Abstract:
A method determines axial alignment between the centroid of an end effector and the effective center of a specimen held by the end effector. The method is implemented with use of an end effector coupled to a robot arm and having a controllable supination angle. A condition in which two locations of the effective center of the specimen measured at 180° displaced supination angles do not lie on the supination axis indicates that the centroid is offset from the actual effective center of the specimen.

Description:
RELATED APPLICATIONS 
   This application is a division of U.S. patent application Ser. No. 10/649,116, filed Aug. 26, 2003, now U.S. Pat. No. 6,898,487, which is a division of U.S. patent application Ser. No. 10/223,075, filed Aug. 15, 2002, now U.S. Pat. No. 6,618,645, which is a division of U.S. patent application Ser. No. 09/920,353, filed Aug. 1, 2001, now U.S. Pat. No. 6,438,460, which is a division of U.S. patent application Ser. No. 09/312,343, filed May 14, 1999, now U.S. Pat. No. 6,275,748, which is a continuation-in-part of U.S. patent application Ser. No. 09/204,747, filed Dec. 2, 1998, now U.S. Pat. No. 6,256,555. 

   FIELD OF THE INVENTION 
   This invention is directed to a specimen handling apparatus and method and, more particularly, to a method of determining the axial alignment of the centroid of a semiconductor wafer robot arm edge gripping end effector to the center of a semiconductor wafer gripped by the end effector. 
   BACKGROUND OF THE INVENTION 
   Integrated circuits are produced from wafers of semiconductor material. The wafers are typically housed in a cassette having a plurality of closely spaced slots, each of which can contain a wafer. The cassette is typically moved to a processing station where the wafers are removed from the cassette, placed in a predetermined orientation by a prealigner or otherwise processed, and returned to another location for further processing. 
   Various types of wafer handling devices are known for transporting the wafers to and from the cassette and among processing stations. Many employ a robotic arm having a spatula-shaped end that is inserted into the cassette to remove or insert a wafer. The end of the robotic arm is referred to as an end effector that typically employs a vacuum to releasibly hold the wafer to the end effector. The end effector typically enters the cassette through the narrow gap between a pair of adjacent wafers and engages the backside of a wafer to retrieve it from the cassette. The end effector must be thin, rigid, and positionable with high accuracy to fit between and not touch the closely spaced apart wafers in the cassette. After the wafer has been processed, the robotic arm inserts the wafer back into the cassette. 
   Unfortunately, transferring the wafer among the cassette, robot arm, and processing stations, such as a prealigner, may cause backside damage to the wafer and contamination of the other wafers in the cassette because intentional engagement as well as inadvertent touching of the wafer may dislodge particles that can fall and settle onto the other wafers. Wafer backside damage can include scratches as well as metallic and organic contamination of the wafer material. Robotic arms and prealigners that employ a vacuum to grip the wafer can be designed to minimize backside damage and particle creation. Even the few particles created with vacuum pressure gripping or any other non-edge gripping method are sufficient to contaminate adjacent wafers housed in the cassette. Reducing such contamination is particularly important to maintaining wafer processing yields. Moreover, the wafer being transferred may be scratched or abraded on its backside, resulting in wafer processing damage. 
   What is needed, therefore, is a specimen gripping end effector that can securely, quickly, and accurately transfer semiconductor wafers while minimizing wafer scratching and particle contamination. 
   SUMMARY OF THE INVENTION 
   An object of this invention is, therefore, to provide a specimen handling device that minimizes specimen damage and the production of contaminant particles. 
   Another object of this invention is to provide a semiconductor wafer handling device that can quickly and accurately transfer semiconductor wafers between a wafer cassette and a wafer processing station. 
   A further object of this invention is to provide a wafer handling device that can be retrofit to existing robot arm systems. 
   Robot arm end effectors of this invention rapidly and cleanly transfer semiconductor wafers between a wafer cassette and a processing station. Embodiments of the end effectors include at least one proximal rest pad and at least two distal rest pads having pad and backstop portions that support and grip the wafer at its peripheral edge or within an annular exclusion zone that extends inward from the peripheral edge of the wafer. The end effectors also include an active contact point that is movable between a retracted wafer-loading position and an extended wafer-gripping position. The active contact point is movable to urge the wafer against the distal rest pads so that the wafer is gripped only at its edge or within the exclusion zone. The end effectors are configured so that wafer edge contact is achieved for end effectors with inclined rest pads. Optical sensors detect retracted, safe specimen loading/gripping, and extended positions of the active contact point. 
   The end effectors are generally spatula-shaped and have a proximal end that is operably connected to a robot arm. The active contact point is located at the proximal end, which allows the end effector to be lighter, stronger, and more slender than end effectors having moving mechanisms that may not fit between adjacent wafers in a cassette. The lack of moving mechanisms that could be located over a wafer further causes the end effector to produce less contamination within the cassette. Additionally, locating the active contact point at the proximal end of the end effector ensures that it is remote from harsh conditions such as heated environments and liquids. 
   A vacuum pressure-actuated piston moves the active contact point between a retracted position, in which the wafer is loaded into the end effector, and an extended position in which the wafer is gripped. A first embodiment of the piston employs vacuum pressure to move the active contact point between extreme positions; a second embodiment of the piston employs vacuum pressure to retract the active contact point and a spring to extend the active contact point; and a third embodiment of the piston adds the above-mentioned optical sensors for detecting retracted, safe specimen loading/gripping, and extended positions of the active contact point. 
   Alternative embodiments of the end effector include flat or inclined, narrow or arcuate rest pads onto which the wafer is initially loaded. The narrow and arcuate inclined rest pad embodiments assist in centering and gripping the wafer between the active contact point and the distal rest pads. The arcuate rest pads more readily accommodate gripping and handling flatted wafers. 
   Embodiments of the end effectors further include fiber optic light transmission sensors for accurately locating the wafer edge and bottom surface. Three alternative embodiments include placing the wafer edge and bottom sensors at the proximal end of the end effector; placing the edge sensors at the proximal end and the bottom sensors at the distal end of the end effector; and placing a combined edge and bottom sensor at the distal end of the end effector. In all three embodiments, the sensors provide robot arm extension, elevation, and positioning data that support methods of rapidly and accurately placing a wafer on and retrieving a wafer from a wafer transport stage or a process chamber, and placing a wafer in and retrieving a wafer from among a stack of closely spaced wafers stored in a wafer cassette. The methods effectively prevent accidental contact between the end effector and adjacent wafers stacked in a cassette or a wafer resting on a processing device while effecting clean, secure gripping of the wafer. 
   Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceed with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a first embodiment of the end effector of this invention shown inserted into a semiconductor wafer cassette to retrieve or replace a wafer. 
       FIG. 2  is a side elevation view of the end effector of  FIG. 1  without the wafer cassette but showing the end effector inserted between an adjacent pair of three closely spaced apart wafers as they would be stored in the cassette. 
       FIG. 3  is an enlarged side elevation view of a flat rest pad embodiment of this invention showing the rest pad engaging an exclusion zone of a wafer. 
       FIG. 4  is an enlarged side elevation view of an inclined rest pad embodiment of this invention showing the inclined rest pad engaging substantially a periphery of a wafer. 
       FIG. 5  is a fragmentary plan view of a portion of the end effector and wafer of  FIG. 1 , enlarged to reveal positional relationships among the wafer and a movable contact point, wafer rest pads, and wafer edge and elevation sensors of the first embodiment end effector of this invention. 
       FIGS. 6A and 6B  are respective side and front elevation views of one of the edge and elevation sensors of  FIG. 5 , further enlarged to reveal the positioning of fiber optic light paths relative to the wafer. 
       FIG. 7  is a plan view of a second embodiment of the end effector of this invention shown gripping a semiconductor wafer and adjacent to a semiconductor wafer in a wafer cassette to sense, retrieve, or replace a wafer. 
       FIG. 8  is a sectional side elevation view of the end effector of  FIG. 7  showing an active contact point actuating mechanism gripping a wafer between adjacent ones of closely spaced apart wafers as they would be stored in the wafer cassette. 
       FIG. 9  is an enlarged isometric view of a distal arcuate rest pad embodiment of this invention mounted on the distal end of the end effector of  FIG. 7 . 
       FIG. 10  is an end perspective view of the end effector of  FIG. 7  showing positional relationships among the movable contact point, arcuate rest pads, and wafer edge and elevation sensors of the second embodiment end effector of this invention. 
       FIG. 11  is a bottom view of the end effector of  FIG. 7  showing fiber optic routing channels for elevation sensors of the second embodiment end effector of this invention. 
       FIG. 12  is a fragmentary plan view of a portion of a third embodiment of an end effector of this invention, showing positional relationships among the wafer, a position sensing active contact point actuating mechanism, and the proximal rest pads. 
       FIG. 13  is a sectional side elevation view of the end effector portion of  FIG. 12  showing the position sensing active contact point actuating mechanism fully extended between adjacent closely spaced wafers as they would be stored in the wafer cassette. 
       FIG. 14  is an overall plan view of the end effector of  FIG. 12  showing alternate wafer gripping and sensing positions. 
       FIGS. 15A and 15B  are respective side elevation and plan views of an exemplary two-arm, multiple link robot arm system from which the end effector of the present invention extends. 
       FIG. 16  is a side elevation view in stick diagram form showing the link components and the associated mechanical linkage of the robot arm system of  FIGS. 15A and 15B . 
       FIG. 17  is an isometric view in stick diagram form showing the rotational motion imparted by the motor drive links of the mechanical linkage of the robot arm system of  FIGS. 15A and 15B . 
       FIG. 18A  is a diagram showing the spatial relationships and parameters that are used to derive control signals provided by, and  FIG. 18B  is a block diagram of, the motor controller for the robot arm system of  FIGS. 15A and 15B . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  show a first embodiment of a spatula-shaped end effector  10  of this invention for transferring semiconductor wafers, such as a wafer  12  (shown transparent to reveal underlying structures), to and from a wafer cassette  14 . End effector  10  is adapted to receive and securely hold wafer  12  and transfer it to and from cassette  14  for processing.  FIG. 2  shows that end effector  10  is particularly adapted for retrieving and replacing wafer  12  from among closely spaced wafers, such as wafers  12 ,  12 A, and  12 B, which are shown as they might be stacked in slots  13  of wafer cassette  14 , or from a lowermost slot  13  of wafer cassette  14 . Wafers having diameters of less than 150 mm are typically spaced apart at a 4.76 mm ( 3/16 inch) pitch distance; 200 mm diameter wafers are typically spaced apart at a 6.35 mm (¼ inch) pitch distance; and 300 mm wafers are typically spaced apart at a 10 mm (0.394 inch) pitch distance. 
   End effector  10  is operably attached to a robot arm  16  (a portion of which is shown) that is programmably positionable in a well known manner. In general, end effector  10  enters wafer cassette  14  to retrieve wafer  12  positioned between wafers  12 A and  12 B. End effector  10  is then finely positioned by robot arm  16  and actuated to grip a periphery  18  of wafer  12 , remove wafer  12  from cassette  14 , and transfer wafer  12  to a processing station (not shown) for processing. End effector  10  may then, if necessary, reinsert wafer  12  into cassette  14 , release wafer  12 , and withdraw from cassette  14 . 
   End effector  10  is operably coupled to robot arm  16  at a proximal end  20  and extends to a distal end  22 . End effector  10  receives wafer  12  between proximal end  20  and distal end  22  and includes on a support surface  10   s  at least two and, preferably, four rest pads upon which wafer  12  is initially loaded. Two distal rest pads  24  are located at, or adjacent to, distal end  22  of end effector  10 ; and at least one, but preferably two proximal rest pads  26  are located toward proximal end  20 . Distal rest pads  24  may alternatively be formed as a single arcuate rest pad having an angular extent greater than the length of a “flat,” which is a crystal structure-indicating feature commonly found on semiconductor wafers. A flat  27  is shown, by way of example only, positioned between proximal rest pads  26 . Of course, wafer  12  may have a different orientation, so periphery  18  is also shown positioned between proximal rest pads  26 . 
   Wafer  12  includes an exclusion zone  30  (a portion of which is shown in dashed lines). Semiconductor wafers have an annular exclusion zone, or inactive portion, that extends inwardly about 1 mm to about 5 mm from periphery  18  and completely surrounding wafer  12 . Exclusion zone  30  is described as part of an industry standard wafer edge profile template in SEMI (Semiconductor Equipment and Materials International) specification M10298, pages 18 and 19. As a general rule, no part of end effector  10  may contact wafer  12  beyond the inner boundary of exclusion zone  30 . It is anticipated that future versions of the specification may allow edge contact only, a requirement that is readily accommodated by this invention. 
   The distance between rest pads  24  and the distance between rest pads  26  each have an angular extent greater than any feature on wafer  12  to guarantee that wafer  12  is gripped only within exclusion zone  30 . Rest pads  24  and  26  may be made of various materials, but a preferred material is polyetheretherketone (“peek”), which is a semi-crystalline high temperature thermoplastic manufactured by Victrex in the United Kingdom. The rest pad material may be changed to adapt to different working environments, such as in high temperature applications. 
     FIG. 3  shows a substantially flat embodiment of distal rest pads  24 . This embodiment can be advantageously, but need not exclusively be, used with wafers having less than about a 200 mm diameter. Distal rest pads  24  include a pad portion  32  and a backstop portion  34 . In the flat embodiment, pad portion  32  is substantially parallel to an imaginary plane  36  extending through wafer  12 , and backstop portion  34  is inclined toward wafer  12  at a backstop angle  38  of up to about 5 degrees relative to a line perpendicular to plane  36 . Alternatively, pad portion  32  may be inclined away from wafer  12  up to about 3 degrees relative to plane  36 . Pad portion  32  has a length  40  that is a function of the depth of exclusion zone  30 , but is preferably about 3 mm long. Wafer  12  typically has a substantially rounded peripheral edge and contacts rest pads  24  only within exclusion zone  30 . Wafer  12  is gripped by urging it into the included angle formed between pad portion  32  and backstop portion  34 . 
     FIG. 4  shows an inclined embodiment of distal rest pads  24 . This embodiment can be advantageously, but need not exclusively be, used with wafers having greater than about a 200 mm diameter. Distal rest pads  24  include an inclined pad portion  42  and a backstop portion  34 . In the inclined embodiment, inclined pad portion  42  is inclined away from wafer  12  at a rest pad angle  44  of about 3 degrees relative to plane  36 , and backstop portion  34  is inclined toward wafer  12  at backstop angle  38  of up to about 3 degrees. Inclined pad portion  42  has a length  40  that is a function of the depth of exclusion zone  30 , but is preferably about 3 mm long. As before, wafer  12  typically has a substantially rounded peripheral edge and contacts rest pads  24  only within exclusion zone  30 . Wafer  12  is gripped by urging it into the included angle formed between pad portion  42  and backstop portion  34 . In the inclined embodiment, there is substantially no contact between rest pad  24  and a bottom surface  46  of wafer  12 . This rest pad embodiment is also suitable for wafer edge contact only. 
   Both the flat and inclined embodiments of distal rest pads  24  have a height  48  that substantially reaches but does not extend beyond the top surface of wafer  12 . 
   Referring again to  FIG. 1 , proximal rest pads  26  are similar to distal rest pads  24  except that each rest pad  26  does not necessarily require a backstop portion and its pad portion has a length of about twice that of length  40 . 
   End effector  10  further includes an active contact point  50  that is located at proximal end  20  of end effector  10  and between proximal rest pads  26 . Alternatively, proximal end contact point  50  is formed as part of a proximal rest pad  26 . Active contact point  50  is movable between a retracted wafer-loading position (shown in dashed lines) and an extended wafer-gripping position (shown in solid lines). As shown, in the retracted position, active contact point  50  is positioned between and behind front side margins  28  of the pad portions of proximal rest pads  26 . 
   Active contact point  50  is operatively connected to a piston  52  for reciprocation between the retracted and extended positions. In a first embodiment, piston  52  reciprocates within a bore  54  and is preferably vacuum pressure operated to extend and retract active contact point  50 . Active contact point  50  is connected to piston  52  by a piston rod  56  that extends through an airtight seal  58 . Bore  54  forms a vacuum chamber in end effector  10  that is divided by piston  52  into a drive chamber  60  and a return chamber  62 . Drive chamber  60  is in pneumatic communication with a vacuum pressure source (not shown) through a first channel  64 , and return chamber  62  is in pneumatic communication with the vacuum pressure source through a second channel  66 . The vacuum pressure acts through drive chamber  60  against the front face of piston  52  to extend active contact point  50  to the wafer-gripping position and acts through return chamber  62  against the back face of piston  52  to retract active contact point  50  as controlled by the programmable control. The vacuum pressure source is routed to first and second channels  64  and  66  through rotary vacuum communication spools in robot arm  16 . Preferred rotary vacuum communication spools are described in U.S. Pat. No. 5,741,113 for CONTINUOUSLY ROTATABLE MULTIPLE LINK ROBOT ARM MECHANISM, which is assigned to the assignee of this application. 
   Piston  52  further includes an annular groove  68  that is in pneumatic communication with a vent (not shown) in piston rod  56 . First and second channels  64  and  66  are connected to, respectively, drive chamber  60  and return chamber  62  at locations that are opened to groove  68  at the travel limits of piston  52 . Therefore, vacuum pressure in first and second channels  64  and  66  is reduced at the travel limits of piston  52 , thereby providing signals to the vacuum controller that active contact point  50  is fully extended or retracted to effect proper loading of wafer  12 . 
   After wafer  12  is loaded onto end effector  10 , active contact point  50  is actuated to move wafer  12  into its gripped position. As active contact point  50  is extended, it urges wafer  12  toward distal rest pads  24  until wafer  12  is gripped within exclusion zone  30  by active contact point  50  and distal rest pads  24 . 
   Proximal rest pads  26  are arranged relative to distal rest pads  24  so that plane  36  of wafer  12  is preferably parallel to end effector  10  when gripped. This arrangement is readily achieved when the flat embodiment of proximal and distal rest pads  24  and  26  is employed. However, when the inclined embodiment is employed, proximal and distal rest pads  24  and  26  are arranged such that the points where wafer  12  contacts pad portions  42  are substantially equidistant from a center  70  of wafer  12  when active contact point  50  is extended and wafer  12  is gripped. For example, when wafer  12  is in the position shown in  FIG. 1 , the pad portions of distal and proximal rest pads  24  and  26  contact wafer  12  at points tangent to periphery  18  such that a line through the center of each pad portion  42  intersects center  70  of wafer  12 . Wafer  12  is, therefore, laterally centered when its peripheral edge is gripped. 
   The location of active contact point  50  at proximal end  20  allows end effector  10  to be lighter, stronger, and more slender than end effectors having moving mechanisms that may not fit between adjacent wafers  12 ,  12 A, and  12 B in cassette  14 . The lack of moving mechanisms further causes end effector  10  to produce less contamination within cassette  14 . Additionally, locating active contact point  50  at proximal end  20  of end effector  10  ensures that active contact point  50  is remote from harsh conditions such as heated environments and liquids. 
   The close spacing of adjacent wafers  12 ,  12 A, and  12 B requires accurate positioning of end effector  10  to enter cassette  14  and positively grip the wafers without touching adjacent wafers. 
     FIGS. 5 ,  6 A, and  6 B show respective top, side, and front views of a first embodiment of wafer edge and elevation sensors that provide accurate wafer  12  positioning data relative to end effector  10 . (Wafer  12  is shown transparent to reveal underlying structures.) The sensors are housed in first and second sensor housings  80  and  82 , which together form three light transmission sensors, each having a fiber optic source/receiver pair. 
   Two wafer edge sensors are implemented as follows. First and second sensor housings  80  and  82  each include a light source fiber  84  and a light receiver fiber  86  that form between them a small U-shaped opening  88  into which periphery  18  of wafer  12  can fit. Fibers  84  and  86  further include mutually facing light path openings  90  that form a narrow light transmission pathway for detecting the presence or absence of periphery  18  of wafer  12 . Fibers  84  and  86  extend through ferrules  92  to a light source/receiver module  94  that is mounted on a convenient location of end effector  10  near its rotary connection to robot arm  16 . Light source/receiver module  94  conventionally detects degrees of light transmission between fibers  84  and  86  and, thereby, accurately senses the positioning of periphery  18  between light path openings  90 . Of course, the relative positions of fibers  84  and  86  may be reversed. 
   One elevation sensor is implemented as follows. First sensor housing  80  further includes a light source fiber  96  (shown in phantom), and second sensor housing  82  includes a light receiver fiber  98  (shown in phantom). Fibers  96  and  98  form between them a wide opening that sights along a bottom surface chord  100  of wafer  12 . Fibers  96  and  98  further include mutually facing light path openings  102  that form a narrow light transmission pathway  104  for detecting the presence or absence of bottom surface chord  100  of wafer  12 . Fibers  96  and  98  extend through ferrules  106  to light source/receiver module  94 . Light source/receiver module  94  conventionally detects degrees of light transmission between fibers  96  and  98  and thereby accurately senses the positioning of bottom surface chord  100  between light path openings  102 . Of course, the relative positions of fibers  96  and  98  may be reversed. 
   Flat  27  may be detected by separating light path openings  102  from each other by distance greater than the length of flat  27 . Flat  27  is present if bottom surface chord  100  is sensed between light path openings  102 , but periphery  18  is not sensed between one of the pairs of light path openings  90 . 
   The procedure by which end effector  10  accesses wafer  12  of a known diameter, such as 200 mm, is described below with reference to  FIGS. 2 ,  5 ,  6 A, and  6 B. 
   Active contact point  50  is placed in its retracted or wafer-releasing position. 
   End effector  10  is inserted in an X direction into cassette  14  between, for example, wafers  12  and  12 B, until periphery  18  is sensed between at least one pair of light path openings  90 . 
   A controller (not shown) associated with robot arm  16  records the extension of robot arm  16  when periphery  18  is sensed, ignoring any sensed flat. 
   End effector  10  is retracted in the -X direction by an amount sufficient to provide clearance between wafer  12  and the edge detectors. 
   Robot arm  16  is moved in a Z direction until bottom surface chord  100  of wafer  12  is sensed. 
   The controller records the Z elevation of the bottom surface of wafer  12 . 
   The controller computes the X distance required to reach into cassette  14  at a Z elevation below the bottom surface of wafer  12  so distal and proximal rest pads  24  and  26  clear wafers  12  and  12 B. End effector  10  in this position defines a space between wafer  12  and the proximal and distal pad wafer contacting surfaces of the distal and proximal pad portions of support surface  10   s.    
   The controller also accounts for: 
   1) a radial distance offset and an elevation distance offset of distal rest pads  24  relative to the Z elevation of light transmission pathway  104 , and 
   2) the radial distance end effector  10  was retracted after sensing periphery  18 . 
   The controller moves end effector  10  in the X direction into cassette  14  and elevates in the Z direction to eliminate the space between and thereby contact wafer  12  on rest pads  24  and  26 . 
   Active contact point  50  is actuated toward a wafer-securing position to move wafer  12  along the proximal and distal pad wafer contacting surfaces of the distal and proximal pad portions of support surface  10   s . This movement of wafer  12  urges its peripheral edge into the included angle between pad and backstop portions  32  and  34  of distal rest pads  24 , thereby gripping wafer  12 . 
   End effector  10  withdraws wafer  12  in the −X direction from cassette  14 . 
     FIGS. 7 and 8  show a second embodiment of a spatula-shaped end effector  110  of this invention for transferring semiconductor wafers, such as wafer  12  (shown transparent to reveal underlying structures), to and from wafer cassette  14  (not shown in this view). End effector  110  is similar to end effector  10  but is further adapted to sense the bottom surface of a wafer stored in wafer cassette  14  without protruding into the cassette.  FIG. 8  shows that end effector  110  is particularly adapted for retrieving and replacing wafer  12  from among closely spaced apart wafers, such as wafers  12 ,  12 A, and  12 B, which are shown as they might be stacked in wafer cassette  14 . 
   End effector  110  is operably attached to robot arm  16 . In general, end effector  110  senses the bottom surface of wafer  12  before entering wafer cassette  14  to retrieve wafer  12  from between wafers  12 A and  12 B. End effector  110  is then finely positioned by robot arm  16  and actuated to grip periphery  18  of wafer  12 , remove wafer  12  from cassette  14 , and transfer wafer  12  to a processing station (not shown) for processing. End effector  110  may then, if necessary, reinsert wafer  12  into cassette  14 , release wafer  12 , and withdraw from cassette  14 . 
   End effector  110  is operably coupled to robot arm  16  at a proximal end  120  and extends to a distal end  122 . End effector  110  receives wafer  12  between proximal end  120  and distal end  122  and preferably includes on a support surface  110   s  at least two and, more preferably, four arcuate rest pads upon which wafer  12  is initially loaded. Two distal arcuate rest pads  124  are located at, or adjacent to, distal end  122  of end effector  110 ; and at least one, but preferably two proximal arcuate rest pads  126  are located toward proximal end  120 . Distal and proximal arcuate rest pads  124  and  126  may have an angular extent greater than flat  27 , which is shown, by way of example only, positioned between proximal rest pads  126 . Of course, wafer  12  may have a different orientation from that shown. 
   Arcuate rest pads  124  and  126 , whether separated as shown, or joined into a single rest pad, have an angular extent greater than any feature on wafer  12  to guarantee that wafer  12  is sufficiently gripped, whether flatted or not, and only within exclusion zone  30 . Like rest pads  24  and  26 , rest pads  124  and  126  may be made of various materials, but the preferred material is peek. 
     FIG. 9  shows the embodiment of distal arcuate rest pads  124  that is suitable for use with flatted or nonflatted wafers. Distal arcuate rest pads  124  include an inclined pad portion  132  and a backstop portion  134 . Referring also to  FIG. 4 , inclined pad portion  132  is inclined away from wafer  12  at rest pad angle  44  of about 3 degrees relative to plane  36 , and backstop portion  134  is inclined toward wafer  12  at backstop angle  38  of up to about 3 degrees. Inclined pad portion  132  has a length  140  that is a function of the depth of exclusion zone  30 , but is preferably about 3 mm long. As before, wafer  12  typically has a substantially rounded peripheral edge and contacts arcuate rest pads  124  by wafer edge contact (and perforce only within exclusion zone  30 ). Of course, the peripheral edge need not be rounded. Wafer  12  is gripped by urging it into the included angle formed between inclined pad portion  132  and backstop portion  134 . 
   Distal arcuate rest pads  124  have a height  148  that substantially reaches but does not extend beyond the top surface of wafer  12 . 
   Referring again to  FIG. 7 , proximal arcuate rest pads  126  are similar to distal arcuate rest pads  124  except that each rest pad  126  does not necessarily require a backstop portion and its pad portion has a length of about twice that of length  140 . 
   End effector  110  further includes an active contact point  150  that is located at proximal end  120  of end effector  110  and between proximal arcuate rest pads  126 . Alternatively, proximal end contact point  150  is formed as part of a proximal rest pad  126 . Active contact point  150  is movable between a retracted wafer-loading position (not shown) and the extended wafer-gripping position shown. 
   Referring again to  FIG. 8 , a second embodiment of an active contact point actuating mechanism  151  is shown employed with end effector  110 . Active contact point  150  is operatively connected to a piston  152  for reciprocation between retracted and extended positions. In this embodiment, piston  152  reciprocates within a bore  154  and is urged by a biasing device or spring  155  to extend active contact point  150  and by a vacuum pressure to retract active contact point  150 . Active contact point  150  is connected to piston  152  by a piston rod  156  that extends through an annular airtight seal  158 . Bore  154  includes an end cap  159  that forms one wall of a vacuum chamber  160 , the other wall of which is movably formed by piston  152 . Vacuum chamber  160  is in pneumatic communication with a vacuum pressure source (not shown) through a vacuum feedthrough  162  and a vacuum channel  164 . Spring  155  presses against the face of piston  152  to extend active contact point  150  to the wafer-gripping position, whereas the vacuum pressure acts through vacuum chamber  160  against the face of piston  152  to overcome the spring force and retract active contact point  150  to the wafer-releasing position. 
   In the second embodiment, active contact point  150  is urged against wafer  12  with a force determined solely by spring  155 . Spring  155  is supported between recesses  166  in piston  152  and end cap  159 . The vacuum pressure source is routed to vacuum channel  164  through rotary vacuum communication seals or spools in robot arm  16 . Thus, in the event of loss of vacuum pressure or other facilities, end effector  110  operates in a failsafe manner with spring  155  applying a biasing force that causes active contact point actuating mechanism  151  to attain its wafer-securing position to hold wafer  12  in its gripped position. 
   Actuating mechanism  151  further includes a vent  168  in pneumatic communication with the atmosphere to allow free movement of piston  152  within the portion of bore  154  not in pneumatic communication with the vacuum pressure source. Actuating mechanism  151  is made “vacuum tight” by O-ring seals  170  surrounding end cap  159  and vacuum feedthrough  162  and by an annular moving seal  172  surrounding piston  152 . O-ring bumper seals  174  fitted to the faces of piston  152  absorb contact shocks potentially encountered by piston  152  at the extreme ends of its travel. 
   After wafer  12  is loaded onto end effector  110 , active contact point  150  is actuated to move wafer  12  into its gripped position. As active contact point  150  is extended by spring  155 , it urges wafer  12  toward distal arcuate rest pads  124  until wafer  12  is gripped by wafer edge contact (and perforce within exclusion zone  30 ) by active contact point  150  and distal arcuate rest pads  124 . Active contact point  150  includes an inwardly inclined face portion  176  to impart to wafer  12  a motive force component that urges wafer  12  toward proximal arcuate rest pads  126 , thereby firmly gripping the peripheral edge of wafer  12 . 
   Proximal arcuate rest pads  126  are arranged relative to distal arcuate rest pads  124  so that the plane of wafer  12  is preferably parallel to end effector  110  when gripped. 
   In a manner similar to end effector  10 , the location of active contact point  150  at proximal end  120  allows end effector  110  to be lighter, stronger, and more slender than end effectors having moving mechanisms that may not fit between adjacent wafers  12 ,  12 A, and  12 B in cassette  14 . The lack of moving mechanisms between its proximal end  120  and distal end  122  further causes end effector  110  to produce less contamination within cassette  14 . Moreover, unlike end effector  10 , which is actuated by two vacuum lines, end effector  110  requires only one vacuum line for actuation. Of course, end effector  10  could be fitted with actuating mechanism  151 . 
   The close spacing of adjacent wafers  12 ,  12 A, and  12 B requires accurate positioning of end effector  110  to enter cassette  14  and positively grip the wafers without touching adjacent wafers. 
     FIGS. 7 ,  10 , and  11  show respective top, end, and bottom views of a second embodiment of wafer edge and elevation sensors that provide accurate wafer  12  positioning data relative to end effector  110 . The wafer edge sensors are housed in first and second sensor housings  180  and  182 , each having a fiber optic source/receiver pair forming a light transmission sensor in each housing. The elevation sensor is housed in distal end  122  of end effector  110 . 
   Two wafer edge sensors are implemented as follows. First and second sensor housings  180  and  182  each include light source fiber  84  and light receiver fiber  86 , as in end effector  10 , that form between them a small U-shaped opening  88  into which periphery  18  of wafer  12  can fit. As before, fibers  84  and  86  include mutually facing light path openings that form a narrow light transmission pathway for detecting the presence or absence of periphery  18  of wafer  12 . The two wafer edge sensors are separated from each other by a distance  183  greater than the length of flat  27  so that a flatted wafer can be detected when only one of the two wafer edge sensors detects periphery  18  of wafer  12 . Of course, wafer  12  must be appropriately oriented in cassette  14  to detect flat  27 . 
   The elevation sensor is implemented as follows. Unlike the first embodiment, first and second sensor housings  180  and  182  do not include light source fiber  96  and light receiver fiber  98 . Rather in this embodiment, light source fiber  96  is routed through a first channel  184  formed in the bottom surface of end effector  110  and running between proximal end  120  and a first distal tine  188  proximal to distal end  122  of end effector  110 . In like manner, light receiver fiber  98  is routed through a second channel  186  formed in the bottom surface of end effector  110  and running between proximal end  120  and a second distal tine  190  proximal to distal end  122  of end effector  110 . Distal tines  188  and  190  are widely spaced apart across a gap  191  that forms a relief region for certain types of processing equipment, such as wafer prealigners. 
   Fibers  96  and  98  terminate in mutually facing light path openings  192  and  194  formed in distal tines  188  and  190 . Fibers  96  and  98  form between them a wide opening that sights along a bottom surface chord  200  of, for example, wafer  12 A. Mutually facing light path openings  192  and  194  form a narrow light transmission pathway  202  for detecting the presence or absence of bottom surface chord  200  of wafer  12 A. In end effector  110 , light transmission pathway  202  extends beyond the portion of distal end  122  that would first contact wafer  12 , thereby further providing an obstruction sensing capability. As before, light source/receiver module  94  conventionally detects degrees of light transmission between fibers  96  and  98  and, thereby, accurately senses the positioning of bottom surface chord  200  between light path openings  192  and  194 . Of course, the relative positions of fibers  96  and  98  may be reversed. 
   The procedure by which end effector  110  accesses a predetermined wafer from among closely spaced apart wafers in a cassette, is described below with reference to  FIGS. 7 ,  8 , and  10 . 
   Active contact point  150  is placed in its retracted position. 
   End effector  110  is moved in an X direction toward cassette  14  until tines  188  and  190  are adjacent to, but not touching, a predicted position for any wafer  12  in cassette  14 . 
   End effector  110  is then scanned in a Z direction such that light transmission pathway  202  intersects the bottom surface chord  200  of any wafer in cassette  14  and, additionally, detects any obstruction projecting from cassette  14  toward end effector  110 . 
   The controller (not shown) records the Z elevations of the bottom surfaces of any wafers and obstructions detected. 
   Robot arm  16  is moved to a Z elevation calculated to access a predetermined wafer, such as wafer  12 A, while also providing clearance for end effector  110  between adjacent wafers. 
   The following optional operations may be performed: 
   End effector  110  may be optionally moved in an X direction toward cassette  14  until tines  188  and  190  are adjacent to, but not touching, wafer  12 A. In this position, light transmission pathway  202  should be adjacent to bottom surface chord  200  of wafer  12 A; 
   robot arm  16  is optionally moved in a Z direction until bottom surface chord  200  of wafer  12 A is sensed; 
   the controller optionally verifies the previously sensed Z elevation of the bottom surface of wafer  12 A; and 
   robot arm  16  is optionally moved in a −Z direction to provide clearance for end effector  110  between adjacent wafers. 
   End effector  110  is inserted in an X direction into cassette  14  between adjacent wafers until periphery  18  is sensed by at least one wafer edge sensor. 
   The controller moves end effector  10  in the Z direction by an amount calculated to contact wafer  12 A on arcuate rest pads  124  and  126 . 
   Active contact point  150  is actuated to urge wafer  12 A into the included angle between pad and backstop portions  132  and  134  of distal arcuate rest pads  124 , thereby gripping wafer  12 A. (In  FIG. 7 , the gripped wafer is shown as wafer  12 .) 
   End effector  110  withdraws wafer  12 A in the −X direction from cassette  14 . 
   End effector  110  combines a very thin Z-direction profile and accurate wafer position sensing to enable clean, rapid, and secure movement of very closely spaced apart wafers in a cassette. 
     FIGS. 12 ,  13 , and  14  show a third embodiment of a preferred fork-shaped end effector  210  of this invention for transferring semiconductor wafers, such as wafer  12  (shown transparent to reveal underlying structures), to and from wafer cassette  14  (not shown in these views). End effector  210  is similar to end effectors  10  and  110  but further includes a position sensing active contact point actuating mechanism  212 , and deletes the proximal end edge and elevation sensors. Rather, end effector  210  employs distal end sensors  214  to accomplish various wafer sensing measurements. Distal end sensors  214  are implemented similarly to the elevation sensor generating light transmission pathway  202  as shown in  FIGS. 7 and 10 . 
     FIG. 13  shows that end effector  210  is particularly suited for retrieving and replacing wafer  12  from among closely spaced apart wafers, such as wafers  12 ,  12 A, and  12 B, which are shown as they might be stacked in wafer cassette  14 . 
     FIG. 14  shows end effector  210  operably coupled to robot arm  16  at a proximal end  216  and extending to forked distal ends  218  and  220 . End effector  210  receives wafer  12  between proximal end  216  and forked distal ends  218  and  220  and preferably includes at least two and, more preferably, four arcuate rest pads upon which wafer  12  is initially loaded. A distal arcuate rest pad  124  is located at, or adjacent to, each of forked distal ends  218  and  220 ; and at least one, but preferably two proximal arcuate rest pads  126  are located toward proximal end  216 . End effector  210  also includes an active contact point  222  that is located at proximal end  216  of end effector  210  and between proximal arcuate rest pads  126 . 
   Referring to  FIGS. 12 and 13 , position sensing active contact point actuating mechanism  212  is a third embodiment of the active contact point actuating mechanism. As in the second embodiment, active contact point  222  is operatively connected to piston  152  for reciprocation between fully retracted, fully extended, and intermediate positions. Piston  152  moves within bore  154  and is urged by a spring ( FIG. 8 ) to extend active contact point  222  and by a vacuum pressure to retract active contact point  222 . Active contact point  222  is connected to piston  152  by piston rod  156  that extends through annular airtight seal  158 . Bore  154  includes end cap  159  that forms one wall of vacuum chamber  160 , the other wall of which is movably formed by piston  152 . Vacuum chamber  160  is in pneumatic communication with the vacuum pressure source (not shown) through vacuum feedthrough  162  and vacuum channel  164 . The spring presses against the face of piston  152  to extend active contact point  222  to wafer-gripping and fully extended positions, whereas the vacuum pressure acts through vacuum chamber  160  against the face of piston  152  to overcome the spring force and retract active contact point  222  to wafer-releasing and fully retracted positions. 
   Actuating mechanism  212  further includes vent  168  in pneumatic communication with the atmosphere to allow free movement of piston  152  within the portion of bore  154  not in pneumatic communication with the vacuum pressure source. Actuating mechanism  212  is made “vacuum tight” by O-ring seals  170  surrounding end cap  159  and vacuum feedthrough  162 , and by an annular moving seal  172  surrounding piston  152 . 
   Unlike the first and second embodiments, actuating mechanism  212  further includes a position indicating shaft  224  attached to piston  152  and extending axially through an annular seal  226  in end cap  159 . A pair of optical interrupter switches  228  and  230  are mounted to a circuit board  232  positioned just behind end cap  159  such that, depending on the position of indicating shaft  224 , it interrupts a pair of light beams  234  and  236  in respective optical interrupter switches  228  and  230 . 
   Optical interrupter switches  228  and  230  sense positions of active contact point  222  corresponding to a retracted position region, a safe gripping operation region, and an extended position region. ( FIGS. 12 and 13  show active contact point  222  in a fully extended position.) 
   The retracted position region ensures that wafer  12  is not gripped and is sensed when position indicating shaft  224  interrupts both of light beams  234  and  236 . 
   The safe gripping operation region is a range of active contact point  222  positions within which wafer loading, gripping, or unloading operation can be safely carried out and is sensed when position indicating shaft  224  interrupts light beam  236  but not light beam  234 . Moreover, when active contact point  222  is extended and comes to rest in the safe gripping operation region, proper wafer gripping is verified. 
   The extended position region is a range of active contact point  222  positions within which wafer  12  is not gripped and is sensed when position indicating shaft  224  interrupts neither of light beams  234  and  236 . 
   Optical interrupter switches  228  and  230  are in electrical communication with the above-referenced controller. The controller coacts with the vacuum pressure source actuating piston  152  to pulse or pressure regulate the amount of vacuum pressure and, thereby, control the positions of active contact point  222 . Of course, various other forms of controllable motive forces may be employed to position active contact point  222 . 
   In an operational example, active contact point  222  is moved to the safe gripping operation region and a wafer  12  is loaded into end effector  210 . After wafer  12  is loaded, active contact point  222  is actuated to move wafer  12  into its gripped position. As active contact point  150  is extended, it urges wafer  12  up inclined pad portions  132  of distal arcuate rest pads  124  away from support surface  110   s  and down the inclined proximal pad portions of proximal arcuate rest pads  126  toward support surface  110   s  until wafer  12  is gripped. Active contact point  222  must be sensed in the safe gripping operating region to ensure that wafer  12  is properly gripped. 
   Wafer  12  is released by retracting active contact point  222  to the retracted position region as sensed by position indicating shaft  224  interrupting both of light beams  234  and  236 . When wafer  12  is released, it slips back on inclined pad portions  132  of distal arcuate rest pads  124 , thereby providing sufficient clearance between wafer  12  and backstop portion  134  for a safe Z-axis elevation move and retrieval of end effector  210 . 
     FIG. 14  shows a top view of the third embodiment of end effector  210  in which the wafer edge sensors of end effectors  10  and  110  have been removed. Distal end sensors  214  of end effector  210  are housed in forked distal ends  218  and  220 . Distal end sensors  214  are implemented as follows. A light source fiber is routed through a first channel  238  (shown in phantom lines) formed in the bottom surface of end effector  210  and running between proximal end  216  and forked distal end  218 . In like manner, a light receiver fiber is routed through a second channel  240  (shown in phantom lines) formed in the bottom surface of end effector  210  and running between proximal end  216  and forked distal end  220 . Forked distal ends  218  and  220  are widely spaced apart across a gap  242  that forms a relief region for certain types of processing equipment, such as wafer prealigners. 
   The light fibers terminate in mutually facing light path openings (not shown) formed in forked distal ends  218  and  220 . The fibers form between them a wide opening that sights along the peripheral edge or the bottom surface chord of a wafer. The mutually facing light path openings form a narrow light transmission pathway  244  for detecting the presence or absence of the periphery or bottom surface chord of a wafer. Light transmission pathway  244  extends beyond the portion of forked distal ends  218  and  220  that would first contact a wafer, thereby further providing an obstruction sensing capability. As before, light source/receiver module  94  conventionally detects degrees of light transmission between the fibers and, thereby, senses any objects that interrupt light transmission pathway  244 . 
   End effector  210  employs distal end sensors  214  to accomplish various wafer sensing measurements including sensing wafer protrusion from a cassette, wafer edge sensing, wafer top and bottom chord sensing, wafer tilt, wafer center determination, wafer thickness, center-to-center distance between the wafer and the robot arm rotational axis, and determining the end effector centroid. The sensing measurements are described with reference to light transmission pathway  244  of end effector  210 , but they can also be accomplished with light transmission pathway  202  of end effector  110 . 
   Three alternative wafer positions are shown in  FIG. 14 . Wafer  12  (shown in phantom) is shown gripped by end effector  210 , wafer  12 A (shown in solid lines) is shown in a wafer edge sensing position, and wafer  12 B (shown in phantom) is shown in a wafer chord sensing position. 
   Sensing wafer  12 B protrusion from a cassette (not shown) entails stepping robot arm  16  up and down in the Z-axis direction while also moving end effector  210  in the X-axis direction until wafer  12 B is detected. Prior robot arm systems typically employed a dedicated protrusion sensor. Robot arm  16  X- and Z-axis movements are preferably in a fine resolution mode. 
   After light transmission pathway  244  is interrupted, indicating detected presence of wafer  12 B, end effector  210  can find wafer  12 B top and bottom surfaces by moving end effector  210  downward in the Z-axis direction until a top surface chord of wafer  12 B interrupts light transmission pathway  244 . End effector  210  continues moving downward until light transmission pathway  244  is restored. This point represents sensing a bottom surface chord of wafer  12 B. End effector  210  is then moved to a Z-axis position midway between the points of interruption and restoration of light transmission pathway. This Z-axis position represents the approximate midpoint of wafer  12 B thickness. While maintaining this Z-axis position, end effector  210  is retracted in the X-axis direction until light transmission pathway  244  is restored, indicating that periphery  18  of the wafer has been detected. Wafer  12 A is shown in this position. 
   When end effector  210  is at the edge detection point represented by wafer  12 A and because the radius of wafer  12 A is known, the controller and position encoders associated with robot arm  16  can determine the X-axis direction distance to a center  246  of wafer  12 A and a downward Z-axis distance required to provide clearance between the bottom surface of wafer  12 A and end effector  210 . Knowing the clearance is necessary when placing and retrieving wafers from the cassette because the wafers are not necessarily parallel to end effector  210  and distances between adjacent wafers in the cassette can be tight. 
   End effector  210  further includes a controllable supination angle  248 , which is the tilt angle about the X-axis of end effector  210  relative to a Y-axis. Wafers stacked in a cassette would have their major surface planes at a predetermined tilt angle, preferably zero degrees, that should be matched by supination angle  248  of end effector  210 . To determine whether supination angle  248  is level with the tilt angle of a wafer, robot arm  16  moves end effector  210  up and down in the Z-axis direction while dithering its supination angle  248  until a minimum wafer thickness is computed, which indicates that end effector  210  and the wafer are in the same datum plane. Robot arm systems can be equipped with two end effectors or multiple arms (see  FIGS. 15A and 15B  for dual arm robot). The technique described above for a controllable supination angle can be extended to such multiple end effector systems by using a single wafer as a reference to determine the X, Y, and Z dimension offsets among them. 
   Light transmission pathway  244  may also be employed to determine the X-axis position of a wafer in the cassette or on a prealigner. This determination entails finding the minimum distance between a shoulder axis  316  of robot arm  16  and the front of a wafer, for example, wafer  12 B. Finding this minimum distance then provides the corresponding robot arm extension and angle values. The determination entails angularly displacing robot arm  16  such that light transmission pathway  244  intersects wafer  12 B at two different chord positions, such as chord positions  254  and  256 . There is a variety of available search routines that can be used to compute this minimum distance. This distance determination is accomplished without any of the teaching fixtures required by prior robot arms and end effectors. If multiple end effectors  210  are employed, the foregoing procedure can be repeated together with determining any Z-axis elevation difference between them. 
   Referring to  FIG. 5 , it should be noted that the U-shaped edge detecting sensors in housing  80  and  82  are useful for determining certain parameters of a flatless 300 mm wafer. For instance, the edge detecting sensors can be employed to determine the center-to-center distance between shoulder axis  316  of robot arm  16  and a wafer center while the wafer is in the cassette or end effector  10  is positioned beneath the wafer. Of course, the Z-axis dimension of U-shaped openings  88  ( FIG. 6A ) presents a potential spacing problem. 
   Referring again to  FIG. 14 , light transmission pathway  244  may also be used in combination with the supination capability of end effector  210  to determine whether a centroid  262  of end effector  210  is axially aligned with center  252  of wafer  12 B. Ideally, centroid  262  is coaxial with the center of gripped wafer  12  and lies on an imaginary line extending between shoulder axis  316  and center  252  of wafer  12 B. However, manufacturing tolerances and the positionings of features creating light transmission pathway  244  may cause a calculated position of centroid  262  to be offset from the supination axis of rotation. Determining whether centroid  262  is offset or coincident entails carrying out the above-referenced robot arm  16  movements and distance calculations to determine the location of center  252  of wafer  12 B, rotating end effector  210  through a supination angle  248  of 180 degrees and repeating the center  252  location calculation. If the centroid is offset, the calculated location of center  252  will be in a mirror image position on the opposite side of the supination axis of rotation. The correct location for centroid  262  is determined by averaging the two calculated locations for center  252  of wafer  12 B. 
   The above-described embodiments are merely illustrative of the principles of the invention. Various modifications and changes may be made thereto by those skilled in the art that will embody the principles of the invention and fall within the spirit and scope thereof. For example, skilled workers will understand that the pistons may be actuated by alternative power sources, such as, for example, by a pulsing solenoid that slows the pistons as wafer  12  is secured. Electric signals may be employed to drive and monitor the positioning of the pistons. The pistons may also be pneumatically operated and monitored, such as in applications where the end effectors are submerged in a liquid. The end effectors may be forked or otherwise include a cutout or be shaped to avoid obstacles, such as a prealigner hub. The sensors preferably employ laser beams from light-emitting diodes and diode lasers, but may also employ incandescent, infrared, and other radiation sources. Moreover, the end effector is usable for handling various types of specimens other than semiconductor wafers, such as compact diskettes and computer memory discs. 
     FIGS. 15A and 15B  and  FIGS. 16 and 17  show a type of multiple link robot arm system  308  to which end effector  210  is mountable.  FIGS. 18A and 18B  present in conjunction with pertinent mathematical expressions characterizing robot arm displacement an example of positioning robot arm mechanism  308  to demonstrate the manipulation of the linear and angular displacement values necessary to compute the parameters associated with the various wafer sensing measurements described above. U.S. Pat. No. 5,765,444 provides a detailed description of the construction and operation of this type of robot arm system. 
     FIGS. 15A and 15B  are respective side elevation and plan views of a two-arm, multiple link robot arm system  308  mounted on and through an aperture in the top surface of a support table  309 . With reference to  FIGS. 15A and 15B , two similar but independently controllable three-link robot arm mechanisms  310 L and  310 R are rotatably mounted at opposite ends of a torso link  311 , which is mounted to the top surface of a base housing  312  for rotation about a central or torso axis  313 . Because they are mirror images of each other, robot arm mechanisms  310 L and  310 R have corresponding components identified by identical reference numerals followed by the respective suffices “L” and “R”. Accordingly, the following discussion is directed to the construction and operation of only robot arm mechanism  310 R but is similarly applicable to robot arm mechanism  310 L. 
   Robot arm mechanism  310 R comprises an upper arm  314 R mounted to the top surface of a cylindrical spacer  315 R, which is positioned on the right-hand end of torso link  311  for rotation about a shoulder axis  316 R. Cylindrical spacer  315 R provides room for the motors and certain other components of robot arm mechanism  310 R, as will be described below. Upper arm  314 R has a distal end  318 R to which a proximal end  320 R of a forearm  322 R is mounted for rotation about an elbow axis  324 R, and forearm  322 R has a distal end  326 R to which a proximal end  328 R of end effector or hand  210 R is mounted for rotation about a wrist axis  332 R. Hand  210 R is equipped at its distal end  334 R with a fluid pressure outlet  336 R that preferably applies vacuum pressure supplied to robot arm mechanism  310 R at an inlet  338  to vacuum channel  164  to securely hold semiconductor wafer  12 , a compact disk, or other suitable specimen (not shown) in place on hand  210 R. As will be described in detail later, each of upper arm  314 R, forearm  322 R, and hand  210 R is capable of continuous rotation about its respective shoulder axis  316 R, elbow axis  324 R, and wrist axis  332 R. 
     FIG. 16  shows the link components and associated mechanical linkage of robot arm mechanism  310 R. With reference to  FIG. 16 , robot arm mechanism  310 R is positioned by first and second concentric motors  350 R and  352 R that operate in response to commands provided by a motor controller  354  ( FIGS. 18A and 18B ). First motor  350 R rotates forearm  322 R about elbow axis  324 R, and second motor  352 R rotates upper arm  314 R about shoulder axis  316 R. 
   More specifically, first motor  350 R rotates a forearm spindle  356 R that extends through an aperture in upper arm  314 R and terminates in an upper arm pulley  358 R. A post  360 R extends upwardly at distal end  318 R of upper arm  314 R through the center of a bearing  362 R that is mounted to a bottom surface  364 R of forearm  322 R at its proximal end  320 R. Post  360 R also extends through an aperture in forearm  322 R and terminates in a forearm pulley  366 R. An endless belt  368 R connects upper arm pulley  358 R and the outer surface of bearing  362 R to rotate forearm  322 R about elbow axis  324 R in response to rotation of first motor  350 R. 
   Second motor  352 R rotates an upper arm spindle  380 R that is mounted to a bottom surface  382 R of upper arm  314 R to rotate upper arm  314 R about shoulder axis  316 R. Coordinated operation of first and second motors  350 R and  352 R in conjunction with the mechanical linkage described below causes hand  210 R to rotate about shoulder axis  316 R. A post  384 R extends upwardly through the center of a bearing  386 R that is mounted to a bottom surface  388 R of hand  210 R. An endless belt  390 R connects forearm pulley  366 R to the outer surface of bearing  386 R to rotate hand  210 R about shoulder axis  316 R in response to the coordinated rotational motions of motors  350 R and  352 R. 
   The mechanical linkage coupling upper arm  314 R and forearm  322 R forms an active drive link and a passive drive link. The active drive link includes belt  368 R connecting upper arm pulley  358 R and the outer surface of bearing  362 R and causes forearm  322 R to rotate in response to rotation of first motor  350 R. The passive drive link includes belt  390 R connecting forearm pulley  366 R and the outer surface of bearing  386 R and causes hand  210 R to rotate about wrist axis  332 R in response to rotation of forearm  322 R about elbow axis  324 R. Rotation of hand  210 R can also be caused by a complex interaction among the active and passive drive links and the rotation of upper arm  314 R in response to rotation of second motor  352 R. 
   A third or torso motor  392  rotates a torso link spindle  394  that is mounted to a bottom surface of torso link  311 , to which robot arm mechanism  310 R is rotatably mounted. A main ring  396  supports a bearing assembly  398  around which spindle  394  rotates. Motor  392  is capable of 360 degree continuous rotation about central axis  313  and therefore can, in cooperation with robot arm mechanism  310 R, move hand  210 R along an irregular path to any location within the reach of hand  210 R. 
   Motor controller  54  ( FIGS. 18A and 18B ) controls motors  350 R and  352 R in two preferred operational states to enable robot arm mechanism  310 R to perform two principal motion sequences. The first motion sequence changes the extension or radial position of hand  210 R, and the second motion sequence changes the angular position of hand  210 R relative to shoulder axis  316 R.  FIG. 17  is a useful diagram for showing the two motion sequences. 
   With reference to  FIGS. 16 and 17 , in the first operational state, motor controller  354  causes first motor  350 R to maintain the position of forearm spindle  356 R and second motor  352 R to rotate upper arm spindle  380 R. The non-rotation of first motor  350 R maintains the position of upper arm pulley  38 R, and the rotation of upper arm spindle  380 R by second motor  352 R rotates upper arm  314 R about shoulder axis  316 R, thereby causing rotation of forearm  322 R about elbow axis  324 R and counter-rotation of hand  210 R about wrist axis  332 R. Because the ratio of the diameters of upper arm pulley  358 R and the outer surface of bearing  362 R are 4:2 and the ratio of the diameters of forearm pulley  366 R and the outer surface of bearing  386 R is 1:2, the rotation of upper arm  314 R in a direction specified by P 2  shown in  FIG. 17  will cause hand  210 R to move along a straight line path  400 . (The diameters of forearm pulley  366 R and the outer surface of bearing  386 R are one-half of the diameters of, respectively, the outer surface of bearing  362 R and upper arm pulley  358 R to streamline the sizes and shapes of forearm  322 R and hand  210 R.) 
   Whenever upper arm  314 R rotates in the clockwise direction specified by P 2 , hand  210 R extends (i.e., increases radial distance from shoulder axis  16 R) along path  400 . Whenever upper arm  314 R rotates in the counter-clockwise direction specified by P 2 , hand  210 R retracts (i.e., decreases radial distance from shoulder axis  316 R) along path  400 . Skilled persons will appreciate that robot arm mechanism  310  in a mirror image configuration of that shown in  FIG. 17  would extend and retract in response to upper arm  314  rotation in directions opposite to those described.  FIG. 15B  shows that when robot arm mechanism  310 R is extended, axes  313 ,  316 R,  324 R, and  332 R are collinear. 
   In the second operational state, motor controller  352 R causes first motor  350 R to rotate forearm spindle  356 R in the direction specified by P 1  and second motor  352 R to rotate upper arm spindle  380 R in the direction specified by P 2 . In the special case in which motors  350 R and  352 R are synchronized to rotate in the same direction by the same amount of displacement, hand  210 R is only angularly displaced about shoulder axis  316 R. This is so because the rotation of forearm  322 R about elbow axis  324 R caused by the rotation of first motor  350 R and the rotation of hand  330 R about wrist axis  332 R caused by rotation of second motor  352 R and the operation of the passive drive link offset each other to produce no net rotation about elbow axis  324 R and wrist axis  332 R. Thus, hand  210 R is fixed radially at a point along path  400  and describes a circular path as only upper arm  314 R rotates about shoulder axis  316 R. By application of kinematic constraints to achieve a desired travel path for hand  210 , motor controller  354  can operate first and second motors  350 R and  352 R to move robot arm mechanism  310 R along non-radial straight line paths, as will be further described below. 
   Skilled persons will appreciate that to operate robot arm mechanism  310 R, first and second motors  350 R and  352 R are coupled by either rotating both of them or grounding one while rotating the other one. For example, robot arm mechanism  310 R can be operated such that forearm  322 R rotates about elbow axis  324 R. Such motion would cause hand  210 R to describe a simple spiral path between shoulder axis  316 R and the full extension of hand  210 R. This motion is accomplished by fixing the position of shoulder  314 R and operating motor  350 R to move forearm  322 R. 
   Motor controller  354  controls the operation of torso motor  392  and therefore the rotation of torso link  311  in a direction specified by P 3  independently of the operational states of motors  350 R and  352 R. 
   The angular positions of motors  350 R and  352 R are tracked by separate glass scale encoders (not shown). Each of the encoders typically includes an annular diffraction grating scale and a light source/detector subassembly (not shown). Such glass scale encoders are known to skilled persons. The angular position of motor  392  is tracked by a glass scale the encoder of a type similar to the encoders for motors  350 R and  352 R. 
     FIG. 18A  is a diagram that specifies a local coordinate axis frame whose axes are defined by the orientation of a semiconductor wafer cassette  168   r  and its location relative to shoulder axis  316 R. With reference to  FIG. 18A , the following description sets forth the mathematical expressions from which are derived the command signals controller  354  uses to retrieve from cassette  168   r  a wafer  170   r  along a vector perpendicular to the opening of cassette  168   r . (Skilled persons will appreciate that similar mathematical expressions can be used for different drive ratios from the above-stated drive ratio on which this example is based.) 
   The following parameters are pertinent to the derivation of the path of travel of hand  210 :
         Θ S =angle of motor  352 R   Θ E =angle of motor  350 R   r=distance between shoulder axis  316 R and elbow axis  324 R and distance between elbow axis  324 R and wrist axis  332 R   β=angle between upper arm  314 R and forearm  322 R   p=length of hand  210 R   E=2r=extension of robot arm   R i =reach of robot arm (i.e., its radius measured from shoulder axis  316 R to the center  172   r  of wafer  170   r  positioned on hand  210 R).       

   Application of the law of cosines provides the following expressions for R i : 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         R 
                         i 
                       
                       = 
                       
                         p 
                         + 
                         
                           
                             ( 
                             
                               
                                 r 
                                 2 
                               
                               + 
                               
                                 r 
                                 2 
                               
                               - 
                               
                                 2 
                                 ⁢ 
                                 
                                   r 
                                   2 
                                 
                                 ⁢ 
                                 cos 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 β 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         p 
                         + 
                         
                           
                             2 
                           
                           ⁢ 
                           r 
                           ⁢ 
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     cos 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     β 
                                   
                                 
                                 ) 
                               
                             
                             . 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   For β=0, equation (1) provides that R i =p and x=0, y=0, Θ S =Θ SR , ΘE=Θ ER . The quantities Θ SR  and Θ ER  represent reference motor angles. The motor angles may be expressed as Θ S =Θ SR +ΔΘ SR , Θ E =Θ ER +ΔΘ ER . The angle β may be expressed as β=2(ΔΘ SR −ΔΘ ER ) because of the construction of the mechanical linkages of robot arm mechanism  310 R. This equation relates the angle β to changes in the motor angles. 
   To retrieve wafer  170   r  from cassette  168   r  along a straight line path, the displacement along the X-axis equals X 0 , which is a constant. Thus, X(t)=X 0 . The quantity X(t) can be expressed as a function of the lengths of the X-axis components of its links:
 
 X ( t )= r  cos Θ 1   +r  cos Θ 2   +p  cos Θ p ,  (2)
 
in which
 
   Θ 1 =angle of upper arm  314 R 
   Θ 2 =angle of forearm  322 R 
   Θp=angle of hand  210 R. 
   Because upper arm  314 R and forearm  322 R are of the same length (r), Θ 1  tracks the angle Θ S  of motor  352 R, and hand  210 R moves in a straight line, the following expressions hold: 
   
     
       
         
           
             Θ 
             1 
           
           = 
           
             Θ 
             S 
           
         
       
     
     
       
         
           
             Θ 
             2 
           
           = 
           
             
               Θ 
               1 
             
             + 
             π 
             - 
             β 
           
         
       
     
     
       
         
           
             Θ 
             p 
           
           = 
           
             
               Θ 
               1 
             
             + 
             
               
                 ( 
                 
                   
                     π 
                     - 
                     β 
                   
                   2 
                 
                 ) 
               
               . 
             
           
         
       
     
   
   Thus, to compute X 0 , one substitutes the foregoing identities for Θ 1 , Θ 2 , and Θ p  into equation (2) for X(t) and finds: 
                       X   0     =       r   ⁡     (       cos   ⁢           ⁢     Θ   1       +     cos   ⁢           ⁢     Θ   2         )       +     p   ⁢           ⁢   cos   ⁢           ⁢     Θ   p           ⁢     
     ⁢     X   0     =       r   ⁡     (       cos   ⁢           ⁢     Θ   1       +     cos   ⁡     (       Θ   1     +   π   -   β     )         )       +     p   ⁢           ⁢   cos   ⁢           ⁢     (       Θ   1     +     π   2     -     β   2       )           ⁢     
     ⁢       X   0     =       r   ⁡     (       cos   ⁢           ⁢     Θ   1       -     cos   ⁡     (       Θ   1     -   β     )         )       -     p   ⁢           ⁢   sin   ⁢           ⁢       (       Θ   1     -     β   2       )     .                   (   3   )               
Equation (3) expresses the constraint that sets out the relationship between the angles Θ S  and Θ E  of motors  352 R and  350 R operating to move equal angular distances to achieve straight line movement of hand  210 R.
 
   Skilled persons can implement constraint equation (3) by means of a servomechanism controller in any one of a number of ways. For example, to achieve high speed operation to implement a given wafer move profile, one can compute from equation (3) command signal values and store them in a look-up table for real-time use. The precomputation process would entail the indexing of Θ S  in accordance with the wafer move profile and determining from equation (3) the corresponding Θ E  values, thereby configuring the displacement of Θ S  and Θ E  in a master-slave relationship. 
   To achieve angular displacement of hand  210 R about shoulder axis  316 R, controller  354  causes motors  350 R and  352 R to rotate in the same direction through the desired angular displacement of hand  330 R to reach the desired destination. The linear extension of hand  330 R does not change during this move. Skilled persons will appreciate that complicated concurrent linear and angular displacement move profiles of hand  330 R could be accomplished by programming controller  354  to operate motors  350 R and  352 R through different angular displacements.  FIG. 6A  shows a second wafer cassette  168   l  positioned so that the center  172   l  of a stored wafer  170   l  is coincident to Y 0 . The parallel arrangement of the openings of cassettes  168   l  and  168   r  demonstrates that the above expressions can be used to retrieve wafers stored in cassettes not positioned a radial distance from shoulder axis  316 . Robot arm mechanism  310  is not restricted to radial placement but can accommodate any combination of distances within its reach. 
     FIG. 18B  is a simplified block diagram showing the primary components of controller  354 . With reference to  FIG. 18B , controller  354  includes a program memory  474  that stores move sequence instructions for robot arm mechanism  310 R. A microprocessor  476  receives from program memory  474  the move sequence instructions and interprets them to determine whether the first or second operational state is required or whether motion of motor  392  is required to position torso link  311 . A system clock  478  controls the operation of microprocessor  476 . A look-up table (LUT)  480  stores corresponding values for Θ S  (motor  352 R) and Θ E  (motor  350 R) to accomplish the straight line motion of the first operational state and the angular displacements of Θ S  and Θ E  to accomplish the angular motion of the second operational state. Because the rotation of torso link  311  is independent of the motions of the robot arm mechanisms mounted to it, the overall coordination of the angular displacement of motor  392  with the angular displacements of motors  350 R and  352 R is carried out in the move sequence instructions, not in LUT  180 . This results in higher speed and more accurate straight line motion because multiple axis servomechanism following errors and drive accuracy errors do not affect the straight line path of hand  210 R. 
   Microprocessor  476  provides Θ S  and Θ E  position signals to a servomechanism amplifier  482 , which delivers Θ S  and Θ E  command signals to motors  352 R and  350 R, respectively. Microprocessor  476  also provides position signals to servomechanism amplifier  476  to deliver a command signal to torso motor  392 . Servomechanism amplifier  482  receives from the three glass scale encoders signals indicative of the angular positions of the respective motors  350 R,  352 R, and  392 . 
   Microprocessor  476  also provides control signals to a vacuum valve controller  484 , which causes a vacuum valve (not shown) to provide from a vacuum source (not shown) an appropriate amount of vacuum pressure to outlet  336  in response to the need to hold a wafer on or release a wafer from hand  210 R. 
   It will be further obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.