Abstract:
The present invention generally relates to an end effector that utilizes a gripping mechanism to grip a peripheral edge of the wafer and secure the wafer to the end effector. In one embodiment, the gripping mechanism includes a pair of gripper arms that pivot between a wafer-loading position and a wafer-engaging position. In another embodiment, an active plunger moves linearly between the wafer-loading and wafer-engaging positions. Both the gripper arms and the plunger device are driven by a motor assembly. A force feedback system monitors the force the gripping mechanism exerts on the wafer and, based on the amount of force, controls the operation of the motor assembly to dynamically adjust the position of the gripping mechanism.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an end effector for acquiring and transporting semiconductor wafers. More particularly, the present invention comprises an end effector that includes a mechanical actuator having the ability to vary the gripping force exerted by a gripping mechanism on a wafer. 
     BACKGROUND OF THE INVENTION 
     Various types of wafer-handling robots are known for transporting the wafers to and from the FOUP and among processing stations. Many such robots employ a robotic arm having a spatula-shaped end that is inserted into the cassette to remove or insert a wafer. The spatula-shaped end of the robotic arm is commonly referred to as an end effector. One type of end effector secures the wafer contact surface to the wafer by a vacuum source. The vacuum source pulls the bottom surface of the wafer into contact with the entire wafer contact surface. 
     With many workpieces, and certainly with semiconductor wafers, the surfaces of the workpieces can be easily damaged if the wafer-handling robot contacts the top or bottom surface of the wafer. Because of this, the wafer-handling robot should preferably contact only the peripheral edge of the wafer, or at most, the bottom surface within a narrow distance from the edge (known within the semiconductor industry as the “edge exclusion zone”). A wafer-handling robot must load wafers into and unload wafers from a wafer processing apparatus with a high degree of precision to avoid contacting the critical surfaces of the wafer. 
       FIG. 1  illustrates a conventional wafer-handling end effector. The wafer-handling robot  10  supports a wafer  12  by a wafer blade  11  that has a proximal end  12  and a distal end  14 . The distal end  14 , in this configuration, has two spaced apart and substantially parallel fingers—a first finger  16  and a second finger  18 . The first finger  16  includes a distal wafer support  20  that contacts the wafer  12  either along the peripheral edge or the Exclusion Zone. The second finger  18  also includes a distal wafer support  22  that contacts the wafer in a similar manner. 
     The wafer blade  11  also includes proximal wafer supports  24  and  26 . The proximal wafer supports  24  and  26  also contact the wafer  12  either along the peripheral edge of the wafer  12  or along the Exclusion Zone. The distal wafer supports  20  and  22  include a backstop portion to prevent the wafer  12  from sliding off the wafer blade  11  during transport. A wafer  12  is not always perfectly positioned on the wafer blade  11  such that the peripheral edge of the wafer rests against the wafer supports  20  and  22  at all times. Sudden movement or high rotational speeds by the wafer handling robot  10  may throw the wafer  12  against the supports  20  and  22  and cause damage to the wafer  12 , or cause the wafer  12  to slip over the wafer supports  20  and  22  and off the blade  11 . An example of an end effector similar to that shown in FIG. 1 is disclosed in U.S. Pat. No. 6,077,026, issued to Schultz, entitled “Programmable Substrate Support For a Substrate Positioning System,” which is assigned to Asyst Technologies, Inc., and is incorporated herein by reference. 
     As the robot speed and acceleration increase, the amount of time spent handling each wafer and delivering each wafer to its next destination is decreased. The desire for speed, however, must be balanced against the possibility of damaging the wafer or the film formed on the wafer. If a robot moves a wafer too abruptly, or rotates the wafer blade too fast, the wafer may slide off the blade, potentially damaging the wafer. Further, particle contaminants may result if the wafer slides around on the end effector. In addition, movement of the wafer on the wafer blade may cause substantial misalignment of the wafer. A wafer that is not aligned may result in inaccurate processing or even additional particle generation. 
