SEMICONDUCTOR DEVICE

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor device including an oxide semiconductor and a method for fabricating the semiconductor device.

[0003] In this specification, a “semiconductor device” refers to a device that can function by utilizing semiconductor characteristics; an electro-optical device, a semiconductor circuit, and an electric device are all included in the category of the semiconductor device.

[0004] 2. Description of the Related Art

[0005] Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The 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 example, an oxide semiconductor has been attracting attention.

[0006] For example, a transistor whose active layer includes an amorphous oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.

• [0007] [Patent Document 1] Japanese Published Patent Application No. 2006-165528

[0008] It is known that an oxygen vacancy in an oxide semiconductor becomes a donor; thus, in the case where the oxide semiconductor is used for a channel formation region of a transistor, an oxide semiconductor layer including as few oxygen vacancies as possible is preferably used.

[0009] However, even when an oxide semiconductor layer includes few oxygen vacancies initially, oxygen vacancies will increase in number from various causes. An increase in oxygen vacancies in an oxide semiconductor layer causes poor electrical characteristics in some cases; for example, the transistor becomes normally-on, leakage current increases, or threshold voltage is shifted due to stress application.

[0010] Therefore, an object of one embodiment of the present invention is to provide a semiconductor device in which an increase in oxygen vacancies in an oxide semiconductor layer can be suppressed. Another object is to provide a semiconductor device with favorable electrical characteristics. A further object is to provide a highly reliable semiconductor device.

[0011] In one embodiment of the present invention, a semiconductor device includes an oxide semiconductor layer in a channel formation region. An oxide insulating film below and in contact with the oxide semiconductor layer and a gate insulating film over and in contact with the oxide semiconductor layer are used to supply oxygen of the oxide insulating film or the gate insulating film to the oxide semiconductor layer. Further, a conductive nitride is used for a metal film of a source electrode layer and a drain electrode layer, whereby diffusion or transfer of oxygen to the metal film is suppressed. Details are described below.

[0012] One embodiment of the present invention is a semiconductor device including an oxide insulating film; an oxide semiconductor layer over the oxide insulating film; a first source electrode layer and a first drain electrode layer in contact with the oxide semiconductor layer; a second source electrode layer and a second drain electrode layer covering the first source electrode layer and the first drain electrode layer, respectively, and being in contact with the oxide semiconductor layer; a gate insulating film over the oxide insulating film, the oxide semiconductor layer, the second source electrode layer, and the second drain electrode layer; a gate electrode layer over the gate insulating film and in a position overlapping with the oxide semiconductor layer; and a protective insulating film over the gate insulating film and the gate electrode layer. The gate insulating film is partly in contact with the oxide insulating film in an exterior region of the second source electrode layer and the second drain electrode layer.

[0013] Another embodiment of the present invention is a semiconductor device including an oxide insulating film; an oxide semiconductor layer over the oxide insulating film; a first source electrode layer and a first drain electrode layer in contact with the oxide semiconductor layer; a second source electrode layer and a second drain electrode layer in contact with the first source electrode layer and the first drain electrode layer, respectively, and in contact with the oxide semiconductor layer; a gate insulating film over the oxide insulating film, the oxide semiconductor layer, the first source electrode layer, the first drain electrode layer, the second source electrode layer, and the second drain electrode layer; a gate electrode layer over the gate insulating film and in a position overlapping with the oxide semiconductor layer; and a protective insulating film over the gate insulating film and the gate electrode layer. The gate insulating film is partly in contact with the oxide insulating film in an exterior region of the first source electrode layer and the first drain electrode layer.

[0014] In each of the above structures, the first source electrode layer and the first drain electrode layer are preferably at least one material selected from Al, Cr, Cu, Ta, Ti, Mo, and W or an alloy material containing any of these as a main component.

[0015] In each of the above structures, end portions of the first source electrode layer and the first drain electrode layer preferably have a staircase-like shape.

[0016] In each of the above structures, the second source electrode layer and the second drain electrode layer are preferably at least one material selected from tantalum nitride, titanium nitride, and ruthenium or an alloy material containing any of these as a main component.

[0017] In each of the above structures, the protective insulating film is preferably a silicon nitride film.

[0018] In each of the above structures, it is preferable that the oxide semiconductor layer contain a crystal, and a c-axis of the crystal be parallel to a normal vector of a surface of the oxide semiconductor layer.

[0019] In one embodiment of the present invention, a semiconductor device in which an increase in oxygen vacancies in an oxide semiconductor layer is suppressed can be provided. A semiconductor device with favorable electrical characteristics can be provided. A highly reliable semiconductor device can be provided.

[0043] Embodiments and examples are described in detail with reference to the drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention should not be limited to the descriptions of the embodiments and examples below. Note that in structures of the present invention described below, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and description thereof is omitted in some cases.

[0044] In this specification, functions of a “source” and a “drain” of a transistor are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Thus, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification.

[0045] In this embodiment, a semiconductor device of one embodiment of the present invention will be described with reference to drawings.

[0046] FIGS. 1A, 1B, 1C, 1D, and 1E are a top view and cross-sectional views which illustrate a transistor of one embodiment of the present invention. FIG. 1A is the top view of the transistor, and a cross section taken along a dashed-dotted line X1-Y1 in FIG. 1A is illustrated in FIG. 1B. A cross section taken along a dashed-dotted line V1-W1 in FIG. 1A is illustrated in FIG. 1C. FIG. 1D illustrates widths of components of the transistor which are illustrated in FIG. 1B. FIG. 1E is an enlarged view of a region 105 illustrated in FIG. 1B. Note that for simplification of the drawing, some components in the top view in FIG. 1A are illustrated in a see-through manner or not illustrated.

[0047] A transistor 150 illustrated in FIGS. 1A, 1B, 1C, 1D, and 1E includes an oxide insulating film 104 formed over a substrate 102; an oxide semiconductor layer 106 formed over the oxide insulating film 104; a first source electrode layer 108a and a first drain electrode layer 108b formed over the oxide semiconductor layer 106; a second source electrode layer 110a and a second drain electrode layer 110b formed over the first source electrode layer 108a and the first drain electrode layer 108b, respectively; a gate insulating film 112 formed over the oxide insulating film 104, the oxide semiconductor layer 106, the second source electrode layer 110a, and the second drain electrode layer 110b; a gate electrode layer 114 formed over the gate insulating film 112 and in a position overlapping with the oxide semiconductor layer 106; and a protective insulating film 116 formed over the gate insulating film 112 and the gate electrode layer 114. Note that another insulating layer, another wiring, or the like may be formed over the protective insulating film 116.

[0048] The substrate 102 is not limited to a simple supporting substrate, and may be a substrate where another device such as a transistor is formed. In that case, at least one of the gate electrode layer 114, the first source electrode layer 108a, the first drain electrode layer 108b, the second source electrode layer 110a, and the second drain electrode layer 110b of the transistor 150 may be electrically connected to the above device.

[0049] The oxide insulating film 104 can have a function of supplying oxygen to the oxide semiconductor layer 106 as well as a function of preventing diffusion of an impurity from the substrate 102; thus, the oxide insulating film 104 is an insulating film containing oxygen. It is particularly preferable that the oxide insulating film 104 be an insulating film containing excess oxygen. An oxide insulating film containing excess oxygen refers to an oxide insulating film from which oxygen can be released by heat treatment or the like. The oxide insulating film containing excess oxygen is preferably a film in which the amount of released oxygen when converted into oxygen atoms is 1.0×10¹⁹ atoms/cm³ or more in thermal desorption spectroscopy analysis. Further, excess oxygen refers to oxygen which can be transferred in the oxide semiconductor layer, silicon oxide, or silicon oxynitride by heat treatment, oxygen in excess of an intrinsic stoichiometric composition, or oxygen which can fill Vo (oxygen vacancy) caused by lack of oxygen. Oxygen released from the oxide insulating film 104 can be diffused to a channel formation region of the oxide semiconductor layer 106, so that oxygen vacancies which might be formed in the oxide semiconductor layer can be filled with the oxygen. In this manner, stable electrical characteristics of the transistor can be achieved.

[0050] Since the oxide insulating film 104 is provided in contact with the oxide semiconductor layer 106, oxygen can be directly diffused to the oxide semiconductor layer 106 from a lower side of the oxide semiconductor layer 106. Moreover, since the oxide insulating film 104 is provided in contact with the gate insulating film 112, oxygen can be diffused to the oxide semiconductor layer 106 from an upper side of the oxide semiconductor layer 106 through the gate insulating film 112. More specifically, oxygen released from the oxide insulating film 104 can enter the upper side region of the oxide semiconductor layer 106, which serves as a channel, by being transferred from the outside of the second source electrode layer 110a (the left side in FIG. 1B) and the outside of the second drain electrode layer 110b (the right side in FIG. 1B) through the gate insulating film 112. In other words, the gate insulating film 112 is partly in contact with the oxide insulating film 104 in an exterior region of the second source electrode layer 110a and the second drain electrode layer 110b.

[0051] Thus, the gate insulating film 112 is provided between the protective insulating film 116, and the second source electrode layer 110a and the second drain electrode layer 110b so that oxygen released from the oxide insulating film 104 can be diffused to the channel of the oxide semiconductor layer 106. Accordingly, a material to which oxygen is not easily diffused or transferred is used for the second source electrode layer 110a, the second drain electrode layer 110b, and the protective insulating film 116. In this manner, diffusion or transfer of oxygen to the source electrode layer and the drain electrode layer at the time when oxygen is diffused to the oxide semiconductor layer through the gate insulating film can be suppressed.

[0052] In a transistor having such a structure, excess oxygen can be supplied from the oxide insulating film 104 and the gate insulating film 112 to the channel formation region of the oxide semiconductor layer 106, whereby the transistor including the oxide semiconductor layer 106 has normally-off characteristics with a positive threshold voltage. Thus, it is possible to provide a semiconductor device in which an increase in oxygen vacancies in the oxide semiconductor layer 106 is suppressed. Further, a highly reliable semiconductor device can be provided.

[0053] Note that in the case where the substrate 102 is a substrate where another device is formed, the oxide insulating film 104 also has a function as an interlayer insulating film. In that case, the oxide insulating film 104 is preferably subjected to planarization treatment such as chemical mechanical polishing (CMP) treatment so as to have a flat surface.

[0054] An oxide semiconductor that can be used for the oxide semiconductor layer 106 preferably contains at least indium (In) or zinc (Zn). Alternatively, the oxide semiconductor preferably contains both In and Zn. Details of a material and a formation method which can be used for the oxide semiconductor layer 106 are to be described in description of a method for fabricating the transistor.

[0055] Note that stable electrical characteristics can be effectively imparted to a transistor in which an oxide semiconductor layer serves as a channel by reducing the concentration of impurities in the oxide semiconductor layer to make the oxide semiconductor layer intrinsic or substantially intrinsic. The term “substantially intrinsic” refers to the state where an oxide semiconductor layer has a carrier density lower than 1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, further preferably lower than 1×10¹³/cm³.

[0056] Further, in the oxide semiconductor layer, hydrogen, nitrogen, carbon, silicon, and metal elements except for those of main components are impurities. For example, hydrogen and nitrogen form donor levels to increase the carrier density. Silicon forms impurity levels in an oxide semiconductor layer. The impurity levels serve as traps and might cause electrical characteristics of the transistor to deteriorate.

[0057] The oxide semiconductor layer can be intrinsic or substantially intrinsic under the following conditions: in SIMS analysis, the concentration of silicon is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³; the concentration of hydrogen is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, still further preferably lower than or equal to 5×10¹⁸ atoms/cm³; and the concentration of nitrogen is lower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

[0058] In the case where the oxide semiconductor layer includes crystals, high concentration of silicon or carbon might reduce the crystallinity of the oxide semiconductor layer. The crystallinity of the oxide semiconductor layer can be prevented from decreasing when the concentration of silicon is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³, and the concentration of carbon is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³.

[0059] A transistor in which a highly purified oxide semiconductor film is used for a channel formation region as described above has an extremely low off-state current, and the off-state current standardized on the channel width of the transistor can be as low as several yoktoamperes per micrometer to several zeptoamperes per micrometer.

