Process for the preparation of partially-substituted fluorosilane

Abstract: A process is provided by the present invention for the preparation of a partially-substituted fluorosilane represented by the following formula: SiH n F 4-n wherein n stands for an integer of 1 to 3. The process comprises converting a corresponding partially-substituted chlorosilane into the partially-substituted fluorosilane by halogen replacement while using a fluorinating agent. The fluorinating agent is anhydrous zinc fluoride having at least 500 Å in terms of the size of crystallites of the (110) plane.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for the preparation of a partially-substituted fluorosilane. More specifically, the present invention is concerned with an improvement in a process for preparing a fluorosilane (e.g., monofluoromonosilane, difluoromonosilane, trifluoromonosilane) by halogen substitution. 2. Description of the Related Art Partially-substituted fluorosilanes are useful for the formation of thin films of fluorinated amorphous silicon. Known processes for the preparation of a partially-substituted fluorosilane include the so-called halogen substitution process in which a corresponding chlorosilane is fluorinated with a fluorinating agent. Use of zinc fluoride (hereinafter referred to as "ZnF₂") as the fluorinating agent is disclosed in Japanese Patent Laid-Open No. 151015/1986. Further, this patent publication discloses that all equipments employed in the reaction, to say nothing of ZnF₂, require full drying immediately before their employment because a chlorosilane or fluorosilane undergoes hydrolysis in the presence of water and also that the drying of ZnF₂ can be achieved sufficiently by heating it, for example at 200°C for 4 hours. It was however found that, when halogen substitution of a partially-substituted chlorosilane is carried out using ZnF₂ treated as described above, fluorosilanes other than the corresponding partially-substituted fluorosilane, silanes and silicon tetrafluoride are byproduced. The present inventors then found that a partially-substituted fluorosilane can be prepared in a high yield and moreover with less analogous impurities when the water content of ZnF₂ is reduced to 0.2 wt.% or less by processing such as heat treatment (Japanese Patent Laid-Open No. 147,920/1993). Even by this process, flurorosilanes other than the corresponding partially-substituted fluorosilane, silane, silicon tetrafluoride and the like are still be byproduced although their amounts are extremely small. SUMMARY OF THE INVENTION The present inventors have attempted to find out a fluorinating agent which features easy handling and good conversion upon preparation of a partially-substituted fluorosilane with high purity by the halogen replacement technique. As a result, it has been found that, by paying attention to physical properties of ZnF₂, especially to a crystallite size capable of showing good crystallinity, use of anhydrous zinc fluoride having 500Å in terms of the size of crystallites of the (110) plane can prepare a partially-substituted fluorosilane represented by the following general formula: SiH n F 4-n (n: integer of from 1 to 3) with high purity in a high yield, leading to the completion of the present invention. The present invention can therefore replace the invention disclosed in Japanese Patent Laid-Open No. 147,920/1993. In one aspect of the present invention, there is thus provided a process for the preparation of a partially-substituted fluorosilane represented by the following formula: SiH n F 4-n wherein n stands for an integer of 1 to 3, said process comprising converting a corresponding partially-substituted chlorosilane represented by the following formula: SiH n Cl 4-n wherein n has the same meaning as defined above into the partially-substituted fluorosilane by halogen replacement while using a fluorinating agent, wherein the fluorinating agent is anhydrous zinc fluoride having at least 500 Å in terms of the size of crystallites of the (110) plane. Owing to the use of anhydrous zinc fluoride having at least 500 Å in terms of the size of crystallites of the (110) plane as a fluorinating agent, the process according to the present invention can prepare the partially-substituted fluorosilane in a far higher yield with less analogous impurities compared with conventional zinc fluoride. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a clinographic projection of zinc fluoride of the tetragonal crystal system; and FIG. 2 is an X-ray diffractometer pattern of zinc fluoride. DETAILED DESCRIPTION OF THE INVENTION The process of the present invention for the preparation of the partially-substituted fluorosilane is a halogen replacement process in which a fluorinating agent is used. As the fluorinating agent, zinc fluoride having 500 Å in terms of the size of crystallites of the (110) plane is employed. A description will now be made of crystals. A crystal generally means a body formed of atoms, ions or molecules arranged regularly in three dimensions. For example, a ZnF₂ crystal is formed of fluorine ions F⁻ and zinc ions Zn²⁺ arranged alternately so that the charges are balanced. It is known that the minimum unit (unit cell) of a ZnF₂ crystal is a cube. The lengths (a,b,c) of respective axes of the unit cell and the values of angles (α,β,γ) between the respective axes are called "lattice constants". All crystals can be defined by such lattice constants. Relying upon the symmetry of each unit cell, all crystals can be classified to seven types of crystal systems. ZnF₂ is known to belong to the tetragonal crystal system shown in FIG. 1. Expressed by lattice constants, the lengths of its axes are (a,a,c) while the angles between the respective axes are (90°.90°,90°). A crystal can be considered to have been formed by repeating or stacking such unit cells. Corners (vertices) of each unit cell are called "lattice points". Planes containing such lattice points are called "lattice planes", which are considered to exist in an infinite number. Such lattice planes extend in parallel at constant intervals therebetween. The type of each lattice plane is expressed by a set of integers called "Miller indices (plane indices)". When a lattice plane extends crossing axes X,Y,Z of rectangular coordinates, the respective intercepts are expressed in terms of the lengths of the corresponding axes and a combination of three numbers obtained as the reciprocals of these numbers are multiplied by the smallest number to make all the reciprocals become integers, whereby Miller indices are obtained. The (110) plane indicates a plane which crosses the X- and Y-axes at the intercepts, respectively, and extends in parallel with the Z-axis. As is understood from the foregoing, Miller indices are generally indicated using symbols (hkℓ) (h,k,ℓ: integers). When a crystal is exposed to X rays, diffraction takes place. The direction of the diffraction and the intensity of X rays so diffracted are inherent to the crystal. It is therefore possible to determine the structure of a crystal by measuring the angle of diffraction and the intensity of X rays. This method is generally called "X-ray diffractometry". Among the wavelength λ of X rays irradiated onto a crystal, an interplanar distance d between certain lattice planes of the crystal and an angle  at which the X rays are reflected to form diffracted X rays, the following formula (1) called "Bragg condition" is established: nλ = 2d sin where n is called an "order of diffraction" and takes a positive integer, As has been described above, each crystal has an inherent interplanar distance d in accordance with the arrangement of atoms therein so that an inherent diffraction pattern is obtained. An X-ray diffractometric pattern of ZnF₂ is shown in FIG. 2. As is shown in the figure, the peak corresponding to the (110) plane is the greatest. It is hence understood that X rays diffracted by the (110) plane has the greatest intensity. The "crystallite size D" as used in the present invention is a kind of particle size of crystals. The crystallite size D can be calculated in accordance with the following formula (2) from the half band width of the integrated intensity of diffracted X rays (peak area) corresponding to each lattice plane in an X ray diffractometric pattern obtained by the X-ray diffraction analysis: D = 0.9λ/βcos. where D is the size of crystallite (Å), λ is the wavelength of X rays used in the measurement,  is a Bragg angle, and β is a half band width (rad) [P. Scherrer, Goettingen Nachr., 98 (1918).] It is envisaged from the above formula that, as the crystallinity becomes better, β approaches toward 0 without limit and the value of D increases infinitely. The size of crystallites obtained by the above-described method is the size as viewed in the direction (vector) perpendicular to the X ray diffracted plane (hkℓ) used for the measurement, and is generally different from a particle size obtained by a method such as the particle size analysis by sieving, a microscope or a settling speed. ZnF₂ used in the present invention has a crystallite size greater than 500 Ä as viewed in the direction of the (110) plane. Use of such ZnF₂ makes it possible to lower byproduction of fluorosilanes other than the corresponding partially-substituted fluorosilane, fluorosilanes, silanes (SiH₄), silicon tetrafluoride (SiF₄) and the like. When SiHF₃ is prepared, for example, such ZnF₂ is effective especially in lowering the byproduction of SiF₄. Described more specifically, upon preparation of a partially-substituted fluorosilane from its corresponding partially-substituted chlorosilane as a starting material, it is possible to suppress the byproduction of a fluorosilane substituted by one more fluorine atom than the formula: SiH n F 4-n (n: integer of 1 to 3). Although its detailed reason is still unknown, the present inventors presume as will be described next. A ZnF₂ crystal is a cubic crystal as described above. On most of its crystal surface, a reaction to form a corresponding fluorosilane to a starting material is considered to take place. In contrast, portions corresponding to the vertices of the cubic crystal are generally considered to have high activity so that the fluorination proceeds further to form a fluorosilane substituted by one more fluorine atom. Accordingly, control of the size of crystallites to a particular value or greater is expected to relatively decrease the number of high-activity points, leading to inhibition to the occurrence of the above-described analogous impurity. To obtain anhydrous zinc fluoride having a crystallite size greater than 500 Å in the direction of the (110) plane, there are several processes. For example, it is preferred to react zinc oxide or zinc carbonate, which has a purity of 99 wt.% or higher and contains no water of crystallization, with hydrofluoric anhydride having a purity of 99 wt.% or higher. Further, it is also suitable to use ZnF₂ which has been obtained by reacting a zinc compound, which has a purity of 99 wt.% or higher and contains no water of crystallization, with fluorine gas having a purity of 99% or higher. If the purities of the compounds are lower than 99 wt.%, the resulting ZnF₂ products tend to have a smaller crystallite size. Illustrative examples of the zinc compound can include ZnO, ZnCO₃, ZnCl₂ and ZnBr₂. When ZnF₂ is synthesized by one of the above reactions, the molar ratio of zinc oxide or zinc carbonate to hydrofluoric anhydride or the molar ratio of the zinc compound to fluorine gas can be set preferably at 1:2 to 1:10 with a range of from 1:3 to 1:5 being more preferred. In the halogen replacement technique, it is preferred to contact the chlorosilane with ZnF₂ by causing the chlorosilane to pass in a gaseous form through a ZnF₂-packed layer as disclosed in Japanese Patent Laid-Open No. 147,920/1993 although the present invention is not limited to the use of such a contacting method. The reaction between the chlorosilane and ZnF₂ is a significantly exothermic reaction. Since the resulting partially-substituted fluorosilane is thermally unstable, consideration for the effective removal of the resulting heat is needed to obtain the fluorosilane with high purity. It is an effective measure for the prevention of side reactions to conduct the reaction by suspending ZnF₂ in an organic solvent such as ethyl ether and then adding the chlorosilane to the suspension. The reaction