Abstract:
An improved conveyor system for transporting a microelectronic workpiece within a processing tool is set forth. The conveyor system includes a transport unit slidably guided on a conveyor rail for transporting and manipulating the workpieces. The transport unit includes a vertical member which is connected to a base end of a two section robot arm. The robot arm includes an end effector at a distal end thereof which is actuated to grip a surrounding edge of a workpiece. A first rotary actuator is arranged to rotate the vertical member about its axis to rotate the entire robot arm. A second rotary actuator is positioned to rotate the second section of the robot arm, via a belt, with respect to the first section of the robot arm. A third rotary actuator is arranged to rotate the end effector about its horizontal axis. The third rotary actuator permits the end effector to flip the microelectronic workpiece between a face up and a face down orientation. In a further aspect of the invention, two transport units are mounted to slide laterally on the conveyor rail. The transport units include a vertical space between respective end effectors and the first sections of the robot arms to allow wafers carried by the end effectors to overlap in plan. Two different end effectors are disclosed, a plunger activated gripping device and a vacuum operated gripping device which uses raised pad areas, vacuum ports and locating pins.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of International PCT Patent Application No. PCT/US99/15567, designating the U.S., filed Jul. 9, 1999, entitled ROBOTS FOR MICROELECTRONIC WORKPIECE HANDLING, which claims priority from U.S. patent application Ser. No. 09/114,105, filed Jul. 11, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     There are a wide range of apparatus types for processing workpieces that ultimately become microelectronic devices. As the microelectronics industry advances toward efficient and economical mass production of the devices, the demands on the apparatus used in processing of the workpieces have increased. Increasingly, automation of the apparatus is being used to meet these ever-increasing demands. More particularly, many of the increased demands relate to automated devices for handling the microelectronic workpieces during processing. 
     An automated apparatus used for processing a microelectronic workpiece, such as a semiconductor workpiece, is disclosed in U.S. Ser. No. 08/991,062, filed Dec. 15, 1997, and titled “Semiconductor Processing Apparatus Having Lift and Tilt Mechanism”, which is hereby incorporated by reference. This apparatus utilizes a plurality of workpiece processing modules or stations for performing various processing steps. Workpiece transport units are used to access workpiece cassettes and transfer workpieces throughout the processing apparatus. A workpiece conveyor supports and guides the workpiece transport units for transferring individual workpieces between workpiece interface modules and the workpiece processing modules or stations. The workpiece conveyor also includes a transport unit guide, such as an elongated rail, which defines a path for one or more workpiece transport units within the apparatus. The workpiece transport units which move along the rail are configured to have a workpiece transfer arm assembly having an end with a vacuum effector for holding a workpiece. The transfer arm assembly can be adjusted in vertical elevation and can be rotated about the vertical axis for precise positioning of the effector and the workpiece. 
     Workpieces are typically handled and stored with the face to be processed (the “front” face) oriented facing upwardly. This orientation avoids contact on the front face by the supporting structure. Some processing modules, on the other hand, require the workpiece to be oriented with the face to be processed facing downwardly. To accommodate such requirements, some processing modules such as electroplating reactors, utilize a processing head which can be “flipped”, i.e., rotated, between a first position in which the processing head is positioned to receive the workpiece with a front side of the workpiece facing up and a second positioned in which the front side of the workpiece faces down for processing. 
     Making provision for each processing module or station to “flip” the workpiece for processing requires complicated head operator mechanisms for rotating the processing heads. Such operator mechanisms can require substantially heavy or large structures for rotating the processing heads, and can require significant overhead operating room for the rotational movement. 
     The present inventors have recognized that reducing or eliminating the requirement for processing modules to turn over or flip a workpiece for processing would simplify the overall workpiece apparatus. The present inventors have also recognized that cost savings and process simplicities would be enhanced by eliminating the requirement for flipping the workpiece. Still further, the inventors have recognized that a wider range of processing stations of different types may be integrated into a single processing tool. Such processing stations may have varying wafer orientation requirements, one station requiring a front-face up orientation for processing while another station requires a front-face down orientation for processing. An apparatus that addresses each of these recognized problems is set forth. 
