JPH0481523B2 - - Google Patents

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.
High purity silicon tetrafluoride gas is required for these uses. (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 produced in the silicon tetrafluoride manufacturing process.
For example, it is thought to be produced by a reaction between the water contained in the raw material and the silicon tetrafluoride produced.
It is extremely difficult and virtually impossible to completely remove water and prevent the formation of siloxanes during the manufacturing process of silicon tetrafluoride. Therefore, the siloxanes in the silicon tetrafluoride gas produced must be separated and removed, but since the critical pressure of silicon tetrafluoride is extremely high (36.66 atm), separation by distillation is also difficult. Regarding the removal of siloxanes, the adsorption removal method using activated carbon (Japanese Patent Publication No. 59-4126) and the adsorption removal method using activated alumina (Japanese Patent Publication No. 59-162122)
etc. 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 adsorption capacity over time. I tracked it in detail and found that these methods
Although it is of course effective in removing siloxanes from silicon tetrafluoride gas as an overall problem, surprisingly, its adsorption capacity is abnormally low at the beginning of ventilation, so this initial outflow gas contains a large amount of siloxanes. Therefore, it was found to be 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 silicon tetrafluoride gas can be easily absorbed by using silica gel that has been pretreated as an adsorbent and by aerating it under specific conditions from the beginning. The inventors discovered that the siloxanes contained therein can be removed extremely efficiently, leading to the completion of 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 without substantially mixing water. 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. Dehydration treatment by heating silica gel is preferably
It is carried out at a temperature of 150-300°C, more preferably 180-200°C. Dehydration treatment temperature by heating silica gel is 150℃
If it is less than that, a considerable amount of water remains in the silica gel. With such silica gel, when silicon tetrafluoride gas is bubbled through, the remaining moisture reacts with silicon tetrafluoride to produce siloxanes, which is undesirable.Hydrogen fluoride is also produced at the same time, so this fluoride This is undesirable because hydrogen can cause corrosion of equipment such as columns. On the contrary, the heat treatment temperature is 300
When the temperature exceeds .degree. C., not only is there a loss of energy, but heat treatment at, for example, 350.degree. C. greatly reduces the adsorption ability of silica gel, which is not preferable. The atmosphere for dehydrating silica gel by heating may be normal air, but since the moisture contained in the silica gel needs to be vaporized and dissipated by heating, it should not contain moisture, such as dry nitrogen gas. It is preferable to carry out the process in a gas stream, and it is also preferable to carry out the process under reduced pressure while sucking the 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 it through a silica gel layer packed in a column, etc.
The ventilation temperature at this time 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 also not particularly limited, and can be carried out within the range of, for example, a vacuum of about 1 Torr to an increased pressure of about 10 atmospheres. Calculating the absolute amount of siloxanes mixed in silicon tetrafluoride gas is difficult because there is no suitable quantitative method for siloxanes, but in order to examine the degree of purification, it is necessary to aerate the column. Before and after the sample was collected into a 100 mm long gas cell and the infrared absorption spectrum was measured.
-Absorption of 2057 cm ^-1 derived from F vibration and Si of hexafluorosiloxane, a type of siloxane.
It is sufficient to estimate the degree of purification by determining the absorption ratio (R) of 839 cm ^-1 derived from F vibration using equation (1). Absorption ratio (R) = log (To/T ₁ )839cm ^-1 /log
(To/T ₁ ) 2057 cm ^-1 ...(1) (Effects of the invention) The present invention utilizes a method for adsorbing and removing siloxanes from silicon tetrafluoride, in which inexpensive silica gel is dehydrated by heating in advance as an adsorbent. After treatment, silicon tetrafluoride gas is aerated in a specific manner, making it possible to virtually completely remove siloxanes from the beginning of aeration. This makes it possible to provide highly pure silicon tetrafluoride, which can be used as a raw material for dry etching agents. Of course, it may be combined with other known adsorbents having a higher adsorption capacity but poorer initial adsorption ability, and the silica gel layer of the present invention may be used as the first layer, and activated carbon or activated alumina may be used as the second layer, and these may be combined in series. (Examples and Comparative Examples) The present invention will be specifically explained below using Examples and Comparative Examples. Examples 1 to 4 Average particle size of 2 mm in a stainless steel column with an inner diameter of 15 mm
After filling with granular silica gel (filling height: 40 cm), the silica gel was dehydrated by heating and silicon tetrafluoride gas was aerated and adsorbed under the conditions shown in Table 1. The absorption ratio (R) of the infrared absorption spectrum before and after the aeration adsorption treatment is as shown in Tables 1 and 2.
It can be seen that higher purity silicon tetrafluoride gas can be obtained than the outflow gas of 0.2/ml at the beginning of aeration.
It is also seen that this effect lasts for at least 4/ml.

【table】

[Table] Comparative Examples 1 to 3 Using a column filled with the same silica gel as in the example, dehydration treatment by heating the silica gel and aeration adsorption treatment of silicon tetrafluoride gas were performed under the conditions shown in Table 3. The results are shown in Tables 3 and 4, and it is clear that unless the treatment is carried out under the conditions specified in the present invention, the effect of removing siloxanes will be insufficient from the beginning.

【table】

【table】

[Table] 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 a column packed with each adsorbent shown in Table 5. The column shape and raw material silicon tetrafluoride gas were the same as in the examples. The results are shown in Table 6,
It was found that the removal of siloxane from silicon tetrafluoride gas was completely poor at the initial stage of aeration (0.2 to 4/ml).

【table】

【table】

Claims (1)

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