SEMICONDUCTOR DEVICE AND OPERATION METHOD THEREOF

A semiconductor device including an amplifier with improved accuracy is provided. The semiconductor device includes a switch, a capacitor, a chopping circuit, and the amplifier. The amplifier includes a non-inverting input terminal, an inverting input terminal, an inverting output terminal, and a non-inverting output terminal. The semiconductor device, with use of the switch and the capacitor, has a function of sampling and holding a first potential and a second potential input in a first period. The chopping circuit is provided on each of the input terminal side and the output terminal side of the amplifier, and the first potential and the second potential are each input to either one of the non-inverting input terminal and the inverting input terminal in a second period. In a third period, the first potential and the second potential are each input to either one of the non-inverting input terminal and the inverted input terminal, which is different from the second period. In a similar manner, the inverting output terminal and non-inverting output terminal are replaced by the chopping circuit in the second period and the third period to be output from the semiconductor device.

TECHNICAL FIELD

The present invention relates to a semiconductor device and an operation method thereof. In particular, the present invention relates to a semiconductor device including an amplifier (also referred to as an amplifier circuit) with improved accuracy.

In this specification and the like, a semiconductor device refers to a device that utilizes semiconductor characteristics, such as a circuit including a semiconductor element (a transistor, a diode, a photodiode, and the like), a device including the circuit, and the like. In this specification and the like, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics; examples of the semiconductor device include an integrated circuit, a chip provided with an integrated circuit, an electronic component in which a chip is incorporated in a package, and an electronic device provided with an integrated circuit.

BACKGROUND ART

As one of techniques of a semiconductor circuit, a technique of a switched capacitor circuit has been known in which a switch (also referred to as a switching element) is combined with a capacitor (also referred to as a capacitor element) and the switch controls charge and discharge of the capacitor. Temperature dependence of the electric characteristics of a switched capacitor circuit is small, and the switched capacitor circuit can be used in place of a resistor (also referred to as a resistor element) in a semiconductor circuit; thus, a semiconductor device having small temperature dependence can be achieved.

In addition, a technique of using a switched capacitor circuit and an amplifier in combination has been known (see Non-Patent Document 1). A semiconductor device in which a switched capacitor circuit and an amplifier are combined (such a semiconductor device is also referred to as a switched capacitor amplifier) can, with a signal (potential) to be input to the semiconductor device being sampled and held in a capacitor, achieve a highly accurate amplifier.

Meanwhile, a transistor including an oxide semiconductor or a metal oxide in a channel formation region (also referred to as an oxide semiconductor (OS) transistor) has been attracting attention. The drain current of an OS transistor in an off state (such a current is also referred to as an off-state current) is extremely low (e.g., see Non-Patent Documents 2 and 3); when an OS transistor is used in a memory cell of a DRAM, for example, electric charge accumulated in a capacitive element can be retained for a long time.

A CAAC (c-axis aligned crystalline) structure and an nc (nanocrystalline) structure, which are neither single crystal nor amorphous, have been found in an oxide semiconductor (see Non-Patent Document 2 and Non-Patent Document 4). Non-Patent Document 2 and Non-Patent Document 4 also disclose a technique for fabricating a transistor using an oxide semiconductor having a CAAC structure.

REFERENCE

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In an amplifier including a non-inverting input terminal and an inverting input terminal, for example, amplifier-induced noise such as offset voltage which is output even when a potential difference between the non-inverting input terminal and the inverting input terminal is 0 V, 1/f noise for which the noise power is inversely proportional to the frequency and which can hardly be filtered out, or thermal noise which is caused when a free electron moves randomly with thermal energy is superimposed on the output even in a period when a switched capacitor amplifier is sampling an input signal and holding the signal in the capacitor, and the removal of such noise is difficult, which has been problematic.

An object of one embodiment of the present invention is to provide a switched capacitor amplifier in which an effect of amplifier-induced noise on an output is reduced. Another object of one embodiment of the present invention is to provide a semiconductor device including an amplifier, in which an effect of amplifier-induced noise on an output is reduced. Another object of one embodiment of the present invention is to provide a semiconductor device including an amplifier, in which the accuracy of the amplifier is improved.

Note that one embodiment of the present invention does not necessarily have to achieve all the above-described objects and only needs to achieve at least one of the objects. The descriptions of the above objects do not preclude the existence of other objects. Objects other than these will be apparent from the descriptions of the specification, the claims, the drawings, and the like, and objects other than these can be derived from the descriptions of the specification, the claims, the drawings, and the like.

Means for Solving the Problems

One embodiment of the present invention is a semiconductor device including a switch, first and second capacitors, first and second chopping circuits, an amplifier, first and second input terminals, and first and second output terminals. The amplifier includes a non-inverting input terminal, an inverting input terminal, an inverting output terminal, and a non-inverting output terminal. In a first period, the semiconductor device electrically connects the first input terminal and one terminal of the first capacitor, electrically connects the second input terminal and one terminal of the second capacitor, electrically connects the other terminal of the first capacitor and the first output terminal, and electrically connects the other terminal of the second capacitor and the second output terminal; the first chopping circuit electrically connects the other terminal of the first capacitor and the non-inverting input terminal and electrically connects the other terminal of the second capacitor and the inverting input terminal; and the second chopping circuit electrically connects the inverting output terminal and the first output terminal and electrically connects the non-inverting output terminal and the second output terminal. In a second period, the semiconductor device electrically connects the one terminal of the first capacitor and the first output terminal and electrically connects the one terminal of the second capacitor and the second output terminal; the first chopping circuit electrically connects the other terminal of the first capacitor and the non-inverting input terminal and electrically connects the other terminal of the second capacitor and the inverting input terminal, and the second chopping circuit electrically connects the inverting output terminal and the first output terminal and electrically connects the non-inverting output terminal and the second output terminal. In a third period, the semiconductor device electrically connects the one terminal of the first capacitor and the first output terminal and electrically connects the one terminal of the second capacitor and the second output terminal; the first chopping circuit electrically connects the other terminal of the first capacitor and the inverting input terminal and electrically connects the other terminal of the second capacitor and the non-inverting input terminal; and the second chopping circuit electrically connects the non-inverting output terminal and the first output terminal and electrically connects the inverting output terminal and the second output terminal.

In the above embodiment, the switch, the first chopping circuit, and the second chopping circuit each include a transistor, and the transistor includes a metal oxide in a channel formation region.

Another embodiment of the present invention is an operation method of a semiconductor device including a switch, first and second capacitors, first and second chopping circuits, an amplifier, first and second input terminals, and first and second output terminals. The amplifier includes a non-inverting input terminal, an inverting input terminal, an inverting output terminal, and a non-inverting output terminal. In a first period, the semiconductor device electrically connects the first input terminal and one terminal of the first capacitor, electrically connects the second input terminal and one terminal of the second capacitor, electrically connects the other terminal of the first capacitor and the first output terminal, and electrically connects the other terminal of the second capacitor and the second output terminal; the first chopping circuit electrically connects the other terminal of the first capacitor and the non-inverting input terminal and electrically connects the other terminal of the second capacitor and the inverting input terminal; and the second chopping circuit electrically connects the inverting output terminal and the first output terminal and electrically connects the non-inverting output terminal and the second output terminal. In a second period, the semiconductor device electrically connects the one terminal of the first capacitor and the first output terminal and electrically connects the one terminal of the second capacitor and the second output terminal; the first chopping circuit electrically connects the other terminal of the first capacitor and the non-inverting input terminal and electrically connects the other terminal of the second capacitor and the inverting input terminal, and the second chopping circuit electrically connects the inverting output terminal and the first output terminal and electrically connects the non-inverting output terminal and the second output terminal. In a third period, the semiconductor device electrically connects the one terminal of the first capacitor and the first output terminal and electrically connects the one terminal of the second capacitor and the second output terminal; the first chopping circuit electrically connects the other terminal of the first capacitor and the inverting input terminal and electrically connects the other terminal of the second capacitor and the non-inverting input terminal; and the second chopping circuit electrically connects the non-inverting output terminal and the first output terminal and electrically connects the inverting output terminal and the second output terminal.

In the above embodiment, the switch, the first chopping circuit, and the second chopping circuit each include a transistor, and the transistor includes a metal oxide in a channel formation region.

Another embodiment of the present invention is a semiconductor device including first to sixth switches, first and second capacitors, first and second chopping circuits, an amplifier, first and second input terminals, and first and second output terminals. The amplifier includes a non-inverting input terminal, an inverting input terminal, an inverted output terminal, and a non-inverting output terminal; the first chopping circuit includes first to fourth terminals; and the second chopping circuit includes fifth to eighth terminals. The first input terminal is electrically connected to one terminal of the first switch, the second input terminal is electrically connected to one terminal of the second switch, the other terminal of the first switch is electrically connected to one terminal of the third switch and one terminal of the first capacitor, the other terminal of the second switch is electrically connected to one terminal of the fourth switch and one terminal of the second capacitor, the other terminal of the first capacitor is electrically connected to one terminal of the fifth switch and the first terminal, and the other terminal of the second capacitor is electrically connected to one terminal of the sixth switch and the second terminal. The third terminal is electrically connected to the non-inverting input terminal, the fourth terminal is electrically connected to the inverting input terminal, the inverting output terminal is electrically connected to the fifth terminal, the non-inverting output terminal is electrically connected to the sixth terminal, the seventh terminal is electrically connected to the other terminal of the third switch, the other terminal of the fifth switch, and the first output terminal, and the eighth terminal is electrically connected to the other terminal of the fourth switch, the other terminal of the sixth switch, and the second output terminal. In a first period, the first chopping circuit has a function of bringing the first terminal and the third terminal into a conduction state and a function of bringing the second terminal and the fourth terminal into a conduction state, and the second chopping circuit has a function of bringing the fifth terminal and the seventh terminal into a conduction state and a function of bringing the sixth terminal and the eighth terminal into a conduction state. In a second period, the first chopping circuit has a function of bringing the first terminal and the fourth terminal into a conduction state and a function of bringing the second terminal and the third terminal into a conduction state, and the second chopping circuit has a function of bringing the fifth terminal and the eighth terminal into a conduction state and a function of bringing the sixth terminal and the seventh terminal into a conduction state.

In the above embodiment, the first to sixth switches, the first chopping circuit, and the second chopping circuit each include a transistor, and the transistor includes a metal oxide in a channel formation region.

Effect of the Invention

According to one embodiment of the present invention, a switched capacitor amplifier in which an effect of amplifier-induced noise on an output is reduced can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device including an amplifier, in which an effect of amplifier-induced noise on an output is reduced, can be provided. Alternatively, according to one embodiment of the present invention, a semiconductor device including an amplifier, in which the accuracy of the amplifier is improved, can be provided.

Note that the descriptions of the above effects do not preclude the existence of other effects. Note that one embodiment of the present invention does not necessarily have to achieve all the above effects and only needs to have at least one of the effects. Effects other than these are apparent from the descriptions of the specification, the claims, the drawings, and the like, and effects other than these can be derived from the descriptions of the specification, the claims, the drawings, and the like.

MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to the drawings. However, the embodiments can be implemented in many different modes, and it will be readily appreciated by those skilled in the art that modes and details thereof can be changed in various ways without departing from the spirit and scope thereof. Thus, the present invention should not be construed as being limited to the following description of the embodiments.

A plurality of embodiments described below can be combined as appropriate. In addition, in the case where a plurality of structure examples are described in one embodiment, the structure examples can be combined as appropriate.

Note that in the drawings attached to this specification, the block diagram in which components are classified according to their functions and shown as independent blocks is illustrated; however, it is difficult to completely separate actual components according to their functions, and it is possible for one component to relate to a plurality of functions.

In the drawings and the like, the size, the layer thickness, the region, or the like is exaggerated for clarity in some cases. Therefore, they are not limited to the illustrated scale. The drawings schematically show ideal examples, and shapes, values, or the like are not limited to shapes, values, or the like shown in the drawings.

In the drawings and the like, the same elements, elements having similar functions, elements formed of the same material, elements formed at the same time, or the like are sometimes denoted by the same reference numerals, and description thereof is not repeated in some cases.

In this specification and the like, the term “film” and the term “layer” can be interchanged with each other. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. As another example, the term “insulating film” can be changed into the term “insulating layer” in some cases.

In this specification and the like, the terms for describing arrangement such as “over” and “below” do not necessarily mean “directly over” and “directly below”, respectively, in the positional relationship between components. For example, the expression “a gate electrode over a gate insulating layer” does not exclude the case where there is an additional component between the gate insulating layer and the gate electrode.

In this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.

In this specification and the like, when a plurality of components are denoted by the same reference signs, and in particular need to be distinguished from each other, an identification sign such as “_1”,“_2”, “[n]”, or “[m, n]” is sometimes added to the reference signs. For example, the second wiring GL is referred to as a wiring GL[2].

In this specification and the like, “electrically connected” includes the case where connection is made through an “object having any electric function”. There is no particular limitation on the “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of the “object having any electric function” include a switching element such as a transistor, a resistor, an inductor, a capacitive element, and other elements with a variety of functions as well as an electrode and a wiring. Furthermore, even when the expression “being electrically connected” is used, there is a case in which no physical connection portion is made and a wiring is just extended in an actual circuit.

In addition, in this specification and the like, the term “electrode” or “wiring” does not functionally limit these components. For example, an “electrode” is used as part of a “wiring” in some cases, and vice versa.

In this specification and the like, a “terminal” in an electric circuit refers to a portion where a current or a potential is input (or output) or a signal is received (or transmitted). Accordingly, part of a wiring or an electrode functions as a terminal in some cases.

In general, a “capacitive element” has a structure in which two electrodes face each other with an insulator (dielectric) therebetween. Furthermore, in this specification and the like, cases where a “capacitive element” is one having a structure in which two electrodes face each other with an insulator therebetween, one having a structure in which two wirings face each other with an insulator therebetween, or one in which two wirings are positioned with an insulator therebetween, are included. In this specification and the like, a “capacitive element” is referred to as a “capacitor” or a “condenser” in some cases.

In this specification and the like, a “voltage” often refers to a potential difference between a given potential and a reference potential (e.g., a ground potential). Thus, a voltage and a potential difference can be interchanged with each other.

In this specification and the like, a transistor is an element having at least three terminals of a source, a drain, and a gate. Further, a channel formation region is included between the source (a source terminal, a source region, or a source electrode) and the drain (a drain terminal, a drain region, or a drain electrode), and a current can flow between the source and the drain through the channel formation region. Note that in this specification and the like, a channel formation region refers to a region through which a current mainly flows.

Furthermore, functions of a source and a drain might be switched when a transistor of opposite polarity is employed or the direction of current flow is changed in circuit operation, for example. Thus, the terms of a source and a drain are interchangeable in this specification and the like.

Furthermore, unless otherwise specified, an off-state current in this specification and the like refers to a drain current of a transistor in an off state (also referred to as a non-conduction state or a cutoff state). Unless otherwise specified, the off state of an n-channel transistor refers to a state where the voltage Vgs of a gate with respect to a source is lower than a threshold voltage Vth, and the off state of a p-channel transistor refers to a state where the voltage Vgs of a gate with respect to a source is higher than the threshold voltage Vth. That is, the off-state current of an n-channel transistor sometimes refers to a drain current at the time when the voltage Vgs of a gate with respect to a source is lower than the threshold voltage Vth.

In the above description of the off-state current, the drain may be replaced with the source. That is, the off-state current sometimes refers to a source current when a transistor is in the off state. In addition, a leakage current sometimes expresses the same meaning as the off-state current. Furthermore, in this specification and the like, the off-state current sometimes refers to a current that flows between a source and a drain when a transistor is in the off state.

