Patent Publication Number: US-8986564-B2

Title: Apparatus and methods for handling workpieces in a processing system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of application Ser. No. 12/376,175, filed Feb. 3, 2009, which is the National Stage of International Application No. PCT/US07/76205, filed Aug. 17, 2007, which claims the benefit of U.S. Provisional Application No. 60/823,175, filed Aug. 22, 2006, the disclosure of each of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to processing systems and, more particularly, to apparatus and methods for handling workpieces in a processing system. 
     BACKGROUND OF THE INVENTION 
     Processing systems, such as plasma processing tools, rely on holding mechanisms to support a workpiece, such as a semiconductor, ceramic, or metal substrate or wafer, inside a process chamber during the performance of a treatment process. Certain holding mechanisms include a plurality of lift pins that are configured to raise or lower in unison for lifting and lowering the workpiece relative to a top surface of a support. In the lowered position, the tips of the lift pins are either flush with, or slightly recessed below, the top surface of the support so that the workpiece at least partially contacts the top surface. In the raised position, the tips of the lift pins contact a bottom surface (backside) of the workpiece and elevate the workpiece above the top surface of the support. Typically, multiple lift pins establish multiple points of contact with the backside of the workpiece. The resulting gap between the lifted workpiece and the surface of the support permits an access space for insertion of an end effector. 
     The requirement for lift pins in conventional processing systems necessitates one or more mechanical feedthroughs in the process chamber for transferring mechanical motion from a location outside of the process chamber to the lift pins. Each mechanical feedthrough requires at least one port extending through the chamber wall of the process chamber. Each port provides a prime location for vacuum leaks. Moreover, contact between the lift pins and the workpiece may damage or contaminate the backside of the workpiece. Further, the process of raising and lowering the lift pins may generate particles that contaminate the process chamber and, if not remediated, eventually, result in contamination of the processed workpieces. 
     It would therefore be desirable to provide a workpiece vertical lift mechanism that addresses these and other deficiencies or challenges of conventional processing systems. 
     SUMMARY 
     In an embodiment of the invention, an apparatus comprises a process chamber including upper and lower electrodes. A pedestal portion of the lower electrode is configured to support the workpiece during processing. A workpiece vertical lift mechanism is disposed in the process chamber. The workpiece vertical lift mechanism includes a vertically movable member disposed between the upper and lower electrodes. The vertically movable member is configured to move vertically relative to the pedestal portion of the lower electrode between first and second positions. In the first position, the vertically movable member holds the workpiece in a non-contacting relationship with the pedestal portion. In the second position, the pedestal portion of the lower electrode projects above the first member so as to transfer the workpiece from the first member to the pedestal portion. 
     In another embodiment, a processing method includes transferring a workpiece to a vertically movable member inside a process chamber and then moving the movable member vertically toward a pedestal portion of a lower electrode so as to transfer the workpiece from the movable member to the pedestal portion of the lower electrode. The method further comprises generating a plasma inside the process chamber using the lower electrode and an upper electrode, and processing the workpiece with the plasma while the workpiece is supported on the pedestal portion of the lower electrode. 
    
    
     
       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 embodiments of the invention. 
         FIG. 1  is a perspective view of a plasma processing system including a workpiece vertical lift mechanism in accordance with an embodiment of the invention. 
         FIG. 2  is a front view of the plasma processing system of  FIG. 1 . 
         FIG. 3  is an exploded view of the enclosure and workpiece vertical lift mechanism of the plasma processing system of  FIGS. 1 and 2 . 
         FIG. 3A  is another exploded view of the workpiece vertical lift mechanism of the plasma processing system of  FIGS. 1 ,  2 , and  3 . 
         FIG. 4  is a cross-sectional view taken generally along line  4 - 4  in  FIG. 2 . 
         FIG. 5  is a cross-sectional view similar to  FIG. 4  in which the lid of the enclosure is in contact with the base of the enclosure. 
         FIG. 6  is an enlarged view of a portion of  FIG. 4 . 
         FIGS. 7-11  are perspective views similar to  FIG. 1  illustrating the workpiece loading process. 
         FIG. 12  is a perspective view showing a workpiece vertical lift mechanism in accordance with an alternative embodiment of the invention. 
         FIG. 13  is a perspective view of a workpiece vertical lift mechanism in accordance with an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1-4 , a plasma processing system  10  generally includes an enclosure  12  having a lid  14  and a base  16  upon which the lid  14  rests, support arms  18 ,  20  depending from the lid  14 , an upper electrode  22 , a lower electrode  24 , and a workpiece vertical lift mechanism  200  ( FIG. 3A ). The plasma processing system  10  further includes a tubular separating member or ring  26  positioned between the upper and lower electrodes  22 ,  24  and contacting confronting faces about the perimeter of the upper and lower electrodes  22 ,  24 . The confronting faces of the upper and lower electrodes  22 ,  24  are generally planar and parallel plates and have approximately identical surface areas. 
