CN1268592C - Method and Application of Purifying Tetrafluoromethane - Google Patents

Abstract

Tetrafluoromethane containing an ethylene compound, a hydrocarbon compound, carbon monoxide and/or carbon dioxide is contacted with a zeolite having an average pore diameter of 3.4 to 11 angstroms and a Si/Al ratio of 1.5 or less and/or a carbonaceous adsorbent having an average pore diameter of 3.4 to 11 angstroms. Therefore, the high-purity tetrafluoromethane which is beneficial to industrial production and has good benefits can be obtained.

Description

Method and Application of Purifying Tetrafluoromethane

Cross References to Related Applications

This application is an application filed pursuant to 35 U.S.C. Section 111(a) required by 35 U.S.C. Section 119(e)(1) on July 9, 2000 pursuant to 35 U.S.C. Section 111(b) Benefit from the filing date of provisional application 60/230,704.

technical field

The present invention relates to a method of purifying tetrafluoromethane (hereinafter may be referred to as "FC-14" or "CF ₄ ") and the use of the purified tetrafluoromethane.

Background technique

FC-14 is used, for example, as an etching gas or a cleaning gas in the manufacture of semiconductor devices, and thus a high-purity product thereof is required.

In order to produce FC-14, various methods have been proposed so far. Specifically, the following methods are known, for example:

(1) A method of reacting dichlorodifluoromethane and hydrogen fluoride in the presence of a catalyst;

(2) A method of reacting chlorotrifluoromethane and hydrogen fluoride in the presence of a catalyst;

(3) A method of reacting trifluoromethane with fluorine gas;

(4) A method of reacting carbon with fluorine gas; and

(5) A method of thermally decomposing tetrafluoroethylene.

However, these methods for producing FC-14 have a problem that intermediates or by-products of FC-14 produced by the reaction or impurities derived from raw materials form an azeotrope or an azeotrope-like mixture with the target product FC-14 and Its separation is extremely difficult. In order to solve this problem, for example, it has been proposed to treat FC-14 containing trifluoromethane (CHF ₃ ) as an impurity with zeolite or a carbonaceous adsorbent (see Japanese Patent No. 2,924,660).

invention disclosure

However, so far there is no FC-14 suitable for industrial production that can purify ethylene compounds, hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities, so that it has good benefits, and at the same time obtains high-purity FC-14 method.

The present invention is produced under the above circumstances, and the purpose of the present invention is to provide a purification method, wherein FC-14 is contacted with an adsorbent to remove the above-mentioned impurities by adsorption, and can be industrialized and obtain high-purity FC-14 with good benefits. 14.

In order to solve the above-mentioned problems, the present inventors conducted extensive studies and found that in the process for producing high-purity FC-14, when FC-14 containing ethylene compounds, hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities is combined with When an adsorbent of zeolite with an average pore size and a specific Si/Al ratio and/or a carbon-containing adsorbent (Molecular Sieving Carbon) with a specific average pore size are contacted, impurities can be selectively adsorbed and removed and almost no High-purity FC-14 containing impurities. The present invention has been accomplished based on this finding.

Therefore, the present invention provides the method of purifying tetrafluoromethane and the use of the purified tetrafluoromethane as described in (1)-(15) below.

(1) A method for purifying tetrafluoromethane, comprising making tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities with an average pore diameter of 3.4-11 angstroms and a zeolite with a Si/Al ratio of 1.5 or less and/or a carbonaceous adsorbent with an average pore size of 3.4-11 Angstroms are contacted to reduce the amount of said impurities.

(2) The method as described in (1) above, wherein the tetrafluoromethane containing the impurity is contacted with the zeolite and/or the carbon-containing adsorbent in a liquid phase.

(3) The method as described in (1) or (2) above, wherein the zeolite is at least one selected from MS-4A, MS-5A, MS-10X and MS-13X.

(4) The method as described in (1) or (2) above, wherein the carbon-containing adsorbent is molecular sieve carbon 4A and/or molecular sieve carbon 5A.

(5) The method as described in any one of (1)-(4) above, wherein one or more vinyl compounds are selected from ethylene, vinyl fluoride, vinyl difluoride and tetrafluoroethylene.

(6) The method as described in (5) above, wherein the one or more vinyl compounds are ethylene and/or tetrafluoroethylene.

(7) The method as described in any one of (1) to (4) above, wherein one or more hydrocarbon compounds are selected from methane, ethane and propane.

