Semiconductor device

A highly reliable semiconductor device which is formed using an oxide semiconductor and has stable electric characteristics is provided. A semiconductor device which includes an amorphous oxide semiconductor layer including a region containing oxygen in a proportion higher than that in the stoichiometric composition, and an aluminum oxide film provided over the amorphous oxide semiconductor layer is provided. The amorphous oxide semiconductor layer is formed as follows: oxygen implantation treatment is performed on a crystalline or amorphous oxide semiconductor layer which has been subjected to dehydration or dehydrogenation treatment, and then thermal treatment is performed on the oxide semiconductor layer provided with an aluminum oxide film at a temperature lower than or equal to 450° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.

It is to be noted that the semiconductor device in this specification refers to all devices that can function by utilizing semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic appliances are all semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor (also referred to as a thin film transistor (TFT)) with the use of a semiconductor thin film formed over a substrate having an insulating surface. Such a transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor material has been attracting attention.

For example, a transistor whose active layer is formed using an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration lower than 1018/cm3is disclosed (see Patent Document 1).

REFERENCE

Patent Document

SUMMARY OF THE INVENTION

However, the electric conductivity of a semiconductor device including an oxide semiconductor changes when hydrogen or moisture forming an electron donor contaminates the oxide semiconductor in a process of forming a thin film of the oxide semiconductor. The electric conductivity might also change when the formed oxide semiconductor thin film includes an oxygen vacancy. Such a phenomenon causes variation in the electric characteristics of a transistor formed using the oxide semiconductor.

In view of the above problem, an object is to provide a highly reliable semiconductor device which is formed using an oxide semiconductor and has stable electric characteristics.

A semiconductor device according to one embodiment of the invention disclosed herein includes an amorphous oxide semiconductor layer including a region containing oxygen in a proportion higher than that in the stoichiometric composition (that is, stoichiometrically excessive oxygen), and an aluminum oxide film provided over the amorphous oxide semiconductor layer. The amorphous oxide semiconductor layer is formed as follows: oxygen implantation treatment is performed on a crystalline or amorphous oxide semiconductor layer which has been subjected to dehydration or dehydrogenation treatment, and then thermal treatment is performed on the oxide semiconductor layer provided with an aluminum oxide film at a temperature which allows the amorphous state to be maintained. The temperature of the thermal treatment is lower than or equal to 450° C. Specifically, the following structure can be employed for example.

One embodiment of the present invention is a semiconductor device which includes an amorphous oxide semiconductor layer including a region containing oxygen in a proportion higher than that in a stoichiometric composition, a source electrode and a drain electrode that are electrically connected to the amorphous oxide semiconductor layer, a gate electrode overlapping with the amorphous oxide semiconductor layer, a gate insulating layer provided between the amorphous oxide semiconductor layer and the gate electrode, and an aluminum oxide film provided over the amorphous oxide semiconductor layer.

Another embodiment of the present invention is a semiconductor device which includes an amorphous oxide semiconductor layer including a region containing oxygen in a proportion higher than that in a stoichiometric composition, a source electrode and a drain electrode that are electrically connected to the amorphous oxide semiconductor layer, a gate insulating layer provided over the amorphous oxide semiconductor layer so as to cover the source electrode and the drain electrode, a gate electrode provided over the gate insulating layer so as to overlap with the amorphous oxide semiconductor layer, and an aluminum oxide film provided over and in contact with the gate electrode.

Another embodiment of the present invention is a semiconductor device which includes a gate electrode, a gate insulating layer provided over the gate electrode, an amorphous oxide semiconductor layer that is provided over the gate insulating layer so as to overlap with the gate electrode and includes a region containing oxygen in a proportion higher than that in a stoichiometric composition, a source electrode and a drain electrode that are electrically connected to the amorphous oxide semiconductor layer, and an aluminum oxide film provided over the amorphous oxide semiconductor layer so as to be in contact with at least part of the amorphous oxide semiconductor layer.

In any one of the above semiconductor devices, the gate insulating layer preferably includes a region containing oxygen in a proportion higher than that in a stoichiometric composition.

In any one of the above semiconductor devices, it is preferable that an oxide insulating film be further provided between the aluminum oxide film and the amorphous oxide semiconductor layer and that the oxide insulating film include a region containing oxygen in a proportion higher than that in a stoichiometric composition.

Note that the term “over” in this specification and the like does not necessarily mean that a component is placed “directly on” another component. For example, the expression “a gate electrode over a gate insulating layer” does not exclude the case where a component is placed between the gate insulating layer and the gate electrode. The same applies to the term “below”.

In this specification and the like, the term “electrode” or “wiring” does not limit the function of a component. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. In addition, the term “electrode” or “wiring” can also mean a combination of a plurality of “electrodes” or “wirings”, for example.

Oxygen implantation is performed so that excess oxygen is contained in an amorphous oxide semiconductor layer, and thermal treatment is performed in a state where an aluminum oxide film is provided over the amorphous oxide semiconductor layer in order to prevent the oxygen in the amorphous oxide semiconductor layer from being released, whereby it is possible to prevent generation or increase of defects in the amorphous oxide semiconductor and at interfaces between the amorphous oxide semiconductor and layers which are over/under and in contact with the amorphous oxide semiconductor. That is, the excess oxygen contained in the amorphous oxide semiconductor layer acts to fill an oxygen-vacancy defect, so that a highly reliable semiconductor device having stable electric characteristics can be provided.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the invention disclosed in this specification will be described in detail with reference to the accompanying drawings. Note that the invention disclosed in this specification is not limited to the following description, and it is easily understood by those skilled in the art that modes and details of the invention can be modified in various ways. Therefore, the invention disclosed in this specification is not limited to the description of the following embodiments and examples.

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

In this embodiment, one embodiment of a semiconductor device and one embodiment of a method of manufacturing the semiconductor device will be described with reference toFIGS. 1A to 1C,FIGS. 2A to 2C, andFIGS. 3A to 3D.

FIGS. 1A to 1Care a plan view and cross-sectional views which illustrate a top-gate transistor510as an example of a semiconductor device.FIG. 1Ais a plan view of the transistor510, andFIGS. 1B and 1Care cross-sectional views along line A-B and line C-D inFIG. 1A, respectively. Note that inFIG. 1A, some components of the transistor510(e.g., a gate insulating layer406) are not illustrated for brevity.

The transistor510illustrated inFIGS. 1A to 1Cincludes, over a substrate400having an insulating surface, a base insulating layer402, an amorphous oxide semiconductor layer404, a source electrode405a, a drain electrode405b, the gate insulating layer406, a gate electrode410, and an insulating layer412.

In the transistor510illustrated inFIGS. 1A to 1C, the amorphous oxide semiconductor layer404includes a region containing oxygen in a proportion higher than that in the stoichiometric composition (hereinafter also referred to as an oxygen-excess region). In general, oxygen, which is one of main components of the amorphous oxide semiconductor layer404, dynamically repeats bonding to and dissociation from metal elements that are other main components in the layer. A metal element from which oxygen is dissociated has a dangling bond; therefore, it is supposed that a certain number of oxygen vacancies caused by dissociation of oxygen exist in the amorphous oxide semiconductor layer. In the transistor according to one embodiment of the present invention, however, excess oxygen contained in the amorphous oxide semiconductor layer404can immediately compensate a defect (oxygen defect) due to an oxygen vacancy in the amorphous oxide semiconductor layer404. Consequently, a highly reliable semiconductor device having stable electric characteristics can be provided.

The amorphous oxide semiconductor layer404has an amorphous structure in whole.

In the transistor510, a layer including an aluminum oxide film is provided as the insulating layer412. Since aluminum oxide has a barrier property and thus is less likely to transmit hydrogen, moisture, oxygen, and another impurity, entry of an impurity such as moisture from the outside after completion of the device can be prevented. Note that the insulating layer412includes at least an aluminum oxide film and may have a stacked structure including the aluminum oxide film and a film containing another inorganic insulating material. In the case where the insulating layer412has a stacked structure including the aluminum oxide film and a film containing another inorganic insulating material, it is preferable that the film containing another inorganic insulating material be positioned on the amorphous oxide semiconductor layer404side and be an oxide insulating film including an oxygen-excess region. For example, the insulating layer412can have a structure in which a silicon oxide film including an oxygen-excess region and the aluminum oxide film are stacked in this order from the amorphous oxide semiconductor layer404side.

It is preferable that the gate insulating layer406include an oxygen-excess region for the following reason. When the gate insulating layer406includes an oxygen-excess region, oxygen can be prevented from moving from the amorphous oxide semiconductor layer404to the gate insulating layer406, and oxygen can be supplied from the gate insulating layer406to the amorphous oxide semiconductor layer404. It is preferable that the base insulating layer402also include an oxygen-excess region.

An insulating layer may be further provided over the transistor510. Further, an opening may be formed in the gate insulating layer406or the like in order that the source electrode405aor the drain electrode405bmay be electrically connected to a wiring. Note that the amorphous oxide semiconductor layer404is not necessarily processed into an island shape.

FIGS. 2A to 2Cillustrate another structure example of a transistor according to this embodiment.FIG. 2Ais a plan view of a transistor520, andFIGS. 2B and 2Care cross-sectional views along line E-F and line G-H inFIG. 2A, respectively. Note that inFIG. 2A, some components of the transistor520(e.g., the gate insulating layer406) are omitted for brevity.

As in the case of the transistor510illustrated inFIGS. 1A to 1C, the transistor520illustrated inFIGS. 2A to 2Cincludes, over the substrate400having an insulating surface, the base insulating layer402, the amorphous oxide semiconductor layer404, the source electrode405a, the drain electrode405b, the gate insulating layer406, the gate electrode410, and the insulating layer412.

A difference between the transistor520illustrated inFIGS. 2A to 2Cand the transistor510illustrated inFIGS. 1A to 1Cis the stacking order of the source and drain electrodes405aand405band the amorphous oxide semiconductor layer404. That is, the transistor520includes the source electrode405aand the drain electrode405bwhich are in contact with the base insulating layer402, and the amorphous oxide semiconductor layer404provided over the source electrode405aand the drain electrode405b. The other components are similar to those of the transistor510, and thus the description of the transistor510can be referred to for the details.

An example of a manufacturing process of the transistor510will be described below with reference toFIGS. 3A to 3D. Note that the transistor520can be manufactured in a process similar to that of the transistor510except for the stacking order of the source and drain electrodes405aand405band the amorphous oxide semiconductor layer404.

First, the base insulating layer402is formed over the substrate400having an insulating surface. There is no particular limitation on a substrate that can be used as the substrate400having an insulating surface as long as it has heat resistance high enough to withstand thermal treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. A single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like; a compound semiconductor substrate of silicon germanium or the like; an SOI substrate; or the like can be used as the substrate400. Alternatively, any of these substrates over which a semiconductor element is provided may be used as the substrate400.

A flexible substrate may be used as the substrate400. In the case of using a flexible substrate, a transistor including an oxide semiconductor film may be directly formed over the flexible substrate, or a transistor including an oxide semiconductor film may be formed over a different manufacturing substrate and then separated to be transferred to the flexible substrate. Note that in order to separate the transistor from the manufacturing substrate and transfer it to the flexible substrate, a separation layer may be provided between the manufacturing substrate and the transistor including the oxide semiconductor film.