     The support arms  18 ,  20  mechanically couple a housing  46  of the lid  14  with a lifting device  28 , illustrated in the representative embodiment as a pneumatic cylinder, which is configured to raise and lower the lid  14  relative to the base  16  between a raised position ( FIGS. 1 ,  2 ,  4 ) and a lowered position ( FIGS. 5 ,  11 ). Alternatively, the lifting device  28  may comprise a linear slide. When the lid  14  is in its lowered position, a conducting member  64  is captured between the respective perimeters of the lid  14  and base  16 , which are metallic. The conducting member  64  supplies a good electrical contact between the lid  14  and base  16  to establish a conductive outer shell surrounding the upper and lower electrodes  22 ,  24 . 
     The upper electrode  22  is suspended from the housing  46  of the lid  14  by a plurality of electrically insulating spacers, of which spacer  42  is visible in  FIG. 4 . As a result, the upper electrode  22  moves along with the housing  46  when the lid  14  is moved by the lifting device between the raised and lowered positions relative to the base  16 . When the lid  14  is lowered into contact with the base  16  as shown in  FIG. 5 , a sealing member  52  is compressed between the separating ring  26  and a perimeter of the lower electrode  24  to define a process chamber  40  ( FIG. 5 ). The process chamber  40  includes the volume or space bounded by the inwardly-facing horizontal surfaces of the upper and lower electrodes  22 ,  24  and the inwardly-facing vertical surface of the sidewall defined by the separating ring  26 . 
     In the raised position, unprocessed workpieces  55  can be transferred to the workpiece vertical lift mechanism  200  and processed workpieces  55  can be removed from the workpiece vertical lift mechanism  200 . The workpiece vertical lift mechanism  200  is operative to facilitate the loading of workpieces  55  onto a pedestal portion  286  of the lower electrode  24  and the unloading of workpieces  55  from the pedestal portion  286  of the lower electrode  24 . In the lowered position of the lid  14  and upper electrode  22 , the process chamber  40  defines an environment that is suitable for plasma processing one of the workpieces  55 . 
     For in-line applications, the plasma processing system  10  may be provided with an input carrier (not shown) that provides unprocessed workpieces  55 , an output carrier (not shown) that receives processed workpieces  55 , and an end effector  280  ( FIG. 7 ) on an articulated arm of a robot (not shown). The robot includes a series of actuators (not shown) that permit controlled, multi-axis articulated motion of the articulated arm and end effector  280 . The end effector  280  is manipulated by the robot for transferring workpieces  55  from the input carrier to the process chamber  40  and from the process chamber  40  to the output carrier. In addition, a plurality of workpieces  55  may be introduced in such a way that each individual workpiece  55  is independently introduced into the plasma processing system  10  or in such a way that multiple workpieces  55  are currently introduced into the plasma processing system  10 . Individual workpieces  55  may also be positioned on a support or carrier and transported thereon into the plasma processing system  10 . The plasma processing system  10  may comprise a single process station among multiple process stations that cooperate to sequentially process multiple workpieces  55  moving in an assembly line fashion among the multiple process stations. 
     A power supply  30 , which is coupled with the upper and lower electrodes  22 ,  24  by shielded coaxial cables or transmission lines  32 ,  34 , respectively, controls the power level and frequency of operation of the upper and lower electrodes  22 ,  24 . The power supply  30  may be an alternating current power supply operating at an extremely low frequency, such as 50 Hz and 60 Hz, at a high radio frequency, such as 40 kHz and 13.56 MHz, at a medium radio frequency, such as 1 kHz, or at a microwave frequency, such as 2.4 GHz. The power supply  30  may also operate at dual frequencies superimposed upon one another. Alternatively, the power supply  30  may be a direct current (DC) power supply in which the plasma is non-oscillating. In other alternative embodiments, power supply  30  may supply a radio frequency (RF) power component that provides a dense plasma and a DC power component that independently increases ion energy without affecting the plasma density. 
     In certain embodiments of the invention, the power supply  30  may operate at one or more radio frequencies and include an impedance matching network (not shown) that measures reflected power from the load represented by the upper and lower electrodes  22 ,  24  and plasma confined therebetween back to the power supply  30 . The impedance matching network adjusts the frequency of operation of power supply  30  to minimize the reflected power. The construction of such matching networks is understood by a person of ordinary skill in the art. For example, the impedance matching network may tune the matching network by changing the capacitance of variable capacitors within the matching network to match the impedance of the power supply  30  to the impedance of the load as the load changes. The power and voltage levels and operating frequency(ies) may vary depending upon the particular application. 
     A vacuum pump  36  continuously pumps byproduct generated by the plasma process and non-reacted process gas from the process chamber  40 , when the plasma processing system  10  is operating, through a vacuum manifold  38 . A sealing member  50  ( FIGS. 4 and 5 ) is compressed between the separating ring  26  and the upper electrode  22 . The vacuum pump  36  is operative to maintain the total pressure in the process chamber  40  at a sub-atmospheric level sufficiently low to promote plasma generation when power is applied by the operation of the upper and lower electrodes  22 ,  24 . Typical pressures suitable for plasma generation range from about twenty (20) millitorr to greater than about fifty (50) torr. The pressure within the process chamber  40  is controlled in accordance with a particular desired plasma process and primarily consists of partial pressure contributions from the process gas, which may comprise one or more individual gas species, supplied to the evacuated process chamber  40 . 
