JP2598433B2 - Exhaust gas discharge treatment equipment - Google Patents

Description

The present invention relates to an apparatus for detoxifying exhaust gas by discharge plasma. More specifically, thin film forming technology using vacuum, for example, various chemical vapor deposition methods, that is, reduced pressure CV
D method (Chemical Vapor Deposition), plasma CVD method,
Exhaust gas discharge treatment equipment that detoxifies the reactive gas discharged in the photo-CVD method or plasma etching method from a small amount to a large amount on an industrial scale of semiconductor manufacturing under vacuum decompression by plasma treatment. About.

[Related Art] Reactive gases used in various CVD methods and plasma etching for forming a thin film using a vacuum are not necessarily consumed in a CVD chamber or an etching chamber when forming the thin film. In some cases, and the generation of by-product gas. In the process of forming a thin film, the source gas may be directly discharged from the pump without going through CVD or etching. Many of these reactive gases can burn or explode if released to the atmosphere without treatment, and some of them are toxic. Have been.

Conventionally, as a method for detoxifying these reactive gases, dilution with a large excess of inert gas, chemical treatment by catalytic reaction, wet absorption and adsorption removal, dry adsorption removal, and the like have been used. All of these methods are performed under normal pressure after the vacuum pump is discharged, and not only have the drawbacks of high equipment costs and operating costs, but also cannot ensure safety if attention is paid to maintenance. Have been reported as some accidents.

On the other hand, as a technique belonging to another system, an exhaust gas discharge treatment method using discharge (discharge treatment method) has been proposed. These are characterized in that the gas to be treated is treated under reduced pressure in vacuum before the gas to be treated is discharged from the pump to the outside of the system. For example, Japanese Unexamined Patent Publication No. 51-129868 discloses that a waste gas containing a toxic substance and an oxidizing agent are brought into contact with each other in a space where plasma is generated to convert the toxic substance into a stable compound. A method of removing is disclosed. Further, Japanese Patent Application Laid-Open No. 58-6231 discloses a waste gas discharge treatment device which is disposed between a reaction tank for discharging reactive waste gas and a discharge device and decomposes and discharges waste gas by discharge. ing. However, in these discharge treatment methods, the load range capable of maintaining the plasma required for detoxifying the waste gas to a predetermined concentration is naturally limited, and a stable plasma state is maintained against a large load change, particularly, a pressure change. However, there is a problem that the applicable range of the exhaust gas treatment device must be limited.

Furthermore, recently, in the discharge treatment method, a method of using a plasma with a superimposed magnetic field in order to enable load fluctuation tracking (magnetic field superposition method) has been proposed (JSPS Plasma Electronics Research Group, January 1986). ). In the magnetic field superposition method, the direction of the electric field formed by the electrodes is approximately 45 °.
By applying a DC or AC magnetic field at an angle of about 135 °, the turning radius of the electrons in the plasma is reduced, and the electrons can turn between the electrodes. It has the characteristic that a stable discharge can be maintained underneath.

[Problems to be Solved by the Invention] An exhaust gas treatment device is required to stably detoxify an unexpected fluctuation of a load condition such as a flow rate, a pressure and a composition of a gas to be treated. Therefore, even in the discharge treatment method,
It is not enough to simply maintain a stable plasma by applying a magnetic field to follow such a load fluctuation, and a desired processing rate must be stably maintained. Since such a load change induces a change in the discharge voltage of the discharge plasma, that is, a change in the plasma resistance value, in order to realize a stable maintenance of the processing rate, these changes are detected and power of a predetermined value or more is supplied. It is necessary. To this end, it is common to apply a constant power supply or a constant current power supply having a fluctuation detection circuit and an output connection circuit. However, since these power supplies have such functions, they usually have problems such as not only superposition and large size, but also high cost, low power factor and large power consumption.

The present inventors have conducted intensive studies on these problems peculiar to the exhaust gas discharge treatment. As a result, if a power supply having drooping characteristics in current-voltage characteristics is applied, a simple and inexpensive power supply can be used with any specific control device. The present invention has found that, by automatically detecting a change in plasma resistance due to a load change and automatically supplying power equal to or higher than a predetermined value, a stable detoxification process of a gas to be processed can be performed. Was completed.

[Means for Solving the Problems] That is, according to the present invention, a discharge tube including at least a pair of electrodes provided in a tubular container having a gas inlet and a gas outlet, and a power supply connected to the electrodes The present invention provides an exhaust gas discharge treatment device, characterized in that a power supply having current-voltage characteristics having drooping characteristics is applied as the power supply.

