Semiconductor memory device capable of preventing malfunction due to disconnection of word select line

Abstract: A column select line (CSL) includes a first layer column select line (CSLB) and a second layer column select line (CSLA) formed above the first layer column select line (CSLB) and connected thereto at any point. Furthermore, clamping circuits (7, 9) each for clamping each word line of paired main word lines (MWL, /MWL) at a constant potential are provided in a semiconductor memory device having main and secondary word line structure. With such a structure, malfunction due to multiselection of memory cells can be avoided even when the column select line (CSL) or the paired main word lines (MWL, /MWL) is disconnected.

Description: The present invention relates to a semiconductor memory device according to the preamble of claim 1. It particularly relates to a clamping circuit which prevents a malfunction when a word line is disconnected by foreign matters or the like. Fig. 7 is a block diagram showing a configuration of a dynamic random access memory (referred to as a "DRAM" hereinafter), a conventional semiconductor memory device. As shown in Fig. 7, the semiconductor memory device is provided with subarrays 1 into which a memory cell array is divided, a row decoder 2 arranged at each subarray 1 for selecting a word line WL, and a column decoder 3 arranged at each subarray 1 for selecting a column select line CSL. Furthermore, the semiconductor memory device shown in Fig. 7 is a DRAM of x4 configuration and has a memory cell array consisting of four subarrays 1. Each subarray 1 has a plurality of word lines WL including a redundant word line, a plurality of pairs of bit lines (not shown) arranged to cross the plurality of word lines WL and including a redundant pair of bit lines, a plurality of column select lines CSL arranged parallel to the plurality of pairs of bit lines and including a redundant column select line for sending a signal for selecting a pair of bit lines, and memory cells (not shown) connected to intersections of word lines WL and the pairs of bit lines. One end of column select line CSL is connected to column decoder 3 and the other end is opened. Thus, when column select line CSL is disconnected due to foreign matters or the like, a redundant column select line is substituted for the column select line CSL while the disconnected portion is floating, causing multiselection of memory cells and hence malfunction of the device. Furthermore, a word line WL also has one end connected to row decoder 2 while the other end open. Thus, when word line WL is disconnected due to foreign matters or the like, a redundant word line is substituted for the disconnected word line WL while the disconnected portion is floating, causing multiselection of memory cells and hence malfunction of the device. EP 0 107 921 A2 discloses a semiconductor memory device, comprising a plurality of word lines, a row decoder for selecting said word lines, and a plurality of clamping means, each connected to a corresponding one of said word lines. EP 0 115 127 A2 discloses a semiconductor memory device according to the preamble of claim 1. An object of the present invention is to provide a semiconductor memory device free from malfunction even when a word line is disconnected due to foreign matters or the like. This object is achieved by a semiconductor memory device according to claim 1. Further developments of the invention are given in the dependent claims. An advantage of the present invention is that in a semiconductor memory device having a layered word line structure, malfunction can be prevented even when a main word line pair is disconnected by foreign matters or the like. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of an embodiment of the present invention when taken in conjunction with the accompanying drawings, of which: Fig. 1 shows a configuration of a semiconductor memory device according to an embodiment of the present invention. Figs. 2 and 3 are circuit diagrams showing configurations of the clamping circuits shown in Fig. 1. Fig. 4 is a circuit diagram showing a configuration of a circuit for generating a signal RXn. Figs. 5A and 5B are timing charts illustrating a relation between a signal /RAS and signal RXn. Fig. 6 is a circuit diagram showing a configuration of a circuit for generating signals p and /p. Fig. 7 shows a configuration of a conventional semiconductor memory device. (Preferred Embodiment) Fig. 1 shows a configuration of a semiconductor memory device according to a preferred embodiment of the present invention. As shown in Fig. 1, the semiconductor memory device includes a main pair of word lines MWL and /MWL transmitting complementary signals, a row decoder 2 for selecting the main pair of word lines MWL and /MWL, a secondary word line SWL, a clamping circuit 7 for pulling down a main word line MWL to a ground potential, a clamping circuit 9 for pulling up a main word line /MWL to a power supply potential, and N channel MOS transistors NT1-NT3. Fig. 2 is a circuit diagram showing a configuration of clamping cirucit 7. As shown in Fig. 2, clamping circuit 7 includes an N channel MOS transistor NT4 connected between main word line MWL and a ground node, an N channel MOS transistor NT5 connected between the gate of N channel MOS transistor NT4 and a ground node and having its gate connected to main word line MWL, and a p channel MOS transistor PT1 connected between the gate of N channel MOS transistor NT4 and a power supply node and having its gate receiving a signal /p. Fig. 3 is a circuit diagram showing a configuration of clamping circuit 9. As shown in Fig. 3, clamping circuit 9 includes a p channel MOS transistor PT2 connected between main word line /MWL and a power supply node, and a p channel MOS transistor PT3 connected between the gate of p channel MOS transistor PT2 and a power supply node and having its gate connected to main word line /MWL, and an N channel MOS transistor NT6 connected between the gate of p channel MOS transistor PT2 and a ground node and having its gate receiving a signal p. A signal RXn applied to the drain of n channel MOS transistor NT2 is generated from a signal /RAS, as shown in Fig. 4, and a relation between signal /RAS and signal RXn is illustrated in a timing chart shown in Figs. 5A and 5B. The signal /p applied to the gate of p channel MOS transistor PT1 shown in Fig. 2 and the signal p applied to the gate of N channel MOS transistor NT6 shown in Fig. 3 are complementary signals and generated from signal RXn, as shown in Fig. 6. Furthermore, it is difficult in manufacturing a semiconductor memory device having a main pair of word lines MWL and /MWL and a secondary word line SWL such as a semiconductor memory device according to the present embodiment to backline a word line formed of polysilicon with aluminum. Thus, clamping circuits 7 and 9 are provided at ends of paired main word lines MWL and /MWL opposite to row decoder 2, as shown in Fig. 1. Operation of a semiconductor memory device according to the present embodiment will now be described. When signal /p input to the gate of p channel MOS transistor PT1 in clamping circuit 7 shown in Fig. 2 is activated (low level), p channel MOS transistor PT1 is turned on and the potential of a node fl attains high level. This causes N channel MOS transistor NT4 to be turned on and the potential of a node el attains low level. Then, when signal /p attains inactive high level, p channel MOS transistor PT1 is turned off and thus the potential of node el is held at low level. Thus, main word line MWL is pulled down to low level. On the other hand, while signal p attains active high level in clamping circuit 9 shown in Fig. 3, N channel MOS transistor NT6 is turned on and the potential of a node hl attains low level. This causes p channel MOS transistor PT2 to be turned on and the potential of a node g1 attains high level. Then, when signal p attains inactive low level, n channel MOS transistor NT6 is turned off and thus the potential of node g1 is held at high level. Thus, main word line /MWL is pulled up to high level. The main word line /MWL pulled up to high level causes n channel MOS transistor NT3 shown in Fig. 1 to be turned on and thus secondary word line SWL is clamped at low level. Thus, floating of paired main word lines MWL and /MWL can be avoided even when they are disconnected by foreign matters or the like. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The scope of the present invention being limited only by the terms of the appended claims.

Claims: A semiconductor memory device, comprising: a plurality of word lines (MWL, /MWL) for transmitting a signal, a row decoder (2) for selecting said word lines, and a plurality of clamping means (7, 9) each connected to one word line (MWL, /MWL) and clamping said word line (MWL, /MWL) at a constant potential in response to an input control signal (/p, p), characterized in that said plurality of word lines is a plurality of word line pairs (MWL, /MWL) for transmitting complementary signals, said row decoder (2) selects word line pairs, said plurality of clamping means comprises a plurality of first clamping means (7) each connected to one word line (MWL) of a corresponding one of said word line pairs and a plurality of second clamping means (9) each connected to the other word line (/MWL) of a corresponding one of said word line pairs, a plurality of control circuits (NT1-3) each connected to one pair of word lines (MWL, /MWL) is provided, and a plurality of secondary word lines (SWL) is provided, each of which is connected to one corresponding control circuit (NT1-3) for controlling a potential applied to the secondary word line (SWL) in accordance with said complementary signals. The semiconductor memory device according to claim 1, wherein said control signal (/p) input to said first clamping means (7) is a signal complementary to said control signal (p) input to said second clamping means (9). The semiconductor memory device according to claim 1 or 2, wherein each of said first clamping means (7) includes: a first transistor (NT4) connected between said one word line (MWL) and a ground node, a second transistor (NT5) connected between the gate of said first transistor (NT4) and said ground node and having its gate connected to said one word line (MWL), and a third transistor (PT1) connected between the gate of said first transistor (NT4) and a power supply node. The semiconductor memory device according to claim 3, wherein said third transistor (PT1) is a p channel transistor. The semiconductor memory device according to one of claims 1 to 4, wherein each of said second clamping means (9) includes a fourth transistor (PT2) connected between said other word line (MWL) and a power supply node, a fifth transistor (PT3) connected between the gate of said fourth transistor (PT2) and said power supply node and having its gate connected to said other word line (/MWL) and a sixth transistor (NT6) connected between the gate of said fourth transistor (PT2) and a ground node. The semiconductor memory device according to claim 5, wherein said sixth transistor (NT6) is an n channel transistor. The semiconductor memory device according to one of claims 1 to 6, wherein each of said first clamping means (7) and said second clamping means (9) is connected at an end of said word lines opposite to said row decoder (2).

Inventors: HAYASHIKOSHI MASANORI, OKIMOTO HIROMI, TOBITA YOUICHI, HAYASHIKOSHI, MASANORI, OKIMOTO, HIROMI, TOBITA, YOUICHI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1997
Main classification: G06F11
IPC codes: G06F11, G11C8, G11C29