Semiconductor Memory Miniaturized by Line Groups and Staggered Cells

1. Field of the Invention

The present invention relates to a semiconductor memory device.

Generally, a semiconductor memory device comprises a word-line selecting and driving circuit (word drive circuit) for decoding word address signals to select a single word line and for driving the selected single word line, a bit-line selecting circuit for decoding bit address signals, a plurality of word lines connected to the outputs of the word-drive circuit, a plurality of bit lines connected to the outputs of the bit-line selecting circuit, and memory cells arranged on the intersections between the word lines and the bit lines.

The present invention more particularly relates to a semiconductor memory device in which the memory cell array can be miniaturized without limitation by the space occupied by the word drive circuit.

2. Description of the Prior Art

Conventionally, the outputs of a word drive circuit are connected to the word lines in a one-to-one correspondence. Recent miniaturization of memory cells has led to an increasingly smaller space between adjacent word lines; however, this has made essential the miniaturization of the word drive circuit which is integrated with the memory cell array on one chip. Miniaturization of word drive circuits, however, is difficult.

As a result, the size of the word drive circuits has limited the degree to which memory cell arrays could be miniaturized. This has been especially true in an erasable programmable read only memory (EPROM) which has a complicated word drive circuit including high voltage driving circuits as well as row decoder circuits. Therefore, in conventional semiconductor memory devices, although the size of the memory cell arrays themselves could theoretically be miniaturized, the degree of their miniaturization has been limited by the size of the word drive circuits. As a result, the total size of semiconductor memory devices could not be miniaturized to the desired extent.

To achieve some miniaturization of semiconductor memory devices, one known system has had the word drive circuits arranged on both sides of the memory cell arrays. While this system does allow the memory cell arrays to be miniaturized, it doubles the area occupied by the word drive circuits compared with a system where the word drive circuits are arranged on only one side of the memory cell array. Therefore, this known system runs counter in principle to the miniaturization of the entire semiconductor memory device.

Accordingly, a primary object of the present invention is to provide a semiconductor memory device in which a memory cell array can be miniaturized without limitation by the word drive circuit.

Another object of the present invention is to provide a semiconductor memory device in which a memory cell array can be miniaturized without requiring additional word drive circuits.

In order to achieve the above objects, there is provided, according to the invention, a semiconductor memory device comprising: a plurality of word lines; a plurality of bit lines cross coupled with said word lines; a word-line selecting and driving circuit having inputs operatively receiving word address signals, said word-line selecting and driving circuit operatively decoding said word address signals and selectively driving the word lines; a bit-line selecting circuit having inputs operatively receiving bit address signals, said bit-line selecting circuit operatively decoding said bit address signals and selecting one of said bit lines; a plurality of memory cells arranged at corresponding intersections between said word lines and said bit lines; further, the word-line selecting and driving circuit having outputs each of which is commonly connected to a set of plural adjacent word lines; and the memory cells are arranged only at intersections between one of the adjacent word lines in each one of the sets and one of the bit lines, and between another of the adjacent word lines in said each of the sets and another of the bit lines.

The above objects and features as well as other features and advantages of the present invention will be more apparent when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of an example of a conventional semiconductor memory device;

FIG. 2 is a block diagram of another example of a conventional semiconductor memory device;

FIG. 3 is a block diagram of a semiconductor memory device, according to an embodiment of the present invention;

FIG. 4 is a plan view of a pattern arrangement of a part of the memory cell array in the device of FIG. 3;

FIG. 5 is a cross-sectional view taken along line VI--VI of FIG. 4, of a physical structure of a floating gate avalanche injection metal oxide semiconductor (FAMOS);

FIG. 6 is an expanded cross-sectional view taken along line VII--VII of FIG. 4, of a physical structure of the FAMOS;

FIG. 7 is a plan view of a pattern arrangement in which adjacent word lines are not commonly connected to an output of a word drive circuit; and

FIG. 8 is a block diagram of a semiconductor memory device, according to another embodiment of the present invention.

Embodiments of the present invention will now be described in comparison with conventional technology and with reference to the accompanying drawings.

