Method of manufacturing a vertical MOS semiconductor device

Abstract: A groove (4) is formed on a semiconductor substrate (1). A mask material layer (3) is so formed on the surface of the semiconductor substrate (1) as to open a groove (4) region. With the mask material layer (3) used as a mask, a semiconductor layer (10) is selectively formed on the semiconductor substrate (1) exposed with the inner wall surface of the groove (4). Then, the mask material layer (3) is removed. An insulating film (5) is formed on the semiconductor layer (10) formed on the inner wall surface of the groove (4) and the surface of the semiconductor substrate (1). The groove (4) is buried with a conductor (6).

Description: This invention relates to a method of manufacturing a MOS type semiconductor device of a vertical structure having an MOS type transistor on an inner wall surface of a groove formed on a semiconductor substrate. In a power insulated gate type field effect transistor (hereinafter referred to as a "power MOSFET"), attempts to reduce the on-resistance by shortening the channel length have heretobefore been conducted. Recently, the development of a MOSFET of a vertical structure has been advanced to further reduce the area of a cell. FIG. 3 shows a section of a MOSFET of a vertical structure. A p type layer 2 is, for example, formed on a surface of an n type semiconductor substrate 1. A groove 4 is formed on the n type semiconductor substrate 1, and a gate electrode 6 is formed on the inner wall surface of the groove 4 through a gate insulating film 5. An n type diffusion layer 7 is formed adjacent to the groove 4 on the surface of the p type layer 2. The potential of the p type layer 2 between the n type semiconductor substrate 1 and the n type diffusion layer 7 is controlled by the gate electrode 6. Such a MOSFET structure is vertically formed on the side face of the groove 4. The channel of the MOSFET is formed near a boundary between the gate insulating film 5 formed on the side face of the groove 4 and the p type layer 2. A conventional method of manufacturing a vertical MOSFET will be described with reference to FIG. 4A to FIG. 4F. FIG. 4A to FIG. 4F are sectional views showing the conventional method of manufacturing the vertical MOSFET. A p type layer 2 is formed on the surface of an n type semiconductor substrate 1. A mask material 3 of, for example, an oxide film is formed on the p type layer of the n type semiconductor substrate 1. FIG. 4A shows a section of a semiconductor device at this stage. The mask material 3 is used as an etching mask at the time of forming a groove on the semiconductor substrate 1. Next, the mask material 3 is etched by using ordinary lithography technology and etching technology. Further, with the mask material 3 used as a mask, the semiconductor substrate 1 is etched to form a groove 4. FIG. 4B shows a section of the semiconductor device at this stage. At this time, the groove 4 is necessarily arrived at the n type semiconductor substrate 1 through the p type layer 2 as shown in FIG. 4B. Thereafter, the mask material 3 is slightly etched by, for example, hydrofluoric acid, the mask material 3 is retracted from the end of the groove 4, and the exposed surface of the semiconductor substrate 1 is slightly etched by using, for example, surface active material and alkaline treating solution. FIG. 4C shows a section of the semiconductor device at this stage. A main object of this etching is to remove contaminant substance adhered to the inner wall surface of the groove 4 at the time of forming the groove 4 and to round corners 11 of the upper portion of the groove 4. For example, the conditions of etching by the above-mentioned alkaline treating solution are so set that the etching speed becomes different according to the orientation of the semiconductor substrate and particularly only the corners 11 of the groove 4 are earlier etched than the other inner wall surface of the groove 4, and the corners 11 of the groove 4 are rounded. Then, the mask material 3 is etched and removed. FIG. 4D shows a section of the semiconductor device at this stage. Further, a gate insulating film 5 of a thickness such as, for example, about 50 nm is formed on the inner wall surface of the groove 4 and the surface of the p type layer 2 by, for example, thermal oxidation. FIG. 4E shows a section of the semiconductor device at this stage. Subsequently, an electrode material 6 of, for example, polycrystal silicon is buried in the groove 4, and an n type diffusion layer 7 is formed adjacent to the groove 4 on the surface of the p type layer 2 by using, for example, lithography technology and ion implantation technology. In this manner, a vertical MOSFET structure is completed. FIG.4F shows a section of the semiconductor device at this stage. As described above, according to the conventional method of manufacturing the vertical MOSFET, the corners of the groove 4 are rounded by using the alkaline treating solution. By so doing, the damage of the gate insulating film 5 by an electric field formed in the gate insulating film according to a potential difference applied between the gate electrode 6 and the n type diffusion layer 7 due to the concentration of the electric field at the corners 11 of the groove 4 is prevented. As a result, the withstand voltage of the gate insulating film is improved. However, the alkaline