JPH0692963B2 - Analytical method for oxygen difluoride in nitrogen trifluoride gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to oxygen difluoride (OF ₂ ) in nitrogen trifluoride (NF ₃ ) gas.
Analysis method. More specifically, gas chromatography (hereinafter gas chromatography is abbreviated as GC)
Is used for the above analysis method.

[Conventional technology]

NF ₃ gas has recently been attracting attention as a dry etching agent for semiconductors and a cleaning gas for a CVD apparatus, but NF ₃ gas used for these applications is required to have a purity as high as possible.

NF ₃ gas can be produced by various methods such as so-called molten salt electrolysis method of electrolyzing NH ₄ F · xHF using acidic ammonium fluoride or ammonium fluoride and hydrogen fluoride as raw materials, and a method of reacting ammonium and fluorine. Although produced, the NF ₃ gas obtained by either method is nitrogen (N ₂ ), dinitrogen difluoride (N ₂ F ₂
It is known that various compounds such as ₂ ), nitrous oxide (N ₂ O), carbon dioxide (CO ₂ ) are contained as impurities. Therefore, in order to obtain high-purity NF ₃ gas, it is necessary to remove and purify these impurities, and various purification methods have been proposed.

However, in the NF ₃ gas, in addition to the above-mentioned impurities, O
It may also contain F ₂ . In particular, it is considered that the NF ₃ gas produced by the molten salt electrolysis method may contain a considerable amount of OF ₂ .

If the NF ₃ in the gas OF ₂ is also contained, in order to obtain a high-purity NF ₃ gases it is necessary to remove the OF _2. In addition, since OF ₂ is an oxygen-containing compound, there is a problem that when NF ₃ gas containing such an oxygen-containing compound is used as a dry etching agent for semiconductors or a cleaning gas for CVD equipment, it has an adverse effect.

However, although the peaks of N ₂ O and CO ₂ can be detected even when the NF ₃ gas is analyzed at normal temperature using a GC, OF ₂
Peak was not detected, and its analysis method is not yet known. Therefore, the presence of OF ₂ in NF ₃ gas has not been confirmed. Of course, there is no known method for removing OF _{2 from} NF ₃ gas.

From the above circumstances, while also be monkey to establish a method for removing OF ₂ containing in the NF ₃ gas, as a premise of establishing removal methods, confirmed the presence of OF ₂ of NF ₃ gas, quantitative It is necessary to establish a method of doing so.

[Means for Solving the Problems]

In view of the above situation, the present inventors have found that O contained in NF ₃ gas.
As a result of extensive studies to establish an F ₂ analysis method, it was found that OF ₂ contained can be accurately quantified by analyzing NF ₃ gas at low temperature using GC, and the present invention was completed. It is a thing.

That is, the present invention provides a method for analyzing nitrogen trifluoride gas containing oxygen difluoride as an impurity by GC,
Using silica gel preheated and dehydrated as a packing material for the column of the GC, cooling the column to a temperature in the range of -30 ° C to -110 ° C, and injecting nitrogen trifluoride gas into the column,
Difluorination in nitrogen trifluoride gas, characterized in that the column is maintained at a constant temperature within the above temperature range or is heated at a rate of 20 ° C / min. Or less at a temperature not exceeding -30 ° C. The present invention provides a method for analyzing oxygen.

[Detailed Disclosure of the Invention]

 Hereinafter, the present invention will be described in detail.

Generally, when a gas is analyzed using a GC, a certain amount of sample gas is injected into the column at a room temperature while feeding a gas such as helium (He) gas as a carrier gas into the column packed with a packing material. The analysis method of the present invention is basically the same as this.

In the present invention, He gas is used as the carrier gas. This He gas may be used for GC, but it is preferable that it has a purity of 99.99% or higher in order to improve analysis accuracy.

Moreover, since OF ₂ is easily hydrolyzed by reacting with water, it is preferable that He gas is dehumidified with a dehumidifying agent such as molecular sieve and then fed into the column.

Next, the filler will be described.

In the present invention, silica gel is used as a packing material for the column, and the silica gel has a particle size of 60 to 80 mesh and is usually commercially available.
The one for GC is used.

However, if water is present in the silica gel,
Since OF ₂ is easily hydrolyzed, it is necessary in the present invention to heat this silica gel for dehydration treatment prior to the analysis of NF ₃ gas.

