GB2187179A - Method of preparing perfluorosilanes from perchlorosilanes - Google Patents

Abstract

A perfluorosilane such as Si2F6 or Si3F8 is prepared by reaction between a perchlorosilane such as Si2Cl6 or Si3Cl8 and a metal fluoride such as ZnF2 or SBF3 the reaction is carried out in a nonaqueous liquid medium such as n-butyl ether. By using the liquid medium the reaction proceeds smoothly, and the desired perfluorosilane is obtained at high yield. Preferably a saturated hydrocarbon such as n-decane is added to the liquid medium to prevent the reaction system from becoming viscous as zinc chloride is formed.

Description

SPECIFICATION Method of preparing perfluorosilanes from perchlorosilanes This invention relates two a method of preparing perfluorosilanes represented by hexafluorodisilane Si2F6 and octafluorotrisilane Si3F8from perchlorosilanes.

At present it is prevailing to use either silane gas or disilane gas as the raw material gas for the deposition of amorphous silicon in film form, which is largely used in solar batteries and electrophotographic materials.

However, great care must be devoted to handling of the raw material gas since both silane gas and disilane gas readily react with oxygen in the air usually to burn bright with flame.

Tetrafluorosilane gas is less dangerous than silane and disilane gases, and amorphous silicon can be deposited from tetrafluorosiiane gas too. Besides, the obtained amorphous silicon films are said to be better in thermal stability than the ones deposited from silane gas or disilane gas by reason of the high strength of bond between silicon and fluorine. However, when tetrafluorosilane gas is used the rate of deposition of amorphous silicon film is very lowerthan in the cases of using silane or disilane gas.

Perfluorosilanes represented by hexafluorodisilane and octafluorotrisilane are also low in dangerfor handling, and high rate deposition of amorphous silicon can be accomplished by selecting the raw material gas from such perfluorosilanes. However, problems yet remain to be soived in respect of the preparation of perfluorosilanes as industrial materials.

According to Schumb and Gamble, J. Am. Chem. Soc., Vol. 54, 583 (1932), hexafluorodisilane is prepared bydirectcontactof hexachlorodisilane with anhydrous zincfluoride ZnF2 in solid phase. However, from a practical point ofviewthis method is inconvenient and has disadvantages in the following respects. First itis difficu It to control the manner or degree of the halogen substitution reaction. Second, the reactantsturn into a lava-like mass as zinc chloride is formed by the reaction. This phenomenon offers difficultyto operation and causes a large portion of hexachlorodisilane to remain unreacted, sothatthe yield of hexafluorodisilane is low.Besides, a considerable amount oftetrafluorosilane is formed as an unwanted by-product because of a considerable probability of severance of the silicon-silicon bond.

It is an object ofthe present invention to provide an improved method of preparing a perfluorosilane, such as hexafluorodisilane oroctafluorotrisilane, from a perchlorosilane, which method is high in the yield ofthe desired perfluorosilane and can be put into practice without difficulty.

According to the invention, there is provided a method of preparing a perfluorosilane by reaction between a perchlorosilane and a metal fluoride, characterized in that the reaction is carried out in a nonaqueous liquid medium.

The method according to the invention is very suited two the preparation of hexafluorodisilanefrom hexachlorodisilane or the preparation of octafluorotrisilane from octachlorotrisilane. Good examples ofthe metal fluoride used in this method as the fluorine source are zincfluoride (anhydrous) ZnF2 and antimony trifluoride (anhydrous) SbF3. It is preferred to use ZnF2which exhibits very high reactivity and is convenient for handling. In the method according to the invention the nonaqueous liquid medium makes an important contribution to smooth proceeding of the desired halogen substitution reaction. It is suitableto use an organic solvent in which the starting perchlorosilane is soluble. Good examples are n-butyl ether, diethyl ether and ethylene glycol dimethyl ether.It is preferred to use n-butyl ether which is convenient for handling and which provides very good yield ofthe desired perfluorosilane.

In n-butyl ether either Si2Cl8 or Si3Cl8 readily reacts with anhydrous zincfluoride. In either case the reaction is carried out attemperatures between about 50"C and about 1 50"C, and preferably at 80-100"C. Zincfluoride is put into n-butyl ether, and the mixture is continuously stirred so as to keep keepzincfluorideuniformly dispersed in the liquid medium. The perchlorosilane is gradually introduced into the slurry prepared by dispersing zincfluoride in n-butyl ether.In preparing Si2F6the reaction gas is passed through a condenserfor refluxing the liquid medium and then passed to a solvent trap which is kept cooled at about -20 Cto completely separate the liquid medium and any other high boiling point matter from the reaction gas. After that the reaction product is captured in a trap kept cooled at -80 to -95 C, and a small amount by-produced tetrafluorosilane is captured in a separate trap cooled with liquid nitrogen. Also in the case of preparing Si3F8 the reaction gas mistreated in substantially the same manner, except that the solvent trap is kept at about 300C and that the reaction product is captured in a trap kept cooled at about -30 C.

