JP2005142445A - Surface treatment method of silicon substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To carry out both cleaning of a silicon (001) surface and flattening in monoatomic order at a temperature lower that a conventionally known temperature. <P>SOLUTION: A silicon substrate 1 where an oxide film 2 is formed in a (001) surface is treated by a hydrogen fluoride water solution comprising a hydroxide ion reducer (hydrochloric acid, for example) and the oxide film 2 is peeled äFig.(c)}. The silicon substrate is subjected to a heating treatment in a gas comprising hydrogen äFig.(d)}. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a processing method for planarizing and cleaning the surface of silicon used as a substrate for various semiconductor integrated circuits, and more specifically, when the oxide film on the surface of the silicon substrate is removed, the surface is cleaned. The present invention relates to a processing method for generating regular atomic steps.

  Currently, most semiconductor elements used in various electronic devices use silicon substrates. Among these semiconductor elements, integrated circuits such as central processing elements, so-called microchips and memory elements, are formed by forming a fine electronic circuit network on the silicon (001) surface. Along with this trend, the demand for planarization and cleaning of the silicon substrate is becoming stricter.

  For example, in the LSI that is mainly used at present, the minimum processing dimension of the electric circuit wiring provided on the silicon surface is about 0.13 μm. If the LSI is miniaturized for further performance enhancement, it is considered that the minimum processing dimension of wiring is required to be about 0.04 μm around 2010. In order to create such a fine network, it is necessary to further improve the flatness on the silicon (001) surface, and the contamination must be suppressed to an extremely low level.

  By the way, in order to clean the silicon surface, an oxide film is first formed on the silicon surface, and this is dissolved and removed with hydrofluoric acid, and at the same time, the bonds of the outermost silicon atoms exposed on the surface are replaced with hydrogen atoms. Termination methods are known. Usually, the silicon surface is covered with an oxide film, and once removed, the newly exposed silicon surface is very active, so it immediately reacts with oxygen in the atmosphere to oxidize the surface and remove contaminants. Adsorption causes surface contamination. As described above, when the outermost silicon atom bond is terminated with a hydrogen atom simultaneously with the removal of the oxide film, the bond between the silicon atom and the hydrogen atom is strong. Even if the surface is inactivated and left in the atmosphere, oxidation and adsorption of pollutants are prevented in a short time.

  However, in the hydrofluoric acid used so far, the silicon (001) surface is eroded by the hydroxide ions present in the solution, and the oxide film is removed during the process of removing the oxide film by the oxygen dissolved therein. Since the silicon surface is oxidized and the portion is further dissolved by hydrofluoric acid, there is a drawback that the flatness of the silicon (001) surface is lowered.

  On the other hand, when the oxide film is dissolved and removed with hydrofluoric acid for cleaning, when the silicon atoms on the silicon (001) surface are terminated with hydrogen atoms, the hydrogen atoms in the dihydride molecules formed on the surface It is inevitable that surface distortion is caused by the repulsive force of the surface or by the interaction between molecules constituting the oxide film when it dissolves, but this surface distortion is alleviated by the unevenness existing on the silicon (001) surface, and the surface energy is reduced It is getting smaller. Therefore, when this surface is flattened and unevenness is removed, distortion accumulates unevenly, and this accumulated part has a larger surface energy than other parts, so the action of hydrogen fluoride, hydroxide ions, and dissolved oxygen It becomes easier to receive, and rather uneven unevenness is generated.

  Therefore, in order to planarize the silicon surface by solution treatment, regular atomic steps are formed by slightly inclining the surface from the correct (00l) orientation so that the repulsive force between dihydride molecules does not accumulate. The present inventors have already proposed a method for reducing the repulsive force at the end of each step by introducing it in advance (see, for example, Patent Document 1).

  However, in this method, only a silicon surface having a limited inclination angle can be flattened. Generally, a silicon surface having a (001) orientation whose surface orientation is almost accurate is generally used. It is not yet known how to perform planarization simultaneously with cleaning throughout.

