SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device is disclosed in which, when a main memory (36) contains an error bit cell, the main memory (36) is switched to an auxiliary memory (37) by cutting off an interconnection wire between a decoder (31) and the main memory (36), and subjecting a high resis­ tance polysilicon (32₁, 32₂, ...) connected between the de­ coder (31) and the auxiliary memory (37) to laser annealing to reduce the resistance of the high resistance polysilicon (32₁,32₂,...) and to connect the decoder (31) and the auxili­ ary memory (37).

Description: The present invention relates to semiconductor memory devices with improved production yields. With the increase of memory capacity in the fabrication of recent semiconductor memory devices, the probability of the occurrence of error bit cells has also increased. Conventionally, semiconductor memory devices containing error bit cells have been discarded as defective devices. The discarding of the defective devices is very uneconomical, and also leads to an increase in the manufacturing cost. Some measures taken to solve this problem have succeeded in overcoming such uneconomical manufacturing methods. One of the successful measures employs a memory device with a redundancy function. Specifically, an auxiliary memory is used in addition to a main memory. When the main memory contains an error bit cell, a drive or select line, on which bit cells including the error bit are arranged, is selected and all of the bit cells including the error bit cell are replaced with the correct bit cells arrayed on a select line of the auxiliary memory. Fig. 1 shows in block form an example of such a semiconductor memory device. In Fig. 1, reference numeral 1 designates an address buffer whose output is applied to a main address decoder 2 and an auxiliary decoder 3. The output signal from the main address decoder 2 is applied to a main memory 4 to select one of the row lines in the main memory 4. Subsequently, data may be written, by a write signal, into specified memory bit cells connected to the selected row line. Similarly, the data may be read out therefrom by a read signal. The main address decoder 2 is controlled by an output signal from the auxiliary address decoder 3. The decoder output of the auxiliary address decoder 3 is applied to an auxiliary memory 5. The signal selects one row line of the auxiliary memory 5. Then, data may be written into memory cells connected to the row line by a write signal, and read out therefrom by a read signal. The auxiliary address decoder 3 is programmed so as to produce an output signal representing an address of a memory area containing an error bit cell in the main memory. A control signal generating circuit 6 generates a control signal for the auxiliary decoder 3 when an error bit cell is found in the main memory 4. The control signal controls the auxiliary address decoder 3 so as to replace a specific number of bit cells including the error bit cell with the corresponding number of correct bit cells in the auxiliary memory 5. The specific number of bit cells are those arrayed on a select or drive line, having the error bit cell connected thereto, of the main or auxiliary memory. This circuit 6 is composed of a nonvolatile memory, so programmed that it drives the auxiliary address decoder 3 to access the auxiliary memory 5 when the error bit cell is found. When the main memory 4 does not contain an error bit cell, this control circuit 6 does not operate, and the main address decoder 2 accesses the main memory 4. On the other hand, when the main memory 4 has an error bit cell and is accessed, the control signal generating circuit 6 drives the auxiliary address decoder 3 to access the auxiliary memory 5, and at the same time to stop the operation of the main address decoder 2. In this way, the error bit cell in the main memory 4 is replaced with the correct bit cell of the auxiliary memory 5. Figs. 2A and 2B show practical circuit diagrams of two examples of the control signal generator 6. In the circuit shown in Fig. 2A, a fuse element F made of polysilicon or the like is inserted between a power source VD terminal and an output terminal OUT. A programming enhancement type MOS transistor 7 is connected between the output terminal OUT and a ground. A depletion type MOS transistor 8 is inserted between the output terminal OUT and a ground. A programming signal P is applied to the gate of the MOS transistor 7. The gate of the MOS transistor 8 is connected to a ground. In the circuit shown in Fig. 2B, the power source VD and the output terminal OUT have a programming enhancement type MOS transistor 7 and a depletion type MOS transistor 8, which are connected in parallel therebetween. A fuse element F is inserted between the output terminal OUT and a ground. A programming signal P is applied to the gate of the MOS transistor 7 and the gate of the MOS transistor 8 is connected to the output terminal OUT. In the circuit shown in Fig. 2A, when the fuse element F is not burned out, a signal level at the output terminal OUT is logical "1" since the resistance ratio of the MOS transistor 8 and the fuse element F is very large. However, when it is burned out, the output terminal OUT is grounded through the MOS transistor 8 and becomes logical "0". In order to burn out the fuse element F, a programming signal P of logical "1" is applied to the gate of the MOS transistor 7. Then, the MOS transistor 7 turns on to allow a large current to flow into the fuse element F. The large current generates Joule heat which in turn burns out the fuse element F. When the fuse element F is burned out, the signal P returns to logical "0" to turn off the transistor 7. The signal at the output