Abstract:
A wafer handling system and a method of retrofitting the system to an existing wafer handling apparatus are provided that make possible a method of handling wafers by contacting only a narrow area of not more than two millimeters wide adjacent the edge of the wafer, which is particularly useful for backside deposition where device side contact defines an area of exclusion that renders the wafer unusable in that area. The system provides a chuck on a wafer transfer arm that holds a wafer by gravity on a segmented, upwardly facing annular surface. A compatible annular surface is provided on an aligning station chuck so that wafers can be transferred by contact only with the exclusion area of the wafer surface. A load arm has two similarly compatible chucks further provided with pneumatically actuated grippers to allow the wafer to be loaded into a vertical processing apparatus. The wafer chucks are retrofitted into a processing apparatus in place of vacuum chucks and the vacuum lines that were provided to activate them are used for wafer detection. The electrical signals that were provided for vacuum chuck commands are utilized to actuate the grippers on the transfer arm so that no control software and little hardware need be altered for the retrofit.

Description:
This invention relates to the wafer handling, and particularly to the transfer and holding, of semiconductor wafer substrates during semiconductor manufacture, more particularly, wafers of diameters of 200 millimeters (mm) and larger. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, many companies manufacture equipment to process semiconductor wafers, particularly silicon wafers, for device production. Semiconductor wafer processing equipment employs automated and robotic wafer handlers for moving the wafers through the processing equipment and for holding the wafers for processing. Wafer handlers in the industry typically involve the use of a vacuum chuck that includes a vacuum-type spatula or end effector on a robotic arm, which makes contact with the backside of the wafer. With the more versatile handlers, contact with the wafer is made by the vacuum chuck in a circular area at the center of the wafer. One such wafer handling system is described in U.S. Pat. No. 5,820,329, hereby expressly incorporated by reference herein. Such wafer handling is typical in many wafer processing machines for the processing of the device sides of the wafers. 
     In semiconductor manufacture, when processing of the device side of a wafer is complete, a backside metallization layer is sometimes applied. For some devices, the metallization layer is often gold. Backside metallization with gold, and wafer holders for such processes, is described in the commonly assigned U.S. Pat. No. 6,258,228, filed Jan. 8, 1999, and hereby expressly incorporated by reference herein. For backside metallization, similar processing equipment is used as is used to process the device sides of wafers, but the wafer orientation is reversed. Reversal of the orientation of a partially processed wafer would expose the devices at the center of the wafer to contact by the vacuum chuck of the wafer transfer arm. The devices usually cannot be subjected to such contact without suffering damage. 
     For this reason, vacuum chucks have been developed to grip the wafer along a 6 mm ring inside of the edge of the wafer. As a result, a 6 mm ring at the edge of the wafer is reserved as an exclusion zone in which the wafer cannot be used for device manufacture. The 6 mm ring of exclusion is needed to provide a surface area that is enough to enable a vacuum chuck to reliably hold the wafer in the vertical, horizontal and inverted orientations that are required of a handler. Typically, for a wafer of 200 mm in diameter, a 6 mm contact area on the wafer engaged by elements of the wafer handler along the edge of the wafer, amounts to an area of over 36 square centimeters or twelve percent of the area of the wafer. A need has been expressed in the industry for the contact areas between the wafers and the wafer handlers to be reduced, preferably to not more than two mm around the edge of a 200 mm or 300 mm wafer. A two mm exclusion zone contains an area of only about 12 square centimeters on a 200 mm wafer and 18 square centimeters on a 300 mm wafer. This need has not been filled in the prior art. 
     Wafer handlers operate and are controlled in conjunction with the operation and control of the machines of which they are a part or with which they interact. Fundamental changes in the nature and operation of wafer handlers, if made, may be incompatible with, and can adversely affect, the operation and control of the semiconductor processing machines. Unless wafer handler changes are accompanied by replacement or redesign of the machines (e.g. via a kit), impact on operating software and on system operation can occur. 
     These contact areas typically prevent use of the portion of the wafer bounded by the contacted area for device manufacture, limiting the per-wafer device yield. As pattern geometries become smaller and demands for higher per-wafer yield become greater, the need for increased useful area of the wafer becomes greater. 