temperature can range preferably from -30°C to 40°C, more preferably from -10°C to 5°C. Reaction temperatures lower than -30°C tend to lower the conversion of the halogen replacement reaction. Conversely, reaction temperatures higher than 40°C tend to induce thermal decomposition of the resulting partially-substituted fluorosilane and hence to lower the conversion. The present invention will hereinafter be described specifically by the following examples and comparative examples. It is however borne in mind that the present invention shall not be limited thereto. Example 1 Anhydrous ZnF₂ was synthesized by reacting anhydrous ZnO and anhydrous HF, whose purities were 99.5 wt.%, at a molar ratio of 1:4. The reaction temperature was 100°C. The anhydrous ZnF₂ was then placed in a polypropylene bag and stored in a desiccator. The X-ray diffraction intensity of the (110) plane of the ZnF₂ was measured by an X-ray diffractometer. From the results, the size of crystallites in the direction of the (110) plane was calculated. It was found to be 546 Å. Five hundreds grams of the ZnF₂ were charged in a 3 ℓ glass flask equipped with a stirrer and the interior of the flask was thoroughly purged with N₂. Thereafter, 100 g of SiHCl₃ were fed at a flow rate of 0.5 g/min to the flask. Incidentally, the flask was immersed in iced water during the reaction so that the reaction temperature was prevented from rising. Gas produced from the flask was guided to a dry ice-acetone trap to remove impurities and was then collected in a liquefied nitrogen trap. Based on the SiHCl₃, the yield of SiHF₃ was 90 vol.%. The purity of the collected gas (SiHF₃) was 99 vol.% and, among analogous impurity gases, the content of SiF₄ was 0.7 vol.% and the contents of the other fluorosilanes were each not higher than 0.1 vol.%, as shown in Table 1. Example 2 Anhydrous ZnF₂ was synthesized by reacting at 25°C 1.2 moles of anhydrous ZnCO₃ having a purity of 99.0 wt.% with an excess amount, i.e., 8 moles of F₂ having a purity of 99.0 wt.%. The anhydrous ZnF₂ was then stored in the same manner as in Example 1. The size of crystallites in the (110) plane of the ZnF₂ was calculated in the same manner as in Example 1. It was found to be 527 Å. One hundreds grams of the ZnF₂ were charged in a 1 ℓ glass flask equipped with a stirrer and suspended in 500 mℓ of anisole. After the interior of the flask was thoroughly purged with N₂, 100 g of SiH₂Cl₂ were fed at a flow rate of 0.5 g/min to the flask under stirring. Incidentally, the flask was immersed in iced water during the reaction so that the reaction temperature was prevented from rising. Further, the flask was also fitted with a reflux condenser to prevent evaporation of the anisole. Reaction gas produced from the flask was guided to a dry ice-acetone trap to remove impurities and was then collected in a liquefied nitrogen trap. Based on the SiH₂Cl₂, the yield of SiH₂F₂ was 95 vol.%. Incidentally, the purity of the collected gas (SiH₂F₂) was 99 vol.% and, among analogous impurity gases, the content of SiHF₃ was 0.7 vol.% and the contents of the other fluorosilanes were each not higher than 0.1 vol.%, as shown in Table 1. Example 3 ZnF₂, which had been obtained by reacting 1.2 moles of anhydrous ZnO having a purity of 99.0 wt.% with an excess amount, i.e., 5 moles of F₂ having a purity of 99.0 wt.%, was stored in a similar manner to Example 1. By the method described in Example 1, the size of crystallites in the (110) plane was calculated. It was found to be 537 Å. Using 200 g of the ZnF₂, conversion of SiH₃Cl to SiH₃F was conducted in a similar manner to Example 2. Based on the SiH₃Cl, the yield of SiH₃F was 92 vol.%. Incidentally, the purity of the collected gas (SiH₃F) was 99 vol.% and, among analogous impurity gases, the content of SiH₂F₂ was 0.7 vol.% and the contents of the other fluorosilanes were each not higher than 0.1 vol.%, as shown in Table 1. Comparative Example 1 ZnF₂ was synthesized by reacting ZnO having a purity of 95.0 wt.% with a 50 wt.