Patent Publication Number: US-2004050327-A1

Title: Vertically translatable chuck assembly and method for a plasma reactor system

Description:
[0001] This is a Continuation application of International Application No. PCT/US01/48851, filed on Dec. 21, 2001, which, in turn, claims the benefit of Provisional U.S. Patent Application No. 60/262,642, filed Jan. 22, 2001, the contents of both of which are incorporated herein in their entirety. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to plasma reactor systems, and in particular relates to chucks for plasma reactor systems.  
       [0004] 2. Background Of The Invention  
       [0005] Ionized gas or “plasma” may be used during processing and fabrication of semiconductor devices, flat panel displays and other products requiring etching or deposition of materials. Plasma may be used to etch or remove material from semiconductor integrated circuit wafers, or sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in manufacturing or fabrication processes typically is done by introducing a low-pressure process gas into a chamber surrounding a workpiece such as an integrated circuit (IC) wafer that resides on a workpiece support member, more commonly referred to as a “chuck.” The molecules of the low-pressure gas in the chamber are ionized into a plasma by a radio frequency energy (power) source after the gas molecules enter the chamber. The plasma then flows over and interacts with the workpiece, which is typically biased by providing RF power to the chuck supporting the workpiece. In this regard, the chuck serves as an electrode, and is thus sometimes referred to as a “chuck electrode.” The plasma gas flowing over the chuck is removed by a vacuum system connected to the chamber.  
       [0006] One of the key factors that determine the yield and overall quality of an IC is the uniformity of the plasma process at the surface of the workpiece. In plasma reactors, the process uniformity is governed by the design of the overall system, and in particular by the physical relationship of the chuck, plasma generation source, radio frequency (RF) tuning electronics, and vacuum pumping system.  
       [0007] The plasma chamber is maintained at the low pressure required for plasma formation through the operation of the aforementioned vacuum pump (e.g., a turbo-mechanical pump) that is in pneumatic communication with the chamber. Conventional plasma reactor system designs have the connection to the vacuum pump provided on the side of the reactor chamber.  
       [0008] U.S. Pat. No. 5,948,704 (“the &#39;704 patent”) discloses a plasma chamber with a chuck connected by a support arm to the side of the chamber in a cantilever fashion. A gas distribution plate issues gas into the chamber and a vacuum pump removes gas along an axis perpendicular to the chuck.  
       [0009] In certain plasma reactor systems, it is beneficial to provide an axially (i.e., vertically) translatable chuck. This allows for the workpiece to be exposed to different regions of the plasma gas, which results in the workpiece being processed differently.  
       [0010] To obtain fast plasma processing of a workpiece, it is necessary to deliver a large amount of RF power to the chuck. To do so, a low-impedance RF circuit is required to efficiently couple the RF power to the chuck. The RF circuit includes an impedance match network and an RF transmission line or stub, which connects the output of the match network to the chuck electrode. The impedance match network maximizes the power transferred to the plasma by matching the output impedance of the match network to that of the load, i.e., the match network impedance becomes the complex conjugate of the load impedance. In turn, the load impedance includes the impedance of the transmission line residing between the output of the match network and the chuck electrode, the chuck electrode itself, and the plasma formed adjacent the chuck.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011] The present invention relates to plasma reactor systems, and in particular relates to chucks for high-flow, high-power plasma reactor systems.  
       [0012] A first aspect of the invention is a chuck assembly capable of vertical translation for supporting a workpiece at different positions within a plasma reactor chamber having an interior region capable of supporting a plasma. The assembly includes a chuck base and at least one support arm extending outwardly from the chuck base perimeter to the chamber sidewalls. The support arm is adapted to support the chuck base within the interior region, while also adapted to provide a path for mechanical, electrical, pneumatic and/or fluid communication with the chuck assembly from outside the chamber. The chuck assembly further includes a workpiece support member, arranged above the chuck base, capable of supporting the workpiece. The workpiece support member is connected to an RF power supply and serves as a chuck electrode. The workpiece support member is supported above the chuck base by one or more vertical translation members arranged between and operatively connecting the chuck base and the workpiece support member. The chuck assembly includes a match network wherein at least a portion of the match network is mounted directly to the workpiece support member, thereby providing a low-impedance path from the RF power supply to the workpiece support member. The match network is designed so that most, if not all, of the match network elements (e.g., variable capacitors and one or more inductors) fit within the chuck assembly itself. Those elements not within the chuck assembly are housed outside of the assembly, and communicate with the other elements via an electrical connection through the support arm. The use of the support arm also allows for the positioning of a vacuum pump system directly beneath the chuck assembly.  
       [0013] A second aspect of the invention is a plasma reactor system for processing a workpiece. The system comprises a plasma reactor chamber having a central axis and sidewalls surrounding an interior region capable of supporting a plasma in an upper part of the interior region. The system includes the chuck assembly described briefly above arranged adjacent the upper part of the interior region and along the central axis. Also included in the system is a vacuum pump system arranged adjacent the chuck assembly opposite the upper part and along the central axis.  
       [0014] A third aspect of the invention is a method of providing for uniform, substantially axially symmetric flow of plasma gas over a workpiece in a plasma reactor chamber having a central axis and capable of containing a plasma in an upper interior region of the chamber. The method includes the step of supporting a chuck assembly within the reactor chamber with at least one support arm. The support arm is arranged such that gas can flow around the chuck assembly from the upper interior region. The next step includes arranging a vacuum pump system along the central axis adjacent the chuck assembly opposite the upper interior region. The next step involves providing the workpiece to the chuck assembly such that the workpiece is supported adjacent the upper interior region. The next step involves flowing a plasma-forming gas into the upper interior region and forming a plasma in the upper interior region. The next step is then activating the vacuum pump system so as to draw gas from the upper interior region over the workpiece and into the vacuum pump system residing directly beneath the chuck assembly. 
     
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING  
     [0015]FIG. 1A is a schematic cross-sectional diagram of the plasma reactor system according to one embodiment of the present invention illustrating its various components and showing one of the three support arms that support the chuck assembly within the reactor chamber;  
     [0016]FIG. 1B is a schematic cross-sectional diagram of the plasma reactor system according to another embodiment of the present invention illustrating its various components and showing one of the three support arms that support the chuck assembly within the reactor chamber;  
     [0017]FIG. 1C is a schematic cross-sectional diagram of the plasma reactor system according to another embodiment of the present invention illustrating its various components and showing one of the three support arms that support the chuck assembly within the reactor chamber;  
     [0018]FIG. 2 is a plan view of the chuck assembly of FIGS.  1 A-C with just the chuck base and support arms present, and also showing the systems connected to the chuck base via the support arms, along with the workpiece load chamber;  
     [0019]FIG. 3 is a schematic cross-sectional view of a portion of the plasma reactor system of FIG. 1 showing the RF power supply and cooling support arm and the elements making up a first embodiment of a chuck RF supply system, and the cooling system;  
     [0020]FIG. 4 is a schematic circuit diagram of the L-type match network of FIG. 3;  
     [0021]FIG. 5 is a schematic cross-sectional view of a portion of the plasma reactor system similar to that of FIG. 3, but showing a second embodiment of a chuck RF supply system;  
     [0022]FIG. 6 is a schematic cross-sectional view of a portion of the plasma reactor system of FIGS.  1 A-C showing the utilities support arm and the elements making up the utility supply system of the present invention; and  
     [0023]FIG. 7 is a schematic cross-sectional view of a portion of the plasma reactor system of FIGS.  1 A-C, showing the mechanical support arm and an embodiment of the vertical translation mechanism of the present invention. 
    
    
     DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS OF THE INVENTION  
     [0024] The present invention relates to plasma reactor systems, and in particular relates to chucks for high-flow, high-power plasma reactor systems.  
     [0025] With reference to FIG. 1A, there is shown a plasma processing system  50  that includes a plasma chamber  60  having an outer wall  62  and an upper wall  64  that encloses an interior region  65  that includes an upper interior region (or “upper part”)  65 U closest to the upper wall, and a lower interior region (or “lower part”)  65 L adjacent the upper region. Surrounding chamber  60  about upper region  65 U is a plasma source generator  80  capable of igniting and sustaining a plasma  82  in upper region  65 U of chamber  60 .  
