Abstract:
A microwave reactor for decomposing waste green house gases resulting from the manufacture of semiconductors and from other industrial processes. The microwave reactor includes a plasma chamber having a gas inflow port spaced apart from a gas outflow port for transporting gases through the plasma chamber. A gas plasma is generated in the plasma chamber to facilitate the gas decomposition. The structure of the microwave reactor includes an insulating cover protruding into the plasma chamber and forming an internal cavity that is isolated from gases in the plasma chamber. A microwave antenna extends into the internal cavity of the plasma chamber to couple the microwave energy into plasma chamber for causing a plasma to form in the gases.

Description:
CROSS REFERENCE 
     PROCESS GAS DECOMPOSITION REACTOR, invented by Bruce Minaee, filed May 17, 2000 and having SC/Ser. No: 09/572,111. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of gas decomposition and, more particularly, to a microwave reactor for decomposing waste gases resulting from the manufacture of semiconductors and from other industrial processes. 
     Certain gases such as Perfluorinated Compounds (PFCs) including nitrogen trifloride, NF 3 , and sulfurhexafloride, SF 6 , and hydrofluorocarbons (HFCs) emitted by industrial processes, such as semiconductor processes, are harmful when released into the atmosphere. PFCs and HFCs are categorized as greenhouse gases because of their strong infrared absorption and long atmospheric lifetimes. PFCs and HFCs act similar to CO 2  in causing the greenhouse effect. Because of their potential long term impact on the global climate, PFC&#39;s, HFC&#39;s, NF 3  and SF 6  have been included in the Kyoto Protocol which is aimed at significantly reducing the release of unwanted gases into the atmosphere. 
     The above-identified, cross-referenced application PROCESS GAS DECOMPOSITION REACTOR describes an improved microwave reactor for removing unwanted gases from industrial processes. In the cross-referenced application, a microwave reactor generates a plasma for decomposition of perfluorinated and hydro fluorocarbon compounds in a gas stream emerging from an industrial process, for example, a semiconductor manufacturing process. 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. 
     In the cross-referenced application, the plasma chamber includes inlet and outlet openings through which reactant and additive gases (such as oxygen, hydrogen or water vapor) enter the chamber and exit the chamber for gas flow-through processing. The openings of the plasma chamber are through flanges which mate with corresponding flanges in exhaust gas lines from the industrial process apparatus. The gases enter the plasma microwave chamber through a standard vacuum flange, are dispersed, and undergo plasma decomposition reactions in the microwave chamber. The decomposition reactions result in hydrofluorocarbonated compounds and perfluorocarbonated compounds and these and other exhaust gases are evacuated from the plasma chamber through directly mounted flanges at the outlet of the plasma chamber. 
     In the cross-referenced application, decomposition reactions occur once the microwave chamber has been energized to cause a plasma and the reactant and additive gases are flowing. A microwave generated field causes ionization of the gas molecules by extracting electrons from them. These electrons are accelerated by the microwave generated field and cause more ionization and cracking of the gas molecules. The cracked reactant molecules and the cracked additive gas molecules react to form by-products that can be scrubbed by a wet scrubber. 
     While the cross-referenced application is a significant improvement over other gas reactors, the embodiments described are constrained by the ability to economically generate microwave-induced plasmas without excessive wear on the microwave components. As semiconductor processes use larger and larger gas-flow tubes for larger and larger semiconductor wafers and other parts, a need exists for larger, more efficient and more easily installed and maintained microwave reactors for removing unwanted gases. 
     Accordingly, there is a need for improved microwave reactors to decompose PFCs, HFCs and other unwanted gases suitable for insertion in the lines of processes used in industry, particularly in the semiconductor manufacturing industry. 
