Patent Publication Number: US-2002007912-A1

Title: Coolant for plasma generator

Description:
REFERENCE TO RELATED APPLICATION  
     [0001] This application is a continuation-in-part of application Ser. No. 09/546,750, filed Apr. 11, 2000, which claims the priority benefit under 35 U.S.C. § 119(e) from provisional Application No. 60/128,859 of Kamarehi et al., filed Apr. 12, 1999. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally to remote plasma generators, for semiconductor processing equipment, and more particularly to microwave power plasma generators for ashing or stripping photoresist and removing polymeric residue from semiconductor substrates.  
       BACKGROUND OF THE INVENTION  
       [0003] In fabricating integrated circuits, photoresist is coated over semiconductor substrates and patterned through selective exposure to developing light and removal of either the developed or undeveloped portions. The patterned resist forms a mask used to extend the pattern into underlying layers, such as oxides or metal layers, by etching through the holes in the mask. Masks are also commonly used to selectively dope regions of the substrate by ion implantation. Once the mask has been employed, it is typically removed by an oxidation process. The oxidizing process is referred to in the industry as resist “stripping” or “ashing.” 
       [0004] Increased throughput is a primary objective in the semiconductor industry, particularly in the current era of single-wafer processing. Any reduction in the time required for processing each substrate serially in single-wafer processing systems can lead to significant cost savings in a highly competitive industry. In the case of resist stripping, the rate of processing can be increased by supplying reactive oxygen free radicals to the substrate. For example, dissociation of oxygen-containing gases, such as diatomic oxygen gas (O 2 ), results in atomic oxygen (O), also known as oxygen free radicals.  
       [0005] The addition of fluorine, in the form of NF 3 , CF 4 , SF 6  or fluorine free radicals (F), often aids the stripping process where resist chemistry has been complicated by prior processing. For example, it is difficult to remove photoresist that has been subjected to ion implantation, such as that employed in electrically doping the semiconductor substrate through the mask. Similarly, reactive ion etch (RIE) through resist masks, particularly where metal is exposed during the etch, tends to form polymeric residue, which is also difficult to remove by oxidation alone. In each of these situations, application of fluorinated chemistries aids cleaning the resist and residue from the substrate. Fluorine is also commonly used in other cleaning or etching steps.  
       [0006] Maximizing the generation of oxygen (and/or fluorine) free radicals positively influences the rate at which resist can be stripped, thus increasing substrate throughput. Such free radicals are commonly produced by coupling energy from a microwave power source to oxygen-containing gas. Remote microwave plasma generators guide microwave energy produced in a magnetron through a waveguide to a resonant cavity or “applicator,” where the energy is coupled to a gas flowing through the cavity. The gas is excited, thereby forming oxygen free radicals (O). Fluorine free radicals (F) are similarly formed when fluorine source gas is added to the flow. Common source gases include O 2  for providing O, and NF 3 , CF 4 , SF 6  or C 2 F 6  for providing F. Nitrogen (N 2 ) or forming gas (N 2 /H 2 ) is often added to the flow to increase particle kinetics and thereby improve the efficiency of radical generation.  
       [0007] While microwave radical generators can lead to significant improvements in ash rates, conventional technology remains somewhat limiting. The plasma ignited by the microwave power, for example, includes highly energetic ions, electrons and free radicals (e.g., O, F, N). While O and F free radicals are desirable for stripping and cleaning resist from the substrate, direct contact with other constituents of the plasma can damage the substrate and the process chamber. Additionally, the plasma emits ultraviolet (UV) radiation, which is also harmful to structures on the substrate.  
       [0008] Direct contact between the plasma and the process chamber can be avoided by providing a transport tube between the microwave cavity or applicator and the process chamber. The length of the tube is selected to encourage recombination of the more energetic particles along the length of the tube, forming stable, less damaging atoms and compounds. Less reactive F and O radicals reach the process chamber downstream of the microwave plasma source in greater proportions than the ions. Because the process chamber is located downstream of the plasma source, this arrangement is known as a chemical downstream etch (CDE) reactor. By creating a bend in the tube, the process chamber is kept out of direct line of sight with the plasma, such that harmful UV radiation from the glow discharge does not reach the substrate.  
       [0009] The tube itself, however, places several limitations on the CDE reactor. Conventionally, both the applicator and the transport tube are formed of quartz. Quartz exhibits advantageously low rates of O and F recombination, permitting these desired radicals to reach the process chamber while ions generated in the plasma source recombine. Unfortunately, quartz is highly susceptible to fluorine attack. Thus, the quartz transport tube and particularly the quartz applicator, which is subject to direct contact with the plasma, deteriorates rapidly and must be frequently replaced. Each replacement of the quartz tubing not only incurs the cost of the tubing itself, but more importantly leads to reactor downtime during tube replacement, and consequent reduction in substrate throughput.  
