JPH0474283B2 - - 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 dinitrogen difluoride from 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. Nitrogen fluoride gas is required to be as pure as possible. Nitrogen trifluoride (NF ₃ ) gas is produced by various methods, but in most cases, the gas obtained by any of these methods is a mixture of nitrous oxide (N ₂ O), carbon dioxide (CO ₂ ), and difluoride. Because it contains a relatively large amount of impurities such as dinitrogen oxide (N ₂ F ₂ ), it is not suitable for high-purity applications.
Purification is necessary to obtain _NF3 gas. As a purification method for removing these impurities from _NF3 gas, the method of adsorbing and removing impurities using an adsorbent such as zeolite is well known as one of the most efficient and simple methods [Chemical
Engineering (Chem.Eng.) 84, 116 (1977)
etc〕. However, in this adsorption-based purification method, the presence of N ₂ F ₂ in the NF ₃ gas causes the following disadvantages. That is, (1) When N ₂ F ₂ is present, other impurities such as CO ₂
The adsorption capacity for N ₂ O, etc. becomes extremely small. (2) The presence of N ₂ F ₂ also makes it easier for NF ₃ to be adsorbed by the adsorbent, thus leading to loss of NF ₃ gas. (3) N ₂ F ₂ that is adsorbed and concentrated on an adsorbent is likely to decompose and generate heat, which in severe cases can cause an explosion. Therefore, using adsorbents such as zeolites to remove _NF3
When adopting a method of adsorbing and removing impurities in the gas, it is necessary to remove N ₂ F ₂ in advance. Methods _for removing _N2F2 from _NF3 gas include KI,
With aqueous solutions such as HI, Na ₂ S, Na ₂ S ₂ O ₄ , Na ₂ SO ₃ , etc.
A method of removing N ₂ F ₂ by reacting it in a reaction tank is conventionally known [J. Massonne, Chem. Ing.
Techn.) 41, (12), 695, (1969)]. However, this method requires a relatively long time to completely remove N ₂ F ₂ and thus requires not only a fairly large reaction vessel but also a large amount of chemicals. Also, as another method to remove N ₂ F ₂ , N ₂ F ₂
NF ₃ gas containing NF 3 gas is filled in a reaction vessel with heated metal pieces or nets of stainless steel, carbon steel, copper, aluminum, zinc, lead, nickel, iron, etc. to form a catalyst packed _layer . There is also a method known in which gas is brought into contact with the packed bed by aeration, and reaction decomposition occurs on the surface of the metal piece or net using the metal piece or net as a catalyst (Japanese Patent Publication No. 1983).
-15081). However, this method
According to our study, metal pieces and N ₂ F ₂ tend to react and form metal fluoride on the surface of metal pieces and nets. In many cases, the generated metal fluoride peels off from the surface of the metal piece and becomes powder, causing a problem of clogging the inside of the packed bed and the piping of the refining device. However, according to our study, when nickel is used as a metal piece, the nickel piece only forms a fluoride film on its surface, and this film is relatively difficult to peel off, so it may cause blockage of pipes. Although the above problem can be prevented, the nickel piece whose surface is covered with fluoride no longer reacts with N ₂ F ₂ and naturally loses its activity as a catalyst, so it is necessary to periodically stop the operation. It is necessary to replace the nickel with a new piece of nickel and refill the catalyst layer, which is not only extremely complicated, but also causes the problem of a considerable drop-out due to the high price of nickel. Furthermore, in order to increase the removal efficiency of N ₂ F ₂ , by increasing the heating temperature of the packed bed of metal pieces,
At temperatures above ℃, the main component NF ₃ also reacts considerably with the metal pieces and decomposes, causing NF ₃ to decompose.
There is also the problem that the yield of (Means for Solving the Problem) As a result of intensive studies by the present inventors on a method for removing N ₂ F ₂ contained in NF ₃ gas, it was surprisingly found that NF ₃ gas containing N ₂ F ₂ They found that N ₂ F ₂ can be efficiently decomposed into nitrogen (N ₂ ) gas and fluorine (F ₂ ) gas simply by heating it to a specific temperature. Also, if the above heating is performed in a specific container,
_NF3 remains the main component even when heated to temperatures above 200℃
This method is advantageous because it does not decompose, and we have also obtained the knowledge that N ₂ F ₂ in NF ₃ gas can be removed efficiently, safely, and economically. The present invention has been made based on this new knowledge discovered by us. That is, the present invention provides nitrogen trifluoride gas containing at least dinitrogen difluoride as an impurity in a metal container, preferably a cylindrical container, whose inner wall is lined with a solid fluoride at a temperature of 150°C to 600°C. This is a method for purifying nitrogen trifluoride gas, which is characterized by heating it to . [Detailed Disclosure of the Invention] The present invention will be described in detail below. First, to explain the equipment such as containers used in the present invention, NF ₃ gas is subject to the High Pressure Gas Control Law. Therefore, the manufacturing equipment must be a high-pressure gas manufacturing equipment, and the outside of the manufacturing equipment generally needs to be made of metal. For the above reasons, the container used in the present invention, preferably a cylindrical container, must be made of metal such as iron or stainless steel. In the following explanation, a cylindrical container, which is easy to manufacture, will be used as a representative container shape, but as is clear from the description of the first claim, this invention does not apply to the present invention as defined in Article 70 of the Patent Law. It should be clearly understood that container shapes other than cylindrical containers, such as rectangular, box-shaped, conical, double-pipe, etc., are not excluded from the technical scope of the invention. The metal cylindrical container (hereinafter simply referred to as cylindrical container) used in the present invention is preferably
