Ferroelectric memory

A ferroelectric memory according to an embodiment of the present invention includes a memory cell array including plural memory cells, and provided with plural word lines, plural bit lines, and plural plate lines, each of the plate lines corresponding to at least two of the word lines, an access control circuit configured to perform an access operation to a selected cell which is selected from the memory cells, and a refresh control circuit configured to perform a refresh operation, in a background of the access operation, on a refresh cell which is selected from the memory cells, the refresh control circuit performing the refresh operation when a plate line connected to the selected cell and a bit line connected to the selected cell are at the same potential after the access operation.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-325195, filed on Dec. 22, 2008, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ferroelectric memory, for example, to a refresh operation in the ferroelectric memory.

2. Background Art

A ferroelectric memory is a semiconductor memory including a ferroelectric capacitor as a component of a memory cell. In general, the memory cell in the ferroelectric memory includes the ferroelectric capacitor and a cell transistor.

In general, the ferroelectric memory is provided with lines such as word lines, bit lines, and plate lines. In the ferroelectric memory, if the word lines are not associated with the plate lines in one-to-one correspondence, charges stored at a node between the ferroelectric capacitor and the cell transistor poses a problem. The stored charges and their leak become a cause of reliability lowering of the ferroelectric memory.

In a conventional ferroelectric memory, a refresh operation of turning on a word line to release the charges is performed periodically as a measure to counter this problem. However, when performing the refresh operation, it is necessary that the plate line and the bit line have the same potential. Therefore, in the conventional art, the refresh operation cannot be performed in situations where accesses for reading and/or writing are performed frequently.

JP-A 2000-11665 (KOKAI) describes an example of a ferroelectric memory which performs a data reading operation to read binary data from a selected cell, a counter data writing operation to write data which is opposite in a logical level to the read binary data into the selected cell, and an identical data writing operation to write data which is identical in a logical level as the read binary data into the selected cell again, as a sequential refresh operation.

SUMMARY OF THE INVENTION

An aspect of the present invention is, for example, a ferroelectric memory including a memory cell array including plural memory cells, and provided with plural word lines, plural bit lines, and plural plate lines, each of the plate lines corresponding to at least two of the word lines, an access control circuit configured to perform an access operation to a selected cell which is selected from the memory cells, and a refresh control circuit configured to perform a refresh operation, in a background of the access operation, on a refresh cell which is selected from the memory cells, the refresh control circuit performing the refresh operation when a plate line connected to the selected cell and a bit line connected to the selected cell are at the same potential after the access operation.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

FIG. 1is a schematic diagram showing a circuit configuration of a ferroelectric memory101according to a first embodiment. The ferroelectric memory101shown inFIG. 1includes a memory cell array111, a sense amplifier112, and a drive circuit113. The drive circuit113is an example of an access control circuit and a refresh control circuit of the present invention.

The memory cell array111includes a plurality of memory cells121. InFIG. 1, a first memory cell1211and a second memory cell1212are shown as examples of the memory cells121. As shown inFIG. 1, each memory cell121includes a ferroelectric capacitor131and a cell transistor132.

The memory cell array111is further provided with plural word lines, plural bit lines, and plural plate lines. As examples of the word lines, the bit lines, and the plate lines, first and second word lines WL1and WL2first and second bit lines BL1and BL2, and a plate line PL are shown inFIG. 1.

InFIG. 1, WL1and WL2are connected to the first memory cell1211and the second memory cell1212, respectively. Furthermore, BL1and BL2are connected to the first memory cell1211and the second memory cell1212, respectively. On the other hand, PL is connected to both the first memory cell1211and the second memory cell1212, and is associated with both WL1and WL2. In this way, in this embodiment, the plate lines are not associated with the word lines in one-to-one correspondence, but are each associated with at least two word lines.

In each memory cell121shown inFIG. 1, the ferroelectric capacitor131and the cell transistor132are connected in series. A first electrode of the ferroelectric capacitor131is connected to a plate line, and a second electrode of the ferroelectric capacitor131is connected to the cell transistor132. Further, one of a source and a drain (main terminals) of the cell transistor132is connected to the ferroelectric capacitor131, and the other of the source and the drain of the cell transistor132is connected to a bit line. A gate (control terminal) of the cell transistor132is connected to a word line.

InFIG. 1, the word lines and the plate lines extend in a first direction, and the bit lines extend in a second direction. Herein, the first direction is the longitudinal direction on paper, and the second direction is the lateral direction on paper. Each memory cell121shown inFIG. 1is provided near an intersection of a word line and a bit line.

As described above, the ferroelectric memory101is further provided with the sense amplifier112and the drive circuit113. The sense amplifier112is configured to read data stored in a selected cell by detecting and amplifying a potential change on the bit lines. The drive circuit113is configured to perform an access operation to the selected cell and a refresh operation on a refresh cell.

In the access operation, a cell to be accessed is selected from the memory cells121, and the access operation to the cell (selected cell) is performed. In the reading, stored data is read from this cell. When data is to be read from a selected cell, the sense amplifier112detects a potential change on a bit line connected to this selected cell. On the other hand, in the refresh operation, a cell to be refreshed is selected from the memory cells121, and the refresh operation on the cell (refresh cell) is performed.

FIG. 2is a timing chart for explaining the access operation and the refresh operation of the ferroelectric memory101shown inFIG. 1.FIG. 2is a waveform diagram in a case where an access target is the first memory cell1211.

When reading data stored in the first memory cell1211, the ferroelectric memory101causes both the potential on WL1and the potential on PL to become H (high), i.e., makes both WL1and PL valid (S1and S2). As a result, the potential on BL1rises (S3).

If the data stored in the first memory cell1211is “0” at this time, the potential on BL1slightly rises (S3A). On the other hand, if the data stored in the first memory cell1211is “1”, the potential on BL1largely rises (S3B). The sense amplifier112reads the data stored in the first memory cell1211by detecting and amplifying the potential on BL1.

Then, the ferroelectric memory101performs a rewriting into the first memory cell1211(S4). If data to be rewritten is “0”, the ferroelectric memory101drops the potential on BL1to a ground potential Vss (S4A). On the other hand, if the data to be rewritten is “1”, the ferroelectric memory101further raises the potential on BL1(S4B).

In this way, the ferroelectric memory101performs the access operation to the first memory cell1211by performing processing S1to S4. InFIG. 2, the access operation is a data read operation (including the rewriting operation). If the data to be rewritten is “1”, the ferroelectric memory101drops the potential on BL1to the ground potential Vss after the access operation is finished (S5).

The potential on PL is dropped to the ground potential Vss at the stage of S4. Therefore, in the ferroelectric memory101, PL and BL1become the same in potential at the time of S5.

The ferroelectric memory101performs processing of S1′ to S5′ in the same way as the processing of S1to S5at the next access operation. InFIG. 2, a time period between S5and S1′ is denoted by T. Over the time period T, PL and BL1become the same in potential.

In this embodiment, the refresh operation is performed during the time period T.FIG. 2is the waveform diagram in the case where a refresh target is the second memory cell1212. The ferroelectric memory101causes the potential on WL2to become H (high) during the time period T (S6). As a result, in the second memory1212, charges stored at a node between the capacitor131and the transistor132is released. InFIG. 1, the node is denoted by “A”.

In this way, in this embodiment, the refresh operation is performed when the plate line and the bit line connected to the selected cell are at the same potential after the access operation. In other words, the refresh operation is performed during the time period T shown inFIG. 2.

In this embodiment, this makes it possible to perform the refresh operation in a background of the access operation without interrupting the access operation. This is because the refresh operation is performed during the time period T which is an idle time period for the access operation.

Therefore, in this embodiment, the refresh operation is performed in the background of the access operation by performing it during the time period T. In this embodiment, this enables the refresh operation to be performed without exerting bad influence upon the performance of the access operation.

In this embodiment, the refresh operation is performed during the time period T which is the idle time period for the access operation. Therefore, even in situations where readings and/or writings are performed frequently, the refresh operation can be performed.

Furthermore, in this embodiment, the refresh operation is performed during a time period between an access operation and the next access operation. Therefore, in this embodiment, the refresh operation can be performed per access operation. Therefore, in this embodiment, the number of times of refreshes can be increased as the number of times of accesses increases. In general, the storage of charges takes place every time the access operations are performed. Therefore, as the access operations become frequent, the refresh operations also need to be performed frequently. In this embodiment, therefore, efficient refresh operations can be implemented by increasing the number of times of refreshes as the number of times of accesses in order to cope with such a situation.

InFIG. 2, the selected cell is set to the first memory cell1211, and the refresh cell is set to the second memory cell1212. Therefore, inFIG. 2, the refresh cell is an unselected cell connected to the same plate line as that of the selected cell.

In this embodiment, the refresh cell can be selected randomly from the memory cells121, for example. At this time, there is a possibility that the refresh cell becomes the selected cell or an unselected cell connected to a plate line which is different from that of the selected cell. However, since the refresh operation can be applied to these cells as well, no problems are posed by such a random selection.

However, if the refresh cell is the unselected cell connected to the plate line which is different from that of the selected cell, the executable time period of the refresh operation can be set wider than the time period T. Such setting will be described later with reference to an embodiment.

InFIG. 2, the refresh operation is performed immediately after the potential on BL1is dropped to the ground potential Vss. In other words, inFIG. 2, the refresh operation is started immediately after the access operation is finished. However, the refresh operation may be performed at other timing.

As described above, in this embodiment, the refresh operation is performed when the plate line and the bit line connected to the selected cell are at the same potential after the access operation, and the refresh operation is performed in the background of the access operation. In this embodiment, this enables the refresh operation to be performed efficiently.

Comparative Example

FIG. 3is a timing chart for explaining an access operation and a refresh operation of a ferroelectric memory according to a comparative example. In this ferroelectric memory, refresh operations of turning on word lines to release charges are performed periodically. When performing a refresh operation, it is necessary that the plate line and the bit line are at the same potential. InFIG. 3, when the potential on the plate line PL is L (low), the plate line PL and the bit line are set to the same potential. The refresh operation of turning on the word line to release the charges is performed under this setting.

In this case, it is difficult to perform the refresh operation in situations where accesses for reading and/or writing take place frequently. A potential at a node “A” and a refresh time TRare shown inFIG. 3. If the number of times of refreshes is small, the potential at the node “A” becomes unstable because of the charge storage, as shown inFIG. 3.

On the other hand, according to the first embodiment, the refresh operation can be performed even in situations where readings and/or writings are performed frequently. As a result, the problem shown inFIG. 3is dissolved.

Ferroelectric memories101according to second to fifth embodiments will be described below. These embodiments are modifications of the first embodiment, and will be described mainly as to points different from those of the first embodiment.

Second Embodiment

FIG. 4is a schematic diagram showing a circuit configuration of a ferroelectric memory101according to a second embodiment. InFIG. 4, a third memory cell1213is shown in addition to the first and second memory cells1211and1212. The third memory cell1213is connected to a third word line WL3and a third bit line BL3which differs from the first and second bit lines BL1and BL2. The third memory cell1213is further connected to a plate line PL′ which differs from the plate line PL.

FIG. 5is a timing chart for explaining an access operation and a refresh operation of the ferroelectric memory101shown inFIG. 4.FIG. 5is a waveform diagram in a case where an access target is the first memory cell1211.

In this embodiment, the refresh operation shown inFIG. 2is performed, when the refresh cell (refresh target cell) is a memory cell121which is connected to the same plate line as that of the selected cell (access target cell).FIG. 2is the waveform diagram in the case when the refresh target is the second memory cell1212.

On the other hand, in this embodiment, the refresh operation shown inFIG. 5can be performed, when the refresh cell (refresh target cell) is a memory cell121which is connected to a plate line which is different from that of the selected cell (access target cell).FIG. 5is the waveform diagram in the case where the refresh target is the third memory cell1213.

InFIG. 2, the refresh operation on the refresh target cell is performed, when the plate line and the bit line connected to the access target cell are at the same potential after the access operation to the access target cell, as described above (S6). In other words, the refresh operation is performed during the time period T shown inFIG. 2.

On the other hand, inFIG. 5, the refresh operation on the refresh target cell is performed, when the plate line and the bit connected to the access target cell are at the same potential or are not at the same potential (S6). In other words, inFIG. 5, the refresh operation may be performed after the plate line and the bit line have been at the same potential or before they are at the same potential. This is because the refresh target cell is connected to the plate line different from that of the access target cell. Furthermore, inFIG. 5, a complete background operation may be implemented by performing the refresh operation at the same time that the word line connected to the access target cell turns on. In other words, inFIG. 5, the refresh operation may be started concurrently with the start of the access operation.

InFIGS. 2 and 5, the refresh operation on the refresh target cell is performed when the plate line and the bit line connected to the refresh target cell are at the same potential. The refresh operation shown inFIG. 5is applied to a memory cell121which is connected to a plate line different from the plate line PL and a bit line different from the bit lines BL1and BL2.

As described above, in this embodiment, if the refresh cell is connected to the plate line different from that of the selected cell, the refresh operation can be performed not only after the access operation but also during the access operation. This embodiment has an advantage that the degree of freedom in setting an execution timing of the refresh operation is high.

On the other hand, the refresh operation may be performed during the time period T no matter which plate line is connected to the refresh cell. Such processing has an advantage that the execution timing of the refresh operation can be unified no matter which plate line is connected to the refresh cell.

Third Embodiment

FIG. 6is a schematic diagram showing a circuit configuration of a ferroelectric memory101according to a third embodiment. The ferroelectric memory101in this embodiment includes a circuit shown inFIG. 6in addition to the circuit shown inFIG. 1.

As shown inFIG. 6, the ferroelectric memory101in this embodiment includes a counter201, a selector202, and a decoder203. The counter201is an example of an address generation circuit in the present invention. The counter201, the selector202, and the decoder203are provided in the drive circuit113shown inFIG. 1.

In this embodiment, an address of the refresh cell is generated independently of an address of the selected cell to refresh all memory cells121without deviation. It is desirable that the refresh operations have been performed on all memory cells121when a predetermined time elapses.

The circuit shown inFIG. 6is an example of a circuit for performing such refresh operations.

InFIG. 6, the address of the refresh cell is generated by the counter201. The counter201is connected to all plate lines in the memory cell array111. The counter201generates the address of the refresh cell by counting the number of times of variation of potential on these plate lines. Therefore, in this embodiment, the address of the refresh cell changes every access operation.

In a case where the number of the word lines in the memory cell array111is m (where m is an integer of at least 2), an m-ary counter is employed as the counter201in this embodiment. This makes it possible to cause all of the m word lines to become refresh targets by performing the access operations m times, which makes it easy to refresh the memory cells121without deviation.

As shown inFIG. 6, an address signal ADD and a refresh address signal ADDRare inputted to the selector202. The address signal ADD is a signal which indicates the address of the selected cell, and is utilized at the time of the access operation. The refresh address signal ADDRis a signal which indicates the address of the refresh cell, and is utilized at the time of the refresh operation. The refresh address signal ADDRis generated and outputted by the counter201. The address signal ADD is an example of a first address signal in the present invention, and the refresh address signal ADDRis an example of a second address signal in the present invention.

A select signal SEL which specifies an address signal to be selected is also inputted to the selector202. When the selector202receives the select signal indicating that ADD should be selected, it selects and outputs ADD. When the selector202receives the select signal indicating that ADDRshould be selected, it selects and outputs ADDR. In this embodiment, it is possible to use the decoder203for both the access operation and the refresh operation due to the selector202.

An address signal selected by the selector202is inputted to the decoder203. When the decoder203is supplied with ADD, it turns on a word line indicated by ADD. As a result, a memory cell121connected to this word line becomes the access target. On the other hand, when the decoder203is supplied with ADDR, it turns on a word line indicated by ADDR. As a result, a memory cell121connected to this word line becomes the refresh target.

As described above, in this embodiment, the address of the refresh cell is generated independently of the address of the selected cell. This makes it possible to perform the refresh operations on respective memory cells121without deviation. Furthermore, in this embodiment, the address of the refresh cell is generated by the counter201. This makes it possible to implement the refresh operations whose deviation is suppressed, by using a comparatively simple circuit configuration.

Fourth Embodiment

FIG. 7is a timing chart for explaining an access operation and a refresh operation of a ferroelectric memory101according to a fourth embodiment. The ferroelectric memory101according to this embodiment has the circuit configurations shown inFIGS. 1 and 6.

FIG. 7(A)shows a potential change on the plate line PL. It is similar to the potential change on the plate line PL shown inFIG. 2.FIG. 7(B)shows the address signal ADD. It is now supposed that the selected cell is the first memory cell1211connected to the word line WL1.

A signal SEN shown inFIG. 7(C)represents a drive signal for the sense amplifier112. The sense amplifier112is brought into an active state to perform the detection and amplification when the drive signal SEN is H (high), i.e., in a valid state. On the other hand, the sense amplifier112is brought into an inactive state to stop the detection and amplification when the drive signal SEN is L (low), i.e., in an invalid state.

In the ferroelectric memory101, the sense amplifier112is brought into the inactive state if the drive signal SEN becomes L. Furthermore, in the ferroelectric memory101, when the sense amplifier112is brought into the inactive state, the potential on the bit line connected to the selected cell is dropped to the ground potential Vss as represented by S5inFIG. 2.

Therefore, in this embodiment, when the drive signal SEN has become L, the select signal SEL is brought to H to change the address signal from ADD to ADDR(FIG. 7(D)). In this embodiment, such a control makes it possible to perform the refresh operation during the time period T shown inFIG. 2. It is now supposed that the refresh cell is the second memory cell1212connected to the word line WL2.FIG. 7(E)shows how the word line WL2turns on in response to the change of the select signal SEL to H.

As described above, in this embodiment, the refresh operation is performed when the sense amplifier112is inactive. In this embodiment, this makes it possible to perform the refresh operation during the time period T.

Fifth Embodiment

FIG. 8is a timing chart for explaining an access operation and a refresh operation of a ferroelectric memory101according to a fifth embodiment. The ferroelectric memory101according to this embodiment has the circuit configurations shown inFIGS. 1 and 6.

Whereas the access operation shown inFIG. 2is a plate line drive type, the access operation shown inFIG. 8is a bit line drive type.

When the data stored in the first memory cell1211is to be read, the ferroelectric memory101of this embodiment raises the potential on BL1(S11), and then turns the potential on WL1H (S12). As a result, the potential on BL1falls (S13).

At this time, if the data stored in the first memory cell1211is “0”, the potential on BL1falls greatly (S13A). On the other hand, if the data stored in the first memory cell1211is “1”, the potential on BL1falls slightly (S13B). The sense amplifier112reads the data stored in the first memory cell1211by detecting and amplifying the potential on BL1.

Then, the ferroelectric memory101performs a rewriting into the first memory cell1211(S14). If data to be rewritten is “0”, the ferroelectric memory101drops the potential on BL1to the ground potential Vss (S14A). On the other hand, if the data to be rewritten is “1”, the ferroelectric memory101raises the potential on BL1again (S14B). At the time of the rewriting, the potential on PL is set to H (S15).

In this way, the ferroelectric memory101performs the access operation to the first memory cell1211by performing the processing S11to S15. InFIG. 8, the access operation is a data read operation (including the rewrite operation). If the data to be rewritten is “1”, the ferroelectric memory101drops the potential on BL1to the ground potential Vss after the access operation is finished (S16). Furthermore, the potential on PL is also dropped to the ground potential Vss. Therefore, in the ferroelectric memory101, PL and BL1are at the same potential at the time of S16.

The ferroelectric memory101performs processing S11′ to S16′ in the same way as the processing S11to S16at the next access operation. InFIG. 8, a time period between S16to S11′ is denoted by T. During the time period T, PL and BL1are at the same potential.

In this embodiment, the refresh operation is performed during the time period T in the same way as the first embodiment. During the time period T, the ferroelectric memory101turns the potential on WL2to H (S17). Therefore, in the second memory cell1212, charges stored at the node between the capacitor131and the transistor132are released. InFIG. 1, the node is represented by “A”.

As described above, this embodiment makes it possible to execute the refresh operation similar to that in the first embodiment, in the ferroelectric memory101of a bit line drive type.

As described above, the embodiments of the present invention enable the refresh operation in the ferroelectric memory to be performed effectively.

Although examples of specific aspects of the present invention have been described with reference to the first to fifth embodiments, the present invention is not restricted to these embodiments.