     Wafer handling robots that grip the wafer by its peripheral edge exist today. By way of example only, one type of robot end effector includes an active contact point that moves between a retracted wafer-loading position and an extended wafer-engaging position that urges the wafer against a pair of distal wafer supports. Another example of a robot end effector includes a pair of arms that grip the peripheral edge of the wafer after the wafer is placed on the blade. The active contact point and the pair of arms retract and extend through a vacuum or pneumatically actuated mechanism. Vacuum and pneumatic actuation mechanisms provide poor control of the maximum force exerted on the wafer, because velocity of the active contact point or the arms cannot be controlled. 
     After the end effector lifts a wafer off a cassette support, the active contact point (or the pair of arms) extends to contact the peripheral edge of the wafer and push the wafer against the distal wafer supports. The active contact point holds the wafer in place on the wafer blade. A vacuum actuated gripping device, for example, cannot stop or vary its speed between the retracted wafer-loading position and the extended wafer-engaging position. 
     An end effector that incrementally controls the motion of the gripping device, which prevents damage to the wafer, is a desirable feature. An end effector having a real-time force feedback system that monitors the force exerted on the wafer is also a highly desirable feature. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is to provide an end effector that creates four points of contact along the peripheral edge of the wafer to improve the centering ability of the end effector. In one embodiment, the gripping mechanism includes two gripping arms and, in combination with two static rest pads, secures the wafer to the end effector. The gripping arms and static rest pads provide four contact points along the peripheral edge of the wafer. In another embodiment, the gripping mechanism includes a plunger having a pair of spaced apart contact pads. The plunger, similar to the gripping arms, provides two contact points along the peripheral edge of the wafer. 
     Another aspect of the present invention is to provide an end effector that reduces or eliminates particles created by securing the wafer to the end effector. In one embodiment, the end effector includes a centering device adapted to center the wafer on the end effector before the end effector picks up the wafer. After the wafer is seated on the end effector, a gripping mechanism secures the wafer on the end effector. If the wafer is pre-centered, the wafer will not slide substantially on the end effector, if at all, when the wafer is secured to the end effector by the gripping mechanism. In another embodiment, the contact pads of the gripping mechanism initially contact the peripheral edge of the wafer in such a manner as to minimize any sliding of the wafer. 
     Yet another aspect of the present invention is to provide an end effector that provides precise motion control of the contact pads. The gripping mechanism includes a motor assembly that drives a carriage, which is operatively coupled to the gripping arms. In one embodiment, a cam engages the carriage such that the cam&#39;s rotational motion drives the translational member along a linear path. The linear motion of the carriage in turn imparts rotational motion to the gripper arms. 
     Still yet another aspect of the present invention is to provide an end effector that includes a real-time force feedback system. The force feedback system minimizes the initial impact force exerted by the gripping mechanism against the wafer. The force feedback system also maintains the force exerted by the gripping mechanism against the wafer at a constant state. In one embodiment, each contact pad includes a force sensing device to detect the amount of force the contact pad is exerting on the wafer. The force sensing device is electrically coupled to the motor assembly to create a closed loop feedback system. In another embodiment, the force feedback system is an open loop system. 
     Another aspect of the present invention is to provide an end effector that has optical means for sensing the presence of a wafer. In one embodiment, thru-beam sensors detect a wafer proximate to the digital end of the end effector. 
     Yet another aspect of the present invention is to provide an end effector that may approach the wafer from above and pick up the wafer. The gripping surfaces of the wafer are vertical (e.g., the peripheral edge of wafer). The force feedback system ensures that the force exerted by the gripping mechanism on the wafer is sufficient to prevent the wafer from falling off the end effector. 
     Still another aspect of the present invention is to provide an end effector that minimizes the initial impact the gripping mechanism places on the wafer. Controlling the speed of the gripping mechanism as it approaches the wafer reduces the large impact force exerted against the wafer that a conventional edge grip device produces (e.g., vacuum actuated plunger). The speed of the gripping mechanism may be controlled—whether the gripping mechanism is a pair of gripper arms or a plunger. 
     Yet another aspect of the present invention is to provide an end effector that recognizes when the gripping mechanism attempts to grip a wafer that is not present on the wafer blade. In one embodiment, an “over-travel” position is identified when the gripping mechanism moves to a location that should contact a wafer and yet no contact is made. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top perspective view of a conventional end effector, according to the prior art; 
         FIG. 2  is an assembly view of an embodiment of an active grip end effector, according to the present invention; 
         FIG. 3  is a top view of an embodiment of the end effector shown in  FIG. 2 , illustrating the gripper arms in a wafer-loading position; 
         FIG. 4  is a top view of an embodiment of the end effector shown in  FIG. 2 , illustrating the gripper arms in a wafer-engaging position; 
         FIGS. 5A-5C  illustrate one embodiment of a motor assembly, according to the present invention; 
         FIG. 6  is an assembly view of the motor assembly shown in  FIG. 5  in conjunction with a motor mounting block; 
         FIGS. 7A-7B  illustrate one embodiment of a carriage, according to the present invention; 
         FIGS. 8A-8B  illustrate an embodiment of a gripper arm, according to the present invention; 
         FIG. 9  is an assembly view of the gripper arm shown in  FIGS. 8A-8B , illustrating a bearing assembly; 
         FIG. 10  is a top view of the end effector shown in  FIG. 2 , illustrating the contact point of the gripper arms against the wafer; 
         FIG. 11  is a perspective view of another embodiment of an end effector, according to the present invention; 
         FIG. 12  is a perspective view of yet another embodiment of an end effector, according to the present invention; and 
         FIG. 13  is a perspective view of the edge gripper plunger motor assembly shown in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An end effector  100  manufactured according to one or more embodiments of the present invention will now be described with reference to  FIGS. 2-13 . In general, the end effector  100  includes a gripping mechanism that contacts the peripheral edge of a wafer in order to secure the wafer to the end effector  100 . 
       FIGS. 2-4  illustrate one embodiment the end effector  100 . In this embodiment, the end effector  100  has a proximal end  112  and a distal end  114 . The distal end  114  of the end effector  102  comprises a wafer blade  111  having two spaced apart fingers  116  and  118 . The distal end of the two fingers  116  and  118  each include a wafer support pad  120  for supporting a portion of a wafer  12  seated on the wafer blade. It is within the scope and spirit of the invention for the wafer blade  111  to comprise a single finger in order to provide a narrower end effector. The wafer blade  111  also includes a proximal wafer support  122  so that a wafer seated on the wafer blade  111  is supported in three areas. As shown in  FIG. 3-4 , the wafer supports  120  and  122  are spaced apart such that the wafer&#39;s geometrical center C is located between the supports. 
     The distal wafer supports  120  include a platform  124  and a backstop  126 . The platform  124  may have several configurations. If the platform  124  is, for example, a substantially flat or horizontal surface, the wafer platform  124  will contact the wafer  12  along the exclusion zone when the wafer  12  is seated on the wafer blade  111 . If the platform  124  is a slanted or sloped surface (e.g., a raked surface), the platform  124  will only contact the peripheral edge of the wafer while it is seated on the wafer blade  111 . The backstop  126  extends upward from the platform  124 . The backstop  126  contacts the peripheral edge of the wafer  12  and prevents the wafer  12  from sliding off the wafer blade  111 . It is within the scope and spirit of the invention for the backstop  126  to comprise other shapes such as, but not limited to, an arcuate shape that conforms to the peripheral edge of the wafer  12  and/or a sloped surface. 
     The end effector  100  picks up a wafer  12  by sliding the wafer blade  111  beneath the wafer  12  until the backstop  126  is located just past the edge of the wafer  112 . The wafer blade  111  is then raised until the platform  124  contacts the wafer  12 . The end effector  100  raises the wafer  12  off a wafer support and the gripper mechanism (described later) secures the wafer  12  to the end effector  100 . Releasing the wafer  12  is accomplished by reversing the steps performed to pick up the wafer  12 . The gripping mechanism releases the wafer  12 , lowers the wafer  12  onto a support surface and withdraws from the proximity of the wafer  12 . 
     It may be useful on occasion to approach the wafer  12  from its top surface to pick up the wafer  12 . In this embodiment, the wafer  12  is supported substantially by the gripping mechanism. An end effector that approaches a wafer from the top to grip it preferably includes a center finding mechanism (described later) that locates the center of the wafer prior to gripping the wafer. The wafer will otherwise slide on the wafer supports when the gripping mechanism contacts the peripheral edge of the wafer—potentially damaging the wafer. An example of a system for positioning an end effector of a wafer handling robot with respect to a wafer is described in U.S. Pat. No. 6,298,280, entitled “Method for In-Cassette Wafer Center Determination,” issued to Bonora, et al., which is assigned to the owner of this invention and is incorporated in its entirety herein. 
     The proximal end  112  of the wafer blade  111  also includes a mounting area  130 . The mounting area  130  is adapted to receive a printed circuit board (“PCB”)  132 , that in one embodiment, includes the controllers for the gripping mechanism. An insulating plate  134  is preferably located between the PCB  132  and the wafer blade  111  to electrically isolate the components on the PCB  132  from the blade  111 . 
     Some of the key components of the gripping mechanism, which will be described in more detail later, include a motor assembly  144 , a carriage  160 , a flexible link  190 , and a pair of gripper arms  200 . In the embodiment shown in  FIGS. 3-4 , the gripping mechanism comprises a pair of gripping arms  200 .  FIG. 3  shows the gripping arms  200  located in a wafer-loading position. In this position, the contact pads  216  of each gripper arm  200  are retracted away from the peripheral edge  13  of the wafer  12  and the end effector  100  may pick up or drop off a wafer  12 .  FIG. 4  shows the gripper arms  200  located in a wafer-engaging position. In this position, the contact pads  216  of each gripper arm  200  contacts the peripheral edge  13  of the wafer  12  and exerts a force on the wafer  12 . The gripper arms  200  may be moved to any position between the wafer-loading and the wafer-engaging positions. 
       FIG. 5A-5C  illustrate one embodiment of the motor assembly  144 . The motor assembly  144 , in this embodiment, includes a motor  146 , a planetary gearhead  148 , and a cam  150 . The motor  146  is, for example, a 6 mm diameter 1.2-watt brushless D.C. motor that operates between 3,000-4,000 rpm. The motor assembly  144  therefore has a very low profile. A small motor diameter is preferred to provide a low profile end effector. It is within the scope and spirit of the invention for the motor  146  to have a different power rating and/or diameter. Other mechanical devices, such as a leadscrew, may be coupled to the motor  146  instead of the planetary gearhead  148 . 
     A planetary gearhead  148  is mechanically coupled to the motor  146  to greatly reduce the speed of the output shaft  152  and provide a greater control over the motion of the cam  150 . The planetary gearhead is preferably coaxially aligned with the motor  146 . A planetary gearhead is well-known in the mechanical areas and does not require further disclosure. In one embodiment, the planetary gearhead  148  provides a 57:1 gear reduction between the speed of the motor shaft and the spread of the output shaft  152 . The planetary gearhead  148 , however, may provide other gear reduction ratios. The output shaft  152  of the planetary gearhead  148 , in this example, rotates fifty-seven times slower than the speed of the motor  146 . The high ratio planetary gearhead  148  allows the motor  146  to precisely control the rotation of the output shaft  152 . For example, the motor must rotate fifty-seven times to impart one revolution of the output shaft  152 . The motor  146  can therefore move each gripper arm  200  a distance equivalent to 1/57 of a rotation of the output shaft  152 . 
     Other embodiments of the motor assembly  144  may not include a planetary gearhead. A motor assembly  144  may use, for example, a motor and a leadscrew to drive the cam  150 . Each embodiment of the motor assembly  144  preferably tracks the motion of the motor by hall-effect sensors. Hall-effect sensors are well-known within the art and do not require further disclosure. The location of each gripper arm  200  may be determined by the hall state of the motor  146 . In one embodiment, each Hall state corresponds to 6 microns of movement of the gripping arms  200 . 
     The cam  150  is coupled to the output shaft  152 .  FIG. 5B  illustrates that the geometrical center E of the cam  150  is offset a distance d from the rotational center G of the output shaft  152 . The offset d allows the outer edge  151  of the cam  150  to extend beyond the outside perimeter  151  of the planetary gearhead  148 . The rotation of cam  150  imparts linear motion to the carriage  162 . In this embodiment, the carriage  160  may move a total distance equivalent to twice the offset d. The carriage  160 , in general, requires a certain range of travel to allow the gripper arms  200  to pick up off center wafers The gripper arms  200 , however, need to do that with a certain amount of force to secure the wafer  12  on the wafer blade  111 . 
       FIG. 5C  illustrates that the cam  150  may rotate through a 180° range of motion. The furthest position to the left (as viewed in  FIG. 5C ) is referred to as the rear position and is shown as position R. The furthest position to the right (as viewed in  FIG. 5C ) is referred to as the forward position and is shown as position F. The total linear travel distance of the cam  150  between the rear position R and the forward position F is shown as distance D. The cam  150  may be rotated to any position located between the rear position R and the forward position F. 
       FIG. 6  illustrates one embodiment of a mounting block  154  that secures or fastens the motor assembly  144  to the wafer blade  111 . In this embodiment, the mounting block  154  has a channel  156  and a bore  158  to accommodate the motor assembly  144 . The channel  156  is adapted to receive the motor  146  and the planetary gear  148 . The motor  146  and the planetary gearhead  148  are preferably secured to the channel  156  to prevent to motor  146  and the planetary gearhead  148  from rotating within the channel  156 . The bore  158  is located at one end of the channel  156  so that the output shaft  152  extends through the bore  158 . The mounting block  154  includes two alignment holes  159  that dowel pins, as an example, pass through to align the mounting block  154  on the wafer blade  111  (See  FIG. 2 ). It is within the scope and spirit of the invention to mount the motor assembly  144  to the wafer blade  111  in another manner. 
     The cam  150  is operatively coupled with a carriage  160  (see  FIGS. 7A-7B ). The rotational motion of the cam  150  drives the carriage  160  along a substantially linear path. In one embodiment, the cam  150  is seated within a drive slot  164  of the carriage  160 . As the cam  150  rotates, the outer edge  151  of the cam  150  drives the carriage  160  either towards or away from the wafer blade  111 . The width W of the drive slot  164  and the diameter D2 of the cam  150  are preferably substantially equal so that the outer edge of the cam  150  is in contact with the drive slot  164  at all times. 
     When the cam  150  rotates, for example, counterclockwise (as viewed in  FIG. 5C ), it contacts a forward face  163  of the slot  164  and pushes the body  162  of the carriage  160  forward (toward the wafer). The cam  150  continues to drive the body  162  forward until the cam  150  rotates into position F (see  FIG. 5C ). Similarly, the cam  150  drives the body  162  rearward by rotating the cam  150  clockwise (as viewed in  FIG. 5C ) towards the rear position R. Since the cam  150  and carriage  160  are in slidable contact with each other, they are preferably manufactured from a low resistance, low wear material to prevent particle generation proximate to the wafer  12 . 
       FIGS. 7A-7B  illustrate one embodiment of the carriage  160  which provides a link between the motor assembly  144  and the gripper arms  200 . One side of the carriage body  162  includes a first guide  170  and a second guide  172  that extend outward from the body  162 . The guides  170  and  172  are preferably spaced apart and substantially parallel to each other. Each guide includes a bore  174 . The bore hole  174  may extend through any portion of the support guides  170  and  172  as long as the geometric center of each bore hole  174  is concentrically aligned with the other. 
     As previously discussed above, the carriage  160  travels along a linear path. The bore holes  174  are adapted to receive a guide rail  182 . The carriage  160  slides along the guide rail  182 , which also prevents lateral motion of the carriage  160 . 
     It is within the scope and spirit of the invention to restrict the motion of the carriage  160  by devices other than a guide rail  182 . Regardless, the carriage  160  slides along the proximal end of the end effector. In one embodiment, the end effector  100  includes a slot  143  that the carriage  160  slides within. The slot  143  allows the carriage  160  to travel freely back and forth, yet constrains the lateral motion of carriage  160 . In another embodiment, the carriage  160  includes multiple support surfaces (not shown) that extend from the bottom of the carriage  160 . Only the support members contact the wafer blade  111 , thus provide a smaller contact surface area between the wafer blade  111  and the carriage  160 . 
     The carriage  160  is coupled with the cam  150 . One side of the carriage  160  includes a drive slot  164  that the cam  150  sits within. The drive slot  164  extends, in this embodiment, vetically through the body  162  to form a front surface  163  and a rear surface  165 . The drive slot  164 , as shown in  FIG. 7A , has a width w that is substantially equal to the diameter d2 of the cam  150  (see  FIG. 5C ). 
     The front portion  166  of the carriage  160  is operatively connected to the flexible link  190 . The front portion  166 , in order to accommodate the flexible link  190 , has a bore  168  extending horizontally through it. In one embodiment, four vertical dowel holes  169  extend from the top surface of the body  162  to the horizontal bore  168 . Dowel pins (not shown), or other fastening devices, may be inserted into the holes  169  to prevent the flexible link  190  from moving laterally (towards or away from the wafer) within the carriage  160 . 
     The motor assembly  144 , through the cam  150 , drives the carriage  160 . Any motion by the cam  150  preferably imparts motion to the carriage  160 . The end of cam travel (e.g., cam  150  is located at 0° or 180° degrees), however, may allow a couple degrees of cam motion to translate into no linear motion of the carriage  160 . The cam  150 , in other words, is not doing useful work during some portion. If the cam  150  is allowed to rotate completely to the 0° or 180° position (defined as an “overtravel”position), the cam  150  further may jam or the end effector  100  will lose track of the precise location of the carriage  160 . 
     A hard stop device  141  (See  FIG. 2 ) prevents the cam  150  from reaching the overtravel position. The hard stop  141  is positioned such that the guides  170  will contact the hard stop  141  before the cam reaches the forward position F or the rear position R. In another embodiment, the carriage  160  includes a bore in the top surface that a dowel pin, or other similar device, fits into. The top portion of the dowel pin travels within a track located above the carriage  160  as the carriage  160  moves between its first and second positions. The track prevents the dowel pin from traveling beyond a certain distance in both the forward and backward directions and will thus stop the travel of the carriage  160  too. When either the guide  170 , or the dowel pin, contacts the hard stop  141  when traveling in the backward direction (away from the wafer), the gripper arms  200  are located at a “home” position. 
       FIG. 8A-8B  illustrate one embodiment of a gripping mechanism—a pair of gripping arms  200  (see also  FIGS. 2-4 ). Each gripper arm  200  preferably comprises a unitary construction and includes a driver portion  208  and a cantilever portion  210 . The driver portion  208  is connected to the flexible link  190 . As shown in  FIG. 8A , the driver portion  208  is fastened to the flexible link  190  by a pair of screws or dowel pins  195 . It is within the scope and spirit of the invention to secure the flexible link  190  to the driver portion  208  by other methods such as, but not limited to, a bolt, a screw, welding and the like. The cantilever portion  210  includes a contact pad  216 , a sensor  214 , and a force sensing device  228 . The sensor  214  preferably includes a pair of transmitters  224  and a pair of receivers  226 . Two optical paths are preferred, because a single optical path may be located over the wafer&#39;s indicial notch when the wafer  12  is placed on the wafer blade  111 , and thus not recognize that a wafer is located on the end effector  100 . 
     A contact pad  216  is secured to the sensor  214  and is the only portion of the gripper arm  200  that contacts the wafer  12 . In this embodiment, the contact pad  216  includes a mounting segment  217  that fastens to the cantilever portion  210  of a gripper arm  200 . It is within the scope and spirit of the invention to integrate the sensor  214  and/or the contact pad  216  into the distal end  218  of the gripper arm  200 . In one embodiment, the contact pads  216  comprise an upper pad  220  and a lower pad  222  to form a “C”-shape contact pad where the wafer  12  sits between the upper and lower pads. 
     The force sensing device  228  is located behind the contact pad  216 . A small gap exists between the arm and the back surface of the contact pad. In operation, the gripper arms  200  grip the peripheral edge  13  of the wafer  12  until the gripper arms  200  are pushes back into the load cell. A load cell converts displacement of the contact pad  216  into a measurable voltage difference. An electrical signal that represents the voltage difference is sent to and processed by a microprocessor that controls the operation of the motor assembly  144 . Alternatively, the load sensing device  228  may comprise a strain gauge. A strain gauge is well known within the art and does not require further disclosure. It is within the scope and spirit of the invention to integrate the load sensing device  228  or strain gauge into a gripper arm  200  or contact pad  216 . 
     Movement by the carriage  160  displaces the flexible link  190 .  FIG. 9  illustrates that, in one embodiment, each gripper arm  200  contains a ball bearing assembly  230  that is secured to, and rotates about, a post  232 . A ball bearing assembly  230  is well-known within the art and does not require further disclosure. Each gripper arm  200  may also rotate by other means. 
     The flexible link  190 , in general, functions as a leaf spring. When the carriage  160  travels away from the wafer  12 , for example, the carriage  160  pulls the center  191  of the flexible link  190  away from the wafer  12  and rotates the gripper arms  200  into the wafer-engaging position ( FIG. 4 ). The flexible link  190  compensates for any amount of wafer offset so that, for example, both gripper arms  200  engage the wafer  12  even if the wafer is not centered on the wafer blade  111 . The flexible link  190  also prevents overloading of the drive mechanism  144  if an operator manually inserts a wafer of the wafer blade  111 . 
     The gripper arms  200  may contact the peripheral edge of the wafer  12  at substantially the same, or at a different, time.  FIG. 2-4  illustrate that the carriage  160  is located substantially equally between the gripper arms  200 . The carriage  160  may also be located in other positions relative to the gripper arms  200 . By way of example only, the carriage  160  may be located closer to one gripper arm than the other. 
     The default or home position of the gripper arms  200  is the wafer-loading position (see  FIG. 3 ). In the wafer-loading position, the gripper arm  200  are retracted, providing space so that end effector  102  may pick-up a wafer  12 . 
     If the wafer  12  is not centered on the wafer blade  111  before the wafer is placed on the end effector  100 , the wafer  12  will slide on the supports as the gripper arms  200  push the wafer  12  against the back stop  126 . The first and second gripper arms  200  rotate towards the wafer  12  and initially contact the peripheral edge  13  of the wafer  12 . The gripper arms  200  minimize the distance the wafer  12  slides on the supports  122  and  124 .  FIG. 10  illustrates that the contact pads  216  preferably contact the peripheral edge  13  of the wafer  12  such that the center of each pad  216  substantially faces the center C of the wafer  12  (indicated by lines A and B in  FIG. 10 ). This contact pattern, in effect, pushes the wafer  12  forward toward each support  120  along a substantially linear line, indicated as arrow  250 . 
     The gripper arms  200  exert a force against the peripheral edge  13  of the wafer  12  when the gripper arms  200  are in contact with the wafer  12 . In a preferred embodiment, a force feedback system monitors, in real time, the force the gripper arms  200  exert on the peripheral edge  13  of the wafer  12 . It is within the scope and spirit of the present invention for the force feedback system to periodically monitor the force the gripper arms  200  exert on the peripheral edge  13  of the wafer  12 . Monitoring the force exerted on the wafer may also be controlled through an open loop control method. A predetermined grip motion sequence, for example, may be programmed into the processor. In this embodiment, the end effector  100  would not require a force feedback system to measure the force exerted on the wafer  12 . Instead, the force exerted on the wafer  12  may be approximated through an algorithm. 
     The load sensing device  228  is one element of the force feedback system. As previously discussed, the load sensing device  228  measures the force the gripper arms exert on the peripheral edge  13  of the wafer  12 . A load sensing device  228  is located behind each contact pad  216 . The contact pad  216 , when in contact with a wafer  12 , is pushed back slightly into the force sensing device  228 . The contact between the contact pad  216  and the force sensing device  228  generates an electrical signal that is sent to a processor through the flex circuit  240 . 
     The processor processes the signal and sends an appropriate control signal to the motor assembly  144 . For example, if a wafer is not gripped, the force sensing device  228  will read a calibrated “zero” force, and the sensor  214  will not see the wafer edge  13 . The motion of each gripper arm  200  is monitored by its physical location and the amount of force it is exerting on the wafer  12 . Until the contact pad  216  contacts a wafer  12 , there is no force. When the contact pad  216  contacts a wafer, the force sensing device  228  detects a force. The processor, upon receiving a force signal from the force sensing device  228 , immediately slows down the motor  146  to achieve a final destination force by the time the gripper arms  200  secure the wafer  12  to the wafer blade  111 . This operation may not occur at the same position every time because wafer sizes may vary and the wafer may be initially seated off-center on the wafer blade  111 . 
     The force sensing device  228  provides real-time data to the motor assembly  144  so that the end effector can determine immediately of there is a malfunction. If, for example, the gripper arms  200  attempt to grip the wafer  12  and a wafer  12  is not present on the wafer blade  111 , or the wafer  12  slips on the wafer blade  111 , the position of motor  146  (as determined by the Hall-effect sensors) will indicate that the gripper arms  200  missed the wafer  12  and moved too far. If, on the other hand, the wafer  12  pops off the support pads  126 , the amount of force measured by the force sensing device  228  will drop immediately and indicate to the end effector that a malfunction has occurred. The thru-beam sensors  222  and  224 , in addition the force sensing device  228 , also detects the edge of a wafer  12 . In general, the force sensing device  228  and the optical sensors provide a double check system to detect a wafer  12  on the wafer blade  111 . 
       FIGS. 12-13  illustrates yet another embodiment of an end effector. The end effector  300 , in general, includes plunger motor assembly  300  that drives a single plunger device  313  to grip a peripheral edge of a wafer. The motor housing assembly  302  preferably employs a 6 mm D.C. brushless motor  346  coupled to a 57:1 planetary gearhead to drive the single plunger. As shown in  FIG. 12 , the motor  346  is secured to the mounting plate  354 . The motor  346  may be secured to the mounting plate  354  by several methods such as, but not limited to, a bolt or screw. 
     A leadscrew  350  is coupled to the output shaft (not shown) of the planetary gearhead. The leadscrew  350  drives a carriage  360  forwards and backwards along a linear path. The leadscrew  350  imparts the rotary motion of the leadscrew  350  into linear motion of the carriage  360  by an acme thread. The function of a leadscrew is known within the art and does not require further disclosure. In one embodiment, the carriage  260  may travel through a range of motion of approximately 6-7 mm. It is within the scope and spirit of the invention to allow the carriage  360  to have a different range of motion. The plunger motor assembly  300  may also drive the plunger  313  by mechanical means other than a leadscrew as long as the mechanical means may incrementally control the position of the plunger  313 . 
     A locating block  306  maintains the alignment of the leadscrew  350  during the operation of the plunger motor assembly  302 . An alignment rod  352  provides a low friction surface to keep the carriage  360  from rotating during its linear motion. 
     The single plunger device  302  comprises a gripping pad assembly  304  that includes a gripping pad  314 . The gripping pad  314  moves forwards and backwards with the carriage  360  and the leadscrew nut  306 . The gripping pad  314  may consist of a single or dual pad arrangement. As shown in  FIGS. 12-13 , the gripping pad  314  includes an upper pad  320  and a lower pad  322 . The gripping pad assembly  304  also preferably includes dual through beam sensor arrangement  216  for sensing the edge of a wafer before the gripping pad  314  contacts the wafer. In one embodiment, the gripping pad assembly  304  houses two thru beam sensors  324  and  326  and a load cell (not shown). The gripping pad assembly  304  preferably comprises a semi-flexible material and mounts against an arm  330 . The gripping pad assembly  304  therefore flexes towards and contacts the load cell when the contact pads  320  and  322  contact the peripheral edge of the wafer. The load cell accordingly measures the force exerted on the wafer while the wafer is gripped. The gripping pad assembly  304  may also include multiple gripping pads  314 . 
     The end effector  300  includes several components that are similar to the previously disclosed embodiments—such as the end effector  100 . The end effector  300  includes, for example, a hard-stop  341 , a printed circuit board  332 , and a guide rail  382 . The motion of the carriage  360  is restricted by the guide rail  382  and the hard-stop  341 —similar to how the motion of the carriage  160  is restricted by the hard-stop  141  and the guide rail  182  concerning the end effector  100 . The printed circuit board  332  also includes a processor for coordinating a force feedback system similar to the force feedback system previously discussed above concerning the end effector  100 . The processor and PCB  132  may be located remotely and do not have to me mounted on the end effector  100 . 
     The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiment and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.