JPH0515740A - Treatment of nitrogen trifluoride-containing exhaust gas - Google Patents

Abstract

PURPOSE:To improve the defect of a nitrogen trifluoride (NF3)containing exhaust gas treatment method provided heretofore and to simply and efficiently treat NF3-containing exhaust gas to reduce the concn. of NH3 to a level not generating a problem from the aspect of air pollution or working environment. CONSTITUTION:Exhaust gas containing nitrogen trifluoride (NF3) is brought into contact with a mixture consisting of activated carbon and metal oxide at 200-600 deg.C to be reacted therewith to be made harmless.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for decomposing and removing exhaust gas containing nitrogen trifluoride (NF ₃ ) used for dry etching or the like.

NF ₃ has hitherto been prized as an oxidizer for high-energy fuels and as a fluorine source for chemical lasers, but in recent years, it has also been used as a dry etching gas for ultra-LSI, which leaves no residue after processing, and CVD (chemical vapor phase). It is a useful gas that has been attracting attention as a cleaning gas for growth equipment.

On the other hand, NF ₃ is a non-flammable gas that is very stable at room temperature and does not react with water, acid or alkaline aqueous solution, but is a toxic gas with an allowable concentration of 10 ppm.

For this reason, the NF ₃ -containing exhaust gas after use cannot be released into the atmosphere as it is, and it is diluted with air to 10 ppm or less and then discharged, or it is discharged after some decomposition treatment. The current situation.

However, the N discharged after dilution with air
Since F ₃ is not decomposed in the atmosphere, it is feared that the use of NF ₃ will increase in the future and that the natural environment, especially the biological system, will be adversely affected.

[0006]

2. Description of the Related Art Conventionally, the following method has been known as a method for decomposing NF ₃ or NF ₃ -containing gas.

(1) Method of reacting with heated metal In this method, when the produced metal fluoride is a solid, the reaction is inhibited because the metal fluoride forms a film on the metal surface, and the reaction is continued within a short time. In addition, the ability to decompose NF ₃ decreases.

Further, since the metal fluoride is a powder, it tends to cause clogging of the reactor or piping.

When the produced fluoride is in a gaseous state, it has to be physically adsorbed or washed with an alkali as a secondary treatment, so that the apparatus becomes large in size and the apparatus is corroded remarkably.

(2) Method of reacting with high temperature activated carbon This method can decompose NF ₃ at a reaction temperature of 400 ° C. or higher with relatively high efficiency, but at the same time, it is more toxic and explosive than NF _3. Nitrous oxide (N ₂ F ₂ ) is produced as a large amount as a by-product. In order to suppress the production of N ₂ F _{2 as} a by-product, the contact time between NF ₃ and activated carbon must be extremely long and the reaction temperature must be 500 ° C. or higher. CF ₄ generated in this case,
There is a risk that fluorocarbons such as C ₂ F ₆ and NF ₃ may cause an explosion due to the explosion of N ₂ F ₂ . Further, if O ₂ is mixed in the gas to be treated, combustion of activated carbon may occur, and in any case, it cannot be said to be an industrially effective method.

[0011]

As described above, several methods have been proposed for the decomposition treatment method of NF ₃ -containing exhaust gas, but there are many problems in terms of equipment and reaction, and in terms of efficiency. At present, the decisive method has not been established.

Therefore, the present invention improves the drawbacks of these NF ₃ -containing exhaust gas treatment methods that have hitherto been proposed, and simply and efficiently treats NF ₃ -containing exhaust gas to a concentration at which there is no problem in air pollution or working environment. It provides a way to render it harmless.

[0013]

The present invention is an industrially extremely effective NF which solves the above-mentioned problems of the conventional method all at once.
₃ provides a method of removing harm. Specifically, NF ₃
Exhaust gas containing activated carbon, MgO, BaO, Al ₂ O ₃ , T
At least one of metal oxides such as iO ₂ , ZnO and CaO
It is made harmless by reacting with a mixture with a seed in a temperature range of 200 to 600 ° C.

The gas composition after treatment according to the invention is N ₂ , C
Since N ₂ F _{2 which} is composed of F ₄ and CO ₂ and has toxicity and explosiveness is not by-produced at all, it is possible to directly discharge the produced gas into the atmosphere without the need for secondary treatment.

The concentration of NF ₃ gas which can be treated by the present invention is 10
Not only 0%, but also He, Ar, N ₂ and a gas diluted with air can be applied.

The reaction temperature is generally 200 to 600 ° C., more preferably 300 to 500 ° C., though it depends on the kind of metal oxide used in combination with the activated carbon. Two
If the temperature is less than 00 ° C, the decomposition of NF ₃ is insufficient.
Deterioration of the reactor material is likely to occur at temperatures above ℃.

The reaction time varies depending on the type of metal oxide used, the mixing ratio with activated carbon, and the NF ₃ concentration in the gas to be treated, but is appropriately selected from 1 second to 10 minutes. If it is less than 1 second, the decomposition of NF ₃ is insufficient, and if it is more than 10 minutes, NOx is by-produced, which is not preferable.

Although the activated carbon may be used in the form of crushed coal or crushed coal, it is desirable that the activated carbon be granular or crushed pieces of 4 to 8 mesh in consideration of both the pressure loss in the catalyst layer and the contact area with gas.

The metal oxide used in combination with the activated carbon may be used as it is as a powder, but it is desirable to use it by molding it into a granular form of 4 to 8 mesh in advance in order to prevent pressure loss or blockage in the reactor.

The activated carbon and the metal oxide may be mixed at a predetermined ratio and used, or each may be finely pulverized and tar pitch or the like may be added thereto and well mixed,
It may be used as an integrated catalyst which is obtained by molding with a granulator into a predetermined particle size, followed by heating and firing.

The mixing ratio of the activated carbon and the metal oxide may be arbitrarily selected within the range of 10 to 90 wt%, but in consideration of the catalyst life and effect, the activated carbon is 50 to 90 wt% and the metal oxide is 10 to 50 wt%. It is desirable to select within the range.

The reactor is not particularly limited in its shape as long as it can catalyze gas and solid. However, if the contact efficiency is a fixed bed flow type, a fluidized bed type forced circulation type or the like, it is further preferable. It is suitable.

As the material, it is preferable to use a fluorine-resistant material such as nickel or monel.

[0024]

The method of the present invention will be further described with reference to the following examples, but the present invention is not limited to these examples.

[0025]

Example 1 In a nickel reaction tube having an inner diameter of 40 mm and a length of 1,000 mm, 160 g of 6-8 mesh granular activated carbon and 4-
80 g of granular magnesium oxide molded into 6 mesh was mixed well and filled, and the reaction tube was heated to 400 ° C by an external heater.
Then, 100% NF ₃ was supplied at 0.5 l / min. As a result of analyzing the composition of the exhaust gas after 2 hours by gas chromatography, NF ₃ was 3 ppm, and the others were N ₂ and CO.
₂ and CF ₄ .

[0026]

Example 2 NF ₃ : 10 vol% and H as feed gas
e: The same reaction as in Example 1 was performed except that the composition was 90 vol%. Exhaust gas composition is N ₂ and CO _{2 in} addition to He
CF ₃ was not detected by itself.

[0027]

Example 3 In a reactor similar to that of Example 1, 160 g of 6-8 mesh granular activated carbon and 4-6 mesh granular zinc oxide 8 were used.
After charging 0 g of the mixture and keeping the reaction tube at 300 ° C., a mixed gas of NF ₃ : 10 vol% and He: 90 vol% was supplied at 1 l / min. Exhaust gas composition after 2 hours is H
Other than e, NF ₃ was not detected with only N ₂ and CO ₂ .

[0028]

Example 4 A reactor similar to that of Example 1 was filled with a mixture of 200 g of 6-8 mesh granular activated carbon and 50 g of 4-6 mesh granular aluminum oxide, and the reaction tube was kept at 300 ° C. and NF ₃ : 5 vol%. , O ₂ : 5 vol% and He: 9
A 0 vol% mixed gas was supplied at 1 l / min. The composition of the exhaust gas after 2 hours was N ₂ and CO ₂ other than He, and NF ₃ was not detected. No abnormal reaction such as combustion of activated carbon due to addition of O ₂ to the supply gas was observed.

[0029]

Example 5 A reactor similar to that in Example 1 was mixed with activated carbon powder and iron oxide powder in a weight ratio of 3: 1 and charged with 300 g of a catalyst molded to a particle size of 4 to 6 mesh. Three
After holding at 00 ° C, NF ₃ : 10 vol% and He: 90v
An ol% mixed gas was supplied at 1 l / min.

After 2 hours, the composition of the exhaust gas was N _{2 in} addition to He.
And CO ₂ alone, no NF ₃ was detected.

[0031]

Comparative Example A reactor similar to that of Example 1 was charged with 200 g of 6-8 mesh granular activated carbon, the reaction tube was kept at 300 ° C., and then a mixed gas of NF ₃ : 10 vol% and He: 90 vol% was added at 1 l / min. Supplied by.

Exhaust gas composition after 2 hours was NF in addition to He
₃ : 25 ppm, N ₂ F ₂ : 11.8 vol% and others N ₂
Met.

[0033]

According to the present invention, the NF ₃ -containing exhaust gas stream is catalytically treated at elevated temperature in a relatively simple apparatus containing a bed of activated carbon / metal oxide mixture to produce NF ₃
Efficiently breaks down ₃ minutes into a safe product, thus obtaining a detoxified exhaust gas stream that can be released directly into the atmosphere. Therefore, the present invention greatly opens up a way to use NF ₃ gas industrially and safely and contributes to environmental protection.

Claims (1)

Claim: What is claimed is: 1. An exhaust gas containing nitrogen trifluoride is reacted with a mixture of activated carbon and a metal oxide at a temperature in the range of 200 to 600 ° C. Method for treating nitrogen-containing exhaust gas.