JPS62143812A - Purification of silicon tetrafluoride - Google Patents

Abstract

PURPOSE:An SiF4 gas is passed through the silica gel layers which have been dehydrated, under specific conditions to remove siloxanes included in the product as contaminants through simple procedures whereby high-efficient purification of SiF4 can be done. CONSTITUTION:An SiF4 gas is passed through silica gel layers which has been preliminarily dehydrated, preferably at about 150-300 deg.C under substantially water-free conditions in a temperature range from -10 to -95 deg.C. Inexpensive silica gel is used as an adsorbent to remove siloxanes completely through simple operations whereby high-purity SiF4 can be supplied for amorphous silicon film and dry etching.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for purifying silicon tetrafluoride.

More specifically, the present invention relates to a method for removing siloxanes from silicon tetrafluoride gas. Silicon tetrafluoride gas has recently attracted attention as a raw material for amorphous silicon thin film semiconductors and as a dry etching agent, but silicon tetrafluoride gas for these uses is required to be of high purity.

(Prior art and its problems) Silicon tetrafluoride gas is produced by various methods, but in most cases it contains siloxane as an impurity.

Known methods for producing silicon tetrafluoride gas include, for example, an acid decomposition method of silicofluoric acid and silicofluoride compounds, a thermal decomposition method of silicofluoride compounds, and a synthesis method by reacting hydrogen fluoride with silicon dioxide. ing.

However, the silicon tetrafluoride gas obtained by these production methods may contain siloxanes in an amount of several thousand ppm, which must be removed.

In particular, when used as a raw material for amorphous silicon thin films, the presence of siloxanes causes a large amount of oxygen to be mixed into the produced silicon thin film, which adversely affects semiconductor properties, so these must be thoroughly removed.

Siloxanes are thought to be produced during the silicon tetrafluoride manufacturing process, for example, by the reaction between water contained in raw materials and the silicon tetrafluoride produced, but the water is completely removed during the silicon tetrafluoride manufacturing process. Preventing the formation of siloxanes is extremely difficult and virtually impossible.

Therefore, the siloxanes in the generated silicon tetrafluoride gas must be separated and removed, but the critical pressure of silicon tetrafluoride is extremely high (36.66 atm), so separation by distillation is difficult.

Regarding the removal of siloxanes, adsorption removal methods using activated carbon (Japanese Patent Publication No. 59-4126) and adsorption removal methods using activated alumina (Japanese Patent Application Laid-open No. 59-162122) are known.

These adsorption removal methods are usually carried out by bubbling silicon tetrafluoride gas through a column packed with activated carbon or activated alumina, but the present inventors individually sampled changes in their adsorption capacity over time. A detailed investigation revealed that these methods are of course effective in removing siloxanes from silicon tetrafluoride gas as an over-rall, but surprisingly, their adsorption capacity is abnormally low in the early stages of ventilation. It was found that this initial effluent gas contained a large amount of siloxanes and was therefore insufficient for applications requiring high purity, such as the production of amorphous silicon thin films.

(Means for Solving the Problems) The present inventors have attempted to adsorb and remove siloxanes from silicon tetrafluoride gas, particularly improving the adsorption effect of siloxanes at the initial stage of silicon tetrafluoride gas ventilation into an adsorption tower. As a result of intensive studies on various adsorbents and aeration conditions, we found that by using silica gel that had been pretreated as an adsorbent and aerating it under specific conditions from the beginning, it was possible to absorb silicon tetrafluoride gas. The inventors have discovered that siloxanes can be removed extremely efficiently, and have completed the present invention.

That is, the method for purifying silicon tetrafluoride of the present invention involves passing silicon tetrafluoride gas through a silica gel layer that has been dehydrated by heating in advance at a temperature in the range of -10 to -95°C in a state where substantially no moisture is mixed. It is characterized by this.

The silica gel used in the present invention is not particularly limited, and any commercially available silica gel can be used, but granular silica gel with a high surface area is more preferable.

The dehydration treatment by heating silica gel is preferably performed at a temperature of 150 to
It is carried out at a temperature of 300°C, more preferably 180-200°C.

When the dehydration temperature of silica gel by heating is less than 150° C., a considerable amount of water remains in the silica gel.

In such silica gel, when silicon tetrafluoride gas is aerated, the remaining moisture reacts with silicon tetrafluoride to produce siloxanes, so it is good for IL<fi<, and hydrogen fluoride is also produced at the same time. This hydrogen fluoride causes corrosion of equipment such as columns, which is not desirable. On the other hand, if the heat treatment temperature exceeds 300'C, this is not preferable because not only is there a loss of energy, but also the adsorption ability of the silica gel is significantly reduced if the heat treatment is carried out at, for example, 650C.

The atmosphere for dehydrating silica gel by heating may be normal air, but since it is necessary to vaporize and dissipate the moisture contained in the silica gel by heating, use a gas that does not contain moisture, such as dry nitrogen gas. It is preferable to carry out this process in an air stream, and it is also preferable to carry out this process under reduced pressure while sucking gas.

The dehydration treatment usually requires 1 to 2 hours at the above heating temperature and atmosphere. After the dehydration treatment, the silica gel is cooled down to room temperature or below by cooling or forced cooling, but in this case it is necessary to avoid contamination with moisture.

Purification of silicon tetrafluoride gas is carried out by venting through a silica gel layer packed in a column or the like, but the venting temperature is important and must be below -10°C.

The lower the ventilation temperature, the higher the removal rate of siloxanes, but since the sublimation temperature of silicon tetrafluoride is -95°C, it is practically difficult to operate below this temperature. Therefore, in the present invention, the ventilation temperature is set in the range of -10 to -95°C. The pressure of silicon tetrafluoride during ventilation is not particularly limited, and can be carried out within the range of, for example, a vacuum of about 1'rorr to an increased pressure of about 10 atm.

Calculating the amount of siloxanes mixed into silicon tetrafluoride gas as a feedstock is difficult because there is no suitable quantitative method for siloxanes, but in order to examine the degree of purification, it is necessary to Before and after the sample was taken into a 100 mm long gas cell, the infrared absorption spectrum was measured, and the 2057 cm'' was derived from the 5i-F vibration of silicon tetrafluoride.
It is sufficient to estimate the degree of purification by calculating the absorption ratio (R) of 839Cm-' derived from the absorption of .

Absorption 1! dR)=l og (To/T□) 839
cm-1/log (To/T,) 2[157cm-
1...(1) (Effects of the invention) The present invention provides a method for adsorbing and removing siloxanes from silicon tetrafluoride as described above. This is an extremely simple method in which silicon tetrafluoride gas is aerated under specific conditions, making it possible to virtually completely remove siloxanes from the beginning of aeration. This makes it possible to provide highly pure silicon tetrafluoride, which is the raw material for the agent.

Of course, the silica gel layer of the present invention is combined with other known adsorbents that have inferior initial adsorption capacity but high adsorption capacity, and the silica gel layer of the present invention is used as the first layer.
Activated carbon or activated alumina may be bonded in series as a second layer.

(Example and comparative example) The present invention will be specifically explained below using Examples and Comparative Examples.

Examples 1 to 4 A stainless steel column with an inner diameter of 15 m+m was filled with granular silica gel having an average particle size of 2 mm (packing height: 4 Q cm).

After that, dehydration treatment by heating the silica gel and aeration adsorption treatment of silicon tetrafluoride gas were performed under the conditions shown in Table 1. Absorption ratio of infrared absorption spectra before and after aeration adsorption treatment (as shown in Tables 1 and 2, the initial value of aeration is 0.2
It can be seen that silicon tetrafluoride gas of high purity can be obtained from the effluent gas of t/me. Moreover, this effect is at least 4
It can be seen that t/rrrl persists.

Table 1 Table 2 Comparative Examples 1 to 3 Using the same column packed with silica gel as in the example,
Dehydration treatment by heating silica gel and aeration and adsorption treatment of silicon tetrafluoride gas were performed under the conditions shown in Table 6. 6th result
As shown in Table and Table 4, it is clear that the removal effect of siloxanes is insufficient from the initial stage unless the treatment is carried out under the conditions specified in the present invention.

Table 6 Table 4 Comparative Examples 4 to 7 In place of the silica gel used in Examples and Comparative Examples 1 to 3,
An attempt was made to purify silicon tetrafluoride gas using columns packed with each adsorbent shown in the table. The shape of the column and the raw material silicon tetrafluoride gas were the same as in the examples. The result is the 6th
As shown in the table, it was found that the removal of siloxane from silicon tetrafluoride gas was completely inferior in all cases at the initial stage of aeration (0.2t to 4t/ml).

Table 6

Claims (2)

[Claims] (1) A silica gel layer that has been dehydrated by heating in advance is heated at a temperature of -10°C to -95°C in a state where substantially no moisture is mixed.
1. A method for purifying silicon tetrafluoride, which comprises aerating silicon tetrafluoride gas in the range of . (2) The method according to claim 1, wherein the heating temperature of the silica gel is 150 to 300°C.