JP3260826B2 - Purification method of nitrous oxide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying nitrous oxide, and more particularly to a method for purifying nitrous oxide capable of removing oxygen contained as an impurity in nitrous oxide to an extremely low concentration. Nitrous oxide is used in the manufacture of semiconductors, such as being used to form a silicon oxide film for a surface insulating film in the manufacture of LSIs.

[0002]

2. Description of the Related Art Nitrous oxide used in the production of semiconductors is generally commercially available as pure nitrous oxide (liquefied gas). These nitrous oxides particularly contain oxygen and moisture as impurities, and the moisture can be removed by a dehumidifier such as synthetic zeolite. Oxygen concentration in commercially available nitrous oxide is usually 20 ppm
As described below, a high purity nitrous oxide of about 5 nines in a cylinder is commercially available with a relatively low oxygen concentration of 0.1 to 1 ppm.

[0003]

However, if the oxygen content is less than 1 ppm, it is not possible to sufficiently cope with the demands in recent semiconductor manufacturing processes.
It is strongly desired that the content be not more than 0.01 ppm, more preferably not more than 0.01 ppm. Also, recently, gases such as silane used together with nitrous oxide during semiconductor production can be purified to high purity, for example, oxygen containing 0.01 ppm or less of oxygen can be obtained. (For example, JP-A-3-12303). For this reason, the demand for nitrous oxide is increasing even though the oxygen content is extremely low. In addition, these nitrous oxides may be contaminated by impurities such as air during the supply process to the semiconductor device such as when a cylinder is connected or a pipe is switched. Therefore, it is desirable to finally remove the impurities immediately before the device. Thus, the demand for high-purity nitrous oxide is increasing year by year,
As a method of removing oxygen contained in nitrous oxide, a method of passing air through an aqueous solution of alkaline dithionate is known. However, since the method is a wet method, the configuration of the apparatus becomes complicated, which is disadvantageous in terms of simplicity. In addition, it is necessary to remove water again. For this reason, it is desirable to use a method in which a deoxygenating catalyst is filled in a purification cylinder as in the case of refining nitrogen or an inert gas, but nitrous oxide is almost decomposed even at room temperature with a deoxidizing catalyst such as reduced nickel. There is a problem.

[0004]

Means for Solving the Problems The present inventors have intensively studied a purification method capable of efficiently removing oxygen contained in nitrous oxide to an extremely low concentration without decomposing nitrous oxide. As a result, oxygen was reduced to 0.1 pp by bringing nitrous oxide into contact with a catalyst containing manganese oxide as a main component.
m, more preferably 0.01 ppm or less, and the decomposition of nitrous oxide was found to be small, thus completing the present invention. That is, the present invention is a method for purifying nitrous oxide, which comprises contacting crude nitrous oxide with a catalyst containing manganese oxide as a main component to remove oxygen contained in the crude nitrous oxide. . The present invention relates to nitrous oxide alone represented by the chemical formula N ₂ O, and nitrous oxide diluted with an inert gas (based on an inert gas) such as nitrogen or argon (hereinafter collectively referred to as crude nitrous oxide). Applies to the removal of oxygen contained therein.

In the present invention, various manganese oxides can be used, and among them, a lower oxide of manganese is generally preferred. The lower oxide has the chemical formula MnO
And manganese monoxide represented by Mn ₂ O ₃ , trimanganese tetroxide represented by Mn ₃ O ₄ , and mixtures thereof. Although there are various methods for obtaining these lower oxides, a method of reducing a higher oxide such as manganese dioxide with hydrogen or carbon monoxide is a typical production method. Alternatively, lower oxides of various compositions such as a mixture of two or more oxides can be obtained. It is also possible to prepare these oxides from manganese salts of the corresponding oxidation number, for example, to obtain manganese monoxide,
There is a method of heating manganese (II) carbonate, manganese (II) hydroxide, manganese (II) oxalate, manganese (II) acetate, etc. in a state where air is shut off. These manganese oxides may be used as they are, or may be used after being crushed to an appropriate size, or may be used in a form supported on a catalyst carrier. As a method of supporting manganese oxide on a carrier, for example, a carrier powder such as diatomaceous earth, alumina, silica alumina, aluminosilicate and calcium silicate is dispersed in an aqueous solution of a manganese salt, and an alkali is further added to add manganese to the carrier powder. The components are deposited, then filtered and, if necessary, washed with water. The resulting cake is dried at 120-150 ° C.
Firing at 0 ° C. or higher, pulverizing the fired product, or an inorganic salt such as MnCO ₃ , Mn (OH) ₂ , MnC ₂ O
_4. There is a method in which an organic salt such as Mn (CH ₃ COO) ₂ is baked in nitrogen and pulverized, mixed with a heat-resistant cement, and baked again in nitrogen. These manganese oxides are usually formed into a molded body by extrusion molding, tablet molding, or the like, and are used as they are or crushed to an appropriate size as needed. As a molding method, a dry method or a wet method can be used, and in that case, a small amount of water, a lubricant, or the like may be used. When using low-grade manganese oxide, it must be handled in a state where it does not come into contact with oxygen, such as in a nitrogen gas atmosphere in a glove box. The content of manganese oxide is mainly 10% by weight or less, preferably 2% by weight, mainly of lower oxide of manganese.
0 wt% or more. Manganese oxide content is 10wt
%, The deoxygenation ability is reduced, and oxygen may not be sufficiently removed.