The base insulating layer402can have a single-layer or stacked structure including one or more films selected from those containing silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, hafnium oxide, gallium oxide, and a mixed material of any of these materials. When the base insulating layer402is formed as a stacked structure including an oxide insulating film, it is preferred that the oxide insulating film be in contact with the amorphous oxide semiconductor layer404formed later. In this embodiment, a silicon oxide film is formed as the base insulating layer402by a plasma CVD method, a sputtering method, or the like.

Further, the base insulating layer402preferably includes an oxygen-excess region because an oxygen vacancy in the amorphous oxide semiconductor layer404can be compensated by excess oxygen contained in the base insulating layer402. In the case of having a stacked structure, the base insulating layer402preferably includes an oxygen-excess region at least in a layer in contact with the amorphous oxide semiconductor layer404. In order to provide the oxygen-excess region in the base insulating layer402, for example, the base insulating layer402may be formed in an oxygen atmosphere. Alternatively, the oxygen-excess region may be formed by implanting oxygen (including at least one of an oxygen radical, an oxygen atom, and an oxygen ion) into the base insulating layer402after its formation. Oxygen can be implanted by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like.

Next, over the base insulating layer402, an amorphous oxide semiconductor layer404ahaving a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm, is formed (seeFIG. 3A).

A metal oxide material containing two or more kinds selected from In, Ga, Zn, and Sn may be used as an oxide semiconductor material. For example, a four-component metal oxide such as an In—Sn—Ga—Zn—O-based material; a three-component metal oxide such as an In—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, or a Hf—In—Zn—O-based material; a two-component metal oxide such as an In—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-based material, a Sn—Mg—O-based material, an In—Mg—O-based material, or an In—Ga—O-based material; an In—O-based material; a Sn—O-based material; or a Zn—O-based material may be used. In addition, any of the above oxide semiconductors may contain an element other than In, Ga, Sn, and Zn, for example, SiO2.

Here, for example, an In—Ga—Zn—O-based oxide semiconductor means an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) and there is no particular limitation on the composition ratio thereof.

In addition, as the amorphous oxide semiconductor layer404a, a thin film of a material represented by the chemical formula, InMO3(ZnO)m(m>0), can be used. Here, M represents one or more metal elements selected from Zn, Ga, Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, or Ga and Co.

In the case where an In—Sn—Zn—O-based material is used as an oxide semiconductor, a target to be used may have a composition ratio of In:Sn:Zn=1:2:2, In:Sn:Zn=2:1:3, In:Sn:Zn=1:1:1, or the like in an atomic ratio.

In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target to be used has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In2O3:ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In2O3:ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomic ratio (In2O3:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O═X:Y:Z, the relation of Z≧1.5X+Y is satisfied.

The amorphous oxide semiconductor layer404ais preferably formed by a sputtering method. In addition, in the formation of the amorphous oxide semiconductor layer404aby a sputtering method, the concentration of hydrogen contained in the amorphous oxide semiconductor layer404ais preferably reduced as much as possible. In order to reduce the hydrogen concentration, a high-purity rare gas (typically argon), high-purity oxygen, or a high-purity mixed gas of a rare gas and oxygen, from which impurities such as hydrogen, water, a compound having a hydroxyl group, and a hydride are removed, is supplied into a treatment chamber of a sputtering apparatus as an atmosphere gas, as appropriate. Evacuation of the treatment chamber is preferably performed using a cryopump having a high capability in evacuating water or a sputter ion pump having a high capability in evacuating hydrogen.

It is preferable that the base insulating layer402and the amorphous oxide semiconductor layer404abe successively formed without exposure to the air. For example, the following steps may be performed: impurities containing hydrogen attached to a surface of the substrate400are removed by thermal treatment or plasma treatment, and then the base insulating layer402and the amorphous oxide semiconductor layer404aare successively formed in this order without exposure to the air. In this manner, impurities containing hydrogen attached to a surface of the base insulating layer402can be reduced and an atmospheric component can be prevented from being attached to an interface between the substrate400and the base insulating layer402and an interface between the base insulating layer402and the amorphous oxide semiconductor layer404a. As a result, it is possible to manufacture the transistor510having favorable electric characteristics and high reliability.

Note that before the amorphous oxide semiconductor layer404ais formed by a sputtering method, powder substances (also referred to as particles or dust) which are attached to the surface of the base insulating layer402are 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, not to a target side, in an argon atmosphere by using an RF power source and plasma is generated in the vicinity of the substrate to modify a surface. Note that instead of an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or the like may be used.

Next, thermal treatment (first thermal treatment) is performed on the amorphous oxide semiconductor layer404ain order to remove hydrogen (including water and a hydroxyl group) (dehydration or dehydrogenation). The thermal treatment is performed at a temperature at which the amorphous oxide semiconductor layer404ais not crystallized, typically, higher than or equal to 250° C. and lower than or equal to 450° C., preferably lower than or equal to 300° C.

Hydrogen, which is an n-type impurity, can be removed from the oxide semiconductor by the thermal treatment; thus, the oxide semiconductor can be highly purified so as to contain as few impurities as possible. For example, the concentration of hydrogen contained in the amorphous oxide semiconductor layer404aafter the dehydration or dehydrogenation treatment can be lower than or equal to 5×1019/cm3or lower than or equal to 5×1018/cm3.

Note that the thermal treatment for dehydration or dehydrogenation is preferably performed before the amorphous oxide semiconductor layer404ais processed into an island shape because oxygen contained in the base insulating layer402can be prevented from being released by the thermal treatment.

Further, it is preferable that in the thermal treatment, water, hydrogen, and the like be not contained in nitrogen or a rare gas such as helium, neon, or argon. The purity of nitrogen or a rare gas such as helium, neon, or argon which is introduced into a thermal treatment apparatus is preferably higher than or equal to 6N (99.9999%), further preferably higher than or equal to 7N (99.99999%) (that is, the impurity concentration is preferably lower than or equal to 1 ppm, further preferably lower than or equal to 0.1 ppm).

Next, the amorphous oxide semiconductor layer404ais processed into the island-shaped amorphous oxide semiconductor layer404in a photolithography step. Note that a resist mask for forming the island-shaped amorphous oxide semiconductor layer404may be formed by an inkjet method. Formation of the resist mask by an inkjet method needs no photomask; thus, the manufacturing cost of the semiconductor device can be reduced.

Next, a conductive film which is to be a source electrode and a drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode) is formed over the amorphous oxide semiconductor layer404and is processed, so that the source electrode405aand the drain electrode405bare formed (seeFIG. 3B).

The conductive film used for the source electrode405aand the drain electrode405bis formed using a material that can withstand a thermal treatment step performed later. For example, a metal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride film containing any of these elements as a component (titanium nitride film, molybdenum nitride film, or tungsten nitride film) can be used. Alternatively, a film of a high-melting-point metal such as Ti, Mo, or W or a metal nitride film of any of these elements (titanium nitride film, molybdenum nitride film, or tungsten nitride film) may be stacked on one or both of a bottom side and a top side of a metal film of Al, Cu, or the like. Further alternatively, the conductive film used for the source electrode and the drain electrode may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), indium oxide-tin oxide (In2O3—SnO2, abbreviated to ITO), indium oxide-zinc oxide (In2O3—ZnO), or any of these metal oxide materials containing silicon oxide can be used.

Note that it is preferable that etching conditions be optimized so that the amorphous oxide semiconductor layer404is not etched and divided when the conductive film is etched. However, it is difficult to obtain conditions under which only the conductive film is etched and the amorphous oxide semiconductor layer404is not etched at all. For that reason, in some cases, part of the amorphous oxide semiconductor layer404is etched to be an amorphous oxide semiconductor layer having a groove (depressed portion) when the conductive film is etched.

Next, the gate insulating layer406which covers the source electrode405aand the drain electrode405band is in contact with part of the amorphous oxide semiconductor layer404is formed.

The gate insulating layer406is preferably formed by a plasma CVD method or a sputtering method, and can be formed as a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, a silicon nitride oxide film, or the like. Alternatively, as a material for the gate insulating layer406, a high-k material such as hafnium oxide, yttrium oxide, lanthanum oxide, hafnium silicate (HfSixOy(x>0, y>0)), hafnium aluminate (HfAlxOy(x>0, y>0)), hafnium silicate to which nitrogen is added, or hafnium aluminate to which nitrogen is added may be used. Use of such a high-k material enables a reduction in gate leakage current. Note that the gate insulating layer406may have either a single-layer structure or a stacked structure.

Note that the gate insulating layer406preferably includes an oxygen-excess region because an oxygen vacancy in the amorphous oxide semiconductor layer404can be compensated by excess oxygen contained in the gate insulating layer406.

Next, oxygen421is implanted into the amorphous oxide semiconductor layer404from above the gate insulating layer406, so that an oxygen-excess region is formed (seeFIG. 3C). By the implantation treatment of the oxygen421, oxygen in the amorphous oxide semiconductor layer404, which is decreased in concentration through the above thermal treatment for dehydration or dehydrogenation, can be supplied. Thus, the amorphous oxide semiconductor layer404can be highly purified and become an electrically i-type (intrinsic) semiconductor. Further, formation of the oxygen-excess region in the amorphous oxide semiconductor layer404enables an oxygen vacancy to be compensated. Thus, charge trapping centers in the amorphous oxide semiconductor layer404can be reduced.

As a method of implanting the oxygen421, a method by which the oxygen421can be implanted into the inside or an interface of the amorphous oxide semiconductor layer404is employed. For example, an ion implantation method or an ion doping method can be used. In an ion implantation method, a source gas is made into plasma, ion species included in this plasma are extracted and mass-separated, and ion species with predetermined mass are accelerated and implanted into an object to be processed as an ion beam. In an ion doping method, a source gas is made into plasma, ion species are extracted from the plasma by application of a predetermined electric field, and the extracted ion species are accelerated without mass separation and implanted into an object to be processed as an ion beam. When the implantation of the oxygen421is performed by an ion implantation method involving mass separation, an impurity such as a metal element can be prevented from being added, together with the oxygen421, to the amorphous oxide semiconductor layer404. On the other hand, an ion doping method enables ion-beam irradiation to a larger area than an ion implantation method; therefore, when the implantation of the oxygen421is performed by an ion doping method, the takt time can be shortened.

Alternatively, as a method of implanting the oxygen421, a plasma immersion ion implantation method may be used. By a plasma immersion ion implantation method, the oxygen421can be implanted efficiently even when the amorphous oxide semiconductor layer404has an uneven shape.

In this embodiment, an example is shown in which the oxygen421is implanted into the amorphous oxide semiconductor layer404through the gate insulating layer406. Implantation of the oxygen421into the amorphous oxide semiconductor layer404through a film stacked over the amorphous oxide semiconductor layer404is advantageous in that the depth at which the oxygen is implanted (implantation region) can be controlled more easily, so that the oxygen421can be efficiently implanted into the amorphous oxide semiconductor layer404. Note that an embodiment of the present invention is not limited to this manner; it is also possible to perform the implantation in a state where a surface of the amorphous oxide semiconductor layer404is exposed (i.e., before the conductive film which is to be the source electrode405aand the drain electrode405bis formed) or after the insulating layer412is formed.

