Abstract:
Apparatus and methods are provided that are adapted to match the impedance of an electrical load to an impedance of an electrical signal generator. The invention includes providing a plurality of electrical components adapted to collectively match the impedance of the electrical load to the impedance of the electrical signal generator. The electrical components are arranged symmetrically and concentrically about an axis. Additionally, the invention may also include a first connector adapted to electrically couple the electrical signal generator to the electrical components. Additionally, the invention may also include a second connector adapted to electrically couple the load to the electrical components. Numerous other aspects are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to currently pending, commonly assigned, U.S. Provisional Patent Application Ser. No. 60/712,190 filed Aug. 29, 2005 and entitled “METHODS AND APPARATUS FOR SYMMETRICAL AND/OR CONCENTRIC RADIO FREQUENCY MATCHING NETWORKS,” which is hereby incorporated herein by reference for all purposes. 

   FIELD OF THE INVENTION 
   The present invention relates generally to high or radio frequency matching networks and specifically to high powered matching networks for plasma processing chambers. 
   BACKGROUND OF THE INVENTION 
   Referring to  FIG. 1 , a plasma processing system may include a high or radio frequency (hereinafter referred to as ‘RF’) matching network  100 , a variable impedance load (e.g. a plasma processing chamber)  102 , an RF generator  104 , and an RF delivery system  106 . The RF matching network  100  is disposed between and electrically coupled to the RF delivery system  106  and the variable impedance load  102 . The RF delivery system  106  is electrically coupled to the RF generator  104 . The RF matching network  100  may include electrical components of known or variable impedance values (e.g., variable capacitors and/or inductors). The RF delivery system  106  may include items such as a high power coaxial cable assembly and connectors. 
   The RF generator  104  may provide RF energy to the variable impedance load  102  via the RF delivery system  106  and the RF matching network  100 . The function of the RF matching network  100  may be to match the impedance of the variable impedance load  102  to the output impedance of the RF generator  104  and RF delivery system  106 . By matching the impedance of the variable impedance load  102  to the output impedance of the RF generator  104  and the RF delivery system  106 , the reflection of the RF energy from the variable impedance load  102  may be reduced. Reducing the reflection of RF energy may effectively increase the amount of RF energy provided to the variable impedance load  102  by the RF generator  104 . 
   A first technique of RF matching may include varying the electrical impedance of the capacitors and/or inductors until the impedance of the variable impedance load matches the output impedance of the RF generator.  FIG. 2  is a more detailed schematic drawing depicting the prior art RF matching network  100 . The depiction shows the asymmetrical arrangements of a tune component  108  and a load component  110  of the RF matching network  100 . In addition, RF matching networks  100  are typically asymmetrical in the arrangement of the tune  108  and load components  110 . 
   A second technique of matching the impedance of the variable impedance load  102  to the impedance of the RF generator  104  may utilize variable frequency matching. The impedance presented by the RF matching network  100  to the output of the variable RF frequency generator  104  may change with the frequency. By outputting a particular frequency from the RF generator  104 , the variable impedance load  102  may match the impedance of the RF generator  104  and the RF delivery system  106 . This technique may be referred to as variable frequency matching. Variable frequency matching may employ the RF matching network  100  that includes fixed value tune components  108  and load components  110  (e.g. fixed value capacitors, inductors and/or resistors). The values of the tune components  108  and load components  110  may be selected to help ensure that the impedance of the RF generator  104  will match the impedance of the variable impedance load  102 . 
   Prior art RF matching networks may help reduce the amount of energy reflected by the variable impedance load. However, the inventors of the present invention have determined that in some circumstances, existing RF matching networks may not reduce the amount of reflected energy sufficiently to avoid problems. Thus, what is needed are improved methods and apparatus for RF matching. 
