Abstract:
A force compensating apparatus is for use in a wafer-handling machine. The wafer-handling machine includes a rotatable rim adapted to be concentric with and external to a wafer, and a plurality of wafer-supporting fingers disposed about the rim. Each finger is supported by a floating arm, part of a support mechanism utilizing first and second fixed arms bracketing the floating arm where the fixed arms attach the support to the rim. Each the finger is preloaded with sufficient force to support the wafer among the plurality of fingers and each the finger is generally positioned in a radial plane with the rim. The force compensating apparatus comprises a fulcrum mechanism on each fixed arm, a first force receiving mass suspended on a rod projecting from the fulcrum in a direction away from the floating arm, a second force receiving mass suspended on a rod projecting from the fulcrum in a radial direction and a force transferring mass suspended on a rod projecting from the fulcrum in a direction toward the floating arm. The fulcrum, first force receiving mass and force transferring mass form a balance with the force transferring mass contacting the floating arm. The second force receiving mass and the force transferring mass form a rigid triangle. The action of the balance transfers a force to the floating arm to balance the centrifugal force on the finger, and the actions of the triangles transfer a force to the floating arm to balance the acceleration and deceleration forces on the finger.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent Application No. 60/158,513 Entitled: CENTRIFUGAL GRIPPER MECHANISM FOR DYNAMIC FORCE COMPENSATION, incorporated herein by reference filed Oct. 8, 1999. 
    