[0060] When the density of localized levels in the film of the oxide semiconductor which can be used for the oxide semiconductor layer 106 is reduced, stable electrical characteristics can be imparted to the transistor including the oxide semiconductor layer 106. Note that to impart stable electrical characteristics to the transistor, the absorption coefficient due to the localized levels in the oxide semiconductor layer 106, which is obtained in measurement by a constant photocurrent method (CPM), is set lower than 1×10⁻³/cm, preferably lower than 3×10⁻⁴/cm.

[0061] For the first source electrode layer 108a and the first drain electrode layer 108b, a conductive material which is easily bonded to oxygen can be used. For example, Al, Cr, Cu, Ta, Ti, Mo, or W can be used. In particular, W with a high melting point is preferably used, which allows subsequent process temperatures to be relatively high. Note that the conductive material which is easily bonded to oxygen includes, in its category, a material to which oxygen is easily diffused or transferred.

[0062] When the conductive material which is easily bonded to oxygen is in contact with the oxide semiconductor layer, a phenomenon occurs in which oxygen of the oxide semiconductor layer is diffused or transferred to the conductive material which is easily bonded to oxygen. Since the fabrication process of the transistor involves some heat treatment steps, the above phenomenon causes generation of oxygen vacancies in a region of the oxide semiconductor layer, which is in contact with the source electrode or the drain electrode, and the region is changed to an n-type. Thus, the n-type region can serve as a source or a drain of the transistor.

[0063] However, in the case of forming a transistor with an extremely short channel length, the n-type region which is formed by the generation of oxygen vacancies sometimes extends in the channel length direction of the transistor. In that case, electrical characteristics of the transistor change; for example, the threshold voltage is shifted or on and off of the transistor cannot be controlled with the gate voltage (i.e., the transistor is on). Accordingly, when a transistor with an extremely short channel length is formed, it is not preferable that the conductive material which is easily bonded to oxygen be used for a source electrode and a drain electrode.

[0064] Thus, in one embodiment of the present invention, the source electrode and the drain electrode have stacked-layer structures, and the second source electrode layer 110a and the second drain electrode layer 110b, which determine the channel length, are formed using the conductive material which is not easily bonded to oxygen. As the conductive material which is not easily bonded to oxygen, for example, a conductive nitride such as tantalum nitride or titanium nitride, or ruthenium is preferably used. Note that the conductive material which is not easily bonded to oxygen includes, in its category, a material to which oxygen is not easily diffused or transferred.

[0065] Note that in the transistor having the structure illustrated in FIGS. 1A to 1E, the channel length refers to a distance between the second source electrode layer 110a and the second drain electrode layer 110b.

[0066] By the use of the above conductive material which is not easily bonded to oxygen for the second source electrode layer 110a and the second drain electrode layer 110b, generation of oxygen vacancies in the channel formation region of the oxide semiconductor layer 106 can be suppressed, so that change of the channel to an n-type can be suppressed. In this manner, even a transistor with an extremely short channel length can have favorable electrical characteristics.

[0067] In the case where the source electrode and the drain electrode are formed using only the above conductive material which is not easily bonded to oxygen, the contact resistance with the oxide semiconductor layer 106 becomes too high; thus, it is preferable that as illustrated in FIG. 1B, the first source electrode layer 108a and the first drain electrode layer 108b be formed over the oxide semiconductor layer 106 and the second source electrode layer 110a and the second drain electrode layer 110b be formed so as to cover the first source electrode layer 108a and the first drain electrode layer 108b.

[0068] At this time, it is preferable that the oxide semiconductor layer 106 have a large contact area with the first source electrode layer 108a or the first drain electrode layer 108b, and the oxide semiconductor layer 106 have a small contact area with the second source electrode layer 110a or the second drain electrode layer 110b. The region of the oxide semiconductor layer 106, which is in contact with the first source electrode layer 108a or the first drain electrode layer 108b, is changed to an n-type region due to generation of oxygen vacancies. Owing to the n-type region, the contact resistance between the oxide semiconductor layer 106 and the first source electrode layer 108a or the first drain electrode layer 108b can be reduced. Accordingly, when the oxide semiconductor layer 106 has a large contact area with the first source electrode layer 108a or the first drain electrode layer 108b, the area of the n-type region can also be large.

[0069] Here, the above-mentioned n-type region is described with reference to FIG. 1E. FIG. 1E is an enlarged view of the region 105 illustrated in FIG. 1B, and in the region of the oxide semiconductor layer 106, which is in contact with the first source electrode layer 108a, oxygen of the oxide semiconductor layer 106 is extracted to the first source electrode layer 108a side, so that an n-type region 106a is formed. Note that the n-type region 106a is a region of the oxide semiconductor layer 106, which includes many oxygen vacancies. Moreover, a component of the first source electrode layer 108a, for example, a tungsten element in the case where a tungsten film is used for the first source electrode layer 108a enters the n-type region 106a. In addition, although not illustrated, a mixed layer might be formed due to entry of oxygen of the oxide semiconductor layer 106 into a region of the first source electrode layer 108a, which is in contact with the oxide semiconductor layer 106.

[0070] Note that although the region 105 has been described with reference to the enlarged view illustrating the oxide semiconductor layer 106 and the first source electrode layer 108a, the above-described n-type region is also formed on the first drain electrode layer 108b side of the oxide semiconductor layer 106.

[0071] Note that the n-type region 106a may be used as a source region or a drain region in the oxide semiconductor layer 106.

[0072] Further, the conductive material which is not easily bonded to oxygen is used for the second source electrode layer 110a and the second drain electrode layer 110b. Thus, when the oxide semiconductor layer 106 is supplied with oxygen of the oxide insulating film 104 from the upper side of the oxide semiconductor layer 106 through the gate insulating film 112, the oxygen is less likely to be diffused or transferred to the second source electrode layer 110a and the second drain electrode layer 110b. Accordingly, oxygen can be favorably supplied to the oxide semiconductor layer 106.

[0073] The gate insulating film 112 can be formed using an insulating film containing one or more of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide. The gate insulating film 112 may be a stack of any of the above materials.

[0074] For the gate electrode layer 114, a conductive film formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used. The gate electrode layer 114 may be a stack of any of the above materials.

[0075] It is preferable that a material to which oxygen is not easily diffused or transferred be used for the protective insulating film 116. Further, a material containing little hydrogen when formed into a film is preferably used for the protective insulating film 116. The hydrogen content of the protective insulating film 116 is preferably lower than 5×10¹⁹/cm³, further preferably lower than 5×10¹⁸/cm³. When the hydrogen content of the protective insulating film 116 has the above value, an off-state current of the transistor can be low. For example, a silicon nitride film or a silicon nitride oxide film is preferably used as the protective insulating film 116.

[0076] Here, distances between the components are described with reference to the cross-sectional view in FIG. 1D.

[0077] The distance (L1) between the first source electrode layer 108a and the first drain electrode layer 108b is set to 0.8 μm or longer, preferably 1.0 μm or longer. In the case where L1 is shorter than 0.8 μm, influence of oxygen vacancies generated in the channel formation region cannot be eliminated, which might cause deterioration of the electrical characteristics of the transistor.

[0078] Even when the distance (L2) between the second source electrode layer 110a and the second drain electrode layer 110b is shorter than L1, for example, 30 nm or shorter, the transistor can have favorable electrical characteristics.

[0079] Further, when the width of the gate electrode layer 114 is referred to as L0, L0≦L1≦L2 (L1 is longer than or equal to L2 and shorter than or equal to L0) is satisfied as illustrated in FIG. 1D so that regions can be formed in which the gate electrode layer 114 overlaps with the source and drain electrode layers (the first source electrode layer 108a, the second source electrode layer 110a, the first drain electrode layer 108b, and the second drain electrode layer 110b) with the gate insulating film 112 provided therebetween. With the use of such a structure, on-state characteristics (e.g., on-state current and field-effect mobility) of a miniaturized transistor can be improved.

[0080] When the width of the oxide semiconductor layer 106 is referred to as L3 and the width of the transistor 150 is referred to as L4, L3 is preferably shorter than 1 μm and L4 is preferably longer than or equal to 1 μm and shorter than or equal to 2.5 μm. When L3 and L4 have the respective values, the transistor can be miniaturized.

[0081] The above is the transistor of one embodiment of the present invention, whose structure can suppress an increase in oxygen vacancies in the oxide semiconductor layer. Specifically, in the transistor, oxygen can be supplied from the oxide insulating film and the gate insulating film which are in contact with the oxide semiconductor layer to the oxide semiconductor layer. It is thus possible to provide a semiconductor device having favorable electrical characteristics and high long-term reliability.

[0082] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0083] In this embodiment, a method for fabricating the transistor 150 described in Embodiment 1 with reference to FIGS. 1A to 1E will be described with reference to FIGS. 2A to 2D, FIGS. 3A to 3D, and FIGS. 4A and 4B.

[0084] For the substrate 102, a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon-on-insulator (SOI) substrate, or the like may be used. Still alternatively, any of these substrates further provided with a semiconductor element may be used.

[0085] The oxide insulating film 104 can be formed by a plasma chemical vapor deposition (CVD) method, a sputtering method, or the like using an oxide insulating film of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or the like or a mixed material of any of these. Further, a stack of any of the above materials may be used, and at least an upper layer of the oxide insulating film 104, which is in contact with the oxide semiconductor layer 106, is formed using a material containing oxygen that might serve as a supply source of oxygen to the oxide semiconductor layer 106.

[0086] Oxygen may be added to the oxide insulating film 104 by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. By addition of oxygen, the oxide insulating film 104 can further contain excess oxygen.

[0087] Then, an oxide semiconductor film is formed over the oxide insulating film 104 by a sputtering method, a CVD method, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, or a pulse laser deposition (PLD) method and selectively etched, so that the oxide semiconductor layer 106 is formed (see FIG. 2A). Note that heating may be performed before etching.

[0088] An oxide semiconductor that can be used for the oxide semiconductor layer 106 preferably contains at least indium (In) or zinc (Zn). Alternatively, the oxide semiconductor preferably contains both In and Zn. In order to reduce fluctuations in electrical characteristics of the transistors including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and Zn.

[0089] As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), zirconium (Zr), and the like can be given. As another stabilizer, lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) can be given.

[0090] As the oxide semiconductor, for example, any of the following can be used: indium oxide, tin oxide, zinc oxide, an In—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, or an In—Hf—Al—Zn oxide.

[0091] Note that an In—Ga—Zn oxide refers to, for example, an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In to Ga and Zn. The In—Ga—Zn oxide may contain a metal element other than In, Ga, and Zn. Further, in this specification, a film formed using an In—Ga—Zn oxide is also referred to as an IGZO film.

[0092] Alternatively, a material represented by InMO₃(ZnO)_m (m>0, where m is not an integer) may be used. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Further alternatively, a material represented by In₂SnO₅(ZnO)_n (n>0, where n is an integer) may be used.

[0093] Note that the oxide semiconductor film is preferably formed by a sputtering method. As a sputtering method, an RF sputtering method, a DC sputtering method, an AC sputtering method, or the like can be used. In particular, a DC sputtering method is preferably used because dust generated in the deposition can be reduced and the film thickness can be uniform.

[0094] An oxide semiconductor film is classified roughly into a single-crystal oxide semiconductor film and a non-single-crystal oxide semiconductor film. The non-single-crystal oxide semiconductor film includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, a polycrystalline oxide semiconductor film, a microcrystalline oxide semiconductor film, an amorphous oxide semiconductor film, and the like.

[0095] First, a CAAC-OS film is described.

[0096] The CAAC-OS film is one of oxide semiconductor films having a plurality of c-axis aligned crystal parts.

[0097] In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.

[0098] According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflecting unevenness of a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film.

[0099] In this specification and the like, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°.

[0100] On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

[0101] From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film.