At first the reaction between Si2CI6orSi3CI8 and Zn F2 in n-butyl ether proceeds very smoothly. Asthe reaction proceeds the viscosity of the reaction system increases because of formation ofzincchloride. If the reaction is continued without any countermeasure the reaction system will entirely become very viscous and gummy, and hard to stir, so that further continuation of the reaction becomes difficult. This problem can easily be solved by introducing a relatively large amount of a suitable saturated hydrocarbon, such as n-decane, n-dodecane or n-tetradecane, into the reaction system. The saturated hydrocarbon may be mixed with n-butyl ether before starting the reaction or, alternatively, may be put into the reaction system when the viscosity is increasing by the formation of zinc chloride. As to the amount of the saturated hydrocarbon, it is suitablethatthe volume ratio of n-butyl etherto the saturated hydrocarbon falls in the rangefrom 2:100to 100:100, and preferably in the range from 7:100 to 50:100. When a saturated hydrocarbon alone is used as the liquid medium the viscosity of the reaction system does not seriously increase even though zinc chloride is formed, but in this case the rate of reaction is very low.When n-butyl ether and a suitable saturated hydrocarbon are jointly used the desired reaction continues to proceed smoothly and ata fairly high rate without suffering from clogging or gelling of the reaction system. After completion of the reaction the liquid medium can be recovered for reusing by removing the solid matter and, if necessary, performing a simple distillation operation. It is likely that hexafluorodisilane or octafluorotrisilane obtained by the above described method contains very small amount of C4 hydrocarbons originated in n-butyl ether. Presumably, a small portion of n-butyl ether undergoes severance of the ether bond during the halogen substitution reaction ofthe perchlorosilane.When the obtained perfluorosilane is for use as the raw material gas for producing amorphous silicon,the presence of even a very small amount of hydrocarbons is undesirable because of offering difficulty in controlling the energy band gap of the deposited amorphous silicon films.

The hydrocarbons can be removed from the perfluorosilane by precision distillation. However, more simple and convenient method for removing the hydrocarbons is washing the perfluorosilane with a suitable solvent. The washing must be accomplished without loss of the perfluorosilane, and the solvent must not remain in the washed product. Accordingly an organic solvent relatively high in boiling point and inactive and incompatible with the perfluorosilane is used. For example, it is suitable to use liquid paraffin, a high boiling point satu rated hydrocarbon such as n-decane ora high boiling point ether such as n-butyl ether or diphenyl ether.

The invention is further illustrated bythe following nonlimitative examples.

Example I Afour-necked glass flask having a capacity of 300 ml was used as the reaction vessel. The flas(twas provided with a stirrer and was connected with a recovery line including a condenser, a trap for removal of liquid reaction medium and anothertrapfor recovery of product.

The fluorinating agent was anhydrous zinc fluoride (moisture content 200 ppm), which was prepared by heating zincfluoridetetrahydratefirst in an electric furnace at200 Cfor 6 hr and then in a hydrogen fluoride gas stream at 350"C for 6 hr. Initially 41.4 g (0.4004 mol) of anhydrous zincfluoride was charged into the flask.

Afterthatthe reaction system was vacuumed and then filled with helium gas, and this procedure was repeated several times to accomplish almost complete removal of air and moisture. Atthe end of a helium gas atmosphere was established throughout the reaction system. Next, 40 ml of a fixed liquid consisting of 20 parts by volume of n-butyl ether and 100 parts by volume of n-decane was put into the flask after dehydrating with molecular sieves. Then the sti rrer was operated to uniformly disperse zincefluoride in the mixed liquid, and the resultant slurry was heated to and kept at 85"C by external heating.

Next, 30.0 g (0.1115 mol) of hexachlorodisilane was slowly dropped into the slurry in the flask by using a dropping funnel provided with a pressure equalizing bypass, while stirring was continued to promote reaction between the dropped starting material and zinc fluoride in the slurry. It took 30 min to introduce the entire quantity of hexachlorodisilane into the flask, and the reaction proceeded smoothly.