    
     S 
     TATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT N/A 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to wafer handling and more particularly to an improved wafer handling system. 
     Processing machinery for handling semiconductor wafers is known. Such machinery typically includes the ability to lift and spin the wafers to bring them in proximate distance to testing machinery. It is generally desirable to move the wafers as quickly as possible while keeping the plane of the wafer as flat as possible. While platters and other support mechanisms have been used, increasingly there is a need to handle the wafers by the edges so that both surfaces are accessible. 
     Handling wafers by the edges has involved compromise among the security of the grip, the speed at which the wafer may be moved and the degree of flatness exhibited by the wafer. The wafer can be securely gripped by increasing the inward force of each finger gripping. However, this action may deform the wafer and cause processing errors due to the distorted profile of the wafer. When a wafer is spun, the centrifugal forces act on the finger grippers. These forces tend to pull the finger grippers away from the wafer. If the pull on the gripper is sufficient, the wafer may slip in the grip causing erroneous measurements or the wafer may be released causing destruction of the wafer. Previous efforts to reduce this effect have included adding additional spring force to grip more tightly and compensate for the centrifugal force. However, this extra force can cause the previously noted deformation. Alternately, an upper speed limit can be placed on the rotation thereby limiting the centrifugal force. This limit slows the production line increasing manufacturing cost. 
     Thus there is a need to minimize and eliminate the undesired effect of centrifugal force on gripping fingers without distorting the shape of the wafer or allowing the wafer to slip within the grip. 
     BRIEF SUMMARY OF THE INVENTION 
     In a wafer processing machine where gripper fingers hold the wafer by the edges and the wafer is spun, the invention compensates for the undesired effect of centrifugal force acting on the gripper fingers. The centrifugal force may reduce the finger gripping force on the wafers to the extent of dropping the wafer. The gyroscopic principle is used to balance out the undesired centrifugal force while maintaining a constant desired minimum gripping force regardless of the rotational speed. In other words the forces generated due to motion of the fixtures is prevented from acting on the wafers. In addition, the invention also compensates for the lessening of force between the wafer and the gripping fingers during the acceleration and deceleration, thereby preventing slipping. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which: 
     FIG. 1 a  is a three dinensional representation of the yoke depicted in FIGS. 1 b ,  4 , and  6 ; 
     FIG. 1 b  is a schematic representation of a wafer-handling machine in which the wafer is gripped by the edges; 
     FIG. 2 a  is a schematic of the mechanism to support the finger; 
     FIG. 2 b  is a detail of the finger/wafer interface; 
     FIG. 3 is schematic detail of the preloading of the finger; 
     FIG. 4 is a schematic of the mounting of the support system in the rotor cavity before the installation of the invention; 
     FIG. 5 a  is a schematic of the dynamic force compensation system and its relation to the support according to the invention; 
     FIG. 5 b  is a diagram of the dynamic force compensation system according to the invention; 
     FIG. 6 is a schematic of the mounting of the support in the rotor cavity with the dynamic force compensation system according to the invention; 
     FIG. 7 is a schematic of one embodiment of the dynamic force compensation system according to the invention, 
     FIG. 8 a  is a detail of the pivot block of FIG. 7; 
     FIG. 8 b  is a detail of the force transmission mass according to FIG. 7; 
     FIG. 8 c  is a detail of the pivot with bearings installed of FIG. 8 a;    
     FIG. 8 d  is a detail of the journaling of the pivot of FIG. 8 c;    
     FIG. 8 e  is a detail of the adjustable weights with springs; 
     FIG. 8 f  is a detail of the adjustable weights of both embodiments; and 
     FIG. 9 is a schematic of a second embodiment of the dynamic compensation system according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 b , a measurement station  11  includes, calibration gauges (not shown), three finger grippers  14   a ,  14   b ,  14   c , air bearing rotor  12 , and supporting drive assemblies. The rotor  12  is a yoke that is kept in a well defined and stable plane of rotation by radial support bearings and axial thrust bearings not shown. For insertion and removal of a wafer  10  by a robot, the finger grippers  14  are positioned to locations that prevent interference with the robot. The finger grippers  14  hold the wafer  10  in a plane that is outbound of the face of the rotor  40 . 
     When wafer  10  is loaded onto the measurement station  11 , the wafer  10  is securely held in a vertical position by the finger grippers  14   a-c . The wafer  10  is then rotationally accelerated at a predetermined rate. When the wafer  10  reaches a target rotation rate, the speed is stabilized and sensors are moved across the wafer  10  in a predetermined pattern to measure wafer characteristics such as conductivity, circuit continuity, and process parameters. After the wafer has been measured, the rotation is stopped and the wafer  10  is removed from the measurement station  11 . 
     The finger grippers  14   a-c  function to hold the wafer  10  in a repeatable position for measurement. In the prior art measurement station, all three finger grippers e.g.  14   a-c  are stationary during measurement. The finger grippers  14  are attached to support  20  shown in FIG. 2 a . The support  20  advantageously allows finger gripper movement only along a single axis  25 . 
