Semiconductor device and manufacturing method thereof

Provided is a novel semiconductor device. A switching element, specifically a transistor having a well potential structure is manufactured by utilizing a structure including at least a composite material in which a first region and a second region are stacked over a base like a superlattice. The thickness of each of the first region and the second region is greater than or equal to 0.5 nm and less than or equal to 5 nm. A band structure can be controlled by adjusting the number of stacks, which enables application to a variety of semiconductor elements.

TECHNICAL FIELD

The present invention relates to a transistor, a semiconductor device, and manufacturing methods thereof, for example. The present invention relates to, for example, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, an imaging device, a processor, and an electronic device. The present invention relates to a method for manufacturing a display device, a liquid crystal display device, a light-emitting device, a memory device, an imaging device, and an electronic device. The present invention relates to a driving method of a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a memory device, and an electronic device.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.

BACKGROUND ART

A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.

In recent years, transistors including oxide semiconductors (typically, In—Ga—Zn oxide) have been actively developed. Oxide semiconductors have been researched since early times. In 1988, there was a disclosure of a crystal In—Ga—Zn oxide that can be used for a semiconductor element (see Patent Document 1).

Non-Patent Document 1 discloses a homologous series represented by In1−xGa1+xO3(ZnO)m(−1≤x≤1, and m is a natural number). Furthermore, Non-Patent Document 1 discloses a solid solution range of the homologous series. For example, in the solid solution range of the homologous series in the case where m is 1, x ranges from −0.33 to 0.08, and in the solid solution range of the homologous series in the case where m is 2, x ranges from −0.68 to 0.32.

Furthermore, a technique for forming a transistor using an In—Ga—Zn-based oxide semiconductor is disclosed (see, for example, Patent Document 2).

DISCLOSURE OF INVENTION

A transistor, which is a typical semiconductor element, is required to have high electrical characteristics (e.g., high on-state current or high field-effect mobility), low power consumption, and high reliability. To achieve high integration of a semiconductor device, the transistor is also required to achieve fine processing, mass production by a relatively simple process, and low manufacturing cost.

A material used for a channel formation region, which influences the characteristics of a transistor most strongly, is amorphous silicon, polycrystalline silicon, single crystal silicon, or an oxide semiconductor, for example.

The transistors including oxide semiconductors have features different from those of the transistors including amorphous silicon or polycrystalline silicon.

There is a problem in that it is difficult to provide a high-performance display device where driver circuits are formed over the same substrate because a transistor including amorphous silicon has low field-effect mobility.

Polycrystalline silicon has a problem of a step of performing high-temperature heat treatment or laser light treatment on amorphous silicon.

Consequently, it is desirable to use a novel oxide semiconductor material to provide a transistor which has higher field-effect mobility than a transistor including polycrystalline silicon, without performing laser light treatment.

A display device in which a transistor including an oxide semiconductor is used is known to have low power consumption. An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used in a transistor included in a large display device. A transistor including an oxide semiconductor has high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.

An object is to provide a transistor with stable electrical characteristics. Another object is to provide a transistor having a low leakage current in an off state. Another object is to provide a transistor with high frequency characteristics. Another object is to provide a transistor having a high on-state current. Another object is to provide a transistor with normally-off electrical characteristics. Another object is to provide a transistor with a small subthreshold swing value. Another object is to provide a highly reliable transistor.

Another object is to provide a semiconductor device including the transistor. Another object is to provide a module including the semiconductor device. Another object is to provide an electronic device including the semiconductor device or the module. Another object is to provide a novel semiconductor device. Another object is to provide a novel module. Another object is to provide a novel electronic device.

Non-Patent Document 1 discloses an example of InxZnyGazOw, and when x, y, and z are set such that a composition in the neighborhood of ZnGa2O4is obtained, that is, when x, y, and z are close to 0, 1, and 2, respectively, a spinel crystal structure is likely to be formed or mixed. A compound represented by AB2O4(A and B are metals) is known as a compound having a spinel crystal structure.

However, when a spinel crystal structure is formed or mixed in an In—Ga—Zn-based oxide semiconductor, electrical characteristics or reliability of a semiconductor device (e.g., a transistor) including the In—Ga—Zn-based oxide semiconductor is adversely affected by the spinel crystal structure in some cases.

Therefore, a semiconductor element including a novel composite material is provided.

According to one embodiment of the present invention, a first region and a second region in contact with the first region are arranged in the thickness direction. The first region contains at least indium, gallium, and zinc, and the second region contains indium and zinc. The second region has a lower gallium concentration and higher carrier mobility than the first region.

The thickness of the first region in the above structure may be greater than or equal to 0.5 nm and less than or equal to 5 nm.

The thickness of the second region in the above structure may be greater than or equal to 0.5 nm and less than or equal to 5 nm.

The first region and the second region in the above structure may be alternately stacked.

The first region in the above structure may be non-single crystal.

The first region in the above structure may include a crystal portion, and have a portion in which a c-axis of the crystal portion is parallel to a normal vector of a formation surface of an oxide semiconductor film.

The second region in the above structure may be non-single crystal.

One embodiment of the present invention is a transistor including the oxide semiconductor having the above structure.

FIG. 1is a schematic diagram illustrating an example of a novel composite material used for a semiconductor element.

As illustrated inFIG. 1, the novel composite material includes at least a composite in which a first region and a second region are stacked over a base. By adjusting the number of stacks, the band structure can be controlled, and application to a variety of semiconductor elements can be achieved. The stacked structure can be referred to as a structure like a superlattice. A transistor having the structure is a completely novel transistor.

The novel composite material illustrated inFIG. 1includes at least a composite material (also referred to as a composite) including a stack in which a first region (specifically, with [In]:[Ga]:[Zn]=4:2:3 or a neighborhood thereof) is sandwiched between second regions (specifically, with [In]:[Zn]=1:m (m is a rational number) or a neighborhood thereof). The first region including an oxide material has a thickness of greater than or equal to 0.5 nm and less than or equal to 5 nm and includes In, Ga, and Zn. The second region including an oxide material has a thickness of greater than or equal to 0.5 nm and less than or equal to 5 nm and includes In and Zn.

Specifically, deposition is performed using a sputtering target having a composition represented by a region including [In]:[Ga]:[Zn]=4:2:3 and a neighborhood thereof, a composition represented by a region including [In]:[Ga]:[Zn]=5:2:5 and a neighborhood thereof, or a composition represented by a region including [In]:[Ga]:[Zn]=6:1:6 and a neighborhood thereof, adjusting the temperature to room temperature or lower than 140° C., and setting the ratio of an oxygen gas flow rate to an argon gas flow rate, that is, O2/Ar to greater than or equal to 5% and less than or equal to 20%, preferably greater than or equal to 7% and less than or equal to 15%, and then oxygen is supplied from above or below the obtained composite material thin film, thus, the composite material including the first region and the second region is obtained. As a method for supplying oxygen from above or below the composite material thin film, oxygen plasma treatment and a method of providing a film containing excess oxygen above or below the composite material thin film, and then diffusing excess oxygen by heat treatment can be given. Excess oxygen is diffused into the second region to reduce oxygen vacancies therein.

In the case where the composite material is over and in contact with the base, the second region is provided to be in contact with the base. Zinc with the lowest atomic weight in the metal elements contained in the sputtering target for an oxide semiconductor is more easily deposited on the substrate side than the other elements with larger atomic weight than zinc, and bonds for forming hexagonal lattices are formed. Thus, the second region represented by [In]:[Zn]=1:m (m is a rational number) is formed in contact with the substrate surface. When the second region is formed, a crystal having a hexagonal crystal structure which has bonds for forming hexagonal lattices in the a-b plane parallel to the substrate surface thereover and in which c-axes are substantially perpendicular to the substrate surface which is substantially parallel to the a-b plane is easily formed. The crystal having the hexagonal crystal structure is a crystalline oxide semiconductor (c-axis aligned crystal; also referred to as CAAC OS) including a c-axis substantially perpendicular to the substrate plane and corresponds to the first region.

The CAAC-OS has c-axis alignment, its nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion.

An a-like OS has a structure between those of the nc-OS and the amorphous oxide semiconductor. The a-like OS includes a void or a low-density region. That is, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS.

In the nc-OS, a microscopic region (for example, 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 arrangement. There is no regularity of crystal orientation between different nanocrystals in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an amorphous-like oxide semiconductor (a-like OS) or an amorphous oxide semiconductor, depending on an analysis method.

One embodiment of the present invention is a semiconductor device including a semiconductor element, specifically a field-effect transistor, in which a first region and a second region in contact with the first region are stacked in the thickness direction. The first region includes at least indium, gallium, and zinc. The second region includes indium and zinc. The second region serves as a well portion of a well potential in the thickness direction. The second region has a smaller bandgap than the first region. The second region is located between the first regions in the thickness direction.

In the above structure, the thickness of each of the first region and the second region is greater than or equal to 0.5 nm and less than or equal to 5 nm. The first region and the second region have different main functions. The first region has a property of bending an energy band, and the second region has a function of making carriers flow. As the thickness of the first region is smaller, higher switching characteristics can be obtained.

In the above structure, a structure in which the first region and the second region are alternately stacked is used. Although the second region tends to be unstable, the second region becomes wholly stable by providing the second region between the first regions which are stable in the vertical direction. Furthermore, division into two layers occurs during deposition, and a structure in which the first region and the second region are alternately stacked by one-time deposition using a sputtering method is obtained. Part of the composite material including the first region and the second region may be periodically stacked.

In the above structure, the number of gallium atoms in the composite material including the first region and the second region is smaller than the number of indium atoms in the composite material and is smaller than the number of zinc atoms in the composite material, for example, the atomic ratio of indium to gallium and zinc is [In]:[Ga]:[Zn]=4:2:3 or neighborhood thereof. The atomic ratio of indium to gallium and zinc in the composite material may be [In]:[Ga]:[Zn]=5:2:5. The atomic ratio of indium, gallium, and zinc in the composite material may be [In]:[Ga]:[Zn]=6:1:6.

Note that in the case where the oxide semiconductor is deposited with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of the target is formed. Especially for zinc, [Zn] in the atomic ratio of a deposited film is smaller than that in the atomic ratio of the target in some cases depending on the substrate temperature during deposition.

Therefore, the oxide semiconductor of one embodiment of the present invention is not uniquely determined by an atomic ratio. Meanwhile, the atomic ratio affects difficulty in forming the structure. Even with the same atomic ratio, the structure is not formed depending on a fabrication method in some cases. Therefore, the illustrated regions each represent an atomic ratio with which an oxide semiconductor has a layered structure, and boundaries of the regions A to C are not clear.

A sputtering apparatus and a deposition apparatus with which the oxide semiconductor is formed are described with reference toFIGS. 34A and 34B,FIGS. 35A and 35B,FIGS. 36A to 36C,FIG. 37,FIG. 38,FIGS. 39A and 39B,FIG. 40, andFIGS. 41A to 41C.

Examples of sputtering apparatuses include a parallel-plate-type sputtering apparatus and a facing-targets sputtering apparatus. Note that deposition using a parallel-plate-type sputtering apparatus can also be referred to as parallel electrode sputtering (PESP), and deposition using a facing-targets sputtering apparatus can also be referred to as vapor deposition sputtering (VDSP).

First, the parallel-plate-type sputtering apparatus is described.FIG. 34Ais a cross-sectional view of a deposition chamber2501including a parallel-plate-type sputtering apparatus. The deposition chamber2501inFIG. 34Aincludes a target holder2520, a backing plate2510, a target2500, a magnet unit2530, and a substrate holder2570. Note that the target2500is placed over the backing plate2510. The backing plate2510is placed over the target holder2520. The magnet unit2530is placed under the target2500with the backing plate2510positioned therebetween. The substrate holder2570faces the target2500. Note that in this specification, a magnet unit means a group of magnets. The term “magnet unit” can be replaced with “cathode”, “cathode magnet”, “magnetic member”, “magnetic part”, or the like. The magnet unit2530includes a magnet2530N, a magnet2530S, and a magnet holder2532. Note that in the magnet unit2530, the magnet2530N and the magnet2530S are placed over the magnet holder2532. The magnet2530N and the magnet2530S are spaced. When a substrate2560is transferred into the deposition chamber2501, the substrate2560is placed on the substrate holder2570.

The target holder2520and the backing plate2510are fixed to each other with a screw (e.g., a bolt) and have the same potential. The target holder2520has a function of supporting the target2500with the backing plate2510positioned therebetween.

The target2500is fixed to the backing plate2510. For example, the target2500can be fixed to the backing plate2510with a bonding member containing a low-melting-point metal such as indium.

FIG. 34Aillustrates a magnetic line of force2580aand a magnetic line of force2580bformed by the magnet unit2530.

The magnetic line of force2580ais one of magnetic lines of force that form a horizontal magnetic field in the vicinity of the top surface of the target2500. The vicinity of the top surface of the target2500corresponds to a region in which the vertical distance from the top surface of the target2500is, for example, greater than or equal to 0 mm and less than or equal to 10 mm, in particular, greater than or equal to 0 mm and less than or equal to 5 mm.

The magnetic line of force2580bis one of magnetic lines of force that form a horizontal magnetic field in a plane apart from the top surface of the magnet unit2530by a vertical distance d. The vertical distance d is, for example, greater than or equal to 0 mm and less than or equal to 20 mm or greater than or equal to 5 mm and less than or equal to 15 mm.

Here, with the use of the strong magnet2530N and the strong magnet2530S, an intense magnetic field can be generated in the vicinity of the substrate2560. Specifically, the magnetic flux density of the horizontal magnetic field in the vicinity of the substrate2560can be greater than or equal to 10 G and less than or equal to 100 G, preferably greater than or equal to 15 G and less than or equal to 60 G, further preferably greater than or equal to 20 G and less than or equal to 40 G.

Note that the intensity of the horizontal magnetic field may be measured when the intensity of the vertical magnetic field is 0 G.

By setting the intensity of the magnetic field in the deposition chamber2501to be in the above range, an oxide with high density and high crystallinity can be deposited. The deposited oxide hardly includes a plurality of kinds of crystalline phases and has a substantially single crystalline phase.

FIG. 34Bis a top view of the magnet unit2530. In the magnet unit2530, the magnet2530N having a circular or substantially circular shape and the magnet2530S having a circular or substantially circular shape are fixed to the magnet holder2532. The magnet unit2530can be rotated about a normal vector at the center of the top surface of the magnet unit2530or a normal vector substantially at the center of the top surface of the magnet unit2530. For example, the magnet unit2530may be rotated with a beat (also referred to as rhythm, pulse, frequency, period, cycle, or the like) of greater than or equal to 0.1 Hz and less than or equal to 1 kHz.

Thus, a region where a magnetic field on the target2500is intense changes as the magnet unit2530is rotated. The region with an intense magnetic field is a high-density plasma region; thus, sputtering of the target2500easily occurs in the vicinity of the region. For example, when the region with an intense magnetic field is fixed, only a specific region of the target2500is used. In contrast, when the magnet unit2530is rotated as shown inFIG. 34B, the target2500can be uniformly used. By rotating the magnet unit2530, a film with a uniform thickness and a film with uniform quality can be deposited.

By rotating the magnet unit2530, the direction of the magnetic line of force in the vicinity of the substrate2560can also be changed.

Although the magnet unit2530is rotated in this example, one embodiment of the present invention is not limited to this example. For example, the magnet unit2530may be oscillated vertically and/or horizontally. For example, the magnet unit2530may be oscillated with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz. Alternatively, the target2500may be rotated or moved. For example, the target2500may be rotated or moved with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz. Further alternatively, the direction of a magnetic line of force in the vicinity of the substrate2560may be changed relatively by rotating the substrate2560. These methods may be combined.

The deposition chamber2501may have a water channel inside or under the backing plate2510. By making a fluid (air, nitrogen, a rare gas, water, oil, or the like) flow through the water channel, discharge anomaly due to an increase in the temperature of the target2500or damage to the deposition chamber2501due to deformation of a component can be prevented in the sputtering. In that case, the backing plate2510and the target2500are preferably adhered to each other with a bonding member because the cooling capability is increased.

A gasket is preferably provided between the target holder2520and the backing plate2510, in which case an impurity is less likely to enter the deposition chamber2501from the outside, the water channel, or the like.

In the magnet unit2530, the magnet2530N and the magnet2530S are placed such that their surfaces on the target2500side have opposite polarities. Here, the case where the pole of the magnet2530N on the target2500side is the north pole and the pole of the magnet2530S on the target2500side is the south pole is described. Note that the layout of the magnets and the poles in the magnet unit2530is not limited to that described here or that illustrated inFIG. 34A.

In the deposition, a potential applied to a terminal V1connected to the target holder2520is, for example, lower than a potential applied to a terminal V2connected to the substrate holder2570. The potential applied to the terminal V2connected to the substrate holder2570is, for example, the ground potential. A potential applied to the terminal V3connected to the magnet holder2532is, for example, the ground potential. Note that the potentials applied to the terminals V1, V2, and V3are not limited to the above description. Not all the target holder2520, the substrate holder2570, and the magnet holder2532are necessarily supplied with potentials. For example, the substrate holder2570may be electrically floating. Note that it is assumed that a power source capable of controlling a potential applied to the terminal V1is electrically connected to the terminal V1. As the power source, a DC power source or an RF power source may be used.

A cation in plasma2540is accelerated toward the target2500by a potential V1applied to the target holder2520. When the cation collides with the target2500, sputtered particles are generated and deposited on the substrate2560.

FIG. 34Aillustrates an example where the backing plate2510and the target holder2520are not electrically connected to the magnet unit2530and the magnet holder2532, but electrical connection is not limited thereto. For example, the backing plate2510and the target holder2520may be electrically connected to the magnet unit2530and the magnet holder2532, and the backing plate2510, the target holder2520, the magnet unit2530, and the magnet holder2532may have the same potential.

To increase the crystallinity of the deposited oxide, the temperature of the substrate2560may be set high. By setting the temperature of the substrate2560high, migration of sputtered particles in the vicinity of the substrate2560can be promoted. Thus, an oxide with higher density and higher crystallinity can be deposited. Note that the temperature of the substrate2560is, for example, higher than or equal to room temperature and lower than 540° C., preferably higher than or equal to room temperature and lower than or equal to 500° C., further preferably higher than or equal to 100° C. and lower than or equal to 450° C., still further preferably higher than or equal to 100° C. and lower than or equal to 400° C.

When the partial pressure of oxygen in the deposition gas is too high, an oxide including a plurality of kinds of crystalline phases is likely to be deposited; therefore, a mixed gas of oxygen and a rare gas such as argon (other examples of the rare gas are helium, neon, krypton, and xenon) is preferably used as the deposition gas. For example, the proportion of oxygen in the whole deposition gas is less than 50 vol %, preferably less than or equal to 33 vol %, further preferably less than or equal to 20 vol %, still further preferably less than or equal to 15 vol %.

The vertical distance between the target2500and the substrate2560is greater than or equal to 10 mm and less than or equal to 600 mm, preferably greater than or equal to 20 mm and less than or equal to 400 mm, more preferably greater than or equal to 30 mm and less than or equal to 200 mm, further more preferably greater than or equal to 40 mm and less than or equal to 100 mm. Within the above range, the vertical distance between the target2500and the substrate2560can be, in some cases, small enough to suppress a decrease in the energy of the sputtered particles until the sputtered particles reach the substrate2560. Within the above range, the vertical distance between the target2500and the substrate2560can be, in some cases, large enough to make the incident direction of the sputtered particle approximately vertical to the substrate2560, so that damage to the substrate2560caused by collision of the sputtered particles can be reduced.

FIG. 35Aillustrates an example of a deposition chamber including a sputtering apparatus different from that inFIG. 34A.

The deposition chamber2501inFIG. 35Aincludes a target holder2520a, a target holder2520b, a backing plate2510a, a backing plate2510b, a target2500a, a target2500b, a magnet unit2530a, a magnet unit2530b, a member2542, and the substrate holder2570. Note that the target2500ais placed over the backing plate2510a. The backing plate2510ais placed over the target holder2520a. The magnet unit2530ais placed under the target2500awith the backing plate2510apositioned therebetween. The target2500bis placed over the backing plate2510b. The backing plate2510bis placed over the target holder2520b. The magnet unit2530bis placed under the target2500bwith the backing plate2510bpositioned therebetween.

The magnet unit2530aincludes a magnet2530N1, a magnet2530N2, the magnet2530S, and the magnet holder2532. Note that in the magnet unit2530a, the magnet2530N1, the magnet2530N2, and the magnet2530S are placed over the magnet holder2532. The magnet2530N1, the magnet2530N2, and the magnet2530S are spaced. Note that the magnet unit2530bhas a structure similar to that of the magnet unit2530a. When the substrate2560is transferred into the deposition chamber2501, the substrate2560is placed in contact with the substrate holder2570.

The target2500a, the backing plate2510a, and the target holder2520aare separated from the target2500b, the backing plate2510b, and the target holder2520bby the member2542. Note that the member2542is preferably an insulator. However, the member2542may be a conductor or a semiconductor. The member2542may be a conductor or a semiconductor whose surface is covered with an insulator.

The target holder2520aand the backing plate2510aare fixed to each other with a screw (e.g., a bolt) and have the same potential. The target holder2520ahas a function of supporting the target2500awith the backing plate2510apositioned therebetween. The target holder2520band the backing plate2510bare fixed to each other with a screw (e.g., a bolt) and have the same potential. The target holder2520bhas a function of supporting the target2500bwith the backing plate2510bpositioned therebetween.

The backing plate2510ahas a function of fixing the target2500a. The backing plate2510bhas a function of fixing the target2500b.

FIG. 35Aillustrates the magnetic line of force2580aand the magnetic line of force2580bformed by the magnet unit2530a.

The magnetic line of force2580ais one of magnetic lines of force that form a horizontal magnetic field in the vicinity of the target2500a. The vicinity of the target2500acorresponds to a region in which the vertical distance from the target2500ais, for example, greater than or equal to 0 mm and less than or equal to 10 mm, in particular, greater than or equal to 0 mm and less than or equal to 5 mm.

The magnetic line of force2580bis one of magnetic lines of force that form a horizontal magnetic field in a plane apart from the top surface of the magnet unit2530aby a vertical distance d. The vertical distance d is, for example, greater than or equal to 0 mm and less than or equal to 20 mm or greater than or equal to 5 mm and less than or equal to 15 mm.

Here, with the use of the strong magnet2530N1, the strong magnet2530N2, and the strong magnet2530S, an intense magnetic field can be generated in the vicinity of the substrate2560. Specifically, the intensity of the horizontal magnetic field in the vicinity of the substrate2560can be greater than or equal to 10 G and less than or equal to 100 G, preferably greater than or equal to 15 G and less than or equal to 60 G, further preferably greater than or equal to 20 G and less than or equal to 40 G.

By setting the intensity of the magnetic field in the deposition chamber2501to be in the above range, an oxide with high density and high crystallinity can be deposited. The deposited oxide hardly includes a plurality of kinds of crystalline phases and has a substantially single crystalline phase.