[0102] Most of the crystal parts included in the CAAC-OS film each fit inside a cube whose one side is less than 100 nm Thus, there is a case where a crystal part included in the CAAC-OS film fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm Note that when a plurality of crystal parts included in the CAAC-OS film are connected to each other, one large crystal region is formed in some cases. For example, a crystal region with an area of 2500 nm² or more, 5 μm² or more, or 1000 μm² or more is observed in some cases in the plan TEM image.

[0103] A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.

[0104] On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZnO₄, six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when φ scan is performed with 2θ fixed at around 56°.

[0105] According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner and observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal.

[0106] Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film.

[0107] Further, distribution of c-axis aligned crystal parts in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the crystal parts of the CAAC-OS film occurs from the vicinity of the top surface of the film, the proportion of the c-axis aligned crystal parts in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, a region to which the impurity is added is altered, and the proportion of the c-axis aligned crystal parts in the CAAC-OS film varies depending on regions, in some cases.

[0108] Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appears at around 31° and a peak of 2θ do not appear at around 36°.

[0109] The CAAC-OS film is an oxide semiconductor film having a low concentration of impurities. The impurity is an element other than the main components of the oxide semiconductor film, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor film, such as silicon, disturbs the atomic arrangement of the oxide semiconductor film by depriving the oxide semiconductor film of oxygen and causes a decrease in crystallinity. Further, a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor film and causes a decrease in crystallinity when it is contained in the oxide semiconductor film. Note that the impurity contained in the oxide semiconductor film might serve as a carrier trap or a carrier generation source.

[0110] The CAAC-OS film is an oxide semiconductor film having a low density of defect levels. In some cases, the oxygen vacancies in the oxide semiconductor film serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

[0111] The state in which the concentration of impurities is low and density of defect levels is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a lower carrier density. Thus, a transistor including the oxide semiconductor film rarely has electrical characteristics with negative threshold voltage (such electrical characteristics are also referred to as normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier traps. Accordingly, the transistor including the oxide semiconductor film has small change in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor film takes a long time to be released, and might behave like fixed electric charge. Thus, the transistor including the oxide semiconductor film having a high concentration of impurities and a high density of defect levels has unstable electrical characteristics in some cases.

[0112] In a transistor including the CAAC-OS film, change in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small.

[0113] Next, a microcrystalline oxide semiconductor film is described.

[0114] In a TEM image of the microcrystalline oxide semiconductor film, crystal parts sometimes cannot be found clearly. In most cases, the size of a crystal part in the microcrystalline oxide semiconductor film is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor film including nanocrystal is referred to as an nc-OS (nanocrystalline oxide semiconductor) film. In an image of the nc-OS film obtained with a TEM, for example, a boundary between crystal parts is not clearly detected in some cases.

[0115] In the nc-OS film, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Further, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor film depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Further, a halo pattern is shown in a selected-area electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 50 nm) larger than a diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm) close to, or smaller than or equal to a diameter of a crystal part. Further, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots are shown in a ring-like region in some cases.

[0116] Since the nc-OS film is an oxide semiconductor film having more regularity than the amorphous oxide semiconductor film, the nc-OS film has a lower density of defect levels than the amorphous oxide semiconductor film. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; hence, the nc-OS film has a higher density of defect levels than the CAAC-OS film.

[0117] Note that an oxide semiconductor film may be a stacked film including two or more films of an amorphous oxide semiconductor film, a microcrystalline oxide semiconductor film, and a CAAC-OS film, for example.

[0118] A CAAC-OS film can be deposited by a sputtering method using a polycrystalline oxide semiconductor sputtering target, for example. When ions collide with the sputtering target, a crystal region included in the sputtering target might be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) might flake off from the sputtering target. In that case, the flat-plate-like sputtered particle reaches a substrate while maintaining its crystal state, whereby the CAAC-OS film can be deposited.

[0119] For the deposition of the CAAC-OS film, the following conditions are preferably employed.

[0120] By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is lower than or equal to −80° C., preferably lower than or equal to −100° C. is used.

[0121] By increasing the substrate heating temperature during the deposition, migration of a sputtered particle occurs after the sputtered particle reaches the substrate. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate, migration occurs over the substrate, so that a flat plane of the sputtered particle is attached to the substrate.

[0122] Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is higher than or equal to 30 vol %, preferably 100 vol %.

[0123] As an example of the sputtering target, an In—Ga—Zn—O compound target is described below.

[0124] The In—Ga—Zn—O compound target, which is polycrystalline, is made by mixing InO_X powder, GaO_Y powder, and ZnO_Z powder in a predetermined molar ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y, and Z are each a given positive number. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target.

[0125] Next, first heat treatment is preferably performed. The first heat treatment may be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 500° C., in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, or a reduced pressure state. Alternatively, the first heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more in order to compensate desorbed oxygen. By the first heat treatment, the crystallinity of the oxide semiconductor layer 106 can be improved, and in addition, impurities such as hydrogen and water can be removed from the oxide insulating film 104 and the oxide semiconductor layer 106. Note that the step of the first heat treatment may be performed before etching for formation of the oxide semiconductor layer 106.

[0126] Then, a first conductive film 108 to be the first source electrode layer 108a and the first drain electrode layer 108b is formed over the oxide semiconductor layer 106 (see FIG. 2B). For the first conductive film 108, Al, Cr, Cu, Ta, Ti, Mo, W, or an alloy material containing any of these as a main component can be used. For example, a 100-nm-thick tungsten film is formed by a sputtering method or the like.

[0127] Next, resist masks 190a and 190b are formed over the first conductive film 108 (see FIG. 2C).

[0128] After that, the first conductive film 108 is etched so as to be divided over the oxide semiconductor layer 106 with the use of the resist masks 190a and 190b as masks, so that the first source electrode layer 108a and the first drain electrode layer 108b are formed; then, the resist masks 190a and 190b are removed (see FIG. 2D).

[0129] At this time, the first conductive film 108 is over-etched, so that the oxide semiconductor layer 106 is partly etched as illustrated in FIG. 2D. However, when the etching selectivity of the first conductive film 108 to the oxide semiconductor layer 106 is high, the oxide semiconductor layer 106 is hardly etched.

[0130] In addition, by over-etching the first conductive film 108, part of the oxide insulating film 104, more specifically, the oxide insulating film 104 in an exterior region of the first source electrode layer 108a and the first drain electrode layer 108b is etched as illustrated in FIG. 2D.

[0131] Then, a second conductive film 110 to be the second source electrode layer 110a and the second drain electrode layer 110b is formed over the oxide semiconductor layer 106, the first source electrode layer 108a, and the first drain electrode layer 108b (see FIG. 3A). For the second conductive film 110, a conductive nitride such as tantalum nitride or titanium nitride, ruthenium, or an alloy material containing any of these as a main component can be used. For example, a 20-nm-thick tantalum nitride film is formed by a sputtering method or the like.

[0132] Next, the second conductive film 110 is etched so as to be divided over the oxide semiconductor layer 106, so that the second source electrode layer 110a and the second drain electrode layer 110b are formed (see FIG. 3B). At this time, as illustrated in FIG. 3B, part of the oxide semiconductor layer 106 may be etched. Although not illustrated, at the time of etching for formation of the second source electrode layer 110a and the second drain electrode layer 110b, part of the oxide insulating film 104, more specifically, the oxide insulating film 104 in an exterior region of the second source electrode layer 110a and the second drain electrode layer 110b may be etched.

[0133] Note that in the case of forming a transistor whose channel length (a distance between the second source electrode layer 110a and the second drain electrode layer 110b) is extremely short, the second source electrode layer 110a and the second drain electrode layer 110b can be formed in such a manner that the second conductive film 110 is etched first so as to cover the first source electrode layer 108a and the first drain electrode layer 108b, and then etched using resist masks that are processed by a method suitable for fine line processing, such as electron beam exposure. Note that by the use of a positive type resist for the resist masks, the exposed region can be minimized and throughput can be thus improved. In the above manner, a transistor having a channel length of 30 nm or less can be formed.

[0134] Next, second heat treatment is preferably performed. The second heat treatment can be performed under a condition similar to that of the first heat treatment. By the second heat treatment, impurities such as hydrogen and water can be further removed from the oxide semiconductor layer 106.

[0135] Next, the gate insulating film 112 is formed over the oxide insulating film 104, the oxide semiconductor layer 106, the second source electrode layer 110a, and the second drain electrode layer 110b (see FIG. 3C). The gate insulating film 112 can be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or the like. The gate insulating film 112 may be a stack of any of the above materials. The gate insulating film 112 can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, a PLD method, or the like.

[0136] It is preferable that the gate insulating film 112 be successively subjected to heat treatment after being formed. For example, the gate insulating film 112 is formed with a PE-CVD apparatus and is successively subjected to heat treatment in a vacuum. The heat treatment can remove hydrogen, moisture, and the like from the gate insulating film 112. By the heat treatment, the gate insulating film 112 can be dehydrated or dehydrogenated to be dense.

[0137] After that, a third conductive film 113 to be the gate electrode layer 114 is formed over the gate insulating film 112; then, a resist mask 192 is formed in a desired region (see FIG. 3D). For the third conductive film 113, Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or an alloy material containing any of these as a main component can be used. The third conductive film 113 can be formed by a sputtering method or the like.

[0138] Then, the third conductive film 113 is etched, so that the gate electrode layer 114 is formed; then, the resist mask 192 is removed (see FIG. 4A).

[0139] Next, the protective insulating film 116 is formed over the gate insulating film 112 and the gate electrode layer 114 (see FIG. 4B). It is preferable that a material to which oxygen is not easily diffused or transferred be used for the protective insulating film 116. Further, a material containing little hydrogen when formed into a film is preferably used for the protective insulating film 116. The hydrogen content of the protective insulating film 116 is preferably lower than 5×10¹⁹/cm³, further preferably lower than 5×10¹⁸/cm³. When the hydrogen content of the protective insulating film 116 has the above value, an off-state current of the transistor can be low.

[0140] For example, a silicon nitride film or a silicon nitride oxide film is preferably used as the protective insulating film 116. The protective insulating film 116 can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method. In particular, for the protective insulating film 116, a silicon nitride film is preferably formed by a sputtering method, in which case the content of water or hydrogen is low.

[0141] Next, third heat treatment is preferably performed. The third heat treatment can be performed under a condition similar to that of the first heat treatment. By the third heat treatment, oxygen is easily released from the oxide insulating film 104 and the gate insulating film 112, so that oxygen vacancies in the oxide semiconductor layer 106 can be reduced.

[0142] Through the above process, the transistor 150 illustrated in FIGS. 1A to 1E can be fabricated.

[0143] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0144] In this embodiment, a transistor having a structure different from that of the transistor described in Embodiment 1 will be described with reference to FIGS. 5A to 5C and FIGS. 6A to 6D.

[0145] FIGS. 5A, 5B, and 5C are a top view and cross-sectional views which illustrate a transistor of one embodiment of the present invention. FIG. 5A is the top view of the transistor, and a cross section taken along a dashed-dotted line X2-Y2 in FIG. 5A is illustrated in FIG. 5B. A cross section taken along a dashed-dotted line V2-W2 in FIG. 5A is illustrated in FIG. 5C. Note that for simplification of the drawing, some components in the top view in FIG. 5A are illustrated in a see-through manner or not illustrated. Note that the same portions as or portions having functions similar to those of the transistor described in Embodiment 1 are denoted by the same reference numerals, and repeated description thereof is omitted.

[0146] A transistor 152 illustrated in FIGS. 5A, 5B, and 5C includes the oxide insulating film 104 formed over the substrate 102; the oxide semiconductor layer 106 formed over the oxide insulating film 104; a first source electrode layer 168a and a first drain electrode layer 168b formed over the oxide semiconductor layer 106; the second source electrode layer 110a and the second drain electrode layer 110b formed over the first source electrode layer 168a and the first drain electrode layer 168b, respectively; the gate insulating film 112 formed over the oxide insulating film 104, the oxide semiconductor layer 106, the second source electrode layer 110a, and the second drain electrode layer 110b; the gate electrode layer 114 formed over the gate insulating film 112 and in a position overlapping with the oxide semiconductor layer 106; and the protective insulating film 116 formed over the gate insulating film 112 and the gate electrode layer 114. Note that another insulating layer, another wiring, or the like may be formed over the protective insulating film 116.

[0147] The transistor 152 described in this embodiment is different from the transistor 150 described in Embodiment 1 in the shapes of the first source electrode layer 168a and the first drain electrode layer 168b. Note that the second source electrode layer 110a, the second drain electrode layer 110b, the gate insulating film 112, the gate electrode layer 114, and the protective insulating film 116 which are formed over the first source electrode layer 168a and the first drain electrode layer 168b have shapes corresponding to the shapes of the first source electrode layer 168a and the first drain electrode layer 168b.

[0148] With the staircase-like shapes of the first source electrode layer 168a and the first drain electrode layer 168b as illustrated in FIG. 5B, the second source electrode layer 110a, the second drain electrode layer 110b, and the gate insulating film 112 can have favorable coverage. When the gate insulating film 112 has favorable coverage, oxygen released from the oxide insulating film 104 is likely to be diffused to an upper region of the oxide semiconductor layer 106, which serves as a channel, through the gate insulating film 112.

[0149] Here, a fabrication method of the transistor 152 will be described with reference to FIGS. 6A to 6D.

[0150] In the fabrication method of the transistor 152, steps before FIG. 6A are performed in a manner similar to those up to FIG. 2C in the fabrication method of the transistor 150 (see FIG. 6A). Note that the cross-sectional structure illustrated in FIG. 6A is the same as that illustrated in FIG. 2C.

[0151] Next, the first conductive film 108 is etched using the resist masks 190a and 190b to form the first source electrode layer 108a and the first drain electrode layer 108b (see FIG. 6B).

[0152] Next, resist masks 194a and 194b are formed by making the resist masks 190a and 190b recede or reducing them by ashing (see FIG. 6C).

[0153] Next, the first source electrode layer 108a and the first drain electrode layer 108b are etched using the resist masks 194a and 194b and then the resist masks 194a and 194b are removed, whereby the first source electrode layer 168a and the first drain electrode layer 168b are formed (see FIG. 6D).

[0154] By alternately performing plural times a step of making the resist masks recede or reducing them by ashing and an etching step, the end portions of the first source electrode layer 168a and the first drain electrode layer 168b can have staircase-like shapes.

[0155] Note that the subsequent steps are performed in manners similar to those of the corresponding steps in the fabrication process of the transistor 150 described in the above embodiment, whereby the transistor 152 described in this embodiment can be fabricated.

[0156] The above is the transistor of one embodiment of the present invention, whose structure can suppress an increase in oxygen vacancies in the oxide semiconductor layer. Specifically, in the transistor, oxygen can be supplied from the oxide insulating film and the gate insulating film which are in contact with the oxide semiconductor layer to the oxide semiconductor layer. It is thus possible to provide a semiconductor device having favorable electrical characteristics and high long-term reliability.

[0157] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0158] In this embodiment, a transistor having a structure different from that of the transistor described in Embodiment 1 will be described with reference to FIGS. 7A to 7D and FIGS. 8A and 8B.

[0159] FIGS. 7A, 7B, 7C, and 7D are a top view and cross-sectional views which illustrate a transistor of one embodiment of the present invention. FIG. 7A is the top view of the transistor, and a cross section taken along a dashed-dotted line X3-Y3 in FIG. 7A is illustrated in FIG. 7B. A cross section taken along a dashed-dotted line V3-W3 in FIG. 7A is illustrated in FIG. 7C. FIG. 7D illustrates widths of components of the transistor which are illustrated in FIG. 7B. Note that for simplification of the drawing, some components in the top view in FIG. 7A are illustrated in a see-through manner or not illustrated. Note that the same portions as or portions having functions similar to those of the transistor described in Embodiment 1 are denoted by the same reference numerals, and repeated description thereof is omitted.

[0160] A transistor 154 illustrated in FIGS. 7A, 7B, 7C, and 7D includes the oxide insulating film 104 formed over the substrate 102; the oxide semiconductor layer 106 formed over the oxide insulating film 104; the first source electrode layer 108a and the first drain electrode layer 108b formed over the oxide semiconductor layer 106; the second source electrode layer 110a and the second drain electrode layer 110b formed over the first source electrode layer 108a and the first drain electrode layer 108b, respectively; the gate insulating film 112 formed over the oxide insulating film 104, the oxide semiconductor layer 106, the second source electrode layer 110a, and the second drain electrode layer 110b; a gate electrode layer 174 formed over the gate insulating film 112 and in a position overlapping with the oxide semiconductor layer 106; and the protective insulating film 116 formed over the gate insulating film 112 and the gate electrode layer 174. Note that another insulating layer, another wiring, or the like may be formed over the protective insulating film 116.

[0161] The transistor 154 described in this embodiment is different from the transistor 150 described in Embodiment 1 in the shape of the gate electrode layer 174. In the transistor 150, the gate electrode layer 114 is provided in a position overlapping with the first source electrode layer 108a, the first drain electrode layer 108b, the second source electrode layer 110a, and the second drain electrode layer 110b; however, in the transistor 154 described in this embodiment, the gate electrode layer 174 is provided in a position overlapping with the second source electrode layer 110a and the second drain electrode layer 110b. In other words, the gate electrode layer 174 is not provided in a position overlapping with the first source electrode layer 108a and the first drain electrode layer 108b.

[0162] Here, distances between the components are described with reference to the cross-sectional view in FIG. 7D.

[0163] The distance (L1) between the first source electrode layer 108a and the first drain electrode layer 108b is set to 0.8 μm or longer, preferably 1.0 μm or longer. In the case where L1 is shorter than 0.8 μm, influence of oxygen vacancies generated in the channel formation region cannot be eliminated, which might cause deterioration of the electrical characteristics of the transistor.

[0164] Even when the distance (L2) between the second source electrode layer 110a and the second drain electrode layer 110b is shorter than L1, for example, 30 nm or shorter, the transistor can have favorable electrical characteristics.

[0165] When the width of the gate electrode layer 174 is referred to as L0, L1≦L0≦L2 (L0 is longer than or equal to L2 and shorter than or equal to L1) is satisfied so that parasitic capacitance which is caused between the gate and the drain and between the gate and the source can be made small as much as possible. Accordingly, the frequency characteristics of the transistor can be improved. For example, L0 can be set to 40 nm. Note that in order to obtain favorable electrical characteristics of the transistor, it is preferable that a difference between L0 and L2 be greater than or equal to 2 nm and less than or equal to 20 nm and a difference between L1 and L2 is greater than or equal to 20 nm and less than or equal to 1 μm.

[0166] Note that in a transistor that does not require high frequency characteristics, L0≦L1≦L2 (L1 is longer than or equal to L2 and shorter than or equal to L0) may be satisfied as illustrated in FIG. 1B. With such a structure, the degree of difficulty in formation steps of the gate electrode can be lowered.

[0167] When the width of the oxide semiconductor layer 106 is referred to as L3 and the width of the transistor 154 is referred to as L4, L3 is preferably shorter than 1 μm and L4 is preferably longer than or equal to 1 μm and shorter than or equal to 2.5 μm. When L3 and L4 have the respective values, the transistor can be miniaturized.

[0168] Here, a fabrication method of the transistor 154 will be described with reference to FIGS. 8A and 8B.

[0169] In the fabrication method of the transistor 154, steps before FIG. 8A are performed in a manner similar to those up to FIG. 3D in the fabrication method of the transistor 150 (see FIG. 8A). Note that the cross section illustrated in FIG. 8A is different from the cross section illustrated in FIG. 3D in the shape of a resist mask 196.

[0170] Note that as the resist mask 196, a mask having a finer pattern which is formed by performing a slimming process on a mask formed by a photolithography method or the like is preferably used. As the slimming process, an ashing process in which oxygen in a radical state (an oxygen radical) is used can be employed, for example. As a result of the slimming process, the line width of the mask formed by a photolithography method or the like can be reduced to a length shorter than or equal to the resolution limit of a light exposure apparatus, preferably less than or equal to half of the resolution limit of a light exposure apparatus, further preferably less than or equal to one third of the resolution limit of the light exposure apparatus. For example, the line width can be greater than or equal to 20 nm and less than or equal to 2000 nm, preferably greater than or equal to 50 nm and less than or equal to 350 nm.

[0171] Then, the third conductive film 113 is etched using a resist mask 196, so that the gate electrode layer 174 is formed; then, the resist mask 196 is removed (see FIG. 8B).

[0172] Note that the subsequent steps are performed in manners similar to those of the corresponding steps in the fabrication process of the transistor 150 described in the above embodiment, whereby the transistor 154 described in this embodiment can be fabricated.

[0173] The above is the transistor of one embodiment of the present invention, whose structure can suppress an increase in oxygen vacancies in the oxide semiconductor layer. Specifically, in the transistor, oxygen can be supplied from the oxide insulating film and the gate insulating film which are in contact with the oxide semiconductor layer to the oxide semiconductor layer. It is thus possible to provide a semiconductor device having favorable electrical characteristics and high long-term reliability.

[0174] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0175] In this embodiment, a transistor having a structure different from that of the transistor described in Embodiment 1 will be described with reference to FIGS. 9A to 9C and FIGS. 10A to 10C.

[0176] First, a transistor 156 illustrated in FIGS. 9A to 9C is described.

[0177] FIGS. 9A, 9B, and 9C are a top view and cross-sectional views which illustrate a transistor of one embodiment of the present invention. FIG. 9A is the top view of the transistor, and a cross section taken along a dashed-dotted line X4-Y4 in FIG. 9A is illustrated in FIG. 9B. A cross section taken along a dashed-dotted line V4-W4 in FIG. 9A is illustrated in FIG. 9C. Note that for simplification of the drawing, some components in the top view in FIG. 9A are illustrated in a see-through manner or not illustrated. Note that the same portions as or portions having functions similar to those of the transistor described in Embodiment 1 are denoted by the same reference numerals, and repeated description thereof is omitted.

[0178] The transistor 156 illustrated in FIGS. 9A, 9B, and 9C includes the oxide insulating film 104 formed over the substrate 102; the oxide semiconductor layer 106 formed over the oxide insulating film 104; the first source electrode layer 168a and the first drain electrode layer 168b formed over the oxide semiconductor layer 106; the second source electrode layer 110a and the second drain electrode layer 110b formed over the first source electrode layer 168a and the first drain electrode layer 168b, respectively; the gate insulating film 112 formed over the oxide insulating film 104, the oxide semiconductor layer 106, the second source electrode layer 110a, and the second drain electrode layer 110b; the gate electrode layer 174 formed over the gate insulating film 112 and in a position overlapping with the oxide semiconductor layer 106; and the protective insulating film 116 formed over the gate insulating film 112 and the gate electrode layer 174. Note that another insulating layer, another wiring, or the like may be formed over the protective insulating film 116.

[0179] The transistor 156 described in this embodiment is different from the transistor 150 described in Embodiment 1 in the shapes of the first source electrode layer 168a, the first drain electrode layer 168b, and the gate electrode layer 174. Note that the second source electrode layer 110a, the second drain electrode layer 110b, the gate insulating film 112, the gate electrode layer 174, and the protective insulating film 116 which are formed over the first source electrode layer 168a and the first drain electrode layer 168b have shapes corresponding to the shapes of the first source electrode layer 168a and the first drain electrode layer 168b.

[0180] In the transistor 150, the gate electrode layer 114 is provided in a position overlapping with the first source electrode layer 108a, the first drain electrode layer 108b, the second source electrode layer 110a, and the second drain electrode layer 110b; however, in the transistor 156 described in this embodiment, the gate electrode layer 174 is provided in a position overlapping with the second source electrode layer 110a and the second drain electrode layer 110b. In other words, the gate electrode layer 174 is not provided in a position overlapping with the first source electrode layer 168a and the first drain electrode layer 168b.

[0181] The transistor 156 described in this embodiment can be fabricated by referring to the fabrication methods of the transistors 152 and 154 described in the above embodiments for the structures of the other components.

[0182] Next, a transistor 158 illustrated in FIGS. 10A to 10C is described.

[0183] The transistor 158 illustrated in FIGS. 10A, 10B, and 10C includes the oxide insulating film 104 formed over the substrate 102; the oxide semiconductor layer 106 formed over the oxide insulating film 104; a first source electrode layer 178a and a first drain electrode layer 178b formed over the oxide semiconductor layer 106; a second source electrode layer 180a and a second drain electrode layer 180b formed over the first source electrode layer 178a and the first drain electrode layer 178b, respectively; the gate insulating film 112 formed over the oxide insulating film 104, the oxide semiconductor layer 106, the second source electrode layer 180a, and the second drain electrode layer 180b; the gate electrode layer 174 formed over the gate insulating film 112 and in a position overlapping with the oxide semiconductor layer 106; and the protective insulating film 116 formed over the gate insulating film 112 and the gate electrode layer 174. Note that another insulating layer, another wiring, or the like may be formed over the protective insulating film 116.

[0184] The transistor 158 described in this embodiment is different from the transistor 150 described in Embodiment 1 in the shapes of the first source electrode layer 178a, the first drain electrode layer 178b, the second source electrode layer 180a, the second drain electrode layer 180b, and the gate electrode layer 174. Note that the second source electrode layer 180a, the second drain electrode layer 180b, the gate insulating film 112, the gate electrode layer 174, and the protective insulating film 116 which are formed over the first source electrode layer 178a and the first drain electrode layer 178b have shapes corresponding to the shapes of the first source electrode layer 178a and the first drain electrode layer 178b.

[0185] With the shapes of the first source electrode layer 178a and the first drain electrode layer 178b as illustrated in FIG. 10B, the second source electrode layer 180a, the second drain electrode layer 180b, and the gate insulating film 112 can have favorable coverage.

[0186] Further, the second source electrode layer 180a and the second drain electrode layer 180b are provided on inner sides than the edges of the first source electrode layer 178a and the first drain electrode layer 178b in the cross section in the channel length direction (FIG. 10B). The first source electrode layer 178a and the first drain electrode layer 178b are not necessarily covered with the second source electrode layer 180a and the second drain electrode layer 180b as long as the second source electrode layer 180a and the second drain electrode layer 180b are provided in this manner at least over a region to be a channel length of the oxide semiconductor layer 106. Note that when the first source electrode layer and the first drain electrode layer are covered with the second source electrode layer and the second drain electrode layer as in any of the transistors described in the above embodiments, a possibility that oxygen might be diffused or transferred to the side faces of the first source electrode layer and the first drain electrode layer is reduced; accordingly, oxygen can be favorably supplied to the oxide semiconductor layer from the oxide insulating film through the gate insulating film.

[0187] The above is the transistor of one embodiment of the present invention, whose structure can suppress an increase in oxygen vacancies in the oxide semiconductor layer. Specifically, in the transistor, oxygen can be supplied from the oxide insulating film and the gate insulating film which are in contact with the oxide semiconductor layer to the oxide semiconductor layer. It is thus possible to provide a semiconductor device having favorable electrical characteristics and high long-term reliability.

[0188] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0189] In this embodiment, an example of a semiconductor device (memory device) which includes a transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles will be described with reference to drawings.

[0190] FIG. 11A is a cross-sectional view of the semiconductor device, and FIG. 11B is a circuit diagram of the semiconductor device.

[0191] The semiconductor device illustrated in FIGS. 11A and 11B includes a transistor 3200 including a first semiconductor material in a lower portion, and a transistor 3202 including a second semiconductor material and a capacitor 3204 in an upper portion. As the transistor 3202, any of the transistors described in Embodiments 1 to 5 can be used, and an example in which the transistor 150 described in Embodiment 1 with reference to FIGS. 1A to 1E is applied to the transistor 3202 is described in this embodiment. One electrode of the capacitor 3204 is formed using the same material as a gate electrode of the transistor 3202, the other electrode of the capacitor 3204 is formed using the same material as a source electrode and a drain electrode of the transistor 3202, and a dielectric of the capacitor 3204 is formed using the same material as the gate insulating film 112 of the transistor 3202; thus, the capacitor 3204 can be formed at the same time as the transistor 3202.

[0192] Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material may be the oxide semiconductor described in Embodiment 1. A transistor including, for example, crystalline silicon as a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor enables charge to be held for a long time owing to its electrical characteristics, that is, the low off-state current.

[0193] Although both of the above transistors are n-channel transistors in the following description, it is needless to say that p-channel transistors can be used. The specific structure of the semiconductor device, such as the material used for the semiconductor device and the structure of the semiconductor device, is not necessarily limited to that described here except for the use of the transistor described in Embodiment 1, which is formed using an oxide semiconductor for storing data.

[0194] The transistor 3200 in FIG. 11A includes a channel formation region provided in a substrate 3000 including a semiconductor material (such as crystalline silicon), impurity regions provided such that the channel formation region is provided therebetween, intermetallic compound regions provided in contact with the impurity regions, a gate insulating film provided over the channel formation region, and a gate electrode layer provided over the gate insulating film. Note that a transistor whose source electrode layer and drain electrode layer are not illustrated in a drawing may also be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode layer may be collectively referred to as a source electrode layer, and a drain region and a drain electrode layer may be collectively referred to as a drain electrode layer. That is, in this specification, the term “source electrode layer” might include a source region.

[0195] Further, an element isolation insulating layer 3106 is formed on the substrate 3000 so as to surround the transistor 3200, and an oxide insulating film 3220 is formed so as to cover the transistor 3200. Note that the element isolation insulating layer 3106 can be formed by an element isolation technique such as local oxidation of silicon (LOCOS) or shallow trench isolation (STI).

[0196] For example, the transistor 3200 formed using a crystalline silicon substrate can operate at high speed. Thus, when the transistor is used as a reading transistor, data can be read at high speed. As treatment prior to formation of the transistor 3202 and the capacitor 3204, CMP treatment is performed on the oxide insulating film 3220 covering the transistor 3200, whereby the oxide insulating film 3220 is planarized and, at the same time, an upper surface of the gate electrode layer of the transistor 3200 is exposed.

[0197] The transistor 3202 is provided over the oxide insulating film 3220, and one of the source electrode and the drain electrode thereof is extended so as to function as the other electrode of the capacitor 3204.

[0198] The transistor 3202 in FIG. 11A is a top-gate transistor in which a channel is formed in an oxide semiconductor layer. Since the off-state current of the transistor 3202 is low, stored data can be retained for a long period owing to such a transistor. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation in a semiconductor memory device can be extremely low, which leads to a sufficient reduction in power consumption.

[0199] Further, an electrode 3150 overlaps with the transistor 3202 with the oxide insulating film 3220 provided therebetween. By supplying an appropriate potential to the electrode 3150, the threshold voltage of the transistor 3202 can be controlled. In addition, long-term reliability of the transistor 3202 can be improved.

[0200] The transistor 3200 and the transistor 3202 can be formed so as to overlap with each other as illustrated in FIG. 11A, whereby the area occupied by them can be reduced. Accordingly, the degree of integration of the semiconductor device can be increased.

[0201] An example of a circuit configuration corresponding to FIG. 11A is illustrated in FIG. 11B.

[0202] In FIG. 11B, a first wiring (1st Line) is electrically connected to a source electrode layer of the transistor 3200. A second wiring (2nd Line) is electrically connected to a drain electrode layer of the transistor 3200. A third wiring (3rd Line) is electrically connected to the other of the source electrode layer and the drain electrode layer of the transistor 3202, and a fourth wiring (4th Line) is electrically connected to the gate electrode layer of the transistor 3202. The gate electrode layer of the transistor 3200 and the one of the source electrode layer and the drain electrode layer of the transistor 3202 are electrically connected to the other electrode of the capacitor 3204. A fifth wiring (5th Line) is electrically connected to the one electrode of the capacitor 3204.

[0203] The semiconductor device in FIG. 11B utilizes a characteristic in which the potential of the gate electrode layer of the transistor 3200 can be held, and thus enables writing, storing, and reading of data as follows.

[0204] Writing and storing of data are described. First, the potential of the fourth wiring is set to a potential at which the transistor 3202 is turned on, so that the transistor 3202 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode layer of the transistor 3200 and the capacitor 3204. That is, a predetermined charge is supplied to the gate electrode layer of the transistor 3200 (writing). Here, one of two kinds of charges providing different potential levels (hereinafter referred to as a low-level charge and a high-level charge) is supplied. After that, the potential of the fourth wiring is set to a potential at which the transistor 3202 is turned off, so that the transistor 3202 is turned off. Thus, the charge supplied to the gate electrode layer of the transistor 3200 is held (holding).

[0205] Since the off-state current of the transistor 3202 is extremely low, the charge of the gate electrode layer of the transistor 3200 is held for a long time.

[0206] Next, reading of data is described. By supplying an appropriate potential (a reading potential) to the fifth wiring while supplying a predetermined potential (a constant potential) to the first wiring, the potential of the second wiring varies depending on the amount of charge held in the gate electrode layer of the transistor 3200. This is because in general, when the transistor 3200 is an n-channel transistor, an apparent threshold voltage V_th—H in the case where the high-level charge is given to the gate electrode layer of the transistor 3200 is lower than an apparent threshold voltage V_th—L in the case where the low-level charge is given to the gate electrode layer of the transistor 3200. Here, an apparent threshold voltage refers to the potential of the fifth wiring which is needed to turn on the transistor 3200. Thus, the potential of the fifth wiring is set to a potential V₀ which is between V_th—H and V_th—L, whereby charge supplied to the gate electrode layer of the transistor 3200 can be determined For example, in the case where the high-level charge is supplied in writing, when the potential of the fifth wiring is V₀ (>V_th—H), the transistor 3200 is turned on. In the case where the low-level charge is supplied in writing, even when the potential of the fifth wiring is V₀ (<V_th—L), the transistor 3200 remains off. Therefore, the data stored in the gate electrode layer can be read by determining the potential of the second wiring.

[0207] Note that in the case where memory cells are arrayed, it is necessary that only data of a desired memory cell be able to be read. The fifth wiring in the case where data is not read may be supplied with a potential at which the transistor 3200 is turned off regardless of the state of the gate electrode layer, that is, a potential lower than V_th—H. Alternatively, the fifth wiring may be supplied with a potential at which the transistor 3200 is turned on regardless of the state of the gate electrode layer, that is, a potential higher than V_th—L.

[0208] When including a transistor having a channel formation region formed using an oxide semiconductor and having an extremely low off-state current, the semiconductor device described in this embodiment can retain stored data for an extremely long period. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long period even when power is not supplied (note that a potential is preferably fixed).

[0209] Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional nonvolatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating film does not arise at all. That is, the semiconductor device according to the disclosed invention does not have a limitation on the number of times data can be rewritten, which is a problem of a conventional nonvolatile memory, and the reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily achieved.

[0210] As described above, a miniaturized and highly-integrated semiconductor device having high electrical characteristics and a fabrication method of the semiconductor device can be provided.

[0211] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0212] In this embodiment, a semiconductor device including a transistor of one embodiment of the present invention, which can retain stored data even when not powered, which does not have a limitation on the number of write cycles, and which has a structure different from that described in Embodiment 6, will be described.

[0213] FIG. 12A illustrates an example of a circuit configuration of the semiconductor device, and FIG. 12B is a conceptual diagram illustrating an example of the semiconductor device. As a transistor 4162 included in the semiconductor device, any of the transistors described in Embodiments 1 to 5 can be used. A capacitor 4254 can be formed through the same process and at the same time as the transistor 4162 in a manner similar to that of the capacitor 3204 described in Embodiment 6.

[0214] In the semiconductor device illustrated in FIG. 12A, a bit line BL is electrically connected to a source electrode of the transistor 4162, a word line WL is electrically connected to a gate electrode of the transistor 4162, and a drain electrode of the transistor 4162 is electrically connected to a first terminal of the capacitor 4254.