  On the other hand, as a method for cleaning and cleaning the silicon surface other than solution processing, a method of heating the silicon surface to 900 ° C. or higher in a gas containing hydrogen is known (for example, see Non-Patent Document 1). . In this method, by increasing the temperature, the diffusion of atoms on the silicon surface is promoted and flattened, and at the same time, the bonds of the outermost silicon atoms are terminated with hydrogen atoms. In this case, unlike the planarization of the silicon (001) surface using solution processing, the surface having any crystal orientation can be cleaned and planarized.

However, it is difficult to directly incorporate such a technique for heating and cleaning the silicon surface at a high temperature of 900 ° C. or higher directly into the manufacture of currently used LSIs. In a normal LSI, it is necessary to make many fine elements in the same chip, and in order to prevent current interference between elements, a thick silicon oxide for element isolation or the like is used between elements. An insulating film is built in. At a high temperature of 900 ° C. or higher, the insulating film for element isolation is dissolved, and the insulation between elements may not be maintained. For this reason, from the viewpoint of use in actual LSI fabrication, a method for performing planarization simultaneously with cleaning over the entire silicon surface having the (001) orientation has not been known yet.
Japanese Patent No. 2659088 J. Vac. Sci. Technol. A16 (3), 1998, pp.1775-1778

  The present invention has been made to solve the above-mentioned problems of the prior art, and the object of the present invention is to first prevent flattening only in a specific inclination angle direction. In other words, it is possible to provide a flattening method that does not depend on the inclination angle. Second, a cleaning and flattening method that does not remove an existing oxide film such as an element isolation film is provided. It is to be able to provide.

To achieve the above object, according to the present invention, there is provided a silicon substrate surface treatment method for planarizing and cleaning a silicon surface,
(1) A silicon substrate having a silicon crystal having a (001) plane with an inclination angle of 4 ° or less as a main surface is treated with a hydrogen fluoride aqueous solution containing a hydroxide ion reducing agent to be formed on the surface of the silicon substrate. A step of dissolving and removing the oxide film,
(2) heating the silicon substrate in gas;
A method for surface treatment of a silicon substrate is provided.
Preferably, a step of forming an oxide film on the surface of the silicon substrate is added prior to the first step (1). Preferably, hydrogen is contained in the gas in the step (2). More preferably, the heating temperature in the step (2) is 900 ° C. or lower.

  According to the present invention, a silicon substrate that is normally available is used without giving a specific tilt angle from the (001) orientation, an easily available processing solution is used, and an available gas is used to simply specify. By combining the removal of the oxide film under the above conditions and the heat treatment in gas, the silicon (001) surface can be easily cleaned and planarized in the single atom order without using a special apparatus. be able to. And, since a surface with a considerably high flatness is obtained by immersion in a hydrogen fluoride aqueous solution containing a hydroxide ion reducing agent, it is said that silicon atoms can be sufficiently diffused at 900 ° C. or higher. Heating is not necessary, and a high degree of flatness can be obtained by slight diffusion at a lower processing temperature. Therefore, since the heating temperature lower than the melting temperature of the insulating film used for element isolation is sufficient, flattening of the atomic order can be realized without damaging the element isolation film.

  FIG. 1 is a schematic view showing a processing apparatus for carrying out the processing method of the present invention. First, the oxide film 2 is formed on the silicon substrate 1. This is formed by thermal oxidation, but either a dry method or a wet method may be used. Moreover, it is not necessary to actively perform thermal oxidation. Although the silicon substrate 1 has a (001) plane as a main surface, the silicon substrate 1 may have an off angle of up to about 4 °. The silicon substrate 1 may also be an SOI substrate having a buried oxide film inside. The silicon substrate 1 is immersed in the processing solution 11 in order to remove the oxide film 2.