Note that the magnet unit2530bforms magnetic lines of force similar to those formed by the magnet unit2530a.

FIG. 35Bis a top view of the magnet units2530aand2530b. The magnet unit2530ahas a structure in which the magnet2530N1having a rectangular or substantially rectangular shape, the magnet2530N2having a rectangular or substantially rectangular shape, and the magnet2530S having a rectangular or substantially rectangular shape are fixed to the magnet holder2532. The magnet unit2530acan be oscillated horizontally as shown inFIG. 35B. For example, the magnet unit2530amay be oscillated with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz.

Thus, a region where a magnetic field on the target2500ais intense changes as the magnet unit2530ais rotated. The region with an intense magnetic field is a high-density plasma region; thus, sputtering of the target2500aeasily occurs in the vicinity of the region. For example, when the region with an intense magnetic field is fixed, only a specific region of the target2500ais used. Meanwhile, as illustrated inFIG. 35B, in the magnet unit2530a, the magnets2530N1and2530N2and the magnet2530S are placed such that their surfaces on the target2500aside have opposite polarities. Here, the case where the pole of each of the magnets2530N1and2530N2on the target2500aside is the north pole and the pole of the magnet2530S on the target2500aside is the south pole is described. Note that the layout of the magnets and the poles in the magnet unit2530ais not limited to that described here or that illustrated inFIGS. 35A and 35B. The same applies to the magnet unit2503b.

By oscillating the magnet unit2530a, the state of the magnetic lines of force in the vicinity of the substrate2560can also be changed. The same applies to the magnet unit2530b.

Although the magnet unit2530aand the magnet unit2530bare oscillated in this example, one embodiment of the present invention is not limited to this example. For example, the magnet unit2530aand the magnet unit2530bmay be rotated. For example, the magnet unit2530aand the magnet unit2530bmay be rotated with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz. Alternatively, the target2500may be rotated or moved. For example, the target2500may be rotated or moved with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz. Further alternatively, the state of magnetic lines of force on the top surface of the substrate2560can be changed relatively by rotating the substrate2560. These methods may be combined.

The deposition chamber2501may have a water channel inside or under the backing plate2510aand the backing plate2510b. By making a fluid (air, nitrogen, a rare gas, water, oil, or the like) flow through the water channel, discharge anomaly due to an increase in the temperature of the target2500aand the target2500bor damage to the deposition chamber2501due to deformation of a component can be prevented in the sputtering. In that case, the backing plate2510aand the target2500aare preferably adhered to each other with a bonding member because the cooling capability is increased. Furthermore, the backing plate2510band the target2500bare preferably adhered to each other with a bonding member because the cooling capability is increased.

A gasket is preferably provided between the target holder2520aand the backing plate2510a, in which case an impurity is less likely to enter the deposition chamber2501from the outside, the water channel, or the like. A gasket is preferably provided between the target holder2520band the backing plate2510b, in which case an impurity is less likely to enter the deposition chamber2501from the outside, the water channel, or the like.

In the magnet unit2530a, the magnets2530N1and2530N2and the magnet2530S are placed such that their surfaces on the target2500aside have opposite polarities. Here, the case where the pole of each of the magnets2530N1and2530N2on the target2500aside is the north pole and the pole of the magnet2530S on the target2500aside is the south pole is described. Note that the layout of the magnets and the poles in the magnet unit2530ais not limited to that described here or that illustrated inFIG. 35A. The same applies to the magnet unit2530b.

In the deposition, a potential applied to the terminal V1connected to the target holder2520aand a potential applied to the terminal V4connected to the target holder2520bmay be alternately switched between a high level and a low level. A potential applied to the terminal V2connected to the substrate holder2570is, for example, the ground potential. A potential applied to the terminal V3connected to the magnet holder2532is, for example, the ground potential. Note that the potentials applied to the terminals V1, V2, V3, and V4are not limited to the above description. Not all the target holder2520a, the target holder2520b, the substrate holder2570, and the magnet holder2532are necessarily supplied with potentials. For example, the substrate holder2570may be electrically floating. Note that the potential applied to the terminal V1connected to the target holder2520aand the potential applied to the terminal V4connected to the target holder2520bare alternately switched between the high level and the low level (i.e., an AC sputtering method) in the example illustrated inFIG. 35A; however, one embodiment of the present invention is not limited thereto.

FIG. 35Aillustrates an example where the backing plate2510aand the target holder2520aare not electrically connected to the magnet unit2530aand the magnet holder2532, but electrical connection is not limited thereto. For example, the backing plate2510aand the target holder2520amay be electrically connected to the magnet unit2530aand the magnet holder2532, and the backing plate2510a, the target holder2520a, the magnet unit2530a, and the magnet holder2532may have the same potential. The backing plate2510band the target holder2520bare not electrically connected to the magnet unit2530band the magnet holder2532in the example, but electrical connection is not limited thereto. For example, the backing plate2510aand the target holder2520bmay be electrically connected to the magnet unit2530band the magnet holder2532, and the backing plate2510a, the target holder2520b, the magnet unit2530b, and the magnet holder2532may have the same potential.

To increase the crystallinity of the deposited oxide, the temperature of the substrate2560may be set high. By setting the temperature of the substrate2560high, migration of sputtered particles in the vicinity of the substrate2560can be promoted. Thus, an oxide with higher density and higher crystallinity can be deposited. Note that the temperature of the substrate2560is, for example, higher than or equal to room temperature and lower than 440° C., preferably higher than or equal to room temperature and lower than or equal to 400° C., further preferably higher than or equal to 150° C. and lower than or equal to 300° C., still further preferably higher than or equal to 100° C. and lower than or equal to 200° C.

When the partial pressure of oxygen in the deposition gas is too high, an oxide including a plurality of kinds of crystalline phases is likely to be deposited; therefore, a mixed gas of oxygen and a rare gas such as argon (other examples of the rare gas are helium, neon, krypton, and xenon) is preferably used as the deposition gas.

For example, the proportion of oxygen in the whole deposition gas is less than 50 vol %, preferably less than or equal to 33 vol %, further preferably less than or equal to 20 vol %, still further preferably less than or equal to 15 vol %.

The vertical distance between the target2500aand the substrate2560is greater than or equal to 10 mm and less than or equal to 600 mm, preferably greater than or equal to 20 mm and less than or equal to 400 mm, more preferably greater than or equal to 30 mm and less than or equal to 200 mm, further more preferably greater than or equal to 40 mm and less than or equal to 100 mm. Within the above range, the vertical distance between the target2500aand the substrate2560can be, in some cases, small enough to suppress a decrease in the energy of the sputtered particles until the sputtered particles reach the substrate2560. Within the above range, the vertical distance between the target2500aand the substrate2560can be, in some cases, large enough to make the incident direction of the sputtered particle approximately vertical to the substrate2560, so that damage to the substrate2560caused by collision of the sputtered particles can be reduced.

The vertical distance between the target2500band the substrate2560is greater than or equal to 10 mm and less than or equal to 600 mm, preferably greater than or equal to 20 mm and less than or equal to 400 mm, more preferably greater than or equal to 30 mm and less than or equal to 200 mm, further more preferably greater than or equal to 40 mm and less than or equal to 100 mm. Within the above range, the vertical distance between the target2500band the substrate2560can be, in some cases, small enough to suppress a decrease in the energy of the sputtered particles until the sputtered particles reach the substrate2560. Within the above range, the vertical distance between the target2500band the substrate2560can be, in some cases, large enough to make the incident direction of the sputtered particle approximately vertical to the substrate2560, so that damage to the substrate2560caused by collision of the sputtered particles can be reduced.

Next, the facing-targets sputtering apparatus is described.FIG. 36Ais a cross-sectional view of a deposition chamber including a facing-targets sputtering apparatus. The deposition chamber illustrated inFIG. 36Aincludes the target2500a, the target2500b, the backing plate2510afor holding the target2500a, the backing plate2510bfor holding the target2500b, the magnet unit2530aplaced behind the target2500awith the backing plate2510apositioned therebetween, and the magnet unit2530bplaced behind the target2500bwith the backing plate2510bpositioned therebetween. The substrate holder2570is placed between the target2500aand the target2500b. The substrate2560is transferred into the deposition chamber, and then the substrate2560is fixed to the substrate holder2570.

As illustrated inFIG. 36A, a power source2590and a power source2591for applying potentials are connected to the backing plates2510aand2510b. It is preferable to use AC power sources, which alternately apply a high-level potential and a low-level potential to the backing plate2510aand the backing plate2510b. When one of the high-level potential and the low-level potential is applied to the backing plate2510a, the other of the high-level potential and the low-level potential is applied to the backing plate2510b. Although AC power sources are used as the power sources2590and2591illustrated inFIG. 36A, one embodiment of the present invention is not limited thereto. For example, RF power sources, DC power sources, or the like can be used as the power sources2590and2591. Alternatively, different kinds of power sources may be used as the power sources2590and2591.

The substrate holder2570is preferably connected to GND. The substrate holder2570may be in a floating state.

FIGS. 36B and 36Ceach show potential distribution of plasma2540along dashed-dotted line A-B inFIG. 36A.FIG. 36Bshows the potential distribution in the case where a high potential is applied to the backing plate2510aand a low potential is applied to the backing plate2510b. In that case, a cation is accelerated toward the target2500a.FIG. 36Cshows the potential distribution in the case where a low potential is applied to the backing plate2510aand a high potential is applied to the backing plate2510b. In that case, a cation is accelerated toward the target2500b. The deposition can be performed by alternating the state inFIG. 36Band the state inFIG. 36C.

The deposition is preferably performed while the plasma2540completely reaches the surface of the substrate2560. For example, the substrate holder2570and the substrate2560are preferably placed in the plasma2540as illustrated inFIG. 36A. It is particularly preferable that the substrate holder2570and the substrate2560be placed in a positive column of the plasma2540. The positive column of the plasma2540is, in each ofFIGS. 36B and 36C, a region where the gradient of the potential distribution is small. When the substrate2560is placed in the positive column of the plasma2540as illustrated inFIG. 36A, the substrate2560is not exposed to a high electric field portion in the plasma2540; thus, damage to the substrate2560due to the plasma2540can be reduced and defects can be reduced.

It is preferable to place the substrate holder2570and the substrate2560in the plasma2540during deposition as illustrated inFIG. 36Aalso because utilization efficiencies of the targets2500aand2500bare increased.

As illustrated inFIG. 36A, the horizontal distance between the substrate holder2570and the target2500ais referred to as L1and the horizontal distance between the substrate holder2570and the target2500bis referred to as L2. It is preferable that the distance L1and the distance L2each be substantially equal to the length of the substrate2560in the horizontal direction inFIG. 36A. In addition, it is preferable that the distances L and L2be adjusted as appropriate so that the substrate2560is placed in the positive column of the plasma2540as described above. The distances L1and L2can each be, for example, greater than or equal to 10 mm and less than or equal to 200 mm.

InFIG. 36A, the target2500aand the target2500bare parallel to each other. Moreover, the magnet unit2530aand the magnet unit2530bare placed so that opposite poles face each other. Magnetic lines of force run from the magnet unit2530bto the magnet unit2530a. Therefore, in the deposition, the plasma2540is confined in the magnetic field formed by the magnet units2530aand2530b. The substrate holder2570and the substrate2560are placed in a region where the target2500aand the target2500bface each other (also referred to as a region between targets). Note that although the substrate holder2570and the substrate2560are placed parallel to the direction in which the target2500aand the target2500bface each other inFIG. 36A, the substrate holder2570and the substrate2560may be inclined to the direction. By inclination of the substrate holder2570and the substrate2560at 30° or more and 60° or less (typified by 45°), for example, the proportion of sputtered particles that perpendicularly reach the substrate2560during deposition can be increased.

A structure illustrated inFIG. 37is different from that illustrated inFIG. 36Ain that the target2500aand the target2500bthat face each other are not parallel but inclined to each other (in V-shape). Thus, the description forFIG. 36Ais referred to for the description except for the arrangement of the targets. The magnet unit2530aand the magnet unit2530bare placed so that opposite poles face each other. The substrate holder2570and the substrate2560are placed in the region between targets. With the targets2500aand2500bplaced as illustrated inFIG. 37, the proportion of sputtered particles that reach the substrate2560can be increased; accordingly, the deposition rate can be increased.

AlthoughFIG. 36Aillustrates a state in which the substrate holder2570and the substrate2560are placed in the plasma2540, one embodiment of the present invention is not limited to this state. The substrate holder2570and the substrate2560may be placed outside the plasma2540as illustrated inFIG. 38, for example. In that case, the substrate2560is not exposed to a high electric field region of the plasma2540, leading to a reduction in damage due to the plasma2540. Note that the utilization efficiencies of the targets2500aand2500bare decreased as the distance between the plasma2540and the substrate2560is increased. It is preferable that the position of the substrate holder2570be adjustable as illustrated inFIG. 38.

The substrate holder2570may be placed either above or below the region between targets. Alternatively, the substrate holders2570may be placed above and below the region. Providing the substrate holders2570above and below the region allows deposition on two or more substrates at once, leading to an increase in productivity. Note that the position above or below the region where the target2500aand the target2500bface each other can also be referred to as the side of the region where the target2500aand the target2500bface each other.

The facing-targets sputtering apparatus can stably generate plasma even in a high vacuum. Thus, deposition can be performed at a pressure higher than or equal to 0.005 Pa and lower than or equal to 0.09 Pa, for example. As a result, the concentration of impurities contained during deposition can be reduced.

The use of the facing-targets sputtering apparatus allows deposition in a high vacuum or deposition with less plasma damage and thus can provide a film with high crystallinity even when the temperature of the substrate2560is low (e.g., higher than or equal to 10° C. and lower than 100° C.).

FIG. 39Aillustrates another example of a facing-targets sputtering apparatus.

FIG. 39Ais a schematic cross-sectional view of a deposition chamber of a facing-targets sputtering apparatus. Unlike in the deposition chamber illustrated inFIG. 34A, a target shield2522and a target shield2523are provided. The power source2591connected to the backing plates2510aand2510bis also provided.

The target shields2522and2523are connected to GND as illustrated inFIG. 39A. This means that the plasma2540is generated by a potential difference between the backing plates2510aand2510bto which a potential of the power source2591is applied and the target shields2522and2523to which GND is applied.

The deposition is preferably performed while the plasma2540completely reaches the surface of the substrate2560. For example, the substrate holder2570and the substrate2560are preferably placed in the plasma2540as illustrated inFIG. 39A. It is particularly preferable that the substrate holder2570and the substrate2560be placed in a positive column of the plasma2540. The positive column of the plasma is a region where the gradient of the potential distribution is small. When the substrate2560is placed in the positive column of the plasma2540as illustrated inFIG. 39A, the substrate2560is not exposed to a high electric field portion in the plasma2540; thus, damage to the substrate2560due to the plasma2540can be reduced and an oxide with a favorable film quality can be obtained.

It is preferable to place the substrate holder2570and the substrate2560in the plasma2540during deposition as illustrated inFIG. 39Aalso because utilization efficiencies of the targets2500aand2500bare increased.

As illustrated inFIG. 39A, the horizontal distance between the substrate holder2570and the target2500ais referred to as L1and the horizontal distance between the substrate holder2570and the target2500bis referred to as L2. It is preferable that the distance L1and the distance L2each be substantially equal to the length of the substrate2560in the horizontal direction inFIG. 39A. In addition, it is preferable that the distances L1and L2be adjusted as appropriate so that the substrate2560is placed in the positive column of the plasma2540as described above.

AlthoughFIG. 39Aillustrates a state in which the substrate holder2570and the substrate2560are placed in the plasma2540, one embodiment of the present invention is not limited to this state. The substrate holder2570and the substrate2560may be placed outside the plasma2540as illustrated inFIG. 39B, for example. In that case, the substrate2560is not exposed to a high electric field region of the plasma2540, leading to a reduction in damage due to the plasma2540. Note that the utilization efficiencies of the targets2500aand2500bare decreased as the distance between the plasma2540and the substrate2560is increased. It is preferable that the position of the substrate holder2570be adjustable as illustrated inFIG. 39B.

The substrate holder2570may be placed above a region where the target2500aand the target2500bface each other as illustrated inFIG. 39B, or may be placed below the region. Alternatively, the substrate holders2570may be placed above and below the region. Providing the substrate holders2570above and below the region allows deposition on two or more substrates at once, leading to an increase in productivity.

In the above-described facing-targets sputtering apparatuses, plasma is confined by magnetic fields between targets; thus, plasma damage to a substrate can be reduced. Furthermore, a deposited film can have improved step coverage because an incident angle of a sputtered particle to a substrate can be made smaller by the inclination of the target. Moreover, deposition in a high vacuum enables the concentration of impurities contained in the film to be reduced.

Note that a parallel-plate-type sputtering apparatus or an ion beam sputtering apparatus may be provided in the deposition chamber.

A deposition apparatus with a deposition chamber including a sputtering apparatus with which the oxide of one embodiment of the present invention can be deposited is described below.

First, a structure of a deposition apparatus that hardly allows the entry of impurities into a film during deposition or the like will be described with reference toFIG. 40andFIGS. 41A to 41C.

FIG. 40is a schematic top view of a single wafer multi-chamber deposition apparatus2700. The single wafer multi-chamber deposition apparatus2700includes an atmosphere-side substrate supply chamber2701including a cassette port2761for storing substrates and an alignment port2762for performing alignment of substrates, an atmosphere-side substrate transfer chamber2702through which a substrate is transferred from the atmosphere-side substrate supply chamber2701, a load lock chamber2703awhere a substrate is carried in and the pressure is switched from atmospheric pressure to reduced pressure or from reduced pressure to atmospheric pressure, an unload lock chamber2703bwhere a substrate is carried out and the pressure is switched from reduced pressure to atmospheric pressure or from atmospheric pressure to reduced pressure, a transfer chamber2704where a substrate is transferred in a vacuum, a substrate heating chamber2705where a substrate is heated, and deposition chambers2706a,2706b, and2706cin each of which a target is placed for deposition. Note that for the deposition chambers2706a,2706b, and2706c, the structure of a deposition chamber which will be described later can be referred to.

The atmosphere-side substrate transfer chamber2702is connected to the load lock chamber2703aand the unload lock chamber2703b, the load lock chamber2703aand the unload lock chamber2703bare connected to the transfer chamber2704, and the transfer chamber2704is connected to the substrate heating chamber2705and the deposition chambers2706a,2706b, and2706c.

Note that gate valves2764are provided in connecting portions between the chambers so that each chamber excluding the atmosphere-side substrate supply chamber2701and the atmosphere-side substrate transfer chamber2702can be independently kept in a vacuum state. In each of the atmosphere-side substrate transfer chamber2702and the transfer chamber2704, a transfer robot2763is provided, which is capable of transferring substrates.

It is preferable that the substrate heating chamber2705also serve as a plasma treatment chamber. In the deposition apparatus2700, substrates can be transferred without being exposed to the air between treatments, and adsorption of impurities to substrates can be suppressed. In addition, the order of deposition, heat treatment, or the like can be freely determined. Note that the number of transfer chambers, the number of deposition chambers, the number of load lock chambers, the number of unload lock chambers, and the number of substrate heating chambers are not limited to the above, and the numbers thereof can be set as appropriate depending on the space for installation or the process conditions.

Next,FIG. 41A,FIG. 41B, andFIG. 41Care a cross-sectional view taken along dashed-dotted line V1-V2, a cross-sectional view taken along dashed-dotted line W1-W2, and a cross-sectional view taken along dashed-dotted line W2-W3, respectively, in the deposition apparatus2700illustrated inFIG. 40.

FIG. 41Aillustrates a cross section of the substrate heating chamber2705and the transfer chamber2704, and the substrate heating chamber2705includes a plurality of heating stages2765which can hold a substrate. Note that the substrate heating chamber2705is connected to a vacuum pump2770through a valve. As the vacuum pump2770, a dry pump and a mechanical booster pump can be used, for example.

As a heating mechanism which can be used for the substrate heating chamber2705, a resistance heater may be used for heating, for example. Alternatively, heat conduction or heat radiation from a medium such as a heated gas may be used as the heating mechanism. For example, rapid thermal annealing (RTA) such as gas rapid thermal annealing (GRTA) or lamp rapid thermal annealing (LRTA) can be used. The LRTA is a method for heating an object by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. In the GRTA, heat treatment is performed using a high-temperature gas. An inert gas is used as the gas.

Moreover, the substrate heating chamber2705is connected to a refiner2781through a mass flow controller2780. Note that although the mass flow controller2780and the refiner2781can be provided for each of a plurality of kinds of gases, only one mass flow controller2780and one refiner2781are illustrated for easy understanding. As the gas introduced to the substrate heating chamber2705, a gas whose dew point is −80° C. or lower, preferably −100° C. or lower can be used; for example, an oxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) are used.

The transfer chamber2704includes the transfer robot2763. The transfer robot2763can transfer a substrate to each chamber. Furthermore, the transfer chamber2704is connected to the vacuum pump2770and a cryopump2771through valves. Owing to such a structure, exhaust is performed using the vacuum pump2770from the atmospheric pressure to low or medium vacuum (approximately 0.1 Pa) and then the valves are switched and exhaust is performed using the cryopump2771from the medium vacuum to high or ultra-high vacuum (0.1 Pa to 1×10−7Pa).

Alternatively, two or more cryopumps2771may be connected in parallel to the transfer chamber2704. With such a structure, even when one of the cryopumps is in regeneration, exhaust can be performed using any of the other cryopumps. Note that the above regeneration refers to treatment for discharging molecules (or atoms) entrapped in the cryopump. When molecules (or atoms) are entrapped too much in a cryopump, the exhaust capability of the cryopump is lowered; therefore, regeneration is performed regularly.

FIG. 41Billustrates a cross section of the deposition chamber2706b, the transfer chamber2704, and the load lock chamber2703a.

Here, the details of the deposition chamber including the sputtering apparatus are described with reference toFIG. 41B. The deposition chamber2706billustrated inFIG. 41Bincludes a target2766a, a target2766b, a target shield2767a, a target shield2767b, a magnet unit2790a, a magnet unit2790b, a substrate holder2768, and a power source2791. Although not illustrated, the targets2766aand2766bare each fixed to a target holder with a backing plate positioned therebetween. The power source2791is electrically connected to each of the target2766aand the target2766b. The magnet units2790aand2790bare placed behind the targets2766aand2766b, respectively. The target shield2767ais provided so as to surround an end portion of the target2766aand the target shield2767bis provided so as to surround an end portion of target2766b. Note that here, a substrate2769is supported by the substrate holder2768. The substrate holder2768is fixed to the deposition chamber2706bby an adjustment member2784. Owing to the adjustment member2784, the substrate holder2768can move to a region between the targets2766aand2766b(a region between targets). Providing the substrate holder2768supporting the substrate2769in the region between targets can reduce damage due to plasma in some cases, for example. Although not illustrated, the substrate holder2768may include a substrate holding mechanism that holds the substrate2769, a heater that heats the substrate2769from the back side, or the like.

