Patent Publication Number: US-2007104559-A1

Title: End-effectors for handling microelectronic workpieces

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
      This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/305,335, filed Jul. 13, 2001, and U.S. Provisional Patent Application Ser. No. 60/305,388, filed Jul. 13, 2001, both currently pending and incorporated herein in its entirety by reference. 
    
    
      The following applications identified in paragraphs (a)-(k) are herein incorporated by reference:  
      (a) U.S. application Ser. No. 09/386,566, filed Aug. 31, 1999, entitled “IMPROVED ROBOT FOR MICROELECTRONIC WORKPIECE HANDLING;” 
      (b) U.S. application Ser. No. 09/386,590, filed Aug. 31, 1999, and entitled “ROBOTS FOR MICROELECTRONIC WORKPIECE HANDLING;” 
      (c) U.S. application Ser. No. 08/990,107, filed Dec. 15, 1997, entitled “SEMICONDUCTOR APPARATUS HAVING LINEAR CONVEYOR SYSTEM;” 
      (d) U.S. application Ser. No. 09/114,105, filed Jul. 11, 1998, entitled “IMPROVED ROBOT FOR MICROELECTRONIC WORKPIECE HANDLING;” 
      (e) U.S. patent application Ser. No. 09/875,428, entitled “INTEGRATED TOOLS WITH TRANSFER DEVICES FOR HANDLING MICROELECTRONIC WORKPIECES,” filed on 5 Jun. 2001  
      (f) U.S. patent application Ser. No. 09/875,304, entitled “DISTRIBUTED POWER SUPPLIES FOR MICROELECTRONIC WORKPIECE PROCESSING TOOLS,” filed on 5 Jun. 2001;  
      (g) U.S. patent application Ser. No. 09/875,365, entitled “ADAPTABLE ELECTROCHEMICAL PROCESSING CHAMBER,” filed on 5 Jun. 2001;  
      (h) U.S. patent application Ser. No. 09/875,424, entitled “LIFT AND ROTATE ASSEMBLY FOR USE IN A WORKPIECE PROCESSING STATION AND A METHOD OF ATTACHING THE SAME,” filed on 5 Jun. 2001;  
      (i) U.S. patent application Ser. No. 09/872,151, entitled “APPARATUS AND METHODS FOR ELECTROCHEMICAL PROCESSING OF MICROELECTRONIC WORKPIECES,” filed on 31 May 2001;  
      (j) U.S. patent application Ser. Nos. 09/866,391 and 09/866,463, each entitled “TUNING ELECTRODES USED IN A REACTOR FOR ELECTROCHEMICALLY PROCESSING A MICROELECTRONIC WORKPIECE,” filed on 24 May 2001; and  
      (k) U.S. patent application Ser. No. 09/875,300, entitled “TRANSFER DEVICES FOR HANDLING MICROELECTRONIC WORKPIECES WITHIN AN ENVIRONMENT OF A PROCESSING MACHINE AND METHODS OF MANUFACTURING AND USING SUCH DEVICES IN THE PROCESSING OF MICROELECTRONIC WORKPIECES,” filed on 5 Jun. 2001.  
     TECHNICAL FIELD  
      The present invention relates to equipment for handling microelectronic workpieces.  
     BACKGROUND  
      Microelectronic devices, such as semiconductor devices and field emission displays, are fabricated on and/or in microelectronic workpieces using several different apparatus (“tools”). Many such processing apparatus have a single processing station that performs one or more procedures on the workpieces. Other processing apparatus have a plurality of processing stations that perform a series of different procedures on individual workpieces or batches of workpieces. The workpieces are generally handled within the processing apparatus by automatic handling equipment (i.e., robots) because microelectronic fabrication requires extremely clean environments, very precise positioning of the workpieces, and conditions that are not suitable for human access (e.g., vacuum environments, high temperatures, chemicals, etc.).  
      An increasingly important category of processing apparatus are plating tools that plate metals and other materials onto workpieces. Existing plating tools use automatic handling equipment to handle the workpieces because the position, movement and cleanliness of the workpieces are important parameters for accurately plating materials onto the workpieces. The plating tools can be used to plate metals and other materials (e.g., ceramics or polymers) in the formation of contacts, interconnects and other components of microelectronic devices. For example, copper plating tools are used to form copper contacts and interconnects on semiconductor wafers, field emission displays, read/write heads and other types of microelectronic workpieces. A typical copper plating process involves depositing a copper seed layer onto the surface of the workpiece using chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating processes, or other suitable methods. After forming the seed layer, copper is plated onto the workpiece by applying an appropriate electrical field between the seed layer and an anode in the presence of an electrochemical plating solution. The workpiece is then cleaned, etched and/or annealed in subsequent procedures before transferring the workpiece to another apparatus.  
      Single-wafer plating tools generally have a load/unload station, a number of plating chambers, a number of cleaning chambers, and a transfer mechanism for moving the microelectronic workpieces between the various chambers and the load/unload station. The transfer mechanism can be a rotary system having one or more robots that rotate about a fixed location in the plating tool. One existing rotary transfer mechanism is shown in U.S. Pat. No. 6,136,163 issued to Cheung, et al., which is herein incorporated by reference in its entirety. Alternate transfer mechanisms include linear systems that have an elongated track and a plurality of individual robots that can move independently along the track. Each of the robots on a linear track can also include independently operable end-effectors. Existing linear track systems are shown in U.S. Pat. No. 5,571,325 issued to Ueyama, et al., PCT Publication No. WO 00/02808, and U.S. patent application Ser. Nos. 09/386,566; 09/386,590; 09/386,568; and 09/759,998, all of which are herein incorporated in their entirety by reference. Many rotary and linear transfer mechanisms have a plurality of individual robots that can each independently access most, if not all, of the processing stations within an individual tool to increase the flexibility and throughput of the plating tool.  
      These robots use end-effectors to grasp the workpiece in moving the workpiece from one processing station to another. The nature and design of the end-effectors will depend, in part, on the nature of the workpiece being handled. In some circumstances, the backside of the workpiece is not overly sensitive and may be contacted by the end-effector. In such circumstances, a vacuum-based end-effector may be used. Such vacuum-based end-effectors typically have a vacuum plenum having a plurality of vacuum outlets.  
      Some workpieces are not tolerant of such contact, though. Such workpieces typically must be handled by their edges and the distance inwardly from the edge of a workpiece which handling equipment may contact is strictly proscribed. This significantly limits the area of contact between the end-effectors and the workpieces, making it more difficult to securely grasp the workpiece during handling. If the workpiece is not grasped adequately, it may slide off the end-effector during movement of the robot in transferring the workpiece from one processing station to another. This problem is particularly acute where the end-effector is rotated to flip the workpiece from one horizontal orientation to an inverse horizontal orientation, e.g., to properly position a semiconductor wafer in an electroplating chamber.  
      It would be advantageous to confirm that a workpiece is properly positioned on and grasped by an end-effector before the end-effector moves the workpiece. International Publication No. WO 00/02808, which is incorporated herein in its entirety by reference, suggests using light reflected off the workpiece to determine the presence of a workpiece. A lack of reflected light indicates that no workpiece is present. While such a system does indicate whether a workpiece is in the proper vicinity, it does not ensure that the end-effector has properly grasped the workpiece.  
      Most current end-effectors use three spaced-apart points of contact with the workpiece to define a plane within which the workpiece will be received. Such three-point contact is able to adapt to minor dimensional differences from one workpiece to the next. Grasping the edge of the workpiece at four locations can lead to a more secure grip of a workpiece which is precisely the anticipated size. If the workpiece falls outside of very narrowly proscribed tolerances, however, it is difficult to ensure that all four contact points are gripping the edge of the workpiece with sufficient force to securely hold the workpiece to the end-effector.  
     SUMMARY  
      The present invention is directed toward various end-effectors for handling microelectronic workpieces and methods of handling microelectronic workpieces. Certain embodiments of the invention provide end-effectors having detectors capable of monitoring operation of the end-effector and, if so desired, generate an error signal if a workpiece is not properly engaged by the end-effector. This can significantly reduce the likelihood that a workpiece will be inadvertently dropped because it is not properly gripped by the end-effector.  
      Other embodiments of the invention provide end-effectors with resiliently actuated abutments to grasp the workpiece. In some specific applications of these embodiments, the end-effector may include four or more abutments, two or more of which are resiliently actuated, which engage the workpiece at spaced-apart locations. Such end-effectors may grasp workpieces by their edges at more than three points, but the use of the resiliently actuated abutments facilitates proper workpiece alignment with respect to all of the abutments.  
