SEMICONDUCTOR MEMORY DEVICE WITH REDUNDANT MEMORY CELL

Abstract: A semiconductor memory device having a redundant memory cell array (12) by means of which faulty memory cells are replaced with redundant memory cells during the production process. The device comprises a forced release circuit (31,32) for releasing or cancelling the replacement of the faulty cells with the redundant memory cells, for example, by applying a voltage different from a normal voltage to one terminal of the device. This is useful for enabling the faulty states, and the causes of the faults, of the device to be determined.

Description: The present invention relates to a semiconductor memory device with a redundant memory cell. The larger the scale of integration of a semiconductor memory device, the higher the probability of the generation of a faulty memory cell, and thus redundant (spare) memory cells are provided. If a faulty memory cell is found during a test, the faulty memory cell is replaced with a redundant memory cell, and thus the production rate of satisfactory devices is increased. The replacement of a faulty memory cell with a redundant memory cell is usually performed at a memory device production step. Namely, in the final step of testing the wafer, a faulty memory cell in each chip is detected by a tester, and when a faulty memory cell or cells is detected, the possibility of the replacement thereof is examined (for example, the number of faulty word lines must not be larger than the number of redundant (spare) word lines available). If replacement is possible, the faulty memory cells are replaced with redundant memory cells and the process advances to detect a faulty memory cell in the next chip. After the faulty memory cells are replaced with the redundant memory cells, the faulty memory cells cannot be distinguished from the outside, since in operation the redundant memory cell is no different from a normal memory cell. That is, when using the memory device, it is impossible to determine by an external examination whether or not memory cells have been replaced by redundant memory cells. This is most inconvenient when attempting to discover the cause of a fault, as it is important to know whether or not a redundant memory cell is being used, and if a redundant memory cell is being used, to know the location (or address) of the faulty memory cell. Therefore, although the extraction of the output of a ROM (read only memory) for memorizing the redundant (faulty) memory cell address or the like has been attempted, nevertheless it cannot be determined whether any one of word lines or bit lines in a block is faulty when a sequential plurality of lines has been replaced as a block. In addition, even if the faulty word line or bit line address is determined by reading the output of the redundant address ROM, it cannot be determined whether a particular memory cell on the word line or bit line is faulty. In addition, the type of fault , e.g., malfunction by voltage fluctuation, malfunction caused by read/write speed, increase of faulty area, cannot be determined. Proposals regarding a semiconductor memory device with a redundant memory cell wherein the address of the replaced faulty memory cells can be known, are disclosed in Japanese Unexamined Patent Publications (Kokai) No. 59-217300 (filed December 7, 1984) and No. 60-151899 (filed August 9, 1985). US-A-4 567 580 discloses a semiconductor memory device in accordance with the preamble of accompanying claim 1. In this device, a faulty memory cell is replaced by permanently disconnecting, using a fuse, the whole row of cells including the faulty memory cell. It is therefore impossible to access the faulty memory cell by means of a forced redundant release technique. EP-A-0 052 481 discloses a semiconductor memory device with redundant cells, having a ROM for storing addresses of replaced faulty cells and a comparator for comparing stored addresses with incoming addresses. It is also disclosed that a ROM can be used as a flag register for recording the use or non-use of redundant cells. According to the present invention, there is provided a semiconductor memory device, comprising:- a plurality of memory cells; a decoder, connected to said memory cells, for selecting the memory cell corresponding to an accessing address; a redundant memory cell to be selected in place of a faulty memory cell of the memory cells; a redundant decoder, connected to said redundant memory cell, for selecting the redundant memory cell; and a forced release circuit, operable to release the replacement of the faulty memory cell by the redundant memory cell, in response to application of a forced redundant release signal, when the accessing address coincides with the address of the faulty memory cell; characterised in that said device further comprises:- a first nonvolatile memory for memorising an address of the faulty memory cell; a second nonvolatile memory for memorising information indicating the use or nonuse of the redundant memory cell; and a comparator means, connected to said first nonvolatile memory, for comparing the accessing address with the address of the faulty memory cell stored in the first nonvolatile memory and for making the redundant decoder active and making the decoder inactive when the accessing address coincides with the address of the faulty memory cell; and in that said forced release circuit is connected to said comparator means, is operable to restore access to said faulty memory cell, and is operable to make the decoder, which was replaced with the redundant decoder, active in response to said forced redundant release signal, thereby restoring access to said faulty memory cell, whereby said faulty memory cell can be selected and accessed for testing purposes. An embodiment of the present invention may provide a semiconductor memory device having a redundant memory cell wherein, even after replacement of a faulty memory cell with the redundant memory cell, the address of the faulty memory cell, and changes with the lapse of time in the faulty memory cells, can be determined, and thus, an improvement can be made to the production processes. In such a memory device having a redundant function, if the release of the replacement of the redundant memory cell is carried out, the states of the faulty memory cells can be easily determined, as follows. Namely, if the memory is read out in the condition where the redundant function is released and an abnormality is not found in the read out data, the memory cell at that address is normal and a redundant memory cell has not been used. If a bit error is found in the read out data, the memory cell at that address is faulty and the cells connected to the word line or the bit line at that address should be replaced with a word line or bit line of redundant memory cells. Reference is made, by way of example, to the accompanying drawings in which:- Figure 1 is a block circuit diagram of a previously proposed semiconductor memory device with a redundant cell array; Fig. 2 is a block circuit diagram of a semiconductor memory device with a redundant cell array according to a first embodiment of the present invention; Fig. 3 is a block circuit diagram of a semiconductor memory device with a redundant cell array according to a second embodiment of the present invention; Fig. 4 is a circuit diagram of a forced release circuit for a redundant cell array in the device shown in Fig. 2; Fig. 5 is a circuit diagram showing a forced release circuit for a redundant cell array in the device shown in Fig. 3; Fig. 6 is a circuit diagram of a circuit for obtaining a forced redundant release signal in the devices shown in Figs. 4 and 5. Prior to the explanation of embodiments of the present invention, a previously proposed semiconductor memory device with a redundant cell array is explained with reference to Fig. 1. This device comprises a normal memory cell array 10 in which memory cells 41 are arranged, a redundant (spare) cell array 12 in which redundant memory cells 42 are arranged, appended to the normal memory cell array 10, a decoder 14 for the cell array 10, a redundant decoder 16 for the redundant cell array 12, a comparator 18, a redundant address ROM 20 (first nonvolatile memory) for memorizing a redundant (faulty) address, and a redundant use/nonuse ROM 22 (second nonvolatile memory) for memorizing the redundant use or nonuse. The replacement of a faulty memory cell with a redundant memory cell is performed in units of a word line (WL) or a bit line (BL) at a time , or a plurality of word lines or bit lines (word line block or bit line block). In this example, the replacement block is a word line block, and therefore, the decoders 14 and 16 are word decoders. In general, the redundant cell array comprises a plurality of word lines and memory cells provided at each cross point of the plurality of word lines and bit lines. In Fig. 1, only one word line is shown, for simplicity. Addresses of the word lines including the faulty memory cells are written into the ROM 20. An address signal applied to the decoder 14 and the comparator 18 is the word line address, which is a portion of the address signal of a plurality of bits for a memory access. The ROM 22 outputs, for example, an H (high) level, if the redundant memory cell is used, and an L (low) level if the redundant memory cell is not used. The comparator 18 generates the output signal S₁ which is, for example, L level, when the address accessing the memory does not coincide with the address stored in the ROM 20. The signal S₁ , being L level, is applied to the redundant decoder 16 and makes the redundant decoder, 16 inactive. Also, the signal S₁ is inverted through an inverter 24, and the inverted signal is applied to,and makes active, the active decoder 14 . Then, the decoder 14 selects the word line of the cell array 10 corresponding to the applied address signal, and thus the memory device operates as usual. On the other hand, the comparator 18 makes the output signal S₁ H level if the address signal coincides with the stored address of the ROM 20 and if an H level signal showing the redundant use is applied to the comparator 18 from the ROM 22. As a result, the decoder 14 is made inactive, and the redundant decoder 16 is made active, and the selection of the word line of the redundant cell array is carried out in response to the applied address signal. If there is only one word line in the redundant cell array, the redundant decoder 16 is not always necessary : the word line selection may be carried out to apply the H level signal S₁ directly or through a word driver. Of course, in this case, only the faulty memory cells belonging to one word line can be replaced, and the address stored in the ROM 20 is a word address of the faulty memory cell. Where a number N of word lines are provided in the redundant cell array, faulty memory cells corresponding to N word lines can be replaced. In this case, the maximum number of faulty addresses stored in the ROM 20 are N word addresses. Where the redundant cell array provides N number of word lines and all of the lines are used, the operation is the same as described above. In this case, the coincidence of the addresses occurs N times and the switching by the comparator 18 is carried out at each coincidence. When the replacement of N word lines with the redundant cell array is carried out, the output S₁ of the comparator 18 becomes N lines in Fig. 1. The number of inverters 24 is increased accordingly. Assuming that the faulty memory cells in the normal cell array replaced by the redundant memory cells are on a sequential plurality of word lines, the number of address bits stored in the ROM 20 can be decreased. For example, assuming that N is 4 and one block consisting of four sequential lines is replaced with that of the cell array 10, 2 bits of the lower figure (address) can be omitted and the comparator 18 need compare only the upper remaining bits. A first embodiment of this invention is explained with reference to Fig. 2. In this device a forced release circuit 31 for the redundant function is added between the redundant use/nonuse ROM 22 (second nonvolatile memory) and the comparator 18. In use, a forced redundant release signal S₃ is applied to the circuit 31. The remaining constitution and operations are the same as for the device shown in Fig. 1. To release (that is, cancel or bypass) the redundant function, the output signal S₁ of the comparator 18 in Fig. 1 must be always at L level. The forced release circuit 31 for releasing the redundant function is shown in Fig. 4. The output S₂ of the redundant use/nonuse ROM 22 is at H level in the redundant use and at L level in the redundant nonuse in this example, and therefore, an inverted signal of the signal S₂ through an inverter 25 is applied to one input of a NOR gate which comprises p channel MOS transistors Q₁ and Q₂ and n channel MOS transistors Q₃ and Q₄. The forced redundant release signal S₃ is applied to the other input of the NOR gate. The signal S₃ is set at H level to release the redundant function. The output S₄ of the NOR gate is applied to the comparator 18. In this circuit, the transistors Q₁ , Q₂ , and Q₄ are connected in series between a V CC terminal as a positive side of the power source and V EE terminal as a negative side of the source, and the inverted signal of the signal S₂ is applied to the gate of the transistor Q₁ and the signal S₃ is applied to the gate of the transistors Q₂ and Q₄. The transistor Q₃ is connected between the V EE terminal and the output (S₄) terminal and, further, the inverted signal of the signal S₂ is applied to the gate of the transistor Q₃. In normal operation, when the redundant function is not to be released, the signal S₃ is at L level, and when the device is in the redundant use state, the signal S₂ is at H level. Therefore, in this state, the signal S₄ is at H level. If the device is in the redundant nonuse state, the signal S₂ is at L level, and thus the signal S₄ is at L level. On the other hand, when the redundant function is released, the signal S₃ is at H level, and the signal S₄ is at L level. Since the redundant memory cells are then not used, if a word line address having a faulty memory cell is accessed, the faulty word line in the cell array 10 is selected, and the data stored in the memory cell on the word line is read out. This data has an error at the portion of the faulty memory cell. The error is found by comparison with the correct data. The address of the faulty memory cell can be determined by the error bit of the data. Figure 3 shows a semiconductor memory device with a redundant cell array according to a second embodiment of this invention. In this device, the output S₁ of the comparator 18 is supplied to a forced release circuit 32, and the output S₆ of the forced release circuit 32 is supplied to the decoder 14 through the inverter 24 and to the redundant decoder 16. The forced redundant release signal S₃ is applied to the circuit 32. The remaining constitution and operations are the same as for the device shown in Fig. 1. The circuit diagram of the circuit 32 is shown in Fig. 5. The output signal S₁ of the comparator 18 is inverted through an inverter 26 and the inverted signal S₅ is applied to one input of an NOR gate which comprises p channel MOS transistors Q₅ and Q₆ and n channel MOS transistors Q₇ and Q₈. The forced redundant release signal S₃ is applied to the other input of the NOR gate. Namely, the transistors Q₅ , Q₆ , and Q₈ are connected in series between the V CC terminal and V EE terminal. The signal S₅ is applied to the gate of the transistor Q₅ and the signal S₃ is applied to the gate of the transistors Q₆ and Q₈. The transistor Q₇ is connected between the V EE terminal and the output (S₆) terminal, and the signal S₅ is applied to the gate of the transistor Q₇. In this circuit, the signal S₃ is at L level in an ordinary state in which the faulty memory cells are replaced with the redundant cells. In addition, when the address for memory accessing coincides with the address stored in the ROM 20 (first nonvolatile memory), the signal S₁ is at H level, and thus the signal S₅ is at L level and the signal S₆ becomes H level. The signal S₆ is applied to and makes active the redundant decoder 16, and is further applied to and makes the decoder 14 inactive, through the inverter 24. When the address for memory access does not coincide with the address stored in the ROM 20, the signal S₁ is at L level; thus the signal S₅ is at H level and the signal S₆ is at L level, the decoder 14 is active, and the redundant decoder 16 is inactive. When (in the released state) the faulty cells are not replaced with the redundant memory cells, the signal S₃ is at H level, thus the signal S₆ is always at L level: the decoder 14 is active, and the redundant decoder 16 is inactive. Figure 6 shows an example of a circuit for generating the forced redundant release signal S₃. Transistors Q₁₁ and Q₁₂ , Q₁₃ and Q₁₄ , and Q₁₅ and Q₁₆ form CMOS inverters, and are connected in cascade. A first stage of the CMOS inverters is connected to an input signal S₇ through a p channel MOS transistor Q₁₀ , the gate and drain of which are interconnected. In this circuit, at an ordinary time, when the voltage of the signal S₇ is less than that of V CC , the output of the first stage inverter is at L level, the output of the second stage inverter is at H level, and the output of the third stage inverter S₃ is at L level. When the replacement of the faulty memory cells is to be released, the voltage of the signal S₇ is made to be sufficiently higher than that of V CC . Making an attempt as above-mentioned, the output level of the first stage inverter increases, the second stage inverter outputs an L level and receives an H level at the input thereof. Therefore, the output S₃ of the third stage inverter becomes at H level, and thus the forced redundant release signal S₃ is obtained. The input signal S₇ may be applied to an appropriate terminal pin of a package of the device. In this case, it is preferable to use one of the terminal pins other than those for address signals, as the memory access would be thereby obstructed. As explained above, in embodiments of the invention, the replacement of faulty memory cells with redundant memory cells can be released, for example, by an application of a high voltage to one of the terminal pins of the device, and the address of a faulty memory cell can be determined by reading out memorized data in this state, and therefore, this is highly advantageous in clarifying the cause of the faulty memory cell or the like. Namely, a detailed aspect of the memory cells on the associated word line is obtained. For example, the causes of the faults in the cells, i.e., writing or reading malfunction, fluctuation of the power source voltage, malfunction during high speed operation, or an increase of the faulty portion of the memory array over a period of time, etc., can be determined.

Claims: A semiconductor memory device, comprising:- a plurality of memory cells (10); a decoder (14), connected to said memory cells, for selecting the memory cell corresponding to an accessing address; a redundant memory cell (12) to be selected in place of a faulty memory cell of the memory cells; a redundant decoder (16), connected to said redundant memory cell, for selecting the redundant memory cell; and a forced release circuit (31, 32), operable to release the replacement of the faulty memory cell by the redundant memory cell (12), in response to application of a forced redundant release signal (S₃), when the accessing address coincides with the address of the faulty memory cell; characterised in that said device further comprises:- a first nonvolatile memory (20) for memorising an address of the faulty memory cell; a second nonvolatile memory (22) for memorising information indicating the use or nonuse of the redundant memory cell (12); and a comparator means (18), connected to said first nonvolatile memory (20), for comparing the accessing address with the address of the faulty memory cell stored in the first nonvolatile memory (20) and for making the redundant decoder (16) active and making the decoder (14) inactive when the accessing address coincides with the address of the faulty memory cell; and in that said forced release circuit (31, 32) is connected to said comparator means (18), is operable to restore access to said faulty memory cell, and is operable to make the decoder (14), which was replaced with the redundant decoder (16), active in response to said forced redundant release signal (S₃), thereby restoring access to said faulty memory cell, whereby said faulty memory cell can be selected and accessed for testing purposes. A semiconductor memory device as set forth in claim 1, wherein said forced release circuit (31) is connected to an output of the second nonvolatile memory (22), and wherein an output (S₄) of said forced release circuit is connected to the comparator means (18). A semiconductor memory device as set forth in claim 1, wherein said forced release circuit (32) is connected to the output of the comparator means (18), and wherein the output (S₆) of said forced release circuit is connected to the decoder (14) and the redundant decoder (16). A semiconductor memory device as set forth in claim 2, wherein said forced release circuit (31) is constituted by an inverter and a NOR gate, and wherein the output (S₂) of the second nonvolatile memory (22) is connected to the input of the inverter, the output of the inverter is connected to one input of the NOR gate, the forced redundant release signal (S₃) is connected to the other input of the NOR gate, and the output (S₄) of the NOR gate is connected to the comparator means (18). A semiconductor memory device as set forth in claim 3, wherein said forced release circuit (32) is constituted by an inverter and a NOR gate and the output (S₁) of the comparator means (18) is connected to the input of the inverter, the output of the inverter is connected to one input of the NOR gate, the forced redundant release signal (S₃) is connected to the other input of the NOR gate, and the output (S₆) of the NOR gate is connected to the decoder (14) and the redundant decoder (16). A semiconductor memory device as set forth in any preceding claim, wherein said forced redundant release signal (S₃) is obtained by applying a voltage different from a normal voltage to one terminal of said semiconductor memory device.

Inventors: TAKEUCHI, ATSUSHI
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: G06F11
IPC codes: G11C29, G11C11, G11C17