JPH0474285B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for purifying nitrogen trifluoride gas.
More specifically, the present invention relates to a method for removing nitrous oxide (N ₂ O), carbon dioxide (CO ₂ ), and nitrogen difluoride (N ₂ F ₂ ) contained in nitrogen trifluoride gas. [Prior Art and Problems to be Solved by the Invention] Nitrogen trifluoride (NF ₃ ) gas has recently attracted attention as a dry etching agent for semiconductors and a cleaning gas for CVD equipment. The NF ₃ gas used in these applications is required to be of high purity. _NF3 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 gaseous fluorine in a molten state, a method of reacting an ammonium complex of a solid metal fluoride with elemental fluorine, NH ₄ F・HF, which is made from ammonium fluoride or acidic ammonium fluoride and hydrogen fluoride, and KF・NH ₄ F・HF, which is made by adding potassium fluoride or acidic potassium fluoride to the raw materials. Examples include molten salt electrolysis. However, in most cases the gases obtained by either method are N ₂ O, CO ₂ , N ₂ F ₂
Since it contains relatively large amounts of impurities such as NF3, it is necessary to purify it in order to obtain high-purity _NF3 gas for the above uses. Conventionally, as a purification method for removing these impurities from NF ₃ gas, a method of adsorbing and removing these impurities using an adsorbent such as synthetic zeolite, activated carbon, or activated alumina is well known. In particular, the synthetic zeolite disclosed in US Pat. No. 4156598 efficiently adsorbs the above impurities, so it is a preferable adsorbent in this respect. However, synthetic zeolites have the problem that, for example, Molecule Sieve 5A has a large adsorption capacity for N ₂ O, but has a low adsorption capacity for CO ₂ , and Molecule Sieve 13X has the opposite problem.
The adsorption ability of impurities differs depending on the type, for example, the adsorption ability of CO ₂ is large, but the adsorption ability of N ₂ O is small. Therefore, the present inventors have discovered that when synthetic zeolite is used as an adsorbent for the above-mentioned impurities, there is a troublesome problem in that two or more types of synthetic zeolite must be used in combination. In addition, it was discovered that synthetic zeolite adsorbs a large amount of the product NF ₃ at the same time as it adsorbs the impurities mentioned above, resulting in a very inconvenient problem of causing a large loss of NF ₃ gas. On the other hand, when activated carbon or activated alumina is used as an adsorbent, although the adsorption of NF ₃ is relatively small, compared to synthetic zeolite, the amount of impurities adsorbed per unit volume of adsorbent is very small.
Loses adsorption capacity very quickly. Therefore, it is recognized that there is a major problem in that the frequency of renewing or regenerating the adsorbent increases, and the loss of _NF3 gas during this renewal or regeneration results in a large overall loss of _NF3 gas. The inventors have discovered. Also, increasing the frequency of sorbent renewal or regeneration
Not only does this require more manpower, but it also greatly impedes the ability to purify NF ₃ gas over time. [Means for Solving the Problems] In view of this situation, the present inventors have developed a method for removing N ₂ O, CO ₂ or N ₂ F ₂ contained in NF ₃ gas using various adsorbents. As a result of extensive research, we surprisingly found that if NF ₃ gas is aerated at a specific temperature through a natural zeolite layer that has been heat-treated to a specific temperature in advance, the amount of NF ₃ adsorbed by natural zeolite can be significantly increased. Less and less N ₂ O in NF ₃ gas,
Since the adsorption capacity for CO ₂ and N ₂ F ₂ is large, the above impurities can be efficiently and economically removed, which led to the completion of the present invention. That is, nitrogen trifluoride gas is heated in advance to a temperature in the range of 250 to 700°C and heated to a natural zeolite layer of -125 to 700°C.