     [0026] Plasma processing system  50  may be any one of a number of plasma processing systems, such as an electrostatically shielded radio frequency (ESRF) plasma system, as shown in FIG. 1A. Such plasma processing systems are described in U.S. Pat. No. 4,938,031 and 5,234,529, which patents are incorporated herein by reference. Thus, in an exemplary embodiment of system  50 , plasma source generator  80  includes an inductive coil  90  that encircles a portion of chamber  60  so as to surround upper region  65 U. Inductive coil  90  may be a helical resonator (i.e. a quarter-wave or half-wave resonator), wherein one coil end is grounded, and the opposite coil end is open. Coil  90  is tapped near the grounded end, whereby the coil is electrically connected to an RF power supply  92  through a match network  92 MN. The latter is used to maximize RF power transfer to plasma  82 . Between inductive coil  90  and chamber wall  62  is a grounded electrostatic shield  98  (also referred to as an E-shield or Faraday shield) comprising an electrically grounded, conductive sheet with slots (not shown) aligned parallel with the axis of revolution A (i.e., the central axis) of chamber  60  (i.e., in the vertical or Y-direction) and are typically equally spaced. E-shield  98  minimizes capacitive coupling between coil  90  and plasma  82  by limiting the area through which the electromagnetic field from the coil can couple to the plasma. Furthermore, the wall  64  additionally includes a dielectric window (not shown) proximate the inductive coil  90  in order to allow the penetration of the RF electromagnetic field into the plasma. Other plasma source configurations can include capacitively coupled plasma (CCP) sources, electron cyclotron resonance (ECR) plasma sources, helicon-type plasma sources, etc., without limiting the scope of the present invention.  
     [0027] For example, in another embodiment according to the present invention, FIG. 1B presents a plasma reactor system  50  comprising an upper electrode  94  electrically coupled to RF power supply  92  through match network  92 MN. Capacitively coupled plasma reactors are well known to those of skill in the art.  
     [0028] Additionally, in another embodiment according to the present invention, FIG. 1C presents a plasma reactor system  50  comprising a grounded wall  64 . In an alternate embodiment, plasma reactor system  50  can further comprises either a stationary or rotating ring of magnets  96 . Reactive ion plasma reactor systems and their design is well known to those of skill in the art.  
     [0029] With reference to FIGS.  1 A-C and  2 , chamber  60  further includes a chuck assembly  110  located within chamber  60  in lower interior region  65 L, for supporting a workpiece (e.g., a wafer)  116  having an upper surface  116 S. Chuck assembly  110  includes a chuck base  130  having an upper surface  132 , a lower surface  134  and a perimeter  136 . Chuck  110  is suspended within lower interior region  65 L by three support arms  150 A- 150 C that connect to the perimeter of chuck base  130  and that are attached to outer wall  62 . Support arms  150 A- 150 C are adapted to provide a path for connecting RF power, utilities and mechanical power to chuck assembly  110 . In particular, and as discussed in greater detail below, support arm  150 A serves as a RF network and cooling support arm, support arm  150 B serves as a utilities support arm, and support arm  150 C serves as a mechanical support arm. Although three support arms  150 A-C are discussed, there can be either more or less.  
     [0030] Chuck assembly  110  further includes a workpiece support member  160  having a lower surface  162  and an upper surface  164 , with the upper surface directly supporting workpiece  116 . The body of workpiece support member  160  includes one or more cavities  160 C (not shown in FIG. 1) through which cooling fluid can flow to cool the support member and workpiece  116 . Workpiece support member  160  is movably supported above chuck base upper surface  132  by one or more vertical translation members  168  arranged between and operatively connected to chuck base  130  and workpiece support member  160 . In alternate embodiments, workpiece support member  160  can be removably mounted and workpiece support members can be configured to accomodate different sized workpieces. Vertical translation members  168  are further operatively connected to a vertical drive motor  170  external to chamber  60 . An exemplary mechanism for vertically translating workpiece support member  160  relative to chuck base  130  is discussed further below with reference to FIG. 7. Chuck assembly  110  also preferably includes a bellows  176  connected at one end to lower surface  162  of workpiece support member  160 , and at the opposite end upper surface  132  of chuck base  130 . Bellows  176  is preferably made from stainless steel and serves to protect the rest of chuck assembly  110  from exposure to gas from plasma  82  as well as maintain the vacuum integrity of chamber  60 . In alternate embodiments, chuck assembly  110  can be removably mounted within chamber  60  and chuck assembly  110  can easily be configured to accommodate different sized workpieces.  
     [0031] With continuing reference to FIGS.  1 A-C, system  50  further includes a chuck RF power supply  180  for supplying RF power to workpiece support member  160 , which also serves as an electrode. A chuck match network  180 MN is provided, in whole or in part, immediately adjacent lower surface  162  of workpiece support member  160  so as to provide an impedance match to the load presented by plasma  82 . This arrangement reduces the RF transmission line and is designed to reduce the load impedance and therefore increase the ion current flowing to workpiece  116 . A more detailed description of embodiments for chuck match network  180 MN is provided below.  
     [0032] System  50  further includes a workpiece load chamber  200  having a sealable door  204 , attached to plasma chamber  60  near chuck assembly  110 . Chamber  200  is in communication with lower interior region  65 L through an aperture  206  in wall  62 . Door  204  is sized to allow workpieces to be placed into workpiece load chamber  200 . Also included is a workpiece handling system  210  in operable communication with load chamber  200  for transporting wafers to and from workpiece support member  160  through the load chamber.  
     [0033] Further included in system  50  is a gas supply delivery system  220 , pneumatically connected to upper interior region  65 U of chamber  60  via a gas line  222 , for delivering a gas (e.g., Argon) for the formation of plasma  82 .  
     [0034] System  50  also includes a vacuum pump system  250  arranged beneath lower surface  134  of chuck base  130  along central axis A. Vacuum pump system  250  includes vacuum pump  256 , such as a turbo-mechanical pump of the kind typically used in plasma reactor systems, and a gate valve  258  arranged between chuck base  130  and the vacuum pump for controlling the vacuum level in interior region  65 . Vacuum pump system  250  can include a roughing pump (not shown) connected to vacuum pump  256  and chamber  60  for initially pumping down chamber interior region  65 . Gate valve  258  can be electromechanical so that it can be remotely operated via an electrical signal from a controller. Vacuum pump system  250  and gas supply system  220  together are capable of reducing the pressure in chamber  60  to between approximately 1 mTorr and approximately 1 Torr, depending on the application. The location of vacuum pump system  250  directly underneath chuck assembly  110  in the present invention is made possible by support arms  150 A- 150 C, and serves to make the flow of gas axially symmetric, or substantially so.  
     [0035] Also included in system  50  is a cooling system  290  fluidly connected, via cooling lines  292 , to chuck assembly  100  and cavities  160 C in workpiece support member  160 , for cooling the workpiece support member and workpiece  116  during plasma processing of the workpiece.  
     [0036] System  50  also includes a main control system  300  electronically connected to RF power supply  92  and match network  92 MN, chuck RF power supply  180  and match network  180 MN, vertical drive system  170 , workpiece handling system  210 , gas supply delivery system  220 , vacuum pump system  250 , and cooling system  290 . Control system  300  controls and coordinates the operation of the above-mentioned systems through respective electronic signals.  
     [0037] In operation, system  300  initiates the placement of workpiece  116  onto upper surface  164  of workpiece support member  160  through load chamber  200  via workpiece handling system  210 . Control system  300  then activates vacuum pump system  250  to pump down interior region  65 . Once the pressure of interior region  65  is reduced to a certain level (e.g., 10 −8  to 10 −4  Torr), control system  300  initiates the flow of gas from gas supply system  290  into upper region  65 U. At about the same time, control system  300  activates RF power supply  92  to activate plasma source generator  80 , thereby forming plasma  82  in upper region  65 U adjacent workpiece  116 . Control system  300  then activates chuck RF power supply  180  to bias workpiece support member  160 , thereby initiating the flow of plasma towards workpiece upper surface  116 S. Because chuck assembly  110  is suspended within interior region  65  with support arms  150 A- 150 C, and because of the location of vacuum pump system  250 , the plasma gas flows towards and over the wafer, around chuck assembly  110  and into vacuum pump  256  in a substantially axially symmetric manner.  
     [0038] With reference now to FIG. 3, a more detailed description of chuck match network  180 MN as part of a chuck RF supply system  330  and chuck assembly  110  is now provided in connection with support arm  150 A. Support arm  150 A is hollow so that it can serve as a conduit for supplying the RF power and cooling fluid to the chuck assembly.  
     [0039] A first embodiment of a chuck RF supply system of the present invention is system  330  of FIG. 3, which includes a first variable capacitor C 1  mounted to lower surface  162  of workpiece support member  160  so as to be in direct electrical contact therewith. Electrically connected to variable capacitor C 1  by means of a first capacitor lanyard  336  is a first capacitor servo  340  for adjusting the capacitance of variable capacitor C 1 . The capacitance of variable capacitor C 1  is adjustable from about 100 to about 500 pF. First capacitor servo  340  is electrically connected to and is controlled by main control system  300 . First capacitor lanyard  336  is designed to accommodate the vertical movement of first variable capacitor C 1  when workpiece support member  160  is vertically translated.  
     [0040] System  330  further includes a first inductor L 1  located immediately adjacent variable capacitor C 1  and electrically connected thereto. First inductor L 1  is thus proximate lower surface  162  of workpiece support member  160 . System  330  further includes a second inductor L 2  in series with the first inductor but external (i.e., not proximate) to chuck assembly  100 , and a second variable capacitor C 2  external (i.e., not proximate) to the chuck assembly and arranged in parallel with the first and second inductors. For most applications, the values of first inductor L 1  and second inductor L 2  typically comprise a total inductance of about 400 nH with usually a 4-turn inductor internal to the chuck assembly (L 1 ) and a 5-turn inductor external to the chuck assembly (L 2 ), and the value of a second variable capacitor C 2  typically ranges from 500 to about 1500 pF. The topology of chuck match network  180 MN is L-type, and is illustrated in the circuit diagram of FIG. 4, with L EFF  being the effective inductance of the arrangement of inductors L 1  and L 2 . Other topologies for the chuck match network, such as a T-network or a π-network, may be employed.  
     [0041] System  330  also includes RF power supply  180  connected across second variable capacitor C 2 . The reason for having two inductors, internal inductor L 1  and external conductor L 2 , for locating second variable capacitor C 2  external to chuck assembly  100 , and for using lanyard  336  for adjusting the internal capacitor C 1  is because of space limitations within chuck assembly  110 .  
     [0042] As mentioned above, because match network  180 MN (or at least a portion thereof) is located immediately adjacent workpiece support member  160 , the need for a transmission line that connects the match network to the support member is eliminated. Accordingly, match network  180 MN can be tuned by selecting the inductance values for the inductor(s) and varying the capacitance of the variable capacitors so as to provide a low impedance path from RF source  180  to workpiece support member  160 .  
     [0043] Also optionally included in system  330  is a housing  370  that partly resides within chuck assembly  110  that encloses match network  180 MN ( 370 A), partly resides outside the chuck assembly  110  that encloses the outer portion of match network  180 MN ( 370 B), and that may also enclose a portion of cooling lines  292 .  
     [0044] With reference now to FIG. 5, a chuck RF supply system  380  as an alternative embodiment to system  330  is now described. System  380  is similar to system  330  and has many of the same elements, except that in system  380 , there is a single inductor L 2 ′ in the position of inductor L 2  of system  330 , and second variable capacitor C 2  is arranged internal to chuck assembly  100  rather than external. Second variable capacitor C 2  is thus proximate workpiece support member lower surface  162 , and is adjusted by an externally located second capacitor servo  386  electrically connected thereto, and also electrically connected to main control system  300 . The electrical connection between capacitor servo  386  and second capacitor C 2  may be by lanyard, such as lanyard  336 , if the second capacitor is situated such that it moves along with workpiece support member  160 . Where there is adequate room in chuck assembly  110 , capacitor servos  340  and  386  may be moved into the chuck assembly, thereby eliminating the need for lanyards. Housing  370  comprised of components  370 A and  370 B may also be used to enclose all or part of system  380 .  
     [0045] With reference again to FIGS.  1  to  3 , support arm  150 A serves as a conduit for cooling inlet line  292  and coolant outlet line  292 B that fluidly connect cooling system  290  to workpiece support member cavities  160 C, inner match network  180 MN cavity within housing  370 A (for external cooling of variable capacitor C 1  and inductor L 1  which are immersed within the coolant contained in  370 A as shown in FIG. 3, or additionally variable capacitor C 2  as shown in FIG. 5), the interior of inductor L 2  as shown in FIG. 3 (inductor coil L 2  can be fabricated from hollow copper tubing) and a (copper) cooling manifold on variable capacitor C 2  as shown in FIG. 3. Cooling system flows cooling fluid to and from cavities  160 C via cooling lines  292  so that fluid can be circulated through workpiece support member  160  during the processing of workpiece  116 .  
