ION BEAM SHIELD FOR IMPLANTATION SYSTEMS

Abstract: An ion implantation system is described having an ion source (330) coupled to a process chamber (318), and a workpiece handling assembly (320) having a workpiece support (348) that is mounted within the process chamber. The system implants ions into a photoresists coated workpiece to change the conductivity of the workpiece. An ion beam shield (410) is provided that is positioned between the ion source and the workpiece support during processing to prevent outgassed photoresist from coating portions of the ion source.

Description: ION BEAM SHIJELD FOR IMPLANTATION SYSTEMS Background of the Invention The present invention relates to processing systems for processing workpieces, and more particularly, to ion implantation systems for implanting workpieces. Ion implantation has become a standard, commercially accepted technique for introducing conductivity-altering dopants into a workpiece, such as a semiconductor wafer or thin film deposition on a glass substrate, in a controlled and rapid manner. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. This beam is directed at the surface ofthe workpiece. Typically, the energetic ions ofthe ion beam penetrate into the bulk ofthe workpiece and are embedded into the crystalline lattice of the material to form a region of desired conductivity. This ion implantation process is typically performed in a high vacuum, gas-tight process chamber which encases a workpiece handling assembly, a workpiece support assembly, and the ion source. This high vacuum environment prevents dispersion ofthe ion beam by collisions with gas molecules and also minimizes the risk of contamination ofthe workpiece by airborne particulates. The process chamber is typically coupled via a valve assembly with a processing end station. The end station can include an intermediate Ioadlock chamber or pressure lock which can be pumped down from atmospheric pressure by a vacuum pumping system. The chamber is selectively closed at a downstream end by the valve assembly, which selectively places the Ioadlock chamber in fluid communication with the process chamber. The Ioadlock chamber is also coupled at an opposite end to an upstream valve assembly. The end station also includes an end effector which transfers workpieces from one or more workpiece cassettes, through the upstream valve assembly, and into the chamber. Once a workpiece has been loaded into the intermediate chamber by the end effector, the chamber is evacuated via the pumping system to a high vacuum condition compatible with the process chamber. The valve assembly at the downstream end ofthe intermediate chamber then opens and the workpiece handling assembly mounted within the process chamber removes the workpiece from the intermediate chamber and transfers the workpiece to the support assembly, which supports the workpiece during processing. For example, a loading arm of the workpiece handling assembly removes the workpiece from the intermediate chamber and places it on a platen ofthe workpiece support structure. The workpiece support then moves the workpiece in the scanning direction past the operating ion source, which implants the workpiece. Prior to implantation of the workpiece, each workpiece can be coated with a masking layer such as a photoresist layer to create a selected pattern on the face of the workpiece according to conventional photolithographic techniques. According to a conventional practice, the photoresist layer is removed in areas where ion implantation is to take place and remains as a mask over the remainder ofthe workpiece face. During the implantation process, the ion beam implants those areas ofthe workpiece surface where the photoresist has been removed to produce the desired doping characteristics. For the remaining photoresist covered regions, the ions ofthe ion beam (which form an ion shower) penetrate the photoresist and undergo collisions with electrons and nuclei ofthe photoresist material and eventually come to rest. Since the photoresist is usually made of an organic polymer, the energetic ions cleave the hydrocarbon chains ofthe polymeric material as the ions travel therethrough. Consequently, the photoresist outgasses hydrogen, water vapor and other residue from the photoresist. The outgassed residue condenses everywhere within line of sight ofthe photoresist coated surface. This residue, which is an insulating material, collects on the ion source, and specifically on the extraction electrode assembly ofthe ion source. Due to the relatively large dimensions ofthe ion beam extracted from the source, it is impractical to prevent overcoating of the electrodes. After prolonged exposure of the electrodes to the outgassed residue, it becomes necessary to dismantle the ion beam assembly in order to clean the electrodes. This accordingly results in down-time for the implantation system, thus decreasing the throughput of the ion implantation system, while concomitantly increasing the costs of operation. Hence, there exists a need in the art for improved ion implantation systems that reduce the amount of outgassed photoresist or impede the coating ofthe extraction electrode assembly by the outgassed photoresist. Summary of the Invention The present invention provides for the use of an ion beam shield to block photoresist outgassed from a coated workpiece from reaching the ion source during processing by an ion implantation system. The ion beam shield prevents the outgassed residue from the photoresist from accumulating on an electrode structure, such as the extraction electrode assembly, ofthe ion source. The shield can further include an opening that is sized to accommodate the beam (e.g., a slotted opening to pass an ion ribbon beam) generated by the ion source. According to one aspect, the ion implantation system ofthe invention includes an ion source for ionizing selected matter to generate an ion beam, and a housing defining a process chamber that is fluidly coupled to the ion source. The ion source is mounted to one side ofthe housing. A workpiece handling assembly is mounted within the process chamber and has a workpiece support surface for supporting the workpiece within the process chamber during the implantation process. According to one practice, the ion beam shield is positioned between the support surface ofthe handling assembly and the ion source for shielding the ion source from residue that is outgassed from the coated surface ofthe workpiece during implantation. According to one aspect ofthe invention, the ion beam shield is positioned between the workpiece and the electrode structure to prevent backsplatter of photoresist coated on the workpiece from collecting on the electrodes during the implantation process. The ion beam shield includes an aperture sized to allow the ion beam to proceed unobstructed to the workpiece. The aperture is in one aspect advantageously configured to allow the passing of an ion ribbon beam therethrough, and is particularly sized to accommodate a ribbon beam having an aspect ratio equal to or greater than about fifty to one. According to another aspect, the ion source and the support surface ofthe workpiece handling assembly are separated by a selected distance D during processing, and the shield is separated from the workpiece handling assembly by a distance of about D/2 or less. According to another aspect, the ion beam shield has a generally rough outer surface that effectively increases the surface area thereof to allow greater amounts of the outgassed residue to collect thereon. The roughness ofthe shield surface is preferably greater than about 5 microns, and more preferably greater than about 10 microns, although other roughness ranges can be used if increased servicing ofthe shield is acceptable. According to still another aspect, a focusing lens can be employed to focus the ion beam at the shield aperture, which can be disposed at the focal plane of the lens. In this aspect, the ion beam shield is positioned at the focal plane ofthe focused ion beam. This positioning further enhances the protection ofthe electrode structure since the size ofthe aperture in the ion beam shield can be reduced, thus minimizing the amount of residue that can escape through the aperture and to the ion source. According to yet another aspect, the workpiece handling assembly mounted within the process chamber removes the workpiece from the Ioadlock and supports the workpiece during implantation by the ion beam generated by the ion source. The process chamber is in selective fluid communication with a Ioadlock assembly, which in turn is mechanically integrated with a workpiece loading or end station. According to another aspect, the workpiece handling assembly includes a workpiece support surface or platen for translating the workpiece in a linear scanning direction during implantation, and structure for moving the position ofthe support surface relative to the Ioadlock. This linear scanning direction can be a horizontal scanning direction, or according to a preferred practice, the scanning direction is along a path transverse or orthogonal to the horizontal longitudinal axis ofthe implantation system. The scanning direction and the longitudinal axis form an angle therebetween that is less than or equal to about 85 degrees. According to another aspect, the ion beam generated by the ion source extends in a direction transverse to the longitudinal axis, and according to one embodiment, is generally parallel to the transverse scanning direction. The translation stage moves the workpiece through the ion beam in the scanning direction. In another aspect, the system further includes a pump assembly, e.g., a pressure regulator, that is coupled to the Ioadlock for disposing the Ioadlock chamber at a selected pressure. In still another aspect, the system includes a temperature control element coupled to the Ioadlock for disposing at least a portion ofthe Ioadlock or workpiece at a selected temperature. According to another aspect, the system includes structure for disposing the floor ofthe Ioadlock chamber at a selected temperature to form a cold deck. A vacuum network can also be coupled to the chamber floor for drawing the workpiece into contact therewith when the workpiece is disposed within the chamber, thereby effecting heat transfer from the workpiece to the chamber floor. The vacuum network can include a plurality of vacuum-applying openings formed win the chamber floor and in fluid communication with a vacuum source for disposing a substantial portion of the backside of the workpiece in contact with the cooling surface. According to still another aspect, the ion implantation system includes structure, such as a fluid conduit in communication with a fluid source, for circulating a cooling fluid through the chamber floor to form a cooling surface. According to yet another aspect, the Ioadlock assembly includes a plurality of loadlocks that are axially positioned relative to each other to form a stacked array of loadlocks. Furthermore, each Ioadlock ofthe array