Nonvolatile semiconductor memory device and write/verify method thereof

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-293870, filed Oct. 6, 2004, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

This invention relates to an electrically rewritable nonvolatile semiconductor memory device and a write/verify method thereof. More specifically, for example, this invention relates to the technique for enhancing the write/verify speed in a flash memory having large memory capacity or a system using the flash memory.

2. Description of the Related Art

The number of memory cells connected to a bit-line is increased with an increase in the memory capacity of a nonvolatile semiconductor memory device and the pitch of the bit lines is narrowed with a reduction in the chip size. Therefore, the capacitance associated with the bit line and the capacitance between the bit lines are steadily increased. The charging/discharging time of the bit line is increased by an increase in the load capacitance of the bit line and time for writing data into a memory cell becomes long. As a result, in a system using the nonvolatile semiconductor memory device, for example, in a system such as a digital camera having a NAND flash memory into which a large amount of data is written, write time will become long.

Further, in the general nonvolatile semiconductor memory device, since it is necessary to verify whether or not data is written to a sufficient extent after the data write operation, longer time is required. The operation of writing data into the NAND flash memory is performed by applying high voltage to the control gate of a cell transistor acting as a memory cell to change (shift) the threshold voltage of the cell transistor. After this, the verify operation is performed to monitor the shifting amount of the threshold voltage of the cell transistor and determine whether or not the shifting amount of the threshold voltage of the cell transistor is sufficiently large. At this time, if the write characteristic of the cell transistor varies, it becomes necessary to increase the number of write and verify operations and it becomes necessary to take further long time to perform the write/verify operation (for example, refer to Jpn. Pat. Appn. KOKAI Publication No. H08-087895).

Recently, a chance of dealing with large capacity data such as moving data, image data of high quality and the like occurs many times and it is desired to enhance the write/verify speed in a flash memory and a system using the flash memory.

According to an aspect of the present invention, there is provided a nonvolatile semiconductor memory device comprising a memory cell array having memory cells arranged in a matrix form, write/verify circuits which are connected to one-side ends of bit lines in the memory cell array and configured to write data into the memory cell and verify the written data, a switching elements which divides the bit lines into plural portions, and a control circuit configured to control the write/verify circuit and switching elements, wherein the control circuit performs a control operation to perform write and verify operations with the switching elements set in an OFF state when a memory cell of an address to be written lies on the write/verify circuit side with respect to the switching elements, writes and saves data into a memory cell lying on the write/verify circuit side with the switching elements set in the-OFF state when the memory cell of the address to be written lies farther apart from the write/verify circuit than the switching elements, and then turns ON the switching elements while the write/verify circuit is not being operated and writes the saved data into a memory cell of an address to be written.

According to another aspect of the present invention, there is provided a write/verify method of a nonvolatile semiconductor memory device comprising determining whether or not a memory cell of an address to be written lies on the write/verify circuit side in a memory cell array, dividing bit lines into plural portions, writing data into a selected memory cell in the memory cell array and verifying the data when it is determined that the memory cell of the address to be written lies on the write/verify circuit side in the memory cell array, dividing the bit lines into plural portions, saving data into a memory cell which lies on the write/verify circuit side in the memory cell array when it is determined that the memory cell of the address to be written lies in position apart from the write/verify circuit in the memory cell array, and connecting the divided bit lines while the write/verify circuit is not being operated and writing data saved by the saving into the memory cell to be written.

According to another aspect of the present invention, there is provided a write/verify method of a nonvolatile semiconductor memory device comprising a first step of determining whether or not a memory cell of an address to be written lies on a write/verify circuit side in a memory cell array, a second step of dividing bit lines into plural portions, writing data into a selected memory cell in the memory cell array and verifying the data when it is determined in the first step that the memory cell of the address to be written lies on the write/verify circuit side in the memory cell array, a third step of dividing the bit lines into plural portions and saving data into a memory cell which lies on the write/verify circuit side in the memory cell array when it is determined in the first step that the memory cell of the address to be written lies in position apart from the write/verify circuit in the memory cell array, and a fourth step of connecting the divided bit lines while the write/verify circuit is not being operated and writing data saved in the third step into the memory cell to be written.

FIG. 1 is a block diagram showing the schematic configuration of a main portion relating to the write and verify operations of a NAND flash memory, for illustrating a nonvolatile semiconductor memory device according to one embodiment of this invention and a write/verify method thereof;

FIG. 2 is a block diagram showing an example of the configuration of a memory cell array and a write/verify circuit in the circuit shown in FIG. 1 ;

FIG. 3A is a symbol diagram showing a NAND string of FIG. 2 ;

FIG. 3B is a circuit diagram showing a concrete example of the configuration of the NAND string of FIG. 2 ;

FIG. 4 is a flowchart for illustrating a write/verify method;

FIG. 5 is a block diagram for illustrating the write verify operation; and

FIG. 6 is a block diagram for illustrating the write verify operation.

FIG. 1 is a block diagram showing the schematic configuration of a main portion relating to the write and verify operations of a NAND flash memory, for illustrating a nonvolatile semiconductor memory device according to one embodiment of this invention and a write/verify method thereof.

As shown in FIG. 1 , the NAND flash memory includes a memory cell array 11 , command input buffer 12 , control circuit 13 , address input buffer 14 , row decoder 15 , column decoder 16 , write/verify circuit 17 and data output buffer 18 .

When a command CMD is input to the command input buffer 12 , the command is decoded by the control circuit 13 and the address input buffer 14 , row decoder 15 , write/verify circuit 17 and the like are controlled according to the command CMD by the control circuit 13 . A row address signal among the address signal ADD input to the address input buffer 14 is supplied to and decoded by the row decoder 15 and a column address signal is supplied to and decoded by the column decoder 16 . One of the memory cells (cell transistors) in the memory cell array 11 is selected by the row decoder 15 and column decoder 16 . Data read out from the selected memory cell is amplified by the sense amplifier of the write/verify circuit 17 and supplied to the exterior via the data output buffer 18 . Further, when data is written into the memory cell, whether data is written to a sufficient extent or not, that is, whether the shifting amount of the threshold voltage of the sell transistor is sufficiently large or not is verified by the write/verify circuit 17 .

As shown in FIG. 2 , in the memory cell array 11 , NAND strings are arranged in a matrix form. The NAND strings arranged on the same row configure one block. In this example, 1024 blocks Block 0 to Block 1023 configure the memory cell array 11 . A corresponding one of word lines WL 0 - 31 _* (* indicates 0 to 1023) and corresponding ones of selection gate lines SGD, SGS are commonly connected to the NAND strings in each of the blocks Block 0 to Block 1023 .

As shown in FIGS. 3A and 3B , an example in which the current paths of 32 cell transistors CT 0 _* to CT 31 _* are serially connected in each of the NAND strings is shown. The current path of a selection gate transistor ST 1 is connected between the source of the cell transistor CT 0 _* and a source line CELSRC and the current path of a selection gate transistor ST 2 is connected between the drain of the cell transistor CT 31 _* and the bit line BL. The selection transistors ST 1 in the NAND strings arranged on the same row are commonly connected to a corresponding one of the selection gate lines SGS, the control gates of the cell transistors CT 0 _* to CT 31 _* are respectively connected to word lines WL 0 _* to WL 31 _* and the selection transistors ST 2 are commonly connected to a corresponding one of the selection gate lines SGD.

The NAND strings arranged on the same column are commonly connected to a corresponding one of the bit lines BL 0 to BL 2047 for each column. In portions of the bit lines BL 0 to BL 2047 which lie in the central portion (between Block 511 and Block 512 ) of the memory cell array 11 , the current paths of MOS transistors Q 0 to Q 2047 acting as switching elements which divide the bit lines BL 0 to BL 2047 into plural portions are connected. The gates of the MOS transistors Q 0 to Q 2047 are supplied with a control signal CS from the control circuit 13 and the MOS transistors are ON/OFF controlled by the control signal. When the MOS transistors Q 0 to Q 2047 are set in the ON state, the bit lines BL 0 to BL 2047 are made continuous, and if a bit line BLi (i=0 to 2047) is selected, 1024 NAND strings are connected to the selected bit line BLi. On the other hand, when the MOS transistors Q 0 to Q 2047 are set in the OFF state, the bit lines BL 0 to BL 2047 are divided into two portions and 512 NAND strings are connected to each divided portion of the selected bit line BLi. That is, the memory cell array 11 is divided into two portions of the blocks Block 0 to Block 511 and the blocks Block 512 to Block 1023 by the MOS transistors Q 0 to Q 2047 . One-side ends of the bit lines BL 0 to BL 2047 are connected to write/verify circuits 17 - 0 to 17 - 2047 .

With the above configuration, if a command CMD is input to the command input buffer 12 to specify a write operation, whether an address to be written specifies a memory cell which lies in the blocks Block 0 to Block 511 or a memory cell which lies in the blocks Block 512 to Block 1023 is determined based on the address signal ADD input to the address input buffer 14 as shown in the flowchart of FIG. 4 . In other words, whether the memory cell lies in position closer to or farther apart from the write/verify circuits 17 than the MOS transistors Q 0 to Q 2047 is determined (STEP 1 ).

For example, suppose that it is determined in STEP 1 that the memory cell of the address to be written lies in position close to the write/verify circuit 17 (in the block Block 0 to Block 511 ), for example, the word line WL 1 _ 1 in the block Block 1 is selected as shown in FIG. 2 . Then, the control signal CS is set to a ground potential VSS level to turn OFF the MOS transistors Q 0 to Q 2047 and divide the bit lines BL 0 to BL 2047 into two portions and data is written into the memory cell selected by the word line WL 1 _ 1 and verified (STEP 2 ). In this case, since the bit line capacitance is reduced by half, the charging and discharging time of the bit line can be substantially reduced by half and time required for the writing and verifying operations can be reduced.

On the other hand, suppose that it is determined in STEP 1 that the memory cell of the address to be written lies in position far apart from the write/verify circuit 17 (in the block Block 512 to Block 1023 ), for example, the word line WL 1 _ 512 in the block Block 512 is selected as shown in FIG. 5 . Then, the control signal CS is set to the ground potential VSS level to turn OFF the MOS transistors Q 0 to Q 2047 and divide the bit lines BL 0 to BL 2047 into two portions and data is saved into the memory cell of an address obtained by changing the most significant address of the block address (STEP 3 ). In the case of FIG. 5 , data is saved into a memory cell selected by the word line WL 1 _ 0 .

In order to enhance the write operation speed, it is important to perform the save operation in the early stage (reduce the number of write operations). Therefore, it is preferable to reduce the number of write operations by performing the write operation by use of the binary write operation and raising the write voltage (even if the threshold voltage control operation of the cell transistor is sacrificed to some extent).

After this, as shown in FIG. 6 , the operation of writing the saved data into a memory cell with the correct address to be written is performed in a manner such as a page copying operation by setting the control signal CS to the power supply voltage VDD level to turn ON the MOS transistors Q 0 to Q 2047 when the write/verify circuit 17 is not used (STEP 3 ).

As timing at which the write operation with respect to the correct address is performed, for example, the following timings (a) to (d) are considered.

(a) Timing at which the write operation is performed into another plane:

(b) Timing at which another chip is accessed in the case of a plural-chip configuration:

(c) Input/output (serial read/data load) time:

(d) Available time of the controller:

That is, the write time for the whole portion of the system can be reduced by writing data into a correct address in available time in which the write time of the system is not limited.

In the above embodiment, a case wherein the bit lines are divided into two portions and the charging/discharging time of the bit line is halved is explained as an example. However, if the bit lines are divided into n (n is a positive integer equal to or larger than 3) portions, the charging/discharging time of the bit line can be reduced to 1/n times and the write and verify operation speeds can be enhanced.

As described above, in the nonvolatile semiconductor memory device such as the NAND flash memory having large load capacitance of the bit lines, the write and verify operation speeds can be enhanced by dividing the bit lines to reduce the bit line capacitance of the bit line and performing the write operation when a memory cell arranged in position close to the write/verify circuit is selected at the write time. On the other hand, when a memory cell arranged in position far away from the write/verify circuit is selected, data is first saved into a memory cell arranged in position close to the write/verify circuit at high speed and then the data is written (copied) into a memory cell of a correct address by operating the write/verify circuit while the write/verify circuit is not being used.

Thus, the charging/discharging time of the bit line at the write time or verify time can be shortened by reducing the load capacitance of the bit lines, saving data into a memory cell arranged in position close to the write/verify circuit and rewriting the data into a memory cell with a correct address during available time. The write and verify speed of the whole chip (system) can be enhanced by performing the operation of rewriting the data into the memory cell with the correct address during another operation (while data is being written into another address, while another chip is being operated in the case of a plural-chip configuration or while data is being input/output).

As described above, according to one aspect of this invention, a nonvolatile semiconductor memory device in which the write and verify speeds can be enhanced and a write/verify method thereof can be attained.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.