# Patent

## Title
Method for driving semiconductor device

## Abstract

In a driving method of a semiconductor device which conducts a multilevel writing operation, a signal line for controlling on/off of a writing transistor for conducting a writing operation on a memory cell using a transistor including an oxide semiconductor layer is disposed along a bit line, and a multilevel writing operation is conducted with use of, also in a writing operation, a voltage which is applied to a capacitor at a reading operation. The potential of a bit line is detected while data writing is conducted, and thereby whether a potential corresponding to the written data is normally applied to the floating gate can be confirmed without a writing verify operation.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The disclosed invention relates to a semiconductor device using a semiconductor element and a method for driving the semiconductor device.
2. Description of the Related Art
Storage devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile storage device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a memory element is selected and electric charge is stored in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost on the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, since leakage current (off-state current) or the like flows between a source and a drain of a transistor included in a memory element when the transistor is in an off state, electric charge flows into or out even if the transistor is not selected, which makes a data holding period short. For that reason, another writing operation (refresh operation) is necessary at given intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile storage device is an SRAM (static random access memory). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that the data holding period is extremely long (almost permanent) and refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a memory element deteriorates by tunneling current generated in writing, so that the memory element stops its function after a given number of writing operations. In order to suppress adverse effects of this problem, a method in which the number of writing operations for memory elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary in order to inject electric charge into the floating gate or remove the electric charge, and a circuit therefor is required. Therefore, there is a problem of high power consumption. Further, it takes a relatively long time to inject or remove electric charges, and it is not easy to perform writing and erasing at higher speed.
Further, as for the above-described flash memory, in order to increase storage capacity, a “multilevel” flash memory is proposed, in which data having more levels than two levels is stored in one memory cell (e.g., see Patent Document 2).
In addition, in a multilevel memory, a “writing verify operation” of detecting a writing state of a memory cell after data is written is conducted in order to precisely control the state of the data writing to the memory cell (e.g., see Patent Document 3).
REFERENCE


    [Patent Document 1] Japanese Published Patent Application No. S57-105889
    [Patent Document 2] Japanese Published Patent Application No. H11-25682
    [Patent Document 3] Japanese Published Patent Application No. H10-214492


SUMMARY OF THE INVENTION
The above multilevel memory uses many different voltage values accompanying the increase in storage capacity and thus the number of circuits needed is increased, which leads to growth in size and cost increase of a semiconductor device. This is a problem for multilevel memories.
Further, when the above writing verify operation is performed, time required for writing becomes longer.
In view of the foregoing problems, an object of one embodiment of the invention disclosed herein is to provide a driving method of a semiconductor device capable of performing a highly reliable writing operation at high speed.
Another object of one embodiment of the invention disclosed herein is to provide a semiconductor device with a novel structure in which stored data can be retained even when power is not supplied, and does not have a limitation on the number of times of writing.
Another object of one embodiment of the invention disclosed herein is to simplify a semiconductor device by employing a novel structure and further to increase the storage capacity per unit area.
In a semiconductor device which conducts a multilevel writing operation, disclosed in this specification, a signal line for controlling on/off of a writing transistor for a writing operation is disposed along a bit line of a memory cell using a transistor including an oxide semiconductor layer. Further, as for a driving method of the semiconductor device, a multilevel writing operation is conducted with use of, also in a writing operation, a voltage which is applied to a capacitor at a reading operation.
In a multilevel memory using a transistor including an oxide semiconductor layer, a writing operation is conducted while an appropriate potential is given to a capacitor of a memory in accordance with data to be written, and thereby a potential corresponding to data to be written can be given to a floating gate without change in writing voltage. In other words, without setting a writing voltage in accordance with data to be written, a voltage given to a capacitor of a memory is controlled, and thereby a multilevel operation can be conducted. Therefore, a circuit for controlling writing voltage can be omitted, so that a circuit configuration can be simplified.
Further, the potential of a bit line is detected while data writing is conducted, and thereby whether a potential corresponding to the written data is normally applied to the floating gate can be confirmed without a writing verify operation. Thus, in a driving method of a semiconductor device according to the invention disclosed herein, a writing operation with high reliability can be performed at high speed.
A transistor used for a memory cell which includes a node to hold a potential is a transistor having, as a semiconductor layer, a material which can decrease off-state current sufficiently, for example, a wide gap semiconductor material (specifically, e.g., a semiconductor material having an energy gap Eg more than 3 eV). When a semiconductor material which can sufficiently decrease off-state current of a transistor is used, data can be held for a long period. As one of such wide gap semiconductor materials, an oxide semiconductor material can be given. In a semiconductor device disclosed in this specification, a transistor including an oxide semiconductor layer using an oxide semiconductor material can be preferably used.
One embodiment of the invention disclosed in this specification is a method for driving a semiconductor device including a first to m-th memory cells which are connected in series between a source line and a bit line, a first selection transistor whose gate terminal is electrically connected to a first selection line, and a second selection transistor whose gate terminal is electrically connected to a second selection line. The first to m-th memory cells each include a first transistor provided on a substrate including a semiconductor material and including a first gate terminal electrically connected to a first signal line, a first source terminal, and a first drain terminal; a second transistor including an oxide semiconductor layer and including a second gate terminal electrically connected to the first signal line, a second source terminal, and a second drain terminal; and a capacitor of which one terminal is electrically connected to one of m word lines. The source line is electrically connected to the first source terminal of the m-th memory cell via the second selection transistor, the bit line is electrically connected to the first drain terminal of the first memory cell via the first selection transistor, and the second source terminal, the first gate terminal, and the other terminal of the capacitor are electrically connected to one another to form a node. The method includes the steps of: in a writing operation for supplying potentials to the nodes in such a manner that a potential is supplied to the second signal line to turn on the second transistors and a potential is supplied to the first signal line, supplying potentials to the first selection line and the second selection line to turn on the first selection transistor and the second selection transistor; and detecting a potential of the bit line.
One embodiment of the invention disclosed in this specification is a method for driving a semiconductor device including a first to m-th memory cells which are connected in series between a source line and a bit line, a first selection transistor whose gate terminal is electrically connected to a first selection line, and a second selection transistor whose gate terminal is electrically connected to a second selection line. The first to m-th memory cells each include a first transistor provided on a substrate including a semiconductor material and including a first gate terminal electrically connected to a first signal line, a first source terminal, and a first drain terminal; a second transistor including an oxide semiconductor layer and including a second gate terminal electrically connected to the first signal line, a second source terminal, and a second drain terminal; and a capacitor of which one terminal is electrically connected to one of m word lines. The source line is electrically connected to the first source terminal of the m-th memory cell via the second selection transistor, the bit line is electrically connected to the first drain terminal of the first memory cell via the first selection transistor, and the second source terminal, the first gate terminal, and the other terminal of the capacitor are electrically connected to one another to form a node. The method includes the steps of: in a writing operation for supplying potentials to the nodes in such a manner that a potential is supplied to the second signal line to turn on the second transistors and a potential is supplied to the first signal line, supplying potentials to the first selection line and the second selection line to turn on the first selection transistor and the second selection transistor; detecting a potential of the bit line; and after the bit line and the source line are electrically connected, turning off the second transistors and completing the writing operation.
Another embodiment of the invention disclosed in this specification is a method for driving a semiconductor device that includes a source line, a bit line, m word lines, a first signal line, a second signal line, a first selection line, a second selection line, a first to m-th memory cells connected in series between the source line and the bit line, a first selection transistor whose gate terminal is electrically connected to the first selection line, and a second selection transistor whose gate terminal is electrically connected to the second selection line. The first to m-th memory cells each include a first transistor including a first gate terminal, a first source terminal, and a first drain terminal, a second transistor including a second gate terminal, a second source terminal, and a second drain terminal, and a capacitor. The first transistor is provided on a substrate including a semiconductor material, and the second transistor includes an oxide semiconductor layer. The source line is electrically connected to the first source terminal in the m-th memory cell via the second selection transistor. The bit line is electrically connected to the first drain terminal in the first memory cell via the first selection transistor. The first signal line is electrically connected to the second drain terminal, and the second signal line is electrically connected to the second gate terminal. The first drain terminal in the l-th memory cell (l is a natural number greater than or equal to 2 and less than or equal to m) is electrically connected to the first source terminal in the (l−1)-th memory cell. The k-th (k is a natural number greater than or equal to 1 and less than or equal to m) word line is electrically connected to one terminal of the capacitor in the k-th memory cell. The second source terminal in the k-th memory cell is electrically connected to the first gate terminal in the k-th memory cell and the other terminal of the capacitor in the k-th memory cell. The second source terminal, the first gate terminal, and the other terminal of the capacitor are electrically connected to one another to form a node. In a writing operation for supplying potentials to the nodes in such a manner that a potential is supplied to the second signal line to turn on the second transistors and a potential is supplied to the first signal line, potentials are supplied to the first selection line and the second selection line to turn on the first selection transistor and the second selection transistor; and a potential of the bit line is detected.
Another embodiment of the invention disclosed in this specification is a method for driving a semiconductor device that includes a source line, a bit line, m word lines, a first signal line, a second signal line, a first selection line, a second selection line, a first to m-th memory cells connected in series between the source line and the bit line, a first selection transistor whose gate terminal is electrically connected to the first selection line, and a second selection transistor whose gate terminal is electrically connected to the second selection line. The first to m-th memory cells each include a first transistor including a first gate terminal, a first source terminal, and a first drain terminal, a second transistor including a second gate terminal, a second source terminal, and a second drain terminal, and a capacitor. The first transistor is provided on a substrate including a semiconductor material, and the second transistor includes an oxide semiconductor layer. The source line is electrically connected to the first source terminal in the m-th memory cell via the second selection transistor. The bit line is electrically connected to the first drain terminal in the first memory cell via the first selection transistor. The first signal line is electrically connected to the second drain terminal, and the second signal line is electrically connected to the second gate terminal. The first drain terminal in the l-th memory cell (l is a natural number greater than or equal to 2 and less than or equal to m) is electrically connected to the first source terminal in the (l−1)-th memory cell. The k-th (k is a natural number greater than or equal to 1 and less than or equal to m) word line is electrically connected to one terminal of the capacitor in the k-th memory cell. The second source terminal in the k-th memory cell is electrically connected to the first gate terminal in the k-th memory cell and the other terminal of the capacitor in the k-th memory cell. The second source terminal, the first gate terminal, and the other terminal of the capacitor are electrically connected to one another to form a node. In a writing operation for supplying potentials to the nodes in such a manner that a potential is supplied to the second signal line to turn on the second transistors and a potential is supplied to the first signal line, potentials are supplied to the first selection line and the second selection line to turn on the first selection transistor and the second selection transistor; a potential of the bit line is detected; and after the bit line and the source line are electrically connected to each other, the second transistors are turned off, so that the writing operation is finished. When the second transistors are turned off, the potentials can be held in the nodes.
In any of the above structures, a potential applied to the first signal line in order to supply potentials to the nodes is preferably increased step-by-step.
Further, in the writing operation, potentials applied to the m word lines are any of potentials including a plurality of different potentials in the first to m-th memory cells and after the second transistors are turned off, supply of the potentials to the m word lines is stopped; thus, potentials held in the nodes after the writing operation is completed are a plurality of different potentials in the first to m-th memory cells.
In any of the above-described embodiments, the first transistor may be configured to include a channel formation region provided in the substrate including the semiconductor material, impurity regions disposed so as to interpose the channel formation region therebetween, a first gate insulating layer over the channel formation region, and a first gate electrode overlapping with the channel formation region and provided over the first gate insulating layer.
Note that in this specification and the like, the term such as “over” or “below” does not necessarily mean that a component is placed “directly on” or “directly under” another component. 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. Further, the terms “over” and “below” are used simply for convenience of explanation.
In addition, in this specification and the like, the term such as “electrode” or “wiring” does not limit a function thereof. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. Furthermore, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” are formed in an integrated manner.
Functions of a “source” and a “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be used interchangeably in this specification.
Note that in this specification and the like, the term “electrically connected” includes the case where components are connected via an “object having some electric function”. There is no particular limitation on an “object having some electric function” as long as electric signals can be transmitted and received between components that are connected. Examples of an “object having some electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions as well as an electrode and a wiring.
In a multilevel memory using a transistor including an oxide semiconductor layer, a writing operation is conducted while an appropriate potential is given to a capacitor of a memory in accordance with data to be written, and thereby a potential corresponding to the written data can be given to a floating gate without change in writing voltage. Therefore, a circuit for controlling writing voltage can be omitted, so that a circuit configuration can be simplified.
The potential of a bit line is detected while data writing is conducted, and thereby whether a potential corresponding to the written data is normally applied to the floating gate can be confirmed without a writing verify operation. Thus, in a driving method of a semiconductor device according to the invention disclosed herein, a writing operation with high reliability can be performed at high speed.
Since the off-state current of a transistor including an oxide semiconductor is extremely low, stored data can be held for an extremely long time with use of the transistor. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed).
Further, a semiconductor device according to one embodiment of the disclosed invention does not need high voltage for writing of data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating layer does not occur at all. In other words, the semiconductor device according to one embodiment of the disclosed invention does not have a limit on the number of times of writing, which is a problem in a conventional non-volatile memory, and thus reliability thereof is drastically improved. Furthermore, data is written depending on the on state or the off state of the transistor, whereby high-speed operation can be easily realized. In addition, there is an advantage that there is no need of operation for erasing data.
Since a transistor including a material other than an oxide semiconductor can operate at sufficiently high speed, a semiconductor device employing a combination of the transistor with a transistor including an oxide semiconductor can perform high speed operation (e.g., reading data) readily. Further, a transistor including a material other than an oxide semiconductor can favorably realize a variety of circuits (such as a logic circuit or a driver circuit) which is required to operate at high speed.
Thus, a semiconductor device having a novel feature can be achieved by being provided with both the transistor including a material other than an oxide semiconductor (a transistor capable of operation at sufficiently high speed, in a wider sense) and the transistor including an oxide semiconductor (a transistor whose off-state current is sufficiently low, in a wider sense).



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a timing chart of a semiconductor device.
FIG. 2 is a circuit diagram of a semiconductor device.
FIG. 3 is a circuit diagram of a semiconductor device.
FIG. 4 is a circuit diagram of a semiconductor device.
FIG. 5 is a circuit diagram of a semiconductor device.
FIG. 6 is a timing chart of a semiconductor device.
FIG. 7 is a timing chart of a semiconductor device.
FIG. 8 is a timing chart of a semiconductor device.
FIGS. 9A and 9B are a cross-sectional view and a plan view of a semiconductor device.
FIGS. 10A to 10D are cross-sectional views of a manufacturing process of a semiconductor device.
FIGS. 11A to 11D are cross-sectional views of a manufacturing process of a semiconductor device.
FIGS. 12A to 12D are cross-sectional views of a manufacturing process of a semiconductor device.
FIGS. 13A to 13C are cross-sectional views of a manufacturing process of a semiconductor device.
FIGS. 14A to 14F are diagrams each illustrating an electronic device including a semiconductor device.
FIGS. 15A to 15D are cross-sectional views of semiconductor devices.
FIGS. 16A and 16B are cross-sectional views of semiconductor devices.
FIGS. 17A to 17C are cross-sectional views of a manufacturing process of a semiconductor device.
FIGS. 18A-1, 18A-2, and 18B are circuit diagrams of a semiconductor device.



DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the disclosed invention are described with reference to the drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments.
Note that the position, size, range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like as disclosed in the drawings and the like.
Note that in this specification and the like, the 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.
Embodiment 1
In this embodiment, a circuit configuration and an operation of a semiconductor device according to one embodiment of the disclosed invention will be described with reference to FIG. 1, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8. Note that in each of circuit diagrams, in some cases, “OS” is written beside a transistor in order to indicate that the transistor includes an oxide semiconductor.
In the semiconductor device and a driving method thereof in this embodiment, a signal line for controlling on/off of a writing transistor for conducting a writing operation on a memory cell is disposed along a bit line and a multilevel writing operation is conducted with use of, also in a writing operation, a voltage which is applied to a capacitor at a reading operation.
First, a basic circuit configuration and its operation will be described with reference to FIGS. 18A-1, 18A-2, and 18B. In a semiconductor device illustrated in FIG. 18A-1, a first wiring (a 1st Line) is electrically connected to a source electrode (or a drain electrode) of a transistor 160. A second wiring (a 2nd Line) is electrically connected to the drain electrode (or the source electrode) of the transistor 160. In addition, a third wiring (a 3rd Line) and a source electrode (or a drain electrode) of a transistor 162 are electrically connected to each other, and a fourth wiring (a 4th Line) and a gate electrode of the transistor 162 are electrically connected to each other. In addition, a gate electrode of the transistor 160 and the drain electrode (or the source electrode) of the transistor 162 are electrically connected to one electrode of a capacitor 164, and a fifth wiring (a 5th Line) and the other electrode of the capacitor 164 are electrically connected to each other.
Here, a transistor including an oxide semiconductor is used as the transistor 162, for example. A transistor including an oxide semiconductor has a characteristic of a significantly small off-state current. For that reason, a potential of the gate electrode of the transistor 160 can be held for an extremely long time by turning off the transistor 162. Provision of the capacitor 164 facilitates holding of electric charge given to the gate electrode of the transistor 160 and reading of stored data.
Note that there is no particular limitation on the transistor 160. In terms of increasing the speed of reading data, it is preferable to use, for example, a transistor with high switching rate such as a transistor formed using single crystal silicon.
Alternatively, a structure in which the capacitor 164 is not provided as illustrated in FIG. 18B can be employed.
The semiconductor device in FIG. 18A-1 utilizes a characteristic in which the potential of the gate electrode of the transistor 160 can be held, whereby writing, holding, and reading of data can be performed as follows.
Firstly, writing and holding of data will be described. First, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode of the transistor 160 and the capacitor 164. That is, a predetermined electric charge is given to the gate electrode of the transistor 160 (writing operation). Here, one of electric charges for supply of two different potentials (hereinafter, an electric charge for supply of a low potential is referred to as a charge QL and an electric charge for supply of a high potential is referred to as a charge QH) is given to the gate electrode of the transistor 160. Note that electric charges giving three or more different potentials may be applied to improve a storage capacity. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the electric charge given to the gate electrode of the transistor 160 is held (storing operation).
Since the off-state current of the transistor 162 is significantly small, the electric charge of the gate electrode of the transistor 160 is held for a long time.
Next, reading operation of data will be described. By supplying an appropriate potential (reading potential) to the fifth wiring while a predetermined potential (constant potential) is supplied to the first wiring, the potential of the second wiring varies depending on the amount of electric charge held in the gate electrode of the transistor 160. This is because in general, when the transistor 160 is an n-channel transistor, an apparent threshold voltage Vth—H in the case where QH is given to the gate electrode of the transistor 160 is lower than an apparent threshold voltage Vth—L in the case where QL is given to the gate electrode of the transistor 160. Here, an apparent threshold voltage refers to the potential of the fifth wiring, which is used to turn on the transistor 160. Thus, the potential of the fifth wiring is set to a potential V0 intermediate between Vth—H and Vth—L, whereby electric charge given to the gate electrode of the transistor 160 can be determined. For example, in the case where QH is applied in writing operation, when the potential of the fifth wiring is set to V0 (>Vth—H), the transistor 160 is turned on. In the case where QL is applied in writing operation, even when the potential of the fifth wiring is set to V0 (<Vth—L), the transistor 160 remains off. Therefore, the stored data can be read by the potential of the second wiring.
Note that in the case where memory cells are arrayed to be used, only data of desired memory cells should be read. When data of a predetermined memory cell is read and data of the other memory cells is not read, fifth wirings in memory cells that are not targets for reading data may be supplied with a potential at which the transistors 160 are turned off regardless of the state of the gate electrodes, that is, a potential lower than Vth—H. On the other hand, fifth wirings may be supplied with a potential with which the transistors 160 are turned on regardless of the state of the gate electrodes, that is, a potential higher than Vth—L.
Next, rewriting operation of data will be described. Rewriting operation of data is performed in a manner similar to that of the writing and holding of data. That is, the potential of the fourth wiring is set to a potential which allows the transistor 162 to be turned on, whereby the transistor 162 is turned on. Accordingly, the potential of the third wiring (potential related to new data) is supplied to the gate electrode of the transistor 160 and the capacitor 164. After that, the potential of the fourth wiring is set to a potential which allows the transistor 162 to be turned off, whereby the transistor 162 is turned off. Accordingly, electric charge related to new data is given to the gate electrode of the transistor 160.
In the semiconductor device according to one embodiment of the invention disclosed herein, data can be directly rewritten by another writing operation of data as described above. Therefore, extracting of electric charge from a floating gate with the use of a high voltage needed in a flash memory or the like is not necessary and thus, reduction in operation speed, which is attributed to erasing operation, can be suppressed. In other words, high-speed operation of the semiconductor device can be realized.
Note that the drain electrode (or the source electrode) of the transistor 162 is electrically connected to the gate electrode of the transistor 160, thereby having an effect similar to that of a floating gate of a floating gate transistor used for a non-volatile memory element. In this specification, a portion in which the drain electrode (or the source electrode) of the transistor 162 is electrically connected to the gate electrode of the transistor 160 is called a floating gate (node FG). When the transistor 162 is turned off, the node FG can be regarded as being embedded in an insulator and electric charge is held in the node FG. The off-state current of the transistor 162 including an oxide semiconductor is smaller than or equal to one hundred thousandth of the off-state current of a transistor including a silicon semiconductor or the like; thus, loss of the electric charge accumulated in the node FG due to leakage current of the transistor 162 is negligible. That is, with the transistor 162 including an oxide semiconductor, a non-volatile memory device which can hold data without being supplied with power can be realized.
For example, when the off-state current of the transistor 162 is 10 zA (1 zA (zeptoampere) is 1×10−21 A) or less at room temperature (25° C.) and the capacitance of the capacitor 164 is approximately 10 fF, data can be stored for 104 seconds or longer. It is needless to say that the holding time depends on transistor characteristics and the capacitance.
Further, the semiconductor device according to one embodiment of the disclosed invention does not have the problem of deterioration of a gate insulating film (a tunnel insulating film), which is a problem of a conventional floating gate transistor. That is, the problem of deterioration of a gate insulating film due to injection of electrons into a floating gate, which has been problematic, can be solved. This means that there is no limit on the number of times of writing in principle. Furthermore, a high voltage needed for writing or erasing in a conventional floating gate transistor is not necessary.
Components such as transistors in the semiconductor device illustrated in FIG. 18A-1 can be regarded as including resistors and capacitors as illustrated in FIG. 18A-2. That is, in FIG. 18A-2, the transistor 160 and the capacitor 164 are each regarded as including a resistor and a capacitor. R1 and C1 denote the resistance and the capacitance of the capacitor 164, respectively. The resistance R1 corresponds to the resistance which depends on an insulating layer included in the capacitor 164. Further, R2 and C2 denote the resistance and the capacitance of the transistor 160, respectively. The resistance R2 corresponds to the resistance which depends on a gate insulating layer at the time when the transistor 160 is turned on. The capacitance C2 corresponds to the capacitance of a so-called gate capacitance (the capacitance formed between the gate electrode and the source electrode or the drain electrode, and the capacitance formed between the gate electrode and the channel formation region).
A charge holding period (also referred to as a data holding period) is determined mainly by off-state current of the transistor 162 under the condition where gate leakage current of the transistor 162 is sufficiently small, R1≧ROS, and R2≧ROS, where ROS is the resistance value (also referred to as effective resistance) between the source electrode and the drain electrode at the time when the transistor 162 is in an off-state.
On the other hand, when the conditions are not met, it is difficult to sufficiently secure the holding period even if the off-state current of the transistor 162 is small enough. This is because a leakage current other than the off-state current of the transistor 162 (e.g., a leakage current generated between the source electrode and the gate electrode of the transistor 160) is large. Accordingly, it can be said that it is preferable that the semiconductor device disclosed in this embodiment satisfy the above relations of R1≧ROS and R2≧ROS.
On the other hand, it is desirable that C1 and C2 satisfy C1≧C2. This is because by increasing C1, the potential of the fifth wiring can be effectively applied to the node FG when the potential in the node FG is controlled by the fifth wiring, and thus the difference between the potentials applied to the fifth wiring (e.g., a potential of reading and a potential of not reading) can be made small.
As described above, when the above relation is satisfied, a more favorable semiconductor device can be realized. Note that R1 and R2 are controlled by the gate insulating layer of the transistor 160 and the insulating layer of the capacitor 164. The same is applied to C1 and C2. Therefore, the material, the thickness, and the like of the gate insulating layer are desirably set as appropriate to satisfy the above relation.
In the semiconductor device described in this embodiment, the node FG operates similarly to a floating gate of a floating gate transistor in a flash memory or the like, but the node FG in this embodiment has a feature which is essentially different from that of the floating gate in the flash memory or the like.
In a flash memory, since a potential applied to a control gate is high, it is necessary to keep a proper distance between cells in order to prevent the potential from affecting a floating gate of an adjacent cell. This is one of inhibiting factors for high integration of the semiconductor device. The factor is attributed to a basic principle of a flash memory, in which a tunneling current flows in applying a high electric field.
In contrast, the semiconductor device according to this embodiment operates by switching of a transistor including an oxide semiconductor and does not use the above-described principle of electric charge injection by tunneling current. That is, a high electric field for electric charge injection is not necessary, unlike a flash memory. Accordingly, it is not necessary to consider an influence of a high electric field from a control gate on an adjacent cell, which facilitates high integration.
In addition, it is also advantageous that a high electric field is unnecessary and a large peripheral circuit (such as a booster circuit) is unnecessary, over a flash memory. For example, the highest voltage applied to the memory cell according to this embodiment (the difference between the highest potential and the lowest potential applied to the terminals of the memory cell at the same time) can be 5 V or lower, preferably 3 V or lower in a memory cell in the case where two levels (one bit) of data are written.
In the case where the dielectric constant ∈r1 of the insulating layer included in the capacitor 164 is different from the dielectric constant ∈r2 of the insulating layer included in the transistor 160, C1 and C2 can easily satisfy C1≧C2 while S1 that is the area of the insulating layer included in the capacitor 164 and S2 that is the area of an insulating layer forming gate capacitance of the transistor 160 satisfy 2·S2≧S1 (desirably S2≧S1). That is, it is easy to satisfy C1≧C2 while the area of the insulating layer included in the capacitor 164 is small. Specifically, for example, a film formed of a high-k material such as hafnium oxide or a stacked-layer structure of a film formed of a high-k material such as hafnium oxide and a film formed of an oxide semiconductor is used for the insulating layer included in the capacitor 164 so that ∈r1 can be set to 10 or more, preferably 15 or more, and silicon oxide is used for the insulating layer forming the gate capacitance of the transistor 160 so that ∈r2 can be set to 3 to 4.
Combination of such structures enables higher integration of the semiconductor device according to one embodiment of the disclosed invention.
Note that in addition to the increase in the degree of integration, a multilevel technique can be employed in order to increase the storage capacity of the semiconductor device. For example, three or more levels of data are written to one memory cell, whereby the storage capacity can be increased as compared to that in the case where two-level (one-bit) data is written. The multilevel technique can be achieved by, for example, giving charge Q, which is different from the charge QL for supplying a low potential and the charge QH for supplying a high potential, to the gate electrode of the first transistor, in addition to the charge QL and the charge QH. In that case, enough storage capacity can be ensured even in a circuit configuration with a relatively large scale.
Next, a more specific circuit configuration to which the circuit illustrated in FIGS. 18A-1, 18A-2, and 18B is applied and an operation thereof will be described with reference to FIG. 1, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8.
FIG. 2 is an example of a circuit diagram of a semiconductor device including m (rows) (in a vertical direction)×n (columns) (in a horizontal direction) memory cells 190. The configuration of the memory cells 190 in FIG. 2 is similar to that in FIG. 18A-1. That is, the first wiring in FIG. 18A-1 corresponds to a bit line BL in FIG. 2; the second wiring in FIG. 18A-1, a source line SL in FIG. 2; the third wiring in FIG. 18A-1, the first signal line S1 in FIG. 2; the fourth wiring in FIG. 18A-1, the second signal line S2 in FIG. 2; and the fifth wiring in FIG. 18A-1, a word line WL in FIG. 2. Note that in FIG. 2, the transistors 160 in the memory cells 190 are connected in series in the column direction. Thus, only the memory cells 190 in the first row are connected to the bit lines BL without other memory cells 190 interposed therebetween, and only the memory cells 190 in the m-th row are connected to the source line SL without other memory cells 190 interposed therebetween. The memory cells 190 in the other rows are electrically connected to the bit lines BL and the source line SL through other memory cells 190 of the same columns.
The semiconductor device illustrated in FIG. 2 includes m (m is an integer greater than or equal to 2) word lines WL; n (n is an integer greater than or equal to 2) bit lines BL; the first signal line S1; n second signal lines S2; a memory cell array having the memory cells 190 arranged in a matrix of m (rows) (in the vertical direction)×n (columns) (in the horizontal direction); a source line SL; a selection line G_1 and a selection line G_2; n selection transistors 180 which are arranged along the selection line G_1 and between the bit lines BL and the memory cells 190 in the first row and whose gate electrodes are electrically connected to the selection line G_1; and n selection transistors 182 which are arranged along the selection line G_2 and between the memory cells 190 in the m-th row and the source line SL and whose gate electrodes are electrically connected to the selection line G_2.
That is, the bit lines BL are electrically connected to the drain electrodes of the transistors 160 in the memory cells 190 in the first row via the selection transistors 180. Further, the source line SL is electrically connected to the source electrodes of the transistors 160 in the memory cells 190 in the m-th row via the selection transistors 182. The first signal line S1 is electrically connected to the drain electrodes of all of the transistors 162, the signal line S2_k in the k-th column (k is a natural number greater than or equal to 1 and less than or equal to n) is electrically connected to the gate electrodes of the transistors 162 in the memory cells 190 in the k-th column. The word line WL in the k-th row is electrically connected to electrodes on one side of the capacitors 164 in the memory cells 190 in the k-th row.
Further, the second signal lines S2 are parallel to the bit lines, and are electrically connected to the transistors 162 in the adjacent memory cells 190.
The node FG in each of the memory cells 190 in the k-th row of the semiconductor device illustrated in FIG. 2 is the same as the structure illustrated in FIG. 18A-1. Here, the transistors 162 including an oxide semiconductor have significantly small off-state current in the k-th row, in the memory cells 190 in the semiconductor device illustrated in FIG. 2, the potentials of the nodes FG can be held for a long time by turning off the transistors 162 as in the semiconductor device illustrated in FIG. 18A-1.
Further, the gate electrodes of the transistors 162 in the memory cells 190 are electrically connected to the second signal line S2, which is parallel to the bit line, and thereby a writing operation using voltage given to the capacitor 164 is possible. Therefore, also in the case where multilevel data is written in the memory cell 190, the peripheral circuit such as the circuit which controls writing voltage can be omitted, because voltage applied to the drain electrodes of the transistors 162 are not necessary to be changed corresponding to writing data.
Note that the selection line G_1, the selection line G_2, the selection transistors 180, and the selection transistors 182 are not necessarily provided. The selection line G_1 and the selection transistors 180 may be omitted. Alternatively, the selection line G_2 and the transistors 182 may be omitted. For example, as illustrated in FIG. 3, a structure may be employed in which a selection line G corresponding to the selection line G_2 and the selection transistors 182 alone are provided.