       [0010] An alternative material for applicators and/or transport tubes is sapphire (Al 2 O 3 ). While highly resistant to fluorine attack, sapphire tubes have their own shortcomings. For example, sapphire transport tubes exhibit much higher rates of O and F recombination as compared to quartz, resulting in lower ash rates. Additionally, sapphire is susceptible to cracking due to thermal stresses created by the energetic plasma, limiting the power which can be safely employed. Lower plasma power means less generation of free radicals, which in turn also reduces the ash rate. While employing single-crystal sapphire somewhat improves the strength of the tube relative to polycrystalline sapphire, safe power levels for single-crystal sapphire are still low compared to those which can be employed for quartz tubes. Moreover, bonding material at the joint between sapphire sections that create the bend in the transport tube, which prevents UV radiation from reaching the process chamber, is typically as susceptible to fluorine ion attack as is quartz.  
       [0011] Other limitations on the production of radicals in a conventional microwave plasma generator relate to the efficiency of the energy coupling mechanism. Much of the microwave power supplied by the magnetron is lost in power reflected back up the waveguide, where it is absorbed by an isolator module designed to protect the magnetron.  
       [0012] Energy also escapes where the applicator carries source gas in and free radicals out of the resonant cavity. The plasma-filled tube acts as a conductor along which microwave energy travels out of the cavity, thus effectively extending the plasma and reducing plasma density. In addition to reducing plasma density, and therefore reducing generation of radicals, the extension of the plasma also increases the risk of ions surviving to reach the process chamber and substrate housed therein. Microwave traps can confine such microwave leakage. For example, U.S. Pat. No. 5,498,308 to Kamarehi et al., entitled “Plasma Asher with Microwave Trap,” discloses a resonant circuit trap. Even employing such traps, however, the plasma expands outside the plasma source cavity along the tube to the outer edges of the traps.  
       [0013] Accordingly, a need exists for more efficient microwave generators to improve resist ash rates.  
       SUMMARY OF THE INVENTION  
       [0014] In satisfaction of this need, a remote plasma generator is provided for coupling microwave frequency energy to a gas and delivering radicals to a downstream process chamber. The plasma generator includes several features which, in conjunction, enable highly efficient radical generation and consequently high photoresist ash rate. Such high ash rates can be achieved for both standard photoresist and chemically more problematic residues, such as those created by ion implantation and reactive ion etching.  
       [0015] In accordance with one aspect of the invention, high power can be coupled to gas flow that includes a fluorinated chemistry. A single-crystal, one-piece sapphire applicator and transport tube resists fluorine attack. The tube can be lengthened and provided with an elbow joint, aiding recombination of ions and protecting the process chamber from UV radiation produced in the plasma discharge.  
       [0016] In accordance with another aspect of the invention, microwave transparent cooling fluid enables high power, high temperature plasma production. While useful for increasing practicable power for applicators of any material, liquid cooling is of particular utility in conjunction with sapphire applicators, which are susceptible to stress cracking.  
       [0017] In accordance with still another aspect of the invention, dynamic impedance matching via a sliding short at the terminus of a microwave cavity reduces power loss through reflected energy. At the same time, a low profile microwave trap produces a more dense plasma to increase radical production.  
       [0018] In accordance with yet another aspect of the invention, different radicals are separately generated and mixed just upstream of the process chamber, enabling individually optimized radical generation of the two species. In the illustrated embodiment, fluorine radicals are generated in a sapphire applicator, while oxygen radicals are generated in a quartz applicator. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019] These and other aspects of the invention will be apparent from the detailed description below, and from the appended drawings, which are intended to illustrate and not to limit the invention, and wherein:  
     [0020]FIG. 1 is a schematic view of a semiconductor reactor incorporating a remote microwave plasma generator, constructed in accordance with a preferred embodiment of the present invention;  
     [0021]FIG. 2 is a right, front and top perspective view of a plasma generator, constructed in accordance with the preferred embodiment;  
     [0022]FIG. 3 is a left, front and top perspective view of the plasma generator of FIG. 2;  
     [0023]FIG. 4 shows a subsystem of the plasma generator of FIG. 3;  
     [0024]FIG. 5 is a right, front and top perspective view of the subsystem of FIG. 4, shown with a protective sheath removed from a carrier tube;  
     [0025]FIG. 6 is an exploded view of the subsystem of FIG. 4, taken from the angle of FIG. 5;  
     [0026]FIG. 7 is a side sectional view of the subsystem, taken along lines  7 - 7  of FIG. 4;  
     [0027]FIG. 8 is a top down sectional view of the subsystem, taken along lines  8 - 8  of FIG. 4;  
     [0028]FIG. 9 is a graph illustrating reflected power against microwave generator power, for both fixed tuning and dynamically or in-situ tuned impedance matching;  
     [0029]FIG. 10 is a graph illustrating reflected power against total gas flow, for both fixed tuning and dynamically or in-situ tuned impedance matching;  
     [0030]FIG. 11 is a graph illustrating reflected power against gas pressure, for both fixed tuning and dynamically or in-situ tuned impedance matching;  
     [0031]FIG. 12 is a cross-sectional view of a component of the subsystem of FIG. 4, including a low profile, co-axial microwave choke;  
     [0032]FIG. 13 is a rear, right perspective view of a semiconductor reactor incorporating dual plasma generators, constructed in accordance with a second embodiment of the present invention;  
     [0033]FIG. 14 is a rear elevational view of the reactor of FIG. 13;  
     [0034]FIG. 15 is a cross-sectional view taken along lines  15 - 15  of FIG. 13; and  
     [0035]FIG. 16 is a pair of graphs illustrating ash rates and uniformity using the plasma generators of the preferred embodiments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0036] While the illustrated embodiments are described in the context of a resist stripping or ashing system, the skilled artisan will readily find application for the devices and methods disclosed herein for other systems. Within the semiconductor industry, for example, plasma or free radical generation is desirable for assisting or enhancing many chemical etch and chemical vapor deposition processes. For many of these processes, remote production of plasmas advantageously avoids damage to the substrate.  