terminal OUT, or the control signal from the control signal generator 6, is logical "1", for example, and at this time the decoding operation of the auxiliary address decoder 3 stops. The decoding operation is performed only when the signal at the output terminal OUT is logical "0". In the circuit of Fig. 2B, when the fuse element F is not burned out, the signal level at the output terminal OUT is kept at logical "0", unlike the circuit of Fig. 2A, since the resistance ratio of the MOS transistor 8 and the fuse element F is very large. On the other hand, when it is burned out, the output terminal OUT is connected to the power source VD terminal through the MOS transistor 8, and is logical "1". In order to burn the fuse element F, a programming signal P of logical "1" is applied to the gate of the MOS transistor 7. At this time, the transistor 7 is turned on, as in the above case, to allow a large current to flow into the fuse element F. In this circuit, when the signal at the output terminal OUT, or a control signal, is logical "0", the decoding operation of the auxiliary address decoder 3 is stopped. When it is logical "1", the decoding operation is executed. Fig. 3 shows a circuit diagram of an example of the auxiliary address decoder 3 when the control signal generator 6 is not used. The auxiliary address decoder 3 is comprised of a depletion type MOS transistor 9 for a load, a plurality of enhancement type MOS transistors 10 for driving the auxiliary memory whose gates are coupled with an address signal or data A0, A0 , ..., An, An , and a plurality of fuse elements FB inserted between each of the transistors 10 and the transistor 9. The auxiliary address decoder 3 is so programmed that when the memory cell in the main memory 4 which is specified by an address signal A0 = A1 = ... = An = 0, is an error bit cell, the decoder 3 produces a signal designating this address. This programming is done by burning the fuse elements FB connected to the transistors 10 coupled with the address signals A0 to An. The auxiliary address decoder 3 thus programmed responds to the address signal A0 = A1 = ... = An = 0 to access the auxiliary memory 5. In the auxiliary address decoder 3 shown in Fig. 3, in order to access the auxiliary memory 5, a plurality of fuse elements FB must be burned out, which are selected according to the code of the address signal applied thereto. A laser beam or Joule heat is used to burn out these fuse elements. This burning means, has some problems, however: welded material attached to its peripheral circuit reduces the reliability of the memory device, the failure of burning causes erroneous programming, and reliability on the burned locations is poor. In this respect, it is evident that the fewer the burning locations of the fuse elements, the better. With the remarkable progress of recent microelectronics technology, memory capacity has increased together with the number of bits of an address input signal. Accordingly, the number of fuse elements burned out in the auxiliary memory also increases. For this reason, there is a pressing need to solve the above problems. There is another method for solving this error bit cell problem of the main memory. In this method, when an error bit cell is found in a memory area specified by, for example, at the most significant bit (MSB) An = 0 in the address signal, the MSB An of an address signal from the address buffer is fixed at "0" or "1" irrespective of the contents of the input address signal to the address buffer. The main memory is used as a memory with half the capacity of the main memory having no error bit cell. According to this measure, when an error bit cell is contained in the memory of 16 kilo bits, for example, it can be used as a memory of 8 kilo bits by fixing the MSB of the output address signal from the address buffer at "1" or "0". Fig. 4 shows a memory array of a memory of 16 kilo bits. The upper half of the memory array is selected by the MSB of the address signal An = "0" and the lower half area by the MSB An = "1". It is assumed that the memory bit cell at point X is defective in Fig. 4. If the MSB An is fixed at "0", An = "0", the memory area specified by An = "1" and containing the error bit cell is not selected. Thus, the memory of 16 kilo bits can be used as a memory of 8 kilo bits. A practical arrangement of an address decoder for fixing the MSB An of an address signal at "0", which has thus far been used, is illustrated in Fig. 5. This circuit is an address buffer of the type which receives an address signal An and produces address signals An and An . In Fig. 5, when the control signals F1 and F2 are both "1", MOSFETs 11 and 12 are both turned on. The input address signal An is inverted by an inverter 15 composed of a load MOSFET 13 and a drive MOSFET 14. The inverted signal is again inverted by an inverter 18 composed of a load MOSFET 16 and a drive MOSFET 17 to be an inverted address signal An. This address signal is further applied to an inverter 21 composed of a load MOSFET 19 and a drive MOSFET 20 to be an address signal An . In case where the address buffer is applied for the above-mentioned memory array, if an error bit cell is found in the memory area selected by An = "1", a control signal F1 is set to "0" and another control signal F2 is set to "1". In this condition, the MOSFET 11 is always in an OFF state, so that An is "1" and An = "0". If the input address signal An to the address buffer takes any logical level, the error bit cell is not selected. As described above, the main memory containing the error bit cell can be used as a memory having only half the capacity of the main memory, by fixing the MSB of the output signal from the address buffer at "1" or "0". Therefore, the