treating solution also anisotropically etched the semiconductor substrate 1 exposed on the inner wall surface of the groove 4. Therefore, the uneven portion generated on the surface of the inner wall of the groove 4 at the time of etching the semiconductor substrate 1 to form the groove 4 is amplified by etching with the alkaline treating solution as shown in FIG. 4C. For example, if the alkaline treatment is so conducted that the radius of curvature of each of the corners 11 becomes 15 nm, the uneven portion becomes about 2 nm. This uneven portion is not removed even after the thermal oxidation for forming the gate insulating film 5 is executed. Since the uneven portion of the boundary between the gate insulating film and the semiconductor substrate when the MOSFET is formed on the surface of the semiconductor substrate 1 is about 0.2 nm, the uneven portion of the vertical MOSFET reaches its ten times as large as the uneven portion of the boundary. Since carrier such as electron moving in the channel is scattered according to such uneven portion of the boundary between the gate insulating film and the semiconductor substrate, the mobility of the carrier is remarkably reduced. FIG. 5 shows the gate voltage dependence of the electron mobility of an n type MOSFET formed on a normal flat surface and the electron mobility of a vertical n type MOSFET. As shown in FIG. 5, the rise of the mobility of the electron is abrupt in the n type MOSFET of a planar structure but the rise of the mobility of the electron is very smooth in the vertical n type MOSFET. In general, a current is proportional to the mobility of the electron. Therefore, it is necessary in the conventional vertical MOSFET in which the rise of the mobility of the electron is smooth to largely vary a gate voltage in order to control the current. That is, since the conventional vertical MOSFET has high on resistance, its high speed operation becomes difficult. Furthermore, in order to prevent the uneven portion generated on the inner wall surface of the groove 4 from being amplified, it is possible not to conduct the alkaline treatment. However, in this case, since the corners 11 of the groove 4 are not rounded, the withstand voltage of the gate insulating film is deteriorated as described above. FIG. 6 shows the dependency of an electric field strength for causing the gate oxide film 5 to dielectric break down with respect to the ratio of the thickness T ox of the oxide film to the radius R of curvature of each of the corners 11 of the groove 4 of the semiconductor substrate 1. The larger the radius R of curvature of the semiconductor substrate 1 to the thickness T ox of the oxide film is, namely, the more the corners of the groove 4 are rounded, the greater the withstand voltage is improved. The withstand voltage of the oxide film formed on the flat surface is generally about 8 MV/cm. As described above, in order to improve the withstand voltage of the gate insulating film 5 in the conventional vertical MOSFET, it is necessary to round the corners 11 of the groove 4. However, there has been a problem that the uneven portion of the inner wall surface of the groove 4 is amplified by this treatment and the mobility of the carrier at the channel section of the MOSFET is reduced. Patent Abstracts of Japan vol. 012, no. 342 (E-658), 14 September 1988 & JP 63 102351 A (Matsushita Electronics Corp.), 7 May 1988 describes a method to form a trench capacitor wherein a trench is provided in a p-type Si substrate using RIE. A p-type epitaxial layer 2 doped with boron is thereafter formed by selective epitaxy. A n-type diffusion layer 3 is formed and an oxide film is grown by thermal diffusion thereon. Document D1 explicitly teaches to first provide a trench and then to provide an epitaxial layer using selective epitaxy. Patent Abstracts of Japan vol. 014, no. 288 (E-0943), 21 June 1990 & JP 02 091976 A (Oki Electric Ind. Co. Ltd.), 30 March 1990 describes a manufacturing method of a vertical and groove type MOSFET. In the sequence of processing steps a groove is formed in a semiconductor substrate covered by an oxide layer 24. Thereafter the layer 24 is removed and thereafter a p-type epitaxial layer 26 is formed by covering the entire surface including the groove by the epitaxial layer. In. a next processing step parts of the epitaxial layer not in the groove are removed and a gate oxide film is formed on the thus provided surface. It is an object of the present invention to provide a method of manufacturing a vertical MOSFET semiconductor device having high withstand voltage of a gate insulating film and large mobility of carrier at a channel section. The object of the present invention is addressed by the following method. According to the invention, there is provided a method of manufacturing a semiconductor device comprising the steps of: (a) forming a mask having an opening on the surface of a silicon semiconductor substrate; (b) forming a groove in said silicon semiconductor substrate by etching and using said mask; (c) selectively epitaxially growing a silicon semiconductor layer on said semiconductor substrate exposed at the surface of said groove, such that the ends of the silicon semiconductor layer adjacent to the opening become a tapered shape; (d) removing said mask after step to expose the remaining surface of said silicon semiconductor substrate; (e) thermally oxidizing said silicon semiconductor layer and said remaining surface to form an oxide film and to round the edge of said groove; and (f) burying a conductor at least on said oxide film in said groove. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIGS. 1A to 1F are sectional views showing a method of manufacturing a semiconductor device according to a first embodiment of the invention; FIGS. 2A and 2B are sectional views showing a method of manufacturing a semiconductor device according to a second embodiment of the invention; FIG. 3 is a sectional view showing a conventional semiconductor device; FIGS. 4A to 4F are sectional views showing a conventional method of manufacturing a semiconductor device; FIG. 5 is a view showing a problem of a conventional semiconductor device; and FIG. 6 is a view showing a problem of the conventional semiconductor device. Embodiments of the present invention will be explained with reference to the accompanying drawings. FIGS. 1A to 1F are sectional views showing a method of manufacturing a semiconductor device according to a first embodiment of the present invention. A p type layer 2 is, for example, formed on the surface of, for example, an n type silicon substrate 1 by using, for example, epitaxial growth technology or ion implantation technology similarly to the conventional examples. A mask material 3 of, for example, an oxide film is formed on the n type semiconductor substrate 1. FIG. 1A shows a section of a semiconductor device at this stage. Next, the mask material 3 is etched by using ordinary lithography technology and etching technology. Further, with the mask material 3 used as a mask, the semiconductor substrate 1 is so etched as to arrive at the n type semiconductor substrate 1 through the p type layer 2 to form a groove 4. FIG. 4B shows a section of the semiconductor device at this stage. Thereafter, a natural oxide film (not shown) formed on the inner wall surface of the groove 4 and a foreign matter such as an oxide film deposited on the inner wall surface of the groove 4 in the etching step for forming the groove 4 are removed by dipping or immersing the semiconductor device in, for example, 1% concentration of hydrofluoric acid different from the conventional example. Subsequently, with the oxide film 3 used as a mask, a semiconductor layer 10 of, for example, silicon is selectively grown on the semiconductor substrate exposed with the inner wall surface of the groove 4 by epitaxial growth technology using raw material such as, for example, SiH 2 Cl 2 and HCl. According to the epitaxial growth technology, silicon is grown on silicon, and nothing is grown on the oxide film. FIG. 1C shows a section of the semiconductor device at this stage. Next, the mask material 3 is removed by using, for example, hydrofluoric acid. FIG. 1D shows a section of the semiconductor device at this stage. Thereafter, a gate oxide film 5 of, for example, 50 nm is formed on the surface of the p type layer 2 and the semiconductor layer 10 by, for example, thermal oxidation. At this time, it is desirable that all the semiconductor layers 10 formed by the selective epitaxial grown are oxidized. FIG. 1E shows a section of the semiconductor device at this stage. Then, a gate electrode material 6 such as, for example, polycrystal silicon is buried in the groove 4 similarly to the conventional example. An n type diffusion layer 7 is so formed in the p type layer 2 as to be disposed adjacent to the groove 4 by, for example, lithography method and ion implantation technology to complete a vertical MOSFET. FIG. 1F shows a section of the semiconductor device at this stage. In this embodiment, after the groove 4 is formed, with the mask material 3 for forming the groove 4 used as a mask, the semiconductor layer 10 such as silicon is selectively grown only on the inner wall surface of the groove 4. In this epitaxial growth, the semiconductor layer 10 is grown in a direction that the end of the semiconductor layer 10 is separated from the mask material 3, namely, toward the bottom of the groove 4. Therefore, as shown in FIG. 1C, the end 12 of the semiconductor layer 10 becomes a tapered shape to obtain the similar effect to the effective rounding of the corners of the groove 4. More specifically, the corners of the groove 4 are rounded by utilizing the tapered shape 12 of the semiconductor layer 10 formed by the selective epitaxial growth to suppress the concentration of the electric field to be applied to the gate insulating film 5 without conventional alkaline treatment, thereby improving the withstand voltage of the gate insulating film 5. Since it is not necessary to conduct the alkaline treatment that has heretofore been conducted to round the corners of the groove 4 in the embodiment of the present invention, the uneven portion of the inner wall surface of the groove 4 by the alkaline treatment is no longer amplified. Furthermore, the uneven portion of the surface of the semiconductor layer 10 becomes smaller than the uneven portion generated at the inner wall surface of the groove 4 by the etching for forming the groove 4. Since the uneven portion of the inner wall surface of the groove 4 is reduced, the uneven portion of the sidewall face of the groove 4 used as the channel section of the vertical MOSFET is reduced, and the surface scattering of the carrier in the channel section is