Patent Publication Number: US-2003230385-A1

Title: Electro-magnetic configuration for uniformity enhancement in a dual chamber plasma processing system

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] Embodiments of the invention generally relate to semiconductor processing, and more particularly, to etch and inductive plasma related semiconductor manufacturing processes and related hardware.  
       [0003] 2. Description of the Related Art  
       [0004] Integrated circuit (IC) substrate processing systems, and in particular, substrate processing systems configured to fabricate VLSI and/or ULSI circuits on silicon substrates, often utilize several processes in order to form the desired circuit features into a die on a substrate. One process generally used in the manufacture of semiconductor devices is an etch process, which may be conducted in a reactive ion etching (RIE) chamber or a magnetically enhanced reactive ion etching (MERIE) chamber, for example. RIE and MERIE chambers are generally effective in etching narrow features into layers formed on a substrate, and therefore, RIE and MERIE chambers are generally preferred for VLSI and ULSI applications.  
       [0005] In an MERIE chamber, for example, features may be etched into a layer formed on a semiconductor substrate via the generation of a reactive plasma configured to react with a material on the substrate surface or the substrate surface itself though a series of photoresist masks. The reactive plasma is generated via the introduction of a reactive gas into the chamber, generally via a showerhead and blocker plate assembly, along with the application of sufficient energy, generally RF energy, to ignite a plasma of the reactive gas. A rotating magnetic field, generally produced by a bank of rotating magnets mounted outside and above the MERIE chamber, may operate to stir the ignited plasma in order to generate more uniform plasma characteristics over the entire substrate surface. However, although the density of the reactive plasma generated in conventional MERIE systems is sufficient for etching, it is desired to provide a more dense plasma for some etch processes.  
       [0006] In response to the need for a high density plasma in etch processes, an externally excited torroidal plasma source was added to an etch chamber. For example, U.S. Pat. No. 6,348,126, which is incorporated herein by reference, illustrates a torroidal plasma source in communication with an etch chamber. The torroidal plasma source operates to communicate a plasma to the processing region, and is generally capable of generating a plasma having a higher density than plasmas generated by conventional MERIE chambers.  
       [0007] However, another challenge associated with conventional semiconductor etch systems is that they are generally configured as single chamber, single substrate-type chambers, i.e., a single chamber is used to conduct an etch process on a single substrate in a one-at-a-time-type fashion. These single chamber-type systems are not able to provide high throughput rates, as each substrate must be sequentially processed in the single chamber. In order to address the throughput issues of single chamber-type systems, batch etch processing-type chambers have been developed. However, batch-type systems have been found to be generally undesirable in semiconductor manufacturing etch processes, as batch etch-type systems have been shown to yield uniformity variations between substrates manufactured in the same batch. Additionally, in other semiconductor processing areas, such as, for example, chemical vapor deposition, tandem processing chambers have been utilized to provide improved throughput while maintaining yield uniformity. For example, U.S. Pat. No. 6,152,070, which is assigned to Applied Materials of Santa Clara, Calif., illustrates a tandem processing chamber that may be used for vacuum processing of two substrates in separate isolated tandem processing regions at the same time. The tandem processing chambers may be accessed simultaneously by a single dual robot blade configured to insert and/or remove substrates from both of the processing regions at the same time. However, conventional semiconductor processing apparatuses and methods generally do not provide an etch chamber capable of providing greater throughput than that provided by single substrate chambers without sacrificing the physical characteristics, such as uniformity, for example, of the substrates produced.  
       [0008] Therefore, there is a need for an etch chamber configured to provide controllable etch uniformity and improved throughput characteristics.  
       SUMMARY OF THE INVENTION  
       [0009] Embodiments of the invention generally provide an etch system configured to provide concurrent transfer of at least two substrates through the etch system simultaneously. The substrates may be processed concurrently in tandem chambers that share common gas supply and pumping systems. Each of the tandem chambers generally includes a processing region having a substrate support member positioned therein, wherein the substrate support member may include heating and/or cooling elements to maintain a desired substrate temperature during processing. Additionally, each of the tandem chambers includes devices configured to generate and control a plasma in each of the respective tandem chambers, as well as a shield member positioned between the respective chambers to prevent magnetic interference. Accordingly, the present etch system is capable of providing the process control features of single substrate etch processing systems, while also providing increased substrate throughput.  
       [0010] More particularly, embodiments of the invention provide a tandem magnetically enhanced inductive source chamber for a semiconductor processing system. The tandem chamber generally includes a first tandem processing chamber, a second tandem processing chamber positioned adjacent the first tandem processing chamber and being separated therefrom by a shared central wall, and a pumping apparatus cooperatively in fluid communication with the first and second chambers. The first tandem processing chamber generally includes a first substrate support member positioned in a first chamber, a first plasma generation device in communication with the first chamber, and a plurality of first selectively actuated electromagnets positioned around the first chamber. The second tandem processing chamber generally includes a second substrate support member positioned in a second chamber, a second plasma generation device in communication with the second chamber, and a plurality of second selectively actuated electromagnets positioned around the second chamber.  
       [0011] Embodiments of the invention further provide a magnetically enhanced inductive source processing system that includes a loadlock chamber, a substrate transfer chamber selectively in communication with the loadlock chamber, and at least one tandem processing chamber selectively in communication with the substrate transfer chamber. The at least one tandem chamber generally includes a first and second adjacently positioned isolated processing chambers, a plurality of electromagnets positioned around the first and second processing regions, and at least one torroidal conduit in communication with each of the first and second adjacently positioned processing chambers. Additionally, the first and second adjacently positioned processing chambers generally share a common wall that magnetically separates the respective processing chambers while allowing fluid communication therebetween.  
