Abstract:
In a semiconductor memory tester in which a test pattern from a pattern generator is applied to a plurality of memory devices installed on a test head and their outputs are each logically compared by a logical comparator with an expected value for each pin, there are provided a plurality of OR circuits each of which obtains the OR of the results of logical comparisons corresponding to predetermined plural pins of each memory device. A plurality of multiplexers, each of which is supplied with the results of logical comparisons for corresponding output pins of the plurality of memory devices and a different one of the plurality of OR outputs, are provided for each group of corresponding output pins and each selectively output any one of the plurality of results of logical comparisons and the OR input thereto in accordance with a select signal. The outputs of these multiplexers are distributed to and stored in a plurality of fail memories.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor memory tester which has an arrangement in which a test pattern is applied to each of a plurality of memory devices under test mounted on a test head, the output from each memory device is subjected to a logical comparison with an expected value for each pin and the result of logical comparison is stored in a fail memory. 
     FIG. 1 shows a portion of a semiconductor memory tester of this kind. Memory devices under test A1 and A2 are mounted on a first test head 11A and memory devices B1 and B2 are mounted on a second test head 11B. A test pattern, i.e. data and an address, from a pattern generator 15 is applied to the memory devices A1, A2, B1 and B2, in which the data is written at the address and from which the thus written data is read out into logical comparators 16 1 , 16 2 , 17 1  and 17 2  for comparison with expected values for each pin. The compared results are output from the comparators 16 1 , 16 2 , 17 1  and 17 2 . 
     Conventionally, these results of comparison are stored in fail memories in the following manner: as depicted in FIG. 2 in which the results of logical comparison for only first pins of the memory devices A1, A2, B1 and B2 are shown, compared outputs A1 1  and A2 1  for the first pins of the memory devices A1 and A2 on the first test head 11A are both applied to multiplexers 18 1  and 182. The multiplexer 181 responds to a select signal S 1  to select and output one of the two inputs thereto, and the multiplexer 18 2  similarly responds to a select signal S 2  to select and output one of the two inputs thereto. Likewise, the results of comparison B1 1  and B2 1   for the first pins of the memory devices B1 and B2 on the second test head 11B are both provided to multiplexers 21 1  and 21 2 . The multiplexer 21 1  responds to a select signal S 3  to select and output one of the two inputs thereto, and the multiplexer 21 2  responds to a select signal S 4  to select and output one of the two inputs thereto. The outputs of the multiplexers 18 1  and 21 1  are input into a multiplexer 23 1 , which responds to a select signal S 5  to select and output one of the inputs. Similarly, the outputs of the multiplexers 18 2  and 21 2  are input into a multiplexer 23 2 , which responds to a select signal S 6  to select and output one of the inputs. The outputs of the multiplexers 23 1  and 23 2  are stored in fail memories 25 1  and 25 2 , respectively. 
     As a result of this, for example, when the select signals S 1  and S 5  are both low, the comparison result Al 1  is stored in the fail memory 25 1 , and when the select signals S 2  and S 6  are high and low, respectively, the comparison result B2 1  is stored in the fail memory 25 2 . Also for handling results of logical comparisons of the memory devices A1, A2, B1 and B2 for other pins, there are provided multiplexers similar to those 18 1 , 18 2 , 21 1 , 21 2 , 23 1  and 23 2  though not shown, and the fail memories 25 1  and 25 2  are adapted to store at respective addresses the results of logical comparisons corresponding to respective pins. 
     In the case where only one of the test heads 11A and 11B, for example, 11A, is used, the results of logical comparisons for the two memory devices A1 and A2 loaded on the test head 11A can concurrently be stored in the fail memories 25 1  and 25 2 . However, when the both test heads 11A and 11B are used, the following problems exist: 
     (a) Since the multiplexers 23 1  and 23 2  each select either one of the comparison results from the test heads 11A and 11B, all the comparison results for the memory devices A1, A2 and B1, B2 carried by the test heads 11A and 11B, respectively, cannot be output concurrently. In other words, the comparison result for only one of the memory devices, for example, A1 loaded on the test head 11A and the comparison result for only one of the memory devices, for example, B1 on the test head 11B are stored in the fail memories 25 1  and 25 2 . 
