Semiconductor Memory Device With Provision of Pseudo-Acceleration Test

1. Field of the Invention

The present invention relates to a semiconductor memory device such as a dynamic random access memory (DRAM) and a static random access memory (SRAM) having word line.

2. Description of the Related Art

In a semiconductor memory device such as a DRAM and an SRAM, a so-called acceleration test is carried out by imposing strict operating conditions on the device, to thereby determine whether the device is normal or abnormal. In one typical acceleration test, a remarkably high power supply voltage is applied to the device, so that a stress is imposed on the various portions of the device, particularly, on the memory cells thereof.

In a prior art semiconductor memory device, when an acceleration test is carried out, the strict conditions are imposed on internal circuits such as a word line driving circuit and an output buffer, which will be explained later. Therefore, in this acceleration test, the internal circuits cannot always be operated normally. As a result, if the internal circuits are not normally operated, data is erroneously output from the memory cells, so that it is impossible to determine whether the memory cells are normal or abnormal.

Note that, in order to carry out an acceleration test, a special instrument and the time therefor are necessary, thus increasing the manufacturing costs thereof.

It is an object of the present invention to minimize the number of acceleration tests performed upon semiconductor memory devices, to thereby reduce the manufacturing costs thereof.

According to the present invention, a pseudo-acceleration test, which is simple, is performed upon an internal circuit such as a word line driving circuit prior to an acceleration test. If an acceleration test is performed upon only the devices which pass the pseudo-acceleration test, the total test time can be reduced, to thereby reduce the manufacturing costs.

The present invention will be more clearly understood from the description as set forth below as compared with the prior art, with reference to the accompanying drawings, wherein;

FIG. 1 is a circuit diagram illustrating a prior art semiconductor memory device;

FIG. 2 is a detailed circuit diagram of the row address decoder and the word line selecting circuit of FIG. 1;

FIG. 3 is a detailed circuit diagram of the word line driving circuit of FIG. 1;

FIGS. 4A and 4B are timing diagrams showing the operation of the circuit of FIG. 3;

FIG. 5 is a detailed circuit diagram of the output buffer driving circuit of FIG. 1;

FIGS. 6A, 6B and 6C are timing diagrams showing the operation of the circuit of FIG. 5;

FIG. 7 is a detailed circuit diagram of the read amplifier and the output buffer of FIG. 1;

FIG. 8 is a circuit diagram illustrating an embodiment of the semiconductor memory device according to the present invention;

FIG. 9 is a circuit diagram of the word line driving circuit of FIG. 8;

FIGS. 10A and 10B and FIGS. 11A and 11B are timing diagrams showing the operation of the circuit of FIG. 9;

FIG. 12 is a circuit diagram of a modification of the circuit of FIG. 9;

FIG. 13 is a circuit diagram of the word line driving circuit determining circuit of FIG. 8;

FIGS. 14A, 14B, 14C, 14D and 14E and FIGS. 15A, 15B, 15C, 15D and 15E are timing diagrams showing the operation of the circuit of FIG. 13;

FIG. 16 is a circuit diagram of the output buffer of FIG. 8;

FIG. 17 is a circuit diagram illustrating a tester connected to the output buffer of FIG. 16; and

FIG. 18 is a circuit diagram illustrating a memory cell of a static type.

Before the description of the preferred embodiment, a prior art semiconductor memory device will be explained with reference to FIGS. 1 through 5.

In FIG. 1, reference numeral 1 designates a memory cell array having memory cells C.sub.ij (i,j=0,1,2, . . .) connected to one word line WL.sub.i and one bit line BL.sub.j. In FIG. 1, note that the memory cells C.sub.ij are of a dynamic type.

The selection of one row, i.e., one word line is carried out by a row address buffer 2, a row address decoder 3 and a word line selecting circuit 4. The row address buffer 2 receives address signals A.sub.1, A.sub.2, . . . , A.sub.m, to generate address signals A.sub.1, A.sub.2, . . . , A.sub.m and their inverted signals A.sub.1, A.sub.2, . . . , A.sub.m (see FIG. 2). The row address decoder 3 receives the row address signals A.sub.1, A.sub.2, . . . , A.sub.m and their inverted signals A.sub.1, A.sub.2, . . . , A.sub.m, to generate one selecting signal and transmit it to the word line selecting circuit 4. As a result the word line selecting circuit 4 activates one of the word lines WL.sub.1, WL.sub.2, . . . , WL.sub.n (n=2.sup.m).