      One specific embodiment of the invention provides an end-effector for handling a microelectronic workpiece including a body. A plurality of spaced-apart abutments are carried by the body and the plurality of abutments may define a workpiece-receiving area. An actuator is also carried by the body and is associated with at least one of the abutments. The actuator is adapted to move such an associated abutment inwardly toward the workpiece-receiving area from a retracted position. A detector is operatively associated with the actuator and is adapted to generate an error signal if the associated abutment fails to engage an edge of a workpiece when the actuator moves the associated abutment inwardly. Such an error signal enables intervention in operation of a transfer device to avoid dropping or misplacement of the workpiece.  
      In accordance with another embodiment of the invention, an end-effector has a body and a plurality of abutments carried by the body at locations adapted to selectively engage an edge of a workpiece. A detector is adapted to detect engagement of the edge of the workpiece by at least one of the abutments. If so desired, the end-effector may also include an actuator associated with at least one of the abutments and the detector may detect a position of the actuator, e.g., by detecting the position of a flag carried by the actuator. In one application of this embodiment, the detector generates an error signal if the actuator moves inwardly a distance greater than a predetermined distance which corresponds to positive engagement of the workpiece by the abutment associated with the actuator. If the actuator moves in too far, this may be an indication that no workpiece is present or, even if it is present, it is not adequately grasped by the abutments to permit safe transfer of the workpiece to another processing station.  
      In accordance with another embodiment, an end-effector for handling microelectronic workpieces includes a body and three abutments carried by the body which together defining a workpiece-receiving area. These abutments include spaced-apart, stationary first and second abutments and a moveable third abutment carried by the body opposite the first and second abutments. The end-effector also includes an actuator comprising a shaft having an inward end carrying the third abutment. The shaft is adapted to move inwardly from a retracted position, e.g., for loading a workpiece in the workpiece-receiving area, to a deployed position wherein the third abutment engages the workpiece. The end-effector also includes a detector having spaced-apart first and second position sensors positioned adjacent a path of travel of the actuator. The first position sensor may generate a first signal when the actuator moves inwardly a predetermined distance from the retracted position and the second position sensor may generate a second signal when the actuator moves inwardly beyond the deployed position. The detector may generate an error signal if the second position sensor generates the second signal. This provides a reliable means for detecting whether the workpiece is properly engaged by the end-effector, avoiding mishaps encountered when workpieces are not gripped adequately during handling.  
      If so desired, any one or more of these end-effectors may be included in a transfer device. The transfer device may include a transport unit configured to move along a transport path, a lift assembly carried by the transport unit, an arm carried by the lift assembly, and at least one end-effector. If so desired, two or more end-effectors may be provided on the arm.  
      Another embodiment of the invention provides a method of grasping a microelectronic workpiece. This method includes providing an end-effector having a plurality of abutments, an actuator, and a detector. At least one of these abutments is a moveable abutment. A microelectronic workpiece is positioned between the abutments of the end-effector. The moveable abutment is moved inwardly using the actuator and action of the actuator is monitored using the detector. An error signal may be generated if the moveable abutment fails to engage an edge of the workpiece.  
      An end-effector in accordance with still another embodiment of the invention employs a body which carries first and second resilient members, first and second rigid members, and an actuator. The first the first resilient member carries a first abutment and the second resilient member carries a second abutment spaced from the first abutment. The first rigid member carries a third abutment and the second rigid member carries a fourth abutment spaced from the third abutment. The actuator is coupled to the first and second resilient members and to the first and second rigid members. The actuator is adapted to move the first, second, third and fourth abutments to selectively grasp or release a workpiece therebetween. If so desired, the body may include a first guide deforming the first resilient member and a second guide deforming the second resilient member.  
      An alternative end-effector in another embodiment includes a body and an actuator. The body carries a first abutment, a second abutment, a third abutment, and a fourth abutment. These abutments are spaced from one another to define a workpiece-receiving area therebetween. The actuator is resiliently connected to the first and second abutments and rigidly connected to the third and fourth abutments. The actuator is adapted to urge each of the abutments inwardly toward the workpiece-receiving area to grip a workpiece received therein. In a further aspect of this embodiment, the actuator may include first and second rotatable members. The first rotatable member may be coupled to the first abutment by a resilient first rod and coupled to the third abutment by a third rod. The second rotatable member may be coupled to the second abutment by a resilient second rod and coupled to the fourth abutment by a fourth rod. These rotatable members may be driven by a common driver, providing for reliable actuation of four separate abutments by a single driver.  
      Yet another embodiment provides an end-effector having an actuator and a body carrying a first channel, a second channel, a first abutment, a second abutment, a third abutment, and a fourth abutment. A first resilient member, which is slidably received in the first channel, couples the first abutment to the actuator. A second resilient member, which is slidably received in the second channel, couples the second abutment to the actuator. In one more specific application of this embodiment, the first resilient member has a relaxed shape and the first abutment is moveable between a retracted position and a workpiece-engaging position. The first channel in this design deflects the first resilient member away from its relaxed shape when the first abutment is in its retracted position. Similarly, the second resilient member may have a retracted position and a workpiece-engaging position and the second channel may deflect the second resilient member away from its relaxed shape when the second abutment is in its retracted position.  
      One additional embodiment of the invention provides a method of grasping a microelectronic workpiece. In this method, an end-effector is provided, this end-effector having an actuator and first, second, third and fourth abutments. The first abutment is coupled to the actuator by a first member and the second abutment being coupled to the actuator by a second member. A workpiece is positioned between the first, second, third and fourth abutments. Using the actuator, at least the first and second abutments are moved inwardly toward the workpiece to grasp the workpiece between the first, second, third and fourth abutments. The actuator urges the first and second abutments against an edge of the workpiece, resiliently deforming the first and second members. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an isometric view of a processing apparatus for processing microelectronic workpieces including a transfer device for handling the workpieces in accordance with an embodiment of the invention. A portion of the processing apparatus is shown in a cut-away illustration.  
       FIGS. 2A and 2B  are isometric views of transfer devices for handling microelectronic workpieces in accordance with embodiments of the invention.  
       FIG. 3  is an isometric view of a transfer device for handling microelectronic workpieces in accordance with an embodiment of the invention in which selected components are shown in cross section and other components are shown schematically.  
       FIG. 4  is a cross-sectional view of a portion of an arm assembly of the transfer device of  FIG. 3 .  
       FIG. 5  is an isolation view of a portion of an end-effector of the arm assembly of  FIG. 4 .  
       FIG. 6  is an isometric view of an end-effector in accordance with one embodiment of the invention.  
       FIG. 7  is an isometric isolation view of the encircled portion of  FIG. 6 .  
       FIG. 8  is an isometric exploded view showing the components of the end-effector of  FIG. 6 .  
       FIG. 9  is an isometric view of an actuator shaft useful in the end-effector of  FIG. 6 .  
       FIG. 10  is a side cross-sectional view taken along line  7 - 7  of  FIG. 6 .  
       FIG. 11  is a cross-sectional isolational view of the encircled portion of  FIG. 10 .  
       FIGS. 12A and 12B  are isometric views of stationary abutments in accordance with an embodiment of the invention which are useful in the end-effector of  FIG. 3 .  
       FIG. 13  is a top view of the stationary abutment of  FIG. 12B .  
       FIG. 14  is a cross-sectional view of the stationary abutment taking along line  14 - 14  in  FIG. 13 .  
       FIG. 15  is top elevation view schematically illustrating an end-effector in accordance with an alternative embodiment of the invention grasping a workpiece.  
       FIG. 16  is a schematic side elevation view of the end-effector and workpiece of  FIG. 15 .  
       FIG. 17  is a schematic bottom isometric view of the end-effector and workpiece of  FIG. 15 .  
       FIG. 18  is a schematic partial cross-sectional view taken along line  18 - 18  of  FIG. 15 .  
       FIG. 19  is a schematic partial cross-sectional view taken along line  19 - 19   FIG. 15 .  
       FIG. 20  is a top view of an abutment housing in accordance with one embodiment of the invention.  
       FIG. 21  is an isometric view of the abutment housing of  FIG. 20 .  
       FIG. 22  is an isometric view of one embodiment of a tensile rod useful in the end-effector of  FIG. 15 .  
       FIG. 23  is a schematic cross-sectional view taken along line  23 - 23  of  FIG. 15 .  
       FIG. 24  is a schematic partial cross-sectional view taken along line  24 - 24  of  FIG. 23 . 
    
    
     DETAILED DESCRIPTION  
      The following description discloses the details and features of several embodiments of end-effectors for handling microelectronic workpieces, and methods for using such devices. The term “microelectronic workpiece” is used throughout to include a workpiece formed from a substrate upon which and/or in which microelectronic circuits or components, data storage elements or layers, and/or micro-mechanical elements are fabricated. It will be appreciated that several of the details set forth below are provided to describe the following embodiments in a manner sufficient to enable a person skilled in the art to make and use the disclosed embodiments. Several of the details and advantages described below, however, may not be necessary to practice certain embodiments of the invention. Additionally, the invention can also include additional embodiments that are within the scope of the claims, but are not described in detail with respect to  FIGS. 1-24 .  
      The operation and features of the transfer devices for handling microelectronic workpieces are best understood in light of the environment and equipment in which they can be used. As such, several embodiments of processing apparatus in which the transfer devices can be used will be described with reference to  FIGS. 1 and 2 . The details and features of several embodiments of transfer devices will then be described with reference to  FIGS. 3-24 .  
      A. Selected Embodiments of Microelectronic Workpiece Processing Apparatus for Use with Automatic Workpiece Transfer Devices  
       FIG. 1  is an isometric view of a processing apparatus  100  having a workpiece handling device  130  in accordance with an embodiment of the invention for manipulating a plurality of microelectronic workpieces  101 . A portion of the processing apparatus  100  is shown in a cut-away view to illustrate selected internal components. In one aspect of this embodiment, the processing apparatus  100  can include a cabinet  102  having an interior region  104  defining an enclosure that is at least partially isolated from an exterior region  105 . The cabinet  102  can also include a plurality of apertures  106  through which the workpieces  101  can ingress and egress between the interior region  104  and a load/unload station  110 .  
      The load/unload station  110  can have two container supports  112  that are each housed in a protective shroud  113 . The container supports  112  are configured to position workpiece containers  114  relative to the apertures  106  in the cabinet  102 . The workpiece containers  114  can each house a plurality of microelectronic workpieces  101  in a “mini” clean environment for carrying a plurality of workpieces through other environments that are not at clean room standards. Each of the workpiece containers  114  is accessible from the interior region  104  of the cabinet  102  through the apertures  106 .  
      The processing apparatus  100  can also include a plurality of processing stations  120  and a transfer device  130  in the interior region  104  of the cabinet  102 . The processing apparatus, for example, can be a plating tool, and the processing stations  120  can be single-wafer chambers for electroplating, electroless plating, annealing, cleaning, etching, and/or metrology analysis. Suitable processing stations  120  for use in the processing apparatus  100  are disclosed in U.S. Pat. Nos. 6,228,232 and 6,080,691, and in U.S. application Ser. Nos. 09/385,784; 09/386,803; 09/386,610; 09/386,197; 09/501,002; 09/733,608; 09/804,696; and 09/804,697, all of which are herein incorporated in their entirety by reference. The processing stations  120  are not limited to plating devices, and thus the processing apparatus  100  can be another type of tool.  
      The transfer device  130  moves the microelectronic workpieces  101  between the workpiece containers  114  and the processing stations  120 . The transfer device  130  includes a linear track  132  extending in a lengthwise direction of the interior region  104  between the processing stations  120 . In the particular embodiment shown in  FIG. 1 , a first set of processing stations  120  is arranged along a first row R 1 -R 1  and a second set of processing stations  120  is arranged along a second row R 2 -R 2 . The linear track  130  extends between the first and second rows of processing stations  120 . The transfer device  130  can further include a robot unit  134  carried by the track  132 .  
      B. Embodiments of Transfer Devices for Handling Microelectronic Workpieces in Processing Machines  
       FIG. 2A  illustrates an embodiment of the robot unit  134  in greater detail. The robot unit  134  can include a transport unit  210 , an arm assembly  230  carried by the transport unit  210 , and first and second end-effectors  300  (identified individually by reference numbers  300   a  and  300   b ) carried by the arm assembly  230 . The transport unit  210  can include a shroud or housing  212  having a plurality of panels attached to an internal frame (not shown in  FIG. 2A ). An opening  214  in a top panel of the housing  212  receives a portion of the arm assembly  230 . It will be appreciated that the transport unit  210  and the housing  212  can have many different configurations depending upon the particular environment in which the robot unit  134  is used. The transport unit  210 , for example, can be a base that can be stationary, rotary, or move in a non-linear manner. The transport unit  210  can also include a guide member configured to move laterally along the track  132 . The particular embodiment of the transport unit  210  shown in  FIG. 2A  includes a guide member defined by a base plate  216  that slidably couples the robot unit  134  to the track  132 . The robot unit  134  can accordingly translate along the track  132  (arrow T) to position the robot unit  134  adjacent to a desired processing station  120  ( FIG. 1 ).  
      The arm assembly  230  can include a waist member  232  that is coupled to a lift assembly (not shown in  FIG. 2A ). The arm assembly  230  can also include an arm  234  having a medial section  235 , a first extension  236   a  projecting from one side of the medial section  235 , and a second extension  236   b  extending from another side of the medial section  235 . The first and second extensions  236   a - b  of the arm  234  can be diametrically opposed to one another as shown in  FIG. 2A , or they can extend at a desired angle to each other. In one embodiment, the first and second extensions  236   a  and  236   b  are integral with another, but in alternate embodiments the first and second extensions  236   a  and  236   b  can be individual components that are fixed to each other.  
      The arm assembly  230  can move along a lift path L-L to change the elevation of the arm  234  for positioning the end-effectors  300  at desired elevations. The lift path L-L generally extends transverse to the track  132 , which as used herein includes any oblique or perpendicular arrangement. The arm assembly  230  can also rotate (arrow R 1 ) about the lift path L-L to position a distal end  238   a  of the first extension  236   a  and/or a distal end  238   b  of the second extension  236   b  proximate to a desired workpiece container  114  or processing station  120 . The first and second extensions  236   a - b  generally rotate about the lift path L-L as a single unit because they are integral or fixed with each other. The motion of the first and second extensions  236   a - b  are accordingly dependent upon each other in this embodiment. In alternate embodiments, the arm  234  can have extensions that are not fixed to each other and can move independently from each other.  
      The end-effectors  300  are rotatably carried by the arm  234 . In one embodiment, the first end-effector  300   a  is rotatably coupled to the first distal end  238   a  to rotate about a first rotation axis A 1 -A 1  (arrow R 2 ). The second end-effector  300   b  can be rotatably coupled to the second distal end  238   b  of the arm  234  to rotate about a second rotation axis A 2 -A 2  (arrow R 3 ). The first and second rotation axes A 1 -A 1  and A 2 -A 2  can extend generally parallel to the lift path L-L, but in alternate embodiments these axes can extend transverse to the lift path L-L. The rotational motion of (a) the arm  234  about the lift path L-L, (b) the first end-effector  300   a  about the first rotation axis A 1 -A 1 , and (c) the second end-effector  300   b  about the second rotation axis A 2 -A 2  can be coordinated so that the first and second end-effectors  300   a  and  300   b  can each be positioned adjacent to any of the workpiece containers  114  and processing stations  120  on either side of the cabinet  102  ( FIG. 1 ).  
      The first end-effector  300   a  can be spaced apart from the arm  234  by a first distance D 1 , and the second end-effector  300   b  can be spaced apart from the arm  234  by a second distance D 2 . In the embodiment shown in  FIG. 2A , the distance D 1  is less than the distance D 2  such that the first end-effector  300   a  is at a different elevation than the second end-effector  300   b . The first end-effector  300   a  accordingly moves through a first plane as it rotates about the first rotation axis A 1 -A 1 , and the second end-effector  300   b  moves through a second plane as it rotates about the second rotation axis A 2 -A 2 . The first and second planes are generally parallel and fixedly spaced apart from each other so that the end-effectors  300   a - b  cannot interfere with each other. The first and second planes, however, can have other arrangements (i.e., nonparallel) so long as they do not intersect in a region over the arm  234 . The first and second end-effectors  300   a  and  300   b  can be fixed at the particular elevations relative to the arm  234  using spacers or other types of devices. For example, the first end-effector  300   a  can be spaced apart from the arm  234  by a first spacer  302   a , and the second end-effector  300   b  can be spaced apart from the arm  234  by a second spacer  302   b . The first and second spacers  302   a - b  can have different thicknesses to space the end-effectors  300  apart from the arm  234  by the desired distances.  
      The first and second end-effectors  300   a - b  and the arm  234  can have different configurations than the configuration shown in  FIG. 2A . For example, as shown in  FIG. 2B , the arm  234  can have only a single extension  236  projecting from the waist member  232  and both of the end-effectors  300   a - b  can be carried by the “single-extension” arm such that the first and second end-effectors  300   a - b  are fixed at different elevations relative to the arm  234 . The end-effectors  300   a - b , for example, can be coupled to the end  238  of the arm and rotate about a common rotation axis A-A.  
       FIG. 3  illustrates one embodiment of the robot unit  134  in greater detail. In this particular embodiment, the transport unit  210  and the arm assembly  230  can operate in a manner similar to that described above with reference to  FIGS. 1-2A , and thus like reference numbers refer to like components in  FIGS. 1-3 . The robot unit  134  can include a lift assembly  510  having a lift actuator  512 , a lift member  514 , and a lift platform  516  coupled to the lift member  514 . The lift actuator  512  can be a servomotor, a linear actuator, or another suitable device that can provide precise control of rotational or linear motion. In the embodiment shown in  FIG. 3 , lift actuator  512  is a servomotor having a driveshaft  518  to which a drive pulley  519  is attached. The lift member  514  in this embodiment is a ball screw or a lead screw having a lower end securely connected to a passive pulley  520 . The lift assembly  510  can also include a guide, such as a guide rail  514   a . The output from the lift actuator  512  is coupled to the passive pulley  520  by a belt  522  around the drive pulley  519  and the passive pulley  520 . The lift assembly  510  can further include a nut  524  that is threadedly coupled to the lead-screw lift member  514  and fixedly coupled to the lift platform  516 .  
      The lift assembly  510  operates to raise/lower the lift platform  516  by energizing the lift actuator  512  to rotate the drive pulley  519  and produce a corresponding rotation of the lead-screw lift member  514 . The nut  524  moves vertically according to the rotation of the lift member  514  to raise/lower the lift platform  516  for adjusting the elevation of the first and second end-effectors  300   a  and  300   b . It will be appreciated that the stroke length of the nut  524  defines the extent of the lift motion of the arm assembly  230 . Additionally, when the nut  524  is positioned at the lower end of the lift member  514 , the lift actuator  512  is received in a cavity  526  in the lift platform  516 . The cavity  526  allows the size of the robot unit  134  to be relatively compact and the length of the lift stroke to be relatively large because the lift actuator  512  can be positioned directly under the lift platform  516 .  
      It will be appreciated that other embodiments of lift assemblies can be used to raise and lower the arm assembly  230 . For example, the lift member can be a scissor lift assembly driven by a servomotor, or the driveshaft of the lift actuator  512  can be the lead-screw lift member  514  to eliminate the pulleys and belts of the embodiment of  FIG. 3 .  
      The arm assembly  230  is carried by the lift assembly  510  to move along the lift path L-L. In the embodiment shown in  FIG. 3 , the arm assembly  230  includes a base  530  carried by the lift platform  516  and a waist motor  532  carried by the base  530 . The waist member  232  is coupled to an output shaft  536  of the waist motor  532  by a boss. The waist motor  532  is fixedly attached to the base  530 , and a rim  538  is fixedly attached to the base  530  to generally enclose the boss. The waist member  232  is fixedly attached to the boss such that rotation of the boss rotates the waist member  232 . A bearing  540  between the boss and the rim  538  allows the waist motor  532  to rotate the boss and the waist member  232  via the output of the driveshaft  536 .  
      The arm assembly  230  can further include a first effector-drive  542   a  and a second effector-drive  542   b  carried in a cavity  543  of the waist member  232 . The first effector-drive  542   a  has an output shaft coupled to a drive pulley  544   a , which is coupled to a passive pulley  560   a  by a belt  546   a . If so desired, a harmonic drive (nor shown) or other gear reduction mechanism may be disposed between the first effector-drive  542   a  and the drive pulley  544   a  to alter the angular relationship between rotation of the first effector-drive  542   a  and the first end effector  300   a . The second effector-drive  542   b  can be operatively coupled to the second end-effector  300   b  by a similar arrangement. The second effector-drive  542   b , for example, can have an output shaft connected to a drive pulley  544   b , which is coupled to a passive pulley  560   b  by a belt  546   b . In the embodiment shown in  FIG. 3 , the first and second effector-drives  542   a  and  542   b  are servomotors. Alternate embodiments of the arm assembly  230 , however, can use linear actuators housed in the arm  234  or other types of actuators to manipulate the end-effectors  300   a  and  300   b . For example, the effector-drives  542  can be servomotors that have output shafts with a worm gear, and the passive pulleys  560  could be replaced with gears that mesh with the worm gears. The rotation of the worm gears would accordingly rotate the end-effectors about the rotation axes.  
      The arm assembly  230  operates by (a) rotating the waist member  232  and the arm  234  about the lift path L-L, and (b) independently rotating the first and second end-effectors  300   a  and  300   b  about the first and second rotation axes A 1 -A 1  and A 2 -A 2 , respectively. The waist motor  532  rotates the waist member  232  and the arm  234  about the lift path L-L to position the first and second extensions  236   a  and  236   b  of the arm  234  at desired locations relative to the workpiece containers  114  ( FIG. 1 ) and/or the processing stations  120  ( FIG. 1 ). The first effector-drive  542   a  rotates the first end-effector  300   a  about the first rotation axis A 1 -A 1 , and the second effector-drive  542   b  rotates the second end-effector  300   b  about the second rotation axis A 2 -A 2 . The effector-drives  542   a - b  operate independently from each other and the waist motor  532  so that the end-effectors  300   a  and  300   b  can move in a compound motion with the arm  234 . This motion can thus translate the workpieces  101  along virtually any desired path. Therefore, the waist motor  532  and the end-drives  542   a - b  can operate serially or in parallel to provide the desired motion of the end-effectors  300 .  
      The robot unit  134  can also include a plurality of amplifiers to operate the motors carried by the robot unit  134 . In this embodiment, the amplifiers can include four servoamplifiers  550  (identified by reference numbers  550   a - d ). The amplifiers  550  operate the lift actuator  512 , the waist motor  532 , and the effector-drives  542   a - b . Additionally, the transport unit  134  can include a servoamplifier  552  for a rail motor (not shown) that moves the transport unit  210  along the track  132  ( FIG. 1 ). The amplifiers  550  and  552  are controlled by a control circuit board (not shown in  FIG. 4 ) that is carried by the transport unit  210  such that much of the wiring and the electronics for the robot unit  134  are carried locally with the transport unit  210 . Some of the internal wiring for the waist motor  532  and the effector-drives  542   a - b  is carried in a flexible cable track  554  that moves vertically with the lift platform  516 . This reduces the number of long wires running through the processing apparatus  100 .  
       FIG. 4  shows the first and second end-effectors  300   a  and  300   b  in a workpiece transport position. In this configuration, the first spacer  254   a  spaces the first end-effector  300   a  apart from the arm  234  by the first distance D 1  and the second spacer  254   b  spaces the second end-effector  300   b  apart from the arm  234  by the second distance D 2 . When the first and second end-effectors  250   a - b  are over the arm  234 , a first workpiece (not shown) carried by the first end-effector  300   a  can be superimposed under a second workpiece (not shown) carried by the second end-effector  300   b  for transportation along the track  132 . It will be appreciated that the first and second end-effectors  300   a  and  300   b  can be spaced apart from the arm  234  by different distances and using different techniques. The particular embodiment shown in  FIG. 4  uses fixed spacers  254   a  and  254   b  to provide a fixed differential in the elevation between the first and second end-effectors  300   a  and  300   b  that mitigates the need for complex collision avoidance algorithms because the first and second workpieces are inherently held at elevations in which they cannot collide with one another or other components of the robot unit  134 .  
       FIG. 6  illustrates the connection between the first end-effector  300   a  and the first extension  236   a  of the arm  234  in greater detail. In this embodiment, the pulley  560   a  is fixedly attached to the spacer  254   a , and a proximal end of the end-effector  300   a  is fixedly attached to the spacer  254   a . The belt  546   a  accordingly rotates the pulley  560   a  about the first rotation axis A 1 -A 1 .  
      The pulley  560   a  illustrated in  FIGS. 4 and 5  includes an upper pulley section  562  and a lower pulley section  564  attached to one another for rotation together. The pulley  560   a  also includes an electrical pass-through including a lower wire sleeve  570  and an upper slip ring assembly  572 . The wire sleeve  570  may be generally hollow and provide a passage for wires  576  upwardly through the center of the pulley  560   a . The slip ring assembly  572  provides a rotatable electrical connection from the first extension  236   a  of the arm  234  and the first end-effector  300   a , permitting power to be delivered to a detector  290  (discussed below). A variety of suitable slip ring assemblies are commercially available, including one sold by Litton Industries as model AC6023-6.  
      In one embodiment of the invention, detailed below in connection with  FIGS. 6-14 , the first end-effector  300   a  includes a pneumatically powered driver  380 . The pulley  560   a  of  FIG. 5  includes a pneumatic pass-through to pneumatically connect the pneumatic supply line  580  (which may be connected to a compressor in the housing  212  of the transport unit  210 , for example) with a pneumatic delivery line  586  connected to the driver  380 . An annular pneumatic space  584  is defined between the exterior of the wire sleeve  570  and an inner surface of the lower pulley section  564 . As a consequence, the annular space  584  is concentric about the wire sleeve  570 . A lower rotary seal  585  may provide a seal between a lower end of the lower pulley section  564  and the wire sleeve  570  and an upper rotary seal  587  may provide a seal between the upper end of the wire sleeve and the lower pulley section  564 . This seals the annular passageway  584  without interfering with relative rotation between the wire sleeve  570  and lower pulley section  564  which define the annular passageway  584 . A pressurized fluid may follow the path designed by arrows in  FIG. 5 , passing from the pneumatic supply line  580 , into the angled pneumatic fitting  582 , into the annular pneumatic space  584 , into an angled channel  565  in the lower pulley section  564 , into a connecting channel  563  in the upper pulley section  562 , through a spacer channel  225  in the spacer  224   a , and finally through a fluid fitting  382  carried in the end-effector  300   a.    
       FIGS. 6-14  illustrate an end-effector  300  in accordance with one embodiment of the invention and  FIGS. 15-24  illustrate an end-effector  400  in accordance with another embodiment of the invention. End-effector  300  or end-effector  400  may be used as the first end-effector  300   a  and/or the second end-effector  300   b  in  FIGS. 1-3 . It should be understood that the end-effectors  300  and  400  can be used in connection with transfer devices different from those shown in  FIGS. 1-3 , though.  
      The end-effector  300  of  FIGS. 6-14  includes a body  310  which is appropriately sized to grasp the size and shape of workpiece with which the end-effector is intended to be used. While the body  310  can take any of a variety of shapes, the body  310  of the illustrated embodiment has a pair of spaced-apart legs  312   a  and  312   b . The two legs  312  may be substantially symmetrical about a centerline of the body  310 . The distal ends  314  of the legs  312  may be adapted to abut an edge of the workpiece. In the illustrated embodiment, the distal end  314   a  of the first leg  312   a  carries a first stationary abutment  320   a  and the distal end  314   b  of the second leg  312   b  carries a second stationary abutment  320   b . As best seen in  FIG. 8 , the body  310  also includes a proximal section  316 . This proximal section  316  may carry the housing  330  and an actuator  350 , as explained in more detail below. The proximal section  316  may include a depression  317  for receiving a channel member  370 , as also explained below.  
      As noted above, the distal ends  314  of the legs  312  of the base  310  may each be adapted to carry a stationary abutment  320 . If so desired, the stationary abutment  320  may be formed integrally with the associated leg  312  and may take any desired shape. In the illustrated embodiment, however, the stationary abutments  320  are formed as separate members which are attached to the distal ends  314  of the legs  312 , such as by press fitting, gluing, soldering or the like.  
       FIGS. 12-14  illustrate one embodiment of stationary abutments  320   a - b  useful in the end-effector  300 . In particular,  FIG. 12A  illustrates the first stationary abutment  320   a  and  FIG. 12B  illustrates the second stationary abutment  320   b . The first and second stationary abutment  320   a - b  have similar structures and generally include a base  322 , a shoulder  324 , an overhang  326  and a recess  328 . The primary difference between the first stationary abutment  320   a  and the second stationary abutment  320   b  lies in the relative orientation of the recess  328 . The orientation of the recess  328  in each of the stationary abutments  320  should be selected to ensure that an edge of the workpiece may be received therein when the stationary abutments  320  are carried on the legs  312 . The end-effector  300  of  FIGS. 6-14  may be used in connection with circular workpieces, such as semiconductor wafers. The first stationary abutment  320   a  of  FIG. 12A  and the second stationary abutment  320   b  of  FIG. 12B , therefore, have recesses  328  oriented as tangents to the circular workpiece to be handled by the end-effector  300 .  
      Given the similarity of the two stationary abutments  320   a  and  320   b ,  FIGS. 13 and 14  only illustrate the second abutment  320   b  of  FIG. 12B . As best seen in  FIG. 14 , the base  322  of the second stationary abutment  320   b  slopes in an upward direction moving toward the recess  328 . As can be seen in  FIG. 8 , for example, the base  322  is oriented toward the proximal section  316  of the body  310  and the housing  330 . As a consequence, the sloped base  322  slopes upwardly in a direction away from the housing  330 . The base slopes upwardly at an incline angle α from horizontal. The shoulder  324  of the stationary abutment  320   b  slopes upwardly from the upper edge of the base at a different incline angle θ. The incline angle θ of the shoulder  324  is greater than the incline angle α of the base  322 . The overhang  326  slopes upwardly from the shoulder  324 , but in the opposite direction, i.e., proximally back toward the housing  330 .  
      As explained in more detail below, when a workpiece is first positioned for grasping by the end-effector  300 , it will be placed in a workplace-receiving area generally defined between the first stationary abutment  320   a , the second stationary abutment  320   b  and a movable abutment  354 . An edge of the workpiece will initially rest on the sloped bases  322  of the stationary abutments  320 . This will help space the workpiece upwardly away from the body  310  of the end-effector. As the actuator  350  moves the moveable abutment  352  inwardly toward the center of the workpiece-receiving area, the workpiece will be urged up the sloped bases  322  of the stationary abutments  320 . The edge of the workpiece will then encounter the shoulder  324  and may slide up the shoulder until an upper edge of the workpiece engages the overhang  326 . This will securely seat the edge of the workpiece in the recess  328  of the stationary abutment  320  at a predictable position. As a consequence, the overhang  326  may be just large enough to securely hold the workpiece to prevent it from falling out of the end-effector  300  when the end-effector  300  is inverted, but allow the workpiece to readily and predictably drop out of the end-effector  300  without undue interference from the overhang  326 .  
      The end-effector  300  may include a housing  330  coupled to the proximal section  316  of the base  310 . This housing  330  may generally include a shroud  332  coupled to a bottom plate  334  by any suitable means, such as bolts  336 . The housing defines an enclosure  338  within which an actuator  350  and detector  390  may be housed.  
      The actuator  350  generally includes an actuator shaft  352 , a guide plate  360 , a channel member  370  and a driver  380 . These elements may act in tandem to selectively control movement of a movable abutment  354  carried by the shaft  352 , as explained below.  
      As best seen in  FIGS. 9 and 11 , the shaft  352  includes a moveable abutment  354  at its distal end. A collar  356  may be provided at the proximal end of the shaft  352  to link the shaft  352  to a driver  380 , as noted below. If so desired, a pair of wings  355  may extend horizontally from opposite sides of the shaft  352 . The wings  355  can be slidably received between the channel member  370  and a lower surface of the guide plate  360  to help guide movement of the shaft  352  during operation. In the illustrated embodiment, the shaft  352  includes a flag  358  extending upwardly therefrom. As explained in more detail below, this flag  358  may cooperate with the detector  390  to monitor the position of the shaft  352  during operation.  
      The path of the shaft  352  is generally constrained by the guide plate  360  and the channel member  370 . The guide plate  360  may be attached to the proximal section  316  by any suitable attachment, such as screws  364 . The channel member  370  is sized to be received in the depression  317  in the proximal section  316  of the base  310 . Attaching the guide plate  360  to the base  310  will, consequently, help retain the shaft  352  and the channel member  370  in place.  
      The guide plate  360  may include an elongate slot  362 . The flag  358  of the shaft  352  may be slidably received within the slot  362  and move along the slot  362  when the driver  380  moves the movable abutment  354 . To further ensure accurate guidance of the shaft  352 , the channel member  370  may include an elongate channel  374  within which a lower portion of the shaft  352  is slidably received. The wings  355  help ensure an accurate vertical position of the shaft  352  and the guide plate slot  362  and the channel  374  together restrict horizontal movement of the shaft  352 . As a consequence, the shaft  352  and its flag  358  are constrained to follow a relatively precise path as the shaft  352  is moved by the driver  380 .  
      The distal end of the channel member  370  may include a ramp  372  which slopes upwardly in a proximal direction (i.e., toward the driver  380 ). This ramp  372  may provide an area on which an edge of the workpiece may initially rest. In a manner similar to the sloped base  322  of the stationary abutments  320 , this ramp can help keep the workpiece spaced above the legs  312  of the base  310  and help guide an edge of the workpiece into a recess defined by the moveable abutment  354 .  
      The driver  380  is adapted to move the shaft  352  of the actuator  350  inwardly toward the stationary abutments  320  and outwardly away from the stationary abutments  320 . Any suitable motor may be employed. For example, an electrically-actuated solenoid may be used. Alternatively, the driver  380  may be a hydraulic or pneumatic piston which is connected to a fluid supply by appropriate fluid fittings  382 . The driver  380  may be operatively connected to the shaft  352  in any desired fashion. In the illustrated embodiment, the driver  380  includes a link  384  having a head  385  sized to be closely received in the collar  356  of the shaft  352 . In one embodiment, the driver  380  normally biases the shaft  352  inwardly toward engagement with a workpiece. In this manner, the end-effector  300  may retain its grip on the workpiece, even if the motive force of the driver  380  is lost. For example, if a pneumatic driver  380  is used, a spring (not shown) may be interposed between the driver  380  body and the actuator shaft  352  to urge the shaft  352  away from the driver  380  and toward the workpiece if pneumatic pressure is lost.  
      One embodiment of the invention includes a detector  390 , which may be received within the enclosure  338  of the housing  330 . This detector  390  is adapted to detect positive engagement of a workpiece by the movable abutment  354 . In one embodiment, the detector  390  directly measures contact of the movable abutment with the workpiece, such as by including a pressure sensor associated with the movable abutment  354 .  
      In the embodiment illustrated in  FIGS. 6-14 , the detector  390  is operatively associated with the actuator  350 . More particularly, the detector  390  includes a first flag sensor  392   a - b  and a second flag sensor  394   a - b . The first and second flag sensors  392   a - b  and  394   a - b  are spaced along the path along which the flag  358  is constrained to travel. In the illustrated embodiment, the flag  358  follows a straight, horizontal path, so the two flag sensors are horizontally spaced from one another. In another embodiment, the moveable abutment  354  is rotatably carried at the distal end of the shaft  352  and the driver  380  rotates the shaft  352  to advance or retract the shaft  352 . In such an embodiment, a flag  358  extending outwardly from the shaft  352  proximally of the moveable abutment  354  may follow a circular or helical path. The flag sensors in such a design may be angularly and/or axially spaced from one another to detect the position of the flag  358  at two discrete positions along the path followed by the flag  358 .  
      Each of the flag sensors may be adapted to detect the presence or proximity of the flag  358  at a particular location along the path of travel of the flag  358 . The first and second flag sensors  392   a - b  and  394   a - b  may detect the flag in a variety of fashions. For example, the flag  358  may carry a magnet (not shown) and the first and second flag sensors  392   a - b  and  394   a - b  may be responsive to proximity of the magnet in the flag  358 . In the illustrated embodiment, however, the first flag sensor includes a first light source  392   a  ( FIGS. 10 and 11 ) and a first light sensor  392   b  ( FIG. 8 ) which are positioned on opposite sides of the path of the flag  358 . Similarly, the second flag sensor comprises a second light source  394   a  ( FIGS. 10 and 11 ) and a second light sensor  394   b  ( FIG. 8 ) which are positioned on opposite sides of the path of the flag  358 . The flag  358  of the actuator  352  is desirably opaque to wavelengths of light emitted by the first and second light sources  392   a  and  394   a . When the opaque flag  358  is positioned between the first light source  392   a  and the first light sensor  392   b , it will interrupt the beam of light passing from the first light source  392   a  to the first light sensor  392   b . This may generate a first flag position signal. Similarly, if the opaque flag  358  is positioned between the second light source  394   a  and the second light sensor  394   b , the flag will interrupt the passage of light from the second light source  394   a  to the second light sensor  394   b , which may generate a second flag position signal. The detector  390  may include a detector plate  396  which carries the first light source  392   a , the first light sensor  392   b , the second light source  394   a , and the second light sensor  394   b  in the desired spatial relationship.  
      In operation, the actuator shaft  352  may be moved by the driver  380  between a retracted position and a deployed, workpiece-engaging position. When the actuator is in its retracted position, the movable abutment  354  is spaced farther away from the stationary abutments  320   a - b , permitting a workpiece to be received between the three abutments. When the actuator shaft  352  is in this position, the first flag  358  may be positioned proximally of both the first light source  392   a  and the second light source  394   a , as shown in phantom lines in  FIG. 11 . As the actuator shaft  352  is moved inwardly from the retracted position to the deployed position (shown in solid lines in  FIG. 11 ), the flag  358  will interrupt the beam of light from the first light source  392   a , generating the first flag position signal. If the driver  380  is energized and no first signal is detected, the detector  390  may generate a first error signal.  
      In normal operation, the movable abutment  354  of the actuator shaft  352  will engage an edge of a workpiece and grip the workpiece between the movable abutment  354  and the stationary abutments  320 . The workpiece will, therefore, limit movement of the actuator  352  inwardly. As shown in  FIG. 11 , when the actuator is in its proper deployed position, the flag  358  may be positioned between the first light source  392   a  and the second light source  394   a . As a consequence, the flag  358  will not interrupt the beam of light from either of the light sources  392   a  and  394   a . As noted previously, the driver  380  may exert a predetermined urging force on the actuator shaft  352 . If the movable abutment  354  fails to properly engage a workpiece and urge it into the recesses  328  of the stationary abutments  320 , the workpiece will not impede further movement of the actuator shaft  352  toward the stationary abutments  320 . As a consequence, the flag  358  may continue along its path until it interrupts the beam of light from the second light source  394   a , generating the second flag position signal. The detector  390  may then generate a second error signal, which may differ from the first error signal, to indicate that the workpiece is not properly grasped by the end-effector  300 .  
      After the end-effector  300  properly grasps a workpiece and moves it to its intended destination, the end-effector  300  may release the workpiece. This may be accomplished by moving the actuator shaft  352  from its deployed position to its retracted position. In doing so, the flag  358  of the actuator shaft  352  will interrupt the beam of light from the first light sensor  392   a , generating the first flag position signal. If the detector  390  fails to receive the first flag position signal during retraction of the actuator shaft  352 , the detector  390  may generate a third error signal, which may differ from the first and second error signals.  
      Hence, the particular embodiment of the invention shown in  FIGS. 6-14  provides a mechanically simple system for reliably engaging a workpiece positioned between the movable abutment  354  and the stationary abutments  320 . This embodiment also includes a detector  390  which can generate an appropriate error signal if the movable abutment  354  fails to properly engage an edge of the workpiece. By generating an error signal, the detector can avoid any mishap associated with moving the end-effector  300  without first properly grasping the workpiece. The error signals generated by the detector  390  may take a variety of forms, including visual or audible alerts to an operator. In an alternative embodiment, if the detector  390  generates the first error signal, the second error signal or the third error signal, operation of the transfer device  130  may be interrupted, preventing the transfer device  130  from moving the end-effector  300  until an operator can insure that the workpiece is properly gripped by the end-effector  300 .  
       FIGS. 15-24  illustrate an end-effector  400  in accordance with an alternative embodiment of the invention. As noted previously, the end-effector  400  may be used instead of the end-effector  300  ( FIGS. 6-14 ) in the transport unit  210  of the robot unit  134  shown in  FIGS. 1 and 2 .  
       FIGS. 15-19  illustrate the end-effector  400  grasping a workpiece  101 . As best seen in the bottom view of  FIG. 17 , the end-effector  400  includes a body  410  which has a shape suitable for use with the workpiece  101 . As best seen in the bottom view of  FIG. 17 , the body  410  may have a structure roughly similar to the body  310  of the end-effector  300 . In particular, the body  410  of the end-effector  400  may include a first leg  412   a  having a distal end  414   a  and a parallel second leg  412   b  having a distal end  414   b . The two legs  412  may be spaced from each other and joined at a proximal section  416 . A mount  418  may extend proximally behind the proximal section  416  of the body  410 . This mount may be used to attach the end-effector  400  to an arm  234  of the robot unit  134 , for example.  
      The body  410  also includes four separate channels  420   a - d , each of which is designed to slidably receive a rod. In particular, the first channel  420   a  is adapted to slidably receive a first resilient rod  430   a , the second channel  420   b  is adapted to slidably receive a second resilient rod  430   b , the third channel  420   c  is adapted to slidably receive a first tensile rod  440   a , and the fourth channel  420   d  is adapted to slidably receive a second tensile rod  440   b . In one embodiment of the invention, the third and fourth channels  420   c - d  are substantially straight, as are the tensile rods  440   a - b  received therein. For reasons explained more fully below, the first and second channels  420   a - b  may be curved or have any other suitable shape adapted to deflect the first and second resilient rods  430   a - b  received therein from their relaxed, natural state. The degree of curvature of the first and second channels  420   a - b  need not be particularly pronounced. Hence, these channels  420   a - b  appear substantially straight in  FIG. 17 . In  FIG. 24 , the curvature of the channels  420   a - b  is exaggerated somewhat to better illustrate the concept.  
      In one embodiment of the invention, the end-effector  400  includes three spaced-apart abutments  425 , with each of the abutments being adapted to engage the edge of the workpiece  101  to grip the workpiece  101 . In the particular embodiment illustrated in  FIGS. 15-24 , though, the end-effector  400  includes a first abutment  425   a , a second abutment  425   b , a third abutment  425   c , and a fourth abutment  425   d . The third abutment  425   c  may be carried adjacent the distal end  414   a  of the first leg  412   a  and the first abutment  425   a  may be aligned with the third abutment  425   c  along the length of the first leg  412   a . Similarly, the fourth abutment  425   d  may be positioned adjacent the distal end  414   b  of the second leg  412   b  and the second abutment  425   b  may be aligned with the fourth abutment  425   d  along the length of the second leg  412   b.    
      The first abutment  425   a  and the third abutment  425   c  carried by the first leg  412   a  may define a cord across the circular workpiece  101  carried by the end-effector  400 . Similarly, the second abutment  425   b  and the fourth abutment  425   d  carried by the second leg  412   b  may define a parallel cord across the workpiece  101 . To facilitate centering of the workpiece  101  with respect to the body  410 , the first abutment  425   a  and the fourth abutment  425   d  may be positioned on diametrically opposite sides of the workpiece  101 . The second and third abutments  425   b - c  may also be positioned on dramatically opposite sides of the workpiece  101 . As a consequence, the first abutment  425   a  and the fourth abutment  425   d  may define a first antipodal pair of abutments  425  and the second and third abutments  425   b - d  may define a second antipodal pair of abutments  425 .  
      At least one of the abutments  425  should be movable with respect to the workpiece  101  to selectively grip or release the workpiece  101 . If four abutments  425   a - d  are employed, at least two of these abutments may be movable. For example, the first and second abutments  425   a - b  may be movable with respect to the body  410  while the third and fourth abutments  425   c - d  may comprise stationary abutments which do not move with respect to the body  410 . Stationary abutments  320  generally as shown in connection with the prior embodiment in  FIGS. 12-14  may be employed as the third and fourth abutments  425   c - d . In such an embodiment, the third and fourth channels  420   c - d  in the body  410  and the first and second tensile rods  440   a - b  may be omitted.  
      In the particular embodiment illustrated in  FIGS. 15-24 , all four of the abutments  425  are movable. In particular, the abutments  425  can be moved from a retracted position wherein they are spaced radially outwardly from a center of the workpiece  101  to a deployed position where they are located closer to the center of the workpiece  101 . When the abutments  425  are in their respective retracted positions a workpiece may be placed in or removed from the wafer-receiver area of the end-effector  400 . When the abutments  425  are in their respective deployed positions, each of the abutments  425  will engage the edge of the workpiece  101  at a different location, gripping the workpiece  101  therebetween.  
      The abutments may be moved between their respective retracted and deployed positions in any desired manner. In one embodiment of the invention, though, all four of the abutments  425   a - d  are coupled to a common actuator  460 . At least two of the abutments  425  may be resiliently coupled to the actuator  460 . Although all four of the abutments  425  may be respectively coupled to the actuator, in the particular embodiment shown in  FIGS. 15-24  the first and second abutments  425   a - b  are resiliently coupled to the actuator  460  while the third and fourth abutments  425   c - d  need not be resiliently coupled to, and may be rigidly coupled to, the actuator  460 .  
      In the illustrated embodiment, the abutments  425  are connected to the actuator  460  utilizing the resilient rods  430   a - b  and the tensile rods  440   a - b . In particular, the first abutment  425   a  may be carried adjacent a distal end of the first resilient rod  430   a , the second abutment  425   b  may be carried adjacent a distal end of the second resilient rod  430   b , the third abutment  425   c  may be carried adjacent a distal end of the first tensile rod  440   a  and the fourth abutment  425   d  may be carried adjacent a distal end of the second tensile rod  440   b.    
       FIG. 18  is a cross-sectional view of the end-effector  400  and workpiece  101  of  FIG. 15  taken through the first resilient rod  430   a . As seen in  FIG. 18 , the first resilient rod  430   a  may include a central length  432  which extends between a tail  434  and the first abutment  425   a . In this embodiment, the first abutment  425   a , the central length  432  and the tail  434  are all integrally formed, e.g., by bending a straight length of a metal rod or the like into the desired shape. As explained in more detail below, the tail  434  may have any shape suitable for coupling the proximal end of the first resilient rod  430   a  to the actuator  460 . Although none of the drawings separately illustrate the second resilient rod  430   b  in detail, the second resilient rod may have substantially the same size and shape as the first resilient rod  430   a.    
      The third and fourth abutments  425   c - d  are carried by first and second tensile rods  440   a  and  b , respectively. The first and second tensile rods  440   a - b  may have substantially the same size and shape.  FIG. 22  shows one such tensile rod  440  separate rest of the end-effector  400 . The tensile rods  440  may have a structure similar to that of the resilient rods  430 . In particular, the abutment  425  of the tensile rod  440  may extend upwardly from a distal end of an elongate central length  442  and a tail  444  or other suitable connection structure may be carried at the proximal end of the central length  442 . If so desired, the abutment  425  may include a notch  446  adapted to engage or receive the edge of the workpiece  101 , as illustrated in  FIG. 19 . Although  FIG. 18  does not illustrate a notch  446  in the first resilient rod  430   a , such notches  446  may also be employed in the first and second abutments  425   a - b  carried by the resilient rods  430   a - b.    
      The abutments  425  may take any suitable shape. In the illustrated embodiments, the abutments are cylindrical and may include notches  446 . Such a simple shape may not reliably steer a workpiece  101  into the workpiece-receiving area of the end-effector  400  if the workpiece  101  is slightly misaligned with the end-effector  400 . If so desired, the end-effector  400  may include an abutment housing  450  for each of the abutments  425 . In particular, a first abutment housing  450   a  may slidably receive the first abutment  425   a , a second abutment housing  450   b  may slidably receive the second abutment  425   b , a third abutment housing  450   c  may slidably receive the third abutment  425   c , and a fourth abutment housing  450   d  may slidably receive the fourth abutment  425   d . As best seen in  FIGS. 18-21 , the abutment housings  450  may be provided with a sloped or hemispherical dome  452  at its upper surface and a harbor  454  for receiving the abutment  425  therein. Some of the abutment housings  450  may also include a through-hole through which the central length  432  of a resilient rod  430  may slidably pass.  
      When an abutment  425  is in its retracted position, i.e., when it is intended to be spaced away from the workpiece  101 , the abutment may be received within the harbor  454  of the abutment housing  450 . If the workpiece  101  is slightly misaligned with respect to the workpiece-receiving area of the end-effector  400 , the edge of the workpiece  101  may engage the dome  452  of the abutment housing  450 . The slope or curvature of the dome  452  will guide the workpiece  101  into the workpiece-receiving area for gripping by the abutments  425 . Once the workpiece  101  is in place in the workpiece-receiving area, the abutments  425  may be moved inwardly to their deployed positions, engaging the edge of the workpiece  101 . When it is desired to release the workpiece  101  from the end-effector  400 , the abutments  425  may be moved back to their respective recessed positions within their respective abutment housings  450 . If the workpiece  101  happens to cling to one or more of the abutments  425  as they are moved outwardly, the workpiece  101  will tend to strike the edge of the abutment housing  450  as the abutment  425  moves into the harbor  454 . In this manner, the abutment housing  450  may facilitate disengagement of the workpiece  101  from the end-effector  400 .  
      As noted above, each of the abutments  425  in the illustrated end-effector  400  is coupled to an actuator  460 . The actuator  460  may take any desired form. For example, the actuator may have four separate mechanisms similar to the actuator  350  shown in  FIG. 6 , with one actuator  350  being associated with each of the abutments  425 . The embodiment shown in  FIGS. 15-24 , however, employs a single actuator  460 , operated by a single drive mechanism, to synchronously move all four of the abutments  425 . Thus particular embodiment of the actuator  460  includes a common drive  462 , a first rotatable member  470   a , and a second rotatable member  470   b . The common drive  462  may interact with the first and second rotatable members  470   a - b  in a variety of fashions. In one embodiment, the common drive  462  includes a drive shaft  464  having a toothed collar  466 . The toothed collar  466  is positioned to engage both the first rotatable member  470   a  and the second rotatable member  470   b . As the common drive  462  is moved, it can, in turn, move the first and second rotatable members  470   a - b . The common drive  462  may move either by axial movement (e.g., in the direction shown by arrow D in  FIG. 24 ) or by rotation about the axis of the drive shaft  464 . If the drive shaft  464  is moved axially, the teeth on the toothed collar  466  may comprise generally parallel ridges which engage teeth on the first and second gears  472   a - b  of the first and second rotatable members  470   a - b . If the drive shaft  464  is instead rotated about its axis, the common drive  462  may resemble a worm gear, with the toothed collar  466  having a helical gear tooth (not shown) which engages the teeth on the first and second gears  472   a - b . Other suitable mechanical connections between the common drive  462  and the rotatable members  470  will be apparent to those skilled in the art.  
      The common drive  462  may be selectively moved by a driver (not shown) received in a driver housing  468 . Any driver adapted to move the common drive  462  may be employed. For example, an electric stepper motor (not shown) retained in the driver housing  468  can rotate the common drive  462  about its axis. In an alternative embodiment, the connection between the common drive  462  and the driver may be directly analogous to the connection between the driver  380  and the actuator shaft  352  in the end effector  300  of  FIGS. 6-14 . The driver housing  468  may also receive a detector, which may be similar to the detector  390  discussed above, which is adapted to monitor movement of the drive shaft  464  of the common drive  462 . This detector may generate an appropriate error signal if the first and second abutments  425   a - b  do not properly engage the edge of the workpiece  101 .  
      As best seen in  FIG. 23 , the first rotatable member  470   a  may include a first hub  474   a  having a first axis A 1 -A 1 . The first gear  472   a  may be retained on the first hub  474   a  by a retaining nut  476   a . In one embodiment, the angular orientation of the first gear  472   a  with respect to the first hub  474   a  may be adjusted by loosening the first retaining nut  476   a , moving the first gear  472   a  with respect to the first hub  474   a , and retightening the first retaining nut  476   a . The first rotatable member  470   a  also includes a first mounting ring  478   a  fixed to the first hub  474   a  for rotation therewith. The first mounting ring  478   a  may be attached to the first hub  474   a  in any suitable fashion, such as with a spline connection or an adhesive joint.  
      The second rotatable member  470   b  may have a structure substantially the same as that of the first rotatable member  470   a . In particular, the second rotatable member  470   b  may have a second gear  472   b  retained on a second hub  474   b  by a retaining nut  476   b . A second mounting ring  478   b  may be carried by the second hub  474   b  for rotation therewith about axis A 2 -A 2 .  
      A bottom cover  480  may be carried on the lower surface of the body  410 . The bottom cover  480  may include a first orifice  482   a  adapted to closely receive a portion of the first hub  474   a  and a second orifice  482   b  sized to closely receive a portion of the second hub  474   b . If so desired, a bearing (not shown) may be disposed between the orifice  482  and the hub  474  received therein to minimize friction as the hub  474  rotates about its axis. The lower edge of the first orifice  482   a  may be provided with a first chamfer  484   a  and the lower edge of the second orifice  482   b  may be provided with a second chamfer  484   b . Each hub  474  may include an enlarged head received in a chamfer  484 . The enlarged heads and the retaining nuts  476  can hold the first and second rotatable members  470   a - b  in place on the bottom cover  480 .  
      Turning to  FIG. 24 , the first resilient rod  430   a  and the first tensile rod  440   a  are coupled to the first rotatable member  470   a  and the second resilient rod  430   b  and the second tensile rod  440   b  are coupled to the second rotatable member  470   b . More specifically, the tail  434  ( FIG. 18 ) of the first resilient rod  430   a  is coupled to the mounting ring  478   a  of the first rotatable member  470   a  at a position spaced radially outwardly from the first axis A 1 -A 1 . The tail  444  of the first tensile rod  440   a  may also be coupled to the first mounting ring  478   a  at a location spaced radially outwardly from the first rotational axis A 1 -A 1 . The first tensile rod  440   a  is coupled to the first mounting ring  478   a  at a location spaced from the connection of the first mounting ring  478   a  to the first resilient rod  430   a . The first tensile rod  440   a  and the first resilient rod  430   a  may be attached to the first mounting ring  478   a  at diametrically opposed positions, for example. The connections of the second tensile rod  440   b  and the second resilient rod  430   b  to the second mounting ring  478   b  of the second rotatable member  470   b  may mirror the connections of the first tensile rod  440   a  and the first resilient rod  430   a  to the first mounting ring  478   a.    
      In use, operation of the single common drive  462  can move each of the first-fourth abutments  425   a - d  inwardly to engage the edge of a workpiece  101 . In the particular embodiment shown in  FIG. 24 , the common drive  462  may be moved proximally (i.e., in the direction of the arrow D) to move the abutments  425  inwardly into their deployed positions. Moving the common drive  462  proximally will rotate the first rotatable member  470   a  clockwise (arrow D a ) and rotate the second rotatable member  470   b  counterclockwise (arrow D b ). As the first rotatable member  470   a  turns clockwise, it will urge the first resilient rod  430   a  distally within the first channel  420   a  and draw the first tensile member  440   a  proximally within the third channel  420   c . In turn, this will move the third abutment  425   c  carried by the first tensile rod  440   a  and the first abutment  425   a  carried by the first resilient rod  430   a  inwardly toward the workpiece  101 . Rotation of the second rotatable member  470   b  in a counterclockwise direction will urge the second resilient rod distally within the second channel  420   b  and draw the second tensile rod  440   b  proximally within the fourth channel  420   d . As a consequence, the second abutment  425   b  carried by the second resilient rod  430   b  and the fourth abutment  425   d  carried by the second tensile rod  440   b  will both be moved inwardly toward the edge of the workpiece  101 .  
      Hence, simply by moving the common drive  462  proximally, all four of the abutments  425   a - d  may be moved synchronously to engage the edge of the workpiece  101 . Moving the common drive  462  in the opposite direction will rotate the first rotatable member  470   a  in a counterclockwise direction and rotate the second rotatable member  470   b  in a clockwise direction. This will move the first and second resilient rods  430   a - b  and first and second tensile rods  440   a - b  such that the abutments  425  carried by these rods move outwardly into their retracted positions within the abutment housings  450 , releasing the workpiece  101 .  
      The first and second resilient members  430   a - b  are adapted to resiliently engage the edge of the workpiece  101 . As noted previously, the first and second channels  420   a - b  may have a curved shape while the resilient rods  430  received therein may have a straight shape when in their natural, relaxed state. Deflecting the resilient rods  430  to conform to the curved channels  420   a - b  in this fashion will reduce the column strength of the central lengths  432  of the resilient rods  430 . Deflecting of the resilient rods  430  in their respective channels  420   a - b , therefore, reduces the stiffness of the resilient rods  430 , lending them resiliency. As a consequence, when the first and second abutments  425   a - b  are urged against the edge of the workpiece  101  by the actuator  460 , the resilient rods  430  may further deflect relatively easily. Resiliently engaging the edge of the workpiece  101  in this fashion allows the first and second abutments  425   a - b  to appropriately engage the edge of the workpiece  101  even if the workpiece  101  deviates from anticipated dimensions. If the workpiece  101  is larger than anticipated, the resilient members  430  may simply deflect more within the first and second channels  420   a - b . If the workpiece  101  is smaller than anticipated, the resilient rods  430  may not deflect as far, but they should still be able to engage the workpiece  101  with sufficient force to secure the workpiece  101  to the end effector  400 .  
      The tensile rods  440  will be placed in tension when the actuator  460  urges the third and fourth abutments  425   c - d  against the edge of the workpiece  101 . Since the central length  442  of the tensile rods  440  will be placed in tension during operation, they may not exhibit the same spring-like resiliency as the resilient rods  430 . If additional resiliency were needed to insure proper gripping of the workpiece  101 , each of the tensile rods  440  may include a central length  442  adapted to stretch when the abutment  425  carried by the tensile rod  440  is urged against the edge of the workpiece  101 .  
      From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.