Method for exhaust gas abatement under reduced pressure and apparatus therefor

Abstract

The present invention provides an energy-efficient method and apparatus that can achieve exhaust gas abatement with a minimum use of diluent nitrogen gas. More specifically, the present invention is directed to a method and apparatus for exhaust gas abatement under reduced pressure, in which an exhaust gas supplied from an exhaust gas source via a vacuum pump is decomposed by heat of a high-temperature plasma under a reduced pressure.

Claims

1. A method for exhaust gas abatement under reduced pressure, comprising decomposing an exhaust gas supplied from an exhaust gas source via a vacuum pump, by heat of a high-temperature plasma under a reduced pressure, the high-temperature plasma being generated in an atmospheric plasma type plasma generation unit by using direct current arc discharge.

2. The method for exhaust gas abatement under reduced pressure according to claim 1, wherein the reduced pressure is in a range of 50 Torr or more and 400 Torr or less.

3. An apparatus for exhaust gas abatement under reduced pressure, comprising: a reaction chamber in which an exhaust gas supplied from an exhaust gas source via a vacuum pump is decomposed by heat of a high-temperature plasma; an atmospheric plasma type plasma generation unit configured to generate the high-temperature plasma by using direct current arc discharge and to eject the high-temperature plasma into the reaction chamber; and a downstream vacuum pump configured to reduce a pressure in a region located downstream of an outlet of the vacuum pump and including the reaction chamber.

4. The apparatus for exhaust gas abatement under reduced pressure according to claim 3, wherein the plasma generation unit comprises a plasma generation fluid supply unit configured to supply at least one selected from the group consisting of nitrogen, oxygen, argon, helium, and water, as a fluid for high-temperature plasma generation.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram showing an overview of an apparatus for exhaust gas abatement under reduced pressure according to an embodiment of the present invention.

(2) FIG. 2 is a partial cross-sectional front view showing an example of a reaction tube of the apparatus for exhaust gas abatement under reduced pressure according to the present invention.

DESCRIPTION OF EMBODIMENTS

(3) Hereinafter, an embodiment of the present invention will be described with reference to FIG. 1 and FIG. 2.

(4) FIG. 1 is a diagram showing an overview of an apparatus 10 for exhaust gas abatement under reduced pressure according to an embodiment of the present invention. As shown in FIG. 1, the apparatus 10 for exhaust gas abatement under reduced pressure according to this embodiment is an apparatus for abatement of an exhaust gas E supplied from an exhaust gas source 12 such as a CVD apparatus via a vacuum pump 14, and mainly includes: a reaction tube 16 having a reaction chamber 18 and a plasma generation unit 20; and a downstream vacuum pump 24.

(5) Here, the embodiment of FIG. 1 shows a silicon oxynitride film CVD apparatus as an example of the exhaust gas source 12. In a typical silicon oxynitride film CVD apparatus, SiH.sub.4/NH.sub.3/N.sub.2O=1 slm/10 slm/10 slm is used as a process gas, and NF.sub.3/Ar=15 slm/10 slm is used as a cleaning gas. It is presumed that SiF.sub.4 as a product of the cleaning reaction is emitted at about 10 slm. A mixture of these spent gases is supplied as the exhaust gas E to the apparatus 10 for abatement under reduced pressure via the vacuum pump 14. It should be noted that in a semiconductor device manufacturing process such as a CVD process for forming silicon oxynitride films, a dry pump is mainly used as the vacuum pump 14. Therefore, N.sub.2 (nitrogen gas) supplied to this vacuum pump 14 is purging N.sub.2 supplied to seal the shaft of the vacuum pump 14.

(6) The reaction tube 16 is formed of a corrosion-resistant metallic material such as Hastelloy (registered trademark), and has an approximately cylindrical casing 16a mounted upright with its axis vertical (see FIG. 2). The inner space of the casing 16a serves as a reaction chamber 18 for decomposing the exhaust gas E, and the upper end portion of the outer circumferential wall of the casing 16a is provided with an exhaust gas inlet 32 communicating with the outlet of the vacuum pump 14 through a pipe 30. On the other hand, the inlet end of a horizontally extending duct 16c is connected to the lower part of the casing 16a, and the outlet end of the duct 16c is provided with an exhaust gas outlet 34 connected directly to the inlet of a downstream vacuum pump 24.

(7) An opening 16b is formed in the top portion of this casing 16a, and the plasma generation unit 20 is connected to the opening 16b (see FIG. 2).

(8) The plasma generation unit 20 is configured to generate a high-temperature (or hot) plasma 22 in the form of a plasma arc or a plasma jet. In the present embodiment, the plasma generation unit 20 includes a plasma jet torch 36 for generating a high-temperature plasma by direct current (DC) arc discharge (see FIG. 2).

(9) This plasma jet torch 36 has a torch body 36a made of a metal material such as brass. An anode 38 is connected continuously to one end (the lower end in FIG. 2) of the torch body 36a, and a rod-shaped cathode 40 is connected to be disposed in the anode 38.

(10) The anode 38 is a tubular nozzle-type electrode made of a highly conductive, high melting point metal such as copper, copper alloy, nickel, or tungsten, and having a plasma generation chamber 38a formed in its cavity. An ejection port 38b for ejecting a jet of the high-temperature plasma 22 generated inside the plasma generation chamber 38a is formed in the central portion of the lower surface of the anode 38.

(11) The cathode 40 is a rod-shaped electrode element made of thorium-doped or lanthanum-doped tungsten or the like and includes a tapered end portion having an outer diameter decreasing toward the tip and disposed in the plasma generation chamber 38a mentioned above.