Furthermore, in this specification and the like, an on-state current sometimes refers to a current that flows between a source and a drain when a transistor is in the on state (also referred to as a conduction state).

In this specification and the like, a metal oxide means an oxide of metal in a broad sense. Metal oxides are classified into an oxide insulator, an oxide conductor (including a transparent oxide conductor), an oxide semiconductor, and the like.

For example, in the case where a metal oxide is used in a channel formation region of a transistor, the metal oxide is referred to as an oxide semiconductor in some cases. That is to say, in the case where a metal oxide has at least one of an amplifying function, a rectifying function, and a switching function, the metal oxide can be referred to as a metal oxide semiconductor. In other words, a transistor including a metal oxide in a channel formation region can be referred to as an “oxide semiconductor transistor” or an “OS transistor”. Similarly, a “transistor using an oxide semiconductor” is also a transistor including a metal oxide in a channel formation region.

Furthermore, in this specification and the like, a metal oxide containing nitrogen is also referred to as a metal oxide in some cases. A metal oxide containing nitrogen may be referred to as a metal oxynitride. The details of a metal oxide will be described later.

In this embodiment, a configuration example and an operation example of a semiconductor device related to one embodiment of the present invention will be described.

<Configuration Example of Semiconductor Device>

FIG. 1Ais a block diagram showing a configuration example of a semiconductor device100according to one embodiment of the present invention. The semiconductor device100includes a switch SW1_1, a switch SW1_2, a switch SW2_1, a switch SW2_2, a switch SW3_1, a switch SW3_2, a capacitor C11, a capacitor C12, a chopping circuit20_1, a chopping circuit20_2, and an amplifier30.

The semiconductor device100also includes an input terminal INP, an input terminal INM, an output terminal OUTP, and an output terminal OUTM. The chopping circuit20_1, the chopping circuit20_2, and the amplifier30each include a first terminal to a fourth terminal. The first terminal to the fourth terminal included in each of the chopping circuit20_1, the chopping circuit20_2, and the amplifier30will be described later.

Note that in this specification and the like, a reference numeral such as “_1” or “_2” is used to distinguish a plurality of components having similar functions. That is, the reference character “SW1” is used to refer to either one of the switch SW1_1and the switch SW1_2without specifying which, and the reference character “SW1_1” or “SW1_2” is used to refer to a specific one of the switch SW1_1and the switch SW1_2.

In this specification and the like, expressions such as “input terminal”, “output terminal”, and “terminal” are used in order to describe input and output of signals and potentials between components; however, in some cases physical connecting portions such as “input terminal”, “output terminal”, and “terminal” do not exist in the actual circuit and the components are just electrically connected to each other via wirings, electrodes, or the like.

In the semiconductor device100, the input terminal INP is electrically connected to one terminal of the switch SW1_1, the input terminal INM is electrically connected to one terminal of the switch SW1_2, the other terminal of the switch SW1_1is electrically connected to one terminal of the switch SW3_1and one terminal of the capacitor C11, and the other terminal of the switch SW1_2is electrically connected to one terminal of the switch SW3_2and one terminal of the capacitor C12.

The other terminal of the capacitor C11is electrically connected to one terminal of the switch SW2_1and the first terminal of the chopping circuit20_1, the other terminal of the capacitor C12is electrically connected to one terminal of the switch SW2_2and the second terminal of the chopping circuit20_1, the third terminal of the chopping circuit20_1is electrically connected to the first terminal of the amplifier30, and the fourth terminal of the chopping circuit20_1is electrically connected to the second terminal of the amplifier30.

The third terminal of the amplifier30is electrically connected to the first terminal of the chopping circuit20_2; the fourth terminal of the amplifier30is electrically connected to the second terminal of the chopping circuit20_2; the third terminal of the chopping circuit20_2is electrically connected to the other terminal of the switch SW2_1, the other terminal of the switch SW3_1, and the output terminal OUTM; and the fourth terminal of the chopping circuit20_2is electrically connected to the other terminal of the switch SW2_2, the other terminal of the switch SW3_2, and the output terminal OUTP.

<Configuration Example of Switch>

The switch SW1can be formed using a transistor11, for example.FIG. 1Bis a diagram showing a symbol representing the switch SW1, andFIG. 1Cis a circuit diagram showing a configuration example of the switch SW1. Note that inFIG. 1Band the like, two terminals of the switch SW1are referred to as a terminal T11and a terminal T12.

As shown inFIG. 1C, the switch SW1includes the transistor11, one of a source and a drain of the transistor11is electrically connected to the terminal T11, and the other of the source and the drain of the transistor11is electrically connected to the terminal T12. A signal S1is input to a gate of the transistor11, and the switch SW1is a switch whose conduction state or non-conduction state is controlled by the signal S10. That is, when the signal S1is at a high level, the terminal T11and the terminal T12are brought into a conduction state, and when the signal S1is at a low level, the terminal T11and the terminal T12are brought into a non-conduction state.

The switch SW2can be formed using a transistor12, for example.FIG. 1Dis a diagram showing a symbol representing the switch SW2, andFIG. 1Eis a circuit diagram showing a configuration example of the switch SW2. Note that inFIG. 1Dand the like, two terminals of the switch SW2are referred to as a terminal T13and a terminal T14. A configuration example of the switch SW2is similar to that of the switch SW1, so the description thereof is omitted. The switch SW2is a switch whose conduction state or non-conduction state is controlled by a signal S2; when the signal S2is at the high level, the terminal T13and the terminal T14are brought into a conduction state, and when the signal S2is at the low level, the terminal T13and the terminal T14are brought into a non-conduction state.

The switch SW3can be formed using a transistor13, for example.FIG. 1Fis a diagram showing a symbol representing the switch SW3, andFIG. 1Gis a circuit diagram showing a configuration example of the switch SW3. Note that inFIG. 1Fand the like, two terminals of the switch SW3are referred to as a terminal T15and a terminal T16. A configuration example of the switch SW3is similar to that of the switch SW1, so the description thereof is omitted. The switch SW3is a switch whose conduction state or non-conduction state is controlled by a signal S3; when the signal S3is at the high level, the terminal T15and the terminal T16are brought into a conduction state, and when the signal S3is at the low level, the terminal T15and the terminal T16are brought into a non-conduction state.

<Configuration Example of Chopping Circuit>

The chopping circuit20can be formed using a transistor21to a transistor24, for example.FIG. 2Ais a diagram showing a symbol representing the chopping circuit20, andFIG. 2Bis a circuit diagram showing a configuration example of the chopping circuit20.

InFIG. 2Aand the like, four terminals of the chopping circuit20are referred to as a terminal T21to a terminal T24. In the chopping circuit20, the terminal T21corresponds to the above-mentioned first terminal, the terminal T22corresponds to the above-mentioned second terminal, the terminal T23corresponds to the above-mentioned third terminal, and the terminal T24corresponds to the above-mentioned fourth terminal.

As shown inFIG. 2B, the chopping circuit20includes the transistor21to the transistor24, the terminal T21is electrically connected to one of a source and a drain of the transistor21and one of a source and a drain of the transistor22, and the terminal T22is electrically connected to one of a source and a drain of the transistor23and one of a source and a drain of the transistor24. The terminal T23is electrically connected to the other of the source and the drain of the transistor21and the other of the source and the drain of the transistor24, and the terminal T24is electrically connected to the other of the source and the drain of the transistor23and the other of the source and the drain of the transistor22.

A signal S4is input to a gate of the transistor21and a gate of the transistor23, and a conduction state or a non-conduction state of each of the transistor21and the transistor23is controlled by the signal S4. A signal S5is input to a gate of the transistor22and a gate of the transistor24, and a conduction state or a non-conduction state of each of the transistor22and the transistor24is controlled by the signal S5.

The signal S5is an inverted signal of the signal S4. When the signal S4is at the high level, the signal S5is at the low level; and when the signal S4is at the low level, the signal S5is at the high level. That is, when the signal S4is at the high level, the terminal T21and the terminal T23are in a conduction state, and the terminal T22and the terminal T24are in a conduction state. When the signal S4is at the low level, the terminal T21and the terminal T24are in a conduction state, and the terminal T22and the terminal T23are in a conduction state.

OS transistors can be used as the transistor11to the transistor13, and the transistor21to the transistor24. An oxide semiconductor has a band gap of 2 eV or more and thus has a characteristic of an off-state current being extremely small. In an OS transistor, for example, a normalized off-state current per micrometer of channel width at a source-drain voltage of 10 V can be less than or equal to 10×10−21A (10 zeptoampere). Note that the details of the OS transistor will be described in Embodiment 2 and Embodiment 3.

An OS transistor has the following features. An OS transistor can be formed by a method such as a thin-film method, and thus can be provided over a semiconductor substrate. An off-state current of an OS transistor does not easily increase even under high temperature environments, so that the switch SW1to the switch SW3can be highly reliable switches, for example. OS transistors can be manufactured by using fabrication tools that are similar to those of transistors with silicon in a channel formation region, which enables OS transistors to be manufactured at low cost.

The transistor11to the transistor13, and the transistor21to the transistor24may each have a back gate (also referred to as a second gate or a bottom gate). In the case where the transistor11has a back gate, for example, the threshold voltage of the transistor11can be increased and decreased by application of a predetermined potential to the back gate of the transistor11. Alternatively, when the back gate of the transistor11is electrically connected to the gate (also referred to as a first gate, a top gate, or a front gate with respect to the back gate) of the transistor11, the on-state current of the transistor11can be increased.

A metal oxide used in the channel formation region of an OS transistor is preferably an oxide containing one or more elements selected from In, Ga, Sn, and Zn. As such an oxide, an In—Sn—Ga—Zn oxide, an In—Ga—Zn oxide, an In—Sn—Zn oxide, an In—Al—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Ga oxide, an In oxide, a Sn oxide, a Zn oxide, or the like can be used.

Alternatively, as each of the transistor11to the transistor13and the transistor21to the transistor24, a transistor other than an OS transistor may be used. The transistor11to the transistor13and the transistor21to the transistor24are each preferably a transistor with small off-state current, and for example, a transistor in which a semiconductor with a wide bandgap is included in a channel formation region can be used. The semiconductor with a wide bandgap refers to a semiconductor whose bandgap is larger than or equal to 2.2 eV in some cases, and examples thereof include silicon carbide, gallium nitride, and diamond.

<Configuration Example of Amplifier>

The amplifier30can be formed using a transistor31to a transistor39and a transistor41to a transistor44, for example.FIG. 2Cis a diagram showing a symbol representing the amplifier30, andFIG. 2Dis a circuit diagram showing a configuration example of the amplifier30.

InFIG. 2Cand the like, four terminals of the amplifier30are referred to as a terminal T31to a terminal T34. In the amplifier30, the terminal T31corresponds to the above-mentioned first terminal, the terminal T32corresponds to the above-mentioned second terminal, the terminal T33corresponds to the above-mentioned third terminal, and the terminal T34corresponds to the above-mentioned fourth terminal. The terminal T31can have a property of a non-inverting input terminal of the amplifier30, the terminal T32can have a property of an inverting input terminal of the amplifier30, the terminal T33can have a property of an inverting output terminal of the amplifier30, and the terminal T34can have a property of a non-inverting output terminal of the amplifier30.

As shown inFIG. 2D, the amplifier30includes a terminal T_VDD, a terminal T_BP, a terminal T_CP, a terminal T_COM, a terminal T_CN, and a terminal T_BN.

As shown inFIG. 2D, the amplifier30includes the transistor31to the transistor39and the transistor41to the transistor44; one of a source and a drain of the transistor31is electrically connected to the terminal T_VDD; and the other of the source and the drain of the transistor31is electrically connected to one of a source and a drain of the transistor32, one of a source and a drain of the transistor33, one of a source and a drain of the transistor34, and one of a source and a drain of the transistor35.

The other of the source and the drain of the transistor32is electrically connected to one of a source and a drain of the transistor41and one of a source and a drain of the transistor43, and the other of the source and the drain of the transistor33is electrically connected to a wiring supplied with a reference potential. The other of the source and the drain of the transistor34is electrically connected to the wiring supplied with the reference potential, and the other of the source and the drain of the transistor35is electrically connected to one of a source and a drain of the transistor42and one of a source and a drain of the transistor44.

One of a source and a drain of the transistor36and one of a source and a drain of the transistor37are electrically connected to the terminal T_VDD, the other of the source and the drain of the transistor36is electrically connected to one of a source and a drain of the transistor38, and the other of the source and the drain of the transistor37is electrically connected to one of a source and a drain of the transistor39. The other of the source and the drain of the transistor38is electrically connected to the terminal T33and the other of the source and the drain of the transistor41, the other of the source and the drain of the transistor39is electrically connected to the terminal T34and the other of the source and the drain of the transistor42, and the other of the source and the drain of the transistor43and the other of the source and the drain of the transistor44are electrically connected to the wiring supplied with the reference potential.

A gate of the transistor31, a gate of the transistor36, and a gate of the transistor37are electrically connected to the terminal T_BP; a gate of the transistor38and a gate of the transistor39are electrically connected to the terminal T_CP; a gate of the transistor32is electrically connected to the terminal T31; and a gate of the transistor35is electrically connected to the terminal T32. A gate of the transistor33and a gate of the transistor34are electrically connected to the terminal T_COM, a gate of the transistor41and a gate of the transistor42are electrically connected to the terminal T_CN, and a gate of the transistor43and a gate of the transistor44are electrically connected to the terminal T_BN.

A power supply potential VDD is input to the terminal T_VDD, and a bias potential that adjusts the operation of the amplifier30is input to each of the terminal T_BP, the terminal T_CP, the terminal T_COM, the terminal T_CN, and the terminal T_BN. A potential at the middle of potentials input to the terminal T31and the terminal T32is preferably input to the terminal T_COM, for example.

Transistors formed on a semiconductor substrate can be used as the transistor31to the transistor39and the transistor41to the transistor44. There is no particular limitation on the semiconductor substrate as long as a channel region of a transistor can be formed therein. For example, a single crystal silicon substrate, a single crystal germanium substrate, a compound semiconductor substrate (such as a SiC substrate or a GaN substrate), an SOI (Silicon on Insulator) substrate, or the like can be used.

As the SOI substrate, the following substrate may be used for example: an SIMOX (Separation by Implanted Oxygen) substrate which is formed in such a manner that after an oxygen ion is implanted into a mirror-polished wafer, an oxide layer is formed at a certain depth from the surface and defects generated in a surface layer are eliminated by high-temperature annealing; or an SOI substrate formed by using a method such as an ELTRAN method (a registered trademark: Epitaxial Layer Transfer) or a Smart-Cut method in which a semiconductor substrate is cleaved by utilizing growth of a minute void, which is formed by implantation of a hydrogen ion, by thermal treatment. A transistor formed using a single crystal substrate includes a single crystal semiconductor in a channel formation region.

In this embodiment, an example in which a single crystal silicon substrate is used as the semiconductor substrate will be described. A transistor formed on a single crystal silicon substrate is referred to as a “Si transistor”. In the configuration example of the amplifier30shown inFIG. 2D, the transistor31to the transistor39are p-channel transistors, and the transistor41to the transistor44are n-channel transistors.

<Operation Example of Semiconductor Device>

FIG. 3is a timing chart showing an operation example of the semiconductor device100. The timing chart inFIG. 3shows potential states (the high level or the low level) of the signal S1to the signal S5from Time T1to Time T10.

At Time T1, the signal S1and the signal S2change from the low level to the high level. The signal S3and the signal S5stay at the low level, and the signal S4stays at the high level. That is, the switch SW1and the switch SW2are in a conduction state, and the switch SW3is in a non-conduction state. In a period from Time T1to Time T2, sampling of the potentials input to the input terminal INP and the input terminal INM is performed.

With the sampling being performed, when a charge +Q11is stored in the one terminal of the capacitor C11, a charge −Q11is stored in the other terminal of the capacitor C11. In a similar manner, when a charge +Q12is stored in the one terminal of the capacitor C12, a charge −Q12is stored in the other terminal of the capacitor C12.

In a period from Time T1to Time T5, the first terminal and the third terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state, and the second terminal and the fourth terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state.FIG. 4Ashows an equivalent circuit of the semiconductor device100in which the states of the switch SW1to the switch SW3, the chopping circuit20_1, and the chopping circuit20_2in the period from Time T1to Time T2are reflected.

In this state, the signal S2changes from the high level to the low level at Time T2, thereby turning the switch SW2into a non-conduction state. The signal S1changes from the high level to the low level at Time T3, thereby turning the switch SW1into a non-conduction state. With the switch SW1and the switch SW2brought into a non-conduction state, the capacitor C11and the capacitor C12turn into a floating state (an electrically floating state).

The signal S3changes from the low level to the high level at Time T4, thereby turning the switch SW3into a conduction state. At this time, the charge +Q11remains stored in the one terminal of the capacitor C11, the charge −Q11remains stored in the other terminal of the capacitor C11, the charge +Q12remains stored in the one terminal of the capacitor C12, and the charge −Q12remains stored in the other terminal of the capacitor C12. Thus, a difference obtained by subtracting a potential output to the output terminal OUTM from a potential output to the output terminal OUTP is equal to a difference obtained by subtracting a potential input to the input terminal INM from a potential input to the input terminal INP in a period from Time T1to Time T2.

FIG. 4Bshows an equivalent circuit of the semiconductor device100which reflects the states of the switch SW1to the switch SW3, the chopping circuit20_1, and the chopping circuit20_2in a period from Time T4to Time T5.

In a period from Time T4to Time T10, the switch SW1and the switch SW2are in a non-conduction state, and the switch SW3is in a conduction state. The signal S4changes from the high level to the low level and the signal S5changes from the low level to the high level at Time T5. In other words, in a period from Time T5to Time T6, the first terminal and the fourth terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state, and the second terminal and the third terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state.

FIG. 4Cshows an equivalent circuit of the semiconductor device100which reflects the states of the switch SW1to the switch SW3, the chopping circuit20_1, and the chopping circuit20_2in the period from Time T5to Time T6.

In the period from Time T4to Time T10, the states of the switch SW1to the switch SW3stay the same, and the states of the chopping circuit20_1and the chopping circuit20_2change. That is, in the period from Time T4to Time T5, in a period from Time T6to Time T7, and in a period from Time T8to Time T9, the first terminal and the third terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state and the second terminal and the fourth terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state. The semiconductor device100has the state of the equivalent circuit shown inFIG. 4B.

In the period from Time T5to Time T6, in a period from Time T7to Time T8, and in a period from Time T9to Time T10, the first terminal and the fourth terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state and the second terminal and the third terminal of each of the chopping circuit20_1and the chopping circuit20_2are in a conduction state. The semiconductor device100has the state of the equivalent circuit shown inFIG. 4C.

As described above, the semiconductor device100performs sampling of the potentials input to the input terminal INP and the input terminal INM in the period from Time T1to Time T2, and outputs the potentials to the output terminal OUTP and the output terminal OUTM in the period from Time T4to Time T10. At this time, a difference obtained by subtracting the potential output to the output terminal OUTM from the potential output to the output terminal OUTP is equal to a difference obtained from subtracting the potential input to the input terminal INM from the potential input to the input terminal INP.

In the period from Time T4to Time T5, in the period from Time T6to Time T7, and in the period from Time T8to Time T9, the chopping circuit20_1electrically connects the other terminal of the capacitor C11and the first terminal of the amplifier30(to be in a conduction state), and electrically connects the other terminal of the capacitor C12and the second terminal of the amplifier30. Similarly, the chopping circuit20_2electrically connects the third terminal of the amplifier30and the output terminal OUTM, and electrically connects the fourth terminal of the amplifier30and the output terminal OUTP (the state of the equivalent circuit shown inFIG. 4B).

In the period from Time T5to Time T6, in the period from Time T7to Time T8, and in the period from Time T9to Time T10, the chopping circuit20_1electrically connects the other terminal of the capacitor C11and the second terminal of the amplifier30(to be in a conduction state), and electrically connects the other terminal of the capacitor C12and the first terminal of the amplifier30. Similarly, the chopping circuit20_2electrically connects the third terminal of the amplifier30and the output terminal OUTP, and electrically connects the fourth terminal of the amplifier30and the output terminal OUTM (the state of the equivalent circuit shown inFIG. 4C).

In other words, the semiconductor device100alternately has the state of the equivalent circuit shown inFIG. 4Band the state of the equivalent circuit shown inFIG. 4C, enabling an effect of offset voltage attributable to the amplifier30to be canceled, for example. In addition, by replacing the polarities of the input terminals of the amplifier30, for example, an effect of 1/f noise, thermal noise, or the like attributable to the amplifier30on an output can be reduced.

The semiconductor device100, in which transistors with a small off-state current such as OS transistors are used as the transistor11to the transistor13and the transistor21to the transistor24, can hold the potentials that are subjected to sampling in the period from Time T1to Time T2for a long time.

With the use of the semiconductor device100, a highly accurate amplifier with a reduced effect of offset voltage or noise attributable to the amplifier30can be achieved, and the sampled potentials can be held for a long time; thus, the semiconductor device100is suitable for highly accurate measurement such as a sensor with high output impedance, for example.

Note that this embodiment can be implemented in appropriate combination with the other embodiments described in this specification.

In this embodiment, structure examples of the transistor included in the semiconductor device100described in the above embodiment will be described. This embodiment has a structure where a layer including an OS transistor is provided to be stacked above a layer including a Si transistor formed on a single crystal silicon substrate.

<Structure Example of Semiconductor Device>

A semiconductor device shown inFIG. 5includes a transistor300, a transistor500, and a capacitive element600.FIG. 6Ais a cross-sectional view of the transistor500in the channel length direction,FIG. 6Bis a cross-sectional view of the transistor500in the channel width direction, andFIG. 6Cis a cross-sectional view of the transistor300in the channel width direction.

For example, the transistor500corresponds to the transistor21described in the above embodiment, and the transistor500includes a second gate (also referred to as a bottom gate or a back gate) in addition to a first gate (also referred to as a top gate or a front gate). Furthermore, the transistor300corresponds to a Si transistor included in the amplifier30, and the capacitive element600corresponds to, for example, the capacitor C11.

The transistor500is a transistor including a metal oxide in its channel-formation region (an OS transistor). The transistor500has an extremely small off-state current; in the above embodiment, the transistor500is used for the switch SW1to the switch SW3and the chopping circuit20, enabling the semiconductor device100to hold the sampled potential for a long time.

As shown inFIG. 5, in the semiconductor device described in this embodiment, the transistor500is provided above the transistor300, and the capacitive element600is provided above the transistor300and the transistor500.

The transistor300is provided on a substrate311and includes a conductor316, an insulator315, a semiconductor region313that is a part of the substrate311, and a low-resistance region314aand a low-resistance region314bfunctioning as a source region and a drain region.

As shown inFIG. 6C, in the transistor300, a top surface and a side surface in the channel width direction of the semiconductor region313are covered with the conductor316with the insulator315therebetween. Such a Fin-type transistor300can have an increased effective channel width, and thus the transistor300can have improved on-state characteristics. In addition, since contribution of an electric field of a gate electrode can be increased, the off-state characteristics of the transistor300can be improved.

Note that the transistor300can be either a p-channel transistor or an n-channel transistor.

It is preferable that a region of the semiconductor region313where a channel is formed, a region in the vicinity thereof, the low-resistance region314aand the low-resistance region314bfunctioning as the source region and the drain region, and the like contain a semiconductor such as a silicon-based semiconductor, further preferably single crystal silicon. Alternatively, these regions may be formed using a material containing Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), GaAlAs (gallium aluminum arsenide), or the like. A structure using silicon whose effective mass is controlled by applying stress to the crystal lattice and changing the lattice spacing may be employed. Alternatively, the transistor300may be an HEMT (High Electron Mobility Transistor) with GaAs and GaAlAs, or the like.

The low-resistance region314aand the low-resistance region314bcontain an element that imparts n-type conductivity, such as arsenic or phosphorus, or an element that imparts p-type conductivity, such as boron, in addition to the semiconductor material used for the semiconductor region313.

The conductor316functioning as a gate electrode can be formed using a semiconductor material such as silicon containing an element that imparts n-type conductivity, such as arsenic or phosphorus, or an element that imparts p-type conductivity, such as boron, or using a conductive material such as a metal material, an alloy material, or a metal oxide material.

Note that since the work function of a conductor depends on a material of the conductor, Vth of the transistor can be adjusted by changing the material of the conductor. Specifically, it is preferable to use a material such as titanium nitride or tantalum nitride for the conductor. Moreover, in order to ensure both conductivity and embeddability, it is preferable to use stacked layers of metal materials such as tungsten and aluminum for the conductor, and it is particularly preferable to use tungsten in terms of heat resistance.

Note that the transistor300shown inFIG. 5is an example and is not limited to the structure shown therein; an appropriate transistor is used in accordance with a circuit structure or a driving method.

An insulator320, an insulator322, an insulator324, and an insulator326are stacked in this order to cover the transistor300.

The insulator320, the insulator322, the insulator324, and the insulator326can be formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, or aluminum nitride.

The insulator322may have a function of a planarization film for planarizing a level difference caused by the transistor300or the like provided below the insulator322. For example, the top surface of the insulator322may be planarized by planarization treatment using a chemical mechanical polishing (CMP) method or the like to improve planarity.

The insulator324is preferably formed using a film having a barrier property that prevents diffusion of hydrogen or impurities from the substrate311, the transistor300, or the like into the region where the transistor500is provided.

For the film having a barrier property against hydrogen, silicon nitride formed by a CVD method can be used, for example. Here, diffusion of hydrogen to a semiconductor element including an oxide semiconductor, such as the transistor500, degrades the characteristics of the semiconductor element in some cases. Therefore, a film that inhibits hydrogen diffusion is preferably used between the transistor500and the transistor300. The film that inhibits hydrogen diffusion is specifically a film from which a small amount of hydrogen is released.

The amount of released hydrogen can be measured by thermal desorption spectroscopy (TDS), for example. The amount of hydrogen released from the insulator324that is converted into hydrogen atoms per area of the insulator324is less than or equal to 10×1015atoms/cm2, preferably less than or equal to 5×1015atoms/cm2, in the TDS analysis in a film-surface temperature range of 50° C. to 500° C., for example.

Note that the dielectric constant of the insulator326is preferably lower than that of the insulator324. For example, the dielectric constant of the insulator326is preferably lower than 4, further preferably lower than 3. The dielectric constant of the insulator326is, for example, preferably 0.7 times or less, further preferably 0.6 times or less the dielectric constant of the insulator324. When a material with a low dielectric constant is used for an interlayer film, the parasitic capacitance generated between wirings can be reduced.

A conductor328, a conductor330, and the like that are connected to the capacitive element600or the transistor500are embedded in the insulator320, the insulator322, the insulator324, and the insulator326. Note that the conductor328and the conductor330have a function of a plug or a wiring. A plurality of conductors functioning as plugs or wirings are collectively denoted by the same reference numeral in some cases. Moreover, in this specification and the like, a wiring and a plug connected to the wiring may be a single component. That is, there are cases where a part of a conductor functions as a wiring and another part of the conductor functions as a plug.

As a material for each of plugs and wirings (the conductor328, the conductor330, and the like), a single layer or stacked layers of a conductive material such as a metal material, an alloy material, a metal nitride material, or a metal oxide material can be used. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is preferable to use tungsten. Alternatively, it is preferable to use a low-resistance conductive material such as aluminum or copper. The use of a low-resistance conductive material can reduce wiring resistance.

A wiring layer may be provided over the insulator326and the conductor330. For example, inFIG. 5, an insulator350, an insulator352, and an insulator354are provided to be stacked in this order. Furthermore, a conductor356is formed in the insulator350, the insulator352, and the insulator354. The conductor356has a function of a plug or a wiring that is connected to the transistor300. Note that the conductor356can be provided using a material similar to that for the conductor328and the conductor330.

For example, like the insulator324, the insulator350is preferably formed using an insulator having a barrier property against hydrogen. The conductor356preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening portion of the insulator350having a barrier property against hydrogen. With this structure, the transistor300and the transistor500can be separated by the barrier layer, so that diffusion of hydrogen from the transistor300into the transistor500can be inhibited.

For the conductor having a barrier property against hydrogen, tantalum nitride is preferably used, for example. In addition, the use of a stack including tantalum nitride and tungsten, which has high conductivity, can inhibit diffusion of hydrogen from the transistor300while the conductivity of a wiring is maintained. In that case, a structure is preferable in which a tantalum nitride layer having a barrier property against hydrogen is in contact with the insulator350having a barrier property against hydrogen.

A wiring layer may be provided over the insulator354and the conductor356. For example, inFIG. 5, an insulator360, an insulator362, and an insulator364are provided to be stacked in this order. Furthermore, a conductor366is formed in the insulator360, the insulator362, and the insulator364. The conductor366has a function of a plug or a wiring. Note that the conductor366can be provided using a material similar to that for the conductor328and the conductor330.

For example, like the insulator324, the insulator360is preferably formed using an insulator having a barrier property against hydrogen. Furthermore, the conductor366preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening portion of the insulator360having a barrier property against hydrogen. With this structure, the transistor300and the transistor500can be separated by the barrier layer, so that diffusion of hydrogen from the transistor300into the transistor500can be inhibited.

A wiring layer may be provided over the insulator364and the conductor366. For example, inFIG. 5, an insulator370, an insulator372, and an insulator374are provided to be stacked in this order. Furthermore, a conductor376is formed in the insulator370, the insulator372, and the insulator374. The conductor376has a function of a plug or a wiring. Note that the conductor376can be provided using a material similar to the materials for the conductor328and the conductor330.

For example, like the insulator324, the insulator370is preferably formed using an insulator having a barrier property against hydrogen. Furthermore, the conductor376preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening portion of the insulator370having a barrier property against hydrogen. With this structure, the transistor300and the transistor500can be separated by the barrier layer, so that diffusion of hydrogen from the transistor300into the transistor500can be inhibited.

A wiring layer may be provided over the insulator374and the conductor376. For example, inFIG. 5, an insulator380, an insulator382, and an insulator384are provided to be stacked in this order. Furthermore, a conductor386is formed in the insulator380, the insulator382, and the insulator384. The conductor386has a function of a plug or a wiring. Note that the conductor386can be provided using a material similar to the materials for the conductor328and the conductor330.

For example, like the insulator324, the insulator380is preferably formed using an insulator having a barrier property against hydrogen. Furthermore, the conductor386preferably contains a conductor having a barrier property against hydrogen. In particular, the conductor having a barrier property against hydrogen is formed in an opening portion of the insulator380having a barrier property against hydrogen. With this structure, the transistor300and the transistor500can be separated by the barrier layer, so that diffusion of hydrogen from the transistor300into the transistor500can be inhibited.

Although the wiring layer including the conductor356, the wiring layer including the conductor366, the wiring layer including the conductor376, and the wiring layer including the conductor386are described above, the semiconductor device of this embodiment is not limited thereto. Three or less wiring layers that are similar to the wiring layer including the conductor356may be provided, or five or more wiring layers that are similar to the wiring layer including the conductor356may be provided.

An insulator510, an insulator512, an insulator514, and an insulator516are provided to be stacked in this order over the insulator384. A substance having a barrier property against oxygen or hydrogen is preferably used for any of the insulator510, the insulator512, the insulator514, and the insulator516.

The insulator510and the insulator514are preferably formed using, for example, a film having a barrier property that prevents diffusion of hydrogen or impurities from the substrate311, the region where the transistor300is provided, or the like into the region where the transistor500is provided. Thus, a material similar to that for the insulator324can be used.

For the film having a barrier property against hydrogen, silicon nitride formed by a CVD method can be used, for example. Here, diffusion of hydrogen to a semiconductor element including an oxide semiconductor, such as the transistor500, degrades the characteristics of the semiconductor element in some cases. Therefore, a film that inhibits hydrogen diffusion is preferably used between the transistor500and the transistor300. The film that inhibits hydrogen diffusion is specifically a film from which a small amount of hydrogen is released.

For the film having a barrier property against hydrogen used as the insulator510and the insulator514, for example, a metal oxide such as aluminum oxide, hafnium oxide, or tantalum oxide is preferably used.

In particular, aluminum oxide has a high blocking effect that inhibits the passage of both oxygen and impurities such as hydrogen and moisture which are factors of a change in electrical characteristics of the transistor. Thus, aluminum oxide can prevent the entry of impurities such as hydrogen and moisture into the transistor500in the fabrication process and after the fabrication of the transistor. In addition, release of oxygen from the oxide included in the transistor500can be inhibited. Therefore, aluminum oxide is suitably used for a protective film of the transistor500.

The insulator512and the insulator516can be formed using a material similar to that for the insulator320, for example. When a material with a relatively low permittivity is used for the interlayer film, the parasitic capacitance between wirings can be reduced. Silicon oxide films, silicon oxynitride films, or the like can be used as the insulator512and the insulator516, for example.

A conductor518, a conductor included in the transistor500(a conductor503), and the like are embedded in the insulator510, the insulator512, the insulator514, and the insulator516. Note that the conductor518has a function of a plug or a wiring that is connected to the capacitive element600or the transistor300. The conductor518can be provided using a material similar to the materials for the conductor328and the conductor330.

In particular, the conductor518in a region in contact with the insulator510and the insulator514is preferably a conductor having a barrier property against oxygen, hydrogen, and water. With this structure, the transistor300and the transistor500can be separated by the layer having a barrier property against oxygen, hydrogen, and water; thus, the diffusion of hydrogen from the transistor300into the transistor500can be inhibited.

The transistor500is provided above the insulator516.

As shown inFIG. 6AandFIG. 6B, the transistor500includes the conductor503positioned to be embedded in the insulator514and the insulator516; an insulator520positioned over the insulator516and the conductor503; an insulator522positioned over the insulator520; an insulator524positioned over the insulator522; an oxide530apositioned over the insulator524; an oxide530bpositioned over the oxide530a;a conductor542aand a conductor542bpositioned apart from each other over the oxide530b;an insulator580that is positioned over the conductor542aand the conductor542band is provided with an opening formed to overlap with a region between the conductor542aand the conductor542b;a conductor560positioned in the opening; an insulator550positioned between the conductor560and the oxide530b,the conductor542a,the conductor542b,and the insulator580; and an oxide530cpositioned between the insulator550and the oxide530b,the conductor542a,the conductor542b,and the insulator580.

As shown inFIG. 6AandFIG. 6B, an insulator544is preferably positioned between the insulator580and the oxide530a,the oxide530b,the conductor542a,and the conductor542b.In addition, as shown inFIG. 6AandFIG. 6B, the conductor560preferably includes a conductor560aprovided inside the insulator550and a conductor560bprovided to be embedded inside the conductor560a.As shown inFIG. 6AandFIG. 6B, an insulator574is preferably positioned over the insulator580, the conductor560, and the insulator550.

Hereinafter, the oxide530a,the oxide530b,and the oxide530cmay be collectively referred to as an oxide530. The conductor542aand the conductor542bmay be collectively referred to as a conductor542.

Note that the transistor500having a structure in which three layers of the oxide530a,the oxide530b,and the oxide530care stacked in the region where the channel is formed and its vicinity is illustrated; however, the present invention is not limited thereto. For example, a single layer of the oxide530b,a two-layer structure of the oxide530band the oxide530a,a two-layer structure of the oxide530band the oxide530c,or a stacked-layer structure of four or more layers may be employed. Furthermore, although the conductor560having a stacked-layer structure of two layers in the transistor500is illustrated, the present invention is not limited thereto. For example, the conductor560may have a single-layer structure or a stacked-layer structure of three or more layers. The transistor500shown inFIG. 5,FIG. 6A, andFIG. 6Bis an example, and the structure is not limited thereto; an appropriate transistor can be used in accordance with a circuit structure or a driving method.

Here, the conductor560functions as a gate electrode of the transistor, and the conductor542aand the conductor542bfunction as a source electrode and a drain electrode. As described above, the conductor560is formed to be embedded in the opening of the insulator580and the region between the conductor542aand the conductor542b.The positions of the conductor560, the conductor542a,and the conductor542bare selected in a self-aligned manner with respect to the opening of the insulator580. That is, in the transistor500, the gate electrode can be positioned between the source electrode and the drain electrode in a self-aligned manner. Thus, the conductor560can be formed without an alignment margin, resulting in a reduction in the area occupied by the transistor500. Accordingly, miniaturization and high integration of the semiconductor device can be achieved.

In addition, since the conductor560is formed in the region between the conductor542aand the conductor542bin a self-aligned manner, the conductor560does not include a region overlapping with the conductor542aor the conductor542b.Thus, parasitic capacitance formed between the conductor560and each of the conductor542aand the conductor542bcan be reduced. As a result, the switching speed of the transistor500can be improved, and the transistor500can have high frequency characteristics.

The conductor560sometimes functions as a first gate electrode. In addition, the conductor503sometimes functions as a second gate electrode. In that case, Vth of the transistor500can be controlled by changing a potential applied to the conductor503independently of a potential applied to the conductor560. In particular, Vth of the transistor500can be higher than 0 V and the off-state current can be reduced by applying a negative potential to the conductor503. Thus, a drain current at the time when a potential applied to the conductor560is 0 V can be lower in the case where a negative potential is applied to the conductor503than in the case where a negative potential is not applied to the conductor503.

The conductor503is positioned to overlap with the oxide530and the conductor560. Thus, when potentials are applied to the conductor560and the conductor503, an electric field generated from the conductor560and an electric field generated from the conductor503are connected, so that the channel-formation region formed in the oxide530can be covered. In this specification and the like, a transistor structure in which a channel-formation region is electrically surrounded by electric fields of a first gate electrode and a second gate electrode is referred to as a surrounded channel (S-channel) structure.

Furthermore, in this specification and the like, the S-channel structure has a feature that the side surface and the vicinity of the oxide530in contact with the conductor542aand the conductor542bfunctioning as the source electrode and the drain electrode are of i-type like the channel-formation region. The side surface and the vicinity of the oxide530in contact with the conductor542aand the conductor542bare in contact with the insulator544and thus can be of i-type like the channel-formation region. Note that in this specification and the like, “i-type” can be equated with “highly purified intrinsic” to be described later. The S-channel structure disclosed in this specification and the like is different from a Fin-type structure and a planar structure. With the S-channel structure, resistance to a short-channel effect can be enhanced, that is, a transistor in which a short-channel effect is less likely to occur can be provided.

The conductor503has a structure similar to that of the conductor518; a conductor503ais formed in contact with an inner wall of an opening in the insulator514and the insulator516, and a conductor503bis formed on the inner side.

The insulator520, the insulator522, the insulator524, and the insulator550each have a function of a gate insulating film.

Here, as the insulator524in contact with the oxide530, an insulator that contains oxygen more than oxygen in the stoichiometric composition is preferably used. That is, an excess-oxygen region is preferably formed in the insulator524. When such an insulator containing excess oxygen is provided in contact with the oxide530, oxygen vacancies in the oxide530can be reduced and the reliability of the transistor500can be improved.

As the insulator including an excess-oxygen region, specifically, an oxide material from which part of oxygen is released by heating is preferably used. An oxide from which oxygen is released by heating is an oxide film in 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 1.0×1019atoms/cm3, further preferably greater than or equal to 2.0×1019atoms/cm3or greater than or equal to 3.0×1020atoms/cm3in TDS (Thermal Desorption Spectroscopy) analysis. Note that the temperature of the film surface in the TDS analysis is preferably within the range of 100° C. to 700° C., or 100° C. to 400° C.

When the insulator524includes an excess-oxygen region, it is preferable that the insulator522have a function of inhibiting diffusion of oxygen (e.g., oxygen atoms and oxygen molecules) (or that the above oxygen be less likely to pass through the insulator522).

When the insulator522has a function of inhibiting diffusion of oxygen or impurities, oxygen contained in the oxide530is not diffused to the insulator520side, which is preferable. Furthermore, the conductor503can be inhibited from reacting with oxygen contained in the insulator524or the oxide530.

For example, the insulator522is preferably formed using a single layer or stacked layers of an insulator containing aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO3), (Ba,Sr)TiO3(BST), or the like. With miniaturization and high integration of transistors, a problem such as leakage current may arise because of a thinner gate insulating film. When a high-k material is used for the insulator functioning as the gate insulating film, a gate potential at the time when the transistor operates can be reduced while the physical thickness is maintained.

It is particularly preferable to use an insulator containing an oxide of one or both of aluminum and hafnium, which is an insulating material having a function of inhibiting diffusion of impurities, oxygen, and the like (through which the above oxygen is less likely to pass). As the insulator containing an oxide of one or both of aluminum and hafnium, aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), or the like is preferably used. In the case where the insulator522is formed using such a material, the insulator522functions as a layer that inhibits release of oxygen from the oxide530and mixing of impurities such as hydrogen from the periphery of the transistor500into the oxide530.

It is preferable that the insulator520be thermally stable. For example, silicon oxide and silicon oxynitride, which have thermal stability, are preferable. Furthermore, when an insulator that is a high-k material is combined with silicon oxide or silicon oxynitride, the insulator520having a stacked-layer structure that has thermal stability and a high dielectric constant can be obtained.

Note that the insulator520, the insulator522, and the insulator524may each have a stacked-layer structure of two or more layers. In such cases, without limitation to a stacked-layer structure formed of the same material, a stacked-layer structure formed of different materials may be employed.

In the transistor500, a metal oxide functioning as an oxide semiconductor is preferably used as the oxide530including the channel-formation region. For example, as the oxide530, a metal oxide such as an In-M-Zn oxide (the element M is one or more kinds selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like) is preferably used. Furthermore, as the oxide530, an In—Ga oxide or an In—Zn oxide may be used.

Note that the metal oxide functioning as an oxide semiconductor may be formed by a sputtering method or an ALD (Atomic Layer Deposition) method. The metal oxide functioning as an oxide semiconductor will be described in another embodiment.

Furthermore, a metal oxide with a low carrier density is preferably used in the transistor500. In order to reduce the carrier density of the metal oxide, the concentration of impurities in the metal oxide is reduced so that the density of defect states can be reduced. In this specification and the like, a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state. Examples of impurities in a metal oxide include hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, and silicon.

In particular, hydrogen contained in a metal oxide reacts with oxygen bonded to a metal atom to be water, and thus forms oxygen vacancies in the metal oxide in some cases. If the channel-formation region in the metal oxide includes oxygen vacancies, the transistor sometimes has normally-on characteristics. In some cases, a defect that is an oxygen vacancy into which hydrogen has entered functions as a donor and generates an electron serving as a carrier. In other cases, bonding of part of hydrogen to oxygen bonded to a metal atom generates electrons serving as carriers. Thus, a transistor using a metal oxide containing a large amount of hydrogen is likely to have normally-on characteristics.

A defect that is an oxygen vacancy into which hydrogen has entered can function as a donor of a metal oxide. However, it is difficult to evaluate the defects quantitatively. Thus, the metal oxide is sometimes evaluated by not its donor concentration but its carrier density. Therefore, in this specification and the like, as the parameter of the metal oxide, the carrier density assuming the state where an electric field is not applied is sometimes used instead of the donor concentration. That is, “carrier density” in this specification and the like can be replaced with “donor concentration” in some cases.

Consequently, when a metal oxide is used for the oxide530, hydrogen in the metal oxide is preferably reduced as much as possible. Specifically, the hydrogen concentration of the metal oxide, which is measured by secondary ion mass spectrometry (SIMS), is lower than 1×1020atoms/cm3, preferably lower than 1×1019atoms/cm3, further preferably lower than 5×1018atoms/cm3, and still further preferably lower than 1×1018atoms/cm3. When a metal oxide with a sufficiently low concentration of impurities such as hydrogen is used for a channel-formation region of a transistor, the transistor can have stable electrical characteristics.

When a metal oxide is used for the oxide530, the carrier density of the metal oxide in the channel-formation region is preferably lower than or equal to 1×1018cm−3, further preferably lower than 1×1017cm−3, still further preferably lower than 1×1016cm−3, yet further preferably lower than 1×1013cm−3, and yet still further preferably lower than 1×1012cm−3. Note that the lower limit of the carrier density of the metal oxide in the channel-formation region is not particularly limited and can be, for example, 1×10−9cm−3.

When a metal oxide is used for the oxide530, contact between the conductor542(the conductor542aand the conductor542b) and the oxide530may make oxygen in the oxide530diffuse into the conductor542, resulting in oxidation of the conductor542. It is highly possible that oxidation of the conductor542lowers the conductivity of the conductor542. Note that diffusion of oxygen in the oxide530into the conductor542can be interpreted as absorption of oxygen in the oxide530by the conductor542.

When oxygen in the oxide530is diffused into the conductor542(the conductor542aand the conductor542b), a layer is sometimes formed between the conductor542aand the oxide530b,and between the conductor542band the oxide530b.The layer contains more oxygen than the conductor542does, and thus presumably has an insulating property. In this case, a three-layer structure of the conductor542, the layer, and the oxide530bcan be regarded as a three-layer structure of a metal, an insulator, and a semiconductor and is sometimes referred to as a MIS (Metal-Insulator-Semiconductor) structure or a diode junction structure having an MIS structure as its main part.

The above layer is not necessarily formed between the conductor542and the oxide530b,but the layer may be formed between the conductor542and the oxide530c,or formed between the conductor542and the oxide530band between the conductor542and the oxide530c.

The metal oxide functioning as the channel-formation region in the oxide530has a bandgap of preferably 2 eV or larger, further preferably 2.5 eV or larger. With the use of a metal oxide having such a wide bandgap, the off-state current of the transistor can be reduced.

Semiconductor materials that can be used for the oxide530are not limited to the above metal oxides. A semiconductor material having a bandgap (a semiconductor material that is not a zero-gap semiconductor) can be used for the oxide530. For example, a single element semiconductor such as silicon, a compound semiconductor such as gallium arsenide, or a layered material (also referred to as an atomic layered material or a two-dimensional material) is preferably used as a semiconductor material. In particular, a layered material functioning as a semiconductor is preferably used as a semiconductor material.

Here, in this specification and the like, the layered material is a general term of a group of materials having a layered crystal structure. In the layered crystal structure, layers formed by covalent bonding or ionic bonding are stacked with bonding such as the Van der Waals force, which is weaker than covalent bonding or ionic bonding. The layered material has high electrical conductivity in a monolayer, that is, high two-dimensional electrical conductivity. When a material functioning as a semiconductor and having high two-dimensional electrical conductivity is used for a channel-formation region, a transistor having a high on-state current can be provided.

Examples of the layered material include graphene, silicene, and chalcogenide. Chalcogenide is a compound containing chalcogen. Chalcogen is a general term of elements belonging to Group 16, which includes oxygen, sulfur, selenium, tellurium, polonium, and livermorium. Examples of chalcogenide include transition metal chalcogenide and chalcogenide of Group 13 elements.

When the oxide530includes the oxide530aunder the oxide530b,it is possible to inhibit diffusion of impurities into the oxide530bfrom the components formed below the oxide530a.Moreover, including the oxide530cover the oxide530bmakes it possible to inhibit diffusion of impurities into the oxide530bfrom the components formed above the oxide530c.

Note that the oxide530preferably has a stacked-layer structure of a plurality of oxide layers that differ in the atomic ratio of metal atoms. Specifically, the atomic proportion of the element M in the constituent elements in the metal oxide used as the oxide530ais preferably higher than the atomic proportion of the element Min the constituent elements in the metal oxide used as the oxide530b.In addition, the atomic ratio of the element M to In in the metal oxide used as the oxide530ais preferably higher than the atomic ratio of the element M to In in the metal oxide used as the oxide530b.Furthermore, the atomic ratio of In to the element M in the metal oxide used as the oxide530bis preferably higher than the atomic ratio of In to the element

Min the metal oxide used as the oxide530a.A metal oxide that can be used as the oxide530aor the oxide530bcan be used as the oxide530c.

The energy of the conduction band minimum of each of the oxide530aand the oxide530cis preferably higher than the energy of the conduction band minimum of the oxide530b.

In other words, the electron affinity of each of the oxide530aand the oxide530cis preferably smaller than the electron affinity of the oxide530b.

The energy level of the conduction band minimum gradually changes at junction portions of the oxide530a,the oxide530b,and the oxide530c.In other words, the energy level of the conduction band minimum at the junction portions of the oxide530a,the oxide530b,and the oxide530ccontinuously changes or is continuously connected. To obtain this, the density of defect states in a mixed layer formed at the interface between the oxide530aand the oxide530band the interface between the oxide530band the oxide530cis preferably made low.

Specifically, when the oxide530aand the oxide530bor the oxide530band the oxide530ccontain a common element (as a main component) in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, in the case where the oxide530bis an In—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like is preferably used as the oxide530aand the oxide530c.

At this time, the oxide530bserves as a main carrier path. When the oxide530aand the oxide530chave the above structure, the density of defect states at the interface between the oxide530aand the oxide530band the interface between the oxide530band the oxide530ccan be made low. Thus, the influence of interface scattering on carrier conduction is small, and the transistor500can have a high on-state current.

As shown inFIG. 6A, a region543(a region543aand a region543b) is sometimes formed as a low-resistance region at and near the interface between the oxide530and the conductor542. In that case, the region543afunctions as one of a source region and a drain region, and the region543bfunctions as the other of the source region and the drain region. Furthermore, the channel-formation region is formed in a region between the region543aand the region543b.

When the conductor542is provided in contact with the oxide530, the oxygen concentration in the region543sometimes decreases. In addition, a metal compound layer that contains the metal contained in the conductor542and the component of the oxide530is sometimes formed in the region543. In such a case, the carrier density of the region543increases, and the region543becomes a low-resistance region.

The insulator544is provided to cover the conductor542and inhibits oxidation of the conductor542. At this time, the insulator544may be provided to cover a side surface of the oxide530and to be in contact with the insulator524.

A metal oxide containing one or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium, and the like can be used as the insulator544.

For the insulator544, it is particularly preferable to use an insulator containing an oxide of one or both of aluminum and hafnium, for example, aluminum oxide, hafnium oxide, or an oxide containing aluminum and hafnium (hafnium aluminate). In particular, hafnium aluminate has higher heat resistance than a hafnium oxide film. Therefore, hafnium aluminate is preferable because it is less likely to be crystallized by heat treatment in a later step. Note that the insulator544is not an essential component when the conductor542is an oxidation-resistant material or does not significantly lose its conductivity even after absorbing oxygen. Design is appropriately set in consideration of required transistor characteristics.

The insulator550functions as a gate insulating film. The insulator550is preferably positioned in contact with the inner side (the top surface and the side surface) of the oxide530c.The insulator550is preferably formed using an insulator from which oxygen is released by heating. For example, the insulator550is an oxide film in 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 1.0×1019atoms/cm3, further preferably greater than or equal to 2.0×1019atoms/cm3or greater than or equal to 3.0×1020atoms/cm3in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably within the range of 100° C. to 700° C.

Specifically, silicon oxide containing excess oxygen, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, or the like can be used. In particular, silicon oxide and silicon oxynitride, which have thermal stability, are preferable.

When an insulator from which oxygen is released by heating is provided as the insulator550in contact with the top surface of the oxide530c,oxygen can be effectively supplied from the insulator550to the channel-formation region of the oxide530bthrough the oxide530c.Furthermore, as in the insulator524, the concentration of impurities such as water or hydrogen in the insulator550is preferably reduced. The thickness of the insulator550is preferably greater than or equal to 1 nm and less than or equal to 20 nm.

To efficiently supply excess oxygen contained in the insulator550to the oxide530, a metal oxide may be provided between the insulator550and the conductor560. The metal oxide preferably inhibits diffusion of oxygen from the insulator550into the conductor560. Providing the metal oxide that inhibits diffusion of oxygen inhibits diffusion of excess oxygen from the insulator550into the conductor560. That is, a reduction in the amount of excess oxygen supplied to the oxide530can be inhibited. Moreover, oxidation of the conductor560due to excess oxygen can be inhibited. For the metal oxide, a material that can be used for the insulator544is used.

Although the conductor560functioning as the first gate electrode has a two-layer structure inFIG. 6AandFIG. 6B, the conductor560may have a single-layer structure or a stacked-layer structure of three or more layers.

For the conductor560a,it is preferable to use a conductive material having a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, a nitrogen atom, a nitrogen molecule, a nitrogen oxide molecule (N2O, NO, NO2, and the like), and a copper atom. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like). When the conductor560ahas a function of inhibiting diffusion of oxygen, it is possible to inhibit a reduction in conductivity of the conductor560bdue to oxidation caused by oxygen contained in the insulator550. As a conductive material having a function of inhibiting diffusion of oxygen, for example, tantalum, tantalum nitride, ruthenium, ruthenium oxide, or the like is preferably used.

The conductor560bis preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. Furthermore, the conductor560balso functions as a wiring and thus is preferably a conductor having high conductivity. For example, a conductive material containing tungsten, copper, or aluminum as its main component can be used. The conductor560bcan have a stacked-layer structure, for example, a stacked-layer structure of any of the above conductive materials and titanium or titanium nitride.

The insulator580is provided over the conductor542with the insulator544therebetween. The insulator580preferably includes an excess-oxygen region. For example, the insulator580preferably contains silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, a resin, or the like. In particular, silicon oxide and silicon oxynitride, which have thermal stability, are preferable. In particular, silicon oxide and porous silicon oxide are preferable because an excess-oxygen region can be easily formed in a later step.

When the insulator580from which oxygen is released by heating is provided in contact with the oxide530c,oxygen in the insulator580can be efficiently supplied to the oxide530through the oxide530c.Note that the concentration of impurities such as water or hydrogen in the insulator580is preferably reduced.

The opening in the insulator580is formed to overlap with the region between the conductor542aand the conductor542b.Accordingly, the conductor560is formed to be embedded in the opening in the insulator580and the region between the conductor542aand the conductor542b.

The gate length needs to be short for miniaturization of the semiconductor device, but it is necessary to prevent a reduction in conductivity of the conductor560. When the conductor560is made thick to achieve this, the conductor560might have a shape with a high aspect ratio. In this embodiment, the conductor560is provided to be embedded in the opening in the insulator580; thus, even when the conductor560has a shape with a high aspect ratio, the conductor560can be formed without collapsing during the process.

The insulator574is preferably provided in contact with the top surface of the insulator580, the top surface of the conductor560, and the top surface of the insulator550. When the insulator574is deposited by a sputtering method, an excess-oxygen region can be provided in the insulator550and the insulator580. Thus, oxygen can be supplied from the excess-oxygen region to the oxide530.

As the insulator574, a metal oxide containing one or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium, and the like can be used, for example.

In particular, aluminum oxide has a high barrier property, and even a thin aluminum oxide film having a thickness greater than or equal to 0.5 nm and less than or equal to 3.0 nm can inhibit diffusion of hydrogen and nitrogen. Thus, aluminum oxide deposited by a sputtering method serves as an oxygen supply source and can also have a function of a barrier film against impurities such as hydrogen.

An insulator581functioning as an interlayer film is preferably provided over the insulator574. As in the insulator524and the like, the concentration of impurities such as water or hydrogen in the insulator581is preferably reduced.

A conductor540aand a conductor540bare positioned in openings formed in the insulator581, the insulator574, the insulator580, and the insulator544. The conductor540aand the conductor540bare provided to face each other with the conductor560therebetween. The structures of the conductor540aand the conductor540bare similar to the structures of a conductor546and a conductor548that will be described later.

An insulator582is provided over the insulator581. A substance having a barrier property against oxygen or hydrogen is preferably used for the insulator582. Therefore, a material similar to that for the insulator514can be used for the insulator582. For the insulator582, a metal oxide such as aluminum oxide, hafnium oxide, or tantalum oxide is preferably used, for example.

In particular, aluminum oxide has a high blocking effect that inhibits the passage of both oxygen and impurities such as hydrogen and moisture which are factors of a change in electrical characteristics of the transistor. Thus, aluminum oxide can prevent the entry of impurities such as hydrogen and moisture into the transistor500in the fabrication process and after the fabrication of the transistor. In addition, release of oxygen from the oxide included in the transistor500can be inhibited. Therefore, aluminum oxide is suitably used for a protective film of the transistor500.

An insulator586is provided over the insulator582. For the insulator586, a material similar to that for the insulator320can be used. When a material with a relatively low permittivity is used for the interlayer film, the parasitic capacitance between wirings can be reduced. A silicon oxide film, a silicon oxynitride film, or the like can be used for the insulator586, for example.

The conductor546, the conductor548, and the like are embedded in the insulator520, the insulator522, the insulator524, the insulator544, the insulator580, the insulator574, the insulator581, the insulator582, and the insulator586.

The conductor546and the conductor548have functions of plugs or wirings that are connected to the capacitive element600, the transistor500, or the transistor300. The conductor546and the conductor548can be provided using a material similar to the materials for the conductor328and the conductor330.

Next, the capacitive element600is provided above the transistor500. The capacitive element600includes a conductor610, a conductor620, and an insulator630.

A conductor612may be provided over the conductor546and the conductor548. The conductor612has a function of a plug or a wiring that is connected to the transistor500. The conductor610has a function of an electrode of the capacitive element600. The conductor612and the conductor610can be formed at the same time.

The conductor612and the conductor610can be formed using a metal film containing an element selected from molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, and scandium; a metal nitride film containing any of the above elements as its component (a tantalum nitride film, a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film); or the like. Alternatively, it is possible to use a conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added.

Although the conductor612and the conductor610having a single-layer structure are illustrated inFIG. 5, the structure is not limited thereto, and a stacked-layer structure of two or more layers may be employed. For example, between a conductor having a barrier property and a conductor having high conductivity, a conductor that is highly adhesive to the conductor having a barrier property and the conductor having high conductivity may be formed.

The conductor620is provided to overlap with the conductor610with the insulator630therebetween. Note that the conductor620can be formed using a conductive material such as a metal material, an alloy material, or a metal oxide material. It is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum, and it is particularly preferable to use tungsten. In addition, in the case where the conductor620is formed concurrently with another component such as a conductor, Cu (copper), Al (aluminum), or the like, which is a low-resistance metal material, is used.

An insulator650is provided over the conductor620and the insulator630. The insulator650can be provided using a material similar to that for the insulator320. The insulator650may function as a planarization film that covers an uneven shape thereunder.

With the use of this structure, a change in electrical characteristics can be inhibited and reliability can be improved in a semiconductor device including an OS transistor. Alternatively, an OS transistor having a high on-state current can be provided. Alternatively, an OS transistor having a low off-state current can be provided. Alternatively, a semiconductor device with low power consumption can be provided. Alternatively, a semiconductor device including an OS transistor can be miniaturized or highly integrated.

<Structure Example of Transistor>

Note that the structure of the transistor500in the semiconductor device described in this embodiment is not limited to the above. Examples of structures that can be used for the transistor500will be described below.

A structure example of a transistor510A is described with reference toFIG. 7A,FIG. 7B, andFIG. 7C.FIG. 7Ais a top view of the transistor510A.FIG. 7Bis a cross-sectional view of a portion indicated by the dashed-dotted line L1-L2inFIG. 7A.FIG. 7Cis a cross-sectional view of a portion indicated by the dashed-dotted line W1-W2inFIG. 7A. Note that for clarity of the drawing, some components are not illustrated in the top view ofFIG. 7A.

FIG. 7A,FIG. 7B, andFIG. 7Cshow the transistor510A and the insulator511, the insulator512, the insulator514, the insulator516, the insulator580, the insulator582, and an insulator584that function as interlayer films. In addition, the conductor546(a conductor546aand a conductor546b) that is electrically connected to the transistor510A and functions as a contact plug, and the conductor503functioning as a wiring are illustrated.

The transistor510A includes the conductor560(the conductor560aand the conductor560b) functioning as a first gate electrode; a conductor505(a conductor505aand a conductor505b) functioning as a second gate electrode; the insulator550functioning as a first gate insulating film; an insulator521, the insulator522, and the insulator524that function as a second gate insulating film; the oxide530(the oxide530a,the oxide530b,and the oxide530c) including a region where a channel is formed; the conductor542afunctioning as one of a source and a drain; the conductor542bfunctioning as the other of the source and the drain; and the insulator574.

In the transistor510A shown inFIG. 7B, the oxide530c,the insulator550, and the conductor560are positioned in an opening portion provided in the insulator580with the insulator574therebetween. Moreover, the oxide530c,the insulator550, and the conductor560are positioned between the conductor542aand the conductor542b.

The insulator511and the insulator512each function as an interlayer film.

As the interlayer film, a single layer or stacked layers of an insulator such as silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, hafnium oxide, hafnium oxynitride, hafnium nitride oxide, hafnium nitride, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO3), or (Ba,Sr)TiO3(BST) can be used. Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobium oxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, or zirconium oxide may be added to these insulators, for example. Alternatively, these insulators may be subjected to nitriding treatment. Silicon oxide, silicon oxynitride, or silicon nitride may be stacked over the insulator.

For example, the insulator511preferably functions as a barrier film that inhibits entry of impurities such as water or hydrogen into the transistor510A from the substrate side. Accordingly, for the insulator511, it is preferable to use an insulating material that has a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, and a copper atom (through which the above impurities are less likely to pass). Alternatively, it is preferable to use an insulating material that has a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like) (through which the above oxygen is less likely to pass). Moreover, aluminum oxide or silicon nitride, for example, may be used for the insulator511. This structure can inhibit diffusion of impurities such as hydrogen and water to the transistor510A side from the substrate side through the insulator511.

For example, the permittivity of the insulator512is preferably lower than that of the insulator511. When a material with a low permittivity is used for the interlayer film, the parasitic capacitance generated between wirings can be reduced.

The conductor503is formed to be embedded in the insulator512. Here, the top surface of the conductor503and the top surface of the insulator512can be substantially level with each other. Note that although the conductor503has a single-layer structure, the present invention is not limited thereto. For example, the conductor503may have a multilayer structure of two or more layers. Note that for the conductor503, a conductive material that has high conductivity and contains tungsten, copper, or aluminum as its main component is preferably used.

In the transistor510A, the conductor560sometimes functions as a first gate electrode. The conductor505sometimes functions as a second gate electrode. In that case, the threshold voltage of the transistor510A can be controlled by changing a potential applied to the conductor505independently of a potential applied to the conductor560. In particular, when a negative potential is applied to the conductor505, the threshold voltage of the transistor510A can be higher than 0 V, and the off-state current can be reduced. Thus, a drain current at the time when a potential applied to the conductor560is 0 V can be lower in the case where a negative potential is applied to the conductor505than in the case where a negative potential is not applied to the conductor505.

For example, when the conductor505and the conductor560are provided to overlap with each other, in the case where a potential is applied to the conductor560and the conductor505, an electric field generated from the conductor560and an electric field generated from the conductor505are connected, so that the channel-formation region formed in the oxide530can be covered.

That is, the channel-formation region can be electrically surrounded by the electric field of the conductor560having a function of the first gate electrode and the electric field of the conductor505having a function of the second gate electrode. In other words, the transistor510A has a surrounded channel (S-channel) structure, like the transistor500described above.

Like the insulator511or the insulator512, the insulator514and the insulator516each function as an interlayer film. For example, the insulator514preferably functions as a barrier film that inhibits entry of impurities such as water or hydrogen into the transistor510A from the substrate side. This structure can inhibit diffusion of impurities such as hydrogen and water to the transistor510A side from the substrate side through the insulator514. Moreover, for example, the insulator516preferably has a lower permittivity than the insulator514. When a material with a low permittivity is used for the interlayer film, the parasitic capacitance generated between wirings can be reduced.

In the conductor505functioning as the second gate, the conductor505ais formed in contact with an inner wall of an opening in the insulator514and the insulator516, and the conductor505bis formed further inside. Here, the top surfaces of the conductor505aand the conductor505band the top surface of the insulator516can be substantially level with each other. Although the transistor510A has a structure in which the conductor505aand the conductor505bare stacked, the present invention is not limited thereto. For example, the conductor505may have a single-layer structure or a stacked-layer structure of three or more layers.

Here, for the conductor505a,it is preferable to use a conductive material that has a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, and a copper atom (through which the above impurities are less likely to pass). Alternatively, it is preferable to use a conductive material that has a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like) (through which the above oxygen is less likely to pass). Note that in this specification and the like, a function of inhibiting diffusion of impurities or oxygen means a function of inhibiting diffusion of any one or all of the above impurities and the above oxygen.

For example, when the conductor505ahas a function of inhibiting diffusion of oxygen, a reduction in conductivity of the conductor505bdue to oxidation can be inhibited.

In the case where the conductor505doubles as a wiring, the conductor505bis preferably formed using a conductive material that has high conductivity and contains tungsten, copper, or aluminum as its main component. In that case, the conductor503is not necessarily provided. Note that the conductor505bis illustrated as a single layer but may have a stacked-layer structure, for example, a stack of any of the above conductive materials and titanium or titanium nitride.

The insulator521, the insulator522, and the insulator524each have a function of a second gate insulating film.

The insulator522preferably has a barrier property. The insulator522having a barrier property functions as a layer that inhibits entry of impurities such as hydrogen into the transistor510A from the surroundings of the transistor510A.

For the insulator522, a single layer or stacked layers of an insulator containing aluminum oxide, hafnium oxide, an oxide containing aluminum and hafnium (hafnium aluminate), oxynitride containing aluminum and hafnium, tantalum oxide, zirconium oxide, lead zirconate titanate (PZT), strontium titanate (SrTiO3), or (Ba,Sr)TiO3(BST), are preferably used, for example. With miniaturization and high integration of transistors, a problem such as leakage current may arise because of a thinner gate insulating film. When a high-k material is used for the insulator functioning as the gate insulating film, a gate potential at the time when the transistor operates can be reduced while the physical thickness is maintained.

It is preferable that the insulator521be thermally stable. For example, silicon oxide and silicon oxynitride, which have thermal stability, are preferable. In addition, a combination of an insulator of a high-k material and silicon oxide or silicon oxynitride allows the insulator521to have a stacked-layer structure with thermal stability and a high dielectric constant.

Note that the second gate insulating film is shown to have a stacked-layer structure of three layers inFIG. 7BandFIG. 7C, but may have a stacked-layer structure of two or less layers or four or more layers. In such cases, without limitation to a stacked-layer structure formed of the same material, a stacked-layer structure formed of different materials may be employed.

The oxide530including a region functioning as the channel-formation region includes the oxide530a,the oxide530bover the oxide530a,and the oxide530cover the oxide530b.Including the oxide530aunder the oxide530bmakes it possible to inhibit diffusion of impurities into the oxide530bfrom the components formed below the oxide530a.Moreover, including the oxide530cover the oxide530bmakes it possible to inhibit diffusion of impurities into the oxide530bfrom the components formed above the oxide530c.As the oxide530, the above-described oxide semiconductor, which is one type of metal oxide, can be used.

Note that the oxide530cis preferably provided in the opening portion provided in the insulator580with the insulator574therebetween. When the insulator574has a barrier property, diffusion of impurities from the insulator580into the oxide530can be inhibited.

One of the conductors542functions as a source electrode and the other functions as a drain electrode.

For the conductor542aand the conductor542b,a metal such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, or tungsten or an alloy containing any of the metals as its main component can be used. In particular, a metal nitride film of tantalum nitride or the like is preferable because it has a barrier property against hydrogen or oxygen and high oxidation resistance.

Although a single-layer structure is illustrated inFIG. 7B, a stacked-layer structure of two or more layers may also be employed. For example, it is preferable to stack a tantalum nitride film and a tungsten film. Alternatively, a titanium film and an aluminum film may be stacked. Alternatively, a two-layer structure where an aluminum film is stacked over a tungsten film, a two-layer structure where a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure where a copper film is stacked over a titanium film, or a two-layer structure where a copper film is stacked over a tungsten film may be employed.

Other examples include a three-layer structure where a titanium film or a titanium nitride film is formed, an aluminum film or a copper film is stacked over the titanium film or the titanium nitride film, and a titanium film or a titanium nitride film is formed thereover; and a three-layer structure where a molybdenum film or a molybdenum nitride film is formed, an aluminum film or a copper film is stacked over the molybdenum film or the molybdenum nitride film, and a molybdenum film or a molybdenum nitride film is formed thereover. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used.

A barrier layer may be provided over the conductor542. The barrier layer is preferably formed using a substance having a barrier property against oxygen or hydrogen. This structure can inhibit oxidation of the conductor542at the time of depositing the insulator574.

A metal oxide can be used for the barrier layer, for example. In particular, an insulating film of aluminum oxide, hafnium oxide, gallium oxide, or the like, which has a barrier property against oxygen and hydrogen, is preferably used. Alternatively, silicon nitride formed by a CVD method may be used.

With the barrier layer, the range of choices for the material of the conductor542can be expanded. For example, a material having a low oxidation resistance and high conductivity, such as tungsten or aluminum, can be used for the conductor542. Moreover, for example, a conductor that can be easily deposited or processed can be used.

The insulator550functions as a first gate insulating film. The insulator550is preferably provided in the opening portion provided in the insulator580with the oxide530cand the insulator574therebetween.

With miniaturization and high integration of transistors, a problem such as leakage current may arise because of a thinner gate insulating film. In that case, the insulator550may have a stacked-layer structure like the second gate insulating film. When the insulator functioning as the gate insulating film has a stacked-layer structure of a high-k material and a thermally stable material, a gate potential at the time when the transistor operates can be reduced while the physical thickness is maintained. Furthermore, the stacked-layer structure can be thermally stable and have a high dielectric constant.

The conductor560functioning as the first gate electrode includes the conductor560aand the conductor560bover the conductor560a.Like the conductor505a,the conductor560ais preferably formed using a conductive material having a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, and a copper atom. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like).

When the conductor560ahas a function of inhibiting oxygen diffusion, the range of choices for the material of the conductor560bcan be expanded. That is, the conductor560ainhibits oxidation of the conductor560b,thereby preventing a decrease in conductivity.

As a conductive material having a function of inhibiting diffusion of oxygen, tantalum, tantalum nitride, ruthenium, or ruthenium oxide is preferably used, for example. For the conductor560a,the oxide semiconductor that can be used as the oxide530can be used. In that case, when the conductor560bis deposited by a sputtering method, the conductor560acan have a reduced electric resistance value to be a conductor. This can be referred to as an OC (Oxide Conductor) electrode.

The conductor560bis preferably formed using a conductive material containing tungsten, copper, or aluminum as its main component. In addition, since the conductor560functions as a wiring, a conductor having high conductivity is preferably used as the conductor560b.For example, a conductive material containing tungsten, copper, or aluminum as its main component can be used. The conductor560bmay have a stacked-layer structure, for example, a stack of any of the above conductive materials and titanium or titanium nitride.

The insulator574is positioned between the insulator580and the transistor510A. For the insulator574, an insulating material having a function of inhibiting diffusion of oxygen and impurities such as water or hydrogen is preferably used. For example, aluminum oxide or hafnium oxide is preferably used. Moreover, it is possible to use, for example, a metal oxide such as magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, or tantalum oxide or silicon nitride oxide, silicon nitride, or the like.

The insulator574can inhibit diffusion of impurities such as water and hydrogen contained in the insulator580into the oxide530bthrough the oxide530cand the insulator550. Furthermore, oxidation of the conductor560due to excess oxygen contained in the insulator580can be inhibited.

The insulator580, the insulator582, and the insulator584each function as an interlayer film.

Like the insulator514, the insulator582preferably functions as a barrier insulating film that inhibits entry of impurities such as water or hydrogen into the transistor510A from the outside.

Like the insulator516, the insulator580and the insulator584preferably have a lower permittivity than the insulator582. When a material with a low permittivity is used for the interlayer film, the parasitic capacitance generated between wirings can be reduced.

The transistor510A may be electrically connected to another component through a plug or a wiring such as the conductor546embedded in the insulator580, the insulator582, and the insulator584.

As a material for the conductor546, a single layer or stacked layers of a conductive material such as a metal material, an alloy material, a metal nitride material, or a metal oxide material can be used, as in the case of the conductor505. For example, it is preferable to use a high-melting-point material that has both heat resistance and conductivity, such as tungsten or molybdenum. Alternatively, it is preferable to use a low-resistance conductive material such as aluminum or copper. The use of a low-resistance conductive material can reduce wiring resistance.

For example, when the conductor546has a stacked-layer structure of tantalum nitride or the like, which is a conductor having a barrier property against hydrogen and oxygen, and tungsten, which has high conductivity, diffusion of impurities from the outside can be inhibited while the conductivity of the wiring is maintained.

With the above structure, an OS transistor having a high on-state current can be provided. Alternatively, an OS transistor having a low off-state current can be provided. Alternatively, in a semiconductor device including an OS transistor, variations in electrical characteristics can be inhibited and the reliability can be improved.

A structure example of a transistor510B is described with reference toFIG. 8A,FIG. 8B, andFIG. 8C.FIG. 8Ais a top view of the transistor510B.FIG. 8Bis a cross-sectional view of a portion indicated by a dashed-dotted line L1-L2inFIG. 8A.FIG. 8Cis a cross-sectional view of a portion indicated by a dashed-dotted line W1-W2inFIG. 8A. Note that for clarity of the drawing, some components are not illustrated in the top view ofFIG. 8A.

The transistor510B is a modification example of the transistor510A. Therefore, differences from the transistor510A will be mainly described to avoid repeated description.

The transistor510B includes a region where the oxide530c,the insulator550, and the conductor560overlap with the conductor542(the conductor542aand the conductor542b). With this structure, a transistor having a high on-state current can be provided. Moreover, a transistor having high controllability can be provided.

The conductor560functioning as the first gate electrode includes the conductor560aand the conductor560bover the conductor560a.Like the conductor505a,the conductor560ais preferably formed using a conductive material having a function of inhibiting diffusion of impurities such as a hydrogen atom, a hydrogen molecule, a water molecule, and a copper atom. Alternatively, it is preferable to use a conductive material having a function of inhibiting diffusion of oxygen (e.g., at least one of an oxygen atom, an oxygen molecule, and the like).

When the conductor560ahas a function of inhibiting oxygen diffusion, the range of choices for the material of the conductor560bcan be expanded. That is, the conductor560ainhibits oxidation of the conductor560b,thereby preventing a decrease in conductivity.

The insulator574is preferably provided to cover the top surface and a side surface of the conductor560, a side surface of the insulator550, and a side surface of the oxide530c.For the insulator574, an insulating material having a function of inhibiting diffusion of oxygen and impurities such as water or hydrogen is preferably used. For example, aluminum oxide or hafnium oxide is preferably used. Moreover, it is possible to use, for example, a metal oxide such as magnesium oxide, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, or tantalum oxide or silicon nitride oxide, silicon nitride, or the like.

The insulator574can inhibit oxidation of the conductor560. Moreover, the insulator574can inhibit diffusion of impurities such as water and hydrogen contained in the insulator580into the transistor510B.

An insulator576(an insulator576aand an insulator576b) having a barrier property may be provided between the conductor546and the insulator580. Providing the insulator576can inhibit oxygen in the insulator580from reacting with the conductor546and oxidizing the conductor546.

Furthermore, with the insulator576having a barrier property, the range of choices for the material of the conductor used as the plug or the wiring can be expanded. The use of a metal material having an oxygen absorbing property and high conductivity for the conductor546, for example, can provide a semiconductor device with low power consumption. Specifically, a material having a low oxidation resistance and high conductivity, such as tungsten or aluminum, can be used. Moreover, for example, a conductor that can be easily deposited or processed can be used.

A structure example of a transistor510C is described with reference toFIG. 9A,FIG. 9B, andFIG. 9C.FIG. 9Ais a top view of the transistor510C.FIG. 9Bis a cross-sectional view of a portion indicated by a dashed-dotted line L1-L2inFIG. 9A.FIG. 9Cis a cross-sectional view of a portion indicated by a dashed-dotted line W1-W2inFIG. 9A. Note that for clarity of the drawing, some components are not illustrated in the top view ofFIG. 9A.

The transistor510C is a modification example of the transistor510A. Therefore, differences from the transistor510A will be mainly described to avoid repeated description.

In the transistor510C shown inFIG. 9A,FIG. 9B, andFIG. 9C, a conductor547a is positioned between the conductor542aand the oxide530b,and a conductor547b is positioned between the conductor542band the oxide530b.Here, the conductor542a(the conductor542b) has a region that extends beyond the top surface of the conductor547a(the conductor547b) and its side surface on the conductor560side and is in contact with the top surface of the oxide530b.For the conductors547, a conductor that can be used as the conductor542is used. It is preferable that the thickness of the conductor547be at least greater than that of the conductor542.

In the transistor510C shown inFIG. 9A,FIG. 9B, andFIG. 9C, because of the above structure, the conductor542can be closer to the conductor560than in the transistor510A. Alternatively, the conductor560can overlap with an end portion of the conductor542aand an end portion of the conductor542b.Thus, the effective channel length of the transistor510C can be shortened, and the on-state current and the frequency characteristics can be improved.

The conductor547a(the conductor547b) is preferably provided to overlap with the conductor542a(the conductor542b). With such a structure, the conductor547a(the conductor547b) can function as a stopper to prevent over-etching of the oxide530bin etching for forming the opening in which the conductor546a(the conductor546b) is to be embedded.

The transistor510C shown inFIG. 9A,FIG. 9B, andFIG. 9Cmay have a structure in which an insulator545is positioned over and in contact with the insulator544. The insulator544preferably functions as a barrier insulating film that inhibits entry of impurities such as water or hydrogen and excess oxygen into the transistor510C from the insulator580side. The insulator545can be formed using an insulator that can be used for the insulator544. In addition, the insulator544may be formed using a nitride insulator such as aluminum nitride, aluminum titanium nitride, titanium nitride, silicon nitride, or silicon nitride oxide, for example.

Unlike in the transistor510A shown inFIG. 7A,FIG. 7B, andFIG. 7C, in the transistor510C shown inFIG. 9A,FIG. 9B, andFIG. 9C, the conductor505may have a single-layer structure. In this case, an insulating film to be the insulator516is formed over the patterned conductor505, and an upper portion of the insulating film is removed by a CMP method or the like until the top surface of the conductor505is exposed. Preferably, the planarity of the top surface of the conductor505is made favorable. For example, the average surface roughness (Ra) of the top surface of the conductor505is less than or equal to 1 nm, preferably less than or equal to 0.5 nm, and further preferably less than or equal to 0.3 nm. This allows the improvement in planarity of an insulating layer formed over the conductor505and the increase in crystallinity of the oxide530band the oxide530c.

A structure example of a transistor510D is described with reference toFIG. 10A,FIG. 10B, andFIG. 10C.FIG. 10Ais a top view of the transistor510D.FIG. 10Bis a cross-sectional view of a portion indicated by a dashed-dotted line L1-L2inFIG. 10A.FIG. 10Cis a cross-sectional view of a portion indicated by a dashed-dotted line W1-W2inFIG. 10A. Note that for clarity of the drawing, some components are not illustrated in the top view inFIG. 10A.

The transistor510D is a modification example of the above transistors. Therefore, differences from the above transistors will be mainly described to avoid repeated description.

InFIG. 10A,FIG. 10B, andFIG. 10C, the conductor503is not provided, and the conductor505that has a function of a second gate is made to function also as a wiring. In addition, the insulator550is provided over the oxide530cand a metal oxide552is provided over the insulator550. In addition, the conductor560is provided over the metal oxide552, and an insulator570is provided over the conductor560. Furthermore, an insulator571is provided over the insulator570.

The metal oxide552preferably has a function of inhibiting diffusion of oxygen. When the metal oxide552that inhibits oxygen diffusion is provided between the insulator550and the conductor560, diffusion of oxygen into the conductor560is inhibited. That is, a reduction in the amount of oxygen supplied to the oxide530can be inhibited. Moreover, oxidation of the conductor560due to oxygen can be inhibited.

Note that the metal oxide552may have a function of part of the first gate. For example, the oxide semiconductor that can be used for the oxide530can be used for the metal oxide552. In this case, when the conductor560is deposited by a sputtering method, the electrical resistance value of the metal oxide552is lowered so that the metal oxide552can be a conductive layer. This can be referred to as an OC (Oxide Conductor) electrode.

The metal oxide552may have a function of part of a gate insulating film. Thus, when silicon oxide, silicon oxynitride, or the like is used for the insulator550, a metal oxide that is a high-k material with a high dielectric constant is preferably used for the metal oxide552. Such a stacked-layer structure can be thermally stable and can have a high dielectric constant. Thus, a gate potential that is applied when the transistor operates can be lowered while the physical thickness is maintained. In addition, the equivalent oxide thickness (EOT) of an insulating layer functioning as the gate insulating film can be reduced.

Although the metal oxide552in the transistor510D is shown as a single layer, the metal oxide552may have a stacked-layer structure of two or more layers. For example, a metal oxide functioning as part of the gate electrode and a metal oxide functioning as part of the gate insulating film may be stacked.

With the metal oxide552functioning as a gate electrode, the on-state current of the transistor510D can be increased without a reduction in the influence of the electric field from the conductor560. With the metal oxide552functioning as the gate insulating film, the distance between the conductor560and the oxide530is kept by the physical thicknesses of the insulator550and the metal oxide552, so that leakage current between the conductor560and the oxide530can be reduced. Thus, with the stacked-layer structure of the insulator550and the metal oxide552, the physical distance between the conductor560and the oxide530and the intensity of electric field applied from the conductor560to the oxide530can be easily adjusted as appropriate.

Specifically, the oxide semiconductor that can be used for the oxide530can also be used for the metal oxide552when the resistance thereof is reduced. Alternatively, a metal oxide containing one kind or two or more kinds selected from hafnium, aluminum, gallium, yttrium, zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium, and the like can be used.

It is particularly preferable to use an insulating layer containing an oxide of one or both of aluminum and hafnium, for example, aluminum oxide, hafnium oxide, or an oxide containing aluminum and hafnium (hafnium aluminate). In particular, hafnium aluminate has higher heat resistance than a hafnium oxide film. Therefore, hafnium aluminate is preferable because it is less likely to be crystallized by heat treatment in a later step. Note that the metal oxide552is not an essential component. Design is appropriately set in consideration of required transistor characteristics.

For the insulator570, an insulating material having a function of inhibiting the passage of oxygen and impurities such as water and hydrogen is preferably used. For example, aluminum oxide or hafnium oxide is preferably used. Thus, oxidation of the conductor560due to oxygen from above the insulator570can be inhibited. Moreover, entry of impurities such as water or hydrogen from above the insulator570into the oxide530through the conductor560and the insulator550can be inhibited.

The insulator571functions as a hard mask. By providing the insulator571, the conductor560can be processed to have a side surface that is substantially vertical; specifically, an angle formed by the side surface of the conductor560and a substrate surface can be greater than or equal to 75° and less than or equal to 100°, preferably greater than or equal to 80° and less than or equal to 95°.

Note that an insulating material having a function of inhibiting the passage of oxygen and impurities such as water or hydrogen may be used for the insulator571so that the insulator571also functions as a barrier layer. In that case, the insulator570does not have to be provided.

Parts of the insulator570, the conductor560, the metal oxide552, the insulator550, and the oxide530care selected and removed using the insulator571as a hard mask, whereby their side surfaces can be substantially aligned with each other and a surface of the oxide530bcan be partly exposed.

The transistor510D includes a region531aand a region531bon a part of the exposed surface of the oxide530b.One of the region531aand the region531bfunctions as a source region, and the other functions as a drain region.

The region531aand the region531bcan be formed by addition of an impurity element such as phosphorus or boron to the exposed surface of the oxide530bby an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or plasma treatment, for example. In this embodiment and the like, an “impurity element” refers to an element other than main constituent elements.

Alternatively, the region531aand the region531bcan be formed in such a manner that, after a part of the surface of the oxide530bis exposed, a metal film is formed and then heat treatment is performed so that the element contained in the metal film is diffused into the oxide530b.

The electrical resistivity of regions of the oxide530bto which the impurity element has been added decreases. For that reason, the region531aand the region531bare sometimes referred to as “impurity regions” or “low-resistance regions”.

The region531aand the region531bcan be formed in a self-aligned manner by using the insulator571and/or the conductor560as a mask. Thus, the conductor560does not overlap with the region531aand/or the region531b,so that the parasitic capacitance can be reduced. Moreover, an offset region is not formed between a channel-formation region and the source/drain region (the region531aor the region531b). The formation of the region531aand the region531bin a self-aligned manner achieves an increase in on-state current, a reduction in threshold voltage, and an improvement in operating frequency, for example.

Note that an offset region may be provided between the channel-formation region and the source/drain region in order to further reduce the off-state current. The offset region is a region where the electrical resistivity is high and the impurity element is not added. The offset region can be formed by the addition of the impurity element after the formation of an insulator575. In this case, the insulator575serves as a mask like the insulator571or the like. Thus, the impurity element is not added to a region of the oxide530boverlapping with the insulator575, so that the electrical resistivity of the region can be kept high.

The transistor510D includes the insulator575on the side surfaces of the insulator570, the conductor560, the metal oxide552, the insulator550, and the oxide530c.The insulator575is preferably an insulator having a low dielectric constant. For example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, porous silicon oxide, a resin, or the like is preferably used. In particular, silicon oxide, silicon oxynitride, silicon nitride oxide, or porous silicon oxide is preferably used for the insulator575, in which case an excess-oxygen region can be easily formed in the insulator575in a later step. Silicon oxide and silicon oxynitride are preferable because of their thermal stability. The insulator575preferably has a function of diffusing oxygen.

The transistor510D also includes the insulator574over the insulator575and the oxide530. The insulator574is preferably deposited by a sputtering method. When a sputtering method is used, an insulator containing few impurities such as water or hydrogen can be deposited. For example, aluminum oxide is preferably used for the insulator574.

Note that an oxide film obtained by a sputtering method may extract hydrogen from the component over which the oxide film is deposited. Thus, the hydrogen concentration in the oxide530and the insulator575can be reduced when the insulator574absorbs hydrogen and water from the oxide530and the insulator575.

A structure example of a transistor510E is described with reference toFIG. 11A,FIG. 11B, andFIG. 11C.FIG. 11Ais a top view of the transistor510E.FIG. 11Bis a cross-sectional view of a portion indicated by the dashed-dotted line L1-L2inFIG. 11A.FIG. 11Cis a cross-sectional view of a portion indicated by the dashed-dotted line W1-W2inFIG. 11A. Note that for clarity of the drawing, some components are not illustrated in the top view inFIG. 11A.

The transistor510E is a modification example of the above transistors. Therefore, differences from the above transistors will be mainly described to avoid repeated description.

InFIG. 11A,FIG. 11B, andFIG. 11C, the conductor542is not provided, and a part of the exposed surface of the oxide530bincludes the region531aand the region531b.One of the region531aand the region531bfunctions as a source region, and the other functions as a drain region. Moreover, an insulator573is included between the oxide530band the insulator574.

The regions531(the region531aand the region531b) shown inFIG. 11Bare regions where an element described below is added to the oxide530b.The regions531can be formed using a dummy gate, for example.

Specifically, a dummy gate is provided over the oxide530b,and the element that reduces the resistance of the oxide530bis added using the dummy gate as a mask. That is, the element is added to regions of the oxide530that do not overlap with the dummy gate, whereby the regions531are formed. As a method of adding the element, an ion implantation method by which an ionized source gas is subjected to mass separation and then added, an ion doping method by which an ionized source gas is added without mass separation, a plasma immersion ion implantation method, or the like can be used.

Typical examples of the element that reduces the resistance of the oxide530are boron and phosphorus. Moreover, hydrogen, carbon, nitrogen, fluorine, sulfur, chlorine, titanium, a rare gas, or the like may be used. Typical examples of the rare gas include helium, neon, argon, krypton, and xenon. The concentration of the element is measured by secondary ion mass spectrometry (SIMS) or the like.

In particular, boron and phosphorus are preferable because an apparatus used in a manufacturing line for low-temperature polysilicon can be used, for example. Since the existing facility can be used, capital investment can be reduced.

Next, an insulating film to be the insulator573and an insulating film to be the insulator574may be formed over the oxide530band the dummy gate. Stacking the insulating film to be the insulator573and the insulating film to be the insulator574can provide a region where the oxide530cand the insulator550overlap with the region531.

Specifically, after an insulating film to be the insulator580is provided over the insulating film to be the insulator574, the insulating film to be the insulator580is subjected to CMP (Chemical Mechanical Polishing) treatment, whereby a part of the insulating film to be the insulator580is removed and the dummy gate is exposed. Then, when the dummy gate is removed, a part of the insulator573in contact with the dummy gate is preferably also removed. Thus, the insulator574and the insulator573are exposed at a side surface of an opening portion provided in the insulator580, and the region531provided in the oxide530bis partly exposed at the bottom surface of the opening portion. Next, an oxide film to be the oxide530c,an insulating film to be the insulator550, and a conductive film to be the conductor560are formed in this order in the opening portion, and then the oxide film to be the oxide530c,the insulating film to be the insulator550, and the conductive film to be the conductor560are partly removed by CMP treatment or the like until the insulator580is exposed; thus, the transistor shown inFIG. 11A,FIG. 11B, andFIG. 11Ccan be formed.

Note that the insulator573and the insulator574are not essential components. Design is appropriately set in consideration of required transistor characteristics.

The cost of the transistor shown inFIG. 11A,FIG. 11B, andFIG. 11Ccan be reduced because an existing apparatus can be used and the conductor542is not provided.

A structure example of a transistor510F is described with reference toFIG. 12A,FIG. 12B, andFIG. 12C.FIG. 12Ais a top view of the transistor510F.FIG. 12Bis a cross-sectional view of a portion indicated by a dashed-dotted line L1-L2inFIG. 12A.FIG. 12Cis a cross-sectional view of a portion indicated by a dashed-dotted line W1-W2inFIG. 12A. Note that for clarity of the drawing, some components are not illustrated in the top view inFIG. 12A.

The transistor510F is a modification example of the transistor510A. Therefore, differences from the above transistors will be mainly described to avoid repeated description.

In the transistor510A, a part of the insulator574is provided in the opening portion provided in the insulator580and covers the side surface of the conductor560. Meanwhile, in the transistor510F, an opening is formed by partly removing the insulator580and the insulator574.

An insulator576(an insulator576aand an insulator576b) having a barrier property may be provided between the conductor546and the insulator580. Providing the insulator576can inhibit oxygen in the insulator580from reacting with the conductor546and oxidizing the conductor546.

Note that when an oxide semiconductor is used as the oxide530, the oxide530preferably has a stacked-layer structure of a plurality of oxide layers that differ in the atomic ratio of metal atoms. Specifically, the atomic proportion of the element M in the constituent elements in the metal oxide used as the oxide530ais preferably higher than the atomic proportion of the element M in the constituent elements in the metal oxide used as the oxide530b.In addition, the atomic ratio of the element M to In in the metal oxide used as the oxide530ais preferably higher than the atomic ratio of the element M to In in the metal oxide used as the oxide530b.Furthermore, the atomic ratio of In to the elementMin the metal oxide used as the oxide530bis preferably higher than the atomic ratio of In to the element M in the metal oxide used as the oxide530a.A metal oxide that can be used as the oxide530aor the oxide530bcan be used as the oxide530c.

The oxide530a,the oxide530b,and the oxide530cpreferably have crystallinity, and in particular, it is preferable to use a CAAC-OS. An oxide having crystallinity, such as a CAAC-OS, has a dense structure with small amounts of impurities and defects (e.g., oxygen vacancies) and high crystallinity. This can inhibit extraction of oxygen from the oxide530bby the source electrode or the drain electrode. This can reduce extraction of oxygen from the oxide530beven when heat treatment is performed; hence, the transistor510F is stable against high temperatures (i.e., thermal budget) in the manufacturing process.

Note that one or both of the oxide530aand the oxide530cmay be omitted. The oxide530may be a single layer of the oxide530b.In the case where the oxide530is a stack of the oxide530a,the oxide530b,and the oxide530c,the energy of the conduction band minimum of each of the oxide530aand the oxide530cis preferably higher than the energy of the conduction band minimum of the oxide530b.In other words, the electron affinity of each of the oxide530aand the oxide530cis preferably smaller than the electron affinity of the oxide530b.In that case, for the oxide530c,a metal oxide that can be used for the oxide530ais preferably used. Specifically, the atomic proportion of the element M in the constituent elements in the metal oxide used as the oxide530cis preferably higher than the atomic proportion of the element M in the constituent elements in the metal oxide used as the oxide530b.Moreover, the atomic ratio of the element M to In in the metal oxide used as the oxide530cis preferably higher than the atomic ratio of the element M to In in the metal oxide used as the oxide530b.Furthermore, the atomic ratio of In to the elementMin the metal oxide used as the oxide530bis preferably higher than the atomic ratio of In to the element M in the metal oxide used as the oxide530c.

The energy level of the conduction band minimum gradually changes at junction portions of the oxide530a,the oxide530b,and the oxide530c.In other words, the energy level of the conduction band minimum at the junction portions of the oxide530a,the oxide530b,and the oxide530ccontinuously changes or is continuously connected. To obtain this, the density of defect states in a mixed layer formed at the interface between the oxide530aand the oxide530band the interface between the oxide530band the oxide530cis preferably made low.

Specifically, when the oxide530aand the oxide530bor the oxide530band the oxide530ccontain a common element (as a main component) in addition to oxygen, a mixed layer with a low density of defect states can be formed. For example, in the case where the oxide530bis an In—Ga—Zn oxide, an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like may be used as the oxide530aand the oxide530c.In addition, the oxide530cmay have a stacked-layer structure. For example, it is possible to employ a stacked-layer structure of an In—Ga—Zn oxide and a Ga—Zn oxide over the In—Ga—Zn oxide, or a stacked-layer structure of an In—Ga—Zn oxide and gallium oxide over the In—Ga—Zn oxide. In other words, the oxide530cmay employ a stacked-layer structure of an In—Ga—Zn oxide and an oxide that does not contain In.

At this time, the oxide530bserves as a main carrier path. When the oxide530aand the oxide530chave the above structure, the density of defect states at the interface between the oxide530aand the oxide530band the interface between the oxide530band the oxide530ccan be made low. Thus, the influence of interface scattering on carrier conduction is small, and the transistor510F can have a high on-state current and high frequency characteristics. Note that in the case where the oxide530chas a stacked-layer structure, in addition to the effect of reducing the density of defect states at the interface between the oxide530band the oxide530c,the effect of inhibiting diffusion of the constituent element of the oxide530cto the insulator550side is expected. More specifically, the oxide530chas a stacked-layer structure and the oxide that does not contain In is positioned at the upper part of the stacked-layer structure, whereby the amount of In that would diffuse to the insulator550side can be reduced. Since the insulator550functions as a gate insulator, the transistor has defects in characteristics when In diffuses. Thus, when the oxide530chas a stacked-layer structure, a highly reliable display device can be provided.

A metal oxide functioning as an oxide semiconductor is preferably used as the oxide530. For example, as the metal oxide to be the channel-formation region in the oxide530, a metal oxide having a bandgap of 2 eV or larger, preferably 2.5 eV or larger is preferably used. With the use of a metal oxide having such a wide bandgap, the off-state current of the transistor can be reduced. With such a transistor, a semiconductor device with low power consumption can be provided.

A structure example of a transistor510G is described with reference toFIG. 13AandFIG. 13B. The transistor510G is a modification example of the transistor500. Therefore, differences from the above transistors will be mainly described to avoid repeated description. Note that the structure shown inFIG. 13AandFIG. 13Bcan be employed for other transistors, such as the transistor300, included in the semiconductor device of one embodiment of the present invention.

FIG. 13Ais a cross-sectional view of the transistor510G in the channel length direction, andFIG. 13Bis a cross-sectional view of the transistor510G in the channel width direction. The transistor510G shown inFIG. 13AandFIG. 13Bis different from the transistor500shown inFIG. 6AandFIG. 6Bin including the insulator402and the insulator404. Another difference from the transistor500shown inFIG. 6AandFIG. 6Bis that the insulator551is provided in contact with a side surface of the conductor540aand the insulator551is provided in contact with a side surface of the conductor540b.Another difference from the transistor500shown inFIG. 6AandFIG. 6Bis that the insulator520is not provided.

In the transistor510G shown inFIG. 13AandFIG. 13B, the insulator402is provided over the insulator512. In addition, the insulator404is provided over the insulator574and the insulator402.

The transistor510G shown inFIG. 13AandFIG. 13Bhas a structure in which the insulator514, the insulator516, the insulator522, the insulator524, the insulator544, the insulator580, and the insulator574are patterned and covered with the insulator404. That is, the insulator404is in contact with the top surface of the insulator574, a side surface of the insulator574, a side surface of the insulator580, a side surface of the insulator544, a side surface of the insulator524, a side surface of the insulator522, a side surface of the insulator516, a side surface of the insulator514, and the top surface of the insulator402. Thus, the oxide530and the like are isolated from the outside by the insulator404and the insulator402.

It is preferable that the insulator402and the insulator404have higher capability of inhibiting diffusion of hydrogen (e.g., at least one of a hydrogen atom, a hydrogen molecule, and the like) or a water molecule. For example, the insulator402and the insulator404are preferably formed using silicon nitride or silicon nitride oxide with a high hydrogen barrier property. This can inhibit diffusion of hydrogen or the like into the oxide530, thereby inhibiting the degradation of the characteristics of the transistor510G. Consequently, the reliability of the semiconductor device including an OS transistor can be increased.

The insulator551is provided in contact with the insulator581, the insulator404, the insulator574, the insulator580, and the insulator544. The insulator551preferably has a function of inhibiting diffusion of hydrogen or water molecules. For example, as the insulator551, an insulator such as silicon nitride, aluminum oxide, or silicon nitride oxide that has a high hydrogen barrier property is preferably used. In particular, silicon nitride is suitably used for the insulator551because of its high hydrogen barrier property. The use of a material having a high hydrogen barrier property for the insulator551can inhibit diffusion of impurities such as water or hydrogen from the insulator580and the like into the oxide530through the conductor540aand the conductor540b.Furthermore, oxygen contained in the insulator580can be inhibited from being absorbed by the conductor540aand the conductor540b.Consequently, the reliability of the semiconductor device including an OS transistor can be increased.

FIG. 14is a cross-sectional view showing a structure example of the semiconductor device in the case where the transistor500and the transistor300have the structure shown inFIG. 13AandFIG. 13B. The insulator551is provided on the side surface of the conductor546.

FIG. 15AandFIG. 15Bshow a modification example of the transistor shown inFIG. 13AandFIG. 13B.FIG. 15Ais a cross-sectional view of the transistor in the channel length direction, andFIG. 15Bis a cross-sectional view of the transistor in the channel width direction. The transistor shown inFIG. 15AandFIG. 15Bis different from the transistor shown inFIG. 13AandFIG. 13Bin that the oxide530chas a two-layer structure of an oxide530c1and an oxide530c2.

The oxide530c1is in contact with the top surface of the insulator524, a side surface of the oxide530a,the top surface and a side surface of the oxide530b,side surfaces of the conductor542aand the conductor542b,a side surface of the insulator544, and a side surface of the insulator580. The oxide530c2is in contact with the insulator550.

An In—Zn oxide can be used as the oxide530c1, for example. For the oxide530c2, it is possible to use a material similar to the material that can be used for the oxide530cwhen the oxide530chas a single-layer structure. For example, as the oxide530c2, a metal oxide with In:Ga:Zn=1:3:4 [atomic ratio], Ga:Zn=2:1 [atomic ratio], or Ga: Zn=2:5 [atomic ratio] can be used.

When the oxide530chas a two-layer structure of the oxide530c1and the oxide530c2, the on-state current of the transistor can be increased as compared with the case where the oxide530chas a single-layer structure. Thus, a transistor can be a power MOS transistor, for example. Note that the oxide530cincluded in the transistor shown inFIG. 6AandFIG. 6Bcan also have a two-layer structure of the oxide530c1and the oxide530c2.

The transistor shown inFIG. 15AandFIG. 15Bcan be employed for the transistor500, the transistor300, or both thereof.

Note that this embodiment can be implemented in appropriate combination with the other embodiments described in this specification.

In this embodiment, an oxide semiconductor that is a kind of metal oxide will be described.

<Classification of Crystal Structure>

First, the classification of the crystal structures of an oxide semiconductor will be described with reference toFIG. 16A.FIG. 16Ais a diagram showing the classification of crystal structures of an oxide semiconductor, typically IGZO (a metal oxide containing In, Ga, and Zn).

As shown inFIG. 16A, an oxide semiconductor is roughly classified into “Amorphous”, “Crystalline”, and “Crystal”. The term “Amorphous” includes completely amorphous. The term “Crystalline” includes CAAC (c-axis-aligned crystalline), nc (nanocrystalline), and CAC (cloud-aligned composite). Note that the term “Crystalline” excludes single crystal, poly crystal, and completely amorphous. The term “Crystal” includes single crystal and poly crystal.

Note that the structures in the thick frame inFIG. 16Aare in an intermediate state between “Amorphous” and “Crystal”, and belong to a new crystalline phase. That is, these structures are completely different from “Amorphous”, which is energetically unstable, and “Crystal”.

A crystal structure of a film or a substrate can be evaluated with an X-ray diffraction (XRD) spectrum.FIG. 16Bshows an XRD spectrum, which is obtained by GIXD (Grazing-Incidence XRD) measurement, of a CAAC-IGZO film classified into “Crystalline”. Note that a GIXD method is also referred to as a thin film method or a Seemann-Bohlin method. The XRD spectrum that is shown inFIG. 16Band obtained by GIXD measurement is hereinafter simply referred to as an XRD spectrum. The CAAC-IGZO film shown inFIG. 16Bhas a composition in the vicinity of In:Ga:Zn=4:2:3 [atomic ratio]. The CAAC-IGZO film shown inFIG. 16Bhas a thickness of 500 nm.

As shown inFIG. 16B, a clear peak indicating crystallinity is detected in the XRD spectrum of the CAAC-IGZO film. Specifically, a peak indicating c-axis alignment is detected at 2θ of around 31° in the XRD spectrum of the CAAC-IGZO film. As shown inFIG. 16B, the peak at 2θ of around 31° is asymmetric with respect to the axis of the angle at which the peak intensity is detected.

A crystal structure of a film or a substrate can also be evaluated with a diffraction pattern obtained by a nanobeam electron diffraction (NBED) method (such a pattern is also referred to as a nanobeam electron diffraction pattern).FIG. 16Cshows a diffraction pattern of the CAAC-IGZO film.FIG. 16Cshows a diffraction pattern obtained with the NBED method in which an electron beam is incident in the direction parallel to the substrate. The composition of the CAAC-IGZO film shown inFIG. 16Cis In:Ga:Zn=4:2:3 [atomic ratio] or the neighborhood thereof. In the nanobeam electron diffraction method, electron diffraction is performed with a probe diameter of 1 nm.

As shown inFIG. 16C, a plurality of spots indicating c-axis alignment are observed in the diffraction pattern of the CAAC-IGZO film.

Here, the above-described CAAC-OS, nc-OS, and a-like OS will be described in detail.

Note that each of the plurality of crystal regions is formed of one or more fine crystals (crystals each of which has a maximum diameter of less than 10 nm). In the case where the crystal region is formed of one fine crystal, the maximum diameter of the crystal region is less than 10 nm. In the case where the crystal region is formed of a large number of fine crystals, the size of the crystal region may be approximately several tens of nanometers.

In the case of an In—M—Zn oxide (the element M is one or more kinds selected from aluminum, gallium, yttrium, tin, titanium, and the like), the CAAC-OS tends to have a layered crystal structure (also referred to as a layered structure) in which a layer containing indium (In) and oxygen (hereinafter, an In layer) and a layer containing the element M, zinc (Zn), and oxygen (hereinafter, an (M, Zn) layer) are stacked. Indium and the element M can be replaced with each other. Therefore, indium may be contained in the (M, Zn) layer. In addition, the element M may be contained in the In layer. Note that Zn may be contained in the In layer. Such a layered structure is observed as a lattice image in a high-resolution TEM image, for example.

The CAAC-OS is an oxide semiconductor with high crystallinity in which no clear crystal grain boundary is observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the crystal grain boundary is unlikely to occur. Moreover, since the crystallinity of an oxide semiconductor might be decreased by entry of impurities, formation of defects, or the like, the CAAC-OS can be regarded as an oxide semiconductor that has small amounts of impurities and defects (e.g., oxygen vacancies). Thus, an oxide semiconductor including the CAAC-OS is physically stable. Therefore, the oxide semiconductor including the CAAC-OS is resistant to heat and has high reliability. In addition, the CAAC-OS is stable with respect to high temperature in the manufacturing process (i.e., thermal budget). Accordingly, the use of the CAAC-OS for the OS transistor can extend the degree of freedom of the manufacturing process.

Next, the above-described CAC-OS is described in detail. Note that the CAC-OS relates to the material composition.

Note that the atomic ratios of In, Ga, and Zn to the metal elements contained in the CAC-OS in an In—Ga—Zn oxide are denoted with [In], [Ga], and [Zn], respectively. For example, the first region in the CAC-OS in the In—Ga—Zn oxide has [In] higher than [In] in the composition of the CAC-OS film. Moreover, the second region has [Ga] higher than [Ga] in the composition of the CAC-OS film. For example, the first region has higher [In] and lower [Ga] than the second region. Moreover, the second region has higher [Ga] and lower [In] than the first region.

Specifically, the first region includes indium oxide, indium zinc oxide, or the like as its main component. The second region includes gallium oxide, gallium zinc oxide, or the like as its main component. That is, the first region can be referred to as a region containing In as its main component. The second region can be referred to as a region containing Ga as its main component.

For example, in EDX mapping obtained by energy dispersive X-ray spectroscopy (EDX), it is confirmed that the CAC-OS in the In—Ga—Zn oxide has a composition in which the region containing In as its main component (the first region) and the region containing Ga as its main component (the second region) are unevenly distributed and mixed.

In the case where the CAC-OS is used for a transistor, a switching function (On/Off switching function) can be given to the CAC-OS owing to the complementary action of the conductivity derived from the first region and the insulating property derived from the second region. A CAC-OS has a conducting function in part of the material and has an insulating function in another part of the material; as a whole, the CAC-OS has a function of a semiconductor. Separation of the conducting function and the insulating function can maximize each function. Accordingly, when the CAC-OS is used for a transistor, high on-state current (Ion), high field-effect mobility (μ), and excellent switching operation can be achieved.

An oxide semiconductor has various structures with different properties. Two or more kinds among the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, the a-like OS, the CAC-OS, the nc-OS, and the CAAC-OS may be included in an oxide semiconductor of one embodiment of the present invention.

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

When the above oxide semiconductor is used for a transistor, a transistor with high field-effect mobility can be achieved. In addition, a transistor having high reliability can be achieved.

A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and thus has a low density of trap states in some cases.

Accordingly, in order to obtain stable electrical characteristics of a transistor, reducing the impurity concentration in an oxide semiconductor is effective. In order to reduce the impurity concentration in the oxide semiconductor, it is preferable that the impurity concentration in an adjacent film be also reduced. Examples of impurities include hydrogen, nitrogen, an alkali metal, an alkaline earth metal, iron, nickel, and silicon.

Here, the influence of each impurity in the oxide semiconductor is described.

Furthermore, when the oxide semiconductor contains nitrogen, the oxide semiconductor easily becomes n-type by generation of electrons serving as carriers and an increase in carrier concentration. As a result, a transistor using as a semiconductor an oxide semiconductor containing nitrogen is likely to have normally-on characteristics. When nitrogen is contained in the oxide semiconductor, a trap state is sometimes formed. This might make the electrical characteristics of the transistor unstable. Therefore, the concentration of nitrogen in the oxide semiconductor, which is obtained by SIMS, is set lower than 5×1019atoms/cm3, preferably lower than or equal to 5×1018atoms/cm3, further preferably lower than or equal to 1×1018atoms/cm3, and still further preferably lower than or equal to 5×1017atoms/cm3.

Note that the composition, structure, method, and the like described in this embodiment can be used in appropriate combination with the compositions, structures, methods, and the like described in the other embodiments and the like.

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