Semiconductor memory device with short circuit identifying means

Abstract: A semiconductor memory device such as a ROM is provided with a word line test circuit and a word line drive circuit. The word line test circuit outputs a high level signal when a test signal is applied and a low level signal when the test signal is not applied. The word line drive circuit drives the respective word lines in the memory cell array, and is connected to the output of the word line test circuit, so that when driving one group of word lines (either the odd-numbered word lines or the even-numbered word lines), a high level signal is applied to the one group of word lines, and when not driving the one group of word lines, a low level signal is applied to the one group of word lines. On the other hand, when driving the other group of word lines, a high level signal is applied to the other group of word lines, and when not driving the other group of word lines, the output signal from the word line test circuit is applied to the other group of word lines, thereby enabling short circuits between adjacent word lines to be identified quickly. A bit line test switch circuit is provided which is connected to the bit lines in the memory cell array. In response to a test signal, the bit line test switch circuit applies signals of different levels to the odd-numbered and even-numbered bit lines respectively, and short circuits between adjacent bit lines can be identified quickly.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to a semiconductor memory device such as a ROM (Read Only Memory). 2. Description of the Prior Art: Generally, semiconductor memory devices are tested by checking whether the intended data can be obtained for input of a particular address signal, thereby distinguishing between good and bad ones. In recent years, with advances in miniaturization techniques, the memory cell area has been further reduced, which in turn has led to an increase in the failure rate attributed to short circuits between adjacent word lines or between adjacent bit lines within the memory array because of processing effects. In the prior art, verification of such failure has been accomplished by sequentially reading every address location and thus determining the result. However, such verification method has the disadvantage of requiring a prolonged test period, which even further increases with the increase of the memory capacity being expanded. EP-A-0 450 632, representing a state of the art under Article 54(3) EPC for the designated states DE, FR, GB, and EP-A-0 223 188 each disclose a semiconductor memory device in which a checkerboard pattern may be written in order to test the operation of the matrix of memory cells. SUMMARY OF THE INVENTION In one aspect this invention, as defined by claim 1, provides a semiconductor memory device comprising a set of parallel drive lines connected to respective rows or columns of memory cells, and line test means operable in a line test operation to apply a line test signal via a supply line thereof to a first group of alternate said drive lines, characterised in that the line test means is adapted to test for short circuits between said drive lines and comprises means for measuring a current flowing in said supply line during said test operation due to short circuits between said first group of alternate drive lines and a second group of alternate drive lines. In another aspect this invention, as defined by claim 8, provides a method of testing a semiconductor memory device having a plurality of parallel drive lines connected to respective rows or columns of memory cells, the method comprising a test operation in which a test voltage is applied via a supply line to a first group of alternate said drive lines, characterised by testing for short circuits between said drive lines by measuring a current flowing in said supply line due to short circuits between said first group of alternate drive lines and a second group of alternate drive lines. The sub-claims 2 to 7 are directed to embodiments of the invention. In the semiconductor memory device of one embodiment of the invention, a word line test means outputs a high level signal when a test signal is applied and a low level signal when the test signal is not applied. A word line drive means drives the respective word lines in the memory cell array, and is connected to the output of the word line test means, so that when driving one group of alternate word lines (either the odd-numbered word lines or the even-numbered word lines), a high level signal is applied to the one group of word lines, and when not driving the one group of word lines, a low level signal is applied to the one group of word lines. On the other hand, when driving the other group of alternate word lines, a high level signal is applied to the other group of word lines, and when not driving the other group of word lines, the output signal from the word line test means is applied to the other group of word lines, thereby enabling short circuits between adjacent word lines to be identified in a short period of time. In the semiconductor memory device of another embodiment of the invention, the same principle is applied for testing for short circuits between adjacent bit lines. Thus, the invention makes possible achieving the objective of providing a semiconductor memory device in which failures attributed to short circuits between adjacent word lines and/or between adjacent bit lines can be identified in a short period of time. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram schematically illustrating a first embodiment of the invention. Figure 2 is a circuit diagram schematically illustrating a second embodiment of the invention. Figure 3 is a circuit diagram schematically illustrating a third embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows an embodiment of the invention. This embodiment is a ROM comprising a memory cell array 1 , a Y-decoder 2 , a sense amplifier 3 , a word line driving unit 4 , and a word line test switch circuit 5 . The memory cell array 1 has a large number of memory cells (not shown) which are arranged in a matrix form. Each memory cell is selected when the word line to which the memory cell is connected is driven by the word line driving unit 4 . The data stored in the selected memory cell is output on the bit line (not shown in Figure 1) connected to the memory cell and is transferred through the Y-decoder 2 to the sense amplifier 3 . The word line driving unit 4 consists of driving circuits C 0 , C 1 , C 2 , ··· which drive the word lines WL 0 , WL 1 , WL 2 , ··· , respectively. Each of the driving circuits has a PMOS transistor Q 1 and an NMOS transistor Q 2 . The source and drain of the PMOS transistor Q 1 are respectively connected to the Vcc level and the drain of the NMOS transistor Q 2 . Each respective one of control signals IN 0 , IN 1 , IN 2 , ··· is applied to the gates of the transistors Q 1 and Q 2 . In the even-numbered driving circuits C 0 , C 2 , ··· , the source of the NMOS transistor Q 2 is connected to the ground. By contrast, in the odd-numbered driving circuits C 1 , C 3 , ··· , the source of the NMOS transistor Q 2 is connected to the output of the word line test switch circuit 5 . The control signals IN 0 , IN 1 , IN 2 , ··· are normally set to the Vcc level, keeping the NMOS transistors Q 2 in conduction and therefore holding the respective word lines either at the ground level or at the same level as the output signal of the word line test switch circuit 5 . In order to select a particular memory cell, the control signal applied to the driving circuit which drives the word line connected to that memory cell is set low, causing the PMOS transistor Q 1 in that driving circuit to conduct and thereby setting the word line to the Vcc level. The word line test switch circuit 5 comprises NMOS transistors Q 3 and Q 4 and an inverter 6 . The source of the transistor Q 4 is connected to the ground, and the source of the NMOS transistor Q 3 is connected to the drain of the NMOS transistor Q 4 . A test signal TEST is applied to the drain of the NMOS transistor Q 3 . An inverted test signal TEST is applied directly to the gate of the NMOS transistor Q 4 , and to the gate of the NMOS transistor Q 3 via the inverter 6 . In the test mode, the test signal TEST and inverted test signal TEST are set at the Vcc and ground levels respectively. Therefore, a high level signal which is given by subtracting the threshold value Vth of the NMOS transistor Q 3 from the Vcc level is output from the circuit 5 in the test mode, while, in non-test mode, a signal of the ground level is output. When testing the thus constructed semiconductor memory for failures resulting from word line shorting, the test signal TEST is set to the Vcc level while holding all the control signals IN 0 , IN 1 , IN 2 , ··· applied to the word line driving circuits C 0 , C 1 , C 2 , ··· at the Vcc level. This causes the NMOS transistors Q 2 in the driving circuits C 0 , C 1 , C 2 , ··· and the NMOS transistor Q 3 in the test switch circuit 5 to be conductive. As a result, the even-numbered word lines WL 0 , WL 2 , ··· are set to the ground level, while on the other hand, the odd-numbered word lines WL 1 , WL 3 , ··· are driven high since the output signal of the test switch circuit 5 is set high. Because of the thus provided difference between the potential applied to the even-numbered word lines and that applied to the odd-numbered word lines, it is expected that a current Itw will flow through the output of the test switch circuit 5 if there is any short circuit between adjacent word lines. Therefore, by measuring this current, it is possible to check in a short period of time for failures resulting from word line shorting. Figure 2 shows a second embodiment of the invention. The embodiment of Figure 2 comprises a memory cell array 1 , a Y-decoder 2 , and a sense amplifier 3 , as in the foregoing embodiment of Figure 1. For the sake of simplicity, the word lines and the word line driving unit are not shown in Figure 2. This embodiment further comprises a bit line test switch circuit 7 comprising NMOS transistors Q i (i = 0, ··· , n). The sources of the NMOS transistors Q i are connected to the ends of respective bit lines BL i . A test signal TEST is coupled to the gates of the NMOS transistors Q i , a signal TEST 1 is coupled to the drains of the transistors connected to the even-numbered bit lines, and a signal TEST 2 is coupled to the drains of the transistors connected to the odd-numbered bit lines. In the test mode, all the word lines are placed in a non-selected state, and the test signal TEST is set to the Vcc level, causing all the NMOS transistors Q i to be conductive. Under this condition, the signal TEST 1 is set to a high level and the signal TEST 2 to the ground level, thereby setting the even-numbered bit lines to a high level and the odd-numbered bit lines to the ground level. Therefore, by measuring the current Itb flowing through the signal line for the signal TEST 1 , it is possible to check for short circuits between adjacent bit lines in a short period of time. Figure 3 shows a third embodiment of the invention. This embodiment is a ROM having the same configuration as that of the ROM of Figure 1 and provided with the bit line test switch circuit shown in Figure 2. As apparent from the above description, the configuration of this embodiment permits the testing for shorts not only between adjacent word lines but also between adjacent bit lines. Furthermore, by combining these two test modes, it is also possible to check for defects in contacts of bit lines and memory cell transistors, the gate oxide film of a memory cell transistor, etc. The invention may be embodied in other specific forms and manners without departure from the scope thereof as defined by the claims.

Claims: A semiconductor memory device comprising a set of parallel drive lines (WL i ;BL i ) connected to respective rows or columns of memory cells, and line test means (5;7) operable in a line test operation to apply a line test signal via a supply line thereof to a first group of alternate said drive lines, characterised in that the line test means (4;7) is adapted to test for short circuits between said drive lines and comprises means for measuring a current (I tw ;I tb ) flowing in said supply line during said test operation due to short circuits between said first group of alternate drive lines and a second group of alternate drive lines. A semiconductor memory device according to claim 1, wherein: said set of parallel drive lines comprises a plurality of word lines (WL 0 ,WL 1 ,WL 2 ) each driven by a respective word line drive means (C 0 ,C 1 ,C 2 ); said line test means comprises word line test means (5) for receiving a test signal and producing an output signal, the level of said output signal being a first level when said test signal is active and a second level when said test signal is not active; and the ones (C 1 ) of said word line drive means for driving said first group of alternate word lines (WL 1 ) are coupled to an output of said word line test means (5) such that, when the word line drive means are not active, said output signal of the first level is applied to the first group of alternate word lines (WL 1 ). A semiconductor memory device according to claim 2, wherein said ones of the word line drive means are connected to the odd-numbered word lines. A semiconductor memory device according to claim 2, wherein said ones of the word line drive means are connected to the even-numbered word lines. A semiconductor memory device according to claim 1, wherein: said set of parallel drive lines comprises a plurality of bit lines (BL n-2 ,BL n-1 ,BL n ); and said line test means comprises bit line test means (7) for receiving a test signal, and for, when said test signal is active, applying a first output signal to odd-numbered ones of said bit lines and a second output signal to even-numbered ones of said bit lines, said first and second output signals being different in level from each other. A semiconductor memory device according to claim 5, wherein said bit line test means comprises switch means (Q n-2 ,Q n-1 ,Q n ) for, when said test signal is active, connecting said bit line test means to said bit lines. A semiconductor memory device according to claim 1, comprising a plurality of word lines (WL 0 ,WL 1 ,WL 2 ) coupled to a word line test means (5) as defined in any of claims 2 to 4, and a plurality of bit lines (BL n-2 ,BL n-1, BL n ) coupled to a bit line test means (7) as defined in claim 5 or claim 6. A method of testing a semiconductor memory device having a plurality of parallel drive lines (WL i ;BL i ) connected to respective rows or columns of memory cells, the method comprising a test operation in which a test voltage is applied via a supply line to a first group of alternate said drive lines, characterised by testing for short circuits between said drive lines by measuring a current (I tw ;I tb ) flowing in said supply line due to short circuits between said first group of alternate drive lines and a second group of alternate drive lines.

Inventors: HOTTA YASUHIRO, HOTTA, YASUHIRO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: G11C29
IPC codes: H01L27, G11C29, H01L21