JPH02164708A - Method for purifying gaseous nitrogen trifluoride - Google Patents

Abstract

PURPOSE:To very efficiently remove N2O and CO2 in gaseous NF3 contg. N2F2, N2O and CO2 for a long time by previously heating the gaseous NF3 in a vessel lined with a solid fluoride when the gaseous NF3 is purified with silica gel. CONSTITUTION:Gaseous NF3 contg. at least N2F2, N2O and CO2 is heated to 150-600 deg.C in a vessel whose inner wall has been coated with a solid fluoride. The vessel may be packed with the solid fluoride. Since the N2F2 is efficiently decomposed into N2 and F2 but NF3 is not decomposed, the N2F2 in the gaseous NF3 can be efficiently removed. A silica gel layer is dehydrated by heating to 150-300 deg.C and the N2F2-free gaseous NF3 is passed through the layer at 0 to -125 deg.C without mixing with moisture. The N2O and CO2 in the gaseous NF3 can be efficiently removed for a long time.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for purifying nitrogen trifluoride gas.

More specifically, dinitrogen difluoride (NJz), nitrous oxide (NtO), and carbon dioxide (
This relates to a method for removing C(h).

[Prior art and problems to be solved by the invention] Nitrogen trifluoride (NFS) gas has recently attracted attention as a dry etching agent for semiconductors and a cleaning gas for CVO equipment. The gas is required to be as pure as possible.

NF3 gas is produced by various methods, but in most cases the gas obtained by any method is N t F 2, N20,
Since it contains a relatively large amount of impurities such as Cot, it is necessary to purify it in order to obtain high purity NF and gas for the above uses.

As a purification method for removing these impurities from NFff gas, the method of adsorbing and removing impurities using an adsorbent such as molecular sheep is well known as one of the most efficient and simple methods.・Engineering (Cheffl, Eng,) 84.116° (19
77) etc.]. However, this fine-tuning force method using adsorption has the extremely inconvenient problem that if N2F2 is present in the NF gas, NF3 is also likely to be adsorbed by the adsorbent, resulting in a loss of NF3 gas.

It is not substantial enough to be used for gas purification.

[Means to solve problems]

In view of this situation, the present inventors have conducted intensive studies on methods for removing NF, NtFl, N20, and CO2 contained in the gas using various adsorbents. If NF3 gas is passed through the dehydrated silica gel layer at a specific temperature, NF will not be adsorbed by the silica gel, and N, F, , N, 0 and CO□ in NF3 gas can be efficiently and economically removed. He discovered that it could be removed and filed a patent application dated December 6, 1988. However, according to subsequent research by the present inventors, this method
It has been found that the presence of N2F2 in the Fz gas has the disadvantage that the removal duration (breakthrough time) of N, F, , NtO and CO□ is short.

Therefore, the present inventors continued various studies and found that NJ2. NF3 gas containing N20 and CO□ is introduced into a container whose inner wall is lined with solid fluoride, or in a state where the container is filled with solid fluoride.
After heating the gas to a specific temperature to remove the NtFx contained therein, if the NFj gas is passed through a silica gel layer that has been heat-treated under specific conditions at a specific temperature, NF3 will not be adsorbed by the silica gel. Extremely efficient and economical NF
They discovered that N, O, and CO□ in gas can be removed over a long period of time, leading to the completion of the present invention.

That is, the first method of the method for purifying nitrogen trifluoride gas of the present invention is to contain at least dinitrogen difluoride, nitrous oxide, and carbon dioxide as impurities in a container whose inner wall is lined with solid fluoride. Nitrogen trifluoride gas at 150~600″C
After removing the dinitrogen difluoride contained by heating to a temperature of
Subsequently, the nitrogen trifluoride gas is heated in advance to a temperature in the range of 150 to 300°C to dehydrate the silica gel layer.
This method is characterized by removing the contained nitrous oxide and carbon dioxide by aeration at a temperature of -125°C and in a state substantially free of moisture. A method as described above, in which a container lined with fluoride is filled with solid fluoride.

[Detailed disclosure of the invention]

The present invention will be explained in detail below.

In the present invention, N2FZ is removed in a container whose inner wall is lined with solid fluoride, or in a bed filled with solid fluoride in which the container is filled with NP.

By heating and venting the gas, it is possible to remove the Nzh contained therein.

