Abstract:
A parallel test circuit is provided in a semiconductor memory chip for use during both a wafer test and a package test. The parallel test circuit operates to automatically reduce the number of test output pins associated with a single package test to thereby increase the number of packages that can be tested simultaneously. The parallel test circuit includes a selector for limiting the number of output pads which may be activated during a package test run. The selector is responsive to a wafer test enable signal, from a selection control circuit, to control output pad selection.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor memory devices, and more particularly to a parallel test circuit for testing memory devices. 
     2. Description of the Related Art 
     The ever increasing complexity of semiconductor components has been matched, in recent years, by similar developments in test and reliability philosophies and practices. 
     Historically, semiconductor memory testing was performed for a variety of reasons. Memory manufacturers perform tests to guarantee performance within predefined specifications and to screen out those components not meeting the specification. 
     Generally a plurality of memory cells are fabricated on a wafer, and then separated so as to be subjected to a packaging process. A self-test circuit, provided within the semiconductor memory device, has been used to evaluate the performance of memory cells. 
     The test of semiconductor memory devices is generally performed in two steps. The first step is performed in what is known as a wafer state (hereinafter referred to as wafer test), and the second step in what is known as a package state (hereinafter referred to as package test). 
     The wafer test is performed before separating the memory cells fabricated on the wafer to detect defective memory cells. An external detection terminal is directly coupled to each output pad of the test circuit which is provided in the memory device and fabricated on the wafer. Accordingly, defective cells are either removed or repaired. 
     After successful completion of the wafer test, memory cells are then subjected to a packaging process to generate an assembled final product. The package test is performed after the packaging process in order to remove those defective memory cells produced during the packaging process. 
     Because input/output pins of the memory device are connected to an output pad associated with a test circuit, a test board is generally used to connect such input/output pins to external detection terminals when performing the package test. Ordinarily, both wafer and package tests use a parallel test procedure which enables a plurality of memory cells to be tested simultaneously. Parallel test modes and practices are known, and have been described in some detail in &#34;Semiconductor Memories: A Handbook of Design, Manufacture, and Application&#34; 2nd Ed., by Betty Prince; John Wiley &amp; Sons, pp. 698-717 (1991). 
     FIG. 2 schematically shows a conventional 16-bit parallel test circuit such as may be found in a 2 Mbyte×8 (i.e., 16 Mbit) dynamic random access memory (DRAM). A data output path is provided having 16 accessible data lines DBi/ DBi (where i=0 to 15), eight 2-bit comparators, a φFTE signal input, 8 output buffers, and 8 output pads. FIG. 1 illustrates a conventional operational timing chart showing the relationship of parallel test enable signal φFTE (a controlling input to the parallel test circuit in FIG. 2) as a function of DRAM read/write control signals WE, CAS and RAS. 
     When row address strobe signal RAS transits from a logic `high` state to a logic `low` state, as would be the case after both column address strobe signal CAS and write enable signal WE are triggered logic `low`, then parallel test signal φFTE is triggered logic `high` from a logic `low` state. In this condition, test data output from memory cells is input, in parallel, to eight 2-bit comparators to perform an appropriate wafer or package test. Hence, parallel test enable signal φFTE serves as an enable control signal for the 2-bit comparators. Output signals from each of the comparators are transferred via output buffers to a corresponding output pad. 
     FIG. 3 shows a possible logic circuit of each of the 2-bit comparators shown in FIG. 2. As provided, each 2-bit comparator includes a first set of NAND gates and a second set of NOR gates. For this example, when input signals DB0 and DB1 are both at the same logic level, an output signal Dcom therefrom will be generated having a logic `high` level. Conversely, when input signals DB0 and DB1 are not the same logic level, output signal Dcom will be at logic `low` level. This provides a way to test for defective cells in a memory cell array quickly and accurately. 
     In the conventional test circuit of FIG. 2, test results will appear along the output pads of the memory device under test (during both the wafer and package tests). The test results at all output pads must necessarily be evaluated. Immediately after a wafer test, defective cells can be repaired if the addresses of defective cells are identifiable. 
     The entire operation of a memory device is tested again during the package test. This proves unnecessary and redundant. Nevertheless, test results at all available output pads are recorded and evaluated because test results are distributed over all output pads. 
     As a result, when a number of packages are subjected to a simultaneous parallel test, the number of memory cells that can be tested simultaneously, is in effect severely limited. Consequently, test costs increase as a result of too great a test time. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a parallel test circuit wherein a large number of memory cells can be tested simultaneously. 
     It is another object of the present invention to provide a parallel test circuit, for use during both a wafer test and a package test, that automatically reduces the number of test output pins associated with a single package test and thereby increase the number of packages that can be tested simultaneously. 
     According to the present invention, a parallel test circuit of a semiconductor memory device includes a selector that limits the number of output pads which may be activated during a package test run. The selector responds to a wafer test enable signal from a selection control circuit to control output pad selection. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     For a better understanding of the invention, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: 
     FIG. 1 is the operational timing diagram for generating a parallel test enable signal φFTE; 
     FIG. 2 is a block diagram of a conventional parallel test circuit for use in a memory device; 
     FIG. 3 is a logic circuit for the conventional 2-bit comparator shown in FIG. 2; 
     FIG. 4 is a block diagram of a parallel test circuit for use in a memory device according to the present invention; and 
     FIG. 5 is a circuit diagram for generating a wafer test enable signal φWTE according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 illustrates a selection control circuit 100 that selectively activates output pads in a parallel test circuit by generating a wafer test enable signal φWTE. 
     Selection control circuit 100 includes a first N-channel metal oxide semiconductor (NMOS) transistor 41 connected between a source voltage Vcc and an input node N1, its gate connected to source voltage Vcc. Also, a first P-channel metal oxide semiconductor (PMOS) transistor 42 connects input node N1 to output node N2, its gate connected to source voltage Vcc. Second NMOS transistor 43 has one terminal connected to output node N2 and its gate to source voltage Vcc. Third NMOS transistor 44 connects the other terminal of second NMOS transistor 43 to ground, its gate also being connected to source voltage Vcc. 
     Selection control circuit 100 also includes an output terminal φWTE, connected to a two-inverter buffer circuit 46 which is connected to output node N2. Input pad IP receives an externally applied voltage. A conducting channel of fourth NMOS transistor 45 connects the input pad to input node N1. The gate of fourth transistor 45 also connects to input pad IP. 
     When the external voltage exceeds Vcc+3 Vt (Vt being the approximate threshold voltage of the NMOS transistors), first NMOS transistor 41 turns off while fourth NMOS transistor 45 turns on. A voltage of Vcc+2 Vt appears at input node N1. In turn, first PMOS transistor 42 turns on. In this state, the voltage at output node N2 is a function of the sizes of the second and third NMOS transistors 43 and 44. The output node N2 will transit to approximately Vcc and appear as a logic `high` signal. 
     Conversely, when the external voltage does not exceed Vcc+3 Vt, input node N1 will settle to voltage level Vcc-Vt, causing first PMOS transistor 42 to turn off. Output node N2 transits to a logic `low` voltage signal because second and third NMOS transistors 43 and 44 turn on in response to source voltage Vcc. 
     The stimulus signal is applied to input pad IP by direct external contact with an appropriate test probe during a wafer test. During the package test, input pad IP is not connected to any such external stimulus. Hence, no super voltage (Vcc+3 Vt) is present at input pad IP during this test period. 
     FIG. 5 is a block diagram of a 16-bit parallel test circuit for testing a 16 Mbit DRAM having 16 accessible data lines DBi (i=0 to 15). The circuit has eight 2-bit comparators 10, each receiving two respective data line inputs DB0, DB1 corresponding inverting signals DB0, DB1. Each comparator also receives control signal φFTE which is initiated by the Write Enable WE and CAS before RAS in the sequence shown in FIG. 1. 
     In addition, the circuit includes four 2-bit comparators 20 responsive to a wafer test enable signal φWTE from selection control circuit 100. Four switches 30 (NMOS switching transistors) controlled by wafer test signal φWTE, bypass the 2-bit comparators 20. A first set of four output buffers 5 receive the outputs of first comparators. A second set of four output buffers 5&#39; receive the outputs of second 2-bit comparators 20, and eight output pads OP respectively connect to the outputs of each of said first and second sets of four output buffers 5 and 5&#39;. 
     During a wafer test, a voltage of Vcc+3 Vt is applied to the input pad IP to generate a wafer test signal voltage φWTE level equal to logic `high` state. In this mode, the parallel test circuit of FIG. 5 will behave as does the conventional parallel test circuit shown in FIG. 2. Accordingly, a logic `high` test signal φFTE will drive first 2-bit comparators 10. 
     In turn, an inverted wafer test signal (logic low) disables the second 2-bit comparators 20. Switches 30 become operational to connect outputs from the first 2-bit comparators 10 to the corresponding output buffers 5&#39;. Consequently, all the outputs of the first 2-bit comparators 10 are transferred to their respective output buffers and generate data output signals at each of the output pads OP. 
     In the package test, wafer test signal φWTE becomes a logic `low` state because input pad IP of selection control circuit 100 is intentionally not supplied with an external voltage signal. As previously, when test signal φFTE becomes logic `high` here again all first 2-bit comparators 10 will become activated. However, second 2-bit comparators 20 become activated. Switches 30 turn off because wafer test signal φWTE is logic `low`. Output buffers 5 are disabled by a logic `low` wafer test signal. Accordingly, disabled output pads 5 produce no signals during a package test. The only signals generated are those from second 2-bit comparators 20 in response to outputs from corresponding pairs of first 2-bit comparators 10. 
     In the parallel test circuit described above in connection with FIG. 5, second 2-bit comparators 20 selectively limit the number of output pads which are necessarily activated during a package test. Second 2-bit comparators 20 are disabled and bypassed during wafer testing so as to make available all existing output pads. As a result, the reduced number of activated output pads during a package test frees up output test pins, which can be used to simultaneously package-test other similarly assembled. Thus, test time is significantly improved. A convenient aspect of this invention is the fact that, additional package testing is possible without the need for any additional complex timing requirements and control signals because wafer test signal φWTE automatically converts to logic `low` state during the package test. 
     FIG. 5 is directed to one embodiment of the present invention wherein the number of output pads is selectively reduced to four in a parallel test circuit capable of 16-bit wafer testing. It should be readily apparent, however, that the number of output pads can be easily reduced to one. In such an embodiment (not shown), numerous stages of second 2-bit comparators 20 and switches 30, can be cascadedly connected to feed a single output buffer/pad. 
     It should also be readily apparent that if the semiconductor memory device under test is a byte-wide memory device having a large number of input/output pins (e.g., ×16 or ×32), then it would not be very difficult to devise a parallel test circuit having `multi-bit` comparators (e.g., 4-bit or 8-bit comparators) as substitutes for the second 2-bit comparators whose operation was described in connection with FIG. 5. This will lead to a yet further reduction in the number of desired output pins for which one can conduct a memory cell test during package testing. Ultimately, the parallel test circuit, according to the present invention, reduces costs associated with testing constraints of highly integrated semiconductor memory devices. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.