nests with an adjacent Ioadlock when stacked together. According to one practice, a bottom surface of one ofthe loadlocks has a first geometric shape and a top surface of an adjacent Ioadlock has a complementary geometric shape such that the loadlocks nest together when stacked. According to one practice, the bottom surface of each Ioadlock has a first bottom-most side, an outwardly stepped second bottom side, and a further outwardly stepped third bottom side. The top surface of each Ioadlock has a first topmost face, a recessed second face which is separated from the first face by a first wall portion, and a further recessed third top face. This stacked array of loadlocks facilitates processing of multiple workpieces by the ion implantation system. The invention will next be described in connection with certain preferred embodiments. However, it should be clear that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope ofthe invention. Brief Description of the Drawings The foregoing and other objects, features and advantages of the invention will be apparent from the following description and apparent from the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings illustrate principles ofthe invention and, although not to scale, show relative dimensions. FIG. 1 A is a perspective view of one embodiment ofthe ion implantation system according to the present invention; FIG. IB is a top view of a portion ofthe ion implantation system of FIG. IA; FIG. 2 is a perspective view of an alternate embodiment of the ion implantation system ofthe present invention; FIGS. 3A-3B are an exploded perspective view of the Ioadlock assembly ofthe present invention; FIGS. 4A-4B are an exploded view of each constituent Ioadlock ofthe Ioadlock assembly of FIGS. 3 A-3B; FIG. 4C is a cross-sectional view of a pair of nested loadlocks of the Ioadlock assembly of FIGS. 3A-3B; FIG. 5 is a cross-sectional view ofthe Ioadlock and process chamber of FIG. 1 A illustrating the sliding seal arrangement of the present invention; FIG. 6 is a cross-sectional view of an alternate embodiment of the sliding seal arrangement ofthe present invention; FIG. 7 shows a perspective view ofthe Ioadlock assembly of FIG. 1 in a serial configuration; FIG. 8 illustrates a detailed view in perspective ofthe pick up arm ofthe workpiece handling assembly of FIG. 1; FIG. 9 schematically illustrates that multiple processes can be performed upon a single panel concurrently; FIG. 10 is a top view of another embodiment of the ion implantation system according to the teachings of the present invention; FIG. 1 1 is a side perspective view ofthe ion implantation system of FIG. 10; FIG. 12 is a schematic cross-sectional view ofthe process housing and chamber ofthe ion implantation system of FIG. 10 illustrating the panel handling assembly ofthe present invention; FIG. 13 is an exemplary perspective view, partly broken away, ofthe process chamber and housing ofthe ion implantation system of FIGS. 10-12 illustrating the panel movement while processing multiple panels according to the teachings ofthe present invention; FIG. 14 is a schematic timing diagram illustrating the particular events which the ion implantation system of FIG. 10 undergoes when processing multiple panels in the stacked Ioadlock array of FIGS. 3 A-4C; FIG. 15 is a side perspective view ofthe ion implantation system of FIG. 10 illustrating the ion beam shield ofthe present invention; FIG. 16 is an exemplary perspective view illustrating the plume of outgassed residue formed from the organic material coated workpiece during the implantation process, as well as illustrating the shielding function ofthe ion beam shield; and FIG. 17 is an exemplary perspective view of the ion beam shield of the present invention employing a focusing lens to create a focused ion beam. Description of Illustrated Embodiments With reference to FIGS. IA and IB. the ion implantation system 10 of the present invention includes a pair of panel cassettes 26, an end effector 24, an assembly of Ioadlock chambers 12, a housing 14 which defines a process chamber 16, and an ion source 18, which communicates with the process chamber 16 through beam aperture 20. An end effector 24 transfers workpieces, such as flat panels P, stacked in cassette 26 to the Ioadlock assembly 12. The illustrated end effector 24 is coupled to a conventional drive and control mechanism which provides power to the end effector and which controls the rate and sequence of panel movement. The panel cassettes 26 are of conventional design and provide a convenient storage facility for the panels. The Ioadlock assembly 12 is also coupled to a linear bearing system and a linear drive system, which provide the desired vertical movement ofthe Ioadlock assembly 12 as well as place and hold the Ioadlock assembly 12 in sealing contact with the process chamber housing 14, as described in further detail below. The linear drive system includes a lead screw 22 and a motor assembly 23. The motor assembly 23 drives the lead screw 22 which in turn positions the Ioadlock assembly 12 at a selected vertical position, as indicated in dashed lines. The linear bearing system includes a pair of stationary linear bearings 28 mounted to the Ioadlock assembly which slide along circular shafts 29. A translation or workpiece holding assembly is preferably mounted within the process chamber 16. The translation assembly includes a pickup arm 27 which is similar in function to the end effector 24. The pickup arm 27 handles the panel P during processing. When the pickup arm 27 initially removes the panel P from the Ioadlock assembly 12, it is oriented in a substantially horizontal position PI . The pickup arm then vertically flips the panel, as denoted by arrow 13, into a substantially vertical position P2. The translation assembly then moves the panel in a scanning or translation direction, from left to right in the illustrated embodiment, across the path of an ion beam emerging from aperture 20, and which is generated by the ion source 18. According to another embodiment, the pickup arm 27 can tilt the panel P relative to its original horizontal position prior to translating the panel. In this embodiment, the ion source 18 is positioned on the process housing 14 to ensure substantially orthogonal bombardment o the workpiece with ions. With further reference to FIGS. 1 A and IB, the process chamber housing 14 includes a front housing portion 14A and a narrower elongated portion 14B. The front housing portion is sized to accommodate the removal ofthe panel P from the Ioadlock assembly 12 in the horizontal position. The panel is then moved from the original horizontal position PI to the vertical position P2 prior to movement along the scanning direction, indicated in FIG. IB by arrow 32. The housing portion 14B has an axial dimension along the scanning direction that allows the panel to pass completely by the ribbon beam generated by the ion source 18. The relatively narrow width of the chamber portion 14B preferably allows the panel to move therealong only when disposed in the vertical position P2 to reduce the total volume of the process chamber 16. This reduction in chamber volume allows faster evacuation of the process chamber. Reducing the time necessary to evacuate the chamber serves to increase the total throughput of the implantation system 10. The illustrated Ioadlock assembly 12 is preferably seaiingly coupled to the front chamber wall 34 ofthe chamber housing 14. The Ioadlock assembly 12 maintains a relatively pressure-tight and fluid-tight seal with the chamber wall 34 during the vertical movement of the Ioadlock assembly 12. This sliding seal-tight arrangement is described in further detail below. Referring again to FIG. 1 A, the ion source 18 in conjunction with the aperture 20 forms a ribbon beam which has dimensions smaller than the dimensions of the panel to be processed. More specifically, the ion source 18 generates a ribbon beam 19 whose width (e.g., axial height) just exceeds the narrow panel dimension and whose length is substantially smaller than the total length ofthe panel, e.g., the larger panel dimension, FIG. 13. The use of a ribbon beam in conjunction with the ion implantation system 10 ofthe present invention provides for several advantages, including (1) the ability to process panel sizes of different dimensions with the same system; (2) achieving a uniform implant which can be controlled by varying the scan velocity ofthe panel and by sampling the current of that portion ofthe ion beam that extends beyond the narrow panel dimension; (3) the size ofthe ion source can be reduced and is thus less expensive and easier to service; and (4) the ion source can be continuously operated. The continuous operation ofthe ion source increases the efficiency ofthe ion implantation system 10 since it produces a more uniform implant by eliminating problems associated with repeatedly turning the ion source off and on, as in prior approaches. These problems typically include beam current density transients which occur upon start-up operation of the source. FIG. 2 is a schematic depiction of an alternate embodiment ofthe ion implantation system 30 according to the present invention. The illustrated system includes a Ioadlock assembly 42 which communicates with a separate service chamber 46 and with the process chamber 40. An ion source 38 is in fluid communication with the process chamber 40, and with a gas box 36 and associated control electronics 44, as is known in the art. The illustrated ion source and gas box assembly are coupled to a power system that includes an isolation transformer 48 and a voltage stack 50. The process chamber 40 can be evacuated by known pumping techniques, such as by the vacuum assembly 52. Any particular exhaust generated during the implantation procedure can be exhausted from the outer housing 56 through exhaust ports 58. The illustrated Ioadlock assembly 42 is intended to encompass any suitable Ioadlock assembly that maximizes throughput by continuously cycling panels through the process chamber 40. For example, the Ioadlock assembly 42 can include the stacked Ioadlock assembly of FIGS. 3A-6 or the dual serial Ioadlock assembly of FIG. 8. Additionally, the illustrated system 30 further shows that the flat panel can be implanted from above while maintained in a substantially horizontal position when translated in the linear scanning direction. FIGS. 3 A through 5 illustrate a first embodiment ofthe Ioadlock assembly 60 ofthe present invention. The illustrated Ioadlock assembly 60 includes a set of individual Ioadlock elements designated as loadlocks 62, 64, 66, 68 and 70. The loadlocks are preferably stackable and, according to a preferred practice, nestable to form a multi-chamber Ioadlock assembly 60 that provides for rapid cycling of panels to form a high throughput ion implantation system. The illustrated stacked set of loadlocks extend along axis 93. The set of loadlocks can also be secured to a front flange 72. The front flange 72 functions as an interface between the multi-chamber Ioadlock assembly 60 and the interface ofthe process chamber, e.g., wall 34 of housing 14. Those of ordinary skill will recognize that the Ioadlock assembly can be directly coupled to the process chamber without utilizing an intermediate flange member, such as front flange 72. With reference to FIGS. 4A through 4C, each Ioadlock element ofthe Ioadlock assembly 60 has a selected configuration that facilitates stacking and nesting of the loadlocks. For example, Ioadlock 62, which is representative of any of loadlocks 62- 70, includes a stepped top surface 74 and a stepped bottom surface 76. The stepped top surface 74 preferably includes a first outermost top surface 74A and an inwardly stepped or recessed surface 74B separated therefrom by a vertically extending wall 74D. The Ioadlock surfaces preferably extend, in a first orientation, along a horizontal axis 91 , and the wall portion extends along axis 93. The top surface includes a further inwardly stepped surface 74C which forms the floor ofthe Ioadlock chamber. The surface 74C is separated from the stepped surface 74B by a second wall portion 74E, parallel to wall portion 74D, and which forms a sidewall ofthe Ioadlock chamber. Similarly, the stepped bottom portion 76 ofthe illustrated Ioadlock 62 includes a vertically bottom-most surface 76C and an outwardly stepped surface 76A, each surface being separated by a wall portion 76D which extends along axis 93. The bottom surface 76 includes a further outwardly stepped surface 76B, which is separated from the adjacent stepped surface 76A by a wall portion 76E. The wall portions 76E and 76D are preferably substantially parallel to each other. The bottom-most surface 76C of the Ioadlock bottom surface forms the roof of the adjacent Ioadlock chamber, e.g., Ioadlock chamber 640, in the stacked array. The illustrated Ioadlock 62 further includes a pair of parallel sidewalls 78A and 78B, which are spaced apart along axis 91. The sidewalls 78A and 78B, as well as the stepped top and bottom surfaces 74, 76, terminate in a frontwall 78D and a rearwall 78C. The front and rear walls have appropriately sized slots 84 A and 84B formed therein and disposed in registration with each other to allow passage of a panel, FIG. 4B. The illustrated Ioadlock 64 also includes a similarly formed stepped top and bottom surface. The Ioadlock 64 includes an outer top surface 64A, an inwardly stepped surface 64B, and a further inwardly stepped surface 64C which forms the bottom of the process chamber 64D. The stepped surfaces 64 A, 64B, 64C are separated by wall portions 64E and 64F, which are axially spaced apart and generally parallel to each other. Loadlock 64 further includes parallel sidewalls 86A, 86B, and back and front walls 86C and 86D, respectively. The front wall 86D has a panel slot 88A formed therein that is in registration with panel slot 88B formed in backwall 86C. These panel slots are also sized to allow passage of a panel, as illustrated by arrow 87. With further reference to FIG. 4A, the stepped top surface 74B of loadlock 62 can have formed therein a peripheral seat or channel which mounts an elastomeric member 90, e.g., an O-ring. The O-ring helps form a pressure tight seal along the periphery ofthe plate surface 74B, as is described in further detail below. The floor portions 74C and 64C of loadlocks 62, 64 preferably include a selectively deployable workpiece support assembly, such as pin assembly 96. The pin assembly includes two linear sets of pins 96A-96C and 98A-98C. The illustrated pin assembly 96 is selectively deployable by appropriate control electronics between a deployed position where the pins outwardly extend from the bottom surface ofthe chamber, as shown, and a stowed position where the pins retract within appropriate passageways. The pin assembly 96 can be actuated into the deployed position to support a panel thereon. In this spaced position relative to the bottom portion 64C, the pickup arm mounted within the process chamber 16 (and shown in FIG. 10) can easily access and remove the panel from the loadlock 64. According to another embodiment, the workpiece support assembly can employ a series of selectively deployable support arms, such as those shown and described in Attorney Docket No. ETE-002, the teachings of which were previously incorporated by reference. The bottom-most surface 64C further includes a vacuum port 94 preferably formed in a central location thereon. The vacuum port can be coupled to a conventional pumping assembly to either pump down or vent the loadlock chamber, or to apply a vacuum pressure to the backside ofthe panel P. The illustrated loadlocks 62, 64 can be stacked or assembled by vertically aligning loadlock 62 with the bottom loadlock 64 such that the outwardly stepped bottom portion 76A of loadlock 62 contacts the O-ring 90 mounted on the first inwardly stepped top portion 64B of loadlock 64. Furthermore, the bottom outermost stepped surface 76B contacts the top surface 64A of loadlock 64. In this arrangement, the bottom most surface 76C of loadlock 62 forms the root ofthe loadlock chamber 64D, FIG. 5, and the recessed top surface 64C of loadlock 64 forms the floor ofthe chamber. As the loadlocks are stacked together, the bottom surface ofthe upper loadlock forms the roof of the chamber disposed therebeneath. Those of ordinary skill will recognize that the loadlocks can have other geometric shapes that promote the stacking and nesting together of a plurality of loadlocks. For example, the loadlocks can be dove-tailed together or fashioned to have a tongue-in groove configuration. FIG. 4C depicts the loadlocks 62 and 64 of FIGS. 4A and 4B stacked and nested together. The elastomeric member 90 seats between the adjacent loadlocks to form a pressure seal therebetween. Furthermore, the mating relationship of the loadlock surfaces further create a pressure seal along these mating surfaces, for example, along surfaces 76A, 648 and 76B, 64A and along the vertical walls ofthe loadlocks that extend between the horizontal sealing surfaces. Thus, the multiple mating surfaces ofthe loadlocks 62, 64 when nested together form a secondary or redundant pressure seal. The telescopic engagement of loadlocks 62 and 64 form loadlock chamber 64D. The chamber 64D preferably has a relatively small volume to facilitate rapid pumping of the chamber 64D. This feature reduces processing time, which increases the throughput of the ion implantation system. With reference to FIGS. 3A and 3B, multiple loadlocks 62-70 can be stacked together to form the stacked loadlock assembly 60. The total number of nested loadlocks can be chosen to maximize the throughput ofthe ion implantation system 10. For example, the number of loadlocks can be selected by one of ordinary skill to minimize processing time to provide for a continuous and rapid cycling of panels. According to a preferred practice, a set of five stacked loadlocks forming four loadlock chambers are employed. Those of ordinary skill will recognize that other stacked arrangements utilizing varying numbers of loadlocks can be employed to facilitate rapid processing of panels. The front faces ofthe loadlocks, for example, front face 78D of loadlock 62 and front face 86D of loadlock 64, can have fastener-receiving apertures 102 formed therein at peripheral locations, as illustrated. The front portion 60A of the loadlock assembly is affixed to front flange 72, which functions as the interface between the loadlock assembly 60 and the housing 14 ofthe process chamber 16, by suitable fasteners, such as bolts 124A-124E, FIG. 5. The illustrated flange 72 includes a rectangular outer portion 104 having a front face 104A, back face 104B and peripheral side portions 104C-104F. The front sealing face 104A has formed thereon three concentric grooves which seat elastomeric seals 108A-108C. The seals preferably have a high wear resistance and can be formed of high molecular weight polyethylene. The three successive concentric seals are provided so that each individual seal is not exposed to a full atmospheric to high-vacuum differential pressure. The grooves formed on the front face 104A are preferably sized to accommodate slight movement ofthe seals within the grooves during the sliding movement of the front surface 104A with surface 34 ofthe process chamber housing 34. Between-seal pumping can be employed to differentially pump between each ofthe seals 108A-108C to lower the drop in pressure that must be achieved across any one particular seal. This between-seal pumping can be provided by a conventional rotary vane, two-stage pumping system which includes pressure ports that are formed between the seals 108 A and 108B, and between seals 108B and 108C. The pressure within the pressure port located between seals 108 A and 108B can be pumped to about 1 Torr, and the pressure within the pressure port located between seals 108B and 108C can be pumped to about 0.1 Torr. With reference to FIGS. 3 A, 3B and 5, the recess 106 ofthe flange 72 terminates in a backwall 1 10 having formed therein a plurality of vertically aligned slots 126A-126E that correspond to the slots formed in the stacked loadlock elements of the loadlock assembly 60. Thus, when the front flange 72 is secured to the loadlock assembly 60, the front flange slots 126A-126E vertically and horizontally align with the respective slots of each ofthe loadlocks 62-70, FIG. 5. Each ofthe flange slots 126A- 126E also have associated therewith a valve assembly 128A-128E. For example, slot 126A ofthe front flange 72 has associated therewith valve assembly 128 A. The valve 128 A is selectively actuatable to seat over the slot 126A to seal the chamber 62D of loadlock 62. Referring to FIGS. IA and 5, the front surface 34 of the process chamber housing 14 preferably has formed therein a panel aperture 1 14 which corresponds in dimensions to slots 126A-126E. The panel aperture 1 14 is preferably in fluid communication with the process chamber 16 and the cavity 118 formed by the front flange recess 106. This cavity 1 18 can be evacuated to match the high vacuum pressure state ofthe process chamber 16, or can be vented to match the lower vacuum pressure state ofthe loadlock chambers 62D-70D. This differential pumping relaxes the pumping and venting requirements of each individual loadlock during panel transfer, since each loadlock need not be pumped down to the high vacuum pressure of the process chamber. The loadlock assembly 60 and the front flange 72 are preferably mounted on the linear bearing system and linear support system of FIG. 1 A. The bearings 28 are preferably coupled to the rear 104B ofthe flange 72. Additionally, the front surface 104A of the front flange is preferably placed in sliding sealing engagement with the chamber wall 34. The illustrated linear bearing system preferably moves the loadlock assembly 60 in the vertical direction, as indicated by arrow 120, to attain a selected vertical position. According to a preferred embodiment, a control system controls the vertical movement ofthe loadlock assembly such that the