[0215] Next, writing and storing of data in the semiconductor device (a memory cell 4250) illustrated in FIG. 12A are described.

[0216] First, the potential of the word line WL is set to a potential at which the transistor 4162 is turned on, and the transistor 4162 is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor 4254 (writing). After that, the potential of the word line WL is set to a potential at which the transistor 4162 is turned off, so that the transistor 4162 is turned off. Thus, the potential of the first terminal of the capacitor 4254 is held (holding).

[0217] In addition, the transistor 4162 including an oxide semiconductor has an extremely low off-state current. For that reason, the potential of the first terminal of the capacitor 4254 (or a charge accumulated in the capacitor 4254) can be held for an extremely long time by turning off the transistor 4162.

[0218] Next, reading of data is described. When the transistor 4162 is turned on, the bit line BL which is in a floating state and the capacitor 4254 are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor 4254. As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor 4254 (or the charge accumulated in the capacitor 4254).

[0219] For example, the potential of the bit line BL after charge redistribution is (C_B×V_B0+C×V)/(C_B+C), where V is the potential of the first terminal of the capacitor 4254, C is the capacitance of the capacitor 4254, C_B is the capacitance component of the bit line BL (hereinafter also referred to as bit line capacitance), and V_B0 is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell 4250 is in either of two states in which the potentials of the first terminal of the capacitor 4254 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case of holding the potential V₁ (=(C_B×V_B0+C×V₁)/(C_B+C)) is higher than the potential of the bit line BL in the case of holding the potential V₀ (=(C_B×V_B0+C×V₀)/(C_B+C)).

[0220] Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read.

[0221] As described above, the semiconductor device illustrated in FIG. 12A can hold charge that is accumulated in the capacitor 4254 for a long time because the off-state current of the transistor 4162 is extremely low. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely low, which leads to a sufficient reduction in power consumption. Moreover, stored data can be retained for a long period even when power is not supplied.

[0222] Next, the semiconductor device illustrated in FIG. 12B is described.

[0223] The semiconductor device illustrated in FIG. 12B includes a memory cell array 4251 (memory cell arrays 4251a and 4251b) including the plurality of memory cells 4250 illustrated in FIG. 12A as memory circuits in the upper portion, and a peripheral circuit 4253 in the lower portion, which is necessary for operating the memory cell array 4251. Note that the peripheral circuit 4253 is electrically connected to the memory cell array 4251.

[0224] In the structure illustrated in FIG. 12B, the peripheral circuit 4253 can be provided under the memory cell arrays 4251a and 4251b. Thus, the size of the semiconductor device can be reduced.

[0225] It is preferable that a semiconductor material of the transistor provided in the peripheral circuit 4253 be different from that of the transistor 4162. For example, silicon, germanium, silicon germanium, silicon carbide, or gallium arsenide can be used, and a single crystal semiconductor is preferably used. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Thus, the transistor enables a variety of circuits (e.g., a logic circuit and a driver circuit) which need to operate at high speed to be favorably obtained.

[0226] Note that FIG. 12B illustrates, as an example, the semiconductor device in which the memory cell array 4251 has a stack of the memory cell array 4251a and the memory cell array 4251b; however, the number of stacked memory cell arrays is not limited to two. For the memory cell array 4251, a stack of three or more memory cell arrays may be used, or only one memory cell array may be used.

[0227] The transistor 4162 is formed using an oxide semiconductor, and any of the transistors described in Embodiments 1 to 5 can be used as the transistor 4162. Since the off-state current of the transistor including an oxide semiconductor is low, stored data can be retained for a long period. In other words, the frequency of refresh operation can be extremely low, which leads to a sufficient reduction in power consumption.

[0228] A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit which includes the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit which includes the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently low). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, an increase in the degree of integration of the semiconductor device can be achieved.

[0229] As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics can be provided.

[0230] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0231] In this embodiment, examples of an electronic device and an electric device which can use any of the transistors described in Embodiments 1 to 5 will be described.

[0232] Any of the transistors described in Embodiments 1 to 5 can be applied to a variety of electronic devices (including game machines) and electric devices. Examples of the electronic devices include display devices of televisions, monitors, and the like, lighting devices, desktop personal computers and notebook personal computers, word processors, image reproduction devices which reproduce still images or moving images stored in recording media such as digital versatile discs (DVDs), portable compact disc (CD) players, radio receivers, tape recorders, headphone stereos, stereos, cordless phone handsets, transceivers, mobile phones, car phones, portable game machines, calculators, portable information terminals, electronic notebooks, e-book readers, electronic translators, audio input devices, cameras such as video cameras and digital still cameras, electric shavers, and IC chips. Examples of the electric devices include high-frequency heating appliances such as microwave ovens, electric rice cookers, electric washing machines, electric vacuum cleaners, air-conditioning systems such as air conditioners, dishwashers, dish dryers, clothes dryers, futon dryers, electric refrigerators, electric freezers, electric refrigerator-freezers, freezers for preserving DNA, radiation counters, and medical equipment such as dialyzers. In addition, the examples of the electric devices include alarm devices such as smoke detectors, gas alarm devices, and security alarm devices. Further, the examples also include industrial equipment such as guide lights, traffic lights, belt conveyors, elevators, escalators, industrial robots, and power storage systems. In addition, moving objects and the like driven by oil engines and electric motors using power from non-aqueous secondary batteries are also included in the category of electric devices. Examples of the moving objects include electric vehicles (EV), hybrid electric vehicles (HEV) which include both an internal-combustion engine and a motor, plug-in hybrid electric vehicles (PHEV), tracked vehicles in which caterpillar tracks are substituted for wheels of these vehicles, motorized bicycles including motor-assisted bicycles, motorcycles, electric wheelchairs, golf carts, boats or ships, submarines, helicopters, aircrafts, rockets, artificial satellites, space probes, planetary probes, and spacecrafts. Specific examples of these electronic devices and electric devices are illustrated in FIG. 13, FIG. 14, FIGS. 15A to 15C, and FIGS. 16A to 16C.

[0233] First, as an example of the alarm device, a structure of a fire alarm is described with reference to FIG. 13. A fire alarm in this specification refers to any device which raises an alarm over fire occurrence instantly, and for example, a residential fire alarm, an automatic fire alarm system, and a fire detector used for the automatic fire alarm system are included in its category.

[0234] An alarm device illustrated in FIG. 13 includes at least a microcomputer 500. Here, the microcomputer 500 is provided in the alarm device. The microcomputer 500 includes a power gate controller 503 electrically connected to a high potential power supply line VDD, a power gate 504 electrically connected to the high potential power supply line VDD and the power gate controller 503, a CPU (central processing unit) 505 electrically connected to the power gate 504, and a sensor portion 509 electrically connected to the power gate 504 and the CPU 505. Further, the CPU 505 includes a volatile memory portion 506 and a nonvolatile memory portion 507.

[0235] The CPU 505 is electrically connected to a bus line 502 through an interface 508. The interface 508 as well as the CPU 505 is electrically connected to the power gate 504. As a bus standard of the interface 508, an I²C bus can be used, for example. A light-emitting element 530 electrically connected to the power gate 504 through the interface 508 is provided in the alarm device described in this embodiment.

[0236] The light-emitting element 530 is preferably an element which emits light with high directivity, and for example, an organic EL element, an inorganic EL element, or a light-emitting diode (LED) can be used.

[0237] The power gate controller 503 includes a timer and controls the power gate 504 with the use of the timer. The power gate 504 allows or stops supply of power from the high potential power supply line VDD to the CPU 505, the sensor portion 509, and the interface 508, in accordance with the control by the power gate controller 503. Here, as an example of the power gate 504, a switching element such as a transistor can be given.

[0238] With the use of the power gate controller 503 and the power gate 504, power is supplied to the sensor portion 509, the CPU 505, and the interface 508 in a period during which the amount of light is measured, and supply of power to the sensor portion 509, the CPU 505, and the interface 508 can be stopped during an interval between measurement periods. The alarm device operates in such a manner, whereby a reduction in power consumption of the alarm device can be achieved compared with that of the case where power is continuously supplied to the above structures.

[0239] In the case where a transistor is used as the power gate 504, it is preferable to use a transistor which has an extremely low off-state current and is used for the nonvolatile memory portion 507, for example, a transistor including an oxide semiconductor. With the use of such a transistor, leakage current can be reduced when supply of power is stopped by the power gate 504, so that a reduction in power consumption of the alarm device can be achieved.

[0240] A direct-current power source 501 may be provided in the alarm device described in this embodiment so that power is supplied from the direct-current power source 501 to the high potential power supply line VDD. An electrode of the direct-current power source 501 on a high potential side is electrically connected to the high potential power supply line VDD, and an electrode of the direct-current power source 501 on a low potential side is electrically connected to a low potential power supply line VSS. The low potential power supply line VSS is electrically connected to the microcomputer 500. Here, the high potential power supply line VDD is supplied with a high potential H. The low potential power supply line VSS is supplied with a low potential L, for example, a ground potential (GND).

[0241] In the case where a battery is used as the direct-current power source 501, for example, a battery case including an electrode electrically connected to the high potential power supply line VDD, an electrode electrically connected to the low potential power supply line VSS, and a housing which can hold the battery is provided in a housing. Note that the alarm device described in this embodiment does not necessarily include the direct-current power source 501 and may have, for example, a structure in which power is supplied from an alternate-current power source provided outside the alarm device through a wiring.

[0242] As the above battery, a secondary battery such as a lithium ion secondary battery (also called a lithium ion storage battery or a lithium ion battery) can be used. Further, a solar battery is preferably provided so that the secondary battery can be charged.

[0243] The sensor portion 509 measures a physical quantity relating to an abnormal situation and transmits a measurement value to the CPU 505. A physical quantity relating to an abnormal situation depends on the usage of the alarm device, and in an alarm device functioning as a fire alarm, a physical quantity relating to a fire is measured. Accordingly, the sensor portion 509 measures the amount of light as a physical quantity relating to a fire and senses smoke.

[0244] The sensor portion 509 includes an optical sensor 511 electrically connected to the power gate 504, an amplifier 512 electrically connected to the power gate 504, and an AD converter 513 electrically connected to the power gate 504 and the CPU 505. The optical sensor 511, the amplifier 512, and the AD converter 513 which are provided in the sensor portion 509, and the light-emitting element 530 operate when the power gate 504 allows supply of power to the sensor portion 509.

[0245] Here, FIG. 14 illustrates part of the cross section of the alarm device illustrated in FIG. 13. In the alarm device, element isolation regions 603 are formed in a p-type semiconductor substrate 601, and an n-channel transistor 719 including a gate insulating film 607, a gate electrode layer 609, n-type impurity regions 611a and 611b, an insulating film 615, and an insulating film 617 is formed. Here, the n-channel transistor 719 is formed using a semiconductor other than an oxide semiconductor, such as single crystal silicon, so that the n-channel transistor 719 can operate at sufficiently high speed. Accordingly, a volatile memory portion of a CPU that can achieve high-speed access can be formed.

[0246] In addition, contact plugs 619a and 619b are formed in openings which are formed by partly etching the insulating films 615 and 617, and an insulating film 621 having groove portions is formed over the insulating film 617 and the contact plugs 619a and 619b.

[0247] Wirings 623a and 623b are formed in the groove portions of the insulating film 621, and an insulating film 620 formed by a sputtering method, a CVD method, or the like is provided over the insulating film 621 and the wirings 623a and 623b. An insulating film 622 having a groove portion is formed over the insulating film 620.

[0248] An electrode 624 functioning as a back gate electrode of a second transistor 717 is formed in the groove portion of the insulating film 622. The electrode 624 can control the threshold voltage of the second transistor 717.

[0249] An oxide insulating film 625 formed by a sputtering method, a CVD method, or the like is provided over the insulating film 622 and the electrode 624, and the second transistor 717 and a photoelectric conversion element 714 are provided over the oxide insulating film 625.

[0250] The second transistor 717 includes an oxide semiconductor layer 606, a first source electrode layer 616a and a first drain electrode layer 616b in contact with the oxide semiconductor layer 606, a second source electrode layer 626a and a second drain electrode layer 626b in contact with upper portions of the first source electrode layer 616a and the first drain electrode layer 616b, a gate insulating film 612, a gate electrode layer 604, and a protective insulating film 618. Moreover, an insulating film 645 and an insulating film 646 cover the photoelectric conversion element 714 and the second transistor 717, and a wiring 649 is formed over the insulating film 646 so as to be in contact with the first drain electrode layer 616b. The wiring 649 functions as the node which electrically connects a drain electrode of the second transistor 717 to the gate electrode layer 609 of the n-channel transistor 719.

[0251] Although the structure in which the connection portion of the second transistor 717 and the wiring 649 is in contact with the first drain electrode layer 616b is shown as an example in this embodiment, without limitation thereon, a structure in which the connection portion is in contact with the second drain electrode layer 626b may be employed, for example.

[0252] Here, any of the transistors described in Embodiments 1 to 5 can be used as the second transistor 717, and the oxide semiconductor layer 606 corresponds to the oxide semiconductor layer 106 described in Embodiment 1. Moreover, the first source electrode layer 616a and the first drain electrode layer 616b correspond to the first source electrode layer 108a and the first drain electrode layer 108b described in Embodiment 1, respectively. The second source electrode layer 626a and the second drain electrode layer 626b correspond to the second source electrode layer 110a and the second drain electrode layer 110b described in Embodiment 1, respectively.

[0253] The optical sensor 511 includes the photoelectric conversion element 714, a capacitor, a first transistor, the second transistor 717, a third transistor, and the n-channel transistor 719. As the photoelectric conversion element 714, a photodiode can be used here, for example.

[0254] One of terminals of the photoelectric conversion element 714 is electrically connected to the low potential power supply line VSS, and the other of the terminals thereof is electrically connected to one of the first source electrode layer 616a and the first drain electrode layer 616b and/or one of the second source electrode layer 626a and the second drain electrode layer 626b of the second transistor 717.

[0255] The gate electrode layer 604 of the second transistor 717 is supplied with an electric charge accumulation control signal Tx, and the other of the first source electrode layer 616a and the first drain electrode layer 616b and/or the other of the second source electrode layer 626a and the second drain electrode layer 626b of the second transistor 717 are/is electrically connected to one of a pair of electrodes of the capacitor, one of a source electrode and a drain electrode of the first transistor, and the gate electrode of the n-channel transistor 719 (hereinafter the node is referred to as a node FD in some cases).

[0256] The other of the pair of electrodes of the capacitor is electrically connected to the low potential power supply line VSS. A gate electrode of the first transistor is supplied with a reset signal Res, and the other of the source electrode and the drain electrode thereof is electrically connected to the high potential power supply line VDD.

[0257] One of a source electrode and a drain electrode of the n-channel transistor 719 is electrically connected to one of a source electrode and a drain electrode of the third transistor and the amplifier 512. The other of the source electrode and the drain electrode of the n-channel transistor 719 is electrically connected to the high potential power supply line VDD. A gate electrode of the third transistor is supplied with a bias signal Bias, and the other of the source electrode and the drain electrode thereof is electrically connected to the low potential power supply line VSS.

[0258] Note that the capacitor is not necessarily provided. For example, in the case where parasitic capacitance of the n-channel transistor 719 or the like is sufficiently large, a structure without the capacitor may be employed.

[0259] Further, as each of the first transistor and the second transistor 717, the transistor having an extremely low off-state current is preferably used. As the transistor having an extremely low off-state current, a transistor including an oxide semiconductor is preferably used. With such a structure, the potential of the node FD can be held for a long time.

[0260] In the structure in FIG. 14, the photoelectric conversion element 714 is electrically connected to the second transistor 717 and is provided over the oxide insulating film 625.

[0261] The photoelectric conversion element 714 includes a semiconductor film 660 provided over the oxide insulating film 625, and the first source electrode layer 616a and an electrode 616c which are in contact with a top surface of the semiconductor film 660. The first source electrode layer 616a is an electrode functioning as the source electrode or the drain electrode of the second transistor 717 and electrically connects the photoelectric conversion element 714 to the second transistor 717. In the photoelectric conversion element 714, the second source electrode layer 626a and an electrode 626c are provided over the first source electrode layer 616a and the electrode 616c, respectively.

[0262] Over the semiconductor film 660, the second source electrode layer 626a, and the electrode 626c, the gate insulating film 612, the protective insulating film 618, the insulating film 645, and the insulating film 646 are provided. Further, a wiring 656 is formed over the insulating film 646 and is in contact with the electrode 616c through an opening provided in the electrode 626c, the gate insulating film 612, the protective insulating film 618, the insulating film 645, and the insulating film 646.

[0263] The electrode 616c can be formed in steps similar to those of the first source electrode layer 616a and the first drain electrode layer 616b, and the wiring 656 can be formed in steps similar to those of the wiring 649.

[0264] As the semiconductor film 660, a semiconductor film which can perform photoelectric conversion is provided, and for example, silicon or germanium can be used. In the case of using silicon, the semiconductor film 660 functions as an optical sensor which senses visible light. Further, there is a difference, between silicon and germanium, in wavelengths of electromagnetic waves that can be absorbed. When the semiconductor film 660 includes germanium, a sensor which mainly senses an infrared ray can be obtained.

[0265] In the above manner, the sensor portion 509 including the optical sensor 511 can be incorporated into the microcomputer 500, so that the number of components can be reduced and the size of the housing of the alarm device can be reduced. Note that in the case where the place of the optical sensor or the photoelectric conversion element needs a high degree of freedom, the optical sensor or the photoelectric conversion element may be externally provided so as to be electrically connected to the microcomputer 500.

[0266] In the alarm device including the above-described IC chip, the CPU 505 in which a plurality of circuits including any of the transistors described in the above embodiments are combined and mounted on one IC chip is used.

[0267] FIGS. 15A to 15C are block diagrams illustrating a specific configuration of a CPU at least partly including any of the transistors described in Embodiments 1 to 5.

[0268] The CPU illustrated in FIG. 15A includes an arithmetic logic unit (ALU) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface 1198, a rewritable ROM 1199, and an ROM interface 1189 over a substrate 1190. A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may be provided over a separate chip. Needless to say, the CPU in FIG. 15A is just an example in which the configuration has been simplified, and an actual CPU may have various configurations depending on the application.

[0269] An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.

[0270] The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 judges an interrupt request from an external input/output device or a peripheral circuit on the basis of its priority or a mask state, and processes the request. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 in accordance with the state of the CPU.

[0271] The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 based on a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.

[0272] In the CPU illustrated in FIG. 15A, a memory cell is provided in the register 1196. As the memory cell of the register 1196, any of the transistors described in the above embodiments can be used.

[0273] In the CPU illustrated in FIG. 15A, the register controller 1197 selects operation of storing data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is stored by a flip-flop or by a capacitor in the memory cell included in the register 1196. When data storing by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196. When data storing by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.

[0274] The power supply can be stopped by a switching element provided between a memory cell group and a node to which a power supply potential VDD or a power supply potential VSS is supplied, as illustrated in FIG. 15B or FIG. 15C. Circuits illustrated in FIGS. 15B and 15C are described below.

[0275] FIGS. 15B and 15C each illustrate an example of the configuration of a memory circuit in which any of the transistors described in the above embodiments is used as a switching element which controls supply of a power supply potential to a memory cell.

[0276] The memory device illustrated in FIG. 15B includes a switching element 1141 and a memory cell group 1143 including a plurality of memory cells 1142. Specifically, as each of the memory cells 1142, any of the transistors described in the above embodiments can be used. Each of the memory cells 1142 included in the memory cell group 1143 is supplied with the high-level power supply potential VDD via the switching element 1141. Further, each of the memory cells 1142 included in the memory cell group 1143 is supplied with a potential of a signal IN and the low-level power supply potential VSS.

[0277] In FIG. 15B, any of the transistors described in the above embodiments is used as the switching element 1141, and the switching of the transistor is controlled by a signal SigA supplied to a gate electrode layer thereof.

[0278] Note that FIG. 15B illustrates the configuration in which the switching element 1141 includes only one transistor; however, without particular limitation thereon, the switching element 1141 may include a plurality of transistors. In the case where the switching element 1141 includes a plurality of transistors which function as switching elements, the plurality of transistors may be connected to each other in parallel, in series, or in combination of parallel connection and series connection.

[0279] Although the switching element 1141 controls the supply of the high-level power supply potential VDD to each of the memory cells 1142 included in the memory cell group 1143 in FIG. 15B, the switching element 1141 may control the supply of the low-level power supply potential VSS.

[0280] In FIG. 15C, an example of a memory device in which each of the memory cells 1142 included in the memory cell group 1143 is supplied with the low-level power supply potential VSS via the switching element 1141 is illustrated. The supply of the low-level power supply potential VSS to each of the memory cells 1142 included in the memory cell group 1143 can be controlled by the switching element 1141.

[0281] When a switching element is provided between a memory cell group and a node to which the power supply potential VDD or the power supply potential VSS is supplied, data can be stored even in the case where an operation of a CPU is temporarily stopped and the supply of the power supply voltage is stopped; accordingly, power consumption can be reduced. Specifically, for example, while a user of a personal computer does not input data to an input device such as a keyboard, the operation of the CPU can be stopped, so that the power consumption can be reduced.

[0282] Although the CPU is given as an example here, the transistor can also be applied to an LSI such as a digital signal processor (DSP), a custom LSI, or a field programmable gate array (FPGA).

[0283] In FIG. 16A, an alarm device 8100 is a residential fire alarm, which is an example of an electric device including a sensor portion and a microcomputer 8101. Note that the microcomputer 8101 is an example of an electronic device including a CPU in which any of the transistors described in the above embodiments is used.

[0284] In FIG. 16A, an air conditioner which includes an indoor unit 8200 and an outdoor unit 8204 is an example of an electric device including the CPU in which any of the transistors described in the above embodiments is used. Specifically, the indoor unit 8200 includes a housing 8201, an air outlet 8202, a CPU 8203, and the like. Although the CPU 8203 is provided in the indoor unit 8200 in FIG. 16A, the CPU 8203 may be provided in the outdoor unit 8204. Alternatively, the CPU 8203 may be provided in both the indoor unit 8200 and the outdoor unit 8204. By using any of the transistors described in the above embodiments as the CPU in the air conditioner, a reduction in power consumption of the air conditioner can be achieved.

[0285] In FIG. 16A, an electric refrigerator-freezer 8300 is an example of an electric device including the CPU in which any of the transistors described in the above embodiments is used. Specifically, the electric refrigerator-freezer 8300 includes a housing 8301, a door for a refrigerator 8302, a door for a freezer 8303, a CPU 8304, and the like. In FIG. 16A, the CPU 8304 is provided in the housing 8301. When any of the transistors described in the above embodiments is used as the CPU 8304 of the electric refrigerator-freezer 8300, a reduction in power consumption of the electric refrigerator-freezer 8300 can be achieved.

[0286] FIGS. 16B and 16C illustrate an example of an electric vehicle which is an example of an electric device. An electric vehicle 9700 is equipped with a secondary battery 9701. The output of the electric power of the secondary battery 9701 is adjusted by a control circuit 9702 and the electric power is supplied to a driving device 9703. The control circuit 9702 is controlled by a processing unit 9704 including a ROM, a RAM, a CPU, or the like which is not illustrated. When any of the transistors described in the above embodiments is used as the CPU in the electric vehicle 9700, a reduction in power consumption of the electric vehicle 9700 can be achieved.

[0287] The driving device 9703 includes a DC motor or an AC motor either alone or in combination with an internal-combustion engine. The processing unit 9704 outputs a control signal to the control circuit 9702 based on input data such as data of operation (e.g., acceleration, deceleration, or stop) by a driver or data during driving (e.g., data on an upgrade or a downgrade, or data on a load on a driving wheel) of the electric vehicle 9700. The control circuit 9702 adjusts the electric energy supplied from the secondary battery 9701 in accordance with the control signal of the processing unit 9704 to control the output of the driving device 9703. In the case where the AC motor is mounted, although not illustrated, an inverter which converts direct current into alternate current is also incorporated.

[0288] Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

[0289] In this example, a conductive film was formed over an oxide semiconductor film and diffusion or transfer of elements which exist between the stacked films was examined by secondary ion mass spectrometry (SIMS), and results thereof will be described.

[0290] FIGS. 17A and 17B each show SIMS analysis results of profiles of an oxygen isotope (¹⁸O) in a depth direction before and after heat treatment in samples which were each fabricated with a stack of an IGZO film and a tungsten film by a sputtering method. Note that the IGZO film was formed by a DC sputtering method with a sputtering target containing In, Ga, and Zn at an atomic ratio of 1:1:1 or 1:3:2 and a deposition gas containing Ar and O₂ (¹⁸O) at a flow rate ratio of 2:1. The tungsten film was formed by a DC sputtering method with a tungsten sputtering target and a 100 percent Ar gas used as a deposition gas. Note that heat treatment was performed at 300° C., 350° C., 400° C., and 450° C. each for one hour, and five samples including a sample which was not subjected to heat treatment were compared with one another.

[0291] Here, the IGZO film formed with the sputtering target containing In, Ga, and Zn at an atomic ratio of 1:1:1 is crystalline, and the IGZO film formed with the sputtering target containing In, Ga, and Zn at an atomic ratio of 1:3:2 is amorphous.

[0292] As shown in FIGS. 17A and 17B, as the temperature of the heat treatment is increased, oxygen of the oxide semiconductor film is taken into the tungsten film despite the composition or crystallinity of the oxide semiconductor film.

[0293] Since the fabrication process of the transistor involves some heat treatment steps, oxygen vacancies are generated in a region of the oxide semiconductor layer, which is in contact with the source electrode or the drain electrode, and the region is changed to an n-type. Thus, the n-type region can serve as a source or a drain of the transistor.

[0294] FIGS. 18A and 18B each show the analysis results by SIMS in samples which were each fabricated using a tantalum nitride film instead of the tungsten film. The tantalum nitride film was formed by a reactive sputtering method (a DC sputtering method) with a tantalum sputtering target and a deposition gas containing Ar and N₂ at a flow rate ratio of 5:1. Note that heat treatment was performed under four conditions similar to the above, and five samples including a sample which was not subjected to heat treatment were compared with one another.

[0295] FIG. 18A shows the analysis results by SIMS in samples which were each fabricated with a stack of the IGZO film whose atomic ratio of In to Ga and Zn was 1:1:1 and the tantalum nitride film. In any of the samples, transfer of oxygen to the tantalum nitride film was not observed and its behavior was different from that of the sample with the tungsten film in FIG. 17A. FIG. 18B shows the analysis results by SIMS in samples which were each formed with a stack of the IGZO film whose atomic ratio of In to Ga and Zn was 1:3:2 and the tantalum nitride film. In any of the samples, transfer of oxygen to the tantalum nitride film was not observed and its behavior was different from that of the sample with the tungsten film in FIG. 17B. Accordingly, it can be said that the tantalum nitride film is a film that is not easily bonded to oxygen or a film to which oxygen is not easily transferred.

[0296] FIGS. 19A and 19B each show the analysis results by SIMS in samples which were each fabricated using a titanium nitride film instead of the tungsten film. The titanium nitride film was formed by a reactive sputtering method (a DC sputtering method) with a titanium sputtering target and a 100 percent N₂ gas used as a deposition gas. Note that heat treatment was performed under four conditions similar to the above, and five samples including a sample which was not subjected to heat treatment were compared with one another.

[0297] FIG. 19A shows the analysis results by SIMS in samples which were each fabricated with a stack of the IGZO film whose atomic ratio of In to Ga and Zn was 1:1:1 and the titanium nitride film. In any of the samples, transfer of oxygen to the titanium nitride film was not observed and its behavior was different from that of the sample with the tungsten film in FIG. 17A. FIG. 19B shows the analysis results by SIMS in samples which were each fabricated with a stack of the IGZO film whose atomic ratio of In to Ga and Zn was 1:3:2 and the titanium nitride film. In either sample, transfer of oxygen to the titanium nitride film was not observed and its behavior was different from that of the sample with the tungsten film in FIG. 17B. Accordingly, it can be said that the titanium nitride film is a film that is not easily bonded to oxygen or a film to which oxygen is not easily transferred.

[0298] Next, transfer of an impurity to an IGZO film was examined by SIMS analysis, and results thereof are described.

[0299] FIGS. 20A and 20B each show SIMS analysis results of profiles of nitrogen in a depth direction before and after heat treatment in samples which were each fabricated with a tantalum nitride film or a titanium nitride film formed over an IGZO film by a sputtering method. Note that the IGZO film was formed by a DC sputtering method with a sputtering target containing In, Ga, and Zn at an atomic ratio of 1:1:1 and a deposition gas containing Ar and O₂ at a flow rate ratio of 2:1. The tantalum nitride film and the titanium nitride film were formed by the above method. Note that heat treatment was performed at 400° C. for one hour, and two samples including a sample which was not subjected to heat treatment were compared with each other.

[0300] As shown in FIGS. 20A and 20B, in either sample, transfer of nitrogen to the IGZO film was not observed. Therefore, nitrogen which serves as a donor in the IGZO film is not widely transferred to the IGZO film from the tantalum nitride film or the titanium nitride film; accordingly, a channel formation region of the transistor is not made to have n-type conductivity.

[0301] FIGS. 21A and 21B show SIMS analysis results of profiles of tantalum and titanium, respectively, in a depth direction in samples similar to those shown in FIGS. 20A and 20B as examples. As shown in FIGS. 21A and 21B, transfer of tantalum or titanium to the IGZO film was not observed. Accordingly, each of titanium and tantalum which might serve as an impurity affecting the electrical characteristics of the transistor is not widely transferred to the IGZO film from the tantalum nitride film or the titanium nitride film.

[0302] The above results showed that a film of a conductive nitride such as tantalum nitride or titanium nitride is a film that is not easily bonded to oxygen or a film to which oxygen is not easily transferred, and nitrogen and a metal element in such a conductive nitride are not easily transferred to the oxide semiconductor film.

[0303] Note that this example can be combined as appropriate with any of embodiments or the other example in this specification.

[0304] In this example, measurement results of sheet resistance values of an oxide semiconductor film after removal of a conductive film which was formed over the oxide semiconductor film will be described.

[0305] FIG. 22 shows measurement results of sheet resistance values of samples each fabricated as follows with respect to a depth to which an IGZO film was etched: the IGZO film was formed by a sputtering method, a tungsten film or a titanium nitride film was stacked over the IGZO film by a sputtering method, and then the tungsten film or the titanium nitride film was removed. For comparison, a sample in which a conductive film was not formed over the IGZO film was also fabricated. Note that the IGZO film was formed by a DC sputtering method with a sputtering target containing In, Ga, and Zn at an atomic ratio of 1:1:1 and a deposition gas containing Ar and O₂ (¹⁸O) at a flow rate ratio of 2:1. The tungsten film was formed by a DC sputtering method with a tungsten sputtering target and a 100 percent Ar gas used as a deposition gas. The titanium nitride film was formed by a reactive sputtering method (a DC sputtering method) with a titanium sputtering target and a 100 percent N₂ gas used as a deposition gas. The tungsten film and the titanium nitride film were etched using hydrogen peroxide water. The IGZO film was etched using a mixed solution of hydrogen peroxide water and ammonia. The remaining thickness of the IGZO film after the etching was measured using spectroscopic ellipsometry before and after the etching to obtain the depth to which the IGZO film was etched.

[0306] In the sample in which the tungsten film was formed over the IGZO film, the resistance of a region of the IGZO film, which was formed to a depth of about 5 nm from the surface of the IGZO film, was reduced as shown in FIG. 22. This suggests that a low-resistant mixed layer of IGZO and tungsten is formed in a region of the IGZO film, which is close to the surface thereof, and that an n-type region is formed due to oxygen vacancies which exist in the above region by transfer of oxygen of the IGZO film to the tungsten film, for example.

[0307] On the other hand, in the sample in which the titanium nitride film was formed over the IGZO film and the sample in which a conductive film was not formed over the IGZO film, the resistance of each of the IGZO films was not reduced. This suggests that elements of titanium nitride are not easily transferred to the IGZO film and that oxygen of the IGZO film is not easily transferred to the titanium nitride film, for example.

[0308] FIG. 23A shows measurement results of sheet resistance values of samples each fabricated as follows with respect to a depth to which an IGZO film was etched: the IGZO film was formed by a sputtering method, a tungsten film or a titanium nitride film was stacked over the IGZO film by a sputtering method, heat treatment was performed, and then the tungsten film or the titanium nitride film was removed. For comparison, a sample in which a conductive film was not formed over the IGZO film was also fabricated. Note that the formation of the IGZO film, and the tungsten film or the titanium nitride film and the removal of the tungsten film or the titanium nitride film were performed in manners similar to those of the above. The heat treatment was performed at 400° C. under a N₂ atmosphere for one hour.

[0309] As shown in FIG. 23A, in any of the samples, the resistance of the IGZO film was reduced. Here, in the sample in which the tungsten film was formed over the IGZO film, the resistance of the IGZO film was most reduced in the region close to the surface thereof and reduced up to the greatest depth. This suggests that the tungsten film takes oxygen of the IGZO film thereinto most easily. Further, the behavior of the sample in which the titanium nitride film was formed over the IGZO film was similar to that of the sample in which a conductive film was not formed over the IGZO film. In other words, in the sample in which the tungsten film was formed over the IGZO film, the resistance of the IGZO film was reduced by transfer of oxygen of the IGZO film to the tungsten film, whereas in the sample in which the titanium nitride film was formed over the IGZO film, oxygen released from the IGZO film was transmitted through the titanium nitride film and released to the upper side. This result well accords with the SIMS analysis results shown in Example 1.

[0310] FIG. 23B shows measurement results of sheet resistance values of samples each fabricated as follows with respect to a depth to which an IGZO film was etched: a silicon oxide film was formed by a sputtering method, the IGZO film was formed over the silicon oxide film by a sputtering method, a tungsten film or a titanium nitride film was stacked over the IGZO film by a sputtering method, heat treatment was performed, and then the tungsten film or the titanium nitride film was removed. For comparison, a sample in which a conductive film was not formed over the IGZO film was also fabricated. The silicon oxide film was formed by a reactive sputtering method (a DC sputtering method) with a silicon sputtering target and a 100 percent O₂ gas used as a deposition gas. Note that the formation of the IGZO film, and the tungsten film or the titanium nitride film and the removal of the tungsten film or the titanium nitride film were performed in manners similar to those of the above. The heat treatment was performed at 400° C. under a N₂ atmosphere for one hour.

[0311] As shown in FIG. 23B, a region of the IGZO film, whose resistance was reduced, had a smaller thickness in a thickness direction than that obtained from the results shown in FIG. 23A. This suggests that oxygen was supplied from the silicon oxide film to the IGZO film by the heat treatment and oxygen vacancies in the IGZO film were reduced; accordingly, the resistance of the IGZO film was increased. With the use of a film which is capable of releasing oxygen and provided below the IGZO film in this manner, the thickness of a region of the IGZO film, whose resistance is reduced, can be controlled.

[0312] As described above, there were the following findings. A conductive film such as a tungsten film, which easily takes oxygen thereinto, is formed in contact with an IGZO film, so that the resistance of a region of the IGZO film, which is in contact with and close to the conductive film, can be reduced. Moreover, the region of the IGZO film, whose resistance is reduced, can be increased in a depth direction by heat treatment. Further, a film capable of releasing oxygen is formed close to the IGZO film, whereby the thickness of the region whose resistance is reduced can be controlled.

[0313] Note that this example can be combined as appropriate with any of embodiments or the other example in this specification.

[0314] This application is based on Japanese Patent Application serial No. 2012-230360 filed with the Japan Patent Office on Oct. 17, 2012, the entire contents of which are hereby incorporated by reference.