In the method of the present invention, the treatment solution used for dissolving and removing the oxide film needs to be an aqueous solution containing hydrogen fluoride, because it is necessary to terminate the bond of silicon atoms in the outermost layer of the silicon surface with hydrogen atoms. There is.
In addition, in order to selectively remove only the oxide film and to remove it equivalent to each crystal structure on the silicon surface without giving erosion that inhibits flattening to silicon, a hydroxide ion reducing agent is used. In addition, it is necessary to reduce the hydroxide ion concentration in the treatment liquid.

As the hydroxide ion reducing agent, a substance having an action of reducing the hydroxide ion concentration in the hydrogen fluoride aqueous solution as much as possible, in other words, an action of increasing the hydrogen ion concentration as much as possible, is used. As such, it is necessary to select an inorganic acid other than hydrofluoric acid and not having an effect of regenerating an oxide film on the silicon surface. For example, hydrochloric acid, hydrobromic acid, Hydroiodic acid or the like is used. Particularly preferred is an aqueous solution containing hydrogen fluoride and hydrogen chloride, for example, an aqueous solution containing hydrogen fluoride and hydrogen chloride at concentrations of about 1% to 25% and 20% to 40%, respectively. In general, since hydroxide ions have the action of eroding the silicon surface, it is preferable to reduce the amount of hydroxide ions in the treatment liquid as much as possible, but a concentration of 10 ⁻¹² mol / l or less is sufficient. This concentration is about 1/10 of that of conventionally used hydrofluoric acid.
Further, as this treatment liquid, it is preferable to use a liquid having a small amount of dissolved oxygen in order to suppress the regeneration of the oxide during dissolution and removal of the oxide film. A preferable dissolved oxygen concentration is 10 ppb or less.

After the oxide film is peeled off in such a manner that the silicon is not eroded as much as possible, the silicon substrate is moved to the processing container 12 for performing the heat treatment. A gas supply device 13 and an exhaust device 14 are connected to the processing container 12 so that a desired gas can be supplied at a desired partial pressure. Although not shown, the gas supply device 13 is provided with a purifier for removing impurities and residual oxygen. Moreover, although not shown in figure, the heating apparatus which heats up the temperature in a processing container is provided. After the inside of the processing container 12 is filled with the gas, the temperature is raised to a predetermined temperature, and the post-processing after a predetermined time is finished.
In the present invention, as the gas for heat-treating the silicon substrate, it is desirable to use a gas containing hydrogen because it is necessary to terminate the dangling bonds of silicon on the surface after the treatment with hydrogen.

A high partial pressure of hydrogen in the gas is desirable to sufficiently hydrogen-terminate and clean the silicon surface during processing, but approximately 0.01 Torr (1.33 Pa) or more is sufficient.
In addition to pure hydrogen gas, this gas may be a gas obtained by mixing hydrogen gas in an inert gas such as argon or helium.
Further, in order to prevent oxidation or erosion of the silicon surface, the gas needs to be highly purified so as not to substantially contain a substance containing oxygen atoms such as oxygen and water.

  In the method of the present invention, since surface erosion during solution processing is prevented in advance, flatness on the atomic order can be achieved by diffusing a small amount of silicon atoms. A temperature lower than 900 ° C. required to sufficiently diffuse silicon atoms is sufficient. If the heat treatment is performed at 900 ° C. or lower, the element isolation film can be left without dissolving the oxide film. However, heat treatment at 900 ° C. or higher can be performed under conditions where there is no dissolved oxide film. In the present invention, since it is necessary to diffuse silicon slightly, it is desirable to perform heat treatment at 650 ° C. or higher.