Moreover, as illustrated in FIG. 4, the source electrode of the transistor 162 in a memory cell 190 and the drain electrode of the transistor 162 in the adjacent memory cell 190 thereto may be connected in series. Note that the selection line G_1, the selection line G_2, the selection transistors 180, and the selection transistors 182 are not necessarily provided. The selection line G_1 and the selection transistors 180 may be omitted. Alternatively, the selection line G_2 and the selection transistors 182 may be omitted. For example, as illustrated in FIG. 5, a structure may be employed in which a selection line G corresponding to the selection line G_2 and the selection transistors 182 alone are provided.
Data writing, holding, and reading operations in the semiconductor device illustrated in FIG. 5 are basically similar to those in the case of FIGS. 18A-1, 18A-2, and 18B. Note that data writing is conducted on each column. This is because a gate electrode of a transistor 162 in a memory cell 190 is connected to a gate electrode of a transistor 162 in the adjacent memory cell 190 by a second signal line S2 and thus it is difficult to conduct writing operation on each memory cell 190. As an example of specific writing operation, an example in which any of potentials V1, V2, V3 and a reference potential GND (VDD>V3>V2>V1>GND=0V) is given to a node FG is explained; however, the relation in the potentials given to the nodes FG is not limited to the example. Data that is held when the potentials V1, V2, and V3 are given to the node FG is referred to as data “1”, “2”, and “3”, respectively, and data that is held when the reference potential GND is given to the node FG is referred to as data “0”.
First, a potential is given to capacitors 164 in the memory cells 190 of a column into which data is written, in accordance with data to be written. A potential V4 (sufficiently high potential, e.g., VDD) is given to the second signal line S2 of the same column so that transistors 162, that are OS transistors, of the memory cells 190 into which data is written are turned on, and thereby data is written. Note that a writing voltage to be used for supplying an electric charge to nodes FG via the transistors 162 from the first signal line S1 is referred to as Von. Here, Von is a voltage sufficiently higher than threshold voltages of reading selection transistors 180 connected to the bit line.
When data “0” is written into the memory cell 190, Von is given to the capacitor 164. When data “1” is written into the memory cell 190, −(V1−Von) is given to the capacitor 164. When data “2” is written into the memory cell 190, −(V2−Von) is given to the capacitor 164. When data “3” is written into the memory cell 190, −(V3−Von) is given to the capacitor 164. At this time, whichever voltage is applied to the capacitor 164, the voltage Von is applied to the node FG at the time of writing.
In this case, when data “1” is written, GND is given to the capacitor 164 for the data writing, and thereby the peripheral circuit can be more simplified. In other words, by employing V1=Von, the number of voltages to be adjusted can be decreased by one, and thereby the peripheral circuit can be more simplified.
Data holding operation is conducted as follows: the potential of the second signal line S2 connected to the memory cells 190 that are targets for holding data is set to GND. When the potential of the second signal line S2 is fixed to GND, the potential of the nodes FG is fixed to the potential at the time of writing. In other words, in the memory cells 190 in which data is written, the potential of the nodes FG is Von in the state where a potential in accordance with data to be written is given to the capacitors 164. Thus, after the potential Von is given to the nodes FG and thus the nodes FG are in a floating state, the potential of the capacitors 164 is turned into GND. At this time, the potential of the node FG in the memory cell 190 in which data “1” is written is V1, the potential of the node FG in the memory cell 190 in which data “2” is written is V2, the potential of the node FG in the memory cell 190 in which data “3” is written is V3, and the potential of the node FG in the memory cell 190 in which data “0” is written is the reference potential GND.
Because GND is supplied to the second signal line S2, the transistor 162 is turned off even when any of data “0”, data “1”, data “2”, and data “3” is written. Since the off-state current of the transistor 162 is significantly small, the electric charge of the gate electrode of the transistor 160 is held for a long time. In this manner, writing into an arbitrary column is completed.
Data reading operation is conducted as follows: the potential of the word line WL connected to the memory cells 190 that are targets for reading data is set to any one of GND, −(V1−Von), and −(V2−Von), and the potential of the word lines WL connected to the memory cells 190 that are not targets for reading data is set to Von, and the potentials of the selection line G_1 and the selection line G_2 are set to V4.
When the potential of the word line WL connected to the memory cells 190 that are targets for reading data is set to GND, the transistors 160 are turned on when any of data “1”, data “2”, and data “3” is supplied to the nodes FG of the memory cells 190 that are targets for reading data. On the other hand, the transistors 160 are turned off when GND for data “0” is supplied to the nodes FG.
Similarly, when the potential of the word line WL connected to the memory cells 190 that are targets for reading data is −(V1−Von) and data “2” or data “3” is given to the nodes FG of the memory cells 190 that are targets for reading data, the transistors 160 are turned on. On the other hand, when the potential of the word line WL connected to the memory cells 190 for reading operation is −(V1−Von) and data “0” or data “1” is given to the nodes FG of the memory cells 190 for reading operation, the transistors 160 are turned off. In addition, when the potential of the word line WL connected to the memory cells 190 that are targets for reading data is −(V2−Von) and data “3” is given to the nodes FG of the memory cells 190 for reading operation, the transistors 160 are turned on. On the other hand, when the potential of the word line WL connected to the memory cells 190 that are targets for reading data is −(V2−Von) and data “0”, data “1”, or data “2” is given to the nodes FG of the memory cells 190 that are targets for reading data, the transistors 160 are turned off.
When the potential of the word line WL connected to the memory cells 190 from which data is not to be read is set to Von, in a case where data “0” is written in the memory cells 190 that are not targets for reading data and in a case where any of data “1”, data “2”, and data “3” is written in the memory cells 190 that are not targets for reading data, the transistors 160 are turned on in any case.
Note that in the configuration illustrated in FIG. 2, writing cannot be conducted on each arbitrary memory cell 190, and thus re-writing should be conducted per column. The reason for that is the same as that for the case where writing is conducted per column. In other words, because a gate electrode of a transistor 162, that is an OS transistor, in a certain memory cell 190 is connected to a gate electrode of a transistor 162, that is an OS transistor, in the adjacent memory cell 190 by the second signal line S2, re-writing to each memory cell 190 is difficult.
FIG. 6 and FIG. 7 are examples of timing charts for more detailed operations of the semiconductor device in FIG. 2. Names such as S, BL, and the like in the timing charts denote the wirings to which the potentials shown in the timing charts are applied. Wirings having similar functions are distinguished by “—1”, “—2”, and the like added to the end of their names.
The timing chart in FIG. 6 shows the relation among the potentials of the wirings in the case where data “1” is written to a first row of an arbitrary memory cell column (k-th column), data “2” is written to a second row of the arbitrary memory cell column (k-th column), data “3” is written to a third row of the desired memory cell column (k-th column), and data “0” is written to a fourth row to an m-th row of the arbitrary memory cell column (k-th column). The timing chart in FIG. 7 shows the relation among the potentials of the wirings in the case where, after the writing operation, data written in an arbitrary i-th row (i is a natural number greater than or equal to 1 and less than or equal to m) is read out. Note that in FIG. 7, V5 is a potential applied to BL at the time of reading.
In the writing operation, in accordance with data to be written in each memory cell 190 in a memory cell column to which data is to be written, a potential corresponding to the writing data is given to the capacitors 164 from the WL, and V4 is given to S2, so that all transistors 162 of the memory cell column to which data is to be written are turned on and Von is given to S1 so that the nodes FG of all of the memory cells 190 to which data is to be written are Von.
After that, the potential given to the capacitors 164 from WL is set to GND so that the potentials of the nodes FG are adjusted. The relation of the potentials of the wirings at this time is shown in FIG. 8. In other words, in accordance with GND given to the capacitors 164 after writing, the potential of the first row on the k-th column is shifted into V1 and thus data “1” is written. In the same manner, the potential of the second row on the k-th column is shifted into V2 and thus data “2” is written, the potential of the third row on the k-th column is shifted into V3 and thus data “3” is written, and the potential of the fourth row to the m-th row on the k-th column is shifted into GND and thus data “0” is written.
Note that in the semiconductor device described in this embodiment, when data is written to the memory cells 190 in the k-th row (k is a natural number greater than or equal to 1 and less than or equal to m), the transistors 162 in the same column should be turned on; therefore, data should be written to the memory cell array per column.
As shown in FIG. 7, in the reading operation, the voltage supplied to the capacitors 164 at the writing operation is used alone, and the reading operation can be completed.
When data reading is performed in the i-th row, S2_1 to S2_m are set to GND so that all the transistors 162 are turned off, and the selection line G_1 and the selection line G_2 are supplied with the potential V4 so that the selection transistors 180 and the selection transistors 182 are turned on. In addition, to WL_i connected to the memory cells 190 of the i-th row as reading targets, GND, −(V1−Von), and −(V2−Von) are sequentially supplied, and the potential of the nodes FG is identified, i.e., written data is identified based on whether BL is in a conduction state or a non-conduction state at each of GND, −(V1−Von), and −(V2−Von). Note that the potential Von is given to the WL connected to the memory cells 190 that are not targets for reading data.
In the driving method of a semiconductor device disclosed in this specification, a writing operation is not adversely affected in conducting data writing on a memory cell even when any potential is applied to the bit line BL. Thus, when the potential of the bit line BL is detected while data writing is conducted, the writing operation can be conducted while whether it is normally performed is confirmed. Therefore, a writing verify operation can be omitted.
One embodiment of a driving method of a semiconductor device disclosed in this specification is described with reference to a timing chart in FIG. 1. In the case where data writing is performed on the k-th column, the selection transistors 180 in the selection line G_1 and the selection transistors 182 in the selection line G_2 are turned on, and data writing is performed while V5 (voltage used for data reading) is applied to the bit line BL_k. Voltage Von applied to the first signal line S1 is stepped up little by little, and the potentials of the nodes FG in all the memory cells in the k-th column on which data writing is conducted are increased. When the potentials of the nodes FG in all the memory cells in the k-th column are each increased to a predetermined potential and the transistors 160 in all the memory cells in the k-th column are turned on, the bit line BL_k and the source line SL (the potential of SL is 0 V) are brought into conduction and the potential of the bit line BL_k becomes 0 V. When it is detected that the potential of the bit line BL_k becomes 0 V, data writing to the next column k+1 starts.
Therefore, in a multilevel memory according to one embodiment of the disclosed invention, a necessary writing verify operation can be omitted; thus, data writing can be performed at high speed. In addition, even when the verify operation is omitted, a writing operation can be conducted on each column with appropriate writing voltage, which leads to a narrow distribution of the threshold voltage of the transistor 160. Consequently, the range of voltage used for data writing can be narrowed, so that low power consumption can be realized.
In the case where a configuration is employed in which the selection line G_1 and the selection transistors 180 are omitted or the selection line G_2 and the selection transistors 182 are omitted and only the selection line G corresponding to the selection line G_2 and the selection transistors 182 are provided as illustrated in FIG. 3 and FIG. 5, data writing, data holding, data reading, and data erasing at one time can also be performed basically in the same manner as in the above operations.
Since the off-state current of the transistor including an oxide semiconductor is extremely small in the semiconductor device described in this embodiment, stored data can be held for an extremely long period owing to such a transistor. In other words, power consumption can be adequately suppressed because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed).
Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating layer does not occur at all. In other words, the semiconductor device according to one embodiment of the disclosed invention does not have a limit on the number of times of writing, which is a problem in a conventional non-volatile memory, and thus reliability thereof is drastically improved. Furthermore, data is written depending on the on state or the off state of the transistor, whereby high-speed operation can be easily realized. In addition, there is an advantage that there is no need of operation for erasing data.
Since a transistor including a material other than an oxide semiconductor can operate at sufficiently high speed, a semiconductor device can perform operation (e.g., reading data) at sufficiently high speed in combination of the transistor with a transistor including an oxide semiconductor. Further, a transistor including a material other than an oxide semiconductor can favorably realize a variety of circuits (such as a logic circuit or a driver circuit) which is required to operate at high speed.
Thus, a semiconductor device having a novel feature can be achieved by being provided with both a transistor including a material other than an oxide semiconductor (a transistor capable of operation at a sufficiently high speed, in a wider sense) and a transistor including an oxide semiconductor (a transistor whose off-state current is sufficiently low, in a wider sense).
In addition, in the semiconductor device in this embodiment, a signal line for controlling on/off of a writing transistor and a bit line are disposed in parallel. When data of more levels than 2 (multilevel) is written to be stored, the potential of a capacitor of a memory cell is shifted in accordance with writing data (the potential of the word line WL is shifted), so that multilevel data can be written into the node FG with one potential for data writing. In conventional art, for writing of multilevel data, potentials for respective levels are needed; however, in this embodiment, one potential is enough for writing of multilevel data. Therefore, conventional circuits for producing potentials for multilevel are not needed, and thereby the peripheral circuit can be simplified so that the memory itself can be downsized.
The configuration, structure, method, and the like described in this embodiment can be combined with any of the configurations, structures, methods, and the like described in the other embodiments as appropriate.
Embodiment 2
In this embodiment, a structure and a manufacturing method of a semiconductor device according to one embodiment of the disclosed invention will be described with reference to FIG. 5, FIGS. 9A and 9B, FIGS. 10A to 10D, FIGS. 11A to 11D, FIGS. 12A to 12D, and FIGS. 13A to 13C.
FIGS. 9A and 9B show an example of a structure of the memory cell 190 of the semiconductor device illustrated in the circuit diagram of FIG. 5. FIG. 9A illustrates a cross section of the semiconductor device, and FIG. 9B illustrates a plan view of the semiconductor device. Note that, in the plan view illustrated in FIG. 9B, the drawing is simplified without the insulating layer 154, the insulating layer 172, the wiring 171, and the wiring 158 illustrated. Here, in FIG. 9A, the direction parallel to the line A1-A2 of FIG. 9B corresponds to the column direction in the circuit diagram of FIG. 5, and the direction perpendicular to the line A1-A2 of FIG. 9B corresponds to the row direction in the circuit diagram of FIG. 5. The semiconductor device illustrated in FIGS. 9A and 9B includes, in a lower portion, a transistor 160 including a first semiconductor material, and in an upper portion, a transistor 162 including a second semiconductor material. Note that although the transistor 160 and the transistor 162 in the first row are illustrated in FIGS. 9A and 9B, as for the transistors 160 in the first to m-th rows, a source electrode (source region) of a transistor 160 and a drain electrode (drain region) of an adjacent transistor 160 are connected in series, and also as for the transistors 162 in the first to m-th rows, a source electrode (source region) of a transistor 162 and a drain electrode (drain region) of an adjacent transistor 162 are connected in series as illustrated in the circuit diagram of FIG. 5.
Here, the first semiconductor material is preferably different from the second semiconductor material. For example, the first semiconductor material may be a semiconductor material (e.g., silicon) other than an oxide semiconductor and the second semiconductor material may be an oxide semiconductor. A transistor using a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor using an oxide semiconductor can hold electric charges for a long time owing to its characteristics.
Although all the transistors are n-channel transistors here, it is needless to say that p-channel transistors can be used. The technical essence of the disclosed invention lies in the use of a semiconductor material with which off-state current can be sufficiently reduced, such as an oxide semiconductor, for the transistor 162 in order to hold data. Therefore, it is not necessary to limit specific conditions, such as a material, a structure, or the like of the semiconductor device, to those given here.
The transistor 160 in FIGS. 9A and 9B includes a channel formation region 116a provided in a substrate 100 including a semiconductor material (such as silicon), an impurity region 120a and an impurity region 120b provided such that the channel formation region 116a is sandwiched therebetween, a metal compound region 124a and a metal compound region 124b in contact with the impurity region 120a and the impurity region 120b, a gate insulating layer 108a provided over the channel formation region 116a, and a gate electrode 110 provided over the gate insulating layer 108a. Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing may be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode are collectively referred to as a “source electrode”, and a drain region and a drain electrode are collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region and the term “drain electrode” may include a drain region.
