JPH0551782A - Production of gaseous nitrogen trifluoride - Google Patents

Abstract

PURPOSE:To suppress revelation of anode effect in a production of nitrogen trifluoride using a carbon electrode. CONSTITUTION:In the production of gaseous nitrogen trifluoride by electrolysis using an ammonium fluoride-hydrogen fluoride two component molten salt as an electrolyte, when the ratio of the maximum current under >=1V and < 4V potential based standard hydrogen electrode potential to the maximum current under 4V potential in potential scanning with a carbonaceous electrode as an anode is >=10, electrolysis is executed in >=50mA/cm<2> anode current density with another carbonaceous electrode from the carbonaceous electrode. In the result, revelation of anode effect is suppressed and dissolution of anode, contamination of electrolyte is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing nitrogen trifluoride gas (NF ₃ ). More specifically, ammonium fluoride
NF by electrolysis of (NH ₄ F) -hydrogen fluoride (HF) binary molten salt
₃ Gas production method.

[0002]

2. Description of the Related Art With the recent rapid development of the electronics industry, the density and performance of semiconductor devices have been increased, and the production of ultra-large-scale integrated circuits has increased. .. Along with this, NF of high purity is used as a gas for dry etching used in the manufacturing process of the integrated circuit and as a gas for a cleaner of a CVD device.
₃ gas is now required.

The method of producing NF ₃ gas is roughly divided into a chemical method and an electrolytic method. The chemical method was obtained in the second step by producing fluorine gas (F ₂ ) by electrolysis as the first step.
NF ₃ gas is produced by reacting F ₂ with a nitrogen-containing raw material. On the other hand, in the electrolysis method, a NF ₃ gas is produced by electrolyzing a non-aqueous solution type molten salt containing a nitrogen content and a fluorine content as an electrolytic solution.

A feature of the industrial electrolysis method is that high-purity NF ₃ gas containing almost no CF ₄ can be produced. Equal the boiling point of CF ₄ is very close to the boiling point of NF ₃ gas, since the physical properties are very similar, be removed by purification of CF ₄ in the NF ₃ gas is extremely difficult, high-purity NF ₃
For gas production, it is important that the process does not have a carbon source.

Next, the electrolysis method will be described in more detail. Anode materials that can be used in the electrolysis method are nickel and carbon. When nickel is used, since the process does not have a carbon source that causes CF ₄ generation, it is possible to produce a high-purity NF ₃ gas containing almost no CF ₄ . However, the nickel anode has the following drawbacks. That is, the nickel anode is electrolyzed at a dissolution current efficiency of only a few percent. However, when the electrolysis is industrially continued for a long period of time, the nickel anode is consumed, and eventually the electrode needs to be renewed. Furthermore, the dissolved nickel accumulates in the molten salt electrolyte as nickel complex salt sludge,
Since the electrolyte is contaminated, the electrolyte needs to be renewed. The frequency of renewal of the electrode and the molten salt electrolyte varies depending on the amount of current and the size of the electrode, but it is the biggest cause of the reduction in operation efficiency industrially and is a major problem.

Since carbon is insoluble, the problem with nickel does not occur when used in the anode. However, since CF ₄ is generated using the electrode itself as a carbon source, high-purity NF
₃ Gas cannot be produced, and there are problems such as electrode collapse. However, since a new type of carbon electrode was recently developed in which the generation of CF ₄ was suppressed and mechanical strength was remarkably improved, the possibility of producing high-purity NF ₃ gas using the new type carbon electrode is increasing.

However, the problem peculiar to the carbon electrode, that is, the so-called anodic effect has not been completely solved, and even the new-type carbon electrode has a problem in using it as an anode.

[0008]

The anodic effect is a phenomenon in which the electric potential suddenly rises abnormally during electrolysis and electrolysis cannot be continued. This occurs because the electrode surface is covered with the generated gas by the film having extremely low surface energy generated on the electrode surface during electrolysis.

[0009] As a result of intensive studies on the manifestation of the anodic effect, the inventors have used a carbonaceous electrode prepared separately from the carbonaceous electrode before conducting electrolysis on the carbonaceous electrode, If the current peak ratio of the single-sweep voltammogram obtained when performing scanning satisfies the predetermined requirements, then, it was found that the anode effect does not appear even if electrolysis with a carbonaceous electrode is performed, and the present invention was completed. It has come to do.