N and F contained in NF3 gas! By simply heating NF3 gas to a specific temperature, Nzh is efficiently decomposed into nitrogen (N2) gas and fluorine (F2) gas. Furthermore, if the above heating is performed in a container whose inner wall is lined with solid fluoride, even if heated to a temperature of 200°C or higher, the main component NF
, is advantageous because it does not decompose, which makes it efficient, safe, and economical to eliminate NF and N2F in gas.
Z can be removed. Furthermore, if the container whose inner wall is lined with solid fluoride is heated while being filled with solid fluoride, N2Fg can be removed even more efficiently.

In the step of removing 82F2 from Np3 gas, which is the first step of the present invention, a container with an inlet pipe and an outlet pipe for NF3 gas and whose inner wall is lined with solid fluoride is required.

In the present invention, the container is made of metal such as iron or stainless steel. There are no particular limitations on the shape of the container, but the shape of the container is easy to manufacture and is lined with solid fluoride, the strength of the lining layer, and the prevention of cracks in the lining layer during use. A cylindrical shape is preferred.

There is no particular limit to the thickness of the lining layer, but it is technically difficult to process a lining that is too thin, and there is a risk that the inner wall of the container will not be completely lined and a portion of the metal surface of the container will be exposed. On the other hand, if it becomes too thick, the container is heated from outside the container using a heater or the like in the present invention, so the heat transfer efficiency during heating decreases, resulting in a loss of thermal energy, which is not preferable. Therefore, the thickness of the lining layer is approximately 1 to 5III11.

In the present invention, the NF, gas is stored in the container at a maximum of 60%
Since it is heated to 0°C, it is desirable that the solid fluoride used for the above-mentioned lining has a melting point of 600°C or higher.
Even if the melting point is less than 600°C, there is no problem in implementing the present invention as long as it is solid at the temperature at which the NFs is heated. Moreover, this solid fluoride is also a fluoride that does not react with NFs. Examples of such solid fluorides include metals of group IA such as lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), rubidium fluoride (RbF), and cesium fluoride (CsF). Fluoride; helium fluoride (BeFz), magnesium fluoride (Mgh
), calcium fluoride (CaF2), strontium fluoride (SrFz), barium fluoride (BaFz), etc.
ffE metal fluoride; aluminum fluoride (AI!, h)
, gallium fluoride (GaFs), indium fluoride (In
Metal fluorides of the H1A group such as Fz); and double salts such as sodium aluminum fluoride (NasA 12 F&). Note that a mixture of these may also be used.

Further, these solid fluorides are preferable because powdered ones tend to line the inner wall of the container.

A method for lining the inner wall of a container with solid fluoride will be described below using a cylindrical container as an example.

Among the solid fluorides exemplified above, when using a metal fluoride having a relatively low melting point, for example, a group IA metal fluoride, a sintering method can be employed.

That is, as shown in FIG. 1, an inner cylinder 3 having a slightly smaller outer diameter toward the inner diameter of the cylindrical container 1 is inserted into the cylindrical container 1 so as to be concentric. Next, after filling the gap between the cylindrical container 1 and the inner tube 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 entire inner wall of the cylindrical container l. After that, the inner cylinder 3 is slowly pulled out, and this cylindrical container l is filled with nitrogen (N
2) Easily line the inner wall of the cylindrical container 1 with the solid fluoride 4 by gradually raising the temperature to the softening point of the solid fluoride 4 in an atmosphere of an inert gas such as helium (He) and then slowly cooling it. be able to.

In this case, it is preferable for the solid fluoride to contain 2 to 3 weight percent water, since the above-mentioned compression molding is easier. This also applies to the case of lining by the high-pressure pressing method described later.

Incidentally, it is preferable to apply a lubricant to the surface of the inner cylinder 3 in advance, since this makes it easier to pull out the inner cylinder 3 after the compression-molded layer of the solid fluoride 4 is formed on the inner wall of the cylindrical container 1.

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 filling the gap between the cylindrical container 1 and the inner tube 3 shown in FIG. 1 with the solid fluoride 4, a load is applied to the press-fit tube 5 and compression molding of the solid fluoride 4 is repeated. This can be carried out by forming a lining layer of solid fluoride 4 on the entire inner wall of cylindrical container 1 and then slowly pulling out inner cylinder 3.

In this case, in order to form a strong lining layer,
The load during the compression molding described above must be 2 t/cJ or more. In addition, if a lubricant is applied to the surface of the inner cylinder 3 in advance as in the case of the sintering method, the inner cylinder 3 can be prevented from being pulled out after the lining layer of the solid fluoride 4 is formed on the inner wall of the cylindrical container l. This is preferred 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 method, after flowing a molten solid fluoride into the gap between the cylindrical container 1 and the inner tube 3 shown in FIG. Methods such as cooling and solidifying can also be adopted.

