Abstract:
A semiconductor memory device includes a redundant memory cell, an electrode and an output circuit. The redundant memory cell is used instead of a memory cell when the memory cell has a defect. The electrode applys with a test signal for setting a test condition from outside in testing the redundant memory cell. The output circuit outputs data read out of the memory cell and the redundant memory cell. When the test signal is applied to the electrode to set the test condition for the redundant memory cell, the output circuit is configured to output data read out of the redundant memory cell at a level different from a signal level of data readout of the memory cell for output.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a semiconductor memory device having redundant memory cells, particularly to the function of testing the redundant memory cells. 
   A semiconductor memory device is manufactured by collectively fabricating a plurality of storage circuits in a succession of wafer processes. The fabricated separate storage circuits are to be selected by address signals. Therefore, when the selectable storage circuits have even a single defect, the semiconductor memory device is an unusable defective item. Particularly, as the individual memory devices are reduced in size and increased in the integration degree to grow storage capacity, it is hard to manufacture flawless semiconductor memory devices. 
   On this account, paying attention that the semiconductor memory device is configured by arranging a plurality of storage circuits having the same patterns, a method is adopted that a substitute storage circuit is prepared beforehand and a storage circuit with a defect is replaced by the substitute storage circuit when the storage circuit originally used has the defect. 
   However, the traditional DRAM has the following problem. When redundant cells are tested, a probe is brought into contact with individual pads to apply high-level signals. However, it could not be confirmed from outside whether to correctly set the conditions for testing the redundant memory cells inside the device. Therefore, even though the redundant memory cells are not tested appropriately because of logic errors in a test circuit, defects in the circuit patterns formed or contact failure of the probe, such a test result sometimes shows that the redundant memory cell is normal. 
   SUMMARY OF THE INVENTION 
   The invention may provide a semiconductor memory device having a test circuit capable of solving the problem of the related art and checking whether to correctly set test conditions inside the device in testing redundant memory cells. 
   The invention provides a semiconductor memory device having: 
   a redundant memory cell for use instead of a memory cell when the memory cell has a defect; 
   an electrode applied with a test signal for setting a test condition from outside in testing the redundant memory cell; and 
   an output circuit for outputting data read out of the memory cell and the redundant memory cell, 
   wherein when the test signal is applied to the electrode to set the test condition for the redundant memory cell, the output circuit is configured to output data read out of the redundant memory cell at a level different from a signal level of data readout of the memory cell for output. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a DRAM illustrating a first embodiment of the invention; 
       FIG. 2  is a circuit diagram of an output buffer illustrating a second embodiment of the invention; 
       FIGS. 3A to 3D  are circuit diagrams of output buffers illustrating a third embodiment of the invention; 
       FIG. 4  is a circuit diagram of an output buffer illustrating a fourth embodiment of the invention; 
       FIG. 5  is a circuit diagram of an output buffer illustrating a fifth embodiment of the invention; and 
       FIG. 6  is a circuit diagram of an output buffer illustrating a sixth embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   First Embodiment 
     FIG. 1  is a block diagram of a DRAM illustrating a first embodiment of the invention. This DRAM has a memory cell array  10  with redundant memory cells. The memory cell array  10  has m lines of word lines Wli (where i=1 to m) arranged in parallel with each other, and a single redundant word line WLr. In addition, the DRAM has n pairs of bit lines BLj, /BLj (where j=1 to n) arranged in parallel with each other and crossing the word lines Wli and WLr, and has a single pair of redundant bit lines BLr, /BLr. 
   At the cross-points of the word lines Wli and the bit line pairs BLj, /BLj, original memory cells  11   i, j  are disposed. Furthermore, at the cross-points of the redundant word line WLr, the bit line pairs BLj, /BLj and the redundant bit line pair BLr, /BLr, redundant memory cells  12   r, k  (where k=1 to n, r) are disposed. Moreover, at the cross-points of the redundant bit line pair BLr, /BLr, the word lines WLi and the redundant word line WLr, redundant memory cells  13   l, r  (where l=1 to m, r) are disposed. 
   Each of the memory cells  11   i, j  is configured of one capacitor and one insulated gate transistor, not shown in the drawing. Then, the word line WLi controls the transistor to turn on/off, and the bit lines BLj, /BLj write or read data in/out of the capacitor. Each of the redundant memory cells  12   r, k , and  13   l, r  is the same. 
   The DRAM has a row-address decoder  20  for selecting the word lines WLi. When row-address signal RAD is applied, the row-address decoder  20  decodes it, turns any one of row-selection signals R 1 , R 2  to Rm to be high level, and outputs it. To the output of the row-address decoder  20 , a row-replacement circuit  30  and a word line driver  40  are connected. 