     A gas inlet plate  106  is fastened to an upper horizontal surface of the upper electrode  22 . The gas inlet plate  106  is coupled by a gas port  112  and a delivery line  113  with a process gas supply  114 . A mass flow controller and a flow measurement device (not shown) may be provided that cooperate to regulate the flow of each process gas from the process gas supply  114  to the gas port  112 . The gas inlet plate  106  includes distribution passages (not shown) and the upper electrode  22  includes passages (not shown) coupled with the distribution passages of the gas inlet plate  106 . The passages in the upper electrode  22  communicate with the process chamber  40  for injecting process gas into the process chamber. 
     The plasma processing system  10  includes a microprocessor-based controller (not shown) that is programmed to control the operation of, among other components, the power supply  30 , the vacuum pump  36 , and the process gas supply  114 . For example, the controller regulates the power levels, voltages, currents and frequencies of the power supply  30  and orchestrates the provision of process gas from process gas supply  114  and the pumping rate of vacuum pump  36  to define a suitable pressure in process chamber  40  in accordance with the particular plasma process and application. 
     During processing of workpiece  55 , the power applied between the upper and lower electrodes  22 ,  24  by power supply  30  produces an electromagnetic field in the process chamber  40  defined between the upper and lower electrodes  22 ,  24  when the lid  14  and base  16  are contacting and an environment suitable for plasma processing is provided. The electromagnetic field excites the process gas present in the processing region to a plasma state, which is sustained by the application of power from power supply  30  for the duration of the plasma treatment. 
     A forced flow of an appropriate cooling fluid may be circulated through the air gaps between the upper and lower electrodes  22 ,  24  and the enclosure  12 , such as air gap  56 , for cooling the plasma processing system  10  and, in particular, for cooling the upper and lower electrodes  22 ,  24 . To that end, a fitting  57  ( FIG. 2 ) may be provided in the lid  14  to define a coolant port for coupling a coolant supply  59  ( FIG. 2 ) with these air gaps. 
     The upper and lower electrodes  22 ,  24  are formed from an electrically-conductive material, such as aluminum. The separating ring  26  is formed from a non-conducting dielectric material and is constructed to be able to withstand the plasma environment inside the process chamber  40  without unduly contaminating the processed workpiece  55 . Generally, this implies that the material forming the separating ring  26  should be substantially resistant to etching by the plasma present in the process chamber  40 . The separating ring  26  defines a vertical sidewall of non-conductive material, in addition to providing the vacuum seal between the upper and lower electrodes  22 ,  24 . 
     Constituent species from the plasma contact and interact with exposed material on the workpiece  55  to perform the desired surface modification. The plasma is configured to perform the desired surface modification of the workpiece  55  by selecting parameters such as the chemistry of the process gas, the pressure inside the process chamber  40 , and the amount of power and/or frequency applied to the upper and lower electrodes  22 ,  24 . The plasma processing system  10  may include an end point recognition system (not shown) that automatically recognizes when a plasma process (e.g., an etching process) has reached a predetermined end point or, alternatively, plasma processes may be timed based upon an empirically-determined process time. 
     With reference to  FIGS. 3 ,  3 A,  4 , and  5  in which like reference numerals refer to like features in  FIGS. 1 and 2 , the workpiece vertical lift mechanism  200  generally includes a lift plate  202 , a workpiece fixture  290 , a set of resiliently-biased supports  240  mechanically coupling the workpiece fixture  290  with the lower electrode  24 , and a set of resiliently-biased push devices  258  projecting from the upper electrode  22  toward the lower electrode  24  and the workpiece fixture  290 . An outer peripheral edge or perimeter  274  of the workpiece fixture  290 , which is positioned between the upper and lower electrodes  22 ,  24 , is encircled by the separating ring  26 . After lid  14  is placed in the lowered position contacting the base  16  to seal the process chamber  40  from the ambient environment and the process chamber  40  is evacuated, the workpiece vertical lift mechanism  200  resides within the evacuated process chamber  40 . 
     The workpiece fixture  290  includes a lift plate  202  and a workpiece ring  204  that are joined, for example, by a pin-in-socket type engagement in which one of the lift plate  202  or workpiece ring  204  carries a set of projecting pins (not shown) and the other of the lift plate  202  or workpiece ring  204  carries a set of sockets (not shown) that register and mate with the pins. The workpiece fixture  290  is automatically moveable in conjunction with opening and closing the lid  14  and without operator intervention between a raised position, when the lid  14  is opened, as best shown in  FIG. 4 , and a lowered position when the lid  14  is in a closed position relative to the base  16 , as best shown in  FIG. 5 . In other words, the workpiece fixture  290  moves toward the lowered position as the upper electrode  22  is moved by the lid  14  toward the lower electrode  24  to seal the process chamber  40  and moves toward the raised position as the upper electrode  22  is moved by the lid  14  away from the lower electrode  24 . 