Next, the solid fluoride in the case of filling the container with the solid fluoride will be described.

The solid fluoride used for this filling is exactly the same as the solid fluoride used for the lining described above.

However, the solid fluoride used for this filling is preferably in the form of particles, but the size is not particularly limited and is determined depending on the size of the reactor, ease of handling, etc. Furthermore, even if the solid fluoride is in powder form, it can be suitably used by forming it into tablets using a tablet machine or the like.

Note that if the solid fluoride contains moisture, the moisture reacts with NF when it comes into contact with NF3 gas, producing nitrogen monoxide (NO). Therefore, the solid fluoride must be dried beforehand. It is desirable to thoroughly remove moisture.

In the present invention, N17. The gas is heated in a heated container whose inner wall is lined with solid fluoride as described above.
A method in which Nh gas containing Ft, NzO and CO□ is vented into the container is preferred. Note that it is a more preferable method to fill the container with solid fluoride in advance, since this further improves the Nzh removal rate.

The container can be easily heated by heating the outside of the container using a heater or the like.

In the present invention, the heating temperature of the NF3 gas containing NzF2, NJ and CO□ is 150 to 600°C, preferably 250 to 600°C.
Performed at 350'C. If the ventilation temperature is less than 150"C, it is almost impossible to decompose and remove N2Fz. On the other hand, if the temperature exceeds 600"C, N and F2 can be almost completely removed, but there is a risk that the lining layer of the container may crack due to the difference in thermal expansion coefficient. This is inconvenient because it causes a loss of thermal energy.It should be noted that at the above heating temperature, NzF
Since the decomposition rate of t is very fast, the residence time of the aerated Nh gas in the container may be short, but it is usually 5 to 1
It is carried out in a range of about 000 seconds.

In the present invention, the NFff gas to be vented into the container may be supplied alone, but N! It may be diluted with an inert gas such as , lle, etc. There are no particular restrictions on the pressure of the ventilation gas, but it is usually 0 to 5.
A pressure of kg/cm''-G is preferred because it is easy to operate.

The method for removing Nzh from NF3 gas, which is the first step of the present invention described above, is described in Japanese Patent Application No. 63-87208 and Japanese Patent Application No. 638721 previously filed by the present applicant.
The method detailed in No. 0 can be applied as is.

Next, in the latter step of the present invention, N and O in the NF+ gas are
The process of removing CO□ and CO□ will be described.

The silica gel used in the present invention is not particularly limited, and any reprinted silica gel can be used, but granular silica gel with a high surface area is more preferable.

The dehydration treatment of silica gel is carried out by heating to a temperature of 150 to 300'C1, preferably 150 to 200C.

If the heating temperature is less than 150'C, water will remain in the silica gel, and when NF or gas is passed through the silica gel layer, N
, O and COz removal ability decreases. On the other hand, a higher temperature than necessary not only causes a loss of energy but also is disadvantageous because heat treatment at a high temperature significantly reduces the adsorption ability of silica gel.

Dehydration treatment by heating silica gel may be performed in air, but since the heating is performed to vaporize and dissipate the moisture contained in the silica gel, for example, an air stream of an inert gas that does not contain moisture such as N2 gas is used. It is best to do it inside.
It is also preferable to conduct the reaction under reduced pressure while sucking gas.

The heating time may be 30 minutes or more at the above-mentioned heating temperature and atmosphere, but it is usually carried out for 1 to 2 hours just to be sure.

The silica gel that has been dehydrated by heating is cooled down to room temperature or below by standing or forced cooling, but in this case, it is necessary to avoid contamination with moisture. Therefore, the method is to dehydrate silica gel by heating.
For example, by filling a column with silica gel,
A preferred method is to cool the silica gel layer after the dehydration treatment and then subsequently pass NFt gas through the silica gel layer.

Purification of NF and gas is carried out by a method of venting through a silica gel layer packed in a column etc. as described above, but the venting temperature at this time is important and must be below O'C.
Low temperatures are preferred. However, the boiling point of NF is -129℃
Therefore, it is virtually difficult to grow at temperatures below this temperature. Therefore, in the present invention, the ventilation temperature of Nh gas is 0 to -12
It is carried out at a temperature of 5°C.

The pressure of NF and gas during ventilation is also not particularly limited, but for example, a pressure of about 0 to 5 kg/cJ-G is preferred because it is easy to operate.