In FIG. 1, a conventional semiconductor memory device comprises a memory cell array MCA on which word lines WL.sub.1 through WL.sub.5 and bit lines BL.sub.1 through BL.sub.5 intersect each other. A memory cell MC is arranged at the intersection of each word line and bit line. The word lines WL.sub.1 through WL.sub.5 are connected, in a one-to-one correspondence, to the outputs of a word drive circuit WD including word drive units WD.sub.1 through WD.sub.5, respectively. The bit lines BL.sub.1 through BL.sub.5 are connected through a column gate CG to a sense amplifier SA. CD is a column decoder with outputs connected to the column gate CG. The column decoder CD and the column gate CG constitute a bit-line selecting circuit.

The miniaturization of memory cells due to recent developments in semiconductor technology has led to increasingly smaller intervals d.sub.1 between adjacent word lines. Since each word drive unit has a complicated circuit structure, however, the width d.sub.2 of each word drive unit cannot be made as small as the smaller interval d.sub.1. Since the word drive units WD.sub.1 through WD.sub.5 are integrated with the memory cell array on one chip, it is not practical to make the interval d.sub.1 between word lines smaller than the width d.sub.2 of each word drive unit. This is because it would complicate the wiring patterns for which connect the word drive units WD.sub.1 through WD.sub.5 and the word lines WL.sub.1 through WL.sub.5. Thus, in the conventional device of FIG. 1, the miniaturization of a memory cell array is restricted by the width d.sub.2 of the word drive unit.

In FIG. 2, another example of a conventional semiconductor memory device is illustrated. The main difference between FIG. 1 and FIG. 2 is that, in FIG. 2 an additional word drive circuit WD', including word drive units WD.sub.6 through WD.sub.10 is arranged on the side of memory cell array MCA opposite to word drive circuit WD including word drive units WD.sub.1 through WD.sub.5. The outputs of the word drive units WD.sub.6 through WD.sub.10 are connected to additional word lines WL.sub.6 through WL.sub.10. Each of the additional word lines WL.sub.6 through WL.sub.10 extends between two adjacent word lines of the word lines WL.sub.1 through WL.sub.5. A memory cell MC is arranged on the intersection between each word line of the word lines WL.sub.1 through WL.sub.10 and each bit line of the bit lines BL.sub.1 through BL.sub.5. The circuit configuration of FIG. 2 is the same as that of FIG. 1 except for the above. The provision of word drive units on both sides of the memory cell array doubles the integration degree of the memory cell array in FIG. 2 over that of FIG. 1. However, it also doubles the area occupied by the word drive units over that of FIG. 1. Therefore, the conventional device of FIG. 2 runs counter in principle to the miniaturization of semiconductor memory devices.

An embodiment of the present invention will now be described with reference to FIG. 3. This embodiment is realized by an EPROM. In FIG. 3 each output, O.sub.1 through O.sub.5, of the word drive units WD.sub.1 through WD.sub.5 is commonly connected to a pair of adjacent word lines (WL.sub.1, WL.sub.2), (WL.sub.3, WL.sub.4), (WL.sub.5, WL.sub.6), (WL.sub.7, WL.sub.8), and (WL.sub.9, WL.sub.10), respectively. Between each of the bit lines BL.sub.1 through BL.sub.10 and one of two word lines in each of the pairs (WL.sub.1, WL.sub.2) through (WL.sub.9, WL.sub.10), a memory cell MC is arranged. That is, between each of the bit lines BL.sub.1, BL.sub.3, BL.sub.5, BL.sub.7, and BL.sub.9 and each of the word lines WL.sub.1, WL.sub.4, WL.sub.5, WL.sub.8, and WL.sub.9, a memory cell MC is arranged. Also, between each of the bit lines BL.sub.2, BL.sub.4, BL.sub.6, BL.sub.8, and BL.sub.10 and each of the word lines WL.sub.2, WL.sub.3, WL.sub.6, WL.sub.7, and WL.sub.10, another memory cell MC is arranged. The bit lines BL.sub.1 through BL.sub.10 are connected to ten outputs of a column gate CG.sub.0. Ten inputs of the column gate CG.sub.0 are connected to ten outputs C.sub.1, C.sub.2, . . . of a column decoder CD.sub.0. The column gate CG.sub.0 is connected to a sense amplifier SA.sub.0.

In operation, the word drive units receive word address signals and decode them to drive one of the five outputs O.sub.1 through O.sub.5, while the column decoder CD.sub.0 receives bit address signals, decodes them and selects one of the outputs C.sub.1 through C.sub.10 l to be a high potential. The selected one of the ouputs C.sub.1 through C.sub.10 is applied to the corresponding gate transistor (not shown in FIG. 3) in the column gate CG.sub.0 to open the gate transistor. Thus, the memory cell arranged between the driven word line and the selected bit line is selected for writing or reading.

In the wiring pattern of FIG. 3, the size of the memory cell array MCA.sub.0 is the same as the size of the array MCA in FIG. 1 or FIG. 2. On the other hand, there are twice the number of word lines, bit lines, and memory cells as in FIG. 1. Therefore, the integration degree of the memory cell array MCA.sub.0 in FIG. 3 is twice as that of the conventional array MCA in FIG. 1. It should be noted that the word drive units WD.sub.1 through WD.sub.5 are the same as those in the conventional example of FIG. 1. No additional word drive circuit is provided in FIG. 3.

It will be noted that the space between two adjacent bit lines in FIG. 3 is half that of FIG. 1. Since column gates are not as complex a word drive circuits and since the memory cells connected to one word line are connected to alternate bit lines, reducing the space between adjacent bit lines is not a problem. The word drive unit WD.sub.1 comprises a row decoder, an output buffer and a high voltage driving circuit. The word drive unit WD.sub.2 comprises a row decoder, an output buffer, and a high voltage driving circuit. Each of these circuits in the word drive units is well known. Briefly, when all of the address signals are "0", the output of the first row decoder is selected. When the word address signals are 1000 . . . 0, the output of the second row decoder is selected.

The output buffer or word drive unit WD, or WD.sub.2 receives an inverted power down signal. When the memory device is in a standby state, the power down signal is at a high potential and the inverted power down signal is at a low potential so that the output buffers of both word drive units, WD.sub.1 and WD.sub.2 ; are inactive.

The high voltage driving circuits supply the high voltages necessary to effect writing into the memory cells MC of the EPROM (PROM erasable by means of ultraviolet rays). When the output of the row decoder of unit WD.sub.1 is selected to be a high potential (H level), the output buffer thereof outputs the H level. Therefore, the high voltage is applied to the output O.sub.1 of the word drive unit WD.sub.1. With respect to the remaining nonselected word drive circuits, for example, the word drive unit WD.sub.2, since the output buffer thereof outputs a low potential (L level), the output O.sub.2 becomes the L level. During reading, the writing signal is not applied to the high voltage driving circuits, so that the high voltage V.sub.pp is not applied to the memory cells MC. No further detailed description is provided here because the components of the word drive units WD.sub.1, WD.sub.2, . . . are well known.

The memory cell array MCA.sub.0 comprises memory cells MC. Each memory cell MC is a FAMOS with stored contents erasable by radiating ultraviolet rays. Each FAMOS has a control gate connected to a word line, a floating gate provided under the control gate and for storing information, a drain connected to a bit line, and a source connected to a ground. Each bit line is connected through a metal oxide semiconductor (MOS) transistor in the column gate CG.sub.0 to the sense amplifier SA.sub.0. The output signals C.sub.1, C.sub.2, C.sub.3, . . . of the column decoder CD.sub.0 are applied to the gates of the MOS transistors in the column gate CG.sub.0.

The output O.sub.1 of the word drive unit WD.sub.1 is commonly connected, in this embodiment, to two word lines WL.sub.1 and WL.sub.2. The output O.sub.2 is similarly commonly connected to two word lines WL.sub.3 and WL.sub.4.

FIG. 4 is a plan view illustrating a pattern arrangement of a part of the memory cell array in the device of FIG. 3. In FIG. 4, the word lines WL.sub.1 and WL.sub.2 extend in parallel, spaced apart at an interval d.sub.1. At one end, the word lines WL.sub.1 and WL.sub.2 are connected to the output O.sub.1 of the word drive unit WD.sub.1 (FIG. 3). Similarly, the word lines WL.sub.3 and WL.sub.4 extend in parallel, spaced apart at the interval d.sub.1, and are connected at one end to the output O.sub.2 of the word drive unit WD.sub.2. The interval between the word lines WL.sub.2 and WL.sub.3 is represented by d.sub.2, which is greater than d.sub.1 because of the presence of contact windows CW. The bit lines BL.sub.1, BL.sub.2, . . . are arranged so as to be perpendicular to the extending direction of the word lines. Each memory cell MC, which is a well known FAMOS, is arranged on an intersecting portion of a word line and a bit line. The contact windows CW are for connecting the bit lines to diffusion regions D.

