SEMICONDUCTOR MEMORY DEVICE

Abstract: A semiconductor memory device having a folded bit line structure (16a, 16b), in which a field oxide film (2) is formed on both sides of a channel region (11) of a transfer gate, a groove isolation region 12 for defining a memory cell region is formed to surround the field oxide film 2, and the side walls of the groove isolation region 12 include a memory cell utilized as a capacitor for storing charges as information.

Description: The present invention relates to a semiconductor memory device, and more particularly, it relates to a memory cell structure in a semiconductor memory device with a folded bit line structure suitable for high integration. More specifically the present invention relates to a semiconductor memory device which is of that type comprising: a substrate; a multiplicity of word-lines; a multiplicity of bit-lines isolated from and arranged to intersect with the multiplicity of word-lines, which bit-lines are paired and are arranged as part of a folded bit-line memory structure; and a multiplicity of memory cells, each located at an intersection of a word-line and a bit-line and each comprised of a capacitor and a field-effect transistor which is defined in the substrate and connects the capacitor to a corresponding one of the bit-lines; wherein each memory cell is isolated from its neighbours by means of an insulative structure including insulator lined grooving that is adjacent to and extends around each capacitor; and each capacitor has a portion (C P ) that is merged with the insulator lined grooving. Conventionally, various memory cell structures have been proposed and put into practice for achieving high integration of a semiconductor memory device. For example, a memory cell structure is known in which adjacent memory cells are isolated by a groove region and the side walls of the groove region are utilised as a capacitor for storing information. Figs. 1A and 1B are diagrams showing a structure of a conventional highly integrated dynamic semiconductor memory device, where Fig. 1A is a top sectional view thereof, and Fig. 1B is a cross sectional view taken along a line X-X' in Fig. 1A. The conventional dynamic memory device as shown is disclosed in an article by F. Horiguchi et al., entitled "1 Mb DRAM with Folded Capacitor Cell Structure", ISSCC85 Digest of Technical Papers, Feb., 1985, pp. 244-245. A memory cell comprises a capacitor portion C S for storing information in the form of charges and a transistor TR for transferring information to and from the capacitor portion C S . A groove isolation region 12 is formed around the memory cell. The groove region 12 is filled with oxide 2 serving as an insulator. Adjacent cells are elecrically isolated by the oxide 2. The transistor TR comprises n⁺-type diffusion regions 5a and 5b formed in a predetermined region of the p-type semiconductor substrate 1 and serving as a source and a drain, and a polysilicon (polycrystalline silicon) layer 7 formed on the p⁺-type semiconductor substrate 1 between the diffusion regions 5a and 5b through a thin insulator film (gate dielectric film) and serving as a gate electrode. The capacitor portion C S comprises a polysilicon layer 3 serving as a cell plate electrode, an n⁺-type diffusion region 5b serving as the other electrode, and a capacitor dielectric film 4 serving as a dielectric between these layers. The diffusion region 5b is also formed in the portion along the side walls of the groove 12. The polysilicon layer 3 is formed so as to extend over and beyond the groove 12. Thus, the capacitor portion C S comprises capacitance C F formed of the diffusion region 5b and the polysilicon layer 3 in a flat portion, and capacitance C P formed of the diffusion region 5b and the polysilicon layer 3 on the side walls of the groove isolation region 12. The n⁺-type diffusion region 5a of the transistor TR is connected to a first aluminium interconnection layer 6 serving as a bit line through a contact hole 10. The gate electrode for turning the transistor TR on/off serves as a part of a word line 9. The word line 9 comprises the polysilicon layer 7 and a second aluminium interconnection layer 8. The polysilicon layer 7 and the second aluminium interconnection layer 8 are electrically connected at a constant pitch, whereby the word line 9 is reduced in resistance. In the foregoing, since the side walls of the groove isolation region 12 provided along the outer peripheral portion of the memory cell are utilised as capacitance C P , sufficiently wide operating margin and sufficient alpha particle immunity are ensured due to capacitance C P , even if the chip area and the cell size are reduced so that capacitance C S in the flat portion is reduced. The longer the peripheral length of the memory cell to be utilised is, the shallower the groove for obtaining the same quantity of capacitance C P can