Abstract:
Specimen edge-gripping prealigners ( 8, 80 ) grasp a wafer ( 10 ) by at least three edge-gripping capstans ( 12 ) that are equally spaced around a periphery ( 13 ) of the wafer. Each edge-gripping capstan is coupled by a continuous synchronous belt ( 14 ) to a drive hub ( 15, 84 ) that is rotated by a drive motor ( 18, 88 ). The belts are tensioned by idler pulleys ( 22, 92 ) that are rotated by a motive force ( 25, 96, 102 ). The edge-gripping capstans and the drive drums are mounted to hinged bearing housings ( 28, 112 ) that are spring biased to urge the capstans away from the drive hub. Deactivating the motive force rotates the idler plates into a belt tensioning position that draws the capstans inward to grip the periphery of the wafer. Once gripped, rotation of the drive hub is coupled through the tensioned belts to the capstans. Driving all the capstans provides positive grasping and rotation of the wafer without surface contact with the wafer and thereby reduces wafer damage and particle contamination.

Description:
FIELD OF THE INVENTION 
     This invention is directed to a specimen prealigning apparatus and method and, more particularly, to an edge gripping semiconductor wafer prealigner that substantially reduces wafer backside damage and particulate contamination. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are produced from wafers of semiconductor material. The wafers are typically housed in a cassette having a plurality of closely spaced slots, each of which can contain a wafer. The cassette is typically moved to a processing station where the wafers are removed from the cassette, placed in a predetermined orientation (prealigned), and returned to another location for further wafer processing. 
     Various types of wafer handling devices are known for transporting the wafers to and from the cassette and among processing stations. Many employ a robotic arm having a spatula-shaped end that is inserted into the cassette to remove or insert a wafer. The end of the robotic arm typically employs vacuum pressure to releasably hold the wafer to the end of the arm. The robotic arm enters the cassette through the narrow gap between an adjacent pair of wafer slots and engages the backside of a wafer to retrieve it from the cassette. After the wafer has been processed, the robotic arm inserts the wafer back into the cassette. 
     U.S. Pat. No. 5,513,948 for UNIVERSAL SPECIMEN PREALIGNER, which is assigned to the assignee of this application, and U.S. Pat. No. 5,238,354 for SEMICONDUCTOR OBJECT PRE-ALIGNING APPARATUS describe prior semiconductor wafer prealigners that include a rotating vacuum chuck on which the wafer is placed by a robot arm for prealigning. 
     Unfortunately, transferring the wafer among the cassette, robot arm, and prealigner may cause backside damage thereto and contamination of the other wafers housed in the cassette because engagement with the wafer may dislodge particles that can fall and settle onto the other wafers. Robotic arms and prealigners that employ a vacuum pressure to grip the wafer can be designed to minimize particle creation. Even the few particles created with vacuum pressure gripping or any other non-edge gripping method are sufficient to contaminate adjacent wafers housed in the cassette. Reducing such contamination is particularly important to maintaining wafer processing yields. Moreover, the wafer being transferred may be scratched or abraded on its backside, resulting in wafer processing damage. 
     What is needed, therefore, is a wafer gripping technique that can securely, quickly, and accurately prealign wafers while minimizing particle contamination and wafer scratching. 
     SUMMARY OF THE INVENTION 
     An object of this invention is, therefore, to provide an apparatus and a method for prealigning semiconductor wafers. 
     Another object of this invention is to provide an apparatus and a method for quickly and accurately prealigning specimens. 
     A further object of this invention is to provide an apparatus and a method for prealigning wafers while minimizing particle contamination and wafer scratching. 
     Specimen edge-gripping prealigners of this invention grasp a wafer by at least three edge-gripping capstans that are preferably equally spaced around the periphery of the wafer. Each of the edge-gripping capstans is coupled by a continuous synchronous belt to an axially centered, grooved drive hub that is rotated by a drive motor. Each of the capstans is also coaxially connected to a grooved drive drum that is coupled to the drive hub by one of the continuous synchronous belts, and each belt is routed in a unique location in a set of grooves in the drive drums and the drive hub. The continuous synchronous belts are tensioned by idler pulleys that are mounted to axially rotatable idler plates that are coupled together for common rotation by a belt tensioning motor or some other form of rotary biasing force, such as a spring, solenoid, or vacuum pressure actuated piston. 
     The edge-gripping capstans and the grooved drive drums are mounted to hinged bearing housings that are pivotally spring biased to preload the grooved drive drums radially away from the axially centered drive hub. The edge-gripping capstans can be driven radially inward to grip the wafer by rotating the belt tensioning motor to apply sufficient tension to overcome the spring preload force on the idler plates. Once gripped, the wafer can be rotated by energizing the drive motor to rotate the drive hub, which rotation is coupled through the tensioned belts and drive drums to the capstans. 
     The edge-gripping specimen prealigner of this invention is suitable for prealigning semiconductor wafers. Simultaneously rotating all the edge-gripping capstans provides positive rotation of the wafer without wafer surface contact, which eliminates wafer backside damage. Synchronously driving of all the capstans prevents slippage between each capstan and the wafer and thereby results in minimized edge contamination. 
     Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional elevation view of a first embodiment of an edge-gripping specimen prealigner of this invention showing internal details of motors, belt drives, capstans, and a specimen peripheral edge scanner. 
     FIG. 2 is a sectional top view taken along lines  2 — 2  of FIG. 1 showing belt driving and tensioning mechanisms coupling a drive motor to three specimen edge gripping capstans. 
     FIG. 3 is a sectional elevation view taken along lines  3 — 3  of FIG. 2 showing internal details of a representative drive drum and specimen edge gripping capstan of this invention. 
     FIG. 4 is an enlarged sectional view of an edge-gripping capstan gripping a wafer periphery in a manner according to this invention. 
     FIG. 5 is a sectional elevation view of a second embodiment of an edge-gripping specimen prealigner of this invention showing internal details of motors, belt drives, and capstans. 
     FIG. 6 is a bottom view of FIG. 5 showing belt driving and tensioning mechanisms coupling a drive motor to six specimen edge gripping capstans that are in a specimen edge-gripping position. 
     FIG. 7 is a bottom view of FIG. 5 showing belt driving and tensioning mechanisms coupling a drive motor to six specimen edge gripping capstans that are in a specimen releasing position. 
     FIG. 8 is an enlarged sectional elevation view showing internal details of a representative drive drum, specimen edge gripping capstan, and specimen peripheral edge scanner of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show sectional side and bottom views of a first preferred embodiment of a specimen edge-gripping prealigner  8  (hereafter “prealigner  8 ”). Prealigner  8  is composed of a frame  9  to which three edge-gripping capstans  12  are movably mounted and positioned to grasp a generally circular specimen, such as a wafer  10  (shown in phantom in FIG.  2 ). The capstans  12  are preferably spaced equally apart and located along a circle generally defined by a periphery  13  (shown in dashed lines in FIG. 2) of wafer  10 . Periphery  13  may include “flat” and “notch” features, which are used for orientating wafer  10 . Prealigner  8  may be adapted for use with any generally circular specimens. 
     Edge-gripping capstans  12  are coupled by continuous synchronous belts  14  to a grooved drive hub  15  that is journaled in bearings  16  for rotation about a rotational axis  17  by a motor  18 , all of which are supported by frame  9 . Edge-gripping capstans  12  are directly coupled to grooved drive drums  20 . Each drive drum  20  is coupled to drive hub  15  by a different one of the three continuous synchronous belts  14 . Each of belts  14  is routed at a different elevation around the same set of associated grooves in its corresponding drive drum  20  and drive hub  15 . The resulting rotation of edge-gripping capstans  12  takes place about capstan axes  21 , which extend parallel to rotational axis  17 . 