     SUMMARY 
     The present invention is a microwave reactor for decomposing waste green house gases resulting from the manufacture of semiconductors and from other industrial. The microwave reactor includes a plasma chamber having a gas inflow port spaced apart from a gas outflow port for transporting gases through the plasma chamber. A gas plasma is generated in the plasma chamber to facilitate the gas decomposition. The structure of the microwave reactor includes an insulating cover protruding into the plasma chamber and forming an internal cavity that is isolated from gases in the plasma chamber. A microwave antenna extends into the internal cavity of the plasma chamber to couple the microwave energy into plasma chamber for causing a plasma to form in the gases. A microwave generator generates microwave power. A microwave connector connects the microwave power from the microwave generator to the plasma chamber. The microwave connector includes a microwave transport for transporting the microwave energy, a first microwave coupler for coupling the microwave energy from the microwave generator to the microwave transport with a matched impedance, and a second microwave coupler for coupling the microwave energy from the microwave transport to the plasma chamber with a matched impedance The second microwave coupler includes the microwave antenna extending into the internal cavity of the plasma chamber. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a microwave reactor positioned in the exhaust gas line of an industrial process chamber for decomposing green house gases. 
     FIG. 2 depicts a schematic representation of the microwave circuit that conducts the microwave energy in the microwave reactor of FIG.  1 . 
     FIG. 3 depicts one embodiment of the microwave reactor of FIG.  1 . 
     FIG. 4 depicts another embodiment of the microwave reactor of FIG.  1 . 
     FIG. 5 depicts a detailed top view of a portion of the microwave reactor of FIG.  1  and FIG.  3 . 
     FIG. 6 depicts a detailed front sectional view along sectional view line  4 - 4 ′ of FIG.  5 . 
     FIG. 7 depicts an enlarged sectional view of a portion of the microwave antenna of the FIG. 5 view. 
     FIG. 8 depicts an alternate embodiment of an enlarged sectional view of a portion of the microwave antenna of the FIG. 5 view. 
     FIG. 9 depicts a representation of the microwave power duty cycles used in controlling the microwave energy in the microwave reactor of FIG.  1 . 
     FIG. 10 depicts an alternate embodiment of the microwave reactor plasma chamber of FIG.  3 . 
     FIG. 11 depicts an alternate embodiment of an antenna with a center cooling hole  92 . 
     FIG. 12 depicts another alternate embodiment of an antenna with a bidirectional center cooling member. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, the process chamber  3  is used for industrial processes that exhaust green house gases such as PFCs and HFCs. Such gases are frequently found in the semiconductor industry. The industrial process chamber  3  includes an input port  4  for receiving input reactants used in the industrial process and an output port  5  for exhausting gases  16 . A turbo pump  6  is connected to pump the gases from the process chamber  3  and deliver the exhaust gases  16  to the gas line  12 . The gas line  12  connects to a microwave reactor  7  which operates to process the exhaust gases  16 . The microwave reactor  7  has a plasma chamber  11  with an inflow port  13 , an outflow port  15  and one or more other ports  14 . The diameter of inflow port  13  conveniently matches the diameter of the outflow port of the turbo pump  6  or is otherwise connected so that exhaust gases  16  enter the inflow port  13  with a minimum of back pressure resulting from the piping connections. The plasma chamber  11  is connected with exhaust piping  12  from the industrial process carried out in process chamber  3 . The microwave reactor  7  includes, or is connected to, a reactant supply  2  which provides reactant gases  17  such as hydrogen, oxygen and water vapor that are used in the plasma chamber  11 . A microwave generator  10  provides microwave energy to the plasma chamber  11 . A control unit  18  provides the measurement and control signals used in operating the microwave reactor  7 . The microwave reactor  7  includes, or is connected to, a cooling unit  93  that provides liquid or gas cooling through line  94  for the chamber  11  and/or other components of the microwave reactor  7 . 
     The microwave reactor  7  causes gases  16  together with reactants  17  in plasma chamber  11  to become ionized by collisions with electrons and ions as a result of the microwave energy supplied to chamber  11 . In such a plasma environment, the PFCs and HFCs decompose. The reactant gases  17 , such as hydrogen, oxygen or water vapor, introduced into the plasma chamber  11  facilitate formation of reactions with the decomposed constituents of the PFCs and HFCs. The reactant gases  17  are metered by flow controllers in reactant supply  2  and are dispersed throughout the plasma chamber  11  by injection under the operation of control  18 . 
