JP2000297393A - Production of high-purity gaseous nitrogen trifluoride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To greatly decrease the gaseous impurities to be formed and to enhance the purity of formed gases by controlling the quantity of carbon which is the gaseous impurity component to be entrained in crude gases among the impurities in a nickel electrode used for an anode. SOLUTION: The gaseous trifluoride is produced by a fused salt electrolysis method using the nickel electrode for the anode and acidic ammonium fluoride as an electrolyte. The quantity of the carbon among the impurity elements included in the nickel used for the anode is limited to <=400 mass ppm, more preferably <=200 mass ppm and further preferably <=100 mass ppm, by which the formation of the gaseous impurities occurring in the carbon element is suppressed. The purity of the nickel is preferably >=98.5 mass %. The restriction of the sulfur component in the nickel to <=20 mass ppm is preferable for the purpose of suppressing the formation of gaseous SF6. The total of C, B, Si, P, As, Mo, Ge and W which are the impurity components is preferably <=400 mass ppm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing high-purity nitrogen trifluoride (NF ₃ ) gas. More specifically, the present invention relates to a manufacturing method for industrially providing high-purity nitrogen trifluoride gas at low cost.

[0002]

2. Description of the Related Art Nitrogen trifluoride gas is used for electronic materials, in particular, as a dry etching gas in the manufacture of semiconductor devices, or a dry cleaning gas in a plasma CVD device, and in the field of liquid crystal display device manufacturing using TFTs. In recent years, attention has been paid to the cleaning of single-wafer type apparatuses, and the production amount has been remarkably growing. In recent years, NF ₃ gas used for these applications has been required to have higher and higher purity.

Hitherto, various methods have been proposed as a method for producing NF ₃ by molten salt electrolysis. For example, for an electrode to be used, a method using nickel as an anode is industrially used for impurities, for example, It is widely used because it hardly produces CF ₄ .

As a raw material used for performing molten salt electrolysis, ammonium acid fluoride is used, but ammonium acid fluoride commercially available as a reagent contains a large amount of ammonium hexafluorosilicate as an impurity. The one described in JP-A-4-56789, that is, one using an ammonium ammonium fluoride prepared from hydrofluoric acid and ammonia as a raw material is preferable in that it has a small amount of impurities. However, in recent years, with the advance of technology, demand for higher purity NF ₃ gas has been increasing, and it is necessary to further purify product gas.

[0005] As a method for performing the purification, a crude gas obtained by molten salt electrolysis (hereinafter, referred to as a crude gas) is adsorbed by a purifying apparatus such as zeolite, activated alumina, silica gel, etc. together with a carrier gas. There is a method of conducting purification by introducing it to one of apparatuses for performing chemical cleaning treatment, plasma decomposition, cryogenic separation, liquefaction rectification of gas, or a method of purifying by combining a plurality of these apparatuses.

The impurities in the crude gas include nitrous oxide (N ₂ ) except for the carrier gas component and water (H ₂ O).
O), carbon dioxide (CO ₂ ), carbon monoxide (CO), dinitrogen difluoride (N ₂ F ₂ ), oxygen difluoride (F ₂ O), sulfur hexafluoride (SF ₆ ), tetrafluoride Carbonized (CF ₄ ), etc., which contain a large amount of these impurities, so that purification is necessary, and the above-described purification apparatus is used to purify the crude gas.

In the case of using these refining devices, it is necessary to control the performance based on the content and dispersion of impurities. For example, in a device for performing an adsorption treatment, the adsorption speed, the renewal of the adsorbent and the frequency of regeneration are changed. It is necessary to change the parameters of various conditions, which requires labor, and the obtained purified gas also requires considerable frequency to investigate variations in product purity. Therefore, the cost is increased.

When industrially producing high purity NF ₃ gas of 4N (purity: 99.99% by volume), 5N (purity: 99.999% by volume) or more, the impurity level in the crude gas is reduced. That is, the impurity content becomes a problem. Purifying a crude gas containing many impurities to increase its purity and purify it is limited in terms of equipment and economy, and it is practically difficult to produce high-purity gas economically. It is.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to reduce impurities in a crude gas and to easily produce a high-purity nitrogen trifluoride gas of 4N or more by a conventionally known purification method and production apparatus. It provides a way to gain.

[0010]

In view of the above problems, the present inventors have studied the causes of the generation of impurities and found that the main cause of the generation of impurity components in nitrogen trifluoride gas is used as an electrode. By controlling the impurity content by using electrodes of a predetermined purity and by controlling the impurity content, it is possible to greatly reduce the generated impurity gas and easily produce high-purity NF ₃ gas. It was found that it was possible to manufacture the product.

That is, the present invention provides a method for producing nitrogen trifluoride gas by a molten salt electrolysis method using ammonium acid fluoride as an electrolytic solution by using a nickel electrode. A high-purity trifluoride, characterized in that the amount of carbon as an impurity gas component entrained in the crude gas is set to a content of 400 mass ppm or less, preferably 200 mass ppm or less, more preferably 100 mass ppm or less. The present invention relates to a method for producing nitrogen iodide gas.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Nickel, which is an electrode material used for producing NF ₃ , usually contains trace amounts of many impurity elements.

[0013] The applicant of the present invention has previously described Japanese Patent Application Laid-Open No. 8-134467.
No. 5 proposes a method for producing a high-purity NF ₃ gas in which the sulfur content in a nickel electrode is regulated to 20 ppm by mass or less to suppress the generation of SF _{6 in} a crude gas. or,
JP-A-8-120475 discloses that hydrofluoric acid and ammonia gas, which are raw materials for preparing ammonium acid fluoride, are each made of a high-purity raw material and have a purity of 9%.
A method of performing molten salt electrolysis using a nickel electrode of 8.5% by mass or more has been proposed.

As a result of further study, of the impurity elements contained in the nickel, the carbon content was reduced to 400 ppm by mass.
It has been found that by limiting to the following, generation of an impurity gas due to a carbon element can be suppressed. Preferably,
The carbon content is 200 ppm by mass or less, more preferably 100 ppm by mass or less.

[0015] The nickel purity of the electrode used for the anode is preferably 98.5% by mass or more. When the nickel purity is less than 98.5% by mass, the high purity NF having a purity of 4N or more
₃ It becomes difficult to obtain gas. Of course,
As disclosed in Japanese Patent Publication No. 134675, it is desirable to reduce the sulfur content in nickel to 20 ppm by mass or less in terms of suppressing the generation of SF ₆ gas. Impurity components C, B, S
The total of i, P, As, Mo, Ge and W is 400 mass p.
It is a preferred embodiment to use a nickel electrode of pm or less.

The raw material ammonium ammonium fluoride is not particularly limited, but one obtained from hydrofluoric acid gas and ammonia gas is economical and industrially preferable.
Particularly, as disclosed in JP-A-8-120475, a hydrofluoric acid gas having a purity of 99.8 mass% or more and a purity of 99.
It is desirable to use ammonium acid fluoride obtained by reacting with 5% by mass or more of ammonia gas because generation of impurity gas derived from raw materials can be suppressed. As the hydrofluoric acid gas, the above-mentioned purity can be obtained by gasifying general industrial grade hydrofluoric anhydride, and as the ammonia gas, the general industrial grade liquefied ammonia is gasified. A product of the above purity is obtained.

Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a flow sheet of a manufacturing process suitable for carrying out the present invention. In the figure, predetermined amounts of hydrofluoric acid gas (HF) and ammonia gas (NH ₃ ) are supplied to a raw material preparation tank to generate ammonium acid fluoride, and the ammonium acid fluoride is supplied to an electrolytic tank to supply molten salt. An NF ₃ gas is generated on the anode side by an electrolytic method. The raw material preparation tank is preferably sealed with an appropriate amount of nitrogen gas, argon gas, helium gas or the like so as not to be affected by the outside air.

The reaction between hydrofluoric acid gas and ammonia gas is as follows:
Because it proceeds very quickly, there is no need for particularly sufficient stirring, etc.
The raw material preparation tank is not particularly limited as long as it can contact hydrofluoric acid gas and ammonia gas without reacting them. Preferably, ETFE or PF
A metal tank lined with a fluororesin such as A is preferable. The reaction between the hydrofluoric acid gas and the ammonia gas is preferably performed so that the molar ratio of HF / NH ₄ F is in the range of 1.5 to 2.0.

FIG. 2 is a conceptual diagram showing an example of an electrolytic cell suitable for the present invention. In the figure, ammonium acid fluoride prepared in a raw material preparation tank is led to an electrolytic tank main body 1 to be used as an electrolytic solution 2. The supply of the electrolytic solution may be either a continuous type or a batch type, but a continuous type is preferable when a constant amount of NF ₃ gas is to be obtained continuously. When the electrolysis is started, NF ₃ gas is generated at the anode 4 and H ₂ gas is generated at the cathode 6. Since there is a danger of explosion when these gases are mixed, the electrolytic cell main body 1 is composed of the anode chamber 3 and the cathode chamber 5.
Are separated by a partition plate 7. The anode 4 is a nickel electrode as defined above, and a similar nickel electrode is attached as the cathode 6. Although FIG. 1 shows a configuration in which one anode 4 and one cathode 6 are provided, a plurality of these anodes and cathodes may be provided in one electrolytic cell. It is general to provide a plurality from the viewpoint of the above. Further, the cathode may be arranged on both sides of the anode.

As for the electrolytic cell main body 1, it is desirable to use the one whose inside is lined with a fluororesin similarly to the raw material mixing tank. Further, the main body 1 is provided with a temperature adjusting mechanism (not shown) for performing heating and cooling, and adjusts the temperature of the electrolytic solution at the time of molten salt electrolysis. In addition, the dissolution of the nickel anode generates nickel complex salt sludge inside the electrolytic cell, and the exchange frequency of the electrolytic solution may increase. For example, as disclosed in Japanese Patent Application Laid-Open No. 8-176873. Alternatively, a method of forcibly convection of the electrolytic solution may be applied, and such a convection mechanism may be provided in the electrolytic cell main body 1.

The conditions for the electrolysis are as follows.
While maintaining the temperature at about 10 to 140 ° C., a direct current is applied to the anode 4 and the cathode 6 in the electrolytic cell main body 1 to perform molten salt electrolysis. The electrolysis voltage is preferably 5 to 10 V, and the current density is preferably about 1 to 15 A / dm ² .

During the electrolysis, an inert gas such as a nitrogen gas, an argon gas, a helium gas or the like is used in order to make the electrolysis reaction proceed mildly and to maintain the pressure in the anode chamber 3 and the cathode chamber 5 as uniform as possible. As carrier gas, conduits 10 and 1 to anode chamber 3 and cathode chamber 5 respectively
Guide through 1. The purity of the carrier gas is preferably a purity that does not affect the purity of NF _3. According to the study of the present inventors, the purity is preferably 4 N or more, and most preferably 6 N or more. As the carrier gas, it is preferable to use nitrogen gas which is industrially inexpensive and can easily obtain a high-purity product.

The NF ₃ gas and the H ₂ gas generated at the electrodes are taken out together with the carrier gas from the individual conduits 8 and 9 so as not to mix. The crude NF ₃ gas taken out in the conduit 8 is led to a purification device. On the other hand, conduit 9
The H ₂ gas taken out of the system is released into the atmosphere via a detoxification device or the like.

High purity NF ₃ gas can be obtained by removing a trace amount of impurities from the crude NF ₃ gas led to the purification device. The purifying apparatus and the purifying method may be a purifying apparatus including a gas cleaning apparatus using a chemical liquid cleaning, an adsorption tower using an adsorbent, a rectification tower, and the like, which have been generally used.

[0026]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 Using a flow sheet and an electrolytic cell shown in FIGS.
NF by molten salt electrolysis _ThreeGas production was carried out.

First, industrial hydrofluoric anhydride (purity 99.8)
Mass% or more) is gasified to obtain hydrofluoric acid gas, and industrial liquefied ammonia (purity of 99.5 mass% or more) is gasified to obtain ammonia gas. These hydrofluoric acid gas and ammonia gas are respectively 2.00 kg / h and 0%. SS at 71 kg / h
-400 lined with fluororesin (ETFE) 5
Guided to a 00L raw material mixing tank, purity 99.9999% by volume
Under an N ₂ gas seal to obtain ammonium acid fluoride having a HF / NH ₄ F molar ratio of 1.7 mol.

Next, a fluororesin (PF) is added to SUS-304.
A) 450L electrolytic cell (lined with 3)
The temperature was adjusted to 122 ° C. while continuously supplying the ammonium acid fluoride obtained in the mixing tank to the above (with set). Next, N 9 having a purity of 99.9999% by volume is placed in the anode chamber 3.
While introducing _two gases as a carrier gas at a flow rate of 0.1 L / min, a voltage of 7.0 V and a current of 200 A were applied between the positive electrode and the negative electrode to perform electrolysis. At this time, the anode 4 and the cathode 6
Low-carbon Ni (JIS H4
551) A plate was used. The nickel electrode had a carbon (C) content of 350 mass ppm as an impurity. This electrolysis is
It was performed continuously for 00 hours.

The crude gas generated from the anode is taken out through a conduit 8, and is subjected to a chemical cleaning of water, sodium sulfite, and potassium hydroxide, a gas cleaning apparatus, an adsorption tower filled with natural zeolite, and a purification apparatus comprising a rectification tower. And a gas meter was installed at the outlet of the apparatus to measure the amount of generated gas. As a result, the generated gas amount was 10 to 11 L / min on average. The purity of the outlet gas was analyzed by using online gas chromatography. The purity and carbon compound content of the obtained NF ₃ gas were as shown in Table 1.

Example 2 Molten salt electrolysis was carried out in the same apparatus and under the same conditions as in Example 1 except that the carbon (C) content in low-carbon Ni used for the electrode was changed to 190 ppm by mass. Purification was performed. The purity and carbon compound content of the obtained NF ₃ gas were as shown in Table 1.

Example 3 The carbon (C) content in low-carbon Ni used for the electrode was 90 mass p.
Except that the pressure was set to pm, the molten salt electrolysis was performed under the same apparatus and conditions as in Example 1, and gas cleaning and purification were performed in the same manner. The purity and carbon compound content of the obtained NF ₃ gas were as shown in Table 1.

COMPARATIVE EXAMPLE 1 A nickel electrode to be used as an electrode was made to have a Ni purity of 99.5% by mass.
Example 1 except that the carbon (C) content was 800 ppm by mass.
Molten salt electrolysis was carried out under the same apparatus and conditions as described above, and gas cleaning and purification were carried out similarly. As shown in Table 1, the purity of the obtained NF ₃ gas and the content of the carbon compound were large, and the purity of the NF ₃ gas was lower than 4N. In addition, since the obtained product purity is low, the production is about 700
Stopped in time.

Reference Example Molten salt electrolysis was carried out under the same apparatus and conditions as in Example 1 except that a carbon electrode was used as the anode, and gas cleaning and purification were carried out in the same manner. As a result, the content of the carbon compound was extremely high, and the purity of the NF ₃ gas was poor.

[0035]

[Table 1]

As described above, according to the present invention, it is possible to produce NF ₃ gas having a high purity of 4N or more.

[0037]

According to the present invention, in a method for industrially producing high-purity nitrogen trifluoride gas at low cost, an electrode, in particular, a predetermined nickel electrode is used as an anode material, and electrolysis is performed by a molten salt electrolysis method. This is a very simple method. By controlling the amount of impurities, particularly the amount of carbon, contained in the nickel electrode, it is possible to produce high-purity nitrogen trifluoride gas which has been difficult to achieve conventionally.

[Brief description of the drawings]

FIG. 1 is an example of a manufacturing flow sheet suitable for carrying out the present invention.

FIG. 2 is an example of an electrolytic cell suitable for carrying out the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electrolyzer main body 2 Electrolyte 3 Anode chamber 4 Anode 5 Cathode chamber 6 Cathode 7 Separator 8 Pipe 9 Pipe 10 Pipe 11 Pipe

Continued on the front page (72) Inventor Hiromi Hayashida 7-1-1, Hikoshimasako-cho, Shimonoseki-shi, Yamaguchi F-term (reference) in Mitsui Chemicals, Inc.

Claims (6)

[Claims] 1. A method for producing a nitrogen trifluoride gas by a molten salt electrolysis method using ammonium acid fluoride as an electrolytic solution by using a nickel electrode, wherein a crude gas among impurity components in the nickel electrode used for the anode is used. A method for producing a high-purity nitrogen trifluoride gas, characterized in that the amount of carbon serving as an impurity gas component entrained therein is set to 400 ppm by mass or less. 2. The method for producing high-purity nitrogen trifluoride gas according to claim 1, wherein a nickel electrode having a carbon content of 200 mass ppm or less is used as an anode. 3. The method for producing high-purity nitrogen trifluoride gas according to claim 2, wherein a nickel electrode having a carbon content of 100 mass ppm or less is used as an anode. 4. An impurity component C, B, S in a nickel electrode.
The total of i, P, As, Mo, Ge and W is 400 mass p.
2. The method for producing high-purity nitrogen trifluoride gas according to claim 1, which is not more than pm. 5. The nickel electrode has a nickel purity of 98.
The method for producing high-purity nitrogen trifluoride gas according to claim 1, wherein the content is 5% by mass or more. 6. The HF / NH ₄ F molar ratio of the ammonium acid fluoride as an electrolytic solution is 1.5 to 2.0.
For producing high-purity nitrogen trifluoride gas.