The target gas to be treated in the present invention is a gas or a vapor that is used in a CVD method or a plasma etching method and may cause some disaster or pollution if released into the atmosphere without treatment. In particular, it is a gas that cannot be easily implemented by a conventional chemical treatment method such as a catalytic reaction or absorption / adsorption. For example, silane-based gases such as monosilane, disilane, and trisilane (these are decomposed into silicon or hydrogenated amorphous silicon and hydrogen by the discharge treatment according to the present invention);
Alkylsilane-based gases such as dimethylsilane (also decomposed into hydrogenated amorphous silicon carbide and hydrogen); Germanic gases (also decomposed into hydrogenated amorphous germane and hydrogen); chlorosilane-based gases; fluorosilane-based gases, etc. No. In addition, the present invention can be applied to a borane-based gas such as diborane; an alkylborane-based gas such as trimethylboron; and a phosphine-based gas. In these cases, by adding oxygen or the like,
More effective processing is possible. Of course, the gas that can be applied is not limited to the above gases, and may be a mixture thereof, or a gas diluted with hydrogen, nitrogen, and an inert gas.

The exhaust gas discharge treatment apparatus to which the present invention is applied basically includes at least a cathode 1 and an anode 2 in a tubular container having at least a gas inlet 5 and a gas outlet 6 as shown in FIG. Any device may be used as long as it is an exhaust gas discharge treatment device including a discharge tube 8 configured by providing a pair of electrodes and a power supply connected to the electrodes.
The present invention can be suitably applied to any of the apparatuses described above and other apparatuses used in a known discharge treatment method. Further, the present invention can be suitably applied to an exhaust gas device using at least one pair of a cathode pair and an anode pair proposed in another application filed on the same date as the present application. In FIG. 1, a magnetic field application device, for example, a permanent magnet 3 is installed on both sides of the cathode in the discharge tube 8 so as to face the N / S, and a DC magnetic field is applied in the cathode facing direction so that the magnetic field can also be superposed. However, the magnetic field applying device is not always an essential requirement.
Incidentally, 9 is a yoke.

According to the present invention, in such an exhaust gas discharge treatment apparatus, a power supply having a current-voltage characteristic having a so-called drooping characteristic is applied as a power supply connected to an electrode to generate plasma.

That is, as shown in FIG. 2, the drooping characteristic means a finite short-circuit current value of, for example, 500 mA and a finite release voltage value of, for example, 2.5.
When a certain load is connected to the power supply, the current and voltage in the power supply are not arbitrarily set, but the current is determined by a characteristic curve connecting the short-circuit current value and the open-circuit voltage value. It means that voltage characteristics are given.
Such a drooping characteristic is a characteristic found in a DC battery such as a solar cell, but in an AC, a power supply using a leakage transformer has the characteristic. In a leakage transformer,
When the impedance of the load is reduced, the characteristic is exhibited by leaking the magnetic field to the outside so that a current of a predetermined value or more does not flow. When the impedance of the load increases, the reverse operation is performed.

In exhaust gas discharge processing using discharge plasma,
When applied industrially, the flow rate, pressure,
Variations in composition and the like are often inevitable. These fluctuations appear as fluctuations in the resistance value or impedance value of the discharge plasma, and as a result, fluctuations in the discharge voltage. In the stable detoxification treatment of exhaust gas, it is necessary to maintain a current equal to or more than a predetermined value against such a load variation, but in a power supply having a drooping characteristic, even if the discharge voltage fluctuates, the current follows the characteristic curve. Since it is determined autonomously, by designing the power supply so as to have a characteristic curve composed of an appropriate short-circuit current value and open-circuit voltage value according to the processing, the current exceeding a predetermined value without adding any other function Can be maintained autonomously. Thereby, in the exhaust gas discharge treatment device of the present invention, even if the conditions of the treated exhaust gas such as the flow rate, the pressure, and the composition fluctuate, a stable current supply is realized,
The stability of the detoxification treatment rate is ensured. Furthermore, since a current of a short circuit current value or more does not flow due to the structure, there is an advantage that protection against damage to a circuit or equipment due to an excessive current can be achieved at the same time.

The power supply used in the present invention may be either DC or AC, but is preferably AC from the viewpoint of preventing arc discharge from continuing. In particular, an AC power supply composed of a leakage transformer has the above-mentioned functions, and has advantages such as easy operation, and the price is as low as a fraction of that of a constant-current or constant-power power supply. It is.

As described above, a power supply having drooping characteristics represented by a leakage transformer is connected to an electrode in an exhaust gas discharge treatment device, and by forming a plasma, stable exhaust gas detoxification processing can be performed. In order to maintain stable plasma generation under a wide range of exhaust gas conditions, it is preferable to further superpose a magnetic field on the plasma to reduce the turning radius of electrons. The pressure range in that case is about 0.1mTorr ~ 10To
It is about rr. More preferably, the superposing magnetic field is applied at an angle of about 45 ° to 135 ° with respect to the direction of the electric field formed by the electrodes.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1 An apparatus as shown in FIG. 1 was used. That is, a 20-cm-long stainless steel vacuum vessel with a 2-liter internal volume equipped with a 2-inch flange for gas introduction and gas desorption was used.
A pair of stainless steel plates having a thickness of 30 cm and a thickness of 2 mm are opposed to each other at an interval of 2 cm to form a cathode pair.
And an anode pair. Then, a capacitance of 4 kVA having a drooping characteristic as shown in FIG. 2, a short-circuit current of 500 mA,
A 2.5 kV open-circuit leakage transformer (H-type dry type, manufactured by Daihen) was connected to each of the cathode pair and the anode pair, and at the same time, a ferrite magnet with a surface magnetic flux density of 500 Gauss was installed on both back surfaces of the cathode pair. Plasma was generated by supplying 50 sccm of 100% monosilane gas from the gas inlet 5, and the pressure was arbitrarily changed within a range of 0.1 Torr to 1.5 Torr by a butterfly valve. The current flowing through the cathode and the voltage between the cathode and the anode were measured with an ammeter and a voltmeter, and the concentration of monosilane gas at the gas outlet 6 was measured with a quadrupole mass spectrometer. The current, voltage and outlet monosilane gas concentration under each pressure condition were as follows. At 0.1 Torr, 400 mA,
1.1kV, 1.8%; At 0.5Torr, 440mA, 1.0kV, 1.2%; 1.
At 5 Torr, it was 475 mA, 0.95 kV, 0.8%. In order to obtain this result, no artificial operation such as changing the set current value during the processing operation was required.

Comparative Example 1 A constant current DC power supply with a withstand voltage of 3 kV (Takasago Seisakusho)
The same measurement was attempted using the same apparatus as in Example 1 at the same monosilane gas flow rate and pressure range, except for using. However, each time the pressure changed, the current value also changed, and accordingly, the supplied power also changed. For this reason, the outlet monosilane gas concentration also fluctuated greatly, and the operation was terminated. Therefore, the set current value was changed every time the pressure was changed so that the supply power was 440 W artificially. Thus, the following results were finally obtained. In other words, when the pressure is 0.1 Torr, 400 m
A, 1.1 kV, 1.9%; at 0.5 Torr, 400 mA, 1.0 kV, 1.1
%; 1.5 Torr, 475 mA, 0.95 kV, 0.75%.

Example 2 In Example 1, when the same measurement was attempted with the magnet removed, the plasma could be stably maintained at a pressure of 0.2 Torr or more.

[Effects of the Invention] According to the present invention, in an exhaust gas discharge treatment device, a flow rate, a pressure,
Provided is an exhaust gas discharge treatment apparatus which realizes a stable treatment rate of a gas to be treated without any artificial adjustment or control system for fluctuations in the gas to be treated such as a composition. Further, by applying a magnetic field, it was possible to stably maintain plasma in a wider pressure range. Therefore, it must be said that the industrial applicability of the present invention is extremely high.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of an exhaust gas discharge treatment device used in an embodiment of the present invention. FIG. 2 is a graph showing an example of a current-voltage drooping characteristic of the leakage transformer according to the present invention. In the figure, the vertical axis indicates a voltage value, and the horizontal axis indicates a current value.

Claims (5)

(57) [Claims] 1. An exhaust gas discharge treatment apparatus comprising a discharge tube constructed by providing at least a pair of electrodes in a tubular container having a gas inlet and a gas outlet and a power supply connected to the electrodes. An exhaust gas discharge treatment device characterized by applying a power supply having a voltage characteristic having a drooping characteristic. 2. The power supply according to claim 1, wherein the power supply is an AC power supply.
An exhaust gas discharge treatment device according to claim 1. 3. The exhaust gas discharge treatment device according to claim 2, wherein the power supply comprises a leakage transformer. 4. The exhaust gas discharge treatment device according to claim 1, wherein a magnetic field is superimposed. 5. The direction of the electric field formed by the electrode and the angle between 45 ° and 135 °
The exhaust gas discharge treatment device according to claim 4, wherein a DC or AC magnetic field is applied at an angle of °.