     As best shown in  FIGS. 3 and 3A , a cover plate  206  includes a cap  208  and a support  210  that underlies the cap  208 . The cap  208  may also be joined with the support  210  by a pin-in-socket type engagement or, alternatively, the cap  208  and support  210  may constitute an integral, one-piece component. The cover plate  206  has a good electrical contact with the lower electrode  24 , as does the workpiece ring  204  and lift plate  202 , when the lid  14  is lowered. As a result, the workpiece fixture  290  and the workpiece  55  are at approximately the same electrical potential as the lower electrode  24  when the plasma processing system  10  is operating to generate a plasma inside the process chamber  40  and to process workpieces  55  inside the process chamber  40  with the plasma. 
     A recess  212  is located near each of the corners of the lower electrode  24 . Each recess  212  has a base  211  that represents a relatively thin wall of the material of lower electrode  24  remaining after the respective recess  212  is formed or machined in the lower electrode  24 . Projecting from the base  211  of each of the recesses  212  is a mounting post  214  with an internally threaded opening  216 . Each mounting post  214  may be positioned to be substantially coaxial with the respective one of the recesses  212 . In the assembly forming the support  240 , a threaded tip  218  of a guide pin  220  is mated with the internally threaded opening  216  of each mounting post  214 . The internally threaded opening  216  of each mounting post  214  is oriented such that the respective guide pin  220  projects in a direction toward the lift plate  202 . 
     Each of the recesses  212  is also bounded peripherally by a substantially cylindrical sidewall  222  extending to the base  211  and a beveled or flared rim  224  disposed between sidewall  222  and a top surface  226  of the lower electrode  24 . The diameter of the flared rim  224 , which intersects the top surface  226 , is greater than the diameter of the sidewall  222  of each recess  212 . 
     Each guide pin  220  includes a substantially cylindrical, non-threaded shank  228  extending from the threaded tip  218  toward a head  230 . The head  230  may include a recessed feature  232  that receives the tip of a tool (not shown) used to generate the mated engagement between the threaded tip  218  of guide pin  220  and the internally-threaded opening  216 . The head  230  of each guide pin  220 , which projects at least partially above the nearby top surface  226  of the lower electrode  24 , carries a flared surface  225  located near the non-threaded shank  228 . The non-threaded shank  228  of each guide pin  220  and the sidewall  222  of the respective recess  212  have a substantially coaxial arrangement. 
     Each of the supports  240  includes a stop block  242  coupled by a respective one of the guide pins  220  with the lift plate  202  of the workpiece fixture  290 . Each stop block  242  includes a body  244  with an enlarged head  246  and a central bore or passageway  248  extending the length of the body  244 . The radially outward projection of enlarged head  246  relative to the body  244  defines an edge or lip  250 , which extends circumferentially about the body  244 . The enlarged head  246  of each stop block  242  further includes a first beveled or tapered exterior sidewall  252  that decreases in diameter with increasing distance from the lip  250  and a second beveled or tapered exterior sidewall  234  that increases in diameter with increasing distance from the lip  250 . The exterior sidewall  234  is disposed between the lip  250  and the tapered exterior sidewall  252 . The passageway  248  includes a substantially cylindrical surface  236  and a beveled or tapered surface  238  that narrows a portion of the substantially cylindrical surface  236 . 
     A flared recess  254  is defined near each of the peripheral corners of the lift plate  202 . The tapered exterior sidewall  252  of each stop block  242  is engaged with a respective one of the flared recesses  254 . The depth of each flared recess  254  is selected such that a respective inclined surface  256  of the flared recess  254  and tapered exterior sidewall  252  of each stop block  242  are contacting when the lift plate  202  is secured with the stop blocks  242 . The inclination angles of each flared recess  254  and the corresponding tapered exterior sidewall  252  of its stop block  242  are matched to assist in securing the stop blocks  242  with the lift plate  202 , yet permit ready removability of the lift plate  202  by a vertical force of sufficient magnitude. 
     When mounted to the lift plate  202 , the tapered surface  238  of passageway  248  in stop block  242  is located generally between one of the recesses  212  in the lower electrode  24  and the workpiece fixture  290 . Disposed in each of the recesses  212  is a spring element  260 , which may have the form of a compression spring formed from a helical coil of wire. Each spring element  260  is confined within the respective recess  212  and is captured between the base  211  and the lip  250  on the respective stop block  242 . 
     As best shown in  FIG. 6 , the spring elements  260  are extended when the workpiece fixture  290  is in the raised position. As a result, the lift plate  202  and workpiece ring  204  of the workpiece fixture  290  are supported in a resiliently floating manner atop the supports  240 . Under the load supplied by the lift plate  202  and workpiece ring  204 , the spring elements  260  collectively have a spring force sufficient to suspend or elevate the lift plate  202  above the top surface  226  of lower electrode  24 . 
     The tapered surface  238  contacts the flared surface  225  on the head  230  of guide pin  220  to provide a positive stop for vertical motion when the workpiece fixture  290  is in the raised position. The inclination angles of the flared surface  225  and the tapered surface  238  are matched so that each stop block  242  is self-centered on the respective guide pin  220  when the workpiece fixture  290  is in the raised position. This permits the workpiece fixture  290  to return to a reproducible spatial location when residing in the raised position. In turn, this provides a reproducible location within plasma processing system  10  for the workpiece  55  carried by the workpiece fixture  290 . 
     As explained in detail below, movement of the lid  14  to a lowered position ( FIG. 5 ) moves the workpiece fixture  290  toward a lowered position and, thereby, compresses the spring elements  260 . As the workpiece fixture  290  is lowered, the head  230  of each of the guide pins  220  moves in its respective passageway  248  toward the lift plate  202 . 
     As best shown in  FIGS. 3 and 3A , the workpiece fixture  290  includes a central opening  270  extending entirely through the lift plate  202  and workpiece ring  204 , and a gap  272  that extends radially from the central opening  270  to the outer perimeter  274  of the workpiece fixture  290 . The cover plate  206  is dimensioned with a width substantially identical to the width of the gap  272 . When the workpiece fixture  290  is lowered to a process position, the cover plate  206  fills the gap  272  so that the central opening  270  is surrounded by a substantially planar surface defined collectively by the top surface  266  of the workpiece ring  204  and a top surface  276  of the cover plate  206 . To promote the requisite coplanar arrangement, the respective thicknesses of the cover plate  206  and workpiece fixture  290  are selected to be approximately equal, which permits the top surfaces  266 ,  276  to be approximately flush when the workpiece fixture  290  is in its lowered position. The central opening  270  is round in the representative embodiment. However, the central opening  270  may have other shapes, such as rectangular. 
     A shoulder or rim  278 , which is defined in the workpiece ring  204 , coaxially encircles the central opening  270 . The radial dimension or width of the rim  278  is selected such that only a thin annular surface area on the workpiece  55  is contacted by the rim  278 . For example, the radial dimension of the rim  278  may be approximately equal to 3 mm. The diameter of the central opening  270  is approximately equal to the diameter of the workpiece  55  less the radial dimension of the rim  278 . If the workpieces  55  are not round, the geometrical shape of the central opening  270  is selected to conform to the shape of the workpiece  55 . The rim  278  is recessed below the top surface  266  of the workpiece ring  204  by an amount related to the thickness of the workpiece  55 . When workpiece  55  is resting on, and supported by, rim  278 , a top surface of the workpiece  55  is approximately coplanar with the top surface  266  of the workpiece ring  204 . 
     The width of the gap  272  is selected such that an end effector  280  ( FIG. 7 ) can pass through the gap  272  and access the central opening  270  for transferring unprocessed workpieces  55  to the workpiece fixture  290  and removing processed workpieces  55  from the workpiece fixture  290 . The end effector  280  is coupled with a robot, such as a selective compliant articulated/assembly robot arm (SCARA) robot, as understood by a person having ordinary skill in the art. 
     The lower electrode  24  further comprises a removable electrode section  284 , which includes a mounting flange  285  situated in a recess defined in the lower electrode  24  and the pedestal portion  286 . The pedestal portion  286 , which defines representative a workpiece support, projects from the mounting flange  285  toward the upper electrode  22 . The electrode section  284  is secured with conventional fasteners to the underlying and surrounding remainder of the lower electrode  24 . The top surface  226  of lower electrode  24  and the top surface  226  of the mounting flange  285  are approximately flush. The surface area of a top surface  288  of the pedestal portion  286 , which is elevated above the surrounding mounting flange  285 , is approximately equal to the open cross-sectional area radially inside the central opening  270 . The diameter of the pedestal portion  286  is approximately equal to the diameter of the central opening  270  of workpiece ring  204 . The electrode section  284  has a good electrical contact with the remainder of the lower electrode  24  so that the pedestal portion  286  and support  210  are at substantially the same potential as the lower electrode  24  when the plasma processing system  10  is operating and a plasma is present in the process chamber  40 . 
     The cover plate  206  comprises another raised region of the electrode section  284  that projects above the plane of the mounting flange  285 . The cover plate  206  and pedestal portion  286  may comprise a single or unitary raised region projecting from the mounting flange  285 . Alternatively, the cover plate  206  may comprise a separate component that is mounted to the electrode section  284  and, in this instance, may include locating pins (not shown) or the like used to automatically position the cover plate  206  relative to the central opening  270  in the workpiece fixture  290 . 
