Abstract:
A plasma treatment system ( 10 ) and related methods for rapidly treating a workpiece ( 56 ) with ions from a plasma having an ion density that is reproducibly uniform and symmetrical. The processing chamber ( 12 ) of the plasma treatment system ( 10 ) includes a chamber ( 14 ) lid having a symmetrical array of apertures ( 192 ) and further includes a vacuum distribution baffle ( 180 ), which are both configured to uniformly disperse a process gas adjacent the surface of the workpiece ( 56 ). The uniform dispersion of process gas and a symmetrical placement of the workpiece within the chamber ( 12 ) contribute to providing a uniformly dense plasma of ions adjacent the workpiece ( 56 ). A treatment system control ( 304 ) automates the operation of the system and controls the flow of process gas, evacuation of the chamber, and the application of the plasma excitation power to minimize the length of a treatment cycle and to optimize the uniformity of the plasma treatment.

Description:
This application claims benefit of Provisional 60/143,577 filed Jul. 13, 1999. 

   FIELD OF THE INVENTION 
   The present invention relates generally to plasma processing, and more particularly to a plasma treatment system configured to enhance plasma uniformity and to increase process throughput. 
   BACKGROUND OF THE INVENTION 
   Plasma treatment is commonly applied for modifying the surface properties of workpieces used in applications relating to integrated circuits, electronic packages, and printed circuit boards. Plasma treatment systems are configured to produce a plasma from a process gas and directly bombard the surface of a substrate or workpiece with energetic ions from the gas plasma to remove surface atoms by physical sputtering or chemically-assisted sputtering. Ion bombardment may be used to condition the surface to improve properties such as adhesion, to selectively remove an extraneous surface layer of a process material, or to clean undesired contaminants from the surface. Plasma treatment is used in electronics packaging, for example, to increase surface activation and/or surface cleanliness for eliminating delamination and bond failures, improving wire bond strength, ensuring void free underfill, removing oxides, increasing die attach, and improving adhesion for encapsulation. 
   The number of impinging ions per unit area, or ion flux, must be precisely and accurately controlled at all positions on the surface of the workpiece so that the time-integrated ion flux is substantially uniform across the surface. Critical parameters for controlling the uniformity of the ion flux include the spatial uniformity of the excitation power and the dispersion of the process gas. A non-uniform ion flux degrades process reliability and reduces the process yield. 
   Plasma treatment systems may be integrated into in-line and cluster systems or batch processes in which groups of workpieces are processed by successive plasma exposures or processing cycles. Workpieces may be supplied within a magazine, individually by a conveyer transport system, or manually. Plasma treatment systems may be provided with automated robotic manipulators that coordinate workpiece exchange for plasma processing operations. 
   Conventional plasma treatment systems have failed to provide adequate process uniformity across the surface of individual workpieces. To achieve workpiece-to-workpiece uniformity, the process gas must be evenly dispersed and uniformly ionized by the excitation power so that the ion flux is spatially uniform across the surface of the workpiece. Conventional plasma treatment systems have likewise failed to achieve adequate reproducibility of the plasma treatment between successive batches of workpieces. Batch-to-batch reproducibility depends on the precise control of process variables and parameters so that successive workpieces are exposed to substantially identical plasma conditions. Moreover, conventional plasma treatment systems are incapable of rapidly processing workpieces with a throughput amenable to automated process lines or fabrication requirements. System throughput and uniformity of the plasma treatment must be maximized for reducing production costs. 
   There is thus a need for a plasma treatment system that can provide a plasma having a uniform density adjacent all points of a surface of a workpiece and that can consistently reproduce that uniformly dense plasma for sequentially processing a series of workpieces. 
   SUMMARY 
   The present invention addresses these and other problems associated with the prior art by providing a plasma treatment system having a processing chamber interfaced with and controlled by a programmable logic control system. The system includes a vacuum chamber having a processing space that surrounds a workpiece-holding portion, which is configured to receive and support the workpiece. Process gas is provided to the processing space and actively pumped through a vacuum port by a vacuum pump. A vacuum distribution baffle provides a uniform flow of process gas adjacent over the workpiece to the vacuum port, while simultaneously affording a high pumping rate for rapidly evacuating the processing chamber. A powered electrode, positioned between the vacuum distribution baffle and the workpiece holding portion and in electrical continuity with the workpiece holding portion, is operably connected to a plasma excitation source for generating a plasma from the process gas. 
   In an aspect of this invention, the vacuum distribution baffle may be composed of a ceramic and serve as a shield for reducing the plasma excitation power required to generate a plasma in the processing chamber. The baffle confines the plasma to a portion of the processing space adjacent the powered electrode and workpiece. 
   In another aspect of this invention, the workpiece holding portion is configured to position the workpiece equidistantly between the powered electrode and the ground electrode. The symmetrical placement contributes to producing a substantially perpendicular electrical field between the electrodes and, thereby, contributing to a highly uniform and symmetrical distribution of plasma density adjacent the workpiece. 
   In another aspect of this invention, the chamber lid of the vacuum chamber features a gas distribution system that includes a process gas inlet port for introducing a process gas into a gas distribution space embedded in the chamber lid. An array of apertures is provided on the interior face of the lid and are arranged to provide a symmetrical and uniform flow of a process gas in two dimensions over the surface of the workpiece held by the workpiece holding portion. When the process gas is ionized by a plasma excitation source, the uniform flow contributes to a uniform plasma density. 
   In another aspect of this invention, the hinge that couples the chamber base and the chamber lid, or access member, includes an obround bearing groove for receiving a hinge pin. The obround bearing groove accommodates a substantially vertical compression of the sealing member or O-ring between the access member and the base when a vacuum pressure exists within the processing chamber. By constraining lateral movement between the lid and base, abrasion of the surface of the sealing member is significantly reduced and the lifetime of the sealing member is substantially extended. Further, the lid and base are uniformly sealed along all points of contact with the sealing member due to the uniform substantially vertical compression. 
   In one embodiment, the plasma treatment system may include a variable position workpiece holding portion with moveable holding structure. By simply repositioning the holding structure, the system can be rapidly reconfigured to adapt to changes in workpiece dimension. 
   In another embodiment, the invention provides a plasma processing system in which a workpiece to be processed is transferred into a processing chamber. The pressure within the processing chamber is reduced, and flows of process gases are initiated into the processing chamber. A first lower magnitude of RF power is applied to electrodes within the processing chamber to create a gas plasma and initiate a plasma treatment cycle. An impedance of an RF system including the electrodes is matched to a desired impedance at the first RF power magnitude. While increasing the RF power to the electrodes, the impedance of the RF system including the electrodes is matched to the desired impedance. After an end of the plasma processing cycle is detected, the flow of process gases to the processing chamber and the application of RF power to the electrodes are terminated. 
   In one aspect of this invention, the decrease in pressure in the processing chamber and the flow of process gases into the processing chamber occur simultaneously. In another aspect of this invention, RF power to the electrodes is increased at the highest rate at which the impedance of the RF system can be continuously matched to the desired impedance. In a still further aspect of this invention, the RF power to the electrodes is decreased after detecting the end of the processing cycle. 
