JP3340273B2 - Composite electrode and method for producing nitrogen trifluoride gas using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to nitrogen trifluoride (NF ₃₎ a method for the production of gas. More specifically, the present invention relates to a method for producing NF ₃ gas by electrolysis of an ammonium fluoride (NH ₄ F) -hydrogen fluoride (HF) -based molten salt.

[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 been increasing. Accordingly, high-purity NF ₃ gas has been required as a gas for dry etching used in the integrated circuit manufacturing process and as a gas for a cleaner of a CVD apparatus.

[0003] The method for producing NF ₃ gas is roughly divided into a chemical method and an electrolytic method. In the chemical method, a fluorine (F ₂ ) gas is produced by electrolysis as a first step, and an NF ₃ gas is produced by reacting the F ₂ gas obtained in the second step with a nitrogen-containing raw material. On the other hand, the electrolysis method
An NF ₃ gas is produced by using a non-aqueous solution type molten salt containing a nitrogen component and a fluorine component as an electrolytic solution and electrolyzing the electrolytic solution.

[0004] The electrolysis method has an advantage that NF ₃ gas can be produced in one step and in a high yield as compared with the chemical method.

In the chemical method, carbon tetrafluoride (CF₄) Is large
F included in₂Inevitably requires a large amount of CF
₄Is NF₃Mix into gas. However, this CF₄Is
NF ₃Is very similar in physical properties to high-purity NF₃Gas
In order to obtain it, use sophisticated purification techniques that are industrially expensive.
I have to apply it. In contrast, the electrolytic method
In the process CF₄Is hardly generated or mixed
Therefore, high-purity NF₃Has the advantage of obtaining gas
ing.

The outline of the industrial synthesis of electrolytic NF ₃ gas is as follows. The electrolytic solution is ammonia or ammonium acid fluoride (NH ₄ HF ₂ ) and anhydrous hydrogen fluoride (HF)
NH ₄ F-HF-based molten salt is used. This is electrolyzed with a nickel anode. NF ₃ gas is generated from the anode, and NF ₃ gas containing impurities is obtained from the anode side. NF ₃ gas purity after the purification operation exceeds 99.99% by volume.

[0007]

The most suitable metal material for the anode is nickel. Other metals are either passivated and no current flows, or they melt violently. However, some dissolution also occurs with nickel,
The electrodes wear out. For this reason, in industrial production, not only frequent electrode replacement is caused, but also frequent replacement of the electrolytic solution contaminated with the nickel salt generated by dissolution is inevitable. To address this problem, the effect of suppressing dissolution of various electrode materials and electrolytes has been studied, but no definitive measures have been found.

[0008]

Means for Solving the Problems The present inventors diligently studied the difference in dissolution behavior between nickel and other metals in order to solve this problem relating to dissolution. As a result, in the case of nickel, the stable conductive oxyfluoride covers the electrode surface, and electrons are exchanged between the electrode and the electrolyte through the film, so that it is lower than other metals. It has been found that electrolysis is carried out in a dissolved amount and without being in a passive state. Further, the present invention is considered to be able to further reduce the amount of dissolution by using a specific electrode in order to positively promote the generation of conductive oxyfluoride on the electrode surface, thereby completing the present invention.

That is, the present invention provides a composite electrode for producing nitrogen trifluoride gas obtained by sintering a nickel-based oxide and dispersed nickel plating or nickel powder, or ammonium fluoride (NH ₄ F) -fluoride. Hydrogen (H
F) A molten salt having a composition molar ratio (HF / NH ₄ F)
The present invention relates to a method for producing a nitrogen trifluoride gas, comprising performing electrolysis using the above-mentioned composite electrode as an anode while maintaining an electrolytic solution having a value of 1 to 3 at 100 to 140 ° C.

Next, the present invention will be described in more detail.
You. The nickel-based oxide used in the present invention includes NiO₂,
NiO, Ni₂O₃, Ni ₃O₄, NiO_{1 + X}, Li
NiO₂, Ni₂O and Ni₂O + nickel oxide
I can do it. Also, it becomes an oxide during sintering, for example, Ni
(OH)₂Etc. can also be used.

