Process gas decomposition reactor

Details Process gas decomposition reactor Process gas decomposition reactor

2000-05-17

2001-07-17

Gorgos, Kathryn (Department: 1741)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

With means applying electromagnetic wave energy or...

C219S678000

Reexamination Certificate

active

06261525

ABSTRACT:

TECHNICAL FIELD
The invention relates to a gas decomposition reactor and, more particularly, to a reactor for decomposing greenhouse gases emitted by semiconductor wafer processing tools.
BACKGROUND ART
Certain organic gases known as perfluorinatedcarbons (PFCs) and hydrofluorocarbons (HFCs), emitted by industrial processes, particularly semiconductor process tools, trap infrared radiation in the atmosphere, contributing to the greenhouse effect. These gases can also decompose in the upper atmosphere where highly reactive fluorine and fluorine compounds are liberated. These interact and react with ozone, generally causing the ozone to become molecular oxygen and enter reactions forming stable compounds. There is a net removal of ozone from the atmosphere in the ozone layer, a region of the atmosphere from 18 to 48 km (12 to 30 mi) above the earth's surface. Some scientists have predicted that foreseeable destruction of the ozone layer will cause increases in ultraviolet radiation with negative health effects, damage to certain crops and to plankton and the marine food web, with an accompanying increase in carbon dioxide due to the decrease in plants and plankton. The increase in carbon dioxide gives rise to global warming because a sufficiently thick layer of carbon dioxide will also trap infrared radiation which is normally radiated away from the earth after solar heating of the earth's surface by the sun, giving rise to the “greenhouse effect”.
PFCs and HFCs are greenhouse gases because of their strong infrared absorption cross sections and long atmospheric lifetimes, when not in the upper atmosphere. Thus, PFCs and HFCs behave similar to CO2 in trapping heat and causing the greenhouse effect. Because of their potential long term impact on the global climate, PFC's, HFC's, NF3 and SF6 have specifically been included in Kyoto Protocol, which aims to significantly reduce the rate of global warming gas emissions into the atmosphere.
In summary, PFCs and HFCs get trapped in the atmosphere causing an increase in the earth's temperature. Secondly, these gases absorb atmospheric UV radiation in the upper atmosphere and break down to elemental materials, including fluorine atoms, which then deplete the ozone layer. Hence, there is a danger of exposure to high levels of UV radiation on the earth's surface, causing a decrease in plants and plankton and increasing levels of atmospheric carbon dioxide, a greenhouse gas.
In U.S. Pat. No. 5,965,786 J. C. Rostaing et al. disclose use of an atmospheric pressure gas plasma for destroying PFCs and HFCs thereby eliminating greenhouse gases before the gases are released to the atmosphere. The plasma is generated by a microwave source, directing energy down a waveguide to a dielectric discharge tube where the plasma resides. Process gas is fed into the discharge tube where the PFCs and HFCs are decomposed by collisions with high energy electrons.
In U.S. Pat. No. 5,902,404 G. Fong et al. disclose use of a chamber that is open to microwave energy from a waveguide. The chamber and waveguide are operated as a resonant cavity to produce an ionized gas plasma. The purpose of the chamber is to excite a process gas for use with semiconductor manufacturing equipment where thin films are deposited on wafers.
An object of the invention was to decompose PFCs and HFCs in a discharge tube compatible with process gas handling, particularly in the semiconductor manufacturing industry.
SUMMARY OF THE INVENTION
The above object has been achieved with a gas flow-though reactor generating a plasma for decomposition of perfluorocarbon and hydrofluorocarbon compounds in a process gas stream emerging from a process tool. The reactor features a pair of magnetrons feeding a pair of launching waveguides to a pair of helical coils forming a microwave induction structure within a plasma chamber coaxial with the gas flow path. Each magnetron has a microwave power source or generator that typically operates at an industrial frequency of 2450 MHz. Each magnetron is electrically coupled to one of a pair of antennae. One of the pair of antennae extends into one waveguide and the second of the pair of antennae extends into the other waveguide.
Each waveguide forms an elongate chamber resonant cavity and within each elongate chamber is a tuning stub. The tuning stubs are aluminum rods that minimize the reflected microwave energy so that the maximal microwave power may be applied to the plasma chamber. The position and size of the tuning stubs may be adjusted depending upon the components and parameters of the system as will be described below. The resonant cavities stabilize the magnetron oscillator frequency.
At the end of each waveguide elongate chamber closest to the plasma chamber a waveguide-to-applicator transition component, a core, is coupled to an impedance transformer. The cores couple the energy from the waveguides into the transformers and thence into the plasma chamber via a pair of intertwined, oppositely wound helical coils forming a helical induction structure.
The plasma chamber has a housing surrounding the coils and is formed of a metal such as aluminum. An insulating jacket is disposed within the outer housing of the chamber. The insulating jacket comprises a ceramic tube that provides a surface for catalytic conversion. In one embodiment the surface of the tube comprises a material that is flame polished. The insulating jacket may be made of other dielectric materials and may have other cross sectional shapes such as square.
The helical induction structure has a first and a second coaxial conductive helical coil. Each coil is intertwined within the other, within the insulating jacket. By “intertwined” is meant that the two coils are wound in parallel but insulated from each other. Thus two separate helical coils exist in side-by-side relation. The two coils carry current in opposite directions. The induction structure is formed from any conductive metallic surface that may be used for catalyzing reactions.
The microwave energy from each of the transformers is directed to the helical induction structure from two microwave oscillator sources via the two cores. Therefore, one microwave energy source does not affect the other source directly, but one source may stabilize the other by a cross-coupling effect explained below.
Since a gas discharge can be sustained under a variety of process conditions, it is inherently more advantageous to initially ignite the plasma at a predetermined condition and then maintain this plasma as operating conditions change based on other parameters of the process, for example, flow, pressure, residence time, etc. This ignition is provided by high voltage sparks applied to each coil through common components of the microwave energy delivery system, namely, waveguides, impedance matching transformers and helical coils.
The plasma chamber also comprises an inlet and outlet openings through which reactant and additive gases enter the chamber and exit the chamber. The openings are through flanges which mate with corresponding flanges in a process gas stream of a process tool.
Examples of reactant gases are additive gases such as oxygen, hydrogen or water vapor. The gases enter the plasma chamber through a standard vacuum flange, are dispersed, and after decomposition reactions of the hydrofluorocarbonated compounds and perfluorocarbonated compounds have taken place the gases are evacuated through directly mounted flanges at the inlet and the outlet of the plasma chamber.
Decomposition reactions occur once the helical induction structure has been energized and the reactant and additive gases have been added. The energized helical induction structure which has microwave energy delivered to it from two sources produces two oppositely directed fields that polarize in opposite directions. As the fields of sufficient strength are built in the microwave cavity, a gas plasma is generated, causing a net gain in high energy electrons. The high energy electrons create large numbers of ion pairs and excited molecules.

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