be. The disclosed device has a folded bit line structure as also is evident from the following figures of the present drawings. Figs. 2A and 2B are diagrams showing a structure of this same memory cell, where Fig. 2A is a plan view thereof and Fig. 2B is a cross sectional view taken along a line Y-Y' in Figs. 2A. Portions corresponding to those in Figs. 1A and 1B have the same reference numerals in Figs. 2A and 2B. In this structure, bit lines 6a and 6b comprising the first aluminium interconnection layer constitute a pair of complementary bit lines. The groove isolation region 12 is formed on both sides of a channel region 11 of the transistor T R , whereby the channel region 11 is isolated from a capacitor portion of an adjacent memory cell. The polysilicon layer 3 serving as a cell plate extends into the groove isolation region 12. Fig. 3 is an enlarged cross sectional view in the neighborhood of the groove isolation region in Fig. 2A. As obvious from Fig. 3, the channel region 11 of the transistor TR is isolated from the capacitor portion of an adjacent memory cell by the groove isolation region 12. However, since a p-type diffusion layer serving as a channel stopper is not formed on the semiconductor substrate 1 in the groove isolation region 12, an inversion layer is liable to be formed in this portion. As a result, a leakage current flows from an edge portion A of the channel region 11 through the inversion layer. In addition, the polysilicon layer 3 serving as the cell plate must be terminated within the groove isolation region 12. Thus, if the groove isolation region 12 is formed with a minimum pattern width, it is difficult to perform the etching so as to terminate the polysilicon layer 3 within the isolation region 12. Furthermore, since the channel region 11 is in contact with the groove region 12, the step in this portion is increased, so that it becomes difficult to pattern the polysilicon layer 7 serving as a word line and being formed to extend over and beyond the channel region 11 and the groove isolation region 12. In order to avoid this, it is necessary to improve the proces of filling the groove isolation region 12 with insulator material. A mention is also made of US Patent No. 4044340 which likewise discloses a semiconductor memory device which has a folded bit-line configuration. The memory cells disclosed are arranged one-by-one, in pairs or in fours, which are connected alternately to one bit-line and to its complement. No details concerning memory cell isolation are given. It is also noted that European Patent Application No. 0181162 discloses a semiconductor memory device which uses trench isolation and includes isolation merged capacitance formed with the trench isolation. The structure is one of open bit line and not folded bit-line structure. Thick oxide film is embedded at the base of grooving and this grooving is provided immediately adjacent to each side of the channels of the charge transfer transistors employed in each memory cell. The problems of this aspect of the structure, in particular, the inherent problems of current leakage and conductor coverage, have already been mentioned above. The present invention is intended as a solution to the problems described above. It is also intended to provide a semiconductor memory device which can be produced by a process in which etching of the polysilicon layer serving as the cell plate need not be a critical step of the process. In accordance with the present invention there is provided a semiconductor memory device which is of that type already defined and which further is characterised in that the insulative structure also includes field oxide insulator members, which members are shallow by comparison with the insulator lined grooving, and extend in a longitudinal direction along the lateral side edges of each side of the channel region of each of the field-effect transistors. Since a field oxide insulator member is formed on each side of each of the channel regions, it is possible to constrain the leakage current in each channel region. The leakage current is in fact constrained to an extent that is typical of a conventional local oxidation isolated (LOCOS) structure. Also a step height reduction can be achieved with a consequent improvement in reliability and yield. In this structure, the conductive layer serving as the cell plate need not terminate in the groove isolation region. The patterning thereof is thus not critical and processing can be simplified. These and other features and advantages of the present invention will become apparent from the appended claims and also from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings in which: Fig. 1A is a plan view showing a structure of a conventional semiconductor memory device; Fig. 1B is a cross sectional view taken along a line X-X' in Fig. 1A; Fig. 2A is a plan view of this same conventional