The target shields2767can suppress deposition of a particle which is sputtered from the target2766on a region where deposition is not needed. Moreover, the target shields2767are preferably processed to prevent accumulated sputtered particles from being separated. For example, blasting treatment which increases surface roughness may be performed, or roughness may be formed on the surfaces of the target shields2767.

The deposition chamber2706bis connected to the mass flow controller2780through a gas heating mechanism2782, and the gas heating mechanism2782is connected to the refiner2781through the mass flow controller2780. With the gas heating mechanism2782, a gas which is introduced to the deposition chamber2706bcan be heated to a temperature higher than or equal to 40° C. and lower than or equal to 400° C., preferably higher than or equal to 50° C. and lower than or equal to 200° C. Note that although the gas heating mechanism2782, the mass flow controller2780, and the refiner2781can be provided for each of a plurality of kinds of gases, only one gas heating mechanism2782, one mass flow controller2780, and one refiner2781are illustrated for easy understanding. As the gas introduced to the deposition chamber2706b, a gas whose dew point is −80° C. or lower, preferably −100° C. or lower can be used; for example, an oxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) are used.

In the case where the refiner is provided near a gas inlet, the length of a pipe between the refiner and the deposition chamber2706bis less than or equal to 10 m, preferably less than or equal to 5 m, further preferably less than or equal to 1 m. When the length of the pipe is less than or equal to 10 m, less than or equal to 5 m, or less than or equal to 1 m, the effect of the release of gas from the pipe can be reduced accordingly. As the pipe for the gas, a metal pipe the inside of which is covered with iron fluoride, aluminum oxide, chromium oxide, or the like can be used. With the above pipe, the amount of released gas containing impurities is made small and the entry of impurities into the gas can be reduced as compared with a SUS316L-EP pipe, for example. Furthermore, a high-performance ultra-compact metal gasket joint (UPG joint) may be used as a joint of the pipe. A structure where all the materials of the pipe are metals is preferable because the effect of the generated released gas or the external leakage can be reduced as compared with a structure where a resin or the like is used.

The deposition chamber2706bis connected to a turbo molecular pump2772and the vacuum pump2770through valves.

In addition, the deposition chamber2706bis provided with a cryotrap2751.

The cryotrap2751is a mechanism which can adsorb a molecule (or an atom) having a relatively high melting point, such as water. The turbo molecular pump2772is capable of stably removing a large-sized molecule (or atom), needs low frequency of maintenance, and thus enables high productivity; on the other hand, it has a low capability in removing hydrogen and water. Hence, the cryotrap2751is connected to the deposition chamber2706bso as to have a high capability in removing water or the like. The temperature of a refrigerator of the cryotrap2751is set to be lower than or equal to 100 K, preferably lower than or equal to 80 K. In the case where the cryotrap2751includes a plurality of refrigerators, it is preferable to set the temperatures of the refrigerators at different temperatures because efficient exhaust is possible. For example, the temperature of a first-stage refrigerator may be set to be lower than or equal to 100 K and the temperature of a second-stage refrigerator may be set to be lower than or equal to 20 K. Note that when a titanium sublimation pump is used instead of the cryotrap, a higher vacuum can be achieved in some cases. Using an ion pump instead of a cryopump or a turbo molecular pump can also achieve higher vacuum in some cases.

Note that the exhaust method of the deposition chamber2706bis not limited to the above, and a structure similar to that in the exhaust method described above for the transfer chamber2704(the exhaust method using the cryopump and the vacuum pump) may be employed. Needless to say, the exhaust method of the transfer chamber2704may have a structure similar to that of the deposition chamber2706b(the exhaust method using the turbo molecular pump and the vacuum pump).

Note that in each of the transfer chamber2704, the substrate heating chamber2705, and the deposition chamber2706bwhich are described above, the back pressure (total pressure) and the partial pressure of each gas molecule (atom) are preferably set as follows. In particular, the back pressure and the partial pressure of each gas molecule (atom) in the deposition chamber2706bneed to be noted because impurities might enter a film to be formed.

In each of the above chambers, the back pressure (total pressure) is less than or equal to 1×10−4Pa, preferably less than or equal to 3×10−5Pa, further preferably less than or equal to 1×10−5Pa. In each of the above chambers, the partial pressure of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 18 is less than or equal to 3×10−5Pa, preferably less than or equal to 1×10−5Pa, further preferably less than or equal to 3×10−6Pa. Moreover, in each of the above chambers, the partial pressure of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 28 is less than or equal to 3×10−5Pa, preferably less than or equal to 1×10−5Pa, further preferably less than or equal to 3×10−6Pa. Furthermore, in each of the above chambers, the partial pressure of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 44 is less than or equal to 3×10−5Pa, preferably less than or equal to 1×10−5Pa, further preferably less than or equal to 3×10−6Pa.

Note that a total pressure and a partial pressure in a vacuum chamber can be measured using a mass analyzer. For example, Qulee CGM-051, a quadrupole mass analyzer (also referred to as Q-mass) manufactured by ULVAC, Inc. may be used.

Moreover, the transfer chamber2704, the substrate heating chamber2705, and the deposition chamber2706bwhich are described above preferably have a small amount of external leakage or internal leakage.

For example, in each of the transfer chamber2704, the substrate heating chamber2705, and the deposition chamber2706bwhich are described above, the leakage rate is less than or equal to 3×10−6Pa·m3/s, preferably less than or equal to 1×10−6Pa·m3/s. The leakage rate of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 18 is less than or equal to 1×10−7Pa·m3/s, preferably less than or equal to 3×10−8Pa·m3/s. The leakage rate of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 28 is less than or equal to 1×10−5Pa·m3/s, preferably less than or equal to 1×10−6Pa·m3/s. The leakage rate of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 44 is less than or equal to 3×10−6Pa·m3/s, preferably less than or equal to 1×10−6Pa·m3/s.

Note that a leakage rate can be derived from the total pressure and partial pressure measured using the mass analyzer.

The leakage rate depends on external leakage and internal leakage. The external leakage refers to inflow of gas from the outside of a vacuum system through a minute hole, a sealing defect, or the like. The internal leakage is due to leakage through a partition, such as a valve, in a vacuum system or due to released gas from an internal member. Measures need to be taken from both aspects of external leakage and internal leakage in order that the leakage rate can be set to be less than or equal to the above value.

For example, an open/close portion of the deposition chamber2706bcan be sealed with a metal gasket. For the metal gasket, metal covered with iron fluoride, aluminum oxide, or chromium oxide is preferably used. The metal gasket realizes higher adhesion than an O-ring, and can reduce the external leakage. Furthermore, with the use of the metal covered with iron fluoride, aluminum oxide, chromium oxide, or the like, which is in the passive state, the release of gas containing impurities released from the metal gasket is suppressed, so that the internal leakage can be reduced.

For a member of the deposition apparatus2700, aluminum, chromium, titanium, zirconium, nickel, or vanadium, which releases a smaller amount of gas containing impurities, is used. Alternatively, for the above member, an alloy containing iron, chromium, nickel, and the like covered with the above material may be used. The alloy containing iron, chromium, nickel, and the like is rigid, resistant to heat, and suitable for processing. Here, when surface unevenness of the member is decreased by polishing or the like to reduce the surface area, the release of gas can be reduced.

Alternatively, the above member of the deposition apparatus2700may be covered with iron fluoride, aluminum oxide, chromium oxide, or the like.

The member of the deposition apparatus2700is preferably formed using only metal when possible. For example, in the case where a viewing window formed with quartz or the like is provided, it is preferable that the surface of the viewing window be thinly covered with iron fluoride, aluminum oxide, chromium oxide, or the like so as to suppress release of gas.

When an adsorbed substance is present in the deposition chamber, the adsorbed substance does not affect the pressure in the deposition chamber because it is adsorbed onto an inner wall or the like; however, the adsorbed substance causes gas to be released when the inside of the deposition chamber is evacuated. Therefore, although there is no correlation between the leakage rate and the exhaust rate, it is important that the adsorbed substance present in the deposition chamber be desorbed as much as possible and exhaust be performed in advance with the use of a pump with high exhaust capability. Note that the deposition chamber may be subjected to baking to promote desorption of the adsorbed substance. By the baking, the desorption rate of the adsorbed substance can be increased about tenfold. The baking can be performed at a temperature in the range of 100° C. to 450° C. At this time, when the adsorbed substance is removed while an inert gas is introduced to the deposition chamber, the desorption rate of water or the like, which is difficult to desorb simply by exhaust, can be further increased. Note that when the inert gas which is introduced is heated to substantially the same temperature as the baking temperature, the desorption rate of the adsorbed substance can be further increased. Here, a rare gas is preferably used as an inert gas. Depending on the kind of a film to be deposited, oxygen or the like may be used instead of an inert gas. For example, in deposition of an oxide, the use of oxygen, which is the main component of the oxide, is preferable in some cases. The baking is preferably performed using a lamp.

Alternatively, treatment for evacuating the inside of the deposition chamber is preferably performed a certain period of time after heated oxygen, a heated inert gas such as a heated rare gas, or the like is introduced to increase a pressure in the deposition chamber. The introduction of the heated gas can desorb the adsorbed substance in the deposition chamber, and the impurities present in the deposition chamber can be reduced. Note that an advantageous effect can be achieved when this treatment is repeated more than or equal to 2 times and less than or equal to 30 times, preferably more than or equal to 5 times and less than or equal to 15 times. Specifically, an inert gas, oxygen, or the like with a temperature higher than or equal to 40° C. and lower than or equal to 400° C., preferably higher than or equal to 50° C. and lower than or equal to 200° C. is introduced to the deposition chamber, so that the pressure therein can be kept to be greater than or equal to 0.1 Pa and less than or equal to 10 kPa, preferably greater than or equal to 1 Pa and less than or equal to 1 kPa, further preferably greater than or equal to 5 Pa and less than or equal to 100 Pa in the time range of 1 minute to 300 minutes, preferably 5 minutes to 120 minutes. After that, the inside of the deposition chamber is evacuated in the time range of 5 minutes to 300 minutes, preferably 10 minutes to 120 minutes.

The desorption rate of the adsorbed substance can be further increased also by dummy deposition. Here, the dummy deposition refers to deposition on a dummy substrate by a sputtering method or the like, in which a film is deposited on the dummy substrate and the inner wall of the deposition chamber so that impurities in the deposition chamber and an adsorbed substance on the inner wall of the deposition chamber are confined in the film. As the dummy substrate, a substrate which releases a smaller amount of gas is preferably used. By performing dummy deposition, the concentration of impurities in a film to be formed later can be reduced. Note that the dummy deposition may be performed at the same time as the baking of the deposition chamber.

Next, the details of the transfer chamber2704and the load lock chamber2703aillustrated inFIG. 41Band the atmosphere-side substrate transfer chamber2702and the atmosphere-side substrate supply chamber2701illustrated inFIG. 41Care described. Note thatFIG. 41Cillustrates a cross section of the atmosphere-side substrate transfer chamber2702and the atmosphere-side substrate supply chamber2701.

For the transfer chamber2704illustrated inFIG. 41B, the description of the transfer chamber2704illustrated inFIG. 41Acan be referred to.

The load lock chamber2703aincludes a substrate delivery stage2752. When a pressure in the load lock chamber2703abecomes atmospheric pressure by being increased from reduced pressure, the substrate delivery stage2752receives a substrate from the transfer robot2763provided in the atmosphere-side substrate transfer chamber2702. After that, the load lock chamber2703ais evacuated into vacuum so that the pressure therein becomes reduced pressure and then the transfer robot2763provided in the transfer chamber2704receives the substrate from the substrate delivery stage2752.

Furthermore, the load lock chamber2703ais connected to the vacuum pump2770and the cryopump2771through valves. For a method for connecting exhaust systems such as the vacuum pump2770and the cryopump2771, the description of the method for connecting the transfer chamber2704can be referred to, and the description thereof is omitted here. Note that the unload lock chamber2703billustrated inFIG. 40can have a structure similar to that of the load lock chamber2703a.

The atmosphere-side substrate transfer chamber2702includes the transfer robot2763. The transfer robot2763can deliver a substrate from the cassette port2761to the load lock chamber2703aor deliver a substrate from the load lock chamber2703ato the cassette port2761. Furthermore, a mechanism for suppressing entry of dust or a particle, such as a high-efficiency particulate air (HEPA) filter, may be provided above the atmosphere-side substrate transfer chamber2702and the atmosphere-side substrate supply chamber2701.

The atmosphere-side substrate supply chamber2701includes a plurality of cassette ports2761. The cassette port2761can hold a plurality of substrates.

The surface temperature of the target is set to be lower than or equal to 100° C., preferably lower than or equal to 50° C., further preferably about room temperature (typified by 25° C.). In a sputtering apparatus for a large substrate, a large target is often used. However, it is difficult to form a target for a large substrate without a juncture. In fact, a plurality of targets are arranged so that there is as little space as possible therebetween to obtain a large shape; however, a slight space is inevitably generated. When the surface temperature of the target increases, in some cases, zinc or the like is volatilized from such a slight space and the space might be expanded gradually. When the space expands, a metal of a backing plate or a metal of a bonding material used for adhesion between the backing plate and the target might be sputtered and might cause an increase in impurity concentration. Thus, it is preferable that the target be cooled sufficiently.

Specifically, to efficiently cool the target, a metal having high conductivity and a high heat dissipation property (specifically copper) is used for the backing plate, or a sufficient amount of cooling water is made to flow through a water channel formed in the backing plate.

Note that in the case where the target includes zinc, plasma damage is alleviated by the deposition in an oxygen gas atmosphere; thus, an oxide in which zinc is unlikely to be volatilized can be obtained.

The above-described deposition apparatus enables deposition of an oxide semiconductor whose hydrogen concentration measured by secondary ion mass spectrometry (SIMS) is lower than or equal to 2×1020atoms/cm3, preferably lower than or equal to 5×1019atoms/cm3, further preferably lower than or equal to 1×1019atoms/cm3, still further preferably lower than or equal to 5×1018atoms/cm3.

Furthermore, an oxide semiconductor whose nitrogen concentration measured by SIMS is lower than 5×1019atoms/cm3, preferably lower than or equal to 1×1019atoms/cm3, further preferably lower than or equal to 5×1018atoms/cm3, still further preferably lower than or equal to 1×1018atoms/cm3can be deposited.

Moreover, an oxide semiconductor whose carbon concentration measured by SIMS is lower than 5×1019atoms/cm3, preferably lower than or equal to 5×1019atoms/cm3, further preferably lower than or equal to 1×1018atoms/cm3, still further preferably lower than or equal to 5×1017atoms/cm3can be deposited.

An oxide having few impurities and oxygen vacancies is an oxide with low carrier density (specifically, lower than 8×1011/cm3, preferably lower than 1×1011/cm3, further preferably lower than 1×1010/cm3, and higher than or equal to 1×10−9/cm3). Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be regarded as an oxide having stable characteristics.

Furthermore, an oxide semiconductor can be deposited in which the released amount of each of the following gas molecules (atoms) measured by thermal desorption spectroscopy (TDS) is less than or equal to 1×1019/cm3, preferably less than or equal to 1×1018/cm3: a gas molecule (atom) having a mass-to-charge ratio (m/z) of 2 (e.g., a hydrogen molecule), a gas molecule (atom) having a mass-to-charge ratio (m/z) of 18, a gas molecule (atom) having a mass-to-charge ratio (m/z) of 28, and a gas molecule (atom) having a mass-to-charge ratio (m/z) of 44.

With the above deposition apparatus, entry of impurities into the oxide can be suppressed. Furthermore, when a film in contact with the oxide is formed with the use of the above deposition apparatus, the entry of impurities into the oxide from the film in contact therewith can be suppressed.

Next, the case where the oxide semiconductor is used for a transistor is described.

FIG. 9shows Id-Vgcharacteristics, which are the electrical characteristics of a transistor.

First, drain current-gate voltage characteristics (Id-Vgcharacteristics) of a transistor are described.FIG. 9illustrates an example of Id-Vgcharacteristics of the transistor. InFIG. 9, the vertical axis and the horizontal axis represent Idand Vg, respectively.

Id-Vgcharacteristics are broadly divided into three regions. A first region, a second region, and a third region are referred to as an off region (OFF region), a subthreshold region, and an on region (ON region), respectively. A gate voltage at a boundary between the subthreshold region and the on region is referred to as a threshold voltage (Vth).

To obtain favorable characteristics of the transistor, it is preferable that the drain current in the off region (also referred to as off-state current or Ioff) be low and the drain current in the on region (also referred to as on-state current or Ion) be high. As an index of the on-state current of the transistor, the field-effect mobility is often used. The details of the field-effect mobility are described later.

To drive the transistor at a low voltage, the slope of the Id-Vgcharacteristics in the subthreshold region is preferably steep. An index of the degree of change in the Id-Vgcharacteristics in the subthreshold region is referred to as subthreshold swing (SS) or an S value.

The S value is a minimum value of the amount of change in gate voltage which is needed for changing a drain current by an order of magnitude in the subthreshold region. As the S value is smaller, switching operation between on and off states can be performed rapidly.

Next, field-effect mobility is described. As an index of current drive capability of a transistor, the field-effect mobility is used. As described above, the on region of the transistor is divided into the linear region and the saturation region. From the characteristics in the regions, the field-effect mobility of the transistor can be calculated on the basis of the analytic formulae of the drain current based on GCA. The field-effect mobility in the linear region and the field-effect mobility in the saturation region are referred to as linear mobility and saturation mobility, respectively, when they need to be distinguished from each other.

The Id-Vgcharacteristics of the transistor inFIG. 9are Id-Vgcharacteristics of the transistor including an oxide semiconductor film which has a composition of [In]:[Ga]:[Zn]=4:2:3 or a neighborhood thereof and is obtained using a target with [In]:[Ga]:[Zn]=4:2:4.1. Note that the proportion of oxygen to the whole deposition gas is referred to as “oxygen flow rate” in some cases. The oxygen flow rate percentage in deposition of the oxide semiconductor film of the transistor illustrated inFIG. 9is 10%.

The transistor has high field-effect mobility as shown inFIG. 9.

Next, a density of shallow defect states (also referred to as sDOS) in an oxide semiconductor film is described. The sDOS of an oxide semiconductor film can be estimated from electrical characteristics of a transistor including the oxide semiconductor film. In the description below, the density of interface states of the transistor was measured. In addition, there is a method for estimating subthreshold leakage current in consideration of the density of interface states and the number of electrons trapped by the interface states, Ntrap.

The number of electrons trapped by the interface state, Ntrap, can be measured by comparing drain current-gate voltage (Id-Vg) characteristics of the transistor that were actually measured and drain current-gate voltage (Id-Vg) characteristics that were calculated. The sDOS in the oxide semiconductor film influences the mobility curve of the field-effect mobility. In particular, in the vicinity of the threshold voltage, the shape of the mobility curve is changed because electrons are trapped by the sDOS.

To measure sDOS in the oxide semiconductor film of the transistor having electrical characteristics inFIG. 9, transistors for measurement of sDOS were fabricated. The transistors were fabricated by the same manufacturing method. The size of each of the transistors is L/W=6/50 μm. Detailed description of a method for measuring sDOS will be made later. The results of sDOS of the transistor for measurement of sDOS show that the peak value of the shallow defect states is extremely low, less than 1.5×1012cm−2eV−1.

The field-effect mobility of the transistor obtained by deposition using the same sputtering target in an oxygen atmosphere (oxygen gas: 100%) is smaller than that of the transistor inFIG. 9. The results of sDOS of the transistor for measurement of sDOS are over 2.5×1012cm−2eV−1.

These results indicate that the oxide semiconductor film of the transistor inFIG. 9includes the composite material inFIG. 1. In other words, when the oxygen flow rate is less than 20%, preferably less than or equal to 10% in deposition, a plurality of stacks each including a first region and a second region are formed, excess oxygen is diffused into the second region, and oxygen vacancies in the second region are reduced. The second region has a main function of making carriers flow, and the first region has a property of bending an energy band. As a result, high electrical characteristics (e.g., high field-effect mobility or a small S value) and small sDOS can be realized. A region where sDOS is small, that is, the density of shallow defect states is less than 1.0×10−12cm−2is provided. The sDOS is a density of shallow defect states of an oxide semiconductor and can be estimated from electrical characteristics of a transistor including the oxide semiconductor film. The sDOS in the oxide semiconductor film influences the mobility curve of the field-effect mobility. In particular, in the vicinity of the threshold voltage, the shape of the mobility curve is changed because electrons are trapped by the sDOS.

Furthermore, when the sputtering target is formed using the composite material inFIG. 1, the composite material film may be formed.

With a novel composite material (also referred to as a composite semiconductor material), high electrical characteristics (e.g., a high on-state current) are realized, and a highly reliable semiconductor device can be provided. The high on-state current value can achieve a further decrease in channel width (W). A highly integrated semiconductor device can be provided. A semiconductor device which can operate at high speed can be provided.

A semiconductor device with low power consumption can be provided. A semiconductor device with high productivity can be provided. A semiconductor device with high manufacturing yield can be provided. A novel semiconductor device can be provided. A module including the semiconductor device can be provided. An electronic device including the semiconductor device or the module can be provided.

Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detail with reference to the drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. Furthermore, the present invention is not construed as being limited to the description of the embodiments given below.

FIG. 1is a conceptual view of a composite material described in this embodiment. The terms of the atomic ratios of indium to gallium and zinc contained in the composite material are denoted by [In], [Ga], and [Zn].

The composite material has a structure in which a plurality of stacks each including a first region11and a second region12are provided over a base10. Each of the first region11and the second region12has a thickness of greater than or equal to 0.5 nm and less than or equal to 5 nm. InFIG. 1, the interface between layers is denoted by a dotted line; however, the dotted line does not indicate a clear boundary and shows that carriers can flow through either layer but are more likely flow in the second region12than in the first region11.

A substrate having an insulating surface (e.g., a glass substrate, a sapphire substrate, or a ceramic substrate), 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, an SOI substrate, or the like may be used as the base10. Alternatively, any of these substrates provided with a semiconductor element may be used as the base10.

A switching element, specifically a transistor including a well potential structure, can be manufactured by utilizing a structure like a superlattice illustrated inFIG. 1. The term “superlattice” means a structure in which thin films are stacked periodically or partly periodically. The structure is not limited to the structure to have a crystal lattice, and includes the case where a crystal lattice cannot be observed clearly. The first region11and the second region12in contact with the first region11are placed in the thickness direction. The first region11contains at least indium, gallium, and zinc, and the second region12contains indium and zinc. The second region12serves as a well portion of the well potential and has a smaller bandgap than the first region11. Thus, a semiconductor element in which the second region12is sandwiched between the first regions11in the thickness direction can be formed.

The first region11preferably contains at least indium, an element M, and zinc. The element M is aluminum, gallium, yttrium, tin, or the like. Other elements that can be used as the element M include boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium. Note that two or more of the above elements may be used in combination as the element M. In this embodiment, an example of the first region11containing indium, gallium, and zinc is shown.