In the present invention, a molded body containing manganese oxide, particularly a lower oxide, as an active ingredient may be filled in a purifying cylinder and purified by flowing crude nitrous oxide as it is.
In order to prevent the decomposition of nitrous oxide, it is preferable to perform a pretreatment with nitrous oxide before purification. In the pretreatment, nitrous oxide alone or a mixed gas with an inert gas such as argon is usually supplied at a linear velocity (LV) of 1 to 10 cm / sec.
This is carried out by flowing the mixture for 30 minutes to 2 hours at c. The pretreatment is performed at a temperature higher than the temperature at the time of purification of the crude nitrous oxide, usually at 50 ° C. or higher. However, when the temperature is too high, the deoxygenation activity as a catalyst due to the reaction with nitrous oxide is rather lowered. Since it may be damaged, it is preferable to carry out the reaction in a temperature range higher by 10 to 100 ° C. than the temperature at the time of purification. If the pretreatment is not performed, the concentration of nitrogen generated by decomposition of nitrous oxide may be close to 3 vol% depending on the conditions at the time of purification. Almost certainly prevented. The pretreatment may be performed before filling the purification cylinder, or may be performed after the filling.However, if pretreatment is performed after manganese oxide is previously filled in the purification cylinder, the crude nitrous oxide Advantageously, purification can be performed.

The purification of crude nitrous oxide is carried out by passing crude nitrous oxide through a purifying cylinder filled with the above-mentioned catalyst containing manganese oxide as a main component, whereby the crude nitrous oxide comes into contact with the manganese oxide catalyst. As a result, oxygen contained as an impurity is removed without decomposing nitrous oxide. The oxygen concentration in the crude nitrous oxide applied to the present invention is usually 100 ppm or less. If the oxygen concentration is higher than this, the calorific value increases, so a heat removal means is required depending on the conditions.

[0008] The filling length of the manganese oxide catalyst filled in the purifying cylinder is determined by the amount of crude nitrous oxide supplied, the oxygen concentration, the characteristics of the catalyst used, the conditions for removing oxygen, and the like. is there. Filling length is 50m
If it is shorter than m, the oxygen removal rate may decrease,
If it is longer than 1500 mm, the pressure loss may increase. In addition, the cylinder linear velocity (LV) of crude nitrous oxide at the time of purification differs depending on the oxygen concentration in the supplied nitrous oxide, operating conditions, and the like, and cannot be specified unconditionally.
Usually 100 cm / sec or less, preferably 30 cm / sec
sec or less. The contact temperature between the nitrous oxide and the manganese oxide catalyst is the temperature of the gas supplied to the inlet of the purification column, and is usually 50 ° C or lower, preferably 0 to 40 ° C. If the contact temperature is too high, nitrous oxide is decomposed, deoxidizing ability is reduced, and nitrogen may be generated as an impurity.
The pressure upon contact of the gas normal pressure, reduced pressure, but it is possible either process the pressure is usually 10 Kg / cm ² abs or less, preferably 0.1Kg~5Kg / cm ² abs. Even if the pressure is too high, nitrous oxide may be decomposed.

In the present invention, even if a small amount of water is contained in nitrous oxide, it does not adversely affect the deoxidizing ability. Further, when a carrier or the like is used,
Depending on the type, moisture is also removed at the same time. It is also possible to appropriately combine a water removal step with a dehumidifier such as a synthetic zeolite as necessary with the oxygen removal step with manganese oxide, whereby water is completely removed, and a highly purified purified suboxide. Nitrogen can be obtained.

[0010]

【Example】

Example 1 (Preparation of purification cylinder) A commercially available manganese oxide catalyst was used.
This is an extruded product of MnO ₂ and is black. This is crushed and sieved to 8-12 mesh 63
ml into a stainless steel refining cylinder having an inner diameter of 16.4 mm and a length of 400 mm (filling density 1.6 g / m3).
l). Nitrogen was added at a temperature of 300 ° C and a flow rate of
After purging the system while preheating by flowing at 0 ml / min for 1 hour, carbon monoxide is supplied at normal pressure at a flow rate of 127 ml / min.
(LV = 1 cm / sec) for 3 hours to carry out a reduction treatment. Thereafter, the system was switched to nitrogen and cooled to room temperature.
Further, as a result of the reduction in a quartz tube under the same conditions, the color of the catalyst changed to green, so that it was confirmed that MnO ₂ was reduced and a lower oxide of manganese was generated. (Purification of Crude Nitrous Oxide) Subsequently, purification was carried out using the above-mentioned stainless steel purification cylinder. 1266 ml / min of 50% nitrous oxide (helium base) containing 0.8 ppm of oxygen as an impurity (LV = 10 c
m / sec), and the oxygen concentration in the outlet gas was measured using a yellow phosphorus emission type oxygen analyzer (measurement lower limit concentration: 0.01 ppm). As a result, no oxygen was detected and the concentration was 0.01 ppm or less. In addition, gas chromatography (detector TC
When the nitrogen concentration in the outlet gas was measured in D), it was 1.3%.
Met. After 100 minutes from the start of the purification, the oxygen concentration in the outlet gas was 0.01 ppm or less, and the nitrogen concentration was 0.55%.

Example 2 (Preparation of a purification cylinder) A purification cylinder was prepared under the same conditions as in Example 1. Prior to purification of nitrous oxide, the purification cylinder was heated to 100 ° C., and 50% nitrous oxide (helium base) was applied at normal pressure at 127 ml / min (LV = 1 cm / sec), 30 m
The pretreatment was performed by flowing in. (Purification of nitrous oxide) In the same manner as in Example 1, 50% nitrous oxide (helium base) containing 0.8 ppm of oxygen as an impurity was added at 1266 ml / min (LV =
When the outlet gas was analyzed at a flow rate of 10 cm / sec), the oxygen concentration was 0.01 ppm or less and the nitrogen concentration was 230 ppm.
ppm. 100 minutes after the start of the purification, the oxygen concentration in the outlet gas was 0.01 ppm or less and the nitrogen concentration was 170 ppm.

Example 3 (Preparation of Purification Cylinder) A commercially available manganese catalyst similar to that of Example 1 was charged into a purification cylinder made of stainless steel, and then hydrogen was supplied at normal pressure and pressure.
At 00 ° C., the flow rate is 127 ml / min (LV = 1 cm / se).
The reduction treatment was performed by flowing for 4 hours in c). Separately from this, reduction was performed in a quartz tube under the same conditions, and as a result, it was confirmed that the color of the catalyst had changed to gray brown. Further, the catalyst in the quartz tube was taken out into the atmosphere and quickly identified by a powder X-ray diffractometer, and it was confirmed that the catalyst was a mixture of manganese oxide mainly composed of manganese monoxide and dimanganese trioxide. (Purification of nitrous oxide) As in Example 1, 50% nitrous oxide (helium base) containing 0.8 ppm of oxygen was passed through this purifying cylinder, and the outlet gas was analyzed. The oxygen concentration was 0.01 ppm or less. And the nitrogen concentration was 360 ppm. After 100 minutes from the start of the purification, the oxygen concentration in the outlet gas is 0.01 ppm or less, and the nitrogen concentration is 180 pp.
m.

Comparative Example 1 (Preparation of a purification cylinder) A commercially available nickel catalyst (manufactured by JGC Corporation)
N-111) was used. Its composition is Ni + NiO
45 to 47 wt% as Ni, Cr 2 to 3
wt%, Cu 2-3 wt%, diatomaceous earth 27-29 wt% and graphite 4-5 wt%, diameter 5 mm, height 4.5
mm. 8 to 10 me of this nickel catalyst
63 ml of the shredded product was packed into the same purification cylinder as used in the examples (packing density: 1.0 g / ml). Hydrogen was added at normal pressure at a temperature of 150 ° C and a flow rate of 595 ml / min.
(LV = 3.6 cm / sec) for 3 hours, followed by cooling to room temperature. (Purification of Nitrous Oxide) When crude nitrous oxide was passed through this purifying cylinder in the same manner as in Example 1, the purifying cylinder generated remarkable heat.
When the outlet gas was analyzed here, the oxygen concentration was 0.01
Although it was less than ppm, nitrous oxide was not detected, only nitrogen was detected, and it was found that most of nitrous oxide was decomposed.

[0014]

According to the present invention, oxygen in nitrous oxide, which has been difficult to remove conventionally, can be removed to an extremely low concentration of 0.1 ppm or less, and even 0.01 ppm or less. At times, contamination of impurity nitrogen by decomposition of nitrous oxide can be prevented, and high-purity nitrous oxide can be obtained.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-58607 (JP, A) JP-B-45-13446 (JP, B1) JP-B-48-37917 (JP, B1) (58) Field (Int. Cl. ⁷ , DB name) C01B 21/22 B01D 53/04 CA (STN)

Claims (4)

(57) [Claims] 1. A method for purifying nitrous oxide, comprising contacting crude nitrous oxide with a catalyst containing manganese oxide as a main component to remove oxygen contained in the crude nitrous oxide. 2. Manganese oxide is manganese monoxide (Mn)
O), manganese sesquioxide (Mn ₂ O _3), trimanganese tetraoxide (Mn ₃ O ₄₎ the purification method according to claim 1 is one or more selected from. 3. The method for purifying nitrous oxide according to claim 1, wherein prior to the purification, the catalyst is pretreated by bringing the catalyst into contact with nitrous oxide at a temperature higher than the temperature at the time of purification. 4. The purification method according to claim 3, wherein the difference between the pretreatment temperature with nitrous oxide and the temperature during purification is 10 to 100 ° C.