STATIC SEMICONDUCTOR MEMORY DEVICE INCORPORATING REDUNDANCY MEMORY CELLS

Abstract: In a static semiconductor memory device incorporating redundancy memory cells (C R0 , C R1 , ...), a connecting/discon­ necting circuit is linked between a power supply terminal (V CC ) and one of the bit lines (B₀, B ₀, ...), thereby reducing or cutting off a current flowing through a defective memory cell.

Description: The present invention relates to a static semiconductor memory device incorporating redundancy memory cells. In a semiconductor memory device, a large number of memory cells are arranged along rows and columns. The density of defects generated in such a semiconductor memory device during the manufacture thereof is relatively independent of the integration density of the device. Rather, it derives from the semiconductor manufacturing technology. In general, the higher the integration density of the device, the greater the ratio of normal memory cells to defective memory cells. This is one of the advantages of increasing the integration density of a semiconductor memory device. Even if a device includes only one defective memory cell, however, the device cannot be operated normally and, therefore, must be scrapped. As a result, despite the lower ratio of defective memory cells, greater integration density means reduced manufacturing yield. To overcome the problem of defective memory cells, use is made of redundancy (redundant) memory cells. When a defective memory cell is detected, a redundancy memory cell row or column is selected instead of the memory cell row or column including the defective memory cell. In general, one or two redundancy memory cell rows or columns are usually provided. A problem in the prior art, however, has been that when the defective memory cell has a direct current (DC) defect, i.e., when the defective memory cell is short-circuited, the DC defect itself is not resolved even when a redundancy memory cell row or column is selected. An article by Uchida et al in IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 5, October 1982, pp 804-809, discloses a low-power resistive load 64 k bit CMOS RAM having two spare rows of memory cells. Fuse elements are programmed so that a spare decoder selects one of the spare rows instead of a failure row. However, there is no disclosure of the above-mentioned problem of a DC defect, and no measure is taken to resolve such a DC defect. An article by Ochii et al in the same journal, pp 798-803, discloses an ultralow power 64 k bit CMOS RAM having spare rows and columns of memory cells. The spare rows or columns are again selected by programming fuses, and in addition, fuses are provided for isolating a failure row electrically by cutting the corresponding power line. However, this necessitates replacing both a failure row and its neighbour by a pair of spare rows so as to complete the power isolation. This is wasteful of spare cell capacity. According to the present invention, there is provided a static semiconductor memory device comprising: a power supply means; a memory cell array having a plurality of normal memory cell rows or columns each comprising a plurality of memory cells; at least one redundancy (redundant) memory cell row or column comprising a plurality of redundancy memory cells, said redundancy memory cell row or column being able to replace one of said normal cell rows or columns including a defective cell; a plurality of selecting line means, each connected to one of said normal memory cell rows or columns, or to a said redundancy memory cell row or column; and connecting/disconnecting means to/from said redundancy memory cell row or column each linked between the corresponding selecting line means and said power supply means; and characterised by: a plurality of connecting/disconnecting means, each linked between one of said selecting line means and said power supply means and operable to temporarily electrically disconnect said selecting line means from said power supply means for at least a non-selected mode of said device, during normal operation thereof; wherein said connecting/disconnecting means comprise transistors linked between said selecting line means and said power supply means, each of said transistors being an enhancement MIS transistor having a drain connected to said power supply means, a source connected to one of said selecting line means, and a gate, the potential of said gate being controlled by pulse generating means comprising an inverter for inverting a chip select signal, so as to turn on and off said MIS transistor. In an embodiment of the present invention, the potential of a clock signal is applied to loads linked between bit line pairs and a power supply at least during a non-selected state (stand-by state) so as to cut off currents flowing through the loads, thereby reducing the load current of bit lines for all memory cells, including a defective memory cell. That is, even when the defective memory cell has a DC defect, power supply currents due to the DC defect are suppressed thereby substantially resolving the DC defect. Reference is made, by way of example, to the accompanying drawings, wherein: Fig. 1 is a block diagram of a prior art static semiconductor memory device; Fig. 2 is a circuit diagram of the memory cell of Fig. 1; Fig. 3 is a block diagram of an embodiment of the static semiconductor memory device according to the present invention; Figs. 4A and 4B are timing diagrams for explaining the clock signal ø in Fig. 3; Fig. 5 is a logic diagram of a circuit for generating the clock signal ø of Fig. 4B; Figs. 6A, 6B, and 6C are other timing diagrams for explaining the clock signal ø in Fig. 3; Fig. 7 is a block circuit diagram of a circuit for generating the clock signal ø in Fig. 6C; Fig. 8 is a logic circuit diagram of the pulse generating circuit PG i of Fig. 7; and Figs. 9A through 9H are timing diagrams of the signals appearing in the circuit of Fig. 8. Figs. 10 and 11 are block diagrams of semiconductor memory devices useful for explaining the present invention. In Fig. 1, which illustrates a prior art static semiconductor memory device, normal static memory cells C ij (i,j = 0, 1, ..., n-1) are arranged in an n-row n-column matrix. Each memory cell is connected to one word line, a hold line and a pair of bit lines. For example, a memory cell C₀₀ is connected to a word line W₀ , to a hold line H₀ , and to bit lines B₀ and B ₀. In addition, a series of memory cells C 0R , C 1R , ..., C n-1,R are arranged along the column direction. These memory cells C 0R , C 1R , ..., C n-1,R serve as redundancy memory cells. Selection of the word lines W₀ , W₁ , ..., W n-1 is performed by the row-selection signals X₀ , X₁ , ..., X n-1 of row address decoders RD. That is, the row address decoders RD receive address signals A₀ , A ₀ , A₁ , A ₁ , ..., A ℓ-1 , A ℓ-1 from row address buffers (not shown). Accordingly, one of the outputs X₀ , X₁ , ..., X n-1 (n = 2 ℓ ) of the decoders RD becomes high, whereby one of the word lines is selected. On the other hand, the hold lines H₀ , H₁ , ..., H n-1 are connected to earth, which serves as one of the power supplies. The bit lines B₀ , B ₀ , B₁ , B ₁ , ..., B n-1 , B n-1 , B R , and B R are connected via load transistors Q L0 , Q L0 , Q L1 , Q L1 , ..., Q LR , Q LR to a power supply V CC . The load transistors are, for example, enhancement metal-insulator-semiconductor (MIS) transistors. Reference CD₁ designates column decoders, and CD₂ designates a decoder for selecting the redundancy memory cells. The decoder CD₂ is comprised of a resistor R, fuses F X , F₀ , F ₀ , F₁ , F ₁ , ..., F n-1 , F n-1 , and transistors Q X0 , Q X1 , ..., Q Xn-1 . Note, SA is a sense amplifier, OB is an output buffer, D out is output data, and A₀', A₀ ', A₁', A ₁', ..., A ℓ-1 , and A ℓ-1 ' are column address signals. Prior to testing of the device of Fig. 1, all the fuses of the decoder CD₂ are in a connected state. Therefore, the gate potentials of the transistors Q X0 , Q X1 , ..., Q X,n-1 are low, and accordingly, these transistors are in an off state. In this state, a test is performed upon the memory cells C ij (i,j = 0, 1, ..., n-1). If the memory cell C₀₀ is determined to be defective, the fuse F X is melted so as to turn on the transistors Q X0 , Q X1 , ..., Q Xn-1 . In addition, the fuses F₀ , F₁' , ..., F n-1 ' are melted. Thus, when the column decoders CD₁ generate a column selection signal Y₀ for selecting the memory cell C₀₀ , the redundancy memory cell C 0R instead of the defective memory cell C₀₀ is actually selected by the fuse F₀'. Note, if no defective memory cell is detected, none of the fuses are melted, and, accordingly, all the fuses are in a connected state. Thus, it is possible to detect (select) a redundancy memory cell instead of a defective memory cell. In the device of Fig. 1, however, when a direct current (DC) defect is generated in the interior of a memory cell, the defect itself is not resolved. For example, as illustrated in Fig. 2, the memory cell C₀₀ is comprised of resistors R₁ and R₂ , driver transistors Q₁ and Q₂ , and transfer transistors Q₃ and Q₄. Therefore, when a DC defect is generated so as to short-circuit the bit line B₀ or B ₀ to the hold line H₀ , a current flows from the power supply V CC via the load transistor Q L and the short-circuited portion to the hold line H₀ even when the redundancy memory cell C 0R is selected instead of the memory cell C₀₀. Therefore, the DC defect itself is not resolved. In Fig. 1, note, the power supply voltage V CC is also applied to the gates of the load transistors Q L0 , Q L0 , Q L1 , Q L1 , ..., Q L,n-1 , Q L,n-1 , Q LR , Q LR . In Fig. 3, which illustrates an embodiment of a first aspect of a static memory device according to the present invention, a clock signal ø is applied to the gates of the transistors Q L0 , Q L0 , Q L1 , Q L1 , ..., Q L,n-1 , Q L,n-1 , Q LR , Q LR . The potential of the clock signal ø is made low at least during a non-selected state, thereby preventing currents from flowing from the power supply V CC to all the memory cells. As shown in Figs. 4A and 4B, the above-mentioned clock signal (pulse signal) ø is obtained as an inversion signal of a chip select signal CS . That is, the clock signal ø is easily obtained by using an inverter G₀ as shown in Fig. 5. As a result, even when the above-mentioned DC defect is generated, no short-circuit current due to the DC defect flows during a non-selected state, since the potential of the clock signal ø is, in this case, low, so that all the load transistors are cut off. Note, a current flowing through the defective portion in the case of the potential of the clock signal ø being high is extremely small as compared with the total power consumption of the semiconductor device. Figures 6A, 6B, and 6C are also timing diagrams for explaining the generation of the clock signal ø of Fig. 3. The clock signal ø as shown in Fig. 6C becomes high for a definite (short) time period only after the chip select signal CS as shown in Fig. 6A is changed or after one ADD of the row address signals A₀ , A₁ , ..., and A ℓ-1 is changed. Therefore, the current flowing from the power supply V CC to the above-mentioned defective portion is of shorter duration than with the signal ø as shown in Fig. 4B. Note, the reason why power supply currents due to the conductive load transistors are supplied to the bit lines after the change of the row address signals A₀ , A₁ , ..., and A ℓ-1 is that, when reverse data opposed to the previous read data is read out from the same bit line pair, the bit line pair must be in a precharging (stand-by) state. The clock signal ø as shown in Fig. 6C can be obtained by a pulse generating circuit PG of Fig. 7. That is, in Fig. 7, the pulse generating circuit PG comprises a plurality of clock pulse generating circuits PG₀ , PG₁ , ..., PG ℓ-1 , and PG ℓ corresponding to each of the row address signals A₀ , A₁ , ..., A ℓ-1 , and the chip select signal CS . Therefore, when one of the signals A₀ , A₁ , ..., A ℓ-1 , and CS is changed, for example, when the signal A₀ is changed, the clock pulse generating circuit PG₀ generates the clock signal ø via an OR circuit OR. That is, when the device of Fig. 3 is changed from a non-selected state to a selected state or vice versa, or when the selected row is changed, the pulse generating circuit PG generates the clock signal ø. In Fig. 8, which is a detailed logic circuit diagram of one of the clock pulse generating circuits PG i of the pulse generating circuit PG of Fig. 7, references G₁ to G₄ are NAND gates, G₅ and G₆ NOR gates, G₇ an OR gate, and C₁ and C₂ capacitors. Operation will be explained below with reference to Figs. 9A to 9H. When the address signal A i (node a) is changed from "1" to "0" as shown in Fig. 9A, the output b of the NAND gate G₁ changes as shown in Fig. 9B and the output d of the NAND gate G₂ is an inverted signal of the signal b which is delayed a little by the capacitor C₁ , as shown in Fig. 9D. As a result, the output f of the NOR gate G₅ is a pulse which is generated by the rise of the address signal A i , as shown in Fig. 9E. Similarly, in the circuit of the NAND gates G₃ and G₄ , the capacitor C₂ , and the NOR gate G₆ , the output g of the NOR gate G₆ is a signal which is generated by the fall of the address signal A i , as shown in Fig. 9G. Therefore, as shown in Fig. 9H the output CP i of the OR gate G₇ , which combines the signal f with the signal g, is a clock signal which is generated by the change of the address signal A₀ , A₁ , ..., A ℓ-1 , or the signal CS . Thus, since the bit line load currents are reduced in duration, the power supply currents flowing through a defective memory cell can be reduced, even when the defective memory cell has a DC defect. Therefore, the DC defect can be substantially resolved. In Fig. 10, which illustrates a semiconductor memory device useful for explaining the present invention, fuses F₁₀, F ₁₀ , F₁₁ , F ₁₁ , ..., F 1,n-1 , F 1,n-1 , F 1R , and F 1R , are added to the elements of Fig. 1. Each fuse is provided between the memory cells and the power supply V CC . Note, each fuse can be provided on the drain side of the load transistor (between drain and V cc )as well as on the source side (between source and bit line) thereof. In either case, the same effect is exhibited. For example, when the memory cell C₀₀ is determined to be defective, in the decoder CD₂ , the fuses F X , F₀ , F₁' , ..., and F n-1 ' are melted in the same way as stated above. In addition, the fuses F₁₀ and F ₁₀ are melted. Therefore, even when the memory cell C₀₀ has a DC defect, no current flows from the power supply V CC via the memory cell C₀₀ to the hold line H₀. This means that the DC defect is completely resolved. In Fig. 11, which illustrates another semiconductor memory device useful for explaining the present invention, fuses F₂₀ , F₂₁ , ..., F 2,n-1 , and F 2R are added to the elements of Fig. 1. This case is equivalent to that wherein each fuse is provided between the memory cells and the power supply V CC . For example, when the memory cell C₀₀ is determined to be defective, in the decoder CD₂ , the fuses F X , F₀ , F₁' , ..., and F n-1 ' are melted in the same way as stated above. In addition, the fuse F₂₀ is melted, thereby producing the same result as in the Fig. 10 device. Thus, since the power supply V CC can be disconnected from a defective memory cell, a DC defect within memory cells is itself resolved. Note that while the above-mentioned embodiments relate to static metal-oxide semiconductor (MOS) memory devices, the invention can be applied in the same way to other memory devices, such as static bipolar transistor memory devices.

Claims: A static semiconductor memory device comprising: a power supply means (V CC ); a memory cell array having a plurality of normal memory cell rows or columns comprising (C ij ; 0 ≦ i, j ≦ n-1) at least one redundancy (redundant) memory cell row or column comprising a plurality of redundancy memory cells (C 0R , C 1R , ..., C n-1,R ), said redundancy memory cell row or column being able to replace one of said normal cell rows or columns including a defective cell; a plurality of selecting line means (B₀, B ₀; B₁, B ₁;..., B n-1 , B n-1 , B R , B R ), each connected to one of said normal memory cell rows or columns, or to a said redundancy memory cell row or column; and connecting/disconnecting means (Q LR , Q LR ) to/from said redundancy memory cell row or column each linked between selecting line means (B R , B R ) and said power supply means (V CC ); and characterised by: a plurality of connecting/disconnecting means (Q L0 , Q L0 ; Q L1 , Q L1 ;...; Q L,n-1 Q L,n-1 ) to/from said normal memory cell rows or columns, each linked between one of said selecting line means (B₀, B ₀;...; B n-1 , B n-1 ) and said power supply means (V CC ) and operable to temporarily electrically disconnect said selecting line means (B₀, B ₀;...; B n-1 , B n-1 ) from said power supply means (V CC ) for at least a non-selected mode of said static semidonductor memory device, during normal operation thereof; wherein said connecting/disconnecting means (Q L0 , Q L0 ;...; Q LN-1 , Q LN-1 ) comprise transistors linked between said selecting line means (B₀, B ₀;...B n-1 , B n-1 ) and said power supply means (V CC ), each of said transistors being an enhancement MIS transistor having a drain connected to said power supply means (V CC ) a source connected to one of said selecting line means, and a gate, the potential of said gate being controlled by pulse (ø) generating circuits (PG i ; i=0...l) corresponding to each of the row address signals (A i ; i=0...l-1) and an inverted inverting a chip select signal ( CS ), so as to turn on and off said MIS transistor. A device as set forth in claim 1, further comprising a plurality of additional selecting line means (W₀, W₁,...W n-1 ), connected to said memory cells and said redundancy memory cells, said additional selecting line means being perpendicular to said selecting line means (B₀, B ₀; B₁, B ₁;...; B n-1 , B n-1 ; B R , B R ). A device as set forth in claim 2, wherein said connecting/disconnecting means (Q L0 , Q L0 ;...;Q L,n-1 , Q L,n-1 ) electrically connects said selecting line means (B₀, B ₀;...; B n-1 , B n-1 )to said power supply means (V CC ) for a definite time period after said static semiconductor memory device is changed from a non-selected mode to a selected mode or after the potential of one of said additional selecting line means (W₀, W₁,...; W n-1 ) is changed. A device as set forth in claim 1, 2 or 3, wherein said connecting/disconnecting means Q L0 , Q L0 ;...;Q L,n-1 , Q L,n-1; Q LR , Q LR ) further comprises: fuse means (F₁₀, F ₁₀,..., F 1,n-1 , F 1,n-1 , F 1R , F 1R ) linked between said selecting line means and said transistors; and a melting means for melting said fuse means connectable to one of said selecting line means ( B₀, B ₀;...;B n-1 , B n-1 ) connected to one or more defective memory cells. A device as set forth in claim 1, 2 or 3, wherein said connecting/disconnecting means (Q L0 , Q L0 ;...; Q L,n-1 , Q L,n-1, Q LR , Q LR ) further comprises: fuse means (F₂₀ , F ₂₁,..., F 2,n-1 , F 2R )linked between said power supply means (V CC )and said transistors; and a melting means for melting said fuse means connectable to one of said selecting line means (B₀, B ₀;...;B n-1 , B n-1 ) connected to one or more defective memory cells. A device as set forth in any preceding claim, wherein each of said selecting line means (B₀, B ₀;B₁, B ₁;...;B n-1 , B n-1 ; B R , B R ) comprises a pair of bit lines.

Inventors: AOYAMA, KEIZO, SEKI, TERUO DAI-2 YAYOI-SO 5, YAMAUCHI, TAKAHIKO
Applicants: FUJITSU LIMITED
Application year: 1984
Main classification: G06F11
IPC codes: G11C29, G11C7, G11C11, G11C8