JPH0891812A - Method for purifying nitrogen trifluoride gas - Google Patents

Abstract

PURPOSE: To efficiently remove SF6 in gaseous NF3 and to efficiently purify the gaseous NF3 by allowing the gaseous NF3 to flow to a zeolite having specified condition under a prescribed condition. CONSTITUTION: The gaseous NF3 removed impurity content such as N2 O, CO2 , N2 F2 by a well-known method preferably and containing the SF6 as an impurity and having >=99.99vol.% purity is allowed to flow at -125 to 50 deg.C and in such a state that water content is not incorporated substantially to a synthetic zeolite (e.g. 5A type zeolite) cooled to about room temp. after being subjected previously to heat treatment at 250-500 deg.C, preferably at 300-400 deg.C. A grain size of the synthetic zeolite is preferably 2-20 mesh.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for purifying nitrogen trifluoride gas. More specifically, it relates to a method for removing sulfur hexafluoride (SF ₆ ) contained in nitrogen trifluoride gas.

[0002]

2. Description of the Related Art Nitrogen trifluoride (NF ₃ ) gas is used as a dry etching agent for semiconductors and CVD.
In recent years, it has attracted attention as a cleaning gas for equipment, and its production volume has increased remarkably. NF ₃ used in these applications
In recent years, gas with higher purity has been required.

NF ₃ gas is produced in various ways. For example, a method of electrolyzing a molten salt of ammonium acid fluoride, a method of reacting ammonium acid fluoride with fluorine in a vapor phase in a molten state, a method of reacting an ammonium complex of a solid metal fluoride with elemental fluorine, NH ₄ F ・ HF using ammonium fluoride or acidic ammonium fluoride and hydrogen fluoride as raw materials, or KF ・ NH ₄ F ・ HF system in which potassium fluoride or acidic potassium fluoride is further added to the raw materials There is a molten salt electrolysis method. However, in most cases, the gas obtained by any of the methods contains a relatively large amount of impurities such as N ₂ O, CO ₂ , and N ₂ F ₂ , so that the high-purity NF ₃ gas for the above-mentioned application is used. Purification is required to obtain.

Conventionally, as a purification method for removing these impurities in the NF ₃ gas, a method of adsorbing and removing these impurities using an adsorbent such as synthetic zeolite, activated carbon, activated alumina and natural zeolite is often used. Are known.

In particular, the methods of US Pat. No. 4,156,598 and the applicants; Japanese Patent Publication No. 4-74285, Japanese Patent Application No. 4-98006, Japanese Patent Application No. 4-175211, etc., efficiently adsorb the above-mentioned impurities. In this respect, it is a preferable adsorbent. However, in recent years, further purification has been required while NF ₃ gas with higher purification is required.

In particular, sulfur hexafluoride (SF ₆ ) has not been regarded as a problem in the past as compared with other impurities,
For example, 4N (meaning 99.99% purity), 5
When purifying N, 6N, etc., the content thereof becomes a problem.

[0007]

In view of the above situation, the inventors of the present invention have conducted extensive studies on various methods of removing SF ₆ contained in NF ₃ gas, and as a result, it was surprising. In particular, if the NF ₃ gas to be treated at a specific temperature is passed through the synthetic zeolite layer that has been heat-treated at a specific temperature in advance, NF ₃ is added to the zeolite layer and SF in the NF ₃ gas is added.
_The present inventors have completed the present invention by discovering that the above-mentioned impurities can be removed efficiently and further industrially and economically because the adsorption capacity of ₆ is also large.

That is, the method for purifying nitrogen trifluoride gas according to the present invention is that synthetic zeolite is preheated to a temperature in the range of 250 to 500 ° C., and then nitrogen trifluoride gas is converted to −12.
The method is characterized in that aeration is carried out at a temperature of 5 to 50 ° C and in a state in which substantially no water is mixed.

[0009]

DETAILED DISCLOSURE OF THE INVENTION The present invention is described in detail below.

There are various kinds of synthetic zeolites, and the zeolite used in the present invention is not particularly limited, but according to the study of the present inventors, the molecular diameter of SF ₆ to be adsorbed ( 5.6 Å), considering the polarity, 4
A, 5A, Y type, 10X, and 13X are suitable.

In order to use this as an adsorbent in the present invention, it is preferable to use zeolite having an appropriate particle size, for example, 2 to 20 mesh.

In the present invention, the zeolite having the desired particle size is then added to the range of 250 ° C to 500 ° C, preferably
Heat to 300-400 ° C. Only by heat-treating the zeolite in the above-specified temperature range, the ability as an adsorbent is remarkably improved, and the adsorbent that achieves the object of the present invention can be obtained.

[0013] heating the treatment temperature is less than this, no matter how long with Cooked adsorption capacity of the zeolite also is operated after the start abruptly decreased breakthrough time is much shorter, NF ₃ gas after venting This is because the SF ₆ content of is significantly increased.

Although the reason is not always clear, in the study by the present inventors, in the case of a zeolite containing water of crystallization, when it was used as the adsorbent of the present invention, the water of crystallization (hereinafter referred to as water) (Noted) remains and enters the zeolite layer
When aerated with NF ₃ gas,
This is probably because the SF ₆ removal capacity is reduced. Therefore, the heat treatment is carried out at the above temperature in order to substantially completely remove the water content in the zeolite.

On the other hand, if the zeolite is heated to a temperature exceeding this, the crystal structure of the zeolite itself changes or collapses. As a result, the adsorbing ability is significantly impaired and the product ends up, adsorption is not carried out, or the gas breaks through in a short time after gas is passed, which is not preferable.

The heat treatment of the zeolite is preferably carried out in a stream of an inert gas such as nitrogen, helium, neon, argon, krypton or xenon gas which does not substantially contain water. The treatment can also be performed in dry air from which water and CO ₂ are removed in advance. Also, under reduced pressure,
The heat treatment may be performed while sucking these gases.

The heat treatment is carried out for 10 minutes to 80 hours, preferably 1 hour to 40 hours in the above heating temperature and gas stream atmosphere.
More preferably, it is carried out for 3 hours to 10 hours.

As a mode of the heat treatment, zeolite having a desired particle size distribution may be thinly spread in a dryer and an inert gas may be flown on the surface of the thin layer.
More preferably, a packed bed is formed of zeolite and the inert gas is heated while ventilating the packed bed.

The zeolite thus heat-treated is cooled or forcedly cooled to a temperature of about room temperature (10-40 ° C.) in preparation for the next adsorption treatment. Of course, it is preferable to avoid mixing of water in the zeolite during cooling.

In the present invention, a packed bed is formed from the heat-treated zeolite, and nitrogen trifluoride gas containing an impurity gas is passed through the packed bed at a temperature of −125 ° C. to 50 ° C. for purification treatment. I do.

In the most preferred embodiment of the present invention, NF ₃ gas from which other impurities (N ₂ 0, CO ₂ , N ₂ F ₂ etc.) have been removed in advance by a known method is used. It is industrially and economically suitable. In addition, N ₂ 0, CO ₂ , N ₂
NF ₃ with a purity of approximately 99.99% or higher, excluding impurities such as F ₂ minutes
Further, it is particularly useful for gas purification, and it is also preferable to perform heat treatment of zeolite and adsorption purification of NF ₃ gas in the same container, and fill a suitable container or column with zeolite having a desired particle size distribution. To form a packed bed, and then an inert gas is passed through the packed bed for heat treatment.

After the heat treatment, the zeolite is cooled as it is without being taken out of the container, and the NF ₃ gas to be purified into the packed bed of this zeolite is continuously cooled to −125 ° C.
Preference is given to directing and aeration at a temperature of -50 ° C.

NF₃Purification of gas can be performed in columns etc. as described above.
Performed by venting the packed zeolite layer
However, the ventilation temperature at this time is important and it must be below 50 ° C.
And are preferred. If this temperature is exceeded, research by the present inventors
Then, NF after ventilation₃SF which is an impurity in gas₆Content of
Does not decrease sufficiently, and NF per unit volume of zeolite
₃ SF in gas₆The adsorption amount of
is there.

Further, the lower the temperature is, the better, but since the boiling point of NF ₃ is −129 ° C., it is practically difficult to operate below this temperature, and it is carried out in the range of −125 ° C. or higher.

As the material of the container or column, SUS-304, SU
Usual materials such as stainless steel such as S-316L, copper, nickel and iron can be used. Since iron is easily corroded on the outer surface of the column that is constantly in contact with air and rusted, it is preferable to perform rust prevention treatment.

The aeration conditions of the zeolite packed bed will be described in more detail below. The layer diameter of the zeolite packed bed is preferably about 1 cm to 1 mφ. If desired, it is possible to use a plurality of packed columns having a small diameter in a bundle. Here, when the layer diameter exceeds 20 cm, for example, in order to efficiently remove the heat of adsorption generated during ventilation,
It is preferable to provide heat transfer fins on the column surface or insert a heat exchanger in the packed bed.

The packed bed height is about 10 cm to 3 m, and the gas flow rate is about 10 cc / min to 100 l / min. The combination of bed diameter, bed height, and gas flow rate is 1 to 500 cm / min, preferably 1 to 500 cm / min.
It is possible to freely select within a range that satisfies the gas linear velocity of 200 cm / min.

The pressure of the NF ₃ gas at the time of aeration is not particularly limited, but a pressure of, for example, 0 to 5 kg / cm ² (gauge pressure) is preferable because it is easy to operate.

The amount of the impurity gas in the NF ₃ gas which is the object of the present invention is not particularly limited, but SF ₆ is 5 to 5000.
It is about ppm.

When the refining treatment of the present invention is applied, the amount of SF ₆ in the purified gas can be set to about ND to 3 ppm. The analysis method is gas chromatography (detector: PID).
ND is below the detection limit (0.
It is less than 1 ppm).

[0031]

The present invention will be described in more detail with reference to the following examples, but it is clear that these are for the purpose of better understanding of the invention, and are not intended to limit the scope thereof. Must be understood by. In the following,% and ppm are based on capacity unless otherwise specified.

In the examples and comparative examples, the breakthrough time means the following. That is, when a gas containing impurities is vented to the adsorbent layer to adsorb and remove the impurities, the content of impurities in the obtained gas is small immediately after the start of the venting of the gas, and the content is constant or gradually increases. Changes in. By the time the adsorbent loses its adsorption capacity, the impurity content begins to increase sharply. The breakthrough time is the ventilation time until the rapid increase.
In Examples and Comparative Examples, SF _{6 in} NF ₃ gas after aeration was used.
The breakthrough time was defined as the aeration time until the time exceeded 5 ppm.

Examples 1 to 4 Granular 5A type zeolite (made by Tosoh; brand SA-500) having a particle size of 24 to 48 mesh was packed in a stainless steel column having an inner diameter of 10 mm (packing height 200 mm), and then the zeolite The heat treatment was performed under the conditions shown in Table 1. Then, the zeolite layer was cooled, and NF ₃ gas having the analysis value shown in Table 1 was aerated under the conditions shown in Table 1 until the breakthrough time.

The breakthrough time and the SF ₆ content in the purified gas are
Is as shown in Table 1, NF ₃ SF ₆ in the gas is very well removed, SF ₆ in NF ₃ was allowed purification it can be seen that the greatly reduced. The NF ₃ gas was analyzed by gas chromatography (PID method). (Same as below) Analysis of SF ₆ in purified NF ₃ gas is performed online.
The measurement was performed every 0 minutes, and the fluctuation range and the average value are also shown in Table 1.

[0035]

[Table 1]

Examples 5-8 Using the stainless steel columns used in Examples 1-4,
The kind of synthetic zeolite was changed to 4A and 13X, and heat treatment of the zeolite and NF were performed in the same manner as in Examples 1 to 4.
Purification by aeration with ₃ gases was performed under the conditions shown in Table 2 until the breakthrough time (the amount of zeolite packed in the stainless steel column was the same as in Examples 1 to 4).

The results are shown in Table 2, and SF ₆ in NF ₃ gas is removed very well as in Examples 1 to 4. Moreover, it can be seen that the breakthrough time is long.

[0038]

[Table 2]

Comparative Examples 1 to 4 Using the same column and synthetic zeolite as in Examples 1 to 4, heat treatment of zeolite and purification by aeration of NF ₃ gas were carried out in the same manner as in Examples 1 to 4, and Table 3 It carried out until the breakthrough time under the conditions shown in (the amount of zeolite packed in the column is the same as in Examples 1 to 4).

The results are shown in Table 3, and when using natural zeolite heat-treated at a temperature lower than the conditions specified in the present invention as in Comparative Example 1 and Comparative Example 2, even if it was heat-treated for a long time, However, it can be seen that the adsorption capacity of zeolite is reduced, the breakthrough time is significantly shortened, and the content of SF ₆ in the NF ₃ gas after aeration is increased.

Further, as in Comparative Example 3 and Comparative Example 4, NF ₃
When the gas aeration temperature to the zeolite layer is higher than the temperature specified in the present invention, the breakthrough time is shortened, though not so much as in Comparative Example 1 and Comparative Example 2, and S in the NF ₃ gas after aeration is reduced.
It can be seen that the content of F ₆ is also high.

Comparative Examples 5 to 6 Comparative Example 5 was carried out in the same manner as Example 5 except that the heat treatment was carried out at a high temperature exceeding the conditions specified in the present invention, and Comparative Example 6 was carried out. The experiment was conducted in the same manner as in Example 8.

The results are shown in Table 3, and when the synthetic zeolite heat-treated at a high temperature exceeding the conditions specified in the present invention as in Comparative Examples 5 and 6 is used, its crystal structure is probably destroyed. Perhaps because of this, the adsorption capacity drops significantly and the breakthrough time is less than 5 minutes, leaving almost no industrial applicability. Further, it is understood that SF ₆ is hardly adsorbed until the breakthrough is reached.

[0044]

[Table 3]

Comparative Examples 7 to 9 As in Example 1, natural zeolite (Comparative Example 7), activated carbon (Comparative Example 8) and activated alumina (Comparative Example 9) were used instead of the synthetic zeolite as the adsorbent. The heat treatment of the adsorbent and the purification by aeration of NF ₃ gas were performed under the conditions shown in Table 4 until the breakthrough time (the amount of adsorbent packed in the stainless steel column was the same as in Examples 1 to 4).

The results are shown in Table 4. In the case of activated carbon and activated alumina, the adsorption capacity of SF ₆ is small,
Therefore, the breakthrough time is short.

[0047]

[Table 4]

[0048]

INDUSTRIAL APPLICABILITY As described in detail above, according to the present invention, SF ₆ in NF ₃ gas to be purified is removed by using an adsorbent, and in a method for producing NF ₃ with higher purity, adsorption is performed. This is an extremely simple method in which synthetic zeolite as an agent is previously heated to a specific temperature for dehydration treatment, and NF ₃ gas is passed through the zeolite under specific conditions.

By the method of the present invention using zeolite, it has become possible to easily carry out a method of producing high-purity NF ₃ which could not be achieved by the conventional method.

As described above, since the method of the present invention can efficiently and economically remove SF ₆ content in NF ₃ by using zeolite, the industrial and economic effects of the present invention are There is an extremely large one.

Claims (2)

[Claims] 1. Synthetic zeolite is preheated at 250 to 500 ° C.
After the heat treatment at a temperature in the range of, the nitrogen trifluoride gas is aerated at a temperature of -125 to 50 ° C and in a state in which substantially no water is mixed. Method. 2. The nitrogen trifluoride gas to be treated has a purity of 9%.
The method for purifying nitrogen trifluoride gas according to claim 1, which is 9.99% by volume or more and contains sulfur hexafluoride as an impurity.