     Accordingly, there is a need for a wafer handler and a wafer handling technique that provides for a smaller contact area or exclusion zone where contact with the wafer is allowed. There is also a need for such improved wafer handling in a way that does not impact upon the operating software and systems operation of the machines with which such an improved handler or handling technique is used. 
     SUMMARY OF THE INVENTION 
     A primary objective of the present invention is to provide for the engaging and transfer of a semiconductor wafer for backside processing while making minimal contact with the device side of the wafer. A particular objective of the invention is to provide for the engagement and transfer of a semiconductor wafer while contacting the wafer in a zone of exclusion adjacent the periphery of the wafer, and more particularly, where the zone of exclusion is not more than two mm wide. 
     A further objective of the present invention is to provide a method and apparatus for the holding and transfer of wafers that is capable of holding and transferring wafers of differing sizes, particularly of diameters of 200 mm and 300 mm, and that is capable among wafers of differing sizes. 
     Another objective of the invention is to provide a wafer holding and transferring system that provides for reduced device side contact for the backside processing of wafers that is compatible with, and can be retrofitted to, processing machines of the prior art that do not otherwise have such capability. A more particular objective of the invention is to provide for the retrofit of such a system to an existing processing machine with minimal impact on the hardware of the processing machine and with little or no impact on the operating software of the processing machine. 
     According to the principles of the present invention, a wafer handling system is provided having a wafer transfer arm chuck, a centering station chuck and a load arm chuck which can hold and exchange wafers between chucks, wafer cassettes and processing machine wafer holders while contacting a reduced area on the surface of the wafer, and which is capable of contacting only an exclusion zone of preferably not more than approximately two mm in width adjacent the periphery of the wafer on the device side of the wafer during backside processing. 
     In certain embodiments, the wafer chucks of the transfer arm and load arm have beveled edge rings or edge ring segments that are of a diameter larger than the diameter of the wafer. The beveled edge rings insure that only a narrow exclusion zone on the edge of the wafer comes into contact with surfaces on the chucks that support the wafer. For some applications, the centering station chuck may also be provided with such a beveled edge ring. 
     In certain embodiments, an end effector chuck is provided on the transfer arm. The end effector is of multiple piece construction to achieve greater flatness, uses an internal vacuum channel to determine wafer presence and has ceramic outriggers that are adjustable to accommodate wafers of differing diameter, for example, diameters of 200 mm and 300 mm. 
     The centering station chuck corrects wafer flat and crystal orientation of the wafer and wafer centering. The chuck is preferably of multiple piece construction and uses a vacuum channel to sense the presence of the wafer and optical sensors to sense the wafer flat orientation as the chuck rotates the wafer. The surface of the centering station chuck has recesses such as clearance grooves to allow the outriggers of the end effector to successfully place or remove a wafer within an acceptable window of the chuck orientation relative to a home position. 
     The load arm chuck is in some respects similar to the centering station chuck in the way that it interacts with the transfer arm chuck but does not rotate to alter the orientation of the wafer. It uses a vacuum channel to determine wafer presence. The load arm has pivotal wafer edge hooks or gripper elements that grip the edge of the wafer within the exclusion zone. The gripper elements are pneumatically operated by electro-pneumatic actuators that can be responsive to the same electrical control signals that operate the vacuum controls as were the vacuum chucks of previous load arms. The gripper elements of the load arm chucks are actuated in response to the motion of the robot transfer arm in relation to the load arm or the load arm in relation to the wafer holder of the processing machine. The gripper elements may be pivotal gripper hooks, or in lieu of the pivotal gripper hooks, may be other types of wafer holding devices such rotary latches, similar to those having pivotal rollers or non-contact tabs of the described prior art, which can be rotated over the edge of the wafer to latch the wafer to the load arm chuck. The gripper elements allow the load arm to hold the wafer in a vertical orientation or facing downwardly and while being held in or moved through some other or different orientations. 
     In certain embodiments of the invention, a vacuum chuck equipped system is retrofitted with mechanical wafer engaging chucks, particularly replacing the vacuum chucks on a transfer arm end effector, a wafer centering or aligning station and a wafer load arm. In such a system, the transfer arm may be operable to pick up and move horizontally disposed wafers, transferring them to and from wafer cassettes, the centering or aligning station and the load arm. Also, the chuck at the centering and aligning station may be operable to receive a wafer from the transfer arm, to orient and align the wafer and to return the wafer to a centered and oriented position on the transfer arm, also in a horizontal orientation. Additionally, the load arm chucks may be operable to move a wafer between the transfer arm chuck and a wafer holder of the wafer processing machine, or exchange one wafer with another, reorienting the wafers between a horizontal orientation on the transfer arm and a vertical orientation in the wafer holder of the processing machine. Wafers are held at least in part by gravity on the horizontally disposed, upwardly facing chucks of the transfer arm and aligning station and are held in part by gravity on the load arm chucks when they are horizontally disposed and upwardly facing during transfer of the wafers to and from the transfer arm, and by positive wafer edge gripper hooks, when being moved in other orientations. 
     In certain embodiments of the invention, the controls of the wafer handling system are compatible with the controls of the vacuum chuck equipped, prior art machine so that the system can be retrofitted thereto without substantial hardware changes and without modification to the control software of the machine. Vacuum chuck control lines are used to sense the presence of wafers on the chucks. Gripper operating pneumatic cylinders are operated by electro-pneumatic solenoids that are controlled by electrical software vacuum command signals that were provided for vacuum chuck operation. 
     Embodiments of the present invention may be provided in the form of a retrofit kit that includes the three chuck assemblies configured to replace vacuum chucks of the transfer arm, aligning station and load arm of existing processing machines. 
     The method and apparatus of the present invention provides the advantage of increasing the useful area of a wafer by approximately four percent, or from 88% to 96% of the area of the wafer, reducing by two-thirds the exclusion zone or unusable area of the wafer, and providing, on average, similar increases in the number of devices produced per wafer, thereby improving the productivity of the semiconductor making processes and machinery. 
     These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective diagram illustrating a wafer handling and processing apparatus of the prior art to which the present invention is applicable. 
     FIG. 1A is an enlarged perspective diagram of a portion of FIG. 1 illustrating movements of chuck assemblies. 
     FIG. 2 is a partially broken away view illustrating a prior art wafer holder of the carrousel processing machine of the apparatus of FIG.  1 . 
     FIG. 2A is a detailed illustration of a prior art alternative embodiment to the latch of FIG. 2 shown in its latched position. 
     FIG. 3 is a cross-sectional view through a portion of the carrousel processing machine of the prior art apparatus of FIG. 1 in relation to the wafer transfer arm of the apparatus of the prior art. 
     FIG. 3A is a perspective view of one embodiment of the vacuum chuck of the transfer arm of FIG. 3 used in the prior art for backside processing. 
     FIG. 4 is a perspective view of a wafer handling system according to one embodiment of the present invention, which is compatible with the prior art apparatus illustrated in FIGS. 1-3A. 
     FIG. 5 is a schematic top view of the end effector of the wafer transfer arm of the wafer handling system of FIG. 4 aligned with the wafer chucks of the aligning station and the load arm. 
     FIG. 5A is a sectional view along line  5 A— 5 A of FIG.  5 . 
     FIG. 5B is a sectional view along line  5 B— 5 B of FIG.  5 . 
     FIG. 5C is a sectional view along line  5 C— 5 C of FIG.  5 . 
     FIG. 5D is a sectional view along line  5 D— 5 D of FIG.  5 . 
     FIG. 5E is a schematic top view, similar to FIG. 5, illustrating the end effector of the wafer transfer arm locating a wafer on an alternative embodiment of the load arm chuck. 
     FIG. 5F is a diagrammatic sectional view along line  5 F— 5 F of FIG.  5 E. 
     FIG. 5G is a diagrammatic sectional view along line  5 G— 5 G of FIG.  5 E. 
     FIG. 6 is a partial top view of the gripper mechanism of the load arm chuck of FIG.  5 . 
     FIG. 7 is a cross-sectional view along line  7 — 7  of FIG. 6 illustrating the load arm chuck grippers in a release position. 
     FIG. 7A is a cross-sectional view similar to FIG. 7 illustrating the load arm chuck grippers gripping a wafer. 
     FIG. 8 is an enlarged partial cross-sectional illustrating in more detail the load arm chuck grippers of FIG.  7 A. 
    
    
     DETAILED DESCRIPTION 
     The environment of the present invention is a semiconductor wafer handling and processing apparatus  100  as diagrammatically illustrated in FIG. 1, which is representative of the prior art. The apparatus  100  includes a high throughput carrousel-type vertical wafer processing machine  10  of the types illustrated and described in U.S. Pat. Nos. 4,915,564 or 5,516,732, both expressly incorporated by reference herein. The invention is also applicable to other types of wafer processing machines, such as typical cluster tool type machines, for example, and other machines in which wafers remain horizontally and upwardly disposed throughout as well as machines in which the wafers are moved through or processed in other orientations. However, the described embodiment is particularly suited for machines of the type of apparatus  100  as described herein, particularly by retrofitting thereto. 
     The apparatus  100  includes an enclosure  102 , illustrated in phantom, having a clean atmosphere contained therein in which is situated the processing machine  10 . The enclosure  102  has contained therein provision for supporting two wafer cassettes  103 ,  104 , each of which contains a rack  105  configured to support a vertical stack of horizontally oriented wafers  35  in parallel spaced relationship for processing in the machine  10  (e.g. FIG.  1 A). The cassettes  103 ,  104  are equipped with elevator mechanisms  106  to move the cassettes  103 ,  104  vertically to bring each of the wafer positions of the rack selectively into a transfer plane so that a wafer  35  therein can be unloaded or loaded into the selected position of the cassette. 
     The enclosure  102  also has a wafer transfer mechanism  110  therein having a wafer transfer arm  112  that is rotatable about a vertical pivot axis  115  and is extendable in the transfer plane  119 . At the free end of the transfer arm  112  is a gripper or end effector  114  adapted to pick up and deposit wafers  35  from and into the cassettes  103 ,  104  and from and onto a centering chuck  116  of a wafer centering or aligning station  118  and a load arm chuck  59  at the end of a loading arm  60 . The cassettes  103 ,  104 , the centering chuck  116  and the load arm chuck  59  are positioned or positionable along a circular arc  117  about the pivot axis  115  of the transfer arm  112 . The load arm chuck  59  rotates on the longitudinal axis  121  of the load arm  60  to rotate a wafer  35  held thereby between a horizontal orientation needed to transfer wafers between it and the transfer arm  112  and a vertical orientation needed to transfer wafers to and from a wafer holder  30  (FIG. 2) in the loadlock station  14  of the wafer processing machine  10 . The arm  60  further pivots about a vertical axis  122  to transfer wafers to and from the wafer holder  30  in the loadlock station  14 . 
     The machine  10  includes main vacuum chamber  11 , which is in the form of a disc-shaped plenum enclosed between two generally circular walls  12 ,  13  with a plurality of, for example, five, stations  14 - 18  spaced at even angular intervals around a central horizontal axis  19 . Within the chamber  11 , mounted for rotary movement on the axis  19 , is a circular index plate  20  having five circular openings  21  therein equally spaced to simultaneously align with each of the processing stations  14 - 18 . 
     As illustrated in FIG. 2, in each of the openings  21 , a seal ring  27  is provided, resiliently supported on three equally angularly spaced leaf springs  28  connected to the index plate  20 . Within each of the seal rings  27  a wafer holder  30  is provided, rigidly supported on a set of three ceramic insulator pins  29  to the seal ring  27 . The wafer holders  30  are each equipped to carry a wafer  35  through each of the processing stations  14 - 18  as the plate  20  is angularly indexed by one-fifth revolution, or 72°. At least one of the five stations  14 - 18 , for example, station  14 , is a loadlock station through which wafers  35  are inserted into and removed from the main chamber  11 . The remaining stations  15 - 18  include any of a number of types of treatment stations, such as sputter coating or etching stations. 
     The main structure of each of the wafer holders  30  is a ring-shaped annular body  31 , typically made of aluminum. The body  31  has a clamp ring  32  resiliently attached to the holder body  31  and biased against the body by a set of three equally angularly spaced leaf springs  36 . The clamp ring  32  has an inner diameter slightly less than a wafer  35  that is to be clamped. A wafer  35  is held in an opening  39  of the holder body  31  by the clamp ring  32 , which overlies the rim of the wafer  35 . The springs  36  press the clamp ring  32  and wafer  35  against a set of three latch clips  33 , one of each of three latch assemblies  65  that are spaced at equal angular intervals around the opening  21 . 
     The latch assemblies  65  may be of any of a number of configurations. Those illustrated in FIG. 2 are pivotally mounted to the space around the holder body  31 , and are of the type described in detail in U.S. Pat. No. 5,820,329. As shown in FIG. 2, latch clips  33  are fixed at one end to one end of a pivot shaft  70  which is rotatably supported in a bearing (not shown) that extends through the body  31  of the holder  30 . At the opposite end of the shaft  70  from the clip  33 , the shaft is rigidly connected to a circular disc  74 , which has a pair of holes  75  therein with flared openings at the rearward facing surface of the disc  74 . The holes  75  are spaced on opposite sides of the shaft  70 , at the same distance therefrom, to receive the pins  62  of an actuator  61  (FIG. 3) on a load arm  60  so that rotation of the actuator rotates the disc  74 , the shaft  70  and the clip  33  about the axis of the shaft  70 . The rotation of the disc  74  on the shaft  70  is limited to 90° of rotational movement. In the loading of the holder  30  having the latch assemblies  65 , the wafer  35  is pressed against the clamp ring  32  to move it away from the holder body  31  so that the latch clips  33  can be rotated between latched (phantom lines) and unlatched (solid lines) positions without the clips  33  abrading the surface of the wafer  33 . 
     An alternative latch assembly  65   a  is illustrated in FIGS. 2 and 2A, which are described in U.S. patent application Ser. No. 09/183,503, expressly incorporated by reference herein. The latch assemblies  65   a , as with latch assemblies  65  described above, clamp the edge of the wafer  35  against the clamp ring  32  around the opening  39  in the wafer holding clamp ring  32 . Each latch assembly  65   a  is, however, pivotally mounted to the clamp ring  32 , rather than to the holder body  31 . The latch assembly  65   a  is so mounted at a mounting post  130  fixed to the clamp ring  32 . The latch assembly  65   a  includes a non-metallic latch body  131  pivotally mounted on the post  130  through a tungsten carbide ball bearing (not shown), and is biased against the clamp ring  32  by a conical spring (not shown) that surrounds the mounting post  130 . The latch bodies  131  each have a pair of actuator pin receiving holes or slots  175  therein that are similar to the holes  75  described in connection with the latch assemblies  65  above, which are equally spaced from the mounting post  130 , to receive actuator pins of an actuator mechanism on a transfer arm, as described below, which operate the latch assemblies  65   a . At opposite ends of the latch body  131  are rotatably mounted a pair of non-metallic rollers, including a front roller  135  and a back roller  136 . 
     FIG. 3 is a cross-sectional view through the processing machine  10 , through the loadlock station  14 . 
     The loadlock station  14  includes a moveable cup-shaped closure  54 , which is actuated to force the seal ring  27  against the front wall  12  of the chamber  11  to form a sealed loadlock chamber  55  at the loadlock station  14 . A door  56  provides access for a wafer  35 , carried by the vacuum chuck  59  of the transfer or load arm  60 . 
     In the manufacture of semiconductor devices, the load arm  60  operates to translate the wafer  35 , device side first, into the loadlock chamber  55 , through the opening in the holder body  31 , to bring the outer rim of the wafer  35  into contact with the underside of the clamping clamp ring  32 . For device side processing, the load arm chuck is a circular chuck  59   a  that engages the center of the backside of the wafer  35 . The vacuum chuck  59   a  is connected to a selectively operable vacuum source through a vacuum line  69  in the load arm  60 . 
     When processing the backside of a wafer  35 , such as where backside metallization is being carried out to deposit a continuous film of a metal such as gold on the wafer, the wafer is inverted and chucks  59 ,  114 ,  116  engage the wafer  35  from the device side. Since contact between the chucks and the devices on the wafer must be avoided to prevent damage or destruction of the devices, it is necessary to restrict devices to a reserved or restricted area of the wafer  35  and to avoiding the placement of devices in an exclusion zone on the surface of the wafer. The exclusion zone has historically been an area within 6 mm of the wafer rim around the perimeter of the wafer. In this area of exclusion, arc-shaped vacuum chucks  59   b , as illustrated in FIG. 3A, contact only the exclusion area at the edge the wafer  35 . The paddle or wafer engaging end of the chucks preferably extends around a major part of the circumference of a 6 mm ring around the perimeter of the wafer. 
     Insertion of the wafer  35  into the holder  30  is carried out with the latch clips  33  rotated out of the path of the wafer  35 . Rotation of the latch clips  33  into and out of position behind the wafer  35  is accomplished by three latch actuators  61  carried by the transfer or load arm  60 , also as illustrated in FIG.  3 . The latch actuators  61  each include a pair of actuator pins  62  on the shaft of a pneumatically actuatable rotary cylinder  63 , which, when the pins  62  are in engagement with a corresponding one of three latching mechanisms  65  on the holder  30 , rotate 90° in one direction to simultaneously move the three clips  33  into a latching position behind the wafer  35 , and 90° in the opposite direction to move the clips  33  to a release position in the holder beyond the rim of the wafer  35 . 
     According to principles of the present invention, a wafer handling and processing apparatus, such as the apparatus  100 , is provided with a wafer transfer mechanism  210 , similar to the transfer mechanism  110  of the apparatus  100 , described in connection with FIG. 1, but equipped with the wafer handling system  200 , one embodiment of which is illustrated in FIG.  4 . The system  200  includes three chucks and related hardware, including a transfer arm chuck or end effector  201  located at the end of the transfer arm  212 , which operates in the manner of the transfer arm  112  described above. The system  200  also includes a centering chuck  216  at the wafer aligning station  118 , which operates in the manner of the centering chuck  116  described above, and a pair of load arm chucks  259  at the end of a load arm  260 , which operates in the manner of the load arm  60  described above. 
     The end effector  201  includes wafer support surfaces that contact a downwardly facing device side of a horizontally disposed wafer  35  only in an exclusion zone  202  within two mm of the edge of the device side of the wafer  35 . The wafer  35  rests under the force of gravity on the end effector  201 , which moves to move the wafers  35  to and from wafer cassettes  103 ,  104 , to and from the centering chuck  216  at the aligning station  118  and to and from the load arm chuck  259  on the load arm  260 . FIGS. 5,  5 A- 5 D illustrate the end effector  201  in detail as including a body in the form of a transfer arm plate  203 , which is fixed by screws  204  to the end of the transfer arm  212 , and a pair of outrigger arms  205 , of trapezoidal cross-section, rigidly attached to the transfer arm plate  203 , by fitting into dove-tail keyways (not shown) in the plate  203 , where they are held by locking washer nuts  211 . The outriggers  205  are adjustable in the keyways to accommodate wafers of different sizes. The arms  205  are ceramic and have upstanding support tips  206  that contact the exclusion area  202  of a wafer  35  being transferred by the transfer arm  212 . The plate  203  and the tips  206  have segments  207 ,  208 , respectively, of a circular shoulder thereon configured to support the wafer  35  at its exclusion edge  202 . Immediately adjacent the shoulder segments  207 ,  208  are respective angularly or circumferentially spaced segments  217 ,  218  of a beveled circular lip which immediately surrounds the peripheral edge of a wafer  35  being carried by the transfer arm  212 . The nuts  211  are located tangent to the lip segment  217  and have conical head surfaces  219  that form an extension of the beveled lip, and function with the lip to guide wafers onto the shoulder segments  207 ,  208 . 
     The segments  217 ,  218  of the beveled circular lip and the pin head surfaces  219  serve to cause a wafer being supported thereon to immediately center on the end effector  201  without coming to rest with the edge of the wafer beyond the lip. The inside diameter of the bevel surrounds an annular surface of the shoulder segments  207 ,  208  of less than two mm in width, on which the exclusion zone  202  of the wafer  35  rests. This inside diameter of the bevel equals the nominal diameter of a standard wafer, typically 200 mm or 300 mm, plus the maximum tolerance in the radial direction of about 0.008 inches, or about 0.2 mm, so that the largest wafer will rest inside of the beveled circular lip. The outriggers  205  are secured to the plate  202  in part by washer nuts  211 . The conical side surfaces of the heads  219  of the nuts  211  extend the bevel of segments  217 ,  218  above the upper surfaces of the plate  203  to assist in guiding a wafer  35  onto the circular area surrounded by the segments  217 ,  218  of the beveled lip when the wafers  35  are picked up from the cassettes  103 ,  104 . The bevels on the chuck rise above the support surface of the chuck a distance of at least the nominal thickness of a wafer and preferably about one mm. The angle of the bevel is preferably about 45°, plus or minus 10° or 15°. The bevel does not have to be all the way around the support surface but can be at a plurality of points, preferably three or more points spanning more than 180°, and preferably four points. The outriggers  205  are sufficiently thin to reach between wafers  35  stacked in the cassettes  103 , 104  for removal and replacement of the wafers. 
     As illustrated in FIG. 5, the outriggers  205  also are dimensioned to fit into grooves  221 , 222  in the surface of the centering chuck  216  at the centering station  118 . The centering chuck  216  has a lower disk shaped body  216   a  and a segmented upstanding edge ring  223 , which is semicircular to receive a wafer  35  from the end effector  201 , that can lift and rotate the wafer on the end effector  201  to bring a flat or notch  235  (FIG. 5) on the wafer  35  to a predetermined orientation corresponding to a flat  224  on the centering chuck  216 . The edge ring  223  may be beveled, as with the transfer arm chuck, to guide the wafer into contact with the supporting surfaces of the chuck only in the exclusion area of the wafer, but the absence of beveling may be more compatible with the centering mechanism and function. In the embodiment shown, the edge ring  223  at the centering station has an inner surface perpendicular to the plane of the wafer surface. At the centering station optical sensors on the centering chuck  216  (not shown) determine the wafer orientation. 
     Also as illustrated in FIG. 5, the load arm chucks  259  each have a chuck plate or body  261  having grooves  262 , 263  therein to receive the outriggers  205  of the end effector  201 . The chucks  259  take the place of the device side engaging vacuum chuck  59   b  of FIG.  3 A. The plate  261  has an upstanding segmented partially circular lip  265  around its edge on which rests an exclusion zone around the edge of the wafer  35 . The lip  265  is beveled, as with the transfer arm chuck and centering station chuck described above, to insure that the device side contact is made only with the exclusion zone that is not more than two mm wide. 
     As illustrated in FIGS. 6,  7 ,  7 A, and  8 , at spaced intervals around the lip  265  is a plurality of grippers or gripper fingers  270 , for example two to four in number, each pivotally mounted on a rod  281  that is supported in bearings to the body  261  of the load arm chuck  259 . The grippers  270  each have a notch  273  therein to capture the peripheral edge of the wafer  35 . The grippers  270  each have an operating lever  271  that is controlled by an actuator  272  mounted on the load arm chuck  259 , which operates in response to the vacuum commands provided through the line  69  to control the vacuum to operate the vacuum chucks  59 ,  59   a  of the prior art system, but which chucks have been removed and replaced with the mechanical load arm chucks  259 . The actuator  272  may be an electrically or pneumatically controlled pneumatic cylinder, an electrical-mechanical solenoid or some other device for moving the grippers  270  between the released and gripping positions of FIGS. 7 and 7A. Further, as best seen in FIG. 8, vacuum ports  295  are also provided in the chuck body  261  to connect to a series of holes (not shown) in the lip  265  to provide for the sensing of a properly seated wafer  35  in the chuck  259 . 
     The actuator  272  has a body  282  fixed to the body  261  of the load arm chuck  259  and a moveable element  283  that reciprocates in a direction perpendicular to the body  261  between a retracted position (FIG. 7) proximate the body  261  in which the grippers  270  are in their released position and an extended position (FIG. 7A) away from the body  261  in which the grippers  270  are in their gripping position. Fixed to the end of the moveable element  283  of the actuator  272  is an actuator bar  284 , which extends parallel to the body  261  and moves with the element  283 . The operating lever  271  of each gripper  270  has a fork  285  thereon having an adjustable set screw  286  threaded therein. The set screw has an eye  287  therethrough in which is hooked a soft spring  288 , which is connected in tension between the set screw  286  and a remote end of the actuator bar  284 . The spring  288  functions to limit the amount of force that can be exerted by the gripper  270  against the backside of the wafer  35  in clamping the wafer  35  against the lip  265  of the chuck  259 . The set screw  286  is provided so that this limit can be manually adjusted. Each operating lever  271  also has a link  289  extending therefrom, which are pivotally joined together at their remote ends with a fork  290  in the end of one pivoting and sliding on a bearing  291  in the end of the other. The interconnection of the ends of the links  289  synchronizes the motion of the grippers  270 . 
     In lieu of the grippers  270 , other elements can be used to latch the wafer to the load arm. Rotatable latches having rollers or fingers, for example, such as those described in connection with the prior art described and incorporated by reference above, can be used. These can also be operated by pneumatically operated actuators  61  or by mechanical or other devices. 
     For example, FIG. 5E illustrates an alternative embodiment  300  to the load arm chuck  259 , that employs support of the wafer  35  on three edge supports that include a fixed pin  301  and two rollers  302 . The load arm chuck  300  of this embodiment is illustrated in a form configured for the support of 300 mm wafers without an edge flat  235  of the embodiment shown in FIG.  5 . Alternatively, a wafer  35  with a flat or notch may be placed on the chuck  300  with the flat or notch oriented to miss the fixed pin  301  and rollers  302 . The chuck  300  may be employed as a load arm chuck or a centering station chuck. The chuck  300  provides contact with three or more points on the edge of the wafer  35 , and includes, for example, a housing  307  to which is mounted a fixed pin  301  and a pair of concave or notched rollers  302 . The rollers  302  are mounted to the housing  307  so as to latch against and retract from the edge of the wafer  35 . The chuck  300  is configured such that the transfer arm chuck or end effector  201  can deliver the wafer  35  to the chuck  300  by advancing the wafer edgewise between the two rollers  302 , when they are in retracted positions, and against the fixed pin  301 . The rollers  302  are located opposite the centerline  303  of the chuck  300  from the fixed pin  301 , with the rollers being spaced at an angle  304  of preferably at least 160° apart, but less than 180° apart. An angle of less than 160° is acceptable depending on the design of the end effector  201 . 
     When the chuck  300  is upwardly facing, the fixed pin  301  has a horizontal surface  311  to support the edge of the wafer  35  by contacting the wafer  35  only within the 2 mm exclusion zone as shown, for example, in FIG.  5 F. The pin  301  has an overhanging inclined and nearly vertical surface  312  which faces radially inwardly toward the center of the wafer  35 . The rollers  302  may be mounted on pivotal levers  302   a  so as to swing, in response to the motion of the actuators  61 , between retracted positions away from the edge of the wafer  35  to latching positions against the edge of the wafer  35 , as illustrated in FIG.  5 E. Alternatively, the rollers  302  can be pivoted in response to other actuators that respond to the signal that was formerly used to control the vacuum chuck, as in the case of the actuators  272  for the gripper fingers  270  in the embodiments discussed above. The rollers  302  have smooth concave surfaces  315  to trap the edge of the wafer  35  when in the latching positions, at which the wafer  35  is picked up from the end effector  201 . The concave surfaces  315  of the rollers  302  and the combined inclined surface  312  and horizontal surface  311  of the fixed pin  301  trap the wafer  35  on the chuck  300  so that the wafer  35  can be rotated to a vertical orientation or downwardly facing orientation, and transferred to and from a wafer holder  30 . 
     The load arm chuck provides for the correct placement of the wafer into the wafer processing machine. After being so positioned into a vacant wafer holder  30  within the loadlock of the processing machine, the grippers or latches release the wafer while the wafer holder simultaneously is grasping the wafer. The opposite sequence is used to remove a wafer from the loadlock. When the load arm chuck withdraws from the wafer holder, the processing machine can index the next wafer holder into the loadlock. With two chucks on the load arm, the load arm checks for the presence of a wafer in the wafer holder at the loadlock and, if one is present, removes the wafer with an empty chuck on the load arm. Then, the load arm rotates 180° to bring a new wafer into position and loads it into the wafer holder in the loadlock. The chuck that deposits the wafer into the loadlock is then available to receive the next wafer from the transfer arm end effector before rotating 180° to transfer the processed wafer on the other chuck of the load arm onto the end effector. 
     While the above description and accompanying drawings set forth various embodiments of the invention, it will be apparent to those skilled in the art that additions and modifications may be made without departing from the principles of the invention.