suppressed. As a result, the mobility of the carrier can be increased. In this embodiment as described above, a vertical MOSFET of high performance, having high withstand voltage of the gate insulating film and large mobility of the carrier in the channel section, can be realized. Next, a second embodiment of the present invention will be explained with reference to FIG. 2A and FIG. 2B. In this second embodiment, the uneven portion of a boundary between a gate insulating film 5 and a p type layer2 is further flattened. Up to the steps of forming a groove on a semiconductor substrate 1 and forming a semiconductor layer 10 by using selective epitaxial growth technology, the process is similar to that of the first embodiment. Thereafter, in this embodiment, before a mask material 3 is removed, the semiconductor device is heat treated in a hydrogen atmosphere at, for example 800°C different from the first embodiment. FIG. 2A shows a section of the semiconductor device at this stage. Thereafter, similarly to the first embodiment, the mask material 3 is removed, and a gate oxide film 5 is formed. Further, a gate electrode material 6 of, for example, polycrystal silicon is buried in the groove 4, an n type diffusion layer 7 is so formed as to be disposed adjacent to the groove 4 to complete a vertical MOSFET. FIG. 2B shows a section of the semiconductor device at this stage. In this second embodiment as described above, there is a feature in that after the semiconductor layer 10 is formed by the selective epitaxial growth technology, the semiconductor device is heat treated. In general, the uneven portion on the surface of the semiconductor layer 10 formed by the selective epitaxial growth technology is alleviated as compared with the uneven portion existing on the surface of the semiconductor substrate before the epitaxial growth is conducted, but the uneven portion is not completely removed. Therefore, in the first embodiment described above, the uneven portion remains on the surface of the epitaxially grown layer 10. However, in this second embodiment, the heat treatment is conducted after the selective epitaxial growth, and hence the surface of the semiconductor layer 10 can be flattened. As compared with the first embodiment, the surface of the channel section is further flattened, and hence the mobility of the carrier of the channel section can be more increased. Furthermore, as shown in FIG. 2A, the corners 13 of the upper portion and the corners 14 of the bottom of the groove 4 can be smoothened by the heat treatment. Not only the facet surface 12 of the epitaxially grown layer 10 is a tapered shape but also the corners 13 and 14 of the groove 4 are rounded, and hence the withstand voltage of the gate insulating film 5 can be further improved as compared with the first embodiment. The heat treatment may be conducted by not limiting to the hydrogen atmosphere but in an inert gas atmosphere of, for example, nitrogen. Even in this case, the same effect can be attained. In the first and second embodiments, all the epitaxially grown layers 10 are oxidized to form the gate oxide film 5. If not all the epitaxially grown layers 10 are oxidized, the unoxidized part of the epitaxially grow layer 10 is used as the channel section of the vertical MOSFET. However, impurities are diffused from the semiconductor substrate 1 or the p type layer 2 to the epitaxially grown layer 10 during the epitaxially growing step or the later heating step. Therefore, there might be the possibility of difficulty in the control of the impurity concentration of the channel section or the control of the channel length. In order that all the epitaxially grown layer 10 are oxidized, it is necessary to reduce the thickness of the semiconductor layer 10 to at least 2/3 or less of the thickness of the gate oxide film 5. For example, when a gate oxide film of, for example, 50 nm, is formed, the thickness of the semiconductor layer 10 is set to 30 nm or less. By so doing, all the semiconductor layers 10 can be oxidized. If the heat treatment is not conducted after the epitaxially growing step like in the first embodiment, the radius of curvature of each of the corners of the groove 4 is determined according to the thickness of the epitaxially grown layer 10 and the taper angle of the facet surface 12. In general, the larger the thickness of the epitaxially grown layer 10 is, the larger the radius of curvature of each of the corners of the groove 4 becomes. For example, in the case that the taper angle is 45 degrees, the radius of curvature of each of the corners of the groove 4 become about the same as the thickness of the epitaxially grown layer 10. When the taper angle becomes larger than 45 degrees, the radius of curvature of each of the corners of the groove 4 becomes larger than the thickness of the epitaxially grown layer 10. Normally, since the taper angle of the faucet surface of the selectively epitaxially grown layer 10 is about 60 degrees, the thickness of the epitaxially grown layer 10 is formed thicker than the desired radius of curvature of each of the corners of the groove 4. As a result, the corner of the groove 4 is so rounded as to have the radius of curvature larger than the desired radius of curvature, and hence the withstand voltage can be assured. In this embodiment as described above, a vertical MOSFET of high performance that the withstand voltage of the gate insulating film is high and the mobility of the carrier of the channel section is large can be realized. In the first and second embodiments of the present invention, there is described that, when the semiconductor layer 10 remains, it is difficult to stably control the concentration of the channel section and the channel length of the MOSFET by the diffusion of the impurities from the semiconductor substrate 1 and the p type layer 2. However, this problem can be avoided by controlling the impurity concentrations of the semiconductor substrate 1 and the p type layer 2 and controlling the impurity control of the epitaxially grown layer 10 at the time of epitaxial growth. For example, the impurity concentration of the epitaxially grown layer 10 is set to lower than that of the n type semiconductor substrate 1 and higher than that of the p type layer 2. In this case, the channel length is determined according to the impurity concentration of the n type semiconductor substrate 1, and the concentration of the channel section is determined according to the impurity concentration of the epitaxially grown layer 10 without influence of the diffusion of the p type layer 2. In the first and second embodiments described above, the concentration of the channel section is determined according to the concentration of the p type layer 2. Since the p type layer 2 is formed at the initial stage of a series of manufacturing steps of the semiconductor device, the impurity concentration of the p type layer 2 is affected by the influence of the step after the p type layer 2 is formed such as, for example, the step of forming the groove 4. When other element is simultaneously formed on the semiconductor substrate formed with the vertical MOSFET, there might be no possibility of freely setting the concentration of the p type layer 2 from the request of the vertical MOSFET. Incidentally as shown in FIG. 6, the larger the radius of curvature of each of the corners of the groove 4 is, the more the withstand voltage of the gate oxide film is improved. It is general that an electric field for breaking down the gate oxide film formed on the surface of the semiconductor substrate is about 8 MV/cm. Therefore, in order to guarantee the withstand . of this degree, it is understood from the graph shown in FIG. 6 that the ratio of the thickness T OX of the. gate oxide film to the radius R of curvature of the corner of the groove 4 may be about 1. In the f irst and second embodiments, the conditions of the thickness of the epitaxially grown layer 10 and the heat treatment after the epitaxial growth are suitably so set that the thickness of the gate insulating film 5 becomes, for example, the same degree as the radius of curvature of the corner of the groove. As a result, the withstand voltage of the gate insulating film of the vertical MOSFET can be set to the same degree as that of the MOSFET of a planar structure. Furthermore, in the first and second embodiments described above, the vertical MOSFET is designated as the example. However, the present invention is not limited to the particular embodiment. For example, the present invention can be applied to all semiconductor devices that have a MOS structure formed with electrodes on the inner wall surface of a groove formed on a semiconductor substrate through an insulating film.

Claims: A method of manufacturing an MOS device comprising the steps of: a) forming a mask (3) having an opening on the surface of a silicon semiconductor substrate (1,2); b) forming a groove (4) in said silicon semiconductor substrate (1,2) by etching and using said mask; c) selectively epitaxially growing a silicon semiconductor layer (10) on said semiconductor substrate (1,2) exposed at the surface of said groove (4), such that, the ends of the silicon semiconductor layer adjacent to the opening become a tapered shape (12); d) removing said mask (3) after step (c) to expose the remaining surface of said silicon semiconductor substrate (1,2); e) thermally oxidizing said silicon semiconductor layer (10) and said remaining surface to form an oxide film (5) and to round the edge of said groove (4); and f) burying a conductor (6) at least on said oxide film (5) in said groove (4). The method according to claim 1, wherein: said silicon semiconductor substrate (1,2) having a first conductivity type region (1) and a second conductivity type layer (2) on said first conductivity type region (1), wherein said second conductivity type is opposite to said first conductivity type ; and a first conductivity type diffusion region (7) is formed in said diffusion layer (2) adjacent to said groove (4). The method according to claims 1 and 2, characterized in that a heat treatment is conducted after the step c) of selectively epitaxially growing the semiconductor layer (10) on said semiconductor substrate and before the step d) of removing of said mask (3). The method according to claim 3, characterized in that inert gas at 800°C or higher is used in said heat treatment. The method according to claim 1, characterized in that the semiconductor layer (10) formed on the inner wall surface of said groove is totally oxidized by said thermal oxidation.

Inventors: BABA YOSHIRO, NARUSE HIROSHI, BABA, YOSHIRO, NARUSE, HIROSHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1997
Main classification: H01L21
IPC codes: H01L29, H01L21