Abstract:
A semiconductor memory device is of a bank switching type having a plurality of memory array banks provided in a memory chip which can be switched from one to another for storage operation. The semiconductor memory device includes: a plurality of memory arrays in the memory array banks; an input/output circuit for transmitting information data between the memory arrays and the outside; a data bus for connecting between the memory arrays and the input/output circuit; and N-channel transistors provided across the data bus. The data bus consists of a plurality of adjacent lines. Each of N-channel transistors is connected at their drain to the corresponding lines of the data bus while at their source to the ground. When a multi-bit test is commenced for writing and reading data on the memory arrays, the N-channel transistors are turned on to connect the lines of the data bus to the ground.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor memory device having a plurality of memory cell arrays, e.g., in the form of DRAMs in a memory chip, and a method of testing the semiconductor memory device.  
           [0003]    2. Description of The Related Art  
           [0004]    In recent years, as highly integral circuit technologies have been introduced in the semiconductor industries, the cost per bit of memories is significantly decreased. However, the increasing demand for memory device now requires an advanced technology for more reduction of the cost. It is however known that as advanced memories for mass storage are developed, their testing process now claims a considerable length of time. This may inhibit the cost reduction, hence hardly permitting more decrease in the cost per bit of memories.  
           [0005]    For overcoming the foregoing drawback, some methods are proposed of reducing the time required for testing a semiconductor memory device of bank switching type where a plurality of memory array banks are mounted in a memory chip, and switched from one to another for storage operation. As one of such methods, a multi-bit test is well known, which is a sort of multi-bit parallel test method, for writing one identical data onto a number of bits at once and examining the data read from their corresponding bits for matching or mismatching. In a normal operation mode of the semiconductor memory device, one-bit data received from a DQ pad is written onto and read from each memory cell. In a multi-bit test mode of the semiconductor memory device, one-bit data received from the DQ pad are written onto and read from a plurality of memory cells at once before returned back to the DQ pad.  
           [0006]    The write/read operation in both the normal mode and the multi-bit test mode of the conventional semiconductor memory device of a bank switching type will now be explained referring to FIGS.  10  to  13 . FIG. 10 is a block diagram schematically showing the conventional semiconductor memory device of a bank switching type. The semiconductor memory device denoted by  80  includes four memory array banks  81 A to  81 D provided in a memory chip. Each of the memory array banks  81 A to  81 D has a plurality of memory arrays (only two shown in FIG. 10). Each memory array is connected by a data bus  82  to a DQ input/output circuit  84 .  
           [0007]    An information data EXTDQ received from the outside is passed through the DQ input/output circuit  84 , sent to the memory array in the memory array bank addressed by an address data EXTA received together with the information data EXTDQ via the data bus  82 , and written onto a corresponding memory cell. The information data in the memory cell is read out in response to the input of reading command for the memory cell, sent to the DQ input/output circuit  84  via the data bus  82 , and output to the outside.  
           [0008]    [0008]FIG. 11 illustrates a detail of the memory array bank  81 B. The memory array banks  81 A,  81 C, and  81 D are identical in the structure to the memory array bank  81 B. The memory array bank  81 B includes four memory arrays  85   a  to  85   d,  four sense amplifiers  86   a  to  86   d  connected to the corresponding memory arrays  85   a  to  85   d,  and four I/O circuits  91   a  to  91   d,  as the transmission paths of information data. Also, the memory array bank  81 B has a column decoder  87 , a column address buffer  88  connected to the column decoder  87 , a row decoder  89 , and a row address buffer  90  connected to the row decoder  89 , as the transmission path of address data. The sense amplifiers  86   a  to  86   d  are connected by sub data buses  92   a  to  92   d  to the I/O circuits  91   a  to  91   d  respectively.  
           [0009]    In the memory array bank  81 B, on the occasion of data writing in a normal mode, a one-bit information data received at the input (not shown) of the DQ input/output circuit  84  is sent via the data bus  82  and written onto a memory cell at the intersection between a row address and a column address in the memory array determined by the address data. For example, in the memory array  85   a,  the information data received via the DQ input/output circuit  84  is written onto the memory cell (denoted by “A”) assigned at the intersection between a word-line (denoted by “WL”) selected by the row decoder  89  and a column-select-line (denoted by “CSL”) selected by the column decoder  87 .  
           [0010]    On the occasion of data writing in the multi-bit test mode, the information data received via the DQ input/output circuit  84  is written simultaneously onto four memory cells (denoted by “A” to “D”) assigned at the intersection between a word-line and a column-select-line, for same row and column address, in memory arrays  85   a  to  85   d.    