The first region11may be included within a range of a region24shown inFIG. 7.FIG. 7is a phase diagram, and in the case where a substance contains an element X, an element Y, and an element Z, the atomic ratio of the elements is denoted by x:y:z where x, y, and z correspond to the number of atoms of the elements X, Y, and Z, and the atomic ratio can be shown as coordinates (x:y:z) in drawings. Note that the proportion of oxygen atoms is not shown inFIG. 7.

In this embodiment, an example is shown in which the first region11has an atomic ratio of [In]:[Ga]:[Zn]=4:2:3 or a neighborhood thereof. The first region11can have a c-axis aligned crystalline oxide semiconductor (CAAC-OS) structure, a polycrystalline oxide semiconductor structure, or a nanocrystalline oxide semiconductor (nc-OS) structure. The CAAC-OS includes a crystal part, and a portion in which a c-axis of the crystal part is aligned parallel to a normal vector of a surface where the film is formed. Among the non-single crystal structures, the CAAC-OS structure has the lowest density of defect states; thus, the CAAC-OS is suitable for the first region11.

In this embodiment, an example is shown in which the second region12does not contain gallium and has a lower concentration of gallium than the first region11.

The second region12may be included within a range of a region22shown inFIG. 6. The second region12may have crystallinity and have a nanocrystalline oxide semiconductor (nc-OS) structure, for example. The second region12itself is unstable but becomes stable when the second region12is sandwiched between the first regions11.

In this embodiment, an example is shown in which the first region11has an atomic ratio of [In]:[Zn]=1:m (m is a rational number).

The first region11and the second region12have different main functions: the first region11has a property of bending an energy band and the second region12has a function of making carriers flow.

FIG. 1shows the structure in which the first region11and the second region12are alternately stacked; however, one embodiment of the present invention is not particularly limited thereto. Indium oxide may be included between the first region11and the second region12. A composite material having a novel periodic structure may be formed by changing the number of layers included in each stack as appropriate.

An example of a method for manufacturing the composite material illustrated inFIG. 1is described.

The temperature at which the composite material thin film is formed is preferably higher than or equal to room temperature and lower than 140° C. Note that room temperature includes not only the case where temperature control is not performed but also the case where temperature control is performed. Oxygen and argon are used as deposition gases for the composite material thin film, and the percentage of oxygen flow rate is higher than or equal to 5% and lower than or equal to 20%, preferably higher than or equal to 7% and lower than or equal to 15%.

As a sputtering target, an In—Ga—Zn metal oxide target having an atomic ratio shown in the region20inFIG. 5(e.g., [In]:[Ga]:[Zn]=4:2:4.1) is used.

In the case where the composite material thin film is deposited by a sputtering method, as a sputtering gas, a rare gas (typically, argon), oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to the rare gas is preferably increased. In addition, increasing the purity of the sputtering gas is necessary. For example, when a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower, is used as the sputtering gas, i.e., the oxygen gas or the argon gas, entry of moisture or the like into the composite material thin film can be minimized.

In the case where the composite material thin film is deposited by a sputtering method, the sputtering gas containing oxygen is preferably used. When the composite material thin film is deposited using the sputtering gas containing oxygen, oxygen can be added to a film under the composite material thin film (here, the insulating film207) at the same time as the deposition of the composite material thin film. Therefore, an oxygen-excess region can be provided in the insulating film207.

In the case where the composite material thin film is deposited by a sputtering method, a chamber in a sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10−7Pa to 1×10−4Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity for the composite material thin film, as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas containing carbon or hydrogen from an exhaust system to the inside of the chamber.

A structure in which the first region11and the second region12are alternately stacked can be obtained by one-time deposition using a sputtering method. By one-time deposition using a sputtering method, part of a composite material including the first region11and the second region12may be periodically stacked.

In this embodiment, an example where a sputtering target containing a polycrystalline oxide semiconductor (having an atomic ratio of [In]:[Ga]:[Zn]=4:2:4.1) is used is shown; however, one embodiment of the present invention is not limited thereto. The deposition may be performed using a sputtering target that is a composite material including the stacks illustrated inFIG. 1and has a structure like a superlattice.

In this embodiment, an example which is partly different from Embodiment 1 is described with reference toFIG. 2.

This embodiment is the same as Embodiment 1 except that the composition of the first region11is different; thus, the detailed description of the same portions is omitted.

As illustrated inFIG. 2, the first region11has an atomic ratio of [In]:[Ga]:[Zn]=1:1:1 or a neighborhood thereof.

Also inFIG. 2, the composite material has a structure in which a plurality of stacks each including the first region11and the second region12are provided over the base10. Each of the first region11and the second region12has a thickness of greater than or equal to 0.5 nm and less than or equal to 5 nm. A stacked structure in which at least two stacks are repeated may be used.

A switching element, specifically a transistor including a well potential structure, can be manufactured by utilizing a structure like a superlattice illustrated inFIG. 2. A three-layer structure in which the second region12which serves as a well portion is sandwiched between the first regions11in the vertical direction is used so that a well portion of a transistor is sandwiched between barrier portions. The first region11and the second region12in contact with the first region11are placed in the thickness direction. The first region11contains at least indium, gallium, and zinc, and the second region12contains indium and zinc. Current is more likely to flow in the second region12than in the first region11. The second region12serves as a well portion of the well potential, and the second region12has a smaller bandgap than the first region11. Thus, a semiconductor element in which the second region12is sandwiched between the first regions11in the thickness direction can be formed.

In this embodiment, an example which is partly different from Embodiment 1 is described with reference toFIG. 3.

This embodiment is the same as Embodiment 1 except that there is a portion in which one more layer is added to the structure in Embodiment 1; thus, the detailed description of the same portions is omitted.

InFIG. 3, the composite material has a structure in which a plurality of stacks each including the first region11, the second region12, and a third region13are provided over the base10. Each of the first region11, the second region12, and the third region13has a thickness of greater than or equal to 0.5 nm and less than or equal to 5 nm. A stacked structure in which at least three stacks are repeated may be used.

The third region13may have a different composition from the first region11, and may be included within the range of the region24shown inFIG. 7.

A switching element, specifically a transistor including a well potential structure, can be manufactured by utilizing a structure like a superlattice illustrated inFIG. 3. A structure in which a well portion of a transistor is sandwiched between two types of barrier portions is used. A three-layer structure in which the second region12serving as a first well portion is sandwiched between the first region11and the third region13is used. The first region11, the second region12in contact with the first region11, and the third region13in contact with the second region12are placed in the thickness direction. The first region11contains at least indium, gallium, and zinc. The second region12contains indium and zinc. Current is more likely to flow in the second region12than in the first region11and the third region13. The second region12serves as a well portion of the well potential, and the second region12has a smaller bandgap than the first region11and the third region13. Thus, a semiconductor element in which the second region12is sandwiched between the first region11and the third region13in the thickness direction can be formed.

In this embodiment, an example which is partly different from Embodiment 1 is described with reference toFIG. 4B.FIG. 4Ais a schematic diagram showing a superordinate concept ofFIG. 4Band shows a structure like a superlattice in which a region with a small amount of gallium and a region with a large amount of gallium are stacked. The interface between the region with a small amount of gallium and the region with a large amount of gallium is denoted by a dotted line, but the boundary is not clear and the regions are partially mixed in some cases.

This embodiment is the same as Embodiment 1 except that the composition of the second region12is different; thus, the detailed description of the same portions is omitted.

The second region12has an atomic ratio of [In]:[Ga]:[Zn]=X:Y:Z (0<Y<Z, 0<Y<X) or a neighborhood thereof and is formed using a material having a lower gallium concentration than that for the first region11. Note that the second region12with a small amount of gallium has a relatively lower concentration of gallium than the first region11with a large amount of gallium. For example, the second region12may have an atomic ratio of [In]:[Ga]:[Zn]=6:1:6 or a neighborhood thereof.

InFIG. 4B, the composite material has a structure in which a plurality of stacks each including the first region11and the second region12are provided over the base10. Each of the first region11and the second region12has a thickness of greater than or equal to 0.5 nm and less than or equal to 5 nm. A stacked structure in which at least two stacks are repeated may be used.

Electric conduction is easily caused in the second region12with a small amount of gallium, whereas electric conduction is hardly caused in the first region11with a large amount of gallium. Therefore, the composite material illustrated inFIGS. 4A and 4Bcan be used as a material with favorable semiconductor characteristics for a transistor, and the transistor can have high field-effect mobility and high switching characteristics.

A switching element, specifically a transistor including a well potential structure, can be manufactured by utilizing such a structure like a superlattice illustrated inFIG. 4B. A three-layer structure in which the second region12which serves as a well portion is sandwiched between the first regions11in the vertical direction is used so that a well portion of a transistor is sandwiched between barrier portions. The first region11and the second region12in contact with the first region11are placed in the thickness direction. Each of the first region11and the second region12contains at least indium, gallium, and zinc. Current is more likely to flow in the second region12than in the first region11. The second region12serves as a well portion of the well potential, and the second region12has a smaller bandgap than the first region11. Thus, a semiconductor element in which the second region12is sandwiched between the first regions11in the thickness direction can be formed.

In this embodiment, an example where a transistor is manufactured as a semiconductor element including the composite material described in any one of Embodiments 1 to 4 in a channel formation region is described. A structure of the transistor is described with reference toFIGS. 8A to 8C, and evaluation results of electrical characteristics of the transistor are shown inFIG. 9.

[Structure Example of Transistor]

FIG. 8Ais a top view of a transistor800A.FIG. 8Bis a cross-sectional view taken along the dashed-dotted line X1-X2inFIG. 8A.FIG. 8Cis a cross-sectional view taken along the dashed-dotted line Y1-Y2inFIG. 8A. For clarity, some components such as an insulating film810are not illustrated inFIG. 8A. As inFIG. 8A, some components are not illustrated in some cases in top views of transistors described below. In addition, the direction of the dashed-dotted line X1-X2may be referred to as the channel length (L) direction, and the direction of the dashed-dotted line Y1-Y2may be referred to as the channel width (W) direction.

The transistor800A illustrated inFIGS. 8A to 8Cincludes a conductive film806over a substrate802, an insulating film804over the conductive film806, a semiconductor film808over the insulating film804, the insulating film810over the semiconductor film808, a conductive film812over the insulating film810, and an insulating film816over the insulating film804, the semiconductor film808, and the conductive film812. The semiconductor film808is a composite material film obtained using an oxide semiconductor target and includes a channel region808ioverlapping with the conductive film812, a source region808sin contact with the insulating film816, and a drain region808din contact with the insulating film816.

The insulating film816contains nitrogen or hydrogen. The insulating film816is in contact with the source region808sand the drain region808d, so that nitrogen or hydrogen that is contained in the insulating film816is added to the source region808sand the drain region808d. The source region808sand the drain region808deach have a high carrier density when nitrogen or hydrogen is added thereto.

The transistor800A may further include an insulating film818over the insulating film816, a conductive film820aelectrically connected to the source region808sthrough an opening841aprovided in the insulating films816and818, and a conductive film820belectrically connected to the drain region808dthrough an opening841bprovided in the insulating films816and818. In addition, an insulating film822may be provided over the insulating film818, the conductive film820a, and the conductive film820b. Although the structure where the insulating film822is provided is shown inFIGS. 8B and 8C, one embodiment of the present invention is not limited thereto, and the insulating film822is not necessarily provided.

In this specification and the like, the insulating film804may be referred to as a first insulating film, the insulating film810may be referred to as a second insulating film, the insulating film816may be referred to as a third insulating film, the insulating film818may be referred to as a fourth insulating film, and the insulating film822may be referred to as a fifth insulating film. The insulating film804functions as a first gate insulating film and the insulating film810functions as a second gate insulating film. The insulating films816and818function as a protective insulating film and the insulating film822functions as a planarization film.

The insulating film810includes an excess oxygen region. Since the insulating film810includes the excess oxygen region, excess oxygen can be supplied to the channel region808iincluded in the semiconductor film808. As a result, oxygen vacancies that might be formed in the channel region808ican be filled with excess oxygen, which can provide a highly reliable semiconductor device.

To supply excess oxygen to the semiconductor film808, excess oxygen may be supplied to the insulating film804that is formed under the semiconductor film808. In that case, excess oxygen contained in the insulating film804might also be supplied to the source region808sand the drain region808dincluded in the semiconductor film808. When excess oxygen is supplied to the source region808sand the drain region808d, the resistance of the source region808sand the drain region808dmight be increased.

In contrast, in the structure in which the insulating film810formed over the semiconductor film808contains excess oxygen, excess oxygen can be selectively supplied only to the channel region808i. Alternatively, the carrier density of the source and drain regions808sand808dcan be selectively increased after excess oxygen is supplied to the channel region808iand the source and drain regions808sand808d, in which case an increase in the resistance of the source and drain regions808sand808dcan be prevented.

Furthermore, each of the source region808sand the drain region808dincluded in the semiconductor film808preferably contains an element that forms an oxygen vacancy or an element that is bonded to an oxygen vacancy. Typical examples of the element that forms an oxygen vacancy or the element that is bonded to an oxygen vacancy include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, and a rare gas. Typical examples of the rare gas element include helium, neon, argon, krypton, and xenon. The element that forms an oxygen vacancy is diffused from the insulating film816to the source region808sand the drain region808din the case where the insulating film816contains one or more such elements. In addition or alternatively, the element that forms an oxygen vacancy is added to the source region808sand the drain region808dby impurity addition treatment.

An impurity element added to the semiconductor film cuts a bond between a metal element and oxygen in the semiconductor film, so that an oxygen vacancy is formed. Alternatively, when an impurity element is added to the semiconductor film, oxygen bonded to a metal element in the semiconductor film is bonded to the impurity element and detached from the metal element, so that an oxygen vacancy is formed. As a result, the semiconductor film has a higher carrier density, and thus, the conductivity thereof becomes higher.

The conductive film806functions as a first gate electrode and the conductive film812functions as a second gate electrode. The conductive film820afunctions as a source electrode and the conductive film820bfunctions as a drain electrode.

As illustrated inFIG. 8C, an opening843is provided in the insulating films804and810. The conductive film806is electrically connected to the conductive film812through the opening843. Thus, the same potential is applied to the conductive film806and the conductive film812. Note that different potentials may be applied to the conductive film806and the conductive film812without providing the opening843. Alternatively, the conductive film806may be used as a light-shielding film without providing the opening843. For example, when the conductive film806is formed using a light-shielding material, light irradiation of the channel region808ifrom the bottom can be reduced.

As illustrated inFIGS. 8B and 8C, the semiconductor film808faces the conductive film806functioning as a first gate electrode and the conductive film812functioning as a second gate electrode and is positioned between the two conductive films functioning as the gate electrodes.

Furthermore, the length of the conductive film812in the channel width direction is larger than the length of the semiconductor film808in the channel width direction. In the channel width direction, the whole semiconductor film808is covered with the conductive film812with the insulating film810placed therebetween. Since the conductive film812is connected to the conductive film806through the opening843provided in the insulating films804and810, a side surface of the semiconductor film808in the channel width direction faces the conductive film812with the insulating film810placed therebetween.

In other words, in the channel width direction of the transistor800A, the conductive films806and812are connected to each other through the opening843provided in the insulating films804and810, and the conductive films806and812surround the oxide semiconductor film808with the insulating films804and810positioned therebetween.

Such a structure enables the semiconductor film808included in the transistor800A to be electrically surrounded by electric fields of the conductive film806functioning as a first gate electrode and the conductive film812functioning as a second gate electrode. A device structure of a transistor, like that of the transistor800A, in which electric fields of a first gate electrode and a second gate electrode electrically surround the semiconductor film808in which a channel region is formed can be referred to as a surrounded channel (S-channel) structure.

Since the transistor800A has the S-channel structure, an electric field for inducing a channel can be effectively applied to the semiconductor film808by the conductive film806or the conductive film812; thus, the current drive capability of the transistor800A can be improved and high on-state current characteristics can be obtained. As a result of the high on-state current, it is possible to reduce the size of the transistor800A. Furthermore, since the transistor800A has a structure in which the semiconductor film808is surrounded by the conductive film806and the conductive film812, the mechanical strength of the transistor800A can be increased.

When seen in the channel width direction of the transistor800A, an opening different from the opening843may be formed on the side of the semiconductor film808on which the opening843is not formed.

Next, details of the components of the transistor inFIGS. 8A to 8Cwill be described.

The substrate802can be formed using a material having heat resistance high enough to withstand heat treatment in the manufacturing process.

Specifically, non-alkali glass, soda-lime glass, potash glass, crystal glass, quartz, sapphire, or the like can be used. Alternatively, an inorganic insulating film may be used. Examples of the inorganic insulating film include a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and an aluminum oxide film.

The non-alkali glass may have a thickness of greater than or equal to 0.2 mm and less than or equal to 0.7 mm, for example. The non-alkali glass may be polished to obtain the above thickness.

As the non-alkali glass, a large-area glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured.

Alternatively, as the substrate802, a single-crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used.

For the substrate802, an inorganic material such as a metal may be used. Examples of the inorganic material such as a metal include stainless steel and aluminum.

Alternatively, for the substrate802, an organic material such as a resin, a resin film, or plastic may be used. Examples of the resin film include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxy resin, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and a resin having a siloxane bond.

For the substrate802, a composite material of an inorganic material and an organic material may be used. Examples of the composite material include a resin film to which a metal plate or a thin glass plate is bonded, a resin film into which a fibrous or particulate metal or a fibrous or particulate glass is dispersed, and an inorganic material into which a fibrous or particulate resin is dispersed.

The substrate802can at least support films or layers formed thereover or thereunder and may be one or more of an insulating film, a semiconductor film, and a conductive film.

The insulating film804can be formed by a sputtering method, a CVD method, an evaporation method, a pulsed laser deposition (PLD) method, a printing method, a coating method, or the like as appropriate. For example, the insulating film804can be formed to have a single-layer structure or stacked-layer structure of an oxide insulating film and/or a nitride insulating film. To improve the properties of the interface with the semiconductor film808, at least a region of the insulating film804which is in contact with the semiconductor film808is preferably formed using an oxide insulating film. When the insulating film804is formed using an oxide insulating film from which oxygen is released by heating, oxygen contained in the insulating film804can be moved to the semiconductor film808by heat treatment.

The thickness of the insulating film804can be greater than or equal to 50 nm, greater than or equal to 100 nm and less than or equal to 3000 nm, or greater than or equal to 200 nm and less than or equal to 1000 nm. By increasing the thickness of the insulating film804, the amount of oxygen released from the insulating film804can be increased, and interface states at the interface between the insulating film804and the semiconductor film808and oxygen vacancies included in the channel region808iof the semiconductor film808can be reduced.

For example, the insulating film804can be formed to have a single-layer structure or stacked-layer structure of a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, a gallium oxide film, a Ga—Zn oxide film, or the like. In this embodiment, the insulating film804has a stacked-layer structure of a silicon nitride film and a silicon oxynitride film. With the insulating film804having such a layered structure including a silicon nitride film as a lower layer and a silicon oxynitride film as an upper layer, oxygen can be efficiently introduced into the semiconductor film808.

For the semiconductor film808, the composite material described in any one of Embodiments 1 to 4 is used. With the composite material described in any one of Embodiments 1 to 4, high electrical characteristics (e.g., high field-effect mobility or a small S value) can be achieved.

The insulating film810has a function of supplying oxygen to the semiconductor film808, particularly to the channel region808i. The insulating film810can be formed to have a single-layer structure or a stacked-layer structure of an oxide insulating film or a nitride insulating film, for example. To improve the interface properties with the semiconductor film808, a region which is in the insulating film810and in contact with the semiconductor film808is preferably formed using at least an oxide insulating film. For example, a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, or a silicon nitride film may be used as the insulating film810.

The thickness of the insulating film810can be greater than or equal to 5 nm and less than or equal to 400 nm, greater than or equal to 5 nm and less than or equal to 300 nm, or greater than or equal to 10 nm and less than or equal to 250 nm.

It is preferable that the insulating film810have few defects and typically have as few signals observed by electron spin resonance (ESR) spectroscopy as possible. Examples of the signals include a signal due to an E′ center observed at a g-factor of 2.001. Note that the E′ center is due to the dangling bond of silicon. As the insulating film810, a silicon oxide film or a silicon oxynitride film whose spin density of a signal due to the E′ center is lower than or equal to 3×1017spins/cm3, preferably lower than or equal to 5×1016spins/cm3may be used.

In addition to the above-described signal, a signal due to nitrogen dioxide (NO2) might be observed in the insulating film810. The signal is divided into three signals according to the N nuclear spin; a first signal, a second signal, and a third signal. The first signal is observed at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039. The second signal is observed at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003. The third signal is observed at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966.

It is suitable to use an insulating film whose spin density of a signal due to nitrogen dioxide (NO2) is higher than or equal to 1×1017spins/cm3and lower than 1×1018spins/cm3as the insulating film810, for example.

Note that a nitrogen oxide (NOx) such as nitrogen dioxide (NO2) forms a state in the insulating film810. The state is positioned in the energy gap of the semiconductor film808. Thus, when nitrogen oxide (NOx) is diffused to the interface between the insulating film810and the semiconductor film808, an electron might be trapped by the state on the insulating film810side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film810and the semiconductor film808, leading to a positive shift of the threshold voltage of the transistor. Accordingly, the use of a film with a low nitrogen oxide content as the insulating film810can reduce a shift of the threshold voltage of the transistor.

As an insulating film that releases a small amount of nitrogen oxide (NOx), for example, a silicon oxynitride film can be used. The silicon oxynitride film releases more ammonia than nitrogen oxide (NOx) in thermal desorption spectroscopy (TDS); the typical released amount of ammonia is greater than or equal to 1×1018molecules/cm3and less than or equal to 5×1019molecules/cm3. Note that the released amount of ammonia is the total amount of ammonia released by heat treatment in a range of 50° C. to 650° C. or 50° C. to 550° C. in TDS.

Since nitrogen oxide (NOx) reacts with ammonia and oxygen in heat treatment, the use of an insulating film that releases a large amount of ammonia reduces nitrogen oxide (NOx).

Note that in the case where the insulating film810is analyzed by SIMS, nitrogen concentration in the film is preferably lower than or equal to 6×1020atoms/cm3.

The insulating film810may be formed using a high-k material such as hafnium silicate (HfSiOx), hafnium silicate containing nitrogen (HfSixOyNz), hafnium aluminate containing nitrogen (HfAlxOyNz), or hafnium oxide. The use of such a high-k material enables a reduction in gate leakage current of a transistor.

The insulating film816contains nitrogen or hydrogen. The insulating film816may contain fluorine. As the insulating film816, for example, a nitride insulating film can be used. The nitride insulating film can be formed using silicon nitride, silicon nitride oxide, silicon oxynitride, silicon nitride fluoride, silicon fluoronitride, or the like. The hydrogen concentration in the insulating film816is preferably higher than or equal to 1×1022atoms/cm3. Furthermore, the insulating film816is in contact with the source region808sand the drain region808dof the semiconductor film808. Thus, the concentration of an impurity (nitrogen or hydrogen) in the source region808sand the drain region808din contact with the insulating film816is increased, leading to an increase in the carrier density of the source region808sand the drain region808d.

