MOS CAPACITOR AND METHOD OF MANUFACTURING THE SAME

Abstract: A MOS capacitor of the structure of a trench-shaped capacitor is manufactured, first, by forming a first trench (19) in a semiconductor substrate (16) of one conductivity type and the trench is filled with an insulator (21) to form an element isolation region (22). Second and third trenches (24, 24) are formed at positions respectively adjacent to both sides of the element isolation region (22) and opposing to each other sandwiching the element isolation region, and after a first polycrystalline silicon film (25) of an opposite conductivity type to that of the semiconductor substrate and an insulating film (27) are sequentially formed on the inner walls of the second and third trenches, the trenches (24, 24) are filled with a second polycrystalline silicon film (28). The trench-shaped capacitor uses the insulating film (27) as a dielectric layer and uses the first and second polcrystalline silicon films (25, 28) as electrodes. In this capacitor, the width of the isolation region (22) can be reduced to about 1 µm and still the isolation dielectric strength can be improved. Further, when the structure is applied to a DRAM, the memory cell can be made in a small size. Moreover, since an insulating film (27) of the capacitor is formed on a polycrys­ talline silicon film (25), the capacitor insulating film (27) is not affected by a defect caused in the silicon substrate (16) due to a work strain and the like, and as a result, a memory cell having high dielectric strength and an improved characteris­ tic of a time (pause time) for the charges to disappear naturally due to leakage can be obtained.

Description: This invention relates to MOS capacitors and their manufacture. Recently, MOS dynamic RAMs (hereinafter referred to as DRAMs) having a large storage capacity on the order of a mega-bit have been realised. In keeping with such a trend, DRAMs have been designed to achieve higher density by reducing the size of the memory cells contained therein. The present invention relates to a MOS capacitor and a method of manufacturing it and in particular, to a MOS capacitor suitable for increasing the density of a DRAM and a method of manufacturing such a MOS capacitor. A similar MOS capacitor has been disclosed in GB-A-2 138 207. With progress in the study of means for increasing the degree of integration of the DRAM, a DRAM having a capacity as large as several mega-bits has been reported. However, since the size of a chip generally increases as the storage capacity is increased, in order to achieve practical use, it is necessary to increase the density by further reducing the size of the memory cell. The area of the memory cell per 1 bit is in a range from 50 to 70 µm² for a DRAM of 256 kb, and the area must be reduced to a range from 20 to 30 µm² or less for 1 Mb. However, taking a soft error and noise margin into consideration, it is necessary that the capacity of a capacitor constituting the memory cell be made substantially comparable to the DRAM of 256 kb. In order to maintain the capacity of the capacitor of the memory cell comparable to that of the DRAM of 256 kb while reducing the memory cell area, there is a method in which the effective film thickness of an insulating film (hereinafter referred to as a capacitor insulating film) which is a constituent element of the capacitor is reduced, or another method in which the effective area of the capacitor insulating film is increased. However, in the former method, since it is difficult to make a silicon dioxide film thinner than about 10 - 15 µm in view of pinholes and the like, there exists a limitation. On the other hand, a trench-shaped capacitor is known in which, in order to eliminate the drawbacks mentioned above, a trench of several µm in depth is formed in the semiconductor substrate, and by forming a capacitor on the inner wall of the trench, the effective area of the capacitor is increased (e.g., Kiyoo Itoh et al, "256 k/l Mb DRAMS-II", 1984, ISSCC, pages 282 - 288). The trench-shaped capacitor of this structure is a kind of MOS capacitor structure in which an element isolation region is formed in the silicon substrate by a selective oxidation method, trenches are formed adjacent to the element isolation region and sandwiching this region, insulating films are respectively formed on inner walls of these trenches, and the trenches are filled up by forming polycrystalline silicon films on the insulating films so that the insulating films constitute dielectric layers and the silicon substrate and the polycrystalline silicon films are used as electrodes. The trench-shaped capacitor mentioned above are designed to reduce the area of the memory cell by forming the trenches in the silicon substrate and forming the capacitor on the inner walls. However, when the trench-shaped capacitors are formed adjacent to both sides of the element isolation region formed by the selective oxidation method, punch-through will be caused between the adjacent capacitors, and reducing the isolation width will become difficult. Moreover, since the selective oxidation method is employed in the element isolation, there arises a problem in that the generation of a "bird's beak" can not be avoided and higher integration can not be achieved. Furthermore, a crystal defect or the like will occur in the silicon substrate during the trench forming process and the heat treatment process will cause a leakage in the substrate and a defect in the capacitor insulating film. These problems pose a large obstacle in manufacturing a large capacity memory. According to one aspect of the present invention there is provided an MOS capacitor comprising a semiconductor substrate having a principal surface and a pair of spaced trenches, an isolation region embedded in said semiconductor substrate between said pair of trenches, first polycrystalline silicon films formed in each of said trenches and serving as a first electrode of said MOS capacitor, insulating films formed on said first polycrystalline films and constituting dielectric layers of said capacitor and a second polycrystalline film formed on said insulating film and serving as a second electrode of the MOS capacitor, characterised in that each of said trenches are defined by side walls, having first and second parts, and a bottom wall, the first parts of the side walls and the bottom of the trenches being inner surfaces of the substrate, said isolation region has a pair of side walls which extend in a direction substantially perpendicular to the principal surface of said substrate and define said second parts of the side walls of the trenches, said isolation region has a depth greater than the bottom of said trenches and said first polycrystalline silicon films are formed directly on said first parts, said second parts and said bottom of said trenches, whereby said first polycrystalline silicon films are disposed entirely between the bottoms of said trenches and said insulating films. According to another aspect of the present invention there is provided a method for manufacturing a MOS capacitor comprising the steps of: forming a first trench in a main surface of a semiconductor substrate of one conductivity type, and after covering a wall of said first trench with a first insulating film, filling the inside of said first trench with one of an insulator and a semiconductor material thereby to form an element isolation region with the substrate surface flattened; forming at least second and third trenches so that said second and third trenches respectively adjoin both sides of said element isolation region and oppose each other sandwiching said element isolation region therebetween and so that the depth of said second and third trenches is smaller than that of said first trench; forming a first polycrystalline silicon film of an opposite conductivity type to that of said semiconductor substrate on the whole of inner walls of said second and third trenches; forming a second insulating film on said first polycrystalline silicon film; and forming a second polycrystalline silicon film respectively on said second insulating film to fill said second and third trenches; thereby to form a capacitor having a dielectric layer constituted by said second insulating film and having electrodes constituted by first polycrystalline silicon film and said second polycrystalline silicon film. In an MOS capacitor of the present invention, the element isolation region can be formed to provide high isolation dielectric strength and in a trench shape having a reduced width. Furthermore, since a polycrystalline silicon film is formed on the inner wall of the trench-shaped capacitor, and an insulating film of the capacitor is formed on the polycrystalline silicon film, the capacitor insulating film can be formed in a region which is not influenced by a crystal defect present in the substrate. The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Figs. 1 to 10 are sectional views illustrating in the order of processes in a method of manufacturing a DRAM applied with a method of manufacturing a MOS capacitor in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a method of manufacturing a DRAM applied in order to obtain a structure of MOS capacitor in accordance with the present invention will be described with reference to a flow chart of the process. First, as shown in Fig. 1, a silicon dioxide film 17 is formed on a surface of a P-type silicon substrate 16, and an opening 18 having a width of 1 µm is formed in a portion intended to be an element isolation region by a photoetching technique. Next, as shown in Fig. 2, a trench 19 having a depth of 5 µm and a width of 1 µm is formed in the silicon substrate 16 by reactive ion etching using CCl₄ and O₂ and by using the silicon dioxide film 17 as a mask. Following this a channel stopper region 20 is formed on the bottom of the trench 19 by ion implanation of boron. Next, as shown in Fig. 3, after oxidising thermally the inner wall of the trench 19 and the surface of the silicon substrate 16, silicon dioxide 21 is deposited by a CVD technique to fill the trench 19 with the silicon dioxide 21, and an element isolation region 22 is formed. In this respect, as a technique for filling the trench 19, another technique may be used in which after the silicon dioxide is formed on the inner wall of the trench 19, a polycrystalline silicon layer is formed by the CVD technique thereby to fill the trench 19 with this polycrystalline silicon layer. Following this, as shown in Fig. 4, the silicon dioxide films 17 and 21 formed on the main surface of the substrate 16 are removed entirely by etching. Next, as shown in Fig. 5, a new silicon dioxide film 23 is formed on the surface of the silicon substrate 16, and further, by the photoetching technique, openings for forming trench-shaped capacitors are formed at areas located at both sides of the element isolation region 22 and adjoining this region 22. Following this, reactive ion etching is applied to silicon substrate portions exposed at the inside of these openings, and trenches 24 and 24 each having a depth of 4 µm and a width of 1 µm are formed in the silicon substrate 16. These trenches 24 and 24 each having a U- or V-shaped cross section are located at both sides of the element isolation region 22 and are of a construction adjoining the element isolation region 22. Next, as shown in Fig. 6, an N-conductivity type polycrystalline silicon film 25 containing is grown to a film thickness of about 200 nm on the inner walls of the trenches 24 and 24, and then N-type diffusion layers 26 and 26 are formed by diffusing the contained in the polycrystalline silicon film 25 silicon substrate 16, and the silicon substrate 16 and the polycrystalline silicon film 25 are electrically connected. Next, after coating a resist (not shown) all over the polycrystalline silicon film 25 to fill the trenches 24 and 24 with the resist, the resist film on the surface is removed by reactive ion etching using an oxygen gas, and leaves the resist only in the inside of the trenches 24 and 24. In this condition, the polycrystalline silicon film 25 and the silicon dioxide film 23 on the surface are sequentially removed. Thereafter, the resist in the inside of the trenches 24 and 24 are removed, and the resulting condition is shown in Fig. 7. Next, as shown in Fig. 8, a silicon dioxide film 27 is formed so that an effective oxide film thickness of 15 nm is attained which functions as a capacitor insulating film. On this silicon dioxide film 27, a polycrystalline silicon film 28 containing phosphorus and intended to constitute