The depth at which the oxygen421is implanted may be controlled by appropriately setting an implantation condition such as acceleration voltage or a dose amount, or the thickness of the gate insulating layer406through which the oxygen passes. The oxygen implantation treatment is performed so that the amount of oxygen contained in the amorphous oxide semiconductor layer404exceeds that in the stoichiometric composition. For example, a peak of the concentration of oxygen in the amorphous oxide semiconductor layer404, which is introduced by the oxygen implantation treatment, is preferably higher than or equal to 1×1018/cm3and lower than or equal to 5×1021/cm3. The oxygen421for implantation includes an oxygen radical, an oxygen atom, and/or an oxygen ion. Note that the oxygen-excess region may exist in part (including an interface) of the amorphous oxide semiconductor layer404.

Oxygen is one of main components of an oxide semiconductor. Therefore, it is difficult to accurately estimate the oxygen concentration of an oxide semiconductor film by a method such as secondary ion mass spectrometry (SIMS). That is, it is difficult to determine whether oxygen is intentionally added to the amorphous oxide semiconductor layer.

It is known that isotopes such as17O and18O exist in oxygen and that the proportions of17O and18O in all of the oxygen atoms in nature are 0.037% and 0.204%, respectively. That is to say, it is possible to measure the concentrations of these isotopes in the amorphous oxide semiconductor layer by a method such as SIMS; therefore, the oxygen concentration of the amorphous oxide semiconductor layer may be able to be estimated more accurately by measuring the concentration of such an isotope. Thus, the concentration of the isotope may be measured to determine whether oxygen is intentionally added to the amorphous oxide semiconductor layer.

Part of the oxygen421added to (contained in) the amorphous oxide semiconductor layer404may have a dangling bond in the oxide semiconductor. This is because such a dangling bond is bonded with hydrogen remaining in the layer so that hydrogen can be fixed (made to be an immovable ion).

Next, a conductive film which is to be a gate electrode (including a wiring formed in the same layer as the gate electrode) is formed over the gate insulating layer406and is processed, so that the gate electrode410is formed. The gate electrode410can be formed by a plasma CVD method, a sputtering method, or the like with the use of a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as a main component. Alternatively, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used for the gate electrode410. The gate electrode410may have either a single-layer structure or a stacked structure.

Next, the insulating layer412which covers the gate electrode410is formed (seeFIG. 3D). A layer including an aluminum oxide film can be used as the insulating layer412. That is, the insulating layer412contains aluminum oxide. An aluminum oxide film has a barrier property and thus is less likely to transmit moisture, oxygen, and another impurity. Therefore, by providing an aluminum oxide film over the amorphous oxide semiconductor layer404, the aluminum oxide film functions as a passivation film and prevents an impurity such as moisture from entering the amorphous oxide semiconductor layer404from the outside after completion of the device. Moreover, release of oxygen from the amorphous oxide semiconductor layer404can be prevented.

Further, the insulating layer412may have a stacked structure including the aluminum oxide film and an oxide insulating film (e.g., a silicon oxide film or a silicon oxynitride film) including an oxygen-excess region.

The insulating layer412has a thickness of at least 1 nm and can be formed by a method by which impurities such as water and hydrogen do not enter the insulating layer412, such as a sputtering method, as appropriate. When hydrogen is contained in the insulating layer412, hydrogen may possibly enter the amorphous oxide semiconductor layer404or abstract oxygen in the amorphous oxide semiconductor layer404. Therefore, it is important to employ a film formation method in which hydrogen is not used so that the insulating layer412does not contain hydrogen as much as possible. It is preferable that a high-purity gas from which impurities such as hydrogen, water, a compound having a hydroxyl group, and a hydride are removed be used as a sputtering gas for the formation of the insulating layer412.

After the insulating layer412is formed, thermal treatment (second thermal treatment) is performed. The thermal treatment is performed at a temperature at which the amorphous oxide semiconductor layer404is not crystallized, preferably higher than or equal to 250° C. and lower than or equal to 450° C. The thermal treatment may be performed in an atmosphere of nitrogen, oxygen, ultra-dry air (air in which the water content is less than or equal to 20 ppm, preferably less than or equal to 1 ppm, further preferably less than or equal to 10 ppb), or a rare gas (such as argon or helium). Note that it is preferable that water, hydrogen, and the like be not contained in the atmosphere of nitrogen, oxygen, ultra-dry air, a rare gas, or the like. The purity of nitrogen, oxygen, or a rare gas which is introduced into a thermal treatment apparatus is preferably higher than or equal to 6N (99.9999%), further preferably higher than or equal to 7N (99.99999%) (that is, the impurity concentration is preferably lower than or equal to 1 ppm, further preferably lower than or equal to 0.1 ppm).

Although the timing of the thermal treatment (second thermal treatment) performed after the oxygen implantation treatment is not limited to that in this embodiment, the thermal treatment needs to be performed at least after the insulating layer412is formed. This is because, since the aluminum oxide film used as the insulating layer412has a high blocking effect and thus is less likely to transmit both oxygen and impurities such as hydrogen and moisture, release of oxygen from the amorphous oxide semiconductor layer404can be prevented by performing the thermal treatment after the insulating layer412is formed.

By the second thermal treatment, oxygen may be supplied to the amorphous oxide semiconductor layer404from the insulating film (e.g., the gate insulating layer406or the base insulating layer402) which contains oxygen and is in contact with the amorphous oxide semiconductor layer404.

Through the above steps, the transistor510is formed (seeFIG. 3D). In the formation of the transistor510, by the thermal treatment for dehydration or dehydrogenation, impurities such as hydrogen, water, a hydroxyl group, and a hydride (also referred to as a hydrogen compound) can be intentionally removed from the amorphous oxide semiconductor layer; by the subsequent oxygen implantation treatment, oxygen, which is decreased in concentration through the thermal treatment for dehydration or dehydrogenation, can be supplied to the amorphous oxide semiconductor layer. Thus, the amorphous oxide semiconductor layer can be highly purified and become an electrically i-type (intrinsic) semiconductor.

Further, an oxygen-excess region is formed by the oxygen implantation treatment, whereby formation of an oxygen vacancy in the amorphous oxide semiconductor layer or at the interface thereof can be suppressed and the number of donor levels in the energy gap due to oxygen vacancies can be reduced or the donor levels can be substantially removed. Therefore, variation in the electric characteristics of the transistor510is suppressed, and the transistor510is electrically stable. The amorphous oxide semiconductor layer has an amorphous structure in whole, and thus has uniform film quality as compared with a partly crystallized oxide semiconductor layer.

According to this embodiment, a semiconductor device which includes an amorphous oxide semiconductor and has stable electric characteristics can be provided. Further, a highly reliable semiconductor device can be provided.

The methods, structures, and the like described in this embodiment can be combined as appropriate with any of the methods, structures, and the like described in the other embodiments.

In this embodiment, a method of manufacturing the transistor510, which is different from that in Embodiment 1, will be described with reference toFIGS. 4A to 4D. Note that the same portions as Embodiment 1 or portions having functions similar to those of Embodiment 1 can be formed as in Embodiment 1, and also the same steps as Embodiment 1 or steps similar to those of Embodiment 1 can be performed as in Embodiment 1; therefore, repetitive description thereof is omitted. In addition, detailed description of the same portions is not repeated.

First, the base insulating layer402is formed over the substrate400having an insulating surface, and then an oxide semiconductor layer401ais formed in contact with the base insulating layer402(seeFIG. 4A).

In this embodiment, the oxide semiconductor layer401acan be formed using a material similar to that described in Embodiment 1. In addition, the oxide semiconductor layer401amay have an amorphous structure or include a crystalline region.

In formation of the oxide semiconductor layer401aby a sputtering method, the concentration of hydrogen contained in the oxide semiconductor layer401ais preferably reduced as much as possible. In order to reduce the hydrogen concentration, a high-purity rare gas (typically argon), high-purity oxygen, or a high-purity mixed gas of a rare gas and oxygen, from which impurities such as hydrogen, water, a compound having a hydroxyl group, and a hydride are removed, is supplied into a treatment chamber of a sputtering apparatus as an atmosphere gas, as appropriate.

In order to reduce the concentration of impurities contained in the oxide semiconductor layer401a, it is also effective to form the oxide semiconductor layer401awhile the substrate400is kept at a high temperature. The temperature at which the substrate400is heated may be higher than or equal to 150° C. and lower than or equal to 450° C.; the substrate temperature is preferably higher than or equal to 200° C. and lower than or equal to 350° C. Note that when the substrate is heated at a high temperature during the film formation, an oxide semiconductor layer including a crystalline region may be formed.

In this embodiment, the oxide semiconductor layer401ais formed to include a crystalline region at least partly, by heating the substrate during the film formation.

Next, the formed oxide semiconductor layer401ais subjected to thermal treatment (first thermal treatment) for dehydration or dehydrogenation. In this embodiment, the first thermal treatment is performed at a temperature higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C. or lower than the strain point of the substrate. The first thermal treatment is preferably performed at a high temperature (e.g., a temperature higher than 400° C.) because release of an impurity from the oxide semiconductor layer401ais enhanced. Note that, when the first thermal treatment is performed at a high temperature, part of the oxide semiconductor layer401amight be crystallized or the crystalline region might be expanded.

Then, the oxygen421is implanted into the oxide semiconductor layer401a. The oxygen421can be implanted as in Embodiment 1. By the oxygen implantation treatment, a crystal structure included in the oxide semiconductor layer401ais broken to be an amorphous structure, so that the amorphous oxide semiconductor layer404aincluding an oxygen-excess region is formed (seeFIG. 4B).

By the implantation treatment of the oxygen421, oxygen in the amorphous oxide semiconductor layer404a, which is decreased in concentration through the above thermal treatment for dehydration or dehydrogenation, can be supplied. Thus, the amorphous oxide semiconductor layer404acan be highly purified and become an electrically i-type (intrinsic) semiconductor. Further, formation of the oxygen-excess region in the amorphous oxide semiconductor layer404aenables an oxygen vacancy therein to be compensated. Thus, charge trapping centers in the amorphous oxide semiconductor layer404acan be reduced.

In this embodiment, an example is shown in which the oxygen421is implanted with a surface of the oxide semiconductor layer401aexposed. Note that an embodiment of the present invention is not limited to this manner; it is also possible to implant oxygen into the amorphous oxide semiconductor layer404through the gate insulating layer406or the insulating layer412.

Next, the amorphous oxide semiconductor layer404ais processed into the island-shaped amorphous oxide semiconductor layer404in a photolithography step. A conductive film which is to be a source electrode and a drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode) is formed over the amorphous oxide semiconductor layer404and is processed, so that the source electrode405aand the drain electrode405bare formed.

Next, the gate insulating layer406which covers the source electrode405aand the drain electrode405band is in contact with part of the amorphous oxide semiconductor layer404is formed (seeFIG. 4C).

Next, a conductive film which is to be a gate electrode (including a wiring formed in the same layer as the gate electrode) is formed over the gate insulating layer406and is processed, so that the gate electrode410is formed. After that, the insulating layer412which covers the gate electrode410is formed (seeFIG. 4D).

After the insulating layer412is formed, thermal treatment (second thermal treatment) is performed. The thermal treatment is performed at a temperature at which the amorphous oxide semiconductor layer404is not crystallized, preferably higher than or equal to 250° C. and lower than or equal to 450° C.

Through the above steps, the transistor510is formed (seeFIG. 4D). Variation in the electric characteristics of the transistor510is suppressed, and the transistor510is electrically stable.

According to this embodiment, a semiconductor device which includes an amorphous oxide semiconductor and has stable electric characteristics can be provided. Further, a highly reliable semiconductor device can be provided.

