Process for producing silicon tetrafluoride

Details Process for producing silicon tetrafluoride Process for producing silicon tetrafluoride

2002-09-10

2004-08-03

Nguyen, Ngoc-Yen (Department: 1754)

Chemistry of inorganic compounds

Silicon or compound thereof

Halogen containing

C423S342000, C423S489000, C423S344000, C423S24000R

Reexamination Certificate

active

06770253

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for producing silicon tetrafluoride of high purity, which is used in the fields of electronics, optics and the like.
Silicon tetrafluoride (SiF
4
) is used, for example, as a fluorine-doping agent of quartz-based optical fibers, as a raw material of photomask materials for semiconductor lithography, and as a chemical vapor deposition gas for producing semiconductors. Silicon tetrafluoride has increasingly been used in recent years. Therefore, there is a demand for more efficient techniques for producing silicon tetrafluoride of high purity.
There are several conventional processes for producing silicon tetrafluoride, as follows. Japanese Patent Unexamined Publication JP-A-63-74910 discloses a first process for producing silicon tetrafluoride by pyrolyzing a metal silicofluoride (e.g., sodium silicofluoride). Japanese Patent Unexamined Publication JP-A-57-17414 discloses a second process for producing silicon tetrafluoride by introducing hydrogen fluoride gas into a system where amorphous silicon dioxide has been dispersed in sulfuric acid. Japanese Patent Unexamined Publication JP-A-7-81903 discloses a third process for producing silicon tetrafluoride by reacting fluorine with a mixture of calcium fluoride and silicon powder at 140° C. Of these processes, the third process is superior to the first and second processes with respect to reducing the amount of industrial wastes. In fact, the third process does almost not generate wastes. In contrast, the first and second processes respectively generate large amounts of a metal fluoride (e.g. sodium fluoride) and sulfuric acid as industrial wastes. Prior to conducting the third process, it is necessary to produce fluorine by electrolyzing hydrogen fluoride. Due to this, the third process is disadvantageous in terms of reducing production cost and energy consumption.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for economically producing silicon tetrafluoride of high purity.
According to the present invention, there is provided a process for producing silicon tetrafluoride. This process comprises the step of (a) reacting at a temperature of 250° C. or higher elemental silicon in the form of solid with hydrogen fluoride in the form of gas, thereby producing a gas product comprising the silicon tetrafluoride
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In course of achieving the present invention, the present inventors have studied the use of hydrogen fluoride, which is superior to fluorine, as the fluorine source in producing silicon tetrafluoride from silicon. In fact, hydrogen fluoride is superior to fluorine by the following points. Firstly, hydrogen fluoride itself is a raw material for producing fluorine by electrolysis. Therefore, the use of hydrogen fluoride is naturally lower than that of fluorine in cost. Secondly, hydrogen fluoride is lower than fluorine in chemical potential. Therefore, a reaction of hydrogen fluoride with silicon is considered to generate a less heat than that of a reaction of fluorine with silicon. Thus, it is easier to conduct a thermal control of the former reaction as compared with the latter reaction. Thirdly, hydrogen fluoride has a boiling point of 20° C., thereby making it possible to store hydrogen fluoride in the form of liquid under normal temperature below its boiling point. In contrast, fluorine is in the form of gas under normal temperature. Therefore, the handling of hydrogen fluoride is much easier than that of fluorine.
In reacting silicon with hydrogen fluoride, it can be understood in theory that any of the following three reactions (1)-(3) can occur since &Dgr;G of each reaction has a negative value.
Si (solid)+4HF (gas)→SiF
4
(gas)+2H
2
(gas)  (1)
Si (solid)+3HF (gas)→SiHF
3
(gas)+H
2
(gas)  (2)
Si (solid)+2HF (gas)→SiH
2
F
2
(gas)  (3)
The present inventors, however, unexpectedly found that, when silicon in the form of solid is actually reacted with gaseous hydrogen fluoride, the above reaction (1) occurs dominantly, thereby efficiently producing silicon tetrafluoride.
Although the above reaction (1) does almost not occur at a temperature around room temperature, it does occur as the temperature increases. In fact, the reaction rate increases rapidly as it passes about 250° C. The reaction rate further increases as the temperature increases further. The inventors found that the above reaction (1) occurs dominantly in a temperature range of from 300° C. to a temperature higher than 1,000° C. by directly reacting silicon with hydrogen fluoride gas.
By a further detailed examination on the reaction between silicon and hydrogen fluoride, the inventors found that the above reaction (2) also occurs in some cases, although the reaction (1) is a dominant reaction. In these cases, the gas product naturally contains SiHF
3
. As the reaction (1) proceeds, H
2
is formed as a by-product in an amount two times that of SiF
4
in molar number, thereby making the reaction system under a strongly reduced atmosphere. The inventors assumed that SiHF
3
would be formed when the amount of HF is insufficient under the H
2
atmosphere. Thus, the inventors have tried to suppress the formation of SiHF
3
by leaving a part of HF (the unreacted HF) in the resulting gas product. With this, the inventors unexpectedly found that SiHF
3
is not formed when the gas product contains HF in an amount of at least 0.02 volume %, preferably at least 0.05 volume %. In other words, according to a first process of a first aspect of the present invention, the reaction is conducted in a manner that the gas product contains at least 0.02 volume % of the unreacted HF, in order to produce silicon tetrafluoride of high purity. Although the upper limit of the HF content of the gas product is not particularly limited, it is preferably 1 volume % or lower in order to conduct a post-treatment for removing HF in an economical way.
In order to control the HF concentration of the gas product in accordance with the first aspect of the present invention, it is preferable to use a so-called perfect mixing type reactor. With this, it becomes possible to obtain a uniform composition of the gas phase through a compulsory stirring of the inside of the reactor. In fact, a large amount of HF gas always exists around Si particles when the gas phase of the reactor is under a perfectly mixed condition. With this, the gas phase HF concentration is less influenced, even if the flow rate of HF gas supplied to the reactor or the reaction temperature fluctuates by certain degrees.
When the gas product contains HF in an amount of 0.02 volume % or higher, such HF can be removed from the gas product by a post-treatment. This removal can be conducted by a purification of the gas product. It is possible to conduct such post-treatment, for example, by bringing the gas product (containing HF) into contact with NaF (in the form of pellets), thereby fixing HF in the form of NaF.HF. It is possible to conduct the post-treatment in an economical way, if the HF concentration of the gas product is in a range of 0.02-1 volume % or 0.05-1 volume %.
When the gas product contains no HF or a very small amount of HF, the gas product may contain SiHF
3
as a by-product. The SiHF
3
concentration of the gas product becomes higher as the HF concentration of the gas product becomes lower. For example, when the gas product contains no HF, the SiHF
3
concentration may reach about 1 volume %. Such condition can be obtained, for example, by conducting a reaction in a manner that HF gas is introduced in a so-called piston flow manner into one end of a cylindrical reactor charged with silicon. In fact, the HF gas introduced into the reactor is consumed by the reaction with silicon as it passes through a fixed bed of Si. Therefore, the HF concentration of the gas phase becomes zero as the HF gas introduced in a piston flow manner reaches the exit of the reactor, provided that silicon exists in a suffi

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