  FIG. 2 is a cross-sectional view in order of steps showing the first processing mode of the present invention. A silicon substrate 1 having a (001) plane with a main surface having a predetermined conductivity type and a predetermined specific resistance is prepared [FIG. 2 (a)], and surface contamination is sufficiently removed by a normal cleaning process. After that, an oxide film 2 is formed on the surface of the silicon substrate by thermal oxidation [FIG. 2 (b)]. The silicon substrate is immersed in a hydrogen fluoride aqueous solution containing a hydroxide ion reducing agent to peel off the oxide film 2 [FIG. 2 (c)]. Thereby, the surface which has the silicon | silicone terminated by the hydrogen atom by which the unevenness | corrugation was suppressed is obtained. After the drying treatment, heat treatment is performed in an atmosphere containing hydrogen gas. This heat treatment terminates the unterminated hands and eliminates unevenness by silicon diffusion. Therefore, a regular step and terrace structure with a single atom step is obtained on the surface of the silicon substrate [FIG. 2 (d)].

  FIG. 3 is a cross-sectional view in order of steps showing the second processing mode of the present invention. A silicon substrate 1 having a (001) plane with a main surface having a predetermined conductivity type and a predetermined specific resistance is prepared [FIG. 2 (a)], and surface contamination is sufficiently removed by a normal cleaning process. After that, an element isolation film 3 for defining an element region is formed [FIG. 2B]. The element isolation film 3 is formed using any of the conventionally used isolation techniques such as the LOCOS method and the shallow trench method. An oxide film 2 is formed on the surface of the silicon substrate by thermal oxidation [FIG. 2 (c)]. The silicon substrate is immersed in a hydrogen fluoride aqueous solution containing a hydroxide ion reducing agent to peel off the oxide film 2 [FIG. 2 (d)]. Thereby, the surface which has the silicon | silicone terminated by the hydrogen atom by which the unevenness | corrugation was suppressed is obtained. At this time, since the difference in film thickness between the oxide film 2 and the element isolation film 3 is sufficiently large, the element isolation film 3 hardly decreases. After the drying treatment, heat treatment is performed in an atmosphere containing hydrogen gas. This heat treatment terminates the unterminated hands and eliminates unevenness by silicon diffusion. Therefore, a regular step and terrace structure with single atom steps is obtained on the surface of the silicon substrate [FIG. 2 (e)].

  As a treatment substrate, a silicon (001) substrate which was made p-type by adding boron was used. The resistivity is about 0.1 Ω · cm. First, after sufficiently removing organic contaminants and heavy metal contamination on the substrate surface, the substrate was heated to 1000 ° C. in a dry oxygen atmosphere using an atmospheric open type thermal oxidation furnace to form an approximately 110 nm oxide film.

As the treatment solution, EL (electronic industry) grade hydrofluoric acid (concentration 50%) and EL (electronic industry) grade hydrogen chloride (concentration 36%), which are generally used for the electronics industry, are approximately 1:20. What was mixed by the volume ratio of this was used. At this time, the concentrations of hydrogen fluoride and hydrogen chloride in the solution were approximately 3% by volume and approximately 35% by volume, respectively, and the hydroxide ion concentration in the solution was approximately 1 × 10 ⁻¹³ mol / l. Dissolved oxygen in the solution was removed, but its content was not controlled.
The silicon substrate was immersed in this processing solution and allowed to stand for about 2 minutes, and then the substrate was pulled up from the processing solution and dried, and the substrate was introduced into the processing container. The direction of immersion of the substrate in the treatment solution was not particularly controlled.

After introducing the sample into the processing container, the container was evacuated to about 1 × 10 ⁻⁹ Torr (1.33 × 10 ⁻⁷ Pa) and heated to about 200 ° C. to remove the adsorbed excess solution. . Thereafter, about 20 Torr (2666 Pa) of hydrogen gas was introduced. The substrate was heated to approximately 750 ° C. in gas and held for 2 minutes. Thereafter, it was cooled in gas and the surface was observed with a scanning tunneling microscope.

  FIG. 4 is a scanning tunneling micrograph of the silicon (001) surface that has been planarized and cleaned by the above-described method, and the observation area is approximately 100 nm square. What is observed stepwise in FIG. 4 is about 0.14 nm when the height is measured from a scanning tunneling micrograph. This is equal to the monoatomic height of silicon (001). That is, this is an atomic step of silicon (001). This structure is the minimum unit of height of the structure present on the silicon (001) surface. Such a clear single-atom step image has never been obtained on a heat-treated surface in a gas of about 750 ° C. These structures were built over most areas of the surface. That is, it can be seen that hydrogen termination on the surface of silicon (001) and planarization on the atomic order were realized at the same time by the method used this time.

The schematic block diagram of the processing apparatus used for the processing method of this invention. Sectional drawing of the process order of the silicon substrate which shows the process sequence of this invention method (the 1). Sectional drawing of the process order of the silicon substrate which shows the process sequence of this invention method (the 2). 1 is a scanning tunneling micrograph of a silicon (001) surface treated according to an example of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2 Oxide film 3 Element isolation film 11 Processing solution 12 Processing container 13 Gas supply apparatus 14 Exhaust apparatus

Claims (18)

A silicon substrate surface treatment method for planarizing and cleaning a silicon crystal surface,
(1) A silicon substrate having a silicon crystal having a (001) plane with an inclination angle of 4 ° or less as a main surface is treated with a hydrogen fluoride aqueous solution containing a hydroxide ion reducing agent to be formed on the surface of the silicon substrate. A step of dissolving and removing the oxide film,
(2) heating the silicon substrate in gas;
A method for surface treatment of a silicon substrate.   2. The method for treating a surface of a silicon substrate according to claim 1, wherein atomic steps having a regular monoatomic layer height are formed on the planarized and cleaned silicon surface.   2. The method for surface treatment of a silicon substrate according to claim 1, wherein a step of forming an oxide film on the surface of the silicon substrate is added prior to the step (1).   4. The hydroxide ion reducing agent is an inorganic acid other than hydrofluoric acid and an acid that does not have an action of forming an oxide on the silicon surface. 5. Silicon substrate surface treatment method.   The surface treatment method for a silicon substrate according to claim 4, wherein the inorganic acid is hydrohalic acid.   6. The surface treatment method for a silicon substrate according to claim 5, wherein the hydrohalic acid is hydrochloric acid.   The method for surface treatment of a silicon substrate according to claim 6, wherein the concentration of hydrogen fluoride in the aqueous hydrogen fluoride solution is 1 to 25% by volume and the concentration of hydrogen chloride is 20 to 40% by volume. 8. The method for surface treatment of a silicon substrate according to claim 1, wherein the hydroxide ion concentration of the aqueous solution of hydrogen fluoride containing a hydroxide ion reducing agent is 10 ⁻¹² mol / l or less.   The method for surface treatment of a silicon substrate according to any one of claims 1 to 8, wherein the dissolved oxygen concentration in the aqueous hydrogen fluoride solution is 10 ppb or less.   The method for treating a surface of a silicon substrate according to claim 1, wherein hydrogen is contained in the gas in the step (2).   The surface treatment method for a silicon substrate according to claim 10, wherein the partial pressure of hydrogen in the gas is 0.01 Torr (1.33 Pa) or more.   12. The silicon substrate surface treatment method according to claim 1, wherein the gas used in the step (2) is highly purified by a purifier.   13. The surface treatment method for a silicon substrate according to claim 1, wherein the heating temperature in the step (2) is 900 ° C. or less.   14. The silicon substrate surface treatment method according to claim 1, wherein in the step (1), the silicon substrate is immersed in the hydrogen fluoride aqueous solution to perform the treatment.   15. The method for surface treatment of a silicon substrate according to claim 1, further comprising a step of drying the silicon substrate prior to the step (2) after the step (1). .   The method for treating a surface of a silicon substrate according to claim 15, wherein the step of drying the silicon substrate is performed in a container in which the step (2) is performed.   The silicon substrate surface treatment method according to claim 1, wherein an element isolation film is formed on the silicon substrate. The silicon substrate surface treatment method according to claim 1, wherein the silicon substrate is an SOI substrate.

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