     Continuous synchronous belts  14  are tensioned by idler pulleys  22  that are mounted to radially extending arms of an axially rotatable idler plate  24 , which is shown in FIG. 2 rotated to a belt tensioning position  24 A (solid lines) and an alternate belt untensioned position  24 B (phantom lines). Idler plate  24  is rotated through a predetermined angular range about rotational axis  17  by a motor  25  or some other rotary biasing force, such as a spring and a solenoid. Motor  25  and idler plate  24  are journaled for rotation about bearings  26 , all of which are supported by frame  9 . 
     Referring to FIG. 3, each of grooved drive drums  20  is journaled for rotation about bearings  27  that are mounted in associated ones of hinged bearing housings  28 . Bearing housing  28  are journaled for pivotal movement about bearings  29 , which are supported by frame  9 . The pivoting of hinged bearing housings  28  allows radial displacement of capstan axis  21  relative to rotational axis  17 . The pivoting of hinged bearing housings  112  allows radial displacement of capstan axis  21  relative to rotational axis  17 . Each of hinged bearing housings  28  includes a coil spring  30  that preloads drive drum  20  away from rotational axis  17 . To ensure proper movement of edge-gripping capstans  12 , each of hinged bearing housings  28  further includes a vane  120 , that protrudes from the end of hinged bearing housing  28  opposite pivot axis  116 ,. Depending on the rotational state of hinged bearing housing  112 , vane  120 , is positioned to alternately interrupt (see FIG. 6 showing this position for an alternative embodiment) or not interrupt (see FIG. 7 showing this position for an alternative embodiment) a light beam within an optical sensor  122 ,. All three of optical sensors  122 , acting together provide a positive electrical indication of whether prealigner  8  is in a wafer gripping state or a wafer releasing state. 
     FIG. 4 shows an enlarged view of a representative one of edge-gripping capstans  12 , which includes a wafer-contacting pulley  31  that may be formed from various materials, and preferably polyetheretherketone (“peek”), a semi-crystalline high temperature thermoplastic manufactured by Victrex in the United Kingdom. The material forming wafer-contacting pulley  31  may be changed to suit the working environment, such as in high temperature applications. Peek material provides a contamination resistant low scratching wafer contacting surface. 
     Wafer-contacting pulley  31  includes a load/unload portion  32  ramped at a shallow angle for supporting wafer  10  when capstan  12  is in its specimen gripping and nongripping positions. Pulley  31  also includes an inwardly inclined ramp-backstop portion  34  that is pressed against the periphery  13  of wafer  10  when capstan  12  is in its specimen gripping position. 
     Load/unload ramp portion  32  has a radial width  36  that allows adequate range for the wafer positioning variation of the mechanism which loads the wafer onto the prealigner. Load/unload ramp portion  32  is angled downwardly from the plane of wafer  10  by an angle greater than 0 degrees, and preferably 1 to 5 degrees. 
     Inwardly inclined backstop portion  34  has a height  38  large enough to capture wafer  10 , preferably between about 1 mm and 2 mm and is angled upwardly from the plane of wafer  10  to secure it by about 3 degrees. 
     Load/unload ramp portion  32  and backstop portion  34  together form an intersecting pair of truncated right conical sections having an included angle for gripping periphery  13  of wafer  10 . 
     When edge-gripping capstans  12  are actuated to press against periphery  13  of wafer  10 , the intersecting inclined conical surfaces formed by load/unload ramp portion  32  and inwardly inclined backstop portion  34  positively grip and maintain wafer  10  in a preferable horizontal attitude, although other attitudes are possible. When edge-gripping capstans  12  are released from gripping wafer  10 , load/unload ramp portion  32  supports the periphery  13  of wafer  10 . 
     A typical operational sequence for prealigner  8  is described below with reference to FIGS. 1 and 2. 
     Prealigner  8  is in an initial state in which no wafer  10  is present and idler plate  24  is in belt untensioning position  24 B. 
