Multilevel memory system

Abstract: A memory system according to the present invention includes a memory portion including a memory cell for storing n-level data (n is an integer equal to or larger than 3, for example, 4) data, wherein the memory cell is operated as an n-level data storing memory cell when the number of times of write-erase sequence is smaller than a predetermined number of times, and the memory cell is operated as an m-level (m is an integer smaller than n, for example, 3) data storing memory cell when the number of times of write-erase sequence has exceeded the predetermined number of times. The number of information items (values) which can be stored in one memory cell is decreased with respect to a predetermined number of times of write-erase sequence. Thus, a memory system including a multi-level data storing memory cell and exhibiting improved durability against write-erase sequence operations is provided.

Description: The present invention relates to a memory system composed of an electrically erasable and programmable multi-level data storage nonvolatile semiconductor memory device (EEPROM). In recent years, a NAND cell EEPROM has been suggested as one of electrically erasable and programmable nonvolatile semiconductor memory devices. A NAND cell EEPROM is arranged such that a plurality of memory cells, each comprising a stacked gate type n-channel FETMOS structure with a floating gate and a control gate, are connected in series. The series-connected memory cells constitute one unit and are connected to a bit line. FIG. 17A is a plan pattern view showing one NAND cell of a memory cell array, FIG. 17B is a circuit diagram of the NAND cell. FIG. 18A is a cross sectional view taken along line XXXIXA-XXXIXA shown in FIG. 17A, and FIG. 18B is a cross sectional view taken along line XXXIXB-XXXIXB shown in FIG. 17A. A p-type silicon substrate (or a p-type well) 11 has a surface having a device region surrounded by a device isolating oxide film 12. A NAND cell is formed in each device region. A plurality of NAND cells form one memory cell array. Referring to FIGS. 17A, 17B, 18A and 18B, one of NAND cells will now be described. A semiconductor apparatus shown in FIGS. 17A, 17B, 18A and 18B has eight memory cells M1 to M8 connected in series to form one NAND cell. Each memory cell has a floating gate 14 (14-1, 14-2, ···, 14-8) formed on a substrate 11 through a gate insulating film 13. A control gate 16 (16-1, 16-2, ···, 16-8) is formed on the floating gate 14 through a second gate insulating film 15. An n-type diffusion layer 19, which is the source/drain of the memory cell is shared by adjacent memory cells. Thus, the eight memory cells are connected in series. The NAND cell has, in the drain portion, first selection gates 14-9 and 16-9, and in the source portion, second selection gates 14-10 and 16-10, formed simultaneously with the floating gates 14-1 to 14-8 and the control gates 16-1 to 16-8. The substrate 11 having the devices, such as the memory cells, is covered with a CVD oxide film 17. A bit line 18 is disposed above the CVD oxide film 17. The control gates 16 of the NAND cell are formed to extend in the row direction to be made common with control gates of the corresponding memory cells of the NAND adjacent in the direction of the row to serve as word lines (control gates CG1, CG2, ..., CG8). The selection gates 14-9 and 16-9 and the selection gates 14-10 and 16-10 are formed to extend in the direction of the row, in a manner similar to the control gates 16-1 to 16-8, to be made common with the corresponding selection gates of a NAND cell adjacent in the direction of the row to serve as selection gates SG1 and SG2. FIG. 19 is a circuit diagram showing a memory cell array having the NAND cells disposed in a matrix configuration. As shown in FIG. 19, the source line is, through a contact, connected to a reference potential line made of aluminum or conductive polysilicon. The contact between the source line and the reference potential line is provided for each 64 bit lines. The reference potential line is connected to a peripheral circuit (not shown) which controls, for example, the potential to be applied to the source line to correspond to the operation mode. The first and second selection gates SG1 and SG2 of the control gates CG1, CG2, ···, CG8 are formed to extend in the direction of the row. A set of memory cells connected to one control gate is, in general, called a page (one page), and a set of the pages held between one set of drain side (the first selection gate) and source side (the second selection gate) selection gates is called a NAND block (one NAND block) or a block (one block). One page is composed of, for example, 256 byte (256 × 8) memory cells. Writing of data is substantially simultaneously performed on the memory cells for one page. One block is composed of, for example, 2048 byte (2048 × 8) memory cells. Data is substantially simultaneously erased from the memory cells for one block. The operation of the NAND EEPROM is performed as follows. Writing of data is sequentially performed while starting at the farthest memory cell from the bit line. Raised writing voltage Vpp (= about 20V) is applied to the control gate of the selected memory cell, an intermediate potential (= about 10V) is applied to the control gates of the non-selected memory cells and the first selection gates and 0V (writing of "0") or an intermediate potential (writing of "1") is applied to the bit line in accordance with data. At this time, the potential of the bit line is transmitted to the selected memory cell. When data is "0", high voltage is applied between the floating gate of the selected memory cell and the substrate so that electrons are tunnel-implanted from the substrate into the floating gate so that the threshold voltage is shifted in the positive direction. When data is "1", the threshold voltage is not changed. Erase of data is performed substantially simultaneously performed in block units. That is, when data is erased, all of the control gates and selection gates included in the blocks from which data is erased are made to be 0V, and then raised potential VppE (about 20V) is applied to the p-type silicon substrate (or the p-type well formed on the n-type substrate). The raised potential VppE is applied to the control gates and selection gates included in the blocks from which data is not erased. As a result, in the memory cells in the blocks from which data is erased, electrons stored in the floating gate are discharged to the p-type silicon substrate (or the p-type well) so that the threshold voltage is shifted in the negative direction. The operation for reading data is performed such that the bit line is precharged, and then the bit line is brought to a floating state. Then, the control gate of the selected memory cell is made to be 0V, the control gates of the other memory cells and the selection gates are made to be power supply voltage Vcc (for example, 3V) and the source line is made to be 0V. As a result, whether or not an electric current flows in the selected memory cell is detected in accordance with change in the potential of the bit line. That is, if data written on the memory cell is "0" (if the threshold value of the memory cell satisfies Vth > 0), the memory cell is turned off so that the bit line maintains the precharge potential. If data is "1" (if the threshold value of the memory cell satisfies Vth < 0), the memory cell is turned on so that an electric current flows to cause the potential of the bit line to be lowered by *1V from the precharge potential. The potential of the bit line is detected by a sense amplifier so that data in the memory cell is read. In recent years, a multi-level data storage cell structured such that information of three or more levels is stored in one cell has been known as one of a method capable of realizing a EEPROM having a large capacity (for example, refer to Japanese Patent Laid-Open No. 7-93979 and Japanese Patent Laid-Open No. 7-161852). FIG. 20 is a graph showing the relationship between the threshold voltages of a memory cell and four writing states (four value data "0", "1", "2" and "3"). The state of data "0" is similar to the state after data has been erased and has, for example, a negative threshold value. The state of data "1" has a threshold voltage in a range from, for example, 0.5V to 0.8V. The state of data "2" has a threshold voltage in a range from, for example, 1.5V to 1.8V. The state of data "3" has a threshold voltage in a range from, for example, 2.5V to 2.8V. Therefore, reading voltage VCG2R is applied to the control gate CG to detect whether the memory cell is turned on or off so as to detect whether data in the memory cell is "0" or "1" and whether the same is "2" or "3". In accordance with a result of the foregoing detection, reading voltage VCG3R or VCG1R is applied so that data in the memory cell is detected. The reading voltages VCG1R, VCG2R and VCG3R are, for example, 0V, 1V and 2V, respectively. The voltages VCG1V, VCG2V and VCG3V are called verify voltages. When data is written, the verify voltages are applied to the control gate to detect the data writing state on the memory cell M, that is, whether or not data has been sufficiently written. The verify voltages VCG1V, VCG2V and VCG3V are, for example, 0.5V, 1.5V and 2.5V, respectively. A flash memory involves a limited number of times of write-erase sequence such that, for example, the number of times of write-erase sequence for a two-level memory cell is limited to 1,000,000 times. The limitation of the number of times of write-erase sequence arises when electrons stored in the floating gate leak from the floating gate of the memory cell in the write state to the substrate if further write-erase sequence is performed. When electrons are leaked from the memory cell in the state of data "1" shown in FIG. 20 and thus the memory cell is brought to the state of data "0", written data is broken. When multi-level data is stored in the memory cell, the difference between the multi-level data (for example, the difference in the voltage between the state "3" and the state "2" shown in FIG. 20) is reduced. As a result, if electrons in a slight quantity are leaked to the substrate, data in the state "3" is unintentionally changed to the state "2". If the memory cell is brought to the multi-level data mode, the threshold value of the highest threshold voltage (the state "3" in a state shown in FIG. 20) must be enlarged. Therefore, the electric field between the floating gate and the substrate is enlarged, thus causing the quantity of electrons leaked from the floating gate to be enlarged. Under these circumstances, the more the number of data levels of the memory cell becomes, the more the reliability to the number of times of write-erase sequence deteriorates. Therefore, the number of permitted times of write-erase sequence is reduced to, for example, 500,000 times. As a result, the durability (the lifetime) of the semiconductor device deteriorates. A conventional memory card (for example, refer to Niijima; IBM J. RES. DEVELOP. VOL. 39, No. 5 SEPTEMBER 1995) has a structure such that the number of times of write-erase sequence is recorded for each block and a block subjected to 1,000,000 times or more write-erase sequence operations is not used. However, also the foregoing method involves the number of times permitted for the memory card to be used being reduced as compared with the 2-level data memory mode when the number of the data levels is increased. In WO95/22146, a family member of which is US-A-5,784,018, there is disclosed a semiconductor memory system as indicated in the preamble of claim 1. In view of the foregoing, an object of the present invention is to provide a memory system including a multi-level data memory cell and exhibiting improved durability against write-erase sequence operations. Another object of the present invention is to provide a memory system including a plurality of new system elements required for the memory system capable. of achieving the foregoing object. According to one aspect of the present invention there is provided a memory system comprising a memory portion including a memory cell comprising a multilevel data memory cell operable at different levels, characterized in that: the memory cell is operated as an n-level data storing memory cell, where n is an integer equal to or larger than 3, until the number of times a write-erase sequence has been performed reaches a predetermined number of times, and said memory cell is operated as an m-level data storing memory cell, where m is an integer smaller than n, when the number of times the write-erase sequence has been performed has exceeded the predetermined number of times. The memory cell may be operated as a 2-level data storing memory cell when the number of times the write-erase sequence has been performed has exceeded the predetermined number of times. The memory cell may be operated as the n-level data storing memory cell until the number of times the write-erase sequence has been performed reaches an n-th number of times comprising the predetermined number of times, said memory cell is operated as an (n-1) level data storing memory cell after the n-th number of times of the write-erase sequence until the number of times the write-erase sequence has been performed reaches an (n-1)th number of times, and said memory cell may be operated as an i-level data storing memory cell, where i is an integer equal to or larger than 2, after an (i+1)th number of times of the write-erase sequence until the number of times the write-erase sequence has been performed reaches an i-th number of times. A change of an n-level data memory mode to an m-level data memory mode may be performed in a memory chip unit including said memory portion, or a change of an n-level data memory mode to an (n-1) level data memory mode or a change of an (i+1) level data memory mode to an i-level data memory mode may be performed in a memory chip unit including said memory portion. The memory portion may include a memory cell block including a predetermined number of memory cells and serving as a unit for collectively writing or erasing data, and a change of n-level data memory mode to m-level data memory mode may be performed in the memory cell block unit, or the memory portion may include a memory cell block including a predetermined number of memory cells and serving as a unit for collectively writing or erasing data, and a change of n-level data memory mode to (n-1) level data memory mode or a change of (i+1) level data memory mode to i-level data memory mode may be performed in the memory cell block unit. Data may be neither written into said memory cell nor erased from said memory cell after the number of times the write-erase sequence has been performed reaches a limited number in an m-level data memory mode after a change of an n-level data memory mode to the m-level data memory mode has been performed, or data may be neither written into said memory cell nor erased from said memory cell after the number of times the write-erase sequence has been performed reaches a limited number in an (n-1) level data memory mode after a change of an n-level data memory mode to the (n-1) level data memory mode has been performed or after the number of times the write-erase sequence has been performed reaches a