panel aperture 114 aligns with one ofthe apertures 126A-126E ofthe front flange 72. During this vertical movement, the seals 108A-108C slidingly engage with the chamber wall 34. This sliding engagement permits the vertical movement ofthe loadlock assembly relative to the chamber wall 34 while maintaining a pressure seal therebetween. To enhance the sliding engagement between the seals 108A-108C and the chamber wall 34, the chamber wall can be lapped to reduce the frictional resistance to lateral motion ofthe loadlock assembly 60 and thus improve wear and vacuum reliability. In operation, the end effector 24, FIG. 1 A, removes a panel P from one of the cassettes 26 and inserts it through a slot of one ofthe respective loadlocks 62-70. The end effector 24 can thus load one or more ofthe loadlocks with a panel. After loading the panels P into the loadlocks. valve assemblies associated with each slot are closed, and the chambers 62D-70D are pumped down to the high vacuum pressure ofthe process chamber 16. In an alternate embodiment, the cavity 1 18 can be maintained at an intermediate pressure between the pressures ofthe loadlock and the process chamber. In this circumstance, the loadlock chamber 62D-70D need only be pumped down to some intermediate pressure higher than the vacuum condition ofthe process chamber 16. The linear bearing system vertically positions the loadlock assembly 60 at a selected vertical position. In this selected position, the panel aperture 114 formed in the sliding surface 34 ofthe process chamber housing 14 aligns with one of the slots 126A- 126E of the flange 72. For example, in the illustrated embodiment, panel aperture 1 14 aligns with slot 126B. Valve 128B is then opened and the pickup arm mounted within the process chamber retrieves the panel P from chamber 64D and places it into the process chamber 16. As previously described, the pickup arm then imparts a vertical movement to the panel to move the panel from the horizontal position PI into the vertical position P2. The pickup arm is then moved in the scanning direction and the panel is implanted by the ion source. After processing, the pickup arm returns the processed panel to the loadlock 64. According to one practice, the processed panel, which has been heated during implantation by the energized ions of the ion beam, is placed on the pin assembly 96 or other suitable support structure coupled to the loadlock 64. The pin assembly, which has been disposed in the deployed position, allows the pickup arm to retrieve as well as return the panel to the loadlock chamber 64D. The linear bearing system then positions the assembly at a different vertical position to allow the pickup arm to access another chamber, which has also been pumped down and its associated downstream valve opened, to continue the cycling of panels through the process chamber. As the second panel is being implanted, chamber 64D can further process the implanted and heated panel. For example, the panel can be convectively cooled by flowing a cooling fluid through the chamber via one ofthe vacuum ports 92A-92E, and in this example, vacuum port 92B. Alternatively, the pin assembly 96 or other support assembly can be moved from the deployed position into the stowed position to place the panel into contact with the floor 64C of the chamber, FIG. 4A. The chamber floor because of its size can form a heat exchanging surface which conductively transfers heat from the panel. The loadlock floor 64C can be cooled by providing a supplemental cooling fluid thereto via appropriate fluid conduits. For example, fluid conduits can be formed within the loadlock to circulate a cooling fluid therethrough, or cooling tubes containing a cooling medium can be mounted about the peripheral or outer portion ofthe loadlock. To enhance the uniformity of cooling ofthe panel, a vacuum pressure can be applied to the backside ofthe panel through the vacuum port 94 (or other suitable vacuum network) formed in the floor 64C of the loadlock 64. This vacuum pressure forces the panel into cooling contact with the floor 64C ofthe loadlock, to attain substantially uniform conductive cooling thereof. During this conductive cooling process, the chamber 64D can be vented close to ambient pressure. The valve assembly at the upstream end can then be opened to allow the end effector 24 to remove the panel therefrom and place it within a storage cassette. Those of ordinary skill will also recognize that a combination conductive/convective cooling process can be employed. During this panel cycling process, the loadlock assembly 60 is positioned at different vertical heights to align the panel aperture 114 with one ofthe loadlock slots. For example, the panel aperture 1 14 can be aligned with slot 126A of the flange 72. The valve 128 opens and the pickup arm accesses the chamber 62D and removes the panel therefrom. The pickup arm then moves the panel downstream in a scanning direction to process the panel. After the panel is implanted, the panel is returned to the loadlock 62D. Similar to that described above, the valve 128A is closed and the chamber is vented. The load bearing system then positions the loadlock assembly 60 at a different selected vertical position to allow the pickup arm to access another chamber. This system thus allows continuous cycling and processing of panels, which in turn significantly increases the throughput ofthe ion implantation system 10. The illustrated ion implantation systems 10, 30 also allow the implantation process to be divided into multiple implant stages, each stage being separated by periods of cooling. The period of cooling can vary in order to allow the workpiece to be sufficiently cooled while it is located within a loadlock. High throughput in this ion implantation system can be maintained by processing workpieces in parallel. In particular, a workpiece 174 is constantly being processed within the process chamber 16 while other workpieces are being cooled within a particular loadlock or loaded and unloaded from the loadlock. The workpiece can thus be held within a loadlock for an indefinite period of time. After the holding period the workpiece can either be transferred back into the process chamber 16 for additional implantation, or the workpiece can be removed from the loadlock through the upstream entry valve. The implantation system ofthe present invention provides for a number of unique and beneficial advantages. One advantage is the high throughput ofthe system which is provided for by the stacked loadlock assembly of the present invention. Another advantage is that the illustrated system allows multiple processing of panels by temporarily storing panels within the loadlock chambers 62D-70D. Thus, for example, after a panel is implanted, it can be stored within a loadlock chamber, cooled, and later retrieved for further processing. The system of the present invention further provides for multiple processes to be performed upon a single panel P. As illustrated in FIG. 9, the housing 14" can be configured to form one or more partitions 148A-148B along the process chamber. These partitions preferably divide the process chamber 16" into contiguous and serial process sections 144A, 144B and 144C which can be maintained at different pressures by suitable pumping assemblies. The illustrated panel P, which is secured to pickup arm 152, moves in the scanning direction represented by arrow 140 by the linear bearing and shaft assembly 154, 156. As the panel P moves in the scanning direction, it passes beneath the partitions 148A-148B and through each particular processing section. For example, section 144A can preheat the panel as it passes therethrough, section 144B can implant the panel, and section 144C can cool the panel. Thus, multiple processes can be performed upon the same panel nearly simultaneously in the same process environment. This serial processing technique allows multiple processes to be performed upon the same panel, thus reducing the overall processing time and increasing the throughput of the implantation system. FIG. 6 illustrates an alternate embodiment ofthe sliding seal arrangement ofthe present invention. In this illustrated embodiment, a strip of polyethylene sealing material 130 is affixed to the chamber wall 34' along its lateral face. This sealing material thus eliminates the need for providing the flange 72 which mounts to the loadlock assembly 60'. The front face of each loadlock further has mounted thereon a sealing material 132A-132F, such as a strip of high molecular weight polyethylene. Furthermore, the sliding surface 34' is formed as an extension ofthe process chamber 16' and ofthe housing 14'. The illustrated loadlock assembly 60' is vertically movable as by the linear bearing system ofthe present invention, as denoted by arrow 134. FIG. 7 shows a serial loadlock assembly formed of a plurality of loadlocks 190A, 190B, and 190C. The serial assembly also includes elevator assemblies 192A, 192B, and 192C positioned for acting upon the loadlocks 190A, 190B, and 190C, respectively. Each ofthe loadlocks 190A, 190B, and 190C include exit valves 194A, 194B, and 194C for loading or unloading workpieces from an exit end ofthe loadlocks, respectively. The loadlocks 190A, 190B, and 190C can further include entry valves 195A, 195B, and 195C for loading or unloading workpieces from the entry end ofthe loadlocks, respectively. Preferably, the loadlocks 190A and 190B are arranged so that a robot (not shown) can readily move a workpiece from loadlock 190A to loadlock 190B. For instance, the loadlocks 190A and 190B can be oriented so that the exit valve 194A of loadlock 190A is adjacent the entry valve 195B of loadlock 190B. A robot situated between the two loadlocks can then move a workpiece from loadlock 190 A to loadlock 190B. Loadlock 190C is arranged with respect to loadlock 190B in an analogous fashion. This allows a serial chain of loadlocks, with robots spaced between the loadlocks, to be formed, as illustrated in FIG. 7. The serial chain provides significant throughput increases when processing the workpieces. In particular, different processes can be performed in the various loadlocks. This allows a plurality of workpieces to be undergoing various steps in the ion implantation sequence concurrently. For example, one loadlock may be loading or unloading workpieces from an external workpiece cassette, one loadlock may be cooling workpieces, and one loadlock may be loading or unloading workpieces from the process chamber 16. According to a further aspect of the invention, each ofthe loadlocks 190A, 190B, and 190C can also include an upper chamber 210 and a lower chamber 212 divided by a separation plate 196. For example, loadlock 190A includes a separation plate 196A dividing the loadlock into an upper chamber 210A and a lower chamber 212A, loadlock 190B includes a separation plate 196B dividing the chamber into an upper chamber 21 OB and a lower chamber 212B, and loadlock 190C includes a separation plate 196C dividing the loadlock into an upper chamber 2 IOC and a lower chamber 212C. Each chamber is sized to hold the workpiece 174, and thus each loadlock is sized to hold more than one vertically spaced apart workpiece. Accordingly, both the lower and upper chamber can concurrently hold workpieces. Preferably, the loadlocks are arranged in a serial chain as described above, with workpieces that move along a direction Dl in the serial chain being loaded and unloaded from the upper chambers in each ofthe loadlocks and with workpieces that move along a direction D2 in the serial chain being loaded and unloaded from the lower chambers in each of the loadlocks. Accordingly, workpieces moving along direction Dl are found only in the upper chambers 210 and workpieces moving along direction D2 are found only in the lower chambers 212. This embodiment provides for a serial chain of loadlocks that does not require complicated robot arms between each loadlock that are capable of moving vertically as well as horizontally. Elevator assemblies 192A, 192B, and 192C are utilized to aide the movement of a workpiece through loadlocks 190A, 190B, and 190C, respectively. Each elevator assembly comprises an actuator 206 capable of movement along the direction of axis 214. The elevators aid in the movement ofthe workpieces by providing structure capable of allowing a robot arm to easily load and unload workpieces from within the loadlocks 190. In particular, the loadlocks 190 can further comprise a set of holes 200 placed in the separation plate 196 and a set of holes 202 placed in the lower plate 198. Furthermore, the elevator assemblies 192 can further include a plate 208 mounted to actuator 206, and a set of pins 204 mounted to the plate 208. The pins 204 are fixedly mounted to the plate 208 such that they are aligned with an equal number of holes 200 and an equal number of holes 202. Preferably, in accordance with this embodiment, actuator 206 is a three position actuator that places the pins 204 either above the level of separation plate 196, above the level of lower plate 198, or below both ofthe plates 196 and 198. Accordingly, when the elevator is properly aligned with a loadlock, the elevator aids in the loading and unloading of a workpiece from the particular loadlock by lifting a workpiece off of or by lowering a workpiece onto either the separation plate 196 or the lower plate 198. For example, as the elevator assembly 192A is activated the pins 204A-204D move along direction 214. When the elevator 192 A is properly aligned with loadlock 190A and the elevator is raised, the pins 204A-204D will first pass through holes 202A-202D and then through holes 200A-200D. The movement of the pins through the holes provides a force for lifting and lowering a workpiece placed upon separation plate 196 and upon lower plate 198. In operation, a workpiece entering a lower chamber 212A of loadlock 190A from along direction D2 is first moved into the lower chamber 212A by a robot arm (not shown). Pins 204A-204D are inserted through holes 202A-202D such that a workpiece placed within chamber 212A rests upon the pins 204A-204D rather than upon the surface 198A. Once the workpiece is moved into the chamber 212A and placed upon the pins 204A-204D the elevator assembly 192 A lowers the pins 204A-204D, thereby allowing the workpiece to rest upon surface 198 A. This advantageously allows a workpiece to be inserted into a loadlock 190A and be cooled once it is lowered upon the lower plate 198 A. Cooling ofthe workpiece by the loadlock 190A can occur through processes described above, such as convection cooling by direct contact ofthe workpiece with the lower plate 212A. When the plate is to be moved out ofthe loadlock along direction D2, the pins 204A-204D are inserted through the holes 202A-202D to raise the workpiece above the level of surface 2198 A. Once the workpiece is raised above the level of surface 198 A a robot arm can be inserted into the chamber 212A to remove the workpiece. An analogous process is used to insert and remove a workpiece from chamber 210A. For example, a workpiece to be inserted into chamber 210A along direction Dl is moved into the chamber 210A by a robot arm. Pins 204A-204D are inserted through holes 200A-200D to provide a support for the workpiece. Once the workpiece has been positioned and is resting upon the pins 204A-204D the pins can be lowered through holes 200A-200D to allow the workpiece to rest upon surface 196A. When the workpiece is to be removed from loadlock 190A the pins 204A-204D are once again inserted into holes 200A-200D thereby raising the workpiece above the surface 196A. This allows a robot arm to be inserted into the chamber 210A between the workpiece and the surface 196A, thereby allowing the robot arm to extract the workpiece from loadlock 190A. FIG. 8 illustrates a pickup arm 216 that can be used to translate the workpiece throughout the ion implantation system 10 or 30. In particular, the pickup arm 216 is preferably used to move the workpiece from the loadlock assembly 12 into the process chamber 16. In addition, the pickup arm 216 can be utilized to hold the workpiece and translate the workpiece as it passes through the system. The pickup arm is formed of a main body 218 having a raised edge on three sides that forms a perimeter wall 220. Accordingly, the pickup arm 216 contains a surface 222 that is recessed relative to the perimeter wall 220. The recessed surface 222 and the wall 220 are shaped to provide a space for securing the workpiece. The illustrated pickup arm 216 can include clamps 224A-224F for securing a workpiece to the pickup arm 216. The clamps 224A-224F each contain an upper flange 223 for applying a force to the workpiece that opposes the force generated by the recessed surface 222. In particular, the upper flange 223 in conjunction with the recessed surface 222 pinches the workpiece so as to secure it in place. The clamps 224 can move along the direction ofthe first axis 230 to allow engagement and disengagement from the workpiece. In addition, the movement ofthe clamps along the first axis 230 allows for the clamps to secure a workpiece to the pickup arm 216 regardless ofthe size ofthe workpiece. One example of such clamps are spring loaded clamps. The illustrated clamps are also configured for movement along axis 232 in order to vary the securing force applied by the clamps to the workpiece. The securing force is advantageously varied in order to generate a force sufficient to hold the workpiece in place, yet not so strong as to damage the workpiece. In order to achieve the proper securing force, either the upper flange 223 or the lower flange can be varied along the direction of axis 232. In order to overcome the difficulties in securing a workpiece that is slightly warped, i.e. varies from a horizontal plane, the clamps 224A-224F can be varied independently. In those cases where the workpiece tends to rise above the surface 222, the flanges 223 and 225 can be raised in height relative to the surface 222 and apply a force to either side ofthe workpiece thereby securing the workpiece. With further reference to FIG. 8, the pickup arm 216 can include a first channel 234 and a second channel 236 that runs through the surface 222. The channel runs through the pickup arm from surface 222 to a bottom surface 238 ofthe pickup arm 216. Accordingly, the pickup arm forms a three-pronged structure. The channels 234 and 236 provide access to the bottom of a workpiece resting upon surface 222. In operation, the pickup arm 216 is inserted into a loadlock containing a workpiece. Typically the workpiece will be raised from the surface ofthe loadlock by the use of pins 204. As the pickup arm 216 is inserted into the loadlock, the pickup arm 216 is aligned such that the pins 204 pass along the channels 234 and 236. This allows the pickup arm to be positioned below the workpiece which is simultaneously being supported by the pins 204. In addition, at this point in time, the clamps 224 are fully retracted to allow the workpiece a higher degree of freedom of movement within the pickup arm 216. Once the pickup arm is positioned, the pins 204 can be lowered thereby allowing the workpiece to come to rest on a recessed surface 222. After the workpiece has come to rest on the surface 222 ofthe pickup arm the clamps 224 can be engaged. After the workpiece has been securely mounted to the pickup arm 216, the pickup arm is removed from the loadlock with the workpiece. The pickup arm detailed above has the further advantage of being able to hold the workpiece to the pickup arm while the pickup arm is rotated about a third axis 240. This allows the workpiece secured to the pickup arm to be positioned under an ion source 18. FIGS. 10 through 13 illustrate another embodiment ofthe ion implantation system ofthe present invention. The illustrated ion implantation system 300 includes, from the input loading end ofthe implantation system to the processing end (e.g., right to left in FIGS. 10 and 11), a plurality of conventional panel cassettes 302 which store a number of flat panels. An end effector 306 is positioned so as to retrieve a panel from a selected cassette 302 and to place the panel in one ofthe loadlock chambers ofthe loadlock assembly 310. The end effector 306 includes a pair of hinged arms 306 A that are coupled to a support bracket 306B that terminates in a panel support table 306C. The end effector 306 is pivotable about a base 307 that translates along axis 31 1 to allow the end effector 306 to access and retrieve panels stored in either cassette 302. The end effector arms 306A, 306A are hinged about fulcrum 306D, 306D and are extendible so as to move the support table 306C into and out ofthe panel cassettes. The end effector is also pivotable about base 307 to allow the support table 306C to insert or retrieve a panel from the loadlock assembly 310. The end effector 306 is also vertically movable, as indicated in phantom in FIG. 1 1, to allow the effector to access panels along the entire vertical height ofthe panel cassettes 302, 302. The illustrated end effector is known in the art, and need not be described further herein. At the input end ofthe loadlock assembly 310 is formed an input gate valve 312, which is selectively actuatable by a controller according to a user-defined sequence. An output gate valve 314 is formed at the opposite end of the loadlock assembly 310 and is in fluid communication with the loadlock assembly and with the process chamber 318 contained within the process housing 316. The illustrated process chamber 318 mounts a single workpiece handling assembly 320, FIG. 12. The illustrated workpiece handling assembly 320 performs a dual function of loading and unloading panels from the loadlock assembly 310 and for supporting the panel during processing. Specifically, the workpiece handling assembly 320 supports the panel and translates the panel in a scanning direction past the ion source 330, which generates an ion beam of prescribed energy. When the workpiece handling assembly 320 translates the panel through the ion beam, the panel is implanted by the energetic ions contained within the beam. The ion source 330 mounts to one side ofthe process housing 316, and preferably to the side ofthe housing located opposite the wall mounting the loadlock assembly 310. The illustrated ion implantation system 300 further includes a gas box 332 which encloses a portion of the ion source, including an RF matching circuit 330A. As is known in the art, the ion source 330 is surrounded by one or more high voltage bushings 330B which function as an insulator for the high voltage source for powering the ion source 330. A generator 334 is coupled at one end to the gas box 332, and at another end to motor 336. The loadlock assembly 310 and the process chamber 18 can be disposed at selected pressure levels by the pump-down assembly 319, which includes pressure conduits 319A and a pump source. The controller for controlling the operation of the system components can be housed in the facility box 321. Peripheral electronics typically employed during use ofthe system 300 can be housed in various locations, including the electronics rack 323. Any particular exhaust generated during the implantation procedure can be exhausted from the enclosure 325 via exhaust conduits 327. Those of ordinary skill will recognize the function and operation ofthe conventional portions ofthe ion implantation system, which include the ion source 330, gas box 332, generator 334 and motor 336, and need not be described further herein. FIGS. 12 and 13 illustrate the workpiece handling assembly 320 mounted within the process chamber 318 ofthe process housing 316. The illustrated process housing 316 has formed along an outer surface a number of structural webs 338 which provide rigid mechanical support for the process housing, and thus help maintain the mechanical integrity ofthe system 300. The ion source 330 is mounted to an outer wall 364 of the process housing 316. The outer wall 364 preferably includes a first substantially vertical wall portion 364A, which extends along axis 304, and a second canted upper wall portion 364B which mounts the ion source 330. The illustrated surface 364B is preferably offset from the vertical axis 304 by a selected amount, which is preferably equal to the offset between the platen 348 ofthe workpiece handling assembly from the axis 304 when disposed in its most vertical position, as described further below. By forming the canted surface 364B of the process housing 316 at an angle substantially equal to the angle between the platen and the vertical axis 304 during scanning, the system ensures that the platen surface will be substantially parallel to the ion source, and thus perpendicular to the ion beam 331 that is emitted from the ion source 330. This creates a substantially uniform implant ofthe panel. The process chamber 318 includes a first chamber portion 318A and a substantially vertically extending chamber portion 318B. The chamber portion 318A is sized to accommodate removal ofthe panel P from one ofthe loadlock chambers ofthe loadlock assembly 310. The other vertically extending chamber portion is preferably smaller and is sized to accommodate transverse scanning ofthe panel in the scanning direction 360, while concomitantly reducing the total chamber volume to relax the pumping constraints ofthe process chamber, e.g., allow faster evacuation of the chamber. The chamber portion 318B also extends far enough in a direction transverse, or even orthogonal, to axis 305 to allow the panel to pass completely by the ion beam 331. The loadlock assembly 310 is mounted to another wall 365 ofthe process housing 316, preferably opposite the wall 364 mounting the ion source 330. The illustrated loadlock assembly 310 is fixedly mounted to the housing wall 365 and overlies a series of vertically aligned slots 390A-390D that are in registration with the openings of the loadlock assembly 310. These openings preferably open onto the loadlock chambers of each loadlock in the loadlock assembly 310. The loadlock assembly is preferably similar to the stackable and nestable loadlock assembly 60 of FIGS. 1 A-6. The illustrated loadlock assembly preferably includes output gate valves 314 that selectively close each one ofthe openings ofthe loadlock chamber. The operation of the output gate valves is known in the art. The loadlock assembly 310 is also seaiingly coupled to the wall 365 ofthe housing 316 to maintain a relatively pressure-tight and fluid-tight seal. With further reference to FIG. 12, the illustrated workpiece handling assembly 320 includes a number of pivotable components which effectuate the loading and unloading ofthe panel from the loadlock assembly 310, as well as to tilt or orient the panel in a substantially vertical orientation to translate the panel therealong during processing. The workpiece handling assembly 320 includes a vertical carriage 322 that is operatively coupled to a vertically extending guide rail 324, e.g., extends along axis 304. The illustrated guide rail 324 includes a groove which seats the vertical carriage 322. The vertical carriage and guide rail are preferably coupled to a driving mechanism, such as a linear drive motor or lead screw, which selectively and variably moves the carriage along the guide rail 324 along axis 304. A support arm 326 is pivotally attached to the vertical carriage 322 at a pivot point 327. Also attached to this pivot point is a pair of altitude control arms 328A and 328B. The altitude control arms are each coupled to an altitude control link 340A and 340B, respectively. As shown, a first end ofthe control link 340A is coupled to one end ofthe control arm 328A. the opposite end of which is coupled to the pivot point 327. The opposite end of the control link 340A is pivotally coupled to pivot point 342 and to an intermediate support arm 344. The intermediate support arm 344 terminates at an end opposite the pivot point 342 at a pivot point which has attached thereto the platen 348. The platen includes a first face 348A and a second recessed face 348B to form a lip 348C therebetween. The lip is sized to mate with one end of a panel P, and serves to hold or support the panel on the platen 348 during processing, as described further below. The illustrated altitude control links 340a, 340b are each operatively coupled to a drive mechanism, such as a linear motor, drive screw or other types of driving structures. The drive mechanisms in conjunction with the components ofthe workpiece handling assembly 320 provide structure for removing a panel P from one of the loadlock constituents ofthe loadlock assembly 310, and to translate the panel in a scanning direction designated by arrow 360 past the ion source 330. In operation, the end effector 306 removes a panel from one of the panel cassettes 302. A controller then opens the upstream (input) gate valve 312 to allow the end effector 306 to place the panel P within one ofthe loadlocks ofthe loadlock assembly 310. The end effector, according to a user-defined sequence, further loads panels or removes panels from a particular loadlock. This user-defined sequence allows the ion implantation system 300 to process multiple workpieces concurrently to effectuate a high throughput ion implantation system. For simplicity, the loading and implantation of a single panel will be described further below, but those of ordinary skill will readily recognize that additional panels can be loaded in the loadlock during the implantation step, or other panels can reside in one ofthe remaining loadlocks where it undergoes further processing, such as cooling. Once a panel P has been loaded in a selected loadlock, the workpiece handling assembly 320 removes the panel P and holds the panel during implantation. According to one sequence, the vertical carriage 322 is moved from position A vertically upward along guardrail 324 to a second vertical position B. When the handling assembly is disposed at position B, the altitude control link 340A pivots about pivot point 342 to move the platen 348 into a loadlock chamber, such as chamber 354. The panel residing within the chamber 354 is preferably vertically spaced from the floor ofthe chamber to allow the platen 348 to slide beneath the panel to remove it from the chamber 354. When the platen 348 is slid beneath the panel P, the end ofthe panel closest the process chamber 318 contacts the lip 348C ofthe platen. The panel P is removed from the chamber of the loadlock 354 by actuating the control link 340A to pivot about the pivot point 342, while concomitantly actuating the vertical carriage 322 to move vertically downward along the guiderail 324 to a position the same as or close to position A. The altitude control arms and links 328A, 328B, 348, 340B are then operatively controlled to move about pivot points 327 and 342 to swing the platen in a direction illustrated by arrow 358 to tilt the workpiece relative to the horizontal plane ofthe implantation system 300. According to a preferred embodiment, the workpiece handling assembly tilts the platen surface 348A relative to the axis 304 to form an angle therebetween that is greater than zero degrees, and is preferably greater than 5 degrees. According to a most preferred embodiment, the platen surface and the axis 304 form an angle of about 5 degrees. This specific orientation allows gravity to hold the panel on the platen. The vertical carriage 322 is then moved vertically upward to position C, and the illustrated pivotable components ofthe workpiece handling assembly 320 are adjusted to translate the platen surface in the scanning direction denoted by arrow 360. During this scanning motion, the vertical carriage 322 and the pivoting components of the workpiece handling assembly maintain the same degree of separation between the platen surface and the chamber wall 364B. That is, the scanning direction 360 is substantially parallel to the wall 364B. This constant separation between the panel and wall 364B during processing ensures a uniform implant ofthe workpiece. The workpiece handling assembly then reverses the motion ofthe panel to place the implanted workpiece into one ofthe loadlock chambers. During the implantation process, a portion ofthe ribbon beam 19 spills over the edge ofthe panel P. A beam monitoring apparatus 398 can be housed within the chamber 318, or can be coupled to the process housing 316 and communicate with the chamber 316, to analyze the ion beam 19, 331. The monitoring apparatus further enables the ion implantation system ofthe invention to achieve increased throughput by measuring the parameters ofthe ion beam 331 while contemporaneously treating/implanting the workpiece with the ion beam. The term parameters refers to those characteristics ofthe ion beam relevant to the implantation of ions in a workpiece. such as the current density ofthe ion beam, the number of neutral particles in the ion beam, and the mass of particles in the ion beam. In comparison to prior art techniques that intermittently measured the current density of the ion beam between the implantation of a predetermined number of workpieces, the beam