The selection of one column, i.e., one bit line is carried out by a column address buffer 5 and a column address decoder 6 which have a similar configuration to the row address buffer 2 and the row address decoder 3. Note that the address signals A.sub.1, A.sub.2, . . . , A.sub.m are multiplexed by a control circuit (not shown) into row address signals and column address signals.

In a read mode, data is output from one cell selected by the word line selecting circuit 4 and the column address decoder 6 to a read amplifier 7 and an output buffer 8.

The word line selecting circuit 4 is driven by a row access signal RA generated from a word line driving circuit 9 which receives a row address strobe signal RAS. Also, the output buffer 8 is driven by an output buffer driving signal .phi. generated from an output buffer driving circuit 10 which receives a column address strobe signal CAS in addition to the row address strobe signal RAS.

Next, each portion of the device of FIG. 1 will be explained in detail with reference to FIGS. 2 to 5.

In FIG. 2, which is a detailed circuit diagram of the row address decoder 3 and the word line selecting circuit 4 of FIG. 1, the row address decoder 3 includes NAND circuits 3-1, 3-2, . . . , 3-n, and the word line selecting circuit 4 includes circuits 4-1, 4-2, . . . , 4-n each formed two MOS transistors such as 41-1 and 42-1 and an inverter such as 43-1. In this case, each of the circuits 4-1, 4-2, . . . , 4-n are connected to one of the word lines WL.sub.1, WL.sub.2, . . . , WL.sub.n, and are driven by the row access signal RA of the word line driving circuit 9. For example, when all of the address signals A.sub.1, A.sub.2, . . . , A.sub.n are made high (="1"), the NAND circuit 3-1 generates a selecting signal and transmits it to the circuit 4-1. Also, when the row access signal RA of the word line driving circuit 9 is high (="1"), the transistor 41-1 is turned ON and the transistor 42-1 is turned OFF, to increase the potential at the word line WL.sub.1. Thus, the word line WL.sub.1 is activated, i.e., selected.

In FIG. 3, which is a detailed circuit diagram of the word line driving circuit 9 of FIG. 1, this circuit 9 includes an inverter 901, a delay circuit 902, a MOS transistor 903, a MOS transistor 904, and an inverter 905. As shown in FIGS. 4A and 4B, when the potential of the row address strobe signal RAS is high, the MOS transistor 904 is turned ON and the MOS transistor 903 is turned OFF, so that the potential of the row access signal RA is low. Conversely, when the potential of the row address strobe signal RAS is low, the MOS transistor 903 is turned ON and the MOS transistor 904 is turned OFF, so that the potential of the row access signal RA is high. When the potential of the row address strobe signal RAS is switched from high to low or vice versa, the potential of the row access signal RA is switched from low to high or vice versa, with a delay time which mainly depends on the delay circuit 902. Note that this delay time is determined in view of the operation speed of the row address buffer 2 and the row address decoder 3.

In FIG. 5, which is a detailed circuit diagram of the output buffer driving circuit 10 of FIG. 4, this circuit 10 includes a delay circuit 101, an inverter 102, NAND circuits 103 and 104 for a flip-flop, a NOR circuit 105, and an inverter 106. As shown in FIGS. 6A, 6B and 6C, when the potentials of the row address strobe signal RAS and the column address strobe signal CAS are high (="1"), the output of the NOR circuit 105 is low, i.e., the output buffer driving signal .phi. is high. In this state, the nodes N.sub.1 and N.sub.2 of the flip-flop are high and low, respectively. Next, when the potential of the row address strobe signal RAS is switched from high to low, the potentials of the nodes N.sub.1 and N.sub.2 of the flip-flop are reversed, however, the output of the NOR circuit 105, i.e., the output buffer driving signal .phi. is not changed. Next, when the potential of the column address strobe signal CAS is switched from high to low, the output of the NOR circuit 105 is switched from low to high, i.e., the potential of the output buffer driving signal .phi. is switched from high to low. Next, when the potential of the column address strobe signal CAS is switched from low to high, the potentials of the nodes N.sub.1 and N.sub.2 of the flip-flop are reversed, however, the output of the NOR circuit 105, i.e., the output buffer driving signal .phi. is not changed. Finally, when the potential of the row address strobe signal RAS is switched from high to low, the output of the NOR circuit 105 is switched from high to low, i.e., the potential of the output buffer driving signal .phi. is switched from low to high. Thus, the output buffer driving signal .phi. is caused to fall by a fall of the column address strobe signal CAS and is caused to rise by a rise of the row address strobe signal RAS.

In FIG. 7, which is a detailed circuit diagram of the read amplifier 7 and the output buffer 8 of FIG. 1, the read amplifier 7 includes a comparator 701 and two inverters 702 and 703. In this case, the comparator 701 compares the potential of read data with a predetermined voltage such as V.sub.CC /2 where V.sub.CC is a power supply voltage such as 5 V.

Also, in FIG. 7, the output buffer 8 includes two NOR circuits 801 and 802 controlled by the output buffer driving signal .phi., an inverter 803, and two N-channel MOS transistors 804 and 805. When the output buffer driving signal .phi. is low, one of the transistors 804 and 805 is turned ON and the other is turned OFF. For example, if the data D of the read amplifier 7 is high (="1"), the transistor 804 is turned ON and the transistor 805 is turned OFF, so that the output data DO is high. Conversely, if the data D of the read amplifier 7 is low (="0"), the transistor 805 is turned ON and the transistor 804 is turned OFF, so that the output data DO is low. On the other hand, when the output buffer driving signal .phi. is high, the outputs of the NOR circuits 801 and 802 are both low, so that the output data DO is in a high impedance state.

In the above-mentioned prior art semiconductor memory device of FIG. 1, however, when an acceleration test is carried out by increasing the power supply voltage V.sub.CC, the word line driving circuit 9 cannot always be operated normally. For example, the delay time of the row access signal RA compared with the change of the row address strobe signal RAS may be increased. At worst, if the delay time is too large, the row access signal RA is reset before its rise by a fall of the row address strobe signal RAS (see FIGS. 4A and 4B), so that the row access signal RA remains at a low level. Therefore, in this case, it is impossible to determine whether or not the memory cells are normal, since data is erroneously output therefrom.

In FIG. 8, which illustrates an embodiment of the present invention, the word line driving circuit 9 of FIG. 1 is modified into a word line driving circuit 9' which also receives a pseudo-acceleration test signal .phi..sub.T. As a result, a pseudo-acceleration test is performed upon the word line driving circuit 9' without increasing the power supply voltage V.sub.CC. Also, a word line driving circuit determining circuit 11 is added to the elements of FIG. 1, to thereby determine whether the word line driving circuit 9' is normal or abnormal. An output Z of the word line driving circuit determining circuit 11 is supplied to an output buffer 8' which is obtained by modifying the output buffer 8 of FIG. 1. In this case, when the word line driving circuit determining circuit 11 determines that the word line driving circuit 9' is abnormal, the output buffer 8' is put in a high impedance state by the determining circuit 11, which state can be recognized by an external tester.

In FIG. 9, which is a detailed circuit diagram of the word line driving circuit 9' of FIG. 8, a delay circuit 902', transfer gates 906 and 906' and an inverter 907 are added to the elements of FIG. 3. In this case, the delay time of the delay circuit 902' is larger than that of the delay circuit 902. In a usual mode where the pseudo-acceleration test signal .phi..sub.T is low, the transfer gate 906 is turned ON and the transfer gate 906' is turned OFF, so that the word line driving circuit 9' operates in the same way as the word line driving circuit 9 of FIG. 3 as shown in FIGS. 10A and 10B. On the other hand, in a pseudo-acceleration test mode where the pseudo-acceleration test signal .phi..sub.T is high, the transfer gate 906' is turned ON and the transfer gate 906 is turned OFF, so that the word line driving circuit 9' operates as shown in FIGS. 11A and 11B.

In FIG. 12, which is a modification of the word line driving circuit 9' of FIG. 9, two MOS transistors 903A and 903B are provided instead of the MOS transistor 903 of FIG. 9, and a P-channel MOS transistor 908 is added to the elements of FIG. 9. That is, in a usual mode where the pseudo-acceleration test signal .phi..sub.T is low, the transistor 908 is turned ON, so that the two transistors 903A and 903B contribute to the rise of the potential of the row access signal RA. Conversely, in a pseudo-acceleration test mode where the pseudo-acceleration test signal .phi..sub.T is high, the transistor 908 is turned OFF, so that only the transistor 903A contributes to the rise of the potential of the row access signal RA. Thus, the delay time of the word line driving circuit 9' can be further adjusted in accordance with the pseudo-acceleration test signal .phi..sub.T.

In FIG. 13, which is a detailed circuit diagram of the word line driving circuit determining circuit 11 of FIG. 8, this circuit 11 includes a delay circuit 1101, a NOR circuit 1102, a NAND circuit 1103, and an inverter 1104. The delay time of the delay circuit 1101 is determined so that the output S.sub.1 of the delay circuit 1101 synchronizes with the row access signal RA.

The operation of the word line driving circuit determining circuit 11, where the word line driving circuit 9' is normal, will be explained with reference to FIGS. 14A, 14B, 14C, 14D and 14E. As shown in FIGS. 14A, 14B and 14C, the row access signal RA and the output S.sub.1 of the delay circuit 1101 are derived from the row address strobe signal RAS. In this case, the high level of the row access signal RA and the low level of the output S.sub.1 of the delay circuit 1101 are synchronized in phase with each other by the delay time of the delay circuit 1101. As a result, as shown in FIG. 14D, the output S.sub.2 of the NOR circuit 1102 remains at a low level. Therefore, the output of the NAND circuit 1103 remains at a high level, and accordingly, the output Z of the circuit 11 remains at a low level as shown in FIG. 14E.

The operation of the word line driving circuit determining circuit 11, where the word line driving circuit 9' is abnormal, will be explained with reference to FIGS. 15A, 15B, 15C, 15D and 15E. As shown in FIG. 15B, the row access signal RA is not generated from the word line driving circuit 9'. Contrary to this, as shown in FIGS. 15B and 15C, the output S.sub.1 of the delay circuit 1101 is derived from the row address strobe signal RAS. As a result, as shown in FIG. 15D, the output S.sub.2 of the NOR circuit 1102 is an inverted signal of the output S.sub.1 of the delay circuit 1101. Therefore, an AND logic between the row address strobe signal RAS and the output S.sub.2 of the NOR circuit 1102 is as shown in FIG. 15E. That is, the output Z of the word line driving circuit determining circuit 11 is high for a definite time period after the row address strobe signal RAS is switched from low to high.

In FIG. 16, which is a detailed circuit diagram of the output buffer 8' of FIG. 8, an inverter 806 and an NOR circuit 807 are added to the elements of the output buffer 8 of FIG. 7. As a result, when the output Z of the word line driving circuit determining circuit 11 is high, the output DO of the output buffer 8' is in a high impedance state regardless of the output buffer driving signal .phi.. Note that, when the output Z of the word line driving circuit determining circuit 11 is low, the output buffer 8' operates in the same way as the output buffer 8 of FIG. 7, so that the output DO of the output buffer 8' is high or low.

Thus, when a pseudo-acceleration test (.phi..sub.T ="1") is performed upon the word line driving circuit 9', so that the word line driving circuit 9' becomes in an abnormal state, the output DO of the output buffer 8' is in a high impedance state for a definite time period after the row address strobe signal RAS is switched from low (active) to high (inactive). This high impedance state of the output buffer 8' can be easily recognized by an external tester connected to the semiconductor memory device of FIG. 8. For example, such a tester has an input portion as illustrated in FIG. 17. In FIG. 17, the tester has resistors R.sub.1 and R.sub.2 and a capacitor C.sub.1. The value of the capacitor C.sub.1 is set in accordance with a predetermined specification, and the values of the resistors R.sub.1 and R.sub.2 are in accordance with a maximum output current. For example, the values of the resigtors R.sub.1 and R.sub.2 are 820.OMEGA. and 290.OMEGA., respectively. When the word line driving circuit 9' is normal, the potential at a node N.sub.3 is high or low. Contrary to this, when the word line driving circuit 9' is abnormal, the potential at the node N.sub.3 is an intermediate level determined by the resistors R.sub.1 and R.sub.2.

Note that, if a semiconductor memory device is determined to be defective in a pseudo-acceleration test, this semiconductor memory device will be scraped without performing an acceleration test thereupon. Thus, the time for carrying out such an acceleration test can be reduced.

In the above-mentioned embodiment, memory cells of a dynamic type are illustrated. However, the present invention can be applied to memory cells of a static type as illustrated in FIG. 18. In FIG. 18, R.sub.3 and R.sub.4 are load resistors, Q.sub.1 and Q.sub.2 are driving transistors, and Q.sub.3 and Q.sub.4 are transfer transistors. In this case, the selection of one column is carried out by selecting a pair of bit lines such as BL.sub.j and BL.sub.j.

As explained hereinbefore, according to the present invention, since a pseudo-acceleration test can be performed upon an internal circuit such as a word line driving circuit without increasing a power supply voltage, the time taken for an acceleration test can be reduced, to thereby reduce the manufacturing costs of semiconductor memory devices.