     A robot arm  50  (fragmentary view shown in FIG. 1) grips wafer  10  by periphery  13  and positions wafer  10  at a wafer position  10 A that is separated apart from but substantially parallel to a plane passing through load/unload ramp portions  32  of edge-gripping capstans  12 . Robot arm  50  performs wafer  10  positioning movements in one of the approximately 120-degree clearance spaces between edge-gripping capstans  12 . A specimen edge-gripping robot arm suitable for use with this invention is described in copending U.S. Pat. application Ser. No. 09/204,747, filed Dec. 2, 1998, for ROBOT ARM WITH SPECIMEN EDGE GRIPPING END EFFECTOR, which is assigned to the assignee of this application. 
     Robot arm  50  lowers wafer  10  to a wafer position  10 B such that wafer  10  is supported by the load/unload ramp portions  32  of edge-gripping capstans  12 . 
     Robot arm  50  disengages from wafer  10  and moves to a wafer disengaged position (shown in dashed lines). Robot arm  50  may stay at the wafer disengaged position during subsequent wafer prealigning operations or it may be withdrawn from prealigner  8 . 
     Motor  25  is actuated to rotate idler plate  24  from untensioned position  24 B to tensioned position  24 A to provide sufficient tension in belts  14  to overcome the preload force applied to grooved drive drums  20  and to draw edge-gripping capstans  12  radially inward to grip periphery  13  of wafer  10 . 
     Once gripped, wafer  10  is rotated by energizing motor  18  to rotate drive hub  15 , which rotation is coupled through tensioned belts  14  and drive drums  20  and, therefore, to edge-gripping capstans  12 . Preferably all of edge-gripping capstans  12  are driven to prevent rotational slippage, even though wafer  10  is gripped with minimal force. 
     During rotation of wafer  10 , a linear charge-coupled device (“CCD”) array  52  receives an image of a slice of periphery  13  of wafer  10 . Periphery  13  is illuminated through a collimating lens  53  by a light source  54  that casts a shadow of the periphery  13  on CCD array  52 . The “terminator” position of the shadow on individual sensors in the CCD array  52  provides a signal from CCD array  52  that accurately represents a radial distance between rotational axis  17  and periphery  13  for each of a set of rotational angles of wafer  10 . CCD array  52  may also sense when wafer  10  is gripped by detecting a lateral movement of periphery  13 . 
     An optical rotary encoder  56  provides feedback to control the rotation of motor  25 . A notch (not shown) in periphery  13  serves as an angular index mark for determining in cooperation with optical rotary encoder  56  the actual rotational angles of wafer  10  since there is uncertainty of the actual effective radii of the wafer  10  and the edge-gripping capstans  12 . 
     Prealigning of wafer  10  may be carried out in the manner described in the above-referenced U.S. Pat. No. 5,513,948 for UNIVERSAL SPECIMEN PREALIGNER. 
     After wafer  10  is prealigned, motor  18  is deactivated, motor  25  rotates idler plate  24  to belt untensioning position  24 B, and robot arm  50  retrieves wafer  10  from prealigner  8 . 
     FIGS. 5,  6 , and  7  show respectively a sectional side view and two bottom views of a second preferred embodiment of a specimen edge-gripping prealigner  80  (hereafter “prealigner  80 ”). Prealigner  80  is composed of a frame  82  to which six edge-gripping capstans  12  are movably mounted and positioned to grasp a generally circular specimen, such as wafer  10  (shown in phantom in FIGS.  6  and  7 ). The capstans are spaced apart and located along a circular plane generally defined by a periphery  13  (shown in dashed lines in FIGS. 6 and 7) of wafer  10 . Periphery  13  typically includes a “notch” feature for identifying a rotational index orientation for wafer  10 . FIGS. 6 and 7 show periphery  13  of wafer  10  respectively gripped and released by edge-gripping capstans  12 . 
     Prealigner  80  may be adapted for use with generally circular specimens, such as wafer  10  having a nominal diameter ranging from about 200 mm to 300 mm, although other diameters would also be applicable. 
     Edge-gripping capstans  12  are coupled by continuous synchronous belts  14  to a drive hub  84  that is journaled in bearings  86  for rotation about rotational axis  17  by a motor  88 , all of which are supported by frame  82 . Edge-gripping capstans  12  are directly coupled to drive drums  90 . Each drive drum  90  is coupled to drive hub  84  by a different one of the six continuous synchronous belts  14 . Each of belts  14  is routed at different elevations around the same set of associated grooves in its corresponding drive drum  90  and drive hub  84 . The resulting rotation of edge-gripping capstans  12  takes place about capstan axes  21 , which extend parallel to rotational axis  17 . 
     Continuous synchronous belts  14  are tensioned by idler pulleys  92  that are mounted at the ends of arms that extend radially from an axially rotatable idler plate  94 , which is shown in FIG. 6 rotated to a belt tensioning position and in FIG. 7 rotated to a belt untensioned position. Idler plate  94  is rotated through an angular range about rotational axis  17  by a vacuum pressure actuated piston  96  acting through a coupling link  98  that is attached to the end of one of the arms of idler plate  94 . Idler plate  94  is journaled in bearings  100  for rotation about rotational axis  17 . 
     When vacuum pressure actuated piston  96  receives no vacuum pressure and/or prealigner  80  is deenergized, a set of springs  102  extending between a rotationally adjustable hub  104  and the arms of idler plate  94  provides a biasing force that rotates idler plate  94  to the belt tensioning position shown in FIG.  6 . This is advantageous because prealigner  80  will remain in a wafer gripping state in the event of a power or vacuum pressure failure. The amount of biasing force is adjustable by rotating adjustable hub  104 . While a single spring  102  could provide the biasing force, multiple springs are preferred because they provide a more uniform and linear biasing force to idler plate  94 . Of course, when moving idler plate  94  to the belt relaxing position shown in FIG. 7, vacuum pressure actuated piston  96  must provide sufficient force to overcome the biasing force of springs  102 . 
     Drive hub  84  and drive drums  90  have unequal diameters that provide about a 3.6:1 drive ratio from drive hub  84  to drive drums  90  in a preferred embodiment. The rotational position of drive hub  84  is sensed by a conventional glass scale rotary encoder  106  and an associated optical sensor  108 . 
     Referring also to FIG. 8, each drive drum  90  is journaled on bearings  110  that are mounted in associated ones of hinged bearing housings  112 . The hinged bearing housings  122  are journaled on bearings  114  for pivoting about a pivot axis  116 . The pivoting of hinged bearing housings  112  allows radial displacement of capstan axis  21  relative to rotational axis  17 . Each of hinged bearing housings  112  further includes a coil spring  118  that preloads drive drum  90  radially away from rotational axis  17 . 
     The preloading force provided by springs  118  is sufficient to move edge-gripping capstans  12  radially away from rotational axis  17  when belts  14  are in the untensioned state, but the preloading force is insufficient when belts  14  are in the tensioned state. Accordingly, edge-gripping capstans  12  alternate between wafer gripping and wafer releasing positions in response to actuation of vacuum pressure actuated piston  96 . To ensure proper movement of edge-gripping capstans  12 , each of hinged bearing housings  112  further includes a vane  120  that protrudes from the end of hinged bearing housing  112  opposite pivot axis  116 . Depending on the rotational state of hinged bearing housing  112 , vane  120  is positioned to alternately interrupt (FIG. 6) or not interrupt (FIG. 7) a light beam within an optical sensor  122 . All six of optical sensors  122  acting together provide a positive electrical indication of whether prealigner  80  is in a wafer gripping state or a wafer releasing state. 
     A typical operational sequence for prealigner  80  is described below with reference to FIGS. 5,  6 ,  7 , and  8 . 
     Prealigner  80  is in an initial state in which no wafer  10  is present and idler plate  94  is in the belt untensioning position shown in FIG.  7 . 
     A robot arm (not shown) grips wafer  10  by periphery  13  and positions wafer  10  similar to the manner described-above for prealigner  8 . 
     The robot arm lowers wafer  10  such that wafer  10  rests on load/unload ramp portions  32  of edge-gripping capstans  12 . 