% aqueous solution of HF at room temperature. By a method similar to that employed in Example 1, the size of crystallites in the (110) plane was calculated. It was found to be 386 Å. A reaction was then conducted in a similar manner to Example 1 except for the use of the ZnF₂. As a result, analogous impurities other than SiHF₃ were formed, as shown in Table 1, in greater amounts compared with Example 1. In particular, byproduction of SiF₄ was observed. The yield was also found to be low. Comparative Example 2 A reaction was then conducted in a similar manner to Example 2 except for the use of the ZnF₂ synthesized in Comparative Example 1. As a result, analogous impurities other than SiH₂F₂ were formed, as shown in Table 1, in greater amounts compared with Example 2. In particular, byproduction of SiHF₃ was observed. The yield was also found to be low. Comparative Example 3 A reaction was then conducted in a similar manner to Example 3 except for the use of the ZnF₂ synthesized in Comparative Example 1. As a result, analogous impurities other than SiH₃F were formed, as shown in Table 1, in greater amounts compared with Example 3. In particular, byproduction of SiH₂F₂ was observed. The yield was also found to be low. Comparative Example 4 After commercial zinc fluoride tetrahydrate (ZnF₂·4H₂O) was dried at 200°C for 4 hours, the resulting zinc fluoride was subjected to further heat treatment at 600°C for 2 hours. Thereafter, the zinc fluoride was allowed to gradually cool down to room temperature in a dry nitrogen gas atmosphere. Weight variations of the ZnF₂ so obtained were measured from room temperature to 700°C by a differential thermobalance in an atmosphere purged with helium gas. The weight loss was found to be 0.14%. In addition, the size of crystallites in the (110) plane of the ZnF₂ was also measured in the same manner as in Example 1. It was found to be 327 Å. Using 200 g of the ZnF₂, conversion of SiH₂Cl₂ to SiH₂F₂ was conducted in a similar manner to Example 1. Based on the SiH₂Cl₂, the yield of SiH₂F₂ was 81 vol.%. This yield is considerably lower compared with the value achieved in Example 2. Further, analogous impurities other than SiH₂F₂ occurred generally in greater amounts compared with Example 2. The purity of the SiH₂F₂ was lower than 99%. The above results are presented in Table 1. Example Comparative Example 1 2 3 1 2 3 4 Yield (%) 90 95 92 78 75 76 81 Analysis value of collected gas (vol.%) SiHF₃ ≧99.0 0.7 ≦0.1 85.2 10.3 4.0 0.4 SiH₂F₂ ≦0.1 ≧99.0 0.7 2.1 78.4 12.5 98.3 SiH₃F ≦0.1 ≦0.1 ≧99.0 1.6 2.2 75.3 0.4 SiF₄ 0.7 ≦0.1 ≦0.1 9.3 6.1 5.5 0.5 SiH₄ ≦0.1 ≦0.1 ≦0.1 1.8 3.0 2.7 0.6 Table 1 indicates that the Comparative Examples produced the analogous impurities in high yields but the target fluorosilanes in a low yield. In contrast, they also indicate that the Examples, which fall within the breadth of the present invention, are superior in these aspects.

Claims: A process for the preparation of a partially-substituted fluorosilane represented by the following formula: SiH n F 4-n wherein n stands for an integer of 1 to 3, said process comprising converting a corresponding partially-substituted chlorosilane represented by the following formula: SiH n Cl 4-n wherein n has the same meaning as defined above into the partially-substituted fluorosilane by halogen replacement while using a fluorinating agent, characterised in that the fluorinating agent is anhydrous zinc fluoride having a size of crystallites of at least 50 nm as calculated from the half-width value of the 110 peak of the X-ray diffractometric pattern. A process of claim 1, wherein the anhydrous zinc fluoride has been obtained by reacting a zinc compound selected from zinc oxide or zinc carbonate, said zinc compound being free of water of crystallization and having a purity of at least 99 wt.%, with hydrofluoric anhydride having a purity of at least 99 wt.%. A process of claim 1, wherein the anhydrous zinc fluoride has been obtained by reacting a zinc compound having a purity of at least 99 wt.%, with fluorine gas having a purity of at least 99 wt.%.

Inventors: ARITSUKA MAKOTO, HARADA ISAO, MIMOTO ATSUHISA, ARITSUKA, MAKOTO, HARADA, ISAO, MIMOTO, ATSUHISA
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1993
Main classification: C01B33
IPC codes: C01B33, C01G9