JPH02263988A - Production of gaseous nitrogen trifluoride - Google Patents

Abstract

PURPOSE:To obtain high purity gaseous NF3 having a low CF4 content by electrolyzing an NH4F.HF-based molten salt while keeping the water content in a prescribed range and replenishing the salt when gaseous NF3 is produced by electrolyzing the salt. CONSTITUTION:A molten salt is prepd. with NH4F and/or NN4F.HF and RF as starting materials. This molten salt is previously dehydrated and electrolyzed to regulate the water content to 0.01-1wt.% and electrolysis is carried out. After this electrolysis, the molten salt is replenished with 0.1-10wt.% of the above-mentioned starting materials basing on the total amt. of the salt and the replenished salt is electrolyzed while keeping the water content in the range of 0.01-1wt.%.

Description

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

More specifically, N11. F
-Relates to a method for producing NF□ gas by IIF-based fused salt solution method.

[Conventional techniques and problems to be solved by the invention]N
F is a colorless gas with a boiling point of -129'C1 and a melting point of 1207°C.

NF and gas have recently attracted attention as a dry etching agent for semiconductors and a cleaning gas for CVD equipment, but NF and gas used for these purposes are required to be of high purity.

However, the produced NFff gas is nitrogen (N2)
, dinitrogen difluoride (NzFz), nitrous oxide (Neo)
, carbon dioxide (CO,), oxygen difluoride (OPT),
Contains relatively large amounts of impurities such as carbon tetrafluoride (CF4), oxygen (0□), and unreacted hydrogen fluoride (IIF),
NFff manufactured by the molten salt electrolysis method targeted by the present invention
The same applies to gas. Therefore, purification is necessary to obtain high-purity NF and gas for the above-mentioned uses.

As a purification method for removing these impurities from NFx gas, the following method is known.

That is, 1) Ngh is 1.1lazs, NaxSzO
A method of contacting with an aqueous solution such as s (J, Massonn
e+Chemie Ingenur Tehenir (Chew
, Ing, Techn, )41+(12), 695
．． (1969)) or a method of contacting with metal at a temperature of 148.9 to 537.8°C (Japanese Patent Publication No. 15081/1981). 2)) IP is too'
It can be easily removed by heating it to around c and contacting it with NaF. 3) OFt is Na1SzOz, 1. It can be removed by contacting with an aqueous solution of Na, SO), 1111NazS, or the like. 4) N2O and C
Components with relatively high boiling points such as O□ can be efficiently removed by contacting with adsorbents such as zeolites (Chew, Eng.

84, 116. (1977) etc.), 5) Nz and O
Low boiling point components such as R can be removed by cooling to a temperature of -150°C to -190°C and liquefying NF.

However, CF4 is not removed by each of the above methods,
An effective method for removing it is not yet known. Also CF
4 has a boiling point of 1128°C, which is very close to the boiling point of NF3, so it is impossible to separate it even by cryogenic distillation of Nf''+.

Under these circumstances, the NFs gas purified by each of the above methods contains about 100 to 200 ppm by volume of CF4. As a result of this, the purity of currently commercially available NF and gases is limited to 99.99% by volume, and the current situation is that they do not satisfy recent demands for even higher purity. In addition, if CP is contained in the NF gas,
When this NF3 gas is used for dry etching of semiconductors or cleaning of CVD equipment, there is also the problem that sufficiently satisfactory results cannot be obtained.

On the other hand, since the raw material NH, F·IIF-based molten salt is always highly hygroscopic, it inevitably absorbs moisture from the air during the raw material preparation stage. When a molten salt containing such moisture is electrolyzed, OF and H2 are produced as by-products due to the influence of this moisture, and this O
Ft and ■ mix into the NF and gas generated from the anode, causing an explosion. Therefore, NF3 by molten salt electrolysis method
In the production of gas, so-called dehydration electrolysis is indispensable, in which a current lower than the current density during electrolysis (main electrolysis) is passed in advance, and after the dehydration electrolysis is completed, the main electrolysis is carried out. This is a batch type electrolysis in which the electrolysis is stopped when the amount of molten salt in the electrolytic cell becomes small, and after the raw materials are prepared again, dehydration electrolysis and main electrolysis are repeated.

Therefore, the operation rate is low and the electric power consumption rate is also deteriorated accordingly, so it is desired to establish a continuous electrolysis method.