     (b) If the comparison results for both of the memory devices, for instance, A1 and A2 on the one test head 11A are simultaneously stored in the fail memories 25 1  and 25 2 , the comparison results for the both memory devices B1 and B2 on the other test head 11B cannot be stored in the fail memories 25 1  and 25 2 . 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a semiconductor memory tester in which the results of tests of a plurality of semiconductor memory devices under test on a desired one of a plurality of test heads can selectively stored in a plurality of fail memories as in the prior art and in which, if necessary, the results of &#34;good/no good&#34; decision on all the memory devices under test can be stored in a desired fail memory for each test pattern. 
     According to the present invention, the results of logical comparisons for a predetermined plurality of pins of each of N (where N is an integer equal to or greater than 2) memory devices under test on a first test head are ORed by a first OR circuit, and the results of logical comparisons for a predetermined plurality of pins of each of N memory devices under test on a second test head are ORed by a second OR circuit. A desired one of N+1 outputs including the results of logical comparisons for the corresponding pins of the N memory devices on the first test head and the output of the first OR circuit, corresponding to one of the N memory devices, is selected by a first multiplexer. Similarly, a desired one of N+1 outputs including the results of logical comparisons for the corresponding pins of the N memory devices on the second test head and the output of the second OR circuit, corresponding to one of the N memory devices, is selected by a second multiplexer. One of the outputs of the first and second multiplexers for each of the corresponding pins is selected by one of third multiplexers, and the outputs of these third multiplexers are stored in a plurality of fail memories. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a portion of a conventional memory tester; 
     FIG. 2 is a block diagram showing multiplexers connected to test heads in the conventional memory tester; 
     FIG. 3 is a block diagram illustrating a portion of a memory tester embodying the present invention; 
     FIGS. 4, 4A, and 4B are block diagrams showing the entire multiplexer circuitry used in the memory tester of the present invention; 
     FIG. 5 is a circuit diagram showing in detail a portion of the multiplexer circuitry depicted in FIG. 4; 
     FIG. 6 is a circuit diagram showing in detail another portion of the multiplexer circuitry depicted in FIG. 4; and 
     FIG. 7 is a table showing selective outputs corresponding to typical select signals of multiplexers in the multiplexer circuitry in FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will hereinafter be described on the assumption that the afore-mentioned integer N is four. 
     As shown in FIG. 3, the first test head 11A carries four memory devices under test A1 to A4 and the second test head 11B carries four memory devices under test B1 to B4. A test pattern composed of data and an address is applied from the pattern generator 15 to the memory devices A1 to A4 and B1 to B4, wherein the data is written at the address. The data thus written in the memory devices under test is read out therefrom and provided to logical comparators 16 1  to 16 4  and 17 1  to 17 4  for logical comparison with expected values for each pin. The results of such logical comparisons are stored in fail memories. In this embodiment the memory devices Al to A4 and B1 to B4 each have four output pins and the results of logical comparisons A1 1  to A1 4 , A2 1  to A2 4 , A3 1  to A3 4 , A4 1  to A4 4  and B1 1  to B1 4 , B2 1  to B2 4 , B3 1  to B3 4 , B4 1  to B4 4  are selectively stored in the fail memories as described below. 
     FIG. 4 illustrates an example of a multiplexer circuit constructed so that the results of logical comparisons output from the logical comparators 16 1  to 16 4  and 17 1  to 17 4  in FIG. 3 can selectively be stored in fail memories 25 1  to 25 4  as in the prior art and that, if necessary, the logical sums (i.e. logical OR&#39;s) of the results of logical comparisons corresponding to the plurality of pins of the memory devices can selectively stored in the fail memories 25 1  to 25 4  according to the present invention. 
     In FIG. 4, first multiplexers 18 1  to 18 4  are supplied with the results of logical comparisons A1 1 , A2 1 , A3 1  and A4 1  corresponding to first pins of the memory devices A1 to A4 loaded on the first test head 11A depicted in FIG. 3, respectively. The results of logical comparisons A1 1 , A1 2 , A1 3  and A1 4  corresponding to the four output pins of the memory device A1 are ORed by a first OR circuit 27 1  and the resulting logical sum ORAl is also provided to the multiplexers 18 1  to 18 4  Based on select signals al, bl and cl applied thereto, the first multiplexer 18 1  selects and outputs any one of the four results of logical comparisons A1 1  to A4 1  and the logical sum ORAl. The first multiplexers 18 2 , 18 3  and 18 4  each select and output any one of the four results of logical comparisons A1 1  to A4 1  and the logical sum ORAl, based on the select signals (a2, b2, c2), (a3, b3, c3) and (a4, b4, c4), respectively. Similarly, first multiplexers 18 5  to 18 8  are each supplied with the results of logical comparisons A1 2 , A2 2 , A3 2  and A4 2  corresponding to second output pins of the memory devices A1 to A4 and a logical sum ORA2 of the results of logical comparisons corresponding to the four output pins of the memory device A2 and provided from a first OR circuit 27 2 , and one of the five inputs to each of the first multiplexers 18 5  to 18 8  is selected and output therefrom in accordance with the select signals (a1, b1, c1), (a2, b2, c2), (a3, b3, c3) and (a4, b4, c4), respectively. First multiplexers 18 9  to 18 12  are each supplied with the results of logical comparisons A1 3 , A2 3 , A3 3  and A4 3  corresponding to third output pins of the memory devices A1 to A4 and a logical sum ORA3 of the results of logical comparisons corresponding to the four output pins of the memory device A3 and provided from a first OR circuit 27 3 , and one of the five inputs to each of the first multiplexers 18 9  to 18 12  is selected and output. First multiplexers 18 13  to 18 16  are each supplied with the results of logical comparisons A1 4 , A2 4 , A3 4  and A4 4  corresponding to fourth output pins of the memory devices A1 to A4 and a logical sum ORA4 of the results of logical comparisons corresponding to the four output pins of the memory device A4 provided from a first OR circuit 27 4 , and the first multiplexers 18 13  to 18 16  each selectively output one of such five inputs thereto in the same manner as referred to above. 
     Likewise, four groups of second multiplexers 21 1  to 21 4 , 21 5  to 21 8 , 21 9  to 21 12  and 21 13  to 21 16  are supplied with the results of logical comparisons B1 1  to B4 1 , B1 2  to B4 2 , B1 3  to B4 3  and B1 4  to B4 4  for the corresponding output pins of the memory devices B1 to B4 loaded on the test head 11B and logical sums ORB1 to ORB4 of the results of logical comparisons corresponding to the four output pins of each of the memory devices B1 to B4 provided from second OR circuits 28 1  to 28 4 , respectively. The second multiplexers each selectively output one of such five inputs thereto. 
     A third multiplexer 23 1  is supplied with the selected outputs from the first multiplexer 18 1  associated with the first test head 11A and the second multiplexer 21 1  associated with the second test head 11B and selectively outputs one of the two inputs in accordance with the select signal S 1 . A third multiplexer 23 2  is supplied with the selected outputs from the first and second multiplexers 18 2  and 21 2  and selectively outputs either one of the two inputs in accordance with the select signal S 2 . A third multiplexer 23 3  is supplied with the selected outputs from the first and second multiplexers 18 3  and 21 3  and selectively outputs either one of the two inputs in accordance with a select signal S 3 . A third multiplexer 23 4  is supplied with the selected outputs from the first and second multiplexers 18 4  and 21 4  and selectively outputs either one of the two inputs in accordance with a select signal S 4 . Similarly, each of third multiplexers 23 5  to 23 8 , 23 9  to 23 12  and 23 13  to 23 16  is also supplied with the Selected outputs from the first and second multiplexers associated with the first and second test heads 11A and 11B, respectively, and selectively outputs either one of the two inputs in accordance with corresponding one of select signals S 1  to S 4 . 