Here, the transistors 160 in the first to m-th rows share the impurity regions 120 and the metal compound regions 124 functioning as source regions and drain regions with each other, and thus are connected in series. That is, the impurity region 120 and the metal compound region 124 functioning as a source region of the transistor 160 in the (l−1)-th row (l is a natural number greater than or equal to 2 and less than or equal to m) also function as a drain region of the transistor 160 in the l-th row. In this manner, the transistors 160 in the memory cells 190 are connected in series, whereby the source regions and the drain regions can be shared by the transistors 160 in the adjacent memory cells 190. Therefore, the planar layout of the transistor 160 can easily overlap with the planar layout of the transistor 162 which is described later; thus, the area occupied by the memory cell 190 can be reduced.
An element isolation insulating layer 106 is provided over the substrate 100 to surround the transistor 160. An insulating layer 128 is provided to cover the transistor 160. Note that in order to realize higher integration, the transistor 160 preferably has a structure without a sidewall insulating layer as illustrated in FIGS. 9A and 9B. On the other hand, when the characteristics of the transistor 160 have priority, the sidewall insulating layer may be formed on a side surface of the gate electrode 110 and the impurity region 120 may include regions having different impurity concentrations.
Here, the insulating layer 128 preferably has a surface with favorable planarity; for example, the surface of the insulating layer 128 preferably has a root-mean-square (RMS) roughness of 1 nm or less.
The transistor 162 in FIGS. 9A and 9B includes a source electrode 142a and a drain electrode 142b which are embedded in an insulating layer 140 formed over the insulating layer 128; an oxide semiconductor layer 144 in contact with part of the insulating layer 140, the source electrode 142a, and the drain electrode 142b; a gate insulating layer 146 covering the oxide semiconductor layer 144; and a gate electrode 148 provided over the gate insulating layer 146 so as to overlap with the oxide semiconductor layer 144. Note that the gate electrode 148 functions as the second signal line S2 in the circuit diagram of FIG. 5.
Here, the oxide semiconductor layer 144 is preferably a highly-purified oxide semiconductor layer by sufficiently removing impurities such as hydrogen or sufficiently supplying oxygen. Specifically, the hydrogen concentration of the oxide semiconductor layer 144 is 5×1019 atoms/cm3 or lower, preferably 5×1018 atoms/cm3 or lower, more preferably 5×1017 atoms/cm3 or lower. Note that the hydrogen concentration of the above-described oxide semiconductor layer 144 is measured by secondary ion mass spectrometry (SIMS). Thus, in the oxide semiconductor layer 144 in which the hydrogen concentration is sufficiently reduced and defect levels in the energy gap due to oxygen deficiency are reduced by sufficient supply of oxygen, the density of carriers due to a donor such as hydrogen is lower than 1×1012/cm3, preferably lower than 1×1011/cm3, further preferably lower than 1.45×1010/cm3. In addition, for example, the off-state current (per unit channel width (1 μm), here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10−21 A) or less, preferably 10 zA or less. In this manner, by using an oxide semiconductor which is made to be an i-type (intrinsic) oxide semiconductor or a substantially i-type oxide semiconductor, the transistor 162 which has excellent off-state current characteristics can be obtained.
Note that a region which is part of a surface of the insulating layer 140 and in contact with the oxide semiconductor layer 144 preferably has a root-mean-square (RMS) roughness of 1 nm or less. In this manner, a channel formation region of the transistor 162 is provided in the extremely flat region having a root-mean-square (RMS) roughness of 1 nm or less, whereby the transistor 162 which can prevent a malfunction such as a short-channel effect and has favorable characteristics can be provided even when the transistor 162 is miniaturized.
The transistors 162 in the first to m-th rows share the source electrodes 142a and the drain electrodes 142b with each other, and thus are connected in series. That is, the source electrode 142a of the transistor 162 in the (l−1)-th row (l is a natural number greater than or equal to 2 and less than or equal to m) and the drain electrode 142b of the transistor 162 in the l-th row are formed of the same conductive layer.
In this manner, the transistors 162 in the memory cells 190 are connected in series, whereby the source electrodes 142a and the drain electrodes 142b of the transistors 162 in the adjacent memory cells 190 can be shared. Thus, only one of the source electrode 142a and the drain electrode 142b of the transistor 162 is included in the planar layout of the memory cell 190. That is, the length in the column direction in the planar layout of the memory cell 190 can be approximately equal to the length in the column direction in the gate electrode 148 and the source electrode 142a 
On the other hand, in the case where the transistors 162 of the memory cells 190 are connected in parallel and the source electrode 142a and the drain electrode 142b are provided for each of the transistors 162 in the memory cells 190, both the source electrode 142a and the drain electrode 142b of the transistor 162 are included in the planar layout of the memory cell 190.
Therefore, the structure illustrated in FIGS. 9A and 9B is employed for the planar layout of the memory cell 190, whereby the area occupied by the memory cell 190 can be reduced. For example, when F is used to express the minimum feature size, the area occupied by the memory cell 190 can be 4 F2 to 12 F2. Accordingly, the degree of integration of the semiconductor device can be enhanced, and the storage capacity per unit area can be increased.
The capacitor 164 in FIGS. 9A and 9B includes the source electrode 142a; the oxide semiconductor layer 144; the gate insulating layer 146; and an insulating layer 150 and an electrode 152 over the gate insulating layer 146. That is, the source electrode 142a functions as one electrode of the capacitor 164, and the electrode 152 functions as the other electrode of the capacitor 164. Here, one electrode of the capacitor 164 in the (l−1)-th row (l is a natural number greater than or equal to 2 and less than or equal to m) is the source electrode 142a of the transistor 162 in the (l−1)-th row (l is a natural number greater than or equal to 2 and less than or equal to m); therefore, the planar layout of the capacitor 164 can easily overlap with the planar layout of the transistor 162, and the area occupied by the memory cell 190 can be reduced. In addition, in the case of forming the electrode 152 over the insulating layer 150, the area of the electrode 152 can be increased more easily in a range of overlapping with the planar layout of the transistor 162 than the case of forming the electrode 152 and the gate electrode 148 in the same layer. Note that the electrode 152 functions as the word line WL in the circuit diagram of FIG. 5.
The insulating layer 150 is provided over the transistor 162, and an insulating layer 154 is provided over the insulating layer 150 and the electrode 152 of the capacitor 164. An opening that reaches the gate electrode 148 is formed in the insulating layer 150 and the insulating layer 154, and an electrode 170 is formed in the opening. By forming the wiring 171 so as to be in contact with the electrode 170 formed to be embedded in the insulating layer 154, over the insulating layer 154, the gate electrode 148 is electrically connected to the wiring 171. The insulating layer 172 is provided over the insulating layer 154 and the wiring 171.
In an opening formed in the gate insulating layer 146, the insulating layer 150, the insulating layer 154, and the insulating layer 172, an electrode 156 is provided. Over the insulating layer 172, the wiring 158 connected to the electrode 156 is formed. The wiring 158 and the metal compound region 124b functioning as a drain region of the transistor 160 are electrically connected to each other through the electrode 156 provided in the opening formed in the gate insulating layer 146, the insulating layer 150, the insulating layer 154, and the insulating layer 172; the drain electrode 142b embedded in the insulating layer 140; and the electrode 126 embedded in the insulating layer 128. Here, the wiring 158 functions as the bit line BL in the circuit diagram of FIG. 5.
With the above structure, in the planar layout of the memory cell 190 including the transistor 160, the transistor 162, and the capacitor 164, the length in the row direction can be approximately equal to the width of the wiring 158, the length in the column direction can be approximately equal to the length of the gate electrode 148 and the source electrode 142a. When such a planar layout is employed, the degree of integration of the circuit in FIG. 5 can be enhanced. For example, when F is used to express the minimum feature size, the area occupied by a memory cell can be expressed as 4 F2 to 12 F2. Accordingly, the storage capacity per unit area of the semiconductor device can be increased.
Note that the structure of a semiconductor device according to one embodiment of the disclosed invention is not limited to that illustrated in FIG. 9A and FIG. 9B. Since the technical spirit of one embodiment of the disclosed invention is formation of a stacked-layer structure formed using an oxide semiconductor and a material other than an oxide semiconductor, details such as an electrode connection or the like can be modified as appropriate.
Next, an example of a method for manufacturing the above-described semiconductor device will be described. First, a method for manufacturing the transistor 160 in the lower portion will be described below with reference to FIGS. 10A to 10D and FIGS. 11A to 11D; then, a method for manufacturing the transistor 162 in the upper portion and the capacitor 164 will be described with reference to FIGS. 12A to 12D and FIGS. 13A to 13C.
First, the substrate 100 including a semiconductor material is prepared (see FIG. 10A). As the substrate 100 including a semiconductor material, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like; a compound semiconductor substrate made of silicon germanium or the like; an SOI substrate; or the like can be used. Here, an example of using a single crystal silicon substrate as the substrate 100 including a semiconductor material is described. Note that in general, the term “SOI substrate” means a substrate where a silicon semiconductor layer is provided on an insulating surface. In this specification and the like, however, the term “SOI substrate” also includes a substrate where a semiconductor layer formed using a material other than silicon is provided over an insulating surface in its category. That is, the semiconductor layer included in the “SOI substrate” is not limited to a silicon semiconductor layer. Moreover, the SOT substrate can be a substrate having a structure in which a semiconductor layer is provided over an insulating substrate such as a glass substrate, with an insulating layer interposed therebetween.
A single crystal semiconductor substrate of silicon or the like is particularly preferably used as the substrate 100 including a semiconductor material, in which case the speed of reading operation of the semiconductor device can be increased.
In order to control the threshold voltage of the transistor, an impurity element may be added to regions which later function as the channel formation region 116a of the transistor 160 and the channel formation region 116b of the selection transistor 182 (not illustrated in FIGS. 9A and 9B, FIGS. 10A to 10D, FIGS. 11A to 11D, FIGS. 12A to 12D, and FIGS. 13A to 13C, see FIG. 5). Here, an impurity element imparting conductivity is added so that the threshold voltage of the transistor 160 and the threshold voltage of the selection transistor 182 (not illustrated in FIGS. 9A and 9B, FIGS. 10A to 10D, FIGS. 11A to 11D, FIGS. 12A to 12D, and FIGS. 13A to 13C, see FIG. 5) become positive. When the semiconductor material is silicon, the impurity imparting conductivity may be boron, aluminum, gallium, or the like. Note that it is preferable to perform heat treatment after adding an impurity element, in order to activate the impurity element or reduce defects which may be generated during addition of the impurity element.
A protective layer 102 serving as a mask for forming an element isolation insulating layer is formed over the substrate 100 (see FIG. 10A). As the protective layer 102, for example, an insulating layer formed using silicon oxide, silicon nitride, silicon oxynitride, or the like can be used.
Next, part of the substrate 100 in a region that is not covered with the protective layer 102 (in an exposed region) is removed by etching with the use of the protective layer 102 as a mask. Thus, a semiconductor region 104 which is separated from another semiconductor region is formed (see FIG. 10B). As the etching, dry etching is preferably performed, but wet etching may be employed. An etching gas and an etchant can be selected as appropriate depending on a material to be etched.
Then, an insulating layer is formed so as to cover the semiconductor region 104, and the insulating layer in a region overlapping with the semiconductor region 104 is selectively removed; thus, the element isolation insulating layer 106 is formed (see FIG. 10C). The insulating layer is formed using silicon oxide, silicon nitride, silicon oxynitride, or the like. As a method for removing the insulating layer, any of polishing treatment such as chemical mechanical polishing (CMP) treatment, etching treatment, and the like can be employed. Note that the protective layer 102 is removed after the formation of the semiconductor region 104 or after the formation of the element isolation insulating layer 106.
Next, an insulating layer is formed over a surface of the semiconductor region 104, and a layer including a conductive material is formed over the insulating layer.
The insulating layer is to be a gate insulating layer later and can be formed by performing heat treatment (e.g., thermal oxidation treatment or thermal nitridation treatment) on the surface of the semiconductor region 104, for example. Instead of the heat treatment, high-density plasma treatment may be employed. The high-density plasma treatment can be performed using, for example, a mixed gas of a rare gas such as He, Ar, Kr, or Xe and a gas such as oxygen, nitrogen oxide, ammonia, nitrogen, or hydrogen. Needless to say, the insulating layer may be formed using a CVD method, a sputtering method, or the like. The insulating layer preferably has a single-layer structure or a stacked-layer structure which includes a film including, silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSixOy (x>0, y>0)), hafnium silicate (HfSixOy (x>0, y>0)) to which nitrogen is added, hafnium aluminate (HfAlxOy (x>0, y>0)) to which nitrogen is added, or the like. The insulating layer can have a thickness of, for example, greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 10 nm and less than or equal to 50 nm.
The layer including a conductive material can be formed using a metal material such as aluminum, copper, titanium, tantalum, or tungsten. The layer including a conductive material may be formed using a semiconductor material such as polycrystalline silicon. There is no particular limitation on the method for forming the layer containing a conductive material, and any of a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, and a spin coating method can be employed. Note that this embodiment shows an example of the case where the layer containing a conductive material is formed using a metal material.
After that, the insulating layer and the layer containing a conductive material are selectively etched, so that the gate insulating layer 108 and the gate electrode 110 are formed (see FIG. 10C).
Then, phosphorus (P), arsenic (As), or the like is added to the semiconductor region 104, whereby the channel formation regions 116 and the impurity regions 120 (the impurity region 120a, the impurity region 120b) are formed (see FIG. 10D). Note that phosphorus or arsenic is added in order to form an n-channel transistor; an impurity element such as boron (B) or aluminum (Al) may be added in the case of forming a p-channel transistor. Here, the concentration of the impurity added can be set as appropriate; the concentration is preferably increased when a semiconductor element is highly miniaturized.
Note that a sidewall insulating layer may be formed in the periphery of the gate electrode 110 so that an impurity region to which an impurity element is added at different concentrations may be formed.
Next, a metal layer 122 is formed so as to cover the gate electrode 110, the impurity regions 120, and the like (see FIG. 11A). Any of a variety of film formation methods such as a vacuum evaporation method, a sputtering method, and a spin coating method can be employed for forming the metal layer 122. The metal layer 122 is preferably formed using a metal material that reacts with a semiconductor material included in the semiconductor region 104 to become a low-resistance metal compound. Examples of such metal materials include titanium, tantalum, tungsten, nickel, cobalt, and platinum.
Next, heat treatment is performed so that the metal layer 122 reacts with the semiconductor material. Thus, the metal compound regions 124 (the metal compound region 124a, the metal compound region 124b) which are in contact with the impurity regions 120 (the impurity region 120a, the impurity region 120b) are formed (see FIG. 11A). Note that when the gate electrode 110 is formed using polycrystalline silicon or the like, a metal compound region is also formed in a region of the gate electrode 110 in contact with the metal layer 122.
As the heat treatment, irradiation with a flash lamp can be employed, for example. Although it is needless to say that another heat treatment method may be used, a method by which heat treatment for an extremely short time can be achieved is preferably used in order to improve the controllability of chemical reaction in formation of the metal compound. Note that the metal compound regions are formed by reaction of the metal material and the semiconductor material and have sufficiently high conductivity. The formation of the metal compound regions can properly reduce the electric resistance and enhance element characteristics. Note that the metal layer 122 is removed after the metal compound regions 124 are formed.
Next, the electrode 126 is formed over and in contact with the metal compound region 124b of the transistor 160 (see FIG. 11B). The electrode 126 is formed in such a manner that a conductive layer is formed by a PVD method such as a sputtering method or a CVD method such as a plasma CVD method and then the conductive layer is etched into a desired shape. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these elements as a component; or the like can be used. Further, one of manganese, magnesium, zirconium, beryllium, neodymium, and scandium or more materials in combination thereof may be used. The details are similar to those of the source electrode 142a, the drain electrode 142b, and the like which are described later.
Through the above steps, the transistor 160 using the substrate 100 including a semiconductor material is formed (see FIG. 11C). Such a transistor 160 is capable of high-speed operation. Thus, when the transistor is used as a reading transistor, data can be read at high speed.
Next, the insulating layer 128 is formed so as to cover the elements formed in the above steps (see FIG. 11C). The insulating layer 128 can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, or aluminum oxide. In particular, a material with a low dielectric constant (a low-k material) is preferably used for the insulating layer 128, because capacitance due to overlap of electrodes or wirings can be sufficiently reduced. Note that the insulating layer 128 may be a porous insulating layer formed using any of those materials. Since the porous insulating layer has low dielectric constant as compared to a dense insulating layer, capacitance due to electrodes or wirings can be further decreased. Further, the insulating layer 128 can be formed using an organic insulating material such as a polyimide or an acrylic resin. Note that although the insulating layer 128 has a single-layer structure here, one embodiment of the disclosed invention is not limited to this structure. The insulating layer 128 may have a stacked-layer structure of two or more layers.
