Abstract:
Circuits and methods that enable screening for defective or weak memory cells in a semiconductor memory device. In one aspect, a semiconductor memory device comprises first and second drivers for a SRAM cell. The first driver is connected between a power supply voltage and the cell, which supplies the power supply voltage into the cell in response to a cell power control signal. The second driver is connected between the power supply signal and the cell, which supplies a voltage lower than the power supply voltage into the cell in response to the cell power down signal. A method for screening for defective or weak cells does not require a time for stabilizing a circuit condition after voltage variation to supply the voltage lower than the power supply voltage from a conventional tester because the cell power down signal activates a driver that causes a supply voltage that is lower than the power supply voltage to be loaded directly to the cell, which results in a reduction of the test time for screening defective cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to Korean Patent Application No. 2002-29200, filed on May 27, 2002, which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention generally relates to semiconductor memory devices and, more specifically, to circuits and methods that enable screening for defective or weak memory cells in a semiconductor memory device. 
   BACKGROUND OF THE INVENTION 
   Due to the demand for increased data storage capacity and density of memory cells of semiconductor memory devices, such devices are being designed with smaller design rules. This demand applies to static random access memory devices (SRAMs), for example, where physical damage and topological miss-alignments in cell areas can generate defective memory cells. Defective or weak memory cells can result in abnormal leakage current flowing through the memory cells during standby states. Efficient and accurate methods for detecting or screening for defective memory cells are important for saving costs of fabrication and testing. 
   One method that has been proposed for screening for defective memory cells is disclosed, for example, in Japanese Patent Publication No. 1996-312097, and is illustrated in  FIG. 1  Referring to  FIG. 1 , a semiconductor switch  1  is connected between a power supply line Vdd and PMOS transistors M 1  and M 2  of a SRAM cell. Data is written into the cell when the switch  1  is turned on. The switch  1  is turned off after the writing operation and then returns to a turn-on state after a predetermined time of the turn-off state has elapsed to perform a read operation for the cell. A cell will be deemed to be defective or weak cells if the cell is not successful in reading data after it has been written with the data during the turn-on state of the switch  1 . 
   Another technique to check defective or weak cells is to test performance of data retention in the cell (referred to as “VDR test”) with a power supply voltage. In the VDR test, a modified power supply voltage level of a tester is applied into a SRAM and the data retention capability is verified after an internal power supply voltage level maintains a constant voltage level. 
   However, the VDR test requires modification of the power supply voltage of the tester, a power recovery time, and a time for settling the internal power supply voltage in the SRAM, which increases the testing time. 
   Therefore, methods and circuits for effectively and efficiently screening for defective or weak memory cells of a semiconductor memory device are highly desirable. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to circuits and methods for efficiently screening for defective or weak memory cells in semiconductor memory devices. 
   According to one embodiment of the invention, a semiconductor memory device comprises a power supply voltage, a memory cell, a first driver for supplying the power supply voltage to the memory cell in response to a cell power control signal, and a second driver for supplying a voltage lower than the power supply voltage to the memory cell in response to a cell power down signal. 
   The first driver is preferably a PMOS transistor that is controlled by the cell power control signal and the second driver is preferably a NMOS transistor that is controlled by the cell power down signal. 
   According to another embodiment of the invention, a semiconductor memory device comprises a power supply voltage, a plurality of sub-memory blocks, each comprising a plurality of memory cells arranged in rows and columns, a plurality of column decoders and bitline sense amplifiers associated with corresponding sub-memory blocks, sub-row decoders disposed between the sub-memory blocks, and drivers for supplying the power supply voltage to the memory cells in response to a cell power control signal. 
   These and other embodiments, aspects, features, and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments, which is to be read in connection with the accompanying drawings, in which like reference characters denote the same or similar components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a SRAM memory cell for illustrating a conventional method for screening for defective memory cells. 
       FIG. 2  is a diagram illustrating a method for screening for defective memory cells according to an embodiment of the invention. 
       FIG. 3  is a circuit diagram of a cell power signal generator according to an embodiment of the present invention. 