   In a still further embodiment, the invention provides a plasma processing system in which a workpiece to be processed is transferred into a processing chamber. The pressure within the processing chamber is reduced to a first pressure value, and flows of process gases are initiated into the processing chamber. RF power is applied to electrodes within the processing chamber to create a gas plasma and initiate a plasma processing cycle, and an impedance of an RF system including the electrodes is matched to a desired impedance at the first RF power magnitude. Evacuation of the processing chamber continues over the plasma processing cycle. After an end of the plasma processing cycle is detected, the flow of process gases and the application of RF power to the electrodes are terminated. 
   In an aspect of this invention, pressure within the processing chamber is allowed to vary between upper and lower boundary pressure limits. The upper pressure limit is determined by adding an incremental pressure value to normally used pressure value, and the lower pressure limit is determined by subtracting the incremental pressure value from the normally used pressure value. 
   These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
       FIG. 1  is a perspective view of a plasma treatment system in accordance with the principles of the present invention; 
       FIG. 2A  is a side schematic and partially broken view of the plasma treatment system of  FIG. 1 ; 
       FIG. 2B  is a side schematic and partially broken view of the plasma treatment system of  FIG. 1  in which the chamber lid is in a closed position; 
       FIG. 2C  is a detailed side view of the plasma treatment system of  FIG. 1 ; 
       FIG. 3  is a front view of the plasma treatment system of  FIG. 1 ; 
       FIG. 4  is a schematic block diagram illustrating a control system for the plasma treatment system of  FIG. 1 ; 
       FIG. 5  is a flow chart illustrating a process of implementing a plasma processing cycle utilizing the control system of  FIG. 4 ; 
       FIG. 6  is a side view of a substrate support in accordance with an alternative embodiment the principles of the present invention; and 
       FIG. 7  is a partial front view of the substrate support of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention, in accordance with the principles and objectives herein, provides an apparatus and a method for processing a workpiece with a plasma. The present invention provides a plasma treatment system configured to provide a uniformly dense plasma, rapid pump-down and venting cycles, reproducible processing conditions, and simplified material handling. The system advantageously requires a reduced excitation power to initiate and sustain a uniformly dense plasma in the processing space, while employing a control algorithm that minimizes the cycle time required to process each successive workpiece. 
   A plasma treatment system  10 , in accordance with the principles of the present invention, is illustrated in  FIGS. 1 ,  2 A– 2 C and  3 . Referring to  FIG. 1 , plasma treatment system  10  includes a processing chamber  12 , a loading station  20 , and an exit station  22 , which are situated on a substantially flat and mechanically stable surface  24  atop an instrument cabinet  26 . Processing chamber  12  includes a chamber lid  14  hingeably connected to a chamber base  18  by a hinge assembly  16 . Chamber lid  14  is selectively positionable between an open position, as shown in  FIGS. 1 and 2A , and a closed position, as shown in  FIG. 2B . Chamber base  18  and chamber lid  14  are preferably formed of an electrically conductive material suitable for high-vacuum applications, such as an aluminum alloy or a stainless steel. 
   Chamber lid  14  includes a domed ceiling  28  and an integral sidewall  30  encircled by a flat rim  32 . A viewport opening  38  is provided in ceiling  28  for holding a viewport  34 . As best shown in  FIG. 2C , viewport  34  is a substantially planar panel attached to chamber lid  14  by a frame  35  and fasteners  36 . An O-ring  40  is received within a groove  42  that circumscribes viewport opening  38 . Viewport  34  compressively engages O-ring  40  to create a vacuum-tight seal, where the sealing force is supplied by the pressure differential between the interior and exterior of processing chamber  12  and fasteners  36 . Viewport  34  is constructed of a dielectric ceramic, such as quartz or alumina, that has a low sputtering coefficient, is gas impermeable, and has a wide transmission range for optical wavelengths. O-ring  40  is preferably formed of an elastomer such as Viton®. 
   Chamber base  18  includes a floor wall  44  integral with a sidewall  46  which is encircled by a flat lip  48 . Lip  48  includes a circumferential groove  50  for receiving a conductive resilient sealing member or O-ring  51  that provides an electrically-conductive pathway and a substantially vacuum-tight seal between chamber lid  14  and chamber base  18 . The dimensions of groove  50  and O-ring  51  are selected for creating a vacuum-tight seal. It may be appreciated that O-ring groove  50 , and therefore O-ring  51 , may be positioned in either chamber lid  14  or chamber base  18  without departing from the spirit and scope of the present invention. Particulates from the surrounding environment are less likely to attach to, and compromise the sealing ability of, O-ring  51  if positioned in chamber lid  14 . 
   O-ring  51  is a conductive elastomer gasket, preferably formed of a composite of a conductive fill powder impregnated in an elastomer binder, such as a powder of silver and aluminum in silicone. An exemplary O-ring  51  is formed of a conductive composite manufactured and marketed under the trade name Cho-seal® by EMI Shielding Products, a division of Parker Hannifin Corp. (Cleveland, Ohio). 
   In another aspect, chamber base  18  further includes a workpiece holding portion or substrate support  64  configured to receive and support a part or workpiece  56 . Generally, workpiece  56  is a rectangular, planar structure that includes a periphery having opposed side edges  58 ,  59  of a predetermined thickness, a leading edge  60 , and a trailing edge  62 . Opposed side edges  58 ,  59  are separated by a predetermined maximum transverse width that is measured perpendicular to a longitudinal axis of workpiece  56 . Workpiece  56  may be a strip type part, such as a ball grid array (BGA) or a metal lead frame, singulated BGA&#39;s carried in an Auer boat, or a pallet carrying multi-chip electronic modules, integrated circuit chips, or the like. 
   As best depicted in  FIG. 2C , substrate support  64  comprises opposed side rails  66   a ,  66   b  that extend vertically from a substantially planar support platform  68 . Side rail  66   a  is in a spaced relationship relative to side rail  66   b  along the longitudinal axis of support platform  68  so that the maximum width between side edges  58  and  59  of workpiece  56  may be accommodated. For convenience, side rail  66   a  will be detailed below with the understanding that side rail  66   b  has an identical structure. Side rail  66   a  protrudes above a horizontal plane that includes lip  48  and incorporates an elongate channel  72 , as best shown in  FIG. 3 , that extends parallel to a longitudinal axis of substrate support  64 . Channel  72  has a U-shaped cross-sectional profile that is dimensioned to slidingly receive side edge  59  of workpiece  56  therein. Opposed extremities of channel  72  include a flared lip  74 , as best shown in  FIG. 3 , that physically captures side edge  59  of workpiece  56  during loading. By way of example and not limitation, substrate support  64  may be configured to accept workpieces  56  having maximum dimensions of 2.7″ (wide)×9.25″ (long)×3/8″ (thick) or maximum dimensions of 6″×12″×1″. 