As the insulating film818, an oxide insulating film can be used. Alternatively, a layered film of an oxide insulating film and a nitride insulating film can be used as the insulating film818. The insulating film818can be formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, or Ga—Zn oxide.

Furthermore, the insulating film818preferably functions as a barrier film against hydrogen, water, and the like from the outside.

The thickness of the insulating film818can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.

The insulating film822has an insulating property and is formed using an inorganic material or an organic material. Examples of the inorganic material include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and aluminum nitride. Examples of the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin.

The conductive films806,812,820a, and820bcan be formed by a sputtering method, a vacuum evaporation method, a PLD method, a thermal CVD method, or the like. As each of the conductive films806,812,820a, and820b, a conductive metal film, a conductive film that has a function of reflecting visible light, or a conductive film having a function of transmitting visible light may be used.

A material containing a metal element selected from aluminum, gold, platinum, silver, copper, chromium, tantalum, titanium, molybdenum, tungsten, nickel, iron, cobalt, palladium, and manganese can be used for the metal film having conductivity. Alternatively, an alloy containing any of the above metal elements may be used.

For the metal film having conductivity, specifically a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a titanium nitride film, a two-layer structure in which a copper film is stacked over a tantalum nitride film, a three-layer structure in which a titanium film, a copper film, and a titanium film are stacked in this order, or the like may be used. In particular, a conductive film containing a copper element is preferably used because the resistance can be reduced. As an example of the conductive film containing a copper element, an alloy film containing copper and manganese is given. The alloy film is favorable because it can be processed by a wet etching method.

Note that a tantalum nitride film is preferably used as the conductive films806,812,820a, and820b. A tantalum nitride film has conductivity and a high barrier property against copper or hydrogen. Because a tantalum nitride film releases little hydrogen from itself, it can be favorably used as a metal film in contact with the semiconductor film808or a metal film in the vicinity of the semiconductor film808.

As the conductive film having conductivity, a conductive macromolecule or a conductive polymer may be used.

For the conductive film having a function of reflecting visible light, a material containing a metal element selected from gold, silver, copper, and palladium can be used. In particular, a conductive film containing a silver element is preferably used because reflectance of visible light can be improved.

For the conductive film having a function of transmitting visible light, a material containing an element selected from indium, tin, zinc, gallium, and silicon can be used. Specifically, an In oxide, a Zn oxide, an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (also referred to as ITSO), an In—Zn oxide, an In—Ga—Zn oxide, or the like can be used.

As the conductive film having a function of transmitting visible light, a film containing graphene or graphite may be used. The film containing graphene can be formed in the following manner: a film containing graphene oxide is formed and is reduced. As a reducing method, a method with application of heat, a method using a reducing agent, or the like can be employed.

The conductive films812,820a, and820bcan be formed by electroless plating. As a material that can be deposited by electroless plating, for example, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. It is further favorable to use Cu or Ag because the resistance of the conductive film can be reduced.

When the conductive film is formed by electroless plating, a diffusion prevention film may be formed under the conductive film to prevent component elements of the conductive film from diffusing outward. A seed layer that can make the conductive film grow may be formed between the diffusion prevention film and the conductive film. The diffusion prevention film can be formed by a sputtering method, for example. For the diffusion prevention film, a tantalum nitride film or a titanium nitride film can be used, for example. The seed layer can be formed by an electroless plating method. For the seed layer, a material similar to the material for the conductive film that can be formed by an electroless plating method can be used.

Note that an oxide semiconductor typified by an In—Ga—Zn oxide may be used for the conductive film812. The oxide semiconductor can have a high carrier density when nitrogen or hydrogen is supplied from the insulating film. In other words, the oxide semiconductor functions as an oxide conductor (OC). Accordingly, the oxide semiconductor can be used for a gate electrode.

The conductive film812can have, for example, a single-layer structure of an oxide conductor (OC), a single-layer structure of a metal film, or a stacked-layer structure of an oxide conductor (OC) and a metal film.

Note that it is suitable that the conductive film812has a single-layer structure of a light-shielding metal film or a stacked-layer structure of an oxide conductor (OC) and a light-shielding metal film because the channel region808iformed under the conductive film812can be shielded from light. In the case where the conductive film812has a stacked-layer structure of an oxide semiconductor or an oxide conductor (OC) and a light-shielding metal film, formation of a metal film (e.g., a titanium film or a tungsten film) over the oxide semiconductor or the oxide conductor (OC) produces any of the following effects: the resistance of the oxide semiconductor or the oxide conductor (OC) is reduced by the diffusion of the constituent element of the metal film to the oxide semiconductor or oxide conductor (OC) side, the resistance is reduced by damage (e.g., sputtering damage) during the deposition of the metal film, and the resistance is reduced when oxygen vacancies are formed by the diffusion of oxygen in the oxide semiconductor or the oxide conductor (OC) to the metal film.

The thickness of the conductive films806,812,820a, and820bcan be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.

[Calculation of Density of Shallow Defect States (sDOS) in Semiconductor Film]

Calculation of a density of shallow defect states (sDOS) in a semiconductor film is described below in detail.

The number of electrons trapped by the interface state, Ntrap, can be measured by comparing drain current-gate voltage (Id-Vg) characteristics of the transistor that were actually measured and drain current-gate voltage (Id-Vg) characteristics that were calculated.

FIG. 10illustrates ideal Id-Vgcharacteristics obtained by calculation and the actually measured Id-Vgcharacteristics of the transistor when a source voltage Vsis 0 V and a drain voltage Vdis 0.1 V. Note that only values more than or equal to 1×10−13A at which drain voltage Idcan be easily measured were plotted among the measurement results of the transistor.

A change of the drain current Idwith respect to the gate voltage Vgis more gradual in the actually measured Id-Vgcharacteristics than in the ideal Id-Vgcharacteristics obtained by calculation. This is probably because an electron is trapped by a shallow interface state positioned near energy at the bottom of the conduction band (represented as Ec). In this measurement, the density of interface states Nitcan be estimated more accurately in consideration of the number of electrons (per unit area and unit energy) trapped by a shallow interface state, Ntrap, with use of the Fermi distribution function.

First, a method for evaluating the number of electrons trapped by an interface trap state, Ntrap, by using schematic Id-Vgcharacteristics illustrated inFIG. 11is described. The dashed line indicates ideal Id-Vgcharacteristics without trap state and is obtained by the calculation. On the dashed line, a change in gate voltage Vgwhen the drain current changes from Id1 to Id2 is represented by ΔVid. The solid line indicates the actually measured Id-Vgcharacteristics. On the solid line, a change in gate voltage Vgwhen the drain current changes from Id1 to Id2 is represented by ΔVex. The potential at the target interface when the drain current is Id1, the potential at the target interface when the drain current is Id2, and the amount of change are represented by ϕit1, ϕit2, and Δϕit, respectively.

The slope of the actually measured values is smaller than that of the calculated values inFIG. 11, which indicates that ΔVexis always larger than ΔVid. Here, a difference between ΔVexand ΔVidcorresponds to a potential difference that is needed for trapping of an electron in a shallow interface state. Therefore, ΔQtrapwhich the amount of change in charge due to trapped electrons can be expressed by Formula 1 shown below.
ΔQtrap=−Ctg(ΔVex−ΔVid)  [Formula 1]

Ctgis combined capacitance of an insulator and a semiconductor per unit area. In addition, ΔQtrapcan be expressed by Formula 2 by using the number of trapped electrons Ntrap(per unit area and per unit energy). Note that q represents elementary charge.
ΔQtrap=−qNtrapΔϕit[Formula 2]

Then, taking the limit zero of Δϕitin Formula 3 gives Formula 4.

In other words, the number of electrons trapped by an interface surface, Ntrap, can be estimated by using the ideal Id-Vgcharacteristics, the actually measured Id-Vgcharacteristics, and Formula 4. Note that the relationship between the drain current and the potential at the interface surface can be obtained by calculation with the device simulator described above.

The relationship between the number of electrons Ntrapper unit area and per unit energy and the density of interface states Nitis expressed by Formula 5.

Here, f(E) is Fermi distribution function. The Ntrapobtained from Formula 4 is fitted with Formula 5 to determine Nit. The conduction characteristics including Id<0.1 pA can be obtained by the device simulator to which the Nitis set.

The actually measured Id-Vgcharacteristics inFIG. 10are applied to Formula 4 and the results of extracting Ntrapare plotted as white circles inFIG. 12. The vertical axis inFIG. 12represents Fermi energy Ef at the bottom of the conduction band Ec of a semiconductor. The maximum value is positioned on the dashed line just under Ec. When tail distribution of Formula 6 is assumed as Nitof Formula 5, Ntrapcan be fitted well like the dashed line inFIG. 12. As a result, the peak value Nta=1.67×1013cm−2eV−1and the characteristic width Wta=0.105 eV are obtained as the fitting parameters.

FIGS. 13A and 13Bshow the inverse calculation results of Id-Vgcharacteristics by feeding back the obtained fitting curve of interface state to the calculation using the device simulator.FIG. 13Ashows the calculated Id-Vgcharacteristics when the drain voltage Vdis 0.1 V and 1.8V and the actually measured Id-Vgcharacteristics when the drain voltage Vdis 0.1 V and 1.8V.FIG. 13Bis a graph in which the drain current Idis a logarithm inFIG. 13A.

The curve obtained by the calculation substantially matches with the plot of the actually measured values, which suggests that the calculated values and the measured values are highly reproducible. Accordingly, the above method is quite appropriate as a method for calculating the density of shallow defect states.

The sDOS in the semiconductor film including the above composite material influences the mobility curve of the field-effect mobility. In particular, in the vicinity of the threshold voltage, the shape of the mobility curve is changed because electrons are trapped by the sDOS. The sDOS in the semiconductor film including the composite material is represented by the product of Ntaand Wtain Formula 6 and tOS.

Providing transistors having different electrical characteristics over the same layer can increase the degree of freedom in design of a semiconductor device and the integration degree in the semiconductor device. In this embodiment, an example of an embodiment where transistors having different electrical characteristics are provided over the same layer while an increase in the number of manufacturing steps is suppressed will be described.

<Structure Example of Semiconductor Device1000>

FIG. 14Ais a cross-sectional view of a semiconductor device1000. The semiconductor device1000includes a transistor100and a transistor200. The transistors100and200have different structures.FIG. 14Aillustrates cross sections of the transistors100and200over the substrate101.FIG. 14Acorresponds to a portion taken along dashed-dotted line L1-L2in the top view inFIG. 15.

FIG. 15is a plan view of the semiconductor device1000.FIG. 16is a cross-sectional view of a portion taken along dashed-dotted line L1-L2inFIG. 15.FIGS. 17A to 17Care cross-sectional views of portions taken along dashed-dotted lines W1-W2, W3-W4, and W5-W6inFIG. 15. InFIG. 16, the cross-sectional view along L1-L2is taken in the channel length direction of the semiconductor device1000, and inFIGS. 17A to 17C, the cross-sectional views along W1-W2, W3-W4, and W5-W6are taken in the channel width direction of the semiconductor device1000.

FIGS. 14B and 14Ceach show an example of a Vg-Idcurve, which is one of the electrical characteristics of a transistor. In the Vg-Idcurves inFIGS. 14B and 14C, the horizontal axis and the vertical axis represent voltage between a gate and a source (Vg) of the transistor and current flowing to a drain (Id) of the transistor on a logarithmic scale, respectively.

The transistor100is a transistor including a back gate.FIG. 14Bshows an example of the Vg-Idcurve of the transistor100when the potential of the back gate is set to the same as that of the source or the gate.FIG. 14Cshows an example of the Vg-Idcurve of the transistor200when the potential of the back gate is set to the same as that of the source or the gate. As shown inFIGS. 14B and 14C, the transistors100and200have different transistor characteristics. The Vgin the Vg-Idcurve of the transistor200is shifted in the positive direction compared with that in the Vg-Idcurve of the transistor100. In other words, the transistor200has higher Vththan the transistor100.

The transistors100and200are described with reference to drawings.

The transistor100is a kind of top-gate transistor. The transistor100includes an electrode105_1a, an electrode105_1b, an electrode105_1c, an insulating layer108, a semiconductor layer109_1a, a semiconductor layer109_1b, a semiconductor layer109_1c, an electrode110_1a, an electrode110_1b, a layer129_1a, a layer129_1b, an insulating layer111_1, an electrode112_1, and an insulating layer113_1(seeFIG. 15,FIG. 16, andFIGS. 17A to 17C).

The transistor100illustrated inFIG. 15,FIG. 16, andFIGS. 17A to 17Cis provided over the substrate101with an insulating layer102and an insulating layer103located therebetween. Specifically, an insulating layer104is provided over the insulating layer103, the insulating layers103and104are partly removed, and the electrode105_1aand the electrode105_1bare embedded. The electrode105_1cis provided over the electrode105_1aand the electrode105_1b, and the insulating layer108is provided over the electrode105_1cand the insulating layer104. The insulating layer108has a projection, the semiconductor layer109_1ais provided over the projection, and the semiconductor layer109_1bis provided over the semiconductor layer109_1a.

The transistor100includes the electrode110_1aover the semiconductor layer109_1band the electrode110_1bover the semiconductor layer109_1b. One of the electrode110_1aand the electrode110_1bcan function as one of a source electrode and a drain electrode, and the other can function as the other of the source electrode and the drain electrode. Thus, a portion of the semiconductor layer109_1bcan function as the source region and another portion can function as the drain region. Furthermore, a portion of the semiconductor layer109_1bbetween the source region and the drain region can function as a channel formation region.

The transistor100further includes the layer129_1aover the electrode110_1aand the layer129_1bover the electrode110_1b. The semiconductor layer109_1ccovering the layer129_1a, the layer129_1b, the electrode110_1a, the electrode110_1b, the semiconductor layer109_1b, and the semiconductor layer109_1ais provided. The semiconductor layer109_1cincludes a region in contact with a side surface of the electrode110_1a, a region in contact with a side surface of the electrode110_1b, a region in contact with the portion of the semiconductor layer109_1b, a region in contact with a side surface of the semiconductor layer109_1b, and a region in contact with a side surface of the semiconductor layer109_1a.

The insulating layer111_1is provided over the semiconductor layer109_1c, and the electrode112_1is provided over the insulating layer111_1. The insulating layer111_1and the electrode112_1include a region overlapping with the portion of the semiconductor layer109_1b.

The transistor100further includes the insulating layer113_1over the electrode112_1. The insulating layers111_1and113_1extend beyond an end portion of the electrode112_1and have a region where the insulating layers111_1and113_1are in contact with each other in the extended portion.

In this embodiment, an insulating layer114is provided over the semiconductor layer109_1cand the insulating layer113_1.

An electrode116_1ais provided in an opening which overlaps with the electrode110_1aand which is provided in the insulating layer114, the semiconductor layer109_1c, and the layer129_1a. An electrode116_1bis provided in an opening which overlaps with the electrode110_1band which is provided in the insulating layer114, the semiconductor layer109_1c, and the layer129_1a. An electrode116_1cis provided in an opening which overlaps with the electrode112_1and which is provided in the insulating layer115, the insulating layer114, and the insulating layer113_1.

In this embodiment, an electrode117_1a, an electrode117_1b, and an electrode117_1care provided over the electrode116_1a, the electrode116_1b, and the electrode116_1c, respectively. The electrode117_1ais electrically connected to the electrode110_1athrough the electrode116_1a. The electrode117_1bis electrically connected to the electrode110_1bthrough the electrode116_1b. The electrode117_1cis electrically connected to the electrode112_1through the electrode116_1c.

In this embodiment, the insulating layer115is provided over the insulating layer114, the electrode116_1c, the electrode116_1b, and the electrode116_1c.

[Gate Electrode and Back Gate Electrode]

One of the electrodes105_1and112_1can function as a gate electrode and the other can function as a back gate electrode. In general, a gate electrode and a back gate electrode are formed using a conductive layer and positioned so that the channel formation region of the semiconductor layer is located between the gate electrode and the back gate electrode. Thus, the back gate electrode can function in a manner similar to that of the gate electrode. The potential of the back gate electrode may be the same as that of the gate electrode or may be a ground potential or a predetermined potential. By changing the potential of the back gate electrode independently of the potential of the gate electrode, the threshold voltage of the transistor can be changed.

The electrode105_1and the electrode112_1can each function as a gate electrode. Thus, the insulating layers106,107,108, and111_1can each function as a gate insulating layer.

In the case where one of the electrode105_1and the electrode112_1is referred to as a “gate electrode” or a “gate”, the other can be referred to as a “back gate electrode” or a “back gate”. For example, in the transistor100, in the case where the electrode105_1is referred to as a “gate electrode”, the electrode112_1is referred to as a “back gate electrode”. In the case where the electrode112_1is used as a “gate electrode”, the transistor100can be regarded as a kind of bottom-gate transistor. Alternatively, one of the electrode105_1and the electrode112_1may be referred to as a “first gate electrode” or a “first gate”, and the other may be referred to as a “second gate electrode” or a “second gate”.

By providing the electrode105_1and the electrode112_1with the semiconductor layer109_1bprovided therebetween and setting the potentials of the electrode105_1and the electrode112_1to be the same, a region of the semiconductor layer109_1bthrough which carriers flow is enlarged in the thickness direction; thus, the number of transferred carriers is increased. As a result, the on-state current and the field-effect mobility of the transistor100are increased.

Therefore, the transistor100has large on-state current for its area. That is, the area occupied by the transistor100can be small for required on-state current. Therefore, a semiconductor device having a high degree of integration can be provided.

Furthermore, the gate electrode and the back gate electrode are formed using conductive layers and thus each have a function of preventing an electric field generated outside the transistor from influencing the semiconductor layer in which the channel is formed (in particular, an electric field blocking function against static electricity and the like). When the back gate electrode is formed larger than the semiconductor layer to cover the semiconductor layer in the plan view, the electric field blocking function can be enhanced.

Since the electrode105_1and the electrode112_1each have a function of blocking an electric field from the outside, charges of charged particles and the like generated over the electrode112_1or under the electrode105_1do not influence the channel formation region in the semiconductor layer109_1b. Thus, degradation due to a stress test (e.g., a negative gate bias temperature (−GBT) stress test in which negative charges are applied to a gate) can be reduced. In addition, the electrode105_1and the electrode112_1can block an electric field generated from the drain electrode so as not to affect the semiconductor layer. Thus, changes in the rising voltage of on-state current due to changes in drain voltage can be suppressed. Note that this effect is significant when a potential is applied to the electrodes105_1and112_1.

The GBT stress test is one kind of acceleration test and can evaluate, in a short time, a change by long-term use (i.e., a change over time) in characteristics of a transistor. In particular, the amount of change in threshold voltage of the transistor between before and after the GBT stress test is an important indicator when the reliability of the transistor is examined. If the amount of change in the threshold voltage between before and after the GBT stress test is small, the transistor has higher reliability.

By providing the electrodes105_1and112_1and setting the potentials of the electrodes105_1and112_1to be the same, the amount of change in threshold voltage is reduced. Accordingly, a variation in electrical characteristics among a plurality of transistors is also reduced.

The transistor including the back gate electrode has a smaller change in threshold voltage by a positive GBT stress test in which positive charges are applied to a gate than a transistor including no back gate electrode.

In the case where light is incident on the back gate electrode side, when the back gate electrode is formed using a light-blocking conductive film, light can be prevented from entering the semiconductor layer from the back gate electrode side. Therefore, photodegradation of the semiconductor layer can be prevented and deterioration in electrical characteristics of the transistor, such as a shift of the threshold voltage, can be prevented.

The transistor200is a kind of top-gate transistor. The transistor200includes an electrode105_2a1, an electrode105_2b1, an electrode105_2c1, an electrode105_2a2, an electrode105_2b2, an electrode105_2c2, the insulating layer108, a semiconductor layer109_2c, an insulating layer111_2, an electrode112_2, and an insulating layer113_2(seeFIG. 15,FIG. 16, andFIGS. 17A to 17C).

The transistor200illustrated inFIG. 15,FIG. 16, andFIGS. 17A to 17Cis provided over the substrate101with the insulating layer102and the insulating layer103located therebetween. Specifically, the insulating layer104is provided over the insulating layer103, the insulating layers103and104are partly removed, and the electrode105_2a1, the electrode105_2b1, the electrode105_2a2, and the electrode105_2b2are embedded. The electrode105_2c1is provided over the electrode105_2a1and the electrode205_2b1, and the electrode105_2c2is provided over the electrode105_2a2and the electrode105_2b2. The insulating layer108is provided over the electrode105_2c1, the electrode105_2c2, and the insulating layer104. The electrode105_2a1, the electrode105_2b1, the electrode105_2c1, the electrode105_2a2, the electrode105_2b2, and the electrode105_2c2can function as one of a source electrode and a drain electrode. When the electrode105_2a1, the electrode105_2b1, and the electrode105_2c1function as one of the source electrode and the drain electrode, the electrode105_2a2, the electrode105_2b2, and the electrode105_2c2function as the other of the source electrode and the drain electrode.

The transistor200includes the semiconductor layer109_2cover the insulating layer108.

A portion of the semiconductor layer109_2cis a first portion overlapping with the electrode105_2c1. Another portion of the semiconductor layer109_2cis a second portion overlapping with the electrode105_2c2. Furthermore, another portion of the semiconductor layer109_2cbetween the first portion and the second portion can function as a channel formation region.

The transistor200includes the insulating layer111_2over the semiconductor layer109_2cand the electrode112_2over the insulating layer111_2. The insulating layer111_2and the electrode112_2include a region overlapping with the portion of the semiconductor layer109_2c.

The transistor200further includes the insulating layer113_2over the electrode112_2. The insulating layers111_2and113_2extend beyond an end portion of the electrode112_2and have a region where the insulating layers111_2and113_2are in contact with each other in the extended portion.

In this embodiment, the insulating layer114is provided over the semiconductor layer109_2cand the insulating layer113_2, and the insulating layer115is provided over the insulating layer114.

In this specification and the like, the electrode105_1a, the electrode105_1b, and the electrode105_1cmay be collectively referred to as an “electrode105”, for example. The semiconductor layer109_1cand the semiconductor layer109_2cmay be collectively referred to as a “semiconductor layer109c”, for example. The electrode105_2aland the electrode105_2a2may be collectively referred to as an “electrode105_2a”. The electrode105_1a, the electrode105_2a1, and the electrode105_2a2may be collectively referred to as an “electrode105a”. In the other structures, similar description may be used.

In the transistor100, a channel is formed in the semiconductor layer109b. In the transistor200, a channel is formed in the semiconductor layer109c. The semiconductor layer109band the semiconductor layer109care preferably formed using semiconductor materials having different physical properties. When the semiconductor layer109band the semiconductor layer109care formed using semiconductor materials having different physical properties, the transistor100and the transistor200can have different electrical characteristics. When semiconductors having different energy bandgaps are used for the semiconductor layers109band109c, for example, the transistors100and200can have different field-effect mobilities.

When a semiconductor having lower electron affinity than that of the semiconductor layer109bis used for the semiconductor layer109c, for example, the transistor200can have higher Vththan the transistor100.

When the semiconductor layer109_2b1is provided in contact with the electrode110_2aand the semiconductor layer109_2c, variation in electrical characteristics of the transistor200can be reduced. When the semiconductor layer109_2b2is provided in contact with the electrode110_2band the semiconductor layer109_2c, variation in electrical characteristics of the transistor200can be reduced.

There is no particular limitation on a material used for the substrate101as long as the material has heat resistance high enough to withstand at least heat treatment performed later. For example, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like or a compound semiconductor substrate made of silicon germanium or the like can be used as the substrate101. Alternatively, an SOI substrate, a semiconductor substrate on which a semiconductor element such as a strained transistor or a FIN-type transistor is provided, or the like can also be used. Alternatively, gallium arsenide, aluminum gallium arsenide, indium gallium arsenide, gallium nitride, indium phosphide, silicon germanium, or the like that can be used for a high-electron-mobility transistor (HEMT) may be used. The substrate101is not limited to a simple supporting substrate, and may be a substrate where a device such as a transistor is formed. In this case, at least one of the gate, the source, and the drain of the transistor100and/or the transistor200may be electrically connected to the device.

Further alternatively, as the substrate101, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, or a sapphire substrate can be used. Note that a flexible substrate may be used as the substrate101. In the case where a flexible substrate is used, the transistor, a capacitor, or the like may be directly formed over the flexible substrate; or the transistor, the capacitor, or the like may be formed over a manufacturing substrate and then separated from the manufacturing substrate and transferred onto the flexible substrate. To separate and transfer the transistor, the capacitor, or the like from the manufacturing substrate to the flexible substrate, a separation layer may be provided between the manufacturing substrate and the transistor, the capacitor, or the like.

For the flexible substrate, for example, metal, an alloy, resin, glass, or fiber thereof can be used. The flexible substrate used as the substrate101preferably has a lower coefficient of linear expansion because deformation due to an environment is suppressed. The flexible substrate used as the substrate101is formed using, for example, a material whose coefficient of linear expansion is lower than or equal to 1×10−3/K, lower than or equal to 5×10−5/K, or lower than or equal to 1×10−5/K. Examples of the resin include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, and acrylic. In particular, aramid is preferably used for the flexible substrate because of its low coefficient of linear expansion.

The insulating layers102to104,108,111, and113to115can be formed with a single layer or a stack of layers of one or more materials selected from aluminum nitride, aluminum oxide, aluminum nitride oxide, aluminum oxynitride, magnesium oxide, silicon nitride, silicon oxide, silicon nitride oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, and aluminum silicate. Alternatively, a material in which two or more materials selected from an oxide material, a nitride material, an oxynitride material, and a nitride oxide material are mixed may be used.

Note that in this specification, a nitride oxide refers to a compound that includes more nitrogen than oxygen. An oxynitride refers to a compound that includes more oxygen than nitrogen. The content of each element can be measured by Rutherford backscattering spectrometry (RBS), for example.

It is particularly preferable that the insulating layer102and/or the insulating layer103be formed using an insulating material that is relatively impermeable to impurities. The insulating layer102and/or the insulating layer103may each be formed to have, for example, a single-layer structure or a stacked-layer structure including an insulating material containing boron, carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon, phosphorus, argon, gallium, germanium, yttrium, zirconium, lanthanum, neodymium, hafnium, or tantalum. Examples of such an insulating material include aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, and silicon nitride.

When the insulating material that is relatively impermeable to impurities is used for the insulating layer102and/or the insulating layer103, impurity diffusion from the substrate101side can be suppressed, and the reliability of the transistor can be improved. When the insulating material that is relatively impermeable to impurities is used for the insulating layer115, impurity diffusion from layers above the insulating layer115can be suppressed, and the reliability of the transistor can be improved.

Note that a stack of a plurality of insulating layers formed with these materials may be used as the insulating layer102and/or the insulating layer103. One of the insulating layer102and the insulating layer103may be omitted.

When a composite material obtained using a target containing an oxide semiconductor is used for the semiconductor layer109, the hydrogen concentrations in the insulating layers are preferably lowered in order to prevent an increase in the hydrogen concentration in the semiconductor layer109. Specifically, the hydrogen concentration in the insulating layer that is measured by SIMS is set lower than or equal to 2×1020atoms/cm3, preferably lower than or equal to 5×1019atoms/cm3, more preferably lower than or equal to 1×1019atoms/cm3, still more preferably lower than or equal to 5×1018atoms/cm3. It is particularly preferable to lower the hydrogen concentrations of the insulating layers104,106to108,111, and114. It is preferable to lower at least the hydrogen concentrations of the insulating layers108,111, and114in contact with the semiconductor layer109.

Furthermore, the nitrogen concentrations in the insulating layers are preferably low in order to prevent an increase in the nitrogen concentration in the semiconductor layer109. Specifically, the nitrogen concentration in the insulating layer, which is measured by SIMS, is set lower than or equal to 5×1019atoms/cm3, preferably lower than or equal to 5×1018atoms/cm3, further preferably lower than or equal to 1×1018atoms/cm3, still further preferably lower than or equal to 5×1017atoms/cm3.

It is preferable that a region of the insulating layer108which is in contact with at least the semiconductor layer109and a region of the insulating layer111which is in contact with at least the semiconductor layer109have few defects and typically have as few signals observed by electron spin resonance (ESR) spectroscopy as possible. Examples of the signals include a signal due to an E′ center observed at a g-factor of 2.001. Note that the E′ center is due to the dangling bond of silicon. As the insulating layer108and the insulating layer111, a silicon oxide layer or a silicon oxynitride layer whose spin density due to the E′ center is lower than or equal to 3×1017spins/cm3, preferably lower than or equal to 5×1016spins/cm3can be used.

In addition to the above-described signal, a signal due to nitrogen dioxide (NO2) might be observed. The signal is divided into three signals according to the N nuclear spin; a first signal, a second signal, and a third signal. The first signal is observed at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039. The second signal is observed at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003. The third signal is observed at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966.

It is suitable to use an insulating layer whose spin density of a signal due to nitrogen dioxide (NO2) is higher than or equal to 1×1017spins/cm3and lower than 1×1018spins/cm3as the insulating layer108and the insulating layer111, for example.

Note that nitrogen oxide (NOx) such as nitrogen dioxide (NO2) forms a state in the insulating layer. The state is positioned in the energy gap of the oxide semiconductor layer. Thus, when nitrogen oxide (NOx) is diffused to the interface between the insulating layer and the oxide semiconductor layer, an electron can potentially be trapped by the state on the insulating layer side. As a result, the trapped electrons remain in the vicinity of the interface between the insulating layer and the oxide semiconductor layer; thus, the threshold voltage of the transistor is shifted in the positive direction. Therefore, a shift in the threshold voltage of the transistor can be reduced when a film with a low nitrogen oxide content is used as the insulating layer108and the insulating layer111.

As an insulating layer that releases little nitrogen oxide (NOx), for example, a silicon oxynitride layer can be used. The silicon oxynitride layer is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide (NOx) in thermal desorption spectroscopy (TDS); the typical amount of released ammonia is greater than or equal to 1×1018/cm3and less than or equal to 5×1019/cm3. Note that the released amount of ammonia is the total amount of ammonia released by heat treatment in a range from 50° C. to 650° C. or a range from 50° C. to 550° C. in TDS.

Since nitrogen oxide (NOx) reacts with ammonia and oxygen in heat treatment, the use of an insulating layer that releases a large amount of ammonia reduces nitrogen oxide (NOx).

At least one of the insulating layers108,111, and114is preferably formed using an insulating layer from which oxygen is released by heating. Specifically, it is preferable to use an insulating layer of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×1018atoms/cm3, preferably greater than or equal to 3.0×1020atoms/cm3in TDS analysis. Note that oxygen released by heating is also referred to as excess oxygen.

The insulating layer containing excess oxygen can be formed by performing treatment for adding oxygen to an insulating layer. The treatment for adding oxygen can be performed by heat treatment under an oxygen atmosphere, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like. As a gas for adding oxygen, an oxygen gas of16O2,18O2, or the like, a nitrous oxide gas, an ozone gas, or the like can be used. In this specification, the treatment for adding oxygen is also referred to as “oxygen doping treatment”.

A heat-resistant organic material such as a polyimide, an acrylic-based resin, a benzocyclobutene-based resin, a polyamide, or an epoxy-based resin may be used to form the insulating layer114. Other than the above organic materials, a low-dielectric constant material (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like can be used. Note that the insulating layer114may be formed by stacking a plurality of insulating layers formed using any of these materials.

Note that the siloxane-based resin corresponds to a resin including an Si—O—Si bond formed using a siloxane-based material as a starting material. The siloxane-based resin may contain, as a substituent, an organic group (e.g., an alkyl group or an aryl group) or a fluoro group. The organic group may contain a fluoro group.

There is no particular limitation on the method for forming the insulating layer114, and any of the following methods which depend on a material thereof can be used: a sputtering method; an SOG method; spin coating; dipping; spray coating; a droplet discharging method (e.g., an ink-jet method); a printing method (e.g., screen printing or offset printing); or the like. When the baking step of the insulating layer114also serves as heat treatment for another layer, the transistor can be manufactured efficiently.

Any of the above insulating layers may be used as the layer129. In the case where the layer129is an insulating layer, an insulating layer which is less likely to release oxygen and/or which is less likely to absorb oxygen is preferably used.

As a conductive material for forming the electrodes105,110,112,116, and117, a material containing one or more metal elements selected from aluminum, chromium, copper, silver, gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten, hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium, indium, and the like can be used. Alternatively, a semiconductor having a high electric conductivity typified by polycrystalline silicon including an impurity element such as phosphorus, or a silicide such as nickel silicide may be used.

A conductive material containing the above metal element and oxygen may be used. A conductive material containing the above metal element and nitrogen may be used. For example, a conductive material containing nitrogen such as titanium nitride or tantalum nitride may be used. Indium tin oxide (ITO), indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, indium tin oxide to which silicon is added, or indium gallium zinc oxide containing nitrogen may be used.

A stack of a plurality of conductive layers formed with the above materials may be used. For example, a stacked-layer structure formed using a material containing the above metal element and a conductive material containing oxygen may be used. Alternatively, a stacked-layer structure formed using a material containing the above metal element and a conductive material containing nitrogen may be used. Further alternatively, a stacked-layer structure formed using a material containing the above metal element, a conductive material containing oxygen, and a conductive material containing nitrogen may be used.

Note that in the case where the above composite material is used for the semiconductor layer and a stacked-layer structure formed using a combination of a material including any of the metal elements listed above and a conductive material including oxygen is used as the gate electrode, the conductive material including oxygen is preferably provided on the semiconductor layer side. By providing the conductive material including oxygen on the semiconductor layer side, oxygen released from the conductive material is likely to be supplied to the semiconductor layer.

The electrode116may be formed using, for example, a conductive material with high embeddability, such as tungsten or polysilicon. A conductive material with high embeddability and a barrier layer (a diffusion prevention layer) such as a titanium layer, a titanium nitride layer, or a tantalum nitride layer may be used in combination. Note that the electrode116may be referred to as a contact plug.

In particular, the electrode105in contact with the insulating layers103and104is preferably formed using a conductive material that is relatively impermeable to impurities. Furthermore, the electrode116in contact with the insulating layers114and115is preferably formed using a conductive material that is relatively impermeable to impurities. As an example of the conductive material that is relatively impermeable to impurities, tantalum nitride can be given.

When the insulating layers103and115are formed using an insulating material that is relatively impermeable to impurities and the electrodes105and116are formed using a conductive material that is relatively impermeable to impurities, diffusion of impurities into the transistors100and200can be further suppressed. Thus, the reliability of the transistors100and200can be further increased.

Any of the above conductive materials may be used for the layer129. In the case where the layer129is formed using a conductive material, a conductive material which is less likely to release oxygen and/or which is less likely to absorb oxygen is preferably used.

For the semiconductor layer109, any one of the composite materials in Embodiments 1 to 4 is used. The composite material has a structure including a plurality of stacks of the first region and the second region. The thickness of each of the first region and the second region is greater than or equal to 0.5 nm and less than or equal to 5 nm. The semiconductor layer109has a stacked structure in which at least three stacks are repeated.

A switching element, specifically a transistor including a well potential structure, can be manufactured by utilizing a structure like a superlattice included in the composite material.

The transistors disclosed in Embodiment 6 can be used in a variety of semiconductor circuits, e.g., logic circuits such as an OR circuit, an AND circuit, a NAND circuit, and a NOR circuit, an inverter circuit, a buffer circuit, a shift register circuit, a flip-flop circuit, an encoder circuit, a decoder circuit, an amplifier circuit, an analog switch circuit, an integrator circuit, a differentiation circuit, a memory element, and the like.

Examples of a semiconductor circuit including the transistor disclosed in this specification and the like are illustrated in circuit diagrams inFIGS. 18A to 18C. In the circuit diagrams and the like, “OS” is given beside the circuit symbol of a transistor which is preferably an OS transistor.

The semiconductor circuit illustrated inFIG. 18Ahas a configuration of an inverter circuit in which the p-channel transistor281and the n-channel transistor282are connected to each other in series and in which gates of the transistors are connected to each other.

The semiconductor circuit illustrated inFIG. 18Bhas a configuration of an analog switch circuit in which the p-channel transistor281and the n-channel transistor282are connected to each other in parallel.

The semiconductor circuit illustrated inFIG. 18Chas a configuration of a NAND circuit including a transistor281a, a transistor281b, a transistor282a, and a transistor282b. A potential output from the NAND circuit changes depending on the combination of potentials input to an input terminal IN_A and an input terminal IN_B.

<Example of Memory Element>

The semiconductor circuit illustrated inFIG. 19Ahas a configuration of a memory element251ain which one of a source and a drain of a transistor262is connected to a gate of a transistor263and one electrode of a capacitor258. The circuit illustrated inFIG. 19Bhas a configuration of a memory element261ain which one of the source and the drain of the transistor262is connected to one electrode of the capacitor258.

In each of the memory elements251aand261a, charges injected through the wiring254and the transistor262can be stored at a node257. The transistor262is an OS transistor, which enables charges to be stored at the node257for a long period.

The memory element251aincludes the transistor263. Although the transistor263is a p-channel transistor inFIG. 19A, the transistor263may be an n-channel transistor. For example, the transistor281or the transistor282may be used as the transistor263. An OS transistor may also be used as the transistor263.

The memory element251aillustrated inFIG. 19Aand the memory element261aillustrated inFIG. 19Bare described in detail here.

The memory element251aincludes the transistor263using a first semiconductor, the transistor262using a second semiconductor, and the capacitor258.

As the transistor262, any of the OS transistors disclosed in the above embodiments can be used. When a transistor having a low off-state current is used as the transistor262, data can be retained at the node257for a long period. In other words, power consumption of the memory element can be reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low.

InFIG. 19A, a wiring252is electrically connected to one of a source and a drain of the transistor263, and a wiring253is electrically connected to the other of the source and the drain of the transistor263. The wiring254is electrically connected to one of the source and the drain of the transistor262, and a wiring255is electrically connected to a gate of the transistor262. The gate of the transistor263, the other of the source and the drain of the transistor262, and the one electrode of the capacitor258are electrically connected to the node257. A wiring256is electrically connected to the other electrode of the capacitor258.

The memory element251ainFIG. 19Ahas a feature that the charges supplied to the node257can be retained, and thus enables writing, retaining, and reading of data as follows.

Data writing and retaining operations of the memory element251awill be described. First, the potential of the wiring255is set to a potential at which the transistor262is on. Accordingly, the potential of the wiring254is supplied to the node257. That is, a predetermined charge is supplied to the node257(writing). Here, one of two kinds of charges providing different potential levels (hereinafter also referred to as a “low-level charge” and a “high-level charge”) is supplied. After that, the potential of the wiring255is set to a potential at which the transistor262is off. Thus, the charge is retained at the node257(retaining operation).

Note that the high-level charge is a charge for supplying a higher potential to the node257than the low-level charge. In the case where the transistor263is a p-channel transistor, each of the high-level and low-level charges is a charge for supplying a potential higher than the threshold voltage of the transistor263. In the case where the transistor263is an n-channel transistor, each of the high-level and low-level charges is a charge for supplying a potential lower than the threshold voltage of the transistor263. In other words, each of the high-level and low-level charges is a charge for supplying a potential at which the transistor263is off.

Next, operation of reading data will be described. A reading potential VRis supplied to the wiring256while a predetermined potential (a constant potential) different from the potential of the wiring253is supplied to the wiring252, whereby data retained at the node257can be read.

The reading potential VRis set to {(Vth−VH)+(Vth+VL)}/2, where VHis the potential supplied in the case of the high-level charge and VLis the potential supplied in the case of the low-level charge. Note that the potential of the wiring256in a period during which data is not read is set to a potential higher than VHin the case where the transistor263is a p-channel transistor, and is set to a potential lower than VLin the case where the transistor263is an n-channel transistor.

For example, in the case where the transistor263is a p-channel transistor, VRis −2 V when Vthof the transistor263is −2 V, VHis 1 V, and VLis −1 V. When the potential written to the node257is VHand VRis applied to the wiring256, VR+VH, i.e., −1 V, is applied to the gate of the transistor263. Since −1 V is higher than Vth, the transistor263is not turned on. Thus, the potential of the wiring253is not changed. When the potential written to the node257is VLand VRis applied to the wiring256, VR+VL, i.e., −3 V, is applied to the gate of the transistor263. Since −3 V is lower than Vth, the transistor263is turned on. Thus, the potential of the wiring253is changed.

In the case where the transistor263is an n-channel transistor, VRis 2 V when Vthof the transistor263is 2 V, VHis 1 V, and VLis −1 V. When the potential written to the node257is VHand VRis applied to the wiring256, VR+VH, i.e., 3 V, is applied to the gate of the transistor263. Since 3 V is higher than Vth, the transistor263is turned on. Thus, the potential of the wiring253is changed. When the potential written to the node257is VLand VRis applied to the wiring256, VR+VL, i.e., 1 V, is applied to the gate of the transistor263. Since 1 V is lower than Vth, the transistor263is not turned on. Thus, the potential of the wiring253is not changed.

By determining the potential of the wiring253, data retained at the node257can be read.

The memory element261aillustrated inFIG. 19Bis different from the memory element251ain that the transistor263is not provided. The other electrode of the capacitor258is electrically connected to a wiring264. The potential of the wiring264may be any potential as long as it is a fixed potential. For example, the wiring264may be supplied with GND. Data can be written to the memory element261ain a manner similar to that of the memory element251a.

Operation of reading data of the memory element261awill be described. When a potential at which the transistor262is turned on is supplied to the wiring255, the wiring254which is in a floating state and the capacitor258are brought into conduction, and the charge is redistributed between the wiring254and the capacitor258. As a result, the potential of the wiring254is changed. The amount of change in the potential of the wiring254varies depending on the potential of the node257(or the charge accumulated in the node257).

For example, the potential of the wiring254after the charge redistribution is (CB×VB0+C×V)/(CB+C), where V is the potential of the node257, C is the capacitance of the capacitor258, CBis the capacitance component of the wiring254, and VB0is the potential of the wiring254before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the node257is V1and V0(V1>V0), the potential of the wiring254in the case of retaining the potential V1(=(CB×VB0+C×V)/(CB+C)) is higher than the potential of the wiring254in the case of retaining the potential V0(=(CB×VB0+C×V0)/(CB+C)).

Then, by comparing the potential of the wiring254with a predetermined potential, data can be read.

When including a transistor using a composite material and having an extremely low off-state current, the memory element described above can retain stored data for a long time. In other words, power consumption of the semiconductor device can be reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be retained for a long time even when power is not supplied (note that a potential is preferably fixed).

In the memory element, high voltage is not needed for data writing and deterioration of the element is less likely to occur. Unlike in a conventional nonvolatile memory, for example, it is not necessary to inject and extract electrons into and from a floating gate; thus, a problem such as deterioration of an insulator is not caused. That is, the memory element of one embodiment of the present invention does not have a limit 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/off state of the transistor, whereby high-speed operation can be achieved.

The transistor262may be a transistor including a back gate. By controlling the potential supplied to the back gate, the threshold voltage of the transistor262can be appropriately changed. A memory element251billustrated inFIG. 19Chas substantially the same circuit configuration as the memory element251a. The memory element251bis different from the memory element251ain that a transistor including a back gate is used as the transistor262. A memory element261billustrated inFIG. 19Dhas substantially the same circuit configuration as the memory element261a. The memory element261bis different from the memory element261ain that a transistor including a back gate is used as the transistor262.

In each of the memory elements251band261b, the back gate of the transistor262is electrically connected to a wiring259. By controlling the potential supplied to the wiring259, the threshold voltage of the transistor262can be appropriately changed.

<Examples of Memory Device>

FIGS. 20A and 20Bare circuit diagrams showing examples of a memory device including any of the above memory elements. A memory device300illustrated inFIG. 20Aincludes a memory circuit310and a voltage retention circuit320. A memory device300aillustrated inFIG. 20Bincludes a memory circuit310aand the voltage retention circuit320. The memory circuits310and310aeach include a plurality of memory elements.FIGS. 20A and 20Billustrate the case where three memory elements261b(memory elements261b_1to261b_3) are provided.

In the memory device300illustrated inFIG. 20A, the memory element261b_1included in the memory circuit310is electrically connected to a wiring255_1and a wiring254_1. The memory element261b_2included in the memory circuit310is electrically connected to a wiring255_2and a wiring254_2. The memory element261b_3included in the memory circuit310is electrically connected to a wiring255_3and a wiring254_3. The memory elements261b_1to261b_3included in the memory circuit310are electrically connected to the wiring264.

In the memory device300aillustrated inFIG. 20B, the memory elements261b_1to261b_3included in the memory circuit310aare electrically connected to the wiring255. The memory element261b_1included in the memory circuit310ais electrically connected to the wiring254_1and a wiring264_1. The memory element261b_2included in the memory circuit310ais electrically connected to the wiring254_2and a wiring264_2. The memory element261b_3included in the memory circuit310ais electrically connected to the wiring254_3and a wiring264_3.

Note that the above description of the memory element261bcan be referred to for configurations, operations, and the like of the memory elements261b_1to261b_3. Thus, detailed description is omitted here.

The voltage retention circuit320includes a transistor323and a capacitor324. InFIGS. 20A and 20B, the transistor323is a transistor including a back gate; however, the transistor323may be a transistor without a back gate. InFIGS. 20A and 20B, one of a source and a drain of the transistor323is electrically connected to a terminal321. The other of the source and the drain, a gate, and the back gate of the transistor323are electrically connected to the wiring259. One electrode of the capacitor324is electrically connected to the wiring259. The other electrode of the capacitor324is electrically connected to a wiring322.