     When the workpiece fixture  290  is lowered to a process position, contact between the workpiece  55  and the top surface  288  of pedestal portion  286  transfers the workpiece  55  from the workpiece ring  204  to the pedestal portion  286 . The transfer of the workpiece  55  is accomplished without any structure on the pedestal portion  286 , the lower electrode  24 , or the base  16  of the enclosure  12  guiding of the workpiece  55  onto the pedestal portion  286 . In the lowered process position of the workpiece fixture  290 , the top surface  266  of workpiece ring  204  is recessed slightly below the top surface  288  of the pedestal portion  286 . During plasma treatment, the workpiece  55  rests on the top surface  288  of the pedestal portion  286 . 
     The electrode section  284  and the lift plate  202  are constructed from an electrical conductor, such as aluminum. The cap  208  on the cover plate  206  and the workpiece ring  204  are constructed from an electrical insulator or dielectric, such as alumina or high-purity alumina. Alternatively, the cap  208  on the cover plate  206  and the workpiece ring  204  may also be constructed from an electrical conductor, such as aluminum. The selection of a constituent material for the cap  208  of the cover plate  206  and the workpiece ring  204  is dictated by the type of plasma performance required in the plasma processing system  10  for a particular plasma process on workpiece  55 . Although not wishing to be bound by theory, it is believed that constructing the cap  208  of the cover plate  206  and workpiece ring  204  from an electrical conductor optimizes etch rate driven plasma processes or treatments and that constructing the cap  208  of the cover plate  206  and workpiece ring  204  from a dielectric optimizes uniformity driven plasma processes. 
     With reference to  FIGS. 3A and 4 , one of the push devices  258  is located spatially near each inside corner  15  of separating ring  26  and, as apparent, near each corresponding outside corner (not shown) of the upper electrode  22 . Each of the push devices  258  includes a pusher block  262 , which is secured with the upper electrode  22  by the cooperation between an insert  261  and a shoulder bolt  263 , and a spring element  264 . Each of the pusher blocks  262  has a substantially overlying relationship with a respective one of the stop blocks  242 . One end of the spring element  264 , which may have the form of a compression spring formed from a helical coil of wire, is captured between an enlarged head  265  of the pusher block  262  and the upper electrode  22 . The pusher block  262  is constructed from an insulating or dielectric material, such as a ceramic, and the insert  261  and shoulder bolt  263  may be formed from a metal, such as a stainless steel. The shoulder bolt  263  has a threaded tip that is fastened in a threaded bolthole in the upper electrode  22 . The pusher block  262  of each push device  258  is movable relative to the shoulder bolt  263  between a first position ( FIG. 4 ) in which the spring element  264  is extended and a second position ( FIG. 5 ) in which the spring element  264  is compressed. The spring element  264  supplies a preloaded bias to each pusher block  262  in the first position. 
     As the lid  14  is moved toward the base  16 , the pusher block  262  of each of the push devices  258  contacts the top surface  266  of workpiece ring  204  and the spring elements  264  begin to compress. As the lid  14  approaches the base  16 , the spring elements  264  are further compressed, which applies an increasing force to the workpiece ring  204  that causes the workpiece fixture  290  to move toward the top surface  288  of the pedestal portion  286  and toward the lower electrode  24 . When the workpiece fixture  290  is in the lowered position, the tapered exterior sidewall  234  on each stop block  242  contacts the flared rim  224  of recess  212  and each pusher block  262  is moved to its second position. 
     The inclination angles of the flared rim  224  and tapered exterior sidewall  234  are approximately equal or matched. When the workpiece fixture  290  is in the lowered position, each of the flared rims  224  is in contact with the respective one of the exterior sidewalls  234 . The contact automatically self-centers each stop block  242  within its respective recess  212 . Consequently, each time that the lid  14  is lowered, the workpiece fixture  290  returns to a reproducible spatial location relative to the lower electrode  24  and removable electrode section  284  when the lid  14  moves the workpiece fixture  290  to the lowered position. In turn, this provides a reproducible location for successive workpieces  55  on the pedestal portion  286  during each sequential plasma treatments. 
     In use and with reference to  FIGS. 1 ,  2 ,  3 ,  3 A, and  4 - 11 , the lid  14  is lifted to an open position relative to the base  16  ( FIGS. 2 ,  7 ) and placed out of contact with the base  16 . With the lid  14  lifted, the components of the workpiece vertical lift mechanism  200  have an initial arrangement as best shown in  FIG. 4 . The spring-biased supports  240  urge the workpiece fixture  290  away from the lower electrode  24  to provide the raised position. When the workpiece fixture  290  is in the raised position and contact is established between the tapered surface  225  and flared surface  238  of each spring-biased support  240 , a reproducible location is defined for the central opening  270  so that the end effector  280  can reproducibly place workpieces  55  on the workpiece fixture  290 . 
     The end effector  280  is manipulated to transport one of the workpieces  55  to a location near to the plasma processing system  10 , as shown in  FIG. 7 . The end effector  280  moves the workpiece  55  from this location, which is outside of the enclosure  12 , to a position between the upper and lower electrodes  22 ,  24  and above the central opening  270 , as shown in  FIG. 8 . When the workpiece  55  is transferred by the end effector  280  from the location outside of the enclosure into the position above the central opening  270  of the workpiece fixture  290 , the workpiece  55  is not guided by any structure associated with the enclosure  12 , such as rails or another type of mechanical constraint, associated with the base  16  of enclosure  12 . 