     The selected outputs from the third multiplexers 23 1  to 23 4  are provided as first bit inputs to fail memories 25 1  to 25 4  ; the selected outputs from the third multiplexers 23 5  to 23 8  are provided as second bit inputs to the fail memories 25 1  to 25 4  ; the selected outputs from the third multiplexers 23 9  to 23 12  are provided as third bit inputs to the fail memories 25 1  to 25 4  ; and the selected outputs from the third multiplexers 23 12  to 23 16  are provided as fourth bit inputs to the fail memories 25 1  to 25 4 . 
     FIGS. 5 and 6 show concrete circuits interconnecting the OR circuits 27 1  and 28 1 , the first multiplexers 18 1  to 18 4 , the second multiplexers 21 1  to 21 4 , the third multiplexers 23 1  to 23 4  and the fail memories 25 1  to 25 4  in FIG. 4. For convenience of description there are shown also in FIG. 6 the same OR circuits 27 1  and 28 1  as those depicted in FIG. 5. 
     As shown in FIGS. 5 and 6, the results of logical comparisons A1 1  to A4 1  corresponding to the first output pins of the memory devices 12 1  to 12 4  carried by the first test head 11A are input into the first multiplexers 18 1 , 18 2 , 18 3  and 18 4 , respectively. The results of logical comparisons A1 1  to A1 4  corresponding to the first to fourth output pins of the first memory device 12 1  mounted on the first test head 11A are ORed by the first OR circuit 27 1  and its output ORAl is applied to each of the first multiplexers 18 1  to 18 4 . The first multiplexer 18 1  responds to the select signals a1 and b1 to select and output any one of the inputs A1 1  to A4 1  and, when the select signal cl is high, outputs the output ORAl from the first OR circuit 27 1 . The first multiplexer 18 2  responds to the select signals a2 and b2 to select and output any one of the inputs A1 1  to A4 1  and, when the select signal c2 is high, outputs the output ORAl from the first OR circuit 27 1 . The first multiplexer 18 3  responds to the select signals a3 and b3 to select and output any one of the inputs A1 1  to A4 1  and, when the select signal c3 is high, outputs the output ORAl from the first OR circuit 27 1 . The first multiplexer 18 4  responds to the select signals a4 and b4 to select and output any one of the inputs A1 1  to A4 1  and, when the select signal c4 is high, outputs the output ORAl from the first OR circuit 27 1 . 
     The results of logical comparisons B1 1  to B4 1  corresponding to the first output pins of the memory devices 14 1  to 14 4  carried by the second test head 11B are input into the second multiplexers 21 1 , 21 2 , 21 3  and 21 4 , respectively. The results of logical comparisons corresponding to the first to fourth output pins of the memory device 14 1  mounted on the second test head first memory device 14 1  mounted on the second test head 11B are ORed by the second OR circuit 28 1  and its output ORB1 is applied to each of the second multiplexers 21 1  to 21 4 . As is the case with the first multiplexers 18 1  to 18 4 , the second multiplexers 21 1  to 21 4  each selectively output any one of the five inputs in accordance With the select signals (a1, b1, c1), . . . (a4, b4, c4). 
     The outputs of the first and second multiplexers 18 1  and 21 1  are input into the third multiplexer 23 1  and either one of the two inputs is selectively output therefrom in accordance with the select signal S 1  ; the outputs of the first and second multiplexers 18 2  and 21 2  are input into the third multiplexer 23 2  and either one of the two inputs is selectively output therefrom in accordance with the select signal S 2  ; the outputs of the first and second multiplexers 18 3  and 21 3  are input into the third multiplexer 23 3  and either one of the two inputs is selectively output therefrom in accordance with the select signal S 3  ; and the outputs of the first and second multiplexers 18 4  and 21 4  are input into the third multiplexer 23 4  and either one of the two inputs is selectively output therefrom in accordance with the select signal S 4 . The outputs of the third multiplexers 23 1  to 23 4  are stored in the fail memories 25 1   and 25 4 , respectively. 
     The connections of the first multiplexers 18 5  to 18 8 , the second multiplexers 21 5  to 21 8  and the third multiplexers 23 5  to 23 8  in FIG. 4 are similar to those shown in FIGS. 5 and 6. Further, the first multiplexers 18 9  to 18 12 , the second multiplexers 21 9  to 21 12 , the third multiplexers 23 9  to 23 12  and the first multiplexers 18 13  to 18 16 , the Second multiplexers 21 13  to 21 16 , the third multiplexers 23 13  to 23 16  are also connected in a manner Similar to those shown in FIGS. 5 and 6. Hence, these circuit connections are not shown, for the sake of brevity. 
     Table I in FIG. 7 shows, by way of example, the selected outputs of the first and second multiplexers 18 1  to 18 4  and 21 1  to 21 4  and the selection of the third multiplexers 23 1  to 23 4  in response to the application thereto of the select signals identified by a, b, c and S. A horizontal column labeled &#34;1st MPX&#34; shows the five inputs A1 1  to A4 1  and ORAl to the first multiplexers 18 1  to 18 4  associated with the first test head 11A and selected outputs corresponding to combinations of the select signals (a, b, c). Another horizontal column labeled &#34;2nd MPX&#34; shows the five inputs B1 1  to B4 1  and ORB1 to the second multiplexers 21 1  to 21 4  associated with the second test head 11B and selected outputs corresponding to combinations of the select signals (a, b, c). A vertical column labeled &#34;3rd MPX&#34; shows the multiplexer outputs which the third multiplexers 23 1  to 23 4  select in accordance with the select signal S. 
     Also in connection with the groups of first multiplexers 18 5  to 18 8 , 18 9  to 18 12 , 18 13  to 18 16 , the groups of second multiplexers 21 5  to 21 8 , 21 9  to 21 12 , 21 13  to 21 16  and the groups of third multiplexers 23 5  to 23 8 , 23 9  to 23 12 , 23 13  to 23 16 , the selected outputs based on the select Signals a, b, c and S can easily be determined in the same manner as described above in respect of Table I. Hence, such selected outputs are not shown. 
     Now, let it be assumed that the select signals are as follows: al=0, bl=0, cl=0, a2=1, b2=0, c2=0, a3=0, b3=1, c3=0, a4=1, b4=1, c4=0, S 1  =0, S2=0, S3=0, S4=0. In the first multiplexers 181, 182, 18 3  and 18 4  the outputs A1 1 , A2 1 , A3 1  and A4 1  are selected, respectively; in the first multiplexers 18 5 , 18 6 , 18 7  and 18 8  the outputs A1 2 , A2 2 , A3 2  and A4 2  are selected, respectively; in the first multiplexers 18 9 , 18 10 , 18 11  and 18 12  the outputs A1 3 , A2 3 , A3 3  and A4 3  are selected, respectively; and in the first multiplexers 18 13 , 18 14 , 18 15  and 18 16  the outputs A1 4 , A2 4 , A3 4  and A4 4  are selected, respectively. Furthermore, since the third multiplexers 23 1  to 23 16  all select the first test head 11 side (the first multiplexer outputs), the results of logical comparisons A1 1  to A1 4  of the memory device 12 1  are stored in the fail memory 25 1  ; the results of logical comparisons A2 1  to A2 4  of the memory device 12 2  are stored in the fail memory 25 2  ; the results of logical comparisons A3 1  to A3 4  of the memory device 12 3  are stored in the fail memory 25 3  ; and the results of logical comparisons A4 1  to A4 4  of the memory device 12 4  are stored in the fail memory 25 4 . In other words, the results of logical comparisons of the memory devices 12 1  to 12 4  are stored individually in the fail memories 25 1  to 25 4 . 
     In the case where the select signals (a1, b1, c1), ... (a4, b4, c4) for the second multiplexers 21 1  to 21 16  have the same values as in the above case and the select signals S 1 , S 2 , S 3  and S 4  are all set to 1, the third multiplexers 23 1  to 23 16  select the second test head 11B side (the second multiplexer outputs), and consequently, the results of logical comparisons B1 1  to B1 4  of the memory device 14 1 , the results of logical comparisons B2 1  to B2 4  of the memory device 14 2 , the results of logical comparisons B3 1  to B3 4  of the memory device 14 3  and the results of logical comparisons B4 1  to B4 4  of the memory device 14 4  are stored in the fail memories 25 1 , 25 2 , 25 3  and 25 4 , respectively. That is, the results of logical comparisons of the memory devices 14 1  to 14 4  are stored individually in the fail memories 25 1  to 25 4 . 
     Setting the select signals a1, b1, a2 and b2 to 0, a3, b3, a4 and b4 to arbitrary values, S 1  to 0, S 2  to 1, S 3  to 0, S 4  to 1, the select signal cl of each of the first multiplexers 18 1 , 18 5 , 18 9  and 18 13  to 1, the other select signals c2, c3 and c4 to 0, the select signal cl of each of the second multiplexers 21 1 , 21 5 , 21 9  and 21 13  to 1 and the other select signals c2, c3 and c4 to 0, the third multiplexer 23 1  provides the output ORAl of the first OR circuit 27 1  and, similarly, the third multiplexers 23 5 , 23 9  and 23 13  provide the outputs ORA2, ORA3 and ORA4 of the first OR circuits 27 2 , 27 3  and 27 4 , respectively. The outputs of the third multiplexers 23 1 , 23 5 , 23 9   and 23 13  are stored in the fail memory 25 1 . Furthermore, the third multiplexer 23 2  provides the output ORB1 of the second OR circuit 28 1 , the third multiplexers 23 6 , 23 10  and 23 14  provide the outputs ORB2, ORB3 and ORB4 of the second OR circuits 28 2 , 28 3  and 28 4 , respectively, and the outputs of the third multiplexers 23 2 , 23 6 , 23 10  and 23 14  are stored in the fail memory 25 2 . In this way, the logical sums ORA1, ORA2, ORA3 and ORA4 of the results of logical comparisons A1 1  to A1 4 , A2 1  to A2 4 , A3 1  to A3 4  and A4 1  to A4 4  of the memory devices 12 1 , 12 2 , 12 3  and 12 4  loaded on the first test head 11A are stored in the fail memory 25 1 . Likewise, the logical sums ORB1, ORB2, ORB3 and ORB4 of the results of logical comparisons B1 1  to B1 4 , B2 1  to B2 4 , B3 1  to B3 4  and B4 1  to B4 4  of the memory devices 14 1 , 14 2 , 14 3  and 14 4  loaded on the second test head 11B are stored in the fail memory 25 2 . Thus, the results of logical comparisons of the memory devices on the first and second test heads 11A and 11B can be stored in the fail memories 25 1  and 25 2   concurrently. In this instance, only the OR of the results of logical comparisons corresponding to the first to fourth output pins of each memory device can be known, that is, the results of logical comparisons for each pin are not stored in the fail memory. However, in the case of a memory device in which a plural-bit word is accessed by one address, i.e. in a memory device of a 4-bit word configuration, it is necessary only to check the memory device for an error for each word (or address); there is no need of making a check for an error for each bit (or pin). 
     The above embodiment has been described to include two test heads, but the number of test heads is not limited specifically thereto but may be one or more than two. While the above embodiment has been described in connection with the case where four memory devices under test are loaded on each test head and each have four output pins, it can also be considered that the memory devices Al and A2 in FIG. 3, for example, are those into which one memory device having eight output pins has been provisionally divided for test. The same is true of the memory devices A3, A4, Bl, B2, B3 and B4. The number of fail memories need not always be four but needs only to be equal to at least the number of test heads used. 
     As described above, according to the present invention, the results of logical comparisons corresponding to a plurality of pins of each memory device are ORed and the OR is selectable. Hence, the results of logical comparisons corresponding to each pin of a plurality of memory device on each test head can be stored in separate fail memories as in the prior art, besides, by selecting the above-mentioned OR, the results of tests of the memory devices on first and second test heads can be stored in separate fail memories concurrently. 
     It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.