     Additionally, the present inventors have recognized that it would be advantageous to provide a workpiece conveyor with transport unit slidable thereon which minimizes the required working space or “footprint” of the conveyor and transport units operating between laterally disposed process units. An apparatus which provides this advantage is set forth. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a workpiece conveyor system that is used for transporting individual workpieces between workpiece processing stations and the/or interface modules in a workpiece processing apparatus. The workpiece conveyor system includes an improved workpiece transport unit that carries the workpieces within the apparatus on, for example, a conveyor rail or the like. The transport unit includes a vertical member extending from a housing. An arm member extends from the vertical member at a base end of the arm member. A workpiece-holding end effector is disposed at a distal end of the arm member and is selectively driven in rotation about a horizontal axis to “flip” the workpiece between a face-up orientation and a face-down orientation. The effector is preferably configured to grip an edge of a workpiece, such as a semiconductor wafer, and can have a workpiece presence sensor for informing a control unit that a workpiece is present on the effector. 
     In accordance with one embodiment of the present invention, the workpiece transport unit provides five “axes” of movement. To this end, the transport unit can be driven linearly on the rail along a horizontal axis (Y). The vertical member can be raised or lowered vertically along a vertical axis (Z 1 ). The arm member can be rotated about the vertical axis (Z 1 ) and a distal portion of the arm member can be rotated about the vertical axis (Z 2 ). The end effector can rotate or “flip” about a horizontal axis (R), for example, to orient the workpiece in either the front-face up or front face down orientation. To execute such rotation, the arm member preferably includes a rotary actuator mounted within the arm member to turn the end effector about the horizontal axis. 
     By providing a workpiece transport unit with increased flexibility of movement, including a rotation about a horizontal axis, more expensive, heavy and complicated mechanisms for flipping workpieces at a plurality of process modules is avoided. Additionally, it becomes possible to integrate processing stations having different workpiece orientation requirements into a single processing apparatus. 
     In a further aspect of the invention, a workpiece transport unit is provided having a vacuum gripping mechanism for holding a workpiece to the end effector. The vacuum gripping mechanism includes a plurality of raised pads for pressing against an edge region of the workpiece, and vacuum ports through the pads for urging the workpiece onto the pads. 
     In a still further aspect of the invention, two workpiece transport units are slidable on opposite lateral sides of a guide rail structure. At least one of the transport units includes a first end effector which is elevated above an adjacent section of its respective first robot arm, providing a vertical space therebetween. The vertical space is sufficiently projected in a horizontal direction for the respective other end effector of the other transport unit, operating at a lower elevation, to pass under the first end effector and over the first robot arm. Thus, wafers held by the two end effectors can be overlapped in plan, and the two transport units can be moved longitudinally along the conveyor rail, together, or individually with respect to each other, without interference between end effectors or wafers held thereby. This arrangement minimizes the lateral footprint needed between opposing process units of the tool. 
    
    
     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings in which details of the invention are fully and completely disclosed as part of this specification. 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a workpiece processing tool incorporating an improved workpiece conveyor system constructed in accordance with one embodiment of the present invention; 
     FIG. 2 is a perspective view of the improved workpiece conveyor system shown in FIG. 1; 
     FIG. 3 is a sectional view taken generally along line  3 — 3  of FIG. 2; 
     FIG. 4 is a perspective view of a workpiece transport unit constructed in accordance with one embodiment of the present invention; 
     FIG. 5 is an exploded perspective view of the workpiece transport unit shown in FIG. 4; 
     FIG. 6A is a partial exploded perspective view of the robot arm components of the transport units of FIG. 5; 
     FIG. 6B is a partial exploded perspective view of the robot arm components of FIG. 6A, FIG. 6B being a continuation of FIG. 6A; 
     FIG. 7 is a side view of the robot arm components of FIGS. 6A,  6 B, as assembled; 
     FIG. 8 is a sectional view taken generally along line  8 — 8  of FIG. 7; 
     FIG. 9 is a sectional view taken generally along line  9 — 9  of FIG. 8; 
     FIG. 10 is an enlarged fragmentary sectional view from FIG. 8; 
     FIG. 11 is an enlarged fragmentary right side view taken from FIG. 7; 
     FIG. 12 is an enlarged fragmentary sectional view taken from FIG. 8; 
     FIG. 13 is an enlarged perspective view of one embodiment of an end effector suitable for use in the workpiece transport unit shown in FIG. 4; 
     FIG. 14 is a rear perspective view of the workpiece transport unit of FIG. 4 in which the arm is in a different rotary position and in which the end effector is holding a workpiece; 