That is, according to the present invention, in the production of nitrogen trifluoride gas by an electrolysis method using an ammonium fluoride-hydrogen fluoride binary system molten salt as an electrolytic solution, a standard hydrogen is used in a potential scan using a carbonaceous electrode as an anode. When the ratio of the maximum current at 4 V or more to the maximum current at 1 V or more and less than 4 V based on the electrode potential is 10 or more, the anode current density is 50 mA / cm in a carbonaceous electrode different from the carbon electrode. ^A method for producing nitrogen trifluoride gas, characterized in that electrolysis is performed at ² or more.

The present invention is intended to suppress the development of the anodic effect, which has been a problem in the past when using a carbon electrode, and it does not matter whether the carbon electrode is a new type or a conventional type. Needless to say.

The present invention will be disclosed in detail below. First, a method for adjusting the molten salt used for electrolysis will be described. The molten salt can be prepared by two methods. The first method is to adjust from ammonium hydrogen difluoride (NH ₄ HF ₂ ) and HF. First, a predetermined amount of NH ₄ HF ₂ is put into a container or an electrolytic cell, and a predetermined amount of gaseous form is added to this. It is what blows HF. The second method is a method in which a predetermined amount of ammonia gas (NH ₃ ) is directly reacted with gaseous HF in a container or an electrolytic cell to prepare a molten salt. In the latter method, it is 5 to 70 vol for both NH ₃ gas and HF gas.
When a dry inert gas of about%, for example, nitrogen, argon, or helium is supplied together, the molten salt can be stably adjusted without backflow of the molten salt into the gas supply pipe. In either case, the molten salt can be easily adjusted. NH ₄ F has remarkable deliquescent property and thermal decomposability, but it can be used as a molten salt raw material.

The temperature of the molten salt used in the present invention is 16
0 ° C or less is desirable. When the temperature of the molten salt exceeds 160 ° C, the vapor pressure becomes extremely high, which not only increases the loss of the molten salt but also causes the problem that volatile components are condensed and solidified near the outlet of the electrolysis product gas, which causes clogging. ..

As described above, it is a feature of the present invention that the potential scanning is performed using the carbonaceous electrode before the electrolysis using the prepared electrolytic solution. As the carbonaceous electrode for performing potential scanning, it is preferable to use a carbonaceous electrode having the following characteristics. That is, in the single sweep voltammogram obtained by potential scanning in concentrated sulfuric acid, the potential giving the peak having the maximum current density is 1.2 V or more in terms of the potential using mercuric sulfate as the reference electrode. The behavior in concentrated sulfuric acid can be used as a scale for measuring the difficulty of HF penetration between microcrystalline layers in a carbon electrode, and those whose potential is less than 1.2 V may be destroyed by potential scanning in concentrated sulfuric acid. Abnormalities may be found.

Other than that, it is acceptable to use a carbon electrode having the following characteristics. Porosity 2 ~ 10%,
The average diameter of the pores is, for example, 1 μm. A bulk specific gravity of about 1.50 to 1.7 g · cm ^−3, and an aggregate raw material having an isotropic structure or shape (for example,
Select a method disclosed in Japanese Patent Publication No. 50-39427) or a molding method in which the raw material aggregate particles are not aligned in a specific direction (for example, Japanese Patent Publication No. 51-2).
No. 0197 disclosed in Japanese Patent Application No. 0197), which is cut out from a carbon block (so-called isotropic carbon) having an anisotropy ratio of specific resistance of 1.2 or less, which is obtained by applying the technique. Further, a material having high mechanical strength is also preferable. For example, the bending strength is 50 MPa or more.

Since these electrodes are installed separately from the original electrodes for electrolysis, the size of the electrodes is not particularly limited. Therefore, the method of providing the hole for mounting the potential scanning electrode in the electrolytic cell lid and installing the potential scanning electrode from this hole is the most simple method for carrying out the present invention. More specifically, 1 cm
If the electrode area is about ^2, it can be sufficiently implemented.