       [0012] Embodiments of the invention further provide a tandem processing chamber having a first processing chamber, a second processing chamber positioned adjacent the first processing chamber and sharing a common wall therewith, at least one power supply in electrical communication with a first coil and a second coil, and a system controller in electrical communication with the power supply, the system controller being configured to regulate the electrical power delivered to the first and second coils. The first processing chamber includes a first substrate support member configured to receive a substrate in a lower portion of the first processing chamber and communicate the substrate to an upper portion of the first processing chamber for processing, and a first plurality of electronically controlled electromagnets positioned around a perimeter of the upper portion of the first processing chamber. Additionally, the first processing chamber generally includes at least one first torroidal plasma conduit in fluid communication with the upper portion of the first processing chamber, and at least one first coil positioned proximate the at least one first torroidal plasma conduit and being configured to generate a field within the at least one first torroidal plasma conduit. The second processing chamber generally includes a second substrate support member configured to receive a substrate in a lower portion of the second processing chamber and communicate the substrate to an upper portion of the second processing chamber for processing and a second plurality of electronically controlled electromagnets positioned around a perimeter of the upper portion of the second processing chamber. Additionally, the second processing chamber generally includes at least one second torroidal plasma conduit in fluid communication with the upper portion of the second processing chamber, and at least one second coil positioned proximate the at least one second torroidal plasma conduit and being configured to generate a field within the at least one second torroidal plasma conduit. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013] So that the manner in which the above-recited features of the invention are obtained may be understood in detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof, which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention, and are therefore, not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
     [0014]FIG. 1 illustrates a plan view of an embodiment of the etch system of the invention.  
     [0015]FIG. 2A illustrates a sectional view of an embodiment of a tandem etch chamber of the invention.  
     [0016]FIG. 2B illustrates a sectional view of another embodiment of a tandem etch chamber of the invention.  
     [0017]FIG. 3 illustrates a plan view of an exemplary tandem processing chamber of the invention.  
     [0018]FIG. 4 illustrates a plan view of another exemplary tandem processing chamber of the invention.  
     [0019]FIG. 5 illustrates a plan view of another exemplary tandem processing chamber of the invention.  
     [0020]FIG. 6 illustrates a sectional view of an alternative embodiment of the tandem processing chamber of the invention.  
     [0021]FIG. 7A illustrates a plan view of dense plasma regions within tandem processing chambers of the invention.  
     [0022]FIG. 7B illustrates a plan view of magnetic field lines in a tandem processing chamber of the invention.  
     [0023]FIG. 7C illustrates a plan view of magnetic field lines in another tandem processing chamber of the invention.  
     [0024]FIG. 8 illustrates a sectional view of an exemplary tandem processing chamber of the invention.  
     [0025]FIG. 9 illustrates a tandem etch processing chamber having cantilever-type substrate support members.  
     [0026]FIG. 10 illustrates a tandem etch processing chamber having rotatable magnets positioned above the lid of the chamber.  
     [0027] FIGS.  11 A- 11 D illustrate an exemplary plasma stirring process that may be implemented by embodiments of the invention. 
    
    
     DETAILED DESCRIPTION  
     [0028]FIG. 1 illustrates a plan view of an exemplary tandem chamber-type etch platform  100  of the invention. Platform  100  is generally a self-contained system having the necessary processing utilities supported on a main frame structure that can be easily installed and provides a quick start up for operation. System  100  generally includes four different regions, namely, a front end staging area  102 , a loadlock chamber  112 , and a transfer chamber  104  in communication with a plurality of tandem processing chambers  106  via isolation valves  209 . Front end staging area  102 , which is generally known as a factory interface or mini environment, generally includes an enclosure having at least one substrate containing cassette  109  positioned in communication therewith via a pod loader configuration. A front end substrate transfer robot  113 , which may generally be a track robot configured to move longitudinally within the enclosure, is generally positioned proximate cassettes  109  and is configured to remove substrates therefrom for processing, as well as position substrates therein once processing of the substrates is complete. Although four cassettes are shown, the present invention is not limited to any particular number of cassettes. For example, embodiments of the invention contemplate using the two outermost substrate cassette positions/pod loaders, while replacing the two interior cassette positions/pod loaders with a stackable substrate cassette feeder assembly (not shown). The stackable substrate feeder assembly may be configured to store a plurality of substrate cassettes in a vertical stack and individually deliver the cassettes to the outer cassette locations/pod loaders when needed. The front end staging area  102  is selectively in communication with the load lock chamber  112  through, for example, a selectively actuated valve (not shown). Additionally, loadlock  112  may also be selectively in communication with the transfer chamber  104  via another selectively actuated valve, for example. Therefore, the loadlock chamber  112  may operate to isolate the interior of the substrate transfer chamber  104  from the interior of the front end enclosure  102  during the process of transferring one or more substrates into the transfer chamber  104  for processing. Loadlock chamber  112  may be a side-by-side substrate type chamber, a single substrate type chamber, or multi-substrate-type loadlock chamber, for example, as is generally known in the art.  
     [0029] A substrate handler  105  may be centrally positioned in the interior portion of the transfer chamber  104 . Substrate handler  105  is generally configured to receive substrates from the loadlock chamber  112  and transport the substrates received therefrom to one of the processing chambers  106  positioned about the perimeter of the transfer chamber  106 . Additionally, substrate handler  105  is generally configured to transport substrates between the respective processing chambers  106 , as well as from the processing chambers  106  back into the loadlock chamber  112 . The substrate handler  105  generally includes a dual blade configured to support two substrates thereon simultaneously. Additionally, the blade of substrate handler  105  is selectively extendable, while the base is rotatable, which allows the blade to access the interior portion of any of the processing chambers  106 , the loadlock chamber  112 , and/or any other chamber positioned around the perimeter of the transfer chamber  104 . A utility supply unit (not shown), which may be positioned in any location that is generally proximate system  100 , generally houses the support utilities needed for operation of system  100 , such as a gas panel, a power distribution panel, power generators, and other components used to support semiconductor etch processes.  
     [0030]FIG. 2A illustrates a sectional view of an exemplary processing chamber  106  of the invention, which may be a tandem magnetically enhanced etch chamber, for example. Processing chamber  106  generally provides a tandem process chamber configuration, wherein each of the tandem process chambers  200 ,  201  includes an individual processing region  202 ,  203  therein. Each of the respective tandem process chambers  200 ,  201  includes sidewalls  205 , a common interior wall  206 , and a bottom  207 . The interior wall  206  may generally be a shared central wall that separates the upper portion of the respective chambers  200 ,  201  from each other. As such, the processing regions  202 ,  203  defined in the respective chambers  200 ,  201  may not be in line of sight contact, but may share a common pressure, as the lower portion of wall  206  may allow the respective chambers  200 ,  201  to communicate with each other. A substrate support member  208 , which may include a substrate lift pin assembly  212 , may be positioned within each of the respective processing chambers  200 ,  201  via extension into chambers  200 ,  201  through bottom  207 . The substrate support members  208  may be movable in a vertical direction, i.e., in the direction along the axis of the supporting stem member, and may be heated and/or cooled through, for example, fluid conduits formed therein or resistive heaters. Additionally, the substrate support member  208  may be in electrical communication with a power supply configured to supply an electrical bias to the substrate support member  200 . The sidewalls  205  of each of the respective chambers  200 ,  201  additionally include an aperture  209  formed therein, wherein the aperture  209  is configured to communicate substrates into and out of the respective chambers. As such, each of the apertures  209  may generally be in selective communication with, for example, a substrate transfer chamber, such as chamber  104  illustrated in FIG. 1. Therefore, in order to maintain a processing region within each of processing chambers  200 ,  201 , a valve  210 , such as a gate or slit valve, for example, may be positioned between each of the apertures and the connecting chamber (as illustrated in FIG. 3), or alternatively, a single valve may be implemented.  