     [0037] With reference initially to FIG. 1, a chemical downstream etch (CDE) reactor  10 , according to a preferred embodiment, is schematically illustrated. The reactor  10  includes a microwave plasma generator  12  upstream of a process chamber  14 . A substrate  16 , typically comprising a monolithic semiconductor wafer, is supported upon a chuck  18  over a pedestal  20  within the chamber  14 .  
     [0038] With reference to FIGS.  1 - 4 , the plasma generator  12  includes a microwave power source  22 , which can be a conventional magnetron. For example, suitable microwave power sources are commercially available under the trade names NL10230 and NL 10250 from Richardson Electronics of LaFox, Ill. The NL10230 magnetron generator is capable of producing about 3,000 W of microwave power at 2,450 MHz (nominal). The skilled artisan will readily appreciate that, in other arrangements, the power source can be of any construction suitable for coupling power to a gas. Other plasma generators, for example, can employ radio frequency power, and energy can be coupled inductively or capacitively to the gas being ionized.  
     [0039] The illustrated plasma generator  12  further includes, adjacent the power source  22 , an isolator module  24 , which can also be of conventional construction. As is known in the field, the isolator  24  protects the magnetron from reflected power by diverting such reflected power to a dummy load. Desirably, the isolator  24  includes an integrated directional coupler, which also measures reflected power in order to match impedance along the microwave energy pathway, as will be discussed in more detail with respect to the section entitled “Impedance Matching,” below.  
     [0040] Microwave power is directed through the isolator  24  to a waveguide  26 , which extends into a microwave cavity or plasma source  28 . As best seen from FIG. 2, the illustrated waveguide  26  is S-shaped, enabling a stacked configuration and reducing reactor footprint on the fabrication floor. The waveguide includes a fixed tuning knob  30  (FIG. 1), which operates in conjunction with an autotuner module  32  to match impedance of the microwave energy path (including the isolator  24 , waveguide  26  and cavity  28 ) to that of the power source  22 . The autotuner module  32  is described in more detail in the section entitled “Impedance Matching,” below.  
     [0041] A gas carrier tube  34  extends from a gas source  36  through the microwave cavity  28 . The tube  34  axis extends transversely to the waveguide axis. In the illustrated embodiment, the gas source includes an oxygen source gas (preferably O 2 ), a fluorine source gas (preferably CF 4  or NF 3 ), and a carrier gas (preferably N 2 ). As discussed in more detail in the section entitled “Single-Crystal Transport Tube,” below, the carrier tube  34  includes an upstream section  38 , an applicator section  40 , and a transport tube section  41 . The transport tube  41  includes a bend or elbow joint  42  between the cavity  28  and the process chamber  14 .  
     [0042] A pair of microwave emission barriers  44 ,  46 , surrounding the applicator  40  immediately upstream and downstream of the cavity  28 , respectively, serve to prevent microwave energy escaping the cavity  28 . The construction of the emission barriers  44 ,  46  is discussed in more detail in the section entitled “Microwave Choke,” below.  
     [0043] Single-crystal Transport Tube  
     [0044] Referring to FIG. 1, as briefly noted above, the gas carrier  34  includes three sections: the upstream  38 , carrying gas from the gas source  36 ; the applicator section  40 , extending through the microwave cavity  28 ; and the transport tube section  41 , extending downstream of the cavity  28  to the process chamber  14 .  
     [0045] In operation, microwave power conducted from the waveguide  26  is coupled to gas flowing through the applicator portion  40  of the tube  34  (within the cavity  28 ), exciting the gas and igniting a plasma. The applicator  40 , including portions of the tube  34  within the cavity  28  as well as sections immediately adjacent the cavity  28 , is directly subjected to energetic particles of the plasma discharge and are consequently subject to faster deterioration than upstream portions, and slightly faster than downstream portions. Since the upstream section  38  of the tube  34  is not subject to the plasma discharge and therefore does not deteriorate rapidly, the upstream section  38  is preferably a conventional stainless steel gas line and is provided separately from applicator  40 , such that it need not be replaced when the applicator  40  is due for replacement.  