     The end effector  280  is lowered so that the outer periphery of the workpiece  55  contacts the rim  278  inside the central opening  270  of workpiece ring  204 , as shown in  FIG. 9 . The workpiece  55  is then fully supported on the rim  278 . After the workpiece  55  is transferred to the workpiece fixture  290 , the end effector  280  is withdrawn through the gap  272  to a location completely outside of the footprint of the lid  14  and base  16 , as shown in  FIG. 10 . The end effector  280  physically fits within the gap  272  during this sequence of motions. The outer periphery of the workpiece  55  is supported by contact with the rim  278  of central opening  270  and is suspended above the lower electrode  24 . 
     The lid  14  is lowered relative to the base  16 , as shown in  FIG. 10 , and eventually contacts the base  16  to establish the closed position, as shown in  FIG. 11 . Lowering the lid  14  moves the upper electrode  22  toward the lower electrode  24 . As the lid  14  is lowered toward the closed position, the pusher blocks  262  projecting from the upper electrode  22  contact the top surface  266  of the workpiece ring  204 . The contact causes the spring elements  264  to compress, which applies a bias force to the workpiece fixture  290 . The applied bias force from the spring elements  264  of the push devices  258  overcomes the opposing spring bias of the supports  240  and moves the workpiece fixture  290  toward the lower electrode  24 . The motion of the workpiece  55  on the workpiece fixture  290  is in a vertical direction relative to the base  16  of the enclosure  12  and toward the lower electrode  24 , and fails to include any significant motion component in a horizontal direction relative to the base  16 . 
     Each guide pin  220  has a non-contacting relationship with the respective stop block  242  as the workpiece fixture  290  moves between the raised and lowered positions. As the workpiece fixture  290  nears the fully lowered position and the spring-biased supports  240  yields toward the lower electrode  24 , the workpiece  55  is transferred from the rim  278  of workpiece ring  204  to the pedestal portion  286  of the lower electrode  24 . At the fully lowered position of the workpiece fixture  290 , the workpiece  55  rests on the pedestal portion  286  of the lower electrode  24  and a small clearance (e.g., 2 or 3 mils) exists between the rim  278  of workpiece ring  204  and the workpiece  55 . 
     The process chamber  40  is evacuated using vacuum pump  36  and process gas is introduced from the gas inlet plate  106  to establish a suitable pressure. The upper and lower electrodes  22 ,  24  are energized by power supply  30  to generate a plasma from the sub-atmospheric pressure of process gas in the process chamber  40 . When the plasma treatment of workpiece  55  is completed, the process chamber  40  is vented and the lid  14  of enclosure  12  is raised. Movement of the upper electrode  22  away from the lower electrode  24  opens the process chamber and eventually removes the contact between push devices  258  and the top surface  266  of workpiece ring  204 . As a result, the force urging the workpiece fixture  290  toward the lower electrode  24  is gradually removed. The spring elements  260  of the spring-biased supports  240  are released to expand to their uncompressed state, which automatically elevates and restores the workpiece fixture  290  to the raised position. The motion of the workpiece  55  on the workpiece fixture  290  is in a vertical direction relative to the base  16  of the enclosure  12  and toward the upper electrode  22 , and does not include any motion component in a horizontal direction relative to the base  16 . The workpiece  55  is transferred from the pedestal portion  286  of the lower electrode  24  back to the rim  278  of workpiece ring  204 . 
     As the raised position is approached, the tapered surface  225  and flared surface  238  of each spring-biased support  240  reestablish contact to again define the reproducible location for the central opening  270 . The separation between the lower electrode  24  and the central opening  270  of the workpiece fixture  290  provides an open space for the end effector  280  to gain access to the backside of each processed workpiece  55 , lift the processed workpiece  55  from the central opening  270 , and withdraw the processed workpiece  55  from the plasma processing system  10 . The end effector  280  physically fits within the gap  272  during this sequence of motions. When the workpiece  55  is retrieved by the end effector  280  from the position within the central opening  270 , the workpiece  55  is not guided by any structure associated with the enclosure  12 . 
     The workpiece vertical lift mechanism  200  maintains a parallel plate design of the plasma processing system  10  while the plasma treatment occurs. The workpiece vertical lift mechanism  200  presents a passive wafer chucking system that eliminates the need for active mechanical assistance, such as mechanically- or electrically-actuated lift pins. This simplifies the design of the plasma processing system  10  because the workpiece vertical lift mechanism  200  eliminates the need for mechanical feedthroughs, and the like, that are characteristic of active wafer chucking systems with a concomitant cost reduction. The design of workpiece vertical lift mechanism  200  readily permits modifications to handle workpieces  55  of different dimensions. The workpiece vertical lift mechanism  200  may be retrofitted to existing plasma processing systems. Device regions of the workpiece  55  are not contacted by lift pins or any other mechanical structure during workpiece exchanges. Instead, the workpiece fixture  290  permits the workpiece  55  to be lifted and lowered by contact only between rim  278  and a narrow annular band near the workpiece&#39;s peripheral edge. 
