Dynamic Semiconductor Memory Device

1. Field of the Invention

The present invention relates to a dynamic semiconductor memory device having memory cells of a one-transistor one-capacitor type and more particularly to a dynamic semiconductor memory device in which the clamping of the potentials of unselected word lines is guaranteed.

2. Description of the Prior Art

Generally, a dynamic semiconductor memory device such as a one-transistor one-capacitor type dynamic random access memory (DRAM) has memory cells each consisting of one metal-insulator semiconductor (MIS) transistor having a drain connected to a bit line, a gate connected to a word line and a source, and also having one capacitor. The capacitor is usually a MOS capacitor, having one electrode connected to the source of the MIS transistor. The potential of another electrode opposed to the above-mentioned capacitor electrode (hereinafter referred to as an opposed electrode) is preferably fixed at the low voltage V.sub.SS of a power supply.

In a conventional technique for simplifying the process of manufacturing opposed electrodes, unselected word lines are used as opposed electrodes because unselected word lines are clamped, at one end thereof, to the low voltage V.sub.SS by word decoders. When the opposed electrodes of the cell capacitors are independently formed without utilizing unselected word lines, and additional conductive layer, for example, a polycrystalline layer, is used for the opposed electrodes, thereby rendering the manufacturing process more complex.

In a conventional one-transistor one-capacitor DRAM, there exists the possibility of the destruction of data stored in a memory cell adjacent to a selected memory cell during memory cell selection (described in detail later with reference to the drawing).

An object of the present invention is to provide a dynamic semiconductor memory device in which data destruction during memory cell selection is prevented, thereby preventing an error in a subsequent reading operation.

To attain the above object, a dynamic semiconductor memory device includes one-transistor one-capacitor type memory cells connected between word lines and bit lines; and flip-flops, each flip-flop being connected between a pair of word lines to clamp an unselected word line in the pair of word lines to the low voltage of a power source.

The above object and advantages thereof will be more apparent from the following description of the embodiments when read in conjunction with the accompanying drawings.

FIG. 1 is a circuit diagram of an example of a conventional DRAM;

FIG. 2 is a waveform diagram showing the operation of the DRAM of FIG. 1;

FIG. 3 is a circuit diagram of a DRAM according to an embodiment of the present invention;

FIG. 4 is a waveform diagram showing the operation of the DRAM of FIG. 3;

FIG. 5 is a block diagram of a DRAM according to another embodiment of the present invention;

FIG. 6A is a plan view of the DRAM of FIG. 3 showing the structure thereof;

FIG. 6B is a cross-sectional view of FIG. 6A taken along the line X-X' of FIG. 6A;

FIG. 6C is a cross-sectional view of FIG. 6A taken along the line Y-Y' of FIG. 6A;

FIG. 7 is a circuit diagram of a DRAM according to yet another embodiment of the present invention.

Before describing the embodiments of the present invention, a conventional DRAM and the problems involved therein are described with reference to FIGS. 1 and 2. FIG. 1 is an example of a conventional DRAM. In FIG. 1, WL.sub.1 , WL.sub.2 , . . . are word lines formed by a polysilicon layer; BL and BL are one pair of bit lines (other bit lines are omitted in the drawing); SA is a sense amplifier; WD.sub.1 , WD.sub.2 , . . . are word decoders; MC.sub.1 and MC.sub.2 are memory cells; Q.sub.1 and Q.sub.2 are metal-oxide semiconductor (MOS) transistors having silicon gates; and C.sub.1 and C.sub.2 are MOS capacitors. One of the transistors and one of the capacitors constitute a memory cell. In this DRAM, the opposed electrode OE.sub.1 of the capacitor C.sub.1 in the memory cell MC.sub.1 is connected to the word line WL.sub.2 adjacent to the word line WL.sub.1. The memory cell MC.sub.1 is selected by raising the word line WL.sub.1 to a high potential. The opposed electrode OE.sub.2 of the capacitor C.sub.2 in the memory cell MC.sub.2 , which is selected by raising the potential of the word line WL.sub.2 , is connected to the next word line WL.sub.3 adjacent to the word line WL.sub.2 , and so on. Thus, the adjacent word line WL.sub.2, WL.sub.3, . . . are used as opposed electrodes OE.sub.1, OE.sub.2, . . . of the MOS capacitors C.sub.1, C.sub.2, . . .