     [0046] In the illustrated embodiment, both fluorine and oxygen source gases are provided to the applicator  40 , generating O and F free radicals as well as a variety of ionic species and electrons. As noted in the Background section above, fluorine is particularly corrosive to quartz tubing. Accordingly, the applicator  40  preferably comprises sapphire for resistance to fluorine attack. Most preferably, the applicator  40  is formed of single-crystal sapphire, providing superior physical strength to withstand the stresses generated by exposure to the plasma.  
     [0047] As discussed in the Background section above, the length of the transport section  41  of the carrier tube  34  is selected to allow recombination of ions prior to introduction of the energized gas to the process chamber  14 . Preferably, the transport section  41 , from the end of the microwave cavity  28  to the process chamber  14 , is at least about 5 inches long, more preferably at least about 10 inches, and is about 14.5 inches in the illustrated embodiment. The total length of the applicator  40  and transport section  41  is about 21.5 inches in the illustrated embodiment. The skilled artisan will understand, however, that shorter lengths of transport tubing can be used where ion content is reduced in alternative manners.  
     [0048] The transport section  41  preferably includes the bend or elbow joint  42 , best seen in FIG. 7, thus avoiding direct line of sight between the glow discharge within the cavity  28  and the process chamber  14  (FIG. 1). Preferably, the bend  42  defines at least a 35° angle, and more preferably greater than about a 45° angle. In the illustrated embodiment, the bend  42  defines a 90° or right angle. The substrate  16  is thus shielded from harmful UV photons released by the glow discharge.  
     [0049] As the transport tube  41  is also subjected to energetic plasma products, including excited fluorine species, this section is also preferably formed of sapphire, and more preferably single-crystal sapphire. Additionally, the transport tube  41  is preferably formed integrally with the applicator tube  40 .  
     [0050] Unfortunately, the crystalline quality of sapphire tends to degrade for tube lengths over 12 inches. As previously noted, the resultant polycrystalline structure of longer lengths of sapphire tubing is susceptible to stress cracking when subjected to high power, high temperature plasmas. The desirability of forming an elbow joint within the transport tube section  41  also necessitates joining at least two sections of single-crystal sapphire. Typical bonding materials, however, are incompatible with fluorine, such that employing such bonds would negate the very advantage of sapphire tubing.  
     [0051] Accordingly, the illustrated transport tube  41  and applicator  40  are provided as sections of single-crystal sapphire, bonded at the elbow joint  42  without bonding materials susceptible to fluorine attack. In particular, the sections  40  and  41  are bonded by eutectic bonding, as disclosed in PCT publication number WO 09 856 575, entitled Eutectic Bonding of Single Crystal Components, published Dec. 17, 1998 (the “PCT application”). The disclosure of the PCT &#39;575 application publication is incorporated herein by reference. Single-crystal sections pre-bonded in the manner of the PCT &#39;575 application are available from Saphikon, Inc. of Milford, N.H.  
     [0052] Applicator Cooling System  
     [0053] The employment of single-crystal sapphire provides resistance to fluorine attack and greater strength than polycrystalline sapphire. Accordingly, the single-crystal sapphire tube, serving as an integral applicator  40  and transport tube  41 , enables coupling relatively high power to the gas, while withstanding fluorine attack for applications such as post-implantation ashing.  
     [0054] Coupling high power to the gas desirably increases the rate of O and/or F radical production, thereby increasing ash rates. However, the kinetic energy created in a high power plasma introduces negative effects as well. Within the cavity  28 , rapid and frequent collisions between energized particles, and between such particles and the applicator walls, raises the temperature of the applicator  40 , creating thermal stresses on the tube. While single-crystal sapphire is more resistant to such stresses than polycrystalline sapphire, sapphire remains subject to stress cracking under high power, high temperature operation, as compared to quartz. Moreover, the high temperature of the applicator  40  encourages recombination of dissociated particles. While recombination of ions and electrons is desirable, recombination of free radicals (F, O) is counterproductive.  
     [0055] Accordingly, the preferred embodiments employ a cooling mechanism within the applicator  40 , thereby reducing kinetically-induced recombination within the applicator  40 . Lengthening the transport tube  41  compensates for reduced ion recombination within the applicator  40 . Energetic ions recombine over the length of the transport tube  41  in greater proportions than radicals, due to additional electrostatic reactions encouraging such recombination. At the same time, cooling the applicator  40  enables use of higher power for a given tolerance for thermal stress. For sapphire applicators, in particular, the cooling mechanism reduces the occurrence of stress cracking while boosting the efficiency of free radical generation.  
     [0056] With reference to FIGS. 7 and 8, a cooling jacket  50  surrounds the applicator section  50  of the carrier tube  34 . The space between the jacket  50  and the applicator  40  is filled with a coolant fluid. Advantageously, the fluid is circulated through the jacket  50 , entering at a fluid inlet  52  (FIG. 7) and exiting at a fluid outlet  54  (FIG. 8). The inlet  52  and outlet  54  are arranged at 90° to one another, to encourage circumferential circulation of the fluid. The cooling jacket  50  preferably extends upstream and downstream of the microwave cavity  28 , as shown.  
     [0057] Desirably, both the jacket  50  and the cooling fluid comprise microwave transparent materials, thus maximizing microwave energy coupling to the gas within the applicator  40 , rather than direct absorption by the jacket  50  and coolant. The cooling jacket  50  preferably comprises quartz.  
     [0058] The coolant fluid is selected to have minimal hydrogen (H) content, which readily absorb microwave energy. Preferably, the coolant contains no hydrogen, and in the illustrated embodiment comprises a perfluorinated, inert heat transfer fluid. Such fluids are available from the Kurt J. Lesker Company of Clairton, Pa. under the trade name Galden™. Advantageously, this liquid coolant is available in multiple formulations having different boiling points. Thus, the most appropriate formulation can be selected for cooling the microwave applicator, depending upon the desired parameters for operating the microwave plasma generator. HT 135, for example, has a boiling point of about 135° C. but remains inert up to about 290° C. Galden™ is described as 1,1,2,3,3,3-hexafluoro, oxidized and polymerized, belonging to the family of fluorocarbons generally, and more particularly perfluorinated polyethers.  
     [0059] Similarly suitable microwave-transparent fluids are available in multiple formulations from 3M Specialty Materials of St. Paul, Minn. under the trade name Fluorinert™ liquids. Like Galden Like Galden™, Fluorinert™ liquids are described as perfluorinated inert fluids generally and more particularly as perfluorinated polyethers. Advantageously, the fluids are preferably completely fluorinated, having no hydrogen atoms. Formulations of Fluorinert™ sold as FC-3283 and FC-8270, with boiling points of about 135° C., are particularly suitable for the preferred embodiments. The skilled artisan will appreciate that the multiple formulations represent different levels of polymerization utilizing the same monomer. I the case of both Galden™ and Fluorinert™, the monomer is C 6 F 14 , although various substitutions may be made in the process of oxidation and polymerization without loss of the cooling function described herein.  
     [0060] Accordingly, recombination of desirable radicals within the applicator  40  is reduced by provision of liquid cooling of the applicator  40 . Moreover, greater power can be coupled to the gas without damage to the applicator  40 . In the illustrated embodiment, the power source  22  can be operated at full power (about 3,000 W) under normal operating conditions (i.e., continuous or intermittent operation while sequentially ashing photoresist from multiple wafers), without inducing stress cracks in the sapphire applicator  40 . It will be understood that improved power tolerance, and hence more efficient radical production, are also applicable to quartz applicators, which are generally more desirable for non-fluorinated chemistries. It will be understood that operable power levels may be considerably higher for quartz applicators with liquid cooling.  
     [0061] To a lesser extent, the downstream transport tube  41  is also heated by exposure to plasma by-products. The downstream tube, however, is not directly contacted by the glow discharge. Rather than liquid cooling the transport tube  41 , therefore, the preferred embodiments provide an insulated shroud  56  around the tube, as shown in FIGS.  2 - 4  and  6 , reducing risk of bums to technicians. Preferably, fans direct air through the insulated shroud  56 , cooling the transport tube  41  by convection.  
     [0062] Impedance Matching  
     [0063] With reference to FIGS.  1 - 3 , the microwave energy generated by the power source  22  is propagated through an energy path including the isolator  24 , waveguide  26  and microwave cavity  28 . The impedance of the various sections of the energy path should be closely matched to avoid energy loss through reflected power. By careful impedance matching, a standing wave or resonant condition is created in the microwave waveguide system  26 ,  28 , where the power is coupled to gas flowing through the applicator  40 . While the power source is protected from reflected energy by the isolator  24 , reflected energy absorbed by the dummy load in the isolator  24  represents wasted power that would otherwise be available for radical generation.  
     [0064] Impedance matching is complicated by the fact that the medium within which the microwaves propagate is of variable composition. The density and conductivity of gas flowing through the microwave cavity  28  varies with different process recipes. Since a reactor is typically utilized repetitively for the same recipe by a semiconductor manufacturer, impedance matching is typically performed for a given process recipe. Tuning the impedance of the waveguide  26  (including the cavity  28 ) is thus necessary when the reactor is first shipped to the manufacturer, as well as each time the process recipe is changed.  