   SUMMARY OF THE INVENTION 
   In some aspects, the present invention provides an apparatus adapted to match the impedance of an electrical load to an impedance of an electrical signal generator. The apparatus includes a plurality of electrical components adapted to collectively match the impedance of the electrical load to the impedance of the electrical signal generator. The electrical components are arranged symmetrically and concentrically about an axis. Additionally, the apparatus also includes a first connector adapted to electrically couple the electrical signal generator to the electrical components. The apparatus also includes a second connector adapted to electrically couple the load to the electrical components. 
   In another aspect, the invention provides a system, comprising an RF power generator, an electrical load, and an RF matching network coupled to the electrical load and the power generator, wherein the RF matching network includes one or more components symmetrically disposed about an axis. 
   In another aspect, the invention provides a method comprising receiving RF energy with a first connector having a first axis, coupling the RF energy from the first connector to an RF matching network having one or more components disposed symmetrically about a second axis, coupling the RF energy from the RF matching network to a second connector having a third axis, and preventing the reflection of RF energy from the second connector, wherein the first axis, the second axis and third axis substantially colinear. 
   Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art RF power system with an RF generator, an RF matching network, and a variable load; 
       FIG. 2  is a schematic drawing illustrating details of the prior art RF matching network depicted in  FIG. 1 ; 
       FIG. 3  is a perspective view of a first example embodiment of a symmetrical and/or concentric RF matching network according to some embodiments of the present invention; 
       FIG. 4  is a drawing depicting a schematic view of the RF matching network of  FIG. 3  according to some embodiments of the present invention; 
       FIG. 5  is a perspective view of a second example embodiment of a symmetrical and/or concentric matching network according to some embodiments of the present invention; and 
       FIG. 6  is a drawing depicting a schematic view of the RF matching network of  FIG. 5  according to some embodiments of the present invention. 
   

   DETAILED DESCRIPTION 
   Substrates processed in plasma processing chambers are becoming larger. Thus, with each successive generation of technology, larger plasma processing chambers are being manufactured to accommodate the larger substrates. Due to the plasma processing chambers increasing in size, the power needed to perform the requisite processing steps (e.g., etch, deposition and/or implant) is increasing. The inventors of the present invention have determined that the increase in RF power needed may lead to localized excessive current densities within existing RF matching network designs and, subsequently, localized heating of the components and/or conductors (often referred to as ‘hotspots’) of existing RF matching network designs. Accordingly, an improved RF matching network is needed to prevent the formation the hotspots. 
   In accordance with the present invention, an inventive RF matching network that includes a fixed valued component or components is provided. The fixed value components are disposed in a symmetrical and/or concentric arrangement about a longitudinal axis. The symmetrical and/or concentric arrangement may include a single component that is fabricated in a concentric fashion. Alternatively, the symmetrical and/or concentric arrangement may include multiple fixed value components arranged in a similar manner. The efficiency of the matching network is improved by symmetrical and/or concentric arrangement about the longitudinal axis because the RF currents are distributed evenly about the components. This reduces the likelihood of hotspots, thereby allowing for fewer and less expensive components. 
   Turning to  FIG. 3 , a perspective view of the inventive RF matching network  300  is depicted. The fixed valued components are disposed in a symmetrically and/or concentric arrangement. In this embodiment, fixed valued load capacitors  204  are disposed between and couple an input ground flange  206  and an input core  202 . The input core  202  is coupled to a capacitor plate  220 . An output disk  228  is coupled to a variable impedance load  302 . Additionally, tune capacitors  222  are disposed between and couple the input core  202  and the output disk  228 . The input core  202  is coupled to an electrically excited conductor of an RF delivery system  305 . The input ground flange  206  is coupled to the grounded portion of the RF delivery system. Also depicted in this figure are coolant inputs and outputs that are coupled to external fluid channels  218 A-D (only four of six shown). 
   Turning to  FIG. 4 , the details of the RF matching network of  FIG. 3  are illustrated schematically. The input core  202  is connected to the RF generator  304  via the electrically excited conductor of the RF delivery system  305 . The input core  202  is coupled to load capacitors  204  of the RF matching network  300 . The load capacitors  204  are coupled to an input ground flange  206  using load capacitor bolts  208 A. Disposed between the input ground flange  206  and the load capacitors  204  are concentric springs  210 A that circumnavigate the load capacitors  204 . Between the input ground flange  206  and the capacitor bolts  208 A are o-rings  212 A. Additionally, the o-rings  212 B are interposed between the load capacitors  204  and the input ground flange  206 . The o-rings  212 A and  212 B for each load capacitor  204  form a fluid reservoir  214 A around each load capacitor  204 . The fluid reservoirs  214 A are connected to each other via fluid channels  216  (only one shown). Additionally, at least two of the reservoirs  214 A (one input and one output) are connected to an external source of fluid (not shown) via external fluid channels  218  (only one shown). Furthermore, a capacitor plate  220  and output disk  228  also include fluid channels  216  and external fluid channels  218  in this embodiment. 
   The input core  202  is additionally coupled to the capacitor plate  220 . The input core  202  is coupled to the tune capacitor plate  220  via the compressive force of the threaded bolts  224  screwed into both the load capacitors  204  and tune capacitors  222 . The coupling of the input core  202  to the capacitor plate  220  is aided by alignment pins  226 . The capacitor plate  220  is also coupled to the tune capacitors  222  via the concentric springs  210 B. Additionally, interposed between the tune capacitors  222  and the capacitor plate  220  are the o-rings  212 D. As before, the o-rings  212 D disposed between the tune capacitors  222  and the capacitor plate  220  in addition to the o-rings  212 C between the load capacitors  204  and the input core  202  form fluid reservoirs  214 B around each tune capacitor  222 . The tune capacitors  222  are coupled to the output disk  228  via the bolts  208 B. As before, the o-rings  212 E interposed between the tune capacitors  222  and output disk  228 , in addition to the o-rings  212 F interposed between the bolts  208  and the output ring  228 , form fluid reservoirs  214 C around the end of each tune capacitor  222 . The tune capacitors  222  are also coupled to the output disk  228  via the concentric springs  210 C. Additionally, the output disk  228  is coupled to the variable impedance load  302  (e.g., the processing chamber). 
   In a first aspect of the invention, the novel RF matching circuit  300 , in a symmetrical and/or concentric configuration, is provided for a variable frequency network. In this embodiment, the tune capacitors  222  and load capacitors  204  are arranged in a symmetrical and/or concentric configuration. The tune capacitors  222  and load capacitors  204  are fixed capacitors arranged in an electrical circuit composing an RF matching circuit  300 . The tune capacitors  222  are in series with the variable impedance load. The load capacitors  204  are in parallel with the variable impedance load to be matched to the impedance of the RF generator. In this embodiment, the RF current is supplied via the core conductor of the coaxial cable to the input core  202  of the novel RF matching network  300 . The input core  202  may be employed as a first connector for the RF matching network  300 . The longitudinal axis of the input core  202  is located approximately collinear to the longitudinal axis of the RF matching circuit  300  to facilitate the symmetrical and/or concentric arrangement of the RF matching network tune capacitors  222  and load capacitors  204 . The input core  202  includes a rod and disk that are electrically coupled to a plurality of tune capacitors  222  and load capacitors  204 . The load capacitors  204 , being the high frequency coupling to earth ground, are arranged in a symmetrical and/or concentric configuration to ‘double back’ to the input ground flange  206 , thereby forming the electrical coupling to ground. The input ground flange  206  is held to be at or close to electrical earth ground by electrical coupling to the RF delivery system. This symmetrical and/or concentric ‘double back’ configuration is accomplished by placing the equal capacitive value load capacitors  204  in an equidistant fashion around the axis of the RF matching network  300 . Electrically, the load capacitors  204  are in parallel thus creating an electrically equivalent lumped capacitance value that may be used in approximate circuit analysis. The lumped value capacitance is an approximation and may be subjected to electromagnetic frequency and time domain analysis for a complete analysis of the electrical characteristic of the design. Alternatively, in some applications that would benefit from a different arrangement of load or tune components, there may be resistors and/or inductors arranged in place of, in series with or in parallel with the load capacitors  204  and/or tune capacitors  222 . Different physical arrangements may alternatively be used for a symmetrical and/or concentric arrangement such as, for example, an inductor alternating concentrically with the capacitors or an inner ring of resistors surrounded by inductors or capacitors. In some embodiments, a single capacitor, inductor or resistor may be formed or manufactured in a symmetrical and/or concentric fashion to create symmetrical and/or concentric arrangement of the passive components and/or resistor, capacitor and inductor. Additionally, various combinations of the symmetrical and/or concentric configuration of fixed and variable components may be employed with or without the variable frequency generator to create an optimal solution for different applications of the RF matching network  300 . For example, there may be benefit from combining variable capacitors and/or inductors in combination with a variable frequency generator to allow for a single matching network to be used on multiple different processes or chambers that do not have similar impedance characteristics. Generally, variable frequency matching networks are used with loads that are approximately stable. Variable components may be employed in applications that have a wider window of impedance. Note that the that symmetrical and/or concentric arrangement of the components may not be limited to matching networks, but may be applied to any circuit that would benefit from the symmetrical and/or concentric configuration of the components. In some embodiments, employment of active components such as diodes or transistors may be used in high power amplifier networks. 
   Returning to the embodiment of  FIG. 4 , the symmetrical and/or concentric configuration of the tune capacitors  222  and load capacitors  204  is facilitated by the use of disk shaped mating surfaces that comprise the input core  202  and input ground flange  206 . These disk shaped mating surfaces may be flat and thick enough to both dissipate heat and accommodate dishes that are created to further facilitate cooling, as discussed below. However, additional embodiments may include other support structure arrangements such as a hub and spoke form that facilitate, for example, fluid being circulated around the tune capacitors  222  and load capacitors  204 . Additionally, for applications that would benefit from a thinner and/or longer structure, support structures may be employed that arrange the components that are in a symmetrical and/or concentric configuration, but asymmetrical along the axis of the RF matching network  300 . 
   Additionally, in the embodiment of  FIG. 4 , the load capacitors  204  are mechanically coupled on one end (e.g., the RF or hot end) to the input core  202  and input ground flange  206 . The mechanical coupling or fastening is accomplished by placing a bolt  224 , with an outside diameter appreciably smaller than the inside diameter of the hole in the input core  202  (e.g., such that there is no contact between the bolt  224  and the input core  202  or capacitor plate  220 ) through which the bolt is placed into the threaded hole on the load capacitor  204  and tune capacitor  222 . In the alternative, other means of coupling or fastening may be employed such as welding, riveting and/or the like. 
   Additionally, in the embodiment of  FIG. 4 , the electrical coupling of the load capacitors  204  and the tune capacitors  222  to the input ground flange  206  and the capacitor plate  220  may be accomplished by placing canted coil springs  210 A-C into hollowed out dishes  230 A-C in the input core  202 , output disk  228 , and/or the capacitor plate  220 . The dishes  230 A-C provide mechanically stable platforms with which to place the circumnavigated canted coil springs  210 A-C and the capacitors  204 ,  222  to ensure a centered configuration of the capacitors  204 , 222 . Additionally, the dishes  230 A-C create a concentric and reliable electrical connection between the load capacitors  204 , the tune capacitors  222 , the capacitor plate  220  and the input core  202 , thereby forming an electrical node. Additionally, the dishes  230 A-C may provide a hollowed out feature that allow o-rings  212 A-F placed therein to define a volume that forms the fluid reservoirs  214 A-C to cool the load capacitors  204  and the tune capacitors  222 , as discussed below. The arrangement and size of the dishes  230 A-C depend on the arrangement and size of the load capacitors  204  and tune capacitors  222  and other components that create the electrical and mechanical coupling. 
   As indicated above, in the embodiment of  FIG. 4 , arrangements may be provided to cool the RF matching circuit  300  to prevent any overheating which may occur. Cooling may be provided by forming the fluid reservoirs  214 A with the body of the bolts  208 A, load capacitors  204 , the dish  230 A and the o-rings  212 A and  212 B. The o-rings  212 A and  212 B are arranged concentrically around the shaft of the bolt and capacitor. By tightening the bolts  208 A, the o-rings  212 A and  212 B are thereby compressed to form a fluid seal. Because the outside diameter of the bolts  208 A are sized to be appreciably smaller than the through hole of the input ground flange  206  or input core  202 , the fluid reservoir  214 A is formed. In addition, the ends of the tune capacitors  222  and load capacitors  204  may offer additional volume for fluid circulation by providing a dish on each end of the capacitor. Alternatively, the o-rings  212 A-F may be situated in a groove formed to hold the o-ring in place while the bolt is being tightened. Other forms of sealing such as flat chemrez o-rings, nickel-brass single compression disk or other such embodiments may be employed to form the seal. The fluid reservoirs  214 A around each load cap  204  are in fluid communication with each other via fluid channels  216  that may be machined or otherwise formed into the input ground flange. The fluid channels  216  are arranged to communicate between each of the fluid reservoirs  214 A,  214 B or  214 C to ensure that cooling fluid is applied, equally, to all of the capacitors to prevent the formation of hotspots. The arrangement in this embodiment is a concentric loop of fluid channels  216  connecting all of the fluid reservoirs  214 A or  214 B or  214 C. Alternative arrangements may be implemented such as a spoke and wheel arrangement of fluid channels, or other communication channels external to the input core or input ground flange such as copper tubing connecting each fluid reservoir or set of reservoirs. Alternatively, the fluid reservoirs may be replaced by a cooling loop arranged in a concentric and/or symmetrical fashion which may be attached to the input core  202  and input ground flange  206 . Such an embodiment may reduce the cost of the matching assembly via the use of techniques such as soldering, glue or other such connection and/or adhesion methods. Alternatively, a combination of a hub and spoke construction of the input core  202  and input ground flange  206  may be employed with forced air or other uncontained fluid immersion to cool the RF matching circuit  300 , thereby reducing costs. Various combinations of contained and uncontained fluids may be employed to optimize the compromise between costs and design goals. De-ionized water is employed as the cooling fluid. However, other fluids may be employed. De-ionized water may be selected because of its high impedance to ground to ensure that the electrically energized portions of the circuit remain electrically decoupled, via the fluid communication, from the grounds such as the input ground flange  206 . 
   Still referring to the embodiment of  FIG. 4 , the load capacitors  204  and tune capacitors  222  are attached to each other with threaded bolts  224 . The outside diameter of the threaded bolts  224  are appreciably smaller than the inside diameter of the holes in the capacitor plate  220  and input core  202  through which the stud is placed. By threading the threaded bolts  224  into one end of the load capacitors  204  through the holes in the input core  202  and the capacitor plate  220  and subsequently threading the tune capacitors  222  onto other end of the threaded bolts  224  the capacitor plate  220  and input core  202  are mechanically coupled to the tune capacitors  222  and load capacitors  204 . The electrical connection between the tune capacitors  222 , load capacitors  204 , the capacitor plate  220  and input core  202  may be considered a single electrical node that is ‘hot’ or ‘energized’ with RF energy. 
   Still referring to the embodiment of  FIG. 4 , the output disk  228  is coupled to the tune capacitors  222 . As with the mechanical coupling mentioned herein, the output disk  228  is mechanically coupled to the tune capacitors  222  via the threaded bolts  208 B that are arranged in a fashion similar to the arrangement described above pertaining to the capacitor plate  220  and input core  202 . The output disk  228  may be employed as a second connector for the RF matching network  300 . A longitudinal axis of the output disk  228  may be approximately collinear with the longitudinal axis of the RF matching network  300 . Additionally, the output disk  228  may be formed to create fluid reservoirs  214 C, channels similar to channels  216  and/or source channels similar to channel  218  in a similar fashion to the input core  202  and the capacitor plate  220 . The alternatives for cooling and electrical connection, as described above for the capacitor plate  220  and input core  202 , may also be applied to the output disk  228 . 
   The electrical connection to the process chamber may be accomplished via a bus bar  229  ( FIG. 5 ) that extends to couple to the variable impedance load  302  (e.g., the interior of the chamber). Other connective means may be employed such as a coaxial connection to a coaxial cable or simple copper cable bolted to the output disk  228 . Because the output disk  228  is connected to the variable impedance load  302 , there may not be a need to maintain a symmetrical shape to provide electrical symmetry as is useful on the RF generator side of the RF matching network  300 . However, the method of connection to the output disk  228  may be robust such as to provide for a large contact surface and tight fastening since the current path to the processing chamber is limited to passing through this connector. Additionally, a method of cooling this connection may be provided. 
   Turning to  FIGS. 5 &amp; 6 , a second embodiment of an RF matching network  500  is depicted. The internal conductor of the RF delivery system  305  is coupled to the input core  202 ′. The input core  202 ′ is coupled to the load capacitors  204  with threaded bolts  224  ( FIG. 6 ). The input core  202 ′ is also coupled to the tune capacitors  222  via threaded bolts  224  ( FIG. 6 ). The input core  202 ′ includes a copper coil  330 B that provides cooling for the input core  202 ′. The load capacitors  204  are connected to the input ground flange  206 ′ via bolts  208 A ( FIG. 6 ). The input ground flange  206 ′ may be at or near electrical earth ground by coupling directly to ground or via the grounded connector of the RF delivery system. The tune capacitors  222  are connected to the output disk  228 ′ via bolts  208 B. The output disk  228 ′ may be electrically energized and conduct the RF current from the RF matching network  500  to the variable impedance load via a bus bar  229 . 
   Referring specifically to  FIG. 6 , a schematic representation of the second embodiment of  FIG. 5  is depicted. The input core  202 ′ is shown connected to the energized conductor of the RF delivery system  305 . The input core  202 ′ is connected to the load capacitors  204  via threaded bolts  224 . The load capacitors  204  are coupled to the input ground flange  206 ′ via the bolts  208 B. The input core  202  is coupled to the tune capacitors  222  via threaded bolts  224 . The tune capacitors  222  are coupled to the output disk  228 ′ via bolts  208 B. The output disk  228  is coupled to the variable impedance load (e.g. the processing chamber) via a bus bar connection  229  ( FIG. 5 ). 
   In the embodiment of  FIGS. 5 and 6 , an input ground flange cooling loop  330 A may be coupled to the input ground flange  206 ′ by, for example, soldering the input ground flange cooling loop  330 A to the input ground flange  206 ′. Likewise an input core cooling loop  330 B may be coupled to the input core  202 ′ by using solder. Additionally, an output cooling loop  330 C may be coupled to the output disk  228 ′ by using solder. The input ground flange cooling loop  330 A, the input core cooling loop  330 B and output disk cooling loop  330 C may be connected to an external source of cooling fluid (e.g. de-ionized water). Each may be independently connected to a source and drain of cooling fluid (e.g. de-ionized water) via two connectors for each cooling loop. The input ground flange cooling loop  330 A may be connected with connectors  320 A and  320 E. The input core cooling loop  330 B may be connected with connectors  320 F and  320 D, and the output disk cooling  330 C may be connected with connectors  320 B and  320 C. One of the two connectors in the connector pairs is for the supply of the cooling fluid. The second connector is for the return of the cooling fluid. Thereby, fluid may be circulated through each the input ground flange cooling loop  330 A, input core cooling loop  330 B, and output disk cooling loop  330 C. In this example embodiment, the material used for the input ground flange cooling loop  330 A, input core cooling loop  330 B and output disk cooling loop  330 C may be copper. However, any practicable materials may be used to provide a contained path through which the cooling fluid may circulate. Such materials may include, for example, stainless steel or aluminum. Additionally, different materials and geometries may be employed in the cooling loop such as an array of tubes or a sandwich grid of holes in a machined surface. 
   Accordingly, while the present invention has been disclosed in connection with exemplary embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.