(8) The method as described in (7) above, wherein the one or more hydrocarbon compounds are methane and/or ethane.

(9) The method as described in any one of the above (1)-(8), wherein one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and carbon dioxide are mixed in the tetrafluoromethane The total content is reduced to 3ppm or less.

(10) The method as described in any one of (1)-(9) above, wherein tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities It is produced by the direct fluorination method of reacting trifluoromethane with fluorine gas.

(11) The method as described in any one of (1)-(9) above, wherein tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities Produced by the direct fluorination method of reacting carbon with fluorine gas.

(12) A tetrafluoromethane product having a purity of 99.9997% by mass or higher obtained by purification according to the method described in any one of (1)-(11) above.

(13) An etching gas comprising the tetrafluoromethane product as described in (12) above.

(14) A cleaning gas comprising the tetrafluoromethane product as described in (12) above.

Briefly, the present invention provides "a method of purifying tetrafluoromethane comprising combining tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities with an average A zeolite with a pore size of 3.4-11 angstroms and a Si/Al ratio of 1.5 or less and/or a carbon-containing adsorbent with an average pore size of 3.4-11 angstroms is contacted to reduce the amount of said impurities", "a method by A tetrafluoromethane product having a purity of 99.9997% by mass or higher obtained by performing the purification" and "an etching gas and a cleaning gas comprising the aforementioned tetrafluoromethane product".

Best Mode for Carrying Out the Invention

For the production of FC-14, for example, a method of reacting trifluoromethane with fluorine gas, a method of reacting carbon with fluorine gas and a method of thermally decomposing tetrafluoroethylene are known. When using these methods, the resulting FC-14 contains one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as an impurity.

The ethylene compound contained may be one or more selected from ethylene (CH ₂ =CH ₂ ), monofluoroethylene (CH ₂ =CHF), difluoroethylene (CH ₂ =CF ₂ ) and tetrafluoroethylene (CF ₂ = CF ₂ ).

The hydrocarbon compound contained may be one or more compounds selected from methane (CH ₄ ), ethane (C ₂ H ₆ ) and propane (C ₃ H ₈ ).

The boiling points of the target product FC-14 and these impurities at normal pressure are shown in Table 1 below.

Table 1 Compound name chemical formula Boiling point (°C) carbon monoxide CO -191.5 methane CH ₄ -161.5 FC-14 CF ₄ -128 Vinyl CH ₂ =CH ₂ -103.7 ethane CH ₃ CH ₃ -89 Vinyl difluoride CH ₂ =CF ₂ -83 carbon dioxide CO ₂ -78.5 Tetrafluoroethylene CF ₂ =CF ₂ -76.3 vinyl fluoride _CH2 =CHF -72 propane _{CH2CH2CH3 _ _} -42.1

Since the target product FC-14 forms an azeotrope-like mixture with these impurities or, as shown in Table 1, their boiling points are close, these impurities are very difficult to separate by distillation. In order to solve this problem, in the conventional distillation operation, the impurity content is reduced as much as possible by increasing the number of distillation towers or increasing the number of distillation towers, but this is not cost-effective, and it is almost impossible to obtain High-purity FC-14 with no impurities.

In the present invention, in order to selectively adsorb and remove these impurities in FC-14, a zeolite having an average pore diameter of 3.4-11 angstroms and a Si/Al ratio of 1.5 or less and/or a zeolite having an average pore diameter of 3.4-11 angstroms are used. A carbon-containing adsorbent (molecular sieve carbon) was used as the adsorbent. In order to measure the average pore diameter, a gas adsorption method using Ar gas can be utilized.

Thus, the adsorbent is (1) a zeolite with an average pore diameter of 3.4-11 angstroms and a Si/Al ratio of 1.5 or less, (2) a carbon-containing adsorbent (molecular sieve carbon) with an average pore diameter of 3.4-11 angstroms, or (3) An adsorbent obtained by adding a carbon-containing adsorbent having an average pore diameter of 3.4-11 angstroms to a zeolite having an average pore diameter of 3.4-11 angstroms and a Si/Al ratio of 1.5 or less. As used herein, the Si/Al ratio is an atomic ratio.

Specific examples of impurities in FC-14 that can be removed by using these adsorbents may be unsaturated compounds such as ethylene, monofluoroethylene, difluoroethylene, and tetrafluoroethylene, hydrocarbon compounds such as methane, ethane, and propane, and oxygen-containing compounds such as carbon monoxide and carbon dioxide. Preferred as impurities are ethylene, tetrafluoroethylene, methane, ethane, carbon monoxide and carbon dioxide, more preferred are ethylene and ethane.

The difference in molecular size between the target product FC-14 and these impurities is small, so the selective adsorption and removal of these impurities in FC-14 can hardly be achieved only by the difference in molecular size. In the present invention, the following three types of adsorbents are used as adsorbents that can selectively adsorb and remove these impurities by considering the polarity and pore size of the adsorbent.

The first adsorbent is a zeolite with an average pore size of 3.4-11 Angstroms and a Si/Al ratio of 1.5 or less. Specific examples thereof include MS-4A. MS-4A has an average pore size of about 3.5 Angstroms and a Si/Al ratio of 1.0. By performing an adsorption operation using this zeolite, the content of ethylene, tetrafluoroethylene, methane, ethane, carbon monoxide, and carbon dioxide as impurities can be reduced. Depending on the type of zeolite, it is even possible to reduce the impurity content to 5 ppm or less, so that high-purity FC-14 can be obtained.

If a zeolite with an average pore diameter of less than 3.4 angstroms, for example about 3.2 angstroms, is used, the reduction of the impurity content cannot be ensured even if the Si/Al ratio is 1.5 or less.

Even if the Si/Al ratio is 1.5 or less, if the average pore diameter of the zeolite is larger than 11 angstroms, the reduction of the impurity content cannot be ensured.

Furthermore, even if the average pore diameter is 3.4-11 angstroms, if the Si/Al ratio of the zeolite exceeds 1.5, the reduction of the impurity content cannot be ensured.

The second adsorbent is a carbonaceous adsorbent (molecular sieve carbon) with an average pore size of 3.4-11 Angstroms. For example, a carbon-containing adsorbent having an average pore diameter of about 4 angstroms can also reduce the impurity content to 5 ppm or less like the above-mentioned zeolite, so that high-purity FC-14 can be obtained.

However, if the average pore diameter of the carbon-containing adsorbent exceeds 11 angstroms, the reduction of the impurity content cannot be ensured. For example, if activated carbon with an average pore diameter of about 35 angstroms, which is generally used and exhibits strong adsorption activity, is used, the reduction of the impurity content can hardly be ensured. reduce.

The third adsorbent is obtained by adding (blending) a carbonaceous adsorbent (second adsorbent) having an average pore diameter of 3.4-11 angstroms to a carbonaceous adsorbent having an average pore diameter of 3.4-11 angstroms and a Si/Al ratio of preferably 1.5 or less Adsorbent obtained from zeolite (first adsorbent). Depending on the kind of the adsorbent, the impurity content can be reduced to even 3 ppm or less, so that higher purity FC-14 can be obtained. The reason for this result is considered to be that zeolite has excellent adsorption especially for carbon monoxide, carbon dioxide, etc., and carbonaceous adsorbent has excellent adsorption especially for unsaturated compounds, etc., and when these two adsorbents are combined When used, brings about the effect caused by this combined use.

The above-mentioned zeolite and carbon-containing adsorbent may be used alone, but two or more adsorbents may also be used in combination in a desired ratio. When using the third adsorbent, the mixing ratio between the zeolite and the carbonaceous adsorbent can be changed according to the concentration of impurities.

The concentration of ethylene compounds, hydrocarbon compounds, carbon monoxide and/or carbon dioxide contained as impurities in FC-14 is not particularly limited, but the concentration is preferably 0.1% by mass or less, more preferably 0.05% by mass or less.

If the target product FC-14 is mixed with impurities other than the above-mentioned impurities, such as perfluorinated compounds such as FC-116 (CF ₃ CF ₃ ) and FC-218 (C ₃ F ₈ ), these perfluorinated compounds can be passed through in-use Distillation operation is performed before or after the above-mentioned adsorbent treatment step for separation and removal.

In the method for purifying FC-14 according to the present invention, the method of contacting FC-14 containing impurities with the adsorbent is not limited, for example, the FC-14 containing impurities can be contacted with the adsorbent in the gas phase, through gas- liquid contact or contact in the liquid phase. Among them, the method of bringing impurity-containing FC-14 into contact with an adsorbent in a liquid phase is effective and preferable.

In order to bring impurity-containing FC-14 into contact with an adsorbent in a liquid phase, a known method such as a batch system or a continuous system can be used, but an adsorption tower of, for example, a fixed-bed type provided with two unit, and when one unit reaches its saturated adsorption limit, the other unit is used and the first unit is regenerated.

When impurity-containing FC-14 is in contact with the adsorbent, the treatment temperature, treatment amount and treatment pressure are not particularly limited, but the treatment temperature is preferably low temperature, suitably -50°C to +50°C. In the case of the liquid phase, the treatment pressure is sufficient as long as it can maintain the liquid phase, and in the case of the gas phase, the treatment pressure is not particularly limited.

As described above, by using the purification method of the present invention, ethylene compounds, hydrocarbon compounds, carbon monoxide and/or carbon dioxide contained in FC-14 can be efficiently adsorbed and removed, whereby high-purity FC-14 can be obtained. The resulting FC-14 has a purity of 99.9997% by mass or higher, and in order to analyze the FC-14 product having a purity of 99.9997% by mass or higher, it is possible to use (1) a TCD method, a FID method (each including a pre-cut method) or Gas chromatography (GC) of the ECD method, or (2) an analytical instrument such as a gas chromatography mass spectrometer (GC-MS).

The resulting high-purity FC-14 can be used as an etching gas in an etching step in a method for producing a semiconductor device. In addition, high-purity FC-14 can be used as a cleaning gas in a cleaning step in a method of producing a semiconductor device. In the production method of semiconductor devices such as LSI and TFT, a thin or thick film is formed using a CVD method, a sputtering method or a vapor deposition method and the film is etched to form a circuit pattern. In the equipment for forming said thin or thick film, cleaning is performed to remove unnecessary deposits accumulated on the inner walls of the equipment, jigs, etc., because unnecessary deposits cause particles and must be removed from time to time Produce high quality membranes.

In etching by using FC-14, etching can be performed under various dry etching conditions such as plasma etching and microwave etching, and FC-14 can be mixed with an inert gas such as He, N ₂ and Ar in an appropriate ratio or mixed with Gases such as HCl, _O2 and _H2 are used in mixture.

The present invention will be further described below by referring to examples and comparative examples, but the present invention is not limited to these examples.

Raw material embodiment 1 of FC-14

The carbon is reacted with fluorine in the presence of a diluent gas, unreacted fluorine is removed and the FC-14 enriched product gas is purified by fractional distillation according to conventional methods. The product gas was then analyzed by gas chromatography, as a result of which the obtained FC-14 had the composition shown in Table 2 below.

Table 2 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9688 carbon monoxide CO 0.0006 carbon dioxide CO ₂ 0.0056 methane CH ₄ 0.0012 Vinyl CH ₂ =CH ₂ 0.0112 Tetrafluoroethylene CF ₂ =CF ₂ 0.0028 ethane CH ₃ CH ₃ 0.0098

Raw material embodiment 2 of FC-14

Difluoromethane (CH ₂ F ₂ ) is reacted with fluorine gas in the presence of diluent gas, and then the reacted gas is introduced into an alkaline cleaning tower to remove generated hydrogen fluoride and a slight amount of unreacted fluorine gas. The FC-14-enriched product gas was purified by fractional distillation according to known methods and analyzed by gas chromatography, with the result that the resulting FC-14 had the composition shown in Table 3 below.

table 3 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9722 carbon monoxide CO 0.0005 carbon dioxide CO ₂ 0.0025 methane CH ₄ 0.0004 vinyl fluoride _CH2 =CHF 0.0056 Vinyl difluoride CH ₂ =CF ₂ 0.0038 Tetrafluoroethylene CF ₂ =CF ₂ 0.0108 Trifluoromethane CHF ₃ 0.0042

Example 1

20 g of zeolite (molecular sieve 4A, produced by Union Showa K.K., average pore diameter: 3.5 angstroms, Si/Al ratio: 1) was filled into a 200 ml stainless steel cylinder and vacuum-dried, and then about 70 g of raw materials were added while cooling the cylinder. 1 of FC-14, stirring the contents occasionally while maintaining the temperature at -20 °C. After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 4 below.

Table 4 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9992 carbon monoxide CO <0.0001 carbon dioxide CO ₂ <0.0001 methane CH ₄ <0.0001 Vinyl CH ₂ =CH ₂ <0.0001 Tetrafluoroethylene CF ₂ =CF ₂ 0.0003 ethane CH ₃ CH ₃ 0.0001

From the results in Table 4, it can be seen that by using zeolite with an average pore diameter of 3.5 angstroms and a Si/Al ratio of 1 as an adsorbent, the amount of impurities in FC-14 can be reduced and the impurity content can be reduced to 10 ppm or less.

Example 2

20 g of zeolite (molecular sieve 13X, produced by Union Showa K.K., average pore diameter: 10 angstroms, Si/Al ratio: 1.23) was filled into a 200 ml stainless steel cylinder and vacuum-dried, and then about 70 g of raw materials were added while cooling the cylinder. 1 of FC-14, stirring the contents occasionally at room temperature (about 18°C). After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 5 below.

table 5 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9991 carbon monoxide CO <0.0001 carbon dioxide CO ₂ <0.0001 methane CH ₄ <0.0001 Vinyl CH ₂ =CH ₂ 0.0003 Tetrafluoroethylene CF ₂ =CF ₂ 0.0002 ethane CH ₃ CH ₃ 0.0001

From the results in Table 5, it can be seen that by using zeolite with an average pore size of 10 angstroms and a Si/Al ratio of 1.23 as an adsorbent, the amount of impurities in FC-14 can be reduced and the impurity content can be reduced to 10 ppm or less.

Example 3

20 g of a carbon-containing adsorbent (molecular sieve carbon, produced by Takeda Yakuhin Kogyo K.K., average pore size: 4 angstroms) was filled into a 200 ml stainless steel cylinder and vacuum-dried, and then about 70 g of FC of Raw Material Example 2 was added while cooling the cylinder -14, Stir the contents occasionally at room temperature (about 18°C). After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 6 below.

Table 6 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9992 carbon monoxide CO 0.0001 carbon dioxide CO ₂ 0.0002 methane CH ₄ 0.0001 vinyl fluoride _CH2 =CHF <0.0001 Vinyl difluoride CH ₂ =CF ₂ <0.0001 Tetrafluoroethylene CF ₂ =CF ₂ <0.0001 trifluoromethane CHF ₃ 0.0001

As can be seen from the results in Table 6, by using a carbon-containing adsorbent (molecular sieve carbon) with an average pore diameter of 4 angstroms as the adsorbent, the amount of impurities in FC-14 can be reduced and the impurity content can be reduced to 10 ppm or lower.

Example 4

15 g of zeolite (molecular sieve 4A, produced by Union Showa K.K., average pore diameter: 3.5 angstroms, Si/Al ratio: 1 ) into a 200ml stainless steel cylinder and vacuum-dried, then add about 70g of FC-14 of Raw Material Example 1 while cooling the cylinder, and stir the contents from time to time at room temperature (about 18°C). After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 7 below.

Table 7 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9994 carbon monoxide CO <0.0001 carbon dioxide CO ₂ <0.0001 methane CH ₄ <0.0001 Vinyl CH ₂ =CH ₂ <0.0001 Tetrafluoroethylene CF ₂ =CF ₂ <0.0001 ethane CH ₃ CH ₃ <0.0001

To determine the content of trace impurities, microanalysis by gas chromatography using the TCD method, the FID method (including the pre-cut method) or the ECD method or microanalysis by an analytical instrument such as a gas chromatography mass spectrometer (GC/MS), and Purity was calculated from the obtained value. The results are shown in Table 8.

Table 8 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9998 carbon monoxide CO ＜0.4ppm carbon dioxide CO ₂ ＜0.4ppm methane CH ₄ <0.3ppm Vinyl CH ₂ =CH ₂ <0.1ppm Tetrafluoroethylene CF ₂ =CF ₂ <0.2ppm ethane CH ₃ CH ₃ <0.2ppm

As can be seen from the results in Table 8, the purity of the obtained FC-14 was 99.9997% by mass or higher.

Comparative example 1

20 g of zeolite (molecular sieve XH-9, produced by Union Showa K.K., average pore diameter: 3.2 angstroms, Si/Al ratio: 1) was filled into a 200 ml stainless steel cylinder and vacuum-dried, and then about 70 g of the raw material was added while cooling the cylinder For FC-14 of Example 1, the contents were stirred occasionally at room temperature (about 18°C). After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 9 below.

Table 9 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9698 carbon monoxide CO 0.0004 carbon dioxide CO ₂ 0.0051 methane CH ₄ 0.0012 Vinyl CH ₂ =CH ₂ 0.0111 Tetrafluoroethylene CF ₂ =CF ₂ 0.0027 ethane CH ₃ CH ₃ 0.0097

From the results in Table 9, it can be seen that even if the Si/Al ratio is 1, the impurity content hardly decreases if the average pore size of the zeolite is less than 3.4 angstroms.

Comparative example 2

20 g of zeolite (H-ZSM-5, produced by N.E. Chemcat K.K., average pore diameter: 6 angstroms, Si/Al ratio: 75) was filled into a 200 ml stainless steel cylinder and vacuum-dried, and then about 70 g was added while cooling the cylinder For FC-14 of Raw Material Example 1, the contents were stirred occasionally at room temperature (about 18°C). After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 10 below.

Table 10 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9733 carbon monoxide CO 0.0003 carbon dioxide CO ₂ 0.0047 methane CH ₄ 0.0009 Vinyl CH ₂ =CH ₂ 0.0098 Tetrafluoroethylene CF ₂ =CF ₂ 0.0021 ethane CH ₃ CH ₃ 0.0089

From the results in Table 10, it can be seen that even if the average pore diameter is 6 angstroms, if the Si/Al ratio of the zeolite exceeds 1.5, the impurity content can hardly be reduced.

Comparative example 3

20 g of a carbon-containing adsorbent (activated carbon, granular SHIROSAGI KL, produced by Takeda Yakuhin Kygyo K.K., average pore size: 35 angstroms) was filled into a 200 ml stainless steel cylinder and vacuum-dried, and then about 70 g of raw materials were added while cooling the cylinder. 2 of FC-14, stirring the contents occasionally at room temperature (about 18°C). After about 8 hours, the liquid fraction was analyzed by gas chromatography. The results of the analysis are shown in Table 11 below.

Table 11 Compound name chemical formula Purity (mass%) FC-14 CF ₄ 99.9795 carbon monoxide CO 0.0004 carbon dioxide CO ₂ 0.0021 methane CH ₄ 0.0003 Vinyl CH ₂ =CH ₂ 0.0038 Vinyl difluoride CH ₂ =CF ₂ 0.0026 Tetrafluoroethylene CF ₂ =CF ₂ 0.0079 Trifluoromethane CHF ₃ 0.0034

From the results in Table 11, it can be seen that if a carbon-containing adsorbent with an average pore diameter exceeding 11 angstroms is used, the content of impurities cannot be reduced.

Industrial Applicability

According to the present invention, it is possible to reduce the amount of impurities contained in tetrafluoromethane which have hitherto been very difficult to remove, especially ethylene compounds, hydrocarbon compounds, carbon monoxide and/or carbon dioxide. Purified high-purity tetrafluoromethane can be used as etching gas or cleaning gas.

Claims (11)

1. A method for purifying tetrafluoromethane, comprising making tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities with an average pore diameter of 3.4-11 angstroms and A zeolite having a Si/Al ratio of 1.5 or less is contacted with a carbonaceous adsorbent having an average pore size of 3.4-11 Angstroms to reduce the amount of said impurities. 2. The method of claim 1, wherein tetrafluoromethane containing said impurity is contacted with said zeolite and said carbonaceous adsorbent in a liquid phase. 3. The method according to claim 1 or 2, wherein the zeolite is at least one selected from MS-4A, MS-5A, MS-10X and MS-13X. 4. The method according to claim 1 or 2, wherein the carbon-containing adsorbent is molecular sieve carbon 4A and/or molecular sieve carbon 5A. 5. The method of claim 1 or 2, wherein the one or more vinyl compounds are selected from the group consisting of ethylene, vinyl fluoride, vinyl difluoride and tetrafluoroethylene. 6. The method of claim 5, wherein the one or more ethylene compounds are ethylene and/or tetrafluoroethylene. 7. The method of claim 1 or 2, wherein the one or more hydrocarbon compounds are selected from methane, ethane and propane. 8. The method of claim 7, wherein the one or more hydrocarbon compounds are methane and/or ethane. 9. The method of claim 1 or 2, wherein the total content of one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and carbon dioxide in the tetrafluoromethane is reduced to 3 ppm or less . 10. The method as claimed in claim 1 or 2, wherein the tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities is passed through trifluoromethane and fluorine It is produced by the direct fluorination method of the reaction. 11. The method as claimed in claim 1 or 2, wherein the tetrafluoromethane containing one or more ethylene compounds, one or more hydrocarbon compounds, carbon monoxide and/or carbon dioxide as impurities is obtained by reacting carbon with fluorine Produced by direct fluorination method.