   The row-replacement circuit  30  is a circuit that when a certain word line WLi or a memory cell  11  connected to this word line WLi has a defect, the word line WLi is inhibited to use and the redundant word line WLr is substituted for the replacement. 
   The row-replacement circuit  30  has a test pad  31  to which a test signal RRT is applied. The pad  31  is pulled down to the ground voltage GND through a resistor  32  and is connected to an inverter  33 . The output of the inverter  33  is connected to one input of a two-input NAND  34 . To the other input of the NAND  34 , control signal XR is applied which is activated and turned to be high level at the same time when the row-address signal RAD is activated. The output of the NAND  34  is connected to a node N 1 . 
   The control signal XR is further applied to the gate of a PMOS  35 . The source and drain of the PMOS  35  are connected to the power supply voltage VCC and a node N 2 , respectively. 
   Between the nodes N 1  and N 2 , m pairs of an NMOS  36   i  and a fuse  37   i  serially connected are arranged in parallel, and the row-selection signal Ri is applied to the gate of each of the NMOSs  36   i . In addition, a retaining circuit formed of two inverters  38   a  and  38   b  serially connected is connected to the node N 2 . Furthermore, an inverter  39  is connected to the node N 2 , and control signal XF is outputted from the output of the inverter  39 . 
   The word line driver  40  has two-input NANDS  41   i  corresponding to the row-selection signals Ri the row-selection signals Ri are applied to one inputs of the NANDs  41   i  and the control signal XF is commonly applied to the other inputs thereof. The outputs of the NANDs  41   i  are connected to the word lines WLi through inverting amplifiers  42   i . Moreover, the word line driver  40  has an inverting amplifier  43  to which the control signal XF is inputted. The redundant word line WLr is connected to the output of the inverting amplifier  43 . 
   The bit line pairs BLj, /BLj are connected to sense amplifiers  51   j , and connected to data lines DL, /DL through switching NMOSs  52   j ,  53   j . In addition, the redundant bit line pair BLr, /BLr is connected to a sense amplifier  51   r , and connected to the data lines DL, /DL through switching NMOSs  52   r ,  53   r . The NMOSs  52   j ,  53   j , and  52   r ,  53   r  are controlled by a column-address decoder  60  and a column switching circuit  70  based on column selection signal CAD. 
   When the address signal CAD is applied, the column-address decoder  60  decodes it, turns anyone of column selection signals C 1 , C 2  to Cn to be high level, and outputs it. The column switching circuit  70  is connected to the output of the column-address decoder  60 . 
   The column switching circuit  70  has a test pad  71  to which a test signal CRT is applied. The pad  71  is pulled down to the ground voltage GND through a resistor  72  and connected to an inverter  73 . The output of the inverter  73  is connected to one input of a two-input NAND  74 . To the other input of the NAND  74 , initial signal INT is applied that is turned to be low level for a fixed time right after the power is switched on and then turned to be high level. The output of the NAND  74  is connected to a node NC 0 , and control signal YFD is outputted to the node NC 0 . 
   The node NC 0  is connected to a node NC 1  through a fuse  75   1 , and the node NC 1  is connected to a node NC 2  through a fuse  75   2 . Subsequently, nodes NC 3 , NC 4  to NCn are similarly connected through fuses  75   2 ,  75   3  to  75   n . The node NCn is connected to the power supply voltage VCC through a PMOS  76 , and also connected to the power supply voltage VCC through a PMOS  77 . The control signal YFD is applied to the gate of the PMOS  76  through an inverter  78 , and the potential of the node NCn is applied to the gate of the PMOS  77  through an inverter  79 . A retaining circuit formed of the PMOSs  76  and  77  and the inverter  79  holds the potential of the node NCn. 
   The column switching circuit  70  has two TGs  80   j ,  81   j  controlled to be turned on/off at the level of the node NCj corresponding to each of column selection signals Cj applied from the column-address decoder  60 . The TG  80 j is in the on state when the node NCj is at low level, and it is in the off state when the node is at high level. On the other hand, the TG  81   j  is in the off state when the node NCj is at low level, and it is in the on state when the node is at high level. 
   The output of the TG  80   j  is connected to the gate of the switching NMOSs  52   j ,  53   j  corresponding to the bit line pair BLj, /BLj. On the other hand, the output of the TG  81   j  is connected to the gate of the switching NMOSs  52   j+1 ,  53   j+1  corresponding to the bit line pair BL j+1 , /BL j+1 . In addition, the output of a TG  81   n  corresponding to the column selection signal Cn is connected to the gate of the switching NMOSs  52   r ,  53   r  corresponding to the redundant bit line pair BLr, /BLr. 
