor device is described with reference to  FIGS. 24A and 24B .  FIG. 24A  is a top view of a panel in which a thin film transistor and a light-emitting element formed over a first substrate are sealed between the first substrate and a second substrate with a sealant, and  FIG. 24B  is a cross-sectional view taken along H-I of  FIG. 24A . 
     A sealant  4505  is provided so as to surround a pixel portion  4502 , signal-line driver circuits  4503   a  and  4503   b , and scanning-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 scanning-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 scanning-line driver circuits  4504   a  and  4504   b  are sealed together with filler  4507  by the first substrate  4501 , the sealant  4505 , and the second substrate  4506 . In this manner, it is preferable that the pixel portion  4502 , the signal-line driver circuits  4503   a  and  4503   b , and the scanning-line driver circuits  4504   a  and  4504   b  be packaged (sealed) with a protective film (such as an attachment film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the pixel portion  4502 , the signal-line driver circuits  4503   a  and  4503   b , and the scanning-line driver circuits  4504   a  and  4504   b  are not exposed to external air. 
     The pixel portion  4502 , the signal-line driver circuits  4503   a  and  4503   b , and the scanning-line driver circuits  4504   a  and  4504   b  provided over the first substrate  4501  each include a plurality of thin film transistors, and a thin film transistor  4510  included in the pixel portion  4502  and a thin film transistor  4509  included in the signal-line driver circuit  4503   a  are illustrated as an example in  FIG. 24B . 
     As the thin film transistors  4509  and  4510 , highly reliable thin film transistors described in Embodiment 3 including the In—Ga—Zn—O-based non-single-crystal films as the semiconductor layers can be employed. Alternatively, the thin film transistors described in Embodiment 1 or 2 may be employed as the thin film transistors  4509  and  4510 . In this embodiment, the thin film transistors  4509  and  4510  are n-channel thin film transistors. 
     Moreover, reference numeral  4511  denotes a light-emitting element. A first electrode layer  4517  which is a pixel electrode included in the light-emitting element  4511  is electrically connected to source and drain electrode layers of the thin film transistor  4510 . Note that although the light-emitting element  4511  has a stacked structure of the first electrode layer  4517 , an electroluminescent layer  4512 , and a second electrode layer  4513 , the structure of the light-emitting element  4511  is not limited to the structure described in this embodiment. The structure of the light-emitting element  4511  can be changed as appropriate depending on a direction in which light is extracted from the light-emitting element  4511 , or the like. 
     A partition  4520  is formed using an organic resin film, an inorganic insulating film, or organic polysiloxane. It is particularly preferable that the partition  4520  be formed using a photosensitive material to have an opening on the first electrode layer  4517  so that a sidewall of the opening is formed as a tilted surface with continuous curvature. 
     The electroluminescent layer  4512  may be formed using a single layer or by stacking a plurality of layers. 
     In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element  4511 , a protective film may be formed over the second electrode layer  4513  and the partition  4520 . As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed. 
     In addition, a variety of signals and potentials are supplied from FPCs  4518   a  and  4518   b  to the signal-line driver circuits  4503   a  and  4503   b , the scanning-line driver circuits  4504   a  and  4504   b , or the pixel portion  4502 . 
     In this embodiment, a connecting terminal electrode  4515  is formed using the same conductive film as the first electrode layer  4517  included in the light-emitting element  4511 . A terminal electrode  4516  is formed using the same conductive film as the source and drain electrode layers included in the thin film transistors  4509  and  4510 . 
     The connecting terminal electrode  4515  is electrically connected to a terminal included in the FPC  4518   a  through an anisotropic conductive film  4519 . 
     The substrate located in the direction in which light is extracted from the light-emitting element  4511  needs to have a light-transmitting property. In that case, a light-transmitting material such as a glass plate, a plastic plate, a polyester film, or an acrylic film is used. 
     As the filler  4507 , an ultraviolet curable resin or a thermosetting resin as well as inert gas such as nitrogen or argon can be used. For example, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. In this embodiment, nitrogen is used for the filler  4507 . 
     In addition, if needed, an optical film such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retarder plate (a quarter-wave plate, a half-wave plate), or a color filter may be provided on an emission surface of the light-emitting element, as appropriate. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment can be performed by which reflected light is diffused in the depression/projection of the surface and glare can be reduced. 
     As the signal-line driver circuits  4503   a  and  4503   b  and the scanning-line driver circuits  4504   a  and  4504   b , driver circuits formed using a single crystal semiconductor film or polycrystalline semiconductor film over a substrate separately prepared may be mounted. In addition, only the signal-line driver circuit or only part thereof, or only the scanning-line driver circuit or only part thereof may be separately formed and mounted. This embodiment is not limited to the structure illustrated in  FIGS. 24A and 24B . 
     Through the above process, a highly reliable light-emitting display device (display panel) as a semiconductor device can be manufactured. 
     This embodiment can be implemented in combination with any of the structures described in the other embodiments, as appropriate. 
     (Embodiment 8) 
     A semiconductor device disclosed in this specification can be applied as electronic paper. Electronic paper can be used for electronic appliances of every field for displaying information. For example, electronic paper can be used for electronic book (e-book), posters, advertisements in vehicles such as trains, display in a variety of cards such as credit cards, and so on. Examples of such electronic appliances are illustrated in  FIGS. 25A and 25B  and  FIG. 26 . 
       FIG. 25A  illustrates a poster  2631  formed using electronic paper. If the advertizing medium is printed paper, the advertisement is replaced by manpower; however, when electronic paper disclosed in this specification is used, the advertisement display can be changed in a short time. Moreover, a stable image can be obtained without display deterioration. Note that the poster may send and receive information wirelessly. 
       FIG. 25B  illustrates an advertisement  2632  in a vehicle such as a train. If the advertizing medium is printed paper, the advertisement is replaced by manpower; however, when electronic paper disclosed in this specification is used, the advertisement display can be changed in a short time without much manpower. Moreover, a stable image can be obtained without display deterioration. Note that the advertisement may send and receive information wirelessly. 
       FIG. 26  illustrates an example of an electronic book  2700 . For example, the electronic book  2700  includes two chassis of a chassis  2701  and a chassis  2703 . The chassis  2701  and  2703  are bound with each other by an axis portion  2711 , along which the electronic book  2700  can be opened and closed. With such a structure, operation as a paper book is achieved. 
     A display portion  2705  is incorporated in the chassis  2701 , and a display portion  2707  is incorporated in the chassis  2703 . The display portions  2705  and  2707  may display a series of images, or may display different images. With the structure where different images are displayed in different display portions, for example, the right display portion (the display portion  2705  in  FIG. 26 ) can display text, and the left display portion (the display portion  2707  in  FIG. 26 ) can display images. 
       FIG. 26  illustrates an example in which the chassis  2701  is provided with an operation portion and the like. For example, the chassis  2701  is provided with a power supply  2721 , an operation key  2723 , a speaker  2725 , and the like. The page can be turned with the operation key  2723 . Note that a keyboard, a pointing device, and the like may be provided on the same plane as the display portion of the chassis. Further, a rear surface or a side surface of the chassis may be provided with an external connection terminal (an earphone terminal, a USB terminal, a terminal which can be connected with a variety of cables such as an AC adopter or a USB cable, and the like), a storage medium inserting portion, or the like. Moreover, the electronic book  2700  may have a function of an electronic dictionary. 
     Further, the electronic book  2700  may send and receive information wirelessly. Desired book data or the like can be purchased and downloaded from an electronic book server wirelessly. 
     (Embodiment 9) 
     A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). As the electronic appliances, for example, there are a television device (also referred to as TV or a television receiver), a monitor for a computer or the like, a digital camera, a digital video camera, a digital photo frame, a cellular phone (also referred to as a mobile phone or a portable telephone device), a portable game machine, a portable information terminal, an audio playback device, a large game machine such as a pachinko machine, and the like. 
       FIG. 27A  illustrates an example of a television device  9600 . A display portion  9603  is incorporated in a chassis  9601  of the television device  9600 . The display portion  9603  can display images. Here, the chassis  9601  is supported on a stand  9605 . 
     The television device  9600  can be operated by an operation switch of the chassis  9601  or a separate remote controller  9610 . The channel and volume can be controlled with operation keys  9609  of the remote controller  9610 , and the images displayed in the display portion  9603  can be controlled. Moreover, the remote controller  9610  may have a display portion  9607  in which the information outgoing from the remote controller  9610  is displayed. 
     Note that the television device  9600  is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver, between receivers, or the like) information communication can be performed. 
       FIG. 27B  illustrates an example of a digital photo frame  9700 . For example, a display portion  9703  is incorporated in a chassis  9701  of the digital photo frame  9700 . The display portion  9703  can display a variety of images. For example, image data taken by a digital camera or the like is displayed, so that the digital photo frame can function in a manner similar to that of a general picture frame. 
     Note that the digital photo frame  9700  is provided with an operation portion, an external connection terminal (such as a USB terminal, a terminal which can be connected to a variety of cables including a USB cable, or the like), a storage medium inserting portion, and the like. These structures may be incorporated on the same plane as the display portion; however, they are preferably provided on the side surface or rear surface of the display portion because the design is improved. For example, a memory including image data taken by a digital camera is inserted into the storage medium inserting portion of the digital photo frame and the image data is imported. Then, the imported image data can be displayed in the display portion  9703 . 
     The digital photo frame  9700  may send and receive information wirelessly. In this case, desired image data can be wirelessly imported into the digital photo frame  9700  and can be displayed therein. 
       FIG. 28A  illustrates a portable game machine including a chassis  9881  and a chassis  9891  which are jointed with a connector  9893  so as to be able to open and close. A display portion  9882  is incorporated in the chassis  9881  and a display portion  9883  is incorporated in the chassis  9891 . The portable game machine illustrated in  FIG. 28A  additionally includes a speaker portion  9884 , a storage medium inserting portion  9886 , an LED lamp  9890 , an input means (operation keys  9885 , a connection terminal  9887 , a sensor  9888  (including a function of measuring force, displacement, position, speed, acceleration, angular speed, the number of rotations, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, tilt angle, vibration, smell, or infrared ray), and a microphone  9889 ), and the like. Needless to say, the structure of the portable game machine is not limited to the above, and may be any structure as long as at least a semiconductor device disclosed in this specification is provided. Moreover, another accessory may be provided as appropriate. The portable game machine illustrated in  FIG. 28A  has a function of reading out a program or data stored in a storage medium to display it on the display portion and a function of sharing information with another portable game machine by wireless communication. The functions of the portable game machine illustrated in  FIG. 28A  are not limited to these, and the portable game machine can have a variety of functions. 
       FIG. 28B  illustrates an example of a slot machine  9900  which is a large game machine. A display portion  9903  is incorporated in a chassis  9901  of the slot machine  9900 . The slot machine  9900  additionally includes an operation means such as a start lever or a stop switch, a coin slot, a speaker, and the like. Needless to say, the structure of the slot machine  9900  is not limited to the above, and may be any structure as long as at least a semiconductor device disclosed in this specification is provided. Moreover, another accessory may be provided as appropriate. 
       FIG. 29A  illustrates an example of a cellular phone  1000 . The cellular phone  1000  includes a chassis  1001  in which a display portion  1002  is incorporated, and moreover includes an operation button  1003 , an external connection port  1004 , a speaker  1005 , a microphone  1006 , and the like. 
     Information can be inputted to the cellular phone  1000  illustrated in  FIG. 29A  by touching the display portion  1002  with a finger or the like. Moreover, operations such as making a phone call or texting message can be performed by touching the display portion  1002  with a finger or the like. 
     There are mainly three screen modes of the display portion  1002 . The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting information such as text. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are mixed. 
     For example, in the case of making a phone call or texting message, the display portion  1002  is set to a text input mode where text input is mainly performed, and text input operation can be performed on a screen. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion  1002 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone  1000 , display in the screen of the display portion  1002  can be automatically switched by judging the direction of the cellular phone  1000  (whether the cellular phone  1000  is placed horizontally or vertically for a landscape mode or a portrait mode). 
     Further, the screen modes are switched by touching the display portion  1002  or operating the operation button  1003  of the chassis  1001 . Alternatively, the screen modes can be switched depending on kinds of image displayed in the display portion  1002 . For example, when a signal for an image displayed in the display portion is data of moving images, the screen mode is switched to the display mode. When the signal is text data, the screen mode is switched to the input mode. 
     Moreover, in the input mode, when input by touching the display portion  1002  is not performed within a specified period while a signal detected by an optical sensor in the display portion  1002  is detected, the screen mode may be controlled so as to be switched from the input mode to the display mode. 
     The display portion  1002  can also function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touching the display portion  1002  with the palm or the finger, whereby personal authentication can be performed. Moreover, when a backlight which emits near-infrared light or a sensing light source which emits near-infrared light is provided in the display portion, a finger vein, a palm vein, or the like can be taken. 
       FIG. 29B  illustrates an example of a cellular phone as well. The cellular phone in  FIG. 29B  includes a display device  9410  having, in a chassis  9411 , a display portion  9412 , and operation buttons  9413 , and a communication device  9400  having, in a chassis  9401 , operation buttons  9402 , an external input terminal  9403 , a microphone  9404 , a speaker  9405 , and a light-emitting portion  9406  which emits light when a phone call is received. The display device  9410  having a display function can be detached from or attached to the communication device  9400  having a phone function in the two directions indicated by arrows. Accordingly, the display device  9410  and the communication device  9400  can be attached to each other along respective short axes or long axes. Alternatively, in the case where only the display function is needed, the display device  9410  is detached from the communication device  9400 , and then the display device  9410  can be used alone. Images or input information can be transmitted and received by wireless or wire communication between the communication device  9400  and the display device  9410 , each of which includes a chargeable battery. 
     This application is based on Japanese Patent Application serial No. 2008-274634 filed with Japan Patent Office on Oct. 24, 2008, the entire contents of which are hereby incorporated by reference.