(12) In order to prevent electrical contact (i.e., short circuit) between the anode 38 and the anode 40 through the torch body 36a, an insulating material (not shown) such as a tetrafluoroethylene resin or a ceramic is disposed therebetween. Cooling water channels (not shown) are provided in the anode 38 and the cathode 40, respectively, to cool these components.

(13) A power supply unit 42 is connected to the anode 38 and the cathode 40 of the plasma jet torch 36 configured as described above, so as to apply a predetermined discharge voltage and form an arc between the anode 38 and the cathode 40. As the power supply unit 42, a so-called switching-type DC power supply device is suitably used.

(14) The plasma generation unit 20 configured as described above includes a plasma generation fluid supply unit 26.

(15) The plasma generation fluid supply unit 26 is configured to supply at least one selected from the group consisting of nitrogen, oxygen, argon, helium, and water, as a fluid for high-temperature plasma generation, into the plasma generation chamber 38a of the anode 38, and includes a storage tank for storing these fluids and a piping system for communicating the storage tank and the plasma generation chamber 38a of the anode 38, although not shown in the figures. This piping system is provided with a flow controller such as a mass flow controller.

(16) The downstream vacuum pump 24 is a pump for reducing the pressure in the region located downstream of the outlet of the vacuum pump 14 and including the reaction chamber 18 of the reaction tube 16 to a predetermined degree of vacuum and drawing the exhaust gas E that has undergone abatement treatment in the reaction chamber 18 to discharge it. In the present embodiment, a water-sealed pump is used as the downstream vacuum pump 24. Therefore, on the outlet side of the downstream vacuum pump 24, a separator 44 such as a gas-liquid separator coalescer is optionally provided to separate the treated exhaust gas E and the seal water in their mixture discharged from the downstream vacuum pump 24 (see FIG. 1).

(17) Here, the reduced pressure created by the downstream vacuum pump 24 for the exhaust gas flow region located downstream of the outlet of the vacuum pump 14 and including the reaction chamber 18 is preferably in a range of 50 Torr or more and 400 Torr or less, and more preferably in a range of 100±40 Torr. When the reduced pressure is lower than 50 Torr, an expensive and elaborate system is needed to achieve a high vacuum environment. In contrast, when the reduced pressure is higher than 400 Torr, which is close to the atmospheric pressure, the exhaust gas E must be diluted with a large amount of nitrogen gas, which is comparable to the amount of nitrogen gas required to dilute the exhaust gas E under atmospheric pressure.

(18) It is needless to say that the apparatus 10 for exhaust gas abatement under reduced pressure according to the present embodiment includes various types of detectors, controllers, and power supplies that are necessary to generate the high-temperature plasma 22 in the plasma generation unit 20 and to operate the downstream vacuum pump 24, and others, although not shown in the figures.

(19) Next, a method for exhaust gas E abatement under reduced pressure using the apparatus 10 for exhaust gas abatement under reduced pressure configured as described above will be described.

(20) The exhaust gas E discharged from the exhaust gas source 12 is delivered to the reaction tube 16 via the vacuum pump 14. When the downstream vacuum pump 24 is operated, the exhaust gas E is introduced into the reaction chamber 18 under a predetermined reduced pressure, where the exhaust gas E is decomposed by the heat of the high-temperature plasma 22 ejected from the plasma generation unit 20.

(21) According to the method for exhaust gas abatement under reduced pressure of the present embodiment, the exhaust gas E is decomposed by the heat of the high-temperature plasma 22 under the reduced pressure, and therefore there is no need to use diluent nitrogen gas or the use of the nitrogen gas can be reduced to a minimum. Since there is no need to dilute with diluent nitrogen gas or the use of the nitrogen gas can be reduced to a minimum, almost all the heat of the high-temperature plasma 22 can be used directly for decomposition/reaction of the exhaust gas E. A combination of these two advantageous effects allow the exhaust gas E abatement apparatus to be configured very compactly.

(22) In addition, since the region located downstream of the outlet of the exhaust gas source and including the abatement treatment unit is under the reduced pressure, even if the exhaust gas E contains toxic substances to humans, there is no risk that the exhaust gas E leaks from the system before being decomposed by the heat of the high-temperature plasma 22.

(23) The following modifications may be made to the embodiment described above.

(24) In the above embodiment, DC arc discharge is used as a technique for generating a high-temperature plasma in the plasma generation unit 20 connected to the reaction tube 16, but any other high-temperature plasma generation technique can be used as long as the plasma generation unit 20 can eject a plasma with a temperature high enough to thermally decomposing the exhaust gas E. For example, other techniques such as inductive coupling and capacitive coupling are suitably used to generate a high-temperature plasma.

(25) In the above embodiment, a water-sealed pump is used as the downstream vacuum pump 24. However, a dry pump or the like may be used instead of this water-sealed pump, for example, when there is no need to wash decomposition products with water after the exhaust gas E abatement treatment.

(26) In the above embodiment, the vacuum pump 14 and the exhaust gas inlet 32 of the reaction tube 16 are connected by the pipe 30. However, the outlet of the vacuum pump 14 and the exhaust gas inlet 32 may be connected directly to each other. Furthermore, in the above embodiment, the exhaust gas outlet 34 of the reaction tube 16 and the inlet of the downstream vacuum pump 24 are connected directly to each other. However, the exhaust gas outlet 34 of the reaction tube 16 and the downstream vacuum pump 24 may be connected by a pipe.

(27) It will be understood that various modifications may be made to the above embodiment within the scope of knowledge of those skilled in the art.

REFERENCE SIGNS LIST

(28) 10: Apparatus for exhaust gas abatement under reduced pressure 12: Exhaust gas source 14: Vacuum pump 16: Reaction tube 18: Reaction chamber 20: Plasma generation unit 22: High-temperature plasma 24: Downstream vacuum pump 26: Plasma generation fluid supply unit E: Exhaust gas