Method and device for dissolving surface layer of semiconductor substrate

Abstract: Disclosed is a method of dissolving a surface of a semiconductor substrate or a thin-film surface layer formed on the semiconductor substrate, with an oxidizing agent and fluorine-series gas. The method is characterized in that an initial dissolution rate is controlled by gradually increasing a concentration of fluorine-series gas introduced in a dissolving solution containing the oxidizing agent.

Description: The present invention relates to a method and a device for uniformly and slightly dissolving a surface of a semiconductor substrate or a thin-film surface layer, such as a polysilicon film, formed on a semiconductor substrate, (hereinafter collectively referred to as "a semiconductor substrate"). The present invention is effective in analyzing the impurities contained in the surface of a semiconductor substrate and in cleaning the surface of a semiconductor substrate by removing the impurities. If metallic impurities, such as Na, K, and Fe, are present in the semiconductor substrate, even in a small quantities, they have adverse effects on electrical characteristics of semiconductor elements, as is known in the art. As a semiconductor device is developed toward a highly-dense and highly-integrated one, the presence of the impurities extremely near the surface of the semiconductor substrate becomes a significant problem. To improve the electrical characteristics of semiconductor elements, it is necessary to prevent contamination with impurities or to remove impurities from the surface of the semiconductor substrate. To achieve this, it is important to know accurate amounts of the impurities contained in the semiconductor substrate and to determine a vertical distribution profile of the impurities near the surface of the semiconductor substrate. Hitherto, to dissolve the surface layer of the semiconductor substrate, a device shown in FIGS. 3 and 4 has been used. In such a conventionally-used device, a semiconductor substrate 1 is mounted on a fluoroplastic holding stage 2. On the semiconductor substrate 1, a fluoroplastic frame 3 is mounted and fixed watertight by means of screws at the four corners, thereby forming a hole surrounded by the frame 3. Into the hole, a dissolving solution A, which is a mixed solution 5 of hydrofluoric acid and nitric acid, is supplied in a volume needed for dissolving a predetermined amount (a predetermined depth) of the surface layer of the semiconductor substrate. Subsequently, the surface layer is allowed to stand for a predetermined time period while being exposed to the dissolving solution. In this manner, a predetermined region in the surface of the semiconductor substrate is dissolved selectively to a predetermined depth. The solution dissolving impurities thus obtained is subjected to a flame-less atomic absorption device to measure the amount of the impurities. In the dissolving solution consisting of nitric acid, which is responsible for oxidization, and hydrofluoric acid, when the concentration of hydrofluoric acid is high, a dissolution rate of the surface layer is controlled by nitric acid. In contrast, when the concentration of nitric acid is high, hydrofluoric acid determines the dissolution rate. To be more specific, when nitric acid is diluted with acetic acid or water, nitric acid is activated. Therefore, the oxidation reaction of nitric acid controls the dissolution rate. If no diluent is added, the dissolution is controlled only by diffusion of hydrofluoric acid. In short, in the case where the concentration of nitric acid is high, the dissolution rate of the surface of the semiconductor substrate is preferentially regulated by the diffusion of hydrofluoric acid contained in a low amount. The crystal orientation and electrical conductivity of the semiconductor substrate do not influence the dissolution rate of the surface of the semiconductor substrate. Hence, it is important to stir the dissolving solution. However, there are the following problems with the conventional methods. First, since the dissolving solution is directly supplied to the surface of the semiconductor substrate present at the bottom of the hole surrounded by the frame in a volume enough to dissolve a predetermined amount (predetermined depth) of the semiconductor substrate, the semiconductor substrate exposed first to the dissolving solution initiates the reaction with the dissolving solution, immediately upon exposure, as indicated by curve a shown in FIG. 2. The portion first exposed to the dissolving solution is therefore dissolved much (dissolved deeper) than the portion finally exposed to the dissolving solution. As a result, the surface of the wafer is dissolved not uniformly in depth, forming a rough surface. Second, since the dissolving solution is supplied to the hole in as a low volume as possible, approximately 3 seconds is required to spread the dissolving solution over the entire surface of a semiconductor substrate if a 150 mm (6-inch) semiconductor substrate is used. As a result, if the dissolving solution is supplied at the same flow rate, longer time is required for spreading the solution over the entire surface of the semiconductor substrate as the diameter of the substrate increases. Third, if a diluent, such as acetic acid, is added to the dissolving solution, the dissolving solution is contaminated with impurities ascribed to the diluent, with the result that the impurity concentration of the background is undesirably increased. Fourth, when nitric acid and hydrofluoric acid is mixed uniformly and sufficiently with stirring manually or mechanically, there is a problem in that the dissolving solution is easily splashed out of the hole, even if the dissolving solution is used in as a low volume as