The electrode used in the present invention is obtained by coexisting nickel by nickel-based oxide and dispersed nickel plating or sintering with nickel powder. For example, in a method in which nickel powder and lithium nickel oxide are mixed and then sintered at a high temperature in an inert gas atmosphere, a sintered electrode mainly made of nickel whose nickel-based oxide is nickel oxide can be obtained.

Further, by dispersing lithium nickelate as dispersed particles in a nickel plating bath and performing electrolysis,
A composite electrode in which lithium nickelate is incorporated in a nickel matrix on a nickel plate can be obtained. The nickel plate used mainly contains nickel, and preferably has a nickel content of about 60% or more. Further, pure nickel, a monel alloy, and the like can be given.

FIG. 1 shows an example of the configuration of the electrolytic cell. The main body 1 and the tank lid 2 have a structure that isolates the electrolytic solution 8 and generated gas from the outside of the system. In general, the connection between the main body 1 and the tank lid 2 is fixed and sealed via packing in order to ensure airtightness. When the inner surfaces of the main body 1 and the tank lid 2 are covered with a fluororesin or the like, the durability is further improved.

The anode 3 and the cathode 4 are separated by a partition wall 5 provided on the tank lid 2. When NF ₃ generated from the anode 3 and H ₂ generated from the cathode 4 are mixed, they easily ignite and explode. Therefore, a partition 5 is provided to prevent this. The length of the partition wall 5 in the downward direction can be appropriately selected on condition that the partition wall 5 does not extremely approach the bottom of the main body 1 and is below the electrolyte surface. Note that the main body 1 can be a cathode or an anode.

The generated gas is led out of the electrolytic cell through an anode gas outlet 6 and a cathode gas outlet 7 provided in the tank lid 2. In the electrolysis, an inert gas such as a nitrogen gas may be supplied as a carrier gas to the anode 3 side and the cathode 4 side, respectively. The material of the main body 1, the tank lid 2, and the partition 5 is usually a metal, but a fluororesin or the like can be used if necessary.

The illustrated electrolytic cell merely shows the basic configuration requirements, and it goes without saying that the electrolytic cell varies in shape, arrangement of electrodes and partition walls, and the like. Although a special electrode is used, there is no need for a specially configured electrolytic cell for that purpose. Further, the effects of the present invention are not affected by the configuration of the electrolytic cell.

The electrolyte is ammonium fluoride (NH)
₄ F) - using a hydrogen fluoride (HF) based salts. Examples of the preparation method include a method of preparing from ammonia gas and anhydrous hydrogen fluoride, a method of preparing from ammonium hydrogen difluoride and anhydrous hydrogen fluoride, a method of preparing from ammonium fluoride and anhydrous hydrogen fluoride.

The electrolytic solution can be prepared, for example, by the following method. A method for preparing ammonium hydrogen difluoride (NH ₄ HF ₂ ) or / and ammonium fluoride (NH ₄ F) and anhydrous HF is as follows. First, NH ₄ HF ₂ or / and NH _{4 is} added to a vessel or an electrolytic cell.
A predetermined amount of F is charged, and a predetermined amount of anhydrous HF gas is blown into this.

Another method is a method in which a predetermined amount of NH ₃ gas and HF gas are directly reacted in a container or an electrolytic bath to prepare an electrolytic solution. In particular, in the reaction of the NH ₃ gas and the HF gas, when a dry inert gas of about 5 to 70 vol%, for example, nitrogen, argon, helium, etc. is supplied, the electrolyte flows back into the gas supply pipe. And can be prepared stably. In any case, the electrolyte can be easily prepared.

The composition of the electrolytic solution is preferably a HF / NH ₄ F molar ratio of 1 to 3. The electrolytic solution having a molar ratio of less than 1 is not preferable because it has thermal decomposability. On the other hand, if the molar ratio exceeds 3, the vapor pressure of HF becomes high, and the loss of HF is large. This loss undesirably increases the fluctuation of the electrolyte composition. The molar ratio is preferably 1 to 3, but if higher composition stability is required, 1.5
The optimum range is from 2.5 to 2.5, and more preferably from 1.8 to 2.2.

The electrolytic current density is preferably 1 to 30 A · d
m- ² . The lower limit of the current density affects the productivity of NF ₃ gas, and has almost no technical restrictions. The heat generated near the electrodes is almost proportional to the current density. For this reason, when the current density becomes extremely high, inconveniences such as a local increase in the temperature of the electrolytic solution and an unstable composition are caused.
Although there is no effect on the effects of the present invention, the range of the current density is generally 1 to 30 A · dm ⁻² , more preferably 5 to ²
A range of 0 A · dm ⁻² is recommended.

As the cathode used for the electrolysis, a material generally used for electrolytic production of NF ₃ gas, for example, iron, steel, nickel, monel, etc. can be used.

[0023]

The present invention will be described more specifically with reference to the following examples. Example 1 1110 g of dried ammonium acid fluoride (NH ₄
After thoroughly mixing HF ₂ ), the mixture was placed in a fluororesin small-mouthed bottle having a capacity of 3 L. Insert a fluororesin tube up to the bottom of this small-mouthed bottle and measure
90 g was blown thereinto to obtain an electrolytic solution (NH ₄ F · 2HF) having a composition ratio of HF / NH ₄ F = 2. Next, 9.8 g of lithium nickel oxide (LiNiO ₂ ) powder and 53 g of nickel powder were mixed well, and then formed into tablets having a diameter of 20 mm. This was sintered at 900 ° C. for 2 hours in an argon atmosphere to obtain a composite electrode. Next, the composite electrode was set in a fluorine resin electrolytic tank having a capacity of 200 cc containing the electrolytic solution, and electrolysis was performed. Temperature 120 ° C, current density 25mA
After performing electrolysis for 70 hours at, the weight of the anode was measured, and a decrease of 0.11 g due to dissolution was observed.

Comparative Example 1 The same experiment as in Example 1 was performed using an electrode obtained by sintering in the same manner as in Example 1 using only nickel powder. As a result, the weight of the anode was reduced by 0.27 g.

[0025] Example 2 Watts bath _{(NiSO 4 · 6H 2 O =} 280g / L, Ni
_{Cl 2 · 6H 2 O = 45g} / L, boric acid = 40 g / L)
LiNiO ₂ (particle size: 10 μm or less) is added to 150 ml
0 g was added and dispersed well. Next, a nickel plate having an effective area of 4 cm ² was immersed in a Watt bath at 40 ° C. and a current density of 20 ° C.
Electroplating was performed at mA / cm ² [2 A / dm ² ]. This electrode was treated with NH ₄ F.
An electrolysis experiment was performed in a 2HF electrolyte. As a result, the weight of the anode was reduced by 0.22 g.

Comparative Example 2 The same experiment as in Example 2 was performed except that LiNiO ₂ was not added to the Watt bath. as a result,
The weight of the anode was reduced by 0.32 g.

[0027]

The electrolysis method is an excellent method for easily obtaining a high-purity nitrogen trifluoride gas, but it has been an industrial problem that a large amount of anode has been dissolved so far. ADVANTAGE OF THE INVENTION According to the method of this invention, it is an epoch-making invention in which the amount of dissolution of nickel can be suppressed significantly, without changing a conventional electrolysis process at all. This not only can reduce the replacement frequency of the electrodes and the electrolyte to less than half, but can also reduce the cost. The effect in industrial production can be said to be extremely large.

[0028]

[Brief description of the drawings]

Fig. 1 Example of electrolytic cell

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Main body 2 Tank lid 3 Anode 4 Cathode 5 Partition wall 6 Anode gas outlet 7 Cathode gas outlet 8 Electrolyte

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-140783 (JP, A) JP-A-3-170687 (JP, A) JP-A-3-236486 (JP, A) 500602 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25B 1/00-15/08 C01B 21/083

Claims (2)

(57) [Claims] 1. A composite electrode for producing nitrogen trifluoride gas obtained by sintering a nickel-based oxide and dispersed nickel plating or nickel powder. 2. An ammonium fluoride (NH ₄ F) -hydrogen fluoride (HF) -based molten salt having a composition molar ratio (HF
/ NH ₄ F) is 1 to 3 with 100 to 140
2. A method for producing nitrogen trifluoride gas, comprising performing electrolysis using the composite electrode according to claim 1 as an anode while maintaining the temperature at a temperature of ° C.