limited number in an i-level data memory mode after a change of an (i+1) level data memory mode to the i-level data memory mode has been performed. The memory cell may not be used after the number of times the write-erase sequence has been performed reaches a limited number in an m-level data memory mode after change of an n-level data memory mode to the m-level data memory mode has been performed, or the memory cell may not be used after the number of times the write-erase sequence has been performed reaches a limited number in an (n-1) level data memory mode after change of an n-level data memory mode to the (n-1) level data memory mode has been performed or after the number of times the write-erase sequence has been performed reaches a limited number in an i-level data memory mode after change of an (i+1) level data memory mode to the i-level data memory mode has been performed. Cycles may be repeated each of which includes a data writing or erasing operation into or from said memory cell and a verify read operation for detecting a state of said memory cell; the number of the repeated cycles of the data writing or erasing operation and the verify read operation may be monitored, and a change of an n-level data memory mode to an m-level data memory mode may be performed after the number of the repeated cycles has reached a predetermined number of times. Alternatively, cycles may be repeated each of which includes a data writing or erasing operation into or from said memory cell and a verify read operation for detecting a state of said memory cell; the number of the repeated cycles of the data writing or erasing operation and the verify read operation may be monitored; and a change of an n-level data memory mode to a 2-level data memory mode may be performed after the number of the repeated cycles has reached a predetermined number of times. Alternatively, cycles may be repeated each of which includes a data writing or erasing operation into or from said memory cell and a verify read operation for detecting a state of said memory cell; the number of the repeated cycles of the data writing or erasing operation and the verify read operation may be monitored; and a change of an n-level data memory mode to an (n-1) level data memory mode or change of an (i+1) level data memory mode to an i-level data memory mode may be performed after the number of the repeated cycles has reached a predetermined number of times. A data write voltage to the memory cell in a data write mode or an erase voltage to the memory cell in a data erase mode may be increased when the memory cell is operated as an m-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the predetermined number of times, or a data write voltage to the memory cell in a data write mode or a data erase voltage in a data erase mode may be increased when the memory cell is operated as a (n-1) level data memory cell after the number of times the write-erase sequence has been performed has exceeded the n-th number of times the write-erase sequence or when the memory cell is operated as a i-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the (i+1)th number of times of the write-erase sequence. A width of a data write pulse to the memory cell in a data write mode or a width of a data erase pulse to the memory cell in a data erase mode may be increased when the memory cell is operated as an m-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the predetermined number of times of the write-erase sequence, or a width of a data write pulse to the memory cell in a data write mode or a width of a data erase pulse to the memory cell in a data erase mode may be increased when the memory cell is operated as a (n-1) level data memory cell after the number of times the write-erase sequence has been performed has exceeded the n-th number of times of the write-erase sequence or when the memory cell is operated as an i-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the (i+1)th number of times of the write-erase sequence. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1 is a graph showing the relationship between the threshold voltages and 4-level data in a multi-level data NAND flash memory according to a first embodiment of the present invention; FIGS. 2A, 2B and 2C are graphs each showing the relationship between the threshold voltages and 3-level data in a multi-level data NAND flash memory according to the first embodiment of the present invention; FIGS. 3A and 3B are graphs each showing the relationship between the threshold voltages and 2-level data in a multi-level data NAND flash memory according to the first embodiment of the present invention; FIG. 4 is a block diagram showing a flash memory according to a second embodiment of the present invention; FIG. 5 is a structure of a chip of the flash memory shown in FIG. 4; FIG. 6 is a block diagram showing a memory system according to a modification of the second embodiment of the present invention; FIG. 7 is a flow chart of the operation of a first memory system according to a third embodiment of the present invention; FIG. 8 is a flow chart of the operation of a second memory system according to the third embodiment of the present invention; FIG. 9 is a flow chart of the operation of a third memory system according to the third embodiment of the present invention; FIG. 10 is a flow chart of the operation of a seventh memory system according to the third embodiment of the present invention; FIG. 11 is a flow chart of the operation of an eighth memory system according to the third embodiment of the present invention; FIG. 12 is a flow chart of the operation of a ninth memory system according to the third embodiment of the present invention; FIG. 13 is a flow chart of the operation of a tenth memory system according to the third embodiment of the present invention; FIG. 14 is a flow chart of the operation of an eleventh memory system according to the third embodiment of the present invention; FIG. 15 is a flow chart of the operation of a twelfth memory system according to the third embodiment of the present invention; FIG. 16 is a flow chart of the operation of a thirteenth memory system according to the third embodiment of the present invention; FIG. 17A is a diagram showing plan pattern of a NAND cell portion of the memory cell array; FIG. 17B is a circuit diagram of the NAND cell portion; FIG. 18A is a cross sectional view taken along line XXXIX-XXXIX shown in FIG. 17A; FIG. 18B is a cross sectional view taken along line XXXIXB-XXXIXB shown in FIG. 17A; FIG. 19 is a circuit diagram showing a memory cell array having NAND cells disposed in a matrix configuration: and FIG. 20 is a graph showing the relationship between threshold voltages and 4-level data in the memory cell. Embodiments of the present invention will now be described such that a multi-level data NAND flash. FIGS. 1, 2A-2C, and 3A-3B are graphs showing the relationship between the threshold voltages and multi-level data in each memory mode of the multi-level data NAND flash memory according to a first embodiment of the present invention. FIG. 1 shows an example of the relationship between threshold voltages and 4-level data. FIGS. 2A to 2C show three examples of the relationship between the threshold voltages and the 3-level data in the 3-level operation mode. FIG. 3A and 3B show two examples of the relationship between the threshold voltages and 2-level data in a 2-level operation mode. Referring to FIGS. 1, 2A to 2C, 3A and 3B, state "0" is an erasing state, and "1", "2" and "3" are writing states. The operation (writing, reading and erasing) in the chip of the multi-level data NAND flash memory is performed as disclosed in Japanese Patent