production yield of the memory devices is improved. The method mentioned above, in which the MSB of the output address signal is fixed at "1" or "0", is ineffective when error bit cells are present in both the memory areas selected by the MSB An = "0" and "1", for example, in the memory area hatched and denoted as Y in Fig. 4. The same thing is true for a case in which the error bit memory is located at point Z in the area selected by the MSB An = "0" and at point Z' selected by the MSB An = "1". Thus, the circuit as shown in Fig. 5 can only cope with a defective memory in which the error bit cell is located at the memory area selected by the MSB An = "0" or "1". Thus, this circuit still involves the problem of low production yield. Prior art document US-A-3940740 describes a method for providing, in addition to desired circuits "redundant" circuits of the same type. Each circuit has included a nonvolatile electrically alterable semiconductor device, which can be set to either conduct or non-conduct power to the circuit. During a test procedure, only a desired number of devices are rendered accessible by "setting" the semiconductor device to conduct. To perform this method, a microelectronic device having repetitive rows and/or columns for memory cells or logical processors is provided with additional rows and columns. An electrically alterable device is placed in each row and column. Upon the successful test of each row, the row enabling device is set to a conductive state. Any row containing a defective device is not enabled. If additional defects exist, and no redundant rows are available, redundant columns are disabled to isolate the defective devices. It is an object of the present invention to provide a semiconductor memory device which, when a main memory contains an error bit cell, can replace a specific number of bit cells including the error bit cell in the main memory with the corresponding number of correct bit cells in an auxiliary memory in a simple manner, thereby improving the production yield of semiconductor devices. Another object of the present invention is to provide a semiconductor memory device which can save a memory containing an error bit irrespective of the location of the error bit cell, or can effectively use such a memory without discarding it, thereby improving the production yield of semiconductor devices. The present invention provides a semiconductor memory device as defined in anyone of claims 1 and 2. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a block diagram of a conventional semiconductor memory device containing an auxiliary memory; Figs. 2A and 2B show two examples of a practical arrangement of a control signal generator included in the memory device shown in Fig. 1; Fig. 3 shows a practical arrangement of a conventional decoder circuit for an auxiliary memory; Fig. 4 schematically illustrates a main memory array containing error bit cells; Fig. 5 is a circuit arrangement of an address decoder for fixing the contents of the output data; Fig. 6 shows a block diagram of an embodiment of a semiconductor memory device containing an auxiliary memory according to the present invention; Fig. 7 shows another example of a burning circuit used in the memory device of Fig. 6; Fig. 8 shows a block diagram of another embodiment of a semiconductor device according to the present invention; Figs. 9A and 9B show high resistance polysilicon resistors for controlling signal levels at points B1 and B2 in the memory device of Fig. 8; Fig. 10 shows a practical arrangement of main and auxiliary address decoders for selecting main and auxiliary memories, respectively; Fig. 11 is a CMOS circuit arrangement for controlling signal levels at points B1 and B2 in the memory device of Fig. 8; Figs. 12 and 13 show practical arrangements of a circuit for connecting a row line with a drive line; and Fig. 14 is a practical arrangement of a buffer used in the memory device shown in Fig. 8. Preferred embodiments of the present invention will be described referring to the accompanying drawings. In Fig. 6, reference numeral 31 designates an address decoder which receives an address signal A0, A0 , ..., An, An and selects one of the output lines R1 to Ri, which is specified by the address signal. The output lines R1 to Ri are connected to a drain terminal B1 of a depletion type MOS transistor 33 via high resistance polysilicon 32₁ to 32i, and also coupled with a main memory, as in the prior art. The drain terminal B1 of the transistor 33 is connected to an auxiliary memory 37 and its gate and source terminals are connected to a power source VS (ground). The output lines R1 to Ri are connected through high resistance polysilicon 34₁ to 34i to the power source VS. In Fig. 6, when no error bit cell is contained in the main memory 36, this memory is selected. At this time, the auxiliary memory 37 is not selected because point B1 is connected through the transistor 33 to the terminal VS. If an error bit cell is found in the memory cells connected to the output line R2, the output line R2 is cut at a point 35 by means of a laser beam, for example, to separate the decoder 31 from the main memory 36. The cut output line R2 is coupled with the VS potential through the high resistance polysilicon 34₂. Therefore, this output line R2 is not selected in the subsequent stage. In this case, if the high resistance polysilicon 34₂ is annealed by irradiation of a laser beam to reduce its resistance, the output line R2 can be reliably connected to the VS terminal. After the output line R2 is separated from the decoder 31, the high resistance polysilicon 32₂ is subjected to the laser annealing process to reduce its resistance so as to be connected to point