The reaction gas was passed through a condenser cooled with ice water and then through a trap kept cooled at -20 C and then passed to a product recovering trap (capacity 100 ml) kept cooled at90 Cto thereby collect hexafluorodisilane formed by the reaction. A small quantity oftetrafluorosilane formed as a by-product was separated from the reaction gas in a cold trap cooled with liquid nitrogen. After completion of the dropping of hexachlorodisilane into the reaction vessel, helium gas was continuously blown into the vessel for 3 her to thereby completely transfer the remaining reaction gas to the product recovering trap. After that, hexafluorodisilane collected in the trap was allowed to evaporate in a separate vacuum vessel.The volume ofthe gasified product was 1.96 NI (0.0875 mol), so that the yield on silicon basis ways calculated to be about78%.

Refining experiment Gas chromatography analysis of hexafluorodisilane obtained in Example 1 revealed inclusion of 0.70% of C4 hydrocarbons. The hexafluorodisilane gas was washed by passing itthrough a washing bottle, which had a ball filter and contained a selected high-boiling point solvent, at a flow rate of8 ml/min. Bythistreatment the C4 hydrocarbons were completely removed to the extent of being undetactable by gas chromatography analysis. As the high-boiling point solvent liquid paraffin (b.p. about 360DC), n-decane (b.p. 1 740C) and diphenyl ether (b.p. 2590C) were alternately used. In every case the resu It was excellent as mentioned above.

Examples 2-5 In these examples the entire process of Example 1 was repeated exceptthatthe mixing ratio of n-butyl ether to n-decane in the liquid reaction medium was varied as shown in the following table. The yield of hexafluorodisilane on silicon basis in each example was as shown in the same table.

n-butyletherln-decane yield (byvolume) (O/oJ Example2 2/100 47 Example3 10/100 75 Example 4 40/100 74 Example 5 100/100 50 Example 6 To prepare octafluorotrisilane, 0.400 mol of octachlorotrisilane was reacted with anhydrous zinc fluoride by the same method and under the same conditions as in Example 1, except that the trap to separatethe liquid reaction mediu m from the reaction gas was kept coo led at 30"C and that the trap to recover the product was kept cooled at -30 C. As the result 1.82 NI (0.0825 mol) of octafluorotrisilane was obtained, so thatthe yield on silicon basis was about 72%.

Claims (18)

1. A method of preparing a perfluorosilane by reaction between a perchlorosilane and a metal fluoride, characterized in that the reaction is carried out in a nonaqueous liquid medium.

2. A method according to Claim 1, wherein said nonaqueous liquid medium is selected from n-butyl ether, diethyl ether and ethylene glycol dimethyl ether.

3. A method according to Claim 2, wherein said metal fluoride is selected from anhydrous zincfluoride and anhydrous antimonytrifluoride.

4. A method according to Claim 1,2 or 3, wherein a saturated hydrocarbon is added to said liquid medium.

5. A method according to Claim 4, wherein said saturated hydrocarbon is selected from n-decane, n-dodecane and n-tetradecane,

6. A method according to Claim 5, wherein the volume ratio of said liquid medium to said saturated hydrocarbon is in the rangefrom 2:100to 100:100.

7. A method according to Claim 6, wherein said volume ratio is in the range from 7:100 to 50:100.

8. A method according to Claim 1, wherein said perfluorosilane is hexafluorodisilane, said perchlorosilane being hexachlorodisilane.

9. A method according to Claim 8, wherein said metal fluoride is anhydrous zinc fluoride and said liquid medium is n-butyl ether.

10. A method according to Claim 9, wherein a saturated hydrocarbon selected from n-decane, n-dodecane and n-tetradecane is added to said n-butyl ether, the volume ratio of said n-butyl ether to said saturated hydrocarbon being in the rangefrom 7:100 to 50:100.

11. A method according to Claim 1, wherein said perfluorosilane is octafluorotrisilane, said perchlorosilane being octachlorotrisilane.

12. A method according to Claim 11,wherein said metal fluoride is anhydrous zinc fluoride and said liquid medium is n-butyl ether.

13. A method according to Claim 12, wherein a saturated hydrocarbon selected from n-decane, n-dodecane and n-tetradecane is added to said n-butyl ether, the volume ratio of said n-butyl ether to said saturated hydrocarbon is being in the range from 7:100 to 50:100.

14. A method according to any one of the preceding claims, wherein the reaction is carried out at a temperature in the range from 50to 1500C.

15. A method according to any one of the preceding claims, further comprising the step of washing the obtained perfluorosilane with an organic solvent having a relatively high boiling point for removing possibly coexisting hydrocarbons.

16. A method according to Claim 15, wherein said organic solvent is selected from liquid paraffin, n-decane, n-butyl ether and diphenyl ether.

17. A method of preparing hexafluorodisilane, substantially as hereinabove described in any one of Examples 1 to 5.

18. A method of preparing octafluorotrisilane, substantially as hereinbefore described in Example 6.