possible. In addition, when a stirrer is used as mechanical stirring means, the dissolving solution is possibly contaminated with impurities due to the stirrer. Consequently, the background is contaminated with impurities. The background impurity contamination occurs also when the entire surface of the semiconductor substrate is dissolved, since the dissolving solution is kept by means of a holding tool consisting of a frame, holding stage and screws. Accordingly, the contamination ascribed to the holding tool is caused since it is exposed to the dissolving solution all the time. The present invention has been made with the view toward solving the aforementioned problems. An object of the present invention is to provide a method and a device capable of dissolving the surface of the semiconductor substrate or a thin-film surface layer, such as a polysilicon film, formed on a semiconductor substrate, slightly, uniformly and accurately. According to the present invention, there is provided a method of dissolving a surface of a semiconductor substrate or a thin-film surface layer formed on the semiconductor substrate by use of a fluorine-series gas (hereinafter, gas includes vapor) selected from hydrofluoric acid and ammoniun fluoride and an oxidizing agent. In this method, an initial dissolution rate is controlled by gradually increasing a concentration of the fluorine-series gas in a dissolving solution containing an oxidizing agent. The surface of the semiconductor substrate or the thin-film surface layer formed on the semiconductor substrate is placed in a dissolving solution containing an oxidizing agent, and exposed to the fluorine-series gas to gradually increase the concentration of the fluorine-series gas to be dispersed in the dissolving solution containing an oxidizing agent, thereby controlling the initial dissolution rate. In the aforementioned method, a dissolving solution containing an oxidizing agent solution is placed on part of the surface of the semiconductor substrate or the thin-film surface layer formed on the semiconductor substrate, and exposed to the fluorine-series gas to gradually increase the concentration of the fluorine-series gas to be dispersed in the dissolving solution containing an oxidizing agent solution, thereby controlling an initial dissolution rate of the part. In the aforementioned method, a dissolving solution containing an oxidizing agent solution is supplied to part or an entire surface of the semiconductor substrate or the thin-film surface layer within a frame immobilized on the surface of the semiconductor substrate or the thin-film surface layer formed on the semiconductor substrate, and exposed to the fluorine-series gas to gradually increase the concentration of the fluorine-series gas to be dispersed in the dissolving solution containing an oxidizing agent solution, thereby controlling an initial dissolution rate of the part or the entire surface of the semiconductor substrate or the thin-film surface layer within the frame immobilized. As the oxidizing agent, use may be made of nitric acid, a nitric acid solution containing a diluent selected from the group consisting of acetic acid, water, hydrogen, and bromine; hydrogen peroxide; and ammonium fluoride. As the surface of the semiconductor substrate or the thin-film surface layer formed on a semiconductor substrate to be dissolved, use may be made of a single-crystalline silicon, polycrystalline silicon, amorphous silicon; a single crystalline silicon, polycrystalline silicon, and amorphous silicon, each containing any one of the group consisting of bromine, phosphorus, and arsenic; and a metal-tungsten silicon (WSi) alloy, a metal-molybdenum silicon (MoSi) alloy, a metal-aluminium silicon copper (AlSiCu) alloy, and a metal-titanium silicon (TiSi) alloy. The initial dissolution rate is larger than 0.02 nm/min and lower than 2 nm/min. Furthermore, there is provided a device for use in dissolving a surface of a semiconductor substrate or a thin-film surface layer formed on a semiconductor substrate, comprising: a holding stage for mounting a semiconductor substrate thereon, a frame having a discharge port, to be provided on the semiconductor substrate, a rack, and an airtight chamber for housing the rack 7, in which the holding stage and the frame are immobilized with screws to form a hole surrounded by the frame; the rack houses the semiconductor substrate formed on the holding stage having a hole filled with nitric acid; and the airtight chamber has a groove containing a hydrofluoric acid solution at the bottom. The present invention thus constituted has the following advantages. First, a surface layer of a semiconductor substrate or a thin-film surface layer, such as a polysilicon film, formed on a semiconductor substrate can be dissolved uniformly to the depth of several nm (tens of angstroms) with an accuracy of ± 3%, by use of a nitric acid solution to which a fluorine-series gas is uniformly diffused. Second, the contamination ascribed to environment can be prevented since the semiconductor substrate is dissolved in an airtight chamber. The dissolving solution of nitric acid and hydrofluoric acid (contained in a low amount) can be stirred without splash since the stirring is performed by use of acoustic waves, Since the dissolving solution is stirred by acoustic waves transmitted to the semiconductor substrate from the rear side thereof, without using any stirring means exposed to the dissolving solution, the impurity contamination due to stirring means in the background of the dissolving solution, can be reduced. Furthermore, in the case where the semiconductor substrate is partially dissolved, since the dissolving solution can be retained at a predetermined region by acoustic waves but not by a retaining member, contamination ascribed to the retaining member is inhibited. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIG. 1A is a schematic cross sectional view of a device according to Embodiment 1 of the present invention; FIG. 1B is a plan view of the construct formed of a semiconductor substrate and a frame immobilized on the substrate with screws; FIG. 2 is a graph showing the relationship between the dissolving amount per time by comparing a method according to Embodiment 1 of the present invention and a conventional method; FIG. 3 is a plan view of a conventional device; and FIG. 4 is a schematic cross-sectional view of a conventional device; Hereinbelow, the preferred embodiment will be explained with reference to the accompanying drawings. (Embodiment 1) The device according to Embodiment 1 of the present invention is shown in FIGS. 1A and 2. FIG. 1A is a schematic cross sectional view of a device according to Embodiment 1 of the present invention. FIG. 2 is a graph showing the relationship between the dissolving amount per time by comparing a method according to Embodiment 1 of the present invention and a conventional method. In Embodiment 1, explained is a method of dissolving 125 mm (5-inch) diameter portion of a 150 mm (6-inch) diameter semiconductor substrate to several nm (tens of angstroms) in depth. In the first place, a semiconductor substrate 1 is mounted on a fluoroplastic holding stage 2. On the semiconductor substrate 1, a fluoroplastic frame 3 is mounted and immobilized watertight by means of screws 4 at the four corners to form a hole surrounded by the frame 3. In the hole, a nitric acid solution 6 (68%) serving as an oxidizing agent is supplied in as a low volume as possible (5-10 ml). Incidentally, if an 200 mm (8-inch) diameter semiconductor substrate is used, 10-15 ml of the nitric acid solution should be supplied. The resultant construct is placed in a rack 7. The rack 7 with the construct is housed in an airtight chamber 8 containing hydrofluoric acid 10 in a groove provided at the bottom. After closed airtight, the airtight chamber 8 is allowed to stand for about 3-5 minutes. During this period, a fluorine-series gas is diffused within the nitric acid solution in a concentration of several tens of ppm and then the surface of the semiconductor substrate 1 is dissolved in a thickness of several nm (tens of angstroms). Thereafter, the rack 7 with the construct is taken out of the airtight chamber 8, in its entirety. Subsequently, the construct shown in FIG. 1B is taken out. After the dissolving solution 12 is discharged by way of a discharging port, the semiconductor substrate 1 with the frame is washed with water. In the method of the present invention, first, the nitric acid solution is supplied dropwise to the hole in a minimum volume required for being spread slightly over the entire surface of the semiconductor substrate at the bottom of the hole. Subsequently, the nitric acid solution spread over the substrate is exposed to a fluorine-series gas. Consequently, the concentration of fluorine-series gas is gradually and increasingly diffused into the nitric acid solution, thereby controlling the initial dissolution rate of the semiconductor substrate as indicated by a curve b shown in FIG. 2. In this way, the surface of the semiconductor substrate can be uniformly and slightly dissolved. Therefore, the present invention made it possible to dissolve the semiconductor surface uniformly and slightly, thereby attaining the dissolution accurately. As the fluorine-series gas used herein, hydrofluoric acid or ammonium fluoride is used. As the oxidizing agent, use may be made of nitric acid; a nitric acid solution containing acetic acid, water, hydrogen, or bromine, as a diluent; hydrogen peroxide; or ammonium fluoride. The oxidizing agent is prepared in advance by mixing the nitric acid solution (68%) and the diluent in the dilution rate of 5:1 to 5:3 by volume. To be more specific, when 5-10 ml of the nitric acid solution is used, a diluent, for example, acetic acid, may be added in a volume of 1 ml to 6 ml in accordance with the aforementioned dilution rate. The surface layers to be dissolved by the oxidizing agent in combination with a fluorine-series gas include a single crystalline silicon, polycrystalline silicon, and amorphous silicon; single crystalline silicon, polycrystalline silicon, and amorphous silicon containing any one of boron, phosphorus, and arsenic; alloys such as a metal-tungsten silicon (WSi) alloy, a metal-molybdenum silicon (MoSi) alloy, a metal-aluminium silicon copper (AlSiCu) alloy, and a metal-titanium silicon (TiSi) alloy. In this method, the initial dissolution rate should be a value larger than 0.02 nm/min and smaller than 2 nm/min. The amount of impurities is determined by collecting the dissolving solution 12 in a previously-washed fluoroplastic beaker 13 and then analyzing by a flame-less atomic absorption device directly or after appropriately concentrated to a desired volume. After the semiconductor substrate is washed with water and dried, the aforementioned procedure is repeated for predetermined times, starting from the supply of a nitric acid solution 6 (68%) to the hole. In this way, the vertical impurity distribution profile of the semiconductor substrate can be determined. Nitric acid and acetic acid used herein must have an impurity concentration of 10 ppt or less. The hydrofluoric acid solution to be used herein must have an impurity concentration of 10 ppb or less. As the tool for immobilizing the semiconductor substrate such as a frame, holding stage, and screws, use may be made of fluoroplastic such as poly-tetra fluoroethylene. The same method can be used for dissolving a thin film surface layer, such as a polysilicon film, formed on the semiconductor substrate.

Claims: A method of dissolving a surface of a semiconductor substrate or a thin-film surface layer formed on the semiconductor substrate by use of a fluorine-series gas selected from hydrofluoric acid and ammonium fluoride and an oxidizing agent, wherein an initial dissolution rate is controlled by gradually increasing a concentration of said fluorine-series gas in a dissolving solution containing an oxidizing agent. The method according to claim 1, characterized in that said surface of a semiconductor substrate or said thin-film surface layer formed on the semiconductor substrate is placed in a dissolving solution containing an oxidizing agent, and exposed to said fluorine-series gas to gradually increase a concentration of the fluorine-series gas to be dispersed in said dissolving solution containing an oxidizing agent, thereby controlling said initial dissolution rate. The method according to claim 1, characterized in that a dissolving solution containing an oxidizing agent is placed on part of said surface of a semiconductor substrate or a thin-film surface layer formed on the semiconductor substrate and exposed to said fluorine series gas to gradually increase a concentration of said fluorine-series gas to be dispersed in said dissolving solution containing an oxidizing agent, thereby controlling an initial dissolution rate of said part. The method according to claim 1, characterized in that a dissolving solution containing an oxidizing agent is supplied to part or an entire surface of the semiconductor substrate or the thin-film surface layer within a frame immobilized on said surface of the semiconductor substrate or said thin-film surface layer formed on the semiconductor substrate, and then exposed to said fluorine-series gas to gradually increase a concentration of said fluorine-series gas to be dispersed in said dissolving solution containing an oxidizing agent, thereby controlling an initial dissolution rate of said part or said entire surface of a semiconductor substrate or said thin-film surface layer within said frame immobilized. The method according to claims 1 to 4, characterized in that said oxidizing agent is selected from the group consisting of nitric acid; a nitric acid solution containing a diluent selected from the group consisting of acetic acid, water, hydrogen, and bromine; hydrogen peroxide; and ammonium fluoride. The method according to any one of claims 1 to 5, characterized in that said surface of a semiconductor substrate or said thin-film surface layer formed on the semiconductor substrate is one selected from the group consisting of a single-crystalline silicon, polycrystalline silicon, amorphous silicon; a single crystalline silicon, polycrystalline silicon, and amorphous silicon, each containing any one of the group consisting of bromine, phosphorus, and arsenic; and a metal-tungsten silicon (WSi) alloy, a metal-molybdenum silicon (MoSi) alloy, a metal-aluminium silicon copper (AlSiCu) alloy, and a metal-titanium silicon (TiSi) alloy. The method according to claims 1 to 6, characterized in that said initial dissolution rate is larger than 0.02 nm/min and lower than 2 nm/min. A device for use in dissolving a surface of a semiconductor substrate or a thin-film surface layer formed on the semiconductor substrate, comprising a holding stage (2) on which a semiconductor substrate (1) is mounted, a frame (3) having a discharge port (11) for mounting on said semiconductor substrate (1) a rack (7), an airtight chamber (8) for housing said rack 7 wherein said holding stage (2) and said frame (3) are immobilized watertight by means of screws (4) at four corners to form a hole surrounded by said frame; said rack houses said semiconductor substrate formed on the holding stage and having a hole in which nitric acid is contained in a minimum volume required for covering said surface of said semiconductor substrate; and said airtight chamber (8) has a groove (9) containing hydrofluoric acid (10) at the bottom.

Inventors: ISHIZAKI ITSURO, KANEKO MINAKO, SHIMAZAKI AYAKO, ISHIZAKI, ITSURO, KANEKO, MINAKO, SHIMAZAKI, AYAKO
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1996
Main classification: H01L21
IPC codes: H01L21, G01N1