In the method of the invention, N! If the earlier step of removing F is omitted, Nzh, Neo and C will be removed in the later step of aerating NF and l gas through the silica gel layer as described above.
The removal duration or breakthrough time of Oo becomes shorter. To explain this specifically, the gas obtained by blowing a gas containing N2Ft, N, O, and CO□ through a silica gel layer initially contained extremely small amounts of Nth, NzO, and CO, which were below the detection limit. However, after a certain period of time, it rapidly increases in a short period of time. The time taken for this phenomenon to increase dramatically is called the breakthrough time.The analysis of Nth, N20, and C(h) mentioned above is carried out by gas chromatography, and the detection limit for each is topple. Since hh is removed in advance in the process, N20, Co,
The time when any of the contents exceeds this detection limit was defined as the breakthrough time.

〔Example〕

Hereinafter, the present invention will be explained in more detail with reference to Examples. Note that in the following, % and ppm represent capacity standards unless otherwise specified.

Examples 1 to 4 A cylindrical container (column) 1 shown in FIG. 1 made of stainless steel with an inner diameter of 10III11 and a length of 300+ am was coated with stearic acid as a lubricant on the surface as shown in FIG. 1, and the outer diameter was 6 m. After inserting the inner tube 3 having a length of 400+ nm concentrically with the column L, 5 weight percent of the cesium fluoride powder is added to the lithium fluoride powder with a water content of 3 weight percent in the gap between the column L and the inner tube 3. After filling the added and mixed powder 4 little by little, a press-fit tube 5 with an outer diameter of 9.6 m and an inner diameter of 6.5 m is inserted into the above-mentioned gap, and a load of 1 t/cj is applied to this press-fit tube 5 to fill the mixed powder. 4 was compression molded.

The operations of inserting the mixed powder 4 and compression molding 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 rate of 200°C/h in an electric furnace under a N2 gas atmosphere, kept at this temperature for 1 hour, and then allowed to cool naturally in the electric furnace to room temperature. A column 1 was obtained whose inner wall was entirely lined with a layer of solid fluoride having a thickness of 2III+1. In addition, the water contained in the lithium fluoride and cesium fluoride and the stearic acid used in forming the lining layer were also evaporated and removed by the above heating.

Thereafter, in this column 1, the contents of each component diluted with approximately the same volume of LIE gas were He 48.5%, NFi4
8.2%, NzFz 2.0%, NZ 0.4%, Ne
o A gas with a composition of O, 3%, CO, 0,6% (NF3 purity! ft 80.4 g) was heated at a ventilation temperature of 300°C, a gas flow of fit 60 NId/min, and a ventilation pressure cuff of 60 T.
The concentration of gas after 0 aeration, which was aerated under the conditions of orr, was 1% by weight.
After bubbling into an aqueous solution of potassium iodide (Kl), the NF was introduced into a collection bomb cooled with liquid nitrogen to liquefy and collect the NF. After stopping the IIF and gas ventilation, the inside of the NF3 collection cylinder was evacuated to remove the lle gas.

The composition of NF in the collection cylinder after ventilation was analyzed by gas chromatography. The results showed that the Nzh content was 10 pp or less for steel and the removal rate was very high. Also N
The loss amount of F3 was also very small at 1.3%.

Note that the content of N2 in the NFff gas was higher after ventilation, but this is considered to be because NzFz was decomposed into N2 and F2 due to heating.

Next, after filling a stainless steel column with an inner diameter of 15 IIll with granular silica gel having a particle size of 24 to 48 mesh (filling height: 300 vw), the silica gel was dehydrated by heating under the conditions shown in Table 1. , NF) gas from which NJ* was removed as described above was bubbled into the silica gel layer under the conditions shown in Table 1.

N in NF3 gas after aeration of Nh gas into the silica gel layer
The contents of 20 and CO□ were analyzed by gas chromatography, and the breakthrough time was measured. The breakthrough time was defined as the time when the content of any one of N, 0, and CO□ in the gas after aeration into the silica gel layer exceeded 10 ppm.

The results are shown in Table 1, and if purified by the method of the present invention, ! bO, COz, N2F! It can be seen that it is removed extremely well and the breakthrough time is also long.

Table ■ Examples 5 to 8 Using the column whose inner wall was lined with solid fluoride as used in Examples 1 to 4, a column with a particle size of 24 mm as solid fluoride was used.
~32 meshes of calcium fluoride (CaFz) (Example 5), aluminum fluoride (Aj2F+) (Example 6)
, potassium fluoride (KF) (Example 7), and sodium fluoride (Nap) (Example 8) (filling height 250 mm) L, heated to a temperature of 200'C, filled with N gas at 100% The water in the solid fluoride was removed by aeration for 1 hour at a flow rate of ml/min.

Next, NF gas having the same composition as that used in Examples 1 to 4, diluted with approximately the same volume of He gas, was added to the column filled with this solid fluoride at a ventilation temperature of 300° C. and 1 gas per 1 gas. Ventilation was carried out under the following conditions: A volume: 60 N ml/min, vent pressure: 60 Torr. The gas after ventilation was bubbled into a potassium iodide (Kl) aqueous solution in the same manner as in Examples 1 to 4, and then NF was liquefied and collected, and then l(e gas was removed).

When the composition of this NFi was analyzed by gas chromatography, the result was that the content of NzFz was 10ρpm.
The removal rate was extremely high. Furthermore, the loss amount of Nh was very small at 1.0.

It should be noted that the content of N2 in the NPj gas was higher after ventilation, but this is because NzFz was increased by heating as in Examples 1-4.
is considered to be decomposed into Nt and F2.

Next, in exactly the same manner as in Examples 1 to 4, the granular silica gel packed in a stainless steel column was dehydrated by heating under the conditions shown in Table 2, and then NFff from which Nth had been removed as described above was applied to the silica gel layer. Gas was passed under the conditions shown in Table '5S2.

NF, N in Nh gas after gas ventilation into the silica gel layer
, 0 and CO□ were analyzed by gas chromatography in the same manner as in Examples 1 to 4, and the breakthrough time was measured.

The results are shown in Table 2, and when purified by the method of the present invention, NJ, COz, and N2FZ in NFt gas can be purified.
It can be seen that it is removed extremely well and the breakthrough time is also long.

Comparative Example 1 in Table 1 In exactly the same manner as in Example 1, a stainless steel column was filled with granular silica gel, and then the granular silica gel was dehydrated by heating.

After that, NF3 gas (the content of each component is Nh93.6) before being diluted with the He gas used in Examples 1 to 4.
%, NiF23.9%, NO0.8%, Neo O,6
%, cot 1.2%) and without removing NzPz, the dehydrated granular silica gel layer was aerated under the same conditions as in Example 1 to measure the breakthrough time.

As a result, the breakthrough time was 4.0 hours, and Examples 1 to 4
The time was significantly shorter than that of

〔Effect of the invention〕

As explained in detail above, the present invention is carried out by heating NFt gas to a specific temperature in a container whose inner wall is lined with a solid fluoride, or in a state where the container is filled with a solid fluoride. After removing the NzF2 contained in it in advance, this NF and gas are passed through a silica gel layer that has been heated to a specific temperature and dehydrated in advance at a specific temperature and in a state where substantially no moisture is mixed. method, by implementing the present invention, NJz, N2 in NFff gas
0 and CO□ can be removed efficiently.

In addition, in the present invention, Nz in NF3 gas is
Since h is removal, the subsequent step of removing NZO and CO2 using silica gel can be carried out for a long time as shown in Examples and Comparative Examples.

Furthermore, in the present invention, a container whose inner wall is lined with a solid fluoride is used in the first step, or a container lined with the solid fluoride and a solid fluoride are used, and silica gel is used in the second step. Since it is also inexpensive,
It is also an economical method. Furthermore, the method of the present invention does not involve adsorption of NF3 to silica gel, which is an adsorbent.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a state in which the inner wall of a cylindrical container (column) is lined with solid fluoride in an example. In the figure, 1-- cylindrical container (column), 2-- NF, gas outlet pipe, 4-- solid fluoride, 6-- auxiliary cylindrical tube, are shown. 3--Inner cylinder, 5--Press-fit pipe, 7-- Fixed plate, Fig. 1

Claims (2)

[Claims] (1) In a container whose inner wall is lined with solid fluoride, nitrogen trifluoride gas containing at least dinitrogen difluoride, nitrous oxide, and carbon dioxide as impurities is heated at 150 to 600°C.
After removing the dinitrogen difluoride contained by heating to a temperature of
Subsequently, the nitrogen trifluoride gas is heated in advance to a temperature in the range of 150 to 300°C to dehydrate the silica gel layer.
A method for purifying nitrogen trifluoride gas, which comprises removing nitrous oxide and carbon dioxide by aeration at a temperature of -125°C and in a state substantially free of moisture. (2) The method for purifying nitrogen trifluoride gas according to claim 1, wherein a container whose inner wall is lined with solid fluoride is filled with solid fluoride.