     [0065] A common method of tuning impedance of the waveguide  26  is by employing three tuning knobs within the waveguide  26 . By adjusting the amount of protrusion of these conductors tranversely across the waveguide  26  at three different locations along the energy propagation axis, impedance of the waveguide  26  can be matched to that of the isolator  24  and power source  22 , thus minimizing reflected power for a given process recipe. This manner of impedance matching is known as a triple stub tuner. While effective in minimizing reflected power, triple stub tuners are expensive.  
     [0066] In the preferred embodiments, however, impedance matching is controlled by the combination of a fixed tuning knob  30  (FIG. 1) within the waveguide  26  and a sliding short  60 , shown in FIG. 8. The tuning knob  30  is preferably factory preset for gross tuning, while the sliding short  60  dynamically fine tunes the impedance matching.  
     [0067] The sliding short  60  is driven by a motor actuator  62  within the autotuner module  32 . The sliding short  60  comprises a conductor which extends across the walls of the microwave cavity  28 , thus providing a movable end wall for the cavity  28 . The position of the sliding short  60  can be changed until impedance of the waveguide  26  (including the microwave cavity  28 ) closely matches that of the isolator  24 , at which point a resonant condition is achieved within the cavity  28 . Moreover, adjustment of the sliding short  60  along the energy propagation axis, in place of transverse movement of a triple stub tuner in the waveguide  26 , facilitates arranging the standing wave pattern to optimize coupling of energy to the gases flowing through the applicator  40 .  
     [0068] Moreover, as the name implies, the autotuner  32  matches impedance dynamically via closed loop control. Reflected power is continually measured at the isolator module  24  (FIGS.  1 - 3 ) and sends signals to an electronic controller (not shown). The controller, in turn, sends signals to the motor actuator  62 , which drives the sliding short  60 . After the sliding short  60  moves, the change in reflected power is recognized by the controller, which then further adjusts the position of the sliding short  60 , and so on until reflected power is minimized.  
     [0069] In the illustrated embodiment, the sliding short  60  is initially positioned one half of the microwave energy wavelength (as measured in the Waveguide system) from the center of the applicator  40  running through the cavity  28 . In this position, the magnetic field is maximum, and a generally low electric field strength reduces damage from ion acceleration downstream. If necessary, the short  60  can be moved to one quarter of the microwave energy wavelength for initial ignition and then moved to one half wavelength after the plasma is essentially self-supporting. The motor actuator  62  preferably enables deviation forward or back from this default position by up to about 0.25 inch, more preferably up to about 0.5 inch, and in the illustrated embodiment, the sliding short  60  is movable by about 0.75 inch to either side of the default (½ wavelength) position. In another embodiment, the actuator  62  enables deviation from the default position of up to a half wavelength in either direction (±1.7 inches), for fully adjustable impedance matching. The autotuner module  32  further includes optical sensors to prevent over-movement of the sliding short  60 . It has been found that, not only does the above-described arrangement (short 60 at λ/2 from applicator center) minimize reflected power, but it also maximizes microwave magnetic field intensity within the applicator.  
     [0070] Dynamic closed loop control of impedance matching thus accommodates fluctuations in parameters in operation of a single process recipe. Additionally, autotuning can accommodate various process recipes. FIGS.  9 - 11 , for example, illustrate the effect of reflected power against various process parameters, including differences in power source output, total gas flow and gas pressure. It will be understood that other process parameters, such as constituent gas makeup, will also affect reflected power. As illustrated, dynamic or in-situ impedance matching reduces losses from reflected power, relative to fixed tuning, without the downtime required for manually tuning for each different process recipe. Such autotuning is particularly advantageous for research and development of new processes, where it is desirable to test many different processes for optimization.  
     [0071] Microwave Choke  
     [0072] With reference to FIG. 1, while the autotuner module  32  minimizes the power reflected back toward the power source  22 , impedance matching does not address the possibility of microwave leakage through the openings in the cavity  28  through which the applicator  40  extends upstream and downstream. Such leakage is disadvantageous for a variety of reasons discussed in the Background section above, and in U.S. Pat. No. 5,498,308 to Kamarehi et al., entitled “Plasma Asher with Microwave Trap” (hereinafter “the &#39;308 patent”). The disclosure of the &#39;308 patent is incorporated herein by reference.  
     [0073] In order to minimize this leakage, therefore, the preferred embodiments are provided with the upstream microwave choke or emission barrier  44  and the downstream microwave choke or emission barrier  46 . FIGS.  4 - 8  show these emission barriers  44 ,  46  in relation to the microwave cavity  28  and the carrier tube  34 , specifically in relation to the applicator section  40  of the carrier tube  34 .  
     [0074]FIG. 12 shows the emission barrier  46  in isolation. As illustrated, the emission barrier  46  comprises an inner conductor  70 , an outer conductor  72 , and a dielectric medium  74 . Each of these components is rectangular (see the exploded view of FIG. 6) and surrounds the applicator  40  just outside the microwave cavity  28 . When assembled, the inner and outer conductors  70 ,  72  define a choke cavity filled with the dielectric medium  74 , having a gap or opening  75  between the inner and outer conductors  70 ,  72  at a distal end of the choke cavity.  
     [0075] The inner conductor  70  and the outer conductor  72  define co-axial conductors selected to have an electrical length of a quarter of a wavelength of the microwave energy of interest. As disclosed, for example, in “Fields and Waves in Communication Electronics,” Ramo, Whinnery and Van Duzer, p. 46, Table 1.23 (hereinafter, “Ramo et al.”) the impedance of an ideal quarter wavelength line is given by the following formula,  
       Z   =       Z   0   2       Z   L                     
 
     [0076] where Z is the impedance of the coaxial line, Z 0  represents the characteristic impedance of the medium through which the electromagnetic waves travel and Z L  represents the load impedance. In the illustrated embodiment, the coaxial line is shorted across the inner conductor  70  and outer conductor  72  at the proximal end  76 . As the load impedance Z L  of a shorted line is ideally zero, the microwave energy propagating from the microwave cavity  28  toward the distal end of the choke  46  meets with an impedance approaching infinity at the opening  75  of the choke cavity, regardless of the characteristic impedance Z 0 .  
     [0077] The high impedance of an open ended, shorted, quarter wave coaxial line can alternatively be shown by using the formula for the impedance of a shorted line, as also disclosed in Ramo et al.:  
       Z=jZ   0  tan (β l )  
     [0078] The phase constant β equals 2π/λ, while the length of the line l has been selected to be a quarter wave, or λ/4. Inserting these values into the formula above, the tangent term (tan βl) becomes the tangent of π/2, which approaches infinity.  
     [0079] Referring again to FIG. 1, while the high impedance microwave chokes  44 ,  46  limit leakage of microwaves past the opening  75  of the choke cavity, the energy still propagates along the inner conductor  70  to the opening  75  of the choke. Energy thus continues to couple to the gas within the applicator  40  to this point, expanding the plasma beyond the confines of the microwave cavity  28 , both upstream and downstream of the cavity  28 . Such plasma expansion is disadvantageous for a number of reasons. As noted above, the expansion of plasma downstream of the cavity  28  increases the likelihood that energetic ions and/or UV radiation from plasma glow discharge will reach the process chamber  14 . Moreover, the expansion of the plasma disadvantageously reduces plasma density. As will be recognized by the skilled artisan, increasing the plasma density facilitates more efficient generation of free radicals for a given power input.  
     [0080] Accordingly, the dielectric medium  74  is selected to have a high dielectric constant. In contrast to air (dielectric constant=1), the illustrated dielectric medium  74  preferably comprises a solid material having a dielectric constant of at least about 3.0, more preferably greater than about 5, and comprises about 9 in the illustrated embodiment. The exemplary material of the illustrated embodiment comprises a ceramic, more particularly Stycast™ Hi K, available from Emerson &amp; Cuming.  
     [0081] Microwaves travel on the surface of conductors within the high dielectric medium  74 , i.e., along the interior of the choke cavity. Thus, the absolute distance of a quarter wave in the exemplary ceramic is much shorter than the absolute distance of a quarter wave in air (about 1.2 inches for 2,450 MHz microwave energy). In the illustrated embodiment, a quarter wave through the ceramic translates to an absolute distance of about 0.4 inch, since the quarter wave length is proportional to the square root of the medium&#39;s dielectric constant.  
     [0082] The effective volume of the plasma generated within the cavity  28  and the leakage out to the opening of the choke cavity is thus reduced with increasing dielectric constant of the dielectric medium  74 . Consequently, the density of the plasma is improved for a given power input, and the efficiency of radical generation improves. Improved radical generation, in turn, results in increased ash rates in the illustrated plasma ash reactor.  
     [0083] Segregated Plasma Sources  
     [0084] FIGS.  13 - 15  illustrate a plasma ash reactor  100  constructed in accordance with a second embodiment of the invention. It will be understand that the reactor  100  preferably includes one or more, and more preferably all of the above-noted features of the invention. As the reactor  100  includes many features which can be similar or identical to those of the previously discussed embodiments, like features will be referred to by like reference numerals, with the addition of the number  100 .  
     [0085] The illustrated reactor includes a first process chamber  114  and a second process chamber  115 . As the two chamber  114  and  115  can have identical construction, the present description will focus on the first chamber  114  and the plasma generators associated therewith.  
     [0086] The chamber  114  has two plasma generators  112  and  112 ′, each of which can have a similar construction as that disclosed above (with particular distinctions noted below). Each of the two plasma generators  112 ,  112 ′ leads generated radicals, via transport tubes  141 ,  141 ′ to the first process chamber  114 , as shown. The free radicals from each generator are mixed, prior to introduction to the process chamber  114 , in a mixer chamber  145 .  
     [0087] With reference to FIGS. 13 and 14, it can be seen that the microwave cavities  128  and  128 ′ of the two generators  112  and  112 ′ are transverse to one another. This arrangement enables closer packing of the modules within the reactor frame, saving footprint on the clean room floor.  
     [0088] With reference to FIGS. 14 and 15, the transport tubes  141 ,  141 ′ each communicate with the mixer chamber  145  via injectors  147 ,  147 ′. Advantageously, the injectors  147 ,  147 ′ are configured to inject radicals tangentially near the circumference of the mixer chamber  145 , thus facilitating mixing of the radicals from the two different sources  128 ,  128 ′. Most preferably, the injectors  147 ,  147 ′ inject with opposite orientation, such as clockwise and counter-clockwise, creating turbulence and aiding the mixture of reactive species from each of the plasma sources  128 ,  128 ′.  
     [0089] The interior walls of the mixer chamber  145  and the injectors  147 ,  147 ′ preferably comprise anodized aluminum, but can also be fabricated of polished sapphire for improved surface smoothness. In either case, the chamber  145  preferably has the same chemical makeup as sapphire (Al 2 O 3 ), which is advantageously resistant to fluorine attack. The mixer chamber  145  has a low profile, preferably less than about  1 . 0  inch in height, more preferably less than about 0.5 inch, and is about 0.22 inch high in the illustrated embodiment. The low profile presents less wall surface to the radicals, and thus reduced recombination of free radicals.  
     [0090] As shown in FIG. 15, mixer chamber  145  includes a relatively small central window  149  in the floor, opening into a first plenum chamber  153  defined above a first perforated baffle plate  151 . A second baffle plate  155  below the first baffle plate  151  defines a second plenum chamber  157  between the baffle plates  151 ,  155 . The second baffle plate  153  includes perforations (not shown) which are misaligned relative to the perforations of the first baffle plate  151 . Together, the baffle plates  151 ,  155  ensure uniform delivery of free radicals to the process chamber  114  below.  
     [0091] The separate plasma generators  112 ,  112 ′ advantageously enable individual optimization for different reactants. For example, the material used for the applicator and transport tubes have individual advantages and disadvantages, which tend to favor one material for a certain process recipe and another material for another process recipe. As noted above, sapphire advantageously exhibits resistance to fluorine attack. The disadvantage of sapphire, however, is that it exhibits undesirable recombination of free radicals. The table below illustrates, by way of example, the recombination coefficient (γ) of various materials.  
                                                   Material   Recombination efficiency (γ)                                                    silver   1.000           copper   0.708           iron   0.150           nickel   0.117           aluminum oxide (sapphire)   0.009           glass (quartz)   0.001           Teflon ™   0.0001                      
 
     [0092] The above table illustrates that sapphire exhibits about nine times more recombination of desirable radicals than quartz. Thus, while desirable for resisting fluorine attack, sapphire significantly reduces the efficiency of radical delivery to the process chamber.  
     [0093] Thus, in the illustrated dual plasma source reactor  100 , the first plasma generator  112  includes a single-crystal sapphire, integral applicator  140  and transport tube  141 , and is optimized for fluorine radical generation. Thus, the sapphire tube withstands fluorine attack. The second plasma generator  112 ′ includes a quartz applicator  140 ′ and transport tube  141 ′, and is optimized for oxygen radical generation. Thus, the quartz tube minimizes undesirable recombination of oxygen radicals, relative to sapphire, and at the same time can withstand higher power and higher temperature plasma production than sapphire.  
     [0094] Accordingly, oxygen radical production is not subjected to the limitations on plasma generation imposed by fluorine corrosion, while both fluorine and oxygen radicals are introduced to the processing chamber  114 . Moreover, if desirable for ashing photoresist at back-end stages of semiconductor fabrication, where the resist has not been implanted with ions, the fluorine plasma generator  112  can be turned off, and only the oxygen plasma generator  112 ′ operated. Similarly, where only fluorine etching is desired, the oxygen plasma generator  112 ′ can be turned off.  
     [0095] The skilled artisan will readily appreciate, in view of the present disclosure, application for the dual plasma source reactor for other plasma- or radical-assisted processes. In such other arrangements, it may be desirable to independently optimize features other than the material of the applicator tube, the source gas and the power level.  
     [0096]FIG. 16 illustrates very high plasma ash rates and uniformity obtained with the features described above. As shown, the preferred embodiments can achieve ash rates greater than about 6 μm/min, with a Shipley resist, with about 2% non-uniformity or less, and ash rates greater than about 8 μm/min with a Sumitomo resist with non-uniformity of about 2.5% or less.  
     [0097] Although the foregoing invention has been described in terms of certain preferred embodiments, other embodiments will become apparent to those of ordinary skill in the art in view of the disclosure herein. Accordingly, the present invention is not intended to be limited by the recitation of preferred embodiments, but is intended to be defined solely by reference to the appended claims.