When a memory cell, for example, MC.sub.1 , is selected, the selected word line WL.sub.1 is driven to a voltage higher than the high voltage V.sub.CC of the power supply by the word decoder WD.sub.1. The unselected word lines WL.sub.2 and WL.sub.3 are substantially fixed to the low voltage V.sub.SS , which is generally equal to 0 V. If a P-type semiconductor substrate is employed, an N-type inversion layer is formed immediately under one part of a word line by means of an ion injection technique and so on to form a MOS capacitor such as C.sub.1 between that part of the word line and the inversion layer. The inversion layer is formed to contact the drain or the source of the MOS transistor. Therefore, the unselected word lines can be used as the opposed electrodes of the MOS capacitors.

In the above-described conventional DRAM, however, there exist some problems, which are explained with reference to FIG. 2. For example, assume that the cell MC.sub.1 is storing a "0", i.e., the node N.sub.1 at the connection of the source of the MOS transistor Q.sub.1 and the MOS capacitor C.sub.1 is 0 V. When the word line WL.sub.1 is selected at time t=t.sub.0 , the node N.sub.1 is charged by the bit line BL, which was precharged to the voltage V.sub.CC. Accompanied by an increease in potential at the node N.sub.1 , the potential of the opposed electrode OE.sub.1 and, therefore, of the word line WL.sub.2 adjacent to the word line WL.sub.1 is increased via the MOS capacitor C.sub.1. It is preferable that the potential of the unselected word line WL.sub.2 not be increased since the word line WL.sub.2 is not being selected at this time. Although the word decoder WD.sub.2 clamps the word line WL.sub.2 at one end thereof to the voltage V.sub.SS (generally 0 V), an increase in potential of the unselected word line WL.sub.2 is caused since the clamping effect is not satisfactorily obtained due to the resistances R.sub.1 , R.sub.2 , . . . (FIG. 1) of the word lines because the word lines are composed of polycrystalline silicon which has a larger resistance than metal wiring. Thus, the potential of the word line WL.sub.2 is increased temporarily and locally by a boost via the above-mentioned capacitor C.sub.1. If the potential of the word line WL.sub.2 increases to more than the threshold voltage of the MOS transistor Q.sub.2 , the memory cell MC.sub.2 is erroneously turned on. If the state of the memory cell MC.sub.2 is "1", i.e., if the node N.sub.2 is at the voltage V.sub.CC , no problem occurs. If, however, the state of the memory cell MC.sub.2 is "0", i.e., if the node N.sub.2 is at the voltage V.sub.SS , when the transistor Q.sub.2 is turned on, a current flows from the bit line BL, which has been precharged to the voltage V.sub.CC , to the node N.sub.2 , resulting in an increase of the potential at the node N.sub.2. This is the reason why the level of the node N.sub.2 in FIG. 2 increases. Even after the potential of the word line WL.sub.2 is lowered to the voltage V.sub.SS , by the word decoder WD.sub.2 , connected to one end of the word line WL.sub.2 , which clamps the word line WL.sub.2 , the potential of the node N.sub.2 does not again assume the voltage V.sub.SS.

The operation of reading the data stored in a memory cell, for example, MC.sub.2 , is, as is well known, carried out by precharging the bit lines BL and BL to the voltage V.sub.CC and then selecting the word line WL.sub.2 to raise the potential thereof. This turns on the MOS transistor Q.sub.2 which has a drain connected to the bit line BL and functions as a transfer gate. Simultaneous with the turning on of the MOS transistor Q.sub.2 , a transfer gate of a dummy cell (not shown) connected to the bit line BL is turned on. The dummy cell stores a reference amount of charge. Thus, the charges on the bit lines BL and BL flow into the dummy cell and the real cell MC.sub.2 , respectively. The difference between the charge which flows into the real cell MC.sub.2 and the charge which flows into the dummy cell, that is, the potential difference between the bit lines BL and BL, is detected by the sense amplifier SA. If, as was previously mentioned, the potential of the node N.sub.2 is erroneously increased, the amount of charge which flows from the bit line BL to the real cell MC.sub.2 is too small to indicate that the memory cell MC.sub.2 is storing "0". Thus, a reading error may occur.