     In an alternative embodiment, the separating ring  26  may be omitted and the interior of the enclosure  12  may be otherwise configured to define a volume that is evacuated by the vacuum pump  36  to define a process chamber similar to process chamber  40 . In this instance, the upper and lower electrodes  22 ,  24  are resident in the process chamber  40 . In another alternative embodiment, the upper electrode  22  and the lid  14  may be consolidated into a unitary structure in which all or a portion of the lid  14  operates as an electrode. 
     The lift plate  202  and workpiece ring  204  of the workpiece fixture  290 , the removable electrode section  284  and its pedestal  286 , and the cover plate  206  may be collectively replaced as a component set to adapt the workpiece vertical lift mechanism  200  to handle workpieces  55  of different sizes and/or geometrical shapes. 
     As shown in  FIG. 12  in which like reference numerals refer to like features in  FIGS. 1-11  and in accordance with an alternative embodiment, a workpiece fixture  290   a  includes a lift plate  202   a  and a workpiece ring  204   a  that are similar in construction to lift plate  202  and workpiece ring  204 . The workpiece fixture  290   a  has a round central opening  270   a  and a rim  278   a  of a different diameter than the respective central opening  270  and rim  278  of workpiece fixture  290  ( FIG. 3 ) for accommodating a workpiece  55   a  with the representative disk shape and characterized by a different diameter than workpiece  55 . The replacement entails removing the assembly consisting of the lift plate  202  and workpiece ring  204  of workpiece fixture  290  from the lower electrode  24  with a lifting force of a magnitude effective to disengage the flared recesses  254  from the corresponding tapered exterior sidewall  252  of the respective stop blocks  242 . The replacement workpiece fixture  290   a  is installed by engaging the flared recesses (not shown) in the lift plate  202   a , which are similar to the flared recesses  254  in lift plate  202 , with the same supports  240 . The guide pins  220  and stop blocks  242  are capable of being universally used with each of the workpiece fixtures  290 ,  290   a . All that is required is to establish a physical connection between the supports  240  and the specific one of the workpiece fixtures  290 ,  290   a  installed in the plasma processing system  10 . 
     A new removable electrode section  284   a  is installed, as a replacement for electrode section  284 , that includes a pedestal portion  286   a  dimensioned diametrically to fit within the central opening  270   a . In the representative embodiment, the outer diameter of pedestal portion  286   a  is slightly less than the inner diameter of the round central opening  270   a . A cover plate  206   a , which is similar to cover plate  206 , is disposed in a gap  272   a , which is similar to gap  272  but slightly wider due to the larger dimensions of the central opening. The pusher blocks  262 , which remain the same, move the workpiece fixture  290   a  between the raised and lowered positions in conjunction with the opening and closing of the lid  14 . When the workpiece fixture  290   a  is in the lowered position, a top surface  266   a  of workpiece fixture  290   a  and a top surface  276   a  of cover plate  206   a  are approximately coplanar and the pedestal portion  286   a  projects slightly above the top surfaces  276   a  and  288   a . As a result, the workpiece  55   a  contacts and is supported by a top surface  288   a  of the pedestal portion  286   a.    
     Additional workpiece fixtures, cover plates, and electrode sections (not shown) may be supplied that are substantially identical to workpiece fixtures  290 ,  290   a , cover plates  206 ,  206   a , and electrode sections  284 ,  284   a , but in which these components cooperate to accommodate round, disk-shaped workpieces of different diameters. For example, a set of four different component sets may be provided for use with the plasma processing system  10  to support 100 mm, 150 mm, 200 mm and 300 mm wafers, respectively, by simply replacing the workpiece fixture, cover plate, and electrode section. 
     With reference to  FIG. 13  in which like reference numerals refer to like features in  FIGS. 1-12  and in an alternative embodiment, a compression ring  300  may be carried by the lid  14  that, when the lid  14  is closed, contacts the workpiece  55  to establish a seal encircling the outer perimeter of the workpiece  55  and press the workpiece  55  downward toward the pedestal portion  286 . The compression ring  300  includes perforations  302  for the passage of gases that permit the process chamber  40  to be evacuated by vacuum pump  36 . 
     References herein to terms such as “vertical”, “horizontal”, etc. are made by way of example, and not by way of limitation, to establish a three-dimensional frame of reference. Terms, such as “upper”, “lower”, “on”, “above”, “below”, “side” (as in “sidewall”), “higher”, “lower”, “over”, “beneath” and “under”, are defined with respect to the horizontal plane. It is understood that various other frames of reference may be employed without departing from the spirit and scope of the invention as a person of ordinary skill will appreciate that the defined frame of reference is relative as opposed to absolute. 
     While the 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 methods, and illustrative examples 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.