B1. Accordingly, when an address signal for selecting the output line R2 is applied to the address decoder 31, the point B1 is selected through the high resistance polysilicon 32₂ (with the row reduced resistance) and selects the auxiliary memory 37. Turning now to Fig. 7, a cut-off circuit is shown which cuts off the output line R2 at the point 35 by current flowing therethrough, not by the laser beam as mentioned above. In the present embodiment, the cut-off point 35 is made of a polysilicon fuse P1 provided between the decoder 31 and the main memory 36. In order to melt the high resistance polysilicon fuse P1, an address signal to select an error bit cell is input to the decoder 31 which in turn produces an output signal "1". Then, the output of the inverter I1 is logical "0" and the transistor 38 is cut off. At this time, a high voltage VP is applied to the transistor 40 through the depletion type MOS transistor 39. Current flows from the high voltage source VP through the transistor 41, the fuse P1, and the transistor 40 to heat and melt the fuse P1. In this instance, the high voltage source VP is used for melting the fuse P1. This high voltage source VP is used in order to cause a large current to flow through the transistors 40 and 41 even if the transistors 40 and 41 are of small size. Therefore, if the transistors 40 and 41 are both large, an ordinary power source may be used in place of the high voltage power source VP. While the present instance employs a single auxiliary memory, the same thing is applied for the case using two auxiliary memories. In this case, another set of high resistance polysilicons 32₁ to 32i, 34₁ to 34i and the transistor 33 must be used. In the embodiments shown in Figs. 6 and 7, when the main memory contains an error bit cell, the interconnection between the decoder and the main memory is cut off at a single point and the high resistance polysilicon connected between the decoder and the auxiliary memory is subjected to laser annealing to reduce its resistance. In this way, the bit cells containing an error bit cell arrayed on a specific output line of a main address decoder in the main memory are replaced with those in the auxiliary memory. In this way, the main memory containing an error bit cell can be saved. Turning now to Fig. 8, another embodiment of a semiconductor memory device containing an auxiliary memory is shown. In Fig. 8, like reference symbols are used to designate like or equivalent portions in Fig. 6. In the present embodiment, output lines R1 to Ri of a decoder 31 are coupled with input terminals of buffer circuits BU1 to BUi, respectively. The output terminals of the buffer circuits BU1 to BUi are connected to a main memory 36 through drive lines W1 to Wi. The output lines R1 to Ri are commonly connected to the input terminals of buffers BU11 and BU12, through high resistance polysilicons P11 to P1i. The output lines R1 to Ri are also commonly connected to the input terminals of buffers BU21 and BU22, through high resistance polysilicons P21 to P2i. The drive lines W1 to Wi are connected to a power source VS (ground), through enhancement type MOS transistors (N channel) 45₁ to 45i, and are also connected to the power source VS through transistors 46₁ to 46i. The gates of the transistors 45₁ to 45i are commonly connected to the output terminal of the buffer circuit BU21. The gates of the transistors 46₁ to 46i are commonly connected to the output terminal of the buffer circuit BU11. The output terminal of the buffer circuit BU22 is connected to a drive line D1 of the auxiliary memory 37, and the output terminal of the buffer circuit BU12 is connected to a drive line D2 of the auxiliary memory 37. The input terminals B1 of the buffer circuits BU11 and BU12 are connected to the drain terminals of the depletion type MOS transistor 47₁ and the input terminals B2 of the buffer circuits BU21 and BU22 are connected to the drain terminal of the transistor 47₂. In operation, when an error bit cell is not contained in the main memory 36, the decoder 31 selects one of the output lines R1 to Ri according to an address signal A0, A0 , ..., An, An , and sets the selected one at logical "1". This logical "1" signal is applied to the buffer circuit associated with the selected output line and then rapidly charges one of the drive lines W1 to Wi to set it at logical "1". The points B1 and B2 are set at the VS level (ground) through the transistors 47₁ and 47₂, and hence the points B1 and B2 are kept at the VS level. Therefore, the drive lines D1 and D2 connected through the buffer circuits BU11 and BU12 to the points B1 and B2 and the gates of the transistors 45₁ to 45i and 46₁ to 46i connected to the points B1 and B2 through the buffer circuits BU12 and BU22, are kept at logical "0". Then, one of the drive lines connected to the main memory 36 is selected. It is assumed that one of the memory cells connected to the drive line W3, for example, is an error bit cell. In this case, the high resistance polysilicon P13 is annealed by laser beam, for example, thereby reducing its impedance or resistance. As a result, the output line R3 and the point B1 are electrically connected to each other. When the decoder 31 renders the output line R3 at logical "1", the point B1 is also logical "1". Incidentally, the transistor 47₁ has a high enough resistance to have no influence on the logical "1" of the output line R3. As a result, the transistors 46₁ to 46i are turned on and the drive lines W1 to Wi are "0". The bit cells of the auxiliary memory connected to the drive line are selected through the drive line D2. In this way, the desired memory cells of the auxiliary memory may be selected by merely laser-annealing one high resistance polysilicon P3. If an error bit cell is found on a drive line other than the drive line W3, all one has to do is to reduce the resistance of one of the high resistance polysilicons P21 to P2i, which corresponds to the drive line connected to the additional error bit cell. While the auxiliary memory has two drive lines in Fig. 8, the drive lines may be increased to any number, if necessary. In the memory device of Fig. 8, the MOS transistors 47₁ and 47₂ may be replaced with high resistance polysilicons 48₁ and 48₂ which are both connected at one end to the points B1 and B2, and at the other end to the VS power source. In this case, if the auxiliary memory is not used, the high resistance polysilicons are annealed to reduce their resistance, and the potential at the points B1 and B2 is at the VS level (logical "0"). These transistors 47₁ and 47₂ and the high resistance polysilicon 48₁ and 48₂ are not always needed. In the integrated circuit, if PN junctions are formed in their place, the points B1 and B2 are always kept at "0" level by the leak current of the PN junction or the like. In this case, the potential at the points B1 and B2 may be reliably kept at logical "0". Fig. 10 shows a circuit diagram of the buffers BU11, and BU22 and BU12 in Fig. 8. In Fig. 10, reference numeral 79 designates a main address buffer and 80 an auxiliary address buffer containing buffer sections 80₁ and 80₂ which correspond to the buffers BU22 and BU12. In Fig. 10, MOS transistors 51 to 54, 59, 61, 63, 65, 67 and 69 are P channel MOS transistors, and transistors 55 to 58, 60, 62, 64, 66, 68 and 70 are N channel MOS transistors. The transistors 52 and 56 functionally correspond to transistors 46₁ to 46i, and similarly, the transistors 53 and 57 correspond to the transistors 45₁ to 45i. The pairs of transistors 51 and 55, 54 and 58, 59 and 60, 61 and 62, 63 and 64, 65 and 66, 67 and 68, and 69 and 70 make up CMOS inverters 71 to 78, respectively. The input terminals of the inverters 73 and 74 are connected to the points B1 and B2 of the memory device of Fig. 8. The output signal of the inverter 73 is applied to the gates of the transistors 56 and 52, and the output signal from the inverter 74 is connected to the gates of the transistors 57 and 53. When no error bit cell is found in the main memory coupled with the buffer circuits thus arranged, the signal level at the points B1 and B2 is "0". Accordingly, the output signals of the inverters 65 and 66 are both logical "1", so that the transistors 56 and 57 are both ON and the transistors 52 and 53 are both OFF. The output line R1 and the drive line W1 are both at the same level. In this manner, the drive line W1 is selected. Let us consider that an error bit cell is connected to the drive line W1, for example. The signal at point B1 is logical "1" and the output A of the inverter 73 is logical "0". Accordingly, the transistor 56 is turned off and the transistor 52 is turned on. Therefore, the drive line W1 is logical "0" and the main memory 36 is not selected. At this time, the point B1 is logical "1". The output of the buffer section 80₁ connected to the drive line D2 in the auxiliary buffer 80 is logical "1" to select the drive line D2 of the auxiliary memory 37. In other words, the bit cells on the drive line W1 containing an error bit cell are replaced with the bit cells on the drive line D2 of the auxiliary memory 37. Fig. 11 shows a CMOS circuit used in place of the MOS transistor 47₁ or 47₂ in Fig. 8. This circuit is comprised of MOS transistors 81 to 90, capacitors 91 to 93, a high resistance polysilicon 94, and resistors 95 to 97. The transistors 81 to 84 are of the P channel type, while the transistors 85 to 90 are of the N channel type. The transistor 81 is connected at the source to a power source VD through a resistor, and at the drain to the power source VS through the capacitor 91. The transistors 82 and 85 form a CMOS inverter. The source of the transistor 82 is connected to the power source VD, via the resistor 96. The source of the transistor 85 is connected to the potential VS. The transistors 82 and 85 are interconnected at the drain and their junction is connected to the drain of the transistor 81. The transistors 83 and 86 constitute a CMOS inverter. The source of the transistor 86 is connected to the ground potential VS via the resistor 97. The gates of the transistors 83 and 86 are interconnected and their junction is connected to the drain of the transistor 81. The drains of the transistors 83 and 86 are interconnected and their junction is connected to the gate of the transistors 82 and 85. The drain of the transistor 87 is connected to the power source VD via the capacitor 92, and its source is connected to the ground potential VS. The drain of the transistor 87 is connected to a drain junction between the transistors 83 and 86. The transistor 88 is connected to the power source VD via the high resistance polysilicon 94, and its source is connected to the ground potential VS. The gate of the transistor 88 is connected to the drain interjunction between the transistors 83 and 86. A capacitor 93 is connected between the junction of the transistor 88 and the polysilicon 94, and the ground potential VS. The transistors 84 and 89 form a CMOS inverter. The source of the transistor 84 is connected to the power source VD and the source of the transistor 89 is connected to the ground potential VS. The gates of the transistors 84 and 89 are connected to the gate of the transistor 87 and the drain of the transistor 88. The transistors 84 and 89 are interconnected at the drains and their junction is connected to the gate of the transistor 81. The drain of the transistor 90 is connected to the point B1 or B2 in the memory device of Fig. 8, and its source is connected to the ground potential VS. The gate of the transistor 90 is connected to the drain interjunction of the transistors 84 and 89. In the operation of this circuit, when the power source VD is turned on, the potential at point E1 becomes high (logical "1"). At this time, the point E2 is logical "0" since the high resistance polysilicon 94 is present. The point E3 is logical "1" and the points B1 and B2 are kept at VS level. When the main memory contains an error bit cell, the high resistance polysilicon 94 is laser-annealed to reduce its resistance. Therefore, the power source VD is turned on, the points E1 and E2 are at VD level and the point E3 is at VS level. Since the point E1 is at VD level, the transistor 88 is turned on. If a resistance ratio of the resistance reduced polysilicon 94 and the transistor 88 is set at a proper value, the point E2 is kept at logical "1". Since the point E3 is "0", the transistors 81 and 87 are both turned on and the potential levels at the points E4 and E1 are inverted to "1" and "0", respectively. Therefore, the transistor 88 is turned off and the point E2 is logical "1" and the point E3 is logical "0". As a result, the transistor 90 is turned off. Therefore, the points B1 and B2 are both logical "1" at the time when it receives an address signal to select an error bit cell. Figs. 12 and 13 show other means for selecting a specific drive line of the auxiliary memory which correspond to the high resistance polysilicons P11 to P1i and P21 to P2i in the memory device shown in Fig. 8. The embodiment shown in Fig. 12 employs a MOS transistor 101 in place of the polysilicons in Fig. 8. This circuit is further provided with a high resistance polysilicon 102 connected between the gate of the transistor 101 and the power source VD, and a depletion type MOS transistor 103 connected between the polysilicon 102 and the ground potential VS. When an error bit cell is contained in the main memory, the polysilicon 102 is subjected to laser annealing, for example, to reduce its resistance. With this reduction of resistance, the transistor 101 is turned on to change its impedance, and the output line R1 and the point B1 are connected to each other. Finally, the auxiliary memory is selected. In the embodiment of Fig. 13, like that of Fig. 12, a MOS transistor 101 is used in place of the high resistance polysilicons P11 to P1i and P21 to P2i. In this example, the MOS transistor 103 is connected at the drain to the power source VD and at the gate and the source to the ground potential VS. The gate of the transistor 101 is connected to the gate and source of the transistor 103. When an error bit cell is present in the main memory, a laser beam is applied to a melting location on the interconnection wire between the junction of the gate and source of the transistor 103 and the ground, thereby cutting off the wire. As the result, the transistor 101 is turned on to select the drive line of the specific bit cells in the auxiliary memory. Fig. 14 shows a circuit arrangement formed when each of the buffers BU1 to BUi in the memory device of Fig. 8 is constructed using N channel E and D (enhancement and depletion) type MOS transistors. The gate of an E type MOS transistor 112 is connected to the row line R1, and its drain is connected to the power source VD through a D type MOS transistor 111. The source of the transistor 112 is connected to the potential VS through an E type MOS transistor 113. The row line R1 is connected to the gate of the E type MOS transistor 115 and its drain is connected to the power source VD through an E type transistor 114. Its source is connected to the ground potential VS through the E type transistor 116. A junction between the transistors 115 and 116 is connected to the drive line W1 of the main memory. The D type transistor 117 and the E type MOS transistor 118 make up an E/D type inverter. The gate of the transistor 118 forms an input terminal of the inverter which is connected to the point B1 in the memory device of Fig. 8. A junction of the transistors 117 and 118 forms an output terminal of the inverter which is connected to the gates of the transistors 113 and 114. In the memory device shown in Fig. 8, when no error bit cell is present in the main memory 36, the output line R1 is selected and becomes logical "1", the point B1 being logical "0". When the output line R1 is logical "1", the transistors 112 and 115 are turned on. When the point B1 is logical "0", the inverter comprised of the transistors 117 and 118 is logical "1". Therefore, the transistors 113 and 114 are turned on. When the transistors 112 and 113 are both turned on, the gate of the transistor 116 becomes logical "0" and is thus turned off. In this way, the transistors 114 and 115 are both turned on and the transistor 116 is turned off, so that the drive line W1 is logical "1" and is selected. In this way, the main memory is accessed. When an error bit cell is present in the main memory, the output line R1 is logical "1" and the point B1 is logical "1". Therefore, the transistors 115 and 116 are both turned on, while the transistors 113 and 114 are both turned off. The transistors 111 and 112 are both turned on, and the transistor 113 is turned off, so that the gate of the transistor 116 is logical "1" and is turned on. In this way, the transistor 114 is turned off and the transistors 115 and 116 are turned on. The drive line is logical "0", and therefore is not selected. In this case, the drive line connecting to specific memory cells in the auxiliary memory is selected. When the buffer arranged as shown in Fig. 14 is applied for the buffers BU1 to BUi, the transistors 45₁ to 45i in the memory device of Fig. 8 are not required.

Claims: A semiconductor memory device comprising: a main memory (36); a decoder (31) for selecting said main memory (36); an auxiliary memory (37); and a plurality of row lines (R1, R2, ...) connected between said decoder (31) and said main memory (36); characterized by further comprising: selecting resistive means (32₁, 32₂, ...) for selecting said auxiliary memory (37) according to the output of said decoder (31), comprising a plurality of selecting resistors (32₁, 32₂, ...) provided for said row lines, one terminals of the selecting resistors being connected to the row lines and the other terminals of the resistors being OR wired to form an ORing circuit at a circuit node (B1) and then therethrough connected to said auxiliary memory (37); resistive changing means for changing the resistance of the selecting resistors of said selecting resistive means (32₁, 32₂, ...) to program the selecting resistors so that, when said main memory (36) contains an error bit cell, said auxiliary memory (37) is accessed; disconnecting means for disconnecting the row lines from said main memory (36), so that, when said main memory (36) contains an error bit cell, the main memory (36) is prevented from being accessed; and inhibiting resistive means (34₁, 34₂, ...) for inhibiting the selection of said main memory (36) when said auxiliary memory (37) is selected, said inhibiting resistive means (34₁, 34₂, ...) comprising a plurality of inhibiting resistive elements (34₁, 34₂, ...) provided for said row lines (R1, R2, ...), the one terminals of the resistive elements being connected to the row lines (R1, R2, ...) and the other terminals of the resistive elements being connected to the reference potential. A semiconductor memory device comprising: a main memory (36); a decoder (31) for selecting said main memory (36); an auxiliary memory (37); and a plurality of row lines (R1, R2, ...) connected between said decoder (31) and said main memory (36); characterized by further comprising: selecting resistive means (P11, P12, ...) for selecting said auxiliary memory (37) according to the output of said decoder (31), comprising a plurality of selecting resistors (P11, P12, ...) provided for said row lines, one terminals of the selecting resistors being connected to the row lines and the other terminals of the resistors being OR wired to form an ORing circuit at a circuit node (B1) and then therethrough connected to said auxiliary memory (37); resistive changing means for changing the resistance of the selecting resistors of said selecting resistive means (P11, P12, ...) to program the selecting resistors so that, when said main memory (36) contains an error bit cell, said auxiliary memory (37) is accessed; inhibiting means (45₁, 45₂, ...; 46₁, 46₂, ...) for inhibiting the selection of said main memory (36) when said auxiliary memory (37) is selected, the inhibiting means comprising a plurality of inhibiting MOS transistors (45₁, 45₂, ...; 46₁, 46₂, ...) provided for said row lines and a plurality of buffer circuits (BU1, BU2, ...) provided for said row lines, the inhibiting MOS transistors being controlled by an output signal of said ORing circuit formed by said selecting resistors so as to bring the deselected row line to a reference potential, the buffer circuits (BU1, BU2, ...) being connected between outputs of said decoder (31) and said inhibiting MOS transistors of the inhibiting means. A semiconductor memory device according to claim 1, characterized by further including level holding means (33; 48₁, 48₂; 81 to 97) which is coupled between said circuit node (B1) and a reference potential (VS), and keeps a signal level at said circuit node (B1) at a non-select level for said auxiliary memory (37) when said main memory (36) is selected, and keeps the signal level at said circuit node (B1) at a select level for said auxiliary memory (37) when said auxiliary memory (37) is selected. A semiconductor memory device according to claim 3, characterized in that said level holding means is made of a MOS transistor (33) of which the drain-source path is connected between said circuit node (B1) and said reference potential (VS). A semiconductor memory device according to claim 4, characterized in that said MOS transistor (33) is of the depletion type. A semiconductor memory device according to claim 3, characterized in that said level holding means is made of high resistance polysilicon (48₁, 48₂). A semiconductor memory device according to claim 2, characterized in that said level holding means includes a first MOS transistor (81) of which the drain-source path is connected at one end to a power source potential (VD), a first inverter (82, 85) provided between the power source potential (VD) and the reference potential (VS), the output terminal of said first inverter (82, 85) being connected to the other end of the source-drain path of said first MOS transistor (81), a second inverter (83, 86) provided between the power source potential and the reference potential, the input terminal of said second inverter (83, 86) being connected to the other end of said first MOS transistor (81), the output terminal thereof being connected to the input terminal of said first inverter (82, 85), a second MOS transistor (88) whose drain-source path is connected at one end to the power source potential via a further provided impedance element (94) and at the other end to the reference potential, the gate of said second MOS transistor (88) being connected to said output terminal of said second inverter (83, 86), a first capacitor element (92) connected between said power source potential (VD) and the gate of said second MOS transistor (88), a third MOS transistor (87) of which the drain-source path is connected between the gate of said second MOS transistor (88) and said reference potential (VS), a third inverter (84, 89) provided between said power source potential (VD) and said reference potential (VS), said third inverter (84, 89) being connected at the input terminal to a junction of said further provided impedance element (94) and said second MOS transistor (88) and to the gate of said third MOS transistor (87), and being connected at the output terminal to the gate of said first MOS transistor (81), and a second capacitor element (93) connected between the input terminal of said third inverter (84, 89) and said reference potential (VS), and a fourth MOS transistor (90) whose drain-source path is connected at one end to a junction of said selecting resistive means (32₁, 32₂, ...) and said auxiliary memory (37) and is connected at the other end to said reference potential (VS), and is connected at the gate to said output terminal of said third inveerter (84, 89). A semiconductor memory device according to claim 1, characterized in that said disconnecting means irradiates a laser beam to the row lines to cut off the row lines (R1, R2, ...,) between said decoder (31) and said main memory (36). A semiconductor memory device according to claim 1, characterized in that said disconnecting means comprises a fusible element (P1) inserted between said decoder (31) and said main memory (36), and circuit means (I1, 38 to 41) for fusing said fusible element (P1) by flowing current to said fusible element (P1). A semiconductor memory device according to claim 9, characterized in that said fusing circuit means includes a first MOS transistor (41) of which the drain-source path is connected between one end of said fusible element (P1) closer to said main memory (36) and a high potential power source (VP) and the gate is connected to said high potential power source (VP), a first inverter (I1) of which the input terminal is connected to the other end of said fusible element (P1) closer to said decoder (31), a second inverter composed of second and third MOS transistors (39, 38) connected in series between said high potential power source (VP) and said reference potential (VS), the input terminal of said inverter being connected to the output terminal of said first inverter (I1), and a fourth MOS transistor (40) of which the drain-source path is connected between one end of said fusible element (P1) and said reference potential (VS) and the gate is connected to the output terminal of said second inverter. A semiconductor memory device according to claim 1, characterized in that said selecting resistive means is made of polysilicon (32₁, 32₂; P11, P12, ...; P21, P22, ...). A semiconductor memory device according to claim 11, characterized in that said resistance changing means changes the resistance of said polysilicon. A semiconductor memory device according to claim 12, characterized in that said resistance changing means irradiates said polysilicon with a laser beam. A semiconductor memory device according to claim 2, characterized in that said selecting resistive means is a first MOS transistor (101) whose drain-source path is connected between said decoder (31) and said auxiliary memory (37), and said resistance changing means includes a second MOS transistor (103) connected between a polysilicon connected between said power source potential (VD) and the gate of said first MOS transistor (101), and said reference potential (VS), and means (102) for reducing a resistance of said polysilicon. A semiconductor memory device according to claim 14, characterized in that said second MOS transistor (103) of said resistance changing means is of the depletion type, and its gate is connected to said reference potential (VS). A semiconductor memory device according to claim 15, characterized in that said means (102) for reducing the resistance of said polysilicon anneals said polysilicon. A semiconductor memory device according to claim 16, characterized in that said annealing means irradiates said polysilicon with a laser beam. A semiconductor memory device according to claim 2, characterized in that said selecting resistive means is a first MOS transistor (101) whose drain-source path is connected between said decoder (31) and said auxiliary memory (37), said resistance changing means includes a second MOS transistor (103) whose drain-source path is connected at one end to said power source potential (VD) and at the other end to the gate of said first MOS transistor (101) and said reference potential (VS), and means (104) for rendering nonconductive a path between said second MOS transistor (103) and said reference potential (VS). A semiconductor memory device according to claim 18, characterized in that said second MOS transistor (103) of said resistance changing means is of the depletion type, and its gate is connected to said reference potential (VS). A semiconductor memory device according to claim 18, characterized in that said means (104) for rendering nonconductive the second MOS transistor reference path cuts off an interconnection wire layer between said second MOS transistor (104) and said reference potential (VS). A semiconductor memory device according to claim 20, characterized in that said interconnection wire layer cutting means irradiates said interconnection wire layer with a laser beam. A semiconductor memory device according to claim 2, characterized by further including an auxiliary buffer (BU11, BU12, ...) connected between said selecting resistive means and said auxiliary memory (37). A semiconductor memory device according to claim 22, characterized in that said auxiliary buffer includes a first inverter composed of first and second complementary MOS transistors (63, 64) provided between said power source potential (VD) and said reference potential (VS), the input terminal of said first inverter being connected to a junction between said first and second MOS transistors (63, 64), and a second inverter composed of third and fourth complementary MOS transistors (65, 66) connected between said power source potential (VD) and said reference potential (VS), the input terminal of said second inverter being connected to the output terminal of said first inverter, the output terminal thereof being connected to said auxiliary memory (37). A semiconductor memory device according to claim 1, characterized in that the resistance of said inhibiting resistive elements (34₁, 34₂, ...) is changed by a laser beam irradiated thereonto to program the resistive elements so that said row lines are selectively connected to the reference voltage via said inhibiting resistive elements (34₁, 34₂, ...). A semiconductor memory device according to claim 2, characterized in that said inhibiting means (BU1, BU2, .., 45₁, 45₂, ..., 46₁, 46₂, ...) comprises a plura lity of buffer circuits (BU1, BU2, ...) provided for said row lines and inserted between said decoder (31) said main memory (36). A semiconductor memory device according to claim 25, characterized in that said buffer circuits (BU1, BU2, ...) each comprises a first inverter (71) connected between first and second power source potentials for receiving an output signal of said decoder (31), a second inverter (72) connected between said first and second power source potentials for receiving an output signal of said first inverter (71), a first MOS transistors (52, 53) connected between said first power source potential and the junction of said first and second inverters, for receiving an inverted signal of an output signal of said ORing circuit formed at said circuit node (B1), a second MOS transistor (56, 57) inserted between said first first inverter and said second power source potential for receiving an inverted signal of an output signal of said ORing circuit formed at said circuit node (B1). A semiconductor memory device according to claim 25, characterized in that said inhibiting means (BU1, BU2, ..., 45₁, 45₂, ..., 46₁, 46₂, ...) further comprises a plurality of inhibiting MOS transistors (45₁, 45₂, ..., 46₁, 46₂, ...) provided for said row lines (R1, R2, ...), the one terminals of the current paths of the inhibiting MOS transistors being connected to said row lines (R1, R2, ...) and the other terminals of the current paths being connected to a reference voltage (VS), the inhibiting MOS transistors being connected at the gates to the ORing circuit and controlled by the output of the ORing circuit so that said row lines are selectively connected to the reference voltage (VS) via said inhibiting MOS transistors.

Inventors: ARIIZUMI, SHOJI, IWAHASHI, HIROSHI
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1983
Main classification: G06F11
IPC codes: G11C29