The physical structure of an FAMOS is illustrated in FIG. 5, which is a cross-sectional view taken along line VI--VI of FIG. 4. The physical structure of two or more FAMOS is illustrated in FIG. 6, which is a cross-sectional view taken along line VII--VII of FIG. 4. Referring to FIGS. 4, 5, and 6, on a semiconductor substrate 1, a first insulating layer 2 is formed. On the first insulating layer 2, floating gates FG are formed, and covered by a second insulating layer 3 formed thereon. The word lines WL.sub.1, WL.sub.2, WL.sub.3, . . . are formed on the layer 3 extending above and spaced thereby from the floating gates FG. The bit lines BL.sub.1, BL.sub.2, . . . are formed on the third insulating layer 4 and spaced thereby from the floating gates FG. The word lines WL.sub.1, WL.sub.2, . . . are formed of polycrystalline silicon and act as control gates. The bit lines BL.sub.1, BL.sub.2, . . . are formed of aluminum and are in contact with diffusion regions D. The diffusion regions D are formed by a self-alignment technique after forming the floating gates FG and the word lines (control gates) WL.sub.1 and, therefore, are not present under the floating gates FG. The broken line in FIG. 5 indicates that the diffusion region D is not actually under the floating gate FG but is rather to its front or rear. It will be understood from FIGS. 4 and 6 that the bit lines BL.sub.1, BL.sub.2, . . . are in contact with the diffusion regions D at the contact windows CW.

As illustrated in FIG. 4, the interval between the two adjacent outputs O.sub.1 and O.sub.2 of the word drive circuits WD.sub.1 and WD.sub.2 is represented by d.sub.3. The relation among the intervals d.sub.1, d.sub.2, and d.sub.3 is d.sub.3 =d.sub.1 +d.sub.2. Thus, the interval between adjacent word lines can be made smaller than the interval between adjacent outputs of the word drive circuits. Since d.sub.1 is nearly equal to d.sub.2, the interval between adjacent word lines can be reduced to about half the interval between adjacent outputs of the word drive circuit, i.e., half the width of a word drive unit.

If, as in the conventional device, adjacent word lines are not commonly connected to an output of a word drive circuit, and if the interval between word lines could be made as small as in FIG. 4, i.e., d.sub.1 or d.sub.2, as illustrated in FIG. 7, then the interval between adjacent outputs of the word drive circuit, i.e., the width of a word drive unit would have to be the same as the interval d.sub.1 or d.sub.2. However, as mentioned before, it is difficult to miniaturize the word drive circuit because each drive unit has a very complex structure. Therefore, it is impossible to realize an interval between the adjacent outputs as short as d.sub.1 or d.sub.2 shown in FIG. 7 without increasing the amount of space required for the word drive circuits.

It will be seen that, in the embodiment illustrated in FIG. 3, in addition to the number of word lines, the number of bit lines is also twice that in the conventional device. This means that the interval between adjacent bit lines is half that in the conventional device when the size of the memory cell array MCA.sub.0 in FIG. 3 is the same as that of the array MCA in FIG. 1. It is not difficult to make the interval between adjacent bit lines shorter than in the conventional devices.

In an EPROM decreasing the interval between adjacent bit lines is less difficult than in other types of memory devices. The size of the memory cell in the EPROM has, as illustrated in FIG. 4, a width x and a length y. Because the floating gate FG extends along the extending direction of the word line (control gate), the length y is greater than the width x. In the conventional devices, the interval between adjacent bit lines was greater than the length y of the memory cell, as can be seen from FIG. 7. In the embodiment, the memory cells connected to one word line are connected to alternate bit lines. Also, the column gate CG.sub.0 connected to the bit lines does not have as complex a circuit structure as the word drive circuit. Therefore, it is easy to shorten the interval between adjacent bit lines.

In the embodiment of FIG. 3, the number of memory cells is twice that of the conventional example of FIG. 1. On the other hand, the number of memory cells connected to each bit line is the same as that in the conventional example of FIG. 1. Therefore, when the number of memory cells in a memory device of the present invention is made the same as that of the conventional device, the number of memory cells connected to each bit line in the embodiment of the present invention can be reduced to less than that in the conventional example of FIG. 1. As a result, with respect to a certain memory capacity, the capacitance of each bit line in this embodiment (the capacitance being derived from the diffusion regions of memory cells connected to the bit line) is reduced to less than the conventional example of FIG. 1. The reduction of the capacitance of each bit line results in an additional effect of high speed operation.

In the construction illustrated in FIG. 3, the number of the outputs C.sub.1, C.sub.2, . . . , and C.sub.10 of the column decoder CD.sub.0 is the same as the number of the bit lines BL.sub.1, BL.sub.2, . . . , and BL.sub.10. The addition of a simple circuit in the column gate CG.sub.0, however, can greatly reduce the number of the outputs of the column decoder. FIG. 8 is a block diagram illustrating another embodiment of the present invention, in which the number of the outputs of the column decoder is reduced. In FIG. 8, the column decoder CD.sub.1 outputs seven signals, i.e., a first bit selection signal BS.sub.1, a second bit selection signal BS.sub.2, and the outputs C.sub.1 through C.sub.5. The column gate CG.sub.1 comprises transistors T.sub.1 through T.sub.5, for respectively receiving the outputs C.sub.1 through C.sub.5, and transistors Q.sub.1 through Q.sub.10. The gates of the transistors Q.sub.1 through Q.sub.5 receive the first bit selection signal BS.sub.1. The gates of the transistors Q.sub.6 through Q.sub.10 receive the second bit selection signal BS.sub.2. The bit lines BL.sub.1, BL.sub.3, BL.sub.5, BL.sub.7, and BL.sub.9 are respectively connected through the transistors Q.sub.1, Q.sub.2, Q.sub.3, Q.sub.4, and Q.sub.5 to the transistors T.sub.1, T.sub.2, T.sub.3, T.sub.4, and T.sub.5. The bit lines BL.sub.2, BL.sub.4, BL.sub.6, BL.sub.8, and BL.sub.10 are respectively connected through the transistors Q.sub.6, Q.sub.7, Q.sub.8, Q.sub.9, and Q.sub.10 to the transistors T.sub.1, T.sub.2, T.sub.3, T.sub.4, and T.sub.5. The sources (or drains) of the transistors T.sub.1, T.sub.2, T.sub.3, T.sub.4, and T.sub.5 are commonly connected to the input of the sense amplifier SA.sub.1. The memory cell array MCA.sub.0 and the word drive units WD.sub.1 through WD.sub. 5 are the same as those in FIG. 3.

In the operation of the device of FIG. 8, when the first bit selection signal becomes the H level, the information on the bit lines BL.sub.1, BL.sub.3, BL.sub.5, BL.sub.7, and BL.sub.9 is transferred to the transistors T.sub.1 through T.sub.5. When the second bit selection signal BS.sub.2 becomes the H level, the information on the bit lines BL.sub.2, BL.sub.4, BL.sub.6, BL.sub.8, and BL.sub.10 is transferred through the transistors Q.sub.6, Q.sub.7, Q.sub.8, Q.sub.9, and Q.sub.10 to the transistors T.sub.1 through T.sub.5. According to the outputs C.sub.1 through C.sub.5 to the column decoder, a transistor, selected from the transistors T.sub.1 through T.sub.5, transfers the information from the selected bit line to the sense amplifier SA.sub.1.

In the constitution of FIG. 8, although there are 10 bit lines, there are seven outputs of the column decoder CD.sub.1. Generally, n bit lines can be selected by (n/2+2) or {(n+1)/2+2} outputs of the column decoder.

The present invention is not restricted to the above-described embodiments. Many other changes and modifications are possible without departing from the spirit of the invention.

For example, as an alternative to an EPROM erasable by means of ultraviolet rays, a mask read only memory (ROM) or a programmable read only memory (PROM) may be used for constituting a memory cell array. When a mask ROM or a PROM is used, the high voltage driving circuits V.sub.1, V.sub.2, . . . for effecting writing, included in the word drive circuit, are not necessary.

Also, instead of connecting two adjacent word lines to one output of a word drive circuit, a set including three or more adjacent word lines may alternatively be connected to one output of a word drive circuit.

From the foregoing description, it will be apparent that, according to the present invention, in a semiconductor memory device, a memory cell array can be miniaturized regardless of the fact that the word drive circuit cannot be miniaturized and without providing a further word drive circuit. Also, with respect to a certain memory capacity, the number of memory cells connected to one bit line is reduced in comparison with the prior art so that the capacitance of the bit line is reduced, resulting in a high speed operation.