Preferably, the container is equipped with an inlet and an outlet pipe for NF ₃ gas, and whose inner walls are lined with solid fluoride. In the present invention, _NF3 gas is the highest in the above cylindrical container.
Since it is heated to about 600°C, the solid fluoride lining the inner wall preferably has a melting point of over 600°C. Examples of such solid fluorides include lithium fluoride (TiF), sodium fluoride (NaF), potassium fluoride (KF), rudybium fluoride (RbF), cesium fluoride (CsF), etc. Metal fluorides of the genus; beryllium fluoride (BeF ₂ ), magnesium fluoride (MgF ₂ ), calcium fluoride (CaF ₂ ), strontium fluoride (SrF ₂ ),
Group A metal fluorides such as vanium fluoride (BaF ₂ ); Group A metal fluorides such as aluminum fluoride (AlF ₃ ), gallium fluoride (GaF ₃ ), and insidium fluoride (InF ₃ ); fluoride Examples include double salts such as sodium aluminum (Na ₃ AlF ₆ ). A mixture of these may also be used. It goes without saying that even with solid fluorides other than those mentioned above, for example, having a melting point of 600°C or lower, the present invention can be carried out without any problem at a temperature below that melting point as long as it has a melting point of about 350°C or higher. do not have. In the present invention, the shape of the container for heating NF ₃ does not necessarily have to be cylindrical, but it is easy to manufacture the container itself and line it with solid fluoride, and the strength of the lining layer and its use are A cylindrical shape is preferable in order to prevent cracks from forming in the lining layer inside. There is no particular limit to the thickness of the lining layer, but if it is too thin, it will be technically difficult to process the lining, and there is also a risk that the inner wall of the cylindrical container will not be completely lined and a portion of the metal surface of the cylindrical container will be exposed. . On the other hand, if it becomes too thick, the cylindrical container is heated by a heater or the like from outside the cylindrical container in the present invention, so the heat transfer efficiency during heating will decrease and thermal energy will be lost, so it is preferable. Therefore, the thickness of the lining layer is approximately 1 to 5 mm. As a method for lining the inner wall of a cylindrical container with a solid fluoride, among the solid fluorides listed above,
When using a fluoride of a group A element having a relatively low melting point, for example, a sintering method can be employed. That is, as shown in FIG. 1, an inner cylinder 3 having an outer diameter slightly smaller than the inner diameter of the cylindrical container 1 is inserted into the cylindrical container 1 so as to be concentric. Next, the cylindrical container 1
After filling the space between the inner tube 3 and the inner cylinder 3 with powdered solid fluoride 4, a load is applied to the press-fit tube 5 to compress and mold the solid fluoride 4. This filling and compression molding of the solid fluoride 4 is repeated to form a compression molded layer of the solid fluoride 4 on the front surface of the inner wall of the cylindrical container 1. After that, inner cylinder 1
The cylindrical container 1 is gradually heated to the softening point of the solid fluoride 4 under an atmosphere of an inert gas such as nitrogen (N ₂ ) or helium (He), and then slowly cooled. The inner wall of the cylindrical container 1 can be lined with solid fluoride 4. In this case, it is preferable that the solid fluoride contains 2 to 3% by weight of water, since the above-mentioned compression molding is easier. This also applies to the case of lining by the high-pressure press method described later. In addition, if a lubricant is applied to the surface of the cylindrical container 3 in advance, the solid fluoride 4 will be applied to the surface of the cylindrical container 1.
This is preferable because it is easy to pull out the inner cylinder 3 after forming the compression molded layer. When lining the inner wall of a cylindrical container with a solid fluoride having a relatively high melting point, a high-pressure press method is suitable. That is, as in the case of the sintering method, after solid fluoride 4 is filled between the cylindrical container 1 and the inner tube 3 shown in FIG. By repeating molding, cylindrical container 1
This can be carried out by slowly pulling out the inner cylinder 3 after forming a lining layer of solid fluoride 4 on the entire inner wall of the inner cylinder. In this case, in order to form a strong lining layer, the load during compression molding is preferably 2 t/cm ² or more. Furthermore, as in the case of the sintering method, if a lubricant is applied to the surface of the inner cylinder 3 in advance, the inner cylinder 3 can be easily pulled out after the solid fluoride lining layer is formed on the inner surface of the cylindrical container 1. This is desirable because it is easy. However, in this case, the lubricant must be removed by evaporation by heating or the like after the solid fluoride lining layer is formed. Furthermore, in the present invention, as a method for lining the inner wall of a cylindrical container with a solid fluoride, in addition to the above-mentioned method, after flowing a molten solid fluoride between the inner tubes 3 of the cylindrical container 1 shown in FIG. It is also possible to adopt methods such as cooling and solidifying. In the present invention, the NF ₃ gas to be purified is thus heated and decomposed in a cylindrical container whose inner surface is lined with solid fluoride. The NF ₃ is heated by preparing in advance a cylindrical container whose inner surface is lined with solid fluoride as described above, keeping it in a heated state, and venting NF ₃ gas containing N ₂ F ₂ into the cylindrical container. The method of doing so is preferable. Heating of the cylindrical container can be easily carried out by heating the outside of the cylindrical container with a heater or the like. In the present invention, the heating temperature of NF ₃ gas containing N ₂ F ₂ is