     After microwave decomposition of PFCs and HFCs in plasma chamber  11 , the resultant exhaust gases  16 ′ are pumped from the plasma chamber  11  by pump  8  which in turn delivers the exhaust  16 ′ to a scrubber  9  that separates components of the exhaust  16 ′ and typically results in, among other things, a non-polluting ash that is easily disposable. 
     In FIG. 2, a schematic representation is shown of the microwave circuit that conducts the microwave energy in the microwave reactor of FIG.  1 . The microwave energy from the microwave generator  10  is conducted through a microwave connector  30  to the plasma chamber  11 . The control  18  controls the generation of power by the microwave generator  10  and senses and controls the resulting reactions in the plasma chamber  11 . The microwave connector  30  includes a microwave coupler  31 , a microwave transport  32  and a microwave coupler  33 . The function of the microwave coupler  31  is to match the impedance (electric and magnetic) input to the microwave transport  32  to the output impedance of the microwave generator  10 . The function of the microwave transport  32  is to efficiently transport the microwave energy over a distance that separates the microwave generator  10  and the plasma chamber  11 . The function of the microwave coupler  33  is to match the electrical impedance output from the microwave transport  32  to the input impedance of the plasma chamber  11  so that microwave energy is efficiently delivered to the plasma chamber  11 . 
     In order to have an efficient transfer of energy from a source such as microwave generator  10  to a load such as the plasma chamber  11 , the impedance of the load is desirably matched to the impedance of the source. Since these impedances are usually not the same, the microwave couplers  31  and  33  require impedance matching to ensure an efficient transfer of microwave energy. 
     In FIG. 3, a waveguide embodiment used in the microwave reactor  7  of FIG. 1 is shown. The microwave generator includes magnetron  27  formed, for example, by a microwave oscillator, not shown, that is coupled to the microwave waveguide  26  through a microwave coupler  31 . The microwave coupler  31  includes an oscillator antenna  29  that matches the impedance of the magnetron  27  to the waveguide  26 . The magnetron  27  delivers microwave energy into the waveguide  26  at the end distal to plasma chamber  11 . The dimensions of the waveguide are selected to provide a resonant cavity at the operating microwave frequency, typically 2450 MHz. The input power to the magnetron is typically from a power supply that generates a high voltage DC which may be in a pulse format. 
     Low cost power supplies, like those used for typical microwave ovens, provide an input to the magnetron using an LC circuit including a transformer, a capacitor and a diode. The power supply provides a 60 Hz half wave DC voltage that is ON for about {fraction (1/120)} of a second and OFF for about {fraction (1/120)} of a second. In the countries where the power line frequency is 50 Hz, then the ON and OFF times are {fraction (1/100)} of a second. Magnetrons used for more precise applications usually are supplied by a constant high-voltage DC power supply. In addition to a high-voltage DC, the filament of the magnetron also has a low-voltage, high-current AC power input (for example, 5 volts at 20 amps). The AC power for the high-voltage DC power supply that feeds the magnetron can be any convenient value, such as 110 V single phase, 208 V single phase, 208 V three phase. 
     At the end of the microwave waveguide that is proximate to plasma chamber  11 , a microwave coupler  33  couples the microwave energy from the waveguide  26  to the plasma chamber  11 . The microwave coupler  33  includes an opening  22  that permits a plasma antenna  19  to connect into the waveguide  26 , through opening  24  and connector  21  into the plasma chamber  11  within a non-conducing cover  25 . The cover  25  has a vacuum seal  23  with the plasma chamber  11 . The plasma antenna  19  couples microwave energy from waveguide  26  into the plasma chamber  11 . In the embodiment of FIG. 3, the antenna  19  is movable within the opening  22  so that the amount of extension of antenna  19  into plasma chamber  11  is adjustable. The adjustment of antenna  19  aids in matching the impedance between the waveguide  26  and the plasma chamber  11 . 