     [0046] With reference now to FIG. 6, utilities support arm  150 B is now described. Utilities support arm has an input end  150 Bi and is hollow so that a utilities supply system  410  of the present invention can communicate with chuck assembly  110  through this supply arm. Utilities supply system  410  includes at input end  150 Bi one or more input/output ports  420  that connect to respective devices located within chuck assembly  110  via utility lines  422 . The input/output ports  420  and the corresponding devices typically include the following: a helium port for a helium line used to supply helium for the wafer  116  back-side to improve the gas gap conductance and in turn improve the wafer-to-chuck heat transfer; a nitrogen port for a nitrogen line for purging the inner chuck cavity and variable capacitor seals to reduce condensation, a thermocouple port for electrically connecting a thermocouple (not shown) within chuck assembly  100  to main control system  300 , for monitoring the chuck temperature; a current monitor port for electrically connecting a current monitor  426  electrically connected to workpiece support member  160  for monitoring the RF current to the chuck electrode; a voltage probe port for electrically connecting a voltage probe  430  electrically connected to workpiece support member  160  for monitoring the chuck electrode voltage; an electrostatic clamp port for electrically connecting an electrostatic clamp (not shown) in operable communication with workpiece support member  160  for electrostatically securing workpiece  116  to upper surface  164  of support member  160 ; and a pneumatic push pin supply port for pneumatically connecting pneumatic push pins  436  located in upper surface  164  of workpiece support member  160 , for lifting workpiece  116  from the support member upper surface.  
     [0047] The various utility lines  422  leading from ports  420  to the respective devices may be gathered into a flexible cable  440  so that the lines occupy as small a volume within chuck assembly  100  as possible. Flexible cable  440  is designed to flex to accommodate the vertical movement of workpiece support member  160 .  
     [0048] With reference now to FIG. 7, mechanical support arm  150 C is hollow and is used to provide a pathway for the mechanical connection between vertical drive motor  170  and workpiece support member  160  as part of a drive mechanism  500 .  
     [0049] Drive mechanism  500  includes drive motor  170  having a drive shaft  508  extending therefrom, with the distal end of the shaft having a first beveled gear  512 . Mechanism  500  further includes two vertical translation screws  520  and  522  each having respective threaded upper portions  530  and  532 . Screws  520  and  522 , which can be considered as the aforementioned vertical translation members  168  (FIG. 1), are arranged parallel to the y-axis and central axis A and are threadedly engaged at their respective upper portions  530  and  532  with respective elongate threaded nuts  536  and  538  that depend from lower surface  162  of workpiece support member  160 . At distal ends of translation screws  520  and  522  are respective sprockets  560  and  562  engagedly connected by a chain  566 . Attached to sprocket  562  and depending a short distance therefrom is a second beveled gear  570 . Vertical translation screw  522  is arranged so that second beveled gear  570  matingly engages first beveled gear  512  of drive shaft  508 .  
     [0050] In operation, to vertically position workpiece support member  160  within interior region  65  of chamber  60 , main control system  300  transmits an electronic signal to drive motor  170 , thereby initiating the rotation of drive shaft  508 . Position sensors (not shown) provide feedback signals to main control system  300 . This rotation causes the rotation of sprocket  562  and thus the rotation of vertical translation screw  522  via the engagement of first and second beveled gears  512  and  570 . Because of the coupling between sprockets  560  and  562  via chain  566 , vertical translation screws  520  and  522  rotate in synchrony. The threaded engagement of translation screws  520  and  522  with corresponding threaded nuts  530  and  532  causes workpiece support member  160  to move vertically upward (i.e., toward upper wall  64 ) or vertically downward, depending on the direction of rotation of drive shaft  508 . In alternate embodiments, other drive mechanisms can be used.  
     [0051] Since numerous modifications and changes to the embodiments described above will readily occur to those of ordinary skill in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention.