Patent Publication Number: US-9842939-B2

Title: Semiconductor device

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof. 
     In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices. 
     BACKGROUND ART 
     In recent years, transistors which are used for many liquid crystal display devices and light-emitting display devices typified by flat panel displays have included a silicon semiconductor such as amorphous silicon or polycrystalline silicon and have been formed over glass substrates. 
     Instead of the silicon semiconductor, a technique in which an oxide semiconductor is used for transistors has attracted attention. 
     For example, techniques by which a transistor is manufactured using zinc oxide which is a single-component metal oxide or an In—Ga—Zn—O-based oxide which is a homologous compound as an oxide semiconductor, and is used as a switching element or the like of a pixel of a display device, is disclosed (see Patent Document 1 to Patent Document 3). 
     REFERENCE 
     Patent Documents 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2006-165528 
         [Patent Document 2] Japanese Published Patent Application No. 2007-96055 
         [Patent Document 3] Japanese Published Patent Application No. 2007-123861 
       
    
     DISCLOSURE OF INVENTION 
     There is a problem in that drain current flows even in the state (Vg=0V) where voltage is not applied to a gate electrode in a transistor whose channel region includes an oxide semiconductor because a threshold voltage (Vth) shifts in the negative direction. 
     In view of the above problems, an object of an embodiment of the present invention disclosed in this specification is to provide a semiconductor device with favorable electric characteristics. 
     In order to achieve the object, an insulating layer with a low hydrogen content is used as an insulating layer being in contact with an oxide semiconductor layer which forms a channel region, whereby diffusion of hydrogen into the oxide semiconductor layer can be prevented. Specifically, an insulating layer in which the concentration of hydrogen is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3  is used as the insulating layer being in contact with the oxide semiconductor layer. 
     An embodiment of the present invention is a semiconductor device which comprises a gate electrode layer, an oxide semiconductor layer which forms a channel region, a source electrode layer and a drain electrode layer being in contact with the oxide semiconductor layer, a gate insulating layer provided between the gate electrode layer and the oxide semiconductor layer, and an insulating layer which faces the gate insulating layer with the oxide semiconductor layer interposed therebetween and is in contact with the oxide semiconductor layer, in which the concentration of hydrogen is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     A transistor having a top-gate structure, in which a gate electrode layer overlaps with an oxide semiconductor layer with a gate insulating layer interposed therebetween has a top-contact type and a bottom-contact type. The top-contact transistor includes an oxide semiconductor layer between source and drain electrode layers and the insulating layer, and the bottom-contact transistor includes source and drain electrode layers between the oxide semiconductor layer and the insulating layer. 
     Another embodiment of the present invention is a semiconductor device including a top-gate/top-contact transistor and a top-gate/bottom contact transistor in which the concentration of hydrogen in an insulating layer being in contact with an oxide semiconductor layer is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     Further, in another embodiment of the present invention, an oxide insulating layer comprising silicon oxide, silicon oxynitride, silicon nitride oxide, hafnium oxide, aluminum oxide, or tantalum oxide can be used as the insulating layer. 
     Further, a gate insulating layer with a low hydrogen content is used as the gate insulating layer provided for the top-gate/top-contact transistor and the top-gate/bottom contact transistor, whereby a semiconductor device having favorable electric characteristics can be obtained. 
     Another embodiment of the present invention is a semiconductor device in which the concentration of hydrogen in the gate insulating layer being in contact with the oxide semiconductor layer is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     According to one embodiment of the present invention, a semiconductor device having favorable electric characteristics can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate a top view and a cross-sectional view of a transistor, respectively. 
         FIGS. 2A to 2D  are cross-sectional views illustrating a method for manufacturing a transistor. 
         FIGS. 3A and 3B  illustrate a top view and a cross-sectional view of a transistor, respectively. 
         FIGS. 4A to 4D  are cross-sectional views illustrating a method for manufacturing a transistor. 
         FIG. 5  is an external view illustrating an example of an electronic book reader. 
         FIGS. 6A and 6B  are external views illustrating respective examples of a television device and a digital photo frame. 
         FIG. 7  is a perspective view illustrating an example of a portable computer. 
         FIG. 8  is a graph showing the concentration of hydrogen contained in an insulating layer. 
         FIG. 9  is a graph showing a measurement result of electric characteristics of a transistor. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the scope and spirit of the present invention. Accordingly, the invention should not be construed as being limited to the description of the embodiments below. In describing structures of the present invention with reference to the drawings, the same reference numerals are used in common for the same portions in different drawings. The same hatching pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases. In addition, an insulating layer is not illustrated in a top view in some cases. Note that the size, the layer thickness, or the region of each structure illustrated in each drawing is exaggerated for clarity in some cases. Therefore, the present invention is not necessarily limited to such scales illustrated in the drawings. 
     Note that when it is described that “A and B are connected to each other”, the case where A and B are electrically connected to each other, and the case where A and B are directly connected to each other are included therein. Here, each of A and B corresponds to an object (e.g., a device, an element, a circuit, a wiring, an electrode, a terminal, a conductive film, or a layer). 
     Note that, functions of “source” and “drain” may become switched in the case that a direction of a current flow is changed during circuit operation, for example. Therefore, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification. 
     Embodiment 1 
     In this embodiment, a semiconductor device which is one embodiment of the present invention is described with reference to  FIGS. 1A and 1B . 
       FIG. 1A  is a top view of a transistor  100  included in a semiconductor device.  FIG. 1B  is a cross sectional view along line A 1 -B 1  of  FIG. 1A . The transistor  100  includes, over a substrate  102 , an insulating layer  104 , a source electrode layer  106   a  and a drain electrode layer  106   b , an oxide semiconductor layer  108  including a channel region, a gate insulating layer  110 , and a gate electrode layer  112 . 
     The transistor  100  is a transistor having a top-gate structure, in which a gate electrode layer  112  is formed so as to overlap with the oxide semiconductor layer  108  with the gate insulating layer  110  interposed therebetween. Further, the transistor  100  is a bottom-contact transistor in which the source electrode layer  106   a  and the drain electrode layer  106   b  are provided between the oxide semiconductor layer  108  and the insulating layer  104 . 
     The transistor  100  is a top-gate/bottom-contact transistor, so that part of the upper surface of the insulating layer  104  and part of the lower surface of the oxide semiconductor layer  108  are in contact with each other. Therefore, in manufacturing steps of the transistor  100 , hydrogen is diffused into the oxide semiconductor layer  108  when a large amount of hydrogen exists in the insulating layer  104 . Hydrogen is diffused into the oxide semiconductor layer  108 , so that excessive carriers are generated in the oxide semiconductor layer  108 . Thus, the threshold voltage (Vth) of the transistor  100  shifts in the negative direction, and drain current flows even in the state (Vg=0V) where voltage is not applied to the gate electrode (normally-on). Therefore, when a large amount of hydrogen exists in the insulating layer  104 , electric characteristics of the transistor  100  are degraded. 
     There is a method in which the oxide semiconductor layer  108  is subjected to heat treatment in order to remove the diffused hydrogen from the oxide semiconductor layer  108 . However, as the manufacturing steps of the transistor are increased, manufacturing cost is increased and yield may be reduced. 
     Thus, an insulating layer in which the concentration of hydrogen is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3  is used as the insulating layer  104  being in contact with the oxide semiconductor layer  108 , whereby diffusion of hydrogen in the oxide semiconductor layer  108  can be prevented and a transistor having favorable electric characteristics can be provided. Accordingly, a transistor having favorable electric characteristics can be provided without increasing the number of manufacturing steps of the transistor. 
     Further, the concentration of hydrogen in the gate insulating layer  110  can be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . In other words, the concentration of hydrogen in each of the insulating layer  104  and the gate insulating layer  110  is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 , whereby diffusion of hydrogen in the oxide semiconductor layer  108  can be suppressed. 
     There is no particular limitation on the substrate  102  as long as it has a resistance for the manufacturing steps performed later. For example, an insulating substrate such as a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate; a semiconductor substrate which is formed using a semiconductor material such as silicon; a conductive substrate which is formed using a conductor such as metal or stainless steel; or a substrate in which the surface of a semiconductor substrate or the surface of a conductive substrate is covered with an insulating material, can be used. Further alternatively, a plastic substrate can be used as the substrate  102  as appropriate. 
     Further, a glass substrate whose strain point is greater than or equal to 730° C. is preferably used in the case where heat treatment at a high temperature is performed in the manufacturing steps of the transistor. A glass material such as aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass is used, for example. In general, by containing a larger amount of barium oxide (BaO) than boric oxide, more practical heat-resistant glass can be obtained. Therefore, a glass substrate containing a larger amount of BaO than B 2 O 3  is preferably used. 
     The insulating layer  104  serves as a base to prevent diffusion of an impurity element from the substrate  102  and also serves as a base to prevent the substrate from being etched by etching in the manufacturing steps of the transistor. There is no limitation on the thickness of the insulating layer  104 ; however, the thickness of the insulating layer  104  is preferably greater than or equal to 50 nm. 
     The insulating layer  104  is formed with a single-layer structure using any of oxide insulating layers of silicon oxide, silicon oxynitride, silicon nitride oxide, hafnium oxide, aluminum oxide, tantalum oxide, and the like; or a stacked structure including two or more layers selected from these layers. In the case where the stacked structure is adopted, an insulating layer being in contact with the substrate  102  is formed using a silicon nitride and the insulating layer  104  being in contact with the oxide semiconductor layer  108  is formed using the above-mentioned oxide insulating layer. An oxide insulating layer in which the concentration of hydrogen is reduced is used as the insulating layer  104  being in contact with the oxide semiconductor layer  108 , whereby, diffusion of hydrogen in the oxide semiconductor layer  108  is prevented and a transistor having favorable electric characteristics can be provided because oxygen is supplied to defects in the oxide semiconductor layer  108  from the insulating layer  104 . At this time, as described above, it is necessary that the concentration of hydrogen in the insulating layer  104  be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     Here, a silicon oxynitride means the one that contains more oxygen than nitrogen and for example, silicon oxynitride includes oxygen, nitrogen, and silicon at concentrations ranging from greater than or equal to 50 atomic % and less than or equal to 70 atomic %, greater than or equal to 0.5 atomic % and less than or equal to 15 atomic %, and greater than or equal to 25 atomic % and less than or equal to 35 atomic %, respectively. Further, silicon nitride oxide means the one that contains more nitrogen than oxygen and for example, silicon nitride oxide includes oxygen, nitrogen, and silicon at concentrations ranging from greater than or equal to 5 atomic % and less than or equal to 30 atomic %, greater than or equal to 20 atomic % and less than or equal to 55 atomic %, and greater than or equal to 25 atomic % and less than or equal to 35 atomic %, respectively. Note that rates of oxygen, nitrogen, and silicon fall within the aforementioned ranges in the cases where measurement is performed using Rutherford backscattering spectrometry (RBS) or hydrogen forward scattering (HFS). In addition, the total of the percentages of the constituent elements does not exceed 100 atomic %. 
     In this embodiment, description is made on the cases where a silicon oxide layer formed by sputtering is used as the insulating layer  104  and a silicon oxide layer formed by a plasma enhanced chemical vapor deposition (plasma CVD) is used as the insulating layer  104 . 
     In the case where the insulating layer  104  is formed by sputtering, a target containing a silicon element is preferably used. That is to say, a Si target or SiO 2  target can be used. Preferably, a SiO 2  target is used in order to reduce the concentration of hydrogen in the obtained oxide silicon layer, more preferably a SiO 2  target in which the concentration of a hydroxyl group contained in the SiO 2  target is less than or equal to 1000 ppm or the concentration of hydrogen measured using secondary ion mass spectrometry (SIMS) is less than or equal to 3.5×10 19  atoms/cm 3  is used. As gases to be supplied for forming the insulating layer  104 , a rare gas such as argon and oxygen are used. Further, it is preferable to use high-purity gas in which impurities such as hydrogen, water, a hydroxyl group, or hydride are reduced to a concentration of a “ppm” level or a “ppb” level as gases to be supplied. 
     Examples of sputtering include RF sputtering in which a high-frequency power source is used for a sputtering power supply, DC sputtering, and pulsed DC sputtering in which a bias is applied in a pulsed manner. 
     A multi-source sputtering apparatus in which a plurality of targets of different materials can be placed may be used for forming the insulating layer  104 . With the multi-source sputtering apparatus, films of different materials can be formed to be stacked in the same chamber, or a film of plural kinds of materials can be formed by electric discharge at the same time in the same chamber. 
     In addition, there are a sputtering apparatus provided with a magnet system inside the chamber, which is for magnetron sputtering, and a sputtering apparatus which is used for ECR sputtering in which plasma produced with the use of microwaves is used without using glow discharge. 
     Further, as sputtering, reactive sputtering in which a target substance and a sputtering gas component are chemically reacted with each other to form a thin compound film thereof, or bias sputtering in which voltage is also applied to a substrate can be used. 
     In this specification, sputtering can be performed while the substrate is heated using the above-described sputtering apparatus and sputtering as appropriate. 
     Thus, the concentration of hydrogen in the obtained oxide silicon layer can be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     In addition to sputtering, plasma CVD can be used for the formation of the insulating layer  104 . Plasma CVD is a method for forming a film by supplying a deposition gas to be raw materials to a reaction chamber of a plasma CVD apparatus to employ plasma energy. 
     As the plasma CVD apparatus, a capacitively coupled high-frequency plasma CVD apparatus using a high-frequency power source, an inductively coupled high-frequency plasma CVD apparatus, a microwave plasma CVD apparatus (an electron cyclotron resonant plasma CVD apparatus) which has magnetron that is a microwave generation source and generates plasma using the microwave, and a helicon wave plasma CVD apparatus are given. In plasma CVD of this specification, a CVD apparatus in which glow discharge plasma is utilized for the formation of the film can be used as appropriate. Further, plasma CVD can be also performed while the substrate is heated. 
     When the insulating layer  104  in which the concentration of hydrogen is reduced is formed by plasma CVD, a gas in which hydrogen is not contained in its molecular structure is needed to be selected as the deposition gas. 
     In other words, as the deposition gas, not SiH 4  but SiF 4  is used. Further, an oxidizing gas of N 2 O or O 2  with a low content of hydrogen and water is also used so that a film to be deposited is an oxide insulating film. Further, a gas with a low content of hydrogen and water is used also as the other gases to be added (a rare gas such as argon) in consideration of the spread of plasma generated in the plasma CVD apparatus. 
     When the oxide silicon layer to be the insulating layer  104  is formed by plasma CVD, impurities such as hydrogen and water which remain in the reaction chamber of the plasma CVD apparatus or adsorb onto the inner wall of the reaction chamber are removed, and then the oxide silicon layer is formed using the above-mentioned gases. In this manner, the concentration of hydrogen in the insulating layer  104  formed by plasma CVD can be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     The source electrode layer  106   a  and the drain electrode layer  106   b  are formed over the insulating layer  104 . The source electrode layer  106   a  and the drain electrode layer  106   b  can be formed with a single layer or a stacked layer using a conductive film of a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, neodymium, or scandium, or an alloy material which contains any of these metal materials as a main component, or a nitride of any of these metals. Note that aluminum or copper can also be used as such a metal material if it can withstand the temperature of heat treatment to be performed in a later process. Aluminum or copper is preferably used in combination with a refractory metal material in order to avoid problems of heat resistance and corrosion. As the refractory metal material, molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium, or the like can be used. 
