Redundancy circuitry for a semiconductor memory device

Abstract: A redundancy circuitry for a semiconductor memory device comprising a matrix of memory elements, comprises a plurality of programmable non-volatile memory registers (1), which are programmable to store addresses of defective memory elements which must be replaced by redundancy memory elements; the redundancy circuitry comprises combinatorial circuit means (3) supplied by address signals (ADD) and supplying the non-volatile registers (1) with an inhibition signal (DIS) for inhibiting the selection of redundancy memory elements when a memory element of the matrix is addressed whose address coincides with the address stored in a non-programmed memory register (1).

Description: The present invention relates to a redundancy circuitry for a semiconductor memory device. In the manufacture of semiconductor memories defects are frequently encountered that afflict a limited number of memory elements in the memory matrix. The reason for the high probability of defects of this type resides in that in a semiconductor memory device the greatest part of the chip area is occupied by the memory matrix; moreover, it is in the memory matrix, and not in the peripheral circuitry, that the manufacturing process characteristics are usually pushed to limits. In order to avoid that the presence of a limited number of defective memory elements on many millions forces the rejection of the entire chip, and therefore to increase the manufacturing process yield, the technique is known of providing for the manufacture of a certain number of additional memory elements, commonly called "redundancy memory elements", to be used as a replacement of those elements that, during testing of the memory device, prove defective; the selection circuits, with which the integrated component must necessarily be provided, and which allow the abovementioned functional replacement of a defective memory element with a redundancy memory element are indicated as a whole with the name of "redundancy circuitry", while the set of redundancy memory elements and circuitry is defined for short as "redundancy". The redundancy circuitry comprises programmable non-volatile memory registers suitable to store those address configurations corresponding to the defective memory elements such registers are programmed once and for all during the memory device testing, and must retain the information stored therein even in absence of the power supply. In practical implementations of redundancy, both rows ("word lines") and columns ("bit lines") of redundancy memory elements are provided in the memory matrix; each redundancy word line or bit line is associated to a respective non-volatile memory register, wherein the address of a defective word line or bit line is stored so that, whenever the defective word line or bit line is addressed, the corresponding redundancy word line or bit line is selected. This implies that each non-volatile memory register must be made up of a number of programmable memory cells at least equal to the number of bits in the row address bus, if the register is associated to a redundancy word line, or in the column address bus, if the register is instead associated to a redundancy bit line. Each memory cell of a memory register is therefore dedicated to store the logical state of a particular address bit, in the row or column address configurations, corresponding to a defective word line or bit line, and comprises at least one programmable memory element, a circuit for programming the memory element, a circuit for reading the information stored in the memory element and a circuit for comparing said information with the current logical state of the address bit associated to the memory cell. If each non-volatile memory register is made up of a number of programmable memory cells exactly equal to the number of bits in the row address bus or in the column address bus, an ambiguous selection can take place. Since in fact even unprogrammed non-volatile memory registers, associated to unused redundancy word lines or bit lines, store that particular address configuration corresponding to the unprogrammed condition of the memory cells, and since said particular address configuration belongs to the set of all the possible address configurations for the memory device, when a non-defective word line or bit line is addressed whose address coincides with the logical configuration of the memory cells in an unprogrammed memory register, the redundancy word line or bit line associated to said unprogrammed register will be selected instead of the non-defective word line or bit line. The situation is even worse if in a memory device two or more redundancy word lines or bit lines are not used: since in fact the unprogrammed condition is the same for all the memory cells of the non-volatile memory registers, addressing the non-defective word line or bit line whose address coincides with the configuration of the unprogrammed memory cells would cause said two or more redundancy word lines or bit lines to be selected simultaneously. To prevent such unacceptable ambiguous or even simultaneous selection, each non-volatile memory register is provided with an additional programmable memory cell (called "guard memory cell" or "control memory cell") which allows the selection of the associated redundancy word line or bit line only in the case it is programmed. This causes a significant increase in the overall chip area. In view of the state of art described, the object of the present invention is to realize a redundancy circuitry wherein it is not necessary to provide each non-volatile memory register with a respective guard memory cell. According to the present invention, as defined in detail in claims, such object is attained by means of a redundancy circuitry for a semiconductor memory device comprising a main matrix of memory elements and a plurality of redundancy memory elements for replacing defective main memory elements, said redundancy circuitry comprising a plurality of programmable non-volatile memory registers, which are programmable to store addresses of defective main memory elements which must be replaced by said redundancy memory elements, characterized by comprising combinatorial circuit means supplied by address signals for generating and supplying to the said non-volatile registers an inhibition signal for inhibiting the selection of redundancy memory elements when a memory element of the said matrix of main memory elements is addressed whose address coincides with the address stored in a non-programmed memory register. Thanks to the fact that the logical state stored in the unprogrammed memory elements is well known, it is possible, by means of a redundancy circuitry according to the invention, to prevent unused redundancy word lines or bit lines from being selected when addressing a non-defective word line or bit line whose address configuration coincides with that stored in the non-programmed non-volatile memory registers by just supplying the redundancy circuitry with an inhibition signal when said address configuration is supplied to the memory device. No guard memory cells are necessary in the non-volatile memory registers, and the chip area is thus reduced. The features of the present invention will be made more evident by the following detailed description of a particular embodiment, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 is an electrical schematic diagram of a part of a redundancy circuitry according to the invention; Figure 2 is a schematic block diagram of a memory cell of a non-volatile memory register. A redundancy circuitry according to the invention is integrated in a memory device chip and comprises a plurality of non-volatile memory registers 1, each associated to a respective redundancy word line or bit line (not shown in the drawings). The plurality of non-volatile memory registers 1 can be divided into two sets, a first set comprising all the non-volatile registers 1, which are associated to redundancy word lines, and a second set comprising all the non-volatile registers which are instead associated to redundancy bit lines. In Figure 1, two non-volatile registers 1 are shown, which belong to a same set comprising i+1 registers 1, that in the following is assumed to be the first set. As regards the second set, the following description is still valid, provided that the terms "row" and "word line" are replaced by the terms "column" and "bit line". Each non-volatile memory register 1 is supplied with row address signals A0-An, which taken as a whole represent a row address bus ADD; the row address bus ADD also supplies row decoding circuits (not shown) for the selection of a particular word line in the memory matrix. Each non-volatile register 1 comprises a plurality of programmable non-volatile memory cells MC0-MCn; each of said cells MCO-MCn is supplied with one of the row address signals A0-An and comprises, with reference to Figure 2, in a per-se known way, at least one programmable non-volatile memory element 6, a first circuit 5 for programming said memory element 6, a second circuit 7 for reading the information stored in the non-volatile memory element 6 and a third circuit 8 for comparing said information with the current logical state of the respective row address signal A0-An (Fig. 2). All the memory cells MC0-MCn of a non-volatile memory register 1 are supplied with a signal PGM supplied by a control circuitry 4 of the memory device to enable the programming of the memory element 6; different non-volatile memory registers 1 are supplied with different signals PGM, so that one register 1 is programmable at a time. Each memory cell MC0-MCn has an output signal CMP0-CMPn which is activated whenever the current logical state of the respective row address signal A0-An coincides with the logical state stored in the non-volatile memory element 6 of the cell MC0-MCn. Each non-volatile memory register 1 further comprises a redundancy word line selection circuit 2 which is supplied with all the signals CMP0-CMPn and generates a signal RS0-RSi used to select one redundancy word line and to deselect a defective word line whose address coincides with the address configuration stored in the non-volatile register 1. The redundancy circuitry also comprises a combinatorial circuit 3 supplied with the row address bus ADD and generating a signal DIS which is individually supplied to all the redundancy word line selection circuits 2. At the end of the manufacturing process of the memory device, all the programmable non-volatile memory elements 6 included in the memory cells MC0-MCn of all the non-volatile memory registers 1 are in a well known and defined logical state, i.e. in the virgin or non-programmed state. In the case of UV EPROM devices, this is assured by an exposure to UV light during the final steps of the manufacturing process; in the case of EEPROM or Flash EEPROM devices, both UV light exposure and electrical erasure can be performed to guarantee that all the non-volatile memory elements 6 of the non-volatile registers 1 are in the same starting condition. During the memory device testing, the address configurations corresponding to defective word lines or bit lines are programmed into respective non-volatile memory registers 1; each time a defective word line or bit line is encountered, the testing machine puts the memory device in a condition such that the control circuitry 4 activates one signal PGM, to enable the programming of the memory cells MC0-MCn of a given non-volatile memory register 1; in this way any successive attempt to address said defective word lines or bit lines will automatically cause redundancy word lines or bit lines to be addressed. At the end of this phase, it is possible that some redundancy word lines or bit lines are left unused, and the associated non-volatile memory registers 1 are therefore left in their unprogrammed state. The combinatorial circuit 3 is designed to recognize if all the address signals ADD currently supplied to the memory device are in a logical state coincident with the well known virgin or non-programmed state of the programmable memory elements 6 in the memory cells MC0-MCn of non-volatile registers 1, and if this happens it activates the signal DIS which inhibits the generation of the redundancy word line or bit line selection signals RS0-RSi. In this way even if no guard memory cells are provided in the non-volatile registers 1, no ambiguous selections of redundancy word lines or bit lines take place when the memory device is supplied with an address coincident with that stored in non-programmed registers 1; this also prevents that, when two or more redundancy word lines or bit lines are not used, the addressing of the non-defective word line or bit line having an address configuration coincident with that stored in the respective non programmed non-volatile registers 1 causes said two or more unused redundancy word line or bit line to be simultaneously selected. Obviously, this solution prevents defective word lines or bit lines having an address configuration coincident with that stored in non-programmed non-volatile registers 1 from being repaired, since redundancy word lines or bit lines selection is inhibited whenever said address configuration is encountered. This however does not constitute a great drawback, since the probability of having defective memory elements in such word lines or bit lines is small. This means that even if memory device chips having defects in such word lines or bit lines are to be rejected, the overall process yield is nevertheless improved, because thanks to the reduction in the chip area a greater number of chips are obtained in each semiconductor wafer. The combinatorial circuit 3 could be a part of the row or column address decoding circuits, the signal DIS being in this case one of the word line or bit line selection signals. It is known that said address decoding circuits are made up of a plurality of identical blocks, each decoding a particular configuration of the address signals. Since in the practice the address configuration stored in non-programmed registers 1 is represented by an "all 0" or by an "all 1" configuration, and since the blocks performing the decoding of such two address configurations are physically located at the periphery of the address decoding circuits, it is simple, from the point of view of the physical layout of the device, to extract the signal DIS from the address decoding circuits and to supply it to the redundancy circuitry.

Claims: Redundancy circuitry for a semiconductor memory device comprising a matrix of main memory elements and a plurality of redundancy memory elements for replacing defective main memory elements, said redundancy circuitry comprising a plurality of programmable non-volatile memory registers (1), which are programmable to store addresses of defective main memory elements which must be replaced by said redundancy memory elements, characterized by comprising combinatorial circuit means (3) supplied by address signals (ADD) for generating and supplying to the said non-volatile registers (1) an inhibition signal (DIS) for inhibiting the selection of redundancy memory elements when a memory element of the said matrix of main memory elements is addressed whose address coincides with the address stored in a non-programmed memory register (1). Redundancy circuitry according to claim 1, said matrix of main memory elements comprising rows and columns of memory elements and redundancy rows and redundancy columns of redundancy memory elements, characterized in that said plurality of programmable non-volatile memory registers (1) is formed by a first set of programmable non-volatile memory registers (1) each associated to one of said redundancy rows, and by a second set of programmable non-volatile memory registers (1) each associated to one of said redundancy columns, each set of programmable non-volatile memory registers (1) being provided with respective combinatorial circuit means (3) respectively supplied with row address signals and column address signals for inhibiting the selection of the redundancy rows and of the redundancy columns respectively when a matrix row or a matrix column is addressed whose address coincides with the address stored in a non-programmed memory register (1) of the first set or of the second set. Redundancy circuitry according to claim 2, characterized in that each programmable non-volatile memory register (1) of said first set and second set comprises a number of programmable memory cells (MC0-MCn) respectively equal to the number of row address signals and to the number of column address signals, each memory cell (MC0-MCn) being supplied with one (A0-An) address signal and generating an output signal (CMP0-CMPn) when the logical state of said address signal correspond to the logical state stored in the memory cell (MC0-MCn), each non-volatile memory register (1) further comprising selection circuits means (2) supplied with the output signals (CMP0-CMPn) of the memory cells (MC0-MCn) for generating a selection signal (RS0-RSi) for selecting the respective redundancy row or redundancy column and inhibiting the selection of the defective row or column whose address is stored in the memory cells (MC0-MCn) of the non-volatile register (1), said selection circuits means (2) being also supplied with an inhibition signal (DIS) supplied by the respective combinatorial circuit means (3) to prevent said selection signal (RS0-RSi) from being generated. Redundancy circuitry according to claim 3, characterized in that each memory cell (MC0-MCn) comprises at least one programmable non-volatile memory element (6), first circuit means (5) for programming said non-volatile memory element (6), second circuit means (7) for reading an information stored in said non-volatile memory element (6), and third circuit means (8) for comparing said information with the logical state of the address signal (A0-An) supplied to the memory cell (MC0-MCn) and generating said output signal (CMP0-CMPn). The redundancy circuitry according to claim 2, characterized in that said combinatorial circuit means (3) provided to the first set and to the second set of non-volatile memory registers (1) are also parts of a row decoding circuit and of a column decoding circuit, respectively, supplied with row address signals and with column address signals to select the addressed row and column of the memory matrix.

Inventors: GOLLA CARLA MARIA, PASCUCCI LUIGI, GOLLA, CARLA MARIA, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: G06F11
IPC codes: H01L21, G11C29, H01L27