NF3 treating process

Details NF3 treating process NF3 treating process

2000-04-07

2002-01-29

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Silicon or compound thereof

Halogen containing

C423S239100, C423S406000, C423S489000

Reexamination Certificate

active

06342194

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for treating NF
3
gas that is useful as a dry etching gas and cleaning gas in processes for producing LSI, TFT, and solar cell and in an electron photographic process.
2. Description of the Invention
NF
3
is a toxic gas having a TLV of 100 ppm that is extremely stable in air and essentially insoluble in water. In the case of using this substance, it is necessary at all times to remove residual NF
3
present in exhaust gas. Since NF
3
is extremely chemically stable at temperatures near room temperature and is also insoluble in water, it cannot be processed by ordinary gas absorption processes in its original state. Consequently, the following process has been proposed in Japanese Patent No. 1538007 (Japanese Patent Provisional Publication No. 61-204025), in which NF
3
is reacted with a substance that converts NF
3
into a fluoride gas that easily reacts with water and alkaline solution, followed by treating the resulting fluoride gas with a normal gas absorption process. The Japanese Patent discloses a process wherein NF
3
is reacted with Si, B, W, Mo, V, Se, Te, Ge and their non-oxidizing solid compounds that are used as the converting substance.
Although the above NF
3
treatment process is effective for converting NF
3
into an easily treated gas compound, a characteristic reactor taking the NF
3
into consideration was not proposed with respect to the reactor for reacting and treating a large amount of NF
3
. Namely, the above Patent only proposes a single flow type of fixed bed reactor as equipment for contacting gaseous NF
3
with a solid compound.
The fixed bed reactor described in the present specification refers to a reactor having a cylindrical outer tube in which a fixed bed, filled with a solid compound such as a metal element that reacts with NF
3
throughout an ordinary cylindrical reactor, is disposed. The fixed bed is heated as necessary followed by introducing gas from one end of the cylinder, contacting and reacting the gas with a metal element and so forth inside the cylindrical tube, and discharging the gas from the other end of the tube. This form of the reactor is that which has been known since long ago. In addition, various types of NF
3
detoxification technologies taking into consideration new reaction systems other than the above reaction system have been disclosed as being disclosed in Japanese Patent Publication No. 2-30731 and Japanese Patent provisional Publication No. 7-155540. In such technologies, the fixed bed reactor of the gas flow type is still used. Thus, the reactors that provide an effective setting for an NF
3
detoxification reaction have not yet discovered.
Now, treatment of NF
3
gas is accompanied by the generation of extremely large amounts of heat from the reaction. Namely, its standard formation enthalpy is −127 kJ/mol (−42 kJ per fluorine atom), and in the case of SiF
4
gas being obtained as the product of the action of metal Si, for example, since the standard formation enthalpy of SiF
4
is −1615 kJ/mol (−404 kJ per fluorine atom). The difference between the two enthalpies are the amount of heat generated accompanying reaction (362 kJ per fluorine atom), which demonstrates that NF
3
detoxification reaction is accompanied by the generation of an extremely large amount of heat. Even though there may be some difference in the amount of heat generated in the case that the other substance than Si such as B or W, or if C is selected as a reacting metal element: however, it is intrinsically a reaction that is accompanied by the generation of a large amount of heat.
In the case of conducting a gas-solid reaction using a reactor or reaction tube of the type in which gas is allowed to flow over a fixed bed, the reaction initially occurs in the zone on the inlet side of the initial reaction tube, and as chemical is consumed, the reaction zone gradually moves to the outlet side. Since the flow of gas inside the reactor is so-called piston flow, there are many cases in which the reaction always occurs in a special location inside the reaction tube in this manner, while other portions of the reaction tube merely fulfill the role of a gas pathway and are not involved in the reaction itself. Moreover, due to the low rate of heat transfer of the fixed bed, it cannot be said to be suited for efficiently discharging the reaction heat generated locally inside the reactor in this manner outside the system.
For these reasons, when an NF
3
detoxification reaction is carried out with a fixed bed gas flow system for a reaction that generates a large amount of heat, the local temperature that results from the reaction ends up becoming extremely high. Consequently, the amount of NF
3
that be treated per unit time cannot be increased relative to the volume of the reactor.
Moreover, there has been proposed a process in which the concentration of supplied NF
3
is diluted with an inert gas (such as N
2
) for the purpose of lowering the temperature of the formed gas. However, this process increases the volumetric flow of all gas resulting in a shortening of retention time, and therefore is not effective as a means of improving the NF
3
treatment rate per reactor volume. Moreover, even if a large reactor is attempted to be designed having a larger NF
3
treatment rate, there is a limit on the size of the reaction tube diameter for ensuring heat transfer in the radial direction. Ultimately, in order to provide NF
3
treatment volume, a plurality of small diameter reactors must be arranged in parallel, and in any case, fixed bed gas flow systems had the problem of being disadvantageous in terms of equipment cost.
In addition, in the case of using Si, for example, in the reaction between NF
3
and Si, a relatively large amount of heat is generated on the order of 1,086 kJ/mol. Consequently, this invites a local temperature rise and overheating in conventional tubular apparatuses of the fixed bed type, thereby placing a limit on the amount (concentration) of NF
3
supplied, and the limit of that supplied concentration is 5 vol %. In addition, the actual limit on the tube diameter of a fixed bed system is 150 A (according to Japanese Industrial Standard) corresponding to an outer diameter of 165.2 mm. Namely, it was necessary to accompany treatment of NF
3
at 5 NL/min with a diluting gas (N
2
) at 100 NL/min. For this reason, fixed bed systems are not suited for treatment of highly concentrated NF
3
or large amounts of NF
3
.
In addition, in the case of fixed bed systems, treatment capacity has been observed to decrease when air or oxygen is present. Consequently, there is a need for an NF
3
treatment process that allows treatment of highly concentrated NF
3
, does not result in a decrease in treatment rate even in the presence of, for example oxygen (air) in the NF
3
, and is able to ensure a certain degree of treatment volume per unit time.
SUMMARY OF THE INVENTION
As a result of conducting earnest studies in consideration of the above-mentioned problems, the inventors of the present invention have found that highly concentrated and large amounts of NF
3
gas can be treated by creating a setting for gas flow that prevents local overheating of the fixed portion of a reactor, rapidly transports generated heat to the wall of the reactor with the flow of gas, and provides as rapid a gas flow as possible along the reactor wall to promote transfer of heat between the gas phase and solid wall in the vicinity of the reactor wall, thereby leading to completion of the present invention.
An aspect of the present invention resides in a process for treating NF
3
, comprising the following step: (a) preparing a first reactor including agitator blades for agitating gas in the first reactor and generating a flow of the gas, and a gas flow guide tube for efficiently circulating and dispersing the gas flow generated by the agitator blades in a space of the first reactor; (b) stationarily placing at least one substance selected from the group cons

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