# Patent

## Title
Semiconductor device and method for manufacturing the same

## Abstract

A highly reliable semiconductor device exhibiting stable electrical characteristics is provided. Further, a highly reliable semiconductor device is provided. Oxide semiconductor films are stacked so that the conduction band has a well-shaped structure. A second oxide semiconductor film having a crystalline structure is provided over the first oxide semiconductor film and a third oxide semiconductor film is provided over the second oxide semiconductor film. The bottom of a conduction band in the second oxide semiconductor film is deeper from a vacuum level than the bottom of a conduction band in the first oxide semiconductor film and the bottom of a conduction band in the third oxide semiconductor film.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a method for manufacturing the semiconductor device.
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 appliance are all semiconductor devices.
2. Description of the Related Art
In recent years, semiconductor devices have been developed to be used mainly for a CPU, or a memory. A CPU is an aggregation of semiconductor elements each provided with an electrode which is a connection terminal, which includes a semiconductor integrated circuit (including at least a transistor and a memory) separated from a semiconductor wafer.
A semiconductor circuit (IC chip) of a CPU or a memory is mounted on a circuit board, for example, a printed wiring board, to be used as one of components of a variety of electronic appliances.
A technique for manufacturing a transistor or the like using an oxide semiconductor film for a channel formation region and applying it to a display device has been attracting attention. Examples of such a transistor include a transistor in which zinc oxide (ZnO) is used as an oxide semiconductor film and a transistor in which InGaO3(ZnO)m is used as an oxide semiconductor film.
A technique for manufacturing transistor including an oxide semiconductor film over a light-transmitting substrate and applying it to a switching element or the like of an image display device is disclosed in Patent Documents 1 and 2.
Patent Document 3 discloses a semiconductor device in which a transistor including an oxide semiconductor is provided over a single crystal substrate.
REFERENCE
Patent Document
[Patent Document 1] Japanese Published Patent Application No. 2007-123861
[Patent Document 2] Japanese Published Patent Application No. 2007-096055
[Patent Document 3] Japanese Published Patent Application No. 2011-109079
SUMMARY OF THE INVENTION
The electrical characteristics of a transistor including an oxide semiconductor film are varied by influence of an insulating film in contact with the oxide semiconductor film, that is, by an interface state between the oxide semiconductor film and the insulating film.
Further, a transistor including an oxide semiconductor film in which many oxygen vacancies have been generated in the manufacturing process has low long-term reliability. Therefore, it is required to manufacture a transistor including an oxide semiconductor film which has as few oxygen vacancies as possible. Further, it is required to reduce damage to the oxide semiconductor film which is caused by exposure of the oxide semiconductor film to plasma during or after deposition of the oxide semiconductor film.
In view of the above problems, an object of one embodiment of the present invention is to provide a highly reliable semiconductor device which exhibits stable electrical characteristics. Another object is to manufacture a highly reliable semiconductor device.
A buried-channel transistor in which two or more oxide semiconductor films are stacked so that an oxide semiconductor having a small number of oxygen vacancies serves as a carrier path is manufactured.
Specifically, a transistor having a multi-layer structure is manufactured in which a second oxide semiconductor film having a crystalline structure is stacked over a first oxide semiconductor film, and at least a third oxide semiconductor film is provided over the second oxide semiconductor film.
The second oxide semiconductor film is separated from a silicon insulating film which is located below the second oxide semiconductor film by the first oxide semiconductor film, and the third oxide semiconductor film reduces damage to the second oxide semiconductor film which is caused at the time of exposure to plasma in deposition of a silicon insulating film located over the second oxide semiconductor film or in etching after formation of a conductive film.
One embodiment of the present invention disclosed in this specification is a semiconductor device including a first oxide semiconductor film, a second oxide semiconductor film having a crystalline structure over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. In the band structure of the semiconductor device, the bottom of a conduction band in the second oxide semiconductor film is deeper from the vacuum level than the bottom of a conduction band in the first oxide semiconductor film and the bottom of a conduction band in the third oxide semiconductor film.
FIG. 1A illustrates an example of a cross section of a transistor. A transistor 410 includes a gate electrode 401, a first gate insulating film 402a which is a silicon nitride film, a second gate insulating film 402b which is a silicon oxide film, a first oxide semiconductor film 403a, a second oxide semiconductor film 403b, a third oxide semiconductor film 403c, and electrode layers 405a and 405b functioning as source and drain electrodes, over a substrate 400 having an insulating surface. Further, protective insulating films 407a and 407b which are silicon oxide films and a protective insulating film 408 which is silicon nitride film are provided over the third oxide semiconductor film 403c. 
A material of the second oxide semiconductor film having a crystalline structure is selected as appropriate so that the conduction band has a well-shaped structure (also referred to as a well structure). FIG. 1B illustrates an example of the well-shaped structure of the conduction band. Note that the schematic view illustrated in FIG. 1B corresponds to an energy band diagram of a cross section taken along line Y1-Y2 in FIG. 1A and illustrates only a portion where the stack of oxide semiconductor films is positioned between the silicon nitride films.
If silicon or carbon, which is an element belonging to Group 14, is contained in the oxide semiconductor film as an impurity, it serves as a donor and the oxide semiconductor film becomes an n-type oxide semiconductor film. Therefore, the concentration of Si contained in each of the first and third oxide semiconductor films (the concentration obtained by secondary ion mass spectrometry (SIMS)) is lower than or equal to 3×1018/cm3, preferably lower than or equal to 3×1017/cm3. Further, the concentration of carbon contained in each of the first and third oxide semiconductor films is lower than or equal to 3×1018/cm3, preferably lower than or equal to 3×1017/cm3. In particular, in order that a large amount of impurities such as elements belonging to Group 14 are not mixed into the second oxide semiconductor film, it is preferable that the second oxide semiconductor film serving as a carrier path be provided between or surrounded by the first and third oxide semiconductor films. In other words, the first and third oxide semiconductor films can also be referred to as barrier layers which prevent mixing of elements belonging to Group 14 such as silicon to the second oxide semiconductor film. Since the barrier layers are provided above and below the second oxide semiconductor film, needless to say, the second oxide semiconductor film hardly contains impurities such as elements belonging to Group 14. For example, the Si concentration of the second oxide semiconductor film is lower than or equal to 3×1018/cm3, preferably lower than or equal to 3×1017/cm3, and the carbon concentration thereof is lower than or equal to 3×1018/cm3, preferably lower than or equal to 3×1017/cm3.
Such a stacked-layer structure enables the second oxide semiconductor film to serve as a carrier path, so that carriers travel through a region having a low content of oxygen vacancies. Since carriers flow through the region which is separated from the silicon insulating films which are located above and below the stack of the oxide semiconductor films, influence of the oxygen vacancies can be reduced.
If hydrogen or moisture is contained as an impurity in the stack of oxide semiconductor films, it serves as a donor and the oxide semiconductor films become n-type oxide semiconductor films; therefore, in order to achieve a well-shaped structure, it is valuable to provide a protective film (a nitride insulating film, typically a silicon nitride film, or the like) for preventing the entry of hydrogen or moisture from the outside, above and below the stack of oxide semiconductor films.
Another embodiment of the present invention disclosed in this specification is a semiconductor device including a first nitride insulating film, a first oxide semiconductor film over the first nitride insulating film, a second oxide semiconductor film having a crystalline structure over the first oxide semiconductor film, a third oxide semiconductor film over the second oxide semiconductor film, and a second nitride insulating film over the third oxide semiconductor film. In the semiconductor device, the bottom of a conduction band in the second oxide semiconductor film is deeper from the vacuum level than the bottom of a conduction band in the first oxide semiconductor film and the bottom of a conduction band in the third oxide semiconductor film. The semiconductor device can have higher reliability by including the first nitride insulating film and the second nitride insulating film.
Each of the oxide semiconductor films included in the multi-layer structure contains at least indium (In) at a concentration higher than or equal to 1×1019/cm3 and is deposited using a sputtering target with which a film can be formed by an AC sputtering method or a DC sputtering method. When the sputtering target contains indium, the conductivity is increased; therefore, deposition by an AC sputtering method or a DC sputtering method is facilitated. A material which can be represented as InM1XZnYOZ (X≧1, Y>1, Z>0, and M1 is a metal element such as Ga or Hf) is used as each of materials of the first and third oxide semiconductor films. Note that in the case where Ga is contained in the materials of the first and third oxide semiconductors, when the proportion of Ga is high, specifically, when a material which can be represented by InM1XZnYOZ where X is larger than 10 is used, powder might be generated in deposition, and it is made difficult to perform deposition by an AC sputtering or a DC sputtering; therefore, such a material is not suitable as a sputtering target.
A material which can be represented as InM2XZnYOZ (X≧1, Y≧X, Z>0, and M2 is a metal element such as Ga or Sn) is used as the material of the second oxide semiconductor film. Further, indium tin oxide that has a composition which does not contain M2, i.e., the composition in which X is 0, or a material which contains indium oxide as a main component can be used for the second oxide semiconductor film.
The materials of the first, second, and third oxide semiconductor films are selected as appropriate so that a well-shaped structure in which the bottom of the conduction band in the second oxide semiconductor film is deeper from the vacuum level than the bottoms of the conduction band in the first and third oxide semiconductor films. Specifically, the second oxide semiconductor film is formed using a material which has a higher indium content than the first and third oxide semiconductor films. The contents of indium, gallium, or the like in the first, second, and third oxide semiconductor films can be compared with each other by time-of-flight secondary ion mass spectrometry (also referred to as TOF-SIMS) or X-ray photoelectron spectrometry (also referred to as XPS). Note that the ionization potential of an oxide semiconductor can be measured by ultraviolet photoelectron spectroscopy (UPS) or the like. Typically, VersaProbe (manufactured by ULVAC-PHI Inc) is used as a measurement apparatus for UPS. Note that electron affinity refers to an energy difference between the vacuum level (E∞) and the bottom of the conduction band (Ec). An energy band gap (Eg) can be measured with a full automatic spectroscopic ellipsometer UT-300. The energy of the conduction band is calculated by deducting the energy band gap from the value of the ionization potential; thus a band structure of a single layer or a stack of layers can be formed. In this way, it can be confirmed that a buried channel is formed using the stacked-layer structure disclosed in this specification. FIGS. 2A and 2B illustrate an example thereof.
FIG. 2A is data which shows energies from the vacuum level to the conduction band which are calculated on the basis of measurement data of a sample by a full automatic spectroscopic ellipsometer UT-300. The sample was formed in the following manner: a 10-nm-thick film was formed in an atmosphere containing oxygen at 100% using a sputtering target of an In—Ga—Zn oxide having an atomic ratio of In:Ga:Zn=1:1:1; a 10-nm-thick film was stacked thereover in an atmosphere containing argon at 100% using a sputtering target of an In—Ga—Zn oxide having an atomic ratio of In:Ga:Zn=3:1:2; and a 10-nm-thick film was stacked thereover in an atmosphere containing oxygen at 100% using a sputtering target of an In—Ga—Zn oxide having an atomic ratio of In:Ga:Zn=1:1:1. FIG. 2B shows a band structure formed on the basis of the data of FIG. 2A. FIG. 2B shows that a well-shaped structure is formed in which the bottom of the conduction band in the second oxide semiconductor film is deeper from the vacuum level than the bottoms of the conduction band in the first and third oxide semiconductor films.
Since the second oxide semiconductor film having a crystalline structure is stacked over the first oxide semiconductor film, they can be referred to as a hetero structure having different crystalline structures.
When the oxide semiconductor film serving as a semiconductor film of the transistor has the above stacked-layer structure, the absorption coefficient due to localized states of a region where a channel is formed or at least the second oxide semiconductor film can be lower than or equal to 3×10−3/cm when measured by a constant photocurrent method (CPM) (lower than or equal to 3×1013/cm3 when converted into density of states).
Although one well-shaped structure is formed using the first, second, and the third oxide semiconductor films as an example of the stacked-layer structure, the structure is not particularly limited thereto; a plurality of well-shaped structures may be formed with the second oxide semiconductor film which has a multi-layer structure, and one example thereof is shown in FIG. 3.
As the first, second, and third oxide semiconductor films, any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, a polycrystalline oxide semiconductor film, a microcrystalline oxide semiconductor film, an amorphous oxide semiconductor film, or the like is used. Note that the second oxide semiconductor film is preferably a CAAC-OS film. In this specification and the like, a CAAC-OS film refers to an oxide semiconductor film which includes a crystal whose c-axis is aligned in a direction substantially perpendicular to the surface of the oxide semiconductor film. A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO4 crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO4 crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.
The CAAC-OS layer is not completely single crystal nor completely amorphous. The CAAC-OS film is one of oxide semiconductor films including a plurality of crystal parts, and most of each crystal part fits inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm. In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.
In each of the crystals included in the CAAC-OS film, a c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film. Note that the directions of the a-axis and the b-axis of one crystal may be different from those of another crystal. In this specification and the like, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.
In the CAAC-OS layer, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS layer, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed, i.e., the crystallinity is increased in some cases. Further, when oxygen is added to the CAAC-OS layer, crystallinity in a region to which the element or the oxygen is added is lowered in some cases.
Since the c-axes of the crystals included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by deposition or by performing treatment for crystallization such as heat treatment after deposition.
The CAAC-OS film is formed using a sputtering target. Here, methods for manufacturing sputtering targets each including an oxide semiconductor having a crystal region in which the direction of the c-axis is parallel to a normal vector of the top surface of the oxide semiconductor will be described (see FIG. 14).
First, raw materials for the sputtering target are weighed (step S101).
Here, an InOX raw material (a raw material of In), a GaOY raw material (a raw material of Ga), and a ZnOZ raw material (a raw material of Zn) are prepared as raw materials for the sputtering target. Note that X, Y, and Z are each a given positive number; for example, X, Y, and Z are 1.5, 1.5, and 1, respectively. It is needless to say that the above raw materials are an example, and raw materials can be selected as appropriate in order to obtain a desired compound. For example, a MOY raw material may be used instead of the GaOy raw material. Note that Sn, Hf, or Al can be used as M. Alternatively, the following lanthanoid may be used as M: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu. Although the case where three kinds of raw materials are used is shown as an example in this embodiment, one embodiment of the present invention is not limited thereto. For example, this embodiment may be applied to the case where four or more kinds of raw materials are used or the case where one or two kinds of raw materials are used.
Next, the InOX raw material, the GaOY raw material, and the ZnOZ raw material are mixed in a predetermined ratio.
For example, the predetermined ratio of the InOX raw material, the GaOY raw material, and the ZnOZ raw material is 2:2:1, 8:4:3, 3:1:1, 1:1:1, 1:3:2, 4:2:3, 1:1:2, 3:1:4, or 3:1:2 in a molar ratio. With the use of a mixed material having such a ratio, a sputtering target including an oxide semiconductor having a crystal region in which the direction of the c-axis is parallel to a normal vector of the top surface of the oxide semiconductor can be easily obtained.
More specifically, in the case of forming a sputtering target of In—Ga—Zn-based oxide having a composition of In:Ga:Zn=1:1:1 [atomic ratio], the raw materials are weighed so that In2O3:Ga2O3:ZnO=1:1:2 [molar ratio].
Note that also in the case where the MOY raw material is used instead of the GaOY raw material, the ratio of the InOX raw material, the MOY raw material, and the ZnOZ raw material is 2:2:1, 8:4:3, 3:1:1, 1:1:1, 1:3:2, 4:2:3, 1:1:2, 3:1:4, or 3:1:2 in a molar ratio.
A method for forming the sputtering target using a wet method is described. The raw materials for the sputtering target are weighed, and then, the raw materials are ground and mixed with a ball mill or the like to obtain compound powder. After the mixing of the plurality of raw materials, first baking is performed to generate a crystalline oxide. Then, the crystalline oxide is ground to obtain compound power. The grain size of the compound powder is greater than or equal to 0.01 μm and less than or equal to 1 μm, preferably greater than or equal to 0.01 μm and less than or equal to 0.5 μm, further preferably greater than or equal to 0.01 μm and less than or equal to 0.3 μm. Ion-exchange water, an organic additive, and the like are further mixed into the compound powder to form slurry (step S111).
Then, the slurry is poured into a mold provided with a moisture-permeable filter, so that moisture is removed. The mold may be formed using a metal or an oxide and the upper shape thereof is rectangular or rounded. The mold can be provided with one or more holes at the bottom. With the plural holes, moisture of the slurry can be removed rapidly. A porous resin, cloth, or the like may be used for the filter.
Moisture is removed from the slurry in such a manner that water is removed under reduced pressure through the hole provided at the bottom of the mold into which the slurry is poured. Next, the slurry from which moisture has been removed under reduced pressure is naturally dried. Thus, the slurry from which moisture has been removed is molded into the internal shape of the mold (step S113).
Then, second baking is performed on the molded body in an oxygen (O2) atmosphere at a temperature of 1400° C. (step S114). Through the above-described steps, the sputtering target can be obtained using a wet method.
Next, a method for forming the sputtering target using a dry method is described. The raw materials for the sputtering target are weighed, and then, the raw materials are ground and mixed with a ball mill or the like to obtain compound powder (step S121).
The compound powder obtained is spread over a mold, and pressure is applied thereto with a pressing machine, whereby the raw material powder is molded to obtain a molded body (step S122).
The obtained molded body is placed in a heating apparatus such as an electric furnace and baked in an oxygen (O2) atmosphere at a temperature of 1400° C. (step S123). Note that in this embodiment, a method in which a molding step and a baking step are separated as in step S122 and step S123 is referred to as a cold press method. As a comparison example of a cold press method, a hot press method in which a molding step and a baking step are concurrently performed is described below.
First, the above-described steps up to step S121 are performed. The compound powder obtained is spread over the mold, and pressure is applied with a pressing machine to the compound powder provided on the inner side of the mold while the mold is heated in an argon (Ar) atmosphere at a temperature of 1000° C. In this manner, pressure is applied to the compound powder with the compound powder baked, whereby the compound powder can be molded to obtain a molded body (step S125).
Here, a method for using a sputtering target containing InGaZnO4 is described.
Deposition of a CAAC-OS film using the sputtering target containing InGaZnO4 is described below in detail. First, an ion collides with a sputtering target to separate a sputtered particle having crystallinity. A crystal grain which is included in the sputtering target has a cleavage plane which is parallel to a surface of the sputtering target. The crystal grain has a portion with a weak interatomic bond. At the time of collision of the ion with the crystal grain, the weak interatomic bond is cut. Accordingly, the sputtered particle is separated along the cleavage plane and the portion with the weak interatomic bond to have a flat-plate-like shape.
Alternatively, part of the crystal grain is separated along the cleavage plane as a particle and exposed to plasma, so that a bond is cut from the portion with the weak interatomic bond; as a result, a plurality of sputtered particles is generated.
When an oxygen cation is used as the ion, plasma damage at the deposition can be alleviated. Thus, when the ion collides with the surface of the sputtering target, a reduction in degree of crystallinity of the sputtering target can be prevented.
It is preferable that the separated sputtered particles be positively charged. There is no particular limitation on a timing of when the sputtered particle is positively charged, but it is preferably positively charged by receiving an electric charge when an ion collides. Alternatively, in the case where plasma is generated, the sputtered particle is preferably exposed to plasma to be positively charged. Further alternatively, an ion which is an oxygen cation is preferably bonded to a side surface, a top surface, or a bottom surface of the sputtered particle, whereby the sputtered particle is positively charged.
Next, a situation where a sputtered particle is deposited on a deposition surface (a surface where a film is to be formed) is described in detail.
In deposition, the deposition surface has a surface on which several sputtering particles are deposited. In the case where the sputtered particles are positively charged, the sputtered particles are deposited in a region of the deposition surface, where no sputtered particle has been deposited yet. This is because the sputtered particles which are positively charged repel with each other.
Further, c-axes of crystals of the sputtered particles deposited in the above manner are aligned in a direction perpendicular to the deposition surface; accordingly, a CAAC-OS film is formed as the oxide film.
To form the CAAC-OS film, it is preferable to increase the substrate temperature in deposition, and the substrate temperature is higher than or equal to 200° C. and lower than or equal to 550° C. Note that the CAAC-OS film is a film which contains a large amount of oxygen and has a reduced number of oxygen vacancies.
In the case where steps for sequentially stacking the three oxide semiconductor films in FIG. 1A in which a buried channel is formed are performed successively without exposure to the air, a manufacturing apparatus a top view of which is illustrated in FIG. 11 may be used.
The manufacturing apparatus illustrated in FIG. 11 is single wafer multi-chamber equipment, which includes three sputtering devices 10a, 10b, and 10c, a substrate supply chamber 11 provided with three cassette ports 14 for holding a process substrate, load lock chambers 12a and 12b, a transfer chamber 13, substrate heating chambers 15 and 16, and the like. Note that a transfer robot for transferring a substrate to be treated is provided in each of the substrate supply chamber 11 and the transfer chamber 13. The atmospheres of the sputtering devices 10a, 10b, and 10c, the transfer chamber 13, and the substrate heating chambers 15 and 16 are preferably controlled so as to hardly contain hydrogen and moisture (i.e., as an inert atmosphere, a reduced pressure atmosphere, or a dry air atmosphere). For example, a preferable atmosphere is a dry nitrogen atmosphere in which the dew point of moisture is −40° C. or lower, preferably −50° C. or lower. An example of a procedure of the manufacturing steps with use of the manufacturing apparatus illustrated in FIG. 11 is as follows. The process substrate is transferred from the substrate supply chamber 11 to the substrate heating chamber 15 through the load lock chamber 12a and the transfer chamber 13; moisture attached to the process substrate is removed by vacuum baking in the substrate heating chamber 15; the process substrate is transferred to the sputtering device 10c through the transfer chamber 13; and a first oxide semiconductor film S1 is deposited in the sputtering device 10c. Then, the process substrate is transferred to the sputtering device 10a through the transfer chamber 13 without exposure to air, and a second oxide semiconductor film S2 is deposited in the sputtering device 10a. Then, the process temperature is transferred to the sputtering device 10b through the transfer chamber 13, and a third oxide semiconductor film S3 is deposited in the sputtering device 10b. If needed, the process substrate is transferred to the substrate heating chamber 16 through the transfer chamber 13 without exposure to air and the heat treatment is performed. As described above, with use of the manufacturing apparatus illustrated in FIG. 11, a manufacturing process can proceed without exposure to air. Further, with of the sputtering devices in the manufacturing apparatus in FIG. 11, a process performed without exposure to the air can be achieved by change of the sputtering target. As the sputtering devices in the manufacturing apparatus in FIG. 11, a parallel plate sputtering device, an ion beam sputtering device, a facing-target sputtering device, or the like may be used. In a facing-target type sputtering device, an object surface is separated from plasma and thus damage in deposition is small; therefore, a CAAC-OS film having high crystallinity can be formed.
A high purity gas having a low concentration of impurities such as hydrogen, water, a hydroxyl group, and hydride is used as a deposition gas for depositing the oxide semiconductor film in each of the sputtering devices 10a, 10b, and 10c. 
The heat treatment may be performed in the substrate heating chamber 16 under reduced pressure, in a nitrogen atmosphere, in an oxygen atmosphere, in ultra-dry air (air in which the moisture amount is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, more preferably less than or equal to 10 ppb, in the measurement with the use of a dew point meter in the cavity ring down laser spectroscopy (CRDS) system), or in a rare gas (argon, helium, or the like) atmosphere. It is preferable that water, hydrogen, and the like be not contained in the nitrogen atmosphere, in the oxygen atmosphere, in the ultra-dry air, in the rare gas atmosphere, or the like. It is also preferable that the purity of nitrogen, oxygen, or the rare gas 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).
The semiconductor device of one embodiment of the present invention includes a transistor including an oxide semiconductor film or a circuit including the transistor. For example, an electronic device which includes, as a component, a semiconductor integrated circuit including an LSI, a CPU, a power device mounted in a power circuit, a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display panel; or a light-emitting display device including a light-emitting element is also included in the category of the semiconductor device.
According one embodiment of the present invention, a highly reliable semiconductor device including an oxide semiconductor exhibiting stable electrical characteristics can be provided. A highly reliable semiconductor device can be manufactured.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are cross-sectional views illustrating one embodiment of the present invention.
FIG. 2A is data which shows energies from the vacuum level to the conduction band and FIG. 2B shows a band structure formed on the basis of the data of FIG. 2A.
FIG. 3 is an example of a band structure illustrating one embodiment of the present invention.
FIGS. 4A and 4B are cross-sectional views illustrating manufacturing steps of one embodiment of the present invention.
FIG. 5 is a cross-sectional view illustrating one embodiment of the present invention.
FIGS. 6A and 6B are each a circuit diagram illustrating one embodiment of the present invention.
FIGS. 7A to 7C are block diagrams illustrating embodiments of the present invention.
FIGS. 8A to 8C are a diagram illustrating a display device of one embodiment of the present invention and circuit diagrams illustrating pixels.
FIG. 9 is a top view illustrating a display device.
FIG. 10 is a cross-sectional view illustrating a display device.
FIG. 11 is a top view illustrating one example of a manufacturing apparatus of a semiconductor device.
FIGS. 12A to 12C illustrate an electronic appliance.
FIGS. 13A to 13C illustrate electronic appliances.
FIG. 14 is a flowchart showing manufacturing steps of a sputtering target of one embodiment of the present invention.



DETAILED DESCRIPTION OF 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 description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments.
Embodiment 1
In this embodiment, one embodiment of a semiconductor device is described with reference to FIGS. 4A and 4B. In this embodiment, a transistor 311 including a first semiconductor material is provided in a lower portion, and a transistor 411 including a second semiconductor material is provided in an upper portion.
Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material such as silicon can easily operate at high speed. On the other hand, a transistor including an oxide semiconductor enables charge to be held for a long time owing to its characteristics.
As a substrate 300 used in the semiconductor device, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, a silicon on insulator (SOI) substrate, or the like can be used. A channel formation region of the transistor can be formed in or over the semiconductor substrate. The semiconductor device in FIGS. 4A and 4B is an example in which the channel formation region is formed in the semiconductor substrate to form the transistor in the lower portion.
In the semiconductor device in FIGS. 4A and 4B, a single crystal silicon substrate is used as the substrate 300, and the transistor 311 is formed using the single crystal silicon substrate. Single crystal silicon is used as the first semiconductor material. The transistor 311 is a p-channel transistor. The transistor 311 is manufactured by a known manufacturing method. In addition, an n-channel transistor can also be manufactured by the known method on the substrate on which the transistor 311 is formed. A complementary metal oxide semiconductor (CMOS) circuit can be formed by combining an n-channel transistor and a p-channel transistor as appropriate, whereby a variety of circuits can be provided.
The transistor 311 includes a channel formation region, a source region, a drain region, a gate insulating film 303, and a gate electrode 301. An element isolation insulating film 302 is provided to surround the transistor 311, and an electrode layer 304 which is electrically connected to the transistor 311 is provided over the element isolation insulating film 302.
A first interlayer insulating film 312 is formed to cover the electrode layer 304 and the transistor 311. After an opening reaching the electrode layer 304 is formed in the first interlayer insulating film 312 and a conductive film is deposited, planarization is performed by polishing treatment (e.g., chemical mechanical polishing (CMP)). A first wiring layer 306 is formed in the opening of the first interlayer insulating film 312. Then, a conductive film is formed over the first wiring layer 306 and the first interlayer insulating film 312. After that, the conductive film is selectively removed using a mask to form a second wiring layer having a desired shape. In this embodiment, the second wiring layer has a three-layer structure in which a first conductive film 307a that is a titanium film, a second conductive film 307b that is an aluminum film, and a third conductive film 307c that is a titanium film are formed in this order.
Next, a second interlayer insulating film 308 covering the second wiring layer is formed and a barrier layer 310 is stacked. Then, an opening reaching the second wiring layer is formed in the second interlayer insulating film 308 and the barrier layer 310. After that, a conductive film is formed and planarized by CMP or the like, so that a third wiring layer 309 is formed in the opening.
The barrier layer 310 is provided between the transistor 311 in the lower portion and a transistor 411 in the upper portion. The barrier layer 310 is provided in order to prevent impurities such as hydrogen contained in the vicinity of the transistor 311 from diffusing into the transistor 411 in the upper portion. As the barrier layer 310, a material film which releases a small amount of hydrogen or does not release hydrogen in later heat treatment is preferably used. Thus, the barrier layer 310 is preferably formed using a dense inorganic insulating film (e.g., an aluminum oxide film or a silicon nitride film) having a high blocking property of impurities and the like. Specifically, a silicon nitride film which is deposited by a plasma CVD method with supply of a mixed gas of silane (SiH4) and nitrogen (N2) is used as the barrier layer 310.
Next, the transistor 411 is formed over the barrier layer 310.
A conductive film is formed over the barrier layer and selectively etched to form a fourth wiring layer 434 and a gate electrode 491.
After that, an insulating film 435 covering the gate electrode 491 is formed and planarized by CMP or the like.
Then, the first gate insulating film 402a and the second gate insulating film 402b are formed. The first gate insulating film 402a and the second gate insulating film 402b each have a thickness greater than or equal to 1 nm and less than or equal to 100 nm and can be formed by a sputtering method, an MBE method, a CVD method, a PLD method, an ALD method, or the like as appropriate. A silicon nitride film is used as the first gate insulating film 402a. The second gate insulating film 402b is preferably formed using an oxide insulating film including silicon oxide, gallium oxide, aluminum oxide, silicon oxynitride, silicon nitride oxide, hafnium oxide, tantalum oxide, or the like. Because the second gate insulating film 402b is in contact with an oxide semiconductor film to be formed later, the second gate insulating film 402b preferably contains oxygen in excess of the stoichiometric composition in the layer (the bulk).
A 20-nm-thick silicon oxide film may be deposited as the second gate insulating film 402b by a plasma CVD method, and after the deposition, the silicon oxide film may be subjected to radical oxidation treatment by microwave plasma treatment for repairing oxygen vacancies. For example, a high-density plasma apparatus is used under a condition where the power is 3800 W using a power source of 2.45 GHz, the pressure is 106.67 Pa, the substrate temperature is 325° C., the flow rate of argon is 900 sccm, and the flow rate of oxygen is 5 sccm. Here, the high-density plasma apparatus refers to an apparatus which can achieve a plasma density higher than or equal to 1×1011/cm3. For example, plasma is generated by applying a microwave power of 3 kW to 6 kW. Before formation of the second gate insulating film 402b, plasma treatment in which nitrous oxide (N2O) and a rare gas are introduced may be performed using a high-density plasma apparatus.
Next, a stack of oxide semiconductor films is formed over the second gate insulating film 402b. 
The first oxide semiconductor film 403a and the second oxide semiconductor film 403b are formed, subjected to heat treatment, and then selectively etched using a mask.
In this embodiment, the first oxide semiconductor film 403a is deposited at a substrate temperature of room temperature using a target having an atomic ratio of In:Ga:Zn=1:3:2. The first oxide semiconductor film 403a has a thickness greater than or equal to 10 nm and less than or equal to 40 nm, preferably greater than or equal to 20 nm and less than or equal to 30 nm. An increase in the thickness of the first oxide semiconductor film 403a can prevent diffusion of Si from the base film (insulating film containing silicon). The second oxide semiconductor film 403b having a crystalline structure is deposited at a substrate temperature of 400° C. using a target having an atomic ratio of In:Ga:Zn=1:1:1. The second oxide semiconductor film 403b is a film including crystals which are c-axis-aligned in a direction substantially perpendicular to the surface and preferably a CAAC-OS film. The second oxide semiconductor film 403b has a thickness greater than or equal to 5 nm and less than or equal to 10 nm.
A deposition temperature of the second oxide semiconductor film 403b is higher than or equal to 400° C. and lower than or equal to 550° C., preferably higher than or equal to 450° C. and lower than or equal to 500° C. Note that the deposition temperature is within a range of temperatures at which the already formed wiring layer can withstand. The heat treatment is performed in reduced pressure or a nitrogen atmosphere at a temperature higher than or equal to 300° C. and lower than or equal to 600° C., preferably higher than or equal to 300° C. lower than or equal to 500° C., further preferably higher than or equal to 350° C. and lower than or equal to 450° C., to remove excess hydrogen (including water and a hydroxyl group) (to perform dehydration or dehydrogenation) in the oxide semiconductor films. Then, a high-purity oxygen gas or ultra dry air (the amount of moisture is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, further preferably less than or equal to 10 ppb, when measured with a dew point meter of a CRDS system) is introduced to the same furnace while the heating temperature is kept or slowly lowered after the heat treatment. Owing the effect of the oxygen gas, oxygen which is a main component of the oxide semiconductor and which has been reduced at the same time as the step for removing impurities by dehydration or dehydrogenation can be supplied.
Next, the third oxide semiconductor film 403c is formed to be in contact with and cover the top and side surfaces of the second oxide semiconductor film 403b. In addition, heat treatment for removing excess hydrogen (including water and a hydroxyl group) (performing dehydration or dehydrogenation) in the third oxide semiconductor film 403c may be performed, and oxygen may be supplied to the third oxide semiconductor film 403c using an oxygen gas.
The third oxide semiconductor film 403c is deposited at a substrate temperature of room temperature using a target having an atomic ratio of In:Ga:Zn=1:3:2. The third oxide semiconductor film 403c is deposited under substantially the same deposition conditions as those of the first oxide semiconductor film 403a and the entire third oxide semiconductor film 403c has substantially uniform film quality. The third oxide semiconductor film 403c has a thickness greater than or equal to 10 nm and less than or equal to 40 nm, preferably greater than or equal to 20 nm and less than or equal to 30 nm. A cross-sectional view at this stage is illustrated in FIG. 4A.
Then, a conductive film is formed over the third oxide semiconductor film 403c and processed to form the electrode layer 405a and the electrode layer 405b (including a wiring formed with the same layer). Then, wet etching is performed using diluted hydrofluoric acid to reduce the thickness of part of the third oxide semiconductor film 403c. 
Next, an insulating film 407 is formed to cover the electrode layers 405a and 405b and a stack 403 of oxide semiconductor films which is exposed. The insulating film 407 can be formed with a single layer or a stack of layers using one or more of the following films formed by a plasma CVD method or a sputtering method: a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, a silicon nitride oxide film, and the like. It is preferable that a first oxide insulating film be formed to cover the stack 403 of oxide semiconductor films under a condition which causes little plasma damage in order to reduce plasma damage, and that a second oxide insulating film be stacked thereover under a deposition condition which allows the film to contain a large amount of oxygen. Note that it is preferable that an oxide insulating film be formed as the insulating film 407 in contact with the stack 403 of oxide semiconductor films because the oxide insulating film can supply oxygen to the stack 403 of oxide semiconductor films. In this embodiment, an oxide insulating film containing nitrogen is used as the insulating film 407.
Next, oxygen is added to the insulating film 407 by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. The timing of supply of oxygen to the stack 403 of oxide semiconductor films is not particularly limited as long as it is after the formation of the stack 403 of oxide semiconductor films. The step of introducing oxygen may be performed a plurality of times.
After the formation of the insulating film 407, heat treatment is performed. The stack 403 of oxide semiconductor films is damaged by being exposed to plasma at the time of etching or deposition and thus includes oxygen vacancies due to the damage; therefore, the heat treatment for repairing the damage to the oxide semiconductor is performed, whereby oxygen is supplied and thus the oxygen vacancies are reduced. The temperature of the heat treatment is typically higher than or equal to 200° C. and lower than or equal to 450° C. The heat treatment allows nitrogen contained in the oxide insulating film containing nitrogen to be released. Note that the heat treatment can eliminate water, hydrogen, and the like from the oxide insulating film containing nitrogen. In this embodiment, the heat treatment is performed in a mixed atmosphere of nitrogen and oxygen at 350° C. for 1 hour. By the heat treatment, hydrogen atoms and oxygen atoms included in the oxide semiconductor films are bonded in the oxide semiconductor films at an interface between the oxide semiconductor film and the oxide insulating film containing nitrogen, in the oxide insulating film containing nitrogen, or at the surface of the oxide insulating film containing nitrogen to generate water molecules, and the water molecules are desorbed from the oxide insulating film containing nitrogen. In the oxide semiconductor films, portions from which oxygen atoms are desorbed become oxygen vacancies; however, a large number of oxygen atoms which are contained in the oxide insulating film containing nitrogen in excess of that of the stoichiometric composition move to the oxygen vacancies to reduce the oxygen vacancies.
Further, it is preferable that the number of defects be small in the oxide insulating film containing nitrogen used as the insulating film 407, and typically, the spin density of a signal at g=2.001 which is due to dangling bonds of silicon by ESR measurement be lower than or equal to 3×1017 spins/cm3. This is because when the density of defects in the oxide insulating film containing nitrogen is high, oxygen may be bonded to the defect and the amount of oxygen that passes through the oxide insulating film containing nitrogen is decreased.
In this manner, nitrogen, hydrogen, or water is desorbed from the oxide semiconductor films by the heat treatment after the formation of the insulating film 407, whereby the nitrogen, hydrogen, or water content of the films can be reduced to approximately one tenth.
Next, the protective insulating film 408 is formed over the insulating film 407. An aluminum oxide film or a silicon nitride film is formed as the protective insulating film 408. The protective insulating film 408 has a role in preventing mixing of impurities such as hydrogen and moisture from the outside.
Through the above manufacturing process, the transistor 411 of this embodiment can be formed.
In the transistor described in this embodiment, the second oxide semiconductor film 403b which functions as a current path (channel) of the transistor is positioned between the first and third oxide semiconductor films 403a and 403c which have lower carrier density than the second oxide semiconductor film 403b. Accordingly, the channel can be separated from the interface of the insulating film in contact with the stack 403 of oxide semiconductor films, so that a buried channel can be obtained. Further, the second oxide semiconductor film 403b can contain as much oxygen as possible; therefore, the number of generated oxygen vacancies is small and thus the reliability of the transistor is improved.
Embodiment 2
In this embodiment, an example which is partly different from Embodiment 1 is described below. The example of this embodiment differs from Embodiment 1 in the way to form the stack 403 of oxide semiconductor films, the way to supply oxygen, and the like.
The steps from formation of the transistor 311 using the first oxide semiconductor material in the lower portion up to formation of the second interlayer insulating film 308, the barrier layer 310, and the third wiring layer 309 are the same as those in Embodiment 1, and thus details of the steps are not described here.
A conductive film is formed over the barrier layer and selectively etched to form the fourth wiring layer 434 and the gate electrode 491.
Then, the insulating film 435 covering the gate electrode 491 is formed and planarized by CMP or the like. As the insulating film 435, an oxide insulating film containing nitrogen is used.
Then, the first gate insulating film 402a and the second gate insulating film 402b are formed. An oxide insulating film containing nitrogen is used as the first gate insulating film 402a. 
A 20-nm-thick silicon oxide film may be deposited as the second gate insulating film 402b by a plasma CVD method, and after the deposition, the silicon oxide film may be subjected to radical oxidation treatment by microwave plasma treatment for repairing oxygen vacancies.
Next, with the use of a manufacturing apparatus illustrated in FIG. 11, a stack of oxide semiconductor films is deposited over the second gate insulating film 402b without exposure to the air
The first oxide semiconductor film 403a, the second oxide semiconductor film 403b, and the third oxide semiconductor film 403c are formed.
A deposition temperature of each of the first, second, and third oxide semiconductor films 403a, 403b, and 403c is higher than or equal to room temperature and lower than or equal to 550° C., preferably higher than or equal to 200° C. and lower than 400° C. In order to make the second oxide semiconductor film 403b a CAAC-OS film, the deposition temperature is set to higher than or equal to 200° C. and lower than or equal to 550° C. In order to make the first and third oxide semiconductor films 403a and 403c have a low degree of crystallinity, a deposition temperature of each of the first and third oxide semiconductor films 403a and 403c is set to higher than or equal to room temperature and lower than 200° C.
Then, selective etching is performed using a mask, so that the island-shaped stack 403 of oxide semiconductor films is formed.
Then, a conductive film is formed over the third oxide semiconductor film 403c and processed to form the electrode layer 405a and the electrode layer 405b (including a wiring formed with the same layer). Then, wet etching is performed using diluted hydrofluoric acid to reduce the thickness of part of the third oxide semiconductor film 403c. 
Next, an insulating film 407 is formed to cover the electrode layers 405a and 405b and the stack 403 of oxide semiconductor films which is exposed. The insulating film 407 preferably has a stacked-layer structure. It is preferable that a first oxide insulating film be formed to cover the stack 403 of oxide semiconductor films under a condition which causes little plasma damage in order to reduce plasma damage, and that a second oxide insulating film be stacked thereover under a deposition condition which allows the film to contain a large amount of oxygen.
Oxygen may be added to the insulating film 407 by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. The timing of supply of oxygen to the stack 403 of oxide semiconductor films is not particularly limited as long as it is after the formation of the stack 403 of oxide semiconductor films. The step of introducing oxygen may be performed a plurality of times.
After formation of the insulating film 407, heat treatment is performed at a temperature higher than or equal to 350° C. and lower than or equal to 450° C. Oxygen contained in the insulating film 435, the first gate insulating film 402a, and the second gate insulating film 402b is supplied to the stack 403 of oxide semiconductor films. Alternatively, the oxide insulating film containing nitrogen which is described in Embodiment 1 may be used as the insulating film 407 and oxygen contained in the insulating film 407 may be supplied to the stack 403 of oxide semiconductor films.
Next, the protective insulating film 408 is formed over the insulating film 407. An aluminum oxide film or a silicon nitride film is formed as the protective insulating film 408. The protective insulating film 408 has a role in preventing mixing of impurities such as hydrogen and moisture from the outside.
Through the above manufacturing process, as illustrated in FIG. 5, a transistor 412 of this embodiment can be formed over the transistor 311.
This embodiment can be freely combined with Embodiment 1.
Embodiment 3
As another example of a semiconductor device including the transistor described in Embodiment 1 or 2, a circuit diagram of a NOR circuit, which is a logic circuit, is illustrated in FIG. 6A. FIG. 6B illustrates a circuit diagram of a NAND circuit.
In the NOR circuit illustrated in FIG. 6A, p-channel transistors 801 and 802 each have a structure similar to that of the transistor 311 in FIG. 4B in that a single crystal silicon substrate is used for a channel formation region, and n-channel transistors 803 and 804 each have a structure similar to that of the transistor 411 in FIG. 4B and that of the transistor 412 in FIG. 5 in that an oxide semiconductor film is used for a channel formation region.
In the NOR circuit in FIG. 6A, conductive layers controlling electrical characteristics of the transistors may be provided to overlap with gate electrode layers with oxide semiconductor films provided therebetween in the transistors 803 and 804. By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors 803 and 804 are increased, so that the transistors can be normally off.
In the NAND circuit in FIG. 6B, p-channel transistors 811 and 814 each have a structure similar to that of the transistor 311 in FIGS. 4A and 4B, and n-channel transistors 812 and 813 each have a structure similar to that of the transistor 411 in FIG. 4B and that of the transistor 412 in FIG. 5 in that an oxide semiconductor film is used for a channel formation region.
In the NAND circuit illustrated in FIG. 6B, conductive layers controlling electrical characteristics of the transistors are provided to overlap with gate electrode layers with oxide semiconductor films provided therebetween in the transistors 812, and 813. By controlling the potential of the conductive layer to GND, for example, the threshold voltages of the transistors 812 and 813 are increased, so that the transistors can be normally off.
By applying a transistor including an oxide semiconductor for a channel formation region and having extremely small off-state current to the semiconductor device in this embodiment, power consumption of the semiconductor device can be sufficiently reduced.
A semiconductor device which is miniaturized, is highly integrated, and has stable and excellent electrical characteristics by stacking semiconductor elements including different semiconductor materials and a method for manufacturing the semiconductor device can be provided.
The NOR circuit and the NAND circuit including the transistors described in Embodiment 2 are described as examples in this embodiment; however, one embodiment of the present invention is not limited to the circuits, and an AND circuit, an OR circuit, or the like can be formed using the transistors described in Embodiment 1 or 2. For example, a semiconductor device (storage device) in which stored data can be held even when power is not supplied and which has an unlimited number of times of writing with the transistors described in Embodiment 1 or 2 can be manufactured.
Embodiment 4
In this embodiment, a central processing unit (CPU) at least part of which includes one of the transistor 411 in FIG. 4B and the transistor 412 in FIG. 5 is described as an example of a semiconductor device.
FIG. 7A is a block diagram illustrating a specific structure of a CPU. The CPU illustrated in FIG. 7A includes an arithmetic logic unit (ALU) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface (Bus UF) 1198, a rewritable ROM 1199, and an ROM interface (ROM I/F) 1189 over a substrate 1190. A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may each be provided over a separate chip. Obviously, the CPU illustrated in FIG. 7A is only an example in which the structure is simplified, and an actual CPU has various structures depending on the application.
An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.
The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 in accordance with the state of the CPU.
The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 based on a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.
In the CPU illustrated in FIG. 7A, a memory cell is provided in the register 1196.
In the CPU illustrated in FIG. 7A, the register controller 1197 selects operation of holding data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is held by a flip-flop or by a capacitor in the memory cell included in the register 1196. When data holding by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196. When data holding by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.
The power supply can be stopped by providing a switching element between a memory cell group and a node to which a power supply potential VDD or a power supply potential VSS is supplied, as illustrated in FIG. 7B or FIG. 7C. Circuits illustrated in FIGS. 7B and 7C are described below.
FIGS. 7B and 7C each illustrate an example of a memory circuit configuration in which one of the transistor 411 in FIG. 4B and the transistor 412 in FIG. 5 is used as a switching element which controls supply of a power supply potential to a memory cell.
The memory device illustrated in FIG. 7B includes a switching element 1141 and a memory cell group 1143 including a plurality of memory cells 1142. Each of the memory cells 1142 included in the memory cell group 1143 is supplied with the high-level power supply potential VDD via the switching element 1141. Further, each of the memory cells 1142 included in the memory cell group 1143 is supplied with a potential of a signal IN and the low-level power supply potential VSS.
In FIG. 7B, one of the transistor 411 in FIG. 4B and the transistor 412 in FIG. 5 is used as the switching element 1141, and the switching of the transistor is controlled by a signal Sig A supplied to a gate electrode thereof.
Note that FIG. 7B illustrates the structure in which the switching element 1141 includes only one transistor; however, without limitation thereto, the switching element 1141 may include a plurality of transistors. In the case where the switching element 1141 includes a plurality of transistors which serves as switching elements, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.
Although the switching element 1141 controls the supply of the high-level power supply potential VDD to each of the memory cells 1142 included in the memory cell group 1143 in FIG. 7B, the switching element 1141 may control the supply of the low-level power supply potential VSS.
In FIG. 7C, an example of a memory device in which each of the memory cells 1142 included in the memory cell group 1143 is supplied with the low-level power supply potential VSS via the switching element 1141 is illustrated. The supply of the low-level power supply potential VSS to each of the memory cells 1142 included in the memory cell group 1143 can be controlled by the switching element 1141.
When a switching element is provided between a memory cell group and a node to which the power supply potential VDD or the power supply potential VSS is supplied, data can be held even in the case where an operation of a CPU is temporarily stopped and the supply of the power supply voltage is stopped; accordingly, power consumption can be reduced. Specifically, for example, while a user of a personal computer does not input data to an input device such as a keyboard, the operation of the CPU can be stopped, so that the power consumption can be reduced.
Although the CPU is given as an example, the transistor can also be applied to an LSI such as a digital signal processor (DSP), a custom LSI, or a field programmable gate array (FPGA).
The methods and structures described in this embodiment can be combined as appropriate with any of the methods and structures described in the other embodiments.
Embodiment 5
In this embodiment, a semiconductor device that is one embodiment of the present invention is described using a liquid crystal display device as an example.
FIG. 8A illustrates a structural example of a semiconductor device. The semiconductor device in FIG. 8A includes a pixel portion 100, a scan line driver circuit 104, a signal line driver circuit 106, m scan lines 107 which are arranged in parallel or substantially in parallel and whose potentials are controlled by the scan line driver circuit 104, and n signal lines 109 which are arranged in parallel or substantially in parallel and whose potentials are controlled by the signal line driver circuit 106. Further, the pixel portion 100 includes a plurality of pixels 101 arranged in a matrix. Furthermore, capacitor lines 115 arranged in parallel or substantially in parallel are provided along the scan lines 107. Note that the capacitor lines 115 may be arranged in parallel or substantially in parallel along the signal lines 109.
Each scan line 107 is electrically connected to the n pixels 101 in the corresponding row among the pixels 101 arranged in m rows and n columns in the pixel portion 100. Each signal line 109 is electrically connected to the m pixels 101 in the corresponding column among the pixels 101 arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each capacitor line 115 is electrically connected to the n pixels 101 in the corresponding row among the pixels 101 arranged in m rows and n columns. Note that in the case where the capacitor lines 115 are arranged in parallel or substantially in parallel along the signal lines 109, each capacitor line 115 is electrically connected to the m pixels 101 in the corresponding column among the pixels 101 arranged in m rows and n columns.
FIG. 8B is an example of a circuit diagram of the pixel 101 included in the semiconductor device illustrated in FIG. 8A. The pixel 101 in FIG. 8B includes a transistor 103 a gate electrode of which is electrically connected to the scan line 107 and a source electrode of which is electrically connected to the signal line 109, a capacitor 105 one electrode of which is electrically connected to a drain electrode of the transistor 103 and the other electrode of which is electrically connected to the capacitor line 115 which supplies a constant potential, and a liquid crystal element 108. A pixel electrode of the liquid crystal element 108 is electrically connected to the drain electrode of the transistor 103 and the one electrode of the capacitor 105, and an electrode (counter electrode) facing the pixel electrode is electrically connected to a wiring which supplies a counter potential.
The liquid crystal element 108 is an element which controls transmission of light by an optical modulation action of liquid crystal which is positioned between a substrate provided with the transistor 103 and the pixel electrode and a substrate provided with the counter electrode. The optical modulation action of liquid crystal is controlled by an electric field applied to the liquid crystal (including a horizontal electric field, a vertical electric field, and a diagonal electric field).
Next, a specific structural example of the pixel 101 of the liquid crystal display device will be described. FIG. 9 is a top view of the pixel 101. Note that in FIG. 9, the counter electrode and the liquid crystal element are omitted.
In FIG. 9, the scan line 107 is provided so as to extend in the direction perpendicular or substantially perpendicular to the signal line 109 (in the horizontal direction in FIG. 9). The signal line 109 is provided so as to extend in the direction perpendicular or substantially perpendicular to the scan line 107 (in the vertical direction in FIG. 9). The capacitor line 115 is provided so as to extend in the direction parallel with the scan line 107. The scan line 107 and the capacitor line 115 are electrically connected to the scan line driver circuit 104 (see FIG. 8A), and the signal line 109 is electrically connected to the signal line driver circuit 106 (see FIG. 8A).
The transistor 103 is provided in a region where the scan line 107 and the signal line 109 cross each other. The transistor 103 includes at least a stack 111 of oxide semiconductor films including a channel formation region, a gate electrode, a gate insulating film (not illustrated in FIG. 9), a source electrode, and a drain electrode.
In the stack 111 of the oxide semiconductor films, as illustrated in FIG. 10, a first oxide semiconductor film 111a, a second oxide semiconductor film 111b, and a third oxide semiconductor film 111c are stacked in this order. Materials of the first to third oxide semiconductor films 111a, 111b, and 111c are selected as appropriate so that a well-shaped structure in which the bottom of the conduction band in the second oxide semiconductor film 111b is deeper than the bottoms of the conduction band in the first and third oxide semiconductor films 111a and 111c is obtained. In this embodiment, the first and third oxide semiconductor films 111a and 111c are formed using a target having an atomic ratio of In:Ga:Zn=1:3:2, and the second oxide semiconductor film 111b is formed using a target having an atomic ratio of In:Ga:Zn=1:1:1. The stack 111 of oxide semiconductor films enables formation of a buried channel and a reduction in oxygen vacancies, thereby improving the reliability of the transistor 103.
Further, when the stack 111 of oxide semiconductor films is deposited or subjected to heat treatment under an appropriate condition, the off-state current of the transistor can be significantly reduced; therefore, the power consumption of the semiconductor device can be reduced.
The scan line 107 also functions as a gate electrode of the transistor 103, and the signal line 109 also functions as a source electrode of the transistor 103. A conductive film 113 functions as a drain electrode of the transistor 103 and is electrically connected to a pixel electrode 121 through an opening 117. Note that pixel electrode 121 is illustrated without hatching in FIG. 9. Further, the scan line 107 is represented as it is even when it indicates the gate electrode of the transistor, and the signal line 109 is represented as it is even when it indicates the source electrode of the transistor.
The capacitor 105 is provided in a region which is in the pixel 101 and surrounded by the capacitor lines 115 and the signal lines 109. The capacitor 105 is electrically connected to the capacitor line 115 through a conductive film 125 provided in and over an opening 123. The capacitor 105 includes a stack 119 of oxide semiconductor films, the pixel electrode 121, and an insulating film (not illustrated in FIG. 9) which is formed as a dielectric film over the transistor 103. The oxide semiconductor film 119, the pixel electrode 121, and the dielectric film transmit light; accordingly, the capacitor 105 transmits light.
Thanks to the light-transmitting property of the stack 119 of oxide semiconductor films, the capacitor 105 can be formed large (in a large area) in the pixel 101. For this reason, the semiconductor device can have charge capacity increased while the aperture ratio is not reduced or is improved. Further, by improving the aperture ratio, a semiconductor device having high display quality can be obtained.
Here, the characteristics of a transistor including an oxide semiconductor will be described. The transistor including an oxide semiconductor is an n-channel transistor. Further, oxygen vacancies in the oxide semiconductor might generate carriers, which might degrade the electrical characteristics and reliability of the transistor. For example, in some cases, the threshold voltage of the transistor is shifted in the negative direction, and drain current flows when the gate voltage is 0 V. The characteristics of a transistor in which drain current flows when the gate voltage is 0 V are referred to as normally-on characteristics, whereas the characteristics of a transistor in which substantially no drain current flows when the gate voltage is 0 V are referred to as normally-off characteristics.
It is preferable that defects in the stack 111 of oxide semiconductor films, typically, oxygen vacancies be reduced as much as possible. For example, it is preferable that the spin density of the stack 111 of oxide semiconductor films (the density of defects in the oxide semiconductor films) at a g-value of 1.93 in electron spin resonance spectroscopy in which a magnetic field is applied in parallel with the film surface be reduced to lower than or equal to the lower detection limit of measurement equipment. When the defects typified by oxygen vacancies in the oxide semiconductor films are reduced as much as possible, the transistor 103 can be prevented from being normally on, leading to improvements in the electrical characteristics and reliability of a semiconductor device.
It is preferable that impurities (e.g., hydrogen, moisture, and elements belonging to Group 14) contained in the stack 111 of oxide semiconductor films be reduced as much as possible, and when the oxide semiconductor films are highly purified to be i-type oxide semiconductor films, the transistor 103 can be prevented from being normally on; as a result, the off-state current of the transistor 103 can be significantly reduced. Therefore, a semiconductor device having favorable electrical characteristics can be manufactured. Further, a highly reliable semiconductor device can be manufactured.
Next, FIG. 10 is a cross-sectional view taken along dashed-dotted lines A1-A2 and B1-B2 in FIG. 9.
A cross-sectional structure of the pixel 101 of the liquid crystal display device is as follows. The liquid crystal display device includes an element portion over a substrate 102, an element portion on a substrate 150, and a liquid crystal layer sandwiched between the two element portions.
First, the structure of the element portion formed over the substrate 102 is described. The scan line 107 functioning as the gate electrode of the transistor 103 and the capacitor line 115 over the same surface as the scan line 107 are provided over the substrate 102. A gate insulating film 127 is provided over the scan line 107 and the capacitor line 115. The stack 111 of oxide semiconductor films is provided over a portion of the gate insulating film 127 which overlaps with the scan line 107, and the stack 119 of oxide semiconductor films is provided over the gate insulating film 127. The signal line 109 functioning as the source electrode of the transistor 103 and the conductive film 113 functioning as the drain electrode of the transistor 103 are provided over the stack 111 of oxide semiconductor films and the gate insulating film 127. An opening 123 reaching the capacitor line 115 is formed in the gate insulating film 127, and the conductive film 125 is provided over the opening 123, the gate insulating film 127, and the stack 119 of oxide semiconductor films. An insulating film 129, an insulating film 131, and an insulating film 132 functioning as protective insulating films of the transistor 103 are provided over the gate insulating film 127, the signal line 109, the stack 111 of oxide semiconductor films, the conductive film 113, the conductive film 125, and the stack 119 of oxide semiconductor films. The opening 117 reaching the conductive film 113 is formed in the insulating film 129, the insulating film 131, and the insulating film 132, and the pixel electrode 121 is provided over the opening 117 and the insulating film 132. An insulating film 158 functioning as an alignment film is provided over the pixel electrode 121 and the insulating film 132. Note that a base insulating film may be provided between the substrate 102, and the scan line 107, the capacitor line 115, and the gate insulating film 127.
In the capacitor 105 of this structure, the stack 119 of oxide semiconductor films formed in the same step as the stack 111 of oxide semiconductor films serves as one of a pair of electrodes, the pixel electrode 121 serves as the other of the pair of electrodes, and the insulating film 129, the insulating film 131, and the insulating film 132 serve as dielectric films provided between the pair of electrodes.
The details of the components of the above structure are described below.
Although there is no particular limitation on a material and the like of the substrate 102, it is necessary that the substrate have heat resistance high enough to withstand at least heat treatment performed in a manufacturing process of a semiconductor device. Examples of the substrate are a glass substrate, a ceramic substrate, and a plastic substrate, and as the glass substrate, an alkali-free glass substrate such as a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, or an aluminosilicate glass substrate is preferably used. As the substrate 102, a quartz substrate, a sapphire substrate, or the like can be used.
The scan line 107 and the capacitor line 115 are formed to have a single-layer structure or a stacked-layer structure using any of metal materials such as molybdenum (Mo), titanium (Ti), tungsten (W), tantalum (Ta), aluminum (Al), copper (Cu), chromium (Cr), neodymium (Nd), or scandium (Sc), or an alloy material which contains any of these materials as its main component.
Examples of the scan line 107 and the capacitor line 115 are a single-layer structure using aluminum containing silicon, a two-layer structure in which titanium is stacked over aluminum, a two-layer structure in which titanium is stacked over titanium nitride, a two-layer structure in which tungsten is stacked over titanium nitride, a two-layer structure in which tungsten is stacked over tantalum nitride, a two-layer structure in which copper is stacked over Cu—Mg—Al alloy, and a three-layer structure in which titanium nitride, copper, and tungsten are stacked in this order.
As a material of the scan line 107 and the capacitor line 115, a light-transmitting conductive material which can be used for the pixel electrode 121 can be used.
The scan line 107 and the capacitor line 115 are preferably formed using aluminum or copper, which are low resistance materials. With the use of aluminum or copper, signal delay is reduced, so that higher image quality can be achieved. Note that aluminum has low heat resistance, and thus a defect due to hillocks, whiskers, or migration is easily generated. To prevent migration of aluminum, a layer of a metal material having a higher melting point than aluminum, such as molybdenum, titanium, or tungsten, is preferably stacked over an aluminum layer. Also in the case where copper is used, in order to prevent a defect due to migration and diffusion of copper elements, a layer of a metal material having a higher melting point than copper, such as molybdenum, titanium, or tungsten, is preferably stacked over a copper layer.
The gate insulating film 127 is formed to have a single-layer structure or a stacked-layer structure using, for example, any of insulating materials such as silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, and a Ga—Zn-based metal oxide. In order to improve the characteristics of the interface between the gate insulating film 127 and the stack 111 of oxide semiconductor films, a region in the gate insulating film 127 which is in contact with at least the stack 111 of oxide semiconductor films is preferably formed using an oxide insulating film.
Further, it is possible to prevent outward diffusion of oxygen from the stack 111 of oxide semiconductor films and entry of hydrogen, water, or the like into the stack 111 of oxide semiconductor films from the outside by providing an insulating film having a barrier property against oxygen, hydrogen, water, and the like as the gate insulating film 127. Examples of the insulating film having a barrier property against oxygen, hydrogen, water, and the like are an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, a hafnium oxynitride film, and a silicon nitride film.
The gate insulating film 127 may be formed using a high-k material such as hafnium silicate (HfSiOx), hafnium silicate containing nitrogen (HfSixOyNz), hafnium aluminate containing nitrogen (HfAlxOyNz), hafnium oxide, or yttrium oxide, in which case gate leakage current of the transistor 103 can be reduced.
The gate insulating film 127 preferably has the following stacked-layer structure. It is preferable that a silicon nitride film having fewer defects be provided as a first silicon nitride film, a silicon nitride film from which less hydrogen and ammonia are released be provided as a second silicon nitride film over the first silicon nitride film, and any of the oxide insulating films listed as those used for the gate insulating film 127 be provided over the second silicon nitride film.
As the second silicon nitride film, a nitride insulating film which releases hydrogen molecules less than 5×1021 molecules/cm3, preferably less than or equal to 3×1021 molecules/cm3, more preferably less than or equal to 1×1021 molecules/cm3, and ammonia molecules less than 1×1022 molecules/cm3, preferably less than or equal to 5×1021 molecules/cm3, more preferably less than or equal to 1×1021 molecules/cm3 by thermal desorption spectroscopy is preferably used. The first silicon nitride film and the second silicon nitride film are used as part of the gate insulating film 127, whereby a gate insulating film which has fewer defects and from which less hydrogen and ammonia are released can be formed as the gate insulating film 127. Thus, the amount of hydrogen and nitrogen contained in the gate insulating film 127 which enter the stack 111 of oxide semiconductor films can be reduced.
In the case where the trap level (also referred to as interface level) is present at the interface between the stack 111 of oxide semiconductor films and the gate insulating film or in the gate insulating film in the transistor including an oxide semiconductor, a shift of the threshold voltage of the transistor, typically, a shift of the threshold voltage in the negative direction, and an increase in the subthreshold swing (S value) showing a gate voltage needed for changing the drain current by an order of magnitude when the transistor is turned on are caused. As a result, there is a problem in that electrical characteristics vary among transistors. Therefore, the use of a silicon nitride film having fewer defects as a gate insulating film and provision of an oxide insulating film in contact with the first oxide semiconductor film 111a can reduce a shift of the threshold voltage in the negative direction and minimize an increase in S value.
The thickness of the gate insulating film 127 is greater than or equal to 5 nm and less than or equal to 400 nm, preferably greater than or equal to 10 nm and less than or equal to 300 nm, further preferably greater than or equal to 50 nm and less than or equal to 250 nm.
In this embodiment, the first and third oxide semiconductor films 111a and 111c are films with a low degree of crystallinity, and the second oxide semiconductor film 111b has a crystalline structure. After a film with a low degree of crystallinity is formed as the first oxide semiconductor film 111a and the second oxide semiconductor film 111b is deposited, heat treatment (at a temperature higher than or equal to 200° C. and lower than or equal to 550° C.) is performed, and then the third oxide semiconductor film 111c is formed. This process is preferably performed without exposure to the air, using a manufacturing apparatus illustrated in FIG. 11. Further, also in the stack 119 of oxide semiconductor films which is formed through a process similar to that of the stack 111 of oxide semiconductor films, the first and third oxide semiconductor films are films with a low degree of crystallinity and the second oxide semiconductor film is a film having a crystalline structure with a high degree of crystallinity and is a CAAC-OS film. The thickness of the stack 111 of oxide semiconductor films is greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 1 nm and less than or equal to 50 nm, further preferably greater than or equal to 3 nm and less than or equal to 20 nm.
When the buried channel is formed in the transistor 103, few oxygen vacancies are generated and the reliability of the transistor is improved.
An oxide semiconductor which can be used for the stack 111 of oxide semiconductor films and the stack 119 of oxide semiconductor films has an energy gap of greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, more preferably greater than or equal to 3 eV. The use of such an oxide semiconductor having a wide energy gap can reduce the off-state current of the transistor 103.
The signal line 109 functioning as the source electrode of the transistor 103, the conductive film 113 functioning as the drain electrode of the transistor 103, and the conductive film 125 electrically connecting the stack 119 of oxide semiconductor films and the capacitor line 115 in the capacitor 105 are formed to have a single-layer structure or a stacked-layer structure using a material which can be used for the scan line 107 and the capacitor line 115.
The insulating films 129, 131, and 132 functioning as the protective insulating films of the transistor 103 and the dielectric films in the capacitor 105 are insulating films each formed using a material which can be used for the gate insulating film 127. It is particularly preferable that the insulating films 129 and 131 be oxide insulating films and the insulating film 132 be a nitride insulating film. Further the use of a nitride insulating film as the insulating film 132 can suppress entry of impurities such as hydrogen and water into the transistor 103 (in particular, the second oxide semiconductor film 111b) from the outside. Note that the insulating film 129 is not necessarily provided.
Further, an oxide insulating film in which the oxygen content is higher than that in the stoichiometric composition is preferably used as one of or both the insulating film 129 and the insulating film 131. In that case, oxygen can be prevented from being released from the oxide semiconductor film, and the oxygen contained in an oxygen excess region can enter the oxide semiconductor film to reduce oxygen vacancies. For example, when an oxide insulating film having the following feature is used, oxygen vacancies in the oxide semiconductor film can be reduced. The feature of the oxide insulating film is that the number of oxygen molecules released from the oxide insulating film is greater than or equal to 1.0×1018 molecules/cm3 when measured by thermal desorption spectroscopy (hereinafter referred to as TDS spectroscopy). Note that an oxide insulating film partly including a region in which the oxygen content is higher than that in the stoichiometric composition (oxygen excess region) may be used as one of or both the insulating film 129 and the insulating film 131. When such an oxygen excess region is present in a region overlapping with at least the stack 111 of semiconductor films, oxygen is prevented from being released from the oxide semiconductor film and the oxygen contained in the oxygen excess region can enter the oxide semiconductor film to reduce oxygen vacancies.
In the case where the insulating film 131 is an oxide insulating film in which the oxygen content is higher than that in the stoichiometric composition, the insulating film 129 is preferably an oxide insulating film through which oxygen penetrates. Oxygen which enters the insulating film 129 from the outside does not completely penetrate through the insulating film 129 to be released and part thereof remains in the insulating film 129. Further, there is oxygen which is contained in the insulating film 129 from the first and is released from the insulating film 129 to the outside. Thus, the insulating film 129 preferably has a high coefficient of diffusion of oxygen.
Since the insulating film 129 is in contact with the third oxide semiconductor film 111c, the insulating film 129 is preferably an oxide insulating film through which oxygen penetrates and which has a low interface state with the third oxide semiconductor film 111c. For example, the insulating film 129 is preferably an oxide insulating film having a lower defect density than the insulating film 131. Specifically, the spin density of the oxide insulating film at a g-value of 2.001 (E′-center) measured by electron spin resonance spectroscopy is lower than or equal to 3.0×1017 spins/cm3, preferably lower than or equal to 5.0×1016 spins/cm3. The spin density at a g-value of 2.001 measured by electron spin resonance spectroscopy corresponds to the number of dangling bonds in the insulating film 129.
The insulating film 129 can have a thickness of greater than or equal to 5 nm and less than or equal to 150 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm, more preferably greater than or equal to 10 nm and less than or equal to 30 nm. The insulating film 131 can have a thickness of greater than or equal to 30 nm and less than or equal to 500 nm, preferably greater than or equal to 150 nm and less than or equal to 400 nm.
In the case where a nitride insulating film is used as the insulating film 132, an insulating film having a barrier property against nitrogen is preferably used as one of or both the insulating film 129 and the insulating film 131. For example, a dense oxide insulating film has a barrier property against nitrogen. Specifically, an oxide insulating film which can be etched at a rate of less than or equal to 10 nm per minute when the temperature is 25° C. and 0.5 wt % of fluoric acid is used is preferably used.
In the case where an oxide insulating film containing nitrogen, such as a silicon oxynitride film or a silicon nitride oxide film, is used as one of or both the insulating film 129 and the insulating film 131, the nitrogen concentration measured by SIMS is greater than or equal to the lower limit of measurement by SIMS and less than 3×1020 atoms/cm3, preferably greater than or equal to 1×1018 atoms/cm3 and less than or equal to 1×1020 atoms/cm3. In that case, the amount of nitrogen which enters the stack 111 of oxide semiconductor films included in the transistor 103 can be reduced and the number of defects in the nitrogen-containing oxide insulating film itself can be reduced.
As the insulating film 132, a nitride insulating film where the hydrogen content is low may be provided. The nitride insulating film is as follows, for example: the number of hydrogen molecules released from the nitride insulating film is less than 5.0×1021 molecules/cm3, preferably less than 3.0×1021 molecules/cm3, more preferably less than 1.0×1021 molecules/cm3 when measured by TDS spectroscopy.
The insulating film 132 has a thickness large enough to prevent entry of impurities such as hydrogen and water from the outside. For example, the thickness can be greater than or equal to 50 nm and less than or equal to 200 nm, preferably greater than or equal to 50 nm and less than or equal to 150 nm, more preferably greater than or equal to 50 nm and less than or equal to 100 nm.
The pixel electrode 121 can be provided using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added. Further, the scan line driver circuit 104 and the signal line driver circuit 106 illustrated in FIG. 8A can be formed over the same substrate. In the case where a transistor including an oxide semiconductor film is formed as a transistor which is provided in the scan line driver circuit 104 or the signal line driver circuit 106, an electrode functioning as a back gate may be formed over the oxide semiconductor film in the same step as formation of the pixel electrode 121. When the electrode functioning as a back gate is provided over the transistor in the scan line driver circuit 104 or the signal line driver circuit 106 to overlap with the transistor, the reliability of the transistor can be improved.
Next, the structure of the element portion provided over the substrate 150 is described. A light-blocking film 152 is provided over the substrate 150, an electrode (a counter electrode 154) is provided over the light-blocking film 152 so as to face the pixel electrode 121, and an insulating film 156 which functions as an alignment film is provided over the counter electrode 154.
The light-blocking film 152 prevents the transistor 103 from being irradiated with backlight or light from the outside. The light-blocking film 152 can be formed using a material such as a metal or an organic resin including a pigment and may be provided in a region outside the pixel portion 100, such as over the scan line driver circuit 104 and over the signal line driver circuit 106 (see FIGS. 8A to 8C), as well as over the transistor 103 in the pixel 101.
Note that a coloring film which transmits light with a predetermined wavelength may be provided between the light-blocking films 152 adjacent to each other. Further, an overcoat film may be provided between the counter electrode 154, and the light-blocking films 152 and the coloring film.
The counter electrode 154 is formed using any of the light-transmitting conductive materials given as those used for the pixel electrode 121 as appropriate.
The liquid crystal element 108 includes the pixel electrode 121, the counter electrode 154, and a liquid crystal layer 160. The liquid crystal layer 160 is sandwiched between the insulating film 158 which is provided in the element portion over the substrate 102 and functions as an alignment film and the insulating film 156 which is provided in the element portion over the substrate 150 and functions as an alignment film. Further, the pixel electrode 121 overlaps with the counter electrode 154 with the liquid crystal layer 160 interposed therebetween.
Here, connection of the components included in the pixel 101 described in this embodiment is described with reference to the circuit diagram in FIG. 8C and the cross-sectional view in FIG. 10.
FIG. 8C is an example of a detailed circuit diagram of the pixel 101 included in the semiconductor device illustrated in FIG. 8A. As illustrated in FIG. 8C and FIG. 10, the transistor 103 includes the scan line 107 including the gate electrode 107a, the signal line 109 including the source electrode 109a, and the conductive film 113 including the drain electrode 113a. 
In the capacitor 105, the stack 119 of oxide semiconductor films connected to the capacitor line 115 through the conductive film 125 functions as one electrode; the pixel electrode 121 connected to the conductive film 113 including the drain electrode 113a functions as the other electrode; and the insulating films 129, 131, and 132 provided between the oxide semiconductor film 119 and the pixel electrode 121 function as dielectric films.
The liquid crystal element 108 includes the pixel electrode 121, the counter electrode 154, and the liquid crystal layer 160 provided between the pixel electrode 121 and the counter electrode 154.
Despite having the same structure as the stack 111 of oxide semiconductor films, the stack 119 of oxide semiconductor films in the capacitor 105 functions as the electrode of the capacitor 105. This is because the pixel electrode 121 can function as a gate electrode, the insulating films 129, 131, and 132 can function as gate insulating films, and a capacitor line 115 can function as a source electrode or a drain electrode, so that the capacitor 105 can be operated in a manner similar to that of a transistor and the stack 119 of oxide semiconductor films can be made to be in a conductive state. In other words, the capacitor 105 can serve as a MOS capacitor, the stack 119 of oxide semiconductor films can be made to be in a conductive state so that the stack 119 of oxide semiconductor films can function as one electrode of the capacitor by controlling a potential to be supplied to the capacitor line 115. In this case, the potential to be supplied to the capacitor line 115 is set as follows. The potential of the pixel electrode 121 is changed in the positive direction and the negative direction in order to operate the liquid crystal element 108 (see FIG. 8C). The potential of the capacitor line 115 needs to be constantly lower than the potential to be supplied to the pixel electrode 121 by the threshold voltage of the capacitor 105 (MOS capacitor) or more in order that the capacitor 105 (MOS capacitor) be constantly in a conductive state. In other words, since the stack 119 of oxide semiconductor films has the same structure as the stack 111 of oxide semiconductor films, the potential of the capacitor line 115 should be lower than the potential to be supplied to the pixel electrode 121 by the threshold voltage of the transistor 103 or more. In such a manner, a channel is formed; thus, the capacitor 105 (MOS capacitor) can be made to be in a conductive state constantly.
When an oxide insulating film through which oxygen penetrates and which has fewer interface states between the third oxide semiconductor film 111c and the oxide insulating film is used as the insulating film 129 over the stack 111 of oxide semiconductor films and the stack 119 of oxide semiconductor films, and an oxide insulating film which includes an oxygen excess region or an oxide insulating film in which the oxygen content is higher than that in the stoichiometric composition is used as the insulating film 131, oxygen can be easily supplied to the stack 111 of oxide semiconductor films, the release of oxygen from the stack 111 of oxide semiconductor films can be prevented, and the oxygen contained in the insulating film 131 can enter the stack 111 of oxide semiconductor films to reduce oxygen vacancies in the stack 111 of oxide semiconductor films. Thus, the transistor 103 can be prevented from being normally on and a potential to be supplied to the capacitor line 115 can be controlled so that the capacitor 105 (MOS capacitor) can be constantly in a conductive state; thus, the semiconductor device can have favorable electrical characteristics and high reliability.
The use of a nitride insulating film as the insulating film 132 over the insulating film 131 can suppress entry of impurities such as hydrogen and water into the stack 111 of oxide semiconductor films and the stack 119 of oxide semiconductor films from the outside. Moreover, the use of a nitride insulating film with a low hydrogen content as the insulating film 132 can minimize variations in electrical characteristics of the transistor 103 and the capacitor 105 (MOS capacitor).
Further, the capacitor 105 can be formed large (in a large area) in the pixel 101. Therefore, a semiconductor device in which the aperture ratio is not reduced or is improved and the charge capacitance is increased can be obtained. Further, with the increased aperture ratio, the semiconductor device can have favorable display quality.
Embodiment 6
A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including game machines). Examples of the electronic appliances include display devices of televisions, monitors, and the like, lighting devices, desktop personal computers and laptop personal computers, word processors, image reproduction devices which reproduce still images or moving images stored in recording media such as digital versatile discs (DVDs), portable compact disc (CD) players, radio receivers, tape recorders, headphone stereos, stereos, cordless phone handsets, transceivers, mobile phones, car phones, portable game machines, calculators, portable information terminals, electronic notebooks, e-book readers, electronic translators, audio input devices, cameras such as still cameras and video cameras, electric shavers, high-frequency heating appliances such as microwave ovens, electric rice cookers, electric washing machines, electric vacuum cleaners, air-conditioning systems such as air conditioners, dishwashers, dish dryers, clothes dryers, futon dryers, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, smoke detectors, radiation counters, and medical equipment such as dialyzers. Further, the examples include industrial equipment such as guide lights, traffic lights, belt conveyors, elevators, escalators, industrial robots, and power storage systems. In addition, oil engines, moving objects driven by electric motors using power from the non-aqueous secondary batteries, and the like are also included in the category of electronic appliances. Examples of the moving objects include electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats or ships, submarines, helicopters, aircrafts, rockets, artificial satellites, space probes, planetary probes, spacecrafts, and the like. Specific examples of these electronic appliances are shown in FIGS. 12A to 12C and FIGS. 13A to 13C.
FIGS. 12A and 12B illustrate a tablet terminal that can be folded. In FIG. 12A, the tablet terminal is opened, and includes a housing 9630, a display portion 9631a, a display portion 9631b, a display-mode switching button 9034, a power button 9035, a power-saving-mode switching button 9036, a clip 9033, and an operation button 9038.
A CPU for performing image processing or arithmetic processing is used in the portable device illustrated in FIGS. 12A and 12B. As the CPU, the CPU described in Embodiment 4 can be used, in which case the CPU described in Embodiment 4 is used, power consumption of the portable device can be reduced.
The display device described in Embodiment 5 can be used for the display portions 9631a and 9631b. The use of the display device leads to an improvement in reliability.
A touch panel area 9632a can be provided in a part of the display portion 9631a, in which data can be input by touching displayed operation keys 9638. Note that FIG. 12A shows, as an example, that half of the area of the display portion 9631a has only a display function and the other half of the area has a touch panel function. However, the structure of the display portion 9631a is not limited to this, and all the area of the display portion 9631a may have a touch panel function. For example, all the area of the display portion 9631a can display keyboard buttons and serve as a touch panel while the display portion 9631b can be used as a display screen.
Like the display portion 9631a, part of the display portion 9631b can be a touch panel area 9632b. When a finger, a stylus, or the like touches the place where a button 9639 for switching to keyboard display is displayed in the touch panel, keyboard buttons can be displayed on the display portion 9631b. 
Touch input can be performed concurrently on the touch panel areas 9632a and 9632b. 
The display-mode switching button 9034 allows switching between a landscape mode and a portrait mode, color display and black-and-white display, and the like. With the power-saving-mode switching button 9036, the luminance of display can be optimized in accordance with the amount of external light at the time when the tablet terminal is in use, which is detected with an optical sensor incorporated in the tablet terminal. The tablet terminal may include another detection device such as a sensor for detecting orientation (e.g., a gyroscope or an acceleration sensor) in addition to the optical sensor.
Although the display portion 9631a and the display portion 9631b have the same display area in FIG. 12A, one embodiment of the present invention is not limited to this example. The display portion 9631a and the display portion 9631b may have different areas or different display quality. For example, one of them may be a display panel that can display higher-definition images than the other.
The tablet terminal is closed in FIG. 12B. The tablet terminal includes the housing 9630, a solar battery 9633, a charge/discharge control circuit 9634, a battery 9635, and a DCDC converter 9636. Note that in FIG. 12B, a structure including a battery 9635 and a DCDC converter 9636 is illustrated as an example of the charge/discharge control circuit 9634.
Since the tablet terminal can be folded in two, the housing 9630 can be closed when the tablet is not in use. Thus, the display portions 9631a and 9631b can be protected, thereby providing a tablet terminal with high endurance and high reliability for long-term use.
The tablet terminal illustrated in FIGS. 12A and 12B can have other functions such as a function of displaying various kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, and a function of controlling processing by various kinds of software (programs).
The solar battery 9633, which is attached on a surface of the tablet terminal, can supply electric power to a touch panel, a display portion, an image signal processor, and the like. Note that the solar battery 9633 can be provided on one or both surfaces of the housing 9630 and the battery 9635 can be charged efficiently. When a lithium ion battery is used as the battery 9635, there is an advantage of downsizing or the like.
The structure and operation of the charge/discharge control circuit 9634 illustrated in FIG. 12B are described with reference to a block diagram of FIG. 12C. FIG. 12C illustrates the solar battery 9633, the battery 9635, the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and the display portion 9631. The battery 9635, the DCDC converter 9636, the converter 9637, and the switches SW1 to SW3 correspond to the charge/discharge control circuit 9634 in FIG. 12B.
First, an example of operation in the case where power is generated by the solar battery 9633 using external light is described. The voltage of power generated by the solar battery 9633 is raised or lowered by the DCDC converter 9636 so that a voltage for charging the battery 9635 is obtained. When the display portion 9631 is operated with the power from the solar battery 9633, the switch SW1 is turned on and the voltage of the power is raised or lowered by the converter 9637 to a voltage needed for operating the display portion 9631. In addition, when display on the display portion 9631 is not performed, the switch SW1 is turned off and a switch SW2 is turned on so that charge of the battery 9635 may be performed.
Here, the solar battery 9633 is shown as an example of a power generation means; however, there is no particular limitation on a way of charging the battery 9635, and the battery 9635 may be charged with another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, the battery 9635 may be charged with a non-contact power transmission module that transmits and receives power wirelessly (without contact) to charge the battery or with a combination of other charging means.
In a television device 8000 in FIG. 13A, a display portion 8002 is incorporated in a housing 8001. The display portion 8002 displays an image and a speaker portion 8003 can output sound.
The display device described in Embodiment 5 can be used for the display portion 8002, and in the case of using the display device, the reliability of the display portion 8002 can be improved.
The television device 8000 may be provided with a receiver, a modem, and the like. With the receiver, the television device 8000 can receive general television broadcasting. Furthermore, when the television device 8000 is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed.
In addition, the television device 8000 may include a CPU for performing information communication or a memory. The CPU described in Embodiment 4 can be used in the television device 8000.
In FIG. 13A, an air conditioner including an indoor unit 8200 and an outdoor unit 8204 is an example of an electronic appliance including the CPU of Embodiment 6. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a CPU 8203, and the like. FIG. 13A shows the case where the CPU 8203 is provided in the indoor unit 8200; the CPU 8203 may be provided in the outdoor unit 8204. Alternatively, the CPU 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. By using the CPU described in Embodiment 4 as the CPU in the air conditioner, power consumption can be reduced.
In FIG. 13A, an electric refrigerator-freezer 8300 is an example of an electronic appliance which is provided with the CPU formed using an oxide semiconductor. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a door for a refrigerator 8302, a door for a freezer 8303, a CPU 8304, and the like. In FIG. 13A, the CPU 8304 is provided in the housing 8301. When the CPU described in Embodiment 4 is used as the CPU 8304 of the electric refrigerator-freezer 8300, power saving can be achieved.
FIG. 13B illustrates an example of an electric vehicle which is an example of an electronic appliance. An electric vehicle 9700 is equipped with a secondary battery 9701. The output of the electric power of the secondary battery 9701 is adjusted by a control circuit 9702 and the electric power is supplied to a driving device 9703. The control circuit 9702 is controlled by a processing unit 9704 including a ROM, a RAM, a CPU, or the like which is not illustrated. When the CPU described in Embodiment 4 is used as the CPU in the electric vehicle 9700, power saving can be achieved.
The driving device 9703 includes a DC motor or an AC motor either alone or in combination with an internal-combustion engine. The processing unit 9704 outputs a control signal to the control circuit 9702 based on input data such as data of operation (e.g., acceleration, deceleration, or stop) by a driver or data during driving (e.g., data on an upgrade or a downgrade, or data on a load on a driving wheel) of the electric vehicle 9700. The control circuit 9702 adjusts the electric energy supplied from the secondary battery 9701 in accordance with the control signal of the processing unit 9704 to control the output of the driving device 9703. In the case where the AC motor is mounted, although not illustrated, an inverter which converts direct current into alternate current is also incorporated.
This embodiment can be implemented combining with another embodiment as appropriate.
This application is based on Japanese Patent Application serial no. 2012-178723 filed with Japan Patent Office on Aug. 10, 2012, the entire contents of which are hereby incorporated by reference.

## Claims

1. A semiconductor device comprising:
a gate electrode;
a gate insulating film over the gate electrode;
a first oxide semiconductor film over the gate insulating film;
a second oxide semiconductor film having a crystalline structure over the first oxide semiconductor film; and
a third oxide semiconductor film over the second oxide semiconductor film,
wherein a bottom of a conduction band in the second oxide semiconductor film is deeper from a vacuum level than a bottom of a conduction band in the first oxide semiconductor film and a bottom of a conduction band in the third oxide semiconductor film.

2. The semiconductor device according to claim 1, wherein a concentration of Si contained in each of the first oxide semiconductor film and the third oxide semiconductor film is lower than or equal to 3×1018/cm3.

3. The semiconductor device according to claim 1, wherein a concentration of carbon contained in each of the first oxide semiconductor film and the third oxide semiconductor film is lower than or equal to 3×1018/cm3.

4. The semiconductor device according to claim 1, wherein the second oxide semiconductor film includes a crystal region which is c-axis-aligned in a direction substantially perpendicular to a surface of the second oxide semiconductor film.

5. The semiconductor device according to claim 1, wherein the first oxide semiconductor film and the third oxide semiconductor film each have an indium content of higher than or equal to 1×1019/cm3.

6. The semiconductor device according to claim 1, wherein the second oxide semiconductor film has a higher indium content than the first oxide semiconductor film and the third oxide semiconductor film.

7. The semiconductor device according to claim 1, wherein the first oxide semiconductor film and the third oxide semiconductor film contain indium, zinc, and gallium.

8. The semiconductor device according to claim 1, wherein the second oxide semiconductor film has an absorption coefficient due to a localized state of less than or equal to 3×10−3/cm.

9. The semiconductor device according to claim 1, wherein the third oxide semiconductor film has a lower degree of crystallinity than the second oxide semiconductor film.

10. The semiconductor device according to claim 1, wherein the first oxide semiconductor film has a lower degree of crystallinity than the second oxide semiconductor film.

11. The semiconductor device according to claim 1,
wherein the first oxide semiconductor film has a thickness greater than or equal to 10 nm and less than or equal to 40 nm,
wherein the second oxide semiconductor film has a thickness greater than or equal to 5 nm and less than or equal to 10 nm, and
wherein the third oxide semiconductor film has a thickness greater than or equal to 10 nm and less than or equal to 40 nm.

12. The semiconductor device according to claim 1, further comprising an oxide insulating film over and in contact with the third oxide semiconductor film.

13. The semiconductor device according to claim 1, further comprising an electrode layer over the gate insulating film and in contact with the third oxide semiconductor film.

14. The semiconductor device according to claim 13, wherein a portion of the electrode layer is overlapped with the gate electrode.

15. A semiconductor device comprising:
a first nitride insulating film;
a first oxide semiconductor film over the first nitride insulating film;
a second oxide semiconductor film having a crystalline structure over the first oxide semiconductor film;
a third oxide semiconductor film over the second oxide semiconductor film; and
a second nitride insulating film over the third oxide semiconductor film,
wherein a bottom of a conduction band in the second oxide semiconductor film is deeper from a vacuum level than a bottom of a conduction band in the first oxide semiconductor film and a bottom of a conduction band in the third oxide semiconductor film.

16. The semiconductor device according to claim 15, wherein a concentration of Si contained in each of the first oxide semiconductor film and the third oxide semiconductor film is lower than or equal to 3×1018/cm3.

17. The semiconductor device according to claim 15, wherein a concentration of carbon contained in each of the first oxide semiconductor film and the third oxide semiconductor film is lower than or equal to 3×1018/cm3.

18. The semiconductor device according to claim 15, wherein the second oxide semiconductor film includes a crystal region which is c-axis-aligned in a direction substantially perpendicular to a surface of the second oxide semiconductor film.

19. The semiconductor device according to claim 15, wherein the first oxide semiconductor film and the third oxide semiconductor film each have an indium content of higher than or equal to 1×1019/cm3.

20. The semiconductor device according to claim 15, wherein the second oxide semiconductor film has a higher indium content than the first oxide semiconductor film and the third oxide semiconductor film.

21. The semiconductor device according to claim 15, wherein the first oxide semiconductor film and the third oxide semiconductor film contain indium, zinc, and gallium.

22. The semiconductor device according to claim 15, wherein the second oxide semiconductor film has an absorption coefficient due to a localized state of less than or equal to 3×10−3/cm.

23. The semiconductor device according to claim 15, wherein the third oxide semiconductor film has a lower degree of crystallinity than the second oxide semiconductor film.

24. The semiconductor device according to claim 15, wherein the first oxide semiconductor film has a lower degree of crystallinity than the second oxide semiconductor film.

25. The semiconductor device according to claim 15,
wherein the first oxide semiconductor film has a thickness greater than or equal to 10 nm and less than or equal to 40 nm,
wherein the second oxide semiconductor film has a thickness greater than or equal to 5 nm and less than or equal to 10 nm, and
wherein the third oxide semiconductor film has a thickness greater than or equal to 10 nm and less than or equal to 40 nm.

26. The semiconductor device according to claim 15, further comprising an oxide insulating film over and in contact with the third oxide semiconductor film.

27. The semiconductor device according to claim 15, further comprising a gate electrode, a gate insulating film including the first nitride insulating film over the gate electrode, and an electrode layer over the gate insulating film and in contact with the third oxide semiconductor film.

28. The semiconductor device according to claim 27, wherein a portion of the electrode layer is overlapped with the gate electrode.

29. A semiconductor device comprising:
a gate electrode;
a gate insulating film over the gate electrode;
a first oxide semiconductor film over the gate insulating film;
a second oxide semiconductor film having a crystalline structure over the first oxide semiconductor film;
a third oxide semiconductor film over the second oxide semiconductor film; and
a source electrode and a drain electrode over the third oxide semiconductor film,
wherein the third oxide semiconductor film is in contact with a side surface of the first oxide semiconductor film and a side surface of the second oxide semiconductor film, and
wherein a bottom of a conduction band in the second oxide semiconductor film is deeper from a vacuum level than a bottom of a conduction band in the first oxide semiconductor film and a bottom of a conduction band in the third oxide semiconductor film.

30. The semiconductor device according to claim 29, wherein the second oxide semiconductor film includes a crystal region which is c-axis-aligned in a direction substantially perpendicular to a surface of the second oxide semiconductor film.

31. The semiconductor device according to claim 29, wherein the second oxide semiconductor film has a higher indium content than the first oxide semiconductor film and the third oxide semiconductor film.

32. The semiconductor device according to claim 29, wherein the first oxide semiconductor film and the third oxide semiconductor film contain indium, zinc, and gallium.

33. The semiconductor device according to claim 29, wherein a portion of the source electrode and a portion of the drain electrode are overlapped with the gate electrode.

34. A semiconductor device comprising:
a gate electrode;
a gate insulating film over the gate electrode;
a first oxide semiconductor film over the gate insulating film;
a second oxide semiconductor film having a crystalline structure over the first oxide semiconductor film;
a third oxide semiconductor film over the second oxide semiconductor film; and
a source electrode and a drain electrode over the third oxide semiconductor film,
wherein each of the first oxide semiconductor film and the second oxide semiconductor film has tapered side surfaces,
wherein the third oxide semiconductor film is in contact with the tapered side surfaces of the first oxide semiconductor film and the tapered side surfaces of the second oxide semiconductor film, and
wherein a bottom of a conduction band in the second oxide semiconductor film is deeper from a vacuum level than a bottom of a conduction band in the first oxide semiconductor film and a bottom of a conduction band in the third oxide semiconductor film.

35. The semiconductor device according to claim 34, wherein the second oxide semiconductor film includes a crystal region which is c-axis-aligned in a direction substantially perpendicular to a surface of the second oxide semiconductor film.

36. The semiconductor device according to claim 34, wherein the second oxide semiconductor film has a higher indium content than the first oxide semiconductor film and the third oxide semiconductor film.

37. The semiconductor device according to claim 34, wherein the first oxide semiconductor film and the third oxide semiconductor film contain indium, zinc, and gallium.

38. The semiconductor device according to claim 34, wherein a portion of the source electrode and a portion of the drain electrode are overlapped with the gate electrode.