           [0011]    On the occasion of data reading in the normal mode, the data are read out from the memory cells assigned at the intersection between a word-line selected by the row decoder  89  and a column-select-line selected by the column decoder  87  in memory arrays  85   a  to  85   d.  The data are transmitted via the sense amplifiers  86   a  to  86   d,  the sub data buses  92   a  to  92   d,  the I/O circuits  91   a  to  91   d,  the data bus  82 , and the DQ input/output circuit  84  and output to the outside.  
           [0012]    On the occasion of data reading in the multi-bit test mode, the data are read out from the memory cells A, B, C, and D shown in FIG. 11 by energizing the four column-select-lines at once similar to the writing operation, passed via the sense amplifiers  86   a  to  86   d,  the sub data buses  92   a  to  92   d,  and I/O circuits  91   a  to  91   d,  and loaded to the data bus  82 .  
           [0013]    As the data bus  82  basically includes a pair of lines DB (data bus) and ZDB, its paired lines are provided on a layout pattern depending on the type of the memory device. For example, when the memory device is of x16 type, the data bus  82  has sixteen pairs of lines. When x8 type, the data bus  82  has eight pairs of lines. While the data bus  82  is patterned in such manner, each of the I/O circuits  91  to  91   d  includes a data bus drive circuit  93  (referred to as a DB drive circuit hereinafter) for DB-ZDB lines as best shown in FIG. 12. When receiving a H level signal from the memory cell, the DB drive circuit  93  provides an “H” drive on the DB line and an “Hi-Z” (high impedance) drive on the ZDB line. On the other hand, when receiving an L level signal from the memory cell, the DB drive circuit  93  provides an “Hi-Z” drive on the DB line and an “H” drive on the ZDB line.  
           [0014]    The data bus  82  performs a wired-or action on each data from the four memory cells and its wired-or signal is transmitted to the DQ input/output circuit  84 . On the occasion of data writing, the same data is simultaneously written on four memory cells. Accordingly, when non of the four memory cells is defective, the four data read out from their respective memory cells are identical thus providing the H level on either the DB or ZDB line. When the read-out data is an H level signal, the DB line draws “H” and the ZDB line draws “(Hi-Z)”. When the read-out data is an L level signal, the DB line draws “L(Hi-Z)” and the ZDB line draws “H”.  
           [0015]    If any of the four memory cells is defective, the read-out data include both H and L levels hence causing both the lines DB and ZDB to draw “H”. When any of the four memory cells is defective and the read-out data include both H and L levels, the “H” level on both the lines DB and ZDB is received by the DQ input/output circuit  84 .  
           [0016]    Upon receiving a result from the four memory cell data, the DQ input/output circuit  84  performs a pass/fail operation. More specifically as shown in FIG. 13, the DQ input/output circuit  84  has a logic that it judges “passed” when the DB line and the ZDB line draw “H” and “L”, or “L” and “H” respectively and “failed” when both the DB and ZDB lines draw “H”. FIGS. 14 and 15 are timing charts of the signals when the multi-bit test mode provides “passed” and “failed” respectively. The symbols (e.g. EXTCLK, EXTA, and EXTDQ) shown in FIGS. 14 and 15 represent the signals announced in the output side of the DQ input/output circuit  84  shown in FIG. 13.  
           [0017]    As described above, the multi-bit test allows the multiple memory cells in different memory arrays to be tested at once thus contributing to the reduction of the time required for testing the semiconductor memory device  80 .  
           [0018]    However, the paired lines DB and ZDB of the data bus  82  are located adjacent to each other in a layout pattern. As the memory cells are improved in the down-sizing and the integral level, the generation of a parasitic capacitance C between the paired lines may hardly be negligible (See FIG. 16). When any paired lines adjacent to each other of lines in the data bus  82  are energized in the multi-bit test mode, their induced parasitic capacitance C may produce a coupling noise which elevates the level of the data bus  82  on “L(Hi-Z)” side. As a result, its level is detected as “H” level and may interrupt the pass/fail operation of the DQ input/output circuit  84  thus leading to a fault decision.  
         SUMMARY OF THE INVENTION  
         [0019]    It is an object to provide a semiconductor memory device which can eliminate any fault result of a multi-bit test resulting from the coupling noises produced between two adjacent lines of a data bus and a method of testing the semiconductor memory device.  
           [0020]    A semiconductor memory device of the present invention is of a bank switching type having a plurality of memory array banks provided in a memory chip which can be switched from one to another for storage operation. The semiconductor memory device basically includes: a plurality of memory arrays incorporated in the memory array banks; an input/output circuit for transmitting information data between the memory arrays and the outside; a data bus for connecting between the memory arrays and the input/output circuit; and N-channel transistors provided across the data bus. The data bus consists of a plurality of adjacent lines. Each of N-channel transistors is connected at their drain to the corresponding lines of the data bus while at their source to the ground. In the semiconductor memory, when a multi-bit test is commenced for writing and reading data on the memory arrays, the N-channel transistors are turned on to connect the lines of the data bus to the ground.  
           [0021]    Accordingly, the “L(Hi-Z)” level on the lines which may undesirably be elevated by the effect of coupling noises resulting from the parasitic capacitance C induced between any two adjacent lines of the data bus, can be maintained to a low level enough not to recognized as the “H” level. As a result, any fault judgment of the multi-bit test caused by the effect of coupling noises generated between the lines of the data bus can successfully be eliminated.  
           [0022]    A set of the N-channel transistors are connected in parallel to each other across each line of the data bus, a switch is provided at the drain of each the N-channel transistor for connecting and disconnecting the line and the source.  
           [0023]    A set of the N-channel transistors are connected in parallel to each other across each line of the data bus, a fuse is provided at the drain of each the N-channel transistor for disconnecting the line when receiving an over-current.  
           [0024]    The semiconductor memory device further includes a delay circuit for delaying by a predetermined length of time the input of a signal to the gate of the N-channel transistor from the start of driving the data via the lines of the data bus.  
           [0025]    The semiconductor memory device still further includes a delay circuit for delaying by a predetermined length of time the input of a signal to the gate of the N-channel transistor when the output side of the input/output circuit on the lines has reached to a CMOS level. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a diagram of an overall arrangement of a semiconductor memory device according to first embodiment of the present invention;  
         [0027]    [0027]FIG. 2 is a diagram showing a part of a memory array bank provided in the semiconductor memory device;  
         [0028]    [0028]FIG. 3 is a diagram showing a control circuit for a DB clamp circuit provided in the I/O circuit;  
         [0029]    [0029]FIG. 4 is a diagram of the DB clamp circuit;  
         [0030]    [0030]FIG. 5 is a diagram of another DB clamp circuit according to second embodiment of the present invention;  
         [0031]    [0031]FIG. 6 illustrates a first modification of the DB clamp circuit of second embodiment;  
         [0032]    [0032]FIG. 7 illustrates a second modification of the DB clamp circuit of second embodiment;  
         [0033]    [0033]FIG. 8 is a diagram of a delay circuit provided in the I/O circuit for making the activation period of the DB clamp circuit variable according to third embodiment of the present invention;  
         [0034]    [0034]FIG. 9 is a diagram of a delay circuit provided in the output side of a DQ input/output circuit for making the activation period of the DB clamp circuit variable according to fourth embodiment of the present invention;  
         [0035]    [0035]FIG. 10 is a diagram of an overall arrangement of a conventional semiconductor memory device;  
         [0036]    [0036]FIG. 11 is a diagram showing a part of a memory array bank provided in the conventional semiconductor memory device;  
         [0037]    [0037]FIG. 12 is a diagram of a DB drive circuit provided in an I/O circuit of the memory array bank in the conventional semiconductor memory device;  
         [0038]    [0038]FIG. 13 is a diagram of the output side of a DQ input/output circuit provided in the conventional semiconductor memory device;  
         [0039]    [0039]FIG. 14 is a timing chart of signals when the multi-bit test mode provides “passed”;  
         [0040]    [0040]FIG. 15 is a timing chart of the signals when the multi-bit test mode provides “failed”; and  
         [0041]    [0041]FIG. 16 is a schematic diagram showing a parasitic capacitance C produced between any adjacent lines of the data bus. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    Some embodiments of the present invention will be described in more detail referring to the accompanying drawings.  
         [0043]    First Embodiment  
         [0044]    [0044]FIG. 1 illustrates an overall arrangement of a semiconductor memory device according to first embodiment of the present invention. The semiconductor memory device denoted by  10  has four memory array banks  1 A to  1 D provided in the form of a memory chip which are switched from one to another for storage operation. Each of the memory array banks  1 A to  1 D includes a plurality of memory arrays (only two shown in FIG. 1). Each memory array is connected by a data bus  2  to a DQ input/output circuit  4  for transmitting data between the memory array and the outside.  
         [0045]    Also, DB clamp circuits  5  are provided across the data bus  2  between the memory array banks  1 A to ID and the DQ circuit  4  for attenuating the effect of coupling noises caused by the parasitic capacitance in their adjacent lines of the data bus  2 , as will be explained later in more detail.  
         [0046]    In an operation of the semiconductor memory device  10 , information data EXTDQ received from the outside are transmitted via an input (not shown) of the DQ input/output circuit  4 , sent to corresponding memory arrays, determined by address data EXRA received with the information data EXTDQ, in the memory array banks  1 A to  1 D through the data bus  2  and the DB clamp circuits  5 , and written onto the memory cells. Any of the information data written on the memory cells can be read out in response to input of data for reading, transmitted via the data bus  2  and DB clamp circuits  5  to the output of the DQ input/output circuit  4 , and output to outside.  
         [0047]    [0047]FIG. 2 is a schematic diagram showing a detail of the memory array bank  1 B. The other memory array banks  1 A,  1 C, and  1 D are identical in the construction to the memory array bank  1 B. The memory array bank  1 B includes four memory arrays  6   a  to  6   d,  four sense amplifiers  7   a  to  7   d  connected to the corresponding memory arrays  6   a  to  6   d,  and four I/O circuits  13   a  to  13   d  which all constitute a transmission path of the information data. Also, the memory array back  1 B includes a column decoder  8 , a column address buffer  9  connected to the column decoder  8 , a row decoder  11 , and a row address buffer  12  connected to the row decoder  11  which all constitute a transmission path of the address data. The sense amplifiers  7   a  to  7   d  are connected to the four I/O circuits  13   a  to  13   d  via four sub data buses  14   a  to  14   d,  respectively.  
         [0048]    As described above, particularly in the semiconductor memory device  10 , the DB clamp circuits  5  are provided across the data bus  2  which connects between the memory array banks  1 A to  1 D and the DQ input/output circuit  4 . The DB clamp circuits  5  provide “leak paths” from the lines of the data bus  2  to the ground GND. As shown in FIG. 2, the I/O circuits  13   a  to  13   d  corresponding to their respective memory arrays  6   a  to  6   d  are connected by a drive bus  15  to the DB clamp circuit  5 . The on/off action of the DB clamp circuit  5  can be controlled by DB drive circuits provided in the I/O circuits  13   a  to  13   d.  For this purpose, each the DB drive circuit includes a specific control circuit for controlling the operation of the DB clamp circuit  5 , shown in FIG. 3, in addition to the conventional arrangement described previously referring to FIG. 11.  
         [0049]    [0049]FIG. 4 illustrates a structure of the DB clamp circuit  5 . The DB clamp circuit  5  allows the line of the data bus  2  to be connected to the ground by an N-channel transistor of which the gate width (W) is smaller than a predetermined value, thus providing a leak path to the ground GND. More specifically, the N-channel transistor is connected between a pair of lines DB and ZDB which is a basic unit of the data bus  2 . The gate of the N-channel transistor is communicated to the drive bus  15  from the I/O circuits  13   a  to  13   d  and the drain of it is communicated to the ground. The DB clamp circuit  5  in first embodiment is switched on in response to a pulse signal received at the gate of the N-channel transistor via the drive bus  15  and held throughout the period for data reading operation in the multi-bit test mode.  
         [0050]    With the DB clamp circuits  5 , in the multi-bit test mode, by grounding adjacent lines of data bus  2  are simultaneously used, a level of lines on “L(Hi-Z)” side, which may possibly be elevated by coupling noises of their parasitic capacitance C between their lines, can be reduced to such a level that can hardly be misjudged as an “H” level. Accordingly, fault judgment in the multi-bit test which may result from the coupling noise between adjacent lines of the data bus  2  will successfully be eliminated.  
         [0051]    Second Embodiment  
         [0052]    As described above, by providing the N-channel transistors between any two relevant lines of the data bus  2 , the level of lines on “L(Hi-Z)” side can be reduced. At the time, on “H” side, a through current can flow from the drain to the ground GND via the lines of the data bus  2 . Accordingly, the multi-bit test may be interrupted by the through current, depending on the gate length (L) and width (W) of the N-channel transistors. For eliminating this drawback, in second embodiment, the DB clamp circuits having a modified arrangement as follows is provided.  
         [0053]    [0053]FIG. 5 illustrates the modified DB clamp circuit according to second embodiment. The modified DB clamp circuit  20  has two or more N-channel transistors provided in parallel between adjacent lines of the data bus  2  (only two shown in FIG. 5). Also, a switch (denoted by “AL SW”) is provided between the drain of each N-channel transistor and the line DB or ZDB.  
         [0054]    The DB clamp circuit  20  allows the operation mode of the N-channel transistors to be switched from one to another by the switch. Accordingly, the gate length (L) and width (W) of the N-channel transistor in the DB clamp circuit  20  can properly be modified, thus optimizing the size of the leak path to the ground GND. This inhibit the through current from the drain to the ground GND on “H” side from interrupting the result of the multi-bit test.  
         [0055]    With the DB clamp circuit  20 , the gate length and width of its N-channel transistors may be modified for each arrangement of DB clamp circuits  20  in the semiconductor memory device. For example, in the semiconductor memory device as shown in FIG. 1, provided that the three DB clamp circuits  5  with a single N-channel transistor are replaced by the DB clamp circuits  20  with two or more N-channel transistors, the gate length and width can be controlled for each DB clamp circuits located at a specific position. By optimizing the size for gate length and width at each position, the multi-bit test will be improved in the reliability.  
         [0056]    In second embodiment, the switch with N-channel transistors is provided, but is not limited to the switch. For example, fuses  32  may be used as shown in a DB clamp circuit  30  of FIG. 6, which are disconnected when an over-current flows through. Alternatively, a combination of the N-channel transistor with switches and the N-channel transistor with fuses  32  may be used in a DB clamp circuit  40  as shown in FIG. 7.  
         [0057]    Third Embodiment  
         [0058]    [0058]FIG. 8 illustrates a delay circuit provided in each of the I/O circuits  13   a  to  13   d  for making the activation period of the DB clamp circuit variable, according to third embodiment of the present invention. The delay circuit denoted by  50  delays by a few nanoseconds the output of a pulse signal to the drive bus  15  shown in FIG. 2 from the start of energizing the line DB of the data bus  2 , that is, from the leading edge of a PDD (i.e. data bus drive signal) in the DB drive circuit (See FIG. 12) incorporated in each of the I/O circuits  13   a  to  13   d.    
         [0059]    As a result, the input of the pulse signal to the gate of the N-channel transistor in the DB clamp circuit is delayed, i.e. the timing of turning the N-channel transistor on is delayed by a few nanoseconds from the start of energizing the line DB of the data bus  2 . This causes the time required for increasing the line DB driven on “H” side to a CMOS level to be shorter than that of first embodiment (where the N-channel transistor is kept on throughout the period of data reading process in the multi-bit test). Accordingly, the multi-bit test can be prevented from declining in the frequency characteristic.  
         [0060]    Fourth Embodiment  
         [0061]    [0061]FIG. 9 illustrates a delay circuit provided in the output side of the DQ input/output circuit for making the activation period of the DB clamp circuit variable in fourth embodiment of the present invention. The arrangement of the output side of the DQ input/output circuit in fourth embodiment is identical to that shown in FIG. 13 except the delay circuit denoted by  60 . The delay circuit  60  delays by a few nanoseconds the output of a pulse signal to the gate of the N-channel transistor in the DB clamp circuit from the time when the line DB driven on H side in the output of the DQ input/output circuit reaches to a CMOS level, that is, from the leading edge of a GATE signal shown in FIG. 13.  
         [0062]    As a result, the timing of turning the N-channel transistor on is delayed by a few nanoseconds from the rise of the GATE signal in the output side of the DQ input/output circuit. This causes the time required for increasing the line DB driven on “H” side to a CMOS level to be shorter than that of first embodiment (where the N-channel transistor is kept on throughout the period of data reading process in the multi-bit test), as well as third embodiment. Accordingly, the multi-bit test can be prevented from declining in the frequency characteristic.  
         [0063]    Although not shown, each DB clamp circuit provided in the semiconductor memory device may include a delay circuit for making the activation period of the DB clamp circuit of third embodiment or 4 so that the turn-on period of the N-channel transistor in the DB clamp circuit can be modified for each DB clamp circuit located at a specific position. This allows a level of line driven on “L(Hi-Z)” side and the time required for increasing the level of lines driven on “H” side to the CMOS level to be precisely controlled. As a result, the multi-bit test can further be improved in the reliability.  
         [0064]    It would be understood that the present invention is not limited to the foregoing embodiments but various changes and modifications may be made without departing the scope of the present invention.