semiconductor memory device and shows its folded bit line structure; Fig. 2B is a cross sectional view taken along a line Y-Y' in Fig. 2A; Fig. 3 is a partially enlarged view of Fig. 2B; Fig. 4A is a plan view showing a structure of a semiconductor memory device according to an embodiment of the present invention; Fig. 4B is a cross sectional view taken along a line X-X' in Fig. 4A; Fig. 4C is a cross sectional view taken along a line Y-Y' in Fig. 4A; Fig. 5A is a plan view showing a structure of a semiconductor memory device according to another embodiment of the present invention; and Fig. 5B is a cross sectional view taken along a line X-X' in Fig. 5A. So that this invention can be better understood, preferred embodiments thereof will now be described in detail. The description that follows is given by way of example only. Figs. 4A, 4B and 4C are diagrams showing a structure of a semiconductor memory device embodying the present invention, where Fig. 4A is a plan view thereof, Fig. 4B is a cross sectional view taken along a line X-X' in Fig. 4A and Fig. 4C is a cross sectional view taken along a line Y-Y' in Fig. 4A. Portions identical or corresponding to those in Fig. 2A and 2B have identical reference numerals in Figs. 4A to 4C. A single memory cell is formed in a U shape. A thick field oxide film 2 is formed on both sides of the channel region 11 of a transistor. A channel region 11 and a capacitor portion C F are isolated by the field oxide film 2 (Figs. 4A and 4C). Capacitors of memory cells connected to a first aluminium interconnection layer 16a (or 16b) serving as a bit line are isolated from each other by a groove isolation region 12. A first polysilicon layer 3 serving as a cell plate is formed to extend over and beyond the groove isolation region 12, and is shared with adjacent memory cells. The first aluminium interconnection layer 16a (or 16b) serving as a bit line is connected to two memory cells through a contact hole 10. A pair of the first aluminium interconnection layers 16a and 16b constitute a pair of complementary bit lines. As obvious from the plan view of Fig. 4A, the polysilicon layer 3 can be shared with adjacent memory cells. During patterning (etching) it is not required to terminate the polysilicon layer 3 in the groove isolation region 12. Accordingly, the manufacturing process is not so critical and therefore can be simplified. In addition, since the groove isolation region 12 does not come into contact with the channel region 11, the channel region 11 does not have an edge discontinuity, so that the leakage current in the transistors is easily constrained and the holding characteristics of charges as information are not degraded. Furthermore, the step in side ends of the channel region 11 is reduced, so that the patterning of the polysilicon layer 7 also is simplified. Furthermore, since this structure can make the best use of the side walls of the groove isolation region 12, more capacitance for storing charges as information can be obtained, as compared with a conventional structure. For example, in order to obtain capacitance of 50 fF if the thickness of a capacitor insulating 10 nm film (100 Å) in design rule, the depth of the groove isolation region 12 may be approximately 2 um if the cell area is 10 um², so that the manufacturing process again can be simplified. Although in the above described embodiment, the capacitor electrodes are formed of a polysilicon layer 3 and an n⁺-type diffusion region 5b, other combinations are possible. For example, if polysilicon is used in place of the diffusion, a memory cell structure immune to radioactive rays such as alpha rays can be obtained. Figs. 5A and 5B are diagrams showing a structure of a semiconductor memory device according to another embodiment of the present invention, where Fig. 5A is a plan view thereof, and Fig. 5B is a cross sectional view taken along a line X-X' in Fig. 5A. Portions identical or corresponding to those in Figs. 4A and 4B have identical reference numerals in Figs. 5A and 5B. In the structure shown in Figs. 5A and 5B, a polysilicon layer 13 is formed on the polysilicon layer 3 and over an insulating film 14 to overlap with the polysilicon layer 3 at least in a flat region. The polysilicon layer 13 is electrically connected to the n⁺-type diffusion region 5b directly by a contact 15. A polysilicon layer 17 serving as a word line is provided to extend over and beyond the polysilicon layer 13. The polysilicon layer 17 constitutes a gate electrode of the transistor. In the above described structure, additional capacitance C S is formed between the polysilicon layer 13 and the polysilicon layer 3. Thus, capacitance C S for storing charges as information comprising the polysilicon layer 3 and the polysilicon layer 13 is added to capacitance C P for storing charges as information comprising the polysilicon layer 3 and the diffusion layer 5b, so that larger capacitance for storing charges as information can be obtained. 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 substrate (1); a multiplicity of word-lines (7); a multiplicity of bit-lines (16a, 16b) isolated from and arranged to intersect with said multiplicity of word-lines (7), which bit-lines are paired (16a & 16b) and are arranged as part of a folded bit-line memory structure; and a multiplicity of memory cells, each located at an intersection of a word-line (7) and a bit-line (16a, 16b), and each comprised of a capacitor (5b & 3) and a field-effect transistor (5a, 5b, 11 & 7) which is defined in said substrate (1) and connects said capacitor (5b & 3) to a corresponding one of said bit-lines (16a, 16b); wherein each memory cell is isolated from its neighbours by means of an insulative structure including insulator lined grooving (12) that is adjacent to and extends around each capacitor (5b & 3); and each capacitor (5b & 3) has a portion (C P ) that is merged with said insulator lined grooving (12); which semiconductor memory device is characterised in that: said insulative structure also includes field oxide insulator members (2), which members are shallow by comparison with said insulator lined grooving (12), and extend in a longitudinal direction along the lateral side edges of each side of the channel region (11) of each of said field-effect transistors (5a, 5b, 11, 7). A device as claimed in claim 1 wherein each capacitor (5b & 3) has an electrode structure comprising: an individual diffused electrode formed by a dopant diffused region (5b) of said substrate (1) which is located at a principal surface of said substrate (1) and at a sidewall surface which is co-extensible therewith and which is adjacent to said insulator lined grooving (12); and a common plate electrode (3) spaced from and overlying said individual electrode (5b) to define them, a capacitor having both a principal surface portion (C F ) and a portion (C P ) that is merged with said insulator lined grooving (12). A device, as claimed in claim 2, wherein the electrode structure of each capacitor (5b & 13) further comprises an individual plate electrode (13) spaced from and overlying said common plate electrode (3), which individual plate electrode (13) is electrically connected to said individual diffused electrode (5b) by means of an interconnect (15) that passes through and is isolated from said common plate electrode (3). A device as claimed in either of claims 2 & 3 wherein in the electrode structure of each capacitor (5b & 13) that portion of said diffused region (5b) of said substrate (1) which is located at the sidewall surface of said substrate (1) extends to the bottom of said insulator lined grooving (12), and said common plate electrode (3) reaches towards said bottom to overlap substantially the whole of said individual diffused electrode (5b). A device as claimed in any one of the preceding claims 2 to 4 wherein said common plate electrode (3) reaches into said insulator lined grooving (12), is continuous, and overlies the sidewall surfaces of the substrate (1) to each side of the insulator lined grooving (12). A device as claimed in any one of the preceding claims wherein the memory cells connected to each pair of paired bit-lines (16a, 16b) are connected in pairs alternately to one bit-line (16a) and to the other bit-line (16b). A device as claimed in claim 6 wherein said pairs of memory cells connected to the one bit-line (16a) of one pair of paired bit-lines (16a, 16b) are located adjacent to the pairs of memory cells connected to the other bit-line (16b) of an adjacent pair of paired bit-lines (16a, 16b) and are separated therefrom by respective ones of said field oxide insulator members (2), each adjacent two pairs of memory cells being entirely surrounded by said insulator-lined grooving (12). A device as claimed in any one of the preceding claims wherein the capacitor (5b & 3) of each memory cell has first and second regions respectively overlapped by one and the other bit-line of a pair of the paired bit-lines (16a, 16b) and a third region integral therewith, said first, second and third regions of said capacitor defining the uprights and the base of a U-shaped structure, which U-shaped structure is formed about one of the two field oxide insulator members (2) each side of the channel region (11) of the memory cell transistor (5a, 5b, 11 & 7), one of said uprights of the U-shape being adjacent also to the other one of the two field oxide insulator members (2), and the remaining sides of the U-shaped structure being surrounded by and merged with the insulator-lined grooving (12).

Inventors: ARIMOTO, KAZUTAMI MITSUBISHI-DENKI K.K., FURUTANI, KIYOHIRO MITSUBISHI-DENKI K.K., MASHIKO, KOICHIRO MITSUBISHI-DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: H01L27
IPC codes: H01L21, G11C11, H01L27