     FIG. 15 is a plan view of the end effector of FIG. 13; 
     FIG. 16 is a sectional view taken generally along line  16 — 16  of FIG. 15; 
     FIG. 17 is an enlarged fragmentary sectional view taken from FIG. 16, shown holding a workpiece; 
     FIG. 18 is an enlarged fragmentary sectional view taken generally along line  18 — 18  of FIG. 15; 
     FIG. 19 is an enlarged fragmentary sectional view taken from FIG. 16; 
     FIG. 20 is an enlarged fragmentary sectional view of an alternative embodiment robot arm; 
     FIG. 21 is an enlarged view taken from FIG. 20; 
     FIG. 22 is an end view of an alternative workpiece processing tool having a workpiece conveyor system using alternative transport units which incorporate the robot arms of FIG. 20; 
     FIG. 23 is an enlarged view taken from FIG. 22; 
     FIG. 24 is a plan view of the workpiece processing tool of FIG. 22; 
     FIG. 25 is an exploded perspective view of an end effector of the robot arm shown in FIG. 20, and a workpiece; 
     FIG. 26 is a plan view of the end effector of FIG. 25; 
     FIG. 27 is a bottom view of the end effector of FIG. 26; 
     FIG. 28 is an enlarged view taken from FIG. 26; 
     FIG. 29 is an enlarged view taken from FIG. 26; 
     FIG. 30 is a sectional view taken along line  30 — 30  in FIG. 26; 
     FIG. 31 is a plan view of the end effector of FIG. 25, holding a workpiece; 
     FIG. 32 is a sectional view taken along line  32 — 32  in FIG. 31; 
     FIG. 33 is a sectional view taken along line  33 — 33  in FIG. 31; and 
     FIG. 34 is a sectional view taken along line  34 — 34  in FIG.  31 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention is susceptible of embodiment in many different forms, there are shown in the drawings and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
     FIG. 1 illustrates an exemplary modular workpiece processing apparatus  10  that may use the improved conveyor system of the present invention. As illustrated, apparatus  10  includes an input/output assembly  12 , and left and right processing modules  14 ,  16 . The apparatus  10  also includes the improved workpiece conveyor system  20 , a top exhaust assembly  24 , and an end panel  25 . As illustrated, left and right processing modules  14 ,  16 , which each include a plurality of workpiece processing stations, may be secured to one another about the workpiece conveying system  20  to form a processing chamber having a longitudinally disposed inlet and outlet. Preferably, workpiece conveyor  20  is disposed in the processing chamber so that it can access each of a plurality of workpiece cassette interface modules within the input/output assembly  12  and, further, can access each workpiece processing station within the left and right processing modules  14 ,  16 . 
     A plurality of the processing modules  14 ,  16  may be secured in an end-to-end configuration to thereby provide an extended processing chamber capable of performing a substantial number of processes on each workpiece or, in the alternative, process a larger number of workpieces concurrently. In such instances, the workpiece conveying system  20  of one apparatus  10  is programmed to cooperate with the workpiece conveying system  20  of one or more prior or subsequent conveying systems  20 . 
     FIG. 2 illustrates further details of the workpiece conveyor  20  for transporting workpieces throughout the processing apparatus  10  of FIG.  1 . As shown, the workpiece conveyor  20  generally includes one or more workpiece transport units  30 , 32  that are coupled for movement along workpiece transport unit guide  26 . The transport unit guide  26  preferably comprises an elongate spine  26   a  mounted on a frame  28 . Alternatively, transport unit guide  26  may be formed as a track or other elongate configuration for guiding workpiece transport units  30 ,  32  thereon. The length and shape of workpiece conveyor  20  and transport unit guide  26  may be varied, and configured to permit the workpiece transport units  30 ,  32  to access each processing station within the apparatus  10 . 
     In the illustrated embodiment, the workpiece transport unit guide  26  includes a spine that supports a pair of upper guide rails  36 ,  38  mounted on opposite sides of the upper portion of spine  26   a  and a pair of lower guide rails  40 ,  42  mounted on opposite sides of the lower portion of spine  26   a.  Each workpiece transport unit  30 ,  32  preferably engages a respective pair of the upper and lower guide rails  36 ,  40  and  38 ,  42 . Each pair of guide rails can mount one or more transport units along the spine  26   a.    
     Each workpiece transport unit  30 ,  32  is powered along the respective path by a suitable driver. More specifically, drive operators  61 ,  64  are mounted to respective sides of transport unit guide  26  to provide controllable axial movement of workpiece transport units  30 ,  32  along the transport unit guide  26 . The drive operator  61 ,  64  may be linear magnetic motors for providing precise positioning of workpiece transport units  30 ,  32  along the guide  26 . In particular, drive operators  61 , 64  are preferably linear brushless direct current motors. Such preferred drive operators  61 ,  64  utilize a series of magnetic segments which magnetically interact with a respective electromagnet  69  mounted on each of the workpiece transport units  30 ,  32  to propel the units along the transport unit guide  26 . 
