JPH0456789A - Production of gaseous nitrogen trifluoride - Google Patents

Abstract

PURPOSE:To produce gaseous nitrogen trifluoride having a reduced impurity content by using a molten salt prepd. from hydrofluoric acid and ammonia as starting material and electrolyzing the molten salt by a molten salt electrolysis method. CONSTITUTION:Molten salt represented by a formula NH4F.2HF is prepd. from hydrofluoric acid and ammonia. This molten salt as starting material is dehydrated by electrolysis with an Ni electrode as the anode, this anode is changed over to a carbon electrode and the dehydrated molten salt is electrolyzed. In continuous operation with the carbon electrode, no anode effect is produced and electrolysis can safely be continued. Since the molten salt having a low water content can continuously be electrolyzed, gaseous nitrogen trifluoride is obtd. while reducing the amts. of impurities such as N2O and OF2 produced by the presence of water.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing nitrogen trifluoride gas (NFS).

More specifically, the present invention relates to a method for producing NF3 gas by electrolysis of NH,F-1 (F-based molten salt) using hydrofluoric acid and ammonia as raw materials.

(Problems to be solved by conventional techniques and inventions) NF,
is a colorless gas with a boiling point of 1129°C and a melting point of 1207°C.

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

However, the produced NF3 gas is nitrogen (N2),
Nitrogen difluoride (NJz), nitrous oxide (N20), carbon dioxide (Co□), oxygen difluoride (OFz), oxygen (
NH4F −
NF3 gas produced by HF-based molten salt electrolysis similarly contains impurities. Therefore, purification is necessary to obtain high-purity NFj gas for the above-mentioned uses.

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

this house,! 'hFz, HP, Cot, NZ, 0□
There are known methods for relatively easily removing or reducing the content.

That is, 1) N2FZ is Kl, Na2S, NazSz
A method of contacting with an aqueous solution of Os, etc. (J, Masson
ne+Chemie Inji x -1-Neil Tehenir (
CheIIl, Ing, Techn, ) 41° (1
2), 695. (1969)) and 148.9-537
．． Method of contacting with metal at a temperature of 8℃ (Special Publication Publication No. 59-1)
No. 5081), etc., and by installing a gas flapper or a pyrolysis device, it can be easily removed industrially. 2) Since OF is easily absorbed into water or a weakly alkaline aqueous solution, it can be easily removed by installing a gas flapper. 3) Since CO□ is well absorbed into an alkaline aqueous solution, it can be easily removed industrially by installing a gas flapper like the liver. 4) Low boiling point components such as N2 and 0□ are -
NFff can be removed by cooling to a temperature of 150 to -190°C and liquefying it.

Although it is possible to remove OL, N, and O by the method described below, it cannot be said to be perfect from an industrial perspective.

That is, 5) opz is NazSzOs, Kl, Naz
It can be removed by contacting with an aqueous solution such as SOz, SOz, NatS, etc. However, since the absorption efficiency is relatively low, it is necessary to install a large flapper or use a highly concentrated aqueous solution in order to increase the contact efficiency between the OF-containing gas and the aqueous solution. There are many problems.

In addition, 6) N20 can be efficiently removed by contacting it with an adsorbent such as zeolite (Chell,
Eng,, 84.116. (1977), etc.), it is necessary to regenerate the zeolite after aeration of the gas containing N20, and there are problems such as the need to renew the zeolite due to deterioration of its adsorption capacity.

Carbon is the anode material with the best corrosion resistance in NH,F-IF-based molten salt electrolysis. However, the occurrence of an anodic effect in carbon electrodes poses an obstacle in actual use.

The anode effect is a secondary phenomenon caused by the formation of a film with extremely low surface energy on the surface of a carbon electrode, which causes the voltage to rise abnormally and almost no current to flow. The upper limit of moisture content to prevent the occurrence of the anodic effect is several 1100 pp.

However, the molten salt prepared by adding anhydrous hydrofluoric acid (IF) to acidic ammonium fluoride (NHJJ) or ammonium fluoride (NH4F) already contains several thousand ppm of water; The prepared molten salt inevitably contains at least several thousand ppm of water, and it is impossible to use this molten salt as it is to perform electrolysis with a carbon electrode because of the occurrence of the anodic effect described above.

Therefore, in such a case - boat fishing, a method is adopted to remove the moisture in the molten salt. A method called dehydration electrolysis is usually used for this, and for this purpose an electrode material that does not cause an anodic effect, ie nickel (Nt), is used as the electrode.

As for the operation mode involving dehydration electrolysis, after the implementation of dehydration electrolysis, the electrode is subsequently switched to a carbon electrode and main electrolysis is performed,
As the molten salt is consumed, the molten salt is replenished and the process returns to dehydration electrolysis in a batch manner, and the molten salt is replenished little by little in proportion to the consumed amount during the main electrolysis using the carbon electrode after the completion of the dehydration electrolysis. There is a method of continuously performing main electrolysis.

Although the former method has a relatively wide allowable range for the amount of water in the molten salt, it is not a practical method due to the sludge problem described below due to the frequent use of Ni electrodes and the need for frequent electrode switching. do not have.

The latter is ideal as an operating mode, but the water content of the molten salt to be replenished must preferably be 500 ppn+ or less, which is impossible with the current water content of the molten salt because an anode effect will occur. . Although the molten salt to be replenished can also be dehydrated by dehydration electrolysis, the problem of sludge, which will be described later, cannot be avoided.

For the above reasons, at present, Ni or an alloy mainly composed of Ni, which has the best corrosion resistance in the molten salt among metal materials, is used as the anode.

Ni does not cause an anode effect. However, the proportion of the current used for dissolving Ni (dissolution current efficiency) among the current supplied for electrolysis is about a few percent, and the wear of the anode Ni electrode is unavoidable. It is necessary to frequently renew the Ni1i electrode or remove sludge in the molten salt generated by dissolving the Ni1i electrode, which is a big problem.

(Means for Solving the Problems) In view of the above situation, the present inventors have made extensive studies and found that NH4F, H
Alternatively, if NH and F are not used, and ammonia with a low water content due to ammonia and hydrofluoric acid are used as the ammonia source in the molten salt, the water content that does not cause an anode effect is 500 ppm or less. It was discovered that continuous electrolysis using the most corrosion-resistant carbon electrode is possible while supplying molten salt, and that impurities derived from moisture such as NzO and OLz can be reduced, and the present invention was completed. This is what we have come to.

That is, in producing nitrogen trifluoride gas by a molten salt electrolysis method, a molten salt prepared from hydrofluoric acid and ammonia is used as a raw material, and nitrogen trifluoride gas is electrolyzed by a molten salt electrolysis method. This invention relates to a method for producing gas.

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

In order to carry out the present invention, first, a sealed molten salt preparation container is prepared so that moisture does not enter the prepared molten salt from the outside. The molten salt preparation container has a structure that allows introduction of hydrofluoric acid gas and ammonia, extraction of the molten salt, and introduction and extraction of dry inert gas for pressure adjustment inside the molten salt preparation container. In addition, thermometers, pressure gauges,
It is more preferable if the structure is such that a liquid level gauge can be attached. In addition, it has a structure that allows heating and cooling to remove heat during molten salt preparation and to maintain heat during molten salt storage. It is preferable to use a Teflon container for preparing the molten salt. Corrosion-resistant metal materials such as nickel can also be used, but containers made of fluororesin or lined or coated with fluororesin are preferable. Incidentally, the inside of the molten salt preparation container must be sufficiently dried immediately before use.

Next, the inside of the molten salt preparation container is sufficiently replaced with dry nitrogen (N2) gas obtained from liquid nitrogen. Argon (Ar),
Although an inert gas such as helium (He) can be used, it is necessary to use a dry gas obtained by vaporizing a liquefied gas, and N2 gas is preferably used in consideration of cost.

The internal pressure of the container is adjusted by a gas seal and a pressure regulator that supplies dry nitrogen, and is preferably kept slightly higher than atmospheric pressure at all times.

Both ammonia (NHs) and hydrofluoric acid (HF) use vaporized gases of N1 (3 and HF).The moisture content in these is generally 100 to 300 ppm or less, and moisture intrusion from the outside is prevented. By using a solid container, the water content of the prepared molten salt can also be kept low.

NHs gas and HF gas are introduced into the container through the gas inlet, but since the reaction between NH3 gas and HF gas proceeds extremely quickly and generates a large amount of heat, introduction of large amounts should be avoided. The supply rate is appropriately selected taking into consideration the size of the container and the heat removal capacity.

Next, the water content of the prepared molten salt was determined. The water content of the molten salt is approximately 500 ppm or less,
It is difficult to quantify moisture using the Karl Fischer method, which is a simple moisture measurement method, so the moisture content in molten salt was announced by Masahiko Hori of Doshisha University and others at the 1988 Fall Conference of the Electrochemical Society. It was measured by the method described in the paper ``Study of method for determining water content in molten fluoride baths.''

In this measurement method, binderless carbon is used as a test electrode as an anode, and potential scanning is performed with respect to a reference electrode potential indicated by platinum/electrolyte. The water content is determined from the ratio of the heights of the 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. The effective surface area of the test electrode is approximately 8 cm and is proportional to the peak peak height. 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.

The water content in the molten salt determined from this was 400 ppm.
and N11. The amount was less than half that of the case prepared from F,)I or NH,F.

Next, electrolysis is carried out, and for the operation mode of NH,F/liver-based molten salt electrolysis using a carbon electrode, a continuous method is applied for the reasons described above.

First, molten salt was charged into the electrolytic tank. The molten salt from NHi gas and HF gas prepared above was used, but since it is okay to perform dehydration electrolysis only at the start of electrolysis, it was prepared using NH, F, or NH, F as raw materials. You may also use a molten salt obtained by

Dehydration electrolysis uses a Ni electrode as an anode and a current density of 20 mA/
Electrolysis was carried out at a current flow rate of 40 A-hr/l-molten salt at a current flow rate of about 1.5 cm. Iron (Fe) was used for the cathode.

After the dehydration electrolysis is completed, the electrode is then switched to a carbon electrode and the main electrolysis is performed. The molten salt supplied during continuous electrolysis is carried out while supplying the molten salt prepared in a molten salt preparation container provided separately from the electrolytic cell, but even if the electrolytic cell itself is considered as the molten salt preparation container. , the present invention can equivalently implement this. That is, NH,
All that is required is to newly install gas and HF gas inlets and equipment for adjusting the pressure inside the tank. As the electrolysis progresses and the molten salt is consumed, it is sufficient to supply NH, gas, and HF gas, but in order to minimize pressure fluctuations in the electrolytic cell, Nl
(It is preferable that the gas and strong gas outlets are provided in the molten salt liquid rather than in the gas phase part of the electrolytic cell. It is preferable because the composition of the molten salt becomes uniform.Also, NH3 gas,
Since both the FIF gas and the molten salt are absorbed very quickly, if the supply amount is small, there is a risk that the gas will flow backward through the supply pipe. For this reason, if about 10 to 20% of dry N2 gas is entrained in NH, l gas, and HF gas, stable supply is possible without causing backflow.

Although the present invention is carried out mainly to electrolyze NH, F-HF based molten salt using a carbon electrode, electrolysis can also be carried out using a metal electrode such as Ni, depending on the moisture content. Since the molten salt can be supplied with a small amount of molten salt, it has the advantage of suppressing the amount of impurities such as opz, N, and O produced at a low level. do. In the following, % and pp+* represent capacity standards unless otherwise specified.

Example 1 A molten salt was prepared using the apparatus shown in FIG.

First, dry N2 gas was supplied for 30 minutes at 342/min to a molten salt preparation container 1 with a capacity of 5N whose inner surface had been sufficiently dried in advance to completely replace the inside of the container with dry N2 gas.

Next, the main body of the molten salt preparation container 1 is immersed in a temperature control bath 5 at 80°C, and the NH3 gas 3 supplied from the liquefied NH3 gas cylinder is heated.
and HF gas 4 supplied from an HF gas cylinder at a rate of 11 g/sin and 39 g/+*in for about 100 minutes to prepare 5 kg of a molten salt having a composition of NH4F.2HF.

During this time, slight fluctuations in the internal pressure of the molten salt preparation container l were observed by the micropressure gauge 6, but the pressure was always maintained at a positive pressure. A part of the prepared molten salt 7 is introduced into a small cell for determining water content,
When the moisture content was measured, it was approximately 300 ppm.

Next, the capacity is about 20! After filling the electrolytic cell 9 with the molten salt 7 prepared in the molten salt preparation container 1 and performing dehydration electrolysis for about 100 OA-hr at a current density of 20 ^/cm'' using an Ntii electrode as an anode, The Ni electrode of the anode was changed to a carbon electrode, and the main electrolysis was carried out at a current density of 10 (laA/cm") for about one month. During this time, the molten salt prepared in the molten salt preparation container 1 was replenished as appropriate, and the electrolysis was carried out in the electrolytic cell. The amount of molten salt was kept constant.

As a result, in continuous operation using carbon electrodes,
No anode effect occurred and electrolysis operation could be continued.

Example 2 Molten salt 2ON initially charged into the electrolytic cell 9 in Example 1
Only N)l, F, HF and IP 17kg and 6.
The same method as in Example 1 was used except that a molten salt with a water content of 0.2% prepared by 5 kg was used.

As a result, in continuous operation using carbon electrodes, no anode effect occurred and electrolysis operation could be continued.

Example 3 The electrolytic cell 9 used in Example 1 was provided with an NH gas inlet 10 and an HF gas inlet 11. The molten salt 7 prepared in the same manner as in Example 1 was charged into the electrolytic cell 9,
Dehydration electrolysis was performed using a Ni anode. After this, switch to the carbon electrode and increase the current density to 10011IA/cl+Ig.
Main electrolysis was performed for one month at . During this period, the consumed molten salt was replenished with liquefied NH gas accompanied by 10% dry N2 gas.
3 NH3 gas supplied from a gas cylinder is transferred to HF supplied from an NH3 gas inlet lO to an anhydrous HF gas pump.
This was carried out by blowing gas directly into the molten salt in the electrolytic cell through the liver gas inlet 11.

As a result, in continuous operation using carbon electrodes,
No anode effect occurred and electrolysis operation could be continued.

Example 4 In Example 1, the NF2 gas generated from the anode 12 during electrolysis was carried out in the same manner as in Example 1 except that the Ni electrode was continued to be used as an anode in the main electrolysis after dehydration electrolysis. A tube 15 was installed, and the gas was collected using a cylinder cooled with liquid nitrogen. After the gas was vaporized in a cylinder at room temperature, it was analyzed by gas chromatography, and the NZO content was approximately 300 ppm, OF
, was below the detection limit.

(Comparative example) Comparative example 1 In Example 1, NH and F were added to the molten salt to be replenished during the main electrolysis.
- Composition N) prepared by Von and HF 1. F・2H
The same method as in Example 1 was conducted using F, a molten salt with a water content of 0.12% and a carbon electrode as the anode.

As a result, an anodic effect occurred on the third day after the start of main electrolysis, making it impossible to continue electrolysis.

Comparative Example 2 In Example 4, NH,
Composition NH,F prepared by F-HF and HF
The same method as in Example 1 was carried out except that a molten salt having a temperature of 28F and a water content of 0.2% was used. The gas generated from the anode during the main electrolysis was collected using a bong cooled with liquid nitrogen. After the gas was vaporized in a cylinder at room temperature, the gas was analyzed by gas chromatography, and the content of Neo was about 2000 ppm, and the content of OF was about 200 ppm.

(Effects of the Invention) As explained in detail above, the present invention eliminates the anode effect, which is a problem when using an insoluble carbon electrode, when producing NF and gas by the electrolytic method of NH, F and liver molten salts. This is a very effective method that allows stable electrolysis to continue without causing any problems.

As mentioned above, carbon electrodes that generate very small amounts of impurities have recently been developed, and the possibility of producing high-purity gas using carbon electrodes has increased. This was one of the major harms.

Furthermore, since electrolysis can be performed continuously using molten salt with a low water content, Nz, an impurity caused by water, can be removed.
It also has the advantage that the production amounts of o, opt, etc. can be kept lower than in the past. Needless to say, this also applies to conventional manufacturing methods that use Ni electrodes as anodes.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a molten salt preparation device and an electrolysis device used in Examples and Comparative Examples. In the figure, 1-m-container for preparing molten salt, 2--N2 gas inlet, NH, gas inlet, liver gas inlet, 5--1 temperature bath, micropressure gauge, 7--molten salt, Molten salt outlet valve, electrolytic cell, NH, gas inlet (for blowing into the electrolytic cell), IF gas inlet (for blowing into the electrolytic cell), anode, I3-m-cathode, bulkhead, anode gas outlet pipe, cathode gas outlet pipe, Temperature control heater and cooler patent applicant Mitsui Toatsu Chemical Co., Ltd.

Claims (1)

[Claims] 1) In producing nitrogen trifluoride gas by a molten salt electrolysis method, nitrogen trifluoride is characterized in that a molten salt prepared from hydrofluoric acid and ammonia is used as a raw material, and electrolysis is performed by a molten salt electrolysis method. Gas production method.