     For example, the following structure is preferable as a two-layer structure of the source electrode layer  106   a  and the drain electrode layer  106   b : a two-layer structure in which a molybdenum film is stacked over an aluminum film; a two-layer structure in which a molybdenum film is stacked over a copper film; a two-layer structure in which a titanium nitride film or a tantalum nitride film is stacked over a copper film; a two-layer structure in which a titanium nitride film and a molybdenum film are stacked; or a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film. As a three-layer structure of the source electrode layer  106   a  and the drain electrode layer  106   b , the following structure is preferable: a stacked structure including an aluminum film, an alloy film of aluminum and silicon, an alloy film of aluminum and titanium, or an alloy film of aluminum and neodymium in a middle layer and any of a tungsten film, a tungsten nitride film, a titanium nitride film, and a titanium film in a top layer and a bottom layer. 
     Further, a light-transmitting oxide conductive film of indium oxide, an alloy of indium oxide and tin oxide, an alloy of indium oxide and zinc oxide, zinc oxide, aluminum zinc oxide, aluminum zinc oxynitride, gallium zinc oxide, or the like may be used for the source electrode layer  106   a  and the drain electrode layer  106   b.    
     The thickness of the source electrode layer  106   a  and the drain electrode layer  106   b  is not particularly limited and can be determined as appropriate in consideration of electric resistance and time required for a manufacturing process of the conductive film serving as the source electrode layer  106   a  and the drain electrode layer  106   b . For example, the source electrode layer  106   a  and the drain electrode layer  106   b  can be formed to have thickness of 10 nm to 500 nm. 
     The oxide semiconductor layer  108  which forms a channel region is formed so as to be in contact with part of the upper surfaces of the source electrode layer  106   a  and the drain electrode layer  106   b  and part of the upper surface of the insulating layer  104 . Because the concentration of hydrogen in the insulating layer  104  is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3  as described above, diffusion of hydrogen in the oxide semiconductor layer  108  can be prevented when the oxide semiconductor layer  108  is formed. The thickness of the oxide semiconductor layer  108  is set to 10 nm to 300 nm, preferably 20 nm to 100 nm. 
     The oxide semiconductor layer  108  is formed using an In—Ga—Zn—O-based non-single-crystal film which contains In, Ga, and Zn and has a structure represented as InMO 3 (ZnO) m  (m&gt;0). Note that M denotes one or more of metal elements selected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), and cobalt (Co). For example, M denotes Ga in some cases; meanwhile, M denotes the above metal element such as Ni or Fe in addition to Ga in other cases. Further, the above oxide semiconductor may contain Fe or Ni, another transitional metal element, or an oxide of the transitional metal as an impurity element in addition to the metal element contained as M. In addition, a metal oxide contained in the metal oxide target have a relative density of higher than or equal to 80%, preferably higher than or equal to 95%, more preferably higher than or equal to 99.9% is used. 
     Specifically, the oxide semiconductor layer  108  can be formed using any of the following oxide semiconductors: an oxide of four metal elements such as an In—Sn—Ga—Zn—O-based oxide semiconductor; oxides of three metal elements such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, and a Sn—Al—Zn—O-based oxide semiconductor; oxides of two metal elements such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, and an In—Mg—O-based oxide semiconductor; and oxides of one metal element such as an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductor. Here, for example, an In—Ga—Zn—O-based oxide semiconductor is an oxide semiconductor containing at least In, Ga, and Zn, and there is no particular limitation on the composition ratio thereof. The In—Ga—Zn—O-based oxide semiconductor may contain an element other than In, Ga, and Zn. Moreover, silicon oxide may be included in the above oxide semiconductor layer. 
     The gate insulating layer  110  is formed so as to cover the source electrode layer  106   a , the drain electrode layer  106   b , and the oxide semiconductor layer  108 . The gate insulating layer  110  is formed using an oxide insulating layer, similarly to the insulating layer  104 . The gate insulating layer  110  is formed with a low hydrogen content, whereby a semiconductor device having favorable electric characteristics can be obtained. Thus, it is preferable that the concentration of hydrogen in the gate insulating layer being in contact with the oxide semiconductor layer be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     The gate electrode  112  is formed so as to overlap with the oxide semiconductor layer  108  with the gate insulating layer  110  interposed therebetween. The gate electrode  112  can have a structure similar to that of the source electrode layer  106   a  and the drain electrode layer  106   b.    
     Although not illustrated in  FIGS. 1A and 1B , an insulating layer serving as a passivation layer or an interlayer insulating layer is preferably formed over the transistor 
     As described above, the concentration of hydrogen in one or both of the insulating layer  104  and the gate insulating layer  110  is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 ; thus, diffusion of hydrogen in the oxide semiconductor layer  108  can be prevented and a semiconductor device having favorable electric characteristics can be obtained. 
     Note that the structure described in this embodiment can be combined as appropriate with any structure described in the other embodiments in this specification. 
     Embodiment 2 
     Note that a method for manufacturing the semiconductor device described in Embodiment 1 is described in this embodiment with reference to  FIGS. 2A to 2D . 
     As illustrated in  FIG. 2A , the insulating layer  104  is formed over the substrate  102 . The material described in Embodiment 1 can be used for the substrate  102  and the insulating layer  104 . In this embodiment, a glass substrate is used for the substrate  102 . As the insulating layer  104 , a silicon oxide layer is formed to have a thickness of 200 nm by RF sputtering using SiO 2  as a target and a rare gas such as argon and oxygen as gases to be supplied when the insulating layer  104  is formed. 
     As described in Embodiment 1, when the insulating layer  104  is formed by plasma CVD, the inner wall of the reaction chamber in the plasma CVD apparatus is heated to release impurities from the inner wall of the reaction chamber and remove impurities remaining in the reaction chamber or adsorbing onto the inner wall of the reaction chamber. Then, SiF 4  as the deposition gas, N 2 O the oxidizing gas, and argon as the gas to be added are supplied to the reaction chamber, whereby the insulating layer  104  is formed using plasma energy. In this embodiment, a plasma CVD apparatus using a high-frequency power source is used. 
     As a method for removing the impurities remaining in the reaction chamber or adsorbing onto the inner wall of the reaction chamber, an exhaust process, plasma cleaning using a fluorine compound such as nitrogen trifluoride, or the like is preferably performed. 
     Next, the conductive film serving as the source electrode layer  106   a  and the drain electrode layer  106   b  is formed. As the conductive film, in this embodiment, a titanium film with a thickness of 150 nm is formed by DC sputtering using a titanium target. Then, the source electrode layer  106   a  and the drain electrode layer  106   b  each having a thickness of 150 nm are formed by performing a first photolithography step and an etching step. 
     Either wet etching or dry etching may be used for the etching of the conductive film. Note that dry etching is preferably used in terms of microfabrication of the element. An etching gas and an etchant can be selected as appropriate depending on a material of layers to be etched. 
     Note that the side surfaces of the source electrode layer  106   a  and the drain electrode layer  106   b  are formed to have a tapered shape. This is in order to prevent disconnection at a step portion because the oxide semiconductor film and the conductive film to be the gate electrode are formed over the source electrode layer  106   a  and the drain electrode layer  106   b  in a later step. In order to form the side surfaces of the source electrode layer  106   a  and the drain electrode layer  106   b  to be tapered, etching may be performed while the resist mask is recessed. 
     Next, the oxide semiconductor film with a thickness of 50 nm is formed by DC sputtering. Oxygen is supplied to defects in the oxide semiconductor layer from the insulating layer  104  because the oxide semiconductor film is formed to be in contact with the insulating layer  104 . Then, an oxide semiconductor layer  107  that is processed into an island shape is formed by performing a photolithography step or an etching step. In this embodiment, DC sputtering is used; however, vacuum evaporation, pulse laser deposition, CVD, and the like may be used. 
     As the oxide semiconductor film, the oxide semiconductor described in Embodiment 1 can be used. In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-based non-single-crystal film with a thickness of 50 nm is formed by sputtering using an oxide semiconductor target including indium (In), gallium (Ga), and zinc (Zn) (In 2 O 3 :Ga 2 O 3 :ZnO=1:1:1 and In 2 O 3 :Ga 2 O 3 :ZnO=1:1:2 in a molar ratio). Further, in this embodiment, DC sputtering is employed, a flow rate of argon is 30 sccm, a flow rate of oxygen is 15 sccm, and a substrate temperature is a room temperature (15° C. to 35° C.). 
     In the case where an In—Zn—O-based oxide semiconductor film is used as the oxide semiconductor film, a target used has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In 2 O 3 :ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In 2 O 3 :ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=15:1 to 3:2 (In 2 O 3 :ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z&gt;1.5X+Y is satisfied. 
     Before the oxide semiconductor film is formed by sputtering, reverse sputtering in which plasma is generated by introduction of an argon gas is preferably performed. The reverse sputtering refers to a method in which an RF power source is used for application of voltage to a substrate in an argon atmosphere and plasma is generated around the substrate to modify a surface. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, or the like may be used. Alternatively, an argon atmosphere to which oxygen, nitrous oxide, or the like is added may be used. Alternatively, an argon atmosphere to which chlorine, carbon tetrafluoride, or the like is added may be used. 
     In forming the oxide semiconductor film, the substrate is held in a treatment chamber that is maintained at reduced pressure and is heated so that the substrate temperature is higher than or equal to 100° C. and lower than 550° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Alternatively, the substrate temperature in forming the oxide semiconductor film may be a room temperature (15° C. to 35° C.). Then, moisture in the treatment chamber is removed, a sputtering gas from which hydrogen, water, or the like has been removed is introduced, and the oxide semiconductor target is used; thus, the oxide semiconductor film is formed. The oxide semiconductor film is formed while the substrate is heated, so that impurities contained in the oxide semiconductor film can be reduced. Moreover, damage due to sputtering can be reduced. In order to remove moisture in the treatment chamber, an entrapment vacuum pump is preferably used. For example, a cryopump, an ion pump, a titanium sublimation pump, or the like can be used. A turbo pump provided with a cold trap may be used. Since it is possible to remove hydrogen, water, or the like from the treatment chamber by evacuating the treatment chamber with a cryopump or the like, the concentration of an impurity in the oxide semiconductor film can be reduced. 
     The structure provided through the steps up to here is illustrated in  FIG. 2B . 
     Then, the oxide semiconductor layer  107  may be subjected to heat treatment in the atmosphere, an inert gas atmosphere (nitrogen, helium, neon, argon, or the like), or in the atmosphere where the dew point under atmospheric pressure is less than or equal to −60° C. and the moisture content is small. Specifically, the oxide semiconductor layer  107  is subjected to heat treatment in the atmosphere at greater than or equal to 100° C. and less than or equal to 400° C. for 10 minutes or more, preferably at 350° C. for 60 minutes. In this embodiment, the oxide semiconductor layer  107  is subjected to heat treatment, whereby the oxide semiconductor layer  108  in which moisture and hydrogen are eliminated is formed. At that time, oxygen is supplied to defects in the oxide semiconductor layer  108  from the insulating layer  104 . 
     Furthermore, rapid thermal annealing (RTA) treatment can be performed in an inert gas atmosphere (such as nitrogen, helium, neon, or argon) at a temperature of higher than or equal to 500° C. and lower than or equal to 750° C. (or a temperature lower than or equal to the strain point of the glass substrate) for approximately 1 minute to 10 minutes, preferably at 600° C. for approximately 3 minutes to 6 minutes. Since dehydration or dehydrogenation can be performed in a short time with RTA treatment, the heat treatment can be performed even at a temperature over the strain point of a glass substrate. Note that in the heat treatment, it is preferable that water, hydrogen, and the like be not contained in the inert gas (nitrogen or a rare gas such as helium, neon, or argon) atmosphere or in the oxygen atmosphere. It is preferable that the purity of nitrogen or the rare gas such as helium, neon, or argon which is introduced into a heat treatment apparatus be set to be 6N (99.9999%) or higher, preferably 7N (99.99999%) or higher (that is, the impurity concentration is 1 ppm or lower, preferably 0.1 ppm or lower). 
     Note that the timing of the above heat treatment is not limited to after formation of the island-shaped oxide semiconductor layer  108 , and the oxide semiconductor film before being processed into the island-shaped oxide semiconductor layer  108  may be subjected to the heat treatment. The heat treatment may be performed more than once after the oxide semiconductor film  107  is formed. 
     In this embodiment, heat treatment is performed for 60 minutes in the atmosphere in the state where the substrate temperature reaches 350° C. Further, heating with the use of an electric furnace, rapid heating such as gas rapid thermal annealing (GRTA) using a heated gas or lamp rapid thermal annealing (LRTA) using lamp light, or the like can be used for the heat treatment. For example, in the case of performing heat treatment using an electric furnace, the temperature rise characteristics are preferably set at higher than or equal to 0.1° C./min and lower than or equal to 20° C./min and the temperature drop characteristics are preferably set at higher than or equal to 0.1° C./min and lower than or equal to 15° C./min. 
     The island-shaped oxide semiconductor layer  108  which has been subjected to the heat treatment in an inert gas atmosphere is preferably in an amorphous state, but may be partly crystallized. 
     Here, plasma treatment using oxygen, ozone, or dinitrogen monoxide may be performed on an exposed surface of the oxide semiconductor layer  108 . By performing the plasma treatment, oxygen can be supplied to defects of the oxide semiconductor layer  108 . 
     Next, the gate insulating layer  110  is formed. Note that the gate insulating layer  110  can be formed in a manner similar to that of the insulating layer  104 . In this embodiment, a silicon oxide layer with a thickness of 200 nm is formed by RF sputtering using SiO 2  as a target and a rare gas such as argon and oxygen as gases to be supplied when the gate insulating layer  110  is formed. 
     The structure obtained through the steps up to here is illustrated in  FIG. 2C . 
     After the gate insulating layer  110  is formed, heat treatment may be performed. The heat treatment is performed in the atmosphere or an inert gas atmosphere (nitrogen, helium, neon, argon, or the like). The heat treatment is preferably performed at a temperature of greater than or equal to 200° C. and less than or equal to 400° C. In this embodiment, the heat treatment is preferably performed at 350° C. for 1 hour in the atmosphere. Alternatively, RTA treatment for a short time at a high temperature may be performed in a similar manner to the heat treatment performed before forming the gate insulating layer  110 . The timing of this heat treatment is not particularly limited as long as it is after the formation of the gate insulating layer  110 , and can be performed without increasing the number of manufacturing steps by doubling as another step such as a heat treatment for reducing the resistance of a transparent conductive film. 
     A conductive film serving as the gate electrode layer  112  is formed over the gate insulating layer  110 , and a third photolithography step and an etching step are performed, whereby the gate electrode layer  112  is formed. The conductive film can have a structure similar to that of the source electrode layer  106   a  and the drain electrode layer  106   b . In this embodiment, a titanium film with a thickness of 150 nm is formed by DC sputtering using a titanium target. Then, the gate electrode layer  112  is formed by performing the third photolithography step and the etching step. 
     The structure obtained through the steps up to here is illustrated in  FIG. 2D . 
     In the foregoing manner, the semiconductor device of Embodiment 1 can be manufactured. 
     Embodiment 3 
     In this embodiment, a semiconductor device which is another embodiment of the present invention is described with reference to  FIGS. 3A and 3B . 
       FIG. 3A  is a top view of a transistor  200  included in a semiconductor device.  FIG. 3B  is a cross-sectional view taken along line A 2 -B 2  of  FIG. 3A . The transistor  200  include, over the substrate  102 , the insulating layer  104 , an oxide semiconductor layer  208  which forms a channel region, a source electrode layer  206   a  and a drain electrode layer  206   b , a gate insulating layer  210 , and a gate electrode layer  212 . 
     The transistor  200  is a transistor having a top-gate structure, in which the gate electrode layer  212  overlaps with the oxide semiconductor layer  208  with the gate insulating layer  210  interposed therebetween. Further, the transistor  200  is a top-contact transistor including the oxide semiconductor layer  208  between the source and drain electrode layers  206   a  and  206   b  and the insulating layer  104 . 
     The transistor  200  is a top-gate/bottom-contact transistor, so that part of the upper surface of the insulating layer  104  and part of the lower surface of the oxide semiconductor layer  208  are in contact with each other. Therefore, in manufacturing steps of the transistor  200 , hydrogen is diffused into the oxide semiconductor layer  208  when a large amount of hydrogen exists in the insulating layer  104 . Hydrogen is diffused into the oxide semiconductor layer  208 , so that excessive carriers are generated in the oxide semiconductor layer  208 . Thus, the threshold voltage of the transistor  200  shifts in the negative direction, and drain current flows even in the state (Vg=0V) where voltage is not applied to the gate electrode (normally-on). Therefore, electric characteristics of the transistor  200  are degraded. 
     There is a method in which the oxide semiconductor layer  208  is subjected to heat treatment in order to remove the diffused hydrogen from the oxide semiconductor layer  208 . However, as the manufacturing steps of the transistor are increased, manufacturing cost is increased and yield may be reduced. 
     Thus, an insulating layer in which the concentration of hydrogen is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3  is used as the insulating layer  104  being in contact with the oxide semiconductor layer  208 , whereby diffusion of hydrogen in the oxide semiconductor layer  208  can be prevented and a transistor having favorable electric characteristics can be provided. Accordingly, a transistor having favorable electric characteristics can be provided without increasing the number of manufacturing steps of the transistor. 
     Further, the concentration of hydrogen in the gate insulating layer  210  can be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . In other words, the concentration of hydrogen in each of the insulating layer  104  and the gate insulating layer  210  is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 , whereby diffusion of hydrogen in the oxide semiconductor layer  208  can be suppressed. 
     The substrate  102  in this embodiment is similar to the substrate  102  described in Embodiment 1. 
     The insulating layer  104  has a structure similar to that described in Embodiment 1. The insulating layer  104  serves as a base to prevent diffusion of an impurity element from the substrate  102  and also serves as a base to prevent the substrate from being etched by etching in the manufacturing steps of the transistor. There is no limitation on the thickness of the insulating layer  104 ; however, the thickness of the insulating layer  104  is preferably greater than or equal to 50 nm. 
     The insulating layer  104  is formed with a single-layer structure using any of oxide insulating layers of silicon oxide, silicon oxynitride, silicon nitride, silicon nitride oxide, hafnium oxide, aluminum oxide, tantalum oxide, and the like; or a stacked structure including two or more layers selected from these layers. In the case where the stacked structure including two or more layers is adopted, an insulating layer being in contact with the substrate  102  is formed using a silicon nitride and the insulating layer  104  being in contact with the oxide semiconductor layer  108  is formed using the above-mentioned oxide insulating layer. An oxide insulating layer in which the concentration of hydrogen is reduced is used as the insulating layer  104  being in contact with the oxide semiconductor layer  208 , whereby, oxygen is supplied to defects of the oxide semiconductor layer  208  from the insulating layer  104 . Thus, a transistor having favorable electric characteristics can be provided. At this time, as described above, it is necessary that the concentration of hydrogen in the insulating layer  104  be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     As the insulating layer  104  in this embodiment, a silicon oxide layer formed by sputtering or a silicon oxide layer formed by plasma CVD can be used as described in Embodiment 1. 
     In the case where the insulating layer  104  is formed by sputtering, a target containing a silicon element is preferably used. That is to say, a Si target or SiO 2  target can be used. Preferably, a SiO 2  target is used in order to reduce the concentration of hydrogen in the obtained oxide silicon layer; more preferably a SiO 2  target in which the concentration of a hydroxyl group contained in the SiO 2  target is less than or equal to 1000 ppm or the concentration of hydrogen measured using secondary ion mass spectrometry (SIMS) is less than or equal to 3.5×10 19  atoms/cm 3  is used. As gases to be supplied for forming the insulating layer  104 , a rare gas such as argon and oxygen are used. Further, it is preferable to use high-purity gas in which impurities such as hydrogen, water, a hydroxyl group, or hydride are reduced to a concentration of a “ppm” level or a “ppb” level as gases to be supplied. 
     The insulating layer  104  can be formed by plasma CVD instead of sputtering. Plasma CVD is a method for forming a film by supplying a deposition gas to be raw materials to a reaction chamber of a plasma CVD apparatus to employ plasma energy. 
     When the oxide silicon layer is formed by plasma CVD, a gas in which hydrogen is not contained in its molecular structure as the deposition gas is needed to be selected. 
     As the deposition gas, not SiH 4  but SiF 4  is used. Further, as a gas for oxidation, N 2 O or O 2  with a low content of hydrogen and water is used. Further, a gas with a low content of hydrogen and water is used also as the other gases to be added (a rare gas such as argon) in consideration of the spread of plasma. 
     Furthermore, the silicon oxide layer is formed by plasma CVD using the gas having the above-described structure after impurities remaining in the reaction chamber or adsorbing onto the inner wall of the reaction chamber are removed. In such a manner, the concentration of hydrogen in the insulating layer  104  formed by plasma CVD can be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     The oxide semiconductor layer  208  which forms a channel region is formed over the insulating layer  104 . The oxide semiconductor layer  208  is similar to the oxide semiconductor layer  108  in Embodiment 1. As described in Embodiment 1, the concentration of hydrogen in the insulating layer  104  is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 , whereby diffusion of hydrogen in the oxide semiconductor layer  208  is prevented when the oxide semiconductor layer  208  is formed. 
     The gate insulating layer  210  is formed so as to cover the source electrode layer  206   a , the drain electrode layer  206   b , and the oxide semiconductor layer  208 . The gate insulating layer  210  is similar to the gate insulating layer  110  in Embodiment 1. Further, an oxide insulating layer with a low hydrogen content, similarly to the insulating layer  104 , is used as the gate insulating layer  210 , whereby a semiconductor device having favorable electric characteristics can be obtained. Thus, it is preferable that the concentration of hydrogen in the gate insulating layer being in contact with the oxide semiconductor layer be less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 . 
     The source electrode layer  206   a  and the drain electrode layer  206   b  are formed over part of the upper surface of the insulating layer  104  and part of the upper surface of the oxide semiconductor layer  208 . The source electrode layer  206   a  and the drain electrode layer  206   b  are similar to the source electrode layer  106   a  and the drain electrode layer  106   b  in Embodiment 1. 
     The gate electrode  212  is formed so as to overlap with the oxide semiconductor layer  208  with the gate insulating layer  210  interposed therebetween. The gate electrode  212  is similar to the gate electrode  112  in Embodiment 1. 
     Although not illustrated in  FIGS. 3A and 3B , an insulating layer serving as a passivation layer or an interlayer insulating layer is preferably formed over the transistor  200 . 
     As described above, the concentration of hydrogen in the insulating layer  104  and the gate insulating layer  210  is less than 6×10 20  atoms/cm 3 , preferably less than or equal to 5×10 20  atoms/cm 3 , more preferably less than or equal to 5×10 19  atoms/cm 3 , whereby diffusion of hydrogen in the oxide semiconductor layer  208  can be prevented and a semiconductor device having favorable electric characteristics can be obtained. 
     Note that the structure described in this embodiment can be combined as appropriate with any structure described in the other embodiments in this specification. 
     Embodiment 4 
     In this embodiment, a method for manufacturing the semiconductor device illustrated in Embodiment 3 is described with reference to  FIGS. 4A to 4D . 
     As shown in  FIG. 4A , the insulating layer  104  is formed over the substrate  102 . The substrate  102  and the insulating layer  104  illustrated in Embodiment 3 can be used. In this embodiment, a glass substrate is used for the substrate  102 . As the insulating layer  104 , a silicon oxide layer with a thickness of 200 nm is formed by RF sputtering using SiO 2  as a target and a rare gas such as argon and oxygen as gases to be supplied when the insulating layer  104  is formed. 
     The insulating layer  104  can be formed as described in Embodiment 2 when it is formed by plasma CVD, 
     Then, an oxide semiconductor film with a thickness of 50 nm is formed by sputtering. Because the oxide semiconductor film is formed to be in contact with the insulating layer  104 , oxygen is supplied to defects in the oxide semiconductor layer from the insulating layer  104 . After that, the oxide semiconductor layer  207  that is processed into an island shape is formed by performing the first photolithography step or the etching step. 
     The oxide semiconductor film can be formed as described in Embodiment 2. 
     The structure obtained through the steps up to here is illustrated in  FIG. 4A . 
     Next, the conductive film serving as the source electrode layer  206   a  and the drain electrode layer  206   b  is formed. As the conductive film, in this embodiment, a titanium film with a thickness of 150 nm is formed by DC sputtering using a titanium target as in Embodiment 2. Then, the source electrode layer  206   a  and the drain electrode layer  206   b  each having a thickness of 150 nm are formed by performing a second photolithography step and an etching step. 
     Etching of the conductive film can be performed in a similar manner to that described in Embodiment 2. 
     Then, the oxide semiconductor layer  207  may be subjected to heat treatment in the atmosphere, an inert gas atmosphere (nitrogen, helium, neon, argon, or the like), or in the atmosphere where the dew point under atmospheric pressure is less than or equal to −60° C. and the moisture content is small. Specifically, the oxide semiconductor layer  207  is subjected to heat treatment in the atmosphere at greater than or equal to 100° C. and less than or equal to 400° C. for 10 minutes or more, preferably at 350° C. for 60 minutes. In this embodiment, the oxide semiconductor layer  207  is subjected to heat treatment, whereby the oxide semiconductor layer  208  in which moisture and hydrogen are eliminated is formed. At that time, oxygen is supplied to defects in the oxide semiconductor layer  208  from the insulating layer  104 . 
     Note the heat treatment is not necessarily performed after the source electrode layer  206   a  and the drain electrode layer  206   b  are formed, and the heat treatment may be performed on the island-shaped oxide semiconductor film  207  formed by performing the first photolithography step and the etching step before forming the source electrode layer  206   a  and the drain electrode layer  206   b . The heat treatment may also be performed plural times after forming the oxide semiconductor layer  207 . 
     In this embodiment, heat treatment is performed at 350° C. for 60 minutes in the atmosphere in the state where the substrate temperature reaches 350° C. 
     Here, plasma treatment using oxygen, ozone, or dinitrogen monoxide may be performed on an exposed surface of the oxide semiconductor layer  208 . By performing the plasma treatment, oxygen can be supplied to defects in the oxide semiconductor layer  208 . 
     Next, the gate insulating layer  210  is formed. Note that the gate insulating layer  210  can be formed in a manner similar to that of the gate insulating layer  104 . In this embodiment, a silicon oxide layer with a thickness of 200 nm is formed by RF sputtering using SiO 2  as a target and a rare gas such as argon and oxygen as gases to be supplied when the gate insulating layer  210  is formed. 
     The structure obtained through the steps up to here is illustrated in  FIG. 4C . 
     After the gate insulating layer  210  is formed, heat treatment may be performed. The heat treatment can be performed by a method which is similar to that in Embodiment 2, and the heat treatment can also be performed at the timing which is the same as the timing described in Embodiment 2. 
     A conductive film serving as the gate electrode layer  212  is formed over the gate insulating layer  210 . Then, the gate electrode layer  212  is formed by performing the third photolithography step or the etching step. The conductive film can have a structure similar to that of the source electrode layer  206   a  and the drain electrode layer  206   b . In this embodiment, a titanium film with a thickness of 150 nm is formed by DC sputtering using a titanium target as in Embodiment 2. Then, the gate electrode layer  212  is formed by performing the third photolithography step and the etching step. 
     The structure obtained through the steps up to here is illustrated in  FIG. 4D . 
     As described above, a semiconductor device illustrated in Embodiment 3 can be manufactured. 
     Note that the structure described in this embodiment can be combined as appropriate with any structure described in the other embodiments in this specification. 
     Embodiment 5 
     The transistor described in the above embodiments is manufactured, and a semiconductor device having a display function (also referred to as a display device) can be manufactured using the transistor for a pixel portion and further for a driver circuit. Further, part of or the entire driver circuit including the transistors can be formed over a substrate where the pixel portion is formed; thus, a system-on-panel can be obtained. Further, a semiconductor device including a memory cell can be manufactured using the transistors in which the oxide semiconductor described in the above embodiments is used. 
     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. The light-emitting element includes, in its category, an element whose luminance is controlled by a current or a voltage, and specifically includes, in its category, an inorganic electroluminescent (EL) element, an organic EL element, and the like. Furthermore, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used. 
     In addition, the display device includes a panel in which the display element is sealed, and a module in which an IC or the like including a controller is mounted on the panel. Furthermore, an element substrate, which corresponds to one embodiment before the display element is completed in a manufacturing process of the display device, is provided with a means for supplying current to the display element in each of a plurality of pixels. Specifically, the element substrate may be in a state where only a pixel electrode of the display element is formed, a state where a conductive film to be a pixel electrode is formed but is not etched yet to form the pixel electrode, or any other states. 
     Note that a display device in this specification means an image display device, a display device, or a light source (including a lighting device). Further, the “display device” includes 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) attached; 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. 
     Embodiment 6 
     A display device using the transistor described in the above embodiments can be used for an electronic paper in which electronic ink is driven to perform display. An electronic paper can be used for electronic devices of a variety of fields as long as they can display data. For example, electronic paper can be applied to an electronic book reader (e-book), a poster, a digital signage, a public information display (PID), an advertisement in a vehicle such as a train, displays of various cards such as a credit card, and the like. An example of the electronic device is illustrated in  FIG. 5 . 
       FIG. 5  illustrates an e-book reader  2700  as an example the electronic device. For example, the e-book reader  2700  includes two housings, a housing  2701  and a housing  2703 . The housing  2701  and the housing  2703  are combined with a hinge  2711  so that the e-book reader  2700  can be opened and closed with the hinge  2711  as an axis. With such a structure, the e-book reader  2700  can operate like a paper book. 
     A display portion  2705  and a photoelectric conversion device  2706  are incorporated in the housing  2701 . A display portion  2707  and a photoelectric conversion device  2708  are incorporated in the housing  2703 . The display portion  2705  and the display portion  2707  may display one image or different images. In the case where the display portion  2705  and the display portion  2707  display different images, for example, text can be displayed on a display portion on the right side (the display portion  2705  in  FIG. 5 ) and graphics can be displayed on a display portion on the left side (the display portion  2707  in  FIG. 5 ). 
       FIG. 5  illustrates an example in which the housing  2701  is provided with an operation portion and the like. For example, the housing  2701  is provided with a power switch  2721 , an operation key  2723 , a speaker  2725 , and the like. With the operation key  2723 , pages can be turned. Note that a keyboard, a pointing device, or the like may also be provided on the surface of the housing, on which the display portion is provided. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, a terminal that can be connected to various cables such as an AC adapter and a USB cable, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Moreover, the e-book reader  2700  may have a function of an electronic dictionary. 
     The e-book reader  2700  may have a configuration capable of wirelessly transmitting and receiving data. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server. 
     Embodiment 7 
     A semiconductor device disclosed in this specification can be applied to a variety of electronic devices (including game machines). Examples of electronic devices are a television device (also referred to as a television or a television receiver), a monitor of a computer or the like, electronic paper, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone (also referred to as a mobile telephone or a mobile phone device), a portable game console, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like. 
       FIG. 6A  illustrates a television set  9600  as an example of an electronic device. In the television set  9600 , a display portion  9603  is incorporated in a housing  9601 . The display portion  9603  can display images. Here, the housing  9601  is supported by a stand  9605 . 
     The television set  9600  can be operated with an operation switch of the housing  9601  or a separate remote controller  9610 . Channels and volume can be controlled with an operation key  9609  of the remote controller  9610  so that an image displayed on the display portion  9603  can be controlled. Furthermore, the remote controller  9610  may be provided with a display portion  9607  for displaying data output from the remote controller  9610 . 
     Note that the television set  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 or between receivers) information communication can be performed. 
       FIG. 6B  illustrates a digital photo frame  9700  as an example of an electronic device. For example, in the digital photo frame  9700 , a display portion  9703  is incorporated in a housing  9701 . The display portion  9703  can display a variety of images. For example, the display portion  9703  can display data of an image taken with a digital camera or the like and function as a normal photo frame 
     Note that the digital photo frame  9700  is provided with an operation portion, an external connection portion (a USB terminal, a terminal that can be connected to various cables such as a USB cable, or the like), a recording medium insertion portion, and the like. Although these components may be provided on the surface on which the display portion is provided, it is preferable to provide them on the side surface or the back surface for the design of the digital photo frame  9700 . For example, a memory storing data of an image taken with a digital camera is inserted in the recording medium insertion portion of the digital photo frame, whereby the image data can be transferred and then displayed on the display portion  9703 . 
     The digital photo frame  9700  may be configured to transmit and receive data wirelessly. The structure may be employed in which desired image data is transferred wirelessly to be displayed. 
       FIG. 7  is a perspective view illustrating an example of a portable computer. 
     In the portable computer of  FIG. 7 , a top housing  9301  having a display portion  9303  and a bottom housing  9302  having a keyboard  9304  can overlap with each other by closing a hinge unit which connects the top housing  9301  and the bottom housing  9302 . Thus, the portable computer illustrated in  FIG. 7  is conveniently carried. Moreover, in the case of using the keyboard for input of data, the hinge unit is opened so that a user can input data looking at the display portion  9303 . 
     The bottom housing  9302  includes a pointing device  9306  with which input can be performed, in addition to the keyboard  9304 . Further, when the display portion  9303  is a touch input panel, input can be performed by touching part of the display portion. The bottom housing  9302  includes an arithmetic function portion such as a CPU or hard disk. In addition, the bottom housing  9302  includes an external connection port  9305  into which another device such as a communication cable conformable to communication standards of a USB is inserted. 
     The top housing  9301  includes a display portion  9307  and can keep the display portion  9307  therein by sliding it toward the inside of the top housing  9301 ; thus, the top housing  9301  can have a large display screen. In addition, the user can adjust the orientation of a screen of the display portion  9307  which can be kept in the top housing  9301 . When the display portion  9307  which can be kept in the top housing  9301  is a touch input panel, input can be performed by touching part of the display portion  9307  which can be kept in the top housing  9301 . 
     The display portion  9303  or the display portion  9307  which can be kept in the top housing  9301  are formed with an image display device of a liquid crystal display panel, a light-emitting display panel such as an organic light-emitting element or an inorganic light-emitting element, or the like. 
     In addition, the portable computer in  FIG. 7 , which can be provided with a receiver and the like, can receive a television broadcast to display an image on the display portion  9303  or the display portion  9307 . The user can watch television broadcast when the whole screen of the display portion  9307  is exposed by sliding the display portion  9307  while the hinge unit which connects the top housing  9301  and the bottom housing  9302  is kept closed. In this case, the hinge unit is not opened and display is not performed on the display portion  9303 . In addition, start up of only a circuit for displaying television broadcast is performed. Therefore, power can be consumed to the minimum, which is useful for the portable computer whose battery capacity is limited. 
     Example 1 
     In the insulating layers described in the above embodiments, the concentration of hydrogen included in the following insulating layers A and B is illustrated with reference to  FIG. 8 . The insulating layer A was formed by sputtering using SiO 2  as a target and the insulating layer B was formed by plasma CVD using SiH 4  as a deposition gas. 
     First, a method for manufacturing samples is described. As the insulating layer A, a silicon oxide layer with a thickness of 200 nm was formed over a silicon substrate by RF sputtering under the following conditions: SiO 2  was used as a target; argon and oxygen were supplied at flow rates of 40 sccm and 10 sccm, respectively; and the power and the pressure were adjusted to 1.5 kW and 0.4 Pa, respectively. At that time, the substrate temperature was 100° C., and the distance between electrodes in a sputtering apparatus was 60 mm. 
     As the insulating layer B, a silicon oxynitride layer with a thickness of 100 nm was formed over a silicon substrate by plasma CVD in which plasma discharge was performed under the following conditions: SiH 4  as a deposition gas and N 2 O as an oxynitride gas were supplied with flow rates of 4 sccm and 800 sccm, respectively to gain stability; the pressure in the treatment chamber was 40 Pa; RF power source frequency was 27 MHz; and the power of the RF power source was 50 W. At that time, the substrate temperature was 400° C., and the distance between electrodes in a plasma CVD apparatus was 15 mm. 
     Next, SIMS measurement results of the insulating layers A and B are shown in  FIG. 8 . In  FIG. 8 , the vertical axis represents the concentration of hydrogen in the insulating layers A and B, and the horizontal axis represents the depth in a direction of the substrate from the surfaces of the insulating layers A and B. Further, the solid line shows a concentration profile of the insulating layer A and the broken line shows a concentration profile of the insulating layer B. In the insulating layer A, the horizontal axis corresponding to 70 nm to 120 nm represents a quantitative range and the horizontal axis corresponding to 200 nm or more represents the silicon substrate. In the insulating layer B, the horizontal axis corresponding to 10 nm to 60 nm represents a quantitative range and the horizontal axis corresponding to 100 nm or more represents the silicon substrate. Note that the quantitative range in this example means a range where high reliability is obtained with the SIMS measurement results (concentration of hydrogen). In other words, the measurement results (concentration of hydrogen) in each quantitative range in insulating layers A and B represent the concentration of hydrogen included in each of the insulating layers A and B. 
     When the quantitative ranges of the insulating layer A and the insulating layer B were compared in  FIG. 8 , the concentration of hydrogen in the insulating layer A was greater than or equal to 4.9×10 19  atoms/cm 3  and less than or equal to 5.2×10 19  atoms/cm 3 , while the concentration of hydrogen in the insulating layer B was greater than or equal to 6.4×10 20  atoms/cm 3  and less than or equal to 9.6×10 20  atoms/cm 3 . 
     It was found that the silicon oxide layer in which diffusion of hydrogen was suppressed was formed because the insulating layer A was formed by sputtering using SiO 2  as the target while being supplied with argon and oxygen. Also, it was found that hydrogen was diffused in the silicon oxynitride layer because the insulating layer B was formed using SiH 4  as the deposition gas. 
     Example 2 
     In this example, in the top-gate/top-contact transistors described in Embodiment 1, electric characteristics of the following transistors (sample A and sample B) illustrated in Example 1 is described. The transistor (sample A) was formed using silicon oxide of the insulating layer A described in Example 1, and the transistor (sample B) was formed using silicon oxynitride of the insulating layer B described in Example 1. The other structures of the transistors are the same in the sample A and the sample B. 
     Manufacturing steps of the sample A and sample B are illustrated with reference to  FIGS. 2A to 2D . A glass substrate (EAGLE XG-2000 manufactured by Corning Incorporated) was used as the substrate  102 . 
     As shown in  FIG. 2A , the insulating layer  104  was formed over the substrate  102 . 
     The insulating layer  104  in each of the samples A and B was formed to have a thickness of 200 nm and formed by the method described in Example 1. 
     Then, the conductive film serving as the source electrode layer  106   a  and the drain electrode layer  106   b  were formed. A titanium film with a thickness of 150 nm was formed by DC sputtering as follows: a titanium target was used; argon with a flow rate of 20 sccm was supplied; and the power and the pressure were adjusted to 12 kW and 0.1 Pa, respectively. At that time, the substrate temperature was room temperature (15° C. to 35° C.), and the distance between electrodes in a sputtering apparatus was 400 mm. 
     After a resist was applied over the titanium film, light exposure was performed using a first photomask. After that, development was performed, so that a resist mask was formed. Then, etching was performed using the resist mask, whereby the source electrode layer  106   a  and the drain electrode layer  106   b  were formed. Here, an inductively coupled plasma (ICP) etching apparatus was used. A first etching was performed in the following conditions: ICP power was 450 W, the bias power was 100 W, the pressure was 1.9 Pa, and boron trichloride at a flow rate of 60 sccm and chlorine at a flow rate of 20 sccm were used for an etching gas. After that, the resist mask was removed. 
     Then, the oxide semiconductor film was formed with a thickness of 50 nm over the insulating layer  104 , the source electrode layer  106   a , and the drain electrode layer  106   b . Here, the oxide semiconductor film containing indium (In), gallium (Ga), zinc (Zn), and oxygen atoms was formed by DC sputtering without heating the substrate. Note that the DC sputtering was performed under the following condition: the target composition in the oxide semiconductor was In 2 O 3 :Ga 2 O 3 : ZnO=1:1:1 (In:Ga:Zn=1:1:0.5); argon with a flow rate of 30 sccm and oxygen with a flow rate of 15 sccm were supplied; and the power and the pressure were adjusted to 0.5 kW and 0.4 Pa, respectively. 
     After a resist was applied over the oxide semiconductor film, light exposure was performed using a second photomask. After that, development was performed, so that a resist mask was formed. Then, etching was performed using the resist mask, whereby the island-shaped oxide semiconductor layer  107  was formed. Here, wet etching was performed using an Al-Etchant (an aqueous solution containing 2.0 wt % nitric acid, 9.8 wt % acetic acid, and 72.3 wt % phosphoric acid) produced by Wako Pure Chemical Industries Co., Ltd. After that, the resist mask was removed. The structure obtained through the steps up to here is illustrated in  FIG. 2B . 
     Then, heat treatment was performed at 350° C. for 60 minutes in the atmosphere, whereby the island-shaped oxide semiconductor layer  108  was obtained. The gate insulating layer  110  was formed over the island-shaped oxide semiconductor layer  108 . As the gate insulating layer  110 , a silicon oxide layer with a thickness of 200 nm was formed by RF sputtering under the following conditions: SiO 2  was used as a target; argon with a flow rate of 25 sccm and oxygen with a flow rate of 25 sccm were supplied; and the power and the pressure were adjusted to 1.5 kW and 0.4 Pa, respectively. At that time, the substrate temperature was 100° C., and the distance between electrodes in the sputtering apparatus was 60 mm. The structure obtained through the steps up to here is illustrated in  FIG. 2C . 
     Then, a conductive film serving as the gate electrode layer  112  was formed after performing heat treatment at 350° C. for 60 minutes in the atmosphere. Here, a titanium film with a thickness of 150 nm was formed by DC sputtering as follows: a titanium target was used; argon with a flow rate of 20 sccm was supplied; and the power and the pressure were adjusted to 12 kW and 0.1 Pa, respectively. At that time, the substrate temperature was room temperature (15° C. to 35° C.), and the distance between electrodes in the sputtering apparatus was 400 mm. 
     After a resist was applied over the titanium film, light exposure was performed using a third photomask. After that, development was performed, so that a resist mask was formed. Then, etching was performed using the resist mask, whereby the gate electrode layer  112  was formed. Here, an ICP apparatus was used and the first etching was performed in the following conditions: ICP power was 450 W, the bias power was 100 W, the pressure was 1.9 Pa, and an etching gas included boron trichloride at a flow rate of 60 sccm and chlorine at a flow rate of 20 sccm. After that, the resist mask was removed. Through the above steps, the transistor was manufactured (see  FIG. 2D ). 
     The measurement result of the samples A and B is shown in  FIG. 9 . The solid line shows current-voltage characteristics and field-effect mobility of the sample A when the drain voltage was 10 V, and the broken line shows current-voltage characteristics and field-effect mobility of the sample B when the drain voltage was 10 V. Note that the transistor of this example was formed so as to have a channel length of 3.0 μm and a channel width of 10 μm. 
       FIG. 9  shows that favorable electric characteristics were obtained in the sample A in which the concentration of hydrogen in the insulating layer was less than 6×10 20  atoms/cm 3 , while electric characteristics were degraded in the sample B in which the concentration of hydrogen in the insulating layer was greater than or equal to 6×10 20  atoms/cm 3  because threshold voltage shifted in the negative direction, and a drain current flowed in a state (Vg=0V) where voltage was not applied to the gate electrode. 
     From the above, it can be seen that the defect was caused in the sample B in which the concentration of hydrogen in the insulating layer was greater than or equal to 6×10 20  atoms/cm 3  because hydrogen was diffused into the oxide semiconductor layer including the channel region in the manufacturing steps of the transistor. 
     From the above, it can be seen that the sample A in which the concentration of hydrogen in the insulating layer was less than 6×10 20  atoms/cm 3  had favorable electric characteristics because diffusion of hydrogen in the oxide semiconductor layer including the channel region was prevented in the manufacturing steps of the transistor. 
     Consequently, the transistor having favorable electric characteristics can be provided by setting the concentration of hydrogen in the insulating layer to less than 6×10 20  atoms/cm 3 . 
     This application is based on Japanese Patent Application serial no. 2010-117086 filed with Japan Patent Office on May 21, 2010, the entire contents of which are hereby incorporated by reference. 
     EXPLANATION OF REFERENCE 
       100 : transistor,  102 : substrate,  104 : insulating layer,  106   a : source electrode layer,  106   b : drain electrode layer,  107 : oxide semiconductor layer,  108 : oxide semiconductor layer,  110 : gate insulating layer,  112 : gate electrode layer,  200 : transistor,  206   a : source electrode layer,  206   b : drain electrode layer,  207 : oxide semiconductor layer,  208 : oxide semiconductor layer,  210 : gate insulating layer,  212 : gate electrode layer,  2700 : e-book reader,  2701 : housing,  2703 : housing,  2705 : display portion,  2706 : photoelectric conversion device,  2707 : display portion,  2708 : photoelectric conversion device,  2711 : hinge,  2721 : power switch,  2723 : operation key,  2725 : speaker,  9301 : top housing,  9302 : bottom housing,  9303 : display portion,  9304 : keyboard,  9305 : external connection port,  9306 : pointing device,  9307 : display portion,  9600 : television set,  9601 : housing,  9603 : display portion,  9605 : stand,  9607 : display portion,  9609 : operation key,  9610 : remote controller,  9700 : digital photo frame,  9701 : housing,  9703 : display portion