monitoring apparatus takes measurements ofthe ion beam during the implantation ofthe workpiece. Accordingly, the ion beam can be constantly monitored during the implantation process without interrupting the implantation process to obtain ion beam measurements. The illustrated ribbon beam 19 that extends along an axis of elongation and has a width that extends along a second path transverse to the elongation axis. The ribbon beam can be formed from an elongated slot in a plasma electrode ofthe ion source having a high aspect ratio, that is a ribbon beam having a length that exceeds the width by fifty (50) times or more. Ribbon beams prove effective in implanting large area workpieces because they can reduce the number of passes ofthe workpiece through the ion beam required to obtain a preselected dosage. For example, prior art techniques required that the ion beam be scanned in two orthogonal directions over the workpiece to completely cover the workpiece. In comparison, when a ribbon beam has a length that exceeds at least one dimension ofthe workpiece, only one scan ofthe workpiece through the ribbon beam is required to completely cover the workpiece. Ribbon beams formed from electrode slots having even higher aspect ratios of 100:1 prove useful in implanting larger workpieces, such as workpieces having dimensions of 550 mm by 650 mm and larger. These ribbon beams, however, are more difficult to control and to keep uniform. Accordingly, continuous measurement and control of the parameters ofthe ion beam prove to be particularly important when using ribbon beams formed from electrode slots having high aspect ratios. The preferred ion beam monitoring apparatus is set forth in Attorney Docket No. ETE-003, the teachings of which were previously incorporated by reference. The multiple processing of panels by the ion implantation system 300 is schematically illustrated in FIG. 13. The panel P removed from the loadlock 372 is moved in the direction indicated by arrow 374. Upon removal ofthe panel P from the loadlock 372, the workpiece handling assembly 320 tilts the platen surface, and thus the panel, in a direction transverse to the substantially horizontal direction from which the panel P is removed from the loadlock. Specifically, the panel P is disposed in a first transverse position 378 and is then moved vertically downward along axis 304 and disposed in a further vertical position 380, as indicated by arrow 381. The workpiece handling assembly 320 then moves the panel upwardly in the translation direction, as indicated by arrows 382 and 384. While the illustrated panel is being implanted, another panel P2 is loaded in an adjacent loadlock, where either it awaits processing, or is cooled via the cooling system ofthe ion implantation system, such as the cooling system associated with the loadlock assembly of FIGS. 1 A-6. Additionally, a further panel P3 can be loaded into another loadlock ofthe loadlock assembly 310. The ion implantation system 300 of the present invention has a number of beneficial advantages. One advantage ofthe present ion implantation system is that it employs a single workpiece handling assembly to both load an implanted panel into and remove a panel from the loadlock assembly 310, while concomitantly supporting the panel during translation along the scanning direction. The use of a single workpiece handling assembly to both manipulate a panel and to support it during implantation reduces the number of mechanical units which must be mounted within the process chamber, as well as reduces the number of mechanical transfers ofthe panel during processing relative to prior art ion implantation systems. For example, prior art systems employed a first robotic arm to remove a panel from a loadlock and to load the panel on a separate translating stage. The translating stage then moves the panel in the scanning direction where it is implanted by the ion source. By integrating these components into a single workpiece handling assembly, the number of workpiece transfers is reduced, thus increasing the throughput ofthe implantation system. Another advantage ofthe present invention is that the workpiece handling assembly 320 translates the panel in a vertical scanning direction, where it is implanted by the ion source 330. Constructing the process housing such that it accommodates movement of the panel in a vertical scanning direction, reduces the overall size and footprint ofthe housing. This results in a reduced overall footprint for the ion implantation system 300. For example, the process housing has a horizontal footprint of less than about 60 inches along axis 304, and has a vertical height of about 102 inches. The dimensions ofthe complete ion implantation system illustrated in FIGS. 10 and 1 1 is approximately 197 inches long and 92 inches wide. Those of ordinary school will recognize that incorporating the end effector and panel cassettes will add to the overall dimensions ofthe ion implantation system, and according to a preferred embodiment, these dimensions are about 290 inches long and about 89 inches wide. The concurrent processing of multiple panels is further illustrated by the control sequence timing diagram of FIG. 14. The timing diagram graphically illustrates the events that occur for various loadlocks ofthe loadlock assembly 310 during the processing of multiple panels. The control sequence table 388 illustrates the particular sequences that occur within the loadlocks of the loadlock assembly 310, designated as loadlock 1, loadlock 2, loadlock 3, and loadlock 4. The sequence numbers 1-4 correspond to particular operational events which occur during the timing sequence illustrated by the timing diagram 390. For example, block 1 corresponds to the operational events that occur between about 0 and 30 seconds, block 2 corresponds to the operational events that occur between about 30 and 60 seconds, block 3 corresponds to the operational events that occur between about 60 and 90 seconds, and block 4 corresponds to the operational events that occur between about 90 and 120 seconds. The operational events that occur during blocks 1 -4 are listed under the event heading at the left-most portion ofthe timing diagram 390. One example of an event timing sequence is as follows. At time 0, the following events occur the front gate valve VF, which is typically adjacent the end effector of the ion implantation system, is opened by any suitable controller, such as a general purpose computer. After the gate valve VF is opened, a panel is loaded therein by the end effector 306. Once the panel is loaded within the loadlock chamber, the valve VF is closed and the chamber is pumped down by any suitable pumping assembly. As illustrated, the pump down ofthe loadlock takes about 25.5 seconds to complete. Upon completion of the pump down ofthe loadlock, the downstream gate valve VR is opened and the workpiece handling assembly 320 removes the panel from the loadlock and translates it in the scanning direction to be implanted by the ion source. The implant portion of this timing sequence takes about 12 seconds to complete. The workpiece handling assembly is then positioned to unload the panel within the loadlock chamber. The gate valve VR is then closed and the loadlock chamber is then vented up to an appropriate pressure, such as atmospheric or any intermediate pressure. Furthermore, during the venting portion ofthe timing sequence, the heated panel is cooled according to the techniques described herein. As part of the vent/cool timing sequence which spans between blocks 3 and 4 ofthe timing sequence, and which takes approximately 49.5 seconds, the workpiece-support assembly mounted within the loadlock chamber is vertically moved between its deployed position where an implanted panel can be loaded thereon, and the stowed position where the implanted panel can be placed in intimate facing contact with the relatively cool floor of the loadlock. The control sequence table 388 thus illustrates that different stages ofthe timing sequence occur at different loadlocks. The events that occur for a particular panel associated with a particular loadlock can thus be manipulated to facilitate the concurrent processing of multiple panels. According to a preferred practice, approximately 80 panels per hour can be processed by the ion implantation system 300 ofthe present invention. In conventional ion implantation systems the ion beam generated by the ion source illuminates an entire workpiece in both dimensions (along the length and width ofthe workpiece) with energetic ions, requiring the region ofthe process chamber between the workpiece and the ion source to be open and free of any obstruction. Prior to implantation of the panel, each panel can be coated with a masking layer such as a photoresist layer to create a selected pattern on the face ofthe workpiece. During the implantation process, the ion beam strikes the photoresist and penetrates therein. Since the photoresist is usually made of an organic polymer, the energetic ions cleave the hydrocarbon chains of the polymeric material as the ions travel therethrough. Consequently, the photoresist outgasses hydrogen, water vapor and other residue from the surface ofthe photoresist. The resultant residue condenses everywhere within line of sight ofthe photoresist coated surface. Since the region between the ion source and the panel is open, e.g., free of obstruction, the photoresist residue has a free path to the ion source, and particularly to the extraction and suppression electrode assembly of the source. The outgassed photoresist residue, which is an insulating material, coats the electrode assembly over time, shorts the electrode such that arcing occurs. In order to clean the photoresist residue from the electrodes, it is necessary to dismantle the ion source of the implantation system. The ion implantation systems 10, 20, 300 ofthe present invention thus contemplate the use of an ion beam or blocking shield 410 that prevents a significant portion ofthe outgassed residue from reaching the electrode assembly ofthe ion source. Figure 15 illustrates the blocking shield 410 mounted within the process chamber 318 of the process housing 316 ofthe implantation system 300. Those of ordinary skill will recognize that the shield can be used with any of the implantation systems disclosed herein, but for purposes of simplicity, will be discussed in conjunction with the ion implantation system 300. The shield 410 is preferably inteφosed between the ion source 330 and the panel P when moved in the scanning direction designated by arrow 360. The placement ofthe shield 410 between the ion source 330 and the panel P prevents the accumulation of photoresist residues on the ion source 330 and its components. Specifically, the illustrated shield 410 prevents photoresist that is outgassed from the panel P when the ion beam strikes the coated front panel face from reaching the electrode assembly (not shown) ofthe ion source. The general construction ofthe ion source is known to those of ordinary skill in the art. The shield 410 preferably has a geometric shape, e.g., rectangular, that allows it to mount within the process chamber 318. The shield 410 includes an opening 412 that allows passage ofthe ribbon beam 19, 331 generated by the ion source. As illustrated in FIG. 13, the ribbon beam 19 extends in a longitudinal direction (beam length) along the width ofthe panel P when the panel is passed through the beam. The second orthogonal direction ofthe beam (beam width) is much smaller that the beam length to provide a high aspect ratio beam. The narrowness ofthe beam along one dimension and the length ofthe beam in the transverse direction means only one dimension between the ion source 330 and the panel P must be free of obstruction, e.g., along the beam length, thus allowing a blocking shield 410 to be placed relatively close to the boundaries ofthe ribbon beam 19. The shield 410 when properly positioned between the panel and the ion source 330 prevents at least a portion ofthe photoresist residue from coating the electrode structure of the ion source 330. The shield 410 is preferably formed of a relatively inflexible material that has a roughened outer surface that promotes the adhesion ofthe outgassed photoresist to the shield. The roughness ofthe outer surface(s) ofthe shield also effectively increases the overall surface area thereof, thus increasing the amount of photoresist that can coat the shield. The roughened surfaces ofthe shield 410 prolongs its useful life by reducing the number of times that the shield needs servicing or replacement. The roughness ofthe shield surface is greater than about 5 microns, and preferably greater than about 10 microns and higher, although other roughness ranges (including roughness values less than 5 microns and significantly greater than 10 microns) can be used if increased servicing ofthe shield is acceptable (when using a relatively smooth surface). According to a preferred practice, the shield 410 has a surface finish of about 128 microinches rms as would be achieved by grip blasting aluminum. The shield can be made of any suitable material, such as aluminum, that is compatible with the processing environment, and is generally not adversely affected thereby. Those of ordinary skill can readily choose the appropriate roughness and select an appropriate relatively inflexible material for use with the teachings ofthe present invention. FIG. 16 illustrates the separation D between the panel P and ion source 330. and the outgassing of photoresist that occurs as the panel is passed beneath the beam in the scanning direction 360. The illustrated ion beam 331, which is configured to be a ribbon beam 19, passes through opening 412 formed in the shield 410 and impacts the panel P. The resulting interaction between the ion beam and the photoresist coated panel generates a photoresist residue plume 414. The opening 412 preferably extends in the longitudinal direction ofthe beam, e.g.. along the length ofthe beam, and has a width that corresponds to the width ofthe ribbon beam. The dimensions of the shield opening 412 are selected to prevent, impede or minimize the amount of outgassed photoresist that reaches the ion source 330 through the opening, without significantly interfering with the passage ofthe ribbon beam 19 through the opening 412. The ordinarily skilled artisan would be able to determine the appropriate dimensions ofthe opening 412 as a function ofthe aspect ratio ofthe ribbon beam and the size ofthe beam. The shield 410 blocks or restricts the upward rise ofthe residue plume 414 to facilitate accumulation of the plume in the volume of space located between the shield 410 and the source 330. The residue adheres to the shield 410 upon contact, and thus is effectively removed from the volume of space beneath the shield before reaching the volume of space located above the shield 410. According to a preferred embodiment, the shield 410 is mounted or supported within the chamber portion 318B at a distance D/2 or about halfway between the ion source 330 and the panel when translated along the chamber 318B in the scanning direction 360. The separation between the ion source and the panel P is selected so as to prevent the ion source from excessively heating the shield such that it expands or warps, thus spalling or flaking the residue coated thereon, which may then fall onto the panel during processing. Those of ordinary skill knowing the type and power ofthe ion source, the process chamber configuration, the distance between the panel and the source and the type of ions being implanted, can readily determine the proper spacing between the ion source and the panel P during processing. Spacings closer to either the ion source or the platen are contemplated by the present invention. FIG. 17 illustrates a further embodiment ofthe shield system for improving the ion source life. The illustrated system employs a focusing lens 420 which is disposed in the path ofthe beam 19 and in the volume of space between the ion source 330 and the shield 410. In operation, the focusing lens 420 causes the ion beam to converge at the plane ofthe shield 410, e.g., at the focal plane, and particularly at the opening 412' formed in the shield 410. The illustrated opening 412' has overall smaller dimensions than the opening 412 of FIG. 16. As a result ofthe smaller diameter opening 412' ofthe shield 410, the photoresist residue 414 produced by the action ofthe ion beam on the panel P is less likely to pass through the opening and interfere with the operation ofthe ion source 18. Specifically, the opening 412' is preferably sized to allow passage ofthe converged beam 422 while concomitantly preventing, impeding or minimizing the amount of residue that passes through the opening and to the ion source. It will thus be seen that the invention efficiently attains the objects set forth above, among those made apparent from the preceding description. Since certain changes may be made in the above constructions without departing from the scope of he invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. Having described the invention, what is claimed as new and desired to be secured by Letters Patent is:

Claims: 1. An ion implantation system for processing a workpiece with a coated surface, comprising an ion source for ionizing selected matter to generate an ion beam, a housing defining a process chamber that is fluidly coupled to the ion source, a workpiece handling assembly mounted within the process chamber and having a workpiece support surface for supporting the workpiece within the process chamber during implantation by the ion beam, and an ion beam shield having an aperture sized for allowing the ion beam to pass therethrough, the ion beam shield being positioned between the support surface ofthe handling assembly and the ion source during processing for shielding the ion source from residue that is outgassed from the coated surface ofthe workpiece during implantation. 2. The ion implantation system of claim 1, wherein the ion beam comprises a ribbon beam. 3. The ion implantation system of claim 1 , wherein the ion beam shield has a rough outer surface that effectively increases the surface area thereof to allow greater amounts ofthe outgassed residue to adhere thereto. 4. The ion implantation system of claim 1, wherein the workpiece is coated with photoresist. 5. The ion implantation system of claim 2, wherein the ion source and the support surface ofthe workpiece handling assembly is separated by a distance D during processing, and wherein the ion beam shield is separated from the workpiece handling assembly by a distance of about D/2 or less. 6. The ion implantation system of claim 1 , wherein the shield aperture is sized to accommodate an ion beam having an aspect ratio equal to or greater than fifty to one. 7. The ion implantation system of claim 1 , further comprising a focusing lens disposed between the ion source and the shield for focusing the ion beam and wherein the shield is disposed at a focal plane ofthe focussed beam. 8. The ion implantation system of claim 1 , wherein the ion beam shield is composed of an inflexible material. 9. The ion implantation system of claim 1, wherein the workpiece handling assembly includes translation means for translating the workpiece in a linear scanning direction during implantation. 10. The ion implantation system of claim 9, wherein the ion implantation system extends along a longitudinal axis, and wherein the ion beam generated by the ion source is directed transverse to the longitudinal axis. 11. The ion implantation system of claim 10, wherein the translation means translates the workpiece through the ion beam in the scanning direction which is disposed generally orthogonal to the ion beam. 12. The ion implantation system of claim 10, wherein the ion beam generated by the ion source and the longitudinal axis form an angle therebetween that is greater than or equal to about 85 degrees. 13. The ion implantation system of claim 1, wherein the system extends along a longitudinal axis, and wherein the ion beam generated by the ion source and the longitudinal axis form an angle therebetween of equal to or greater than about 5 degrees. 14. The ion implantation system of claim 1, wherein the workpiece handling assembly further includes means for moving the position ofthe support surface relative to the loadlock. 15. The ion implantation system of claim 1, further comprising an end station assembly having a workpiece cassette for storing a plurality of workpieces, and an end effector for transporting the workpieces between the cassette and the loadlock. 16. The ion implantation system of claim 1, further comprising temperature control means coupled to the loadlock for disposing at least a portion ofthe loadlock at a selected temperature. 17. The ion implantation system of claim 1, wherein the loadlock includes a chamber having a floor, the system further including cooling structure for disposing the chamber floor at a selected temperature to form a cold deck. 18. The ion implantation system of claim 1, wherein the loadlock includes a chamber having a floor, the system further including vacuum means coupled to the floor for drawing the workpiece into contact therewith when the workpiece is disposed within the chamber, thereby effecting heat transfer from the workpiece to the chamber floor. 19. The ion implantation system of claim 18, wherein the chamber floor has a plurality of vacuum-applying openings formed therein, the openings being in fluid communication with the vacuum means for disposing a substantial portion ofthe backside ofthe workpiece in contact with the cooling surface. 20. The ion implantation system of claim 1 , further comprising a plurality of loadlocks having a top surface and a bottom surface, the loadlocks being axially positioned relative to each other to form a stacked array of loadlocks.

Inventors: BLAKE JULIAN G, BLAKE, JULIAN, G.
Applicants: EATON CORP, EATON CORPORATION
Application year: 1997
Main classification: H01J37
IPC codes: H01J27, H01J37, H01L21