The methods, structures, and the like described in this embodiment can be combined as appropriate with any of the methods, structures, and the like described in the other embodiments.

In this embodiment, another embodiment of a semiconductor device and another embodiment of a method of manufacturing the semiconductor device will be described with reference toFIGS. 5A to 5C,FIGS. 6A to 6C, andFIGS. 7A to 7D. Note that the same portions as the above embodiment or portions having functions similar to those of the above embodiment can be formed as in the above embodiment, and also the same steps as the above embodiment or steps similar to those of the above embodiment can be performed as in the above embodiment; therefore, repetitive description thereof is omitted. In addition, detailed description of the same portions is not repeated.

FIGS. 5A to 5Care a plan view and cross-sectional views which illustrate a bottom-gate transistor530as an example of a semiconductor device.FIG. 5Ais a plan view, andFIGS. 5B and 5Care cross-sectional views along line I-J and line K-L inFIG. 5A, respectively. Note that inFIG. 5A, some components of the transistor530(e.g., the insulating layer412) are omitted for brevity.

The transistor530illustrated inFIGS. 5A to 5Cincludes, over the substrate400having an insulating surface, the gate electrode410, the gate insulating layer406, the amorphous oxide semiconductor layer404, the source electrode405a, the drain electrode405b, and the insulating layer412.

In the transistor530illustrated inFIGS. 5A to 5C, the amorphous oxide semiconductor layer404has been subjected to oxygen implantation treatment and includes an oxygen-excess region. By performing oxygen implantation treatment, a sufficient amount of oxygen can be contained in the amorphous oxide semiconductor layer404, so that the transistor530can have higher reliability.

In the transistor530illustrated inFIGS. 5A to 5C, the gate insulating layer406which is an insulating layer in contact with the amorphous oxide semiconductor layer404preferably includes an oxygen-excess region. When the gate insulating layer406includes an oxygen-excess region, oxygen can be prevented from moving from the amorphous oxide semiconductor layer404to the gate insulating layer406, and oxygen can be supplied from the gate insulating layer406to the amorphous oxide semiconductor layer404.

Similarly, it is preferable that the insulating layer412have a stacked structure including an aluminum oxide film and an oxide insulating film (such as a silicon oxide film or a silicon oxynitride film) which is in contact with the amorphous oxide semiconductor layer404and includes an oxygen-excess region. When the oxide insulating film includes an oxygen-excess region, an oxygen vacancy in the amorphous oxide semiconductor layer404can be compensated by excess oxygen contained in the oxide insulating film.

FIGS. 6A to 6Cillustrate another structure example of a transistor according to this embodiment.FIG. 6Ais a plan view of a transistor540, andFIGS. 6B and 6Care cross-sectional views along line M-N and line O-P inFIG. 6A, respectively. Note that inFIG. 6A, some components of the transistor540(e.g., the insulating layer412) are omitted for brevity.

As in the case of the transistor530illustrated inFIGS. 5A to 5C, the transistor540illustrated inFIGS. 6A to 6Cincludes, over the substrate400having an insulating surface, the gate electrode410, the gate insulating layer406, the amorphous oxide semiconductor layer404, the source electrode405a, the drain electrode405b, and the insulating layer412.

A difference between the transistor540illustrated inFIGS. 6A to 6Cand the transistor530illustrated inFIGS. 5A to 5Cis the stacking order of the source and drain electrodes405aand405band the amorphous oxide semiconductor layer404. That is, the transistor540includes the source electrode405aand the drain electrode405bwhich are in contact with the gate insulating layer406, and the amorphous oxide semiconductor layer404which is provided over the source electrode405aand the drain electrode405band is at least partly in contact with the gate insulating layer406. The other components are similar to those of the transistor530, and thus the description of the transistor530can be referred to for the details.

FIGS. 7A to 7Dillustrate an example of a method of manufacturing the transistor530. Note that the transistor540can be manufactured in a process similar to that of the transistor530except for the stacking order of the source and drain electrodes405aand405band the amorphous oxide semiconductor layer404.

First, a conductive film is formed over the substrate400having an insulating surface, and then the gate electrode410is formed in a photolithography step.

Note that an insulating layer serving as a base film may be provided between the substrate400and the gate electrode410. 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 structure including one or more films selected from a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film.

Next, the gate insulating layer406is formed over the gate electrode410, and the amorphous oxide semiconductor layer404ahaving a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm, is formed over the gate insulating layer406(seeFIG. 7A). The amorphous oxide semiconductor layer404acan be formed as in the step inFIG. 3A.

Note that it is preferable that the gate insulating layer406and the amorphous oxide semiconductor layer404abe successively formed without exposure to the air. For example, the following steps may be performed: impurities containing hydrogen attached to a surface of the substrate400and a surface of the gate electrode410are removed by thermal treatment or plasma treatment, and then the gate insulating layer406and the amorphous oxide semiconductor layer404aare successively formed in this order without exposure to the air. In this manner, impurities containing hydrogen attached to a surface of the gate insulating layer406can be reduced and an atmospheric component can be prevented from being attached to an interface between the gate insulating layer406and the substrate400or the gate electrode410and an interface between the gate insulating layer406and the amorphous oxide semiconductor layer404a. As a result, it is possible to manufacture the transistor530having favorable electric characteristics and high reliability. Also in the case where the insulating layer serving as a base film is formed, it is preferable that the insulating layer, the gate insulating layer406, and the amorphous oxide semiconductor layer404abe successively formed without exposure to the air.

Next, thermal treatment (first thermal treatment) is performed on the amorphous oxide semiconductor layer404ain order to remove hydrogen (including water and a hydroxyl group) (dehydration or dehydrogenation). The thermal treatment is performed at a temperature at which the amorphous oxide semiconductor layer404ais not crystallized, typically, higher than or equal to 250° C. and lower than or equal to 450° C., preferably lower than or equal to 300° C.

Then, the oxygen421is implanted into the amorphous oxide semiconductor layer404a(seeFIG. 7B). The oxygen421can be implanted as in Embodiment 1. In this embodiment, the implantation treatment of the oxygen421is performed with a surface of the amorphous oxide semiconductor layer404aexposed; therefore, plasma treatment may be carried out so that the amorphous oxide semiconductor layer404ais exposed in an atmosphere of the plasmatized oxygen421, instead of the above implantation method. Alternatively, these methods may be combined.

By the implantation treatment of the oxygen421, oxygen in the amorphous oxide semiconductor layer404a, which is decreased in concentration through the above thermal treatment for dehydration or dehydrogenation, can be supplied. Thus, the amorphous oxide semiconductor layer404acan be highly purified and become an electrically i-type (intrinsic) semiconductor. Further, formation of an oxygen-excess region in the amorphous oxide semiconductor layer404aenables an oxygen vacancy to be compensated. Thus, charge trapping centers in the amorphous oxide semiconductor layer404acan be reduced.

In this embodiment, an example is shown in which the oxygen421is implanted with the surface of the amorphous oxide semiconductor layer404aexposed. Note that an embodiment of the present invention is not limited to this manner; it is also possible to implant oxygen into the amorphous oxide semiconductor layer404through the insulating layer412.

Next, the amorphous oxide semiconductor layer404ais processed into the island-shaped amorphous oxide semiconductor layer404in a photolithography step. After that, a conductive film which is to be a source electrode and a drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode) is formed over the amorphous oxide semiconductor layer404and is processed, so that the source electrode405aand the drain electrode405bare formed (seeFIG. 7C).

Next, the insulating layer412which covers the source electrode405aand the drain electrode405bis formed (seeFIG. 7D).

After the insulating layer412is formed, thermal treatment (second thermal treatment) is performed. The thermal treatment is performed at a temperature at which the amorphous oxide semiconductor layer404is not crystallized, preferably higher than or equal to 250° C. and lower than or equal to 450° C.

Through the above steps, the transistor530is formed (seeFIG. 7D). Variation in the electric characteristics of the transistor530is suppressed, and the transistor530is electrically stable.

According to this embodiment, a semiconductor device which includes an amorphous oxide semiconductor and has stable electric characteristics can be provided. Further, a highly reliable semiconductor device can be provided.

The methods, structures, and the like described in this embodiment can be combined as appropriate with any of the methods, structures, and the like described in the other embodiments.

In this embodiment, a method of manufacturing the transistor530, which is different from that in Embodiment 3, will be described with reference toFIGS. 8A to 8D. Note that the same portions as the above embodiment or portions having functions similar to those of the above embodiment can be formed as in the above embodiment, and also the same steps as the above embodiment or steps similar to those of the above embodiment can be performed as in the above embodiment; therefore, repetitive description thereof is omitted. In addition, detailed description of the same portions is not repeated.

First, a conductive film is formed over the substrate400having an insulating surface, and then the gate electrode410is formed in a photolithography step. Next, the gate insulating layer406is formed over the gate electrode410, and the oxide semiconductor layer401ahaving a thickness greater than or equal to 2 nm and less than or equal to 200 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm, is formed over the gate insulating layer406(seeFIG. 8A). The oxide semiconductor layer401acan be formed as in the step inFIG. 4A. The oxide semiconductor layer401amay have an amorphous structure or include a crystalline region.

In this embodiment, the oxide semiconductor layer401ais formed to include a crystalline region at least partly, by heating the substrate during the film formation.

Next, the formed oxide semiconductor layer401ais subjected to thermal treatment (first thermal treatment) for dehydration or dehydrogenation. In this embodiment, the first thermal treatment is performed at a temperature higher than or equal to 250° C. and lower than or equal to 700° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C. or lower than the strain point of the substrate. The first thermal treatment is preferably performed at a high temperature (e.g., a temperature higher than 400° C.) because release of an impurity from the oxide semiconductor layer401ais enhanced. Note that, when the first thermal treatment is performed at a high temperature, part of the oxide semiconductor layer401amight be crystallized or the crystalline region might be expanded.

Next, the oxide semiconductor layer401ais processed into an island-shaped oxide semiconductor layer401in a photolithography step. After that, a conductive film which is to be a source electrode and a drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode) is formed over the oxide semiconductor layer401and is processed, so that the source electrode405aand the drain electrode405bare formed.

Next, the insulating layer412which covers the source electrode405aand the drain electrode405band is in contact with part of the oxide semiconductor layer401is formed (seeFIG. 8B).

Then, the oxygen421is implanted into the oxide semiconductor layer401through the insulating layer412(seeFIG. 8C). By the oxygen implantation treatment, a crystal structure included in the oxide semiconductor layer401is broken to be an amorphous structure, so that the amorphous oxide semiconductor layer404including an oxygen-excess region is formed.

By the implantation treatment of the oxygen421, oxygen, which is decreased in concentration through the above thermal treatment for dehydration or dehydrogenation, can be supplied. Thus, the amorphous oxide semiconductor layer404can be highly purified and become an electrically i-type (intrinsic) semiconductor. Further, formation of the oxygen-excess region in the amorphous oxide semiconductor layer404enables an oxygen vacancy therein to be compensated. Thus, charge trapping centers in the amorphous oxide semiconductor layer404can be reduced.

After the oxygen421is implanted, thermal treatment (second thermal treatment) is performed. The thermal treatment is performed at a temperature at which the amorphous oxide semiconductor layer404is not crystallized, preferably higher than or equal to 250° C. and lower than or equal to 450° C.

Through the above steps, the transistor530is formed (seeFIG. 8D). Variation in the electric characteristics of the transistor530is suppressed, and the transistor530is electrically stable.

