US3818845A

US3818845A - Low temperature plasma incinerator and method of stabilizing impedance therein

Info

Publication number: US3818845A
Application number: US00209879A
Authority: US
Inventors: H Nakane; A Uehara
Original assignee: Tokyo Ohka Kogyo Co Ltd
Current assignee: Tokyo Ohka Kogyo Co Ltd
Priority date: 1970-12-21
Filing date: 1971-12-20
Publication date: 1974-06-25
Anticipated expiration: 1991-06-25
Also published as: JPS4840437B1

Abstract

Rapid incineration of organic materials in a low temperature plasma incinerator for generation into a plasma by an applied high frequency electric field supplied with oxygen gas and maintained at a high vacuum by a vacuum pump is promoted by introducing a secondary gas between the incinerator and the vacuum pump at a rate sufficient to maintain a generally constant vacuum in the incinerator and thus avoid deviations in the electric impedance balance of the system.

Description

"United States Patent- 191 s U t 111 3,818,845

Nakane et al. I [4511 June 25,- 1974 [54] LOW TEMPERATURE PLASMA 3,173,388 3/1965 Menrath et a] 110/8 E [NCINERATOR AND METHQD 0 3,357,376 12/1967 Miller 110/8 E Marr, .11. et al l R [75] Inventors: Hisashi Nakane, Kawasaki; Akira Uehara, Yokohama, both of Japan Prima'y Examinelr Meyer Peru Assistant Examiner-Ronald C. Capossela Assignee! Tokyo Ohka Kogyo Attorney, Agent, or Firm-William J. Daniel Kanagawa-ken, Japan [22] Filed: Dec. 20, 1971 5 [2]] Appl. No.: 209,879 7] ABSTRACT Rapid incineration of organic materials in a low tem- I 30 F A perature plasma incinerator for generation into a 1 Drug pphcauon nomy Data plasma by an applied high frequency electric field sup- DCC. 21, I970 Japan; yg g and maintained at a uumby a vacuum pump is promoted by introducing a b51050? Secondary gas between the incinerator and the 9 l 8 E uum pump at a rate sufficient to maintain a generally 0 care constant vacuum in the incinerator and thus avoid deviations in the electric impedance balance of the sys [56] References Cited tem UNITED STATES PATENTS 3,027,445 3/1962 Johnson 110/8 E 6 Claims, 1 Drawing Figure I FROM Pfi/MARY GAS SOURCE FROM szco/vomr 64.5 SOURCE H- T0 HIGH WCUUM SOURCE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for stabilizing impedance in a plasma low-temperature incinerator.

2. Summary of the Prior Art In a low-temperature plasma incinerator, organic materials which are otherwise difficult to incinerate can be readily incinerated into ashes within a plasma at a low temperature of about 100 to 200 C. Consequently, this type of oven is widely used in quantitative analysis for the pre-treatment of metals in an organic substance or for the incineration of photo-resists used for the manufacture of semi-conductors, et cetera.

The operation of the usual plasma low-temperature incinerator may be summarized as follows. A cylindrical reaction tube of quartz or glass is evacuated to a pressure less than about Torrs with a vacuum pump and a high-frequency high voltage is applied to electrode plates disposed on the outside of the reaction tube by a high-frequency oscillator connected to the electrode plates, whereby a plasma of the gas remaining in the reaction tube is generated. If oxygen gas is introduced under such conditions into the reaction tube containing organic material, an oxygen plasma is produced, the material is incinerated to ashes by the action of this oxygen plasma.

In such a system, theincineration rate is affected by various factors such as, for example, the level of voltage applied, the kind of gas introduced (such as oxygen, nitrogen, hydrogen and the like), the rate of introduction of such gas, and the nature of its flow (either laminar flow or turbulent flow). The most important factor, however, is the matching of the impedance at the, electrical output of the oscillator with the impedance at the incineration reaction tube. The effect of such matching of impedance is great, particularly in the case of an apparatus of large processing capacity.

One of the causes of variation of such impedance is a change of the capacitance on the output side relative to a prescribed capacitance on the reactiontube side. Thus, when such changes occur, it becomes necessary to adjust the grid voltage of the output tube and this necessarily results in a shift in impedance. However, such a shift is usually small and can be easily corrected by a variable condenser.

Another cause of a variation in impedance is a change in the flow rate of the gas introduced. It often occurs that the incineration operation is carried out by changing the flow rate of the gas introduced while keeping the power output constant. For instance, where the amount of the material to be treated is large, the gas may be initially introduced in a great quantity and gradually reduced in amount as theprocess proceeds. Also, when treating a wafer, the gas flow rate is adjusted according to the thickness of the photo-resist layer thereupon. In general, impedance decreases in proportion to increases in the level of vacuum, resulting in increased consumption of electric power. On the other hand, if the level of vacuum is lowered, the impedance increases and the power consumption is reduced. There exists, however, a level of vacuum below which it may become impossible to generate any plasma. 7 I

It is known that in operating a plasma incinerator, a significant variation in impedance may be produced when a small amount of gas is introduced into a reaction tube held at a high vacuum so that the level of vacuum in the tube is reduced, i.e., the pressure in the tube increasesQTherefore, impedance matching usually is accomplished by a manual adjustment of the condenser in accordance with the gas flow rate. However. if this technique of impedance matching is used for a high vacuum treatment, there can arise a situation where the plasma cannot be retained in the reaction tube but is released from the inlet or outlet, resulting in a condition similar tothat of a neon tube. If such a condition occurs, it is no longer possible to make correction by a variable condenser. Thus, a simple method for preventing variations in impedance caused by change of the gas flow rate in a plasma low-temperature incinerator has long been sought.

SUMMARY OF THE INVENTION GENERAL DESCRIPTION OF THE INVENTION According to the present invention, there is provided a method and apparatus for preventing variation in the impedance of the system in question characterized in that a suitable flow of a secondary gas is introduced through a secondary gas inlet provided between the exhaust port of the reaction tube and the vacuum pump, in accordance with any increase or decrease of the rate of primary gas flow into the reaction tube, while the interior of the reaction tube is kept in a high vacuum condition by use of a vacuum pump having a large dis charge capacity, thereby maintaining constant the level of vacuum in the reaction tube by maintaining the total flow of the two gases through the system substantially constant.

According to the method of the present inventiomit is possible to substantially avoid any variation in impedance even if the primary gas flow rate to the reaction tube is varied from zero to the maximum level for practical use.

In practicing the concept of the present invention, no particular difficulty is encountered. It is only required to provide a secondary gas inlet between the exhaust port of an incineration reaction tube and a vacuum pump in a conventional plasma low-temperature incinerator, and introduce a secondary gas, such as, for example, air, through such. secondary gas inlet into the system. During operation of the incinerator in this manner, if the flow rate of the primary gas fed to the gas inlet of the incineration reaction tube is reduced, the flow rate of the secondary gas from the secondary gas inlet is increased; while, if the gas flow rate to the incineration reaction tube is increased, the gas flow rate from the secondary gas inlet is decreased. In this way, variations in the primary gas flow are compensated or balanced by an opposite variation in the secondary gas flow and the pressure in the incineration reaction tube is maintained substantially constant to prevent any change in impedance.

The incinerator reaction tube may include a cover or tion. The feature of the invention has the further ad vance that even if the operator inadvertently neglects to disconnect the vacuum pump to restore normal pressure after completion of the incineration operation, no back flow of oil to the vacuum pump will result since air or other secondary gas is allowed to flow in gradually through the inlet.

In practicing the present invention, it is preferred to use a vacuum pump having a large discharge capacity and the intake of the secondary gas is controlled so that the amount of secondary gas is larger than the amount of primary gas fed into each incineration reaction tube. In order to obtain an excellent plasma, the level of vacuum in each incineration reaction tube should be less than Torrs. Below 8 Torrs, a perfectly stabilized plasma can be obtained. For the case of cylindrical incineration reaction tubes each having a diameter of 75 mm and a length of 400 mm, the flow rate of primary gas introduced thereinto is usually 1 l/min per tube at most.