This is a method for purifying nitrogen trifluoride gas, which is characterized by aeration at a temperature of 50°C and in a state substantially free of moisture. [Detailed Disclosure of the Invention] The present invention will be described in detail below. There are various types of natural zeolite mineralogically, and the type of natural zeolite used in the present invention is not particularly limited. However, due to its abundant reserves and low mining costs, louftite (analcime or analcine),
clinoptilolite, mordenite, ferrierite,
Preferred are phillipsite, chabazite, erionite, laumontite, and the like. Furthermore, among these, clinoptilolite (Na ₆ [AlO ₂ ) ₆ (SiO ₂ ) ₃₀ ]・24H ₂ O) and mordenite (Na ₈ [AlO ₂ ) ₈ (SiO ₂ ) ₄₀ ]・24H ₂ O) is of the Na type, which is particularly preferable because it adsorbs a large amount of impurities in the NF ₃ gas per unit volume of the adsorbent. In addition, other minerals include harmotome, gismondine, garronite, levyne, faujasite,
scolesite, thomsonite, edingtonite, dachiardite,
epistilbite, heulandine, stilbite, barrerite, cowlesite,
wairakite, ashcrofine, merlinite, amicite, paulingite, yugawaralite,
offretite, mazzite, gmelinite, natrolite, mesolite,
Gonnardite, bikitite, stellerite, brewsterite, pollucite, etc. can also be used as equivalent materials. These natural zeolites are minerals produced in moraine rocks, so in order to use them as adsorbents in the present invention, natural zeolites collected in the form of rocks must be mixed with an appropriate particle size, e.g.
It is preferable to grind it into meshes, preferably about 8 to 60 meshes. In the present invention, the thus crushed natural zeolite having the desired particle size distribution is then heated at 250°C.
Heat treatment at ~700°C, preferably 250-500°C. Only by heat-treating zeolite in the above-specified temperature range, its ability as an adsorbent can be significantly improved, and an adsorbent that achieves the object of the present invention can be obtained. If the heat treatment temperature is lower than this, no matter how long the heat treatment is, the adsorption capacity of natural zeolite will decrease rapidly after the start of operation, the breakthrough time will be significantly shortened, and the N in the _NF3 gas after ventilation will decrease. _2O , _CO2 ,
This is because the content of impurities such as N ₂ F ₂ becomes significantly high. The reason is not necessarily clear, but one is that
In the case of natural zeolite containing water of crystallization, when it is used as an adsorbent in the present invention, the water of crystallization (hereinafter referred to as water) remains, and when NF ₃ gas is aerated into the natural zeolite layer, This is thought to be due to a decrease in the removal ability of N ₂ O, CO ₂ and N ₂ F ₂ per unit volume of zeolite. Therefore, heat treatment at the above temperature is performed in order to substantially completely remove the moisture in the natural zeolite. On the other hand, when zeolite is heated to temperatures exceeding this temperature, the crystal structure of natural zeolite changes or collapses. As a result, the adsorption capacity is significantly impaired, and adsorption is not performed or breakthrough occurs in a short period of time after gas ventilation. Heat treatment of natural zeolite uses nitrogen, helium, neon, argon,
It is preferable to carry out in an inert gas flow such as krypton or xenon gas. The treatment can also be carried out in dry air from which CO ₂ has been removed beforehand. Alternatively, the heat treatment may be performed under reduced pressure while sucking these gases. The heat treatment is carried out at the above heating temperature and gas flow atmosphere for 10 minutes to 80 hours, preferably 1 hour to 40 hours, and more preferably 3 hours to 10 hours. Note that the heat treatment may be carried out by spreading a thin layer of crushed natural zeolite having a desired particle size distribution in a dryer and flowing an inert gas over the surface of this thin layer, but more preferably, A packed bed is formed of zeolite, and an inert gas is passed through the packed bed while heating. The natural zeolite that has undergone the heat treatment is allowed to cool or is forced to cool to a temperature of 30° C. or lower in preparation for the next adsorption treatment. Of course, it is preferable to avoid mixing water into the zeolite during cooling. In the present invention, a packed bed is formed from the heat-treated zeolite, and nitrogen trifluoride gas containing impurity gas is passed through the packed bed at a temperature of -125°C to 50°C to perform a purification treatment. The most preferred embodiment of the present invention is one in which heat treatment and adsorption purification of NF ₃ gas are performed in the same container. That is, a suitable container or column is filled with pulverized natural zeolite having a desired particle size distribution to form a packed bed. Next, heat treatment is performed while an inert gas is passed through the packed bed.
After the heat treatment, the zeolite is cooled as it is without taking it out of the container, and then NF ₃ gas is poured into the packed bed of natural zeolite from -125℃ to 50℃.
A method of aeration at a temperature of . Purification of NF ₃ gas is carried out by aeration through a natural zeolite layer packed in a column or the like as described above, but the aeration temperature at this time is important and is preferably a temperature of 50° C. or lower. Once this temperature is exceeded,
The content of N ₂ O, CO ₂ and N ₂ F ₂ in the NF ₃ gas after ventilation does not decrease sufficiently, and the content of N ₂ O, CO ₂ and N ₂ in the NF ₃ gas per unit volume of natural zeolite This is disadvantageous because the amount of F ₂ adsorbed is greatly reduced. Further, the lower the temperature, the more preferable it is, but since the boiling point of NF ₃ is -129°C, it is practically difficult to operate below this temperature, so it is carried out in the range of -125°C or higher. As the material for the container and column, common materials such as stainless steel, copper, nickel, and iron can be used. Note that iron is susceptible to corrosion and rust on the outer surface of the column, which is constantly in contact with air, so it is preferable to perform rust prevention treatment. The ventilation conditions for the zeolite packed bed will be described in more detail below. The diameter of the zeolite packed bed is preferably about 1 cm to 1 mφ. If desired, it is also possible to use a plurality of narrow-diameter packed columns in a bundle. If the bed diameter exceeds 50 cm, heat transfer fins should be installed on the column surface or a heat exchanger should be inserted into the packed bed to efficiently remove adsorption heat generated during ventilation. is preferred. Filled bed height is 10cm to 3m
The gas flow rate is about 10cc/min to 100/min.
It is about min. The combination of layer diameter, layer height, and gas flow rate is 1 to 500 cm/min, preferably 1 to 200 cm/min.
It is possible to freely select a gas linear velocity of min within a range that satisfies the gas linear velocity. The pressure of NF ₃ gas during ventilation is not particularly limited, but
For example, a pressure of about 0 to 5 kg/cm ² -G is preferred because it is easy to operate. The amount of impurity gases in the _NF3 gas targeted by the present invention is _N2O 0.1-2.0%, _CO2 0.3-2.0%, _{N2F2 0.2}
It is about ~0.6%, and after processing this, respectively,
It is requested that it be less than 10ppm. When subjected to the purification treatment of the present invention, the amount of impurities in the gas is
_N2O ND _~ 5ppm, _CO2ND ~5ppm, _N2F2ND
~5ppm can be achieved. The analysis method is
This is a value determined by gas chromatography (detector: PID), and ND indicates that it is below the detection limit (less than 1 ppm). [Examples] The present invention will be explained in more detail with reference to Examples below, but it is clear that these examples are for the purpose of better understanding the invention and are not intended to limit the scope of the invention. must be understood. Note that in the following, % and ppm represent capacity standards unless otherwise specified. Moreover, in Examples and Comparative Examples, breakthrough time means the following. That is, when a gas containing impurities is vented through an adsorbent layer to adsorb and remove impurities, the impurity content in the obtained gas is small immediately after the gas venting starts, and the content remains constant or increases slightly. It will remain at . When the adsorbent loses its adsorption capacity, the impurity content begins to increase rapidly. The time until this sudden increase in ventilation begins is called the breakthrough time. In Examples and Comparative Examples, except for Comparative Examples 1 to 4, the breakthrough time is the ventilation time until any of N ₂ O, CO ₂ , or N ₂ F ₂ in the NF ₃ gas after ventilation exceeds 10 ppm. did. In addition, in Comparative Examples 1 to 4, any of the above impurities
The aeration time until the temperature exceeded 20 ppm was defined as the breakthrough time. Examples 1-4 Particle size 24-48 in a stainless steel column with an inner diameter of 10 mm
After filling with mesh granular natural zeolite (mordenite) (filling height 200 mm), the mordenite was heat-treated under the conditions shown in Table 1. Thereafter, the mordenite layer was cooled, and NF ₃ gas having the analysis values shown in Table 1 was added under the conditions shown in Table 1.
Aerated until breakthrough time. Breakthrough time, _N2O , _CO2 , _N2F2 up to breakthrough _time
The amount of adsorption of NF 3 and NF ₃ is as shown in Table 1. If the method of the present invention is used for purification, the amount of loss due to NF ₃ adsorption will be the same as that of synthetic zeolite, as can be seen from the comparison with Comparative Examples 5 to 7 described later. The amount of N ₂ O, CO ₂ and N ₂ F ₂ in the NF ₃ gas can be removed much better than when using a type of molecular sieve. Furthermore, compared to the use of activated carbon or activated alumina, the breakthrough time is much longer, and NF ₃
It can be seen that the amount of loss due to adsorption is even smaller. Note that the analysis of NF ₃ gas was performed using gas chromatography. (Similarly below) Examples 5 to 8 Using the stainless steel columns used in Examples 1 to 4, changing the type of natural zeolite to granular clinobutyrolite with a particle size of 24 to 48 mesh,
In the same manner as in Examples 1 to 4, the clinobutyrolite was purified by heat treatment and aeration of NF ₃ gas until the breakthrough time under the conditions shown in Table 2. The amount packed into the column is the same as in Examples 1 to 4). The results are shown in Table 2, and Example 1
Similar to ~4, the amount of loss due to adsorption of NF ₃ is small;
Moreover, N ₂ O, CO ₂ and N ₂ F ₂ in the NF ₃ gas are removed extremely well. Furthermore, it can be seen that the breakthrough time is also long. Comparative Examples 1 to 4 Using natural zeolite of the type shown in Table 3 and having a particle size of 24 to 48 mesh, the natural zeolite was purified by heat treatment and aeration of NF ₃ gas in the same manner as in Examples 1 to 4. The test was carried out under the conditions shown in Table 3 until the breakthrough time (the amount of natural zeolite packed into the stainless steel column was 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 Examples 1 and 2.

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[Table] shows that even if heat treated for a long time, the adsorption capacity of natural zeolite decreases, the breakthrough time becomes significantly shorter, and the content of impurities in the NF ₃ gas after aeration increases. In addition, when the temperature at which NF ₃ gas is aerated into the natural zeolite layer is higher than the temperature specified in the present invention as in Comparative Examples 3 and 4, the breakthrough time is not as high as in Comparative Examples 1 and 2, but the breakthrough time is It can be seen that the length becomes shorter and the content of impurities in the NF ₃ gas after ventilation also becomes higher. Comparative Examples 5-6 Comparative Example 1 except that the heat treatment temperature was 800°C
Alternatively, an experiment similar to Comparative Example 2 was conducted. The results are shown in Table 3, and when using natural zeolite heat-treated at a high temperature exceeding the conditions specified in the present invention as in Comparative Examples 5 and 6,
Possibly due to the destruction of its crystal structure, the adsorption capacity is significantly reduced and the breakthrough time is less than 5 minutes, making it almost impossible to use it industrially. It can also be seen that almost no impurities are adsorbed until breakthrough occurs. Comparative Examples 7 to 9 Molecular sieve 5A and 1, which are a type of synthetic zeolite, were used instead of natural zeolite as adsorbents.
Heating the adsorbent in the same manner as in Examples 1 to 4 using a mixture of 3 Processing and purification by aeration of _NF3 gas are carried out in the fourth stage.
The experiment was carried out under the conditions shown in the table until the breakthrough time (the amount of adsorbent packed into the stainless steel column was the same as in Examples 1 to 4). The results are shown in Table 4, and when molecular sieve was used as the adsorbent, NF ₃
The loss due to adsorption is large. In the case of activated carbon and activated alumina, the ability to adsorb impurities is small, and therefore the breakthrough time is short. It can also be seen that the loss due to NF ₃ adsorption is greater than when using the natural zeolite of the present invention. As explained in detail above, the present invention is a method for removing N ₂ O, CO ₂ and N ₂ F ₂ from NF ₃ gas using an adsorbent, in which inexpensive natural zeolite is used as an adsorbent in advance in a specified manner. It is an extremely simple method that involves heating the natural zeolite layer at a high temperature and then passing NF ₃ gas through the natural zeolite layer under specific conditions. Conventional methods using synthetic zeolites produce NF ₃
The adsorption amount of NF 3 was large, and the loss of expensive NF ₃ due to this adsorption was large. Furthermore, in order to remove the above-mentioned impurities, it was necessary to use two or more types of synthetic zeolites in combination. In addition, if the adsorbent is activated carbon or activated alumina, NF ₃
Although the adsorption amount of adsorbent is small, the ability to adsorb impurities is small, and therefore the breakthrough time is short and the adsorbent has to be renewed or regenerated frequently. On the other hand, the method of the present invention using natural zeolite has a large adsorption capacity for N ₂ O, CO ₂ and N ₂ F ₂ .
Moreover, the loss due to adsorption of NF ₃ is much lower than in the method using natural zeolite. Furthermore, the loss of NF ₃ is lower than in methods using activated carbon or activated alumina. As described above, the method of the present invention uses inexpensive natural zeolite.

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〔Effect of the invention〕

As explained in detail above, the present invention is a method for removing N ₂ O, CO ₂ and N ₂ F ₂ from NF ₃ gas using an adsorbent, in which inexpensive natural zeolite is used as an adsorbent in advance in a specified manner. Dehydrated by heating to a high temperature,
This is an extremely simple method in which NF ₃ gas is aerated through this natural zeolite layer under specific conditions. Conventional methods using synthetic zeolites produce NF ₃
The adsorption amount of NF 3 was large, and the loss of expensive NF ₃ due to this adsorption was large. Furthermore, in order to remove the above-mentioned impurities, it was necessary to use two or more types of synthetic zeolites in combination. In addition, if the adsorbent is activated carbon or activated alumina, NF ₃
Although the adsorption amount of adsorbent is small, the ability to adsorb impurities is small, and therefore the breakthrough time is short and the adsorbent has to be renewed or regenerated frequently. On the other hand, the method of the present invention using natural zeolite has a large adsorption capacity for N ₂ O, CO ₂ and N ₂ F ₂ .
Moreover, the loss due to adsorption of NF ₃ is much lower than in the method using natural zeolite. Furthermore, the loss of NF ₃ is lower than in methods using activated carbon or activated alumina. As described above, the method of the present invention can efficiently and economically remove N ₂ O, CO ₂ and N ₂ F ₂ in NF ₃ using inexpensive natural zeolite, and can also remove expensive natural zeolite. Since the loss of NF ₃ is also small, the economic effects of the present invention are extremely large.

Claims (1)

[Claims] 1.Nitrogen trifluoride gas is heated to -125 to 50℃ to a natural zeolite layer that has been heated to a temperature in the range of 250 to 700℃ in advance.
1. A method for purifying nitrogen trifluoride gas, which comprises aeration at a temperature of approximately 100 liters and in a state substantially free of moisture.