   Referring to  FIG. 1 , loading station  20  and exit station  22  are proximate to respective opposed ends of processing chamber  12  and are adapted for shuttling workpieces  56 ,  56 ′ into and out of processing chamber  12 . Loading station  20  includes a substantially planar support platform  76  and opposed loading side rails  78   a  and  78 . Loading side rail  78   a  is in a spaced relationship relative to loading side rail  78   b  along the longitudinal axis of support platform  76  so that the maximum width of workpiece  56  may be accommodated. For convenience, side rail  78   a  will be detailed below with the understanding that side rail  78   b  is substantially identical. Loading side rail  78   a  protrudes above a horizontal plane that includes lip  48  and incorporates an elongate channel  82 . Channel  82  has a U-shaped cross-sectional profile that is dimensionally adapted to slideably receive one opposed side edge  58  or  59  of workpiece  56  therein. Opposed extremities of channel  82  include a flared lip  80  that physically captures the side edges  58 ,  59  of workpiece  56 . Support posts  84  extend from a bottom surface of the support platform  76  to surface  24 . 
   Exit station  22  is configured similarly to loading station  20 . Exit station  22  includes opposed unloading side rails  86   a ,  86   b  that extend upwardly and outwardly from a planar support platform  88 . For convenience, side rail  86   a  will be detailed below with the understanding that side rail  86   b  has an identical structure. Side rail  86   a  protrudes above a horizontal plane that includes rim  48  and incorporates a longitudinal channel  90 . Channel  90  has a U-shaped cross-sectional profile that is dimensioned to slideably receive one of the two peripheral edge  58 ′,  59 ′ of processed workpiece  56 ′ therein. Opposed extremities of channel  90  include a flared lip  91  that aids in physically capturing the side edges  58 ′,  59 ′ of processed workpiece  56 ′ during unloading. Support posts  92  extend from a bottom surface of support platform  88  to surface  24 . 
   Plasma treatment system  10  further includes pinch wheels  99  attached to loading station  20  and exit station  22  and a positioning lever  94 . Pinch wheels  99  are operable to make fine adjustments in the positioning of workpiece  56  or workpiece  56 ′. Lever  94  is operable to move along the length of a slot  96  defined in the top surface  24  of instrument cabinet  26  and to also translate vertically. A driving mechanism (not shown) is attached to lever  94  and is operable to move arm  94  vertically and longitudinally in slot  96 . Lever  94  is positioned entirely outside of processing chamber  12  during a plasma processing cycle. 
   Positioning lever  94  further includes a rod  97  having a first finger  98   a  that selectively abuts a rear edge  62  of workpiece  56  held between loading side rails  78   a,b  and a second finger  98   b  that selectively abuts a rear edge  62  of second workpiece  56  held between sides rails  66   a,b . It may be appreciated that fingers  98   a,b  can be resiliently biased relative to rod  97  and, in addition, that fingers  98   a,b  may further include a sensor for detecting resistance in the linear movement of positioning lever  94  due to, for example, a workpiece misaligned with a set of side rails. 
   During a workpiece loading operation, workpiece  56  is delivered by an automated conveying system (not shown) and positioned in loading side rails  78   a,b  on loading station  20 . Pinch wheels  99  of loading station  20  are used to move the workpiece  56  short distances for proper positioning. After chamber lid  14  is opened, positioning lever  94  is lowered from its initial position and linearly actuated so that finger  98   a  will engage rear edge  62  and push workpiece  56  along loading side rails  78   a,b  toward substrate support  64 . The front edge  60  of workpiece  56  will traverse the gap between loading side rails  78   a,b  and side rails  66   a,b . Opposed side edges  58 ,  59  of workpiece  56  will be slideably received by side rails  66   a,b . Thereafter, the positioning lever  94  will continue to push the workpiece  56  until it is suitably and accurately positioned on substrate support  64 . Preferably, the center of workpiece  56  is positioned coaxial with the central vertical axis or centerline of the processing chamber  12 . Positioning lever  94  then translates vertically so that finger  98   b  will clear the leading edge of workpiece  56  as lever  94  is retracted to its initial position. 
   If processed workpiece  56 ′ resides on substrate support  64  during the workplace loading operation, finger  98   b  engages rear edge  62 ′ and positioning lever  94  sweeps the processed workpiece  56 ′ toward exit station  22 . Front edge  60 ′ of processed workpiece  56 ′ will cross the gap between the processing chamber  12  and exit station  22 . Side edges  58 ′,  59 ′ of processed workpiece  56 ′ are captured by unloading side rails  86   a,b . With continued linear movement, processed workpiece  56 ′ is completely removed from processing chamber  12 . Pinch wheels  99  of exit station  22  are used to move the workpiece  56 ′ short distances for proper positioning in preparation for transport to the next processing station. 
   Hinge assembly  16  is adapted so that chamber lid  14  may be selectively pivoted relative to chamber base  18  between an open position, as best illustrated in  FIG. 2A , and a closed position, as best illustrated in  FIG. 2B . Hinge assembly  16  includes at least two brackets  100 , as best shown in  FIG. 1 , that are disposed in a spaced relationship along the non-vacuum side of sidewall  46 . When chamber lid  14  is cantilevered into a closed position, chamber lid  14  and chamber base  18  bound a vacuum-tight processing space  102 , as shown for example in  FIG. 2B . 
   Each bracket  100  includes a V-shaped brace  104  and a nub  106  mounted with fasteners  108  to a non-vacuum side of sidewall  46 . Each brace  104  is carried by a hinge pin  110  received within an aperture  112  near the bend in brace  104  and within a coaxial aperture  124  in nub  106 . As shown in  FIG. 1 , hinge pin  110  is shared by both brackets  100 . Returning to  FIG. 2A , one end of brace  104  is connected to the non-vacuum side of sidewall  30  of chamber lid  14 . A second end of each brace  104  includes an aperture  114  that receives a connecting rod  116  that is also shared by both braces  104 . 
   Connecting rod  116  is further attached to a rod end  118  that is threadingly carried by one end of a piston rod  120  of a bi-directional pneumatic cylinder or lid actuator  122 . Rod end  118  further includes an aperture (not shown but similar to, and collinear with, aperture  114 ) with an inner diameter sized to slideably receive connecting rod  116  therein. Piston rod  120  is adapted for reciprocating linear, vertical motion so that brace  104  can pivot about hinge pin  110  to cantilever chamber lid  14  between an open position and a closed position. As shown in  FIG. 2C , the opposed end of lid actuator  122  is affixed via a mounting block  126  to a structural support (not shown) within instrument cabinet  26 . 