By controlling the potential of the wiring322, the potential of the wiring259can be changed. In the reading operation and the writing operation of the memory devices300and300a, a potential is supplied to the wiring322so that the potential of the wiring259is higher than a negative potential (a potential lower than GND) described later and lower than a potential corresponding to Vthof the transistor262(a potential at which the transistor262is turned on).

In the case where gates of the transistors262in the memory elements261b_1to261b_3are electrically connected to the wiring255as in the memory device300a, the potential of the wiring259may be higher than a potential corresponding to Vthof the transistor262.

By controlling the potential of the wiring259, the operation speed of the transistor262can be increased. Furthermore, apparent Vthof the transistor262can be decreased. Thus, the data writing speed and the data reading speed can be increased.

In the retention operation of the memory circuit310, a fixed potential is supplied to the wiring322. For example, GND is supplied. After that, a negative potential (a potential lower than GND) is supplied to the terminal321. When a negative potential is supplied to the terminal321, the gate potential of the transistor323becomes relatively high, so that the transistor323is turned on. Consequently, the negative potential is supplied to the wiring259through the transistor323. More accurately, the wiring259is supplied with a potential higher than the negative potential by Vth. Note that the wiring259is supplied with the negative potential in this embodiment and the like for easy understanding.

When the wiring259is supplied with a negative potential, the potential of the back gate of the transistor262is decreased, and the transistor262is turned off; thus, data written to the memory circuit310can be retained. Furthermore, by supplying the negative potential to the back gate of the transistor262, apparent Vthof the transistor is increased. Thus, even when the potential of the gate of the transistor262is changed, data written to the memory circuit310can be retained.

Next, a potential higher than or equal to GND is supplied to the terminal321. For example, GND is supplied. Since the potential of the wiring259is the negative potential, the potential of the gate of the transistor becomes the negative potential. Accordingly, the transistor323is turned off. Even when power supply to the memory device300(memory device300a) is stopped after that, the transistors323and262can remain off.

The voltage retention circuit320has a function of suppressing a change in the potential of the wiring259in the retention operation of the memory device300(memory device300a). The voltage retention circuit320has a function of suppressing a change in the potential of the wiring259even when power supply to the memory device300(memory device300a) is stopped. In other words, the voltage retention circuit320has a function of retaining the voltage of the wiring259. The transistor323is preferably a transistor having a low off-state current to retain the voltage of the wiring259. For example, when the capacitance of the capacitor324is 10 pF and an acceptable increase in the potential of the wiring259is 0.5 V, a period during which the potential of the wiring259is increased by 0.5 V is an hour in the case where the off-state current of the transistor323is 1.39×10−15A, a day in the case where the off-state current of the transistor323is 5.79×10−17A, a year in the case where the off-state current of the transistor323is 1.59×10−19A, and ten years in the case where the off-state current of the transistor323is 1.59×10−20A. When the off-state current of the transistor323is lower than or equal to 1.59×10−20A, data written to the memory circuit310can be retained for ten years or more.

For example, by using an OS transistor as the transistor323, an extremely low off-state current of the transistor can be achieved. In order to reduce the off-state current, the transistor323preferably has a long channel length. Alternatively, the transistor323preferably has a short channel width. Alternatively, the transistor323preferably has a channel length longer than a channel width.

The transistor323is particularly preferably a transistor having a low off-state current at Vgof 0 V. Thus, a transistor having high Vthis preferably used as the transistor323. As the transistor having high Vth, the transistor200or the like described above can be used. Since the transistor262writes or reads data, a transistor having low Vthis preferably used as the transistor262. Furthermore, a transistor having a high on-state current and high field-effect mobility is preferably used as the transistor262. The transistor100or the like described above can be used as the transistor262.

In this embodiment, examples of CPUs including semiconductor devices such as the transistor of one embodiment of the present invention and the above-described memory device will be described.

For a semiconductor layer of the transistor, any one of the composite materials described in Embodiments 1 to 4 is used. With the composite material including the stack of the first region and the second region, the first region bends an energy band and carriers mainly flow in the second region; thus, high electrical characteristics (e.g., a high on-state current value or high field-effect mobility) are obtained, and high-speed switching can be realized. Since carriers more hardly flow in the first region of the composite material than the second region thereof, the transistor of one embodiment of the present invention is a normally-off transistor which is in a non-conduction state (also referred to as an off state) even when the gate voltage is 0V.

A semiconductor device400shown inFIG. 21includes a CPU core401, a power management unit421, and a peripheral circuit422. The power management unit421includes a power controller402and a power switch403. The peripheral circuit422includes a cache404including cache memory, a bus interface (BUS I/F)405, and a debug interface (Debug I/F)406. The CPU core401includes a data bus423, a control unit407, a program counter (PC)408, a pipeline register409, a pipeline register410, an arithmetic logic unit (ALU)411, and a register file412. Data is transmitted between the CPU core401and the peripheral circuit422such as the cache404via the data bus423.

The semiconductor device (cell) can be used for many logic circuits typified by the power controller402and the control unit407, particularly to all logic circuits that can be constituted using standard cells. Accordingly, the semiconductor device400can be small. The semiconductor device400can have reduced power consumption. The semiconductor device400can have a higher operating speed. The semiconductor device400can have a smaller power supply voltage variation.

When p-channel Si transistors and the transistor described in any of Embodiments 1 to 4 which includes the composite material (a material having a structure in which a region with a small amount of gallium and a region with a large amount of gallium are stacked like a superlattice) in a channel formation region are used in the semiconductor device (cell) and the semiconductor device (cell) is used in the semiconductor device400, the semiconductor device400can be small. The semiconductor device400can have reduced power consumption. The semiconductor device400can have a higher operating speed. Particularly when the Si transistors are only p-channel ones, the manufacturing cost can be reduced.

The control unit407has functions of totally controlling operations of the PC408, the pipeline register409, the pipeline register410, the ALU411, the register file412, the cache404, the bus interface405, the debug interface406, and the power controller402to decode and execute instructions contained in a program such as input applications.

The ALU411has a function of performing a variety of arithmetic operations such as four arithmetic operations and logic operations.

The cache404has a function of temporarily storing frequently-used data. The PC408is a register having a function of storing an address of an instruction to be executed next. Note that although not shown inFIG. 21, the cache404is provided with a cache controller for controlling the operation of the cache memory.

The pipeline register409has a function of temporarily storing instruction data.

The register file412includes a plurality of registers including a general purpose register and can store data that is read from the main memory, data obtained as a result of arithmetic operations in the ALU411, or the like.

The pipeline register410has a function of temporarily storing data used for arithmetic operations of the ALU411, data obtained as a result of arithmetic operations of the ALU411, or the like.

The bus interface405has a function as a path for data between the semiconductor device400and various devices outside the semiconductor device400. The debug interface406has a function as a path of a signal for inputting an instruction to control debugging to the semiconductor device400.

The power switch403has a function of controlling supply of a power source voltage to various circuits included in the semiconductor device400other than the power controller402. The above various circuits belong to several different power domains. The power switch403controls whether the power supply voltage is supplied to the various circuits in the same power domain. In addition, the power controller402has a function of controlling the operation of the power switch403.

The semiconductor device400having the above structure is capable of performing power gating. A description will be given of an example of the power gating operation sequence.

First, by the CPU core401, timing for stopping the supply of the power supply voltage is set in a register of the power controller402. Then, an instruction of starting power gating is sent from the CPU core401to the power controller402. Then, various registers and the cache404included in the semiconductor device400start data storing. Then, the power switch403stops the supply of a power supply voltage to the various circuits other than the power controller402included in the semiconductor device400. Then, an interrupt signal is input to the power controller402, whereby the supply of the power supply voltage to the various circuits included in the semiconductor device400is started. Note that a counter may be provided in the power controller402to be used to determine the timing of starting the supply of the power supply voltage regardless of input of an interrupt signal. Next, the various registers and the cache404start data recovery. Then, the instruction is resumed in the control unit407.

Such power gating can be performed in the whole processor or one or a plurality of logic circuits forming the processor. Furthermore, power supply can be stopped even for a short time. Consequently, power consumption can be reduced finely in terms of a space or time.

In performing power gating, data held by the CPU core401or the peripheral circuit422is preferably restored in a short time. In that case, the power can be turned on or off in a short time, and an effect of saving power becomes significant.

In order that the data held by the CPU core401or the peripheral circuit422be restored in a short time, the data is preferably restored to a flip-flop circuit itself (referred to as a flip-flop circuit capable of backup operation). Furthermore, the data is preferably restored to an SRAM cell itself (referred to as an SRAM cell capable of backup operation). The flip-flop circuit and the SRAM cell which are capable of backup operation preferably include transistors including a composite material (the material having a structure in which a region with a small amount of gallium and a region with a large amount of gallium are stacked like a superlattice) in a channel formation region. Consequently, the transistor has a low off-state current; thus, the flip-flop circuit and the SRAM cell which are capable of backup operation can retain data for a long time without power supply. When the transistor has a high switching speed, the flip-flop circuit and the SRAM cell which are capable of backup operation can save and restore data in a short time in some cases.

An example of the flip-flop circuit capable of backup operation is described usingFIG. 22.

A semiconductor device500shown inFIG. 22is an example of the flip-flop circuit capable of backup operation. The semiconductor device500includes a first memory circuit501, a second memory circuit502, a third memory circuit503, and a read circuit504. As a power supply voltage, a potential difference between a potential V1and a potential V2is supplied to the semiconductor device500. One of the potential V1and the potential V2is at a high level, and the other is at a low level. An example of the structure of the semiconductor device500when the potential V1is at a low level and the potential V2is at a high level will be described below.

The first memory circuit501has a function of retaining data when a signal D including the data is input in a period during which the power supply voltage is supplied to the semiconductor device500. Furthermore, the first memory circuit501outputs a signal Q including the retained data in the period during which the power supply voltage is supplied to the semiconductor device500. On the other hand, the first memory circuit501cannot retain data in a period during which the power supply voltage is not supplied to the semiconductor device500. That is, the first memory circuit501can be referred to as a volatile memory circuit.

The second memory circuit502has a function of reading the data held in the first memory circuit501to store (or restore) it. The third memory circuit503has a function of reading the data held in the second memory circuit502to store (or restore) it. The read circuit504has a function of reading the data held in the second memory circuit502or the third memory circuit503to store (or return) it in (to) the first memory circuit501.

In particular, the third memory circuit503has a function of reading the data held in the second memory circuit502to store (or restore) it even in the period during which the power supply voltage is not supplied to the semiconductor device500.

As shown inFIG. 22, the second memory circuit502includes a transistor512and a capacitor519. The third memory circuit503includes a transistor513, a transistor515, and a capacitor520. The read circuit504includes a transistor510, a transistor518, a transistor509, and a transistor517.

The transistor512has a function of charging and discharging the capacitor519in accordance with data held in the first memory circuit501. The transistor512is desirably capable of charging and discharging the capacitor519at a high speed in accordance with data held in the first memory circuit501. Specifically, the transistor512desirably contains crystalline silicon (preferably polycrystalline silicon, further preferably single crystal silicon) in a channel formation region.

The conduction state or the non-conduction state of the transistor513is determined in accordance with the charge held in the capacitor519. The transistor515has a function of charging and discharging the capacitor520in accordance with the potential of a wiring544when the transistor513is in a conduction state. It is desirable that the off-state current of the transistor515be extremely low. Specifically, the transistor515desirably contains a composite material (the material in which a region with a small amount of gallium and a region with a large amount of gallium are stacked like a superlattice) in a channel formation region.

Specific connection relations between the elements are described. One of a source and a drain of the transistor512is connected to the first memory circuit501. The other of the source and the drain of the transistor512is connected to one electrode of the capacitor519, a gate of the transistor513, and a gate of the transistor518. The other electrode of the capacitor519is connected to the wiring542. One of a source and a drain of the transistor513is connected to the wiring544. The other of the source and the drain of the transistor513is connected to one of a source and a drain of the transistor515. The other of the source and the drain of the transistor515is connected to one electrode of the capacitor520and a gate of the transistor510. The other electrode of the capacitor520is connected to the wiring543. One of a source and a drain of the transistor510is connected to a wiring541. The other of the source and the drain of the transistor510is connected to one of a source and a drain of the transistor518. The other of the source and the drain of the transistor518is connected to one of a source and a drain of the transistor509. The other of the source and the drain of the transistor509is connected to one of a source and a drain of the transistor517and the first memory circuit501. The other of the source and the drain of the transistor517is connected to a wiring540. Furthermore, although a gate of the transistor509is connected to a gate of the transistor517inFIG. 22, the gate of the transistor509is not necessarily connected to the gate of the transistor517.

The transistor described in the above embodiment as an example can be applied to the transistor515. Because of the low off-state current of the transistor515, the semiconductor device500can retain data for a long time without power supply. The favorable switching characteristics of the transistor515allow the semiconductor device500to perform high-speed backup and recovery.

In this embodiment, an example of an imaging device including the transistor or the like of one embodiment of the present invention will be described.

An imaging device of one embodiment of the present invention will be described below.

FIG. 23Ais a plan view illustrating an example of an imaging device2200of one embodiment of the present invention. The imaging device2200includes a pixel portion2210and peripheral circuits for driving the pixel portion2210(a peripheral circuit2260, a peripheral circuit2270, a peripheral circuit2280, and a peripheral circuit2290). The pixel portion2210includes a plurality of pixels2211arranged in a matrix with p rows and q columns (p and q are each an integer of 2 or more). The peripheral circuit2260, the peripheral circuit2270, the peripheral circuit2280, and the peripheral circuit2290are each connected to the plurality of pixels2211, and a signal for driving the plurality of pixels2211is supplied. In this specification and the like, in some cases, a “peripheral circuit” or a “driver circuit” indicate all of the peripheral circuits2260,2270,2280, and2290. For example, the peripheral circuit2260can be regarded as part of the peripheral circuit.

The imaging device2200preferably includes a light source2291. The light source2291can emit detection light P1.

The peripheral circuit includes at least one of a logic circuit, a switch, a buffer, an amplifier circuit, and a converter circuit. The peripheral circuit may be formed over a substrate where the pixel portion2210is formed. A semiconductor device such as an IC chip may be used as part or the whole of the peripheral circuit. Note that as the peripheral circuit, one or more of the peripheral circuits2260,2270,2280, and2290may be omitted.

As illustrated inFIG. 23B, the pixels2211may be provided to be inclined in the pixel portion2210included in the imaging device2200. When the pixels2211are obliquely arranged, the distance between pixels (pitch) can be shortened in the row direction and the column direction. Accordingly, the quality of an image taken with the imaging device2200can be improved.

Configuration Example 1 of Pixel

The pixel2211included in the imaging device2200is formed with a plurality of subpixels2212, and each subpixel2212is combined with a filter (color filter) which transmits light in a specific wavelength band, whereby data for achieving color image display can be obtained.

FIG. 24Ais a top view showing an example of the pixel2211with which a color image is obtained. The pixel2211illustrated inFIG. 24Aincludes a subpixel2212provided with a color filter that transmits light in a red (R) wavelength band (also referred to as a subpixel2212R), a subpixel2212provided with a color filter that transmits light in a green (G) wavelength band (also referred to as a subpixel2212G), and a subpixel2212provided with a color filter that transmits light in a blue (B) wavelength band (also referred to as a subpixel2212B). The subpixel2212can function as a photosensor.

The subpixel2212(the subpixel2212R, the subpixel2212G, and the subpixel2212B) is electrically connected to a wiring2231, a wiring2247, a wiring2248, a wiring2249, and a wiring2250. In addition, the subpixel2212R, the subpixel2212G, and the subpixel2212B are connected to respective wirings2253which are independently provided. In this specification and the like, for example, the wiring2248and the wiring2249that are connected to the pixel2211in the n-th row are referred to as a wiring2248[n] and a wiring2249[n]. For example, the wiring2253connected to the pixel2211in the m-th column is referred to as a wiring2253[m]. Note that inFIG. 24A, the wirings2253connected to the subpixel2212R, the subpixel2212G, and the subpixel2212B in the pixel2211in the m-th column are referred to as a wiring2253[m]R, a wiring2253[m]G, and a wiring2253[m]B. The subpixels2212are electrically connected to the peripheral circuit through the above wirings.

The imaging device2200has a structure in which the subpixel2212is electrically connected to the subpixel2212in an adjacent pixel2211which is provided with a color filter transmitting light in the same wavelength band as the subpixel2212, via a switch.FIG. 24Bshows a connection example of the subpixels2212: the subpixel2212in the pixel2211arranged in the n-th (n is an integer greater than or equal to 1 and less than or equal to p) row and the m-th (m is an integer greater than or equal to 1 and less than or equal to q) column and the subpixel2212in the adjacent pixel2211arranged in an (n+1)-th row and the m-th column. InFIG. 24B, the subpixel2212R arranged in the n-th row and the m-th column and the subpixel2212R arranged in the (n+1)-th row and the m-th column are connected to each other via a switch2201. The subpixel2212G arranged in the n-th row and the m-th column and the subpixel2212G arranged in the (n+1)-th row and the m-th column are connected to each other via a switch2202. The subpixel2212B arranged in the n-th row and the m-th column and the subpixel2212B arranged in the (n+1)-th row and the m-th column are connected to each other via a switch2203.

The color filter used in the subpixel2212is not limited to red (R), green (G), and blue (B) color filters, and color filters that transmit light of cyan (C), yellow (Y), and magenta (M) may be used. By provision of the subpixels2212that sense light in three different wavelength bands in one pixel2211, a full-color image can be obtained.

The pixel2211including the subpixel2212provided with a color filter transmitting yellow (Y) light may be provided, in addition to the subpixels2212provided with the color filters transmitting red (R), green (G), and blue (B) light. The pixel2211including the subpixel2212provided with a color filter transmitting blue (B) light may be provided, in addition to the subpixels2212provided with the color filters transmitting cyan (C), yellow (Y), and magenta (M) light. When the subpixels2212sensing light in four different wavelength bands are provided in one pixel2211, the reproducibility of colors of an obtained image can be increased.

For example, inFIG. 24A, in regard to the subpixel2212sensing light in a red wavelength band, the subpixel2212sensing light in a green wavelength band, and the subpixel2212sensing light in a blue wavelength band, the pixel number ratio (or the light receiving area ratio) thereof is not necessarily 1:1:1. For example, the Bayer arrangement in which the pixel number ratio (the light receiving area ratio) is set at red:green:blue=1:2:1 may be employed. Alternatively, the pixel number ratio (the light receiving area ratio) of red and green to blue may be 1:6:1.

Although the number of subpixels2212provided in the pixel2211may be one, two or more subpixels are preferably provided. For example, when two or more subpixels2212sensing light in the same wavelength band are provided, the redundancy is increased, and the reliability of the imaging device2200can be increased.

When an infrared (IR) filter that transmits infrared light and absorbs or reflects visible light is used as the filter, the imaging device2200that senses infrared light can be achieved.

Furthermore, when a neutral density (ND) filter (dark filter) is used, output saturation which occurs when a large amount of light enters a photoelectric conversion element (light-receiving element) can be prevented. With a combination of ND filters with different dimming capabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel2211may be provided with a lens. An arrangement example of the pixel2211, a filter2254, and a lens2255is described with reference to cross-sectional views inFIGS. 25A and 25B. With the lens2255, the photoelectric conversion element can receive incident light efficiently. Specifically, as illustrated inFIG. 25A, light2256enters a photoelectric conversion element2220through the lens2255, the filter2254(a filter2254R, a filter2254G, and a filter2254B), a pixel circuit2230, and the like which are provided in the pixel2211.

As indicated by a region surrounded with dashed dotted lines, however, part of the light2256indicated by arrows might be blocked by some wirings2257. Thus, a preferable structure is such that the lens2255and the filter2254are provided on the photoelectric conversion element2220side as illustrated inFIG. 25B, whereby the photoelectric conversion element2220can efficiently receive the light2256. When the light2256enters the photoelectric conversion element2220from the photoelectric conversion element2220side, the imaging device2200with high sensitivity can be provided.

As the photoelectric conversion element2220illustrated inFIGS. 25A and 25B, a photoelectric conversion element in which a p-n junction or a p-i-n junction is formed may be used.

The photoelectric conversion element2220may be formed using a substance that has a function of absorbing a radiation and generating charges. Examples of the substance that has a function of absorbing a radiation and generating charges include selenium, lead iodide, mercury iodide, gallium arsenide, cadmium telluride, and cadmium zinc alloy.

For example, when selenium is used for the photoelectric conversion element2220, the photoelectric conversion element2220can have a light absorption coefficient in a wide wavelength band, such as visible light, ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel2211included in the imaging device2200may include the subpixel2212with a first filter in addition to the subpixel2212illustrated inFIGS. 24A and 24B.

Configuration Example 2 of Pixel

An example of a pixel including a transistor including silicon and a transistor including a composite material (a material in which a region with a small amount of gallium and a region with a large amount of gallium are stacked like a superlattice) is described below. A transistor similar to any of the transistors described in the above embodiments can be used as each of the transistors.

FIG. 26is a cross-sectional view of an element included in an imaging device. The imaging device illustrated inFIG. 26includes a transistor2351including silicon over a silicon substrate2300, transistors2352and2353which include a composite material and are stacked over the transistor2351, and a photodiode2360provided in the silicon substrate2300. The transistors and the photodiode2360are electrically connected to various plugs2370and wirings2371. In addition, an anode2361of the photodiode2360is electrically connected to the plug2370through a low-resistance region2363.

The imaging device includes a layer2310including the transistor2351provided on the silicon substrate2300and the photodiode2360provided in the silicon substrate2300, a layer2320which is in contact with the layer2310and includes the wirings2371, a layer2330which is in contact with the layer2320and includes the transistors2352and2353, and a layer2340which is in contact with the layer2330and includes a wiring2372and a wiring2373.

In the example of the cross-sectional view inFIG. 26, a light-receiving surface of the photodiode2360is provided on the side opposite to a surface of the silicon substrate2300where the transistor2351is formed. With this structure, a light path can be secured without an influence of the transistors and the wirings. Thus, a pixel with a high aperture ratio can be formed. Note that the light-receiving surface of the photodiode2360can be the same as the surface where the transistor2351is formed.

In the case where a pixel is formed with use of only transistors including a composite material, the layer2310may include the transistor including a composite material. Alternatively, the layer2310may be omitted, and the pixel may include only transistors including a composite material.

Note that the silicon substrate2300may be an SOI substrate. Furthermore, the silicon substrate2300can be replaced with a substrate made of germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, or an organic semiconductor.

Here, an insulator2380is provided between the layer2310including the transistor2351and the photodiode2360and the layer2330including the transistors2352and2353. However, there is no limitation on the position of the insulator2380. An insulator2379is provided under the insulator2380, and an insulator2381is provided over the insulator2380.

Conductors2390ato2390eare provided in openings formed in the insulators2379and2381. The conductors2390a,2390b, and2390efunction as plugs and wirings. The conductor2390cfunctions as a back gate of the transistor2353. The conductor2390dfunctions as a back gate of the transistor2352.