FIG. 3 illustrates an embodiment of the present invention, wherein the same portions as in FIG. 1 are denoted by the same reference numerals and symbols. In FIG. 3, the element enclosed within the broken lines is a clamping circuit CLP, which is an element in this embodiment not found in the conventional circuit of FIG. 1. The structure of the clamping circuit CLP is simple, that is, between the left end of the word line WL.sub.1 and the left end of the word line WL.sub.2 (the open ends of the word lines WL.sub.1 and WL.sub.2 in FIG. 1) and the voltage V.sub.SS , cross-coupled transistors Q.sub.11 and Q.sub.12 , comprising a flip-flop, are connected. Similarly, between the left end of the word line WL.sub.2 and the left end of the word line WL.sub.3 and the voltage V.sub.SS , transistors Q.sub.13 and Q.sub.14 , comprising a flip-flop, are connected. In such a structure, when the word line WL.sub.1 is selected, the transistor Q.sub.12 is turned on so that the left end of the unselected word line WL.sub.2 adjacent to the word line WL.sub.1 is clamped at the voltage V.sub.SS and the other end thereof is clamped at the voltage V.sub.SS by the word decoder WD.sub.2.

Accordingly, as can be seen in FIG. 4, although the changes in potential of the selected word line WL.sub.1 and the node N.sub.1 are the same as those in FIG. 2, the potential increase of the unselected word line WL.sub.2 can be kept small so that the MOS transistor Q.sub.2 is not turned on, thereby preventing inversion of the data stored in the memory cell MC.sub.2. Similarly, when the word line WL.sub.2 is selected, the transistor Q.sub.14 is turned on so that the left end of the word line WL.sub.3 is clamped at the voltage V.sub.SS , thereby preventing inversion of the data stored in a memory cell connected to the word line WL.sub.3.

It is apparent that the effectiveness of this clamping circuit CLP can be increased by providing a plurality of clamping circuits CLP along each word line. Therefore, a second embodiment provides one clamping circuit for each of the sections of a word line, as illustrated in FIG. 5. FIG. 5 is a block diagram of a 64K DRAM having a memory-cell matrix of 256.times.256 bits, the memory-cell matrix being divided into 16 groups G.sub.1 through G.sub.16 in the word line direction. Clamping circuits CLP.sub.1 through CLP.sub.16 are inserted between adjacent groups and on the side of the DRAM opposite the word drivers WD.sub.n. Since one clamping circuit, including one flip-flop consisting the transistors Q.sub.11 and Q.sub.12 (FIG. 3), for example, can be formed within a 12 .mu.m width, the total width of the clamping circuits is only about 200 .mu.m.

FIGS. 6A through 6C are diagrams of the structure of the memory cell MC.sub.1 in the DRAM of FIG. 3. FIG. 6A is a plan pattern, FIG. 6B a cross section along the line X-X' of FIG. 6A and FIG. 6C a cross section along the line Y-Y' of FIG. 6A. In these figures, 1 is a P-type silicon substrate, 2 and 3 are N.sup.+ -type diffusion layers which act as source and drain regions of the transistor Q.sub.1 , 4 is a gate oxidation film, 5 is a field oxidation film, 6 is an insulating layer between layers, 7 is an N-type inversion layer formed by means of an ion implantation technique and so on, 8 represents a boundary of the field oxidation film 5 and 9 is a contact hole connecting the bit line BL of aluminum to the N.sup.+ -type diffusion layer 2. The word lines WL.sub.1 and WL.sub.2 are polycrystalline silicon layers. The word line WL.sub.2 adjacent to the word line WL.sub.1 is used as the opposed electrode OE.sub.1 of the MOS capacitor C.sub.1. The bit line BL is an aluminum (Al) layer which is in ohmic contact with one of the N.sup.+ -type layers, i.e., the diffusion layer 2, of the transistor Q.sub.1. To obtain the desired capacitance of the capacitor C.sub.1 , the inversion layer 7 is extended under the word line WL.sub.2.

Note the present invention is not restricted to the DRAM of the above-described embodiments. For example, the present invention can also be applied to a DRAM in which an independent opposed electrode of a memory cell is provided but in which, due to parasitic capacitances, the level of an unselected word line is increased. FIG. 7 is an equivalent circuit diagram of part of a DRAM according to yet another embodiment of the present invention, wherein C.sub.S represents parasitic capacitances which cause unselected word line WL.sub.0 and WL.sub.2 , for example, to have an increased potential, and A is an independent opposed electrode layer. Each flip-flop in the clamping circuit CLP is connected between the adjacent word lines WL.sub.0 and WL.sub.1 or WL.sub.1 and WL.sub.2. Instead of connecting the flip-flop in the clamping circuit between adjacent word lines, the flip-flop may be connected between any pair of word lines having parasitic capacitances, such as those illustrated in FIG. 7.

As was described above, in a DRAM according to the present invention, there is an advantage in that the possibility of information in unselected cells being destroyed is greatly reduced by adding simple circuits to the memory device whereby the potentials of unselected word lines in a one-transistor one-capacitor type DRAM can effectively be clamped to the unselected level.