It is carried out at 150-600°C, preferably 250-350°C. If the ventilation temperature is less than 150°C, almost no N ₂ F ₂ can be decomposed and removed. On the other hand, at temperatures exceeding 600°C, although N ₂ F ₂ can be almost completely removed, there is a risk that cracks may occur in the lining layer of the cylindrical container due to the difference in coefficient of thermal expansion, which is disadvantageous. It also leads to loss of thermal energy. In addition, at the above heating temperature,
The decomposition rate of N ₂ F ₂ is very fast, so ventilate it.
The residence time of _NF3 gas in the vessel (ratio of reactor volume to gas volume velocity) can be short, but usually
It is carried out in a range of about 5 to 1000 seconds. In the present invention, the NF ₃ gas to be vented into the cylindrical container may be supplied alone, or may be diluted with an inert gas such as N ₂ or He.
There is no particular restriction on the pressure of the ventilation gas, but a pressure of 0 to 5 kg/cm ² -G is usually preferred because it is easy to operate. [Effects of the Invention] As explained in detail above, the present invention is a method for removing N ₂ F ₂ from NF ₃ gas by heating NF ₃ gas at a specific temperature in a cylindrical container whose inner wall is lined with solid fluoride. Since it is a very simple method of heating to Furthermore, as shown in the examples described later, the removal rate of N ₂ F ₂ is excellent, so the NF ₃ gas purified by the method of the present invention can be purified using a conventionally known purification method, for example, using an adsorbent such as the zeolite mentioned above. As shown in Reference Example 1, if the NF 3 gas is re-purified, it is possible to easily obtain a highly purified NF ₃ gas suitable as a raw material for a semiconductor dry etching agent, which is a remarkable effect.
Furthermore, the method of the present invention allows NF ₃ gas to be obtained in high yield with almost no loss of NF ₃ and is a safe method. (Example) Hereinafter, the present invention will be explained in more detail with reference to Examples. In addition, % and in Examples, Comparative Examples and Reference Examples
ppm indicates capacity unless otherwise specified. Examples 1 to 3 A stainless steel cylindrical container (column) 1 with an inner diameter of 10 mm and a length of 300 mm is coated with stearic acid as a lubricant on the surface as shown in Fig. 1, and the outer diameter is 6 mm.
After inserting the inner cylinder 3 with a length of 400 mm concentrically with the column 1, between the column 1 and the inner cylinder 3, 5 weights of cesium fluoride powder is added to lithium fluoride powder with a water content of 3% by weight. After filling the mixed powder 4 little by little, a press-fit tube 5 with an outer diameter of 9.6 mm and an inner diameter of 6.5 mm is inserted between the above spaces.
was inserted into the press-fit tube 5, and a load of 1 t/cm ² was applied to the press-fit tube 5 to compression mold the mixed powder 4. The insertion and compression molding operations of the mixed powder 4 were repeated to form a compression molded body of the mixed powder 4 on the entire inner wall of the column 1, and then the inner cylinder 3 was slowly pulled out. Next, this column 1 was heated to 850°C at a heating rate of 200°C/h in an electric furnace under a _N2 gas atmosphere, and after being held at a temperature of 850°C for 1 hour, it was allowed to naturally air in the electric furnace. The column 1 was cooled to room temperature to obtain a column 1 whose inner wall was entirely laminated with a solid fluoride layer having a thickness of 2 mm. NF ₃ gas containing N ₂ F ₂ diluted with approximately the same volume of He gas was passed through this column 1 under the conditions shown in Table 1. After aeration, the gas was bubbled into a potassium iodide (KI) aqueous solution with a concentration of 1%.
The NF ₃ was introduced into a collection cylinder cooled with liquid nitrogen, where it was liquefied and collected. After stopping the ventilation of NF ₃ gas, the above
The inside of the NF ₃ collection cylinder was evacuated to remove He gas. The composition of NF ₃ gas before ventilation and the composition of NF ₃ in the collection cylinder after ventilation were analyzed by gas chromatography. The results are shown in Table 1.
N ₂ F ₂ had a high removal rate. Furthermore, there was almost no disappearance of NF ₃ . Note that the reason that the N ₂ gas content is higher after ventilation in Table 1 is considered to be that N ₂ F ₂ was decomposed into N ₂ and F ₂ due to heating. Furthermore, in Example 3, the condition of the lining surface of column 1 after aeration of NF ₃ gas was observed, but no abnormalities such as cracks or damage were observed. Examples 4 to 6 The same stainless steel column 1 with an inner diameter of 10 mm and a length of 300 mm as used in Examples 1 to 3, and an outer diameter of 6 mm,
Using an inner cylinder 3 with a length of 400 mm, moisture was added between the column 1 shown in Fig. 1 and the inner cylinder 3 whose surface was coated with stearic acid as a lubricant, as in Examples 1 to 3.

【table】

[Table] After filling powder of solid fluoride 4 of the type shown in Table 2 with a content of 3% by weight little by little, 2t/cm was added to a press-fit tube 5 with an outer diameter of 9.8 mm and an inner diameter of 6.2 mm between the above spaces. The powder of solid fluoride 4 was compressed by applying a load of ² . This solid fluoride 4 powder is repeatedly filled and compressed,
A lining layer of solid fluoride powder was formed on the entire inner wall of the column 1, and then the inner cylinder 3 was slowly pulled out. Next, this column 1 was heated to 300°C at a temperature increase rate of 200°C/h in an electric furnace in a N ₂ gas atmosphere, and the stearic acid was evaporated off by maintaining the temperature at 300°C for 1 hour. The column was cooled to room temperature by natural cooling in an electric furnace to obtain a column 1 whose inner wall was entirely lined with solid fluoride having a thickness of 2 mm. NF ₃ gas containing N ₂ F ₂ diluted with approximately the same volume of He gas as in Examples 1 to 3 was passed through the thus obtained column 1 under the conditions shown in Table 2.
The gas after ventilation had a concentration of 1% as in Examples 1 to 3.
After bubbling into the KI aqueous solution, the NF 3 was introduced into a collection cylinder cooled with liquid nitrogen to liquefy and collect NF ₃ .

[Table] After stopping the ventilation of NF ₃ gas, the inside of the NF ₃ collection cylinder was evacuated to remove He gas. The composition of NF ₃ gas before ventilation and the composition of NF ₃ in the collection cylinder after ventilation were analyzed by gas chromatography. The results are shown in Table 2.
N ₂ F ₂ had a high removal rate. Furthermore, there was almost no disappearance of NF ₃ . Note that in Table 2, the content of N ₂ gas is higher after ventilation, which is considered to be because N ₂ F ₂ was decomposed into N ₂ and F ₂ due to heating. Furthermore, the condition of the lining surface of column 1 after the NF ₃ gas ventilation was observed, but no abnormality such as cracks or damage was observed. Comparative Examples 1 to 3 A cylindrical container (column) made of the material shown in Table 3 (dimensions: inner diameter 6 mm, length 300 mm) was used as it was without lining the inner wall with solid fluoride, and the column was made of the material shown in Table 3. Contains N ₂ F ₂ under the conditions
NF ₃ gas was added in approximately the same volume as in Examples 1 to 3.
It was diluted with He gas and vented. The vented gas was bubbled into a 1% KI aqueous solution in the same manner as in Examples 1 to 3, and then introduced to a collection cylinder cooled with liquid nitrogen in the same manner as in Examples 1 to 3 to liquefy and capture _NF3 . collected. After stopping the ventilation of NF ₃ gas, the inside of the NF ₃ collection cylinder was evacuated to remove He gas. The composition of NF ₃ gas before ventilation and the NF ₃ composition in the collection cylinder after ventilation were analyzed by gas chromatography. The results are shown in Table 3, and it can be seen that although N ₂ F ₂ is removed, the yield of NF ₃ is also poor. Reference Example 1 After filling a stainless steel column with an inner diameter of 10 mm and a length of 300 mm with commercially available zeolite (pore diameter 5 Å) (granular product of 24 to 28 meshes) (packed bed 250 mm),
The NF ₃ gas obtained in Example 3 from which N ₂ F ₂ had been removed was bubbled through this zeolite layer. Ventilation conditions include room temperature (approximately 20°C), _NF3 gas flow rate of 20Nml/min,
The ventilation pressure was 760 Torr. The composition of _NF3 gas after ventilation was analyzed by gas chromatography. The result contains impurities.

【table】

[Table] Amounts are N ₂ F ₂ 20ppm or less, N ₂ O 20ppm or less,
The amount of CO ₂ is 20 ppm or less, and if NF ₃ gas from which N ₂ F ₂ has been removed in advance by the method of the present invention is purified using a conventionally known adsorbent, other substances other than N ₂ F ₂ such as N ₂ O and CO ₂ can be removed. High purity with extremely high impurity removal rate.
It is understood that NF ₃ can be obtained.

[Brief explanation of drawings]

Figure 1 shows a cylindrical container (column) in an example.
FIG. 3 is an explanatory diagram showing a state in which the inner wall of the fuel cell is lined with solid fluoride. In the figure, 1... Metal cylindrical container (column), 2... NF ₃ gas outlet pipe, 3... Inner cylinder, 4...
... solid fluoride, 5 ... press-fit tube, 6 ... auxiliary cylindrical pipe, 7 ... surface plate.

Claims (1)

[Claims] 1. Nitrogen trifluoride gas containing at least dinitrogen difluoride as an impurity is heated at 150 to 600°C in a metal container whose inner wall is lined with a solid fluoride excluding nickel fluoride. A method for purifying nitrogen trifluoride gas, characterized by heating it to a certain temperature. 2. The method according to claim 1, wherein the metal container is a cylindrical container.