     Cable guards  72 ,  73  may be connected to respective workpiece transport units  30 ,  32  and frame  28  for protecting communication or power cables therein. Cable guard  72 ,  73  may comprise a plurality of interconnected segments to permit a full range of motion of workpiece transport units  30 ,  32  along transport unit guide  26 . 
     As shown in FIG. 3, the workpiece transport unit  30  is coupled with a first side of the spine  26   a  of guide  26 , and the workpiece transport unit  32  is coupled to a second side of the spine  26   a.  Each workpiece transport unit  30 ,  32  can include four linear bearings  136 ,  140 ,  138 ,  142  for engagement with linear guide rails  36 ,  40 ,  38 ,  42  respectively. 
     FIG. 4 illustrates a workpiece transport unit  30  which is substantially identical to the workpiece transport unit  32 . For simplicity, only the transport unit  30  will be described in detail. The transport unit  30  includes a robot arm or arm member  100  extending horizontally from a transport unit housing  106  at a base end of the arm member, to an edge-grip end effector  108  at a distal end of the arm member. The arm member  100  includes a first arm section  110  rotatably connected to a second arm section  1   14 . The first arm section  110  is rotatable about a vertical axis Z 1  with respect to the housing  106 . The second rotatable arm section  114  is rotatable about a vertical axis Z 2  with respect to the first arm section  110 . The end effector  108  is rotatable about a horizontal axis (or “flip” axis) R, perpendicular to the vertical axes Z 1 and Z 2 . 
     The housing  106  includes a vertically arranged base plate  120 , a first top cover plate  122 , a second top cover plate  124 , a bottom cover plate  126  and a U-shaped shroud  128 . The U-shaped shroud  128  comprises side walls  129 ,  130  and a back wall  132 . 
     Mounted to the base plate are the four linear bearings  136 ,  138 ,  140 ,  142  which receive the guide rails as shown in FIG.  3 . Arranged between the upper linear bearings  136 ,  138  and the lower linear bearings  140 ,  142  is a brushless motor  69 , which acts on the drive operator  61  of the guide  26  (shown in FIGS.  2  and  3 ). A head reader linear encoder  149  provides a position signal corresponding to the position of the transport unit  30  on the guide  26 , to a control unit used to control the transport unit. 
     FIG. 5 illustrates the various components that are disposed inside of the housing  106 . As illustrated, a lift assembly  154  and cooperating components of arm assembly  100  are disposed within the housing  106 . 
     The lift assembly  154  includes the various components used drive the arm assembly  100  along vertical axis Z 1 . To this end, the lift assembly  154  includes a lead screw motor  156  which turns a threaded lead screw  158  that, and turn, is disposed for rotation within a lift bracket  160 . A lead screw nut  162  is threaded onto the lead screw  158  and fastened to a lift nut adaptor  164 . Vertical movement within the lift assembly  154  is guided by a linear rail  170 . Thus, rotation of the lead screw  158  about its axis will advance the nut  162  and the adaptor  164  upwardly, axially along the lead screw  158 . Reverse rotation of the lead screw motor  156  will lower the nut  162  and adaptor  164  along the lead screw  158 . A signal corresponding to the vertical position of the arm assembly  100  along the vertical axis Z 2  is provided by an absolute position sensor  165 . 
     The arm member  100  is connected to vertical rail  176  for movement along the vertical axis Z 2 . A vertical linear bearing assembly  170  having a track  172  and a sliding element  174  is arranged adjacent to the lift assembly  154 . The vertical member includes at a base end thereof a carrier plate  180  which is connected to the moving element  174  and the adaptor  164  such that the vertical rail  176  and the arm member  100  can be vertically raised and lowered by the adaptor  164  through actuation of the lead screw motor  156 . The linear bearing assembly  170  ensures a precise and stable vertical lifting of the vertical member. A lift encoder  177  is connected to the driven shaft of the lead screw motor  156  to send a precise lift position signal to a control for the transport unit. 
     FIGS. 6A and 12 illustrate a first rotational movement motor  200  which, by rotation of an output shaft  201 , effects rotation of the vertical member  176  and the first arm section  110  about the vertical axis Z 1  with respect to the housing  106 . The motor  200  is connected by a motor mount  202  to a lower housing  206 . The lower housing is connected by screws  210  to the carrier plate  180 . A coupling  214  connects the output shaft  201  of the motor  200  to an input shaft  218  of a tube assembly  220 . Between the tube assembly  220  and the lower housing  206  are arranged a bearing retainer  224 , a resolver sensor  226 , a roller bearing  230  (shown schematically), and a lower bearing retainer  232 . The resolver sensor  226  sends a precise rotary position signal of the tube assembly  220  with respect to the housing  106  to a control of the transport unit. 
     FIGS. 6B,  8  and  9  illustrate the connection of tube  220  to a lower housing  242  of the first arm section  110 . Rotation of the tube  220  rotates the lower housing  242  and the first arm section  110  about the vertical axis Z 1 . A top cover  245  fits over the lower housing  242  to form a substantially closed volume  244  in which these components are held. 
     FIGS. 6B and 8 through  10  illustrate components for imparting rotation of the second arm section  114  about the vertical axis Z 2 . As shown, a second rotational motor  240  is housed within the tube  220  and the lower housing  242 . The motor  240  is vertically supported by a motor flange  248  which is fastened to a bottom wall  242   a  of the housing  242  and to the tube  220 . The flange  248  is also fastened to a top of the motor  240  as shown in FIG. 8, by fasteners (not shown). An output shaft  250  of the motor  240  receives a pulley flange  252 , a drive pulley  254  and a pulley clamp  256  which together constitute a driven pulley arrangement as shown assembled in FIG.  8 . The second rotation motor  240  includes a rotary position encoder (not shown) integrated therewith. The encoder sends a rotary position signal to a control unit for control of the transport unit operation. 
     As shown more clearly in FIG. 10, a wrist torque tube  260  is mounted for rotation within the lower housing  242  and is wrapped by an arm belt  290 . The arm belt  290  is driven by the drive pulley  254 . A bearing  264  (shown schematically) held by a bearing retainer  266 , and a torque tube retainer  272  support and guide the torque tube  260 . Upper and lower retaining rings  262 , 263  fit on the torque tube  260  and vertically retain the belt  290  circulating on the torque tube  260 . A read head mount  268  is mounted with a rotary absolute encoder  270  to the lower housing  242 . The rotary absolute encoder generates a rotational position signal of the second arm section  114  with respect to the first arm section  110 . The position signal is provided to a control for the transport unit. An absolute encoder cover  274  mates with the bottom of the lower housing  242 . 
     Located above the lower housing  242  is a robot wrist housing  280  fastened to the lower housing  242 , and a bottom cover  282  fastened to the torque tube  260 . Also held within the volume of the lower housing  242  is a flip axis amplifier  292 , and a spring loaded belt tensioner  294 . 
     Referring to FIG. 9, the tensioner  294  includes an idler pulley  295  for maintaining tension on the arm belt  290 . The idler pulley is carried by a plate  297  which is pivoted about a pin  296  with respect to the lower housing  242 . The plate is spring loaded by a spring (not shown) stretched between a fixed point on the lower housing  242  and a spring pin carried by the plate  297 . The force of the spring rotates the plate to press the idler pulley  295  against the belt  290 . 
     The second rotational motor  240  is selectively actuated to circulate the belt  290  which is wrapped around the wrist torque tube  260 . This actuation swings the second arm section  114  about the vertical axis Z 2 . 
     FIGS. 6B and 10 illustrate the flip axis components which allow rotation of the effector  108  about the horizontal axis R. Located beneath a flip axis cover  300  within the second arm section  114  is a flip axis motor  302 . The flip axis motor  302  is selectively actuated to rotate the end effector  108  about the horizontal axis R. The flip axis motor is connected to an actuator mount  304 . A bearing housing  306  is located within the cover  300  and holds a bearing  308  (shown schematically) together with a retainer  310 . A flip axis hub  312  is mounted to the end effector  108 . 
     The flip axis motor includes an output shaft  350  connected, at a back end of the motor  302 , to two rotary position encoders  351 . The redundant rotary position encoders provide a signal to a control unit of the transport unit that corresponds to the rotary position of the effector  108  about the horizontal axis R with respect to the second arm section  114 . The output shaft  350  is clamped to the flip axis hub  312  by the action of a clamp ring  352  and an interacting pressure flange clamp  354  which are squeezed between the flip axis hub  312  and a rear flange  356  of the effector  108 . The rear flange  356  is attached by fasteners to the flip axis hub  312  (registering fastener holes shown in FIG.  6 B). 
     The flip axis hub  312  includes an annular bearing surface  360  which is journaled for rotation by the bearing  308 . The bearing  308  is held in place by the bearing retainer  310  which is attached by fasteners to the bearing housing  306  (registering fastener holes shown in FIG.  6 B). The bearing housing  306  includes a base portion  362  which is fastened to the wrist torque tube  260  and to the bottom cover  282  by fasteners  364 . The actuator mount  304  is attached by fasteners  305  to a rear side of the bearing housing  306 . The actuator mount  304  is attached by fasteners to a front side of the motor  302  (registering fastener holes are shown in FIG.  6 B). 
     As illustrated in FIG. 10, a pneumatic cylinder  414  includes a spring  470  which exerts a thrusting force on a piston  472  which is connected to the plunger  434  via a threaded socket  473 . Pressurized air introduced into the port  422  acts on the piston  472  in opposition to the force of expansion of the spring and retracts the plunger  434  (to the left as shown in FIG.  10 ). 
     As can be seen in FIG. 10, an annular space  600  is provided around the pneumatic cylinder  414  and beneath the flip axis cover  300  for the purpose of containing pneumatic tubing and signal and power conductors wound in a loose fashion to allow for rotation of the end effector  108 . This pneumatic tubing as well as the conductors can be routed from the space  600  backwardly, partly through the second arm section  114 , and downwardly through a central passage  260   a  of the torque tube  260 . Other conductors, such as from the motor  302  and the encoders  351  are routed via printed circuit cables disposed in cavities  260   b.  This arrangement winds up or unwinds these cables about torque tube  260  to thereby allow rotation of arm section  114  about axis Z 2 . The tubing and conductors can then be routed through the encoder housing  224 , upwardly into the volume  244  provided by the lower housing cover  245 , and down through the vertical member  176 , to exit the tube  220  at the opening  604  as shown in FIG.  6 A. To allow sufficient flexibility for the relative rotation between the first and second arm sections  110 ,  114 , the conductors and tubing can be loosely coiled within the torque tube  260  before exiting. 
     FIGS. 13 through 16 illustrate one embodiment of the edge-gripping end effector  108 . As illustrated, the end effector  108  includes a paddle  400  extending from a base portion  400   a  (shown in FIG. 19) located over a bracket  402 . The paddle  400  is substantially Y-shaped with two substantially parallel prongs, a first prong  401  and a second prong  403 . A gripper body  404  is connected by fasteners  408  to the bracket  402  and acts to clamp the base portion  400   a  of the paddle  400  between the gripper body  404  and the bracket  402 . The pneumatic actuator  414  is connected to an upstanding leg  410  of the bracket  402 , connected by a plurality of fasteners  416 . The pneumatic actuator  414  is connected to the rear flange  356  of the effector  108 , by fasteners (not shown). The pneumatic actuator  414  includes the pressurized air inlet port  422  which can be a threaded opening for receiving a tube fitting of an air supply line (not shown). 
     The gripper body  404  includes a guide tab  428  at a front end thereof, overlying the paddle  400 . The guide tab includes, on a top surface thereof, a semicylindrical groove  430 . A plunger  434  is fit within a longitudinal bore through the gripper body  404 , in registry with the groove  430 . The tab  428  includes a ramp surface  440  on a front end thereof, declined downwardly in a forward direction toward a surface of the paddle  400 . 
     On a front surface of the gripper body  404  is a workpiece sensor  442 . The workpiece sensor is a light emitting and receiving sensor which emits a light beam and, if a workpiece is present on the paddle  400 , receives a light reflection from the workpiece. If no workpiece is present the reflection is not received, and a “no workpiece” signal or condition is transmitted. Preferably, the sensor  442  emits an infrared light beam. 
     At a front end of the paddle  400  are located two identical workpiece edge-gripping pins  450 ,  452 . The pins are preferably formed from plastic material. For simplicity, only the pin  452  will be described. As shown in FIG. 17, the pin  452  has a cylindrical body  456  with a radially extending top flange  458  and an intermediate base  460 . The base  460  fits onto a stepped region  462  of the prong  403  of the paddle  400 . A lower portion of the cylinder  456  is held within an aperture  464  through the prong  403 , by friction, bonding, or by adhesive. The intermediate base  460  has an outwardly declined, surrounding top surface  466 . When the workpiece is placed onto the paddle  400 , initially before being gripped by the pins, the declined surface  466  ensures that only an edge of the workpiece will be in contact with the effector, on the declined surface  466 . 
     FIG. 18 illustrates the workpiece W (shown solid) initially resting on an edge  467  thereof on the declined surface  466 . When the effector grips the workpiece against the pins  450 ,  452  by means of the plunger  434 , an inclined annular radius  468  of the pin will vertically raise the workpiece W to be in edge contact with a vertical contact surface  456   a  of the pin  452 . This ensures that the workpiece W is contacted by the pin substantially only on an outside edge  469  of the workpiece. In addition to the gripping force, the workpiece W is also retained vertically by the flange  458 , particularly during the flipping operation. 
     As shown in FIG. 19 the plunger  434  includes a conical tip  434   a  which has an inclined portion  474  that pushes and overlies an edge  475  of the workpiece W to vertically retain the workpiece on the paddle  400 . The ramp surface  440  ensures that the workpiece is only contacted on its edge  475 , and does not rest on its flat back surface. When the end effector  108  is rotated about the horizontal axis R by the flip motor  302 , the flanges  458  of the pins  450 ,  452  and the conical tip  434   a  of the plunger  434  ensure that the workpiece does not fall from the paddle  400 . 
     The plunger includes a cylindrical slender forward extension  434   b,  which includes the tip  434   a,  and a cylindrical, thicker barrel portion  434   c  extending rearwardly therefrom. Connected to the barrel portion  434   c  is a cylindrical tool gripping portion  434   d  having opposing flat surfaces  434   e,    434   f  for engagement of the portion  434   d  with a wrench. A threaded connecting end portion  434   g  is screwed into the threaded socket  473 . The plunger  434  fits into a stepped bore  476 . The stepped bore  476  includes a forward slender bore  476   a  for guiding the slender forward extension  434   b  and a rear larger bore  476   b  for guiding the rear barrel portion  434   c.    
     Thus, in operation, when a workpiece W is placed onto the paddle  400  as shown in FIG. 14, air is released from the pneumatic cylinder  414  and the spring  470  thrusts the plunger  434  forwardly (to the left in FIG.  19 ). The conical tip  434   a  pushes the workpiece edge into the pins  450 ,  452 . The workpiece edge is pressed into the vertical contact surface  456   a  of the pins and between the ramp surface  440  and the inclined surface  474 . The workpiece can be released by introduction of pressurized air into the pneumatic cylinder  414 , to retract the plunger  434 . 
     FIG. 20 illustrates an alternative robot arm assembly  500 . The robot arm assembly shares many common features with the robot arm assembly described, for example, in FIG. 8 except as described below. A first rotatable arm section  5   10  includes the electric motor  240  and the belt  290  for turning a wrist tube  540  about the vertical axis Z 2 . A vacuum chamber cap  546  is fastened to the wrist tube  540  by a plurality of vertically oriented fasteners (not shown). An end effector  562  is fastened to the vacuum chamber cap  546 . Thus, turning the wrist tube  540  turns the end effector  562 . 
     As shown more clearly in FIG. 21, the first arm section  510  includes a housing  560  which surrounds the rotary absolute encoder  270 . A pneumatic fitting  564  is exposed outside of the housing  560  for being connected to a source of vacuum, and is in flow communication with a channel  570  through the wrist tube  540 . The channel is in flow communication with an indented region  572  of the wrist tube  540 . The vacuum chamber cap  546  includes an inlet portion  574  which extends down into the indented region  572 . The inlet portion  574  includes a plurality of ports  576  and an internal inlet nozzle  578 . The inlet nozzle  578  extends upwardly into an axial channel  580  which is in flow communication with a vacuum channel  760  (described below) within the end effector  562 . 
     FIG. 22 illustrates a processing tool  600  having a central workpiece conveyor system  620 . The workpiece conveyor system  620  includes a workpiece transport unit guide  26  as previously described, and transport units  630 ,  632 , one slidably mounted on each side of the guide as previously described. The workpiece transport unit  630 , 632  incorporate the robot transfer arm  500  as described in FIGS. 20 and 21. 
     FIG. 23 illustrates a compact lateral arrangement of the transport units  630 ,  631  having a lateral outside dimension  640  for compact mutual sliding along the guide rail  26 . The lateral dimension  640  can be minimized because the caps  546  allow a sufficient vertical clearance, projected horizontally, between the end effectors  562  such that when the (right) robot arm  500  is maintained at a slightly lower elevation than the (left) robot arm  500 , the (right) end effector  562  and wafer W held thereby can underlie the (left) end effector  562  and wafer W held thereby in close proximity to the (left) vacuum chamber cap  546 . The (left) end effector  562  and wafer W held thereby can overlie the (right) end effector  522  and wafer W held thereby. The transport unit  630 ,  632  can both be moved along the rails of the guide rail  26  in this configuration, or can be moved separately. 
     FIG. 24 illustrates the (left and right) transport units  630 ,  632  in this compact, retracted arrangement with the wafers W at slightly different elevations. The transport units can deliver wafers to the laterally arranged process vessels  650 . 
     The design of FIGS. 22-24 allows for simultaneous linear transfer of wafers by both robots in either direction along the rail without interference by passing one end effector and wafer over the top of the respective other robot end effector and wafer. This is accomplished by setting a safe travel zone vertically for each robot. The vacuum cap  546  of the robot arm assembly has an axially length which elevates the end effector above the first arm section  510  a distance sufficient to allow the adjacent robot end effector and wafer held thereby to pass between the first arm section  510  and respective end effector. 