   The data lines DL, /DL are connected to a read amplifier  90 . The read amplifier  90  amplifies the signals of the bit line pairs BLj, /BLj connected to the data lines DL, /DL to output data signal DB at high level or low level. An output buffer  100 A is connected to the output of the read amplifier  90 . 
   The output buffer  100 A has a two-input negative OR gate (hereafter, it is called ‘NOR’)  104 , and the data signal DB is applied to a first input of the NOR  104 . On the other hand, a four-input AND gate (hereafter, it is called ‘AND’)  105  is connected to a second input of the NOR  104 . The control signal YFD and test signals RRT and CRT are applied to the inputs of the AND  105 , and the control signal XF is applied thereto through an inverter  106 . A CMOS inverter formed of a PMOS  102  and an NMOS  103  is connected to the output of the NOR  104 , and output data DOUT is outputted from the CMOS inverter. 
   Next, the operation in testing will be described. 
   Prior to testing the redundant memory cells, probes are brought into contact with the pads  31  and  71  in the DRAM simultaneously, and high-level signals are applied to check a test circuit. At this time, when there are not logic errors in the test circuit, defects in the circuit patterns formed and contact failure of the probe, the test signals RRT and CRT and the control signal YFD are turned to be high level and the control signal XF is turned to be low level. Consequently, the output signal of the AND  105  of the output buffer  100 A is turned to be high level, and the output data DOUT is always at high level regardless of the data signal DB outputted from the read amplifier  90 . 
   When there are logic errors in the test circuit, defects in the circuit patterns formed or contact failure of the probe, the output signal of the AND  105  is turned to be low level and the output data DOUT is varied to be high level or low level in accordance with the data signal DB outputted from the read amplifier  90 . 
   In checking the test circuit, the output data DOUT is confirmed to be high level all the time, and then the redundant memory cells are tested. As described above, in testing the redundant memory cells, high-level signals are applied to one of the pads  31  and  71 , and thus the output signal of the AND  105  of the output buffer  100 A is turned to be low level. Consequently, the output data DOUT is varied to be high level or low level in accordance with the data signal DB outputted from the read amplifier  90 , and then the redundant memory cells are tested traditionally. 
   As described above, the DRAM of the first embodiment has the output buffer  100 A for fixing the output data DOUT at high level when the two test signals RRT and CRT are applied simultaneously. Accordingly, the first embodiment has the advantage that can check whether to correctly set the test conditions for the redundant memory cells inside the device. 
   Second Embodiment 
     FIG. 2  is a circuit diagram of an output buffer illustrating a second embodiment of the invention. This output buffer  100 B is disposed instead of the output buffer  100 A shown in  FIG. 1 . The same components as those shown in  FIG. 1  are designated the same numerals and signs. 
   The output buffer  100 B has a four-input NAND  107  to which the test signals RRT and CRT, the control signal YFD and the control signal XF inverted by an inverter  106  are inputted. The output of the NAND  107  is connected to one input of a two-input NAND  108 , and the data signal DB outputted from the read amplifier  90  is applied to the other input of the NAND  108 . ACMOS inverter formed of a PMOS  102  and an NMOS  103  is connected to the output of the NAND  108 , and the CMOS inverter outputs the output data DOUT. 
   In the output buffer  100 B, only when the test signals RRT and CRT and the control signal YFD are at high level and the control signal XF is at low level, the output signal of the NAND  107  is turned to be low level. The output data DOUT is at low level all the time regardless of the data signal DB. The other operations are the same as those of the first embodiment, and thus the second embodiment has the same advantage. 
   Third Embodiment 
     FIG. 3A to 3D  are circuit diagrams of output buffers illustrating a third embodiment of the invention. These output buffers  100 C to  100 F are disposed instead of the output buffer  100 A shown in  FIG. 1 . The same components as those shown in  FIG. 1  are designated the same numerals and signs. 
   In the output buffer  100 C shown in  FIG. 3A , the AND  105  in the output buffer  100 A shown in  FIG. 1  is replaced by a three-input AND  105 A to omit inputting the control signal YFD. In addition, in the output buffer  100 D shown in  FIG. 3B , the AND  105  in the output buffer  100 A shown in  FIG. 1  is replaced by a three-input AND  105 B and the inverter  106  is removed to omit inputting the control signal XF. Both of them cannot confirm the control signals XF and YFD at the same time, but the other operations are almost the same as those of the first embodiment, having the same advantage. 
   In an output buffer  100 E shown in  FIG. 3C , the NAND  107  in the output buffer  100 B shown in  FIG. 2  is replaced by a three-input NAND  107 A to omit inputting the control signal YFD. Furthermore, in an output buffer  100 F shown in  FIG. 3D , the NAND  107  in the output buffer  100 B shown in  FIG. 2  is replaced by a three-input NAND  107 B and the inverter  106  is removed to omit inputting the control signal XF. Both of them cannot confirm the control signals XF and YFD at the same time, but the other operations are almost the same as the second embodiment, having the same advantage. 
   Fourth Embodiment 
     FIG. 4  is a circuit diagram of an output buffer illustrating a fourth embodiment of the invention. This output buffer  100 G is disposed instead of the output buffer  100 A shown in  FIG. 1 . The same components as those shown in  FIG. 1  are designated the same numerals and signs. 
   The output buffer  100 G has an inverter  106  to which the control signal XF is applied. The output of the inverter  106  is connected to one input of a two-input NAND  109 . The test signal RRT is applied to the other input of the NAND  109 . Furthermore, the test signal CRT and the control signal YFD are applied to a two-input NAND  110 . The outputs of the NANDs  109  and  110  are connected to the input of a two-input AND  111 . The output of the AND  111  is connected to one input of an exclusive NOR gate (hereafter, it is called ‘ENOR’)  112 . The data signal DB is applied to the other input of the ENOR  112 . The output of the ENOR  112  is connected to a CMOS inverter formed of a PMOS  102  and an NMOS  103 , and the CMOS inverter outputs the output data DOUT. 
   In the output buffer  100 G, when a high-level potential is applied to the test signal RRT to turn the control signal XF to be low level in testing the redundant memory cells, the output signal of the NAND  109  is turned to be low level and the output signal of the AND  111  is turned to be low level. Consequently, the data signal DB is inverted and outputted as the output data DOUT. Similarly, when a high-level potential is applied to the test signal CRT to turn the control signal YFD to be high level, the output signal of the NAND  110  is turned to be low level and the output signal of the AND  111  turned to be low level. Consequently, the data signal DB is inverted and outputted as the output data DOUT. 
   On the other hand, when the redundant memory cells are not tested and both the test signals RRT and CRT are at low level, the output signals of the NANDs  109  and  110  are turned to be high level. Consequently, the output signal of the AND  111  is turned to be high level, and the data signal DB is outputted as the output data DOUT, not inverted. 
   As described above, the output buffer  100 G of the fourth embodiment is configured in which the data signal DB is inverted to output the output data DOUT in testing the redundant memory cells. Accordingly, the fourth embodiment has the advantage that can test the redundant memory cells as similar to regular memory cells and can check whether to correctly set the test conditions inside the device from the test result. 
   Fifth Embodiment 
     FIG. 5  is a circuit diagram of an output buffer illustrating a fifth embodiment of the invention. The same components as those shown in  FIG. 1  are designated the same numerals and signs. This output buffer  100 H has a two-input NAND  113  to which the test signal RRT and the control signal XF are applied, and a two-input NAND  115  to which the test signal CRT and the control signal YFD inverted by an inverter  114  are applied. The outputs of the NANDs  113  and  115  are connected to the inputs of a two-input NAND  116 , and the output of the NAND  116  is connected to a node N 11 . 
   Furthermore, the output buffer  100 H has a PMOS  117  and an NMOS  118  connected in parallel with each other between the power supply voltage VCC and a node N 12 . The gate of the PMOS  117  and the NMOS  118  is connected to a node N 11 . Moreover, a CMOS inverter formed of a PMOS  102  and an NMOS  103  is connected between the node N 12  and the ground voltage GND. The data signal DB is applied to the input of the CMOS inverter through an inverter  101 , and the output data DOUT is outputted from the output thereof. 
   In the output buffer  100 H, only when the control signals XF and YFD are not turned to be correct level in testing the redundant memory cells, that is, only when the test signal RRT is at high level and the control signal XF is at high level, or the test signal CRT is at high level and the control signal YFD is at low level, the output signal of the NAND  116  (that is, the node N 11 ) is turned to be high level. Consequently, the PMOS  117  is in the off state, the NMOS  118  is in the on state and the potential of the node N 12  is VCC−Vt (where Vt is the threshold voltage of the NMOS  118 ). Therefore, the potential of the output data DOUT at high level is not increased to the power supply voltage VCC, which is turned to be VCC−Vt. 
   On the other hand, when the test configurations for the redundant memory cells are set correctly, or in case of normal memory access, the node N 11  is turned to be low level. Consequently, the PMOS  117  is in the on state, the NMOS  118  is in the off state, and the node N 12  is the power supply voltage VCC. The potential of the output data DOUT at high level is increased to the power supply voltage VCC. 
   As described above, the output buffer  100 H of the fifth embodiment is configured in which the power supply voltage of the output CMOS inverter circuit is decreased when the internal control signals are not set correctly in testing the redundant memory cells. Accordingly, the fifth embodiment has the advantage that the potential of the output data DOUT at high level is checked to allow checking whether to correctly set the test conditions inside the device. 
   Sixth Embodiment 
     FIG. 6  is a circuit diagram of an output buffer illustrating a sixth embodiment of the invention. This output buffer  200  is adapted to an output buffer for a traditional synchronous DRAM (hereafter, it is called SDRAM). The portion surrounded by an alternate long and short dash line is a traditional output timing control circuit  210 . 
   The output timing control circuit  210  is configured of flip flops (hereafter, it is called ‘FF’)  211  and  212  for sequentially delaying the data signal DB in synchronization with clock signal CLK, and TGs  213 ,  214  and  215  for selecting and outputting the data signal DB or delayed data signals. 
   In the output timing control circuit  210 , in the case of latency  1 , control signal LT 1  is turned to be high level and the TG  213  is in the on state for output in the same clock cycle as the clock signal CLK. In addition, in the case of latencies  2  and  3 , control signals LT 2  and LT 3  are turned to be high level and the TGs  214  and  215  are in the on state for output as delayed by one to two clock cycles from the clock signal CLK. 
   In the output buffer  200  of the sixth embodiment, in addition to the test signals RRT and CRT and the logics of the control signals XF and YFD, a circuit is added for outputting the output data DOUT at the timing corresponding to latency  4  when the test conditions are correctly set for the inside of the redundant memory cells. 
   More specifically, the output buffer  200  has a two-input NAND  222  to which the test signal RRT and the control signal XF inverted by an inverter  221  are applied, and a two-input NAND  223  to which the test signal CRT and the control signal YFD are applied. The outputs of the NANDs  222  and  223  are connected to a two-input NAND  224 , and control signal LT 4  outputted from the NAND  224  is applied to one inputs of NORs  225  to  227 . Control signals L 1  to L 3  are applied to the other inputs of the NORs  225  to  227 , and the control signals LT 1  to LT 3  are applied to the output timing control circuit  210  from the NORs  225  to  227 . 
   Furthermore, the output buffer  200  has an FF  228  for delaying further the output signal of the FF  212  by one clock cycle, and a TG  229  controlled by the control signal LT 4  is connected to the output of the FF  228 . The outputs of the TGs  213  to  215  and  229  are connected to a CMOS inverter formed of a PMOS  231  and an NMOS  232  through an inverter  230 , and the CMOS inverter outputs the output data DOUT. 
   In the output buffer  200 , when the internal conditions are set correctly in testing the redundant memory cells, the control signal LT 4  outputted from the NAND  224  is turned to be high level, and the TG  229  is in the on state to output the output data DOUT at the timing of latency  4 . 
   As described above, the output buffer  200  of the sixth embodiment is configured to output the output data DOUT at the timing of latency  4  when the internal control signals are set correctly in testing the redundant memory cells. Accordingly, the sixth embodiment has the advantage that the timing to output the output data DOUT is checked to allow checking whether to correctly set the test conditions inside the device. 
   Moreover, the invention is not limited to the embodiments, which can be modified variously. As the modified examples, for instance, the following are named. 
   The configuration of the row-replacement circuit  30  shown in  FIG. 1  is not limited to that shown in the drawing. It is acceptable to use a row switching circuit similar to the column switching circuit  70 . In addition, it is fine to use a column-replacement circuit similar to the row-replacement circuit  30  instead of the column switching circuit  70  shown in  FIG. 1 . 
   The configuration of the logic gate in the output buffer  100 A is not limited to that shown in the drawing. It is acceptable that the logic gate is configured of any combinations when the same conditions can be set. 
   The examples adapted to DRAMs have been described, but the invention can be similarly adapted to the semiconductor memory devices of other systems such as SRAMs (Static Random Access Memory). 
   The DRAM with a set of the redundant memory cells in both the column direction and the row direction has been exemplified. However, for example, the invention can be similarly adapted to semiconductor memory devices with redundant memory cells only in the column direction. Furthermore, the invention can be similarly adapted to semiconductor memory devices with a plurality of redundant memory cells in the same direction 
   As described above in detail, the second invention has the output circuit for outputting data read out of the redundant memory cells at a level different from the normal level when the test conditions for the redundant memory cells are correctly set. Accordingly, the second invention can check whether to correctly set the test conditions inside the device.