The primary gas will ordinarily be oxygen to effect incineration of the organic materials but, where other gases are known for use for this purpose in the art, they are equally suitable in the context of this invention. The

secondary gas is of less importance since, being introduced downstream of the incineration Zone, it does not affect the reaction. Air is extremely convenient for this purpose as it can be merely drawn from the atmosphere but other gas, which do not deleteriously affect the system, could be substituted if desired.

DESCRIPTION OF PREFERRED EMBODIMENT A plasma low-temperature incinerator constructed as shown in the drawing was used. Three incineration reaction tubes 1, 1' and 1" (each 75 mm in diameter and 400 mm in length) having individual gas inlets, 2, 2' and 2" and

gas discharge pipes

3, 3 and 3" were connected together through a distributing manifold 4 and a collecting manifold 5. A gas feed inlet port 6 of the distributing manifold 4 was connected through a pipe 9 to an oxygen gas bomb (not shown), with a flowmeter 8 having a needle valve 7 arranged at a suitable location along pipe 9, so as to allow oxygen gas to flow into the system as a primary gas. The exhaust port 10 of the collecting manifold 5 is connected to a vacuum pump preferably having a discharge capacity of about 150 l/min (now shown). Between the exhaust port 10 and the vacuum pump is provided a secondary gas inlet 11 to which a flow-meter 13 having a needle valve 12 is connected and through which a secondary gas (for example air) 14 is introduced. Along the circumference of each incineration reaction tube 1, l' and 1" are provided pairs of opposed metal plates l5, l6, l5, l6 and 15", 16" to serve as electrodes, to which a 3.000- volt, 13.56-MI-Iz high-frequency oscillator is connected.

With the mechanism assembled in this manner, each needle valve is closed and the vacuum pump is operated until the pressure in each incineration reaction tube is reduced to 0.2 Torrs in about 3 minutes. Upon reaching this stage, a high-frequency output power of about 300 W is applied to the electrodes of each incineration reaction tube, whereupon generation of plasma is observed and the impedances at the electrical output of the oscillator and at the incineration reaction tubes are matched by adjusting a variable condenser in the oscillator, If the output is increased to the level of 1,000 W under this condition, plasma spreads out to gas inlet and outlet ports of each incineration reaction tube (as in the case of a neon tube) and, when this occurs, it is no longer possible for the variable condenser to control the plasma so as to retain it inside the reaction tubes. Therefore, the output power is usually reduced to the level of 500 W in operation, but this is not economical. When under such low output condition oxygen gas is introduced into each incineration reaction tube at a flow rate of 200 to 300 ml/min, the pressure in each tube slightly varies within the range of 0.2 to 0.5 Torrs, causing decrease of the output to 400 W. When the flow rate is increased to 500 ml/min, the pressure is increased to 1 Torr, causing a further decline in the output to 300 W. The matching impedance condition could be restored by operating the variable condenser manually in accordance with the change of the flow rate to thereby obtain the approximately initial output power. In this case, the amount of control by the variable condenser is proportional to'the flow rate.

Then, in'accordance with the invention, before generating plasma, air was introduced from the secondary gas inlet, the flow rate of air controlled by the needle valve so that the flow rate is gradually increased from zero and the shift of the impedance matching point, the condition of plasma in the tubes, and the output were observed. The results are shown in the Table below.

Effect on Secondary Gas Introduction on Conditions of Incineration Gas flow rate into Air flow incin. re- Impedance Incinerarate I/ Pressure Temp I act. tube Output matching tion time min) (Torr) Condition of plasma (C) (ml/min) (Watts) point (Min.)

0 0.2 Spread to outside of tubes 0 1700 15 I 60 0 1600 2 60 0 1500 Shifted 3 60 0 1400 5 4 1 0-300 1300 5 I 100 0-300 1300 Slightly 3 6 2 100 0-300 1100 Shifted 7 3 Plasma at outlet vanishes & is fixed in tubes I60 0-300 1000 1.5-2 8 3 Plasma at inlet also vanishes & is fixed in tubes... 0-300 1000 1.5-2 9 3 160 0-300 1000 10 3 160 0-300 1000 ll 3 160 0-300 1000 Q 12 3 160 0-300 1000 Shift 13 3. 160 0-300 1000 14 "3. 160 0-300 1000 15 4 I60 0-300 I000 Note 1: The air flow rate is the value for three incineration reaction tubes combined.