Laid-Open No. 7-93979, Japanese Patent Laid-Open No. 7-161852, Japanese Patent Application No. 7-295137, Japanese Patent Application No. 8-61443 and Japanese Patent Application No. 8-61445. In the EEPROM, the number of times the write-erase sequence can be performed decreases as the number of data levels increases. For example, a 4-level data memory cell having the 4-level data storage state as shown in FIG. 1 is limited to 500,000 times of write-erase sequence, a-3-level data memory cell having the 3-level data storage state as shown in FIGS. 2A to 2C is limited to 800,000 times of write-erase sequence, and a 2-level data memory cell having the 2-level data storage state as shown in FIGS. 3A and 3B is limited to 1,000,000 times of write-erase sequence. According to the present invention, the memory cell is used as a 4-level data memory cell having a storage state as shown in FIG. 1. After 500,000 times of write-erase sequence have been performed, the memory cell is used as a 3-level data memory cell as shown in FIGS. 2A to 2C. After 800,000 times of write-erase sequence have been performed, the memory cell is used as a 2-level data memory cell as shown in FIGS. 3A and 3B. To simplify the operation method of the semiconductor device, the structure may be arranged such that the memory cell is used as the 4-level data memory cell until 500,000 times of write-erase sequence is performed and is used as the 2-level data memory cell after 500,000 times of write-erase sequence have been performed without being used as the 3-level data memory cell. If the number of times the write-erase sequence has been performed for the 4-level data memory cell exceeds the limited number of times, the memory cell is sequentially used as the 3-level data memory cell and the 2-level data memory cell or as the 2-level data memory cell. Thus, the number of times of write-erase sequence permitted for the flash memory can be increased. Therefore, the multi-level data storage flash memory according to the present invention is able to improve the durability against write-erase sequence as compared with the conventional multi-level data storage flash memory. The present invention improves the durability of a single flash memory, that is, the durability of a semiconductor chip and that of a memory device by including the flash memory into a variety of memory devices (for example, a memory card). Whether the memory cell is operated as the 4-level data memory cell or a 3-level data memory cell or a 2-level data memory cell may be controlled by inputting a command to the flash memory from outside of the chip. That is, the flash memory is provided with the following operation modes consisting of a writing operation mode (or an operation method)/reading operation mode (or an operation method) for 4-level data, a writing operation mode (or an operation method)/reading operation mode (or an operation method) for 3-level data and a writing operation mode (or an operation method)/reading operation mode (or an operation method) for 2-level data. Any one of the operation modes (or the operation methods) may be selected by inputting a command. Whether the memory cell is operated as the 4-level data memory cell or the 3-level data memory cell or the 2-level data memory cell may be controlled in accordance with write data supplied from a controller for controlling the flash memory. That is, a structure may be employed in which the internal operation of the flash memory is not changed but the memory cell is sequentially operated as the 4-level data memory cell, the 3-level data memory cell and the 4-level data memory cell or as the 2-level data memory cell after it has been operated as the 4-level data memory cell in accordance with the contents of write data supplied from the outside. When the memory cell is operated as the 4-level data memory cell, 4-level data "0", "1", "2" and "3" is supplied. When the memory cell is operated as the 3-level data memory cell, 3-level data "0", "1" and "2" is supplied. When the memory cell is operated as the 2-level data memory cell, 2-level data "0" and "1" is supplied. As described above, the value of input data may be controlled in accordance with the operation mode of the memory cell. If the number of times of the write-erase sequence for the 2-level data memory cell exceeds the limited number of times, use of the memory cell may be inhibited. If the number of times of the write-erase sequence for the 2-level data memory cell exceeds the limited number of times, writing of data into the memory cell or erasing of data from the memory cell may be inhibited. Since in such a case write-erase sequence of data to the memory cell is inhibited, the memory cell may be used as a ROM. When the memory cell is used as a ROM, particularly, the degree of degradation of the tunnel oxide film determines the time period in which data can be stored. Even if the number of times of the write-erase sequence reaches the limited number of times, the memory cell generally has reliability to serve as the ROM. The reason for this is that the limited number of times of the write-erase sequence has been set to have a certain margin to the moment at which the tunnel oxide film can no longer be used. A memory system according to a second embodiment will now be described. FIG. 4 is a block diagram showing a flash memory according to a second embodiment of the present invention. As shown in FIG. 4, a controller 100 controls k (k is an integer) chips 101-1 to 101-k of a multi-level data NAND flash memory. FIG. 5 is a block diagram showing one chip, for example, chip 101-1, of the multi-level data NAND flash memory shown in FIG. 4. Each chip shown in FIG. 4 includes a plurality of memory cells each of which has a threshold voltage distribution corresponding to the multi-level data storage level, like those according to the first embodiment of the present invention. The threshold voltage distribution is similar to that shown in FIGS. 1, 2A to 2C, 3A and 3B such that "0" is the erasing state, and "1", "2" and "3" are writing states, as shown in FIGS. 1, 2A to 2C, 3A and 3B. The operation (writing, reading and erasing) in the chip of the multi-level data NAND flash memory is performed as disclosed in Japanese Patent Laid-Open No. 7-93979, Japanese Patent Laid-Open No. 7-161852, Japanese Patent Application No. 7-295137, Japanese Patent Application No. 8-61443 and Japanese Patent Application No. 8-61445. One page, which is a unit on which data is written simultaneously, is composed of 528-byte pieces of memory cells. Among 528-byte pieces of memory cells, 512-byte pieces of memory cell constitute a data region, while remaining 16-byte pieces of memory cell store an address conversion table showing the correspondence between the logical addresses and physical addresses, error correcting codes (Error Correcting Code: ECC). Referring to FIG. 5, one chip is composed of 512 blocks from block 0 to block 511. One block, which is a unit from which data is simultaneously erased, is composed of 16 pages from page 0 to page 15. For example, leading end block Block 0 is used as a system region. That is, Block 0 is required to store the number of times of write-erase sequence in each block, information of broken block, or block sequence number S. The block sequence number S and the like have been disclosed in Niijima; IBM J. RES. DEVELOP. VOL. 39, No. 5 SEPTEMBER 1995. In this case, whenever writing and erasing are repeated, the number of write-erase sequence is rewritten in the leading-end block which stores the number of write-erase sequence. Since write-erase sequence is, as described above, frequently performed in the system block, the lifetime of the memory cell in the system region is sometimes shortened as compared with the lifetime of the memory cell in the other data region. FIG. 6 is a block diagram showing a modification of the memory system according to the second embodiment. If the lifetime of the system region is shorter than that of the memory cells in the other regions, a DRAM 102 for storing the system region, such as a region for recording the number of write-erase sequence, may be provided, as shown in FIG. 6. When the power supply is turned on (i.e., in operation), data in the system region of the flash memory is read and stored in the DRAM. Thereafter, write-erase sequence of the system region is performed with respect to data write, data erase, or data write-erase sequence. At this time, the system region stored in the DRAM is subjected to the write-erase sequence. In accordance with data in the system region in the DRAM, data in the system region in the flash memory is subjected to the data write-erase sequence when the power source is turned off or every predetermined period of time. As described above, write-erase sequence of the system region during the operation is performed such that data in the DRAM is subjected to the data write-erase sequence. Therefore, frequent write-erase sequence of the system region in the flash memory can be prevented. If the address conversion table must be rewritten when data is written, erased or rewritten, the contents of the address conversion table may be stored in the DRAM and also write-erase sequence may be performed in the DRAM. In this embodiment, a 4-level data flash memory will be described as an example. For example, a 4-level data memory cell structured as shown in FIG. 1 can be written 500,000 times, while a 2-level data memory cell structured as shown in FIGS. 3A and 3B can be written 1,000,000 times. According to this embodiment, the memory cell is used in a 4-level data mode to serve as a 4-level data memory cell as shown in FIG. 1 until write-erase sequence is performed 500,000 times. The number of times of the write-erase sequence for each block is stored in a region for storing the number of write-erase sequence in the system region on the flash memory when the power supply is turned off. When the power supply is turned on (i.e., in operation), it is stored in the region for recording the number of write-erase sequence in the DRAM. After 500,000 times of write-erase sequence have been performed, the memory cell is used in the 2-level data mode to serve as a 2-level data memory cell. A change between the 4-level mode and the 2-level mode may be performed in one page unit or one block unit or in a unit of plural blocks or chip unit. A level holding means for storing information whether each page, each block or each chip is in the 2-level data mode or the 4-level data mode stores information about the operation mode in the system region of the flash memory when the power supply is turned off. When power supply is turned on (i.e., in operation), the information is stored in the DRAM. When electric power has been supplied, the level holding means is read to determine the level at which the memory cell is operated. As described above, the memory cell is used as the 2-level data memory cell if the number of times of the performed write-erase sequence exceeds the limited number of times for the 4-level data memory cell. Thus, the number of times of the write-erase sequence permitted for the flash memory to be used can be increased. Whether the memory cell is operated as the 4-level data memory cell or the 2-level data memory cell may be controlled by inputting a command to the flash memory from the controller 100. That is, the flash memory is provided with the following operation modes consisting of a writing operation mode (or an operation method)/reading operation mode (or an operation method) for 4-level data and a writing operation mode (or an operation method)/reading operation mode (or an operation method) for 2-level data. Either of the provided operation mode (or the operation method) may be selected by inputting a command. Whether the memory cell is operated as the 4-level data memory cell or the 2-level data memory cell may be controlled in accordance with write data supplied from a controller 100 for controlling the flash memory. That is, a structure may be employed in which the internal operation of the flash memory is not changed but the memory cell is sequentially operated as the 4-level data memory cell or the 2-level data memory cell in accordance with the contents of write data supplied from the outside. When the memory cell is operated as the 4-level data memory cell, 4-level data "0", "1", "2" and "3" is supplied. When the memory cell is operated as the 2-level data memory cell, 2-level data "0" and "1" is supplied. The number of memory cells in the data region in one page may be made to be different between the 4-level mode and the 2-level mode. In either case of the 4-level mode or the 2-level mode, for example, 512-byte pieces of memory cells of the 528-byte pieces of memory cells may form the data region and 16-byte memory cells, which are the remaining memory cells, may store the address conversion table showing the correspondence between the logical address and the physical address and the error correcting code (ECC). As an alternative to this, the regions for the address conversion table and the ECC region may be optimized for each of the 4-level mode and the 2-level mode. A structure may be employed in which the 4-level mode is arranged such that the regions for the address conversion table and the ECC use 16-byte pieces of memory cells and in which the 2-level data mode uses 24-byte pieces of memory cells or 8-byte pieces of memory cells. In any case, regions except the address conversion table and the ECC may be used as the data region. Since data in the region for recording the number of write-erase sequence is updated whenever the write-erase sequence in each block is performed, monitoring of the region for storing the number of times of the write-erase sequence enables the level at which the memory cell is operated to be determined. That is, the region for recording the number of times of the performed write-erase sequence in the DRAM is read before a write-erase sequence is performed. If the number of write-erase sequence in the block in which data is written is 500,000 times or smaller, writing is performed by making the block as a 4-level data memory cell. If the number of write-erase sequence is larger than 500,000 times, the contents in the level holding means in the DRAM are changed from the 4-level data memory cell to the 2-level data memory cell and data is written in the block as the 2-level data memory cell. If the number of times of write-erase sequence exceeds 1,000,000 times, use of the block may be inhibited or writing and erasing of data with respect to the block may be inhibited. The second embodiment has the structure such that the number of times of write-erase sequence is recorded for each block and the level at which each block is operated is determined. The unit, the level of which is determined, is not limited to the block. For example, one region for recording the number of times of write-erase sequence may be provided for the plural blocks and the level at which a unit of plural blocks is operated may be determined. As an alternative to this, one region for recording the number of times of write-erase sequence may be provided for each chip of the flash memory and the level at which a unit of the chips is operated may be determined. Another structure may be employed in which the number of times of write-erase sequence may be monitored for each page unit and the level at which each page unit is operated may be determined. Although the second embodiment has the structure such that the number of times of write-erase sequence is monitored and the fact that the level at which the memory cell is operated is determined. As an alternative to this, time taken from, for example, shipping may be monitored to determined the level at which the memory cell is operated. A memory system according to a third embodiment of the present invention will now be described with reference to FIGS. 7 to 16. A plurality of memory systems according to the present invention will now be described as the memory systems according to the third embodiment. FIG. 7 is a flow chart showing the operation of a first memory system according to the third embodiment. The first memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). The first memory system is structured such that the memory cell is operated as an n-level data memory until the number of times of write-erase sequence is not more than a predetermined number of times of write-erase sequence (for example, 500,000 times), as shown in FIG. 7. When the number of times of write-erase sequence is larger than 500,000, the memory cell is operated as an m-level (m is an integer smaller than n) memory cell. As described in the second embodiment, the number of times of write-erase sequence may be recorded on the region for storing the number of times of write-erase sequence for each block or each chip and the region for storing the number of times of write-erase sequence is monitored so as to determine the level at which the memory cell is operated. FIG. 8 is a flow chart showing the operation of a second memory system according to the third embodiment. The second memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). The second memory system is structured such that the memory cell is operated as an n-level data memory until the number of times of write-erase sequence is not more than a predetermined times of write-erase sequence, as shown in FIG. 8. When the number of times of write-erase sequence is larger than the predetermined times of write-erase sequence, the memory cell is operated as a 2-level data memory cell. FIG. 9 is a flow chart showing the operation of a third memory system according to the third embodiment. The third memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). As shown in FIG. 9, the third memory system is structured such that the memory cell is operated as an n-level data memory until the number of times of the performed write-erase sequence reaches an n-th number of times of write-erase sequence. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an (n - 1)th number of times of write-erase sequence, the memory cell is operated as a (n - 1) level data memory cell. After performing (i+1)th number of times of write-erase sequence (i is an integer larger than 2), until the number of times of performed write-erase sequence reaches an i-th number of times of write-erase sequence, the memory cell is operated as an i-level data memory cell. Note that the predetermined number of times of write-erase sequence set for the n-level data memory cell, the (n - 1) level data memory cell and the i-level data memory cell respectively are defined as n-th number of times of write-erase sequence, (n - 1)th number of times of write-erase sequence and i-th number of times of write-erase sequence which are similarly applied hereinafter. A fourth memory system is a semiconductor device including a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). The fourth memory system includes a level holding means for storing the level of data stored by the memory cell. The level holding means is required to be structured such that the contents of storage are stored in, for example, the first block of the flash memory chip, for example, Block 0 shown in FIG. 5. As an alternative to this, a structure may be employed in which the level holding means is read when electric power has been supplied and the stored contents are stored in the DRAM 102 shown in FIG. 6. When the region for storing the number of times of write-erase sequence is read and a determination has been made that the number of times of write-erase sequence has reached 500,000 times, the contents of the level holding means are changed so as to cause the memory cell to operate as a 2-level memory cell. Then, the level holding means is read to cause the memory cell to operate as the 2-level data memory-cell and writing is performed. That is, this embodiment has the structure such that the level holding means is read when, for example, electric power has been supplied so that the level at which the memory cell is operated is determined. A fifth memory system is a semiconductor storage device including a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). A predetermined number of memory cells form a memory cell block so that writing or erasing is performed in units of the memory cell blocks. In the fifth memory system, memory cells in the memory cell block are operated as n-level memory cells until the number of times of write-erase sequence reaches a predetermined number of times. If the number of times of performed write-erase sequence exceeds the predetermined number of times, all of the memory cells in the memory cell block are operated as an m (m is an integer small than n) level memory cells. The fifth memory system has a region for storing the number of times of write-erase sequence in the memory cell block provided for each memory cell block. The level at which the memory cell block is operated may be determined in accordance with the number of times of write-erase sequence stored in the region for storing the number of times of write-erase sequence. The memory cell block is not limited to a so-called "block" which is a set of a plurality of pages interposed between one drain side selection gate and one source side selection gate. The memory cell block is a memory cell group to and from which data is write-erase sequence or erased simultaneously. Since the NAND EEPROM is generally structured such that writing is performed in page units and erasing is performed in block units, the pages and blocks correspond to the memory cell blocks. A sixth memory system is a semiconductor storage device including a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). The memory cells in the sixth system are operated as n-level memory cells until the number of times of performed write-erase sequence exceeds a predetermined times of write-erase sequence. If the number of times of the performed write-erase sequence exceeds the predetermined number of times, all of the memory cells in the chip including the memory cell are operated as an m (m is an integer smaller than n) level memory cells. The sixth memory system has a region on the chip for storing the number of times of the performed write-erase sequence. The region for storing the number of times of the performed write-erase sequence is provided for each chip. The level at which the chip is operated may be determined in accordance with the number of times of the performed write-erase sequence stored in the region for storing the number of times of the performed write-erase sequence. FIG. 10 is a flow chart showing the operation of a seventh memory system according to the third embodiment. The seventh memory system includes a memory cell for storing n-level data (n is an integer not smaller than 3 and is, for example, 3 or 4 or 8 or 16), as shown in FIG. 10. The seventh memory system is arranged such that the memory cells are operated as n-level memory cells until the number of times of performed write-erase sequence to the memory cell reaches an n-th number of times. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches m-th number of times of write-erase sequence, the memory cells are operated as m (m is an integer smaller than n) level memory cells. If the number of the times of the performed write-erase sequence exceeds the m-th number of times of write-erase sequence, the memory cells are not used. FIG. 11 is a flow chart of the operation of an eighth memory system according to the third embodiment. The eighth memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). The eighth memory system is structured such that the memory cells are operated as n-level memory cells until the number of performed write-erase sequence reaches the n-th number of times, as shown in FIG. 11. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches a second number of times of write-erase sequence, the memory cells are operated as 2-level memory cells. If the number of times of the performed write-erase sequence exceeds second number of times of write-erase sequence, the memory cells are not used. FIG. 12 is a flow chart of the operation of a ninth memory system according to a third embodiment. The ninth memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example 3 or 4 or 8 or 16). As shown in FIG. 12, the ninth memory system has the structure such that the memory cells are operated as n-level memory cells when the number of times of performed write-erase sequence reaches the n-th number of write-erase sequence. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an (n-1)th number of times of write-erase sequence, the memory cells are operated as (n-1) level memory cells, as shown in FIG. 12. After performing an (i+1)-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an i-th number of times of write-erase sequence (i is an integer larger than 2), the memory cells are operated as i-level memory cells. Thereafter, when the number of times of performed write-erase sequence exceeds a second number of times of write-erase sequence, then the memory cells are not used. FIG. 13 is a flow chart of the operation of a tenth memory system according to the third embodiment. The tenth memory system includes memory cells for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). As shown in FIG. 13, the tenth memory system has the memory cells which are operated as n-level memory cells until the number of times of performed write-erase sequence to the memory cell reaches an n-th number of times. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches m-th number of times of write-erase sequence, the memory cells are operated as m (m is an integer smaller than n) level memory cells. If the number of the times of the performed write-erase sequence exceeds the m-th number of times of write-erase sequence, data is not written or erased to and from the memoy cells. FIG. 14 is a flow chart of the operation of an eleventh memory system according to the third embodiment. The eleventh memory system includes memory cells for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). The eleventh memory system has a structure such that the memory cells are operated as n-level memory cells until the number of performed write-erase sequence reaches the n-th number of times, as shown in FIG. 11. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches a second number of times of write-erase sequence, the memory cells are operated as 2-level memory cells. If the number of times of the performed write-erase sequence exceeds second number of times of write-erase sequence, data in the memory cells is output erased and data is not written on the memory cells. FIG. 15 is a flow chart of the operation of a twelfth memory system according to the third embodiment. The twelfth memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). As shown in FIG. 15, the ninth memory system has the structure such that the memory cells are operated as n-level memory cells when the number of times of performed write-erase sequence reaches the n-th number of write-erase sequence. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an (n-1)th number of times of write-erase sequence, the memory cells are operated as (n - 1) level memory cells, as shown in FIG. 15. After performing an (i+1)-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an i-th number of times of write-erase sequence (i is an integer larger than 2), the memory cells are operated as i-level memory cells. Thereafter, when the number of times of performed write-erase sequence exceeds a second number of times of write-erase sequence, then data in the memory cells is not erased or data is not written on the same. FIG. 16 is a flow chart of the operation of a thirteenth memory system according to the third embodiment. The thirteenth memory system includes a memory cell for storing n-level data (n is an integer equal to or larger than 3 and is, for example, 3 or 4 or 8 or 16). As shown in FIG. 16, the ninth memory system has the structure such that the memory cells are operated as n-level memory cells when the number of times of performed write-erase sequence reaches the n-th number of write-erase sequence. After performing the n-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an (n-1)th number of times of write-erase sequence, the memory cells are operated as (n - 1) level memory cells, as shown in FIG. 16. After performing an (i+1)-th number of times of write-erase sequence, until the number of times of performed write-erase sequence reaches an i-th number of times of write-erase sequence (i is an integer larger than 2), the memory cells are operated as i-level memory cells. Thereafter, until the number of times of performed write-erase sequence reaches a third number of times of write-erase sequence, the memory cells are operated as 3-level memory cells. After that, until the number of times of performed write-erase sequence reaches a second number of times of write-erase sequence, the memory cells are operated as 2-level memory cells. After the number of times of performed write-erase sequence has exceeded the second number of times of write-erase sequence, erase and writing of data to and from the memory cells may be inhibited or use of the memory cells may be inhibited. The first to third embodiments have the structure such that the multi-level data semiconductor storage device is arranged to reduce the number of information items (values) which are stored in one memory cell as the number of times of write-erase sequence increases. For example, a 4-level memory cell is operated as the 4-level memory cell when the number of times of write-erase sequence is smaller than 500,000 times. Then, the memory cell is operated as a 2-level memory cell. As a result, the number of times of write-erase sequence of the memory system can be enlarged as compared with that of the conventional memory system. In addition to the NAND EEPROM and a NOR flash memory, the present invention may be applied to an AND (K.Kume et al.; IEDM Tech. Dig., Dec. 1992, pp. 991-993), a DINOR (S.Kobayashi et al.; ISSCC Tech. Dig., 1995, pp. 122) and a virtual ground array (R.Cema et al.; ISSCC tech. Dig., 1995, pp. 126). As a matter of course, the present invention may be applied to a multi-level data DRAM or a multi-level data mask ROM or a multi-level data SRAM. The present invention may, of course, be applied to a 5-level memory cell, an 8-level memory cell or a 16-level memory cell as well as the 3-level memory cell or the 4-level memory cell. As described above, according to the present invention, there is provided a memory system including a multi-level data storing memory cells and exhibiting satisfactory durability against write-erase sequence operation.

Claims: A memory system comprising a memory portion (1) including a memory cell (M1) comprising a multilevel data memory cell operable at different levels, characterized in that : the memory cell is operated as an n-level data storing memory cell, where n is an integer equal to or larger than 3, until the number of times a write-erase sequence has been performed reaches a predetermined number of times, and said memory cell is operated as an m-level data storing memory cell, where m is an integer smaller than n, when the number of times the write-erase sequence has been performed has exceeded the predetermined number of times. A memory system according to Claim 1, characterized in that said memory cell is operated as a 2-level data storing memory cell when the number of times the write-erase sequence has been performed has exceeded the predetermined number of times. A memory system according to Claim 1, characterized in that the memory cell is operated as the n-level data storing memory cell until the number of times the write-erase sequence has been performed reaches an n-th number of times comprising the predetermined number of times, said memory cell is operated as an (n-1) level data storing memory cell after the n-th number of times of the write-erase sequence until the number of times the write-erase sequence has been performed reaches an (n-1)th number of times, and said memory cell is operated as an i-level data storing memory cell, where i is an integer equal to or larger than 2, after an (i+1)th number of times of the write-erase sequence until the number of times the write-erase sequence has been performed reaches an i-th number of times. A memory system according to Claim 1, characterized in that a change of an n-level data memory mode to an m-level data memory mode is performed in a memory chip unit including said memory portion. A memory system according to Claim 3, characterized in that a change of an n-level data memory mode to an (n-1) level data memory mode or a change of an (i+1) level data memory mode to an i-level data memory mode is performed in a memory chip unit including said memory portion. A memory system according to Claim 1, characterized in that : said memory portion includes a memory cell block including a predetermined number of memory cells and serving as a unit for collectively writing or erasing data, and a change of n-level data memory mode to m-level data memory mode is performed in the memory cell block unit A memory system according to Claim 3, characterized in that : said memory portion includes a memory cell block including a predetermined number of memory cells and serving as a unit for collectively writing or erasing data, and a change of n-level data memory mode to (n-1) level data memory mode or a change of (i+1) level data memory mode to i-level data memory mode is performed in the memory cell block unit. A memory system according to Claim 1, characterized in that : data is neither written into said memory cell nor erased from said memory cell after the number of times the write-erase sequence has been performed reaches a limited number in an m-level data memory mode after a change of an n-level data memory mode to the m-level data memory mode has been performed. A memory system according to Claim 3, characterized in that : data is neither written into said memory cell nor erased from said memory cell after the number of times the write-erase sequence has been performed reaches a limited number in an (n-1) level data memory mode after a change of an n-level data memory mode to the (n-1) level data memory mode has been performed or after the number of times the write-erase sequence has been performed reaches a limited number in an i-level data memory mode after a change of an (i+1) level data memory mode to the i-level data memory mode has been performed. A memory system according to Claim 1, characterized in that : said memory cell is not used after the number of times the write-erase sequence has been performed reaches a limited number in an m-level data memory mode after change of an n-level data memory mode to the m-level data memory mode has been performed. A memory system according to Claim 3, characterized in that : said memory cell is not used after the number of times the write-erase sequence has been performed reaches a limited number in an (n-1) level data memory mode after change of an n-level data memory mode to the (n-1) level data memory mode has been performed or after the number of times the write-erase sequence has been performed reaches a limited number in an i-level data memory mode after change of an (i+1) level data memory mode to the i-level data memory mode has been performed. A memory system according to Claim 1, characterized in that cycles are repeated each of which includes a data writing or erasing operation into or from said memory cell and a verify read operation for detecting a state of said memory cell; the number of the repeated cycles of the data writing or erasing operation and the verify read operation is monitored, and a change of an n-level data memory mode to an m-level data memory mode is performed after the number of the repeated cycles has reached a predetermined number of times. A memory system according to Claim 2, characterized in that cycles are repeated each of which includes a data writing or erasing operation into or from said memory cell and a verify read operation for detecting a state of said memory cell; the number of the repeated cycles of the data writing or erasing operation and the verify read operation is monitored; and a change of an n-level data memory mode to an 2-level data memory mode is performed after the number of the repeated cycles has reached a predetermined number of times. A memory system according to Claim 3, characterized in that cycles are repeated each of which includes a data writing or erasing operation into or from said memory cell and a verify read operation for detecting a state of said memory cell; the number of the repeated cycles of the data writing or erasing operation and the verify read operation is monitored; and a change of an n-level data memory mode to an (n-1) level data memory mode or change of an (i+1) level data memory mode to an i-level data memory mode is performed after the number of the repeated cycles has reached a predetermined number of times. A memory system according to Claim 1, characterized in that a data write voltage to the memory cell in a data write mode or an erase voltage to the memory cell in a data erase mode is increased when the memory cell is operated as an m-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the predetermined number of times. A memory system according to Claim 3, characterized in that a data writing voltage to the memory cell in a data write mode or a data erase voltage in a data erase mode is increased when the memory cell is operated as a (n-1) level data memory cell after the number of times the write-erase sequence has been performed has exceeded the n-th number of times the write-erase sequence or when the memory cell is operated as a i-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the (i+1)th number of times of the write-erase sequence. A memory system according to Claim 1, characterized in that a width of a data write pulse to the memory cell in a data write mode or a width of a data erase pulse to the memory cell in a data erase mode is increased when the memory cell is operated as an m-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the predetermined number of times of the write-erase sequence. A memory system according to Claim 3, characterized in that a width of a data write pulse to the memory cell in a data write mode or a width of a data erase pulse to the memory cell in a data erase mode is increased when the memory cell is operated as a (n-1) level data memory cell after the number of times the write-erase sequence has been performed has exceeded the n-th number of times of the write-erase sequence or when the memory cell is operated as an i-level data memory cell after the number of times the write-erase sequence has been performed has exceeded the (i+1)th number of times of the write-erase sequence.

Inventors: TAKEUCHI KEN, TANAKA TOMOHARU, TAKEUCHI, KEN, TANAKA, TOMOHARU
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: G11C11
IPC codes: G11C17, G06F11, G11C16, G11C11