       FIG. 4  is a diagram illustrating a method for screening for defective memory cells according to another embodiment of the invention. 
       FIG. 5  is a circuit diagram illustrating a cell power signal generator according to another embodiment of the invention. 
       FIG. 6  is an exemplary timing diagram illustrating a method for screening for defective memory cells according to an embodiment of the invention. 
       FIG. 7  schematically illustrates an architecture of a memory cell array in a SRAM memory device in which screening methods of the present invention may be implemented. 
       FIG. 8  is a schematic diagram of a memory cell array that is capable of performing screening of a block of memory cells according to an embodiment of the invention. 
       FIG. 9  is a schematic diagram of a memory cell array that is capable of performing screening of a block of memory cells according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the following description, exemplary embodiments of the invention are discussed in detail to provide a thorough understanding of the present invention. It is to be understood, however, that the description of preferred embodiments is for purposes of illustration and that nothing herein should be construed as placing any limitation of the invention. Indeed, it will be apparent to those of ordinary skill in the art that present invention may be practiced without these specific details 
     FIG. 2  illustrates a circuit construction that is used for screening a SRAM cell according to an embodiment of the present invention. Referring to  FIG. 2 , a SRAM cell comprises a CMOS-type cell, including PMOS transistors M 1  and M 2 , and NMOS transistors M 3 ˜M 6 . The transistors M 1 ˜M 4  are cross-coupled with their gates, drains, and sources to form latch circuits  210 ,  220 , as is well known in the art. Each NMOS transistor M 5  and M 6  is connected to a respective bitline BL and BLB, and has a gate that is coupled to a common wordline WL. 
   The sources of PMOS transistors M 1  and M 2  are connected to a first internal voltage VDDC through a driver  10 . Preferably, the driver  10  is a PMOS transistor which connects the first internal voltage VDDC to the sources of the PMOS transistors M 1  and M 2  (i.e., connects the first internal voltage VDDC to the SRAM cell) in response to a cell power control signal CPENB. With this circuit, a screening method according to the invention can use the cell power control signal CPENB to perform either a wafer-level test or a package-level test. 
     FIG. 3  illustrates a control circuit according to an embodiment of the present invention which is preferably used for generating the cell power control signal CPENB that controls the driver  10  (FIG.  2 ). The control circuit comprises first and second test pads  22  and  24 , a cell power control circuit  26 , an AND gate  28 , and a NOR gate  30 . The first test pad  22  receives an input signal for a wafer-level test mode while the second test pad  24  receives an input signal for a package-level test mode. The cell power control circuit  26  (which is preferably a type of JTAG (Joint Test Action Group) test mode circuit) generates a cell power-off signal CPZ to set a cell power-off mode for test sources to test a semiconductor memory device in response to the input signal supplied to the second test pad  24 . 
   The AND gate  28  receives as input the cell power-off signal CPZ and the input signal from the second test pad  24 , and the NOR gate  30  outputs the cell power control signal CPENB in response to an output of the AND gate  28  and the input signal from the first test pad  22 . 
   The cell power control signal CPENB is active with a “low” logic level when the input signal from the first test pad  22  is a “high” logic level or when the output of the AND gate  28  is a “high” logic level. In  FIG. 2 , the driver  10  supplies the first internal voltage VDDC into the SRAM cell when the cell power control signal CPENB input to the driver  10  is a “low” logic level. 
   More specifically, in a wafer-level test mode, the first internal voltage VDDC is supplied into the SRAM cell in response to the cell power control signal CPENB, in response to an input pulse signal of a “high” logic level to the first test pad  22 , regardless of the output of the cell power control circuit  26 . In a package-level test mode, the cell power-off signal CPZ output from the cell power control circuit  26  affects the activation of the cell power control signal CPENB to supply the first internal voltage VDDC into the SRAM cell. 