     When the environment within plasma chamber  11  is suitable, a plasma is generated and operates to decompose gases flowing through the chamber  11 . The cover  25  permits the opening  22  and the antenna  19  to extend into the interior of the plasma chamber  11  without actual contact with the gases that are present. In this way, corrosion of the antenna by the gases is avoided. Also, the microwave components including the waveguide  26  and the antenna  19  are all located external to locations where a vacuum is required. 
     Conditions within the plasma chamber  11 , such as temperature, pressure and plasma operation are sensed by transducers inserted through sensor ports  14 - 1 ,  14 - 2  and  14 - 3  penetrating through the housing  63  of plasma chamber  11 . The housing  63  is typically made of a solid block of aluminum. An outer wall of housing  63  can be stainless steel to protect the interior from damage. The ports  14 - 1 ,  14 - 2  and  14 - 3  typically each have flanges that resemble standard vacuum flanges for vacuum chambers. 
     The housing  63  has an inflow port  13  and an outflow port  15  which have screens  13 ′ and  15 ′, respectively, across the openings of the inflow port  13  and the outflow port  15  through which the gas  16  of FIG. 1 flows. The screens  13 ′ and  15 ′ are in good electrical contact with the housing  63  that encloses the microwave chamber  11  and therefore “close” the plasma microwave region at either end of chamber  11 . Also, the screens  13 ′ and  15 ′, in some embodiments, are coated with a catalyst that is useful in the breakdown of the input gas. 
     Typically, the port  14 - 1  is used for optical fiber to observe the optical emission spectra of the plasma within plasma chamber  11 . Typically, the port  14 - 2  is used for an optical diode for detecting the presence of the plasma. Typically, the port  14 - 3  is used for a pressure switch to cause an alarm if pressure in the plasma chamber exceeds a maximum level. 
     In FIG. 3, the impedance of the right side of the waveguide  26  is matched to the impedance of the magnetron  27  by the geometry of the waveguide and the antenna  29 . The dimensions of the waveguide  26  are selected to carry the microwave energy efficiently to the other side. Tuning rods  35  are inserted and adjustable for the amount of extension into waveguide  26 , further or less, for tuning the waveguide  26 . On the other side, the antenna  19  picks up the energy in the waveguide and delivers it to the plasma chamber  11 . The impedance of the antenna  19  is matched to the waveguide  26 . A plate  28  inside the waveguide  26  is movable to tune the waveguide  26  and match the impedance of the antenna  19 . 
     FIG. 4 depicts a coaxial cable embodiment used in the microwave reactor  7  of FIG. 1. A coaxial cable  41  functions as a transmission line for conducting microwave energy from the microwave generator to the plasma chamber  11 . The length of coaxial cable  41  is selected for efficiently transporting microwave energy and has a length that can be varied in multiples of ½ of the wavelength, λ, of the microwave that is transmitted by microwave generator  10 . Typically, the frequency of the microwave generator is 2.45 GHz having a wavelength, λ, of about 4.8 inches (about 12 cm). The length of the coaxial cable typically includes a length, having a value used for matching impedance, in addition to the length measured in multiples of ½ of the wavelength, λ. 
     In FIG. 4, a microwave coupler  33  includes a fitting  42  that attaches the coaxial cable  41  perpendicularly to antenna  19  to couple microwave energy from the coaxial cable  41  through antenna  19  to the plasma chamber  11 . Alternatively, microwave coupler  33  attaches the coaxial cable  41 ′, shown in alternate location relative to cable  41 , in line with the long direction of antenna  19  to couple microwave energy from the coaxial cable  41 ′ through antenna  19  to the plasma chamber  11 . The angle that the fittings make with the antenna  19  are selected to achieve good mechanical support and good microwave coupling. The microwave coupler  33  includes an antenna  19  that connects from the fitting  42  of the coaxial cable  41  or directly from the cable  41 ′ into the plasma chamber  11  at a position within a non-conducing cover  25 . The cover  25  has a vacuum seal  23  with the plasma chamber  11 . In the embodiment of FIG. 4, the antenna  19  is fixed in length but alternatively can be adjustable, as shown in other embodiments, for tuning. 
     When the environment within plasma chamber  11  is suitable, a plasma is generated and operates to decompose gases flowing through the chamber  11 . The cover  25  permits the antenna  19  to extend into the interior of the plasma chamber  11  without actual contact with the gases that are present. In this way, corrosion of the antenna by the gases in chamber  11  is avoided. 
     FIG. 5 depicts a detailed top view of a portion of the FIG. 3 waveguide embodiment of the microwave reactor  7 . The waveguide  26  is supported by a frame  51  and is attached to a housing  63  that contains the plasma chamber  11 . The opening  22  is open to provide access for adjusting the position of the antenna that extends into the plasma chamber. A sectional view line  4 - 4 ′ extends along the center of the waveguide  26 . 
     FIG. 6 depicts a detailed front sectional view along sectional view line  6 - 6 ′ of FIG.  5 . The microwave generator  10  is coupled to the microwave waveguide  26  through a microwave coupler  31 . The microwave coupler  31  includes antenna  29  that matches the impedance of the microwave generator  10  to the waveguide  26 . The waveguide  26  has openings  61 - 1  and  61 - 2  for receiving turning stubs, like turning stubs  35 - 1  and  35 - 2  of FIG. 3, for turning the waveguide  26 . 
     At the end of the microwave waveguide that is proximate to plasma chamber  11 , a microwave coupler  33  couples the microwave energy from the waveguide  26  to the plasma chamber  11  within the housing  63 . The microwave coupler  33  includes a hollow tube  78  that connects through the waveguide  26  into the plasma chamber  11  within a non-conducing cover  25 . The tube  78  is made of Teflon®, ceramic, quartz or other material transparent to microwaves. The cover  25  has a vacuum seal  23  with the plasma chamber  11 . The vacuum seal is made by bolting or otherwise fixing the connector  21  to the housing  63  of the plasma chamber  11 . The tube  78  encloses a plasma antenna  19  that couples microwave energy from waveguide  26  into the plasma chamber  11 . In FIG. 6, the antenna  19  is movable within the tube  78  so that the amount of extension of antenna  19  into plasma chamber  11  is adjustable. In the position shown in FIG. 6, the antenna  19  is retracted from the interior of the plasma chamber  11 . The adjustment of antenna  19  aids in matching the impedance between the waveguide  26  and the plasma chamber  11 . The cover  25  permits the tube  78  and the antenna  19  to extend into the interior of the plasma chamber  11  without actual contact with the gases that are present. The cover  25  is typically made of a one-piece ceramic material such as aluminum oxide and has a flange on one end for forming a tight seal to the connector  21 . 
     In FIG. 6, the screen  13 ′ is in good electrical contact with the housing  63  that encloses the microwave chamber. The screen  13 ′, for example, is formed of a metal sheet having holes of a few millimeters in diameter closely space with offsets of a millimeter or more. The object of the screen is to provide a good microwave barrier without impeding the flow of the gas to be reacted. Also, the screen  13 ′ is a good location to deposit a catalyst for the reaction in the microwave chamber. A catalyst can be located at other locations in the plasma chamber  11 , for example, as a lining  90 , on the interior wall of the plasma chamber  11 . 
     FIG. 7 depicts an enlarged view of a portion of the FIG. 6 view of the microwave reactor  7 . In FIG. 7, an outline of the plasma chamber  11  is shown. A plasma antenna  19  is slidably engaged for insertion into and retraction from the plasma chamber  11  through tube  78 . Tube  78  is located in the center of the connector  21  and is surrounded by an air pocket  72 . Tube  78  includes a slide member  73  made of Teflon® or other material transparent to microwaves and providing a good surface for sliding engagement. The plasma antenna  19 , typically made of aluminum, couples microwave energy from the waveguide  26 . The waveguide  26  has a conducting wall  50  which is typically aluminum and about {fraction (1/32)} inch (0.8 mm) thick. In the position shown in FIG. 7, the antenna  19  is retracted from the interior of the plasma chamber  11  with the tip  74  of antenna  19  upward in the Z axis direction. By vertical adjustment, the antenna  19  is movable to any position in the Z axis direction, for example, to a position as shown by tip  74 ′. The antenna  19  fits within tube  78  and tube  78  is typically formed of aluminum or other metal. In the FIG. 7 embodiment, the elevation of the antenna  19  is adjustable by movement of the antenna extension  71 . In other embodiments, the antenna is fixed and not movable. The core of antenna  19  is typically solid and formed of aluminum having a diameter of about 0.5 inch (1.3 cm). The height, T h , of the core of antenna  19  and the extender  71  is about 4 inches (10 cm). The height, P h , of the core  19  is about 1.8 inch (4.6 cm). The height, A a , of the antenna bottom above the plasma chamber  11  is about 1.8 inches (4.6 cm). The height, A o , of the extension of cover  25  into the plasma chamber  25  is about 2 inches (5.1 cm). The diameter of the plasma chamber  11  is about 4 inches (10 cm). The connector  21  has an outer diameter of about 2.4 inch (6.1 cm) and an inner diameter of about 1.8 inch (4.6 cm). In order to have efficient transfer of energy from the waveguide  26  to the plasma chamber  11 ′, the flange structure of the connector  21  and the waveguide  26  the antenna  198  and cover  25  act to impedance match the antenna and the plasma in the chamber. 
     FIG. 8 depicts an alternate embodiment of an enlarged view, analogous to the FIG. 7 view, of a portion of a microwave reactor  7 . In FIG. 8, an outline of the plasma chamber  11  ′ is shown. A plasma antenna  19   8  is slidably engaged for insertion into and retraction from the plasma chamber  11 ′. Plasma antenna  19   8  extends into the center of the connector  21  and is surrounded by an air pocket  72 . An opening  22  in the wall of waveguide  26  receives a vertical adjustment member  91  through a grommet  22  typically made of Teflon®, ceramic or other material transparent to microwaves and providing a good surface for sliding engagement with member  91 . The vertical adjustment member  91  is attached to plasma antenna  19   8  and is used for adjusting the vertical position, along the Z axis, of the antenna  19   8 . The plasma antenna  19   8 , typically made of aluminum or other good microwave conductor, couples microwave energy from the waveguide  26  into the plasma chamber  11 ′. The waveguide  26  has a conducting wall  50  which is typically aluminum and about {fraction (1/32)} inch (0.8 mm) thick. In the position shown in FIG. 8, the antenna  19   8  is inserted into the interior of the plasma chamber  11 ′. In the FIG. 8 embodiment, the elevation of the antenna  19   8  is adjustable along the Z axis by Z axis movement of the antenna extension  91 . In other embodiments, the antenna  19   8  is fixed and not movable. The core of antenna  19   8  is typically solid and formed of aluminum having a diameter of about 0.5 inch (1.3 cm). The height, T h , of the core of antenna  19   8  is about 4 inches (10 cm). The height, A o , of the extension of cover  25  into the plasma chamber  11 ′ is about 2 inches (5.1 cm). The diameter, D 11′ , of the plasma chamber  11 ′ is about 4 inches (10 cm). The ceramic cover  21  has an outer diameter, C o , of about 0.8 inch (2 cm) and an inner diameter, C i , of about 0.6 inch (1.5 cm). In order to have efficient transfer of energy from the waveguide  26  to the plasma chamber  11 , the structure and dimensions of the connector  21  and the waveguide  26  together with antenna  19   8  and cover  25  impedance match antenna  19   8  to the chamber  11 ′. The antenna  19   8  is moved in the vertical, Z axis, direction to further tune the impedance matching. 
     In FIG. 8, in order to quickly start the plasma operation, high voltage ignitor electrodes  86  and  87  are optionally provided for suppling a high voltage path into plasma chamber  11 . In many embodiments, such electrodes are not required. When used, the electrodes are coupled to a high voltage supply  88  and cause a spark inside of plasma chamber  11 . The spark within the plasma chamber  11  ignites a gas plasma as the result of an arc within the chamber. A plasma is ignited when a sufficient number of gas particles are present in a cloud within the central region of plasma chamber  11 . 
     In FIG. 9, the signals used for a pulsed power embodiment are shown. The C 1  waveform represents a power full ON operation with amplitude, A f , which is typically employed at the start of plasma operation to help initiate generation of the plasma in the gas. The C 2  waveform represents an ON/OFF duty cycle of about 30/70 with medium power amplitude, A m , during the ON portion of the cycle. The C 3  waveform represents a low ON/OFF duty cycle of about 10/90 with low power amplitude, A l , during the ON portion of the cycle. When using a low duty cycle, the microwave energy tends to be used in production of electrons and not in heating of the gas. The electrons perform the cracking of the molecules and facilitate the chemical reactions. In certain structures when the power is ON continuously, the majority of the energy of the electrons is used to heat the gas and not concentrated on the chemical reactions. Also, many reactions prefer a lower gas temperature than occurs at maximum microwave power. With an average power some value less than 100% of a continuous power source, better results are achieved in some embodiments. The ON/OFF duty cycle is adjustable to reduce the power supplied. The actual value of the duty cycle is achieved by experimentation for any particular embodiment. The control of the power and duty cycle has the advantages of requiring less consumption of electricity, less heating of the gases while permitting load control and flexible set-up and processing that tolerates wide changes in the process parameters. 
     In FIG. 10 an alternate embodiment of the microwave reactor plasma chamber  11  of FIG. 3 is shown where the cover  25 ′ extends all the way through the plasma chamber  11 . The opening  24  and connector  21  of the microwave coupler  33  receive an antenna  19   10  that penetrates into the plasma chamber  11  within the non-conducing cover  25 ′. The cover  25 ′ has vacuum seals  23  and  23 ′ with the plasma chamber  11 . The cover  25 ′ encloses plasma antenna  19   10  that couples microwave energy from waveguide  26  (see FIG. 8) into the plasma chamber  11 . In the embodiment of FIG. 10, the antenna  19   10  is movable within the cover  25 ′ so that the amount of extension of antenna  19   10  into plasma chamber  11  is adjustable. The cover  25 ′ permits the antenna  19   10  to extend into and through the interior of the plasma chamber  11  without actual contact with the gases that are present in chamber  11 . The microwave components including the interior to cover  25 ′ and the antenna  19   10  are all located external to locations where a vacuum is required. 
     In FIG. 11, an alternate embodiment, antenna  19   11 , is shown with a center cooling hole  92 . The cooling hole  92  mates with the cooling line  94  that connects to the cooling unit  93  of FIG.  1 . In connection with the embodiment of FIG. 8, air or other cooling gas is injected into the hole  92  by the cooling unit  93  into the opening of cover  25  and passes through a clearance distance between the antenna  19  into the interior  72  of connector  21  and out through leakage holes (not specifically shown) in waveguide  26  to the atmosphere. Accordingly, the embodiment of FIG. 11, when used in FIG. 8, tends to cool both the antenna  19  and the waveguide  26 . The waveguide  26  is specifically designed not to be air or other gas tight, the only design objective is to be a good microwave conductor without substantial microwave leakage for human safety considerations. The cooling material can be air, nitrogen or any other cooling material suitable for microwave environments. 
     In FIG. 12, an alternate embodiment, antenna  19   12 , is shown with center cooling holes  96  and  97  which provide for bidirectional flow of a cooling gas or liquid. The cooling holes  96  and  97  mate with corresponding holes in the cooling line  94  that connects to the cooling unit  93  of FIG.  1 . In connection with the embodiment of FIG. 8, air, water or other cooling gas or liquid is injected into the inner hole  97  and extracted from the outer hole  96 , or vice versa, by the cooling unit  93  by means of line  94 ′ and connector  95 . Accordingly, the embodiment of FIG. 12, when used in FIG. 8, tends to cool antenna  19   12  by flow into hole  97  and out from hole  96 . 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention.