Method for maintaining the memory of non-volatile memory cells

Abstract: The method includes restoring the charge lost from memory cells, such as to restore the original voltage levels, within a time equivalent to the retention time. The decision concerning when the memory is to be restored is taken for example when the memory is switched on, based on the time elapsed since the previous programming/restoration, or based on the difference between the present threshold voltage of the reference cells and the original threshold voltage of the (suitably stored) reference cells, or when predetermined operating conditions occur. This makes it possible to prolong the life of nonvolatile memories, in particular of multilevel type, wherein the retention time decreases as the number of levels (bits/cell) is increased.

Description: The present invention relates to a method for maintaining the memory content of non-volatile memory cells, in particular flash-EEPROM cells. As known, the problem of charge retention in flash memory cells storing more than one bit is becoming increasingly important, as the number of levels to be stored in a single memory cell gradually increases. In general, it is found that the retention time (maximum time after which there is no longer any certainty of correctly reading the stored data) decreases when the number of levels to be stored in a single flash cell increases (see for example ISSCC'98, SA 21.2: "1M-Cell 6b/Cell Analog Flash Memory of Digital Storage", P.L.Rolandi et al., page 334). When there is one bit per cell (two levels), the existing technologies allow retention times of more than 10 years; however if there are 4 bits/cell (16 levels), even when the used reference elements are reference flash cells storing the various voltage levels such as to (obviously partially) compensate for the losses, the maximum times of use decrease to approximately 4-5 years, or to as little as approximately 6 months when there are 6 bits/cell. Figures 1a and 1b show the effect of charge losses from the floating gate regions of a flash memory, in case of storing 2 bits/cell (four levels). In particular, figure 1a shows the distribution of the cells, i.e. the number of cells N versus the cell threshold voltage Vth for four stored levels, immediately after programming the memory, whereas figure 1b shows the distribution of the same cells after a time close to the retention time. As known, the charge loss modifies the cell distribution, both such as to displace towards lower threshold voltages the centre of each bell-shaped curve (from voltages R1, R2, R3, R4 to voltages R1', R2', R3', R4' for the four stored levels), and to cause extension of at least some distributions. Further charge losses subsequently prevent discrimination between the various levels, and thus correct reading of the cells. EP-A-0 791 933 discloses a refreshing method for a non-volatile memory according to the preamble of claim 1. US-A-4, 218, 764 discloses another refreshing method wherein the gap between adjacent memory level, as measured, is compared with reference gap values to decide when refresh is needed. GB-A-2 136 992 teaches a different method based on redundancy wherein the cells that are supposed to be susceptible to become faulty are copied in auxiliary memory location that replace the original cells when faults are detected. The object of the invention is to provide a method for maintaining the charge stored in non-volatile memory cells, such as to allow retention of the data for long periods, even in multilevel memories. According to the present invention, a method is provided for maintaining the memory content of non-volatile memory cells, as defined in claim 1. In practice, the method is based on the finding that as long as it is possible to read the cells correctly, it is also possible to restore the original voltage levels (including those of the reference cells), thereby eliminating the effects caused by charge losses. Taking into consideration for example a flash memory using 4 bits/cell, having a data retention time equivalent to 5 years, if, 5 years after initial programming, a process to restore the levels is carried out, data is preserved unchanged for a further 5 years, after which it is possible to carry out the restoration process again, and prolong retention of the data for a further 5 years. This is made possible also because the analog precision used for memory reading is far greater than the stored digital precision; in fact, to store 4 bits/cell, now reading systems with precision greater than 7bits/cell are used. The decision concerning the moment at which the process of restoring the memory is to be carried out, can be taken for example when the memory is switched on, based on the difference between current reference values (current threshold voltage of the reference cells) and original reference values (original threshold voltage of the reference cells). The threshold voltages corresponding to the various original levels are stored in the data memory in a digital manner, and can then be used when necessary to restore all the memory cells (including the reference cells and the cells themselves that store the references). To assist understanding of the present invention, preferred embodiments are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein: figures 1a and 1b show memory cell distributions, respectively after programming, and after a time close to the retention time; figure 2 shows a block diagram of a memory used according to a method that does not belong to the invention; figure 3 shows a block diagram of a memory used in an embodiment of the invention; figure 4 shows a flow chart of the present method; figure 4a show the detail of a block in figure 4, according to an embodiment of the present method; figure 4b shows the detail of a block in figure 4, according to an embodiment of the present method; and figure 5 shows the detail of two blocks of figure 4. Figure 2 shows the architecture of a memory device 1 that does not belong to the invention. The memory device 1 comprises a memory array 2, preferably of flash type, comprising a plurality of data cells 3. Memory array 2 additionally comprises reference cells 4, used for comparison reading of data cells 3. Memory array 2 is connected to a memory control unit 5, generally integrated in a single chip with the memory array 2, and comprising address circuits and management circuits of the memory array 2, in a known manner. Memory control unit 5 additionally comprises a small permanent memory (ROM memory 6), for storing the original reference levels. The memory control unit 5 is also connected to the exterior through a control unit 7 of a user. Figure 3 shows a different architecture, wherein some data cells 3 of memory array 2, indicated at 10, store the date of the latest restoration of memory array 2, and some data cells, indicated at 11, store the original threshold voltages corresponding to the various preset digital levels. In this case, memory control unit 5 dispenses with an ROM memory for the original threshold voltages. According to an aspect of the invention, memory control unit 5 of figures 2 and 3 begins a test regarding the need to carry out memory refreshing or restoration, at switching on, as described in detail hereinafter. In particular, with reference to figure 4, upon switching on (block 20), memory control unit 5 checks whether it is necessary to carry out refreshing, for example according to the method shown in figure 4b, block 21. In figure 4a, not representing the present invention, memory control unit 5 requests from the exterior (for example from user control unit 7) the current date DATT, and reads from data cells 10 the stored date DMEM of the latest refreshing or programming, block 23. Memory control unit 5 then checks whether the difference between the current date DATT and the stored date DMEM is equal to, or greater than, a predetermined threshold value K1 (for example 5 years), block 24. In figure 4b, memory control unit 5 reads the current threshold voltage RIFATT of reference cells 4 and the original threshold voltage RIFMEM from data cells 11 , block 26. Memory control unit 5 then checks whether the difference between the current threshold voltage RIFATT and the stored threshold voltage RIFMEM, is equal to, or greater than, a predetermined threshold value K2, corresponding to the separation between the various levels, block 27. If the result of the check in block 24 or block 27 is negative (NO outputs), no refreshing is carried out, and memory control unit 5 continues with the operations requested (block 30). On the other hand, if the result of the check in block 24 or block 27 is positive, the refreshing process is started. The refreshing process begins with resetting an address counter I, block 32; the threshold voltage THi of the i-th data cell 3, block 33, is then read; threshold voltage THi is compared with threshold value THr corresponding to the level stored in the i-th data cell 3, block 34; if the difference between THi and Thr in absolute value exceeds a predetermined error K3, reprogramming of the i-th cell itself is carried out (block 35). The check according to block 34 is carried out to take into account any cells originally overprogrammed, which, on completion of the latest programming, had a threshold voltage greater than the reference voltage, and therefore, even after long-term charge losses, have maintained a substantially correct threshold value. At the end of programming according to block 35, or for a NO output from block 34, counter I is incremented, block 36, and a check is made as to whether all the memory array 2 has been examined, block 37, if not (still cells to be checked and possibly reprogrammed), the method returns to block 33; if the check is positive (YES output), the reference cells are reprogrammed, block 40. Subsequently the current date is stored according to operation DMEM<--DATT, if the information on the reprogramming date is to be retained for other reasons, and the operations requested by user control unit 7 are continued, block 30. Programming according to blocks 35 and 40 must be carried out as soft programming which does not overprogram the cell. For example, use can advantageously be made of the algorithm described in European patent application 97830566.2 (EP 0913832) of 3.11.97., entitled "Method for multilevel programming of a nonvolatile memory, and a multilevel nonvolatile memory", and also referred to in the aforementioned article by Rolandi et al. This algorithm is summarised hereinafter for reference purposes. In detail, the following algorithm is based on the fact that when a series of programming voltage pulses with a linearly increasing value is applied to the gate terminal of a nonvolatile cell, at equilibrium, the threshold voltage increases in the same manner as the programming voltage. Reading the present threshold voltage of the cell to be programmed with high precision, once set the increase speed of the cell gate voltage (gradient of the ramp) based on the required threshold voltage value, the time necessary to reach the required value, and thus the number of programming pulses (of constant, predetermined duration) are determined. In practice, the number of pulses is calculated from the following equation: N - (V p - V i )/ΔV G wherein N is the number of programming pulses to be applied; V p is the required threshold voltage, V i is the initial (measured) threshold voltage, and ΔV G is the amplitude of each programming pulse, set on the basis of the required precision. In practice, since calculation of the number of needed pulses is correct only after equilibrium has been achieved, and considering