the other electrode of a capacitor is formed to a thickness of 500 nm, and the surface is flattened by filling the trenches each having a width of 1 µm. Next, as shown in Fig. 9, the polycrystalline silicon film 28 on the surface is selectively removed, and an electrode pattern for the capacitor is formed. Following this, a gate oxide film 29 of an access MOS transistor is formed, and further, a layer insulating film 30 for insulating the capacitor electrode from a word line is formed. Then, after a gate electrode 31 made of tungsten or aluminum and the word line connected to the gate electrode 31 are formed, N-type diffusion regions 33 and 34 intended to serve as a source and a drain region are formed. Next, as shown in Fig. 10, a layer insulating film 35 for insulating the word line 32 from a bit line is formed, and herinafter, an opening for forming an electrode is formed in the N-type diffusion region 33, and aluminum wiring is selectively formed so that one end of the bit line 36 is connected to this opening. Lastly, a memory cell of a DRAM is completed by forming a protective coat (not shown). In this respect, the N-type diffusion layer 26 formed on the substrate side of the inner wall of each trench 24 shown in the embodiment may be omitted, and in this case, although the capacitor capacity decreases by about 10%, the effect of reduction of the memory cell area is not changed. The memory cell of the DRAM formed by the MOS capacitor and the manufacturing method thereof in accordance with the present invention provides a capacity of 0.06 pF although the element isolation width and the trench width of the capacitor are reduced to 1 µm. Moreover, even when the element isolation trench width is made 1 µm, the substrate resistivity is about 4 - 5 Ω cm, and the dielectric strength between elements of 20 V can also be obtained. Further, when this manufacturing method is used for a DRAM, since an insulating film is formed on a polycrystalline silicon film after the polycrystalline silicon film is formed on the inner wall of a trench for forming a capacitor, the capacitor insulating film for the DRAM is not influenced by a defect present in the silicon substrate due to work strain or the like. Thus, a DRAM having high dielectric strength, and still having an improved time characteristic (pause time) since the charges stored in the capacitor naturally disappear due to leakage, can be obtained.

Claims: An MOS capacitor comprising a semiconductor substrate (16) having a principal surface and a pair of spaced trenches (24), an isolation region (22) embedded in said semiconductor substrate between said pair of trenches, first polycrustalline silicon films (25) formed in each of said trenches and serving as a first electrode of said MOS capacitor, insulating films (27) formed on said first polycrystalline films and constituting dielectric layers of said capacitor and a second polycrystalline film (28) formed on said insulating film and serving as a second electrode of the MOS capacitor, characterised in that each of said trenches (24) are defined by side walls, having first and second parts, and a bottom wall, the first parts of the side walls and the bottom of the trenches being inner surfaces of the substrate, said isolation region (22) has a pair of side walls which extend in a direction substantially perpendicular to the principal surface of said substrate and define said second parts of the side walls of the trenches, said isolation region has a depth greater than the bottom of said trenches and said first polycrystalline silicon films (25) are formed directly on said first parts, said second parts and said bottom of said trenches, whereby said first polycrystalline silicon films are disposed entirely between the bottoms of said trenches and said insulating films. An MOS capacitor according to claim 1, wherein said first polycrystalline silicon films (25) have a conductivity type opposite to that of said semiconductor substrate (16). An MOS capacitor according to claim 1 or claim 2, wherein said second polycrystalline film (28) formed on the insulating film (27) within at least one of said trenches is in the form of a column. An MOS capacitor according to any preceding claim, wherein said entire isolation region (22) is formed of silicon dioxide. An MOS capacitor according to claim 1, wherein said isolation region (22) comprises: an inner core portion of polycrystalline silicon; and an outer portion of silicon dioxide, said outer portion surrounding said inner core portion. A method for manufacturing a MOS capacitor according to any of the preceding claims comprising the steps of: forming a first trench (19) in a main surface of a semiconductor substrate (16) of one conductivity type, and after covering a wall of said first trench with a first insulating film, filling the inside of said first trench with one of an insulator (21) and a semiconductor material thereby to form an element isolation region (22) with the substrate surface flattened; forming at least second and third trenches (24) so that said second and third trenches respectively adjoin both sides of said element isolation region and oppose each other sandwiching said element isolation region therebetween and so that the depth of said second and third trenches is smaller than that of said first trench; forming a first polycrystalline silicon film (25) of an opposite conductivity type to that of said semiconductor substrate on the whole of inner walls of said second and third trenches (24); forming a second insulating film (27) on said first polycrystalline silicon film; and forming a second polycrystalline silicon film (28) respectively on said second insulating film to fill said second and third trenches; thereby to form a capacitor having a dielectric layer constituted by said second insulating film (27) and having electrodes constituted by first polycrystalline silicon film (25) and said second polycrystalline silicon film (28). A method according to claim 6, wherein diffusion regions (26) of an opposite conductivity type to that of said semiconductor substrate are respectively formed in said semiconductor substrate at the inner sides of said second and third trenches.

Inventors: UEDA, SEIJI
Applicants: MATSUSHITA ELECTRONICS CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L29