Note 2: The gas flow rate into incineration reaction tube is the value for a single incineration reaction tube.

Note 3: The output is the value for the three incineration reaction tubes combined.

As apparent from the results shown in the above table, an ideal stabilized gas balance effect is produced when the air flow rate from the secondary gas inlet is made higher than the gas flow into the incineration reaction tubes, particularly when it is maintained at. 8 to 14 l/min.

Then, a piece of wafer made of OMR81 (photo-resist manufactured by Tokyo Applied Chemicals Co.) having a film thickness of l p was placed in each of the three incineration reaction tubes and plasma was generated under the same conditionsas described above for determining the time required for incineration. The results are also shown in the above table.

. It is evident from these results that the incineration rate is high where good impedance matching exists.

What is claimed is:

1. in a method of incinerating organic material in a low temperature plasma incinerator wherein a plasmaforming incinerating gas is fed into an enclosed incinerating zone containing said material and is converted into a low temperature plasma by means of a high frequency electric field passing through said zone while the zone is maintained under a high vacuum by communication with a vacuum source, and the amount of such gas is varied during the course of incineration, the improvement of introducing downstream of said zone a'secondary gas in an amount sufficient to maintain in said incinerator zone a substantially constant level of vacuum during the incineration.

2. The methodof claim 1 wherein said incineration temperature is between about lOD200 C.

3. The method of claim 1 wherein said high vacuum is not higher than about 10 Torr.

4. The method of claim 1 wherein the amount of the secondary gas introduced downstream of said incinerating zone exceeds that of the oxygen supplied to the incinerating zone.

5. In a low temperature plasma incinerator for organic materials which includes at least one ceramic incinerator reaction tube for containing the material to be incinerated and having inlet and outlet ports, means for supplying gaseous oxygen to said inlet port, conduit means for connecting said outlet port to a high vacuum source, and meansfor creating a high frequency electric field within each such tube for converting said oxygen to low temperature plasma and comprising at least one pair of electrode plates arranged on opposite exterior sides of said tube, each such pair of plates being in opposed spaced relation with said tube therebetween, and means for connecting said plates to a high he quency high voltage electrical source, the improvement of a secondary inlet port in said conduit between said reaction tube outlet port and said high vacuum source connected to a source of secondary gas, and adjustable valve means for controlling the amount of secondary gas admitted through said secondary inlet port, whereby variations in the quantity of oxygen supplied to said tube can becompensated by admission of the secondary gas to maintain the tube under substantially constant vacuum.

ments of said tube.

Claims

1. In a method of incinerating organic material in a low temperature plasma incinerator wherein a plasma-forming incinerating gas is fed into an enclosed incinerating zone containing said material and is converted into a low temperature plasma by means of a high frequency electric field passing through said zone while the zone is maintained under a high vacuum by communication with a vacuum source, and the amount of such gas is varied during the course of incineration, the improvement of introducing downstream of said zone a secondary gas in an amount sufficient to maintain in said incinerator zone a substantially constant level of vacuum during the incineration.

2. The method of claim 1 wherein said incineration temperature is between about 100*-200* C.

5. In a low temperature plasma incinerator for organic materials which includes at least one ceramic incinerator reaction tube for containing the material to be incinerated and having inlet and outlet ports, means for supplying gaseous oxygen to said inlet port, conduit means for connecting said outlet port to a high vacuum source, and means for creating a high frequency electric fielD within each such tube for converting said oxygen to low temperature plasma and comprising at least one pair of electrode plates arranged on opposite exterior sides of said tube, each such pair of plates being in opposed spaced relation with said tube therebetween, and means for connecting said plates to a high frequency high voltage electrical source, the improvement of a secondary inlet port in said conduit between said reaction tube outlet port and said high vacuum source connected to a source of secondary gas, and adjustable valve means for controlling the amount of secondary gas admitted through said secondary inlet port, whereby variations in the quantity of oxygen supplied to said tube can be compensated by admission of the secondary gas to maintain the tube under substantially constant vacuum.

6. The incinerator of claim 5 wherein each such tube is generally tubular shaped and said electrode plates are arcuately curved around opposite peripheral segments of said tube.