According to this embodiment, a semiconductor device which includes an amorphous oxide semiconductor and has stable electric characteristics can be provided. Further, a highly reliable semiconductor device can be provided.

The methods, structures, and the like described in this embodiment can be combined as appropriate with any of the methods, structures, and the like described in the other embodiments.

A semiconductor device having a display function (also referred to as a display device) can be manufactured using any of the transistors described in Embodiments 1 to 4. Moreover, some or all of driver circuits which include transistors can be formed over a substrate where a pixel portion is formed, whereby a system-on-panel can be obtained.

InFIG. 9A, a sealant4005is provided so as to surround a pixel portion4002provided over a first substrate4001, and the pixel portion4002is sealed by using a second substrate4006. InFIG. 9A, a signal line driver circuit4003and a scan line driver circuit4004which are each formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared are mounted in a region that is different from the region surrounded by the sealant4005over the first substrate4001. Various signals and potentials which are provided to the pixel portion4002through the signal line driver circuit4003and the scan line driver circuit4004are supplied from flexible printed circuits (FPCs)4018aand4018b.

InFIGS. 9B and 9C, the sealant4005is provided so as to surround the pixel portion4002and the scan line driver circuit4004which are provided over the first substrate4001. The second substrate4006is provided over the pixel portion4002and the scan line driver circuit4004. Consequently, the pixel portion4002and the scan line driver circuit4004are sealed together with a display element, by the first substrate4001, the sealant4005, and the second substrate4006. InFIGS. 9B and 9C, the signal line driver circuit4003which is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate separately prepared is mounted in a region that is different from the region surrounded by the sealant4005over the first substrate4001. InFIGS. 9B and 9C, various signals and potentials which are provided to the pixel portion4002through the signal line driver circuit4003and the scan line driver circuit4004are supplied from an FPC 4018.

AlthoughFIGS. 9B and 9Ceach illustrate an example in which the signal line driver circuit4003is formed separately and mounted on the first substrate4001, one embodiment of the present invention is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted.

Note that there is no particular limitation on the method of connecting a separately formed driver circuit, and a chip on glass (COG) method, a wire bonding method, a tape automated bonding (TAB) method, or the like can be used.FIG. 9Aillustrates an example in which the signal line driver circuit4003and the scan line driver circuit4004are mounted by a COG method.FIG. 9Billustrates an example in which the signal line driver circuit4003is mounted by a COG method.FIG. 9Cillustrates an example in which the signal line driver circuit4003is mounted by a TAB method.

Note that the display device includes a panel in which the display element is sealed, and a module in which an IC including a controller or the like is mounted on the panel.

Specifically, 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 not only a panel in which the display element is sealed but also the following modules in its category: a module to which a connector such as an FPC, a TAB tape, or a TCP is attached; a module having a TAB tape or a TCP at the tip of which a printed wiring board is provided; and a module in which an integrated circuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit which are provided over the first substrate include a plurality of transistors; any of the transistors described in Embodiments 1 to 4 can be applied thereto.

As the display element provided in the display device, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. The light-emitting element includes, in its category, an element whose luminance is controlled by current or voltage, and specifically includes 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 an electronic ink display (electronic paper), can be used.

As illustrated inFIG. 10,FIG. 11, andFIG. 12, the semiconductor device includes a connection terminal electrode4015and a terminal electrode4016. The connection terminal electrode4015and the terminal electrode4016are electrically connected to a terminal included in the FPC 4018 through an anisotropic conductive film4019.

The connection terminal electrode4015is formed using the same conductive film as a first electrode4030, and the terminal electrode4016is formed using the same conductive film as source electrodes and drain electrodes of transistors4010and4011.

The pixel portion4002and the scan line driver circuit4004which are provided over the first substrate4001include a plurality of transistors.FIG. 10,FIG. 11, andFIG. 12illustrate the transistor4010included in the pixel portion4002and the transistor4011included in the scan line driver circuit4004as an example. InFIG. 10,FIG. 11, andFIG. 12, an insulating layer4020and an insulating layer4024are provided over the transistors4010and4011. InFIG. 11andFIG. 12, an insulating layer4021is further provided. Note that an insulating layer4023inFIGS. 10 to 12is an insulating layer serving as a base film.

Any of the transistors described in Embodiments 1 to 4 can be applied to the transistor4010and the transistor4011.

Each of the transistor4010and the transistor4011includes a highly purified amorphous oxide semiconductor layer including an oxygen-excess region. Therefore, variation in the electric characteristics of the transistor4010and the transistor4011is suppressed, and the transistor4010and the transistor4011are electrically stable.

Thus, as the semiconductor devices of this embodiment illustrated inFIG. 10,FIG. 11, andFIG. 12, a highly reliable semiconductor device can be provided.

In this embodiment, a conductive layer4037is provided over the insulating layer so as to overlap with a channel formation region of the amorphous oxide semiconductor layer in the transistor4011for the driver circuit, whereby the amount of change in the threshold voltage of the transistor4011can be further reduced. The potential of the conductive layer4037may be the same as or different from that of a gate electrode4039of the transistor4011. The conductive layer4037can also function as a second gate electrode. The potential of the conductive layer4037may be GND or 0V, or the conductive layer4037may be in a floating state.

The conductive layer4037functions to block an external electric field, that is, to prevent an external electric field (particularly, to prevent static electricity) from affecting the inside (a circuit portion including a thin film transistor). The blocking function of the conductive layer4037can prevent variation in the electric characteristics of the transistor4011due to the influence of an external electric field such as static electricity.

The transistor4010provided in the pixel portion4002is electrically connected to a display element to form a display panel. A variety of display elements can be used as the display element as long as display can be performed.

An example of a liquid crystal display device using a liquid crystal element as a display element is illustrated inFIG. 10. InFIG. 10, a liquid crystal element4013includes the first electrode4030, a second electrode4031, and a liquid crystal layer4008. Note that an insulating layer4032and an insulating layer4033which function as alignment films are provided so that the liquid crystal layer4008is interposed therebetween. The second electrode4031is provided on the second substrate4006side. The first electrode4030and the second electrode4031are stacked with the liquid crystal layer4008interposed therebetween.

A columnar spacer denoted by reference numeral4035is obtained by selective etching of an insulating layer and is provided in order to control the thickness (cell gap) of the liquid crystal layer4008. Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. These liquid crystals may be any of a compound with a low molecular weight and a polymer. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which appears just before a cholesteric phase changes into an isotropic phase while the temperature of a cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral agent is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes a liquid crystal exhibiting a blue phase and a chiral agent has a short response time, and has optical isotropy, which contributes to the exclusion of the alignment process and reduction of viewing angle dependence. In addition, since an alignment film does not need to be provided and rubbing treatment is unnecessary, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. Thus, productivity of the liquid crystal display device can be improved.

The specific resistivity of the liquid crystal material is higher than or equal to 1×109Ω·cm, preferably higher than or equal to 1×1011Ω·cm, further preferably higher than or equal to 1×1012Ω·cm. Note that the specific resistivity in this specification is measured at 20° C.

The size of a storage capacitor formed in the liquid crystal display device is set considering the leakage current of the transistor provided in the pixel portion4002or the like so that charge can be held for a predetermined period. The size of the storage capacitor may be set considering the off-state current of the transistor or the like. By using a transistor which includes a highly purified amorphous oxide semiconductor layer including an oxygen-excess region, it is enough to provide a storage capacitor having a capacitance that is ⅓ or less, preferably ⅕ or less, of liquid crystal capacitance of each pixel.

In the transistor used in this embodiment, which includes a highly purified amorphous oxide semiconductor layer in which formation of an oxygen vacancy is suppressed, the current in an off state (off-state current) can be decreased. Accordingly, an electric signal such as an image signal can be held for a longer time, and a writing interval can be set longer. Accordingly, the frequency of refresh operation can be reduced, which leads to suppression of power consumption.

The transistor used in this embodiment, which includes a highly purified amorphous oxide semiconductor layer in which formation of an oxygen vacancy is suppressed, can have relatively high field-effect mobility and thus can operate at high speed. For example, when such a transistor is used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. In addition, by using such a transistor in a pixel portion, a high-quality image can be provided.

A normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or an ASV mode can be employed. Furthermore, this embodiment can be applied to a VA liquid crystal display device. The VA liquid crystal display device utilizes one of the alignment modes of liquid crystal molecules of a liquid crystal display panel. In the VA liquid crystal display device, liquid crystal molecules are aligned in a vertical direction with respect to a panel surface when no voltage is applied. Moreover, it is possible to use a method called domain multiplication or multi-domain design, in which a pixel is divided into some regions (subpixels) and molecules are aligned in different directions in their respective regions.

In the display device, a black matrix (light-blocking layer), an optical member (optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, and the like are provided as appropriate. For example, circular polarization may be obtained by using a polarizing substrate and a retardation substrate. In addition, a backlight, a side light, or the like may be used as a light source.

As a display method in the pixel portion, a progressive method, an interlace method, or the like can be employed. Further, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, R, G, B, and W (W corresponds to white); or R, G, B, and one or more of yellow, cyan, magenta, and the like can be used. Further, the sizes of display regions may differ between respective dots of color elements. Note that one embodiment of the invention disclosed herein is not limited to the application to a display device for color display; one embodiment of the invention disclosed herein can also be applied to a display device for monochrome display.

Alternatively, as the display element included in the display device, a light-emitting element utilizing electroluminescence can be used. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter is referred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and current flows. The carriers (electrons and holes) are recombined, and thus 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.

In order to extract light emitted from the light-emitting element, at least one of the pair of electrodes has a light-transmitting property. A transistor and the light-emitting element are formed over a substrate. The light-emitting element can have a top emission structure in which light emission is extracted through a surface opposite to the substrate; a bottom emission structure in which light emission is extracted through a surface on the substrate side; or a dual emission structure in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side. A light-emitting element having any of these emission structures can be used.

An example of a light-emitting device using a light-emitting element as a display element is illustrated inFIG. 11. A light-emitting element4513is electrically connected to the transistor4010provided in the pixel portion4002. The structure of the light-emitting element4513is not limited to that shown inFIG. 11, which has a stacked structure including the first electrode4030, an electroluminescent layer4511, and the second electrode4031. The structure of the light-emitting element4513can be changed as appropriate depending on the direction in which light is extracted from the light-emitting element4513, or the like.

A partition wall4510is formed using an organic insulating material or an inorganic insulating material. It is particularly preferable that the partition wall4510be formed using a photosensitive resin material to have an opening over the first electrode4030so that a sidewall of the opening has a tilted surface with continuous curvature.

The electroluminescent layer4511may be formed using a single layer or a plurality of layers stacked.

A protective film may be formed over the second electrode4031and the partition wall4510in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, and the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC film, or the like can be formed. In a space which is formed with the first substrate4001, the second substrate4006, and the sealant4005, a filler4514is provided for sealing. In this manner, the light-emitting element is packaged (sealed) with the protective film (such as a laminate film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the light-emitting element is not exposed to the outside air.

As the filler4514, an ultraviolet curable resin or a thermosetting resin can be used as well as an inert gas such as nitrogen or argon. For example, poly(vinyl chloride) (PVC), an acrylic resin, a polyimide, an epoxy resin, a silicone resin, poly(vinyl butyral) (PVB), or a copolymer of ethylene with vinyl acetate (EVA) can be used.

In addition, as needed, an optical film such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (quarter-wave plate or half-wave plate), or a color filter may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed.

Electronic paper in which electronic ink is driven can be provided as the display device. The electronic paper is also referred to as an electrophoretic display device (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.

An electrophoretic display device can have various modes. An electrophoretic display device includes a plurality of microcapsules dispersed in a solvent, and each microcapsule contains 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 a pigment and do not move in the absence of an electric field. The first particles and the second particles have different colors (which may be colorless).

A dispersion of the above microcapsules in a solvent is referred to as electronic ink. The use of a color filter or particles that have a pigment enables color display.

As the electronic paper, a display device using a twisting ball display system can be used. In the twisting ball display system, spherical particles each colored in black and white are arranged between a first electrode and a second electrode which are electrodes used for a display element, and a potential difference is generated between the first electrode and the second electrode to control the orientation of the spherical particles, so that display is performed.

FIG. 12illustrates active matrix electronic paper as one embodiment of a semiconductor device. The electronic paper inFIG. 12is an example of a display device using a twisting ball display system.

Between the first electrode4030connected to the transistor4010and the second electrode4031provided on the second substrate4006side are provided spherical particles4613each including a black region4615a, a white region4615b, and a cavity4612which is filled with liquid around the black region4615aand the white region4615b. A space around the spherical particles4613is filled with a filler4614such as a resin. The second electrode4031corresponds to a common electrode (counter electrode). The second electrode4031is electrically connected to a common potential line.

Note that inFIG. 10,FIG. 11, andFIG. 12, a flexible substrate as well as a glass substrate can be used as the first substrate4001and the second substrate4006. For example, a plastic substrate having a light-transmitting property can be used. As plastic, a fiberglass-reinforced plastics (FRP) plate, a poly(vinyl fluoride) (PVF) film, a polyester film, or an acrylic resin film can be used. In addition, a sheet with a structure in which an aluminum foil is interposed between PVF films or polyester films can be used.

In this embodiment, a silicon oxide film is used as the insulating layer4020, and an aluminum oxide film is used as the insulating layer4024. The insulating layer4020and the insulating layer4024can be formed by a sputtering method or a plasma CVD method. The insulating layer4020in contact with the amorphous oxide semiconductor layer preferably includes an oxygen-excess region.

The aluminum oxide film provided as the insulating layer4024over the amorphous oxide semiconductor layer has a high blocking effect and thus is less likely to transmit both oxygen and impurities such as hydrogen and moisture.

Accordingly, the aluminum oxide film functions as a protective film that prevents entry of impurities such as hydrogen and moisture, which can cause variation, into the amorphous oxide semiconductor layer and release of oxygen from the amorphous oxide semiconductor layer during and after the manufacturing process.

Each of the transistor4010and the transistor4011includes a highly purified amorphous oxide semiconductor layer in which an oxygen-excess region is provided for suppression of formation of an oxygen vacancy. In addition, the transistor4010and the transistor4011each include a silicon oxide film as a gate insulating layer. The amorphous oxide semiconductor layer included in each of the transistor4010and the transistor4011is obtained in such a manner that a region containing oxygen in a proportion higher than that in the stoichiometric composition is formed by oxygen implantation treatment, and thermal treatment is performed after the implantation in a state where the aluminum oxide film or a film including the aluminum oxide film is provided as the insulating layer4024over the amorphous oxide semiconductor layer; therefore, oxygen can be prevented from being released from the amorphous oxide semiconductor layer by the thermal treatment. Accordingly, the obtained amorphous oxide semiconductor layer can be a film which includes a region containing oxygen in a proportion higher than that in the stoichiometric composition.

The amorphous oxide semiconductor layer included in each of the transistor4010and the transistor4011is a highly purified film which is dehydrated or dehydrogenated and in which an oxygen vacancy is compensated. Accordingly, by using the amorphous oxide semiconductor layer for each of the transistor4010and the transistor4011, it is possible to reduce variation in the threshold voltage Vthof the transistors due to an oxygen vacancy and suppress a shift of the threshold voltage.

The insulating layer4021functioning as a planarizing insulating layer can be formed using an organic material having heat resistance, such as an acrylic resin, a polyimide, a benzocyclobutene-based resin, a polyamide, or an epoxy resin. Other than such organic materials, it is also possible to use a low-dielectric constant material (low-k material) such as a siloxane-based resin, phosphosilicate glass (PSG), or borophosphosilicate glass (BPSG). Note that the insulating layer4021may be formed by stacking a plurality of insulating layers formed using any of these materials.

There is no particular limitation on the method of forming the insulating layer4021, and the following method can be used depending on the material: a sputtering method, an SOG method, spin coating, dipping, spray coating, a droplet discharge method (such as an inkjet method), screen printing, offset printing, or the like.

Alternatively, the first electrode4030and the second electrode4031can be formed using one or more materials selected from metals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag); an alloy of any of these metals; and a nitride of any of these metals.

A conductive composition including a conductive polymer can be used for the first electrode4030and the second electrode4031. As the conductive polymer, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, and a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof can be given.

A protective circuit for protecting the driver circuit may be provided. The protective circuit is preferably formed using a nonlinear element.

As described above, by using any of the transistors described in Embodiments 1 to 4, a semiconductor device having a variety of functions can be provided.

A semiconductor device having an image sensor function of reading information on an object can be manufactured using any of the transistors described in Embodiments 1 to 4.

FIG. 13Aillustrates an example of a semiconductor device having an image sensor function.FIG. 13Ais an equivalent circuit diagram of a photosensor, andFIG. 13Bis a cross-sectional view illustrating part of the photosensor.

One electrode of a photodiode602is electrically connected to a photodiode reset signal line658, and the other electrode of the photodiode602is electrically connected to a gate of a transistor640. One of a source and a drain of the transistor640is electrically connected to a photosensor reference signal line672, and the other of the source and the drain of the transistor640is electrically connected to one of a source and a drain of a transistor656. A gate of the transistor656is electrically connected to a gate signal line659, and the other of the source and the drain of the transistor656is electrically connected to a photosensor output signal line671.

Note that in the circuit diagram in this specification, a transistor including an amorphous oxide semiconductor layer is denoted by a symbol “OS” so that it can be identified as a transistor formed using an amorphous oxide semiconductor layer. InFIG. 13A, the transistor640and the transistor656are each a transistor including an amorphous oxide semiconductor layer in which an oxygen-excess region is formed by oxygen implantation treatment, like the transistors described in Embodiments 1 to 4.

FIG. 13Bis a cross-sectional view of the photodiode602and the transistor640in the photosensor. The photodiode602functioning as a sensor and the transistor640are provided over a substrate601(TFT substrate) having an insulating surface. A substrate613is provided over the photodiode602and the transistor640with the use of an adhesive layer608.

An insulating layer631, an insulating layer632, an interlayer insulating film633, and an interlayer insulating film634are provided over the transistor640. The photodiode602is provided over the interlayer insulating film633. In the photodiode602, a first semiconductor film606a, a second semiconductor film606b, and a third semiconductor film606care sequentially stacked from the interlayer insulating film633side, between electrodes641aand641bformed over the interlayer insulating film633and an electrode642formed over the interlayer insulating film634.

The electrode641bis electrically connected to a conductive layer643formed over the interlayer insulating film634, and the electrode642is electrically connected to an electrode645through the electrode641a. The electrode645is electrically connected to a gate electrode of the transistor640, and thus the photodiode602is electrically connected to the transistor640.

Here, a pin photodiode in which a semiconductor film having p-type conductivity as the first semiconductor film606a, a high-resistance semiconductor film (i-type semiconductor film) as the second semiconductor film606b, and a semiconductor film having n-type conductivity as the third semiconductor film606care stacked is illustrated as an example.

The first semiconductor film606ais a p-type semiconductor film and can be formed using an amorphous silicon film containing an impurity element imparting p-type conductivity. The first semiconductor film606ais formed by a plasma CVD method with the use of a semiconductor source gas containing an impurity element belonging to Group 13 (e.g., boron (B)). As the semiconductor source gas, silane (SiH4) may be used. Alternatively, Si2H6, SiH2Cl2, SiHCl3, SiCl4, SiF4, or the like may be used. Further alternatively, an amorphous silicon film which does not contain an impurity element may be formed, and then an impurity element may be introduced into the amorphous silicon film by a diffusion method or an ion implantation method. Heating or the like may be conducted after introducing the impurity element by an ion implantation method or the like in order to diffuse the impurity element. In this case, as a method of forming the amorphous silicon film, an LPCVD method, a vapor deposition method, a sputtering method, or the like may be used. The first semiconductor film606ais preferably formed to have a thickness greater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor film606bis an i-type semiconductor film (intrinsic semiconductor film) and is formed using an amorphous silicon film. As for formation of the second semiconductor film606b, an amorphous silicon film is formed by a plasma CVD method with the use of a semiconductor source gas. As the semiconductor source gas, silane (SiH4) may be used. Alternatively, Si2H6, SiH2Cl2, SiHCl3, SiCl4, SiF4, or the like may be used. The second semiconductor film606bmay be formed by an LPCVD method, a vapor deposition method, a sputtering method, or the like. The second semiconductor film606bis preferably formed to have a thickness greater than or equal to 200 nm and less than or equal to 1000 nm.

The third semiconductor film606cis an n-type semiconductor film and is formed using an amorphous silicon film containing an impurity element imparting n-type conductivity. The third semiconductor film606cis formed by a plasma CVD method with the use of a semiconductor source gas containing an impurity element belonging to Group 15 (e.g., phosphorus (P)). As the semiconductor source gas, silane (SiH4) may be used. Alternatively, Si2H6, SiH2Cl2, SiHCl3, SiCl4, SiF4, or the like may be used. Further alternatively, an amorphous silicon film which does not contain an impurity element may be formed, and then an impurity element may be introduced into the amorphous silicon film by a diffusion method or an ion implantation method. Heating or the like may be conducted after introducing the impurity element by an ion implantation method or the like in order to diffuse the impurity element. In this case, as a method of forming the amorphous silicon film, an LPCVD method, a vapor deposition method, a sputtering method, or the like may be used. The third semiconductor film606cis preferably formed to have a thickness greater than or equal to 20 nm and less than or equal to 200 nm.

The first semiconductor film606a, the second semiconductor film606b, and the third semiconductor film606care not necessarily formed using an amorphous semiconductor, and may be formed using a polycrystalline semiconductor or a microcrystalline semiconductor (semi-amorphous semiconductor: SAS).

The microcrystalline semiconductor belongs to a metastable state of an intermediate between amorphous and single crystalline when Gibbs free energy is considered. That is, the microcrystalline semiconductor is a semiconductor having a third state which is thermodynamically stable and has a short range order and lattice distortion. Columnar-like or needle-like crystals grow in a normal direction with respect to a substrate surface. The Raman spectrum of microcrystalline silicon, which is a typical example of a microcrystalline semiconductor, is located in lower wavenumbers than 520 cm−1, which represents a peak of the Raman spectrum of single crystal silicon. That is, the peak of the Raman spectrum of microcrystalline silicon exists between 520 cm1which represents single crystal silicon and 480 cm1which represents amorphous silicon. In addition, microcrystalline silicon contains at least 1 at. % or more of hydrogen or halogen in order to terminate a dangling bond. Moreover, microcrystalline silicon contains a rare gas element such as helium, argon, krypton, or neon to further promote lattice distortion, so that stability is increased and a favorable microcrystalline semiconductor film can be obtained.

This microcrystalline semiconductor film can be formed by a high-frequency plasma CVD method with a frequency of several tens of megahertz to several hundreds of megahertz or using a microwave plasma CVD apparatus with a frequency of 1 GHz or higher. The microcrystalline semiconductor film can be typically formed using a dilution of silicon hydride such as SiH4, Si2H6, SiH2Cl2, or SiHCl3with hydrogen. With a dilution of a silicon hydride with one or plural kinds of rare gas elements selected from helium, argon, krypton, and neon in addition to hydrogen, the microcrystalline semiconductor film can be formed. In that case, the flow ratio of hydrogen to silicon hydride is greater than or equal to 5:1 and less than or equal to 200:1, preferably greater than or equal to 50:1 and less than or equal to 150:1, further preferably 100:1. Further, a hydrocarbon gas such as CH4or C2H6, a germanium gas such as GeH4or GeF4, F2, or the like may be mixed into the gas containing silicon.

The mobility of holes generated by the photoelectric effect is lower than the mobility of electrons. Therefore, a pin photodiode has better characteristics when a surface on the p-type semiconductor film side is used as a light-receiving plane. Here, an example in which light received by the photodiode602from a surface of the substrate601, over which the pin photodiode is formed, is converted into electric signals is described. Light from the semiconductor film having a conductivity type opposite to that of the semiconductor film on the light-receiving plane is disturbance light; therefore, the electrode is preferably formed using a light-blocking conductive film. Note that the n-type semiconductor film side may alternatively be a light-receiving plane.

With the use of an insulating material, the insulating layer632, the interlayer insulating film633, and the interlayer insulating film634can be formed, depending on the material, using a method such as a sputtering method, a plasma CVD method, an SOG method, spin coating, dipping, spray coating, a droplet discharge method (such as an inkjet method), screen printing, or offset printing.

In this embodiment, an aluminum oxide film is used as the insulating layer631. The insulating layer631can be formed by a sputtering method or a plasma CVD method.

The aluminum oxide film provided as the insulating layer631over the amorphous oxide semiconductor layer has a high blocking effect and thus is less likely to transmit both oxygen and impurities such as hydrogen and moisture.

Accordingly, the aluminum oxide film functions as a protective film that prevents entry of impurities such as hydrogen and moisture, which can cause variation, into the amorphous oxide semiconductor layer and release of oxygen from the amorphous oxide semiconductor layer during and after the manufacturing process.

In this embodiment, the transistor640includes a highly purified amorphous oxide semiconductor layer in which an oxygen-excess region is provided for suppression of formation of an oxygen vacancy. In addition, the transistor640includes a silicon oxide film as a gate insulating layer. The amorphous oxide semiconductor layer included in the transistor640is obtained in such a manner that a region containing oxygen in a proportion higher than that in the stoichiometric composition is formed by oxygen implantation treatment, and thermal treatment is performed after the implantation in a state where the aluminum oxide film is provided as the insulating layer631over the amorphous oxide semiconductor layer; therefore, oxygen can be prevented from being released from the amorphous oxide semiconductor layer by the thermal treatment. Accordingly, the obtained amorphous oxide semiconductor layer can be a film which includes a region containing oxygen in a proportion higher than that in the stoichiometric composition.

The amorphous oxide semiconductor layer included in the transistor640is a highly purified film which is dehydrated or dehydrogenated by thermal treatment performed after formation of the amorphous oxide semiconductor layer. Accordingly, by using the amorphous oxide semiconductor layer for the transistor640, it is possible to reduce variation in the threshold voltage Vthof the transistor due to an oxygen vacancy and suppress a shift of the threshold voltage.

The insulating layer632can be formed using an inorganic insulating material and can have a single-layer or stacked structure including any of oxide insulating films such as a silicon oxide layer, a silicon oxynitride layer, an aluminum oxide layer, and an aluminum oxynitride layer; and nitride insulating films such as a silicon nitride layer, a silicon nitride oxide layer, an aluminum nitride layer, and an aluminum nitride oxide layer.

For reduction of surface roughness, an insulating layer functioning as a planarizing insulating layer is preferably used as each of the interlayer insulating films633and634. For the interlayer insulating films633and634, for example, an organic insulating material having heat resistance, such as a polyimide, an acrylic resin, a benzocyclobutene-based resin, a polyamide, or an epoxy resin, can be used. Other than such organic insulating materials, it is possible to use a single layer or stacked layers of any of low-dielectric constant materials (low-k material) such as a siloxane-based resin, phosphosilicate glass (PSG), and borophosphosilicate glass (BPSG).

With detection of light that enters the photodiode602, information on an object to be detected can be read. Note that a light source such as a backlight can be used at the time of reading information on an object to be detected.

As described above, a transistor which includes a highly purified amorphous oxide semiconductor layer containing excess oxygen that compensates an oxygen vacancy has less variation in electric characteristics and is electrically stable. Thus, by using the transistor, a highly reliable semiconductor device can be provided.

A semiconductor device disclosed in this specification can be applied to a variety of electronic appliances (including a game machine). Examples of electronic appliances are a television device (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone (also referred to as a cellular phone or a mobile phone device), a portable game machine, a personal digital assistant, an audio reproducing device, and a large-sized game machine such as a pachinko machine. Examples of electronic appliances each including the semiconductor device described in any of the above embodiments will be described.

FIG. 14Aillustrates a laptop personal computer which includes a main body3001, a housing3002, a display portion3003, a keyboard3004, and the like. The semiconductor device described in any of the above embodiments is applied to the display portion3003, whereby the laptop personal computer can be highly reliable.

FIG. 14Billustrates a personal digital assistant (PDA) which includes a main body3021provided with a display portion3023, an external interface3025, an operation button3024, and the like. A stylus3022is provided as an accessory for operation. The semiconductor device described in any of the above embodiments is applied to the display portion3023, whereby the personal digital assistant (PDA) can be highly reliable.

FIG. 14Cillustrates an example of an e-book reader. For example, the e-book reader includes two housings, a housing2701and a housing2703. The housing2701and the housing2703are combined with a hinge2711so that the e-book reader can be opened and closed using the hinge2711as an axis. With such a structure, the e-book reader can 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 structure where different images are displayed on different display portions, for example, text can be displayed on the right display portion (the display portion2705inFIG. 14C) and graphics can be displayed on the left display portion (the display portion2707inFIG. 14C). The semiconductor device described in any of the above embodiments is applied to the display portion2705and the display portion2707, whereby the e-book reader can be highly reliable. In the case of using a transflective or reflective liquid crystal display device as the display portion2705, the e-book reader may be used in a comparatively bright environment; therefore, a solar cell may be provided so that power generation by the solar cell and charge by a battery can be performed. When a lithium ion battery is used as the battery, there are advantages of downsizing and the like.

FIG. 14Cillustrates 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, and a speaker2725. With the operation key2723, pages can be turned. Note that a keyboard, a pointing device, or the like may also be provided on the surface of the housing, on which the display portion is provided. Further, an external connection terminal (such as an earphone terminal or a USB terminal), a recording medium insertion portion, or the like may be provided on the back surface or the side surface of the housing. Further, the e-book reader may have a function of an electronic dictionary.

The e-book reader may transmit and receive data wirelessly. Through wireless communication, desired book data or the like can be purchased and downloaded from an e-book server.

FIG. 14Dillustrates a mobile phone which includes two housings, a housing2800and a housing2801. The housing2801includes a display panel2802, a speaker2803, a microphone2804, a pointing device2806, a camera lens2807, an external connection terminal2808, and the like. In addition, the housing2800includes a solar cell2810having a function of charge of the portable information terminal, an external memory slot2811, and the like. Further, an antenna is incorporated in the housing2801. The semiconductor device described in any of the above embodiments is applied to the display panel2802, whereby the mobile phone can be highly reliable.

Further, the display panel2802is provided with a touch panel. A plurality of operation keys2805displayed as images is illustrated by dashed lines inFIG. 14D. Note that a boosting circuit by which voltage output from the solar cell2810is increased to be sufficiently high for each circuit is also provided.

In the display panel2802, the display direction can be appropriately changed depending on a usage pattern. Further, the mobile phone is provided with the camera lens2807on the same surface as the display panel2802, and thus it can be used as a video phone. The speaker2803and the microphone2804can be used for videophone calls, recording and playing sound, and the like as well as voice calls. Moreover, the housings2800and2801in a state where they are developed as illustrated inFIG. 14Dcan shift by sliding so that one is lapped over the other; therefore, the size of the mobile phone can be reduced, which makes the mobile phone suitable for being carried.

The external connection terminal2808can be connected to an AC adapter and various types of cables such as a USB cable, and charge and data communication with a personal computer or the like are possible. Moreover, a large amount of data can be stored by inserting a storage medium into the external memory slot2811and can be moved.

Further, in addition to the above functions, an infrared communication function, a television reception function, or the like may be provided.

FIG. 14Eillustrates a digital video camera which includes a main body3051, a display portion A3057, an eyepiece portion3053, an operation switch3054, a display portion B3055, a battery3056, and the like. The semiconductor device described in any of the above embodiments is applied to the display portion A3057and the display portion B3055, whereby the digital video camera can be highly reliable.

FIG. 14Fillustrates an example of a television device. In the television device, a display portion9603is incorporated in a housing9601. The display portion9603can display images. Here, the housing9601is supported by a stand9605. The semiconductor device described in any of the above embodiments is applied to the display portion9603, whereby the television device can be highly reliable.

The television device can be operated by an operation switch of the housing9601or a separate remote controller. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller.

In this example, properties of an aluminum oxide film used as a barrier film in a semiconductor device according to one embodiment of the invention disclosed herein were evaluated. The results are shown in FIGS.15A1,15A2,15B1, and15B2, FIGS.16A1,16A2,16B1, and16B2,FIGS. 17A to 17D, andFIGS. 18A to 18D. As evaluation methods, secondary ion mass spectrometry (SIMS) and thermal desorption spectrometry (TDS) were used.

First, evaluation by SIMS analysis is described. As a comparative example, a comparative sample A was fabricated in such a manner that a silicon oxide film was formed to a thickness of 100 nm over a glass substrate by a sputtering method. An example sample A was fabricated in such a manner that a silicon oxide film was formed to a thickness of 100 nm over a glass substrate by a sputtering method, and an aluminum oxide film was formed to a thickness of 100 nm over the silicon oxide film by a sputtering method.

For each of the comparative sample A and the example sample A, the silicon oxide film was formed under the following conditions: a silicon oxide (SiO2) target was used as a target, the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the power source was 1.5 kW, the atmosphere was an oxygen atmosphere (the oxygen flow rate was 50 sccm), and the substrate temperature was 100° C.

For the example sample A, the aluminum oxide film was formed under the following conditions: an aluminum oxide (Al2O3) target was used as a target, the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the power source was 1.5 kW, the atmosphere was a mixed atmosphere containing argon and oxygen (the argon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), and the substrate temperature was 250° C.

The comparative sample A and the example sample A were each subjected to a pressure cooker test (PCT). In the PCT in this example, the comparative sample A and the example sample A were held for 100 hours under the following conditions: the temperature was 130° C., the humidity was 85%, the atmosphere was an atmosphere where H2O (water):D2O (heavy water)=3:1, and the atmospheric pressure was 2.3 atm (0.23 MPa).

As SIMS analysis, substrate side depth profile (SSDP) SIMS was used to measure concentrations of an H atom and a D (deuterium) atom in the comparative sample A and the example sample A before and after the PCT.

FIG.15A1shows H-atom and D-atom concentration profiles of the comparative sample A before the PCT, and FIG.15A2shows H-atom and D-atom concentration profiles of the comparative sample A after the PCT, which were obtained using SIMS analysis. In FIGS.15A1and15A2, an expected D-atom concentration (D expected) profile is a calculated concentration profile of the D atom, which was obtained using the H-atom concentration profile on the assumption that the abundance ratio of the D atom thereto is 0.015%. Therefore, the amount of the D atom absorbed in the sample by the PCT equals the difference between the measured D-atom concentration (D profile) and the expected D-atom concentration (D expected). FIG.15B1shows a D-atom concentration profile before the PCT, which was obtained by subtracting the expected D-atom concentration from the measured D-atom concentration; FIG.15B2shows a D-atom concentration profile after the PCT, which was obtained by subtracting the expected D-atom concentration from the measured D-atom concentration.

In a similar manner, FIG.16A1shows H-atom and D-atom concentration profiles of the example sample A before the PCT, which was obtained by SIMS; FIG.16A2shows H-atom and D-atom concentration profiles of the example sample A after the PCT, which was obtained by SIMS. In addition, FIG.16B1shows a D-atom concentration profile before the PCT, which was obtained by subtracting the expected D-atom concentration from the measured D-atom concentration; FIG.16B2shows a D-atom concentration profile after the PCT, which was obtained by subtracting the expected D-atom concentration from the measured D-atom concentration.

Note that all the results of SIMS analysis in this example were quantified using a standard sample of a silicon oxide film.

As shown in FIGS.15A1,15A2,15B1, and15B2, while the measured D-atom concentration profile overlaps with the expected D-atom concentration profile before the PCT, the measured D-atom concentration greatly increases after the PCT; accordingly, it is found that the D atom was absorbed into the silicon oxide film. Therefore, it is confirmed that the silicon oxide film of the comparative sample has a low bather property with respect to moisture (H2O and D2O) from the outside.

In contrast, as shown in FIGS.16A1,16A2,16B1, and16B2, as for the example sample A in which the aluminum oxide film was stacked over the silicon oxide film, it is found that only a slight amount of D atom entered a surface of the aluminum oxide film by the PCT and that the D atom entered neither the aluminum oxide film at a depth around 30 nm or greater nor the silicon oxide film. Therefore, it is confirmed that the aluminum oxide film has a high barrier property with respect to moisture (H2O and D2O) from the outside.

The following shows evaluation by TDS analysis. As an example sample, an example sample B was fabricated in such a manner that a silicon oxide film was formed to a thickness of 100 nm over a glass substrate by a sputtering method, and an aluminum oxide film was formed to a thickness of 20 nm over the silicon oxide film by a sputtering method. Further, as a comparative example, a comparative sample B was fabricated in such a manner that after the example sample B was measured by TDS analysis, the aluminum oxide film thereof was removed, and only the silicon oxide film was left over the glass substrate.

For each of the comparative sample B and the example sample B, the silicon oxide film was formed under the following conditions: a silicon oxide (SiO2) target was used as a target, the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the power source was 1.5 kW, the atmosphere was an oxygen atmosphere (the oxygen flow rate was 50 sccm), and the substrate temperature was 100° C.

For the example sample B, the aluminum oxide film was formed under the following conditions: an aluminum oxide (Al2O3) target was used as a target, the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the power source was 1.5 kW, the atmosphere was a mixed atmosphere containing argon and oxygen (the argon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), and the substrate temperature was 250° C.

Other three types of the comparative sample B and the example sample B were fabricated by further performing thermal treatment at the respective temperatures of 300° C., 450° C., and 600° C. The thermal treatment was performed for 1 hour in a nitrogen atmosphere for each sample.

TDS analysis was performed on the four types of the comparative sample B and the four types of the example sample B fabricated under the respective four conditions (without thermal treatment, with 300° C. thermal treatment, with 450° C. thermal treatment, and with 600° C. thermal treatment).FIG. 17Ashows a result of TDS analysis at M/z=32 (O2) of the comparative sample B subjected to no thermal treatment,FIG. 17Bshows that of the comparative sample B subjected to thermal treatment at 300° C.,FIG. 17Cshows that of the comparative sample B subjected to thermal treatment at 450° C., andFIG. 17Dshows that of the comparative sample B subjected to thermal treatment at 600° C.FIG. 18Ashows a result of TDS analysis at M/z=32 (O2) of the example sample B subjected to no thermal treatment,FIG. 18Bshows that of the example sample B subjected to thermal treatment at 300° C.,FIG. 18Cshows that of the example sample B subjected to thermal treatment at 450° C., andFIG. 18Dshows that of the example sample B subjected to thermal treatment at 600° C.

As shown inFIGS. 17A to 17D, it can be seen inFIG. 17Athat oxygen was released from the silicon oxide film of the comparative sample B which was not subjected to thermal treatment. However, the amount of oxygen released was greatly decreased in the comparative sample B subjected to the thermal treatment at 300° C. as shown inFIG. 17B. Furthermore, the amount of the released oxygen was smaller than or equal to a background level of TDS measurement in the case of comparative samples B subjected to the thermal treatment at 450° C. and 600° C. as shown inFIGS. 17C and 17D, respectively.

The results inFIGS. 17A to 17Dindicate that 90% or more of excess oxygen contained in the silicon oxide film was released outside the silicon oxide film by the thermal treatment at 300° C. and that substantially all of the excess oxygen contained in the silicon oxide film was released outside the silicon oxide film by the thermal treatment at 450° C. and 600° C. Therefore, it is confirmed that the silicon oxide film has a low bather property with respect to oxygen.

In contrast, as shown inFIGS. 18A to 18D, substantially the same amount of oxygen was released from every type of the example sample B, in which the aluminum oxide film was formed over the silicon oxide film, regardless of whether the thermal treatment was not performed or was performed at 300° C., 450° C., and 600° C.

The results inFIGS. 18A to 18Dindicate that when the aluminum oxide film was formed over the silicon oxide film, the excess oxygen contained in the silicon oxide film was not easily released to the outside by thermal treatment and the state where the excess oxygen was contained in the silicon oxide film was held to a considerable extent. Therefore, it is confirmed that the aluminum oxide film has a high barrier property with respect to oxygen.

The above results confirm that the aluminum oxide film has both a high barrier property with respect to hydrogen and moisture and a high barrier property with respect to oxygen, and functions suitably as a barrier film with respect to hydrogen, moisture, and oxygen.

Accordingly, the aluminum oxide film can function as a protective film that prevents entry of impurities such as hydrogen and moisture, which can cause variation, into an amorphous oxide semiconductor layer and release of oxygen from the amorphous oxide semiconductor layer during and after the manufacturing process of a transistor including the amorphous oxide semiconductor layer.

Therefore, the amorphous oxide semiconductor layer has high purity because impurities such as hydrogen and moisture do not enter the amorphous oxide semiconductor layer, and includes a region containing oxygen in a proportion higher than that in the stoichiometric composition of the amorphous oxide semiconductor layer in a crystalline state because oxygen is prevented from being released. Accordingly, by using the amorphous oxide semiconductor layer for a transistor, it is possible to reduce variation in the threshold voltage Vthof the transistor due to an oxygen vacancy and suppress a shift of the threshold voltage.

In this example, the crystalline state of an oxide semiconductor layer was observed.

An example sample C1 was fabricated as a sample as follows: a 300-nm-thick silicon oxide film was formed over a glass substrate by a sputtering method, a 100-nm-thick In—Ga—Zn—O film was formed over the silicon oxide film by a sputtering method, and a 100-nm-thick aluminum oxide film was formed over the In—Ga—Zn—O film by a sputtering method.

The silicon oxide film was formed under the following conditions: a silicon oxide (SiO2) target was used as a target, the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the RF power source was 1.5 kW, the atmosphere was a mixed atmosphere containing argon and oxygen (the argon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), and the substrate temperature was 100° C.

The film formation conditions of the In—Ga—Zn—O film were as follows: the target was an oxide target having a composition ratio of In2O3:Ga2O3:ZnO=1:1:2 [molar ratio], the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the RF power source was 0.5 kW, the atmosphere was a mixed atmosphere containing argon and oxygen (the argon flow rate was 30 sccm and the oxygen flow rate was 15 sccm), and the substrate temperature was 250° C.

Note that the silicon oxide film and the In—Ga—Zn—O film were successively formed without exposure to the air, and then thermal treatment (first thermal treatment) was performed thereon at 400° C. for 30 minutes in a reduced pressure atmosphere.

Then, the aluminum oxide film was formed under the following conditions: an aluminum oxide (Al2O3) target was used as a target, the distance between the glass substrate and the target was 60 mm, the pressure was 0.4 Pa, the power of the RF power source was 2.5 kW, the atmosphere was a mixed atmosphere containing argon and oxygen (the argon flow rate was 25 sccm and the oxygen flow rate was 25 sccm), and the substrate temperature was 250° C.

Next, an example sample C2 was fabricated by implanting oxygen into the In—Ga—Zn—O film through the aluminum oxide film of the example sample C1. In the example sample C2, an oxygen (18O) ion was implanted into the In—Ga—Zn—O film through the aluminum oxide film by an ion implantation method. The oxygen (18O) ion was implanted under conditions of an acceleration voltage of 80 kV and a dose of 1.0×1016ions/cm2.

Further, an example sample C3 was fabricated by performing thermal treatment (second thermal treatment) on the example sample C2 at 450° C. for 1 hour in a nitrogen atmosphere.

End planes were cut out from the example samples C1 to C3 obtained through the above steps, and cross sections of the In—Ga—Zn—O films thereof were observed with a high resolution transmission electron microscope (TEM) (“H9000-NAR” manufactured by Hitachi High-Technologies Corporation) at an acceleration voltage of 300 kV.FIGS. 19A to 19Care TEM images of the example samples C1 to C3, respectively.

FIG. 19Ais a TEM image at 8 million-fold magnification of the interface between the In—Ga—Zn—O film and the aluminum oxide film of the example sample C1.FIG. 19Bis a TEM image at 8 million-fold magnification of the interface between the In—Ga—Zn—O film and the aluminum oxide film of the example sample C2.FIG. 19Cis a TEM image at 8 million-fold magnification of the interface between the In—Ga—Zn—O film and the aluminum oxide film of the example sample C3.

A lattice image can be observed in the In—Ga—Zn—O film inFIG. 19A, which indicates that the example sample C1 includes a crystalline region.

In contrast, a lattice image, which is seen inFIG. 19A, is not observed in the In—Ga—Zn—O film inFIG. 19Bthat was subjected to the oxygen implantation treatment.

As in the case ofFIG. 19B, a lattice image is not observed in the In—Ga—Zn—O film inFIG. 19Cthat was subjected to the oxygen implantation treatment and the thermal treatment at 450° C. This result indicates that the In—Ga—Zn—O film of the example sample C3 is amorphous.

The following can be confirmed from the above results: oxygen implantation on an oxide semiconductor layer including a crystalline region transforms the crystalline region into an amorphous state, and subsequent thermal treatment at a temperature lower than or equal to 450° C. allows the amorphous state to be maintained.

This application is based on Japanese Patent Application serial no. 2011-103592 filed with the Japan Patent Office on May 6, 2011, the entire contents of which are hereby incorporated by reference.