Since the above heating is carried out in order to substantially completely remove the water content in the silica gel, the heating temperature is usually 100 to 200 ° C., and the heating time is 5 to 12 hours, although it depends on the heating temperature. In addition, it is a preferable method to carry out the above heating while circulating an inert gas such as N ₂ gas or He gas.

Thus, the silica gel dehydrated by heating is packed in a column after being cooled to room temperature, and moisture absorption in the air must be prevented during the cooling and packing.

From the above, if silica gel is packed in a column in advance and then dehydrated by heating while feeding an inert gas, it does not absorb moisture in the air during cooling after heating, Since it is possible to immediately shift to the analysis operation of NF ₃ gas later, it is advantageous in operation.

In the present invention, the column, piping, etc. that come into contact with the sample NF ₃ gas must be made of a material that does not react with OF ₂ in the NF ₃ gas, and Teflon (fluorine-based resin) is usually used. However, if it is difficult to use the Teflon pipe, use a pipe such as a nickel pipe or a stainless pipe, thoroughly degrease the inner wall of the pipe, and then ventilate fluorine gas or the like to clean the inner wall of the pipe. It is necessary to fluorinate and prevent OF ₂ from reacting with the piping.

Next, the analysis of OF ₂ in NF ₃ gas will be described.

When the dehydration treatment by heating the silica gel is completed as described above, a fixed amount of He gas as a carrier gas is continuously fed into the column. Next, the calm is cooled to a temperature in the range of −30 to −110 ° C., and then a fixed amount of NF ₃ gas as a sample gas is injected into the column. Thereafter, by the column temperature is raised at 20 ° C. / min. Following rates or to maintain a constant temperature, or at -30 ° C. below the temperature in the above temperature range, OF ₂ chromatogram of low-temperature GC The peak of can be detected.

In the present invention, the temperature of the column is the most important, and it is necessary to cool the temperature of the column to a temperature in the range of −30 to −110 ° C. before injecting the NF ₃ gas. If the column temperature is -30 ° C or higher, the OF ₂ peak is not detected in the above chromatogram. Further, if the temperature of the column is −110 ° C. or lower, the boiling point of NF ₃ is −129 ° C., which makes it practically difficult to carry out the analytical operation, which is inconvenient.

Keep the column cooled to the above temperature at this temperature,
Alternatively, raise the temperature at a rate of 20 ° C / min. Or less at a temperature not exceeding -30 ° C. However, raising the temperature makes the operation a little complicated, so it is preferable to maintain a constant temperature. If the temperature rising rate exceeds 20 ° C./min., The OF ₂ peak will be inaccurate or not detected.

When NF ₃ gas containing impurities was analyzed at normal temperature using GC, OF ₂ was not detected in the chromatogram as described above, and N ₂ , NF ₃ , N ₂ F ₂ , N ₂ O, Peaks are detected in the order of CO ₂ . However, when the method of the present invention in which the column temperature is analyzed at −30 to −110 ° C. is adopted, N ₂ peak and
The OF ₂ peak is detected between the NF ₃ peaks.
Of course, peaks of N ₂ F ₂ , N ₂ O and CO ₂ are also detected.

As described above, by adopting the method of the present invention, OF ₂ in NF ₃ gas can be satisfactorily analyzed. However, in the method of the present invention, since the temperature of the column is analyzed at a low temperature, it takes a longer time than the usual method in which analysis is performed at room temperature. Therefore, in the case of simultaneously analyzing impurities other than OF ₂ by the method of the present invention, after OF ₂ is detected, the column is taken out from the refrigerant bath described below and heated to room temperature,
Subsequent analyzes can be performed in about the same time as conventional methods.

In the method of the present invention, the content of OF _{2 in} NF ₃ gas can be quantified by the area percentage from the peak detected by the above method and the peak of the standard sample.

In the present invention, it is necessary to cool the column to a temperature of −30 to −110 ° C. as described above, but such cooling is performed by immersing the column in a liquid nitrogen-ethanol-based or dry ice-ethanol-based refrigerant bath. Can be implemented easily. Also, the cooling temperature can be adjusted by changing the mixing ratio of liquid nitrogen and ethanol and the mixing ratio of dry ice and ethanol.