(Means for Solving the Problems) In view of the above situation, the present inventors have developed a method in which continuous electrolysis is possible when producing NF3 gas by the molten salt electrolysis method.
Moreover, the content of CF4 in the generated Nh gas is small, and as a result of intensive studies on how to obtain extremely high-purity Nh gas by purifying it with each of the above methods, we have found that the water content in the molten salt can be reduced. By limiting the amount to 0.01 to 1.0% by weight and replenishing the raw material continuously or intermittently, CF, NF with a small content, and gas can be continuously obtained.
Moreover, they discovered that molten salt electrolysis can be carried out continuously rather than batchwise, leading to the completion of the present invention.

That is, when producing nitrogen trifluoride gas by a molten salt electrolysis method using ammonium fluoride or/and acidic ammonium fluoride and anhydrous hydrofluoric acid as raw materials, the water content in the molten salt is set to 0.01 to 1. This is a method of electrolyzing in the range of .0% by weight, in which the molten salt electrolytic bath is dehydrated and electrolyzed in advance, then the main electrolysis is performed, and when the raw material is replenished, the amount of the raw material supplied is equal to the total electrolysis amount. This is a method for producing nitrogen trifluoride gas, characterized in that nitrogen trifluoride gas is supplied continuously or intermittently in a range of 0.1 to 10% by weight.

(Detailed disclosure of the invention) The present invention will be explained in detail below.

FIG. 1 is a diagram showing an example of an electrolyzer, in which NF3 gas is generated from the anode 3 and lh gas is generated from the cathode 4.
If PI gas and 11□ gas mix, they will explode, so a partition plate 5 is provided between the anode 3 and cathode 4 to prevent this mixing. In addition, for electrolysis, three anodes and four cathodes were used.
In some cases, an inert gas such as N2 gas is introduced into each side as a carrier gas.

When obtaining NF3 gas as a NIl, P/IIF-based molten salt material, if water is present in the molten salt, OF
, and 11□ are produced as by-products, which are mixed into the NF3 gas generated from the anode and cause an explosion. This is not desirable from this point of view, but if the water in the molten salt is 1. [1st generation% (hereinafter referred to as
The inventors have confirmed that the amount of by-products of ops and I+ is small and there is no danger of explosion.

On the other hand, the present inventors have found that there is a relationship between the water content in the molten salt and the content of NF and CF in the gas. That is, when some moisture is present in the molten salt, the content of CF4 in the NF and l gases decreases.

The reason for this is not clear, but it is likely that the organic matter in the raw material and F generated by the oxidation of IIF react to form CF, or the islands generated by water decomposition react with the organic matter to produce COt. It is considered that this is preferential, and as a result, the content of CF decreases.

In the present invention, dehydration electrolysis is performed until the water content in the molten salt falls within a specific range, and then main electrolysis is performed.

As this electrolysis continues for a long time, the electrolytic solution decreases, so it is necessary to replenish the molten salt.

However, a batch method in which electrolysis is stopped when the amount of molten salt becomes small, molten salt is prepared again, dehydration electrolysis is performed again, and then main electrolysis is performed is not preferable because the work efficiency is significantly reduced. .

Therefore, the present inventors have determined that the water content in the molten salt is 0.0.
If it is 1 to 1.0%, as mentioned above, the amount of by-products of OF and 112 is small, and it has been confirmed that there is no danger of explosion. If the water content is less than 0.01%, the reduction in CF content will be insufficient, and if it exceeds 1.0%, there will be a risk of the NF3 gas generated as described above exploding, which is disadvantageous. It is.

In the present invention, as described above, the water content in the molten salt is 0.0.
If it is 1 to 1.0%, the operation can be carried out safely without the risk of the generated NFi gas exploding. Therefore, if the molten salt is replenished so that the water content in the molten salt does not deviate from the above range, not only can safety be ensured, but also CF,
A reduction in the content of is also achieved.

Usually, molten salt absorbs moisture from the air during preparation and contains 1 to 2% moisture. Therefore, if this newly prepared molten salt is continuously or intermittently supplied to the electrolytic cell in small amounts during electrolysis, the water content in the molten salt can be maintained within the above range, and Molten salt can be replenished. Therefore, dehydration electrolysis only needs to be carried out at the beginning of the first electrolysis, and the main electrolysis can be carried out continuously thereafter.

In the present invention, the replenishment of molten salt is 0.1-
It is preferable to replenish in a range of 10%; if it is less than 0.1%, the water content in the molten salt will hardly increase, but it will require frequent replenishment, making the work complicated and undesirable. 1
If it exceeds 0%, there will be a sudden increase in moisture content, fluctuations in liquid level, and temperature changes, which is undesirable.

Furthermore, in the present invention, it is more preferable to continuously replenish the molten salt because this will reduce the rapid increase in water content, fluctuations in liquid level, and temperature changes.

In the present invention, the water content in the molten salt is 0.01 to 1.0%.
It is possible to continue electrolysis by replenishing the electrolytic salt while maintaining the W1 range.

Therefore, the water content of the molten salt in the electrolytic cell must be measured during electrolysis. However, the water content in the molten salt is 0.01 to 1.0%, which is a small amount to in! Therefore, the Karl Fischer method, which is a simple moisture measurement method, cannot be applied.Therefore, in the present invention, the moisture content in the molten salt is It was measured by the method described in the paper "Study of method for determining water content in molten fluoride baths" published by Masahiko et al.

That is, using binderless carbon as the anode as a test electrode, a potential scan is performed with respect to the base f$ electrode potential shown by the platinum/electrolyte solution, and the water content is determined from the ratio of the heights of two current peaks that appear at this time. That is, the peak height that appears around 2V is proportional to the amount of water in the molten salt that is in contact with the effective surface area of the test electrode.

Test 7B, the effective surface area of Pfx is proportional to the peak height appearing around 8■. Therefore, since the ratio of the two peak heights is proportional to the water content in the molten salt, it is possible to obtain a calibration curve for the water content in the electrolytic solution.

Incidentally, the NF3 gas produced by the method of the present invention can be purified by the above-mentioned known methods to easily obtain extremely high purity INF and gas with a low CF4 content.

[Example] Hereinafter, the present invention will be explained in more detail with reference to Examples. Note that in the following, ppl represents a capacity standard unless otherwise specified.

Example 1 N by molten salt electrolysis using the electrolyzer shown in Figure 1.
F. Gas was produced.

That is, acidic ammonium fluoride (NH,F) I is placed in the electrolytic cell 1.
F) 24kg preparation, 120°C temperature TI-
Molten salt 2 was prepared by gradually adding 6 kg of anhydrous IIF to the mixture while it was still warm (IIF/HaF molar ratio 1.7, water content 1.8%).

Thereafter, a current of 20 amperes (A) was passed from the anode 3 to the cathode 4 to perform dehydration electrolysis for 72 hours. At this point? Since the water content of 6 n+ salt 2 was 0.32%, the current was increased to 80 A and the main electrolysis was started.

As the main electrolysis continues, the molten salt 2 in the electrolytic cell 1 decreases, so the II P/N If prepared in advance in the raw material tank 9,
Molten salt 1 with an F molar ratio of 1.7 and a water content of 1.8%
The main electrolysis was carried out for 300 hours by intermittently replenishing 0.5 kg of 0.0 to the electrolytic cell 1 every 12 hours.

The water content (2) of the molten salt 2 during the main electrolysis was measured every 5 hours, and the water content was 0.03 to 0.07%.

In addition, N in the NF3 gas generated from the anode 3 during the main electrolysis
The concentrations of F+ and CF were also measured every 5 hours and the results are shown in Table 1.

Examples 2 to 4 Dehydration electrolysis and subsequent main electrolysis were carried out under the same conditions as in Example 1, except that the water content of the molten salt 10 prepared in the raw material tank 9 and the dehydration electrolysis time were changed to the values shown in Table 1. I did this.

Moisture content of molten salt 2 in electrolytic cell 1 during main electrolysis, anode 3
The concentrations of NF generated from the gas, NF in the gas, and CPa were as shown in Table 1.

Example 5 N by molten salt electrolysis using the electrolyzer shown in Figure 1.
F3 gas was produced.

That is, acidic ammonium fluoride (NH, F・1
1F) was charged, and 6 kg of anhydrous IIF was gradually added thereto while raising the temperature to 120°C to prepare molten salt 2 (IIP/N11.F molar ratio 1.8, water content 1). .9%).

Thereafter, a current of 20 amperes (A) was passed from the anode 3 to the cathode 4 to perform dehydration electrolysis for 70 hours. Since the water content of the molten salt 2 at this point was 0.35%, the current was increased to 80 A and the main electrolysis began.

As the main electrolysis continues, the molten salt 2 in the electrolytic cell 1 decreases, so the molten salt 10 with an IIF/NH,F molar ratio of 1.8 and a water content of 1.9% prepared in advance in the raw material tank 9 is The main electrolysis was carried out for 300 hours using a diaphragm pump to continuously replenish the feed rate at a feed rate of 42 g/h, and the moisture content of the molten salt 2 during the main electrolysis was measured every 5 hours. was in a very stable state at 0.07-0.09%. Furthermore, the concentrations of NF and CFa in the NFI gas generated from the anode 3 during the main electrolysis were measured every 5 hours, and the results were as shown in Table 1.

Comparative Example 1 (Conventional Method) Molten salt electrolysis was performed without replenishing molten salt 10 to electrolytic cell 1 during main electrolysis.

That is, in the same manner as in Example 1, 2 NIl and F liver were placed in the electrolytic cell 1.
Molten salt 2 was prepared by charging 4 kg of anhydrous IIF and 6 kg of anhydrous IIF (IIF-I1.F molar ratio 1.7, water content 1.7 kg).
8%), a current of 20 A was applied to perform dehydration electrolysis for 96 hours.

At this point, the water content of molten salt 2 was 0.005%, so the current was increased to 80A and the main electrolysis started.
Main electrolysis was performed for 50 hours.

'01 of NF in NF3 gas produced by this electrolysis
The concentration was 54% by volume, and the concentration of CFa was 110 ppm.

[Effects of the Invention] As described above in detail, the present invention has the following effects! +11. F.I.
When producing NF3 gas by the IF-based molten salt solution method, electrolysis is carried out continuously for a long period of time while maintaining the water content in the molten salt in the range of 0.01 to 1.0% and replenishing the electrolytic salt. As a result, Nh gas containing less CF4, which could not be removed by purification, can be obtained. Therefore, if the NF gas produced by the method of the present invention is purified by the method described above, it will have a low CF content and extremely high purity, which is suitable as a dry etching agent for semiconductors or a cleaning gas for CvD equipment. Nl? ff gas can be obtained.

Furthermore, since the method of the present invention makes it possible to perform molten salt electrolysis continuously for a long period of time, as in the conventional molten salt electrolysis method, dehydration electrolysis is performed, followed by main electrolysis, Since this is not a batch method in which electrolysis is stopped when the amount of molten salt becomes small, molten salt is prepared again, dehydration electrolysis is performed again, and then main electrolysis is performed, the operating rate and power consumption rate can be improved. It is possible. Another advantage is that electrolytic work can be simplified.

As described above, the NPs and gas produced by the method of the present invention are C
Since the content of F is small, extremely high purity NFff gas can be obtained by purification using the conventionally known method described above, and the fact that electrolysis can be performed continuously makes it economically valuable. There is something very big about it.

In addition, the method of the present invention uses ammonia and anhydrous hydrofluoric acid as raw materials and electrolyzes them by i8 molten salt electrolysis method.

Of course, the present invention can also be applied to methods of producing gas.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an electrolyzer K used in Examples and Comparative Examples. In the figure, 1------ Electrolytic cell, 8---1 Heating device, 2...--1111 Molten salt, 9---1--
Raw material tank, 3----one anode, 10----
-molten salt, 4--11--cathode, 11-
--1 Raw material supply pipe, diaphragm, valve, F3 gas outlet pipe, ... Heating device ----)1! Gas outlet pipe, shown.

Claims (1)

[Scope of Claims] 1) When nitrogen trifluoride gas is produced by a molten salt electrolysis method using ammonium fluoride or/and acidic ammonium fluoride and anhydrous hydrofluoric acid as raw materials, the water content in the molten salt A method for producing nitrogen trifluoride gas, which comprises electrolyzing nitrogen trifluoride gas in a range of 0.01 to 1.0% by weight. 2) When producing nitrogen trifluoride gas by the molten salt electrolysis method using ammonium fluoride or/and acidic ammonium fluoride and anhydrous hydrofluoric acid as raw materials, the molten salt electrolytic bath is dehydrated and electrolyzed in advance, and then After carrying out the main electrolysis, when replenishing the raw material, the raw material is replenished continuously or intermittently within a range of 0.1 to 10% by weight based on the total amount of the electrolytic bath. A method for producing nitrogen trifluoride gas, characterized by: