Semiconductor device and method for manufacturing the same

It is an object to provide a highly reliable semiconductor device including a thin film transistor whose electric characteristics are stable. In addition, it is another object to manufacture a highly reliable semiconductor device at low cost with high productivity. In a semiconductor device including a thin film transistor, a semiconductor layer of the thin film transistor is formed with an oxide semiconductor layer to which a metal element is added. As the metal element, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum is used. In addition, the oxide semiconductor layer contains indium, gallium, and zinc.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.

2. Description of the Related Art

A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor using amorphous silicon has low field effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor using polycrystalline silicon has high field effect mobility, but a crystallization step such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate.

In view of the foregoing, attention has been paid to a technique by which a thin film transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film, and such a transistor is used as a switching element or the like of an image display device.

REFERENCE

Patent Document

SUMMARY OF THE INVENTION

The field effect mobility of a thin film transistor using an oxide semiconductor for a channel formation region is higher than that of a thin film transistor using amorphous silicon. An oxide semiconductor film can be formed by a sputtering method or the like at a temperature of 300° C. or lower. Its manufacturing process is easier than that of a thin film transistor using polycrystalline silicon.

Such an oxide semiconductor is expected to be used for forming a thin film transistor over a glass substrate, a plastic substrate, or the like, and to be applied to a liquid crystal display, an electroluminescent display, electronic paper, or the like.

It is important that electric characteristics of a thin film transistor, such as threshold voltage, should not be varied. In particular, when the photosensitivity of a semiconductor layer is high, electric characteristics of the thin film transistor are varied, which could lower the reliability of a semiconductor device.

It is an object to provide a highly reliable semiconductor device including a thin film transistor whose electric characteristics are stable. In addition, it is another object to manufacture a highly reliable semiconductor device at low cost with high productivity.

In a semiconductor device including a thin film transistor, a semiconductor layer of the thin film transistor is formed using an oxide semiconductor layer containing at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum.

As an oxide semiconductor film used in this specification, a thin film of a material denoted by InMO3(ZnO)m(m>0) is formed, and a thin film transistor in which the thin film is used as a semiconductor layer is manufactured. Note that M denotes one or more metal elements selected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), and cobalt (Co). In addition to the case where only Ga is contained as M, there is a case where Ga and any of the above metal elements other than Ga, for example, Ga and Ni or Ga and Fe are contained as M. Moreover, in the oxide semiconductor, in some cases, a transition metal element such as Fe or Ni or an oxide of the transition metal element is contained as an impurity element in addition to the metal element contained as M. In this specification, the metal element denoted by M and the above impurity element are contained during deposition of the oxide semiconductor film. For example, a target containing the metal element denoted by M and the impurity element is used, and a film of InMO3(ZnO)m(m>0) is formed by a sputtering method.

In this specification, when M is gallium (Ga) in the thin film (layer) of InMO3(ZnO)m(m>0), this thin film is also referred to as an In—Ga—Zn—O-based non-single-crystal film. An amorphous structure is observed as a crystal structure of the In—Ga—Zn—O-based non-single-crystal film by X-ray diffraction (XRD). Note that heat treatment is performed on the In—Ga—Zn—O-based non-single-crystal film of the examined sample at 200° C. to 500° C., typically 300° C. to 400° C., for 10 minutes to 100 minutes after the film is formed by a sputtering method. In addition, a thin film transistor having electric characteristics such as an on/off ratio of 109or more and a mobility of 10 or more at a gate voltage of ±20 V can be manufactured. An In—Ga—Zn—O-based non-single-crystal film formed by a sputtering method using a target in which In2O3:Ga2O3:ZnO is 1:1:1 has a photosensitivity at a wavelength of 450 nm or lower.

At least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum is added to a thin oxide semiconductor layer such as the above-described film of InMO3(ZnO)m(m>0). By selecting a condition and a method for adding the metal element, a region where the metal element is added and a concentration distribution of the metal element in the oxide semiconductor layer can be controlled.

By addition of the metal element to the oxide semiconductor layer, the metal element serves as a recombination center in the oxide semiconductor layer; therefore, the photosensitivity of the oxide semiconductor layer can be lowered. With the low photosensitivity, electric characteristics of the thin film transistor can be stable, which prevents an increase in off current due to variation in threshold voltage or the like.

The metal element becomes a recombination center in the film of InMO3(ZnO)m(m>0) by being added thereto, whereby the photosensitivity of the film can be lowered. As the metal element, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. By adding these metals regardless of the kind of a metal element contained in the film of InMO3(ZnO)m(m>0), electric characteristics of the thin film transistor can be stabilized. In addition, a region where the metal element is added or a concentration distribution of the metal element in the oxide semiconductor layer can be controlled, whereby the photosensitivity can be lowered and electric characteristics of the thin film transistor can be stabilized efficiently.

A method for adding the metal element to the oxide semiconductor layer is not limited to a particular method, and any of various methods, such as a dry method or a wet method (coating method) can be used. Preferably, an ion implantation method or an ion doping method is used.

One embodiment of the structure of the invention disclosed in this specification includes a gate electrode layer; a gate insulating layer; an oxide semiconductor layer containing indium, gallium, and zinc; a source electrode layer; and a drain electrode layer, wherein the oxide semiconductor layer containing indium, gallium, and zinc contains at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum.

Another embodiment of the structure of the invention disclosed in this specification includes a gate electrode layer over a substrate having an insulating surface; a gate insulating layer over the gate electrode layer; an oxide semiconductor layer containing indium, gallium, and zinc over the gate insulating layer; and a source electrode layer and a drain electrode layer over the oxide semiconductor layer containing indium, gallium, and zinc, wherein the oxide semiconductor layer containing indium, gallium, and zinc contains at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum.

The metal element may be added to the entire surface of the oxide semiconductor layer or may be added selectively. For example, a structure may be employed in which the oxide semiconductor layer has a region with a thin thickness, and the metal element is contained in the region with a thin thickness of the oxide semiconductor layer. The metal element is also added to a film other than the oxide semiconductor layer, depending on when the metal element is added in the process or a mask condition. For example, in an inverted staggered thin film transistor, when a metal element is added to an exposed region of an oxide semiconductor layer by using a source electrode layer and a drain electrode layer as masks, the metal element is also added to the source electrode layer and the drain electrode layer.

Another embodiment of the structure of the invention disclosed in this specification includes a gate electrode layer over a substrate having an insulating surface; a gate insulating layer over the gate electrode layer; a source electrode layer and a drain electrode layer over the gate insulating layer; and an oxide semiconductor layer containing indium, gallium, and zinc over the source electrode layer and the drain electrode layer, wherein the oxide semiconductor layer containing indium, gallium, and zinc contains at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum.

Another embodiment of the structure of the invention disclosed in this specification includes the steps of: forming a gate electrode layer over a substrate having an insulating surface; forming a gate insulating layer over the gate electrode layer; forming an oxide semiconductor layer over the gate insulating layer; forming a source electrode layer and a drain electrode layer over the oxide semiconductor layer; and adding a metal element to a region which is not covered with the source electrode layer or the drain electrode layer in the oxide semiconductor layer.

Another embodiment of the structure of the invention disclosed in this specification includes the steps of: forming a gate electrode layer over a substrate having an insulating surface; forming a gate insulating layer over the gate electrode layer; forming an oxide semiconductor layer over the gate insulating layer; adding a metal element to the oxide semiconductor layer; and forming a source electrode layer and a drain electrode layer over the oxide semiconductor layer to which the metal element is added.

Another embodiment of the structure of the invention disclosed in this specification includes the steps of: forming a gate electrode layer over a substrate having an insulating surface; forming a gate insulating layer over the gate electrode layer; forming a source electrode layer and a drain electrode layer over the gate insulating layer; forming an oxide semiconductor layer over the source electrode layer and the drain electrode layer; and adding a metal element to the oxide semiconductor layer.

A semiconductor device and a method for manufacturing a semiconductor device which are disclosed in this specification achieve at least one of the above objects.

In addition, a source region may be provided between the oxide semiconductor layer (also referred to as a first oxide semiconductor layer) and the source electrode layer, and a drain region may be provided between the oxide semiconductor layer and the drain electrode layer. As the source region and the drain region, an oxide semiconductor layer having n-type conductivity (also referred to as a second oxide semiconductor layer) can be used.

In addition, the second oxide semiconductor layer used for the source region and the drain region of the thin film transistor is preferably thinner than the first oxide semiconductor layer used for a channel formation region and preferably has higher conductivity (electrical conductivity) than the first oxide semiconductor layer.

Further, the first oxide semiconductor layer used for the channel formation region has an amorphous structure and the second oxide semiconductor layer used for the source region and the drain region includes a crystal grain (nanocrystal) in an amorphous structure in some cases. The crystal grain (nanocrystal) in the second oxide semiconductor layer used for the source region and the drain region has a diameter of 1 nm to 10 nm, typically about 2 nm to 4 nm.

As the second oxide semiconductor layer used for the source region and the drain region (n+layer), an In—Ga—Zn—O-based non-single-crystal film can be used.

An insulating film may be formed so as to cover the thin film transistor including the oxide semiconductor layer and the source and drain electrode layers and so as to be in contact with the oxide semiconductor layer including the channel formation region.

Since a thin film transistor is easily broken by static electricity and the like, a protection circuit for protecting a driver circuit is preferably provided over the same substrate as a gate line or a source line. The protection circuit is preferably formed using a nonlinear element in which an oxide semiconductor is used.

Note that ordinal numbers such as “first” and “second” are used for convenience. Therefore, they do not denote the order of steps, the stacking order of layers, and particular names which specify the invention in this specification.

Moreover, as a display device including a driver circuit, a light-emitting display device in which a light-emitting element is used and a display device in which an electrophoretic display element is used, which is also referred to as electronic paper, are given in addition to a liquid crystal display device.

In the light-emitting display device in which a light-emitting element is used, a plurality of thin film transistors are included in a pixel portion, and in the pixel portion, there is a region where a gate electrode of a thin film transistor is connected to a source wiring or a drain wiring of another thin film transistor. In addition, in a driver circuit of the light-emitting display device in which a light-emitting element is used, there is a region where a gate electrode of a thin film transistor is connected to a source wiring or a drain wiring of the thin film transistor.

Note that the semiconductor device in this specification means all the devices which can operate by using semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic appliance are all included in the semiconductor devices.

A thin film transistor with stable electric characteristics can be obtained and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor with high electric characteristics can be provided.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description, and various changes for the modes and details thereof will be apparent to those skilled in the art unless such changes depart from the spirit and the scope of the invention. Therefore, the present invention should not be interpreted as being limited to what is described in the embodiments. In the structures of the present invention to be described below, identical portions or portions having similar functions may be marked by the same reference numerals throughout the drawings so as to omit repeated explanation.

A semiconductor device and a method for manufacturing a semiconductor device will be described with reference toFIGS. 1A and 1BandFIGS. 2A to 2D.

FIG. 1Ais a top view of a thin film transistor470of a semiconductor device, andFIG. 1Bis a cross-sectional view along line C1-C2ofFIG. 1A. The thin film transistor470is an inverted staggered thin film transistor and includes, over a substrate400which is a substrate having an insulating surface, a gate electrode layer401, a gate insulating layer402, a semiconductor layer403, n+layers404aand404beach serving as a source region or a drain region, and source or drain electrode layers405aand405b. In addition, an insulating film407is provided so as to cover the thin film transistor470and be in contact with the semiconductor layer403.

The semiconductor layer403is an oxide semiconductor layer containing a metal element, to which at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum is added. In the drawings of this specification, a shaded region of the semiconductor layer403is a region to which the metal element is added.

When the metal element is added to the oxide semiconductor layer, the metal element becomes a recombination center in the oxide semiconductor layer, whereby the photosensitivity of the oxide semiconductor layer can be lowered. The low photosensitivity can stabilize electric characteristics of the thin film transistor and can prevent an increase in off current due to variation in threshold voltage or the like.

FIGS. 2A to 2Dare cross-sectional views illustrating steps of manufacturing the thin film transistor470.

InFIG. 2A, the gate electrode layer401is provided over the substrate400which is a substrate having an insulating surface. An insulating film serving as a base film may be provided between the substrate400and the gate electrode layer401. The base film has a function of preventing diffusion of an impurity element from the substrate400, and can be formed to have a single-layer or stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. The gate electrode layer401can be formed to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these materials as its main component.

For example, as a two-layer structure of the gate electrode layer401, the following structures are preferable: a two-layer structure in which a molybdenum layer is stacked over an aluminum layer, a two-layer structure in which a molybdenum layer is stacked over a copper layer, a two-layer structure in which a titanium nitride layer or a tantalum nitride layer is stacked over a copper layer, and a two-layer structure in which a titanium nitride layer and a molybdenum layer are stacked. As a three-layer structure, it is preferable to stack a tungsten layer or a tungsten nitride layer, a layer of an alloy of aluminum and silicon or an alloy of aluminum and titanium, and a titanium nitride layer or a titanium layer.

The gate insulating layer402is formed over the gate electrode layer401.

The gate insulating layer402can be formed using a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a silicon nitride oxide layer to have a single-layer or stacked-layer structure by a plasma CVD method, a sputtering method, or the like. Alternatively, the gate insulating layer402can be formed using a silicon oxide layer by a CVD method in which an organosilane gas is used. As the organosilane gas, a silicon-containing compound such as tetraethoxysilane (TEOS: chemical formula, Si(OC2H5)4), tetramethylsilane (TMS: chemical formula, Si(CH3)4), tetramethylcyclotetrasiloxane (TMCTS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS), triethoxysilane (SiH(OC2H5)3), or trisdimethylaminosilane (SiH(N(CH3)2)3) can be used.

Over the gate insulating layer402, a first oxide semiconductor film430and a second oxide semiconductor film431are stacked in this order. The first oxide semiconductor film430and the second oxide semiconductor film431are island-shaped oxide semiconductor layers patterned by a photolithography step.

Note that before the first oxide semiconductor film430is formed by a sputtering method, dust attached to a surface of the gate insulating layer402is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering is a method in which voltage is applied to a substrate side using an RF power source to generate plasma on the substrate side in an argon atmosphere without applying voltage to a target side, so that a surface is modified. Nitrogen, helium, or the like may be used instead of the argon atmosphere. Alternatively, oxygen, hydrogen, N2O, or the like may be added to the argon atmosphere. Further alternatively, Cl2, CF4, or the like may be added to the argon atmosphere.

A region where the second oxide semiconductor film431and a conductive film432are in contact with each other is preferably modified through plasma treatment.

The plasma treatment may be performed using nitrogen, helium, or the like instead of the argon atmosphere. Alternatively, oxygen, hydrogen, N2O, or the like may be added to the argon atmosphere. Further alternatively, Cl2, CF4, or the like may be added to the argon atmosphere.

An In—Ga—Zn—O-based non-single-crystal film is used as each of the first oxide semiconductor film430and the second oxide semiconductor film431. The first oxide semiconductor film430and the second oxide semiconductor film431are formed under different conditions, and the second oxide semiconductor film431has higher conductivity and lower resistance than the first oxide semiconductor film430. For example, the second oxide semiconductor film431is formed using an oxide semiconductor film obtained by a sputtering method in which the argon gas flow rate is set to be 40 sccm. The second oxide semiconductor film431has n-type conductivity and has an activation energy (ΔE) of 0.01 eV to 0.1 eV inclusive. Note that the second oxide semiconductor film431is an In—Ga—Zn—O-based non-single-crystal film and includes at least an amorphous component. In some cases, the second oxide semiconductor film431has a crystal grain (nanocrystal) in an amorphous structure. The crystal grain (nanocrystal) in this second oxide semiconductor film431has a diameter of 1 nm to 10 nm, typically about 2 nm to 4 nm.

By providing the second oxide semiconductor film431serving as an n+layer, the conductive film432formed using a metal layer and the first oxide semiconductor film430serving as a channel formation region have a favorable junction, which allows more thermally-stable operation than in Schottky junction. In addition, willing provision of the n+layer is effective in supplying carriers to the channel (on the source side), stably absorbing carriers from the channel (on the drain side), or preventing a resistance component from being formed at an interface with the wiring. Further, by the decrease in resistance, high mobility can be maintained even at high drain voltage.

The conductive film432is formed over the gate insulating layer402, the first oxide semiconductor film430, and the second oxide semiconductor film431.

As the material of the conductive film432, there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, and the like. If heat treatment at 200° C. to 600° C. is performed, it is preferable that the conductive film have heat resistance enough to withstand the heat treatment. Since aluminum alone has the disadvantages of low heat resistance, being easily corroded, and the like, it is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance which is combined with aluminum, an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, or a nitride containing any of these elements as its component is used.

The gate insulating layer402, the first oxide semiconductor film430, the second oxide semiconductor film431, and the conductive film432may be formed successively without being exposed to the air. When the gate insulating layer402, the first oxide semiconductor film430, the second oxide semiconductor film431, and the conductive film432are formed successively without being exposed to the air, an interface between the stacked films can be formed without being contaminated with atmospheric components or impurity elements contained in the air. Thus, variation in characteristics of the thin film transistor can be reduced.

The first oxide semiconductor film430, the second oxide semiconductor film431, and the conductive film432are etched by an etching step to form a semiconductor layer433, n+layers404aand404b, and source or drain electrode layers405aand405b(FIG. 2B). Note that the semiconductor layer433is a semiconductor layer obtained by being only partly etched to have a groove (depression) and an end portion which is also partly etched and exposed.

Next, a metal element434is added to the semiconductor layer433which is an oxide semiconductor layer, whereby a semiconductor layer403is formed (FIG. 2C).

As the metal element434, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. For example, iron is used as the metal element434and added by an ion implantation method. Iron is added with the use of an ion implantation apparatus which is provided with an ion source chamber including a heater capable of vaporizing a solid state source.

Since the source or drain electrode layers405aand405bare formed over the semiconductor layer433, the source or drain electrode layers405aand405bserve as masks and the metal element434is added to an exposed region of the semiconductor layer433. When the metal element is selectively added to the semiconductor layer433in this manner, the concentration of the added metal element has a distribution and a region to which the metal element is not added also exists in the semiconductor layer403.

In addition, the metal element is also added to a film other than the oxide semiconductor layer, depending on when the metal element is added in the process or a mask condition. For example, in an inverted staggered thin film transistor, when a metal element is added to an exposed region of an oxide semiconductor layer by using a source electrode layer and a drain electrode layer as masks, the metal element is also added to the source electrode layer and the drain electrode layer. In a similar manner, since a metal element is added to an exposed region, the metal element is also added to an exposed region of a gate insulating layer. In addition, depending on the condition of adding the metal element, the metal element is also added to a base film below the gate insulating layer in some cases. Of course, when a mask is formed, a region covered with the mask can be a region to which the metal element is not added; thus, the metal element can be selectively contained only in the oxide semiconductor layer.

By addition of the metal element, the photosensitivity of the oxide semiconductor layer is lowered. Further, by addition of the metal element only in the vicinity of a surface of the oxide semiconductor layer, photosensitivity of the oxide semiconductor layer is lowered while degradation of mobility in the oxide semiconductor layer is suppressed. Therefore, variation in electric characteristics such as an increase in off current can be prevented.

After that, heat treatment is preferably performed at 200° C. to 600° C., typically 300° C. to 500° C. For example, heat treatment is performed at 350° C. for 1 hour under a nitrogen atmosphere. By this heat treatment, rearrangement at an atomic level of the In—Ga—Zn—O based oxide semiconductor included in the semiconductor layer403and the n+layers404aand404bis performed. This heat treatment (including photo annealing and the like) is important because this heat treatment enables release of distortion which interrupts transfer of carriers in the semiconductor layer403and the n+layers404aand404b. Note that there is no particular limitation on timing of the above heat treatment as long as the heat treatment is performed after formation of the first oxide semiconductor film430and the second oxide semiconductor film431.

In addition, the depression of the exposed part of the semiconductor layer403may be subjected to oxygen radical treatment. By performing the oxygen radical treatment, the thin film transistor in which the semiconductor layer403is a channel formation region can be normally off. Further, by performing the radical treatment, damage to the semiconductor layer403due to etching can be repaired. The radical treatment is preferably performed in an atmosphere of O2or N2O, or an atmosphere of N2, He, Ar, or the like which contains oxygen. In addition, the radical treatment may be performed under the above atmosphere to which Cl2or CF4is added. Note that the radical treatment is preferably performed without application of a bias voltage to the substrate400side.

Through the above process, as illustrated inFIG. 2D, the inverted staggered thin film transistor470in which the semiconductor layer403to which the metal element is added serves as a channel formation region can be manufactured. In addition, the insulating film407is formed so as to cover the thin film transistor470and be in contact with the semiconductor layer403.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

In this embodiment,FIGS. 30A to 30Eillustrate an example of a semiconductor device including a thin film transistor in which a metal element is added to a semiconductor layer before formation of source and drain electrode layers, which is different from Embodiment 1.

FIGS. 30A to 30Eare cross-sectional views illustrating a manufacturing process of a thin film transistor440.

InFIG. 30A, a gate electrode layer401is formed over a substrate400, and a gate insulating layer402is formed over the gate electrode layer401.

A first oxide semiconductor film435is formed over the gate insulating layer402.

Next, a metal element434is added to the first oxide semiconductor film435, whereby a first oxide semiconductor film436is formed (FIG. 30B).

As the metal element434, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. For example, iron is used as the metal element434and added by an ion implantation method. Iron is added with the use of an ion implantation apparatus which is provided with an ion source chamber including a heater capable of vaporizing a solid state source.

The metal element434is added to the entire surface of the first oxide semiconductor film435. Although an example in which the metal element is added entirely in the thickness direction of the first oxide semiconductor film435is shown, the metal element may also be added only in the vicinity of the surface of the first oxide semiconductor film435.

By addition of the metal element, the photosensitivity of the oxide semiconductor film is lowered, which can prevent variation in electric characteristics such as an increase in off current.

Over the first oxide semiconductor film436, a second oxide semiconductor film is formed. The first oxide semiconductor film436and the second oxide semiconductor film are patterned by a photolithography step into island-shaped first oxide semiconductor film437and second oxide semiconductor film431, respectively.

A conductive film432is formed over the gate insulating layer402, the first oxide semiconductor film437, and the second oxide semiconductor film431(FIG. 30C).

The first oxide semiconductor film437, the second oxide semiconductor film431, and the conductive film432are etched by an etching step to form a semiconductor layer438, n+layers404aand404b, and source or drain electrode layers405aand405b(FIG. 30D). Note that the semiconductor layer438is a semiconductor layer obtained by being only partly etched to have a groove (depression) and an end portion which is also partly etched and exposed.

By addition of the metal element, the photosensitivity of the oxide semiconductor layer438is lowered, which can prevent variation in electric characteristics such as an increase in off current.

After that, heat treatment is preferably performed at 200° C. to 600° C., typically 300° C. to 500° C. For example, heat treatment is performed at 350° C. for 1 hour under a nitrogen atmosphere.

Through the above process, as illustrated inFIG. 30E, the inverted staggered thin film transistor440in which the semiconductor layer483to which the metal element is added serves as a channel formation region can be manufactured. In addition, an insulating film407is formed so as to cover the thin film transistor440and be in contact with the semiconductor layer438.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

In this embodiment,FIGS. 31A to 31Dillustrate an example of a semiconductor device including a thin film transistor in which source and drain electrode layers are in contact with a semiconductor layer without n+layers interposed therebetween, which is different from Embodiment 1.

FIGS. 31A to 31Dare cross-sectional views illustrating a manufacturing process of a thin film transistor471.

InFIG. 31A, a gate electrode layer401is formed over a substrate400, and a gate insulating layer402is formed over the gate electrode layer401.

Over the gate insulating layer402, a first oxide semiconductor film430is formed. The first oxide semiconductor film430is an island-shaped oxide semiconductor layer patterned by a photolithography step.

A conductive film432is formed over the gate insulating layer402and the first oxide semiconductor film430.

The first oxide semiconductor film430and the conductive film432are etched by an etching step to form a semiconductor layer433and source or drain electrode layers405aand405b(FIG. 31B). Note that the semiconductor layer433is a semiconductor layer obtained by being only partly etched to have a groove (depression) and an end portion which is also partly etched and exposed.

Next, a metal element434is added to the semiconductor layer433which is an oxide semiconductor layer, whereby a semiconductor layer403is formed (FIG. 31C).

As the metal element434, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. For example, iron is used as the metal element434and added by an ion implantation method. Iron can be added with the use of an in implantation apparatus which is provided with an ion source chamber including a heater capable of vaporizing a solid state source.

Since the source or drain electrode layers405aand405bare formed over the semiconductor layer433, the source or drain electrode layers405aand405bserve as masks and the metal element434is added to an exposed region of the semiconductor layer433. When the metal element is selectively added to the semiconductor layer433in this manner, the concentration of the added metal element has a distribution and a region to which the metal element is not added also exists in the semiconductor layer403. Although an example in which the metal element is added only in the vicinity of the surface of the semiconductor layer433is shown, the metal element may also be added entirely in the thickness direction of the semiconductor layer433.

By addition of the metal element, the photosensitivity of the oxide semiconductor layer is lowered, which can prevent variation in electric characteristics such as an increase in off current.

After that, heat treatment is preferably performed at 200° C. to 600° C., typically 300° C. to 500° C. For example, heat treatment is performed at 350° C. for 1 hour under a nitrogen atmosphere.

Through the above process, as illustrated inFIG. 31D, the inverted staggered thin film transistor471in which the semiconductor layer403to which the metal element is added serves as a channel formation region can be manufactured. In addition, an insulating film407is formed so as to cover the thin film transistor471and be in contact with the semiconductor layer403.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

A semiconductor device and a method for manufacturing a semiconductor device will be described with reference toFIGS. 3A and 3BandFIGS. 4A to 4D.

FIG. 3Ais a top view of a thin film transistor460included in a semiconductor device, andFIG. 3Bis a cross-sectional view along line D1-D2ofFIG. 3A. The thin film transistor460is a bottom gate thin film transistor and includes, over a substrate450which is a substrate having an insulating surface, a gate electrode layer451, a gate insulating layer452, source or drain electrode layers455aand455b, n+layers454aand454beach serving as a source region or a drain region, and a semiconductor layer453. In addition, an insulating film457is provided so as to cover the thin film transistor460and be in contact with the semiconductor layer453. For the semiconductor layer453and the n+layers454aand454b, an In—Ga—Zn—O-based non-single-crystal film is used.

In the thin film transistor460, the gate insulating layer452exists throughout the region including the thin film transistor460, and the gate electrode layer451is provided between the gate insulating layer452and the substrate450which is a substrate having an insulating surface. Over the gate insulating layer452, the source or drain electrode layers455aand455band the n+layers454aand454bare provided. Further, over the gate insulating layer452, the source or drain electrode layers455aand455b, and the n+layers454aand454b, the semiconductor layer453is provided. Although not illustrated, in addition to the source or drain electrode layers455aand455b, a wiring layer is provided over the gate insulating layer452, and the wiring layer extends beyond the peripheral portion of a semiconductor layer in the semiconductor device.

The semiconductor layer453is an oxide semiconductor layer containing a metal element, to which at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum is added. In the drawings of this specification, a shaded region of the semiconductor layer453is a region to which the metal element is added.

When the metal element is added to the oxide semiconductor layer, the metal element becomes a recombination center in the oxide semiconductor layer, whereby the photosensitivity of the oxide semiconductor layer can be lowered. The low photosensitivity can stabilize electric characteristics of the thin film transistor and can prevent an increase in off current due to variation in threshold voltage or the like.

FIGS. 4A to 4Dare cross-sectional views illustrating steps of manufacturing the thin film transistor460.

The gate electrode layer451is provided over the substrate450which is a substrate having an insulating surface. An insulating film serving as a base film may be provided between the substrate450and the gate electrode layer451. The base film has a function of preventing diffusion of an impurity element from the substrate450, and can be formed to have a single-layer or stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. The gate electrode layer451can be formed to have a single-layer or stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these materials as its main component.

The gate insulating layer452is formed over the gate electrode layer451.

The gate insulating layer452can be formed using a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, or a silicon nitride oxide layer to have a single-layer or stacked-layer structure by a plasma CVD method, a sputtering method, or the like. Alternatively, the gate insulating layer452can be formed using a silicon oxide layer by a CVD method in which an organosilane gas is used.

Over the gate insulating layer452, a conductive film and a first oxide semiconductor film are stacked in this order. The conductive film and the first oxide semiconductor film are patterned into island-shaped source or drain electrode layers455aand455band oxide semiconductor films481aand481b, respectively, by a photolithography step (FIG. 4A). The oxide semiconductor films481aand481bserve as n+layers and are thus formed in a manner similar to the second oxide semiconductor film431in Embodiment 1.

As the material of the source or drain electrode layers455aand455b, there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, and the like. If heat treatment at 200° C. to 600° C. is performed, it is preferable that the conductive film have heat resistance enough to withstand the heat treatment. Since aluminum alone has the disadvantages of low heat resistance, being easily corroded, and the like, it is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance which is combined with aluminum, an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, or a nitride containing any of these elements as its component is used.

Then, a second oxide semiconductor film is formed over the gate insulating layer452, the source or drain electrode layers455aand455b, and the oxide semiconductor films481aand481b, and an island-shaped semiconductor layer483and n+layers454aand454bare formed by a photolithography step (FIG. 4B).

The semiconductor layer483serves as a channel formation region and is thus formed in a manner similar to the first oxide semiconductor film430in Embodiment 1.

Note that before the semiconductor layer483is formed by a sputtering method, dust attached to a surface of the gate insulating layer452is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated.

Next, a metal element484is added to the semiconductor layer483which is an oxide semiconductor layer, whereby a semiconductor layer453is formed (FIG. 4C).

As the metal element484, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. For example, iron is used as the metal element484and added by an ion implantation method. Iron is added with the use of an ion implantation apparatus which is provided with an ion source chamber including a heater capable of vaporizing a solid state source.

The metal element484is added to the entire surface of the semiconductor layer483which is exposed. Although an example in which the metal element is added entirely in the thickness direction of the semiconductor layer483is shown, the metal element may also be added only in the vicinity of the surface of the semiconductor layer483.

By addition of the metal element, the photosensitivity of the oxide semiconductor layer is lowered, which can prevent variation in electric characteristics such as an increase in off current.

After that, heat treatment is preferably performed at 200° C. to 600° C., typically 300° C. to 500° C. For example, heat treatment is performed at 350° C. for 1 hour under a nitrogen atmosphere. There is no particular limitation on timing for this heat treatment as long as it is performed after formation of the oxide semiconductor film.

Further, the semiconductor layer453may be subjected to oxygen radical treatment.

Through the above process, as illustrated inFIG. 4D, the bottom gate thin film transistor460in which the semiconductor layer453to which the metal element is added serves as a channel formation region can be manufactured. In addition, an insulating film457is formed so as to cover the thin film transistor460and be in contact with the semiconductor layer453.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

FIGS. 32A to 32Dillustrate an example of a semiconductor device including a thin film transistor in which source and drain electrode layers are in contact with a semiconductor layer without n+layers interposed therebetween, which is different from Embodiment 4.

FIGS. 32A to 32Dare cross-sectional views illustrating a manufacturing process of a thin film transistor461.

A gate electrode layer451is formed over a substrate450which is a substrate having an insulating surface. An insulating film serving as a base film may be provided between the substrate450and the gate electrode layer451.

A gate insulating layer452is formed over the gate electrode layer451.

Over the gate insulating layer452, a conductive film is stacked and patterned into island-shaped source or drain electrode layers455aand455bby a photolithography step (FIG. 32A).

Then, an oxide semiconductor film is formed over the gate insulating layer452and the source or drain electrode layers455aand455b, and patterned into an island-shaped semiconductor layer483by a photolithography step (FIG. 32B).

The semiconductor layer483serves as a channel formation region and is thus formed in a manner similar to the first oxide semiconductor film430in Embodiment 1.

Note that before the semiconductor layer483is formed by a sputtering method, dust attached to a surface of the gate insulating layer452is preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated.

Next, a metal element484is added to the semiconductor layer483which is an oxide semiconductor layer, whereby a semiconductor layer485is formed (FIG. 32C).

As the metal element484, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. For example, iron is used as the metal element484and added by an ion implantation method. Iron is added with the use of an ion implantation apparatus which is provided with an ion source chamber including a heater capable of vaporizing a solid state source.

The metal element484is added to the entire surface of the semiconductor layer483which is exposed. Although an example in which the metal element is added only in the vicinity of the surface of the semiconductor layer483is shown, the metal element may also be added entirely in the thickness direction of the semiconductor layer483.

By addition of the metal element, the photosensitivity of the oxide semiconductor layer is lowered, which can prevent variation in electric characteristics such as an increase in off current.

After that, heat treatment is preferably performed at 200° C. to 600° C., typically 300° C. to 500° C. For example, heat treatment is performed at 350° C. for 1 hour under a nitrogen atmosphere. There is no particular limitation on timing for this heat treatment as long as it is performed after formation of the oxide semiconductor film.

Further, the semiconductor layer485may be subjected to oxygen radical treatment.

Through the above process, as illustrated inFIG. 32D, the bottom gate thin film transistor461in which the semiconductor layer485to which the metal element is added serves as a channel formation region can be manufactured. In addition, an insulating film457is formed so as to cover the thin film transistor461and be in contact with the semiconductor layer485.

In the thin film transistor461, the gate insulating layer452exists throughout the region including the thin film transistor461, and the gate electrode layer451is provided between the gate insulating layer452and the substrate450which is a substrate having an insulating surface. Over the gate insulating layer452, the source or drain electrode layers455aand455bare provided. Further, over the gate insulating layer452and the source or drain electrode layers455aand455b, the semiconductor layer485is provided. Although not illustrated, in addition to the source or drain electrode layers455aand455b, a wiring layer is provided over the gate insulating layer452, and the wiring layer extends beyond the peripheral portion of a semiconductor layer in the semiconductor device.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

A manufacturing process of a semiconductor device including a thin film transistor will be described with reference toFIGS. 5A to 5C,FIGS. 6A to 6C,FIG. 7,FIG. 8,FIG. 9,FIG. 10, FIGS.11A1to11B2, andFIG. 12.

InFIG. 5A, for a substrate100having a light-transmitting property, a glass substrate formed of barium borosilicate glass, alumino-borosilicate glass, or the like can be used.

After a conductive layer is formed over the entire surface of the substrate100, a resist mask is formed by a first photolithography step. Then, unnecessary portions are removed by etching, thereby forming wirings and electrodes (a gate wiring including a gate electrode layer101, a capacitor wiring108, and a first terminal121). At that time, etching is performed so that at least the edge of the gate electrode layer101is tapered. A cross-sectional view at this stage is illustrated inFIG. 5A. Note thatFIG. 7is a top view at this stage.

Each of the gate wiring including the gate electrode layer101, the capacitor wiring108, and the first terminal121at a terminal portion is preferably formed using a heat-resistant conductive material such as an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc); an alloy containing any of these elements as its component; an alloy film containing a combination of any of these elements; or a nitride containing any of these elements as its component. In the case of using a low-resistant conductive material such as aluminum (Al) or copper (Cu), the low-resistant conductive material is used in combination with the above heat-resistant conductive material because Al alone has problems of low heat resistance, being easily corroded, and the like.

Then, a gate insulating layer102is formed throughout the surface over the gate electrode layer101. The gate insulating layer102is formed to a thickness of 50 nm to 250 nm by a sputtering method or the like.

For example, as the gate insulating layer102, a silicon oxide film is formed to a thickness of 100 nm by a sputtering method. It is needless to say that the gate insulating layer102is not limited to such a silicon oxide film, and other insulating films such as a silicon oxynitride film, a silicon nitride film, an aluminum oxide film, and a tantalum oxide film may be used to form a single-layer structure or a stacked-layer structure.

Note that reverse sputtering in which an argon gas is introduced and plasma is generated is preferably performed before the formation of an oxide semiconductor film, in order to remove dust attached to the surface of the gate insulating layer. Nitrogen, helium, or the like may be used instead of the argon atmosphere. Alternatively, oxygen, hydrogen, N2O, or the like may be added to the argon atmosphere. Further alternatively, Cl2, CF4, or the like may be added to the argon atmosphere.

Next, a first oxide semiconductor film (a first In—Ga—Zn—O-based non-single-crystal film) is formed over the gate insulating layer102. It is effective to deposit the first In—Ga—Zn—O-based non-single-crystal film without exposure to the air after the plasma treatment because dust and moisture are not attached to the interface between the gate insulating layer and the semiconductor film. Here, the first In—Ga—Zn—O-based non-single-crystal film is formed under the following condition: the target is an oxide semiconductor target including In, Ga, and Zn (In2O3:Ga2O3:ZnO=1:1:1) with a diameter of 8 inches, the distance between the substrate and the target is 170 mm, the pressure is 0.4 Pa, the direct current (DC) power is 0.5 kW, and the atmosphere is argon or oxygen. A pulsed direct current (DC) power source is preferable because dust can be reduced and film thickness becomes uniform. The thickness of the first In—Ga—Zn—O-based non-single-crystal film is set in the range of 5 nm to 200 nm. Here, the thickness of the first In—Ga—Zn—O-based non-single-crystal film is 100 nm.

Next, a second oxide semiconductor film (a second In—Ga—Zn—O-based non-single-crystal film) is formed without exposure to the air by a sputtering method. Here, sputtering deposition is performed under the following condition: the target is an oxide semiconductor target including In, Ga, and Zn (In2O3:Ga2O3:ZnO=1:1:1), the pressure is 0.4 Pa, the amount of electric power is 500 W, the deposition temperature is room temperature, and the argon gas flow rate is 40 sccm. Although the target in which In2O3:Ga2O3:ZnO is 1:1:1 is used intentionally, an In—Ga—Zn—O-based non-single-crystal film including a crystal grain which has a size of 1 nm to 10 nm just after the deposition is obtained in some cases. By adjusting the target composition ratio, the deposition pressure (0.1 Pa to 2.0 Pa), the amount of electric power (250 W to 3000 W: 8 inches φ), the temperature (room temperature to 100° C.), the deposition condition of reactive sputtering, and the like as appropriate, the presence or absence of the crystal grains and the density of the crystal grains can be controlled and the diameter of the crystal grain can be adjusted within the range of 1 nm to 10 nm. The thickness of the second In—Ga—Zn—O-based non-single-crystal film is 5 nm to 20 nm. Needless to say, in the case where the film includes the crystal grain, the size of the crystal grain does not exceed the film thickness. Here, the thickness of the second In—Ga—Zn—O-based non-single-crystal film is 5 nm.

The first In—Ga—Zn—O-based non-single-crystal film and the second In—Ga—Zn—O-based non-single-crystal film are formed under different conditions from each other. For example, the flow ratio of an oxygen gas to an argon gas under the deposition condition of the first In—Ga—Zn—O-based non-single-crystal film is higher than that under the deposition condition of the second In—Ga—Zn—O-based non-single-crystal film. Specifically, the second In—Ga—Zn—O-based non-single-crystal film is formed in a rare gas (such as argon or helium) atmosphere (or an atmosphere including an oxygen gas at 10% or less and an argon gas at 90% or more), and the first In—Ga—Zn—O-based non-single-crystal film is formed in an oxygen atmosphere (or an atmosphere where an argon gas-to-oxygen gas flow ratio is 1 to 1 or more).

The second In—Ga—Zn—O-based non-single-crystal film may be formed in the chamber where reverse sputtering has been performed previously, or in a different chamber from the chamber where reverse sputtering has been performed previously.

Examples of a sputtering method include an RF sputtering method in which a high-frequency power source is used for a sputtering power source, a DC sputtering method, and a pulsed DC sputtering method in which a bias is applied in a pulsed manner. The RF sputtering method is mainly used in the case of forming an insulating film, and the DC sputtering method is mainly used in the case of forming a metal film.

In addition, there is also a multi-source sputtering apparatus in which a plurality of targets of different materials can be set. With the multi-source sputtering apparatus, films of different materials can be deposited to be stacked in the same chamber, or a plurality of kinds of materials can be deposited by electric discharge at the same time in the same chamber.

In addition, there are a sputtering apparatus provided with a magnet system inside the chamber and used for a magnetron sputtering method, and a sputtering apparatus used for an ECR sputtering method in which plasma generated with the use of microwaves is used without using glow discharge.

Furthermore, as a deposition method by a sputtering method, there are also a reactive sputtering method in which a target substance and a sputtering gas component are chemically reacted with each other during deposition to form a thin compound film thereof, and a bias sputtering method in which voltage is also applied to a substrate during deposition.

Next, a second photolithography step is performed to form a resist mask, and the first In—Ga—Zn—O-based non-single-crystal film and the second In—Ga—Zn—O-based non-single-crystal film are etched. For example, by wet etching using a solution in which phosphoric acid, acetic acid, and nitric acid are mixed, unnecessary portions are removed, thereby forming an oxide semiconductor film109which is the first In—Ga—Zn—O-based non-single-crystal film and an oxide semiconductor film111which is the second In—Ga—Zn—O-based non-single-crystal film. Note that etching here is not limited to wet etching and may be dry etching. Note that a top view at this stage corresponds toFIG. 8.

As the etching gas for dry etching, a gas containing chlorine (chlorine-based gas such as chlorine (Cl2), boron chloride (BCl3), silicon chloride (SiCl4), or carbon tetrachloride (CCl4)) is preferably used.

Alternatively, a gas containing fluorine (fluorine-based gas such as carbon tetrafluoride (CF4), sulfur fluoride (SF6), nitrogen fluoride (NF3), or trifluoromethane (CHF3)); hydrogen bromide (HBr); oxygen (O2); any of these gases to which a rare gas such as helium (He) or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching) method or an ICP (inductively coupled plasma) etching method can be used. In order to etch the films into desired shapes, the etching condition (the amount of electric power applied to a coil-shaped electrode, the amount of electric power applied to an electrode on a substrate side, the temperature of the electrode on the substrate side, or the like) is adjusted as appropriate.

As an etchant used for wet etching, a solution obtained by mixing phosphoric acid, acetic acid, and nitric acid, an ammonia peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), or the like can be used. In addition, ITO07N (produced by KANTO CHEMICAL CO., INC.) may also be used.

The etchant used in the wet etching is removed by cleaning together with the material which is etched off. The waste liquid including the etchant and the material etched off may be purified and the material may be reused. When a material such as indium included in the oxide semiconductor layer is collected from the waste liquid after the etching and reused, the resources can be efficiently used and the cost can be reduced.

The etching conditions (such as an etchant, etching time, and temperature) are appropriately adjusted depending on the material so that the material can be etched into a desired shape.

Next, a third photolithography step is conducted to form a resist mask, and unnecessary portions are removed by etching to form a contact hole which reaches an electrode layer or a wiring made of the same material as the gate electrode layer. The contact hole is provided for direct connection with a conductive film to be formed later. For example, a contact hole is formed when a thin film transistor whose gate electrode layer is in direct contact with the source or drain electrode layer in the driver circuit portion is formed, or when a terminal that is electrically connected to a gate wiring of a terminal portion is formed.

Next, over the oxide semiconductor film109and the oxide semiconductor film111, a conductive film132formed from a metal material is formed by a sputtering method or a vacuum evaporation method. A cross-sectional view at this stage is illustrated inFIG. 5B.

As the material of the conductive film132, there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, and the like. Further, in the case of performing heat treatment at 200° C. to 600° C., it is preferable that the conductive film have heat resistance enough to withstand the heat treatment. Since aluminum alone has the disadvantages of low heat resistance, being easily corroded, and the like, it is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance which is used in combination with Al, any of the following materials may be used: an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of these elements as its component, an alloy containing a combination of any of these elements, and a nitride containing any of these elements as its component.

Here, the conductive film132has a single-layer structure of a titanium film. Alternatively, the conductive film132may have a two-layer structure: an aluminum film and a titanium film stacked thereover. Still alternatively, the conductive film132may have a three-layer structure: a Ti film, an aluminum film containing Nd (Al—Nd) which is stacked over the Ti film, and a Ti film formed over these films. The conductive film132may have a single-layer structure of an aluminum film containing silicon.

Next, a resist mask131is formed by a fourth photolithography step, and unnecessary portions are removed by etching, thereby forming source or drain electrode layers105aand105b, n+layers104aand104beach serving as a source region or a drain region, and a second terminal122. Wet etching or dry etching is used as an etching method at this time. For example, when an aluminum film or an aluminum-alloy film is used as the conductive film132, wet etching using a mixed solution of phosphoric acid, acetic acid, and nitric acid can be carried out. Here, by wet etching using an ammonia hydrogen peroxide mixture (with the ratio of hydrogen peroxide to ammonia and water being 5:2:2), the conductive film132which is a Ti film is etched to form the source or drain electrode layers105aand105b, and the oxide semiconductor film111is etched to form the n+layers104aand104b. In this etching step, an exposed region of the oxide semiconductor film109is also partly etched to be a semiconductor layer133. Thus, a channel region of the semiconductor layer133between the n+layers104aand104bhas a small thickness. InFIG. 5C, the etching for forming the source or drain electrode layers105aand105band the n+layers104aand104bis performed at a time by using an etchant of an ammonia hydrogen peroxide mixture. Accordingly, an end portion of the source or drain electrode layer105aand an end portion of the source or drain electrode layer105bare aligned with an end portion of the n+layer104aand an end portion of the n+layer104b, respectively; thus, continuous structures are formed. In addition, wet etching allows the layers to be etched isotropically, so that the end portions of the source or drain electrode layers105aand105bare positioned more inwardly than those of the resist mask131.

Next, a metal element134which is at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum is added to the semiconductor layer133which is an oxide semiconductor layer, whereby a semiconductor layer103is formed (FIG. 6A). As the metal element134, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. For example, iron is used as the metal element134and added by an ion implantation method. Iron is added with the use of an ion implantation apparatus which is provided with an ion source chamber including a heater capable of vaporizing a solid state source.

Since the source or drain electrode layers105aand105bare formed over the semiconductor layer133, the source or drain electrode layers105aand105bserve as masks and the metal element134is added to an exposed region of the semiconductor layer133. When the metal element is selectively added to the semiconductor layer133in this manner, the concentration of the added metal element has a distribution and a region to which the metal element is not added also exists in the semiconductor layer103.

By addition of the metal element, the oxide semiconductor layer is stabilized and the photosensitivity of the oxide semiconductor layer is lowered, which can prevent variation in electric characteristics such as an increase in off current.

Through the above steps, a thin film transistor170including the semiconductor layer103to which the metal element is added as a channel formation region can be manufactured.FIG. 6Ais a cross-sectional view at this stage. A top view at this stage is illustrated inFIG. 9.

Then, heat treatment is preferably performed at 200° C. to 600° C., typically, 300° C. to 500° C. For example, heat treatment is performed using a furnace at 350° C. for one hour in a nitrogen atmosphere. By this heat treatment, rearrangement at an atomic level of the In—Ga—Zn—O-based non-single-crystal film is performed. This heat treatment (including photo annealing and the like) is important because this heat treatment enables release of distortion which interrupts transfer of carriers. Note that there is no particular limitation on the timing of heat treatment as long as it is performed after formation of the second In—Ga—Zn—O-based non-single-crystal film, and for example, heat treatment may be performed after formation of a pixel electrode.

Furthermore, the exposed part of the channel formation region of the semiconductor layer103may be subjected to oxygen radical treatment, so that a normally-off thin film transistor can be obtained. In addition, the radical treatment can repair damage due to the etching in the semiconductor layer103. The radical treatment is preferably performed in an atmosphere of O2or N2O, preferably an atmosphere of N2, He, or Ar which contains oxygen. The radical treatment may also be performed under the above atmosphere to which Cl2or CF4is added. Note that the radical treatment is preferably performed with no bias applied.

In the fourth photolithography step, the second terminal122that is made of the same material as the source or drain electrode layers105aand105bremains in the terminal portion. Note that the second terminal122is electrically connected to a source wiring (a source wiring including the source or drain electrode layers105aand105b).

If using a resist mask having a plurality of regions with different thicknesses (typically, two kinds of thicknesses) formed using a multi-tone mask, the number of resist masks can be reduced, resulting in a simplified process and lower cost.

Next, the resist mask131is removed and a protective insulating layer107is formed to cover the thin film transistor170. The protective insulating layer107can be formed using a silicon nitride film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, a tantalum oxide film, or the like which is obtained by a sputtering method or the like.

Then, a fifth photolithography step is performed to form a resist mask, and the protective insulating layer107is etched to form a contact hole125reaching the source electrode layer or drain electrode layer105b. In addition, a contact hole127reaching the second terminal122and a contact hole126reaching the first terminal121are also formed in the same etching step. A cross-sectional view at this stage is illustrated inFIG. 6B.

Next, the resist mask is removed, and then a transparent conductive film is formed. The transparent conductive film is formed of indium oxide (In2O3), indium oxide-tin oxide alloy (In2O3—SnO2, abbreviated to ITO), or the like by a sputtering method, a vacuum evaporation method, or the like. Such a material is etched with a hydrochloric acid-based solution. However, since a residue is easily generated particularly in etching ITO, indium oxide-zinc oxide alloy (In2O3—ZnO) may be used to improve etching processability.

Next, a sixth photolithography step is performed to form a resist mask, and unnecessary portions are removed by etching, thereby forming a pixel electrode layer110.

In the sixth photolithography step, a storage capacitor is formed between a capacitor wiring108and the pixel electrode layer110with the gate insulating layer102and the protective insulating layer107in the capacitor portion as dielectrics.

Furthermore, in the sixth photolithography step, the first terminal and the second terminal are covered with the resist mask so that transparent conductive films128and129in the terminal portions remain. The transparent conductive films128and129serve as an electrode or a wiring connected to an FPC. The transparent conductive film128formed over the first terminal121is a connecting terminal electrode serving as an input terminal of a gate wiring. The transparent conductive film129formed over the second terminal122is a connecting terminal electrode serving as an input terminal of a source wiring.

Then, the resist mask is removed. A cross-sectional view at this stage is illustrated inFIG. 6C. Note thatFIG. 10is a top view at this stage.

FIGS.11A1and11A2are respectively a cross-sectional view and a top view of a gate wiring terminal portion at this stage. FIG.11A1is a cross-sectional view along line E1-E2of FIG.11A2. In FIG.11A1, a transparent conductive film155formed over a protective insulating film154is a connecting terminal electrode serving as an input terminal. Furthermore, in the terminal portion of FIG.11A1, a first terminal151made of the same material as a gate wiring and a connection electrode layer153made of the same material as a source wiring overlap with each other with a gate insulating layer152interposed therebetween, and are electrically connected to each other through the transparent conductive film155. Note that a part ofFIG. 6Cwhere the transparent conductive film128is in contact with the first terminal121corresponds to a part of FIG.11A1where the transparent conductive film155is in contact with the first terminal151.

FIGS.11B1and11B2are respectively a cross-sectional view and a top view of a source wiring terminal portion which is different from that illustrated inFIG. 6C. FIG.11B1is a cross-sectional view along line F1-F2of FIG.11B2. In FIG.11B1, a transparent conductive film155formed over a protective insulating film154is a connecting terminal electrode serving as an input terminal. Furthermore, in the terminal portion of FIG.11B1, an electrode layer156made of the same material as a gate wiring is formed under a second terminal150electrically connected to the source wiring and overlaps with the second terminal150with a gate insulating layer152interposed therebetween. The electrode layer156is not electrically connected to the second terminal150, and a capacitor to prevent noise or static electricity can be formed if the potential of the electrode layer156is set to be a potential different from that of the second terminal150, such as floating, GND, or 0 V. The second terminal150is electrically connected to the transparent conductive film155and the protective insulating film154is interposed between the second terminal150and the transparent conductive film155.

A plurality of gate wirings, source wirings, and capacitor wirings are provided depending on the pixel density. Also in the terminal portion, a plurality of the first terminals at the same potential as the gate wirings, a plurality of the second terminals at the same potential as the source wirings, a plurality of the third terminals at the same potential as the capacitor wirings, and the like are arranged. The numbers of the first to third terminals may be each a given number and determined by a practitioner as appropriate.

Through these six photolithography steps, a pixel thin film transistor portion including the thin film transistor170which is a bottom-gate n-channel thin film transistor, and a storage capacitor can be completed using the six photomasks. When these pixel thin film transistor portion and storage capacitor are arranged in a matrix corresponding to their respective pixels, a pixel portion can be formed and one of the substrates for manufacturing an active matrix display device can be obtained. In this specification, such a substrate is referred to as an active matrix substrate for convenience.

In the case of manufacturing an active matrix liquid crystal display device, an active matrix substrate and a counter substrate provided with a counter electrode are bonded to each other with a liquid crystal layer interposed therebetween. Note that a common electrode electrically connected to the counter electrode on the counter substrate is provided over the active matrix substrate, and a fourth terminal electrically connected to the common electrode is provided in the terminal portion. The fourth terminal is provided so that the common electrode is fixed to a predetermined potential such as GND or 0 V.

An embodiment of the present invention is not limited to the pixel structure ofFIG. 10, and an example of the top view different fromFIG. 10is illustrated inFIG. 12.FIG. 12illustrates an example in which a capacitor wiring is not provided and a pixel electrode overlaps with a gate wiring of an adjacent pixel with a protective insulating film and a gate insulating layer interposed therebetween to form a storage capacitor. In that case, the capacitor wiring and the third terminal connected to the capacitor wiring can be omitted. Note that inFIG. 12, the same portions as inFIG. 10are denoted by the same reference numerals.

In an active matrix liquid crystal display device, pixel electrodes arranged in a matrix are driven to form a display pattern on a screen. Specifically, voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, so that a liquid crystal layer provided between the pixel electrode and the counter electrode is optically modulated and this optical modulation is recognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has a problem in that a long response time of liquid crystal molecules themselves causes afterimages or blurring of moving images. In order to improve the moving-image characteristics of a liquid crystal display device, a driving method called black insertion is employed in which black is displayed on the whole screen every other frame period.

Alternatively, a driving method called double-frame rate driving may be employed in which frequency of the vertical cycle is 1.5 times or more, preferably, 2 times or more as long as that of a usual vertical cycle to improve the moving-image characteristics.

Further alternatively, in order to improve the moving-image characteristics of a liquid crystal display device, a driving method may be employed in which a plurality of LED (light-emitting diode) light sources or a plurality of EL light sources are used to form a surface light source as a backlight, and each light source of the surface light source is independently driven in a pulsed manner in one frame period. As the surface light source, three or more kinds of LEDs may be used or an LED emitting white light may be used. Since a plurality of LEDs can be controlled independently, the light emission timing of LEDs can be synchronized with the timing at which a liquid crystal layer is optically modulated. According to this driving method, LEDs can be partly turned off; therefore, an effect of reducing power consumption can be obtained particularly in the case of displaying an image having a large part of black display on one screen.

By combining these driving methods, the display characteristics of a liquid crystal display device, such as moving-image characteristics, can be improved as compared to those of conventional liquid crystal display devices.

The n-channel transistor disclosed in this specification includes an oxide semiconductor film to which a metal element is added for a channel formation region and has good dynamic characteristics. Accordingly, these driving methods can be applied in combination to the n-channel transistor disclosed in this specification.

In the case of manufacturing a light-emitting display device, one electrode (also referred to as a cathode) of an organic light-emitting element is set to be a low power source potential such as GND or 0 V; thus, a terminal portion is provided with a fourth terminal for setting the cathode to a low power source potential such as GND or 0 V. In addition, in the case of manufacturing a light-emitting display device, a power supply line is provided in addition to a source wiring and a gate wiring. Accordingly, a terminal portion is provided with a fifth terminal electrically connected to the power supply line.

The use of an oxide semiconductor for a thin film transistor leads to reduction in manufacturing cost.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

This embodiment can be combined with the structure disclosed in other embodiments, as appropriate.

Instead of the oxide semiconductor layer (InMO3(ZnO)m(m>0) film) which is the In—Ga—Zn—O-based non-single-crystal film in Embodiments 1 to 6, an InMO3(ZnO)m(m>0) film in which M is a different metal element may be used.

As an oxide semiconductor used in this specification, a thin film of a material denoted by InMO3(ZnO)m(m>0) is formed, and a thin film transistor in which the thin film is used as a semiconductor layer is manufactured. Note that M denotes one or more of metal elements, selected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), and cobalt (Co). There is a case where Ga and the above metal elements other than Ga, for example, Ga and Ni or Ga and Fe are contained as M. Moreover, in the oxide semiconductor, in some cases, a transition metal element such as Fe or Ni or an oxide of the transition metal is contained as an impurity element in addition to the metal element contained as M. Note that the metal element denoted by M and the above impurity element are contained during deposition of the oxide semiconductor film, so that the InMO3(ZnO)m(m>0) film is obtained.

At least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum is added to the thin oxide semiconductor layer such as the InMO3(ZnO)m(m>0) film. By selecting a condition and a method for adding the metal element, a region to which the metal element is added or a concentration distribution of the metal element in the oxide semiconductor layer can be controlled.

When the metal element is added to the oxide semiconductor layer, the metal element becomes a recombination center in the oxide semiconductor layer, whereby the photosensitivity of the oxide semiconductor layer can be lowered. The low photosensitivity can stabilize electric characteristics of the thin film transistor and can prevent an increase in off current due to variation in threshold voltage or the like.

The metal element becomes a recombination center of the film of InMO3(ZnO)m(m>0) by being added thereto, whereby the photosensitivity can be lowered. As the metal element, at least one of metal elements of iron, nickel, cobalt, copper, gold, manganese, molybdenum, tungsten, niobium, and tantalum can be used. By adding these metals regardless of the kind of a metal element contained in the film of InMO3(ZnO)m(m>0), electric characteristics of the thin film transistor can be stabilized. In addition, a region to which the metal element is added or a concentration distribution of the metal element in the oxide semiconductor layer can be controlled, whereby the photosensitivity can be lowered and electric characteristics of the thin film transistor can be stabilized efficiently.

A thin film transistor with stable electric characteristics can be obtained by adding a metal element to an oxide semiconductor layer, and a thin film transistor with favorable dynamic characteristics can be manufactured. Therefore, a semiconductor device including a highly reliable thin film transistor can be provided.

An example will be described below, in which at least a part of a driver circuit and a thin film transistor arranged in a pixel portion are formed over the same substrate in a display device which is one example of a semiconductor device.

The thin film transistor to be disposed in the pixel portion is formed according to Embodiments 1 to 7. Further, the thin film transistor described in Embodiments 1 to 7 is an n-channel TFT, and thus a part of a driver circuit that can include an n-channel TFT among driver circuits is formed over the same substrate as the thin film transistor of the pixel portion.

FIG. 14Ais an example of a block diagram of an active matrix liquid crystal display device which is an example of a semiconductor device. The display device illustrated inFIG. 14Aincludes, over a substrate5300, a pixel portion5301including a plurality of pixels that are each provided with a display element; a scan line driver circuit5302that selects a pixel; and a signal line driver circuit5303that controls a video signal input to the selected pixel.

The pixel portion5301is connected to the signal line driver circuit5303by a plurality of signal lines S1to Sm (not illustrated) which extend in a column direction from the signal line driver circuit5303, and to the scan line driver circuit5302by a plurality of scan lines G1to Gn (not illustrated) that extend in a row direction from the scan line driver circuit5302. The pixel portion5301includes a plurality of pixels (not illustrated) arranged in matrix so as to correspond to the signal lines S1to Sm and the scan lines G1to Gn. Each pixel is connected to a signal line Sj (one of the signal lines S1to Sm) and a scan line Gj (one of the scan lines G1to Gn).

In addition, the thin film transistor described in Embodiments 1 to 7 is an n-channel TFT, and a signal line driver circuit including the n-channel TFT is described with reference toFIG. 15.

The signal line driver circuit illustrated inFIG. 15includes a driver IC5601, switch groups5602_1to5602_M, a first wiring5611, a second wiring5612, a third wiring5613, and wirings5621_1to5621_M. Each of the switch groups5602_1to5602_M includes a first thin film transistor5603a, a second thin film transistor5603b, and a third thin film transistor5603c.

The driver IC5601is connected to the first wiring5611, the second wiring5612, the third wiring5613, and the wirings5621_1to5621_M. Each of the switch groups5602_1to5602_M is connected to the first wiring5611, the second wiring5612, and the third wiring5613, and the wirings5621_1to5621_M are connected to the switch groups5602_1to5602_M, respectively. Each of the wirings5621_1to5621_M is connected to three signal lines via the first thin film transistor5603a, the second thin film transistor5603b, and the third thin film transistor5603c. For example, the wiring5621_J of the J-th column (one of the wirings5621_1to5621_M) is connected to a signal line Sj−1, a signal line Sj, and a signal line Sj+1 via the first thin film transistor5603a, the second thin film transistor5603b, and the third thin film transistor5603c, respectively, which are included in the switch group5602_J.

A signal is input to each of the first wiring5611, the second wiring5612, and the third wiring5613.

Note that the driver IC5601is preferably formed over a single crystal substrate. Further, the switch groups5602_1to5602_M are preferably formed over the same substrate as the pixel portion is. Therefore, the driver IC5601and the switch groups5602_1to5602_M are preferably connected through an FPC or the like.

Next, an operation of the signal line driver circuit illustrated inFIG. 15is described with reference to a timing chart inFIG. 16. The timing chart inFIG. 16illustrates the case where the scan line Gi of the i-th row is selected. A selection period of the scan line Gi of the i-th row is divided into a first sub-selection period T1, a second sub-selection period T2, and a third sub-selection period T3. In addition, even when a scan line of another row is selected, the signal line driver circuit inFIG. 15operates in a manner similar toFIG. 16.

Note that the timing chart inFIG. 16shows the case where the wiring5621_J of the J-th column is connected to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 via the first thin film transistor5603a, the second thin film transistor5603b, and the third thin film transistor5603c, respectively.

Note that the timing chart inFIG. 16shows timing at which the scan line Gi of the i-th row is selected, timing5703aof on/off of the first thin film transistor5603a, timing5703bof on/off of the second thin film transistor5603b, timing5703cof on/off of the third thin film transistor5603c, and a signal5721_J input to the wiring5621_J of the J-th column.

In the first sub-selection period T1, the second sub-selection period T2, and the third sub-selection period T3, different video signals are input to the wirings5621_1to5621_M. For example, a video signal input to the wiring5621_J in the first sub-selection period T1 is input to the signal line Sj−1, a video signal input to the wiring5621_J in the second sub-selection period T2 is input to the signal line Sj, and a video signal input to the wiring5621_J in the third sub-selection period T3 is input to the signal line Sj+1. In addition, the video signals input to the wiring5621_J in the first sub-selection period T1, the second sub-selection period T2, and the third sub-selection period T3 are denoted by Data_j−1, Data_j, and Data_j+1.

As shown inFIG. 16, in the first sub-selection period T1, the first thin film transistor5603ais turned on, and the second thin film transistor5603band the third thin film transistor5603care turned off. At this time, Data_j−1 input to the wiring5621_J is input to the signal line Sj−1 via the first thin film transistor5603a. In the second sub-selection period T2, the second thin film transistor5603bis turned on, and the first thin film transistor5603aand the third thin film transistor5603care turned off. At this time, Data_j input to the wiring5621_J is input to the signal line Sj via the second thin film transistor5603b. In the third sub-selection period T3, the third thin film transistor5603cis turned on, and the first thin film transistor5603aand the second thin film transistor5603bare turned off. At this time, Data_j+1 input to the wiring5621_J is input to the signal line Sj+1 via the third thin film transistor5603c.

As described above, in the signal line driver circuit inFIG. 15, by dividing one gate selection period into three, video signals can be input to three signal lines from one wiring5621in one gate selection period. Therefore, in the signal line driver circuit inFIG. 15, the number of connections between the substrate provided with the driver IC5601and the substrate provided with the pixel portion can be approximately ⅓ of the number of signal lines. The number of connections is reduced to approximately ⅓ of the number of the signal lines, so that reliability, yield, etc., of the signal line driver circuit inFIG. 15can be improved.

Note that there are no particular limitation on the arrangement, the number, a driving method, and the like of the thin film transistors, as long as one gate selection period is divided into a plurality of sub-selection periods and video signals are input to a plurality of signal lines from one wiring in the respective sub-selection periods as shown inFIG. 15.

For example, when video signals are input to three or more signal lines from one wiring in three or more sub-selection periods, it is only necessary to add a thin film transistor and a wiring for controlling the thin film transistor. Note that when one gate selection period is divided into four or more sub-selection periods, one sub-selection period becomes shorter. Therefore, one gate selection period is preferably divided into two or three sub-selection periods.

As another example, one selection period may be divided into a precharge period Tp, the first sub-selection period T1, the second sub-selection period T2, and the third sub-selection period T3 as shown in a timing chart ofFIG. 17. The timing chart ofFIG. 17shows the timing at which the scan line Gi of the i-th row is selected, timing5803aof on/off of the first thin film transistor5603a, timing5803bof on/off of the second thin film transistor5603b, timing5803cof on/off of the third thin film transistor5603c, and a signal5821_J input to the wiring5621_J of the J-th column. As shown inFIG. 17, the first thin film transistor5603a, the second thin film transistor5603b, and the third thin film transistor5603care tuned on in the precharge period Tp. At this time, precharge voltage Vp input to the wiring5621_J is input to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 via the first thin film transistor5603a, the second thin film transistor5603b, and the third thin film transistor5603c, respectively. In the first sub-selection period T1, the first thin film transistor5603ais turned on, and the second thin film transistor5603band the third thin film transistor5603care turned off. At this time, Data_j−1 input to the wiring5621_J is input to the signal line Sj−1 via the first thin film transistor5603a. In the second sub-selection period T2, the second thin film transistor5603bis turned on, and the first thin film transistor5603aand the third thin film transistor5603care turned off. At this time, Data_j input to the wiring5621_J is input to the signal line Sj via the second thin film transistor5603b. In the third sub-selection period T3, the third thin film transistor5603cis turned on, and the first thin film transistor5603aand the second thin film transistor5603bare turned off. At this time, Data_j+1 input to the wiring5621_J is input to the signal line Sj+1 via the third thin film transistor5603c.

As described above, in the signal line driver circuit ofFIG. 15to which the timing chart ofFIG. 17is applied, the video signal can be written to the pixel at high speed because the signal line can be precharged by providing a precharge selection period before a sub-selection period. Note that portions ofFIG. 17which are similar to those ofFIG. 16are denoted by common reference numerals and detailed description of like portions and portions having similar functions is omitted.

Further, a structure of a scan line driver circuit is described. The scan line driver circuit includes a shift register and a buffer. Additionally, the scan line driver circuit may include a level shifter. In the scan line driver circuit, when a clock signal (CLK) and a start pulse signal (SP) are input to the shift register, a selection signal is generated. The generated selection signal is buffered and amplified by the buffer, and the resulting signal is supplied to a corresponding scan line. Gate electrodes of transistors in pixels of one line are connected to the scan line. Since the transistors in the pixels of one line have to be turned on all at once, a buffer which can supply a large current is used.

One mode of a shift register used for a part of the scan line driver circuit will be described with reference toFIG. 18andFIG. 19.

FIG. 18illustrates a circuit structure of the shift register. The shift register illustrated inFIG. 18includes a plurality of flip-flops: flip-flops5701—ito5701—n. The shift register is operated with input of a first clock signal, a second clock signal, a start pulse signal, and a reset signal.

The connection relationship of the shift register ofFIG. 18will be described. In the i-th stage flip-flop5701—i(one of the flip-flops5701_1to5701—n) in the shift register ofFIG. 18, a first wiring5501shown inFIG. 19is connected to a seventh wiring5717—i−1, a second wiring5502shown inFIG. 19is connected to a seventh wiring5717—i+1, a third wiring5503shown inFIG. 19is connected to a seventh wiring5717—i, and a sixth wiring5506shown inFIG. 19is connected to a fifth wiring5715.

Further, a fourth wiring5504shown inFIG. 19is connected to a second wiring5712in flip-flops of odd-numbered stages, and is connected to a third wiring5713in flip-flops of even-numbered stages. A fifth wiring5505shown inFIG. 19is connected to a fourth wiring5714.

Note that the first wiring5501of the first stage flip-flop5701_1which is shown inFIG. 19is connected to a first wiring5711. Moreover, the second wiring5502of the n-th stage flip-flop5701—nwhich is shown inFIG. 19is connected to a sixth wiring5716.

Note that the first wiring5711, the second wiring5712, the third wiring5713, and the sixth wiring5716may be referred to as a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. The fourth wiring5714and the fifth wiring5715may be referred to as a first power source line and a second power source line, respectively.

Next,FIG. 19illustrates details of the flip-flop shown inFIG. 18. A flip-flop shown inFIG. 19includes a first thin film transistor5571, a second thin film transistor5572, a third thin film transistor5573, a fourth thin film transistor5574, a fifth thin film transistor5575, a sixth thin film transistor5576, a seventh thin film transistor5577, and an eighth thin film transistor5578. Each of the first thin film transistor5571, the second thin film transistor5572, the third thin film transistor5573, the fourth thin film transistor5574, the fifth thin film transistor5575, the sixth thin film transistor5576, the seventh thin film transistor5577, and the eighth thin film transistor5578is an n-channel transistor and is turned on when the gate-source voltage (Vgs) exceeds the threshold voltage (Vth).

Next, connection structures of the flip-flops shown inFIG. 18will be described below.

A first electrode (one of a source electrode and a drain electrode) of the first thin film transistor5571is connected to the fourth wiring5504. A second electrode (the other of the source electrode and the drain electrode) of the first thin film transistor5571is connected to the third wiring5503.

A first electrode of the second thin film transistor5572is connected to the sixth wiring5506. A second electrode of the second thin film transistor5572is connected to the third wiring5503.

A first electrode of the third thin film transistor5573is connected to the fifth wiring5505, and a second electrode of the third thin film transistor5573is connected to a gate electrode of the second thin film transistor5572. A gate electrode of the third thin film transistor5573is connected to the fifth wiring5505.

A first electrode of the fourth thin film transistor5574is connected to the sixth wiring5506. A second electrode of the fourth thin film transistor5574is connected to the gate electrode of the second thin film transistor5572. A gate electrode of the fourth thin film transistor5574is connected to a gate electrode of the first thin film transistor5571.

A first electrode of the fifth thin film transistor5575is connected to the fifth wiring5505. A second electrode of the fifth thin film transistor5575is connected to the gate electrode of the first thin film transistor5571. A gate electrode of the fifth thin film transistor5575is connected to the first wiring5501.

A first electrode of the sixth thin film transistor5576is connected to the sixth wiring5506. A second electrode of the sixth thin film transistor5576is connected to the gate electrode of the first thin film transistor5571. A gate electrode of the sixth thin film transistor5576is connected to the gate electrode of the second thin film transistor5572.

A first electrode of the seventh thin film transistor5577is connected to the sixth wiring5506. A second electrode of the seventh thin film transistor5577is connected to the gate electrode of the first thin film transistor5571. A gate electrode of the seventh thin film transistor5577is connected to the second wiring5502. A first electrode of the eighth thin film transistor5578is connected to the sixth wiring5506. A second electrode of the eighth thin film transistor5578is connected to the gate electrode of the second thin film transistor5572. A gate electrode of the eighth thin film transistor5578is connected to the first wiring5501.

Note that the point at which the gate electrode of the first thin film transistor5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor5575, the second electrode of the sixth thin film transistor5576, and the second electrode of the seventh thin film transistor5577are connected is referred to as a node5543. The point at which the gate electrode of the second thin film transistor5572, the second electrode of the third thin film transistor5573, the second electrode of the fourth thin film transistor5574, the gate electrode of the sixth thin film transistor5576, and the second electrode of the eighth thin film transistor5578are connected is referred to as a node5544.

Note that the first wiring5501, the second wiring5502, the third wiring5503, and the fourth wiring5504may be referred to as a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. The fifth wiring5505and the sixth wiring5506may be referred to as a first power source line and a second power source line, respectively.

In addition, the signal line driver circuit and the scan line driver circuit can be formed using only the n-channel TFTs described in Embodiment 1. The n-channel TFT described in Embodiment 1 has a high mobility of the transistor, and thus a driving frequency of a driver circuit can be increased. Further, parasitic capacitance is reduced by the source or drain region which is an In—Ga—Zn—O-based non-single-crystal film; thus, the n-channel TFT described in Embodiment 1 has high frequency characteristics (referred to as f characteristics). For example, a scan line driver circuit using the n-channel TFT described in Embodiment 1 can operate at high speed, and thus a frame frequency can be increased and insertion of black images can be realized.

In addition, when the channel width of the transistor in the scan line driver circuit is increased or a plurality of scan line driver circuits are provided, for example, higher frame frequency can be realized. When a plurality of scan line driver circuits are provided, a scan line driver circuit for driving scan lines of even-numbered rows is provided on one side and a scan line driver circuit for driving scan lines of odd-numbered rows is provided on the opposite side; thus, an increase in frame frequency can be realized. Furthermore, the use of the plurality of scan line driver circuits for output of signals to the same scan line is advantageous in increasing the size of a display device.

Further, when an active matrix light-emitting display device which is an example of a semiconductor device is manufactured, a plurality of thin film transistors are arranged in at least one pixel, and thus a plurality of scan line driver circuits are preferably arranged.FIG. 14Billustrates an example of a block diagram of an active matrix light-emitting display device.

The light-emitting display device illustrated inFIG. 14Bincludes, over a substrate5400, a pixel portion5401including a plurality of pixels each provided with a display element, a first scan line driver circuit5402and a second scan line driver circuit5404for selecting a pixel, and a signal line driver circuit5403for controlling a video signal input to the selected pixel.

When the video signal input to a pixel of the light-emitting display device illustrated inFIG. 14Bis a digital signal, a pixel emits light or does not emit light by switching a transistor on/off. Thus, grayscale can be displayed using an area grayscale method or a time grayscale method. An area grayscale method refers to a driving method in which one pixel is divided into a plurality of sub-pixels and each sub-pixel is driven independently based on a video signal so that grayscale is displayed. Further, a time grayscale method refers to a driving method in which a period during which a pixel emits light is controlled so that grayscale is displayed.

Since the response time of a light-emitting element is higher than that of a liquid crystal element or the like, the light-emitting element is more suitable for a time grayscale method than the liquid crystal element. Specifically, in the case of displaying with a time grayscale method, one frame period is divided into a plurality of sub-frame periods. Then, in accordance with video signals, the light-emitting element in the pixel is brought into a light-emitting state or a non-light-emitting state in each sub-frame period. By dividing one frame period into a plurality of sub-frame periods, the total length of time of a period, in which a pixel actually emits light in one frame period, can be controlled by video signals so that grayscale can be displayed.

Note that in the example of the light-emitting display device illustrated inFIG. 14B, when two switching TFTs are arranged in one pixel, the first scan line driver circuit5402generates a signal which is input to a first scan line serving as a gate wiring of one of the two switching TFTs, and the second scan line driver circuit5404generates a signal which is input to a second scan line serving as a gate wiring of the other of the two switching TFTs; however, one scan line driver circuit may generate both the signal which is input to the first scan line and the signal which is input to the second scan line. In addition, for example, there is a possibility that a plurality of scan lines used for controlling the operation of the switching element are provided in each pixel, depending on the number of the switching TFTs included in one pixel. In this case, one scan line driver circuit may generate all signals that are input to the plurality of scan lines, or a plurality of scan line driver circuits may generate signals that are input to the plurality of scan lines.

Also in the light-emitting display device, a part of a driver circuit that can include an n-channel TFT among driver circuits can be formed over the same substrate as the thin film transistor of the pixel portion. Alternatively, the signal line driver circuit and the scan line driver circuit can be formed using only the n-channel TFTs described in Embodiment 1 to 7.

Moreover, the above-described driver circuit can be used for electronic paper that drives electronic ink using an element electrically connected to a switching element, without being limited to applications to a liquid crystal display device or a light-emitting display device. The electronic paper is also referred to as an electrophoretic display device (an electrophoretic display) and is advantageous in that it has the same level of readability as plain paper, it has lower power consumption than other display devices, and it can be made thin and lightweight.

Electrophoretic displays can have various modes. Electrophoretic displays contain a plurality of microcapsules dispersed in a solvent or a solute, each microcapsule containing first particles which are positively charged and second particles which are negatively charged. By applying an electric field to the microcapsules, the particles in the microcapsules move in opposite directions to each other and only the color of the particles gathering on one side is displayed. Note that the first particles and the second particles each contain pigment and do not move without an electric field. Moreover, the first particles and the second particles have different colors (which may be colorless).

Thus, an electrophoretic display is a display that utilizes a so-called dielectrophoretic effect by which a substance having a high dielectric constant moves to a high-electric field region. An electrophoretic display device does not need to use a polarizer or a counter substrate, which is required in a liquid crystal display device, and both the thickness and weight of the electrophoretic display device can be reduced to a half of those of a liquid crystal display device.

A solution in which the above microcapsules are dispersed in a solvent is referred to as electronic ink. This electronic ink can be printed on a surface of glass, plastic, cloth, paper, or the like. Furthermore, by using a color filter or particles that have a pigment, color display can also be achieved.

In addition, if a plurality of the above microcapsules are arranged as appropriate over an active matrix substrate so as to be interposed between two electrodes, an active matrix display device can be completed, and display can be performed by application of an electric field to the microcapsules. For example, the active matrix substrate obtained by the thin film transistors described in Embodiment 1 to 7 can be used.

Note that the first particles and the second particles in the microcapsules may each be formed of a single material selected from a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material, or formed of a composite material of any of these.

Through the above process, a highly reliable display device can be manufactured as a semiconductor device.

When a thin film transistor is manufactured and used for a pixel portion and further for a driver circuit, a semiconductor device having a display function (also referred to as a display device) can be manufactured. Furthermore, when a part or whole of a driver circuit using a thin film transistor is formed over the same substrate as that for a pixel portion, a system-on-panel can be obtained.

The display device includes a display element. As the display element, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. Light-emitting elements include, in its category, an element whose luminance is controlled by current or voltage, and specifically include an inorganic electroluminescent (EL) element, an organic EL element, and the like. Furthermore, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used.

In addition, the display device includes a panel in which the display element is sealed, and a module in which an IC or the like including a controller is mounted on the panel. The present invention also relates to an element substrate, which corresponds to one mode before the display element is completed in a manufacturing process of the display device, and the element substrate is provided with means for supplying current to the display element in each of a plurality of pixels. Specifically, the element substrate may be in a state after only a pixel electrode of the display element is formed, a state after a conductive film to be a pixel electrode is formed and before the conductive film is etched to form the pixel electrode, or any of other states.

Note that a display device in this specification means an image display device, a display device, or a light source (including a lighting device). Furthermore, the display device also includes the following modules in its category: a module to which a connector such as an FPC (flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package) is attached; a module having a TAB tape or a TCP at the tip of which a printed wiring board is provided; and a module in which an IC (integrated circuit) is directly mounted on a display element by a COG (chip on glass) method.

The appearance and a cross section of a liquid crystal display panel, which is one embodiment of a semiconductor device, will be described with reference to FIGS.22A1,22A2, and22B. FIGS.22A1and22A2are each a top view of a panel in which highly reliable thin film transistors4010and4011formed over a first substrate4001and a liquid crystal element4013are sealed between the first substrate4001and a second substrate4006with a sealant4005. The thin film transistors4010and4011each include, as a semiconductor layer, an oxide semiconductor film to which a metal element is added as described in Embodiment 1.FIG. 22Bis a cross-sectional view along line M-N of FIGS.22A1and22A2.

The sealant4005is provided to surround a pixel portion4002and a scan line driver circuit4004that are provided over the first substrate4001. The second substrate4006is provided over the pixel portion4002and the scan line driver circuit4004. Therefore, the pixel portion4002and the scan line driver circuit4004are sealed together with a liquid crystal layer4008, by the first substrate4001, the sealant4005, and the second substrate4006. A signal line driver circuit4003that is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region different from the region surrounded by the sealant4005over the first substrate4001.

Note that there is no particular limitation on the connection method of a driver circuit which is separately formed, and a COG method, a wire bonding method, a TAB method, or the like can be used. FIG.22A1illustrates an example of mounting the signal line driver circuit4003by a COG method, and FIG.22A2illustrates an example of mounting the signal line driver circuit4003by a TAB method.

The pixel portion4002and the scan line driver circuit4004provided over the first substrate4001each include a plurality of thin film transistors.FIG. 22Billustrates the thin film transistor4010included in the pixel portion4002and the thin film transistor4011included in the scan line driver circuit4004. Insulating layers4020and4021are provided over the thin film transistors4010and4011.

As each of the thin film transistors4010and4011, a highly reliable thin film transistor including an oxide semiconductor film to which a metal element is added as a semiconductor layer as described in Embodiment 1 can be employed. Alternatively, the thin film transistor described in Embodiments 2 to 7 may be employed. In this embodiment, the thin film transistors4010and4011are n-channel thin film transistors.

A pixel electrode layer4030included in the liquid crystal element4013is electrically connected to the thin film transistor4010. A counter electrode layer4031of the liquid crystal element4013is formed on the second substrate4006. A portion where the pixel electrode layer4030, the counter electrode layer4031, and the liquid crystal layer4008overlap with one another corresponds to the liquid crystal element4013. Note that the pixel electrode layer4030and the counter electrode layer4031are provided with an insulating layer4032and an insulating layer4033, respectively, each of which functions as an alignment film. The liquid crystal layer4008is sandwiched between the pixel electrode layer4030and the counter electrode layer4031with the insulating layers4032and4033therebetween.

Note that the first substrate4001and the second substrate4006can be made of glass, metal (typically, stainless steel), ceramic, or plastic. As plastic, an FRP (fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride) film, a polyester film, or an acrylic resin film can be used. Alternatively, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used.

A columnar spacer denoted by reference numeral4035is obtained by selective etching of an insulating film and is provided in order to control the distance (a cell gap) between the pixel electrode layer4030and the counter electrode layer4031. Note that a spherical spacer may also be used. The counter electrode layer4031is electrically connected to a common potential line provided over the same substrate as the thin film transistor4010. With the use of a common connection portion, the counter electrode layer4031can be electrically connected to the common potential line through conductive particles provided between the pair of substrates. Note that the conductive particles are contained in the sealant4005.

Alternatively, a liquid crystal showing a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of the liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while the temperature of cholesteric liquid crystal is increased. Since the blue phase is only generated within a narrow range of temperatures, a liquid crystal composition containing a chiral agent at 5 wt % or more is used for the liquid crystal layer4008in order to broaden the temperature range. The liquid crystal composition which includes a liquid crystal showing a blue phase and a chiral agent has a short response time of 10 μs to 100 μs, has optical isotropy which makes the alignment process unneeded, and has a small viewing angle dependence.

An embodiment of the present invention can also be applied to a reflective liquid crystal display device or a semi-transmissive liquid crystal display device, in addition to a transmissive liquid crystal display device.

An example of the liquid crystal display device is described in which a polarizing plate is provided on the outer surface of the substrate (on the viewer side) and a coloring layer and an electrode layer used for a display element are provided on the inner surface of the substrate; however, the polarizing plate may be provided on the inner surface of the substrate. The stack structure of the polarizing plate and the coloring layer is not limited to that described in this embodiment and may be set as appropriate depending on materials of the polarizing plate and the coloring layer or conditions of manufacturing steps. Furthermore, a light-blocking film serving as a black matrix may be provided.

In addition, in order to reduce the surface roughness of the thin film transistor and to improve the reliability of the thin film transistor, the thin film transistor obtained by Embodiment 1 is covered with the insulating layers (the insulating layer4020and the insulating layer4021) serving as a protective film or a planarizing insulating film. Note that the protective film is provided to prevent entry of impurities contained in the air, such as an organic substance, a metal substance, or water vapor, and is preferably a dense film. The protective film may be formed by a sputtering method to be a single-layer film or a stack using any of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, and an aluminum nitride oxide film. Although an example in which the protective film is formed by a sputtering method is described in this embodiment, an embodiment of the present invention is not limited to this method and a variety of methods may be employed.

Here, the insulating layer4020having a stack structure is formed as the protective film. As a first layer of the insulating layer4020, a silicon oxide film is formed by a sputtering method. The use of the silicon oxide film as the protective film has the effect of preventing a hillock of an aluminum film used for the source and drain electrode layers.

In addition, an insulating layer is formed as a second layer of the protective film. In this embodiment, as a second layer of the insulating layer4020, a silicon nitride film is formed by a sputtering method. The use of the silicon nitride film as the protective film can prevent mobile ions such as sodium ions from entering a semiconductor region, thereby suppressing variations in electric characteristics of the TFT.

After the protective film is formed, the semiconductor layer may be subjected to annealing (300° C. to 400° C.).

The insulating layer4021is formed as the planarizing insulating film. As the insulating layer4021, an organic material having heat resistance such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. Note that the insulating layer4021may be formed by stacking a plurality of insulating films formed of these materials.

Note that a siloxane resin is a resin formed from a siloxane material as a starting material and having a Si—O—Si bond. As a substituent, an organic group (e.g., an alkyl group or an aryl group) or a fluoro group may be used. The organic group may include a fluoro group.

There is no particular limitation on the method for forming the insulating layer4021, and the insulating layer4021can be formed, depending on the material, by using a sputtering method, an SOG method, a spin coating method, a dipping method, a spray coating method, a droplet discharge method (e.g., an inkjet method, screen printing, offset printing, or the like), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. In the case where the insulating layer4021is formed using a material solution, the semiconductor layer may be annealed (at 300° C. to 400° C.) at the same time as a baking step. The baking step of the insulating layer4021also serves as the annealing step of the semiconductor layer, whereby a semiconductor device can be manufactured efficiently.

A conductive composition containing a conductive high molecule (also referred to as a conductive polymer) can be used for the pixel electrode layer4030and the counter electrode layer4031. The pixel electrode made of the conductive composition preferably has a sheet resistance of 10000 ohms per square or less and a transmittance of 70% or more at a wavelength of 550 nm. Furthermore, the resistivity of the conductive high molecule contained in the conductive composition is preferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called t-electron conjugated conductive polymer can be used. For example, it is possible to use polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, or a copolymer of two or more kinds of them.

In addition, a variety of signals and potentials are supplied to the signal line driver circuit4003that is formed separately, and the scan line driver circuit4004or the pixel portion4002from an FPC4018.

A connection terminal electrode4015is formed from the same conductive film as the pixel electrode layer4030included in the liquid crystal element4013, and a terminal electrode4016is formed from the same conductive film as source and drain electrode layers of the thin film transistors4010and4011.

The connection terminal electrode4015is electrically connected to a terminal included in the FPC4018through an anisotropic conductive film4019.

Note that FIGS.22A1,22A2, and22B illustrate an example in which the signal line driver circuit4003is formed separately and mounted on the first substrate4001; however, the present invention is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only a part of the signal line driver circuit or a part of the scan line driver circuit may be separately formed and then mounted.

FIG. 23illustrates an example of a liquid crystal display module which is formed as a semiconductor device by using a TFT substrate manufactured according to a manufacturing method disclosed in this specification.

FIG. 23illustrates an example of a liquid crystal display module, in which a substrate2600and a counter substrate2601are bonded to each other with a sealant2602, and a pixel portion2603including a TFT or the like, a display element2604including a liquid crystal layer, and a coloring layer2605are provided between the substrates to form a display region. The coloring layer2605is necessary to perform color display. In the case of the RGB system, respective coloring layers corresponding to colors of red, green, and blue are provided for respective pixels. Polarizing plates2606and2607and a diffusion plate2613are provided outside the substrate2600and the counter substrate2601. A light source includes a cold cathode tube2610and a reflective plate2611. A circuit board2612is connected to a wiring circuit portion2608of the substrate2600through a flexible wiring board2609and includes an external circuit such as a control circuit or a power source circuit. The polarizing plate and the liquid crystal layer may be stacked with a retardation plate interposed therebetween.

Through the above process, a highly reliable liquid crystal display panel can be manufactured as a semiconductor device.

An example of electronic paper will be described as a semiconductor device.

FIG. 13illustrates active matrix electronic paper as an example of a semiconductor device. A thin film transistor581used for the semiconductor device can be manufactured in a manner similar to that of the thin film transistor described in Embodiment 1 and is a highly reliable thin film transistor including an oxide semiconductor film to which a metal element is added as a semiconductor layer. The thin film transistor described in Embodiments 2 to 7 can also be used as the thin film transistor581of this embodiment.

The electronic paper inFIG. 13is an example of a display device using a twisting ball display system. The twisting ball display system refers to a method in which spherical particles each colored in black or white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed.

The thin film transistor581provided for a substrate580is a bottom gate thin film transistor, and a source or drain electrode layer thereof is in contact with a first electrode layer587through an opening formed in insulating layers583,584, and585, whereby the thin film transistor581is electrically connected to the first electrode layer587. Between the first electrode layer587and a second electrode layer588provided for a counter substrate596, spherical particles589each having a black region590a, a white region590b, and a cavity594which is filled with liquid around the regions are provided. A space around the spherical particles589is filled with a filler595such as a resin (FIG. 13). The first electrode layer587corresponds to a pixel electrode, and the second electrode layer588corresponds to a common electrode. The second electrode layer588is electrically connected to a common potential line provided over the same substrate as that for the thin film transistor581. With the use of a common connection portion, the second electrode layer588can be electrically connected to the common potential line through conductive particles provided between a pair of substrates.

Instead of the twisting ball, an electrophoretic element can also be used. A microcapsule having a diameter of about 10 μm to 200 μm in which transparent liquid, positively- or negatively-charged white microparticles, and negatively- or positively-charged black microparticles (having polarity opposite to that of the white microparticles) are encapsulated, is used. In the microcapsule which is provided between the first electrode layer and the second electrode layer, when an electric field is applied by the first electrode layer and the second electrode layer, the white microparticles and the black microparticles move to opposite sides from each other, so that white or black can be displayed. A display element using this principle is an electrophoretic display element and is generally called electronic paper. The electrophoretic display element has higher reflectance than a liquid crystal display element, and thus, an auxiliary light is unnecessary, power consumption is low, and a display portion can be recognized in a dim place. In addition, even when power is not supplied to the display portion, an image which has been displayed once can be maintained. Accordingly, a displayed image can be stored even if a semiconductor device having a display function (which may be referred to simply as a display device or a semiconductor device provided with a display device) is distanced from an electric wave source.

Through this process, highly reliable electronic paper can be manufactured as a semiconductor device.

An example of a light-emitting display device will be described as a semiconductor device. As a display element included in a display device, a light-emitting element utilizing electroluminescence is described here. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element.

In an organic EL element, by application of a voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and a current flows. Then, the carriers (electrons and holes) recombine, so that the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Owing to such a mechanism, this light-emitting element is referred to as a current-excitation light-emitting element.

The inorganic EL elements are classified according to their element structures into a dispersion-type inorganic EL element and a thin-film inorganic EL element. The dispersion-type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission which utilizes a donor level and an acceptor level. The thin-film inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that description is made here using an organic EL element as a light-emitting element.

FIG. 20illustrates an example of a pixel structure as an example of a semiconductor device which can be driven by a digital time grayscale method.

The structure and operation of a pixel which can be driven by a digital time grayscale method will be described. An example is described here in which one pixel includes two n-channel transistors each using an oxide semiconductor layer (an In—Ga—Zn—O-based non-single-crystal film) for a channel formation region.

A pixel6400includes a switching transistor6401, a driving transistor6402, a light-emitting element6404, and a capacitor6403. A gate of the switching transistor6401is connected to a scan line6406, a first electrode (one of a source electrode and a drain electrode) of the switching transistor6401is connected to a signal line6405, and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor6401is connected to a gate of the driving transistor6402. The gate of the driving transistor6402is connected to a power source line6407through the capacitor6403, a first electrode of the driving transistor6402is connected to the power source line6407, and a second electrode of the driving transistor6402is connected to a first electrode (pixel electrode) of the light-emitting element6404. A second electrode of the light-emitting element6404corresponds to a common electrode6408. The common electrode6408is electrically connected to a common potential line provided over the same substrate.

Note that the second electrode (common electrode6408) of the light-emitting element6404is set to be a low power source potential. The low power source potential is lower than a high power source potential which is supplied to the power source line6407. For example, GND, 0 V, or the like may be set as the low power source potential. The difference between the high power source potential and the low power source potential is applied to the light-emitting element6404so that a current flows through the light-emitting element6404, whereby the light-emitting element6404emits light. Thus, each potential is set so that the difference between the high power source potential and the low power source potential is greater than or equal to a forward threshold voltage of the light-emitting element6404.

When the gate capacitance of the driving transistor6402is used as a substitute for the capacitor6403, the capacitor6403can be omitted. The gate capacitance of the driving transistor6402may be formed between a channel region and a gate electrode.

Here, in the case of using a voltage-input voltage-driving method, a video signal is input to the gate of the driving transistor6402to make the driving transistor6402completely turned on or off. That is, the driving transistor6402operates in a linear region, and thus, a voltage higher than the voltage of the power source line6407is applied to the gate of the driving transistor6402. Note that a voltage greater than or equal to (power source line voltage+Vthof the driving transistor6402) is applied to the signal line6405.

In the case of using an analog grayscale method instead of the digital time grayscale method, the same pixel structure as inFIG. 20can be employed by inputting signals in a different way.

In the case of using the analog grayscale driving method, a voltage greater than or equal to (forward voltage of the light-emitting element6404+Vthof the driving transistor6402) is applied to the gate of the driving transistor6402. The forward voltage of the light-emitting element6404refers to a voltage to obtain a desired luminance, and includes at least a forward threshold voltage. By inputting a video signal to enable the driving transistor6402to operate in a saturation region, a current can flow through the light-emitting element6404. In order that the driving transistor6402can operate in the saturation region, the potential of the power source line6407is higher than a gate potential of the driving transistor6402. With the analog video signal, a current in accordance with the video signal flows through the light-emitting element6404, and the analog grayscale driving method can be performed.

Note that the pixel structure is not limited to that illustrated inFIG. 20. For example, the pixel inFIG. 20can further include a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like.

Next, structures of the light-emitting element will be described with reference toFIGS. 21A to 21C. Here, a cross-sectional structure of a pixel will be described by taking an n-channel driving TFT as an example. Driving TFTs7001,7011, and7021used for semiconductor devices illustrated inFIGS. 21A to 21Ccan be manufactured in a manner similar to that of the thin film transistor described in Embodiment 1 and are highly reliable thin film transistors each including an oxide semiconductor film to which a metal element is added as a semiconductor layer. Alternatively, the thin film transistor described in Embodiments 2 to 7 can be employed as the driving TFTs7001,7011, and7021.

In order to extract light emitted from the light-emitting element, at least one of an anode and a cathode is required to transmit light. A thin film transistor and a light-emitting element are formed over a substrate. A light-emitting element can have a top emission structure in which light is extracted through the surface opposite to the substrate, a bottom emission structure in which light is extracted through the surface on the substrate side, or a dual emission structure in which light is extracted through the surface opposite to the substrate and the surface on the substrate side. The pixel structure can be applied to a light-emitting element having any of these emission structures.

A light-emitting element having a top emission structure will be described with reference toFIG. 21A.

FIG. 21Ais a cross-sectional view of a pixel in the case where the driving TFT7001is of an n-type and light is emitted from a light-emitting element7002to an anode7005side. InFIG. 21A, a cathode7003of the light-emitting element7002is electrically connected to the driving TFT7001, and a light-emitting layer7004and the anode7005are stacked in this order over the cathode7003. The cathode7003can be made of a variety of conductive materials as long as they have a low work function and reflect light. For example, Ca, Al, CaF, MgAg, AlLi, or the like is preferably used. The light-emitting layer7004may be formed as a single layer or a plurality of layers stacked. When the light-emitting layer7004is formed as a plurality of layers, the light-emitting layer7004is formed by stacking an electron-injecting layer, an electron-transporting layer, a light-emitting layer, a hole-transporting layer, and a hole-injecting layer in this order over the cathode7003. Note that not all of these layers need to be provided. The anode7005is made of a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added.

The light-emitting element7002corresponds to a region where the light-emitting layer7004is sandwiched between the cathode7003and the anode7005. In the case of the pixel illustrated inFIG. 21A, light is emitted from the light-emitting element7002to the anode7005side as indicated by an arrow.

Next, a light-emitting element having a bottom emission structure will be described with reference toFIG. 21B.FIG. 21Bis a cross-sectional view of a pixel in the case where the driving TFT7011is of an n-type and light is emitted from a light-emitting element7012to a cathode7013side. InFIG. 21B, the cathode7013of the light-emitting element7012is formed over a light-transmitting conductive film7017which is electrically connected to the driving TFT7011, and a light-emitting layer7014and an anode7015are stacked in this order over the cathode7013. Note that a light-blocking film7016for reflecting or blocking light may be formed to cover the anode7015when the anode7015has a light-transmitting property. For the cathode7013, various materials can be used, as in the case ofFIG. 21A, as long as they are conductive materials having a low work function. Note that the cathode7013is formed to have a thickness that can transmit light (preferably, approximately 5 nm to 30 nm). For example, an aluminum film with a thickness of 20 nm can be used as the cathode7013. Similarly to the case ofFIG. 21A, the light-emitting layer7014may be formed using either a single layer or a plurality of layers stacked. The anode7015is not required to transmit light, but can be made of a light-transmitting conductive material as in the case ofFIG. 21A. As the light-blocking film7016, a metal which reflects light can be used for example; however, it is not limited to a metal film. For example, a resin to which a black pigment is added can also be used.

The light-emitting element7012corresponds to a region where the light-emitting layer7014is sandwiched between the cathode7013and the anode7015. In the case of the pixel illustrated inFIG. 21B, light is emitted from the light-emitting element7012to the cathode7013side as indicated by an arrow.

Next, a light-emitting element having a dual emission structure will be described with reference toFIG. 21C. InFIG. 21C, a cathode7023of a light-emitting element7022is formed over a light-transmitting conductive film7027which is electrically connected to the driving TFT7021, and a light-emitting layer7024and an anode7025are stacked in this order over the cathode7023. As in the case ofFIG. 21A, the cathode7023can be made of a variety of conductive materials as long as they have a low work function. Note that the cathode7023is formed to have a thickness that can transmit light. For example, an aluminum film with a thickness of 20 nm can be used as the cathode7023. As inFIG. 21A, the light-emitting layer7024may be formed as either a single layer or a plurality of layers stacked. The anode7025can be made of a light-transmitting conductive material as in the case ofFIG. 21A.

The light-emitting element7022corresponds to a region where the cathode7023, the light-emitting layer7024, and the anode7025overlap with one another. In the case of the pixel illustrated inFIG. 21C, light is emitted from the light-emitting element7022to both the anode7025side and the cathode7023side as indicated by arrows.

Although an organic EL element is described here as a light-emitting element, an inorganic EL element can also be provided as a light-emitting element.

Note that the example is described in which a thin film transistor (a driving TFT) which controls the driving of a light-emitting element is electrically connected to the light-emitting element; however, a structure may be employed in which a TFT for current control is connected between the driving TFT and the light-emitting element.

Note that the structure of the semiconductor device is not limited to those illustrated inFIGS. 21A to 21Cand can be modified in various ways based on techniques disclosed in this specification.

Next, the appearance and a cross section of a light-emitting display panel (also referred to as a light-emitting panel), which is one embodiment of the semiconductor device, will be described with reference toFIGS. 24A and 24B.FIG. 24Ais a top view of a panel in which a thin film transistor and a light-emitting element formed over a first substrate are sealed between the first substrate and a second substrate with a sealant.FIG. 24Bis a cross-sectional view along line H-I ofFIG. 24A.

A sealant4505is provided to surround a pixel portion4502, signal line driver circuits4503aand4503b, and scan line driver circuits4504aand4504b, which are provided over a first substrate4501. In addition, a second substrate4506is provided over the pixel portion4502, the signal line driver circuits4503aand4503b, and the scan line driver circuits4504aand4504b. Accordingly, the pixel portion4502, the signal line driver circuits4503aand4503b, and the scan line driver circuits4504aand4504bare sealed together with a filler4507, by the first substrate4501, the sealant4505, and the second substrate4506. It is preferable that a display device be thus packaged (sealed) with a protective film (such as a bonding film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the display device is not exposed to the outside air.

The pixel portion4502, the signal line driver circuits4503aand4503b, and the scan line driver circuits4504aand4504bformed over the first substrate4501each include a plurality of thin film transistors, and a thin film transistor4510included in the pixel portion4502and a thin film transistor4509included in the signal line driver circuit4503aare illustrated as an example inFIG. 24B.

As each of the thin film transistors4509and4510, the thin film transistor including an oxide semiconductor film to which a metal element is added as a semiconductor layer which is described in Embodiment 1 can be employed. Alternatively, the thin film transistor described in Embodiments 2 to 7 can be employed. The thin film transistors4509and4510are n-channel thin film transistors.

Moreover, reference numeral4511denotes a light-emitting element. A first electrode layer4517that is a pixel electrode included in the light-emitting element4511is electrically connected to a source electrode layer or a drain electrode layer of the thin film transistor4510. Note that the structure of the light-emitting element4511is, but not limited to, the stack structure which includes the first electrode layer4517, an electroluminescent layer4512, and the second electrode layer4513. The structure of the light-emitting element4511can be changed as appropriate depending on the direction in which light is extracted from the light-emitting element4511, or the like.

A partition wall4520is made of an organic resin film, an inorganic insulating film, or organic polysiloxane. It is particularly preferable that the partition wall4520be formed of a photosensitive material to have an opening over the first electrode layer4517so that a sidewall of the opening is formed as an inclined surface with continuous curvature.

The electroluminescent layer4512may be formed as a single layer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer4513and the partition wall4520in order to prevent oxygen, hydrogen, moisture, carbon dioxide, or the like from entering the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed.

A variety of signals and potentials are supplied to the signal line driver circuits4503aand4503b, the scan line driver circuits4504aand4504b, or the pixel portion4502from FPCs4518aand4518b.

A connection terminal electrode4515is formed from the same conductive film as the first electrode layer4517included in the light-emitting element4511, and a terminal electrode4516is formed from the same conductive film as the source and drain electrode layers included in the thin film transistors4509and4510.

The connection terminal electrode4515is electrically connected to a terminal of the FPC4518athrough an anisotropic conductive film4519.

The second substrate4506located in the direction in which light is extracted from the light-emitting element4511needs to have a light-transmitting property. In that case, a light-transmitting material such as a glass plate, a plastic plate, a polyester film, or an acrylic film is used.

As the filler4507, an ultraviolet curable resin or a thermosetting resin can be used, in addition to an inert gas such as nitrogen or argon. For example, PVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate) can be used. For example, nitrogen is used for the filler.

The signal line driver circuits4503aand4503band the scan line driver circuits4504aand4504bmay be mounted as driver circuits formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared. Alternatively, only the signal line driver circuits or part thereof, or only the scan line driver circuits or part thereof may be separately formed and mounted. This embodiment is not limited to the structure illustrated inFIGS. 24A and 24B.

Through the above process, a highly reliable light-emitting display device (display panel) can be manufactured as a semiconductor device.

A semiconductor device disclosed in this specification can be applied to electronic paper. Electronic paper can be used for electronic appliances of a variety of fields as long as they display data. For example, electronic paper can be applied to an electronic book device (electronic book), a poster, an advertisement in a vehicle such as a train, or displays of various cards such as a credit card. Examples of the electronic appliances are illustrated inFIGS. 25A and 25BandFIG. 26.

FIG. 25Aillustrates a poster2631using electronic paper. In the case where an advertising medium is printed paper, the advertisement is replaced by hands; however, by using electronic paper disclosed in this specification, the advertising display can be changed in a short time. Furthermore, stable images can be obtained without display defects. Note that the poster may have a configuration capable of wirelessly transmitting and receiving data.

FIG. 25Billustrates an advertisement2632in a vehicle such as a train. In the case where an advertising medium is printed paper, the advertisement is replaced by hands; however, by using electronic paper disclosed in this specification, the advertising display can be changed in a short time with less manpower. Furthermore, stable images can be obtained without display defects. Note that the advertisement in a vehicle may have a configuration capable of wirelessly transmitting and receiving data.

FIG. 26illustrates an example of an electronic book device2700. For example, the electronic book device2700includes two housings, a housing2701and a housing2703. The housing2701and the housing2703are combined with a hinge2711so that the electronic book device2700can be opened and closed with the hinge2711as an axis. With such a structure, the electronic book device2700can operate like a paper book.

A display portion2705and a display portion2707are incorporated in the housing2701and the housing2703, respectively. The display portion2705and the display portion2707may display one image or different images. In the case where the display portion2705and the display portion2707display different images, for example, text can be displayed on a display portion on the right side (the display portion2705inFIG. 26) and graphics can be displayed on a display portion on the left side (the display portion2707inFIG. 26).

FIG. 26illustrates an example in which the housing2701is provided with an operation portion and the like. For example, the housing2701is provided with a power switch2721, an operation key2723, a speaker2725, and the like. With the operation key2723, pages can be turned. A keyboard, a pointing device, and the like may be provided on the same surface as the display portion of the housing. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, a terminal that can be connected to various cables such as an AC adapter and a USB cable, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Moreover, the electronic book device2700may have a function of an electronic dictionary.

The electronic book device2700may have a configuration capable of wirelessly transmitting and receiving data. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server.

A semiconductor device disclosed in this specification can be applied as a variety of electronic appliances (including amusement machines). Examples of electronic appliances include television sets (also referred to as televisions or television receivers), monitor of computers or the like, cameras such as digital cameras or digital video cameras, digital photo frames, cellular phones (also referred to as mobile phones or mobile phone sets), portable game consoles, portable information terminals, audio reproducing devices, large-sized game machines such as pachinko machines, and the like.

FIG. 27Aillustrates an example of a television set9600. In the television set9600, a display portion9603is incorporated in a housing9601. Images can be displayed on the display portion9603. Here, the housing9601is supported by a stand9605.

The television set9600can be operated with an operation switch of the housing9601or a separate remote controller9610. Channels and volume can be controlled with an operation key9609of the remote controller9610so that an image displayed on the display portion9603can be controlled. Furthermore, the remote controller9610may be provided with a display portion9607for displaying data output from the remote controller9610.

Note that the television set9600is provided with a receiver, a modem, and the like. With the receiver, a general television broadcast can be received. Furthermore, when the television set9600is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed.

FIG. 27Billustrates an example of a digital photo frame9700. For example, in the digital photo frame9700, a display portion9703is incorporated in a housing9701. Various images can be displayed on the display portion9703. For example, the display portion9703can display data of an image shot by a digital camera or the like to function as a normal photo frame.

Note that the digital photo frame9700is provided with an operation portion, an external connection portion (a USB terminal, a terminal that can be connected to various cables such as a USB cable, or the like), a recording medium insertion portion, and the like. Although they may be provided on the same surface as the display portion, it is preferable to provide them on the side surface or the back surface for the design of the digital photo frame9700. For example, a memory storing data of an image shot by a digital camera is inserted in the recording medium insertion portion of the digital photo frame, whereby the image data can be transferred and displayed on the display portion9703.

The digital photo frame9700may have a configuration capable of wirelessly transmitting and receiving data. Through wireless communication, desired image data can be transferred to be displayed.

FIG. 28Aillustrates a portable amusement machine including two housings: a housing9881and a housing9891. The housings9881and9891are connected with a connection portion9893so as to be opened and closed. A display portion9882and a display portion9883are incorporated in the housing9881and the housing9891, respectively. In addition, the portable amusement machine illustrated inFIG. 28Aincludes a speaker portion9884, a recording medium insertion portion9886, an LED lamp9890, an input means (an operation key9885, a connection terminal9887, a sensor9888(a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), or a microphone9889), and the like. It is needless to say that the structure of the portable amusement machine is not limited to the above and other structures provided with at least a semiconductor device disclosed in this specification can be employed. The portable amusement machine may include other accessory equipment as appropriate. The portable amusement machine illustrated inFIG. 28Ahas a function of reading a program or data stored in a recording medium to display it on the display portion, and a function of sharing information with another portable amusement machine by wireless communication. The portable amusement machine illustrated inFIG. 28Acan have various functions without limitation to the above.

FIG. 28Billustrates an example of a slot machine9900which is a large-sized amusement machine. In the slot machine9900, a display portion9903is incorporated in a housing9901. In addition, the slot machine9900includes an operation means such as a start lever or a stop switch, a coin slot, a speaker, and the like. It is needless to say that the structure of the slot machine9900is not limited to the above and other structures provided with at least a semiconductor device disclosed in this specification may be employed. The slot machine9900may include other accessory equipment as appropriate.

FIG. 29Aillustrates an example of a cellular phone1000. The cellular phone1000is provided with a display portion1002incorporated in a housing1001, an operation button1003, an external connection port1004, a speaker1005, a microphone1006, and the like.

When the display portion1002of the cellular phone1000illustrated inFIG. 29Ais touched with a finger or the like, data can be input into the cellular phone1000. Furthermore, operations such as making calls and composing mails can be performed by touching the display portion1002with a finger or the like.

There are mainly three screen modes for the display portion1002. The first mode is a display mode mainly for displaying images. The second mode is an input mode mainly for inputting data such as text. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are combined.

For example, in the case of making a call or composing a mail, a text input mode mainly for inputting text is selected for the display portion1002so that text displayed on a screen can be input. In that case, it is preferable to display a keyboard or number buttons on almost all the area of the screen of the display portion1002.

When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone1000, display on the screen of the display portion1002can be automatically switched by determining the orientation of the cellular phone1000(whether the cellular phone1000is placed horizontally or vertically for a landscape mode or a portrait mode).

The screen mode is switched by touching the display portion1002or operating the operation button1003of the housing1001. Alternatively, the screen mode may be switched depending on the kind of images displayed on the display portion1002. For example, when a signal of an image displayed on the display portion is of moving image data, the screen mode is switched to the display mode. When the signal is of text data, the screen mode is switched to the input mode.

Furthermore, in the input mode, when input by touching the display portion1002is not performed for a certain period of time while a signal is detected by an optical sensor in the display portion1002, the screen mode may be controlled so as to be switched from the input mode to the display mode.

The display portion1002may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touching the display portion1002with the palm or the finger, whereby personal authentication can be performed. Furthermore, by providing a backlight or a sensing light source emitting a near-infrared light for the display portion, an image of a finger vein, a palm vein, or the like can also be taken.

FIG. 29Billustrates another example of a cellular phone. The cellular phone inFIG. 29Bhas a display device9410in a housing9411, which includes a display portion9412and operation buttons9413, and a communication device9400in a housing9401, which includes operation buttons9402, an external input terminal9403, a microphone9404, a speaker9405, and a light-emitting portion9406that emits light when a phone call is received. The display device9410which has a display function can be detached from or attached to the communication device9400which has a phone function by moving in two directions represented by arrows. Thus, the display device9410and the communication device9400can be attached to each other along their short sides or long sides. In addition, when only the display function is needed, the display device9410can be detached from the communication device9400and used alone. Images or input information can be transmitted or received by wireless or wire communication between the communication device9400and the display device9410, each of which has a rechargeable battery.

This application is based on Japanese Patent Application serial no. 2008-286384 filed with Japan Patent Office on Nov. 7, 2008, the entire contents of which are hereby incorporated by reference.