     The robot arm disengages from wafer  10  and moves to a wafer disengaged position. The robot arm may stay at the wafer disengaged position during subsequent wafer prealigning operations or it may be withdrawn from prealigner  80 . 
     Vacuum pressure actuated piston  96  is deactuated to rotate idler plate  94  from the belt untensioned position shown in FIG. 7 to the belt tensioned position shown in FIG. 6, thereby drawing edge-gripping capstans  12  radially inward to grip periphery  13  of wafer  10 . 
     Once gripped, wafer  10  is rotated by energizing motor  88  to rotate drive hub  84 , which rotation is coupled through tensioned belts  14  and drive drum  90  and, therefore, to edge-gripping capstans  12 . Preferably all of edge-gripping capstans  12  are driven to prevent rotational slippage, even though wafer  10  is gripped with minimal force. 
     During rotation of wafer  10 , a linear charge-coupled device (“CCD”) array  124  receives an image of a slice of periphery  13  of wafer  10 . Periphery  13  is illuminated through a collimating lens  126  by a light source  128  that casts a shadow of the periphery  13  on CCD array  124 . The “terminator” position of the shadow on individual sensors in the CCD array  124  provides a signal from CCD array  124  that accurately represents a radial distance between rotational axis  17  and periphery  13  for each of a set of rotational angles of wafer  10 . CCD array  124  may also sense when wafer  10  is gripped by detecting a lateral movement of periphery  13 . 
     Rotational axis  17  is substantially coaxial with the effective center of wafer  10  because of the angular spacing of edge-gripping capstans  12  around periphery  13 . Edge-gripping capstans  12  are arranged in two groups of three, with the groups on opposite sides of a first imaginary line  130  extending through rotational axis  17  and CCD array  124 . Adjacent capstans  12  in each group are angularly spaced apart from each other, with the center capstan in each group having its capstan axis  21  lying in a second imaginary line  132  that extends perpendicular to the first imaginary line  130  and through rotational axis  17 . 
     The amount of angular rotation imparted by edge-gripping capstans  12  to wafer  10  is sensed by rotary encoder  106  and optical sensor  108  that is coupled to drive hub  84 . A notch (not shown) in periphery  13  serves as an angular index mark for determining in cooperation with rotary encoder  106  and optical sensor  108  the actual rotational angles of wafer  10 . Because the diameter of wafer  10  is a variable and wafer periphery  13  may be square, chamfered, or rounded, an angular encoding calibration is carried out as follows. Wafer  10  is rotated until CCD array  124  senses the notch. Wafer  10  is rotated one complete revolution until CCD array  124  again senses the notch. During one complete notch-to-notch revolution of wafer  10 , the distance travelled is measured by the optical sensor  108 . The total distance measured is divided by one revolution in the appropriate unit system to derive the appropriate relationship between the distance units of optical sensor  108  and wafer rotational units. During a subsequent notch-to-notch rotation of wafer  10 , a set of radius measurements made at predetermined angular intervals by CCD array  124  sensing periphery  13  of wafer  10  as described above. 
     Thereafter, rotational prealigning of wafer  10  may be carried out in the manner described in the above-referenced U.S. Pat. No. 5,513,948. 
     After wafer  10  is prealigned, vacuum pressure actuated piston  96  is activated to rotate idler plate  94  to belt untensioned position shown in FIG. 7, and the robot arm retrieves wafer  10  from prealigner  80 . 
     Skilled workers will recognize that portions of this invention may be implemented differently from the implementations described above for preferred embodiments. For example, different drive hub to capstan ratios may be employed. Three and six capstan embodiments are shown, but many embodiments with more than three capstans are envisioned can be implemented. Also, the capstans necessarily require neither equal angular spacing around the specimen nor the spacings shown and described in the above-described embodiments. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. Accordingly, it will be appreciated that this invention is also applicable to specimen handling applications other than those found in semiconductor wafer processing. The scope of the present invention should, therefore, be determined only by the following claims.