Next, potential scanning is performed using this potential scanning electrode. As the conditions for the potential scanning, the scanning speed and the scanning potential range can be increased. Scanning speed is from 1 mV / sec to several hundred mV
It can be implemented in the range of up to / sec. Of course, it does not matter if it goes out of this range. However, when the scanning speed is extremely slow, the absolute value of the current becomes small, which may make the measurement difficult depending on the ammeter. Further, even when the scanning speed is extremely high, the absolute value of the current becomes large, and the same is true. As described above, the scanning speed should be appropriately selected according to the capability of the device for performing potential scanning. The scanning potential range is usually from the natural immersion potential to the standard hydrogen electrode potential (hereinafter, SH
It is sufficient to carry out the voltage up to 10 V with reference to (abbreviated as E). It is meaningless in the present invention because a reducing current flows below the natural immersion potential. Although there is no problem in implementing it in the range of 10V or more, it is empirically not necessary to implement it in the range of 10V or more.

The potential scanning is performed as described above. Here, the scanning speed is 100 mV / sec, the scanning potential range is more than the natural immersion potential.
It was performed at 10V vs. SHE. Usually, two current peaks are observed in the voltammogram obtained by the potential scanning as shown in FIG. When the peak current value at 1 to 4 V vs. SHE is Ip1, and the peak current value or the maximum current value appearing at a potential higher than 4 V vs. SHE is Ip2, the current peak ratio is represented by Ip2 / Ip1. When the current peak ratio is less than 10, the anode effect is generated when switching to the electrode for electrolysis thereafter. On the other hand, when the ratio is 10 or more, the anode effect does not occur.

[0019]

The present invention will be described in more detail with reference to the following examples.

Example 1 The NH ₄ F-HF binary molten salt was prepared in an electrolytic cell having a volume of about 2 liters. About 2200 g of vacuum-dried NH ₄ HF ₂ was charged into the electrolytic cell, and about 800 g of HF gas was fed into the electrolytic cell at 50 g / min for preparation. The molten salt composition was approximately NH ₄ F · 2HF. This was used as an electrolytic solution, first the potential scanning carbonaceous electrode was used as the anode, the potential scanning speed was 100 mV / sec, and the scanning potential range was more than the natural immersion potential.
A potential scan was performed at 10 V vs. SHE. As a result, the obtained current peak ratio was 14. Then, electrolysis was continued for 1 hour at an anode current density of 70 mA / cm ² , but the anode effect did not occur.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the molten salt was prepared by using NH ₄ HF ₂ which was not vacuum dried.
The current peak ratio obtained as a result of the potential scanning was 7. Further, when electrolysis was continued with the carbon electrode, the anode effect occurred within 1 hour, an abnormal increase in voltage occurred, and continuation of electrolysis became impossible.

[0022]

The carbon electrode is an electrode that solves the drawback of the nickel electrode, namely the dissolution, but it has the drawback of the anode effect. Although it is important to surely suppress this, the present invention has found that the use of an extremely simple method has a great effect on suppressing the anode effect. It can be said that there is great significance in giving a new possibility to solve the conventional problems in the electrolytic production of NF ₃ gas, that is, the dissolution of the anode, the contamination of the electrolytic solution and the like.

[0023]

[Brief description of drawings]

FIG. 1 is a current peak obtained from an example of a voltammogram by potential scanning of the present invention.

Claims (2)

[Claims] 1. In the production of nitrogen trifluoride gas by an electrolytic method using an ammonium fluoride-hydrogen fluoride binary molten salt as an electrolytic solution, a potential scanning using a carbonaceous electrode as an anode,
When the ratio of the maximum current at 4 V or more to the maximum current at 1 V or more and less than 4 V at the potential based on the standard hydrogen electrode potential is 10 or more, the carbonaceous electrode different from the carbon electrode has an anode current density of 50 mA. A method for producing nitrogen trifluoride gas, characterized in that electrolysis is performed at a pressure of not less than / cm ² . 2. The potential of a carbonaceous electrode for performing potential scanning, which gives a peak having the maximum current density in a single-sweep voltammogram obtained by potential scanning in concentrated sulfuric acid, uses mercuric sulfate as a reference electrode. 1.2 V at electric potential
The method according to claim 1, which is the above.