     [0031] Additionally, each of the respective tandem chambers  200 ,  201  may include an upper and lower portions, wherein the upper portion generally includes the processing regions  202 ,  203 , and wherein the lower portion generally includes a loading region  211 . The loading region  211  may generally be defined as the region positioned below the electromagnets  218  (assuming electromagnets  218  are each a unitary rectangular magnet with a solid center), which will be further discussed herein. In this configuration, the substrate support members  208  may be lowered into the loading region  211  below the lower surface of electromagnets  218 . In this position, a substrate may be positioned on the substrate support member  208  via aperture and gate valve  210 , which are formed into the sidewalls of the chambers below the electromagnets  218 . More particularly, when the substrate support member  208  is lowered, the lift pin assembly  212  may operate to lift a substrate off of the upper surface of the substrate support member  208 . Thereafter, a robot blade may enter into the loading region  211  and engage the substrate lifted by the lift pin assembly  212  for removal therefrom. Similarly, with the substrate support member  208  in a lowered positioned, substrates may be placed thereon for processing. Thereafter, the substrate support member may be vertically moved into a processing position, i.e., a position where the upper surface of the substrate support member  208  is positioned proximate the upper or top portion of the respective chamber.  
     [0032] In another embodiment of the invention, magnets  208  may be rectangular in shape and have a hollow central portion. In this configuration the substrate support member may be configured to have an upper substrate support surface that corresponds with the hollow central portion of the magnets  208 , and similarly, the aperture  209  and valve  210  may be located to correspond with the hollow central portion of the magnet  208 . Thus, in this configuration the substrate support member may not need to be movable in the vertical direction in order to load and unload substrates. In yet another embodiment, the rectangular magnets  208  having a hollow central portion may again be used, however, the aperture may again be positioned below the lower surface of the magnet  208 . As such, the substrate support member  208  may be movable between a processing position (where the upper surface of the substrate support member  208  is generally positioned proximate the middle hollow portion of the magnet  208 ) and a loading position (where the upper surface of the substrate support member  208  is positioned below the lower surface of the lowest portion of the magnet  218 ).  
     [0033] The upper portion of the respective chambers  200 ,  201  generally define the respective isolated processing regions  202 ,  203  for each of the respective chambers. Additionally, the upper portions of the respective chambers provides the devices and/or apparatuses necessary to support plasma generation. For example, processing chamber  106  may generally include a unitary top or lid member  215  that defines the upper boundary of the respective processing regions  202 ,  203 . The lid member  215  may optionally include a gas distribution assembly  216 , such as, for example, a showerhead and blocker plate assembly configured to dispense a processing gas into the respective processing regions  202 ,  203 . The shower head assembly, which may be manufactured from an electrically conductive material, may be in electrical communication with a power source (not shown) configured to supply an electrical bias thereto, as is known in the art. Additionally, the substrate support members  208  may be in electrical communication with a power supply. Therefore, once a plasma is generated in the respective processing regions, the power supply in communication with the substrate support member may be used to control bombardment of the ions in the plasma on the substrate support member. The upper portions of the respective isolated chambers may also include a circumferentially positioned pumping channel  217 , wherein pumping channel  217  is in fluid communication with a common vacuum source (not shown), through, for example, vacuum lines  237 . Therefore, the respective pumping channels  217  are generally configured to maintain the respective chambers  200 ,  201 , and more particularly, the respective processing regions  202 ,  203 , at a pressure desired for semiconductor processing.  
     [0034] As briefly noted above, the upper portions of the respective chambers  200 ,  201  also include a plurality of electromagnets  218 A,  218 B (generally referred to as electromagnets  218 ) positioned around the perimeter of the respective processing regions  202 ,  203 . As illustrated in FIG. 2A, electromagnets  218  may be positioned radially outward of the circumferential pumping channels  217 , and as such, electromagnets  218  may generally surround processing regions  202 ,  203 . Electromagnets  218 , which may be in electrical communication with a system controller  250  configured to control the operation thereof, are generally positioned and configured to generate a quasi-static magnetic field in the respective processing regions  202 ,  203 . The system controller, which may be a micro-processor based controller, for example, may be configured to electronically control both the electrical power applied to each of the respective electromagnets  218 , as well as various other system parameters, such as gas flows, chamber pressures, and other parameters generally controlled in a semiconductor processing system. However, inasmuch as each of the electromagnets  218  are individually and cooperatively controlled by system controller  250 , the cumulative magnetic field generated by the respective electromagnets  218  may be modified and or controlled by the system controller  250 , for example, in accordance with a semiconductor processing recipe. Furthermore, inasmuch as the present invention implements a tandem etch processing chamber configuration, the inwardly positioned electromagnets  218 B may generate interfering magnetic fields. Therefore, in order to prevent interfering fields from entering into the adjacent processing chamber, a field insulating shield  219 , i.e., a shield manufactured from a material configured to prevent the transmission of magnetic fields therethrough, may be positioned between the respective chambers, and more particularly, may be positioned between the respective adjacent electromagnets  218 B. Shield member  219  may, for example, be manufactured from a number of dense metals known to shield magnetic fields, such as, for example, steel, aluminum, and/or iron. Additionally, shield member  219  may be manufactured from various alloys, rubbers, and plastics, which may also have metal dispersed therethrough to assist in the magnetic shielding properties. Regardless of the actual composition, shield member  219 , which may be of varying thicknesses, is generally manufactured from one or more materials known in the art to shield magnetic fields. As such, a magnetic field generated by the respective electromagnets  218 B will be directed towards the interior of the respective processing chambers  200 ,  201 , while the magnetic field emanating in the opposite direction from the adjacent electromagnet  218 B may be absorbed and/or canceled by the magnetic insulating shield  219 .  