     The result of the described configuration is a reduced tool footprint, when viewed in plan view, of approximately nine inches in width. 
     The embodiment shown in FIG. 8 could also be modified to extend the torque tube  260  to provide a clearance between the first arm section  110  and the end effector  108  in a similar fashion. 
     FIG. 25 illustrates an alternative embodiment end effector  700  for gripping a workpiece such as a wafer W. The end effector  700  includes a paddle member  706  and a link member  708 . The paddle member  706  is fastened to the link member  708 . The paddle member  706  includes vacuum channel  740  on a bottom side thereof, which can be closed by a vacuum closeout  710 . The paddle member includes four holes which receive locator pins or buttons  714  which locate the wafer W onto the paddle  706 . A link member vacuum closeout  716  closes the vacuum channel  760  arranged on a bottom side of the link member (shown in FIG.  32 ). 
     FIG. 26 illustrates a top surface  706   a  of the paddle  706 . The paddle  706  includes parallel prongs  722 ,  724 . At the distal end of the prongs are raised wafer supporting ridges or pad areas  726 ,  727 . The locator pins  714  are located adjacent to the pad areas  726 ,  727 . At the base end of the paddle  706  is an elongated wafer supporting ridge or pad area  730 . Locator pins  714  are located at opposite ends of the pad area  730 . The pad areas  726 ,  727 ,  730  circumscribe a portion of a circle which corresponds to an edge region of a wafer supported on the paddle. 
     FIG. 27 illustrates the bottom of the paddle member  706  which includes the elongate vacuum channel  740  which is surrounded by a recessed ledge  742  which corresponds to the shape of the vacuum closeout  710  shown in FIG.  25 . Additionally, within the vacuum channel  740  are located vacuum ports or holes  744  which open the vacuum channel through a thickness of the paddle member  706  to vacuum openings in the pad areas. 
     FIG. 28 illustrates the pad area  727  including a vacuum port  744  therethrough which is in communication with the vacuum channel  740 . 
     FIG. 31 illustrates the wafer W located between the four locator pins  714  and covering the pad areas  726 ,  727 ,  730 . 
     FIG. 32 shows the link member vacuum closeout  716  which closes the elongate vacuum channel  760 . The closeout  716  includes an inlet opening  764  and an outlet opening  766 . The inlet opening  764  communicates with the vacuum chamber cap  546  as shown in FIG.  21 . The opening  766  communicates with the vacuum channel  740 . 
     FIGS. 33 and 34 illustrate one of the locator pins  714  in more detail. The locator pin  714  includes a beveled surface  714   b  which guides downward loading movement of the wafer W to arrive at its precisely located position adjacent to a base of the beveled surface  714   b.    
     The end effector assembly of FIGS. 25-34 provides a vacuum manifold which communicates vacuum pressure to the three vacuum pad areas  726 ,  727 ,  730  elevated above the remaining portions of the paddle top surface  706   a.  The differential vacuum pressure acting on each of the vacuum pad areas provides a force to hold the wafer stationary relative to the paddle. Advantageously, the elevated vacuum pad areas contact the wafer surface only in a preselected, defined exclusion zone of 3 mm, for example. Additionally, the four buttons or locator pins  714  provide guide “furniture” with angled lead-in to precisely locate the wafer relative to the raised pad areas to assure contact only on the wafer exclusion zone. 
     A tool system provides the controlled vacuum source to the end effector vacuum pneumatic fitting  564  such that a vacuum pressure sensor (not shown) in the tool can detect the presence of a wafer. 
     The vacuum gripping end effector of FIGS. 25-34 may offer some advantages over the plunger wafer gripping mechanism of FIGS.  13  and  15 - 19 . The plunger which actuates against the wafer may cause the wafer to slide relative to the paddle. To prevent the wafer from interfering with features in the carrier or process heads during this motion the robot must first lift the end effector up then back then actuate the plunger. The vacuum edge grips of FIGS. 25-34 simplifies robot movement by only requiring a lift up to attach the vacuum pad areas to the wafer. Additionally, the plunger type edge grip requires a wafer presence sensor system separate from the grip mechanism. This includes an electrical/optic sensor such as described with the previous embodiment, which requires wire routing through the wrist axis. Such wire routing limits a 360° rotation of the wrist. 
     Numerous modifications may be made to the foregoing system without departing from the basic teachings thereof. Although the present invention has been described in substantial detail with reference to one or more specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims.