Method for recovering failed memory devices

Abstract: The invention relates to a method of recovering faulty non-volatile memories. This method can be applied to an electrically programmable semiconductor non-volatile memory device being set up as a multi-sectors memory matrix and including selection circuitry for selecting words or individual bytes of the memory. According to this method, the memory matrix is addressed by byte, rather than by memory word, by acting said selection circuitry, whenever the device fails an operation test. The use of a Hamming code of error correction to remedy malfunctions due to manufacture allows the method to be applied to those devices which fail their test and would otherwise be treated as rejects.

Description: Field of the Invention This invention relates to a method of recovering faulty non-volatile memories. More particularly, but not exclusively, the invention concerns such a method as can be applied to an electrically programmable semiconductor non-volatile memory device of the type which is set up as a multi-sectors memory matrix and includes selection circuitry for selecting words or individual bytes of the memory. The invention further relates to a non-volatile memory device which can be programmed and erased electrically and comprises a multi-sectors memory cell matrix. As is well known, of major concern in the large-volume manufacture of memory devices integrated on a semiconductor is the yield from their manufacturing process. A matrix of memory cells often emerges from the manufacturing process in a faulty condition, such that the device will be unsuitable for use. In fact, the cell matrix taking up most of the device circuit area, the probability of a serious manufacturing fault occurring in the circuit portion occupied by the matrix is quite high. Specifically, in this field of application, the manufacture of non-volatile memory devices of the so-called Flash type shows a comparatively low yield rate. Background Art There have been prior proposals for improving the yield from the manufacturing process of Flash memories. The solution adopted heretofore provided a circuit-oriented remedy. In essence, the structure of the matrix of memory cells is given additional rows and/or columns, known as redundancy rows/columns, which can be utilized in place of any faulty rows/columns that have developed malfunctions after the device testing. The skilled person in the art knows well the criteria for designing and utilizing redundancy rows/columns, but also the limitations of such a circuit-oriented approach. It is presently anticipated that a continually evolving technology may lead in the near future to such processing improvements as will make row redundancy an unnecessary measure; however, no comparable improvements are anticipated in the respect of column redundancy. As an example, and for an easier understanding of the aspects of this invention, the advantages and drawbacks of column redundancy will be reviewed briefly herein below, keeping in mind that a primary objective of this technique is to remedy malfunctions brought about by individual cells whose electrical characteristics divert from their design specifications. It is indeed at the designing stage that the ideal number of redundancy columns must be decided upon, as a compromise between the safety level sought and the circuit area increase involved by the presence of redundancy columns. It should be considered that most of the circuit feature-size is due to circuitry needed for controlling the inclusion of redundancy columns; moreover, the size of such control circuitry is bound to increase with the overall size of the memory device. This is explained by that a redundant element must be selected by comparing a memory address outside the device with the contents of a specific internal register whereinto the locations of the columns (or the rows) to be put in exchange are stored. In the particular instance of Flash memories, wherein the matrix is set up in sectors, column redundancy commands special attention at the designing stage. In fact, with the columns being assigned to individual sectors rather than to the device as a whole, it often occurs that an integrated device must be rejected wherein one sector cannot be repaired due to its supply of redundant elements having been exhausted, while other sectors of the same device may have a surplus of redundant elements available to them. One example of the prior art, EP 0 448 970A2, includes an error correcting circuit for restoring data retrieved from a set of memory cells that included one or more faulty cells. If an error check reveals that an error in data occurred, the memory device uses a correction circuit to recreate the original data by use of a Hamming code. This solution is limited to a few number of corrupt cells. If more cells than the maximum number of corrupt cells are present in the memory device, the address of the memory location can be changed. This solution is not completely satisfactory because, as described above, the number of extra available addresses in a particular sector may be exceeded, necessitating the disposal of an otherwise useable device. Another approach is seen in US 5,282,311. In this solution a DRAM can be reformatted from a 1Mx4 to a 1Mx2 if defective memory cells are found in the matrix. This solution uses a series of fuses that can be selectively blown to redirect the necessary signals, which causes additional manufacturing time for testing and converting the memory device. Further, the additional hardware circuitry that causes the resteering of the signals must be present in the device. Additionally, this solution is directed to an DRAM, rather than the non-volatile memories of the present invention. A further approach is seen in EP 0 590 809A2, which divides a cell array into two separate areas with a common word line. Data can be written to or read from the first area depending on information stored in the second area. This solution is unattractive because of the necessity of changing the layout of a standard memory matrix, along with including all of the additional communication overhead necessitated thereby. The underlying technical problem of this invention is to provide a method of recovering faulty non-volatile memories, which method can turn memory devices that have shown malfunctions upon testing into properly operating, and hence marketable, products. The method should have such features as to make the presence or absence of redundant elements inconsequential. This would allow the limitations and drawbacks of prior techniques to be successfully overcome. Summary of the Invention The solution idea on which this invention stands is one of having the memory matrix addressed by the byte, rather than by the word, through the operation of appropriate selection circuitry incorporated to the memory device, in the event of the device failing the test. Stated otherwise, the solution idea of this invention consists of obtaining the redundant feature from the memory word itself, the stipulation being that only one byte can be used for storing data. The recovery method of this invention obviously results in a memory device of halved capacity compared to that of the original device. This should, however, be weighed against the fact that the original device is one that has failed its test, and hence a total reject, whereas the issue of the inventive method is a properly operating product. Based on this solution idea, the technical problem is solved by a method as indicated above and defined in the characterizing portion of Claim 1 foll. appended hereto. The technical problem is also solved by a memory device of the kind outlined above and defined in Claim 8 foll. The features and advantages of the method and device according to the invention will be apparent from the following detailed description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawing. Brief Description of the Drawing Figure 1 is a block diagram illustrating schematically a non-volatile memory device according to this invention. Detailed Description Referring to Figure 1, the structure will be described of a memory device to be recovered by the method of recovering faulty non-volatile memories according to this invention. In fact, the inventive method is particularly, though not exclusively, intended for application to a non-volatile memory device which is programmable and erasable electrically and set up as a memory matrix having multiple sectors and memory words of sixteen bits. However, the principles of this invention could also be applied to memories of thirty-two bits, or for that matter, having memory words which are multiples of one byte (eight bits). The device 1 is of the monolithically integrated type on a semiconductor, and comprises a matrix 2 of memory cells arranged in rows and columns. The structure of the cell matrix is known per se, and is addressable through a row decoder 3 and a similar column decoder 4. The columns of the matrix 2 are connected bi-directionally to a selector 5 which communicates them to a series of sense amplifiers incorporated to a block 6. With a sixteen-bit memory device, sixteen sense amplifiers are needed in order to simultaneously write or read the sixteen bits. Thus far, the structure of the device 1 may be regarded as quite conventional. The added circuit portions of this invention will be described next. The device 1 includes a logic unit 10 which contains information concerning the mode of operation and generates suitable enable signals to the other circuit portions of the device. The device 1 further includes a set 15 of error-correcting components, described hereinafter, which are tied operatively to the control unit 10. In particular, the unit 1 is connected with its output side to a data input selector 7 and a data output selector 8. The input selector 7 operates with sixteen bits and addresses to the matrix 2 memory words which comprise the required bits for correcting in accordance with the inventive method. The selector 7 has outputs connected to the column selector 5. An encoder 9 is input eight-bit digital data D(7:0) of the least significant byte which correspond to a memory word, and has its output side communicated to the input selector 7 for the latter to receive said digital data D(15:0). The encoder 9 functions to calculate so-called parity bits, as explained hereinafter. Also provided is a calculation block 11 arranged to calculate an "error syndrome". This block 11 is connected after the block 6 that contains the sense amplifiers. Provided after the calculation block 11 are two cascade-coonnected correction blocks 12 and 13. The blocks 12 and 13 are arranged to effect a correction of the faulty bits, as explained hereinafter. The block 13 is connected to the data output selector 8 to send an eight-bit byte D(7:0) containing said parity bits to the selector 8. Both the correction block 13 and the selector 8 are further input the sixteen-bit output from the sense amplifier block 6. The output selector 8 effects the routing of two bytes to be output. In the interest of a clearer understanding of the aspects of this invention, the theory of error correction codes will be recalled briefly herein below. These codes ensure correctness of a digital word by adding a given number of bits, called parity bits, to the information to be safeguarded. An error correction code known for its unique simplicity and small size is the Hamming code C(7,4), which can be described by a first generating matrix G and a second parity matrix H, as follows: and A correspondance exists between the words of the source alphabet and those of the encoded alphabet; this correspondance is arrived at through an encode operation which can also be represented in chart form, as in the following table: A code alphabet is created using the parity bit, i.e. a set of admissible words is formed wherein the minimum Hamming spacing is adapted to allow of correction of any errors which may damage a number of bits smaller than or equal to the corrective capacity of the code. The spacing of the code C(7,4) utilized here is equal to three, meaning that this code is capable of correcting single errors. Advantageously, the method of this invention employs the Hamming code to remedy any malfunctions in the memory device. For example, for a Flash memory device addressable by sixteen-bit memory words, as is that shown in Figure 1, it has been elected of obtaining the redundancy bits from the memory word itself. The only stipulation made here is that of using one byte (eight bits) to contain the data, the other byte of the memory word being used for storing the parity bits. In this way, a memory of halved capacity compared to the original one is obviously obtained. Thus, having decided on the use of a Hamming code C(7,4) which operates with four-bit words, for protecting an eight-bit byte, the byte must be split in two portions, and the six parity bit calculated separately, three for each portion. In this way, one erroneous bit can be corrected for every four adjacent bits aligned to the portion. An exemplary application of the method will now be described. The instance of a 4-Mbyte Flash memory having multiple sectors and a sixteen-bit memory word will be assumed. If this device reveals on testing the inclusion of a sector with more than five faulty columns, then it is a reject, because no sufficient number of redundancy columns are found therein to permit of its recovery. But the device can still be recovered using the method of this invention. It will suffice for the purpose that the control unit 10 be programmed for enabling the circuit portions 11, 12, 13, as well as the selectors 7, 8 and the encoder 9. Accordingly, the encoder 9 calculates the parity bits according to the Hamming code, the calculation block 11 evaluating the "syndrome" of the error and activates the blocks 12 and 13 for the correction of faulty bits. The selector 8 routes the two bytes for output, the one containing the corrected byte and the other containing the original sixteen-bit word. Thus, on completion of the encoding process, the word that is actually stored has sixteen bits, but the eight most significant bits are taken up entirely by the error correction code. All this occurs in a fully transparent manner to the user, who will be denied the faculty of driving a data into the most significant byte of the memory word, because the encoder block has already determined its value according to that taken by the least significant byte. If the memory device, thus modified, does pass subsequent tests, it can be regarded as recovered in the form of a 2-Mbyte device capable of operating with eight-bit words. Upon testing, that is when the memory devices are to pass an EWS (Electrical Wafer Sort) test, all those devices which reveal malfunctions are found. These will be applied the recovery method of this invention and again submitted for testing. For all the integrated memory circuits where the correction arrangement of the invention has been enabled, it will be necessary to specify that the address space is now one-half of the original, and that the width of the memory word is of one byte only. As for the properly operating devices, the resident control unit 10 of each of them shall have to be programmed for inhibition of the set 15 of error correction blocks. In fact, the inventive method is unsuitable for a properly operating device, and can only be applied to devices that have failed their test and are, on this account, regarded as a total loss. The invention does solve the technical problem and affords a major advantage. The pin layout in an integrated memory circuit, as recovered in accordance with the invention, corresponds exactly to that of any circuit from the same "family", but with a smaller storage capacity. This feature avoids packaging problems with the devices recovered in accordance with the invention. It should be further considered that space requirements for the additional, error-correcting circuit portion are quite moderate, while the overall integration area taken up by the memory device is only negligibly larger. An accurate estimate gives an added area of just 0.5 mm 2 .

Claims: A method of recovering a faulty electrically programmable semiconductor non-volatile memory device (1) being set up as a multi-sector memory matrix (2) and which includes selection circuitry (10, 15) to select either complete words or individual bytes for output of the memory matrix, characterized in that the memory matrix (2) is addressed by byte, rather than by memory word, by acting on said selection circuitry (10, 15) whenever the memory device (1) fails an operation test. A method according to Claim 1, characterized in that said memory device (1) is a Flash memory. A method according to Claim 1, characterized in that a Hamming code of error correction is used to remedy malfunctions of the memory device (1). A method according to Claim 1, characterized in that one byte of the memory word is used for containing data, and the other byte of the memory word is used for storing parity bits. A method according to Claim 1, characterized in that the memory of a recovered device has its capacity halved from that of the original device. A method according to Claim 1, characterized in that redundancy bits are obtained, in said memory device (1), from the memory word itself. A method according to Claim 3, characterized in that , by using said Hamming code, the memory word comprises eight most significant bits which are utilized by the error correction code and eight least significant bits which are utilized for storing data. A non-volatile memory device integrated on a semiconductor, being programmable and erasable electrically and comprising a multi-sector matrix (2) of memory cells and selection circuitry for selecting either complete words or individual bytes of the memory matrix, said circuitry comprising row and column decoders (3,4), a column selector (5), and a sense amplifier block (6), the device being characterized in that it further comprises a control unit (10) and a set (15) of error-correcting components (7,8,9,11,12,13) to select addressing of the memory matrix (2) by byte rather than by memory word, by acting on said selection circuitry whenever the device fails an operation test. A device according to Claim 8, characterized in that the control unit (10) is connected with its output side to a data input selector (7) and a data output selector (8), the input selector (7) having outputs connected to the column selector (5). A device according to Claim 8, characterized in that said set (15) includes a calculation block (11), connected after the block (6) that contains the sense amplifiers, which is operative to calculate an "error syndrome". A device according to Claim 10, characterized in that provided after the calculation block (11) are two cascade-connected correction blocks (12,13) for applying a correction to the faulty bits in the memory word, the second (13) of said correction blocks (12,13) communicating to the data output selector (8) in order to send a byte (D(7:0)) containing the corrected byte to said selector (8).

Inventors: BRANCHETTI MAURIZIO, CONCI ARMANDO, GOLLA CARLA, BRANCHETTI, MAURIZIO, CONCI, ARMANDO, GOLLA, CARLA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G06F11
IPC codes: G06F11