JPH0151445B2 - - Google Patents

Description

BACKGROUND OF THE INVENTION The present invention relates to the production of pure trichlorosilane for the production of silicon for electronic devices, and more particularly to the production of pure trichlorosilane containing trace amounts of electron-donating impurities, particularly phosphorus and other The present invention relates to a novel method for removing compounds of group elements.

For highly advanced electronic applications such as semiconductors and transistors, ultra-high purity silicon is required. As is well known, even trace amounts of impurities can significantly impair the performance of silicon-containing electronic components.

Generally, elemental silicon for semiconductors is a silicon halide such as silicon tetrachloride (SiCl ₄ ), trichlorosilane (HSiCl ₃ ) or dichlorosilane (H ₂ SiCl ₂ ).
is produced by reducing it with hydrogen, zinc, sodium or metal hydride. Silicon can also be obtained by thermal decomposition of silane (SiH ₄ ), but silane is difficult to process because it burns explosively when it comes into contact with air.

One of the most difficult impurities to remove from high purity silicon is phosphorus. Other impurities, such as copper, iron, and manganese, can be removed relatively easily by conventional methods (e.g., zone melt refining, crystal pulling, etc.), but phosphorus has physical properties similar to silicon, so these impurities can be removed relatively easily. The only way to separate them is through repeated trials. Furthermore, since phosphorus forms compounds with similar properties to starting materials such as chlorosilanes, purification and concentration of these starting materials is similarly difficult.

Methods proposed to date for removing impurities such as phosphorus from silicon or silicon halide compounds typically involve adsorption of the impurities by conjugation with a solid hydrated metal oxide complexing agent. or, forming a stable addition compound and then precipitating or distilling pure silicon or silicon halide. For details of such processing, see, for example, U.S. Pat.
No. 3188168 (Bradley) and British Patent No.
Please refer to No. 929696 (Siemens-Suchkertwerke Ltd.). Also, the applicant's U.S. patent application [Applicant's Docket 339-1696 (60SI-609/
614)] and [Applicant Docket 339-1698 (60SI-
708/710/711/716)] disclose an improved method. However, these methods also have problems such as impurity regeneration and difficulty in large-scale processing.

The above patents and patent applications are incorporated herein by reference.

Summary of the invention It has been discovered that phosphorus compounds and other compounds containing these elements can be removed from a chlorosilane solution by taking advantage of the property that group elements of the periodic table can be pentavalent as well as trivalent. . The normally trivalent impurities present in the ultrapure chlorosilane solution can be oxidized to the pentavalent state, by conventional distillation, or preferably by further combining the impurities with a complexing agent and then distilling. Therefore, it has been found that chlorosilane can be easily separated from these impurities. For example, phosphorus trichloride (PCl ₃ )
is a common residual impurity in ultrapure trichlorosilane. In the present invention, trichlorosilane is treated to remove phosphorus impurities from phosphorus oxychloride (POCl ₃ , boiling point
105.3℃), but this phosphorus oxychloride is
It is easier to separate by distillation from trichlorosilane (boiling point 31.8°C) than from PCl ₃ (boiling point 75.5°C).
Alternatively, this POCl ₃ can be further reacted with some complexing agent according to the present invention to form a more thermally stable complex to facilitate subsequent distillation.

It is thus an object of the present invention to provide a new method for the purification of chlorosilanes such as dichlorosilane, trichlorosilane, silicon tetrachloride or mixtures thereof.

Another object of the present invention is to provide a method for removing phosphorus compounds and other n-type impurities from chlorosilane solutions.

Yet another object of the present invention is to provide a purification method that is irreversible and applicable to large-scale purification systems.

Another object of the present invention is to provide a novel method for oxidizing phosphorus and other n-type impurities in chlorosilane solutions.

Furthermore, another object of the present invention is to provide a new means for removing pentavalent phosphorous impurities from chlorosilanes.

These and other objects are achieved by the method provided by the present invention, ie, the purification of chlorosilane solutions contaminated with impurities containing trivalent compounds of elements of Group Group of the Periodic Table. The method of the present invention includes the following steps. That is, (A) a step of oxidizing the impurities to obtain a compound in which the element is in a pentavalent state, and a subsequent step (B) of extracting and recovering purified chlorosilane by distillation.

In a preferred embodiment of the invention, the means for oxidizing the group element-containing impurity is by contacting it with an oxidizing agent selected from chromium trioxide or manganese dioxide.

In another preferred embodiment of the invention, the means for oxidizing Group element impurities is contacting the chlorosilane solution with oxygen under ultraviolet (UV) irradiation.

Other embodiments contemplated by the invention include an additional step after the step of oxidizing the impurity to the pentavalent state.
In this step, impurities are brought into contact with transition metal compounds or Lewis acids, which act as complexing agents to form thermally stable complexes, which are then used during the distillation of chlorosilane. Residues are removed.

DETAILED DESCRIPTION OF THE INVENTION The method of the invention comprises the step of oxidizing a compound of a trivalent Group element, such as phosphorus, which may be present as an impurity in a chlorosilane solution to form a compound in which the element is in the pentavalent state. . Impurities can be easily and efficiently removed by treating the chlorosilane thus treated by fractional distillation or distillation of pure chlorosilane after reaction with a complexing agent. The process of the invention is particularly effective for removing traces of phosphorus and other common n-type impurities such as arsenic, antimony, bismuth, etc. from chlorosilanes, especially trichlorosilanes. According to the present invention, for example, the phosphorus content in a trichlorosilane solution can be reduced to 0.5 ppb or less.

The process of the present invention is useful for removing phosphorus and other Group elements from chlorosilanes that are then reduced to crystalline silicon for electronic applications. Phosphorus and its sister group elements such as arsenic, antimony or bismuth are of particular importance in the production of silicon for electronics because they donate an excess of free electrons. When these elements are added into the silicon matrix, the excess electrons change the (neutral) electrical properties of the silicon crystal and also interfere with the doping agents it contains to give the crystal semiconducting properties. do. Since these excess electrons contribute as negative charges, impurities such as phosphorus are called "n"-type (negative) impurities.
As used herein, the term also refers to impurities that may be present in substantially pure chlorosilane, namely phosphorus compounds, compounds of group elements, and other electron-rich compounds.

The Group elements to be oxidized by the present invention are typically trichloride or trivalent chlorine in chlorosilanes of particularly high purity (e.g. less than 200 ppb total residual n-type impurities). Found in hydride state. These impurities have the general formula
AH _n Cl _o in which A is selected from phosphorus, arsenic, antimony and bismuth, m and n are 0, 1, 2 or 3 and m+n is 3 or 5. Although other forms of Group element compounds may be removed by the process of the present invention, the most common form of impurity remaining in the chlorosilane is that represented by the above formula.

A common property of the group elements that are the object of the method of the invention is that they all have five electrons in their outermost electron orbitals. This is explained in Hackh's Chemical Dictionary.
It is described in ``The Periodic Chain,'' pages 500-501 of McGraw-Hill, 1st edition. The most common valence of these elements is +3 (trivalent), but since there are 5 electrons in the outer shell, +5
That is, it can take on a pentavalent state. When a trivalent group element compound is oxidized, it can donate free electrons in its outermost orbit to oxygen (valence -2), which is an acceptor.

In the first step of the present invention, that is, the oxidation step, the chlorosilane solution is brought into contact with an oxidizing agent or oxidation or air is introduced under conditions that allow oxidation of impurities to proceed. Many oxidizing agents are known to those skilled in the art and are suitable for use in the present invention as long as they are compatible with the chlorosilane solution and can effectively oxidize residual impurities. However, it has been found that chromium trioxide and manganese dioxide are particularly suitable for the purification of the present invention and are therefore preferably used in embodiments of the invention that use oxidizing agents. In embodiments where oxygen is introduced into the system in gaseous form, ie as air, mixed with pure O ₂ or other gases, ultraviolet (UV) radiation is preferably applied to promote oxidation. When used in conjunction with UV irradiation, the reaction rate increases, and therefore sufficient oxygen can be supplied even with air, which is cheaper than pure oxygen gas. UV-oxidation is more selective for impurities than chlorosilanes, thus increasing the efficiency of the operation.

The amount of oxidizing agent used in the present invention will depend on the type of agent, the concentration of impurities, the chemistry of the particular oxidation reaction utilized, etc. In general, the use of excess is undesirable from a material balance point of view and because of the inevitable partial oxidation of the chlorosilane. On the other hand, it is desirable that impurities be completely oxidized. For this reason, it is necessary to experimentally confirm the optimal balance between excess and deficiency of oxidation under given conditions. For complete oxidation of impurities in a trichlorosilane sample, when using the preferred formulations i.e. chromium trioxide or manganese dioxide, about 7 to 10 and about 10 to 14
It has been found that molar equivalents of oxidizing agent are required. The oxidizing agent is typically added directly to the chlorosilane and mixed. To determine the degree of oxidation, samples are taken at appropriate intervals and analyzed using well-known chromatography.

When group element compounds are oxidized by contacting them with air (or oxygen) along with UV irradiation, the amount of air required depends on the concentration of impurities, the degree of agitation, the extent of UV irradiation, and several other factors. . Fundamentally, however, it has been found that UV irradiation can reduce the amount of air passing through the system for oxidation. This point will be explained in detail later. UV irradiation may be applied directly to the reaction vessel receiving the irradiation, but of course
The strength of the source must be such as to promote oxidation.

Once the oxidation step is complete, it is advantageous to selectively complex impurities to form stable compounds.
This stable compound will remain in bulk distillation of pure chlorosilane. Any complexing agent suitable for this purpose may be used as long as it is compatible with the chlorosilane and selectively binds to the oxidized group elements present in the chlorosilane to form a complex. It is easy to separate the chlorosilane from the impurity/complexing agent complex by distillation. Complexing agents include transition metal halides and Lewis acid compounds, which react with the electron-rich Group elements present in the chlorosilane matrix.

Preferred transition metal halides found to be suitable for the purposes of the present invention are zirconium tetrachloride and hafnium tetrachloride. It has been found that compounds such as phosphorus oxychloride form a 1:1 complex with zirconium tetrachloride in trichlorosilane and that this complexing agent can remove more than 99.9% of such impurities from chlorosilane solutions. It was done. Other transition metal halides of this type have also been found to be compatible with chlorosilanes without unnecessary experimentation and to efficiently complex residual common group elements, and these are also suitable for use in the present invention. The purpose can be fully achieved.

"Lewis acids" are those that accept a pair of electrons to form a covalent bond (ie, "electron pair acceptors") and may be used to accomplish the objectives of the present invention. This encompasses the Lowry-Brensted definition of acid, i.e., the general meaning of "proton donor." For example, boron trifluoride (BF ₃ Q) has only 6 electrons in the outermost electron orbital, so it is a typical Lewis acid.
BF ₃ has a tendency to accept free electron pairs to complete the eight-electron orbital. Although a wide variety of Lewis acids can be used to bond with the specific Group element compounds of interest in this invention, Lewis acids such as boron trichloride (BCl ₃ ) are difficult to remove impurities in the chlorosilane system. I will bring it in. In particular, the use of boron should generally be avoided as it poses problems of being difficult to purify in the production of silicon for electronics. For this reason, Lewis acid complexing agents suitable for the purpose of the present invention preferably contain elements that are easier to remove from chlorosilane. Most preferred are ferric oxide (FeCl ₃ ) and aluminum chloride (AlCl ₃ ).

The amount of complexing agent added to the contaminated chlorosilane solution is such that sufficient reaction occurs between this compound and the already oxidized impurities. In terms of reaction time and complete removal of impurities, best results are obtained with a molar excess of, for example, 2 to 50 times relative to the concentration of contaminants. However, it will be understood that the amount may be sufficient as long as it effectively binds to impurities present in the solution.

After the complexing agent is mixed into the solution, the mixture may be heated to promote the reaction between the oxidized impurities and the complexing agent compound. Too high a temperature, i.e. above 150°C, will cause some decomposition of the complex formed, and too low a temperature may not be sufficient to efficiently remove all impurities. For this reason, the reaction temperature ranges from 0°C to approx.
125°C is preferred. However, elevated temperatures above the above range may be used as long as the reaction products are not distilled in the same fraction as the chlorosilane and are therefore not contaminated during the purification process. Best results were obtained at temperatures below about 100°C. Pressurization may be applied within the reaction vessel to prevent premature distillation of the chlorosilane from occurring. Simple experimentation can be used to determine the optimum reaction temperature and pressure under given conditions.

As previously described, the reaction is continued until substantially all of the impurities have combined to form a thermally stable complex. Of course, reaction times will vary depending on the materials used, temperature, pressure, etc. Simple experimentation will determine the optimal reaction time required for a given purification.

The final step in the purification method of the present invention is to distill pure chlorosilane from the reaction solution. This final distillation step is possible because of the reduced volatility of the impurities relative to the chlorosilane.

Distillation may be carried out at atmospheric pressure or under pressure, provided that the temperature of the liquid does not exceed the decomposition temperature of the impurities or impurity complexes formed in the previous steps of the process. Preferably, the temperature of the solution is kept below about 200°C.

To assist those skilled in the art in practicing the invention, the following examples are provided for illustrative purposes, but the invention is not limited thereto.

Example 1 Conversion of _PCl3 to _POCl3 using an oxidizing agent A standard solution of trichlorosilane containing 500 ppm phosphorous trichloride was placed in a reaction vessel. Manganese dioxide was added to the solution with vigorous stirring, and samples of the reaction mixture were taken periodically to determine the respective concentrations of phosphorus trichloride and phosphorus oxychloride by chromatography. It was found that 10-14 molar equivalents of manganese dioxide were required for complete oxidation and that phosphorus was quantitatively oxidized to the pentavalent state within 2 hours.

Example 2 Conversion of _PCl3 to _POCl3 by photooxidation A standard solution of trichlorosilane containing 5000 ppm phosphorus trichloride was placed in a transparent flask equipped with a dry ice condenser and a gas inlet. Dry air was introduced from the inlet at 50 cm ³ /min and stirred to rapidly disperse the mixture. A dry ice condenser was sufficient to cool and condense the trichlorosilane in the gas stream. Samples of the trichlorosilane solution were taken periodically and the relative concentrations of phosphorus trichloride and phosphorus oxychloride were determined by chromatography. 1 when a 16 molar excess of oxygen (in air) is passed through the system.
It was found that only ~5% of _PCl3 was oxidized to _POCl3 .

The experiment was continued by introducing air and irradiating the sample while stirring and dispersing it. 2.4 for safe oxidation of PCl ₃
A molar excess of oxygen has been found to be sufficient.

The reduction in the amount of oxygen required means that
This does not imply that the stoichiometry of PCl ₃ is 2.5, but rather suggests that the molar ratio was determined by mass action.

Example 3 Using a transition metal halide complexing agent
Complexation of POCl ₃ To a solution of 450 parts by weight of trichlorosilane containing 1.5 parts by weight of phosphorous oxychloride, 4.6 parts by weight of zirconium tetrachloride were added. After stirring the solution overnight, chromatographic analysis showed that POCl ₃ was quantitatively removed from the trichlorosilane solution.

Example 4 Complexation of POCl ₃ using a Lewis acid complexing agent 5.0 parts by weight of aluminum trichloride were added to a solution of 402 parts by weight of trichlorosilane containing 1.7 parts by weight of phosphorous oxychloride. Put the mixture into reaction container 1.
Heated to 100°C. Take samples regularly
The concentration of _POCl3 was measured. After 1 hour, no POCl ₃ was detected in the upper gas.

Claims (1)

[Claims] 1. A method for removing trichloride of one or more elements selected from the group consisting of phosphorus, arsenic, antimony, and bismuth from ultra-high purity trichlorosilane, comprising the following steps: (A) The above-mentioned An excess amount of oxidizing agent relative to the molar concentration of the trichloride is applied to the trichloride for a period of time effective to completely oxidize the element to the pentavalent oxychloride. (B) the molar concentration of the _oxychloride relative to the trichlorosilane; by adding a 2 to 50 times excess of a complexing agent relative to the complexing agent, wherein said complexing agent is selected from the group consisting of zirconium tetrachloride, hafnium tetrachloride, ferric chloride and aluminum chloride; C) heating the trichlorosilane to a temperature of from about 0°C to about 125°C for a period of time sufficient to promote the formation of a thermally stable complex between the oxychloride and the complexing agent. while maintaining pressure conditions effective to prevent evaporation of said trichlorosilane; and (D) to about 100°C while avoiding temperature and pressure conditions that would lead to decomposition of said thermally stable complex. Purified trichlorosilane is removed by distillation at the following temperatures: 2. The method of claim 1, wherein in the oxidation step A, about 7 to 10 molar equivalents of chromium trioxide or about 10 to 14 molar equivalents of manganese dioxide are brought into contact with the trichloride. 3. The method according to claim 1, wherein in the oxidation step A, O ₂ is introduced into trichlorosilane in the presence of ultraviolet light. 4. The method according to claim 1 for removing PCl ₃ impurities from an ultra-high purity trichlorosilane solution, which comprises the following steps: While exposing the trichlorosilane solution to ultraviolet rays, air is introduced into this solution to remove the PCl 3 impurities from the solution. Oxidize substantially all of ₃ to POCl ₃ ; while adding a complexing agent in an excess amount of 2 to 50 times with respect to the molar concentration of POCl ₃ (however, the complexing agent may be zirconium tetrachloride, hafnium tetrachloride, , ferric chloride and aluminum);
heating the solution to a temperature of 0 to 125°C to form a thermostable complex between the complexing agent and substantially all of the _POCl3 ; Purified trichlorosilane is removed by distillation at a temperature below about 100°C while avoiding decomposition.