conditions of inaccuracy, a first sequence of pulses is initially applied, to bring the cells into an equilibrium condition; a verify step is then carried out, to identify the value of the present cell threshold voltage; the number of pulses necessary to obtain the required threshold value is calculated; a number of pulses close to the number of pulses just calculated is applied, to prevent the cell from being overwritten; the present threshold value is verified out again; the number of pulses necessary to obtain the required threshold value is calculated; the number of pulses just calculated is applied; and a verify step is carried out again, optionally followed by further steps of applying programming and verify pulses. More specifically, with reference to figure 5, initially (block 100), the memory control unit reads the data necessary for programming a predetermined data cell 3 of the array. In particular, the following data are supplied: the intermediate voltage value V int to which the threshold voltage of the data cell 3 is to be brought to arrive into the linear zone, the amplitude ΔV G of the programming pulses, and the required final threshold voltage V p . In addition, memory control units 5 acquires the initial threshold voltage V i corresponding to the just measured value THi for data cell 3, or controls reading of the threshold voltage of reference cell 4 (block 110). Subsequently (block 120), memory control unit 5 calculates the number of pulses N1 necessary to reach the intermediate threshold voltage V int on the basis of (1), and controls that N1 pulses are supplied (block 130). Subsequently, a further verify step is carried out, and the output voltage V eff representing the present threshold voltage of the data cell 3 (block 140) is acquired. From the acquired value V eff , memory control unit 5 calculates the number of pulses N2 necessary to reach the final threshold voltage V p , less one, for safety (block 150) , and controls supplying of the N2 pulses (block 160). A further verify step is then carried out, the value of the present threshold V eff is acquired (block 170), and the number of pulses N3 necessary to reach the final threshold voltage V p is calculated (block 180). The N3 pulses are applied (block 190), and a further verify step is carried out, acquiring the present threshold value V eff (200). Then it is checked whether the present threshold value V eff is equal to the required threshold value, within the required precision level (block 210); if yes, the programming process ends; otherwise the method returns to block 180, for calculating the number of pulses still necessary to reach the threshold, and verifying. The advantages of the described method are apparent from the preceding description. The fact is now simply emphasised that the present method can easily be implemented by memory control unit 5, which is already present in memory device 1, without requiring changes to the structure thereof. Finally, it is apparent that many modifications and variations can be made to the method described and illustrated here, all of which come within the scope of the invention, as defined in the appended claims. In particular, although the preceding description relates to application to a flash-EEPROM memory, the present method can also be applied to other types of nonvolatile memory. In addition, the decision as to whether or not to restore the memory can depend on criteria and needs which are different from those indicated by way of example.

Claims: A method for maintaining the memory content of memory cells (3) of a nonvolatile memory (2), the memory cells (3) being subjected to long-term charge losses, after a programming step wherein said memory cells have been programmed to predetermined threshold voltage values, comprising the steps of: checking whether said memory cells (3) require refreshing; and restoring said predetermined threshold voltage values, wherein the step of checking whether said memory cells (3) require refreshing includes the steps of: reading predetermined threshold voltage values; reading present threshold voltage values of said memory cells (3); characterized in that the step of checking whether said memory cells (3) require refreshing further includes the steps of: determining the difference between said predetermined threshold voltage values and said present threshold voltage values; and checking whether said difference exceeds a predetermined value; and in that said predetermined threshold voltage values are read from memory locations (11) of said nonvolatile memory (2) where they are stored in digital manner and said step of restoring further comprises refreshing said memory locations (11). A method according to claim 1, wherein said step of checking whether said memory cells (3) require refreshing is carried out at switching on of said nonvolatile memory (2). A method according to claim 1, for a memory array comprising data cells (3) and reference cells (4), characterised in that said step of reading said present threshold values comprises the step of reading said reference cells (4). A method according to any one of the preceding claims, characterised in that said step of restoring said predetermined threshold voltage values comprises the steps of: determining a present threshold voltage value of one cell (3) of said memory cells; acquiring a predetermined threshold value for said one cell; calculating a first number of programming pulses with regularly increasing amplitude, as necessary to bring said present threshold value to said predetermined threshold value; and applying a second number of pulses correlated to the first number of pulses.

Inventors: LHERMET FRANK, PASOTTI MARCO, ROLANDI PIER LUIGI, LHERMET, FRANK, PASOTTI, MARCO, ROLANDI, PIER LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: G11C11
IPC codes: G11C16, G11C11