Semiconductor Memory Device

Related, copending application of particular interest to the instant application is U.S. Ser. No. 094,647, entitled "Dynamic-Type Semiconductor Memory Device", filed Sept. 9, 1987 assigned to the same assignee of the instant application.

1. Field of the Invention

The present invention relates to a semiconductor memory device and, more specifically, it relates to improvements in the arrangement of the memory cells, in the electrical connection structure and in the structure of the capacitor region.

2. Description of the Prior Art

A semiconductor memory device comprises capacitors in which information is stored, transistors which are switched by corresponding word lines for inputting (writing) and outputting (reading) information to and from the corresponding capacitors, and bit lines connected to the transistors for transmitting the information.

FIG. 4A shows a schematic plan view of a conventional dynamic type semiconductor memory device. FIG. 4B is a cross sectional view taken along a line B--B of FIG. 4A. In these figures, a source region 6a and drain regions 6b of transistors 6 are formed on a main surface of a silicon substrate 1 and capacitor regions 4a are provided adjacent to the drain regions 6b. These regions are surrounded by an isolating region 7 and a channel cut 8 is formed below the isolating region 7. Over channel regions 3a provided between the source region 6a and the drain regions 6b, word lines 3 are formed with corresponding gate insulating films 3b interposed therebetween. A capacitor electrode 9 is formed over the capacitor regions 4a with a capacitor insulating film 4b interposed therebetween. The area on which the capacitor electrode 9 is formed is shown by the hatching of broken lines in FIG. 4A. These word lines 3 and the capacitor electrode 9 are covered with an insulating layer 10. A bit line 5 formed on the insulating layer 10 is connected through a contact hole 2 to the source region 6a which is common to the two transistors 6. Namely, two capacitors 4a are connected to one bit line 5 via one contact hole 2 through respective switching transistors 6.

As can be seen from FIG. 4A, a contact hole 2 formed on a source region 6a of another memory connected to the adjacent bit line 5 exist in the vicinity of the outer periphery of one capacitor region 4a. Therefore, if a trench is formed around the capacitor region 4a so as to employ the side wall of the trench also as a capacitor region, the capacitor region on the side wall of the trench is close to and opposed to the transistor region of the adjacent member cell, affecting the characteristics of the transistors.

In view of the foregoing, an object of the present invention is to provide a semiconductor memory device which is improved in the arrangement of the memory cells, in electrical connection structure and in the structure of the capacitor region.

A semiconductor memory device in accordance with the present invention comprises: a semiconductor substrate having a main surface; and four memory cells arranged in point symmetry on the main surface, each of the memory cells comprising one transistor having a source region formed around the point of symmetry and a drain region formed therearound, and one capacitor adjacent to the outside of the drain region, the capacitor comprising a surface capacitor region parallel to the main surface of the substrate and a trench capacitor region parallel to a side wall of a trench formed in the main surface of the substrate along the outer periphery of the surface capacitor region, and the the semiconductor memory device further comprises an insulating layer covering the memory cells and having one contact hole arranged on the center of the point symmetry, with the contact hole enabling electrical connection to the source region of each transistor.

In the semiconductor memory device in accordance with the present invention, transistors arranged in point symmetry around one contact hole are surrounded by respective capacitors formed adjacent to the outside thereof. Therefore, the trench capacitor region of each capacitor is not opposed to the transistor region in the vicinity, whereby a highly integrated semiconductor memory device of high performance can be provided without affecting the characteristics of the transistors.

These object and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic plan view of a semiconductor memory device in accordance with one embodiment of the present invention;

FIG. 2 is a schematic cross sectional view taken along the line X--X of FIG. 1;

FIG. 3 is a cross sectional view similar to FIG. 2 showing another embodiment with some portions changed;

FIG. 4A is a schematic plan view of a conventional semiconductor memory device; and

FIG. 4B is a cross sectional view taken along the line B--B of FIG. 4A.

FIG. 1 shows a schematic plan view of a dynamic type semiconductor memory device in accordance with one embodiment of the present invention. FIG. 2 is a schematic cross sectional view taken along a line X--X of FIG. 1. In these figures, source regions 6a and drain regions 6b of transistors are formed on a main surface of a semiconductor substrate 1 and surface capacitor regions 4a parallel to the main surface of the substrate 1 are formed adjacent to the drain regions 6b. Trenches 40 are formed around the surface capacitor regions 4a, the side wall of a trench 40 being used as a trench capacitor region 40a continuous to the surface capacitor region 4a. Over a channel region 3a between the source region 6a and the drain region 6b, a word line 3 is formed with a gate insulating film 3b interposed therebetween. The surface capacitor region 4a and the trench capacitor region 40a are opposed to the capacitor electrode 9 through the capacitor insulating films 4b and 40b, respectively. The areas on which the capacitor electrodes 9 are formed are shown by the hatching of broken lines in FIG. 1. More specifically, under the region of the capacitor electrode 9 shown by the hatching of broken lines in FIG. 1, the outer periphery of the surface capacitor region 4a is surrounded by the capacitor trench 40. The word lines 3 and the capacitor electrode 9 are covered with an insulating layer 10. A pair of bit lines 5 formed on the insulating layer 10 are connected to the source regions 6a through a contact hole 2. Four transistors 6 arranged in point symmetry around the contact hole 2 are isolated from each other by isolating trenches 70. The isolating trench 70 may be filled with an insulating material 70a. The capacitors of the memory cells are isolated from each other by the isolating regions 7 formed at the bottom portions of the capacitor trenches 40. As is apparent from FIG. 1, the isolating trenches 70 and the capacitor trenches 40 can be simultaneously formed. Channel cuts 8 are formed below these isolating regions 70 and 7. It would be understood that the source regions 6a of two transistors which are connected to the same bit line 5 are not necessarily be isolated from each other in the contact hole 2.

As described above, in a semiconductor memory device in which four memory cells are arranged in point symmetry around one contact hole 2, capacitors can be always arranged on both sides of one capacitor trench 40. Namely, the problem as in the conventional device, in which is one side of the capacitor trench is placed adjacent to the transistor region thereby affecting the characteristics of the transistor, can be eliminated. Therefore, a highly integrated semiconductor memory device of high performance can be provided without degrading the characteristic of the memory cells.

FIG. 3 is a cross sectional view similar to FIG. 2, showing another embodiment with some portions changed. In this embodiment, four transistors arranged in point symmetry around the contact hole 2 are isolated from each other by isolating oxide films 7 formed by selective oxidation and the like and by the channel cuts 8 therebelow, instead of the isolating trenches 70. In this case, the trench 40 in the capacitor electrode region 9 shown by the hatching of the broken lines in FIG. 1 comes to an end at the border of four transistor regions surrounded by a circle around the contact hole 2.

As described above, according to the present invention, four memory cells are structured by forming capacitors outside four transistors arranged in point symmetry with the capacitors connected to respective transistors, so that even if a trench capacitor region is provided around the surface region of each capacitor, the transistor region do not opposed close to the trench capacitor region. Therefore, a highly integrated semiconductor memory device of high performance can be provided without affecting the characteristics of the transistors.

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 spirit and scope of the present invention being limited only by the terms of the appended claims.