Hydrogen in an insulator provided in the vicinity of a channel formation region of the transistor2351terminates dangling bonds of silicon; accordingly, the reliability of the transistor2351can be improved. In contrast, hydrogen in the insulator provided in the vicinity of the transistor2352, the transistor2353, and the like becomes one of factors generating a carrier in the composite material. Thus, the hydrogen may cause a reduction of the reliability of the transistor2352, the transistor2353, and the like. For this reason, in the case where the transistor including a composite material is provided over the transistor including a silicon-based semiconductor, it is preferable that the insulator2380having a function of blocking hydrogen be provided between the transistors. When hydrogen is confined in layers below the insulator2380, the reliability of the transistor2351can be improved. In addition, hydrogen can be prevented from diffusing from the layers below the insulator2380into layers above the insulator2380; thus, the reliability of the transistor2352, the transistor2353, and the like can be increased. The conductors2390a,2390b, and2390ecan prevent hydrogen from diffusing into the layers provided thereover through the via holes formed in the insulator2380, resulting in improvement in the reliability of the transistors2352and2353and the like.

In the cross-sectional view inFIG. 26, the photodiode2360in the layer2310and the transistor in the layer2330can be formed so as to overlap with each other. Thus, the degree of integration of pixels can be increased. In other words, the resolution of the imaging device can be increased.

Part or the whole of the imaging device may be bent. The bent imaging device enables the curvature of field and astigmatism to be reduced. Thus, the optical design of lens and the like, which is used in combination of the imaging device, can be facilitated. For example, the number of lenses used for aberration correction can be reduced; accordingly, a reduction in size or weight of electronic devices using the imaging device, and the like, can be achieved. In addition, the quality of a captured image can be improved.

In this embodiment, a semiconductor wafer, a chip, and an electronic component of one embodiment of the present invention will be described.

FIG. 27Ais a top view illustrating a substrate1711before dicing treatment. As the substrate1711, a semiconductor substrate (also referred to as a “semiconductor wafer”) can be used, for example. A plurality of circuit regions1712are provided over the substrate1711. The semiconductor device according to one embodiment of the present invention, a CPU, an RF tag, an image sensor, or the like can be provided in the circuit region1712.

The plurality of circuit regions1712are each surrounded by a separation region1713. Separation lines (also referred to as “dicing lines”)1714are set at a position overlapping with the separation regions1713. The substrate1711can be cut along the separation lines1714into chips1715including the circuit regions1712.FIG. 27Bis an enlarged view of the chip1715.

A conductive layer or a semiconductor layer may be provided in the separation regions1713. Providing a conductive layer or a semiconductor layer in the separation regions1713relieves ESD that might be caused in a dicing step, preventing a decrease in the yield of the dicing step. A dicing step is generally performed while letting pure water whose specific resistance is decreased by dissolution of a carbonic acid gas or the like flow to a cut portion, in order to cool down a substrate, remove swarf, and prevent electrification, for example. Providing a conductive layer or a semiconductor layer in the separation regions1713allows a reduction in the usage of the pure water. Therefore, the cost of manufacturing semiconductor devices can be reduced. Thus, semiconductor devices can be manufactured with improved productivity.

For a semiconductor layer provided in the separation regions1713, a material having a bandgap greater than or equal to 2.5 eV and less than or equal to 4.2 eV, preferably greater than or equal to 2.7 eV and less than or equal to 3.5 eV is preferably used. The use of such a material allows accumulated charges to be released slowly; thus, the rapid move of charges due to ESD can be suppressed and electrostatic breakdown is less likely to occur.

FIGS. 28A and 28Bshow an example where the chip1715is used to make an electronic component. Note that the electronic component is also referred to as a semiconductor package or an IC package. This electronic component has a plurality of standards and names depending on a terminal extraction direction and a terminal shape.

The electronic component is completed when the semiconductor device described in any of the above embodiments is combined with components other than the semiconductor device in an assembly process (post-process).

The post-process will be described with reference to a flowchart inFIG. 28A. After an element substrate including the semiconductor device described in any of the above embodiments is completed in a pre-process, a back surface grinding step in which a back surface (a surface where a semiconductor device and the like are not formed) of the element substrate is ground is performed (Step S721). When the element substrate is thinned by grinding, warpage or the like of the element substrate is reduced, so that the size of the electronic component can be reduced.

Next, the element substrate is divided into a plurality of chips (chips1715) in a dicing step (Step S722). Then, the separated chips are individually picked up to be bonded to a lead frame in a die bonding step (Step S723). To bond a chip and a lead frame in the die bonding step, a method such as resin bonding or tape-automated bonding is selected as appropriate depending on products. Note that the chip may be bonded to an interposer substrate instead of the lead frame.

Next, a wire bonding step for electrically connecting a lead of the lead frame and an electrode on the chip through a metal wire is performed (Step S724). As the metal wire, a silver wire or a gold wire can be used. Ball bonding or wedge bonding can be used as the wire bonding.

The wire-bonded chip is subjected to a molding step of sealing the chip with an epoxy resin or the like (Step S725). Through the molding step, the inside of the electronic component is filled with a resin, so that a circuit portion incorporated in the chip and a wire for connecting the chip to the lead can be protected from external mechanical force, and deterioration of characteristics (decrease in reliability) due to moisture or dust can be reduced.

Subsequently, the lead of the lead frame is plated in a lead plating step (Step S726). This plating process prevents rust of the lead and facilitates soldering at the time of mounting the chip on a printed circuit board in a later step. Then, the lead is cut and processed in a formation step (Step S727).

Next, a printing (marking) step is performed on a surface of the package (Step S728). After a testing step (Step S729) for checking whether an external shape is good and whether there is a malfunction, for example, the electronic component is completed.

FIG. 28Bis a perspective schematic diagram of a completed electronic component.FIG. 28Bis a perspective schematic diagram illustrating a quad flat package (QFP) as an example of the electronic component. An electronic component1750inFIG. 28Bincludes a lead1755and a semiconductor device1753. As the semiconductor device1753, the semiconductor device described in any of the above embodiments can be used.

The electronic component1750inFIG. 28Bis mounted on a printed circuit board1752, for example. A plurality of electronic components1750are combined and electrically connected to each other over the printed circuit board1752; thus, a substrate on which the electronic components are mounted (a circuit board1754) is completed. The completed circuit board1754is provided in an electronic device or the like.

In this embodiment, an example of a display device that includes the transistor described in Embodiment 5 will be described below with reference toFIG. 29,FIG. 30,FIG. 31, andFIG. 32.

FIG. 29is a top view illustrating an example of a display device. A display device700illustrated inFIG. 29includes a pixel portion702provided over a first substrate701; a source driver circuit portion704and a gate driver circuit portion706provided over the first substrate701; a sealant712provided to surround the pixel portion702, the source driver circuit portion704, and the gate driver circuit portion706; and a second substrate705provided to face the first substrate701. The first substrate701and the second substrate705are sealed with the sealant712. That is, the pixel portion702, the source driver circuit portion704, and the gate driver circuit portion706are sealed with the first substrate701, the sealant712, and the second substrate705. Although not illustrated inFIG. 29, a display element is provided between the first substrate701and the second substrate705.

In the display device700, a flexible printed circuit (FPC) terminal portion708electrically connected to the pixel portion702, the source driver circuit portion704, and the gate driver circuit portion706is provided in a region different from the region which is surrounded by the sealant712and positioned over the first substrate701. Furthermore, an FPC716is connected to the FPC terminal portion708, and a variety of signals and the like are supplied to the pixel portion702, the source driver circuit portion704, and the gate driver circuit portion706through the FPC716. Furthermore, a signal line710is connected to the pixel portion702, the source driver circuit portion704, the gate driver circuit portion706, and the FPC terminal portion708. The variety of signals and the like are applied to the pixel portion702, the source driver circuit portion704, the gate driver circuit portion706, and the FPC terminal portion708via the signal line710from the FPC716.

A plurality of gate driver circuit portions706may be provided in the display device700. An example of the display device700in which the source driver circuit portion704and the gate driver circuit portion706are formed over the first substrate701where the pixel portion702is also formed is described; however, the structure is not limited thereto. For example, only the gate driver circuit portion706may be formed over the first substrate701or only the source driver circuit portion704may be formed over the first substrate701. In this case, a substrate over which a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver circuit board formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be formed on the first substrate701. Note that there is no particular limitation on the method of connecting a separately prepared driver circuit substrate, and a chip on glass (COG) method, a wire bonding method, or the like can be used.

The pixel portion702, the source driver circuit portion704, and the gate driver circuit portion706included in the display device700include a plurality of transistors.

The display device700can include any of a variety of elements. As examples of the elements, electroluminescent (EL) element (e.g., an EL element containing organic and inorganic materials, an organic EL element, an inorganic EL element, or an LED), a light-emitting transistor element (a transistor which emits light depending on current), an electron emitter, a liquid crystal element, an electronic ink display, an electrophoretic element, an electrowetting element, a plasma display panel (PDP), a micro electro mechanical systems (MEMS) display (e.g., a grating light valve (GLV), a digital micromirror device (DMD), a digital micro shutter (DMS) element, or an interferometric modulator display (IMOD) element), and a piezoelectric ceramic display can be given.

An example of a display device including an EL element is an EL display. Examples of display devices including electron emitters are a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). Display devices having electronic ink or electrophoretic elements include electronic paper and the like. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes. Thus, the power consumption can be further reduced.

As a display method in the display device700, a progressive method, an interlace method, or the like can be employed. Furthermore, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel may be included. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ among color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Furthermore, the size of a display region may be different depending on respective dots of the color components. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used to obtain a full-color display device in which white light (W) is used for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp). As the coloring layer, red (R), green (G), blue (B), yellow (Y), or the like may be combined as appropriate, for example. With the use of the coloring layer, higher color reproducibility can be obtained than in the case without the coloring layer. In this case, by providing a region with the coloring layer and a region without the coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without the coloring layer, a decrease in luminance due to the coloring layer can be suppressed, and 20% to 30% of power consumption can be reduced in some cases when an image is displayed brightly. Note that in the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light of their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption can be further reduced as compared to the case of using the coloring layer in some cases.

As a coloring system, any of the following systems may be used: the above-described color filter system in which part of white light is converted into red light, green light, and blue light through color filters; a three-color system in which red light, green light, and blue light are used; and a color conversion system or a quantum dot system in which part of blue light is converted into red light or green light.

In this embodiment, a structure including a liquid crystal element and an EL element as display elements is described with reference toFIG. 30,FIG. 31, andFIG. 32. Note thatFIG. 30andFIG. 31are each a cross-sectional view taken along dashed-dotted line Q-R shown inFIG. 29and show a structure including a liquid crystal element as a display element.FIG. 32is a cross-sectional view taken along dashed-dotted line Q-R inFIG. 29and illustrates the structure including an EL element as a display element.

[Common Portions in Display Devices]

The display device700illustrated inFIG. 30,FIG. 31, andFIG. 32includes a lead wiring portion711, the pixel portion702, the source driver circuit portion704, and the FPC terminal portion708. Note that the lead wiring portion711includes the signal line710. The pixel portion702includes a transistor750and a capacitor790. The source driver circuit portion704includes a transistor752.

The transistor750and the transistor752each have a structure similar to that of the transistor800A described above. Note that the transistor750and the transistor752may each have the structure of any of the other transistors described in the above embodiments.

The transistors used in this embodiment each include the composite material described in any one of Embodiments 1 to 4. The transistor can have low off-state current. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, the frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption.

In addition, the transistor including the composite material in the semiconductor layer, which is used in this embodiment, can have relatively high field-effect mobility and thus is capable of high-speed operation. For example, with such a transistor which can operate at high speed used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced. In addition, the transistor which can operate at high speed can be used also in the pixel portion, whereby a high-quality image can be provided.

The capacitor790includes a lower electrode that is formed through a step of processing the same conductive film as a conductive film functioning as a first gate electrode of the transistor750and an upper electrode that is formed through a step of processing the same conductive film as a conductive film functioning as a source electrode or a drain electrode of the transistor750. Furthermore, between the lower electrode and the upper electrode, an insulating film that is formed through a step of forming the same insulating film as an insulating film functioning as a first gate insulating film of the transistor750and an insulating film that is formed through a step of forming the same insulating film as an insulating film functioning as a protective insulating film of the transistor750are provided. That is, the capacitor790has a stacked-layer structure in which the insulating films functioning as a dielectric film are positioned between a pair of electrodes.

AlthoughFIG. 30,FIG. 31, andFIG. 32each illustrate an example in which the transistor750included in the pixel portion702and the transistor752included in the source driver circuit portion704have the same structure, one embodiment of the present invention is not limited thereto. For example, the pixel portion702and the source driver circuit portion704may include different transistors. Specifically, a structure in which a top-gate transistor is used in the pixel portion702and a bottom-gate transistor is used in the source driver circuit portion704, or a structure in which a bottom-gate transistor is used in the pixel portion702and a top-gate transistor is used in the source driver circuit portion704may be employed. Note that the term “source driver circuit portion704” can be replaced by the term “gate driver circuit portion”.

The signal line710is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors750and752. In the case where the signal line710is formed using a material including a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.

The FPC terminal portion708includes a connection electrode760, an anisotropic conductive film780, and the FPC716. Note that the connection electrode760is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors750and752. The connection electrode760is electrically connected to a terminal included in the FPC716through the anisotropic conductive film780.

For example, a glass substrate can be used as the first substrate701and the second substrate705. A flexible substrate may be used as the first substrate701and the second substrate705. Examples of the flexible substrate include a plastic substrate.

A structure body778is provided between the first substrate701and the second substrate705. The structure body778is a columnar spacer obtained by selective etching of an insulating film and provided to control the distance (cell gap) between the first substrate701and the second substrate705. Note that a spherical spacer may be used as the structure body778.

Furthermore, a light-blocking film738functioning as a black matrix, a coloring film736functioning as a color filter, and an insulating film734in contact with the light-blocking film738and the coloring film736are provided on the second substrate705side.

[Structure Example of Display Device Using Liquid Crystal Element]

The display device700illustrated inFIG. 30includes a liquid crystal element775. The liquid crystal element775includes a conductive film772, a conductive film774, and a liquid crystal layer776. The conductive film774is provided on the second substrate705side and functions as a counter electrode. The display device700illustrated inFIG. 30can display an image in such a manner that transmission or non-transmission of light is controlled by the alignment state in the liquid crystal layer776which is changed depending on the voltage applied to the conductive film772and the conductive film774.

The conductive film772is electrically connected to the conductive film that functions as a source electrode and a drain electrode included in the transistor750. The conductive film772is formed over the planarization insulating film770to function as a pixel electrode, i.e., one electrode of the display element.

A conductive film that transmits visible light or a conductive film that reflects visible light can be used as the conductive film772. For example, a material including one kind selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material including aluminum or silver may be used for the conductive film that reflects visible light.

In the case where a conductive film that reflects visible light is used as the conductive film772, the display device700becomes a reflective-type liquid crystal display device. In the case where a conductive film that transmits visible light is used as the conductive film772, the display device700becomes a transmissive liquid crystal display device.

When a structure over the conductive film772is changed, a driving method of the liquid crystal element can be changed. An example of this case is illustrated inFIG. 31. The display device700illustrated inFIG. 31is an example of employing a transverse electric field mode (e.g., an FFS mode) as a driving mode of the liquid crystal element. In the structure illustrated inFIG. 31, an insulating film773is provided over the conductive film772and the conductive film774is provided over the insulating film773. In such a structure, the conductive film774functions as a common electrode, and an electric field generated between the conductive film772and the conductive film774through the insulating film773can control the alignment state of the liquid crystal layer776.

Although not illustrated inFIG. 30andFIG. 31, the conductive film772and/or the conductive film774may be provided with an alignment film on a side in contact with the liquid crystal layer776. Although not illustrated inFIG. 30andFIG. 31, an optical member (optical substrate) or the like, such as a polarizing member, a retardation member, or an anti-reflection member, may be provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a side light, or the like may be used as a light source.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer-dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has a short response time and optical isotropy, which makes the alignment process unneeded. An alignment film does not need to be provided and rubbing treatment is thus not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced. Moreover, the liquid crystal material which exhibits a blue phase has a small viewing angle dependence.

Furthermore, a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or an advanced super view (ASV) mode can be employed.

The display device700illustrated inFIG. 32includes a light-emitting element782. The light-emitting element782includes the conductive film772, an EL layer786, and a conductive film788. The display device700shown inFIG. 32is capable of displaying an image by light emission from the EL layer786included in the light-emitting element782. Note that the EL layer786contains an organic compound or an inorganic compound such as a quantum dot.

Examples of materials that can be used for an organic compound include a fluorescent material and a phosphorescent material. Examples of materials that can be used for a quantum dot include a colloidal quantum dot material, an alloyed quantum dot material, a core-shell quantum dot material, and a core quantum dot material. The quantum dot containing elements belonging to Groups12and16, elements belonging to Groups13and15, or elements belonging to Groups14and16, may be used. Alternatively, a quantum dot material containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.

The above-described organic compound and the inorganic compound can be deposited by a method such as an evaporation method (including a vacuum evaporation method), a droplet discharging method (also referred to as an ink-jet method), a coating method, or a gravure printing method. A low molecular material, a middle molecular material (including an oligomer and a dendrimer), or a high molecular material may be included in the EL layer786.

In the display device700inFIG. 32, an insulating film730is provided over the planarization insulating film770and the conductive film772. The insulating film730covers part of the conductive film772. Note that the light-emitting element782has a top emission structure. Therefore, the conductive film788has a light-transmitting property and transmits light emitted from the EL layer786. Although the top-emission structure is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. A bottom-emission structure in which light is emitted to the conductive film772side, or a dual-emission structure in which light is emitted to both the conductive film772side and the conductive film788side may be employed.

The coloring film736is provided to overlap with the light-emitting element782, and the light-blocking film738is provided to overlap with the insulating film730and to be included in the lead wiring portion711and in the source driver circuit portion704. The coloring film736and the light-blocking film738are covered with the insulating film734. A space between the light-emitting element782and the insulating film734is filled with a sealing film732. Although a structure with the coloring film736is described as the display device700inFIG. 32, the structure is not limited thereto. In the case where the EL layer786is formed by a separate coloring method, the coloring film736is not necessarily provided.

In this embodiment, electronic devices including the transistor or the like of one embodiment of the present invention will be described.

The semiconductor device of one embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can be equipped with the semiconductor device of one embodiment of the present invention are mobile phones, game machines including portable game consoles, portable data terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio units and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines.FIGS. 33A to 33Fillustrate specific examples of these electronic devices.

FIG. 33Aillustrates a portable game machine including a housing1901, a housing1902, a display portion1903, a display portion1904, a microphone1905, a speaker1906, an operation key1907, a stylus1908, and the like. Although the portable game machine inFIG. 33Ahas the two display portions1903and1904, the number of display portions included in a portable game machine is not limited to this. Integrated circuits such as a sensor and a CPU are provided in the housing1901or the housing1902. When the composite material described in any one of Embodiments 1 to 4 is used for a semiconductor element such as a sensor or a CPU, a device having high electrical characteristics (e.g., high on-state current) and high reliability can be provided.

For semiconductor elements used in pixels and driver circuits thereof which are used in the display portion1903and the display portion1904, the composite material described in any one of Embodiments 1 to 4 can be used. Consequently, a display portion with low power consumption can be provided.

FIG. 33Billustrates a portable data terminal including a first housing1911, a second housing1912, a first display portion1913, a second display portion1914, a joint1915, an operation key1916, and the like. The first display portion1913is provided in the first housing1911, and the second display portion1914is provided in the second housing1912. The first housing1911and the second housing1912are connected to each other with the joint1915, and the angle between the first housing1911and the second housing1912can be changed with the joint1915. An image on the first display portion1913may be switched in accordance with the angle at the joint1915between the first housing1911and the second housing1912. A display device with a position input function may be used as at least one of the first display portion1913and the second display portion1914. Note that the position input function can be added by provision of a touch panel in a display device. Alternatively, the position input function can be added by provision of a photoelectric conversion element called a photosensor in a pixel portion of a display device. Integrated circuits such as a sensor and a CPU are provided in the housing1911or the housing1912. When the composite material described in any one of Embodiments 1 to 4 is used for a semiconductor element such as a sensor or a CPU, a device having high electrical characteristics (e.g., high on-state current) and high reliability can be provided.

For semiconductor elements used in pixels and driver circuits thereof which are used in the display portion1913and the display portion1914, the composite material described in any one of Embodiments 1 to 4 can be used. Consequently, a display portion with low power consumption can be provided.

FIG. 33Cillustrates a laptop personal computer including a housing1921, a display portion1922, a keyboard1923, a pointing device1924, and the like. Integrated circuits such as a sensor and a CPU are provided in the housing1921. When the composite material described in any one of Embodiments 1 to 4 is used for a semiconductor element such as a sensor or a CPU, a device having high electrical characteristics (e.g., high on-state current) and high reliability can be provided.

FIG. 33Dillustrates an electric refrigerator-freezer including a housing1931, a door for a refrigerator1932, a door for a freezer1933, and the like. In the housing1931, a temperature sensor and a reader are provided, data on a product taken in and out is read by the reader, the data on the product is sent to an arithmetic device in the refrigerator, and the arithmetic device makes the memory device store data on the product as necessary. When the composite material described in any one of Embodiments 1 to 4 is used for a semiconductor element in a temperature sensor, a reader, an arithmetic device, or a memory device, a device having high electrical characteristics (e.g., high on-state current) and high reliability can be provided.

FIG. 33Eillustrates a video camera including a first housing1941, a second housing1942, a display portion1943, operation keys1944, a lens1945, a joint1946, and the like. The operation keys1944and the lens1945are provided in the first housing1941, and the display portion1943is provided in the second housing1942. The first housing1941and the second housing1942are connected to each other with the joint1946, and the angle between the first housing1941and the second housing1942can be changed with the joint1946. Images displayed on the display portion1943may be switched in accordance with the angle at the joint1946between the first housing1941and the second housing1942. In the first housing1941, a memory device, an image processing circuit, and an imaging element are provided. When the composite material described in any one of Embodiments 1 to 4 is used for a semiconductor element used in the memory device, the image processing circuit, or the imaging element, a device having high electrical characteristics (e.g., high on-state current) and high reliability can be provided.

FIG. 33Fillustrates an automobile including a car body1951, wheels1952, a dashboard1953, lights1954, and the like. A variety of integrated circuits such as sensors or control circuits are provided in the car body1951in the automobile. When the composite material described in any one of Embodiments 1 to 4 is used for a semiconductor element used in the integrated circuits such as sensors or control circuits, a device having high electrical characteristics (e.g., high on-state current) and high reliability can be provided.

EXPLANATION OF REFERENCE

This application is based on Japanese Patent Application serial no. 2016-042358 filed with Japan Patent Office on Mar. 4, 2016 and Japanese Patent Application serial no. 2016-042749 filed with Japan Patent Office on Mar. 4, 2016, the entire contents of which are hereby incorporated by reference.