   Referring to  FIG. 2B , in one aspect of the present invention, an obround bearing  128  is slidingly received within aperture  124  of nub  106 . Obround bearing  128  has an exterior, annular surface of an outer diameter chosen to frictionally fit within aperture  124  and an interior bore  130  that is dimensioned to receive hinge pin  110 . Bore  130  has a substantially oval cross-sectional profile with a vertical major axis, as viewed normal to the longitudinal axis of bore  130 . When chamber lid  14  is in an open position, as shown in  FIG. 2B , a length of one end of hinge pin  110  will contact a lower interior surface of bore  130 . As chamber lid  14  is pivoted by the lid actuator  122 , hinge pin  110  rotates about a longitudinal axis thereof. During rotation, the outer surface of hinge pin  110  remains in contact with the lower interior surface of bore  130 . When lip  32  contacts the surface of O-ring  51 , as shown in  FIG. 2B , lid actuator  122  will continue to extend so that the chamber lid  14  moves downward to compress O-ring  51 . Due to the presence of obround bearing  128 , hinge pin  110  is free to translate vertically upward in bore  130 . 
   Referring to  FIG. 2C , in which the chamber lid  14  resides in a closed position, the interior peripheral surface of the chamber lid  14  and chamber base  18  bounds processing space  102 . The vacuum seal is enhanced by the further compression of O-ring  51  between chamber base  18  and chamber lid  14 . The additional compression of O-ring  51  results from the pressure differential between atmospheric pressure acting on the exterior of chamber lid  14  and the vacuum within processing chamber  12  that applies a force that urges chamber lid  14  vertically downward towards chamber base  18 . Hinge pin  110  translates vertically and with minimal transverse motion due to the presence of obround bearing  128 . 
   Bore  130  within obround bearing  128  affords an additional degree of vertical freedom for hinge pin  110 , as compared with a conventional bearing having a bore of a circular cross-sectional profile. Chamber lid  14  is free to move vertically in response to the forces that compress O-ring  51 . As a result, the vacuum-tight seal between lip  32  and O-ring  51  is uniform about the circumference of groove  50 . In a preferred embodiment, the presence of obround bearing  128  provides approximately 50 mils of vertical movement for hinge pin  110 . 
   A pressure gauge  52  is connected via tubing  53  to an opening provided in sidewall  46 . Pressure gauge  52  is operable to sense the vacuum pressure within processing space  102  and provides a pressure feedback signal. An exemplary pressure gauge  52  is a capacitance manometer, such as the Baratron® Capacitance Manometer manufactured by MKS Instruments (Andover, Mass.). A bleed valve  54  is connected via tubing  55  to another opening provided in sidewall  46 . Bleed valve  54  is operable to vent processing chamber  12  with ambient air or a supplied gas, such as nitrogen. 
   Referring to  FIG. 3 , plasma treatment system  10  is connected for fluid communication with a vacuum pumping system  134  through a large, centrally located exhaust port  136  in bottom wall  44  of chamber base  18 . Vacuum pumping system  134  includes a conical reducing nipple  138 , a vacuum valve  140 , an exhaust vacuum conduit (not shown), and a vacuum pump  144 . 
   Opposing ends of conical reducing nipple  138  carry a first vacuum flange  146  and a second vacuum flange  166 . First vacuum flange  146  is connected to exhaust port  136  via a screened centering ring  148  circumscribed by O-ring  150  and a plurality of bulkhead clamps  152 . Bulkhead clamps  152  are symmetrically disposed about the periphery of first vacuum flange  146 . Each bulkhead clamp  152  has a tapered segment  154  that is adapted to engage a complementary lower surface of first vacuum flange  146  and a block portion  156  that further includes bores (not shown) for removably receiving fasteners  160 . Preferably, fasteners  160  are threaded bolts attachable to openings having complementary internal threads (not shown) in bottom wall  44 . To create a vacuum-tight seal, fasteners  160  are tightened to a preselected torque in a patterned sequence so as to uniformly compress O-ring  150 . 
   Vacuum valve  140  carries an upper vacuum flange  162  connected for fluid communication via a vacuum fixture  164  with second vacuum flange  166  which is carried by conical reducing nipple  138 . Vacuum fixture  164  comprises a removable clamshell clamp  168  with a wingnut closure  170  and a through-bore centering ring  172 . When wingnut closure  170  is tightened, an O-ring  174  carried by centering ring  172  is compressed to created a vacuum-tight seal. Vacuum valve  140  also is further connected for fluid communication with vacuum pump  144 . 
   Vacuum pump  144  may comprise one or more vacuum pumps as would be apparent to one of ordinary skill in the art of vacuum technology. A preferred vacuum pump  144  is a single rotary-vane vacuum pump of the type manufactured by, for example, Alcatel Vacuum Technologies Inc. (Fremont, Calif.), that has a pumping rate of about eleven cubic feet per minute and which, due to the high conductance of processing chamber  12 , can evacuate processing space  102  to a vacuum pressure of about 200 mTorr in less than about six seconds. Alternative vacuum pumps  144  include dry pumps and turbomolecular pumps. 
   In another aspect of the present invention, a vacuum distribution baffle  180  is positioned on a shoulder  178  on the interior of chamber base  18 . Vacuum distribution baffle  180  is a flat elongate plate  182  perforated by a plurality of orifices  184 . Orifices  184  restrict the flow of process gas toward the inlet of vacuum pumping system  134  so as to divert the pressure differential. As a result, the entire processed surface of workpiece  56  will be uniformly exposed to the plasma while simultaneously allowing high-speed evacuation of process gas and sputtered contaminant species during a plasma processing operation. Vacuum distribution baffle  180  also prevents gas flow to vacuum pump  144  from disturbing the position of workpiece  56  upon substrate support  64 . 
   Preferably, vacuum distribution baffle  180  is formed of an electrically-insulating material, such as a machinable ceramic, having a minimal out-gassing potential. Suitable machinable ceramics include an aluminum oxide or a glass-bonded mica composite, such as Mykroy/Mycalex® or Macor®. 
   In one aspect of the present invention, chamber lid  14  integrates a gas distribution system that is configured to symmetrically and evenly distribute the flowing stream of process gas over the surface of workpiece  56 . Specifically, ceiling  28  of chamber lid  14  includes an embedded cavity  186 , a process gas inlet port  190 , and a plurality of apertures  192 . As best shown in  FIG. 2C , gas inlet port  190  is positioned in chamber lid  14  and is coupled via gas line  194  to a gas manifold  308  ( FIG. 4 ) for providing a process gas to processing space  102 . As best shown in  FIG. 3 , the vacuum side of ceiling  28  includes apertures  192  for injecting process gas from cavity  186  into processing space  102 . Preferably, apertures  192  are symmetrically distributed in a two dimensional array about the longitudinal axis of processing chamber  12  so that process gas will flow uniformly over the surface of workpiece  56 , and therefore, contribute to improving plasma uniformity. 
   In another aspect, chamber base  18  further includes a power distribution system that transfers electrical power from a plasma excitation source, such as radio-frequency (RF) generator  302  ( FIG. 4 ), to ionize the process gas confined within processing space  102 . The power distribution system includes a power distribution bar  198  operably connected to the RF generator  302 , a pair of power feedthroughs  200 , a bottom electrode  202 , and substrate support  64 . The RF generator  302  is operably connected by feedthroughs  200  to the substrate support  64 , which serves as a powered electrode for capacitively coupling excitation energy with the process gas in processing chamber  12  to initiate and sustain a plasma in processing space  102 . Chamber lid  14  and chamber base  18  collectively form an unpowered, ground electrode. 
   Floor wall  44  of chamber base  18  further includes two openings  204  that receive power feedthroughs  200 . A circular groove  208  is concentrically disposed about the central, longitudinal axis of each opening  204  for receiving an O-ring  210  therein. Power feedthrough  200  includes an electrical tie rod  212  coaxially surrounded by a shield insulator washer  214 , a chamber insulator washer  216 , and a bottom insulator washer  218 . Preferably, washers  214 ,  216 ,  218  are composed of a gas-impermeable ceramic dielectric, such as quartz or alumina, and each tie rod is formed of an electrical conductor, such as copper, aluminum, or alloys thereof. Power feedthrough  200  is electrically isolated from processing chamber  12 . 
   Electrical tie rod  212  includes a flanged head  222  and an opposed threaded end  226 . Flanged head  222  is received within a complementary recess  228  disposed in the upper surface of bottom electrode  202  for electrical continuity therewith and mechanical securement to inhibit downward movement. Tie rod  212  extends downward through the central bores in shield insulator washer  214 , chamber insulator washer  216 , and bottom insulator washer  218 . Threaded end  226  protrudes beyond bottom wall  44  for connection with the excitation power supply. 
   Bottom insulator washer  218  includes an annular lower portion  232  of a first outer diameter continuous with an annular upper portion  234  of a lesser second outer diameter. Upper portion  234  is received within opening  204  so that an upper surface of lower portion  232  abuts O-ring  210  for a vacuum-tight seal with the non-vacuum surface of floor wall  44 . A frustoconical portion  236  of bore  230  is adapted to receive an O-ring  238 . Frustoconical portion  236  is sized and configured so that O-ring  238  can be compressed via fastener  239  to provide a vacuum seal between the circumference of tie rod  212  and bottom insulator washer  218 . 
   Shield insulator washer  214  is interposed between the lower surface of bottom electrode  202  and the upper surface of vacuum distribution baffle  180 . Shield insulator washer  214  includes an annular lower portion  242  of a first diameter integral with an annular upper portion  244  of a greater second outer diameter. Upper portion  244  abuts vacuum distribution baffle  180  and lower portion  242  protrudes downward into an opening therein. 
   Chamber insulator washer  216  is interposed between the inner, bottom surface of the chamber base  18  and the lower surface of the vacuum distribution baffle  180 . Chamber insulator washer  214  has opposed parallel surfaces  248 ,  250 . Surface  248  includes a first recess that is adapted to fit over a length of upper portion  234  of bottom insulator washer  218 . Opposed surface  250  includes a second recess of a diverse diameter that receives a length of lower portion  242  of chamber insulator washer  216 . 
   Fastener  239  has a threaded bore adapted to mate with the threaded end  226  of tie rod  212 . When fastener  239  is tightened, an upper surface of bottom insulator washer  218  compressively engages O-ring  210  and is urged upwardly thereagainst to create a vacuum-tight seal between the exterior of the chamber base  18  and bottom insulator washer  218 . An upper surface of fastener  239  compressively engages O-ring  238  disposed in frustoconical taper  234  to create a vacuum-tight seal between the circumference of tie rod  212  and the inner diameter of bottom insulator washer  218 . 
   Power distribution bar  198  is attached to threaded end  224  of tie rod  212  by two fasteners  256 ,  258 . The top surface of bottom electrode  202  engages the lower surface of substrate support  64  in close contact so as to provide electrical continuity. Therefore, electrical power applied to the power distribution power  198  is transferred via tie rod  212  to substrate support  64 , which itself functions as a portion of the powered electrode. Bottom electrode  202  and substrate support  64  are preferably formed of an electrically-conductive material, such as aluminum. In an alternative embodiment, bottom electrode  202  may be composed of a ceramic such that substrate support  64  alone constitutes the powered electrode. 
   Vacuum baffle  180 , described in detail above, also functions as a plasma shield that reduces the RF field strength between the underside of bottom electrode  202  and chamber base  18 . As a result, the plasma will be intensified near the surface of the workpiece  56  held by substrate support  64  and the power and time to perform a plasma treatment each workpiece  56  will be minimized. Further, the configuration of powered and ground electrodes produce an electric field substantially perpendicular to a workpiece  56  residing on substrate support  64  such that ion trajectories are substantially perpendicular to the surface normal of the workpiece  56 . 
   Workpiece  56  is advantageously positioned in processing chamber  12  having a vertical position substantially in a plane half-way between the ceiling  28  of chamber lid  14  and the top surface of support platform  68 . Relative to known plasma treatment systems, minimization of the volume of chamber  12  for a high pumping rate and precise positioning of workpiece  56  permit rapid plasma processing at a reduced power level. 
   Referring to  FIG. 4 , the plasma treatment system  10  includes a gas flow control  300  and an RF generator  302  connected to the processing chamber  12 . A treatment system control  304  receives input signals from various devices within the plasma treatment system  10  and provides output signals to operate the gas flow control  300  and RF generator  302 . The control  304  is also connected to a programmable graphics user interface  306 . The interface provides user input devices, for example, pushbuttons, switches, etc., and further, has output devices, for example, lights and a display screen, thereby allowing the user to follow the status of the operation of the plasma treatment system  10  and control its operation. The control  304  may be any type of microprocessor based control having both logic and arithmetic capabilities. For example, a programmable logic controller such as Model Direct Logic  205  manufactured by Koyo and commercially available from Automation Direct of Cummings, Georgia. Further, the graphics user interface  306  is also manufactured by Koyo for the Direct Logic  205  and is also commercially available from Automation Direct. 
   Normally, during a plasma processing operation within the processing chamber  12 , a plurality of process gases are mixed within a manifold  308 . Exemplary process gases include Ar, He, CO 2 , N 2 , O 2 , CF 4 , SF 6 , H 2 , and mixtures thereof. Each process gas has an independent gas supply system  309  comprised of a gas source  310 , a mass flow controller  312 , an isolation valve  314  and a solenoid valve  315 . In the example where two gases, for example, Ar and O 2 , are used, there would be two independent gas supply systems  309   a ,  309   b  comprised of gas sources  310   a ,  310   b , mass flow controllers  312   a ,  312   b , isolation valves  314   a ,  314   b  and solenoid valves  315   a ,  315   b . As will be appreciated, any number of additional gas supplies  309   n  may be connected to the manifold  308  and each additional gas will have its own gas source  310   n , mass flow controller  312   n , isolation valve  314   n  and solenoid valve  315   n.    
   In addition to independent gas supplies, the gas flow control  300  includes vacuum pump  144 , vacuum valve  140 , solenoid valve  341  and pressure gauge  52 . The plasma treatment system  10  is highly responsive to changes in processing parameters. Therefore, pressure gauge  52  is placed in close proximity to the chamber  12  and is fluidly connected to the chamber  12  with tube  55  of an advantageously large diameter, for example, a 0.500 inch diameter tube. The gas flow control  300  further includes bleed valve  54  and its solenoid  357  for bringing the processing chamber  12  back to atmospheric pressure at the end of a plasma processing cycle. Again, to minimize the depressurization process, bleed valve  54  is normally in close proximity to the processing chamber  12  and has a relatively large fluid communication opening therewith. Thus, the bleed valve  54  has the capability of returning the processing chamber  12  to atmospheric pressure in approximately one second. 
   The RF generator  302  is comprised of an RF power supply  318  providing RF power to an L-network tuner or impedance matching device  320 , for example, a pair of variable air capacitors. RF power supply  302  operates at a frequency between about 40 kHz and about 13.56 MHz, preferably about 13.56 MHz, and a power between about 0 watts and about 300 wafts, preferably about 60 wafts to about 150 watts. RF power from the variable air capacitors  320 ,  324  is applied over an output  328  to substrate support  64  ( FIG. 3 ) within the processing chamber  12 . A phase capacitor  320  includes a movable plate connected to a motor  321  and further has a phase control  322  that provides an analog feedback signal on an input  323  of the control  304 . A magnitude capacitor  324  has a movable plate connected to a motor  325  and further has a phase control  326  that provides an analog feedback signal on an input  327  of the control  304 . The control  304  utilizes a known PID control loop to provide analog command signals on outputs  328 ,  329  to the respective motors  321 ,  325  to move the plates of the variable air capacitors  320 ,  324  in a known manner. 
   The PID control loop of the present invention utilizes a control algorithm that automatically provides a variable gain to improve performance at the boundary conditions. The magnitude of the feedback signal on the input  323  has a range of from −5 volts to +5 volts; and with a constant gain system, as the magnitude of the feedback signal moves close to and through the zero crossing, accurate and stable system control is difficult. Traditionally, the gain is set to a fixed value that is a compromise between that needed to handle lower signal levels while not letting the control system saturate at higher signal levels. The result is a generally compromised or lower level of system responsiveness and performance, that is, the time required for the control system to stabilize is longer. The present invention continuously recalculates, and dynamically sets, a gain value as a function of the signal strength of the feedback signal on the input  323 . Thus, the PID loop is critically damped, that is, it reaches a stable state quickly with a minimum of overshoot. In other respects, the tuning network  320  functions in a known manner to match an impedance of an RF system comprised of an RF output of the RF power supply  318 , the tuning network  320  and the RF load presented by the RF circuit within the processing chamber  12  to a desired impedance value, for example, 50 ohms. 
   As will be appreciated, various limit or proximity switches  330  are utilized in association with the operation of the processing chamber  12 . For example, limit switches are utilized to detect the respective opened and closed positions of chamber lid  14  ( FIG. 1 ) of the processing chamber  12  and provide a state feedback signal on a respective input  331  of the control  304 . Those limit switches may be connected to the lid actuator  122  ( FIG. 2C ) operating the lid  14 , may be mounted on the lid  14 , or otherwise detect the position of the lid  14 . A proximity switch is also used to detect the desired position of a workpiece  56  within the processing chamber  12 . There are many different commercially available limit switch devices that utilize magnetism, mechanical contact, light, etc., to detect the proximity or position of an object. The choice of a particular type of commercially available limit switch is dependent on the application and preference of the designer. 
   An end point of a plasma processing cycle may be determined in several ways. The plasma treatment system of the present invention has a very high level of control; and therefore, the plasma processing cycle is highly repeatable. Hence, with the plasma treatment system of the present invention, the control  304  normally utilizes an internal timer to measure the duration of the plasma processing cycle. In some applications, an end point detector  334  is operatively connected with the processing chamber  12 . The end point detector  334  is normally a photoelectric switch that changes state in response to detecting a desired and particular wavelength of the light of the plasma generated within the processing chamber  12 . Visual communication between the end point detector  334  and the interior of the processing chamber  12  may be achieved by directing the end point detector  334  through the viewport  34  ( FIG. 1 ) or mounting the end point detector  334  within an opening or hole (not shown) in a wall of the processing chamber  12 . Creation of the gas plasma within the processing chamber  12  produces light. Further, the wavelength of that light changes with the composition of the different materials within the gas plasma in the chamber  12 . For example, with an etching process, as the gas plasma etches different materials from the surface of the workpiece, the wavelength of the light created by the plasma will be a function of a combination of the gas plasma and atoms of those materials. After any coatings and impurities have been etched from the surface, continued etching will result in a combination of atoms of the native material of the workpiece and the gas plasma. That combination produces a unique wavelength of light which is detected by the end point detector  334 , and the detector  334  provides a binary feedback signal on an output  336  back to the control  304 . Thus the control  304  is able to detect when the plasma processing cycle is completed when that feedback signal changes state. 
     FIG. 5  is a flowchart illustrating the operation of the control  304  in implementing a typical plasma processing cycle. At  602 , a part transfer cycle is initiated. During that process, the control  304  provides command signals to a controller (not shown) that causes the positioning lever  94  to move an unprocessed workpiece  56  into the chamber  12  between the side rails  78   a,b . As the part  56  is moved into position, one of the limit switches  330  detects the loaded position of the part and provides a state feedback on a respective output  331  to the control  304 . Upon the control, at  604 , detecting a change in the switch state indicating that the part is loaded, the control  304  provides a command signal on an output  337  to open a solenoid valve  338 . The open solenoid  338  directs pressurized air from a pneumatic source, for example, shop air,  340  to the lid actuator  122  in a direction causing the lid actuator  122  to move the lid  14  to its closed position. One of the limit switches  330  detects the closed position, changes state and provides a state feedback signal on a respective input  331  to the control  304 . 
   Upon detecting the lid closed position, at  608 , the control  304  then, at  610 , provides a signal over an output  342  commanding the solenoid  341  to open the vacuum valve  140 . Simultaneously, at  612 , the control  304  establishes a pressure set point equal to PR PROCESS  and initiates operation of a process pressure monitor. Normally, in a plasma treatment system, the chamber  12  is evacuated to a desired and fixed partial vacuum pressure prior to the start of a plasma processing cycle. However, the initial evacuation of the chamber  12  is a time consuming process. Applicants discovered that high quality plasma processing can be undertaken within a range of pressures above and below a normally used processing pressure within the chamber  12 . The permissible pressure range has been determined by processing many parts under different conditions within the chamber  12 . Thus, with the plasma treatment system of the present invention an upper pressure boundary limit, for example, 250 mTorr, is determined by adding an offset pressure, for example, 50 mTorr, to the normally used processing pressure, for example, 200 mTorr. Further, a lower pressure boundary limit, for example, 150 mTorr, is determined by subtracting the offset pressure, for example, 50 mTorr, from the normally used processing pressure, for example, 200 mTorr. In this example, the pressure monitor system establishes the normally used processing pressure of 200 mTorr as the pressure set point, but the pressure monitoring system will not set an alarm or otherwise impact the operation of the plasma treatment process as long as the pressure remains between the upper and lower boundary limits of 250 mtorr and 150 mtorr, respectively. Therefore, as long as the vacuum pump  144  is running, the control  304  is monitoring the input  348  which is providing a pressure feedback signal from the pressure gauge  52 . When the control  304  detects that the chamber  12  is evacuated to 250 mtorr, the gas plasma is started. 
   Simultaneously with starting the pressure monitor at  612 , the control  304 , at  614 , provides command signals over the outputs  344 ,  346  to operate respective mass flow controllers  312  and isolation valves  314 . Process gas is introduced through process gas inlet port  190  at a predetermined flow rate, such as 5–100 standard cubic centimeters per minute (“sccm”) for Ar. The flow rate of gas provided by the mass flow controllers  312  and the pumping rate of the vacuum pump  144  are adjusted to provide a processing pressure suitable for plasma generation so that subsequent plasma processing may be sustained. Processing pressures within the chamber  12  are typically on the order of 50 to 1000 mTorr and preferably in the range of 125 to 250 mTorr. In contrast to prior systems, the processing chamber  12  is continuously evacuated simultaneously with the introduction of the process gases which are initially used to purge ambient air from the chamber  12 . In one embodiment, the mass flow controllers  312  are operated to provide a flow rate of 30 sccm to the processing chamber which has a volume of approximately 0.50 liters. Thus, fresh gases are exchanged within the processing chamber  12  approximately four times per second. More traditional plasma treatment systems exchange the gas in the processing chamber approximately once every five seconds. The higher gas flow rate of the system of the present invention improves the removal of etched materials and other contaminants from the processing chamber and also minimizes the deposition of etched materials on the walls and tooling within the chamber  12 . 
   The control  304  continuously monitors the feedback signal on the input  348  from the pressure gauge  54  which is continuously measuring the pressure or partial vacuum within the processing chamber  12 . At  616 , the control  304  detects when the pressure in the processing chamber  12  is equal to an initial pressure, that is, the normally used processing pressure plus the offset pressure value, which, in the example above is 250 mTorr. The control then, at  618 , provides a command signal on an output  350  to turn on the RF power supply  318 . However, instead of providing full power from the RF power supply  318 , the control  304  commands the RF power supply to supply only a minimum power level, for example, 30 watts. Traditional plasma treatment systems initially apply full power to the processing chamber  12  via the tuning network  320 . Creating the gas plasma at full power often results in plasma spikes, electric arcs, energy hot spots, other anomalies and a very unstable gas plasma. Further, since changes in the gas plasma result in a different RF load in the processing chamber  12 , the unstable gas plasma makes it very difficult for the tuning network  320  to match the impedance of the RF system to a desired value. Consequently, by initially creating the gas plasma at full RF power, a substantial amount of time is consumed waiting for the plasma to stabilize within the processing chamber  12  and thereafter, operating the tuning network  320  until the desired impedance match is established. With the plasma treatment system of the present invention, initially applying a lower or minimum level of power, for example, 30 watts, to the system permits the plasma in the chamber  12  to stabilize very quickly when compared to traditional systems. 
   After turning on the RF power supply  318  to the minimum power level, the control  304 , at  620 , executes a 200 millisecond delay. This delay period permits the plasma at the minimum power level to stabilize. Thereafter, at  622 , the control  304  initiates the operation of an automatic tuning cycle or autotune control by which the variable air capacitors are used to match the RF impedance of the output of the power supply  318  and the RF impedance of the input of the processing chamber  12  to a desired impedance, for example, 50 ohms. During that process, analog feedback signals from the phase magnitude controls  322 ,  326  are provided on respective inputs  323 ,  329  of the control  304 . The control executes a PID control loop and provides command signals on the outputs  328 ,  329  to operate the respective motors  321 ,  325  such that the variable air capacitors  320 ,  324  provide the desired impedance match. 
   The control then, at  624 , determines whether the tuning network  320  has achieved the desired impedance match. When that occurs, the control  304 , at  626 , begins to ramp the power from its minimum level to a maximum level; and as the power is increased, the control, at  628 , continues to operate the tuning network  320  with each successive power level. Thus, as the control moves from its minimum power level to the maximum power level, the variable air capacitor  320  is continuously adjusted so that the impedance presented to the RF power supply  318  remains matched to the desired 50 ohm load. Applicants have discovered that by maintaining the impedance match while ramping the RF power up to the maximum level, a stabilized gas plasma is achieved at full power in less time than if the RF power supply  318  were initially turned on to its maximum power level and the impedance matching operation executed. 
   It should be noted that as the power is ramping up to its maximum level, the process gases are flowing through the processing chamber  12  at their desired flow rates and the vacuum pump  144  is continuing to depressurize the processing chamber. As previously described, a range of operating pressure was determined by processing many workpieces using different process parameters. Using similar empirical methods, the maximum rate at which RF power can be increased while maintaining a tuned RF system was also determined; and that maximum rate of RF power increase provides a reduced plasma treatment cycle. 
   If the control  304 , at  630 , determines the RF power is not at its maximum level, the control, at  628 , again increments the power level and operates the tuning network  320  to match the impedance to the desired value. If, at  630 , the control  304  determines that the power is now at its maximum value, the control then, at  632 , begins monitoring for an endpoint of the plasma treatment cycle while the power remains at its maximum value and the plasma treatment process continues. During a plasma treatment operation, contaminant species sputtered from the surface of workpiece  56  will be evacuated from processing space  102  via exhaust port  136  along with the flowing stream of process gas. Plasma treatment system  10  is optimized to enhance both the spatial uniformity of plasma treatment and system throughput. 
   The control  304 , at  634 , checks the state of the feedback signal on the input  352  from the end point detector  334  to determine whether the plasma processing cycle is complete. In the described embodiment, the endpoint of the processing cycle is determined by the endpoint detector  334  detecting a particular wavelength of light of the plasma and providing a signal representing such to the control  304 . As will be appreciated, by processing a large number of workpieces using different processing parameters, the amount of time required to process a workpiece can be determined. In an alternative embodiment, the control  304  can start an internal timer at the same time that the autotune control is started at  622 . The timer is set to the amount of time required to process a workpiece as was empirically determined. Therefore, when the internal timer expires indicating an end of the plasma processing cycle, the control at  304  detects the expiration of the timer as the endpoint of the plasma treatment cycle. 
   Upon the control, at  634 , detecting a state of the end point feedback signal on the input  352  representing an end of the plasma treatment cycle, the control  304 , at  636 , provides a command signal on its output  350  to cause the RF power supply  318  to decrement or ramp down the RF power from its maximum level to its minimum level. Normally, the power is ramped down from its maximum level to its minimum level at the same rate and thus, over an identical time period, as is required to ramp the power up from its minimum level to its maximum level. Upon the control  304  detecting, at  638 , that the RF power supply  318  is providing power at the minimum level, the control  304  then, at  640 , the control  304  checks that the RF system is tuned at the minimum power level. Thereafter, at  642 , the control  304  turns off the autotune control and executes a 200 millisecond delay which permits the plasma at the minimum power level to stabilize. 
   Traditional plasma processing cycles simply turn the RF generator off at the end of a processing cycle, and the tuning network is in a state corresponding to a processing power output from the RF power supply. Hence, when the next cycle is started, which may be at a different power level, some time is required to for the tuning network  320  to match the impedance. In contrast, with the present invention, at the end of a cycle, the tuning network is tuned to minimum power. Thus, at the start of the next processing cycle, when the RF power supply  318  is turned on to minimum power, the tuning network  320  is in a state such that, either, the desired impedance match already exists, or it can be quickly tuned to a match. Minimizing tuning of the RF system can result in cycle time savings of up to 15 seconds. 
   Next, the control  304 , at  644 , stops the operation of the pressure monitor and provides command signals on the outputs  342  and  346  to cause respective solenoid valves  341  and  315  to close the respective vacuum valve  140  and isolation valves  314 . Further, the control  304  provides a command signal on output  344  to terminate the flowrate of gases through the appropriate mass flow controllers  312 . In addition, the control  304  provides a command signal over an output  356  to cause solenoid valve  357  to open the bleed valve  54 , thereby depressurizing the processing chamber  12 . At  646 , the control  304  determines that the pressure within the processing chamber  12  is substantially equal to atmospheric pressure. This determination is normally made by the control using an internal timer to measure a period of time required to depressurize the processing chamber  12  with the bleed valve  54 . Thereafter, at  648 , the control  304  provides a command signal on the output  337  causing the solenoid valve  338  to change state and reverse the operation of the lid actuator  122 . Thereafter, at  650 , the control  304  detects that the lid  14  is raised to its opened position and initiates a successive part transfer cycle  602 . The above process is then repeated for successive workpieces. 
     FIGS. 6 and 7  depict an alternative embodiment of the processing chamber  12  according to the principles of the present invention which includes a variable-width substrate support  260 . Support  260  advantageously permits workpieces of variable dimension to be received thereon. Referring to  FIG. 6 , substrate support  260  includes an elevated platform  262  that slideably carries two moveable opposed side rails  264 ,  266  and a flat plate  267  that is attached to bottom electrode  202  by the downward force applied by each tie rod  212 . Elevated platform  262  is mechanically and electrically attached by a plurality of fasteners  269  to flat plate  267 . As shown by arrows  268 ,  270 , side rails  264 ,  266  are moveable between an extreme position near the perimeter of support platform  262  to a central position along the longitudinal axis of elevated platform  262 . As a result, the separation distance between sides rails  264 ,  266  may be varied to accommodate a workpiece  272  of a predetermined transverse width. 
   Side rail  264  and side rail  266  are identical structures that will be described with reference to side rail  266 . Referring to  FIG. 7 , side rail  266  comprises a horizontal member  274  flanked at each opposed end by an integral vertical post  276 . A channel  278  extends longitudinally along the entire length of horizontal member  274  and has a U-shaped cross-section with a predetermined width that accepts a peripheral edge of workpiece  272 . Each opposed end of channel  278  includes a flared lip  280  that facilitates slideable capture of side edges of the workpiece  272 . 
   Each vertical post  276  includes an upper prong  282  with a threaded bore  284  for receiving a set screw  286  and a beveled lower prong  288 . The lower surface of upper  282  prong is displaced vertically from the upper surface of lower prong  288  to create an indentation  290  of a width that is slightly less than the thickness of elevated platform  262 . The indentation  290  slideably receives a peripheral edge of elevated platform  262 . Accordingly, each side rail  264 ,  266  may be independently moved to a predetermined transverse position and affixed with set screw  286 . 
   The plasma treatment system described herein provides an exceptionally efficient, high quality and repeatable plasma treatment process. First, by turning the RF system off and on a lesser power, the plasma is started and stabilized in a shorter time than if the plasma were started at full RF power. In addition, by turning the system off and on at the lesser power, very little time, for example, one second, is used to initially tune the RF system. In traditional systems, up to fifteen seconds may be required to initially tune the system. Further, the plasma treatment system of the present invention initiates a flow of process gases at an upper pressure boundary well before the processing chamber has been evacuated to a normal processing pressure. Once again, processing cycle time is minimized with no compromise of plasma treatment quality. In addition, the plasma treatment system of the present invention allows the pressure in the processing chamber to vary over wide pressure limits without interruption to the treatment process or a loss of quality. The plasma treatment system described herein continuously operates the vacuum pump with the flow of process gases throughout the plasma treatment cycle such that the processing pressure is maintained but process gases flows are up to an order of magnitude greater than in known systems. Such flows minimize impurities and the deposition of material on components inside the processing chamber. 
   The plasma treatment system of the present invention includes a vacuum distribution baffle positioned between the workpiece and vacuum port that provides uniformly, symmetric flow lines of process gas across the surface of a workpiece undergoing plasma treatment. The substantially uniform delivery of process gas enhances the uniformity of the plasma density. The vacuum distribution baffle, if comprised of a ceramic, further restricts the portion of the processing space within the vacuum chamber in which the plasma is generated. The volume reduction advantageously reduces the plasma excitation power required to initiate and maintain a plasma and focuses the power on the workpiece. Further, the plasma treatment system of the present invention includes a gas distribution system that distributes process gases through an array of apertures, which are symmetrically positioned in a facing relationship to a surface of the workpiece. The uniformity of gas delivery enhances the uniformity of the plasma density. The plasma treatment system of the present invention has a workpiece holding portion that positions the workpiece equidistantly and symmetrically between the powered and ground electrodes. Such positioning enhances the uniformity of the plasma density and constrains the electric field lines to be substantially perpendicular to the surface of the workpiece. In addition, the plasma treatment system of the present invention includes a hinge coupling with an obround bearing that confines the chamber lid, when closed, to move substantially vertical relative to the chamber base when a vacuum pressure exists within the vacuum chamber. In addition to enhancing the quality of the vacuum seal between the chamber lid and the chamber base, the lifetime of the sealing member therebetween is maximized by the absence of relative transverse movement. 
   In combination, the above features of the plasma treatment system described herein provide exceeding fast processing cycle times and greater productivity than known machines. For example, with the plasma treatment system of the present invention, plasma processing times are in a range of from approximately eight seconds to approximately thirty seconds. In contrast, with known systems, plasma processing times are in a range of from approximately two minutes to approximately 10 minutes. Further, processing quality is substantially improved. For example, with known plasma processing cycles in a high volume batch production environment, using a contact angle measuring device, surface cleanliness and/or surface activation is typically considered acceptable with a 30° contact angle. Further, known plasma systems normally produce a surface cleanliness and/or surface activation measured by contact angles of from approximately 22° to approximately 30°. With the plasma treatment system of the present invention, in a high volume, batch processing environment, a contact angle of 12° +/− 2° can be maintained across the entire area of the workpiece. Further, that high quality is over 99% repeatable. 
   While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept. The scope of the invention itself should only be defined by the appended claims, wherein