     FIG. 4  illustrates a circuit construction that is used for screening a SRAM cell according to another embodiment of the present invention. In  FIG. 4 , two drivers  10  and  40  are connected in parallel between the first internal voltage VDDC and the SRAM memory cell. The driver  10  is the same as in FIG.  2 . The second driver  40  is preferably an NMOS transistor which connects the first internal voltage VDDC into the SRAM cell (i.e., the sources of the PMOS transistors M 1  and M 2 ) in response to a cell power down signal CPDN. With the second driver  40 , the first internal voltage VDDC is lowered by a threshold voltage (Vt) of the NMOS transistor. 
     FIG. 5  is illustrates a control circuit according to another embodiment of the present invention, which is used for generating the cell power control signal CPENB that controls the driver  10  and the cell power control signal CPENB that controls driver  40 . The control circuit comprises a buffer  45  connected to a first test pad  41 , a JTAG test circuit  46  including a cell power control circuit  47  and an internal voltage trimming circuit  48  that are connected to a second test pad  42 , a NAND gate  51  which receives the output signal CPZ of the cell power control circuit  47  and an input from the second test pad  42 , and first and second internal voltage converters  49  and  50 , which convert first and second external voltages  43  and  44 , into first and second internal voltages VDDC and VDD, in response to an output of the internal voltage trimming circuit  48 . An output of the buffer  45  is the cell power down signal CPDN and an output of the NAND gate  51  is the cell power control signal CPENB. The cell power down signal CPDN is essentially the voltage level of a signal supplied through the first test pad  41 . The cell power control signal CPENB is activated with a “low” logic level in response to an input of a “high” logic level from the second test pad  42  and the output CPZ of a “high” logic level from the cell power control circuit  47 . A cell power down signal CPDN having a “high” logic level turns on the second driver  40  to supply a voltage VDDC-Vt into the cell, wherein Vt is a threshold voltage of the NMOS transistor of the second driver  40 . A cell power control signal CPENB having a “low” logic level turns on the first driver  10  to supply the first internal voltage VDDC into the cell. 
   Since the cell power control signal CPDN causes the cell to be connected directly with the voltage of VDDC-Vt, there is no waiting time for turning a voltage level lower in a tester. This advantageously provides a reduction of the test time for screening defective memory cells. 
     FIG. 6  is an exemplary timing diagram illustrating a method for screening defective memory cells according to an embodiment of the present invention. Referring to  FIG. 6 , a test signal from the second test pad  42  maintains a “high” logic level during clock cycles C 2 ˜C 7  of clock signal XCLK of the JTAG test circuit  46 , and maintains a “low” logic level during clock cycles C 8 ˜C 15 . The test signal of the second test pad  42  pulses with a “high” level every two clock cycles during C 16 ˜C 24 . The output CPZ of the cell power control circuit  47  becomes active with a “high” logic level during the clock cycles C 2 ˜C 24  which the test signal appears at the second test pad  42 . The cell power control signal CPENB is generated in accordance with the test signal of the second test pad  42 . When the cell power control signal CPENB is at a “high” logic level, the first internal voltage VDDC is not supplied into the cell. 
   Before the clock cycle C 2  (period TACW), the cell power control signal CPENB of a “low” level causes the first internal voltage VDDC to be supplied into the cell and a writing operation is thereby performed for all cells. During the clock cycles C 2 ˜C 7  (period TWCS) when the cell power control signal CPENB maintains a “high” logic level, the first internal voltage VDDC is not supplied into the cell, which provides a time window for performing a screening operation for defective or weak cells. After that, during the clock cycles C 8 ˜C 15  (period TACR) when the cell power control signal CPENB is set to a “low” logic level, a read operation is performed for all cells. 
   Sub-sequent to clock cycle C 16  (period TWC), read and write operations are performed for every cell. The writing operation (i.e., 1-cell writing) is performed at clock cycles C 16 , C 18 , C 20 , C 22 , and C 24  while the cell power control signal is at a “high” level. At this time, each cell is compulsively put into the write mode even without a supply of the first internal voltage VDDC. Moreover, after each 1-cell writing, 1-cell read operations are carried out at clock cycles C 17 , C 19 , C 21 , and C 23 . During such operations, the read-out data is compared with the written data and defective cells are found when two data of a given cell are different from each other. 
     FIG. 7  schematically illustrates an architecture of a memory cell array in a SRAM memory device in which screening methods of the present invention may be implemented. In  FIG. 7 , a SRAM device comprises first to fourth sub-memory cell blocks SCB 0 ˜SCB 3 , sub-row decoders SRD, column decoders YPATH, write drivers WDRV, bitline sense amplifiers BSA, drivers  61 ˜ 64 , a power supply pad  65 , and a power supply line  66 . In the sub-memory cell blocks SCB 0 ˜SCB 3 , the cells are arranged in a matrix of row and columns. The column decoders YPATH, write drivers WDRV, and bitline sense amplifiers BSA are located the bottom of the sub-memory cell blocks SCB 0 ˜SCB 3 . The sub-row decoders SRD are interposed between the first and second sub-memory cell blocks, SCB 0  and SCB 1 , and between the third and fourth sub-memory cell blocks, SCB 2  and SCB 3 . A unit block  60  comprises two sub-memory cell blocks (SCB 0  and SCB 1 , or SCB 2  and SCB 3 ), a sub-row decoder SRD, two column decoders YPATH, two write drivers WDRV, two bitline sense amplifiers BSA, and two drivers ( 61  and  62 , or  63  and  64 ). The power supply pad  65  is connected to the drivers  61 ˜ 64  through the power supply line  66 . The drivers  61 ˜ 64  preferably each comprise a PMOS transistor (such as the driver  10  shown in  FIG. 2 ) that is disposed between the sub-row decoder SRD and the bitline sense amplifier BSA. The drivers  62  and  64  supply the first internal voltage VDDC into the memory cells arranged in the lower part of the sub-memory cell blocks. The drivers  61  and  63  are disposed at the upper side of the sub-row decoder SRD and supply the first internal voltage VDDC into the memory cells arranged in the tipper part of the sub-memory cell blocks. 
     FIGS. 8 and 9  are schematic diagrams of a memory cell arrays that are capable of performing screening of memory cells blocks according to embodiments of the invention, wherein a power control signal CPENB is used as decoding signals for blocks of memory cell. In  FIGS. 8 and 9 , the unit blocks  60  of  FIG. 7  are arranged in main memory cell blocks MCB 0 ˜MCB 3 . Each main memory cell block is coupled to a main row decoder MRD for selecting the sub-memory cell blocks SCB 0 ˜SCB 3  embedded in the main memory cell block. 
   More specifically, in  FIG. 8  the cell power control signal CPENB is applied to a pre-decoder  70  for performing a screening process for all the main blocks. An output of the pre-decoder  70  is applied to the main row decoders MRD. When the cell power control signal CPENB is enabled, the main row decoders MRD are activated and the first internal voltage VDDC is supplied into the main memory cell blocks MCB 0 ˜MCB 3 . 
   On the other hand, in  FIG. 9 , the cell power control signal CPENB is applied to each of a plurality of block decoders BDC. When the cell power control signal CPENB is enabled to supply the first internal voltage VDDC into each of the sub-cell blocks SCB 0 ˜SCB 3 , the main row decoders MRD select the sub-memory cell block to be supplied with the first internal voltage VDDC. Thus, the decoding of the cell power control signal CPENB may allow different screen modes, one mode for screening entire cell blocks or another mode for screening one or more cell block units. 
   As described above, circuits and methods for screening for defective or weak memory cells according to the present invention provide a reduced test time for screening since no time is needed for settling a test voltage lower than the power supply voltage in a tester. 
   Moreover, the invention enables screening operations in either a wafer-level test mode or a package-level test mode in accordance with a test signal supplied through a test pad. Furthermore, screen operations can be performed for either entire memory cell blocks or for memory cell block units, which provides flexibility of the number of memory cell blocks to be tested. 
   Although preferred embodiments of the present invention have been described for illustrative purposes, those of ordinary skill in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention. For example, the present invention may be applicable to screen operations for other type memory cells not the SRAM cells. It is to be understood that all such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.