In the method of the present invention, it is not necessary to heat silica gel in advance for dehydration treatment each time it is analyzed, and it is possible to perform analysis several times in one dehydration treatment.

〔Example〕

 Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1 Using a GC (model GC-8A) manufactured by Shimadzu Corporation, NH
NF ₃ gas containing impurities obtained by electrolysis of ₄ F / HF molten salt was analyzed.

That is, commercially available silica gel for GC with a particle size of 60-80 mesh 2 m
l was packed in the Teflon column of the above GC (Teflon tube having an inner diameter of 2 mm wound in a coil shape and having a length of 680 mm), and this column was attached to the GC. Next, molecular sieve 5A in advance
The He gas dehumidified in 1. was fed to the column at a flow rate of 30 ml / min., And the column was heated to a temperature of 180 ° C. for 10 hours for dehydration treatment.

The column after the dehydration treatment was cooled to room temperature while continuously feeding He gas at the above flow rate. The column cooled to room temperature is then immersed in a dry ice-ethanol-based refrigerant bath to maintain the state of being cooled to a temperature of -50 ° C, and then 5 Nml of the above NF ₃ gas is injected into the column. Analysis was performed to obtain the chromatograph shown in FIG.

In the analysis, after the OF ₂ peak was completely detected, the refrigerant bath was removed from the column and the analysis was continued.

The content of OF _{2 in} the NF ₃ gas was calculated from the peaks of the chromatogram shown in FIG.
It was confirmed to be 1.1% by volume.

Example 2 A column packed with silica gel, which had been dehydrated by heating in the same manner as in Example 1 and cooled to room temperature, was immersed in a liquid nitrogen-ethanol-based refrigerant bath and cooled to a temperature of -100 ° C. did. Thereafter, 5 Nml of the same NF ₃ gas as that used in Example 1 was injected into the column, and then the temperature of the column was analyzed at a rate of about 10 ° C./min.
The chromatogram shown in the figure was obtained.

In the analysis, as in Example 1, after the OF ₂ peak was completely detected, the refrigerant bath was removed from the column and the analysis was continued.

The temperature rising rate of the column was adjusted by a method of continuously adding a fixed amount of ethanol to the refrigerant bath.

From the peak of the chromatogram shown in FIG. 2 and the peak of the standard sample, it was confirmed by the area percentage that the content of OF ₂ in the NF ₃ gas was 1.1% by volume, as in Example 1.

Comparative Example 1 A column packed with silica gel, which has been dehydrated by heating in the same manner as in Example 1 and cooled to room temperature, is at room temperature (about 25 ° C.) without being cooled in a refrigerant bath, and the others are in Example 1 The analysis was performed in exactly the same manner as in.

The results are shown in FIG. 3, and no OF ₂ peak was detected in the obtained chromatogram.

Comparative Example 2 Example 2 except that the heating rate of the column was set to about 30 ° C./min.
The NF ₃ gas was analyzed by the same method as in.

The results are shown in FIG. 4, and as in Comparative Example 1, no peak of OF ₂ was detected in the chromatogram.

〔The invention's effect〕

As described in detail above, by the method of the present invention, it was possible to confirm the presence of OF ₂ in the NF ₃ gas, which has not been confirmed although it was thought to exist in the past, and further to be quantified. Became possible.

Therefore, the method of the present invention for producing high-purity NF ₃ gas such that removal of OF ₂ contained can be confirmed by a method of contacting NF ₃ gas with an aqueous solution of sodium thiosulfate, sodium sulfite, etc. Plays a big role.

[Brief description of drawings]

1 is Example 1, FIG. 2 is Example 2, FIG. 3 is Comparative Example 1, and FIG. 4 is a diagram showing respective chromatograms obtained in Comparative Example 2.

Claims (1)

[Claims] 1. A method for analyzing a nitrogen trifluoride gas containing at least oxygen difluoride as an impurity by gas chromatography, wherein silica gel dehydrated by preheating the packing material of the column of the gas chromatography is used. Used, after cooling the column to a temperature in the range of -30 ° C to -110 ° C and injecting nitrogen trifluoride gas into the column, maintaining the column at a constant temperature within the above temperature range or -30 A method for analyzing oxygen difluoride in nitrogen trifluoride gas, characterized in that the temperature is raised at a rate not exceeding 20 ° C / min.