Then, as a pretreatment for the formation of the transistor 162 and the capacitor 164, CMP treatment is performed on the insulating layer 128 to expose the upper surfaces of the gate electrode 110 and the electrode 126 (see FIG. 11D). As the treatment for exposing the upper surfaces of the gate electrodes 110, etching treatment may be employed as an alternative to CMP treatment. Note that it is preferable to planarize the surface of the insulating layer 128 as much as possible in order to enhance the characteristics of the transistor 162. For example, the surface of the insulating layer 128 preferably has a root-mean-square (RMS) roughness of 1 nm or less.
Note that before or after the above steps, a step for forming an additional electrode, wiring, semiconductor layer, or insulating layer may be performed. For example, a multilayer wiring structure in which an insulating layer and a conductive layer are stacked is employed as a wiring structure, so that a highly-integrated semiconductor device can be provided.
Next, a conductive layer is formed over the gate electrode 110, the electrode 126, the insulating layer 128, and the like, and the source electrode 142a and the drain electrode 142b are formed by selectively etching the conductive layer (see FIG. 12A).
The conductive layer can be formed by a PVD method such as a sputtering method or a CVD method such as a plasma CVD method. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these elements as a component; or the like can be used. Further, one of manganese, magnesium, zirconium, beryllium, neodymium, and scandium or more materials in combination thereof may be used.
The conductive layer can have a single-layer structure or a stacked-layer structure including two or more layers. For example, the conductive layer can have a single-layer structure of a titanium film or a titanium nitride film, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, or a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order. Note that in the case where the conductive layer has the single-layer structure of a titanium film or a titanium nitride film, there is an advantage that it can be easily processed into the source electrode 142a and the drain electrode 142b having a tapered shape.
Alternatively, the conductive layer may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), an indium oxide-tin oxide alloy (In2O3—SnO2, which is abbreviated to ITO in some cases), an indium oxide-zinc oxide alloy (In2O3—ZnO), or any of these metal oxide materials in which silicon or silicon oxide is included can be used.
Although either dry etching or wet etching may be performed as the etching of the conductive layer, dry etching with high controllability is preferably used for miniaturization. The etching may be performed so that the source electrode 142a and the drain electrode 142b can have a tapered shape. The taper angle can be, for example, greater than or equal to 30° and less than or equal to 60°.
The channel length (L) of the transistor 162 in the upper portion is determined by the distance between an upper end portion of the source electrode 142a and an upper end portion of the drain electrode 142b. Note that for light exposure for forming a mask used in the case where a transistor with a channel length (L) of less than 25 nm is formed, it is preferable to use extreme ultraviolet rays whose wavelength is as short as several nanometers to several tens of nanometers. In the light exposure by extreme ultraviolet rays, the resolution is high and the focus depth is large. For these reasons, the channel length (L) of the transistor to be formed later can be set to less than 2 μm, preferably greater than or equal to 10 nm and less than or equal to 350 nm (0.35 μm), in which case the circuit can operate at higher speed. Moreover, miniaturization can lead to low power consumption of a semiconductor device.
Note that an insulating layer serving as a base may be provided over the insulating layer 128. The insulating layer can be formed by a PVD method, a CVD method, or the like.
Next, the insulating layer 140 is formed so as to cover the source electrode 142a and the drain electrode 142b. Then, in order to planarize the insulating layer 140, chemical mechanical polishing (CMP) treatment is performed so that the source electrode 142a and the drain electrode 142b are exposed (see FIG. 12A).
The insulating layer 140 can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, or aluminum oxide. It is particularly preferable that the insulating layer 140 is formed using silicon oxide because the oxide semiconductor layer 144 formed later is in contact with the insulating layer 140. Although there is no particular limitation on the forming method of the insulating layer 140, in consideration of the contact with the oxide semiconductor layer 144, a method in which hydrogen is sufficiently reduced is preferably employed. Examples of such a method include a sputtering method and the like. Needless to say, another deposition method such as a plasma CVD method may be used.
The chemical mechanical polishing (CMP) treatment is performed so as to expose at least part of surfaces of the source electrode 142a and the drain electrode 142b. In addition, the CMP treatment is preferably performed under such conditions that the root-mean-square (RMS) roughness of the surface of the insulating layer 140 becomes 1 nm or less (preferably 0.5 nm or less). By the CMP treatment performed under such conditions, the planarity of a surface where the oxide semiconductor layer 144 is formed later can be improved, and the characteristics of the transistor 162 can be enhanced.
Note that the CMP treatment may be performed only once or plural times. When the CMP treatment is performed plural times, first polishing is preferably performed with a high polishing rate followed by final polishing with a low polishing rate. By performing polishing at different polishing rates, the planarity of the surface of the insulating layer 140 can be further improved.
Then, after an oxide semiconductor layer is formed in contact with part of the top surfaces of the source electrode 142a, the drain electrode 142b, and the insulating layer 140, the oxide semiconductor layer is selectively etched to form the oxide semiconductor layer 144.
As the oxide semiconductor layer 144, an In—Sn—Ga—Zn—O-based oxide semiconductor which is a four-component metal oxide; an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxide semiconductor which is a three-component metal oxide; an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, or an In—Mg—O-based oxide semiconductor which is a two-component metal oxide; an In—O-based oxide semiconductor; a Sn—O-based oxide semiconductor; a Zn—O-based oxide semiconductor layer; or the like can be used. Further, SiO2 may be contained in the above oxide semiconductor.
In particular, an In—Ga—Zn—O-based oxide semiconductor material has sufficiently high resistance when there is no electric field and thus off-state current can be sufficiently reduced. In addition, with high field-effect mobility, the In—Ga—Zn—O-based oxide semiconductor material is suitable for a semiconductor material used in a semiconductor device.
As a typical example of the In—Ga—Zn—O-based oxide semiconductor material, one represented by InGaO3(ZnO)m (m>0) is given. Using M instead of Ga, there is an oxide semiconductor material represented by InMO3(ZnO)m (m>0). Here, M denotes one or more metal elements selected from gallium (Ga), aluminum (Al), iron (Fe), nickel (Ni), manganese (Mn), cobalt (Co), and the like. For example, M can be Ga, Ga and Al, Ga and Fe, Ga and Ni, Ga and Mn, Ga and Co, or the like. Note that the above-described compositions are derived from the crystal structures that the oxide semiconductor material can have and are only examples.
In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target therefor has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In2O3:ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In2O3:ZnO=10:1 to 1:2 in a molar ratio), further preferably In:Zn=15:1 to 1.5:1 in an atomic ratio (In2O3:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.
As an oxide target used for forming the oxide semiconductor layer 144 by a sputtering method, a target having a composition ratio of In:Ga:Zn=1:x:y (x is 0 or more and y is from 0.5 to 5) is preferably used. For example, a target having a composition ratio of In:Ga:Zn=1:1:1 [atomic ratio] (x=1, y=1) (i.e., a composition ratio of In2O3:Ga2O3:ZnO=1:1:2 [molar ratio]) can be used. In addition, a target having a composition ratio of In:Ga:Zn=1:1:0.5 [atomic ratio] (x=1, y=0.5), a target having a composition ratio of In:Ga:Zn=1:1:2 [atomic ratio] (x=1, y=2), or a target having a composition ratio of In:Ga:Zn=1:0:1 [atomic ratio] (x=0, y=1) can also be used.
In this embodiment, an oxide semiconductor layer having an amorphous structure is formed as the oxide semiconductor layer 144 by a sputtering method with the use of an In—Ga—Zn—O-based metal oxide target. The thickness ranges from 1 nm to 50 nm, preferably from 2 nm to 20 nm, more preferably from 3 nm to 15 nm.
The relative density of the metal oxide in the metal oxide target is 80% or more, preferably 95% or more, and more preferably 99.9% or more. The use of the metal oxide target with high relative density makes it possible to form an oxide semiconductor layer having a dense structure.
The atmosphere in which the oxide semiconductor layer 144 is formed is preferably a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas (typically, argon) and oxygen. Specifically, it is preferable to use a high-purity gas atmosphere, for example, from which an impurity such as hydrogen, water, a hydroxyl group, or hydride is removed to a concentration of 1 ppm or less (preferably, 10 ppb or less).
In forming the oxide semiconductor layer 144, for example, an object to be processed is held in a treatment chamber that is maintained under reduced pressure, and the object to be processed is heated to a temperature higher than or equal to 100° C. and lower than 550° C., preferably higher than or equal to 200° C. and lower than or equal to 400° C. Alternatively, the temperature of an object to be processed in forming the oxide semiconductor layer 144 may be room temperature (25° C.±10° C.). Then, moisture in the treatment chamber is removed, a sputtering gas from which hydrogen, water, or the like is removed is introduced, and the above-described target is used; thus, the oxide semiconductor layer 144 is formed. By forming the oxide semiconductor layer 144 while heating the object to be processed, an impurity in the oxide semiconductor layer 144 can be reduced. Moreover, damage due to sputtering can be reduced. In order to remove the moisture in the treatment chamber, it is preferable to use an entrapment vacuum pump. For example, a cryopump, an ion pump, a titanium sublimation pump, or the like can be used. A turbo pump provided with a cold trap may be used. By evacuation with a cryopump or the like, hydrogen, water, or the like can be removed from the treatment chamber, whereby the concentration of an impurity in the oxide semiconductor layer can be lowered.
For example, conditions for forming the oxide semiconductor layer 144 can be set as follows: the distance between the object to be processed and the target is 170 mm, the pressure is 0.4 Pa, the direct current (DC) power is 0.5 kW, and the atmosphere is an oxygen (100% oxygen) atmosphere, an argon (100% argon) atmosphere, or a mixed atmosphere of oxygen and argon. Note that a pulsed direct current (DC) power source is preferably used because dust (such as powder substances generated in film formation) can be reduced and the film thickness can be made uniform. The thickness of the oxide semiconductor layer 144 is set in the range of 1 nm to 50 nm, preferably 2 nm to 20 nm, more preferably 3 nm to 15 nm. By employing a structure according to the disclosed invention, a short-channel effect due to miniaturization can be suppressed even in the case of using the oxide semiconductor layer 144 having such a thickness. Note that an appropriate thickness differs depending on an oxide semiconductor material used, the usage of a semiconductor device, or the like; therefore, it is also possible to set the thickness as appropriate depending on the material to be used, the usage, or the like. Note that when the insulating layer 140 is formed in the above manner, a surface of a portion where the channel formation region is to be formed in the oxide semiconductor layer 144 can be sufficiently planarized; thus, the oxide semiconductor layer can be suitably formed even when having a small thickness. As illustrated in FIG. 12B, the portion corresponding to the channel formation region in the oxide semiconductor layer 144 preferably has a planar cross-sectional shape. By making the cross-sectional shape of the portion corresponding to the channel formation region of the oxide semiconductor layer 144 flat, leakage current can be reduced as compared to the case where the cross-sectional shape of the oxide semiconductor layer 144 is not flat.
Note that before the oxide semiconductor layer 144 is formed by a sputtering method, a substance attached to a surface where the oxide semiconductor layer 144 is to be formed (e.g., the surface of the insulating layer 140) may be preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. Here, the reverse sputtering is a method by which ions collide with a surface to be processed so that the surface is modified, in contrast to normal sputtering by which ions collide with a sputtering target. An example of a method for making ions collide with a surface to be processed is a method in which high-frequency voltage is applied to the surface to be processed in an argon atmosphere so that plasma is generated in the vicinity of the object to be processed. Note that an atmosphere of nitrogen, helium, oxygen, or the like may be used instead of an argon atmosphere.
After the oxide semiconductor layer 144 is formed, heat treatment (first heat treatment) is preferably performed on the oxide semiconductor layer 144. Excessive hydrogen (including water and a hydroxyl group) in the oxide semiconductor layer 144 can be removed by the first heat treatment, so that the structure of the oxide semiconductor layer 144 can be ordered and defect levels in the energy gap can be reduced. The temperature of the first heat treatment is, for example, higher than or equal to 300° C. and lower than 550° C., preferably higher than or equal to 400° C. and lower than or equal to 500° C.
The heat treatment can be performed in such a way that, for example, an object to be heated is introduced into an electric furnace in which a resistance heating element or the like is used and heated, under a nitrogen atmosphere at 450° C. for one hour. During the heat treatment, the oxide semiconductor layer is not exposed to the air to prevent the entry of water and hydrogen.
The heat treatment apparatus is not limited to the electric furnace and may be an apparatus for heating an object by thermal radiation or thermal conduction from a medium such as a heated gas. For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus is an apparatus for heating an object to be processed by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon, is used.
For example, as the first heat treatment, a GRTA process may be performed as follows. The object is put in an inert gas atmosphere that has been heated, heated for several minutes, and taken out from the inert gas atmosphere. The GRTA process enables high-temperature heat treatment for a short time. Moreover, the GRTA process can be employed under a condition that a process temperature exceeds the upper temperature limit of the object. Note that the inert gas may be switched to a gas including oxygen during the process. This is because defect levels in energy gap due to oxygen deficiency can be reduced by performing the first heat treatment in an atmosphere including oxygen.
Note that as the inert gas atmosphere, an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like is preferably used. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is greater than or equal to 6N (99.9999%), preferably greater than or equal to 7N (99.99999%) (that is, the concentration of the impurities is less than or equal to 1 ppm, preferably less than or equal to 0.1 ppm).
In any case, impurities are reduced by the first heat treatment so that the i-type (intrinsic) or substantially i-type oxide semiconductor layer is obtained. Accordingly, a transistor having significantly excellent characteristics can be realized.
The above heat treatment (first heat treatment) can be referred to as dehydration treatment, dehydrogenation treatment, or the like because of its effect of removing hydrogen, water, and the like. The dehydration treatment or the dehydrogenation treatment can also be performed at the following timing: after the formation of the oxide semiconductor layer 144, after the formation of the gate insulating layer 146, after the formation of the gate electrode, or the like. Such dehydration treatment or dehydrogenation treatment may be conducted only once or plural times.
The etching of the oxide semiconductor layer 144 may be performed before or after the heat treatment. In view of miniaturization of elements, dry etching is preferably used; however, wet etching may be used. An etching gas and an etchant can be selected as appropriate depending on a material of a layer to be etched. Note that in the case where leakage current in an element does not cause a problem, the oxide semiconductor layer may be used without being processed to have an island shape.
An oxide conductive layer serving as a source region and a drain region may be provided as a buffer layer between the oxide semiconductor layer 144 and the source and drain electrodes 142a and 142b. 
As the formation method of the oxide conductive layer, a sputtering method, a vacuum evaporation method (an electron beam evaporation method or the like), an arc discharge ion plating method, or a spray method can be used. As a material for the oxide conductive layer, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, or the like can be used. The thickness of the oxide conductive layer is set as appropriate in a range of from 50 nm to 300 nm. Further, silicon oxide may be contained in the above material.
The shape of the oxide conductive layer can be processed in the same photolithography process as the source electrode 142a and the drain electrode 142b. Also, the shape of the oxide conductive layer may be further processed in the photolithography process for forming the oxide semiconductor layer 144, with use of the same mask as the oxide semiconductor layer 144.
By providing the oxide conductive layer as the source region and the drain region between the oxide semiconductor layer 144 and the source and drain electrodes 142a and 142b, reduction in resistance in the source region and the drain region can be achieved and the transistor 162 can operate at high speed.
Further, by employing the structure of the oxide semiconductor layer 144, the oxide conductive layer, and the drain electrode 142b, withstand voltage of the transistor 162 can be increased.
It is also effective to use the oxide conductive layer for a source region and a drain region in order to enhance frequency characteristics of a peripheral circuit (a driver circuit). This is because the contact between a metal electrode (molybdenum, tungsten, or the like) and an oxide conductive layer can reduce the contact resistance as compared with the contact between a metal electrode (molybdenum, tungsten, or the like) and an oxide semiconductor layer. The contact resistance can be reduced by interposing an oxide conductive layer between an oxide semiconductor layer and source and drain electrode layers; accordingly, frequency characteristics of a peripheral circuit (a driver circuit) can be enhanced.
Next, the gate insulating layer 146 is formed so as to cover the oxide semiconductor layer 144 (see FIG. 12B).
The gate insulating layer 146 can be formed by a CVD method, a sputtering method, or the like. The gate insulating layer 146 preferably includes silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, hafnium oxide, yttrium oxide, hafnium silicate (HfSixOy (x>0, y>0)), hafnium silicate (HfSixOy (x>0, y>0)) to which nitrogen is added, hafnium aluminate (HfAlxOy (x>0, y>0)) to which nitrogen is added, and the like. In addition, the gate insulating layer 146 may have a single-layer structure or a stacked-layer structure. There is no particular limitation on the thickness; however, in the case where a semiconductor device is miniaturized, the thickness is preferably small in order to ensure operation of the transistor. For example, in the case where silicon oxide is used, the thickness can be set to greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 10 nm and less than or equal to 50 nm.
When the gate insulating layer is thin as in the above description, a problem of gate leakage due to a tunnel effect or the like is caused. In order to solve the problem of gate leakage, a high dielectric constant (high-k) material such as hafnium oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSixO3, (x>0, y>0)), hafnium silicate (HfSixO3, (x>0, y>0)) to which nitrogen is added, or hafnium aluminate (HfAlxOy (x>0, y>0)) to which nitrogen is added is preferably used for the gate insulating layer 146. By using a high-k material for the gate insulating layer 146, electrical characteristics can be ensured and the thickness can be large to prevent gate leakage. For example, the relative permittivity of hafnium oxide is approximately 15, which is much higher than that of silicon oxide which is 3 to 4. With such a material, a gate insulating layer where the equivalent oxide thickness is less than 15 nm, preferably 2 nm to 10 nm, can be easily formed. Note that a stacked-layer structure of a film containing a high-k material and a film containing any one of silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, and the like may be employed.
Further, a metal oxide film is preferably used for the film in contact with the oxide semiconductor layer 144 like the gate insulating layer 146. The metal oxide film is formed using a material such as silicon oxide, silicon nitride, silicon oxynitride, or silicon nitride oxide, for example. Alternatively, a material including an element of Group 13 and oxygen can be also used. For example, as the material including an element of Group 13 and oxygen, a material including one or more of gallium oxide, aluminum oxide, aluminum gallium oxide, and gallium aluminum oxide is given. Here, aluminum gallium oxide refers to a material in which the amount of aluminum is larger than that of gallium in atomic percent, and gallium aluminum oxide refers to a material in which the amount of gallium is larger than or equal to that of aluminum in atomic percent. The metal oxide film can be formed with a single-layer structure or a stacked-layer structure using the above-described materials.
After the gate insulating layer 146 is formed, second heat treatment is desirably performed in an inert gas atmosphere or an oxygen atmosphere. The temperature of the heat treatment is set in the range of 200° C. to 450° C. inclusive, preferably 250° C. to 350° C. inclusive. For example, the heat treatment may be performed at 250° C. for one hour in a nitrogen atmosphere. The second heat treatment can suppress variation in electric characteristics of the transistor. Further, in the case where the gate insulating layer 146 contains oxygen, oxygen is supplied to the oxide semiconductor layer 144 to cover oxygen deficiency in the oxide semiconductor layer 144, so that an i-type (intrinsic) or substantially i-type oxide semiconductor layer can be formed.
Note that in this embodiment, the second heat treatment is performed after the gate insulating layer 146 is formed; the timing of the second heat treatment is not limited thereto. For example, the second heat treatment may be performed after the gate electrode is formed. Alternatively, the second heat treatment may be performed following the first heat treatment, the first heat treatment may double as the second heat treatment, or the second heat treatment may double as the first heat treatment.
As described above, at least one of the first heat treatment and the second heat treatment is applied, whereby the oxide semiconductor layer 144 can be highly purified so that impurities that are not main components of the oxide semiconductor are contained as little as possible.
Next, the gate electrode 148 is formed over the gate insulating layer 146.
The gate electrode 148 can be formed in such a manner that a conductive layer is formed over the gate insulating layer 146 and then selectively etched. The conductive layer to become the gate electrode 148 can be formed by a PVD method such as a sputtering method or a CVD method such as a plasma CVD method. The details are similar to those in the case of the source electrode 142a, the drain electrode 142b, or the like; thus, the description thereof can be referred to.
Through the above steps, the transistor 162 including the highly-purified oxide semiconductor layer 144 is completed (see FIG. 12C). Such a transistor 162 has a characteristic of sufficiently reduced off-state current. Therefore, with the use of the transistor as a writing transistor, electric charge can be held for a long time.
Then, the insulating layer 150 is formed over the gate insulating layer 146 and the gate electrode 148 (see FIG. 12D). The insulating layer 150 can be formed by a PVD method, a CVD method, or the like. The insulating layer 150 can be formed so as to have a single-layer structure or a stacked structure using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, or aluminum oxide.
Note that for the insulating layer 150, a material with a low dielectric constant may be preferably used or a structure with a low dielectric constant (e.g., a porous structure) may be preferably employed. This is because by reducing the dielectric constant of the insulating layer 150, capacitance between wirings, electrodes, or the like can be reduced, so that operation speed can be increased.
Next, the electrode 152 is formed over the insulating layer 150 so as to overlap with the source electrode 142a (see FIG. 13A). The method and materials for forming the gate electrode 148 can be applied to the electrode 152; therefore, the description of the gate electrode 148 can be referred to for the details of the electrode 152. Through the above steps, the capacitor 164 is completed.
Next, the insulating layer 154 is formed over the insulating layer 150 and the electrode 152. After an opening reaching the gate electrode 148 is formed in the insulating layer 150 and the insulating layer 154, the electrode 170 is formed in the opening and a wiring 171 in contact with the electrode 170 is formed over the insulating layer 154 (see FIG. 13B). The opening is formed by selective etching with the use of a mask or the like.
Next, the insulating layer 172 is formed over the electrode 152 and the wiring 171. An opening reaching the drain electrode 142b is formed in the gate insulating layer 146, the insulating layer 150, the insulating layer 154, and the insulating layer 172, and then the electrode 156 is formed in the opening and the wiring 158 is formed over the insulating layer 172 so as to be in contact with the electrode 156 (see FIG. 13C). The opening is formed by selective etching with the use of a mask or the like.
Like the insulating layer 150, the insulating layer 154 and the insulating layer 172 can be formed by a PVD method, a CVD method, or the like. The insulating layer 154 and the insulating layer 172 can be formed so as to have a single-layer structure or a stacked structure using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, or aluminum oxide.
Note that for the insulating layer 154 and the insulating layer 172, a material with a low dielectric constant may be preferably used or a structure with a low dielectric constant (e.g., a porous structure) may be preferably employed. This is because by reducing the dielectric constant of the insulating layer 154 and the insulating layer 172, capacitance between wirings, electrodes, or the like can be reduced, so that operation speed can be increased.
Note that the insulating layer 154 and the insulating layer 172 are desirably formed so as to have flat surfaces. This is because when the insulating layer 154 and the insulating layer 172 have a flat surface, an electrode, a wiring, or the like can be favorably formed over the insulating layer 154 and the insulating layer 172 even in the case where the semiconductor device is miniaturized, for example. The insulating layer 154 and the insulating layer 172 can be planarized by a method such as chemical mechanical polishing (CMP).
The electrode 170 and the electrode 156 can be formed in such a manner that, for example, a conductive layer is formed by a PVD method, a CVD method, or the like in a region including the openings and then part of the conductive layer is removed by etching treatment, CMP, or the like.
Specifically, it is possible to employ a method, for example, in which a thin titanium film is formed in a region including the openings by a PVD method and a thin titanium nitride film is fanned by a CVD method, and then, a tungsten film is formed so as to be embedded in the openings. Here, the titanium film formed by a PVD method has a function of reducing an oxide film (such as a natural oxide film) over which the titanium film is to be formed, and thereby lowering contact resistance with lower electrodes or the like (the drain electrode 142b, here). The titanium nitride film formed after the formation of the titanium film has a barrier function of preventing diffusion of the conductive material. A copper film may be formed by a plating method after the formation of the barrier film of titanium, titanium nitride, or the like.
The wiring 171 and the wiring 158 are formed in such a manner that a conductive layer is formed by a PVD method such as a sputtering method or a CVD method such as a plasma CVD method and then the conductive layer is etched into a desired shape. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these elements as a component; or the like can be used. Further, one element selected from manganese, magnesium, zirconium, beryllium, neodymium, and scandium or more elements in combination thereof may be used. The details are similar to those of the source electrode 142a or the like.
Note that a variety of wirings, electrodes, or the like may be formed following the above steps. The wirings or the electrodes can be formed by a method such as a so-called damascene method or dual damascene method.
Through the above steps, the semiconductor device having the structure illustrated in FIG. 5 and FIGS. 9A and 9B can be manufactured.
Since the oxide semiconductor layer 144 is highly purified in the transistor 162 illustrated in this embodiment, the hydrogen concentration thereof is 5×1019 atoms/cm3 or lower, preferably 5×1018 atoms/cm3 or lower, more preferably 5×1017 atoms/cm3 or lower. In addition, the value of the carrier concentration of the oxide semiconductor layer 144 is sufficiently low (e.g., lower than 1×1012/cm3, preferably lower than 1.45×1010/cm3) in comparison with that of a general silicon wafer (approximately 1×1014/cm3). Accordingly, the off-state current of the transistor 162 is also sufficiently small. For example, the off-state current (per unit channel width (1 μm), here) of the transistor 162 at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10−21 A) or lower, preferably 10 zA or lower.
With the use of the highly purified intrinsic oxide semiconductor layer 144, the off-state current of the transistor 162 can be sufficiently reduced easily. Further, with the use of such a transistor 162, a semiconductor device capable of holding stored data for an extremely long time can be obtained.
In the semiconductor device described in this embodiment, the transistors each including an oxide semiconductor in memory cells of the semiconductor device are connected in series; thus, the source electrode of the transistor including an oxide semiconductor in a memory cell and the drain electrode of the transistor including an oxide semiconductor in an adjacent memory cell can be shared in the memory cells. Therefore, the area occupied by the memory cell can be reduced, whereby the degree of integration of the semiconductor device can be enhanced and the storage capacity per unit area can be increased.
The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.
Embodiment 3
In this embodiment, an example of a transistor that can be applied to a semiconductor device disclosed in this specification will be described. There is no particular limitation on the structure of the transistor that can be applied to a semiconductor device disclosed in this specification; for example, a staggered type or a planar type having a top-gate structure or a bottom-gate structure can be employed. The transistor may have a single-gate structure in which one channel formation region is formed, a double-gate structure in which two channel formation regions are formed, or a triple-gate structure in which three channel formation regions are formed. Alternatively, the transistor may have a dual gate structure including two gate electrode layers positioned over and below a channel formation region with a gate insulating layer provided therebetween.
FIGS. 15A to 15D each illustrate an example of a cross-sectional structure of a transistor that can be applied to a semiconductor device disclosed in this specification (for example, the transistor 162 in Embodiments 1 and 2). In each example of FIGS. 15A to 15D, the transistor is provided over an insulating layer 400; however, the transistor may be provided over a substrate such as a glass substrate. Note that in the case where any of the transistors illustrated in FIGS. 15A to 15D is applied to the transistor 162 in Embodiments 1 and 2, the insulating layer 400 corresponds to the insulating layer 128.
A transistor 410 illustrated in FIG. 15A is a kind of bottom-gate thin film transistor, and is also referred to as an inverted-staggered thin film transistor.
The transistor 410 includes, over the insulating layer 400, a gate electrode layer 401, a gate insulating layer 402, an oxide semiconductor layer 403, a source electrode layer 405a, and a drain electrode layer 405b. An insulating layer 407 covering the transistor 410 is stacked over the oxide semiconductor layer 403. An insulating layer 409 is formed over the insulating layer 407.
A transistor 420 illustrated in FIG. 15B has a kind of bottom-gate structure referred to as a channel-protective type (channel-stop type) and is also referred to as an inverted-staggered thin film transistor.
The transistor 420 includes, over the insulating layer 400, the gate electrode layer 401, the gate insulating layer 402, the oxide semiconductor layer 403, an insulating layer 427 functioning as a channel protective layer which covers a channel formation region of the oxide semiconductor layer 403, the source electrode layer 405a, and the drain electrode layer 405b. The insulating layer 409 is formed to cover the transistor 420.
A transistor 430 illustrated in FIG. 15C is a bottom-gate thin film transistor and includes, over the insulating layer 400 which is a substrate having an insulating surface, the gate electrode layer 401, the gate insulating layer 402, the source electrode layer 405a, the drain electrode layer 405b, and the oxide semiconductor layer 403. In addition, the insulating layer 407 covering the transistor 430 is provided in contact with the oxide semiconductor layer 403. The insulating layer 409 is formed over the insulating layer 407.
In the transistor 430, the gate insulating layer 402 is provided over and in contact with the insulating layer 400 and the gate electrode layer 401, and the source electrode layer 405a and the drain electrode layer 405b are provided over and in contact with the gate insulating layer 402. Further, the oxide semiconductor layer 403 is provided over the gate insulating layer 402, the source electrode layer 405a, and the drain electrode layer 405b. 
A transistor 440 illustrated in FIG. 15D is a kind of top-gate thin film transistor. The transistor 440 includes, over the insulating layer 400, an insulating layer 437, the oxide semiconductor layer 403, the source electrode layer 405a, the drain electrode layer 405b, the gate insulating layer 402, and the gate electrode layer 401. A wiring layer 436a and a wiring layer 436b are provided in contact with and electrically connected to the source electrode layer 405a and the drain electrode layer 405b, respectively.
In the case of forming each of the bottom-gate transistors 410, 420, and 430 over a substrate, an insulating film serving as a base film may be provided between the substrate and the gate electrode layer. The base film has a function of preventing diffusion of an impurity element from the substrate, and can be formed to have a single-layer structure or a stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film.
The gate electrode layer 401 can be formed with a single-layer structure or a stacked-layer structure using a metal material such as molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material which contains any of these materials as its main component.
The gate insulating layer 402 can be formed with a single-layer structure or a stacked-layer structure using any of a silicon oxide layer, a silicon nitride layer, a silicon oxynitride layer, a silicon nitride oxide layer, an aluminum oxide layer, an aluminum nitride layer, an aluminum oxynitride layer, an aluminum nitride oxide layer, and a hafnium oxide layer by a plasma CVD method, a sputtering method, or the like. For example, by a plasma CVD method, a silicon nitride layer (SiNy (y>0)) with a thickness of greater than or equal to 50 nm and less than or equal to 200 nm is formed as a first gate insulating layer, and a silicon oxide layer (SiOx (x>0)) with a thickness of greater than or equal to 5 nm and less than or equal to 300 nm is formed as a second gate insulating layer over the first gate insulating layer, so that a gate insulating layer with a total thickness of 200 nm is formed.
As the conductive film used for the source electrode layer 405a and the drain electrode layer 405b, for example, a film of an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W, a film of an alloy containing any of these elements as a component, an alloy film containing these elements in combination, or the like can be used. The conductive film may have a structure in which a high-melting-point metal layer of Ti, Mo, W, or the like is stacked over and/or below a metal layer of Al, Cu, or the like. When an Al material to which an element (e.g., Si, Nd, or Sc) which prevents generation of hillocks and whiskers in an Al film is added is used, heat resistance can be increased.
A material similar to that for the source electrode layer 405a and the drain electrode layer 405b can be used for a conductive film used for the wiring layer 436a and the wiring layer 436b which are respectively connected to the source electrode layer 405a and the drain electrode layer 405b. 
Alternatively, the conductive film to be the source and drain electrode layers 405a and 405b (including a wiring layer formed using the same layer as the source and drain electrode layers) may be formed using a conductive metal oxide. As the conductive metal oxide, indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO), an indium oxide-tin oxide alloy (In2O3—SnO2; abbreviated to ITO), an indium oxide-zinc oxide alloy (In2O3—ZnO), or any of these metal oxide materials in which silicon oxide is contained can be used.
As the insulating layers 407, 427, and 437, an inorganic insulating film, typical examples of which are a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, and an aluminum oxynitride film, can be used.
As the insulating layer 409, an inorganic insulating film such as a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film can be used.
In addition, a planarization insulating film may be formed over the insulating layer 409 in order to suppress surface unevenness due to the transistor. As the planarization insulating film, an organic material such as polyimide, an acrylic resin, or a benzocyclobutene-based resin can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. Note that the planarization insulating film may be formed by stacking a plurality of insulating films formed using some of these materials.
Note that an oxide conductive layer serving as a source and drain regions may be provided between the oxide semiconductor layer 403 and the source and drain electrode layers 405a and 405b, as a buffer layer. FIGS. 16A and 16B illustrate transistors 441 and 442, respectively, each of which is obtained by providing an oxide conductive layer in the transistor 440 in FIG. 15D.
The transistors 441 and 442 in FIGS. 16A and 16B are each provided with oxide conductive layers 404a and 404b serving as a source and drain regions between the oxide semiconductor layer 403 and the source and drain electrode layers 405a and 405b. The transistors 441 and 442 in FIGS. 16A and 16B are different from each other in the shapes of the oxide conductive layers 404a and 404b depending on a manufacturing process.
In the transistor 441 in FIG. 16A, a stack of an oxide semiconductor film and an oxide conductive film is formed and processed through the same photolithography process, so that the oxide semiconductor layer 403 and the oxide conductive layer are formed to have island shapes. After the source electrode layer 405a and the drain electrode layer 405b are formed over the oxide semiconductor layer and the oxide conductive layer, the oxide conductive layer having an island shape is etched using the source electrode layer 405a and the drain electrode layer 405b as masks so that the oxide conductive layers 404a and 404b to be a source and drain regions are formed.
In the transistor 442 in FIG. 16B, an oxide conductive film is formed over the oxide semiconductor layer 403, a metal conductive film is formed thereover, and then the oxide conductive film and the metal conductive film are processed through the same photolithography process, so that the oxide conductive layers 404a and 404b to be a source and drain regions, the source electrode layer 405a, and the drain electrode layer 405b are formed.
In performing etching to process the oxide conductive layer, etching conditions (the kind and the concentration of an etching material, etching time, and the like) are appropriately adjusted so that the oxide semiconductor layer is not excessively etched.
As a formation method of the oxide conductive layers 404a and 404b, a sputtering method, a vacuum evaporation method (an electron beam evaporation method or the like), an arc discharge ion plating method, or a spray method can be used. As a material of the oxide conductive layers 404a and 404b, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, gallium zinc oxide, an alloy of indium oxide and zinc oxide, or the like can be used. In addition, the above materials may contain silicon oxide.
When the oxide conductive layers are provided as the source and drain regions between the oxide semiconductor layer 403 and the source and drain electrode layers 405a and 405b, the resistance of the source and drain regions can be lowered, resulting in high-speed operation of the transistors 441 and 442.
Including the oxide semiconductor layer 403, the oxide conductive layer 404b, and the drain electrode layer 405b, the transistors 441 and 442 can each have a higher withstand voltage.
This embodiment can be combined with the structure or the configuration of any of the other embodiments as appropriate.
Embodiment 4
One embodiment of an oxide semiconductor layer which can be used as any of the semiconductor layers of the transistors in Embodiments 1 to 3 will be described with reference to FIGS. 17A to 17C.
The oxide semiconductor layer of this embodiment has a stacked structure including a first crystalline oxide semiconductor layer and a second crystalline oxide semiconductor layer. The second crystalline oxide semiconductor layer is stacked over the first crystalline oxide semiconductor layer and is thicker than the first crystalline oxide semiconductor layer.
An insulating layer 437 is formed over an insulating layer 400. In this embodiment, an oxide insulating layer with a thickness greater than or equal to 50 nm and less than or equal to 600 nm is formed as the insulating layer 437 by a PCVD method or a sputtering method. For example, a single layer selected from a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon oxynitride film, an aluminum oxynitride film, and a silicon nitride oxide film or a stack of any of these films can be used.
Next, a first oxide semiconductor film with a thickness greater than or equal to 1 nm and less than or equal to 10 nm is formed over the insulating layer 437. The first oxide semiconductor film is formed by a sputtering method, and the substrate temperature in the film formation by a sputtering method is set to be higher than or equal to 200° C. and lower than or equal to 400° C.
In this embodiment, the first oxide semiconductor film is formed to a thickness of 5 nm in an oxygen atmosphere, an argon atmosphere, or an atmosphere including argon and oxygen under conditions where a target for deposition of an oxide semiconductor (a target for deposition of an In—Ga—Zn—O-based oxide semiconductor including In2O3, Ga2O3, and ZnO at 1:1:2 [molar ratio]) is used, the distance between the substrate and the target is 170 mm, the substrate temperature is 250° C., the pressure is 0.4 Pa, and the direct current (DC) power is 0.5 kW.
Next, first heat treatment is performed under a condition where the atmosphere of a chamber in which the substrate is set is an atmosphere of nitrogen or dry air. The temperature of the first heat treatment is higher than or equal to 400° C. and lower than or equal to 750° C. Through the first heat treatment, a first crystalline oxide semiconductor layer 450a is formed (see FIG. 17A).
Although depending on the temperature of the first heat treatment, the first heat treatment causes crystallization from a film surface and crystal grows from the film surface toward the inside of the film; thus, c-axis aligned crystal is obtained. By the first heat treatment, large amounts of zinc and oxygen gather to the film surface, and one or more layers of graphene-type two-dimensional crystal including zinc and oxygen and having a hexagonal upper plane are formed at the outermost surface; the layer(s) at the outermost surface grow in the thickness direction to form a stack of layers. By increasing the temperature of the heat treatment, crystal growth proceeds from the surface to the inside and further from the inside to the bottom.
By the first heat treatment, oxygen in the insulating layer 437 that is an oxide insulating layer is diffused to an interface between the insulating layer 437 and the first crystalline oxide semiconductor layer 450a or the vicinity of the interface (within ±5 nm from the interface), whereby oxygen deficiency in the first crystalline oxide semiconductor layer is decreased. Therefore, it is preferable that oxygen be included in (in a bulk of) the insulating layer 437 used as a base insulating layer or at the interface between the first crystalline oxide semiconductor layer 450a and the insulating layer 437 at an amount that exceeds at least the amount of oxygen in the stoichiometric composition ratio.
Next, a second oxide semiconductor film with a thickness more than 10 nm is formed over the first crystalline oxide semiconductor layer 450a. The second oxide semiconductor film is formed by a sputtering method, and the substrate temperature in the film formation is set to be higher than or equal to 200° C. and lower than or equal to 400° C. By setting the substrate temperature in the film formation in the range of from 200° C. to 400° C., precursors can be arranged in the oxide semiconductor layer formed over and in contact with the surface of the first crystalline oxide semiconductor layer and so-called orderliness can be obtained.
In this embodiment, the second oxide semiconductor film is formed to a thickness of 25 nm in an oxygen atmosphere, an argon atmosphere, or an atmosphere including argon and oxygen under conditions where a target for an oxide semiconductor (a target for an In—Ga—Zn—O-based oxide semiconductor including In2O3, Ga2O3, and ZnO at 1:1:2 [molar ratio]) is used, the distance between the substrate and the target is 170 mm, the substrate temperature is 400° C., the pressure is 0.4 Pa, and the direct current (DC) power is 0.5 kW.
Next, second heat treatment is performed under a condition where the atmosphere of a chamber in which the substrate is set is a nitrogen atmosphere, an oxygen atmosphere, or dry air. The temperature of the second heat treatment is higher than or equal to 400° C. and lower than or equal to 750° C. Through the second heat treatment, a second crystalline oxide semiconductor layer 450b is formed (see FIG. 17B). The second heat treatment is performed in a nitrogen atmosphere, an oxygen atmosphere, or a mixed atmosphere of nitrogen and oxygen, whereby the density of the second crystalline oxide semiconductor layer is increased and the number of defects therein is decreased. By the second heat treatment, crystal growth proceeds in the thickness direction with the use of the first crystalline oxide semiconductor layer 450a as a nucleus, that is, crystal growth proceeds from the bottom to the inside; thus, the second crystalline oxide semiconductor layer 450b is formed.
Preferably, steps from the formation of the insulating layer 437 to the second heat treatment are successively performed without exposure to the air. The steps from the formation of the insulating layer 437 to the second heat treatment are preferably performed in an atmosphere which is controlled to include as little hydrogen and moisture as possible (such as an inert gas atmosphere, a reduced-pressure atmosphere, or a dry-air atmosphere); in terms of moisture, for example, a dry nitrogen atmosphere with a dew point of −40° C. or lower, preferably a dew point of −50° C. or lower may be employed.
Next, the stack of the oxide semiconductor layers, the first crystalline oxide semiconductor layer 450a and the second crystalline oxide semiconductor layer 450b, is processed into an oxide semiconductor layer 453 including a stack of island-shaped oxide semiconductor layers (see FIG. 17C). In the drawing, the interface between the first crystalline oxide semiconductor layer 450a and the second crystalline oxide semiconductor layer 450b is indicated by a dotted line, and the first crystalline oxide semiconductor layer 450a and the second crystalline oxide semiconductor layer 450b are illustrated as a stack of oxide semiconductor layers; however, the interface therebetween is actually not distinct and is illustrated for easy understanding.
The stack of the oxide semiconductor layers can be processed by being etched after a mask having a desired shape is formed over the stack of the oxide semiconductor layers. The mask can be formed by a method such as photolithography. Alternatively, the mask may be formed by a method such as an ink-jet method.
For the etching of the stack of the oxide semiconductor layers, either dry etching or wet etching may be employed. Needless to say, both of them may be employed in combination.
A feature of the first crystalline oxide semiconductor layer and the second crystalline oxide semiconductor layer obtained by the above formation method is that they have c-axis alignment. Note that the first crystalline oxide semiconductor layer and the second crystalline oxide semiconductor layer include an oxide including a crystal having c-axis alignment (also referred to as c-axis aligned crystal (CAAC)), which has neither a single crystal structure nor an amorphous structure. The first crystalline oxide semiconductor layer and the second crystalline oxide semiconductor layer partly include a crystal grain boundary.
In order to obtain CAAC, it is important to form hexagonal crystals in an initial stage of deposition of an oxide semiconductor film and to cause crystal growth from the hexagonal crystals as cores. In order to achieve this, it is preferable that the substrate heating temperature be 100° C. to 500° C., more preferably 200° C. to 400° C., still preferably 250° C. to 300° C. In addition to this, the deposited oxide semiconductor film is subjected to heat treatment at a temperature higher than the substrate heating temperature in the deposition. Therefore, microdefects in the film and defects at the interface of a stacked layer can be compensated.
Note that the first crystalline oxide semiconductor layer and the second crystalline oxide semiconductor layer are each formed using an oxide material including at least Zn. For example, a four-component metal oxide such as an In—Al—Ga—Zn—O-based material or an In—Sn—Ga—Zn—O-based material; a three-component metal oxide such as an In—Ga—Zn—O-based material, an In—Al—Zn—O-based material, an In—Sn—Zn—O-based material, a Sn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or a Sn—Al—Zn—O-based material; a two-component metal oxide such as an In—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-based material, or a Zn—Mg—O-based material; a Zn—O-based material; or the like can be used. Further, an In—Si—Ga—Zn—O-based based material, an In—Ga—B—Zn—O-based material, or an In—B—Zn—O-based material may be used. In addition, the above materials may include SiO2. Here, for example, an In—Ga—Zn—O-based material means an oxide including indium (In), gallium (Ga), and zinc (Zn), and there is no particular limitation on the composition ratio. Further, the In—Ga—Zn—O-based material may include an element other than In, Ga, and Zn.
Note that an alkali metal is not an element included in an oxide semiconductor, and therefore, is an impurity. Also, an alkaline earth metal is an impurity in the case where an alkaline earth metal is not included in an oxide semiconductor. An alkali metal, in particular, Na becomes Na+ when an insulating film in contact with the oxide semiconductor film is an oxide and Na diffuses into the insulating layer. In addition, in the oxide semiconductor film, Na cuts or enters a bond between metal and oxygen which are included in an oxide semiconductor. As a result, for example, deterioration of characteristics of the transistor, such as a normally-on state of the transistor due to shift of a threshold voltage in the negative direction, or reduction in mobility, occurs. In addition, variation in characteristics also occurs. Such deterioration of characteristics of the transistor and variation in characteristics due to the impurity remarkably appear when the hydrogen concentration of the oxide semiconductor film is very low. Therefore, when the hydrogen concentration of the oxide semiconductor film is less than or equal to 5×1019/cm3, particularly less than or equal to 5×1018/cm3, the concentration of the above impurity is preferably reduced. Specifically, a measurement value of a Na concentration by secondary ion mass spectrometry is preferably less than or equal to 5×1016/cm3, more preferably less than or equal to 1×1016/cm3, still more preferably less than or equal to 1×1015/cm3. In a similar manner, a measurement value of a Li concentration is preferably less than or equal to 5×1015/cm3, more preferably less than or equal to 1×1015/cm3. In a similar manner, a measurement value of a K concentration is preferably less than or equal to 5×1015/cm3, more preferably less than or equal to 1×1015/cm3.
Without limitation to the two-layer structure in which the second crystalline oxide semiconductor layer is formed over the first crystalline oxide semiconductor layer, a stacked structure including three or more layers may be formed by performing once or plural times a process of film formation and heat treatment for forming a third crystalline oxide semiconductor layer after the second crystalline oxide semiconductor layer is formed.
The oxide semiconductor layer 453 including the stack of the oxide semiconductor layers formed by the above formation method can be used as appropriate for a transistor (e.g., the transistors 162 in Embodiment 1 and Embodiment 2, the transistors 410, 420, 430, 440, 441, and 442 in Embodiment 3) which can be applied to a semiconductor device disclosed in this specification.
In the transistor 440 in Embodiment 3 in which the stack of the oxide semiconductor layers of this embodiment is used as the oxide semiconductor layer 403, an electric field is not applied from one surface to the other surface of the oxide semiconductor layer and current does not flow in the thickness direction (from one surface to the other surface; e.g., in the vertical direction in FIG. 15D) of the stack of the oxide semiconductor layers. The transistor has a structure in which current mainly flows along the interface of the stack of the oxide semiconductor layers; therefore, even when the transistor is irradiated with light or even when a BT stress is applied to the transistor, deterioration of transistor characteristics is suppressed or reduced.
By forming a transistor with the use of a stack of a first crystalline oxide semiconductor layer and a second crystalline oxide semiconductor layer, like the oxide semiconductor layer 453, the transistor can have stable electric characteristics and high reliability.
This embodiment can be combined with the structure of any of the other embodiments as appropriate.
Embodiment 5
In this embodiment, the cases where the semiconductor device described in any of the above embodiments is applied to electronic devices will be described with reference to FIGS. 14A to 14F. The cases where the above-described semiconductor device is applied to electronic devices such as a computer, a mobile phone set (also referred to as a mobile phone or a mobile phone device), a portable information terminal (including a portable game machine, an audio reproducing device, and the like), a camera such as a digital camera, a digital video camera, electronic paper, a television set (also referred to as a television or a television receiver), and the like are described in this embodiment.
FIG. 14A illustrates a notebook personal computer, which includes a housing 701, a housing 702, a display portion 703, a keyboard 704, and the like. The semiconductor device described in any of the above embodiments is provided in at least one of the housings 701 and 702. Thus, a notebook personal computer with sufficiently lowered power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.
FIG. 14B illustrates a portable information terminal (PDA). A main body 711 is provided with a display portion 713, an external interface 715, operation buttons 714, and the like. Further, a stylus 712 for operation of the portable information terminal or the like is provided. The semiconductor device described in any of the above embodiments is provided in the main body 711. Thus, a portable information terminal with sufficiently lowered power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.
FIG. 14C illustrates an electronic book reader 720 incorporating electronic paper, which includes two housings, a housing 721 and a housing 723. The housing 721 and the housing 723 include a display portion 725 and a display portion 727, respectively. The housing 721 is connected to the housing 723 by a hinge 737, so that the electronic book reader 720 can be opened and closed using the hinge 737 as an axis. In addition, the housing 721 is provided with a power switch 731, operation keys 733, a speaker 735, and the like. At least one of the housings 721 and 723 is provided with the semiconductor device described in any of the above embodiments. Thus, an electronic book reader with sufficiently lowered power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.
FIG. 14D illustrates a mobile phone set, which includes two housings, a housing 740 and a housing 741. Moreover, the housings 740 and 741 in a state where they are developed as illustrated in FIG. 14D can be slid so that one is lapped over the other. Therefore, the size of the mobile phone set can be reduced, which makes the mobile phone set suitable for being carried around. The housing 741 includes a display panel 742, a speaker 743, a microphone 744, operation keys 745, a pointing device 746, a camera lens 747, an external connection terminal 748, and the like. The housing 740 includes a solar cell 749 for charging the mobile phone set, an external memory slot 750, and the like. An antenna is incorporated in the housing 741. The semiconductor device described in any of the above embodiments is provided in at least one of the housings 740 and 741. Thus, a mobile phone set with sufficiently lowered power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.
FIG. 14E illustrates a digital camera, which includes a main body 761, a display portion 767, an eyepiece 763, an operation switch 764, a display portion 765, a battery 766, and the like. The semiconductor device described in any of the above embodiments is provided in the main body 761. Thus, a digital camera with sufficiently lowered power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.
FIG. 14F is a television set 770, which includes a housing 771, a display portion 773, a stand 775, and the like. The television set 770 can be operated with a switch included in the housing 771 or with a remote controller 780. The semiconductor device described in any of the above embodiments is mounted in each of the housing 771 and the remote controller 780. Thus, a television set with sufficiently lowered power consumption, in which writing and reading of data can be performed at high speed and data can be stored for a long time, can be realized.
As described above, the electronic devices described in this embodiment each include the semiconductor device according to any of the above embodiments. Therefore, the electronic devices with lowered power consumption can be realized.
This application is based on Japanese Patent Application serial no. 2010-201267 filed with Japan Patent Office on Sep. 8, 2010, the entire contents of which are hereby incorporated by reference.

## Claims

1. A method for driving a semiconductor device,
the semiconductor device comprising:
a first to m-th memory cells which are connected in series between a source line and a bit line;
a first selection transistor whose gate terminal is electrically connected to a first selection line; and
a second selection transistor whose gate terminal is electrically connected to a second selection line,
wherein each of the first to m-th memory cells comprises:
a first transistor provided on a substrate including a semiconductor material, the first transistor comprising a first source terminal, a first drain terminal, and a first gate terminal electrically connected to a first signal line;
a second transistor including an oxide semiconductor layer, the second transistor comprising a second source terminal, a second drain terminal, and a second gate terminal electrically connected to a second signal line; and
a capacitor of which one terminal is electrically connected to one of m word lines,

wherein the source line is electrically connected to the first source terminal of the m-th memory cell via the second selection transistor,
wherein the bit line is electrically connected to the first drain terminal of the first memory cell via the first selection transistor, and
wherein the second source terminal, the first gate terminal, and the other terminal of the capacitor are electrically connected to one another to form a node,

the method comprising:
detecting a potential of the bit line by supplying potentials to the first selection line and the second selection line to turn on the first selection transistor and the second selection transistor in a writing operation where a potential is supplied to the second signal line to turn on the second transistors, and a potential is supplied to the first signal line to supply a potential to the nodes.

2. The method for driving a semiconductor device, according to claim 1, wherein a potential applied to the first signal line to supply the potential to the node is increased step-by-step.

3. The method for driving a semiconductor device, according to claim 1, wherein the potential in the node is held by turning off the second transistor.

4. The method for driving a semiconductor device, according to claim 1,
wherein in the writing operation, a plurality of different potentials are applied to the in word lines so that a plurality of different potentials are included in the first to m-th memory cells, and
wherein after the second transistors are turned off, supply of potentials to the in word lines is stopped.

5. The method for driving a semiconductor device, according to claim 4, wherein potentials held in the nodes in the first to m-th memory cells after the writing operation is finished include a plurality of different potentials.

6. A method for driving a semiconductor device,
the semiconductor device comprising:
a first to m-th memory cells which are connected in series between a source line and a bit line;
a first selection transistor whose gate terminal is electrically connected to a first selection line; and
a second selection transistor whose gate terminal is electrically connected to a second selection line,
wherein each of the first to m-th memory cells comprises:
a first transistor provided on a substrate including a semiconductor material, the first transistor comprising a first source terminal, a first drain terminal, and a first gate terminal electrically connected to a first signal line;
a second transistor including an oxide semiconductor layer, the second transistor comprising a second source terminal, a second drain terminal, and a second gate terminal electrically connected to a second signal line; and
a capacitor of which one terminal is electrically connected to one of in word lines,

wherein the source line is electrically connected to the first source terminal of the m-th memory cell via the second selection transistor,
wherein the bit line is electrically connected to the first drain terminal of the first memory cell via the first selection transistor, and
wherein the second source terminal, the first gate terminal, and the other terminal of the capacitor are electrically connected to one another to form a node,

the method comprising:
detecting a potential of the bit line by supplying potentials to the first selection line and the second selection line to turn on the first selection transistor and the second selection transistor in a writing operation where a potential is supplied to the second signal line to turn on the second transistors, and a potential is supplied to the first signal line to supply a potential to the nodes; and
after the bit line and the source line are brought into conduction, turning off the second transistors to finish the writing operation.

7. The method for driving a semiconductor device, according to claim 6, wherein a potential applied to the first signal line to supply the potential to the node is increased step-by-step.

8. The method for driving a semiconductor device, according to claim 6, wherein the potential in the node is held by turning off the second transistor.

9. The method for driving a semiconductor device, according to claim 6,
wherein in the writing operation, a plurality of different potentials are applied to the in word lines so that a plurality of different potentials are included in the first to m-th memory cells, and
wherein after the second transistors are turned off, supply of potentials to the in word lines is stopped.

10. The method for driving a semiconductor device, according to claim 9, wherein potentials held in the nodes in the first to m-th memory cells after the writing operation is finished include a plurality of different potentials.

11. A method for driving a semiconductor device,
the semiconductor device comprising:
a first memory cell and a second memory cell which are connected in series between a first line and a second line,
wherein each of the first memory cell and the second memory cell comprises:
a first transistor;
a second transistor, the second transistor includes a semiconductor layer including an oxide semiconductor; and
a capacitor whose first terminal is electrically connected to a gate terminal of the first transistor and a first terminal of the second transistor to form a node,

wherein the first line is electrically connected to a first terminal of the first transistor in the first memory cell,
wherein a second terminal of the first transistor in the first memory cell is electrically connected to a first terminal of the first transistor in the second memory cell, and
wherein the second line is electrically connected to a second terminal of the first transistor in the second memory cell via a third transistor,

the method comprising:
detecting a potential of the first line by turning on the third transistor in a writing operation where the second transistor is turned on, and a potential is supplied to the node.

12. The method for driving a semiconductor device, according to claim 11, wherein the potential supplied to the node is increased step-by-step.

13. The method for driving a semiconductor device, according to claim 11, wherein the potential in the node is held by turning off the second transistor.

14. The method for driving a semiconductor device, according to claim 11,
wherein a second terminal of the capacitor in the first memory cell and a second terminal of the capacitor in the second memory cell are electrically connected to a third line and a fourth line, respectively,
wherein in the writing operation, different potentials are applied to the third line and the fourth line so that different potentials are included in the first memory cell and the second memory cell, and
wherein after the second transistors are turned off, supply of potentials to the third line and the fourth line is stopped.

15. The method for driving a semiconductor device, according to claim 14, wherein potentials held in the nodes in the first memory cell and the second memory cell after the writing operation is finished include different potentials.