     The support, as previously described in U.S. Pat. No. 5,456,561, issued Jun. 24, 1997 and commonly assigned, herein incorporated by reference, includes two outer arms  22  and  24  that are adapted to be disposed on the rotor yoke  12  by mounting holes  28  at a first end  23 . A floating arm  26  is connected to the outer arms at a second end  27  and is connected to the gripper finger  14  at its first end, thereby forming the W-shaped support  20 . When sufficient inward pressure is applied to the floating arm  26 , the finger  14  is depressed and the wafer  10  is secured in position. The profile of finger gripper  14  is shown in FIG. 2 b . The fingertip  17  encompasses a groove  15  sized so that the rounded edge  13  of the wafer  10  is securely held by the finger tip  17 . The wafer gripping edges of the finger gripper  14  are typically provided with grooves to facilitate repeated and reliable gripping. 
     Mounting bracket  30  secures the support to the inner rim of the rotor yoke  12  as shown in FIG.  4 . The mounting bracket, as shown in FIG. 3, is secured to the two outer arms  22  and  24  leaving floating arm  26  free. A captive spring  34  is disposed through a hole  32  in the mounting bracket  30 . This spring  32  bears against floating arm  26 , deflecting it to provide sufficient force to engage the finger gripper  14  with the wafer  10 . 
     Rotor yoke  12  is a U-shaped annular ring. Its front surface  40  is generally in the same plane as a wafer being tested. The two legs  44  and  46  and the outer rim  45  of the rotor yoke  12  define an inner cavity  42 . Support  20  is mounted to the inside face  48  of the front leg  44  by the mounting plate  30 . The rotor yoke  12  rotates causing the wafer  10  to rotate. 
     The dynamic force compensation device of the invention is designed to be a fully passive device that automatically adjusts the force exerted on the finger/wafer interface. It is capable of being retrofit in the prior art system previously described as well as in other systems where edge gripping is utilized. Because the compensation system is finger specific, it may be utilized on all or a lesser number of fingers in such an application. When the fingers are displaced to accommodate differently sized wafers, the compensation system may be moved with the fingers. 
     The dynamic force compensation system  80  according to the invention is mounted on support  20  inside cavity  42  as shown in FIG.  6 . The compensation system  80  is thereby subjected to forces proportional to those experienced by the finger gripper/wafer interface but at a slightly greater distance from the center of the wafer  10 . FIG. 5 a  illustrates the placement of compensation system components in relation to the support  20 , while FIG. 5 b  more clearly illustrates the components of the system  80 . 
     During operation, as the yoke  12  undergoes continually increasing/decreasing speeds or is subjected to an acceleration or deceleration (linear or angular) the weighted arms  66  lean or tilt to one of their sides resulting in a pushing or increase in the gripping force of the wafer. This increase helps to overcome the incipient slipping motions between the gripping finger  14  and the wafer  10 . Then as the yoke settles at a steady angular speed (spinning speed), centrifugal forces acting on the finger  14  try to move the finger  14  away from the gripping direction. This attempted movement reduces the gripping force on the wafer  10 . At the same time, the weighted arms  64  are acted on by the centrifugal force and try to move away from the wafer by pivoting on hinges  50 . The result of this action is that the pushers  52  in turn press or push the finger  14  back towards the wafer. The actions described here are dynamic and act to compensate for the forces on the wafer continuously at all spinning speeds and angular acceleration/deceleration of yoke  12 . 
     The dynamic compensation system  80  will now be explained in detail. It consists of two structures that are mirrored about the center of floating arm  26 . The sides operate symmetrically, except that one side compensates for angular acceleration and the other compensates for angular deceleration. Operation of one side will be explained. A fulcrum  50   a , is mounted on outer arm  22  at a predetermined position. This fulcrum  50   a  supports three masses—M a    56   a , M c ,  54   a  and M f    52   a , with M c  and M f  in a straight-line relationship and M a  perpendicular to the other two. Mass M a    56   a  creates the dynamic compensating force for acceleration (deceleration on the other side). Mass M c    54   a  creates the dynamic compensating force for centrifugal force. Mass M f    52   a  is in contact with and transmits the compensating forces to the floating arm  26 . Springs  32 , having a spring constant K weak , represent the constant force used to maintain contact between the finger  14  and the wafer  10  as in the prior art. Springs  70  and  72 , having spring constants K 1  and K 2  respectively, assure that mass  52  maintains contact with the floating arm  26 . 
     When the mechanism  80  is rotating in the direction indicated on FIG. 5 b , centrifugal force pushes against floating arm  26  and against M c    54   a . The force on M c  lifts the mass in the direction of arrow Fc. Since M c  and M f  are rigidly held in a straight-line relationship, the lifting of M c  exerts a downward force at M f . The derivation in the Appendix shows how the values for the various masses and arm lengths are calculated for various configurations. When the counterpart mechanism ( 52   b  and  54   b ) is considered, each transfers half the force needed to compensate for the centrifugal force. 
     When the system  80  is accelerated from no rotation to the steady state rotational speed, acceleration forces act to cause the wafer  10  to slip in the finger&#39;s grip. Mass M a    56   a  acts to counteract this lifting force. As the wafer/mechanism starts rotating in the direction shown about leg  66   a , M a    56   a  resists the motion by trying to rotate in the opposite direction, toward floating finger  26 . This causes M f    52   a  to press downward against floating finger  26 , exerting more force against the finger/wafer interface. When the system  80  is decelerated from the steady state rotational speed rest, deceleration forces act to weaken the grip on the wafer  10 . M a    56   b  acts to counteract this lifting force. As the wafer/mechanism starts decelerating in the direction shown about leg  66   b , M a    56   b  resists the motion by trying to rotate in the opposite direction, toward floating finger  26 . This causes M f    52   b  to press downward against floating finger  26  exerting more force against the finger/wafer interface. 
     FIG. 7 illustrates one implementation of the invention. Here, the fulcrum  50  is implemented using a pivot  90 , further illustrated in FIGS. 8 c  and  8   d . Threaded arms  92  and  96  allow positioning of weights  94  and  98 , implemented as an adjustable weight with a locking screw. Spring  104  provides tension to the arms  96 , places a fixed bias on the pushers  100  and ensure that the mechanism  80  is chatter and backlash free. Mounting bracket  31  provides a mounting platform for the mechanism  80  and allows the entire mechanism  80  to be added to an existing mounting plate such as plate  30  for ease of installation. 
     FIG. 8 a  illustrates the central rocker  122  of pivot  90 . Central rocker  122  may be made of machined or molded material and exhibits a rounded bottom surface to facilitate pivoting about a central shaft  112  (FIG. 8 d ). A central passageway  130  is provided to accommodate the central shaft  112 . Three branches  140 ,  142 ,  144  are disposed at 90° increments about the top half of the central rocker  122 . Each branch is tapped providing a socket  132  for the reception of a rod. The sockets may exhibit threads for securing and adjusting the positioning of the rods and may be outfitted with setscrew taps  134  for use of a setscrew for the securing and adjusting the positioning of the rods. 
     To facilitate pivoting, a ball bearing may be inserted in central passage  130  as illustrated in FIG. 8 c . The bearing is sized such that the inner race  114  tightly fits against central shaft  112 . Alternately, a journal bearing may be used in the pivot  90 . The central shaft  112  rests in grooves in the pivot housing  110  as shown in FIG. 8 d . Pusher weight  100  is at the end of shaft  102 , spaced such that the pusher  100  rests on floating arm  26  inboard of the attachment point of the finger gripper  14 . The weights  94  and  98  on the other two shafts  92  and  96 , may be implemented as threaded split collars  140  as in FIG. 8 f , the threading being used to adjust the location of the weight on the shaft. Spring  104  between shafts  96  is secured by looping the spring around a thread in shaft  96  as shown in FIG. 8 e.    
     In a second aspect, the pivot  90  is implemented as a living hinge or flexure  150  as illustrated in FIG.  9 . The hinge  150 , made of flexure material that flexes in the direction indicated in FIG. 9 when force is placed on it, is particularly useful for areas where particulate contamination is of concern. Weight  94  causes the pusher  100  to be depressed when centrifugal force is present, while weight  98  causes the pusher  100  to be depressed when acceleration or deceleration is experienced. Because the pusher is not attached to the floating arm  26 , there is no effect on floating arm  26  when the pusher  100  is lifted from the floating arm  26 . 
     Both aspects described have the advantage that the positioning of the weights on the arms is adjustable such that their distance from the pivot  22  can be varied to make fine adjustments. The use of threaded shafts and/or set screws to position the weights further facilitates these adjustments. 
     While the invention has been described in relation to wafer holding by the edges, the principled described herein may be applied to other configurations of workpieces held in a rotary chuck. 
     Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims. 
     APPENDIX 
     The attached derivation may be understood with reference to FIG. 5 b  that illustrates the components of the invention in schematic form. Two forces need to be countered, the lifting centrifugal force and the tangential force from acceleration and deceleration. Since forces are additive, each may be analyzed separately. 
     The mechanism uses the principle of the lever arm to compensate the forces generated due to the centrifugal force. 
     Legend 
     m a  is the mass to generate compensating forces to angular acceleration and deceleration. 
     m f  is the mass equivalent of the arm  62  (Force Arm) which would impart the required stabilizing force on the gripper finger. 
     m c  is the mass to generate compensating forces to varying centrifugal force on the gripper finger. 
     K 1  and K 2  are stiffness of the priming springs which will make sure that the ‘Force Arms’ are always in contact with the back of the gripper finger. It would be sufficient to have either one of them depending upon the mechanical convenience they provide. 
     K weak  is the stiffness of the gripper pre-load spring, which ensures that there exists a minimum amount of gripping force between the finger and the wafer. 
     x is the pre-compression in the priming springs. 
     α is the angular acceleration/deceleration. 
     F c  is the centrifugal force action on m c . 
     F=centrifugal Force. 
     m=mass of a finger (single) [also it could be the mass of the floating arm]. 
     r f =the effective radius of the finger and floating arm center of mass from the center of the wafer (Near top of the finger). 
     r m =the radius of the finger center of mass from the center of the wafer (&lt;r f ). 
     g=32.3 ft/sec2—acceleration due to gravity. 
     s=rotational speed (rpm). 
     R=the nominal radius of rotation of the gripper mechanism (finger tip) from the center of the wafer R=(r f +r m )/2. 
     ω=angular speed of the wafer rad/sec:=       ω   :=     s          2      πrad       1                 sec                                
     F f =Centrifugal force on the finger:= 
     
       
           F   f   =m   finger ω 2   r   f   
       
     
     F bare =The minimum force to support a wafer without distortion.          F   bare     :=           +     K   1       ·     (   x   )            (       l   66       l   62       )       +     2        (       K   weak          ∂     pre   -   comp         )                                
     Case A: Balancing the Angular Acceleration &amp; Deceleration 
     Here the goal is to have the gripper impart a bare minimum force to the wafer in the form of supporting it, and when the wafer is subject to an angular acceleration or deceleration the gripping force is increased slightly or appropriately so that there is no slippage between the wafer and the gripper finger. 
     During the acceleration and deceleration of the wafer it is necessary that the gripper force be increased from F base  to F sufficient  to avoid slippage. The difference between F bare  and F sufficient  is generated by the action of m a  alone. Using the moment balance equation, mass m a  can be determined as follows: 
     At the instant of acceleration the F sufficient  can be given by the following equation 
     
       
           F   sufficient   :=m   a   ·α·R ·1 66   +F   bare   
       
     
     The 1 st  term determines the additional force that is required during acceleration and deceleration. Neglecting the infinitesimally small deflections in the springs K 1  and K weak  at the instant of acceleration/deceleration and hence the forces generated due to them, and also the inertial effects of the spring connectors,          m   a     :=         (       F   sufficient     -     F   bare       )          (     l   62     )         [     α   ·   R   ·     (     l   62     )       ]                              
     Note: In the event of Case A only one of the mass m a  will be generating the compensating force, while the other one will be contributing no force in the mechanism, although there might be some pulling action on the spring K 1  For example, if F bare =1 lbf 
     F sufficient  is the minimum gripping force required to avoid slippage, it depends on three elements: 
     the inertial mass of the wafer I wafer , 
     angular acceleration and α 
     coefficient of friction between the μ wafer-finger    
     gripper finger and the wafer edge it counteracts the tangential force at the periphery of the wafer and can be approximated as follows: 
     At the instant of acceleration the inertial torque generated by the wafer would be 
     
       
           T   a   :=I wafer·α 
       
     
     where I wafer  is the mass moment of inertial of a 300 mm wafer given by α=6 πn rad/sec 2           I   wafer     :=           (     1   2     )     ·         m   wafer          (     r   wafer     )       2                       if                     M   wafer                   0.14                   kg                     and                     r   wafer       =     0.150                   m                                
     then I wafer  is 1.575·10 −3  kg-m 2    
     and                T   a     =         I   wafer        α     :=         (     1   2     )     ·         m   wafer          (     r   wafer     )       2          α                   =     0.03                   N m                                    
     Based on this torque, the tangential force at the periphery of the wafer is:          F   tan     =       T   a       r   wafer                 F   tan     =       1   2          m   wafer          r   wafer        α               F   tan     =     0.198                   N                              
     Considering the coefficient of friction between the finger and the wafer,          F   sufficient     =         F   tan     μ     =         1     2      μ            m   wafer          r   wafer       =     0.66                   N                                  
     For gripping force of F bare  the resisting force for slippage would be 
       F   slippage   :=F   bare ·μ wafer     —     finger   5    F   slippage =1.334· N   
     At the current configuration the F slippage &gt;F sufficient  and hence there will be no slippage 
     Assuming a reasonable values for some of the variables For example: assume that F sufficient  is little higher than that is calculated above. 
     
       
           F   sufficient     —     new :=2 ·F   sufficient   
       
     
     
       
         1 62 :=25.4 mm 
       
     
     
       
         1 66 :=25.4 mm        α   =     18.85        1     s   2                               
      
     
     
       
           R =0.179 m 
       
     
     
       
           F   sufficient     —     new =1.319 N 
       
     
     The value of m a  shows that it would be impossible for slippage to occur. 
     If still decide to try a mass m a  on link  1   66 . The wafer would see the following magnitude of the force. m anew :=20 gm          F       force   —          on   —        wafer       :=         M   aneq     ·   α   ·   R   ·       (       l   66       l   62       )     30       +     F   bare                              F   force     —     on     —     wafer =1.015 lbf 
     This means we can increase the mass even more if required. 
     Case B: Balancing the Centrifugal Force 
     Here we desire a constant equilibrium between the disturbing centrifugal force and the balancing or compensating force generated by the two masses m c . When there is no centrifugal force, F actual =F bare  and when the centrifugal force is balanced out, 
     
       
           F   actual   =F   bare   +F   comp , 
       
     
     Since the fulcrum dictates that m f l 62  =m c l 64 , the compensating force generated by the masses m c  must balance out the Centrifugal force. 
     
       
         2 m   c [·ω 2   ·R ·(1 64 )]:= m   finger ω 2   r   f   l   62   
       
     
     From the moment balance equation:          m   c     :=         (       F   actual     -     F   bare       )     ·     (     l   62     )         [     2   ·     ω   2     ·   R   ·     (     l   64     )       ]                              
     It should be noted that here both the masses m c  will be contributing to counteract the F actual . 
     From the above equations one can determine the magnitude of masses m a  and m c  with reasonable approximation. If 
     F actual —13.327N—The centrifugal force generated by the mass of the floating arm and the finger at 6 r.p.s 
     F bare =4.448—The bare gripping force desired, independent of speed. 
     From the above, it is clear that there is a force imbalance at 6 r.p.s. 
     Assuming reasonable values for some of the variables in the Equations. 
     
       
         R=179.22 mm 
       
     
     
       
         1 62 =25.4 mm 
       
     
     
       
         m c :=20 gm 
       
     
     if s=6, ω=37.699 Hz 
     F actual =13.327 N The actual centrifugal force acting on the Finger including Mechanism 
     F bare =4.448 N          l   64     :=         (       F   actual     -     F   bare       )       (     2   ·     m   2     ·   ω   ·   R     )       ·     (     l   62     )                              
     1 64   =22.135° m