     [0035]FIG. 3 illustrates a plan view of an exemplary tandem processing chamber  106  of the invention. An example of the positioning of the respective electromagnets  218  around the respective chambers  200 ,  201  is illustrated in FIG. 3. Additionally, the interstitially positioned magnetic shield member  219  is also illustrated. However, it is to be noted that embodiments of the present invention are in no way limited to the configuration of electromagnets  218  illustrated in FIG. 3. For example, it is contemplated that each of electromagnets  218  may be radial or arc shaped electromagnets configured to mirror a portion of the perimeter of the respective processing regions  202 ,  203 , as illustrated in the exemplary configuration of FIG. 4. In this configuration, a plurality of the arc shaped electromagnets  218  may be positioned around the perimeter of the respective chambers to form a generally circularly shaped electromagnet configured to generate a magnetic field within each of the respective processing regions surrounded by the arc shaped electromagnet. Additionally, although the embodiments of the invention illustrated in FIGS. 2, 3, and  4  utilize four electromagnets surrounding each of the respective chambers, the invention is in no way limited to using any particular number of electromagnets. For example, the adjacent magnets  218 B positioned between the respective chambers  200 ,  201  in FIG. 3 may be replaced by a unitary magnet  218 B configured to generate a magnetic field on one side for the first chamber  200  and on another opposite side for a second chamber  201 , as illustrated in FIGS. 2B and 3A. In this embodiment, a unitary electromagnet  218 B is positioned between the respective chambers and is configured to supply a magnetic field to both chambers  200 ,  201  simultaneously. Additionally, as will be discussed herein, the magnetic field output of unitary electromagnet  218 B may be controlled by a system controller so that a plasma generated in the respective chambers  200 ,  201  may be stirred through cooperative control of the magnetic field output of the respective electromagnets  218 A,  218 B. Additionally, when the unitary electromagnet  218 B is utilized, the shield member  219  may be removed from the central portion of the chamber where the electromagnet  218 B is positioned. However, the shield member may still be positioned outward of the central electromagnet  218 B so that fields from the other electromagnets  218 A may be prevented from crossing over into the adjacent chamber. Further, although the electromagnets are illustrated in a square-type configuration using four magnets per chamber, embodiments of the invention contemplate utilizing any number of magnets to surround the respective chambers. For example, linear or straight magnets may be utilized in an octagon type configuration, wherein eight magnets are positioned around the perimeter of a chamber. Alternatively, the arc shaped magnets noted above may be utilized to surround a chamber, wherein any number of magnets from about 2 to about 24 or more magnets may be used, as illustrated in FIG. 4. Regardless of the shape or configuration of the electromagnets utilized, embodiments of the invention contemplate that any number of electromagnets may be used to surround a processing chamber, and further, that the electromagnets may be configured in various shapes and configurations that may surround a chamber.  
     [0036] Although the combination of the biased substrate support members  208  and the electrically biased showerhead assemblies  216  may operate to generate a plasma within the respective processing regions  202 ,  203 , embodiments of the invention provide additional assemblies for communicating a plasma into the respective processing regions  202 ,  203 . More particularly, as illustrated in FIG. 2A, each of the respective chambers  200 ,  201  may include an optional torroid assembly  220  configured to generate a plasma in the respective processing regions. Each of the torroid assemblies  220  includes one or more hollow torroid conduits  221  that are in fluid communication with a processing region on opposing sides thereof. As illustrated in FIG. 2A, the torroid conduit  221  connects to a first side of a processing region  200  via a first aperture  222 . The torroid conduit  221  then extends over the top portion  215  of the processing chamber  200  and returns to fluid communication with the processing region  202  on the opposite side thereof via a second aperture  222 . The torroid conduit  221  may generally be manufactured from an electrically conductive material, and therefore, in order to reduce eddy currents generated therein during plasma generation, an insulating member  225  may be positioned inline with the torroidal conduit  221 . The insulating member  225  may generally operate to separate the conduit  221  into two separate electrically isolated sections and prevent electrical current from flowing therethrough. Inasmuch as a torroid conduit  221  is generally configured to generate a plasma and communicate the plasma to a processing region, each torroid conduit  221  may also include a gas supply conduit  223  and at least one electrically biased coil  224  positioned proximate thereto. However, it is understood that the gas supply conduit  223  may not be necessary for proper plasma generation, as the gas supplied to the respective processing regions  200 ,  201  may be communicated into the respective torroids for plasma generation, which eliminates the need for the additional gas supply  223 . Each coil  224  may be wound around a corresponding conduit  221  so that a field generated therefrom may generally intersect and pass through the hollow interior portion of the corresponding conduit  221 . Each of the individual coils  224  may be in electrical communication with a power supply  226 , which may be, for example, an RF power supply configured to drive the respective coils  224 . As such, the combination of the application of electrical power to the respective coils  224  and the process gas in the torroids causes a plasma to be generated within the torroid conduit  221 .  
     [0037] Additionally, although the apertures  222  of torroid conduit  221  are illustrated as entering into the respective processing regions  202 ,  203  via the top or lid portion thereof (see FIG. 2A), the present invention also contemplates that the torroid conduit apertures may enter into the processing regions from the sidewall  205  of the chamber. As illustrated in FIG. 5, the respective electromagnets  218  may be spaced apart slightly at their distil ends, thus forming a region where the aperture  222  of the torroid conduit  221  may communicate with processing regions  202 ,  203 . As a result of this configuration, the plasma generated within the respective torroid conduits  221 , which may number two or more, for example, is communicated to the respective processing region and distributed over the surface of the respective substrate positioned therein for processing. Furthermore, although each of the respective processing chambers are illustrated as including two of the individual torroidal conduits  221 , embodiments of the invention are not limited to any specific number of torroidal conduits  221 . However, if two torroidal conduits are used, generally, the conduits will extend above each of the respective processing chambers and intersect or cross over each other at a generally right angle. Although not required, this configuration generally provides for an even distribution of the plasma generated within the torroidal conduit  221  into the respective processing regions, as placement of the torroidal conduits  221  at right angles to each other provides for an aperture in the respective processing chamber at 90 degree increments, and therefore, provides for a generally uniform plasma to be distributed within the respective processing region. However, embodiments of the invention contemplate that three or more torroidal conduits may be utilized, and as such, the corresponding number of plasma apertures may be positioned radially around the respective processing regions in equal radial spacing.  
     [0038] Furthermore, although embodiments of the invention illustrated in FIG. 2A and FIG. 6 show both a showerhead assembly and a torroidal plasma generation assembly, embodiments of the invention contemplate that either one or both of the respective plasma generation assemblies may be implemented in the tandem etch chambers of the invention. More particularly, embodiments of the invention generally contemplate that the showerhead assembly may be omitted, while the torroidal plasma conduits may be implemented in order to generate a plasma in the respective processing regions.  
     [0039] In another embodiment of the invention, the tandem processing chambers illustrated in FIG. 2A or FIG. 6 may be implemented without the torroidal plasma conduits, as illustrated in FIG. 8. As such, the tandem processing chamber implemented without the torroidal plasma conduits may generally operate as a tandem MERIE chamber. In this configuration a plasma may be capacitatively generated through introduction of a processing gas via the showerhead and the application of an electrical bias between the showerhead and the substrate support member. The plasma may be stirred and/or controlled via the selective actuation of a plurality of electromagnets positioned around the respective processing regions. The shield member positioned between the adjacent tandem processing regions may operate to prevent cross over of magnetic fields intended for one processing region into the adjacent processing region. Therefore, in this tandem MERIE configuration, two substrates may be simultaneously processed in the tandem processing regions, thereby doubling the throughput provided by conventional MERIE chambers, while not sacrificing the control and uniformity provided by single MERIE chambers. Alternatively, however, as noted above, the chamber may also be configured to implement the torroidal plasma conduits and not the showerhead assembly.  
     [0040]FIG. 6 illustrates an alternative configuration of the processing system of the invention. More particularly, FIG. 6 illustrates an embodiment of the invention wherein the torroidal conduits  221  are configured to enter into the respective processing regions  202 ,  203  via the sidewall  205 . Further still, the embodiment of the invention illustrated in FIG. 6 utilizes a central pumping aperture  230  centrally located within the bottom portion of the respective chambers. The central pumping aperture  230 , which may be in fluid communication with a vacuum pump  235 , generally operates to communicate a negative pressure to the respective chambers  200 ,  201 . As such, inasmuch as the respective chambers are in fluid communication with each other as a result of the central wall not extending completely to the bottom portion of the respective chambers, a single pump in fluid communication with the respective chambers via aperture  230  may be utilized to maintain both of the respective chambers at a desired common processing pressure. As a result of this configuration, processing conditions in both chamber  200  and chamber  201  may be identical, and therefore, variations between substrate processes within the respective chambers may be minimized. It is to be noted, however, that both the sidewall entrance configuration for the torroidal conduits  221 , as well as the central pumping configuration, may be implemented individually or in combination into each of the embodiments of the invention. Alternatively, the chambers  200 ,  201  may be separated/isolated from each other, i.e., aperture  230  may be eliminated, and therefore, the pressure in the respective chambers  200 ,  201  may be individually controlled. Aside from the above noted distinctions, the exemplary tandem processing chamber illustrated in FIG. 6 is similar to the tandem processing chamber illustrated in FIG. 2A, and therefore, the structural description of the chamber illustrated in FIG. 2A may be generally applied to FIG. 6 for the common elements. As such, the chamber illustrated in FIG. 6 again includes a system controller  650  configured to control the electromagnets, plasma generation in the torroidal conduits, gas flows into the chambers and conduits, pressures in the respective chambers, electrical biases applied to generate plasmas, and other parameters generally associated with a semiconductor processing system.  
     [0041]FIG. 9 illustrates a tandem etch processing chamber  900  having cantilever-type substrate support members  908  positioned therein. An exemplary cantilever mounted substrate support member that may be used in the present invention may be found in U.S. Pat. No. 6,001,267 entitled Plasma Enhanced Chemical Method, which is hereby incorporated by reference. Chamber  900 , which is structurally similar to the tandem chamber illustrated in FIG. 6 (and therefore, the structural description of FIG. 6 may be applied to the description of FIG. 9 where applicable), generally replaces the centrally mounted stem-type substrate support members  208  with the cantilevered substrate support members  908 . With the exception of the replacement of the substrate support members, the chamber configuration and features may be similar to the exemplary chamber illustrated in FIGS.  2  or  6 . The cantilevered substrate support members  908  utilized in the present exemplary embodiment generally attach to the sidewall  905  of the respective chambers via one or more support arms extending radially outward from the substrate support member to a mounting plate on the outer wall  905 . Inasmuch as the cantilevered substrate support members  908  do not utilize a bottom mounted stem portion to support the substrate platen, the bottom portion of the respective chambers is generally open. As such, the cantilevered substrate support members  908  allow for a central pumping configuration, which may, for example, include a shared central pumping aperture  930  in communication with a vacuum pump  935 . The use of the cantilevered substrate support member, and in particular, the elimination of the stem portion of the conventional substrate support members, may provide for improved gas flow around the substrate support members  908 . Additionally, the cantilevered substrate support members  908  allow for the individual processing chambers to both have central pumping apertures formed therein, i.e., each chamber may have a central pumping aperture formed therein immediately below each of the respective cantilevered substrate support members  908 . In this configuration, each of the pumping apertures formed directly below the cantilevered substrate support members  908  may be in fluid communication with a common vacuum pump.  
     [0042]FIG. 10 illustrates an exemplary embodiment of a tandem etch processing chamber  1000  having rotatable magnet assemblies  1001  positioned above the lid of the chamber  1000 . The rotatable magnet assembly is generally configured to generate a magnetic field in the respective processing regions of the tandem chambers positioned below. Generally, chamber  1000  is similar in construction to the exemplary tandem etch chamber illustrated in FIG. 2A, and therefore, the structural description of the chamber illustrated in FIG. 2A may generally be applicable. However, the electromagnets  218  illustrated in FIG. 2A are removed from the perimeter of the respective processing regions and replaced by the rotatable magnet assemblies  1001  of the present exemplary embodiment. Additionally, inasmuch as the rotatable magnet assemblies  1001  of the present exemplary embodiment are positioned above the respective chambers/processing regions  902  and not beside them, as with the electromagnets  218  illustrated in FIG. 2A, the shield member  919  may generally extend above the top portion of the respective chambers/processing regions  902  so that the magnetic fields generated by the respective rotatable magnet assemblies  1001  do not interfere with the magnetic fields in the adjacent processing region. Shield member  919 , therefore, may be configured to absorb, cancel, or reflect the magnetic field lines passing therethrough so that the field lines do not interfere with adjacent chambers. In similar fashion to previous embodiments, the rotatable magnet assemblies  1001  are generally configured to generate rotating or movable magnetic fields in the processing regions of the chambers positioned below the rotating magnets. The rotating or movable magnetic fields may generally operate to stir and/or control a plasma generated in the processing region therebelow. The embodiment of FIG. 10 may also include a torroidal plasma source in communication with each of the respective processing regions.  
     [0043] In operation, embodiments of the invention generally provide a processing system configured to conduct etch processes on at least two semiconductor substrates simultaneously. More particularly, using the exemplary embodiment of the invention illustrated in FIG. 1 as an example, substrates to be processed may be placed into substrate processing system  100  via cassettes  109 . Then substrates, generally two, may be transported into loadlock chamber  112  via robot  113 , and loadlock chamber  112  may be sealed from the chamber containing cassettes  109 , through, for example, a selectively actuated gate valve positioned between the respective chambers. Thereafter, the loadlock chamber  112  may be brought to a predetermined pressure and opened up to the substrate transfer chamber  104 . Once the two chambers are in communication with each other, the two substrates in the loadlock chamber  112  may be simultaneously transported into the substrate transfer chamber  104  via substrate transfer robot  105 , which generally includes a robot blade configured to simultaneously support two substrates. The two substrates are generally supported in a side-by-side configuration in the same horizontal plane by the robot blade. A pair of the gate valves  210  positioned between the transfer chamber  104  and the processing chamber  106  may be opened and the two substrates may be inserted into a processing chamber  106 , wherein an etch process may be conducted thereon.  
     [0044] Once the robot blade is inserted into the processing chamber  106 , the substrates may be simultaneously placed into the respective tandem chambers  200 ,  201 . The receiving process for the respective tandem chambers  200 ,  201  generally includes, for example, lowering of the respective substrate support members  208  into a loading position, i.e., a position where the substrate support members  208  engage a lift pin assembly  212 , and are generally positioned below a plane through which the robot blade may enter into the respective chambers via gate valve  210  and entrance aperture  209 . Thus, the robot blade may deposit the substrates into the respective chambers  200 ,  201  by lowering the substrates onto the lift pin assemblies  208 . Once the substrates are positioned on the lift pin assemblies  212 , the robot blade may be retracted from the respective chambers  200 ,  201  and the gate valves  210  may be closed to seal the chambers  200 ,  201  from the transfer chamber  104 .  
     [0045] Once the loading process is complete, the respective substrate support members  208  may be moved from a loading position to a substrate processing position. The transition from the loading position to the substrate processing position generally includes raising the substrate support member vertically within the respective chambers  200 ,  201 , such that the distance from the upper surface of the substrate support member  208  to the lower surface of the showerhead assembly  216  is minimized. This movement of the substrate support member  208  also operates to define the respective processing regions  202 ,  203  within chambers  200 ,  201 , as the upper surface of the substrate support member  208  defines the lower portion of the respective regions  202 ,  203 . Additionally, the vertical movement of the respective substrate support members  208  generally causes the lift pin assemblies  212  to lower the substrates onto the upper surfaces of the respective substrate support members  208  as the substrate support members  208  disengage with the portion of lift pin assembly  212  positioned in the lower portion of the respective chambers. Additionally, the process of raising the substrate support members  208  to the upper position, generally referred to as a processing position, also operates to position the upper surface of the substrate support member on approximately the same plane as the electromagnets  218  positioned around the respective processing regions  202 ,  203 . As such, the magnetic fields generated by the respective electromagnets  218  will generally be concentrated in the processing regions  202 ,  203  immediately above the substrate support members  208 . Further, the process of bringing the respective substrate support members  208  into the processing position may further include bringing the respective chambers to a processing pressure, which generally includes evacuating ambient gases from the respective chambers via the aforementioned vacuum pump.  
     [0046] Once the respective substrates are loaded, moved into the processing position, and the pressure in the respective chambers  200 ,  201  is brought to a desired processing pressure, a plasma may be generated within both of the respective processing regions  202 ,  203 . More particularly, a plasma may be generated via application of a bias between substrate support member  208  and the showerhead assembly  216 , which then generates a plasma from a process gas introduced into the respective processing region, or a plasma may be generated within the torroidal conduits  221  and communicated to the respective processing regions  202 ,  203  via apertures  222  at the terminating ends of torroidal conduits  221 . Additionally, if desired, both the showerhead and torroidal conduits may be cooperatively utilized to generate a plasma in the respective processing regions.  
     [0047] In order to generate a plasma within the respective torroidal conduits  221 , a process gas must first be present therein. Therefore, process gases from the respective processing regions  202 ,  203  may be communicated into the respective torroidal conduits  221 , or alternatively, process gases may be delivered directly into the respective torroidal conduits via a gas supply  223 . Once the process gas is present within the respective torroidal conduits  221 , a field may be applied thereto in order to ionize the process gas within the torroidal conduits  221  into a plasma. The field required to ionize the process gases may be generated by coils  224 , which are in electrical communication with power supply  226 , which may be an RF power supply, for example. The plasma generated within the torroidal conduits  221  generally circulates through the torroidal path that extends through the respective processing regions  202 ,  203  via apertures  222 , and therefore forms a continuous plasma path and extends over the surface of the substrate.  
     [0048] Once the plasma is generated in the respective processing regions, the density of a plasma may be manipulated and/or controlled by the selective activation of the individual electromagnets  218 . More particularly, when each of the individual electromagnets  218  are activated, the magnetic field generated by the respective electromagnet  218  intersects the processing region proximate thereto, as each of electromagnets  218  are positioned proximate the perimeter of a processing region. Therefore, each of the electromagnets  218  may be used to vary the magnetic field intensity exerted on a particular portion of the processing region positioned proximate thereto, which operates to confine or control the plasma generated or communicated to that particular portion of the processing region. For example, if a particular electromagnet  218  is supplied with an increased electrical power, the magnetic field generated therefrom, which intersects the processing region proximate thereto, will proportionally increase, and therefore, the magnetic field density in the processing region proximate the respective electromagnet  218  will correspondingly increase. As such, through cooperative control of the individual electromagnets  218 , the present invention provides for control over the magnetic field in intensity through the entire processing region, which inherently provides for control over the plasma density over the entire processing region.  
     [0049]FIG. 7A illustrates a schematic representation of exemplary tandem processing chambers  200 ,  201  of the invention during processing, and more particularly, during the time period when system controller  250  is operating to generate and control a quasi-static, multi-directional magnetic field in each of the respective processing regions  202 ,  203 . Referring primarily to FIGS.  2 - 5  and  7 , opposing coil pairs  218  (coils positioned on opposite sides of the respective processing regions  202 ,  203 ) cooperatively operate to form mutually perpendicular magnetic field vectors B y  and B x , respectively, which are generally parallel to the substrate support member and the surface of the substrate positioned thereon. In order to generate and control the mutually perpendicular magnetic field vectors, the magnitude and direction of the current supplied to each of the individual electromagnets may be controlled by system controller  250 . The perpendicular field vectors B y  and B x  generated by the coil pairs may be defined by the following equations:  
       B   x   =B ·cos(θ)  
       B   y   =B ·sin(θ)  
     [0050] Therefore, given the desired or required values of the magnetic field B, (which is the resultant vector illustrated in FIG. 7), along with its angular orientation, (which is angle θ in FIG. 7), system controller  250  may independently solve the above noted equations to obtain associated magnetic field vectors B y  and B x , which provide the desired strength of field and orientation. Thereafter, system controller  250  may selectively regulate the application of electric currents to the individual electromagnets, and in particular the electromagnet pairs, to provide the desired magnetic field in the respective processing chambers  200 ,  201 . Additionally, the angular orientation and magnitude of the generated magnetic fields may be independently altered as quickly or as slowly as desired by changing the current supplied to the electromagnets. The time that the field is on at each angular position and the direction of angular stepping may be varied, as well as the field intensity, since these parameters are solely a function of changing the currents to the electromagnets and are readily controlled by the system controller  250 .  
     [0051] Therefore, as a result of the field control features provided by system controller  250 , the magnetic field in each of the processing regions  202 ,  203  may be moved or stirred around the respective processing region using selected orientation and time increments, as illustrated by arrows A and B in FIG. 7. If desired, the magnitude of the resultant field By may be changed as the process or reactor configuration requires, or a constant field strength may be used. In short, the electrical current-controlled system provides the versatility of a fast or slow moving, constant or varying strength magnetic field of constant or varied angular velocity. In addition, the orientation of the field need not be stepped or changed sequentially, but can be instantaneously switched from any given orientation (or field strength) to another. This versatility in independently controlling the direction and magnitude of the magnetic field is distinct from existing commercially useful rotating magnetic fields, which typically rotate at a fixed relatively high frequency such as the standard rate of 60 Hertz. In addition, the ability to “rotate” slowly, at a rate, for example, as low as 2 to 5 sec./revolution (12 to 30 cycles/min.) or slower avoids problems, such as the eddy current losses associated with the use of higher frequencies in metal chambers. Furthermore, embodiments of the invention contemplate that either DC or pulsed-type, RF for example, power supplies may be used in conjunction with the controller and electromagnets of the invention. In embodiments where opposing coil pairs are used, for example, the magnetic field may be rotated in 90-degree increments by successively and periodically connecting a DC power supply to a first coil pair with positive polarity, then to a second coil pair with positive polarity, then to the first coil pair with negative polarity, and then to the second coil pair with negative polarity. Alternatively, for example, the magnetic field may be continuously rotated via the use of low frequency (in the range of 0.1 to 10 Hz, for example) power supply having quadrature outputs connected to provide current to the first coil pair offset in phase by 90 degrees from the current provided to the second coil pair  32 .  
     [0052]FIG. 7B illustrates exemplary magnetic field lines for an embodiment of the invention wherein four electromagnets  218 A and  218 B are positioned orthogonally around each of the processing regions and a shield member  219  is positioned between the electromagnets that share the common central wall. Shield member  219  and the common central wall are shown as a unitary member in FIG. 7B, however, the invention is not limited to this configuration, as the shield and wall members may be separate or unitary. In order to improve the spatial uniformity over the surface of a substrate being processed, adjacent sets of electromagnets orthogonally positioned may be configured to augment the strength of the magnetic field near the perimeter of the substrate closest to the intersection of the adjacent electromagnets (designated point Q in FIG. 7B) to reduce the rate at which the magnetic field strength declines from a point on the opposing side of the substrate (designated point P) to point Q. In this configuration, the total magnetic flux produced by one electromagnet pair (in this embodiment an electromagnet pair is defined as two electromagnets positioned adjacent each other, i.e., two electromagnets that both terminate at one end at the same corner) may be set to be sufficiently less than the total magnetic flux produced by the adjacent electromagnet pair so that the combined magnetic field from the two electromagnet pairs declines in strength from point P to point Q across the surface of the substrate. In other words, the use of adjacently positioned opposing electromagnet pairs may operate to reduce the rate of decline, but does not eliminate or reverse the decline, from point P to point Q in the magnetic field strength. The ratio R (where R&gt;1) of the total magnetic flux produced by one electromagnet pair to the total magnetic flux produced by the other electromagnet pair may be adjusted to maximize the spatial uniformity of the ion flux over the surface of the substrate being processed. Shield member  219  operates to magnetically isolate the respective tandem chambers from each other, and therefore, the magnetic field generated by electromagnets for one processing region does not cross over into the adjacent processing region and interfere with the controllability of the field strength in that particular processing region.  
     [0053]FIG. 7C illustrates a plan view of the magnetic field lines generated by another embodiment of the invention. In this embodiment, the respective tandem processing chambers  200  and  201  share a common central electromagnet  218 B. As such, electromagnet  218 B is configured to generate a magnetic field that may be simultaneously used for processing in both tandem chamber  200  and tandem chamber  201 . Therefore, the system controller in electrical communication with the respective electromagnets will generally be configured to adjust the magnitude and direction of the field generated by the central electromagnet  218 B cooperatively with the remaining electromagnets positioned around the remaining three sides of the respective processing regions. For example, as illustrated in FIG. 7C, the magnetic field generated by the central electromagnet  218 B and the lowermost electromagnets  218 A is in a clockwise direction, and therefore, assuming that a contributory magnetic field effect is desired, the magnetic field cooperatively generated by electromagnets  218 C and  218 D may be in a counter-clockwise direction. This configuration may generate a uniformly dense plasma area in a particular area of each of the processing regions, and in the exemplary embodiment, the dense plasma area would be in the area denoted by an “X” in FIG. 7C, as the area proximate the “X” is where the respective field lines converge. Similarly, the field direction of electromagnet  218 B may be switched to a counterclockwise direction, and therefore, the associated magnetic fields generated by the remaining electromagnets may also be switched in direction to maintain the contributory field effect.  
     [0054] Furthermore, given that the system controller in each of the above noted embodiments may selectively control the electrical current supplied to each of the individual electromagnets, the region of dense plasma generated by the electromagnets may be selectively moved or stirred within the respective processing regions. In short, the magnetic field control features of the present invention provides the versatility of a fast or slow moving, constant or varying strength magnetic field of constant or varied angular velocity within each of the respective tandem processing regions. In addition, the orientation of the field need not be stepped or changed sequentially, as it may be instantaneously switched from any given orientation (or field strength) to another, i.e., the plasma confining magnetic field may be switched from one quadrant in the processing region to another quadrant in the processing region, where the respective quadrants are not adjacent each other. Further still, inasmuch as embodiments of the present invention are not limited to any particular number of electromagnets or processing region/chamber shapes, the number of sectors or regions may be varied in accordance with the number of magnets and processing region chamber shape. Further, generally, a sector defined within the processing region may correspond to an area where the magnetic field generated therein is primarily controlled by a single one of a plurality of electromagnets  218  or pairs of electromagnets operating cooperatively.  
     [0055] FIGS.  11 A- 11 D illustrate an exemplary plasma stirring process that may be implemented by embodiments of the invention through selective control of electromagnets positioned around processing regions. In the exemplary embodiment, a tandem magnetically enhanced etch chamber using a unitary central electro magnet (designated electromagnet  1 ) and six surrounding electromagnets (designated electromagnets  2 - 7 ) is used to simultaneously stir a plasma in tandem processing regions. In FIG. 11A, which may be a first step of a plasma stirring process, the magnetic field is configured to generate a dense plasma region in the left side tandem processing region near the upper left hand corner of the region, i.e., proximate the corners of magnets  2  and  6 , while simultaneously generating a dense plasma in the right side tandem region near the lower left corner of the processing region, i.e., proximate the corners of magnets  1  and  5 . In this configuration the magnetic field between magnet  1  and magnet  2  is set up to be in a clockwise direction, while the magnetic field between magnets  4  and  6  is set up to be in a counterclockwise direction, as shown by the arrows in FIG. 11A. Similarly, the magnetic field between magnets  3  and  1  is set up to be in a clockwise direction, while the magnetic field between magnets  7  and  5  is set up to be in a counterclockwise direction.  
     [0056]FIG. 11B illustrates an exemplary second step of a magnetic field stirring process, wherein a dense plasma region is generated in an upper right hand corner of the left hand side tandem processing region, while a dense plasma region is generated in a lower right hand corner of the right side tandem processing region. In this configuration, for the left side tandem processing chamber, the magnetic field between magnet  6  and magnet  2  is generally in a counter clockwise rotation, while the magnetic field between magnet  1  and magnet  4  is in a clockwise rotation. Similarly, for the right hand side tandem chamber, the magnetic field between electromagnets  3  and  7  is in a counterclockwise direction, while the magnetic field between electromagnets  1  and  3  is in a clockwise direction.  
     [0057]FIG. 11C illustrates an exemplary third step of a magnetic field stirring process, wherein a dense plasma region is generated in the lower right hand corner of the left side tandem processing region, while a dense region is generated in an upper right hand corner of the right side tandem processing region. In this configuration, for the left side tandem processing region, the magnetic field between electromagnet  2  and electromagnet  1  is in a counterclockwise direction, while the magnetic field between electromagnet  6  and electromagnet  4  is in a clockwise direction. For the right side tandem processing region, the magnetic field between electromagnet  1  and electromagnet  3  is generally in a counterclockwise direction, while the magnetic field between electromagnet  5  and electromagnet  7  is generally in a clockwise direction.  
     [0058]FIG. 11D illustrates an exemplary fourth step of a magnetic field stirring process, wherein a dense plasma region is generated in a lower left hand corner of the left side tandem processing region, while a dense plasma region is generated in an upper left hand corner of a right side tandem processing region. In this configuration, for the left side tandem processing region, the magnetic field between electromagnet  2  and electromagnet  6  is in a clockwise direction, while the magnetic field between electromagnet  1  and electromagnet  4  is in a counterclockwise direction. For the right side tandem processing region, the magnetic field between electromagnet  7  and electromagnet  3  is in a clockwise direction, while the magnetic field between electromagnet  5  and electromagnet  1  is in a counterclockwise direction. Therefore, through the sequential application of the magnetic fields illustrated in FIGS.  11 A- 11 D, a dense plasma region in each of the respective processing regions may be simultaneously circulated through each of the respective processing regions. Additionally, although a circular circulation has been illustrated in the exemplary embodiment, the invention is not limited to this configuration. Rather, embodiments of the invention contemplate that various plasma circulation patterns may be implemented, including, for example, criss-cross patterns, z-shaped patterns, and box patterns.  
     [0059] Once the individual processing recipe step is completed, plasma generation may be terminated and the individual substrates may be removed from the respective processing chambers  200 ,  201 . The unloading process generally includes lowering of the substrate support member  208  from the processing position to the substrate loading/unloading position. Once the substrate support member is in the loading/unloading position, valves  210  may be opened in order to allow a robot blade to access the respective processing chamber and remove the processed substrates therefrom. Once the substrates are removed, they may be transferred to another set of processing chambers so that another processing recipe step may be conducted thereon. Similarly, two additional substrates may be brought into the processing chambers where the two substrates were just removed therefrom so that a processing step may be conducted thereon. As such, the exemplary configurations of the present invention, which are generally illustrated in FIGS. 1, 2, and  6  allows for the simultaneous processing of two substrates in the tandem processing chambers.  
     [0060] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.